Comprehensive Chemical Kinetics, Volume 19: Simple Processes at the Gas-Solid Interface

COMPREHENSIVE CHEMICAL KINETICS

COMPREHENSIVE Section 1 .

THE PRACTICE AND THEORY OF KINETICS ( 3 volumes)

Section 2.

HOMOGENEOUS DECOMPOSITION AND ISOMERISATION REACTIONS ( 2 volumes)

Section 3.

INORGANIC REACTIONS ( 2 volumes)

Section 4.

ORGANIC REACTIONS ( 6 volumes)

Section 5.

POLYMERISATION REACTIONS ( 3 volumes)

Section 6.

OXIDATION AND COMBUSTION REACTIONS ( 2 volumes)

Section 7 .

SELECTED ELEMENTARY REACTIONS ( 1 volume)

Section 8.

HETEROGENEOUS REACTIONS ( 4 volumes)

Section 9.

KINETICS AND CHEMICAL TECHNOLOGY (1 volume)

Section 10. MODERN METHODS, THEORY, AND DATA

CHEMICAL KINETICS EDITED BY

C.H. BAMFORD M.A.,Ph.D.,Sc.D. (Cantab.),F.R.I.C.,F.R.S. Formerly Campbell-Brown Professor of Industrial Chemisiry, University of Liverpool

The late C.F.H. TIPPER Ph.D. (Bristol), D.Sc. (Edinburgh) Senior Lecturer in Physical Chemistry, University of Liverpool AND

R.G. COMPTON M.A., D.Phil. (Oxon.) Lecturer in Physical Chemistry; University o f Liverpool

VOLUME 19

SIMPLE PROCESSES AT THE GAS-SOLID INTERFACE

ELSEVIER AMSTERDAM-OXFORD-NEW 1984

YORK-TOKY 0

ELSEVIER SCIENCE PUBLISHERS B.V.

Molenwerf 1, P.O. Box 211,1000 AE Amsterdam, The Netherlands

Distributors for the United States and Canada

ELSEVIER SCIENCE PUBLISHING COMPANY INC.

52 Vanderbilt Avenue New York, N.Y. 10017

ISBN 0-444-41631-5 (Series) ISBN 0-444-42287-0 (Vol. 19) with 117 illustrations and 8 tables

@ Elsevier Science Publishers B.V., 1984 All rights reserved. No part of this publication may be reproduced, stored in a retrieval system o r transmitted in any form o r by any means, electronic, mechanical, photocopying, recording or otherwise, without the prior written permission of the publisher, Elsevier Science Publishers B.V., P.O. Box 330, 1000 AH Amsterdam, The Netherlands Printed in The Netherlands

COMPREHENSIVE CHEMICAL KINETICS

ADVISORY BOARD Professor S.W. BENSON Professor SIR FREDERICK DAINTON Professor G. GEE Professor G.S. HAMMOND Professor W. JOST Professor G.B. KISTIAKOWSKY Professor K.J. LAIDLER Professor M. MAGAT Professor SIR HARRY MELVILLE Professor S. OKAMURA Professor N.N. SEMENOV Professor Z.G. SZABO Professor 0. WICHTERLE

Volumes in the Series Section 1. Volume 1 Volume 2 Volume 3

The Practice of Kinetics The Theory of Kinetics The Formation and Decay of Excited Species Section 2.

Volume 4 Volume 5

SELECTED ELEMENTARY REACTIONS (1 volume

Selected Elementary Reactions Section 8.

Volume 19 Volume 20 Volume 21 Volume 2 2

OXIDATION AND COMBUSTION REACTIONS ( 2 volumes)

Liquid-phase Oxidation Gas-phase Combustion Section 7.

Volume 18

POLYMERISATION REACTIONS ( 3 volumes)

Degradation of Polymers Free-radical Polymerisation Non-radical Polymerisation Section 6.

Volume 16 Volume 17

ORGANIC REACTIONS (6 volumes)

Proton Transfer Addition and Elimination Reactions of Aliphatic Compounds Ester Formation and Hydrolysis and Related Reactions Electrophilic Substitution at a Saturated Carbon Atom Reactions of Aromatic Compounds Section 5.

Volume 14 Volume 14A Volume 15

INORGANIC REACTIONS

Reactions of Non-metallic Inorganic Compounds Reactions of Metallic Salts and Complexes, and Organometallic Compounds Section 4 .

Volume 8 Volume 9 Volume 10 Volume 12 Volume 1 3

HOMOGENEOUS DECOMPOSITION AND ISOMERISATION REACTIONS

Decomposition of Inorganic and Organometallic Compounds Decomposition and Isomerisation of Organic Compounds Section 3.

Volume 6 Volume 7

THE PRACTICE AND THEORY OF KINETICS

HETEROGENEOUS REACTIONS ( 4 volumes)

Simple Processes at the Gas-Solid Interface Complex Catalytic Processes Reactions of Solids with Gases Reactions in the Solid State

Section 9. Volume 23

KINETICS AND CHEMICAL TECHNOLOGY (1 volume)

Kinetics and Chemical Technology Section 10. MODERN METHODS, THEORY, AND DATA (1 volume)

Volume 24

Modern Methods in Kinetics

Contributors to Volume 19 M. BOWKER

New Sciences Group, Imperial Chemical Industries, Runcorn, Cheshire. Gt. Britain

J. CUNNINGHAM

Department of Chemistry, University College, Cork, Ireland

C.T. FOXON

Philips Research Laboratories, Solid State Electronics Division, Redhill, Surrey, Gt. Britain

B.A. JOYCE

Philips Research Laboratories, Solid State Electronics Division, Red hill, Surrey, Gt. Britain

D.A. KING

Donnan Laboratories, The University, Liverpool, Gt. Britain

M.A. MORRIS

Department of Chemistry, Imperial College of Science and Technology, London, Gt. Britain

Section 8, which comprises the four volumes 19-22, deals with reactions which occur at gas-solid and solidsolid interfaces other than the degradation of solid polymers which has already been reviewed in Volume 14A. Reactions at the liquidsolid interface are not considered, but those involving electrochemical processes will be covered in subsequent volumes. With respect to chemical processes at gassolid interfaces, it has been necessary to discuss surface structure and adsorption as a lead-in t o the consideration of the kinetics and mechanisms of catalytic reactions. Volume 1 9 is devoted to considering simple processes occurring at the gas solid interface. Chapter 1 serves as an introduction and deals with the methodology of experimental surface science. Experimental results for metal surfaces on both adsorption and desorption kinetics and surface diffusion are discussed in terms of the current theories of these processes. Chapter 2 deals in the same way with these processes on semi-conductor surfaces. Finally, Chapter 3 is concerned with radiation and photoeffects at gassolid interfaces. The editors thank Professor D.A. King for invaluable advice.

C.H. Bamford The late C.F.H. Tipper R.G. Compton

Liverpool January 1984

This Page Intentionally Left Blank

Preface

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ix

Chapter I (M.A. Morris. M . Bowker and D.A. King) Kinetics of adsorption. desorption and diffusion at metal surfaces . . . . . . . . . . 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.1 The development of the science of solid surfaces . . . . . . . . . . . . . . . 1.2 The gassolid surface interaction potential . . . . . . . . . . . . . . . . . . . 1 . 3 Order4isorder phenomena in adsorbed layers . . . . . . . . . . . . . . . . 1.3.1 Dipole-dipole interactions . . . . . . . . . . . . . . . . . . . . . . . . . 1.3.2 Indirect coupling interaction . . . . . . . . . . . . . . . . . . . . . . . . 1.3.3 Direct coupling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.3.4 Substrate atom sharing interactions . . . . . . . . . . . . . . . . . . . . 1.3.5 Importance of lateral interactions . . . . . . . . . . . . . . . . . . . . . 1.4 Adsorbate-induced static distAacements of substrate atoms . . . . . . . . 2. Experimental techniques . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1 Surface crystallography. chemical composition and electronic structure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1.1 Low energy electron diffraction (LEED) . . . . . . . . . . . . . . . . 2.1.2 Auger electron spectroscopy (AES). . . . . . . . . . . . . . . . . . . . 2.1.3 Photoemission . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1.4 Vibrational spectroscopies . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2 Adsorption kinetics and absolute coverages . . . . . . . . . . . . . . . . . . 2.2.1 Uptake techniques . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2.2 Temperature-programmed desorption . . . . . . . . . . . . . . . . . . 2.2.3 Radiotracer techniques. . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2.4 Nuclear reaction method . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2.5 Microbalance techniques . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2.6 Relative coverage measurement techniques . . . . . . . . . . . . . . . 2.2.7 Reflection detector techniques . . . . . . . . . . . . . . . . . . . . . . . 2.2.8 Absolute random flux technique. . . . . . . . . . . . . . . . . . . . . . 2.3 Desorption kinetics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.3.1 Temperature-programmed desorption . . . . . . . . . . . . . . . . . . 2.3.2 Isothermal desorption. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.3.3 Electron.. photon-, ion- and field-stimulated desorption . . . . . . 2.4 Surface diffusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.4.1 Scanning methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.4.2 Field emission and field ion microscopies . . . . . . . . . . . . . . . . 2.4.3 Other methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 . Adsorption kinetics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1 The data base . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1.1 Zero coverage sticking probabilities . . . . . . . . . . . . . . . . . . . . 3.1.2 Variations of s with surface coverage . . . . . . . . . . . . . . . . . . . 3.2 Mechanisms and rate laws in adsorption . . . . . . . . . . . . . . . . . . . . . 3.2.1 Energy accommodation and trapping. . . . . . . . . . . . . . . . . . . 3.2.2 Precursor states in reactive gas-olid interactions. . . . . . . . . . .

1 1 1

3 6 7 7 7 8 8 9 10 10 10 13 15 16 17 17 20 21 21 22 23 24 26 27 27 29 29 31 31 34 39 41 41 41 55 57 58 62

3.2.3Models for adsorption kinetics . . . . . . . . . . . . . . . . . . . . . . . 3.2.4Activated adsorption . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4. Desorption kinetics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.1 Theory and analysis of desorption spectra . . . . . . . . . . . . . . . . . . . 4.1.1Theoretical aspects of thermal desorption. . . . . . . . . . . . . . . . 4.1.2 Integral order desorption with coverage-independent parameters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.1.3Systems with variable desorption energies. . . . . . . . . . . . . . . . 4.1.4Systems with variable pre-exponentials . . . . . . . . . . . . . . . . . 4.1.5 Desorption order. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.1.6The effect of precursor states . . . . . . . . . . . . . . . . . . . . . . . . 4.1.7The influence of lateral interactions . . . . . . . . . . . . . . . . . . . 4.2 The data base . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2.1Crystal plane orientation . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2.2Surface cleanliness . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2.3 Temperature-programmed reaction spectroscopy . . . . . . . . . . . 4.3 The desorption data bank . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.4 Interstate conversion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5. Surface diffusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.2 Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.3 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

64 82 84 86 86 90 94 97 98 101 104 108 108 108 122 125 141 142 142 149 163 163

Chapter 2 (B.A. Joyce and C.T. Foxon) Adsorption. desorption and migration on semiconductor surfaces . . . . . . . . . . . 1 . Introduction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.1 Justification for the subject matter . . . . . . . . . . . . . . . . . . . . . . . . 1.2 The “charge-transfer’’ model . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2. Experimental approach. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1 Surface crystallography . Diffraction techniques. . . . . . . . . . . . . . . . 2.1.1 Low energy electron diffraction (LEED) . . . . . . . . . . . . . . . . 2.1.2 Reflection high energy electron diffraction (RHEED). . . . . . . . 2.2 Surface compositional analysis. Auger electron spectroscopy (AES) . . 2.3 Surface electronic structure. Photoelectron spectroscopies . . . . . . . . 2.4 Surface kinetic measurements . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.4.1 Modulated molecular beam methods . . . . . . . . . . . . . . . . . . . 2.4.2Thermal desorption . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3. Atomically clean semiconductor surfaces . . . . . . . . . . . . . . . . . . . . . . . 3.1 Electronic structure of semiconductor surfaces . . . . . . . . . . . . . . . . 3.1.1Self-consistent pseudo-potential calculations . . . . . . . . . . . . . . 3.1.2 Realistic tight-binding calculations . . . . . . . . . . . . . . . . . . . . 3.2 Crystallography of semiconductor surfaces . Relaxation and reconstruction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3 Preparation of clean surfaces . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3.1 Silicon . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3.2 Gallium arsenide . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.4 Determination of the crystallographic and electronic structure of clean semiconductor surfaces . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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3.4.3 GaAB{ 110) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4. Gas- emiconductor surface interactions . . . . . . . . . . . . . . . . . . . . . . .

181 181 181 182 183 183 183 187 189 190 192 193 195 197 197 199 200 200 201 202 204

206 206 210 215 221

4.1 Adsorption of hydrogen on silicon . . . . . . . . . . . . . . . . . . . . . . . . 4.1.1 Introduction and survey of early work . . . . . . . . . . . . . . . . . . 4.1.2 Hydrogen atom adsorption on Si 111 . . . . . . . . . . . . . . . . . 4.1.3 Hydrogen atom adsorption on Si 110 . . . . . . . . . . . . . . . . . 4.1.4 Hydrogen atom adsorption on Si 100 . . . . . . . . . . . . . . . . . 4.1.5 Direct observation of hydride surface phases . . . . . . . . . . . . . . 4.1.6 Theoretical considerations . . . . . . . . . . . . . . . . . . . . . . . . . . 4.1.7 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2 Oxygen adsorption on silicon . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.3 Chlorine adsorption on silicon . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.4 Oxygen adsorption on GaAs . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.4.1 Oxygen adsorption on clean { 110) GaAs surfaces, . . . . . . . . . . 4.4.2 Oxygen adsorption on other orientations . . . . . . . . . . . . . . . . 5 . Metal interactions with semiconductor surfaces . . . . . . . . . . . . . . . . . . . 5.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.2 Metalsemiconductor interfaces . . . . . . . . . . . . . . . . . . . . . . . . . . 5.2.1 Goldsilicon . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.2.2 Silver-ilicon .................................. 5.2.3 Group I11 metals (Al, Ga, In)--silicon . . . . . . . . . . . . . . . . . . . 5.2.4 Caesium-silicon . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.2.5 Metals+roup 111-V compounds . . . . . . . . . . . . . . . . . . . . . 5.2.6 Mechanisms of metabsemiconductor interface interactions . . . . 5.2.7 “Classical” models of metal desorption from semiconductor surfaces . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.3 Semiconductorsemiconductor interfaces . . . . . . . . . . . . . . . . . . . 5.4 Interaction of Group V elements with GaAs surfaces . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

11

221 221 223 227 229 230 231 232 233 242 246 247 252 253 253 254 255 258 259 260 260 269 270 275 277 280

Chapter 3 (J. Cunningham) 291 Radiation and photoeffects at gaslsolid interfaces . . . . . . . . . . . . . . . . . . . . . 1. General introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 291 1.1 Scope . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 291 293 1.2 Origins of radiation sensitivity . . . . . . . . . . . . . . . . . . . . . . . . . . . 293 1.2.1 Collective-electron models . . . . . . . . . . . . . . . . . . . . . . . . . . 1.2.2 Active-site models and their sensitivity to radiation . . . . . . . . . 2 9 6 1.2.3 Combinations of collective-electron and active-site models . . . . . 301 303 1.2.4 Surface-state models . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.3 Spectroscopic aspects of irradiated gas/solid interfaces . . . . . . . . . . . 310 1.3.1 Electron spectroscopy of surfaces . . . . . . . . . . . . . . . . . . . . . 311 325 2 . Photoeffects at gas/solid interfaces . . . . . . . . . . . . . . . . . . . . . . . . . . . 327 2.1 Photophysical effects. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 327 2.1.1 Experimental aspects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 333 2.1.2 Results and interpretations . . . . . . . . . . . . . . . . . . . . . . . . . 354 2.2 Photochemical effects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 354 2.2.1 Experimental aspects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 359 2.2.2 Results and interpretations . . . . . . . . . . . . . . . . . . . . . . . . . 2.2.3 Modifying effects of surface dopants. . . . . . . . . . . . . . . . . . . 394 3 Effects induced by irradiation with high-energy photons or particles . . . . . 397 3.1 Energy deposition and localisation at the gas/solid interface . . . . . . . 398 399 3.2 Experimental aspects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 401 3.3 Results and interpretations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3.1 Effects on adsorption-desorption processes during irradiation . . 401 405 3.3.2 Chemical effects during irradiation . . . . . . . . . . . . . . . . . . . .

.

3.3.3 Effects persisting at the interface after irradiation . . . . . . . . 4 . Perspectives and prospectus . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Index

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413 417 419 429

Chapter 1

Kinetics of Adsorption ,Desorption and Diffusion at Metal Surfaces M.A. MORRIS, MICHAEL BOWKER and DAVID A. KING

1. Introduction 1.1THE DEVELOPMENT O F THE SCIENCE OF SOLID SURFACES

torr, approximately one monolayer of gas molAt a pressure of ecules collides with a smooth surface every second. For most gas-metal combinations, adsorption is a remarkably efficient process; the probability that a normal background constituent of a vacuum system, such as O,, CO, N, or H,, will become adsorbed during a single collision with a clean metal surface (the sticking probability) is high, often between 0.1 and 1, and the contamination rate is therefore prohibitively high at these ambient pressures. Studies of the gas-solid interface under reproducible and controlled conditions were therefore contingent on the developments in ultrahigh vacuum technology which occurred several decades ago, providing methods for routine access to experimental environments with background pressures in the range lo-'' t o lo-" torr. Subsequently, a powerful array of surface-sensitive, surface-specific analytical, crystallographic, dynamical and spectroscopic techniques has been developed, providing no less of a revolution in the study of solid surfaces than occurred in most other areas of physical chemistry in the 1930s and 1940s. The literature which this revolution has spawned, and the consequential developments in the science of solid surfaces, is both formidable and daunting (particularly to the reviewer!). In this chapter, we attempt a comprehensive survey of the impact of these developments on the study of kinetic processes at metal surfaces. These studies have highlighted both the important relationship between kinetics and structure in the surface layer and the role of weakly held, short-lived adsorption states (usually described as precursor states) in the formation of stable adsorbed layers. To place the kinetic studies in context, we therefore include in this introduction a brief survey of adsorption potential energy curves and of structural phenomena at surfaces. In the period prior to the development of ultrahigh vacuum (UHV) technology, it was widely believed that the process of forming a strongly held, chemisorbed layer was activated [ 11, although even then there was a suspicion that this might be due to the difficulty of attaining and maintaining clean surfaces. Several research groups during this period did, however, manage t o work under conditions where clean surfaces could be References p p . 163-1 79

2

produced, noteworthy among these being Langmuir in the U.S.A. and Roberts in the U.K. Thus, the early work of Taylor and Langmuir [2] on the adsorption of Cs on polycrystalline tungsten provided accurate, absolute measurements of adsorption rates and surface coverages which still form the basis of modern developments in kinetic schemes for adsorption. The work of Roberts [3, 41 on accommdoation coefficients and the adsorption of H 2 , N2 and O2 on tungsten filaments was equally noteworthy in establishing the high efficiency of adsorption on clean metal surfaces, the non-activated nature of the process, and the role of lateral interactions and adsorbate configuration on the coverage dependence of the rate of adsorption. The development of the inverted Bayard-Alpert gauge in the 1950s (the first device capable of measuring low pressures and still the most commonly used) was followed closely by the flash filament technique for thermal desorption and sticking probability measurements, pioneered by Ehrlich [5] and Redhead [ 6 ] . At the same ,time, the field emission microscope, developed by Muller [7] in the later 1930s, began t o be used extensively in adsorption studies, particularly by Becker [ 81, Gomer [91 and Ehrlich and Hudda [ 101 , This work demonstrated in dramatic fashion the crystal plane specificity of the adsorption process. From that time on, research workers could not be content with preparing clean metal surfaces for study; the crystal plane exposed at the surface had to be specified as well. This led to a resurgence of interest in a technique discovered by Davison and Germer [ 111 in 1927 and practised in splendid isolation over the intervening period by Farnsworth [ 121 : low energy electron diffraction (LEED). Germer himself returned to this method in the later 1950s and his adaptation of Ehrenberg’s technique [14] of displaying the backscattered electron diffraction pattern on a fluorescent screen [ 131 is now the sine qua non of every surface science laboratory. Even then, however, the state of cleanliness of the surface was largely a matter of (sometimes misguided) faith; only tungsten surfaces, for which simple cleaning procedures had been established from the flash filament and field emission microscopy studies, could be examined with any confidence; results from the early single crystal studies of Delchar and Ehrlich [15] and of Estrup and Anderson [16] have stood up well to the test of time. In the 1960s, a spectroscopic method, based on a suggestion by Lander [17] was developed [ 181 for determining the chemical composition, and hence the state of cleanliness, of solid surfaces: Auger electron spectroscopy (AES). Only then could the surface science community proceed with the confidence to study the interaction of gases with crystallographically and chemically well-defined solid surfaces. From this period on, numerous powerful spectroscopic and crystallographic techniques, often confusing the literature with their acronyms, have been developed for surface studies: photoelectron spectroscopy; ion scattering and ion channeling; electron energy loss and reflectionadsorption infrared vibrational spectroscopies;

3

atomic and molecular beam scattering; etc. Rather late in the day, the science of solid surfaces had come of age, The state of the field is summarised in a recent series of review articles [ 191 . 1 . 2 THE GAS-SOLID SURFACE INTERACTION POTENTIAL

A major breakthrough in the study of gassolid interactions was the development by Lennard-Jones [20] in 1924 of a potential curve for the interaction. For a gas-metal system where the interaction is strong enough to form a chemisorbed species, it was shown that an incoming molecule passes through two minima, the first a broad, shallow well (the physisorption well attributed to van der Waals forces) and the second a deeper well corresponding to the formation of a chemical bond. The physisorption well was originally calculated by summing the “6 : 12” potentials between the incoming species and each surface atom

f = c(?+$) i

where r is the interatomic distance. Potential wells have been simulated using an estimation of the London constant [21] K , and choosing a range of values of K 2 to fit the data; good approximations to K, are available for many systems [22] and have been applied to adsorption data with apparent success [23]. However, it is now recognised that the physisoprtion well on metals is not produced solely by dispersion forces. Ehrlich [24] found that the heats of adsorption of the rare gases Ar, Kr and Xe on tungsten were 8, 18 and 35 kJ mole-’, respectively, compared with condensation heats (i.e. rare gas atom on rare gas solid) of 6.7, 9.0 and 12.7 kJ mole-’, respectively. The disparity is larger the larger the polarisability of the adatom. Engel and Gomer [25] found that the adsorption heats of rare gases on W single crystal planes were not as predicted by simple pairwise addition of dispersive forces. And it was also discovered that the rare gases induced surprisingly large changes in work function, of the order of 1 eV for Ar, Kr and Xe [25-271. The large electric field gradient at the surface must be expected to cause a polarisation of the adatom, giving an additional energy term %Fa2,where F is the field strength at the equilibrium distance from the surface and a the volume polarisability of the adsorbed species. This theory has been supported [28-301 and has been used to explain the surface roughness effect on adsorption heat q , the rougher the surface, the larger is F and hence q. Engel and Gomer [25], however, pointed out that the surface field falls off very rapidly with distance. It should be noted that, due to electron overspill at a clean metal surface, a surface dipole exists at the surface with a negutiue charge outwards from the surface. This is well established, both theoretically [ 311 and experimentally [32]. Yet physisorbed species on metal surfaces invariably produce a decrease in the work References p p . 163-1 79

4

A

Fermi level ( p )

/

Fig. 1. Electron potential against distance from surface. (From ref. 33.)

function, suggesting that a dipole is induced in the physisorbed species with the positive charge away from the surface, in the opposite sense to that anticipated from the surface electrostatic field. The solution is hinted at in Fig. 1, which shows the electron potential at a metal surface versus distance from the surface as computed by Lang [33], corresponding to a Fermi wavelength of 8 . 6 6 a . It is seen that the electrostatic surface potential barrier $J is only a small part of the total effective barrier Veff, at the surface. Although $J is positive, and electrons are repelled and nuclei attracted by the electrostatic field, Veff is negative and IV,,, I > /$I. The electrons of an adatom physisorbed t o the surface thus tend to move into the region between the adatom nucleus and the surface. Clearly, this effect contributes to both the depth of the physisorption well and the size and sign of the induced dipole. (Theoretical treatments of physisorption potential energy wells are reviewed by Kasperma [34], Steele [35] and Gerahsimenko [36], but do not take account of this last point .) Two further experimentally determined properties of physisorbed species should be mentioned. (i) LEED studies on single crystal planes have shown that physisorbed layers may be ordered, as first noted by Palmberg [37] for Xe on P d ( l l 0 ) and by Chesters and Pritchard [38] for Xe on Cu(100). These ordered layers appear t o be dominated by adatomadatom interactions. Photoemission also provides evidence for such interactions [ 391 . (ii) Activation energies for diffusion of physisorbed species have been determined by field emission methods, as described in detail later, and it is concluded that, in general, the diffusion barrier is about 40% of the desorption barrier. There are, therefore, preferred site configurations for physisorbed species at a metal surface, the diffusion barrier referring t o the saddle point between sites. Although such barriers have been elegantly calculated for rare gas atoms on rare gas solids [40], all theoretical treatments t o date of the interaction

between gases and metal surfaces have treated the surface as a continuum. Figure 1 does, however, provide an intuitive basis for understanding this site preference and also the larger observed heats of adsorption on rough surfaces. A t an open site, the adsorbed species can approach closer to the surface, where Veff is large; between such sites, or on a smooth surface, the adatom sits proud of the surface, where V,,, is small. The potential energy well for chemisorption is associated with the more familiar chemical bond, although the valence band of the solid provides many unique features, and is reviewed by Grimley [41].The experimental distinction between physisorbed and chemisorbed states is now readily made by photoemission studies of the combined adsorbateadsorbent system, thus (thankfully) committing the otherwise rather theological discussions of borderline cases to past history. Chemisorption heats (see ref. 42) usually lie within the range 30 < q < 600 kJ mole-' and measured adatomadsorbent atom equilibrium distances are usually very close to those observed in solid state of molecular analogues. (Such measurements are obtained by LEED or, more accurately, by surface EXAFS.) Two schematic combined potential energy wells for the interaction of a gaseous species with a surface are shown in Fig. 2, illustrating the importance of the crossover point of the chemisorption and physisorption wells for adsorption and desorption kinetics. In the first case, adsorption is activated; in the second, it is non-activated. (There are, in fact, only a few well-documented cases of activated chemisorption.) Recently, Lundqvist et al. [43] have made detailed calculations of the potential interaction between H, and a magnesium surface which substantiate the presence of two minima. Their work is reviewed elsewhere [44]. It must be borne in mind that diagrams such as Fig. 2 grossly oversimplify the

I

I

Distance from surface

___)

Fig. 2. Crossed potential energy curves for physisorption and chemisorption. (a) Nonactivated adsorption; (b) activated adsorption.

References p p . 163-1 79

6

interaction. For example, a distinction must be made between the physisorbed species at a site filled by a chemisorbed species and the species at an empty site. And more than one chemisorption well should be envisaged at a given site configuration: for example, a molecular, upright bridged diatomic; a molecular, “lying-down” diatomic; and the dissociated adatoms of the original diatomic molecule, As discussed later, the physisorption well and (to a lesser extent) other intermediate wells, almost invariably play a decisive role in adsorption and desorption kinetics. 1.3 ORDER-DISORDER PHENOMENA IN ADSORBED LAYERS

If there were no lateral interactions between adsorbed species, equilibrium adsorption would occur with random occupation of preferred site configurations. However, since the advent of LEED as a tool for determining the long-range structural order of a surface it has been known that disorder in thermally equilibrated chemisorbed overlayers is the exception rather than the rule, even at relatively low surface coverages. In general, therefore, the lateral interaction energy between adsorbed species, w , is large compared with thermal energy, kT. The importance for kinetic processes, originally suggested by Roberts [4] and substantiated in kinetic studies of adsorption, desorption and diffusion kinetics on metal single crystal planes by King and co-workers [45-481, lies in both the effect that w has on the activation energy for these processes and its effect on the configuration of the adlayer and of empty sites, and hence on the probability of formation of the required activated complex for the kinetic process. The long-range order in an equilibrated adlayer at submonolayer coverages at low temperatures, in a system where no static displacement of adsorbent atoms is induced by the adsorbate (see Sect. 1.4), is entirely determined by the range, strength and sign of the lateral interactions between adsorbed species, provided that each species occupies an identical site. Once the preferred sites are filled, further adsorption can take place into less preferred sites, or the site configuration of the entire overlayer may be switched. In addition t o a coverage dependence of the overlayer configuration, there is a strong temperature dependence. For example, in the simplest cases an ordered overlayer can be disordered by random occupation of a range of sites. Order-disorder transitions in chemisorbed layers have been documented in several instances and in some cases the surface phase diagrams have been determined [49-53] . Phase transitions in physisorbed layers on exfoliated graphite have recently excited much interest [ 541. The origin of lateral interactions between adsorbed species has been widely discussed and it seems unlikely that an adsorbate/adsorbent system exists where these interactions are not important. A number of contributory effects have been identified in recent years, the dominant effect for individual systems being determined by the range over which

7

the interaction occurs and by the nature of the adsorbateadsorbent bond.

1.3.1 Dipole-dipole interactions [ 4 , 55, 561 If the surface dipole is strong and the distance between dipoles small, the repulsive force between two dipoles aligned parallel to each other may be dominant. This is, however, only likely to be the case for ionic adsorption, as in alkali metals adsorbed on transition metals. Despite the relatively weak nature of this interaction for most systems, the importance of dipole-dipole coupling in vibrational spectroscopy is now well established E571.

1.3.2 Indirect coupling interaction The importance of this effect was first recognised by Koutecky [ 581 for adsorption on semiconductor surfaces, and the work of Grimley [ 591 and of Einstein and Schrieffer [60] has firmly established its influence in chemisorption on metal surfaces. It arises from the interaction between the adsorbed species and the itinerant valence electrons of the adsorbent. The effect is illustrated by considering two H atoms separated in vacuum by a distance such that the wave functions of the 1s electrons are zero between them; there is no direct interaction, On the surface, however, the valence band electrons of the solid provide a path which allows coupling between the H atoms, at the same separation distance. This interaction is relatively long range, is oscillatory (it may be repulsive for nearestneighbours, attractive for next nearest-neighbours, etc.) and is dependent on crystallographic direction in the surface. It is widely regarded as a dominant effect in the formation of ordered structures at low fractional surface coverages.

1.3.3 Direct coupling This effect arises from the interaction between orbitals on adjacent adsorbed species and is clearly only important over very short distances. Energy dispersion, observed by angle-resolved photoemission, in adsorbate energy levels for Xe on Pd(100) [39] has been attributed t o this effect and these effects in high-coverage chemisorbed layers have been theoretically examined by Batra and Ciraci [61]. The contribution to the total lateral interaction energy, even in crowded overlayers, is small. The direct interaction between adsorbed species is dominated by the repulsive interaction between neighbouring molecular orbitals: thus, the saturation coverage of CO on various metal single crystal planes suggests a close-packing intermolecular separation which is close to that found in solid CO. At favourable separation distances between adsorbed species, the van der Waals forces will provide an attractive interaction. References p p . 163-1 79

8

1.3.4 Substrate atom sharing interactions If an adsorbed species, such as a H atom, is adsorbed in a bridged site between two substrate atoms, a second adatom adsorbed in a bridge with one substrate atom common to the first will produce a direct bonding interaction. This effect has been theoretically examined by Grimley and Torrini [62] and may be energetically dominant. It is clearly a specific, short-range interaction.

1.3.5 Importance of lateral interactions Le Bosse et al. [63] have made a theoretical comparison of the importance of the first three of these interactions in chemisorption, with particular relation to the adsorption of sodium on copper. Experimentally, lateral interactions have been determined by observing orderdisorder transitions using LEED [ 50-531 and from desorption [45,641 and diffusion kinetics [48] as documented in Sects. 3.2.3 and 4.1.6 of this review. LEED is a useful structural tool in this respect, because the width and intensity of fractional-order diffracted beams provide a direct measure of the degree of order in the overlayer. To obtain lateral interaction energies from the data requires the use of a two-dimensional lattice gas formalism, which enables the Hamiltonian for the system to be reduced to one equivalent to a magnetic spin system of the Ising type, as solved in one dimension by Onsager [65].(The equivalence was shown by Lee and Young [66].) These analyses are therefore restricted to systems where (i) the substrate is undisturbed by the adsorbate (no static displacements of substrate atoms) and (ii) all adsorbed species occupy identical surface sites and are in the same configuration. For a double-spaced, i.e. c(2 x 2) structure, on a square twodimensional lattice (surface net), where only nearest-neighbour repulsive interactions, wnn, exist, the solution is

where T, is the order4isorder transition temperature. This simple theory, based on only pairwise nearest neighbour (n.n.) repulsive interactions, predicts a phase diagram which is symmetrical about a fractional site coverage of 0.5;this has never been observed. Thus, for one of the simplest systems examined to date, oxygen adatoms on a W { l l O } surface, it has been found necessary to take into account n.n., next nearest neighbour (n.n.n.) and three-body (trio) interactions to explain the results [67, 681. The existence of trio interactions removes the equivalence between filled and empty sites, thus also removing the symmetry about the half-monolayer coverage. The consequential theoretical complexities have to some extent been overcome by the use of Monte Car10 techniques [ 69-73 ] . Pairwise interaction energies, repulsive and attractive, for

9

various chemisorption systems have been determined with values up t o 10-20 kJ mole-’. As an example, the order- disorder transition temperature for O/W{llO}at half monolayer coverage occurs at 700 K. 1.4 ADSORBATE-INDUCEDSTATIC DISPLACEMENTS OF SUBSTRATE ATOMS

The two-dimensional lattice statistics approach described in the previous section is restricted to a “checkerboard”mode1 of the adsorption process, in which a rigid surface provides fixed sites for the adsorbate and is itself undisturbed by the adsorbate. However, there are a number of established examples of systems where the substrate atoms are structurally rearranged in the process of adsorption. The uptake of oxygen by a clean metal surface often results in the formation of a thin oxide film at the surface, with a structure replicating that of a bulk oxide. Here, clearly, a substantial rearrangement of substrate metal atoms is involved. Even at low oxygen adatom coverages, however, substrate reconstruction may occur as, for example, when oxygen is chemisorbed on W(100) [74] or Cu [75] single crystal surfaces. In several instances, oxygen chemisorption on to a unreconstructed surface at low coverages is followed by oxide nucleation as the coverage is increased; for oxygen on Ni films and single crystal surfaces, this occurs at around 0.25 of a monolayer [ 76-78] . Reconstruction is not limited to oxygen adsorption: for example, the annealed state of CO on W{OOl} appears to result from surface reconstruction [ 791. A further complication occurs for several systems where the adsorbate is taken into an underlayer and does not occupy surface sites. This occurs for N atoms on Ti{0001} [80] and on W{llO) [81]; N adatoms form an overlayer at coverages up to half a monolayer on W { 001) and at temperatures below 900 K, but at higher temperatures or coverages they are taken into the bulk [82]. In several cases, adsorption can result in the lifting of a surface reconstruction present on a clean surface. For example, the stable structures of the {loo} and (110) surfaces of Pt and Ir are not the truncated bulk structures. Chemisorption of CO, NO, oxygen or hydrogen, under certain conditions, causes a removal of non-integral order diffraction beams from the clean surface LEED patterns [83, 841. This is widely interpreted to indicate the stabilisation of the truncated bulk structure by the adsorbate and clearly involves a static displacement of substrate atoms from their stable positions in the clean surface. In the case of the (100)surfaces of these metals, which are believed to form a hexagonal close-packed surface layer structure when clean [ 851, this involves a substantial rearrangement of substrate surface atoms. The surface layer density is lowered during chemisorption and this can only result from a significant transport of metal atoms across the surface, with the formation of defects such as steps and kinks. The kinetics of these processes themselves form a significant challenge to the experimentalist, which have hardly been References p p . 163-1 79

10

touched on. Their existence provides an added dimension of complexity t o the study of adsorption, desorption and diffusion in these systems. A further category of adsorbate-induced substrate atom displacements has been revealed in recent studies by the groups at Liverpool University in the U.K. and at Brown University in the U.S.A. on the W{lOO} and Mo{100} surfaces. On these surfaces, the surface atoms are inherently unstable to small lateral displacements from their bulk lattice positions, as first revealed by the observation of reversible surface phase changes which occur on cooling below -370 K [83,841. On the W(1OOj surface, the low temperature phase is formed by alternate static displacements of surface atoms, by -0.2& in the ( n o ) and (110) directions, forming a zig-zag chain-like structure [83].Hydrogen adsorption, at adatom coverages between 0.1 and 0.2 of a monolayer (defined in terms of the number of W atoms in the surface), causes a symmetry switch in the W surface layer, with atoms alternately displaced in the (TOO) and (100) directions, producing a dimer structure [85].Further adsorption to saturation with two monolayers of hydrogen adatoms results in a return of surface tungsten atoms to bulk lattice positions. Adsorption of nitrogen adatoms at very low coverages (- 2% of a monolayer) completely inhibits the low temperature zig-zag structure [ 861. At a N adatom coverage of 0.4monolayer two-dimensional compressed islands are formed in which the interatomic spacing between surface W atoms is reduced compared with that in the bulk [87].And at exactly 0.5 monolayer, the surface W atoms are returned to bulk lattice positions: only at this coverage is the checkerboard model operative. Clearly, lattice statistical models are an oversimplification in the treatment of these systems; the work at Liverpool was originally instigated as a check on the applicability of these models.

2. Experimental techniques 2.1 SURFACE CRYSTALLOGRAPHY, CHEMICAL COMPOSITION A N D ELECTRONIC STRUCTURE

This section is intended as a brief introduction to some of the more widely used techniques for surface characterisation. More detail is available in a number of books [88-911. 2.1.1 Low energy electron diffraction (LEED)

A collimated, primary beam of electrons with a diameter of 0.5-1 mm at energies typically within the range 15-350 eV is impinged on a surface and the elastically back-scattered electrons, after travelling through a field-free region, are spatially analysed. In the commonly used display LEED system, this is achieved by passing the scattered electrons through

11

two (or more) hemispherical grids, the second at a retarding voltage t o eliminate inelastic electrons, and the diffracted beams are then accelerated into a hemispherical phosphor screen. The LEED pattern displayed on the screen is the reciprocal lattice (or net, in two dimensions) of the real space surface net. Figure 3(a) is a schematic representation of the LEED process, demonstrating the relationship between the real set, the reciprocal set and the diffracted beams. The resultant LEED photograph is shown as a typical example in Fig. 3(b). In the energy range 15-250 eV, the escape depth of the back-scattered electrons is 5-10 A and the observed pattern thus represents the combined symmetry parallel to the surface of the top few atomic layers. In the majority of cases, clean single crystal planes of metals produce LEED patterns with the symmetry of the bulk, although there are notable exceptions, as mentioned in Sect. 1.4. Superstructures formed by adsorbed layers, with a real space unit cell (or mesh, in two dimensions) larger than that of the substrate, are readily observed through the appearance of the corresponding additional fractional order beams in the LEED pattern. The information from the LEED pattern itself is limited by the fact that it is the reciprocal net of the surface diffraction grating: the nature of diffraction centres (e.g. adsorbate atoms or molecules) and the registry between overlayer and substrate (e.g. whether adsorbed species are in on-top sites, bridged sites, or four-fold hollow sites on a {loo} surface) are not provided by the pattern. The periodicity normal to the surface is, in fact, weakly felt by the combined incoming and outgoing wavefield of the electron beam and this forms the basis of the so-called intensity-voltage, or I-V, spectral analysis. The intensity of a given diffracted beam is related, albeit in a complex way (due t o multiple scattering or dynamical effects), to the energy of the primary electron beam and analyses of these spectra can provide the spacings normal to the surface required for a complete structural analysis [ 911 . Two sets of notation are commonly used to describe overlayer structures observed in diffraction experiments, the Wood notation [92] and a matrix notation. Although the latter is more flexible, the former is more widely used and we shall restrict ourselves to it in this review. The nomenclature is based on a comparison between the unit mesh of the topmost layer, the overlayer, and that of the second, unreconstructed, substrate layer. If a and b are the unit mesh vectors of the substrate layer and a, and b, the unit mesh vectors of the overlayer, then Wood’s notation for an overlayer of adsorbed species A on the {hkl} plane of a crystal M is

M{hkl}-

i‘::‘Y) -x-

-Rr#~-[[8]

(3)

where $I is the angle of rotation of the overlayer mesh relative t o the substrate mesh. For example, the structures shown in Fig. 4, representing a half-monolayer of adsorbate on a {loo} surface, are described as References p p . 163-1 79

12

bea rn

Fig. 3. (a) The relation between the surface net and the reciprocal surface net as observed by LEED. (b) The LEED photograph.

13

I

Fig. 4. Adsorbate structures on a b.c.c.{100} surface which would produce a c ( 2 X 2) pattern in LEED.

0

0

0

0

0

0

0

0

0

0

0

0

0000000 0000000

0000000 0000000

(a)

(b)

0

0

OO0O 0 ooooooooooooo 0 0 0 0 0000000 0 0 0 0 0 0 0 (C)

(d)

mmmo ooooo~o 0000000 (el

0000000 ( f )

Fig. 5. Six possible substrate-adsorbate structures that result in the same LEED pattern. (From ref. 714.)

M{100}- ( f l x f l ) R 4 5 ' ; M(100) - ~ ( x 22).

alternatively, they are also often given as

All three structures in Fig. 4 produce the same LEED pattern. This is more explicilty shown in Fig. 5; all these structures are equivalent in LEED display. Note, however, that they would all produce different I-V spectra.

2.1.2 Auger electron spectroscopy ( A E S ) An electron or photon incident at a surface with sufficient energy may result in the ejection of an electron from a core level (X) of an atom in References p p . 163-1 79

14

the surface region. Auger electron ejection is an auto-ionisation process of the ion so created: an electron in a lower binding energy level (core or valence) rapidly s) falls into the core level vacancy, in the process ejecting a second electron, the Auger electron, from a lower binding energy core or valence level. Provided the atom is close to the surface, this electron may be ejected into the vacuum, with a kinetic energy characteristic of the energy level structure of the atom (and independent of the primary electron or photon beam energy), hence providing a means of chemically identifying the atom. The alternative decay process, the emission of a photon, is inefficient when the binding energy of the core level X is <- 1500 eV. Experimentally, a major advantage of AES is that it can be performed with the same electron optical system used for display in LEED, but operated in the retarding field mode [93]. In practice, the Auger peaks are relatively small features on a large secondary electron background and electronic double differentiation of the signal is commonly used to accentuate the peak positions and intensity [ 9 4 ] . The development of the cylindrical mirror electron energy analyser for AES [951 was a further major step forward in the applicability of AES: double focussing on to an electron multiplier provides sufficient sensitivity for oscilloscope display of the Auger system. Scanning Auger microscopes are now in widespread use [ 961 , particularly in the microelectronics industry, for the spatial chemical analysis of surfaces. AES is clearly an important technique for characterising the state of cleanliness of crystal surfaces and hence for determining the efficiency of cleaning procedures. The procedure depends critically on the metal and the impurity. For example, for tungsten [ 971, repeated flashing t o -2000K in torr of oxygen is sufficient to remove the major bulk impurity, carbon, which segregates to the surface and is removed in the presence of adsorbed oxygen as carbon monoxide during the flash. Subsequently, oxygen can be removed by flashing to 2500 K in vacuo. For many metals, such as nickel, copper or platinum, however, these chemical cleaning procedures are inadequate and high-energy argon ion bombardmentlhigh temperature annealing cycles are required to remove impurities and return the surface to its crystalline state [ 981 . AES is also used as a quantitative tool to monitor surface coverages of adsorbed species and hence to follow kinetic processes. Provided that (i) the adsorbate is confined to the overlayer at all coverages and (ii) the Auger peak shape is unaltered as the coverage changes, the detected Auger peak intensity is proportional to the surface coverage. The relative coverage is defined as a proportion of monolayer coverage; absolute coverages can only be obtained by calibration against an independent technique. AES peak shape changes can occur if there is a coveragedependent change in the nature of the adsorbate. (This arises from chemical shifts in the core and valence level energies of the component

-

15

atoms of the adsorbates. The lineshape of the Auger peak from an adsorbate atom can be used as a fingerprint to identify the nature of the adsorbate [ 991 .) Absolute calibrations are conveniently expressed as a ratio of the peak-to-peak height M , in the second derivative mode, of the adsorbate Auger signal, to that of a substrate peak. For example, Housley and King [ 1001 report the following calibration figures, obtained using a CMA, for C, N and 0 adatoms on W{OOl}, ratioed to the W Auger peak at 350 eV, and normalized to a coverage of 5 x 1014 atoms cm-* : M(C)/,M(W) = 3.43; d ( N ) / d ( W ) = 1.69; M ( O ) / M ( W )= 1.68 (4) It is often erroneously believed that signal strength is independent of the element adsorbed; clearly, the signal from a given atom is dependent on the binding energy of the initially created core vacancy [ l o l l , in the above cases the K shell. As shown, the cross-section decreases with increasing K-shell binding energy in the series C, N, 0. A warning note should be sounded in the use of AES for kinetic or coverage measurements. It has long been understood that an electron beam can give rise to significant desorption of an adsorbed layer [102, 1031 or to transformation from one adsorbate state to another [102, 1041 . The process of electron-stimulated desorption has been extensively studied and used as a tool for distinguishing different adsorbate states [ 104-1061 ; more recently, photon-stimulated desorption has also been observed from adsorbates on metal surfaces [ 107-1151 . The cross-sections for these processes vary considerably from one system to another. As a general rule, molecular states such as CO [102--106, 116-1201 have relatively high cross-sections, but strongly held atomic states may also have high-cross-section, particularly when they exist on the surface in the form of a maximal valence compound such as tungsten oxide [121, 1221. This effect can be a problem with any technique involving the use of a primary beam of electrons in the energy range above -50eV, but arises particularly in relation to AES because of the high incident beam current densities commonly employed.

2.1.3 Photoemission In photoemission, the electrons directly ejected by photons from the surface region of a solid into the vacuum are energy analysed and the spectrum is then related back to the electron energy levels of the system [123]. A wide range of available photon energies is utilised from the threshold energy, determined by the work function of the surface (i.e. 4-5eV), through the ultraviolet range (He', He", Ne' resonance lamps being commonly employed as photon sources) and up into the X-ray region (using, for example, MgK, or AM, line sources). Synchrotron radiation, being a polarised, tuneable light source covering the entire useful energy range, has given much impetus to this work [129]. References p p . 163-1 79

16

In the vacuum ultraviolet range of photon energies, using angular resolving energy analysers, the technique has become the most accurate available for the determination of the electronic structure of surfaces and of solids [ 1241 . Band structures for clean and adsorbate-covered surfaces have been determined, mapping out the dispersion of electronic states [ 125-1331 . Moreover, adsorbate-induced energy levels provide valuable information concerning the degree of fragmentation of the original gas-phase molecule [ 133-1361 and relative shifts of adsorbate orbitals indicate those which are involved in surface bonding [137-1411. In the X-ray region, the core levels of the adsorbate are determined. Chemical shifts in the core levels of different states of adsorbates (e.g. molecular and dissociated CO) [142-1441 can be identified and this provides a useful and accurate means of following thermally induced interconversions between such adsorbate states [ 145-147 J . Relative surface coverages can be obtained from the intensities of core level peaks; as with AES [144, 148-1501, care must be taken that adsorption is restricted t o a monolayer. Several groups have published methods for obtaining absolute coverages directly from the core level intensities [151], although calibration against another technique is generally to be preferred. 2.1.4 Vibrational spectroscopies Two important techniques have been developed for the determination of the vibrational spectra of adsorbates, viz. reflection--absorption infrared spectroscopy (RAIRS) and high-resolution electron energy loss spectroscopy (HREELS). In the former, a monochromatised IR light source is focussed on the crystal surface a t close to grazing incidence and the intensity of the reflected radiation recorded as a function of wavelength. The adsorbate bands so observed are generally weak and various phasesensitive-detection methods have been employed, with particular success for the strongly absorbing C-0 stretching vibration in adsorbed CO [ 152-1581 . In HREELS, a monochromatised beam of low-energy electrons (1--5 eV) is impinged on the surface and the scattered electrons are energy analysed t o locate the adsorbate vibration absorption bands. With the analyser set in the specular direction, the selection rules for HREELS are identical t o those in RAIRS [159] : only those adsorbate vibrational modes with a component of their dynamic dipole moments normal to the surface produce appreciable absorption. HREELS is the more sensitive of the two techniques, although the best resolution so far attained [160] (-3.5meV, or 28cm-') does not approach that of RAIRS. Vibrational spectroscopy is clearly a very powerful technique for identifying the nature of a chemisorbed species and its local structure. Despite the experimental difficulties inherent in both these techniques, they are, therefore, coming into relatively widespread use [ 161-1681 . With improvements in sensitivity, they will undoubtedly be developed for

17

time-resolved studies of kinetic processes at metal surfaces, including adsorption, desorption, interstate conversion and heterogeneous catalysis. HREELS, in particular, is unrivalled in its potential for identifying chemical intermediates. 2.2 ADSORPTION KINETICS AND ABSOLUTE COVERAGES

Rates of adsorption are most conveniently expressed as the sticking probability, s, defined by

s =

rate of adsorption rate of bombardment

(5)

The sticking probability is usually a sensitive function of the surface coverage, N, which may be expressed in units of adsorbed species cm-*, and in most techniques the measurement of these two quantities is strongly interrelated. At the outset, it must be noted that the definition for s given above is ambiguous and needs further qualification. It is generally the case that several adsorbate states can exist on a surface, as discussed in Sect. 1.2, with widely different adsorption heats. At a given crystal temperature, it is common for a molecule to be adsorbed into a weakly held state with a short lifetime before being scattered back into the gas phase and into a more strongly held state with, effectively, an infinite lifetime. For most techniques, the measured sticking probability refers to the overall rate of adsorption into the more strongly held state. Provided that the lifetime of adsorbed species is very long or very short compared with the experimental time period, the definition is unambiguous. If, however, appreciable desorption occurs from the strongly adsorbed layer, the measured sticking probability refers to a net rate of adsorption. Since heats of adsorption sometimes fall appreciably with increasing coverage, this effect can be particularly important at high coverages. As will be discussed in Sect. 3 of this review, numerous values of s and N have been reported using a wide variety of techniques. The reliability of these techniques varies considerably and the following is intended as a critical appraisal to provide a guide for the treatment of the available data and for the intending experimentalist. 2.2.1 Uptake techniques If the effective pumping speed of the adsorbent in an ultrahigh vacuum cell is similar t o that of the pumps themselves, a comparison of the pumping speed of a given gaseous species with and without adsorption occurring yields the adsorption rate. This method was originally developed by Ehrlich [ 51 in conjunction with the flash filament method for desorption kinetics (Sect. 2.2), but has also been widely applied to single crystals. It is associated with glass UHV systems where pumping speeds are small References p p . 163-1 79

18

-

(typically 3 1 s-’ ) and is not normally applicable to stainless steel systems with pumping speeds several orders of magnitude higher. Even with unit s, the pumping speed of a 1cm diameter crystal is 10 1s-l. With gas leaking into an adsorption cell of volume V at a rate L , pumped by the pumps at a rate R p , the rates of adsorption, RA, and desorption, RD, are related to the pressure change in the cell, dP/dt, by the expression

At the low pressures required for these experiments (
RA = L - F P where L has units molecules

(7)

s-l. The collision frequency at the surface is obtained from gas kinetic theory [ 1691 as vc = ZP, where Z = ( 27rrnhT)-1’2 and m is the molecular mass. From its definition, s = RA / A v c , where A is the geometric surface area of the adsorbent and we therefore have

s=- L-FP ZAP The surface coverage N at a given time t after the start of adsorption, expressed in molecules cm-2, is obtained from the integral t

N = -11 s Z P d t

(9) A . and the required plot of s versus N can be constructed from P versus t plots provided that L, F and A are accurately determined (not a simple matter) and provided that the pressure-measuring device is accurately calibrated. The method cannot be applied where substantial reversible adsorption occurs on the vessel walls; pumping by the pressure gauge may be accounted for, but is a further problem. The difficulties in calibration and system design are enumerated below. (a) The system pumping speed F and the leak mte L This parameter is usually measured in situ with the adsorbent rendered inert, either by saturating it or by maintaining it at a temperature sufficiently high that the steady state coverage is effectively zero. (The latter cannot be applied where fragmentation of the gas molecules occurs at

19

the hot surface, since the fragments are often efficiently pumped a t the system walls.) In either case, R A = R , , and eqn. (6) reduces t o V d_p _

kT d t

=

L-FP

If the inflow of gas t o the cell is suddenly terminated (by closing a valve), L = 0 and the pressure should fall exponentially with time, yielding F from the slope. The volume V must therefore be accurately known. Very large systematic errors in the published values of s and N can be traced back to errors in F . A major difficulty arises in defining the volume V [ 1701 : should it include the tubulation between cell and pumps, including traps? V is only a well-defined quantity in systems where a major constriction exists a t the outlet to the system and this is seldom the case. This problem has been circumvented in the system described by Goymour and King [171]. The gas flow rate into the adsorption cell is accurately determined by following the rate of fall in gas pressure in an accurately determined volume behind a fine capillary leading to the adsorption cell. The pumping speed on the main cell is then obtained at steady state, i.e. L = FP, and the system volume V does not enter the calibration. In the method of Ehrlich, the leak rate L is obtained from this steady-state expression and is thus subject also to the large errors in F . ( b ) The collision frequency vc

The gas kinetic theory expression for vc is based on the assumption that molecules are in random motion with a Maxwellian distribution of velocities. Any directional flow in the adsorption cell produced by direct inflow of gas into it via a capillary or orifice [172] or by a high pumping speed (either of the pumps or, more importantly, of the adsorbent itself) can therefore produce significant systematic errors in s and N . Where the sticking probability and the adsorbent surface area are low [173], the difficulties are reduced; as a rule of thumb, where the uptake technique has been used on single crystals, wires or ribbons without great care in system design, sticking probabilities above 0.1 are very unreliable. The difficulties with uptake techniques are amplified when used to study adsorption rates on metal films; early results underestimated sticking probabilities by a factor of about 50 due to inadequacies in system design. In the system described by Hayward et al. [174], the film is deposited on the walls of a spherical bulb and the gas is introduced to the cell through a diffuser placed a t the centre of the cell and designed t o produce an equal flow towards the film in every direction. The method is described in Sect. 2.2

( c )Pressure measurement Pressures have been measured with either inverted Bayard-Alpert References p p . 163-1 79

20

ionisation gauges or mass spectrometers. Several problems can arise from the use of these devices [ 6 ] , particularly from the following: their pumping action; difficulties in obtaining accurate sensitivity measurements; atomising the ambient gas at the hot cathode (particularly H,, but this can be avoided using LaB, or Tho, coated cathodes); the production of excited neutrals or ionic species which may have a high sticking probability on the adsorbent as well as the walls of the vessel. In particular, a number of literature values for oxygen where an unmodulated ionisation gauge has been used can be discounted, for two reasons: (i) CO is readily produced a t a hot filament (or on collision of 0 atoms produced at the filament with the vessel walls) and can be the major constituent in the vessel when oxygen is introduced; and (ii) high ESD currents from the grid of the gauge can give rise to spurious pressure readings [175]. Clearly, a mass spectrometer operated with a coated cathode a t low emission currents is t o be preferred in these measurements.

2.2.2 Temperature-programmed desorption This method has not been superseded as the most convenient method for measuring coverages for those adsorption systems where the adsorbate is desorbed quantitatively and reversibly. The technique is described in detail in Sect. 2.3. In brief, after adsorption for a given exposure, the adsorbent temperature is ramped, resulting in a desorption pulse, or pulses: the desorption spectrum. If the pressure is recorded as a function of time in a system with pressure-independent pumping speed F , the original coverage is obtained by integration of the desorption pulse: the amount N desorbed in a time interval t, - t , is simply t^

A

tl

where P is the instantaneous pressure and Po the background pressure of the system. Sticking probabilities may be obtained from plots of N against exposure J:’Pdt (where t’ is here the adsorption time) since

1 d N s = --

Z d(Pt)

This method is subject t o the same problems as the closely related uptake technique described above: in particular, exposures are seldom reported to an accuracy better than 50%, with little reliability in derived values of s. Nevertheless, since multiple pulses may be derived from different adsorbate states on the surface, desorption spectra provide the added advantage of providing a means of measuring the relative coverage in each state, in addition t o the absolute total coverage, provided that the different pulses can be reliably deconvoluted. However, care must be taken in the interpretation : interconversion between states can occur

21

during termperature programming, so that the desorption spectrum may not necessarily reflect the state of the adsorbate prior t o the programme being applied. In particular, we believe that the practice of deriving sticking probability curves for each adsorbate “state” is a rather meaningless exercise.

2.2.3 Radiotracer techniques The measurement of enhanced radioactivity at the adsorbent when exposed to a radioactive gas to obtain absolute surface coverages was proposed many years ago by Beck [ 1761 and is one of the most accurate available. Its early use with single crystal surfaces is best exemplified by the work of Dillon and Farnsworth on CI4O, adsorption on N i ( l l 0 ) and (100) [177]. In this work, a Geiger-Muller tube was inserted into the adsorption cell; in the later work of Crowell [178], the measurements were made through a mica window. The arrangement of Crowell and Matthews [179] is shown in Figure 6 and the technique, which is restricted by the availability of radioactive isotopes, has been applied to adsobates containing C, H or S [ 180]. 2.2.4 Nuclear reaction method This is a highly specialised technique requiring the availability of a veryhigh-energy (MeV) ion source, but it is the most accurate in use today for

Fig. 6. The experimental arrangement for measurement of enhanced radioactivity. GM is a GeigerMuller tube. (From ref. 179.) References p p . 163-1 79

22

absolute coverage measurements. It is applicable to systems where the adsorbent undergoes a nuclear reaction with a well-known cross-section. Provided the ion-beam-induced desorption cross-section can be measured, then the coverage can be calculated as a function of exposure. Such a method was used to measure the coverage of D on W(OO1) using the nuclear reaction D(3He3H)4He[230]. This was able to detect coverages as low as 1 0 ’ ~atoms cm-2. 2.2.5 Microbalance techniques Two techniques are in use involving weighing the adsorbate: the vacuum microbalance and the quartz crystal microbalance. The former were developed from thermogravimetric balances; their development and utilisation is described by Czanderna [ 1811. The torsional design, in which a weighing beam is attached t o a wire or ribbon secured at both ends, is commonly used; deflections in the wire support are proportional to the measured mass. This type of system has been used, for example, to study oxygen adsorption on a relatively high area (- 50 cm’) titanium substrate [182]. The measurement of frequency changes in a quartz crystal to obtain absolute coverages was pioneered by Levenson [183] and uses a thinly cut quartz crystal vibrating at 10 MHz [184] as a substrate on to which a thin film of the adsorbent is evaporated. While sensitivities of -0.02 monolayers of Fe adsorbed on Si [185] were reported in the early literature, the recent careful work of Kasemo and co-workers [186] has shown that the technique can even be extended to H2 chemisorption with a sensitivity of -0.05 monolayer. It is, however, limited t o adsorbents in the form of vacuum-deposited metal films. The principal of the vibrating quartz method is summarised simply. The mass, m ,of the vibrating section of the crystal is

-

m = Arp (13) where A is the area of the vibrating sector, r the thickness of the crystal and p the density of the quartz. The increase in mass due t o the adsorbate is

Am

MBN,A L

=-

where M is the molecular (atomic) mass per mole of the adsorbate, N, the number of sites per unit area, 8 the surface coverage and L the Avogadro number. The induced frequency change is

where v o for a clean quartz crystal is 10MHz. The mass of a deposited film, as well as the mass of adsorbate subsequently taken up, is readily

23

obtained from this expression. The surface area is, however, undetermined; N, must therefore be determined from estimates of surface roughness, frequently introducing large errors.

2.2.6 Relative coverage measurement techniques The methods lumped together under this heading can be divided into two groups: those in which a spectroscopic property of the adsorbate is measured, such as Auger peak intensities, photoemission intensities due t o an adsorbate core (or, less often, valence) levels, vibrational absorbance, and ellipsometric A and functions; and those in which a change in a surface property is measured, such as the work function or the resistivity. With the exception of XPS, for which semiquantitative formulae have been derived for the derivation of absolute surface coverages, all of these methods require independent calibration. Provided that the adsorbate is confined to an overlayer and submonolayer coverages, AES and XPS intensities may be linearly dependent on coverage. The remaining properties are seldom lin’ear functions and calibration is therefore required over the full coverage range. For example, although the vibrational spectroscopies described in Sect. 2.1 are powerful methods for distinguishing between different states of the adsorbate, adsorption peak intensities, in the normal dipole selection rule mode, are dependent on the square of ; depolarisation and the dynamic dipole moment, ( a ~ / a Q ) ~mutual coverage-induced changes in da* backbonding, for example, can produce a strong coverage dependence in ap/pQ, [57, 187-1891 and coupling between dipoles can cause intensity stealing into high frequency bands. In the case of CO on Cu{OOl}, a bridged species present at high coverage is not observed in the vibrational spectrum [156, 160, 1901, presumably due to intensity stealing into the higher frequency C-0 stretch of the linearly adsorbed species. Non-linearity in work function changes may arise from mutual depolarisation at high coverages, as described by Topping [191], or from the population of different binding sites as the coverage is increased [ 1921, or from electronic changes resulting from indirect interactions [193], as described in Sect. 1.4. The advantages of many of these methods are simply summarised: they may be available in the apparatus, though intended primarily for another purpose (AES, XPS, work function), or they are convenient t o use due to their nondestructive nature (work function, ellipsometry, vibrational spectroscopies), or they are inexpensive to install (work function). Changes in resistivity are restricted to thin films or thin wires [194] in which the surface layer contributes a measureable fraction of the overall conductivity and such measurements are often run alongside measurements of heats of adsorption [ 1951. Resistance changes due to adsorption have been attributed t o a “demetallisation” of the surface layer of metals [ 1961 and t o diffuse scattering of metal conduction electrons from virtual References p p . 163-1 79

24

charges in the metal [ 1971, resulting in a decrease of the electron mean free path. A high coverages, specular reflection of conduction electrons may occur at the surface virtual charges if the adsorbate spacing is close to the electron wavelength [198] and the resistance may decrease. The influences of grain boundaries, pores, film thickness and morphology are an added complication with this technique. Ellipsometry is an attractive non-destructive technique for monitoring surface coverage and can be applied to any surface with a significant reflection coefficient [ 1991. It is based on changes in the optical properties of a reflecting surface and has been specifically developed by Bootsma et al. [ 2001 for coverage measurements. 2.2.7 Reflection detector techniques Each of the methods described in Sect. 2.2.1-2.2.5 requires an accurate absolute determination of the exposure to derive sticking probabilities and this is a notoriously difficult parameter to determine quantitatively. The reflection detector techniques, which involve a direct measurement of the reflection coefficient for incident particles at a solid surface, avoid this difficulty and are therefore considerably more reliable for systems where the sticking coefficient is high (20.05). The first technique in this category was developed by Bell and Gomer [ 2011 . A beam of gas was allowed t o impinge on a single crystal substrate and the refelcted gas was monitored by means of a field emission tip pointing towards the crystal and placed a few mm from it. Reflection from the vessel walls back to the tip was avoided by cryopumping, the vessel being immersed in liquid hydrogen. The experiment is performed dosewise, the tip being rotated towards a screen to measure the voltage increment required to maintain a constant emission current. Wang and Gomer [ 2021 further refined the technique for absolute coverage measurements by incorporating a means of exposure calibration. However, it is highly specialised and tedious in operation. A reflection detector technique was developed for measurements with metal films [76, 203, 2041 in which the detector was an ionisation gauge or mass spectrometer. The system is shown schematically in Fig. 7 and is unique amongst these techniques as it is accurate over a very wide A baffle between the range of sticking probabilities (1 < s < centrally positioned gas diffuser and the detector prevents gas molecules from entering the detector before collision with the film. With a mass spectrometer as detector, the method has been used by Horgan and King [ 2041 for measurements of displacement processes or catalytic reactions occurring between two different gaseous species. Absolute sticking probabilities for each impinging species, their surface coverages, and absolute rates of reaction were simultaneously obtained. King and Wells [205] developed a molecular beam technique specifically for the determination of absolute sticking probabilities on single

25

J’!

-Gas

supply

I

Vabe

Fin. 7 . The exnerimental arraneement for the reflection detector technique for metal films as used by Horgan and King [ 761. --r

Gas line

0

Pur

s

Pumps

Pumc

Getters

Faraday cup

Electron gun

Fig. 8. Molecular beam reflection detector technique 88 used by King and Wells [205].

References p p . 163-1 79

26

crystal surfaces. As shown in Fig. 8, a highly collimated molecular beam is formed by a series of differentially pumped chambers and the integrated back-scattered flux is continuously measured with a mass spectrometer or ionisation gauge. The flux from the crystal measured as a partial pressure, P , is compared in situ with that from an inert surface interposed intermittently between the crystal and the beam, measured as P o . The sticking probability at a time t , after initiating beaming is given directly as

and is clearly independent of the pressure calibration factor. Absolute errors in s for Nz gas have been estimated as k 0.01. Surface coverages rely on a calibration of the beam flux Q (molecules s - l ) and are obtained from the relationship t

The technique has also been developed by Madey [ 2061 in a simpler form, readily added to stainless steel systems, in which a non-collimated flux of gas is provided by an orifice or capillary array. Here, only a fraction, f , of the gas emerging from the orifice is incident on the crystal, f being determined by the orifice dimensions and the orifice-to-crystal distance. The sticking probability is obtained from an expression similar to eqn. (16),viz. s =-Po - P

fP0 and the estimated absolute accuracy is k 0.05. A clear advantage of molecular beam techniques is the relative ease of independently controlling the beam temperature, the incidence angle and the crystal temperature, providing a means of studying adsorption dynamics. 2.2.8 Absolute random flux technique Morris and King [207] have established an absolute method for the determination of incidence-angle-averaged sticking probabilities. The adsorption cell is shown schematically in Fig. 9. I t consists of a crystal facing a small orifice leading to a pressure measuring device and an inert (zero s) disc placed in front of an identical orifice leading to a second pressure-measuring device. Gas is allowed to flow into the continuously pumped cell and the two pressures, P and P o , directly compared. The sticking probability is given again by eqn. (18), but in this case f is the fraction of molecules which would have entered the orifice at the crystal but are blocked from doing so by the crystal; f is obtained from the

27 TO

pumps TO

pumps

Thermocouple

v

Fig. 9. The experimental arrangement used by Morris and King [207] to measure angle-integrated sticking probabilities.

crystal-to-orifice distance and the crystal dimensions. Steckelmacher [208] has described the evaluation of f for various experimental geometries. Absolute accuracies in s for this technique have been reported as kO.03; it has the advantage of simplicity, and could be readily adapted t o stainless steel systems. 2.3 DESORPTION KINETICS

There are two commonly employed techniques for studying thermal desorption and several variations on each of them. The interpretation of kinetic data is fraught with difficulties and to obtain kinetic parameters such as kinetic order and activation energy from desorption requires some knowledge of the desorption mechanism. We therefore leave discussion of data analysis until a later section. 2.3.1 Temperature-programmed desorption In this very widely used technique, gas is adsorbed on the adsorbent and a temperature programme is subsequently applied to it. Desorption is monitored either by determining the pressure change in the continuously pumped cell, as a desorption pulse (the desorption spectrum) or by following a change in some adsorbate-sensitive physical property of the surface, such as the work function, the secondary electron yield, or the intensity in a photoemission peak. The temperature programme may be hyperbolic, i.e. 1 / T = l / T o i- b t , where To is the initial temperature and T the temperature at time t , or, more commonly, linear, i.e. T = T o 4- b t . Originally, the method was developed for adsorbents in the form of References p p . 163-1 79

28

filaments [ 51 and became known as the flash filament technique, and the desorption spectrum for a given gas/metal combination was generally found to show a series of peaks, attributed t o different states of the adsorbate. For example, if N, is adsorbed to saturation on a polycrystalline tungsten wire at 77 K [209,2101, the desorption spectrum shows a peak a t 150 K, ascribed to a dinitrogen “7” state, a peak at 350 K, attributed to a more strongly held “a” state, and several peaks at -1400K, due to desorption from N adatoms in a “0” state. The y, a, 0 terminology, which derives from these early studies on the N, /Wsystem, is commonly used with other systems. Roughly speaking, y is reserved for desorption peaks below room temperature, CY for peaks occurring between 300 and 600 K, and 0for peaks observed at higher temperatures. Substates are designated by subscripts, e.g. PI and pz. Clearly, one of the advantages in temperature-programmed desorption lies in the ability t o distinguish readily between different states on the surface. However, as discussed later, lateral interactions between identical adsorbed species can also produce multiple peaks in desorption spectra. The peak temperature, Tp, and the shape or half-width, wl/,,of a desorption peak are dependent on both the pumping speed of the system and the heating rate, as discussed in detail in Sect. 4. In early work, very rapid heating rates were employed in an attempt to avoid conversion between states during the heating cycle. However, if such interconversion is to occur, it is generally considered to be unavoidable and the higher resolution obtained with slower heating rates, and relative ease of analysis which follows if the pumping speed is sufficiently high that the desorption rate is directly proportional to the observed pressure rise, are very clear advantages [ 2111. In order to obtain good quality desorption spectra, careful attention must be paid to the method of supporting the crystal in the vacuum system and to the means used for heating the crystal. If electron bombardment heating is employed, a tungsten helix (the cathode) is mounted within a few mm of the crystal face so that reasonable heating rates can be generated at bombardment potentials in the region of 500-1000eV. The crystal itself is then spot-welded t o a wire or rod (again usually tungsten), the dimensions being a compromise between the requirements of (a) rapid and efficient cooling, so that the adsorption temperature can be re-attained after heating within a short period and (b) ease of heating and avoidance of temperature gradients across the crystal. The cathode can be heated without applying a voltage to the crystal, to avoid confusing desorption from the cathode with that from the crystal, but spurious effects can also arise due to desorption from the crystal supports or other components of the vacuum system [ 2121.To avoid these effects, Housley et al. [213] designed a system for determining desorption spectra in which the crystal and the pressure-measuring device are separated by a small diameter orifice and a differential pumping chamber; the crystal

-

-

29

is aligned so that only gas desorbing from the front face can directly enter the pressure-measuring chamber. Clearly, this method has the further advantage that desorption is not monitored from the crystal edges, which are not representative of the crystal face under study.

2.3.2 Is0 thermal d esorp tion In this technique, adsorption is allowed to take place at one temperature and the crystal is then rapidly heated to the desired desorption temperature. This requirement for rapid heating is experimentally very demanding and for this reason, the technique is not often used. The desorption rate can be monitored by measuring the desorption flux as a function of time; Kohrt and Gomer [ 2141 used a field emission tip as a flux detector. Alternatively, an adsorbate-sensitive physical property of the surface, such as electron-stimulated desorption [215] or work function [216] , can be used. An elegant method of obtaining desorption parameters involves the use of molecular beams. Here, the crystal is maintained at the desorption temperature and the requirement for rapid heating is avoided. Two methods are used. If the beam is chopped, the decay rate of the desorbed particles can be directly monitored [ 217, 2181. Alternatively, the beam may be modulated and the phase shift between the input and output measured [219]. These methods were originally used t o determine lifetimes for species desorbing as ions, such as Ba' and Cs' from tungsten [217, 219, 2201 and for metal atoms [221], but COY0, and H, desorption have also been examined in this way [223-2251.

2.3.3 Electron-, photon-, ion- and field-stimulated desorption Alternative methods t o thermal desorption may be used to effect desorption. Although these methods are somewhat beyond the scope of the present review, we mention them briefly here for completeness. A beam of electrons incident on an adsorbed layer may cause desorption of both neutral and ionic species, fragmentation of the adsorbate or rearrangement of the adlayer [102, 1041. Redhead [ 1031 and Menzel and Gomer [ 1021 independently proposed that these effects were brought about by Franck-Condon transitions to be excited states, with reneutralization from ionic states by electron tunneling from the metal Fermi sea playing an important role. The excited, neutralised or ionic fragment may be ejected from the surface with energies up to -10eV. The threshold energy for these processes is about 15-25eVY reaching maximum efficiency at a primary beam energy of about 100eV. The efficiency is in the region of 1 particle per lo4 t o lo8 incident electrons for adsorbates such as H, 0 and CO, but is negligibly small for metallic adatoms and for N and C adatoms on metals. Where techniques such as LEED or AES are used to study adlayers, electron-stimulated desorption References p p . 163-1 79

30

can cause a major perturbation t o the adlayer. However, electron-stimulated desorption (ESD) has itself been widely used as an analytical tool. Crosssections for different states of adsorbates can differ widely, making ESD a useful probe for distinguishing between such states. And the observation of anisotropies in the spatial distribution of desorbing ions by Madey and co-workers [226] has led to the development of ESDIAD as a structural tool. Recently, there has been a revised interest in ESD resulting from the work of Knotek and Fiebelman [121], who showed that ionic desorption from maximal valency compounds, such as transition metal compounds, occurred by a cross-transition, a core hole being initially created in the metal ion, the chalcogenide subsequently forming a positive ion by loss of electrons t o the metal and being ejected from the surface by a coulombic explosion. Desorption of 0' from an oxygen layer on metals can also proceed by this mechanism. In recent years, it has been found that ionic desorption from metal surfaces can also be induced by photons [107-1151 with threshold energies in the same range as for electron-stimulated desorption. The mechanism would appear t o be the same although, as the photon energy is ramped through the threshold, the cross-section rises continuously t o a plateau at 100 eV. In practice, this means that photon-stimulated effects a t any given photon energy are rather less severe than with ESD. In the field ion microscope, very high fields can be applied t o a surface, sufficient to cause a significant distortion to the potential energy barrier experienced by an atom at the surface, resulting in desorption or field stripping. The method is used to produce the perfect surface t o be imaged in the microscope and attempts have also been made t o correlate the field required for desorption with the adatom bond energy to the surface [ 2271 . High-energy ions, such as Ar' at 3000 eV, are commonly employed t o produce clean surfaces by sputtering away the surface layers. The mechanism here is based on momentum transfer, with the incident particle often burying itself a few layers into the surface before losing momentum t o a substrate atom. This results in a phonon transfer cascade, which may result in desorption of neutral, ionic or excited fragments when the effect reaches the surface. The technique of secondary ion mass spectrometry (SIMS) is based on mass analysis of the desorbing ions [228]. For example, when CO is adsorbed on a metal M, species such as M', MCO', MzCO', MO' and MC' [229] have been detected. Since the method effectively monitors the results of an explosion, which causes damage to the surface over a region corresponding t o many atom diameters, the derivation of structural information from the observed fragments is hazardous. Similarly, as an analytical tool it suffers from the fact that cross-sections vary very widely for different adsorbates and substrates.

-

31

2.4 SURFACE DIFFUSION

2.4.1 Scanning methods

( a )Primary photon beam The first use of photons in the analysis of diffusion mechanisms was by Bosworth in the 1930s who investigated the diffusion of sodium and potassium on tungsten ribbons [231] (see Fig. 10). A strip of the alkali metal was deposited on the surface from a collimated evaporation source and the coverage was monitored by measuring the ion current arriving at the sample. An accurately moveable light beam (positioned outside the vacuum system) was focused to a spot on the ribbon surface. In the region of the surface where the alkali atoms were adsorbed, there was a large decrease in work function (@) and a large enhancement in the photoemission of electrons from the surface. Thus a profile of the adsorbed patch concentration across the surface could be obtained. This was originally obtained in terms of photoemission current versus distance moved by the light beam and a calibration curve of alkali metal coverage versus photoemission current was obtained for conversion of the data. Bosworth used vacua of < 10-4Pa, but that was all that was known (low pressure measuring devices not being available at that time) and it is likely that UHV conditions were not obtained (the surface was certainly in the form of tungsten oxide since it had a very high work function). However, recently, another group of workers has used the scanning photon beam method to look at the diffusion of caesium on tungsten surfaces under UHV conditions [ 2321. ( b )Primary electron beam

Scanning AES has been used recently to look at adsorbate diffusion. The scanning is carried out simply by applying a linear voltage ramp of a few volts magnitude between two of the four deflector poles (or plates) while holding the potential between the other two constant. In this way, a particular Auger transition of the adsorbate under investigation can be used as a monitor of relative concentration versus distance scanned across the surface (proportional t o interplate potential). Profiles such as this can be taken after heating periods t o observe the change in concentration profile with time of heating. As an example of this method, Polak and Ehrlich [233] have recently observed N atoms on W ( 1 1 0 ) (using the K L 2 ,3 L 2 , Auger electronic transition as a measure of N concentration) diffusing over an activation energy barrier of 88 kJ mole-' in the temperature range 800-900 K. The secondary electron emission properties of surfaces have been used in combination with scanning techniques in this laboratory [45, 2341. References p p . 163-1 79

32

Fig. 10. Experimental arrangement for photon beam scanning as used by Bosworth [231]. AB is the W ribbon used in the experiments; C is the source of alkali ions; D is a shield to direct alkali ions; wire E is used to measure ion currents; and F is an earthed can.

Effects on the secondary electron emission can be assessed very simply by measuring the total drain current, It, t o the crystal surface

It = Ip

+ I,

(19) where I, is the secondary electron current (all back-scattered electrons) and Ip is the primary beam current; Ip and I, are opposite in sign and, depending on their relative magnitude, there is a drain current t o the

33

crystal from earth or vice versa. Usually, the secondary electron yield, 6 (the ratio of secondary to primary electron currents), is greater than one for metals [235] and so there is a net drain current from earth to the crystal. True secondaries constitute the great majority of secondaries at E , > 200 eV and are of relatively low energy. Such electrons originate from only the top -3-4 atomic layers of the surface due to escape energy considerations 12361 and so these slow electrons are extremely sensitive to change in the state of the metal surface. King and Wells [46] used a scanning deflection method [237] to traverse an electron beam across the surface of a W{ loo} single crystal in order to attempt a study of oxygen surface diffusion. Oxygen adsorption increases 4 for tungsten [238] which, in effect, adds to the energy requirement for secondaries to escape the metal; this chops the lowerenergy electrons originally produced out of the total secondary spectrum (there is negligible emission from the adsorbate) thus markedly attenuating the drain current. The surface could be dosed with a “patch” of oxygen deposited from a molecular beam source and, by measuring the drain current while scanning the crystal, a coverage profile of oxygen across the surface was obtained (the change in drain current, and hence secondary electron emission, was found to be a linear function of coverage). A study of oxygen diffusion was attempted, though, in this case and in the temperature range studied, desorption proved to be the dominating process. Another novel method described recently involves the use of the electron-stimulated desorption (ESD, EID) of species from a surface. Lichtmann and Campuzano [239] used a scanning electron beam to induce desorption of F+ ions from a fluoridised molecule and obtained traces of F+ current (measured in a mass spectrometer) versus distance scanned across the sample. Only qualitative results were obtained but this again illustrates the strength of the scanning technique for boundary diffusion studies. ( c )Other techniques

A further innovation was made in the field of diffusion profile studies by Butz and Wagner [240] who observed the diffusion of 0 atoms on W { l l O } . They deposited a sharp boundary of oxygen on the surface and measured the spatial variation in contact potential using a vibrating capacitor technique [241]. The reference plate used was a 6 x lop6m diameter wire situated 10-20 x m above the surface; this could be moved parallel to the surface and the distance resolution of the probe m. Thus, a plot of surface coverage (caliwas estimated as 50x brated in terms of work function change during adsorption) versus distance traversed by the wire could be obtained. They deposited a boundary of oxygen by placing a shield very close to the crystal and depositing W atoms on the exposed surface. Thus the sticking probability in the References p p . 163-1 79

34

roughened region was enhanced (the smooth (110) plane has a relatively low s value) and also the conductance to gas was low in the shielded region. A sharp step in coverage (within the resolution) was obtained by this method and negligible amounts of oxygen were adsorbed in the shielded area. The shield was then removed and the sample heated for periods of many minutes, the coverage profile being subsequently measured with the sample at room temperature. The results of this work are described later. 2.4.2 Field emission and field ion microscopies

( a ) Field emission microscopy (FEM) Certainly the most common property of surfaces used in the area of surface diffusion studies is the field-induced emission of electrons. The property of field emission under vacuum conditions was first observed as early as 1897 [242] and in 1923 Schottky [243] attempted t o explain field-emission from cold metals theoretically. The most significant practical advance came in 1937 with the invention of the field-emission microscope by Miiller [7]. The most common arrangement of the field-emission. microscope is shown in Fig. 11. B is a thin wire which has a fine point (facing the screen) from which emission takes place. To obtain the field emission of electrons, the tip is held at a high negative potential relative to the screen. Potentials used depend on the radius of the tip end and on the tip-to-screen distance, but are normally in the range 2-10kV. With the potential applied an image of the tip surface is seen on the screen. The magnification obtained is around l o 5 to lo6 and, again, is dependent on geometrical factors. The radius of tips used is lo-’ m. Electrons are emitted from the cold metal surface because the large field strengths used (10’ to 10’’ V m-’)are able to distort the potential energy barrier at the surface to such an extent that electrons can “tunnel” through the barrier (see Fig. 12). In rough areas of the tip, the local field strength is highest and so the emission tends to be highest from these regions. Further, smooth planes (such as the body centred cubic {110} plane) have high work functions, providing a bigger barrier to electron emission and so such planes appear relatively dark on the surface (see Fig. 13). Adsorption usually alters the work functions of individual planes and so alters the distribution of intensity in the FEM pattern. Thus, for example, the adsorption of N atoms on tungsten is known to decrease 9 for the {loo}-type planes, whereas for most other planes, @ is increased [244]; as seen in Fig. 13, this effect is indicated in the FEM pattern since the {loo}plane is the most highly emitting compared with the rest of the tip after adsorption of nitrogen [245]. By far the commonest method of dosing the tip for surface diffusion studies is “shadowing”. The adsorbate is emitted from some source

-

35

Fig. 11. The arrangement of the field emission microscope. A is a gas bottle, B is the emitter assembly, C and D are ionisation gauges, and E is a liquid-nitrogen-cooled trap and F is the willemite screen on which patterns are observed (taken from ref. 5).

situated opposite one side of the tip and thus impinges on only about half of the hemispherical surface. In papers of the late 1950s [246-2481 , for instance, Gomer et al. used an effusion source for depositing gaseous species on one side of a tungsten tip. This depended on the immersion of the whole cell in liquid hydrogen so that any gaseous entities not impinging on the tip were pumped on to the walls with 100% efficiency. Thus, although gaseous species were entering the cell, these workers could be sure that the out-of-beam parts of the tip were remaining clean (vacua, Pa). Other when the cell was cooled in this way, were estimated as sources of adsorbates are commonly filaments or ribbons of the particular material, placed line-of-sight with one side of the tip, which are resistively heated t o their evaporation temperature t o deposit atoms on the tip surface. For example, in 1954, Klein [249] used a carbon filament as a source of atomic carbon. The carbon could be evaporated in varying

-

References p p . 163-1 79

36

>

k

Schottky saddle: 3.79 F’I‘eV

Zero field

? nl x

e

F

I

\ 0.

I X5

(3.60/F)”2 A

1

x/A Distance from image plane

Fig. 12. Diagram showing the bending of the potential energy barrier (image potential above) under applied electric field. The distortion allows electrons t o “tunnel” through the barrier.

amounts on t o the tip by simply altering the time of heating of the carbon filament source operated at 2600 K. Other materials can be evaporated from coatings on high melting point metal wires. Mirinova and Zubenko [ 2501 evaporated ytterbium and neodymium from a tungsten basket containing the substance, the basket being heated by electron bombardment. Nonnura and Sugata [251] dosed a tungsten tip with caesium by heating a nickel tube, which contained a mixture of caesium chromate and silicon powder, near the tip; from the reaction, caesium was evaporated into the cell. FEM has been widely used for surface diffusion studies, but is somewhat limited in that several effects are superimposed on the measurements, particularly the effects of coverage-dependent diffusion parameters [252,253] (due to inter-species lateral interactions) and differing types of adsorption sites on the surface. Probe-hole FEM [254] (a method for investigating the emission from individual planes on the tip surface) goes some way towards separating these two effects. A further improvement in the FEM method, which is particularly helpful in separating out coverage dependencies, is the so-called “flicker noise” techpique [255--2571. This method utilises the fact that, when the adsorbate is in a mobile state on the surface, localised variations in adsorbate density occur with time. These show u p in FEM as rapidly appearing and disappearing points of light on the screen in an FEM tube, or in a current-measuring system as a spikey noise level superimposed on the signal. The level and frequency of the noise is strongly temperature, coverage and plane dependent. The adsorbate density fluctuations build u p and decay with a characteristic time given by 7 = r 2 / 4 0 where r is the radius of the region probed. With r = 50 8,diffusion

37

Fig. 13. Field emission micrographs from a (1lO)oriented W tip, all taken at an accelerating potential of 4.7 kV and photographed under identical conditions. (a) Clean pattern after flashing to 2500K. (b) After deposition of 1 dose of W, from direction indicated. (c) After deposition of 7 doses of W. (d) Annealed tip exposed to N2 for 2 X lo-’ Torr s. (e) Tip shown in ( c ) after exposure to N2 for 2 X lo-’ Torr s, N2 pumped away, and then shadowed with 7 doses of W. (9) Tip shown in (f) after annealing in vacuum at “700K for 5 6 . (h) Tip shown in (g) after annealing in vacuum at “ 800 K for 5 s. (i) Tip shown in (e) after annealing in vacuum at -800 K for 5 s. (Taken from ref. 245.)

coefficients between and lo-’’ cm2 s-l yield 0.65 > T 2 0.00065. The amplitude of the flicker noise is related t o the mean square displacement, Z, of diffusing species. This method is particularly useful in that it extends the substrate temperature range downwards to temperatures where other kinetic processes such as desorption or adsorption can be avoided. One problem with the flicker noise technique, however, is that References p p . 163-1 79

38

measurements can only be made in the presence of a very high electronic field gradient at the surface, which may influence the diffusion rate. A final, important drawback to FEM that must be mentioned is that it is limited by the materials which can be conveniently made into tips which are thermally stable, the refractory metals being the most simple to form and the most commonly used. ( b )Field ion microscopy (FZM)

The apparatus for FIM is basically the same as that for FEM. However, the potential polarity between tip and screen is reversed and experiments are carried out in the presence of a rare gas (usually 0.1 Pa of helium). Because of the high field strengths employed (- 5 x 10" V m-'), atoms incident on the surface (after making several "hops" across the tip) are slowed down and are eventually ionised by quantum mechanical tunnelling of an electron into the tip; after ionisation, the positive ion is accelerated away from the surface by the field gradient and causes a spot of light when incident on the fluorescent screen. Resolution is improved by minimising the parallel component of the velocity of the ion leaving the surface. Thus FIM is usually carried out < 80 K and distance resolution of about 3 8 can be obtained. The surface tends to show up as a series of well-defined spots (see Fig. 14) corresponding to atomic positions on the surface, since these are the points of highest field strength. Thus ledges, steps and protruding atoms on the surface tend to show up brighter because of their high local field strength. Unfortunately, because of the high field strengths employed, adsorbates

-

Fig. 14. Field ion micrograph from a W tip. (Taken from ref. 5.)

39

such as 0 or N tend to be stripped from the surface as ions and so their microscopic diffusion cannot be investigated by this method. Study is usually made of transition metal atoms on the surfaces. The detailed “hopping” of single atoms is examined and so, prior t o study, the sample is heated to remove evaporated adatoms until only one or two remain on the plane of interest. When such an atom (or combination of atoms) is present, then diffusion is examined by first recording its “position” at the low temperature (photographically) then removing the field and warming the tip to -300K for a suitable time (usually found by trial and error). The sample is then cooled, the field re-applied and the field-ion image examined to see if the atom has moved t o a neighbouring site. Obviously, many such experiments have to be carried out to obtain useful values for diffusion rates. The diffusion can be examined at various temperatures (over a fairly narrow temperature range) to determine the activation energy barrier to surface diffusion. 2.4.3 Other methods

One of the first thorough studies of surface diffusion was by the pioneer of surface studies, Langmuir. With Taylor [ 2581, he examined the diffusion of caesium adsorbed at low coverages on polycrystalline tungsten. The sample was a wire and was situated as shown in Fig. 15. Caesium vapour was admitted to the vacuum system. The coverage ( N ) versus time curve (for a particular pressure of Cs) was obtained by biassing the cylinder Co negative and assuming that every particle emitted from the surface is an ion and is collected at this cylinder. Coverages up to 3.6 x 1017 atom m-2 were measured in this way (the sensitivity of this technique was estimated as monolayers). The Cs film was formed on the wire with all three cylinders biassed positively. The Cs was then removed by immersing the cell in liquid air. With the filament at temperature T,,the central cylinder was now changed from 2 2 V to - 2 2 V and caesium escaped from the central part of the wire as ions, thus leaving a boundary on the surface. Thus with all the cylinders again at 2 2 V , the wire temperature was raised for a specific time and a certain amount of Cs diffused into the central region. This amount was subsequently measured

+

+

In

I=

Fig. 15. The arrangement used by Langmuir and Taylor [258] to study surface diffusion. C I , Cz and Co are three concentric cylinders around a tungsten wire, W. A, B, C are three potential leads.

References p p . 163-1 79

40

by biassing all cylinders at - 22 V and measuring the ion current to Co when the filament was heated t o evaporate all the species. Assuming that the diffusion coefficient was concentration-independent, the flux of diffusing atoms (calculated from the number evaporated from the central region) was equal to - Ddo/dt, and D could be determined. The activation energy fo diffusion, Em, was found from the variation of D with temperature. In 1933, Brattain and Becker [259] measured the thermionic emission from a polycrystalline tungsten ribbon and its variation with adsorbate coverage to investigate thorium diffusion. Thorium was evaporated on to one side of the ribbon and they measured the thermionic emission (at 1274 K) from the other side after periods of holding the ribbon at an even higher temperature. As the thorium diffused t o the reverse side, the thermionic emission from that side increased (since the work function is decreased) and from the rate of diffusion the diffusion activation energy was estimated. Recently, Abramenkov et al. [ 2601 investigated the diffusion of copper atoms on the surface of a polycrystalline M o sample. They deposited the copper by electron-bombardment heating of a M o bowl filled with copper shavings. The material was collimated through several slits before impinging on the sample, so that a sharp cleanlcovered boundary was formed on the surface. The film deposited was 500 thick and so was a multilayer of copper. Their method of analysis was surface “sputtering”; using a primary beam of Ar’ ions ( 4 keV), they could analyse metal ions knocked off the surface in a mass spectrometer. The ion beam was 0.4 x m in diameter at the surface, was statically positioned m away from the edge of the copper film and was of low current density Am-2) to minimise the depletion of material in the area sampled. They locked in on the Cu’ secondary ion evolution during heating in the region 800-1100K and so they could monitor the build-up rate of copper at this position outside the original boundary and measure the diffusion coefficient at various temperatures. A t least in the early stages, the results appeared to correspond to surface diffusion of copper atoms and analysis of the coverage change as a function of time yielded values for D and the diffusion activation energy to be presented later in this chapter. Renard and Deloche [261] examined the surface diffusion of physisorbed tritium on a single crystal N i { l l l } surface. The gas was deposited as a patch with the crystal held at 4K and the concentration profile across the surface was determined by collection of the p radiation emitted from tritium in a channeltron electron multiplier. For the diffusion experiments, the collector was positioned so as t o collect radiation from a point well outside the original patch area and the sample was then heated t o temperatures in the range 13-20K. Desorption was also appreciable from- the physisorbed layer and so they derived the coveragetime relation (at fixed temperature)

-

a

41

8 = 0

t~__

...

0

t

+ to

[

r2 t to)

- 4D(t

-.,I

for data analysis where 8 is the coverage after time t , to is the effective zero of the experiment (a little diffusion taking place during heating to the desired temperature), 8, is the maximum coverage in the original deposit, D is the diffusion coefficient, (Y is the desorption coefficient and r is the distance between the centre of deposition and the observation point. This equation was found to fit the data reasonably well and the resultant diffusion coefficients used for the fitting were found t o give a linear Arrhenius plot yielding an Em value of 0.84 kJ mole-'.

3. Adsorption kinetics 3.1 THE DATA BASE

Prior to a discussion of adsorption kinetic rate laws and mechanistic theories, we review the very large amount of data that has been gathered in recent years. We consider first the values reported in the literature for the sticking probabilities at zero coverage, so, for a variety of gas-metal systems; secondly, the variations of so with gas temperature, Tg,and surface temperature, T,;and thirdly, the dependence of s on surface coverage. 3.1.1 Zero coverage stickingprobabilities

Zero coverage sticking probabilities are usually measured at substrate temperatures such that desorption from the chemisorbed species can be ignored, i.e. the adsorbate lifetime is long by comparison with the time taken to make the measurement. The sticking probability so is therefore unambiguously the probability that an incident molecule is finally chemisorbed on impact with the surface. Of course, trapping into more weakly bound, short-lived states may occur, but the trapping probability into these states is not measured in these experiments. Values reported in the literature for so are presented in Table 1. In many cases, the reported values involve rather large error margins, as so has been obtained as a byproduct of other investigations; only where dedicated techniques were used, as described in Sect. 2.2, can the values be trusted to better than 20%. Nevertheless, Table 1gives some indication of trends in reactivity. The initial sticking probability is sensitively dependent on the gasmetal system, spanning the range 1< so < although for H,, 0, and CO and N2 on metals and at substrate temperatures where chemisorption does occur, sticking probabilities are generally between 0.1 and 1.Trends across the periodic table are examined in Fig. 16 and 17, where the most reliable data for polycrystalline ribbons, films or foils of the transition References p p . 163-1 79

TABLE 1

Ip

N

Adsorption parameters : initial sticking probability, so, (at 300 K unless otherwise indicated) and saturation coverage, N,, The variation of s with N is indicated by the letters A-E, illustrated in Fig. 22 (p. 56). Substrate

s versus N

Ref.

C

0.43

492 294 622 62 3 497 624 503 506 504 296 506 509 510 626 626 626 511 515 628 196

-1

517 520 516 516 627

SO ~

Hydrogen co(ooo1j Fe(fi1m) Fe (filament) Fe(100) Ir(11O) Nb (polycryst.) Nb (polycryst.) Nb (polycryst. )

0.045 1 0.05 0.03

Pt(100 Pt(100

Pt(ll0

3.5 x 1014

B

I

0.13(720 K) 0.13 0.023(90 K) 0.056(90 K) 0.06 0.06 0.01

2

Pd(po1ycryst.) Pt (filament) Pt (foil) Pt (polycryst.)

0.13

D B 3.2 x 1014 9 x 1014 3.3 x 1014

0.9 4 x 1014 2.7 x 1014

0.7 0.35 0.7 0.13 4.5 x 1 0 - ~ 0.09(273 K) 0.43(77 K) 0.17 0.07 0.15(78 K) 0.2(78K) 0.31(78 K)

(Two states)

0.39 0.2 7.4 x 1014 4.6 x 1014 8.2 x 1014 1.75 x 1015 1.25 x 1015

B

Pt(ll0 Pt(ll1

Pt(s)6(111 X 111) Pt(s)9(111 x 111) Re (polycryst.) Rh(tip) Ru (000 1} Ta (polycryst. ) Ta(po1ycryst.) Ta(100) Ti(polycryst. ) W(fi1ament) w(100

w(100 W{lOO

w(111

0.33 0.016 0.1 0.2 0.1 (78 K) 0.07 0.4(78 K) 0.25(78 K ) 0.14 0.36 0.34(120 K) 0.4 1 0.4 (100 K) 0.1 0.48 (300 K ) 0.6 (600 K) 0.04 2 0.1 0.2 0.65 0.51 0.65 0.56 0.6

w(112

1 0.22 0.57 0.57

Oxygen Ag (film) Ag (film)

1

8.4 x 1014 8.2 x 1014 6 X 1014 1.5 x 1015 1.5 X 10”

B C

B

1.8 x 1015 1.25 x 1015 6.6 X l O I 4

C A

1 4 x 1014 6 X lOI4

A C 1.6

1x

1oIS

)

1

1 x 1015 2.5 X 1 O I 6 9.6 x 1014 2.3 x 1 0 ’ ~ 2 x 10’~ 2 x 10’~ 2.3 X 1 0 ” 10x 1014 1.9 x 1015 2.0 x 1015 1.5 x 1015 9.4 x 1014 9.1 x 1014 8.1 x 1014 9.0 x 1014 6

X

lox4

1d4

526 526 303 27 5 516 353 516 516 520 353 521 522 633 524 525 503

C C C

624 6 30 631 674 634 230 16 632 364 475 277 632 632 632 192

D A

635 636

A

I&

TABLE 1(continued)

Substrate Ag{llO

Al (films)

:[::I

c o 1000 c u {loo} c u {111}

Cu{l11} Cu (film) Fe (polycryst. ) Fe (film)

s versus N

SO

-

3x 10-~ 3x 5.9 x 3x 10-~

4.2 x 1014

e = 0.5

e = 0.06

5)

0.03(77 1.5 X 103x 0.02 (290 K) l(30K) 0.02(290 K) 0.07 (80 K) 0.8 0.7 0.27 0.03 (300 K) 0.17(720 K) 2.4 x 1 0 - ~ (420 K) 0.95 x 1 0 - ~ (552 9 x 100.9

C E B

e = 0.45 3 x 10'~ 1 6 x 1014

e = 1.7

C B B C

1

Fe(100 Fe(ll0)

I

0.16

460 288 637 200(c) 638 634 640 640 642 642 64 3 644 64 5 646 64 6

e = 0.5

w4

1 0.3

B

Ref.

D

200(b) 636 648 636 649 650 651 652

2 2 m

s %cr 0

0) 0

I

Fe(llO} Ga( film) Ir(ll1 Ir{llO/ Mo(fim) Mo(ribbon) Mo{llO} Nv( film) Nb(fi1m) Ni(100 Ni(100 Ni(ll0 Ni(ll1 Pd(ribbon) Pd (polycryst.)

I

11111

Pd 111 Pt (filament) Pt (filament) Pt (foil) Pt (sheet) Pt (ribbon) Pt (polycryst.) Pt (Plycryst. 1 Pt (ribbon) Pt{llO}- ( 1 x 1) - (5 X 20)

Pt - (S)9(111)X (111)

0.2 1 0.28 0.05 1 0.095 to 1 0 - ~ 1 1 1 1 1 0.35 0.8 (463 K) 0.4(420 K) 0.3 0.14 0.16 0.4 0.01 0.19 0.27

0.05 0.23 0.1 < 1x lo-* 4 x 10-~ 0.4 0.135 0.25 0.08(523K) 0.2 0.1

1.6 X lol: 17 X 10 2.4 x 1014

C

e = 0.5

1 2 x 1014

C C

6=1 18 x 1015 1 4 x 1014

C

E D C

0.8 x 1 0 ' ~

D C C A

e = 1.0 6 = 1.0 e = 0.25 6 = 0.25 3.7 x 1014 1014

C C B C C

e = 0.4

3.9 x 1014

3 x 1014

e = 0.35 6 = 0.01

3.2 x 1014 1 4 x 1014 7 x 1014 7.6 x 1014

i

C

653 636 463 654 636 103 655 636 636 77 656 657 657 468 658 224 659 660 661 662 663 469 664 470 322 665

C C A

e = 0.45

B

e = 0.5

B

471 666 473 474 368 667 668

&

01

TABLE 1 (continued)

rp Q,

Substrate

SO

Pt - (s) 12(111) x (111) Re (film )

0.4 1 0.53 0.16 0.6or 1 0.53(47’3 K) 4 x 100.07 0.17(2033 K) 1 0.86 1 1

Ta(strip) Ta(film) Ta(po1ycryst.) W(fi1m) (polycryst. ) W (polycryst. ) W (polycryst. ) (polycryst.) W (polycryst.)

w w

w(100 W{lOO

wtloo

w 100 w(100 w(100 w(100

W{llO w(110 W(ll0 w - ( s ) [10(11O)X (Oll)]

0.7

1 1 l(77 K ) 0.65 0.75 1 1 1 1 0.17 0.28 0.3 0.45 0.6

Nm,, atom cm-’

emax

s versus

e =48 15 x 1014 7 x 1014

C A

0 = 0.5or 1 = 0.22 e = 0.9

e 29 x 1014

C B C C

10 x 1014

F

17 x 1014 11.4x 1014 3 x 10’~

C

e=1 12 x 1014

C C

e = 1.4

F A A

F

11.4x 1014

e = 0.5 e = 0.55 e = 0.6

e = 0.5

C C C

N

Ref.

476 636 669 477 670 671 479 480 480 636 672 636 673 170 674 675 676 16 677 678 486 367 679 407 680 488 240 680

5 23

W - (s)[8(110) x (112)] W - (s)[8(110)x (112)] Zr(film)

1 0.97 1

e = 1.0 11x 1014

F

C

636 681 636

C C C C C C C C C C C C C C

532 662 5 35 5 36 537 5 38 683 684 665 223 342 550 686 224 68 7 470 150 557 556 557 627 560 557 561 688 187 275 565 5 66 557 5 68

0

2

$

Carbon monoxide

co(ooo1)

c 0)

0

I c1 v

rg

Mo(filament) M o (polycryst.)

Pt (polycryst.)

0.9 0.9 1 0.87(520 K ) 0.6 4 x lo-* 0.13 - 0.16 0.3(370 K ) 1 1 1 1 0.91 0.97 1 0.5 0.6 0.24 1 0.64 1 1 0.34 1

Re( polycryst.)

1 0.67 0.45 (78 K) 0.85 0.737 0.27 0.045

0.33 0.58 0.66

0.5 1 0.42 2 x 1015 0.57 0.5 0.5 0.25 0.75 2 x 1015 1 1 1.5 X 10” 1 4 x 1014 1 4 x 1014 20 x 1014 0.5

2 x 1015 22 x 1014

0.75

Ip

4

+ OD

TABLE 1 (continued)

Substrate

I

M o (film) M o (film) Mo(film) Mo(filament) Mo(filament) M o (110) Nb(fim) N b (polycryst. ) Nb(100)

Ref.

9 x 1014 10x 1014 1.1 x 1015

C C C C

213 572 670 573 689 202 363 306 335 690

2 x 1014 2 x 1014 2 x 1014

A A A

Nm, atom cm-*

0.1 0.5 0.85 0.6 0.4(400 K) 0.89(69 K) 0.85 0.52 0.9

10.2 x 1014 2.1 x 1015 2.1 x 1oI5

Zr (polycryst.) Nitrogen Ag(film) Cu(film) Fe(film) Fe(100 Fe(ll1 Fe(ll1

s versus N

SO

--x 4

0.66 6.5 x 1014 2 x 1014

1.1

10-~

2x 10-~ (743 K) 0.75 1 1 0.2 0.27 0.8 1 0.24 (300 K) 0.3(195 K) 0.57 (360 K)

emax

0.5 2.9 x 1014 1 2 x 1014 1 2 x 1014 2.4 x 1014 4.4 x 1014 ? Y 1014 1 x 1015 1.1

1.3

376 636 636 691 692 693 588 636 502 694 582 502 636 503 503 695

2 -2

2 z2 'E1 ?

; I

~r

2:

Ni(film) Pt(ll1) Re (filament) R e (film) Ta(film) Ta(po1ycryst.) Ta(po1ycryst.) Ta(film) Ta(wire) Ta(100} Ti(film) V(fh) W (film) W (filament) w(100

wtloo w

100

w(110 w(110 w(111 W(310 W(320 W{411

0.15(1205) 0.22 x 10-

lo-*

1 0.42(320 K) 0.45 (195 K) 0.8 0.33 0.16 1 1

0.7 10-~ 0.41 0.59 0.32, 100.22(85 K)

2 x 1014 2.2 x 1014 2 x loJ4 High

> 1015

1.78 2 2.1

1

5 x 10j4 5 x 1014 8x

lOI4

2 x 1014 2.5 x 1 0 ' ~ 8x lOI4 6 X lOI4 2 x 1014

C C C

5 x loJ4 7.5 x 1 0 ' ~ High High High 6 x lOI4

0.1(123 K) 0.08 0.72 0.735 0.4

A

C C C C C C C

E C A C C C

A

636 695 591 636 6 36 503 503 697 698 629 636 636 636 205 629 205 700 701 234 596 702 504 504 505 505 505

50

0.5-

0

0

Y

01

1

TI Zr Hf

I

V Nb Ta

I

Cr Mo W

l

fin Tc Re

C

Fe Ru

0s

I

CO Rh Ir

!

NI

Pd Pt

Fig. 16. The variation of SO across the periodic table for CO ( 0 ) and N2

m

(0).

I

v

I

'

V Nb Ta

Cr MO

W

Mn TC Re

Fe RU 05

Co Rh

Ir

Ni

Cu

Pd Pt

%

Fig. 17. Variation of so across the periodic table. Shown are points for 0 2

(v)

( 0 )and H2

51

metals are plotted as a function of group across the periodic table. The behaviour of the isoelectronic CO and N2 molecules is remarkably similar: hlgh reactivity (with so approaching unity) t o the left, a minimum in the middle (around Mn, Ru and Fe) and higher values to the right. The accumulation of evidence from a variety of studies indicates a further trend; both CO and N2 are dissociated on metals to the left of Fe, and non-dissociatively adsorbed on metals t o the right, with heats of adsorption decreasing from left t o right. A full discussion of these trends has been made by Broden et al. [351] and recently Nieuwenhuys [352] has extended the discussion t o correlate the change in work function across the periodic table with the amount of electron back-donation between CO and N2 and the metal. The magnitude of this back-donation is, of course, a measure of the dissociation tendency since the back-donation is the cause of the C-0 or N-N bond weakening. For oxygen, high so values, close t o unity, are reported for metals in Groups 2-6, only decreasing significantly with the filled d-band metals Cu, Ag and Au. For H,, the reactivity trend appears t o be the reverse of that for CO and N,, with a maximum around the centre of the periodic table. Adsorption heats and heats of formation of the metal hydrides, on the other hand, again fall from left to right. Dissociative adsorption does not occur on Cu, Ag and Au. Studies on single crystals have revealed that the sticking probability may be a sensitive function of crystal plane; this appears t o be more important for H, and N2 than for CO and 02, although similar trends are observed for all these gases on the same metal. Of the body centred cubic (b.c.c.) metals, tungsten is the most extensively studied and the most accurate available values for so are plotted for the principal zones of tungsten in Fig. 18. The range of so values for N2 is illustrated by the extremes, from 0.70 on W(320) to 3 x on W(llO}. It would appear that the closest-packed plane, { l l o ) , is the least reactive for N2, H,, CO and 0 2 ,although the trend is clearly less important for CO and 0,. We have chosen Pt t o illustrate the trends for an face centred cubic (f.c.c.) metal, (Fig. 19), although here there is some difficulty arising from the fact that the {110} and (100) surfaces of these metals are reconstructed in the stable state. Both surfaces can, however, be prepared in a metastable (1x l) state and, where available, so values are plotted for both states of these surfaces. For H,, O2 and COY maximum reactivity is observed for the relatively open (110) face, even in the (1x 2) reconstructed state. Very marked differences in the reactivity between the {loo} (5 x 20) and (1x 1) surfaces have been reported. Interestingly, it is widely accepted that the (5 x 20) reconstructed form of this surface, which has a low sticking probability for H2 and 0,, possesses a hexagonal, close-packed surface structure. For both f.c.c. and b.c.c. metals, therefore, it appears that the more open faces are more reactive. However, this conclusion must be treated with caution; for example, the very open References p p . 163-1 79

52

c 8

8

8

8

0 0

0 0

0 0

0

0

0

(211)

poly.

0

0 0 (100)

(110)

(111)

Fig. 18. Variation of sticking probability with Pt crystal plane for

HZ.

.,

C O ; 0 , O 2 and 0,

{Ill} plane of tungsten is one of the least reactive for nitrogen chemisorption [ 461 . Much has been written about the high reactivity of steps at metal surfaces and there are several examples in chemisorption. For example, the {llO} plane of W has a low sticking probability (- 3 x for nitrogen chemisorption at 300 K, but on the stepped {320) plane, with (110) terraces, so = 0.70. On the other hand, a singular, step-free W(lOO} surface is also very reactive (so = 0.59) and it has been suggested that the reactivity of stepped planes such as {320} is related t o the structural similarity of the steps t o the {loo}plane itself [47]. A further dramatic illustration of the dependence of so on the presence of steps is demonstrated by the work of Salmeron et al. [353] on the stepped Pt surface. Using a moJecular beam source, they demonstrated that the reactivity for dissociative chemisorption of H2 (as indicated by the formation of HD from a mixture of H2 and D 2 )was 7 times higher when the beam was directed “at” t h e steps than when it was directed over the steps, as shown in Fig. 20. Several studies have been made of the dependence of so on crystal and gas temperature. Since these results are critically important in establishing the adsorption mechanism, the detailed presentation of results is left until

53

l.op1.0-

0 0

0

0

0

0 0

0

ul

0

-

0.5

0 0 0

0 o

n

0 (111)

.,

(110)

(211)

Fig. 19. Sticking probability versus W crystal plane for the adsorption of CO.

0 , H2 ; 0,N2 ;and

0,

0,;

the following section of this chapter. Here, we simply note several categories of adsorption systems. (i) For several well-studied systems, such as N 2 / W [46, 3541 and 0 2 / P t [355], so decreases with increasing crystal temperature, although in both systems a low temperature limiting value, less than unity, is reached (0.62 for N2 on W { 100)) and recent results also show that there is a high temperature asymptotic value greater than zero (-0.05 for N2 on W{lOO}) [46]. For these systems, it has also been found that so decreases with increasing gas temperature [46, 3551. (ii) In some cases, so is insensitive t o crystal temperature. For example, King et al. [170] found that the reactive sticking probability for 02,i.e. the probability that an incident O 2 molecule is desorbed as atoms or oxide molecules, was the same a t 2200 K on a polycrystalline W filament as so a t 300K (-0.7). For H2/W{100},so has been reported [356] as independent of crystal temperature over the range 150- 300 K. For these systems, comparable studies of the gas temperature dependences have not been reported. (iii) In a minority of cases, so is found t o increase with increasing crystal temperature; this is true for H2 on Cu [358] and for N2 on Fe [357]. In these cases, an increase in so with gas temperature can be anticipated and the process is properly described as activated adsorption. References p p . 163-1 79

64

'1

:I z

0.0

-90 Pt {332} f = 1 0 H ~

05

O 0 1:

0

Pt{lll}

-80 -60 -40 - 2 0

0

20

40

60

80

Angle of incidence. B / D E G

Fig. 20. H D production as a function at angle of incidence (6)of the molecular beam normalised t o the incident Dz intensity. (a) Pt(332) with steps perpendicular t o the beam. ( b) Pt(332) with step edges parallel t o the beam. (c) P t { l l l } surface. (From ref. 353.)

A few studies have been made of the dependence of so on incidence angle using molecular beam techniques. Apart from the study on a stepped surface illustrated in Fig. 20, relatively small effects have been noted. King and Wells [46]found no dependence (within ? 0.01) for N2 on polycrystalline W and on W(100). Steinbriichel and Schmidt [ 359 J found significant variations only for H2 on W{lOO} (Fig. 21) and, in this case, it is probably again reasonable to assume that the results are dominated by step effects. Recently, Bickley et al. [360] have compared accurate

55

W(100)

08

O L 60

0

30

90

lLLLuAL 0

Angle of incidence, 8

30

60

90

Angle of Incidence. 8

Fig. 21. The variation of sticking probability with angle of incidence 8. s ( 0 ) is the sticking probability at 8 = 0 . (From ref. 359.)

values obtained for angle-integrated measurements with molecular beam values for normal incidence and conclude that the differences imply a significant variation with incidence angle for N,, H, and D, on W{OOl}, but only at angles close to grazing incidence. 3.1.2 Variations of s with surface coverage Provided that the adsorbate is not absorbed into the bulk, as with H, on Ti, and is not reversibly desorbed, all the surface sites will eventually become blocked and s therefore falls to zero. In considerations of the coverage dependence of s, the precise definition must be borne in mind: it is the probability that an incident particle is eventually adsorbed into a state with a lifetime 7 greatly in excess of the experimental time period. For example, even when s is zero at saturation coverage in a strongly bound state, the trapping probability into weakly bound adsorbed states may still be high. In a number of cases, such as CO on Ni [361, 3621 or W { 110) [ 3631 at 300 K, the heat of adsorption falls strongly with coverage and the desorption rate from the chemisorbed layer at high coverages becomes hgh. In these cases, what is experimentally measured is the net sticking probability, given by the ratio (adsorption - desorption rate)/bombardment rate, which will become zero even when the absolute s for adsorption into the chemisorbed state is finite. The shape of plots for s against coverage is a further important indicator of adsorption mechanism. The coverage may either be signified in absolute terms by N , the number of adsorbed species cm-’, or by 8,the fraction of sites filled. The use of the latter must be treated with caution; it is only unambiguously defined when the number of sites is equal t o the number References p p . 163-1 79

56

of substrate atoms cm-*, although even in that case surface reconstruction is an obvious complication, and we have confined our usage t o this definition. Since specific site orientations or symmetries are often required by the adsorbate (which may vary with coverage), the coverage often reaches a saturation value well below 8 = 1;we refer here t o this value as N , or 8,. In a consideration of the very wide range of results published t o date (Table l), we have proposed six different categories of adsorption in terms of the sticking probability-coverage profiles. The six categories are illustrated in Fig. 22, where relative sticking probabilities are schematically plotted against relative coverages. In category A, s falls linearly with coverage; this is anticipated for non-dissociative Langmuir adsorption, i.e. a simple site-blocking mechanism, but typical examples are the dissociative chemisorption of H2 [ 364, 3651 and 0, [ 366, 3671 on W{lOO}. In category B, the curve is convex with respect t o the origin; this is anticipated for dissociative Langmuir adsorption and has been observed for O 2 on P t { l l l } [368] and NO on Ag [369, 3701. Category C is the most common; here the curve is concave to the origin, with s initially almost independent of coverage; examples are N2 on various W single crystal planes and CO on a wide range of transition metals. This is usually taken t o indicate a mechanism in which trapping can take place into a precursor state at a filled chemisorbed site, with subsequent diffusion t o an empty site. In category D, s falls to a minimum at relatively

A

D

B

N

N

E

N

N

C

F

N

N

Fig. 22. Experimentally observed variations of s with N . The letters refer to the listing in Table 1.

57

low coverages, subsequently rising to a plateau before falling again at saturation. First observed by Horgan and King [371] for O 2 on Ni films, this characteristic shape is taken as an indication of chemisorption followed by an adsorbate-induced reconstruction, involving incorporation of the adatoms, at the coverage corresponding t o the minimum. In category E, s rises from an initial relatively low value to a smooth maximum and then falls again. This has only been observed for adsorption into relatively weakly bound states, e.g. for the formation of a y-N2 layer on W { l l O } at low temperatures [372]. In category F, the sticking probability profile is characterised by one or more distinct steps at characteristic coverages, observed, for example, with O 2 on polycrystalline W wires at very high exposures [170, 3731. The steps indicate several distinct stages in the formation of the overlayer, each with a characteristic initial sticking probability. Where coverage dependences have been reported in the literature, the shape is indicated in Table 1 by the lettering A-F, referring to Fig. 22. Clearly, a wide range of different chemisorption rate laws is indicated. 3.2 MECHANISMS AND RATE LAWS IN ADSORPTION

The data base discussed in the previous section provides an indication of the complexity and variety of the steps involved in the formation of a chemisorbed layer when an initially clean surface is exposed t o a gas. The questions posed by these results include the following. What are the important mechanistic steps or processes involved when a molecule strikes a clean surface? How is the excess kinetic energy of the incident particle dissipated? Why is one metal, or one crystal plane of a given metal, more reactive than another? What time-dependent electronic processes occur during the formation of a chemisorbed species? How can the presence of steps be accounted for? And what are the factors governing the variation of sticking probability with surface coverage, surface temperature, gas temperature and incidence angle? In this chapter, we attempt t o survey some of the answers that have been given in the literature, although it should be stressed at the outset that considerable experimental and theoretical work is still required before definitive answers can be provided, even in relation to a single adsorption system. In Sect. 1.2, we discussed briefly the potential energy surfaces for the electronic ground state of a system in which a molecule approaches a surface and is chemisorbed. An attractive potential energy well always exists for a physisorbed state and we therefore begin by examining the question of trapping into this state and energy accommodation in general. We then move on to a discussion of the evidence for, and the properties of, precursor states before considering specific rate laws in adsorption kinetics. The application of these rate laws is exemplified by the adsorption of N2 on W{lOO}, which has been most extensively studied. During Referencesp p . 163-1 79

58

the discussion, we consider also the electronic processes involved in chemisorption and include the relevance of such phenomena as exoelectron emission and chemiluminescence. 3.2.1 Energy accommodation and trapping Where only a physisorption potential energy well exists at a surface for an incident particle, such as helium, three processes can be distinguished: (i) The particle may be elastically scattered back into the gas phase, i.e. without energy loss. Such a collision may lead to diffraction. (ii) During a single collision, the particle may lose kinetic energy and be inelastically scattered. (iii) The particle may lose sufficient kinetic energy to be trapped in the potential energy well with a lifetime T determined by the well depth 4 , i.e. 7 = T~ exp(q/RT) where T~ = 1/v and v is a frequency factor. It is subsequently inelastically scattered into the gas phase. Transfer of kinetic energy during an inelastic collision must take place t o excitations of the combined system, solid plus adsorbate. These excitations are vibrational modes, of which phonons are the low lying ones, electronic modes, two-dimensional translation, and, for polyatomic incident molecules, rotational modes. Each of these energy-loss processes is considered in turn. The loss of kinetic energy by excitation of lattice phonons has received considerable theoretical attention. In the model of McCarroll and Ehrlich [ 3741 , the lattice is simulated as a semi-infinite chain of particles connected via a characteristic force constant and the gassolid interaction is described using a modified harmonic potential. In all these models, energy transfer increases with an increase in (a) the mass ratio p , (incident particle mass)/(substrate atoms mass), and ( b ) the depth of the adsorption potential energy well. Electronic excitations have only received theoretical treatment in recent years [ 375-3781. Kinetic energy is lost through the production of “electron+ole pairs” in the substrate. For metal substrates, the electron-hole pair excitation energies are continuous from zero upwards and, even for rare gases, this may be an important loss mechanism. For insulators, on the other hand, there is a threshold for electron-hole pairs, with excitation modes below it. (Interestingly, for insulators and semiconductors, sticking probabilities are generally considerably lower than for metals, as discussed in Chap. 2 of this volume.) Brako and Newns [377] have used an electron-hole pair model t o determine the trapping probability as a function of incoming particle velocity, reporting a limiting value for low velocity particles. The implication is that, at low gas temperatures, the process may be dominated by losses other than electron-hole pairs. Boato et al. [3791 have discussed evidence, based on molecular beam studies of H2 on LiF surfaces, for the transfer of

59

kinetic energy to rotational and translational modes of the combined system. Thorman and Bernasek [ 3821 have recently developed a method to measure the vibrational and rotational temperatures of a desorbing gas, in this instance N, from a polycrystalline iron surface. Their method basically consists of exciting the desorbing gas molecules with an electron beam and monitoring the subsequent fluorescence to yield the internal energy distribution. Their results indicated that there was a large steric factor controlling nitrogen atom recombination and that when sulphur was present on the surface it caused the formation of a large energy barrier perpendicular to the surface. There are no gas-metal systems for which the dominant loss mechanism has been determined. However, it can be anticipated that developments in angle-resolved inelastic atom beam scattering experiments, exemplified by the recent work of Feuerbacher and Allison [380] with scattering from LiF(100}, will make good this deficiency. In cases where single surface phonons are responsible for the inelasticity in He scattering, timeof-flight measurements with the detector scanned away from the molecular beam enable the dispersion curves for surface phonons to be constructed. Two experimental parameters relating to energy transfer have been quite widely measured, the accommodation coefficient, ac, and the trapping probability a. In the former, all loss processes are integrated; it is defined by E-EE, ac = ES - 4 where E is the average energy of molecules re-emitted from the surface at an effective temperature TL, E g is the average energy of molecules in the incident gas (temperature T g ) ,and E, is the average energy of molecules emitted with the temperature of the solid, T,. For a monatomic gas, the accommodation coefficient is then simply given by

Experimentally, ac may be determined using a metal filament by measuring the incremental electrical energy that has to be applied to the filament to maintain a constant temperature when gas is introduced to the experimental chamber [381]. The trapping probability, a, is the probability of adsorption into a weakly held state on the surface and is measured under conditions where the adsorbate lifetime is small compared with the experimental time period. It is most readily measured by molecular beam techniques; a typical experimental arrangement is depicted in Fig. 23.Depending on the sophistication of the equipment, the parameters which may be measured include the velocity distribution of incident and scattered particles and References p p . 163-1 79

60

zr$z I -to

mect-anical pump

port

15cm

Fig. 23. Typical molecular beam system as used by Somorjai and co-workers [ 3531.

the intensity distribution of scattered particles at variable incidence angles. High fluxes can be achieved using bundles of fine capillaries possessing a large length-to-radius ratio [383, 3841. Wharton and coworkers [ 3851 have measured velocity and angular distributions for Ar atoms scattered from polycrystalline tungsten for supersonic incident beam energies over the range 300-2000K and surface temperatures between 350 and 1900K. They find that direct inelastic scattering involving a single encounter of the gas atom with the surface is the most important process; no elastic or quasi-elastic scattering was observed. At the lowest surface temperatures, trapping-desorption scattering was also observed. Direct inelastic scattering for an incident angle of 45' could be characterised, for all energies and surface temperatures, by the simple expression (KE)e = 0.83(KE)i + 0.20(KETs) (23) where (KE)d, (KE)i and (KE,,) are the kinetic energy of the scattered Ar, the incident Ar, and Ar in equilibrium at the surface temperature, respectively. This work has clearly established the importance of the direct inelastic scattering channel, involving the loss of insufficient incident particle kinetic energy for trapping to occur. However, it is probably difficult to distinguish experimentally between trappingdesorption and inelastic scattering at high surface temperatures, when the lifetime of the trapped species approaches the time of a single collision.

61

Merrill and Weinberg [ 3861 measured the intensity distribution of scattered particles in molecular beam experiments and distinguished between lobular and diffuse (cosine law) scattering. They estimated the fraction of trapped species to be given by the fraction of diffusely scattered particles. For room temperature Ar atoms incident on W ( 1 0 0 ) at 370 K, they report a = 0.45 using this criterion. Wharton and co-workers [ 3851 , analysing velocity distributions of scattered particles, find a 0.43 for low incident Ar atom kinetic energies (analogous to an effusive source temperature of 150 K) on a polycrystalline tungsten ribbon at 393 K and suggest that Merrill and Weinberg overestimate a ( a decreases with increasing incident energy) since inelastic scattering and surface roughness also contribute t o diffuse scattering. Menzel [ 3871 interpreted isothermal energy accommodation experiments in terms of a model which assumes that trapping leads to full equilibration while inelastic scattering leads t o a fixed fractional energy loss t o the surface; for Ar atoms incident on polycrystalline tungsten, both at room temperature, a value for a of 0.20 is derived by this method. In Fig. 24 experimental values of ac and a values for rare gases on tungsten are plotted as a function of the adsorption heat. Both ac and a increase in proportion to the well depth and we also note that ac and a have similar values, at least for the range of experimental conditions used by Merrill and Weinberg [ 3861 . Impurity adatoms, such as H, N and 0, have a dramatic influence on measured accommodation coefficients; for example, a monolayer of oxygen adatoms on polycrystalline tungsten raises the ac for helium from 0.02 for a clean surface to 0.6. Roberts [388] and co-workers made use of this sensitivity to the presence of adsorbates t o determine sticking probabilities for reactive gases on tungsten. West and Somorjai [ 3891 have used the extent of He elastic scattering as a sensitive measure of surface cleanliness. From these experiments, therefore, we can conclude that the trapping probability is a sensitive function of (a) potential energy well depth, q , in the adsorbed state; (b) the mass ratio p of the incident particle t o the substrate atom; and (c) the kinetic energy of the incident particle (slower atoms having a higher trapping probability than fast atoms). A number of theoretical models have been developed to encompass these results, which we only briefly summarise here. They include (a) continuum theories [390] , in which the solid is treated as a continuum and perturbation theory is used to simulate the interaction; (b) classical lattice models [391]; and (c) quantum models, in which an attractive potential well is associated with the gassolid interaction [ 386, 3891 . Hard cube theories, developed by Logan and Keck [392] and by Stickney [ 3921 , have had some success in reproducing experimentally observed dependencies. A detailed discussion of these theories is given in a recent review by Goodman and Wachman [420]. References p p . 163-1 79

62 He Ne

0.e

.

0

Kr

Ar

Xe

1

.

I

I

I

10

20

30

Heat of physisorption

/ kJ

mole-'

Fig. 24. Variations of accommodation coefficients and trapping probabilities for rare gases on polycrystalline W (open data points) and W(110) (filled data points). Target temperatures: A, 375 K; 0 , 575 K;0 , 7 7 5 K;and 0,1300 K. (From ref. 386.)

3.2.2 Precursor states in reactive g a s s o l i d interactions Even when the result of a gassolid collision is the formation of a stable chemisorbed species, weakly bound precursor states can play a major role in the kinetic process. Evidence for such precursor states has recently been reviewed by Cassuto and King, [21]who draw a distinction between intrinsic precursor states, which exist at empty surface sites, and extrinsic precursor states, which exist over sites filled with chemisorbed species. The ability of colliding species to be trapped in these states and to be efficiently transported across the surface is an important mechanistic feature in adsorption. A confusion in nomenclature can arise when a metastable, or "virgin", chemisorbed state can be formed on the surface as an intermediate between physisorbed and stable chemisorbed states: for example, at low temperatures, a virgin, nondissociatively chemisorbed state of CO is formed on tungsten which can be converted t o a dissociatively bound state on heating [ 1021.In the few cases that have been investigated,

63

however, it has been found that these states are virtually immobile over their temperature range of stability [396]. In kinetic formulations involving precursor states, an essential feature is that they are transitory, since the assumption is made that the steady state population is negligibly small. Clearly, at temperatures where a virgin state is frozen in, this does not apply; it is then the stable chemisorbed state. At higher temperatures, however, these states may qualify as precursors. Extrinsic precursor states, as defined above, have been observed and characterised in low temperature shadowed field emitter tip experiments [9]. Diffusion from a low temperature (20K) multilayer deposit of adsorbates such as H,, N2, 0, and CO on tungsten proceeds, on raising the temperature, with a sharp boundary resulting from efficient transfer to the chemisorbed state as second layer, or precursor, species reach empty sites. At higher temperatures, the boundary advances a fixed distance ( Y ~ ) ~and / ~then moves no further, independent of the amount of adsorbate in the original deposit. Following Gomer [9], this distance is approximately related to the desorption energy E i from the precursor and its diffusion energy E L by the expression

E i - E A = RTln ( ( ~ ~ ) ' / ~ / a ) (24) where a is the root mean square jump distance. The average number of hops made before desorption is simply ( x 2 > / a 2 For . example, for O2 on W, E A 4 kJmole-' and E i N 12 kJ mole-' [396] ; thus, at 300 K, where the precursor lifetime is only -lO-'Os, the extrinsic precursor is capable of making 600 hops over filled sites. Recent low temperature (-30K) UPS studies of 0,on A l { l l l } [297] show clear evidence for an extrinsic precursor. The striking kinetic consequence of the mobile extrinsic precursor adsorption rates which may be effectively coverage-independent over a wide range of coverage - in fact constituted the first experimental evidence for its existence [2, 101. It is not, however, the only evidence, as has recently been suggested [ 2971 . Experimental evidence for the existence of intrinsic precursor states is rather more difficult to come by. The common observation that the initial sticking probability, so, often decreases with increaing substrate temperature is consistent with the existence of such a state, as discussed here. Indirect evidence is also provided by molecular beam studies, for example, Hayward and Walters [401] (for H2 on W{OOl}) and Engel [402] (for 0, on Pd{lll}) have observed scattered particle intensity distributions which, even at a fractional coverage in the chemisorbed layer close to zero, exhibit a strong directional lobe in the specular direction superimposed on a cosine law distribution. The specular lobe clearly contains molecules scattered at the first collision, while the cosine law component is most readily attributed to the particles which are trapped in the precursor state and then scattered back into the gas phase. Of

-

Reference8 p p . 163-1 79

64

course, a further fraction of the incident particles are transferred to the chemisorbed state; this may occur by direct transfer into the chemisorbed state, without trapping, or after trapping. For H, on W{lOO} at 300 K, dayward and Walters [401] estimate that, at zero coverage, 3% of the incident particles are reflected at the first collision, 16%are trapped and then re-emitted with a cosine distribution, and 80% pass into the chemisorbed state. The existence of a mobile intrinsic precursor state can also be inferred from the variation of the zero-coverage sticking probability, so, with stereographic angle across the [ 1001 zone for nitrogen on tungsten single crystal planes reported by King and co-workers [ 471. The (110) plane is relatively inactive (so < lo-,), but planes with {110}terraces and “(lOO}” steps were found to have very high sticking probabilities (e.g. on W{320}, about 80% of incident particles will strike these terraces) are efficiently trapped and transported to the step sites. Finally, in systems where trapping into the precursor state is efficient, relative to the process of direct transfer on impact into the chemisorbed state, it would be possible to freeze the oncoming molecule into the precursor state by cooling to very low temperatures ( 5 30 K); spectroscopic techniques could then be employed to examine the state of the precursor and its conversion on heating. In early kinetic models, it was assumed that all molecules incident at the surface are trapped into a precursor state. However, the loss of sufficient excess kinetic energy of the incident particle to result in trapping at the first collision is not necessarily efficient and values lower than unity may be anticipated. Since we wish to reserve the term sticking probability for the probability that an incident particle is finally chemisorbed, the probability of capture into precursor states is described as the trapping probability, a. Values of a are difficult to estimate from experiments with reactive gases, although the velocity distribution has been measured for unreactive gases [32] and, more recently, for nitrogen on polycrystalline tungsten [ 4031 . In molecular beam experiments with rare gases on tungsten, the total scattering intensity could be represented as a superimposition of directional scattering (the quasi-specular portion) and diffuse scattering (the trapped portion), allowing for an estimate of a , As discussed in Sect. 3.2.1, both a and the thermal accommodation coefficient rise monotonically with increasing heat of physisorption, from -0.02 for He to -0.04 for Kr and -0.7 for Xe. By inference, it might be expected that values for reactive gases would cover the same range, as indicated in ref. 429. It is difficult at this time t o obtain reliable theoretical estimates for trapping probabilities. 3.2.3 Models for adsorption kinetics The simplest kinetic model for adsorption is that proposed by Langmuir [422], in which it is assumed that (i) the surface is homogeneous; (ii) every

65

adsorbed species occupies identical sites to every other species at all coverages; (iii) there are no interactions between adsorbed species, other than preclusion of occupation of a site by more than one adsorbed species; and (iv) there is no mechanism for the transport of impinging gas molecules across the surface. The adsorption rate is then simply the product of the impingement rate at the surface, an activation energy term, and a term representing the probability that a site or array of sites is empty, i.e

r,

=

ZP exp (- E , / R T ) f ( 0 )

(25)

where Z is the Knudsen collision factor ( 2 1 ~ r n k T ) - ”For ~ . non-dissociative adsorption, f ( 6 ) = 1 - 8 ; for dissociative adsorption of a diatomic mol-e) ecule f ( 0 ) is (1- f3)2 if the adsorbed layerismobileandZ(1- e)2/(2 if the layer is immobile, where Z is the number of nearest neighbour sites on the surface. Many other cases have been statistically solved; for example, for nondissociative adsorption of a molecule which prevents adsorption at nearest neighbour sites

f(e) = 1 - 3 6 =

+:e2

3 [I - 6 )

+ $ e 3

-q1- el3

+ 5(1 - e ) 4 1

o < e < 0.5 0.5 < e < 1

(26) (27)

Expressed in terms of the sticking probability, the Langmuir expression is simply s = s,f(e)

(28)

and we note that very few adsorption systems follow Langmuir adsorption kinetics (Sect. 3.1.2). The most serious deficiency of this model is its failure to describe the initial virtual independence of s on 8,observed for a large number of systems, particularly at low temperatures. Taylor and Langmuir [2] recognised the importance of introducing a second weakly held adsorbed layer, with a short lifetime, in order to account for their observation that the sticking probability for Cs on W is close to unity, even at coverages approaching saturation in the chemisorbed layer. Kinetic models including a mobile precursor state were derived by Morrison and Roberts [423], Becker and Hartman [424] Ehrlich [425] and Kisliuk [426]. Two distinct approaches have been used to model precursor state kinetics. (1) A successive site statistical model, introduced by Kisliuk [426] for adsorption and adapted by King [298] for desorption. (2) The chemical reaction kinetics approach, involving rate coefficients and the stationary state approximation, followed by Becker and Hartman [ 4241 , Ehrlich [425] and recently developed by Gorte and Schmidt [297] and Cassuto and King [421]. It has recently been shown by Schonhammer [427] and Cassuto and King [421] that the two approaches produce the same kinetic expressions. Variants of these models have References p p . 163-1 79

66

been produced by Kohrt and Gomer [214], Kieffer and Bootsma [428], Lopez-Sancho and de Segovia [373] and King and co-workers [45-481, the latter introducing the influence of lateral interactions between adsorbed species through order-disorder theory. The most general expressions are those recently derived by Cassuto and King [421] for both adsorption and desorption kinetics, incorporating the influence of lateral interactions in the adlayer and the precursor state. We follow their derivation here. Representing chemisorbed species as A,, the intrinsic precursor as A*(A;), the extrinsic precursor as A’(A;) and the gas phase species as A,(A2,) we have (a) for nondissociative adsorption adsorption

desorption

(b) for dissociative adsorption adsorption

67

desorption

Here, k,, kd and k , are the rate coefficients for adsorption, desorption, and migration from the intrinsic precursor state, k k and k i are the rate coefficients for migration and desorption from the extrinsic precursor state, kD is the rate coefficient for transfer from the chemisorbed state to the intrinsic precursor state, and a* and a' are the trapping probabilities for molecules incident at intrinsic and extrinsic precursor sites, respectively. Direct transfer from gas phase to chemisorbed state or vice versa is included through the probability, s, for adsorption and the rate coefficient, k c , for desorption [427]. In order to generalise the rate expressions, we now introduce a group of terms F(8) which are only functions of the surface coverage 8. For a particular case, such as nondissociative adsorption, these terms may be evaluated and inserted into the appropriate rate expression. F, is the occupation probability that a site (or sites) exists in a configuration which can lead to desorption (= 8 for nondissociative adsorption). F, is the probability that an intrinsic precursor, in hopping, moves to a site configuration where an extrinsic precursor state can exist (= 8 for nondissociative adsorption). FL is the probability that an extrinsic precursor, in hopping, moves to a site configuration where an intrinsic precursor state may exist [ = (1- 8) for non-dissociative adsorption] . Fa is the probability that an intrinsic precursor state is at a site configuration where chemisorption can occur (= 1 for non-dissociative adsorption). F* is the probability that a collision takes place at an intrinsic precursor site [= (1- 8 ) for nondissociative adsorption]. F' is the probability that a collision takes place at an extrinsic precursor site (= 8 for nondissociative adsorption). It follows from these definitions that F, + F d = 1 ; F * +F' = 1;FL = F*;F,

= F'

(29)

These last two relations arise from the fact that the probability of finding References p p . 163-1 79

68

an intrinsic or extrinsic precursor position does not depend on the origin of the atom or molecule (gas phase or surface). Applying the stationary state approximation for both desorption and adsorption to the kinetic schemes (Rl-R4) with the assumption that A* and A’ (or A: and A;) coverages are negligibly small yields

+ kaFa + k,F,)[A* or A*,] + k k F ’ [ A ’ o r A;] [A* or A*,] - (k& + k k F k ) [ A ’ o r A;] = 0

k D F D- (kd k,F,

= 0

(30)

and (kd (k&

+ kaFa + k,F,)[A* + k k F k ) [ A ‘ o r A;]

or A*,] - k k F ’ [ A ’ o r A;] = a * Z F * -k,F, [A* or A*,] = a’ZF’

(31) where Z [ = p/(21rrnkT)1’2] is the collision rate per unit area. Since the rate of desorption, rd, and the rate of adsorption, ra, can be written as rd = kd[A* or A*,]

+ k&[A’or A;] + k,FD

(32)

and ra = kaFa[A* or A*,]

+ scZF*Fa

(33)

the general rate expressions are obtained as

ra =

g k a F a [a*F*

haFa

+ kd

+ {a’F’Fkk k /(k’ + k k F k ) } ] + scgF*Fa {k, F, k&/(hh 4-k k FL )}

Using the relationships (29), these are further simplified to

and

Under equilibrium conditions, the kinetic schemes become k,

A,, ( o r 2 A , )

A’ ( o r

- kd

*.

.. (or ,

(34) (35)

69

Application of detailed balancing gives = kkF;[A’orA’,]

k,[A*orA*,,]F,

kd [A* or A*,] = a*gF*

kh [A’or A;] = a’gF’ from which, together with relations (29), we have

The general form of the equilibrium isotherm is readily obtained from the equalities FDkD= kaFa[A* or A;], kd [A* or A*,] = a*gF* as

In the simplest cases, where there is no interaction between adsorbed species, eqn. (40) reduces to the Langmuir isotherm form, for dissociative or non-dissociative adsorption; with pairwise interactions between adsorbed species, eqn. (40) is equivalent to the Fowler-Guggenheim [320] isotherm. From the equilibrium kinetic scheme, it can also readily be shown that

The relationship (39) allows us to further simplify the general rate expression, far from equilibrium, as

and 0, =

+ +

+ +

kd}] gkaFaF*a*[1 {kmFm/((l- F,)a*k,/a’) t s,gF*Fa (43) kaFa kd [ 1 {k, F,/((l - F,)a*km/a’> kd}]

+

Thus, apart from the term a‘,the rate expressions can be reduced to a form which only contains rate coefficients referred to the intrinsic precursor state. If transfer from the gas phase to the precursor state is non-activated the energy diagram in Fig. 2 is applicable. Writing rate coefficients k as the product v exp (- E/RT) and noting from Fig. 2 that Em-Ed

= EL-8;

we have, from eq. (39) References p p . 163-1 79

(44)

70

Thus, only if the frequency factor ratio was unity would we have a* = a'. In the models of Kisliuk [426] and King and Wells [46], this assumption is implicitly made (Kisliuk originally assuming that both trapping probabilities were unity). The physical significance of this assumption is that the trapping probability, and hence energy transfer, are the same for molecules incident on bare sites and on sites occupied by chemisorbed species. While this may appear to be unrealistic on the basis of a phonon energy transfer mechanism, since energy transfer would be substantially increased if the mass ratio of the incident particle to adsorbent atom were increased, there are several examples in the literature (see Table 1) where the sticking probability is less than unity but accurately independent of coverage over a wide range of coverage at low substrate temperatures; this result requires that a* = a'. (On the other hand, there are several recent examples in the literature where s has been found to increase with coverage at sufficiently low coverages and temperatures [ 3221 .) Making the approximation and also assuming, following Kisliuk [426] and King [46, 2981, that ac = 0 (i.e. ignoring direct transfer), eqn. (39) is transformed to

Further simplications can be achieved by using normalised rate coefficients, which are similar to the probability terms used in Kisliuk's statistical method of kinetic analysis, viz.

P, = km/Zk Pa = k$Zk

PL = k k / Z k '

(47)

+ +

+

where ZP = ZP' = 1, Zk = k, kd k, and Zk' = k & k k . The advantage of such a formulation is that the rates of adsorption and desorption can now be expressed in terms of only one of the precursor states, since we can write

P& = Pm/(pm+ Pd) and Pi = The result is rd

=

pd/(pd

k D ( 1 -pa)PdFd papmFa(1 - Fm) +pd [1 - p a ( l

+ P,) - Fa)]

(48)

(49)

71

These expressions give an exact identity with rate laws derived using the statistical approach by Kisliuk [426] and King [46, 2981. Lateral pairwise interactions between nearest neightbour (n.n.) occupied sites can be treated in the quasichemical approximation [45, 46, 641 by considering the equilibrium

2OA+00+AA (51) for which the energy change is o,the pairwise interaction energy. OA, 00 and AA represent n.n. pairs of sites, where 0 is an empty site and A is a filled site. Using material and energy balance at equilibrium in the overlayer for a fractional surface coverage, 0 , the approximation yields

eAA

=

eoo = eOA =

e - 2e (1 - e)/(o+ 1) i-e-2e(i-e)/(~+i) 4 e ( i - e ) / ( ~+ 1)

(52)

where D = [ 1 - 40(1 - O ) ( l - exp w/RT)] 1’2. for nondissociative adsorption is

The desorption energy

and for dissociative adsorption

where z is the number of n.n.6 to a given site in the surface mesh and

E: is the desorption energy when the n.n. occupancy approaches zero. ( a )Non-dissociative adsorption with single site occupancy If the gaseous species is adsorbed intact and occupies only a single site at the surface and if the adsorption rate into an empty site is independent of n.n. occupancy, the only term affected by lateral interactions is the desorption energy Ed [eqn. (53)] in the rate coefficient kD. We have Fa = 1 ; F * = l--B;andF, = which yields, from eqns. (13) and (14)

and

References p p . 163-1 79

FD

= 0,

(55)

72

(b ) D issoc ia tive adso rp t io n

For dissociative adsorption t o occur, it is assumed that a n.n. empty pair site configuration (00)is required, while the prerequisite for desorption is assumed to be a filled n.n. pair site configuration (AA). Although not explicitly stated, two different models have been used in the literature to describe the nature of the intrinsic precursor state; not surprisingly, these model assumptions lead to different rate expressions. Kisliuk [ 4261 and Gorte and Schmidt [297 ] assume that the intrinsic precursor occupies a single empty chemisorption site, while King and Wells [46] assume that is occupies an n.n. pair of empty chemisorption sites. (i) If the intrinsic precursor occupies a single site, the functions F, defined in Sect. 4, become FD =

I

eAA

I

F* = 1 - 8

(We note that F, is the probability than an intrinsic precursor, on a single empty site, is in a n.n. configuration to a second empty site.) Thus, from eqns. (49), (50) and (52), we have

(ii) If the intrinsic precursor occupies a pair of n.n. empty sites, the coverage-dependent functions, F, become

FD = ~ A Fa = 1 F,

=

F* =

A

i-eoo

coo-

(63)

73

These expressions transform the desorption and adsorption rate expressions to

and

In this model, the rates of adsorption, migration and desorption from the intrinsic precursor are ka[A*,], h , [A*,],and k,[A*,], respectively, and it follows that the normalised rate coefficients defined earlier are identical with the probabilities f,, f, and f d defined by King and Wells [46] and equation (65) is readily transformed into their rate expression for dissociative adsorption derived by the statistical method. The influence of the precursor and of lateral interactions between adatoms is demonstrated for dissociative adsorption on a symmetric surface in the theoretical curves of sticking probability as a function of fractional coverage shown in Fig. 25, taken from King and Wells [46]. Increasing the influence of the precursor state leads to a curve which tends to be initially coverage-invariant, while increasing the ratio of the repulsive n.n. interaction energy w t o T tends t o result in a termination of adsorption at half a monlayer. The latter results from the tendency of the overlayer to form a double-spaced structure at half a monolayer coverage; if completely ordered, i.e. if w / T is large, at 0 = 0.5 there would be no n.n. empty pair sites in the surface where dissociative adsorption could occur. King and co-workers [45-48] have examined the applicability of these kinetic models t o the adsorption of nitrogen on tungsten single crystal planes, The nitrogen-tungsten system is a good example of crystallographic anisotropy in adsorption kinetics at T = 300 K. Thus, so on the {110} plane is < and on the (320) plane is -0.74. Adams and Germer [430] proposed a simple model t o account for this. They suggested that the N adatom can only be adsorbed into the fourfold symmetric, fivefold coordinate site characteristic of the (100) plane, with occupation of alternate sites. Thus, the reactivity of any W crystal plane for N adsorption is expected to be proportional t o the number of (100) sites on that plane. All planes on the crystallographic triangle defined by the zones containing the {Ill}, (110) and {211} planes, being devoid of {loo} sites, should, therefore, be inactive. This model was particularly consistent with LEED and work function data for the planes on the (001) zone studied. Singh-Boparai et al. [47] studied the adsorption kinetics and surface coverages for N, on a variety of tungsten single-crystal planes. A References p p . 163-1 79

74

I

1

1

I

I

I

1.0.

I

1

I

-

-

0.8

4 o.60.40.2I

Fig. 25. Upper curves: computed sticking probability profiles for a fixed degree of short-range order in the overlayer (B = 0.99) and various values of the precursor state parameter K. Lower curves: computed sticking probability profiles for a fived value of K (= 0.05) and variable B, illustrating the effect of short-range order in the chemisorbed overlayer. (From King and Wells [ 461.)

collection of sticking probability data is shown in Fig. 26 and a comparison with the number density of [loo] sites on each plane indicated that the prediction of the Adams and Germer model is not fully confirmed. The {110}, {111} and {411} planes are clearly less reactive than the remaining planes studied; although the (411j plane contains [loo] sites, they are not in nearest-neighbour relationship to each other and the (110) and {Ill} are devoid of [loo]sites. Thus, the {loo},(310)and

75

\

\ \ \ I

8 Surface coverage/atom c m - '

Fig. 26. The variation of s with N for various tungsten single crystal planes under N 2 exposure. T,= Tg= 300 K. (From ref. 47.)

(320) planes, all containing [ 1001 sites in nearest-neighbour positions, are the most reactive, but the order of reactivity and saturation coverages for these planes is not as predicted. It was concluded [ 471 that the dissociation of the nitrogen molecule into the state occurs only at the [ l o o ] site pairs, but the chemisorbed atoms thus formed may subsequently migrate out on t o the [110] sites and terraces, thus populating other areas of the crystal with 0 adatoms. King and Wells [46] developed the Kisliuk model for dissociative adsorption t o take account of ordering in the chemisorbed overlayer resulting from the existence of lateral interactions between chemisorbed species in nearest-neighbour positions. s could then be expressed as

the terms being defined above. To extend the model to stepped planes, Singh-Boparai et al. [47] introduced a parameter {, which is the probability that a physisorbed species is in a trap in the vicinity of a [ l o o ] site pair, i.e. the ratio of [ 1001 sites t o total sites on the surface. This yields the general expression appropriate to N, adsorption on all W planes [47] References p p . 163-1 79

76

(

s = a 1+-

l+k--

(ce:o

I)]-’

Best fit parameters for the planes studied were obtained: with these parameters, eqn. (67) is a quantitative description of all the data over a wide range of substrate temperatures. The only significant variation is in the experimentally determined parameter a. It was concluded that the trapping probability is highest for planes with the highest surface density of W atoms. This parameter thus increases across the (100) zone, from [ 1001 to [ 1101, while the [ 1001 nearest-neighbour site pair density decreases in the same direction. These two factors together produce a maximum in so across the (001) zone. King and co-workers derived their models on the assumption that a is independent of substrate temperature but sensitive t o gas temperature. With this assumption, the fit t o the data for the {loo} [46] and stepped planes [47] was found to be good. These kinetic formulations have recently been brought into question by two independent sets of investigations. (i) In a recent elegant study of the velocity distribution of nitrogen back-scattered from a polycrystalline tungsten foil, Auerbach and co-workers [ 4031 were able t o distinguish direct inelastic scattering from trapping-desorption scattering and concluded that their data were consistent with a significant decrease in the precursor state trapping probability with increasing substrate temperature and inconsistent with both previous kinetic formulations [46, 3541. (ii) Top layer tungsten atoms on W(100) have been shown t o be unstable t o arbitrary lateral displacements, the displacement vector being determined by the surface condition [ 83, 841 . During nitrogen adsorption, for example, contracted domains are formed at a fraction coverage 0 = 0.4, with surface W atoms returned t o their bulk lattice positions; at higher coverages, Winters et al. [431] have produced evidence that N adatoms are incorporated into a reconstructed surface layer. Thus, the “checkerboard” surface model on which the order-disorder model of King and Wells [ 461 was based can no longer be considered to give an accurate description of the site availability on the surface. Alnot and King [432] have returned to the measurement of sticking probabilities. for N, on W {loo} in order to extend the earlier data base t o higher temperatures, as a test of the kinetic models in the light of the recent work described above, and to check for anomalous effects at 0 = 0.4 which could be attritmhd to the presence of contracted domains. In particular, whereas the model of King and Wells [46] predicts a high temperature limiting value of s at low surface coverages, the analysis of Auerbach and co-workers [403] predicts that s should approach zero at high temperatures. The molecular beam technique was extended in this study to the measurement of “reactive” sticking coefficients, s,, by In observing the rate of isotopic mixing from mixtures of 30N2and 28N2.

77

contrast t o the measurements of s, these measurements were made at high temperatures where the steady state coverage in the adsorbate is low, thus allowing an extension of the range of substrate temperatures which can be investigated. (In principle the method is similar t o that used for polycrystalline wires by Yates and Madey [433].) In the beam experiment, it is readily shown that for an equimolar mixture of iostopes

where PZ9 and P,, are the steady state pressures of 29N2and ’ON, in the adsorption cell during beaming on the crystal. As with the measurement of s, the absolute accuracy of this method is simply determined by the accuracy in measuring a pressure ratio; no calibration factors are involved. From the work by Alnot and King, it is apparent that there is no kinetic effect associated with the phenomena of surface reconstruction. It seems likely that dissociative chemisorption proceeds in the channels between islands, and N atoms then “sew” the islands together, finally removing the surface layer contraction at 8 = 0.5. Thus, despite contracted domains, the number of adsorption sites effectively remains constant as adsorption proceeds, which explains the success of the “checkerboard” model. The results are illustrated in Figs. 27 and 28. The authors then apply their results t o the King and Wells model [46] with considerable success. The data provide support for the suggestion that CY is virtually independent of T,. The Alnot and King study contradicts some of the conclusions reached by Auerbach and co-workers et al. [403] from their time-of-flight study of N, scattered from polycrystalline W. The following facts are conclusively demonstrated in the Auerbach investigation. (i) Unreacted N, is scattered into only two channels, a direct channel which is entirely inelastic and a trapping4esorption channel: no elastic peak is observed. (ii) At high beam and surface temperatures (> l o o O K ) , the average accommodation coefficient is 0.46, in agreement with an earlier study of Cassuto et al. [429]. The distinction between the direct inelastic and trapping-desorption channels is most clearly demonstrated for a beam temperature of 3400 K at T , = 400 K, where a lobular distribution is found for the former and a cosine distribution for the latter. However, Alnot and King [432] note several unresolved contradictions in the analysis of their data; in particular, their assumption that the trapping-desorption velocity distribution is Boltzmann cannot be justified. It can be seen that, for the N2-W system, the “trapping-precursor” model for adsorption is all-important. It should not be expected that this is so all the time. This is clearly shown by the work of Hayward and Walters [ 4011 who measured the angular distribution of hydrogen scattered from W ( 1 0 0 ) at 300K as a function of the amount of hydrogen References p p . 163-1 79

78

I"--7 10'' N Coverage /atoms cm-'

Fig. 27. The variation of s with surface coverage, N , when N2 is adsorned on W{lOO), for various substrate temperatures, T,. A comparison with the work of King and Wells [46] isalsoincluded.., T s = 3 0 0 K [ 4 6 ] ; . , T s = 4 3 3 K [ 4 6 ] ; A , T s = 3 0 0 K [ 4 3 2 ] ; 0 , T,= 320K [432];0, T s = 4 0 5 K [432].(FromAlnotandKing [432].)

chemisorbed on the surface. Of the molecules incident on the clean surface, 3% were elastically reflected, 16%were inelastically scattered and the remainder were chemisorbed. A polar plot of H, scattered from the clean surface is shown in Fig. 29, which illustrates the clear distinction between lobular and cosine law scattering. The elastic component was found to be a sensitive function of the hydrogen coverage, rising to 17% in the specular beam alone (i.e. excluding diffraction peaks) a t saturation. Two states of hydrogen on W{lOO}, denoted 0, and PI, have long been identified, the former being observed alone at low coverages. It was found that the elastic component only rose above 3% at coverages where the peak was first observed in the desorption spectrum, i.e. when portions of the overlayer are saturated with adatoms. The implication is that, either the trapping probability, 01, is smaller (- 0.82) on hydrogen-saturated areas of the surface than on clean areas (-0.97), or a proportion of the incident molecules which would have been elastically reflected in the absence of the chemisorbed potential energy will pass directly into the chemisorbed state on the clean or pzcovered surface. The latter is probably the correct explanation, particularly as the physisorption well depth for H, is small, so that one would anticipate

79

0

4 GO

800

1200

1600

C r y s t a l temperature, T, / K

Fig. 28. Variation of S O with the crystal temperature T, for nitrogen on W{lOO]. The beam temperature, T B , is also varied. 0, T B= 191 K;n,o, T B= 400 K;*, T B= 533 K; o,isotopic experiments; -, theory. (From Alnot and King [432 I.)

a very small value for a. In general, it is concluded that where stickmg probabilities greater than about 0.05 are observed for hydrogen adsorption, both so > a and so > ac and the process is dominated by a direct passage through the physisorption well, without trapping. [It may be argued that, even at saturation coverage, the proportion of molecules diffusely scattered in the experiment of Hayward and Walters [401] (-0.82) is much higher than anticipated for H,; however, it is well known that adatoms cause an appreciable increase in the accommodation coefficient.] The usual precursor state mechanisms are therefore not a good basis for the description of hydrogen adsorption on metals. The systems discussed above are, in many ways, “ideal” in that adsorption is very site-specific or limited to the surface layer. Many systems are known t o absorb as well as adsorb, This effect is sometimes reflected in sticking probability versus coverage profiles. These may show an increase in s because of a sudden freeing of surface sites due t o the absorption process. One example is O2 on Al(111) [434]. However, the adsorbed species may form at all coverages and the s versus N profiles look like those of a typical mobile precursor-trapping model. Fromm [435] has proposed a model t o fit this absorption-adsorption mechanism. His References p p . 163-1 79

80

O0

I

\

'

1

Surface normal

\

30"

/

/ I 0.05

0.00

u.u3 Molecular flux,

U.IU

u.13

1 dN

N dn

Fig. 22. A polar plot of hydrogen scattered from clean W{lOO} at an incidence angle of 34 . (From ref. 401.)

model is based on the assumption that, if there are large gradients of the chemical potential from the surface t o the bulk, then gas atoms should be able t o migrate freely until a certain point below the surface. This produces an s versus N profile which shows an initial plateau. After this, there will be an exponential decay of s correlated with an activation energy for the initial chemisorption; the position has been reached where the energy released by chemisorption can no longer compensate for the energy needed t o push atoms into relatively high-energy interstial sites. Equating free energy expressions for the processes, Fromm obtained

where A G$(eff) is the effective Gibbs activation energy for adsorption from the gas phase t o the chemisorbed state and AG: is the activation energy for diffusion. 8, as usual, represents relative coverage, and Oi the is equal t o AG$efo (the coverage value where s begins to fall. If AG!!(,,,,

81

effective activation energy between different sites in the metal), then it was found that S _ -

I."--

kT

SO

e -8,

8, being the coverage at the surface. This model was used t o fit the adsorption of 0, and N, on various metal films [435, 4361 and good agreement is claimed between the experimmental and theoretical work. The authors also note that the length of the plateau is a measure of the ratio of heat of solution to activation energy for diffusion. Thus, for a given metal, the length of plateaus for 0 and N adsorption/adsorption should be rationally related and the ratio of the two plateau lengths calculable. However, the model is not as broadly applicable as implied by Fromm, if it is applicable at all. In adsorption on metal films, the porous structure and the rough outer surface of the film play an important role in determining the shape of the sticking probability versus coverage profile [175]. In many instances, there is no possibility of absorption into the bulk, such as molecular N, adsorption on Ni films [ 5861, but the profiles still have the shape discussed by Fromm. For example, King and Tompkins [ 5841, in treating data for N, adsorption on molybdenum and titanium films, distinguish clearly between adsorption on the outer film surface, with a sticking pmbability s,, and adsorption by gaseous diffusion into the film pores, where surface adsorption occurs. The sticking probability on the outer surface is related to the value for an ideally smooth surface, sa, by the expression s,

=

Sa

(1- (1- S a ) > { l - (1/R)J where R is the outer surface roughness factor, The measured sticking probability, s, is a sum of s,, s, and a term relating t o adsorption into the porous microstructure, sp. For example, it is estimated that, for N, adsorption on molybdenum films at 3 0 0 K , the initial measured sticking probability of 0.75 is composed of s, = 0.70, sp = 0.15 (averaged over all collisions) and, with R = 3, s, = 0.44. For N, on titanium films, the corresponding values are s, = 0.46, sa = 0.22 a t 300 K. In a comparison of N, adsorption on nickel and palladium films, King [586] found that the zero coverage sticking probabilities were dependent on sintering temperature and arrived at smooth surface values of 0.56 and 0.67, respectively. Moreover, with nickel, the characteristic distinction between outer surface and porous structure adsorption was clearly noted in the sticking probability profiles, with the extent of porous structure adsorption decreasing dramatically with increasing sintering temperture. With palladium, however, this distinction could not be made, and this was References p p . 163-1 79

82

Fig. 30. Potential energy wells for activated and interstate conversion in adsorbed layers. (a) Shows that the stable state is reached via an intermediate “virgin state” but passage to it is non-activated. (b) Shows the case where passage from one well to the next is activated.

attributed t o rapid surface diffusion of nitrogen on palladium at 78K, providing an efficient means of adsorption into the pores.

3.2.4 Activated adsorption Here, it is necessary t o distinguish between true activated adsorption

83

(where the PE cross-over point lies above the PE zero, as defined by gas molecule and surface being an infinite distance apart) and inter-stake conversion where a metastable chemisorbed state (the “virgin” state) may be formed a t low temperatures and conversion t o the stable chemisorbed state occurs on warming. The latter may o r may not be true activated adsorption, depending, as illustrated in Fig. 30, on the energies at the two PE cross-over points. Contrary to the view expressed elsewhere [ 4371 , activated adsorption can be distinguished by the dependence of sticking probability on surface or gas temperature, depending on the mechanistic circumstances. Both types of information are useful. If adsorption is trapping-dominated, the T, dependence is given by the King and Wells [46] model where eXp ( - E d / R T ) / N , eXp (-Ea/RT)]-’

(70) and so increases with T, for activated adsorption (E, > E d ) . However, if adsorption proceeds by direct transfer through the precursor state, as concluded above for the case of H, on W{lOO}, a very small T, dependence might be anticipated. For trapping-dominated activated adsorption, higher gas temperatures lead t o lower trapping probabilities and hence lower so; for direct transfer, so will increase with T,. Early studies of H, on polycrystalline Cu [438] show that it is an activated process with an activation energy of roughly 20-40 kJ mole-’. Balooch et al. [358] have reported a molecular beam study of hydrogen adsorption on Cu{lOO), (110) and (310) in which the sticking probability is inferred from the reaction rate between H, and atomic D t o form HD at high crystal temperatures. The data show that the HD production rate increases significantly as the energy of the molecules increases and as the angle of incidence decreases towards the surface normal; it is concluded that there are substantial activation energies (12-20 kJ mole-’ ) for adsorption, the barriers depending on crystallographic orientation and acting essentially perpendicular t o the surfaces. Interestingly though, the H, sticking probability was found t o approach a non-unity limiting value (0.14 for Cu(llO), 0.10 for Cu{lOO) and (310)) at high impinging beam energy. A systematic study of the T, dependence for this system would be useful, but this work has established the H,/Cu system as a case of true activated adsorption which proceeds by direct transfer through the precursor state. If proving whether an adsorption process is activated or non-activated is difficult having only the temperature dependence of s, these difficulties have now been removed. There has been a recurrent interest in the angular dependence of the flux and energy distribution of molecules desorbing or scattered from surfaces. Methods for measuring the 8 dependence (8 is the polar angle measured to the surface normal) of the desorption flux, Nd(d), the average kinetic energy and the normalised speed ratio have been established. These methods are usually molecular beam scattering So

a[l

+vd

References p p . 163-1 79

84

experiments [358, 439, 4401 in which Nd(@ is measured at steady state, or permeation experiments [447-4491 in which Nd(@is again measured at steady state after permeation of the gas through the crystal. A method which, unlike the methods above, measures only truly desorbing particles has been developed [ 4501 . The measurements of N d ( 6 ) performed for heavier gases (N,, 0,) show a distribution which normally adheres t o a cosine law [451]. However, in permeation experiments with hydrogen [ 447-4491, strong deviations have been noted. The results are usually expressed as cosxO, where x is > 1. This enhancement of desorption flux along the surface normal is due to the presence of an activation energy for the adsorption process. Recently, Cosser [ 4521 has developed a theoretical model for the desorption from a surface through a Lennard-Jones potential field for both the activated and non-activated cases. The work by Cosser et al. [450] has clearly shown that the low sticking probability of N, on a perfect W {110} plane is an activated process and that chemisorption occurs at step and defect sites followed by diffusion and occupation of [ 1101 sites. They arrive at a value of 17.4 kJ mole-' for the activation energy for adsorption on the (110) terraces. If the s versus N profile for a mobile precursor is typically initially independent of coverage, then a typical profile for the case of activated adsorption follows an exponential decay and s can be written [273, 4531

s

= uf(0) exp

(-E/RT)

(71) where u is the condensation coefficient, E is the activation energy and f (0) as usual introduces the site-dependent coverage term. Activated adsorption processes are often investigated using transition state theory and rate laws and examples of this are common in the literature [4544561. Analyses of this kind lead to expressions which are qualitatively useful but which often contain a number of terms which are difficult to evaluate. 4. Desorption kinetics In this section, the details of thermal desorption from surfaces will be considered. The rate of the process can be represented in an ideal form by the Polanyi-Wigner equation

dN

-- =

dt

vNmexp( - E d / R T )

(72)

where N is the surface coverage, v is the frequency factor and m is the order of the desorption process. The usual routine of desorption experiments is t o adsorb gas on to the sample at a temperature a t which the

85 I 100

1

I

I

I

I

I

I

I SE/

v

I 5-’

48

96

14 4

-

n 200

I

I

I

220

240

260

I

1

280 300 t /m5

I

I

I

320

340

360

Fig. 31. The effect of pumping speed ( S E / V on ) the shape of a desorption trace. (After Ehrlich [ 51 ).

desorption rate is low and then ramp the temperature while monitoring the gas phase for desorbing products. Clearly, from eqn. (72), in such an experiment there are two time-dependent variables affecting the desorption rate; as the exponent increases, the surface coverage decreases and the resulting convolution gives a peaked form to the desorption trace (see Fig. 31). Section 4.1.1 describes the desorption characteristics of systems obeying eqn. (72). I t must be stressed that eqn. (72) represents an ideal desorption process, where both v and Ed are coverage-independent parameters. Unfortunately, very few systems behave in this ideal fashion: desorption is the reverse process of adsorption and, as has been described above for adsorption, several properties of the adlayer severely affect the kinetics of the basic desorption process. Thus, in the following sections, the effects on desorption kinetics of surface inhomogeneity, changes in desorption mechanism, precursor states and lateral interactions between adspecies, will be considered. The effects which these parameters have are considerable References p p . 163-1 79

86

and may result in the broadening or narrowing of the widths of desorption peaks, unexpected shifts in the peak temperature for different coverages and may even produce multiple peaks in the desorption spectrum. Finally in this section, a comprehensive compilation of desorption data which has appeared in the surface science literature over the last 10 years or so, is presented in tabular form; sample spectra for a variety of adsorption systems are also given as an overview of the desorption process. By the way of an introduction to desorption kinetics, a fuller, more theoretical discussion of the type of rate equation for thermal desorption presented above is given in the following subsection. 4.1 THEORY AND ANALYSIS OF DESORPTION SPECTRA

4 . 1 . 1 Theoretical aspects of thermal desorption

Theory has concentrated on trying to predict the more difficult parameter in the basic kinetic equation, i.e. the pre-exponential factor. Two major approaches have been used. ( i ) Collision theory. The older of the two approaches attempts to describe the collisional event between two reacting particles. The likelihood of collision is determined by a parameter known as the reaction cross-section. This quantity is invariant at all but the lowest collision energies where the time of a collision may be long enough t o allow some electronic interaction between particles. There will also be a threshold energy below which a reaction will not occur. Analytically, this is modelled by assuming the reactants are hard spheres, but an extra factor (a steric factor) has often to be introduced to account for pre-exponentials lower than the theories can predict; i.e. the direction of collision is important. This method has now largely been replaced, its major weaknesses being that it cannot predict values for the steric factor and that internal changes (other than rotational) cannot be included in the model. (ii) Transition state theory. This theory is based on the model of an activated complex being the intermediate state between reactants and products. For surface processes the following can be envisaged. PB Molecular adsorption

TB

TB

PB

i B

TB

AB

AB AE

M M M M M M

M M M M M M

M M 4!t

M M M

Molecular desorpt ion Transition sites

Dissociotive adsorption

A

B

B A

B

I I I I I I

A---B

M M M M M M

M M M M M M

A

A---0

A---E

I I I I I I

AB

AB

+B M

M

h

M M

Recombination

M

87

The simple form of the Polanyi-Wigner equation is based on the assumption that any particle possessing the requisite activation energy desorbs during the period of a single vibration. If the recombination mechanism is written as

A(a) + B(a)

2

(AB)C) -P A%

where the rate-determining step in desorption is the formation of the activated complex and, furthermore, assuming that this is in equilibrium with the adsorbed species, then

where the f values are molecular partition functions. Separating out zero point energy differences

If the total number of sites available for adsorption is N,, then [A] [B] = N,f (0) where f(0) is some function involving coverage. Thus

assuming that R d = K#[AB#], that is that the transmission coefficient to gas phase products from the activated state is unity. For the molecular desorption case

and so, in comparison with the Polanyi-Wigner equation pre-exponential

kT q# v(l) = - h qAB (77) kT q * 1 u ( z ) = - ___ h qAqBNs Such relationships indicate two types of behaviour. First, considering the non-dissociative adsorption: (a) if the activated complex and adsorbed species have the same degrees of freedom, then v kT/h, i.e. around 6 x 10l2 s-' at 300 K; (b) if the activated complex is less strongly bound than the adsorbate, then it may possess more degrees of freedom and so q#/qAB > 1. If the activated complex gains translational freedom, i.e. becomes delocalised, then this ratio could be as high as lo4 and so pre-exponentials as high as l O I 7 s-l could be expected.

i

-

-

References p p . 163-1 79

88

For the dissociatively adsorbed case, several possibilities exist: (a) if all particles are localised, then the partition function ratio is close to unity and pre-exponentials would be approximately kT/N, h or 6 x 1 O - j cm' s-' at 300 K; (b) if the activated complex is mobile, then pre-exponentials may be as high as 60; (c) if the adsorbates are mobile but the transition state is less so (due t o its strained configuration), then qP/qAqB< 1. For both cases, we note that lateral interactions in the adlayer will affect the mobility of the adsorbed species and so, particularly at high coverages, anomalous pre-exponential values may be anticipated. Also, and this factor would transmission coefficients may be as low as need to be included in eqn. (75), resulting in desorption at a slower rate than anticipated from the activation energy value. Alternatively, the rate equation can be written as [338, 3391

Rd = N,f(0)

kT - exp (- AG4/RT) h

(78)

where AG' is the Gibbs free energy change for the process, given by Ed - TA@. In this formulation, for first-order desorption, we now have

v

=

kT -exp h

(A@/R)

(79)

and the entropy term is implicit in the pre-exponential factor. The upper limit of AS' would correspond with desorption from a completely localised layer to a completely non-localised transition state and this is the translational entropy of a 2D gas. As;#,,, for such a system is given by Kemball [340] as

As#,,,

= Rln(AHTA)

+ 275

-

(80)

where A is the surface area per adsorbate, giving a figure of 130 J K-' mole-' at room temperature. Many desorption experiments have now shown v values which are well away from the kT/h value. The desorption of CO from Ru(00l) [341] gave v 1 O l s s-' . This prompted Ibach et al. [342] to investigate the CO/Ni{ 111) system and they pointed out that a value of l O I 3 s - l can only be expected if the adsorbate is mobile and has free rotation or has states of low excitation energy. The model they used to elucidate the kinetics was a modification of an earlier theoretical treatment derived by Landau and Lifshitz [343]. It is to be expected that the chemical potentials of gas phase and adsorbed species will be equal (pa = p g ) . From Fermi statistics, the probability of site occupation on the surface is

-

+

W = a0 = {exp [(ea - pa)/kT] I}-' where E , is the energy of the site. Thus

89

pa = E ,

+ kTln [ a 0 / ( 1- a d ) ]

and from Landau and Lifshitz pg = E ,

+ k T { ( h 2 / 2 n m k T ) 3 / 2 ( R T f rInf vP)}

(82)

where P is pressure, f, and f, are the complete rotational and vibrational partition functions and E , is the ground-state energy of the gas phase. For the atomic case, f, = fv = 1and, equating pa with p, P =

ae

(1- a0)[FzT(2nmkT)3/2h-3 1 exP [- (Eg

- Ea)/kTl

where E , - E , is simply the heat of adsorption; P/(2nmhT)'/2is the bombardment rate and if S(0, T ) are the fraction adsorbed into chemisorbed states, then at equilibrium

Rd = R , = S ( d , T ) P ( 2 ~ r n k T ) - ' / ~

(83)

Thus Rd

= (Ye(l-&!0)-'

kT 2nmkT h h2

s ( 6 , T)exP (-

E d / RT )

(84)

and so the pre-exponential factor is given by kT 2nmkT S(0, T ) h h2 Ns Thus, since the sticking probability term is present, precursor state kinetics can be introduced into the desorption. Furthermore, high values of v could be predicted and it is both coverage and temperature dependent. If f, is not unity, as for the molecular adsorption case, then provided hW > k T fr = (U/h2)kT

v =

a(1-aq-1 -

~

~

where I is the reduced mass and so

Using isothermal desorption of CO from Ni{ 111)Ibach et al. [ 3 4 2 ] found that v = l O I 7 s-l, and eqn. (86) indicated a value of 6 x 10l6s-' at low coverages, in good agreement with the experiment. Bauer et al. [ 3 4 4 ] have used a similar formalism, but included a transmission coefficient. Petermann [ 3451 purported to show that the temperature dependence of the transmission coefficient for the system H,/Ni{ 100) decreases from 0.1 to 0.02 between 690 and 8 3 0 K . Such temperature dependence is not completely surprising since it relates to the efficiency of energy transfer and Suhl et al. [ 3461 have shown that there is a change in the transmission coefficient near a paramagnetic to ferromagnetic transition. References p p . 163-1 79

90

Such discussions then, allow deviations of a few orders of magnitude s-’, but for even greater from the normal pre-exponential value of l o L 3 deviations, the basic postulate of the above argument appears t o break down, i.e. equilibrium between the reactants and transition state is not achieved. A rate coefficient defining energy transfer from adsorbent to adsorbate must then be introduced and Kramers [ 3471 has treated this case; if this process is rate-limiting, it causes the pre-exponential factor t o be drastically reduced. The model can be modified by the inclusion of an additional rate coefficient t o account for the relaxation of the surface population from a non-equilibrium t o an equilibrium state and so it is to be expected that v is strongly temperature-dependent [ 348, 3491 . The model has been successfully applied t o the desorption of neon from xenon a t very low temperatures for which v was found t o be lo5 s-’ [ 3501 .

-

4.1.2 Integral order desorption with coverage-independent parameters Even in the simplest desorption systems (those for which there is n o influence of lateral interactions or precursor states, for instance), multiple desorption states are generally observed. These can arise from states of different bonding geometry, and hence binding energy, on the surface. Unfortunately, the literature in this field has become littered with a variety of different symbolisms for different states which make comparisons between spectra confusing. The Greek letters y, a and p are commonly used, referring t o states with increasing heat of adsorption in that order; metastable states, which can be converted t o more strongly bound states by heating, are designated as “virgin” or v states. Even within these categories, there can be sub-states and these are differentiated by numerical subscripts, 0, and pz for instance, the higher number indicating the more strongly bound state. Such complications in desorption spectra clearly complicate their analysis; in order t o simulate a spectrum, each separate state thought t o be present has t o be assigned a theoretical shape; these can be then summed and adjusted for a good “fit”. I t has been shown [263] that the fitting peaks is relatively insensitive to the model chosen for the surface layer. It is assumed that the peaks in such spectra reflect exactly the equilibrium population in the adsorbed layer and that on heating there is n o mixing of population between states; this latter phenomenon can be termed “interstate conversion” and will be discussed later. Once again, in the simplest cases, v and Ed are assumed to be constant over the whole coverage and temperature regime. For a number of states, eqn. (72) can be written in a summed form, for i desorption states, as

The aim in the early 1960s was t o produce a method of treating desorption

91

spectra quickly and easily t o produce accurate determinations of these parameters and such work was pioneered by Ehrlich [5] and Redhead [6] whose methods are still commonly used. Redhead presents a number of methods of analysis of desorption peaks and these are given below. ( a ) Peak temperature analysis

This is the simplest and probably the least accurate method of determining the kinetic parameters involved in thermal desorption processes. T o minimise inaccuracies, the desorption peak should be measured using a linear heating ramp given by T = To Pt (88) where T is the temperature a t time t , T o is the starting temperature and 0 the heating rate (aTlat). Furthermore, the system should have a high and constant pumping speed t o avoid peak distortion. ( A means of experimentally evading this restriction has been presented by Kneringer and Netzer [ 2641 who perform two separate experiments, one where desorption is carried o u t with direct line of sight to a mass spectrometer and one without, the difference in pressures then being representative of the undistorted desorption from the front face of the sample.) In general, if the pressure at saturation of the surface is Pes,then L = KFP,,

(89)

The terms having been defined earlier. If re-adsorption during desorption is negligible, then ARd

+L

=

k S P + hV(dP/dt)

(90)

where A is the sample area. If we write P*

>P

-

Peq,then

(dP*/dt) -k P*/Y = aRd

(91) where a = A/kV and Y = V/S; Y is known as the characteristic pumping time of the system. Thus two extremes of experimental conditions can prevail. Firstly, if S is small, i.e. if Y > desorption time, then Rd a dP*ldt, but secondly, if the pumping speed is high, then Rd P* and so the desorption curve of pressure versus time does represent the desorption rate. For a linear heating rate, eqn. (72) can be represented in the form Q:

which can be differentiatied t o give

References p p . 163-1 79

[ p = aT/at; eqn. (SS)]

92

Since at T = T,, (dZN/dt2)= 0, then in general

In particular, for the first-order case

and so clearly, if rn = 1, the peak temperature is independent of initial coverage; for the second-order case, there is an inverse relationship with coverage, i.e. higher coverages give lower peak temperatures. The values of Ed can be calculated from the peak temperature above using eqn. (94) and assuming v = 1013s-l, of the order of molecular vibrational frequencies. However, much better determinations can be made by varying the heating rate, 0, and plotting ln(p/T;) versus 1/T, (for the first-order case) for the various peak temperatures obtained; as the heating rate increases, so also does the peak temperature. Such a method was originally proposed by Booth [ 2651, but it has been pointed out by Lord and Kittelburger [266] that, for accurate determination, P must be varied by a t least an order of magnitude and preferably by two orders of magnitude (though this is often not experimentally possible due t o sensitivity problems a t low heating rates and distortion and peakmerging effects at very high rates as discussed in detail by Chan et al. [ 2671 ). For other orders, the peak temperature is coverage-dependent and so Ed can be determined from the variation in T , with N o from a plot In (N," T,' ) versus 1/T,. ( b ) Line shape analysis

This method is useful for well-defined, non-overlapping peaks. Equation (72) can be integrated in the form

(96) N1

TI

The left-hand side = In (Nl/N,) for rn = 1

(97)

and = (l/N2)- ( l / N l )f o r m = 2

(98)

The right-hand side may be integrated by parts using a substitution u = Ed/RT and the mathematical identity

I U

-m

(- t , dt =

t

U

x H(u) for ( u ) % 1

(99)

93

+

where H(u) = 1 ( l / u ) the derivation yields

+ ( 2 / u Z ) t. . . Using only two terms in this series,

- T:

exP (-Ed/RTi

)I

For the first-order case and utilising eqns. (72), (97) and

(100)

(loo), then

and the desorption rate curve is asymmetric about T,. For the secondorder case

(102) and so, when T/T,

+

1,then

and the peak is symmetrical about T,. Despite its wide use, the Redhead model has inherent difficulties such as the heating rate problems mentioned above. An alternative analysis method has been proposed and is outlined below.

( c ) Line width analysis. Edwards [ 2681 performed an analysis based on desorption peak widths similar to a method developed by Schmidt [269]. The analysis showed that the type of heating program had very little effect on peak width. The first-order case has been treated similarly. At the half-peak maximum values

dN

1dN tP

Under these conditions, a solution of eqn. (72) is given by In [N(t)/No] =

(105)

where u = - &/RT and A ( T o )is a small constant close to zero. Thus N(tp) = No exp {-j(a)) References p p . 163-1 79

94

+

--

where Q = E d / R T p and j ( a )= 1 - ( 2 / a ) ( 6 / a 2 )- ( 2 4 / a 3 ) .For a linear heating rate [ eqn. (SS)] j ( Q ) = exp

(-3) -(YTp (1

2T'

+

6(T')2 ~ u T ' ) ~ )

1 n 2 + ~ - E-d 7 + j ( ~ ) . .. RT Using a power series technique where s = 01 s = so

- ( E / R T )and E =

1/a

+ SE + S 2 E 2 + . . .

(108)

If AW is the peak width, inserting eqn. (108) into eqn. ( 1 0 7 ) and requiring that eqn. ( 1 0 7 ) is true for arbitrary values of E , the solution yields a value for so, sl, s 2 . . . and these values can then be used to write AW TP

-

2.464RTp

il-

1.40RTp Ed

~

3.53RT, *

Ed

Ed

. .)

or, on inversion

This type of analysis is simple to carry out but suffers from the assumption that there is no static overlap and so, in overlapping curves, deconvolution has t o be used. 4.1.3 Systems with variable desorption energies

In many desorption experiments, the desorption activation energy depends on coverage and examples of the kinds of variation observed are shown in Fig. 32. Desorption of the type shown in curves D and E can also be produced by the presence of two states on the surface. There are several sources of desorption energy variation but the two main causes are (i) lateral interactions and (ii) surface inhomogeneity. The latter has been treated for the case of small uniform patches [270] ; if they are of equal surface area dA, then En

=

E:

(111)

- C,

where En is the value of Ed for the nth patch and C is a constant equal to dE,/dA. In terms of the general rate equation, for the first-order case N=Nt

N , (exp E- (Ed0 - c,)/RTI dA

-~Wt/dt = v N=O

=

[ ( v R T / C )exp (- Eg/RT)][exp ( C N , / R T )- 13

(112)

95

N

A

B

N

C

N

D

c

c N

E

N

F

N

G

N

H

N

Fig. 32. Experimentally observed variations of Ed with coverage. The letters below the examples are used in Table 2 to indicate the shape, if investigated, in the experiments.

and since exp ( C N , / R T )% 1,eqn. (112) simplifies to

- cWt/dt = b exp ( C N t / R T )

(113)

where b is the contents of the first bracket on the right-hand side of eqn. (112). Integration yields

(1 - N t )

=

RT -In

C

(t + to/bo)

where bo = R T / C b , and so values for E: and C can be evaluated from a plot of In (dN,/dt) versus reciprocal temperature giving (E: - C N , ) and, if contant temperature measurements are made, then a plot of (1- N , ) versus In ( t t o ) will give a value for C which can be back-substituted t o give E!. This type of treatment has been used to explain the applicability of the Elovich equation to many metal-gas systems [271] ; the equation can be written as

+

dlv, dt

=

a exp ( b N t )

where a and b are constants. Brunauer et al. [270] and Winter [272] have used this relationship for a linear activation energy dependence on coverage and for patches of varying Ed values. Winter showed that only three types of patches are enough to produce the right form for a uniform surface provided that the adsorption is activated [272]. It must be noted that lateral interactions are unlikely to produce variations as large as References p p . 163-1 79

96

those considered in the above treatments, being usually only of the order of 1 0 kJ mole-' or so. The effect of lateral interactions will be dealt with in Seot. 4.1.6, but here the modifications of the basic equations used for fitting variable desorption energies will be introduced. A linear variation of the energy is given by Ed

= E:

+

C8

(115)

where 8 is the fractional surface coverage and c is usually negative. Such variations in desorption energy can yield ambiguous results since Redhead [ 6 ] has shown that a good fit using the simple second-order analysis could be due to a first-order mechanism with a falling E d . Thus, for the firstorder desorption

Rd = v exp [- (E: - cO)/RT]' 8 (116) The slope between 8 and 8 2 (in an order plot of In R d versus In 8 ) can be written as 1 + [ C ( d , - e , ) / R T I ln(82/81) (117) The slope should be 1 for the first-order case, but if a value for c of 20 kJ mole-' is considered, then between 8 = 0.1 and 0.2, the value of the coverage-dependent term in eqn. (117) is about 0.1 and the order is apparently high. The order approaches an apparent value of 2 for a straight line between two points at 8 = 0.1 and 0.5 at a temperature of 1500 K. Variable Ed values and their coverage dependence can be determined graphically by making the usual isothermal order plot for a set of desorption spectra at different initial coverages and then taking rate values from that plot at constant 8 values for the various isotherms. Such treatments have been used extensively by Falconer and Madix [ 2741 . McCabe and Schmidt [275] have used a revised form of the Redhead equation, eqn. (116), including coverage variations of E d , viz.

If the coverage variations are significant, cO/RT % 1but c8 usually is small compared with E:, then, at least approximately

and so a plot of In (BT,,) versus reciprocal temperature gives a line of slope E:; in the second-order case, the ordinate plotted would be ln(8Ti). It was pointed out that, since changes in T , are usually fairly small, changes in In (T p )and In (Ti) are even smaller and so such plots are very insensitive and will invariably give straight lines even if the wrong order is assumed. A line width analysis by Chan et al. [ 2 6 7 ] revealed that, for the system CO/R {Ilo}, there was an apparent decrease in Ed with coverage.

97

Like the Edwards line shape analysis, the mathematics involved is fairly complex and for brevity is not repeated in detail here. They began by making the general desorption equation dimensionless, viz. d8 R ( Z ) = - = VPexp ( - E / Z )

dz where Z = T/T* and T* = l K , N* is the saturation coverage and E = E d / R T * . This equation then allowed the authors to calculate the relative coverage 8 and the desorption rate R at the peak maximum. These could be written

RM = drn)8; exp ( - E M )

The desorption rate could then be expanded using a Taylor series about 2, (peak temperature) and eventually the half-width could be written as

+

A U 112 = 22, [ EM (EM ZM)] 1’2 (122) The authors make the point after reaching this stage that expansion in terms of a Taylor series is only valid when 2 - ZM is small, something that is only true when desorption occurs over very small temperature ranges. This inherent weakness is emphasised by the authors using a numerical analysis to calculate parameters for CO/Ni (111) and CO/Pt (110). For the CO/Pt{llO) system, the authors found a 10%variation of Ed with coverage and this magnitude of variation has been shown to cause significant deviations from linearity in simple Arrhenius plots. The analysis was fairly successful in that it produced good agreement with previous data for a number of wide-ranging systems. But it emphasised that line width analyses, while yielding parameters from small amounts of experimental data not available via normal methods, are particularly prone t o errors arising from successive approximations. 4.1.4 Systems with variable pre-exponentials

It has been shown that variations in Ed with coverage can be used t o explain the non-linearity of kinetic plots. However, such deviations can also be explained by pre-exponential variations and, in most cases, a shift of Ed or v values, or more rarely, a change in both together, can be used t o fit experimental data successfully. A further problem with a simple analysis is that compensation effects between parameters can make the system appear simple, while give misleading values for kinetic terms. Most experiments assume a constant v, because of the difficulties of making accurate measurements of the pre-exponential. Experiments which have assumed a dependence of Ed on 8 can also be explained by applying a Referencesp p . 163-1 79

98

variation of the v term as shown by Pisani et al. [263]. Tamm and Schmidt [ 2761 have used a coverage-dependent pre-exponential fit t o their second-order desorption spectra for CO on W single crystals. The desorption was considered to occur by molecular desorption, rate-limited by recombination of diffusing atomic species. This model was derived formally in an earlier paper [ 2771 . The pre-exponential v2 is given by

where v is the vibrational frequency, a is the lattice unit diffusing distance and a, is the minimum separation of an atom pair during collision. Using a and a, as variable parameters, the desorption spectra could be fitted successfully. It is believed that v and Ed can be linked together by a compensation effect and Cassuto et al. [ 2781 have shown this intrinsically, They used the relationship V = Vo

exp [-Ed/RT]

to fit their data.

4.1.5 Desorption order In the discussion so far, orders of one or two have been considered. Surface processes with other orders (and fractional orders) have been found and changes in Arrhenius plot slopes can be caused by changes in desorption mechanism. Equation (94) shows the general form of the dependence of the kinetic parameters on the desorption order in terms of initial coverages. Another important relationship can be derived from the work of Falconer [ 2801 who showed that N i m - 1 ) = m-lNbm-l) (125) where N , is the coverage of material on the surface at the desorption peak maximum, and so, by substitution in eqn. (94), we have

As pointed out above, the desorption order markedly effects the shape of the desorption curve and the behaviour of the peak temperature with variation of initial coverage. Zero-order kinetics are shown by an increase in peak temperature with coverage and zero-order surface processes have now been observed for many systems [281-2861. Schwartz et al. [287] performed isothermal desorption measurements on the H2/Ti system and determined an order of 1.5, explaining this finding in terms of surface compound formation, with a stoichiometry of TiH1.,. A very good example of the confusion which can reign in this field is exhibited by the

99

system O2(Ag{llO}. Orders between zero and three have been postulated [ 2881 for the desorption process, although it is known that the adsorbate is in the atomic form. Bowker [289] has shown that the observed desorption effects can be explained in terms of the expected second-order process, but that the order term in the kinetic relationship is obscured by the effects of attractive lateral interactions between adatoms. (Such effects will be discussed in detail below.) The balance between two competing reactions of different order has been discussed by Klein and Yates [ 2901 in relation t o results which were obtained for the NO/W (110) combination. They consider two possible states of dissociated N atoms, N, being more strongly bound than Nx, there being an activation energy barrier between the two states. Thus kl

N X + O ' k z NYkand therefore

3N2W

where y is the concentration of the intermediate state. If the adsorption (N, + f N,) proceeds over a very low activation energy barrier 2dY-

dt

- k , x = 2ky2 - k 2 ( l - [O]

-X)Y

(128)

where [O] is the concentration of adsorbed oxygen. Assuming that y has a steady state coverage during most of the desorption process, i.e. (dyldt) C= 0, then at low coverages ( x >> y 2 and 1 % [ O ] x ) and

+

k 1%

Y = _k2

Thus

Hence, at low coverages, the kinetics will be essentially second order. A t higher coverages and ignoring the presence of oxygen

In general, for NO d"21 = dt 2k2(1- [O] - ~ ) ' [ 1 - { 1

+ 8klkx/k$(l- [O] - x ) ~ } ~ " ]+ 8klk2x 8k (132)

References p p . 163-1 79

100

It has been shown, in general, that zero-order desorption is likely from desorbing layers of surface compounds [ 2911, as has been seen for Xe on C(OOO1) [ 2 9 2 ] and for oxide films on W [ 2 9 3 ] ( W 0 2 oxide states are observed in the desorption spectra). Following the Venables and Bienfait analysis [ 2911, the coverage, N , is made up of atoms in a solid phase, S, in the first layer which covers a fraction, A , of the substrate and an adsorbed gas phase, 1, is in the first layer while 2 denotes second layer adsorbed gas species. If the atoms arrive from the vapour phase at a rate R (per unit area) and are accommodated and the layer evaporates at a rate R e , then

RdN = l - R e (R-R,)(l-A)+(R-R,)A (133) where R l and R 2 are the evaporation rates of phases 1 and 2 and N is the absolute coverage. The phase S only evaporates via phase 1 and/or phase 2 . Now R = P(2~rnkT)”~ (134) If the deposit is in equilibrium with the vapour, R = R e and the four phases are in equilibrium; thus, using chemical potentials

+ RTlnP

(135) where po is the standard chemical potential of the vapour. At equilibrium also 11s

= P1 = p2 = pv = po

Re = R1 = R2 = R (136) If R is reduced from its equilibrium value, the latter two relationships do not apply, but if interchange with the vapour is the slow step, then Ps = P l =

(137)

112

and

(138) Re = R , = R 2 Under these circumstances, the desorption order is zero and from eqns. ( 1 3 3 ) and ( 1 3 4 ) D -

D

where Ps is the equilibrium vapour pressure of solid phase S at temperature T . The zero-order form of eqn. ( 1 3 9 ) seems reasonable since compound desorption leaves an identical material behind and so the rate is independent of the surface density of material. Venables and Bienfait [ 2911 also showed that, under such circumstances, experiment should yield high values of v due to the presence of an extra entropy term (see Sect. 4.1.1) in the pre-exponential factor.

101

4.1.6 The effect o f precursor states

In Sect. 3.2.2, the effects of precursor states on adsorption kinetics have been discussed. Since even the earliest adsorption experiments show evidence for the influence of weakly bound intermediate states, following the principle of microscopic reversibility it might be expected that desorption kinetics would show the influence of such species. However, desorption experiments are usually carried out at considerably higher temperatures and average lifetimes in such states will be much lower. Shanabarger [294, 2951 was the first to use a model based on such effects, studying the systems H,/Fe (films) and H,/Ni (films). He proposed that hydrogen is dissociated on these surfaces and that the formation of a molecular precursor bottle-necks the desorption process. This results in isothermal desorption techniques measuring only the desorption energy of this precursor. This is obviously a misconception as the rate-limiting step in the process has to be desorption from the chemisorbed state and thus kinetics are measured principally from this state, although values may be affected by the precursor to a small extent. The first work to examine precursor effects in a quantitative manner was performed only recently by King [298]. It is based on the reversal of the kinetic model described in Sect. 3.2.2. The rate of transfer from the chemisorbed state to the precursor is given by

ri = Vee-EIRT (140) Because of the reasons discussed earlier, it is necessary to write that

rd = Fri

(141) F being a fraction since not all the molecules reaching the precursor desorb. By describing the possible events for a molecule in terms of probabilities, F was found to be given by

and so the effect of F on desorption spectra could be examined. This was done by computer simulation and showed that, for many chemisorption systems, precursor states have little effect on desorption kinetics; an example is shown in Fig. 33 for illustration. These systems are those for which initial sticking probabilities are low near the desorption temperature or fall off rapidly with coverage. Many other systems, however, exhibit significant mobility of the precursor state when desorption from the chemisorbed state is occurring. These systems are revealed in adsorption experiments by So being independent of coverage at low coverages. These systems were shown to produce broad desorption spectra shifted to higher temperatures than might be expected. Prompted by King’s work, Gorte and Schmidt [297] prepared a model, References p p . 163-1 79

102

o.oe

A"

c

vl

k

-2

0.06

0 C

?i

b

% 0.04 D I

0.02

0 1

Temp ./K

Fig. 33. A series of spectra, computer simulated from eqn. (143), illustratin the influence of the precursor state on spectra. All curves correspond to v = 1 0 ' s - I , f d = 38.3 kJmole-' and einitial = 7 , with heating rate 1 0 Ks-I. ( 1 ) F = 1; (2) K = 0 * 5 , F,=10/12, F d = 1 / 2 , S o / a = 1 ; ( 3 ) K = F d = 0 . 1 , F , = l , so/CU=l; (4) K = 0.01, Fd = l o + , F, = 1, so/& = 1. (After King [298] .)

f

also invoking a precursor as an important step in the desorption process. They adapted an approach based on rate laws. Writing

-

kd

;k

A*(a)-

A(€!)

(144)

and assuming a steady state in the precursor state A*(a) Rd

=

k*k,0 k* -t ka(l - 0 )

If k* 3 k,, then the kinetics are first order, but if k, % k*, then

(145)

103

and higher orders and more complicated desorption spectra are to be expected. The authors continued the development t o examine dissociative adsorption and differentiated between precursors existing over filled and over empty sites. These two precursors have now been assigned the names of extrinsic (filled) and intrinsic (empty) [421]. A new rate law mechanism can now be drawn up, viz.

A2

It is then possible to show that the rate of desorption is given by

so that the “apparent order” observed in experiments is dependent on the relative magnitudes of the rate coefficients. This is exactly the same conclusion that had been reached by King earlier. In many ways, the recent paper by Cassuto and King [421] is the definitive paper of its type. The model presented there includes all effects that may complicate “ideal” desorption spectra. From the reaction scheme (R4) drawn up earlier (p. 67) and applying detailed balancing, it is possible to write

Thus, apart from the term a’,this is now in a form which contains rate coefficient referred t o the intrinsic precursor. As discussed earlier in Sect. 3.2.3, a* = a’ and a, = 0 (i.e. ignoring direct transfer) so that

References p p . 163-1 79

104

kkk&

(149)

and, using normalised rate coefficients, rd can now be written

The Ps represent normalised rate coefficients. If the intrinsic precursor occupies a single site, for dissociative adsorption and no lateral interactions, the function F simply become Fd = 0 2 , F, = 6 , and Fa = (1- 6)z as shown previously (Sect. 3.2.3).Thus

which is a simplified version of the expression derived by Gorte and Schmidt [eqn. (147)] [297], The work by Cassuto and King is readily adapted to any adsorption system, but it must be noted that in order to obtain reasonable expressions, it is always necessary t o make a number of approximations using a detailed knowledge of the adsorption system under consideration. 4.1.7 The influence of lateral interactions

It has been shown that lateral interactions, like precursor states, play a fundamental role in determining adsorption kinetics (see Sect. 3.2.3) and they are of similar importance in thermal desorption measurements. This has been studied theoretically [ 300-3021. The existence of multiple peaks in desorption spectra may, in some cases, be explained by the presence of lateral interactions between adspecies, as first described by Toya [315] in discussing the two peaks seen for hydrogen desorption from W(100) (previously considered to be due t o two distinct binding sites). Goymour and King [ 451 used the quasi-chemical approximation t o obtain a good quantitative description of the effect of lateral interactions on second-order thermal desorption and applied their model t o the desorption of CO from W. Typical desorption spectra for this system are shown in Fig. 34. (A different model has been applied to the problem by Adams [ 3141 .) Strong nearest-neighbour repulsive interactions dominate the spectra and result in the appearance of the low-temperature peak, well separated from the higher-temperature, low-coverage desorption peak. The presence of repulsive interactions between adatoms lowers the heat of adsorption as the coverage increases and for recombinative desorption, the pre-exponential factor takes on a complicated coverage dependence, obscuring the meaning of desorption order. Goymour and King [ 451 have given the desorption rate in that case as

105 I

22 -

I

lA

20-

Y

-3 -0

>E s a

18-

161412-

L

0

2

10-

U r

0

8-

0,

c

6? .r

yip

4-

2-

Fig. 34. 0-CO desorption spectra from polycrystalline tungsten. The spectra shown are best fits t o the experimental data using computer simulation of eqn. (152). Because of the presence of two states, the coverage term, 8, was replaced by (8, OB). For the spectra shown, the values of (8, 8,) are A, 0.1; B, 0.2;C,0.3; D,0.48; E,0.68; F, 0.75;G, 0.88. (From ref. 45.)

+

+

where D = (1- 48(1 - 8 ) [ l - exp ( w / R T ) }1'2 ] and contains the interaction energy w . E is also dependent on the coverage and magnitude of lateral interactions, viz .

E , being the zero coverage desorption energy. The peak seen at 950K for CO adsorbed on tungsten is observed at coverages above 8 = 0.5 when (provided the interaction energy is high) the pre-exponential term becomes (28 - l), i.e. first-order dependence (and so the peak temperature is coverage independent). The desorption energy appropriate to the peak is E , Zw and so the peak appears well separated from the low-coverage peak by eight times the interaction energy (in the case of a four-fold symmetric surface). For the CO/W case (Fig. 34), the repulsive interaction energy was found to be 20 kJ mole-'. At low coverages, the interactions have little effect since the adatoms locate in high-energy sites and so the desorption peak is more nearly second order in character and at very low coverages the desorption energy is Eo.

+

References p p . 163-1 79

106 I

'

I

I

I

1

I

I

Fig, 35. Desorption of O2 from A g ( l l 0 ) . The data points are the experimental values at various coverages. The solid lines are predicted from eqn. (152). (From ref. 289).

An example of recombinative desorption with attractive interactions has recently been reported for the O,/Ag(llO) adsorption system and, again, the desorption order is obscured by the interactions [ 2891 . Figure 35 shows desorption spectra for this system which look more like those for first-order desorption (asymmetric with the desorption integral to the left of the peak being much greater than that to the right). The interaction energy is attractive and has a value of 14 kJ mole-' and so has a large value over most of the coverage range, leaving a near first-order dependence of the desorption rate, while at the maximum possible coverage in this case, 8 = 0.5, E = E , 2 0 (since the (110) is a two-fold symmetric surface). The situation is simpler for the first-order desorption systems since, in that case, only the desorption energy is affected by lateral interactions; the criterion of occupied nearest sites being essential for desorption is not needed. An example of the effect of attractive lateral interactions on desorption can be seen in Fig. 36, taken from the work of Jones and Perry [ 4571 on the Hg/W (100) system. These workers initially concluded that the desorption was zero order [457] since, as Fig. 36 shows, the peak temperature shifts to higher temperature with increasing coverage. However, this conclusion was in stark contrast to the adsorption heat with increasing coverage. Subsequently, Jones and Perry [ 4581 intepreted their

+

107

Fig. 36. Desorption of Hg from W{lOO}. (After Jones and Perry [457].)

desorption data in terms of attractive lateral interactions between the mercury adatoms amounting t o 6 kJ mole-' between pairs of atoms. From these examples, it is clear that much care needs to be taken with thermal desorption data and that ambiguous results may be obtained. When lateral interactions are a possibility in an desorption system, parallel measurements on adsorption or LEED determinations can help eliminate such ambiguities. In the previous discussions on adsorption, with respect t o eqns. (152) and (153), and in a consideration of surface diffusion mechanisms which is t o follow, the role of lateral interactions in the kinetics of these processes has been elucidated using the quasi-chemical approximation of Fowler and Guggenheim [ 3201 . Adams and Germer [ 3191 have also considered the effect of lateral interactions on thermal desorption measurements, but quantify it in terms of the Bethe-Peierls approximation [318]. Using this method, Adams and Germer [ 3191 compute desorption spectra which are qualitatively similar to those shown in Fig. 34 for repulsive lateral ,interactions. This is not surprising since, as Fowler and Guggenheim have discussed [ 3201 , the quasi-chemical and Bethe-Peierls approximations, although derived from different bases, give identical formulations [ 3201 . However, the quantitative fit t o the desorption spectra for CO desorption from W(210) was poor, probably due to the fact that, in contrast to Goymour and King [45], Adams and Germer used a first-order model for the CO desorption; it is now generally accepted that the &states of CO desorption are from dissociated states. References p p . 163-1 79

108

Other systems which have been simulated with these types of model are now manifold. King [ 3211 has extended his work to hydrogen desorption from various tungsten planes. On the (110) plane, o was found to be repulsive to the extent of 6 kJ mole-', but for the (100) and (111) planes the results could not be fitted exactly, possibly due to the presence of different binding energy sites. Lateral interaction effects have been inferred to explain the presence of two /3 states in the desorption of H, from P t { l l l ) [303]. Other examples of systems exhibiting these effects are CO/Ru{lOiO) [307] and Ru(001) [308], O,/W(lOO) [309], H,/Ni{100) [310] and N i { l l l ) [306] and CO/Mo{100) [311]. 4.2 THE DATA BASE

In Fig. 37, a large number of representative desorption spectra are presented for simple adsorption systems in which the adsorbate desorbs from the surface without decomposition or reaction. Even a simple molecule such as NO can undergo disproportionation reactions on surfaces to yield N, into the gas phase. The influences of surface structure and surface cleanliness on desorption spectra are discussed below. Section 4.2.3 deals with a specialised branch of thermal desorption known as temperature programmed reaction spectroscopy. 4.2.1 Crystal plane orientation In general, desorption spectra are very dependent on the types of site exposed at a surface and, in principle, it should be possible to determine the area ratios of different types of crystallography on inhomogeneous surfaces in this way. Investigations of this kind, but in terms of differing amounts of step sites on a well-defined surface, have been conducted by Collins and Spicer [324]. Figure 38 shows the desorption of CO from Pt{ l l l } , Pt [6{111} x {lOO}] and Pt [6{111} x {111}] and clearly shows a high-temperature state for the stepped surfaces over the whole coverage range; Collins and Spicer separated out these contributions from steps and terraces. Figure 39 shows the results for hydrogen desorption which exhibit similar features. These results indicate that the step density on a surface can be determined, if unknown, by desorption from the sample. 4.2.2 Surface cleanliness Once again, Pt will be used as a sample system to illustrate the effect of one adspecies on the desorption of another. Single crystals of Pt often contain large surface impurities of S, P, Ca, C, C1 and 0 and these adatoms are often very difficult to remove. The influence of S at the surface on the desorption of CO from Pt is shown in Fig. 40 from the work of Bonze1 and Ku [325]. Clearly, S substantially affects the desorption, decreasing both the desorption energy of the adsorbate and the amount adsorbed. Oxygen adsorbs on platinum t o form an oxide and McCabe and

109

Fig. 37. (a)

References p p . 163-1 79

110

Fig. 37. (b)

111

100

500 100

Fig. 37. (c)

References p p . 163-1 79

600

100

500-

112

Fig. 37. (d)

113

114

1100

1500

2500

Fig. 37. (g) and (h)

573

115

T/K

623 400

323

1800

300

900

1200

Fig. 37. ( i )

Referencesp p . 163-1 79

700 400

700

500

600

1300

1000

900

293

1300

573

c

116

Fig. 37. (j)

117

373

300

673

600

273

400

Fig. 37. (k)

References p p . 163-1 79

673 300

573 343

1200

300

600

118

200

Fig. 37. (1)

1550

119

N2/MOi100)

154

400

100

T/K 100

200

400

c

800

T/K

373

a1

I

673

c

180

I

200

Fig. 37. (m)

References p p . 163-1 79

1100

120 (n) N

2/

Re(1iloment)

I/K C

900

1500

300

120

1100

1500

Fig. 37. Some desorption spectra from the simpler adsorbate/substrate combinations. References are given with the spectra. (See Table 2.)

300

I“’

bl

(a1

500 T/K

700

300

500

T/K

700

300

n

500

700

T/ K

Fig, 38. TDS of CO as a function of CO exposure (1 L = 1 Langmuir = 10-6Torr s). (a) Pt (111). Heating rate % 1 5 Ks-’. The small amount of desorption for T 500 K is attributed to desorption from a small density of defects present on the {lll}surface, as well as to desorption from the edges of the crystal. (b) Pt6\111) X (100). Heating rate 1 2 K s - I . (c) Pt6(111) X (111). Heating rate ‘v 1 4 K s - . The desorption for T 500 K in (b) and (c) is attributed to adsorption at steps, while that at T 500 K is attributed to adsorption on the terraces. (After Collins and Spicer [324]).

>

>

<

121

I

I

300

400

E

I

I

300

400

E

I

I

300

400

T/K

T/ K

5'

T/K

17 K s-' . Temperature Fig. 39. TDS of Hz from the Pt single crystals. Heating rate of Hz exposure"200K. (a)Pt{111};(b)Pt6(111) x (100);(c)Pt6(111) x (111).The desorption for T 400 K in (b) and (c) is attributed to adsorption at steps, while that at T < 400 K is attributed to adsorption on the terraces. (After Collins and Spicer [3241).

>

7r

300

T/DC

40

1pc

Fig. 40. Desorption spectra of CO from Pt {Ill}.Curves A are the spectra from a clean surface with coverage increasing from (a) to (h). Curves B show the effect of increasing the S coverage: the relative sulphur concentrations are (a) 0.56, (b) 1.02, (c) 1.30, (d) 2.48, (e) 3.18 and (f)4.02. (After Bonze1 and Ku [325].)

References p p . 163-1 79

co

80 100

200

300

400

500

T/ K

Fig. 41. CO and Hz desorption spectra as a function of the surface coverage of oxygen. The ratio o f oxygen to Pt is given. (After McCabe and Schmidt [275].)

Schmidt [275] have looked at hydrogen and CO adsorption with this system. As shown in Fig. 41, there is a significant change in shape for the hydrogen desorption and the binding energy for both gases is reduced considerably while the total coverage remains the same. On the (110) plane, as the oxide level is increased, the 0,and Pz states present on the clean surface are altered, the eventually disappearing while the P2 is increased in the binding energy. Clearly, then, impurities on surfaces substantially affect the desorption spectra observed and many results for the surface chemistry of Pt in particular have been affected by such experimental uncertainties. 4.2.3 Temperature-programmed reaction spectroscopy This technique is a variant of thermal desorption in which products from a surface reaction are desorbed and separated o u t mass spectrometrically. This field has been pioneered by Madix and good reviews of this type of work are available [ 326, 3271. A few examples of such data

123 I

I

I

I

TernP./K

Fig. 42. The product distribution seen in TPRS for DCOOH adsorbed on C u ( l l 0 ) at 140 K: - - -, desorption from the clean surface; -, desorption from a surface predosed with oxygen. (From ref. 327.)

will be presented here. When formic acid is adsorbed on a Cu(ll0) surface [328], the desorption pattern observed (Fig. 42) shows the desorption of molecular acid species at low temperatures, hydrogen at 300 K and coincident peaks of H2 and C 0 2 at 480 K. The latter illustrates a basic tenet of this technique - coincident peaks usually originate from the decomposition of a common intermediate. In this case, the stoichiometry of the species (from the desorption intensities) is HCOO, an adsorbed formate. Thus a total reaction mechanism can be written

-

-

Adsorption HCOOYg)

HCOOH;a)

Dissociation HCOOYa,

HCOO(a)+ &a)

Molecular desorption HCOO&,)HCOOY,) Hydrogen recombination 2 &a)H2W Surface decomposition HCOO,a) C02(,) + 3 H2W

-

On the Ru(10iO) surface, Larsen and Dickenson [329] found the chemistry somewhat different. The major differences were that large amounts of H 2 0 and CO were evolved and a mechanism involving the formation of an anhydride intermediate (proposed earlier by Falconer and Madix L?ferences p p . 163-1 79

124

-

[330] for the reaction on Ni) was invoked, viz. HC00qg)

2 HCOOYg) HCOOOCHta) Coca)

-

HC00qa) HCOOOCH(a, + HZO(g1 CO,,g)

+ 243.) + CO(a)

CO(b9

2 %) HZW A further system which Madix and co-workers [331] have studied in detail is methanol adsorbed on C u ( l l 0 ) . Isotopic labelling was used to ease the interpretation of the reaction mechanism on the surface by differentiating between C-bonded hydrogen and hydroxyl hydrogen and by using labelled 1 8 0 2 to pre-dose the surface and distinguish it from the oxygen in methanol. The reactions observed in this case are shown in Fig. 1

1

"

I

1

1

I

I

4

0 C

.-0 n

L

c aJ

r

aJ n n

a

$

r

I

2 00

300

I 400

Temp./K

Fig. 43. Thermal desorption product distribution after methanol adsorption on a C u ( l l 0 ) face pre-dosed with 2 L of I8O2. (0, = (From ref. 331.)

i).

125

CH3OD(g)

___)

CH3OD(a)

(1C)

+ CH30D(,) + l80D(,,-

CH3O(a) + ‘*OD(a)

(2C)

C H 3 0 + Di80(,,

(3C)

Di80(a)

D

(4C)

H2CO(t3) + &a)

(5C)

CH3O&g,

(6C)

CH,OD(,)

CH3O(a) CH3O(a) + Y a )

2%) H2k) (7C) Steps 2C and 3C show the reaction of adsorbed alcohol with pre-dosed oxygen; D i 8 0 is the only water product evolved and is desorbed at low temperatures leaving two methoxy species for every pre-dosed oxygen atom. The products evolved at 350 K are then desorbed in decompositionlimited peaks from the break-up of the methoxy. The work of Wachs and Madix [331] showed further reaction to produce C 0 2 desorption from the surface at 480 K, but such a strongly bound species could not be observed by Bowker and Madix [331] or by Sexton [332] and so some impurity adsorption must be inferred in the earlier work. One further example, which indicates a very unusual surface reaction, is the work by Falconer and Madix [ 3301, mentioned earlier, on the HCOOHI N i ( l l 0 j system. The anhydride intermediate which was formed was observed to decompose autocatalytically (a “surface explosion”); that is, once the decomposition begins, it accelerates rapidly until all material is used. Thus, if the surface, with anhydride present, was heated t o just below the desorption peak and then held at that temperature, an exponential increase in COz evolution with time was observed isothermally. These results were explained in terms of an island mechanism. The adsorbate is held on the surface in islands and cannot decompose except at vacant sites; a good fit t o the data was obtained with the relationship

where N is the coverage of anhydride, Ni is the initial coverage, k the first-order rate coefficient and f the fraction of initiation sites within the island. f was found to be 0.004 and may represent the defect site level on the surface. In the temperature-programmed mode, the C 0 2 desdrption peak manifested these effects by being anomalously narrow (+ 5 K wide). 4.3 THE DESORPTION DATA BANK

Table 2 gives a summary of the kinetic parameters determined for the common gases on many metal surfaces. The table includes a differentiation of desorption binding states, coverages observed and the type of References p p . 163-1 79

CL

TABLE 2

p5

Q,

Desorption parameters: desorption order, pre-exponential constant, desorption activation energy, Ed, saturation coverage, Nmaxrand adsorption state (a, etc) The variation of Ed with N is indicated by the letters A-H. illustrated in Fig. 32 (p. 95). Substrate Hydrogen Co (film)

State

Order

Pre-exponential (s-’ or ern's-' )

N,, or

(atom crn-’)

em,

(Y

0-

co {OOOl} Cu (filament) c u (polycryst .) Fe(fi1ament) Fe (film) Fe (100) Ir (filament) Ir{llO)

P’ Pi

e = 0.13

1 4.5 x 10-l6 1.5 X 2.2 x 1.5 X

P1

2.8 x 1014 3.5 x 1014

lo-’

5.6 x 1014

lo-’

2.3 x 1014

P2

Mn (film) M o (filament) Mo (100)

1013 5x 5x

Ni (100) Ni (100)

(Y

2 2

3.2 x 1014 9 x 1014 3.3 x 1014 5x

B A

G

1.6 x 1014 1 2

4.2 8.8 78.7 67 182 38.5 85.0 30.5 85.8 100.4 96.2 50.2 50.2 29.3

20 x 1014

Mo(l10) Nb (filament) Nb (100)

Ed Ed versusN (kJmole-I)

A A 117.1 142.3 326.3 110.9 96.6 89.1

Ref.

491 491 491 491 492 493 494 495 294 496 498 497 497 499 500 501 501 501 502 502 503 504 5 04 310 505

5

ff1

3

ff2

r,

2

Ni(100)

%

Ni (100)

PI

Ni(ll0)

g2

(D

2

co

P1 P2

2 2 2 2 2

69.4 49.4 96.2 83.7 96.2

2

123.4 81.5 98.3

ff

I

Lr

v

2.6 x

loL4

2.7 x

loJ4

505 505 506 506

507

P* P2

Ni(ll1) Pd(fi1ament)

PI P2

03

1 2 2 2 2

2x

lo-'

e = 0.39

ff

Pd (110) Pd(111) Pt(film)

1 2

2

8X

Y 1 2 x 1014

PI

P2

Pt (foil) Pt(fi1ament) Pt (110)

Ps P4

03

P2

ptrool Pt 110

P1

02 PI

FJt(ll1)

7.4 x 1014

e = 0.2

03 02

PI

1 1 1 1 1 2 1 1 1 2 2

10'~ 1013 3 x 10" 3 x 10" 1.5 X 10"

17.5 x 1 0 ' ~ 4.6 x 1014

3 x 10" 3 x 10" 3 x 10"

12.5 x 1 0 ' ~

90.0 95.0 92 104.6 146.6 54.4 96.2 87.8 33.5 50.3 92.0 67.0 108.8 115.1 102.5 58.6 51.0 38.1 62.8 54.8 41.0 73.2

1.8 x 1 0 ' ~ 63-84

D

507 508 508 509 507 507 520 511 511 511 511 512 512 512 513 513 513 514 515 516 516 516 516 516 517 516 516 516 516 516

F

EJ -J

TABLE 2 (continued)

Substrate

State

Order

Pre-exponential (s-l o r cm's-l)

N,, or

(atom cm-')

emax

2 P1

1.2x 1014

PZ

Pt{lll}

01 PZ P3

P4

P3 02

15 x 1014 1 1 2 1

3 x 10" 2.7 X 10ls

Ed

Ed

125-1 34 26.8 39.4 32.6 52.8 A 73.2 65.2 73.2

2 x 10"

2 2

18 x loL4

01

Pt (211)

PZ

P1

Pt (332) Pt (997) Re (ribbon) Re (polycryst.)

1 1 1

2.5 X 10" 4.0X 10'' 8 x lo4

2

1 2

6X 6 x 10" 7.5 x

2

1.3x

2 2

2.3 X lo-'

15 x 1014 14 x 1014

1 2

P

8.8 41.2 54.4 54.4 28.0 115.1 127.6

B

a

Rh (filament) Ru (0001) Ta (polycryst .)

75.3

P1

02

01

10 x 1014

92.08 100.4

02

Ta(fi1ament) Ti (POlYmyst.) W(fi1ament) w (filament)

(31

2 1.5 2 1

50 x 10'~ 2x 10-~

versus N

Ref.

(kJmole-' )

343.1 87.9 146.5 111-121

B

518 303 303 275 275 275 519 520 516 516 516 516 516 516 519 521 522 522 523 523 498 524 524 525 525 503 526 498 498

P2

2 9.6 x 1 0 ' ~

PI

8.34 x 1014

P2 P3 P4

PI

4.2 x

P2

2.3 x 1 0 ' ~

PI P2 PI

P2

03 P4

Ys w(111)

Y1 Y2

PI 82 P3

P4

w(211)

PI

P2

2 2 2 2 2 2 1 1 1 2 2 2 2 2 2

[lo-2] 1.4 X

1

4 x 1014 2 x 1013

1

[ 3 x 1012] [3x [ [lo;2 10-

1 9 x 1014

0

Ag (111) Ir(ribbon)

Ir (110)

PI P?

2 2 2

2.7 X 1.4 x 3.5 x

lo-' 2.4 x 1014

117.1 150.6 129.3 138.5 193.7 271.1 110.0 97.5 135.3 133.9 113.0 136.0 59.0 90.8 127.0 153 31.8 33.9 50.2 79.5 104.6 104.6 66.9 146.4

498 528 529 5 29 529 529 276, 277 276, 277 16 5 30 276 276 276 276 276 276 276 531 531 531 531 531 531 192 192

151 147 173 171.5 146-168 160 304.6 268

459 289 460 258 461 462 462 463

A

A

CL (D N

TABLE 2 (continued)

c.

w

0

Substrate Ir(111) Mo( filament) M o (ribbon) M o (100) Pd(ribbon) Pd (111) pt(ribbon)

State

P Q2

Pt(100)

Y a

P PI P2

Pt (110)

03 PZ PI Q

Molecular Pt(s)- [9(111}x (111)l Pt(s)- [12{111}x { l l l ) ]

Preexponential (s-' or em's-'

(atom

lo3

2

5x

2 1 2

lo3

1 1 1

2 2 2 2 2 1

1.95 X [1013]

3x 1 0 ' ~

3.2 x 1014 6.5 x 1.7 X

lo-'

2 1 1 1 1 1

2.4 X [10131

2 2 2 1

2.5 x 1 0 - ~ 2.3 X lo-' p0-21 2 x lo-'*

1 [10'~]

Re (filament)

Ta(strip)

N,, or B,

7.8 x 10'~

Q1

pt(ribbon) Pt(po1ycryst. 1

Order

3.5 x 1 0 ' ~

Ed

Ed

versus N

Ref.

(kJ mole-' ) 27 2 132 260 493.7 209.2 12.5 195 138 146.5 44 167 187 260 290 125.5 134 21 3 15.9 171.5 205 186.6 165 127 235.1 180.0 125.5 552.3

H

464 465 466 467 468 224 469 469 469 322 470 470 471 471 471 472 472 472 473 474 47 5 475 476 476 465 477 478 479 480

5

1 g 9 0)

'c

P L

W(fi1ament) "(filament) W(fi1ament) W(fi1ament) W(po1ycryst.) W(p0lycryst.)

1 x 1015

1

A

m

0

I

c

w (poly cryst. )

[10l3il3 3x10 3 x 1Ol6 1 x 10l2

2 x 1013 3 x lo9 7

x 1o1O

3 x 10l2

4.8 x 10'~ 6 = 0.5

Carbon monoxide co (0001)

B

B , C A

H

E

103.0

Fe (100)

5.6 x 1014

Fe (S) [ 3 (111) X (11l)]

6 = 0.67 M o (filament)

610.0 581.6 564.8 443.5 494.1 543.1 505.8 458.1 610.0 571.5 551.9 774.9 581.6 385.0 188.0 598.3 513.4

1 2

220 105 85 105 94.1 182.0 196.7 133.9 113.0 132.6 150.6 163.2 19.2 85.3

C

481 465 482 483 214 338 338 338 684 685 486 487 485 214 488 489 490 532 532 496 496 496 496 533 533 533 534 535 535 536 537 538 5 38

I-J

w Y

TABLE 2 (continued)

Substrate

State

M o (polycryst.)

3 4 1 2 3 Q

Order

1 1 1 1

Pre-exponential (s-' or cm's-l)

2.8 x l o 9 1.4 x 1015 0.4 x 1014 [10131

N,, or

(atom cm-2) Ed Ed versus N (k J mole- )

em,

10x 1014

128.0 276.1 121.3 225.9 301.2

81 82

Mo (100)

M o (100)

83

81

[lo131 [ 1013J

82 03

Mo (110) Nb (110)

Ni filament) Nib001 Ni 100

I loi4 lo9

5x 5x 4.5 x 10l2 7 x 10" 2.6 X 10" 2 x 1o'O 2.35 x 10' 2 x los

1 2 1 2 3 4 5 6 2 Q

ff2

1

9 x 1Ol2 [ 10'3 J

e = 0.67

241.8 275.7 295.4 313.8 324.7 342.7 259.4 330.5 359.8 414.2 209.2 292.5 239.3 267.36 141.4 62.8 28.9 125.5 109.2 15.7

Ref. 538 538 539 539 539 540 541 541 541 542 542 542 542 542 542 543 543 543 544 544 545 545 545 545 545 545 546 505 547

s

2

'

02

m

Y

P

PI

Ni{llO}

1 2 3

b

P

cu

N Ni{llO ip101

&

Ni 111

'

v

1 1

1

11 x

[1013] 1.6 X 10'' 5 x lo4 2.5 x l o 3 2.5 x 1015

loJ4

e=1 11 x 1014

(0

[1013] Pd(ribbon) Pd{llO}

1 2 3 4

1

e = 0.5 % = 0.7 % = 1.0 % = 1.5

Pd 210 Pd 13101 Pt(foi1)

P

02

1 1 1 1 ? 1 1 1 1

P3

?

Q

Pt (polycryst. )

PI

02

Pt (polycryst. )

Q1

a2 Q3

PI Pt{lOO}-

7.6 X 10" 5.3 x lo9 2.6 x lo4 7 x lo2 2.5 x 1014

[5 x 201

Q1

Q3

a3

1 1 1

5 x 10" 5 x 10" [1012]

1

54.5 119.2 92.0 191.6 58.1 16.3 138.1 106.3 151.5 110.8 108.8 234.3 232.6 190.0 376 20.5 133.9 142.2 150.6 167.4 146.4 154.8 133.9 104.6 108.8 104.6 92.0 107.5 119.2 132.6 140.2 167.4 117.6 132.2 13R.5

C B B

A B A F A B

547 547 547 545 545 545 223 549 342 5 50 551 552 545 545 545 545 402 553 553 553 553 553 470 470 554 554 555 555 555 555 555 555 556 556 556

+

W W

TABLE 2 (continued)

Substrate

State

Order

Pre-exponential (s-' or ern's-' )

N, or

(atom cm-' )

em,

Pt (100)

1 20 x 1014

Pt(ll0)

1 1 1 1 1 1 1 1 1

13 x 1014

Pt (110)

Pttllol Pt 110

14 x 1014

Pt 111 P t11 Pt p1j

1 1 1 1 1 1 1

Pt(ll1) Pt(ll1)

15 x 1014 20 x 1014 14.8 x 1014 1 5 x 1014

1

1 1 1

Re(fi1ament)

P

1 1

11 x

lox4

Ed

E d versus N

Ref.

(kJ mole-' ) 98.7 110.9 121.7 133.5 105.1 131.4 106.2 130.5 125.5 82.8 108.8 138.1 156.1 95.0 123.8 93.3 126.4 121.3 113.4 131.8 130.1 138.9 125.1 100.4-113.0 138.1 138.1 150.6 225.9

B

A A A C C C

557 557 557 557 558 558 560 560 559 557 557 561 299 557 557 275 275 562 562 563 564 565 566 567 567 567 568 568

2 i

Re(0001) Rh(F.E.M. tip)

PI

z 0)

7 :

284.5

&I-2

P2

Ru(001) Ru(10iOj

(0

1 2 a!

13

w(filament)

1 2 1 2 3 4

W(fi1ament)

1 1 1 1 1

[10131 [1013] [1013] [10131 [10131 1.2 x l o 8

1 1 1 1

p o l 31

1 1 1 1 1 1

[10131 [1013] [ 1 0l3 i12 3 x 10 5 x loll 1.7 X lo8

[1013]

6.8 X

PZ

w (polycryst.)

P1 P2 P1 PZ

03 P4

Ps P6

W (ribbon)

12 x 10'~

81

03 a!

1 1 1

B

569

5.7 x 1014

P1-3

1

[lo121

[lO'Z] 1 1 1 1 1 1 2

[ 10121

I,,

2x10 7 x 10l2 [10131 100 [lo121

lOI4

132 121.3 124.7 117.1 98.3 160.0 126.0 90.0 106.3 119.2 205.0 84.4 325.5 283.7 216.7 78.5 221.7 313.8 418.4 101.2 223.0 425.5 205.2 221.7 251.0 280.3 311.7 326.3 101.25 223.0 425.5

570 571 571 572 572 341 307 307 572 573 574 526 526 575 575 575 575 576 576 576 577 577 577 578 578 578 578 578 578 45 45 45

+

0 UI

TABLE 2 (continued)

Substrate

w(oo1)

State a! PI

Pz

w (001)

03 a!

PI 82

P3

w(100)

Order 1 1 2 211 1 1 1 2

Pz

01

wt"ol 210

Nitrogen Ir (filament) Mo (filament) Mo(fi1ament) Mo (film) Mo(fi1m Mo(100)

e = 10

2.6 X lo-' 1 2 x 10l2 2.8 X 10" 5 x lo5 [1013] [1013]

P3

w

N,, (atom or emax

Ed

V V

Pz

PI

2.3 x 1014

9 x 1014

a!

E

2 1 2

7

3

2.5 x 1014

Ed

versus N

Ref.

(kJ mole-' ) 88.7 238.5 259.4 389.1 113.0 238.5 267.7 343.0 361.1 290.8 230.1 41.8 111.3 318.0 251.0

367 367 367 367 543 543 543 543 579 579 579 363 363 363 335 304 304

242.7 260.7 299.2 253.1 334.7 251-283.7 406

580 581 581 582 583 584 501

P1 P2

w(110)

Pre-exponential (s-' or cm2s-' )

5 a 2 $

Mo(l10) Nb(fi1ament) Ni(fi1m)

b G, 0)

Ni(ll0)

I v,

2 2

3x

0

4x

>

Y1 Y2 Y3

P

L-

P

Pd(fi1m) pt (polycryst.)

1 2 07

Re(fi1ament) Ru (1010) Ta (filament) W(fi1ament)

1 2

E

110131

1.1x 1014

8.9 x 1014

2 1 1

10 l3 i12 5x10

G

1 2

3 x 106 4 x lo2

104.6 284.1-313.4 35.6 500

>

2.9 x 1014

ff

P W(fi1ament) W (filament)

167.3

4.5 x 1014

ff

6 W(filament) W (filament)

A

<

ff1 ff2

Pt(ll1) Pt-(s)[9(111)x ( l l l ) ]

364.0 339.0 502.0 29.3 25.1-41.8 37.7-58.6 109.6 180.0 30 357.7

Y PI

02

W (filament) w (polycryst.)

P1

w(100)

P? Y-

1 2 or 1

1 2 1

1.4 x or 1013

lo-'

1.6 X 10" 8.5 x lo-'

9 x 1014

15.1 12.5 334.7 83.7 37.7 38.9 389.1 334.7 343.1 or 372.4 188.3-251 .O 285.8 376.9 43.9

501 585 583 586 586 586 587 587 586 588 588 588 589 590 590 591 478 503 581 581 581 503 55 5 5 592 593 593

593 263 263 354

CI W -3

TABLE 2 (continued) ~~

Substrate

State

Y+ 132

w (1OO} w(110)

w 110) W[lll)

131

Order

Preexponential (s-' or cm2s-l

emax

~

Ed

1

PO9]

2

5x

33.0 37.7 66.9 313.8

1 1 1 1 1 or 2

[1013] [10'~]

103 104 103 100.4 196.7or 192.5 96.2 194.5 276.9 104.6 217.6 113.8 95.4 134 114.2

p'Y a

[1013] or

Ni (100)

1.1 x 1013 [lo161

[10131 Pt (110)

1

pol3]

1

[10l6] [1013]

~~

Ed versus N

~

Ref.

(kJ mole-' )

2.3 X lo-'

P Nitric oxide Adpolwrys)

(atom em-')

1 2 1

Y+

p'

N,, or

38.5 307.5 205.0 41.8 46.0 313.8 26.8 A

354 354 354 15 15 15 595 595 596 15 15 15 369 369 370 614 614 615 615 616 617 617 618 619 619 619

619 605 605 605 605 621 621 323 323 323 307 307 307 307 307 621 621 290 290

117.6

1

110.0 87.9 118.4 284.5 198.7 295.0 125.5 150.6

1 1

125.5 1 1

102.5 119.2

1

151

1 1 1

97.9 209.0 305.4 140.0 217.0 305.4 253.1 198.7

1 2

c12/cu {lll} Clz /Pd (111) C12/Pt (111) Clz/Rh{lll) Cl2/W(100) FZ/Pt (111) NH3/Fe(l10}

1 1 1

[10~~1

6 = 0.43 e = 0.51

2 x 10'

2.7 x 1015 1x 1015

ff

4

1 1

PI

1

3.4 368.1 222 41.8

Invariant

597 598 599 600 600 598 601 601 602 602 603 604 305

+ W

CD

TABLE 2 (continued) ~

Substrate

State P2

NH3/Fe{111}

Order

Pre-exponential (s-' or em's-')

N, or

(atom cm-2)

emax

1

PI

1

[1013]

02

1

[1013]

12( 111)X (ill)] 1 2 NH3/Pt- (s)[ 6{111} X {ill}] NH3/Ru {OOOl} Y1 NH3/Pt

-

~

~

Ed

Ed

versus N

Ref.

(kJ mole-')

71.1 41.8 104.6

305 137 137

77 96 36 30.5 43.35 198.7 72.1

605 605 606 607 607 469 608

130.0 115.0 180.0

608 608 609

125.5 389.1 48.1 63.2 48.5 66.1 73.2 65.0

610 610 611 611 612 612 612 613

-s [

Y2

N20/Pt (ribbon) HNCO/Pt (110)

1

1

1 1 1

2

[1013]

[10~~] 1.85X lo2

(NH3)

[10'~1

(CO) (N2)

[10~~] [1013]

3.6 x loi4 (molecules)

C2N2/Cu{lll} SO2/W(poly-yst.

1

1

HzO/Fe (100)

PI

H~O/Ir{llO}

?

1 1 1

[1013]

p o l 31 [1013] 110~~1 [10131 [1013]

Y1

72

H2O/Pt{111}

3.1 x 1014 (molecules)

0

141

desorption seen is indicated for some of the systems which have been studied in detail.

4.4 INTERSTATE CONVERSION

Different binding states have been observed for many systems; the peak separations observed are distinct from those produced by repulsive laterial interactions. Earlier, it has been shown that chemisorbed-precursor state interchange can be important in desorption phenomena and this is one example of interstate conversion. The term interstate conversion is more usually applied - to conversion between higher binding energy states. This transfer may be of an equilibrium type or may be achieved by thermal activation or electron bombardment. The surface situation can be pictured by reference to Fig. 30; states may be present which have a very shallow potential well, a very deep well as seen for many atomically adsorbed species and wells of intermediate depth as expected for molecularly chemisorbed entities. Thermal transfer between states can occur if the cross-over of curves is below the potential energy zero, otherwise desorption will predominate. The effects of interstate conversion are largely ignored since such a process usually only effects the relative areas under the desorption peaks, but this will give a false indication of relative state populations in the adsorbed layer prior to heating. Tamm and Schmidt [276] investigated the H,/W system and found that the different states were in equilibrium. This was done by sequentially filling states with H, or D,; definite mixing was observed but the peak shapes were undistorted. Interstate conversion has been observed many times, most of the work having been done on the CO/W system. Many states can co-exist on the surface at the same time including virgin (v), a l and a2,and p1 and pz. In 1963, Swanson and Gomer [ 3331 noticed conversion from v to p states and they proposed that an a state could only be produced after this process had taken place. Using electron stimulated desorption (ESD) Yates and King [lo41 disputed this idea and proposed that the v state of Swanson and Gomer was a combination of a1and (11, states and that they interconvert during desorption. These states are quite distinct in ESD since the al yields CO' and a2 yields 0' from the surface. Yates and King [ 1041 and others [ 3341 have not observed a+ p state interconversion, but interchange between and p2 states does occur readily at 900 K [144]. v-CO conversion to other states (aand 0) has been observed by Bowker and King [ 3351 and by Steinbruchel and Gomer [ 1441 on W {110}, where the virgin is the only state below 400K. The conversion has also been observed by UPS [ 3371 and XPS [ 1471.

References p p . 163-1 79

142

5. Surface diffusion 5.1 INTRODUCTION

As mentioned previously, surface diffusion is an important process in adsorption. Mobility in the physisorbed state can improve the efficiency of chemisorption, while migration in the chemisorbed layer enables the surface species to populate the sites of highest binding energy, leading t o the formation of ordered adlayers. Activation energies for desorption from the physisorbed state are usually < 25 kJ mole-’ and so E m (the activation energy to surface diffusion) will be 10 kJ mole-’ or less. Using these figures, the lifetime in the physisorbed state a t room temperature is lo-’’ s and the hopping rate is 10” s-’ (using a pre-exponential value of the order of a vibrational frequency, l O I 3 s-’); thus, the mobile species can visit about 10 adsorption sites before desorbing, hence seeking out sites in which to chemisorb. In the chemisorbed state, the value of E m for nitrogen adatoms on the { 110)plane [233], for example, is 88 kJmole-’ and Do (the diffusion constant, pre-exponential) is 0.014 cm2 s-’ , which leads t o approximately one hop per minute across the surface at 300K. For this reason, FIM studies of individual atomic “hops” are made in the room temperature region, since “hopping” takes place a t a rate which can be followed individually in the course of an experiment. Diffusion is an activated process and is observed t o obey an Arrhenius relationship of the form (where only one diffusion mechanism is involved - single site hopping, for example)

-

-

D

D oexp

-

(- E , / R T )

(155) where D is the diffusion coefficient at temperature T , defined by the Fick’s Law relationship between diffusion rate and coverage gradient, i.e. =

This relationship is used in experiments and Emis determined by measuring

D at different temperatures. In terms of absolute rate theory, provided the transmission coefficient a t the activation barrier is unity, the diffusivity Do is given by the expression

Do =

X2 kT

- --

2a h

exp ( A S g l R )

where h is the jump distance, a is the symmetry number for the surface (1 for a 1-D surface and 2 for the 2-D case) and AS: is the activation entropy for the surface diffusion. D is usually found (particularly in FEM experiments) using the approximation [ 2461 for diffusion in a particular direction

143

3c = (Dt)”2

(157)

The average distance X travelled after time t at a particular temperature is then used as the measure of diffusion coefficient. In FEM, the marker used to measure such diffusion is usually some kind of adsorbate “boundary” observed on the tip; the resolution in an FEM pattern is 20 and so a boundary is observed whenever there is a sharp change in coverage (or a sharp change in work function with coverage) across < 20 8.Equation (157) is a “random walk” equation and assumes no concentration dependence of D when observing such a boundary. Thus, in such cases, D is a measure o f an average diffusion coefficient over the range o f coverage o f the boundary. Likewise, Em is a coverage-average activation energy barrier. As discussed previously, many chemisorption systems form ordered structures and lateral interaction energies between adspecies have been determined in some cases [50, 691. In connection with this, Bowker and King have recently investigated the effects of lateral interactions on surface diffusion computationally [ 48, 234, 4071. For simplicity, a completely homogeneous surface of four-fold symmetric sites was used. Such a surface was simulated in a computer and was set up with the surface completely covered on one side of the initial boundary and empty on the other. With these conditions, the diffusion could be initiated; a simple regime of single site hopping from filled sites into empty nearest neighbour sites was used in this simulation model. The whole array of sites (- 25,000) was scanned and when a filled site was found, then the number of filled nearest neighbour (n.n.) sites (2’)next to each filled site was determined. A value of 0.1 was usually used as the hopping probability (r)to any one empty site. Thus

-

a

v = ( z +)r (158) where v is the total probability of a hop and z is the number of n.n. sites. Note that eqn. (158) applies only for diffusion with no lateral

interactions, where r is unaffected by n.n. occupation. The result of running the programme for 1000 time units (one time unit being one full scan of every surface site) on the coverage profile is shown in Fig. 44. This profile has a centre of symmetry and crosses the original boundary at 50% coverage, just as expected for coverage-independent diffusion. Nearest neighbour lateral interactions have been introduced into this programme by appropriately weighting the values of I’ depending on the value of 2’. For repulsive interactions, r is higher the higher z’ is, whereas the reverse is true for attractive interactions. The effects of such interactions on the surface potential energy barrier t o diffusion are shown schematically in Fig. 45. The effect of n.n. interactions is considered t o be simply additive in this method, that is

E L = Em + z ’ w References p p . 163-1 79

(159)

144

olo--

90 -

80 -

3- 7 0 * 60* 0'

5040 30 -

20 10-

0Distance ( l a t t i c e units)

Fig. 44. Coverage versus distance profile for an initial square boundary diffusing across a surface with no interactions between adspecies. 0 , one dimension; 0, two dimensions. (From ref. 234.) : :

E f f e c t of repulsive interactions

Case 1

E f f e c t of a t t r a c t i v e interactions

Case

@

, .

Repulsive interactions decrease the activation e n e r g y b a r r i e r !o migration 0

, I,

. . ,I

Case

@

A t t r a c t i v e i n t e r a c t i o n s increase the activation energy b a r r i e r to inigration

Fig. 45. The effect of lateral interactions on the energy barrier to surface diffusion. (From Bowker and King [ 2 3 4 ] . )

where E L is the activation energy barrier for the particle in question, Em is that value for z' = 0 and w is the lateral interaction energy (negative for repulsive interactions). The effects of .applying such constraints to the diffusion process are to totally change the rate of diffusion and alter the shape of the diffusion

145

Distance ( l a t t i c e uni ts)

Fig. 46. Diffusion profiles from a species diffusing across a surface where attractive interactions dominate. (From ref. 234.)

Distance [ L a t t i c e u n i t s )

Fig. 47. As for Fig. 46, but the interaction between adspecies is repulsive. (From ref. 234.)

profile. Examples of such profiles obtained for repulsive and attractive interactions are shown in Figs. 46 and 47. It isgenerally the case, that when considering only n.n. interactions, the boundary cross-over value is > 50% for repulsive and < 50%for attractive lateral interactions. In diffusion studies, determinations are usually made s f D as a measure References p p . 163-1 79

146

of the diffusion rate and to determine the activation energy for surface migration. In order to measure D from the curves above, the simplifying assumption of a coverage-independent D must be abandoned. Thus, in terms of the one-dimensional form of Fick’s law at =

2ax (D(N)!y)

where N is the coverage, t is the diffusion time, x is the distance moved at coverage N and D ( N ) is the concentration-dependent diffusion coefficient. D ( N ) cannot be taken outside the brackets as is usually done. Boltzmann [ 405, 4061 derived a general solution for D ( N ) in this situation, with the boundary conditions pertaining in the cases above, as

where 17 is a reduced variable and equals ~ t - ” Matano ~ . [408] used this equation t o analyse diffusion profiles from the bulk interdiffusion of nickel and copper. He used profiles produced after a particular time of diffusion t and used eqn. (161) in the form

1 d x D ( N ) = -- 2t div

1 N

XdN

N1

where rxdN = 0 N1

and when t = 0 and x > 0, N = N o and when t = 0 and x < 0, N = N I . The latter condition is the “conservation of matter” rule during diffusion; the process involved must be solely surface diffusion and the equation cannot be used accurately in the above form when competing processes such as desorption or bulk diffusion take place within the diffusion zone. Diffusion coefficients obtained in this way are shown in Fig. 48 for the results of diffusion in two dimensions with and without lateral interactions. D is much reduced at high coverages for attractive interactions and is increased for repulsive interactions. Dilute layers tend t o the value of D for no lateral interactions. In the random walk situation for the fourfold symmetric surface, D is related t o r by [409]

D

=

ix2rz

(163)

Since z = 4 and X = 1 (the jump distance A is one lattice unit) then D = I‘ at low coverage. Thus, when z’ = 3 (75% coverage) r = 0.34 (the value assigned) and D is approximately the same value (Fig. 48). Similar results have been obtained using an analytical expression t o include the effects of lateral interactions. Making use of the work of

147

a

i

0.3t

i

100

Coverage

(%I

Fig. 48. The variation of D with coverage. Three examples are shown: (a) attractive, (b) repulsive and (c) no interactions existing between adspecies. (From Ref. 234.)

Fowler and Guggenheim and the quasi-chemical approximation [ 3201

i

I

(1-26) [I - 4 e ( i - e ) ~ 1 1 / 2 where E m ( 6 ) is the diffusional activation energy at coverage 8, Em is that a t zero coverage and B = 1-exp ( w / R T )(known as the short-range order parameter). Figure 49 shows the variation in Em(6)with 6 for various interaction energy values. Now eqn. (155) becomes E m ( 6 ) = Em

+-zw2

1-

o(e) = D~ exp [ - E , ( ~ ) / R T ]

(165)

and thus, in Fick’s law

Irdroducing thereducedvariablesx’ = x / l and t’ = [Do exp ( - E m / R T ) ] t / 1 2 where x is the unit surface distance and 1 is the total distance across the surface considered ( t is the time unit) and differentiating the right-hand side of the reduced equation

References p p . 163-1 79

148

1401

----

2

n L7 .:.

loo'

(C)

._. -. _. ._._. .. ...- -. . ....... ........... ... ....... . .......- -....._.

0:1

d.2

013

d.4

d.5

d.6

017

0!8

d.9

0

Coverage (inonoloyer)

Fig. 49. The variation of Em@) with coverage for different values of the lateral interaction energy. w = (a) 4- 4 kJ mole-' ; (b) 4- 2 kJ mole-' ; (c) 0; ( d ) - 2 kJ mole-' ; (e) - 4 kJ mole-'. (From ref. 234.)

This expression was numerically solved in terms of 8 and x using the Crank-Nicolson finite difference method. With w = 0, symmetric profiles (as in Fig. 44) were obtained, yielding concentration-independent D values on analysis using the Boltzmann-Matano method. Profiles similar to those from the simulation were obtained for finite values of w and diffusion coefficients showed the same trends (see Fig. 48). From the analytical results, a plot of B C (the boundary cross-over value) was made as a function of the interaction parameter ( w / R T )as shown in Fig. 50. It is possible that this curve could be used to estimate lateral interaction energies directly from diffusion profiles providing next nearest neighbour (n.n.n.) lateral interactions are negligible. When n.n.n. interactions do occur, if they are of the same sign as the n.n. interactions, then the effect will be to push the OC value to a greater extreme, indicating a nearest neighbour w value which would be somewhat high. The extension of the simulation programme to include n.n.n. lateral interactions is described in the following section in connection with experimental work on O/W (110). These methods have been applied on the assumption of a perfectly flat surface. However, surfaces may have steps at specific intervals across them

149

,

1

* 1.5 ’

O

r

I

T

1

\

i t

4 I

-0.5

-1.0

-2.0

1 1 d

Fig. 50. The position of the boundary cross-over value with the interaction parameter. (From ref. 234.)

and the step-sites may have “deeper” potential wells associated with them; thus, once the adspecies is trapped in such a sites, Em will be higher, reducing the rate of diffusion from them. Such features could be simulated in the difffusion programme, though as yet this has not been attempted. The determination of diffusion properties can also be carried out by static observation at a particular point on the surface and by following the change in concentration with time, rather than by the fuller method of profile analysis. Such methods have been used by Renard and Deloche [ 2611 (equation given earlier) and Abramenkov et al. [ 2601 ;these methods of analysis are detailed elsewhere [406, 4101 and will not be discussed here. 5.2 RESULTS

This survey is not meant to include all the measurements in this field of study; instead, those works which the authors consider to be the most significant to -date are presented together with results which illustrate a particular investigative method. In section 5.3,a table is presented which References p p . 163-1 79

150

contains most of the results obtained as regards the surface diffusion of adsorbates on non-supported metal samples. Some of the most elegant work in this area was that that done by Langmuir and Taylor [258] in the very early days of surface science and the method adopted was described in Sect. 2.4.3. They deposited low coverages of caesium on a polycrystalline wire and found a strong dependence of the diffusion coefficient on the initial Cs coverage. At a fixed temperature of 812 K, they found D = 3.45 x lo-’ cm2 s-’ with an initial coverage (on the high value side of the original boundary) of 2.73 x l o ” atom m-’, whereas with a coverage of 1.73 x 1017 atom m-’, D was lower a t 1.4 x cm2 s-l. These workers considered this variation to be due to repulsive mutual interactions between the Cs adatoms on the surface. Shortly after this, Brattain and Becker [259] investigated the diffusion of thorium on a polycrystalline tungsten ribbon using the method described in Sect. 2.4.3. They, too, found that the diffusion could not be fitted at all coverages with one diffusion coefficient, but D decreased with decreasing coverage by a t least a factor of two. In 1935, Bosworth [231] examined the surface diffusion of sodium and potassium on tungsten using the method given earlier in Sect. 2.4.l(a). He found that, with only one “dose” of sodium on the surface, although the band diminished in size, no matter was seen to leave the patch area (see Fig. 51). However, after carrying o u t this procedure many times without actually cleaning the surface (just allowing the patch signal t o decay), eventually he could deposit a patch which would spread with some semblance of a surface diffusion mechanism. Using various maximum coverages (at the centre of the patch) from 2 x l o L 9t o 6 x l O I 9 atom m-’, he found little variation in D with coverage. From an Arrhenius plot of the temperature variation of the diffusion coefficient, he found an activation energy for diffusion of only 24 kJ mole-’. He postulated that the behaviour of the sodium in the initial doses was due t o its diffusion into a “micro-structure” in the polycrystalline surface, which after many doses eventually became saturated. With potassium, no diffusion was apparent at room temperature but on heating, there was less tendency to diffuse into this “micro-structure” and he measured diffusion over a large coverage range. The profiles were more like those which would be expected for a true diffusion process, the coverage decreasing continuously with distance from the peak (see Fig. 52). He found a strong coverage dependence of E m , being 69 kJ mole-’ a t the lowest coverage measured (0.06 x 10l8atom m-’) and 28 kJ mole-’ a t 4.8 x 10l8atom m-’. He attributed this variation to repulsive dipole-dipole interactions between adatoms, making the diffusion easier at higher coverages. By far the major proportion of the surface diffusion data available at present has been obtained using the field-emission microscope. Not only can this technique yield valuable quantitative results concerning this process, but a layer of atoms can actually be seen to move over

151

Distance along strlp/cm

Fig. 51. Diffusion profiles for sodium adsorbed on tungsten. (From Bosworth [ 2 3 1 ] . ) A, 5 rnin at 295 K ; B, 1 0 min at 295 K ; C, 20 rnin at 295 K ; D, 40 min at 295 K; E, 60 min at 295 K ; F, 1 rnin at 415 K ; G , 3 rnin at 415 K ; H , 5 rnin at 415 K ; I , 1 1 rnin at 415 K.

the surface in a most dramatic fashion. Some of the most significant contributions to this field of study were made by Gomer and co-workers in the later 1950s [246-2481. In 1957, Gomer and Hulm [248] investigated the diffusion of oxygen over a field emission tip by “shadowing” it with oxygen and they discovered several modes of migration. First of all, at high coverages and very low temperatures (27-70K) there was diffusion with a “moving boundary” due to migration of the physisorbed oxygen species over the chemisorbed layer, with precipitation and chemisorption on to the surface where the physisorbed species encountered a clean region : this mechanism did not operate above 8 0 K because of the short lifetime of the physisorbed species at that temperature. Diffusion took place in the chemisorbed layer above 400K, again in a moving boundary fashion: the activation energy for diffusion of the sharp boundary was 95 (f 4)kJ mole-’ . The third References p p . 163-1 79

152 A

2 1.0

D i s t a n c e in c m

Fig. 52. The diffusion of potassium adsorbed o n tungsten. The curves show the increase in patch width as a function of heating time at 480 K. (From Bosworth [ 231.)

fi

[Ill1 n

4

.

, ,

,.

e

(C)

,7

~

S u r face c h onnels

)

Fig. 53. Hard sphere model of three interchangeable configurations for the atom pairs of Mo on the W(211) plane. Small open circles on each surface channel indicate the adsorption sites for adatoms, separated, respectively, by 2.7 A. (From ref. 414.)

153

mode was observed after this boundary reached the { l l O } plane; a new boundary was then seen to radiate outwards from the {llO}, but this stopped after a short distance unless the dose was initially directed on t o the (110) surface: Em for this process was found to be 104 (+ 4) kJ mole-’. Finally, if the temperature was further raised to 600K at this stage, a boundary-free migration process occurred over an activation energy barrier of 1 2 6 (+ 6) kJ mole-’ (this being the value a t low surface coverages). They explained this variation in mechanism as due t o the presence of different proportions of 2, 3, 4 and 5 coordinate sites on the different crystallographic regions of the tip. On the {lOOj-type regions, the sites they proposed are mainly 2 and 5 coordinate sites whereas on (110)types, they are 3 and 4. Thus the lowest energy diffusion is from 2 coordinate sites, which are filled at the highest coverages (least binding energy, less bonds to be broken in diffusion); low coverages exhibit diffusion from the tightrbinding 5-coordinate sites and the middle range of diffusion is in the (110} region. Theoretical calculations using the density functional approximation have recently been performed by Huntington and co-workers [411] and lend support to these usually accepted ideas of plane dependence of surface diffusion. They investigated the variation of diffusional activation energies with surface crystallography and the adatom binding site on the surface. They found a general decrease in activation energy for migration with increasing close packing (smoothness) of the surface. For an atom sitting in a four-fold or three-fold hollow in the surface, this change was considerably more dramatic than for an atom binding directly above a surface atom. Chen and Gomer [412] have recently advanced their earlier work by investigating the diffusion of oxygen on the {110} plane of a tungsten field emission tip using the “flicker-noise” technique and a probe-hole attachment as described earlier in Sect. 2.4.2(a). They studied the diffusion between 300 and 800K and coverages from 0.15 t o 0.56 of a monolayer. At intermediate temperatures, they found predominantly a mechanism of single particle diffusion and found Ed to be 60 kJ mole-’ a t 8 < 0.2, increasing t o around 94 kJ mole-’ at 8 = 0.56, while Do ranged cm2 s-’. In the high temperature range, there was to from evidence f o r - t h e diffusion of multiples of atoms in clusters while a t the lowest temperatures, diffusion was by the mechanism of adatoms “flipping” from one adsorbed state of a certain work function to another state of different work function, rather than by density fluctuations inducing local differences in 4 values. An idea of the presence of lateral interactions between adspecies can be gained from such studies since, in a situation with attractive interactions, the current fluctuations should decrease with increasing temperature, while the reverse is true for repulsive interactions [413]. In fact, Chen and Gomer observed the latter, which is in apparent contradiction References p p . 163-1 79

154

t o the increasing values of the diffusion activation energy which they also found. Field ion microscopy provides us with a facility for observing the diffusion of individual atoms on an FEM tip. The results with heavy metal atoms show clearly that lateral interactions have an important effect on the diffusion of individual species on the surface: indeed, this technique reveals some rather surprising aspects of sub-microscopic diffusion. On highly channelled planes such as (211jW and {321jW, diffusion of M o adatoms is observed to take place along the [ l l l ] channels [414, 4151 and not perpendicular to them; furthermore, when two atoms approach each other in adjacent channels within a couple of atomic spacings, they “lock” into each other’s motion and then diffuse as a dimer (though with single atomic “hops”) as shown in Fig. 53. It is intuitively rather surprising, however, that the activation energy barrier t o diffusion for this entity was only 26 kJ mole-’ on the (211) plane, half that for the single atoms. Reed and Ehrlich [416] found that, for Ir adatom dimers on the same plane, diffusion proceeded with E m = 64 kJmole-’, whereas for the single adatoms it was 54 kJ mole-’. They conclude that the interactions, as might be expected, are very dependent on adatom size. They observed chains of atoms as long as 4 units all moving in correlated motions in adjacent channels. It is important for a more detailed understanding of the surface diffusion process that experiments are conducted with single crystals which have a well-defined surface structure and stoichiometry. On polycrystalline surfaces, there appears to be a surface substructure and in FEM microscopic samples, the diffusion effects are dominated by the close proximity

-1

0.1

0.2

0.3

0.4

I

I

I

I

I

0.5

0.6

0.7

0.0

0.9

Coverage,

e

Fig. 54. The variation of D with surface coverage, for O2 on W ( l l O } . 76OoC. (From Butz and Wagner [ 2 4 0 ] .)

0,

880°C; X I

155

of planes with differing activation energies for diffusion. Experiments with single crystal planes should reveal the effect of any lateral interactions between adspecies on the diffusion parameters. Very little work in this respect has been carried out to date. However, Love and Wiederick [417] have recently compared the diffusion of caesium on polycrystalline and (110) single crystal samples of tungsten. They used a similar method t o that of Bosworth, but had good UHV conditions. For the polycrystalline samples, they found a similar effect t o that of Bosworth (the adsorption of photoactivity by the surface) except that they separated two contributing processes, one with activation energy of 170 kJ mole-’ and the other of 17 kJ mole-’. The former they attributed t o diffusion into the bulk, down grain boundaries, and the latter t o migration into some “surface structure”. Again, after several doses, this structure could be saturated and surface diffusion observed. For the (110) single crystal, this effect was also seen, but was much less extensive, the adsorption effect being saturated after the adsorption of 0.1 monolayer of Cs: subsequently, they found Em was 57 kJ mole-’ for small coverages between lo-’ and 5 x monolayers. Butz and Wagner [ 2401 investigated the diffusion of oxygen on (110) W in the monolayer region using the technique described in Sect. 2.4.l(c). The diffusion of the atomic 0 species was examined in the range 10401150 K and they found Em t o be 113 (k 8) kJ mole-’ and D t o be approximately cmz s-’ at a coverage of half a monolayer and a temperature of 11OOK. D showed a strong dependence on the oxygen coverage and this is illustrated in Fig. 55. The coverage profile results were analysed using the Boltzmann-Matano method described earlier, but it must be noted that matter was lost during their diffusion sequences as shown in

Distance (pm)

Fig. 55. The coverage versus distance profiles obtained by Butz and Wagner [ 2 4 0 ] for 02 diffusion on W { l l O ) at 88OoC.

References p p . 163-1 79

156

t 80-

,2.70 B

m

m-

U

$

0

'

50-

L

40-

10 0

40

Distance ( l a t t i c e units)

Fig. 56. The diffusion profiles observed after 1000 time units when the interaction between molecules extend t o allow both nearest neighbour and next nearest neighbour interactions. 0 , Diffusion in one dimension; 0 , diffusion in two dimensions. (From Bowker and King [ 2341 .)

Fig, 56: the area under the left-hand side of the profile (the matter diffused out from the original boundary) is considerably less than the area above the curve on the right-hand side (matter lost from the original covered area during diffusion). It appears that another process may be taking place a t the same time as the surface diffusion, at least on the low concentration side of the profile. However, prompted by this work and some results obtained recently by Lagally and co-workers [ 501, Bowker and King [407] have extended their simulation model of surface diffusion t o include next-nearest neighbour lateral interactions. Lagally and co-workers [ 501 investigated the p ( 2 x 1) low-energy electron diffraction (LEED) pattern for oxygen adsorbed on {llO}W, measuring the decrease in intensity of the adsorbateinduced spots with increasing substrate temperature. Using a model they devised t o fit this variation a t different coverages, they deduced the presence of repulsive n.n. interactions of 14.4 kJ mole-' and attractive n.n.n. interactions of 6.7 kJmole-' (this oscillating nature of the sign of the interaction energy is a result of its indirect nature, effected via the conduction band electrons [ 591 ). Using these lateral interaction values, Bowker and King [407] obtained the profiles shown in Fig. 56 for diffusion in two dimensions and D values found using the Boltzmann-Matano analysis are presented as a function of coverage in Fig. 57. The qualitative agreement with Butz and Wagner's results produced using the independent data of Lagally and co-workers is encouraging and

157

01

0

I 10

I

I

I

I

20

30

40

50

I

60

I

I

70

80

I

90

I

100

Coverage (%)

Fig. 57. The variation o f D with coverage for diffusion in two dimensions as found by the computational studies of Bowker and King [234].

indicates that the simple ideas of the surface diffusion process encompassed in this model bear a reasonable semblance t o reality. These experimental data have also been qualitatively fitted by Asada and Masuda [ 2551 using the kind of statistical-mechanical treatment outlined in Sect. 4.1.6, adapted to take account of more distant lateral interactions than simply nearest neighbours, and using the Bethe-Peierls approximation [ 3181. Continuing their diffusion studies using scanning electron microscopy, Butz and Wagner have reported elegant work on the surface diffusion of gold and palladium on tungsten planes [418, 4191. Many fascinating details of surface diffusion were observed in this study and, once again, interatomic lateral interactions seemed to affect the results in a marked way. Thus on the {110}plane, kinks in the diffusion profile between zero and one monolayer coverage are produced, corresponding t o ordered phases on the surface (although these kinks are diminished at high temperature and long times due t o the great surplus of material, in excess of one monolayer, deposited in the original adsorbate patch). Such profiles are very similar to those shown in Fig. 57, obtained under the influence of repulsive and attractive interactions. The dominating influence on the diffusion rate was, however, the presence of second layer adatoms since, as found in the FEM studies cited earlier, diffusion is then in the “boundary” regime. This is because the Pd-W or Au-W bond strength is relatively stronger than that for Pd-Pd or Au-Au and thus, on reaching the limit of the second layer boundary, the adatoms drop into more immobile trap sites on the tungsten surface. On stepped W( llO} surfaces, it was found that diffusion was very anisotropic [ 4191 ; palladium could not scramble over steps (which were {loo}type sites and so act as “traps”) as well as over the close-packed { l l O } terraces. As a result, a fascinating phenomenon References p p . 163-1 79

TABLE 3 Surface diffusion parameters Adsorbent (PI-)

Adsorbate

Temp. range

Em

(K)

(kJ mole-’ )

DT (cm s-’

DO )K

Coverage

(cm2s-’)

Coverage dependence

Method

Ref. ~

W(po1ycrystalline wire)

CS

650-812

58.5

(3.4 x 10-5)812

0.2

3 x 10”atom m-’

Strong increase in D with increase in coverage

Desorption sampling

258

550-850

55

( 6 x 10-5)m

0.23

Very low to 5 x monolayer

Dilute layer

Photoemission profile

232

x l0”atom m-’

E m is an average over coverage range

Photoemission profile

231

-40

0.12 x 1 0 ’ ~ a t o mm-’ 1 . 2 x 10i8atom m-’ 4.8 x 10’”atorn m-’

Photoemission profile

231

1.6 x 1.3 x -32

0.5 x 10’8atom m - 2 1.5 x 10”atom m-’ 2.7 x 1O”atom m-’

Strong increase in D with increase in coverage Increase in E m (decrease in D ) with increase in coverage

FEM

715

Average Em for coverage range and for diffusion over different planes of tip

FEM

716

W(oxidised polYcrYddine ribbon)

Na

300-800

24

K

295-750

63 50 28

W (over several planes of tip sufrace)

K

-600

21 40

470-540

40 80

2-6

( 2 x 10-9Mx, (4 x 10-8)Mx, (8 x 10-6),,

3x 1.5 x

-

1 monolayer

-0

?

LI Q,

W((Oll}--t { E l } )

Ti

YI

500-750

106 79 64 24

800-960

180 180

-

700

500-600 530-600 310-340

0.64 0.18 0.03 0.002

-

-(3 X 10-

46 96

(7 x (10- 11)-

< 31

(1O'3)ml5 )TOO

-

-10-l 107 x lo-' 1.4 x

Au

W(across one side of tip)

Hg

2OHOO

W({110}-*{loo}) W(around {211} region)

Yb

600-1 100

43 68

W({110}+ {loo}) W(amund (111) region)

Nd

600-1100

73 62

w(P0lYclYstdine ribbon)

Th

1535-1655

450

Scanning AES

7 35

Low coverage

E m is an

FEM

717

Em has maximum at

FEM

718

E m is an

FEM

719

Decrease in Em with in crease in 0

FEM

720

Decrease in Em with in crease in 0

FEM

721

FEM

250

0.3 monolayer 0.6 monolayer 1.O monolayer

-

1 monolayer

6

lo-'

e * 0.5 average

Probably about 1 monolayer

6(t150

W({101}+{100))

Strong increase in D with increase in coverage average

-

-(2 x

96 120 86

0.25 monolayer 0.50 monolayer 1-7 monolayers

0.2 monolayer to 10-30.5-1 monolayer

-

1 monolayer

Average Em over coverage range of moving bound-

-

1 monolayer

Average E m as above

FEM

250

Increase in D with increase in tl

Thermionic emission

259

X Y

I-

cn W

TABLE 3 (continued)

Adsorbent (plane)

Adsorbate

Mo

Ir

Ge

Temp. range (K)

Em

DT

Do

(kJmole-’)

( C ~ S - ’ ) ~

(em2 s-’ )

235-290

55 25 53

( 2 x 10-:;)270

50 65

(10-16)%

-

300

350-750 -700

W({llO}-+ {loo})

Si

Si

1000-1200

-750

-

1100

Coverage

Coverage dependence

Method

Ref.

9 x lo-’ 2 x lo-’ 1x

Single adatom Dimer Single adatom

Very low coverage only

FIM

414, 415

5 x lo-’ 9x

Single adatom Dimer

Very low coverage only (onedimensional diffusion)

FIM

416

22 24 165 125

High coverage

Migration in second Ge layer

FEM

722

150

“Small amounts”, < monolayer

Em dependent

FEM

723

Large coverage Lower coverage

Diffusion in 2nd Si layer?

FEM

724

Average value for Em

FEM

249

Coverage average E m

FEM

.725

Coverage average E m

Scanning AES

233

Average E , over coverage range boundary

FEM

248

FEM

248

(3 X 10-18)z70 (5 lo- )270 (6x 10-

1300

Low coverage

72 92 152 172

C

850-1100

230

N

-650

85 -150

N

80*900

88

(2 x

10-8)m

0.014

0

500-55 0 560-650

105 126

(3 x 10-l~ (8 x 10-10;::

-0.03 82

0

400-480

65

-

-

-

1 monolayer

Low coverage

-

1 monolayer

on amount Si deposited and region observed

(probe hole)

e=1 e = 0.5

Increase in D with decrease in 09 from 1 to 0.5

Scanning electron beam (secondary electron emission)

234

113

Average for Em given: maximum in D at 0 = 0.4

Scanning work function probe

240

1040-1450

220

High coverage value for Em

co

-

150

-

Scanning electron beam (secondary electron emission)

1 monolayer

FEM

726

co

-750

210

-

Average over all planes, over coverage range

1 monolayer

From dissociation of C 0 2: average over planes and coverage

FEM

727

- 1 . 8 ~ 1 0 --1monolayer ~

E , averaging over varying surface geometry and boundary coverage range

FEM

728

Multilayer

Diffusion in 2nd layer

FEM

729

Multilayer

Diffusion in 2nd layer

FEM

730

< 1 monolayer

First stage of investigated diffusion-average E m over low coverage range

SIMS

260

0

918-1303

106 99

0

1030-1150

0

-(I x~o-~),,,,

-0.04 -(i4 x 10-6)lz13 -0.25

k

0,

cu

W(across half tip)

W (across half tip)

coz

(physisorbed)

W( 110 + 100 ) WJllOI+ [Ill),

H

W(across tip)

Ar

650

60-70

-

-

180

25

18-21

-

-

< 3.8 > 4.5

(physisorbed)

W((310)’

(100))

Ar (physisorbed) KI (physisorbed) Xe (physhorbed)

Mo (polycrystalline) Cu

20 20

10

2.5

-80

16

800-1100

52

(4 x 10-6),070

- lo-)

~

Q,

U

TABLE 3 (continued) Temp. range

Em

(K)

(kJmole-')

Adsorbent @be)

Adsorbate

Mo(211) --* (100)

Si

Ni(across tip)

H

250-280

30

Ni((ll1))

Tritium (physisorbed)

13-20

0.84

H

-

Ni(cornpresed powder sample) Ta(around (110)) Ta((010}+ (111))

Hg

DT (cms-l)K

Do (cm' s-')

covemge dependence

Method

Ref.

1 monolayer

Coverage, average

FEM

731

.- 1 monolayer

Coverage, plane average

FEM

732

1 monolayer

Coverage average

Radio activity sampling

261

G S now rate

733

FEM

124

Coverage

215

(14 x 10-6),s

3x

5

300

88-120

i!,

5

1 monolayer

DecreaseinE, by half in going from

e=oto 8=1

163

could be observed in 2D Auger scans; originally deposited circular patches of palladium became ovals, spreading faster in the direction parallel with the steps; on the (110) plane itself, diffusion was isotropic. 5.3 SUMMARY

A summary of the results to date is given in Table 3. Where the data are available, column 5 lists diffusion coefficient values a t a particular temperature T. It is apparent from the table that by far the majority of work has been done with tungsten. This is because of the ease of forming FEM tips from this material, its activity to adsorption and its ease of cleaning (in polycrystalline and single crystal samples the major impurity is carbon, which is easily removed by heating the sample in oxygen). There is evidently a great need for data from other materials and particularly from singlecrystal samples. With the availability of many powerful new tools in surface science, there will certainly be an upsurge in this field of work and many more fascinating aspects of surface diffusion will be exposed to view. References 1

7 8

9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25

H.S. Taylor, J. Am, Chem. SOC.,53 (1931)578;Trans. Faraday SOC.,28 (1932) 181. J.B. Taylor and I. Langmuir, Phys. Rev., 44 (1933)423. J.K. Roberts, Proc. R. SOC.London Ser. A, 142 (1933)518. J.K. Roberts, Proc. R . SOC.London Ser. A, 152 (1935)445. G. Ehrlich, J . Appl. Phys., 32 (1961)4;Adv. Catal., 14 (1963)255. P.A. Redhead, Vacuum, 12 (1962) 203. P.A. Redhead, J.P. Hobson and E.V. Kornelsen, The Physical Basis of Ultra-high Vacuum, Chapman and Hall, London, 1968. E.W.Miiller, Z. Phys., 106 (1937)541. J.A. Becker, Solid State Phys., 7 (1958)379. R. Gomer, Discuss. Faraday SOC.,28 (1959)23. G. Ehrlich and F.G. Hudda, J. Chem. Phys., 35 (1961)1421. C.J. Davison and L.H. Germer, Phys. Rev., 30 (1927)705. H.E. Farnsworth, Phys. Rev., 34 (1929)679. R.E. Schlier and H.E. Farnsworth, J. Appl. Phys., 25 (1954)1333. L.H. Germer and C.D. Hartman, J. Phys. Chem. Solids, 14 (1960)75. W.Ehrenberg, Philos. Mag., 18 (1934)878. T.A. Delchar and G. Ehrlich, J. Chem. Phys., 42 (1956)2686. P.J. Estrup and J. Anderson, J. Chem. Phys., 45 (1966)2254. J.J. Lander,Phys. Rev., 91 (1953)1382. R.F. Weber and W.T. Peria, J. Appi. Phys., 38 (1967)4355. D.A. King and D.P. Woodruff (Eds.) The Chemical Physics of Solid Surfaces and Heterogeneous Catalysis, Vols. 1-4, Elsevier, Amsterdam, 1981-1983. J.F. Lennard-Jones, Proc. R. SOC.London Ser. A, 106 (1924)463. F. London, Trans. Faraday SOC.,33 (1937)8. A. Miiller, Proc. R . SOC.London Ser. A, 154 (1936)624. R.A. Pierotti and G.D. Halsey, J. Phys. Chem., 63 (1959)680. G. Ehrlich, Trans. 8 t h Vac. Symp., Pergamon Press, Oxford 1961. T. Engel and R. Gomer, J. Chem. Phys., 52 (1970)5572.

164 26 27 28 29 30 31

32 33 34 35 36 37 38 39 40 41

42 43 44

45 46 47 48 49 50

51 52 53 54 55 56 57 58 59 60 61 62 63 64 65

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Chapter 2

Adsorption, Desorption and Migration on Semiconductor Surfaces B.A. JOYCE and C.T. FOXON

1. Introduction 1.1 JUSTIFICATION FOR THE SUBJECT MATTER

The general topic of chemisorption on semiconductors, which encompasses the processes described in the title, has generated a vast literature for which Peshev et al. [l] have provided a bibliography of almost 3000 papers covering the period 1946-1972. If we examine the nature of much of the work cited there, we find it to be predominantly chemically orientated, both in the description of the materials studied and in the mechanisms proposed. Discussion of the crystallography and electronic structure of the semiconductor surface tends to be in bulk terms and measurement techniques for electronic effects are often simple adaptations of those devised for the determination of bulk properties. In large part, this somewhat unsatisfactory approach stems from the use of oxide semiconductors as vehicles for study. From a chemical viewpoint, their choice is entirely logical since they are widely used as catalysts, but in every other respect they are unsuitable. Stone [ 2 ] has pointed out some of the limitations, which include the poor quality of oxide crystals in terms of structural perfection, stoichiometry and purity, the virtual absence of any theoretical or experimental evaluation of their surface electronic and crystallographic structure, except in the most elementary terms [ 3 ] , and a very limited technology for clean surface preparation, such that part of the reported work may not have related to semiconductor surfaces at all because of the level of contamination. Given this framework, it is perhaps not surprising that work on such systems has been largely empiric, providing a vast catalogue of specific information, not necessarily very reproducible, but little in the way of a general understanding. We will include a brief summary of the type of theory which has developed from such an approach, but the remainder of this review will follow a quite different pattern. We will deal only with elemental (Si and Ge) and III-V compound semiconductors whose crystal perfection and purity is extremely high, where atomically clean surfaces can be routinely prepared and where there is a good basis of understanding of surface electronic structure and crystallography. After a brief discussion of experimental techniques, we will discuss the fundamental concepts of surface electronic states and surface reconstruction in References p p . 280-289

182

semiconductors and the preparation of clean surfaces, and to complete this section we will give examples of measurements and calculations on specific clean surfaces. We will then deal with the adsorption of simple gases on well-characterized surfaces to indicate the extent t o which theory and experiments are beginning to converge. Finally, we will treat the adsorption and desorption of metals and use these processes to illustrate instability effects on clean, reconstructed surfaces. In one sense, therefore, this review will be much more limited in scope than most previous reviews on this subject, but we hope t o show how the application of the more recently developed experimental and theoretical methods of surface physics is leading to results of general significance. First, though, we will summarize very briefly the more conventional chemical approaches. 1.2 THE “CHARGE-TRANSFER” MODEL

The concepts of charge transfer adsorption and catalysis have been formulated and developed by Wagner and Hauffe [4],Garner et al. [ 51, Wolkenstein [ 3, 61, Hauffe [ 71 Aigrain and Dagas [ 81, Weisz [ 91, Garrett [ l o ] and several others referred to in those publications. The basic idea is simply that electrons are transferred to or from the semiconductor from or t o the reactants, so that some parts of the overall interacting system are donors and some acceptors. If we consider first the adsorption stage, there is electronic equilibrium between the semiconductor and the adsorbate, which is partially ionized and some fixed amount of charge will be transferred, forming a surface dipole. Where a complete surface reaction is involved, each separate charge-transfer equilibrium will be upset to some extent by the simultaneous presence of other adsorbates, but the net flow of charge to or from the semiconductor is zero. There is also a steady state flux of neutral atoms or molecules between the surface and the gas phase. It is, however, the process of charge transfer which is of particular interest in this discussion, since even though it may not be the rate-limiting reaction step, it is the one which depends on the electronic properties of the semiconductor. The original version of the model assumes a semiconductor adsorbent with no intrinsic surface states, so that before adsorption occurs the bands are flat to the surface. Wolkenstein [6] even refers to “weak” and “strong” chemisorption; with the former an adsorbed molecule is bound only by covalent forces, whilst, with the latter, charge exchange with the semiconductor takes place. The important point is that the model does not stipulate that the chemisorption bond must be completely ionic. The approach most in keeping with current ideas was developed by Krusemeyer and Thomas [ 111, who considered intrinsic surface states, which produced band bending, and showed that following adsorption they are replaced by states characteristic of the adsorbed material.

183

Whilst in a qualitative sense this theory has a certain basic validity, it does not provide very real physical insight into the electronic and structural effects now known to be associated with adsorption on semiconductors. In the following sections, we shall attempt to show how the subject has been developed from the much greater understanding of semiconductor surfaces which is now available.

2. Experimental approach The general topic of surface analysis has already generated several books and some hundreds of review articles, so it is clearly outside the scope of this chapter t o provide a comprehensive discussion of all of the techniques presently available for the study of clean surfaces and adsorption/desorption processes. Instead, we shall consider a few of the methods most relevant to semiconductors which have perhaps been treated less extensively elsewhere. We shall also concentrate on the type of information available rather than the details of its production. In Fig. 1, we have attempted t o illustrate the complete range of surface evaluation techniques, from which we see that a surface can be probed by electrons, ions, photons, neutral particles and phonons (temperature programming), and the products of the resulting interactions analysed in various ways. Fortunately, the bulk of information on semiconductor surfaces has been obtained from the application of a very limited selection from this range of techniques, which we summarize below. They include Auger electron spectroscopy (AES), low energy electron diffraction (LEED), reflection high energy electron diffraction (RHEED), ultraviolet and X-ray photoelectron spectroscopy (UPS and XPS), molecular beam scattering and thermal desorption. For semiconductor device structures, the various modes of operation of the scanning electron microscope (SEM) have been used extensively, but this is outside the scope of this article. References 12-18 are to selected books and review articles which give general surveys of most methods of surface analysis. 2.1 SURFACE CRYSTALLOGRAPHY. DIFFRACTION TECHNIQUES

We shall be concerned principally with electron diffraction, but recently some important results have been obtained using neutral atom (He) beam diffraction, to which we make brief reference in Sect. 3. The applications of interest are t o the determination of the crystallography of clean surfaces and also of surface-adsorbate systems. 2.1.1 Low energy electron diffraction ( L E E D )

This is by far the most widely used method of obtaining surface crystallographic information and as such has been extensively reviewed [19261. Details of experimental techniques are included in most of these References p p . 280-289

184 R BS LEIS SlMS \

ellipsometry Ramon scattering

n

‘I

Moleculair beam scatter i rm reactive and non-teactive (including diffraction)

Fig. 1 . Experimental techniques available for surface studies. SEM = Scanning electron microscopy (all modes); AES = Auger electron spectroscopy; LEED = low energy electron diffraction; RHEED = reflection high energy electron diffraction; ESD = electron stimulated desorption; X ( U)PS = X-ray ( U V ) photoelectron spectroscopy; ELS = electron loss spectroscopy; RBS = Rutherford back scattering; LEIS = low energy ion scattering; SIMS = secondary ion mass spectrometry; INS = ion neutralization spectroscopy.

articles and Fig. 2 shows typical arrangement. The evaluation of surface structures has two aspects: the determination of the space group symmetry (Bravais lattice) and the determination of the unit cell which occupies the individual points of the Bravais lattice. In principle, the former can be obtained simply from the directions of the diffracted beams, whilst an intensity analysis is required t o obtain the unit cell structure. This is valid for X-ray diffraction from solids, but for surfaces the geometric information is complicated by the loss of translational symmetry normal t o the surface. The periodicity parallel to the surface, however, is reflected in the law of conservation of parallel momentum

k;

=

kj

+g

where k; is the surface parallel component of the propagation vector ko which has magnitude 277/X ( X = electron wavelength), k/; is the surface parallel component of the propagation vector of a diffracted beam and g is a reciprocal lattice vector of the Bravais net characterizing the translational symmetry parallel t o the surface. The spot pattern relating t o

185

Fi Qment

Fig. 2. Arrangement of a display-type LEED system. Primary electron energy range 20-500 eV ( hX 2.7-0.55 A). The incident beam is at near normal incidence and the diffracted beams are transmitted through spherical grids biassed to remove inelastically scattered electrons. The screen is biassed to 5 kV to enable the elastically scattered electrons to be accelerated sufficiently to produce spots on the fluorescent screen.

eqn. (1)is therefore a manifestation of the space group symmetry of the lattice parallel to the surface. For the usual case of a normally incident primary beam (k9 = 0), the directions of the diffracted beams correspond directly to the reciprocal lattice directions. If we combine the definition of elastic scattering

k; = ( 2 r n E / h 2 ) 1 ' 2sin 0

(2) where E is the primary beam energy and 0 the exit angle, with eqn. (l), we can derive an expression for the exit angles as a function of primary beam energy and unit cell spacing parallel to the surface. The cell dimensions parallel to the surface are therefore obtained directly from the diffraction pattern, but there may not be a unique third cell dimension, since not only may the structure not repeat normal to the surface, but also the surface normal plane spacings may vary into the bulk, i.e. there may be surface dilation or contraction. The determination of the packing sequence and layer spacing of the top few atomic layers (the distance over which LEED information is produced) does, in fact, require the measurement and analysis of the diffracted beam References pp. 280-289

186

intensities. In addition, if a valid intensity analysis is available, the position of atoms within the unit cell, and hence the complete surface structure, can be obtained. This analysis is, however, a very complex problem, because there is both a very strong inelastic interaction between the incident electrons and the valence electrons of the upper 2-5 layers of the solid, and very high elastic electron-ion core cross-sections, which make multiple elastic scattering phenomena important in analysing diffracted intensities. This dynamical effect makes the use of single scattering models (the Born approximation) inappropriate and means that the theoretical basis of X-ray crystallography [27] cannot be used in the evaluation of LEED intensities. Several reviews of the dynamical theory of LEED have been published 119, 28-30] and we will not pursue it here beyond stating the problem which has been tackled and the basis of its solution. An incident plane wave with wave vector k is incident on a flat, infinitely extended surface in which the atomic structure is perfectly two-dimensionally periodic. The extension normal t o the surface into the crystal is infinite. The problem is t o find the intensities of the back-scattered electron waves and the solution is obtained by first considering scattering by single atoms and then the multiple scattering between atoms. It has three steps ( i ) calculation of the multiple scattering inside a single atom, (ii) calculation of the multiple scattering inside an atomic layer, and (iii) calculation of the multiple scattering between atomic layers. Alternative approaches t o the intensity analysis problem have been proposed, all of which involve some degree of approximation. The quasidynamic method [31] includes all orders of multiple scattering between layers, but only single scatterings from atomic centres within a layer. The kinematic (single scattering) method [ 321 includes only one scattering from each atomic centre. These approximation techniques tend to be used, in the interest of computational economy, where highly complex surface structures are involved, but it must be emphasized that almost all surface structures so far determined have been evaluated on the basis of the dynamical method. However, for some semiconductor surfaces where there is a comparatively small e l e c t r o n a t o m scattering cross-section, some approximation methods may give reasonable results. Although in principle it is possible to analyse the intensity data using one of these methods t o find an unknown surface structure, in practice this cannot be achieved. Instead, the LEED data is used t o decide whether a particular proposed model of the surface structure is correct by comparison with theoretical intensity data derived for that model, i.e. any acceptable model must be able t o pass the LEED test. In addition to attempting to unravel basic surface crystallography, the other main application of LEED is the identification of surface defect structures [15, 331, including point defects, arrays of atomic steps, domain structures and facets.

187

For random distributions of point defects, such as vacancies or adsorbed atoms, the effect is for an increase in the overall background diffracted intensity at the expense of intensity in the pattern. Where the distribution is non-random, no uniform background is found, there is only additional intensity in the vicinity of the diffracted beams. Surface steps are revealed because LEED is only sensitive t o a few atomic layers, so that interference occurs between beams diffracted from neighbouring terraces due to horizontal and vertical shift. Depending on primary beam energy and direction, and the direction of the diffracted beam, the interference will be constructive or destructive. In the former case, there is no change of spot shape, but if the interference is destructive, spot splitting or broadening occurs. Consequently, it is important t o study the variation of spot shape with experimental conditions (especially primary beam energy) to derive the number and distribution of atomic steps. Additional diffraction spots can also be the manifestation of superstructures and domains where the periodicity of the surface atom arrangement differs from that of the substrate. A superstructure may be due t o regular, perhaps partial, occupation of available surface sites, or to periodic displacement of surface atoms with respect to bulk positions. An area containing a superstructure arrangement and having perfect periodicity within this area is called a domain. Finally, during certain processes such as film growth or thermal etching, new, essentially low index planes may be formed on part of the surface. They are referred t o as facets and make definite angles with flat portions of the surface. If they are large enough, their diffraction pattern is that of an independent, inclined face. The problem is more complex when they are small, but they can then be effectively treated as steps or domains. 2.1.2 Reflection high energy electron diffraction (RHEED) The alternative electron diffraction technique is referred to as reflection electron diffraction and uses a comparatively high energy primary beam (typically 10-50 keV) at a grazing incidence of = 1-3". There is, however, very little difference between LEED and RHEED in the depth of material probed, since, for example, a 50 keV electron at 3" incidence angle will have approximately the same momentum perpendicular t o the surface as a normally incident 150 eV electron. To obtain the complete space group symmetry with RHEED, however, it is necessary to use at least two primary beam directions. The geometric arrangement is illustrated in Fig. 3 and the information available from the angular distribution and intensity of the diffracted beams is similar in almost every respect to that obtained from LEED. The use of a grazing incidence beam does, however, reveal more details of surface topography, particularly if there are small asperities on the surface. References p p . 280-289

188

lncide beam Crystal

I

Fig. 3 . Geometric arrangement of a RHEED system showing zeroth- and first-order diffracted beams. Primary Zlectron energy range z 10-50 keV ( A z 0.12-0.055 8). Incident beam angle X 1-3 . In most cases, all of the electrons which are scattered (elastic and inelastic) into the angular range of observation are allowed t o reach the screen.

When such features exist, they are penetrated by the electron beam so the material is represented by a three-dimensional point lattice and diffraction only occurs when the Ewald sphere intersects a point. This produces a transmission-type spot pattern. For smooth surfaces, the diffraction pattern appears as a set of streaks normal t o the shadow edge on the fluorescent screen, due t o the interaction of the Ewald sphere with the rods projecting orthogonally t o the plane of the two-dimensional reciprocal lattice of the surface. The reciprocal lattice points are drawn out into rods because of the very small beam penetration into the crystal (2-5 atomic layers). We would emphasize, however, that despite contrary statements in the literature, the appearance of a streaked pattern is a necessary but not sufficient condition by which to define an atomically flat surface. Several other factors, such as the size of the crystal surface region over which the primary wave field is coherent and thermal diffuse scattering effects (electron-phonon interactions) can influence the intensity modulation along the streaks. The evaluation of crystal structure is complicated by precisely the same multiple scattering and inelastic scattering processes that occur in LEED. The experimental and theoretical aspects of RHEED have been reviewed elsewhere [23, 25, 341 and, in general, with a few notable exceptions [35--371, the technique has not been used very extensively for semiconductors. We have included it here, however, by virtue of its geometric compatibility with molecular beam scattering, which is being used increasingly t o obtain kinetic data from reactions on semiconductor surfaces (see Sects. 2.4 and 5). The combined arrangement makes it possible to

189

obtain crystallographic and kinetic information simultaneously during the course of a surface reaction. 2.2 SURFACE COMPOSITIONAL ANALYSIS. AUGER ELECTRON SPECTROSCOPY (AES)

This particular technique has been the subject of so many reviews and articles (see,,for example, refs. 12-17) that no details of its theory or practice need be repeated here. The important feature lies in its ability t o measure surface composition with a sensitivity of 1-0.176 for any particular surface atom (except H). In absolute terms, the accuracy is probably not better than ? 20% (unless very well-defined standards are available), but the relative accuracy can be better than k 576, with approximately the same reproducibility. AES has been applied in three areas of semiconductor surface studies. The simplest is the assessment of amounts of contamination present on a surface, and the absence of peaks other than those associated with the substrate is used effectively to define an atomically clean surface. I t should be realised, however, that there could still be up to 0.01 monolayer, or x 1013 atomscm-* of any element (hydrogen cannot be detected) on the surface, even though no additional Auger features are present. The second application is to the direct measurement of adsorptiondesorption processes using the Auger peak height of the particular element as a monitor. The principal limitation is the possible influence of the electron beam on the adsorbate, which can result in beam-induced desorption, adsorption or dissociation. The basis of electron-stimulated desorption (ESD) was established some time ago independently be Menzel and Gomer [38] and Redhead [ 3 9 ] . Electron impact causes Franck-Condon transitions of bound electrons in the adsorbed species into excited states. There is, therefore, a probability of dissociation with subsequent desorption of the particular species involved. As an example of these effects on semiconductor surfaces, Joyce and Neave [ 401 have reported results on silicon, while Ranke and Jacobi [ 411 have discussed the electron-stimulated oxidation of GaAs. AES has also been used to assess the surface composition (stoichiometry) of 111-V compounds and alloy semiconductors [ 421 . In principle, the relative peak heights from transitions involving the constituent elements can provide this information, but again some caution must be exercised in the interpretation. The information depth, particularly for the higher energy (2 1keV) electrons, is not restricted to the surface layer, and in materials such as GaAs the thermal history of the substrate can have a serious effect on the cation :anion ratio in the outermost layers [43]. Unless any treatment prior t o the AES measurement is precisely defined, the ratio has no significance and, in general, AES measurements of this type do not provide fundamental information and can only be used to monitor specific surface and sub-surface effects. References p p . 280-289

190

2.3 SURFACE ELECTRONIC STRUCTURE. PHOTOELECTRON SPECTROSCOPIES

The various forms of photoelectron spectroscopy presently available permit a straightforward determination of occupied and unoccupied surface states. The most comprehensive and authoritative collection of reviews is in the book edited by Feuerbacher et al. [44],while Ertl and Kiippers [ 151 also provide useful information. Here, we will only attempt to summarize how the principal versions of the technique can be used in the determination of surface electronic structure. In this context the crucial factor is that photoemission spectra represent a direct manifestation of the initial and final density of states of the emitting system. Because selection rules (matrix element effects) can be involved in the transition, the state densities may not always correspond to those derived from the band structure, but in practice there is frequently a rather close correspondence. If we consider first photoelectron energy distribution spectra, in which radiation of a fixed wavelength is used t o generate photoelectrons which are detected over wide (up to 2 n steradians) angular ranges (“angle integrated”) and energy analysed, it is simply occupied electron states which are being probed. The general geometric arrangement for photoemission experiments is shown in Fig. 4, where the direction of emission is defined by the polar angle 4, and the azimuthal angle, 8. If now the direction, or momentum, as well as the energy of the photoemitted electrons is measured (“angle resolved photoelectron spectroscopy”, ARPES), the wave vector of the free photoelectron is determined. To relate this photoelectron t o its initial and final state in the solid, the surface parallel momentum component, k , , is conserved during photoemission, and thus k , of the initial state is determined by

k , = (2rnEk/h2)1’2sin q5

(3)

hw

Fig. 4. Direction of photoemitted electrons, defined by the polar (@)and azimuthal

(8)angles, which provide information on the momentum distribution of electronic states.

191

where Ek, the measured kinetic energy of the electron is given by E k = hv Ei, in which hu is the incident photon energy, CP the work function of the solid and Ei the initial state energy ($I is the polar angle). For occupied surface states the two-dimensional band structure E (ki) can therefore be determined. Occupied surface states are detected with much greater sensitivity by angle resolved measurements than by angle integrated, since by measuring only those electrons having an energy with kl matching that of a surface state, there is strong emission from the surface state but bulk emission is suppressed. The alternative approach involves the so-called photoemission yield spectroscopies, which enable empty surface states t o be probed. In these, the incident photon energy is varied while the electron energies are or are not resolved. The technique can firstly be used t o investigate transitions between core levels and empty states, usually by using synchrotron radiation so that sufficient photon energy is available for the excitation. Core levels have negligible dispersion in k-space, so the measurement reveals the unoccupied conduction band and surface state transition state densities, since electrons are generated by optical transitions from a core level t o either empty surface states or conduction band states. By using much lower energy photons, Guichar e t al. [45] have developed a high sensitivity technique which probes transitions between occupied surface and bulk states and states at or above the vacuum level. The partial yield of electrons in an energy window A E at a fixed final state energy E , as a function of photon energy, where E is fixed at < 5 eV so that only secondary electrons are measured, is referred t o as partial yield spectroscopy. When E > 5 eV, although the technique is experimentally identical, it is used t o study initial state and excitonic effects and is known as constant final state spectroscopy. Constant initial state techniques require the photon energy and electron energy analyser t o be scanned synchronously so that h v - E is kept constant. In this way, the photon energy dependent partial yield of electrons in an energy window A E a t a fixed initial state energy E, = E - hv is measured. Core level t o empty surface state transitions are enhanced by selecting the appropriate Ei corresponding t o a minimum in the valence band emission. Considered from a very simple mechanistic viewpoint, the various photoemission yield techniques can be rationalized as follows. A core state (occupied state) is excited by photoadsorption and de-excited by one or more of three possible processes, viz. Auger transitions, direct recombination of surface excitations or direct recombination processes of excitons involving the valence bands. The electrons resulting from these processes are inelastically scattered and so generate secondary electrons. Therefore, by separating the optical excitations from Auger de-excitation - CP

+

References pp. 280-289

192

and recombination processes, yield spectroscopy measurements of either the secondary electrons (partial yield spectrum), the non-inelastically scattered electrons (constant initial state spectrum) or the total number of emitted electrons (total yield spectrum) show core level to surface state excitation features. Surface sensitivity is an intrinsic property of photoemission measurements. The incident light penetrates far into the solid, but the escape depth of excited electrons is very short (Fig. 5), although there are local variations related to direction-dependent band structure effects. Surface sensitivity can be further enhanced by appropriate choice of experimental parameters such as photon energy, angles of incidence and emission, etc., which take advantage of selection rules favouring surface processes. Finally, we can summarize general features of photon energy ranges and sources. Threshold (yield) spectroscopy uses energies around 5 eV to probe the region dominated by the emission threshold and to determine work functions and surface states. The UV photoelectron spectroscopy (UPS) range is effectively defined by resonance light sources with energies from 1 0 to X 4 2 e V , which spans the valence band of most materials. The upper energy range uses characteristic X-ray lines (0.1-5keV) to excite the electrons. The most important source is, however, synchrotron radiation [44,461, which provides a continuous spectrum over a wide energy range. The experimental distinction between UPS and XPS is therefore no longer necessary. In addition, the light is highly polarized, so it may be used to check wave function symmetry selection rules. 2.4 SURFACE KINETIC MEASUREMENTS

The two techniques which have provided most of the direct surface rate measurements of adsorption and desorption on semiconductors are

$ I , , , ,

=

I

1

10

lo2

o3

1

lo4

Kinetic energy (eV)

Fig. 5. Energy dependence of the escape depth of excited electrons showing the mean free path as a function of kinetic energy.

193 Substrate

Source

Detector (a)

(a)

Substrate

Source

Detector (b)

1

Identification of desorbed species Desorption r a t e Sticking coefficients j O r d e r s of chemical r e a c t)i o n s ) Thermal accommodation COeff IcientS Surface lifetimes Binding energies

Fig. 6 . Principle of modulated molecular beam measurements.

modulated molecular beam investigations and temperature-programmed thermal desorption. We will describe briefly the essential features of each.

2.4.1 Modulated molecular beam methods The basic principles are illustrated in Fig. 6 . A molecular beam produced from a Knudsen source is directed on t o a substrate and part of the desorbed flux is detected using a mass spectrometer. In surface scattering/ desorption studies, it is essential t o distinguish between signals in the mass spectrometer arising from background gases in the vacuum system and those produced by the desorbed species. This can be achieved by modulating either the adsorbed or the desorbed molecular beam and measuring the resulting time-dependent signal in the mass spectrometer. The simplest method of modulation consists of opening or closing an appropriately positioned beam shutter which produces a step function change in molecular beam intensity. This has been used by a number of workers [47491 but has severe limitations when used for volatile species because the resulting pressure fluctuations in the vacuum system can produce low frequency time-dependent signals in the mass spectrometer which d o not relate to the desorption process. This problem can be overcome by using periodic modulation of the molecular beams together with a synchronous detection system [ 50-521 . The technique can be further improved because of the fundamental connection between time and frequency domains so that signal averaging and Fourier transform technqiues can be combined in order t o extract the maximum possible information from the signal in the mass spectrometer [ 531. In analysing data from modulated molecular beam experiments, it is References p p . 280-289

194

assumed, often implicitly, that the behaviour can be treated as a timeinvariant linear system. In such cases, it is well established (see, for example, ref. 54) that the response, y ( t ) , to an arbitrary stimulus, x ( t ) , is given by

where h (7)is the response of the same system t o a unit impulse at time -7. An important consequence of this relationship is that only those frequency components present in the stimulus x ( t ) will be observed in the output y ( t ) .This can be seen directly by Fourier transforming eqn. (4) to give

(5) where Y(f), H(f) and X(f) are the Fourier transforms of y ( t ) , h ( 7 ) and x ( t ) ,respectively. The complex convolution integral is reduced t o a simple product and it is obvious that an excitation at a frequency f only results in a response at the same frequency. If, as in the case of gassurface studies, the overall response is determined by a number of processes, then the one of interest may be extracted by deconvolution techniques, which are particularly simple in the frequency domain [ 531 . In particular, attenuation and phase shift of the signal produced by the flight time of molecules from the modulator t o the detector and any non-ideal response of the detector may be taken into account [ 551 . Using these techniques, the following kinetic parameters can be determined. (i) In principle, the accommodation coefficient of an adsorbed species can be deduced by studying the effect of substrate temperature on the amplitude and phase of the signal in the detector. In practice, however, difficulties may be encountered if the detector used is non-ideal [ 561 . (ii) The surface lifetime of an adsorbed gas may be deduced since this will give rise t o a characteristic frequency-dependent attenuation and phase shift of the desorbed pulse. The shortest lifetime which can be measured is typically to s depending on the molecular flight times involved and the accuracy of the measurements. (iii) The sticking coefficient of incident molecules can also be measured from the frequency-independent attenuation of the signal in the mass spectrometer. The accuracy of these measurements is usually determined by signal-to-noise considerations and is typically within 1%.(iv) The order of chemical reactions can be studied either by using a small perturbation of the incident flux and changing the average surface concentration with a second unmodulated source, or by modulating the desorbed flux and measuring directly its intensity as a function of adsorption rate. Olander and Ullman [ 561 have shown that, even when a non-linear surface process is involved in the overall chemical reaction, an effective linearization is frequently imposed Y(f) = H ( f ) * X ( f )

*

195

by other reactions steps. The degree of non-linearity can be assessed directly when the various Fourier components of the signal are measured simultaneously [ 53 J . 2.4.2 Thermal desorption

Thermal desorption studies have the attraction of comparatively simple experimentation, but face severe problems in the evaluation of unambiguous, unique rate parameters from the measurements. The subject has been reviewed several times recently (see, for example, refs. 57-61), particularly in relation to gas-metal systems, so here we will concentrate on its specific applications to semiconductors, where it has been used almost exclusively to study metal absorbatesemiconductor surface interactions. Since this topic provides the subject matter for Sect. 5, we will limit the discussion in this section t o the basic experimental approach and available methods of data analysis. We will leave t o Sect. 5 the critical appraisal of the validity of these methods as applied to solid adsorbates, and the interaction models which have been postulated. The two primary objectives of thermal desorption measurements from semiconductors are the detailed physical understanding of surface kinetics and the derivation of practical mechanisms of adsorbatesurface interactions with a view to technological application (metalsemiconductor contacts, Schottky barriers, doping, etc.). Following adsorption at a particular temperature, thermal desorption may be performed either isothermally or by application of a programmed temperature rise, usually linear with time. Isothermal desorption involves a temperature jump, which is very difficult to achieve in practice, whereby the substrate temperature is rapidly raised from the adsorption temperature to some constant temperature at which desorption is monitored, the procedure being repeated for several different desorption temperatures. Alternatively, a programmed temperature rise is applied and desorption measured continuously with time (and hence temperature). This is experimentally simpler than the isothermal method, but the detailed analysis of desorption spectra is complex, often leading to ambiguities. In principle, adsorption energies, adsorption state populations and details of kinetic processes can be obtained. The basic problem, by whatever means the experiment is performed, is t o establish the correct form of the rate equation which describes the desorption. The usual procedure is to postulate the simple, empiric form described as the Polanyi-Wigner equation

-dn = n x v , exp dt

(-z) AE

where n is the surface coverage, x is the order of the desorption process, u, is a frequency factor and A E is the desorption activation energy. For a References pp. 280-289

196

first-order process, there is an analogous rate equation, deduced by Frenkel [62], in which the inverse specific rate is defined as a time constant, 7, and the inverse frequency factor is T~ ; eqn. (6) then becomes 7 = T~

exp ( A E I k T )

(7)

Given such postulated, but not general, rate equations, a variety of methods is available for evaluating the rate parameters. We will consider the uniqueness or otherwise of the rate parameters subsequently, but first we will describe briefly the analytical procedures with specific reference t o their application t o metal-semiconductor systems. The simplest, and in many cases most accurate, is the peak temperature method first described by Redhead [63]. A E can be directly obtained by measuring the temperature, T p , at which the desorption rate is a maximum, then by setting d 2 n / d t 2 = 0, an expression for AE can be derived in terms of v, Tp and the known heating rate 0;v is usually assumed t o be 1013 s-l . A second approach is an initial state analysis, in which the high coverage, low temperature part of the spectrum is fitted t o a large linear constant and an exponential raised to a constant. The object is t o obtain the desorption activation energy for the high coverage limit before the surface is changed by the rapid heating. Not only is this method less accurate than Redhead’s, but, because with most metalsemiconductor systems the adatom-adatom interaction dominates the adatomsurface interaction, both of the above analyses only determine the enthalpy of evaporation of the metal. An alternative approach, referred t o as a “complete” analysis, has been discussed by King [ 591 and Bauer et al. [ 6 4 ] . The desorption spectra are analysed by a quasi-isotherm method, where the isotherms are obtained by measuring desorption rates and coverages at selected temperatures from a family of desorption curves corresponding t o different initial coverages. Although this method can be used at the low coverage, high temperature end of the desorption spectra, it then only gives information on the final state reached by the adsorbate during the application of the programmed temperature rise. No assumptions are made concerning the coverage independence of v and A E , but the Polanyi-Wigner rate equation is assumed, and in addition there is the implicit assumption that the order is constant at any fixed coverage, irrespective of the initial coverage. It will be seen in Sect. 5 that this is frequently not the case for solid absorbates. If we return now t o the question of the uniqueness of the rate parameters determined from thermal desorption measurements, we see that all of the analytical methods depend on the assumption of a rate equation whose validity, in general, is not tested. In particular, when there are adsorbate-adsorbate (lateral) interactions, or where desorption occurs via a precursor state, the coverage dependence in the pre-exponential term is not a simple function and the concept of reaction order is not meaningful.

197

Other effects which can influence the form of the rate equation include a coverage dependence of the activation energy and an apparent order which changes with coverage as a function of initial coverage. It is clear that great caution must be exercised in the interpretation of desorption spectra if a unique, unambiguous solution is to be obtained. Unfortunately, this is all too rare an occurrence, but the reader is referred t o the article by Petermann [ 571 for an excellent appraisal of the overall situation.

3. Atomically clean semiconductor surfaces To perform any meaningful adsorption, desorption or surface diffusion studies, the nature of the surface on which these processes are occurring must be adequately characterized in terms of structure, composition and electronic state. Ideally, it should be free from extraneous impurities and exhibit no segregation of bulk dopant atoms. Crystallographically, it should be of a single orientation, although it may well not have the same structure as the equivalent plane in the bulk, i.e. the surface may undergo some reconstruction. In addition, it is an advantage if it is smooth on an atomic scale and essentially free from steps. A detailed understanding of chemical bonding mechanisms in adsorption requires a knowledge of the electronic energy levels of the surface, which will, of course, be modified with respect t o the bulk, so the existence of specific surface states must be recognized. Such a surface provides a basis for the study of its interactions with other species; the effect of particular surface perturbations, e.g. atomic steps, can be considered once the ideal surface behaviour is understood. This section will begin with a discussion of the fundamental concepts of the electronic and crystallographic structure of semiconductor surfaces, followed by a description of the methods used to prepare surfaces in as ideal a state as possible experimentally. The emphasis will be on Si and GaAs as typical examples of elemental and compound semiconductor, respectively, and with which the great majority of published work has been carried out. We will conclude with some examples of the determination, experimentally and theoretically, of the electronic and crystallographic structure of specific surfaces of elemental and compound semiconductors. 3.1 ELECTRONIC STRUCTURE OF SEMICONDUCTOR SURFACES

We shall only attempt to introduce some of the more important physical concepts involved, since several excellent and comprehensive reviews already exist [65-711. The electron states in an infinite periodic solid are described by Bloch functions, and are therefore non-localized, extending over all of real space. The introduction of a surface imposes a spatial restriction in one direction References p p . 280-289

198

and the electron wave functions are modified close to the solid-vacuum interface. New electron states may then be allowed at the surface having energies corresponding t o a band-gap in the bulk. Since periodicity in the directions parallel t o the surface is not affected by the one-dimensional restriction, the electron state in the presence of a surface can be characterized by a two dimensional Bloch wave vector k,, which is uniquely defined within a polygon representing the surface Brillouin zone. The surface states may thus be delocalized in the directions parallel to the surface, but they are localized at the vacuum-crystal interface, and have wave functions which decay into the bulk. All electron states at a surface, comprising those bulk bands which extend t o the surface plus localized states, are described by a local density of states (LDOS) which is given by

where $ E is an eigenstate of the system at energy E . If the real space co-ordinate, r , is limited to the surface, eqn. (8) defines the surface LDOS. For any particular k i , a surface state may only occur in a bulk band gap, and its dispersion relation ~ ( k y is) defined by following it as a function of k l . I t may well overlap with bulk states for other parallel wave vectors, since kl is a good quantum number. A state can therefore be a surface state for a limited range of kl and be degenerate with bulk bands for other values of k, . In relation t o the localization of charge in a surface state, or how its charge is distributed between the surface region and the tail in the bulk, if a surface state is near the middle of a wide gap, it will be highly surface localized, whilst if the gap is small or the state has its energy near the edge of the gap, a significant fraction of the charge will be in the decaying tail. Where an electron state exists as a surface state for a limited range of k,, being degenerate with bulk bands for other k, , the state can decay into the bulk, and is called a surface resonance, since kl is not a good quantum number. In calculations of surface states, self-consistency is an important consideration, i.e. whether the charge distribution arising from the calculated electron states is consistent with the potential put into the calculation. Results of self-consistent calculations give true local state densities, including bulk and surface states, and the distinction between surface resonances and surface states becomes irrelevant. The methods used to calculate surface states need not concern us here in any detail, but i t will be instructive to give a brief indication of the two approaches currently employed (self-consistent calculations of the electronic energy and surface potential and realistic tight binding models), since this will provide some insight into semiconductor surface bonds and hence into chemisorption.

199

3.1.1 Self-consistentpseudo-potential calculations In principle, surface atomic and electronic structures are both available from self-consistent calculations of the electronic energy and surface potential. Until recently, however, such calculations were rather unrealistic, being based on a one-dimensional model using a square well crystal potential, with a semi-infinite lattice of pseudo-ions added by first-order perturbation theory. This treatment could not adequately describe dangling bond surface bands. Fortunately, the situation has improved enormously as the result of an approach due t o Appelbaum and Hamann (see ref. 70 and references cited therein), which is based on the following concepts. (i) As a result of their diatomic, low co-ordination number structure, semiconductors tend t o behave like macromolecules, especially with regard to valence bands, which determine the energy position of occupied surface states and hence the self-consistent surface potential. The molecular character of semiconductors becomes especially significant at the surface, as Shockley [ 721 originally indicated. He pointed out that the occupied valence states of a semiconductor were separated from the unoccupied conduction band states by an energy gap and that in diamond structure materials the valence states corresponded to an s p 3 hybridization configuration different from the s 2 p* atomic valence configuration. A necessary consequence is that surface states would occur in the energy gap and the ratio of the number of surface state bands to the number of bulk valence bands is equivalent to the ratio of the number of dangling bonds per unit cell to the total number of bonds in the corresponding unit cell. These concepts have the additional benefit of linking chemical bonding and surface states. (ii) The energy bands of semiconductors are known in considerable detail over an energy range of several Rydbergs and for the technologically important materials they are better known than for all molecular systems except H:. Consequently, bulk energy bands can be used t o construct atomic pseudo-potentials which are very accurate in the bulk, so that at the surface it is only necessary to determine the self-consistent valence contribution to the crystal potential. Since both bulk and surface states are molecular in character, the wave functions of atoms in both types of position can be calculated by the same method. Appelbaum and Hamann [ 701 assume two-dimensional periodicity along the surface and make the same Fourier expansion of the pseudo-wave function as for the bulk, except that at each of a set of discrete surface normal co-ordinates a different set of expansion coefficients is used. These sets can be integrated from outside the surface into the bulk. Well inside the bulk, these wave functions are matched t o bulk states of similar lateral symmetry and the matching condition determines energies and wave functions. References p p . 280-289

200

The result of such calculations show that, on a clean semiconductor, surface atomic sites in equilibrium always differ substantially from those of a semi-infinite lattice and there is an inward force on these surface atoms, since the presence of a dangling bond on a surface atom strengthens the back bonds to atoms in the second layer. This means that the back bonds assume some double bond character (i.e. the bond order becomes greater than unity). The consequent change in bond length leads to so-called surface relaxation (see Sect. 3.3). 3.1.2 Realistic tight-binding calculations

Tight binding (TB) and linear combination of atomic orbitals (LCAO) methods represent the more chemical approach to the problem of surface state calculations. They are basically fitting techniques, but, given a reasonable choice of parameters, they can add considerable detail to the basis provided by self-consisten t calculations. The method, as applied to surfaces, was initiated by Hirabayashi [73] and developed into a useable form by Pandey and Phillips [ 74, 751. The major problem with tight-binding calculations is the change of parameters from bulk values when the surface relaxes, causing first and second nearest neigh b o w bond distances to change. This is resolved, however, by the availability of self-consistent calculations for relaxed surfaces. The method as developed by Pandey and Phillips assumes that the wave functions of a thin slab can be written as

where k, is the surface Bloch wave vector, Gj ( r - R m ) is an orthoganalized atomic orbital of either s- or p-symmetry about an atomic site R m . Seven parameters then characterize the matrix elements of the Hamiltonian and they are calculated from the mean squared error between TB and pseudopotential energy levels for the bulk at many points in the zone for the valence and lowest conduction bands, varying them to find the absolute minimum. To incorporate relaxation, they assume that the matrix elements M j k ( R m) between nearest neighbour orbitals $ j ( r ) and $ k ( r -+ R , ) depends on R m through a Hiickel relationship

Mjk(Rrn) = Mjk(R", exP [P(IRO,I - IRm I)] (10) where P is an empiric overlap parameter and RZ is the separation of orbitals j , k before relaxation. is fixed from self-consistent calculations. 3.2 CRYSTALLOGRAPHY OF SEMICONDUCTOR SURFACES. RELAXATION AND RECONSTRUCTION

A plethora of electron diffraction results amply testifies to the fact that clean surfaces of semiconductors undergo relaxation or reconstruction;

201

that is, the surface structure is characterized by a two-dimensional net with primitive unit meshes different from the bulk as the result of a phase transformation in which the surface atoms are displaced from the positions they would occupy if the bulk lattice were simply terminated. The references to the subject are far too numerous to quote individually, so we simply list a selection [76--811 in which the treatment is fairly general and where many references are given t o specific materials. In covalent materials, forming a surface by division along a lattice plane breaks directional bonds, so atoms close to the surface cannot recover the energy to form stable tetrahedral s p 3 orbitals by bonding to their neighbours. These atoms therefore rearrange themselves to obtain more covalent bonding energy. With ionic materials, the corresponding effect is the generation of unbalanced forces on ions close t o the surfaces, which also leads to surface reconstruction. Substantial reconstruction of semiconductor surfaces is therefore to be expected, a prediction well substantiated in practice. This contrasts with metals, where the spatially extended and much less directional bonding makes reconstruction an unusual occurrence. In elemental semiconductors and the polar faces of compound semiconductors, an odd number of electrons is formed per surface atom by the creation of a surface. The solid therefore undergoes a metal-insulator phase transition [ 821 t o produce an even number of electrons per surface unit cell, thus reducing its symmetry in the plane of the surface. For non-polar faces of compound semiconductors, the simple truncated bulk geometry is already insulating in character because anionic and cationic species are electronically inequivalent. No distortions which reduce the symmetry are therefore necessary to provide stability, but the unbalanced ionic forces and unsaturated covalencies can produce quite large (“ 0.5 8 ) atomic movements (“surface relaxation”). Thus to define the atomic geometry of a clean semiconductor surface, it is necessary to determine (1)the depth of the reconstructed layer, ( 2 ) its structure, and ( 3 ) its registry with respect to the underlying substrate. 3.3 PREPARATION O F CLEAN SURFACES

Three methods have been used to produce clean surfaces, with varying degrees of success. The simplest involves heating to some prescribed temperature under UHV conditions. An alternative, if heating alone is inadequate, is rare gas ion bombardment followed by annealing in UHV to remove the crystallographic damage introduced by the bombardment. Finally, to obtain a specific crystal plane, in situ cleaving in UHV, sometimes followed by annealing, can be used. The degree of cleaning achieved is most commonly measured by AES, although both LEED and SIMS have also been used. As a general rule, surfaces are defined as being atomically clean when the level of contamination is less than 0.01-0.001 monolayers, References p p . 280-289

202

i.e. the detection limit of the assessment methods. Each of the cleaning techniques has various limitations depending on which semiconductor is being considered and it will be instructive to consider silicon (germanium is closely similar) and GaAs as particular examples.

3.3.1 Silicon It is comparatively simple to obtain a clean silicon surface by heat treatment alone. The usual contaminants are carbon and oxygen; the latter is removed as SiO, at temperatures > 1100 K according to the reaction SiO,(,,

+ Si,,,

+

2 SiO,,,

although oxygen is not desorbed as such in either the atomic or molecular form [83]. T o remove carbon, it is necessary t o raise the temperature rapidly t o a value a t which the diffusivity and solid solubility of carbon in silicon are high enough t o enable it t o diffuse into the bulk and not precipitate as 0-Sic a t the surface. Typically, temperatures necessary t o remove carbon are 2 1450 K [ 84,851 . Although heat treatment can produce surfaces with < 0.001 monolayers of residual impurity, two effects can occur during the cleaning process which modify the surface significantly, one being topographic, the other electrical. Dealing first with topographic changes, usually referred t o as thermal etching, three apparently different effects have been reported, but they can probably be rationalized. Thus, either n o changes occur, or irregular pits are produced, or the pits assume a crystallographic form which depends on the particular face being cleaned [86-901. It has been found [ 911, however, that pits are only formed if carbon is present on the surface for fairly lengthy periods during heat treatment (i.e. the temperature is too low). They become crystallographic, usually with (311) facets when the partial pressure of oxygen and/or water vapour exceeds torr during heating. Heat treatment at temperatures 2 1450 K generally does not introduce topographic changes. Uncontrolled electrical changes occurring in the surface regions of silicon substrates during heat treatment in vacuum are rather more difficult t o avoid and are also less readily detected than topographic changes. It was first noted by Allen [92] that silicon surfaces became strongly p-type on heating in vacuum system constructed from borosilicate glass, irrespective of the initial characteristics of the silicon used. For temperatures above 1270 K, a surface layer could contain between lo', and 1015 acceptor atoms ern-,. The mechanism suggested by Law [93] and confirmed in greater detail by Allen e t al. [94] is that boron is transported from the glass to the silicon surface as volatile H3BO3 or HBO, by reaction between water vapour and B 2 0 3 in the glass. Acceptor concentrations as high as 1019 cm-3 have been measured after heat treatment.

-

203

However, the effect is not confined t o borosilicate glass vacuum systems, but has also been observed with stainless steel systems [ 95, 961 , although it can be avoided if the bakeout stage is omitted. This phenomenon is frequently ignored because, unless a direct electrical assessment of the surface region is made, it may not be apparent. In the work quoted above with stainless steel systems, the effect was evaluated by forming either Schottky diodes [96] or MOS capacitors [95] and performing electrical measurements in situ. Inert gas ion bombardment was widely used as a cleaning technique in much of the early work on silicon surfaces [86, 88, 93, 971. Ion beam current densities between 1 and lo3 pA c m - 2 , with energies from 200 eV t o sz 1.3 keV were used, combined with a wide range of outgassing, bombardment times and annealing treatments. Substantial crystallographic damage is introduced by ion bombardment, but an annealing temperature of 1000 K was fairly commonly believed t o be sufficient t o remove it. However, this is almost certainly too low a temperature, since it has been shown by transmission electron microscopy that many dislocation loops resulting from vacancy condensation are still present after this treatment [ 9 8 ] . Statements t o the effect that “sharp” LEED patterns are produced by annealing for 1 0 min at 1200 K have been made [ 9 9 ] , but the “sharpness” of a LEED pattern is not really an adequate criterion by which t o judge crystal perfection. In addition t o the direct crystal damage, inert gas ions become trapped in the surface region and, although many are desorbed on annealing, it is an activated process and those bound in deeper states may not be released. There is no question that ion bombardment can produce a clean surface; it is the crystallographic perfection and the extent t o which the defects may introduce electronic states that is open t o doubt. A final problem which has been reported [ l o o ] is that bombardment and annealing can cause surface roughening. Cleavage involves creating a fresh surface in the UHV chamber and, provided that the base pressure is low (
-

References pp. 280-289

204

t o the use of L-sectioned samples. The surfaces produced show cleavage steps ranging in height from 50 t o 500A, separated by a distance of l p m . The areas between steps are flat within the limits of resolution of Pt-C replica electron micrographs. An alternative technique [ 1021 uses a 30" wedge into a notch in the crystal which has a similar notch in the opposite face, backed by a larger angle wedge. The cleavage occurs along the (112) direction in a {111}plane for Si. No details are given of the structure and topography of the surfaces produced. The structure, as determined by LEED, of a freshly cleaved {111}Si surface is different from that of a clean (111)surface produced thermally and the cleaved surface structure appears t o be metastable in that it can be irreversibly transformed to the heat-treated structure. This will be discussed in Sect. 3.4. An associated technique, used t o produce a large exposed surface area for electron spin resonance (ESR) or adsorption studies involved crushing the silicon in UHV [ 103, 1041. However, this obviously introduces a large amount of mechanical damage and it is by no means certain that effects due t o this can be separated from pure surface effects. For the sake of completeness, field desorption should be included, although it is limited in its application to FEM/FIM studies. Allen [lo51 was able t o clean a silicon emitter tip with a combination of field desorption (a field strength of 1-2 x lo8 V cm-' ) and heating to 1275 K.

-

-

3.3.2Gallium arsenide In principle, the same basic methods of heat treatment, ion bombardment and cleavage which are used t o produce clean silicon surfaces can be used t o generate clean GaAs surfaces, and the same general reservations apply. However, the fact that GaAs is a compound whose surface stoichiometry is potentially variable introduces additional problems for those techniques which depend on removal of material. Cleavage is not subject to these effects, however, and the cleavage plane is {110},which contains equal numbers of Ga and Aa atoms. Furthermore, the cleaved surface structure does not appear t o be metastable, at least in terms of the LEED patterns produced [106],so in some ways it represents a simpler case than silicon. Ion bombardment could result in the preferential sputtering of one element, but there is n o direct evidence for this [ 1071 . In order t o anneal out the damage, however, temperatures > 1OOOK are necessary, so stoichiometric changes can still occur and for this reason, ion bombardment with much lower temperature anneals (- 750 K) has frequently been used (e.g. ref. 107). It is known, however, that this treatment produces a surface with substantially modified properties, leading, for example, to changed evaporation behaviour [ 1081. There is n o doubt that heat treatment can result in the preferential loss

205

0

800

900

1000 Temp.( K )

0

1100

Fig. 7 . Temperature dependence of the ( 7 5 A ~ + / 6 9 G aratio + ) from GaAs heated under Langmuir evaporation conditions (after Foxon et al. [ 1091 ).

of the Group V element and in the case of GaAs and most of the 111-V compounds this occurs at temperatures lower than those required to produce a clean surface. 111-V compounds dissociate on heating and under conditions of free (i.e. Langmuir) evaporation, the species evolved are the monomer of the Group I11 element and the dimer of the Group V element [ l o g , 1101. Thus Ga and As, are produced from GaAs. The evaporation rate is not constant with time, probably due to surface roughening causing an increase in area, but up to a certain temperature, the evaporation is congruent, i.e. gallium and arsenic are lost stoichiometrically. This is shown in Fig. 7 where the ratio "AS' to 69Ga+ion currents as measured in a mass spectrometer from the flux evaporating from a GaAs crystal is plotted as a function of the crystal temperature [ 1091. An a.c. technique was used t o remove background effects occurring as a result of the volatility of arsenic. It can be seen that up t o 930 K the evaporation is congruent, but above that temperature arsenic is lost preferentially. Once this begins t o happen, major topographic changes occur, resulting in the formation of large gallium droplets and crystallographic etch pits. Some topographic effects can occur at even lower temperatures, notably the

-

References p p . 280-289

206

formation of (110) facets [ 1071 t o which the (TIT) B surface is the most susceptible, and the (111)A the least. An alternative technique for producing clean surfaces of any orientation of 111-V compounds and alloys is t o grow a thin epitaxial film (> 2000 A ) in situ from molecular beams of the elements generated from Knudsen sources inside the UHV system. The growth process has become known as molecular beam epitaxy or MBE [ l l l ] . Any impurities remain at the filmsubstrate interface and the freshly created surface is very clean. A more detailed account of this process will be given in Sect. 5. Finally, as with silicon, field desorption has been used on GaAs to produce clean field emitters [112] with fields of 1.4-2.7 x lo8 Vcm-' , followed by annealing at temperatures up t o 620 K. Temperatures higher than this produced considerable surface roughening and evidence of surface migration, occuring most readily on (TTT)B faces, was observed at 520 K.

-

3.4 DETERMINATION O F THE CRYSTALLOGRAPHIC AND ELECTRONIC STRUCTURE OF CLEAN SEMICONDUCTOR SURFACES

The aim of this section is to illustrate with definite examples the type of information which can be obtained on semiconductor surface structures. We will consider in some detail experimental and theoretical results for three specific surfaces, Si{111}, Si(100) and GaAs{llO}, which typify most of the important effects in relation t o both preparation and subsequent adsorption behaviour.

3.4.1Si (100) Early LEED studies [97] indicated that the basic structure of a clean (100) surface was 2 x 1. (For a discussion of the nomenclature of surface crystallography, see ref. 113.) Three different models have been proposed to account for this form of reconstruction: the vacancy model [78, 79, 1141 assumes that half the surface atoms are missing, while the other two suggestions invoke surface dimerization [ 80, 1141 and the formation of complex conjugated chain structures [ 115, 1161, respectively. We will consider each of these in a little more detail. The vacancy, dimer and conjugated chain models are illustrated in Fig. 8, by reference t o a non-reconstructed (100) surface [Fig. 8 ( a ) ] , where, in each case, the top two layers of atoms are shown. The two-fold periodicity then results either from removal of alternate rows of first layer atoms, which can be parallel [Fig. 8(b)] or perpendicular [Fig. 8(c)] t o the plane of the first-to-second layer bonds, or by displacement of adjacent rows of first layer atoms in opposite directions [Fig. 8(d)]. The conjugated chain model [Fig. 8(e)] described by Jona et al. (1161 derives from an idea proposed by Seiwatz [115] in which each atom forms u bonds with neighbouring atoms in the chain, a bent bond with the nearest

207

(C)

Fig. 8. Models of Si{lOO} surfaces. Surface unit mesh shown by broken lines. (a) Unreconstructed surface; ( b ) vacancy model ( i ) ; ( c ) vacancy model ( i i ) ; ( d ) dimer model; ( e ) conjugated chain model.

neighbour in the second layer and contributes a fourth electron t o a IImolecular orbital along the chain length. The conjugated chains run along one of the symmetry directions in the surface plane and are separated by twice the bulk distance along the orthogonal symmetry direction. With all of these model structures, multiple domains can, in principle, exist on the surface since their symmetry is lower than a bulk {loo)plane. A 2 x 1 reconstruction represents a rather small surface net, so that Si(100) 2 x 1 could be a very suitable choice for a LEED structure analysis. It was established by Ignatiev et al. [117] that good agreement on quantitative LEED measurements could be obtained between different workers for surfaces prepared either by thermal cleaning at 1520 K or by ion bombardment and annealing. An important point, however, is that in these results only half-order reflections were observed, whereas some References p p . 280-289

208

workers have, in addition, seen faint quarter-order reflections [1 1 8 ,1 1 9 ], indicative of a c ( 4 x 2) periodictiy, and a 2 x 2 structure may also exist. If we restrict our attention initially to the detailed analysis of the 2 x 1 intensity data using either a full dynamical treatment [116] or constant momentum transfer averaging [120], then none of the three possible models provides a really satisfactory fit. The most encouraging agreement from LEED per se is with the conjugated chain model, but as we indicate below, this structure is inconsistent with photoemission results. The alternative structure showing some agreement is the Appelbaum and Hamann modification of the dimer model [ 801 , in which the stress produced by the top layer reconstruction is relieved by elastic distortions in the sub-surface layers. Jona et al. [121] published dynamical LEED calculations for this structure and showed that, while normal incidence LEED data gave better agreement with this model than with any other tested, there is virtually no correspondence at non-normal incidence. It must be concluded that none of the “ 2 x 1 models” adequately describes a Si(100) surface prepared either by thermal cleaning or by ion bombardment, but it should be realized that they are both very drastic treatments. If we now consider the surface electronic structure, self-consistent calculations have been carried out for all of the proposed models. Kerker et al. [ 1221 treated the conjugated chain model (as well as the ideal surface formed by simple termination of the bulk), but in spite of the good LEED agreement with this model, i t did not give any consistency between their calculations and the experimental photoemission data [ 1231 . This lack of agreement derives directly from the essential feature of the model, the formation of atomic chains in the surface plane, and is manifest in two ways. (i) Broken bonds are formed which are associated with atoms in different layers and so cannot take advantage of an attractive bonding potential. They introduce overlapping surface states in the energy gap, and so the surface becomes semi-metallic, contrary to experiment [124]. (ii) States derived from bonds in the conjugated chain occur 5 eV below the bottom of the VB edge and cause a peak in the LDOS, but the experimental photoemission spectrum shows a dip in this energy range [123]. Appelbaum et al. [ 125-1291 have made detailed self-consistent calculations for both the vacancy and bond-pairing models and their results are summarized in Fig. 9, which compares the calculated LDOS for the two models with photoemission data [123], from which the bond-pairing model is apparently more appropriate. Experimentally, the suppression of emission from the s-band region below - 5eV is a matrix element effect, but in the p-band region from 0 t o - 5 eV, the experimental spectrum shows a rapid rise in emission below threshold, a symmetric peak at - 2.5 eV and a shoulder a t - 4 eV, which provide a closer fit t o the pairing model. In addition, the - 7 eV peak is reproduced by the pairing model but split by the vacancy model. The physical origin of the features was determined by calculating the DOS at several points within

209

Photoe misson

e

0

Pairing model

I

I

-12

1

1

-10

-8

I

-6

I

-4

I

-2

I

0

Energy ( e V )

Fig. 9. Calculated surface density of states for two Si (100) 2 X 1 models compared with photoelectron density after subtraction of secondaries (after Appelbaum et al. [1261).

the surface region. The surface enhancement in the 0-1 eV range derives from the broken bonds being pulled down by the formation of a II-band and the - 7 eV peak also has a large back bond contribution. The pairbond spectrum peaks strongly at - 2.5 eV and - 9.5 eV (because there is charge in the II-band states, the pair bond is stronger than asingle bond, which causes a shortening of the bond length). However, the band structure for the gap states showed them to overlap in energy, resulting in a metallic surface band structure, which even further shortening of the pair bond did not eliminate. Not only is this energetically unfavourable (which Appelbaum et al. [130] suggested could lead to charge density wave formation and account for the additional quarter order reflections), but more important, it is quite contrary t o very recent ARPES data [ 1241. Chadi [ 1311 has attempted to resolve these inconsistencies by testing many different surface structural models using an energy minimization scheme. He finds the basic idea of surface dimerization to be correct, but the simplest “symmetric” model in which the in-plane displacements of the atoms forming the dimer are equal and opposite is unstable, even though sub-surface relaxations are included. A considerable lowering of energy (by as much as 0.23eVatom-’) is, however, obtained by the formation of asymmetric, partially ionic dimers. This structure also gives semiconducting surface bands, with the highest filled surface states References p p . 280-289

210

0.18 eV below the bulk VBM and the lowest empty state 0.6 eV above it. Both states are predominantly p r -like, with some s-like contribution. This result is much closer t o the experimental data, but still shows some differences. In fact, no simple model is in complete accord with all experimental results. Using a quite different experimental technique, thermal He atom diffraction, Cardillo and Becker [ 119, 1321 have shown that both thermal cleaning and ion bombardment surface preparation techniques produce a secondary reconstruction, which is quite consistent with LEED results showing additional quarter-order reflections. I t is not possible t o order the surface and there is a near degeneracy for the different configurations of the secondary reconstruction, i.e. co-existing domains of 2 x 1, 2 x 2 and c ( 4 x 2). It thus appears most probable that conventional surface preparation techniques do not produce a simple, ordered (100 \ Si surface, so the lack of agreement between experimental results and theoretical calculations based on rather ideal models is not unexpected. High temperature treatment and ion bombardment are both very severe treatments, however, and it would be illuminating to investigate a reconstructed surface grown in situ, and then re-examine the models, but this has not yet been done. 3.4.2 Si (111 } The cleavage plane of silicon is {111)and a cleaved surface formed under UHV conditions is reconstructed in a 2 x 1 surface lattice. Above 750 K, this converts irreversibly t o a 7 x 7 structure, which is the same as that produced if a clean surface is generated by ion bombardment and annealing or by simple heat cleaning. Much of the earlier work on clean (111)silicon surfaces has been reviewed by Monch [ 1331, but it is worth noting here that of the many reported structures for this surface, these two are now firmly believed to be intrinsic, and not impurity stabilized. Several models bearing close similarities t o those formulated for (100) surfaces have been proposed t o account for the two structures. Lander and Morrison [ 1341 described the surface in terms of warped conjugated rings with 25% of the top layer of atoms missing. The vacancies formed an ordered pattern which reproduced the observed periodicity. The Seiwartz modification [ 1151 has conjugated chains rather than rings, while the thermodynamic arguments advanced by Phillips [ 781 in favour of vacancy-type models apply as well t o (111)surfaces as to (100). Bond pairing models for the 2 x 1 structure were first put forward by Lander e t al. [135], in which the pairing occurred either by one of the back bonds breaking and a double bond between surface atoms forming, or by just saturating the dangling bonds by a simple pairing in an ordered fashion, without breaking back bonds. A third type of model, involving only small vertical displacements of

21 1

surface atoms, has been proposed by Haneman et al. [ 136-1381. They pointed out that, because the tetrahedral symmetry is broken at the surface, the dangling bond cannot retain its ideal sp3 nature, but must undergo some dehybridization. A possible route by which this could occur would be for some surface atoms to be raised so that their dangling bonds becomes more s-like while others are lowered with their dangling bonds assuming more p z character. If this takes place in a regular fashion, then either a 2 x 1 or a 7 x 7 structure could be realized. Appelbaum and Hamann [139] have criticized the idea of an s-like dangling bond on energetic grounds (the s, p splitting of atomic Si is % 6 eV) and suggest instead that the dangling bond states associated with the raised atoms are sp3 like, thus making the surface ionic. A fourth possibility was suggested by Tosatti and Anderson [ 1401 , the basis of which was that reconstructions derive from an electronic instability of the ideal surface, leading either to a charge density wave plus lattice distortions (a kind of Jahn-Teller effect) or possibly to a surface density wave. More recently, Levine et al. [141] have obtained much more detailed LEED data which negates most, if not all, of the models for the 7 x 7 structure discussed above. They obtained intensity measurements at very small intervals over a wide range of primary electron energies and found the fractional order patterns t o change their intensity distribution markedly at 5 V intervals and to have a distinct three-fold rotational symmetry which reverses at % 20 V intervals, with an almost six-fold symmetry at some intermediate value. The observations are quite inconsistent with surface models involving a single terminating plane and Levine et al. advanced a new structural model to account for their results based on interference between two rippled surface double layers. The surface is almost ideally terminated and the model requires no surface vacancies or adatoms. The reasoning behind it is illustrated in Fig. 10. The dangling bonds which result from the sp3 hybridization of the bulk crystal are unstable and the surface atoms contract by relaxing inwards to create a more planar bond, approximating to s p 2 . This produces a weakening of the II back-bonds, so the bonding of the second layer to the third is relaxed, allowing in turn the third layer to relax towards the fourth, and the whole process continues with decreasing magnitude into the crystal. As a result of this partial change in hybridization, excess energy is stored in the surface region and produces a uniform hydrostatic pressure which can provide the driving force for surface deformation. The type of deformation proposed by Levine et al. is a periodic surface ripple which is damped into the bulk, mathematically analogous to standing surface waves and electronic surface states, and perhaps also to the distortions proposed for soft phonons and charge density waves [ 1401. This model, used as the basis for kinematical LEED calculations, successfully accounts for the variation of fractional order intensities References p p . 280-289

212

J"sr

1

bll]

7 atom p

t

F12]--

e

r

i

o

Layer

d

d

Fig. 10. Si (1 11 } surface models after Levine et al. [ 1411. (a) Ideally terminated lattice; ( b ) relaxation of surface atoms; ( c ) relaxation of sub-surface atoms; ( d ) periodic ripple (shown greatly exaggerated in amplitude) formed from residual stress in surface layers.

at 5 V primary energy intervals, for the periodic rotational symmetry reversals and for certain additional details in the LEED patterns. It probably represents the most self-consistent model presently available for the 7 x 7 structure. N o such detailed LEED observations have been made for the 2 x 1 reconstruction, however, and there is insufficient evidence on which to accept or reject the various models described earlier. It is nevertheless worth noting that there are certain close similarities between the above model for the 7 x 7 surface and those of Haneman et al. [ 136-1381 for the 2 x 1,so on this basis alone the Haneman ideas might be preferred. Turning now to the surface electronic structure of (111)silicon, it will be convenient to consider theoretical treatments which attempt to predict the nature of surface states that can occur. The first successful calculations (by Appelbaum and Hamann [144, 1451) dealt only with ideal and relaxed S i ( l l l } 1x 1surfaces, for both of which dangling bond states near the band gap were predicted. In addition, for the relaxed structure produced by an inward movement of the surface atoms, they found further bands of surface states near the bottom of the lowest valence band, a result subsequently confirmed by Pandey and Phillips [ 751 and by Schluter et al. [ 1461 . These latter two groups [ 75, 1461 , together with Batra and Ciraci

213

Energy (eV)

Fig. 11. Calculated density of states for a S i { l l l } 2 X 1 surface, (after Schluter et al. [149]).The energy zero is taken at the bulk valence band edge. clout are the states associated with atoms moving upwards out of the surface plane and d h those from inward-moving atoms.

[147, 1481 have also carried out more realistic treatments on 2 x 1 reconstructed surfaces, basing their calculations on various versions of the Haneman model [136-1381 for this particular structure. The effect of reconstruction is to split the dangling bond states into two, those states associated with the atoms which move upwards out of the surface plane falling in energy, while those associated with the downwards moving atoms are raised in energy (see Fig. 11, taken from Schluter e t al. [ 1491 ). Deeper surface states related to both transverse and longitudinal back bonds are also formed. A final point to have been considered theoretically by several groups [150, 1511 is the influence of surface steps on surface states and, more generally, the possible stabilizing influence of steps [ 1391 . Since steps are usually associated with cleaved surfaces, it is 1 x 1 and 2 x 1 structures which have been treated in this context, Schluter et al. [151], using a self-consistent pseudo-potential method, showed that dangling bond surface states are stongly affected when surface steps are present. New states whose wave functions are localized along the step edges are formed at lower energy, which gives rise to additional structures in the density of states near the fundamental gap. The effect occurs for quite moderate step densities, in that flat terraces 50 wide separate the steps. Because the dangling bonds associated with the step edge atoms have lower energy than the terrace atom dangling bonds, electrons are attracted t o the edge dangling bonds, creating metallic zones near the edges if no additional reconstruction occurs parallel to them. Terrace regions away from the edges retain their 2 x 1 reconstruction. Using a modification of the Haneman model in which the bonds between first and second layer atoms are alternatively lengthened and shortened as atoms are raised and lowered, Appelbaum and Hamann [139] point out that, although this is consistent with angle-resolved

a

References p p . 280-289

214

photoemission results (see below), it is both energetically unstable with respect t o the ideal 1 x 1surface and thermodynamically inaccessible once the transformation from 2 x 1to 7 x 7 has occurred. The 2 x 1structure is formed only by cleavage and they suggest that the stabilizing energy might be provided by steps in some unspecified way. The first experimental determination of surface states on clean S i ( l l 1 ) 2 x 1 surfaces was by Allen and Gobeli [152,153] and they found strong Fermi level pinning at the surface (EF - E , x 0.3 eV, where E , is the valence band maximum). Again, for the 2 x 1 surface, Eastman and Grobman [154] observed, by angle integrated UPS, a single surface state band centred x 0.5eV below the valence band maximum with a tail of states extending up t o the Fermi level and this result has subsequently been confirmed by several groups [ 155-1 581 . The basic result is illustrated in Fig. 12. Traum e t al. [159] have extended this work on 2 x 1surfaces by using angle-resolved UPS. The strong three-fold symmetry pattern (Fig. 13) which they observe is inconsistent with direct emission from a pure p z dangling bond orbital, and the surface state dispersion E versus k , in a (112) direction shows a band splitting inconsistent with a simple dangling bond state. I t seems probable, therefore, that the surface state features near the valence band maximum involve both dangling and back bond orbitals. Rowe et al. [ 1601 have also demonstrated a direct relationship between surface states and surface steps by identifying a new peak x 0.4 eV higher in energy than the main peak when the step density on a cleaved surface exceeds 10%. This observation is completely consistent with the calculations of Schluter et al. [ 1511 .

Filled surface

Energy distribution N(E , h v - 12eV)

-6

-5

-4 -3 -2 -1 Energy below E,(eV)

0.E"

Fig. 12. Photoelectron energy distribution for Si {111}2 x 1 . The filled surface state curve represents the difference between clean and oxidized surface curves and depicts the optical density of intrinsic surface states (after Eastman and Grobman [ 1641).

215

Fig. 13. Radial plot of azimuthal dependence of photoemission intensity from a Si {I 11 } 2 x 1 surface (after Traum et aI. [ 159] ). Surface state emission intensity from a clean surface is shown by the open circles (in the energy window - 1 . 7 eV to E F ) . The filled triangles show, on a 10 X reduced scale, the remainder of the photoemission intensity.

For the 7 x 7 reconstructed surface, two occupied surface state bands near the valence band maximum have been identified by angle-integrated UPS [155--158, 1611. This contrasts with the single structure on the 2 x 1 surface. The dominant feature is about 1eV below the Fermi level and, in addition, there is a metallic edge-like structure at EF . Thus, even for the simplest elemental semiconductor, a complete picture is not yet available for the surface crystallography or the surface electronic structure. Somewhat surprisingly, the cleavage plane of a 111-V compound semiconductor, i.e. (110) GaAs, is much more completely characterized and we will consider this next as our final specific example of a clean semiconductor surface. 3.4.3 GaAs (110)

Surfaces are prepared by direct, in situ cleavage, with no ion bombardment and anneal or heat treatment. However, the quality of the cleave is of vital importance to the electronic structure, as we show subsequently. The characterization begins with an evaluation of the crystal structure of the clean (110)GaAs surface. The first work in this area was by MacRae and Gobeli [ 1061 who observed, in common with all subsequent LEED investigators a unit mesh at the surface having the same dimensions as that of the bulk. However, a strong asymmetry in the intensity of the hk and hE diffracted beams showed that the detailed atomic arrangement at the surface could not be the same as in the bulk crystal. This asymmetry was ascribed to bond rotation by Levine and Freeman [162] and more recent work has developed this basic idea in considerable detail [163--1671. The essential feature of all of the models is the occurrence References p p . 280-289

216

of surface buckling, such that the cations move into the surface plane, while the anions move out slightly. Thus, the Ga atoms assume an almost planar sp2 co-ordination with th,eir nearest As neighbours, while the uppermost As atoms move outwards into a pyramidal structure with their three nearest Ga neighbours. Details of the exact configuration are subject to further refinement, but two slightly different versions have been proposed. In one [166], the surface buckling is achieved by a rather large rotation, w , of the Ga-As surface bonds (34.8' < w < 27"), and slightly expanded back bonds (34.8' corresponds to the maximum possible rotation without change of surface bond length). In the other [167],o is rather smaller (" 20') and the back bonds are contracted. The important aspects of the surface structure models are shown in Fig. 14. Fortunately, Ideal

Relaxed

(a)Top view

(b)Side view

Ga atoms As atoms

0surface layer

0 second layer

O s u r f a c e layer 0 second layer W =tilt angle

0

third layer

0

third layer

Fig. 14. Structure of ideal and relaxed GaAs{110}1 x 1 surfaces. The angle w is the angle between the (110) surface and the plane passing through nearest neighbour surface Ga and As atoms. I t is zero f o r the ideal surface. The t o p view has the [ 1101 direction normal to the paper. The broken lines show the 2-D unit cell. As a result of the relaxation, the spacing between the surface and second layer decreases by the amount 6.

217

the detailed differences do not have important consequences in relation to adsorption, but it is critically important t o take the general concept of relaxation into account in surface state calculations (see below). If now we consider the electronic structure of the surface, as early as 1967 van Laar and Scheer [168], using contact potential difference (CPD) and photoelectron spectral distribution (PSD) measurements provided conclusive evidence that there was no pinning of the Fermi level at the surface by intergap states on {110) cleavage surfaces, i.e. there was not band bending. This fact was further substantiated by the same group [ 169-1721, who added ELS measurements t o their previous techniques. In spite of this work of van Laar et al., a number of groups subsequently published data in which it was claimed that both empty and filled states existed in the gap. Eastman and Grobman [154] asserted, from photoemission measurements, that filled states existed in the bottom half of the gap, while Dinan et al. [173], Gregory et al. [174] and Eastman and Freeouf [ 1751, also using photoemission, found n o filled intergap states, but apparently observed empty states in the upper half of the band gap, pinning the Fermi level at midgap in n-type material. This model was perpetuated in subsequent publications by Spicer e t al. [176, 1771, but it is now known t o be incorrect and it is generally recognised that, in accordance with the results of van Laar and co-workers, there are no intrinsic gap states on { l l O ) surfaces prepared by good quality cleavage. In particular, Spicer et al. [ 1781, after further photoemission measurements, have modified their previous model t o that shown in Fig. 15.

Filled surface states (As)

2e

b i’

2e

Fig. 15. The (11O)GaAs surface showing both electronic and atomic surface rearrangement (after Spicer et al. [178]).* indicates the location of the empty surface state.

References p p . 280-289

218

Results showing complete agreement with those of van Laar e t al. have also been published recently by Luth e t al. [ 1791 and Gudat and Eastman [ 1801. The reason for the experimental variations which give rise t o the different results is precisely that which was originally advanced by van Laar and Scheer, that intergap states can be induced extrinsically by, for example, surface steps formed by poor cleavage, or by very small amounts of adsorbed impurities. Where a (110) surface is prepared by ion bombardment and annealing, the Fermi level is always pinned a t the surface by surface states which are damage-induced [ 1691. We have laboured this point quite deliberately, with the aim of demonstrating that, while modern techniques can provide detailed characterization of the electronic structure of semiconductor surfaces, careful surface preparation is critical if meaningful results are t o be obtained. This problem is, of course, more acute with compounds and alloys than with elemental semiconductors. Given adequately prepared surfaces, angle-resolved photoemission and the various yield spectroscopies have been used to investigate filled and empty surface states, respectively. Results of angle-resolved photoemission measurements have been published by Knapp and Lapeyre [181], Williams et al. [182], Knapp et al. [183] and Huijser e t al. [184]. A typical set of angle-resolved photoelectron energy distributions ( AREDCs) due t o Huijser et al. [184] is shown in Fig. 16, in which four structures labelled B 1 , S 1 , S2 and B, are observed. They are ascribed t o emission from filled intrinsic states since they disappear on exposure t o FZ lo5 L of HT. As we shall see below, B, , S 1 and S2 are primarily As-derived, while B2 is mainly a Ga s-like state bonded t o As p-states. Partial yield spectroscopy has been applied t o obtain information on empty surface states by studying transitions from Ga and As 3d core states t o empty states [180, 1851. All spectra show peaks a t hv = 19.5 and 20.0 eV, corresponding t o transitions from the spin-orbit split Ga 3d levels t o empty surface states. This excitation energy is less than would be required for a one-electron excitation into the condction band minimum and the original interpretation was therefore of a transition into empty gap states. Since these are now known to be absent, it can be concluded, in agreement with an earlier suggestion [186] that a surface exciton having a binding energy 2 0.6 eV must be involved in the transition. It can further be shown that these empty states are mainly Gaderived by comparing yield spectra using photon energies corresponding t o Ga 3d t o surface state excitation with those where the photon energy is appropriate for As 3d t o surface state excitation. It is found that there are no transitions into empty surface states in the second case. Similar results have been obtained with ELS [ 1871 .

* Dose rates are given in Langmuir (L). 1 L =

torrs.

219

I

-7

1

-6

I

I

I

-5 -4 -3 -2 Initial energy (eV)

I

-1

VBM.0

Fig. 16. AREDCs for (6 @)sets corresponding to the X - ' point in the SBZ for Ei at VBM (after Huijser et al. [ 1 8 4 ] ) . -, @ = 0 0 . - _ _ - , @ = 180'.

Concomitant with the experimental work described above, several attempts have been made t o calculate the surface electronic structure of (110) GaAs, and we can now consider how the theoretical models compare with and explain the experimental results. The first point t o make is that where calculations consider only an unrelaxed, i.e. ideal, surface, one or two bands of intergap states are predicted, from either a tight binding approach [ 167, 188, 1891 or a self-consistent pseudo-potential treatment [190]. There is an occupied As dangling bond band and an unoccupied band corresponding t o Ga dangling bonds. However, LEED observations clearly indicate such a surface structure t o be unrealistic and the effect of relaxation on the energy position of the surface states can be treated. Basically, on relaxation, the dangling bond References pp. 280-289

220

orbitals of surface As atoms become more s-like while those of Ga become more p-like. The effect of this is to move the occupied As dangling bond peak in the LDOS to lower energies and the unoccupied Ga dangling bond peak t o higher energies. Self-consistent pseudo-potential calculations, based on a relaxed surface demonstrating this movement, were carried out by Chalikowsky e t al. [191], while Mele and Joannopoulos [188], Calandra et al. [192] and Pandey et al. [167] used a tight-binding approach and obtained similar results, but in general there were still intrinsic gap states, contrary t o experimental data. The most detailed calculations, in relation t o the different surface structure models, have been performed very recently by Chadi [193, 1941 with the tight-binding method, and Chalikowsky and Cohen [ 1951 using self-consistent pseudo-potentials. The first important point is that both calculations show a complete absence of intergap states, both filled and empty, for a preferred, although not unequivocally established surface structure in which the angle of inward rotation of the Ga atoms is 27', not the maximum possible of 34.8'. Further, the bond lengths t o the second layer are only slightly contracted (see Fig. 14). Chadi's results for the model of Kahn e t al. [166] are shown in Fig. 17 and may be compared with experimental data of Huijser e t al. [ 1841 shown in Fig. 16. For the filled states, the calculation identifies five major surface-related structures in the local density of states (LDOS) below the valence band maximum E,. The two states near E,, B, and S , , are both primarily

q

I

$12 f

10

0

Energy (eV)

Fig. 1 7 . Local density of surface states (solid line) for the Kahn et al. [ 1661 model of the (110) surface of GaAs. Electronic states localized o n the first two layers of Ga and As atoms are shown. The broken line is the bulk density of states, shown for comparison. Only strongly localized surface states are shown. The densities of states have been Gaussian broadened (after Chadi [ 1941).

221

As-related, B, is dangling bond-like, i.e. the charge distribution projects into the vacuum, while S , is back-bond associated, so that its charge distribution is directed towards the bulk layers. Of the deeper-lying states, B2 is derived from spu bonding, with Ga s states bonding t o As p states, the orbital character being Ga-derived. The empty states, which for the relaxed surface are above the conduction band minimum, are primarily Ga-related. B( has a localized Ga dangling bond-type charge distribution and is predominantly p-like, whereas S l is back-bond-like. The calculated position of B; gives a surface excitonic binding energy of 1eV. The most important feature, however, is that their (virtual) charge is located behind the surface Ga, which has a strong influence on the reactivity of surface Ga with oxygen. Finally, since the LDOS has clearly defined Ga- and As-derived parts, the filled and empty states must contain significant .mixing of Ga and As characteristics. I t may reasonably be concluded that, on most of the important points, there is good agreement between theory and experiment on the structure and electronic configuration of the clean GaAs(110) surface. From the point of view of chemisorption behaviour, the vital factor is that two dangling bond electrons are associated with each surface As atom, but there are none associated with Ga. The relevance of this will become apparent in the discussion of oxygen adsorption on GaAs, which we consider in Sect. 4.4.

4. Gas-semiconductor

surface interactions

Detailed understanding of the interaction of gas with semiconductor surfaces is effectively limited t o two, or at most three, gases (hydrogen, oxygen and possibly chlorine) and two semiconductors, Si and GaAs. This may a t first sight seem an unreasonably restricted statement, but as we have previously indicated, the semiconductor limitation relates principally t o the necessary evaluation of clean surface behaviour and the gases are the only ones to have been seriously studied using clean surface adsorbents. In the following sections we will consider, in some detail, siliconhydrogen, silicon-oxygen, silicon-chlorine and gallium arsenide-oxygen systems. 4.1 ADSORPTION OF HYDROGEN ON SILICON

4.1.1 Introduction and survey of early work

The adsorption of atomic hydrogen on silicon is probably the simplest gas-semiconductor system and could provide the basis for a realistic attempt t o relate theory and experiment in chemisorption on semiconductors. Early work by Law and Francois [196] and Law [197] established that atomic hydrogen is adsorbed to monolayer coverage, with an References p p . 280-289

222

initial sticking coefficient close t o unity. Molecular hydrogen is adsorbed only in very small quantities, saturating at a few per cent of a monolayer. From photoemission measurements, Eisinger [ 1981 determined the effective dipole moment of a hydrogen atom adsorbed on Si to be 0.5 x cm. while Becker and Gobeli [199] observed infrared vibrational spectra of adsorbed atomic hydrogen close to the Si-H stretching frequency in SiH, . These results suggest that H atoms are adsorbed on top of surface Si atoms by a single Si-H bond. Following the rapid development of surface electron spectroscopy, the first systematic study using the newer techniques was by Ibach and Rowe in 1974 [200]. They used UPS and ELS combined with LEED to investigate hydrogen adsorption on three different silicon surfaces: the cleaved surface structure, Si (111) 2 x 1, together with Si{111)7 x 7 and Si(100) 2 x 1 , both prepared by argon ion bombardment and annealing. They were unable t o measure the hydrogen atom pressure directly, but peaks in UPS and ELS arising torrmin to from H atom adsorption saturated at an exposure of molecular hydrogen in which a heated tungsten filament was maintaining a partial pressure of H. This saturation coverage was assumed to correspond to a monolayer from the work of Law and Francis [196, 1971. On exposure to molecular H, alone, there was no change in the UPS or ELS spectra. Ibach and Rowe found that, on all three surface structures they investigated, the adsorption of atomic hydrogen removed the dangling bond surface states [201], with the corresponding formation of a new peak in the photoemission spectrum at E - E,,, = - 11eV. In addition, transitions from lower-lying surface states, associated with silicon back bonds [201], were also suppressed. These deeper surface states occur at the bottom of the s- and p-like valence bands on a clean surface and arise because the lattice spacing between the first and second layers is reduced as a result of the strengthening of the back bonds. Assuming this type of electronic structure for a clean surface, it is reasonable t o deduce that hydrogen is bonded by the dangling bond surface states and the peak at - 11eV corresponds to the energy of the electron in the Si-H bond. The presence of this bond then causes a charge density redistribution approaching that of bulk silicon, so that surface states associated with the back bonds disappear. Ibach and Rowe found the reconstructed surface structures to be very stable to hydrogen adsorption, in that on{ 11117 x 7 and{ 100}2 x 1 surfaces, no change in LEED patterns occurred up to saturation coverage. The favoured interpretation was based on the model of clean surface reconstruction proposed by Lander [ 761 and extended by Phillips [ 2021, whereby surface vacancies produce “warped benzene ring” structures in the first and second surface layers, and such distortions are too large t o be removed by hydrogen adsorption. The (111) 2 x 1 structure was found to be somewhat less stable, however, and the fractional order spots disappeared on hydrogen adsorption. This was related to the Haneman clean

223

surface model [ 1361, in which one half of the surface atoms are raised above their average positions and the other half are depressed below this “average” plane because of more s-like and p-like bonds, respectively. The adsorption of hydrogen might then restore bulk sp3 hybridization of the surface silicon atoms. More recent and detailed experiments by Hagstrum and co-workers [ 203-2081 , together with theoretical calculations by Appelbaum and Hamann [209, 2101 and Pandey [211] have, however, considerably modified the results and conclusions reached by Ibach and Rowe. The reasons for this are related t o control of substrate temperature and surface coverage, which have been found t o be very important t o the UPS spectra and LEED patterns observed. The work has also been extended t o include Si {110)surfaces [ 2061 . 4.1.2 Hydrogen a t o m adsorption on S i {I 11 } Sakurai and Hagstrum [ 2031, using principally UPS and LEED, combined with some additional ion neutralization spectroscopy and work function experiments, confirmed Ibach and Rowe’s finding that the dangling bond surface state disappears on exposure t o atomic hydrogen. However, their UPS measurements indicated that chemisorbed hydrogen produces two sharp peaks in the surface density of states spectrum at approximately - 9.8 and - 1 2 e V from the vacuum level. This is contrary t o the single peak at - 11.5 eV observed by Ibach and Rowe [ 2001, but in accord with the theoretical predictions of Appelbaum and Hamman [ 2091 and Pandey [211]. The interpretation is that the peak a t - 9.8eV arises from the Si-H bond and the (smaller) peak at - 1 2 eV arises from the enhancement of an d i k e bulk band at the surface by hydrogen chemisorption. The corresponding measurements of work function change (A$) as a function of hydrogen exposure produced a complex curve (Fig. 18),suggesting that hydrogen atoms can occupy more than one type of site on the S i { l l l ) surface. This is consistent with the thermal desorption results obtained by Joyce and Neave [212], who observed up to three peaks in desorption spectra of hydrogen from S i { l l l } . (The desorption was as H, molecules, but adsorption probably occurred via atomic H). Sakurai and Hagstrum’s LEED results showed that, up t o half saturation coverage, corresponding t o the minimum A$ value, n o change occurs in the {ill)7 x 7 pattern, but beyond this coverage the non-integral spots weaken and eventually most of them disappear. Comparing these observations with those of Ibach and Rowe [200], it can only be concluded that the maximum coverage reached by the latter workers was < 0.5 monolayer. T o a limited extent, these results support the contention that the H(111)Si chemisorption system is simple, in that theory and experiment agree that H atoms bond t o the single dangling orbital on each Si atom t o References p p . 280-289

224

-01 -

4

0

10

20

30

40

H exposure (rnin)

Fig. 18. Change in work function (A$) of the Si (1 11 )surface as a function of hydrogen exposure (after Sakurai and Hagstrum [ 2031 ).

form what, Pandey e t al. [204] subsequently referred to as the monohydride phase, S i { l l l ) : H . However, these authors were the first t o report the existence of a second hydrogenated surface phase of S i { l l l ) where, instead of H, atoms, SiH, radicals are attached t o the single dangling orbitals. This phase is referred t o as the trihydride, Si (111): SiH,. Both phases are formed from surfaces which are initially clean but which have different structures. The monohydride phase derives from the Si (111)7 x 7 structure, while the trihydride phase originates from a surface giving a 1 x 1 LEED pattern in which the relative intensity of background t o diffraction spots is greater than in the 7 x 7 pattern. It is obtained by heating a substrate giving the 7 x 7 pattern t o x 11OOK for 10min and then quenching t o room temperature [ 2041 . The UPS spectrum obtained on initial exposure of this surface to hydrogen atoms is similar t o that for Si (111):H, but increasing H atom exposure completely changes the spectrum. Peaks then appear at E - E,,, = - 11 and - 15 eV and the time taken t o reach saturation is six times longer than for the {111)7 x 7 structure. During thermal desorption these new peaks decrease monotonically, without reappearance of the originals, so the first stage of adsorption must differ fundamentally from the later stages. The surface structure model for which the calculated UPS spectrum showed closest agreement with experiment was the trihydride phase (Fig. 19) and its existence, although unexpected, gives support to the surface vacancy models for S i ( l l 1 ) 7 x 7 [76, 2021. The trihydride phase is energetically more favourable than the monohydride because of the relative strengths of Si-H and Si-Si bonds, the former being 50% stronger. However, in order t o form Si (111):SiH,, the outermost Si layer has t o be removed, which can only occur by

-

225

-20

-12

-16

-8

-4

E - E V A c( e V )

Fig. 19. UPS spectra for the trihydride phase compared with calculated spectra for a monohydride and a trihydride surface phase (after Pandey et al. [ 2041).

breaking Si-Si bonds. The proposed mechanism for this bond breaking involves an extension of the {111)7 x 7 surface vacancy model to the 1 x 1 surface whereby, in the latter structure, the vacancies are disordered. Vacancy clustering will then occur and this allows access of some H atoms to adsorption by the three dangling orbitals of Si atoms in the second layer. The energy released in this process can contribute to the removal of Si atoms at the edges of vacancy clusters by further bonding t o incident H atoms to form SiH4, which desorbs. The mechanism leads eventually to the removal of a surface monolayer of Si and allows the formation of S i { l l l ) : SiH, by adsorption of three H atoms per Si atom in what was the second layer. The following points were considered to provide essential support for the model: (i) the sequential appearance of the photoemission spectra of S i ( l l 1 ) : H and Si{lll}:SiH, on adsorption of H atoms; (ii) the relatively large amount of H required to produce Si{lll}:SiH,; (iii) the simple decay of spectral features on thermal desorption. References pp. 280-289

226

In addition, recent calculations by Ho et al. [ 2131 using self-consistent pseudo-potentials basically confirm the existence of the trihydride phase. They suggest that relatively minor discrepancies between the structural model and the calculations can be accounted for by strong hydrogenhydrogen interactions, which become important in the dense hydrogen monolayer of the trihydride phase. Partial monolayers of adsorbed H atoms may also exhibit disorder and the effects of this have been investigated theoretically and experimentally by Appelbaum et al. [205] for H atom adsorption on Si{111}7 x 7 surfaces. The experimental variable related t o disorder is the temperature of the substrate during adsorption, so the evolution of UPS spectra with increasing exposure up to saturation was followed for a substrate at room temperature and at 425K. The results are illustrated in Fig. 20. The spectra obtained at saturation are essentially identical, irrespective of substrate temperature, and closely correspond with theoretical calculations for H chemisorbed on S i { l l l } as an ordered monolayer. However, the

E-EVA, ( e V )

E-EVA, ( e V )

Fig. 20. (a) UPS spectra for Si{111}7 X 7 surfaces during exposure t o atomic hydrogen at 425 K. Exposure time: curve 1, clean surface; curve 2, 1 min; curve 3, 2 min; curve 4, 4 min; curve 5, 8 min (saturated). (b) UPS spectra during thermal desorption from the H-saturated surface. Curve 6, saturated; curve 7, 30s at 590 K; curve 8, 30s at 670 K; curve 9, 30 s at 770 K; curve 10, 30 s at 970 K (clean). (c) UPS spectra for Si {111}7 x 7 surfaces during exposure to atomic hydrogen at 300 K. Exposure time: curve 1, clean surface; curve 2, 0.5 min; curve 3, 1 min; curve 4, 2 min; curve 5, 4 min; curve 6, 8 min (saturated) (after Appelbaum et al. [ 2 0 5 ] ) .

227

evolution of the saturated spectra is quite different at the different temperatures. At 425K, all of the spectra have similar line shapes, but at room temperature the spectral form is a strong function of coverage. Corresponding behaviour was observed for spectral sequences obtained during desorption. These results were related t o disorder in sub-monolayer coverages in the following way: A t 425K, adsorbed H atoms were assumed t o be mobile, with a weak attractive interaction between them; this allows the formation of island clusters of almost complete coverage within the area of a cluster and so spectra from the clusters should be the same as from a complete monolayer, as observed. At room temperature, H atoms will be less mobile and chemisorb in a disordered array, including isolated atoms at low coverages. Close t o saturation, the spectral features of the ordered monolayer will develop. To substantiate these somewhat speculative ideas, a model calculation was made of the electron density of states for a fractional monolayer of H chemisorbed on S i ( l l 1 ) at room temperature, taking into account the disordering. The basis of the model is a two-dimensional tight-binding simulation of the H monolayer in which H atoms occupy random sites on a 2-D hexagonal lattice. Disorder is introduced by using a large unit cell (50-100 lattice sites) with a particular random occupation corresponding to a specific H coverage. The model successfully predicts the line shape evolution with coverage observed in the room temperature spectra, as a consequence of the progressive formation of the complete E - k dispersion of the Si-H surface resonance in the surface Brillouin zone. Finally for Si {lll}, Appelbaum and Hamann [ 2091 calculated a Si-H bond force constant of 0.175 a.u. and the corresponding surface phonon was observed experimentally using ELS by Froitzheim et al. [214]. By treating the Si-H complex as a binary molecule, they derived a bond force constant from their measured data of 0.158 a.u., in reasonable agreement with the predicted value.

4.1.3Hydrogen atom adsorption on Si(110) Sakurai et al. [ 2061, using UPS, established the importance of substrate temperature during H atom adsorption on clean Si (110) 5 x 1 surfaces. Two distinct surface phases are formed, depending on temperature, both giving a 1 x 1LEED pattern at saturation. This in itself is taken to indicate that the 5 x 1 reconstruction is due t o relaxation of surface Si atoms and not t o vacancies. As with (111)surfaces, the {llO)surface has one dangling orbital per surface atom, but the UPS spectra from surfaces saturated with H at room temperature are quite different from the two orientations. Furthermore, a theoretical spectrum for the saturated (110) surface based on the saturation of dangling orbitals to produce Si-H bonds differs significantly from the experimental spectrum. Closest agreement with the calculated spectrum is, in fact, obtained from either an unsaturated References p p . 280-289

228

surface obtained by adsorption at room temperature or by heating the Hsaturated surface t o 575K, a temperature too low to break Si-H bonds. These results imply the presence of weakly bound hydrogen atoms, with more H atoms than dangling orbitals at saturation, a concept substantiated by the adsorption behaviour at 625K, a temperature too high for the weakly bound phase t o form. Under these conditions, the UPS spectrum corresponded to the exact saturation of all dangling orbitals, but on reducing the temperature t o 300K in the presence of H atoms, further uptake occurred and the final UPS spectrum was identical t o that obtained from a surface saturated at room temperature. It was proposed that the extra H atoms were non-directionally chemisorbed at the centre of the hexagonal ring made up of the first and second layers of Si atoms. By using NeI (16.8 eV) and He1 (21.2 eV) radiation in UPS, which showed that at room temperature weakly chemisorbed H atoms are present uniformly within at least 10a of the surface, Sakurai and Hagstrum [207] were able t o offer some evidence for this model. The structure envisaged is shown in Fig. 21. From measurement of adsorption kinetics (using the rate of change of UPS spectra) Sakurai e t al. [215] confirmed that adsorption is always of (b)

(0)

51 (110) 1x1

SI {IIO} 5x1

monohydride

(C)

Monohydride

+

non-bonding (H)

2

Top view 0 first layer second layer H

Fig. 21. Schematic diagrams of ( a ) the clean Si{llO } surface, ( b ) the monohydride surface phase Si {110}1 X 1 :H, formed at 620 K and (c) the 300 K H-saturated phase in which excess atomic hydrogen is present at the centres of the hexagonal Si atom rings (after Sakurai and Hagstrum [ 2071 ).

229

atomic H, not vibrationally excited H,, and that, at room temperature, the initial sticking coefficient of H on a clean Si{llO} 5 x 1surface is close to unity. 4.1.4 Hydrogen a t o m adsorption on S i { 100)

Recently, Sakurai and Hagstrum [208], using UPS and LEED, and White and Woodruff [ 2161 using only LEED, have considerably modified the original conclusions which Ibach and Rowe [200] made about H adsorption on this surface. Two surface phases, a monohydride, Si{lOO} 2 x 1: H, and dihydride, Si{100)1 x 1: 2H, have been identified using techniques identical t o those employed for {ill} and (110) surfaces. The starting surface is always the clean, reconstructed Si{loo} 2 x 1, but it is important to note that with sufficient exposure the non-integral spots completely disappear [ 208, 2161, contrary to the observation of Ibach and Rowe [200]. This leads to a model of the clean surface reconstruction being formed by adjacent rows of surface atoms moving closer together (row dimerization), as originally proposed by Schlier and Farnsworth [ 971 . Hydrogen atoms can then bond to the dangling orbitals to produce a Si{loo} 2 x 1: H monohydride phase as shown in Fig. 22. SI I 1 0 0 1

Unreconstructed

1X I

Reconstructed H

2x1 H

H

1x1

72

H

HH

H

H

H

H

H'H

H

HH

HH

H

(C)

Fig. 22. (a) Schematic of the unreconstructed Si(lO0) surface, with two dangling orbitals per surface atom. ( b ) Si{100}2 X 1 reconstruction based on a pairing model. Atomic hydrogen can bond t o the dangling orbitals without changing the ( 2 x 1) reconstruction. Bonding to all available orbitals produces the Si{lOO) 2 X 1:H monohydride phase. (c) showing adsorption of atomic hydrogen beyond the saturated Si {loo}2 x 1 : H phase to produce the dihydride phase Si { 100) 1 x 1:: 2H. Also shown is H2 desorption. (After Sakurai and Hagstrum [ 2081 .)

References p p . 280-289

230

It has been postulated by Appelbaum et al. [ 2171 that, while the clean Si(100) 2 x 1surface is almost certainly row dimerized, the monohydride phase is not (although the equivalent monohydride on Ge is). This conclusion is based on the lack of agreement between UPS data for Si{100) 2 x 1:H and a self-consistent calculation assuming the pairing model of surface geometry, while there is complete agreement between theory and experiment for the clean Si(100)2 x 1 surface and the S i ( l l 1 ) :H monohydride phase. N o alternative t o row dimerization has been proposed for Si{100) 2 x 1: H, however. Further hydrogen adsorption causes rupture of the Si--Si bond between surface atoms with a hydrogen atom bonded to each additional dangling orbital so produced t o give the dihydride phase. This is the sequence for room temperature adsorption. Thermally stimulated desorption from this dihydride phase occurs by the association of adjacent H atoms to give H, as the desorption product, the surface phase remaining being the monohydride. Adsorption at a substrate temperature of 500 K produces only the monohydride phase, but if the substrate is cooled to room temperature in the presence of H, it is converted completely to the dihydride. These results make the idea that the (100)2 x 1reconstruction results from a high concentration of surface vacancies [78, 2021 or a "canted ridged" structure [ 791 rather improbable, since in both models significant atom migration would be required for the change to (100) 1 x 1 t o occur.

4.1.5 Direct observation o f hydride surface phases Sakurai et al. [218] have used the atom probe field ion microscope [219] t o make a direct study of hydride phases on silicon. Continuous field evaporation from (111) and (110) Si planes in the presence of hydrogen produced Si", SiH" and SiH:. Some Si atoms evaporate without forming hydrides to give Si"; SiH' derives simply from the monohydride phase and SiHi from the dihydride. However, this latter comes from the (111) surface, whereas UPS data consistent with the presence of a dihydride came from (100) 1x 1. This apparent difference can be reconciled by the fact that some kink site atoms on {lll) planes have two dangling orbitals per Si atom (as on (100) surfaces) and field evaporation occurs primarily at kink sites. With a disordered (311) surface, d.c. field evaporation produced Si", SiH" and, importantly, SiHi. If it is assumed that the disordered {311) surface has vacancies similar to those on the disordered {ill) 1 x 1,from which UPS data suggest Si atoms evaporate by the reaction

then the observation of SiHi is consistent with a surface trihydride phase, since (SiH,),d, is not stable in the presence of €3. It seems a reasonable conclusion, therefore, t o suggest that these results confirm the presence of mono-, di-, and tri-hydride surface phases on Si.

231

4.1.6 Theoretical considerations The apparent simplicity of the H-Si system has enabled a few realistic theoretical treatments of chemisorption on semiconductors t o be made. The details of the calculations are outside the scope of this article, but it will be useful to summarize some of the principal conclusions. Appelbaum and Hamann [ 2091 produced a fully self-consistent “first principles” calculation for the chemisorption of H on Si{111),which showed that the Si-H bond potential is considerably greater than that for Si-Si. The force on the H atom is small and inward, with a bond length of 2.73 ? 0.02 a.u. The Si- H bond force constant is 0.175 a.u. compared with the measured value of 0.173a.u. for SiH,. The corresponding surface phonon, as mentioned previously, has been observed by ELS [ 2141 . In the calculated electronic structure of the Si--H surface, the most notable feature is the disappearance of states in the fundamental band gap and the corresponding appearance of a band of states, clearly connected with the Si- H bond, in the gap between the second and third valence bands. Although Appelbaum and Hamann [ 2091 suggested that a tight-binding (TB) approach could not be used to calculate a photoelectron density of states spectrum, Pandey [ 2111 succeeded, using what he called a realistic TB model calculation of the surface energy bands of S i ( l l 1 ) 1 x 1 covered with a monolayer of hydrogen. The model is based on the Hiickel approximation (valence orbitals between constituent atoms are orthogonalized), with the essential assumption that the chemisorption bond between Si and H at the surface is similar t o the corresponding bond in SiH,, where the molecular energy levels are known. Good agreement between theory and experiment is obtained for S i { l l l } : H . The only significant discrepancy is an experimentally observed peak at E - Eva, = - 8 eV, which does not appear in the calculated spectrum. It could arise if surface reconstruction is maintained on chemisorption, producing some “abnormal” Si--H bonds which are not considered in the calculation. Ho e t al. [213] have also obtained reasonable agreement for the trihydride phase using selfconsistent pseudo-po ten tials. Very recently, Appelbaum and Hamann [210] performed a fully selfconsistent calculation of the electronic properties of a monolayer of H interacting with a Si(100) 1 x 1 surface, a highly unsaturated structure having two broken bonds per surface atom. This clean surface structure results in two bands of surface states within the band gap of bulk Si [74, 127, 1281. One is dangling-bond like, energetically narrow and almost completely filled, the other is almost empty with a spatial distribution centred on the interaction of the fixed Si atomic plane with the plane containing the bonds broken in forming the surface. This has been called the bridge bond and is essentially non-bonding. By introducing H above the last Si plane, the dangling bond band is replaced by two bands of states derived from the I s orbital of H and the dangling orbital of Si. References p p . 280-289

23 2

To accommodate the H-donated electron, the clean surfaces dangling bond orbital must promote an electron into the bridge bond orbital which is empty for the clean surface, lying just above the dangling bond orbital. Within the framework of this model, Appelbaum and Hamann also considered charge transfer and work function changes occurring on H chemisorption, which lowers the clean surface value by > 1eV. Conventionally, this implies significant charge transfer between H and Si, which would be quite contrary t o the electronegativity difference between them. However, the Si- H bond is totally covalent and the work function change is not a function of Si-H separation. It arises simply from the promotion of the dangling bond electron to the bridge-bond band, which does not extend so far into the vacuum and so causes an initial dipole moment, but this is not charge transfer between Si and H. Appelbaum et al. [ 2201 have also predicted the existence of a new type of chemisorption bond as the result of fully self-consistent calculations using the local density approximation for exchange and correlation and a model potential for the Si4 core [ 2101 . These calculations show that, for Si{100) 1 x 1, H forms a strong chemisorption bond with Si atoms in the layer below the surface, despite their complete saturation. The bond is non-activated and weakens the Si-Si bond between this layer and the surface. It may well be analogous to the multicentred bonds found in boron hydrides and can be considered t o be formed from valence bond orbitals centred on the charge between surface and second layer Si atoms and the H Is state. The bonding combination appears as a surface state at - 10.5 eV, the antibonding combination as a partially occupied gap state and the non-bonding combination is primarily responsible for the weakened Si-Si back bonds. Although the calculation was for Si{lOO}, it may be applicable to any Si surface whose second layer is roughly tetrahedrally coordinated. This type of bond could also be the precursor state for the corrosive modification of hydrogenated Si surfaces. +

4.1.7 Conclusions

Recent experimental and theoretical work has produced a rather clear understanding, albeit with some speculation, of the electronic processes involved in the chemisorption of H atoms on the low index surfaces of silicon. Even these very simple systems exhibit complicating features, however, and it is probably unrealistic to expect such detailed models to be available for any other gassemiconductor combination. Nevertheless, we will discuss in the following three sections the results which have been obtained at the next level of complexity, viz. oxygen on Si and GaAs, and chlorine on Si.

233

4 . 2 OXYGEN ADSORPTION ON SILICON

The silicon-silicon dioxide system is of paramount importance in the technology of metal-oxidesemiconductor (MOS) and bipolar devices, and SiO, thin film properties, the Si-Si02 interface and oxygen-silicon interactions have all been extensively investigated. A great deal of the work is outside the scope of this article, but selected general reviews have been produced by Kooi [221], Lamb [222], Nicollian [223], Revesz [224] and Sugano e t al. [225], while Agajanian [226] has published a comprehensive bibliography up t o 1977. The collection of articles edited by Pantelides [227] provides an excellent recent survey of most of the more fundamental solid state and device physics involved. We will not consider these topics further here, nor will we deal with the oxidation kinetics of silicon in conventional atmospheric pressure flow systems, which has been reviewed by Revesz and Zaininger [228]. The work discussed there was largely directed towards device fabrication and, in general, neither the silicon surface condition nor the gas phase composition was controlled, while rate processes were evaluated only from oxide thickness measurements as a function of system parameters (time, temperature, oxygen pressure, etc.). We shall concern ourselves here with a review of more controlled oxygen adsorption studies on clean silicon surfaces, but even with this restriction, a vast literature exists and we cannot hope to be comprehensive. Most of the work prior t o 1970 was almost exclusively concerned with oxygen adsorption kinetics on ostensibly atomically clean Si surfaces and has been summarized by Joyce and Neave [ 2291. It was characterized by two features; firstly, the range of methods used t o determine the rate and extent of oxygen uptake including gas volumetry, LEED, field emission, INS, ellipsometry, proton activation analysis of adsorbed l 8 O9 and p'n junction currents; secondly, the range of values for the oxygen sticking coefficient ( S ) and its pressure and coverage dependence. Reported and, while values for S at zero coverage ( S o )varied from l o - ' t o there was agreement on the pressure independence of S, several empirical forms of its functional dependence on coverage ( 0 ) were proposed. Subsequent work has shown the Si-O2 interaction to be extremely complex and t o be dependent on many experimental parameters, some of which are difficult t o control, such as surface topography and step density, while others are directly involved in the measurement itself, e.g. the electron beam. The way in which the oxygen adsorption or oxidation is performed also has a profound influence on the type of S i - 0 bonding and it is not realistic t o propose unique bonding models unless the processing conditions and measurement techniques are precisely specified. Although there has been a great deal of controversy in recent literature on this subject, there appears t o be n o a priori reason t o accept one model as being References p p . 280-289

234

more correct than another, since it is not yet possible t o control all of the parameters concerned. We will deal first, therefore, with recent kinetic data and the experimental variables which influence it. From this, we will attempt t o evaluate the most probable coverage, pressure and temperature dependence of the sticking coefficient on an ideally clean, step-free surface. We list below, with brief comments, the known effects which influence kinetic data. ( a ) Oxygen contamination. When oxygen is admitted to UHV systems to pressures of < torr, a very large amount of CO can be produced by desorption from various parts of the system. In extreme cases, the CO partial pressure can exceed that of oxygen and it is clearly important to have a reliable assessment of the gas phase composition during adsorption. This has not always been the case in reported work. ( b ) Ionization gauge (hot filament) effects. Not only the purity, but also the state of the oxygen can influence adsorption kinetics. Archer and Gobeli [230] and Ibach et al. [231] reported an approximate ten-fold increase in the initial sticking coefficient of oxygen when a nude ion gauge operated in the system. The state of the oxygen produced in this way has not been conclusively established, but it has been variously suggested t o be ions, atomic, ozone or molecular oxygen excited into the singlet state, any of which may adsorb and/or dissociate more readily on the silicon surfaces. Whatever the oxygen species formed, the effect is clearly large. ( c ) Electron beam effects. Ibach et al. [ 2311 found that the adsorption process was not affected by electron irradiation of a clean silicon surface prior to oxygen exposure. When a partially oxygen covered surface was irradiated, however, the adsorbed oxygen was converted to an “oxide” structure and new adsorption sites were created so that the saturation coverage increased two- or three-fold. ( d ) Oxygen pressure effect. Garner et al. [232] have reported that the pressure of oxygen influences the saturation coverage at the completion of the fast adsorption state (see below). The effect has also been observed by Joyce and Neave [229], but in addition, Garner et al. claim that successive small oxygen doses at low pressures (< lo-’ torr) followed by a 9OOL dose (at torr) produces a different oxygen adsorption state from a single lo3 L exposure (at torr). ( e ) Influence of surface steps. Ibach et al. [ 2311 were able t o show that the initial sticking coefficient of oxygen was a very strong function of the surface step density (as measured by an optical reflection technique using a laser beam [ 2331 ). The value of So can vary by more than two decades with changing step density as shown in Fig. 23 (which also shows the influence of a hot filament on S o ) . If the increase in So with step density simply corresponded to adsorption at step edges followed by diffusion of the oxygen or growth of the adsorption layer outwards from the step edges, So would be linearly dependent on the step density (measured as the parameter tan (Y by the optical method). In fact, the dependence is

235 10-1

Id2

In c

C

E

._

z 8

10.’

m c_

Y

u Fl

I15 Fig. 23. Oxygen sticking coefficient on (11112 x 1 silicon surfaces as a function of step density (expressed as tan&).The data also show the influence of a hot filament on the sticking coefficient (after Ibach et al. [ 2 3 1 ] ) . m, Auger spectroscopy; 0,surface vibrations; A, ellipsometry; P , on 7 x 7 surface.

exponential, as seen in Fig. 23, and this may possibly result from some kind of cooperative effect between atoms in different steps which would influence adsorption probability. An alternative speculation, favoured by Ibach et al. [231], is thermally activated adsorption with the activation energy dependent on tan a, viz. So = k

- E A - E l (tana)) + D (tan a ) exp (_________ kT

where u is a coverage-independent condensation coefficient. ( f ) Effects of substrate orientation and reconstruction. It is not clear whether or not the crystallographic state of the silicon has any significant influence on adsorption, since no definitive experiments have been performed in which all of the other experimental variables were adequately controlled, although Ibach et al. [231] observed no difference between Si{111)7 x 7 and {111)2 x 1. Almost all of the recent, more detailed References p p . 280-289

236

work has, however, been concerned exclusively with the 1111)2 x 1 surface formed by cleavage and we shall therefore concentrate on this surface in what follows. Having established some, at least, of the variables involved in kinetic measurements, we will attempt to evaluate the most probable adsorption model from currently available data. There appears t o be general agreement that adsorption occurs in two stages, a fast initial uptake with exposures of lo2--lo4 L, followed by a much slower process. The initial sticking coefficient at room temperature on a clean, step free surface is within the range but the coverage ( 0 ) dependence of S is more of a problem. If it is assumed that the adsorption of an oxygen molecule requires two nearest neighbour Si surface sites and that the adsorbed species are not mobile, then S = f(0) should be of the form where z is the number of equivalent nearest neighbour sites. This model has been proposed by several authors (see Ibach et al. [231] for details), but it leads to a rather rapid decrease of S with increasing coverage as a direct consequence of the implicit assumption that, once a site is occupied, the local sticking coefficient is zero. Such behaviour is not observed experimentally and S is found to decrease slowly with coverage up t o 0 x 0.6--0.7 and then to fall off rapidly. This may suggest a mobile precursor state for oxygen adsorption. To obtain greater understanding of the adsorption process, we must look at the possible binding states between adsorbed oxygen and silicon. To this end, the system has been investigated by XPS, UPS, ELS and very high resolution ELS. Most of the earlier reports interpreted the fast initial stage as non-dissociative adsorption of molecular oxygen to an approximate monolayer coverage, followed by the much slower formation of SiOz by a dissociative process involving atomic oxygen. However, this convenient explanation is almost certainly too simple. From AES and XPS measurements, Garner et al. [232] suggested that, even in the fast adsorption stage, two different processes could occur. For very low pressure ( is an approximate 2.0 eV shift of the Si 2p levels, although the magnitude of the shift is dependent on the quality of the cleave used to produce the clean surface. This is adsorption state “2”. From UPS measurements, it was found that emission from gap states which are present on the clean silicon surface is suppressed by both adsorption states. There is also a sharp increase in emission between - 4

237

and - 9eV for state 2; this is present, but much less intense, for state 1. The valence band structure of state 2, although it is part of the initial fast adsorption regime, is in fact close to that of heavily oxidized silicon and consequently is attributed by Gamer et al. [ 2321 to silicon bonded to atomic oxygen, i.e. dissociative chemisorption. This distinction between adsorption as molecular or atomic oxygen has been a point of considerable contention in the literature, but other authors have not considered the possibility that the oxygen pressure, as opposed to exposure, can inffuence the result. It is therefore not possible to relate the two fast adsorption regimes described by Garner e t al. to other results. Thus, Ludeke and Koma [234], on the basis of LEELS observations and total energy calculations for various possible complexes, proposed that adsorption was dissociative, with an oxygen atom doubly bonded to a silicon surface atom. Meyer and Vrakking [235], using ellipsometry and AES also favoured dissociative adsorption, but in their model, an oxygen bridge is formed between two neighbouring surface atoms, requiring the breaking of back bonds. This model therefore retains the oxygen bridge concept originally proposed by Green and Maxwell [ 2361, but allows for atomic rather than molecular adsorption. The alternative viewpoint, of molecular adsorption, has been strongly advanced by Ibach et al. [231, 237-2391 who used a combination of photoelectron spectroscopy, ellipsometry and very high resolution ELS. From the presence of (i) four O(2p)-like peaks in UPS results, (ii) a 1.1eV smaller O(1.s) core level binding energy for chemisorbed oxygen than for SiO,, and, most definitively, from (iii) surface phonon spectra which showed three Si -. 0 stretching modes, they proposed a model of molecular adsorption via an asymmetric peroxide bridge. Very recently, Stohr et al. [240,241] have made surface-extended X-ray absorption fine structure (SEXAFS) measurements [ 2421 above the 0 K absorption edge at 535eV. The fine structure arises because the matrix element between initial and final states for core-level excitation by a photon contains details of the local environment of the core through back diffraction of the photo-emitted electrons from neighbouring atoms. The adsorption state of oxygen was characterized by a Si 2p shift of 2.5 eV. The 0-Si bond length was found to be very close to that in SiO, (1.65 f 0.03 8,cf. 1.61 in SiO, ) and the effective coordination number of the oxygen atom on a (111)Si surface was 1.1 0.5 (cf. 2 in SiO,). It should be noted that this is an effective coordination number, N*, not the true coordination number, N, and is given by

a

N*

*

N

=

3

cos20j i

where the sum extends over all neighbouring atoms j and Oi is the angle between the electric field vector of the photons at the central atom site and the vector rj from the central atom to the j t h atom. Equation (13) References p p . 280-289

-1 2

-8

-4

E,=O

Energy (eV)

Fig. 24. Calculated LDOS of a six-layer S i ( l l 1 ) slab with chemisorbed oxygen molecules (c) and atoms (a) compared with the UPS spectrum of a cleaved {111}Si surface exposed to lo2 L of O 2 (b) (after Chen et al. [ 2431 ).

can be evaluated for any assumed model geometry and experimentally derived and calculated values of N* compared. Stohr et al. used this technique to eliminate several possible models of this particular adsorption state (2.5 eV Si 2p shift). Finally, Chen et al. [ 2431 compared results of their non-self-consistent LCAO (extended tight binding) calculation, which simulated 0 and 0, adsorption on {111}Si, with UPS and XPS measurements. The calculated LDOS for both atomic and molecular adsorption is shown together with the UPS spectrum of a cleaved surface exposed to 10, L 0, in Fig. 24. The calculations revealed very large differences between atomic and molecular oxygen. For the former, there is one major peak due to r-bonded 0 ( p x , p y ) at -- 7 eV, with much weaker features from o-bonded O ( p , ) at higher energies. In contrast, the LDOS for molecularly chemisorbed O 2 has two major peaks at - 12.5 and - 4.5eV from 0(2p,), with smaller peaks at - 3.5, - 10.3 and - 10.8eV which are primarily O ( p , , p y ) derived. It is clear that there is much better agreement between the measured UPS spectrum and the LDOS resulting from atomic, rather than molecular, adsorptioil; the dominant intensity [from O(2p)l lies between - 6.5 and 9.5 eV in each case. From their XPS measurements, Chen et al. [243] also established that the O(1s) binding energy at sub-monolayer coverage was less than in

239

SiO, , whereas if the adsorption were molecular, it would be higher. There are two possible reasons for the lower value, according to Hollinger et al. [244] ; either additional screening from the silicon substrate, due t o its proximity, gives increased extra-atomic final state relaxation for a photoionized oxygen atom or, alternatively, the Fermi level moves closer to the valence band edge due to localized interface states. The Si 2p shift was in accord with that previously observed for fast adsorption state 2 [ 2321. We are now in a position to summarize the proposed adsorption models, to comment on their validity and to draw some possible conclusions to explain the widely divergent views expressed in the literature. It appears most probable that there is a molecular adsorption stage (state l), which only occurs for very low pressure exposures. For any torr, the adsorption is primarily dissociative. exposure at pressures Z It is most unlikely, however, that the transition is sharp and a mixed molecular-atomic adsorption state could be expected to occur over a comparatively wide range of exposure pressures. The fundamental reason for producing different states as a function of pressure is not clear, but it may relate to lateral interaction effects. Ibach and his co-workers have been the chief protagonists of molecular adsorption per se, but, as pointed out by Chen et al. [243], the most crucial evidence for this was a very broad, weak 135 meV vibrational feature in the high resolution ELS spectrum, assigned to an 0-0 stretch [239], which could certainly be accounted for by a mixed adsorption state. The two strong features in the spectrum at 95 and 135meV can be assigned to either molecular or atomic vibrational modes, provided the 0 atom is allowed to occupy an asymmetric site. Although the exposure pressure is not usually specified, it is probable that most investigators have worked in the experimental regime where adsorption is predominantly dissociative. The simplest evidence for this appears to be the frequently reported Z 2eV shift in the Si 2 p level. The bonding models for atomic adsorption proposed by Ludeke and Koma [ 2341 and Meyer and Vrakking [ 2351 are shown in Fig. 25, together with those of Green and Maxwell [236] and Rowe et al. [239] for molecular adsorption. It is not possible to discriminate between them on the basis of available evidence, since the experimental conditions used by the various investigators have not been adequately specified in view of the known factors influencing adsorption. They may each be correct for specific adsorption conditions. The remaining aspect of the Si-0, system that requires discussion concerns the formation of SiO, during oxygen adsorption. Several pertinent questions can be asked; the first is simply what criteria are used to establish the presence of Si02 ; secondly, whether or not Si02 can be formed at room temperature from non-excited oxygen molecules, and finally what is the compositional abruptness and the Si-0 bonding configuration at the Si-SO2 interface. None of these questions can be References p p . 280-289

240

0

51 atoms

0 atoms I,

Original position of Si atoms in model (d)

Fig. 25. Bonding models for oxygen chemisorbed on (111)Si. ( a ) and ( b ) represent molecular adsorption, ( c ) and ( d ) atomic. Model ( a ) is due to Green and Maxwell [ 2 3 6 ] ( b ) to Rowe et al. [ 2 3 9 ] , ( c ) to Ludeke and Koma [ 2 3 4 ] and ( d ) to Meyer and Vrakking [ 2351.

answered unequivocally at this stage, but sufficient information is available to indicate trends. The presence of SiO, is normally related to a shift of 2 3.0 eV in the Si 2p level [ 232, 2441 , illustrated in Fig. 26. The energy of the core level shift does not uniquely define the local Si-0 bonding, however, so Bauer et al. [ 2451 used constant find state partial yield spectroscopy and compared the spectra with soft X-ray absorption spectra of bulk SiO,. They identified the Si 3d derived inner shell resonances characteristic of SiO, tetrahedra, in conjunction with a > 3 eV 2p level shift.

241

1

si

h v =130eV

2p

+

L

I

2.6eV 3.3e~ 3.8e~

I

100 105 Binding energy (eV)

5

Fig. 2 6 . Si 2 p core level shifts related to oxide formation (a) a clean silicon surface ( b ) adsorption of atomic oxygen ( c ) first indication of S O z formation following exposure to excited oxygen (d) continued exposure of ( c ) to non-excited O 2 (after Garner et al. [ 2321 ).

From observations of either the 2 p shift or this more definitive altemative, there is reasonable agreement that SiOz is only formed at elevated temperatures, or with excited oxygen or by electron irradiation of adsorbed oxygen. It may also be formed by exposure to high pressures of nonexcited molecular oxygen at room temperature, but this is less clear. The abruptness and bonding configuration at the interface are obviously References p p . 280-289

242

closely related and in general, there is agreement that the interface region spreads over no more than X 4 a . The broadening, which occurs in addition to the shift of the Si 2p peak, can be interpreted, according t o Garner et al. [232] as arising from the presence of three, or possibly four, bonding states in the interface. Apart from Si bonded to four oxygen atoms, i.e. the usual tetrahedral arrangement of S O 2 , Si is bonded to three, two and perhaps also one oxygen atom. Even with thicker oxides, these same non-Si02 states are still present in the same proportion, although relatively weaker than the Si02 contribution. Thus, the connecting link between oxide and silicon is formed by Si--O bonds which produce core level shifts that are neither bulk Si nor SiO, -like. There is some evidence [ 2451 from inter-atomic soft X-ray absorption across the interface that oxides formed at high temperatures ( 2 1250 K ) may have slightly more abrupt interfaces than those formed at room temperature, even with excited oxygen, but differences appear marginal. 4.3 CHLORINE ADSORPTION ON SILICON

Chlorine is the only other gas whose interaction with silicon surfaces has been studied in a reasonably detailed and systematic manner. Florio and Robertson [246] reported on the kinetics of the reaction of C1, with S i { l l l }7 x 7 over a decade ago and, more recently, several groups [ 247-2501 have used various types of photoelectron spectroscopy to evaluate the chemical binding sites and electronic states which occur when a saturation coverage of C1, is chemisorbed on S i l l l l ) 2 x 1 , S i ( l l 1 ) 7 x 7 and Si{lOO) 2 x 1. These results, relating to C1-induced surface energy bands, have been compared with self-consistent pseudo:potential [ 247, 2501 and tight-binding [ 2491 calculations. The kinetic measurements [ 2461 were obtained using a combination of LEED, AES and mass spectrometry to monitor changes in surface structure and composition during adsorption, and the nature of desorbing species. The initial sticking coefficient on a 1111)7 x 7 surface is X 0.1, a value not very sensitive to temperature over the range 300-850 K. Adsorption on the 7 x 7 surface at 300K produces a disordered monolayer coverage of chlorine, as shown by a decrease in the first and fractional order LEED beam intensities, with a corresponding increase in diffuse background intensity, but no change in periodicity. This result has subsequently been confirmed by Pandey et al. [ 2491, although Rowe et al. [248] apparently observed only a primitive 1 x 1 pattern at saturation coverage. Heating to x 650 K produces ordering and some loss of chlorine frolr, the surface and eventually a 1 x 1structure is formed, corresponding to a Z= 0.75 monolayer of chlorine. The major desorption product at all temperatures is SiC14, but there is evidence for two distinct adsorption sites for the chlorine. At low temperatures (<650 K), there is a rapid initial decrease in the surface chlorine

243

concentration combined with a large signal from SiC14 in the mass spectrometer, after which the chlorine surface population decreases very slowly with time. For higher temperatures (up to l o o O K ) , there is further desorption of SiCl,, the process obeying second-order kinetics with an activation energy of 1.5eV. A negligible amount of molecular C1, desorbs, but consistent with the observation that SiC1, is the major product, crystallographic (triangular) etch pits are formed during desorp\ c1 tion, which is suggested to occur via the surface complex , Si( C1‘ This “chemical” picture of the silicon surface-chlorine interaction has been augmented by UPS [ 2491 and angle-resolved photoemission measurements using synchrotron radiation [ 248, 2501 . These experiments have provided rather detailed information on the atomic position of adsorbed chlorine and the chemical binding states involved, although no attempt has been made t o relate the results to the kinetic data. There are two reasonable possibilities for the adsorption site of chlorine on { l l l } S i . Either it occupies a one-fold coordination position directly above a surface silicon atom or it enters the three-fold site formed by the space between three neighbouring surface silicon atoms. LDOS and energy band calculations have been made for the two sites, the results of the former, due to Schluter et al. [247], being shown in Fig. 27 where the insets illustrate the site geometry. However, in the {111)Si- C1 system, simple chemical reasoning can also provide a useful guide. In the onefold site, the C1 pz orbital will overlap the Si dangling bond orbital to form a (covalent) u bond of lower energy than the Clp, andp, orbitals. Consequently, this position is often referred to as the covalent site. Conversely, no u bonds exist for the three-fold site, which is more ionic in character . Turning to the LDOS calculations, the four peaks labelled A, B, C and D in Fig. 27 can be identified in the following ways. For the one-fold site, peak A is due to Si s states, B arises from the u-bond between Si sp3 and C1 pz states, C results from non-bonding C1 p, and py states (r-bonding states), while D corresponds t o p-like Si--Si bonds which have been perturbed by a C1 atom. In the three-fold site, where no u-bonds are involved, peak C is broadened and contains all three C1 p states (p,, p, and pr ). The pz states are not completely degenerate with p, and p, , however, and the relative energy positions of C1 p z states with respect t o those of C1 p, and p, is the main distinguishing feature between the two possible adsorption sites. Schluter et al. [247] used polarization selection rule effects with sand p-polarized photons, together with energy distribution measurements, t o show a good level of agreement between experiment and calculation for the one-fold site. This result was confirmed by Pandey et al. [249] and extended by Larsen et al. [250] t o include the derivation of the twodimensional energy bands from measurements of the energy positions of References p p . 280-289

244 Si{lll}+CI - t h e o r y

C

l-fold

covalent site

* I

I

-12

1

I

I

I

-6

I

1

O=E,

Energy (eV) Fig. 27. Calculated LDOS for a chlorine monolayer coverage of S i ( l l 1 ) l X 1 surface. The upper curve is for the one-fold coordination site and the lower curve for the three-fold geometry (see inset sketches). (After Schluter et al. [247] ).

the Cl-induced peaks as a function of polar angle. Again, there is good agreement between the experimentally determined bands and those calculated for the one-fold site using a pseudo-potential method. The results of Pandey et al. [249] applied t o a (111)7 x 7 surface, but the more detailed results of Schluter et al. [ 2471 and Larsen et al. [ 2501 were almost certainly obtained from a 1111) 2 x 1 reconstructed surface, although simultaneous LEED measurements were not reported. The precise form of reconstruction was, however shown to be important by Rowe et al. [ 2481, who studied C1, adsorption on { 111)7 x 7, { 111)2 x 1 and (100) 2 x 1 surfaces by photoemission polarization effects with synchrotron radiation. The first point is an apparent difference between their LEED results and those of Florio and Robertson [ 2461 and Pandey et al. [249]. Rowe et al. [248] found that the saturation coverage of chlorine on (111)7 x 7 and (111)2 x 1 surfaces produced a primitive 1x 1 pattern, whereas the other workers observed that the 7 x 7 periodicity

245

was maintained. The reason for this discrepancy is not apparent. The { 100) 2 x 1 surface has only been investigated by Rowe e t al. [ 2481, who found this reconstruction t o be maintained up t o saturation coverage. Of more fundamental importance is the different saturation coverage adsorption behaviour of C1, on (111)7 x 7 and (111)2 x 1silicon surfaces. This difference is clearly shown by the angle-integrated energy distribution curves at different photon energies for both s- and p-polarizations [Fig. 28(a) and ( b ) ] . Much more pronounced polarization effects occur for

hu32eV

26

22

18

32

26

l I

-12

I

I

-6 Energy (eV)

22

I

18 I

0.E"

Fig. 28. Angle-integrated EDCs for chlorine-saturated Si 1111) surfaces at different photon energies and for both s and p polarizations. (a) The Si{111}2 X 1 surface, curves normalized to have equal intensities for peak C. ( b ) The S i { l l l ) 7 X 7 surfaces, the curves are non-normalized. (After Rowe et al. [ 2481).

References p p . 280-289

246

adsorption on the 2 x 1 structure than on the 7 x 7. For the former, a p,-like character for peak B can be deduced from its weaker intensity for s-than p-polarization over a wide photon energy range, which is the condition resulting from adsorption on the one-fold covalent site, as we have discussed above. However, this does not occur for adsorption on a 7 x 7 reconstruction, where polarization effects are rather weak, although the peak energy positions correspond fairly closely to those from the 2 x 1 surface. Rowe et al. [248] have suggested that, on a 7 x 7 surface, C1 adsorption occurs on both one-fold and three-fold sites, which could be rationalized on the basis of the vacancy model [76] for this reconstruction. Chlorine atoms adsorbed in surface vacancies would be ineffective three-fold coordination sites, while the remainder of the surface is available for chlorine to occupy the covalent sites. This two site idea is also consistent with the desorption behaviour observed by Florio and Robertson [ 2461 from a C1-saturated 7 x 7 surface. At saturation coverage of chlorine on (100) 2 x 1 surfaces, a 2 x 1 LEED pattern is retained and, although no strong polarization effects are observed for angle-integrated EDCs, a t normal emission peaks B and C are enhanced for p-polarization. It has been proposed that the dangling bond dimerization tendency of the { 100) surface can account for these effects, since C1 will occupy sites between the first layer Si atoms, not sites above them. The C1 pr and pr orbitals will then have more bonding character, i.e. the C1 p z orbitals are partially mixed with the p , and p y orbitals, and while the Si-Cl bonds are still covalent, they are at an angle to the surface normal. In summary, it seems clear that chlorine is dissociatively chemisorbed on silicon and for (111)2 x 1 surfaces it occupies only the one-fold coordination site above the surface silicon atoms. On (111)7 x 7 surfaces, there is evidence that both this one-fold site and also the three-fold coordination site formed by the space between three neighbouring silicon surface atoms are occupied. A disappointing feature is that none of the groups using photoemission made any attempt t o relate their results t o the previously published kinetic data. 4.4 OXYGEN ADSORPTION ON GaAs

We will treat this system in some detail because it represents an excellent example of how the combination of kinetic measurements, photoemission, electron spectroscopic and surface structure studies in conjunction with theoretical calculations can produce a rather complete understanding of an adsorption process. However, the work has effectively been restricted to { 110) surfaces for two reasons. Firstly, from an experimental viewpoint, it is the surface obtained by in situ cleavage and, until very recently, with the advent of molecular beam epitaxy [ill], there was no alternative technique for preparing clean, damage-free surfaces. From a theoretical

247

standpoint, this surface does not reconstruct, there is only bond relaxation, and this enables more realistic calculations of the surface electronic structure to be made. Some results for other orientations will be described for completeness, however, including recent work on MBE prepared surfaces.

4.4.1 Oxygen adsorption on clean (110)GaAs surfaces The basic effects with which we will be concerned are the manner in which the surface electronic structure influences adsorption and how it is modified by the adsorption process, thus defining the adsorption site. In purely kinetic terms, the important parameters are the sticking coefficient a t zero coverage, the saturation coverage and the transition from oxygen adsorption t o oxide growth. The amount of kinetic information is very limited, but this is more than compensated by the extent of ELS, UPS and XPS studies. The first values for the zero coverage sticking coefficient ( S o ) of oxygen on well-defined surfaces were obtained by Dorn e t al. [252], following some initial observations by Liith and Russell [ 2511. Using ellipsometry, Dorn e t al. measured So values of 3 x lo-’ and 1.4 x for n- and p-type material, respectively, with a saturation coverage independent of doping of 0.6 monolayer of oxygen atoms (1monolayer two oxygen atoms for every surface molecule of GaAs). They also found from AES results that oxygen adsorption to saturation coverage was faster on n-type substrates than on p-type and confirmed that the behaviour was not influenced by electron beam effects. However, at the same saturation level (0.6 monolayer), LEED patterns from p-type surfaces showed little change from the original clean surface, whereas almost all diffraction features were extinguished on n-type surfaces. These differences suggest that the cleaves were not perfect and that some intergap states were still present, possibly partially filled states in the upper half of the gap which could induce a bond with oxygen on n-type surfaces but which would be completely empty on p-type material and so could not form additional bonds. The coverage dependence of the sticking coefficient could be described by an Elovich-type equation, viz.

where 0 = 0.077eV, 2, the number of nearest neighbours surrounding each site, = 4,and e / O , is the surface coverage expressed as a function of the saturation coverage of 0.6 monolayer; i.e. the pre-exponential term describes the probability of adsorption. Clearly, on the basis of eqn. (14), adsorption is an activated process with an activation energy which References pp. 280-289

248

increases linearly with coverage, i.e. every adsorbed molecule increases the potential barrier, E,, which is initially present on the clean surface and whose value is < 0.3 eV. In a later paper [179], this group note that band bending effects start to show up at exposures lo3 times lower than those required to produce 0.01 monolayer coverage as determined by AES. More importantly, they concluded that the band bending was entirely associated with oxygen adsorption; there was n o evidence for the presence of intrinsic surface states in the energy range of the band gap. For all of the work, it was implied that only non-excited oxygen molecules were involved, since filaments were not switched on after cleavage, but they do not comment on any effects that a hot filament would have. The only other kinetic work on well-defined {110)surfaces is due t o Pianetta e t al. [253] who used soft XPS t o follow oxygen adsorption. They found a zero coverage sticking coefficient, So, of 8 x lo-’’ for both n- and p-type material provided that stringent precautions were taken t o exclude excited oxygen species. The saturation coverage of % 0.5 monolayer was only achieved after exposures of lo9- 10l2 L (there is some uncertainty between continuous and sequential exposures). However, with an ion gauge operating even on 0.4 mA emission current, the adsorption rate was increased by a factor of at least 500, with changes in the fundamental adsorption behaviour (see below). Although there is no detailed kinetic analysis, it may be concluded that the cleaves were sufficiently perfect to exclude gap states and a clear differentiation was made between excited and non-excited oxygen. These results consequently represent the best available values for “ideal” surfaces and defined oxygen states. If we now turn t o electron spectroscopic studies (UPS, XPS and ELS) aimed at elucidating the mechanisms of oxygen adsorption and oxidation, we find an enormous literature. However, much of it is repetitive, so this discussion will be synoptic rather than exhaustive. The vast majority of earlier soft XPS results derive from Spicer’s group [253-- 2581 and their principal findings can be summarized as follows. Experimentally, they used synchrotron radiation in the energy range 32 < hv < 350 eV and their important results are shown in Fig. 29(a), in which the spectra are recorded as a function of exposure to non-excited molecular oxygen. Initially, they reported no major shift (< 0.03eV) and a symmetric broadening of only 0.4 eV of the Ga 3 d level for exposures up t o 10l2 L. With the As 3 d level, on the other hand, a chemically shifted peak with a 2.9 eV higher binding energy due t o charge transfer from surface As atoms t o adsorbed oxygen appears. This effect is first observed at lo6 L exposure, when the coverage is W 0.02 monolayer, and the peak continues t o grow, with a corresponding reduction in the As 3 d peak, up to the saturation coverage of 0.5 monolayer at 10l2 L exposure. At the same time, the 0 2p resonance level develops a t a binding energy of w 5 eV.

249 Go 3d

(0)

Exposure

h v = 100eV

40

30 20 10 Binding energy (eV)

0

Binding energy (eV)

Fig. 29. (a) The effect o f non-excited oxygen exposure o n n-type cleaved (11O)GaAs (exposure in Langmuirs, n o hot filament operating). Note the well-defined arsenic shift and the broadening of the Ga 3 d peak (after Spicer et al. [2581). (b) The comparative effects of exposure to excited oxygen on {110) GaAs (after Pian-etta e t al. [2531).

References pp. 280-289

250

As 3d

Ga 3d

! 20

I

25

I

I

40

III

1

I

I

I

I

I

I

I

45

Fig. 30. (a) Ga 3d and As 3d XPS spectra of (11O)GaAs at different exDosures to molecular oxygen. (b) Difference spectra corresponding to (a). The positions of the Ga and As 3 d levels in bulk Ga z0 3 and AszO3, respectivelJ, are indicated. (After Brundle and Seybold [ 2591 ). ,Freshcleave;----,lo LOz;*, 3 X 10'oLOz; 0, 6.6 x 1013 LOz ; A , 48 h in air.

Very recently, however, Brundle and Seybold [259] have shown that there are chemical shifts associated with both Ga and As 3d levels over the exposure range 105-10'4 L of ground state molecular oxygen, which corresponds to coverages between 0.2 and 0.8 monolayer (Fig. 30). As a result of more careful measurement and assessment, the Spicer group [260] also now confirm that there is a shift in the Ga 3d level. From ELS measurements, Ludeke and Koma [ 261-2631 observed that transitions from the Ga 3d level t o empty cation derived states near the conduction band edge were shifted uniformly in energy as a function of oxygen exposure.

251

In all of their earlier publications, Spicer and his co-workers interpreted the apparent absence of a Ga 3 d level shift as absolute confirmation that adsorbed oxygen bonded only to surface As atoms, while Ludeke and Koma strongly emphasized the alternative viewpoint, based on their ELS results, that oxygen bonding to surface Ga atoms did occur, while not, however, ruling out bonding to As as well. Now that the Ga 3 d level shift has been measured and accepted, the simplest interpretation is that a mixture of Ga, 0, and As, Q3 is formed [ 2591, but Ludeke’s idea [ 2631 of a GaAsO, structure cannot be dismissed on the basis of available evidence. At least his contention that some type of Ga-O bond is involved has been completely vindicated, a result in keeping with bulk thermodynamics, which favours the formation of Ga oxides over As oxides. Somewhat different behaviour is observed if excited oxygen species are present (produced by a hot filament) as shown by the results in Fig. 24(b). The chemically shifted peak ( A E = 2.9 eV from the As 3 d level) is present as before, but it does not increase beyond an exposure of 5 x lo5 L excited oxygen. Instead, a second peak with a binding energy shift of 4.5eV appears at about this exposure and eventually becomes the dominant As-associated peak. The Ga 3 d peak also begins to broaden at this same exposure due to the formation of a shifted ( A E = 1eV) peak, which increases concurrently with the shifted As peak. Finally, for very heavily oxidized surfaces, the shifted Ga peak is present together with two As peaks shifted by 0.4 and 3.2 eV with respect t o the clean surface As peak, which is totally absent. The first step in the chemisorption of excited oxygen is identical to that for non-excited oxygen, but it occurs at a much higher rate. The binding energies involved suggest there are four oxygens bound to the arsenic and that the bonds have some double bond character (from comparison with molecular shifts), while the Ga shift corresponds to that in Ga,O,. Since oxides of Ga and As are forming together, this must involve the breaking of back bonds, so that oxidation (as opposed to simple oxygen adsorption) is occurring. However, the initial chemisorption step is apparently essential so that the strain energy involved can be coupled to the energy carried by the excited oxygen and enable the back bonds to be broken. (It is worth noting that cleaved (11O)InP surfaces behave exactly as GaAs, while GaSb is oxidized immediately on exposure t o non-excited oxygen.) We must next ask whether the adsorbed species is atomic or molecular (when the gas phase species is ground state molecular oxygen). Experimental evidence is based on UPS results of oxygen-induced structure in the valence band regions. Simple considerations suggest that if molecular oxygen is adsorbed, several strong features resembling 0, M.O. energy levels should extend 15eV below E F , whereas for atomic oxygen, References p p . 280-289

252

there should be only one dominant feature, respresenting the 2p atomic levels, at x 5--7 eV below E F . Mele and Joannopoulos [264--2661 have performed extensive calculations of the electronic structure of oxygen chemisorbed on { 110)GaAs in various possible bonding configurations using a localized orbital (tight binding) representation, which confirm that these are the major differences t o be expected. Early UPS results [ 2671 showed that, following an exposure of x lo6 L 0 2 , which corresponds to about 5% of saturation, the Fermi level becomes pinned at midgap. Thus 1013 adsorbed oxygen species cm-* effect pinning; this is approximately the number of states needed to pin the Fermi level at the surface, i.e. every chemisorbed oxygen gives rise to one new surface state. This same exposure also produces considerable smearing in the top 5eV of the EDC, which, considering the coverage involved, implies a long-range effect of the oxygen in producing the necessary rearrangement of all the surface atoms, not just the adsorption sites. Simultaneous growth of peaks at - 11,- 9 and - 5 eV below E F also occur with the main oxygen-induced feature at x - 5 eV. Although Piannetta et al. [267] tentatively ascribed these results to adsorption of molecular oxygen, Brundle and Seybold [259] have now shown conclusively that a single broad peak centred at % 5 eV below EF is the only oxygen-induced feature, thus confirming that adsorption is dissociative. Chye et al. [ 2601 have also shown that the earlier data of Piannetta et al. [ 2671, which apparently showed the additional features, was in fact nonreproducible. That oxygen adsorbs atomically was further substantiated by Stohr et al. [268] from their analysis of the amplitude data from surface-extended X-ray absorption fine structure (SEXAFS) measurements. Turning finally to the mechanism of adsorption, it is considered that molecular 0, may be present on the surface at some stage, but in concentrations too low to be detected by the available techniques. It is then suggested by Brundle and Seybold [259] that molecular oxygen is adsorbed into a weakly bound precursor state from which it either desorbs or migrates to an active (defect) site where dissociation occurs. A low density of such sites would then account for the very low sticking coefficient. This interpretation also receives support from the higher reactivity of a damaged surface to oxygen. Barton et al. [269] have proposed a somewhat similar model on the basis of their calculations using the generalized valence bond ab initio method on small clusters. 4.4.2 Oxygen adsorption o n other orientations Work on other orientations is much less complete, but it raises some interesting points and we will indicate how differences can occur between polar and non-polar surfaces. The principal reason is that Ga-stable polar

253

surfaces contain partially filled dangling cation orbitals, which would be expected to react strongly with oxygen, so that G a - 0 bonds should form very readily during adsorption. Unfortunately, most of the experimental results must be treated with caution in terms of general applicability. Ranke and Jacobi [ 2701 investigated oxygen adsorption on (111) Ga, { i i i ) A s and {loo) surfaces. They proposed a model whereby oxygen adsorbs in two forms, one non-dissociatively in a weakly bound state on t o surface As atoms, the other dissociatively t o Ga atoms, such that back bonds are broken on adsorption. However, the surfaces, although showing reconstruction, were prepared by ion bombardment and annealing, while their UPS results were obtained with He I radiation (21.2 eV). This is not paticularly suitable for reconstructed surfaces with large surface meshes, since the high energy contracts the angular range that covers the surface Brillouin zone, so the resolution is poor. Better results are obtained with Ne I(hv = 16.8 eV) [ 2711. They also failed to indicate whether adsorption occurred with an ion gauge operating and their TDS results could have been confused by simultaneous desorption from the substrate heater and holder. Ludeke and Koma [261] studied the same surface orientations using ELS and their preparative technique also involved ion bombardment, but the annealing was carried out in the presence of an As4 flux to minimize surface dissociation and so produced reconstructed surfaces. The form of the oxygen (excited or non-excited) was not specified, but the initial sticking coefficient on a {100)c(2 x 8) As stable surface was 8 x lo-’, with an increasing adsorption rate as the surface concentration of Ga atoms increased. From electron loss spectra, the peak assigned as Ga 3 d to conduction band state transitions decreased rapidly with oxygen coverage and a new oxide-related structure appeared. Loss peaks associated with As 3 d transitions were completely insensitive to oxygen exposure and they concluded that oxygen adsorption was confined to Ga surface atoms. Until better defined surfaces, grown in situ by MBE, have been studied by a combination of techniques with careful control of the state of the oxygen, it is not possible t o draw any firm conclusions about oxygen adsorption on polar surfaces. At present, reliable data (and conclusions) are confined to the {110} surface.

5. Metal interactions with semiconductor surfaces 5.1 INTRODUCTION

This section is based largely on considerations of the kinetic and electronic processes controlling the interaction of metals with semiconductor surfaces. However, it also includes some discussion of the References p p . 280-289

254

corresponding interactions with group VA elements and also interactions between semiconductors, since there are many close similarities. This also encompasses the phenomenon of epitaxy, or the oriented overgrowth of films, which is outside the scope of this article, but interested readers are referred to the book edited by Matthews [ 2721 and the review articles by Voorhoeve [273] and Joyce [274], the latter dealing specifically with semiconductors. Similarly, we have also decided t o exclude the process of thin film deposition by a chemical reaction at the substrate surface, known as chemical vapour deposition (CVD) or vapour phase epitaxy (VPE), since this is generally carried out in systems operating at atmospheric pressure, with processes being controlled by fluid dynamic effects. This is a subject of considerable interest in its own right and comprehensive reviews have been written by Powell e t d. [275] and Feist e t al. [276]. However, some reactions which result in the growth of semiconductor films have been studied by molecular beam techniques and this work will be considered here. 5.2 METAL-SEMICONDUCTOR INTERFACES

Any study of the adsorption, desorption or migration of metal adatoms on semiconductor surfaces involves forming an interface, even a t the submonolayer level. Until quite recently, it was always treated as a simple, chemically abrupt, ideal planar junction. However, during the past two years, it has been firmly established, mainly but not exclusively by photoemission experiments, that extensive interpenetration over distances of several hundred units occurs even at room temperature for a very wide range of material combinations, provided that the semiconductor surface is clean prior t o metal deposition. This effect has largely negated the fundamental significance of much of the earlier, and some more recent, work for the following reason. If condensation occurred on a contaminated surface, an abrupt interface may have been formed, but the surface would not then have been characteristic of the semiconductor. Conversely, if the surface was clean initially, an abrupt interface would not have been formed, but invariably the results were interpreted on the basis of such a model. Voorhoeve [ 2731 has reviewed the earlier work and we will not make further reference t o it here. Rather we will concentrate on producing a synopsis of results and models from those publications in which there is an awareness of the complexity of the interactions. The difficulty facing the reviewer is the almost complete incompatibility between direct kinetic measurements and electronic/compositional studies of metal deposits. Kinetic results have been used to construct models of the surface metal phase with varying degrees of geometric complexity and/or thin film growth mode, but in all cases the metal is assumed a priori to be “on top” of the semiconductor and to desorb from it in some classical way. Unfortunately, direct measurement of the interface chemistry

a

255

shows this t o be very unrealistic in many systems. It must be emphasized, therefore, that if direct adsorption- desorption measurements are not complemented by an evaluation of the interface, desorption models must be treated with extreme caution. In the following sections, we will first consider metalsilicon and metal/III-V compound systems, covering electronic and chemical interface effects. We will then present a general treatment of the adsorptiondesorption models which have been proposed, including some discussion of the interpretation of thermal desorption spectra, and finally we will discuss to what extent the two sets of data can be reconciled. We will not be concerned with metal films in device fabrication, nor the formation of additional phases by heat treatment. We shall only deal with interactions and interface chemistry which is directly relevant to adsorptiondesorption behaviour. 5.2.1 G o l d s i l i c o n This system has been extensively investigated by AES, LEED and photoemission over a range of adsorption temperatures. In general, there seems to be reasonable agreement between the various authors [27'72811 on the structure and composition of the interface. The basic phenomenon, even at room temperature, is the formation of silicide phases at both the Au-Si and the Au-vacuum interfaces, separated by a gold film containing some dissolved silicon. Qualitatively, the effect is the same for S i ( l l 1 ) 2 x 1, (111) 7 x 7 and (100) 2 x 1 surfaces, although the stoichiometry of the silicide phases may differ. When the silicides are formed at room temperature, they lack long-range order and are effectively amorphous, but they crystallize on heating. The gold film is in parallel orientation to the substrate and, depending on the heating schedule, may agglomerate at temperatures > 700 K. The precise stoichiometry and orientation relationships of the silicides need not concern us further here, but it is crucially important to be aware of their presence and their influence on surface analytical investigations. Their formation has been carefully studied by AES combined with depth-composition profiling [ 277, 2801 and it was shown to be essential to follow the development of the Si peaks due to silicides at 88.5 and 95eV. It is not sufficient t o monitor only the main Au (69.5eV) and Si (92 eV) peaks to assess accurately the surface composition [ 278, 2791 , since this does not take account of compound formation. The most detailed information of Au-Si room temperature interactions has been obtained by Braicovich et al. [281] for S i { l l l } 2 x 1 cleaved surfaces. They used photoemission with synchrotron radiation covering the photon energy range from 10 to 200eV and studied Au coverages from 6 = 0.15 monolayer to 6 = 160 monolayers. We give a brief summary of their results, which not only indicate the degree of complexity of some References p p . 280-289

256

s

7t

0

1

I

I

1

I

I

1

2

3

4

5

6

1

7

c = 0/0.15 Fig. 31. Reduced intensity of Au 4 f emission as a function of reduced coverage on a Si substrate. The broken line is the expected result if there is no Au-Si admixing at the interface (after Braicovich et al. [281]).

metal-semiconductor surface interactions, but also provides a basis for the discussion of desorption behaviour. In the low coverage region (0 = 0.15-2.0 monolayers), the major effect is the rapid reduction, as a function of coverage, of emission from Si dangling bond states present on the clean surface. The mechanism for this can be seen if emission from the Au 4f levels is also followed as a function of coverage. If the Au film simply grows on the Si surface with no intermixing, this emission should be proportional t o coverage. In practice, it is not, but is indicative of a substantial gold deficiency for all coverages (Fig. 31), which can only be explained by intermixing to depths > 5 from the surface. 5 is the approximate escape depth of the photoelectrons, but the actual intermixed depth has been shown t o be = 1 5 8 . The disappearance of surface states can therefore be attributed to surface disruption by Au atoms and the formation of a wide interface. The Au 5 d emission is consistent with the Au being dispersed (i.e. atomic-like) and interacting electronically with Si, suggesting the possibility of alloy formation. With increasing coverage up to 15 monolayers, emission from Au becomes less atomic-like and more similar to that from bulk material, but at the same time the energy separation between the Au 4f and Si 2 p

a

a

257 84.1I

99.9;

'

'

'

I

I

I

I

10 I 20 30 40 Au coverage, 0 (monolayers)

I

I

50

Fig. 32. Au 4 f and Si 2 p binding energes (referred to the Fermi level) as a function of Au coverage (after Braicovich et al. [ 281 ] ).

-

_______ -_ _ _ _ _ -_____ Au silicide

Possible surface segregation of SI

Au + dissolved SI Au silicide

Si substrate

Fig. 3 3 . Schematic representation of the overall effects occurring during Au deposition on Si at room temperature.

levels increases (Fig. 32). A progressive change is therefore taking place in the Au-Si bonding and since it is the Si binding energy which is increasing, the reaction is probably occurring predominantly near the metal-vacuum interface. For coverages beyond 15 monolayers, there is still no pure Au present, but only a Au film containing dissolved Si between two regions of silicide. There is, additionally, a surface enrichment of silicon. The effect of heating is to diffuse more Au further into the Si substrate, leaving a surface more silicon-rich. Figure 33 is a schematic representation of the composition of a room temperature deposit. References p p . 280-289

258

5.2.2 Siluel--silicon

There seems t o be a general consensus [282--2851 that this is a nonreactive interface, i.e. while Ag might be chemically bonded to Si, the interface is essentially planar with no dissociation of the semiconductor and no anomalous diffusion. However, no definitive results have been reported to support this belief and there is evidence which suggests the formation of a silicide phase at room temperature [ 2831. Some difference in condensation behaviour apparently occurs above and below = 470K. Below this temperature, the growth follows a layerby-layer two-dimensional mode, while above it exhibits so-called StranskiKrastanov characteristics in which an epitaxial mono- or multi-layer is first formed on the surface and three-dimensional crystals of the deposit then grow on top of (or from within) this layer. Clearly, when the number density of these crystals becomes high, the difference between the two growth modes is semantic and some workers take the viewpoint that all growth in this system follows the Stranski-Krastanov mode. Four groups [ 282-284, 2861 have studied deposition on (111)7 x 7 Si surfaces prepared by thermal cleaning, which may not have been atomically clean, and although McKinley et al. [ 2851 used a (111)2 x 1 surface prepared by in situ cleavage, their evaporation systems (W spiral and Ta foil) were potentially sources of large amounts of contamination. Venables et al. [286] have made direct observations of the film morphology by UHV-SEM, but the composition and planarity of the interface have been deduced from AES, ELS and UPS. Using AES, Le Lay et al. [282] followed the decay of the Si 92eV peak and the growth of the Ag 353eV peak with increasing deposition and, from the shape of the curves as a function of coverage, deduced the growth mode above and below 470K substrate temperature. They did not, however, consider other peaks which could have been produced from a separate Ag- Si phase. Strictly, they should only have interpreted the results they presented in terms of elemental Ag and Si, but they assumed that the peaks they measured were direct indications of the total amounts of Ag and Si. Housley et al. [283] also used AES, but in addition t o observing the main peaks, they computed difference spectra and showed that a new peak appeared at 82eV. Although they were not able t o follow its height as a function of coverage, they suggest it derived from a bond having Si 3 p Ag 5s character which occurred at the interface. Alternatively, it could be produced by peak shifting/splitting due to silicide formation, as in the Au-Si system. From ELS measurements, Derrien et al. [284] observed a bulk Ag spectrum after three monolayers deposition at room temperature, but only after 30 monolayers at 470K. They attributed this t o the different

259

growth modes at the two temperatures, but did not consider the possibility of chemical interaction. In the only work reported on a (111)2 x 1surface prepared by in situ cleavage [ 2851 , there appears to be evidence for Stranski-Krastanov growth mode with pronounced three-dimensional crystallite formation even at room temperature. From UPS measurements on these films, however, the most noticeable features are the rapid attenuation of photoemission from the Si dangling bond state and the growth of Ag d state emission. This increased emission is not proportional to coverage, however, but becomes asymptotic after x 1 monolayer, an effect attributed t o the Stranski--Krastanov growth mode. However, in the Au-Si case, similar observations [ 2811 were interpreted in terms of intermixing, with the disappearance of the dangling bond states then being caused by surface disruption and the formation of a wide interface region. From the available evidence, it seems reasonable t o conclude that the ideality of the Ag Si interface is not proven, but that any interaction effects are significantly less than those occurring with Au- Si. 5.2.3 Group III metals (Al, Ga, In)--silicon Work on these combinations is in large part attributable t o Rowe et al. [ 287-2921 with some theoretical input from Chalikowsky [ 2931 . Again, the interface is not easy to define, but t o quote Rowe et al. [ 2911 “results clearly show that the electronic states at the metal semiconductor interfaces are different from those of the clean silicon surface, or from those one would expect at an abrupt junction”. The experimental results have been obtained almost entirely from LEED, UPS and LEELS measurements for coverages ranging from a fraction of a monolayer to x 20 monolayers, but an important point has been the use of molecular beam techniques for metal deposition to minimze contamination effects. (The metal was effused from clean Knudsen sources.) If we consider first the LEED observations, the initial state is a thermally produced (111] 7 x 7 structure. The 7 x 7 periodicity is retained up to x 1 monolayer of metal deposit, but as an extrinsic 7 x 7 pattern induced by the metal rather than a simple decrease in intensity of the Si 7 x 7 reflections. This was quoted as evidence of an essentially two-dimensional morphology for the metal deposit, since the formation of three-dimensional nuclei with clean silicon between them would have only reduced the intensity of the intrinsic 7 x 7 pattern. Beyond one monlayer, growth could follow a Stranski-Krastanov mode, however. There are two important effects of metal deposition on the electronic structure of the silicon surface. The first is the saturation of the dangling bonds and removal of the associated back bond states by a very small metal coverage so that the Fermi level is not pinned by dangling bond References pp. 280-289

260

states but by new metal-induced states. Secondly, the main one-electron transition related t o the bulk Si band structure is virtually unchanged, which means that the metal- silicon bonding a t the interface is similar t o bulk Si covalent bonding. This is consistent with Group I11 metals being substitutional impurities, but the interface cannot then be described by a sharp metal- silicon boundary. In the proposed model, initial metal atoms are chemisorbed either as substitutional impurities in the silicon lattice, with strongly localized bonds, or t o fill free surface vacancies. With further deposition, the bonds formed are still much more covalent than could occur for a pure metallic region, but nevertheless the interface region formed at this stage has considerable metallic character, as shown by the high electron density and the tailing of states into the gap. Its effect is t o introduce a high density of new states in the gap and pin the Fermi level in a new position. 5 . 2 . 4 Caesuim-silicon

This system has been studied by Goldstein [294] and Levine [295], and seems to be an example of very site-specific adsorption in which the Cs atoms occupy four-fold coordination sites above the uppermost Si atoms. There is some similarity with other systems in that the Si dangling bond states are removed by Cs deposition t o be replaced by Cs-induced gap states. There is, however, no evidence for interface instability effects.

5.2.5 Metals--Group III- -V compounds Many of the interface phenomena which occur with metal-Si systems occur in a more exaggerated way for metals-- Group III-V compounds and a very large literature has been generated in the past few years. Much of it is concerned with a detailed study of the electronic properties of interfaces in relation t o Schottky barrier formation, t o which we will only make passing reference. However, the chemical effects are very pronounced and we will concentrate on this aspect as providing a particularly good insight into the complexity of the interactions. A useful starting point is t o consider the room temperature deposition of a single metal, Au, on the in situ cleaved (110) surfaces of GaSb, GaAs and InP, studied in detail by the Spicer group [296-3001 and in specific aspects by Brillson e t al. [301, 3021. The experimental techniques used were principally UPS/SXPS (with synchrotron radiation) and AES in combination with sputter profiling. Gold coverages from a fraction of a monolayer t o over 100 monolayers were investigated. Since the interpretation of interface behaviour from such measurements is critically dependent on the morphology of the metal film, we will deal with this problem first. No electron micrographs are available, so structure must be deduced from spectroscopic data. For very thin films, the development of the Au 5 d bands provides the relevant information. The

261

spin-orbit splitting between the d S l 2 and d3,2 levels is X 1.5 eV in atomic gold and = 2.3 eV in bulk gold. For coverages < 0.2 monolayer, the measured splitting using UPS is very close to 1.5eV, indicating that the gold is dispersed and atomic-like; it has not clustered into three-dimensional islands. Nevertheless, this coverage of gold (0.2 monolayer) is sufficient t o pin the Fermi level at the surface and we will discuss this point in more detail subsequently. For Au coverages 2 4 monolayers, the splitting reaches the bulk metal value, but the valence bands still do not resemble bulk Au because the formation of alloys or compounds with the semiconductor components prevents the Au atoms from assuming the lattice structure of gold metal. We cannot, however, use these observations t o confirm that growth occurs by a two-dimensional layer-by-layer process. The same splitting would be observed for large islands separated by areas of clean semiconductor and we must turn to the SXPS results t o provide a model for thicker films. The spectra obtained by the Spicer group for GaSb and GaAs are shown in Figs. 34 and 35, respectively (InP is qualitatively similar to GaAs). They are outer core-level spectra of Au and the semiconductors taken for increasing Au coverages from 1 monolayer to > 100 monolayers, and since the photoelectron escape depths at

rAu-4f

Ga Sb + Au

Sb-4d

hw =120eV

I I I I I . . 85 30 20 10 VBV Binding energy

(eV)

Fig. 34. Photoemission spectra for different Au coverages on GaSb (after Chye et al. P O 0 1 ).

References p p . 280-289

262 ____-----

Ga As + A u

h w = 165eV

Binding energy

(eV)

Fig. 35. Photoemission spectra for differen [3001).

4u coverages on GaAs (after Chye et al.

a),

the energies concerned are very short (5-10 the information in each of the spectra relates only to the surface region. Before extracting morphological data from these spectra, we will consider the most important feature, the presence of peaks from the Group I11 and V elements, even after 100 monolayers of Au have been deposited, despite the x 5a photoelectron escape depth. With GaSb, there is a preferential segregation of Sb to the surface, but for GaAs and InP, the Group I11 and V elements are present in roughly equal amounts at the surface. In all cases, this must involve considerable interaction and possible dissociation at the interface followed by migration through a continuous Au film, since if growth occurred in the form of three-dimensional islands dispersed on a clean semiconductor surface, the spectra would all show equal amounts of Group I11 and V elements. More information on the compositional profiles through the Au films can be obtained from AES combined with sputtering and two examples from the work of Chye et al. [300]are shown in Figs. 36 and 37 for GaSb and GaAs, respectively. It is quite clear that substantial amounts of both Group I11 and Group V elements become distributed throughout the whole of gold films at least 200a thick deposited at room temperature and in the case of Sb, there is additionally considerable surface segregation. The nature and composition of any separate phases which form is not known, but it is abundantly clear that

263 1

Depth 100

(A) 200

I

I

f

0

5 10 15 Sputtering time (min)

Sb

20

Fig. 36. Compositional profile of an Au -GaSb interface obtained by AES and ionmilling (after Chye et al. [ 3001 ).

that these interfaces are very unstable and cannot be treated in a simple geometric fashion. A second materials combination which has attracted a large amount of interest and generated a substantial literature [ 303-3111 is GaAs-Group I11 metals (Ga, A1 and In). Again, the surface in question is an in situ cleaved {llO} and we will only be concerned with room temperature deposition. Starting from a clean surface with no band bending, the first effect of A1 deposition, at x 0.1 monolayer coverage, is an upwards band bending of ,> 0.5eV which pins the Fermi level at the surface near mid-gap. The chemical effects occurring as a function of coverage have been followed by observing photoemission from A1 2 p , Ga 3 d and As 3 d core states. The results of Skeath et al. [307]are shown in Fig. 38 and illustrate a number of important features: from the shifts in A1 References p p . 280-289

264

Depth ( A ) 0 I

100

200

I

I

, Ga A s t Au

3 Sputtering time (min1 Fig. 31. Compositional profile of an Au--GaAs interface obtained by AES and ionmilling (After Chye et al. [300]).

energy at different coverages, it can be seen that two sequential low coverage states occur, both of which are different from bulk Al, although by the time 8 has been deposited, the 2 p binding energy has assumed its bulk value. The Ga and As 3 d levels both shift towards lower binding energies, which simply corresponds to the upwards band bending, and although this is the only effect observed for As, difference curves show a substantial increase in emission on the low binding energy side for the Ga 3 d level. This is consistent with the emission expected from free metallic Ga and it occurs at A1 coverages 2 1 monolayer. For coverages < 1 monolayer, however, the emission is shifted t o higher binding energies, which is indicative of very small Ga clusters dispersed on the surface (i.e. more atomic-Ga-like). The proposed interaction model involves the formation of an A1 coverage to = 0.1 monolayer, with very little penetration of A1 into the lattice, but with increasing A1 deposition there is an increasing tendency for A1 to replace Ga. The thermodynamic driving force is the higher heat of formation of AlAs than GaAs (117 kJ mole-', cf. 71 kJ mole-' ). It is speculated that the initial surface accumulation arises because a Ga surface vacancy, created thermally by A1 adsorption, will more probably be refilled by a Ga atom until a comparatively large A1 population has formed. In

a

265

Initial s t a t e e n e r g y (eV below E, 1

Fig. 38. A1 2 p , As 3 d and Ga 3d core level spectra for several A1 overlayer thicknesses (in 8) on GaAs (after Skeath et al. [ 3 0 7 ] ) .

spite of this first stage stability, however, extensive diffusion of both Ga and As occurs and over 200 of A1 are required before the As 3d level is n o longer observable with 130 eV radiation [ 3081 . Mele and Joannopoulos [ 312, 3131 have performed tight-binding calculations for ordered half-monolayer coverages of A1 chemisorbed on GaAs based on a model in which A1 displaces Ga, which then bonds t o surface As atoms as though the lattice were continued as normal, i.e. a simple exchange reaction. Surface densities of states were computed for various chemisorption configurations and t o account for the observed upwards band bending it is necessary t o postulate a relaxation effect whereby the A1 atom rotates away from its “bulk” position, which lowers the surface electronic energy by 0.3eV per unit cell as the As-Al-As bond angle is reduced. In this new structure, the dangling bond orbitals of the threefold coordinated cation are slightly rehybridized, gaining in s character and shifting t o lower energies. The interaction of Ga with GaAs shows some differences from that with Al. There is a tendency at sub-monolayer coverage t o form two-. dimensional clusters, which do not greatly disturb the GaAs surface lattice electronic structure. When more than one monolayer is deposited, the coverage tends t o become uniform with some As diffusing into this overlayer. The bonding of Ga t o the surface raises an interesting point; the simplest arrangement puts the Ga atom in the position it would occupy if it were part of the next layer. Total energy calculations [309] show this t o correspond t o a local minimum and, somewhat surprisingly, t o a surface geometry which is ideal and unrelaxed. If Ga is adsorbed into a relaxed site, the total energy is appreciably higher. This model is consistent with surface Fermi-level pinning by Ga and with LEED observations.

a

References p p . 280-289

266

However, there is an alternative possibility, which additionally produces better agreement with UPS results. This is a two-fold coordination geometry in which the chemisorbed Ga is bonded to a surface Ga and a surface As atom with roughly equal bond lengths. The vertical distance of the adsorbed Ga atom from the substrate surface plane is then only about half that which it would be in the one-fold site. This configuration requires the repulsive Ga ion-Ga ion interaction energy t o be 4.3eV (i.e. the value of a Ga-Ga atom pair). The actual value is not known, but is estimated t o be % 4 eV, so the two models cannot be distinguished on the basis of total energy calculations. Finally, we may consider the GaAs-Cs system, which is of considerable technological importance for negative electron affinity photocathodes, i.e. where there is n o potential barrier between the conduction band minimum in the bulk of the solid and the vacuum level at the surface. This condition is achieved by downward band bending at the surface brought about by heavy p-type bulk doping and unfilled electronic states at the GaAs-Cs interface. We will not be concerned with device aspects here, but a useful review, emphasizing the physics, has been written by Spicer [314]. The device importance has, however, gven rise t o wide ranging investigations [315--3211 and we will summarize the salient features. We will again restrict our comments t o results with { l l O ) surfaces since, although other orientations have been used, it is not clear to what extent extrinsic effects (damage, contamination, etc.) may have had an influence. Only comparatively low coverages (5 4 monolayers) have been investigated, with surfaces prepared either by in situ cleavage or ion bombardment and annealing, and the critical factor is the nature of the adsorbed species. Since the ionization energy of a free Cs atom (Ei = 3.89 eV) is lower than the work function of clean, cleaved GaAs, caesium should be adsorbed as positive ions with compensating negative charge located in a space charge layer or in surface states, i.e. a dipole layer is formed. In spite of this apparent difference, the effect. of Cs on the surface electronic structure of {11O}GaAs is very similar t o that of Au in that the Fermi level is pinned at the surface ( z 0.45eV above the valence band edge) at coverages well below one monolayer (= 0.2 monolayer). Such pinning can only occur if a large number of surface states is present in the gap, but since there are no gap states with the clean surface, they must be created by the adsorbed Cs. This point was first made by Scheer and van Laar in 1969 [315]. Contrary t o the case with Au, however, with Cs adsorption there is n o suggestion of, or evidence for, interface instability. The adsorption mechanism has been studied by a combination of LEED, AES and work function changes [317, 319-3211. At room temperature, Cs adsorbs on (110) GaAs in an ordered c ( 4 x 4 ) structure for coverages around one monolayer [ 317, 3211 and although the symmetry

<

267

is identical for cleaved and ion bombarded and annealed surfaces, the diffracted intensity is higher on the former [ 3 1 7 ] . At coverages < 0.2 monolayer, a p ( 3 x 2 ) structure is formed which becomes c ( 6 x 2 ) at intermediate coverages (" 0.5 monolayer). By measuring the Cs Auger signal as a function of time for different Cs fluxes at different substrate temperatures, it was found that the coverage increased linearly with time and saturated at an equilibrium value dependent on flux and temperature. The sticking coefficient is unity up to monolayer coverage, but decreases with further adsorption. The deposit grows in a layer-by-layer fashion (curve of coverage vs. time has welldefined break points [ 3221 ) and this AES data was used by Derrien and Arnaud d'Avitaya [321] t o construct a series of adsorption isobars (0 vs. T, a t various Cs pressures, p ) and from them isosteres ( p vs. T, at constant 0 ) . The molar isostenc heat of adsorption, q s t , can then be calculated as a function of coverage from

(15)

qst = R T , ~ @ logPiaT,),

which, for low coverages, represents a reasonable approximation t o the bond energy of Cs on GaAs. The results are shown in Fig. 39, from which it is clear that beyond one monolayer coverage, the value is very close t o

0 Coverage

I

I

1

2 ( monolayers)

Fig. 39. Isosteric heat of adsorption (solid line) and calculated bond energy (broken line) vs. coverage of Cs on GaAs (after Derrien and Arnaud d'Avitaya [321]).

References p p . 280-289

268

the bulk heat of sublimation of Cs, while in the early stages of adsorption Cs is strongly bound to the surface. The authors treated the intermediate region as indicative of two-phase behaviour because of the slope change at 8 x 0.5, which may represent an over-interpretation, but the binding energy clearly decreases with coverage. The adsorption process has also been followed by measuring the change in work function with increasing Cs coverage [320, 3211 and the results of both groups fit on the same curve even though Clemens et al. [320] used a cleaved surface while Derrien and Arnaud d’Avitaya [ 3211 prepared their substrate by ion bombardment and annealing. There is a decrease of > 3 eV by monolayer coverage and Clemens et al. [320] claim there are six linear segments of the curve up t o 1/3 monolayer; however, from inspection of their data, it is difficult t o imagine that the points do not fit a smooth curve. A simple model [321] consistent with the isosteric adsorption, LEED and work function results is that chemisorbed Cs atoms are positively ionized, with the compensating negative charge located either in a space charge layer or in surface states. This gives ionic character to the Cs-GaAs bond, which consequently is strong in the initial stages of adsorption. There is, however, a repulsive lateral interaction between adsorbed Cs atoms as the result of an electrostatic dipole-dipole interaction, so that at low coverages, the Cs adsorbs in rows with wide separation. With increasing coverage, mutual depolarization between adatoms lowers the Cs charge transfer, accounting for the reduced Cs-GaAs bond energies. By considering the surface electronic structure in rather more detail, a further development of the model is to postulate that initially the Cs bonds to surface Ga atoms by transferring negative charge into empty, Ga-associated surface states (the As-associated states are filled). This means that Cs is bonded t o one surface atom a t low coverage, saturating empty dangling bond surface states, but as the coverage increases, a bridge bond is formed with three substrate atoms [320], The broken line in Fig. 39 shows the Cs- GaAs bond energy as a function of coverage calculated from the Levine-Gyftopoulos theory [ 3231 by Derrien and Arnaud d’Avitaya [ 3211. The theory treats the bond as partially ionic, partially covalent and shows similar trends t o the experimental data. Thermal desorption spectroscopy data [318, 3211, which show a single high temperature peak for 8 < 0.5 and additional very broad peaks as the coverage increases t o 8 = 1, can also be interpreted in terms of a decreasing binding energy with increasing coverage on a very simple first-order kinetics desorption model. As we show below (Sect. 5.2.7), however, the evaluation of TDS data from metalsemiconductor systems is subject t o many ambiguities, so simple analysis must be treated with caution. We may conclude, however, that probably as the result of a strong ionic contribution t o the bonding, this system appears more straightforward than other metalsemiconductor combinations, at least with regard to interface instability.

269

5.2.6 Mechanisms of metalsemiconductor interface interactions It is evident from the preceding sections that, with a few exceptions, adsorption of a metal on a clean semiconductor surface leads to pronounced solid state reactivity. We may reasonably ask, therefore, whether it is possible t o establish a general mechanism for this interface instability. Specifically, an explanation is required for solid state reactions and transport processes which are occurring at comparatively high rates at room temperature, where they might have been expected to be negligible. Two plausible models have been advanced, one by Tu et al. [324, 3251 and the other by the Spicer group [298,326, 3271. Tu suggested that the first step is an interstitial diffusion jump of a metal atom into the semiconductor, since the movement of a semiconductor atom would be unfavourable at room temperature. An interstitial defect increases the number of nearest neighbours of its surrounding host atoms, so that electrons involved in the semiconductor lattice bonds are no longer localized, but are shared with the interstitial atom, i.e. the bonds become saturated or metallic-like and correspondingly weaker than the fully saturated covalent bond. Thus an interface in which there is a high concentration of interstitials has two important characteristics: firstly, it modifies the semiconductor bonding so that atoms can dissociate from their lattice sites with a low energy, and also the growth front of the new phase(s) has a high mobility as a consequence of interstitial diffusion. Brillson et al. [302] also propose that metal indiffusion can be the dominating mechanism for some, if not all systems (e.g. Au/GaAs). The Spicer group treat the problem of obtaining sufficient energy to dissociate the semiconductor surface by relating it to the heat of adsorption of the metal, which they equate t o the heat of bonding of the metal t o the free semiconductor surface. With the exception of Cs on GaAs (vide supra), no values of this parameter are known, but they point out that the heat of condensation of a metal on itself can be very high (e.g. x 3.8eV atom-' for Au). A t the adsorption site, this excess energy is released in a localized volume and the neighbouring semiconductor atoms are momentarily excited, creating point defects, i.e. mass transport. A similar thermodynamic expression of this idea relates the heat of reaction for the formation of a specific interface phase to the surface reactivity of a particular metal condensate, as discussed by Williams et al. [328]. With this modeI, however, it is not clear why equivalent effects are not seen during the condensation of semiconductor films (see Sect. 5.3). A t this stage, there is insufficient evidence to decide which, if either, of these models is correct, or indeed whether aunified explanation is possible. We should point out that Tu's model was derived principally for Si, whilst Spicer's proposals were based on work on Group 111-V compounds, but it is quite evident that there is a common feature of high interface mobility. As an aside, it is interesting t o note that these interface effects make the classical theory of Schottky barriers basically untenable. A complete References p p . 280-289

270

discussion of the problem is outside the scope of this article, but it is worth indicating how an increased knowledge of surface and interface state behaviour in relation to adsorption has resulted in substantial modifications to existing theories. Bardeen [329] originally proposed that surface states at the free semiconductor surface, which remained after deposition of the metal, pinned the Fermi level at the interface and fixed the barrier height. For some semiconductor surfaces such as cleaved { l l O ) GaAs, however, there are no intrinsic gap states, and Heine [330] postulated that interface gap states are due to tails of the metallic wave functions penetrating into the semiconductor. Although this model is more in keeping with known surface state behaviour, it is still based on the assumption of an ideal planar junction between metal and semiconductor, which in the light of recent work is clearly inappropriate. Spicer et al. [ 298, 3271 have recently proposed an alternative explanation for Group 111-V compounds which is more consistent with the known facts. They point out that the Fermi level pinning position on a particular semiconductor is effectively constant for a wide range of metals (and oxygen), although different semiconductors have quite different pinning positions. In addition, the pinning is complete after adsorption of no more than 0.2 monolayer, which cannot possibly be explained using ideal interface concepts since the metal is still atomic-like and does not possess bulk properties. The new model is based on the idea that the states responsible for the pinning are defect states introduced by the adatomsemiconductor interactions, which remove anions and/or cations from the Group 111-V compound. The corresponding explanation for silicon, advanced by Ottaviani e t al. [ 3251 , is that the specific reactive interface (i.e. silicidesilicon) determines the barrier height, but again it is the result of a metalsemiconductor interaction.

5.2.7 “Classical” models o f metal desorption from semiconductor surfaces We will conclude this section with some discussion of the conventional treatment of metal desorption, which ignores the interface effects we have been describing. Our reason for doing this is to try to bring the two sets of work into perspective and to investigate the “classical” models in the light of our present knowledge of interfaces. We will also attempt to clarify the analysis of thermal desorption spectroscopy data from metalsemiconductor systems since we believe the presently accepted procedures t o be very misleading. The most plausible models to have been developed for metalsemicondutor systems are all based on the original concepts of Arthur [331], with Kern and his co-workers [279, 282, 3321 being principally responsible for more recent extensions and elaborations. Arthur assumed the metal film to be in the form of two-dimensional

271

islands with an atomically sharp interface between them and the semiconductor surface. Any dissociation of the islands could only occur at their periphery and atoms produced by dissociation entered a weakly adsorbed precursor state from which they could either desorb or be re-incorporated into islands by surface diffusion. This total process can be described by two coupled differential equations n dn - - nkN1l2 i- krN1l2 7 dt and

where n is the number of atoms in the precursor state having surface lifetime 7,N is the number of atoms in two-dimensional islands, k is the rate coefficient for the incorporation of adatoms into islands and k ’ the rate coefficient for island dissociation. At steady state, d n / d t = 0, i.e. at constant temperature the precursor state population is constant, and this simplification allows the desorption rate, - dn/dt, to be written as

There are, however, three very important implicit assumptions in this model, apart from those of an ideal interface. Firstly, since desorption is only allowed t o occur from a constant precursor state population ( d n / d t = 0), it is effectively always a zeroth-order process. If a different order is observed, desorption is not the rate-limiting step. The second point is that this treatment is only appropriate for cases where the metalmetal bond energy (around the peripheries of the islands) is less than that for the metal- semiconductor, since for the opposite case the weaker adsorbate-surface bond will not prevent an atom desorbing once it has acquired sufficient energy to break the (stronger) metal-metal bond. Thirdly, no provision is made for possible diffusion of the adsorbate into the substrate during desorption. Some generalizations can be made t o the model and, in particular, the morphology of the deposit can be allowed to assume m y form. If dissociation is rate limiting, the kinetic order can then be used to determine the morphology. In this context, the desorption rate can be written dn = - n: -_ dt

r(E, T )

( x = 1)

where n, is the number of atoms in a position t o desorb and E and T refer to desorption activation energy and temperature respectively. n, can then References p p . 280-289

212

be rewritten in terms of the total number of atoms appropriate t o a specific morphology. For example, n, can be replaced by for flat discs or n2I3 for hemispheres, where the fractional exponent represents the kinetic order. Allowing a gaussian distribution of radii makes only a marginal difference t o the exponent, since the largest assemblies always dominate the process [333]. To avoid the restrictions imposed when desorption can only occur from the precursor state, Le Lay et al. [279] allowed for a direct desorption flux, which of course always gives fractional-order kinetics within the framework of this model if this process is rate-controlling. Le Lay e t al. thus defined two possible rate limitations, the zero-order case when desorption occurs from a steady state precursor population and fractional order with direct desorption. These more general models appear t o have complete validity in the sense that they describe the ideal case, but are negated by any solid state transport processes at the interface and these, as we have seen, occur with almost all metal--semiconductor combinations. Finally, we will consider in this section the analysis of thermal desorption spectra of metals from semiconductors. In general, the techniques have been developed for gas desorption from metals, but there are several important differences, which are not always realized, when the adsorbate is a solid. We gave a brief outline of the basic aspects of TDS in Sect. 2.4.2 and for metalsemiconductor systems, spectra are almost always single peaked with the peak (i.e. maximum desorption rate) moving t o higher temperatures with increasing initial coverage; in addition, the spectra have a common leading edge, implying a coverage-independent initial desorption rate, i.e. a zeroth-order process. A typical data set, for Pb-GaAs, is shown in Fig. 40. We have indicated previously (Sect. 2.4.2) that the spectra can, in principle, be analysed by (i) fitting exponentials t o the initial desorption rate using a Frenkel-type equation [7 = 70 exp (Ed/kT)] , (ii) measuring the temperature, Tp, corresponding to the maximum desorption rate, and (iii) using the quasi-isotherm method (“complete analysis”). Method (i). This is a purely empirical approach which can give moderately accurate values of Ed, but a small error in the exponent causes a very large error in the pre-exponential term. Although Ed values are numerically accurate, it is not possible t o relate the energy t o a particular physical process and the method is of limited value. Method (ii). Redhead [63] derived equations for desorption rate maxima for various orders of reaction, but the expressions are rather insensitive t o order so, again, evaluation of the pre-exponential term does not necessarily provide physically useful information. Because Tp can be obtained accurately, however, the energy term can also be precisely estimated and where the adsorbatesurface interaction dominates the adsorbate-adsorbate interaction, it provides a reliable value for the

273

Temperature

(K )

Fig. 40. Temperature-programmed thermal desorption spectra for various coverages of Pb on GaAs.

desorption energy. In general, thermal desorption spectra shapes are insensitive t o rate parameters and the most reliable information for a particular system is contained in the absolute and relative peak positions, so that methods relying on T p t o estimate Ed should be a good first approximation, however crude the model. Method (iii). Application of the quasi-isotherm method to, for example, the Pb-GaAs system, where interface effects are believed to be small, produces from the analytic formalism a coverage-dependen t desorption energy which at first sight seems reasonable, decreasing from 1.6eV at multilayer coverage to 0.4 eV for submonolayer quantities (Fig. 41). Closer inspection, however, shows these values t o be spurious; the apparently rapid decrease in desorption energy in the low coverage region is an artefact, since such a sudden reduction in Ed would lead t o a massive increase in desorption rate and produce a step function change in the desorption spectra, which is obviously not observed in practice. These results are not peculiar to Pb--GaAs and several other semiconductormetal (and metal-metal) systems [64, 331, 3341 produce corresponding disparities when analysed in the same way. The basic reason for this seems t o be in the implicit assumption that the kinetic order is constant at any paticular coverage, irrespective of the initial coverage. This is seldom the case, however, since the deposit morphology changes drastically as the References p p . 280-289

->

16-

aJ

v

>,

P aJ 5

14-

12-

c

0

5 10-

3

U

08-

.e

c

aJ

I I. 1

0

-i

--.I-L

I

1

10 20 30 Pb c o v e r a g e (monolayers)

-

40

Fig. 41. Apparent desorption energy for Pb from GaAs as a function of coverage, deduced using the quasi-isotherm method of analysis.

substrate is heated. (The change is typically from uniform coverage to isolated islands, even when the total amount of deposit remains nearly constant.) Analysis in terms of a single kinetic order of desorption, related to a specific deposit geometry, is therefore not appropriate. It appears that the first stage is a zeroth-order process, corresponding to desorption from a steady state adatom population provided by detachment of atoms from island peripheries, which gradually depletes with increasing temperature. It is therefore necessary to consider a desorption order which varies continuously with temperature (i.e. coverage), starting from an initial value of zero. It is not obvious, however, what final value to choose or what functional form the variation should take. For the PbbGaAs system, it nevertheless proved possible to construct theoretical desorption curves which showed all of the essential features of the measured spectra by assuming an order which varied sigmoidally from zero to 2/3 and a constant desorption energy of 2.1 eV (the enthalpy of vapourisation of Pb). The value 2/3 corresponds to desorption from hemispherical-shaped islands which SEM observation indicated to be appropriate. The calculated spectra are shown in Fig. 42 and may be compared with the experimental data of Fig. 40. We may conclude that, as a result of the morphological changes which occur during thermal desorption of metal deposits, it is essential to use a model in which the kinetic order is variable. Analyses which produce an

27 5

Temperature ( K )

Fig. 42. Calculated thermal desorption spectra for Pb from GaAs with a constant desorption energy of 2.1 eV but with varying desorption order.

apparent decrease in Ed with decreasing coverage are probably spurious. In most cases, Ed will be constant and relate t o single adatom desorption energy, since desorption is effectively occurring from a mobile population between metal islands. Where interface instability and diffusion are significant, the technique of TDS is inappropriate. 5 . 3 SEMICONDUCTOR-SEMICONDUCTOR INTERFACES

We will treat this topic rather briefly since no kinetic data as such are available, but it will be of interest t o compare the behaviour of semiconductor heterojunctions with metal-semiconductor interfaces. The material combinations which have been treated in any detail are limited t o GaAs--Ge [335-3391, Si-Ge [340] and GaAs-A1As [341,3421. In general, the deposits are epitaxial, in parallel orientation with the substrate, so we are considering single crystal films as well as single crystal substrates. In addition to this crystallographic aspect, the other major difference from metal deposits is the nature of the interface, which for the systems investigated is essentially ideal, with little or n o interdiffusion References p p . 280-289

276

up t o quite elevated temperatures (at least 550 K ) . It is this feature which casts doubt on the model proposed by Spicer et al. [298, 326, 3271 for interdiffusion across metal--semiconductor interfaces. As we described in Sect. 3.2.6, they suggested that the heat of condensation of the metal provided the excess energy required t o promote the process, but if this were so it is difficult to understand why comparable behaviour is not observed with semiconductor condensation. Heats of condensation would not be expected to be significantly different for metals and semiconductors. The experimental evidence for ideal behaviour has largely been obtained from XPS and LEELS, although high resolution transmission electron microscopy (TEM) has been used to investigate GaAs-A1As interfaces. As an example, Bauer and McMenamin [335] studied the Ge-GaAs interface formed by depositing Ge from an atomic beam onto in situ cleaved { l l O ) GaAs substrates over the temperature ranges 620-800 K. They monitored Ga, As and Ge 3 d core emission, so that with kinetic energies of x 60 eV involved, the probing was limited to the environment of only those atoms within three atom layers of the free surface. By determining the attenuation of the Ga and As emission caused by the Ge deposit, a sensitive indication of the sharpness of the interface was therefore obtained. From the comparison of photoelectron escape depths, they concluded that the composition changed from (110) planes containing only Ga and As to complete Ge layers over one bonding distance, even at a substrate temperature of 620K. By 800K, however, there is considerable interdiffusion, with the transition to a diffuse interface occurring at 700K. Within the temperature range 620-800K, growth is twodimensional, but when deposition is carried out at room temperature, discrete islands are formed. Bauer and Mcmenamin [ 3351 measured a valence band discontinuity of 0.7!:::’ eV for an n-type GaAs substrate where the interface was ideal, but with interdiffusion this was reduced t o 0.2 k 0.1 eV. From similar measurements with a single deposition temperature of = 700 K, Grant et al. [336] deduced a valence band discontinuity of 0.3 f 0.3 eV, a range which spans both types of interface. From both experimental observation and theoretical calculation, the situation with (100) interfaces appears to be slightly different from that with (110}, since at least one mixed atom layer is formed where alternate atoms are Ge. For example, Harrison et al. [337] concluded that at least two mixed layers are required t o obtain charge neutrality and to suppress long-range dipole fields. Similarly, self-consistent pseudo-potential calculations by Baraff et al. [ 3391 , based on an ideal (100) interface, predicted the existence of empty states in the lower half of the gap, i.e. a metallic interface, whereas in practice it is semiconducting. If a certain amount of interface mixing is assumed, however, the calculated result is in ageement with experiment. It is important to emphasize that this type of intermixed

217

heterojunction is a direct result of overlayer bonding in relation t o the (100) GaAs surface structure and does not involve interdiffusion. Nannarone et al. [340] investigated the Ge--Si interface formed by evaporating Ge on in situ cleaved ( 1 l l ) S i surfaces. The thickness ranged from a small fraction of a monolayer t o ten monolayers and the substrate was at room temperature during deposition. They were able t o show that, a t monolayer coverage assuming no interdiffusion, there was reasonable agreement between their LEELS data and a tight-binding calculation of the energy bands, with Ge atoms in one- and three-fold coordination positions on the substrate surface. Finally, the more direct technique of transmission electron microscopy (TEM) has been used t o observe interfaces in (GaAs),-(AlAs), superlattices prepared by molecular beam epitaxy [341, 3421. Here, m and n are the layer thicknesses in monolayers and the range studied was 0.9 < n < 4 and 0.98 < m < 5. At n = m = 1, ordered regions involving atomically sharp interfaces were observed for substrate temperatures up t o 880 K. Thus, although the examples are rather limited, it appears that the large amount of interdiffusion which characterizes many m e t a l s e m i conductor systems does not occur with semiconductor heterojunctions. This would imply that the mechanism proposed by Spicer e t al. [298, 326, 3271 in terms of the heat of condensation of the overlayer is not universally applicable. The fundamental difference between semiconductor and metal deposits is that the latter induce instability in the covalent bonding of the semiconductor substrate, perhaps by their ability t o screen Coulomb interactions due t o their mobile free electrons. 5.4 INTERACTION OF GROUP V ELEMENTS WITH GaAs SURFACES

The impetus for this topic has been provided by the development of molecular beam epitaxy (MBE) as a viable thin film deposition process [ 1111, As a result, the approach has concentrated more on investigations of reaction kinetics than on electronic effects, since kinetic parameters are directly available from modulated molecular beam measurements (see Sect. 2.4.1). We will summarize here only the results for beams of As4 and As, interacting with {100}GaAs surfaces, but closely similar behaviour is observed for other Group V elements and other Group III-V compound surfaces. The choice of tetramer and dimer beams is dictated by the evaporation behaviour of Group V elements in that elemental sources produce tetramers and Group III-V compound sources produce dimers. Monomeric species are not readily available. If we consider first the case of an incident As4 beam, the essential features of the model proposed by Foxon and Joyce [343] are illustrated in Fig. 43. As4 is adsorbed into a mobile precursor state and in the absence of a surface Ga population has a zero sticking coefficient but a References p p . 280-289

278 As, incident flux

f

Chemisorbed state \ \I

$?

2nd order reaction

AS4

Go stabilized Go As surface

Fig. 43. As4 interaction on a (100)GaAs surface.

measurable surface lifetime in the temperature range 300-450 K. From the temperature dependence of the lifetime, a desorption energy of = 0.4 eV may be determined. With a coincident Ga flux, AS, has a temperature-independent sticking coefficient between 450 and 600 K, but it is a function of the Ga beam intensity, JGa. The crucial results are that the sticking coefficient of AS, , SAs, , is always < 0.5, even when J G a % JAs, (the As4 flux); secondly, when JGa Q J A s , , one As atom sticks for every Ga atom supplied; and finally, when J G a > J A s , , the desorption rate of AS, is second order with respect to its adsorption rate, but for J G a < JAs, a first-order dependence is observed. These results can be explained by a process of dissociative chemisorption with a pairwise interaction of AS, molecules adsorbed on adjacent Ga lattice sites. When the AS, surface population is small compared with the number of Ga sites, the rate-limiting step is the encounter/reaction probability between As, molecules, leading t o second-order kinetics. As the AS, surface population is increased, there is an increasing probability that an arriving molecule will find adjacent sites occupied and the desorption rate becomes proportional to the number of molecules being supplied, i.e. a first-order process. This model has been criticized by Jewsbury and Holloway [344], who suggested on theoretical grounds that the second-order behaviour arose not from a pairwise interaction of As, molecules, but simply because an AS, molecule could only desorb from an As site on the surface. The problem with this interpretation is that the sticking coefficient of AS, should become unity for a Ga-stable (100) surface, whereas experimentally its maximum value is 0.5 and then only for a Ga-saturated surface. The second approach t o the growth of GaAs by MBE is t o use beams of Ga and As, (from a GaAs source) and the interaction kinetic model for

279

Dissociat i v e coefficient < I

C

?///////////////A Go stabilized Ga As surface

Fig. 44. AS:, interaction o n a {100}GaAs surface.

these two species on a (100) GaAs surface is represented schematically in Fig, 44. At 600 K, the sticking coefficient of As,, SAs,, is a function of the arrival rate of Ga and when J G a < J A s , , one As atom sticks for each Ga atom supplied, while for 256, > J A s , , SAs, is unity (cf. S A ~=, 0.5 for the equivalent situation). Below 600 K, the behaviour of AS, becomes more complex in that it undergoes a surface association reaction leading t o the desorption of As, by a first-order process with respect t o J A s , [ 3451. At substrate temperatures > 600 K, an additional process occurs whereby a Ga adatom population is created by desorption of As, from the substrate, the basic chemistry remaining otherwise unchanged. As, lost in this way from the surface can be replaced by dissociation of the incident As, or As, beams t o maintain an "arsenic-stabilized" surface under steady state conditions. This process can most readily be demonstrated with an incident As, flux and in Fig. 45 the results obtained by modulating either this incident flux or the desorption flux are presented [345]. For the latter, the total As, flux from the surface is independent of substrate temperature in the range 600--900K, while for the former the amplitude of the correlated response decreases with increasing temperature, from which the temperature-dependent sticking coefficient of As, can be calculated. (This tends t o unity, compared with the limiting value of 0.5 for AS,, and all processes obey first-order kinetics.) Thus, as As, is lost by desorption creating a Ga adatom population, it is replaced by the incident flux, leading to a temperature-dependent sticking coefficient as measured with respect t o this flux. The desorption flux, References p p . 280-289

280

1

.o

3.8

0.6 In

4 I/)

0.4

0.2

200

0.0 400

600

Substrate temperature

800

:000

Ts ( K )

Fig. 45. Desorption flux and sticking coefficient of As2 on a {100)GaAs surface as a function of temperature. 0,Desorbed flux modulated; 0 , incident flux modulated; A*SAs,.

therefore, is composed of two parts, the evaporation flux from GaAs plus the fraction of the incident beam which desorbs. The sum of these two parts is a constant, independent of temperature over the temperature range 600-900 K. The surface composition can be maintained constant provided that the incident flux intensity is high enough to compensate for the evaporation flux. As a final point, which illustrates the importance of surface processes t o semiconductor behaviour, Neave e t al. [346] and Kunzel and Ploog [347] have shown that deep level incorporation during the growth of GaAs films from beams of Ga and arsenic is dependent on the arsenic species used. The deep levels are believed t o be associated with intrinsic defects and films prepared from As4, in which a pairwise interaction is involved, contain a higher concentration of three specific deep centres than those prepared from As, where only simple dissociative chemisorption occurs.

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Chapter 3

Radiation and Photoeffects at Gas / Solid Interfaces J. CUNNINGHAM

1. General introduction 1.1SCOPE

The diversity of chemical and physical phenomena which occur at the surfaces of solids under irradiation by photons or energetic particles and the relevance of these phenomena to surface science, spectroscopy, radiation physics and catalysis have been amply demonstrated during the past decade by many scientific meetings and review articles devoted to various aspects of these phenomena [ 1-6, 260,2611. The aspects of particular interest for this chapter are those concerned with the manner in which radiation incident on a gas/solid interface either alters the extent of coverage of the solid (the adsorbent) by adsorbed molecules (the adsorbate) or modifies reactions of adsorbates with one another or with the surface on which they are adsorbed. Modification of the catalytic activity and selectivity of solid surfaces by radiation comes into this category and this topic has been considered in several review articles [ 7-10,2621, partly because of interest in possibilities for improving the activity or specificity of catalysts by irradiation. Coverage of this topic will be brought up-todate as part of the present chapter, but the treatment of radiation-induced surface processes will not be limited to considerations of heterogeneous catalysis. Rather, an attempt will be made to illustrate by representative examples for gas/inorganic solid interfaces the extent to which radiation has been shown to modify surface physical processes (such as physical adsorption and surface voltage or conductance), as well as surface chemical processes (such as catalysis and surface reduction or oxidation). No coverage is attempted of radiation effects at surfaces of organic or other molecular solids [ 111. Possible relationships between physical processes and chemical processes in reactions occurring by heterogeneous catalysis at the gas/solid interface can be appreciated by separating the multistage catalytic process, as follows, into a sequence of events capable of continuously converting gaseous reactants AB(g) and CD(g) into gaseous product CB(g) and AD(g) through activation of reactant on the surface of a solid catalyst, MX. (a) Diffusion of reactant from distant regions of the gas phase to close proximity with the interface. References p p . 4 1 9 4 2 7

292

AB(g)

+ CD(g)

ABi

+ CDi

(la)

where the subscript i denotes proximity t o the interface. (b) Adsorption of reactant(s) onto the surface. ABi

+ CDi

-

AB(ads)

+ CD(ads)

(1b)

(c) Conversion of at least one adsorbed species to an active surface intermediate through chemisorption at a catalytically active site. AB( ads)

+ S/MX

-

( AB-S)'

/MX'

(1c)

where S/MX denotes an active site on the surface of the solid MX and the superscripts, f and T , allow for electron transfer between adsorbate and adsorbent. ( d ) Chemical reaction between the activated surface intermediate and (an)other reactant present at the interface in either adsorbed or gaseous form. Either CD(ads)

+ (AB-S)'/MX'

or CDi

+ (AB-S)'/MX'

-

-

(CB-S)'/MX'

(CB-S)'/MX'

+ AD(ads)

-I- AD,

(Id)

(Id')

(e) Regeneration of active sites through removal of chemisorbed product with reversal of electron transfer. (CB--S)'/MX'

-

CB(ads)

+ S/MX

(le)

( f ) Desorption of physically adsorbed product.

-

CB(ads) CBi (g) Diffusion of product into the gas phase.

(If)

CBi CWg) (1g) The need for some such sequence of steps in heterogeneous catalysis of chemical reactions is widely recognised, although the detailed nature and degree of separability of the individual steps can be differently represented in various mechanisms [ 121 . In principle, possibilities for the modification of heterogeneously catalysed reactions by irradiation may arise through radiation-induced alterations in the rates of the individual steps (la)-( l g ) , although catalytic reactors and reaction conditions are usually designed t o ensure that ( l a ) and ( l g ) are fast and in reversible equilibrium, so that they are not the rate-determining processes for the overall sequence of events. An influence of diffusional constraints on rates of radiation-induced isotopic exchange on porous solids has, however, been noted [ 1 3 ] . Processes ( l b ) and ( I f ) usually also correspond t o fast reversible equilibria [14] so that any influence of radiation thereon

293

may become manifest through shifts in the equilibria to the right (radiationinduced adsorption) or to the left (radiation-induced desorption). The ways in which radiation sensitivity can be expected to arise for the various steps will be illustrated by reference to various models for chemisorption and catalysis at the gaslsolid interface. Such models can, in general be classified as collective electron or active site or surface state in character and each type will be outlined in turn with a view to identifying possibilities for its modification by irradiation. 1.2 ORIGINS OF RADIATION SENSITIVITY

1.2.1 Collective-electron models In general, these have sought t o correlate chemisorptive capacity of semiconducting solids with collective-electron energy bands close t o the surface, i.e. bands of closely spaced electronic energy levels which arise from the crystal periodicity in similar manner to those of the bulk lattice, but which experience modifications of energy and extent of delocalisation depending upon proximity to the semiconductor/vacuum or semiconductor/gas interface [ 15-18] . One limiting approximation underlying many treatments has been the assumption that no discontinuity (uncoupling) occurs between the band of energy levels allowed within the bulk and those at the surface (which does not, of course, prevent the identification of different Fermi levels with the bulk and with the surface). Chemisorption of an acceptor-type adsorbate, i.e. one with a suitably low-lying LUMO (lowest unoccupied molecular orbital) is then envisaged to require that at least one electron from the collective pool of delocalised electrons becomes localised at the interface in the proximity of the adsorbate. In the notation adopted by Volkenshtein [15] , this would correspond to “strong” acceptor-type chemisorption. Conversely, the delocalisation of one or more electrons from a suitably high-lying HOMO (highest occupied molecular orbital) of a donor-type adsorbate into an empty or partially filled collective electron band of the solid may be termed strong donor-type chemisorption. Electron transfer between adsorbate and collective electron states of the solid takes place in different directions in the two cases. Clearly, the relative positioning of the Fermi level for collective electrons of the solid vis-8-vis the LUMO or HOMO of the adsorbate will inter alia decide whether acceptor- or donor-type chemisorption predominates [ 15, 161. The effects of such chemisorption upon the collective-electron states of the solid in surface and sub-surface layers have frequently been illustrated in the manner of Fig. l ( a ) [17]. The solid lines in Fig. l ( a ) represent the situation that can arise when an electron acceptor at the surface provides an energy level, E,,, below the bulk Fermi level, with the result that electrons transfer from the bulk of the solid on to the surface acceptors and give rise t o a negative surface References p p . 4 19-427

294 (a)

(b)

I

Gas

I

Solid

- v e c h a r g e on surface

I

Fig. 1. Two complementary representations of t he effects of irradiation o n electronic processes and energy levels of a GAS/SOLID interface, based o n electronic models. (a) Electronic energy levels adjacent to non-irradiated (solid lines) and irradiated (broken lines) interface and their relationships t o energy levels within the bulk. The magnitude of t h e Schottky barrier voltage and upward band bending resulting from excess negative charge o n t h e surface is shown as V , for the non-irradiated and V,* for the irradiated interface. T h e energy level corresponding to t h e bottom of the conduction band a t t h e surface, E,, is located above t h at in the bulk, Ecb, and similar band bending is shown for the valence band at t h e surface, E,. Energies of t h e band gap, ( E g ) , Fermi level, (Ef), surface acceptor (Esa)and bulk donors (Ebd)are also depicted. (b) (i) Radiationinduced processes at an irradiated surface free of electron-accepting surface states o r adsorbed species and showing just a small Dember voltage due to greater range of electrons, e, than holes, h , generated by photons, (hv). Quasi-fermi levels Efeand Efh for t h e electrons and holes are shown and recombination is indicated by R. (ii) Additional processes at t h e irradiated surface with adsorbed oxygen acting as an electron acceptor: localisation of electrons by 0 from the conduction band across the reduced Hole capture by 0; is indicated by 0 .ReSchottky barrier, V,*,is indicated by produced with permission from refs. 1 7 and 132.

6.

charge [ 181. [It is worth noting here that the gradual upward bending of the electronic energy levels depicted in Fig. l ( a ) implicitly assumes equilibration of electrons between the surface acceptor states and bulk states, so that questions as to the validity of such models will arise for any case where evidence emerges for isolation or uncoupling of energy levels at the surface from those in the bulk.] The upward band bending of Fig. l ( a ) can arise at the surfaces of n-type semiconducting solids, such as ZnO or TiO,, either as a consequence of electron-accepting surface defects such as oxygen vacancies, or through electron localisation on electron-accepting gases such as 0, or N 2 0 . The latter case is referred to as “depletive chemisorption”, since the number of charge carriers in the conduction band is thereby depleted. It is supported by many observations of reduced electrical conductivity in n-type semiconductors upon chemisorption of

295

oxygen or other adsorbances with appreciable electron affinity [ 19-21] . Growth of a double layer of charge, comprising the negative charges localised at surface acceptors plus the diffuse distribution of partially ionized bulk donors in subsurface regions, makes this process of depletive chemisorption a self-limiting one, which tends to an equilibrium situation of the type depicted in Fig. l(a). In the absence of irradiation, such equilibria involve a balance between localisation and delocalisation of only thermally generated charge carriers, so that establishment of the “dark” equilibrium envisaged by Fig. l ( a ) involves transfer of majority charge carriers (electrons) towards the surface becoming progressively inhibited by the growth of a negative surface potential, V,. The latter conversely increases the nett rate of transfer of electrons back into the bulk until it balances the reduced electron transfer from the conduction band. It will be appreciated that downward bending of collective-electron energy levels at the surface can result in converse fashion whenever holes are the majority charge carriers and suitable donor-type centres exist at the surface. Sensitivity of the extent of depletive chemisorption towards radiations which create free electrons and/or free holes within sub-surface regions stems, in part, from this double layer of charge in surface regions of the semiconductor, since positively charged holes formed by radiation are attracted towards the negatively charged surface and their arrival there first reduces V, to V,*. Subsequently, a reduction in the surface concentration of depletively chemisorbed species occurs through chargeneutralisation reaction with the radiation-generated minority carriers [ 181 . The decreased extent of band bending a t gaseous acceptor/n-type semiconductor interfaces which follows from radiation-induced neutralisation of surface acceptors is indicated by the broken lines in Fig. l ( a ) and should, in general, be accompanied by radiation-induced reductions in the extent of coverage of the surface by chemisorbed acceptor species. Evidence for contributions by such radiation-induced neutralisation processes t o effects at O,/CdS, O,/ZnO and O,/TiO, interfaces will be considered in detail in Sect. 2 for UV illumination and in Sect. 3 for ionising and high-energy radiations. A further general point may, however, be noted here in relation to a different response to radiation to be expected for interfaces carrying a double layer of charge inverted relative t o that in Fig. l(a). Such inversion becomes probable if electronic energy levels associated with metal-excess surface species (typically Ti3+ on TiO, or Zno/Zn+ on ZnO after loss of oxygen) donate sufficient electrons into the bulk to overwhelm any depletive localisation of electrons by surface oxygen vacancies o r chemisorbing acceptors. Early evidence for quite different radiation sensitivities of stoichiometric and non-stoichiometric surfaces of zinc oxide towards radiation-induced oxygen adsorptiondesorption processes was summarised by Cropper [22] and more recent evidence of such effects at various gas/semiconductor interfaces under irradiation will be presented in Sect. 2. Referencespp. 4 1 9 4 2 7

296

Many of the factors which favoured the development of collectiveelectron theories for chemisorption and catalysis on semiconductors operate much less efficiently for chemisorption on metals, e.g. the percentage changes in conductivity accompanying chemisorption are very much less for metals because of the larger number of charge carriers already present. The extent of separation of charge in a double layer close to the gaslsolid interface is thus greatly diminished for metals, with the consequence that possibilities for radiation sensitivity similar to that outlined above for gaslsemiconductor interfaces are correspondingly reduced. Considerations of collective-electron factors in chemisorption on to metals, e.g. in the dissociative chemisorption of hydrogen on to various transition metals, have often been concerned instead with the relative importance of the d-electron character versus s- o r p-electron character of collective-electron states at the surface [23-26,2531. The densities of states and relative occupancies of states in the d-electron band within the solid were originally considered t o influence the percentage d-character of metal-hydrogen bonds in chemisorption. However, difficulties have been experienced with this concept of a simple relationship between catalytic activity of the metal and percentage d-character of the metallic bond. These difficulties have recently been restated [ 271 together with the growing recognition that formation of a localised surface metal-adsorbate bond may lead to decoupling of metal orbitals of surface atoms from the quasi-continuum of delocalised collective-electron states of the other metal atoms. Various workers have considered the role of localised electronic energy levels, e.g. of individual surface metal atoms [28] or of pairs [29] or clusters 1301 of surface metal atoms, in the formation of localised surface bonds. The next subsection outlines some recent work envisaging such local interactions of the chemisorbing species with small numbers of surface metal atoms as predominating over interactions with a quasi-continuum of delocalised states of the metal lattice. Possibilities for radiation-induced modification of chemisorption involving such strongly localised interactions on metals would appear to be limited t o “direct-hit” of incident particle irradiation on to the localised bonds o r to their photochemical rupture. Unfavourable relative alignments of incident radiation and localised bond may, furthermore, lead to small effective cross-sections for such modifications.

1.2.2 Active-site models and their sensitivity to radiation Models which regard solid surfaces as non-uniform, in the sense of containing different sub-sets of surface sites distinguishable from each other by virtue of their local structure and topography, and which associate widely different activities with these subsets, have long been termed active-site models [31-331. The applicability of this type of model to individual catalysed reactions has been the subject of controversy [ 1 2 b ] ,

297

(c) P t - (679) Fig. 2. LEED patterns and schematic representations of the surface configurations of platinum single-crystal surfaces: (a) P t ( i l 1 ) containing less than 10l2 defects cm-2 ; (b) Pt(p57) face containing 2.5 x l O I 4 step atoms cm-2 with an average spacing between steps of 6 atoms; and (c) Pt(679) containing 2.3 x l O I 4 step atoms cm-2 and 7 x l O I 4 kink atoms crn-’, with an average spacing between steps of 7 atoms and between kinks of 3 atoms. Reproduced with permission from ref. 36(a).

References p p . 419-427

298

some reactions being stated to be “structure-sensitive” whilst others appear t o be structure-insensitive [34, 351. Evidence in favour of a positive identification of step sites as active sites for reactions involving hydrogen on platinum surfaces has recently been summarised by Somorjai [ 361 . This evidence has come from the application of an extensive range of surface spectroscopy techniques (such as described in Chap. 2 of this volume) t o the preparation and detailed structural characterisation of surfaces of platinum single crystals. Somorjai and co-workers have thereby demonstrated that high catalytic activity of the Pt single crystals for hydrogen-handling reactions correlate well with the presence of a particular set of active sites, viz. platinum atoms located at steps, as illustrated in Fig. 2. Detailed but not unanimously accepted analysis of LEED patterns of the type shown in Fig. 2(a) and (b) led Somorjai et al. to the view that the topography of platinum atoms at such sites corresponds in certain crystals to steps between narrow terraces, within which Pt atoms have { 111)Miller indices. Such narrow terraces, displaced on average from one another by one atomic spacing per step, were shown to form when the platinum single crystals were cut on a high index plane. The catalytic activity of such stepped surfaces for various hydrogenation reactions was greater than for (111) terraces. Convincing evidence for the role of Pt atoms at these surface defect sites of low coordination number in the hydrogendeuterium exchange reaction was given by Bernasek and Somorjai [ 3 7 ] . They proposed that at temperatures between 300 and 1300K, these active stepped sites serve to dissociate hydrogen in a fast non-activated process, which in their view is not the rate-determining process (r.d.p.) for isotopic exchange. They elaborated a two-branch mechanism, the low temperature (<600 K ) branch of which may be represented as

in which the individual reaction steps are numbered in accordance with the generalised multistep description given on p. 292. The mechanism in this scheme with ( I d ) as the slow rate-determining process (r.d.p.) may in one sense be thought of as Langmuir-Hinshelwood in type, but in another sense resembles VCI mechanisms proceeding via interaction between one strongly chemisorbed species and one molecular species only weekly perturbed by van der Waals-type forces. In the limiting case of OD2 < 1, scheme (2) with step ( I d ) as the slow r.d.p. corresponds formally t o a Langmuir-Hinshelwood mechanism and yields eqn. ( 3 ) as the

299

appropriate rate expression. H(ads)/step] [D2(ads)/terrace]

(3)

Bernasek and Somorjai proposed that, as the temperature increased and the surface concentration of the D2(ads) species decreased, an EleyRideal-type mechanism became important, involving direct reaction of an incident gas phase D2 molecule with H atoms present at the stepped active sites, having (Id‘) as the r.d.p. and obeying the rate expression

Both mechanisms require that active surfaces contain the particular structural feature of stepped sites and reaction was found t o be at least one order of magnitude slower on regular P t ( l l 1 ) surfaces. In this sense, the hydrogen-deuterium exchange reaction may be described as “structuresensitive”. Somorjai and co-workers have likewise claimed that the structure sensitivity of the platinum-catalysed conversion of cyclohexene to benzene may be attributed, in part, t o the requirement that platinum atoms with the low coordination number characteristic of stepped sites be present t o act as active sites. A slightly different hypothesis has been advanced for the structure sensitivity of hydrogenolysis, viz. that this originates from kink sites in the atomic steps [cf. Fig. 2 (c)] . An important role was also indicated for carbonaceous overlayers which form on the Pt catalyst surfaces upon exposure to hydrocarbons plus hydrogen [36]. In relation t o the unique chemistry at surface irregularities, steps and kinks, Somorjai [ 36a] suggested that local atomic structure, local surface composition and local bonding between adsorbates and surface sites each exert a controlling influence. Thus, in relation to the extents and strengths of hydrocarbon chemisorption on Pt, Au and Ir and the markedly different catalytic activity of these metals, he considered that it was not chemisorption per se which was important but rather the possibility for juxtaposition of a chemisorbed species and an active region of the surface which tailors reactivity of the adsorbate t o the needs of reaction. [The formulation given above for eqn. ( l c ) was intended t o include such participation of active sites in the generation of reactive surface intermediates.] The implications of the highly specialised manner of operation of such active sites for the possibility of radiation sensitivity in the catalytic activity of metal surfaces appear clear: efficient radiationinduced enhancement in catalytic activity cannot be expected except in cases where energetic particles generate additional active sites by radiation damage effects. Examples of such effects are considered in Sect. 3. Conversely, and subject t o the low cross-section mentioned above for directhit processes, some radiation-induced decreases in catalytic activity of References p p . 4 19-427

300

gas/metal interfaces may result through radiation-induced desorption of chemisorbed reactant from pre-existing active sites [ 71 . Evidence, mainly of a more traditional and less direct nature than that just described for platinum surfaces, has accumulated in recent years for an important role of active sites on the surfaces of non-metallic catalysts [38-47, 254, 2551. Some similarity of "active site" concepts for nonmetallic catalysts to those for metals is well illustrated by a model advanced by Boudart et al. [38] for hydrogen4euterium exchange catalysed by MgO at 78 K. They associated the catalytically active site with a specified crystallographic plane [a metastable (111)plane], with a nearby proton, and with a surface electronic defect consisting of a triangular array of O, ions. The presence of anions and/or metal ions at positions of high coordinative unsaturation, e.g. O&, on surfaces of metal oxides has been envisaged by other workers [39-47, 254, 2551. Such coordinatively unsaturated (cus) swface ions merit consideration here as types of active site paralleling those just described for metals, albeit often based upon much less direct evidence. Formation of O,;: has been associated with the loss of the elements of water from two adjacent surface hydroxyl groups during outgassing in vacuo at high temperatures [ 421 as in

For strongly ionic metal oxides, such as MgO and other alkaline earth oxides, the reduced Madelung potential of oxide ions existing on the surface with a high degree of co-ordinative unsaturation [42, 431 has been identified as one factor conferring measurably different properties than for other oxide ions possessing higher coordinative saturation. Outgassing of powdered samples in vacuo at high temperatures is thought to favour the process depicted in eqn. (5) which leads, however, to a range of active sites with varying energies and degrees of cus [44]. Optical absorption and emission extending in such samples of powdered alkalineearth metal oxides to much longer wavelengths than for single crystals has been associated with optical transitions involving ions at surface sites of high cus [39, 43, 46, 254, 2551. Higher than usual activity as Lewis bases in electron transfer processes has also been associated with oxide ions in positions of high cus [42, 471. Against this background, and relative to single-crystal platinum surfaces, the following enhanced possibilities for surface sensitisation by irradiation may be discerned for metal oxide surfaces where 0:ls exist; firstly, direct activation of these cus species will result from exposure to radiations which cause optical transi. , : Such activation may enhance rates of tions involving O:is and/or M electron-transfer processes or of chemical reaction involving adsorbate species at those sites [39c]. It may be anticipated that such radiation-

301

enhanced surface activity should vary with pressure, e.g. in a manner similar t o eqn. (4), and examples of this behaviour are given in Sect. 2; secondly, the release of mobile charge carriers within the solid and their migration t o the interface under the influence of V , may directly affect the activity of the sites through increases or decreases in their effective charge. Changes in reactivity of gas/solid interfaces due t o radiationinduced localisation of charge at surface sites will be illustrated for metal oxides in Sects. 2 and 3. 1.2.3 Com binations of collective-electron and active-site models During the past two decades, the collective-electron and active-site models briefly outlined above have each had their strong proponents and have each enjoyed limited success in rationalising variations in the catalytic activity experimentally observed for sequences of related solids [ 35, 48, 491. However, the goal of predicting accurately the catalytic activity of various solids has proved an elusive one, even for collective-electron-type models which have received much the greater attention [48, 491. An initially popular method for evaluating the role of collective-electron factors was t o dope a host metal oxide with an altervalent cationic species, with the objectives of thereby raising or lowering the Fermi level and bringing about alterations in surface processes controlled by electronic factors. Kinetic tests of this approach using the heterogeneously catalysed dissociation of nitrous oxide as a test reaction [50] indicated, however, that such additives may also strongly affect the density of active sites on the surfaces of the doped solids. A similar ambiguity between changes due t o electronic factors and changes due to surface geometry or structure has emerged in studies on the effect of alloying upon the catalytic activity of metals [ 3 5 ] . In the past few years, such difficulties have led t o a noticeable decline in the number of publications interpreting catalytic activity solely in terms of collective-electron factors. Some combination or alternation between active-site concepts and collective-electron factors has, however, been favoured recently by various authors in selected cases [ 51541. Morrison [ 511, for example, has suggested (i) that oxidation reactions on chromia catalysts illustrate a case where active-site-type formation of local surface bonds between adsorbing molecules and coordinatively unsaturated chromium ion sites predominates as the rate-limiting process, (ii) that oxygen chemisorption on to zinc oxide is dominated by collectiveelectron effects (termed the “rigid” band model by Morrison), but (iii) that consideration of both active site and collective-electron aspects are necessary to account for the catalytic activity of bismuth molybdate in the oxidation of propene. Both collective-electron and active-site aspects may, however, be recognised in an alternative model developed by Lagowski et al. [52] for the activated chemisorption of oxygen on zinc oxide: electronic factors control the approach of electrons t o the surface, References p p . 419-427

302

but active sites determine the efficiency with which oxygen molecules or molecules striking the surface from the gas phase undergo the thermal activation process required for their conversion into metastable states capable of capturing an electron at the ZnO surface. It has been suggested by Cunningham et al. [53, 541 that cases which require the participation of an active surface site for the completion of an adsorbatelsurface charge transfer process should be distinguished from cases in which charge transfer occurs by electron tunnelling between adsorbate and the conduction or valence bands. Cases of the former type will herein be referred t o as active-site charge transfer (ASCT) t o distinguish them from collectiveelectron charge transfer (CECT) which does not specify the intermediacy of a specially active site on the surface. An important distinction between these two cases arises in respect of their possible modification by irradiation: CECT can only be expected t o operate efficiently during irradiation, due t o the short lifetimes of radiation-induced minority carriers, whereas ASCT may remain altered after irradiation, due to the possibility that charge carriers trapped at appropriate active sites during irradiation may remain trapped and capable of contributing t o surface activity for a long time after irradiation. Many examples of ASCT after exposure of metal oxides t o ionising radiations will be illustrated in Sects. 2 and 3. A common feature of the models outlined above is that exposure of gas/semiconductor or gas/insulator interfaces t o ionising radiations can be expected, through operation of electronic factors, t o lead to changes in the amount of electrical charge localised on, o r adjacent to, these interfaces. In seeking possible origins of radiation sensitivity, it is instructive t o consider how such radiation-induced changes in surface charge may influence the overall sequence of steps listed in eqns. (1b)-( If) for a heterogeneously catalysed process. Firstly, it may be recognised that such changes in surface charge will often be accompanied directly by changes in processes ( l c ) or ( l e ) , since chemisorption itself can involve localisation of charge at the interface. A multistep heterogeneously catalysed reaction involving such steps may therefore experience a radiationinduced enhancement of its rate in cases where surface coverage by charged chemisorbed intermediates is rate-limiting and is increased by irradiation (or vice versa). In the absence of irradiation, the rates [55] of such catalysed processes are expected t o be exponentially dependent upon the magnitude of the equilibrium surface potential V,. Reduction to V,* by irradiation [cf. Fig. l ( a ) ] , which is expected on the basis of electronic models, may alter the rate by altering the surface concentration of electrons in equilibrium with those in the bulk. Secondly, it may be noted that the rate coefficients for processes ( I d ) or ( I d ' ) may be sensitive to the extent of surface charge and so be susceptible to radiation-induced changes in this charge. Thus Copeland [ 561 has argued, in the context of very large electric field gradients (ca. lo8 V cm-' ) which can arise at the

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surfaces of metals, that the rates of reactions of molecules/ions adsorbed thereon may be greatly modified relative t o the no-field case. He based this conclusion on the orienting effect of large field gradients upon adsorbed polar molecules and predicted decreases in entropy and rotational energy leading to increased equilibrium constants for the formation of activated complexes in the presence of high field gradients. Although field gradients of the magnitude required for such effects appear less likely at gas/semiconductor interfaces in terms of collective electron models, such field gradients may not so readily be discounted for gas/ insulator interfaces at which high densities of radiation-induced charge carriers become localised by surface states. Models based on surface states are considered next. 1.2.4 Surface-state models

The selection on an empirical basis of collective-electron factors or active-site concepts o r some combination thereof in order to account for the activity of surfaces in catalysing various processes has obvious disadvantages. Possibilities for a more systematic approach t o the integration of collective-electron and localised-state aspects of surface structure have developed from theoretical treatments of intrinsic and extrinsic surface states, respectively. Models based on such developments, by reason of their relative novelty, have not yet been as widely applied as collective electron o r active-site models t o interpret catalytic activity of various surfaces and still less t o considerations of sensitivity t o irradiation. However, an abbreviated consideration of such surface state models is deemed essential here both as a basis for assessing their possible relevance in the explanation of radiation-induced effects and as an illustration of the integration of electronic and localised state aspects into a common framework. (a) Intrinsic surface states The relationship of modern concepts of surface states t o collectiveelectron treatments is most readily apparent from the physicist’s approach in which the Bloch waves of an infinite system are regarded as becoming non-current-carrying standing waves when the spatial discontinuity represented by the surface is taken into account [ 5 7 ] . The energy and spatial distribution of such states can be described, as in eqn. ( 6 ) in terms of a local density of states, N ( E ,n ) if J/, is an energy eigenfunction with eigenvalue E , and ,~)I a localised atomic-like basis function of symmetry type i centred on the nth lattice site, viz.

References p p . 419-427

304

The summation in eqn. (6) is understood to include not only the surface states analogous to Bloch states, but also localised surface states which can be split off from the continuum by the strong perturbation introduced by the surface. Such states can extend across the surface of a perfect crystal, but their wavefunctions inward into the solid decay roughly exponentially. Calculated densities of state are shown in ref. 58 for bulk TiO, and T i 2 0 3 and for the first, second, and third layer of a five-layer TiO, (001) film. The exponential decay of surface states with energies in the band gap moving inward from the surface layer is nicely illustrated by those calculations. A relationship of intrinsic surface states to localised bond concepts emerges more readily from the complementary view of the surface as featuring “dangling bonds”, i.e. coordinatively unsaturated lattice atoms or ions at regular surface positions [ 591. Thus formation of the (111)face of a silicon crystal may be visualised through rupture along the crystal plane of one of the four tetrahedrally coordinated bonds originally linking each atom in the rupture plane to its immediate neighbours in the crystal structure. Each surface silicon at a regular surface so formed may then be regarded as possessing one dangling bond. Overlap between dangling-bond states can lead to one or more bands of energy levels extending over the surface. (It is probable that surface species at steps, kinks, or other surface defects may feature more than one dangling bond and so contribute extrinsic surface states, cf. below.) Convincing evidence for the general correctness of this model for the regular S i ( l l 1 ) surface has come from a surface optical absorption measured by Chiarotti et al. [59b] at longer wavelengths than the usual absorption edge. This absorption has been interpreted quantitatively by Betteridge and Heine [ 59c] as involving electronic transition between filled and empty surface states. It is energetically favourable for atoms in the surface layer t o relax backwards towards the second layer and so reconstruct t o an arrangement different from the bulk lattice. Such relaxation has the following important influences on the dangling-bond description and on theoretical descriptions developed to achieve self-consistency in the electronic potential set up by electrons in silicon and their allowed wavefunctions at the surface: (i) bond-hybridisation is somewhat altered (from the sp3 hybrid orbital characteristic of Si-Si bonds within the bulk) towards a p-type dangling bond and some sp2-type hybridisation in the bonds linking the surface Si atom to the three Si atoms beneath it; (ii) the single band of surface states 0.7eV wide lying in the upper half of the Ec.b t o Ev,b band-gap for an unrelaxed surface is split into three bands of surface states for the relaxed surface, with two bands within the energy range of the valence band and a third band, of width 0.8 eV, lying in the band-gap. Since surface states are more sensitive to changes due to adsorption than are bulk properties, features associated with surface states have often been distinguished in photoemission experiments through differences between the photoelectron

305

[ioioi

_1 I00011

VO

Vzn

EleV

Zn 5

0 0

-5

Fig. 3. Illustrations of (a) the geometrical arrangement of zinc and oxygen species on the ZnO(1010) non-polar surface, and (b) the electronic energy levels adjacent to the E c , Ef,and E , denote, respectively, the energy level of electrons in surface. E,, vacuum, at the bottom of the bulk conduction band, at the Fermi level, or at the t o p of the valence band. Discrete energy levels associated with oxygen or zinc vacancies are respectively. Bands of surface states associated with denoted by E v o and E v @’ dangling bonds on surface zinc or surface oxygen are also indicated. Reproduced with permission from ref. 61(a).

References p p . 419-427

306

spectra of clean and contaminated surfaces (sometimes termed photoelectron difference spectroscopy, PEDS). Features of the Si (111) surface states revealed by PEDS are well summarised in ref. 59a which also illustrates the extent of agreement with theories based on intrinsic surface states. The successes achieved in the interpretation of surface properties of S i ( l l 1 ) in terms of surface-state models [59] has naturally led to efforts to account for the surface properties of many other solids in similar manner. The efforts of this type which are of most immediate relevance t o the present chapter concern solids, such as ZnO and TiO,, whose surface properties can be strongly influenced by radiation. Comprehensive reviews of results relating to the reactivity, electronic structure and geometry of various surfaces of ZnO have recently been given by Hirschwald et al. [ 6 0 ] , including evidence for intrinsic and extrinsic surface states. A qualitative representation of energy levels associated with dangling-bond-type surface states on the non-polar surfaces of zinc oxide was given by Gopel [61a] and is reproduced in Fig. 3. A geometric model of these surfaces and of subsurface layers is also included in the figure, depicting a small contraction [AZ(cation) = 0.3 x 10-'om] of the zinc sublattice spacing in the uppermost layer and also a contraction [AZ(anion) = 0.1 x lo-'' m] in the spacing of the oxygen sublattice in that layer, as derived from dynamical LEED intensity calculations. Zinc oxide is partly covalent and this small surface reconstruction of the surface ions has been attributed t o the need t o minimise covalent bond energy for the surface zinc and oxygen species [61b]. Two features of the qualitative model in Fig. 3 have been confirmed by recent calculations [62] : (i) the absence of any significant density of intrinsic surface states with energy levels lying in the band gap, and (ii) subdivision of the dangling-bond states into one group mainly originating from 2p orbitals of surface oxygen anions and having energies overlapping with the valance band, plus another mainly originating from 4s orbitals of surface zinc cations and having energies overlapping with the conduction band. Similar results have been reported for other binary solids, e.g. GaAs for which surface As species contribute filled intrinsic surface states lying below the maximum of the valence band whilst Ga species contribute empty surface states lying within the conduction band [63]. For ZnO, as for GaAs, the dangling-bond electrons associated with the band of surface states within the valence band are regarded as pairing-up on the more electronegative surface species, viz, O2-, which would account for the experimental observation that no ESR signal attributable to paramagnetic surface 0 - species could be detected on ZnO surfaces cleaned in UHV [ 61a] . Recent theoretical calculations on semi-infinite layer models for the polar ZnO(0001) zinc face and Zn(oO0i) oxygen face [62] also appear broadly consistent with the differing work functions of these

307

faces and with observations made on these polar faces by surface spectroscopic techniques. ( b ) Extrinsic surface states

Efforts have been made to account for certain surface properties of ZnO powders in a similar manner t o that adopted for single crystal Si, i.e. in terms of a quasi-equilibrium distribution of electrons, under the control of a surface double layer potential, into intrinsic surface states having an assumed density of states located in the band gap [64, 651. This assumption does not agree with the positioning of the zinc dangling-bond states above E,, envisaged in Fig. 3(b). Furthermore, other workers have concluded that such quasi-equilibrium distribution of electrons between intrinsic surface states cannot provide an adequate basis for accounting for experimental data on spectra and charge transfer at real surfaces and have argued that energy levels associated with extrinsic surface states must be taken into account [61a, 64, 651. One type of extrinsic surface state, viz. that associated with a surface oxygen vacancy on ZnO, is tentatively represented by Evo in Fig. 3(b). Unspecified topographical defect features of surfaces of GaAs cleaved in vacuo have been suggested by some workers as introducing acceptor-type surface states, which are detected by surface potential measurements but are not predicted by computational calculations on electronic energy levels of the atomically flat GaAs surface [66]. Mark et al. [67] have concluded that agreement between computational and experimental information on the existence of empty surface states in the band gap for evacuated non-polar surfaces of the compound semiconductors GaAs, ZnSe and CdTe is also poor and have suggested that the extent of surface order must be taken into account. Further indications of extrinsic surface states have come from mounting evidence for widely differing time dependences in the growth of additional oxygen-related surface states following exposure of various initially evacuated semiconductor surfaces to oxygen, e.g. differences of several orders of magnitude have recently been reported [68] in the sticking coefficient for oxygen adsorption on single crystal surfaces of GaSb, InP and GaAs. Oxygen adsorption on disordered surfaces of ZnSe has been shown to commence at much lower exposure and t o proceed much more rapidly than for initially ordered surfaces, although similar oxygen coverage on both surfaces was ultimately attained [67a]. Important roles of the degree of ionicity of the bonding in compound semiconductors, and of surface structural defects as active sites for oxygen adsorption, have been suggested as contributors t o these large differences [67b]. It appears probable from these results that surface defects can introduce low, but significant, densities of extrinsic surface states with energy levels which enable them to modify gas/solid interactions at such surfaces. Various theoretical approaches with differing degrees of rigour are emerging for the characterisation of extrinsic surface states and their effects upon surface properties. Thus the successful application of lattice References p p . 4 19-42 7

308

simulation techniques for the computation of point defects within the bulk of an ionic solid [ 69a] and of ab initio molecular orbital methods for similar purposes in various divalent oxides [ 69b] have pointed the way to the possibility for the detailed computation of the energies of formation, aggregation, and interconversion of point defects in the surface monolayer of crystalline solids [69c, 2631. Less elaborate calculations, which take into account the decreased Madelung potential associated with ions at the surface, and more particularly with ions at surface steps or comers where they exhibit unusually high degrees of coordinative unsaturation (cus), have also been used to indicate the different energy levels associated with such surface defects at the surfaces of ionic solids [254,2551. Related points which emerge from such calculations, and which are of particular interest for the effects of irradiation upon such solid surfaces, include: (i) possibilities for the formation of surface excitons as excited states of such defect systems, wherein a radiation-induced electron-hole pair is localised upon such coordinatively unsaturated surface species, and (ii) possibilities for surface excitons of differing energies depending upon the degree of cus. A rather different computational approach based upon model clusters has also been developed and appears inherently more likely than semiinfinite models to reveal any differences between surface species in regular terrace-like surface locations and surface species at edge and corner positions, since the latter represent a sizable fraction of all the atoms present in clusters of sufficiently small size to be treated by the discrete vibrational X , method employed. Cluster models can be selected with a view to examining various surface features. Thus, in an application of the method to zinc oxide, Tsukada et al. [70b, 70cl carried out computations on clusters with the formulae Zn4013,ZnloOlo(B)and ZnloOlo(S).Structures of the last two differ by possessing one face simulating a small region of the zinc-rich (0001) face of zinc oxide or of the oxygen-rich ( O O O i ) surface, respectively. Differing energy levels associated with zinc or oxygen species located at the edges or corners of the faces could be distinguished in the calculations.

( c ) Possibilities for radiation sensitivity in surface state models Several possibilities for modifying the electronic and catalytic properties of surfaces by irradiation follow from the surface state approach. Direct modification by irradiation should occur in cases where high energy radiations create additional surface defects with an associated increase, A[SS], in the density of surface states, as in eqn. (7), or in cases where incident radiation interacts directly with pre-existing surface states to ionise them, as in eqn. ( 8 ) , or to promote electrons from full to empty surface states as in eqn. (9), viz.

TABLE 1 Incident radiations used and information on gaslsolid interfaces obtainable by surface spectroscopies Adapted from ref. 7 1(a). Incident radiation

Surface spectroscopy

Type of interaction

Output@)

Information on adsorbed species

Monoenergetic electrons (- 10 eV)

Characteristic loss (low energy)

Inelastic scattering

Energy distribution of scattered electrons

Vibrations

Monochromatic uv photons (- 20 eV)

UV photoelectron (UPS)

Photoejection of electrons from valence states

Energy distribution, angular distribution of photoelectrons

Valence energy levels

Monoenergetic electrons (- 100 eV)

Low energy electron

Elastic scattering

Diffracted intensity

Ordering

Monochromatic X-rays (- 1.5 keV)

X-Ray photoelectron (XPS) X-Ray-excited Auger electron (XEAES)

Electron photoejection

Energy distribution of emitted electrons

Core energy levels and atom identification

Electron emission and scattering

Auger electron energy distribution

Core energy levels and atom identification

Electron ejection from from core levels

Energy distribution of of Auger electrons

Core energy levels and atom identification

X-Ray photon beam (- 1.5 keV) Monoenergetic electrons (- 3 keV)

diffraction (LEED)

Electron-excited Auger electron (EEAES)

W

0 CD

310

-

The symbol in these equations denotes “under the influence of radiation”; ( SS)f and (SS),, denote full and non-full surface state bands and [(SS,) . . . (SS),,] denotes an energetically separated pair of surface states. Possibilities for indirect activation of surface states by irradiation can also be distinguished. Firstly, minority charge carriers created within the bulk valence/conduction band by irradiation may become localised at surface states through electron tunnelling between them and the bulk, as in eqn. (10) or eqn. (ll),provided that there is a good match between the energy levels involved in the bulk and at the surface, viz. h;/MX-

+ (SS1 )f-h+l

eJMX+

+ (SSl )nf -e-

+ MXG I (SS,),f + MX;

(SS,),f

(10) (11)

Secondly, energy deposited within the bulk by excitation of a correlated electron-hole pair (exciton) may transfer by Forster-type energy transfer to, and become localised on, a pair of adjacent surface states as represented by

-

+ [(SSl)f . . (SS*)nf] [ ( h + l ( S S , ) .. . e-(SS,)]* t MX

(h*),*/MX

(12)

Such processes need to be taken into account since they may make important contributions to the high efficiencies noted in Sect. 3 for processes at interfaces exposed to ionising radiations. There are other more general reasons why serious consideration of processes (7)-( 11)and others involving electronically excited states at the gas/solid interface will be important throughout this chapter: firstly, in helping to correct a bias towards interpretations based on electronic ground states, which might otherwise carry over from the framework used in Sect. 1.2 in identifying possible origins of radiation sensitivity, and secondly, in recognising possibilities for reaction via homolytic bond rupture and resultant freeradical pathways, which can arise naturally from application of the oneexcited-electron approximation to electronically excited states. 1.3 SPECTROSCOPIC ASPECTS OF IRRADIATED GASlSOLID INTERFACES

Experimental procedures utilised in the study of radiation-induced processes at gas/solid interfaces differ widely with the nature of the radiation and with the objective of the study. Generally, high radiation fluxes and/or high total radiation doses are employed when objectives include post-irradiation modifications of catalytic activity through changes in the number of active sites or in the number of charge-carriers stably trapped by ASCT at the interface. Such effects are considered in Sect. 3 where relevant details are given of irradiation procedures and methods used to monitor changes in surface processes after irradiation. Different experimental procedures are often appropriate when ultraviolet

311

photons are utilised with the objective of causing changes in surface processes during illumination. Relevant details of illumination procedures and method used t o monitor changes during illumination are given in Sect. 2. In both cases, spectroscopic techniques figure prominently among those utilised to monitor radiation-induced changes in the number or identity of species at the gaslsolid interface. Spectroscopic techniques have the further general relevance that the information they can supply on energy levels of electronically excited states, or on the identity and structure of ground-state species at the interface, are essential components of the overall framework within which radiation-induced changes must be viewed. A brief general consideration of spectroscopic aspects of the gas/solid interface is therefore presented here prior to detailing nonspectroscopic aspects of experimental procedures within Sects. 2 and 3.

1.3.1 Electron spectroscopy o f surfaces The situation which, ideally, should prevail during spectroscopic investigations of the gas/solid interface is that the radiation(s) incident upon the interface with a view t o obtaining information thereon should not significantly perturb the interface. Table 1 gives a partial listing of the wide range of “surface spectroscopies” for which commercial instrumentation has become available in the past decade. Information from such techniques has already had an enormous impact upon the investigation and interpretation of the catalytic activity of welldefined surfaces of metallic single crystals. Such impact will undoubtedly extend to studies of radiation and photo-induced processes but has, at the time of writing, been much less significant than for thermally assisted processes. Information in the table illustrates the wide range of radiation types and energies employed in these surface spectroscopic techniques. Details of these techniques can be found in Chap. 2 of this volume and will not be repeated here. Figure 3 has already illustrated one representation of energy levels of ZnO surfaces which has emerged from such studies. Further illustrations of the power of these modern surface spectroscopic techniques for the detailed characterisation of gaslsolid interfaces has come from studies of chemisorption o r physisorption on t o a well-characterised surface of metal single crystals. Early fears that physisorbed atoms o r molecules might not be detectable by LEED because of the anticipated disturbance of the adsorbate layer even at the low energy of the LEED electrons (cf. Table 1)have largely been dispelled by successful LEED studies upon more firmly bound physisorbed systems [71b] such as Xe, Kr, Br, or I, on metals. Landman and Kleiman [71c] have recently summarised the theoretical models and experimental results for such systems together with evidence for some desorption stimulated by the LEED beam in more weakly bound systems, such as those involving neon or argon on metals. At the higher energies of the electron beam in References p p . 419-427

312

AES studies (cf. Table l), evidence for electron-stimulated desorption (ESD) in firmly bound physisorbed systems, such as xenon on nickel or platinum at 77 K, has emerged from comparisons of the apparent coverages deduced from AES with true equilibrium coverages. In cases where the solid adsorbent is composed predominantly of a non-metallic solid, MX, conventional spectroscopic techniques, such as optical spectroscopy in the ultraviolet visible and infrared regions of the spectrum, or electron spin resonance and nuclear magnetic resonance spectroscopies a t microwave frequencies, can also yield valuable information concerning the gaslsolid interface. An important practical distinction between these spectroscopic techniques and some listed in Table 1 is the ability to employ samples of high surface area, whereas techniques such as LEED o r angle-resolved UPS are applicable only to well-ordered surfaces of appropriate samples, which are usually single crystals. This difference should be kept in mind when attempting to compare results from the two sets of techniques, since samples of high surface area are likely t o exhibit much higher densities of defect sites and of ions with unusually high degrees of coordinative unsaturation.

( a ) Electronic spectroscopy with photons of energies 2-8 e V , High surface area solids In considering electric dipole-induced transitions in gas/solid systems under the action of visible or UV photons with energies insufficient t o produce photoionisation, the following additional possibilities for absorption of incident radiation can be distinguished, over and above those possible for regions of the gaseous, AB(g), o r solid, MX,, phases distant from the interface: (i) absorption via an electric dipole-induced transition between energy levels (possibly perturbed by the surface) of an adsorbate, AB, thereby directly producing an adsorbed electronically excited species AB*(ads) as in eqn. (13a) and may in turn lead t o a surface exciplex as in eqn. (13b). AB(ads) + hv AB*(ads)

-

+ MX

AB*(ads)

__

(13a)

[AB . . . MX]*

U3b)

(ii) absorption by an adsorbate-catalyst surface complex [AB . . . MXIi, in a spectral region where neither AB nor MX absorb, leading to an excited state [AB . . . MX]?, which differs from AB* or (MX)* and will often involve a degree of charge transfer between adsorbate and adsorbent, as represented in eqn. (14) hv

+ [(AB(ads) . . . MX)],

-

[(AB6'(ads).

. . MX)&-]*

(14)

(iii) electric dipole-induced transitions, such as in eqn. (9),between surface states of the catalyst producing an electronically excited state of the surface, which differs from electronic states of the bulk lattice. In

313

1964, Terenin [ 721 reviewed examples of absorption bands originating mainly from processes of types (13) and (14) involving molecules adsorbed on to non-metallic surfaces from the gas phase. He stressed the difficulties encountered by many workers in observing such absorption bands except by use of high surface area solids. Thus a species with an extinction

1.o

(b)

--

0.8

CaO

/

0.6

Sro

0.A

I

0.2

60

20

40

61 2

. 1 0 3 )

Fig. 4. (a) Emission spectra of hydrated MgO, under 2 5 4 nm excitation in vacuo at 300 K, outgassed at -, 300 K; * *, 4 7 3 K ; - - -, 5 7 3 K ; -*-, 973 K; and , 1 0 7 3 K . Reproduced with permission and minor adaptation from ref. 3 9 ( b ) . ( b ) Reflectance spectra in the UV-visible range from alkaline earth oxide powders with high surface areas. Reproduced with permission from ref. 48.

_. ._

Referencespp. 419-427

-

314

I

I 4 00

I

0

I

Excitation

wavelength

.

.

0

.

0

.

.

0

.

0

I

I

500

0

0 0

I

600

/ nm

.

0

.

.

0

.

0

0

.

0

.

.

0

0

0 . - .-0-. -0 -. -

-

Fig. 5. (a) Emission spectra at 300K of virgin and etched MgO smoke excited by 274 light. (1)Virgin smoke after outgassing at 1200 K for 1 h; (2)after contact with 1.33kNm-2 Hz for 10min followed by outgassing at 300K for 10min; (3) etched smoke after contact with water vapour for 20 h at 300 K followed by outgassing at 1200 K for 1 h; (4) etched smoke after H2 treatment as in (2).(b) Model of the MgO surfaces indicating low MgigO& pairs which can be formed on the stepped surface of the smoke particles, particularly after etching. Reproduced with permission from ref. 39c.

315

coefficient lo3 1mole-’ cm-’ , when distributed as a monolayer, could only reduce transmitted intensity by ca. 0.01% for a single pass of radiation through the monolayer. This difficulty can be overcome to some extent by the use of very finely divided adsorbents, such that photons incident thereon experience several passes through the gas/solid interface by multiple reflection and scattering events prior to their entry t o the detector. It is implicit in this approach that the application of wellestablished techniques for reflectance spectroscopy in the UV-visible region [73a] can prove valuable for the characterisation of electronic transitions involving electronic states of the finely divided solids themselves, as well as for molecular species adsorbed in large numbers due to the high surface area of such samples. A representative sequence of reflectance spectra of the former type [cf. eqn. (9)] as summarised by Stone et al. for alkaline earth oxides is reproduced in Fig. 4(b). The important qualitative features which emerge from Fig. 4 ( b ) are (i) that the reflectance, R , begins to decrease (equivalent to increasing the absorbance mainly within the surface regions of the finely divided samples) at energies which are ca. 1-2eV lower than the known absorption edges of single crystal or low surface area samples; (ii) that the gradual decrease in reflectance exhibits a series of shoulders (denoted by I, I1 and I11 in Fig. 4). Both qualitative features have been attributed to “surface exciton”-type transitions, each involving the electric dipole-induced transfer of an electron between the anion and cation of ion pairs, so located adjacent t o surface defects as to result in varying degrees of coordinative unsaturation and hence varying Madelung potential. According t o this interpretation, the reflectance giving rise to the shoulder at the lowest energy is associated with surface ion pairs characterised by the greatest degree of coordinative unsaturation, whilst the shoulders at progressively higher energies are to be associated with surface ion pairs with lesser coordinative unsaturation. Photoluminescence spectra of high surface area samples of alkaline earth oxides have been reported by Tench et al. [39] and interpreted in terms of the radiative decay of surface excitons. Representative data obtained by Tench et al. [39c] with microcrystalline MgO “smoke” (obtained by combustion of the metal in oxygen) are reproduced in Fig. 5, together with their “schematic” model for surfaces of the cubic MgO microcrystallites and their modification by exposure to water vapour. Photoluminescence arising from excitation in vacuo of the MgO smoke samples at energies corresponding to absorbance mainly into feature I11 of Fig. 4, i.e. to yield surface excitons with the highest degree of coordinative unsaturation, is shown in curve 1 of Fig. 5(a). In an earlier paper, Tench et al. [39c] showed small but significant differences in the spectral distribution of emission from evacuated MgO smoke when excited at an energy corresponding mainly to absorbance into feature I1 of Fig. 4. Interpretation of such differences in terms of surface exciton-type emission from ion pairs with differing degrees of coordinative unsaturation References p p . 419-427

316

gained indirect support in that earlier work from differing effects of exposure to water vapour. This diminished the luminescence excited by 230 nm, but increased that excited at 274 nm. Curve 3 of Fig. 5(a) depicts this latter enhancement by exposure t o water vapour whilst Fig. 5(b) schematically represents their interpretation, viz. that erosion of surface steps causes an increase in surface density of ion pairs whose anions are located at corner positions where they experience coordinative unsaturation (and lowest Madelung potential). Both reflectance and luminescence measurements in the UV-visible region can yield information on the presence of other molecular species adsorbed on the surfaces of high surface area samples and on their degree of interaction/reaction on these strongly defective surfaces. Thus, Colluccia e t al. [39] who directly observed a reversible quenching of fluorescence from finely divided MgO upon admission of oxygen, suggested that the formation of a weak charge-transfer exciplex [cf. eqn. (14)] between a coordinatively unsaturated ion-pair site and reversibly adsorbed oxygen could provide radiationless paths replacing surface luminescence. Indirect evidence for quenching of MgO luminescence by CO, CZHz,NO and N,O has been given by Stone and Zechinna [46] who envisaged in its stead the initiation of novel clustering reactions of these gases at ion-pair sites of low coordination. Other lines of evidence also point t o the conclusion that ions/molecules located at surface positions of high coordinative unsaturation can exert a strong influence on both radiative and non-radiative processes at the surfaces of finely divided solids of high surface area. Thus the absorption spectra [ 741 of transition metal ions V5+, Cr6+, Cr5+, Cr3+, COz+ and Ni2+ dispersed on finely divided SiO, or A1,0,, and the phosphorescence spectra of Fe3+ upon amorphous silica alumina [ 7 5 ] , have been interpreted as involving excited states of these cations located at defect surface sites which cause vacancies in their coordination sphere. In the cases of vanadium, molybdenum, nickel o r cobalt dispersed as small particles on the surfaces of high surface area SiO, or A1,03, Kazansky [ 741 has argued that illumination of these catalysts promotes metal ions at positions of high cus into charge-transfertype excited states, as in the reaction 02-

/

0-

\*

Reactions of hydrocarbons or the gases H, and NH, when adsorbed and illuminated on these high surface catalysts have been interpreted [ 74b] in terms of reaction with the 0 - (cus) species produced by the intramolecular charge-transfer process indicated in eqn. (15). ( b ) Other conventional spectroscopic techniques. High surface area solids

Whilst information on short-lived excited states at interfaces has come

317

predominantly from studies of the type just described upon absorption and emission in the UV-visible region, other conventional spectroscopic techniques have been applied with varying success to characterise longerlived species at gas/solid interfaces. ESR spectroscopy has been particularly valuable in this respect, not only because of its extremely high sensitivity, which allows it t o detect as little as 0.0001 of a monolayer in the most favourable cases, but also because it has the capability of distinguishing whether the unpaired electron of a surface paramagnetic centre is associated primarily with the adsorbate or with the adsorbent. Applications of ESR techniques t o the study of gas/solid interfaces have been reviewed [76, 2641 and only a few representative examples are given here t o illustrate their value in studying effects of radiation on such interfaces. The availability of ESR cavities which allow in situ illumination of the gas/ solid samples has greatly facilitated observations on photo-assisted growth in intensity of ESR signals from surface paramagnetic species. Definitive characterisation of the oxygen anion radicals formed by chemisorption of molecular oxygen or nitrous oxide on to diamagnetic adsorbents has come from ESR studies [77, 781 in which the chemisorbed gas was labelled with oxygen-17. These and related ESR studies have produced a substantial body of evidence [79] t o support the conclusion (to be expected from the electronic description of strong chemisorption) that 0; is the dominant oxygen radical anion produced at O,/metal oxide interfaces on contacting low pressures of oxygen with vacuum outgassed surfaces of n-type semiconducting metal oxide in the dark at room temperature. The ESR signal of 0; exhibits strong anisotropy in its g factors (typically ,g = 2.021, g, = 2.01 and g,, = 2.004), giving rise t o a broad resonance with 3 maxima for 0; dispersed on the surface of a polycrystalline powdered support. This signal and the growth in its intensity on irradiation of 0, /metal oxide systems has been extensively studied [79-821, as illustrated by Fig. 6 with (a) illustrating this for an 0,/A1203 system exposed to y-irradiation [81] and (b) for an 0 2 / Z r 0 2 system under UV illumination [82]. Evidence is also shown in the figure that the radiation-induced growth in the surface concentration of 0; radicals on ZrO, was accompanied by a loss of oxygen from the gas phase. Kwan [82] concluded, from correlations between the changes in the number of 0; radicals detected by ESR and changes in oxygen pressure over TiO, surfaces, that illumination produced photodesorption accompanied by decreases in 0; under illumination at very low pressures, but produced photoadsorption with increases in 0, at higher oxygen pressures. It has also been reported that outgassed MgO or A1203 powders additionally activated by an exposure to H2 developed a stronger 0; signal on exposure to UV illumination [ 421 . Chemisorption of nitrous oxide at room temperature on to metal oxides previously outgassed at high temperatures in vacuo has been developed [ 83-85] into a useful method for production of the monatomic Referencespp. 419-427

318

oxygen anion radical, 0-.Production of 0- has been attributed to dissociative attachment of an electron, i.e. N,O e- -+ N2 i0- and this has been supported by observations [87] that contacting N21 7 0 with MgO yielded hyperfine splitting (hfs) consistent with "0-. Figure 6(c) illuswhich Wong et al. [85] observed trates (i) the ESR attributable to l60on contacting either N2l60or N, 1 7 0 with ZnO, and (ii) the growth in intensity of the signal which resulted from UV illumination of the N 2 0 / ZnO interface. Absence of any signal with hfs consistent with 1 7 0 - led to the suggestion that, since such species initially approximate to a hole trapped upon a surface 0'- species, surface "0- species may be conwithin the ZnO lattice by hole migration from the surface verted to l60into the bulk [ 851 . This 0- species is reactive towards many other species co-adsorbed on metal oxide surfaces [54, 861. Conversion of 0- into ozonide anion radicals, 0;, has been reported on MgO surfaces under low oxygen pressures. The ESR parameters of the 0; radical produced MgO with 1 7 0 2 , and the fact that no scrambling of the by reaction of l60oxygen isotopes between the surface and the gas phase accompanies this species having process, have been interpreted in terms of bent 160170170 only the l60end on the surface [86]. An ozonide radical of different structure and having two equivalent oxygens has been suggested on surfaces of silica-supported VzO, which promote oxygen isotope exchange [87] . Activity for oxygen isotope exchange on silica-supported TiO, has been correlated with surface concentration of 0- radicals and interpreted [88] in terms of an 0; intermediate. The abilities of ESR to measure quantitatively the concentration of paramagnetic defects or impurities in surface regions of diamagnetic host lattices, and thence to detect any changes in concentration produced by irradiation and on contact with gases, are also relevant. Illustrative examples of the trapping of radiation-induced holes at surfaces are pro-

+

4.01 ' t

I

I

I

I

I

I

I

I

I

1.5 3.0 45 6.0 7.5 9.0 10.0 12.0 Dose, D x 10-20(eV g-

g = 2,008

319

L

L

0 +J

(b)

10 10-

Y

m

0

9-

c

-- 1

8-

* ~ r

ap

on I

I

I

I

_

(i)

n(g1=2.021

-

(ii) h

-dark

--light

o c-

on

-dark

-+light -dark on

4

- 3

rE

g

:e

e m

- v

Fig. 6. Evidence from ESR for the growth of oxygen anion radicals at gas/solid interfaces exposed to various radiations. (a) (i) Kinetic curves for chemisorption of oxygen (1)andothe formation of ion radicals 0, 2) on A1203 under the action of y-irradiation. T = 25 C, dose rate = 0.5 X 1017eVg-' min-' ; (ii) ESR spectrum of 0;adsorbed on y-Al203. Reproduced with permission and minor adaptation from ref. 81. ( b ) Changes in intensity of ESR signal of 0, at g 2.0, Z2.0, and in oxygen pressure on illumination of an 0 2 / Z r 0 2 sample. Reproduced with permission and minor adaptation from ref. 82. (c) (i) ESR spectrum of 0-and (ii) its growth at an N2O/ZnO interface on exposure to UV illumination. Reproduced with permission from ref. 85.

-

References p p . 4 1 9 4 2 7

320

vided by powders of A1203 or aluminosilicates irradiated in vacuo, for which observed ESR signals have been interpreted as trapped hole-type centres. Such surface V-centres could be removed through reaction with H, adsorbed on to the irradiated solids [89]. A converse effect has been observed with irradiated MgO surfaces, which upon irradiation in vacuo do not lead to measurable concentrations of F+-type centres (i.e. electrons trapped at divalent anion vacancies), but upon which ESR signals attributable to F+-type appeared in readily measurable concentration upon illumination in the presence of H, [ 901 . The widespread application of IR absorption techniques for the study of adsorbed molecules and for the characterisation of reactive surface intermediates in situ on suitable but non-irradiated catalyst surfaces [ 911 is outside the scope of this chapter. Applications of such techniques to characterise effects at irradiated gas/solid interfaces have been much less extensive but, in principle, these techniques are uniquely capable of providing essential information, e.g. when all intermediate or product species in a radiation-induced reaction at the interface are diamagnetic and remain strongly bound to the surface with the result that they are unobservable by ESR or by gas phase studies. Finely divided diamagnetic oxides of high surface area, such as SiOz A1203 or aluminosilicates, are frequently pressed into thin self-supporting discs which transmit sufficiently in the range 4000-600cm-' to permit detection of additional changes in transmission brought about by adsorbates. Representative illustrations of such applications are provided by Russian studies [ 92, 931 of the effects of irradiation on the IR spectra of oxides and silica gels. Thus Ermatov and Koserov [92] found that the adsorption of H, on to reactor-irradiated silica gel brought about the appearance of additional bands in the IR spectrum, whereas the adsorption of 0, brought about a displacement of the band at 3750 cm-' . The reactor irradiation brought about a strong dehydration of the surface of the silica gel sample. After adsorption of CH,OH and H,O vapour on to samples of SiO, or BeO, X-irradiation brought about additional bands in the IR spectrum, which indicated products of decomposition or esterification of the adsorbed methanol [ 921 . Photocatalytic oxidation of isopropanol, methanol, heptane and methane on ZnO, TiO,, A1203 and SiO, have been studied by Filmonov [ 931 using IR spectra of the adsorbed species. A recent example [ 941 which illustrated possibilities for combining IR with NMR concerned adsorption of isobutyl alcohol on y-A1203. IR studies were interpreted by Knozinger et al. in terms of distinct types of alcohol-related adsorbed species on different active sites, a molecular alcohol species coordinated to initially coordinatively unsaturated Al:;, ions within anion vacancies, and a hydrogen-bonded species in which the alcoholic hydroxyl group was the donor group. Subsequent studies [ 94b] of this system by proton NMR were interpreted in terms of translational motion of the adsorbed isobutyl alcohol being strongly inhibited, probably

321

because its interaction with As:l locked it into a fixed orientation a t a Lewis acid-base pair site, of which the base partner was a terminal OH on an adjacent surface A13+. Information on the mobility of protons at the interface was also inferred from the NMR and IR spectra. Kazansky [ 741 has also described the application of NMR techniques t o study activation of molecules by surfaces. The extension of such applications to irradiated systems can be confidently anticipated.

( c ) Electron spectroscopic techniques. High surface area samples Valuable and comprehensive reviews have appeared recently on the UPS of molecular species adsorbed on metal oxide, usually single crystal, surfaces [ 9 5 ] . These topics are not covered again here. Attention is directed, however, to the increasing use of EXAFS for exploring the environment immediately surrounding a metal atom or ion with characteristic and accessible X-ray absorption. The technique has found application in such widely differing systems as metal alloy particles and small metal clusters dispersed on high surface area oxide supports, or extracts from photosynthetic chloroplasts. The EXAFS technique involves sweeping through X-ray absorption of the metal under study (often by selecting continuously increasing X-ray energies from those available in synchrotron radiation) and so constructing a plot of absorption coefficient vs. X-ray energy. This exhibits fine structure on the high energy side of the absorption edge, attributable to scattering of the photoelectron by electrons on the other species in the immediate environment. Although there does not, as yet, appear t o be general agreement on how information may be extracted in a rigorous and self-consistent manner from the observed fine structure, many examples of the type of information which is claimed to emerge from semi-empirical analysis has been given by Sinfeld [ 961 . In relation t o platinum highly dispersed as particles with sizes of the order of 108, Sinfeld makes the points (i) that X-ray diffraction is of little use for such cases and (ii) that semi-empirical analysis of the observed platinum EXAFS indicates the average coordination number of the platinum atoms in such catalysts t o be lower than for bulk platinum metal [96a]. Furthermore, Sinfeld et al. have prepared bimetallic Ru/Cu clusters of very small size dispersed on oxide supports of very large surface area. The resultant high proportion of “surface” relative to “bulk” metal atoms represented favourable conditions in which surface metal atoms and their environment would dominate EXAFS. Semi-empirical analysis of the EXAFS led Sinfeld et al. to conclude that Cu atoms in surface layer(s) of the small bimetallic Cu/Ru clusters had (on average and in contrast to microscopic non-miscibility) several near-neighbour Ru atoms [ 96b] . EXAFS has also been applied recently to a long standing and previously intractable problem in natural photosynthetic systems viz. the form in which manganese occurs at the oxygen-evolving site. X-Ray spectra were References p p . 419-427

322

taken on chloroplasts capable of oxygen production (“active” chloroplasts containing “loosely bound Mn”) and on chloroplasts which had the loosely bound pool of Mn completely removed and were incapable of oxygen evaluation. Detailed EXAFS studies on the two chloroplasts and comparison with 0x0-bridged Mn dimer models, led to proposals for the local structure of the “loosely bound pool” of Mn in chloroplasts. Analysis was compatible with a bridged transition metal dimer, with oxygen suggested as the most likely bridging ligands and another manganese as the transition metal partner [ 971 .

( d ) Conventional spectroscopic techniques. Low surface area samples Consideration will now be switched from spectroscopic techniques particularly suited to the study of finely divided solids, where surface defects and sites of high coordinative unsaturation exert a strong influence, and will be transferred to spectroscopic techniques applicable t o solid samples having well-annealed surfaces of relatively low surface area. Beginning with surface reflectance spectroscopy studies, it is encouraging to note the considerable progress made since Terenin’s review. This has, for example, allowed for differential reflectance spectra from rare gas atoms adsorbed on metallic and oxide surfaces down t o sub-monolayer coverages, and theoretical considerations of local field effects have been given for such cases [ 73bl. Spectroscopic studies in the UV-visible range on molecules adsorbed on to solid samples having well-annealed surfaces of low surface area can yield information dominated by electronic transitions within molecules adsorbed on to normal locations and less prone to perturbation by strong adsorbate-adsorbent interactions at defect sites. For example, Bach and Brauer [ 981 reported that the adsorption spectrum of formaldehyde adsorbed on an evaporated layer of LiF was very similar to that in the gas phase. Information on optical absorption under similar conditions has come from spectroscopic studies on strongly absorbing dye molecules adsorbed on evaporated metal layers or on well-annealed glass surfaces [99, 1001. Little evidence of significant shifts in adsorption maxima has emerged from extensive studies by Kuhn et al. [99] for dye molecules transferred as organised monomolecular layers on to glass slides. Spectra reported by Gerischer [ 1001 for molecules of Rose Bengal adsorbed on t o either glass or evaporated gold layers show a red shift relative t o the spectrum in solution [cf. Fig. 7(a)]. The long wavelength tail observed on this absorption band for the dye molecule adsorbed on gold led Gerischer to dual hypotheses of an overlap of energy levels and of an associated high probability for charge transfer between electronically excited adsorbate and a large density of states within the wide conduction band of the evaporated gold layer. Gerischer has further argued that such chargetransfer interactions may be rapidly reversed, and the excited adsorbate

3 23

1

1.01 N

9 X c

C

L, .-c

.-U *-

c,

0 U

C

0 ._ c U

.-cC K

w I

I

..

x.

550

\ 650

Wavelength/n m

metal

sem icond

semicond.

insulator

(b)

Fig. 7. Interactions of electronically excited adsorbate with adsorbent. (a) Spectroscopic evidence from absorption spectra of Rose Bengale (i) in 0.5 M K N 0 3 aqueous solution; (ii) adsorbed on glass; (iii) adsorbed on transparent gold film. (b) Chargetransfer possibilities at illuminated interfaces: ( i ) Reversible for adsorbate*/metal; (ii) nett electron donation from adsorbate* for adsorbate/semiconductor I ; (iii) nett electron acceptance by adsorbate* for adsorbate/semiconductor 11; (iv) low probability of charge transfer from bulk energy bands of adsorbate/insulator interfaces, Reproduced with permission and with minor adaptation from ref. 100.

thereby quenched, by back-transfer of an electron from the metal to a hole in the lower electronic level of the excited adsorbate, as summarised by part (i) of Fig. 7(b). Quenching of the luminescence of species adsorbed on metals may occur in this manner without any observable nett transfer of charge between adsorbate and adsorbent. Parts (ii) and (iii) of Fig. 7(b) illustrate that quenching of electronically excited adsorbate can result in nett transfer of charge at adsorbate/semiconductor interfaces, viz. with nett photoinjection of electron and holes for energy levels as in part (ii) and (iii), respectively. Non-spectroscopic consequences of such photoinjection of charge at the interface will be considered in detail in Sect. 2

References p p . 419-427

324

and it suffices to note here that the expected quenching of luminescence of dyes adsorbed on single crystals of ZnO has been observed and attributed t o surface-to-bulk charge transfer, following a type (9) transition initiated by Forster-type energy transfer from excited adsorbate to surface states [ 1011 . A related phenomenon on silver halide particles, viz. sensitisation and supersensitisation by adsorbed dyes, has received much attention. A valuable technique developed in those studies involved the systematic variation of the lowest vacant or highest filled electronic energy level of the dye(s) in order to determine photographic thresholds for processes induced in the silver halide systems by injection of photoholes or photoelectrons [ 1021. Studies of photoluminescence from single crystals of AgCl have given evidence that surface defects and/or impurities can play an important role in the formation/activation of surface luminescence centres. Thus Vacek linked cation vacancies and/or dislocation jogs with luminescent centres [ 1031 . Vacek and co-workers [ 1041 later reported that preparation/ annealing of the crystals in air or oxygen produced a new red luminescence which was assigned t o 0;-type surface species. Further changes in photoluminescence produced by annealing in HC1 were tentatively assigned to surface C102 species. A converse experimental observation, viz. quenching of the luminescence intensity from single crystals of CdS or from polycrystalline C u 2 0 upon adsorption thereon, has been described by Wolkenstein et al. [ 1051 who have interpreted it in terms of a more rapid annihilation of surface excitons through recombination at surface recombination centres related to the adsorbed gases. Large red shifts reported by Lendvay in the fluorescence spectra of dye molecules adsorbed from solution on t o polycrystalline metal oxides o r hydroxides have been interpreted in terms of stronger adsorbate-adsorbent interactions and hydrogen-bonding in the excited singlet state than in the ground state of the adsorbed dye [ 1061. Valuable insight into species present as gas/solid interfaces and into the transitions they undergo under excitation can thus come from applications of conventional spectroscopic techniques t o polycrystalline solid samples. However, it will be important in any attempts t o infer mechanisms of chemical and physical changes produced as gas/solid interfaces by irradiation t o retain a critical appreciation of the following limitations and possibilities for misinterpretation which can attach t o such spectroscopic observations in unfavourable cases. (i) Since the surfaces of finely divided solids usually present many subsets of surface sites with non-uniform activity, the observed absorption or emission spectra can be a weighted average from species at a variety of sites. Furthermore, the spectroscopic weighting attaching to particular subsets may depend upon factors other than their number density on the surface (e.g. when the strength of the transition varies with alignment or AHadsfor various sites); (ii) It is often difficult to know whether surface impurities such as alkali ions, halogen

325

ions or “carbon” play an important role in determining surface coverage and reactivity of other species, since sub-monolayer coverages by these impurities will not show up in conventional spectroscopic techniques, whilst ESCA techniques, which can detect them, are difficult to apply to the entire surface area of powdered samples. (iii) The “kinetic” weighting t o be attached t o various surface species in effecting chemical and physical changes at the surface may be quite different from their “spectroscopic” weightings. In unfavourable cases, spectroscopic data on their own can be misleading. One such example arises when O f i s on a metal oxide is kinetically very important, but fails to show up in ESR or IR or electronic spectroscopy, which do, however, reveal 0;. Another arises when the rate-determining process involves encounter with a molecular species from the gas phase or from a physisorbed layer, whilst the spectroscopic data emphasise only those molecular species chemisorbed on the solid.

2. Photoeffects at gasholid interfaces Physical and chemical changes induced a t various interfaces through the absorption of photons with energies 1-8 eV are treated in this section, whilst effects of ionising radiations at higher energies are considered separately in Sect. 3. Electric dipole-induced absorption of photons at such energies may impart sufficient energy t o produce electronically excited states of the adsorbate or adsorbent, but the deposited energy is usually insufficient to cause the ejection of an electron into vacuum from either adsorbed species or the surface of the solid (but see ref. 107 for evidence of long wavelength photoelectron phenomena at surfaces). Whilst the driving force for the photophysical and photochemical effects considered in this section usually derives from the formation of electronically excited states, the detailed nature and efficiency of such photoeffects will depend inter alia on the manner of localisation of excitation energy at the interface. The first column of Table 2 distinguishes four different ways in which energy absorbed from a flux of UV-visible photons can effect energy localisation at the interface: firstly, through the promotion of electronic transitions between energy levels originating mainly from molecular orbitals of the adsorbate, listed as leading t o adsorbate-initiated photoeffects; secondly, through photon absorption in the direct promotion of electronic transitions between energy levels (e.g. surface states) characteristic of the surface, listed as leading t o surface-initiated photoeffects; thirdly, through dual processes of absorption and energy migration within sub-surface regions of the adsorbent, listed as leading t o adsorbentinitiated photoeffects; fourthly, through promotion of electronic transitions between new energy levels originating from strongly bonded surface complexes, listed as leading t o surface-complex initiated photoeffects. Within this section, the physical and chemical changes brought References p p . 4 1 9 4 2 7

TABLE 2

w

tu Q,

Modes of energy deposition or localisation at gaslsolid interfaces and classification of resultant photoeffects Energy deposition/localisation

General classification of resultant photoeffect

Photophysical : example

Mode

Examples

Optically induced transition within adsorbate; see eqn. (13)

Absorption and luminescence of adsorbed dyes [78,79]

Adsorbate-initiated (AI)

Dye-sensitised charge transfer into ZnO [ 8 0 ]

Optically induced transition between surface states; see eqn. (9) Optically induced transitions between energy levels characteristic of the bulk lattice, followed by transfer of energy or charge totheadsorbate

Optical absorption of Si and MgO outside the band edge [46,591

Surface-state-initiated (SSI)

Quenching by

Absorption of CdS, ZnO, Ti02 etc. inside their band edges [ 531 Action spectra similar to Ni( C O ) , at Ni surfaces exposed t o CO [ 1251

Adsorbent-initiated (ANI)

Oxygen photosorption at 02/metal oxide interfaces

Surface-complex initiated (SCI)

CO photodesorption from CO/metal system

Optically induced transitions within adsorbateadsorbent surface complexes [see eqn. (14)] followed by energy localisation within adsorbate

0 2

[45]

327

about at the interface by these processes of energy localisation will be subdivided into photophysical and photochemical effects. 2.1 PHOTOPHYSICAL EFFECTS

Photophysical effects will here be taken to include light-induced changes in the extent of adsorption as well as photoelectronic effects involving the localisation or delocalisation of electrons at the illuminated interface. They will be differentiated from photochemical effects by the phenomenological criterion that photochemical effects involve rupture and/or rearrangement of bonds other than those between adsorbate and adsorbent.

2.1.1 Experimental aspects Illumination-induced increases or decreases in the extent of surface coverage by adsorbate species shall here be referred, in general, as photosorption effects, although this term has been used by some workers in the sense of photoadsorption only. Observations on photosorption require methods for detecting and measuring the extent of displacement of preexisting adsorbate -adsorbent equilibria, of the form

+

,AB(g) MX(s)

, AB(ads) 2

to the right (photoadsorption) or to the left (photodesorption) on illumination of the interface. Prior to the advent of modern surface spectroscopic techniques (such as those listed in Table 1)which can directly observe the extent of coverage of the interface by adsorbate, the range of experimental techniques applied for photosorption studies was very limited and often consisted solely of observations of changes in the partial pressure of AB(g) over the illuminated interface. An early illustration of the sensitivity claimed for this approach was provided by the studies of Haas et al. [ 108J on the photoadsorption of oxygen on to a single crystal specimen of CdS. Outgassing of the CdS single-crystal surfaces at 670-820 K under N m-2 residual pressure was thought at that time to be a good approximation to an initially clean surface, which was then equilibrated with oxygen for 24 h in the dark at room temperature at pressures between lo4 and 7 N m-* prior to illumination. During this dark pre-equilibration, a rapid but small initial uptake of oxygen was detected by monitoring the system pressure. This was followed by a very slow continuing decline in pressure. Illumination of the interface then produced a more rapid loss of oxygen from the gas phase, the extent of which depended upon the pretreatment, e.g. it corresponded to the accumulation after illumination for 1h of 0.1 monolayer of 0,-related species upon CdS previously outgassed at 670 K or of 0.4monolayer after outgassing at 820 K. In more recent studies, mass spectrometry has commonly been used to supplement or replace total pressure measurement as the means of directly References p p . 419-427

3 28

observing photosorption effects, a good example being studies of oxygen photoadsorption on ZnO by Lisachenko and Vilesov jl09J.These workers subjected ZnO surfaces to a prolonged cleaning procedure t o reduce surface impurities to <0.001% of a monolayer and then observed photoadsorption not exceeding 0.01% monolayer under 1.3N m-' of oxygen. Gas photoadsorbed at room temperature could be thermally desorbed at 470 K. Periodic mass spectrometric analysis of the gas phase, which was continuously leaked to the analyser at < 2% h-' during 10-15 cycles of photoadsorption and thermal desorption, confirmed that impurities such as CO o r CO, were not evolved to the gas phase. The small magnitude of the photoadsorption effect reported by those Russian workers and the slow rate of approach to photostationary equilibrium, which required ca. 3 h in their study, appear typical of photosorption effects in many systems. Effects at this level impose severe demands on the sensitivity and reproducibility of the experimental procedures and critical views have been expressed as to whether some effects reported as photoeffects may not rather be thermal desorption effects [ 110, 1111. Experimental criteria by which true photosorption effects might be distinguished from possible thermal effects have been proposed, involving tests of the dependence of the proposed photosorption effects on the intensity of incident radiation at fixed wavelength, or on the wavelength normalised t o constant incident flux [ 112, 1131. A linear dependence of

n T

FA 0

\/ QMS

D

IP

E

1

S(FT)

(a1

Fig. 8. (a) Dynamic mass spectrometer system for the study of changes in gas phase pressure and composition caused by flash-initiated surface processes. The system consists of: (i) a high-vacuum system, comprising inlet leak valve, I; pressure measuring gauges, B; glass-walled photoreactor, C; metal oxide layer, MO; high-conductance tubing, E. quadrupole mass spectrometer, QMS; 14-stage electron multiplier, EM; ion pump, IP; and liquid nitrogen cooled baffle, LNB. (ii) Fast detection circuitry comprising trigger unit, T; variable delay line, D; quartz flash-tube, FT; oscilloscope, 0; and fast amplifier, FA. (iii) Appropriate electronic supplies: S(EM), S(QMS) and S(FT). Reproduced with permission from ref. 114.

3 29

m/e

= 36

(b)

r n l e = 34

Fig. 8. (b) Photographs of oscilloscope traces showing the mass spectrum of ‘*Oenriched oxygen [trace (i)] present 2 X 10s4Nm-’ over T i 0 2 and time profiles of flashinitiated changes in signal level at m/e = 32, 34 and 36 for this ‘80z/TiOg system. (i) N m-2 la 0 2 Mass spectrum in range m/e = 50 to 0 for steady-state pressure of 2 x present over Ti02 prior to flash. (ii) Time profile of lattice breakdown transient at m/e = 32 at high sensitivity with lSOz present over TiO2. (iii) Time profile of flashinitiated processes monitored at m/e = 36 and comprising rapid initial desorption of la02followed by uptake of 1 8 0 2 onto surface. (Photo shows three overlapping traces from 3 flashes at 1 min intervals). (iv) Time profile of transient monitored at m/e = 34. Photo shows traces after 2 flashes. Reproduced with permission from ref. 114.

the rate of photosorption upon light intensity over an adequately wide range of intensities has been proposed as evidence contrary to “thermal heating”, since strongly non-linear effects would be expected from Boltzmann-type factors, exp {- AH(ads)} and exp {- AH(des)}, respectively, for activated chemisorption and desorption. The second criterion is based on wavelength dependence and applies only to materials whose fundamental absorption edge occurs in the region of study, so that the interface can be illuminated inside or outside this band edge. Incident infrared and visible radiation outside the band edge should, according to this criterion, cause surface heating without genuine photosorption and so be experimentally distinguishable from effects with light inside the band edge. Since strong surface heating would not be expected for IR and visible light outside the band edge, which is mainly reflected and scattered, this criterion appears less secure than that based on the linearity of intensity effects for light inside the band edge. The two criteria together, however, can provide a useful test of the validity of photoeffects. A further 0

References pp. 419-427

330

criterion of possible value where pulsed or intermittent illumination is employed is the response time of the photoeffect, since effects depending only on photoinitiated localisation/delocalisation of electrons are expected to exhibit much faster response times than those of thermally assisted processes [ 1141. An experimental difficulty possibly associated with mass spectrometric methods for the study of oxygen photosorption processes has been detailed by Nobbs [115] in relation t o his studies of photosorption on thin films of ZnO. He estimated that the pumping speed of an on-line mass spectrometer itself (ca. 5 1 s-l in his case) could be much higher than the expected low rates of oxygen photodesorption, with the result that unequivocal evidence for oxygen photodesorption may not readily be obtained with a static photosorption cell shut off from the pumps but open t o the full pumping speed of the mass spectrometer ion source. This difficulty, and the related one of breakdown of oxygen to carbon oxides in the ion sources of the mass analyser, may not always have been taken fully into account in cases where CO,, rather than oxygen is the apparent result of photodesorption in closed photoreactors [ 1161 . Such difficulties can be largely overcome by employing a continuous flow of reactant oxygen at very low pressure through the photosorption cell and past the inlet t o a mass analyser en route to a continuously operating UHV pump, which serves to avoid any build-up of residual gas impurities. In this way, oxygen in a pure form a t pressures up t o N m-2 should be maintained in a dynamic equilibrium throughout the UHV system and continually mass analysed. Such an arrangement is illustrated by Fig. 8(a), which depicts its use in conjunction with a pulsed source of UV/visible light. With provision for fast read-out from this dynamic mass spectrometer (DMS) arrangement, two further advantages can be realised over static sealed photoreactor arrangements: firstly, the attainable photosorption effects can be compressed into a much shorter time scale through use of short-duration light pulses of high intensity, and secondly, the detailed time profiles for flash-initiated changes in partial pressure of the reactant gas (or of gaseous products if so desired) can be individually displayed by tuning the mass analyser t o continuously monitor an appropriate mass peak before, during, and after flash illumination. Typical results from the application of this system to the study of flash-initiated photosorption effects of oxygen from metal oxide surfaces are illustrated in Fig. 8(b), which demonstrate that, following flash illumination, a flash-initiated desorption process occurred first, but that a flash-initiated adsorption occurred later in some systems. This ability to time-resolve individual effects is a major advantage over studies employing continuous illumination incident on sealed photoreactor systems. The main worry with such pulsed methods must, of course, be lest genuine photoeffects brought about in this manner be distorted or hidden by possible heating effects accompanying the rapid deposition of energy into surface regions of the

331

sample by the flash illumination. The criteria described above for distinguishing thermal effects from photoeffects under continuous illumination can, however, also be applied in this case t o check whether flashinitiated effects are mainly photoeffects rather than thermal effects. Recent publications serve to re-emphasise that vacuum microbalance techniques, and particularly those involving quartz microbalances, possess adequate sensitivity for observing changes corresponding t o small fractions of a monolayer [ 1171 . Consequently, such techniques are capable, in principle, of directly observing photosorption processes. For example, the use of a vacuum microbalance to detect the changes in weight of an illuminated AB(ads)/MXi interface relative t o a similar but non-illuminated interface can be envisaged but this technique appears to have been little exploited. Characterisation of the interface by surface spectroscopic techniques, such as XPS or AES, prior to its illumination by the flux of photons which induce photosorption, followed by detection of surface changes using the same spectroscopic technique after illumination, can also provide a direct experimental probe for the build-up or removal of species from the surface by illumination. In this way, Shapira et al. [116] claim t o have detected a lowering of surface carbon impurities on metal oxide surfaces after their exposure to UV illumination which was also accompanied by photodesorption of carbon dioxide. It should be noted that those studies were advisedly made with metal oxide surfaces carrying significant surface carbon contamination. Consequently, marked differences can be expected from studies with “low-carbon” surfaces such as studied in ref. 109. Widespread use of electronic models during the decade 1966-1975 as a framework for considering electron localisation/delocalisation at gas/ semiconductor interfaces led t o extensive application of electrical measurements for the study of photosorption in those systems. The collectiveelectron-type assumptions underlying many of those applications were embodied in Fig. 1. It follows from the assumptions of CECT models that parallel measurements on the partial pressure of the adsorbate in the gas phase and on changes in surface potential during illumination are desirable, since each set of measurements should yield complementary and mutually consistent information. An early experimental study featuring such parallel measurements has already been referred to. It involved photosorption of oxygen on to n-type CdS single crystals accompanied by changes in surface potential [ 1081 . Interpretation of that early study was complicated, however, by differing observations on crystals subject t o mild outgassing at room temperature or to rigorous outgassing at 670820 K. The former were reported to yield photodesorption of pre-adsorbed oxygen and a corresponding decrease in surface photovoltage, both these effects being explicable in terms of the localisation of holes photogenerated within the semiconductor, MX, on chemisorbed oxygen which then desorbed according t o O;(ads)

+ h+

-

References p p , 41 9 4 2 7

0: (ads)

-

O,(g)

(16)

332

The switch from photodesorption in these conditions to photoadsorpt-ion of oxygen and to different photoelectronic effects of oxygen pressure upon more rigorously outgassed CdS crystals were interpreted in terms of the intervention of another process, viz. photoassisted uptake of oxygen at defect sites created on the surface during rigorous outgassing. Thus, that early experimental study not only illustrated the merits of parallel measurements on photosorption and photoelectrical surface effects, but also served an early warning, through the need to postulate an ASCT process, that CECT factors alone can be insufficient t o predict or explain the range of photoeffects observed experimentally at gas/semiconductor interfaces. Measurement of photovoltaic effects with semiconducting materials have usually been made with the specimen capacitatively coupled to the external measuring circuit via a semi-transparent field electrode situated just above the illuminated front surface. Usually, an ohmic contact is provided to the back non-illuminated surface and the photovoltage is the open circuit potential difference observed between the illuminated and non-illuminated surfaces of the semi-conducting specimen. In addition to their long-established use for studying the transport of photogenerated charge carriers and the establishment of electronic pseudo-equilibria under illumination, these techniques have more recently been applied t o investigate the surface potential barrier created by the photoinitiated exchange of charge between the surface states and bulk energy bands [ 1181. Parallel photoconductivity measurements on semiconducting specimens frequently suffer from the disadvantage that photogenerated charge carriers represent only a small perturbation on large equilibrium conductivity in the dark, with the result that light-to-dark ratios are small. This disadvantage can be overcome, to some extent, through the use of insulating specimens [ 1191. Parallel photovoltage measurements with capacitative coupling of the front and back faces of the sample to the external measuring circuit by means of two field electrodes can then yield complementary information on the photopotentials associated with the Dember effect and with changes in surface barriers originating from photogenerated charge exchange t o surface states. The possible advantages of pulsed techniques for the experimental determination of the nature and kinetics of photoelectronic effects and their decay at oxygen/CdS interfaces have been examined by Many et al. [120]. Pulsed light from a xenon discharge tube was directed on t o the CdS surface via a semi-transparent field-plate and the transient photovoltage thereby developed between the CdS surface and the field plate was displayed on a C.R.O. Two important advantages were found for such flash illumination relative t o continuous low-intensity illumination: (i) saturation of the surface photoboltage was more readily achieved and (ii) post-illumination decay of the photogenerated holes was considerably faster, with the result that pseudo-equilibrium conditions were restored

333

after a shorter waiting time. It was explicitly recognised by those workers that the effective barrier height, V , , at an O,/CdS interface in the absence of illumination would normally not correspond t o the equilibrium barrier height required by true dynamic equilibration of electrons between the bulk and the surface across a Schottky barrier, since barrier height will approach its true equilibrium value logarithmically with time, i.e. slowly and with Elovich-type kinetics. Direct measurements of oxygen photosorption under flash illumination were unfortunately not made in parallel with measurements on the photoelectronic effects. However, an inference that oxygen photoadsorption on CdS is both faster and more irreversible than photodesorption has emerged in this and other related studies from comparisons of the times taken for an illuminated O,/CdS interface t o adjust to a new photostationary value of the surface barrier height on changing the gas pressure upwards or downwards. Relaxation techniques based on the perturbation of gas/solid interfaces by the application of a large electrostatic field (usually via a field electrode capacitatively coupled to the surface) have been utilised by some workers in the study of photophysical effects [ 120, 1211. Thus Shappir and Many [120b] utilised the application of a perturbing electrostatic field, together with pulsed illumination of O,/CdS interfaces and measurement of photovoltage transients, as a means of determining experimentally what applied field sufficed to cause a “flat-band” situation at the O,/CdS interface. Studies of field-assisted adsorption, desorption and surface electronic effects at non-illuminating interfaces can also be valuable in characterising rates of majority-carrier localisation/delocalisation processes at the interface [121]. Prospects for useful correlation between photosorption and parallel measurements on photoelectronic effects are more problematic for solids other than semiconductors. Thus, for gas/metal interfaces, no significant separation of charge in a double layer can be expected, with the result that photosorption, t o the extent that it occurs at all on metals with photons of energies < 8 e V , may not be accompanied by measurable surface photoelectronic effects in those systems. Photosorption effects may likewise be possible without measurable photoelectronic effects at illuminated interfaces between gases and insulating solids which are capable of binding adsorbate species through electron-pair interaction with electrons already localised in surface states or on Lewis base-type active sites such as 02-(cus) [122a].

2.1.2 Results and interpretations

A formalism, thought to be a suitable basis for the theoretical treatment of photodesorption as the physical outcome of interaction between a photon and an adsorbed species, and a solid surface, has been outlined by Grimley [ 1231 together with indications of the substantial obstacles to References p p . 419-427

334

ab initio calculations. In view of such obstacles, an empirical approach has frequently been adopted in developing various approximate models to interpret results observed for photosorption and related photoelectronic processes a t gaslsolid interfaces. Consequently, in the following subsections, effects are grouped into those interpreted by the original workers as being initiated by electronic transitions within the adsorbate o r within adsorbate-adsorbent surface complexes (cf. processes labelled as A1 o r SCI in Table 2) or, alternatively, into a group interpreted as being initiated by electronic transitions within the adsorbent or involving surface states (cf. processes labelled as AN1 or SSI in Table 2).

( a ) Results interpreted in terms of adsorbate-initiated ( A I ) or surfacecomplex-initia ted ( X I ) photoph ysical processes A review of the extensive Russian work on photosorption processes on metal oxide adsorbents has been given by Solonitsyn [124]. Strong emphasis was placed in much of this work upon direct manometric measurements of the amount adsorbed at the illuminated interface, since results of several Russian workers appeared t o cast serious doubts on the existence of any reliable correlation between photoinduced changes in conductivity and the extent of adsorption. Indeed, Solonitsyn concludes in his review that “it is quite impossible t o draw an unequivocal conclusion on simultaneous photosorption processes on the basis of only single measurements of conductivity and photoconductivity of an adsorbent under different illumination conditions (oxygen and vacuum). This conclusion applies equally to other indirect methods.” Among the conflicting observations which led Solonitsyn t o this conclusion were (i) the contrast between direct manometric measurements, showing only an irreversible photoadsorption of oxygen on TiO,, and indirect photoelectronic measurements on the photoconductivity of T i 0 2 , which would lead to expectations of reversible photodesorption [125] and (ii) the conclusion invariably drawn from indirect studies that photodesorption predominates, whereas manometric measurements on O2/ZnO systems reveal both photoadsorption and photodesorption processes [ 1261. The latter observations led t o postulates that chemisorbed oxygen may exist in differing ionised forms, e.g. O;, 0:- (or 2 0 3 , and that photoinduced desorption may originate from such species by direct photoinduced loss of an electron. For O;, such loss of an electron may be represented, with accompanying photodesorption, as

-

+

-

O;(ads) + kv e; O,(ads) 0 2 (g) However, photoinduced change in the extent of ionisation of 0;- (or 2 0 3 may occur, as in eqns. (17b) and (57c) without the accompaniment of photosorption, viz. Oi-(ads)

+ hu

-

e;

+ O;(ads)

335

2O-(ads)

+ hv -e;

+ O;(ads)

(17c)

Solonitsyn further envisaged that subsequent localisation of electrons set free in these photoinduced processes would tend to increase the extent of surface coverage by 0; through the process 02(ads)

+ e;

-

O;(ads)

(17d)

Reversible photodesorption might thus be accounted for by the predominance of the process (17a), whereas photoadsorption could result from processes (17c) plus (17d). The possibility was also recognised in Solonitsyn’s review that photoelectrons released at the interface by (17a) and (17c) may remain localised on the surface and so be readily accessible for contributions to photoadsorption by ( 17d) but without actually becoming delocalised into the conduction band and capable of participating in the bulk conductivity of the adsorbent. Detailed mechanisms for the photoionisation processes were not established but agreement with experimental data was considered t o emerge from assumptions that photon absorption occurred at chemisorbed oxygen (i.e. A1 in Table 2) or at surface regions of the adsorbent in the immediate proximity of the chemisorbed oxygen (i.e. SCI in Table 2). Solonitsyn claimed that his data on the kinetics of oxygen photoadsorption on carefully oxidised ZnO samples under the influence of monochromatic illumination accurately obeyed an Elovich-type equation

Q = Aln(B1t

+ 1)

where Q is the amount of oxygen photoadsorbed, I the light intensity, t the duration of illumination, and A and B parameters dependent on the history of the sample and photon flux. This kinetic behaviour, together with the observation that photoassisted isotopic exchange accompanied photoadsorption at ( 1602+ “O,)/ZnO interfaces, was considered t o be qualitatively consistent with the following series of reactions, at that illuminated interface.

+ + - + + o+o- o + o , -0-+o, 0 - + hv

e; 0,

(18a)

0;

hv

0 e-

0,

e;

0;

W C )

0;

(18d)

(18b)

(18e) This reaction scheme was explicitly recognised [ 1241 as differing basically from “electronic” models based on the photoinduced disturbance of collective-electron energy levels of the bulk adsorbent. Spectral dependence of the photoadsorption of oxygen on powdered samples of pure zinc oxide o r sinc oxide doped with lithium or aluminium has been studied by Zakharenko et al. [127] whose results provided References p p . 4 1 9 4 2 7

336

evidence for maximum response of photoadsorption under illumination of the O,/ZnO(pure) interfaces at wavelengths corresponding to 0.40.8eV outside the band edge [cf. Fig. 9(a)]. The quantum efficiencies reported for oxygen photoadsorption were quite high for pure ZnO (10% under a flux of 9.5 x 1013 photons s-l of energy 2.8 eV; or 24% at a flux of 1.8 x l O I 3 photonss-’), but lower for lithium-doped ZnO (ca. 2% under a flux of 9.5 x 1013 photons s-’ of energy 2.4eV) and were too low for accurate measurement with aluminium-doped ZnO. The sensitivity of lithium doped zinc oxide towards photoadsorption under the action of photons well outside the band edge was attributed t o optical transitions of the type illustrated by

which corresponds to a surface-complex initiated photoeffect. This involves photoionisation of surface oxide ions situated adjacent t o a lithium, Li;, substituted for Zn2+in the surface lattice of ZnO and having an adjacent interstatial Li;. It was further suggested that the sensitivity of the “extra pure” ZnO samples to photons at ca. 0.4 eV outside the band edge was associated with photoionization-type transitions involving surface ions, denoted by 0:- in 0:-

hv

0;

+ e-

(20)

Unfortunately, the diffuse reflectance spectra measured for the powdered zinc oxides provided no definitive spectroscopic support for the occurrence of transitions of the types envisaged in eqns. (19) and (20) nor for the suggestion that surface 0; species produced by such processes contribute to photoadsorption in the manner of eqn. (21) [cf. Fig. 9(a)]. 20;

hv + 0 2 ( a d s )& 2O;(ads)

(21)

The relatively high quantum efficiencies (up to 0.24) reported by these Russian workers and identified as an adsorbate-initiated or a surface complex-initiated photoeffect raise two interesting points. Firstly, the reported efficiencies are surprisingly high in an absolute sense, in view of the fact that < 1%of incident light could be absorbed by even the most strongly allowed optical transition within a molecule distributed as a monolayer on the surface. Secondly, the efficiencies are orders of magnitude greater than those reported for another photoinitiated surface process, which exhibits some of the characteristics expected for a surface complex-initiated photoeffect, viz. the photodesorption of CO from metal surfaces [128]. For example, a yield of 2 x CO molecule photodesorbed per photon incident at 270 nm has been reported from illuminated interfaces between CO and metallic nickel. The spectral dependence reported for this latter process resembles published absorption spectra

337

[129] for Ni(C0)4 and the resemblance would be consistent with the photoinitiating step being absorption of a photon by a surface complex of the form Ni-CO. Shapira and co-workers [130] have remarked on the lack of any fully satisfactory explanation of the anomalously low value (ca. lo-’’ cm2 ) of the cross-section for photon absorption by this surface complex, which may be inferred from the observed efficiency of -2 x for CO photodesorption, whereas cross-sections ca. lo-’’ cmz , similar to those observed for the gas phase carbonyl, might be expected. A strong diminution of the apparent cross-section for the photodesorption process could, however, be expected if the initially excited surface complex was strongly quenched by reversible electron transfer with the metallic support in the manner depicted in Fig. 7(b)(i).Any such quenching would be less efficient on the semiconducting zinc oxides, but even an assumption of zero quenching leaves the question of how absorption efficiencies of < 1%expected for A1 or SCI processes could produce the efficiencies of up to 24% observed in oxygen photoadsorption on zinc oxides, viz. was adsorption sufficiently enhanced by multiple processes, or did oxygen clusters form around each photo-adsorbed 0;. Adsorption of dye molecules on to the surfaces of insulating or semiconducting solids such as AgBr or ZnO, which do not strongly quench excited states of adsorbed species nor themselves strongly absorb photons in the visible region of the spectrum, has been widely utilised for sensitisation of the adsorbent towards photophysical effects at wavelengths adsorbed initially by the adsorbed dye. Resultant dye-sensitisation phenomena, such as the observations of photoconductance and photovoltage effects under the action of visible light which is absorbed only by the adsorbed dye represent readily accessible adsorbate-initiated photophysical effects. Such measurements of surface photovoltage and photoconductance were made recently by Lagowski et al. [ l o l l with ZnO single-crystal surfaces on which the dyes fluorescein, erythrosine B or rose-bengal had been pre-adsorbed from methanolic solutions. Measurements of surface photovoltage by a contact potential difference (cpd) method clearly revealed processes of photoinduced electron injection from the illuminated (air/adsorbed dye/ZnO) or (vacuum/adsorbed dye/ ZnO) interfaces. The spectral response curves of charge photoinjection were very similar to the optical absorption bands of the dye molecules but with relatively small red shifts due to adsorption. However, the number of electrons photoinjected from the surface into the bulk were derived from the cpd measurements as ca. 3 x 10” cm-’, which contrasted with values of ca. loi3 cm-2 for the density of dye molecules pre-adsorbed on to the surface. Furthermore, increasing the surface concentration of preadsorbed dye did not significantly affect the number of electrons photoinjected. These observations led Lagowski et al. to conclude that “direct” injection of charge from an electronically excited state of the adsorbed dye molecule into the conduction band of ZnO was not occurring, but References p p . 4 1 9 4 2 7

338 DLi, 1.5

0.2

s

CI

K.% 10

8 1.0

0.2

0

c

6

a

\ v F

,

h

3.5

0.1

4

0 -

0 2.0

2.5

2

3.5

3.0 hv (eV)

0"O0

0

0 L

a

2 a

A

-

A A

4 4

A I

I lo-'

I

I

I 10-2

D a r k pressure ( t o r r )

I

I

I 10-1

339 I

I

I

I

2

40 -

- 40

( C )

-

-

0

~0

I

I

I

I

I

I

I

I

.

a

-> E

h

b

50 -

- 50 -

0

1

Time

2

(s)

Fig. 9. Evidence from direct experimental observations on the gas phase for photosorption processes at illuminated 02/metal oxide interfaces. (a) Comparison of the spectral dependence of the quantum yield of oxygen photoadsorption (curve 4) on polycrystalline zinc oxide under continuous illumination with spectral dependence for diffuse reflectance (curve 1) and for quantum yield of carbon monoxide photooxidation (curves 2 and 3 at different photon fluxes). Reproduced by permission from ref. 127. ( b ) Dependence of the sign of the observed oxygen photosorption processes at 0 2 / S n 0 2 interfaces on the state of reduction of the S n 0 2 surface: the upper plot illustrates photodesorption from an SnO, sample vacuum outgassed at 470 K (0, CE); the lower plot shows negative values of &/AT, i.e. photoadsorption, from the same material and another sample after prereduction at 870 K (+, reduced CE; A, reduced BDH). Reproduced with permission from ref. 134. (c) Pressure-independent time profile for the photodesorption of “ 0 2 initiated from “ 0 2 / Z n 0 interfaces by a 5 0 p s flash of photons at wavelengths 3 4 0 - 6 4 0 n m and detected with a DMS system at pressures of lo-’ to N m-*. Reproduced with permission from ref. 135.

rather that energy transfer occurred from the excited dye molecule to surface states of the ZnO surface, followed by injection of an electron into the conduction band from the resultant excited surface state. An important role of pre-adsorbed oxygen in determining the surface density of states capable of dye-sensitised electron photoinjection by this latter “indirect” process was suggested, in order to account for observations that References p p . 4 1 9 4 2 7

340

reducing the system pressure from 1atm t o N rn-, led eventually to a big decrease in photovoltage, although the adsorbed dye was not removed by such evacuation. The specific nature of the oxygen-related surface complex was not established. However, the possibility was recognised that the surface oxygen complexes involved in dye sensitisation could differ from those associated with chemisorption on ZnO in the absence of dyes [ 1011.

( b ) Results interpreted in terms of adsorbent-initiated (ANI) or surfacestate initiated (SSI)processes Surface photoeffects originating from energy deposition by incident photons into subsurface regions of the solid should be favoured relative to adsorbate-initiated processes by the much greater efficiency of energy deposition but disfavoured by the need to “back-transfer” such energy to the surface by processes such as (lo)-( 12). Interpretations developed by various workers for photophysical processes at 02/CdS, 0,/Ti02 and 0 2 / Z n 0 interfaces and based on energy deposition in the adsorbent are utilised in this subsection to illustrate how an emphasis initially placed upon collective-electron factors has been followed by growing recognition of the need to take into account such surface-localised features as active sites and surface states. An early model advanced by Mark [ 1311 t o account for photophysical effects as gas/CdS interfaces was reproduced in Fig. l ( b ) (p. 294) and was based on a representation of the CdS surface as effectively uncharged, except under the combined action of light inside the band edge ( h v > 2.4 eV), plus contact with an electron-accepting gas, such as 0 2 ,N,O or I,. The assumed absence of significant numbers of chemisorbed 0; (or N 2 0 - or I;) on the surface of the insulating single crystals of CdS prior to illumination led to representation of the interface at the start of illumination as in part (i) of Fig. l ( b ) , i.e. in an approximately “flat band’’ situation with only a small Dember potential originating from the greater range of photogenerated electrons than photogenerated holes. The omission of any significant surface charge due t o filled surface states was not rigorously justified, although Many et al. [120] later concluded that surface states were not intrinsic to the CdS surface but could arise from adsorbed species. Recombination as per part (i) of Fig. l ( b ) may therefore be represented as the dominant pathway for the disappearance of photogenerated charge carriers in the absence of adsorbed species. Part (ii) of Fig. l ( b ) illustrates additional features, which arise whenever electron localisation becomes possible at the illuminated 0, /CdS interface, viz. photoassisted uptake of oxygen on to the surface as 0; ions and an accompanying growth of a Schottky barrier layer voltage (i.e. the photovoltage) in surface regions, due to the field between adsorbed ions and trapped holes. Such adsorbent-initiated ( ANI) photoadsorption must

34 1

progressively experience competition through removal of photogenerated carriers by new recombination processes, one of which will be a photodesorption process caused by Schottky barrier-assisted migration of photoholes to the illuminated interface where they neutralise 0; ions and cause their desorption as 0 2 . The recognition that ANI-type photosorption proceeding via models similar to that in Fig. 1(b), represents the net outcome of opposing photoassisted adsorption and desorption processes initially found expression in equations of the type

dM; = dt

cA,N,*M, -cA,BPdMI

for describing the kinetics expected for photoassisted coverage of the surface, M;,by chemisorbed ions at time t . The first term in this equation described photoadsorption depending on: A,, , the cross-section for electron capture by a physically adsorbed molecule; M , , the surface concentration of physically adsorbed molecules available for electron capture under gas pressure P ; N ; , the surface concentration of electrons, related to that in the bulk, N , , by the collective-electron-type expression N: = N , exp ( q V * / h T ) ,where V * is the height of the Schottky barrier under illumination; and c, the thermal velocity of electrons in the conduction band. The second term of eqn. (22) relates to photoassisted desorption depending on: A , the cross-section for capture of a free hole by a chemisorbed ion; BP;, the surface density of free holes under illumination, where 8 expresses the fraction of such photoholes which are free (i.e. not trapped). Many et al. [120] have pointed out that the photostationary state which would ultimately result from operation of the opposing processes envisaged in eqn. ( 2 2 ) is unlikely to be reached in convenient experimental time scales for CdS single crystals. Consequently, eqn. ( 2 2 ) has instead been used as a basis for comparison between predicted and observed initial kinetics of photosorptio; or related photophysical effects. Thus Mark [131] measured Ai, the changes in conductivity of 02,’CdS and N,O/CdS interfaces under the action of band-gap illumination and found agreement with Elovich-type kinetics in the form

Ai = h In { ( t / t o) 1)) over the time interval 3-100s. He argued that this was consistent with eqn. (22) in the limits of strong predominance by the first term and constant number of trapped holes over this time interval. Such kinetic checks, based solely upon photoelectronic effects, provided only limited tests of eqn. (22) and of the model in Fig. l ( b ) . A further criticism of this treatment arises for the N,O/CdS interface, where the dissociative electron capture process N20

+ e-

-

References p p . 4 19-427

Nz

+ 0-(ads)

342

is indicated by other work [83-851 as a more likely (and less reversible) photoinduced process than the reversible photosorption of N 2 0 assumed by Mark [132]. A related phenomenological model proposed later by Many et al. [ 120, 1331 to explain their observations on surface photovoltage, photoconductance and field-assisted conductance at interfaces between 0, and single crystals of n-type CdS may be viewed as introducing some of the features needed to treat systems influenced, to some extent, by surface states. However, in this treatment, only surface states related to adsorbed oxygen were considered t o be involved in charge localisation and delocalisation at the interface. It was proposed that, following the application of an electrostatic field in the absence of illumination, the net rate of electron localisation at the interface would be described by

1 dM; = A,N,M, c dt -__

-NIM;

the first term of which expresses the rate of electron localisation at unoccupied surface states (physically adsorbed oxygen), whilst the second term corresponds to the emission of electrons back into the conduction band from occupied surface states (chemisorbed oxygen molecules). The symbols A , , N,, M, and M; have similar significance as for eqn. (22) and the only new symbol, N1, corresponds to an emission constant for loss of an electron from M- back into the conduction band given by

(24) Here, N, is the effective density of states in the conduction band and AEt is the depth of occupied surface states below E c , . In the treatment of N1 = N,exp(-AEt/kT)

Many et al., the only acceptor-type surface states considered to be present in appreciable concentration and involved in eqns. (23) and (24) originated from adsorbed oxygen. It was further argued that the density of such unoccupied states (physically adsorbed oxygen) was determined by the ambient pressure but was not proportionately reduced by oxygen chemisorption. Although no direct measurements were made upon rates of oxygen adsorption or desorption, experimental observations upon relaxation of the surface voltage were interpreted as being in agreement with a simplified form of eqn. (23) in which the second term, which represents the rate of thermally assisted reconversion of chemisorbed oxygen t o physisorbed oxygen, was considered to be extremely low. When this latter simplifying assumption is not made in treating the surface localisation of majority and minority carriers at illuminated O,/CdS interfaces, the nett rate of electron localisation upon adsorbed oxygen should be approximated by

1 dM;

- ___ =

c

dt

A,N$M,

-

343

Again, no direct measurements of rates of oxygen chemisorption were made which would allow a rigorous test of this equation, but indirect observations o n surface photovoltages were interpreted as being in agreement with a simplified form of eqn. ( 2 5 ) in which the last term was omitted [cf. eqn. (22)]. Such interpretations led initially t o excessively low values of ca. 10-27cmz for the effective cross-section, OA,, for capture of AN1 photoholes by occupied surface states (chemisorbed oxygen). Later attempts were made by other workers, including the present author, t o use the phenomenological models and equations just described as a basis for interpreting direct manometric and mass spectrometric measurements of photosorption at various oxygenln-type semiconductor interfaces. Thus Petrera et al. [134] examined O,/SnO, and O,/TiO, systems and made measurements of pressure changes, AP,with sensitivity down to N m-,. This allowed plots to be made of photoinduced changes in pressure over a given illumination interval, i.e. @/At, as a function of the oxygen pressure with which the interface was equilibrated prior to illumination. Such plots demonstrated that @/At could change from photoadsorption to photodesorption on surface reduction. For a surface of SnO, or TiO, pretreated in a particular manner, the sign of @/At changed from positive (i.e. photodesorption) to negative (photoadsorption) at some inversion pressure, P,, in the pressure range lo-, to 1N m-’ [cf. Fig. 9 ( b ) ] . The pressure at which inversion occurred for a particular sample depended o n the extent of prereduction of the surface by its prior outgassing treatment, e.g. a sample of TiO, outgassed at 473K exhibited nett photodesorption under an oxygen pressure of ca. 3 x lo-’ N m-* , whereas the same sample after pre-outgassing at 673 K exhibited nett photoadsorption under 6 N m-,. Similar dependence on the degree of prereduction was also observed for SnO, samples, with the result that Petrera et al. [134] were led to revise the approach of Many et al. in order to take into account the effects of acceptor-type surface traps (states) resulting from surface prereduction. In their modified treatment [ 1 3 4 ] , an upper limit on M ; , the number of oxygen ions chemisorbed, was expressed as M ; = + M & , where is a fraction of the theoretical equilibrium surface coverage, Meq. The rate of oxygen adsorption at a particular pressure prior t o illumination could then be expressed as

+

where the exponential term relates the number of electrons per cm2 at the surface to those in the bulk across band-bending, v = qV/ht. This equation allows two limiting cases for oxygen adsorption in the dark: either chemisorption can proceed at low oxygen pressures and low Referencespp. 4 1 ~ - 4 2 7

344

density of occupied acceptors u p to an equilibrium value corresponding to v < 1 and a true chemisorption limit, in which coverage is limited by the availability of gas molecules, o r the attainment of chemisorption equilibrium can be "pinched-off" for a high density of acceptor states a t a limiting value set b y th e exponential term with u > 1. A different sign of t he nett photosorption effect may then be obtained by starting illumination from each of these two regimes, nett reversible photodesorption being t he likely outcome at low gas pressures (i.e. with $ < l ) ,whilst net irreversible photoadsorption is the likely outcome in t h e pinch-off regime. The influence of pre-reduction of the SnO, and TiO, surfaces could be qualitatively understood, partly in terms of increased hole-trapping in th e bulk and partly in terms of th e effects of extrinsic surface states a t the reduced surface upon th e magnitude of u for a given coverage by oxygen. The possibility that an approach similar to those underlying eqns. (22)-(26) might be applicable to photosorption kinetics has been examined in th e author's laboratories through observations o n the time profiles for the appearance of I8O2in the gas phase following exposure of various "O,/metal oxide interfaces to flash illumination by a 50 ps light pulse [135, 1 3 6 a I . It was known, from studies of th e release of I6O2 species from flash-illuminated 1 8 0 2 / Z n ' 6 0interfaces [ 1 3 6 b 1 , th a t surface photolysis (and resultant possibility for oxygen isotope exchange a t the interface) was reduced below detection through utilisation of a filter, transmitting photons at 360--640nm, between the flash lamp and t h e previously outgassed ZnO samples situated within a cylindrical quartz vacuum envelope. Moderate sample outgassing temperatures at 6 2 3 K and clean high vacuum conditions (residual system pressures routinely 7 x lo-' N m-2 within an ion-pumped SS system) were utilised in efforts to achieve an approximation to initially well-outgassed surface conditions without excessive reduction o r formation of an accumulation layer. Surfaces were then pre-equilibrated with 4 x N m - 2 of 1802 a t room temperature t o establish a surface depletion layer with a view to causing holes photogenerated in ZnO by the 5 0 p s light flash to move rapidly towards a negatively charged interface under th e action of upward band bending of the type depicted in Fig. 1. The kinetic consequences of the so-designed arrival of a pulse of photogenerated AN1 holes from subsurface layers to the "0, /ZnO interface (as viewed experimentally by monitoring with the dynamic mass spectrometer system of Fig. 8 ( a ) the ) are time-profiles of flash-initiated changes in gas phase pressure of 1802 illustrated in Fig. 9 ( c ) . The appearance in those conditions of positive *4P("02) changes which were at least one, but usually two, orders of magnitude greater than any *AP(" 0 , ) observed in blank experiments involving identical flash illumination of an "0, /quartz substrate interface, established flash-initiated desorption of molecular " 0 , as a real property of " 0 2 / Z n 0 and 1X02/'I'i0, interfaces. Both t h e apparent rise times ( T : , ~ * 1 0 0 m s ) and quantum efficiencies ($"O, % 2 x for

5

TABLE3

2 2

Relative values for flash-initiated surface photolysis, oxygen desorption and isotopic exchange at I8O2 metal oxide interfacesa Gas/solid interfaces

Photodesorption 18 O2 (photo)b via 38A

tw

I8o2/ c r 2 0 3

u

18 0 2 /Fed34

2.3 x 1014 1.4 x 1014 <1.2 x 1Ol3 6.2 x 1013 3.7 x loi3 ~ 1 . x2 l o i 3 2.5 X 10l3 1.2 x 1014 3.7 x 1013 <4.9 Y 1Ol2

A k

1

/Ti02 l8Oz w 2 0 5 16 O2 /ZnO-In " 0 2

l 8 0 2 /&304 18

0 2

/cuo

/ZnO "02/NiO "07 /ZnO-Li

Exchange

Desorption

Photolysis

O2 (chemisorbed)c

l 6 0 I 8 O (photo)d via quartz

1802 (photo) via quartz

I6O2 (photo) via quartz

7.5 x 1017

4.5 x 1014 1.1 x 1014

4.3 x 1.1 x 2.0 x 1.5 x 1.2 x 1.2 x 3.7 x 9.8 x 1.2 x 4.9 x

2.0 x 4.4 x 8.0 x 9.5 x 4.9 x 1.7 X 3.8 x 5.5 x 2.8 x 2.0 x

8.1 x 1Ol6 2.8 x loJ6 2.4 X 10l6 5.1 X 10Is <2 x 1OlS

3.5 x 8.6 X 2.3 x 3.1 x

1013

loi3 1013 1013

N rn-' of 1 8 0 2 prior to flash illumination. Measured under a stable dynamic pressure of 3 x Molecules desorbed following incidence of 6.5 X 10'' photons a t 340-550 nm. ' Oxygen molecules irreversibly chemisorbed onto the sample weight by exposure to 02 at 13.3 N m-'. Molecules released to the gas phase following incidence of 5.4 x 10" photons at 200-550 nm. a

1014 lot4 1014 lok4 1013 1013 1013 1013 1013 1Ol2

l0lS 10IS 1014 1014 1OlS 1OIs 1014 1013 1013 1013

346

ZnO and < 2 x for T i 0 2 ) could be determined by conventional procedures from time profiles of the type shown in Fig. 9(c). These carry implications as to the possible operation and relative importance of the terms in eqns. (22)-(26) as r.d.p. for "02desorption. For example, the observation that *AP( never took negative values under illumination at 360-640 nm argued against predominance of the "pinch-off" situation envisaged as one limit of eqn. (26). Furthermore, the unexpectedly high value of T : , ~ of l o o m s , which could not have arisen from electronic response time nor from the transit times (< 1ms) of gas molecules from interface t o ion source of the mass spectrometer, would not be consistent with the arrival of photogenerated holes (or, indeed, excitons) being the slow ratedetermining process. Various models were considered as possible origins of T x 100 ms. Model A would attribute this to the low effective cross-section, OA,, for the capture of photoholes by chemisorbed as emerged from the photoelectronic studies in relation to the second term of eqns. (22) o r (25). Low overall quantum efficiencies of photodesorption (cf. Table 3) could readily be understood on this basis, since the absence of effective hole capture by 0; would permit most photogenerated holes to disappear via electron-hole recombination processes at other surface (or subsurface) states. One further prediction based initially on this model [here termed Model A to distinguish it from eqns. (27) below], which was confirmed experimentally [135], was that the extent of flash-initiated photodesorption of 1802increased with the amount pre-adsorbed as I8O;. Data relating t o this point are summarised in the second and third columns of Table 3 and qualitatively support a correlation of *AP("O,) with O("O;), but only for pure and doped zinc oxides of known n-type conductivity and conduction band densities. The dynamic mass spectrometer system [cf. Fig. 8(a)] allowed only a limited test of the pressure dependence of At',,,,o n Po,, viz. in the range of pressures from lo-' to N rn-, of " 0 , with which the "O,/ZnO interfaces were equilibrated prior to (and during) the flash. The results of such tests were illustrated by Cunningham and Samman [114] and showed that At',,, was independent of the dynamic oxygen pressure in this range. This result also appeared consistent with a true photoinitiated desorption process of the type envisaged by the second term of eqn. (25). It would not be consistent with a thermally initiated desorption of physically adsorbed since extent of physical adsorption should increase with system pressure and would, in any event, appear too small on the metal oxide surfaces at room temperature and at these low pressures to account for the reproducible amount of " 0 , desorbed after each flash. Another observation which argued against purely thermal desorption was the absence of any detectable AP at m/e = 36 whenever filters were used excluding photons with energies greater than the band gap of ZnO. Such negative results with photons of 450 < h > 650 nm also argued against a dominant role of direct photoionisation of "O;(ads), since that is

347

reportedly possible for gas phase 0; with photons of X < 500 nm [ 1371. However, eqns. (27) represent one alternative mechanism which, with step 27(c) as a slow thermally assisted process, would be equally consistent with the experimental observations and is here denoted as model B.

hv h:

0:

--

+ MO + “O;(ads) + e;

e-, h’, (e--h)

(27a)

l80* 2i

(27b)

O;(ads)

(27d)

Mechanism (27) distinguishes fast photoinitiated processes, viz. (a) and (b) leading t o vibrationally or electronically excited 0: from the slower thermally assisted desorption in step (c) and from the recapture of electrons by 0; , which together would account for T 100 ms and low @. Direct mass spectrometric observations by French workers on the composition of the gases photodesorbed from TiO, (anatase) samples under an initial residual vacuum of 5 x N m-2 showed comparable amounts of C 0 2 , CO and O,, which built up t o a maximum only after about 10 min continuous illumination at moderate intensity by bandgap photons [ 1381. Oxygen previously adsorbed on the O,/TiO, interface by exposure for variable times t o pressures < N rn-, was shown t o undergo subsequent photodesorption with particularly rapid evolution during the first 3 s of intense illumination. Linear Elovich-type plots of the amounts so photodesorbed versus the logarithm of prior contact time between 0, and TiO, were obtained and were interpreted as indicative of a linear variation in activation energy for adsorption as surface coverage increased. Linear log-log plots of the maximum amount so photodesorbed versus pressure during the prior equilibration with oxygen pressures < 1 O - j N m-, had slopes of 0.5 and were interpreted as evidence that oxygen at these low pressures experienced dissociative chemisorption as 0- or 0. Later French studies of the oxygen pressure dependence of the photoconductivity of TiO, , ZrO, , V 2 0 5 , ZnO, SnO, , Sb,O,, CeO, and W 0 3 also utilised the slope of log 6* vs. log Po, plots as a means of determining whether surface photoconductivity (as distinct from dark conductivity) was determined predominantly by equilibria between gas phase 0, and either 0- (slope = - 0.5) or 0; (slope = - 1) [ 1391. Only V,O, of the oxides examined in that study failed t o give photoconductance evidence for “ionosorption” of oxygen species. By contrast, TiO, gave photoconductance evidence for a predominance of ionosorption as 0- at low P o , , giving way to 0; at higher Po, . Control of the equilibria by 0- surface species was indicated for illuminated O2/ZrO, and O,/ZnO interfaces [ 1391 . Direct monometric studies of oxygen uptake on illuminated TiO, surfaces by Bickley et al. indicated, on the other hand, that residual

-

References p p . 4 19-427

348

surface impurities, viz. surface hydroxyls, could play a dominant role in oxygen photoadsorption at hydroxylated O2/Ti02 interfaces under subband-gap illumination [140]. Those workers found that the activity for photoadsorption was progressively diminished when the TiO, was exposed t o prolonged outgassing at high temperatures, followed by reoxidation in dry oxygen prior t o photosorption measurements. The progressive loss of activity was interpreted in terms of gradual removal of residual hydroxyl impurities, since activity for photoadsorption of oxygen could be partially restored through exposure of the outgassed sample to water vapour. The mechanism of oxygen photoadsorption envisaged by those workers represented surface hydroxyls as efficient traps for photogenerated holes, whilst adsorbed oxygen acted as the electron-trapping surface species needed to maintain charge balance a t the illuminated surface [140]. Boonstra and Mutsaers [ 1411 later concluded that the mechanism describing the photoreactions on TiO, in terms of the formation of OH radicals was not probable. The question has been reexamined in a series of papers by Munuera et al. [ 142,1431 who observed both fast photoadsorption and slow photodesorption of oxygen over “fully hydrated” TiO, surfaces at 10 N m-’ and room temperature. Oxygen photodesorption vanished after outgassing the samples a t T > 300°C overnight and an involvement of H,O, in this process was envisaged following protonation of photoadsorbed 0; to HO, according

-

to HO, HOZ

-

+ HOZ + OH

hu

H,Oz

H2O

+ 0,

(2W

+02

O H + OH HzO2 Pre-adsorption of H 2 0 2 was found to greatly enhance the slow 0, photodesorption which exhibited diffusion kinetics for low (or zero) preadsorbed H,O,, but tended towards a zero-order reaction, with dPOz/dt constant, for hydrated surfaces having higher H 2 0 2 coverage. The extent of retention and nature of the bonding of H, 0, were found t o be strongly dependent upon whether the TiO, surface was fully hydrated, or dehydrated at 200°C [Ti0,-(20O0)] o r also dehydroxylated in vacuo at 400°C [TiO, -( 400°)] . Inner-sphere peroxo complexes of H , 0 2 were envisaged for H 2 0 , on the latter surfaces, whereas on Ti0,-(200) it was suggested that at least part of the adsorbed H , 0 2 formed outer-sphere complexes rather than displacing hydroxyls. Excitons produced in TiO, by illumination were suggested to diffuse to surface locations (extrinsic surface states) involving Ti4+ with a basic OH group in its coordinationi sphere. These sites may thereupon be converted into a Ti3+ion and a free OH radical (by localisation of the electron o r the hole of the electronhole pair, respectively) and thereby become photoactivated for H 2 0 , adsorption and decomposition. Such a mechanism is strictly outside the scope of this subsection concerned with purely photophysical processes,

349

but merits presentation here because of strong similarities to mechanism (29) assumed by those workers t o be responsible for fast photoadsorption of 0, (in the absence of H,O,). H

H

Y

Whilst thus reaffirming that basic surface hydroxyl groups are essential for all photoprocesses, Munuera et al. [ 1431 express the view that the degree of hydration of the surface determines, through the ease of protonation, whether 0; becomes HO, or, instead, produces 0;- surface species on extensively dehydroxylated surfaces [cf. eqn. (29)]. This latter process may also account for the secondary uptake of I8O2 shown in Fig. 8 ( b ) (iii) after flash photolysis of "0, /Ti02 interfaces. The need to take surface states of various kinds into account has also developed in 0, /ZnO interfaces using either direct techniques for observing photosorption or indirect techniques based on photoelectronic effects. Thus IR studies of ZnO surfaces [ 1441 have shown that surface hydroxyl groups represent one abundant surface species on samples vacuumoutgassed at temperatures < 520 K. Outgassing at temperatures > 670 K, which can serve t o reduce the surface concentration of hydroxyl impurities, produces, instead, some metal-excess non-stoichiometry in surface layers [ 791. The resultant density of electron-donating surface sites, such as Zno or Zn', are capable of strongly influencing the surface reactivity. Zinc oxide is not alone among metal oxides in posing the problem of identifying outgassing conditions severe enough to remove residual surface impurity species without simultaneously producing a degree of surface reduction through thermolysis: for example, TiO, and Cu,O surfaces pose similar problems [58, 79, 1451. However, examination of the very extensive literature on direct and indirect studies of oxygen sorption on zinc oxide [ 146-1561 reveals a particularly marked influence of surface pretreatment upon observed photoeffects, which suggests that the 0, /ZnO system poses the problem in a particularly apparent form, presumably because of the relatively low temperatures (>320°C) at which thermolysis of the ZnO surface becomes appreciable [ 146,1471. Several workers [ 147-1491 have concluded that electron-donating surface states can exist on prereduced ZnO surfaces and can give rise to an inversion in the sign of the surface double layer and band-bending, since injection of electrons into the bulk from donor surface states can leave the surface positively charged relative to the bulk (i.e. an accumulation-type surface layer). Eger et al. [148] concluded that such accumulation layers were formed during oxygen photodesorption f;om ZnO and that the rate of photodesorption varied exponentially with the excess surface concentration of electrons. An important role of donor surface species in oxygen photosorption at References p p . 4 19-42 7

350

0 2 / Z n 0 interfaces was also envisaged by Arijs and Cardon [149] in their theoretical and experimental investigations of the effect of a surface accumulation Iayer. They concluded that “very satisfactory qualitative agreement” emerged between their indirect electrical measurements and a model for oxygen chemisorption and photosorption based on electron transfer between bulk and surface states [ 1491. Shapira et al. [ 1161 reported excellent agreement of their experimental results on photoconductivity and photodesorption at 0, /ZnO interfaces with predictions of a charge transfer-type model. A novel feature of their model was the role proposed for impurity carbon atoms as the sites near which physically adsorbed oxygen captured an electron from the conduction band t o form surface CO, species. Desorption was proposed to result from the capture of a photogenerated hole by CO,, but otherwise the treatment of charge localisation/delocalisation at the surface and its intercharge with the bulk were treated in similar fashion to that detailed for eqn. (25), above. It was predicted, and observed experimentally, that the rate of change of surface conductivity decreased reciprocally with the square root of the time of exposure to band-gap illumination, whilst parallel behaviour was observed for photosorption of carbon dioxide. The species CO, is, however, paramagnetic with a characteristic EPR signal, and postulates of surface CO, as an important intermediate lack the expected experimental support from results of many observations on O,/ZnO or O,/TiO, interfaces under UV illumination in situ in EPR spectrometers [61a, 791. However, the formation of diamagnetic carbon- or carboxylate-type surface species are extensively reported for “carbon contaminated” metal oxide surfaces. Their photodissociation could account for the observed release of CO, without need t o invoke the paramagnetic CO, intermediate (see scheme 57, p. 397). Other workers [ 5 2 , 6 4 , 101, 150-1531 have commented unfavourably, however, upon the limited and rather qualitative nature of the agreement attainable between observed photoelectronic effects at 0, /ZnO interfaces and the predictions of models based upon charge-transfer mediated solely by collective-electron factors. A variety of additional factors have been identified by these workers as also exerting important influences on observable photophysical effects. Thus Beekmans [ 1501 has elaborated a model in which the conductivity of a ZnO powder layer is controlled by the conductance at narrow “necks” connecting adjacent ZnO grains, and has reported a fair measure of agreement between requirements of this model and photoconductivity measurements at UV-illuminated 0, /ZnO interfaces [ 1501. The spectral dependence of the photoconductivity of (lOi0) faces of ZnO single crystals at 77 K has, on the one hand, been interpreted by Luth and Heiland [64] in terms of photoexcitation of electrons from two sets of surface states a t 170 and 450meV below the conduction band. Since 110 peak occurred in t h e spectral response curve corresponding to the 170 rneV surface states whenever a surface depletion

351

layer was present at the O2/ZnO interface, they concluded that occupancy of this surface state was strongly dependent upon band bending. On the other hand, observations on surface photovoltage on ZnO single crystals, and particularly those on the inversion of surface voltage by illumination, have been interpreted by Sproules and co-workers [118] in terms of photo-excitation of electrons from the valence band into surface states having a wide distribution of energies within the forbidden band gap. Recently, these latter workers have argued that the dominant surface states at O,/ZnO interfaces are associated primarily with adsorbed oxygen and that occupancy of these states does not follow simple electronic theories for electron equilibration [ 521. Rather, they argue that these surface states are quasi-isolated and that operation of both electronic and non-electronic factors must be taken into account in attempting to calculate the probability of electron localisation as chemisorbed oxygen. They propose eqn. (30) to allow for the dual requirement that electrons must penetrate the surface barrier, q V , / k T , in order to reach the surface and that oxygen species at the interface must overcome an activation energy E , in order to be bonded t o the surface.

Other symbols in this equation have the same significance as in eqns. (22) and (26). The first bracketed term in eqn. (30) corresponds to the operation of an electronic factor in charge localisation at the surface, viz. the penetration of electrons from the bulk through the Schottky surface barrier, V,, at a rate exponentially related to - V,. The second bracketed term allows for the operation of a thermally activated nonelectronic factor, viz. the transformation of physisorbed molecular oxygen into a metastable activated form capable of localising electrons with “normal” cross-section. Lagowski et al. attempted to “separate out” the nonelectronic factor by measurements at fixed surface potential and interpreted their results as indicative of an activation energy of 0.72eV and a cross-section of cm2 for oxygen chemisorption. It is worth noting here that eqn. (30) and the ideas underlying it are the converse of mechanism (27) discussed above in relation to flash-initiated ISO, desorption from ZnO. That mechanism saw the need for photogenerated holes to penetrate to the surface (rather than electrons) and envisaged activated desorption (rather than chemisorption) as the non-electronic factor controlling the rate. The complexities considered in the previous paragraph in relation t o surface states at non-illuminated 0 2 / Z n 0 interfaces can be further exacerbated under UV illumination through creation of new surface states by surface photolysis. Recent evidence in support of this additional complication has been provided by observations of order-of-magnitude References pp. 419-427

352

enhancements of the surface conductivity of lithium-doped ZnO single crystals upon prolonged illumination in vacuo by a xenon arc lamp [ 1531 . The single crystal surfaces had been outgassed and partially reduced in vacuo of N m - ? , prior to surface re-oxidation which produced very low initial surface conductivity. Subsequent enhancement of the surface conductivity by prolonged illumination was attributed to photolytic formation of excess zinc in the surface region with growth of an accompanying accumulation layer. The latter could gradually be removed through interaction with oxygen and a cubic equation was developed t o relate conductivity to the number densities of chemisorbed oxygen, ionised surface donors and total surface charge. Agreement with the cubic equation was qualitative rather than quantitative [ 1531 . One final related photophysical process at O,/ZnO interfaces deserves brief mention, viz. photoassisted “place exchange” between I8O2 in the gas phase and 1602previously chemisorbed on the surface during exposure at 673K and during cooling t o room temperature. When such O,/ZnO interfaces were briefly evacuated to < torr and an isotopically pre-equilibrated mixture of ( 2 5 % 1602 50% I6O1’O t 25% ’80,) was admitted at low pressures, UV illumination produced a rapid rise in the mole fraction of 1602in the gas phase, with a corresponding such that AX3,= - (AX3, AX3y)[1 5 4 ]. decline in 1 8 0 2 and 160’80 The surface conditions which emerge from the foregoing considerations as those most likely to minimise the effects of surface states arising from impurities or defects at the O,/metal oxide interface would appear to be: (i) outgassing at temperatures sufficiently high to remove chemisorbed impurities such as OH- or CO,, but not so high as t o cause extensive surface decomposition; (ii) repeated re-oxidation in pure oxygen at sufficiently high temperatures and for sufficiently long times to “burn o f f ” any residual carbon impurities; (iii) UHV sample-handling procedures t o minimise surface recontamination and (iv) use of such low photon fluxes that possibilities for surface photolysis are negligible during photosorption measurements. Unfortunately, these criteria have not been satisfied simultaneously for any of the studies of oxygen photodesorption made by direct mass spectrometric observations on the release of gases into the gas phase from illuminated 0, /metal oxide interfaces. Not surprisingly, therefore, a measure of disagreement has emerged between results obtained in such widely differing sample conditions that different surface states may be expected to dominate the effects observed in different studies. Results with 0, /ZnO illustrate this difficulty. Thus, surface carbon impurities predominated in the studies by Shapira et al. [ 1161 leading t o C 0 2 as the only photoadsorbed product at room temperature with 0.25 s flashes of photons at 365 nm with flux density 3.3 x 10l6 cm-, s-’ ; photodesorption of I8O2 was detected as the major product released from 180,/Zn0 interfaces flash-illuminated by a 5 0 p s pulse consisting of 10l8 photons with wavelengths between 360 and 640nm, but an influence of surface states

+

+

353

related to metal-excess surface non-stoichiometry appeared probable for those flash-illuminated lSO2/ZnO interfaces due to some surface reduction during prior outgassing [ 1361 ; on the other hand, direct observations by Steinbach and Harborth [154] and by Hirschwald and Thule [155] on the products released t o the gas phase from ZnO surfaces maintained at temperatures of about 7 7 0 K during exposure t o very high light fluxes showed oxygen atoms to be the dominant evolved species. Here again, a marked initial influence of the degree of prior surface reduction was noted but the volume of C 0 2 produced was very low. In view of claims that increasing temperature at the O2/ZnO interface causes the predominant chemisorbed oxygen species t o shift from 0; t o 0-,the apparent conflict between the release of monatomic oxygen at high temperatures but O2 at 300 K may be explicable, in part, because photogenerated holes neutralise mainly 0; at 300K but 0- or 02-at high temperatures, and in part because high temperatures are needed to desorb monatomic oxygen prior t o its dimerisation on the surface.

( c ) Photoeffects whose origins are unresolved Several interpretations of photophysical processes considered above envisaged individual photons being instrumental in causing localisation/ delocalisation of an electron between an adsorbate-related state and a collective-electron state arising from bands in the bulk of the adsorbent or at its surface. Some impetus towards consideration of alternative collective excitations, e.g. of small surface regions of the absorbent (as distinct from localised activation of an adsorbate or its bonding t o a surface ion within an incomplete coordination sphere) is developing from a variety of phenomena observed under laser illumination including the following: (i) photon-initiated field ion mass spectrometry (PIFIMS) on the C2 H4 /Ag system [ 1561 ; (ii) laser-induced desorption studies [ 1571 ; and (iii) long wavelength photoemission of electrons from 0 2 / M g and related systems [ 1071. The PIFIMS technique differs from other photodesorption methods described above by obtaining the photoassisted removal of an adsorbate as ions rather than as neutral species and by observing such removal from an extremely small and well-characterised area of adsorbate, viz. the tip of a metal field emitter as used in a field ion microscope. Mass analysis of the ions being emitted from selected regions of the tip and imaged on the display screen is accomplished by time-of-flight mass spectrometry when the ion beam is passed through an aperture in the screen. PIFIMS was observed when laser pulses of 2-5ns pulse length and wavelengths of about 400nm became incident on a silver emitter a t which the field gradient had been reduced t o such a level that ion emission was negligible in the dark with the emitter tip carrying ethylene adsorbate. Below the onset of normal field ionisation, hydrocarbon ions like C,H:, ( 2 < n < 6) were observed by Referencespp. 419-427

354

the PIFIMS technique and attributed to a direct electronic excitation mechanism whose exact nature is as yet unclear. Dynamic mass spectrometry has been used t o detect and identify species desorbed by incidence of an intense laser probe on selected small regions of various solid substrates [157]. Either positive or negative ions (but not neutral species) were detected with appropriate ion optics. A notable feature is the small amount of ion fragmentation seen in the mass spectra. Commercial instrumentation for examining surfaces by laser-assisted microscopy with mass analysis (LAMMA) has been developed. Incidence of photons at 410 and 520 nm but not at 610 nm has been reported to cause photoelectron emission from magnesium surfaces during the early stages of oxidation [107].The criteria applied to distinguish these long wavelength photoelectrons (LWPE) from processes of exoelectron or photoexoelectron emission were: (i) that LWPE should be detectable even when the base pressure was reduced to 5 x lo-'' torr; and (ii) that the LWPE component should reduce to zero when the incident light was turned off. A marked contrast emerged between aluminium and magnesium foil surfaces pretreated in similar manner on an UHV system, since aluminium produced no LWPE with incident light of wavelengths between 600 and 350 nm, whereas LWPE with 4 eV energy spread from Mg was readily observed under excitation at 500, 520, and 410 nm. Photoelectric work functions as low as 0.3 eV were observed and attributed to the development during oxidation of patches having low work functions (termed exopatches). The exact nature of energy levels on such surfaces and of the manner in which photons interact with them remain to be resolved. 2.2 PHOTOCHEMICAL EFFECTS

Photons of energies 2--8eV incident on gas/solid interfaces may produce, in addition to the photophysical processes considered above, the rupture and/or formation of chemical bonding within adsorbates or between them on the surface. These photochemical processes at the interface may be further subdivided into: (i) photoassisted surface reactions yielding products which remain at the interface and so irreversibly alter its chemical composition and reactivity in the selected reaction conditions; and (ii) photocatalytic processes wherein the products from photoinitiated reactions at the interface are continuously removed to the gas phase in the reaction conditions (e.g. by thermally assisted or photoassisted desorption) with the result that the active surface is continuously regenerated and can become responsible for high turnover accomplished per photoactivated surface site (t.a.p.s.*).

2.2.1 Experimental aspects Different experimental approaches and equipment are frequently appropriate depending on whether the photochemical effect under study

355

UV source

t =

-- 6

2 6 0 torr

m

- 5 111

2 4

z

- 3

2

6 0 torr

K

> 2 1 I

1

100

200

References pp. 4 1 9 4 2 7

P(torri

0

100

200

300

356

G.C.

carrier

Ka‘

I I V \ourcis

e

.

outlet

1

i

reactor

reactor

ty

G.c

I

Continuous reactant flow

Pulsed reactant flow

I

(C)

Fig. 10. (a) Photoreactor developed for the study of selective photo-odixations in conditions of continuous illumination with a continuous flow of (carrier gas hydrooxygen) through a thin layer of finely powdered photocatalyst. Reproduced carbon with permission and minor adaptation from ref. 158. (b) Results obtained with photoreactor showing pressure dependence of the photoassisted steady-state rate of acetone 0 2 He)/ formation under continuous UV-illumination of a dynamic (isobutane TiOz ) interface. (c) Comparison of flow diagrams and positions of sampling valve for utilisation of photoreactor in “pulsed-reactant” versus “continuous reactant flow” conditions.

+

+

+

+

at an illuminated gas/solid interface is truly a photocatalytic process or is, instead, a noncatalytic surface reaction. True photocatalytic processes are here understood t o be those in which the photoinitiating making and/or breaking of internal chemical bonds within the adsorbate(s) proceeds to t.a.p.s.* > 100 with the assistance of the surface, but does not lead to significant permanent chemical alteration of the catalyst surface. In such cases, the emphasis in experimental measurements has normally centred on the nature and kinetics of changes in the composition of the gas phase, allied occasionally t o studies of reactant and product species on the surface using spectroscopic techniques. Relatively straightforward kinetic behaviour is often assumed for pure photocatalysed reactions on the basis that the light intensity and the surface concentration of active sites remain constant throughout the photoprocess. Such simplifying assumptions can predict proportionality between the rate of the photocatalysed reaction and the percentage of the corresponding photoproduct present when a photodynamic steady state is reached, i.e. when reactant gas(es) passing at constant flow rate through a fixed and illuminated bed of catalyst emerge with a percentage conversion to products which no longer varies with duration of illumination. Such dynamic flow photoreactors employing gas chromatographic techniques to separate and quantitatively measure the photoproducts have been widely used in combination with continuous

351

illumination to study processes thought t o be true photocatalysed reactions [158--1601. An example of this type of photoreactor, as developed by Teichner and his co-workers [158] for studying the photo-oxidation of alkanes over very finely divided powdered samples of TiO,, specially prepared in a flame reactor which yielded non-porous particles of high surface area, is shown in Fig. 10. Hydrocarbon products indicative of selective partial photo-oxidation were measured at high sensitivity with a flame ionisation detector, whilst CO, product from oxidation was measured with a thermal conductivity detector. N o conversion to products was obtained except when the alkane reactant simultaneously encountered TiO, , UV illumination and appreciable pressures of oxygen gas. Similar prerequisites for the successful continuing operation of photocatalysed oxidation over TiO, and ZnO have been reported by other research groups using dynamic flow photocatalytic reactors [ 158-1601 with continuous UV illumination and oxygen pressures of 5 x lo3 to 5 x lo4 N m-2 . These techniques allow valuable information on the dependence of any steady state extent of conversion to various products on the pressure of reactant or of oxygen and on the flow rate and light intensity to be accumulated readily. They are less satisfactory for describing any build-up or decay of the activity of the solid catalyst under illumination [ 161,2561 or for providing data on the following points: the nature of adsorbed species present at the UV-illuminated interface; the identity of adsorption sites and their extent of coverage by reactant and/or product; the nature of the active sites on which photoassisted surface reaction takes place; details of the manner in which incident radiation activates the surface sites on the adsorbate. Further, the high sensitivity attainable with flame ionisation detection allows small conversions to be readily observed, and it is essential, as stressed recently by Childs and Ollis [ 1621 to check that activity persists and remains constant throughout turnover to product at the illuminated interface of numbers of reactant molecules orders of magnitude greater than the number of surface sites activated by illuminated (i.e. t.a.p.s.* > 100). The foregoing criterion of constant activity of the illuminated gaslsolid reaction throughout large turnover of the photoinduced chemical change at the interface does not apply to non-catalytic photochemical changes. Since such non-catalytic photochemical reactions are frequently accompanied by significant chemical alteration in the surface layer of the solid, marked variations in rate of reaction can occur as the extent of surface reaction increases, e.g. Elovich-type kinetics, with rate falling off exponentially with extent of reaction, can be expected if each non-reversible surface reaction inhibits subsequent reaction events at the illuminated interface. Work by Hemminger et al. [163] illustrated how UHV procedures and electron spectroscopic techniques could be combined with gas chromatographic techniques in order t o delineate (i) the growth of methane product to a limiting value from a photoassisted reaction involving References p p . 4 1 9 4 2 7

A'

C r o s s section A A '

,

I I

4

t

30

7 E

2

m Y

0 4

a 3

C

zoo % x X

0

300

0

2

4 6 Time (h)

8

Fig. 11. (a) Schematic of closed photocatalytic reactor with provision for tumbling powdered catalyst past a continuously operating source of UV illumination and for sampling an illuminated mixture of (alcohol vapour oxygen) to a gas chromatograph for analysis. (b) Results obtained with photoreactor showing photoinduced evolution of acetone (0, left scale), pressure fall for isopropanol (m, left scale) and oxygen uptake (0, m, right scale) during photo-oxidation of isopropanol at 300 K on TiO, outgaeed at 600 K. (1) From a monolayer of isopropanol but without isopropanol in the gao phase (circles); (2)as ( l ) , but also with an initial pressure of 56 N m-z of isopropanol in the gas phase (triangles); (3) as (I), but also with isopropanol initially in the gas phase (squares). Initial oxygen pressure in all experiments was 1.33 kN Reproduced with permission from ref. 161.

.,.,

+

.,

359

H,O and CO, over a single crystal of SrTi03 and (ii) what changes occurred at the surface of the SrTi03 crystal as a consequence of such reaction. Whilst such parallel observations upon the surface by kinetic and non-kinetic techniques carry particular force for non-catalytic radiation-induced surface reactions, they are also highly desirable in the early stages of photocatalysed reactions t o reveal possible variations in the rate of reaction with increasing t.a.p.s.*, such as would result from photoformation of active sites o r from their destruction by illumination [256]. Various types of “sealed” or recirculatory reactions have been devised which allow for periodic or continuous sampling of gases for analysis, following the introduction of gaseous reactants over the catalyst and initiation of illumination. One such sealed photoreactor, featuring continuous UV illumination and constant tumbling of the TiO, particles in the light flux, is illustrated later in Fig. 11 and was utilised by Stone and co-workers [ 1611 together with gas chromatographic analysis t o follow the photo-oxidation of propan-2-01 to acetone. Another common modification of sealed catalytic reactors involves the inclusion of some means (usually magnetically driven glass-enclosed pistons) for the recirculation of gaseous reactants through the sealed system and over the surface of the catalyst [ 164-166, 265, 2661 . Within this context of being able to follow the variation of surface activity with the extent of exposure to, and conversion of, reactant, it is also worth noting modifications of dynamic flow reactors into a pulsed-reactant mode as distinct from the continuousreactant flow normally employed. Such pulsed reactant systems can make it possible to follow the adsorption on the non-illuminated solid prior t o the study of conversions brought about by illumination. It is also possible in these systems to limit the duration of illumination t o the transit time of the reactant pulse and to follow for successive pulses any variation in the percentage conversion with pulse number. Relatively minor variation of the positioning of gas sampling values relative to dynamic flow photoreactors of the type shown in Fig. lO(c) allows their conversion for operation in this pulsed reactant mode [256] . Photoassisted reactions under much lower pressures of reactant can be made by utilising mass spectrometric detection allied to either dynamic flow or sealed photoreactors [ 541.

2.2.2 Results and interpretations Reactions involving molecular oxygen as one of the reactants which undergo photoassisted bond cleavage and rearrangement over illuminated gas/solid interfaces represent the category of photochemical reactants which has received most attention. Photochemical processes falling into this category are considered first whilst those not involving oxygen as a reactant are treated later in this section. References p p . 419-427

360

( a ) Oxygen isotope exchange ( O I E )

Photoassisted scrambling of oxygen isotopes between molecular oxygen species over an illuminated interface is conceptually the simplest representative of this category of reaction since, apart from adsorbate-adsorbent bonds, it involves rupture/rearrangement of only oxygen-oxygen bonds. Equation (31) represents a surface-assisted scrambling process involving redistribution of oxygen isotopes between molecular oxygen species which originate from and return to the gas phase, viz. '60,(g)i

+ l"io,(g),

-

2 '"o'80(g)i

(31)

where the use of the subscript i denotes that species become involved in this equilibration whilst present at the 0, /solid interface. The term homophase has been suggested for this OIE process since it causes variations in isotopic composition only of the gas phase and proceeds without significant exchange between lattice and gas [ 167-1701. Two features useful for distinguishing this scrambling process from other OIE events are ( i ) that it does not produce any change in the atom percentages of l 6 0 o r l 8 0 in the gas phase, since it does not involve loss of either oxygen isotope from the gas phase through incorporation into surface layers of solid catalyst, and (ii) that it may be possible on catalysts which do not have oxygen in their surface layers. The latter feature has recently been reported [ 167 1 as being satisfied over zinc sulphide catalysts which were shown to catalyse process (31) with an optimum rate at 180 K . Operation of OIE in accordance with eqn. (31) has likewise been reported for several metal oxides at temperatures < 300 K in the absence of illumination, but only when surfaces were prereduced. N o photoeffect was detected, however, on illumination of ZnS, despite a report [168] that oxygen does photoadsorb on ZnS. Additional possibilities for exchange of oxygen isotopes over metal oxide catalysts become possible since exchange may occur between oxygen from the gas phase and from the lattice. A classification of oxygen isotope exchange processes as R o , R' or R2 in character has been suggested according to the number of monatomic oxygen lattice species, 01, which transfer into the gas phase for each gaseous oxygen molecule experiencing isotopic scrambling at the surface [ 1691. Thus, eqn. (31) described Ro-type exchange, which has also been termed homomolecular or homophase exchange and which proceeds without oxygen exchange between lattice and gas-phase oxygen [ 1701. On the other hand, type R' and R2 exchange can be represented respectively by the equations

+

{'802(g)}y-

160;-

{'SO*(g)}y-t

' 6 0 nI -

= {'60180(g)},n-

+ 1 6 0 In -

+ 180;+ '80;- + 180;-

= {'6O2(g)}y-

(32)

(33)

which involve exchange between lattice oxygen, Or-, and molecular , The superscripts n- and oxygen present at the interface, ( ( 0 (g)}m-.

361

m - are used for generality here to indicate that lattice oxygen, 01,and molecular oxygen may carry different charges (usually from 0 to -2) while at the interface. The symbol (g) denotes species which originate from o r return to the gas phase. It is possible experimentally, by the choice of appropriate isotopic compositions of the mixture admitted into contact with the metal oxide surface, to favour Ro-type processes over R' and R2 or vice versa. Thus, if a starting mixture of (I6O2 l80,)which is far from equilibrium in relation t o process (31) is employed, illumination of the (I6O, 1802)/ metal oxide interface may provide sites and a reaction pathway on the catalyst surface which facilitates rapid approach of the system to the equilibrium situation defined by

+

+

Such isotopic exchange over non-illuminated TiO, samples with partially reduced surfaces had been studied by Russian workers [171] and a correlation established between the activity and the extent of reduction, which was controlled by pre-heating in H, or CO and monitored by the magnitude of the ESR signal of the Ti3+ species [ 1611. The activity did not correlate with the number of paramagnetic 0, species detected at the O,/TiO, interface by ESR and the suggestion was made that other oxygen anion radicals were involved in the sites active for oxygen exchange. Activity fell off with increasing oxygen exposure and preoxidised TiO, was inactive. These results are quoted since they define the Ro-type activity expected for prereduced TiO, towards isotopically enriched oxygen in the absence of illumination. Similar effects have been reported recently from the author's laboratories for prereduced samples of TiO,, ZnO and MgO, but with the additional observation that exposure t o low light intensities prevented the decay in surface activity noted by Russian workers or restored activity t o samples from which initial activity had decayed in the dark [172a]. The kinetics of the variation of isotopic composition after contacting the ( 1 6 0 2 ''0,) gas phase with a freshly reduced surface accurately obeyed reversible second-order kinetics over zinc oxide surfaces a t 300 and 77 K [172b, 2671. Data for TiO, surfaces exposed t o illumination indicated a small contribution from R' and/or RZ in addition t o an Ro contribution. Following the illumination-induced restoration of Ro -type exchange activity t o dark-decayed samples of TiO, , ZnO or MgO, pronounced "memory" effects took the form of another gradual decline in activity in the dark. These memory effects strongly resemble those reported by Russian workers for oxidised samples of ZnO, MgO and lithium-doped zinc oxide [127]. Each set of observations has been interpreted on the basis that incident light promotes electrons from relatively deep traps into very shallow or untrapped states at the interface. The Russian workers envisaged the deeper traps as lattice 0; type species

+

References p p . 4 1 9 4 2 7

362

and accounted for photoadsorption plus OIE in terms of

q-

+

1802

-

'8o;(ads)

(34b).

Promotion of electrons between surface electronic energy levels of the types depicted in Fig. 3 ( b ) forms the basis of an alternative model developed by Cunningham et al. [154] for photoassisted Ro-type processes. According t o this explanation, activity requires surface locations of the type e;I(O-Zn,,,), i.e. surface ion pairs, in which the zinc ions are at positions of high coordinative unsaturation and near which an electron has been localised. In terms of Fig. 3(b), this would correspond to localisation/ promotion of electrons into the band of surface states associated with zinc dangling bonds, although that band should be extended towards lower energies to allow for the lower Madelung potential of zinc ions with high coordinative unsaturation. The availability of e; I (0-Zncus ) allows activation of molecular 1802(or 1 6 Q 2 ) from the gas phase by the tendencies of these sites both to donate an electron towards the antibonding orbitals of O2 and to reduce the high coordinative unsaturation of Zn;,, through interaction with a lone pair on one of the oxygen atoms. 'Two limiting cases of activation may be distinguished: strong activation, whereby the molecular oxygen becomes dissociatively adsorbed into one bound and one non-bonded monatomic surface oxygen species [cf. step (a) of eqns. (35) ]; and weak activation, whereby bonding in the molecular oxygen is weakened and redistributed in a weak surface donor-acceptor [cf. eqn. (35'd)l. Either type of complex between Q2 and e- I(Q-Zn,,,) activation could be envisaged, as per schemes (35) and (35'), respectively, to lead t o Ro -type OIE exhibiting the reversible second-order kinetics observed experimentally when ( 1 6 0 2 "02)was contacted a t 295 or 77 K with prereduced zinc oxide surfaces in the absence of illumination.

+

363

In this scheme, the initial dissociative adsorption is followed by fast formation of a triatomic O3 intermediate (with equal probability from 1602 o r l8OZ of the isotopically non-equilibrated 1 6 0 2 "O2 gas phase) which serves as the propagator of step (c). The latter has the character of a chain reaction, scrambling the gas phase into isotopic equilibration with reversible second-order kinetics as observed experimentally. Another aspect of experimental observations at 295K readily accounted for by eqns. (35) was the diminution of activity as successive doses of the isotopically nonequilibrated (1602 1 8 0 2 ) were introduced, since step (a) shows destruction of Zncus.The alternative weak-activation case is

+

+

0 2 (g)

+ e-l (O-Zncus)

+

-

(35'd)

O2(O-Zncus)-

2

(g) [ 1 6 ~ 1 8 ~ ( ~ ~ n c7 ,,)]I6Ol8O(g) [1602(o-z~,,,)]-

1602

+

(35'f)

In this scheme, the activated but non-dissociated oxygen species formed in step (d), with equal probability from 1 6 0 2 o r 1 8 0 2 ,serve as the propagator of steps (e) and (e'), each of which is reversible second order and chain-reaction in character [cf. step (f)] . Since low temperatures should enhance surface concentrations of the weak complex envisaged in step (d), which furthermore avoids any significant energy of activation, this scheme offers advantages in accounting for the OIE observed at 77 K. Figure 3(b) made it clear that the requirement, inherent in both schemes (35) and (35'), for electron localisation as e-I(O-ZnC,,) was unlikely to be satisfied in the absence of illumination unless the surface itself featured an excess of donors, such as can arise from the zinc-excess non-stoichiometry of surface layers observed for zinc oxide surfaces outgassed in vacuo at high temperatures. The fact that such surface nonstoichiometry was known to be drastically reduced by oxidation in oxygen at 6 7 0 K , then provided an explanation for the lack of Ro-type activity of pre-oxidised zinc oxide surfaces in the dark, i.e. that oxidation strips electrons from e - ] (0-Zncus) locations by localising them on new surface 0'- species. Interfaces between ( r602 1 8 0 2 ) and these preoxidised ZnO surfaces were found, however, t o be remarkably lightsensitive, since even room light restored Ro-type activity with apparent quantum efficiencies having values up to 30 depending upon the light

+

References p p . 419-427

364

intensity and degree of surface pre-oxidation. Photoinduced reformation of e- I( O-Zncus) situations by promotion of electrons from deeper traps, which could be F: or M,-type centres involving electron localisation on surface anion vacancies V,, , could account for the restoration of Ro-type activity via schemes (35) or (35') on illumination. When the isotopic composition of the oxygen gas initially admitted into contact with a metal oxide surface is already in equilibrium in relation t o eqn. (31a), then no alteration in the isotopic composition of the gas phase can be expected under illumination unless illumination opens up a type R' o r R2 pathway for exchange between oxygen from the lattice with that from the gas phase. Recent French work [ 1731 on photoassisted OIE over TiO, , ZnO, S n 0 2 and ZrO, has provided convincing kinetic evidence for isotopic scrambling via an R'-type mechanism whenever UV light at high intensity became incident on the interface between each of these metal oxides and gaseous oxygen which was initially in isotopic equilibrium in relation t o eqn. (31a). A striking contrast thus emerges between the predominance of an R'-type mechanism (with a need for monatomic oxygen intermediates on the illuminated interface) under the conditions of those French experiments and the predominance of an Ro-type mechanism reported by many Russian workers. The contrast may derive in part from the use of isotopically equilibrated gaseous oxygen by the French workers and in part from the higher light intensities employed in their studies. The intensity data quoted by the French workers indicate a photon flux at least two orders of magnitude greater than employed by Russian workers [ 1271 . Consequently, the possibilities for the formation of a monatomic oxygen species at the interface by photolysis of the surface layer of the solids were greater in the French work. A possible mechanism by which photolysis may contribute to OIE is depicted schematically as

pnotoiysls

Photolysis of the surface of ZnO in vacuo during illumination by photons of energies greater than the bulk band gap of 3.2eV has been reported by several workers [153-1551. The relationship of such photolysis to OIE was re-examined recently in the author's laboratories using the full UV output from a xenon flash tube (ca. 4.7 x 10l8 photons at 200-340 nm per flash of duration 50 ps). The time profiles for rise, with t:,, 0.20s of transient pulses at m / e = 32 (corresponding t o an additional partial pressure of molecular oxygen product, A 1 6 0 2 , appearing in the gas phase as a consequence of surface photolysis), were measured

365

using the dynamic mass spectrometer system (cf. Fig. 8). With no gaseous oxygen admitted over the ZnO surface, which was instead maintained under the residual pressure of lo-' N m-' achieved by the ion pump, the magnitude of the pressure pulses at m / e = 32, initiated by single flashes incident on the vacuum/ZnO interface without the intervention of filters, diminished progressively for a series of equal pulses delivered at 2 0 s intervals. This progressive decline was taken as indicative of photolysis of surface layers of the ZnO, with each pulse leading to a pulse-related growth in metal-excess non-stoichiometry of the surface. In terms of a model advanced by Hirschwald and Stolze [146] for the photolysis of ZnO, increasing surface concentrations of excess zinc can cause decreasing efficiency of photolysis by acting as recombination centres and so competing against surface oxide ions for photogenerated holes as in the final steps of + h' ZnO t hv-

(e

h+ /

+of--00,-

+ h)/ZnO* -------(,

lO,(g)

(37) hi

Previous work illustrated the observation that non-reproducibility in magnitude of the photolytic I6O2 with flash number could be overcome N m-' over by maintaining oxygen at dynamic pressures of lo-' t o the metal oxide surface during the flash. The last column of Table 2 summarises the results of a comparison of the sensitivity of various metal oxides t o surface photolysis, which was made using the DMS system and by introducing I8O2 at a dynamic pressure of 3 x N m-* over flash UV-illuminated metal oxide surfaces as a means of minimising variations of 1602in successive pulses. The very fact that the presence of "0, at these low pressures transformed the system from non-reproducible pulse heights at rnle = 32 suggested a role for I8O2 in preventing or reversing surface photolysis. The manner of its action in this regard was indicated by the observation of a parallel flash-initiated OIE process. Data in the fourth column of Table 3 summarises the measurements made on this flash-initiated OIE, as detected with the DMS system for various 1802/metal oxide interfaces, through integration of the area under the l 6 0 l 8 O transient at m / e = 34. These data show that a flash-initiated transient at mle = 34 was readily detected for 180,/Cr20, and "02/ ZnO-In and was more than an order-of-magnitude larger than the trace (ca. 177 ) present in the isotopically equilibrated m o u n t of 160180 reactant gas (ca. 99% I8O2).N o significant flash-initiated enhancement of content occurred, however, with this highly enriched 1 8 0 2 gas the 160180 present over V,O,, TiO? or NiO. Neither did any enhancement occur when the enriched I8O2 gas but no metal oxide was present within the cylindrical quartz jacket of the photoreactor. These observations ruled out any possibility that the OIE process might have originated from isotopic References p p . 4 19-- 4 2 7

366

or place exchange processes involving residual I6O2 retained on walls of the photoreactor or samples (all of which had been outgassed at 670 K). A model such as that shown in eqn. (36) and based on the interaction between photoactivated l8O5 and I6O produced from the lattice by photolysis can provide a better explanation of this OIE process. Such a model would predict qualitative correlation between the extent of 160180 detected and the arithmetic product (amount of I6O2 released by photolysis x amount of I8O2 released by photodesorption) because the former term should reflect the rate of formation of I6O from photolysis i.e. as a precursor of I6O2) and the latter should reflect the rate of photoassisted activation of l x 0 it o "0;. Values of the product of these two terms, as evaluated from the flash-initiated transients at m / e = 32 and 36 listed in columns five and six of Table 3 would predict the sequence C r , 0 3 > Fe, O4 > V2 0, > Co, 0, > ZnO > CuO > T i 0 2 > NiO > ZnO-Li and the sequence observed experimentally by measurements o n m/e = 34 correlates well with that sequence (V, 0, being the single exception), when values of < 3 x l O I 3 are recognised as below the limit for accurate comparison. ( b) Pho to-oxida t ions in uo lv ing m olecu lar oxygen

The literature on photochemical oxidation of other species with molecular oxygen as the oxidant at illuminated interfaces is very much less extensive than reported for dye-sensitised photo-oxidations involving singlet molecular oxygen ( ' A ) as an important intermediate [ 1741 . Indeed, it is difficult as yet to identify any photoassisted reaction at the gaslsolid interface which can be unequivocally attributed to 0, ( ' A ) , although its possible involvement has been considered by some workers [ 175, 2681 . Published investigations of photoassisted surface reactions with groundstate oxygen species as reactant may be subdivided instead into three categories: (i) investigations of photo-oxidation of carbon monoxide by molecular oxygen, which has served as a model reaction for exploring photoactivation of oxygen at the illuminated surface into a state reactive towards CO; (ii) investigations of selective photo-oxidations of alkanes to partially oxidised products, which has been studied with a view t o identifying selective and efficient photoassisted pathways t o important partially oxidised chemicals, e.g. acetone, ethylene oxide o r phenol; (iii) tests of the contributions by photoassisted redox processes at surfaces towards selective photo-oxidation of oxidisable species (such as alcohols), which can act as hole-trapping species and so complement the electrontrapping action of 0, at the illuminated interfaces. Developments in these three areas will be separately outlined.

( i ) Carbon monoxide photo-oxidation. Investigations of carbon monoxide photo-oxidation prior t o the decade covered by this chapter had established that it was possible t o detect an increase in the rate of oxidation on the illumination of mixtures of CO and 0, in contact with metal oxide

361

catalysts which had been pretreated with oxygen [176, 1771. In the temperature range 470 k 100 K , the reaction under illumination had been variously reported as a true photochemical reaction with no temperature coefficient or activation energy [ 1 7 8 ] , as zero-order with respect t o carbon monoxide or dioxide pressure but first-order with respect t o oxygen pressure [ 1791, and as proceeding via photochemisorption of 0; followed by (C0)i

+ (0i)i

(C0i)i + ( 0 ) i

(38)

Other early investigations by Steinbach [ 180,1811 of the heterogeneously catalysed oxidation of carbon monoxide examined experimentally the then widely held view that electronic factors, in particular the position of the Fermi level at the gas/semiconductor interface, should exert a strong influence on the rate of reaction through modifications of Eact,the activation energy for the surface-catalysed reaction. Following observations consistent with the dependence of the activation energy of carbon monoxide oxidation upon the position of the Fermi level at the non-illuminated surfaces of the semiconducting oxides ZnO, NiO and Co,O, [ l 8 0 ] , Steinbach [ 1811 utilised similar techniques to determine the effects of UV illumination upon E,,, with NiO. Modifications in position of the Fermi level at the surfaces of lOOnm thick NiO grains were sought through their deposition as a thin layer on t o transparent evaporated films of the metals Ag, Au, Pd and Pt. Since the work functions, $, of these < GPd < @ p t < $NiO,it was metals vary in relation t o NiO as $Ag < argued that depletion of electrons from surface layers of particles of NiO in contact with these metals should be greatest on Ag layers and least on Pt, with corresponding shifts of the Fermi level upwards and away from the upper edge of the valence band of this p-type semiconductor. With the exception of Ag, the expected increase in Eactfor CO oxidation in the dark was observed, since Eact increased from 90 kJ mole-' over NiO t o 124.2, 133.7 and 163.1kJmole-' for NiO/Pt, NiO/Pd and NiO/Au, respectively. Illumination through the transparent metal support brought Eact for the latter systems back down again t o 81.9, 86.6 and 94.5kJ mole-', as would be consistent with photoinduced decreases in the net transfer of charge from NiO t o metal and with diminution of the upward shift of the Fermi level. Subsequent kinetic investigations of this supposedly ideal reaction in the temperature range 600-733 K showed that illumination influenced not only an apparent activation energy, but also the pre-exponential factor in a manner considered t o be indicative of parallel thermal and photocatalysed reactions [ 1821. With illumination at increasing light intensities but fixed wavelength, the activation energy decrease followed a parabolic law. Light of progressively shorter wavelengths produced progressively stronger decreases in apparent activation energy. However, a striking observation was made that the efficiency of any wavelength diminished upon simultaneous illumination at other References p p . 419-427

368

wavelengths. This latter effect, together with the non-linear dependence on intensity, was interpreted as evidence that the overall light reaction was not the sum of simultaneous, independent reactions on differently excited surface sites. Activation energy effects were, rather, considered to arise from the effects of illumination on the strength of the zinc-axygen bonds at the surface. This concept of surface bond strength being dependent upon the intensity and energy of the photons incident on the surface contrasts with the interpretations of other workers. Investigations of the spectral dependence of the photocatalytic oxidation of carbon monoxide by pure zinc oxide and its solid solutions with lithium oxide ( ZnO-Li) or aluminium oxide (ZnO-Al) led Zakharenko et al. [ 1271 more recently to the conclusion that the quantum yield of the photoassisted oxidation depended largely on the surface charge present on the surface of the zinc oxide. Differences [cf. Fig. 9(a)] in the spectral dependences of the photoadsorption of oxygen (with a maximum at 2.5eV) and of the photo-oxidation of CO (which showed the same spectral dependence as the band-edge absorption) for pure ZnO led those Russian workers to conclude, contrary t o eqn. (38), that 05, which would reach maximum surface coverage under illumination by photons at 2.5 eV, cannot be the form of oxygen active in the oxidation of CO on the illuminated surfaces. They concluded, rather, that lattice 0;species, produced by hole-trapping on surface 0 2 -ions, were the species active in photo-assisted oxidation. Measurements of photo- and thermoelectric work functions of these materials were interpreted as indicating the location of the Fermi level approximately at the mid-point of the band gap for pure and lithium-doped ZnO, but as being displaced into the top half of this gap for ZnO-Al. Migration of photogenerated holes from within the bulk of ZnO to surface 0'- ion sites was thus suggested as being favoured by a greater negative surface potential than would exist on ZnO-A1. The Russian workers found much larger quantum efficiencies (ca. 0.24-0.3) over ZnO than over ZnO-Al (ca. 0.018) under illumination by photons inside the band edge. The lithium-doped ZnO samples presented a more complex picture, since photocatalytic act,ivity for CO activation exhibited a well-defined peak at 2.5 eV (i.e. 0.7 eV outside the band edge). Quantum efficiencies were 0.032, 0.024 and 0.016 for illumination intensities of 2.2 x l o i 3 , 5.3 10i4 and 4.7 x photons s-' , and this process was attributed to photogeneration of surface 0- sites from surface defect centres involving lithium [cf. eqn. ( 1 9 ) ] . That Russian work, like many of the earlier studies. was carried out with oxidised ZnO surfaces and there are unresolved differences between the results, e.g. Russian claims to observe the effect at room temperatures, in contrast t o the higher temperatures reported elsewhere as necessary. Orders of reaction with respect to carbon monoxide and oxygen were different from those reported by earlier workers [177-1791, but their report of first-order dependence on P,, agreed with Steinhach and Barth [ 1821. 'I

369

100

700

400

Pco ( t o r r )

torr

Fig. 12. Dependence of t h e photoassisted rates of carbon monoxide oxidation over TiOz (upper plots) and of isopropanol oxidation over ZnO (lower plots) on reactant pressure. Observed rate photo-oxidation at each pressure is subdivided, in the manner of eqns. (39) in the text, into a rate V , displaying a Langmuir-Hinshelwood pressure dependence and another rate, V , , increasing linearly with reactant pressure and consistent with an Eley-Rideal process. Reproduced with permission from refs. 183 and 257.

Comparisons of the activity of TiOz and G a z 0 3 with that of ZnO for promoting photo-oxidations of carbon monoxide at room temperature have been made by Thevenet et al. [183] using illumination inside the band edge (210-390 nm) and a gas chromatographic method for monitoring the rather small extents of conversion (ca. 1.3%)attained in one pass through a dynamic photoreactor. The photocatalytic activities of TiO, and G a z 0 3 were stated to be greater than for ZnO, S b z 0 3 o r SnO, . Kinetic analysis of the dependence of the extent of photoconversion on oxygen pressure at constant Pco yielded linear plots in the format of eqn. (39a), which could correspond to the LangmuirHinshelwood-type dependence of rate on surface coverage, O 0 , controlled by eqn. (39b), viz. Referencespp. 4 1 9 4 2 7

370

Dependence on Pco for fixed Po, was not fitted by a single function but was separable empirically into two processes with photoassisted velocities, V1 and V , which obeyed eqns. (39c) and (39d) respectively. Agreement with this analysis is illustrated in Fig. 12(a). The component described by eqn. (39c) was attributed to a Langmuir-Hinshelwood dependence on Oc0 at low Pco . The Eley-Rideal type described by eqn. (39d) became more important a t higher Pco .

v,

=

k 1 Kco pco1 + KcoPco

Thenevet et al. considered that the relationships (39) might originate from any one of three mechanisms, two of which would involve chemisorbed oxygen as the oxidant whilst the third would involve lattice oxygen. Lattice oxygen was favoured on the basis of a single mass spectrometric experiment showing that the CO, photoproduct from CO I8O2 was mainly CI6O2 and not C 1 6 0 1 8 0Largely . on the basis of this experiment, these authors favoured a mechanism previously suggested by Mars and Kreveleri [184] as being responsible for their results and kinetics. That mechanism envisages reaction occurring via collision of CO from the gas phase with lattice oxygen, followed by restoration of lattice oxygen through surface re-oxidation by oxygen from the gas phase. The component of the photocatalysed kinetics expressed by eqn. (39d) was tentatively identified as a mechanism involving collision of CO from the gas phase with adsorbed and dissociated oxygen, i.e. Eley-Rideal in character. Photo-oxidation of carbon monoxide has been re-examined by Steinbach and Harborth [ 1541 using UHV techniques for the preparation of clean surfaces of ZnO single crystals and mass spectrometry for monitoring reaction thereon. Photoassisted oxidation t o CO, over ZnO single-crystal surfaces at temperatures between 670 and 720K was observed with a quadrupole mass spectrometer (QMS) in the presence of N m-’ of 0, and CO. It was argued that true photoassisted processes should be distinguishable from thermally catalysed oxidation on the basis that only the former should respond rapidly to changes in illumination intensity. This led t o the incident photolytic light being chopped and a lock-in amplifier was used to select only those components of the output from the QMS which rapidly followed the intermittent light

+

371

intensity. Steinbach and Harborth [154] concluded that only the partial pressures of C 0 2 and atomic oxygen, i.e. Pcoz and P o , responded sufficiently rapidly t o be classified as genuine photoproducts. Slower changes in other gas-phase components, e.g. O,, H 2 0 , CO, and zinc vapour, were attributed to thermally assisted processes, some of which were promoted by the very high flux of photons focussed on the ZnO crystal from a 1 O O O W lamp. [However, the features envisaged above in relation t o mechanism (27) for photodesorption at I8O2/Zn0 interfaces also merit consideration here, viz. that the photoassisted surface reaction may be fast, but that desorption of observable products may be thermally activated and be the slow rate-determining process.] In the absence of gas phase CO, atomic oxygen was the dominant photoproduct above the interface between loT6Nm-2 of O 2 and the ZnO single crystal surface, provided the latter was at > 670 K. Unfortunately, no direct evidence was obtained in this study as to the sites on the ZnO surface from which this atomic oxygen originated, e.g. whether it resulted from removal of surface 0- from normal lattice sites or from oxygen atom-like species at surface defects. However, Hirschwald [ 1551 , who also studied ZnO photolysis in conditions of UHV, has commented on the much greater ease of photolysis t o atomic oxygen from a nearly stoichiometric oxygen-rich surface layer prepared on polycrystalline ZnO by prior oxidation ( 1 5 h , l O O O K , 1 0 4 N m - 2 0,) than from zinc-rich surfaces produced by vacuum treatment similar t o that used by Steinbach. In Hirschwald's view, the absence of significant photolysis of ZnO below ca. 623K may be attributed t o a retention of zinc (which has an activation energy of 32 kcalmole-' for desorption). Consequently, zinc formed in limited initial photolysis or thermolysis remains on the surface and acts as efficient recombination centres for reconverting adjacent surface 0- (i.e. holes at the illuminated surface) into ZnO. Despite some unresolved points, which undoubtedly stem from differences in the pretreatment of the solid and from differing light intensities, the above interpretations tend to converge on a description of carbon monoxide photo-oxidation on metal oxide catalyst as involving (i) photogeneration from the lattice of a monatomic oxygen species, which Russian workers identify as O;, and (ii) reaction of this species with CO. Since transfer of charge between the bulk and surface regions of the illuminated adsorbent is thus envisaged in the rate-determining step, some sensitivity of this photo-oxidation towards the extent of bandbending would be expected and has been claimed recently by Van Damme and Hall [185] over various perovskites. Those workers report a photocatalytic (PC) enhancement over SrTiO, . Studies of the temperature dependence of this effect revealed that, for temperatures < 573 K, activation energy for the process under illumination was lower than for the reaction in the dark. At higher temperatures, the results obtained over SrTiO, in the light were not experimentally distinguishable from those in References p p . 419-427

372

the dark. However, as the temperature was lowered, irradiation produced a measurable enhancement and an Arrhenius plot of the dark reaction remained linear, whilst that for reaction under illumination levelled off. This latter behaviour was seen as “characteristic” of a photocatalytic process. Both TiOz and BaTi0, also responded t o illumination, whereas LaCo0, and Ba(Fe0,33Ti0,67)Oz.67, which were much more active for CO oxidation in the dark, did not respond t o illumination. The temperature dependence over BaTiO, was remarkable, since in the dark it evidenced those features quoted earlier as “characteristic” of a photocatalytic process and levelled off at temperatures below ca. 6 0 0 K . The title ferrocatalytic (FC) was suggested for this effect, since it was attributed to spontaneous polarisation in surface layers of ferroelectric BaTi0, and t o enhancing effects of associated charge fields on the transfer of charge from the bulk to the surface of BaTiO,. Destruction of the polarised surface layers, or compensation of their effects, by hole-electron pairs generated by illumination were advanced as explanations of the observation that FC effects were found to be quenched after exposure to bandgap illumination. The interrrelationships between PC and FC effects suggested by this work are interesting and deserving of detailed kinetic study t o check the validity of an underlying assumption that the same rate-determining step operated for CO oxidation in all their systems. Interestingly, parallel studies on the oxidation of H, by O2 over the perovskite catalysts (and ZnO) failed t o give evidence of any PC or FC effects on that reaction. Recently, Anpo et al. “61 have observed that the introduction of carbon monoxide at low pressures over the oxides V2OS,MOO, and CrO, dispersed on porous Vycor glass (PVG) diminished the phosphorescence attributable to a triplet state of a charge-transfer excited state e.g. (M o 6 + - 0 2

-

)

9 hv

(Mo”-O-)*

Quenching of the luminescence was accompanied by photoadsorption of CO and photoformation of CO, as detailed in Table 4. Anpo et al. consider: (i) that these results support the conclusion that photoreduction of the oxides with CO molecules proceeds via their charge transfer triplet excited states; and (ii) that this is the first report connecting the photoreactivity of oxide catalysts with the lifetime of (M-0) excited states (but see Sect. 3.3.2). On the basis of a further observation that none of the oxides NiO, Co304, Cr203, FeZO3 or CuO showed activity towards photoreduction with CO, they tentatively advance the idea that such activity may arise only in oxides (such as V z O s , MOO, or CrO,) in which metalboxygen double bonds arise, since they argue that only there would the oxygen of the charge-transfer excited state be similar to a “free 0- ion”. However, that argument ignores the possibilities for photoformation of 0-

373

TABLE 4 Relationship between the lifetimes of excited triplet states and the initial rates of photoreaction at 300 K

Lifetime of excited triplet states (ps) Initial rate of C02 photoformation (lo-'' moles-') Initial rate of CO photoadsorption (lo-'' moles-' ) Quantum yield of C02 photoformation Quantum yield of C02 photoadsorption a

V2 0,/PVGa

Moo3/PVGa

Cr03/PVGa

218

63

2.9

1.44

0.43

0.01

4.33

1.22

0.01

0.043

0.01

0.11

0.03

PVG = porous Vycor glass.

species similar to free 0- from oxide ions existing at surface sites of high coordinative unsaturation.

(ii) Alkane photo-oxidation. A gas chromatographic technique similar t o that employed for their study of carbon monoxide photo-oxidation over TiOz has been utilised by Teichner and co-workers [187, 1881 t o establish the general features of alkane photo-oxidation for a mixture of helium, oxygen and hydrocarbon passed over TiO, at ca. l a t m . total pressure. Studies were made with non-porous finely divided particles having the anatase crystal structure and particle sizes of 6-100 pm. Photoassisted reactions were initiated by the incidence of UV illumination from a 125W medium pressure Hg arc lamp on the interface between these anatase particles and the continuously flowing mixture of helium and reactants. Emergent gases from the dynamic photoreactor were sampled at intervals for separation on appropriately packed columns and were analysed by GLC using either a flame ionisation detector (for hydrocarbon photoproducts) or, at lower sensitivity, by a thermal conductivitytype detector (for permanent gas products, CO,, etc). N o significant amounts of photoproduct emerged continuously from the dynamic photoreactor except when TiO,, oxygen and UV illumination were present simultaneously. Under the latter conditions, small percentage conversions of reactant hydrocarbon (typically 1-5%) could be photoinduced continuously for reactants undergoing one pass over an illuminated layer of the anatase particles dispersed as a thin layer on a membrane permeable to the reactant gases and photoproducts. Despite rather low percentage conversions, the photoassisted processes were classified as photocatalytic, apparently on the basis that stationary activity could be maintained for several hours under illumination, following an initial rise of the References p p . 4 1 9 4 2 7

314

photoassisted activity during the initial 10-30 min of illumination. These features were illustrated in Fig. 10. A very rapid decline in activity occurred when illumination ceased. Quantum efficiencies of 0.1-1 for alkane photo-oxidation have been claimed for the process under optimum conditions. For all the alkanes investigated, except methane and ethane, partial photo-oxidation to aldehyde and ketone products represented a significant fraction of the total photoproducts detected. Results expressed as the ratio of the number of moles of particular product formed, to the number of moles of alkane consumed in the reaction (termed the selectivity, S) showed C 0 2 t o be the dominant product. Ketones and aldehydes were present as products of selective photooxidations. Based on an implicit assumption that similar factors will determine product distribution in photo as in thermally assisted processes, it was argued that distribution of photoproducts among the ketones, aldehydes, etc. indicated photoassisted attack on each carbon atom of the parent alkane. This latter deduction was made for all the alkanes investigated from the distribution of photoproducts. Teichner and co-workers [ 187, 188, 1901 considered that a modified selectivity criterion, S , , (representing the ratio of the number of moles of alkane required to form a particular product to the total number of moles of the alkane consumed in the reaction) formed a better basis for the comparison of various pathways of photooxidation. Such S , values should total loo%, e.g. the followingS, values were observed during isobutane photo-oxidation: C 0 2 23%; acetone 61%; 2-methylpropanol 7%; and t-butanol 9%. These values were considered t o favour a consecutive reaction scheme of the form isobutane + t-butanol + acetone. Indirect support for photo-oxidation via a route involving tbutanol as an intermediate was deduced from the equality in rates of the photoassisted conversion of isobutane to acetone or of t-butanol to acetone. Parallel reactions leading, respectively, t o acetone or t o t-butanol via a hydroperoxide (such as might be expected t o result by attack of 0; or singlet molecular oxygen) were considered less probable in view of the fact that feeding the photocatalytic reactor with the hydroperoxide of isobutane yielded only minor amounts of acetone and no trace of tbutanol. The tentative hypothesis of an alcohol intermediate in this and other selective photo-oxidations raised the interesting question as t o how a UVilluminated (alkane O,)/TiOz interfaces could achieve insertion of oxygen into C-H bonds. No definitive evidence as to the feasibility or mechanism of such photoassisted oxygen insertion was forthcoming from that study, although Teichner mentioned the possibility that the alkoxy radicals (CH,), CO may initially form at the interface in the photoassisted conversion of isobutane t o acetone and t-butanol. Later work by Courbon et al. [ 1731, which has already been discussed above in relation t o OIE at lSOz/Til60g interfaces, showed that. the photoassisted oxidation of

+

315

+

isobutane inhibited any OIE in {**O, (CH,), CH}/Ti160: systems, thereby indicating that the same monatomic oxygen species at the TiO, surface could participate in both photochemical processes. Since dissociated oxygen species were favoured as the sites of OIE under the conditions of the experiments of Courbon et al., the conclusion was drawn that monatomic oxygen also represents the photoactivated surface species which attacks isobutane in its photo-oxidation over TiO,. The nonclassical nature of the selective oxidation products formed over the UV illuminated (hydrocarbon 0,)/TiO, interfaces has also been confirmed through the observations of alkylbenzaldehydes as major photoproducts from alkyltoluenes [ 1891. Rate expressions (40a) and (40b) which are formally similar t o those derived by Mars and van Krevelen for hydrocarbon oxidation on nonilluminated oxide catalysts, were advanced by Formenti et al. [188] as a basis for linearising their experimental data for reactant pressure dependencies in isobutane conversion t o acetone over TiO, under continuous UV illumination at fixed intensity.

+

1 Rate

--

-

1 koPt

1 + K,KoP, -~

1

tkr

In these equations, K c represented an equilibrium constant for the physical adsorption of isobutane (or other hydrocarbon) according to a Langmuir isotherm; k o was a rate constant for oxygen adsorption (irreversible below 300°C) at a rate given by k o c (1- O o ) , where N was set equal t o 0.5 for dissociative adsorption; the rate of oxygen adsorption was assumed equal to the rate of its consumption, which in turn was set equal to acetone formation, koOoO,; and Pc or Po were the pressures of the hydrocarbon or oxygen reactants, respectively. The rate expression (40a) and the assumptions underlying it have recently been criticised by Childs and Ollis [ 1621 who put forward a re-interpretation of the experimental data of Formenti et al. [188] on isobutane and of the data of Walker et al. [ 1901 on photo-oxidation of 2-methyl-2-butyl alcohol over Ti0,. Teichner et al. had reported comparable photoassisted rates of acetone formation from the alkane and the alcohol, leading to the postulate of a common rate-determining process, viz. dehydration of an assumed alcohol intermediate, for the two photo-oxidations. Childs and Ollis adopted this idea t o arrive at the following parallel reaction schemes.

Scheme (41)for isobutane

(1) R2CHR’(g) ( 2) 2 s + o ,

+ S ,--”(R,CHR’.S)

+

References p p . 41 9-42 7

2(0*S)

376

(3a) (R,CHR’*S)

+ (0.S)

k3

- +S

(R2COHR S)

Scheme (42) for alcohol KROH

(1) R,COHR’(g)

+S 5(R2COHR‘.S)

KO

(2) 2 s + o 2 2 2 ( 0 . S ) Kr (3a) (R,COHR*S) S ( R 2 C = R ‘ * S )+ (H2O.S)

-

+ (3b) (R,COHR’-S) + S k ’ -

+ (H20*S)

[R=C(R)R‘*S]

Further rapid reactions applicable to ( 4 1 ) and ( 4 2 ) (4) (R,C=R’) (5a) O=R’ (5b) O=R’

+ 0,

-

A

R2C=0

+ O=R’

desorbed product (if R’ > CH,)

CO,

+ H,O (if R’ = CH,)

The kinetics of each process are determined by the slower processes (3a) and (3b). These lead to an olefin-type surface intermediate (or its precursor) via a dehydration step requiring two sites for its completion, one t o accommodate the olefin-related species and one a water molecule. The requirement for a second, vacant site is characteristic of the model, leading in the case of increasing alcohol pressure to autoinhibition due to decreasing availability of vacant sites. With additional assumptions of (i) Langmuir-type adsorption isotherms for both alcohol and oxygen adsorption, and (ii) the absence of inhibition by product, rate expression (42a) was deduced for acetone formation from alcohol.

If, as a first approximation, oxygen pressure dependence was assumed to be weak, this could be simplified t o

Satisfactory agreement was shown between this expression and the experimental results of Walker et a]. [190] on alcohol pressure dependence. No adequate test of oxygen pressure dependence was possible on the basis of that study. Re-adoption of the assumptions of zero inhibition by products and of Langmuir-type adsorption of the alkane reactant, together with a similar pseudo-equilibrium expression for surface coverage by oxygen, was shown t o lead to rate expressions (41a) and (41b) for the photoassisted production

of acetone from isobutane. Rate =

(

2J=P

k,C$P,NPC [1+(K0Po)N K,P,

+

=

I,(P{)

+P{Pc]2

+ S,P,NP,

+

In these expressions, a = k 3 / ( k - 3 k 4 ) , 0 = KrK,, I is the intercept and S the slope of plots according to eqn. (41b). The fact that linear plots resulted forN = 0.5 provided support for the involvement of dissociatively adsorbed oxygen in the alkane photo-oxidation. In general, the experimental data of Formenti et al. could be adequately accounted for.

( i i i ) Partial photo-oxidation of isopropanol. Partly as a follow-up to their observation that alcohols appeared as minor products and probable intermediates in the photocatalysed oxidation of hydrocarbons over TiO,, French workers have applied similar gas chromatographic techniques t o study photo-oxidation of various alcohols over TiO, and have observed selective photo-oxidations to ketone and aldehyde products [ 1901 . Investigations of alcohol vapour/metal oxide interfaces have also been reported utilising a range of other techniques including a recirculating rotary photoreactor [ 1611 , IR analysis [ 9 3 ] , mass spectrometry and thermal desorption [ 1911, dynamic mass spectrometry [ 192, 1931 and oxygen-labelling techniques [ 1941 . Isopropanol photo-oxidation has been particularly widely studied and was found by IR observations on the composition of the vapour phase to be sensitised by W03, SnO, and ZrO,, as well as by conventional ZnO and Ti02 photocatalysts [ 9 3 ] . IR observations on adsorbed species showed that surfaces of silica gel and y-Al,03 also acted t o promote photo-oxidation of isopropanol to acetone. In addition to the bands at ca. 1700cm-' attributable to carbonyl frequencies of adsorbed acetone, other IR bands appeared at ca. 1600 and 1400 em-' , indicative of surface compounds similar to acetate ions. Photo-oxidation of methanol produced a surface photoproduct with formate-like absorption. Whilst unreacted alcohol could be largely removed by heating and evacuation, these latter carboxylate-like bands were not completely removed even at 6 2 3 K [ 921 and would appear t o originate from a photoassisted surface reaction rather than true photocatalysed processes. IR results thus indicate a slow build-up of strongly held carboxylate species as products of a photoassisted surface reaction proceeding in parallel with photocatalysed oxidative dehydrogenation of alcohol. The composition of the gas phase over an isopropanol/TiOz system illuminated in a rotary photoreactor has been examined with a monolayer of isopropanol pre-adsorbed on the powdered TiO, [ 1611 . Acetone References p p . 419-427

378

photoproduct was not released in significant and continuously increasing amount unless oxygen gas was also present at the illuminated interface and this process is illustrated in Fig. 11, together with the converse photoassisted depletion of oxygen and of isopropanol from the gas phase. The figure also illustrates an effectively constant photoassisted rate of uptake of oxygen, which appears independent of the amount of isopropanol present in the gas phase. This contrasted with an enhancing effect of increased isopropanol partial pressure on the rate of release of acetone to the gas phase, and the contrast led to suggestions that isopropanol could dislodge acetone from the TiO, surface in a 1:l molar ratio [161]. Admission of water vapour to the TiO, surface, after outgassing at 600 K and after contact with isopropanol, did not significantly change the course of reaction from that shown in Fig. 11. However, when water was admitted t o the TiO, surface after outgassing at 6 0 0 K and prior t o admission of isopropanol, a “hydrated surface” resulted, displaying lower rates of oxygen depletion from the gas phase and of acetone production. The latter was not dependent on the amount of isopropanol in the gas phase over the hydrated surface. Bickley and Jayanty [191] considered that these differences could be accounted for in terms of a dependence of the rates of acetone and isopropanol evolution to the gas phase on complex equilibria involving adsorption/desorption and mutual displacement of these species and water a t the surface. The positions of these equilibria were envisaged t o vary as photo-oxidation proceeded with the stoichiometry 0 2 (g)

+ 2 C3H70H = 2 (CH,),CO + 2 H2O

The nature and reactivity of the photoactivated surface site or species could not readily be inferred from kinetic data such as those in Fig. l l ( b ) . A mechanism for the photo-oxidation was, nevertheless, advanced which represented the trapping of photogenerated holes by pre-existing surface hydroxyls as the initiation step, followed by electron capture on adsorbed molecular oxygen and by reaction of the resultant 0; with adsorbed alcohol via either proton or H atom transfer. The choice of surface OH; as the photoactivated site for the initiation of the reaction appeared to rest mainly upon the linkage of two qualitative observations on the effects of high temperature outgassing on the activity of TiO, surfaces: (i) that the activity for alcohol photo-oxidation was reduced by outgassing at 1073 K; (ii) that the activity for oxygen photoadsorption (in the absence of alcohol) declined progressively as the extent of dehydroxylation of the surface was reduced by outgassing at temperatures between 600 and 1073K. A mechanism based on hole trapping by surface hydroxyls was adopted by Bickley and Jayanty [191] in interpreting the results of a re-examination of isopropanol photo-oxidation over fully oxidised T i 0 2 . This employed programmed thermal desorption (PTD) to remove species from the T i 0 2 surface and mass spectrometry to identify them. The hydroxyl-based mechanism was complicated by observations indicating

379

that a strongly ,held form of adsorbed isopropanol was in competition with water for sites capable of yielding surface hydroxyls. Another difficulty which attached to the use of thermal desorption was the occurrence of extensive thermally assisted dehydration and dehydrogenation processes (see also ref. 54), which made it difficult to disentangle photoassisted from thermally assisted products. The photoassisted oxidation of isopropanol and another secondary alcohol, butan-2-01, over TiO, and ZnO has been re-examined recently in the author’s laboratory [ 2561 employing a Lyon-type photoreactor either with a continuous flow of reactants or with pulses of reactant admitted as desired to a surface not otherwise exposed to reactant gases. Results either with intermittent reactant pulses and illumination or with continuous dynamic flow and illumination showed that, with a fixed pressure of 0, but varying pressures of secondary alcohol admixed with N, carrier gas, photoassisted oxidative dehydrogenation of the secondary alcohols was the dominant photocatalysed process with a rate dependent on alcohol pressure in a manner similar to eqns. (39c) and (39d), i.e. the observed rate could be separated into one process displaying a LangmuirHinshelwood-type dependence on alcohol pressure at low PROHand another which increased linearly with alcohol pressure [cf. Fig. 12(b)] and so would be consistent with an Eley-Rideal-type photoassisted process. The latter process was attributed to a photoactivation of surface oxide ion sites by hole capture, producing 0- surface intermediates which reacted during their short lifetime with physisorbed alcohol or alcohol directly striking the photoactivated site from the gas phase. The Langmuir-Hinshelwood-type process was considered t o involve alcohols chemisorbed on a subset of particularly active O:& sites which, on photoactivation by hole capture, activated the chemisorbed alcohol for attack by adsorbed 0,. On ZnO surfaces, this latter process dominated at alcohol pressures < 500 N m-*, but levelled off at ca. 2.5 x lo3 N m-?. The EleyRideal type processes made increasingly important contributions at pressures >2.5 x lo3 Nm-’ and exceeded that from the LangmuirHinshelwood-type processes at alcohol pressures of 5 x lo3 N rn-, [cf. Fig. 12(b)]. Kinetic data did not suffice t o determine whether these increasing contributions involved reaction of physisorbed alcohol or alcohol colliding from the gas phases with photoactivated oxygencontaining locations such as that envisaged in eqn. (36). Pulsed-reactant procedures [ 192-1961 allowed pulsing of the alcohol and/or the photon flux in systems involving alcohol vapour flowing over a powdered sample of the metal oxide (cf. Fig. 10). Surfaces of the metal oxide were preconditioned by heating to 623K either in the N2 stream (referred t o as mildly reduced surfaces) or in a stream of oxygen (referred t o as pre-oxidised surfaces) and cooled prior t o contact with a pulse of reactant(s). Such reactant pulses then passed through the packed GLC column for separation and subsequent quantitative analysis by the flame ionisation detector. Adsorption of reactant alcohol on the preconditioned

-

References p p . 4 1 9 - 4 2 7

380

sample, even in the absence of UV illumination, showed up as a diminution of the amount reaching the detector [195]. With pulses of 0.5 ml total capacity and composition [N,: (CH,),CHOH: 0 , 0.45 atm; 0.05 atm: 0.5 atm] , the amount adsorbed on the metal oxide increased linearly with the number of reactant pulses for the first 1 2 pulses but then levelled off gradually t o a limiting value. Alcohol adsorption in the dark a t 350 K was not accompanied by the release of a detectable amount of products which would correspond t o surface-assisted oxidative dehydrogenation or dehydration of the alcohol. Illumination of the (alcohol 02)/metal oxide interfaces during passage of the reactant pulses did result in the detection of pulses of such products. The dominant product pulses from secondary alcohols corresponded to the prompt formation and release of ketone products requiring photoassisted oxidative dehydrogenation of the reactant alcohol, e.g. butan-2-one from butan-2-01 [ 195, 2561 . Product pulses corresponding t o photoassisted dehydration t o alkene were < 5 % of the total photoproduct yield. However, a lower aldehyde corresponding t o photo-initiated C,-C, bond cleavage in the alcohol did form at significant rates. Depending upon the identities of the secondary alcohol and of the catalyst, the rate of such - (C,-C, )* processes varied from 40 to 160% that of the (-H,) process, the higher yields being obtained for longer-chain alcohols. For all the (alcohol O2 )/metal oxide systems studied, photoassisted conversion t o any of the indicated products was greatest for the first reactant pulse and then declined on admission and analysis of subsequent reactant pulses, this decline being greatest and occurring over the smallest number of pulses for very finely divided samples of high surface area and high initial photoactivity (e.g. a decline t o 12%of initial activity occurred from pulse 1 t o pulse 6 for a TiO, sample of area 170 m2 g-'). Some decay in the level of photocatalytic activity of ZnO or TiOz from an initially high value to some intermediate but reproducible level after use for 0.5-3 h, is not unique t o operation in a pulsed-reactant mode nor t o one laboratory. It seems appropriate, therefore, t o recall that changes in the electronic properties of such systems can be brought about at such gas/semiconductor interfaces on this time scale by UV illumination (cf. Sect. 2.1.2 and Fig. 1) and that such electronic changes can affect rate coefficients and surface concentrations for reaction (cf. Sect. 1.2.3). Changes in photoelectronic factors thus appear likely t o make significant contributions to variations in the rate of photocatalysed reactions with time of illumination. Other likely contributory factors were the poisoning/ inactivation of a sub-set of particularly active surface sites by: (i) photoassisted surface reactions, such as formation of carboxylate or carbonatetype surface centres via a side reaction accompanying the predominant (- H2 )* or (-H,O)* process from the alcohols; or (ii) by photoassisted removal from the surface of some essential component of the specially active sites, such as removal of Of& or OH; as water vapour during

+

+

381

exposure t o alcohol and t o UV illumination. Evidence for the operation of the former of these processes at (CH3),CHOH/Zn0 interfaces came from observations that prior exposure of the ZnO surface t o acetic acid or C 0 2 , which were likely t o produce carboxylate and carbonate-type species, respectively, did cause the (- H, ) photoactivity t o decrease in proportion t o the extent of such prior exposure. Consequently, the decline in activity noted experimentally represents, in all probability, the combined influent? of site poisoning and photoelectronic factors. In such circumstances, quantitative analysis of kinetics during the initial decline in activity would require that the rates of change be independently monitored, both for the surface electronic properties and for the destruction or blockage of active sites. This problem appears not yet t o have been rigorously treated, particularly in dynamic flow photoreactions with gas chromatographic detection, where the working assumption usually adopted has been that rates of conversion attained after the initial decline in activity provide a good measure of stable photocatalytic activity. Despite widespread recognition of the necessity for some gas-phase pressure of 0, if the photocatalytic activity for partial oxidation of alcohols is t o continue t o high t.a.p.s.* over ZnO and TiO,, definitive evidence on the role or roles played by 0, is still awaited. The various suggestions made include: (i) 0, (ads) acting as an electron trap, thereby complementing the hole-trapping roles of surface OH- or surface alcohol species and so diminishing the rate of electron-hole recombination; (ii) entry of 0; into the sequence of reactions, e.g. by abstraction of H+ or H atom from an alcohol-related surface intermediate produced in the primary photoactivation step; (iii) involvement of O,, or of surface oxygen species produced by chemisorption/desorption equilibria, in the formation of charge-transfer-type exciplexes, e.g. as per eqns. (43), in which all square-bracketted or asterisked species exist only on the surface. ht

+ [ \/ CHOH . . .O,]--

[

\ C H O H . . . O,]* /

The possibilities of surface exciplex formation recognised by these equations are: hole capture by surface complexes between alcohol and 0; preformed at the dark interface [cf. eqn. ( 4 3 a ) l ; production of a lowlying electronically excited state of O;, e.g. by hole capture on 0; and subsequent encounter with adsorbed or gas-phase alcohol [cf. eqn. ( 4 3 b ) l ; and adsorbent-initiated excitation of adsorbed alcohol, plus subsequent encounter with O2 from the gas phase or a weakly adsorbed state. Available kinetic data for ZnO and TiO, point t o an approximately first-order Refe:ances p p . 419-427

382

dependence on Po, at pressures of 20-400 torr, but this does not provide definitive discrimination between the roles of molecular oxygen suggested under (i), (ii) and (iii) above, in cases where photo-oxidation proceeded to high t.a.p.s.* in those gas chromatographic experiments. Some insight into the probable roles of molecular 0, at low t.a.p.s.* has emerged from experiments employing low pressures (ca. 0.1 torr) allied to mass spectrometric monitoring of the of isopropanol and of 1802, gas phase composition, over various 3d transition metal oxides at room temperature [196]. Even in the absence of UV illumination or of any gas phase O,, conversions t o acetone product equivalent t o the (-H2) reaction of isopropanol with the first surface monolayer of metal oxide (or a significant fraction thereof) were observed over pre-oxidised surfaces of C r 2 0 3 , F e 2 0 3 , V 2 0 5 and TiO, . Support for the interpretation of these conversions as originating from surface reactions at t.a.p.s.* < 1 came from observations that the extent of conversion diminished sharply over Cr203, Fe203 and Ti02 after outgassing in vacuo a t 675 K, a pretreatment which yielded extensively dehydroxylated and slightly oxygendeficient surface layers. Whenever partial pressures of ca. 0.1 torr each of 1802and isopropanol were established over the indicated oxides or over ZnO and the systems were exposed to the output (at 3 0 0 < X < 800 nm) of a 500 W medium pressure Hg-arc lamp, photoassisted growth of acetone products was observed, except in the case of V 2 0 5 . Acetone photoproduct was enriched t o 28 ? 10% in "0 over the other oxides except high purity TiO,, the enrichment being influenced by the extent of surface pre-oxidation/outgassing. Since no comparable enrichment was found from the "02 (CH3)2CH'60H systems in the dark, nor from "02 (CH3)2 C l 6 0 systems under illumination, it could beqoncluded that oxygen isotope exchange between molecular "02 and ,CHOH or C 'O could not be responsible for the observed incorporation. Photo/ assisted exchange between H, l 8 0 and (CH,), CI6O was observed, however, over ZnO and Cr203, with rates more than adequate t o account for the incorporation of oxygen-18 into the acetone photoproduct from isopropanol over those oxides [196]. One two-step mechanism for l80 incorporation consistent with these observations would envisage: (i) reaction of oxygen-18 surface species derived from 1802with surface hydrogen fragments from the (- H,)* process; and (ii) subsequent rapid oxygen exchange between the resultant H, l80and the acetone fragment from the (-H,) process. Additional hypotheses would be necessary in this mechanism to account for limitation of the l80 incorporation t o 28 t 10% over most of the oxides [e.g. that the (-H2) fragments can react also with surface lattice '60;u,species as well as l80from and for the lack of any significant incorporation of l80into acetone over one high purity TiOz sample or over V 2 0 , [e.g. that reaction of (- H,) with surface oxygen species on those surfaces does not produce the particular H2 "0 surface species which can engage in OIE with acetone]. Unproven

+

+

383

hypotheses would likewise appear necessary t o reconcile experimental observations with an alternative two-step mechanism involving: (a) dehydration yielding surface H2 l6O and propene-like intermediates; (b) attack of l8O species derived from "02 on the newly formed or incipient double bond of the alkene. Since tests with the rate of oxidation of propene by I8O2 over illuminated ZnO and Ti02 indicated rates of "0 incorporation too low to account for those observed from [I8O2 i(CH3)2CH160H]/ZnO systems, one unproven hypothesis necessary t o reconcile observations with this latter mechanism would be an alcoholrelated surface intermediate having an incipient double bond more susceptible to attack by "02than surface propene. Neither (i) and (ii) nor (a) and (b) can be regarded at this stage as providing a fully satisfactory explanation of oxygen-18 incorporation into the acetone photoproduct from isopropanol under low pressure conditions at t.a.p.s.* < 1. When gaseous oxygen was present together with alcohol vapour at partial pressures 2 10 torr, mass spectroscopy studies confirmed the occurrence of continuing photoassisted dehydrogenation of secondary alcohols up to large t.a.p.s.* over ZnO or Ti02 (anatase, Degussa P25). Experiments with a gas chromatographic continuous reactant flow technique, which were weighted towards photocatalytic processes continuing to high t.a.p.s.* and were rather insensitive for photoassisted surface reactions proceeding only to t.a.p.s.* < 1 , again demonstrated photocatalytic activity over ZnO and Ti02 [ 1961. However, similar experiments with VzOs, CrzO3, Fe203, Co304, NiO and CuO or Cu20, failed to yield unequivocal evidence for photodehydrogenation (- H2)*, photodehydration (- H,O)* or photoinitiated C,-C, bond cleavage - (C,-C, )* continuing t o high t.a.p.s.* over these oxides featuring cations with partially filled d shells. Cunningham et al. [196] have suggested that this is a consequence of photogenerated holes losing their predominantly 0--like character in transition metal oxides having partially filled d levels (or d bands). Holes in these latter systems come, instead, to have the character of oxidised cation sites and consequently fail to show the selective reactivity of 0 - towards alcohols seen in ZnO and Ti02. [It is worth noting here that similar arguments would be expected to apply to carbon monoxide oxidation via photogenerated 0species and that SrTi03 and TiOz and BaTi03 did show the expected photocatalytic activity [185], whereas LaCo03 and Ba(Fe,.,, Ti2.0)02.6, featuring cations with partially filled d shells did not.] A recently published kinetic analysis of data on the partial photooxidation of oxygenated isopropanol vapour over a high-purity zinc oxide powder (NJZ-SP500) at 350 K, yielded

as the rate expression most consistent with photoassisted conversions to References p p . 419-427

384

acetone in both continuous and pulsed reactant conditions [256]. The possibilities for dependence on the square root of the intensity of the UV illumination (310-390 nm) were recognised as attaching to processes, such as the following, initiated by localisation of photogenerated holes at the surface.

..

h+

\

+ / CHOH(ads) \

h+ + [ CHOH . . . 0 2 ] -

/

\

CHOH+(ads)

’

(45c)

\

[ C H O H . . .Oz]*

/

(45d)

In high intensity conditions such that (i) the number of holes reaching the surface t o engage in such reactions is a small fraction of those produced within ZnO by Zabs and (ii) most photogenerated holes recombine with the higher concentration of photogenerated electrons rather than with [ e-] in the dark, then [ h+] becomes proportional t o Z . As required by this idea that the driving force of the photoassisted reaction is the small fraction of photogenerated holes which arrive a t the surface having avoided recombination, the quantum efficiencies were low, e.g. 3 x for a flux of UV photons of 7.6 x 10l8 s-’ on to the catalyst [256]. Although there is evidence in homogeneous systems for the selective abstraction of a-hydrogen by 0- (or OH) from secondary alcohols, and although eqn. (45a) gives rise t o 0--type species, operation of this abstraction process alone as the rate-determining step was not considered t o explain adequately the appearance in eqn. (44) of both OROH and Oo,. Rather, this second-order dependence was considered t o point t o the operation of one or more of the following bimolecular or termolecular surface reactions involving surface species activated by hole capture.

’”

--

+ O2 + \CHOH ‘CO + OH; + HOz / \ / \ CO + OH; + HO, OiUs+ [ CHOH . . . 02] / \’ 0: + ,CHOH ‘CO + H z 0 2 / \ \ ( CHOH)’ + 0; CO + H 2 0 2 / / OiUs

/

(45e) (45f) (45g)

Dependence on Z’’2 and on 8 R O H would arise naturally from the operation of these processes as the rate-determining steps. Dependence on Po, can also be readily accommodated, viz. directly in eqn. (45e), or via dark equilibria which determine the surface concentrations of alcohol-O2 complexes [cf. eqn. (45f)], or 0; species [cf. eqns. (45g) and ( 4 5 h ) l . The successful deconvolution of the alcohol pressure dependence into a Langmuir-Hinshelwood component predominating at low PRoHand an

385 TABLE 5 Extent of adsorption of alcohols on to pre-oxidised rutile layers and rate constants for subsequent photoreaction with "Oz Alcohol

Preadsorbed alcohola

ko (- ROH)b (molecules-' )

k l (- " 0 ~ )ko(R1 ~ Rz CO)d (s-l) (molecules-' )

Propan-2-01 Butan-2-01 2-Methylpropan-2-01

1 x 1019 2 x 1019 2 x lOI9

6.7 x 1013 1.0 x 1014 2.7 x 1014

1.8 x 1 0 - ~ 3.3 x 1 0 - ~ 3.2 x 1 0 - ~

2.3 x 1014 3.2 x 1014 5.2 x 1014

a Values expressed as molecules pre-adsorbed in

2000 s-l on to standard weight of pre-oxidised rutine on quartz support in static reactor. Pseudo equilibrium was reached with 66 N m-2 of alcohol vapour in the reactor. Pseudo zero-order rate constant, k o (- ROH), expressed as molecules lost to the surface of the rutile sample per second, corresponding to continuous slow adsorption of alcohols. Pseudo first-order rate constant for photo-initiated loss of I8O2 from the gas phase, as evaluated from first-order plots of log P ( I8O2)against time. Pseudo zero-order rate constant for photo-initiated appearance of ketone photoproduct in the gas phase, expressed as molecules released into reactor s-l under UV illumination.

Eley-Rideal or VCI component predominating at high PRO,may also be accommodated within this set of possible surface reactions [e.g. eqns. (45f) and (45h) are predominantly Langmuir-Hinshelwood in character, whereas eqns. (45c) and (45g) could introduce Eley-Rideal or VCI character]. It is worth noting that no comparably successful explanation of the dependence upon isopropanol pressure could be achieved using models of the Mars-van Krevelen type [256].

(iv) Photoassisted conversions of other alcohols. An approach frequently adopted in mechanistic studies of elimination reactions from alcohols in the absence of irradiation is the comparison of rates and/or selectivities of thermally assisted conversions for alcohols whose differing structures could be expected t o favour one or other of the classical E, , E, or EIcb pathways t o elimination reactions. For example, comparisons of isopropanol with t-butanol and neopentyl alcohol might be undertaken to test the relative importance and rate-determining character of Cp-H and C,-H bond rupture. However, comparison of photocatalysed conversions of isopropanol with the available scanty data for photoconversions and other alcohols suggests that such arguments and approaches may be inadequate and misleading if based solely on heterolytic bond ruptures, with loss of H', H- and OH- from alcohols in E l , E, or EIcb pathways. Some inadequacy of such conventional mechanisms was noted in relation t o data from ref. 194 reproduced in Table 5 summarising comparisons made in closely similar conditions between three different alcohols undergoing photoassisted conversions over a high-purity rutile sample ( TiOz MR128 References p p . 4 1 9 4 2 7

386

from New Jersey Zinc), Rates of an approximately first-order photoassisted depletion of lSO2 from the gas phase greatly exceeded the measured rates of appearance of ketone as the major photoproduct in each case. Acetone, whose formation as major photoproduct from the tertiary alcohol required photoinitiated cleavage of a C,--C, bond, had nevertheless a rate of photoassisted conversion comparable with the rates of simple (- H2)* from the secondary alcohols. The observed (- C,-C, )* rate was, furthermore, much greater than the photoassisted oxidation of butenes in similar conditions, thereby again posing difficulties for pathways based on dehydration and subsequent oxidative scission of a butene intermediate from t-butanol. The ketone product evolved during UV illumination of [alcohol " 0 2 ] reactants over Ti02 surfaces preoxidised by 1 6 0 2 showed very low (<5%) incorporation of oxygen-18. Incorporation was greater, but the overall rate of conversion was diminished, over the TiO, surfaces when these had been extensively dehydroxylated and slightly prereduced by outgassing overnight in vacuo at 675 K. A feature of those experiments not adequately stressed in the original publication was their weighting towards an extent of photoassisted surface reaction approximating to one monolayer equivalent (i.e. t.a.p.s.* < 1).Participation of 160-containing surface anions would be strongly favoured in those conditions for samples pre-oxidised by I6O2. Such participation may be represented schematically for pre-oxidised Ti02 surfaces as

+

The selection of process (46) as the ratedetermining step in the photoinitiated sequence leading t o ketone formation (instead of, for example, the abstraction of an alpha hydrogen) appeared t o offer advantages in accounting for comparability in rates of photoassisted conversions of the tertiary with the secondary alcohols. Further supporting evidence from (46) was adduced in the tertiary alcohol case from observations that, whenever oxygen-18-labded (CH3J3C'80H was illuminated over preoxidised TiO, in the presence of 1602, the oxygen-18 label was completely lost, as represented in eqn. (46). Photoproducts whose formation necessitates rupture of a C,-C, bond in primary or secondary parent alcohols have also been observed on UV

387

illumination of the oxygenated alcohol vapour over Ti02, e.g. the photoassisted conversion of butan-2-01 t o acetaldehyde exceeded that t o butan2-one, Data relevant t o the formation of such -(a-p)* products were usually obtained by GC continuous reactant flow experiments and corresponded t o steady-state conversions proceeding t o t.a.p.s.* 2 10. Comparisons of such reported photocatalysed conversions to - (a-p)* photoproducts with parallel conversions t o (- Hz )* products suggest increasing selectivity towards the former with increasing number of carbon atoms for secondary alcohols [ 190, 2561. Other indications of - (a-P)* bond cleavage were noted by Teichner and co-workers [187, 1881 in respect of a tendency of initial C, -aldehyde photoproducts from primary alcohols t o be progressively degraded t o C,-l, C, - 2 aldehydes etc. t o acetaldehyde. No fully satisfactory mechanism based on heterolytic bond rupture has yet been elaborated for these photoassisted processes yielding C,-Cp bond rupture [ 1971. The inadequacies outlined in the preceding paragraphs prompt a widening of the mechanistic considerations t o include pathways t o photoassisted conversions of alcohols based on photoinitiated homolytic bond cleavage with free-radical intermediates, e.g. as in

I

R-COH

I

>CHOH

-

hu/MX

hulMX

I

R-CO

I

+ A-MX

\ COH + A-MX /

Alternatively, an alkoxy-type surface species may form an inner-sphere complex with a surface cation, eqn. (47c), and later experience photoassisted homolytic C, -Cp bond cleavage, eqn. (47d).

I I

R-COH

. -

+ MX 2 R-CO-I I

*

MX

+ H+

XM

(47c)

Promotion of such homolytic bond rupture by direct excitation of an electron from the bonding orbital involved would be unexceptional, but would be probable only at shorter wavelengths than usually present in the illumination employed (A 2 310 nm). More feasible routes to the photo-

Referencespp. 419-427

388

initiation of homolytic bond cleavage would be via adsorbent-initiated excitation by photons inside the band edge of ZnO or T i 0 2 (X < 390 nm) o r via excitation of an adsorbate-catalyst surface complex [cf. eqn. (14), but note that, for oxygenated alcohol vapour over the metal oxide surface, excitation may occur into either a preformed or into a contactcharge-transfer surface complex involving alcohol and oxygen] . Continuing and more detailed consideration of parallel pathways based on the photoinitiation of homolytic bond cleavage may thus be fruitful, not only in ‘evolving the detailed mechanism of - (C,-C, ) conversions of alcohols, but also in achieving a balanced appreciation of the roles of both “ionic” and free-radical intermediates at the illuminated alcohol vapour/metal oxide interfaces.

( c ) Photoreactions without dioxygen reactant ( i ) Hydrogenldeuterium exchange. Hydrogen-deuterium exchange on metal oxide catalysts, and particularly on magnesium oxide, has been shown t o be sensitised by UV illumination and a measure of agreement has emerged between various workers that, in general, the photoactivated exchange proceeds at surface sites involving a trapped hole [ 198-2001 . The exact identity of the trapped-hole centre and the mechanism by which it promotes H2/D2 exchange has been differently represented by various workers. Thus Boudart et al. [38] located the trapped hole at a triangular array of 0 - ions on the (111)plane of MgO and envisaged activation of an adsorbed D2 molecule at this site leading to exchange with a neighbouring hydroxyl, as represented in OH-

OH-

-0......o-

-0.. ..-. ..0 I.,

,

?

/’

-t

D,

A

S

7-

~

I

D-D,’

\ \

0D -0...... -0-

,

*

\

,o

,

‘0’

’\

\

L ,.I

,

,

,‘

+

HD

(48)

*O’

An earlier study by Harkins et al. [198] had distinguished between Hz/D2 exchange on MgO via a photosensitive mechanism, termed mechanism 11, and a light-insensitive mechanism, termed mechanism I. Only the latter was observed for samples previously outgassed in vacuo at temperatures > 8 7 3 K , whereas mechanism I1 was readily observed with MgO samples outgassed at 570K. This was attributed t o direct or indirect trapping of photogenerated holes on residual surface hydroxyls t o produce OHo centres which then promoted exchange by the interface reactions

+ OH? D.4 + OH: Di + HOHi Hi + DOHi Hzi

-

+ Hi DOHi + Di HOHi OH: OH:

+ HDi + HDi

(49a) (49b) (49c) (49d)

389

A more recent Russian study [199] of H2/D2 exchange on MgO samples preheated in vacuo at 570K confirmed its sensitisation by UV illumination of such interfaces. However, these workers concluded, on the basis of kinetics and IR data, that not only did OH groups take part in the exchange, but also other centres created during irradiation. The exact nature of the centres created by irradiation was not identified in the study, but other Russian work [200] on the photoinduced H2/D2 exchange over zeolites a t 7 7 K has been interpreted in terms of lightinduced formation of a charge transfer-type surface exciplex and subsequent reaction of the exciplex with H2 as represented by Mn+ + 0

H,

2 -

hv_

+ {(M("-l)+ . . . O-)}*

. . . 0-)}*

-

{(M(n-I)+

Mn+H-

(50a)

+ 02-H+

(50b)

It was suggested that the hydride ions produced in the second of these processes gave rise t o photoinduced exchange, which persisted at 77 K for a long time after illumination [200]. Comparison of reaction schemes (49) and (50) shows that, in the former the photoassisted exchange proceeds via free-radical intermediates, whereas ionic intermediates predominate in the latter. ( i i ) Photoreactions involving nitrous oxide. Illumination of the N2O/ZnO interface by UV light has been shown by various techniques t o lead to a photoassisted enhancement in the rate of dissociation into gaseous nitrogen and an adsorbed oxygen fragment. Thus, Wong et al. [85] used ESR spectroscopy t o observe enhanced rates of production of the spectrum of l6O- radical species whenever the N 2 0 / Z n 0 interface at 1 9 2 K was illuminated by photons of X = 254nm. A weak signal was observed with the system in the dark, the intensity of which increased rapidly with irradiation but became constant after ca. 90 min illumination from the low pressure mercury lamp [cf. Fig. 6(c)]. Observations on the release of gaseous product from N2O/ZnO interfaces irradiated at room temperature have yielded complementary evidence for the release of N2 as the predominant photoproduct [83b]. In the absence of illumination, it was concluded from these studies that N 2 0 dissociation to N2 0- on ZnO was essentially non-catalytic, in view of the accumulation of electrically charged oxygen product on the surface at room temperature and agreement with Elovich-type kinetics [83a]. Production of 0- and 0; by photoassisted non-catalytic reaction of N 2 0 with MgO surfaces has likewise been reported by Aika and Lunsford [86]. A photocatalysed process of nitrous oxide dissociation a t higher temperatures has been reported by Tanaka and Blyholder [ 2011 , supported by kinetic analysis, showing agreement of the rate of this process with

+

kPN20

Rate =

1 + KIPN,O

+ KzP02

(51)

Temperatures of 644-704 K were employed and the photocatalysed References p p . 419-427

390

reaction occurred in parallel with an extensive thermal catalytic dis302,The kinetics of the latter process, which sociation of N 2 0 t o N, was first order in PNIOand zero order in Poz, differed from the photocatalysed process and the differences were attributed t o the operation of different mechanisms for the thermally assisted and photoassisted processes leading to N 2 0 dissociation on ZnO. The atomic and molecular mechanism summarised by

+

N,O(g)

+

N,O(ads)

slow

N,O(ads)

A

(52a)

O(ads) + N2 (g)i

(52b)

was suggested for the thermally assisted process and envisaged dissociation of adsorbed neutral N,O (52b) as the slow rate-determining step. An activation energy of ca. 146 kcal mole-' was attributed t o this process, minus the heat of adsorption for (52a). Mechanism (52) differed from earlier proposals that desorption of oxygen was the rate-determining step, but this departure was considered necessary in view of the observed insensitivity of the thermally assisted process t o added oxygen and the fact that the photoassisted process experienced strong inhibition by oxygen. This difference was accounted for in terms of mechanism (53) for the photoassisted process, based on the dissociation of negatively charged N 2 0 - as the slow rate-determining step.

2L

N20(ads) N,O-(ads)

h*

O-(ads),

h'

N,O-(ads)

. slow

e-

+

N,(g),

+ 0-(ads)

0 (ads)

(53a) (53b) (53c)

302(g)i (53d) Electron scavenging by oxygen species in competition with eqn. (53a) was identified as the origin of the sensitivity of this photoassisted redoxtype mechanism towards added oxygen. Photoassisted reaction of N 2 0 and CO. Gas chromatographic analysis using a molecular sieve 5A column was employed by Tanaka and Blyholder [204] t o follow the conversion of a mixture of CO and N 2 0 into N 2 + C 0 2 products over dark and illuminated zinc. oxide powders a t temperatures of ca. 470K. The thermal catalytic reaction was zero order in PN,Oand first order in Pco and was markedly retarded by oxygen. The photoassisted conversion t o nitrogen product exhibited quite different kinetics, characterised by ads)

Rate a ~ ~ ~ o P ~ $ and the absence of any retarding effect of oxygen. These differences led

391

to the suggestion that the thermal catalytic reaction at 470K ceased to occur over UV illuminated ZnO due to the photoassisted conversion into non-active ZnZ+ of any surface Zni or Zn: ions, which were normally thought to act as active sites for the thermally assisted process. The photoassisted reaction was envisaged as proceeding, rather, on ions of the surface lattice following photoinitiated charge-transfer with surface N,O and CO species as depicted in

[N20-(ads)

I+ 1I

(54)

+ CO+(ads)]5IC02(ads)

The observed lack of inhibition by 0, of the rate of photoassisted oxidation of CO by N 2 0 was attributed, in their scheme, to different ratedetermining processes for the two photoassisted redox reactions in the UV-illuminated ZnO surface. Later studies of the photoassisted oxidation of CO by N,O over a supported molybdenum catalyst also concluded that 0- intermediates were not involved. The photoassisted formation of nitrous oxide, rather than its removal from the gas phase, has been reported for UV-illuminated NO(g)/TiO, systems. Thus a recent report by Pichat et al. [205] reconfirmed a much earlier report by Kennedy et al. [ 2061 that if nitric oxide was equilibrated over TiO, in the dark, subsequent UV illumination caused a significant decrease in pressure accompanied by the appearance of NzO as one photoproduct, which predominated for low initial PNo, whereas N z O predominated for higher PNo.Kennedy et al. considered that the stoichiometry of the photoassisted process produced one NzO molecule per four nitric oxide molecules consumed and speculated that the other product was N z 0 3 , which was not directly observed. Pichat et al. observed increased rates of photoassisted conversion of nitric oxide in the presence of alcohols and attributed this to a hole-trapping role for the alcohols, thereby allowing greater electron attachment with formation of NO-. It was suggested that ready dissociation to monatomic nitrogen and oxygen fragments then occurred and that the results corroborate the importance of dissociated oxygen in photocatalytic oxidation [ 2051. Further evidence for photoassisted interconversions between nitrous oxides, nitric oxide and other nitrogen oxides on metal oxide surfaces emerged from studies with ftre dynamic mass spectrometer system of flash-initiated changes at '*NfsiNOJZnO interfaces. Fig. 13(a) illustrates mass spectra of the gas mixture adjacent to an ''N'5N'60/Zn0 interface prior to a flash and then 100ms after a flash, and the broken line demonstrated that a quite different mass spectrum was detected by the DMS system 100 ms after flash illumination. The large enhancement of m / e = 30 (14N'60+), smaller enhancement of m/e = 31 (15N'60+), and absence of References p p . 419-427

392

>

E

20.0 I I

II

30

29

31

"/"

l------

30.0

20.0

rye, .\

:\ (ii) O

0

.\

Lo-o-o-.

I

I 4.0 Pressure "m-2

8.0

x 10-4)

32

393

0.36

9 m

0.31

.. 61

al

F

\ 0 m ,I

< F 0.2(

t 0.21

I

I

Fig. 1 3 (a) Evidence obtained from the DMS system for the release of nitric oxide as the major product observable in the gas phase 100 ms after exposure of an 1 4 N 1 5 N ' 6 0 / ZnO interface to an intense 50ps pulse of photons of wavelengths 340-640 nm. The solid line depicts a fast (10ms) scan across m / e = 28-38 at a dynamic pressure of 1 4 N 1 5 N ' 6 0of N m-2. The broken line depicts a similar scan made 1 0 0 ms after a flash illumination. ( b ) The effects of additions of gases known t o react with 0 - on the magnitude of the nitric oxide transients depicted in (a). ( i ) Additions of isopropanol; (ii) additions of carbon monoxide. (c) Time profiles on commencing continuous illumination of alcohol/ZnO interfaces and also showing temporary enhancing effect of NzO on photodehydrogenation. ( i ) Time profile of acetone photoproduct from (CH3)zCHOH over zinc oxide, showing rapid decay of photoactivity to zero; (ii) as for (i), but showing that a small dynamic pressure of NzO enhanced acetone photoproduct. Reproduced with permission from refs. 136b and 193.

any detectable enhancement of m / e = 29 (14N"N+ ) or m / e = 32 (l60:), all pointed to the presence of nitric oxide, rather than nitrogen or oxygen, as the major photoproduct in the gas phase at 100 ms after flash illumination. The data shown in Fig. 13(a) were taken with photons of X = 340-640 incident on the ''N'5N'60/Zn0 interface. Experiments at References p p . 4 1 9 4 2 7

394

different flash intensities established that nitric oxide formation varied with the square of the intensity and suggested that the nitric oxide product could stem from operation of a new photoactivation process requiring the interaction of preexisting surface species or sites on ZnO with two photogenerated intermediates. On the basis of evidence [indicated in Fig. 13(b)] that 0; intermediates took part in the flash-initiated production of both 14Ni60 and l S N L 6 Oproducts, the mechanism represented by eqns. (55) was proposed t o account for these results and for the observed second-order dependence on flash intensity, the latter coming from the requirement of the process for two photogenerated holes in steps (b) and (d), respectively. ZnO h,+ +

+ 2 hv

-

160;-

1 6 0 1 4 ~ 1 + 5 ~1 6~0 ;

2 h:/ Z n 0 2 -

(553)

160-

(55b)

1

-

__c

h: + (16014N15N160)-

(16014N15~160),1

16014N

+

(55c)

15N160

(55d) Figure 13(b) illustrates supporting evidence for the occurrence of m 1 6 0 1 intermediate a t the illuminated interface, as required by reaction (55c). This takes the form of a plot showing the progressive diminution of the mass spectral peaks of nitric oxide on the addition of species known t o compete for 0-, viz. carbon monoxide [202] or isopropanol [203], into the flow of 1 4 N ' 5 N ' 6 0 over ZnO a t low pressures. Those results under flash illumination not only confirmed the importance of 0; as a surface intermediate on UV-illuminated ZnO, but also pointed t o the important changes in product distribution and in intensity dependence which could accompany the high instantaneous flux of UV photons in the 5 0 p pulses attained with 2005 flash lamp, relative t o studies under continuous illumination. Studies of the influence of nitrous oxide on photodehydration of isopropanol over ZnO under continuous UV illumination showed N2 as the main product from N 2 0 at pressures of 1-102 N m T 2 .Figure 13(c) illustrates (i) a transient increase in the (-H2) product from isopropanol on commencing continuous illumination of the (CH3)2CHOH/Zn0interface and (ii) an enhancing effect of N,O on the size of this transient. i

2.2.3 Modifying effectso f surface dopants The neaessity for mabti&ung dioxygen, or another.gas-phase oxidant, at significant pressure emerged clearly in the work reported above, if continuing photocatalysed conversions of alkanes or alcohols were to be achieved over undoped T i 0 2 or other single-component solid catalysts. Recently, however, it has been shown that the presence of small particles of platinum dispersed on the surfaces of TiO, catalysts can make it possible t o dispense with the need for O2 or other gas-phase oxidant and

395

can promote photodehydrogenation of methanol vapour over the Pt/Ti02 catalysts at room temperature [207]. Mass spectral analysis of the gas phase in the static photoreactor demonstrated a photoassisted formation of H2 and HCHO as the major photoproducts forming with a quantum efficiency of ca. 0.45 from CH,CH vapour a t an initial pressure of 1-3 torr. Parallel and much more extensive studies on the production of H2 and other products from CI-C4 alcohols in the liquid state confirmed that the H2 originated predominantly via (-H2)* from primary or secondary alcohol and that such conversion proceeded to t.a.p.s.* > 100 only when T i 0 2 particles in the illuminated suspension had Pt (or Pd) dispersed on their surfaces. Pichat et al. [258] considered that Pt/TiO, fulfilled a dual role: (i) in transferring free electrons from the Ti02 support (presumably into collective-electron states of the Pt particle); and (ii) in assisting H2 production via migration of alcohol-related H-atom fragments between the support and the metal. Similar ideas have often been employed t o account for the electrocatalytic activity of Pt for H2evolution reactions. In a further extension of this analogy, Pichat et al. [ 2071 have remarked that the low oxidation-reduction potential of the HCHO/CH3OH, CH3CHO/C2H, OH couples, viz. approx. 0.19 V, make the dehydrogenation of alcohols a much less demanding reaction than dehydrogenation of water (see below). Large decreases in the limiting extents and rates attaching t o the photoassisted splitting of water, H 2 0 + H2 402,over UV-illuminated semiconductor surfaces have been noted for water vapour relative t o those attainable at liquid water/semiconductor interfaces [ 208-2141 . For example, Van Damme and Hall [214] estimate that the turnover number per surface OH group can be lower by four orders of magnitude at the gas/solid interface. They point t o evidence implicating direct photodecomposition of surface OH groups on TiO, as the process responsible for the production of H2 with low efficiency and low t.a.p.s.*. As yet, the origins of reportedly large enhancements in photoassisted water splitting when liquid water or aqueous electrolyte are in contact with the illuminated semiconductor surfaces appear unclear and, in any event, are outside the scope of this chapter. One aspect of such reports, viz. claims that surface-doped semiconductors, such as Pt/Ti02 or Pt/SrTi03 or R u 0 2 / T i 0 2 , are capable of yet greater efficiency, has, however, been tested for some gas/solid systems. Thus, Sato and White [215] have reported that the photoassisted water-gas shift reaction H 2 0 CO + H2 -I-C 0 2 , is genuinely photocatalytic over platinised T i 0 2 catalysts with a quantum efficiency of 5 x at 25°C. Comparisons of photoassisted water splitting over Pt/Ti02 with those over Ti02 showed that platinisation gave moderate enhancement ( x 3) in H2 production from water vapour. The mechanistic scheme (56) proposed for the water-gas shift reaction implicated such water splitting,

+

+

h+ -t H 2 0

-

References p p . 41 9-42 7

OH(ads)

+ H+

(56a)

396

h+

+ OH(ads)

CO(g)

-

-

O(ads) + H+

CO(ads)

-

+ OH(ads) CO(ads) + O(ads) H+ + eH(ads) CO(ads)

Pt

2H(ads)

Pt

(56b)

-

C02(g)

+ H(ads)

C02(g) (56f)

H2(g)

(56g)

It was noted that Pt/Ti02 prepared without H2 pretreatment showed much lower photocatalytic activity than Pt/H2-doped T i 0 2 . Recent studies by UPS and XPS of the effects of UV illumination on the chemisorption of 02,H2 and H 2 0 on reduced and stoichiometric SrTiO, (111) surfaces further highlight the crucial role of metal-excess non-stoichiometry within surface layers in determining photoactivity [ 2161. Thus, stoichiometric SrTiO, (111)surfaces were chemically inert towards H2, O2 or H 2 0 and were not photolysed t o surfaces with detectable Ti3+ after UV illumination for 17 h. This contrasted strongly with surfaces of black non-stoichiometric SrTi03 crystals having Ti3+ in the surface layers, which reacted very rapidly and extensively with O2 but less strongly with H, or H 2 0 . Photoregeneration of Ti3+ by subsequent exposure (at a flux of 1014-1010 photons ion-2) t o photons inside the band edge was observed after exposure t o O2 or H2 but, unfortunately, not after exposure to H 2 0 . The unfortunate aspect of this latter observation is the difficulty it introduces into envisaging continuing photocatalytic activity of H 2 0 ( g ) / SrTiO, interfaces through repeated photoregeneration of Ti3+, such as would be required for repeated photoassisted oxidation of H2O(g) over illuminated SrTiO, . The limited production of methane on UV illumination of an [H,O(g) C02(g)]/SrTi03 single crystal interface has been reported, but only in the rather special circumstances that a Pt foil was in contact with the rear non-illuminated face of the SrTi0, single crystal [163]. The attainment in such systems of continuing photoassisted production of methane, which is uphill in energy by 842 kJ mole-' relative t o H 2 0 ( g ) C02(g),would be of great importance as an artificial inorganic analogue of the endoenergic photosynthetic process in biological systems. However, the limited yield of methane so far reported from the [H20(g) C02(g)]/ (SrTi03-Pt) system was comparable in magnitude with the number of surface sites and so fell far short of the ideal of a photosynthetic process proceeding t o high t.a.p.s.*. The sensitising effect of platinisation for heterogeneous photocatalysis continues, however, to excite interest, e.g. photo-oxidation of hydrocarbons on platinised Ti02 has been reported [ 2171 . Consideration of strong metal support interactions (SMSI) in such cases is outside the scope of this chapter except t o note that metals other than Pt have been contacted with T i 0 2 .

+

+

+

397

Substances other than platinum have been contacted with TiO, surfaces during illumination in search of alternative promoters of efficient photocatalysed conversions or of endoenergic reactions at interfaces. Carbon has been advocated for this purpose on the grounds that, by acting as a scavenger for oxygen species from water splitting, wasteful recombination of oxygen with hydrogen may be decreased and more efficient photoassisted production of H2 achieved. On this basis, comparable photoassisted rates of production of H, and carbon oxides would be expected and have been claimed by Kawai and Sakata [218] from liquid water or water vapour over mixed catalysts containing RuO,, TiO, and amorphous carbon. Recent re-examination of photoassisted water vapour conversion in these systems utilised H, "0 and carbon-13 t o explore the origins of H, and CO, as the major products [219]. Analysis of the isotopic composition of the CO, product, which contained all possible isotopic species from 12C1602t o 13C1802, and of a minor yield of CO, led to scheme (57) as a representation of a sequence of events photoinitiated at a coordinatively unsaturated O2 - lattice sites and leading t o a carbonicacid like intermediate. f ?+\

13-

.

.

"nu-

.=

__

"OH-

/

Since mixtures of Cl6O, and H, ''0 admitted to the TiO, surface underwent insignificant isotopic exchange in the dark but were scrambled rapidly under illumination, this supported the idea that some such carbonic acid-like surface species could act as an intermediate for photoassisted scrambling of oxygen-18 between CO, and H 2 0 . If correct, mechanism (57) illustrates how photoinitiated interactions between a promoter and the primary photocatalyst can play a central role in photocatalysed reactions, just as thermally assisted interactions between promoters and primary catalysts can be important in normal catalysis. However, from their study of the reactions of water vapour with carbon and ethylene over illuminated Pt/TiO,, Sato and White [215] conclude that the mechanism and the sites involved in the photocatalysed oxidation of carbon observed in that system are not clear and that 0- may be attached t o carbon not titania. An interesting difference between the results from the carbon-promoted Pt/TiO, and Ru"+-TiO, systems was that the latter did not exhibit photoactivity for the water-gas shift reaction.

3. Effects induced by irradiation with high-energy photons or particles It is important t o recognise from the outset of this section that the majority of photoeffects considered above should also be capable of References p p . 41 9-42 7

398

operation under the influence of the higher-energy radiations now to be considered. This follows from the twin facts that (i) the photoeffects were initiated by excitation of the solid catalyst and/or the gaseous reactants into their respective electronically excited states and that (ii) such excitations account, in most cases, for about one half of the energy deposited into the systems by the higher-energy radiations. Generally speaking, any new radiation-induced phenomena will instead have their origins in’ different types of energy-depositing events or in differing spatial distribution of energy deposition. 3.1 ENERGY DEPOSITION AND LOCALISATION AT THE GAS/SOLID INTERFACE

Relative t o the photoassisted surface processes considered in Sect. 2, additional features can be expected t o arise under the action of highenergy radiations (such as y-rays, electrons, neutrons or a-particles at MeV energies) or other ionising radiations (such as X-rays, ions or electrons at keV energies). This expectation stems from the following new features in modes of energy deposition [220, 2211 t o gas/solid systems from these types of radiation: (i) all of these radiations produce ionisation as well as electronic excitation of atoms, molecules or ions located in regions of energy deposition. Consequently, additional features stemming from ionised states can be expected; (ii) the spatial distribution of energy deposition events along the path of an incident beam of highenergy radiation is much more inhomogeneous than from a beam of photons of energy < 8 eV, which typically generates one electronically excited state per photon absorbed and does not produce such states in close proximity, except in conditions of very intense illumination. With many high-energy radiations, on the other hand, each energy deposition event inherently produces many electronically excited and many ionised states in close proximity in localised regions of the irradiated medium which can be referred t o as “spurs”. Processes involving interactions between two or more radiation-activated states can occur within such spurs. Consequently, additional features stemming from such second- or higher-order processes can be expected if spurs occur a t the gas/solid interface; (iii) possibilities for selectively depositing energy either into the adsorbate or the adsorbent appear less favourable under high-energy radiations in view of the twin facts that the gas phase will not be transparent to high-energy radiations, whilst great depths of penetration into the solid are possible with such radiations. The extent of this difficulty can be expected to vary according t o the rate of linear energy transfer (LET) per unit length of path of the radiation. Radiations of high LET (such as a-particles or ion beams) are likely t o produce short columns of dense and overlapping spurs within the solid, and to a lesser extent in the gas phase, whereas radiations of low LET (such as 1MeV y-rays o r electron beams) penetrate deeply into the solid and produce spurs well

399

separated from one another; (iv) possibilities for displacement of atoms or ions from their original positions in surface regions of the solid lattice are greatly enhanced via momentum transfer from particle irradiations of high LET, threshold energies being low for such displacements, except for electrons where thresholds are ca. 1MeV. The occurrence of such displacements a t catalyst surfaces can be expected to alter the surface densities of active sites and so t o bring about changes in catalytic activity, which persist after exposure to particle irradiation. Energy-loss events of the types just enumerated initiate radiationinduced processes not only at the gaslsolid interface, but also in regions of the solid or gas phases distant from this interface [ 2591 . No general coverage of these latter processes is attempted in this section, which limits consideration to changes brought about by irradiation in the nature or kinetics of processes actually occurring at irradiated gas/solid interfaces. This limitation on the coverage to be attempted means that radiation-induced processses in regions of the gaseous or solid phases distant from the interface are of interest here only in as much as properties of the interface thereby experience modification, e.g. by migration thereto of energetic states or species produced by energy-deposition events distant from the interface. The extent of the contribution by processes of the latter type will, in turn, depend upon the efficiency, range and time span required for transporting t o the interface those excited/ionised states of the solid or gaseous phases which have the capability of altering surface processes. The relative weighting t o be given t o energy-deposition events distant from the interface will be strongly dependent upon assumptions concerning those factors and so will be difficult t o evaluate. An empirical approach often adopted t o estimate the extent of their contribution t o radiation-induced processes at the gaslsolid interface involves, as a first step, the calculation of the total energy deposited into the solid, ADsolid, e.g. expressed either as total electron volts (eV) deposited or as roentgen adsorbed dose (rad, where 1rad. s-' = 100 erg g-' s-' ). The total number of events of a particular kind brought about at the gas/solid interface by deposition of this total dose throughout the solid, e.g. AN(diss) for radiolysis of an adsorbed species, is used to calculate an apparent G value for the process, viz. 100 x AN(diss)/AD, expressed as eV deposited. Where such apparent G values exceed unity, significant contributions by distant energy-deposition events are indicated. Since the fraction of energy deposited at the interface is usually very small, it would be much more usual for the G value, which has the significance of the number of events of a gwen kind achieved per 100 eV deposited, t o have values 4 1 when calculated thus on the basis of energy deposited throughout the solid. 3.2 EXPERIMENTAL ASPECTS

Because of the elaborate precautions and attendent experimental difficulties arising from the need to protect personnel and equipment from Referencespp. 4 1 9 4 2 7

400

harmful effects of penetrating radiations such as neutrons, y-rays and high-energy (>1MeV) electron or ion beams, investigations of changes induced by such radiations have been evaluated much more frequently by measurements made after, rather than during, irradiation. Relevant techniques include: (i) ESR for the detection of paramagnetic centres produced by trapping of radiation-induced holes or electrons at the gas/ solid interface during irradiation; (ii) studies of the adsorptive capacity of the sample as determined by the amount taken up on the surface after irradiation; (iii) studies of the catalytic activity of the irradiated surface through measurements on the rate of a test reaction. Such techniques are favoured by powdered samples of high surface area and this experimental constraint resulted in many of the early studies on surface effects produced by high-energy radiations being made with powdered specimens [ 71 . Almost invariably, the surface conditions of such powdered samples were less well characterised than the single-crystal specimens examined by more recent surface spectroscopic techniques. Consequently, some uncertainties can arise as t o the possible roles of adsorbed impurities in modifying radiation-induced effects on powdered samples. In favourable cases, however, measurements made after irradiation can yield useful information on new post-irradiation interactions at gaslsolid interfaces, recent examples being provided by ESR observations reported on the formation of 0:- at the surface of powdered Ti02 samples on contact with low pressures of O2 after X-irradiation at 77 K [222] and on the formation of a variety of surface molecular ion radicals on contacting CO, oxygen or C 0 2 with H-Y type zeolites previously exposed to yirradiation [ 2231. The latter experiments yielded information on the location and reactivity of trapped-hole type centres present at the surface after y-irradiation and illustrate the application of conventional spectroscopic techniques to study post-irradiation effects of high-energy radiation. Conventional procedures of this type are not ideally suited t o the selective study of changes produced at gas/solid interfaces, since energy is deposited throughout the gaseous and solid phases. Furthermore, considerable complexities related t o the protection of personnel and equipment can arise when high radiation fluxes are employed and information is desired on effects during irradiation. By a fortunate coincidence, these difficulties can, in some cases, be overcome with the aid of equipment and procedures developed for modern ESCA techniques of surface analysis (Table 1). The primary objective of many of the incident radiations listed in Table 1 is ionisation and, in some cases, arrangements can be made for energy deposition t o be concentrated in regions close t o the surface. Small escape depths for emitted electrons mean, in any event, that these carry information mainly about species in the topmost few layers of the solids. The high sensitivities attainable in energy analysis of such electrons means that information can be obtained without the need t o

401

use very high fluxes of ionising radiations. Furthermore, the stainless steel containment vessels utilised as a vacuum envelope can, with appropriate precautions, reduce radiation levels to acceptable values during irradiation. Consequently, the UHV systems marketed commercially for surface spectroscopic examination of single-crystal specimens (cf. Chap. 2 of this volume) have resulted in marked increases in the ease and frequency of measurements detailing the changes produced in surface properties during exposure t o the radiations listed in Table 1. One typical application of these facilities is their use t o study the desorption of molecular species or ions from such surfaces as stimulated by an incident electron beam and detected simultaneously with a suitably placed mass spectrometer [ 2241 . The degree of surface coverage at galsolid interfaces on exposure t o small pressures of chemisorbing gases has been examined in this way [ 41, as also has the spatial distribution of adsorbed species over an inhomogeneous surface using a scanning electron beam [225]. It is also possible in iondesorption and UPS studies t o deduce the directionality of adsorbate bonding via analysis of the angular distribution of desorbed ions or photoelectrons, respectively. The desorption of adsorbed impurities through exposure t o argon-ion bombardment is another routine procedure in these UHV systems [ 2271 and can be monitored during irradiation by secondary ion mass spectrometry (SIMS) or, after irradiation, by AES examination of the surface. Changes in the degree of ordering of the surface after exposure t o ion bombardment can also routinely be monitored through LEED studies. Other radiation-induced changes in the physical properties of the surfaces of single-crystal specimens can be studied through the incorporation of special probes; for example, changes in work function can be detected after irradiation via contact potential difference measurements, whilst changes in surface conductance can be monitored with electrodes evaporated on t o the surface. A continuing expansion in the application of such techniques seems certain in view of their versatility in detecting and characterising radiation-induced changes in the physical properties of initially well-defined surfaces. 3.3 RESULTS AND INTERPRETATIONS

3.3.1 Effects o n adsorption-desorption processes during irradiation In a recent review of the extensive Russian literature on the effects of ionising and high-energy radiations on processes a t gaslsolid interfaces, Sokol’skii et al. [2] surveyed reports of enhanced chemisorption of hydrogen, oxygen and carbon monoxide on A1203 and other oxides during irradiation by X- or y-rays. They concluded that electrons and holes were the main agents for the enhancement of adsorptive capacity during irradiation and cited the large increase in conductivity which accompanied y-ray-induced chemisorption of hydrogen on ZnO at room References p p . 419-427

402

temperature as evidence for the capture of radiation-induced holes by the chemisorbing H2. Ionising and high-energy radiations can readily produce conduction-band electrons and valence-band holes, even for such wide band-gap oxides as Alz 0 3 .The view of these Russian workers was that migration of such non-equilibrium charge carriers to the surface could influence chemisorption. The initial rate of oxygen chemisorption was directly proportional t o the quantity of A1203 catalyst and radiation intensity but was independent of oxygen pressure. Chemisorption stopped when yirradiation ceased. Radiation-enhanced chemisorption of hydrogen obeyed the expression

which appeared consistent with activation of surface sites by radiation. Here, p is the pressure of hydrogen at time t, a is the surface density of hydrogen adsorption centres, Z is the dose rate of the y-rays, S is the total area of the catalyst and 8 is the fraction of adsorption centres occupied. 5.5 x 10” cm-’ was A maximum density of adsorption centres of attained under a dose rate of 350 rad. s-’. At similar dose rates, the quantity of ethylene taken up by the y-alumina surface increased during y-irradiation, whilst a volume of carbon monoxide was taken up almost completely. These differences demonstrate some degree of specificity in the gas-irradiated A1203 interactions. Effects reported on silica gels during irradiation by X-rays or y-rays offer evidence that factors other than radiation enhancement of the density of charge carriers may influence the adsorptive capacity in those systems. Thus Puncocharova et al. [ 2261 have considered that irradiation can cause vapour nucleation centres in capillary-condensed water and can cause a transition in the adsorbate from a metastable t o a stable state within pores of certain dimensions. Infrared studies by Ermatov and Koserov [ 921 indicated dissociative adsorption of water in H,O/silica gel systems under X-ray irradiation and, conversely, that irradiation with X-rays, y-rays and neutrons had dehydroxylating and dehydrating effects on the surface of the silica gel. The OH or H groups thus liberated from the surface may initiate chemical reactions (see Sect. 3.3.2) as well as leaving behind sites capable of enhanced adsorption. Results contrasting markedly with the foregoing and showing net radiation-induced desorption rather than adsorption, have been obtained not only in containment vessels subject t o intense beams of ionising radiations [ 1] , but also with well-characterised single-crystal surfaces exposed t o the radiations employed in modern surface spectroscopic techniques (cf. Table 1).The phenomenon of radiation-induced desorption from the walls of containment vessels has acquired new technological interest from the probability that plasma-induced desorption from the

-

403

walls of Torus or Tomahawk type accelerators for controlled fusion may present a serious source of contamination of the plasma [ l ] . This could include desorption through the action of electrons, X-rays and high-energy optical photons and those aspects of the topic have been treated in ref. 1. Desorption stimulated by electron beams a t high energy has also been reviewed recently [ 227-2291 showing its wide occurrence, drawing attention t o the very poor agreement on quantitative cross-section values, and indicating general agreement that the process involves direct interaction of the bombarding electron with the adsorbed species. In view of these recent reviews, coverage of those aspects of desorption from surfaces during exposure t o electron-beam irradiation is not again attempted here. Consideration is given, however, t o desorption effects during exposure t o other ionising radiations and particularly to important new results on the desorption of ions from surfaces under synchrotron radiation. A technique widely employed in the study of surfaces by UHV techniques is the use of a beam of rare gas ions at moderately low energies (300-1500 eV) t o remove adsorbed surface species, together with atoms in the top layer of the solid, by sputtering. The effects of such ion bombardment on the stay-time (i.e. mean residence time) of physisorbed xenon has recently been investigated [227] using a pulsed molecular beam of xenon incident on nickel surfaces simultaneously with 300 eV ions at a flux of - 3 n A . The stay-time of xenon from the pulsed molecular beam on the cooled nickel surface a t temperatures of 92-125K was reduced by ion bombardment, typically from 2 x t o 2.5 x s at 111K by exposure t o He' or Ar+ ions for 1h. The argument was made that, since this flux corresponded to only one in ten surface atoms being struck over the 1h period, the probability for direct knock-on displacement of xenon would be too small t o account for the significant reduction in stay-time. The result was considered to be more consistent with the removal of surface oxide by sputtering, since higher binding energies for xenon had been reported for oxide-covered tungsten than for the clean metal. Just as the greatly increased use of collimated electron beams for LEED and Auger studies of surfaces has allowed many observations on electronstimulated desorption, so t o o has the development of beams of low-energy X-rays for X-ray photoelectron spectroscopy (XPS) led to observations on resultant desorption processes. Recently, Franchy and Menzel [ 2281 reported the detection of ionised desorption products, including H+ and O', entering a quadrupole mass spectrometer as desorption products from stainless steel or tungsten surfaces during irradiation by AlK, X-rays of energies < 12 keV. Desorption probabilities were < ions photon-' for these ionic species and a preliminary observation was made on the desorption of H, as a neutral species released during the X-ray irradiation. Observations of neutral species desorbing under ionising radiations encounter greater difficulties and are less well characterised than for ions. References p p . 419-427

404

A mechanism for the desorption of positive ions from adsorbed layers on surfaces has been proposed by Knotek and Fiebelman [229] which predicts similar ion desorption from ionically bonded species at surfaces by either electrons or photons of sufficient energy t o create core holes. Interatomic Auger decay then produces a positive charge on the surface species, which experiences strong repulsion from substrate cations in their maximum valency state, e.g. 0' ions repelled from W6' ions of W 0 3 . Recent studies by Madey et al. [230, 2311 have demonstrated that ionyield plots of 0' from W ( 1 1 1 ) shows similar correlation with core-hole binding energies for tungsten atoms whenever stimulated either by photons of energies 20-120 eV from a synchrotron (photon-stimulated ion desorption, PSID) or by electrons of energies ca. 550eV (electronstimulated ion desorption, ESID). Furthermore, these studies show similar angular distribution of the 0' ions from PSID or ESID. Plots of 0' ion yield versus energy of the photons selected from the synchrotron radiation were dominated by peaks at - 4 5 and -55eV, irrespective of whether PSID of 0' originated from a W 0 3 oxide layer, from a monolayer of oxygen on W ( 1 1 1 ) or from 0.5 monolayer coverage. This led Madey et al. t o conclude that W6' species were present even in monolayers or fractional monolayers of oxygen, since the Auger decay model of ion desorption requires maximal valency for the cationic species. For the desorption of 0' from the oxide layer, ion yields were low both for PSID and ESID, viz. 3 x ions photon-' at hv = 55 eV in PSID and ions electron-' at 500 eV. A twenty-fold reduction in the 1x PSID yield of 0' (or possibly OH', which was not distinguishable from 0' by the time-of-flight method used t o identify ions) from an oxide layer resulted from exposure of the clean oxide t o H atoms. Photoexcitation of these surfaces gave PSID of H+ with a dominant threshold at lower values (220 eV) than for 0' but the angular distribution patterns were similar. This led to suggestions of a linear surface W-0-H species with H' desorption proceeding via excitation of a surface OH bond, possibly by an 0 (2s) core hole (- 22 eV) with excitation of the W substrate playing a lesser role than for 0' (or OH') [ 2301 . Madey [231] in an elegant set of experiments with a polyhedral tungsten crystal having a central W(110) facet and four flats with orientations close t o that close-packed surface, has also examined the role of steps and defects in the ESID of 0' at various oxygen coverages. He concluded from the study that: (i) there was little 0' ESID from a flat W ( 1 1 0 ) surface even at high coverages; (ii) the surface sites from which ESID originated were predominantly located at steps and defects; (iii) ESID of 0' was very sensitive t o local site geometry; (iv) the temperature of adsorption exerted a major influence on the details of the adsorbed structures and resultant ESID patterns. These conclusions are strongly supportive of those drawn by Somorjai [36] concerning the importance of several aspects of adsorbate-active site interactions in activating stepped crystals of Pt for various chemisorption and catalytic processes.

-

405

3.3.2 Chemical effects during irradiation Just as prompt adsorption or desorption effects during irradiation served to signal rapid conversions of radiation energy in the systems considered above, so should observations of prompt chemical reactions under irradiation serve as signals for rapid transformations of radiation energy into Eact. Prompt radiation-induced chemical effects should ideally be distinguished from slower effects by the use of short radiation pulses and fast detection techniques. However, the majority of reported observations on chemical effects at gas/solid interfaces during irradiation derive from procedures lacking any time resolution. As such, they represent the net effects, not only of primary and secondary radiation-induced chemical changes, but also of any subsequent changes capable of operation during the period for which the system is continuously exposed t o irradiation. Notwithstanding this deficiency, experimental observations on the overall chemical effects during irradiation have sometimes been made the basis for interpretations seeking t o establish the nature of very early events in the conversion of radiation energy into EaCt.An interesting case in point concerns the interpretations advanced for correlations claimed t o exist between the efficiencies of radiation-induced chemical changes at gas/solid interfaces and the width of the forbidden band gap of the solid [ 21. Thus, large differences have been reported in the relative extent t o which radiolysis of such adsorbates is promoted by various metal oxide surfaces, e.g. ZnO with a band gap of 3.2 eV significantly accelerated the y-radiolysis of adsorbed H 2 0 , whereas V 2 0 5 with a band gap of 0.5 eV produced little or no enhancement under similar conditions [ 2321 . Such observations have led t o claims of a useful empirical correlation between the band gap of the solid and the extent t o which it can promote radiolysis of adsorbates. Indeed, Sokol’skii et al. have alluded in their review [ 2 ] t o a “universal character” of this correlation. They have instanced the sequence SiO, : Al, O3 : ZrO, : MgO > ZnO: ( Al, 03,SiO, ) > NiO > Ni > Pt/SiO, , in which yield of cyclohexane dehydrogenation on the various solids under continuous y-irradiation (3.4 x 10l6 eV g-’ s - l ) was reported t o decrease as the band gap decreased. This sequence is inverted relative t o order-ofmagnitude activities of the catalysts in thermally assisted dehydrogenation. Despite the deficiency noted above, postulates advanced as reasons for existence of such correlations were: (i) that the activation of an adsorbed cyclohexane molecule proceeds from the recombination of a free electron with a radiation-generated hole, initially trapped by the adsorbate; and (ii) that the energy released in such a recombination should relate t o the width of the forbidden band gap minus the heat of adsorption. Relevant radiation-initiated processes may be summarised as MX

-

C6H,,(ads)

+ h+

Referencespp. 419-427

+ h)/MX

-

e-, h’, (e

C6H:,(ads)

(59b)

e; 4-C6H:2(adS)

-

C6H:,(adS)

-

C6H10

+ HZ/MX

(59d)

and are likely t o be completed within s. It has been argued that eqn. (59d) would result in the release of the largest recombination energies to the cyclohexane molecule on large band gap dielectrics (e.g. SiO,, A1203, MgO), the smallest recombination energy on metallic catalysts, and intermediate energies on semiconductors of medium band gap (e.g. ZnO). Whilst espousing the universal character of such mechanisms, Sokol’skii et al. [2] noted the possibility of serious competition between them and charge recombination a t other local surface centres, such as impurities or defects, Overall efficiencies would undoubtedly be affected by competition from other electron-hole recombination processes [ 1961 , but it is improbable that the empirical correlation between the efficiency and the band gap of the solid could arise in that way. A more probable origin can be suggested from a more detailed consideration of processes similar t o (59d) and in particular from considerations of how non-radiative decay of the excited state produced by electron-hole recombination may vary with band gap. A basis for these considerations is provided by the “energy-gap law” for radiationless transitions for which there is theoretical and experimental support in molecular systems [ 233, 2341. According t o this energy-gap law, the radiationless decay constants, k,,, for a series of excited states based on the same chromophore are determined by the vibrational overlap between ground and excited state and can increase exponentially with decreasing energy gap between these states if certain conditions are satisfied [233]. Good agreement with the law, with k,, varying over two orders of magnitude, has been demonstrated recently for a series of osmium(I1) complexes in which the relevant transitions had metal-to-ligand charge transfer character [234]. By analogy with such systems, if electron-hole recombination a t the surface of an irradiated metal oxide is envisaged to produce an excited state a t the surface as represented in the first step of (59e), it is reasonable t o suggest, that k,, for the radiationless decay represented in eqn. (59e) will increase exponentially with decreasing energy gap for charge transfer between an oxygen anion and the metal cation (i.e. the band gap of non-transition metal oxides). e-

+ (M:’

-0;)

--+

(M,Z+-o:-)*

k,_ (M:+. . . OZ-)

(59e) The lifetime of the excited state shown in eqn. (59e) should, in consequence, be much larger for large band-gap oxides such as A1203 or SiOz on MgO, than for oxides having smaller band gaps. The former will thus have greater possibilities for interaction of the excited state (prior to its non-radiative decay) with an adsorbate, e.g. with cyclohexane via an

407

exciplex as in

( M F + . . . Of-)*

+

Z +-

of-) *

(M:+L

0:-1

(59f)

Whilst the intervention of radiationless decay in the manner of eqns. (59e) and (59f) would be consistent with the correlations claimed by Russian workers, such ideas must be regarded as tentative until a more searching test can be made, preferably using techniques with built-in time resolution. However, it is worth noting, that if the radiationless decay process were itself t o be selected as the vehicle by which the energy needed t o drive dehydrogenation was transferred into an adsorbate such as cyclohexane (in analogous fashion t o the transfer into vibrational modes of the ligands in the series of osmium (11) complexes mentioned above [ 2341 ) the efficiency of that process would be predicted t o increase with decreasing band gap of the oxides. Various sequences of radiation-initiated processes leading t o the dissociation of adsorbed methanol by ionising radiations have been proposed by Russian workers [81, 2351. Zhabrova et al. [81] suggested that positively charged species created by the ionising radiation became localised on chemisorbed methanol and that annihilation of this centre through recombination with an electron resulted in the formation of hydroxymethyl (CHzOH) radicals which could either combine to yield ethylene glycol or dissociate t o yield formaldehyde. Their comparisons of the enhancing effect of various solids upon methanol radiolysis, including a conceptually interesting comparison of graphite and diamond, were interpreted in terms of this proposed mechanism. A one hundred-fold intensification of the radiolysis by diamond (band gap 7eV) and the lack of significant enhancement by graphite were interpreted in terms of the aforementioned correlation of radiation catalytic activity and band gap. An important point not fully resolved in that study was the identification of what exceptionally favourable conditions existed at the diamond surfaces t o make them ten times more effective than materials of comparable band gap (e.g. SiOz or A1203 on which the decomposition of methanol under condinuous y-irradiation exceeded the rate of homogeneous radiolysis by a factor of only ten). It is also puzzling in the context of the claimed correlation with band gap that the activity of the wide-band oxide MgO (band gap 8.7 eV) was rather similar t o ZnO or NiO, which have much smaller band gaps. Infrared studies of silica gels exposed t o y-irradiation in the presence of methanol or water vapour have been differently interpreted by Ermatov and Koserov [ 921 who proposed radiation-induced dehydroxylation and dehydration of the surfaces followed by the reaction of adsorbate with the surface sites so produced. This References p p . 419-427

408

concept of reaction between adsorbates and surface sites activated by ionising radiation also emerged from the work of Vedrine e t al. [225], who in their ESR studies of y-irradiated zeolites of the H-Y type, found that the presence of adsorbed H,O or NH3 on the zeolites during y-irradiation prevented the build-up of detectable concentrations of trapped-hole (V-type) centres, but that, instead, surface OH or NH, radicals appeared. The radicals were not observed except when H 2 0 or NH3, respectively, was present during irradiation. The intensity of the corresponding ESR signals was stated t o be a t a maximum for monolayer coverage. Adsorption of gaseous reactant on, or immediately adjacent to, the precursors of the V centres and subsequent reaction with the V centres during irradiation was envisaged as the mode of formation of surface OH and NH2 radicals. The absence of adsorbent-related paramagnetic species from H2 O/SiO2 systems after y-irradiation has likewise been interpreted in terms of the reactions of radiation-induced surface centres with adsorbed H 2 0 . This gains support from reports of the expected converse process, viz. increases in the yield of hydrogen product from these H20/Si02 systems relative to homogeneous systems similarly irradiated. In the terminology introduced in Sect, 2.1, initiation of these processes may be attributed t o active site charge-transfer (ASCT). Photoinitiated oxygen isotope exchange (OIE) in mixtures of (1602 I8O2) over an irradiated metal oxide was considered in Sect. 2.2.2.(a) as one of the simpler representative cases of the radiation-induced cleavage and rearrangement of bonds other than the adsorbate-adsorbent bonds. Recent work by Trokhimets e t al. [237] showed that exposure of (I6O2 1802)/A1203interfaces t o soft X-irradiation a t room temperature produced a rapid constant rate of OIE which was greatly enhanced relative t o non-irradiated interfaces. Those results were interpreted in terms of the creation of a radiation-induced, steady-state concentration of surface sites active for exchange. They would be consistent with a mechanism, such as eqn. (35), driven by ASCT following localisation of radiationinduced carriers on pre-existing defects. The activity of a pre-irradiated interface diminished rapidly after the cessation of irradiation or could immediately be eliminated by contact with H 2 , as would be consistent with spontaneous decay of the trapped charges through recombination or, alternatively, through reaction with H2. Dehydrogenation and dehydration processes at irradiated alcohol/ semiconductor interfaces has been envisaged by Vol’kenshtein and coworkers [8,15, 2361 as being dominated not by ASCT but by collectiveelectron properties of the solid adsorbent. The essential first step towards dehydrogenated product consists, in their view, in the cleavage of the RO-H bond of the alcohol, which they class as an acceptor-type reaction (i.e. one which will, in terms of a collectiveelectron theory, be accelerated by increasing availability of electrons at the surface). The essential first step towards dehydrated product is envisaged as cleavage of the R - O H

+

+

409

bond, which is classed as donor-type reaction (i.e. one accelerated by the availability of holes at the surface). Contact potential difference measurements were utilised by Spitsyn et al. [236] as a means of evaluating radiation-induced increases in the availability of electrons at the surface of an yttrium oxide semiconducting catalyst, as caused by the incorporation of radioactive 91 Y isotopes. Parallel measurements on the influence of the incorporated 9 1 Y (which emitted 0 rays at 1.55meV and X-rays at 1.2 meV) on dehydrogenation and dehydration indicated that the rate of the former was progressively enhanced by increasing the specific activity of t h e . sample, whereas the rate of the latter progressively declined. Qualitative agreement was claimed with the requirements of what would be classed here as a collectiveelectron charge-transfer mechanism for simultaneous alcohol dehydration and hydrogenation. However, the following should be noted: (i) that experiments were carried out at 640-690 K, since only in that region was the static charge formed during radioactive decay considered t o flow entirely from the sample, and that many metal oxides exhibit high catalytic activity for thermally assisted elimination reactions of alcohols in that temperature range via more conventional Lewis acid-base catalysed mechanisms [ 2371 ; (ii) that since severe difficulties can arise, as detailed elsewhere, in distinguishing definitively between the operation of ASCT and CECT, the data available on the yttrium oxide d o not exclude the possibility that ASCT predominates in charge transfer a t the irradiated oxide surface, whilst collective electron factors exert an important influence on the occupancy of active sites by radiation-induced electrons or holes and on the lifetimes of such occupied states. The enhancing effects of y-irradiation on the methanation of carbon oxides over supporting ruthenium catalysts have been reported by Gupta et al. [239]. Using a microcatalytic reactor with H2 carrier gas and carbon oxides introduced in either pulsed-reactant or continuous-flow modes [e.g. see Fig. 10(b)] they demonstrated that in situ y-irradiation enhanced the turnover of CO or C 0 2 t o methane for temperatures 400600K. Figure 14 illustrates their results in continuous flow mode for methanation of 2%COz in H2 over two different catalysts, viz. ruthenium supported on alumina (RA) or on molecular sieve (RM). Progressive slow growth during 30-90 min irradiation characterised the effect and resembled similar slow growths to steady-state photocatalytic activity in other systems. Enhancement by y-irradiation depended upon temperature and the ratio of steady-state activity with and without y-irradiation was greater at lower temperatures (e.g. the ratio equalled 18 over RM at 400K, but 3.5 at 425K) in a manner consistent with a reduction in activation energy of the overall process under irradiation (e.g. a reduction of Eact from 7.3 t o 4.2 kJ mole-' over RM). Gupta et al. concluded that radiolysis of COz was not important. They favoured the radiation-induced acceleration of the rate of reaction of COz with H,, possibly through References p p . 41 9-42 7

410

Time ( m h l

Fig. 14. Data illustrating effect of y-irradiation o n t h e methanation of carbon dioxide over supported Ru catalysts comprising ruthenium o n alumina (Ru/AI) o r ruthenium in molecular sieve (Ru/M). Effects of 7-irradiation a t the indicated temperature on the growth of methane product observed from a continuous flow of C 0 2 in a Hz carrier gas over (i) Ru/M and (ii) Ru/AI. Note the growth in methane yields from C02-H2 reaction at different temperatures as a function of y dose and its decay with time subsequent to removal of the catalyst from t h e y-source.

some weakening of the bonding of a surface ( R u - € 0 , ) complex via a mechanism not fully resolved, following the thermally induced transport of radiation energy from the support material t o the ruthenium. The question of energy transfer from deep within the adsorbent to the surface to effect chemical changes in the adsorbate, emerges yet more strongly from recent studies of the y-radiolysis at 7 7 K of methane mainly physisorbed on a range of y-alumina samples previously outgassed above 5 7 0 K . Norfolk has interpreted these observations in terms of the existence of exposed lattice ions on to which methane can become chemisorbed during y-irradiation, via a VCI or Eley--Rideal-type process, to produce surface precursors of methane or of C, and C3 hydrocarbons [ 2381 . Subsequent thermal desorption of hydrocarbon material from the irradiated y-Al,03 then yielded a mixture of C, t o C, alkanes and alkenes, which was expressed as total product carbon (TPC) given by

411

+

+

+

[chemisorbed CH4 2(C2H4 C2H6) 3(C3H6 iC3H8)].Typical plots of the yield of TPC desorbed t o the gas phase as a function of radiation dose and of coverage by physisorbed methane are illustrated in Fig. 15(a) and (b): respectively. Evidence that the radiation energy deposited within the bulk contributed to the conversion of physisorbed methane came from observations that plots of desorbed TPC as a function of the adsorbed dose had slopes which corresponded t o a yield of G(TPC) 2.0, i.e. to the radiation activation of 2 sites per lOOeV absorbed from the y-rays by the entire bulk of the A1203 sample. (If only energy absorbed directly be adsorbate had yielded TPC, that would have required one conversion per 0.02 eV.) An important feature of the proposed radiationinduced reaction sequence was the migration t o the surface of free charge carriers or excitons created within the bulk by irradiation. The formation of activated sites through hole capture on exposed oxygen sites and electron localisation on exposed cations was envisaged as in eqn. (60a) followed by dissociative chemisorption of methane as in eqn. (60b) during irradiation and by desorption of TPC during subsequent heating as in eqn. ( ~ O C ) viz. ,

-

A diminution in TPC yield when methane was added after, rather than during, irradiation [cf. Fig. 15(c)] was taken t o indicate that the first step and at least part of the second take place during irradiation, as would be consistent with a radiation-assisted Eley-Rideal or VCI-type process. Qualitative comparison of the yield G(TPC) = 2.0 with G(ion pair) X 3 led to the conclusion that, in favourable circumstances, up t o two-thirds of the excited charge carriers generated throughout the Al,03 sample by y-irradiation could be available for adsorbed phase radiolysis, thereby implying efficient energy transfer from y-A1203 t o adsorbed methane under y-irradiation at 77 K. Factors identified as affecting the yield from adsorbed methane radiolysis included the surface area, the surface density and distribution of exposed anion sites, the rate at which excited charge carriers generated in the solid reach the surface, and the availability of sufficient adsorbed methane t o react with such charge carriers. The operReferences p p . 4 1 9 4 2 7

412

1c

8 c

Y

-

u)

0

Y

0

x

(U

0

E -5 U

a

+

0

I

2

6

4

Radiation

dose

0

(Mrad)

I

300

700 500 Desorption temperature ( K )

413

ation of similar factors had been suggested by previous workers who also noted high efficiencies for the chemical changes brought about in adsorbates by high-energy irradiation of high-surface-area adsorbents. Whilst such results on radiation-induced processes imply efficient energy transfer t o the gas/solid interface from within the solid where much of it was deposited, definitive evidence is still lacking as to the nature of the energy transfer process(es), e.g. whether by processes (lo), (11)or (12) or others. The resolution of this question represents a challenging problem. 3.3.3 Effects persisting at the interface after irradiation The changes in physical properties of surfaces which persist after irradiation for times much longer than the expected decay times of the primary radiation-induced species have sometimes been referred t o as “memory” effects and are considered in this subsection for samples exposed t o ionising and high-energy radiations. It is instructive t o recall that memory effects were rare for photoinduced processes at gas/solid interfaces and generally were explained as a consequence of the trapping of photoinduced charge carriers by surface defects where they remain available t o promote charge-transfer processes with gases subsequently admitted (e.g. the promotion of oxygen isotope exchange by surface-trapped electrons). Sokol’skii et al. [ 21 concluded in their review article that similar processes are the main agents for post-radiation enhancements of the adsorptive capacity of oxides, including alumina and silica gel. Thus the appreciable ability of silica gel to adsorb hydrogen, which persists after y-irradiation, has been correlated with the persistence of hole-type colour centres which were removed on hydrogen adsorption. The adsorptive capacity of irradiated silica gel was reported t o increase with the extent of its contamination by aluminium. A subsequent study of y-irradiated aluminosilicates in the form of the H-Y zeolites confirmed the ready formation of V-type paramagnetic surface sites under irradiation and their neutralisation when H2 was later admitted [ 2231 . Enhanced post-irradiation adsorption of 0,’ CO and CO, could be interpreted in terms of their adsorption on the pre-

Fig. 15. Radiolysis of methane adsorbed on y-alumina expressed in terms of total product carbon (TPC, chemisorbed CH4 4- 2(C2H4 C2H6) + 3(C3H6 + C ~ H S ) ] . (a) Variation in TPC yield with radiation dose delivered to the CH3/A1203system for samples previously outgassed at V , 623 K ; 0,673 K ; A, 723 K ; or 0, 928 K. ( b ) Variation in TPC yield with coverage by physisorbed methane on similarly pretreated A1203 samples dosed with methane and then y-irradiated at two different dose rates of 0,1.1 and 0 , 0.29 M h-’ . (c) Comparison of TPC desorption curves for 3 equivalently pretreated A1203 samples to which methane was added: 0 , before y-irradiation at 77 K ; 0 , after y-irradiation at 77 K but before warm-up; after y-irradiation at 77 K and warm-up to 300 K for 1h.

+

*,

References p p . 419-427

414

cursors of these V-type centres, either a t the lattice oxygen or in its immediate proximity. The resultant ESR spectra have been assigned to 0; and GO+ radicals with the structures

Results of the type just described for previously irradiated powdered samples containing SiOz and/or A1203 thus appear consistent with the operation of mechanisms involving charge localisation a t surface sites during irradiation and with subsequent charge transfer t o chemisorbing gases. The rate of publication of papers dealing with the changes in catalytic activity of powdered metal oxide samples after their exposure t o ionising radiations had declined in the past decade relative t o that comprehensively detailed by Taylor [7] through July 1967. Two of the most striking and reproducible post-irradiation effects evident a t that time concerned a greatly enhanced rate of H2/D2 exchange on silica gels after irradiation and a concomitant decrease in activation energy from 37 mJ mole-' with unirradiated t o 8 k J mole-' with irradiated samples. Criticisms levelled at other systems, on the basis that surface poisons were inadequately removed prior t o irradiation, did not appear to apply t o the silica gel system, since the enhancement of activity for the H2/Dz exchange could be reproduced on samples outgassed at high temperatures. Taylor [7] set out the pros and cons of assigning the enhanced activity t o the trapping of radiation-generated holes at surface locations where they remained available for catalysing Hz /D2 exchange after irradiation. Clarification of the nature of trapped-hole-type centres involving surface 0- species and transition metal ions dispersed on the surfaces of silica gels has come from subsequent studies by Kazanskii et al. [74]. The formation of an active surface 0- species by hole capture during y-irradiation of Mo6+ so dispersed was represented as

which includes a second metal ion of unspecified coordination as the corresponding electron trap. The ESR signals of the 0- species thus produced from V205/SiOz,Mo03/Si02 and W03/SiOz has similar magnetic parameters and structures t o radicals produced by the decomposition of nitrous oxide on prereduced surface sites according t o 0 0 ,,oNM06+.' *M05' ... + N20 + N2 '0

'0

-

'0

' 0

415

The radical anion 0- when produced in the gas phase is characterised by high reactivity for H-atom abstraction reactions and the species formed by the above reactions react rapidly in this manner with H2, NH3, CH,OH, CH4. It has been proposed that the reaction with H2 produces H-atom-like species which are active for H2/D2 exchange. Some characteristics of adsorption chemiluminescence on y-irradiated silica gel, alumina gel, NaY zeolite and other silicates have been reported by Russian workers [ 240, 2411 who utilized samples activated in vacuo a t 670 or 770K. A high dose of y-irradiation (13Mrad) was delivered at room temperature to the vacuum-activated samples, after which gaseous 02,C 0 2 , H2 or NH3 was admitted rapidly to pressures typically in the range 660-8 x lo3 Nm-2. The total light emission from a particular material was greatest on admission of H2 and was usually a t least one order of magnitude less with 02.However, very low efficiency of the emission process was inferred from the fact that the number of photons collected were factors of t o lo-'' lower than the number of paramagnetic species detected in the irradiated samples by ESR. Factors other than the trapping of radiation-generated charge carriers at pre-existing surface defects have been emphasised in the interpretations of catalytic effects persisting for long times after irradiation by neutron or high-energy particles. Sokol'skii e t al. [242, 2431 compared the catalytic activity for the hydrogenation of ethynyldimethyl methanol and butynediol exhibited by palladium and platinum blacks prepared by H2 reduction of irradiated and non-irradiated samples of the corresponding metal oxides. Higher activity was found for blacks prepared from the neutron-irradiated samples even more than a month after irradiation. The differences later noted between the electrochemical charging curves of electrodes coated with blacks prepared from neutron-irradiated or non-irradiated oxides were interpreted in terms of the increased dispersion and increased heat of H2 adsorption for the pre-irradiated samples [ 2441. Associated increases in extent of surface coverage by hydrogen were postulated as the origin of enhanced catalytic activity. The manifestation of these effects, even after H2 reduction of the neutron-irradiated oxides, could, in part, be understood on the basis of the extensive production of vacancies and other structural effects in the oxides by neutron irradiation, with consequent influence of such defects on the dispersion and density of defects on the Pd-black or Pt-black formed from the neutron-irradiated oxides. The radiolytic oxidation of porous reactor moderator graphite in C 0 2 represents another effect induced by neutron irradiation in which the radiation-induced formation of structural defects is proposed t o play an important role [245]. It has, however, been demonstrated that neutron irradiation also affects the electronic properties of various carbons [ 2461 and one interpretation placed on such effects was that a dose of 1neutron per cm2 created four electronic holes in the valence band per cm3. The References p p . 419-427

416

resulting complex possibilities for the radiation-induced modification of the surface (including, on the one hand, energy deposited in the bulk but transferred t o the surface quickly by electronic transfer processes or slowly by defect migration and, on the other hand, energy deposition into surface regions) makes this a difficult area of investigation on which specialist texts should be consulted. Interpretations envisaging that highenergy radiations not only lead t o the ’trapping of charge carriers at preexisting defects, but also generate additional surface defects which can further enhance post-irradiation adsorption at the gas/solid interface, appear consistent with several correlations between defect sites and chemisorption : (i) sulphur ion vacancies on CdS surfaces have been identified as sites for radiation-enhanced oxygen chemisorption [ 2471 ; (ii) increased nucleation of “islands” of Au or Ag from the gas phase on t o previously irradiated alkali halide single crystals [248] ; (iii) initial sticking coefficiencies of oxygen on tungsten (110) faces have been shown t o be increased by a factor of 3.6 in the presence of monatomic steps [ 2491 ; (iv) defects or other extraneous sites contribute to the higher binding energies a t 6’ < 0.2 for argon on tungsten [ 7 1 ] . Reliable evidence concerning the types and surface densities of defects and how these are affected by irradiation will, in general, require the use of singlecrystal samples and more sophisticated procedures than utilised for the foregoing studies on powdered solids. In conclusion, the advantages of a coordinated application of modern surface spectroscopic techniques t o study changes in the physical properties of the surfaces of ZnO single crystals after irradiation may be illustrated by recent results. Thus, Margoninski and Eger [ 2501, by their comparisons of the surface conductance or the LEED patterns from the (000%)oxygen-rich or the (0001) zinc-rich surface before and after Ar-ion bombardment, showed parallel decreases in surface resistance and degree of order in the surface layer t o limiting values. Bombardment with He’ Or H+ ions extinguished the LEED pattern and further diminished surface resistance. This added efficiency of the lighter ions was interpreted in terms of their greater penetration into the ZnO lattice with the creation of donor centres and disruption of order. Characteristic low-energy loss spectra at electron energies 2-30eV were also taken on the (OOOi)/‘O’ face and (0001)/‘Zn’ face before and after argon-ion bombardment and/or exposure to hydrogen atoms. The strong similarities of the ELS spectra from the two different faces a t 2.8 eV were interpreted as indicative of a vibration characteristic of surface ZnO molecules rather than of a dangling bond state on a surface ion. Changes in the ELS spectra were difficult t o disentangle from a marked sensitivity of the surfaces towards very small exposures t o hydrogen in the initial study. A more recent study [251] sought correlations between the changes brought about in the surface

417

conductivity of polar ZnO faces by exposure t o H2 or O2 and the simultaneous changes in the corresponding ELS spectra in the range 3-20eV. Correlations were found for the ELS peaks at 5.3 and 14.3 eV, indicating that surface states at these energies contribute electrons t o the surface conduction band, but no correlation was found for the band at 3eV. In another study of the post-irradiation changes in the physical properties of ZnO single crystals, Holmstrom et al. [252] detected changes, after prolonged exposure t o 3 keV electrons, in the AES spectrum and also in the work function of the surface. Radiation-induced changes in the Auger peaks for carbon and zinc were interpreted as evidence for the ESD of carbon and for the production of excess zinc through radiolysis of the ZnO surface. The latter was attributed t o the production within O.1pm of the surface of ca. 100 electron-hole pairs per incident 2 keV electron and t o surface decomposition by the high density of radiation-generated holes. Post-irradiation decreases in the work function were likewise interpreted in terms of the accumulation of positive charge in the surface layers.

4. Perspectives and prospectus In the author’s view, the work surveyed allows identification of the following advances during the past decade. Firstly, from the point of view of experimental procedures brought t o bear on the problem, there has been a marked and welcome increase in the applications of UHV procedures and surface spectroscopic techniques. This has allowed better definition of the surface condition of the solid catalysts prior t o and after irradiation and has made possible, in favourable single-crystal cases, the identification of surface defects, impurities or topographical features which may act as active sites. The expanded use of such techniques is desirable in future work in the search for a definitive correlation between the radiation-induced activity of surfaces and the measured concentrations of defects on the surfaces. Secondly, from the point of view of the models utilised t o account for radiation-induced processes at the gaslsolid interface, early and somewhat antagonistic use of electronic or active-site concepts of surface activity has been supplanted by the recognition and simultaneous utilisation of both concepts as mutually complementary. Thus, changes in surface activity persisting after irradiation have generally been explained in terms of “active-site charge transfer”, which envisages electronic-type localisation of radiation-induced carriers at special surface traps which, however, also serve as active sites for charge-transfer to adsorbing gases. Expanded use of these “active site charge transfer” concepts should prove profitable for effects during irradiation. Thirdly, it References p p . 41 9-427

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has been increasingly recognised that the usually high degree of coordinative unsaturation associated with non-regular surface sites carries with it strong implications for the modification of electronic energy levels and reactivity associated with such sites. For metal oxides, and some other binary non-metallic solids, irradiation can promote such sites into excited states with new and interesting surface reactivities. The emergence of surface-state models and theories of electronic configurations and energy bands at surfaces appears t o offer a new framework within which the enhanced reactivities of such sites in their ground and electronically excited states will increasingly be understood. Fourthly, from the kineticist’s point of view, attention has increasingly been drawn t o mechanisms in which the rate-determining process a t pressures approaching 1atm involves interaction between a radiation-activated site and a gaseous reactant encountering the activated site either directly from the gas phase or whilst in a weakly adsorbed molecular form. Such radiation-induced Eley-Rideal or VCI mechanisms contrast with the predominance of Langmuir-Hinshelwood-type photoassisted processes involving strongly chemisorbed species at low reactant pressures. Further kinetic study and exploitation of such photoassisted Eley-Rideal-type mechanisms at appreciable pressures can be expected in view of the indications already received that, relative to Langmuir-Hinshelwood processes, a greater fraction of surface sites can become activated for Eley-Rideal processes under irradiation and that these may be less susceptible t o poisoning. Fifthly, although progress in characterising and understanding the radiationinduced desorption of neutral species has been disappointing, a satisfactory mechanism for the desorption of positive ions has been developed and gives ground for optimism that radiationless processes involved in the desorption of neutrals may eventually become better understood. A lack of comparable advances in other important areas may also be discerned and it is to be hoped that these may receive increased attention along the following lines in future work: (i) efforts t o time-resolve, and t o display simultaneously, changes in electronic and in other surface processes through the use of pulsed irradiation procedures coupled t o fast detection techniques. Some particular difficulties which can attach t o such studies on heterogeneous systems were indicated above; (ii) more explicit consideration of the manner of initiation by radiation and of the relative importance, not only of mechanisms based on heterolytic bond rupture, but also of radical-type surface processes at the iiradiated interface; (iii) increased emphasis on systematic programmes for evaluating the lifetimes of singlet and triplet electronically excited states a t interfaces and for correlating lifetime changes with details of surface structure and composition of the interface region. Advances will be needed in these areas t o allow reliable correlations t o be established between the efficiencies of radiative and radiationless processes at the interface and early events in the complex sequence of steps which can be initiated by irradiation a t gaslsolid interfaces.

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Index

A accommodation coefficient, 59, 61, 62, 64 active sites, 293, 296, 298-303, 340, 417,418 adsorption/desorption processes, 295, 296, 299-301, 304, 307, 320, 324, 325, 327, 329, 331, 340, 343, 376, 401--403, 405, 406-408, 414, 416, 418 AES, see Auger emission spectroscopy AI, 334,339 aluminium, as doping agent in zinc oxide, 335,368 -, on gallium arsenide, 263-265, 275, 276 -, on silicon, 259 aluminium oxide, as catalyst, 402 -, band gap, 406,407 -, chemisorption of gases on, 401 -, decomposition of methanol, 407 -, finely divided, 320 -, irradiated sample, 414 -, outgassed, 319 -, oxygen interface, 320, 409 -, photo-oxidation, 377 -, radiation-induced surface holes, 320 ammonia, adsorbed on aluminium oxide, 316 -, - iron, 139 -, - platinum, 140 -, - rubidium, 140 -, - silica, 316 -, hydrogen abstraction from, 415 -, radical formation from, 408 ANI, 340, 341, 343 antimony, photocatalytic activity of antimony(II1) oxide, 369 -, photoconductivity of antimony(V) oxide, 347 AREDC, 218 argon, adsorbed on tungsten, 3, 161 -, bombardment with Ar', 416, 417 -, weakly bonded to metals, 311

ARPES, 190, 203 arsenic, formation of gallium arsenide from As2, 278, 279 -, interaction of A& with gallium arsenide surfaces, 277, 279 ASCT, 300, 310, 3 3 2 , 4 0 8 , 4 0 9 Auger electron spectroscopy, 2, 13, 23, 31,183,189-192,201,236,242,255, 258, 260, 262, 266, 267, 312, 330, 401,403,404,417

B barium, on tungsten, 158 Bayard--Alpert gauge, 2 benzene, formation from cyclohexene, 399 beryllium oxide, with adsorbed methanol, 320 bismuth molybdate, catalysed oxidation of propene, 301 Bravais lattice, 184 Brillouin zones, 198, 227, 253 bromine, physisorbed on metals, 139, 31 1

C cadmium sulphide, CdS/gas interface, 340, 34 1 -, CdS/oxygen interface, 295, 332, 333, 339,340,342 -, photoadsorption of oxygen, 327 -, photosorption of oxygen, 331 -, sulphur in vacancies, 416 cadmium telluride, empty surface states, 307 canted ridges, 230 carbon, impurity in clean silicon surfaces, 202 -, on tungsten, 160 carbon dioxide, as impurity, 328, 352

430

-, -, -, -,

on nickel, 21 on tungsten, 1 6 1 photodesorption of, 331, 352 production in oxidation of alkanes, 357,374 -, reaction with hydrogen, 409 -, use as moderator, 416 carbon monoxide, adsorbed, 30, 41, 50-52 -, as impurity, 328 -, chemisorbed, 401 -, contamination of silicon surface, 233 -, on chromium, 4 7 , 1 1 5 , 1 3 1 -, on cobalt, 4 7 , 1 1 5 , 1 3 1 -, on copper, 23 -, on iron, 115, 1 3 1 -, on molybdenum, 4 7 , 1 0 8 , 1 1 5 , 1 3 1 -,on nickel, 47, 88, 89, 97, 115, 132, 155 -, on palladium, 47, 97, 108, 177 -,on platinum, 47, 97, 108, 117, 120, 121,133 -, on rhenium, 47,116, 134 -, on rhodium, 48, 117, 135 -, on ruthenium, 48, 88, 108, 117, 135 -, on titanium, 117, 135 -,on tungsten, 9, 48, 62, 63, 98, 104, 111,118,135,141,161 -, photo-assisted reaction, 390 -, photodesorption from metal surfaces, 336-339,347 -, photo-oxidation of, 366, 373 -, reaction with HzO, 395 catalysis, 292, 296, 298, 299, 301, 308, 310, 311, 320, 354, 356, 357, 367, 369, 371, 374, 377, 383, 390, 391, 394, 395, 397, 400, 402, 404, 406, 407,409,417 -, non-metallic, 300 CECT, 302, 331,332,409 CeOz, photoconductivity of, 347 cesium, adsorbed o n gallium arsenide, 266-269 -, - silicon, 260 -, --tungsten, 39, 155, 158 chemiluminescence, 41 5 chemisorption, 182, 198, 221, 223, 226228, 231, 232, 237, 242, 246, 251, 252, 260, 265, 266, 268, 278, 280, 293-296, 298-302, 311, 317, 319, 325, 329, 331, 335, 340, 342-344, 352, 360, 379, 401, ,402, 404, 407, 416,418

chlorine, absorbed on copper, 139 - gold, 139 - palladium, 139 - platinum, 139 -, - rhodium, 139 -, - silicon, 242-246 -, - tungsten, 139 chromium, metal ion dispersed o n aluminium oxide, 316 -, - silicon dioxide, 316 chromium(II1) oxide, interface with oxygen, 335 -, photoreduction of carbon monoxide, 372 -, pre-oxidised surface, 382 clean surfaces, 1 , 2, 108, 183, 201-206, 210, 214, 215, 222, 224, 229, 230, 232-234, 236, 246, 247, 251, 254, 262, 2 6 3 , 2 6 6 , 2 6 9 , 3 2 6 , 3 2 7 , 4 0 4 -, preparation of, 1 , 14, 258, 259, 329, 403 clusters, copper/ruthenium, 321 -, zinc/oxygen, 308 -, zirconium/oxygen, 308 co-axial filament flow reactor, 354 cobalt(1V) oxide, Fermi levels of, 367 -, photoreduction, 372 collective electron, 293, 296, 301-303, 340,398,408,409 condensation coefficient, 8 4 copper, absorbed on molybdenum, 161 -, - - tungsten, 159 copper(I) oxide, surface reduction through thermolysis, 349 copper/ruthenium clusters, 321 C.R.O., 332 crystallographic etch pits, 205, 243 CVD, 254

-, -, -,

D Dember effect, 332, 340 demetallisation, 23 density of states, 296, 304, 308, 342 depolarisation, 268 desorption mechanisms, 308-31 7 deuterium, exchange with hydrogen, 298, 299, 3 8 8 , 3 8 9 , 4 1 4 , 4 1 5 dipole-dipole interaction, 7 direct coupling, 7 direct hit, 296 disorder/order, 6, 226, 227, 307, 320

431 E

-, photoemission spectra, 261 -, semiconductorsemiconductor inter-

electron tunnelling, 302, 310 Eley-Rideal mechanism, 298, 370, 379, 385,412, 418 ellipsametry, 24, 233, 237, 247 Elovich equation, 247 Elovich kinetics, 333, 335, 341 ELS, 227, 234, 236, 237, 239, 247, 248, 250,251,253, 258,417 ESCA, 325, 400 ESD, 312,417 ESID, 404 ESR, 204, 306, 317-319, 325,361,400, 408,414,415 ethane, quenching of magnesium oxide luminescence, 316 Ewald sphere, 198 EXAFS, 5, 237,321, 322

-, topological defects of surface, 307

F F+ centres, 320, 364 Fick’s law, 146 field emission microscopy, 2, 34-38, 142, 154-163 field ion microscopy, 38, 39, 230 flash filament method, 17, 28 fluorescein, adsorbed on zinc oxide, 339 fluorine, adsorbed on platinum, 139 formic acid, adsorbed on copper, 1 2 3 -, - nickel, 124 Franck-Condon transitions, 189

face, 275, 276

-, TPD spectra, 273, 274 -, with adsorbed cesium, 266, 267 -, with adsorbed gold, 262 gallium arsenide { 100) face, interaction with Group V metals, 279 -, oxygen adsorbed on clean faces, 253 -, semiconductor-semiconductor interface, 276 gallium arsenide { 110) face, cesium adsorbed on, 266 -, cleaning of, 215-221 -, gold adsorbed on, 266 -, oxygen adsorbed on clean faces, 247.25 2 -, reaction with oxygen, 251 -, semiconductor interface interaction, 270 -, semiconductor-semiconductor interface, 276, 277 gallium( 111) oxide, photocatalysis, 369 gassemiconductor interface, 295, 296, 301, 307,310, 332,343 germanium, on tungsten, 160 gold, adsorbed on gallium antimonide, 260-262 -, - gallium arsenide, 260-262, 269 -, -silicon, 255-257, 259 -, - tungsten, 157 -, hydrogen chemisorbed on, 299 -, interface with nickel oxide, 367 -, islands of, 416

G

H gallium, adsorbed on gallium arsenide, 263-267 -, - silicon, 254 gallium antimonide, gold film on, 262 -, oxidation of, 251 -, oxygen adsorbed on, 301 -, spectra of, 251 -, spectra of gold+aSb, 263 gallium arsenide, cleaning of surface, 204-206 -, electron stimulated oxidation, 199 -, energy levels of, 306 -, interaction with aluminium, 265 -, interaction with Group V elements, 277,278

HREELS, 1 6 hydrogen, activating outgassed aluminium oxide, 319 -, - magnesium oxide, 319 -, adsorbed, 41, 5+52 -, adsorbed on chromium, 42,109, 126 -, - cobalt, 42, 109, 126 -, - copper, 53, 83, 109, 126 -, - iridium, 42, 109,126 -, -iron, 42, 101, 109, 126 -, - magnesium, 126 -, -molybdenum, 4 2 , 1 0 9 , 1 2 6 - ,_ nickel, 40, 42, 56, 63, 77, 83, 89, 104,108,128,162

432

-, -niobium, 42, 109, 126 -, -palladium, 42, 110, 127

krypton, adsorbed on tungsten, 3, 161

-, physisorbed on metals, 31 1

-, --platinum, 42, 110, 121, 124

-, -rhenium, 43,110, 128 -, -, -, -, -, -,

-rhodium, 43, 110, 128 -ruthenium, 43, 110, 128 -silica gel, 320, 414 -tantalum, 43, 110, 128 -titanium, 97, 109, 126 - tungsten, 2, 10, 43, 54, 56, 63, 77, 8 3 , 1 0 4 , 1 0 8 , 1 1 2 , 1 2 8 , 161 -, chemisorbed on transition metals, 296 -, desorption of, 403 -, deuterium exchange, 298, 299, 414, 415 -, - catalysed by magnesium oxide, 298, 300 -, formation of palladium black, 415 -, formation of platinum black, 415 -, reaction on platinum surfaces, 298 -, reaction with carbon dioxide, 408 -, - carbon monoxide, 408 -, - hydrogen atoms, 415 hydrogen peroxide, pre-adsorption of, 349,368

I indirect coupling interaction, 7 indium, adsorbed on gallium arsenide, 263 -, - silicon, 259 -, - tungsten, 159 indium phosphide, analysis of surface for gold, 262 -, spectra of gold-InP, 261 -, with adsorbed gold, 260 -, with adsorbed oxygen, 307 interstate conversion, 141 iodine, physisorbed on metals, 311 indium, adsorbed on tungsten, 1 6 0 -, - zinc oxide, 365 -, with chemisorbed hydrocarbons, 299 iron, pre-oxidised surface, 382 iron(111) oxide, photoreduction of carbon monoxide, 372 isothermal desorption, 29

K kinks, 230 Kisluik model, 70, 74

LaCo03, photocatalysis of, 383 Lagowski model, 301 LAMMA, 354 Langmuir adsorption, 2, 39, 56, 64, 65, 6 9 , 1 5 0 , 376 Langmuir evaporation, 205, 249 Langmuir-Hinshelwood recombination mechanism, 298, 369,370, 385,418 LDOS, 198, 208,220,221, 238, 241 Lead, adsorbed on gallium arsenide, 273, 274 LEED, 2, 4 - 6 , 8-11, 73, 107, 156, 183, 185-188, 201, 203, 204, 206-208, 210, 215, 219, 222, 233, 242, 244, 247, 255, 259, 265, 266, 268, 297, 298, 3 1 1 , 4 0 1 , 4 0 3 , 4 1 6 LEELS, 237, 259, 277 Lennard-Jones potential, 3, 8 4 LET, 398,399 lithium, zinc oxide doped with, 335, 352, 361, 368 lithium fluoride, with adsorbed formaldehyde, 322 LWPE, 354

M M+ centre, 364 magnesium oxide, band gap, 406,407 -, catalyst for deuterium/hydrogen exchange, 300, 388, 389 -, F+ type centres, 320 -, isotropic exchange over, 361 -, luminescence of, 316 -, outgassed, 319 -, reaction with N20,389 --,“smoke”, 315 magnesium-oxygen interface, 353 mass spectroscopy, 327, 330, 347, 353, 365,370,377,401 MBE, 206, 247,253, 272,277, 278 mercury, adsorbed on tantalum, 162 -, -tungsten, 106, 159 metal excess surface species, 295 metal oxide layers, 9, 233, 234, 242, 247, 251, 253, 331, 352, 367, 377, 379, 403,404-409,415, 416,418

433

methanol, adsorbed o n beryllium oxide, 320 -, - silicon dioxide, 320 -, hydrogen abstraction, 415 microbalance, 331 -, techniques, 22 molybdenum, adsorbed o n tungsten, 152, 154,160 molybdenum(II1) oxide, ESR signal, 415 --,photoreduction of carbon monoxide, 372 N N i Z + ,dispersed o n alumina, 316 - silica, 316 nickel, photodesorption of carbon monoxide from, 336 -, with physisorbed xenon, 403 nickel oxide, band gap of, 407 -, enriched "0 passed over, 365 -, Fermi level of, 367 -, NiO/metal interface, 367 Ni(C0)4, 338 nitrogen;adsorbed, 41, 50, 51 -, adsorbed o n copper, 48 -, - indium, 136 -, - iron, 48, 5 3 -, -molybdenum, 48,81,119, 136 -, -nickel, 48, 81, 119, 137 -, - niobium, 48, 131 -, -palladium, 119, 137 -, -platinum, 48, 119, 137 -, -rhenium, 48,119, 137 -, - ruthenium, 120 -, - silver, 48 -, -tantalum, 48, 137 -, - titanium, 48, 81 -, - tungsten, 9, 28, 31, 34, 49, 53, 54, 57, 63, 73, 120, 137, 160 -, - vanadium, 48 nitrogen monoxide, adsorbed o n nickel, 138 -, - platinum, 138 -, - rhenium, 138 -, - rhodium, 138 -, - ruthenium, 138 -, -silver, 56, 138 -, -tungsten, 56, 138 -, quenching luminescence of magnesium oxide, 316 nitrous oxide, adsorbed o n platinum, 140

-,

-, causing band bending, 294 -, interface with cadmium sulphide, 341

-, photoreduction, 389-394 -, quenching luminescence of magnesium oxide, 316 NMR, 320

0 osmium(I1) complexes, 407, 408 oxidation of alcohols, 366-373 oxygen, adsorbed, 41,50-52 -, adsorbed on aluminium, 44, 63, 79 -, - copper, 9 , 4 4 -, - gallium antimonide, 307 -, - gallium arsenide, 307 -, -indium, 44, 130 -, - indium phosphide, 3, 91 -, - iron, 44 -, -molybdenum, 44, 130 -, -nickel, 9 , 4 5 , 57 -, -niobium, 113 -, -palladium, 45,63,113,129 -, - platinum, 45, 53,56, 113,114, 130 -, - rhenium, 46, 130 -,-rhodium, 46, 1 1 4 , 130 -, -ruthenium, 46, 1 1 4 , 130 -, -silica gel, 320 -, -silver, 43, 99, 106, 113, 129 -, - tantalum, 46, 130 -, - tungsten, 2, 8 , 9, 33. 46, 63, 100, 108,114, 131, 160, 161 -, -zinc selenide, 307 -, -zirconium, 48 -, anion radical formation, 317 -, aluminium oxide /02 system, 319 -, causing band bending, 294 -, chemisorption on aluminium oxide, 401 -, -zinc oxide, 301 -, contamination on silicon surfaces, 234 -, interaction with magnesium, 353 -, interface with cadmium sulphide, 295, 332, 333, 336, 340, 342, 350-352, 371, 374 -, - tin(1V) oxide, 295, 332, 333, 336, 340,342,350-352,371,372 -, - titanium oxide, 295, 332, 333, 336, 340,342,350-352,371,372 -, -zinc oxide, 295, 332, 333, 336, 340, 342,350-352,371,372 -, irreversible photoadsorption o n titanium oxide, 347, 348

434

-, isotropic exchange, 360-366,

374, 375,380,409 -, photoadsorption on cadmium sulphide 331 -, -zinc oxide, 3 3 5 -, photodesorption from titanium oxide, 347,348 -, photo-oxidation of carbon monoxide, 370-373 -, reaction with gallium antimonide, 251 -, silicon d i o x i d e / 0 2 , 239-242 -, silicon-oxygen bond length, 257 -, sorption o n zinc oxide, 349 -, sticking coefficient o n silicon, 234 oxygen-oxygen interaction, 353 oxygen-xygen stretch, 2 3 9 oxygen pressure effect, 234 oxygen/zirconium(IV) oxide system, 319 P palladium, adsorbed o n tungsten, 1 5 7 palladium black, 416 -, preparation of, 415 Pd/nickel oxide interface, 367 PEDS, 306 photo-oxidation, of alkanes, 373-376 -, of carbon monoxide, 366-373 -, of isopropanol, 377--412 -, of other alcohols, 385-388 photoadsorption/desorption, 327-335, 339, 343, 348, 3 5 2 , 3 7 1 photoelectronic effect, 331, 3 3 2 photoemission, 15, 214, 217, 218, 223, 225, 243-246, 2 5 5 , 2 6 3 , 304 photogenerated holes, 332, 340, 343, 365,384 photoluminescence, 315, 324 PIFIMS, 3 5 3 platinum, { 111)surfaces, 298 -, activating crystals of, 404 -, chemisorption of hydrocarbons on, 299 -, desorption of chemisorbed species from, 300 -, reaction of active sites, 299 -, reaction with hydrogen, 298 -, single crystal of, 298, 300 platinum black, 416 -, preparation of, 415 platinumltitanium oxide, 395-397 Polanyi-wigner equation, 8 4 , 8 7 , 195, 196

potassium, adsorbed o n tungsten, 1 5 2 , 158 potential energy curves, 5 , 57, 8 2 pre-adsorption, 339, 348, 377 precursor states, 6 2 4 9 , 72, 77, 84, 8 5 , 1 0 1 , 236, 252, 271, 272, 2 7 1 propene, oxidation of using bismuth molybdate, 3 0 1 PSID, 404 PTD, 3 7 8 pulsed techniques, 332, 4 1 8

R radiation-induced modifications, 296, 299, 3 0 2 , 3 1 1 , 3 9 9 RAIRS, 1 6 Resistivity, 2 3 RHEED, 187-189 Rose Bengal dye, adsorbed o n glass, 322 -, - gold, 322 -, -zinc oxide, 339 -, spectra o f , 3 2 3 S scanning electron microscope, 1 8 3 , 258, 274 SCI, 3 3 4 , 3 3 5 , 3 3 9 sensitisation, 342 SEXAFS, 252 silica, see silicon dioxide silicon, adsorbed with cesium, 260 -, - gold, 255-257, 260 -, -indium, 259 -, -molybdenum, 1 6 2 -, - silver, 258-259 -, - tungsten, 1 6 0 -, cleaning of surface, 202-204 -, germanium silicate interface, 277 -, intrinsic surface state, 3 0 3 , 306 -, reaction with arsenic, 278 -, -chlorine, 242-246 -, - gallium, 259 -, -hydrogen, 223-232 -, - oxygen, 234-242 -,{loo} face, hydrogen adsorbed o n , 2 29-230 -, { l l O } face, hydrogen adsorbed on, 2 2 7-2 2 9 -, { 111) face, effect of substrate orientation and reconstruction, 235 -, -, hydrogen adsorbed o n , 223-227

435

-, -, reaction with aluminium, 259 -, -, -chlorine, 244 -, -, - gallium, 259 -, -, - gold, 255 -, -, - indium, 2 5 9 -, -, -silver, 258 silicon dioxide, band gap, 406 -, decomposition of methanol, 407 -, finely divided, 3 2 0 -, irradiated sample, 408, 409 -, methanol adsorbed o n , 320 -, molybdenum(V1) oxide/Si02 interface, 415 -, photo-oxidation on, 3 2 0 -, tungsten( VI) oxide/Si02 interface, 415 -, vanadium(V) oxide/SiOz interface, 415 -, water absorbed o n , 3 2 0 , 408 Silver, adsorbed o n silicon, 258-259 silver/ethane interface, 353 silver halides, adsorption of dyes o n silver bromide, 339 -, photoluminescence of, 324 silver islands, 416 silver/nickel oxide interface, 367 SIMS, 30 sodium, adsorbed o n tungsten, 1 5 8 spurs, 398 SSI, 340 steps, 298, 2 9 9 , 3 0 4 , 3 0 8 , 3 1 6 , 404 sticking coefficient, 1 9 4 , 222, 229, 233, 234, 236, 242, 247, 252, 253, 267,278,279 sticking probability, 1, 1 7 , 24. 41, 76, 81, 101 -, zero coverage, 41-55 strontium titanate, photocatalysis, 383 -, photocatalytic enhancement, 3 7 1 -, stoichiometric { 111)surface, 396 -, use in catalysis, 359 structure-sensitive reactions, 298, 299 sulphur arsenide, interaction with gallium arsenide surfaces, 275, 276 sulphur dioxide, adsorbed on tungsten, 140 surface diffusion, 31-41, 82, 143, 1 9 7 surface enhancement, 209 surface-insensitive reactions, 298 surface roughness, 8 1 , 205 surface state, 303-310, 340, 3 4 3 -, extrinsic, 307-308, 3 4 8 -, intrinsic, 303-307

surface steps, 187. 213, 234 symmetric model, 2 0 9

T TEM, 2 0 3 , 2 1 6 , 277 temperature-programmed desorption, 20, 27-29, 9 1 , 109-120, 253, 268, 272, 273, 275 terraces, 52, 6 4 , 213 thorium, adsorbed o n tungsten, 1 5 9 tin(1V) oxide, interface with oxygen, 3 4 3 -, photocatalysis of, 369, 377 -, photoconductivity of, 347 titanium, adsorbed o n tungsten, 1 5 9 titanium oxide, band bending of, 294 -, density of states, 304 -, ESR, 3 1 9 , 4 0 0 -, gases desorbed from, 347 -, interface with oxygen, 295, 340, 343, 344, 350 -, interpretation of surface properties by surface state model, 306 -, irreversible photoadsorption of oxygen on, 334 -, isotropic exchange over, 3 6 1 -, photocatalysis, 369, 372, 378, 380383 -, photoconductivity, 347 --,photo-oxidation of alkanes, 357, 359, 373 -, surface reduction through thermolysis, 349 -, undoped, 394 -, with metal excess surface species (Ti3+),3 9 5 titanium oxidelnitric oxide system, 3 9 1 titanium oxide/platinum, 395-397 Tomahawk accelerator, 4 0 3 Torus accelerator, 4 0 3 TPC, 4 1 1 , 4 1 2 TPRS, 1 2 2 trapping probability, 59, 6 4 , 7 8 tungsten oxide, ESR signal, 4 1 5 -, photoconductivity of, 347 -, sensitisation, 415

U UPS, 1 8 3 , 1 9 0 , 222,226-228, 2 3 0 , 2 3 6 , 238, 247, 248, 251, 255, 258, 259, 261, 2 6 6 , 2 7 6 , 3 1 2 , 3 2 1 , 3 9 6 , 4 0 1

436 V

Z

vanadium, dispersed on aluminium oxide, 316 -, -silicon dioxide, 316 vanadium(V) oxide, ESR signal, 4 1 5 -, photoconductivity, 347 -, pre-oxidised surface, 382 -, with carbon monoxide introduced over, 3 7 2 -, with enriched l8 O2 passed over, 3 6 5 van der Waals, 294 V centres, 3 2 0 , 4 0 8 , 414 VCI, 2 9 8 , 3 8 5 , 4 1 2 , 4 1 8 virgin states, 9 0 VPE. 254

zinc oxide, adsorption of dyes, 339 band bending of, 394 band gap, 4 0 5 4 0 7 chemisorption of hydrogen, 4 0 1 -oxygen, 3 0 1 , 3 4 4 doped with aluminium, 335, 352, 361, 368 -, -lithium, 335, 352, 361, 368 -, extrinsic surface state, 307 -, Fermi level of, 367 -, interface with nitrous oxide, 389, 390, 3 9 3 , 3 9 4 , 401 -, interface with oxygen 295, 336, 350, 352 -, interpretation of surface properties by surface state model, 306 W -, isotropic exchange over, 361 -, lattice, 417 water, o n iridium, 1 4 0 -, photoadsorption of oxygen, 335 -, on iron, 1 4 0 -, photocatalysed reaction, 357, 368, -, o n platinum, 1 4 0 377,380,381,383 Wood notation, 11 -, photoconductivity, 347, 350 work function, 3 , 223, 232, 266, 268, -, photodesorption, 3 7 1 367,368,401,418 -, photolysis of, 364, 365, 3 7 1 -, photo-oxidation, 320 -, photosorption of thin films on, 330 ” A -, pre-oxidised surface, 3 6 3 -, single crystal, 324, 351, 371, 416, 417 -, sorption of oxygen, 349 xenon, adsorbed on copper, 4 -, with metal excess surface species -, - palladium, 4 , 7 (Zn’/Zn+), 295 -, -tungsten, 3 , 1 1 , 6 2 zinc selenide, adsorbed oxygen, 307 -, discharge, 332 -, disordered surface, 307 -, physisorbed o n metals, 4 1 1 -, empty surface states, 307 - - nickel, 4 0 3 XPS, 1 8 3 , 1 9 0 , 222,226-228,230, 236, zirconium(1V) oxide, interface with oxygen, 347 238, 247, 248, 251, 255, 258, 259, -, photoconductivity, 347 261, 266.276, 331. 396. 403 X-ray crystallography,~l86 -, sensitisation of, 377 ’

-, -, -, -, -,