Diamond electrochemistry

Preface It has been nearly ten years since we began to build an international consortium in the area of diamond electro...

Author: Akira Fujishima | Yasuaki Einaga

35 downloads 1776 Views 22MB Size Report

This content was uploaded by our users and we assume good faith they have the permission to share this book. If you own the copyright to this book and it is wrongfully on our website, we offer a simple DMCA procedure to remove your content from our site. Start by pressing the button below!

Report copyright / DMCA form

DOWNLOAD PDF

Preface

It has been nearly ten years since we began to build an international consortium in the area of diamond electrochemistry, with our First International Mini-Symposium, held in Tokyo in 1997. Since that time, we have tried to keep this tradition going. In addition, there have been International Symposia on Diamond Materials every two years, held under the auspices of the Electrochemical Society, with a strong complement of presentations in the area of electrochemical apphcations of conductive diamond. These symposia, together with others, such as the European Conferences on Diamond and Diamond-Like Materials and the International Conferences on New Diamond Science and Technology, held in the Eastern Hemisphere, have kept this field growing at a rapid rate. Almost every aspect of electrochemistry has been impacted by the diamond electrode, from electroanalysis to electrolysis. Recently also, the field has started to mature, with the development of many practical apphcations of diamond electrodes. Some of these are being commercialized at present. Two examples are the diamond electrochemical detector for liquid chromatography and the large-scale diamond electrode for industrial wastewater treatment. For the present volume, we have invited representatives fi^om nearly every group in the world that has been active in the field, and we are very pleased that many of these groups have responded with chapters devoted to both their own work as well as that of others.

VI

Certainly we realize that it is virtually impossible to capture everything that is going on in any given field at a particular time, but our group of authors has tried hard to accompUsh the impossible. In Chapter 1, Rao, et al., have provided a historical introduction to the area, which got its start in 1983 in Japan in a pubUcation by Iwaki et al. In Chapter 2, Ivandini, et al., provide fiirther historical perspective and introduce the basics of the preparation

and

characterization of chemical vapor-deposited (CVD) diamond films. In Chapter 3, Martin, et al., discuss several fimdamental aspects of diamond electrochemistry, including the large working potential range ("wide potential window"), aspects of the reactivity, the optical transparency, semiconductor aspects, and the surface conductivity phenomenon.

In Chapter 4, Pleskov gives a fuU account of the

semiconductor aspects of diamond electrochemistry.

In Chapter 5,

Levy-Clement focuses on the role of the boron doping level in determining the electrochemical properties, together with Raman spectroscopy as a useful diagnostic tool in estimating the effective doping level. In Chapter 6, Yoshimura et al. examine the factors that determine the potential working range for various non-aqueous solvent/electrolyte systems, including theoretical molecular orbital calculations. In Chapter 7, Yagi, et al., examine the use of a novel technique, time-of-flight electron-stimulated desorption, as a means of understanding the interactions of the diamond surface with hydrogen, the most important of the surface terminations. In Chapter 8, Kondo, et al., examine the electrochemistry of single-crystaMike homoepitaxial diamond

films,

particularly

as

nearly

ideal

electrodes

for

electroanalytical apphcations. In Chapter 9, Tryk, et al., review the various techniques available for the chemical modification of the

Preface

diamond surface, including ways of attaching DNA strands.

vii

In

Chapter 10, Notsu, et al., focus on the oxidized diamond surface, which is the most common form of chemically modified diamond surface. In Chapter 11, Einaga, et al., present several different ways of producing functional diamond surfaces, including diamond microelectrode arrays, diamond surfaces ion-implanted with metals to impart catalytic activity, and ultrasmooth diamond surfaces produced by the glow discharge technique.

In Chapter 13, Spataru, et al., focus on the

advantages of the diamond electrode for the oxidative determination of various types of biologically active compounds. In Chapter 14, Shin, et al., discuss the use of the boron-diamond electrode as a detector for capiUary zone eletrophoresis, which is quickly becoming a powerful technique for the detection of a number of different types of compound mixtures, for example, explosives, as well as biologically active compounds such as neurotransmitters. In Chapter 15, Orawon, et al., discuss the use of diamond electrodes for the determination of the biologically important suLfur-containing compounds. In Chapter 16, Manivannan, et al., examine the diamond electrode for use in the detection of trace concentrations of toxic metals.

In Chapter 17,

Suryanarayanan, et al., examine several diverse examples of analytical apphcations of boron-doped diamond electrodes for industrially important chemicals. In Chapter 18, Ohvia, et al., present the topic of boron-doped diamond microelectrodes, which are highly interesting and analytically useful, because they combine the advantages of diamond with those of the microelectrode, including efficient mass transport. In Chapter 19, Honda and Fujishima discuss the highly interesting nanotextured diamond surfaces, along with possible apphcations of such electrodes. In Chapter 20, ComnineUis, et al..

Vlll

discuss the use of hydroxyl radicals generated at the diamond surface to carry

out various types

of oxidation

reactions,

including

electrosynthetic processes, and the electrochemical "combustion" of organic compounds. In Chapter 21, Vatistas, et al., examine a highly useful approach to the use of diamond for wastewater treatment, i.e., involving the electrogeneration of hydroxyl radicals, followed by the reaction of these radicals with inorganic ions such as sulfate to produce active oxidants, circumventing the mass transport problems associated with the direct reaction of hydroxyl radicals with pollutants.

In

Chapter 22, Cho, et al., focus on the use of diamond electrodes for the electrogeneration of ozone, which is an important oxidant and potential replacement for chlorine. In Chapter 23, Furuta, et al., provide a very interesting account of the practical use of diamond electrodes in ordinary tap water to produce oxidants that are capable of destroying the bacteria that cause Legionnaires' Disease. In Chapter 24, Arihara and Fujishima provide an additional account of how diamond electrodes, specifically, free-standing ones, can be used successfully to produce ozone-water, which is an environmentally fidendly decolorizing and antibacterial agent. Finally, in Chapter 25, Rao, et al., provide a summary and perspective on the fundamental and apphed aspects of diamond electrodes. Lastly, we would very much hke to acknowledge the great contribution of Dr. Ivandini Tribidasari in assembhng this volume, which could not have been completed otherwise. Akira Fujishima

IX

The Editors

Professor Akira Fujishima Professor Fujishima was born in 1942 in Tokyo. He received his Ph. D. in AppHed Chemistry at the University of Tokyo in 1971. He taught at Kanagawa University for four years and then moved to the University of Tokyo, where he became a Professor in 1986. In 2003, he retired from this position and took on the position of Chairman at the Kanagawa Academy of Science and Technology. His main interests are in photocatalysis, photoelectrochemistry and diamond electrochemistry. [Kanagawa Academy of science and Technology, KSP 3-2-1 Sakado, Kawasaki 213-0012, Japan, E-mail- [email protected]]

Professor Yasuaki Einaga Professor Einaga was born in Niigata Prefecture, J a p a n in 1971. He received his Ph.D degree in 1999 from The University of Tokyo under the direction of Prof. Akira Fujishima. He joined the Department of Chemistry at Keio University as an Assistant Professor in 2001. In 2003, he was promoted to Associate Professor. His research interests include photo-functional materials science and diamond electrochemistry. [Department of Chemistry, Keio University, 3-14-1 Hiyoshi, Yokohama 2238522, Japan, E-mail- [email protected]]

Dr. Tata Narasinga Rao Dr. Rao was born in India in 1963. He received his Ph.D. degree in 1994 from Banaras Hindu Unversity, India. After working at IIT Madras, he moved to The University of Tokyo as a J S P S Postdoctoral Fellow and became an Assistant Professor in 2001. Presently, he is a senior scientist at the International Advanced Research Centre for Powder Metallurgy and New Materials (ARCI) in Hyderabad, India. His research interests include diamond electrochemistry, nanomaterials synthesis and their applications for environmental remediation. [International Advanced Research Centre for Powder Metallurgy and New Materials. Balapur PO, Hyderabad 500005, India, E-mail : tatanrao@yahoo. com]

Dr. Donald A. Tryk Dr. Donald Tryk was born in California (USA) in 1948 and received his Ph. D. in Chemistry from the University of New Mexico in 1980. He was with the Yeager Center for Electrochemical Sciences at Case Western Reserve University in Ohio (USA) before joining Prof. Fujishima's group in 1995. After two 2^^^ years at Tokyo Metropolitan University, he is now at the University of Puerto Rico. His interests are diamond electrochemistry and electrocatalysis. [Department of Chemistry, University of Puerto Rico, Rio Piedras Campus, San J u a n , Puerto Rico 00931-3346, E-mail •' [email protected]]

Special Thanks for Contribution

Dr. Ivandini Tribidasari Anggraningrum Dr. Ivandini was born in Indonesia in 1970 and received her Ph. D. from the University of Tokyo in 2003. She is a lecturer in the Department of Chemistry, Mathematics and Science Faculty, University of Indonesia in Jakarta, Indonesia. Now, she is doing post-doctoral research supported by a JSPS award at the Department of Chemistry, Keio University, Japan. Her interest is in diamond electrochemistry.

XI

List of Authors John. C. Angus Case Western Reserve University, USA

Kazuki Arihara Central J a p a n Railway Company, J a p a n

Oraw^on Chailapakul Chulalongkorn University, Thailand

Eun-In Cho Chungbuk National University, Korea

Christos Comninellis Swiss Federal Institute of Technology, Switzerland

Ilaria Duo Swiss Federal Institute of Technology, Switzerland

Sally C. Eaton Case Western Reserve University, USA

Yasuaki Einaga Keio University, J a p a n

Akira Fujishima Kanagawa Academy of science and Technology, J a p a n

Tsuneto Furuta Permelec Electrode Ltd., J a p a n

Werner Haenni Centre Suisse d'Electronique et de Microtechnique SA (CSEM), Switzerland

Olivia Herlambang Canon Inc., J a p a n

Kensuke Honda Yamaguchi University, J a p a n

Tribidasari A. Ivandini University of Indonesia, Indonesia

Takeshi Kondo Tokyo University of Science, J a p a n

Uziel Landau

Xll

Case Western Reserve University, USA

Claude LevyClement CNRS, France

Ayyakannu Manivannan West Virginia University, USA

Beatrice Marselli Swiss Federal Institute of Technology, Switzerland

Heidi B. Martin Case Western Reserve University, USA

Hideki Masuda Tokyo Metropolitan University, J a p a n

Pierre "Alain Michaud Swiss Federal Institute of Technology, Switzerland

Yoshinori Nishiki Permelec Electrode Ltd., J a p a n

Hideo Notsu The University of Tokyo, J a p a n

Soo-Gil Park Chungbuk National University, Korea

Su-Moon Park Pohang University of Science &; Technology, Korea

Jong-Eun Park Chungbuk National University, Korea

Gebriele Prosper! University of Pisa, Italy

Yuri V. Pleskov Frumkin Institute of Electrochemistry, Russia

Laurent Pupunat Centre Suisse d'Electronique et de Microtechnique SA (CSEM), Switzerland

Tata N. Rao International Advanced Research Centre for Powder Metallurgy and New Materials, India Philippe Rychen Centre Suisse d'Electronique et de Microtechnique SA (CSEM), Switzerland

List of Authors

xiii

Bulusu V. Sarada The University of Tokyo, J a p a n

Roberto Massahiro Serikawa Ebara Research Co. Ltd., J a p a n

Dongchan Shin National Institute of Advanced Industrial Science and Technology, Japan

Nicolae Spataru Institute of Physical Chemistry of the Roumanian Academy, Romania

Vembu Suryanarayanan Utsunomiya University, J a p a n

Hozumi Tanaka Permelec Electrode Ltd., J a p a n

Tetsu Tatsuma The University of Tokyo, J a p a n

Chiaki Terashima GL Sciences Inc., J a p a n

Donald A. Tryk University of Puerto Rico, Puerto Rico

Kazuyuki Ueda Hokkaido University, J a p a n

Kohei Uosaki Hokkaido University, J a p a n

Nicolaos Vatistas University of Pisa, Italy

Joseph Wang New Mexico State University, USA

Ichizo Yagi Hokkaido University, J a p a n

Sachio Yoshihara Utsunomiya University, J a p a n

Mikiko Yoshimura Matsushita Electric Industrial Co. Ltd., J a p a n

Yanrong Zhang Utsunomiya University, J a p a n

1. Historical Survey of Diamond Electrodes Tata N. Rao, Akira Fujishima and John C. Angus

1.1. Introduction Conductive boron-doped diamond is an alternative to traditional carbon electrodes that provides superior chemical and dimensional stability, low background currents, and a very wide potential window of water stability (Fig. l.l). In this Chapter we describe the historical development of these unique electrodes. Traditional carbon electrodes, such as glassy carbon, carbon fiber, carbon cloth, carbon nanotubes, various forms of disordered carbon, and graphite are important in electrochemistry because of low cost, simple preparation methods, possibility of achieving large surface area, and a relatively wide potential window of water stability. They have many applications, ranging from Li-ion batteries and double layer capacitors to electrochemical sensors. Carbon also plays an important role in fuel cells as a substrate for dispersal of a small amount of precious metal catalyst over a large area. Despite their advantages, traditional carbon electrodes still suffer drawbacks. For example, electrode fouling limits their long term stability and leads to frequent polishing or disposal of the electrode after a few uses. The limited potential window for water Tata N. Rao e-mail: [email protected]

electrolysis prevents the detection of compounds that oxidize at relatively high anodic potentials.

Electrodes exhibiting better

stability and wider potential window are desired for such applications.

lU - i

Glassy carbon

5-

!

>>

!

Diamond u/

/:>^ <—• 1

<

1 -5-

^j

1•

i

Hg electrode

3

10-

1

1

\

1

Voltage (V vs. SCE) Fig. 1.1. Cyclic voltammogram for diamond in 0.1 M H2SO4. The range of potential windows for glassy carbon and Hg electrodes are shown for comparison. Chemical and dimensional stability and wide potential window are also important for the electrochemical treatment of wastewater. Traditional carbon electrodes oxidize and are not a good choice for this application.

A wide potential window for

oxygen evolution permits electrogeneration of oxidants such as hydroxyl radicals from water discharge. Metal oxides, e.g., Sn02 and Pb02, meet these requirements to some extent; however, electrodes with improved environments, technology.

are

stability, especially in

desired

for

electrochemical

aggressive treatment

1. Historical Survey of Diamond Electrodes

1.2. Origin of Diamond Electrochemistry In 1983, Japanese scientists at the Institute of Physical and Chemical Research (RIKEN) in Saitama reported the use of ionimplanted diamond as an electrochemical electrode [l].

Their

electrode had a wider potential window in the cathodic direction and lower background than glassy carbon.

However, their

diamond was made conductive by ion implantation of Zn, which resulted in a damaged surface. Although this was the first report on diamond electrochemistry, it did not lead to other work. In 1987, Pleskov in Russia used semiconducting diamond electrodes for photoelectrochemistry [2].

His work generated

interest by photoelectrochemists because the conduction band of semiconducting diamond is near the vacuum level, leading to the possibility that diamond can drive photoelectrochemical reduction reactions requiring very negative potentials such as nitrogen fixation

or

deposition

of

electronegative

metals

(Fig. 1.2).

Fujishima and coworkers studied the photoelectrochemistry of diamond in the early nineties [3] and later

demonstrated

excitation across the band gap with an Ar-ion laser (193 nm) [4]. This group also showed that changes in the flat-band potential are due to different surface terminations (oxygen or hydrogen) that change the band edge positions [5]. In 1993 Tenne et al. reported the application of diamond for the electroreduction of nitrate to ammonia [6]. Also in 1993, Ramesham and Swain reported the suitability of diamond for analytical applications [7], and Ramesham and Loo [8] indicated the advantage of diamond for dimensionally stable anodes in

electrochemical waste treatment.

Subsequently, Martin and

coworkers [9-11] showed that high quality boron-doped electrodes had an extremely wide potential window of water stability. Miller et al. [12] reported on the use of implanted diamond as an electrode, and Carey [13] at Eastman Kodak used diamond electrodes for the anodic destruction of organic wastes.

-3.04 Li+/Li -2.70 Hp/e,^. 2.40 N2/NH3OH+

-0.27 H+/H,

Potential vs. SHI

Fig. 1.2. Energy band diagram for diamond showing for comparison the energies for the various redox couples (at pH 4.5). These early reports triggered

aggressive research

that

resulted in a rapid increase in the number of publications, from 4 in 1992 to 121 in 2003 (Fig. 1.3). By the end of 2003, there were over

386

research

reports,

mostly

on

fundamentals

and

1. Historical Survey of Diamond Electrodes

applications

related

to

electroanalysis

and

electrochemical

treatment. The first patent on electrochemical treatment using diamond electrodes appeared in 1995 [13].

By 2003 the total

number of patents on diamond electrodes reached 25. The early work on diamond electrodes is summarized in several prior reviews [l4"2l].

140 M I I I I I > I • I • I > I > I > I > I < I • I M > I < I > I < I > I > I > M

120 52 100 OH

a 80 2 60 £

40 20 1^1 I I I I I U . U . I I I I l^^ll 0 0 0 0 0 0 0 0 0 0 0 0 ' > 0 O S O \ O S O S O S O S O \ O S C N O N C D C D C D C D

.^^.^^_H,-i^-i__

.^^.^^_,r-i,-<^^,—i,-4.^^^^csrNrsr-q

Year Fig. 1.3. Yearly research publications on diamond electrochemistry.

1.3. Recent Directions The wider potential window (in comparison to glassy carbon) on the anodic side has enabled the detection of a wide range of compounds in the anodic region [22-31], and it has allowed the generation of oxidants such as hydroxyl radicals with high yields

[32-36]. The potential window on the cathodic side, comparable to that of mercury, allows the detection of trace metals [37-40]. This enables the replacement of toxic, mercury-based electrodes for trace metal detection.

Furthermore, being chemically stable,

diamond can be used in highly aggressive liquid media [41]. The electrochemical properties of diamond are sensitive to the surface termination (oxygen or hydrogen) permitting its application in modified electrodes [42-45]. Conducting, transparent diamond provides a tool for studying electrode processes.

They have been used for in situ infrared

studies of changes at diamond surfaces during polarization [46] and for spectroelectrochemical studies of redox systems such as chlorpromazine and cytochrome c [47]. There is great interest in electrochemical treatment of waste water using diamond electrodes. In addition to the stability of these electrodes in corrosive electrolytes, they also exhibit a large overpotential for oxygen evolution. This permits the production of strong oxidants, such as ozone and hydroxyl radical [32-36].

1.4. Summary Diamond electrodes are a major innovation enabling many new applications in electrochemistry.

However, fundamental issues

must be resolved before their full potential can be realized. In the following Chapters, recent investigations on the properties of these unique electrodes are summarized.

1. Historical Survey of Diamond Electrodes

References 1. M. Iwaki, S. Sato, K. Takahashi and H. Sakairi, Nucl.

Instrum.

Methods Phys. Res,, 209 (1983) 1129. 2. Y. V. Pleskov, A. Ya. Sakharova, M. Krotova, L. L. Bouilov and B. V. Spitsyn, J. ElectroanaL 3.

Chem., 228 (1987) 19.

K. Patel, K. Hashimoto and A. Fujishima, Denki

Kagaku,

60

(1992) 659. 4.

L. Boonma, T. Yano, D. A. Tryk, K. Hashimoto and A. Fujishima, J. Electrochem.

5.

T. N. Rao, D. A. Tryk, K. Hashimoto and A. Fujishima, Electrochem.

6.

Soc, 144 (1997) L142. J.

Soc, 146 (1999) 680.

R. Tenne,

K. Patel,

K. Hashimoto

ElectroanaL

Chem., 347 (1993) 409.

and A. Fujishima,

J.

7.

G. M. Swain and R. Ramesham, Anal. Chem., 65 (1993) 345.

8.

R. Ramesham, R. F. Askew, M. F. Rose and B. H. Loo, J. Electrochem.

9.

Soc, 140 (1993) 3018.

H. B. Martin, A. Argoitia, J. C. Angus, A. B. Anderson and U. Landau,

in

"Apphcations

of Diamond

Films

and

Related

Materials* Third International Conference," (A. Feldman, et al., Eds.), pp. 91-94. NIST Special Publication 885, U.S. Dept. of Commerce. (1995). 10. H. B. Martin, A. Argoitia, U. Landau, A. B. Anderson and J. C. Angus, J. Electrochem.

Soc 143 (1996) L133.

11. H. B. Martin, J. C. Angus and U. Landau, J. Electrochem.

Soc

146 (1999) 2959. 12. B. Miller, R. Kalish, L.C.Feldman, A. Katz, N. Moria, K. Short and A.E. White, J. Electrochem.

Soc, 141 (1994) L41.

13. J. J. Carey, C. S. Christ and S. N. Lowery: US Patent b,^m (1995) 247. 14. G.M. Swain, A.B. Anderson and J.C. Angus, MRS Bull. 23 (1998) 56. 15. R. Tenne and C. Levy-Clement, Isr. J. Chew. 38 (1998) 57. 16. J.C. Angus H.B. Martin, U. Landau., Y.E. Evstefeeva, B. Miller and N. Vinokur, New Diamond Front

Carbon TechnoL 9 (1999)

175. 17. K. Kobashi, Editor, New Diamond Front

Carbon TechnoL 9, Nos.

3&5 (1999). 18. Y. V. Pleskov, Russ. Chem. Rev. 68 (1999) 381. 19. T. N. Rao and A. Fujishima, Diamond Relat Mater. 9 (2000) 384. 20. A. Fujishima and T. N. Rao, Diamond

Relat

Mater.

10 (2001)

1799. 21. T. N. Rao, T. A. Ivandini, C. Terashima, B. V. Sarada and A. Fujishima, New Diamond Front

Carbon TechnoL 13 (2003) 79.

22. T. N. Rao, I. Yagi, T. Miwa, D. A. Tryk and A. Fujishima, AnaL Chem., 71 (1999) 2506. 23. J. Xu and G. M. Swam, AnaL Chem., 70 (1998) 1502. 24. M. D. Koppang, M. Witek, J. Blau and G. M. Swain, AnaL

Chem.,

71 (1999)1188. 25. B. V. Sarada, T. N. Rao, I. Yagi, T. Miwa, D. A. Tryk and A. Fujishima, AnaL Chem. 72 (2000) 1632. 26. C. Terashima, T. N. Rao, B. V. Sarada, D. A. Tryk and A. Fujishima, AnaL Chem., 74 (2002) 895. 27. T. N. Rao, B. H. Loo, B. V. Sarada, C. Terashima and A. Fujishima, Anal Chem., 74 (2002) 1578. 28. F. Marken, C. A. Paddon and D. Asogan, Electrochem.

Comm. 4

1. Historical Survey of Diamond Electrodes

(2002) 62. 29. N. S. Lawrence, M. Thompson, C. Prado, L. Jiang, T. G. J. Jones and R. G. Compton, Electroanalysis,

14 (2002) 499.

30. C. Prado, G. Flechsig, P. Grundler, J. S. Foord, F. Marken and R. G. Compton, Analyst

(Cambridge UK), 127 (2002) 329.

31. C. Terashima, T. N. Rao, B. V. Sarada and A. Fujishima,

Anal.

Chem., 75 (2003) 1564. 32. M. Panizza, P. A. Michaud, G. Cerisola and Ch. Comninellis, J. ElectroanaL

Chem. 507 (2001) 206.

33. M. A. Rodrigo, P. A. Michaud, I. Duo, M. Panizza, G. Cerisola and Ch. ComnineUis, J. Electrochem.

Soc, 148 (2001) D60.

34. B. Boye, P. A. Michaud, B. Marselh, M. M. Dieng, E. Brillas and Ch. ComnineUis, New Diamond Front

Carbon TechnoL 12 (2002)

63. 35. W. Haenni, J. Gobet, A. Perret, L. Pupunat, P. Rychen, Ch. Comninelhs and B. Correa, Ibid., 12 (2002) 83. 36. I. Troster, L. Schafer and M. Fryda, Ibid., 12 (2002) 89. 37. C. Prado, S. J. Wilkins, F. Marken and R. G. Electroanalysis,

Compton,

14 (2002) 262.

38. A. J. Saterlay, D. F. Tibbetts and R. G. Compton, Anal

ScL, 16

(2000) 1055. 39. A. Manivannan, D. A. Tryk and A. Fujishima, Electrochem.

Solld-

StateLettl{X^^^)A^^. 40. A. Manivannan, M. S. Seehra, D. A. Tryk and A. Fujishima,

Anal

Lett 35 (2002) 355. 41. G. M. Swain, J. Electrochem.

Soc, 141 (1994) 3382.

42. H. Notsu, I. Yagi, T. Tatsuma, D. A. Tryk and A. Fujishima, J. ElectroanaL

Chem., 492 (2000) 31.

43. J. Xu, Q. Chen and G. M. Swain, Anal

Chew., 70, 3146 (1998).

44. T. A. Ivandini, B. V. Sarada, T. N. Rao and A. Fujishima,

Analyst,

128 (2003) 924. 45. T. C. Kuo, R. L. McCreery and G. M. Swain, Electrochem. StateLett,

Solid-

2 (1999) 288.

46. H. B. Martin and P. W. Morrison Jr., Electrochem.

SolidState

Lett.,Ai20{)l)E-ll. 47. J. Stotter, S. Raymond, J.K. Zak, Y. Show, Z. Cvackova and G. M. Swain, Interface Spring 33 (2003).

10

2. Preparation and Characterization of Poly crystalline Chemical Vapor Deposited Boron-doped Diamond Thin Films Tribidasari A. Ivandini, Yasuaki Einaga, Kensuke Honda and Akira Fujishima

The development of low-pressure synthesis methods for diamond, such as the chemical vapor deposition (CVD) technique, has generated enormous and increasing interest and has extended the scope of diamond applications.

Highly efficient methods have

been developed for the economical growth of poly crystalline diamond films on non-diamond substrates.

Moreover, these

methods allow the controlled incorporation of an impurity such as boron

into

diamond,

which

in this

case

forms

a

p-type

semiconductor. By doping the diamond with a high concentration of boron (B/C = 0.01), conductivity can be increased, and semimetallic behavior can be obtained, resulting in a new type of electrode material with all of the unique properties of diamond, such as hardness, optical transparency, thermal conductivity and chemical inertness [1,2]. The low-pressure synthesis involves the conversion of a gasphase carbon-containing species into a solid cubic crystalline form, i.e., diamond.

The first reproducible low-pressure diamond

Tribidasari A. Ivandini e-mail: [email protected] 11

synthesis was achieved by use of carbon monoxide as a source gas to precipitate diamond by Eversole [3]. In 1956, Spitsyn proposed the growth of diamond at low pressure through the decomposition of carbon tetraiodide [4], and in 1959, Angus proposed the growth of diamond at relatively low temperatures, where it is in a metastable state [5].

The growth of diamond on non-diamond

substrates was first accomplished in 1976 in a chemical transport reaction occurring in a closed system at a pressure below atmospheric and a substrate temperature of the order of 1000°C [6]. Rapid growth of diamond at low pressure was then reported by NIRIM (the National Institute for Research on Inorganic Materials) in Japan using a hot-filament technique to activate a CH4/H2 gas mixture [7]. Hydrogen is needed for the continuous growth of diamond, as atomic hydrogen etches away the graphite that is coproduced. Oxygen has also found been found to have a role in etching graphite. In the work described in this Chapter, a mixture of methane and acetone was used as the carbon source. For the growth of boron-doped diamond films, the boron doping agent is generally added as small amounts of diborane, trimethyl boron, or organic borates in the gas phase [8]; solid-state boron sources such as boron oxide are also used [9]. Generally, CVD techniques can be classified into three groups' plasma-assisted CVD, hot

filament-assisted

CVD, and

combustion flames, as well as combinations of these [l]. This Chapter will focus on the hot-filament and microwave plasma-assisted CVD techniques, which are the most commonly employed methods to grow diamond thin films and were used for most of the experiments described in this book. The major

12

2 Preparation and Characterization of Poly crystalline Chemical Vapor Deposited Borondoped Diamond Thin Films

advantage of MPACVD is that by simple operation it can easily give high quality diamond. However, since the plasma area is limited by the wavelength of the microwave radiation, it is difficult to achieve a large-area deposition.

For example, the

optimum wafer size used in our equipment is 3.5 cm x 3.5 cm squares, although depositions on 3-to 5-inch (7.6 to 12.7-cm) wafers with a thickness variance of 10% have been reported [lO]. However, for the deposition of large-area films, which are necessary

for

the

electrochemical

treatment

of

industrial

wastewater, hot-filament CVD deposition is more suitable. Thin films with areas as large as 0.5 m^ were deposited on Nb mesh substrates by use of the hot-filament

CVD technique. The

equipment used for the preparation of diamond thin

film

electrodes and for their characterization will be discussed.

2.1. Microwave Plasma-Assisted CVD (MPACVD) In the plasma-assisted technique, diamond growth involves various forms of plasma-assisted CVD processes with carboncontaining species mixed in low concentrations with hydrogen. The main types of plasmas used are (a) the DC plasma, (b) the RF plasma, (c) the microwave plasma, (d) the electron cyclotron resonance microwave plasma, and (e) the high-pressure plasma. The role of the plasma is to generate atomic hydrogen and to produce the appropriate carbon precursors for the growth of diamond. Plasma-assisted deposition is a combination of two processesa homogeneous process in the plasma bulk and a heterogeneous

13

process at the plasma-surface boundary. The processes start in the dissociation of the hydrogen

source and the formation of

intermediate species, through the transport to the substrate and nucleation processes following the stabilization of the sp^ diamond phase on the surface of the growing film. The plasma exists when a significant number of atoms or molecules are electrically charged or ionized by applying an electromagnetic field across a gas. The energy of the applied electric field arises mainly from free electrons, and the plasma achieves a thermodynamic non-equilibrium condition based on the high electron temperature and low gas temperature. Excitation, ionization and dissociation of atoms and molecules of the reactants and of the carrier gases to various radical and ionradical species are achieved as a result of a sequence of inelastic electron-neutral collisions. 2.1.1. Equipment Microwave (from magnetron)

Quanz Window

\ J Sample Stage

|4-.H,

MFC p - — - ® " ^

Rotary pump

^ t ^ i

Fig. 2.1. Schematic diagram for CVD equipment

14

H:

2 Preparation and Characterization doped Diamond Thin Films

of Poly crystalline Chemical Vapor Deposited

Boron-

Figure 2.1 shows a schematic diagram of the CVD equipment. A commercial low-pressure microwave (2.45 GHz) plasma reactor, Model AX5400 (ASTeX Corp., Woburn, MA) was used (Fig. 2.2). This unit consists of a vacuum system, a microwave generating system and a gas supply system.

Control unit

Bubbling unit for carbon source

Fig. 2.2. Illustration of ASTeXCVD System The vacuum system consists of a stainless vacuum chamber, rotary pump (2EM12F, Edwards, Co. Ltd.), inline valve (manual valve) control valve (248A, MKS Instruments, Inc.), pressure controller

(250C, MKS Instruments,

Inc.)

manometer (127A, MKS Instruments, Inc.).

and

capacitance

On the top of the

chamber is a quartz window, which is connected to the magnetron, which is the microwave source, through the wave-guide. Between the wave-guide and the quartz window, there is a fixed symmetric

15

p l a s m a coupler, a n d t h i s plays a role in generating t h e p l a s m a a s a sphere.

Inside the chamber, t h e r e is a sample stage, whose

height can be changed by m e a n s of a drive-motor. S u b s t r a t e s are heated by h e a t irradiation from the plasma, and to m a i n t a i n the t e m p e r a t u r e of t h e stage, a cooling system is utilized. Table 2.1. Condition for a deposition of diamond using a microwave plasma CVD system

Substrate: Carbon Source: Boron Source: Excitation Source:

n-Si(lll) Acetone/Methanol (9:1) Boron trioxide Plasma

Substrate Temperature:

Ca. 900-1000°C llSTorr

Total Pressure: C/H ratio:

0.03

Time Deposition: FilmThickness : Sample Size :

8 hours 40-50 Mm 35 X 35 mm

The microwave generating system consists of wave generator (AX2040, ASTex Corp., Woburn, MA), microwave guide turners,

circular/directional

coupler

(HS5000, Aster Corp., Woburn, MA).

and

microwave

and

source

The microwave generator

can generate a microwave p l a s m a with a power up to 5 kW. The circular/directional coupler protects t h e microwave p l a s m a source from reflecting microwave radiation.

An e n d T a u n c h system is

used w h e n samples exist n e a r t h e bottom of the p l a s m a ball. The gas supply system consists of m a s s flow controllers (1159B, MKS

16

2. Preparation and Characterization of Poly crystalline Chemical Vapor Deposited Borondoped Diamond Thin Films

Instruments, Inc.), a power supply (274C, MKS Instruments), mechanical valves and cylinders. The operating conditions for the microwave plasma are given in Table 2.1.

2.1.2. Gas Sources Although the vapor deposition process for diamond utilizes temperature and pressure conditions under which graphite is the stable form of carbon, kinetic factors allow diamond to be produced by a commonly used net chemical reactionCH4(g)

^

C (diamond)

+

2H2

(2.1)

Generally, methane is used as the carbon source and diborane as the boron source.

In addition to methane, other hydrocarbon

gases can be used. In the present study, a mixture of acetone and methanol (9-1 volume ratio) was used as the carbon source, in a recipe

first

reported by Okano et al. [ l l ] .

Diborane

is

advantageous since the boron/carbon ratio (B/C) can be changed by controlling the respective mass flow rates.

However, the

explosively reactive nature of this system in the presence of oxygen requires a special security system. Therefore, it is safer to use B2O3 than diborane. Methanol is used in the carbon source since the solubility of B2O3 in acetone is very low. B2O3 is added to reach a B/C atomic ratio of 0.01 and the mixture is then ultrasonicated for complete dissolution.

The boron concentration in a film prepared under

these conditions was ca. 1.5 x lO^i cm"3 as estimated by Notsu et al., based on nuclear reaction measurements [iiB(p,a)^Be], carried out by means of 1-meV proton bombardment and subsequent comparison of the alpha spectrum in the 6-8 MeV region with a

17

BN standard [12]. When dissolved in the solution, B2O3 reacts with methanol, to form a boron ester. B2O3 + 6CH3OH

^

2B(OCH3)3 + 3H2O

(2.2)

This boron ester has a very high vapor pressure, similar to that of methanol, and therefore boron is introduced easily.

To

introduce acetone and B2O3 more efficiently, hydrogen gas was used as a carrier gas, by bubbling through the carbon source liquid.

2.1.3. Pretreatment of Substrates Many substrates can be used for the growth of diamond. Many have the characteristic of forming carbides at the interface between the substrate and the diamond crystal. Si, Mo, W, Ti, Nb are commonly used substrates for the growth of diamond films. Si is the most commonly used substrate, because it has a structure similar to diamond. Nucleation of the substrate is necessary for the deposition of diamond. Generally, the substrate is either scratched with abrasive diamond powder or is pretreated with the abrasive powder under ultrasonic generation. For example, the Fujishima group at the University of Tokyo used n-type silicon (lOO) wafers (phosphorus-doped, with a thickness of 0.625 mm, and a resistivity of 0.005-0.018 Q cm). Surface pretreatment of the substrate is needed for the efficient growth of diamond due to the mismatch of the lattice constants for diamond and silicon.

Although no conclusive data has been

published on the mechanism of the nucleation enhancement, it is widely argued that either sub-micromolar scratches or other

18

2. Preparation and Characterization of Poly crystalline Chemical Vapor Deposited Borondoped Diamond Thin Films

structural defects at the substrate surface, induced by the polishing, produce active sites for nucleation [13]. Prior to deposition, silicon substrates were pretreated by surface polishing with 0.5-^im diamond powder (Type Micron+ SND, De Beers). After polishing, the substrates were rinsed in 2-propranol with ultrasonication.

2.2. Hot Filament-Assisted CVD Hot

filament-assisted

CVD

has

been

attracting

extensive

attention due to its low expense, the simplicity of apparatus required and the large area of the diamond films that may be produced. When the decomposition temperature of the gas phase is much higher than the growth temperature, a super-equilibrium of excited species results near the growth region. This is easily achieved by contacting the gas phase with a filament of refractory metal, which is heated by an electrical current. A filament form was chosen to heat the gas phase locally and to avoid overheating the deposition region and of the equipment. In hot-filament CVD, the tungsten filament, which is usually heated above 2000°C, is placed above the substrate heated at 6001000°C, on which diamond film is deposited. The research groups at the Fraunhofer Institute for Surface Engineering and Thin Films at Braunschweig, CONDIAS GmbH at Itzehoe in Germany, together with a group at the Swiss Center of Electronic and Microtechnology (CSEM) at Neuchatel in Switzerland use the HFCVD method for the deposition of their diamond electrodes. These research groups have developed the HF-CVD technology for

19

the production of electrodes with a areas up to 0.5 m^ (see Fig. 2.3). The above research groups produce diamond electrodes mainly on Si or Nd substrates. CSEM produces diamond films on Si discs of 100-mm diameter and

1-mm

thickness. These

electrodes are preferred in electrochemical cells with electrode stacks that are sensitive to the distance between electrodes. For efficiency and lifetime testing in comparison with standard dimensionally

stable

anodes

(DSA),

metal-based

diamond

electrodes, such as those based on Nb and recently also titanium, are used. These are referred to as DiaChem electrodes. Diachem electrodes are manufactured methane

and

dopant

gas

(diborane

by HFCVD with or

trimethylboron)

concentrations of up to 3.0 vol% in hydrogen. Typical filament temperatures

are

between

2300°C

and

2800°C.

Substrate

temperatures are between 700 and 925 °C. The films were deposited at a pressure of 10-150 mbar. Growth rates of 0.2 jum/h to 2 jLim/h are usually achieved in these systems, with boron concentrations in the films ranging between 500 and 8000 ppm.

20

2. Preparation and Characterization of Poly crystalline Chemical Vapor Deposited Borondoped Diamond Thin Films

^^€.

t%

t: w

I

1

^—^--SJ

Fig. 2.3. Photograph of polycrystalline film made by the hot

filament

CVD method.

2.3. Characterization The morphology of the films was characterized by scanning electron microscopy (SEM) with a JEOL JSM-5400 microscope. The surface morphology produced under the conditions used in this

research

consists

in

general

of

a

highly

polycrystalline film and uniformly deposited 5-|im

faceted, diameter

diamond crystallites can be obtained with good reproducibility from film to film. Fig. 2.3 shows a typical SEM image of a CVD boron-doped diamond film deposited in this way. However, some deviations in this faceting were sometimes observed as a function

of both sample position

and

gas

concentration in the feed gas. For example, in the region more

21

than 2.5 mm from the sample edge, less faceting was sometimes found.

Fig. 2.3. SEM micrograph of the polycrystalline diamond film Bachmann et al. have summarized diamond CVD results from a number of authors. The data were plotted in the form of the concentrations of carbon, hydrogen and oxygen in a triangular diagram [l4,15].

It is also clear that crystal growth is highly

affected by the substrate temperature [16-19]. Since CVD diamond is synthesized under conditions in which graphite synthesis is competitive, in many cases, a non-diamond component may be incorporated in its amorphous form. The film quality was confirmed by Raman spectroscopy because it is very sensitive to graphitic and amorphous carbon (a-C) componentsthe Raman scattering intensity of graphite or a-C is about 30 times greater than that of diamond.

Raman spectroscopic

measurements were carried out using an Ar^ laser (wavelength =

22

2. Preparation and Characterization of Poly crystalline Chemical Vapor Deposited Borondoped Diamond Thin Films

514.5 nm) in a Renishaw Raman Imaging Microscope System (Renishaw System 2000).

1600 1400 1200 1000 800 600 400 20O

800

1000

1200

1400

1600

1800

Raman shift (cm"^)

Fig. 2.4. Typical Raman spectrum of a polycrystalline CVD diamond film. A sharp characteristic

peak was observed of diamond

and

at

reflects

crystallinity, as shown in Fig. 2.4.

1332 cm'i, which a

high

degree

is of

In addition, a broad peak

centered at approximately 1200 cm'i was observed, which is usually attributed to either amorphous diamond or extremely small diamond crystallites [20]. The absence of a peak at -^1500 cm"i indicates that the polycrystalline films are free from sp^ carbon impurities. This type of peak was observed only in poor quality

films.

The

details

of

Raman

characterization

of

diamond thin films are discussed in detail in Chapter 5.

23

References 1. E. Bustarret, E. Gheeraert and K. Watanabe, Phys. Status

Solidi

A, 199, 1 (2003) 9. 2.

M. Nesdalek and K. Haenan, Semiconduct

Semimet,

76 (2003)

325. 3.

W. G. Eversole, U.S. Patent, 3030187 and 3030188 (1962)

4.

B. V. Derjaguin, B. V. Fedseev, B. V. Spytsyn, D. V. Lukyanovich, B. V. Ryabov and A. V. Lanentev, J. Cryst. Growth, 2 (1986) 380.

5. J. C. Angus, H. A. Will and W. S. Stanko, J. Appl. Phys., 39 (1968) 2915. 6.

B. V. Derjaguin, B. V. Spytsyn, L. L. Builov, A. A. Klochkov, A. E. Gorodetski and A. V. Smolyaninov, Dokl. Akad. Nauk. SSSR, 231 (1976) 333.

7.

S. Matsumoto, Y. Sato, M. Tsutsumi and N. Satake, J. Mater. ScL, 17 (1982) 3106.

8.

J. C. Angus, H. B. Martin, U. Landau, Y. E. Estefeeva, B. Miller and N. Vinokur, New Diamond Front. Carbon Technol,

9 (1999)

175. 9.

Diamond Electronic Properties

and Application,

ed. S.L. Pan, D.

R. Kania, Kluwer Academic Publisher, 1995. 10. M. Kamo, F. Takamura and Y. Sato, Abst New

Diamond

Sci. & Technol,

of P^ Int. Conf. on the

J a p a n New Diamond Forum

(1988). U . K . Okano, H. Naruki, Y. Akiba, T. Kuraso, M. lida and Y. Hirose, Jpn. J. Appl. Phys., 27 (1988) L173. 12. H. Notsu, I. Yagi, T. Tatsuma, D.A. Tryk and A. Fujishima, J.

24

2. Preparation and Characterization of Poly crystalline Chemical Vapor Deposited Borondoped Diamond Thin Films

Electroanal

Chem., 49 (2000) 31.

13. M. Kamo, Y. Sato, S. Matsumoto and N. Setaka, J. Cryst

Growth

62 (1983) 642. 14. P. K. Bechamnn, D. Leers and H. Lydtin, Diamond Relat

Mater.

1 (1991) 1. 15. T. H. Chein, Y. Wei and Y. Tzeng, Diamond Relat Mater 8 (1999) 1686. 16. Z. Sun, X. Shi, X. Wang, B. K. Tay, H. Yang and Y. Sun,

Diamond

Relat Mater, 7 (1998) 939. 17. J. W. Lindsay, J. M. Larson and S. L. Girshick, Diamond

Relat.

Mater. 6 (1997) 481. 18. Z. Sun, X. Shi, B.K. Tay, D. Flynn, X. Wang, Z. Zheng and Y. Sun, J. Cryst Growth 173 (1997) 402. 19. Y. Einaga, G-S Kim, S-G Park and A. Fujishima, Diamond

Relat.

Mater. 10 (2001) 306. 20. K. Honda, T. N. Rao, D.A. Tryk, A. Fujishima, M. Watanabe, K. Yasui and H. Masuda, J. Electrochem.

Soc. 147 (2000) 659.

25

3. Electrochemical Effects on Diamond Surfaces: Wide Potential Window, Reactivity, Spectroscopy, Doping Levels and Surface Conductivity Heidi B. Martin, Sally C. Eaton, Uziel Landau, and John C. Angus

3.1. Introduction Diamond electrodes have an extremely wide potential window of water stability, low background currents, chemical and mechanical stability, resistance to fouling, lack of a surface oxide, and controllable surface termination. These characteristics have led to application of diamond electrodes as electrochemical sensors, for electroanalysis, for electrochemical synthesis, and for anodic destruction of organic wastes. Boron-doped diamond also can be used as a transparent, conducting medium for analytical chemistry and

photoelectrochemistry

applications.

Furthermore,

a

fundamental understanding of the origin of the adsorbate-induced p-type surface conductivity of diamond may lead to devices and sensor applications.

John C. Angus 26

e-mail: [email protected]

3. Electrochemical Effects on Diamond Surfaces: Wide Potential Window, Reactivity, Spectroscopy, Doping Levels and Surface Conductivity

3.2. Early Studies of Diamond Electrodes The earliest study of diamond electrodes showed that diamond implanted with Zn had an overpotential for hydrogen evolution that was significantly greater than for diamond implanted with Ar and for glassy carbon [l]. This result indicated that the electrodes implanted with Zn may have exhibited some of the inherent electrochemical properties of the diamond rather than of an amorphous surface layer. However, zinc gives high overpotentials for hydrogen evolution.

Therefore, the zinc itself, if present in

sufficient amount at the surface, may have been responsible for the observed hydrogen overpotential. Pleskov et al. made the first extensive study of diamond electrodes [2]. The electrodes were un-doped, but had sufficient conductivity,

most

measurements.

likely

from

defects,

for

electrochemical

They found a photoresponse at sub-bandgap

wavelengths, which they attributed to excitation of electrons from mid-gap states to the conduction band.

Sakharova et al. made

early impedance studies of diamond electrodes [3]. Patel studied the photoresponse of diamond [4, 5], and Tenne et al. reported the reduction of nitrate to ammonia on boron-doped diamond electrodes [6]. Swain and co-workers described the low capacitance

and featureless

background

current of diamond

electrodes, which are desirable for electroanalytical and sensor applications [7-11].

Martin and coworkers showed that high

quality diamond electrodes had an extremely wide potential window of water stability [12-14]. Miller et al. reported on the use of implanted diamond as an electrode [15], and Carey used

27

diamond electrodes for the anodic destruction of organic wastes [16]. The early work has been summarized in reviews by Swain [17], Tenne and LevyClement [18], Swain et al. [19], Angus et al., [20],Kobashi [21], and Pleskov [22].

3.3. Properties of Diamond Electrodes 3.3.1. Wide potential window The surfaces of as-grown chemical vapor deposited diamond electrodes that have been cooled to room temperature under atomic hydrogen are terminated with hydrogen.

These surfaces are

hydrophobic and not attractive for adsorption. Therefore, electrode reactions that involve adsorbed intermediates may be strongly inhibited on diamond.

One result, initially attributed to the

hydrogen termination, is a very wide potential range over which the rate of water electrolysis is negligible [12*14]. Fig. 3.1 displays voltammograms of hydrogen and oxygen evolution from acid solution on various types of electrodes. Negligible electrochemical activity is observed on high quality polycrystalline diamond (or single crystal, not shown) over a very wide range, extending almost four volts. Lower quality polycrystalline diamond electrodes, which contain significant amounts of sp^ carbon have a potential window similar to that of glassy carbon and highly oriented pyrolytic graphite.

Within

the

"window"

of water

stability,

other

electrochemical reactions may be observed, which are important for sensor applications. Also, this wide window allows the generation of species with oxidation potentials much greater than molecular

28

3. Electrochemical Effects on Diamond Surfaces: Wide Potential Window, Reactivity, Spectroscopy, Doping Levels and Surface Conductivity

oxygen, for example, the persulfate ion [23]. These data in Fig. 3.1a are typical of diamond electrodes that have been scanned many times in electrolyte. As discussed further below, including Section 3.3.4 on surface reactivity, it is likely that some of the hydrogen termination has been replaced with oxygen containing groups. This suggests that other factors besides a hydrogen termination contribute to the wide potential window.

(a) H i ^ Quality Polycrystalline Diamond j

|(b)Ijow-QiiaIityPoIycrystalline Diamond O2 Ewlution

r

-2 /

4-^- • -0

-1

i

I

J,

-25

HzEwlution

ibaseliiie< 0.1 mA/cm

Vonse. = - 1 2 5 V -50^ Potential v s . SHE(V)

|(c) Platinum

-50^ Potential v s . SHE(V)

(d)HOPG (basal and edge) 25T

?

-4

-25 I iiMiseiiiie< 1 mA/cm

-sol Potential v s . SHE(V)

Fig. 3.1. Voltammograms for water electrolysis on four electrodes* a) high quality polycrystalline diamond; b) low quality polycrystalline diamond; c) platinum; and d) highly oriented pyrolytic graphite (HOPG). The supporting electrolyte was 0.5 M H2SO4; scan rate = 200 mV s i . Note the very wide potential window for water stabiHty on high-quaUty diamond, (a). Electrode potentials are given versus the standard hydrogen electrode.

29

Oxygen-terminated, boron-doped diamond has been reported to have a wider window of water stability and lower background currents than hydrogenated, boron-doped diamond [24, 25, 26]. Martin et al. [27] observed an irreversible oxidation at +1.4 V (SHE, 0.5M H2SO4) during the first positive scan on a hydrogenated diamond, which they attributed to oxidation of the surface. For a 0.1 M KH2PO4 electrolyte, oxygen evolution was reported at approximately +1.6 V vs. SHE for hydrogen-terminated diamond, + 1.9 V for electrochemically oxidized diamond, and +2.6 V for diamond exposed to an oxygen plasma [28].

3.3.2. Nano-crystalline and amorphous materials Nano-crystalline diamond has a wide potential window of water stability, similar to that of diamond [29]. The current peak at +1.7 V vs. SHE, often found in voltammograms of polycrystalline diamond, is absent from these films. Yoo and co-workers reported that tetrahedral amorphous carbon, ta-C*N,

had a chemical

stability similar to that of boron-doped diamond and a wider potential window for water stability [30]. The ta-C-N electrodes were more catalytic for electron-transfer reactions than diamond electrodes, e.g., they exhibited reversible behavior with outersphere couples. These properties are believed to arise from the presence of sp^ carbon and nitrogen within the

amorphous

structure.

3.3.3. Example of diamond as a sensor- detection of plating additives The voltammetric detection of typical copper plating additives, polyalkylene glycol (PG) and disodium (bis(3-sulfopropyl)) disulfide 30

3. Electrochemical Effects on Diamond Surfaces: Wide Potential Window, Reactivity, Spectroscopy, Doping Levels and Surface Conductivity

(SPS), provides an example of the utility of diamond electrodes for sensor applications voltammograms

[31].

measured

Figs. 3.2a and on

platinum

3.2b show cyclic

and

poly crystalline

diamond of CUSO4/H2SO4 solutions containing PG and SPS, respectively.

The insets of Figs. 3.2a and 3.2b show scans for

platinum and diamond on a 1-mA scale. On platinum, oxygen evolution occurs at +1.4V (SHE).

No

peaks corresponding to oxidation or reduction of the glycol (PG) or disulfide (SPS) are visible, because the potentials for these reactions are more positive than the potential for water electrolysis on platinum, Le., they are outside the "window" of water stability. However, on diamond, two oxidative peaks are observed for each additive: +2.05V and +2.4 V (SHE) for PG (Fig. 3.2a), and +1.65 V and +2.1 V (SHE) for SPS (Fig. 3.2b).

The oxygen evolution

observed at +2.4V on diamond does not interfere with detection of these compounds.

Moreover, the relationship between the peak

heights and the additive concentration is linear [31] as shown in Fig. 3.3.

31

1mA !^

Diamond

LAJ

Platinum

Diamond

.—--:>• 0,5

1.5

•:

2.5

^^

;•

Platinum

1 QJ5 ^

:'

.' [

(200mg/L) ; :

'Wi

I' -

/

.'^

/I

/lOO

'^ 0.05 -

/

/^—

/

^

yOmg/L i

0.5 -0.05 ~

1.5 2.5 Potential vs. SHE(V)

^ ImA Platinum

0|5

Diamond

1.5

:

2-5

Diamond

|

w

;•

ill

Platinum '• (80mg/L) ;

1 0.015

80

1

u u

1

U 0.005 ^

/ 40 ^

-0.005^*^

•"-•"

-••-'

Poteiitial vs .SHE(V)

/

yOmg/L 2.5

Fig. 3.2. Cyclic voltammograms of varying concentrations of (a) polyalkylene glycol (PG) and (b) disodium (bis(3-sulfopropyl)) disulfide (SPS) in CUSO4/H2SO4 on platinum and polycrystalline diamond [31]. The insets show scans with a 1-mA full scale. The oxidation peaks of the two organic compounds are detectable on diamond, but not on platinum. Note that in Fig. 3.2a the concentrations of PG are 0, 20, 50, 100, 200 and 400 mg LK

32

3. Electrochemical Effects on Diamond Surfaces: Wide Potential Window, Reactivity, Spectroscopy, Doping Levels and Surface Conductivity

0.20 T

R^ = 0.988

Peak Currents Measured for Oxidation Peak at E„ = +2.1 V J

100

200

I

I

1

300

I

I

L_l

|_B

1

1

1

400

1

500

[Polyalkylene Glycol] (mg/L)

Fig. 3.3. Oxidation peak currents for polyalkylene glycol at Ep=+2.1 V v s . SHE (from Fig. 3.2) increase linearly with increasing glycol concentration [31].

3.3.4. Reactivity Diamond has remarkable chemical stability compared to other electrode materials.

Li et al. [32] liberated Li from Li+ in a

polymeric electrolyte on a diamond cathode. No intercalation of lithium or any reactivity with the diamond was found.

Diamond

electrodes can easily oxidize cyanide under conditions where traditional anodes such as Pb, Ir02, and Ru02 fail [33]. Diamond

electrodes are, however,

not completely

inert.

Chemically bound oxygen appears on a diamond surface after anodic polarization or oxygen plasma treatment, and the electrode surface changes from hydrophobic to hydrophilic [2, 14, 27, 28, 34, 35, 36]. This is observed on both poly crystalline and single crystal

33

diamond.

However, cyclic voltammetry shows the presence of a

redox couple at +1.7 V vs. SHE on polycrystalline diamond that does not appear on single crystal electrodes [27], which implies some grain boundary reactivity not found on single crystal diamond. Electrodes with significant sp^ carbon in the grain boundaries are less stable under anodic polarization and eventually disintegrate. Fluorination [37, 27] or anodic treatments in basic solution [38, 25] have been shown to either passivate or etch the sp^ carbon in the grain boundaries, as evidenced by the absence of the redox couple at+1.7 V (SHE).

3.3.5. Transparency Diamond's relative transparency, even when doped, permits it to be used as a transparent electrode. Martin and Morrison [39] have exploited this phenomenon to detect changes on the diamond surface

during polarization. They found

evidence that

OH

functional groups appeared on the diamond surface, primarily ( i l l ) facets, during anodic polarization. In Fig. 3.4, one can see that the 3240 cm 1 0-H and 1100 cm ^ C"0 stretch peaks increase with increasing polarization. Notsu et al. [40], using a chemical method, independently observed hydroxyl groups on ( i l l ) films, through preferential coupling to the -OH. Zak et al. [41] have also reported transparent

diamond

electrodes

for

electroanalysis-

ferri/ferrocyanide and methyl viologen were electrooxidized in a thin layer transmission cell with a free-standing diamond disk as the transparent electrode.

34

3. Electrochemical Effects on Diamond Surfaces: Wide Potential Window, Reactivity, Spectroscopy, Doping Levels and Surface Conductivity

IJ

o > o o

1-

1.0 V-forward 995- 2.0 V - forward 3.0 V - forward 1.5 V - reverse QQ—

1

\

\

1

\

5000

4000

3000

2000

1000

.W

Wavenumber (cm-1) Fig. 3.4. Infrared spectra taken during step-wise polarization of a diamond electrode at various potentials [39]. The feature at 3240 cm'^ is assigned to O'H stretching while the feature at 1100 cm'i is assigned to C-0 stretch. Both features increase with increased polarization.

3.4. Semiconducting Diamond Electrodes 3 . 4 . 1 . D o p i n g of d i a m o n d Boron-doped diamond has been widely studied, and some of the properties of relevance to electrochemistry have been reviewed [20, 42, 43].

Substitutional boron at low concentrations gives an

acceptor level at 0.37 eV above the valence band [44]. At very high concentrations of boron (> lO^o cm'^), a dopant band is formed [4449].

The resistivity ranges from about 10^ Q cm at a boron

concentration of lO^^ cm'^ to tenths and thousandths of an Q cm for boron concentrations of the order of lO^i cm'^. At high boron levels,

35

the potential window of water stability decreases and

the

crystalline quality decreases [50]. High levels of boron incorporation are desired for applications where low resistivity is required. The boron incorporation on ( i l l ) faces is approximately ten times greater than on (lOO) faces [51, 52]. Also, higher boron levels are achieved in hot-filament reactors than in microwave plasma reactors [53]. The presence of oxygen in the reaction gas greatly reduces the concentration of boron incorporated in the diamond, presumably because of the formation of stable oxides of boron [54-57].

These results on boron

incorporation are summarized in the review by Angus et al. [20]. Nitrogen and phosphorus give deep donor levels in diamond, 1.6 eV and 0.6 eV below the conduction band, respectively. Sulfur has been reported to give n-type conductivity [58, 59]. However, other work indicated that the samples contained boron and were ptype [60]. Eaton et al. found that sulfur incorporation in diamond was facilitated by the presence of boron [61-63]. They obtained diamond with n-type conductivity by co-doping with sulfur and small quantities of boron; however, the sulfur was concentrated in the near surface region [63]. Density functional calculations by Albu et al. [64] predict that substitutional S and BS centers are deep donors, each with a level about 1.5 eV below the conduction band, which is too deep to provide significant thermal excitation at room temperature. However, they also found more complex B/S/H centers that produced midgap states that might lead to impurity band conduction at sufficiently high concentration.

Eaton et al

[65] performed electrochemical measurements on the B/S co-doped n-type diamond. Mott-Schottky measurements showed a positive

36

3. Electrochemical Effects on Diamond Surfaces: Wide Potential Window, Reactivity, Spectroscopy, Doping Levels and Surface Conductivity

slope of the C^ vs. V curve, consistent with the presence of donor centers.

The observed flat band potential was consistent with

conduction through midgap impurity states.

3.4.2. Electrode potentials and electron energies The relationship between electrode potentials and electron energies is shown in Fig. 3.5. The connection between these scales was made by Gurevich and Pleskov [66] and by Bard et aL [67]. The relationship between the electrode potential, E, in volts and the electron energy, 8 , in electron volts is eE = 4.44 + 8

(3.1)

where e = - 1 is the charge on an electron. The electron energy, 8, is referenced to the electron at rest in vacuum and E is referenced to the standard hydrogen electrode. The potentials of several common electrochemical couples and the estimated positions of the band edges of hydrogen-terminated diamond [68] determined by electron photoemission spectroscopy are shown in Fig. 3.5. The estimated positions of the band edges of diamond in contact with an aqueous solution determined by measuring the flat'band potential using electrochemical methods is also shown.

The flat-band potential,

Efb , gives the position of Fermi level, Ep , on the electrode potential scale. Hence, knowing Ef^ , one can obtain the energy, Ep, of the Fermi level from Eq. (3.1).

37

Reversible Potential

E [VL

£ Bectron Energy

m

Diamond

Hydrogen Terminated

in 0.5M 1^804 Solution

Diamond

1.3 eV

-0.04eV Li + e = Li

5.5 eV

E = -1.39eV 5.5 eV

2hr + 2e' = K

0 "

0^ + 4H* + 4e=2h^oJ_

.£VBM = -4.2 eV

£ = -4.44 eV -1 e = ^.83eV(pH--<4^ J f^TOiJ^E = -5.e6eV(pH=0) -6

-5.54eV

Fig. 3.5. Relationship of electrode potentials, E, and electron energies, £. Also shown are t h e band edges of hydrogen-terminated diamond in vacuum [68] and the approximate position of the band edges of diamond in contact with an aqueous solution. The electrode potentials and t h e electron energies for t h e water oxidation couple O2 + 4H+ + 4e = 2H2O are shown for p H = 0 and p H = 14. These energies straddle the estimated position of the Fermi level in diamond in contact with aqueous solution, which is approximately - 5 . 4 eV. Note- electrode potentials, E, are versus the standard hydrogen electrode; electron energies, e, are measured with respect to the electron a t rest in vacuum,' ECBM = energy of conduction band maximum; EVBM = energy of valence band m a x i m u m .

Yagi a n d coworkers [69] found t h e flat-band potential of asgrown, hydrogen t e r m i n a t e d diamond to be approximately 0.2 V vs. S H E . Oxidation of the diamond surface, either anodically or in a n oxygen p l a s m a

[35, 70], shifts t h e

flat-band

potential

toward

positive values (1.2 to 1.7 V vs. SHE). I n Fig. 3.5, t h e b a n d edge energies in aqueous solution were placed using E p = Ef^ » 1 . 0 e V (SHE), a typical value for diamond in contact with

38

aqueous

3. Electrochemical Effects on Diamond Surfaces: Wide Potential Window, Reactivity, Spectroscopy, Doping Levels and Surface Conductivity

solutions [71- 77]. From Eq. (3.1) the energy of the Fermi level is given

by

Ep = eEp - 4.44 « ~1.0(l)- 4.44 = -5.44eV

.

For

relatively heavily doped samples, we estimate that the energy of the valence band maximum is approximately 0.10 eV lower than ep; therefore eyBM ^ - 5 . 5 4 e V . Since the bandgap of diamond is 5.5 eV, this puts the conduction band minimum slightly below the vacuum level, which is significantly lower than the position shown in Fig. 3.5 for the conduction band minimum of hydrogen terminated diamond in vacuum [68]. The lowering of the band edge energy of diamond in contact with water may be caused by partial oxidation of the diamond surface, and may also be influenced by the dipole induced by an adsorbed water layer at the surface [78].

Different extents of

oxidation and different polar groups on the surface will result in changes of the position of the band edges from the positions shown in Fig. 3.5. Vinokur et al. used well-defined electrochemical couples to probe the band structure of diamond [79]. They showed that, for lightly doped diamond electrodes, the irreversibility of the electrode increased as the potential of the couple became more negative, i.e., moved

higher

in

the

bandgap,

for

the

series

Fe(o-

phenanthroline)Cl3 (1.08 V); ferrocene (1,1') dimethanol (0.42 V); Ru(NH3)6Cl3 (-0.01 V); methyl viologen (-0.45 V). (The electrode potentials are versus SHE.)

The increased irreversibility was

attributed to a decreased number of available charge carriers for couples with electron energies higher in the gap, i.e., at more negative

electrode

potentials.

Heavily

doped,

semimetallic

39

diamond

showed

no

depletion

of charge

carriers

and

httle

overpotential.

3.5. Surface Conductivity of Diamond 3.5.1. Background L a n d s t r a s s a n d Ravi first reported t h e p ' t y p e surface conductivity of diamond [80, 81]. Gi et al. [82, 83] showed t h a t the surface conductivity

increased

when

exposed

to

acidic

vapors

and

decreased when exposed to basic vapors. This near-surface p"type conductivity is characterized by a high carrier sheet density of about 1013 c m - from 150 - 400 K, an activation energy of less t h a n 50 meV, and a low density of surface s t a t e s [78, 84-86].

These

a t t r i b u t e s have led to its application in electronic devices such as field effect t r a n s i s t o r s [84-86]. An electrochemical transfer doping m e c h a n i s m w a s proposed by Maier et al. [87], which w a s further discussed by Garrido et al. [88], Foord et al [89], a n d Ristein et al. [90]. An adsorbed w a t e r film on the diamond surface will have a n electrochemical potential fixed by the dissolved components, for example, O2 a n d CO2. If the chemical potential of t h e electrons in t h e film is less t h a n t h e F e r m i level of the diamond, electrons can transfer diamond to t h e

film.

from

the

The result is a positively charged, p-type

space-charge region in the diamond t h a t is compensated by t h e excess negative ions in the film. The basis of the effect can be seen in Fig. 3.5 in which t h e electrochemical potential of t h e couple O2 + 4H+ + 4e = 2H2O

40

3. Electrochemical Effects on Diamond Surfaces: Wide Potential Window, Reactivity, Spectroscopy, Doping Levels and Surface Conductivity

is shown at both pH = 0 and pH - 14 at an oxygen partial pressure of 0.21 atm. At pH=0, the chemical potential of electrons in the film is less than the Fermi level in the diamond and the p-type conducting surface layer is formed.

At pH=14, the chemical

potential of electrons in the film is greater than the Fermi level, electrons transfer into the diamond, and the p"type conductivity of the diamond surface region is suppressed. The remarkable effect of the gas phase ambient on the surface conductivity of diamond is shown in Fig. 3.6 [91]. dramatic

changes

in

the

surface

conductivity

of

Note the diamond

immediately upon exposure to an acidic or basic vapor.

Added NH.^ vapor (pH 11.3)

Added HCl vapor (pH 1.4)

logi

0

0.5

1.0 time, hours

Fig. 3.6. Changes in the surface conductance of diamond upon exposure to HCl and NH3 vapors from HCl and NH3 solutions of the indicated pH [91]. Plotted as the log of the current versus time.

41

3.5.2. Processes during electrochemical transfer doping The processes that take place during electrochemical transfer doping have been described by Chakrapani et aL [92] and are listed below. We assume that equilibrium is established and that the acidity arises from dissolved CO2. 1. Formation of a water film on the diamond in equilibrium with CO2 a n d O2.

^ 2 ^ . = H,0^,^

O, ,^, = O,,,,

CO,,,

= CO, ^,^

2. Formation of hydronium ions and anions in the water film

3. Transfer of electron from the valence band of diamond to the film forming a hole \^ h^)di3 -^efiin,+hjj^ dia 4. Equilibrium of the electrochemical couple,

The overall net reaction is obtained by summing the above processes using the appropriate stoichiometric coefficients.

One

obtains

The overall process summarized in the above equation results in the formation of a space charge layer of holes, h^-^, in the diamond and compensating anions in the adsorbed film. At moderate pH, if

42

3. Electrochemical Effects on Diamond Surfaces: Wide Potential Window, Reactivity, Spectroscopy, Doping Levels and Surface Conductivity

the diamond is exposed to air containing CO2, the anion will be the bicarbonate ion, HCO3 ; if exposed to HCl, the anion will be CI . There will be an electrostatic attraction between the positive space charge in the diamond and the excess solvated anions in the water film. These solvated anions provide a mechanism for binding the water film to the hydrogen-terminated, hydrophobic diamond surface.

3.6. Summary Diamond is finding application as an electrode material because of its extremely wide potential window in aqueous solution, low background currents, resistance to fouling, chemical stability in aggressive environments, lack of surface oxide, transparency, and relative ease of functionalization. These attributes make diamond especially attractive for sensors, for the generation of highly reactive intermediates, for electrosynthesis, and for spectroscopic studies of surface processes. Furthermore, the inter-relationship between surface

conductivity and the chemical

environment

provides another means for the study of diamond and may serve as the basis for sensors.

References 1. M. Iwaki, S. Sato, K. Takahashi and H. Sakairi, NucL Instrum. Methods in Phys. Res., 209-210 (1983) 1129.

43

2. Y. V. Pleskov, A. Y. Sakharova, M. D. Krotova, L. L. Bouilov and B. V. Spitsyn, J. Electroanal 3.

Chem, 228 (1987) 19.

A. Sakharova, L. Nykos and Y. V. Pleskov, Electrochim.

Acta, 37

(1992) 973. 4.

K. Patel, K. Hashimoto and A. Fujishima, J. Photochem.

Photobiol

^.•C6e/72, 65(1992)419. 5.

K. Patel, K. Hashimoto and A. Fujishima, Denki

Kagaku,

60

(1992) 659. 6.

R. Tenne, K. Patel, K. Hashimoto and A. Fujishima, J.

Electroanal.

Chem., 347 (1993) 409. 7.

G. M. Swain and R. Ramesham, Anal

Chem., 65 (1993) 345.

8.

G. M. Swain, Adv. Mater., 6 (1994) 388.

9.

G. M. Swain, J. Electrochem.

Soc, 141 (1994) 3382.

10. S. Alehashem, F. Chambers, J. W. Strojek, G. M. Swain and R. Ramesham, Anal

Chem., 67 (1995) 2812.

11. M. Awada, J. W. Strojek, and G. M. Swain, J. Electrochem.

Soc,

142 (1995) L42. 12. H. B. Martin, A. Argoitia, J. C. Angus, A. B. Anderson and U. Landau, in "'Applications of Diamond Films and Related Third International

Conference

Materials-

(A. Feldman, et al, Eds.) NIST

Special Publication 885, U.S. Dept. of Commerce (1995) 91. 13. A. Argoitia, H. B, Martin, E. J. Rozak, U. Landau and J. C. Angus, in ''Diamond for Electronic Applications

(D.L. Dreifus, et al., Eds.),

Mater. Res. Soc. Proc. 416 (1996) 349. 14. H. B. Martin, A. Argoitia, U. Landau, A. B. Anderson and J. C. Angus, J. Electrochem.

Soc, 143 (1996) L133.

15. B. Miller, R. Kahsh, L. C. Feldman, A. Katz, N. Moriya, K. Short and A. E. White, J. Electrochem.

44

Soc, 141 (1994) L41.

3. Electrochemical Effects on Diamond Surfaces: Wide Potential Window, Reactivity, Spectroscopy, Doping Levels and Surface Conductivity

16. J. J. Carey, C. S. Christ Jr. and S. N. Lowery, U.S.

Patent,

5399247 (1995). 17. G. M. Swain, Adv. Mater., 6 (1994) 388. 18. R. Tenne and C. Levy-Clement, IsraelJ.

Chem., 38 (1998) 57.

19. G. M. Swain, A. B. Anderson and J. C. Angus, MRS Bulletin

23

(1998) 56. 20. J. C. Angus, H. B. Martin, U. Landau, Y. E. Evstefeeva, B. Miller and N. Vinokur, New Diamond Front. C Tech. 9 (1999) 175. 21. K. Kobashi, Collection of articles from New

Diamond

Front.

Carbon TechnoL, 9 Nos. 3 & 5 (1999). 22. Y. V. Pleskov, Russ. Chem. Rev., 68 (1999) 381. 23. P. A. Machaud, E. Mahe, W. Haenni, A. Ferret and C. Comninellis, Electrochem.

SolidState

Lett,

3 (2000) 77.

24. T. Yano, D. A. Tryk, K. Hashimoto and A. Fujishima, Electrochem.

J.

Soc, 145 (1998) 1870.

25. T. Yano, E. Fopa, D. A. Tryk, K. Hashimoto and A. Fujishima, J. Electrochem.

Soc, 146 (1999) 1081.

26. I. Yagi, H. Notsu, T. Kondo, D. A. Tryk and A. Fujishima, J. Electroanal.

Chem., 473 (1999) 173.

27. H. B. Martin, J. C. Angus and U. Landau, J. Electrochem.

Soc,

146 (1999) 2959. 28. D. A. Tryk, K. Tsunozaki, T. N. Rao and A. Fujishima, Relat. Mater,

Diamond

10 (2001) 1804.

29. Q. Chen, D. M. Gruen, A. R. Krauss, T. D. Corrigan, M. Witek and G. M. Swain, J. Electrochem.

Soc, 148 (2001) E44.

30. K. Yoo, B. Miller, R. KaUsh and X. Shi, Electrochem.

Solid-State

Lett, 2 (1999) 233. 31. H. B. Martin, J. J. D'Urso, P. W. Morrison, U. Landau and J. C.

45

Angus, in Diamond

Materials

VI (J.C. Angus, et al,

Eds.)

Electrochem. Soc. Proc. 99-32 (1999) 491. 32. L. F. Li, D. Totir, B. Miller, G. Chottiner, A. Argoitia, J. C. Angus and D. A. Scherson, J. Am. Chem. Soc, 119 (1997) 7875. 33. A. Perret, W. Haenni, N. Skinner, X. M. Tang, D. Gandini, C. Comninellis, B. Correa and G. Foti, Diamond.

Relat

Mater.,

8

34. T. N. Rao, D. A. Tryk, K. Hashimoto and A. Fujishima,

J.

(1999) 820.

Electrochem.

Soc, 146 (1999) 680.

35. H. Notsu, I. Yagi, T. Tatsuma, D. A. Tryk and A. Fujishima, Electrochem.

Solid-State

Lett, 2 (1999) 522.

36. H. Notsu, I. Yagi, T. Tatsuma, D. A. Tryk and A. Fujishima, J. Electroanal

Chem., 492 (2000) 31.

37. A. Argoitia, H. B. Martin, J. C. Angus and U. Landau, in Diamond Materials

1^(J.L. Davidson, et al, Eds.) Electrochem. Soc. Proc.

97-32 (1998) 364. 38. R. DeClements and G. M. Swain, J. Electrochem.

Soc, 144 (1997)

3806. 39. H. B. Martin and P. W. Morrison Jr., Electrochem.Solid-State

Lett,

4(2001)E17. 40. H. Notsu, T. Fukazawa, T. Tatsuma, D. A. Tryk and A. Fujishima, Electrochem.

Solid-State

Lett, 4 (2001) H I .

41. J. K. Zak, J. E. Butler and G. M. Swain, Anal Chem., 73 (2001) 908. 42. C. Levy-Clement, F. Zenia, N. A. Ndao and A. Deneuville, Diamond Front

Carbon Technol, 9 (1999) 189.

43. T. Inushima, T. Matsushita, S. Ohya, H. Shiomi, Diamond Mater,

46

New

9 (2000) 1066.

Relat

3. Electrochemical Effects on Diamond Surfaces: Wide Potential Window, Reactivity, Spectroscopy, Doping Levels and Surface Conductivity

44. A. T. Collins, Trans. Boy. Soc. A, 342 (1993) 233. 45. Y. F. Tsai, K. P. Ananthanarayanan, P. J. Gielisse and S. S. Mitra, J. Appl. Phys., 43 (1972) 3677. 46. J. Mort, K. Okumura and M. A. Machonkin, Phil. Mag. B, 63 (1991) 1031. 47. D. Narducci, C. R. Guarnieri and J. J. Cuomo, J. Electrochem.

Soc,

138 (1991) 2446. 48. K. Nishimura, K. Das and J. T. Glass, J. Appl. Phys., 43 (1991) 3142. 49. P. Gonon, E. Gheeraert, A. Deneuville, F. Fontaine, L. Abello and G. Lucazeau, J. Appl. Phys., 78 (1995) 7059. 50. A. N. Ndao, F. Zenia, A. Deneuville, M. Bernard Clement, DiamondBelat

and C. Levy

Mater., 9 (2000) 1175.

51. B. V. Spitsyn, L. L. Bouilov and B. V. Deryagin, J. Cryst.

Growth.,

52 (1981) 219. 52. G. Janssen, W. J. P. van Enckevort, W. VoUenberg and L. J. Giling, Diamond Bel. Mat, 1 (1990) 789. 53. M. C. Polo, J. Cifre, J. Puigdollers and J. Esteve, Thin Solid

Films,

253 (1994) 136. 54. H. Kawarada, H. Matsuyama, Y. Yokota, T. Sogi, A. Yamaguchi and A. Hiraki, Phys. Bev. B, 47 (1993) 3633. 55. T. Tachibana, Electrochem.

Y. Yokota, K. Hayashi,

and

K. Kobashi,

J.

Soc, 146 (1999) 1996.

56. I. Sakaguchi, M. Nishitani-Gamo, K. P. Loh, K. Yamamoto, H. Haneda, and T. Ando, Diamond Belat. Mat, 7 (1998) 1144. 57. M. L. Hartsell and L. S. Piano, in Diamond Materials III (K.E. Spear, et al., Eds.) Electrochem. Soc. Proc. 93-17 (1993) 462. 58. I. Sakaguchi, M. Nishitani-Gamo, Y. Kukuchi, E. Yasu,

H.

47

Haneda, T. Suzuki and T. Ando, Phys. Rev. B, 60 (1999) 2139. 59. M. Nishitani-Gamo, C. Xiao, Y. Zhang, E. Yasu, Y. Kukuchi, L Sakaguchi and T. Ando, Thin Solid Films, 382 (2001) 113. 60. R. Kalish, A. Resnick, C. Uzan-Saguy and C. Cytermann,

App.

Phys. Lett, 76 (2000) 757. 61. S. C. Eaton, A. B. Anderson, J. C. Angus, Y. E. Evstefeeva and Y. V. Pleskov, in Diamond

Materials

VII (G. M. Swain, et al,

Eds.)

Electrochem. Soc. Proc. 2001-25 (2001) 139. 62. S. C. Eaton, A. B. Anderson, J. C. Angus, Y. E. Evstefeeva and Y. V. Pleskov, Electrochem.

Solid-State

Lett,

5 (2002) G-65.

63. S. C. Eaton, A. B. Anderson, J. C. Angus, Y. E. Evstefeeva and Y. V. Pleskov, DiamondRelat

Mater., 12 (2003) 1627.

64. T. V. Albu, A. B. Anderson and J. C. Angus, J. Electrochem.

Soc,

149 (2002) E143. 65. S. C. Eaton, Y. E. Evstefeeva, J. C. Angus, A. B. Anderson and Y. V. Pleskov, Elektrokhimiya QQ. Y. Y. Gurevich

and

39 (2003) 2 (In Russian). Y. V. Pleskov,

(translation of Elektrokhimiya),

Russ.

J.

Electrochem.

18 (1982) 1477.

67. A. J. Bard, R. Memming and B. Miller, Pure Appl

Chem., 63

(1991) 569. 68. J. B. Cui, J. Ristein and L. Ley, Phys. Rev. Lett, 81 (1998) 429. 69. I. Yagi, K. Tsunozaki, D. A. Tryk and A. Fujishima, Solid-state

Electrochem.

Lett, 2 (1999) 457.

70. C. H. Goeting, F. Marken, A. Gutierrez-Sosa, R. G. Compton and J. S. Foord, Diamond Relat Mater,

9 (2000) 390.

71. Y. V. Pleskov, A. Y. Sakharova, E. V. Kasatkin and V. A. Shepelin, J. Electroanal

Chem., 344 (1993) 401.

72. Y. V. Pleskov, V. Y. Mishuk, M. A. Abaturov, V. V. Elkin, M. D. 48

3. Electrochemical Effects on Diamond Surfaces: Wide Potential Window, Reactivity, Spectroscopy, Doping Levels and Surface Conductivity

Krotova, V. P. Varnin and I. G. Teremetskaya, J.

ElectroanaL

Chem,, 396 (1995) 227. 73. A. Y. Sakharova, Y. V. Pleskov, F. Di Quarto, S. Piazza, C. Sunseri, I. G. Teremetskaya and V. P. Varnin, J.

Electrochem.

Soc, 142 (1995) 2704. 74. Y. V. Pleskov, V. V. Elkin, M. A. Abaturov, M. D. Krotova, V. Y. Mishuk,

V.

P.

Varnin

and

I.

G.

Teremetskaya,

J.

ElectroanaL Chem., 413 (1996) 105. 75. Y. V. Pleskov, A. Y. Sakharova, A. V. Churikov, V. P. Varnin and I. G.

Teremetskaya,

Elektrokhimiya),

Russ.

J.

Electrochem.

(translation

of

32 (1996) 1075.

76. L. Boonma, T. Yano, D. A. Tryk, K. Hashimoto and A. Fujishima, J. Electrochem.

Soc, 144 (1997) L142.

77. A. D. Modestov, Y. V. Plesko, V. P. Varnin and I. G. Teremetskaya, Buss. J. Electrochem.

(translation

of Elektrokhimiya),

33 (1997) 55.

78. G. Piantanida, A. Breskin, R. Chechik, O. Katz, A. Laikhtman, A. Hoffman and C. Coluzza, J. Appl Phys., 89 (2001) 8259. 79. N. Vinokur, B. Miller, Y. Avyigal and R. Kalish, J.

Electrochem.

Soc, 143 (1996) L238. 80. M. I. Landstrass and K. V. Ravi, Appl Phys. Lett,

55 (1988) 975.

81. M. I. Landstrass and K. V. Ravi, Appl. Phys. Lett,

55 (1989) 1391.

82. R. S. Gi, T. Mizumasa, Y. Akiba, Y. Hirose, T. Kurosu and M. lida, Jpn. J. Appl. Phys. Part 1, 34 (1995) 5550. 83. R. S. Gi, T. Ishikawa, S. Tanaka, T. Kimura, Y. Akiba and M. lida, Jpn. J. Appl. Phys. Part 1, 36 (1997) 2057. 84. K. Tsugawa, K. Kitatani, H. Noda, A. Hokazono, K. Hirose, M. Tajima and H. Kawarada, Diamond. Relat Mater,

8 (1999) 927.

85. A. Aleksov, A. Denisenko, U. Spitzberg, T. Jenkins, W. Ebert and E.

49

Kohn, DiamondRelat

Mater., 11 (2002) 382.

86. H. Ishizaka, H. Umezawa, H. Taniuchi, T. Arima, N. Fujihara, M. Tachiki and H. Kawarada, Diamond Relat Mater., 11 (2002) 378. 87. F. Maier, M. Riedel, B. Mantel, J. Ristein and L. Ley, Phys.

Rev.

Lett, 85 (2000) 3472. 88. J. A. Garrido, C. E. Nebel, M. Stutzmann, E. Snidero, and P. Bergonzo, Appl. Phys. Lett, 81 (2002) 637. 89. J. S. Foord, C. H. Lau, M. Hiramatsu, R. B. Jackman, C. E. Nebel and P. Bergonzo, Diamond Relat Mater., 11 (2002) 856. 90. J. Ristein, M. Riedel and L. Ley, to appear J. Electrochem.

Soc. 151

(2004) E315. 91. S. C. Eaton, Ph.D. thesis, Case Western Reserve University (2003). 92. V. Chakrapani, S. C. Eaton, A. B. Anderson, M. Tabib-Azar and J. C. Angus, submitted

50

to Electrochem.

SolidState

Lett (2004).

4. Electrochemistry of Diamond: Semiconductor and Structural Aspects Yuri V. Pleskov

4.1. Introduction Being extraordinarily stable and corrosion-resistant, diamond is an attractive electrode material for use in theoretical and applied electrochemistry [l]. In particular, diamond electrodes have been reported to be stable and effective electrodes for environmentallyoriented [2] and analytical [3,4] purposes. As with many insulators, diamond can be transformed into a wide-gap semiconductor by appropriate doping. Boron practically is the only "shallow" acceptor dopant that makes diamond conducting at room temperature (most other dopants have too large an ionization energy) [5]. Recently, sulfur was suggested as an equally shallow donor dopant [6]. Depending on the doping level, diamond exhibits properties either of a semiconductor (e.g., at boron content from 10 to 1000 ppm) or a "poor metal" (with up to 10,000 ppm of B or even higher). It is the heavily doped diamond that is used as an electrode material in electrosynthesis, electroanalysis, etc. Yuri V. Pleskov e-mail: [email protected] 51

However, in this chapter we shall focus our attention on moderately doped diamond, because it is most suitable for revealing effects of the semiconductor nature and crystal structure on the electrochemical properties of this material.

4.2. Effects of the Semiconductor Nature of Diamond 4.2.1. Current—Voltage Curves It

is

known

that

one

of

the

characteristic

features

of

semiconductor/metal contacts is the rectification of electric current. The

asymmetry

of

current—voltage

semiconductor/metal contact is

curves

at

a

quite pronounced- the "direct"

current passing the contact can be rather large, while the reverse (blocking)

current

is

very

small.

The

current—voltage

characteristics at some "ideal" semiconductor electrodes do follow this law (e.g., anodic dissolution of n-type germanium, etc.) [7]. For p-type boron-doped diamond electrodes, the blocking direction of current is cathodicJ for n-type sulfur-treated diamond, anodic. The asymmetrical current—voltage characteristics have indeed occasionally been observed in redox electrolytes with lightly doped diamond samples [l, 8 , 9 ] (Fig. 4.1). However, more abundant are symmetrical cyclic voltammograms, the more so for heavily doped diamond (Fig. 4.2 [lO]). Different redox systems differ in their peak-to-peak potential difference AEp on the cyclic voltammograms- the faster is the electrochemical reaction, the smaller is AEp. For

52

reversible

4. Electrochemistry of Diamond: Semiconductor and Structural Aspects

reactions, AEp « 59 mV [ l l ] . An extensive qualitative study of the dependence of AEp on the

I CiA)

>o E ^/)

jg (mA c m ' ) 0,15

0,00

0,7

1,4

2,1

2,8

E (V vs. Ag/AgCI)

-0,30

j ^ (mA cm' )

Fig. 4.1. (a.) Cyclic voltammogram for a boron-doped diamond electrode in 0.008 M K3Fe(CN)6 solution [l]," (b) potentiodynamic curves for a sulfur-treated diamond electrode- cathodic in 0.1 M K3Fe(CN)6, anodic in 0.1 M K4Fe(CN)6 solution, at a potential scan rate of (l) 5, (2) 10, (3) 20, (4) 50, and (5) 100 mV s'l [9]. Supporting electrolyte: 0.5 M H2SO4.

53

1000

500-^

£ o < ZJL

-500 J

"o!o"

— I —

0,2

0,4

0,6

E(V) Fig. 4.2. Cyclic voltammogram for a ( i l l ) face of a HTHP single crystal in 0.5 M H2SO4 + 0.01 M Fe(CN)63 + 0.01 M Fe(CN)64- solution. The potential scan r a t e was 5 mV s"^ [10]. properties of diamond a n d electrolyte [12] showed t h a t (l) outersphere reactions are more reversible t h a n inner-sphere ones^* (2) redox systems with more positive equilibrium potentials are more reversible

than

those having negative equilibrium

potentials,

which is but n a t u r a l for a p-type semiconductor electrode (see [7]); and (3) on heavily doped (metaMil^e) diamond electrodes, t h e

^ We recall t h a t outer*sphere reactions involve t h e outer coordination sphere of reacting ions; t h u s , little if any change occurs inside t h e ion solvate shell. These reactions proceed without b r e a k i n g up intramolecular bonds, w h e r e a s in inner-sphere reactions, involving the i n n e r coordination sphere, the electron transfer is accompanied by t h e breal^ing up or formation of such bonds. Inner-sphere reactions are often complicated by the adsorption of r e a c t a n t s and/or reaction products on t h e electrode surface. 54

4. Electrochemistry of Diamond: Semiconductor and Structural Aspects

reactions proceed in a more reversible manner than on moderately doped electrodes (which demonstrate semiconductor behavior). The latter

statement

juxtaposing

the

diamond/redox resistivity

was

corroborated

charge-transfer electrolyte

p. This

quantitatively

(faradaic)

interfaces

resistance

can

and be

[ 13 ] by

resistance the

RF of

diamond

measured

using

bulk the

electrochemical-impedance method, as a low-frequency cut-off on the real-component axis in the complex-plane presentation of impedance

spectra

taken

in

redox

solutions

of

different

concentrations (Fig. 4.3a [13])2. It can be also determined from the slope of the current—voltage curves recorded in these solutions. We emphasize that both the impedance spectra and the voltammetric curves should be taken at or near the reversible potential of the redox systems. The faradaic resistance is essentially equal to the reciprocal of the exchange current jo (up to a factor of RT / F), which is a measure of the reaction rate* RF = RT/(nFjo)

(4.1)

Here R is the gas constant, T is the absolute temperature, and F is the Faraday constant. The RF VS. p dependence is presented in Fig. 4.4. We conclude that the reaction proceeds at a higher rate at readily conducting diamond electrodes.

2 More precisely, the low-frequency cut-off equals RF + Rs where Rs is the "series" (Ohmic) resistance in the equivalent circuit of the electrode (Fig. 4.6 below). 55

a

o ^

•

•

•

1 .

5

•

•

• 28

•

4 o

° .-2

••

I

^1400 _.

8 o

128

••O680

~Z—

^1

_

.

1

§

2

0.1

>l'

S

.28

I

8000 0.1

0.2

03

0.05

%^*

290'

8000

0.05

0.1

0.15

Re Z / k n cii|2

^ -1

-2 log ( c / mol |-i )

Fig. 4.3. (a) Complex-plane plots of impedance spectra measured for a polycrystalline diamond electrode at the equilibrium potential in 1 M KCl + X M K3Fe(CN)6 + x M K4Fe(CN)6 solution at a value of x: (l) 3.3 10 3; (2) 10 2; (3) 5 10 2; (4) 10 i; (5) 2 lOi. Frequency f (Hz) is shown in the figure, (b) Dependence of the faradaic resistance on the K4Fe(CN)6 concentration (the K3Fe(CN)6 concentration being kept constant) [13].

56

4. Electrochemistry of Diamond: Semiconductor and Structural Aspects

E

a H

•

^ 4H

3H

4

5

6 log ( p / Q cm)

Fig. 4.4. Dependence of the faradaic resistance measured at the equiUbrium redox potential for polycrystalline diamond electrodes on their resistivity for (l) Fe(CN)6^"/4' and (2) quinone/hydroquinone redox systems [13]. At first glance, t h e proportionality between R F a n d p reflects the

fundamental

law

of

the

electrochemical

kinetics

semiconductor electrodes, namely, t h e exchange c u r r e n t semiconductor

electrode

must

be proportional to the

at of a

surface

concentration of charge carriers participating in t h e redox reaction. However, a closer look into t h e problem, based on reference [14], allows one to conclude t h a t it is t h e potential distribution at t h e diamond/redox

electrolyte

solution

interface,

rather

than

the

surface concentration of t h e charge carriers, t h a t depends on t h e diamond doping level, the more so, for more heavily doped samples.

57

More precisely, the potential drop in the Helmholtz layer in a redox electrolyte

increases with

increasing

doping. Therefore,

the

reaction rate reflects the Helmholtz potential drop, rather than the charge carrier concentration. This is the reason why the behavior of semiconductor diamond is far from "ideal"; in particular, no current rectification is typically observed on (rather heavily doped) diamond electrodes, as mentioned above. In addition to the AEp quantity, the electrode kinetics can be characterized by the transfer coefficients a (for cathodic reactions) and (3 (for anodic reactions). The transfer coefficients can be found, e.g., from the slope of the dependence of the voltammetric current peak potential Ep on the logarithm of the potential scanning rate v (compare Fig. 4.1b)' Ep = const - (RT / 2anF) In v

(4.2)

They can also be obtained from the slope of the dependence of the faradaic resistance on the logarithm of the redox electrolyte concentration at the equilibrium potential (see Fig. 4.3b). Indeed, as shown above, the faradaic resistance RF, measured in a redox solution at its equilibrium potential, equals the reciprocal of the exchange current jo. By measuring the dependence of jo on the concentration of the oxidized form Cox in the solution, while the concentration of the reduced form Cred has been kept constant (or vice versa), the transfer coefficient a (or, respectively, (3) can be calculated by the formula [ll]J0-nFk0Coxl"Cred",

where k^ is the rate constant of the reaction.

58

(4.3)

4. Electrochemistry of Diamond: Semiconductor and Structural Aspects

On metal electrodes, the transfer coefficients typically approach 0.5. Generally, the transfer coefficients for redox reactions on moderately doped diamond electrodes are smaller than 0.5?' their sum a +p, less than 1. We recall that an ideal semiconductor electrode must demonstrate a rectification effect; in particular, on p-type semiconductors, reactions proceeding via the valence band have the transfer coefficients a = 0, (3 = 1, and thus, a +(3 = 1 [7]. Actually, the ideal behavior is rarely the case even with single crystal semiconductor materials manufactured by use of advanced technologies ( like germanium, silicon, gallium arsenide, etc.). The departure from the "ideal" semiconductor behavior is likely to be caused by the fact that the interfacial potential drop appears essentially localized, even in part, in the Helmholtz layer, due, e.g., to a high density of surface states, or the surface states directly participate in the electrochemical reactions. As a result, the transfer coefficients a and p have intermediate values, between those characteristic of semiconductors (O or l) and metals (-0.5). Semiconductor diamond falls in with this peculiarity. However, for heavily doped electrodes, the redox reactions often proceed as reversible, and the transfer coefficients approach 0.5 ( "metaMike" behavior).

4.2.2. Impedance Spectroscopy Another

specific

feature

of semiconductor

electrodes is the

characteristic potential dependence of their differential capacitance. The measuring procedure is known to consist in applying a perturbing

harmonic voltage

signal

of frequency

f to

the 59

electrochemical cell and m e a s u r i n g the cell response at t h e same frequency.

The

independent. impedance

differential

capacitance

rarely

is

frequency-

Generally, t h e complex-plane p r e s e n t a t i o n of a n

spectrum

(Fig. 4.5a)

has

a linear

high-frequency

segment inclined to a vertical line (Fig. 4.5b [ 15 ]). Such a dependence can be described by introducing a

constant-phase

element (CPE) into the electrode's equivalent circuit (Fig. 4.6). The i n h e r e n t impedance of a C P E is [16]- ZCPE = o"^ (i(o)^, where i = (l)i/2; (JO = 2jtf is t h e ac a n g u l a r frequency; the power a d e t e r m i n e s t h e character of t h e frequency dependence, a n d the

frequency-

i n d e p e n d e n t q u a n t i t y a is m e a s u r e d in F^Q^^cm^ u n i t s . W h e n a approaches 1, t h e frequency-independent capacitance C can be substituted for a w i t h reasonable accuracy. This capacitance can be used in calculating the acceptor (or donor) concentration N A (respectively, N D ) in the semiconductor diamond, by using the Schottky theory of semiconductor interfaces [7].

60

4. Electrochemistry of Diamond: Semiconductor and Structural Aspects

400

800 Rez/Qcm^

Qcm^ 20b 151015 5-

30>^ 200^^^

0 ()

/ ^

1

1

2

3 4 Rez/Qcm^

5

Fig. 4.5. Complex-plane presentation of (a) impedance spectrum for a HTHP single crystal in 2.5 M H2SO4 solution; (b) its high-frequency p a r t [15]. The frequencies (kHz) are shown in t h e figure.

61

Csc

HI-

CPE

Rs 1

1

Fig. 4.6. Equivalent circuits for semiconductor electrodes with (a) frequency-independent capacitance Csc and (b) constant-phase element CPE. Rs is t h e series resistance, and R F is the faradaic resistance. To d e t e r m i n e N A (ND), the reciprocal of differential capacitance squared h a s been plotted versus t h e electrode potential E; such a g r a p h is called a Mott—Schottky plot (Fig. 4.7). F r o m the slope of the line, NA (ND) can be calculated* N D , A - ± (2 / eeoe) [d(C-2) / d E ] i .

(4.4)

Here e a n d eo are the permitivity of diamond a n d free space, respectively, a n d e is the electron charge; t h e "+" a n d "-" signs relate

to donors

and

acceptors,

respectively.

We recall

that

equation (4.4) reflects the potential dependence of t h e thickness of t h e space-charge layer in the semiconductor. By extrapolating the line to C^ -^ 0, the flat-band potential of the diamond electrode can be found. We note t h a t the slopes of t h e two lines in Figs. 4.7a and 4.7b have opposite signs, t h u s unambiguously evidencing the

p-

type and n-type conductance in the boron-doped a n d sulfur-treated diamond, respectively.

62

4. Electrochemistry of Diamond: Semiconductor and Structural Aspects

10' 'c'lF'cm') 3,0 2,5

2,0-1

1,5^

i,oJ

0,5-1 0,0 0,0

0,4

0,8

1,2

E(V)

0^^ C' ()xF'm) • /•

2,0-

/

1,5-

y 1,0-

/m

/

0,5-

X •

•

My/

m

/

n n. 0,0

0,5

1,0

E(V)

Fig. 4.7. Mott—Schottky plots for (a) (llO) -oriented boron-doped ptype and (b) ( i l l ) oriented sulfur-treated n-type CVD epitaxial diamond thin-film electrodes in 0.5 M H2SO4 solution [8, 29]. The Mott—Schottky plot approach is now widely applied to diamond electrodes as a nondestructive method in studying the bulk and surface properties (through the doping level or flat-band 63

potential, respectively) of synthetic diamond. It should be noted, however, t h a t often t h e doping level in t h e diamond m a y be too high to meet t h e essential a s s u m p t i o n of t h e

Mott—Schottky

analysis (primarily, a non-degenerate semiconductor behavior). If such

is

the

case,

the

values

of

the

acceptor

(or

donor)

concentrations obtained m u s t be referred to as a p p r o x i m a t i o n s . It is worth mention t h a t , even being frequency-dependent, the Mott—Schottky plot usually r e m a i n s linear, t h u s reflecting the potential dependence of t h e space-charge layer thickness, for any p a r t i c u l a r frequency f [17]. This fact allows one to conclude that, w h a t e v e r is the reason for the frequency

dependence of the

capacitance, it relates to t h e space-charge layer, r a t h e r t h a n the diamond surface proper. T h u s , a slow ionization, in t h e space charge region of a diamond crystal, of a t o m s w i t h a relatively deeplying energy level w a s a s s u m e d [18] as a t e n t a t i v e explanation for t h e frequency dispersion of t h e capacitance of d i a m o n d electrodes. In addition to the Mott—Schottky plot method, based on impedance

measurements,

the

dopant

concentration

linear

can

be

determined using the a m p l i t u d e demodulation method (ADM), a version oi nonlinear

im^edsince

techniques [19]. Unlike t h e above-

discussed linear-impedance method, here t h e p e r t u r b i n g signal applied to the cell is a high-frequency (co) current signdil, modulated in its amplitude at a low frequency Q (Fig. 4.8, inset);

i]\Qvoltage

response E Q of t h e cell is m e a s u r e d a t t h i s lower

frequency.

According to the theory of t h e method, the response is proportional to t h e d(C 2)/dE value. T h i s allows direct d e t e r m i n i n g NANA = - IO2 / 2eeoEa)2EQ , 64

(4.5)

4. Electrochemistry of Diamond: Semiconductor and Structural Aspects

w h e r e lo is t h e amplitude of t h e applied c u r r e n t signal. The potential-independent ADM signal (see Fig. 4.8) is t h e equivalent of a constant-slope Mott—Schottky plot; both reflect {versus

depth)

distribution

of

the

a

dopant

constant in

the

semiconductor.

0

10

0.2

0.4

0.6

1

1

1

08

E/V

-

:o

Fig. 4.8. Potential dependence of the ADM signal for a { i l l } lateral face of a HTHP single crystal (see Fig. 4.10 below). Inset: form of the perturbing current signal appUed to the cell; co = 0.5 MHz, Q = 2 kHz [10].

The ADM m e a s u r e m e n t s were used in determining t h e doping level in n u m e r o u s semiconductors, including diamond particular

application

temperature,

of t h e ADM

high-pressure)

diamond

method single

to H T H P crystals

[18]. A (highwill

be

touched on below.

65

4.2.3. Photoelectrochemistry of Diamond Already in the first paper on the photoelectrochemistry of the diamond electrode [20], it was shown that the semiconductor nature of diamond manifests itself in generating photocurrent and photopotential. When light is absorbed in diamond, excess charge carriers (electrons and holes) are generated. The electron—hole pairs are separated in the electric field of the space-charge layer; the minority carriers either cross the diamond/solution interface, provided an appropriate electron or hole acceptor (e.g., hydrogen ion) is present in the solution, or, when the interface is of a blocking nature, charge up the electrode surface. Correspondingly, photocurrent or photopotential is observed in the cell. At the first stage, the photoelectrochemistry of diamond was studied using subband light, whose quantum energy was less than the diamond band gap width (~ 5.5 eV); therefore, the excess charge carriers were generated via photoionization of some light-sensitive impurities, rather than valence-electron excitation to the conduction band [20, 21]. Later, supraband light with higher quantum energy was used [ 2 2 , 23], which provided band-to-band electron excitation in diamond.

66

4. Electrochemistry of Diamond: Semiconductor and Structural Aspects

E/V

Fig. 4.9. Potential dependence of the photocurrent squared for a polycrystalline diamond electrode in 1 M KCl solution [20]. When light is absorbed deep in a semiconductor, that is, the light penetration depth exceeds the (potential-dependent) thickness of the space-charge layer near the electrode surface, the layer where the charge separation proceeds widens with increasing potential applied to the electrode. The photocurrent Iph thus varies with the electrode potential E in much the same way as the differential capacitance C- the photocurrent squared vs. potential plot is linear (Fig. 4.9), resembling the Mott—Schottky plot (compare Fig. 4.7). At the flat-band potential, the electrode is not charged;

hence, no charge

separation

is possible

and

the

photocurrent is zero. Therefore, by extrapolating the plot in Fig. 4.9

67

to Iph^ -> 0, the flat-band potential can be determined, just as in the Mott—Schottky-plot

approach.

Other

photoelectrochemical

methods for the flat-band potential determination, namely, by measuring the photocurrent onset potential or the limiting opencircuit photopotential yielded by very intense illumination, are described in [22].

4.3. Effects of the Crystal Structure The electrode behavior of diamond can be affected by its crystal structure. Our comparative studies of chemical-vapor-deposited (CVD) single-crystal (homoepitaxial) and polycrystalline diamond thin-film electrodes, as well as amorphous diamond-like carbon electrodes, gave insight into the role of intercrystalline boundaries in the electrochemical behavior of diamond. By comparing the impedance spectra and kinetics of various redox reactions, we concluded that the single-crystal and polycrystalline diamond electrodes are similar in their electrochemical properties. Both the impedance characteristics (the differential capacitance, the CPE parameters a and a) and the kinetic parameters (the transfer coefficients a and (3, the rate constant ko) are similar for the singlecrystal and polycrystalline diamond electrodes. By contrast, the wide-gap diamond-like carbon (assumed to be a model material for the intercrystalline boundaries) appeared to be an electrode in performing Therefore,

68

we

concluded

inactive

the electrochemical redox reactions. that

the

electrode

behavior

of

4. Electrochemistry of Diamond: Semiconductor and Structural Aspects

polycrystalline diamond films is entirely determined by the diamond crystallites proper, rather than by the intercrystalline boundaries, at least, at moderate electrode polarization [ 2 4 ] . However, at high anodic potentials (just prior to the onset of oxygen evolution), a minor current peak was observed in the potentiodynamic curves taken with polycrystalline electrodes in a supporting electrolyte solution, which cannot be observed with single-crystal films. The peak was ascribed to the anodic oxidation of sp2-carbon of the intercrystalline boundaries [25], which turned out to be less oxidation-resistant than the diamond crystallites. In the last part of this chapter, our attention will be focused on the electrochemical properties of individual crystal faces of HTHP diamond single crystals, as well as single-crystal (homoepitaxial) CVD-diamond films. Our preliminary studies showed that the HTHP single crystals, on the whole, are similar to the CVD polycrystalline films in terms of their electrode behavior. In particular, both the polycrystalline thin-film electrodes and the HTHP single crystal electrodes are equally characterized by the special type of frequency-dependent capacitance described by the CPE. We dealt with single crystals grown from a Ni—Fe—C—B melt, at pressures and temperatures within the diamond thermodynamic stability range. The growth was performed, using a seed, by temperature gradient techniques (for details, see ref. 26). Boron, an acceptor dopant, was added to the source melt as ferroboron (the boron concentration in the batch was 0.1 wt. %). At the conclusion of the synthesis, the solidified metal was dissolved in a Cr207 + 69

H2SO4 mixture at 80-100^0 and the diamond crystals extracted. This oxidative treatment obviously made the diamond surface oxygen-terminated, unlike as'grown CVD-diamond surfaces which are hydrogen-terminated. The near-seed region of the crystals was then ground off and polished by pressing the crystals against the surface of a rotating cast-iron polishing wheel. One of the crystals is schematically presented in Fig. 4.10. It is a cubo-octahedron with unevenly developed ( i l l ) and (lOO) faces. The orientation of the rear (polished) face also approached ( i l l ) .

444 Fig. 4.10. A HTHP-diamond single crystal (top view). The octahedral and cubic faces (including the polished rear face of the crystal) were consecutively exposed to the electrolyte solution, the rest of the crystal's surface (including the crystal edges), the ohmic contacts,

and

the

current

lead

being insulated. To

distinguish between like faces, we designated them as "central", "left", "right", "bottom" (see Fig. 4.10), and "rear" (polished). Typical Mott—Schottky plots of the individual faces are given in Fig. 4.11 [10]. The rather positive flat-band potential is due to the

70

4, Electrochemistry of Diamond: Semiconductor and Structural Aspects

fact that the diamond surface had been oxidized during its abovedescribed

processing.

From

the

slope

of

the

lines,

the

uncompensated acceptor concentration was calculated. The NA values thus obtained (coinciding with those determined by the ADM method, see Fig. 4.8) are given in Table 4.1.

0,05n

c'• (M

F'cm')

C"' (|i F ' c m ' ) 2,5

(111)

A2,0

•\^

\ 1,5

•

0,04-

\

0,030,02-

1,0

(100) 0,010,00-0,4

• \.

1

0,5

q

—1

1

0,0

1

1

0,4

1

1

0,8

1

r^^^—1

0,0

1,2

E(V) Fig. 4.11. Mott—Schottky plots for two faces of the crystal shown in Fig. 4.10 in 0.5 M H2SO4 solution [lO]. We now turn our attention to the kinetic data. Anodic and cathodic potentiodynamic curves were taken at the faces of the single crystals in the 0.5 M H2SO4 + 0.01 M Fe(CN)63- (or Fe(CN)64) solution. The current vs. potential curves passed through a maximum whose potential depended on the potential scanning rate V (compare the cathodic curves in Fig. 4.1b). The

transfer

71

coefficients for the cathodic (a) and anodic (p) reactions in the Fe(CN)63-^4- system were determined by using equation (2), as discussed above. The a and p values are also shown in Table 4.1. Table 4.1. Properties of individual faces of the single crystal (Fig. 4.10) [10]. NA (cm-')

a

P

(111) central

2 X 10'"

-

0.11

(111)rear

9 X 10^*

0.5

0.25

1.3 X 10-'

0.6

0.25

(111) lateral (left)

3 X 10-°

0.33

0.43

(100) right

6 X 10-'

-

-

(100) left

4.8 X 10-'

0.6

0.38

Face

(111) lateral (bottom)

From these data the following conclusions were drawn* (i) The cast'iron-wheel surface polishing does not affect

the

capacitance significantly, as no marked difference in C or NA was found between the central (as-grown) and the rear (polished) ( i l l ) faces of the crystal shown in Fig. 4.10. In this respect, the semiconductor diamond differs from "traditional" semiconductors (germanium, silicon, etc.), whose mechanical processing, by using abrasives, results in the formation of a so-called damaged layer rich in dislocations, point defects, and other distortions of the crystal lattice. (Therefore, chemical or electrochemical etching is required to remove the damaged layer, which is necessary for revealing the semiconductor properties.) By contrast, the specific

72

4. Electrochemistry of Diamond: Semiconductor and Structural Aspects

abrasive-free processing of diamond we used, evidently, does not distort its crystal lattice. (ii) The (lOO) faces appeared more heavily doped than the central ( i l l ) face. 3 Interestingly, the lateral ( i l l ) faces appeared more heavily doped than the central ( i l l ) face of the crystals. These peculiarities may reflect the complicated conditions in the source melt. (iii) Generally, the higher doping level, the less irreversible is the redox reaction. On the most heavily doped (lOO) "left" face, the reaction kinetics approach that at "metaMike" diamond electrodes. Thus, the known sectorial character of the HTHP diamond crystals

[ 27 ] well manifests

itself in the

electrochemical

measurements. On the whole, the difference in the electrochemical behavior of the individual faces can be primarily ascribed to different boron concentration in their adjacent growth sectors, resulting from the different ability of the diamond crystal faces to incorporate the boron dopant during the growth process, rather than to different surface atomic densities or other purely surface properties. We now turn

to the homoepitaxial

CVD'diamond

films

deposited onto dielectric single crystal diamond substrates. The three crystal faces, that is, (ill)-, (llO)-, and (lOO)-orientated, were studied by differential capacitance measurements [28]. They differ in their capacitance markedly. The Mott—Schottky plots (see, e.g.,

3 We note that this particular finding is in contrast to the known ability of the ( i l l ) face to incorporate boron, during the growth process, more intensely than the (lOO) face. 73

Fig. 4.7a for the (110)-oriented film) allowed us to estimate the acceptor concentration in the films (Table 4.2). Their different doping level can be explained by

Table 4.2. Acceptor concentration in differently oriented homoepitaxial CVD films [28]. Face

(100)

(110)

(111)

Polycrystalline

NA (cm-')

(2-3)X

1.3 X 10-°

(5-7)X

5 X 10-°

10"

10-°

the above-mentioned different intensity of boron incorporation into differently orientated diamond crystal faces during the film growth. The results of the kinetic measurements agree with the capacitance data. The voltammetric curves for the polycrystalline film and ( i l l ) and (110) faces, taken in Fe(CN)63-/4 and Ru(NH3)2+/3+ redox solutions, are typical of irreversible, however, rather fast electrode reactions. By contrast, on the lightly doped (lOO) face, the current is very low; the process is under kinetic, rather than diffusion, control (Fig. 4.12). Thus, the electrochemical reactions in the Fe(CN)63^^ and Ru(NH3)6-^^^^ systems are slowed down in the following sequence* polycrystalline « ( i l l ) > (llO) > (lOO). These findings

agree with the above-discussed

dependence

of the

electrochemical reaction rate on the diamond doping level. Similar results, for the (ill)- and (lOO)- faces, were obtained in ref. 29. On the whole, the results obtained for the homoepitaxial CVD-films are in agreement with those found for the individual faces of the HTHP-single crystals. 74

4. Electrochemistry of Diamond: Semiconductor and Structural Aspects

J(A cm"') 0.008 ^

{111}

0,006 0.004 -

/ / //{1im 0,002-

y^{loc

0.000 -0.002-0,004 -0,006-0 008 -

1

1

r

-1,0

-0,5

1

X

0,0

0,5

1

1

1,0

1

1

1,5

E(V) Fig. 4.12. Cyclic voltammograms taken for (llO), (lOO), and ( i l l ) oriented crystal faces of homoepitaxial CVD-diamond films in 0.1 M NaCl -h 0.005 M Ru(NH3)6Cl2 + 0.005 \M Ru(NH3)6Cl3 solution. The potential scan rate was 10 mV s'^ [28].

4.4. Conclusions Moderately doped diamond electrodes are well suited for revealing the

semiconductor

and

structural

aspects

of the

diamond

electrochemistry. The structural effects often boil down to a difference in the acceptor concentration in the diamond, rather than reflecting the surface atomic density or other purely surface properties. To reveal these fine effects, the electrochemical behavior

75

of individual faces m u s t be compared u n d e r t h e condition of equal doping level. Other

studies

into

the

structural

effects

involved

the

comparison of t h e growth a n d nucleation surfaces of free-standing polycrystalline electrodes

diamond

[31,32],

and

films the

[30],

nanodiamond

electrochemical

thin-film

polishing

of

Science

and

polycrystalline diamond electrode surfaces [33].

References 1. Yu.

V.

Pleskov,

Engineering,

Advances

in

Electrochemical

vol. 8, Weinheim: Wiley-VCH, 2003, p. 209.

2. L. Schafer, M. Fryda, D. Herrmann, 1. Troster, W. Haenni and A. Perret, in'- Proc. 6th App. Diamond

Conf./2nd Frontier

Carbon

TechnoL Joint Conf. (ADC/FCT2001),

Ed. Y. Tzeng et a l , p. 158.

3. A. Fujishima, Ibid., p. 150. 4. R. G. Compton, J. S. Foord and F. Marken, Electroanalysis,

15

(2003) 1. 5. The Properties

of Natural

and Synthetic

Diamond,

Ed. J. Field,

Academic Press, London, 1992. 6. M. Nishitani-Gamo, C. Xiao, Y. Zhang, E. Yasu, Y. Kukuchi, I. Sakaguchi and T. Ando, Thin Solid Films, 382 (2001) 113. 7. Yu. V. Pleskov and Yu. Ya. Gurevich, Semiconductor electrochemistry,

Photo-

Consultants Bureau, New York, 1986.

8. S. C. Eaton, A. B. Anderson, J. C. Angus, Yu. E. Evstefeeva and Yu. V. Pleskov, Electrochem.

76

Solid-State

Lett,

5 (2002) G65.

4. Electrochemistry of Diamond: Semiconductor and Structural Aspects

9 S. C. Eaton, A. B. Anderson, J. C. Angus, Yu. E. Evstefeeva and Yu. V. Pleskov, Elektrokhimiya,

39 (2003) 170.

10. Yu. V. Pleskov, Yu. E. Evstefeeva, M. D. Krotova, V. Ya. Mishuk, V. A. Laptev, Yu. N. Palyanov and Yu. M. Borzdov, J,

Electrochem.

Soc, 149 (2002) E260. 11. P. Delahay, New

Instrumental

Methods

in

Electrochemistry,

Interscience Publishers, New York (1954). 12. N. Vinokur, B. Miller, Y. Avyigal and R. Kalish, J.

Electrochem.

Soc, 143 (1996) L238. 13. A. D. Modestov, Yu. E. Evstefeeva, Yu. V. Pleskov, V. M. Mazin, V. P. Varnin and I. G. Teremetskaya, J. Electroanal.

Chem.,

431

(1997) 211. 14. A. M. Kuznetsov, Electrochim. Acta, 13 (1968) 1293. 15. Yu. V. Pleskov, Yu. E. Evstefeeva, M. D. Krotova and V. A. Laptev, Electrochim. Acta, 44 (1999) 3361. 16. J. R. Macdonald and W. B. Johnson, in: Impedance

Spectroscopy,

Ed. J. R. Macdonald, Wiley, New York, 1987, p . l . 17. Yu. E. Evstefeeva, M. D. Krotova, Yu. V. Pleskov, V. M. Mazin, V. V. Elkin, V. Ya. Mishuk, V. P. Varnin and I. G. Teremetskaya, Elektrokhimiya,

34 (1998) 1493.

18. Yu. V. Pleskov, V. Ya. Mishuk, M. A. Abaturov, V. V. Elkin, M. D. Krotova, V. P. Varnin and I. G. Teremetskaya, Elektrokhimiya,

33

(1997) 67. 19. M. A. Abaturov, V. V. Elkin, M. D. Krotova, V. Ya. Mishuk, Yu. V. Pleskov and A. Ya. Sakharova, Elektrokhimiya,

31 (1995) 1214.

20. Yu. V. Pleskov, A. Ya. Sakharova, M. D. Krotova, L. L. Bouilov and B. V.Spitsyn, J.Electroanal

Chem., 228 (1987) 19.

77

21. Yu. V. Pleskov, A. Ya. Sakharova, E. V. Kasatkin and V. A. Shepelin, J. ElectroanaL

Chew., 344 (1993) 401.

22. Yu. V. Pleskov, V. M. Mazin, Yu. E. Evstefeeva, V. P. Varnin, I. G. Teremetskaya and V. A. Laptev, Electrochem.

SolidState

Lett., 3

(2000) 141. 23. L. Boonma, T. Yano, D.A. Tryk, K. Hashimoto and A. Fujishima, J. Electrochem.

Soc, 144 (1997) L142.

24. Yu. V. Pleskov, Yu. E. Evstefeeva, M. D. Krotova, V. V. Elkin, V. M. Mazin, V. Ya. Mishuk, V. P. Varnin and I, G. Teremetskaya, J. ElectroanaL

Chew., 455 (1998) 139.

25. H. B. Martin, A. Argoitia, U. Landau, A. B. Anderson and J. C. Angus, J. Electrochew.

Soc, 143 (1996) L133.

26. Yu. N. Palyanov, A. F. Khokhryakov, Yu. M. Borzdov, A. G. Sokol, V. A. Gusev, G. M. Rulov and N. V. Sobolev, Russ.

Geology

and

Geophysics, 38 (1997) 920. 27. V. A. Laptev, A. V. Pomchalov and Yu. N. Palyanov, Proc. 3rd Int. Conf.

'Crystals'

Applications',

Growth,

Properties,

Real

Structures,

and

Aleksandrov, Vladimir district (1997), vol. 2, p. 119.

28. Yu. V. Pleskov, Yu. E. Evstefeeva, V. P. Varnin and I. G. Teremetskaya, Elektrokhimiya,

in press.

29. T. Kondo, Y. Einaga, B. V. Sarada, T. N. Rao, D. A. Tryk and A. Fujishima, J. Electrochew.

Soc, 149 (2002) E179.

30. Yu. V. Pleskov, Yu. E. Evstefeeva, M. D. Krotova, V. G. Ralchenko, I. I. Vlasov, E. N. Loubnin and A. V. Khomich, J. Electrochew.,

33 (2003) 909.

31. Y. Show, M. A. Witek, P. Sonthaha and G. M. Swain, Chew. 15 (2003) 879.

78

Appl.

Mater,

4. Electrochemistry of Diamond: Semiconductor and Structural Aspects

32. L. C. Hian, K. J. Grehan, R. G. Compton, J. S. Foord and F. Marken, J. Electrochem.

Soc, 150 (2003) E59.

33. M. Panizza, G. Sine, I. Duo, L. Quattara and C. Comninellis, Electrochem.

Solid-State

Lett, 6 (2003) D17.

79

5. Semiconducting and Metallic BoronDoped Diamond Electrodes Claude Levy-Clement

Boron-doped

diamond

electrodes

exhibit

interesting

electrochemical properties, which include a wide potential window, stability in aqueous and non-aqueous media, low background current

density and

high resistance

against chemical

and

electrochemical corrosion [1-3]. These make the diamond electrode an attractive candidate for various electrochemical applications. Polycrystalline conducting boron-doped diamond films with a wide range of boron doping levels, from semiconductors (lOi^< [B] < lO^o cm^) to heavily doped films with metallic conductivity ([B] > 3 x 10^0 cm 3), are used as electrodes (Fig. 5.1). For electrochemical synthesis or breakdown reactions, highly conductive (metallic) electrodes are necessary, whereas for sensors or analytical purposes, semiconductor electrodes are desirable. Several papers recently addressed the importance of studying the

relationship

existing

between

the

basic

macroscopic

electrochemical properties of boron-doped diamond electrodes and their characterization at the microscopic level, in order to understand which factors influence the electrochemical reactivity [4, 5]. It was found that the boron doping level in the diamond and the presence of graphitic impurities play a major role in the basic Claude Levy-Clement e-mail: [email protected] 80

5. Semiconducting

and Metallic Boron -Doped Diamond

Electrodes

electrochemical properties. The analysis of t h e R a m a n spectra appears

to

be

the

most

suitable

tool

for

the

physical

characterization of boron-doped diamond electrodes. It can provide spatial information

about diamond

film

microstructure,

non-

diamond carbon impurity p h a s e s , boron doping level a n d optical properties.

This

method,

a

non-destructive

technique,

is

recommended because of its simplicity. In this chapter, typical R a m a n spectra of diamond electrodes are discussed a s a function of the boron doping level, together with the relationship between t h e physical and electrical properties a n d t h e

electrochemical

activity.

Fig. 5.1. SEM micrographs: (left) highly ([B] = 8 x IQi'^ cm-3) borondoped semiconducting diamond film; (right) heavily ([B] = 2 x lO^i cm" 3) boron-doped metallic diamond film.

5.1. Boron Precursors Polycrystalline diamond electrodes are currently synthesized in m a n y laboratories around the world by microwave plasma-assisted chemical vapor deposition (MPACVD) a n d hot

filament-assisted

81

CVD

(HFACVD)

methods.

Homoepitaxial

growth

of

monocrystaUine diamond films on single crystal diamond has been demonstrated, affording better control over the morphological, structural and electronic parameters of the films [6-8]. Although intrinsic diamond films are insulating, boron doping renders the films conductive (p"type semiconductor or metallic conductivity). However, since the boron acceptor level is situated 0.37 eV above the valence band, only 10 ^ of the boron atoms are actually ionized at room temperature. Therefore, a large density of B impurities (ca. lO^^-lO^i cm 3) must be introduced into the lattice in order to lend the film sufficient conductivity. Such high concentrations of boron impurities have a number of deleterious effects on the properties of the material, which are discussed below. Diamond is a metastable phase at ambient conditions, consisting of sp^ carbon. Consequently, the synthesis of films free from sp^ (graphitic) carbon is rather difficult. Whereas a diamond film containing 1% sp"^ carbon may not be a great impediment for various applications, like heat management, such a concentration of graphite may preclude its use in electrochemical systems, since due to its low resistivity, graphitic domains will shunt the currents through the electrode and short-circuit the electrochemical process occurring on the diamond. The deposition methods will not be further described here, nor the substrates and details of their preparation

before

deposition of diamond films. However, it appears that

the

MPACVD deposition technique lends itself to good control of the film composition (<1% sp^ carbon) and the doping process over relatively large surface areas (l cm^ and more). Hence, this is

82

5. Semiconducting

and Metallic Boron -Doped Diamond

Electrodes

currently the technique of choice used for many electrochemical studies. Furthermore, the HFACVD process may lead to various sources of contaminants, since W inclusions from the hot filament and diffusion of alkali metal impurities from the sample holder, cannot be fully avoided. Though the HFACVD method presents some drawbacks, it is presently the only technique that allows the production of electrodes with large areas (up to 0.3 m2) for industrial applications. In fact, since the electrochemistry of diamond films is so sensitive to some of these impurities, it can serve as a gauge to the film quality, as will be discussed below. The substrate

should be desirably conductive, because

otherwise the back contact must be established on the periphery of the diamond film, which results in large polarization losses under high current densities. Silicon is often used as a substrate and consequently boron-doped Si is preferable, since it minimizes the phenomenon of cross contamination of the dopant atom, and it would probably form a non-blocking contact with the top diamond film [3]. Refractory metals, such as Ti, Ta, Nb or W are also used as suitable substrates for diamond electrodes, but so far the best quality films are obtained on Si substrates. Columnar growth of the crystallites in the diamond film leads to coalescence of the crystallites as the film thickness increases. To reduce the contribution of the grain boundaries on the electrochemical processes, crystallites of a few |xm in size, and hence film thickness of this order, are preferable. The boron doping agent is usually added through small amounts of gas in the CVD chamber. A review of the boron

83

precursors used

h a s been published

by Morooka

et al. [8].

Diborane (B2H6) is the most frequently used for boron doping. However it is extremely poisonous and explosive. A few groups used boron oxide (B2O3) a s the boron source. It w a s dissolved in m e t h a n o l to form volatile trimethylborate B(CH30)3. However, it is clear t h a t t h e effect of oxygen-containing compounds is not completely understood, especially on t h e formation of epitaxial diamond. Trimethylboron (B(CH3)3) is not reported to be highly toxic a n d is frequently used. BCI3 and solid boron sources have also been used [5, 9].

5.2. Boron Concentration in Diamond Electrodes The boron incorporation from t h e gas to t h e solid p h a s e and all the processes leading up to t h i s are highly d e p e n d e n t on t h e a p p a r a t u s a n d on t h e details of t h e p r e p a r a t i o n . I t s incorporation in diamond depends on t h e growth sectors, a n d is about t e n t i m e s higher for the ( i l l ) face t h a n for t h e (lOO) face [10-12]. In monocrystalline films, the boron concentration can be deduced from t h e s t r e n g t h of t h e absorption coefficients a t some specific infrared wavelength based on several points of calibration by secondary ion m a s s spectrometry (SIMS) [13, 14]. Only t h e order of m a g n i t u d e of the boron concentration can be deduced in polycrystalline films. The boron content can be derived from various experiments [SIMS, boron nuclear reaction analysis or elastic recoil detection analysis (ERDA), etc.]. However its significance for polycrystalline would

films

not be clear, because, according to t h e technique

of

m e a s u r e m e n t , it would be a m e a n value from t h e different growth

84

5. Semiconducting

and Metallic Boron -Doped Diamond

Electrodes

sectors in each crystallite and for all crystallites of the films (infrared absorption) or a total mean concentration including the grains and the grain boundaries, where boron segregation might occur (SIMS). In order to understand the relationships between the physical properties of boron-doped diamond electrodes, LevyClement et al. studied a set of samples in which the boron concentration has been varied over a wide range [4]. The films were prepared by microwave (2.45 GHz) plasma-assisted chemical vapor deposition [15] of mixtures containing 99.5 % H2/0.5 % CH4 and B2H6. The B/C ratio in the gas phase has been varied from 200 to

14000 ppm. The films were deposited

at 880 °C

simultaneously on (lOO) highly boron-doped silicon substrates exhibiting a resistivity in the range 0.1 - 0.01 Q cm and on (lOO) highly resistive silicon substrates. From infrared and some SIMS measurements, it was found that the order of magnitude of the concentration of boron in the solid phase, [B], varies roughly as the square of the B/C ratio in the gas phase, from about 8 x 10^' cm"*^ at 200 ppm, through 2 x 10^0 cm"^ at 2000 ppm, up to 1.5 x 1022cm"3 at 14000 ppm (Fig. 5.2). The films are considered to be highly doped in the lO^^-lO^o B-cm'3 range and heavily doped when the boron concentration is larger than 10^^ B-cm'^ (^ 2800 ppm).

85

1000

10'

B/C ratio in the gas phase (ppm) Fig. 5.2. Mean value (see text) of the boron concentration in the films as a function of the B/C ratio in the gas phase. The resistivity of t h e films at room t e m p e r a t u r e from coplanar measurements

(by the four probe technique) v e r s u s t h e B/C

content in the gas p h a s e is given in Table 5.1. The resistivity decreases from 5 x 10^ to 3 x 10"2 Q cm as the B/C ratio increases from 200 to 8000 ppm. At t h e same time, the activation energy at high t e m p e r a t u r e of t h e resistivity decreases from 0.68 eV (200 ppm) to 0.38 eV (800 ppm), r e m a i n s at this value up to 1600 ppm, t h e n continuously decreases from 0.3 eV (2000 ppm) to 0.12 eV (8000 ppm). The activation energy for the highly doped samples (0.68-0.3 eV) coincides w i t h t h e energy of excitation of a hole from a n isolated acceptor state into the valence band. On t h e other hand, t h e close proximity of the acceptors in the heavily doped samples give rise to a n overlap of the hole wavefunction 86

and

5. Semiconducting

and Metallic Boron -Doped Diamond

Electrodes

consequently an extended miniband is formed having a width greater than 0.2 eV. This explains the low activation energy (0.12 eV) in the case of heavily doped materials. In other words, when the boron concentration increases, there is a first threshold when the electronic levels of the holes associated with boron begin to interact to form the "boron impurity band", then a second threshold when there is metallic conductivity in this "boron impurity" band. These thresholds have been detected in borondoped monocrystalline films [16] around 3x101^ and 10^^ B-cm"^ respectively, and the metallic conductivity threshold is thus very close to its predicted value of 2 x 10^0 cm ^ [17 ]. Table 5.1. Characteristics of the boron-doped electrodes examined Samples B/C in the gas phase (ppm) 200 800 1200 1600 2000 2800 6000 6500 8000 10000 12000 14000

Solid phase [B] (cm-^)

Resistivity (Q cm)

Thickness (wm)

8 X 10'^ 2 X 10'' 5 X 10" 9 X 10'" 1 X 10^" 4 X 10-° 2 X 10-' 3 X 10^' 5 X 10" 7 X 10" 1 X W^ 1.5 X 10^^

4200 1800 250 60 11

9.5 9 9 9.2 8.2 7.5 7.0 6.8 4.6 4 4 4

0.06 0.06 0.06 = 0.1 = 0.1

Average grain size (wm) ~6 ~4 -3.5 -2.5 -2.8 -2 -1.3 -1 -0.1

~1

1

87

5.3. Raman Diffusion Spectroscopy As we previously mentioned, the total concentration of boron from SIMS is not a good parameter to interpret the efficiency of electrochemical

reactions. This parameter

can, however

be

coherently interpreted from Raman spectra of their electrodes. Raman spectroscopy provides information on the various carbon phases present (amorphous carbon, graphite, diamond or intermediary phases) and on the concentration of diamond defects. Information on the evolution of the lattice structure can also be obtained when the boron concentration increases.

n,K CD',k' Fig. 5.3. Scheme of the Raman process. An incident photon (co, k) produces an inelastically scattered photon (co', k') and generates a phonon (Q, K). Basic

principles.

In

Raman

spectroscopy,

a

laser

beam

(monochromatic light) is directed perpendicularly at the sample. The major part of the light is subject to the common optical laws (absorption, reflection and transmission). A very small part (less than 10'^ of the incident beam) is reflected via an inelastic scattering of the photons by the lattice phonons. This is the firstorder Raman scattering. This is the only Raman scattering effect observed for diamond at the excitation energies used. The

5. Semiconducting

and Metallic Boron 'Doped Diamond

Electrodes

conservation of the wavevector implies the following relationships (Fig. 5.3): (o = o)' ± Q and k - k' ± K At the laser wavelengths used (generally 514.5 and 632.8 nm), the K and K' wavevectors are very weak and the generated wavevector of the phonon is also very weak. This means that the Raman process involves a phonon of the Brillouin zone center. A crystalline defect (breaking of the periodicity or impurity) will modify the scattering spectrum compared to the spectrum of a perfect crystal in two ways^ - Either a vibrational mode localized around the defect with its own frequency will be generated and a Raman peak characteristic of this defect will be observed. • Or, the characteristics of the vibrational mode of the perfect lattice will be modified, and the Raman peak of the perfect lattice will be enlarged and shifted in energy. The films used in electrochemistry are poly crystalline and the phonon confinement within one crystallite allows phonons with wavevectors slightly different but close to the Brillouin zone center to eventually participate in the observed Raman scattering process. Finally, the finite size of the crystallites can induce a partial breaking of the selection rules and allows the scattering of incident photons by phonons situated near the Brillouin zone center. Some species present in the films, such as monocrystalline graphite, are characterized by two peaks at 1350 and 1580 cm'i. At a greater level of disorder, all of the selection rules can be broken and the total density of phonons can be observed. Characteristic phonons that can be observed are the following: 520 cm"!: phonon of silicon at the Brillouin zone

89

1332 cm 1- phonon of diamond at the Brillouin zone center (termed the "zone-center peak optical phonon line") 1580 cm"!- graphite 1350 and 1580 cm^- pair of peaks indicative of the presence of microcrystalline graphite 1500 cm'i* amorphous carbon 1260 and 1600 cm i- pair of peaks indicative of the presence of defects at grain boundaries 1560 and 1610 cm i- pair of peaks due to the stretching of C-C diamond bonds, which corresponds to an intermediary phase between graphite and diamond. In practice, the important spectral features to be studied include- (l) The frequency and intensity of the diamond line, (2) The diamond linewidth which is indicative of crystalline quality, (3) The scattering intensity in the 1400-1600 cm ^ region due to non-diamond

carbon

impurities

and

(4)

The

background

photoluminescence intensity measured around 1650 cm'^. For a deep understanding of the quality of the films, two different laser wavelengths, 514.5 and 632.8 nm, are generally used, because the response of the diamond and parasitic phases at each wavelength is different. The 632.8-nm excitation line of the He-Ne laser is more sensitive to the parasitic phase, whereas the 514.5-nm excitation line is more sensitive to the "diamond doping". The choice of the wavelength depends on the goal of the measurementdetection of parasitic phases, doping or quality of the film. Semiconducting

diamond films

- low and high boron

doping.

Two essential sharp features appear in the Raman spectrum at

90

5. Semiconducting and ISdetallic Boron -Doped Diamond Electrodes

1332 a n d 520 cm i, corresponding to the diamond line, exhibiting a Lorentzian shape, a n d t h e silicon s u b s t r a t e , respectively (Fig. 5.4).

1

IJUU

'

1

'

1

r

T—

I

. Si phonon (1st order)

1

I

1

1332 cm" diamond phonon

1250

_

1000 3

-

% 750 Si phonon (2nd order)

2 500

1

_jj

\

250 0 1 -

250

1

1

500

1

1

,

750

1

1,..,.

1000

1 —1

1250

1—

L-

1500

Wavenumber (cm )

Fig. 5.4. Typical Raman spectrum of a semiconducting boron-doped diamond electrode, showing the 1332 cm'^ zero center diamond Une with a typical Fano-hke shape and the Si phonon hnes[514.5-nm excitation line from Ar+ laser (B/C ratio = 2000 ppm)]. Surface

parasitic

generally

the

phases.

crystalline

S E M analysis of the films shows t h a t quality

of t h e

film

improves

with

increasing boron content up to lO^^ cm ^ [4]. However, they still contain a high concentration of defects a n d poor homogeneity. On monocrystalline diamond, only the 1332 cm"i diamond line is observed (Fig. 5.5). For a low boron doping concentration (B/C ratio = 7 ppm, [B] = 1015 cm-3), additional p e a k s a t 1180, 1260, 1370, 1480, 1520 c m i are observed (Fig. 5.5). These p e a k s may be due to p h a s e s

91

different from cubic diamond or parasitic p h a s e s at t h e surface not totally eliminated by t h e hydrogen p l a s m a a t t h e end of t h e deposition. The concentration of parasitic p h a s e s at the surface decreases with increases doping level. Above a 1500-ppm B/C ratio in t h e gas p h a s e (in our experiments), no signal related to their presence is noticed in t h e R a m a n spectrum (Fig. 5.6).

Monocryslalline (type Il-a) Polycrystalline ^ v

20000

. ^

16000

•^

'2000

» • » » » • • • •

1000

1100

1200

>•* ' ^ > « « » • «

1300

»>»>»»••»»>»»<

150

1600

Wavenumber (cm"^) Fig. 5.5. Raman spectra of a monocrystaUine diamond (type IJ-a) and lightly doped polycrystalline diamond film (B/C ratio in the gas phase = 7 ppm), after removal of the background due to the fluorescence (632.8 nm excitation line).

Crystalline

quality. The addition of boron increases t h e crystalline

quality of diamond [18, 19]. A commonly used criterion to evaluate t h e crystalline quality is t h e full width a t half-maximum (FWHM) of t h e 1332 c m i d i a m o n d line (Fig. 5.7). It is equal to 2.6 a n d 10.5 cm"! for crystals with the best and poorest (B/C in t h e gas p h a s e = 7 ppm) crystallinity, respectively. It decreases to 9.5 cm^ for B/C = 1920 ppm. However t h i s value increases again for B/C > 2000 ppm. This new increase is not due 92

5. Semiconducting

and Metallic Boron -Doped Diamond

Electrodes

to a degradation of the crystalline quality but to the appearance of the Fano effect which is a complex phonon-electron coupling [12, 20, 21].

v

1

|-1600ppm CO

/

OS

|L____ •

,.J|L_______ : „---—^11^———-_ • L ^ 1 . - ^ "^ "V 1

[1200 ppm

"

£

I :

1

[ 2000 ppm

^800 ppm

CO

- 200 ppm

'-

\ 900

^ ] _, 1000

.

1100

1 . . . .

1200

1 . . . .

1300

1 . . . .

1400

1 . . . .

1500

1 . .

1600

1 11

1700

W a v e N u m b e r (cm"'')

Fig. 5.6. Raman scattering spectra of semiconducting boron-doped diamond films with different boron concentrations (200 ppm < B/C < 2000 ppm; 632.8 nm excitation line).

93

p 11.5 ; •

i 11 io

^

'X> las i

10 ;~

0

i

"^

500

1000

1500

2000

2500

3000

B/C ratio in the gas phase (ppm)

Fig. 5.7. Variation of the full width at half-maximum of the 1332-cm i diamond peak versus the boron doping level.

Boron

doping

concentration.

The F a n o line shape is correlated

with t h e boron doping. It is characterized by a n u p w a r d shift on t h e high w a v e n u m b e r side of the p e a k (Fig. 5.5). A slight variation in t h e intensity of t h e u p w a r d shift of the 1332 cm ^ line is observed with increase of t h e boron concentration [2]. Metallic

diamond

Glms

(semiconducting)/metallic

- heavily transition

boron-doped. has

An

been

insulating

predicted

by

Williams et al. to occur a t [B] = 2 x lO^o c m ^ [17]. I n our experiments, this occurs for a B/C ratio in the gas p h a s e e q u a l to 2800 p p m ([B] = 4.5 x IO20 cm-3. The s h a p e s of the R a m a n spectra of semiconducting a n d metallic diamond are very different. At the transition,

t h e absorption

coefficient

of diamond

increases sharply a t the energy of t h e incident light a n d as a consequence, t h e silicon line at 520 cm'i d i s a p p e a r s from t h e R a m a n spectrum. The R a m a n spectrum of metallic diamond is characterized by four features (Fig. 5.8). The 1332 cm 1 diamond 94

5. Semiconducting

and Metallic Boron -Doped Diamond

Electrodes

line widens, decreases in intensity, and its position shifts to lower wavenumbers. A wide signal at lower wavenumbers with two maxima at 500 and 1200 cm"! and a secondary feature around 1000 cm-i is associated with the Fano effect [12, 21-24]. Although there is ample evidence that the wide signal and the maxima result from the interaction between a continuum of electronic transitions within the impurity band (or between excited boron levels) and one or more optical phonons, simple Fano'like expressions failed to fit the data [25, 26].

First structure (strong)

/ Second structure (weak) Third structure (medium) 1332 cm-i Phonon

i4(X)

\m)

Wavenumber (cm"^) Fig. 5.8. Typical spectrum of a diamond film showing metalUc conductivity (514.5 nm excitation line).

Effect of the boron dovinsr level. The evolution of the Fano effect with the boron doping level increase is a very important observation. The diamond line at 1332 cm"! decreases in intensity

95

and is shifted to lower w a v e n u m b e r s (1308 cm"i for B/C = 6000 ppm) [27] w h e n t h e boron concentration increases, w h e r e a s t h e t h r e e s t r u c t u r e s in t h e c o n t i n u u m vary differently (Fig. 5.9)-

Wave Number (cm"^)

Fig. 5.9. Raman scattering spectra of metallic heavily boron-doped diamond fihns (6000 ppm < B/C < 14000 ppm) 514.5-nm excitation line). • The intensity of the s t r u c t u r e centered around 1200 cm'i increases, w h e r e a s the intensity of the 1332 cm i diamond line decreases. F r o m the theoretical phonon density of

states

showing a m a x i m u m a r o u n d 1200 cm i, m a t c h i n g t h i s line position, it h a s been concluded t h a t the 1200-cm i b a n d is related to disorder within the diamond lattice [26]. However, other possibilities such a s boron-related electronic t r a n s i t i o n s or defect-activated scattering by accoustic a n d optical phonons away form t h e zone center have also been mentioned. 96

5. Semiconducting and Metallic Boron -Doped Diamond Electrodes

- The position and intensity of the lowintensity band centered around 1000 cm"! do not vary with the doping density. • The maximum of the intense band centered around 500 cm"! is shifted toward lower energy. The signal of this band has been modeled based on two hypotheses. It may originate from a phonon whose lifetime is limited by the excitation of the laser or may be due to a repartitioning of phonons. In the first case, the shape of the signal would be a Lorentzian, and in the second case, the representative shape of the peak will be a Gaussian. The SOO-cm'i peak has been modeled by the linear combination of a Gaussian and a Lorentzian (Fig. 5.10) [28].

Wavenumber (cm*^) Fig. 5.10. Deconvolution of the 500-cm"i band into two components^ Lorentzian (narrow curve) and Gaussian (broadcurve)

The analysis of the position of the maximum and FMHW of the two components of the 500-cm"i peak for various doping levels showed that the Lorentzian component varies regularly with the doping concentration, which was not the case for the Gaussian

97

component. The position of t h e Lorentzian is progressively shifted toward lower energy with increased doping level (Table5.2) and follows t h e empirical logarithmic law* logio [B] =

30.9-0.02X

(with [B] in cm ^ a n d x t h e m a x i m u m of the Lorentzian component in cm 0. These results, shown in Fig. 5.11, are in a g r e e m e n t with those published by Pruvost et al. on epitaxial monocrystalline diamond films [26]. Table 5.2. Characteristics of the Lorentzian peak for various doping levels Sample B/C in the gas phase (ppm) 2800 4000 4800 6000 6500 6800 8000 10000 12000

98

[B] in diamond (cm-^)

Peak position (cm')

FWHM

4 X 10^° 1 X 10^' 1.5 X 10-' 2 X 10" 3 X 10-' 3 X 10-' 5 X 10-' 7.1x0-' 1 X 10-'

500 483 475 464 461 461 458 442 432

123 128 169 140 155 174 172 179 130

(cm-')

5. Semiconducting and Metallic Boron -Doped Diamond Electrodes

440

450

460

470

480

490

500

Position of the Lorentzian fit (cm-1)

Fig. 5.11. Variation of the position of the Lorentzian component as a function of the boron doping concentration for monocrystalline and poly crystalline diamond films (632.8 nm excitation line).

After calibration, the carrier concentration for metallic diamonds can therefore be more conveniently derived from Raman measurements, from the precise position of the 500"cm'i peak than by Hall-effect measurements, which require metallic contacts and a magnetic field, or by SIMS, which destroys the films and measures the total concentration of boron in the grains as well as in the grain boundaries. Graphitic impurities.

In semiconducting diamond electrodes, the

concentration of carbon parasitic phase is very low and cannot be detected with the 514.5-nm excitation. However, using a 632.8-nm excitation line, which is more sensitive to these phases [13], it was found that the concentration of this parasitic phase decreases as

99

t h e boron content in t h e films increases up to a B/C ratio of 6000 p p m ([B] = 2 X 1021 cm"3) [4]. However, for B/C values larger t h a n t h i s (using t h e 514.5-nm laser excitation line), a b a n d a p p e a r s a r o u n d 1540 cm i, which h a s been ascribed to a n parasitic phase,

whereas

a crystalline

graphite

unspecified impurity

is

detected in the 14000-ppm film (our experiment), which exhibits t h e 1350-1580-cm 1 p e a k couple (Fig. 5.9). This m e a n s t h a t , with a controlled a m o u n t of boron doping around [B] = 2 x lO^i cm 3, good quality diamond films with metallic conductivity can be used for electrochemical applications. Non-homogeneity

of boron doping.

Non-uniformity in the boron

doping level within a sample w a s noticed using micro-Raman spectroscopy [5, 27]. This w a s found in semiconducting as well as in metallic films. This can be observed in the shape of t h e F a n o line for semiconducting diamond films. A slight variation in t h e intensity of the u p w a r d shift of the 1332-cmi p e a k observed in spectra recorded a t different locations on the s a m e sample h a s been interpreted as a non-homogeneity in the doping level in the diamond

film

[5].

In

the

case

of

metallic

diamond,

the

nonhomogenous doping level is responsible for the evolution of t h e s h a p e and the position of t h e 1332-cm ^ diamond line a n d t h e intensity of the b a n d s in t h e associated continuum [27]. Figure 5.12 shows a n example of spectral e x t r e m e s observed on t h e s a m e sample with metallic conductivity. The two regions correspond to a doping level close to 3 x lO^o cm"3 a n d a n o t h e r between 3 x lO^o and 1 X 1021 cm'^ [27]. Cases of non-uniform boron doping have been reported for samples grown by MPACVD in t h e presence of a solid boron source [5] a n d trimethylborate (B2O3 dissolved in 100

5. Semiconducting

and Metallic Boron -Doped Diamond

Electrodes

methanol) [27]. Raman mapping experiments made on metallic diamond electrodes grown by HFACVD and MPACVD in the presence of trimethylboron and diborane, respectively, showed that the boron doping was spatially homogeneous over the films [27].

1

1

1

1

1

1

1

1

1

C3

H

f^__._(l) 4>

.s

[y v r^

— I — 1

300

iX)^""*^^*^-^^^

1 _

600

11

1

1 , 1

900

i . "^ J

1200

L

1

1_J

1500

Wavenumber (cm ) Fig. 5.12. Raman spectra of a polycrystalUne boron-doped diamond electrode, showing non-homogenous boron doping.

5.4. Electrochemical Properties Water decomposition. The voltammograms of semiconducting and metallic diamond electrodes, with the same

electrochemical

history, show qualitatively the same gross features in neutral electrolytes (KCl, Na2S04 and KNO3) with a low background current density (Fig. 5.13). The potential window is slightly smaller for metallic electrodes than for the

semiconducting

101

electrodes, and remains large when the diamond electrodes are free of graphite impurities [4. 29]. The major difference between the two types of electrodes is that the anodic and cathodic currents are three orders of magnitude larger for the metallic electrodes (current density in the mA cm-2 range) compared to the semiconducting ones (current density in the ^A cm-^ range). When the voltammograms of the metallic diamond electrodes are recorded in acidic solutions (HCl, H2SO4 and HNO3), the cathodic and anodic currents are ten times larger than in neutral solutions, which confirms the high sensitivity of the diamond surface to hydrogen, in the form of H+ ions.

90

/i

e™

E

i}

=1 0

0,1 M Na^SO^

I

1200 ppm

60

..'•'J

/

14000 ppm

u -2.0

-1.5

-1.0

-0.5

0.0

0.5

1.0

1.5

2.0

Potential (V/SCE) Fig. 5.13. CycUc voltammograms of the 200- and 1200-ppm electrodes (see inset) and 6000- and 14000-ppm electrodes (potential scan -2 V to -^ 2 V) in 0.1 M Na2S04 (scan rate, 100 mV sO.

102

5. Semiconducting

and Metallic Boron -Doped Diamond

Electrodes

The diamond electrodes supply similar current densities of electrons (for the reduction of hydrogen or nitrate) and holes (for the oxidation of oxygen or chlorine). This is assigned to carrier hopping in the boron impurity band, and tunneling assisted by localized states of electrons or holes through the space charge zone(s). Increasing the boron doping level in the semiconducting diamond decreases the defect concentration but also the width of the space charge layer. The electrochemical current increases slowly, whereas the 1332-cmi Raman diamond line remains Lorentzian. When heavy boron doping is reached, the conductivity of the diamond electrodes becomes metallic. The jump of three orders of magnitude in electrochemical current density is ascribed to a percolation in the localized levels in the bulk diamond and across the space charge layer, through the (metallic) continuum of the boron impurity band. The onset of the high electrochemical activity of the diamond electrodes is easily probed by the appearance of the Fano effect and the associated wide signal and especially of the new 500-cm i Raman peak. The electrochemical efficiency can be checked by the position of this band. One-electron redox couples. Fe(CN)6^^^ is a redox system that is often used to probe the reactivity of electrodes. It is often presumed that this redox couple undergoes electron transfer via a simple outer-sphere mechanism, which implies that it is not sensitive to the physical, chemical and electronic properties of the electrode surface. Reversible to quasi-reversible kinetics of the inorganic redox analyte Fe(CN)6^'/^' were reported for diamond electrodes, and it was found that films with no extensive electrochemical history can retain a high degree of activity for 103

Fe(CN)63^^ (1, 30-33). However, recent work by G r a n g e r a n d Swain [34] on diamond electrodes suggests t h a t the redox reaction might proceed via a n inner-sphere route t h r o u g h a specific surface interaction

at

the

hydrogen-terminated

surface.

Despite

its

complexity, t h e Fe(CN)6^^^ system can advantageously be used to evaluate the performances of the diamond electrodes a n d study the influence of t h e doping level of the diamond electrodes on the charge-transfer

kinetics. The

electrochemical

activity

in

the

presence of lO'^ M Fe(CN)6^^^ in 1 KCl was examined on virgin electrodes and also after having been submitted to extensive voltammetry studies in various electrolytes. As observed by Swain and R a m e s h a m (30), the diamond electrodes show d a r k discolored regions on t h e surface after exposure to the

ferri/ferrocyanide

solution. To regain the characteristic color of t h e diamond surface (light gray color), cyclic voltammetry between - 2 a n d + 2 V w a s performed. The i-E curves of t h e semiconducting electrodes (800, 1200 and 2000 ppm) are characterized by large peak-to-peak potential differences

(peak separations), AEp = 575, 270 and

182 mV,

respectively, (Fig. 5.14, Table 5.3). This shows t h a t AEp decreases w h e n t h e doping level increases. The heterogeneous

electron

transfer r a t e constant, k^, is typically around lO'^ cm s'^ [29]. For the metallic diamond electrodes, the AEp values are smaller (ca. 120 mV (Table 5.3)) a n d reflect a quasi-reversible behavior (Fig. 5.15). The c u r r e n t s of anodic and cathodic p e a k s (few rtiA cm-2) are one order of m a g n i t u d e larger t h a n for t h e semiconducting electrodes. The k^ value is one order of m a g n i t u d e larger, a r o u n d 10^ cm s"i [29].

104

5. Semiconducting

0.30

-i^

1

and Metallic Boron -Doped Diamond

1

'

1

•

1

•

1

'

1

0.20

/ / / /: /

..

"'"••••.,

u

1

•

Electrodes

"*

' -

-0.10

•^.---

^

•••'

-•'*

'

/ .-"••

-

'

'

I'''

-0.50

-0.25

0.00

(

•

\

1200 ppm

/ >

-

/

^

1

-0.75

'y

0.25

0.50

1...

I

0.75

Potential (V/SCE)

Fig. 5.14. Cyclic voltammetric i-E curves (total current) of two semiconducting diamond electrodes (800 and 1200 ppm) in 1 mM Fe(CN)6 3/ 4 /I M KCl (scan rate, 100 mV s 0. Table 5.3. Data of the cyclic voltammetric responses of virgin diamond electrodes with various boron doping levels (10"2 M Fe(CN)63" /4' in 1 M KCl) and after electrochemical experiments (O.l V s'^ scan rate). Samples

Virgin electrodes

After extensive electrochemical Studies

B/C (ppm)

AEpimV)

AEpimV)

800

575

1100

1200

270

762

2000

182

443

6000

124

96

10000

102

70

12000

308

106

14000

120

106

105

Similar experiments done after extensive electrochemical experiments show an increase of AEp for the semiconducting electrodes, whereas a slight decrease is observed for the metallic electrodes (70 - 106 mV). This shows that electron transfer at semiconducting electrodes is extremely sensitive to the chemical nature of the electrode surface, which is not the case for the metallic diamond electrodes [35].

14000 ppm

< c^

0h

'a

U 0.00

0.13

0.25

0.38

0.50

Potential (V/SCE) Fig. 5.15. CycUc voltammetric i-E curves (total current) of two diamond electrodes with metaUic conductivity (6000 and 14000 ppm) in 1 mM Fe(CN)6-3/ ^ /i M KCl (scan rate, 100 m V s'l).

Nitrate

reduction.

Similar to water oxidation-reduction,

the

reduction of nitrate is a multistep electron-transfer reaction, which necessitates, at least in one step of the reaction, penetration of the redox species through the Helmholtz layer and adsorption of a reaction intermediate on the electrode surface. In such multistep

106

5. Semiconducting

and Metallic Boron -Doped Diamond

Electrodes

processes, the inner Helmholtz layer, which is the first atomic layer of the solution adsorbed at the electrode surface, is usually perturbed to some extent by the redox reaction. The large overpotential towards hydrogen reduction on diamond electrodes permits reduction of various redox couples that cannot be efficiently reduced at metallic electrodes. This is the case with the reduction of nitrate ions, and previous work showed

that

polycrystalline B-doped diamond electrodes reduce nitrate ions to ammonia in basic solutions [36, 37].

s

u u

1 M KCl 1 M KNO3 H

U

-1.5 -1.0 -0.5 Potential (V/SCE)

0.0

Fig. 5.16. Voltammograms of a diamond electrode with metallic conductivity (B/C = 6000 ppm) in 1 M KCl (dotted line) and 1 M KNO3 (full line). We found that semiconducting diamond electrodes are much less active toward the reduction of nitrate compared to metallic electrodes. When the

electrolyte

contains nitrate

ions,

an

additional cathodic electrochemical activity is noticed which is

107

attributed to their reduction (Figs. 5.16 and 5.17). It was found that the best electrode for nitrate reduction is the 6000-ppm ([B] = 2.4 X 1021 cm"3) diamond film, which contains an extremely low concentration of carbon parasitic phase.

1

1

^i «-

1

1

1

1

1

1

1

_ ^ ^ _ ^ , _ _ „ .

u

/ / / / / / / / //

1 -^^ >^

•t-^

2 -80 es a> -o

1/

g 120

/ i/

u

0u

jj

U -160

/

n

-2.0

1 M HNO3 -1.5

-1.0

-0.5

0.0

Potential (V/SCE)

Fig. 5.17. Voltammograms of a diamond electrode with metallic conductivity (B/C = 6000 ppm) in 1 M HCl (dotted line) and 1 M HNO3 (full line). The electrochemical reduction of nitrate ions consists of multistep reactions, which might give the following overall reactions* NO3 + H2O + 2e ^ NO2 + 2 OH NO3 + 3 H2O + 5e -^ 1/2 N2 + 6 OH NO3 + 7 H2O + 8e -* NH4OH + 9 OH The rate-limiting step in the sequence of reactions involves a weakly adsorbed N03' at the surface of the cathode. The NO2" nitrite ions formed can further be reduced to N2, NH3 or NH2OH (hydroxylamine). Quantitative analysis of the compound formed

108

5. Semiconducting

and Metallic Boron -Doped Diamond

Electrodes

during the reduction of nitrate ions showed that the pH of the solution and the potential applied to the diamond electrode have a great influence on the efficiency of the reduction and on the nature of the nitrogenous products formed. Details of the reduction of nitrate ions in acidic and neutral solutions have been published [29, 38]. In 1 M KNO3, the constant value for the NOs" reduction (10% after a 16-hr electrolysis), with only the formation of gaseous products when the applied potential is between "1.5 and -1.7 V, contrasts with its increasing value but with nitrite production - for potentials more negative than -1.7 V (Table 5.4). As the beginning of the increase just corresponds to the onset of hydrogen evolution, this suggests that between -1.5 < V <-1.7 V, the mechanism of NOs" reduction (-30%) is purely electrochemical, giving only gaseous products, whereas at applied potentials more negative than -1.7 V, an additional chemical reduction of NO3" by the evolved hydrogen might be involved, giving a higher reduction ratio but a main influence of another set of reactions, resulting in the formation of gaseous products and nitrite ions. In IM HNO3 solution, the efficiency of nitrate reduction is larger than in neutral solution, and at -1.5 V, 20% of the nitrate is reduced, mostly to gaseous products. When the applied potential is •2 V, 65% of the nitrate ions are reduced to 40% gaseous products and 12% ammonium ions. It is interesting to note that in acidic medium,

water

decomposition

(hydrogen

evolution)

occurs

simultaneously with the reduction of nitrate. It is obvious that systematic work is needed to understand the detailed mechanism of the reduction process, which at -2 V involves both NOs" and

109

hydrogen absorbed intermediates, which might compete for the same sites. Table 5.4. Concentration of reduced nitrates and nature of nitrogenous products formed when a negative potential was applied at the 6000-ppm electrode a period of 16 hr. Electrolyte IM

Applied potential (V/SCE)

Reduced NO3 (%)

NO2 formed (%)

NH4^ formed (%)

KNO3 KNO3 HNO3 HNO3

-1.6 -2 -1.5 -2

10 29 9 55

0.5 14 0.1 1.3

0 0 0 10

Nitrogen gas formed (%) 9 16 8.6 38

The elimination of nitrate ions by electrochemical reduction with metallic diamond electrodes is a promising and powerful method if the reduction is carried out until the wastewater can be biologically treated. Development of a clean electrochemical process to eliminate high concentrations of nitrate ions and transform them into nitrogen gas is an important challenge for environmental science. Graphitic

impurities.

A shown in

[4], the

electrochemical

efficiency of the films depends also on the concentrations of residual parasitic phases (amorphous graphitic and /or crystalline graphite phase) located at the grain boundaries, which in particular increase rapidly with very high boron concentration ([B] > 3 X 1021 cm 3) [4]. Crystalline graphite impurities in metallic diamond electrodes have an important influence on the reduction of water and nitrate ions. Graphite impurities have a catalytic 110

5. Semiconducting

and Metallic Boron -Doped Diamond

Electrodes

effect on the decomposition of water, and hydrogen evolution is enhanced [4], whereas they partially quench the reduction of nitrates due to a fast poisoning of the electrode and a high activity for water reduction [27].

5.5. Conclusions Diamond conductivity

electrodes depending

exhibit on

the

semiconducting boron

doping

or

metallic

level.

The

electrochemical activity of diamond electrodes is mainly controlled by the boron doping level and more precisely by the occurrence and behavior of the boron impurity band. It can supply either electrons or holes to the electrolyte through hopping between its electronic levels. The anodic and cathodic currents jump by three orders of magnitude when metallic conductivity is reached in this impurity band when [B] ^ 2 x lO^i cm'^, the charge transfer kinetics increasing by one order of magnitude. Diamond electrodes showing metallic conductivity are very efficient cathodes to reduce nitrate ions in highly concentrated wastewater, provided there are no crystalline graphitic impurities in the film. Raman spectroscopy is the ideal tool to probe the boron doping level and presence of non-diamond carbon impurity phases in diamond electrodes.

Ill

References 1.

G. M. Swain, A. B. Anderson and J. C. Angus, MRS Bulletin,

Sept.

98, p. 56 and references therein. 2.

R. Tenne and C. Levy Clement, Isr. J. Chew., 38 (1998) 57 and references therein.

3.

Y. V. Pleskov, Usp. Khi., 68 (1999) 416.

4.

C. Levy-Clement, F. Zenia, N. A. Ndao and A. Deneuville, New Diamond Front

5.

Carbon Technol, 9 (1999) 189.

J. Wang, G. M. Swain, M. Mermoux, G. Lucazeau, J. Zak and J. M. Stojek, New Diamond Front Carb. Technol, 9 (1999) 317.

6.

R. Ramesham, T. Roppel, C. Ellis and B. H. Loo, J.

Electrochem.

Soc, 138 (1991) 2981. 7.

M. W. Geis and J. C. Angus, ScL Am., October (1992) pp. 64-69.

8.

Morooka, T. Fukui, K. Semoto, T. Tsubota, T. Saito, K. Kusakabe, H. Maeda, Y. Hayashi and T. Asano, Diamond Relat

Mater,

8

(1999) 42. 9.

M. Werner and M. Loher, Rep. Prog. Phys., 61 (1998) 1665.

10. B. V. Spitsyn, L. L. Bouilov and B. V. Derjaguin, J. Cryst

Growth,

52 (1981) 219. 11. R. Locher, J. Wagner, F. Fuchs, M. Maier, P. Gonon and P.Koidl, Diamond Relat Mater,

4 (1981) 678.

12. K. Ushizawa, K. Watanabe, T. Ando, I. Sakaguchi, M. NishitaniGamo, Y. Sato and H. Kanda, Diamond

Relat

Mater,

7 (1998)

1719. 13. J. Wagner, C. Wild and P. Koidl, Appl. Phys. Lett,

59 (1991) 779.

14. K. Miyata, K. Kumagai, K. Nishimura and K. Kobashi, J. Mater. Res., 8 (1993) 2845.

112

5. Semiconducting and Metallic Boron -Doped Diamond Electrodes

15. P. Gonon, E. Gheeraert, A. Deneuville and L. Abello, Thin

Solid

Films, 256 (1995) 3. 16. E. Gheeraert, A. Deneuville and J. Mambou, Diamond

Relat

Mater., 7 (1998) 1509. 17. A. S. W. Williams, E. C. Lightowlers and A. T. Collins, J, Phys.

C,

3 (1970) 1727. 18. P. Wurzinger, P. Prongratz, P. Hartmann, R. Haubner and B. Lux, Diamond Relat Mater., 7 (1997), 763. 19. E. Colineau, E. Gheeraert, A. Deneuville, J. Mambou, F. Brunet and J. P. Lagrange, Diamond Eel Mater,

6 (1997) 778.

20. U. Fano, Phys. Rev., 124 (1961) 1866. 21. E. Gheeraert, P. Gonon, A. Deneuville, L. Abello and G. Lucazeau, Diamond

Relat. Mater,

2 (1993) 742.

22. R. J. Zhang, S. T. Lee and Y. W. Lam, Diamond Relat. Mater,

5

(1996) 1288. 23. M. Werner, O. Dorsch, H. U. Baerwind, E. Obermeier, L. Haase, W. Siefert, A. Ringhandt, C. Johnston, S. Romani, H. Bishop and P. R. Chalker, Appl. Phys. Lett, 64 (1994) 595. 24. J. W. Ager III, W. Walukiewicz, M. McCluskey, M. A. Piano and M. I. Landstrass, Appl. Phys. Lett, ^^ (1995) 616. 25. A. Deneuville, Semiconductors

and Semimetals,

Vol. 76,

Thin-

Film Diamond 1. Eds. C. E. Nebel and J. Ristein. Publishers. Acad. Press (2003) 183-237. 26. F. Pruvost, E. Bustarret and A. Deneuville, Diamond Mater,

Relat.

9 (2000) 295.

27. C. Levy-Clement, N. A. Ndao, A. Katty, M. Bernard, A. Deneuville, C. Comninellis and A. Fujishima, Diamond

Relat. Mater.

12

(2003) 606.

113

28. M. Bernard, A. Deneuville and P. Muret, Diamond Relat

Mater.,

13 (2004) 282. 29. N. A. Ndao, PhD thesis, University of Paris XII (2002). 30. G. M. Swain and R. Ramesham, Anal

Chew. 65 (1993) 345.

31. R. Ramesham and M.F. Rose, Diamond 32. R. Ramesham, Sens. ActuorsB,

Relat Mater. 6 (1997) 17.

50 (1998) 131.

33. Y. Yano, D. A. Tryk, K. Hashimoto and A. Fujishima, Electrochem.

J.

Soc. 145 (1998) 1870.

34. M. G. Granger and G. M. Swain, J. Electrochem.

Soc, 146 (1999)

4551. 35. C. Levy-Clement and N. A. Ndao, to be pubhshed. 36. R. Tenne, Electroanal.

K.

Patel,

K. Hashimoto

and

A. Fujishima,

J.

Chem., 347 (1993) 409.

37. C. Reuben, E. Galun, H. Cohen, R. Tenne, R. Kalish, Y. Muraki, K. Hashimoto, A. Fujishima, J. M. Butler and C. LevyClement, J. Electroanal.

Chem., 396 (1995) 233.

38. C. Levy-Clement, B. Trachh, N. A. Ndao, A. Katty and A. Deneuville, to be published.

114

6. Electrochemical Properties and Application of Diamond Electrodes in Non-Aqueous Electrolytes Mikiko Yoshimura, Kensuke Honda, Hideki Masuda and Akira Fujishima

In this chapter, the electrochemistry of diamond electrodes in non-aqueous electrolytes will be introduced.

In the beginning

section, the fundamental electrochemical behavior for diamond electrodes in various non-aqueous electrolytes will be clarified.

In

the subsequent section, the practical application of diamond electrodes for electrochemical energy storage devices will be described.

6.1 Factors Controlling the Electrochemical Potential Window for Diamond Electrodes in Non-Aqueous Electrolytes In non-aqueous electrolytes,

carbon and metal electrodes have

been reported to exhibit wider potential windows than those in aqueous electrolytes.

In this section, the potential windows for

diamond electrodes were examined and the reactions controlling the potential window were tentatively identified by comparison of the relevant potentials and the highest occupied molecular orbital Mikiko Yoshimura

e-mail- [email protected] 115

(HOMO) and lowest unoccupied (LUMO) energies.

6.1.1. Electrochemical potential windows in non-aqueous Electrolytes Fig. 6.1 shows the cycUc voltammograms (CVs) obtained for an as"deposited diamond electrode in several non-aqueous electrolytes and an aqueous electrolyte (l M H2SO4).

In this study, the

working potential window is defined as the range between the potentials at which the anodic and cathodic current densities reach 2 niA cm^ [l].

The non-aqueous electrolytes examined exhibit

potential windows (Fig. 6.1 (a)-(e)) that are 1.5-2.5 times wider than that obtained in the aqueous acid electrolyte (Fig. 6.1 (f)).

In

aqueous electrolytes, the potential window observed for diamond electrodes is wider than that for the other carbon-based electrodes. In non-aqueous electrolytes, however, those values are very similar to those reported for the other carbon-based electrodes [4]. The potentials for the onsets of anodic currents significantly

(1.4 to 4.1 V vs. Ag/Ag+) for the

electrolytes examined.

vary

non-aqueous

The anodic currents appear to be caused

by the decomposition of the organic solvent. As mentioned above, there are no differences of the potentials for the onsets of anodic currents between the diamond electrodes.

and the other

carbon-based

Therefore, this decomposition does not appear to

involve adsorbed intermediates, which are involved in the hydrogen and oxygen evolution reactions from aqueous electrolytes, but instead may involve outer-sphere one-electron transfers.

116

6. Electrochemical Properties Non-Aqueous Electrolytes

and Application

of Diamond

Electrodes

in

^ 4 mAcm^ (a) DMF (b)AN (c) DEC-PC (d)PC (e) GBL (f)lMHjSO^

-5

-2.5 0 2.5 Potential (Vvs. Ag/Ag')

5

Fig. 6.1. Cyclic voltammograms obtained for an as-deposited diamond electrode. Non-aqueous supporting electrolyte- 0.5 M Et4BF4. In outer-sphere

electron transfers,

the electron

transfers

efficiently just w h e n the electrode potential is close to the HOMO a n d LUMO energies of t h e r e a c t a n t molecule [2].

The potential for

oxidation should be well correlated with t h e H O M O energy of a molecule

[3].

We therefore

compared the calculated

HOMO

energies with the potentials for the onsets of anodic c u r r e n t on the assumption

that

the

oxidation

reactions

in

non-aqueous

electrolytes involve simple one-electron electron transfers as first steps.

6.1.2. Theoretical consideration for reactions Fig. 6.2 p r e s e n t s the correlation between the calculated HOMO 117

energies obtained from the Hartree-Fock method and the potentials for the onset of oxidation both on diamond electrodes and on glassy carbon (GC) electrodes, the latter as reported earlier by Ue et al. [4]. The oxidation potentials obtained in the CV measurements were converted to absolute potentials (Eabs) using the formula Eabs (V vs. vacuum) = - E (V vs. SHE) - 4.44 [5]. The correlation between the potentials

for oxidation

and

the

HOMO energies is

fairly

convincing, with a calculated correlation coefficient of 0.82, which is reasonably close to unity [6]. According to reports published earlier [7], the potentials (Eox) for oxidation of an organic molecule with respect to vacuum are related to the HOMO energy (EHOMO) by the following equationEHOMO

- Eox - AEsoi + constant, where AEsoi is the difference in the

solvation energies of the neutral molecule and the radical cation [8] [9]. An additional effect is that the HOMO energy level changes upon ionization due to changes in electron interactions, including correlation and repulsion. and

electron

interaction,

Ignoring the effects of both solvation one

could

expect

a

one"to-one

correspondence between Eox and EHOMO, producing a straight line with a slope of 1 eVA^. This line is shown as curve b in Fig. 6.2.

118

6. Electrochemical Properties Non -Aqueous Electrolytes

and Application

of Diamond

Electrodes

in

_.'' (b) UleoreticaJ-''

3

-9

>

.--

O

^^-12

(a) ExpeiTineiit ii -15 E^^/Vvs. vacuum

Fig. 6.2. Correlation of HOMO energies oforganic solvents calcurated by the H F method with the experimental oxidation potentials, o-carbonated, •'lactones, o-nitriles, •'formamides, A:oxazolidinones, A-nitroalkanes, ••sulfer-containing compound. The actual Eox •

EHOMO

values range from about 2.5 to 5.0 eV

below curve b, which can be explained by the effects of solvation and electron interaction [7].

The slope of the least-squares fit to

t h e d a t a is 0.85 eVA^, which is close to t h e ideal value (1.0 eVA^) for curve b [8]. In any case, a significant correlation, which is approximately shown in t h e equation, w a s obtained for the Eox a n d EHOMO results. Thus,

it

is proposed

that

the

anodic

currents

observed

in

non-aqueous electrolytes result from outer-sphere electron transfer from t h e organic solvents, with fast foUowup chemical reactions. On

the

other

hand,

the

potentials

for

reduction

differ 119

according to the type of supporting electrolyte cation.

It has been

demonstrated that there is a correlation between the potentials for reduction and the LUMO energies of the supporting electrolyte cations, with a correlation coefficient of 0.91.

Thus, it was

proposed that, for diamond electrodes, the onset of cathodic current in non-aqueous electrolytes is controlled by the outer-sphere decomposition of the cation of the supporting salt [lO].

6.1.3. Electrical double-layer capacitance In this section, the double-layer capacitance values, which were obtained by means of ac impedance, were examined for the various solvents and supporting electrolyte salts. Table 6.1 shows the double-layer capacitance values for diamond electrolytes in various non-aqueous electrolytes.

The

value for diamond electrodes is 15.2 |aF cm-2 (in 0.5M Et4BF4/PC), which is lower than that for GC electrodes (44.7 (iF cm"2). The value of the double-layer capacitance is in inverse proportion to the thickness of the double-layer, and thus it is expected that larger ions (considered to be solvated) should show smaller capacitance values at constant electrode charge.

In Table

6.1. , the calculated molecular volumes of the supporting electrolyte cations and the organic solvents are summarized, together with the values of capacitance.

The molecular volumes were calculated by

means of the Hartree-Fock method.

120

6. Electrochemical Properties Non-Aqueous Electrolytes

and Application

of Diamond

Electrodes

in

Table 6.1. Comparison of differential double-layer capacitance for diamond electrode. Molecular volume

Differential c apacitance

/10"^nm^ ( j ^ Si^orting electrolyte cation^)

Et4l>f

15.7

15.2

V^lC

18.0

13.2

BU4N*

22.9

11.8

HeX

32.5

9.06

(B) No n-aciuecus solv ent ^'

AN

3.47

19.5

DMF

8.77

16.6

PC

9.70

15.2

DEC

9.88

14.0

GBL

9.92

13.6

^W alws obtained using BF/ as a cation of supporting salt and PC as a solvent ^J Values obtained using 0.:5 MEt^NBF^ as a supporting salt

As shown in Table 6.1 (a), where the supporting electrolyte anion and solvent are constant, a decrease in the capacitance was observed with increasing number of alkyl carbons, following the trend mentioned above. In Table 6.1 (b), the capacitance values decreased with increasing solvent volume. These results clearly indicate that the electrical double-layer capacitance values for diamond

electrodes decrease, whereas the thickness of the

Helmholz layer increases, with increasing volume of the solvated cations. The

number

of possible

combinations

of solvents

and

supporting salts in non-aqueous electrolytes is so large that it

121

should be possible to optimize the electrochemical properties for any given purpose.

Therefore, we have high expectations for

electrochemical applications of diamond electrodes in non-aqueous electrolytes.

6.2. Electrochemical Characterization of an sp^ - sp3 Composite as a Hybrid Anode for Li-ion Batteries and Super Capacitors In this Section, we discuss the electrochemical applications of diamond electrodes in non-aqueous electrolytes. As stated above, the background current density observed for diamond electrodes was lower than those for the other types of carbon-based

electrodes in non-aqueous as well as

electrolytes.

Utilizing this characteristic, the electrochemical

sensing processes have been investigated.

aqueous

For example, the Ceo

molecule, which shows five reversible reductions at the GC electrode, has been reported to exhibit six waves at the diamond electrode [ll]. Moreover, ascorbic acid, an essential vitamin, has been shown to exhibit a particular reaction in non-aqueous electrolytes.

A quasi-reversible redox reaction to produce the

ascorbic acid radical anion occurs in non-aqueous electrolytes, while only an irreversible anodic oxidation reaction occurs in aqueous electrolytes [12]. In addition to sensing applications, diamond electrodes have been examined for use in energy storage devices. A diamond electrode with nano-porous structures on its surface can exhibit a high

122

double-layer

capacitance

in

both

aqueous

[13]

and

6. Electrochemical Properties Non-Aqueous Electrolytes

and Application

of Diamond

Electrodes

in

non-aqueous electrolytes [14]. In a non-aqueous high-conductivity electrolyte (0.5 M Et4NBF4/AN), the double layer capacitance for a nano-porous diamond surface with 60-nm diameter pores was found to have a high value (2.1 mF cm'^), similar to that in 1 M H2SO4 solution (1.8 mF cm"2). Nano-porous diamond films have the possibility of realizing a high-power, high-energy density double layer capacitor in non-aqueous electrolytes. In this Section, a functional sp^-sp^ carbon composite material using this nano-porous diamond film (NANO) has been introduced. The electrochemical interface on this composite surface was designed such that the sp^ portion, which was composed of carbon nanotubes (CNT), and the sp3 portion operate as a Li+ ion battery and a double-layer capacitor, respectively.

6.2.1. P r e p a r a t i o n method of sp^-sp^ composite electrode Nano-porous structures were prepared by oxygen plasma etching through anodic alumina masks with 400-nm diameter pores on polished diamond films [16][17].

The average diameter and

average depth of the pores were 400 nm and 1.8 pim, respectively. The sp2 carbon nanotubes were prepared by pyrolysis of phthalocyanine with a Fe catalyst using chemical vapor deposition [18].

The CNT density could be controlled by varying the

deposition time.

6.2.2. Images of the sp^-sp^ composite electrode Fig. 6.3 shows SEM images of two types of CNT-NANO composite films. The nano-honeycomb pores formed on the diamond surface were

arranged

uniformly

in

hexagonal

closet

packing.

123

Multi-walled CNTs were located mainly in the nano-honeycomb pores. The inner and outer diameters of the CNTs are estimated to be 10-20 nm and 30-60 nm, respectively.In Fig. 6.3A, the CNTs were tightly deposited in the nanopores, which are termed nano-porous diamond densely deposited CNTs (HD CNT-NANO), and 36 bundles of CNT were found per pore on the average.

The

number of CNTs deposited per unit geometric area was calculated to be 144 \xm^.

On the other hand, the top views for the

nano-porous diamond modified with CNTs at low density (LD CNT-NANO) (Fig. 6.3B) clearly shows that 9 bundles of CNT were located in one pore on the average and the CNT density per unit geometric area was estimated to be 45 \im-.

Fig. 6.3. Top view of SEM images for CNT-NANO. (A) HD CNT-NANO and (B) LD CNT-NANO. 6.2.3. G e n e r a t i o n of functions as t h e hybrid anode of a Li-ion battery a n d a super capacitor CVs for the as-deposited diamond (AD), CNT-AD and CNT-NANO are presented in Fig. 6.4.

For AD (Fig. 6.4A), neither Li+

intercalation nor deintercalation was observed.

124

The electrical

6. Electrochemical Properties Non-Aqueous Electrolytes

and Application

of Diamond

Electrodes

in

double-layer capacitance of 200 ^iF cm'^ w a s calculated from t h e background c u r r e n t density.

This result suggests t h a t at the

diamond surface, the charge-discharge process of t h e electrical double-layer can only proceed without Li+ intercalation, even in a non-aqueous electrolyte including Li+ ions.

- 0 . 4 y-

•3.0

-2.0

-1.0

Potential / V \s. Ag/Ag^

Fig. 6.4. CycUc voltammograms for (a) AD, (b) CNT-AD, (c) HD CNT-NANO and (d) LD CNT-NANO electrodes in 1 M LiC104/PC. In contrast, t h e CNT-modified diamond electrodes exhibited voltammograms

associated

with

a

cathodic

current

for

Li+

intercalation in t h e negative scan in t h e potential region from - 3.5 to - 4.0V (vs. Ag/Ag+).

The charge consumed for Li+ intercalation

for each CNTs modified diamond electrode w a s almost a t the same 125

level as Li+ deintercalation, , suggesting that these reactions were totally reversible.

The specific capacity for HD CNT-NANO was

found to be 894 mAh g i which is higher than that obtained for commercialized carbon materials.

These results demonstrate that

CNT'NANO materials have the possibility to be hybrid electrode materials, working as both a Li^ battery and a double-layer capacitor. Based on the pore dimensions and the pore density, the true surface area of the NANO was a factor of 3.2 times the geometric area (roughness factor).

The CNT density for HD CNT'NANO

was 3.6 times larger than that for CNT-AD from the SEM images. Therefore, the discharge capacity should be ca. 11.5 times larger for HD CNT-NANO than that for CNT-AD. However, the value for HD CNT-NANO was only ca. 7.0 times of that for CNT-AD.

From

impedance analysis using equivalent circuits (not shown here), the penetration depth for Li^ intercalation was estimated to be 0.45 ^im, which is about 25 % of the real pore depth (1.8 |im).

For HD

CNT-NANO, the occupation ratio of CNTs per nanopore was so high that there would be a low void volume in the pores, into which the non-aqueous electrolytes could penetrate. Fig. 6.5 shows the constant-current discharge curves.

To

make clear the function of the power assistance due to discharging from the double layer of the CNT-NANO electrodes, the discharge curves were obtained at the same level of current density.

All

CNT modified diamond electrodes exhibited discharge curves shaped like a parabola, which suggests that Li+ deintercalation occurred at a potential around - 3.2 V (vs. Ag/Ag+).

126

6. Electrochemical Properties Non-Aqueous Electrolytes

and Application

of Diamond

Electrodes

in

(a) 1=2.4 mAcm-2

30 60 Time /sec

10 15 20 Time/sec

Fig. 6.5. Constant current discharge curves for (A-a) CNT-AD, (A-b) HD CNT-NANO, (Ba) CNT-AD, (B-c) LD CNTNANO in 1 M LiC104/PC In the case of a higher value of the electrical double layer capacitance at the nanopores, the IR drop due to the resistance for Li+ intercalation can be relieved by the discharge from the electrical double layer. As a result, it was supposed that the slope of curves at the beginning of the discharge would becomelower, and the specific power would be increased. In Fig. 6.5A, for HD CNT-NANO, the discharge curve exhibited only the plateau for Li+ deintercalation, indicating that the behavior for the pure electrical double layer discharging could not be observed. As the density of the CNTs in the nanopores for HD CNT-NANO was high, the double layer capacitance for pure double layer charging on the surface of the NANO was too low to contribute to a decrease in the potential drop. In contrast, in Fig. 6.5B, for the curves of LD CNT-NANO, a

127

linear slope w a s found at the beginning of t h e discharge, followed by a p l a t e a u .

This linear portion is ascribed to t h e discharge from

t h e double layer capacitance on the NANO surface, indicating that, by decreasing t h e CNT density, the hybrid function of the Li+ ion b a t t e r y a n d t h e double layer capacitor were clearly observed. 101 (l>-2) 100 (c) c

(a-2)

10-1

A A

^ A 4

AAAAAJ/V

QO CD(D QO C»

(b-1)

u

s

10-2

(»-l)* 10-3 10-5

10"^

ia3

10-2

10-1

Power density / W cm'^

Fig. 6.6. Ragone plots obtained from galvanostatic measurements for (a) C N T A D ; (b) HD CNTNANO; (c) LD CNTNANO in 1 M LiC104/PC. (a-1) and (b-l) were observed in 0.3 M Et4NBF4/PC. Fig. 6.6 shows the Ragone plots. The specific power P and energy density E were calculated

from

the discharge

curves

obtained at various c u r r e n t densities I using t h e average potential V ave a n d the formulas P = - V ave * I a n d E - 2 ( - V ave * I * A t ) .

HD CNT-NANO exhibited a n energy density E ave of 1.18 J cm 2, which w a s 7 times higher t h a n t h a t for CNT-AD.

128

However, t h e

6. Electrochemical Properties Non-Aqueous Electrolytes

and Application

of Diamond

Electrodes

in

maximum specific power Pmax obtained for HD CNT-NANO (l.lO x 10"2 W cm"2) was in the same range as CNT-AD.

On the other

hand, the P max for LD CNT-NANO (1.45 x 10 2 W cm 2) was 1.4 times greater than that for CNT'AD due to the hybrid function. As a result, by adjusting the CNT density in the nanopores, the ratio of the discharge from the double layer capacitance and the Li+ deintercalation can be controlled and the performance of the electrochemical cell can be designed for any purpose, for example, high energy density or high specific power. In this section, CNT-NANO was shown to be a hybrid electrode material, working as both a supercapacitor and a Li+ ion battery. In the case of the actual use of this hybrid electrode, the ratio of the combination of sp2 and sp^ carbon must be selected according to the requirements of the application.

Recently, a diamond membrane

with nanometer-order through-holes was reported

[19].

By

combining through-hole diamond films and CNTs, we can proceed in developing a hybrid electrode with higher energy density. Moreover, to make this hybrid electrode for practical use, an easier fabrication process for the porous diamond material is needed.

An

activated carbon powder is normally used as the electrode material for the commercialized double layer capacitor. A conductive porous diamond powder is thought to be a promising host material for the practical application of the hybrid electrode. Considering

the

electrochemical

applications

using

non-aqueous electrolytes, the advantage compared to aqueous electrolytes is the wide potential window. In these high voltage regions, the diamond electrodes seem to have possibility of the inertness and stability above those for the other carbon-based

129

electrodes.

The electrochemical properties of diamond electrodes

are expected to be utilized in an even wider range of fields, in addition to the sensing a n d energy device applications introduced above.

References 1.

K. Honda, T. N. Rao, D. A. Tryk, A. Fujishima, M. Watanabe, K. Yasui and H. Masuda, J. Electrochem.

2.

Electrochemical

Methods,

Soc, 147 (2000) 659.

ed. A. J. Bard, and L. R. Faulkner,

Marcel Dekker, Inc. New York, 2001. 3.

T. Tani and K. Ozeki, J. Electrochem.

Soc, 138 (1991) 1411.

4.

M. Ue, K. Ida and S. Mori, J. Electrochem.

5.

A. J. Bard, R. Memming and B. Miller, Pure Appl

Soc, 141 (1994) 2990. Chem., 63

(1991) 569. 6.

L. K. Steffen, B. F. Plummer, T. L. Braley, W. G. Reese, K. Zych, G. V. Dyke and M. Gill, J. Phys. Org. Chem., 10 (1997) 623.

7.

H. Yilmaz, E. Yurtsever and L. Toppare, J. Electroanal.

Chem.,

261 (1989) 105. 8.

E. S. Pysh and N. C. Yang, J. Am. Chem. Soc, 85 (1963) 2124.

9.

T. Tani, Photogr Sci. Eng., 14 (1970) 72.

10. M. Yoshimura, K. Honda, T. Kondo, R. Uchikado, Y. Einaga, T. N. Rao, D. A. Tryk and A. Fujishima, Diamon Relat. Mater,

11 (2002)

67. 11. Z. Wu, T. Yano, D. A. Tryk, K. Hashimoto and A. Fujishima, Lett,

Chem.

(1998) 503.

12. M. Yoshimura, K. Honda, T. Kondo, T. N. Rao, D. A. Tryk and A.

130

6. Electrochemical Properties and Application of Diamond Electrodes in Non-Aqueous Electrolytes

Fujishima, Electrochim. Acta., 47 (2002) 4387. 13. K. Honda, T. N. Rao, D. A. Tryk, A. Fujishima, M. Watanabe, K. Yasui and H. Masuda, J. Electrochem.

Soc, Ul (2000)659.

14. M. Yoshimura, K. Honda, R. Uchikado, T. Kondo, T. N. Rao, D. A. Tryk, A. Fujishima, Y Sakamoto, K. Yasui and H. Masuda, Diamond. Relat Mater., 10 (2001) 620. 15. K. Honda, M. Yoshimura, K.Kawakita, A. Fujishima, Y. Sakamoto, K. Yasui, N. Nishio and H. Masuda, J. Electrochem.

Soc,

151

(2004) A532. 16. H. Masuda, M. Watanabe, K. Yasui, D. A. Tryk and A. Fujishima, Adv. Mater,

12 (2000) 444.

17. H. Masuda, K. Yada and A. Osaka, Jpn. J. Appl. Phys., 37 (1998) L1340. 18. G. Che, B. B. Lakshmi, C. R. Martin and E. R. Fisher, Mater,

Chem.

10 (1998) 260.

19. H. Masuda, K. Yasui, M. Watanabe, K. Nishio, M. Nakano, T. Tamamura, T. N. Rao and A. Fujishima, Electrochem.

SolidState

Lett, 4 (2001) GlOl.

131

7. Electrochemical Hydrogen and Oxygen Evolution Mechanisms at B-doped Diamond Electrodes Investigated by TOF-ESD Methods Ichizo Yagi, Kazuyuki Ueda and Kohei Uosaki

The boron-doped diamond thin film has attracted much interest, as it shows several important and interesting electrochemical properties, including an extremely large potential window in both the negative and positive directions in aqueous solutions [1-3]. The electrochemical characteristics are strongly affected by the surface composition. It is known that the diamond surface is electrochemically oxidized in the oxygen evolution potential region, and the electrochemical properties of the surface are significantly changed after oxygen evolution [4-9]. This is caused by the conversion of the H"termination, which is originally present on the surface of as-deposited diamond films, to 0-termination [6]. The hydrogen evolution reaction (HER) is one of the most important electrochemical reactions, and its mechanism has been studied in detail using a wide variety of metal electrodes, but it is still not completely understood. One of the most important issues for HER is the intermediate state. At a metal electrode surface, HER is known to proceed as follows [lO]Ichizo Yagi e-mail: [email protected] 132

7. Electrochemical Hydrogen and Oxygen Evolution Mechanisms at B-doped Diamond Electrodes Investigated by TOF-ESD Methods

M + H^ + e--> Hads-M

(7.1)

Hads-M + H+ + e- ^ M + m(g)

(7.2)

2 H a d s - M ^ 2M + H2(g)

(7.3)

where M denotes a metal atom on the electrode surface. First, atomic hydrogen directly adsorbed on a metal atom (Hads) is formed as an intermediate as a result of the discharge process (7.1). H2 gas is formed either by an electrochemical mechanism (7.2) or by a catalytic mechanism (7.3). The diamond surface, however, is already terminated by a hydrogen, and the question is whether the terminal hydrogen may or may not take part in the HER. The mechanism for HER at the diamond electrode has not been

experimentally

clarified,

but

was

treated

quantum

chemically [ l l ] . The proposed mechanism [ll] predicts the formation of a carbon radical at the surface by abstraction of the surface hydrogen, as followsH^aq) + =C-H + e- ^ H2(g) + - C -

(7.4)

In this mechanism, ^ C * operates the same as M in eqs. (7.l)"(7.3) and thus, the substitution of the surface hydrogen by protons on the solution side should be possible. This mechanism has already been

verified

for

other

p-type

semiconductors,

including

germanium [12]. On the other hand, if =C-H operates the same as M in eqs. (7.l)-(7.3), the surface hydrogen could not be substituted.

7.1. TOF-ESD Method: the "Protoscope" 7.1.1. H y d r o g e n detection by T O F - E S D The highly sensitive detection of the hydrogen at the diamond surface is essential to clarify the HER mechanism. Various

133

detection techniques to analyze the surface hydrogen have been developed [13-15], and electron-stimulated desorption (ESD) is the most suitable among them from the viewpoint of sensitivity, focusing, and selection of the incident energy. Ueda and coworkers have developed a scanning time-of-flight (TOF) ESD system to detect the two-dimensional hydrogen distribution at solid surfaces with a spatial resolution of 1 pim. This system has been termed the "protoscope" [16, 17]. ESD measurements have already been carried out at diamond surfaces by several groups, but their interests concentrated on the ESD mechanism [18-20], negative electron affinity (NEA) of the H-terminated diamond [21], and surface patterning [22]. Here, the substitution of the H-termination (D-termination) on

boron-doped

poly crystalline

diamond

electrodes

during

electrochemical deuterium (hydrogen) evolution was confirmed using an ex situ TOF-ESD technique [16]. In addition, the effect of the oxygen evolution reaction on the surface distributions of oxygen and hydrogen species was monitored by "protoscope" imaging and briefly introduced.

7.1.2. Equipment for TOF-ESD IVEeasurements The TOF-ESD measurement was carried out in the TOF-ESD protoscope analyzer (Fig. 7.1). Details of the protoscope were previously reported [16, 17]. In the present study, the off-axis electron gun (LEED gun, spot size, 100 jum) was mainly used for the TOF-ESD measurements on the submillimeter scale to roughly estimate the change in the surface concentration of hydrogen at the diamond surfaces. A second,

134

pencil-type,

7. Electrochemical Hydrogen and Oxygen Evolution Mechanisms at B-doped Diamond Electrodes Investigated by TOF-ESD Methods

electron gun (spot size, less t h a n 300 n m at 600 eV), which is normally used for field effect-scanning electron microscopic (FESEM) imaging, w a s used for t h e TOF-ESD m e a s u r e m e n t on the submicrometer scale to e s t i m a t e t h e local distribution of hydrogen. In t h e T O F spectrum, desorbed species, i.e., H+ a n d 0+, a p p e a r as a function of flight time in ^ s . SCREEN MCP

|««<<<<<1

45°

r- 1 , Sample

¥ Fig. 7.1. Schematic diagram of the TOF-ESD microscopy system, termed the "protoscope". A pencil-type electron gun for SEM and conventional low energy electron diffraction (LEED) gun for LEED, Auger electron spectroscopy (AES), and (electron stimulated desorption ion angular distribution (ESDIAD) are combined with an ion detector consisting of microchannel plates (MCPs) and a phosphor screen.

7.1.3. Sample Preparation The details of t h e growth of the boron-doped

polycrystalline

diamond t h i n films using a high p r e s s u r e microwave plasmaassisted chemical vapor deposition system (ASTeX) have been

135

previously reported [5, 6, 8]. The substrate was a conducting n-Si(lll) (< 0.01 Qcm), which was poHshed with 0.5-jLim diamond powder (De Beers) to provide nucleation sites. Heavily doped diamond films (boron concentration greater than 1020 cm'^) were used in the present study. Deuteration of the diamond surface was carried out using two different methods. In the first treatment, the diamond film was heated to 900 °C in a gold image furnace with continuous deuterium flow under atmospheric pressure. In the second method, electron beam impact from the rear of the Si substrate was used to heat the diamond surface up to 950 °C under D2 dosing in the vacuum chamber. After the thermal treatments, the presence of D'termination was confirmed as a small peak on the TOF-ESD spectrum, which was obtained by a more sensitive TOF-ESD measurement system than the one we used in other experiments. The electrochemical cathodic polarization treatment at the diamond surface was carried out using a conventional threeelectrode configuration with a platinum counter electrode and an Ag/AgCl (sat'd NaCl) reference electrode. The electrode potential was held at -2.5 V vs. Ag/AgCl in 0.1 M H2SO4/H2O or 0.1 M D2SO4/D2O solution for 30 minutes. Since H2 or D2 evolution significantly occurred at the diamond electrode surfaces during the cathodic polarization treatment, the bubbles trapped on the surface were removed by the occasional flow of solution from the pipette positioned right above the electrode surface.

136

7. Electrochemical Hydrogen and Oxygen Evolution Mechanisms at B-doped Diamond Electrodes Investigated by TOF-ESD Methods

7.2. Effect of Heating Pretreatment Fig. 7.2 shows the 400change in the TOF-ESD (a) 350 i spectra measured at an 1 300 250 as-deposited B-doped 200 diamond surface (a) 150 \ before and (b) after 100 P 50heating to 400°C in the OS ^\ vacuum chamber. The 400- (b) starting point of the 350 11+ flight time of the ion is 300k 250noted as position, P [\ 200- P \ which indicates the 150 photon signal generated 1001 \ 0^ 50by the primary electron ' \.^ 0irradiation. While only Flight Time / jus the peak of H+ was recognized at the unFig. 7.2. TOF spectra measured at an as-deposited diamond film (a) before treated sample, two and (b) after heating treatment. types of small 0+ peaks These spectra were obtained at the appeared after heating. specimen bias, Vs = 0, and the primary energy, Ep = 250 V. Since the presence of oxygen at the asdeposited diamond surface was also confirmed by XPS measurement [8], this result indicated that the heat treatment was necessary to avoid the effects of adsorbed water and contamination of the TOF-ESD spectra at the sample surfaces. Thus, all of the TOF-ESD spectra shown below were those measured at surfaces for which a constant spectrum had been obtained after repetitive heat treatment in the protoscope v.,.-^.V--^.^W

-i

, > -V' I'l'-- •^~~-

/

L

137

chamber. The temperature for the heat treatment was limited to 500°C to avoid the thermal decomposition of the H- or O" termination at the diamond [23].

7.3. Macroscopic Measurements 7.3.1. Effect of deuterium evolution treatment at Ht e r m i n a t e d diamond electrode Fig. 7.3 shows the change in the TOF-ESD spectra

250 n(a)

H^

200 H

{

measured on a diamond

)

P 150H

electrode surface (a) before 100-

and

(b)

after

cathodic 3

polarization treatment in

0^

'

50-

"^*

0.1 M D2SO4/D2O solution.

0-

"'•'•••"'•;,,,-

'•.-w.,v..,_,,^,,. .^ 1

(b)

250-

To compare the effects of

200-

the cathodic polarization 150-

on both the H'termination and

O'termination,

P 100- 1

the

50-

sample, which showed a relatively yield

in

spectrum,

large the was

oxygen

measured

n0

A A i j\ ^ ^^-^^ 5

TOF-ESD

10

15

0^ .^IWAJI

20

25

.

r::^^ 30

35

Flight Time / |LIS

selected.

The spectra in Fig. 7.3 were

H'

at

the

same sample surface, but

Fig. 7.3. TOF spectra measured at a diamond film (a) before and (b) after cathodic polarization in 0.1 M D2SO4/D2O solution at -2.5 V for 30 min. Vs = 0, Ep = 250 V.

at different positions, (a) with

138

and

(b) without

contact

to the

electrolyte

solution,

7. Electrochemical Hydrogen and Oxygen Evolution Mechanisms at B-doped Diamond Electrodes Investigated by TOF-ESD Methods

respectively. Therefore, the yields of the species at both the treated and untreated surfaces can be directly compared. The yield of H+ was apparently decreased by the cathodic polarization treatment in the deuterated solution, indicating the substitution of the H'termination by deuterium. However, the small yields of the 0+ species did not show much change after the cathodic polarization treatment. The stability of the O'termination with respect to cathodic polarization has already been confirmed by the XPS measurements at pre-oxidized diamond electrodes [8].

7.3.2. Effect of hydrogen evolution t r e a t m e n t at D-terminated diamond electrode In Fig. 7.4, the results of a comparative experiment, in which a pre-deuterated sample electrode was cathodically polarized in a 0.1 M H2SO4 solution, are shown. The pre-deuteration treatment was

carried

out

at

atmospheric

pressure

and

continuous

deuterium flow, resulting in the conversion of almost all of the H and 0-terminated diamond surface to a Determinated one, because the temperature of 900 °C, used for the treatment, is known to be higher than the desorption temperature of the H and O species at diamond surfaces [23]. The sharpness of the H^ peaks in Fig. 7.4 , compared to those in Fig. 7.3 , can be attributed to the specimen bias in the TOF-ESD measurement. Although the disappearance of the 0+ yield in Fig. 7.4(a) supported the surface substitution by deuterium, the yield of H+ was still clearly observed. The subtraction of the background H+ yield was quite difficult, because the surface was not prepared under UHV, and normal Ar+ bombardment/annealing

cycles could not be used to maintain

139

the surface

termination

formed under atmospheric pressure

or in

solution.

Also, the desorption of D+ cannot be detected in the present equipment, since the

yield

of

D+

from

diamond by ESD is known to be much lower than that of H+. Thus,

only

the

change in the H+ yield can be useful to qualitatively estimate the effects of the cathodic

polarization

treatment. The increase in H+ yield in the cathodically treated area at the predeuterated

surface

was

Flight Time / jiis Fig. 7.4. TOF spectra measured at a pre-deuterated diamond film (a) before and (b) after cathodic polarization in 0.1 M H2SO4/H2O solution at -2.5 V for 30 min (Vs = 0; Ep = 250 V).

indeed observed, as expected.

7.4. Microscopic Measurements In both Figs. 7.3 and 7.4 , the estimation of the area where the cathodic polarization treatment was carried out was difficult because of the uncertainty of the measured area. For the TOFESD measurement using the LEED gun, the two dimensional coordination on the sample was defined as the X" and Ystage positions based on the observation by the SEM imaging with the

140

7. Electrochemical Hydrogen and Oxygen Evolution Mechanisms at B-doped Diamond Electrodes Investigated by TOF-ESD Methods

other, pencil-type e-gun. The use of the different e-guns between the SEM and the TOF-ESD measurements directly caused a gap between the measured positions. Although such a gap was corrected by the detailed mapping of the surface by measuring the position-dependent TOF-ESD spectra at the samples on the millimeter scale, the direct TOF-ESD measurement at the observed position by SEM was desired. A TOF-ESD measurement using the pencil-type e-gun was carried out with a more carefully prepared sample. The predeuterated diamond surface was prepared in a vacuum chamber, and then the cathodic polarization treatment was carried out by dipping half of the sample surface into the electrolyte solution, as illustrated in Fig. 7.5(a). After a polarization for 10 minutes at 2.5 V, the sample was rinsed with Milli-Q water and placed in the protoscope chamber and then heated to 500°C. A TOF-ESD line scan of 128 x 2 points (the scanned area corresponded to 182 x 5.68 |im2) around the positions marked in Fig. 7.5(d) was carried out with the pencil-type e-gun. A typical TOF-ESD spectrum obtained by irradiation from the pencil-type e-gun is shown in Fig. 7.5(b), and the peak assigned for H+ was integrated at each position. Although the fine structure of the TOF-ESD spectra cannot be obtained by use of the measurements with the penciltype e-gun because of the small cross section and the higher incident energy, the 2D distribution of the H+ yield can be sufficiently estimated. The integrated yields were plotted versus the y-axis in Fig. 7.5(b). At y = 16.5 mm, an extremely large H+ yield was observed and was assigned to the H+ desorbed from the tungsten electrode, which was placed on the sample edge to hold

141

and heat the sample surface. The H+ yields increased around y = 9.0 mm, which corresponded to the boundary between the cathodically treated and untreated areas. About a 30% increase in the H+ yield was estimated as a result of the hydrogen evolution at the pre-deuterated diamond surface. Although the change in Fig. 7.5(c) seems smaller as compared with the increase in the H+ yield shown in Fig. 7.4, it can be explained by the shorter treatment period and the smaller current efficiency for hydrogen evolution at the diamond electrode, since the rear Si substrate surface also operated as an electrode. Also, the proper estimation of the background H+ yield was not established at the present time.

Flight time / ^s

Fig. 7.5. (a) Schematic arrangement of the cathodic polarization treatment of a pre-deuterated diamond fihn. (b) TOF-ESD spectra measured with the pencil-type e-gun(Vs = 20 V, Ep = 600 V). (c) H+ yield profile at various positions on the cathodically polarized diamond sample. The measured positions are illustrated in (d).

142

7. Electrochemical Hydrogen and Oxygen Evolution Mechanisms at B-doped Diamond Electrodes Investigated by TOF-ESD Methods

7.5. Hydrogen Evolution Mechanism at the B-doped Diamond Electrode The substitution of the H(D)-termination by the D+(H+) in the solution at the extreme cathodic potential was confirmed from the above-mentioned results[24]. The hydrogen evolution at the Bdoped diamond electrode does proceed via the carbon radical formation at the surface, as shown in eq.(7.l). However, it is difficult to estimate the surface concentration of the substituted sites due to the lack of the inspected correlation between the H^ yield obtained and the surface concentration of the surface hydrogen. The absorption of atomic hydrogen or deuterium in the subsurface and bulk of the B-doped diamond may also confuse this problem [25, 26]. Since the surface roughness affects the H+ yield, TOF-ESD measurements at epitaxially grown B-doped diamond single crystalline electrodes [27, 28] are desirable in order to estimate the kinetics of the cathodic substitution of the surface hydrogen.

7.6. Effect of Oxygen Evolution at the B-doped Diamond Electrode on Protoscope Images An

additional

investigation

concerning

theoxygen-evolution

reaction at B-doped diamond electrodes was carried out with the microscopic protoscope approach. By use of the pencil-type FESEM gun, the oxygen peak can also be observed. Figure 6 shows the TOF-ESD spectra measured at the same B-doped diamond sample (a) before and (b) after oxygen evolution treatment at +2.8

143

V in 0.1 M H2SO4 solution for 1 hour. After the anodic oxygen evolution treatment, a peak corresponding to the oxygen clearly appeared. To visualize the 2-dimensional distribution of oxygen species, a TOF-ESD line scan of 64 x 64 points (the scanned area corresponded to 150 x 150 |im-) was carried out with the penciltype e-gun. The peaks corresponding to the hydrogen and oxygen species were integrated at each measurement point and are shown as 2-dimentional images, respectively.

>H

I L

k...._l 10

15

Flight Time / (J.s

Fig. 7.6. TOF-ESD spectra measured by the pencil-type e-gun at (a) as-deposited and (b) oxygen evolution treated diamond samples (Vs = 20 V, Ep = 600 V). Fig. 7.7 shows (a) SEM, (b) hydrogen and (c) oxygen imaged over the same area (150 x 150 \imr) at the oxygen evolution treated diamond surface. Since the hydrogen and oxygen images seem to be vague, all of the images (a), (b) and (c) were binary-

144

7 Electrochemical Hydrogen and Oxygen Evolution Mechanisms at B-doped Diamond Electrodes Investigated by TOF-ESD Methods

filtered to produce black and white images and are shown in Figs. 7.7 (d), (e) and (f), respectively.

Fig. 7.7. (a) FE-SEM image and (b),(c) protoscope images of H+ and 0+ yields, respectively, in an 150 x 150 ^m2 area at an oxygen evolutiontreated diamond electrode (Vs = 20 V, Ep = 600 V). Images (d) (e) and (f) are the binary BAV filtered images of (a), (b) and'(c) respectively. Comparing with the SEM image, the sizes of the black and white areas in theoxygen distribution seem to correspond to those in the microcrystalHne surface, while those in the hydrogen image seem to reflect the surface roughness. The observation that the oxygen distribution is comparable to the grain sizes of the diamond crystallite could be due to the heterogeneous distribution of the reactivities of the crystalline surfaces for electrochemical oxidation. This could be possible, since the boron concentrations in the diamond microcrystallites are well known to be dependent on the crystaUine orientation. For example, the boron concentrations in (111) oriented diamond films have been found to be much larger 145

t h a n those in (100) oriented diamond films, although these films were grown u n d e r the same conditions. In the p r e s e n t study, we used polycrystalline diamond surfaces, and the formation r a t e of O'termination

can depend on the surface

orientations.

Such

experiments should be carried out at single-crystalline diamond electrodes

to

clarify

the

mechanism

of

the

0-termination

formation.

References 1. J. Xu, M.C. Granger, Q. Chen, J.W. Strojek and G.M. Swain, Anal Chew. News & Features (1997) 591A. 2. A. Fujishima, T.N. Rao, E. Popa, B.V. Sarada, I. Yagi and D.A. Tryk, J. Electroanal

Chem., 473 (1999) 179.

3. R. Tenne and C. LevyClement, Isr. J. Chem., 38 (1998) 57. 4. I. Yagi, K. Tsunozaki, D.A. Tryk and A. Fujishima, Solid-state

Electrochem.

Lett, 2 (1999) 457.

5. D.A. Tryk, K. Tsunozaki, T.N. Rao and A. Fujishima,

Diamond

Relat Mater., 10 (2001) 1804. 6. I. Yagi, H. Notsu, T. Kondo, D.A. Tryk and A. Fujishima, Electroanal.

J.

Chem., 473 (1999) 173.

7. H. Notsu, I. Yagi, T. Tatsuma, D.A. Tryk and A. Fujishima, J. Electroanal.

Chem., 492 (2000) 31.

8. H. Notsu, I. Yagi, T. Tatsuma, D.A. Tryk and A. Fujishima, Electrochem.

Solid-State

Lett, 2 (1999) 522.

9. E. Popa, H. Notsu, T. Miwa, D.A. Tryk and A. Fujishima, Electrochem.

146

Solid-State

Lett, 2 (1999) 49.

7. Electrochemical Hydrogen and Oxygen Evolution Mechanisms at B~doped Diamond Electrodes Investigated by TOF-ESD Methods

lO.J.O.M. Bockris and S.U.M. Khan, in "Surface Electrochemistry", Plenum, NewYork, 1993, p310. 11. A.B. Anderson and D.B. Rang, J. Phys. Chem. A 102 (1998) 5993. 12. D.R. Turner, J. Electroanal

Chem., 103 (1956) 252.

13.Y.J. Chaval, G.S. Higashi, K. Ragharachari and V.A. Burrows, J. Vac. ScL TechoU A7 (1990) 2104. 14. R.J. Cullbertson, L.C. Feldman and P.J. Silverman, J. Vac. Sci. Tech., 20 (1982) 868. 15. H. Kobayashi, K. Edamoto, M. Onchi and M. Nishijima, J. Chem. Phys., 78 (1983) 7429. 16. K. Ishikawa, M. Yoshimura, K. Ueda and Y. Sakai, Rev. Instrum.,

Sci.

id^ (1997) 4103.

17. K. Ueda, J. Cryst Growth, 210 (2000) 416. 18. C. Goeden, G. Dollinger and P. Feulner, DiamondRelat.

Mater., 9

(2000) 1164. 19. A. Hoffman, A. Laikhtman, S. Ustaze, M.H. Hamou, M.N. HedhiH, J.P. Guillotin, Y. Le Coat, D.T. Billy, R. Azria and M. Tronc, Phys. i?eF. ^ 6 3 0 4 ( 2 0 0 1 ) 5401. 20. A. Hoffman, S. Ustaze, M.H. Hamou, M.N. HedhiH, J.P. Guillotin, C.Y. Le, R. Azria and M. Tronc, Phys. Rev. B, 63 (2001) 5417. 21. H.J. Hopman, J. Verhoeven and P.K. Bachmann, Diamond

Relat.

Mater., 9 (2000) 1238. 22. C.H. Goeting, F. Marken, C. Salter, R.G. Compton and J.S. Foord, Chem. Commun., (1999) 1697. 23. R.E. Thomas, R.A. Rudder and R.J. Markunas, J. Vac. Sci. Technol., ^ 1 0 (1992) 2451. 24. I. Yagi, K. Ogai, T. Kondo, A. Fujishima, K. Ueda and K. Uosaki, Chem. Lett, 32 (2004) 1050.

147

25. R. Zeisel, C.E. Nebel and M. Stutzmann, Appl. Phys. Lett,

74

(1999) 1875. 26. J. Chevallier, B. Theys, A. Lusson and C. Grattepain, Phys.

Rev.

B, 58 (1998) 7966. 27. M. Yanagisawa, L. Jiang, D.A. Tryk, K. Hashimoto and A. Fujishima, DiamondRelat.

Mater., 8 (1999) 2059.

28. H.B. Martin, A. Argoitia, J.C. Angus and U. Landau, Electrochem.

148

Soc, 146 (1999) 2959.

J.

8. Single-Crystal Homoepitaxial Diamond Electrodes Takeshi Kondo, Kensuke Honda, Yasuaki Einaga, Donald A. Tryk and Akira Fujishima

In much of the research on boron-doped diamond (BDD) electrodes published thus far, for both fundamental and applied aspects, polycrystalline thin films have been used, because continuous BDD thin films of high quality and purity can be obtained easily and useful research results can be obtained. However, in order to understand the electrochemical properties of diamond electrodes in greater detail, especially concerning the relationships between the crystal structure and the electrode properties, it is becoming essential to carry out studies with single-crystal

diamond

electrodes. In spite of the importance of single-crystal diamond electrodes, the number of reports on their electrochemistry [1-7] is still small, much smaller than that for polycrystalline diamond electrodes. The main reason for this situation is the limited availability of the single "crystal diamond samples.

Generally, in the case of CVD

film preparation, the area of the polycrystalline diamond thin film depends on the capability of the deposition apparatus (e.g., power), because large silicon wafers can be used as substrates. In contrast, in the case of single-crystal homoepitaxial diamond thin films, the Takeshi Kondo e-mail: [email protected] 149

substrates are also single-crystal diamond, which are relatively expensive, especially if the cost of polishing is included. Thus, the available area of a single-crystal homoepitaxial BDD film depends on that of the substrate (a maximum of several mm^). Moreover, as a practical problem, the tolerances for the deposition conditions are much more rigid for homoepitaxy than for polycrystalline diamond deposition, and that situation would be exacerbated by increasing the area of homoepitaxial film to be deposited. In this chapter, we describe the preparation and fundamental electrochemical properties of single-crystal homoepitaxial BDD electrodes. We then introduce some examples of applications involving single-crystal BDD electrodes, including electroanalysis, surface modification and scanning probe nanolithography.

8.1. Preparation of Homoepitaxial Diamond Electrodes Single-crystal homoepitaxial BDD thin films are deposited on natural or synthetic single-crystal diamonds, which are typically prepared by high pressure-high temperature (HPHT) synthesis. A 4x4x1.5-mm sample is shown in Fig. 8.1. Homoepitaxy which is carried

out with

a CVD system,

similar to the case of

polycrystalline BDD, but it requires an atomically flat substrate surface, because growth must conform to the substrate crystal structure. In order to obtain epitaxial films, the growth should proceed with a step-flow mechanism [8,9]; carbon atoms adsorbing from the gas phase onto the substrate surface are incorporated into the growing crystal at the kinks of atomic surface steps.

150

8. Single-Crystal Homoepitaxial Diamond Electrodes

Step-flow growth can be ensured by using substrates whose surfaces are polished with a slight off-axis angle; e.g., 4° in the <110> direction for the (lOO) surface [5-7,9-11].

Fig. 8.1. Single-crystal type homoepitaxial BDD electrodes. It has been reported that the carbon concentration in the gas phase should also be controlled during deposition in order to achieve step-flow growth. When the carbon concentration in the gas phase exceeds the ability of the surface steps to incorporate the carbon atoms, abnormal nucleation occurs on the surface terraces, and this results in non-epitaxial growth [12]. For a (lOO) diamond substrate, non-epitaxial growth can be seen as a pyramidal hillock, which consists of ( i l l ) facets [9,12]. In order to obtain homoepitaxial BDD thin films of high quality, the carbon concentration in the gas phase should be lowi however, low carbon concentration also leads to a low growth rate.

Therefore, the

carbon concentration should be optimized and controlled [13]. In the case of a microwave-assisted plasma (MP) CVD system, it is also known that the substrate temperature

[14] and

microwave power [13] during deposition affect the quality of the deposited diamond crystal.

In general, for both polycrystalline

151

and single-crystal diamond, higher crystal growth rates lead to lower crystal quality. To estimate the quality of homoepitaxial BDD thin films, optical microscopy, scanning electron microscopy (SEM), atomic force microscopy (AFM) [5,15], scanning tunneling microscopy (STM) [16-20], transmission electron microscopy (TEM) [21], among others, are used to observe the surface morphology, and Raman

spectroscopy

[22,23]

and

cathodoluminescence

(CL)

spectroscopy [24] are used to estimate the crystal quality. Reflection high-energy electron diffraction (RHEED) [15,25] and low-energy electron diffraction

(LEED) [25,26] are used to

examine the surface crystal structure, and secondary ion mass spectrometry (SIMS) [27,28] is used to analyze the concentrations of specific elements such as boron and to create depth profiles. We note here the relationships between the particular crystal faces of diamond and the characteristics of the crystal growth. It has been reported that contaminants can be incorporated into the growing ( i l l ) face to a greater extent than into the (lOO) face during CVD diamond crystal growth [29]. This explains why, first, the dopant (e.g., boron) tends to be incorporated into the (lOO) face at lower concentrations compared to the ( i l l ) face. For example, when the relative boron concentration in the carbon source feedstock is 10,000 ppm (atomic ratio, i.e., 1 at%), which would correspond to lO^i cm"3 if incorporated into the diamond crystal, the

actual

boron

concentrations

in

the

(lOO)

and

(ill)

homoepitaxial BDD thin films were found to be ca. 101^ cm"3 and 10^9 cm"3, respectively [6]. These values correspond to materials that would exhibit electrical properties somewhere between

152

8. Single-Crystal Homoepitaxial Diamond Electrodes

semiconducting

and

conducting.

Second, the

tendency

to

incorporate contaminants causes increasing roughness of the deposited surfaces. It has been reported that atomically smooth (lOO) homoepitaxial BDD thin films can be obtained relatively easily, but smooth ( i l l ) homoepitaxial BDD thin films are difficult to prepare [lO, 25]. This is considered to be due to the ability of ( i l l ) diamond faces to incorporate contaminants (e.g., hydrogen atoms) in the gas phase during the deposition process. In any case, it can be concluded that difficulties remain in preparing high quality homoepitaxial BDD thin films, and there is room for further developments.

8.2. Electrochemical Properties of Homoepitaxial Diamond Electrodes BDD electrodes have several very attractive electrochemical properties, such as wide potential window in aqueous [30-33] and non- aqueous [34-36] media, very low capacitance [37-39] and fast electron transfer for typical redox systems, e.g., Fe(CN)6^'^^and Ru(NH3)62^'/3+ [40]. In this section, we discuss the electrochemical properties of single-crystal homoepitaxial BDD electrodes. Fig. 8.2 shows representative cyclic voltammograms (CVs) for polycrystalline and single-crystal homoepitaxial BDD electrodes in 0.1 M H2SO4, showing the useful electrochemical potential windows.

The potentials at which the hydrogen and oxygen

evolution reactions become significant were found to be nearly the same for the single-crystal homoepitaxial BDD electrodes and for the polycrystalline BDD electrodes. Furthermore, no significant

153

difference can be seen between the two types of diamond crystal faces. This indicates that the potential windows for high quality polycrystalline

BDD electrodes

are based

on the

intrinsic

properties of diamond. In the present case, the possible presence of a non-diamond carbon phase, which could be present at grain boundaries, is at a level low enough that there is a negligible effect on the CV. In the case of low quality polycrystalline BDD electrodes, the CV in sulfuric acid may exhibit a pair of redox peaks at a potential of +1.8 V vs. SHE [3]. These peaks are considered to be due to a redox reaction involving sp^ carbon, because it is not observed for high quality polycrystalline and single-crystal BDD electrodes. However, even though no significant differences have been observed for the potential window for the polycrystalline and single-crystal BDD electrodes, it has been found that the doublelayer capacitance is lower at single-crystal BDD electrodes compared

to polycrystalline

electrodes.

This difference

is

considered to be mainly due to the surface roughness of polycrystalline BDD thin films. The roughness factor has been estimated to be 2-3, and this factor agrees approximately with the difference in the double-layer capacitances. We should also note that the capacitance depends on the doping level, and differences can be observed for various epitaxial films with differing doping levels. For example, as already mentioned, the doping levels for the (100) face are typically somewhat lower than those for the (111) face.

154

8. Single-Crystal Homoepitaxial Diamond Electrodes

polycrystalline ^-^-.... - w - ^

-^-w-fc-'^

/^

_y

(100) single-crystal

Jo.5 mA cm" 1 1.5

1 -1.0

\ -0.5

1 0.0

1 1.0

\ 0.5

\ 1.5

r2.0

Potential / V vs. Ag/AgCl

Potential / V vs. Ag/AgCl

Fig. 8.2. (a) CVs for 0.1 M H2SO4 at (dashed line) polycrystalline and (solid line) (lOO) homoepitaxial BDD electrodes; (b) magnified portion of (a) for the potential range of +0.1 to +0.7 V vs. Ag/AgCl. Potential sweep rate- 100 mV s^. The electron transfer for typical redox systems, such Fe(CN)63/4 a n d

Ru(NH3)62+/3+

at

hydrogen-terminated

as

single-

crystal homoepitaxial BDD electrodes vv^as found to be fast, and the

behavior

v^as

similar

to

that

for

polycrystalline

BDD

electrodes as v^ell a s to t h a t for noble m e t a l s and for other types of carbon electrodes.

This fact indicates t h a t the electron transfer 155

occurs on the diamond surface, and not at grain boundaries and non-diamond carbon impurities! the latter appear not to be essential for fast electron transfer. The experimentally obtained results relating the electron transfer rate (^), estimated from CV simulation, and the self-exchange rate (Jcexc) were found to follow the line based on Marcus theory (Fig. 8.3) [6]. Thus, the electron transfer process for these redox systems is considered to be of the outer-sphere electron type. 10^-

10°-

theoretical ^^^^ 10-^-

10-^-

•

^ ^

10-^10"

• ^o'-

-

^

10-^

10-^

10°

^

1 10^

.

[10^

1 ^-h 10g[k,,e/M-^S-^]

Fig. 8.3. Log-log plot for estimated electron-transfer rate constant (i^) vs. homogeneous self-exchange rate constants (iexd for redox systemsFe2+/3+ (ixlO-3 M-i s'l), Ru(NH3)62+/3-^ (4x103 M^ sO, Fe(CN)63-/4- (2x10^ M-i s-i) and IrCl62-/3- (2x10^ M^ sO. (o) ( i l l ) , (•) (lOO) and (x) polycrystalUne BDD electrodes. The soUd line was calculated from the simple Marcus relation (ref. 6) It has been known for some time that the electrochemical properties depend on the exposed crystal face for noble metal electrode materials such as platinum [41] and for sp metals such

156

8. Single-Crystal Homoepitaxial Diamond Electrodes

as silver [42,43]. Also, as an example of a carbon material, it is known that the electrochemical properties are different for the basal and edge planes of highly ordered pyrolytic graphite (HOPG) [44].

The electrochemical properties for given crystal faces of

diamond may also be estimated by use of single "crystal type diamond electrodes. As shown in Fig. 8.3, the electron transfer rates tend to be larger at ( i l l ) diamond electrodes compared to (100) electrodes. This is most likely due to the differing dopant (boron) concentrations in the different crystal faces, as mentioned above. Particularly for semiconductor electrodes, electron transfer rates are well known to depend upon the carrier concentrations in the electrode phase. The poly crystalline BDD electrode surface has various exposed facets, so that the electrochemical activity may

not be 2-dimensionally

homogeneous.

However,

the

relationships between the surface crystal structure of diamond and the electrochemical properties have not been clarified in detail yet, certainly not to the degree that they have noble metal electrode materials. The main reason for this difficulty may be that BDD surfaces are basically inactive for adsorption, contrary to the case for noble metal surfaces, which have well understood catalytic properties. Thus far, in addition to the effect of carrier concentration, the ways in which the surface crystal structure contributes to the electrochemical properties of BDD may involve the variation and density of surface functional groups, which can be generated by surface modification treatments. This topic will be discussed later in this chapter.

157

8.3. Applications in Electroanalysis As mentioned above, single-crystal type BDD electrodes exhibit superior electrochemical properties, especially low background current for electrochemical measurements, as well as wide potential window, fast electron transfer and chemical and physical stability.

These properties are very attractive for an electrode

material for electroanalysis [6].

As one example that single-

crystal homoepitaxial BDD electrodes can be applied to an electrode material for electroanalysis of a redox species that undergoes multielectron transfer, electrochemical detection of uric acid

(UA) was

examined.

Fig. 8.4

shows

linear

sweep

voltammograms (LSVs) for UA at a (lOO) homopitaxial BDD electrode. A peak based on the oxidation of UA was observed at ca. 0.75 V vs. Ag/AgCl in the concentration range of 0.1-1.0 [xM. The peak current was found to be directly proportional to the UA concentration in that range, and this means that the boron-doped (100)

surface

can

be

used

as

an

electrode

material

electrochemical detection of UA in this concentration range.

158

for

8. Single-Crystal Homoepitaxial Diamond Electrodes

: polycrystalline : (100) homoepitaxial 0 0.5

0.6

0.7

0.8

0.9

1.0

500

1000

1500

UA concentration / nM

Potential / V vs. Ag/AgCl

Fig. 8.4. (Left) LSVs for 0.1 M HCIO4 containing various UA concentrations at a (lOO) homoepitaxial BDD electrode. The potential sweep rate was 20 mV s'^ , and the electrode surface area was 0.03 cm^ Fig. 8.5. (Right) UA oxidation LSV peak currents vs. UA concentration for polycrystalline and single-crystal diamond electrodes. Measurement conditions were the same as those for Fig. 8.4. As another factor in determining the quality of the electrode material for electroanalysis, the absence of surface defects [45] is important. Polished polycrystalline BDD electrodes, which have a mirror-like

finish,

can exhibit low background currents in

voltammetric measurements, which lead to high S/B ratios, as for single-crystal homoepitaxial BDD electrodes. However, in the CV for serotonin (Fig. 8.6), one of the important bioamines, there is a 159

p a i r of redox p e a k s in addition to t h e peak for t h e oxidation of serotonin at t h e polished poly cry staUine BDD electrode (Fig. 8.6c), a n d this p e a k p a i r is t h o u g h t to be due to a redox reaction involving a n adsorbed quinone t h a t is a n oxidation product of serotonin. zu -

^—-^1

10-

0-10-20-30-

'

f 1

u

^

-0.4

•

^ 1

'

0.0

^

(a)

1

'

0.4

1—'—\—'—r

1

0.8

Potential / V vs. Ag/AgCl

-0.4

0.0

0.4

0.8

Potential / V vs. Ag/AgCl

20-r

"I -0.4

0.0

'

I 0.4

'

r 0.8

Potential / V vs. Ag/AgCl

Fig. 8.6. CVs for 10 \iM. serotonin in 0.1 M phosphate buffer at (a) polycrystalhne, (b) ( i l l ) homoepitaxial and (c) mirror-poUshed poly cry staUine BDD electrodes," potential sweep rate^ 100 mV s'l (ref. 6).

The p e a k p a i r could not be observed clearly at polycrystalhne (Fig. 8.6a) a n d single-crystal homoepitaxial BDD electrodes (Fig. 8.6b).

160

The presence of defects most likely provides adsorption

8. Single-Crystal Homoepitaxial Diamond Electrodes

sites for the quinone.

Conversely, the absence of defects, and

particularly an absence of sp^-type carbon on the as-deposited homoepitaxial surface, should result in a lower tendency for adsorption of many types of chemical species, particularly those that can form charge-transfer complexes with aromatic rings, such as quinones.

This property is also highly important

for

electroanalysis.

8.4. Surface Modification of Homoepitaxial Diamond Electrodes As mentioned in the previous chapter, surface

termination

contributes greatly to the electrochemical properties of BDD electrodes.

For example, the ferri/ferrocyanide redox system

exhibits slow electron transfer (-lO'^ cm sO at oxygen-terminated surfaces, while it exhibits fast electron transfer (-lO"^ cm sO at hydrogen-terminated surfaces [40]. This fact is very important, especially for applying BDD electrodes to electroanalysis (see Chapter 10). One possible origin of this different electrochemical behavior for the two types of surface termination is thought to be a change in the electrostatic interactions between charges or dipoles of reacting compounds and surface dipoles of the electrode. However, detailed discussion of such a mechanism has not been reported thus far. By use of single-crystal BDD electrodes, it may be possible that the mechanism can be investigated systematically in terms of variation and concentration of surface groups on diamond, because single-crystal homoepitaxial BDDs have highly ordered surface faces.

In this section, we discuss the effect of

161

surface oxidation on the electrochemical behavior of single-crystal homoepitaxial BDD electrodes. The electrochemical properties of single-crystal homoepitaxial BDD electrodes for hydrogen- and oxygen-terminated surfaces are essentially the same as those of the corresponding polycrystalline BDD electrodes. However, as mentioned in the previous section, single-crystal homoepitaxial BDD electrodes may have lower boron concentrations, and this leads to p-type semiconducting or insulating properties for oxygen-terminated surfaces [5-7], even though this phenomenon is not observed for heavily boron-doped polycrystalline diamond electrodes.

Such a change in electrical

properties is thought to be due to two types of roles of hydrogen atoms, which can exist in the near-surface region of diamond [7,28,46] because of the deposition process. When the hydrogen concentration in the region is high enough, these atoms can act as acceptors and provide surface conductivity to the film.

On the

other hand, they can also form H-B pairs with boron atoms in the diamond film and passivate the activity of boron, which operates as an acceptor. Contribution of the latter role of hydrogen may predominate where the concentration of hydrogen atoms in the film is low, and this may be the situation arising in the case of surface oxidation (e.g., anodic treatment). Surface oxidation can modify the surface groups on diamond surfaces [47,48], as well as the surface conductivity.

Surface

groups generated on diamond may depend on the type of surface crystal face [49] (Fig. 8.7). For example, the ideal (lOO) diamond surface has two chemical bonds for a single carbon atom in the first surface layer, and thus carbonyl and bridging ether groups

162

8, Single-Crystal Homoepitaxial Diamond Electrodes

can be generated on the surface. On the other hand, because the ideal unreconstructed ( i l l ) diamond surface has one chemical bond for a single carbon atom in the first surface layer, carbonyl and ether groups are not expected to form, and thus the hydroxyl group

should

dominate.

That

idea

has

been

indicated

experimentally on the basis of XPS analysis and surface chemical modification with appropriate reagents as chemical probes [49]. Surface groups on diamond show trends in stability and reactivity that are similar to those of ordinary organic compounds. Thus, the reactivity of the surface enables BDD to be applied to modified electrodes immobilizing functional molecules, such as enzymes and DNA, on the surfaces. In addition, single-crystal BDD should be useful for fundamental studies of such a technique, especially the estimation of surface structure.

(100) surface

side view

top view

(111) surface

t^A^s 0. >^^ %^. M L i i

• • • • • •

H 0

c(r*layer)

C(2"*Mayer) C(3'*' layer) C(4**' layer)

Fig. 8.7. The expected oxygen-containing surface functional groups on diamond depend on the surface crystal face.

163

8.5. Nanolithographic Modification of Diamond with AFM Techniques Differences in the surface conductivity with surface termination of diamond can be applied to the nanolithographic modification of diamond surfaces by use of atomic force microscopy (AFM) techniques [50-52].

Modification can be carried out by applying

an electrical bias to the sample surface via a conductive cantilever tip, e.g., Au-coated Si (Fig. 8.8). Surface modification using such an AFM technique is relatively general, and has been achieved for semiconductor materials such as Si [53], GaAs [54] and metals such as Ti [55]. Recently, Tachiki et al. and Kondo et al. have applied this technique to single-crystal homoepitaxial diamond thin films, undoped and boron-doped, respectively. In this section, we discuss the properties of diamond surfaces modified via AFM techniques and possible applications. Au-coated AFM tip

Single-crystal BDD thin film Fig. 8.8. A schematic drawing of nanolithography on homoepitaxial diamond with a conductive AFM tip. As already mentioned in the previous section, a hydrogenterminated surface of diamond exhibits surface conductivity, and we can also observe this with a current-mapping AFM image. For example, a topographic AFM image of a hydrogen-terminated

164

8. Single-Crystal Homoepitaxial Diamond Electrodes

(100) homoepitaxial BDD thin film surface shows an almost atomically

flat

surface

on the

micrometer

scale, and

the

simultaneous current mapping image indicates 2-dimensionally uniform surface conductivity for the measured region. surface

conductivity

can

be

observed

for

an

Such

undoped

homoepitaxial diamond thin film, as well as a boron-doped one. However, after scanning with an applied high positive bias voltage (e.g., + 2 V) via an AFM tip on a hydrogen-terminated diamond surface, the current mapping AFM image reveals the area scanned with high bias voltage has become very much less conductive (Fig. 8.9a).

In addition, the topographic image

obtained

with

simultaneously

the

current

image

shows

a

topographic elevation of ca. 1 nm at the very area scanned with high bias voltage (Fig. 8.9b). Interestingly, it was found that a pattern consisting of lines with a width of ca. 30 nm could be created with this technique (Fig. 8.10). By use of Auger electron spectroscopy, the amount of oxygen on the diamond surface was found to have increased after the modification [51].

This indicates that the diamond

surface

scanned with high positive bias voltage has been oxidized, i.e., has become oxygen-terminated. Furthermore, the AFM modification method requires moderate atmospheric humidity (e.g. 55%), and thus the modification should be due to a surface anodic reaction involving water condensing on the AFM tip as an electrolyte. The idea that condensed water exists between an AFM tip and a sample under ambient humidity conditions is well accepted.

165

Fig. 8.9. (a) Current map and (b) simultaneous topographic AFM image obtained at a (lOO) homoepitaxial BDD thin film, measured at a sample vs.. tip bias voltage of +1.0 V. The central 200X200 nm2 area had been subjected to a bias of +2.0 V.

Fig. 8.10. Pattern formation on a hydrogen-terminated (lOO) homoepitaxial BDD thin film. Four lines were produced by scanning the Au-coated AFM tip with a sample-tip bias of +2.0 V.

166

8. Single-Crystal Homoepitaxial Diamond Electrodes

As discussed in the previous section, anodic treatment on diamond can cause modification of surface functional groups and a decrease of acceptor hydrogen atoms existing in the near-surface region of the diamond surface. The former contributes to a change in the hydrophobicity of the surface, as well as the latter leading to a change in surface conductivity. Therefore, the modification with this nanolithographic technique using AFM involves the conductivity, and at the same time, the hydrophobicity. Modification of hydrophobicity on the diamond surface can be thought to cause the topographic elevation observed in the topographic AFM image (Fig. 8.9b). The origin of the elevation has not yet been well elucidated, but one possible reason may be the change of hydrophilic-hydrophobic interactions.

It is well

known that the hydrogen- and oxygen-terminations on diamond yield hydrophobic and hydrophilic surfaces, respectively [40, 56]. The AFM tip coated by gold itself is expected to be hydrophobic, and thus there should be a smaller attractive force between tip and surface for the oxidized portion. Such a difference

in

attractive force would result in an apparent increase in elevation. Another possible reason may be a difference in thickness of adsorbed water layers on the surface. Water can adsorb on the hydrophilic oxidized portion more selectively. Indeed, current voltage measurements with AFM in air and under vacuum conditions indicate the presence of adsorbed water on both hydrogen-terminated and oxidized diamond surfaces. Nanolithography on the diamond surface, as a patterning of conductivity, is expected to be applied to nanoscale electronic devices and ultrahigh density memory devices with greater

167

tolerance to environmental extremes, e.g., high temperature and radiation, than silicon.

Moreover, as a chemical patterning

technique, it can be applied to selective immobilization of biomolecules such as DNA, enzymes and proteins, utilizing differences in hydrophobicity or surface functional groups.

8.6. Conclusions Although diamond is an extremely inert, stable material, the very surface itself can take on various types of relatively stable terminations, such as hydrogen, oxygen, and so on.

It is very

interesting that the surface termination can greatly affect the electrical,

chemical

experimentally

and

observe

electrochemical and

understand

properties. such

To

interesting

properties of diamond surface should be one important theme for single-crystal diamond, although the production of bulk single crystals remains an important recent theme. Research on singlecrystal diamond in electrochemistry is still developing, but it should begin to play an important role in both fundamental and applied aspects of diamond electrodes, because single-crystal diamond is after all an ideal material for both.

168

8. Single-Crystal Homoepitaxial Diamond Electrodes

References 1. R. Ramesham, Sens. Actuators B, 50 (1998) 131. 2. Y. V. Pleskov, Y. E. Evstefeeva, M. D. Krotova, V. V. Elkin, V. M. Mazin, V. Y. Mishuk, V. P. Varnin and I. G. Teremetskaya, J. Electroanal 3.

Chem., 455 (1998) 139.

H. B. Martin, A. Argoitia, J. C. Angus and U. Landau, Electrochem.

J.

Soa, 146 (1999) 2959.

4. J. van de Lagemaat, D. Vanmaekelbergh and J. J. Kelly, J, Electroanal 5.

Chew., 475 (1999) 139.

M. Yanagisawa, L. Jiang, D. A. Tryk, K. Hashimoto and A. Fujishima, DiamondRelat

6.

8 (1999) 2059.

T. Kondo, Y. Einaga, B. V. Sarada, T. N. Rao, D. A. Tryk and A. Fujishima, J. Electrochem.

7.

Mater,

Soc, 149 (2002) E179.

T. Kondo, K. Honda, D. A. Tryk and A. Fujishima,

Electrochim.

Acta, 48 (2003) 2739. 8.

H. Okushi, K. Hayashi, S. Yamanaka, H. Watanabe, K. Kajimura and T. Sekiguchi, J. Cryst. Growth, 183 (1998) 338.

9.

D. Takeuchi, H. Watanabe, S. Yamanaka, H. Okushi and K. Kajimura, Diamond. Relat. Mater,

9 (2000) 231.

10. G. Janssen, W. J. P. van Enckevort, W. Voilenberg and L. J. Giling, Diamond Relat. Mater,

1 (1992) 789.

11. S. -G. Ri, H. Yoshida, S. Yamanaka, H. Watanabe, D. Takeuchi and H. Okushi, J. Cryst. Growth, 235 (2002) 300. 12. N. Lee and A. Badzian, Diamond Relat. Mater,

6 (1997) 130.

13. T. Teraji, S. Mitani, C. Wang and T. Ito, J. Cryst. Growth., 235 (2002) 287.

169

14. C. Findeling-Dufour, A. Vignes and A. Gicquel, Diamond

Relat.

Mater., 4 (1995) 429. 15. T, Takami, K. Suzuki, I. Kusunoki, I. Sakaguchi, M. NishitaniGamo and T. Ando, Diamond. Relat Mater., 8 (1999) 701. 16. T. Tsuno, T. Imai, Y. Nishibayashi, K. Hamada and N. Fujimori, Jpn. J. Appl. Phys., 30 (1991) 1063. 17. Y. Kuang, N. Lee, A. Badzian, T. T. Tseng, T. Badzian and C. Chen, Diamond Relat. Mater,

4 (1995) 1371.

18. H. Kawarada, H. Sasaki and A. Sato, Phys. Rev. B, 52 (1995) 11351. 19. C. Niitzenadel, O. M. Kiittel, L. Diederich, E. Maillard-Schaller, O. Groning and L. Schlapbach, Surf. Sci., 369 (1996) LI 11. 20. H. Murakami, M. Yokoyama, S. M. Lee and T. Ito, Appl. Surf. Sci., 175 (2001) 474. 21. H. Sawada, H. Ichinose, H. Watanabe, D. Takeuchi and H. Okushi, Diamond Relat. Mater,

10 (2001) 2030.

22. F. Pruvost, E. Bustarret and A. Deneuville, Diamond.

Relat.

Mater, 9 (2000) 295. 23. Y. Wang, H. Li, Y. Xiong, S. Zhang, Z. Lin and K. Feng, Diamond Relat. Mater., 9 (2000) 1708. 24. C. L. Wang, A. Hatta, N. Jiang, J. H. Won, T. Ito, A. Hiraki, Z. S. Jin and G. T. Zou, Diamond Relat. Mater., 7 (1998) 748. 25. M. Nishitani-Gamo, K. P. Loh, I. Sakaguchi, T. Takami, I. Kusunoki and T. Ando, J. Vac. Sci. Technol A, 17 (1999) 2991. 26. Y. M. Wang, K. W. Wong, S. T. Lee, M. Nishitani-Gamo, I. Sakaguchu, K. P. Loh and T. Ando, Phys. Rev. B, 59 (1999) 10347. 27. I. Sakaguchi, M. Nishitani-Gamo, K. P. Loh, K. Yamamoto, H. Haneda and T. Ando, Diamond Relat. Mater,

170

7 (1998) 1144.

8. Single-Crystal Homoepitaxial Diamond Electrodes

28. J. Chevallier, B. Theys, A. lusson, C. Grattepain, A. Deneuville and E. Gheeraert, Phys. Rev. B, 58 (1998) 7966. 29. K. Ushizawa, K. Watanabe, T. Ando, I. Sakaguchi, M. NishitaniGamo, Y. Sato and H. Kanda, Diamond

Relat

Mater,, 7 (1998)

1719. 30. N. Vinokur, B. Miller, Y. Avyigal and R. Kalish, J.

Electrochem.

Soc, 143 (1996) L238. 31. F. Bouamrane, A. Tadjeddine, J. E. Butler, R. Tenne and C. LevyClement, J. Electroanal

Chem., 405 (1996) 95.

32. H. B. Martin, A. Argoitia, U. Landau, A. B. Anderson and J. C. Angus, J. Electrochem.

Soc, 143 (1996) L133.

33. G. M. Swain, A. B. Andersonand J. C. Angus, MRS Bull,

23

(1998) 56. 34. Z. Y. Wu, T. Yano, D. A. Tryk, K. Hashimoto and A. Fujishima, Chem. Lett, (1998) 503. 35. L. -F. Li, D. Totir, B. Miller, G. Chottiner, A. Argoitia, J. C. Angus and D. A. Scherson, J. Am. Chem. Soc, 119 (1997) 7875. 36. M. Yoshimura, K. Honda, T. Kondo, R. Uchikado, Y. Einaga, T. N. Rao, D. A. Tryk and A. Fujishima, Diamond

Relat. Mater.,

11

(2002) 67. 37. J. Xu, Q. Chen and G. M. Swain, Anal

Chem., 70 (1998) 3146.

38. R. Tenne, C. LevyClement, Isr J. Chem., 38 (1998) 57. 39. G. M. Swain, J. Electrochem.

Soc, 141 (1994) 3382.

40. I. Yagi, H. Notsu, T. Kondo, D. A. Tryk and A. Fujishima, J. Electroanal.

Chem., 473 (1999) 173.

41. F. J. Huerta, E. Morallon, J. L. Vazques and A. Aldaz, Surf. Sci, 396 (1998) 400.

171

42. B. M. Jovic, V. D. Jovic and G. R. Stafford, Electrochem.

Com., 1

(1999) 247. 43. N. Hoshi, M. Kato and Y. Hori, J. ElectroanaL

Chew., 440 (1997)

283. 44. K. K. Cline, M. T. McDermott and R. L. McCreery, J. Phys.

Chem.,

98 (1994) 5314. 45. K. Honda, T. N. Rao, D. A. Tryk, A. Fujishima, M. Watanabe, K. Yasui and H. Masuda, J. Electrochem.

Soc, 147 (2000) 659.

46. D. A. Tryk, K. Tsunozaki, T. N. Rao and A. Fujishima,

Diam.

Relat. Mater., 10 (2001) 1804. 47. H. Notsu, I. Yagi, T. Tatsuma, D. A. Tryk and A. Fujishima, J. ElectroanaL

Chem., 492 (2000) 31.

48. H. Notsu, T. Fukazawa, T. Tatsuma, D. A. Tryk and A. Fujishima, Electrochem.

Solid-State

Lett, 4 (2000) H I .

49. T. Kondo, K. Honda, D. A. Tryk and A. Fujishima,

submitted.

50. M. Tachiki, T. Fukada, K. Sugata, H. Seo, H. Umezawa and H. Kawarada, Jpn. J. Appl. Phys., 39 (2000) 4631. 51. M. Tachiki, T. Fukada, K. Sugata, H. Seo, H. Umezawa and H. Kawarada, Appl. Surf. Sci, 159-160 (2000) 578. 52. T. Kondo, M. Yanagisawa, L. Jiang, D. A. Tryk and A. Fujishima, Diam. Relat. Mater,

11 (2002) 1788.

53. J. A. Dagata, J. Schneir, H. H. Harary, C. J. Evans, M. T. Postek and J. Bennet, Appl. Phys. Lett, 56 (1990) 2001. 54. P. Moriarty, P. H. Beton and D. A. Woolf, Appl. Phys. Lett,

66

(1995) 1515. 55. H. Sugimura, T. Uchida, N. Kitamura and H. Masuhara, J. Phys. Chem., 98 (1994) 4352.

172

8. Single-Crystal Homoepitaxial Diamond Electrodes

56. Y. Kaibara,

K. Sugata,

Kawarada, DiamondRelat

M. Tachiki, H. Umezawa

and

H.

Mater., 12 (2003) 560.

173

9. Chemical, Photochemical and Electrochemical Modification of Diamond Donald A. Tryk, Takeshi Kondo and Akira Fujishima

9.1. Introduction The attractive characteristics of electrodes based on conductive diamond films have led a number of research groups around the world to use these electrodes for electroanalytical applications. As a way to extend the analytical capabilities of diamond electrodes, researchers have also become interested in chemically modifying the diamond surface. One of the principal motivations is to impart selectivity for analytical purposes. Closely associated with this is the

desire

to

impart

electrocatalytic

activity

for

specific

electrochemical reactions, making use of diamond as a highly robust support. One of the more interesting recent applications of the modified diamond surface is the fabrication of DNA arrays. The reports that have appeared thus far can be classified into the following categories- l) chemical modification; 2) photochemical modification; 3) electrochemical modification; 4) ion implantation techniques; and 5) combined methods, for example, electrochemical modification followed by chemical modification.

All of these

methods have their respective advantages and disadvantages, which we will examine in this chapter. Donald A. Tryk e-mail: [email protected] 174

9. Chemical, Photochemical and Electrochemical Modification of Diamond

Another way to group the published reports on the covalent modification of the diamond surface is as follows- l) conversion of the hydrogen termination to oxygen, chlorine or fluorine?* 2) cycloaddition reactions of alkenes with the bare diamond surface, either after high temperature vacuum annealing or during UV illumination, resulting in carbon-carbon bonds?* 3) reactions with functional group-specific reagents, such as those that react with either hydroxyl groups or carbonyl groups; and 4) radical reactions with

reagents,

directly

without

activation,

or

after

either

electrochemical or photochemical activation . In general, the features that are desired for the modification of the diamond surface include the followingChemical or electrochemical selectivity for a particular species Chemical inertness Chemical stability Electrochemical stability Mechanical robustness Electrical contact (specifically for electrochemical applications) Ability to be patterned,

particularly

at the micrometer

to

nanometer scales Speed of modification

9.2. Chemical Modification Methods Some of the chemical modification methods have been investigated for many years, even though the analytical aspects were not envisaged initially. The simplest technique involves the treatment with an oxidizing acid solution such as nitric acid or chromic acid.

175

This type of treatment can convert the hydrogen termination to oxygen termination.

An additional benefit is that possible non-

diamond carbon impurities, as well as metallic impurities, can be removed fi:om the surface in this way. The chemical oxidation of diamond is closely related to electrochemical oxidation, discussed later, and which is also discussed in Chapters 8 and 10. The

carbon-oxygen

surface

functional

groups

that

are

produced via chemical oxidation, irrespective of the details of the reaction, include carbonyl and ether groups, which can form predominantly on the diamond (lOO) surface, and hydroxyl groups, which can form predominantly on the ( i l l ) surface [1-12]. It has also been found that hydroxyl groups can be stabilized on the (lOO) surface, particularly if they are hydrogen-bonded to each other [5]. Thus, on this surface, the three principal types of functional groups can exist in various proportions, depending on the coverage! this uncertainty exists even if the (lOO) surface is crystallographically perfect. In many cases, chemical oxidation has been used as a standard preparation technique for certain types of experimental measurements, particularly those that involve semiconducting properties, because it removes hydrogen from the surface and/or subsurface, which can impart metallic conductivity [13].

There

have been several reports in which the electronic properties of hydrogen-terminated vs. oxygen-terminated diamond have been compared [l, 3]. Briefly, the carbon-oxygen surface functional groups that are produced possess a strong dipole, in which the negative end points outward from the surface [14, 15]. Depending on the details of the

176

9. Chemical, Photochemical and Electrochemical Modification of Diamond

surface crystal s t r u c t u r e a n d t h e coverage with various functional groups, this dipole can be a s large as - 3 . 6 eV (carbonyl group) or 2.6 eV (ether group), which is enough to affect t h e placement of the energy b a n d edges with respect to the v a c u u m level (Evac), i.e., pulling the conduction b a n d (CB) edge below Evac. The dipole can also affect the electrochemical behavior significantly, leading to a sizable repulsion of anions, with a n accompanying decrease in the electron

transfer

(ET)

rate

(see

section

on

electrochemical

oxidation). Oxidation can also be carried out via gas-phase reaction with various forms of oxygen, including molecular oxygen, (molecular)

oxygen,

and

atomic

oxygen.

The

reaction

singlet with

molecular oxygen h a s been studied extensively. It begins a t a r o u n d 500°C a n d leads to the formation of carbonyl, ether, hydroxyl a n d carboxylic

acid

groups,

but

there

can

also

be

significant

graphitization [2, 11]. The reaction with atomic oxygen t a k e s place without t h e r m a l activation [l]. Initial oxidation can be used in conjunction with a s u b s e q u e n t chemical modification step.

This is very similar to t h e approach

discussed later for initial electrochemical oxidation followed by chemical modification.

For example, U s h i z a w a et al. s t a r t e d with

a n oxidized diamond powder surface, containing carboxylic acid groups, and, via the acid chloride, made use of a n esterification reaction with hydroxyl groups on ribose moieties attached to the DNA s t r a n d s [16]. W e n m a c k e r s et al. used this approach to a t t a c h DNA s t r a n d s to diamond films [17]. In a n o t h e r example, Krysinski et al. chemically oxidized a poly crystalline diamond film to produce hydroxyl groups, converted these to acid chlorides, a n d t h e n used

177

esterification to attach aminopyrene moieties [18]. In connection with the latter work, there is an apparent discrepancy with other work, in that the surface coverage of oxygen is stated to be quite low. This is a question that requires further examination, because, for most purposes, it is desirable to maximize the surface coverage of both the oxygen-containing surface groups and the subsequently attached moieties. Halogenation reactions have also been studied for many years [19-31]. Freedman found that molecular fluorine and chlorine do not react with the diamond surface without activation, whereas atomic fluorine and chlorine do react.

The coverage of fluorine

after treatment with atomic fluorine was about 0.75 of a monolayer, and this was stable at temperatures up to 700 K. The stabilities of the halogenated surfaces are experimentally less than predicted theoretically, as pointed out by Hukka et al. [23]. Fluorination [22] and chlorination [21] of diamond powders were carried out by Ando et al. without thermal activation for fluorine and with thermal activation for chlorine. Comparing the work with diamond films with that for powders shows that there are definite differences in reactivity.

This is to be expected,

because the surfaces of nanoparticles (or even micr op articles) present a variety of crystallographic planes, as well as edges and corners. A more efficient means of fluorinating the diamond surface is the plasma. Several groups have used CF4 as a fluorine source [27, 29-31]. The electrochemical behavior of the resulting surface has been examined by these same groups. In an early report, there was no significant effect on the voltammetric background of the

178

9. Chemical, Photochemical and Electrochemical Modification of Diamond

fluorination

compared to the as-deposited, hydrogen-terminated

surface [27]. However, recently, there has been renewed interest in this topic with the finding that heavily fluorinated surfaces provide a 5-V potential working range [29].

At present, there is no

explanation of the difference in potential window between the earlier and later reports, but the degree of fluorination may have been greater in the latter.

The electrochemical behavior of the

heavily fluorinated surfaces is interesting, because the rates of various redox reactions are affected quite differently from each other.

For example, hydrogen evolution is shifted by about two

volts, and the rate of ferrocyanide oxidation is decreased by three orders of magnitude, but the rates of reactions involving several aquo complexes are only decreased by factors of around five [30]. Thus, it appears that the ET is sensitive to the intimate details of the approach of the redox species to the diamond surface. The halogenated diamond surface, specifically, the chlorinated surface, can be used for further chemical modification, for example, to produce amine-covered or thiol-covered surfaces. This approach has been developed for diamond powders [24], as well as for films; the latter will be described in more detail later, in the section on photochemical modification. The chemical reactions of the halogens with diamond are usually thermally

activated

in order to produce

significant

quantities of the halogen atoms, e.g., chlorine atoms. This is a recurring theme- the very low reactivity of the diamond surface often requires that reactions be initiated by radicals, as halogen atoms are. As already mentioned, the plasma is an efficient means of generating radicals.

As we shall see later, radicals can be

179

generated photochemically. There has also been a sustained effort to make use of a solution-phase, ambient temperature approach to initiate radical reactions [32-36]. This work has involved various types of organic peroxides as radical initiators.

These workers

have succeeded in attaching several different organic compounds, such as carboxylic acids, to the surfaces of diamond powders. Another solution-phase approach has been examined, with the use of sulfuryl chloride, a nucleophilic reagent [25]. In this work, the surfaces of diamond powders were chlorinated and butylated. Next, we will treat cycloaddition reactions of alkenes with the bare diamond surface, after high temperature vacuum annealing, which results in the formation of carbon-carbon bonds [39-41]. For example, if the diamond (lOO) surface is heated in vacuum to 1000°C, hydrogen desorbs, leaving surface C-C dimers. These have appreciable double-bond character and can react with alkenes under conditions appropriate for the Diels-Alder cycloaddition reaction. Either the [2+2] or the [2+4] product can be formed, with the latter being the energetically favored pathway.

This type of

modification can also be carried out photochemically, and this approach is the one that is used more commonly, as discussed in the next section.

9.3. Photochemical JModification Methods There are two principal types of photochemical

modification

techniques- l) cycloaddition reactions of alkenes with the bare diamond surface under UV illumination, resulting in carbon-carbon

180

9. Chemical, Photochemical and Electrochemical Modification of Diamond

bonds; and 2) radical reactions with reagents that are activated photochemically. As mentioned in the previous section, alkenes react with C-C dimers on the clean diamond (lOO) surface, which are produced during high temperature treatment in vacuum. This reaction can also be activated photochemically.

This approach can be used to

attach alkyl chains that are terminated with carboxylic acid or primary amine groups, for example, which are useful for further functionalization [42]. These groups must be protected during the UV illumination and then subsequently deprotected. Hamers and coworkers have used this technique to attach DNA strands to the diamond surface, and they found that the stability of the attachment is excellent, much better than that to other surfaces, such as silicon or gold [43-45]. UV illumination can be used to activate radical-type reactions, for example, chlorination, as first shown by Miller and Brown [46]. They

also

showed

that

the

chlorinated

surface

can

be

photochemically converted to an amine-covered surface [28,46] and to a thiol-covered surface, the latter being accomplished also directly from the hydrogen-terminated surface [28].

9.4. Electrochemical JVIodification Methods Electrochemical

modification

methods

include

l)

anodic

polarization in aqueous acid or base; and 2) radical reactions with reagents that are activated electrochemically.

Both of these

approaches can provide a surface that can be further functionalized. In addition, the electrochemical approach leads to the possibility of

181

p a t t e r n i n g the surface down to the n a n o m e t e r scale t h r o u g h the use

of

scanning

electrochemical

microscopy

(SECM)

or

of

conductive atomic force microscopy (CAFM). One of the motivations for using electrochemical oxidation, compared to chemical oxidation, is t h a t the oxidizing power can be immediately

controlled

over a wide potential

range.

Other

motivations, compared to p l a s m a oxidation, are t h a t the process is simple to implement and, since it does not involve high kinetic energy, leads to negligible surface d a m a g e .

In addition,

the

a m o u n t of charge t h a t is passed in the oxidation process can be monitored.

The

electrochemical

oxidation

approach

has

been

studied in detail by t h e A n g u s group [27], by t h e Fujishima group [l], by t h e Swain group [56, 57], as well a s others [58-60]. As with chemical oxidation, electrochemical oxidation of the polycrystalline surface produces a mixture of several types of carbon-oxygen functional groups, which can reasonably be expected to include the following* carbonyl, e t h e r and hydroxyl on t h e (lOO) surface and principally hydroxyl on the ( i l l ) surface, based on the previously cited surface characterization and theoretical studies. The

presence

of

the

carbonyl

group

has

been

confirmed

unambiguously by work with polycrystalline samples [52] and on single-crystaMike homoepitaxial samples [61]. The presence of the hydroxyl group h a s also been confirmed unambiguously by work with polycrystalline samples [54] a n d homoepitaxial samples [558, 61]. J u s t as in t h e case of

fluorination,

the oxygenation of t h e

surface, due to the presence of the strong dipoles

mentioned

already, leads to a variety of effects on different redox couples. E T

182

9. Chemical, Photochemical and Electrochemical Modification of Diamond

to anions, such as the members of the ferro/ferricyanide redox couple, is often slowed down considerably, compared to the hydrogen-terminated surface, while it is either speeded up or there is little effect for cations [50, 55-57, 62]. For neutral compounds, the effects are subtler, probably involving dipole-dipole interactions as well as other types of interactions. The selective inhibition of ET due to electrochemical preoxidation can lead to enhanced selectivity in the analytical determination of components of mixtures.

One example is the

determination of dopamine (DA) in the presence of ascorbic acid (AA) [46, 47], which is important for patients with Parkinson's disease and the determination of uric acid (UA) in the presence of AA [53]. Unfortunately, the determination of DA in the presence of AA at oxidized diamond can only be carried out successfully at low pH values (0-2), where DA is protonated and thus positively charged. Thus, in vivo analysis is not possible. In the case of UA, this is not a drawback, and electrochemical sensors for UA in urine have been developed. Surface dipole-related effects can be accentuated if there are insufficient charge carriers near the diamond surface, either due to a somewhat low intrinsic boron doping level {<10^^ cm'^) or to a passivation of the existing boron dopant. The latter can occur as a result of hydrogen donors compensating boron acceptors. In either case, the relatively small number of charge carriers cannot support a normal level of ET to species that are at a distance from the electrode surface, particularly those that are sensitive to the number of charge carriers or the density of states (DOS) at the Fermi level, such as ferrocyanide.

183

There are other examples in which analytical determinations have been enhanced with the use of the electrochemically oxidized surface- l) the determination of chlorophenols [63]; and the determination of sulfur-containing organic compounds [64, 65]. In the latter case, the negatively charged surface attracts the cationic disulfide compound. In both cases, the use of the oxidized surface is convenient, because electrochemical oxidation is actually used periodically to clean the surface, which can slowly become fouled with polymeric oxidation products of the analytes.

The fouling

process proceeds much more slowly than it does on glassy carbon, but it can still occur. These topics will be treated in greater detail in subsequent chapters (12 and 15). The electrochemical oxidation approach has also been carried out at the nanometer scale on homoepitaxial films, through the use of conductive AFM [51, 66-69]. With this technique, although there is not a conventional electrochemical cell involved, there is a small amount of water present from the ambient air, and this condenses at the gold-coated AFM tip.

If the tip is biased negative with

respect to the diamond surface, by 2 to 3 volts, oxidation of the diamond surface occurs.

The actual presence of carbon-oxygen

functional groups has not been confirmed yet, due to the extremely small scale of the modification, however. The principal effect is that the conductivity of the homoepitaxial film is effectively decreased, by several orders of magnitude.

In the work of the

Fujishima group [51, 69, 70], the films were boron-doped, in the 10^9 cm 3 region; in work of the Kawarada group, the films were undoped crystals, but these contained hydrogen on or near the surface, due to treatment in a hydrogen plasma [67, 68, 71-73]. In

184

9. Chemical, Photochemical and Electrochemical Modification of Diamond

both cases, the CAFM polarization serves to remove hydrogen, leading to extremely low conductivity. In the case of the borondoped films, the conductivity may also be quite low, due to the passivation phenomenon mentioned earlier.

In any case, the

principal application of this technique at present is the fabrication of nanoscale electronic devices, as reported by the Kawarada group [68]. In the future, it may be quite interesting to try to expand the range of electronic devices that can be fabricated. The other major electrochemical modification approach has been that in which aromatic diazonium salts in an electrolyte solution are reduced at a diamond electrode?* this leads to the formation of an aryl radical, which can then attach to the diamond surface [74]. This work is based on a series of papers in which the same technique was applied to the surface modification of glassy carbon and highly ordered pyrolytic graphite (HOPG) [75-78]. This approach may also be quite fruitful for the covalent modification of diamond surfaces, if the attachment is as robust as it is on glassy carbon surfaces.

9.5. Combined IVLethods- Electrochemical/ Chemical methods Although there are other examples of combined methods, such as photochemical—chemical, which have already been mentioned briefly, as in the case of the photochemical-chemical attachment of DNA strands, we shall focus on that of electrochemical-chemical method. This combined approach has been used recently to attach a protein to the diamond surface [59], using first electrochemical

185

oxidation to produce hydroxyl groups, followed by esterification to attach biotin, with subsequent attachment of streptavidin.

Even

though these workers used electrochemical oxidation, they found that chemical oxidation with singlet oxygen was more effective in achieving high hydroxyl coverage. We shall continue by discussing in greater detail a recent study, in which we made use of electrochemical oxidation of singlecrystal-like homoepitaxial films and then used specific reagents to bind to the resulting carbonyl and hydroxyl groups [61]. In this study, we found that the diamond (lOO) surface contains a mixture of carbonyl and hydroxyl groups, while the ( i l l ) surface contains mainly hydroxyl groups. The presence of other types of groups has not examined. In

previous

modification

of

work, carbonyl

aminopropyltriethoxysilane

2,4-dinitrophenylhydrazine (C=0) (APTES)

groups

[52]

modification

(DNPH) and

3-

of hydroxyl

(-0H) groups [54] were carried out on oxidized poly crystalline diamond electrodes.

The latter is particularly important for

possible applications, because the terminal amino group can be used to form a covalent bond, e.g., via amidization, with a variety of functional species, including examples already mentioned, such as DNA and proteins. In this study, the surface functional groups generated by anodic treatment on (lOO) and ( i l l ) homoepitaxial single-crystal boron-doped diamond electrodes were first examined with X-ray photoelectron spectroscopy (XPS). Subsequently, DNPH and APTES modifications of the anodically treated surfaces were carried out and were characterized again with XPS, as well as with cyclic voltammetry and electrochemical impedance measurements.

186

9. Chemical, Photochemical and Electrochemical Modification of Diamond

The original work can be consulted for the experimental details, which will be given here briefly. boron-doped homoepitaxial diamond

The single-crystaMike

films

were prepared by

microwave plasma-assisted chemical vapor deposition on synthetic high pressure-high temperature single-crystal substrates (3-4° ofl"axis polished). Hydrogen-terminated surfaces were prepared from as-deposited or oxidized surfaces by heating to 900 °C in hydrogen ambient for 1 h.

For the anodic treatment, a single positive

potential sweep (O to +3 V vs. Ag/AgCl), with sweep rate 100 mV s'l in 0.1 M sulfuric acid, was used unless otherwise noted.

The

procedures used for the surface modification with DNPH and APTES were the same as those in described in the previous references. First, the XPS spectra for the oxidized surfaces will be presented. Figure 9.1 shows the C Is spectra for the hydrogen-terminated, anodically treated and oxygen-plasma treated (lOO) and ( i l l ) surfaces. For the hydrogen-terminated surfaces (a and b), the main peak is assigned to carbon in the diamond bulk; its binding energy was found to be 285.0 ± 0.2 eV, and this was used as the binding energy shift (BES) reference. The small peaks found at ca. +1.6 and +1.8 eV BES for the (100) and ( i l l ) surfaces are similar to those that have been assigned to carbon bonded to multiple hydrogens [79] or to subsurface hydrogen [80]. There is negligible oxygen present on the hydrogen-terminated surfaces.

For the

oxidized surfaces (both anodic and plasma treatment), shoulderlike peaks at ca. +2 eV appeared (c-f). These peaks are assigned to carbon singly bonded to oxygen [11, 81, 82]. For the anodically treated (lOO) diamond surface, another shoulder-like peak, at ca.

187

+4 eV BES, was observed (c). This peak is assigned to doubly bonded carbon-oxygen [11, 83].

(100)

(111)

Binding Energy Shift / eV

Binding Energy Shift / eV

(111)

Binding Energy Shift / eV

^

a.

15 -

(100)

— , — - f ^

iij / \

Binding Energy Shift / eV

Binding Energy Shift / eV

(111)

Binding Energy Shift / eV

Fig. 9.1. XPS C Is spectra for (a, b) hydrogen-terminated, (c, d) anodically-treated and (e, 0 oxygen-plasma treated (lOO) and ( i l l ) single-crystal diamond thin films. 188

9. Chemical, Photochemical and Electrochemical Modification of Diamond

For the anodically oxidized (lOO) surface (c), both singly and doubly bonded C-0 are present, whereas, for the ( i l l ) surface (d), only singly bonded oxygen appears to be present. As already mentioned above, the possible oxygen-containing surface functional groups for diamond (lOO) include the hydroxyl group (C-OH), the ether structure (C-Q-C) and the carbonyl (C=0) group [4, 7, 9]. The hydroxyl group is the only one that could reasonably exist on the unreconstructed ( i l l ) surface [10, 84]. The results for the XPS C Is peak

fittings

for

the

homoepitaxial surfaces are summarized in Table 9.1, together with previously reported results for the polycrystalline surfaces.

It can

be concluded from these results that the polycrystalline surfaces consisted of a slightly greater amount of ( i l l ) than (lOO) faces for the first electrode and just the reverse for the second electrode. From the areas of the O Is and C Is peaks in the XPS survey spectra (not shown), corresponding to Fig. 9.1, the overall 0/C ratios have been estimated?' these are summarized in Table 9.2. For the (lOO) surface, the 0/C ratios tend to be in the 0.1 to 0.2 range, while those for ( i l l ) tend to be around 0.2. The latter value is probably rather close to a full monolayer, based on the following argument-

the sampling depth is expected to lead to an

enhancement in the carbon spectrum of between a factor of 3.0 and a factor of 8.8, depending on the specific model used, [85] for the present conditions (Mg Ka radiation. X-ray incidence angle, 45°), so that we have chosen a compromise value of five.

189

Table 9.1. Atomic concentration on modified single-crystal diamond surface estimated from XPS analysis.

N/C ratio at DNPH-modified

N/0 ratio at APTES-

surface

modified surface

(100)

0.015-0.025

0.16

(111)

negligible

0.45

Table 9.2. Estimated ratio of surface functional groups to the total surface oxygen on anodicallytreated single-crystal diamond surfaces.

C=0

COH

poly

0.05a

0.6b

(100)

0.19

0.48

(111)

negligible

~1

a Notsu et aL, J. Electroanal Chem 492 (2000) 31; ^ Notsu et aL, Electrochem. SolidState Lett, 4 (2001) H I . For oxygen-plasma t r e a t e d surfaces (e a n d £), a shoulder a t ca. +2 eV was observed for both the (lOO) and ( i l l )

single-crystal

diamond surfaces, with no evidence of a peak at +4 eV on the (lOO) surface.

This result suggests t h a t the higher kinetic energy

involved with t h e oxygen p l a s m a t r e a t m e n t somehow leads to a surface on which there is little carbonyl coverage.

Direct evidence

for t h e effect of t h e p l a s m a t r e a t m e n t is found in t h e significantly enlarged shoulders at -1.8 to -2.0 eV; for spectra a-d, the p e a k s in t h i s position can be assigned to carbon dimers [79] or to crystal defects [82]. For spectra d a n d f, t h e most reasonable a s s i g n m e n t is surface graphite [79], which can be produced by the energetic oxygen atoms.

190

9. Chemical, Photochemical and Electrochemical Modification of Diamond

In the original paper, linear sweep voltammograms (LSV) are shown for the hydrogen-terminated (lOO), ( i l l ) single-crystal and polycrystalline diamond electrodes in 0.1 M sulfuric acid. The potentials found for the anodic peaks were ca. +1.5 V for the (lOO) surface and +1.65 vs. Ag/AgCl for ( i l l ) surface. In addition, for the polycrystalline surface, both peaks were observed in a single LSV. At +1.8 V vs. Ag/AgCl, current for oxygen evolution reaction began to be seen. These anodic peaks could be seen in the first positivegoing potential sweep but not in further sweeps. These peaks were assigned to the formation of oxygen-containing groups on the electrode surface. These results were compared to those reported by Martin et al. (2:^ Differences in the LSV behavior for the (lOO) and ( i l l ) surfaces were also reported by these authors. However, they observed anodic currents during the first sweep that were on the order of 1 to 2 )xA cm'2, whereas those observed in the later work were on the order of 100-200 |LIA cm 2 [79]. At present, there is no clear explanation for this large difference in the oxidation currents. Based on an integration of the experimental currents and comparison with the theoretically calculated ones, it can be concluded that the LSV curves correspond to conversion of the hydrogen-terminated surfaces to nearly fully oxygen-terminated surfaces. For the theoretical estimates, we made the assumption that the (lOO) surface was initially (2x1)^ H, and the ( i l l ) surface was initially (lxl):H. Furthermore, as a result of anodic oxidation, we assumed that every carbon atom on the (lOO) surface produces a carbonyl (C=0) group, and every carbon on the ( i l l ) surface produces a hydroxy 1 (C-OH) group. Thus, the anodic oxidation of

191

the (lOO) (2x1)*.H surface to a (ixl)-O surface would require three electrons per oxygen atom, yielding 7.64 x lO'^ C cm'^, based on a spacing of 2.508 A between surface carbon atoms. For one (lOO) sample (A), the oxidation charge was less than this value, but for sample B, the charge was slightly greater, but still in good agreement.

For the ( i l l ) (lxl).*H surface, oxidation to (lxl)*OH

requires two electrons per OH group, yielding a value of 7.84 x 10 ^ C cm 2, (2.508 A spacing). In this case also, the integrated charge was slightly greater, but still in good agreement.

B C/D

Cls

15000

D, O

>^

0 1s

&n C
^

j

10000

\1 5000

L„

1

wV^«^^v\^>»-nA..,.^v-^~^-«»S.*s7\ i I

^*-*--*^^«w..<-'-^

V^ *

n 800

600

400

Binding energy / eV

Fig. 9.2. XPS survey spectra for DNPHmodified single-crystal diamond surfaces* (A) (lOW and (B) ( i l l ) homoepitaxial boron-doped diamond films.

192

9. Chemical, Photochemical and Electrochemical Modification of Diamond

B c

Binding energy / eV

Binding energy / eV

D

Binding energy / eV

Binding energy / eV

Fig. 9.3. XPS Nls spectra for single-crystal diamond surfaces- (A, B) before and (C, D) after DNPH-modification; (A, C) (lOO) and (B, D) ( i l l ) homoepitaxial boron-doped diamond films. The XPS survey spectra obtained after the DNPH modification are shown in Fig. 9.2.

In spectrum a (lOO), there is a small N Is

peak, whereas, in spectrum b ( i l l ) , there is no discernable peak. The N Is region is shown in detail in Fig. 9.3; from these results, it is rather clear that the (lOO) surface was partially modified with DNPH, indicating the presence of carbonyl groups, but the ( i l l ) surface was not, indicating their absence. The N/C atomic ratio for the DNPH-treated

single-crystal diamond

estimated to be 0.015-0.025 (Table 9.3).

(lOO) surface

was

These values can be

compared to values that would be obtained if DNPH were at full coverage. For a vertical orientation, the DNPH molecule would occupy approximately 30 A^, compared to the area occupied by one carbon atom on the (lOO) surface (6.29 A2), so that one DNPH 193

would cover 4.77 surface carbons (see Fig. 9.4). Assuming some disorder, one could assume that a limiting coverage might be one DNPH per 10 surface carbons. The N/C ratio for this situation should be 0.071, taking into account the number of carbon and nitrogen atoms (6 and 4, respectively) for an attached DNPH molecule and the fact that several layers of carbon atoms underlying the surface are also being sampled (so that the carbon signal is enhanced by a factor of four to five) [52, 85]. Thus, the experimentally obtained N/C ratios would correspond to 21-35% of "full" coverage. This level of coverage seems reasonable, since the surface oxygens on the anodically treated (lOO) face are expected to include a mixture of carbonyl, hydroxyl and ether groups. In fact, the

experimental

XPS

results

(Fig.

9.1c)

indicated

that

approximately 19% of the surface oxygens (at essentially one full monolayer) were carbonyl groups for the unmodified anodically treated (lOO) surface.

On the other hand, the N/C ratios

correspond to only one DNPH to every 30-50 surface carbons, which means that only 10-17% of the carbonyl groups reacted with DNPH.

Table 9.3. Atomic concentration ratios on modified single-crystal diamond surfaces estimated from XPS analysis.

Surface (100) (111)

194

N/C ratio (DNPHmodified) 0.015-0.025 negligible

N/0 ratio (APTESmodified) 0.16 0.45

9. Chemical, Photochemical and Electrochemical Modification of Diamond

top

side Fig. 9.4. Representation of the attachment of DNPH moieties to the diamond (lOO) surface.

195

The APTES coverage can also be estimated from the atomic concentrations obtained by XPS, shown in Fig. 9.5. For a diamond surface fully covered by APTES, the N/0 atomic ratio would be 0.33, assuming one APTES molecule reacts with three hydroxyl groups on the surface (see Fig. 9.6). In the present study, the N/0 ratios for the (100) and ( i l l ) APTES-modified diamond surface were estimated to be 0.16 and 0.45, respectively. Given the uncertainty in the results, they indicate that the coverage of hydroxyl on the (100) surface is around 50%, while that on the ( i l l ) surface is around 100%. It was also seen in the C Is spectrum for anodically treated (lOO) diamond (Fig. 9.1c) that the surface could contain well over 50% hydroxyl groups.

Biding energy / eV

Biding energy / eV

405

Biding energy / eV

403

401

399

397

395

Biding energy / eV

Fig. 9.5. XPS Nls spectra for single-crystal diamond surfaces^ (A, B) before and (C, D) after APTES-modification; (A, C) (lOO) and (B, D) ( i l l ) homoepitaxial boron-doped diamond films.

196

9. Chemical, Photochemical and Electrochemical Modification of Diamond

Fig. 9.6. Representation of the attachment of APTES moieties to the diamond ( i l l ) surface. Taken together with the earher results for poly crystalline diamond electrodes [79], the APTES coverage versus oxygen on the diamond surface exhibits a trend of ( i l l ) > poly crystalline (N/0 = 0.15—0.25) > (lOO).

This is quite reasonable, because the

polycrystalline surface contains a mixture of the two crystal faces. From the results of the covalent surface modification of the singlecrystal diamond surfaces, we can conclude that anodically treated diamond (lOO) surfaces consist of a mixture of carbonyl and hydroxyl groups, although the presence of ether groups is not ruled out. On the diamond ( i l l ) surface, the hydroxyl group appears to be present at around 100%. We should emphasize that the ether structure (C'O-C) is also possible on the (lOO) diamond surface, given the predominance of singly bonded oxygen on this surface indicated by the C Is spectrum. In fact, even though most of the work that has appeared 197

concerning the oxidation of the (lOO) surface has discussed mainly ether and carbonyl functionalities [4, 6, 7, 9], it has been shown theoretically that a hydroxyl-covered diamond (lOO) surface can be greatly stabilized if these groups exist side-byside on C'C dimers and can undergo hydrogen bonding. [5]. We now turn to the electrochemical behavior.

Figure 9.7

shows cyclic voltammograms (CVs) obtained for the Fe2+/3+ redox couple

at

subsequently

hydrogen-terminated, DNPH-modified

(100)

anodically and

treated,

(ill)

and

single-crystal

diamond electrode surfaces. It has been reported that the electrontransfer rate for this redox couple tends to be affected by surface termination [50, 86]. For polycrystalline diamond electrodes, the electron-transfer rate is larger at oxygen-terminated surfaces than at hydrogen-terminated ones. In the present case, the electrontransfer rate became greater after anodic treatment of the electrode surfaces, as evidenced by decreases in the CV anodiccathodic peak separations (AEp) (Fig. 9.8).

Following DNPH

treatment, the AEp value obtained for the (lOO) electrode surface was found to be greater than that for the corresponding anodically treated surface, but, in contrast, that for the ( i l l ) surface showed almost no change. These results support the presence of carbonyl groups on the (lOO) surface and their absence on the ( i l l ) surface. For glassy carbon electrodes, it has been reported that carbonyl groups on the surface, possibly in the form of quinones, can act as catalysts for the Fe2+/3+couple [50, 52, 86]. Given the fact that graphitic carbon is essentially not present on the diamond surfaces being studied, there could be few quinone groups.

198

9. Chemical, Photochemical

and Electrochemical

Modification

of Diamond

a.

-0.5

0.0

0.5

1.0

1.5

Potential / V vs. Ag/AgCl

a <

100-

/

50-

A

Vj# ^-

. y

/ / . •

0 -

^y

-a

^ -50-

//

.•7/

U 100 -

1

1

1

1

Potential / V vs. Ag/AgCl Fig. 9.7. CVs for 1 mM Fe(C104)2 in 0.1 M H2SO4 at (a) (lOO) and (b) ( i l l ) single-crystal diamond electrodes; dotted line, hydrogenterminated; dashed Une, anodically-treated; and soUd line, DNPHmodified after anodic treatment. On the oxidized surface of polycrystaUine diamond, carbonyl groups can certainly exist, b u t they are of a n aliphatic type, which behave quite differently from t h e quinone type. Indeed, t h i s type of

199

carbonyl also appears to accelerate the ET for the Fe2+/3+couple, but this is due to the previously mentioned attraction of the positively charged ions to the negatively charge electrode surface. The DNPH molecule reacts selectively with surface carbonyl groups, effectively blocking them.

Therefore, for electrode surfaces containing

carbonyl groups, DNPH treatment should cause a decrease in the ET rate for the Fe-"^/3+ couple, whereas, for surfaces containing no carbonyl groups, this treatment should not produce any effect. Thus, we can conclude that the anodically treated (lOO) diamond surface does include a significant coverage of carbonyl groups, but that the anodically treated ( i l l ) surfaces contain essentially no carbonyl groups. This conclusion is consistent with the XPS results. 1700-

• (100)

1600 1500 H

> S

1400-

<

1300120011001000-4

Hydrogenterminated

Anodically -treated

DNPHmodified

Fig. 9.8. Variation of peak separations (AEp) of CVs in Fig. 9.7. For the APTES modification, the surfaces were characterized by recording the CVs for the Fe(CN)63''^" redox couple (Fig. 9.9). From the variation of AEp, it was found that the ET behavior was nearly reversible at the hydrogen-terminated diamond electrode

200

9. Chemical, Photochemical and Electrochemical Modification of Diamond

surfaces, but, at the corresponding oxygen-terminated surfaces, it became much more irreversible, i.e., with a tendency opposite to that for Fe2+/3+. This is also likely to be due to an electrostatic effect, in which the surface dipoles of the C-0 functional groups tend to repel the negative charges of both members of the redox couple [50, 55, 89].

201

j-

200-

u

< d. >^

a

/"•^>...._^^

/ ^•

0-

C/2

- '•*«s;r**'

C

A

100 -

-100-

• - - _ . - ' '

^ ^
>

^ ''

/ 7

-200-

3

u

1 -0.2

1 00

1 02

1 04

1 0.6

1 0.8

1 10

Potential / V vs. Ag/AgCl

<

-a

3

u Potential / V vs, Ag/AgCl

<

T3

3

u Potential / V vs. Ag/AgCl Fig. 9.9. CVs for 1 mM K3Fe(CN)6 in 0.1 M H2SO4 at (a) polycrystaUine, (b) (100) and (c) ( i l l ) single-crystal diamond electrodes; dotted line, hydrogen-terminated; dashed line, anodically-treated; and solid line, APTES-modified after anodic treatment.

202

9. Chemical, Photochemical and Electrochemical Modification of Diamond

It should pointed out that the shapes of the CVs for the anodicallytreated

single-crystal

asymmetric,

the

i.e.,

cathodic

electrodes peak

in Fig. 9.9

currents

were

were either

significantly smaller than the anodic peak currents, on the ( i l l ) surface (c), or could not be observed at all, on the (lOO) surface (b). This behavior may be due to semiconductor-type behavior. As mentioned above, the surface dipole effects are accentuated in cases in which the carrier concentrations fall below the 10^9 cm"^ level.

We have also shown that the acceptor density near the

surface for as-deposited, hydrogen-terminated diamond electrodes can be much higher than that expected from the boron doping level, and can decrease to a density lower than the doping level after surface oxidation [62]. The differences in the electrical properties are also reflected in the ET behavior on the electrode surface. Thus, in the case of the hydrogen-terminated

(lOO)

single-crystal

diamond electrode, the ET for Fe(CN)63/4- (Fig. 9.9b) showed reversible character, which may be due to higher accepter density in the near-surface region than that expected based on the doping level.

Similar behavior has been reported for lightly-doped

polycrystalline diamond electrodes [55]. It should also be noted that the voltammetric behavior for the Fe2+/3+j-edox couple (Fig. 9.7) did not exhibit significant asymmetry. This difference is probably due to the fact that this redox couple is not as sensitive to the number of charge carriers as is the ferro/ferricyanide couple. For

the

polycrystalline

oxidized (Fig. 9.9a)

and and

subsequently (ill)

APTES-treated

single-crystal

diamond

electrodes (Fig. 9.9c), the CV shapes for the ferro/ferricyanide

203

couple became almost the same as those for the respective hydrogen-terminated surfaces. The result for the APTES-modified polycrystalline diamond electrode is similar to that previously reported by Notsu et al. [54]. In that paper, it was proposed that the protonated amino group of the APTES moiety was responsible for electrostatically attracting the anionic redox species. For the diamond (lOO) surface (Fig. 9.9b), the shape of the CV for the APTES-modified surface did not return to that for the hydrogenterminated

surface,

but

the

anodic peak

potential

shifted

significantly negative of that for the anodically treated surface. The fact that it did not return to the initial behavior is probably due to the fact that the doping level for the (lOO) epitaxial film was lower than those for the polycrystalline and ( i l l ) films [55, 62]. In

the

original

paper,

electrochemical

AC

impedance

measurements were also carried out, and these results also tend to support those obtained in the cyclic voltammetry.

Thus, in this

study, by use of XPS in conjunction with chemical

surface

modification techniques, new information was obtained on the surface functional groups generated on oxidized diamond surfaces. In particular, it was found that the variation of oxygen-containing surface functional groups strongly depends on the exposed crystal face, as well as on the type of oxidative treatment, e.g., anodic or oxygen plasma. To understand the dependence of the coverages of oxygen-containing surface functional groups on treatment and crystal face is especially important, for two reasons- one is that the oxygen termination can include various types of surface functional groups, and thus a variety of functional surfaces can be created by utilizing these groups.

204

The other is that diamond electrode

9. Chemical, Photochemical and Electrochemical Modification of Diamond

surfaces typically become oxidized during practical use as an electrode material, because, as we have seen, only a single positivegoing potential sweep may be enough to completely oxidize the diamond.

9.6. Metal and Metal Oxides on Diamond Surfaces As a separate topic, we now discuss metals and metal oxides deposited on diamond surfaces. The topic of metals on diamond has been studied for a number of years from the standpoint of semiconductor devices [3, 88-105]. Many of these papers discuss the effect of hydrogen vs. oxygen termination and the resulting surface dipoles.

As mentioned above, this work has received

attention over the years due to the great interest in electron emission. However, electrochemical

our

interest

here

characteristics.

is

more

The

related

often-cited

to

the

intrinsic

electrochemical characteristics of diamond, including extreme stability, low background current and large potential working range, are also advantages for the fundamental study of the electrochemical

characteristics

of

solid

materials.

The

electroanalytical applications of electrochemical metal deposition will be treated in Chapter 16. The materials of interest in the present chapter are those with applications as battery electrodes, capacitor electrodes and electrocatalysts. There may even be cases in which diamond can play a role as a practical support material. We begin with electrocatalysts. Platinum has been deposited on diamond surfaces electrolytically and then examined with

205

various techniques [106-109]. The Pt deposit has been found to be quite stable on the polycrystalhne

diamond

surface

during

potential cycling, more so than on conventional carbon or graphite substrates.

This is most likely due to stability of the diamond

substrate itself, particularly at highly positive potentials, at which graphitic carbon undergoes irreversible oxidation. Other metals that have been of interest include copper [llO] and

nickel

[ill].

These have been studied due to their

electrocatalytic activity for glucose oxidation.

The copper was

deposited electrochemically, whereas the nickel was deposited via ion implantation.

The behavior of these metals in the form of

nanoparticles on diamond is highly attractive in terms of glucose determination, because the current for glucose oxidation is increased dramatically without increasing the background current substantially. There has been some interest in the possibility that diamond electrodes could be intercalated or inserted with lithium, because there are still problems associated with graphitic materials in terms of stability.

Li et al. showed that there is essentially no

"underpotential deposition" of lithium on homoepitaxial diamond [ i l l ] surfaces under highly controlled conditions, particularly the absence of sp^ carbon-containing grain boundaries [112]. In work of Pleskov and coworkers, they also concluded that there is neglibible Li intercalation into diamond nanoparticles

[113].

However, there is recent work that does suggest that significant insertion of Li into a diamond film grown on carbon is possible [114].

206

9. Chemical, Photochemical and Electrochemical Modification of Diamond

One of the first metal oxides to be examined electrochemically on a diamond substrate was ruthenium dioxide [115, 116]. This material is important both for electrochemical capacitor and electrocatalytic applications (chlorine evolution). Another example is cobalt hydrous oxide, which has catalytic activity for oxygen evolution [117]. A very recent example is lead dioxide [118]. A metal oxide (V2O3) has also been supported on particulate diamond as a catalyst for an organic gas-phase reaction [119].

9.7. Conclusions The

highly

attractive

characteristics

of conductive

diamond

electrodes continue to bring attention. Recently, it has also become clear that the possibilities for chemical modification can take advantage of these fundamental characteristics and build upon them, imparting novel selectivity and catalytic activity.

The

possible applications are as diverse as those for all other types of electrodes, because diamond can be a useful substrate for all. Moreover, all of the modification techniques that have been worked out over the past several years for other surfaces, such as carbon and gold, can now be transplanted onto diamond. Now is just the beginning of the blossoming of this field.

207

References 1.

R. E. Thomas, R. A. Rudder, and R. J. Markunas, J. Vac. Sci. Techno!., A 10 (1992) 2451.

2.

T. Ando, M. Ishii, M. Kamo, and Y. Sato, J. Chem. Soc, Farad. 7V5725., 89(1993)1783.

3.

J. van der Weide and R. J. Nemanich, Phys. Rev. B, 49 (1994) 13629.

4.

J. L. Whitten, P. Cremaschi, R. E. Thomas, R. A. Rudder, and R. J. Markunas, Appl. Surf. ScL, 75 (1994) 45.

5.

S. Skokov, B. Weiner, and M. Frenklach, Phys. Rev. B, 55 (1997) 1895.

6.

M. Z. Hossain, T. Kubo, T. Aruga, N. Takagi, T. Tsuno, N. Fujimori, and M. Nishijuma, Surf. Sci., 436 (1999) 63.

7.

P. E. Pehrsson and T. W. Mercer, Surf Sci., 460 (2000) 49.

8.

P. E. Pehrsson and T. E. Mercer, Surf Sci., 460 (2000)74.

9.

H. Tamura, H. Zhou, K. Sugisako, Y. Yokoi, S. Takami, M. Kubo, K. Teraishi, A. Miyamoto, A. Imamura, M. N."Gamo, and T. Ando, Phys. Rev. B, 61 (2000) 11025.

10. K. P. Loh, X. N. Xie, S. W. Wang, and J. C. Zheng, J. Phys.

Chem.

B, 106 (2002) 5230. 11. P. John, N. Polwart, C. E. Troupe, and J. I. B. Wilson, J. Am. Chem. Soc. 125, 6600 (2003). 12. A. Laikhtman, A. Lafosse, Y. L. Coat, R. Azria, and A. Hoffman, J. Chem. Phys., 119 (2003) 1794. 13. Y. V. Pleskov, Russ. Chem. Rev., 68 (1999) 381.

208

9. Chemical, Photochemical and Electrochemical Modification of Diamond

14. M. J. Rutter and J. Robertson, Phys. Rev. B, 57 (1998) 9241. 15. J. Robertson and M. J. Rutter, Diamond Relat. Mater., 7 (1998) 620. 16. K. Ushizawa, Y. Sato, T. Mitsumori, T. Machinami, T. Ueda, and T. Ando, Chem. Phys. Lett,

351 (2002) 105.

17. S. Wenmackers, K. Haenen, M. Nesladek, P. Wagner, L. Michiels, M. vandeVen, and M. Ameloot, Phys. Status Solidi A, 199 (2003) 44. 18. P. Krysinski, Y. Show, J. Stotter, and G. J. Blanchard, J. Am. Chem. Soc, 125 (2003) 12726. 19. A. Freedman and C. D. Stinespring, Appl

Phys. Lett,

57 (1990)

1194. 20. A. Freedman, J. Appl Phys., 75 (1994) 3112. 21. T. Y. K. Ando, S. Suehara, M. Kamo, Y. Sato, S. Shimosaki, and M. Nishitani-Gamo, J. Chin. Chem. Soc. (Taipei), 42 (1995) 285. 22. T. Ando, K. Yamamoto, M. Kamo, Y. Sato, Y. Takamatsu, S. Kawasaki, F. Okino, and H. Touhara, J. Chem.

Soc,

Farad.

Trans., 91 (1995) 3209. 23. T. I. Hukka, T. A. Pakkanen, and M. P. D'Evelyn, J. Am.

Chem.

Soc, 99 (1995) 4710. 24. T. Ando, M. Nishitani-Gamo, R. E. Rawles, K. Yamamoto, M. Kamo, and Y. Sato, Diamond Relat Mater., 5 (1996) 1136. 25. Y. Ikeda, T. Saito, K. Kusakabe, S. Morooka, H. Maeda, Y. Taniguchi, and Y. Fujiwara, Diamond Relat Mater., 7 (1998) 830. 26. B. Ohtani, K. Young-Ho, T. Yano, K. Hashimoto, A. Fujishima, and K. Uosaki, Chem. Lett,

(1998) 953.

209

27. H. B. Martin, A. Argoitia, J. C. Angus, and U. Landau, Electrochem.

J.

Soc, 146 (1999) 2959.

28. J. B. Miller, Surf Sci., 439 (1999) 21. 29. S. Ferro and A. DeBattisti, Anal

Chem., 75 (2003) 7040.

30. S. Ferro and A. DeBattisti, J. Phys. Chem., B107 (2003) 7567. 31. G.

Sine,

Electrochem.

L.

Ouattara, SolidState

M.

Panizza,

and

C.

Comninellis,

Lett, 6 (2003) D9.

32. T. Tsubota, O. Hirabayashi, S. Ida, S. Nagaoka, M. Nagata, and Y. Matsumoto, Diamond Relat Mater., 11 (2002) 1360. 33. T. Tsubota, O. Hirabayashi, S. Ida, S. Nagaoka, M. Nagata, and Y. Matsumoto, Diamond Relat. Mater., 11 (2002) 1374. 34. T. Tsubota, O. Hirabayashi, S. Ida, S. Nagaoka, M. Nagata, and Y. Matsumoto, Phys. Chem. Chem. Phys., 4 (2002) 806. 35. T. Tsubota, S. Ida, O. Hirabayashi, S. Nagaoka, M. Nagata, and Y. Matsumoto, Phys. Chem. Chem. Phys., 4 (2002) 3881. 36. T. Tsubota, S. Tanii, S. Ida, M. Nagata, and Y. Matsumoto, Phys. Chem. Chem. Phys., 5 (2003) 1474. 37. S. Ida, T. Tsubota, S. Tanii, M. Nagata, and Y. Matsumoto, Langmuir,

19 (2003) 9693.

38. S. Ida, T. Tsubota, O. Hirabayashi, M. Nagata, Y. Matsumoto, A. Fujishima, Diamond Relat. Mater., 12 (2003) 601. 39. D. R. Fitzgerald and D. J. Doren, J. Am. Chem. Soc, 122 (2000) 12334. 40. J. S. Hovis, S. K. Coulter, R. J. Hamers, M. P. D'Evelyn, J. N. Russell, Jr., and J. E. Butler, J. Am. Chem. Soc, 122 (2000) 732. 41. G. T. Wang, S. F. Bent, J. N. Russell, Jr., and J. E. Butler, J. Am. Chem. Soc, 122 (2000) 744.

210

9. Chemical, Photochemical and Electrochemical Modification of Diamond

42. T. Strother, T. Knickerbocker, J. N. Russell, Jr., J. E. Butler, L. M. Smith, and R. J. Hamers, Langmuir,

18 (2002) 968.

43. W. Yang, O. Auciello, J. E. Butler, W. Cai, J. A. Carlisle, J. E. Gerbi, D. M. Gruen, T. Knickerbocker, T. L. Lasseter, J. John N. Russell, L. M. Smith, and R. J. Hamers, Nature Mater., 1 (2002) 253. 44. T. Knickerbocker, T. Strother, M. P. Schwartz, J. N. Russell, Jr., J. Butler, L. M. Smith, and R. J. Hamers, Langmuir,

19 (2003) 1938.

45. W. Yang, J. E. Butler, J. N. Russell, Jr., and R. J. Hamers, Langmuir,

20 (2004) 6778.

46. J. B. Miller and D. W. Brown, Langmuir,

12 (1996) 5809.

47. H. Notsu, I. Yagi, T. Tatsuma, D. A. Tryk, and A. Fujishima, Electrochem.

SolidState

Lett, 2 (1999) 522.

48. E. Popa, H. Notsu, T. Miwa, D. A. Tryk, and A. Fujishima, Electrochem.

SolidState

Lett,

2 (1999) 49.

49. I. Yagi, K. Tsunozaki, D. A. Tryk, and A. Fujishima, Solid-state

Lett,

Electrochem.

9 (1999) 457.

50. I. Yagi, H. Notsu, T. Kondo, D. A. Tryk, and A. Fujishima, J. Electroanal

Chem., 473 (1999) 173.

51. M. Yanagisawa, H. Tai, I. Yagi, D. A. Tryk, A. Fujishima, and L. Jiang, in "Sixth International Symposium on Diamond Materials, Extended Abstracts, Joint Meeting of the Electrochemical Society and the Electrochemical Society of Japan". The Electrochemical Society, Honolulu, Hawaii, 1999. 52. H. Notsu, I. Yagi, T. Tatsuma, D. A. Tryk, and A. Fujishima, J. Electroanal

Chem., 492 (2000)31.

53. E. Popa, Y. Kubota, D. A. Tryk, and A. Fujishima, ^173^7. Chem., 72, (2000) 1724.

211

54. H. Notsu, T. Fukuzawa, T. Tatsuma, D. A. Tryk, and A. Fujishima, Electrochem.

Solid State Lett, 4(2001) H I .

55. D. A. Tryk, K. Tsunozaki, T. N. Rao, and A. Fujishima,

Diamond

Relat Mater., 10 (200l)Hl. 56. M. C. Granger and G. M. Swain, J. Electrochem.

Soc, 146 (1999)

4551. 57. M. C. Granger, M. Witek, J. Xu, J. Wang, M. Hupert, A. Hanks, M. D. Koppang, J. E. Butler, G. Lucazeau, M. Mermoux, J. W. Strojek, and G. M. Swain, Anal. Chem., 72 (2000) 3793. 58. H. B. Martin and J. Philip W. Morrison, Electrochem.

SolidState

Lett, 4 (2001) E17. 59. D. Delabouglise, B. Marcus, M. Mermoux, P. Bouvier, J. ChaneTune, J.-P. Petit, P. Mailley, and T. Livache, Chem.

Commun.,

(2003) 2698. 60. W. C. Poh, K. P. Loh, W. D. Zhang, S. Triparthy, J.-S. Ye, and F.-S. Sheu, Langmuir,

20 (2004) 5484.

61. T. Kondo, K. Honda, D. A. Tryk, and A. Fujishima, J.

Electrochem.

6bc., in press (2004). 62. T. Kondo, D. A. Tryk, and A. Fujishima, Electrochim.

Acta., 48,

(2003) 2739. 63. C. Terashima, T. N. Rao, B. V. Sarada, D. A. Tryk, and A. Fujishima, Anal. Chem., 74 (2002) 895. 64. O. Chailapakul, B. V. Sarada, C. Terashima, T. N. Rao, D. A. Tryk, A. Fujishima, Analyst

(Cambridge,

U.K.), 127 (2002) 1164.

65. C. Terashima, T. N. Rao, B. V. Sarada, Y. Kubota, and A. Fujishima, Anal. Chem., 75 (2003) 1564.

212

9. Chemical, Photochemical and Electrochemical Modification of Diamond

66. We should note that the nanoHthographic AFM modification of diamond was apparently discovered independently and nearly simultaneously by the Fujishima and Kawarada groups. 67. M. Tachiki, T. Fukuda, K. Sugata, H. Seo, H. Umezawa, and H. Kawarada, Jpn. J. Appl Phys., 39 (2000) 4631. 68. M. Tachiki, H. Seo, T. Banno, Y. Sumikawa, H. Umezawa, and H. Kawarada, AppL Phys. Lett,

81 (2002) 854.

69. T. Kondo, M. Yanagisawa, L. Jiang, D. A. Tryk, and A. Fujishima, Diamond Relat Mater., 11 (2002) 1788 70. M. Yanagisawa, L. Jiang, D. A. Tryk, K. Hashimoto, and A. Fujishima, Diamond Relat Mater., 8 (1999) 2059. 71. M. Tachiki, T. Fukuda, K. Sugata, H. Seo, H. Umezawa, and H. Kawarada, in "Third International Symposium on the Control of Semiconductor Interfaces", p. 578, Karuizawa, Japan, 1999. 72. H. Seo, M. Tachiki, T. Banno, Y. Sumikawa, H. Umezawa, and H. Kawarada, Jpn. J. Appl. Phys., Part I Al (2002) 4980. 73. K. Sugata, M. Tachiki, T. Fukuda, H. Seo, and H. Kawarada, Jpn. J. Appl Phys., Part 141 (2002) 4983. 74. T.-C. Kuo, R. L. McCreery, and G. M. Swain, Electrochem.

Solid-

State Lett, 2 (1999) 288. 75. M. Delamar, R. Hitmi, J. Pinson, and J. M. Saveant, J. Am.

Chem.

Soc, 114(1992)5883. 76. C. Bourdillon, M. Delamar, C. Demaille, R. Hitmi, J. Moiroux, and J. Pinson, J. Electroanal

Chem., 336 (1992) 113.

77. P. AUongue, M. Delamar, B. Desbat, O. Fagebaume, R. Hitmi, J. Pinson, and J.-M. Saveant, J. Am. Chem. Soc, 119 (1997) 201. 78. C. Saby, B. Ortiz, G. Y. Champagne, and D. Belanger,

Langmuir,

13, 6805 (1997).

213

79. R. Graupner, F. Maier, J. Ristein, L. Ley, and C. Jung, Phys.

Rev.

B, 57 (1998) 12397. 80. K. Bobrov, A. Mayne, G. Comtet, G. Dujardin, L. Hellner, and A. Hoffman, Phys. Rev., ^^ (2003) 195416. 81. T. N. Rao, D. A. Tryk, K. Hashimoto, and A. Fujishima, Electrochem.

J.

Soc, 146 (1999) 6870.

82. C. H. Goeting, F. Marken, A. Gutierrez-Sosa, R. G. Compton, and J. S. Foord, Diamond Relat. Mater., 9 (2000) 390. 83. C. D. Wagner, A. V. Naumkin, A. Kraut-Vass, J. W. Allison, C. J. Powell, and J. R. Rumble, Jr.; U. S. Department of Commerce, 2000. 84. F. K. de Theije, M. F. Reedijk, J. Arsic, W. J. P. van Enckevort, and E. Vlieg, Phys. Rev. B, 64 (2001) 085. 85. C. J. Powell and A. Jablonski, in "NIST Standard Reference Database 82". National Institute of Standards and Technology, Gaithersburg, MD, 2000. 86. P. Chen, M. A. Fryling, and R. L. McCreery, Anal.

Chem., 67

(1995) 3115. 87. T. Kondo, Y. Einaga, B. V. Sarada, T. N. Rao, D. A. Tryk, and A. Fujishima, J. Electrochem.

Soc, 149 (2002) E179.

88. G. S. Gildenblat, S. A. Grot, C. R. Wronski, A. R. Badzian, T. Badzian, and R. Messier, Appl. Phys. Lett,

53 (1988) 586.

89. M. C. Hicks, C. R. Wronski, S. A. Grot, G. S. Gildenblat, A. R. Badzian, T. Badzian, and R. Messier, J. Appl

Phys., 65 (1989)

2139. 90. S. A. Grot, G. S. Gildenblatt, C. W. Hatfield, C. R. Wronski, A. R. Badzian, T. Badzian, and R. Messier, IEEE Electron Dev. 11 (1990) 100.

214

Lett,

9. Chemical, Photochemical and Electrochemical Modification of Diamond

91. G. S. Gildenblat, S. A. Grot, and A. R. Badzian, Proc. IEEE, 79 (1991) 647. 92. Y. Mori, H. Kawarada, and A. Hiraki, AppL Phys. Lett,

58 (1991)

940. 93. K. Miyata, D. L. Dreifus, and K. Kobashi, AppL Phys. Lett,

60

(1992) 480. 94. T. Tachibana, J. T. Glass, and R. J. Nemanich, J. AppL Phys., 73 (1993) 835. 95. M. Aoki and H. Kawarada, Jpn. J. AppL Phys., 33 (1994) L708. 96. H. Kiyota, E. Matsushima, K. Sato, H. Okushi, T. Ando, M. Kamo, Y. Sato, and M. lida, AppL Phys. Lett, 67 (1995) 3596. 97. H. Kawarada, M. Itoh, and A. Hokazono, Jpn. J. AppL Phys., 35 (1996)L1165. 98. P. K. Baumann, S. P. Bozeman, B. L. Ward, and R. J. Nemanich, Diamond Relat Mater., 6 (1997) 398. 99. P. K. Baumann and R. J. Nemanich, in "The 24th Conference on the Physics and Chemistry of Semiconductor Interfaces", Vol. 15, p. 1236. AVS, Research Triangle Park, North Carolina (USA), 1997. 100. K. Hayashi, S. Yamanaka, H. Watanabe, T. Sekiguchi, H. Okushi, and K. Kajimura, J. AppL Phys., 81 (1997) 744. 101. P. K. Baumann and R. J. Nemanich, Diamond

ReLat Mater., 7

(1998) 612. 102. P. K. Baumann and R. J. Nemanich, J. AppL Phys., 83 (1998) 2072. 103. A. DeneuviUe, C.R Acad. Sci., Ser. IV, 81 (2000), 104. R. Zeisel, C. E. Nebel, and M. Stutzmann, Diamond ReLat

Mater.,

9 (2000) 413.

215

105. J. E. Gerbi, O. Auciello, J. Birrell, D. M. Gruen, B. W. Alphenaar, and J. A. Carlisle, AppL Phys. Lett, 83 (2003) 2001. 106.J. Wang, G. M. Swain, T. Tachibana, and K. Kobashi, Solid-state

Lett,

Electrochem.

3 (2000) 286.

107. K. Honda, M. Yoshimura, T. N. Rao, D. A. Tryk, A. Fujishima, K. Yasui, Y. Sakamoto, K. Nishio, and H. Masuda, J.

Electroanal.

Chem., 514 (2001) 35. 108. O. Enea, B. Riedo, and G. Dietler, Nano Letters, 2 (2002) 241. 109. J. Wang and G. M. Swain, Electrochem.

Solid-State

Lett,

5 (2002)

E4. IIO.R. Uchikado, T. N. Rao, D. A. Tryk, and A. Fujishima, Chem.

Lett,

(2001) 144. l l l . K . Ohnishi, Y. Einaga, H. Notsu, C. Terashima, T. N. Rao, S.-G. Park, and A. Fujishima, Electrochem.

SolidState

Lett,

5 (2002)

Dl. 112. L.-F. Li, D. Totir, B. Miller, G. Chottiner, A. Argoitia, J. C. Angus, and D. Scherson, J. Am. Chem. Soc,

119 (1997) 7875.

113. T. L. Kulova, Y. E. Evstefeeva, Y. V. Pleskov, A. M. Skundin, V. G. Ral'chenko, S. B. Korchagina, and S. K. Gordeev, Phys.

Solid

State, 46 (2004) 726. 114. N. G. Ferreira, L. L. Mendonca, V. J. T. Airoldi, and J. M. Rosolen, Diamond Rel Mater., 12 (2003) 596. 115. A. DeBattisti, S. Ferro and M. D. Colle, J. Phys. Chem. B, 105 (2001) 1679. 116. S. Ferro and A. DeBattisti, J. Phys. Chem. B, 106 (2002) 2249. 117. N. Spataru, C. Terashima, K. Tokuhiro, I. Sutanto, D. A. Tryk, S.M. Park, and A. Fujishima, J. Electrochem.

216

Soc, 150 (2003) E337.

9. Chemical, Photochemical and Electrochemical Modification of Diamond

118.M. E. Hyde, R. M. J. Jacobs, and R. G. Compton, J. Phys. Chem. B, 108 (2004) 6381. 119. K. Okumura, K. Nakagawa, T. Shimamura, N. Ikenaga, M. Nishitani-Gamo, T. Ando, T. Kobayashi, and T. Suzuki, J. Phys. Chem. B, 107 (2003) 13419.

217

10. Characterization of Oxygenated Diamond Electrodes Hideo Notsu, Tetsu Tatsuma and Akira Fujishima

Diamond surfaces are of great interest in many fields, e.g., in the CVD synthesis of diamond, and possible applications as adsorbents, electron emission devices, and so on. In particular, in the field of electrochemistry, the surface structure of diamond electrodes is important,

because electrochemical reactions proceed

at

the

interface between electrode surfaces and electrolyte solutions. As

the

simplest

and

most

effective

surface

structure

modification, we are interested in surface oxidation treatment. There are two important merits in the surface oxidation treatment of the diamond electrode.

First, surface oxidation treatment

changes the electrochemical character of diamond electrodes by means of electrostatic interactions between the electrode surface and redox ions. Second, surface oxidation treatment facilitate the subsequent functionalization of diamond electrodes.

Actually,

surface functional groups, which are introduced onto the diamond electrode by the

surface

oxidation treatment,

facilitate

the

introduction of many kinds of functional materials. In this chapter, the electrochemical character of surface oxidized diamond electrode is described. Then, the introduction of new

functionalities

and

characterization

Hideo Notsu e-mail: [email protected] 218

of surface

oxygen-

10. Characterization of Oxygenated Diamond Electrodes

containing functional groups by use of several additional k i n d s of surface modifications is described.

10.1. Surface Oxidation of Diamond There are several m e t h o d s for t h e oxidation of diamond surfaces. (1)

Vapor-phase oxidation in O2 The diamond surface is oxidized in a t u b u l a r flow reactor using O2 gas at high t r e a t m e n t t e m p e r a t u r e s (300 • 1000 °C).

(2)

Liquid-phase oxidation The diamond surface is immersed in a boiling acid mixture (H2S04-HN03).

(3)

Oxygen-plasma t r e a t m e n t The diamond surface is etched by a n oxygen-plasma in specific reactor.

(4)

Electrochemical oxidation The diamond surface is subjected to a high positive potential or applied potential cycle in positive potential range.

(5)

Oxidizing agent or radical oxidation As a n o t h e r new method, t h e oxidation by hydroxyl radicals from t h e F e n t o n reaction or Ti02 should be possible.

As well as for method (l), t h e necessity of specific i n s t r u m e n t a t i o n is a disadvantage of method (3), b u t the a d v a n t a g e of method (3) is the shorter t r e a t m e n t time required compared to method (l).

As

treatment

the

can

be

performed

in

the

same

system

as

electrochemical m e a s u r e m e n t s , method (4) is convenient for t h e use of electrochemistry.

219

10.2. Contact Angle of Water Droplets One of the obvious differences between

hydrogen-terminated

diamond and oxygenated diamond can be seen in the values of the water contact angle. The contact angle of a sample depends on the polarity of the functional groups on the sample surface.

The

contact angle for as-deposited diamond of about 90° decreases to nearly 0° after surface oxidation by oxygen plasma treatment for 1 min.

After the other surface oxidation methods, e.g., oxygen

annealing and electrochemical oxidation, the contact angles for diamond also decrease but do not reach the value of the oxygenplasma treated sample. This difference of contact angle between the treated diamond surfaces may be due not only to the different amounts of hydrophilic functional groups but also to differences in roughness after the treatments.

10.3. Electrochemical Character of Oxidized Diamond Electrodes The potential windows of the oxygenated diamond electrodes became somewhat wider than that of the as-grown electrode (Fig. 10.1a). The background currents for the electrochemically polarized and oxygen-plasma treated surface became 1-2 and 3-5 times larger than that of the as-grown electrode, respectively (Fig. 10.1b). The change of the chemical termination did not appear to cause a remarkable change in the electrochemical properties. In the case of heavily boron-doped diamond films, the effect of the removal of the surface conductive layer [l] on the surface

220

10. Characterization

of Oxygenated Diamond

Electrodes

conductivity seems not to be serious. It was also confirmed by the four-point probe method that the film resistivity of the diamond surface (~10'3 Q cm) did not change after either surface oxidation treatment. -|

1

r

As-grown Electnocluemically polarized —

-1.2 -1.5

-1

-0.5

Ox3/^rL-plasma treated

0

0.5

1

1.5

2

2.5

Potential/V VS. Ag|AgCl 1.5 As-grown 1.0

-0.5

-1.0 -1.5 -0.5

Ox3/^n-plasma treated

0

Electrochemically polarized

0.5

1

Potential / V vs. Ag|AgCl

Fig. 10.1. Cyclic voltammograms in 0.1 M sulfuric acid solution for asgrown, electrochemically polarized and oxygen-plasma treated diamond electrodes,' scan rate- 100 mVs"i.

221

10.4. Electrochemical Responses to Several Redox Systems A

120-mV

anodic'cathodic

peak

separation

(AEp),

which

corresponds to a quasi-reversible electron transfer, was observed in the CV of K4[Fe(CN)6] for the as-grown electrode.

However, a

larger AEp value, e.g., > 300 mV, was obtained for the oxygenated diamond electrode (Fig. 10.2a).

The anodic and cathodic peak

potentials symmetrically shifted from their initial values in the positive and negative directions, respectively, indicating a large decrease in the heterogeneous electron transfer rate constants. A large decrease in the apparent electron transfer rate constant due to the oxygen-plasma treatment was also observed for the [IrCle]^^^' redox couple [2]. In the case of the CVs in 0.1 M Na2S04 solution containing 0.5 mM Ru(NH3)6Cl2, almost no change in the AEp value was observed after the surface oxidation treatment (Fig. 10.2b). In the case of the CVs obtained in 0.1 M NaC104 solution containing 1 mM Fe(C104)3, the AEp value became smaller after surface oxidation treatment (Fig. 10.2c). This fact indicates that the apparent electron transfer at the diamond surface accelerated by the surface oxidation treatments.

was

These results

clearly suggest that the change of the apparent electron transfer rate should be attributed to causes other than a decrease in the boron concentration as a result of plasma sputtering or the removal of the surface conductive layer.

222

10. Characterization of Oxygenated Diamond Electrodes

20

As-grown —

10 0

^^^~/

-10 20

Oxj^^n-plasma treated -0.5

-j— Electrocherrucally j polarized \.

0 0,5 Potential/V vs. Ag/AgCl As-grown

-0.4

-0.2

0

0.2

Potential / V vs. Ag^AgCl

0 0.5 1.0 Potential / V vs. Ag/AgCl

Fig. 10.2. Cyclic voltammograms of (a) [Fe(CN)6]4-/3-, (b) [Ru(NH3)6]2+/3+ and (c) Fe2+/3+ fQj. as-grown, electrochemicaUy polarized and oxygenplasma treated diamond electrodes; scan rate- 100 mVs'^ The present results are summarized in Table 10.1, comparing the behavior for diamond and GC [3-6] for

oxygen-plasma

treatment times of 1 minute [2]. The three groups of redox systems identified for diamond electrodes can be characterized simply by the charges of the redox species involved. The redox species with negative charge are sensitive to the surface oxygen species, exhibiting decreased apparent electron transfer rate constants after the oxygen-plasma treatment.

On the other hand, the

apparent electron transfer rate constants for the redox species with

223

positive charge did not change significantly after t h e oxygenplasma treatment. Table 10.1. Anodic-cathodic peak potential separations for various redox species.

AEp/mV As-grown diamond

Electrochemically Ox5^n-plasma polarized diamond treated diamond

120 93 117 Ru(Na)'"'

1072 667 135 1879 466

610 380 94 514

Polished GC

Oxygen-plasma treated GC

162 87 86

103 87 89

327

84

Although the various redox species can be classified into three groups for both electrodes, as is clearly seen in Fig. 10.3, the respective classifications are quite different. The differences in the classifications of the redox couples at diamond electrodes compared to t h a t for GC electrodes can be a t t r i b u t e d to the difference of the effect of surface oxygen species on sp^ carbon (diamond) from t h a t on sp2 carbon (GC).Thus, the reason for the difference can be a t t r i b u t e d to t h e adsorption of the redox species.

As one more

reason, it is proposed t h a t oxygen-containing functional

groups

should orientate a n d should exhibit more negative polarity on the flat

surfaces

electrodes.

of diamond Therefore,

microcrystallites t h a n comparatively

strong

those on

electrostatic

interactions may be expected between the diamond surface a n d t h e redox species.

224

GC

electrode

10. Characterization of Oxygenated Diamond Electrodes

Fe2+/3. Yes E^2./3.

Glassy Car b o n

V2-H/3^

Pe2./3. Yes

E^2+/3+ Y2+/3+

y2+/3+

[Co(en)3]2*^3*

j

Oxide Sensitive? No

JFe(CN)6]^

Surface Sensitive?

[IrCle]^"^No

[IrCle]^-^^[Fe(CN)6]^-^^-

[Co(phen)3]2*^3*^ J

Diamond Slow Down

[IrCle]^-'^-

[IrClgl^-^^-

^

[Fe(CN)e]^-^^Effect of Slightly Accelerated

[Ru(NH3)6]2^''* Oxygen

[Ru(NH3)6]2*^3^ [Co(en)3]2^'^^

[Co(en)3]2*^3* Fe2-/3.

Accelerated

Fe2-/3.

Fig. 10.3. Categorization of redox couples for GC [3] and diamond electrodes [2].

10.5. XPS Spectra of Oxidized Diamond Electrodes Figure 10.4 shows typical XPS spectra for as-deposited, oxygenplasma-treated (l min), and electrochemically polarized (60 min) diamond films. A well-defined O Is peak and an O KLL Auger peak were observed after both the oxygen-plasma and anodic treatments. This surface oxygen is almost certainly in the form of carbon-oxygen surface groups.

225

As-grown

Oxygen-plasma treated

Ele ctro chemic ally polarized

Cls 0 1s

UvM

hJwJSJ

p*««H*w^h^

I \jy-^j

0

200

400

600

800 10000

200

400

600

800 1000 0

200

400

600

800 1000

Binding energy / eV

Fig. 10.4. Typical XPS spectra for as-grown and oxidized-diamond electrodes. The various other types of oxygen functional groups were also characterized by means of XPS.

Figure 10.5 depicts C Is core level

XPS spectra for the oxidized diamond electrodes.

The observed

peak consists of several peaks, corresponding to carbon atoms in different states [7,8]. The peak corresponding to bulk diamond i.e., carbon bonded only to carbon, is observed at ca. 284 eV. The peak observed at ca. 285-286 eV is attributed to carbon singly bonded to oxygen (C-OH and C-Q-C). Carbon atoms involved in carbonyl and carboxyl groups exhibit a peak at ca. 287 eV.

Both of the

oxygenated diamond surfaces exhibited similar peaks. By use of only XPS measurements, it is difficult to distinguish each of the oxygen-containing surface functional groups.

226

10. Characterization of Oxygenated Diamond Electrodes

284

Binding energy / eV

286

Binding energy / eV

Fig. 10.5. C Is core level XPS spectra for oxygenated diamond electrodes. Deconvoluted peaks are also depicted.

10.6. Carboxyl Groups on the Diamond Surface One of the confirmations of the presence of carboxyl groups on the electrode surface is the observation of the pH dependence of the electron transfer rates of redox species [9]. In the case of higher pH values, carboxyl groups are deprotonated and negatively charged, and can act to repel redox species with negative charges.

The

negative charges of carboxyl groups decrease with decreasing pH by protonation, and thus the electrostatic interaction between these groups and the redox species would change with pH, and the electron transfer rate should change. On the other hand, electrostatic attraction between carboxyl groups and positively charged redox species [Ru(NH3)6]2'^^^^ should increase with increasing pH. In fact, the redox behavior for these at GC depends on pH, and this effect is an example of the Frumkin effect [10].

However, the rate constants for redox reactions at

oxygenated diamond electrodes do not obey the Frumkin effect (Fig. 227

10.6). Therefore, it is supposed that there is a negligible amount of carboxyl groups on oxygenated diamond electrodes. 7

16

• • • ••

14 12

• Ru(NH5)c'"''

10 8 6

• •

5 -

•

# •

•

•

i

1

1

•

1

•

2 1 -

1

Fe(CN)''''" • •

3

2 0

•

4

•

4

6

1

0 2 4 6 8 10 12 1<4 PH

•••

0 ' 0 2 4 6 8 10 12 14 pH

Fig. 10.6. pH dependence of rate constant for two redox species at (•) GC, (•) oxygen-plasma treated and (A) electrochemically polarized diamond electrodes

10.7. Carbonyl Groups on the Diamond Surface It is expected that carbonyl groups (C=0) and ether groups (C-Q-C) are generated on the diamond (lOO) surface [ l l ] , and hydroxyl groups (C-OH) are generated on the ( i l l ) surface by these surface oxidation methods (Fig. 10.7). Surface carbonyl groups existing on the GC electrode catalyze the redox reaction of [Fe]2+/3+ [12-14]. It is possible that similar catalytic effects could occur with the oxidized diamond electrode surfaces. Therefore, to probe for the existence of carbonyl groups and their effects on the [Fe]2+/3+ redox reactions, diamond electrodes were treated with DNPH [12].

228

10. Characterization of Oxygenated Diamond Electrodes

DNPH covalently binds to carbonyl [15]. The changes in the AEp values obtained before and after the DNPH treatment

are

summarized in Table 10.2 [12]. Only in the case of the oxygenated diamond electrodes, the DNPH treatment increased the AEp value, indicating that the [Fe]2+/3+ redox reactions were decelerated. The electrochemically oxidized diamond electrode exhibited

larger

increases in the AEp values on the average than the oxygenplasma-treated

diamond electrode.

This indicates that

the

electrochemically oxidized diamond surface has a higher coverage of carbonyl groups that are reactive with DNPH than does the surface oxidized by means of the oxygen plasma.

(111) surface

(100) surface

carbon: O

oxygen: Q

hydrogen:

Fig. 10.7. Termination structure of oxygen-terminated diamond for the (100) and (111) faces.

229

Table 10.2. Anodic-cathodic peak potential separation values for Fe2+/3+ at diamond electrodes before and after DNPH treatment. A&/V DNPH treatment

As-deposited

Oxygen-plasma pretreated"

Electro chemically pretreated*"

before

0.95 ±0.12

0.58±0.10

0.52 ±0.05

after

0.95 ±0.05

0.70 ±0.08

0.80 ±0.13

0.00

0.12

0.28

the difference (after - before)

• Oxygen-plasma treatment (70 W, 1 min). ^ eectrochemical polarization (0.1 M H2SO4, +2.4 V, 60 min). ' Mean standard error, (n = 4)

XPS measurements also supported these results. The DNPH treatment resulted in larger N Is peaks due to the nitrogen atoms of the DNPH molecules, and in particular, the anodically polarized surface exhibited a larger increase in the peak area than did the surface treated with oxygen plasma (Fig. 10.8).

Oxygen-plasma

|_ Electrochemical

treatwent+DNPH

pokrization + hDNPH

DNPH untreated 396

398

400

402

404

Binding Energy / eV

Fig. 10.8. N Is core level spectra of XPS for DNPH-treated and untreated oxygenated diamond electrodes.

230

10. Characterization of Oxygenated Diamond Electrodes

10.8. Hydroxyl Groups on the Diamond Surface In addition to confirming the presence

of carbonyl groups, the

redox behavior of [Fe(CN)6]4/3- at the APTES'treated diamond electrodes was observed in order to confirm the presence of hydroxyl groups.

The APTES treatment significantly decreased

the AEp value for the oxygenated diamond electrodes to ca. 150 mV (Table 3) [16].

The increase in the redox reaction rate for

[Fe(CN)6]^"^^' was presumably caused by the electrostatic attraction [17] between the positively charged protonated amino groups of the surface-attached APTES molecules and the [Fe(CN)6]4/3- anions. It is concluded that hydroxyl groups are generated by the oxidative treatments. Table 10.3. Anodic-cathodic peak potential separation values for [Fe(CN)6]2+/3+ at diamond electrodes before and after APTES treatment.

AEp/V treatment

x^s-deposited

Oxygen-plasma preti-eated"

Electro chemically pretreated*"

before

0.11 ±0.03

0.55±0.11

0.43 ±0.12

after

0.12 ±0.03

0.15 ±0.04

0.14±0.03

0.01

-0.40

-0.29

the difference (after - before)

° Oxygen-plasma treatment (70 W, 1 min). "" Electrochemical polarization (0.1 M H2SO4, +2A V, 60 min). " Mean standard error, (n = 4)

Results of contact angle measurements also support this conclusion. Subsequently, the introduction of APTES moieties by

231

means of XPS was also examined. The APTES treatment gave rise to an N Is peak only for the oxygenated diamond electrodes. The peak areas of the two different oxygenated diamond surfaces were almost same; \ t h i s means that almost the same amounts of APTES were introduced, and thus there were almost same amounts of hydroxyl groups on both surfaces. Efectochemical

Oxygpn-pfesma

polarization

tieatrt«nt+

+ APTES

APTES

APTES untiedted 1

1

396

1

1

398

1

400

402

1

1

404

Binding Energy / eV

Fig. 10.9. N Is core level spectra of XPS for APTEStreated and untreated oxygenated diamond electrodes.

10.9. Correlation between Redox Behavior and Surface Groups The changes in the redox behavior of the inorganic compounds at the

diamond

electrode

after

surface

oxidation

are

almost

completely explained by electrostatic interactions between the electrode surface and the redox species.

232

10. Characterization of Oxygenated Diamond Electrodes

However, the difference

in the redox behavior

changes

between the oxygen-plasma treated diamond electrode and the electrochemically polarized diamond electrode cannot be explained only by electrostatic interactions. For example, in the case of the AA and the [Fe]2+/3+ redox reactions, even though the oxygenplasma treatment introduced a larger amount of oxygen, the anodic polarization changed the reaction rates to almost the same or a greater degree than the oxygen-plasma treatment. As described previously, the electrochemical polarization introduced a larger amount of C=0 groups than the oxygen-plasma treatment. difference

This

in the amount of active C=0 groups should be

responsible for the different behavior between [Fe(CN)6]^''^' and the other two redox reactions. It is known that C=0 groups catalyze the [Fe]2+/3+ redox reaction. Thus, it is found that the amounts of C-OH groups are almost the same on both oxidized surfaces, that the amount of C=0 groups introduced by the anodic polarization is larger than that introduced Table 10.4. Summary of oxygen-containing groups on various diamond electrode surfaces [2, 18]. 0/C ratio (%) As-deposited

< 1

Oxygen-plasma pretreated"

~

Electrochemically pretreated'"

~

Carbo xyl Carbonyl Hydroxyl X

X

X

X

O

O

X

©

O

° Oxygen-plasma treatment (70 W, 1 min). ' Oectrochemical polarization (0.1 M H2SO4, +2.4 V, 60 min).

233

by the oxygen-plasma treatment, and that there are negligible amounts of carboxyl groups on both surfaces. In addition, oxygenplasma treatment may introduce a larger amount of ether groups than does the electrochemical polarization.

10.10. Applications of Oxidized Diamond Electrodes The selective determination

of dopamine

(DA) in

solutions

containing ascorbic acid (AA) is the first example of an application that resulted from the introduction of enhanced selectivity to diamond electrodes by use of surface oxidation. In the case of the determination of DA, because the oxidation peak potentials of DA and AA are very close, it is important to decrease the interference due to AA. After the anodic polarization treatment, the oxidation peak potential of DA did not change greatly (~ 0.8 V vs. Ag | AgCl), but at the same time, that of AA shifted greatly (from 0.8 V to 1.4 V vs. AglAgCl). Therefore, by use of the oxygenated diamond electrode as a working electrode, by application of a potential of 0.8 V vs. AglAgCl, DA can be oxidized selectively, without oxidizing AA. This result shows that new selectivity can be introduced to the diamond electrode by use of surface oxidation. Moreover, even in a solution containing 1 mM AA, DA can be determined at nM concentrations [19]. As a second example, we will present the determination of phenol derivatives. As most phenol derivatives are compounds that are harmful to the human body, the determination of these

234

10. Characterization of Oxygenated Diamond Electrodes

compounds in drinking water, river water, tobacco pipe filters, and so on, is important.

The determination of phenol derivatives by

direct oxidation has some disadvantages, e.g., adsorption of intermediates, and interference by coexisting compounds [20]. There is another effective method for the determination of phenol derivatives that makes use of enzymes. The enzyme tyrosinase oxidizes phenol derivatives to benzoquinone derivatives by consuming dissolved oxygen in the solution. derivatives

can be

easily

reduced

Benzoquinone

electrochemically

in

the

moderately negative potential range. By using an electrode with immobilized tyrosinase, the phenol derivatives can be detected from

the response due to the electrochemical reduction of

benzoquinone derivatives. Because the hydrogen-terminated diamond electrode has low chemical reactivity and high hydrophobicity, it is difficult to immobilize enzymes onto its surface.

However,

oxygenated

diamond electrodes have hydroxyl groups on the surface, and with these, amino groups can be introduced by use of subsequent chemical modification with APTES. By use of glutaraldehyde or carbodiimido moieties, covalent linkages between the amino groups on the electrode surface and the amino groups of the enzyme can be made. Tyrosinase-modified diamond electrodes can detect not only simple phenol derivatives, such as phenol, p-cresol and catechol, but also more complicated phenol derivatives, such as bisphenol'A and 17(3-estradiol. The order of the detection limits for phenol, pcresol and bisphenol-A is 10 nM, 100 nM and 1 ^iM, respectively [21].

235

These r e s u l t s indicate t h a t , by use of surface oxidation a n d subsequent A P T E S t r e a t m e n t , any enzymes t h a t contain a n amino group can be immobilized onto a diamond electrode in t h e same way. In other words, any desired type of catalytic activity can be introduced

to

diamond

mentioned above.

by

use

of the

modification

scheme

One additional i m p o r t a n t point is t h a t these

modifications do not decrease the well known a d v a n t a g e s of t h e diamond electrode for electrochemical detection. This fact promises to increase t h e n u m b e r of potential applications of the diamond electrode.

References 1.

B. Miller, R. Kahsh, L. C. Feldman, A. Katz, N. Moriya, K. Short and A. E. White, J. Electrochem. Soc, 141 (1994) L41.

2.

I. Yagi, H. Notsu, T. Kondo, D. A. Tryk and A. Fujishima, J. Electroanal. Chem., 473 (1999) 173.

3.

P. Chen and R.L. McCreery, Anal. Chem., 68 (1996) 3958.

4.

P. Chen, M.A. Fryking and R.L. McCreery, Anal. Chew., 61 (1995) 3115.

5.

M.A. Fryking, J. Zhao and R.L. McCreery, Anal. Chew., 67 (1995) 967.

6.

R. DeClements, G.M. Swain, T. Dallas, M.W. Holtz, R.D. Herrick II and J.L. Stickney, Langmuir,

7.

12 (1996) 6578.

C. H. Goeting, F. Marken, S. A. Gutierrez, R. G. Compton and J. S. Foord, New Diamond Front. Carbon TechnoU 9 (1999) 207.

8.

236

M. R. Deakin, K. J. Stutts and R. M. Wightman, J.

Electroanal.

10. Characterization of Oxygenated Diamond Electrodes

Chem., 182 (1985) 113. 9.

M.R. Deakin, K.J. Stutts and M. Wightman, J. Electroanal.

Chem.,

182(1985) 113. 10. F. G. Gonon, F. Navarre and M. J. Buda, Anal. Chem., 56 (1984) 573. 11. R. E. Thomas, R. A. Rudder and R. J. Markunas, J. Vac. Sci. Technol. A, 10 (1992) 2451. 12. H. Notsu, I. Yagi, T. Tatsuma, D. A. Tryk and A. Fujishima, J. Electroanal. Chem., 492 (2000) 31. 13.

P. Chen, M. A. Fryhng and R. L. McCreery, Anal.

Chem., 67

(1995) 3115. 14. P. Chen and R. L. McCreery, Anal. Chem., QS (1996) 3958. 15. M. A. Fryhng, J. Zhao and R. L. McCreery, Anal. Chem. 67 (1995) 967. 16. H. Notsu, T. Fukazawa, T. Tatsuma, D. A. Tryk and A. Fujishima, Electrochem.

SolidState.

Lett, 4 (3) (2000) H I .

17. M. J. Rutter and J. Robertson, Phys. Rev. B., 57 (1998) 9241. 18. H. Notsu, I. Yagi, T. Tatsuma, D. A. Tryk and A. Fujishima, Electrochem. SolidState

Lett, 2 (1999) 522.

19. E. Popa, H. Notsu, T. Miwa, D. A. Tryk and A. Fujishima, Electrochem. SolidState.

Lett, 2 (l) (1999) 49.

20. C. Terashima, T. N. Rao, B. V. Sarada, D. A. Tryk and A. Fujishima, Anal. Chem. 74 (2001) 895. 21. H. Notsu, T. Tatsuma and A. Fujishima, J. Electroanal.

Chem.,

523 (2002) 86.

237

11. Diamond Electrodes with Functional Structures and Surfaces Yasuaki Einaga, Tribidasari A. Ivandini, and Akira Fujishima

The outstanding properties of diamond make it a very attractive material for use in many potential applications. superior

electrochemical

properties

of

In particular, the

highly

boron-doped

conductive diamond films, prepared by the CVD process, have received attention from electrochemists.

This article reports the

fabrication of boron-doped diamond (BDD) electrodes, creating various functional structures or functional surfaces such as microdisk array (MDA) electrodes, ion-implanted BDD electrodes, and electrodes with ultrasmooth surfaces. Studies have been made of the electrochemical properties of each system and

their

applications in electroanalysis arediscussed.

11.1. Boron-Doped Diamond Microdisk Array Electrodes Microdisk array (MDA) electrodes of boron-doped diamond (BDD) were fabricated on structured silicon substrates.

The BDD-MDA

electrodes exhibited sigmoidal voltammetric curves, which show that they function as assemblies of single microelectrodes.

The

microelectrode behavior was also confirmed with biologically Yasuaki Einaga 238

e-mail: [email protected]

11. Diamond Electrodes with Functional Structures and Surfaces

important

species

such

3,4-dihydroxyphenylacetic

as

ascorbic

acid

and

acid, indicating its applicability in

electroanalysis [l]. Microelectrodes

have

attracted

electroanalysis, due to their small size.

much

attention

in

They possess many

advantages, such as steady-state response, small IR drop and small capacitance [2-4]. The small size of microelectrodes gives them unique properties, for example, an increased rate of mass transport, which results in improved signal-to-background current ratio in comparison to theirplanar counterparts. Generally, arrays of microelectrodes are often used in order to increase the current signal, because the current measured at a single microelectrode is very small.

Micro-array electrodes such as microdisk array

(MDA) electrodes and interdigitated array electrodes can be obtained by use of various micro-fabrication technologies [5,6]. Combining the advantages of diamond and microelectrode arrays makes them more attractive for electroanalytical applications.

In

this work, microelectrode arrays of BDD were fabricated on silicon substrates. A schematic diagram of the procedure for fabricating the BDD-MDA electrode is shown in Fig. 11.1, A Si(lOO) surface was masked with patterned photoresist and etched with a mixture of HF, HNO3 and H2O. The structured silicon surface was seeded with 10-nm diamond powder.

BDD was deposited using a

microwave plasma-assisted chemical vapor deposition system. The details of the diamond deposition have been reported elsewhere [7]. After the deposition of diamond, polyimide varnish was spin-coated on the diamond surface. The polyimide layer was

239

mechanically polished until the diamond tips were just exposed. Electrochemical

measurements

were

carried

out

at

temperature using a potentiostat and an X-Y recorder.

room A

hree-electrode configuration was used for the electrochemical measurements, with Ag/AgCl (sat. KCl) electrode as the reference electrode and platinum wire as the counter electrode.

Electrical

contact from the microelectrode array was taken from the silicon substrate at the backside.

I

1

Fig. 11.1. Schematic diagram of the preparation of BDD -MDA electrodes (a) photoresist pattern formed on a siUcon substrate,* (b) isotropic etching of the substrate,* (c) Deposition of BDD,* (d) spin-coating and mechanical etching of the polyimide film. First, the distance between each microelectrode in the array and the size of each microelectrode must be considered so that the array may realize the properties of a single microelectrode.

That

is, if the packing density is low, the diffusion layers will overlap and the array will behave as a macro-sized electrode [4]. In the method used in this work, the distances between the microdisk electrodes were controlled by the mask pattern, and the electrode size was controlled by the sharpness of the etched silicon substrate tips, which depend on etching conditions. A diamond array with 240

11. Diamond Electrodes with Functional Structures and Surfaces

the tip size of 25 to 30 ^m in d i a m e t e r w a s used for electrochemical measurements.

The distance between tips w a s 250 ^ m .

-0.2

0

0.2

0.4

0.6

0.8

Potential /V vs Ag/AgCl Fig. 11.2. CycUc voltammogram at BDD-MDA electrodes for the oxidation of 1 mM K4Fe(CN)6 in 0.1 M Na2S04; potential sweep rate 10 mV s'O. The insert is a laser microscope image of the BDD-MDA electrodes. Fig. 11.2 shows cyclic v o l t a m m o g r a m s for t h e oxidation of ferrocyanide at the BDD-MDA electrode.

The

voltammogram

exhibited a sigmoidal curve, indicating t h a t the a r r a y functions as a microelectrode.

The half-wave potential w a s +0.23 V vs Ag/AgCl,

which agrees well with t h e value for macro-sized BDD electrodes. The calibration curve for t h e limiting c u r r e n t for oxidation of ferrocyanide w a s linear over a wide r a n g e of concentrations, from 1 jLiM to 1 mM.

The a r e a of t h e electrode exposed to t h e electrolyte

w a s 0.13 cm2, in which approximately 200 microelectrodes exist. Assuming

that

each

of

these

electrodes

functions

as

an

i n d e p e n d e n t microdisk electrode, t h e r a d i u s of each microelectrode 241

can be estimated from the equation, Iiim - 4nFDCr, where Iiim is the limiting current, n is the number of electrons, F is the Faraday constant, D is the diffusion coefficient, C is the concentration, and r is the radius of the electrode. for the diffusion

By using the value 6.5 x 10^ cm^ s"i

coefficient of ferrocyanide, the radius was

calculated to be 14 |im, which is consistent with the tip size of the electrode observed by SEM. The limiting current at MDA electrodes is known to be independent of potential sweep rate for pure spherical diffusion without cross-talk between neighboring microelectrodes.

The

limiting current for the oxidation of ferrocyanide measured at the potential of +0.5 V vs Ag/AgCl was constant up to 200 mV s"^. Above this sweep rate, an increase in the limiting current was observed and the voltammogram began to acquire a peak-shape. The microelectrode behavior in this study is similar to that reported for single BDD microelectrodes [8], except that the observed currents were two orders of magnitude higher. This indicates our success to increase the detection current, while the properties as microelectrodes are retained. BDD-MDA electrodes are useful for electrochemical detection of various biologically and environmentally important chemical species

with

high

sensitivity

and

stability.

Sigmoidal

voltammograms could be obtained for (A) ascorbic acid and (B) 3,4-dihydroxyphenylacetic acid (DOPAC), as shown in Fig. 11.3. Steady-state current was observed even at potentials as high as + 1.2 V vs Ag/AgCl, due to the wide potential window of BDD.

242

11. Diamond Electrodes with Functional Structures and Surfaces

Electrochemical oxidation

sensing

potentials

and

of chemical trace

metal

species with analysis

high

are

major

applications for both microelectrodes and BDD electrodes.

These

electrodes are also expected to exhibit high stability, similar to their planar counterparts [9]. Further work on electroanalysis using BDD-MDAs is in progress in our laboratory.

(A)

1 0.5 !JiA

y ...L

r

(B)

1 1

0

1

1

0.4 0.8 Potential/V vs Ag/AgCl

1

1.2

Fig. 11.3. CycUc voltammograms at BDD-MDA electrodes for the oxidation of (A) 1 mM ascorbic acid in 0.1 M Na2S04 and (B) 1 mM DOPAC in 0.1 M Na2S04; potential sweep rate 10 mV sO.

11.2. Ion-Implanted Boron-Doped Diamond Electrodes Nickel-implanted boron-doped diamond electrodes (Ni-DIA) were fabricated in view of their application for carbohydrate detection. This

electrode

produced

well-defined

and

reproducible

243

voltammograms for 1 mM glucose in alkaline media. The electrode exhibited excellent electrochemical stability with low background current, even after ultrasonic treatment, indicating the strong bonding of nickel with carbon. These results indicate the promising use of Ni-DIA for the detection of carbohydrates and amino acids, and thus, an application of ion implantation, as this surface modification method is effective for controlling the electrochemical properties of polycrystalline diamond thin films [lO]. Electrochemical detection of carbohydrates

is also very

attractive due to the possibility of high sensitivity and wide dynamic range. Conventionally, metal electrodes such as nickel and copper are known to oxidize carbohydrates in alkaline solution [ll].

The advantage of nickel and copper electrodes is that they

produce quite stable responses. These electrodes have been widely used in liquid chromatography and capillary electrophoresis. However, dispersion of metallic particles within an organic polymer or simply on an inert surface results in a drastic increase in the catalyic activity and sensitivity of the electrode [12" 14]. A stable, inert electrode with low background current would be the best choice for the deposition of metal electrocatalysts. Metal-modified diamond electrodes appear to be well suited to overcome these problems. As one example, we recently reported on nickel-modified BDD electrodes, demonstrating their use for the detection of carbohydrates [15]. The electrodes were prepared by depositing Ni(N03)2 solution on the surface of the boron-doped diamond. Although the electrochemical stability was relatively good (at least one week), higher stability is required when stringent conditions,

244

11. Diamond Electrodes with Functional Structures and Surfaces

such as sonication or high flow rates are appUed, for example, to clean the electrode. Ion implantation has been successfully

used in doping

semiconductors such as silicon and gallium arsenide. In particular, applications of ion-implanted diamond have recently come to light. In these studies, the electrical conductivity and other physical properties could be controlled by ion-implanting diamond. However, only a few applications for electrochemical uses by preparing conductive electrodes have been reported [16,17]. Here, the ion implantation method was applied to prepare electrodes with better electrochemical stability. As a result, we have shown the promising use of nickel-implanted BDD electrodes (Ni-DIA) for glucose detection. The

detailed

preparation

of boron-doped

polycrystalline

diamond thin films has been described elsewhere [7]. These films were implanted with 750 keV Ni^^ with a dose of 5 x lO^^ cm'2 (Tandetron 4117-HC, HVEE). The annealing was performed at 850^0 for 10 min in an H2 ambient (80 Torr). Raman spectroscopy was carried out with Ar+ laser illumination (wavelength = 514.5 nm) in a Renishaw Raman imaging microscope system (Renishaw System 2000). A scanning electron microscope (SEM, model JMS-6100, JEOL) was used for imaging the surface morphology. Electrochemical measurements were carried out in 0.2 M NaOH aqueous solution with a single-compartment cell. An Ag/AgCl (saturated KCl) electrode was used as the reference electrode (+0.199 V vs. SHE) and a Pt wire was used as the counter electrode. Current-potential curves were recorded using a potentiostat (Hokuto Denko, Hz-3000). The flow-injection analysis (FIA) system

245

for amperometric measurement in the present study consisted of a binary pump

(GL Sciences, Inc., PU611), an

(Spark-Holland, Triathlon) for constant

auto-sampler

10 |iL injections,

a

thin-layer flow cell (GL Sciences, inc.), an amperometric detector (Bioanajyiical Systems, LC-4C), and a data acquisition system (EZChrom Elite Scientific Software, Inc.). The wall-jet-type flow cell consisted of the Ag/AgCl reference electrode and a stainless steel (type 316) counter electrode. The geometric area of the electrode in the cell was estimated to be 0.1 cm^. The mobile phase for FIA consisted of a 0.2 M NaOH aqueous solution, and the flow rate was set at 0.2 mL min i. Fig. 11.4 shows the Raman spectra of (a) as-deposited, (b) as-implanted, spectrum

and

in Fig.

(c) annealed 11.4a

samples,

exhibits

a

sharp

respectively.

The

first-order

peak

(single-phonon scattering) at 1332 cm'i, which provides strong evidence for well-crystallized diamond. After implantation, not only the peak at 1332 cm ^ but also a peak at 1550 cm'i, related to sp2-carbon, were observed (Fig. 11.4b). Although no changes in surface morphology or color were observed after implantation in the microscope, an SEM image of the surface after implantation indicates the presence of small holes. After 10 min. of annealing at 850^0, the scattering intensity of the characteristic peak is comparable to that of the as-deposited samples (Fig. 11.4c). This implies that the strained in the diamond film generated by irradiation has been partially relieved by annealing. Usually, this may be achieved by the recombination of excess interstitials and vacancies. The presence of nickel on the

246

11. Diamond Electrodes with Functional Structures and Surfaces

diamond surface was further confirmed by X-ray photoelectron spectroscopy (XPS) spectra, which showed a clear peak at a binding energy 855 eV corresponding to Ni 2pi/2. -j-T—r-\—r I I I I — 1 ~ | — r i l l

} i—i i i—j—i i »—r-\—r—n—i—|" i i—i i J

(a)

JV(b)

. 1

" ^ '#:p---^^'**"«'^^»*' ^''•^ V

«>

«-..,p*j-^_^^^^

..^^^r^^^mM^-^^ -^ ^^ ^ . -<

(c) -J

•

«

I

'

•

'

'

I

'

'

'

•

1

r

•

'

'

> '

'

'

'

1

I

I

I

LJ—uj—I—I

I

I

I—i—t-

1000 1100 1200 1300 1400 1500 1600 1700 1800 Raman Shift / cm'^ Fig. 11.4. Raman spectra of (a) as-deposited; (b) as-implanted; and (c) annealed diamond.

247

/

(b)

C/3

d) TD -j-^

G (L) UH v^ p

(c)

1

.^

u 0

(a)

•^0

0.2 0.4 0.6 0.8 Potential / V vs. Ag/AgCl

1

Fig. 11. 5. (a) Cyclic voltammogram for 1 mM glucose at as-deposited diamond electrode in 0.2 M NaOH. (b) Background voltammogram at Ni-DIA in 0.2 M NaOH. (c) CycUc voltammogram for 1 mM glucose at Ni-DIA in 0.2 M NaOH. The potential sweep rate was 100 mV/s. Fig. 11.5a shows a cyclic voltammogram (CV) obtained for an as-deposited diamond electrode in 0.2 M NaOH solution containing 1 mM glucose. No Faradaic response was observed within the potential window. Furthermore, the background current was very low,

as

mentioned

previously

[18-20].

Ni-DIA

produced

no-peak-shaped voltammogram, which shows very low background current at less than +0.7 V vs. Ag/AgCl, in the absence of glucose (Fig. 11.5b). A large increase in the current at about +0.7 is due to the catalytic evolution of oxygen. However, in the presence of 1 mM glucose, a significant increase in the anodic peak current at +0.70 V

248

11. Diamond Electrodes with Functional Structures and Surfaces

VS. Ag/AgCl was observed, which is attributable to redox mediation by the Ni(ll)/Ni(III) couple (Fig. 11. 5c). In previous studies with Ni-modified electrodes, anodic and cathodic peaks were observed at +0.48 and +0.36 V vs. SCE, respectively, which were attributed to the Ni(II)/Ni(lll) couple [11,12,15]. The fact that we do not observe the peaks corresponding to Ni(II)/Ni(lll) in this work (Fig. 11. 5a) is probably due to the very small concentrations of Ni on the diamond surface (Ni/C = 0.1 %) as determined by XPS. However, the large catalytic current for glucose indicates the high catalytic activity of the oxidized form of Ni on the diamond. No peaks for glucose were observed at neutral pH. The voltammograms obtained in the presence of glucose were very reproducible. The observed higher peak

voltage

(+0.70

V

vs.

Ag/AgCl)

in

comparison

to

electrochemically modified Ni-diamond [15] was due to the lower electrical conductivity (15 Q cm). That is, Ni'DIA contains a low boron concentration

(lOO ppm). The presence

of a

metal

oxide/hydroxide film with two different oxidation states at the metal surface appears to be a prerequisite for the electro-oxidation of glucose [21]. In Ni-DIA, Ni(III) acts as a strong oxidant, reacting with the organic compound in a rate-limiting step by abstraction of a hydrogen atom to yield a radical. Further reaction of the radical with additional surface sites results in product formation. Thus, it has been domonstrated that the implanted metal shows promising characteristics for electrochemical sensors, while the properties of the BDD electrode, with chemical stability and low background current, etc., were also demonstrated. Fig. 11.6 shows the amperometric response of Ni-DIA for a 10-^L injection of 1 mM glucose in 0.2 mM NaOH solution, with 0.2

249

mM NaOH as the mobile phase. The operational potential of+0.54 V vs. Ag/AgCl was selected from the hydrodynamic voltammogram for these measurements. A highly reproducible response, with peak variability less than 9% was observed. The background current for Ni'DIA in Figure 11.6 is as low as 80 nA. This value is lower than that for the bulk nickel electrode, with the response for glucose also being higher for the Ni'DIA electrode. The lowest experimental detection limit was estimated to be 500 nM. The Ni-DIA electrode showed excellent stability, at least for five months with regular use, even with sonication. We have presented the advantages of the ion implantation technique to prepare

highly stable metal-modified

diamond

electrodes. Although only the electrochemical application for glucose detection was shown here, the present work offers new perspectives into functional materials derived from ion-implanted diamond. The most important advantage of ion implantation is that we can design highly stable metal-modified materials by choosing the individual target elements. We have also succeeded in controlling

the

electrodes [22].

electrochemical

properties

of

nitrogen-BDD

In that case, implanted nitrogen made the

conductive diamond insulating. Recently there has been an increasing interest in studying the potential application of diamond film, for example, in the electronics field; p- and n-type diamond films are required for these technologies [23]. For these purposes also, the ion implantation technique is thought to have great potential. Thus, further efforts to apply the ion implantation method for preparing stable composite materials will be likely to

250

11. Diamond Electrodes with Functional Structures and Surfaces

open up many possibilities in the development of new superior functional materials.

500 400

Signal current 300

Q 200

Bacl^ound current

100

0

10

20

30 40 Time / min.

50

60

70

Fig. 11.6. Amperometric response of a Ni-DIA for repetitive injections of 1 mM glucose in a FIA system. The mobile phase was 30 mM NaOH, and the applied potential was + 0.54 V vs. Ag/AgCl.

11.3. Boron-Doped Diamond Electrodes with Smoothed Surfaces We have focused on the surface modification of diamond electrodes in order to improve their electrochemical properties.

Surface

modification at the atomic level is a well-known phenomenon, in that the electrochemical properties of the electrodes are found to be quite sensitive to the chemical termination on the surface. For example, the electrochemical responses to several different redox

251

systems

for

oxygen-terminated

diamond

electrodes

and

hydrogen-terminated diamond electrodes are remarkably different [25]. As described above, hybrid electrodes, such as metal"modified diamond electrodes, have been prepared by electrochemical deposition methods or ion-implantation methods [lO] to realize novel multi-functional electrodes. Next, we focus on the effects of surface morphology. We have reported that the initial rough surface of polycrystalline BDD could be smoothed very easily by use of a radio-frequency glow discharge optical emission spectroscopy (rf-GDOES) technique [26]. Here, we examine the differences in the electrochemical properties between the rough, as-deposited surface and the smoothed surface and discuss the electrochemical properties of the ultrasmooth diamond electrodes from the point of view of a novel electrode material [24]. The initial rough, faceted, as-deposited BDD surfaces were smoothed by Ar+ ion sputtering at very low energy (50 eV). A lower background current was measured at these mirror-like modified electrodes than at the initial polycrystalline electrodes. The electrochemical responses to several redox systems also showed a morphological dependence in some cases. Polycrystalline

BDD

electrodes

were

deposited

onto

Si

substrates using a microwave plasma-assisted chemical vapor deposition system. The detailed procedures for the preparation have been described elsewhere [7]. After the diamond was deposited, it was sputtered using a GDOES instrument at an Ar pressure of 0.51 Torr by applying an rf power of 40 W at 13.56 MHz. The values of the gas pressure and the applied power relate to the plasma per se. The surface of the diamond became mirror-like in

252

11. Diamond Electrodes with Functional Structures and Surfaces

appearance. The smoothed surfaces of the polycrystalline diamond films

were

characterized

by

Raman

spectroscopy,

X-ray

photoelectron spectroscopy (XPS) and atomic force microscopy (AFM). Electrochemical measurements were carried out in a single-compartment cell. An Ag/AgCl electrode was used as the reference electrode, and a Pt wire was used as the counter electrode. Current-potential curves were recorded using a potentiostat. The electrochemical

properties

were

studied

for

both

hydrogen-terminated and oxygen-terminated electrodes. Although the

as-deposited

diamond

electrodes

were

terminated

with

hydrogen, we were able to oxidize the electrodes so that they became oxygen-terminated by employing anodic oxidation, i.e., + 3.0 V for 30 min. Fig. 11.7 shows AFM images and their corresponding Raman spectra for samples before and after sputtering. Over the area that we examined, the maximum peak-to-valley height was 1.49 jum and 267 nm, respectively for the two types of surfaces, while the average surface roughnesses Ra were 238 nm and 30 nm, respectively. The Raman spectra of both samples, i.e., before and after sputtering, exhibited sharp peaks for the sp^ carbon-related band at 1331 cm^. This shows that the diamond retained the sp3-carbon structure even after Ar+ ion-sputtering with the.

253

1000

1200 1400 1600 Raman Shift / cm"^

1800

Fig. 11.7. AFM images and Raman spectra of BDD electrode surface (a) before and (b) after Ar+ sputtering. The XPS spectrum for the sputtered BDD shows a clear Ar 2pi/2 peak at 250.6 eV, which indicates the presence of argon on the diamond surface (not shown). Because argon atoms were physically adsorbed at the surface, it showed less surface conductivity. In order to remove the argon atoms from the surface and to increase the surface conductivity, the sample was annealed at 800 °C in an H2 ambient. The Ar 2pi/2 peak disappeared, and the surface conductivity was recovered after annealing, which indicates that the surface was H-terminated. After the anodic oxidation, a sharp O Is peak and an O KLL Auger peak clearly appeared in the XPS spectrum (not shown); the calculated 0/C ratio of the 0-terminated diamond was 0.23, while the calculated

254

0/C ratio of the

11. Diamond Electrodes with Functional Structures and Surfaces

H-terminated diamond was 0.021. Therefore, it can be confirmed that the surface became 0-terminated.

6

TD CD VH

u u

Potential / V vs. Ag/AgCl Fig. 11.8. CVs for 0.1 M H2SO4 at BDD electrodes (a) before and (b) after Ar+ sputtering; potential sweep rate, 100 mVs"^. First, we measured the CV for a 0.1 M H2SO4 solution at both of the BDD electrodes before and after sputtering. The background current was lower for the smoothed electrode than it was for the initial poly crystalline electrode (Fig. 11.8). Determination by AFM gave a surface area of 1.21 cm^ for the poly crystalline electrode and 1.04 cm2 for the smoothed electrode per unit apparent area. Next, we studied the electrochemical responses for several redox couples. The morphological changes in the surface did not appear to cause a notable change, within experimental error, in the electrochemical behavior when the H-terminated surfaces were used. On the other hand, the electrochemical responses for several redox couples changed when the electrodes were 0-terminated. Fig. 11.9 shows CVs in a 0.1 M Na2S04 solution containing 1 mM

255

K3Fe(CN)6 before and after sputtering.

A 540-mV anodic-cathodic

peak separation was observed in the CV for the electrode before sputtering, and a smaller peak separation (320 mV) was obtained for the electrode after sputtering.

This fact indicates an increase

in the heterogeneous electron transfer rate constant at the electrode with the smoothed surface compared to the electrode with the rough surface. An increase in the apparent electron transfer rate constant due to the sputtering was also observed for the IrCle^^^

redox

couple.

The

results

of the

electrochemical

measurements are summarized in Table 11.1. However, we never observed any changes for the Ru(NH3)6^'^^^^ and Fe3+/2+ couples at the smoothed surfaces. As described above, it is known that the electrochemical properties of diamond electrodes are quite sensitive to the surface termination [25]. That is, a negative surface-charge density due to oxygen termination will affect the potential at the reaction plane. As a result, the negative charge of the ionized carboxyl group can act as an electrostatically repulsive site with respect to a redox species with a negative charge. When we measured the electrochemical responses for as-grown electrodes under the same conditions, including the

surface

terminated species, such differences (bold in Table l l . l ) were not observed.

This

indicates

that

the

observed

differences

of

electrochemical response must be explained in terms of the morphological dependences, as follows. When the surface is rough and has an O'terminated surface, there are more repulsive carboxyl sites for redox species with a negative charge because of the three-dimensional roughness. The roughness of the surface was

256

11. Diamond Electrodes with Functional Structures and Surfaces

decreased by sputtering, so the amount of surface oxygen decreased in parallel. Indeed, the surface roughness of the electrode before sputtering (Ra = 238 nm) was 8 times greater than it was after sputtering (Ra - 30 nm). This is consistent with the results of the electrochemical measurements. 800 1

uuu

s

•o

400

^ >% 'S c
^^-> a

200 0 -200 -400

7^

u

-600

-800 -0.8 -0.6 -0.4 -0.2 0 0.2 0.4 0.6 0.8 Potential / V vs Ag/AgCl

1

1.2

Fig. 11.9. CVs for 1 mM K3Fe(CN)6 at BDD electrodes (a) before and (b) after Ar+ sputtering. We conclude that an O'terminated smoothed surface only accelerates the apparent electron transfer rate constant for redox species with a negative charge. The present work offers new insights into how the surface morphology of poly crystalline diamond electrodes can affect the electrochemical properties. Also, diamond electrodes with a smoothed surface may not only be useful for electrochemical applications, but also for the study of basic electrochemical properties.

257

Table 11.1. Comparison of anodic-cathodic peak potential separations for several redox species at diamond electodes before and after Ar+ sputtering. AEP/mv H-termination

AEP/mvo-termination

before

after

before

after

Ru(NH3)6Cl3

140

140

206

206

Fe(CI04)2 . 6H2O

972

811

682

672

KslrCle

348

296

530

331

K4Fe(CN)6

170

169

540

320

11.4. Conclusions Several functional BDD electrodes have been investigated. system exhibited superior electrochemical properties.

Each

Efforts to

improve the electrochemical properties of BDD electrodes will open up many possibilities in the development, not only of sensing applications using electrolysis and electroanalysis, but also in the design of novel electrode materials.

References 1. K. Tsunozaki, Y. Einaga, T. N. Rao and A. Fujishima, Chem. Lett, (2002) 502. 2. M. Bond, Analyst (Cambridge, UK), 119 (1994) Rl. 3. M. Wightman, C. Amatore, R. C. Engstrom, P. D. Hale, E. W. Christensen, W. G. Kuhr and L. J. May, Neuroscience (Oxford, UKl 25 (1988) 513.

258

11. Diamond Electrodes with Functional Structures and Surfaces

4.

"Microelectrodes"

Theory

and

Applications",

ed.

by

M.

I.

Montenegro, M. A. Queiros and J. L. Daschbach, Kluwer Academic Publishers, The Netherlands (1991). 5.

K. Wittkampf, K. Cammann, M. Anrein and R. Reichelt,

Sensors

and Actuators B, 40 (1997) 79. 6.

C. Fiaccabrino, X. M. Tang, N. F. de Rooij and M. Koudelka-Hep, Sensors and Actuators B, 35-36 (1996) 247.

7.

T. Yano, E. Popa, D. A. Tryk, K. Hashimoto and A. Fujishima, J. Electrochem.

8.

Soc, 146 (1999) 1081.

B. V. Sarada, T. N. Rao, D. A. Tryk and A. Fujishima, Electrochem.

J.

Soc, 146 (1999) 1469.

9. A. Fujishima, T. N. Rao, E. Popa, B. V. Sarada, I. Yagi and D. A. Tryk, J. Electroanal.

Chem., 473 (1999) 179.

10. K. Ohnishi, Y. Einaga, H. Notsu, C. Terashima, T. N. Rao, S.-G. Park and A. Fujishima, Electrochem.

Solid-State

Lett,

5 (2002)

Dl. 11. W. Buchberger, Fresenius J. Anal

Chem., 354 (1996) 797.

12. G. Casella, E. Desimoni and T. R. I. Cataldi, Anal

Chem.

Acta.,

248(1991) 117. 13. J. M. Zadeii, J. MarioU and T. Kuwana, Anal

Chem., 63 (1991)

649. 14. F. Luo and T. Kuwana, Anal

Chem., Qe> (1994) 2775.

15. R. Uchikado, T. N. Rao, D. A. Tryk and A. Fujishima, Chem.

Lett,

(2001) 144. 16. K. Takahashi, M. Iwaki and H. Watanabe, J. Electroanal

Chem.,

396 (1995) 541. 17. B. Miller, R. Kahsh, L. C. Feldman, A. Katz, N. Moriya, K. Short and A. E. White, J. Electrochem.

Soc, 141 (1994) L41.

259

18. T. N. Rao and A. Fujishima, DiamondRelat 19. G. M. Swain and R. Ramesham, Anal

Mater., 9 (2000) 84.

Chem., 65 (1993) 345.

20. S. Alehashem, F. Chambers, J. W. Strojek, G. M. Swain and R. Ramesham, Anal

Chem,, 67 (1995) 2812.

21. M. Fleischmann, K. Korinek and D. Fletcher, J.

Electroanal.

Chem., 31 (1971) 39. 22. Y. Einaga, K. Ohnishi and A. Fujishima, unpubhshed results. 23. J. F. Prins, Phys. Rev. B, 61 (2000) 7191. 24. R. Sato, T. Kondo, K. Shimizu, K. Honda, Y. Shibayama, K. Shirahama, A. Fujishima and Y. Einaga, Chem. Lett,

32 (2003)

972. 25. I. Yagi, H. Notsu, T. Kondo, D. A. Tryk and A. Fujishima, J. Electroanal.

Chem., 473 (1999) 173.

26. K. Shimizu, Y. Einaga, K. Ohnishi, A. Fujishima, H. Habazaki, P. Skeldon and G. E. Thompson, Surf. Interface Anal., 33 (2002) 35.

260

12. Electroanalytical Applications of Highly Boron-Doped Diamond Electrode Bulusu V. Sarada, Chiaki Terashima, Tribidasari A. Ivandini, Tata N. Rao and Akira Fujishima

12.1. Introduction Electrochemical methods have been very attractive for the analysis of biologically and environmentally important chemical species compared to the other detection methods, not only because of their higher sensitivity, long-term reliability and rapidity, but also due to their high accuracy, precision and low cost [l]. In recent years, electrochemical sensors have experienced rapid growth in terms of electroactive materials. Mercury electrodes were initially used for electroanalytical determinations [2]. However, they have been found to be hazardous and not applicable for real sample analysis, including in vivo detection. Although several types of metal electrodes fulfill the requirements for electroanalysis, including biocompatibility, inertness and nontoxicity, such detection methods have not become as popular as others due to certain apparently unavoidable problems, such as metal oxide formation, resulting in electrode deactivation, with the necessity of frequent pretreatment and other procedures to reactivate the electrodes [3-5]. Glassy carbon (GC), one of the Bulusu V. Sarada e-mail: [email protected]

261

widely used electrodes for electrochemical detection, due to its relatively wide potential window and low cost, is very susceptible to contamination and fouling. Several studies on the reactivation of GC electrode surface have been carried out [6-8]. A new class of sensors with diamond-based detectors has been rapidly developing recently and has been outperforming other sensor systems. Highly boron-doped diamond electrodes, due to their unique electrochemical electrochemical

properties,

have

applications

been

used

including

for

several

electroanalysis,

electrosynthesis and electrolysis. Diamond electrodes are superior to glassy carbon electrodes and metal electrodes, particularly for electroanalysis,

due

to their

greater

stability

and

higher

sensitivity [9-12]. The extraordinarily low catalytic activity of diamond for both hydrogen and oxygen generation, resulting in a wider potential window [13-17], allows these electrodes to be used to study and detect molecules oxidizing at high potentials. Inertness toward the adsorption of reactants and products [18] and insensitivity to the presence of oxygen dissolved in aqueous solutions, both in acidic and alkaline media [19,20], are other qualities that make diamond a stable electrode for electroanalysis. The properties mentioned above also make diamond an ideal sensor for the analysis of biological samples, including blood, urine and cerebral fluids. Its very low background current [18,21], i.e., about an order less than that observed at metal and glassy carbon electrodes, makes the diamond electrode superior to other electrode materials, with enhancements in the sensitivity for the detection of several environmentally and biologically important compounds that exist at nanomolar to picomolar concentrations.

262

12. Electroanalytical Applications of Highly Boron-Doped Diamond Electrode

Changes in the surface

termination

of diamond

from

hydrogen to oxygen were found to bring about drastic changes in the

electrochemical

behavior

[22-14]. As-deposited

diamond

electrodes, whose surface is hydrogen-terminated, have shown electrochemical activity for a broad range of compounds and ionic species, whereas oxygen-terminated diamond shows selectivity (either enhancement or suppression) for the detection of specific species. The present chapter summarizes the electroanalytical applications of diamond electrodes prepared with boron doping (on the order of 0.01 mol B/mol C) by use of the CVD technique [25], with typical resistivities less than 0.1 Q cm.

12.2. Electroanalysis with As —Deposited BoronDoped Diamond Electrodes As-deposited

diamond

electrodes

are

originally

hydrogen-

terminated and are highly stable for analysis of a number of chemical species. A summary of the performance and superiority of these electrodes for the detection of several selected compounds is given below.

12.2.1. Detection of NADH NADH is a cofactor in a large number of dehydrogenase-based biosensors. However, bare glassy carbon (GC) and other electrodes deactivate rapidly during the determination of NADH due to the irreversible and strong adsorption of NAD"^, an oxidation product [26]. A disadvantage with the modified-electrodes is the influence of oxygen present in the solution. The use of the diamond

263

electrode as t h e detector h a s successfully overcome all of these difficulties for the detection of NADH. Rao et al. have reported the oxidation of NADH at BDD electrodes and also have d e m o n s t r a t e d the

sensitivity

of

an

alcohol

dehydrogenase/NADH/diamond

electrode assembly as a n ethanol sensor [27]. Cyclic voltammetric studies performed by t h e s e workers have yielded excellent cyclic voltammetric s h a p e s (oxidation p e a k at - 0 . 6 V vs. SCE), with high stability and no shift in t h e oxidation p e a k during several days of experimentation, w h e r e a s t h e surface of GC electrodes deactivated rapidly, with a shift of - 2 0 0 mV in t h e voltammetric peak within one hour (Fig. 12.1). Absence of adsorption of t h e highly polar molecule AQDS^^ w a s d e m o n s t r a t e d by Swain a n d his coworkers.

N A D H = N A D + + 2e- + H+ ^^^^ 1 ^

Fresh Glassy carbon

4 0.5 ^A Fresh

^

After 20 h

Diamond

+ 0!2 0.4 0.6 Potential ( V v s SCE)

-TJ^

Fig. 12.1. Cyclic voltammograms at GC and BDD electrodes in airsaturated 0.1 M phosphate buffer containing 50 fiM NADH at a potential sweep rate of 20 mV s"i.

264

12. Electroanalytical Applications of Highly Boron-Doped Diamond Electrode

The

sensitivity

determination

of

the

diamond

electrode

of NADH was demonstrated

for

the

by the use of

amperometry at 0.58 V vs. SCE. The Umit of detection was 10 nM (signal-to-noise S/N ratio = 3). The responses at BDD electrodes remained stable for three monthswhile the electrode was exposed to the laboratory atmosphere. The diamond electrode has also exhibited good performance as an ethanol sensor with an alcohol dehydrogenase-impregnated membrane placed in contact with the electrode and the NADH oxidation current providing a measure of the ethanol concentration.

12.2.2. Detection of biogenic amines Biogenic amines act as important chemical messengers in biological systems, and they are also present in several food products. Therefore, the detection of these biogenic amines is important for monitoring both the freshness of food and also elevated cerebral and urinary concentrations. Higher levels of polyamines are found in cancer patients [28]. Abnormalities in the concentrations of serotonin (5-hydroxytryptamine, or 5-HT), and other

biogenic

Traditionally,

amines

indicate

these

compounds

spectrofluorometric

detection

psychiatric are

methods after

disorders detected

[29]. using

chromatographic

separation. The major disadvantage with these methods is the requirement of pre-column or post-column derivatization [30]. Although electrochemical methods are economical and sensitive, not much work has been carried out, probably due to the relatively high oxidation potential for histamine (HI) and other polyamines. Swain et al. reported the detection of several polyamines

265

(ethylenediamine,

putrescine,

cadaverine,

spermine,

and

spermidine) by use of BDD electrodes at pH 10 [3l]. Well resolved oxidation peaks were observed with E1/2 values of -0.9 V vs. Ag/AgCl. A mechanism was proposed where the polyamine oxidation occurred by oxygen transfer from OH radicals produced at grain boundaries. These workers have obtained a limit of quantification of about - 1 (iM (S/N, 3) and three orders of linear dynamic range. Simultaneously, Sarada et al. have reported the detection of HI and 5-HT at pH 7, with a view of demonstrating diamond as a superior detector for real sample analysis [32]. They have clearly shown the absence of an influence of the oxygen evolution reaction for the oxidation of HI at BDD, whereas at GC, an ill-defined peak was observed due to the rapid increase in background current caused by oxygen evolution (Fig. 12.2). The experimental limit of detection for histamine was 500 nM, with an S/N of 13.8. by use of flow injection analysis (FIA). 5-HT, with an oxidation peak at 0.42 V vs. SCE, is known to deactivate electrode surfaces due to the strong adsorption of its oxidation products. However, the BDD surface has shown high inertness for the adsorption. This was demonstrated by studying cyclic voltammograms at both GC and BDD electrodes. The redox couple observed at -0.15 V (due to quinone adsorption) has shown a AEp value of ~ 35 mV at GC, indicative of a 2-electron process involving oxidation of an adsorbed species [33]. At BDD, broad and asymmetric peaks were observed with a value of -110 mV, indicating a

diffusion-

controlled process. Using FIA-ED with BDD, the experimental detection limit for 5-HT was 10 nM. These detection limits for HI

266

12. Electroanalytical Applications of Highly Boron-Doped Diamond Electrode

and 5HT are on the order of concentrations present in real samples.

Glassy caibon

W 1/

60 ~ 40 -

/ ^ /

20 ~

<

n1 Ii

a

6L\

"C

y

/Dmrnrd

0 ~:i-

-1.8

n <

.1.6

I

-12

-0.8

— i —

-0.4

i^

~

0

Potential(VvsSCE) Fig. 12.2. Linear sweep voltammograms for 100 JAM HI in 0.1 M phosphate buffer and CHCI3 at pH 7 at GC and BDD electrodes.

12.2.3. Detection of azide Sodium

azide

is

reactive

compound

that

is widely

used

commercially as a propellant in automotive airbags. The azide anion is highly toxic and can present health hazards at high levels, in the form of headaches, cytochrome oxidase inhibition and vasodilation. Usage of azide in airbags increases contamination in 267

ground water. The exposure limits are set to 0.1 ppm for gaseous HNs and

to 0.3 ppb for

chromatography,

ion

solid

NaNs [34]. Although

chromatography

and

gas

electrophoresis

methods can detect azide at quantifiable levels, better sensitivity is required for reproducibility in the low ppb range. Xu et al. reported the detection of azide at diamond electrodes using cyclic voltammetry and flow injection analysis^^ At ~ 1 V, where the azide oxidation peak appeared, the background current for diamond was very low. Therefore, signal-to-background ratios were about 50 times higher than those observed for GC electrodes. The detection limit obtained with the diamond electrode was 8 nM (0.3 ppb) for S/N - 3, with a linear dynamic range of 5 orders of magnitude. The response of the diamond electrode was stable for periods up to 12 h. Thus, diamond electrodes outperformed GC in terms of linear dynamic range, detection limit and response stability for azide detection.

12.2.4. Detection of caffeine and theophylline Caffeine and theophylline are methylxanthine derivatives that are widely distributed in plant products and beverages. Theophylline and caffeine have been widely used for the treatment of asthmatic manifestations, neonatal apnea and bronchial spasms. However, these compounds produce the biological effect of dieresis, and excessive intake leads to many undesired side effects, with symptoms including tremors, excessively fast heartbeat and gastrointestinal difficulties [35]. Therefore, it is very important to determine accurately the content of these alkaloids in foods and pharmaceutical preparations. 268

12. Electroanalytical

Applications

of Highly Boron-Doped

Diamond

Electrode

Because of the complex matrices of beverages and serum samples, the determination of caffeine

and theophylline

is

typically carried out by chromatographic separation methods. Caffeine and theophylline have usually been detected by gas chromatography and HPLC equipped with a UV detector. GC electrodes have been used for amperometric detection?* however, due to the

high potentials

for oxidation

of caffeine

and

theophylline, it is difficult to detect these compounds sensitively. BDD is expected to be an excellent choice for the determination of these alkaloids. The Fujishima group has studied the detection of these compounds using BDD after chromatographic separation in acidic media. An initial study on the mechanism and voltammetric behavior was reported by Spataru et al. [36]. As'deposited diamond electrodes were used for the amperometric detection of caffeine and theophylline after these compounds were subjected to HPLC separation. The detection limits for S/N = 3 were 225 nM and 82 nM for caffeine and theophylline. The advantages of diamond electrodes were demonstrated by using the detector for the analysis of real blood samples by collecting rabbit serum samples at time intervals after injecting theophylline. The operating potential of 1.5 V vs. Ag/AgCl was selected on the basis of the hydrodynamic voltammogram. The BDD electrode has given reproducible results for these samples, with no adsorption of proteins on the surface, and the negligible variation in the results for standard sample before and after the analysis of serum samples indicated the absence of adsorption of other blood components .

269

12.2.5. Chromatographic detection of tricyclic a n t i d e p r e s s a n t (TCA) drugs TCAs are widely used for the treatment of psychiatric disorders such as depression. These drugs block the reuptake of the neurotransmitters serotonin and norepinephrine in the central nervous system [37]. Continuous monitoring of these drugs is essential during the treatment of psychiatric patients, because high plasma concentrations of these drugs cause adverse effects, including cardiovascular complications, convulsions and coma, whereas low concentrations have no therapeutic effect. BDD electrodes have been examined for the detection of several TCAs (imipramine,

desiramine,

clomipramine,

amitryptiline,

nortryptiline and doxepin) [38]. A limit of detection as low as 0.5 nM was achieved for clomipraine, with detection limits on the order of nM for the other drugs. A linear dynamic range of three orders of magnitude was obtained. During the analysis of the drugs, the stability of the background current was examined for BDD electrodes and was compared with those observed for commercial GC electrodes. As already mentioned in the previous section, BDD stabilized very quickly and maintained a highly stable background current for --700 minutes, while GC required about one hour of initial stabilization time, and the background current

continuously

changed

during the several hours of

observation. In order to demonstrate the practical use of BDD electrodes, the performance of these electrodes was examined during the detection of these drugs in real plasma samples. No adsorption of blood components onto the surface of the electrode was observed.

270

12. Electroanalytical Applications of Highly Boron-Doped Diamond Electrode

12.2.6. Detection of c a r b a m a t e pesticides N-methylcarbamate pesticides comprise an important class of pesticides widely used as insecticides for the protection of a large variety of crops. The presence of their residues in foodstuffs poses a potential hazard to consumers due to their toxicity. Therefore, the maximum residue levels (MRLs) of pesticides in food samples are regulated by government agencies of most countries. For example, the MRLs for carbaryl, carbofuron and methiocarb are in the 0.2 to 1.0 ppm range [39], depending on the particular pesticide/commodity combination.

Numerous HPLC methods

have been developed to analyze carbamates, and only in some instances

are

electrochemical

methods

used.

However,

electrochemical methods can offer such advantages as greater sensitivity

and

derivatization

selectivity

procedures

without as in

the

need

fluorescence

for

various

measurements.

Nehring et al. used a Kelgraf (Kel-F resin plus graphite) composite electrode for pesticide detection at high oxidation potentials, while Diaz et al. hydrolyzed carbamate pesticides to their

corresponding phenolic

derivatives,

which

were

then

detected electrochemically at relatively low potentials, where the interference of oxygen evolution reaction is minimized. Diamond is unique for the detection of carbamate pesticides as well as their phenolic derivatives to achieve high sensitivity and long-term stability [40].

While the wide electrochemical

potential window is advantageous for the direct detection of carbamate

pesticides

at

high

oxidation potential,

the

low

adsorption behavior of diamond improves the stability of the hydrolyzed phenolic derivatives of these pesticides. The phenolic 271

derivative produced by the alkaline hydrolysis of carbaryl oxidizes at a much lower potential than the underivatized carbaryl. The lower detection potential leads to improved sensitivity and less interference from other components of the sample. We have obtained a chromatogram with well-separated peaks for a 10-nM mixture of three carbamate pesticides.

The fact that a well-

defined chromatogram was obtained for a 10-nM concentration of each pesticide in the standard mixture indicates the potential utility of diamond electrode for pesticide analysis. Furthermore, diamond is very stable for the detection of phenols [12].

12.2.7. Detection of DNA The detection of DNA is very important in analyzing genetic information and in detecting genetic disorders. Chromatographic or electrophoretic separation methods coupled with UV-absorption or fluorescence detectors are generally used for the detection of nucleic

acids

[41]. Although

electrochemical

methods

are

relatively simple and promising, these methods are not preferred due to the lack of an electrode material that can give sensitive, reproducible results. The reason is the high oxidation potentials for the purine and pyrimidine bases of nucleic acids, where oxygen evolution current interferes. Diamond was found to be the best suited material in this regard due to its wider potential window in comparison to other electrodes. Prado et al. have reported the voltammetric detection of several nucleic acids at polished commercial diamond electrodes (De Beers) [42]. Although they found the basic features for DNA in the voltammogram, due to presence of the oxygen-terminated surface, the voltammetric 272

12. Electroanalytical Applications of Highly Boron-Doped Diamond Electrode

peaks were not sufficiently well defined to perform a detailed study. In a subsequent report, the behavior of purine and pyrimidine bases was studied at hydrogen-terminated and oxygenterminated diamond electrodes. DNA is negatively charged due to the phosphate groups present in the sugar backbone. There is electrostatic

repulsion

of these

molecules

at

the

oxygen-

terminated diamond due to the presence of carbon-oxygen dipoles. However, at the hydrogen-terminated surface, the positive dipolar field

created

attracts DNA, facilitating

the

electrochemical

reaction [24]. Superior results were observed compared to those for GC electrodes. Amperometric detection of nucleic acids was performed by flow injection analysis using diamond electrodes. The electrodes exhibited fast and sensitive detection of trace nucleic acids without any derivatization steps. Excellent stability, with low detection limits (11.66, 2.27 and 1.56 \ig L'l for ss-DNA, ds-DNA and t-RNA, respectively), was obtained. High peak current stability and baseline stability indicated the absence of deactivation of the diamond electrode surface during detection.

12.2.8. Detection of oxalic acid High oxalate concentrations in human urine and blood cause primary and secondary hyperoxaluria, chronic renal failure and formation of nephrocalcinosis. Therefore, precise and sensitive methods are required to detect oxalate in urine or blood, and in food for quality analysis. Measurement of oxalate in biological matrices requires separation methods due to the presence of other interfering species. Direct electrochemical detection methods have been proven to be simple and economical for the detection of

273

oxalate. Although glassy carbon electrodes are not highly active for the detection of oxalic acid, chemically-modified and metalmodified carbon electrodes have shown catalytic properties for oxidation

of

deactivation

oxalate,

increasing

is a major

the

disadvantage

sensitivity. with these

However, catalytic

electrodes. The uniqueness of hydrogen-terminated highly borondoped diamond electrodes for the oxidation of oxalic acid and its amperometric detection after chromatographic separation has been reported by the Fujishima group. Cyclic voltammograms for 0.1 M phosphate buffer (pH 7.1) in the presence and absence of 500 \xM oxalate, at as-deposited, oxidized and GC electrodes are shown in Fig. 3. In the presence of oxalate, a well-defined voltammogram was observed at -1.0 V vs. SCE for as-deposited i.e., hydrogen terminated diamond (Fig. 3a), whereas at oxidized diamond, the electrode response for oxalate totally disappeared (Fig. 12.3b), indicating the necessity of the hydrogen-terminated surface for the oxidation of oxalate. As shown in Fig. 12.3c, GC shows very low electroactivity towards oxalate oxidation. Further studies were carried out by use of Hterminated diamond electrodes. From the chromatogram, the elution times for oxalate (OA), ascorbic acid (AA) and uric acid were found to be at 1.57, 1.82 and 3.3 min, respectively. A calibration plot was obtained for OA with a linear range between 100 nM and 50 ^iM with a correlation coefficient of 0.999. The limit of detection with S/N = 3 was 46 nM. The injection loop was 20 |xL. It is interesting to note that the detection limit observed using the present method was much better than those reported previously by use of other analytical 274

12. Electroanalytical Applications of Highly Boron-Doped Diamond Electrode

techniques. The

relative

standard

deviation

(RSD) for

30

injections of 500 nM OA was below 5%, indicating the stability of the electrode surface and the absence of adsorption of reaction products on the electrode surface.

50 ^lA

As-deposited BDD

1

Oxidized BDD

;3

Freshly polished GC 10 ^lA

i =L 0.4 0.8 1.2 Potential (V vs. SCE)

1.6

Fig. 12.3. CycUc voltammograms for 500 \JM oxalate in 0.1 M phosphate buffer at (a) as-deposited diamond, (b), oxidized diamond and (c) GC electrodes. Background voltammograms are also shown. The sweep rate was 100 mV s"i. To demonstrate the usefulness and simplicity of the present method, OA determination was performed in human

urine

samples collected after the subject consumed spinach and vitamin C drink. The samples were collected and diluted by a factor of 100

275

after filtering with a 0.2-jAm pore filter paper prior to injection. The amount of oxalate in urine samples was found to be -450 \xM. In order to prove the necessity of H-terminated diamond for the detection of oxalate, the oxygen-terminated surface was treated by use of hydrogen-plasma in order to achieve Hterminated

surface.

The

voltammetric

disappeared

at the 0-terminated

surface,

responses, reappeared

which after

reconverting the surface to H-termination.

12.3. Electroanalysis at Oxidized Diamond Electrodes The hydrogen-terminated diamond surface can be converted into an oxygen-terminated surface by oxidizing the surface with the application of highly positive potentials or by use of oxygen plasma treatment. These oxidized electrodes can selectively detect certain compounds and are highly stable and suitable for the detection of several other compounds. A report on these results is summarized below. 1 2 . 3 . 1 . Selective detection of uric acid (UA) in the presence of ascorbic acid (AA) Uric acid is the principal final product of purine metabolism in the human body. Abnormal levels of uric acid are symptoms of several diseases, including gout and hyperurichemia. In a healthy human being, normal concentrations of uric acid in the blood are 120-450 (iM and in urine are about 2 mM. Ascorbic acid is the major interfering electroactive species, which oxidizes at a potential 276

12, Electro analytical Applications of Highly Boron-Doped Diamond Electrode

similar to that of uric acid. Several electrochemical techniques based on modified electrodes offer selective, economical and rapid determination of UA acid compared to other techniques, However, these electrodes require surface renewal after each measurement due to the adsorption phenomena. Popa et al. have reported the use of anodically oxidized diamond electrodes for the selective determination

of UA in

the

presence

of AA by use

of

chroamperometry [43]. The advantage of the present method is the selective determination of UA in the presence of AA. Diamond electrodes were oxidized by applying a potential of 2.6 V vs. SCE for 75 minutes in 0.1 M KOH. At as-deposited diamond, the oxidation potentials for AA and UA were similar. The

detection

of

UA

in

the

presence

of AA

required

chromatographic separation. The oxidation potential for AA shifted to more positive values as compared to that of UA at the oxidized diamond electrode, thus minimizing interference from AA during the detection of UA in acidic media. Fig. 12.4 shows the cyclic voltammogram for a mixture of UA and AA at as-deposited and oxidized BDD electrodes. UA was detected with simple chronoamperometry

with

no

requirement

of

complicated

procedures. These authors have successfully determined UA in diluted urine samples (by a factor of lOOOO), and the electrodes have shown stability for about 3 months.

277

10 /\

<

6

51

/

U

0.1

0.4

0.7 1 1.3 Potential (V vs. SCE)

1.6

Fig. 12. 4. Cyclic voltammograms for a mixture of 0.2 mM UA and 1 mM AA at as-deposited(dashed line) and oxidized (solid line) diamond electrodes in 0.1 M HCIO4 solution.

12.3.2. Detection of disulfides at anodically oxidized diamond Thiols and disulfides are widely distributed in living cells, and the tripeptide glutathione exists both as gutathione (GSH) and oxidized glutathione (GSSG). Under oxidative stress, GSH is oxidized to GSSG in living cells. Thus, the ratio of these two compounds serves as an indicator of oxidative stress. The detection methods should exhibit high sensitivity because of the low availability of GSSG in plasma. Electrochemical methods are versatile compared to other methods due to the reasons already mentioned. The oxidized form of glutathione is a positively

278

12. Electroanalytical Applications of Highly Boron-Doped Diamond Electrode

charged molecule and oxidizes at higher potentials compared to thiols. A detailed study on the oxidation reactions of cysteine, a thiol, and their mechanisms was reported by Spataru et al [44]. The oxidized surface of diamond was preferred to the as-deposited BDD surface to study GSSG, because the cyclic voltammograms obtained have shown two well-defined peaks (at 1.3 and 1.75 V ) compared to the ill-defined single peak observed at as-deposited BDD (Fig. 12.5). Terashima et al. have performed a detailed set of experiments and reported the limits of detection and the reaction products of both GSH and GSSG by chromatography

and

electrolysis, respectively [45]. The detection limits were 0.02 and 0.034 pmol for GSH and GSSG, respectively. The variation in dayto-day response was found to be below 3%. Another impressive advantage

with the

anodically

reactivation of the electrode

oxidized electrode was

surface

the

in cases of potential

deactivation after several uses.

279

fS

/

S

To.2mAcm-2

/

<

s

/

^ i

(a)

y

QJ

^

/ To.2inAcm-2

k«

_

J

/ w

/

/A /

^i^

3

/ \J

U

(b) y 1

• . . • 1 . • •. 1 . • • • 1 J l

0.0 0.5 1.0 1.5 2.0

Potential / V vs. SCE Fig. 12.5. Linear sweep voltammograms for 1 mM GSSG in acidic buffer (pH 2.2) at (a) as-deposited and (b) anodized diamond electrodes 12.3.3. D e t e c t i o n of chlorophenols Phenol and substituted phenols such as chlorinated phenols (CPs) and related aromatic compounds are known to be common components of industrial waste. Most of them are carcinogenic, and their presence can be harmful to humans in general. Therefore, phenols are considered to be priority pollutants by the United States Environmental Protection Agency (USEPA). CPs are present also in flue gas from waste incinerators and are noted to be the precursors of polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/F). 280

12. Electroanalytical Applications of Highly Boron-Doped Diamond Electrode

Although organic compounds related to phenols in general can be oxidized at numerous electrode materials, the oxidation of phenolic compounds at solid electrodes produces phenoxy radicals, which couple to form a passivating polymeric film on the surfaces of the electrodes. Electrochemical anodic pretreatment is known to change the diamond surface from hydrophobic to hydrophilic by introducing oxygen functional groups onto the surface. Anodically pretreated diamond is suitable for the selective detection of dopamine and uric acid in the presence of a large amount of ascorbic acid, because the oxidation potential of ascorbic acid shifts to more positive potentials relative to the dopamine or uric acid oxidation potentials at the pretreated electrode.

In the

present study, the advantages of anodically treated diamond for the oxidation of chlorophenols are discussed. A positive shift

in the voltammetric wave for 2, 4-

dichlorophenol was observed on the anodized diamond compared to that at the as-deposited surface [46]. However, relatively stable voltammograms were obtained at anodized diamond, in contrast to those at the as-deposited diamond and GC electrodes, which were deactivated completely by the fifth cycle. The outstanding stability of the diamond electrode was further demonstrated by the electrode response in flow injection analysis. The FIA results for 100 injections of 5 mM 2,4-dichlorophenol at the GC electrode and the anodized diamond electrode were examined at detection potentials of 1.2 V and 1.4 V, respectively. A highly stable detection peak was observed at the anodized diamond; the RSD of the peak heights was 2.3 %, and the decrement was 10 %. In contrast, repetitive injections of concentrated 2,4-dichlorophenol

281

at GC resulted in a 70 % reduction of the peak height. After this durability experiment, a passivating layer was observed on the GC surface. These results clearly demonstrate the

outstanding

stability of the anodized diamond electrode for CP detection. The unique stability of the diamond electrode is not clear yet; however, polar oxygen functional groups generated by oxidation probably repel the phenoxy radicals generated during phenol oxidation. A chromatogram obtained for a standard sample mixture with each CP at the 1 ng ml i (ppb) level (not shown) was welldefined, with a well-behaved baseline. This chromatogram was obtained by means of the column-switching technique, which allows pre-concentration of the samples. column-switching

technique,

which

By coupling with the enabled

on-line

pre-

concentration (by a factor of 50), the detection limit for 2,4dichlorophenol was lowered to 0.4 nM (S/N = 3) from 20 nM. The detection limits were estimated to be in the range of 0.038 ng mL'i (0.23 nM) for 2,6-DCP to 0.361 ng mL i (2.23 nM) for 3,5-DCP (S/N = 3).

12.4. Summary Conductive boron-doped diamond is gaining popularity as a unique electrode material for electroanalytical applications. While its wide electrochemical potential window allows the detection of compounds oxidizing at high potentials, its resistance to the adsorption of chemical species on its surface allows the stable electrochemical detection of a number of different chemical species. Electrochemical pre-treatment of the electrode surface to convert 282

12. Electroanalytical Applications of Highly Boron-Doped Diamond Electrode

it to oxygen t e r m i n a t i o n w a s also found to play a key role in the achievement of b e t t e r stability in t h e electroanalysis of such compounds as uric acid, dopamine a n d chlorophenol. In contrast, t h e hydrogen-terminated

surface

of diamond is the

primary

r e q u i r e m e n t for t h e analysis of certain compounds such as oxalic acid. Several compounds were detected with high stability with no influence of t h e surface t e r m i n a t i o n of of diamond on the oxidation potential and stability. This w a s examined by slight oxidation of t h e surface in order to show t h e stability of diamond electrode for long-term use.

References 1.

J. L. Anderson, L. A. Caury Jr. and J. Leddy, Anal

Chem., 70

(1998) 519R. 2.

I. M. Kolthoff, W. Stricks and N. Tanaka, J. Am. Chem. Soc, 11 (1955) 5215.

3.

D. G. Davis and E. Bianco, Analyst

(Cambridge,

UK), 119 (1994)

309. 4.

J. A. Raynaud, B. Maltoy and P. Canessan, J.

Electroanal.

Chem., 114(1980) 195. 5.

H. Miyama, Y. Nosaka and T. Fukushima, J. Electrochem.

Soc.

133 (1986) 336. 6. 7.

J. F. Rustling, Anal

Chem., 56 (1984) 575.

D. T. Fagan, I. F. Hu and T. Kuwana, Anal Chem., 57 (1985) 2759.

8.

I. F. Hu, D. H. Karweik and T. Kuwana, J. Electroanal.

Chem.,

283

188 (1985) 59. 9.

E. Popa, H. Notsu, T. Miwa, D. A. Tryk and A. Fujishima, Electrochem. Solid-State Lett, 2 (1999) 49.

10.

B. V. Sarada, T. N. Rao, D. A. Tryk and A. Fujishima,

J.

Electrochem. Soc. 146 (1999) 1469. 11. S. JoUey, M. Koppang, T. Jackson and G. M. Swain, Anal.

Chem.,

69 (1997) 4099. 12.

J. Xu and G. M. Swain, Anal. Chem., 70 (1998) 1502.

13.

N. Vinokur, B. Miller, Y. Avyigal and R. Kalish, J.

Electrochem.

Soc, 143 (1996) L238. 14

H. B. Martin, A. Argotia, U. Landau, A. B. Anderson and J. C. Angus, J. Electrochem. Soc, 143 (1996) L133.

15.

G. M. Swain, A. B. Anderson and J. C. Angus, MRS Bull,

23

(1998) 56. 16.

Z. Y. Wu, T. Yano, D. A. Tryk, K. Hashimoto and A. Fujishima, Chem. Lett, (1998) 503.

17.

F. Bouamrane, A. Tadjeddine, J. E. Buttler, R. Tenne and C. Levy Clement, J. Electrochem. Soc, 405 (1996) 95.

18.

J. Xu, Q. Chen and G. M. Swam, Anal. Chem., 70 (1998) 3146.

19.

T. Yano, D. A. Tryk, K. Hashimoto and A. Fujishima,

J.

Electrochem. Soc, 145 (1998) 1870. 20.

T. Yano, E. Popa, D. A. Tryk, K. Hashimoto and A. Fujishima, J. Electrochem. Soc, 146 (1999) 1081.

21.

R. Tenne and C. Levy-Clement, Isr. J. Chem., 38 (1998) 57.

22.

I. Yagi, H. Notsu, T. Kondo, D. A. Tryk and A. Fujishima, J. Electroanal. Chem., 473 (1999) 173.

23.

H. Notsu, I. Yagi, T. Tatsuma, D. A. Tryk and A. Fujishima, J. Electroanal. Chem., 492 (2000) 31.

284

12. Electroanalytical Applications of Highly Boron-Doped Diamond Electrode

24.

T. N. Rao, T. A. Ivandini, C. Terashima, B. V. Sarada and A. Fujishima, New Diamond Front Carbon Technoly, 13 (2003) 79

25.

T. Yano, D. A. Tryk, K. Hashimoto and A. Fujishima,

J.

Electrochem. Soc. 145 (1998) 1870. 26

J. Moiroux and P. J. Elving, J. Electroanal.

Chem., 102 (1979) 93.

27.

T. N. Rao, I. Yagi, T. Miwa, D. A. Tryk and A. Fujishima,

Anal

Chem. 71 (1999) 2506. 28.

D. H. Russel, C. C. Levy, S. C. Schimpff and I. H. Hawk, Cancer Research, 31 (l97l) 1555.

29.

B. P. Jackson, S. M. Dietz and R. M. Wightman, Anal.

Chem.,

67(1995) 1115. 30.

K. Thorre, M. Pravda, S. Sarre, G. Ebinger and Y. Michotte, Anal

31.

Technol. Life Sci., 694 (1997) 297.

M. D. Koppang, M. Witek, J. Blau and G. M. Swain Anal.

Chem.

71 (1999) 1188. 32.

B. V. Sarada, T. N. Rao, D. A. Tryk and A. Fujishima,

Anal.

Chem., 72 (2000) 1632. 33.

M. Z. Wrona and G. Dryhurst, J. Org. Chem. 52 (1987) 2817.

34.

A. Dalmia, R. F. Savinell and C. C. Liu, J. Electrochem.

Soc,

143 (1996) 1827. 35.

B. Stavric, Food Chem. Toxicol, 26 (1988) 645.

36.

N. Spataru, B. V. Sarada, D. A. Tryk and A. Fujishima, Electroanalysis,

37.

14 (2002) 721.

G. M. Greenway and S. J. L. Dolman, Analyst

(Cambridge,

UK),

124 (1999) 759. 38.

T. A. Ivandini, B. V. Sarada, C. Terashima, T. N. Rao, D. A. Tryk, H. Ishiguro, T. Kubota and A. Fujishima, J.

Electroanal

Chem., 521 (2002) 117.

285

39.

A. Abad, M. J. Moreno, R. Pelegri, M. I. Martinez, A. Saez, M. Ganon and A. Montoya, J. Chromatogr. A, 833 (1999) 3.

40.

T. N. Rao, B. H. Loo, B. V. Sarada, C. Terashima and A. Fujishima, Anal

Chem., 74 (2002) 1578.

41.

M. Preston, J. Chromatography,

42.

C. Prado, S. J. Wilkins, F. Marken and R. G. Compton, Electroanalysis,

43.

275 (1983) 178.

14 (2002) 262.

E. Popa, Y. Kubota, D. A. Tryk and A. Fujishima, Anal

Chem.,

72 (2002) 1724. 44.

N. Spataru, B. V. Sarada, E. Popa, D. A. Tryk and A. Fujishima, Anal Chew., 73 (2001) 514.

45.

C. Terashima, T. N. Rao, B. V. Sarada and A. Fujishima,

Chew.

Lett, 32 (2003) 136. 46.

C. Terashima, T. N. Rao, B. V. Sarada, D. A. Tryk and A. Fujishima, Anal

286

Chew., 74 (2002) 895.

13. Anodic Voltammetry at Conductive Diamond Electrodes and Its Analytical Applications Nicolae Spataru, Donald A. Tryk and Akira Fujishima

Cyclic voltammetry is considered to be a versatile electrochemical technique that provides the means to investigate many aspects of an electrode process. Thus, a detailed study of the voltammetric behavior usually affords the best chance of obtaining reliable data concerning the mechanism of the reaction system of interest, the corresponding

kinetic

parameters

and

the

nature

of

the

intermediates involved in the overall process. This technique has been also described as a valuable tool to be employed for the evaluation of an electrode surface.

Because of its relative

experimental simplicity, cyclic voltammetry is perhaps the most popular electrochemical method, and it is usually the

first

experiment

any

to

be

performed

electrochemically active species.

when

dealing

with

Furthermore, voltammetry is

becoming routinely used in electroanalysis, because, in many cases, it has been demonstrated to be a promising approach both for qualitative and quantitative analytical determinations. The working electrode plays a prominent part in the voltammetric experiments, and, in order to obtain pertinent information, stable, conductive and chemically inert electrode Nicolae Spataru e-mail: [email protected] 287

materials are required. Several different materials are currently employed as working electrodes, including mercury, platinum, gold, silver and carbon. Nevertheless, the selection of the most appropriate electrode to be used for a particular voltammetric investigation is not always as obvious as might be hoped, because the limitations of each material are to be taken into account. Thus, besides its toxicity, mercury cannot be used

at

potentials more positive than -0.3 to +0.4 V versus the saturated calomel electrode (SCE) because of the ease with which Hg is oxidized.

At relatively high anodic potentials, noble metal

electrodes are also subject to a loss of activity as a result of surface oxide formation [1-3] and adsorption of partially oxidized reactant molecules [4,5]. Glassy carbon (GC) is normally deactivated after a long-time exposure to electrolyte solutions, although many methods for its reactivation have been suggested (see ref. 6 and references therein). The use of carbon (or graphite) electrodes in the anodic voltammetry is also rendered difficult by interference from oxidation background currents that are not precisely reproducible. Conductive diamond represents an electrode material that has attracted great interest, especially in electroanalysis, due to its outstanding electrochemical features- wide potential window in aqueous solutions [7], low background current [8,9], long-term stability of the response [10,11], low sensitivity to dissolved oxygen [12], and inertness to adsorption [13].

Apart from

electroanalytical purposes, the use of conductive diamond as an electrode material for many other electrochemical applications is well substantiated.

288

From the

standpoint

of voltammetric

13. Anodic Voltammetry at Conductive Diamond Electrodes and Its Analytical Applications

investigations, the extended potential window on the anodic side is perhaps the most useful feature of diamond electrodes, because it provides access to information that is not available with other electrodes. The present chapter aims at summarizing some of the main voltammetric applications of boron-doped diamond (BDD) films, deposited on Si(lOO) wafers by means of microwave plasmaassisted chemical vapor deposition [9].

In order to put these

results into better perspective, the advantages of using BDD for anodic voltammetry are correlated with the unique properties of this electrode material.

In addition, analytical applications of

anodic voltammetry at conductive diamond electrodes are also discussed.

13.1. Wide Potential Window in Aqueous Solutions It is well stated that the hydrogen-terminated surface of as-grown diamond is not favorable for adsorption. This is why conductive diamond electrodes exhibit very high overpotentials for hydrogen, oxygen and halide evolution, leading to a wide potential window in which the background current is very low [14-16]. Although the width of this window is dependent to some extent on the quality of the film (see ref. 17 and references therein), potential values as high as ca, +2.15 V {vs, SCE) are usually required for oxygen evolution to commence on diamond electrodes. particularly

useful

for

studying

This feature is

electrochemical

processes

289

occurring at high anodic potentials, because on the cathodic side, mercury is already known for its extended potential window. Most

of the

voltammetric

investigations

performed

at

diamond electrodes have been devoted to the study of the anodic behavior of organic compounds that oxidize at high potential, including phenols [18] and chlorophenols [19], benzoic acid [20], and carbamate pesticides [21].

Anodic voltammetry at BDD

electrodes was also found to be a valuable tool for the elucidation of the mechanism of in situ generation of powerful oxidants, such as ferrate [22] or hydroxyl radicals [23,24].

In some cases,

although not requiring very high potentials, the investigated process could be hindered by the presence of an oxide film covering the surface of a noble metal electrode such as platinum. For the study of these processes, the use of diamond is also a suitable approach, as illustrated by the work of Swain and coworkers concerning the oxidation of the azide anion [25]. BDD electrodes were used to examine the electrochemical oxidation of xanthine and its naturally occurring N-methyl derivatives, theophylline, theobromine and caffeine [26].

The

voltammetric studies showed that the mechanism of the overall reaction is similar to that of the oxidation of purine derivatives at the pyrolytic graphite electrode. It was observed that, for all of the investigated compounds, acidic media (pH < 3.0) guarantee voltammetric

responses

well

suited

both

investigations and for analytical applications.

for

mechanistic

At low pH, the

main advantages are- high reproducibility of the voltammograms, relatively high peak current, relatively little interference from background current, and freedom from extraneous peaks due to

290

13. Anodic Voltammetry at Conductive Diamond Electrodes and Its Analytical Applications

possible adsorption.

Nevertheless, it should be noted that

decreasing pH also results in a positive shift of the peak potential; depending on the particular compound and on its concentration, at pH values lower than ca. 2.0, the oxidation peaks lie within the potential range +1.1 to +1.3 V vs. SCE. At these relatively large positive potentials the use of carbon (or graphite) and platinum electrodes is rendered difficult by interference from oxygen evolution and oxidation background currents. As shown in Figure 13.1, conductive diamond electrodes are free of such problems. It can be observed that at BDD, the voltammogram was well defined, with a signal-to-background (S/B) ratio of 92, whereas at GC, the oxygen evolution reaction interfered with the oxidation curve, obsuring it. The S/B ratio at GC was 0.85. This value is ca. 100 times lower than that obtained at BDD.

J l JXA

1

0.0

0.4 0.8 1.2 1.6 Potential / V (SCE)

0.0

1

1

1

1

0.4 0.8 1.2 1.6 Potential / V (SCE)

Fig. 13.1. CycUc voltammograms for theophylline at (a) BDD and (b) GC electrodes. Experimental conditions- concentration, 50 [iMJ electrolyte, Britton-Robinson buffer,* sweep rate, 20 mV s'l; electrode area, 0.07 cm^. (Dashed lines represent background current.)

291

Based upon these results, a simple, rapid, reproducible and accurate voltammetric method was proposed for the determination of xanthine, theophylline, theobromine and caffeine

in the

micromolar concentration range [26]. The analytical performance characteristics of the method are comparable to those reported for the determination of xanthines by the use of chemically modified electrodes,

biosensing

voltammetry.

The

techniques

excellent

and

results

differential

obtained

for

pulse caffeine

determination in commercially available products, with very simple sample preparation, involving only dilution in electrolyte, demonstrates the practical analytical utility of the method. The high overvoltage that diamond electrodes exhibit for oxygen evolution is also an advantage when studying the electrochemical formation of metal oxides. processes

are

of

great

importance

for

Many of these various

types

of

electrochemical devices, such as water electrolyzers, fuel cells, secondary metal / air batteries and metal electrowinning cells. In this respect, the oxidation of Co(II) at BDD electrode was investigated in alkaline media by use of anodic voltammetry [27]. It was found that this reaction takes place by a mechanism similar to that of cobalt metal oxidation, and Fig. 13.2 illustrates these results.

Thus, it can be observed that, within the

investigated potential range (O.O to 1.9 V), the voltammogram recorded after Co(II) addition (curve 2 in Fig. 13.2) shows a small step (peak I) at ca. 0.85 V, followed by a well defined peak (labeled II) at 1.07 V. In addition to the above two peaks, another anodic peak (peak III) was observed at 1.68 V, just prior to strong oxygen evolution. In agreement to previously reported data [28], peaks I,

292

13. Anodic Voltammetry at Conductive Diamond Electrodes and Its Analytical Applications

II and III were ascribed to the formation of C03O4, CoOH and C0O2, respectively. The shape of the voltammetric curve indicates that, at potential values above ca. 1.25 V, the anodic process responsible for the appearance of peak III occurs together with oxygen evolution. Because at this potential, oxygen evolution at BDD is insignificant (see curve 1 in Fig. 13.2), it is reasonable to assume that this process is strongly enhanced by the presence of cobalt oxides at the electrode surface, since these compounds are known to have excellent electrocatalytic properties.

The above

findings show that, at the BDD electrode, the voltammetric behavior of Co(II) is significantly different from that observed at noble metal electrodes. Thus, for Au or Pt electrodes, under very similar conditions, cyclic voltammetric measurements showed that prior formation of the Co(II)-glycine complex was a prerequisite for Co(II) oxidation [29].

0.0

0.4 0.8 1.2 1.6 Potential/V(SCE)

2.0

Fig. 13.2. Cyclic voltammograms recorded at a sweep rate of 50 mV s'l in 0.5 M KHCO3 solution. Co(II) concentrations were: l) 0 mM, 2) 0.5 mM; electrode area 0.5 cm^.

293

It is worth noting t h a t t h e extended potential window on t h e anodic side of diamond electrodes allows in some cases the optimization of t h e voltammetric response, which could be of importance for electroanalysis. In t h i s respect, a good example is provided by the investigation of t h e electrochemical oxidation of aniline a t BDD electrodes [30].

b)pH12.0 <240

2

/ 1 •

^160 g 80 0 0.0 0.8 1.6 Potential/V(SCE)

0.0 0.8 1.6 Potential/V(SCE)

Fig. 13.3. CycUc voltammograms in Britton-Robinson buffer at (a) pH 5.2 and (b) pH 12.0. Experimental conditions- aniline concentration, 1 mM; sweep rate, 100 mV s"i; electrode area 1 cm^. (Numbers indicate first, second and t h i r d sweeps.) T h u s , Fig. 13.3 shows voltammetric curves recorded for t h r e e consecutive r u n s a t p H 5.2 (Fig. 13.3a) a n d p H 12.0 (Fig. 13.3b). It can be observed t h a t , d u r i n g t h e first r u n a t p H 5.2, t h e v o l t a m m o g r a m exhibits two well-shaped, well-separated p e a k s . Nevertheless, it a p p e a r s t h a t , a t this p H value, aniline oxidation r e s u l t s in blocking t h e electrode surface with reaction products, as indicated by curves 2 a n d 3 in Fig. 13.3a.

A different type of

behavior w a s found for aniline oxidation at BDD electrodes in strongly alkaline media.

294

As shown in Fig. 13.3b, u n d e r these

13. Anodic Voltammetry at Conductive Diamond Electrodes and Its Analytical Applications

experimental conditions, the voltammetric response was highly reproducible, and no fouling of the electrodes was observed during consecutive runs.

These results are noteworthy, because they

indicate the possibility of using BDD electrodes for accurate voltammetric

determination

of aniline.

Furthermore,

the

relatively high peak current of the anodic peak located at ca. 1.6 V (Fig. 13.3b) could ensure better sensitivity of the method, which is obviously not available with other electrode materials.

13.2. Inertness to Adsorption Another interesting feature of diamond is its inertness with respect to numerous chemical species. This property is very much in contrast

to the

GC electrode, in which case

frequent

pretreatments are required in order to renew the surface, because the electrode often fouls during voltammetric experiments. The most likely explanation for this behavior is provided by a detailed study reported by Swain and coworkers [13] concerning the adsorption of quinones on various pretreated carbon, highly oriented pyrolytic graphite (HOPG), and diamond surfaces.

By

observing differences between GC and BDD electrodes, they attributed the inertness of the diamond surface to hydrogen termination. This point was further supported by these authors by intentional hydrogenation of the GC surface in order to obtain diffusion-controlled voltammograms. In the case of GC electrodes, polar oxygen functional groups were believed to be responsible for the strong adsorption on the surface.

295

The low susceptibility to adsorption exhibited by diamond electrodes h a s a t t r a c t e d great interest in studying electrochemical processes that, due to t h e adsorption of r e a c t a n t s or reaction products, are more difficult to investigate by use of other electrode materials.

T h u s , several reports have appeared concerning the

voltammetric behavior of m a n y organic compounds, including dopamine and reduced nicotinamide adenine dinucleotide [31], histamine

and

serotonin

[32],

glutathione

and

glutathione

disulfide [33], a n d homocysteine [34].

a)BDD

b)GC

t

s

t

3 ^

b 3

o ^ 1

_U..

I

>

t2^A

...i

0.0 0.4 0.8 1.2 Potential / V (SCE)

0.0 0.4 0.8 1.2 Potential/V (SCE)

Fig. 13.4. CycHc voltammograms for CySH oxidation- a) BDD (five consecutive sweeps); b) GC (the numbers indicate first, second and third sweeps). Experimental conditions'- electrolyte, 0.5 M KHCOaJ CySH concentration, 1 mM; sweep rate, 20 mV s'^* electrode area, 0.07 cm2. The study of the anodic oxidation of L-cysteine illustrates feature

the

advantageous

of diamond electrodes

utilization [35].

of t h i s

T h u s , Fig.

(CySH)

outstanding 13.4

shows

v o l t a m m o g r a m s recorded during consecutive r u n s in a

1-mM

CySH solution, both for BDD (Fig. 13.4a) a n d for GC (Fig. 13.4b). Before each r u n , t h e solution w a s mixed and w a s allowed to s t a n d 296

13. Anodic Voltammetry at Conductive Diamond Electrodes and Its Analytical Applications

for 1 min without mixing. It can be seen that, unlike BDD, CySH oxidation results in a deactivation of the GC electrode, the reaction being suppressed after the first run.

This behavior

indicates that the GC surface is blocked by reaction products. The variation of the sweep rate for the BDD electrodes showed that peak currents are linearly proportional to the square root of the sweep rate within the range 5 - 300 mV s'^.

This linearity,

together with the zero intercept, suggest that the current is limited by semiinfinite linear diffusion of CySH in the interfacial reaction zone and that rate-limiting adsorption steps and specific surface

interactions

can be neglected.

Voltammetric

and

polarization studies showed that the cysteine oxidation reaction takes place at the BDD electrode by a mechanism involving the dissociation of the proton from the thiol group, followed by the electrochemical oxidation of CyS' species. It was observed that, in the case of BDD, the electrochemical oxidation reaction is the ratedetermining step, whereas at GC the overall CySH oxidation reaction is controlled by the desorption of reaction products. For that reason, BDD clearly outperforms GC as an electrode material both for the investigation of the anodic oxidation of cysteine, and for its voltammetric determination. Thus, the use of BDD electrodes resulted in a wide dynamic range (O.l - 200 ^iM) and in a relatively high sensitivity (12 - 20 nA ^IMO, which could be adjusted to some extent by changing the sweep rate [35]. The limit of detection for a signal-to-noise (S/N) ratio of 3 was found to be as low as 0.9 [xM, which favorably compares to that obtained by liquid chromatography. It is worth noting that the stability for cysteine response was examined for

297

four days without observing any fouling of the diamond electrode. The zero value of the intercept for the calibration plot obtained for the voltammetric determination of CySH at BDD electrodes enabled the use of the single standard addition method. This is an advantage when compared to some methods involving mercury electrodes, in which case the calibration plot crosses the abscissa, presumably owing to the interference of trace heavy metals in the supporting electrolyte. The above findings prompted us to investigate the possibility of using BDD electrodes in order to study the voltammetric oxidation of other sulfur-containing organic compounds.

To

illustrate these results, preliminary data concerning thiourea (TU) behavior are summarized below.

Thus, it was found that at

diamond electrodes, the overall TU oxidation reaction is rather complicated and takes place via two steps* a very slow electrontransfer process yielding the corresponding free radical, followed by further oxidation of this radical, prior to its dimerization. Detailed results will be published elsewhere.

We shall note,

however, that the first step of TU oxidation requires a higher overpotential at BDD, compared to GC electrodes. This was not surprising because it was observed that, at the GC surface, this process is facilitated by adsorption. Due to the almost complete lack of adsorption, the use of BDD electrodes enabled us to study the effect of TU concentration on the voltammetric response; the results are shown in Fig. 13.5. It was found that at pH 1.8, peak I is less evident at concentrations below ca. 80 jiM. This behavior could indicate that further oxidation within the range of peak II occurs with a higher reaction rate than the first step of TU

298

13. Anodic Voltammetry at Conductive Diamond Electrodes and Its Analytical Applications

oxidation.

The height of peak I increases Knearly with TU

concentration within the range 4 (iM to 1 mM. Conversely, the peak II current is a non-linear function of the concentration and the effect of increasing TU content is less significant above ca. 0.1 mM.

From the standpoint of TU determination, the most

important concentration range lies below 1 mM, suggesting the possible use of the anodic peak I for analytical applications. The well-shaped, reproducible voltammetric response available with BDD electrodes is an advantage as compared with voltammetric methods using noble metal electrodes. This is because the use of Pt and Au electrodes for the investigation of the anodic behavior of most sulfur compounds is limited as a result of the loss of electrode

activity

produced

by

accumulation

of

sulfurous

adsorbates at the surface [36].

n )3

L

<

I 1 2 )1

£2

1

0.0

1

1

0.4 0.8 Potential/V(SCE)

1

1.2

Fig. 13.5. The effect of TU concentration on the voltammetric response of the BDD electrode at pH 1.8. TU concentrations' l) 40 \xM, 2) 60 |iM, 3) 90 ^xM. Experimental conditions- electrolyte, Britton-Robinson buffer,* sweep rate, 50 mV s"^' electrode area, 0.07 cm^

299

13.3. High Chemical and Electrochemical Stability A highly interesting characteristic of diamond electrodes is their stability

compared

to

conventional

carbon

electrodes.

Nevertheless, it was observed that, after long-time polarization in the

oxygen-evolution

region,

the

0(ls)/C(ls)

ratio

slightly

increases, suggesting the formation of surface oxides [17]. The nature of these oxides is an issue that remains to be addressed; their formation has been ascribed to the oxidation of sp^ carbon sites at the electrode surface [37]. By appropriate control of the preparation conditions, it is possible to deposit poly crystalline diamond layers with very low concentration of sp- bonds, which will result in a high electrochemical stability of the electrodes. Indeed, it was found that diamond films are stable during anodic polarization in acidic fluoride, alkaline, acidic chloride and neutral chloride media (see ref. 17 and references therein). It is worth noting that Comninellis and coworkers have reported the use of the same BDD electrode for more than 400 hours at a current density of 30 mA cm -, for the oxidation of isopropanol in sulfuric acid, without any indication of corrosion or loss of activity [38]. In order to assess the electrochemical stability of BDD electrodes, alternating current voltammetric measurements were performed in a Britton-Robinson buffer solution (pH 1.18), in the absence of electroactive species. GC electrodes were also used for comparison, and typical voltammetric patterns are shown in Fig. 13.6a and 13.6b, for BDD and GC, respectively. It can be observed that, within the investigated potential range (O.O to 1.4 V),

300

13. Anodic Voltammetry at Conductive Diamond Electrodes and Its Analytical Applications

diamond electrodes exhibit low, stable background

current,

indicating a high resistance to morphological changes and the absence of surface oxidation processes during anodic polarization. Conversely, the background current for GC was found to be much higher and less reproducible, its increase at potential values above ca. 0.8 V indicating the presence of slow processes in which oxygen-containing functional groups from the surface are involved. Explaining the shape of the ac voltamograms recorded for GC is a difficult task and is beyond the scope of the present work. We shall note, however, that the products of these surface processes are believed to be responsible for the enhancement on the reverse scan of the voltammetric peak located at ca. 0.18 V (see Fig. 13.6b). Conversely, in the case of BDD, the hump at about 0.9 V exhibited in the absence of electroactive species was ascribed to double layer effects.

t

1 o

]3.5 M-A •,

o

1 0.5 ^lA •

"-• • "

•™"

+ - •

a) BDD

^|3.5^A +,

0.0

,

,

,

1

1

0.4 0.8 1.2 Potential/V(SCE)

1

'

^ 19.0 JiA b)GC 1, , , , 0.0 0.4 0.8 1.2 Potential/V(SCE) 1

1

1

1

Fig. 13.6. Alternating current voltammograms in the absence of electroactive species for (a) BDD and (b) GC electrodes. Experimental conditions* electrolyte, Britton-Robinson buffer (pH 1.8); sweep rate, 17 mV s"i; ac frequency, 60 Hz,* amplitude, 15 mV; electrode area, 0.07 cm2.

301

The above findings enabled us to investigate the anodic behavior of nitrite and nitrogen oxides, with an eye toward analytical application [39]. It was found that, in neutral and weakly acidic media, the overall nitrite anodic oxidation process at the BDD electrode takes place by a mechanism that involves the formation of nitric dioxide, as in the case of platinum electrodes. In strongly acidic media, nitrite oxidation at conductive diamond involves two electrons and three protons. It was also found that the anodic peak for nitrite is well defined and suitable for analytical applications. Thus, at pH > ca. 4.0, the linear range of the BDD electrode response was found to extend over a concentration range of about three orders of magnitude, from 0.002 to 1 mM. Within this concentration range, at pH 5.2 and at a sweep rate of 20mVs ^ the least-squares analysis yielded from the calibration graph (/ = aC), SL sensitivity of 0.036 |iA ^iM i, with R2 = 0.9995 [39]. The results also showed that anodic voltammetry at BDD is a promising method for the determination of nitrogen oxides in air and for NO determination in solution. It is worth noting that bare carbon and platinum electrodes are not suitable for nitrite or nitrogen oxides anodic determination, due to oxide formation and passivation, respectively. Although diamond is known to have weak

adsorption

properties, in some cases during anodic voltammetry experiments, an important decrease in the electrode activity appears due to the deposition of adherent polymeric products on the Furthermore,

the electrode

remains deactivated

surface.

even

after

washing with organic solvents. However, the diamond surface can regain its initial activity by an anodic polarization in the potential

302

13. Anodic Voltammetry at Conductive Diamond Electrodes and Its Analytical Applications

region of water decomposition, as a result of the production of active intermediates (probably hydroxyl radicals) that oxidize the polymeric film on the surface [18,20]. Fig. 13.7 illustrates the advantageous use of this behavior for the investigation of the electrochemical oxidation of aniline at BDD electrodes. It can be seen that, during the first forward run in acidic media (pH 2.2), aniline oxidation results in the occurrence of two wave-shaped peaks, labeled I and II. In addition, another ill-defined peak (peak III) was observed within the potential range positive of peak II. Further potential cycles result in a gradual decrease of the voltammetric response (see curve 2 in Fig. 13.7), indicating that the electrode is blocked by polymeric reaction products. In the context of this work, the mechanism of polyaniline film formation has only a subsidiary connotation. However, it is interesting to note that, at low pH values, aniline oxidation is affected in the early stages by the low conductivity of the polyaniline films, even during the first anodic scan [30]. We observed that these films could be completely removed from the BDD surface by applying a high anodic potential (3.0 V) for a few minutes in the same working solution.

After this anodic

treatment, the cyclic voltammetric response is the same as in the first

run

(see curve 3 in Fig. 13.7).

This procedure of

electrochemical cleaning was repeated more than 20 times without observing any loss of the electrode activity. This behavior allowed us to investigate in detail the effect of the pH and that of the aniline concentration on the anodic voltammetric response, in order to establish the best experimental conditions for aniline electrochemical destruction [30].

303

0.4

0.8 1.2 1.6 Potential/V(SCE)

Fig. 13.7. Cyclic voltammograms for aniline oxidation at BDD recorded during the (l) first and (2) second sweeps and (3) after anodic cleaning of the electrode. Experimental conditions* electrolyte, Britton-Robinson buffer (pH 1.8),* aniline concentration, ImM; sweep rate, 100 mV s i,* electrode area, 1 cm^. The high chemical and electrochemical stability of poly crystalline diamond, together with t h e extremely low background current, strongly

recommend

this

material

as

a

substrate

for

electrocatalysts. T h e r m a l deposition of Ir02 and R u 0 2 at the diamond electrodes w a s reported, with possible applications for oxygen and chlorine evolution and for electrochemical reduction of carbon dioxide [40-42]. substrate

F u r t h e r m o r e , diamond is a favorable

to electrochemically

deposit

m e t a l or m e t a l

oxide

clusters discontinuously, due to its inhomogeneity of nucleation sites [27,43,44]. This allows the deposition of isolated particles or discontinuous

films,

thus

maximizing

the

utilization

of

the

catalyst by avoiding the need for thick films. In this respect, T e r a s h i m a et al. have shown t h a t cyclic voltammetry could be a highly versatile

method

for the electrodeposition of hydrous

iridium oxide on BDD for catalytic sensor applications [44]. It is noteworthy t h a t t h e use of conductive diamond as a s u b s t r a t e for 304

13. Anodic Voltammetry at Conductive Diamond Electrodes and Its Analytical Applications

electrocatalysts also results in negligible s u b s t r a t e effects.

This

behavior enabled anodic v o l t a m m e t r y to be used as a valuable tool for t h e reliable estimation of active layer characteristics, such a s the concentration of active sites, specific capacitance, and surface a r e a [27, 44]. For

analytical

applications,

the

high

chemical

and

electrochemical stability of diamond electrodes also play a key role by e n s u r i n g low a n d reproducible background c u r r e n t voltammetric d e t e r m i n a t i o n s

[26, 35, 39].

during

Furthermore,

the

excellent reproducibility of t h e p e a k c u r r e n t for voltammetric oxidation (with essentially no need for electrode p r e t r e a t m e n t or maintenance) is also to be ascribed to the high stability of t h e BDD electrodes. over

one

T h u s , d u r i n g continuous daily e x p e r i m e n t s for

month,

performances

were

negligible usually

changes observed

in

the

for the

electrochemical same

diamond

electrode.

References 1.

M.E. John, D.G. Williams and D.C. Johnson, Electroanalysis,

9

(1997) 1397. 2.

D.G. Davis and E. Bianco, Analyst

(Cambridge,

UK), 119 (1994)

309. 3.

R. Guidelh, F. Pergola and G. Rossi, Anal. Chem., 44 (1972) 745.

4.

N.L. Weinberg and H.R. Weinberg, Chem. Rev., 68 (1968) 449.

5.

I.H. Yeo and D.C. Johnson, J. Electroanal

Chem., 495 (2001)

110.

305

6.

D.T. Fagan, I.F. Hu and T. Kuwana, Anal

Chew., 57 (1985)

2759. 7.

J.W. Strojek, M.C. Granger, T. Dallas, M.W. Holtz and G.M. Swain, Anal

8.

Chew., 68 (1996) 2031.

S. JoUey, M. Koppang, T. Jackson and G.M. Swain, Anal

Chew.,

69 (1997) 4099. 9.

T. Yano, D.A. Tryk, K. Hashimoto and A. Fujishima, Electrochew.

10.

Soc, 145 (1998) 1870.

E. Popa, H. Notsu, T. Miwa, D.A. Tryk and A. Fujishima, Electrochew.

11.

J.

Solid-State

Lett, 2 (1999) 49.

T.N. Rao, I. Yagi, T. Miwa, D.A. Tryk and A. Fujishima,

Anal

Chew., 71 (1999) 2506. 12.

T. Yano, E. Popa, D.A. Tryk, K. Hashimoto and A. Fujishima, J. Electrochew.

13.

Soc, 146 (1999) 1081.

J.S. Xu, Q.Y. Chen and G.M. Swain, Anal

Chew., 70 (1998)

3146. 14.

N. Vinokur, B. Miller, Y. Avyigal and R. Kalish, J.

Electrochew.

Soc, 143 (1996) L243. 15.

Q. Chen, M.C. Granger, T.E. Lister and G.M. Swain, Electrochew.

16.

J.

Soc, 144 (1997) 3086.

N. Vinokur, B. Miller, Y. Avyigal and R. Kalish, in Proc 5^^ Int. Sywp. Diawond Materials,

edited by J. Davidson, W.D. Brown,

A. Gicquel, B.V. Spitsyn and J.C. Angus (The Electrochemical Society, Pennington, N.Y. 1995) p. 103. 17.

G.M. Swain, A.B. Anderson and J.C. Angus, MRS Bull,

23

(1998) 56. 18.

J. Iniesta, P.A. Michaud, M. Panizza, G. Cerisola, A. Aldaz and C. Comninellis, Electrochiwica

306

Acta, 46 (2001) 3573.

13. Anodic Voltammetry at Conductive Diamond Electrodes and Its Analytical Applications

19.

C. Terashima,

T.N. Rao, B.V. Sarada, D.A. Tryk and A.

Fujishima, Anal

Chem., 74 (2002) 895.

20. F. Montilla, P.A. Michaud, E. Morallon, J.L. Vazquez and C. Comninellis, Electrochimica

Acta, 47 (2002) 3509.

21. T.N. Rao, B.H. Loo, B.V. Sarada, C. Terashima and A. Fujishima, Anal

Chem., 74 (2002) 1578.

22. J. Lee, D.A. Tryk, A. Fujishima and S.-M. Park, Chew.

Cowwun.,

4 (2002) 486. 23. M. Panizza, P.A. Michaud, G. Cerisola and C. ComnineUis, Electrochem,

Cowmun., 3 (2001) 336.

24. M.A. Rodrigo, P.A. Michaud, I. Duo, M. Panizza, G. Cerisola and C. Comninelhs, J. Electrochem, 25. J.Xu and G.M. Swain, Anal 26. N.

Spiaru,

B.V.

Electroanalysis,

Soc, 148 (2001) D60.

Chew., 70 (1998) 1502.

Sarada,

D.A.

Tryk

and

A.

Fujishima,

14 (2002) 721.

27. N. Sp i a r u , C. Terashima, K. Tokuhiro, I. Sutanto, D.A. Tryk, S.M. Park and A. Fujishima, J. Electrochem.

Soc, 150 (2003) E337.

28. L.D. Burke, M.E. Lyons and O.J. Murphy, J. Electroanal

Chem.,

132 (1982) 247. 29. K. Nakaoka, M. Nakayama and K. Ogura, J. Electrochem.

Soc,

149 (2002) C159. 30. M. Mitadera, N. Sp Saru and A. Fujishima, J. Appl.

Electrochem.,

34 (2004) 249. 31. A. Fujishima, T.N. Rao, E. Popa, B.V. Sarada, I. Yagi and D.A. Tryk, J. Electroanal

Chem., 473 (1999) 179.

32. B.V. Sarada, T.N. Rao, D.A. Tryk and A. Fujishima, Anal

Chem.,

72 (2000) 1632. 33. C. Terashima,

T.N. Rao, B.V. Sarada, Y. Kubota and A.

307

Fujishima, Anal

Chem., 75 (2003) 1564.

34. O. Chailapakul, W. Siangproh, B.V. Sarada, C. Terashima, T.N. Rao, D.A. Tryk and A. Fujishima, AnaJyst (Cambridge,

UK), 127

(2002) 1164. 35. N. Spataru, B.V. Sarada, E. Popa, D.A. Tryk and A. Fujishima, Anal. Chem., 73 (2001) 514. 36. R.N. Adams, ''Electrochemistry

at Solid

Electrodes,

Marcel

Dekker, Inc., New York 1969, p. 369. 37. A. Argoitia, H.B. Martin, J.C. Angus and U. Landau, in Proc. 5^^ Int. Symp. Brown,

Diamond

A.

Materials,

Gicquel,

B.V.

edited by J. Davidson, W.D.

Spitsyn

and

J.C.

Angus

(The

Electrochemical Society, Pennington, N.Y. 1995) p.364. 38. D. Gandini, P.A. Michaud, I. Duo, E. Mahe, W. Haenni, A. Perret and

C. Comninellis,

New

Diamond

and

Frontier

Carbon

Technology, 9 (1999) 303. 39. N.

Spataru,

Electrochem.

T.N.

Rao,

D.A. Tryk

and

A. Fujishima,

J.

Soc, 148 (2001) E l 12.

40. I. Duo, P.A. Michaud, W. Haenni, A. Perret and C. Comninellis, Electrochem.

SolidState

Lett, 3 (2000) 325.

41. S. Ferro and A. Di Battisti, J. Phys. Chem. B, 106 (2002) 2249. 42. N. Spataru, K. Tokuhiro, C. Terashima, T.N. Rao and A. Fujishima, J. AppL Electrochem.,

33 (2003) 1205.

43. K. Honda, M. Yoshimura, T.N. Rao, D.A. Tryk, A. Fujishima, K. Yasui, Y. Sakamoto, K. Nishio and H. Masuda, J.

Electroanal.

Chem., 514 (2001) 35. 44. C. Terashima, T.N. Rao, B.V. Sarada, N. Spataru and A. Fujishima, J. Electroanal

308

Chem., 544 (2003) 65.

14. Diamond Electrochemical Detector in Capillary Electrophoresis Dongchan Shin, Donald A. Tryk, Akira Fujishima, and Joseph Wang

14.1. Introduction Capillary electrophoresis (CE) is a micro-column

separation

technique that can separate target analytes on the basis of differences in electrophoretic mobilities via the application of high electric fields (several hundreds of V cmO. It has been shown to be a very powerful instrumental technique, resulting in fast, highly efficient separation and analysis of complex liquid-phase mixtures [ll. Moreover, the fabrication of micro-total analysis systems (jiTAS) involving CE has recently made substantial progress [2,3]. Several detection methods have been successfully introduced to CE analysis, which are absorption and fluorescence spectroscopy, electrochemistry, and more recently, mass spectrometry. Electrochemical detection (ED) has proven to be very useful for conventional and microchip CE, in which extremely small volumes of analytes are monitored [4-6]. Although one of the most commonly used detection methods adapted for CE analysis is UVvisible absorbance detection, its sensitivity is limited due to the short light path-length of the capillary. Lower detection limits can Dongchan Shin e-mail: [email protected] 309

be obtained with the use of laser-induced fluorescence (LIF) detection. However, this requires derivatization of the analytes as well as complex and expensive instrumentation. As an attractive alternative to optical CE detection, electrochemical detection (e.g., amperometric) not only achieves high sensitivity, approaching that of LIF detection, but also operates with relatively simple, compact, lowcost, and low-power instrumentation. The inherent miniaturization of electrochemical detection and its compatibility with advanced micromachining technologies make it extremely attractive for CE microchip systems. Fig. 14.1 shows the schematic diagram of a CE-ED amperometric detection system.

High Voltage

Fused silica capillary

Buffer reservoir

Potentiostat

Electrochemical Cell

Fig. 14.1. Schematic diagram of CE-ED amperometric detection system Nevertheless, up to now, several obstacles remain that have stood in the way of routine, practical CE-ED measurements. Nearly all conventional CE-ED electrodes, including carbon fibers and pastes, are susceptible to strong adsorption of reaction intermediates, reaction products, or other contaminants, and can easily be deactivated. Moreover, the carbon surface microstructure

310

14. Diamond Electrochemical Detector in Capillary Electrophoresis

and chemistry can change dynamically over time as a result of surface oxidation processes which tend to occur at the relatively positive working potentials. In CE-ED analysis, the need for polishing and replacement of the electrode due to such surface interference is a very important concern, because this factor not only can limit the reproducibility of the analytical performance, but also can prevent CE-ED analysis from becoming a robust, routine measurement method, due to the involvement of timeconsuming and user-dependent procedures. Highly boron-doped diamond thin film (BDD) electrodes can be favorably applied to the end-column CE-ED system. Chemical vapor deposited (CVD) conductive diamond films have recently received increasing attention for electrochemical applications [7-9]. They possess many attractive electrochemical properties, such as low, stable background currents, a wide potential window in aqueous media, poor adsorption of most types of organic molecules, robust microcrystalline surfaces even at extreme cathodic and anodic potentials, and long-term stability of the response. Such merits of diamond electrodes have been successfully applied to conventional CE [10,12] and microchip CE [11,13,14] systems. These recent studies indicate that the diamond electrode offers improved CE detector performance, including enhanced sensitivity, favorable

signal-to-background

characteristics

and

greatly

improved stability, characteristics that are essential for practical CE-ED measurements. While the diamond electrode has not been easily adapted to CE-ED systems, mainly due to the difficulties of the fabrication of an appropriate CE-ED setup that makes use of the planar CVD 311

thin film shape, diamond microline electrodes were firstly utilized with a conventional CE system, based on a fiised silica capillary for the determination of neurotransmitters [lO]. 90

<

80

70

U

60

50 300

400

600

500

Time (sec) Fig. 14.2. Electropherogram for three neurotransmitters* (l) dopamine, (2) norepinephrine, and (3) epinephrine at diamond CE detector (reprinted from [10]).

14.2. Preparation of Capillary Electrophoresis The diamond microline electrode was prepared by sandwiching freestanding diamond CVD films between two glass slides with UV adhesive. In order to obtain a structure with dimensions appropriate

for

the

inner

diameter

of

the

fused

silica

electrophoresis capillaries, the cross section (W50 x L300 - 500 ^im) of the diamond thin film was exposed as an electrode surface area by polishing the glass-diamond-glass sandwich structure. The separation efficiency and analytical performance of the diamond microelectrode in end-column CE-ED was evaluated for the determination of a catecholamine mixture (see Table 14.1).

312

14. Diamond Electrochemical Detector in Capillary Electrophoresis

Table 14. 1. Separation efficiency in CE and analytical performance of the diamond CE detector

Sensitivity Detection limit N (pA-^iM"^) Dopamine 155,000 86.9 Norepinephrine 152,000 62.3 Epinephrine 138,000 76.1

0.020 0.023 0.019

RSD (%) 4.4 3.2 4.1

Fig. 14.2 illustrates the determination of an equimolar low concentration (O.l jiM) of three neurotransmitters with the diamond-based CE detector. The very low and stable noise levels in the background current enabled us to attain the lowest detection limits from the diamond-based electrochemical detector. Under the same CE separation conditions, the diamond microline electrode showed lower noise levels (0.5 - 1 pA), and a more stable background current, than that of the carbon fiber electrode (whose minimum noise level was 2 pA), even though its surface area was 25 times larger. Variations in the background current (lowfrequency noise) were also much smaller and less irregular than those for the carbon fiber microelectrodes. In addition, the diamond

microelectrodes

exhibited

greater

stability

in

the

amperometric response compared to that for the carbon fiber microelectrodes. Meanwhile, diamond microfiber electrodes prepared via chemical vapor deposition on platinum wires have been fabricated and also reported as an amperometric detector in CE-ED analysis. Cvacka et al. [12] reported that boron-doped polycrystalline diamond thin films were deposited on electrochemically sharpened platinum wires, and similarly confirmed the advantages of the 313

diamond electrode as a CE detector, which include a stable and reproducible

response

and

background

current,

negligible

electrode deactivation, and the lack of required pretreatment for electrode activation. The BDD detector coupled with micromachined chip CE system also exhibited significantly improved detector performance, compared to other commonly used carbon electrodes [11,13,14].

14.3. Applications Fig. 14.3 shows representative electropherograms for phenols (A), organophosphate nerve agents (B) and nitroaromatic explosives (C), recorded with the screen-printed carbon (a) and diamond band (b) working electrodes [ l l ] . In all cases, the BDD electrode displayed higher sensitivity and a lower noise level under the same operational conditions. The enhanced signal-to-background characteristics of the BDD electrode are coupled to sharper peaks and, hence, to enhanced resolution. The influence of separation voltage on the initial baseline current (first 25 sec) at the diamond electrode is significantly smaller than that for screen-printed carbon electrodes. A highly reproducible current response for repetitive injections was also observed for the measurements of TNT (RSD = 0.8 %, n=60) [ l l ] . The improved analytical performance of a diamond electrode for CE-ED analysis has broadened the range of target chemicals to include

many

biologically

and

environmentally

important

compounds that exhibit high oxidation potentials and serious fouling problems. The CE'ED analysis of aromatic amines [13]

314

14. Diamond Electrochemical Detector in Capillary Electrophoresis

and purines and related compounds [14] with a diamond-based electrochemical detector has been performed with high sensitivity and reproducibility.

(a) r— t 1 t 1 1 1 1

1 SnA c

9nA

i •*! 1 2

If,

1

a O

vjV\

C 1 1 1

1

1

t J \

100

1

200 0

100

0

f 5 nA

1•

\1

^

Ai t^..

w V . ,.^

1

c

'2

... -

0

..*/^

\r-

100

200

Time / $

(

b)

II

III

B 1 •••••.......A

10J

G1

20

40

60

Run number

Fig. 14.3. (a) Electropherograms for phenols (I), organophosphate nerve agents (II), and nitroaromatic explosives (III), and (b) Response stability for repetitive flow injection measurements with the screenprinted carbon (A) and diamond (B) electrodes. Sample mixture I* phenol (1), 2-chlorophenol (2), 2,4-dichlorophenol (3), and 2,3dichlorophenol (4). Sample W- paraoxon (l), methyl parathion (2) Sample III: DNB (l), TNT (2), and DNT (3). (reprinted from [11]). Amino'derivatives

of

aromatic

hydrocarbons

are

used

extensively in the manufacturing of dyes, as additives to polymers

315

and rubber, or as intermediates in the manufacturing of industrial chemicals, e.g., pesticides, medicines, dyestuffs, polymers and surfactants [15]. Unfortunately, many aromatic amines are highly toxic,

and

are

also

suspected

to

be

carcinogenic.

The

electrochemical detection of aromatic amines at conventional electrodes is susceptible to considerable difficulties, including low sensitivity, high overpotentials, and surface adsorbed

polymeric

byproducts,

ultimately

passivation by leading

to

the

formation of polyaniline films [16,17]. A previous CE-ED report on aromatic amine analysis has also pointed out that conventional carbon electrodes can not be favorably utilized for the detection of chlorinated anilines, due to low sensitivity [16], and require a regular surface activation process because of fouling [18]. However, the diamond electrode detector is expected to favorably address these limitations, based on its attractive electrode characteristics. Fig. 14.4 compares electropherograms recorded with the boron-doped diamond (a), screen-printed carbon (b) and glassy carbon (c) electrode microchip detectors for a mixture of several aromatic amines that included 4-aminophenol (4-AP) (l), 1,2phenylenediamine (1,2-PDA) (2), 2-aminonaphthalene (2-AN) (3), 2-chloroaniline (2-CA) (4) and craminobenzoic acid (6rABA)(5) [13]. The improved detection is attributed to the fact that a higher working potential can be utilized at the diamond-based detector simultaneously with no adverse effect on the intrinsically lownoised

baseline.

In

particular,

the

significantly

increased

sensitivity for 2-CA and crABA certainly displays such improved analytical

performance.

measured

as

316

decreases

Also,

the

tendency

to

deactivate,

in

peak

current

for

successive

14. Diamond Electrochemical Detector in Capillary Electrophoresis

electropherograms, w a s quite small for t h e diamond electrode- 2.4 a n d 9 . 1 % for 1,2-PDA a n d 2-CA, respectively, (compared to 21.8 a n d 41.0% at t h e screen-printed carbon electrode and 28.3 and 34.1% at t h e glassy carbon electrode, respectively). This is because the

diamond

electrode

shows

little

vulnerability

toward

passivation via adsorption, due to inert surface characteristics, while

5p2-based

carbon

electrodes

are

easily passivated

via

irreversible adsorption of analytes, reaction intermediates, a n d other surface-active compounds contained in real samples.

ij *-•

^ V * * » V K W H J »J

>y V».v»-/ ^''^,.»M>fa»>.»r

c 3 O

^^ 1 5nA

IIIIIIA-^/V^I,

i\Uu

1 c)

60

120

180

Time / s

Fig. 14.4. Electropherograms of five aromatic amines detected at with boron-doped diamond (a), screen-printed carbon (b) and glassy carbon (c) electrodes. Sample mixture4-aminophenol (l), 1,2phenylenediamine (2), 2-aminonaphthalene (S), 2-chloroanihne (4), and craminobenzoic acid (5).The working potentials were +1.1 V (a), +0.9 V (b), and +0.9 V (c) vs. Ag/AgCl (reprinted from [13]). O t h e r examples of favorably chosen t a r g e t s for the diamond electrode are t h e detection of bioactive p u r i n e s and

related

compounds [14]. The separation and detection of p u r i n e b a s e s a n d 317

purine-containing

compounds

represents

a

challenging

and

i m p o r t a n t t a s k with respect to a variety of biochemical processes. Shown in Fig. 14.5 are typical electropherograms for a mixture containing guanine (a), hypoxanthine (b), guanosine (c), x a n t h i n e (d), a n d uric acid (e) at t h e screen-printed carbon (A) a n d diamond (B) detectors. The result of the diamond electrode shows t h a t superior baseline c u r r e n t behavior a n d sensitivity can be achieved with higher working potentials. In contrast, commonly

used

working electrode m a t e r i a l s for chip-based ED detection have not been suitable in t e r m s of detection sensitivity and reproducibility, because

the

electrochemical

oxidation

of p u r i n e s

containing

a n a l y t e s suffers from quite high oxidation potentials and surface fouling.

5nA

0)

J

1

,

^

100

>

1

200

'

300

Time/s Fig. 14.5. Electropherograms for mixtures containing guanine (a), hypoxanthine (b), guanosine (c), xanthine (d), and uric acid (e) at the screen-printed carbon (A) and BDD (B) electrodes. The working potential is +1.3 V vs. Ag/AgCl (reprinted from [14]). 318

14. Diamond Electrochemical Detector in Capillary Electrophoresis

14.4. Conclusions The diamond electrode h a s proven useful in overcoming the limitations of conventional carbon electrodes as detectors for CE. Based on the attractive performance of BDD electrodes, one may expect

more

diverse

analytical

applications,

including

field"

deployable diamond-based microchip detection systems for on-site bioanalysis a n d e n v i r o n m e n t a l monitoring.

References 1. J. P. Landers, Handbook

of Capillary

Electrophoresis

2nd Ed,

CRC Press LLC, Boca Raton, Florida, U.S.A., 1997. 2.

D. J. Harrison, K. Fluri, K. Seiler, C. S. Effenhauser and A. Manz, Science, 261 (1993) 895.

3.

V. Dolnik, S. Liu and S. Jovanovich, Electrophoresis

4.

R. P. Baldwin, Electrophoresis,

5.

J. Wang, Talanta, 56 (2002) 223.

6.

W. R. Vandaveer IV, S. Pasas, R. S. Martin and S. M. Lunte, Electrophoresis,

7.

21 (2000) 41.

21 (2000) 4017.

23 (2002) 3667.

G. M. Swain, A. B. Anderson

and J. C. Angus, MBS Bull,

9

(1998) 56. 8. Y. V. Pleskov, Y. E. Evstefeeva, M. D Kvotova and V. Laptev, Electochim. Acta, 44 (1999) 3361. 9.

T. N. Rao and A. Fujishima, DiamondBelat

Mater., 9 (2000) 384.

10. D. Shin, B. V. Sarada, D. A. Tryk, A., Fujishima and J. Wang, Anal

Chew., 75 (2003) 530.

319

11. J. Wang, G. Chen, M. P. Chatrathi, A. Fujishima, D. A. Tryk and D. Shin, Anal. Chew., 75 (2003) 935. 12. J. Cvacka, V. Quaiserova, J. Park, Y. Show, A. Muck, Jr. and G. M. Swain, Anal

Chem., 75 (2003) 2678.

13. D. Shin, A. Fujishima, A. Muck Jr., G. Chen and J. Wang, Electrophoresis,

in press (2004).

14. J. Wang,G. Chen, A. Muck Jr., D. Shin and A. Fujishima, J. Chromatogr. A, 1022 (2004) 207. 15. W. Gerhartz, (Ed.), 5th ed., UUmanns Encyclopedia

of

Industrial

Chemistry, Vol. 2A, VCH, Weinheim, 1985. 16. A. Asthana, D. Bose, A. Durgbanshi, S. K. Sanghi and W. Th. Kok, J. Chromatogr. A, 2000, 895, 197. 17. E. H. Seymour, N. S. Lawrence, E. L. Beckett, J. Davis and R. G. Compton, Talanta, 2002, 57, 233. 18. X. Huang, T You, T. Li, X. Yang and E. Wang, 1999, 11, 969.

320

Electroanalysis,

15. Determination and Electrooxidation of Sulfur-Containing Compounds at Boron-Doped Diamond Electrodes Orawon Chailapakul, Donald A. Tryk and Akira Fujishima

15.1. Introduction In

this

Chapter,

electrochemical

and

electrooxidation

investigations of sulfur-containing compounds at boron-doped diamond (BDD) electrodes are reviewed. First, the general aspects of BDD in electrochemistry are briefly described. Then, we summarize the use of BDD for the determination of these compounds, as well as for the study of their electrooxidationbehavior. The

BDD

chemicaFvapor-deposited

several distinctive

characteristics

that

thin

film

make it

possesses particularly

interesting for electroanalytical applications, including extreme hardness and chemical inertness, excellent electrical conductivity and extraordinarily low catalytic activity for both hydrogen and oxygen gas generation. The latter gives a wide potential working range.

Furthermore,

the

near

absence

of

intrinsic

electrochemistry and quite low capacitance provide a very low background current within the working potential range.^'^ BDD is superior to glassy carbon (GC) as a result of the very low, stable Orawon Chailapakul e-mail: [email protected]

321

voltammetric background current. Thus, the signal-to-background (S/B) ratios for various analytes are enhanced in voltammetric measurements. Many

studies

have

been

devoted

to

sulfur-containing

compounds, because they play a very important role in living systems. These compounds are also widespread in nature and the environment, for example, being found in proteins, amino acids, foodstuffs, anti-microbial drugs and pesticides. Thus, the analysis of sulfur-containing compounds is very important. In this chapter we report the use of BDD to detect several types of sulfurcontaining compounds, including sulfa drugs, thiol drugs and specific

important

sulfur-containing

compounds,

such

as

homocysteine and cysteine. Moreover, a new finding, the use of anodized BDD to detect disulfide compounds, is described. The

detection

of

sulfur-containing

compounds

by

spectrophotometric detection is not effective, because these sulfurcontaining

groups

do

not

preferentially

absorb

light

[l].

Derivatization of sulfur-containing compounds is an alternative to solve this problem; however it increases the cost and the complication of analysis [2, 3].

Electrochemical methods of

analysis are a more attractive option, because they are low-cost, highly sensitive and have long-term reliability and reproducibility. The electrochemical detection of sulfur-containing compounds has been described, using carbon, platinum, mercury and gold as working electrodes [4, 5]. However, the severe detection conditions can ruin the electrode and cause fluctuating background currents [l].

Some research groups have alleviated these problems by

modifying the electrode, or by using special electrochemical

322

15. Determination and Electrooxidation of Sulfur-Containing Compounds at BoronDoped Diamond Electrodes

detection techniques, such as pulsed electrochemical detection. BDD can overcome the limitations that have been mentioned here, without any pretreatment of the electrode, due to the stable surface characteristics, which rely to a large extent on and the surface terminatation by hydrogen [6]. Thus, the BDD surface is relatively non-polar and suffers little proclivity for adsorption of polar molecules.

15.2. Detection of Sulfur-Containing Compounds Recently, it was first reported that, with a BDD electrode, four important sulfur-containing compounds, including two biologically important amino acids (homocysteine and glutathione (GSH)), one vitamin (2-mercapto ethanesulfonic

acid) and one antibiotic

(cephalexin) can be determined by cyclic voltammetry.

Their

structures are shown in Fig. 15.1. The electrooxidation of these sulfur-containing compounds exhibited well-defined irreversible responses. This preliminary study has shown that BDD has better sensitivity than GC, as shown in Fig. 15. 2. The concentration dependence has been studied and has indicated the promise of using BDD electrodes for quantitative determination. All of the compounds displayed

recognizable

oxidation peaks at BDD electrodes at millimolar concentration levels.

The sequence of the highest to lowest sensitivity was-

homocysteine

> GSH

> 2-mercapto

ethanesulfonic

acid

>

cephalexin. It was expected that the stability of the intermediate (free radical cation, ---S.^-'-R) limits the sensitivity. The higher the

323

stability of this radical cation, the higher should be the sensitivity. Cephalexin has a sulfur atom in the ring, and hence, the dimer may not be produced, and the oxidation mechanism must be different from that of the others. It was suspected that steric hindrance could also affect the sensitivity of this compound.

(A)

1^

«-s^

0

0

(B)

^

NH

^0

NH2

1

H - ^ ^

0 (C) ^

(D)

0

0 1' S—OH 1' 0

H2N

1

y

I

J

n

1 r

1

Fig. 15.1. Structures of sulfur-containing compounds- (A) homocysteine; (B) GSH; (C) 2-mercapto ethanesuLfonic acid; (D) cephalexin.

Furthermore, the scan rate dependence GSH electrooxidation has been examined. It was shown that there was no adsorption on the surface of the BDD electrode for low concentrations. 324

This

15. Determination and Electrooxidation of Sulfur-Containing Compounds at BoronDoped Diamond Electrodes

advantage should be valuable for the electroanalysis of other sulfur-containing

compounds

without

pretreatment

of

the

electrode.

-.2

0.0

.2

.4 .6 .8 E(VvsAg/AgCl)

1.0

1.2

-.2

0.0

.2

.4 .6 .8 E(VvsAg/AgCl)

1.0

1.2

-.2

0.0

.2

.4 .6 .8 E(VvsAg/AgCl)

1.0

1.2

-.2

0.0

.2

.4 .6 .8 E(VvsAg/AgCl)

1.0

1.2

-.2

0.0

.2

.4 .6 E(VvsAg/AgCl)

E(VvsAg/AgCl)

Fig. 15.2. Cyclic voltammograms for (I) homocysteine, (II) 0.67 mM glutathione, (III) 1.0 mM 2-mercapto ethanesulfonic acid, (IV) 10.0 mM cephalexin, at (A) BDD and (B) GC electrodes vs. Ag/AgCl in 0.1 M carbonate buffer (pH 9.2) in the presence (soUd lines) and absence (dotted lines) of the analytes (scan rate, 50 mV s*i; area of electrode, 0.07 cm2).

325

15.2.1. Detection of cysteine The electrochemical behavior of cysteine h a s been

intensively

studied in the course of t h e last 60 years [7]. This compound h a s often been studied as t h e reversible cysteine/cystine couple, and the mechanism for the electrochemical oxidation of this couple w a s found to be as follows [7]CH2SH

CH2SSCH2 CHNH.

2CHNH2

CHNH,

2H"

+

2e"

(15.1)

CO.H CO2H

Cysteine

CO.H

Cysteine

A peak-shaped oxidation response w a s observed for both BDD and GC electrodes at about 0.65 V, as shown in Fig. 15.3.

0.2

0.4

0.6

0.^

E/V(vs SCE)

0.2

0.4 0.6 0.8 EA^(vs SCE)

Fig. 15.3. CycUc voltammograms for (A) BDD and (B) GC electrodes in L-cysteine + 0.5 M KHCO3 (soUd lines) and 0.5 M KHCO3 (dashed lines). The potential sweep rate was 20 mV s~^ and the electrode area was 0.07 cm2.

326

15. Determination and Electrooxidation of Sulfur-Containing Compounds at BoronDoped Diamond Electrodes

However, the peak was somewhat more clearly defined for the BDD electrode than for the GC electrode. It also has been observed that there is a slight adsorption of either reactant or product, as evidenced by a slight change in the background voltammogram. The results presented in this work have shown that it is very promising to use diamond electrodes for cysteine detection, without the use of mercury or surface modification. The

concentration

dependence

for

cysteine

was

also

investigated at a sweep rate of 200 mV s~i on BDD and GC electrodes. The response was in general higher and more linear for the BDD electrode.

As seen in the inset for Fig. 15.4A, the

voltammetric response was almost perfectly linear, while that for GC was not inset. Fig. 15.4B. The slope for the BDD calibration curve was a factor of -1.75 greater than that for GC. The sweep rate dependence for both was also examined- that for BDD was again almost perfectly linear, passing very close to the origin, while that for GC, while linear, did not pass close to the origin. The ideal behavior for the BDD electrode indicates that the mechanism does not follow the same type of mechanism as that proposed for the other compounds examined in the present study, i.e., with the oxidation being mediated by •OH. Also, the electrode response is much higher, indicating that the electrode reaction takes place on the diamond surface itself rather than on trace sp^ carbon impurities Spataru et al. [8] used BDD electrodes to examine L-cysteine (CySH) oxidation in alkaline media with voltammetric

and

polarization measurements. For BDD electrodes. It was found that the overall CySH oxidation reaction was controlled by the initial

327

electrochemical step, i.e., t h e oxidation of t h e CyS electroactive species. Conversely,

at

GC electrodes, the

same

reaction

is

controlled by the desorption of the reaction products.

0.6

0.8

Emvs SCE)

0.4

0.6

0.8

Emvs SCE)

Fig. 15.4. CycUc voltammograms for (A) BDD and (B) GC electrodes in 0.5 M KHCO3 for a series of Lcysteine concentrations" 0, 0.10, 0.50, 1.01, 2.01, 5.04 and 10.07 mM. The potential sweep rate was 200 mV s"^ and the electrode area was 0.07 cm^. The caUbration curves are shown as insets. The correlation coefficients were 0.9997 and 0.9946, respectively. The oxidation of cysteine by ferricyanide at the BDD electrode w a s studied by t h e Compton group [9]. It w a s shown t h a t , onBDD, unlike,

for

example,

platinum

electrodes,

the

voltammetric

responses of t h e ferrocyanide wave and t h a t of t h e t a r g e t were sufficiently different, with t h e former at a lower oxidizing potential.

328

15. Determination and Electrooxidation of Sulfur-Containing Compounds at BoronDoped Diamond Electrodes

to permit examination of the kinetics of the

homogeneous

oxidation via voltammetric methods.

15.2.2. Detection of homocysteine Homocysteine is an important sulfhydryl thiol (RSH) found in the human body, and a Hnk between the presence of this compound and

cardiovascular

diseases

has

been

suggested

[10, 11].

Specifically, the quick, accurate determination of homocysteine is critical, because its concentration indicates the risk of heart attrack.

Chailapkul et al.[\2] have studied the electrochemical

oxidation of homocysteine both at as-deposited and anodized (oxidized) BDD thin film electrodes with cyclic voltammetry, flow injection analysis and high-pressure liquid chromatography with amperometric detection. electrodes

provided

It was found that the anodized BDD

highly

reproducible,

well-defined

cyclic

voltammograms for homocysteine oxidation in acidic media, while as-deposited diamond did not provide a detectable signal.

In

alkaline media, however, the oxidation response was obtained, both at as-deposited and anodized diamond electrodes, as shown in the Fig. 15.5. In the flow system, BDD exhibited a highly reproducible amperometric response, with a peak variation less than 2%, as shown in Fig. 15.6.

329

0.8

A)

B)

Wf\

OJO 0.6 0.25

! 1

^OJO |o.4

(b)//

I

S 0.15

5

\

u 0.2

1 , / •

0.10

(a)

0.05

(a)

0.0 OXM 1

OJ

0.4

1

0.6

0.8

^__

1.0

1.2

1.4

OD

OJ

Potential ( V ) v$ A g / A g Q

Fig. 15. 5. A) Cyclic voltamograms for 1 mM homocysteine in 0.1 M phosphate buffer (pH 9) at (a) as grown BDD(soUd Une) and (b) oxidized BDD thin film electrodes (dotted line). B) CycUc voltamograms for 1 mM homocysteine in 0.05 M phosphate buffer (pH 2.7) at (a) asgrown BDD (solid Une) and (b) oxidized BDD thin film electrodes (dotted line).

:

5jxM

o o -Tt

Isi «- m c

«3 u

1 ^M o(S—-

, 1 1

o o-

" -" 1

280

0.5 ^M

1 llllllll.ll

1

300 Time (min)

1

.

320

Fig. 15.6. Flow injection with amperometric detection of homocysteine at various concentrations in 2% acetonitrile in 0.05 M phosphate buffer (pH 2.7). The flow rate was 1 mL min'i.

330

15. Determination and Electrooxidation of Sulfur-Containing Compounds at BoronDoped Diamond Electrodes

An extremely low detection limit (1 nM) w a s obtained at 1.6 V vs. Ag/AgCl. In addition, t h e d e t e r m i n a t i o n of homocysteine in a s t a n d a r d mixture with aminothiols a n d disulfide compounds by means

of isocratic

reverse-phase

HPLC

with

amperometric

detection a t diamond electrodes h a s been investigated. The r e s u l t s showed excellent separation, w i t h a detection limit of 1 pmol a n d a linear r a n g e of t h r e e orders of m a g n i t u d e . Nekrassova et al. [13] have also studied the electrochemical oxidation of 5-thio-2-nitrobenzoic acid (TNBA) a t a BDD electrode for t h e indirect detection of cysteine, homocysteine a n d glutathione. The reaction w a s shown to proceed via a CEC reaction process at lower pH, in which the thiol moiety of t h e TNBA species m u s t undergo deprotonation before oxidation. reported t h a t 5,5-dithiobis(2-nitrobenzoic

These a u t h o r s

also

acid) (DTNB) can be

used to develop the total thiol detection methodology

using

chronoamperometry with BDD electrodes. The detection limits of cysteine, homocysteine a n d glutathione were found to be 5.7 ^M, 4.4 \xM and 5.8 |xM respectively.

15.2.3. Detection of sulfide and disulfide Lawrence et al.[l4] have developed a method to determine sulfide at

the

BDD

electrode

To

avoid

a

problems

with

solvent

decomposition a n d to detect low (micromolar) concentrations, t h e BDD electrode were used to examine the electrocatalytic reduction of

ferricycanide

by

sulfide

using

cyclic

voltammetry

and

chronoamperometry. The recovery of sulfide in spiked sewage effluents w a s reported to be 102 ± 4.5%.

331

oo -

A

00

o a

X O

§

«

(>

•

A J lOnA 111 12 Timc/min

-

i

—

14

,

10

—

,

—

,

—

12

.

—

.

14

B 1§

c

X en O

J

^8

11 l.Al

l^

i 6

L

8 Time/min

10

,

,

12

1

r-

,

14

Fig. 15.7. Chromatograms of whole r a t blood (1*80 dilution) showing peaks for GSH and GSSG: (A) no addition of standard GSH or GSG; (B) 82.5 ^iM each, and (C) 825 (xM each of GSH and GSSG was added prior to deproteinization. The conditions were as foUows- separation column, Inertsil ODS-3 (4.6-mm i.d. x 75 mm, dp = 3 |xm); mobile phase, MeCN/0.1% TFA = 2/98; temperature, 2 5 t : ; flow rate, 0.7 mL min"i; injection volume, 20 ^xL. An anodically oxidized BDD electrode was used at 1.50 V vs. Ag/AgCl.

Disulfides such a s oxidized glutathione can be detected a t anodized BDD electrodes.

332

This development w a s found

by

15. Determination and Electrooxidation of Sulfur-Containing Compounds at BoronDoped Diamond Electrodes

Terashima et al. [15]. Under oxidative stress, GSH is oxidized to GSSG in living cells. Thus, the ratio of these two compounds serves as an indicator of oxidative stress. It is well known that the detection of GSSG is difficult with conventional metal or graphite electrodes.

However, the results exhibit high sensitivity, even

with the low availability of GSSG in plasma. The results of electrolysis experiments of disulfides and thiols showed that the oxidation reaction mechanism of glutathione (GSH) and GSSG involves oxygen transfer. Following separation by liquid chromatography (LC), the determination of both GSH and GSSG in whole rat blood was achieved at a constant potential of 1.50 V vs. Ag/AgCl, as shown in Fig. 15.7. Very low limits of detection for GSH and GSSG were found, 1.4 nM (0.028 pmol) and 1.9 nM (0.037 pmol), respectively, with a linear calibration range up to 0.25 mM. Another important advantage with the anodically oxidized

electrode

was that

the

electrode

surface

can

be

reactivated if deactivation at surface occurs after several potential scans.

15.3. Detection of Sulfur-Containing Drugs 15.3.1. Detection of sulfa drugs The sulfonamide class of antibiotics is generally used for the treatment of urinary tract infections, chronic bronchitis and other bacterial infections [16]. The electroanalysis of three sulfa drugs, sulfadiazine, sulfamerazine and sulfamethazine at BDD thin film electrodes was first reported by Rao et al. [17]. Cyclic voltammetry, flow

injection

analysis

and

liquid

chromatography

with

333

electrochemical detection were used to study the reactions and quantitative analysis.

oxidation

At diamond electrodes, all

three drugs provided highly reproducible and well-defined cyclic voltammograms, all with signal to background (S/B) ratios of a factor often greater than that obtained at GC electrodes, as shown in Fig. 15.8. An experimental detection limit of 100 nM (2 pmol) was obtained with a response variability of - 5 % (n=15). A linear dynamic range from 0.05 to 500 M was obtained.

< =3.

-0.4

-0.2

0.2 0.4 0.6 EA^ vs. SCE

0.8

1.0

1.2

Fig. 15.8. Cyclic voltammograms for 50 M sulfadiazine in 0.1 M phosphate buffer (pH 7.1) at (a) a BDD electrode (area, 0.12 cm2) and (b) GC electrode (area, 0.07 cm2). The sweep rate was 100 mV s-i. Background voltammograms are also shown in the figure.

334

15. Determination and Electrooxidation of Sulfur-Containing Compounds at BoronDoped Diamond Electrodes

One of the advantages of BDD over GC is the stabiHty of response. The comparison results (BDD, and GC from two companies, BAS and Tokai) are shown in Fig. 15. 9.

A rapid

stabilization of the background current was achieved, within 10 min after the application of the operating potential under flow conditions, unlike the case of GC electrodes, which required more than 30 min for reasonable stabilization.

300

15 20 Time/min

Fig. 15.9. Background current vs. time profiles for BDD and GC electrodes at 1 V under flow conditions. The mobile phase was 85% 0.1 M phosphate buffer (pH 7.1) + 15% methanol (v/v).

15.3.2. Detection of thiol drugs (D-penicillamine, captopril and tiopronin) D-Penicillamine(3-mercapto"

D-valine)

is

a

pharmaceutically

significant thiol compound used as a medicinal agent against a number of diseases, e.g.,

rheumatoid arthritis [18], cystinuria ,

liver diseases, certain skin conditions, heavy metal poisoning and Wilson's disease [19].

335

Captopril belongs to the group of anti-hypertensive drugs that affect the renin-angiotensin system and are commonly referred to as angiotensin-converting enzyme (ACE) inhibitors. The chemical name of captopril is (A^-l-(3-mercapto-2-methyl-l "oxo-propyl)- -Dproline, and it is widely used for the treatment of arterial hypertension. Recent studies suggest that it may also act as a scavenger of free radicals because of its thiol group. Tiopronin [A^(2"mercaptopropionyl)-glycine] is a drug with a free thiol group that is used in clinical applications. It is effective in the treatment of cystinuria, rheumatoid arthritis, as well as hepatic disorders, and as an antidote to heavy metal poisoning [20, 21]. Many methods have been employed for the detection of these compounds such as high-performance liquid chromatography with pre or post column derivatization fluorometry

[22-24], calorimetry [25],

[20, 26], chemiluminescene [27], capillary electro-

phoresis [28, 29] and NMR spectrometry [30].

Electrochemical

methods are other choices for their determination because they are inexpensive, simple, fast and sensitive. Experiments using mercury, mercury amalgam and a chemically modified electrode as the working electrode [31-33] have been performed for thiol-compound determination.

However, mercury has restrictions due to its

toxicity and the fast deterioration of the electrode response. Using a chemically modified electrode [34, 35] is not practical for use in flow injection analysis and liquid chromatography. Therefore, the BDD thin film unique

electrode has been introduced it is known as a

electrode

material

applications [12, 36, 37].

336

for

a number

of electrochemical

15. Determination and Electrooxidation of Sulfur-Containing Compounds at BoronDoped Diamond Electrodes

The

first

use

of BDD

thin

film

electrodes

for

the

electroanalysis of D-penicillamine [38], captopril [39]and tiopronin [40]has been reported.

It was observed that BDD electrodes

exhibited excellent performance for the oxidative detection of these compounds.

Comparison experiments were performing

using a GC electrode. The BDD electrode exhibited a well-resolved and irreversible oxidation voltammogram, but the GC electrode provided only an ill-defined response. Moreover, the outstanding capabilities of the BDD electrode were demonstrated by coupling with flow injection analysis (FIA). The results indicate that all these three of these compounds can be detected amperometrically without derivatization or the use of a pulse waveform. The FIA results showed a significantly lowered detection limit.

The

analytical figures of merit of the diamond electrode for thiol drug determination are shown in Table 15. 1. FIA with amperometry using BDD as the working electrode was also applied to determine these drugs in dosage form; the results obtained in the recovery study were comparable to those given by the manufacturer. Table 15.1. Analytical figures of merit of the BDD electrode for thiol drug determination using FIA coupled with amperometric detection.

Thiol drugs D-penicillamine Captopril Tiopronin

Linear dynamic range (nM) 0.5 - 50 0.5 - 100 0.5 - 50

Detection limit (nM) 10 10 10

Precision (%R.S.D.)

Ref

1.5-2.1 1.2-2.1 1.1-1.6

[38] [39] [40]

We can draw the conclusion that BDD electrodes (both a s deposited and anodized) exhibit excellent performance for the 337

oxidative detection of sulfur-containing compounds. Well-defined voltammograms exhibited advantages

high over

were

obtained

sensitivity, the

GC

at

the

and

BDD electrode,

demonstrated

electrode,

because

which

significant

of its

superior

electrochemical properties. Moreover, t h e use of t h e BDD electrode is a simple analytical procedure. required.

No chemical modification is

Therefore, boron-doped diamond is one of t h e most

attractive m a t e r i a l s t h a t can be used for the detection of other important

sulfur-containing

compounds,

for

example,

thiol-

containing degradation products of t h e highly toxic V-type nerve agents.

References 1. G. S. Owens and W. R. LaCourse, Current Sep., 14 (1996) 82. 2. N. Adam and J. R. Kramer, Aqua. Geochem., 5 (1999) 1. 3. L. Gallo-Martinez, A. Sevillano'Cabeza, P. Campins-Falco and F. Bosch-Reig, Anal. Chim. Acta., 370 (1998) 115. 4. F. G. Banica, A. G. Fogg and J. C. Moreira, Analyst

(Cambridge,

UK), 119(1994)2343. 5. M. Heyrovsky and S. Vavricka, Bioelectrochem.

& Bioenerg.,

48

(1999) 43. 6. J. C. Angus and C. C. Hayman, Science, 241 (1988) 913. 7. L. Eberson and K. Nyberg, in "Encyclopedia of Electrochemistry of the Elements" (A. J. Bard and H. Lund, eds.). Vol. vol. XII, Chs. XII. Marcel Dekker, New York, 1978.

338

15, Determination and Electrooxidation of Sulfur-Containing Compounds at BoronDoped Diamond Electrodes

8. N. Spataru, B. V. Sarada, E. Popa, D. A. Tryk and A. Fujishima, Anal

Chew., 73 (2001) 514.

9. O. Nekrassova, G. D. Allen, N. S. Lawrence, L. Jiang, T. G. J. Jones and R. G. Compton, Electroanalysis,

14 (2002) 1464.

10. H. G. Koch, M. Goebeler, T. Marquardt, J. Roth and E. Harms, Eur. J. Pediatr., 157 (1998) S102. 11. A. Gupta, A. Moustapha, D. W. Jacobsen, M. Goormastic, E. M. Tuzcu, R. Hobbs, J. Young, K. James, P. McCarthy, F. V. Lente, R. Green and K. Robinson, Transplantation,

65 (1998) 544.

12. O. Chailapakul, W. Siangproh, B. V. Sarada, C. Terashima, T. N. Rao, D. A. Tryk and A. Fujishima, Analyst

(Cambridge,

UK), \TJ

(2002) 1164. 13. O. Nekrassova, N. S. Lawrence and R. G. Compton,

Electro-

analysis, 15 (2003) 1501. 14. N. S. Lawrence, M. Thompson, C. Prado, L. Jiang, T. G. J. Jones and R. G. Compton, Electro analysis, 14 (2002) 499. 15. C. Terashima, T. N. Rao, B. V. Sarada, Y. Kubota and A. Fujishima, Anal

Chem., 75 (2003) 1564.

16. T. G. Diaz, A. G. Cabanillas, M. I. A. Valenzuela and F. Salinas, Analyst

(Cambridge, UK), 121 (1996) 547.

17. T. N. Rao, B. V. Sarada, D. A. Tryk and A. Fujishima, Electroanal

J.

Chem., 491 (2000) 175.

18. H. A. Kim and Y. W. S. Rheumatol, Int., Yl (1997) 5. 19. C. Smolarek and W. Stremmel, Zeitchrift

Fur

Gastroenterolagie,

374 (1999) 293. 20. T. P. Ruiz, C. M. Lozano, V. Tomas and C. S. Cardona, J. Pharmaceut

Biomed. Anal,

15 (1996) 33.

339

21. T. P. Ruiz, C. M. Lozano, W. R. G. Baeyens, A. Sanz and M. T. San-Miguel, J. Pharmaceut

Biomed. Anal,

17 (1998) 823.

22. J. Russell, J. A. McKeown, C. Hensman, W. E. Smith and J. Reglinski, J. Pharmaceut.

Biomed. Anal,

15 (1997) 1757.

23. E. P. Lankmayr, K. W. Budna, K. Muller, F. Nachtmann and F. Rainer, J. Chromatogr., 222 (l98l) 249. 24. D. Beales, R. Finch, A. E. M. McLean, M. Smith and I. D. Wilson, J. Chromatogr., 226 (1981) 498. 25. A. A. Al-Majed, J. Pharm. Biomed. Anal., 21 (1999) 827. 26. A. A. Al-Majed, Anal. Chim. Acta, 408 (2000) 169. 27. Z. Zhang, W. R. G. Baeyens, X. Zhano and G. V. D. Weken, Anal. Chim. Acta, 347 (1997) 325. 28. M. Wronski, J. Chromatogr. B (Anal. Technol. Biomed. Life Sci.), 676 (1996) 29. 29. J. Russell and D. L. Rabenstein, Anal. Biochem., 242 (1996) 136. 30. S. E. Ibrahim and A. A. Al-Badr, Spectrosc. Lett, 31. D. L. Rabenstein and G. T. Yamashita, Anal.

13 (1980) 471. Biochem.,

180

32. G. T. Yamashita and D. L. Rabenstein, J. Chromatogr.,

491

(1989) 259.

(1989) 341. 33. H. M. A. Killa and D. L. Rabenstein, J. Chromatogr,

465 (1989)

359. 34. S. Zhang, W. Sun, W. Zhang, L. J. W. Qi, K. Yamamoto, S. Tao and J. Jin, Anal. Chim. Acta, 386 (1999) 21. 35. G. Favaro and M. Fiorani, Anal. Chim. Acta, 332 (1996) 249. 36. T. N. Rao, I. Yagi, T. Miwa, D. A. Tryk and A. Fujishima, Chem., 71 (1999) 2506.

340

Anal

15. Determination and Electrooxidation of Sulfur-Containing Compounds at BoronDoped Diamond Electrodes

37. O. Chailapakul, E. Popa, H. Tai, B. V. Sarada, D. A. Tryk and A. Fujishima, Electrochem.

Commun., 2 (2000) 422.

38. N. Wangfuengkanagul and O. Chailapakul, Talanta, 58 (2002) 1213. 39. W.

Siangproh,

P.

Ngamukot

and

O.

Chailapakul,

Sens.

Actuators B-Chemical, 91 (2003) 60. 40. W. Siangproh, N. Wangfuengkanagul Analytica

and O.

Chailapakul,

Chimica Acta, 499 (2003) 183.

341

16. Boron-Doped Diamond Electrodes for the Analysis of Trace Metals Ayyakannu Manivannan, Donald A. Tryk, and Akira Fujishima

16.1. Introduction The analysis of trace metals has been gaining importance over the past several decades due to growing concerns about their toxicity, and there is a great need to monitor them in a variety of matrices, including air, water, and soil, as well as in physiological tissues and fluids. The most hazardous heavy metals include lead, cadmium, mercury, arsenic, thallium and selenium. Electrochemical stripping analysis is an attractive, powerful tool for detecting trace metals due to its simplicity and sensitivity in the

simultaneous

measurement

of multiple

elements

at

detection levels from ppb to ppt. In addition, the added features of portability, low power requirements, and the suitability

for

automatic on-line monitoring emphasize its great power for rapid, inexpensive analysis of trace metals in applications such as environmental The

field-testing.

detection

voltammetry

(ASV)

of

trace is

a

elements well

by

established

anodic

stripping

electroanalytical

technique, in which the accumulation of the metals at the electrode plays a major role in the improvement of detection limits [1-3]. In Ayyakannu Manivannan e-mail: [email protected] 342

16. Boron-Doped Diamond Electrodes for the Analysis of Trace Metals

this Chapter, we review the topic of electrochemical trace metal analysis at the boron-doped CVD diamond electrode and try to indicate some of the possible advantages of the use of this type of electrode. Being aware of their superior electrochemical characteristics, as described in other Chapters, we have long believed that borondoped diamond (BDD) electrodes could be highly useful electrochemical trace metal analysis.

in

A particularly attractive

feature was envisioned, i.e., the avoidance of the use of mercury. The use of mercury has been extensive in ASV due to its usefulness in avoiding complicated interactions between multiple metals. However, we as well as others have shown that it is possible to achieve excellent analytical performance

on BDD electrodes

without the use of mercury. Of course, there are other advantages in addition to the avoidance of mercury, such as the much wider potential working range, the low background current, and the freedom from fouling problems. As we will show, other advantages have also come to light.

16.2. Mercury-Film Assisted Analysis Trace metal analysis of mM concentrations of lead, copper, and cadmium using differential pulse voltammetry at a BDD electrode in a solution containing intentionally added Hg2+[4-5]. Here, the latter deposits onto the diamond surface as a film. Mercury films are typically coated on electrode surfaces such as glassy carbon or iridium to improve the response [6, 7]. In the conventional

343

procedure, metals are accumulated in dissolved form in the liquid mercury.

16.3. Mercury-Free Metal Detection 16.3.1. General aspects Recently, Manivannan et al. [8] and Ramesham et. al [9] demonstrated for the first time that diamond electrodes can be used for toxic trace analysis without the help of mercury. This approach leads to a mercury-free electrode for trace metal detection. With the BDD electrodes, the metals are deposited in solid form on the electrode. The mercury-free detection of lead (below 1 ppb), cadmium and mercury itself has been demonstrated at a bare diamond electrode [8,10-12]. In

initial

concentrations,

work, simple

carried

out

with

CVs at the

relatively

diamond

high

lead

electrode

were

measured. As we can see in Fig. 16.1, a large cathodic deposition peak and a small anodic stripping peak were observed. This indicates that the deposited lead metal was not completely stripped from the diamond surface. Moreover, consecutive scans show further decreases in the magnitude of the cathodic deposition peak. This work thus concluded that, under some circumstances, specifically, at high metal concentrations in solution, it is difficult to use the anodic stripping technique for the quantification of metals at diamond electrodes. Our results, in which the Pb was not completely stripped from the electrode during a single anodic scan, are in agreement with results reported by others [6, 13]. All have reported that there is excess charge for the cathodic 344

16. Boron-Doped Diamond Electrodes for the Analysis of Trace Metals

deposition compared to the anodic oxidation. Figure 16.1 is a good example for a similar situation. Although this phenomenon has not yet been completely explained, one likely explanation is that the deposited particles may only be weakly attached to the hydrophobic, hydrogenterminated surface [14] or only well attached at specific nucleation sites [15,16]. We have found that, if the particles are large enough to be visible by scanning electron microscopy (SEM) {i.e., > 0.1 fim), they are too large to be completely oxidized during the positive voltammetric sweep. However, at relatively low lead concentrations, i.e., those of most interest for public health, we have shown that is possible to deposit and quantitatively strip small amounts of lead, demonstrating the feasibility of trace analysis.

-0.020

-1.0

-0.8

-0.6

-0.4

-0.2

Potential (V VS. SCE)

Fig. 16.1. CycUc voltammogram for Pb (N03)2 (10 « M) in 0.1 M KCl (pH 1) at a boron-doped diamond (BDD) electrode. This CV indicates a significant unbalanced charge of the cathodic anodic waves. A smaU amount of Pb is being stripped compared to the amount deposited.

345

An SEM image (Fig. 16.2) shows the morphology of the metallic lead that deposits on the diamond surface when the solution concentration is relatively high. Lead was deposited for 2 minutes (Fig. 16.2c) and 10 minutes (Fig. 16.2d) at -0.7 V vs. SCE. It is clear that metal islands deposit on the BDD crystal planes as well as on the grain boundaries. These metallic deposits are of the type that cannot be quantitatively stripped.

Fig. 16.2. (a,b) SEM images of bare BDD; (c) electrochemical nucleation of lead metal is observed; (d) lead deposited electrochemically all over the diamond facets. In anodic stripping voltammetry, however, it is a requirement that the deposited metal be completely removed from the electrode surface. As we found subsequently, it is possible to achieve quantitative stripping when the solution concentration is small. As

346

16. Boron-Doped Diamond Electrodes for the Analysis of Trace Metals

an example, Fig. 16.3 shows consecutive stripping scans for lead on BDD for a solution of 0.1 M KCl (pH = 5). The first scan was performed after the deposition of lead at -1 V vs. SCE for three minutes, and the second scan was performed immediately following the first scan, without deposition. It is clear that the lead deposited on the BDD surface has been mostly removed. The small peak that appears in the second scan is due to the deposition of some lead when the scan starts from -1 V vs. SCE. 4n

-0.6 -0.4 Potential Vvs.SCE

-0.2

Fig. 16.3. DPV scans of 1.5 x 106 M lead on BDD (a) after 3 minutes of deposition at -1.0 V vs. SCE (sat. calomel electrode), (b) without deposition.

16.3.2. Trace lead detection The new approach of mercury-free analysis of trace metals using BDD electrodes proposed by Manivannan et al. indicated in the previous section will be discussed in detail in this section. In the first implementation of this approach, a differential pulse anodic stripping voltammetry (DPASV) experiment was carried out at

347

submicromolar Pb(N03)2 concentrations at diamond electrodes in 0.1 M KCl (pH 1) [11]. Deposition involved holding the potential at -1.0 V vs. SCE for 2 min. Figure 16.4 shows the DPASV curves obtained for several Pb concentrations from 4 x 10 '^ to 2 x 10 ^ M in 0.1 M KCl (pH 1) following deposition.

-0.6

-0.5

-0.4

-0.3

0.2

Potential, V vs. SCE

Fig. 16.4. DPASV for Pb(N03)2 in 0.1 M KCl (pH l), obtained for the BDD electrode after holding the potential at -1.0 V vs. SCE for 2 min. Concentrations were (a) 2 x lO'S M; (b) 1.6 x lO^ M; (c) 1.2 x lO^ M; (d) 8 X 10'^ M (e) 4 X 10'^ M; and (f) background; sweep rate, 20 mV s'l; pulse amplitude, 100 mV; sampling time, 10 ms. A monotonic increase in the stripping peak current was observed with increasing concentration (Fig. 16.5). The linearity observed here and in previous studies on Pb validates the potential usefulness of this technique for quantification. A concentration of 4 X 10 9 M was detected with different measurement conditions (Fig. 16.6), specifically, linear sweep voltammetry (LSV) instead of DPV. Three differences in the two experimental results should be noted. First, although the peak potentials are similar (-0.53 to -0.51 V in 348

16. Boron-Doped Diamond Electrodes for the Analysis of Trace Metals

Fig. 16.6 and "0.52 V in Fig. 16.4), the onset of current occurs at a more negative potential, and the current minimum occurs at a much more positive potential in Fig. 16.4 compared to Fig. 16.6, i.e., the peak width is much larger (0.125 V in Fig. 16.6 vs. 0.045-0.065 V in Fig. 16.4). Second, although the concentration in Fig. 16.6 is a factor of 100 lower than the lowest concentration in Fig. 16.4, the background-corrected peak current ('^0.3 JLIA) is only a factor of ~5 lower (-1.4 |xA). There is only a factor of 2.5 difference in the deposition times (2 vs. 15 min). Both of these differences are related to the higher sweep rate. The increased peak width may be ascribed to kinetic factors [17], and the increased relative current can be ascribed to the simple linear relationship expected for peak current (sweep voltammetry) vs. concentration for a surface-bound redox couple [18].

0.5

1

1.5

2

Concentration (Xlo'^M)

Fig. 16.5. Linear plot of the differential current vs. concentration of Pb2+ (data from Fig. 16.4). Finally, there is a lower magnitude, broader peak at more positive potentials in both figures. This peak is shifted to more positive potentials in Fig. 16.4, probably also due to the increased 349

sweep r a t e . The origin of t h i s p e a k is, at present, u n k n o w n . Both LSV and DPV are equally sensitive techniques. However, in the case of LSV, we have found t h a t it is sometimes difficult to strip all of t h e m e t a l deposited on t h e BDD surface.

-I

-0,8 -(K6 -0.4 -0.2 I'olcniiul, V vs. SCK

0

Fig. 16.6. Linear sweep voltammetric scans for lead (4 x 10^ M Pb) in water. There are several possible reasons to expect lower detection limits on diamond electrodes in comparison to mercury electrodes(i) the intrinsic double-layer capacitance for diamond (~2 to 15fiF cm-2) [19,20] is significantly lower t h a n t h a t for mercury (-10 to SOjLiF cm-2); [21] (u)t\ie

p e a k c u r r e n t dependence upon sweep rate

is usually to the square root with mercury, due to diffusion within t h e mercury itself, w h e r e a s it should be linear with sweep r a t e for a solid electrode like diamond [22] a n d (Hi) due to t h e m u c h greater degree of resistance to oxidation of diamond, it is possible to use much more severe pulsing conditions, e.g., larger pulse a m p l i t u d e s a n d sweep r a t e s

350

16. Boron-Doped Diamond Electrodes for the Analysis of Trace Metals

Figure 16.7 shows ASV results for several Pb concentrations obtained with the LSV method [23]. The detection limit was ca. 2 nM (ca. 400 ppt). This is sufficiently sensitive for public health applications,

considering

that

the

maximum

allowable

concentration in drinking water is 50 ppb.

-0.6

-0.4

E (V) vs SCE

Fig. 16.7. Lead detection by use of the Unear sweep voltammetric technique shows anodic stripping voltammetric curves for several Pb concentrations. The detection Umit is ca. 2 nM (ca. 400 ppt); stripping conditions* accumulation potential, -I.IV, accumulation time, 15 min.; sweep rate, 200 mV s i.

16.3.3. Mercury detection Mercury detection is also extremely important from the standpoint of public health.

We have recently carried out studies on the

detection of Hg itself at a BDD electrode using cyclic voltammetry (CV) and DPASV. The nature of the electrode material is an important consideration for trace detection.

In the past, the

determination

anodic

of

mercury

in

water

by

stripping 351

voltammetry h a s been carried out on several types of electrodes, including graphite [24], carbon p a s t e [25, 26], glassy carbon [27,28], modified glassy carbon [29], gold [30,31] p l a t i n u m [32], modified gold [33] and gold-plated glassy carbon [34]. In some of these cases, electrode p r e p a r a t i o n is necessary, for example, by polishing or chemical modification. A detection limit of 10^^ y[ h a s

been

reported for a modified GC electrode [29]. A detection limit of 5 x 10 14 M h a s been reported for GC r o t a t i n g disk electrodes w i t h the addition of thiocyanate a s a m e a n s of facilitating both deposition a n d stripping [27, 28]. In t h i s case, careful polishing is required. Reproducibility can be an i m p o r t a n t issue if electrode p r e t r e a t m e n t is used. In the absence of anions t h a t form soluble compounds, the relevant reactions are a s follows, where S H E is the

standard

hydrogen electrode [35]* Hg22+ + 2e -> 2Hg; Eo = +0.79 V vs. SHE, +0.55 V vs. SCE

(16.l)

2Hg2+ + 2e- ^ Hg22+; EO = + 0 . 9 2 V vs. S H E , + 0 . 6 8 V vs. SCE (16.2) Hg2+ + 2e- -^ Hg; EO = +0.85 V vs. SHE, +0.61 V vs. SCE

(16.3)

substantially in t h e negative direction* Hg2Cl2 + 2e ->2Hg + 2C1; EO = +0.27 V vs. SHE, +0.03 V vs. SCE

(16.4)

Cyclic voltammetric sweeps for various concentrations of Hg2+ at a BDD electrode are shown in Fig. 16.8, together with the background trace, which clearly shows t h e wide working potential r a n g e . The curves for solutions containing mercury show cathodic a n d anodic p e a k s due to t h e reduction of Hg2+ a n d t h e oxidation of deposited metallic Hg respectively. The a p p a r e n t charge associated with the cathodic deposition p e a k is much less t h a n t h a t for the

352

16. Boron-Doped Diamond Electrodes for the Analysis of Trace Metals

oxidation. This is due to the fact that the deposition process continues out to a relatively negative potential limit (-0.6V vs. SCE) and is enhanced by mass transport as a result of hydrogen evolution.

0.0

0.1

0.2

0.3

0.4

0.5

0.6

Potential, V vs. SCE

Fig. 16.8. CVs for various concentrations of Hg2+ at a BDD electrode [Hg(N03)2 in 0.1 M KNO3 (pH = l)]. The concentrations were (a) 1.5 x 10-'^; (b) 2 X 10-7; (c) 2.5 X lO'^; (d) 3 x lO^; (e) 3.5 x 10-7; (£) 4 x 10"^ (g) 4.5 X 10 "^ (h) 5 X 10"^ M. The background current is also shown. The sweep rate was 20 mV s^. For a concentration of 3.2 x lO'^M, the cathodic peak appeared at ca. 0.12 V vs. SCE and the oxidation peak at ca. 0.23 V vs. SCE. As the concentration was increased to 3.2 xlO'^ M, a simultaneous stepwise increase in the anodic and cathodic peak currents was observed (Fig. 16.8). The shift in the anodic peak potentials may be attributed to a kinetic limitation. The shift in the cathodic peak potentials is explained by the fact that, for low dissolved mercury concentrations, the trace level of chloride present tends to shift the deposition in the negative direction due to reaction 3 above. As the 353

mercury concentration increases, there is proportionately chloride available, which shifts direction.

These

effects

less

t h e potential in t h e positive

a r e less

important

in t h e oxidation

direction, because t h e local concentration of mercury would be significantly higher t h a n t h e trace chloride concentration next to t h e electrode d u r i n g stripping. These results show t h e typical oxidation/reduction processes. T h e well-defined anodic oxidation p e a k n e a r 0.25 V is d u e to stripping of t h e deposited Hg^, w h e r e a s t h e cathodic p e a k n e a r 0.15 V is d u e to reduction of Hg2+ ions to Hgo.

4

a

I

si <

=i 1.0

=3.

2\

K

/i go.5 -

/

i

/ \ / \

o

O -1

r—"" 0.0

-

-0.5 h

' - ^ • ^ . j ^ ^ ' ^ 0.1

0.2

0.3

0.4

Potential, V vs. SCE

0.5

-0.2

0.0

0.2

0.4

0.6

Potential, V vs. SCE

Fig. 16.9. CycUc voltammetry for (a) high-purity mercuric nitrate [Hg(N03)2 solution (2 xlO s M) in 0.1 M KNOs (pH = 1)1 as weU as (b) the effect of the addition of 1.8 mM of chloride. The mercury stripping peak appearing a t 0.4 V vs. SCE in (a) is shifted to 00.18 V vs. SCE in (b).

I n t h e p r e s e n t case, in which chloride is not specifically added to t h e solution, t h e r e will be a n intermediate reversible potential t h a t depends on t h e concentration of chloride a s a n impurity [36]. It is well known t h a t chloride is a ubiquitous impurity. A s e p a r a t e e x p e r i m e n t w a s carried o u t with high-purity mercury (II) n i t r a t e . 354

16. Boron-Doped Diamond Electrodes for the Analysis of Trace Metals

with an upper limit of 0.002% of chloride. Figures 16.9a and b show the CVs for the high-purity mercuric nitrate as well as the effect of the addition of chloride (l.8mM KCl). In the high-purity solution, a mercury stripping peak (Hg^ to Hg2+) is obtained ca. 400 mV. Integration of the cathodic and anodic charge

shows agreement within ca.

10%, indicating

quantitative stripping. In the presence of a known chloride concentration of 1.8 mM, an anodic peak appears at ca. 180 mV, due to reaction 16.4. This peak can no longer be considered a stripping peak, because the product remains on the electrode surface. Because a non-negligible, adventitious concentration of chloride was present in the measurements carried out in this work (0.002%), the anodic peak was assigned to reaction 16.4. It should be noted here that, because the oxidation process involves the formation of highly insoluble product IIg2Cl2 (calomel), the electrode must be cleaned carefully with concentrated HNO3, followed by rinsing with high-purity water.

This procedure can

perhaps be avoided by use of a thiocyanate-containing electrolyte [37].

The presence of ionic species such as chloride and nitrate also influences the sensitivity, especially for mercury. The presence of chloride ions actually improves the sensitivity, compared to the case of nitrate

alone. We have found

the sensitivity

and

reproducibility can be improved by gold co-deposition method, as explained later. It is important to note that the oxidation process involves the formation of the highly insoluble product Hg2Cl2 (calomel), which can adversely affect the reproducibility of the

355

experiment unless the calomel is cleaned from the surface between measurements. Even without the addition of gold, however, the analysis of trace mercury on BDD electrodes is feasible, however.

For

comparison with glassy carbon (GC), we have carried DPASV on both BDD and GC electrodes. Figure 16.10 shows clear evidence for the observation of the mercury stripping peak at the BDD electrode surface, whereas no peak was observed for the GC electrode.

-0.2

0.0

0.2

0.4

0.6

Potential, V vs. SCE

Fig. 16.10. Differential pulse voltammetry curves for Hg(N03)2 solution (6.4 xlO? M) in 0.1 M KNO3 (pH = l) electrolyte for a) diamond and b) glassy carbon electrodes. Deposition time was 2 minutes at -0.5 V vs. SCE," pulse ampUtude, 50 mV; sweep rate, 100 mV s i . In order to validate the ASV method for mercury determination on

BDD,

a

comparison

with

cold-vapor

atomic

absorption

spectrometry (CVAAS) has also been carried out using a real sample (KCl impinger solution) obtained from the flue gas of a coal-

356

16. Boron-Doped Diamond Electrodes for the Analysis of Trace Metals

fired power p l a n t . Figure 16.11 shows t h e DPASV analysis of t h e real sample using a BDD electrode. The concentration of mercury in t h e sample w a s estimated a s 120 ±7 ppb using t h e s t a n d a r d addition method.

A comparison of t h e same solution by CVAAS

indicated a mercury level of 115 ±5 ppb, which agrees well with t h e value e s t i m a t e d using the diamond electrode.

< +^

c 1.

O

-200

0

200

400

600

800

Hg Concentration (ppb) Fig. 16.11. Differential pulse voltammetry for a real sample [suppUed by the National Energy Technology Laboratory (NETL)] with three standard additions of mercury solutions, yielding final concentrations of 300, 500, and 700 ppb. An intercept value of 120 ppb was obtained for the unknown mercury concentration. Deposition time, 120 s, at 0.5 V vs. SCE; pulse width, 40 ms; pulse delay, 160 ms; pulse amplitude, 50 mV; sweep r a t e , 20 mV s'^. We have t h u s d e m o n s t r a t e d the feasibility of using BDD electrodes for t h e analysis of mercury a n d other trace metals. The effect of n i t r a t e a n d chloride ions is very clearly evident from t h e cyclic voltammetric analysis. The presence of chloride e n h a n c e s t h e stripping p e a k currents, b u t it forms insoluble calomel at t h e 357

surface of the electrode. In order to avoid this problem, we followed a new approach in which a small amount (1-3 ppm) of gold was added to the analyte solution and was co-deposited on the BDD electrode surface. The calibration plots for 10-50 ppb and 2-10 ppb of mercury in the presence of a 3-ppm gold standard solution (not shown) were found to be quite linear.

16.3.4. Lead-cadmium mixtures In real-world samples, it is common to encounter multiple trace metals of the type that can be detected with anodic stripping voltammetry. In the analysis of multiple elements with ASV, the use

of

metallic

mercury-electrodes

has

traditionally

been

recommended because of the absence of interactions between the metals dissolved in the liquid mercury. With solid electrodes, there is always the possibility of interactions between the deposited metals, because they are deposited in solid form and can thus form alloys, intermetallic compounds, or other types of intermetallic species. Experiments interactions

were performed

between

Pb

and

to try Cd

to understand

during

anodic

the

stripping

voltammetric analysis at BDD electrodes. Figure 16.12 shows the DPASV stripping peaks for Cd deposited from solutions with various concentrations of Cd in acetate buffer (pH 5), with a peak potential of ca. -0.85 V vs. SCE. A linear increase in the differential current with concentration is apparent.

358

16. Boron-Doped Diamond Electrodes for the Analysis of Trace Metals

20

<

-OjiM 16

1 \M 2\M —V- -Z\M —-tr- 4^M —0-- - 5nM --D-

..-A-

a(xm\\tmmyfmw?. -0.2

-0.6 -0.4 Potential, V vs. SCE

0.0

Fig. 16.12. Differential pulse anodic stripping voltammetry curves for the stripping of Cd deposited from solutions containing 1-5 pM Cd(N03)2 in 0.2 M acetate buffer (pH = 5.0); the deposition time was 2 minutes at -1.0 V vs. SCE.

Figure 16.13 shows the stripping behavior of Cd deposited from various solution concentrations of Cd in the presence of 5 ^iM of Pb(N03)2. In addition to the Cd stripping peak at -0.85 V, the Pb stripping peak can be seen at -0.65V vs. SCE. In the presence of 5 ^iM Pb, the peak currents for Cd were consistently ca. 55% smaller than those obtained in pure Cd^^ solutions. The decreases in the peak currents for Cd were explained on the basis of the proposed formation of an alloy of Pb and Cd [12].

359

-0.8

-0.6

-0.4

Potential, V vs. SCE

Fig. 16.13. Differential pulse anodic stripping voltammetry curves for the stripping of Cd and Pb from solutions containing Cd(N03)2 in 0.2 M acetate buffer with 5 mM Pb(N03)2J the deposition time was 2 minutes at -1.0 V vs. SCE.

In order to understand the interactions more fully, we constructed

three-dimensional

currents were plotted

plots, in which the

as a function

of both

stripping

Pb and

Cd

concentrations (Fig. 16.14). With the 3D plots, it is possible to carry out a quantitative analysis for a Pb-Cd mixed solution, even though there is a significant cross-interference. This can be done by means of successive approximations, working back and forth between the plots for Cd stripping (Fig. 16.14a) and Pb stripping (Fig. 16.14b) [12].

360

16. Boron-Doped Diamond Electrodes for the Analysis of Trace Metals

r12

• 10-12

Cd peak Height

|io

• 8-10

r

06-8

(M)

is

Cd Cone. ipM)

L4

n4-6

2

B2-4

0 sO-2

Pb Cone. (nM) • 16-18

B r18 16 14 12

Pb peak Height (MA)

4'^M/^•Bi^^^^^

10 8 6 4 T2 0

Cd Cone. (MM)

n 14-16 G12-14 • 10-12 •8-10 •6-8 •4-6 •24 •0-2

Pb Cone. (fiM)

Fig. 16.14. Three-dimensional calibration curves for the differential pulse anodic stripping currents for A) Cd and B) Pb in acetate buffer solutions containing both metals.

16.3.5. Lead-copper mixtures A commonly encountered combination of metals in water is lead and copper, both being present as impurities due to the water having passed through pipes made from these two metals. Recently, Prado et al,, reported the simultaneous detection of lead and copper in aqueous solution using anodic stripping voltammetry (ASV) at BDD [38]. Voltammetry and AFM imaging were used to show that, 361

while both metals nucleate as their pure phases on BDD, the copper nuclei, which form more easily than those of lead, act as favorable sites for the subsequent nucleation and growth of lead; the latter act to inhibit hydrogen evolution on the copper surface. ASV at BDD electrodes provides the basis for a method of independent detection of Cu and Pb via conventional standard addition procedures [39, 40]. Measurements were also carried out in our laboratory to examine the interactions of lead and copper [23]. In our work, which is complementary to that of Prado et al., we have used much lower solution concentrations of the respective metals, down to ca. 10 nM lead, in the presence of various concentrations of copper (O to 100 nM). Contrary to the results of Prado et al., [38] we observed no evidence for hydrogen evolution catalyzed by copper deposits and thus no evidence for the inhibition thereof with the subsequent deposition of lead. This discrepancy is most likely due to the lower metal concentrate.ions used in our work. The most interesting finding from this work was that, even though lead'copper alloy does indeed form during electrodeposition, it does so predominantly at higher pH, and the mutual interference can largely be avoided by adjusting the pH to low values (pH 1S). The effect of pH on the anodic stripping linear sweep voltammetry curves for Pb and Cu is shown in Fig. 16.15. We also experimented with the use of so-called masking agents, which are designed to form complexes selectively with copper, so that the electrodeposition of the latter is impeded. This strategy was found to be promising, with the most effective masking agent

362

16. Boron-Doped Diamond Electrodes for the Analysis of Trace Metals

being a water-soluble, metal-free porphyrin, which can indeed form a complex with copper much more easily than with lead [23].

Fig. 16.15. Linear-sweep voltammograms recorded in BrittonRobinson buffer solution at several pH values, for equal concentrations (10 nM) of lead and copper. pH- 1.8, 2.9, 4.1, 5.3, 6.4, 7.5 and 8.7. Stripping conditions" accumulation potential, -1.1 V vs. SCE; accumulation time, 15 min.J sweep rate, 200 mV s'^.

16.4. Special Techniques In this section, we will describe some of the special techniques that are used for trace metal detection in order to improve the sensitivity. These techniques are based on the use of the following modifications- {i) the rotating ring disk electrode; {ii) ultrasonic enhancement of mass transfer; and (iiiy^ microwave enhancement.

363

16.4.1. Rotating disk electrodes Rotating disk electrodes have been used for many years in electrochemistry in order to enhance and better control the hydrodynamics [41]. The use of such electrodes in ASV analysis has also been pursued for a number of years, because it can lead to both higher sensitivity and higher precision. However, during the stripping, the electrode rotation can also help to improve the sensitivity by enhancing the mass transport of the oxidized metal ions into the bulk solution. Here, the stripped metal ions from the disk electrode can be monitored again, i.e., by measuring the reduction current, as they pass over the ring by laminar flow. This method further increases the detection sensitivity. Electrodes such as gold, platinum, glassy carbon have been used widely as RDEs and RRDEs. Currently, BDD has also been used as a disk electrode [41-43]. Vinokur et. al. showed that a diamond disk combined with gold rotating ring-disk electrode (RRDE) confirmed the formation of higher valent silver in a silver nitrate-nitric acid solution. This was performed when the diamond potential scan was positive while the ring potential was held at values above the oxygen reduction potential. During this time higher valent silver species alone were detected [44]. Our

group

also used

diamond

RDE electrodes for

the

determination of lead. The use of rotating electrodes showed that a factor of ca. 10 enhancement in the stripping current was realized. Linear sweep voltammetry curves for 2 nM lead solution using a RDE are shown in Fig. 16.16. The increase in the stripping peak current is evident with an increase in the rotation speed [45].

364

16. Boron-Doped Diamond Electrodes for the Analysis of Trace Metals

lOOnA X 1 : 0 rpm 2 : 100 rpm 3 : 900 rpm 4 : 1600 rpm 5 : 2500 rpm

-1.0

;///

-0.6

-0.4

-0.2 0 E(V) vs. SCE

Fig. 16.16. Linear sweep voltammtery curves for 2 nM lead solution using a rotating disk electrode.

16.4.2. Acoustic techniques Saterlay

et

al,

investigated

ultrasonicallyassisted

cathodic

stripping voltammetry (CSV) at a boron-doped diamond electrode, for the detection of lead. [46]. Square-wave sono-CSV was employed to obtain a response for Pb^^ in river sediment. Additions of an aqueous lead standard were then performed. Extrapolation of the stripping currents resulting from the lead additionsy facilitated the quantification of lead in the acid digest, and hence a lead content of 187.1 mg/kg was calculated for the sediment sample. It has been demonstrated that the application of power ultrasound both increases the efficiency of the electrochemistry taking place and, through continuous electrode activation, reduces the chances of electrode fouling from possible adsorption of organic species. 365

Similarly, a study of the sono-electroanalysis of silver at a highly boron-doped diamond electrode was presented [47]. Both cathodic and anodic stripping voltammetry have been investigated in terms of their analytical suitability towards silver detection. Cathodic stripping voltammetry, via electrodeposited silver oxide, was affected by the unusual chemistry of the highly oxidizing Ag^^ species and the characterization of this system is discussed in detail. Anodic stripping, via deposition of metallic silver on the bare boron-doped diamond electrode surface under ultrasound, coupled with square-wave voltammetry, was successfully employed in the development of a sensitive technique for the analysis of trace silver ions. A detection limit for Ag+ of 10 ^ M for a 300-second deposition, with a linear range of at least two orders of magnitude, and the beneficial effects of controlling the speciation of Ag+ via complexation with chloride ions, are reported. Current responses for silver stripping were clearly observed at concentrations above 10^ M (the practical lower limit of detection for

a

300-seconds

deposition),

with

the

corresponding

voltammograms shown in Fig. 16.17a. A linear relationship was observed between Ag^ concentration and signal response over at least two orders of magnitude-(Fig. 16.17b). As another example, sono-CSV was applied for the detection of manganese in instant tea [46]. Differential-pulse voltammetry was used to give the analytical signal from the cathodic stripping of electrodeposited Mn02J linearity was observed from 10 ii M to at least 3 X 10'' M, with 10"^^ M being the detection limit for a 2-min deposition. deposition

366

The procedure of Mn02

and

involves both

ultrasonic-cathodic

ultrasonic-anodic stripping.

The

16. Boron-Doped Diamond Electrodes for the Analysis of Trace Metals

differential-pulse sono-cathodic stripping voltammetric technique was used to successfully determine the manganese content of two instant tea samples, giving excellent agreement with independent AAS analyses. using

atomic

The samples were also analyzed independently absorption

spectrometry

(AAS), with

a

good

correlation being observed between the sono'DPCSV technique and AAS data [46].

-0.3

-0.1

O.I 0.3 EA^ (vs. S C E )

0.5

0.7

20

AQ GO 80 (Ag*J/ 10-^ M

100

T20

Fig. 16.17. (a) Linear sweep voltammograms (O.O to +1.0 V (vs. SCE) at 100 mV s-i; step potential, 5 mV) of (i) lO'^ M, (ii) 3.7 x lO^ M, (iii) 8.0 X 0-9 M, (iv) 1.5 X 10 8 M, (v) 2.7 x 10 « M, (vi) 5.1 x 10 « M and (vii) 1.0 X 10 7 M Ag+ solutions in 0.5 M HNO3 and 12.5 mM KCl, after 300 s insonated deposition (10 mm horn to electrode separation, 14 W cm" 2) at -0.5 V (vs. SCE); (b) plot of reduction current (J) vs. Ag+ concentration for Unear scan sono-anodic stripping voltammetry, experimental parameters as in (a). Ultrasound was also applied for short periods of time to prevent the temperature of the solution in the electrochemical cell from rising above room temperature (T = 25 ±5 °C) during gold deposition/stripping and gold colloid formation [48].

367

16.4.3. Microwave techniques Application of microwaves in electrochemical processes has been recently demonstrated to allow rapid heat pulses [49, 50] or stable high temperature conditions [51] to be applied locally at the working electrode/electrolyte interface in electrochemical systems. Tsai et

al. have

adopted

the

microwave

activation

of

electrochemical processes and have employed it to improve the analytical detection of Pb [52,53]. The combination of microwave enhancement and boron-doped diamond electrode material has been introduced and explored for analytical procedures recently [49-54]. It was discovered that both the deposition of Pb metal at negative potential followed by anodic stripping, and the deposition of Pb02 at positive potential followed by cathodic stripping are enhanced in the presence of microwave activation. This exploratory work is extended here in order to develop a novel electroanalytical technique for the analysis of Pb. The new technique was applied to determine the Pb content in a contaminated river sediment sample. A special microwave working electrode was constructed from a 150 mm diameter tungsten rod coated with a boron-doped diamond film. For microwave activation experiments, a modified multimode microwave oven with modified power supply, a water energy sink, and a special port [49-54] for the electrochemical cell were used. A high temperature region exists about 1-20 p.m from the electrode), and a convective flow of liquid through the high temperature region towards the electrode surface is induced by density changes. The convective flow accounts for ca. 50% of the observed increase in mass transport.

368

16. Boron-Doped Diamond Electrodes for the Analysis of Trace Metals

Stripping voltammetry experiments at a boron-doped diamond electrode with 2 mM Pb2+ in 0.1 M HNO3 were carried out in order to study the effect of microwave radiation on this process. Microwave activation has a considerable effect on the square-wave stripping response, with signal-to-noise improvements of more than one order of magnitude. A plot of Pb2+ concentration vs. the anodic stripping current (not shown) was linear and demonstrated the applicability of the BDD electrode in the presence of microwave radiation. It was also apparent that the current response for the lead stripping process was enhanced by application of microwave radiation. The limit of detection by square-wave voltammetry after a 20-s deposition time was found to be 0.1 mM and 1.0 mM with and without microwave activation, respectively. The Pb content in a water sediment sample detected by anodic stripping voltammetry at boron-doped diamond electrodes is shown to be in good agreement with two other independent analytical procedures based on

ICP

mass

spectroscopy

and

on

sono'cathodic

stripping

voltammetry. Similarly, the effect of microwave on the detection of cadmium in the presence of a nonionic surfactant (Triton X) was also reported [49-54]. In the presence of the surfactant, Triton X-100, which blocks the electrochemical Cd^^ response under conventional conditions, microwave activation is shown to have a considerable effect in enhancing the sensitivity for Cd2+ detection. Square "wave voltammograms were obtained for a range of Cd2+ concentrations in the presence of Triton X-100. A plot of cadmium concentration versus peak height (not shown) was linear

369

over a wide r a n g e of concentrations a n d therefore d e m o n s t r a t e s the applicability of t h e microwave activation approach.

16.5. Conclusions and Future Prospects The future a p p e a r s to be bright for the further application of BDD electrodes in the analysis of trace metals. Some of the challenges for t h e future include t h e following- l) extension of the ASV technique to a wider range of metals, including those at even more negative potentials a n d even more positive potentials; 2) achieving a b e t t e r u n d e r s t a n d i n g of the interactions between

deposited

metals; 3) development of ways to deposit m e t a l s in a controlled fashion

in

order

to

avoid

co-deposition;

4)

increasing

the

u n d e r s t a n d i n g of the role of the BDD itself in the deposition process; and 5) development of practical analytical systems for both anodic stripping analysis a n d cathodic stripping analysis.

References 1.

J. Wang, D. Larson, N. Foster, S. Armalis, J. Lu, X. Rongrong, K. Olsen, and A. Zirino, Anal

2.

J. Wang, N. Foster, S. Armalis, D. Larson, A. Zirino and K. Olsen, Anal

3.

Chem., 67 (1995) 1481.

Chim. Acta., 310 (1995) 223.

J. Wang, J. Lu, D. Luo, M. Jiang and B. Tian, Anal

Chem., 69

(1997) 2640. 4.

Z. Peilin, Z. Jianzhong, Y. Shenzhong, Z. Xikang and Z. Guoxiong, FreseniusJ.

370

Anal

Chem., 353 (1995) 171.

16. Boron-Doped Diamond Electrodes for the Analysis of Trace Metals

5.

A. Mohamad, J. W. Strojek and G. M. Swain, J.

Electrochem.

Soc, 3 (1995) 142. 6.

E. Fischer, M.G. Constant, and Van den Berg, Anal. Chim. Acta. 385 (1999) 273 and references therein; H. P. Wu, Anal. Chem., 68 (1996) 1639 and references therein.

7.

M.A. Nolan, S.P. Kounaves, Anal.

Chem., 71 (1999) 3567 and

references therein. 8.

A. Manivannan, D. A. Tryk, and A. Fujishima, Solid-state

Electrochem.

Lett, 2 (1999) 455.

9.

R. Ramesham, J. Mater. Sci, 34 (1999) 1439.

10.

A. Manivannan, M. S. Seehra and A. Fujishima, Fuel

Process

Tech. 85 (2004) 513. 11.

A. Manivannan, M. S. Seehra, D. A. Tryk and A. Fujishima, Anal. Lett,

12.

35 (2002) 355.

A. Manivannan, R. Kawasaki, D. A. Tryk and A. Fujishima, Electrochim. Acta, 49 (2004) 3313.

13.

F. Marken, Y. C. Tsai, A. J. Saterlay, B. A. Coles, D. Tibbetts, K. B. Holt, C. H. Goeting, J. S. Foord and R. G. Compton, J. Solid State Electrochem.,

14.

5 (2001) 313.

N. Vinokur, B. Miller, Y. Avyigal and R. Kalish, J.

Electrochem.

Soc, 143 (1996) L238. 15.

L. L. Feng, D. Totir, B. Miller, G. Chottiner, A. Argoitia, J. C. Angus and D. Scherson, J. Am. Chem. Soc, 119 (1997) 7875.

16.

G. Gunawardena, G. Hills and I. Montenegro, J. Chem. Interfac. Electrochem.,

17.

138 (1982) 241.

A. J. Bard and L. R. Faulkner, Fundamentals

Electroanal

and Applications,

Electrochemical

Methods-

New York- John Wiley & Sons,

Inc., (1980) 412.

371

18.

A. J. Bard and L. R. Faulkner, Fundamentals

and Applications,

Electrochemical

Methods-

New York* John Wiley & Sons,

Inc., (1980) 410. 19.

G. M. Swain, Adv. Mater., 6 (1994) 388.

20.

K. Honda, T. N. Rao, D. A. Tryk, A. Fujishima, M. Watanabe, K.

21.

Yasui and H. Masuda, J. Electrochem.

Sac, 147 (2000) 659.

A. J. Bard and L. R. Faulkner,

Electrochemical

Fundamentals

and Applications,

Methods'

New York* John Wiley & Sons,

Inc., (1980) 502. 22.

A. J. Bard and L. R. Faulkner, Fundamentals

and Applications,

Electrochemical

Methods-

New York* John Wiley & Sons,

Inc., (1980) 413. 23.

D. Drago, N. Spataru, R. Kawasaki, A. Manivannan, D. A. Tryk and Fujishima, A. (in preparation)

24.

E. A. Viltchinskaia, Electroanalysis,

L. L. Zeigmann

and

S. G.

Morton,

7 (1995) 264.

25.

Z. Navratilova and P. Kula, Anal

Chim. Acta., 273 (1993) 305.

26.

P. Hernandez, E. Alda and L. Hernandez, Fresenius

J.

Anal

Chem., 327 (1987) 676. 27.

S. Meyer, F. Scholz and R. Trittler, Fresenius J. Anal

Chem., 356

(1996) 247. 28.

D. Jagner, Anal

Chim. Acta., 105 (1979) 33.

29.

I. Turyan and D. Mandler, Electroanalysis,

30.

I. Svancara, M. Matousek, E. Sikora, K. Schachl, K. Kalcher, K. Vytras, Electroanalysis,

6 (1994) 838.

9 (1997) 827.

31.

M. Korolczuk, Fresenius J. Anal. Chem., 357 (1997) 389.

32.

R. Z. Hassan, D. F. Untereker and S. J. Bruckenstein, Electroanal.

372

Chem., 42 (1973) 161.

J.

16. Boron-Doped Diamond Electrodes for the Analysis of Trace Metals

33.

C. Faller, N. Yu. Stojko, G. Henze and Kh. Z. Brainina,

Anal

Chim. Acta., 396 (1999) 195. 34.

EPA Method

7472, Mercury in Aqueous Samples and Extracts

Anodic Stripping 35.

Voltammetry

P. W. Atkins, Physical

by

(ASV).

Chemistry,

Oxford: Oxford University

Press, 6 ed. (1998). 36.

P. K. Wrona and Z. Galus, Encyclopedia the Elements,

of Electrochemistry

of

ed. A. J. Bard., New York: Marcel Dekker, Vol. IXA.

(1976). 37.

M. A. Nolan and S. P. Kounaves, Anal

38.

C. Prado, S. J. Wilkins, F. Marken and R. G. Compton, Electroanalysis,

39.

Chem., 71 (1999) 1176.

14 (2002) 262.

J. S. Gao, T. Arunagiri, J. J. Chen, P. Goodwill and O. Chyan, Chem. Mater,, 12 (2000) 3495.

40.

M. C. Granger, J. Xu, J. W. Strojek and G. M. Swain, Anal

Chim.

Acta,. 397 (1999) 145. 41.

Yu. V. Pleskov and V. Y. Filinovskii, Rotating

Disc

Electrode

(Book written in Russian)., Moscow: Nauka. (1972) 344. 42.

V. A. Myamlin and Pleskov, Electrochemistry

of

Semiconductors.

Plenum, New York, 1967. 410. 43.

V.

Y.

Filinovskii

Electrochem, 44.

and

Yu.

V.

Pleskov,

Compr.

Treatise

9 (1984) 293.

N. Vinokur, B. Miller, Y. Avyigal and R. Kalish, J.

Electrochem.

Soc, 146 (1999) 125. 45.

A. Manivannan, D. A. Tryk and A. Fujishima,

(unpublished

results). 46.

A. J. Saterlay, J. S. Foord and R. G. Compton,

Analyst

(Cambridge, U.K.), 124 (1999) 1791.

373

47.

A. J. Saterlay, F. Marken, J. S. Foord and R. G. Compton, Talanta, 53 (2000) 403.

48.

K. B. Holt, S. Genevieve, R. G. Compton, J. S. Foord and F. Marken, Electroanalysis,

49.

14 (2002) 797.

R. G. Compton, B. A. Coles and F. Marken, Chem, Commun.,

23

(1998) 2595. 50.

R. P. Akkermans, S. L. Roberts, F. Marken, B. A. Coles, S. J. Wilkins, J. A. Cooper, K. E. Woodhouse and R. G. Compton, J. Phys. Chem. B., 103 (1999) 9987.

51.

F. Marken, S. L. Matthews, R. G. Compton and B. A. Coles, Electroanalysis,

52.

12 (2000) 267.

F. Marken, S. L. Matthews, R. G. Compton, B. A. Coles and R. G. Compton, New J. Chem, 24 (2000) 653.

53.

Y. C. Tsai, B. A. Coles, K. B. Holt, J. S. Foord, F. Marken and R. G. Compton, Electroanalysis,

54.

Y. C. Tsai, B. A. Coles, R. G. Compton and F. Marken, Electroanalysis,

374

13 (2001) 831.

13 (2001) 639.

17. Industrial Applications of Boron-Doped Diamond Electrodes: Detection of Sodium Thiosulfate, Naproxen and Nickel Ions and Electrocatalysis of Oxygen Reduction Vembu Suryanarayanan, Yanrong Zhang and Sachio Yoshihara

A number of research reports have appeared recently describing the emergence of boron-doped diamond (BDD) as a unique electrode material in electrochemistry [1,2]. Such interest has resulted

from

the

widespread

electrochemical

systems,

electrocatalysis

[6,7],

for

use e.g.,

waste-water

of

this

material

electroanalysis treatment

[8,9]

in

[3-5], and

electrosynthesis [lO]. It is to be noted that various characteristics, such as the presence of carbon-oxygen

functionalities,

the

hydrophobicity, the microstructure, as well as the electronic properties, have an influence on their electrode kinetics and reaction mechanisms.

Diamond is one of the best insulators in the

world, and when doped with boron, it can possess either metallic or semimetallic properties, depending on the doping level. Diamond thin films prepared by microwave plasma-assisted chemical vapor deposition (MPACVD) can be doped as high as 10^ ppm B/C, resulting in low resistivities, rendering them useful electrode materials in electrochemistry. The doping level and the hydrogen Sachio Yoshihara

e-mail: [email protected]

375

content in the film will influence the carrier concentration. During deposition,

the

boron

impurity

atoms

may

be

inserted

substitutionally into the diamond lattice, and these boron atoms are considered to be responsible for the conduction [ll]. Also, the properties of BDD materials, having sp^'type carbon, based on a tetrahedrally bonded structure, are unique and totally different from other, sp^-type trigonally bonded carbon materials. Diamond electrodes appear to be well suited for electroanalysis because of their well known properties- (i) wide working potential window in both aqueous and non-aqueous electrolyte solutions, (ii) negligible adsorption of polar organic molecules, (iii) extreme electrochemical stability, (iv) insensitivity to dissolved oxygen, (v) low and stable voltammetric background current and very low capacitance, resulting in high signal to background (S/B ratio) and low detection limit, (vi) long term response stability during the exposure to air and (vii) excellent morphological stability at high temperatures and current densities. These properties have spurred our efforts to study and develop diamond electrodes for application as amperometric detectors for industrially important compounds, for example, sodium thiosulfate and naproxen. Additionally, a novel method of detecting nickel ions in electroless deposition solutions using diamond electrodes will also be reported. Finally, the electrocatalytic activity of gold nanoparticles deposited on diamond for oxygen reduction will be evaluated. BDD films were grown on p-Si ( i l l ) substrates by the MPCVD technique (ASTeX Ax2115); the preparation method has been described in our earlier paper [12]. After the deposition, the film

376

17.Industrial Applications of Boron-Doped Diamond Electrodes: Detection of Sodium Thiosulfate,Naproxen and Nickel Ions and Electrocatalysis of Oxygen Reduction

was sonicated in ethanol and deionized in water prior to use. Ohmic contacts between the electrodes and the lead wires were made using gold paste. The electrodes were covered with masking tape in order to define an exact area for measuring the current.

17.1.

Electroanalysis of Sodium Thiosulfate [13]

Sodium thiosulfate, also called "hypo," is an important compound that finds application in chemical and biological fields, especially in the photographic industry. It is becoming a prime pollutant in that industry, and it is therefore very important to detect its presence in waste materials. The electrochemical method used at present involves

amperometric

analysis

using

glassy

carbon

(GC)

electrodes modified with either cobalt or nickel hexacyanoferrate as the electrode material [14,15]. On the other hand, it is found that the increase in anodic current after the addition of thiosulfate is only marginal, and the oxidation potential of thiosulfate could not be determined in the Cyclic Voltammogram (CV) due to the presence of high catalytic background current as a result of modification. Further, the detection limit is only on the order of 10"^ M. However, BDD can be used to eliminate these problems, without any modification, for the amperometric determination of this analyte. In addition to measurements with the BDD electrode, the GC electrode was also used in the present work for the comparison of electrochemical parameters in the voltammetry of thiosulfate.

17.1.1. Cyclic voltammetry (CV) Fig. 17.1 (A) and (B) show typical CV in the presence (curves b) and

377

absence (curves a) of sodium thiosulfate on BDD and GC electrodes in 0.1 M phosphate buffer solution (pH 6.7). The BDD electrode showed a well-defined irreversible oxidation peak at 1.1 V vs. Ag/AgCl, whereas the voltammogram obtained for the GC electrode was poorly defined. Moreover, the voltammograms obtained on the BDD electrode exhibited a higher S/B ratio than that on the GC electrode. The electrocatalytic oxidation of thiosulfate may be represented as follows2S2032-

• S4062- + 2e

(17.1)

0.18

500

1000

1500

E / mV vs Ag/AgCl

Fig. 17.1. Cyclic voltammetry of (A) BDD and (B) GC electrodes in the (a) absence (b) and presence of 0.5 mM of sodium thiosulfate in 0.1 M phosphate buffer solution (pH 6.78) at a sweep rate of 60 mV s"l.

378

U.Industrial Applications of Boron-Doped Diamond Electrodes: Detection of Sodium Thiosulfate,Naproxen and Nickel Ions and Electrocatalysis of Oxygen Reduction

A plot of the square root of the scan rate (10 to 60 mV s'^^ vs. peak current density for the electrochemical oxidation was linear, and this shows that the oxidation current is diffusion-controlled, with negligible adsorption of the electroactive species on the electrode surface. The peak potential value of the irreversible wave was also observed to shift positively with increasing sweep rate. A linear increase of peak current density was found for the concentration range from 0.01 mM to 0.5 mM of analyte in 0.1 M phosphate buffer (pH 6.7) at a constant sweep rate of 60 mV s"i- A signal to background ratio of 3 was obtained in this case at a concentration as low as 5 ^M. 17.1.2. Flow-injection analysis (FIA) The flowinjection analysis system consisted of a home-built thin layer flow cell, an injection port with a 20-^L injection loop, a peristaltic pump (EYELASMP-23) and an electrochemical detector. The thin-layer flow cell consisted of the Ag/AgCl reference electrode and a stainless steel counter electrode, which also served as the outlet or inlet for the solution phase. A 0.5-mm"thick silicon rubber gasket was used as a spacer in the cell; a rectangular groove (10 x 2.2 X 0.5), cut from the gasket, defined the channel flow. The detection potential for the FIA measurements was established

by

means

of

hydrodynamic

voltammetry.

The

hydrodynamic voltammogram exhibited a sigmoidal shape, and beyond a potential of 1.2 V, it reached a limiting current. From a plot of the S/B ratios (obtained from the above measurements), as a function of peak potential, a maximum S/B ratio was obtained at 1.2 V vs. Ag/AgCl, and this potential was selected as the

379

amperometric detection potential for the FIA technique. Figure 17.2 shows an FIA measurement for a series of repetitive 20 |LIL injections of various concentrations of thiosulfate under identical experimental conditions at a detection potential of 1.2 V vs Ag/AgCl. Well defined

amperometric

signals were found

out

at

all

concentrations, and a dynamic linear range of current vs. concentration was also obtained in the calibration curve (r > 0.99, inset of Fig. 17.2), which showed that the BDD electrode could detect this analyte at a concentration as low as 0.2 ^M. The precision was obtained from three concentrations of added solutions with five injections (75, 50 and 25 |im), which gave values of 1.7 to 2.0% for the relative standard deviation (RSD).

=3. 0

50

100

150

Concentration( MM)

100 /i M 75/^M

3

20s

tr

50//M

Time(s)

Fig. 17.2. Flow injection analysis results for the amperometric detection of 20 fiL of thiosulfate at various concentrations. The mobile phase was 0.1 M phosphate buffer (pH 6.7) at a flow rateof 1 mL min i. The inset shows the calibration curve.

380

17.Industrial Applications of Boron-Doped Diamond Electrodes: Detection of Sodium Thiosulfate,Naproxen and Nickel Ions and Electrocatalysis of Oxygen Reduction

Thus, the above results indicate that the FIA technique using the boron doped diamond electrode will be applicable towards the detection of thiosulfate in photographic waste effluents at very low concentration levels.

17.2. Electroanalysis of Naproxen Naproxen, [(S)-6-methoxy-a-methyl-2-naphthalene acetic acid] is a non-steroidal anti-inflammatory (NSAID), indicated for the treatment of rheumatoid, osteo and juvenile arthritis, as well as ankylosing spondylitis [16]. It is very useful for the relief of mild and moderate pain. Naproxen has also caused kidney problems and has sometimes caused blood pressure increases, especially for older people [17]. In the amperometric method, mercury [18] and Pt [19] have been used as the electrode materials of choice. However, mercury electrodes have some limitations as they are toxic and there is rapid deterioration of the electrode response. Conversely, the use of Pt electrodes shows high background current in the CV resulting in low S/B ratio with the addition of analyte and the linear dynamic range of concentrations is also very narrow. During the course of the synthesis of naproxen, other conditions such as pH, light and temperature may favor the formation of impurities such as 2-acetyl-6-methoxy naphthalene (AMN) (Scheme l) in addition to naproxen, and it is very important to detect this compound precisely in both raw materials and final products.

381

CH^ HCOOH

CH^ CH3

Naproxen

2-acetyl-6-methoxy naphthalene (AMN)

Scheme 17.1. 17.2.1. Voltammetric study A well-defined irreversible anodic peak was noted in the CV at 1.44 V vs. Ag/AgCl, for naproxen in 0.1 M LiC104 containing CH3CN on BDD. The electrochemical process is diffusion-controlled, and the number of electrons involved in the rate-determining step is equal to one. A slope of 30.8 mV per decade was obtained from the plot of log Ep vs. log sweep rate, which indicates that the rate control is first

order

following

electron

transfer.

From

this,

the

electrochemical reaction can be expressed by a EC type mechanism, where one-electron oxidation of naproxen proceeds with the formation of a cation radical intermediate in the aromatic rings, followed by the rapid protonation from the side chain. Since

differential

pulse

voltammetry

(DPV)

offers

the

advantage of high sensitivity, and resulting signals obtained with

382

1 l.Industrial Applications of Boron-Doped Diamond Electrodes: Detection of Sodium Thiosulfate,Naproxen and Nickel Ions and Electrocatalysis of Oxygen Reduction

this technique are very sharp and well-defined,

quantitative

determination of naproxen was undertaken with this technique. A linear calibration curve was obtained for six concentrations of naproxen from 0.5 to 50 ^M at a potential of 1.44 V vs. Ag/AgCl in the same solvent-supporting electrolyte system in DPV, with a scan rate of 50 mV s'^, pulse amplitude of 50 mV and pulse height of 20 ms (Fig. 17.3). The calibration plot is described by the linear regression analysis using least squares- Ip (^A) = 0.4094 C (^M) + 0.0849. The detection limit (LOD, 3o/m) and the limit of quantification (LOQ, 10 o/m), which in this case were 30 and 97 nM, respectively, where o is the standard deviation of the intercept and m is the slope.

0.6

0.8

1 1.2 E/Vvs.Ag^AgCl

1.4

1.6

Fig. 17.3. Differential pulse voltammograms for naproxen oxidation in 0.1 M IACIOAI CH3CN on a BDD electrode for a series of concentrations (fiM ) (a) 0.5 (b) 1 (c) 5 (d) 10 (e) 30 and (£> 50. The inset shows the corresponding calibration curve.

383

The accuracy obtained from the determination of naproxen in a real sample (Naprosyne®)was also assessed using the diamond electrode. The declared amount of naproxen in Naprosyne® is 500 mg. From this study, a value of 498 mg (mean - RSD of 1.4%) was obtained, which is in close agreement with the stated content. The analysis exhibited a mean recovery of 99.7% and a relative standard deviation of 2.15%, indicating adequate precision and accuracy for this electrode. This result also indicates that the excipients are electrochemically inactive and have no interference effects on the analysis of naproxen.

17.2.2. Interference study As mentioned earlier, AMN (2-acetyl-6-methoxy naphthalene) is an important degradation compound, and its presence must be monitored during the course of the analysis of naproxen. AMN shows an irreversible oxidation peak at 1.54 V vs. Ag/AgCl in 0.1 M LiC104/ CH3CN on the BDD electrode, in which the potential is more anodic than the oxidation potential of naproxen. In order to determine the effect of interference of AMN on the anodic oxidation of naproxen, DPV signals were recorded for solutions containing both naproxen (12.7 ^M) and AMN (having concentrations in various percentages with respect to the naproxen concentration) under identical experimental conditions, as shown in Fig. 17.4. When the concentrations of AMN were increased up to 5%, there was only a slight increase in the peak current of naproxen, and the error was only minimal (not shown in the figure). On the other hand, apparent errors (increases in peak current of naproxen) of 2.5%, 8% and 14% were noted, corresponding to the increases in the

384

17.Industrial Applications of Boron-Doped Diamond Electrodes: Detection of Sodium Thiosulfate,Naproxen and Nickel Ions and Electrocatalysis of Oxygen Reduction

AMN concentrations , in percentages of 10, 20 and 35, respectively (Fig. 17.4, curves a, b and c).

0.7

0.9

1.1

1.3

1.5

1.7

1.9

EA^vs.Ag/AgO Fig. 17.4. DPV voltammograms showing the influence of the addition of AMN at various concentrations in terms of percentages with respect to the naproxen concentration (12.7 fiM): a) 10 b) 20 c) 35 d) 40 and e) 60 in 0.1 M LiC104 in CH3CN. The inset shows the corresponding caUbration curve.

With the further additions of AMN, the measurement errors in the peak current of naproxen will be increasing linearly. This can be clearly seen from the plot of added AMN concentrations in percentage versus relative error in peak current for naproxen, which follows a linear relationship, with r = 0.996. The regression equation can be written based on the linear plot with respect to AMN as percentage of AMN added = 0.4519 (slope) x (percentage error in the peak current of naproxen)-1.7585 (intercept). It is also noteworthy that, because of this interference, the plot of peak

385

current of AMN vs. concentration, although it was linear, it did not match the exact concentration of AMN in this case. Hence, this equation may also be used to determine the exact amount AMN present along with naproxen in solution, since the naproxen concentration was kept constant. However, this method of calibration should be dealt with carefully in cases where the formulations

contain

other

substituted

naphthalene

species,

having potentials very close to this oxidation peak.

17.3. Electrochemical Detection of Nickel Ions in Solution Electroless nickel (EN) deposits have been used commercially in many diverse fields, such as the aerospace, automotive, electronics, machinery, oil and gas production and valve industries [20,21]. The detection of nickel in EN deposition baths is very important. The present study includes the analysis of Ni ions based on cathodic stripping of electrogenerated

Ni(III) to Ni(II). Initially, the

electrodeposition of nickel ions was carried out at -2.0 V under hydrodynamic conditions, and the deposited nickel was converted to Ni(III) (NiOOH) by switching the electrode potential positively to 1.0 V. Under rest conditions, the peak current corresponding to the cathodic stripping of Ni(III) to Ni(II) (Ni(0H)2) was measured. The formed nickel was also removed by electrochemical cleaning in an acidic solution, and thus the electrode could be used for further analyses.

386

17.Industrial Applications of Boron-Doped Diamond Electrodes: Detection of Sodium Thiosulfate,Naproxen and Nickel Ions and Electrocatalysis of Oxygen Reduction

17.3.1. Differential pulse voltammetric (DPV) study The DPV responses of the BDD electrode for 2.3 pM Ni ions contained in 0.1 M NaOH / 0.1 M NH4NO3 alkaline solution showed a sharp signal at 0.6 V vs Ag/AgCl, which was attributed to the reduction of Ni(III) to Ni(II) (Fig. 17.5). However, it is noted that the sensitivity was higher under rotating conditions than stirred conditions (Fig. 17.5), which may be due to enhanced mass transfer in the former method (see below). After the detection of both stock and sample solutions, the nickel hydroxide layer was cleaned completely by maintaining the potential at +1.0 V for 60 s in a sulfuric acid solution, where the cleaning efficiency was greater than that in alkali.

Background

<

-10

•15

-20 0.4

0.8

1.2

^/Vvs.SCE Fig. 17.5. Differential pulse voltammograms for the cathodic strippingresponse of 2.3 fiM nickel at BDD electrodes. The interference of lead in the sample solution was also evaluated. The equivalent molar ratio of Ni/Pb did not show any significant effect on the detection of Ni. Below a ratio of 1, the peak current increased and the peak potential shifted positively with 387

further addition of lead. In the EN deposition bath, the ratio of Ni/Pb was higher than 10^, and the lead content did not elicit any appreciable influence on the stripping signal.

-20

-3 0

-40 0.4

^

0.8

1.2

0

-5

^ Sam

•10

(b)

•15

0.4

0.8

1.2

E/Yvs.SCE

Fig. 17.6. Differential pulse voltammograms obtained for sample and stock solutions of Ni(II) by RDE (a) and in a stirred solution (b).

For comparative study, Ni-ion detection was performed by two types of hydrodynamic methods. One was conducted in a stirred condition (Fig. 17.6a), with stirring rate of 350 rpm, while the other was conducted with an RDE (Fig. 17.6b), with a rotation rate of 3000 rpm. The detection limit was 55 nM for the former method,

388

17.Industrial Applications of Boron-Doped Diamond Electrodes: Detection of Sodium Thiosulfate,Naproxen and Nickel Ions and Electrocatalysis of Oxygen Reduction

whereas it was 33 nM for the latter one. The electrode response was found to be linear over the range of 0.3 ~ 5 ^M {y= -1.63 x[V3] + 0.52, R2 = 0.990) by use of the former method and 0.1 -- 4 ^M CF = -9.92 x[V4] + 2.55, R2 = 0.995) by use of the latter one. 17.3.2. Standard analysis The analytical applicability was also evaluated through the detection of nickel ions in an electroless nickel-boron deposition solution. By the electrochemical stripping method, under the stirring and rotating conditions, the content of nickel was found to be 3.00 ^M (with 0.3 mL sampling) and 1.95 jaM (with 0.2 mL sampling) respectively. These correspond to 0.10 and 0.098 M Ni contents in the original baths, for which the Ni content was 0.099 M, as measured by ICP-AES. It is clear that the electrochemical technique could provide a fast and economical method for the nickel-ion detection with the EN deposition bath.

17.4. Electrocatalysis of Oxygen [22] The electrocatalysis of the oxygen reduction reaction is of theoretical and practical interest because of its paramount importance for electrochemical energy conversion and industrial electrolysis. Au nanoparticles deposited on a BDD electrode show excellent activity towards catalytic reduction of oxygen [23], which also depends on the particle size, the nature of the support and the preparation method [24].

389

17.4.1. Deposition of gold particles on BDD Au nanoparticles were deposited on diamond films in 5 xlO-^ M of KAuCh contained in 0.1 M H2SO4 medium by maintaining the potential at 0 V. In the first cycle of the CV, a pair of peaks defined the reduction of the Au complex and the oxidation of the deposited Au, at peak potentials of 0.3 V and 1.16 V, respectively. The additional reductive peaks observed during the second cycle at potentials of 0.910 and 0.550 V were attributed to the reduction of Au oxide and the reduction of AuCU' on the deposited Au surface. The crossover potential of the forward and reverse sweep was at 0.67 V, which is very close to the standard potential of the three-electron reduction of AuCU", represented by the standard equation AuCU" + 3e -> Au (0) + 4C1'

(17.2)

£%iii)/Au(0) = 1.001 VV5.NHE and the calculated value was 0.667 V. The roughness factor for the dissolution of Au oxide was also evaluated, in which a corrected theoretical value of 482 |iC cm 1 was observed for the reduction of a monolayer of divalent oxygen on the gold surface. The electrodeposited gold was distributed randomly as small spherical particles, with an average diameter of 60 nm, and these nanoparticles were dispersed both at the grain boundaries and on the facets, as observed in SEM studies (Fig. 17.7 a & b). The active sites for gold nucleation were inhomogeneously distributed on the surface of diamond.

390

17.Industrial Applications of Boron-Doped Diamond Electrodes: Detection of Sodium Thiosulfate,Naproxen and Nickel Ions and Electrocatalysis of Oxygen Reduction

^

(a)

^

l.Ofim

^'^

(b)

gold Fig. 17.7. SEM images of electrochemically deposited nanoparticles on an as-grown diamond film, (a) The sample was prepared in 0.5 M 0.5 M H2SO4 solution containing 5.0 x 10"5 M KAuCU, where the potential was held at 0 V for 14 s,* (b) after the deposition of Au the sample was oxidized at a potential of 1.300 V for 7 s, then swept linearly negatively from 1.3 to 0.5 V with a scan rate of 0.030 V s"i in 0.1 M H2SO4 solution saturated with oxygen gas.

17.4.2. Catalytic reduction of oxygen Well defined cathodic peaks were noted in the voltammetry at a potential of-0.3 V in the aerated and oxygen-saturated solutions of the electrolyte, which contained 0.1 M HCIO4 + 0.01 M NaC104, corresponding to the electrocatalytical reduction of oxygen (Fig. 17.8). Furthermore, the catalytic potential shifted positively, and the current increased, with the increasing coverage of Au deposition.

The

catalytic

efficiency

of

a

nanop articulate

Au-deposited BDD with a coverage of 0.06 was nearly 20 times higher than that of polycrystalline gold, and it was clear that hydrogen evolution exhibited a more negative onset on the 391

Au-deposited BDD than that on the polycrystalline gold.

From

impedance studies, it was observed that the charge transfer rate for oxygen reduction at a nanoparticulate Au-deposited BDD surface with an Au coverage of 0.12 was ca. 10 times higher than that at the polycrystalline Au surface at a potential of -0.25 V, whereas at as-grown diamond, oxygen reduction was insignificant at this potential.

—In aerated solutions -0.4

•In oxygen saturated solutions

-0.8

-1.2 -800

— -400

0

^ 400

E/(mV vsSCE)

Fig. 17.8. CycHc voltammograms measured in aerated 0.1 M 0.1 M HCIO4 containing 0.01 M NaC104 on a gold nanoparticle-deposited BDD film at a scan rate of 0.03 V s"i. The electrode was prepared in 0.5 M H2SO4 containing 5X10"^ M AuCU", and the potential was held at 0 V for 14 s. The

rotating

disk

electrode

measurements

for

the

nanoparticle-deposited diamond film under identical experiments conditions revealed that there was no well-defined limiting current for oxygen reduction, which merges with hydrogen evolution at -0.46 V. From the Koutecky-Levich (K-L) equation, the number of electrons n involved in the reduction was calculated to between 2

392

17.Industrial Applications of Boron-Doped Diamond Electrodes: Detection of Sodium Thiosulfate,Naproxen and Nickel Ions and Electrocatalysis of Oxygen Reduction

and 4. F r o m this, it can be noted t h a t oxygen reduction may lead to a complete 4-electron reduction to H2O or to a n

incomplete

2-electron reduction to H2O2 [25,26]. The incomplete reduction w a s explained in t e r m s of a n e n d ' o n adsorption model, a n d

the

complete reduction resulted from bridged orientations.

References 1.

G. M. Swain, A. B. Anderson and J. C. Angus, MRS Bull, 23 (1998) 56.

2.

T. N. Rao, T. A. Ivandini, C. Terashima, B. V. Sarada and A. Fujishima, New Diamond Front. Carbon. Techno!., 13 (2003) 79.

3.

M. C. Granger, J. Xu, J. W. Strojek and G. M. Swain, Anal.

Chim.

Acta., 397 (1999) 161. 4.

C. Terashima, T N. Rao, B. V. Sarada, Y. Kubota and A. Fujishima, Anal. Chem., 75 (2003) 1564.

5.

A. J. Saterly F. Marken, F Frank and R. G. Compton, Talanta, 53 (2000) 403.

6.

C. Reuben,E. Galun, H. Cohen, R. Tenne, R. Kalish, Y. Muraki, K. Hashimoto, A.Fujishima and J. M. Butler, J. Electroanal.

Chem.,

396 (1995) 233 7.

S. Yoshihara, K. Shinozaki, T Shirakashi, K. Hashimoto, D. A. Tryk and A. Fujishima, Electrochim. Acta., 44 (1999) 2711

8.

F. Montilla, R -A. Michaud, E. Morallon, J. L. Vazquez and Ch. Comninellis, Electrochim. Acta., 47 (2002) 3509.

393

9.

B. Boys, R-A. Michaud, B. Marselli, M. M. Dieng, E. Brillas and Ch. Comninellis, New Diamond Front

Carbon TechnoL, 12 (2002)

63. 10. M.S. Saha, T. Furuta and Y. Nishiki, Electrochem,

SolidState

Lett, 6 (2003) D5. 11. R. R Mamin and T. Inushima, Phys. Rev., ^ 6 3 (2001) 33201. 12. Y. Zhang, S. Asahina, M. Suzuki, S. Yoshihara and T. Shirakashi, Surf. Coatings Tech., 169-170 (2003) 303. 13. V. Suryanarayanan, Sens. Actuators

Y Zhang,

S. Yoshihara and T. Shirakashi,

B- Chemical, in press(2004).

14. S.-M. Chen, J. Electroanal.

Chem., All (1996) 145.

15. X. Zhou, S. Wang, Z. Wang and M. Jiang, Fresenius J. Anal.

Chem.,

345 (1993) 424. 16. C. Boynton, C. Dick and G. Mayer, J. Clin. Pharmacol, 28 (1988) 512. 17. M.A.Perazella. Am. J. Med., 109 (2000) 307. 18. G. Kanoute, F. Nivaud, P. Boucly and M. Guernet, Bull. Soc. Chim. France, 1-2 (1983) 49 19. N. Adhoum, L. Monser, M. Toumi and K. Boujel, Anal. Chim. Acta., 495 (2003) 69. 20. R. E. Miler. Plating Surf. Finishing

14. (1987) 52.

21. R. R Tracy and G. J. Shawhan. J. Mater Perform., 1990, 29, 65. 22. Y Zhang, S. Asahina, S. Yoshihara and T. Shirakashi,

Electrochim.

Acta., 48 (2003) 741. 23. K. Honda, M. Yoshimura, T. N. Rao, D. A. Tryk and A. Fujishima,, J. Electroanal.

Chem., 514 (2001) 35.

24. M. T. Giacomini,

E. A. TicianeUi,

Balasubramanian, J. Electrochem.

394

J.

McBreen

Soc, 148 (2001) A323.

and M.

17.Industrial Applications of Boron-Doped Diamond Electrodes: Detection of Sodium Thiosulfate,Naproxen and Nickel Ions and Electrocatalysis of Oxygen Reduction

25. S. M. Sayed and K. J u t t e r , Electrochim.

Acta., 28 (1983) 1635.

26. M. Alvarez-Rizatti and K. J u t t n e r , J. Electroanal.

Chem.,

144

(1983) 351.

395

18. Diamond Microelectrodes Herlambang Olivia, Bulusu V. Sarada, Tata N. Rao and Akira Fujishima

18.1. Introduction Microelectrodes

have

attracted

much

attention

recently

in

electrochemistry due to their superior properties, which enable them to outperform conventional macroelectrodes and extend the experimental range to several new fields, such as fast-scan measurements and analysis in poorly conducting media [1-4]. The history of microelectrodes actually started more than 60 years ago, when 1942, Davies and Brink [5] reported the use of platinum microdisk electrodes for the measurement of oxygen in muscle tissues. In their work, microelectrodes were used to minimize the damage to the muscle, and to limit the current flowing through the electrode. Since then, several reviews [6-8] and books [9,10] about microelectrodes have been published. Denuault, in his review [8], defined the term "microelectrode" as an electrode with at least one dimension in the range of 0.1 to 50 [im. The small size of microelectrodes makes them possible to be used for in vivo detection, which is usually performed with very small volumes of samples, such as those for neurotransmitter monitoring in the brain. Moreover, due to its small size, at Herlambang Olivia e-mail: [email protected] 396

18. Diamond Microelectrodes

relatively long experimental timescales, the thickness of the diffusion

layer

is

comparable

to

the

dimensions

of

the

microelectrode, and a spherical (or hemi-spherical) diffusion field controls the fast mass transport of reactants and products to and from the electrode surface. Accordingly, a steadystate response (or pseudo-steady

state

response)

can be observed with

cyclic

voltammetry at low sweep rates. Another interesting feature of microelectrodes is their small interfacial capacitance. Capacitance decreases with electrode area, and therefore, due to its small area, microelectrodes have a reduced capacitance and hence small charging current, allowing fast and sensitive response. Furthermore, voltammetry using microelectrodes often

completely eliminates IR

drop, which

enhances the use of media such as organic solvents [ll], nonelectrolyte solutions [12-14], and even gases and solids [15,16], which are generally excluded from any measurements using macroelectrodes. The various geometries of microelectrodes include microdisks, microfibers, microarrays, microbands, and microrings. Among these, the microdisk is the most popular geometry, because of its simple fabrication and the possibility of treatment by polishing. However, the current response at microdisk electrodes is often small enough that it limits the range of measurements, augmenting the need for techniques to fabricate for microfiber, microband and microarray electrodes, which provide larger signals. The

most

commonly

used

electrode

materials

for

microelectrodes include platinum, gold and carbon. Carbon fiber microelectrodes are widely used for electroanalysis in aqueous

397

media, as they exhibit a relatively wide potential window. However, similar to metal electrodes, carbon has several serious limitations, including high background current and deactivation via fouling, especially during the detection of compounds in complex biological fluids, as reported by Baur et al.[l7]. It is an inherent property of carbon to undergo deactivation upon exposure to the laboratory environment or working solution, which is due to factors such as surface oxidation and adsorption of contaminants and reaction products. Diamond is one of the more recent of the carbon allotropes that has been examined as an electrode material. It exhibits several superior properties, including low background current, wide potential window, long-term stability, relative insensitivity towards the presence of dissolved oxygen in the solution, and biocompatibility [18-20]. Thus, diamond is becoming an interesting material to consider for electroanalysis. Cooper et al. (1998) reported for the first time the fabrication and the use of boron-doped diamond (BDD) microelectrodes in nonaqueous electrolytes [21]. Considering the advantages of BDD mentioned above, the Fujishima group undertook the application of BDD microelectrodes, especially BDD microdisk [22], microfiber [23] and microdisk array [24] electrodes in aqueous solutions.

18.2. Preparation of Diamond Microelectrodes 18.2.1. Fabrication of diamond microdisk and microfiber electrodes Diamond microfibers were prepared by depositing boron-doped

398

18. Diamond Microelectrodes

diamond on electrochemically polished t u n g s t e n fibers. Diamond deposition w a s carried out using a microwave plasma-assisted chemical vapor deposition

(MPACVD) system at a

hydrogen

p r e s s u r e of 50-80 Torr a n d microwave power of 1500-3000 W for 38 h on t u n g s t e n fibers. Different powers a n d deposition times resulted in the variation of t h e crystal size a n d t h e film thickness, respectively. The crystal size varies from 5 to 40 ^im, while t h e film thickness varies from 5 to 20 pim. Prior to deposition, the tips of the t u n g s t e n wires (([)= 30 ^im) were etched in 2 M N a O H at 3 V for 45 s in order to reduce t h e d i a m e t e r of the fiber to '^10 ^im, a n d these tips were nucleated by ultrasonicating in solution containing a suspension of 100-nm diamond particles for 60 min. The diamond-deposited t u n g s t e n wire w a s t h e n inserted into a pre-puUed glass capillary {^- 50-100 |im) a n d w a s sealed using epoxy. The ohmic contact to t h e diamond fiber w a s m a d e using a copper wire with either mercury or silver p a s t e . In t h e case of the microdisk electrode [22], t h e diamond fiber w a s preliminarily fully sealed by the use of epoxy, a n d the tip w a s t h e n polished until t h e diamond w a s j u s t exposed, while for t h e micro fiber electrode [23], a ~300-^m length of fiber w a s left exposed.

18.2.2. Characterization Successfully fabricated diamond fibers were characterized by use of scanning electron microscopy (SEM) a n d R a m a n spectroscopy, while t h e roughness factor of t h e diamond fiber w a s calculated based on double-layer capacitance m e a s u r e m e n t s . S E M images of diamond fibers are shown in Figure 18.1. Figure 18.1a shows a suitable diamond fiber for microdisk electrode fabrication, while

399

for

the

microfiber

electrode,

full

coverage

of

diamond

polycrystallites on the tungsten fiber was necessary (Fig 18.1b). Raman spectra (not shown) indicated the high quality and purity of the diamond.

iSkU

K38e

90)AM e a e a Q i

Fig 18.1. Suitable diamond for (a) microdisc electrode and (b) microfiber electrode The double layer capacitance of a diamond microelectrode is calculated based on the equation Ic = U Cd where Ic is the charging current, v is the potential sweep rate, and Cd is the double layer capacitance. By plotting Ic as a function of u, the double-layer capacitance Cd can be obtained from the slope. The Cd value obtained for a diamond microfiber electrode was 8 nF, and the capacitance density was calculated to be 7.02 \xF cm'^. Considering

the

capacitance

density

of

a

smooth

(lOO)

homoepitaxial diamond electrode {ca. SfiF/cm^) (unpublished [tml] result), the roughness factor of the diamond fiber was estimated to be 2.34.

400

18. Diamond Microelectrodes

18.3. Electrochemical Behavior 18.3.1. Electrochemical behavior of diamond microdisk electrodes The simplest way to investigate the electrochemical behavior of an electrode is by studying its cyclic voltammetric curves. Figure 18.2 shows cyclic voltammograms for the oxidation of ferrocyanide at BDD microdisk electrodes with two different radii in aqueous electrolyte.

;3

o

-200

0

200

400

600

800

1000

PotentiaymV vs. SCE Fig 18.2. CycUc voltammograms at diamond micro electrodes for the oxidation of 1 mM K4Fe(CN)6 in 0.1 M KCl (potential sweep rate, 10 mV SOJ electrode radii : (a) 20 and (b) 6 Mm The sigmoidal shapes of the curves and lack of hysteresis, i.e., steady state-type behavior, is characteristic of voltammetry at low potential sweep rates for microelectrodes [1,3]. The half-wave potential was +0.210 vs. SCE. This value agrees well with that

401

reported at conventional macro-type diamond electrodes by JoUey et al. (+0.230 V) [25]. The radius of each microelectrode was calculated from the equation 7iim = 4nFDCr

(18.1)

where iiim is the limiting current, C is the concentration, D is the diffusion coefficient, r is the radius of the electrode, F is the Faraday constant, and n is the number of electrons, in this case, one. The radii of the microelectrodes were calculated to be 20 and 6 ^m using a value of 6.5 xlO^ cm^s'i for the diffusion coefficient for ferrocyanide [26]. Similar steady-state type voltammograms were also obtained for the oxidation of Ru(NH3)6^^, for which a diffusion coefficient of 6.0 xlO ^ cm^s i was used [27]. Owing to the steady-state nature of the spherical diffusion at the microelectrode, the limiting current should be independent of potential sweep rates at lower sweep rates. As the sweep rate increases, the contribution of planar diffusion increases. The value of sweep rate at which planar diffusion begins to significantly interfere depends on the size of microelectrode. One of the most promising features expected for BDD microelectrodes is very low background

current,

due to a

combination of the effect of the microelectrode size [l] plus the intrinsic properties of diamond [28]. One way this effect can be tested is by examining the detection limit for a relatively simple redox couple at slow sweep rates. Figure

18.3a

shows

a

voltammogram

for

a

BDD

microelectrode (r=20 |im) in a 200 nM ferrocyanide (O.l M KCl) solution,

compared

with

the

background

current.

The

voltammogram is very well defined, even at this low concentration.

402

18. Diamond Microelectrodes

indicating its potential use for electrochemical sensor applications. Limiting currents increased linearly with increasing ferrocyanide concentration up to 1.2 \xM (Fig 18.3b). zu

<

b

^15

y^

fl 0

fa

gio

bo C3

• s •^' -100

0

100

200

300

400

500

a

3

/

04

^

1

1

1

0.2 0.4 0.6 0.8

1

j^_

1

1.2

1.4

Concentration (fi M) Potential (mV vs. SCE) Fig. 18.3(a). Cyclic voltammogram for a diamond microelectrode of radius 20 ^Jim for the oxidation of 200 nM K4Fe(CN)6 in 0.1 M KCl (sweep rate, 2 rnVsO. (b) Cahbration curve for K4Fe(CN)6oxidation in 0.1 M KCL In

contrast,

the

high

background

at

glassy

carbon

microelectrodes did not allow well-defined voltammograms to be observed at low analyte concentrations. For example, for a ferrocyanide concentration of 200 nM, the increment in the current due to the analyte was only -25% of the background current, whereas for the BDD microelectrode of similar radius, the corresponding value was ~200%.

18.3.2. Electrochemical behavior of diamond microfiber electrodes A BDD microfiber (BDDMF) electrode was characterized by performing voltammetric experiments using an outer-sphere redox couple. Fig. 18.4 shows the cyclic voltammogram for 1 mM ruthenium hexaamine trichloride at a BDDMF electrode in 0.1 M

403

phosphate buffer (pH 7.1) at a sweep rate of 10 mV s^. The voltammogram

shows

the

pseudo-steady

state

response,

a

characteristic of microfiber electrodes. For a sweep rate of 100 mVs ^ a peak-shaped voltammogram was observed, indicating that planar diffusion is dominating the mass transport in the vicinity of the electrode at relatively high scan rates.

Potential (vs SCE) / -400

-300

-200

-100

0

100

200

300

400

7

F i g 18.4. Cyclic v o l t a m m o g r a m for (A) 1 m M r u t h e n i u m h e x a a m i n e t r i c h l o r i d e in 0.1 M p h o s p h a t e buffer a n d (B) 0.1 M p h o s p h a t e buffer a t a d i a m o n d microfiber electrodes. Sweep r a t e 10 m V s'l.

The current density of the fiber electrode was estimated from the following equation,

given for linear sweep voltammetry

at

cylindrical microelectrodes[29,30]• I = (n2F2CaWRT)(0.446p-i + 0.335p i ss)

i8.2

where I is the diffusion current density, a is the microelectrode radius, v is the potential sweep rate, and p=(nFa2u/RTD)i/2 is a dimensioless parameter that characterizes the type of diffusion. In the theoretical calculation, he value of 6.0x10^ cm^ s"i was used

404

18. Diamond Microelectrodes

[27] for

the

diffusion

coefficient

of ruthenium

hexaamine

trichloride and 25 iim for the fiber radius, giving a current density of 1380 nA mm"2. The experimental current density, calculated by considering the fiber length of 0.8 mm, is 3916.34 nA mm"2. The difference between measured and calculated current density (ca. 2.8) can be mainly attributed to the roughness factor. The roughness factor of the diamond fiber calculated from the doublelayer capacitance measurement was 2.34. The other possibility is that the rough surface of the electrode does not conform to the microfiber model, and therefore, the formula above is not strictly valid for diamond microfiber electrodes.

18.4. Electroanalytical Applications of Diamond Microelectrodes 18.4.1. Detection of H2O2 at metal-modified

diamond

microelectrodes Despite its several superior properties, as mentioned above, diamond has several limitations compared to metal electrodes, such as slow kinetics for reactions involving adsorption and multielectron transfer

processes, including hydrogen and

oxygen

evolution reactions. However, since the low rates of the hydrogen and oxygen evolution reactions result in the wide potential window[31,32] this can be considered to be an advantage of using diamond, especially in aqueous media. Another important multielectron transfer reaction is the oxidation and reduction reaction of H2O2, which is generally enzymatically generated from

the

oxidation reactions of biological materials, such as glucose, lactate,

405

pyruvate, a n d cholesterol. Therefore, the detection of H2O2 is i m p o r t a n t for a wide range of applications in t h e electroanalytical field. Since diamond is inactive for the oxidation a n d reduction reactions of H2O2, modification of t h e electrode is required to m a k e diamond suitable for t h e

enzyme-based biosensor application.

T a t s u m a , et al. [33] reported t h e use of heme peptide

and

horseradish peroxidase, types of redox enzymes, based on the direct electron transfer between t h e diamond electrode a n d t h e redox enzyme. Another promising approach is the deposition of m e t a l nanoparticles t h a t have catalytic activity for t h e H2O2 oxidation- reduction reaction. The

modification

of a BDDMF

electrode

with

platinum

nanoparticles a n d its use for H2O2 detection are discussed in the p r e s e n t chapter, based on t h e following reactionsH2O2

P^

^

02+2H%2e"

H 2 0 2 + 2 H V 2 e " __Pt

^

2H2O

Platinum

deposition

on

diamond

microelectrodes

was

performed electrochemically in 0.1 M H2SO4 containing 100 \iM K2PtCl6 by cycling between the potentials o f - 0 . 2 V and 1.2 V at 50 mVs 1. The electrode w a s t h e n dipped into 0.1 M H2SO4 a n d the same

cycling

potentials

were

applied

until

a

stable

cyclic

v o l t a m m o g r a m w a s achieved, indicating the complete cleaning of t h e F t active a r e a . The P t active a r e a w a s calculated from t h e charge density for the hydrogen desorption reaction [34] between 0 and

-0.2

V, using

a

standard

value

of 210

piC cm^

for

polycrystalline P t [35]. We found t h a t t h e P t active a r e a increased

406

18. Diamond Microelectrodes

linearly

with

deposition

time

(Fig

18.5), a n d

the

signal-to-

background ratio (s/b) for 1 m M H2O2 achieved its m a x i m u m value for a 20-min P t deposition (Fig 18.6).

20

30 40 deposition time/min

50

60

Fig 18.5. Plots of s/b value at 0.6 V as a function of P t deposition time, calculated from t h e cyclic voltammogram for 1 mM H2O2 at Pt-BDDMF electrode. 12 y = 0.0236x-0.3995 R^ = 0.9946

1
0.8

^

0.4

••p

«

02

Uu

0

-Q^ 1P

-02

30

40

60

50

deposition time /min

Fig 18.6. Plots of P t active area a s a function of P t deposition time. P t active area w a s calculated from t h e charge density of hydrogen desorption reaction in 0.1 M H2SO4.

This can be explained from t h e S E M images below. Figure 18.7(a)

and

microfiber

(b) show electrodes

SEM after

images an

1-h

of Pt-modified and

20

min

diamond deposition,

407

respectively. It can be seen from both images that Pt nanoparticles were distributed uniformly on the diamond surface. From Fig. 18.7(a), after 1 h deposition time, the amount of Pt loading was large, with an average diameter (Dih) of 500 nm and the number of Pt particles deposited per unit real area (Nih) of ca. 8.0 xlO'^ particles cm 2 [taking into account the roughness factor of the diamond microfiber electrode [23] (ca. 2.34)]. The number of exposed surface Pt atoms was estimated to be 5.82 x lO^^ Pt atoms cm'2 from the background CV. As the average distance between each particle (estimated to be 1.12 \xm from Nih) isbecoming small, the diffusion layers of reactant were assumed to overlap each other [Fig 18.8(a)], resulting in decreased electrochemical activity.

Fig 18.7. SEM images of Pt-BDDMF electrodes for (a) 1 hour and (b) 20 min Pt deposition time. (a)

(b)

Fig 18.8. (a) Overlapping diffusion layers and (b) Ideal spherical diffusion layers

408

18. Diamond Microelectrodes

In contrast, after a 20"min deposition [Fig 18.7(b)], the amount of Pt loaded on BDDMF was less (3.22 x IQi^ Pt atoms cm-2) with D20min = 400 nm and N2o^iv - 6.0 x 10^ particles cm-2. The average distance between each particle for this electrode (4.09 ^im from N20niv) is 3.7 times greater than that for a 1-h deposition, and this distance is considered to be close to the optimum value [36], allowing an ideal spherical diffusion field to occur in the vicinity of each particle. In this case, the electrode can be considered as a micro-array of platinum particles that are uniformly distributed on the diamond microfiber electrode (Fig 18.8(b)). Comparison

experiments were performed

using a

bare

platinum microelectrode, A Pt-deposited diamond macroelectrode, and a bare Pt macroelectrode. The s/b values for each electrode taken for 1 mM H2O2 in 0.1 M PBS are summarized in Fig 18.9.

Pt-dia Pt microelectrodes Fig. 18.9. Comparison of S/B values at 0.6V calculated from cyclic voltammogram for 1 mM H2O2 at Pt-diamond microfiber, Pt microfiber electrodes, Pt-diamond macroelectrodes and Pt macroelectrodes.

409

The Pt-modified and bare Pt microelectrodes show higher s/b values

than

the

Pt-modified

and

bare

Pt

macroelectrodes

respectively. This can be attributed to the properties of the microelectrodes* non-planar diffusion

profile, low background

current and high sensitivity. Depositing the proper amount of metal catalyst on the microelectrode further effectively enhances the sensitivity of microelectrode. Moreover, with the use of diamond as a stable supporting material for metal deposition, the resulting

microelectrode

overwhelmingly

outperforms

the

platinum microelectrodes.

18.5. Summary Microelectrodes, due to their unique properties, such as small size, non-planar diffusion, small capacitance, and small current, have provided

a major

breakthrough

in electrochemistry.

Taking

advantage of the superior properties of diamond, the diamond microelectrode is a promising new device for several applications in electrochemistry. Furthermore, modification of the diamond microelectrode with appropriate metal nanoparticles increases the quality of the measurements, in the terms of sensitivity, stability, and selectivity.

References 1.

R.J. Forster, Chem. Soc. Rev., 23 (1994) 289.

2.

D.J. Wiedemann, K.T. Kawagoe, R.T. Kennedy, E.L. Ciolkowski

410

18. Diamond Microelectrodes

and R.M. Wightman, Anal

Chem., 63 (1991) 2965.

3. A.M. Bond, M. Fleischmann and J. Robinson, J.

Electroanal.

Chem., 168 (1984) 299. 4. K. Pihel, Q.D. Walkel and R.M. Wightman, Anal. Chem., ^% (1996) 2084. 5.

RW. Davies and R Brink, Rev. Sci. Instum.,

6.

R.M. Wightman, Anal. Chem., 53 (1981) 1125A.

7.

A.M. Bond, Analyst

8.

G. Denuault, Chem. Ind., (1996, 16 September) 678.

9.

Ultramicroelectrodes,

(Cambridge.

13 (1942) 524.

U.K.), 119 (1994) R l .

ed. M. Fleischmann, S. Pons, D.R. Rohson

& P.O. Schmidt, Morganton, NC: Datatech Systems, 1987. 10.

Microelectrodes'

Theory and Applications,

ed. M.I. Montenegro,

M.A. Queiros & J.L. Daschbach, Dortrecht^ Kluwer Academic Press, NATO AST series E, Volume 197. 11.

J.O. Howell and R.M. Wightman, J. Phys. Chem., 88 (1984) 3915.

12.

D.L. Luscombe and A.M. Bond, Talanta, 38 (1991) 65.

13.

J.B. Cooper and A.M. Bond, Anal

14.

A.A. Bumber,

Y.N. Nechitailov,

Chem., 65 (1993) 2724. LA. Profatilova

and

E.G.

Ignatenko, J. Anal. Chem., 55 (2000) 666. 15.

R. Brina, S. Pons and M. Fleischmann, J. Electroanal.

Chem.,

244 (1988) 81. 16.

T.T. Wooster, M.L. Longmire, H. Zhang, M. Watanabe and R.W. Murray, Anal. Chem., 64 (1992) 1132.

17.

J.E. Baur, E.W. Kristensen, L.J. May, D.J. Wiedemann and R.M. Wightman, Anal. Chem., 60 (1988) 1268.

18.

M. C. Granger, J. Xu, J.W. Strojek and G.M. Swain, Anal.

Chim.

Acta, 397(1999) 145. 19.

C. Prado, G. Flechsig, P Grundler, J. S. Ford, F. Marken and R.

411

G. Compton, Analyst 20.

(Cambridge.

U.K.), 127 (2002) 329.

T. N. Rao, L Yagi, T. Miwa, D. A. Tryk and A. Fujishima,

Anal.

Chem., 71 (1999) 2506. 21.

J.B. Cooper, S. Pang, S. Albin, J. Zheng and R.M. Johnson, Anal. Chem., 70 (1998) 464.

22.

B.V. Sarada, Electrochem.

23.

J.

Soc, 146 (1999) 1469.

H. OHvia, B.V. Sarada, D. Shin, T.N. Rao and A. Fujishima, Analyst

24.

T.N. Rao, D.A. Tryk and A. Fujishima,

(Cambridge.

U.K.), 127 (2002) 1572.

K. Tsunozaki, Y. Einaga, T.N. Rao and A. Fujishima, Chem.

Lett,

(2002) 502. 25.

S. Jolley, M. Koppang, J. Jackson and G.M. Swain, Anal.

Chem.,

69 (1997) 4099. 26.

J. Koryta, J. Dvorak and L. Kavan, Principles chemistry

27.

of

Electro-

p. 116, John Wiley & Sons, Chichester, 1993.

K.R. Wehmeyer and R.M. Wightman, Anal.

Chem., 57 (1985)

1989. 28.

G.M. Swain and R. Ramesham, Anal. Chem., 65 (1993) 345.

29.

L. Agui, L., J.E.L. Guzman, A. Gonzales, A., R Yanez and J.M. Pingarron, Anal. Chim. Acta, 385 (1999) 241.

30.

K. Aoki, K. Honda, K. Tokuda and H. Matsuda, J.

Electroanal.

Chem., 182 (1985) 267. 31.

T. Yano, E. Popa, D.A. Tryk, K. Hashimoto and A. Fujishima, J. Electrochem.

32.

Soc, 146 (1999) 1081.

C.H. Goeting, J.S. Foord, F. Marken and R.G. Compton, Diamond Relat. Mater, 8 (1999) 824.

33.

T. Tatsuma, H. Mori and A. Fujishima, Anal. Chem., 72 (2000) 2919.

412

18. Diamond Microelectrodes

34.

K. Honda, M. Yoshimura, T.N. Rao, D.A. Tryk, A. Fujishima, K. Yasui, Y Sakamoto, K. Nishio and H. Masuda, J.

Electroanal

Chem., 514 (2001) 35. 35.

T. Biegler, D.A.J. Rand and R. Woods, J. Electroanal.

Chem., 21

(1971) 269. 36.

H.J. Lee, C. Beriet, R. Ferrigno and H.H. Girault, J.

Electroanal

Chem., 502 (2001) 138.

413

19. Electrochemistry at Nanostructured Diamond Electrodes : Characterization and Applications Kensuke Honda and Akira Fujishima

19.1. Introduction Nanomaterials have many possible applications for analytical chemistry [l], and for electronic, optical, and mechanical devices [2]. In particular, nanomaterials and electrochemistry have a long shared history (e.g., the use of finely dispersed Pt particles as catalysts in fuel cell electrodes). This Chapter deals specifically with electrochemical applications of the template-synthesized nanostructured diamond. We begin with the basic electrochemical properties of nanostructured diamond electrodes.

Two possible

electrochemical applications are discussed.

19.2. Fabrication of Nanostructured Diamond 19.2.1. Template synthesis of nanostructured materials There

are

numerous

chemical

methods

for

preparing

nanomaterials [2, 3]. A number of researchers have been studying a method termed "template synthesis" [3].

Traditionally, this

method has entailed synthesizing the nanoordered structure of a desired compounds or material by use of a nanoscale template. Kensuke Honda e-mail: [email protected] 414

19. Electrochemistry at Nanostructured Diamond Electrodes: Characterization and Applications

Recently, the template method has been used with the pores in a microporous solid as a nanoscopic mold [3]. Many materials are available for the template materials [3, 4]. Pore diameter sizes range from A to micrometers. Out of the many available template materials, anodic alumina (AI2O3) has been commonly used [5, 6]. When grown on high-purity aluminum, anodic alumina has a hexagonal pattern of cylindrical pores. Pore diameters from -10 to -400 nm can be synthesized.

Recent improvements in the

degree of ordering obtainable for a hole array has increased the attractiveness of such materials for nanofabrication.

19.2.2. Fabrication procedure of nanostructured diamond Fig. 19.2.1. shows the procedure for the fabrication of nano-porous diamond films (diamond nanohoneycomb) by template synthesis with porous alumina membranes. Ordered thorough-hole anodic porous alumina membranes were laid on the top of the synthetic diamond films, and then deep holes were etched into the film by use of an oxygen plasma treatment.

19.2.3. Polishing of polycrystalline diamond films Nanohoneycomb

structures

were

fabricated

from

polished

polycrystalline films. The polishing of the as-deposited films was carried out by the Namiki Precision Jewel Co., Ltd., Tokyo, Japan, by use of a proprietary process. The films polished by this process are extremely smooth, with height variations on the order of ca. 1 nm.

415

Thorough-hole porous anodic alumina mask J>OOOOOOC / O O O O O O O/ ,3 0 0 0 0 0 0 C

Oxygen Plasma

i i i i i i a•a•no a Polished boroxi-^doi^ diamond tliiii Sim

oooooo^ O O O O O O/ ooooooc

Nano-honeycomb diamond

Fig. 19.1. Schematic diagrams of the fabrication procedure for the nano-honeycomb diamond electrode

19.2.4. Preparation of the anodic alumina mask Anodic porous a l u m i n a is formed via t h e anodization of Al in an appropriate solution. The p r e p a r a t i o n of the thorough-hole porous anodic a l u m i n a m a s k h a s been described [7]. The pore interval of porous alumina, in other words, t h e cell size, w a s determined by the applied voltage used for anodization [?]• the cell size h a s a good linear relationship with the applied voltage, where

the

proportionality constant of cell size per u n i t applied voltage is approximately 2.5 n m V"i. In a previous survey, self-ordering h a s been observed to occur u n d e r limited voltage conditions, which were specific to the solution used for anodization?' self-ordering t a k e s place at 25 V in sulfuric acid solution with a 6 5 - n m cell size, at 40V in oxalic acid solution with a 100-nm cell size, a n d at 195 V in phosphoric acid with 500-nm cell size [7].

416

19. Electrochemistry at Nanostructured Diamond Electrodes: Characterization and Applications

An aluminum sheet (10 X 50 X 30 mm-' 99.999%; Nilaco) was electropolished in a mixed solution of perchloric acid (60%) and ethanol (1-4 in volume) at constant current conditions of 100 mA cm"2 at a temperature below 10°C for 4 min. Anodization was conducted under constant voltage conditions (40 V in a 0.3 M oxialic acid solution for 10 h) using a DC source (Metronix 410A350). The temperature of the electrolyte was maintained at 0 °C during anodization with a cooling system (EYELA CTP-20). After anodization the surface was protected against etching using a coating layer made of a mixture of nitrocellulose and polyester resin in ethyl acetate, butyl acetate and heptane. The Al layer was removed in a saturated HgCb solution. Then the bottom part of the anodic porous alumina membrane was removed in 5 wt% phosphoric acid at 30°C for 60 min, after which the coating layer was dissolved in acetone, to form a thorough-hole membrane.

19.2.5. Oxygen plasma etching process The oxygen plasma etching of the diamond films was conducted with an RF- driven (13.56 GHz) plasma etching apparatus (Samco BP-1, Japan) [8]. The diamond specimen with mask was placed on one of the planar electrodes in the plasma chamber.

Oxygen

plasma etching was carried out for 15 min. The operating oxygen pressure was 20.0 Pa, and the plasma power was 150 W.

417

19.3. Impedance Characteristics of the Nanoporous Honeycomb Diamond and Application as an Electrical Double- Layer Capacitor 19.3.1. Fabrication of nanostructured diamond Nanoporous materials [8-10] have attracted much recent interest, including

that

stemming

from

possible

electrochemical

applications [11, 12]. The electrochemical capacitor [13, 14] is a natural application for nanoporous structures. Activated carbons have been the most extensively examined capacitor materials over the past decade [13, 15]. Another performance

possible of

approach

activated

involves

carbon-based

improving

capacitors

the

through

modification of the electrolyte. In order to increase the specific energy, organic electrolytes have been examined due to the larger available operating voltage range (ca. 2.5 V) [13], however, the discharge performance of such capacitors is much lower than those obtained with aqueous electrolytes, due to the high resistance of the electrolyte.

The conductivity of aqueous

electrolytes is at least one order of magnitude greater than those of organic electrolytes.

Thus, it would be desirable to have an

electrode material with high capacitance and a wide working potential range in highly conductive aqueous electrolytes.

The

most promising material thus far considered appears to be diamond. Diamond possesses a wide potential window in aqueous [16, 17] and nonaqueous [18] media and extreme electrochemical stability [19].

418

Although as-deposited polycrystalline diamond

19. Electrochemistry at Nanostructured Diamond Electrodes: Characterization and Applications

exhibits very low capacitance [17], here we have demonstrated that the capacitance can be increased drastically by producing high-aspect-ratio cylindrical pores in the electrode

through

oxidative etching. In the present work, we have carried out the electrochemical

characterization

of the

diamond

honeycomb

electrodes using cyclic voltammetry and impedance measurements.

19.3.2. Film characterization Scanning electron microscopy—Fig. 19.2 shows SEM images of the three types of diamond nanohoneycomb films.

Highly uniform,

well-ordered arrangements of holes, with a hexagonal closepacked pattern, are clearly seen in these

figures.

Nanoporous

boron-doped diamond films with various pore diameters (30 nm to 400 nm) and pore depths (50 nm to 3 fxm) were fabricated by etching polished polycrystalline diamond films through porous alumina masks with an oxygen plasma. Among the three honeycomb films that we have fabricated, the film with a pore diameter of 60 nm and depth of 500 nm has the most highly ordered structure, in terms of both the shapes of the individual pores as well as the overall arrangement (Fig. 19.3. IB, honeycomb pore dimension type 60 x 500 nm). average pore density was 1 x lO^^ cm'2.

The

Based on the pore

dimensions and pore density, the surface area was estimated to be a factor of 10.5 times larger for the honeycomb film compared to a flat, polished surface.

The film with 30-nm pores has a lower

porosity (i. e., roughness factor), due to the small diameter of the

419

237 nm W ^ H ^ ^IKf^ ^tK^

^t^

^ 1 ^

' ^ ^ _ ""^.. "^'^ „

i»«»^

"^

<

-flWK

•

I • • # # ••A^APJ

^OOniy <0t-

^.s mp Fig. 19 2 SEM images of a highly boron-doped nanohoneycomb diamond electrode.(a) top view, (b) oblique view at a 45° tilt angle for pore types (A) 30 x 50 nm, and (B) 60 nm x 500 nm and (C) 400 nm x 3 Jim. Nanohoneycomb films observed by SEM were fabricated from tree-standing polished diamond 420

19. Electrochemistry Applications

at Nanostructured

Diamond

pores, the larger intervals and the

Electrodes:

Characterization

shallower pore

and

depth

(Fig.l9.2A, pore type 30 x 50 nm). The average pore density of this film is 2.78 x lO^o cm"2. Based on the pore dimensions and pore density, the surface area for this film was estimated to be only a factor of 2.11 times larger compared to a flat, polished surface.

However, in the case of the honeycomb with 400-nm

diameter pores, the latter are very closely spaced, and some pores have merged to form larger ones (Fig.l9.2C, pore type 400 nm x 3 ^im). In this case, due to the larger pore depth, the porosity is much greater than that of the other two films. Although the pore density is only 4 x 10^ cm 2, this film has a high roughness factor (15.6). ^

4inA( (a) As-deposited diamond

S ' (b) Pore type 30 X 50 nm

(c) Pore type 60 X 500 n

(d) Pore type 400nmX3 nj -0.5 V 0.4 V

-2.5 -1.5 -0.5 0.5 1.5 2.5 Potential (V vs. Ag/AgCI) Fig. 19.3. CycUc voltammograms for (a) as-deposited diamond and pore types (b) 30 x 50 nm, (c) 60 x 500 nm and (d) 400 nm x 3 jim; electrolyte, 1 M H2S04>* sweep rate, 100 mV s"i. Arrows indicate the potentials at which the impedance measurements were carried out.

421

Cyclic voltammetry

- Because the advantage of diamond in the

double-layer capacitor application is its wide working potential window, we have examined the current-potential behavior for the honeycomb films (Figure 19.2A).

Interestingly, the working

potential window for the honeycomb films remained essentially the same as that for the as-deposited film, even after extended oxygen plasma treatment. Table 19.1. Comparison of double-layer capacitance and specific energy for various types of carbon-based electrodes. Potential window A V, V from cyclic Roughness voltammetry factor (Ercd, E„x) ' As-deposked diamond fikn

4.0

Cdi, ^iF cm"' (geometric) C a , , F g ' Edi, ml cm " from from hpedance impedance ' (geometric)

3.04 (-1.24, 1.80)

12.9

5.94 X 10"^

Gbssy carbon GC-20

2.47 (-1.03, 1.44)

55.1

1.68 X l o '

HOPG ZYA

1.93 (-0.64, 1.29)

7.02

Activated carbon

1.0 (-0.7, 0.3)

_

.-Id

Edi,Jg

Edi,Jg' '

1.26 X 10"^ 50-200

100-400

Pore type 30 x 50 nm

2.11

2.70 (-1.12, 1.58)

129

9.12

0.469

33.3

150.9

Pore t>pe 60 x 500 nm

10.9

2.62 (-1.05, 1.57)

1.83 X 10^

14.5

6.29

49.9

63.0

Pore tvpe 70 x 750 nm

16.7

2.61 (-1.05, 1.56) 2.90 X 10^

17.9

9.12

61.1

72.8

Pore type 400 nm x 3 ^m

15.6

2.46 (-0.85, 1.60) 3.91 X 10^

74.6

11.8

224.8

185.1

4.0

3.17 (-1.34, 1.83)

Direct etched dianrond (no mask)

238

1.20

a Values obtained from cyclic voltammograms measured at 100 mV s-1 . The definition of potential window is AV < 2 mA V-1 cm-2 (data from Fig. 19.3). b Values obtained by AC impedance analysis at 0.4 V vs. Ag/AgCl (data from Fig. 19.3). c The specific capacitance for a hypothetical through-hole diamond membrane, d The specific energy was estimated from the equation, Edl = 1/2 x Cdl ^ (AV)2. e The specific energy for a thorough-hole membrane estimated from pore parameters and the differential capacitance of 200 ^F cm-2.f Etched for 1 min. SEM showed no significant roughening of the surface.

422

19. Electrochemistry at Nanostructured Diamond Electrodes: Characterization and Applications

We have chosen the criterion for the definition of potential window to be that the slope of the CV at 100 mV s i is < 2 mA V i cm"2. The potential windows for various electrodes, estimated in this manner, are summarized in Table 19.1.

The potential

windows for as-deposited diamond (3.04 V) and the 30 x 50-nm pore honeycomb (2.70 V) are appreciably larger than those for either GC (2.47 V) or HOPG (1.93 V) [17, 20]. The values for the honeycomb diamond electrodes were somewhat smaller (340 to 580 mV) than that for as-deposited diamond due in part to the less negative potential limits (Table 19.1). As a result, these porous structures exhibited wide electrochemical potential windows (ca. 3.0 V) in aqueous electrolytes, being somewhat smaller than unetched, as-deposited diamond electrodes, independent of pore structure. The double layer capacitive current for the diamond honeycomb was a factor of 18 to 20 larger than that for the asdeposited diamond electrode due to the surface roughness of the nanohoneycomb structure. We shall next explore this difference in greater detail using impedance measurements.

19.3.3. Impedance Measurements Impedance plots - Fig. 19.4 shows experimental impedance plots (complex plane representation) obtained for both the as-deposited and the honeycomb diamond electrodes at 0.4 V. The plots for the pore types, 60 x 500 nm (Fig. 19.4c), 70 x 750 nm (not shown), and 400 nm x 3 mm (Fig. 19.4d), exhibit two distinct domains- a high frequency domain, where the impedance behavior is that expected for a cylindrical pore electrode, with a characteristic linear portion at a 45° angle, and a low frequency domain, where the behavior is

423

t h a t expected for a flat electrode [21].

12.0

~^oiio

I C3

a

8.0 \-o- —

o O 0.025

N 4.0 \o ) S )0.050 JO.IO,

0.0 i ^ i ^ 0.0 4.0 8.0 Re Z (10^ Q cm^)

4.0

8.0

Re Z (103 Q cm2)

22.5 S a 15.0 h N

0.0 7.5 15.0 Re Z (10^ Q cm2)

3.0 6.0 Re Z (10^ Q cm2)

Fig. 19.4. Complex-plane plots of the impedance for electrodes of (a) as-deposited diamond and pore types (b) 30 x 50 nm, (c) 60 x 500 nm, and (d) 400 nm x 3 [xm, a t -1-0.4 V vs. Ag/AgCl.Experimental data points (O) and simulated curves (sohd lines) calculated on the basis of equivalent circuits involving modified transmission line models (see text), are shown. The parameters used in the calculated curves are given in Table 19. 2.

The impedance plots for the pore type 30 x 50 n m electrode, however,

exhibit

only

a

high

frequency

domain,

characteristic linear portion at a 45° angle (Fig. 19.4b).

424

with

a

In this

19, Electrochemistry at Nanostructured Diamond Electrodes: Characterization and Applications

case, even at low frequencies, the potential oscillations have negligible influence beyond a certain depth (penetration depth). At cylindrical-pore electrodes, the capacitance tends to reach an intrinsic limiting value at very low frequencies.

The values

were calculated in the low frequency limit (O.Ol Hz) from the imaginary

component of the

impedance with the relation Z =

•//(coC). The results are summarized in Table 19.1. The double layer capacitance values per unit area discussed in this paper are based on the geometric area, except where explicitly stated otherwise. The capacitance values were found to increase with increasing roughness factor, based on the pore dimensions. Among the electrodes examined, the honeycomb with 400 nm x 3 (im pores yielded a maximum capacitance value of 3.91 x 10^ mF cm"2, which is a factor of ca. 400 larger than that for the asdeposited surface. For the porous film with 30-nm diameter pores, there was only a very small effect of the pore structure on the capacitance due to the high pore impedance. Table 19.1 shows that the specific capacitance value (74.6 F gO estimated for the 400 nm x 3 (a.m pore type honeycomb is comparable to those typical for activated carbon electrodes, which range from 100 to 400 F g i [22]. In terms of device applications, the ability to store energy is important, and the larger available potential range for diamond (> 3.0 V) compared to those for other forms of carbon (ca. 1.0 V for activated carbon [37]) becomes an advantage. Energy densities have been calculated for all of the various types of electrodes examined in the present work in terms of the geometric areas (Table 19.1).

Taking the capacitance values (Cdi) from the

425

impedance m e a s u r e m e n t s a n d t h e potential window values ( A V ) from

t h e CV m e a s u r e m e n t s ,

the energy densities

(per

unit

geometric area) for t h e actual diamond honeycomb double-layer capacitors for a full cell were calculated by use of t h e formula Edi = 0.5 X Cdi X (AV)2.

A s s u m i n g t h a t the free-standing diamond honeycomb with

though-holes

were

available

for

the

pore

films

geometries

examined here, we have e s t i m a t e d hypothetical values for the specific capacitance for t h e various honeycomb samples (i. e., per unit mass) (Table 19.3.1). These range from 33.3 to 224.8 J g i . Due to t h e large working potential range, the specific energies for t h e honeycomb diamond electrodes fall nearly in t h e same range as t h a t for typical activated carbon-based capacitors (50 - 200 F g 0.

Because of the wide electrochemical potential window in

aqueous

electrolytes

and

the

high

capacitance,

honeycomb

diamond electrodes are promising candidates for electrochemical capacitor applications. Numerical

simulations

- The double-layer charging process for a

porous electrode consisting of cylindrical pores can be simulated with the use of the t r a n s m i s s i o n line model [24-26].

If the

cylindrical pores are characterized by r a d i u s r, length 1 a n d n u m b e r of pores n, the m a t h e m a t i c a l form for t h e t r a n s m i s s i o n line model is Z^WcotMyl)

(19.1)

where W and y are defined as (RZ)!/^ and (R/Z)!/^^ respectively. Here, 1/Z is jcoC, a n d R and C are the resistance a n d capacitance per u n i t pore d e p t h and are expressed by lAnjtr^K) a n d 2jtrnCdP°^®, respectively.

426

K is the electrolyte conductivity a n d CdP°^^ is the

19. Electrochemistry at Nanostructured Diamond Electrodes: Characterization and Applications

differential double-layer capacitance in the pores. The impedance can be simulated by use of the geometric parameters of the cylindrical pores observed by SEM. ' R ^^^ Reaction

R^ext

O

I Series 1 resistance

Rpore

C^ lit

1

Pore depth /

Vd/";" T ; / \ Electrolyte '; cionductivity

Pore diaifietqr

-O

d

'\ Transmission line model

Fig. 19.5. Equivalent circuit based on the transmission line model, including both a Faradaic charge-transfer reaction and double-layer charging in the honeycomb diamond electrode The calculated impedance curves for the various honeycomb electrodes are shown in Fig. 19.4, together with the experimental curves.

Figure 19.5 shows an equivalent circuit employed to

reproduce the impedance plots for honeycomb diamond electrodes. Table 19.2 summarizes the values of the fitting parameters and the average relative errors for the calculated curves.

The

calculated curves are in good agreement with the experimental curves. The areal capacitances of the pore walls (CdP^^®), falling in the range 120 to 230 mF cm"2, were on the same order as that of the 1min direct-etched diamond surface

(see Table 19.1).

This

capacitance enhancement for the plasma-etched surfaces is due to contributions from

oxygen-containing

functional

groups

and 427

various types of defects generated on the surface during the plasma treatment. Usually, the electrolyte conductivities inside the honeycomb pores, as determined by impedance, range from 15 to 180 mS cm 1, which are of the same order of magnitude as the bulk sulfuric acid conductivity. However, in the case of the pore type 30 X 50 nm film, the electrolyte conductivity was estimated to be only 70 mS cm i, based on the fitting (Table 19.2). For the equivalent circuit used for the porous electrodes, the pore impedance is usually determined only by the value of the electrolyte conductivity. In the case of the 30-nm pore diameter nanohoneycomb, the pore impedance has drastically increased. Using a transmission-line model for double-layer charging within the pores, we were able to simulate the experimental impedance curves.

The diamond honeycomb structures appear to be good

approximations to an ideal cylindrical pore-type electrode. Table 19. 2. Parameters used for fitting the impedance results in the complex plane (Fig. 19.4), based on the modified transmission Une model (Fig. 19.5). Type

of

Series

Differential

Time

Reaction

Series

Differet\tial

Time

Reaction

Pore

Poi-e

Pore

Electrolyte

Average

equivalent

resistance

capacitance

consent

resistance

resistance

capacitana

constant for

resistance

diameter,

depth,

density,

conductivity,

relative

cifcuil

for external

for

for

for external

for

for

pore,

for pore.

d, nm

1, nm

n, cm 2

KmScm'

en-or, i%)

surface,

surface,

external

surface,

R.'",ncm'

CQcm^

Q,"

surface.

R(",Qcm^

As'deposited

85.2

external

uRm'

12.9

pores.

pores,

cr,

I^por.S

R,'~,Qcm'

uFcm"^

1.10

diamond Pure

140

60

500

l.OxlO'

15

transnusskon line model Pore type 30

35.5

29

2.69

1.42x10^

120

53.58

30

50

2.8xl0'0

0.07

13,1

213

60

9.16

71.0

140

5.15

60

500

1.0x10'

15

9.75

639

160

50.8

3.20x103

230

4.75

400

3000

4.8x108

180

8.94

X50nm Pore type 60 XSOOnm Pore type 400nmX 3/xm

428

19. Electrochemistry at Nanostructured Diamond Electrodes: Characterization and Applications

19.4. Electrochemical Properties of Pt-Modified Nanohoneycomb Diamond and Applications as a Size Selective Sensor Material Diamond possesses morphological stability at extreme anodic and cathodic potentials and corrosion resistance in both acidic and alkaline

conditions,

without

any

evidence

of

structural

degradation [27]. Polycrystalline diamond is ideally suited as a current collector for batteries [28] or as an electrocatalyst support for fuel cells [29] and for electrosynthesis. Diamond, because of its extremely high packing density, is almost completely impervious to insertion of ions. In order to achieve high catalyst loadings and large

surface

areas,

use

of porous

diamond

supports

is

advantageous for applications in electrocatalysis. In this section, we report the use of conductive nanoporous honeycomb diamond as a support for Pt nanoparticles for electrocatalytic applications. In the present work, nanohoneycomb diamond electrodes with various pore diameters were modified with Pt nanoparticles and their size-selective electrocatalytic properties were studied. The catalytic activity and reaction kinetics for oxygen reduction and alcohol oxidation were found to be dependent on the pore dimensions.

19.4.1. Film characterization Scanning

electron

microscopy - Platinum nanoparticles were

deposited in the pores of the diamond nanohoneycomb film using the following method. The nanohoneycomb films were immersed

429

^3 um^

^600 nm

300 nm

300 nm < >

y J.

'iHTlf.t *m Mim^

600 n

5^

600 nm < >

Fig. 19.6. SEM images of Pt-modified highly boron-doped diamond electrodes- (A) top view for Pt-modified as-deposited diamond electrode at (a) low and (b) high magnification,' (a) top view; (b) obHque view at a 45° tilt angle for pore types (B) 60 x 500 nm, and (C) 400 nm x 3 [im

430

19. Electrochemistry at Nanostructured Diamond Electrodes: Characterization and Applications

in a 73-mM H2PtCl6 aqueous solution for 8 hours.

After

immersion, the film was dried in air, and the Pt ions were reduced to the metal by a 3-h exposure to flowing H2 gas at 580<^C. This process results in the incorporation of platinum nanoparticles on the external surface and on the pore walls. Figure 19.6 shows SEM images of three types of Pt-modified diamond films that were fabricated from as-deposited diamond and nanoporous diamond films. Fig. 19.6A (a and b) shows images of

the

as-deposited

diamond

surface

with

dispersed

Pt

nanoparticles (as-deposited diamond/Pt ). The Pt nanoparticles, located mainly at the grain boundaries, have diameters from 10 to 150 nm. There are also very small Pt deposits (10-50 nm) on the grain surface. The two nanohoneycomb films (Fig. 19.6, B and C) are shown both as top views (left) and oblique views (right). The top views show highly uniform, well-ordered arrangements of holes, with a hexagonal

close-packed

pattern

[26]. The

Pt

deposits

are

predominantly present in the pores rather than on the external surface, as seen by comparing the top and oblique views.

The

oblique views of the edges of the honeycomb films clearly show the well-defined cylindrical pores, with relatively large numbers of Pt deposits on the pore walls.

In the SEM images of both nano-

honeycomb/Pt films (honeycomb pore dimension type 60 x 500 nm/Pt and 400 nm x 3 ^im/Pt), the homogeneous distribution of Pt nanoparticles on the inner walls of the honeycomb pores is clearly evident. For pore type 60 x 500 nm, due to the small pores, Pt deposits as small as 10 to 40 nm were obtained. In contrast, on the as-deposited diamond surface, the Pt deposits ranged up to

431

150 nm. Hence, honeycomb films provide better dispersion of Pt deposits. Table 19.3. Comparison of the number of exposed surface Pt atoms for Pt-modified as-deposited diamond and Pt-modified nanohoneycomb diamond electrodes Desorptionof Number of surfifice Pt atom Pt surface area Roughness hydrogen fiictor /mCcm' (geo.) /lO cm" (geo.) /en/(real) As-deposited diamond / Pt

3

Pore type 60X500 nm/Pt Pore type 400 nmX 3 ^m/Pt

15.9

Polyciystalline Pt

Background

cyclic voltammetry

0.61

3.77

188

1.89

11.8

9.02

2.84

17.8

13.6

0.68

4.21

3.21

- Background cyclic voltammo-

grams were obtained in 1 M H2SO4 solution at a sweep rate of 50 mV s i .

The voltammetric features of Pt-modified diamond are

characteristic of Pt metal, with Pt oxide formation in the +0.7 to +1.2 V region, the reduction of Pt oxide at ca. +0.5 V, and the adsorption and desorption of hydrogen between 0 and -0.18 V (not shown). Integration of the oxidation charge associated with the desorption of hydrogen between 0 and -0.18 V yielded a value of 1.89 mC cm'2 for pore type 60 x 500 nm/Pt. This charge can be used to calculate the number of exposed surface Pt atoms, which was estimated to be 1.18 x 10^^ cm'^ (geometric area) using a standard value of 210 mC cm 2, which corresponds to a calculated value of 1.30 x lO^^ atoms cm'2 for poly crystalline Pt. The values determined for the diamond/Pt and polycrystalline Pt from the cyclic

voltammograms

are

summarized

in

Table

19.4.1.

Interestingly, the number of exposed surface Pt atoms per unit

432

19. Electrochemistry at Nanostructured Diamond Electrodes: Characterization and Applications

geometric a r e a observed on the as-deposited diamond/Pt w a s close (ca. 90%) to t h a t for poly crystalline Pt. T h u s , this as'deposited/Pt film w a s expected to exhibit similar electrocatalytic

activity

compared to P t metal. However, this w a s not t h e case,

as

discussed later.

19.4.2. Electrocatalysis with Pt-modified diamond- cyclic voltammetry S <

•^^ u u

-0.5

0.0 0.5 1.0 Potential /Vvs.Ag/AgCl

u u

U

1.5 0.0 0.5 1.0 Potential /Vvs.Ag/AgCl Fig. 19.7. Cyclic voltammograms for (A) as-deposited diamond before Pt deposition (dotted Hne), after Pt deposition (solid line) and (B) Ptnanoparticle-filled nano-honeycomb 60 x 500 nm (dot-dashed Une), 400 nm x 3 ^im (soUd line)," electrolyte, oxygen-saturated 1 M H2SO4J sweep rate, 50 mV s ^' geometric surface area, 0.07Icm^.

433

0.0 0.5 1.0 Potential / V v s . A g / A g C I

1.5

0.0 0.5 1.0 Potential / V vs. Ag/AgCl

1.5

in

C

u

U

Fig. 19.8. Cyclic voltammograms for (A) as-deposited diamond before Pt deposition (dotted line), after P t deposition (solid line) and (B) Pt" nanoparticle-filled nano-honeycomb 60 x 500 nm (dot-dashed line), 400 nm x 3 pim (solid line); electrolyte, 2 M methanol + 1 M H2SO4; sweep rate, 50 mV s i; geometric surface area, 0.071 cm^. The effectiveness of t h e Pt-modified diamond electrodes for t h e electrocatalysis of fuel cell reactions w a s examined. We have tested their electrocatalytic activities for O2 reduction a n d alcohol oxidation. Figure 19.7 compares O2 reduction c u r r e n t s for t h e asdeposited diamond, t h e as-deposited diamond/Pt, honeycomb 60 x 500 n m / P t a n d the 400 n m x 3 \xmfPt electrodes in 1 M H2SO4 434

19. Electrochemistry at Nanostructured Diamond Electrodes: Characterization and Applications

saturated with oxygen at a sweep rate 50 mV s'^. In the absence of the Pt nanoparticles, essentially no O2 reduction is observed over this potential range, as diamond is known to have low catalytic activity for O2 reduction.

In contrast, at diamond/Pt

composite electrodes, large O2 reduction current is observed at potentials characteristic for Pt electrocatalysis in this solution. The cathodic current density for the 400 nm x 3 ^im/Pt electrode (ca. -1.8 mA cm 2, geometric) was nearly twice as large as that for the as-deposited diamond/Pt electrode (ca. "1.0 mA cm 2, geometric), and this is due to the high surface area. Based on the number of surface Pt atoms per unit geometric area, which was ca. five times greater for pore type 400 nm x 3 ^im film than for the as-deposited diamond, a similar factor could be possible for the peak current, but this is clearly not expected, due to mass transport limitations. For methanol oxidation

(2 M in

1 M H2SO4), cyclic

voltammograms were obtained at the as-deposited diamond, the as-deposited diamond/Pt, 60 x 500 nm/Pt and the 400 nm x 3 ^m/Pt electrodes (Fig. 19.8). At an as-deposited diamond film, no methanol oxidation was observed; diamond is known to have low activity for methanol oxidation.

In the case of the nonporous

diamond/Pt electrode, a large anodic peak was observed at ca. 0.9 V, attributable to methanol oxidation. The Pt-containing film is known to be electroactive for methanol electrooxidation [30, 31]. The Pt nanoparticles supported on the diamond electrode provide the catalytic activity for methanol oxidation in acid solution. The oxidation current for the 400 nm x 3 jim/ Pt electrode (ca. 7.0 mA cm"2, geometric) was greatly enhanced compared to the as-

435

deposited diamond/Pt (ca. 1.1 mA cm 2, geometric) and w a s found to be ca. 16 times higher t h a n t h a t for t h e P t polycrystalline electrode (ca. 0.44 mA cm'2, geometric) (Fig. 19. 8).

-0.5

0.0 0.5 1.0 Potential / V vs. Ag/AgCI

1.5

B 0.8 r-"—^—.—,—r^—.—,—^n—'—-—^—'—r—^

u

(B)

y ^

S 0.4

p.^^^^,^^^ F'I<j*^^''^

>» •^ 0.0

" • " ^

-

/

^^^^^'^

i'/^^'^'"

c: (U T3

I w*

--0.4

~

if

a> u

3-0 8 ^ -0.5 L_J

.

L ^ ,

1

,

>

.

l _ l

,

.

.

>

I

L_

_, _,

0.0 0.5 1.0 Potential / V vs. Ag/AgCI

Fig. 19.9. CycUc voltammograms for (A) ethanol oxidation and (B) 2propanol oxidation for as-deposited diamond / Pt (dotted Hne), Ptnanoparticle-fiUed nano-honeycomb 60 x 500 nm (dot-dashed Une), and 400 nm x 3\im (solid Une); electrolyte, 2 M ethanol or 2 M 2propanol + 1 M H2SO4; sweep rate, 50 mV s i; geometric surface area, 0.071 cm2. At t h e as-deposited diamond/Pt electrode, the p e a k c u r r e n t is proportional to the square root of t h e scan rate, indicating t h a t the oxidation of m e t h a n o l a t t h i s electrode is controlled by diffusion. In contrast, at both t h e 60 x 500 n m P t and t h e 400 n m x 3 ^im/Pt

436

19. Electrochemistry at Nanostructured Diamond Electrodes: Characterization and Applications

electrodes, the current densities deviate from the linear curve at higher sweep rates. This behavior is thought to be caused by the nanoporous structure effect for methanol mass transport inside the pores.

This effect is expected to be dependent on the size of

the reacting molecules.

Therefore, the oxidation reactions of

larger size alcohols were also investigated. For example, ethanol oxidation was examined.

Cyclic

voltammograms were obtained for the as-deposited diamond / Pt, 60 X 500 nm/Pt and 400 nm x 3 ^im/Pt electrodes in 2 M ethanol in 1 M H2SO4 (Fig. 19. 4. 4A). The Pt-modified diamond electrodes show elecrocatalysis for ethanol oxidation [32]. It can be seen that the oxidation current for pore type 400 nm x 3 pim/Pt was ca. 4 times higher than that for as-deposited diamond/Pt, but the oxidation current for pore type 60 x 500 nm/Pt was suppressed, being only ca. 0.6 times of that for as-deposited diamond/Pt. The expected

current

enhancement

due

to the

nanohoneycomb

roughness was not observed for this pore type. In addition, 2-propanol oxidation was examined.

Figure

19.4.4B shows cyclic voltammograms obtained for as-deposited diamond/Pt, 60 x 500 nm/Pt and 400 nm x 3 ^im/Pt electrodes in 2 M 2-propanol in 1 M H2SO4. In this case [33], it can be seen that the oxidation currents for as-deposited diamond/Pt, 60 x 500 nm/Pt and 400 nm x 3 mm/Pt electrodes are all similar, and therefore, there was no enhancement due to the honeycomb roughness for either nanohoneycomb/Pt electrode. In order to better illustrate the nanostructure effect for the electrocatalytic reactions examined here, peak current ratios were used. (Fig. 19.10) These values {R^ are the ratios of the peak

437

current densities for the honeycomb diamond/Pt electrodes {Ip^ to that for the as-deposited diamond/Pt (7^^), normalized by the ratio of number of surface Pt atoms exposed (A^ and A^, respectively) using the formula Rp = (Ip^ I Ip^) x (A^^ / A^). This could be considered to be an indicator of the fraction of surface Pt atoms that are actually actively involved in the electrocatalytic reaction.

In the case of methanol oxidation, at

both honeycomb diamond/Pt electrodes, approximately all of the surface Pt atoms appear to be available for the catalytic reaction.

Methanol Ethanol 2-PropanoI Oxygen Oxidation Oxidation Oxidation Reduction

Fig. 19.10. Relationships of peak current ratios for oxygen reduction and alcohol oxidation for Pt-nanoparticles-filled nanohoneycomb (D) 60 • 600 nm and (O) 400 nm X 3 jim electrodes. The peak current ratio is defined as the ratio of the peak current density for the honeycomb/Pt to that for the as-deposited diamond/Pt and normalized by the ratio of the number of surface Pt atoms exposed. In contrast, for ethanol oxidation, the apparent fraction of active Pt atoms for 60 x 500 nm/Pt was only 0.2, which is three times lower than that for pore type 400 nm x 3 (xm/Pt. This result indicates that there is a limitation on the ability of the ethanol molecules to access the Pt atoms located within the 60-nm pores.

438

19. Electrochemistry at Nanostructured Diamond Electrodes: Characterization and Applications

This effect is even more evident for the larger molecule, 2propanol, which yielded an active Pt atom ratio of 0.1, even for 400 nm X 3 ^im/Pt.

These results clearly indicate an effect of

molecular size for the honeycomb/Pt electrodes for the catalytic oxidation of alcohols.

The electrocatalytic activities of the Pt-

modified nanohoneycomb films were found to be dependent on the structural parameters of the honeycomb pores and the molecular sizes of the alcohols, indicating that the selectivity of the electrodes can be controlled by variation of the pore dimensions. Both

nanohoneycomb/Pt

electrodes

showed

electrocatalytic activity for oxygen reduction and

high

methanol

oxidation. Hence, these electrodes have potential application in fuel cell development.

19.4.3. Electrocatalysis with Pt-modified diamondimpedance measurements In order to understand the characteristics of the electrocatalysis reaction inside the nanoporous electrodes, additional analysis of the ac impedance behaviour was carried out, and the penetration depths of reactant molecules in the nanohoneycomb pores for catalytic reactions and the reaction parameters for different pore structures were estimated. The ac impedance measurements for Pt-modified diamond electrodes

439

-5 2.0

a

0.5

O •-^

0.3

(A)

10< k H z ^ 0.23

1 Q J J ^ ^ ^ tegO.l

^P;

0^

BtDc^'^^l

g^m kHz ioo| 0.0^^ ^ 0.01

0J3

0.0

2.0 4.0 Re Z /103 Q cm^

0.0

0.6 1.2 Re Z / 10^ Q cm^

1.3 2.6 Re Z / 103 Q cm^

0.6

0.9

3.9

Fig. 19.11. Impedance plots for electrocatalytic reactions^ (A) the oxidation of methanol at 0.9 V vs. Ag/AgCl; (B) the oxidation of ethanol at 0.9 V vs. Ag/AgCl; and (C) the reduction of oxygen at 0.4 V vs. Ag/AgCl. Experimental data points are shown as open symbols for (A) as-deposited diamond / Pt, (D) 60 X 500 nm/Pt, and (O) 400 nm X 3 jim/Pt. The simulated curves, calcidated on the basis of the equivalent circuit in Fig. 19.3.4, are shown as solid Unas. The parameters are summarized in Tables 19.4 and 5.

440

19. Electrochemistry at Nanostructured Diamond Electrodes: Characterization and Applications

during catalytic reactions were carried out at the peak potentials obtained in the CV measurements (Fig. 19.11A-C). The impedance plots for methanol oxidation (Fig. 19.11A) consist mainly of parallel RC'type semicircles whose diameters (and thus the corresponding resistances) decrease with increasing roughness factor (ca. 3 for as-deposited diamond, 10.9 for the 60-nm pores and 15.9 for the 400-nm pores). The diameters of the semicircles (Q cm2, based on the geometric area) decreased in order for the asdeposited diamond/Pt, pore type 60 x 500 nm/Pt, and pore type 400 nm X 3 pim/Pt, roughly estimated to be 2.9 x 10^ Q cm^, 6.5 x 102 Q cm2, and 5.0 x 10^ Q cm^, respectively. These can be related to the charge transfer resistances (discussed later in detail), which decrease with increasing effective surface area for the charge transfer reaction. In contrast, for ethanol oxidation and oxygen reduction (Fig. 19.11 B and C) the diameters no longer follow the same order as the roughness. The impedance plots for the pore type 60 X 500 nm/Pt electrode trace the largest semicircles for both ethanol oxidation and oxygen reduction. The charge-transfer resistance per unit area for pore type 60 x 500 nm/Pt is now larger than that for pore type 400 nm x 3 ^im/Pt due to a mass transfer effect, as discussed later. The impedance of a porous electrode can be simulated with the transmission line model, and the penetration depth can be evaluated [24].

For the non-porous Pt-modified as-deposited

surface, the methanol oxidation reaction can be simulated as a simple

Randies

equivalent

circuit

comprising

a

parallel

combination of a double layer capacitance and a semi-infinite Warburg impedance in series with a charge transfer resistance.

441

For oxygen reduction, a simple Randies equivalent circuit was also used because the reaction mechanism for oxygen reduction for the Pt electrode can be described by mass

transport-controlled

kinetics. The simulated curves are shown in Fig. 19.1l(A-C). The fits are reasonably good, with the charge-transfer resistances (based on geometric area) Rr values shown in Table 19.4. Table 19.4. Parameters used for fitting the impedance results for an as-deposited diamond/Pt electrode in the impedance plots (Fig. 19.11), based on the Randies circuit.

Series resistance

Electroatalytic reaction R,/Qcm^ Methanol Oxidation 38.2

Differential capacitance Cd /fiFcm^

Reaction resistance R, / Q cm^

Dintision resistance 6 / Q cm^

70

1.90 X 10^

Ethanol Oxidation

50.2

90

3.60 X 10^ 1.45 X 10^

Oxygen Reduction

28.2

80

9.00 X 10^ 3.55 X 10^

4.60X 10^

The impedance of a charge-transfer reaction at a porous electrode consisting of cylindrical pores is given in the previous section by Eq. (19.l) [23-26].

To simplify the calculations, the

Faradaic impedance per unit real surface area was assumed to be potential-independent over a range of values that would exist along the entire pore, e. g., < 0.25 V, and thus consists only of a parallel combination of the charge transfer resistance and a double-layer capacitance, without a Warburg impedance. For a charge transfer-controlled process, 1/Z = 1/Rct + jcoC, and the reaction resistance Ret and capacitance C per unit pore depth are expressed by RrPo^e/(2jtr) and ja)2jtrnCdP°^^, respectively. Here, RrP^^^ is the charge transfer resistance with respect to the 442

19. Electrochemistry at Nanostructured Diamond Electrodes: Characterization and Applications

real surface a r e a on t h e pore walls. A distinction between Rr and RrP°^® h a s been made, because we wish to apply the

latter

specifically to t h e pores only, where most of the P t particles are located.

F r o m t h e geometric p a r a m e t e r s of the cylindrical pores

(i.e., diameter, d e p t h and n u m b e r density), which are obtainable from S E M observation, the impedance can be evaluated. Table 19.5. Parameters used for fitting the impedance results in the impedance plots (Fig. 19.11), based on the equivalent circuit in Fig. 19.5. Series resistance for external surface

Differential capacitance for external surface

/Qcm'

/I^Fcm'

Series resistance for pore

Differential capacitance for pore

Cd"" Type of Electrodes

/ I^F cm ^

Pure transmission line Methanol Oxidation

60x500nni/Pt 400nmx3

V-ml?i

166

Reaction resistance Electrolyte for pore conductivity k / mScm'

/ Q cm^ 1.80 X 10^

Penetration Average depth relative error IV-m

/%

80

400

120

400

166

1.80 X 10^

80

0.46

3.02

130

600

200

900

4.50 X 10^

162

2.69

6.75

200

3.50 X 10^

0.7

Pure transmission line Ethanol

60 x 5 0 0 n m / P t

350

100

350

200

3.50 X 10^

0.7

0.19

22.48

Oxidation

4 0 0 n m x 3 ^^l/Pt

120

480

180

820

1.40 X 10^

830

2.87

12.09

400

1.80 X lO-*

0.5

Pure transmission line Oxygen

60x500 nm/Pt

Reduction 4 0 0 n m x 3 ^ m / P t

The

400

240

400

400

1.80 X lO"*

0.5

0.36

8.76

140

650

100

260

6.20 X 10^

100

2.49

8.01

calculated

electrodes

are

shown

experimental curves.

impedance in

Fig.

curves 19.1l(A-C)

for

the

together

honeycomb with

Fig. 19.5 shows t h e equivalent

the

circuit

employed to simulate the impedance plots for the honeycomb / Pt electrodes.

Table 19.8 s u m m a r i z e s t h e values of t h e

fitting

p a r a m e t e r s and t h e average relative errors for t h e calculated curves.

For 60 x 500 nm/Pt, t h e electrolyte conductivity in the

pores of 80 m S cm'i, m e a s u r e d for m e t h a n o l oxidation, decreased

443

to that for ethanol oxidation (0.7 mS cm i, Table 19.5). This result suggests that the conductivity associated with the

alcohol

molecule in the 60-nm nanohoneycomb pores decreases with increasing molecular size. By use of the transmission line model, the penetration depth for the reaction can be calculated.

The penetration depth is

defined in previous section by Eq. (19.2) [24]. X= I Zt 11/2 R-1/2 sec l/2cp

(19.2)

where | Zt I and cp are the amplitude and the phase angle for the impedance of the transmission part, respectively.

Table 19.5

summarizes the penetration depths for the various catalytic reactions.

For pore type 400 nm x 3 ^im/Pt, the penetration

depths X for all of the catalytic reactions were close to the actual pore depth of 3 ^im, with the lowest value being 2.49 i^m for O2 reduction (ca. 80 % of the pore depth). For pore type 60 x 500 nm/Pt, with 460 nm for methanol oxidation, >^ was also close to the actual depth, indicating that almost all of the pore surface is available. In contrast, the X value for ethanol oxidation was 190 nm, which is only 40 % of the total pore depth.

This result

suggests that pore type 60 x 500 nm is sensitive to the size of the alcohol molecule, so that X decreases with increasing reactant size. For O2 reduction, X. decreases due to its low concentration. However, even so, half of the pore depth for type 60 x 500 nm was still available for ethanol oxidation and oxygen reduction. It is interesting to note that the charge transfer resistances Rr calculated for methanol oxidation for the Pt-modified diamond electrodes (ca. 1.8 • 4.5 kQ cm^) are of the same order. In contrast, the Rr values calculated for ethanol oxidation and oxygen

444

19. Electrochemistry at Nanostructured Diamond Electrodes: Characterization and Applications

reduction for the honeycomb/Pt electrodes are significantly larger than that for as-deposited diamond/Pt.

Also, an increase in Rr is

observed with decreasing pore size. A possible explanation for the increase of the reaction resistance could be the relatively low concentration of the reactant near the active catalytic sites because of the limitation of mass transport by the nanoporous structure.

In order to clarify the contribution of the ethanol

concentration to the Rr values, we have examined the concentration dependence of the impedance behavior. A series of impedance plots for the Pt-modified as-deposited diamond electrode in C2H5OH + 1 M H2SO4 solution were obtained (not shown). The ethanol was varied in concentration from 0.02 to 2 M. The fact that the Rr value (l.O kQ cm2) obtained for 0.2 M ethanol for the as-deposited diamond / Pt electrode is close to the value (1.4 kQ cm2) for 400 nm x 3 ^im/Pt in 2 M ethanol (Table 19.4.3) indicates that the ethanol concentration in the pores for the latter is one order of magnitude less than that in the bulk. Similarly, for pore type 60 x 500 nm/Pt, the Rr value (3.5 kQ cm^) was of the same order as that for as-deposited diamond/Pt (2.8 kQ cm^) in

0.02

M

ethanol.

This

result

suggests

that

the

concentration inside the 60 x 500 nm pores is a factor of 200 lower (ca. 0.01 M) than that in the bulk.

445

References 1.

Science (Special Issue on Nanomaterials) 254 (1990) 1300.

2.

E. A. Medcalf. D. J. Newman, E. G. Gorman and C. P. Price, Clin. Chem., 36, (1990) 446.

3.

C. R. Martin, Science, 266 (1994) 1961.

4.

A. Imhof and D. J. Pine, Nature, 389 (1997) 948.

5.

D. Routkevitch et al., IEEE

Trans. Electon. Devices, 43 (1996)

1646. 6.

G. L. Hornyak, C. J. Patrissi and C. R. Martin, J. Phys. Chem. B, 101 (1997) 1548..

7.

H. Masuda and M. Satoh, Jpn. J. Appl. Phys., 35 (1996) L126.

8.

H. Masuda, M. Watanabe K. Yasui, D. A. Tryk and A. Fujishima, Adv. Mater,

9.

12 (2000) 444.

S. lijima, Nature, 354 (1991) 56.

10. C. R. Martin, Chem. Mater,

8 (1996) 1739.

11. J. M. Planeix, N. Coustel, B. Coq, V. Protons, P. S. Kumbhar, R. Dutartre, P. Bernier and P. M. Ajayan, J. Am. Chem. Soc, 116 (1994) 7935. 12. M. Nishizawa, V. P. Menon and C. R. Martin, Science, 268 (1995) 700. 13. I. Tanahashi, A. Yoshida and A. Nishino, Denki

Kagaku,

56

(1988) 892.. 14. C. Niu, E. K. Sichel, R. Hoch, D. Moy and H. Tennent, Appl. Lett,

Phys.

70 (1997) 1480.

15. S. Sarangapani, B. V. Tilak and C. P. Chen, J. Electrochem.

Soc,

143(1996)3791. 16. H. B. Martin, A. Argoitia, U. Landau, A. B. Anderson and J. C.

446

19. Electrochemistry at Nanostructured Diamond Electrodes: Characterization and Applications

Angus, J. Electrochem.

Soc, 143, (1996) L133.

17. G. M. Swain, A. B. Anderson and J. C. Angus, MRS Bulletin,

23

(1998) 56. 18. Z. Y. Wu, T. Yano, D. A. Tryk, K. Hashimoto and A. Fujishima, Chem. Lett, (1998) 503. 19. G. M.Swain, J. Electrochem.

Soc, 141 (1994) 3382.

20. G. M. Swain and R. Ramesham, Anal. Chem., 65 (1993) 345. 21. J.-P. Candy, P. Fouilloux, M. Keddam

and

H.

Takenouti,

Electrochim. Acta, 36 (1981) 1029. 22. H. Shi, Electrochim. Acta, 41, (1996) 1633. 23. T. Ohmori, T. Kimura and H. Masuda, J. Electrochem.

Soc, 144,

(1997) 1986. 24. R. De Levie, Advances Engineering,

in Electrochemistry

and

Electrochemical

Vol. 6, P. Delahay, Editor, pp. 329-397, John Wiley

& Sons, New York (1967). 25. H. Reiser, K. D. Beccu and M. A. Gutjahr, Electrochim.

Acta, 21,

(1979) 539. 26. K. Honda, T. N. Rao, D. A. Tryk, A. Fujishima, M. Watanabe, K. Yasui and H. Masuda, J. Electrochem.

Soc, 148 (2001) A668..

27. Q. Chen, M. C. Granger, T. E. Lieser and G. M. Swain, J. Electrochem.

Soc, 144 (1997) 3806.

28. I. G. Brown, A. Anders, M. R. Dickinson, R. A. MacGill and O. R. Monterio, Surf. Coat. Technol,

112 (1999) 271.

29. J. Wang, G. M. Swain, T. Tachibana Electrochem.

SolidState

Lett,

and

K.

Kobayashi,

3 (2000) 286.

30. H. Yang, T. Lue, K. Xue, S. Sun, G. Lu and S. Chen,J. Perez, A. A. Tanaka, E. R. Gonzalez, and A. A. Tanaka, J. Electrochem.

Soc,

144 (1997) 2302.

447

31. A. S. Arico, H. Kim, A. K. Shukla, M. K. Ravikumar, V. Antonucci and N. Giordano, Electrochim.

Acta, 39 (1994) 691.

32. V. M. Schimidt, R. lanniello, E. Pastor and S. Gonzalez, J. Phys. Chem., 100 (1996) 17901. 33. J. Wang, S. Wasmus and R. F. Savinell, J. Electrochem. (1995) 4218.

448

Soc, 142

20. Application of Synthetic Boron-Doped Diamond Electrodes in Electrooxidation Processes Christos Comninellis, Ilaria Duo, Pierre-Alain Michaud, Beatrice Marselli and Su-Moon Park

20.1. Introduction Boron-doped diamond (BDD) thin films, grown on a suitable substrate (p-Si, Ti, Nb, W and Mo) by chemical vapor deposition, is a new electrode material that possesses a number of unique electrochemical

properties,

distinguishing

them

fi:om

other

traditional electrodes [1-4]. One of the most useful properties of the BDD diamond electrode is the possibility to electrogenerate hydroxyl radicals under polarization at high anodic potentials (eq.20.1). H^O ^

HO* + H" + e"

(20.1)

Recently, evidence for the formation of hydroxyl radicals on BDD has been reported [5] by means of spin-trapping using 5,5dimethyl-l-pyrroline-N'Oxide (DMPO).

Christos Comninellis e-mail: [email protected] 449

HgC. r^^^N^^^"

+ HO.

DMPO spin trap

(20.2)

DMPO hydroxyl radical spin adduct

The main characteristics of these hydroxyl radicals are- low electrochemical activity (high overpotential of anodic discharge) and high chemical reactivity (fast chemical reactivity). This has been attributed to the weak interaction of these hydroxyl radicals with the BDD surface [5]. The capabiUty of forming these chemically reactive hydroxyl radicals on the BDD surface has made possible an important new electrochemical

technology. The main

applications

are- the

electrochemical oxidation of aromatic compounds for synthetic applications, the production of strong oxidants, and the electroincineration of organic pollutants for wastewater treatment. In this work, we present recent results concerning the application of BDD electrodes in the electrooxidation of organic and inorganic compounds, with applications in electrosynthesis and in the destruction of pollutants for wastewater treatment

20.2. Application of BDD in Electrosynthesis Although much work has been done on organic and inorganic electrosyntheses, few processes have been applied on an industrial scale. In our opinion, the main reason for this is the low

450

20. Application of Synthetic Boron-Doped Diamond Electrodes in Electrooxidation Processes

electrochemical stability of the available electrode

material,

especially in acid solutions at high current densities. Electrode deactivation and fouling during operation also cause serious problems. Boron-doped diamond electrodes have high electrochemical stability, are not deactivated during operation, exhibit high overpotentials for both oxygen and hydrogen evolution, and furthermore produce active hydroxyl radicals under polarization at high anodic potentials. These properties of BDD can open up new possibilities in industrial electrosynthesis. Hereafter,

we present

results

concerning

electroorganic

synthesis (benzoquinone, nicotinic acid and hydroxy salicylic acid) and electroinorganic synthesis (production of powerful oxidants).

20.2.1. Electroorganic synthesis using BDD anodes Three examples are given to demonstrate the feasibility of electroorganic synthesis on BDD anodes. The first example is the oxidation of phenol to benzoquinone [6], an important intermediate in fine organic synthesis. The second example is the oxidation of 3" methylpyridine

(3-MP) to nicotinic

acid

[7], an

important

pharmaceutical intermediate, and the third example is the hydroxylation of salicylic acid [5], a typical example of an electrochemical hydroxylation. These

electrosynthetic

processes

have

been

reported

previously using classical electrodes like lead dioxide. However, the main problems with these electrodes are the low anodic stability under the operating conditions used and the deactivation of the electrode due to fouling.

451

20.2.1.1. Oxidation of phenol to benzoquinone [6] The electrochemical behavior of phenol in acid m e d i u m (l M HCIO4) a t t h e BDD electrode h a s shown t h a t the

oxidation

products of phenol in t h e potential region of w a t e r decomposition depend on t h e applied c u r r e n t density, phenol concentration and extent of phenol conversion. In fact, t h e p r e p a r a t i v e bulk electrolysis of phenol on BDD anodes u n d e r galvanostatic conditions h a s shown t h a t , depending on t h e experimental conditions, it is possible to obtain p a r t i a l oxidation

of phenol to aromatic

compounds or its

complete

oxidation to CO2 [6]. In particular, at low-current density and low phenol

conversion,

only

aromatic

compounds

(benzoquinone,

hydroquinone and catechol) are formed during phenol oxidation [6]. The

electrochemical

oxidation

of

phenols

to

aromatic

compounds on BDD involves highly active i n t e r m e d i a t e s formed by w a t e r discharge, like hydroxyl radicals (eq.20.1), which react with phenol (eq. 20.3) in a fast reaction close to t h e electrode surface. O

OH

OH 2 0H-

2 0H-

H2O

(20.3) OH

O

A typical example of r e s u l t s obtained d u r i n g phenol oxidation carried out in a one-compartment cell a t low c u r r e n t density (5 mA cm"2) is given in Fig. 20.1. U n d e r these conditions, a n d for a low phenol conversion percentage (< 20 %), t h e concentration of phenol decreases linearly

452

20. Application of Synthetic Boron-Doped Diamond Electrodes in Electrooxidation Processes

with the specific charge, forming mainly benzoquinone, with a small amount of hydroquinone and catechol. It is worth noting that the electrode potential remains almost constant during electrolysis (2.5 ± 0.1 V vs. SHE), and there is no indication of electrode deactivation under these conditions.

Fig. 20.1. Concentration trends during phenol electrolysis on a BDD anode* (D) phenol, (A) benzoquinone, (0) hydroquinone, (O) catechol, and (x) total organic carbon (TOC). The experimental conditions were^ electrolyte, 1 M HC104>' initial phenol concentration, 20 mM; temperature, 25**C>* current density, 5 mA cm"2; anode potential, 2.5 ± 0.1 V vs. SHE. Reprinted with permission from'J. Iniesta, P.-A. Michaud, M. Panizza, G. Gerisola, A. Aldaz and Ch. Comninellis. Electrochim. Acta., 46, 3573 (2001); Copyright 2001, Elsevier Science, Ltd. In confirmation of the partial oxidation of phenol to aromatic compounds (benzoquinone, hydroquinone and catechol), Fig. 20.1 also shows that the total organic carbon (TOC) in the solution remains almost constant during electrolysis. This indicates that

453

the oxidation of phenol to CO2 does not occur under these conditions. 20.2.1.2. Oxidation of 3-methylpyridine to nicotinic acid [7] Bulk electrolysis of 3-MP in 0.5 M HCIO4 in a one-compartment cell at low current density (2.5 mA cm-) and for low 3-MP conversion has shown that partial oxidation of 3-MP to nicotinic acid can be achieved [7]. A typical example for the partial oxidation of 3-MP is given in Figure 20.2. This figure shows also that the TOC of the electrolyte remains almost constant during electrolysis, confirming the partial oxidation of 3-MP. As in the case of phenol oxidation, hydroxyl radicals formed by water discharge on the BDD anode (eq. 20.1) participate in the oxidation of 3-MP to nicotinic acid (eq. 20.4)* OH + 6 OHN^

r

N

^O

(20.4)

- 4H2O

Furthermore, there is no indication of electrode deactivation during 3-MP oxidation under these experimental conditions.

454

20. Application of Synthetic Boron-Doped Diamond Electrodes in Electrooxidation Processes

Fig. 20.2. Concentration trends during 3-MP electrolysis on a BDD anode* (n) 3-MP; (A) nicotinic acid; (o) oxidation intermediates; and (x) TOC; Experimental conditions^ electrolyte, 0.5 M HCIO4; initial 3MP concentration, 5 mM; temperature, 25**C; current density, 2.5 mA cm-2; anode potential, 2.7 ± 0.1 V vs. SHE. Reprinted with permission from'- J. Iniesta, P.-A. Michaud, M. Panizza, and Ch. Comninellis. Electrochemistry Communications 3 (2001) 346; Copyright 2001, Elsevier Science, Ltd. 20.2.1.3. Electrochemical hydroxylation of salicylic acid [5] The anodic hydroxylation of salicylic acid at the BDD anode leads to the formation of dihydroxylated products (eq. 20.5). The same reaction products have been obtained using OH radicals produced by H202/Fe2+ (Fenton reaction). HO

(20.5) HO 2,3-Dihydroxybenzoic acid

2,5-Dihydroxybenzoic acid

However, the distribution of the isomers obtained is different. In fact, in the electrochemical hydroxylation, the 2,5-isomer

455

predominates, in contrast to chemical hydroxylation, in which t h e m a i n isomer is 2,3.

20.2.2. Preparation of powerful oxidants The unique properties of BDD electrodes (high anodic stability and high oxygen overpotential) can allow the production of powerful oxidants, with high redox potential. Two examples have been treated below • a) The oxidation of Ag(I) to Ag(II) in concentrated HNO3 (Eo=1.98 V vs. SHE). This redox couple can be used as mediator in the p a r t i a l oxidation of organic compounds (applications in synthesis), or for t h e electrochemical combustion

of organic

compounds

(applications in w a s t e w a t e r t r e a t m e n t ) . b)

The oxidation of sulfate to peroxodisulfate in concentrated

H2SO4. [S2O8 2/SO4 2 (Eo=2.0 V vs. S H E ) ] . For t h e m a n y applications of peroxodisulfate, t h e two most i m p o r t a n t a r e in etching printed circuits and in acrylonitrile polymerization. O t h e r applications are w a s t e w a t e r t r e a t m e n t , dye oxidation, a n d fiber whitening.

20.2.2.1. Oxidation of Ag (I) to Ag (H) in concentrated HNO3 The anodic oxidation of Ag(I) to Ag(II) (eq. 20.6) can be performed on p l a t i n u m , gold a n d antimony-doped S n 0 2 electrodes. However, these

electrodes

concentrated

suffer

HNO3,

and

from

limited

anodic

low

current

efficiency

stability for

in

Ag(n)

formation. Ag(I) ^ A g ( n ) + e

456

(20.6)

20. Application of Synthetic Boron-Doped Diamond Electrodes in Electrooxidation Processes

Fig. 20.3 shows typical cyclic voltammetric curves for BDD in 10 M HNO3 (curve a), a n d in 10 M HNO3 in t h e presence of different concentrations of Ag(I) (curves h't) [8]. I n t h e presence of Ag(I), a n anodic c u r r e n t p e a k w a s observed a t ca. 2.2 V vs. S H E due to t h e oxidation of Ag(I) to Ag(II) according to eq. 4. The c u r r e n t oxidation p e a k m a x i m u m is directly proportional to Ag(I) concentration (Fig. 20.3, inset) [8].

100 n 80R^ = 0.9997

80

/(f)

j>

~g60

^„ 600

£40

-30&20 10 0

/(e

()

100

200

300

/(c /(b /(a

AgNOg cone. [mM] 20

0-

(1

-20

1.2

~iJi

1.4

1.6

1.8

EIVvsSHE]

Fig. 20.3. CycUc voltammetric behavior of BDD at a scan rate of 100 mV s"i in 10 M HNO3 with different Ag(I) concentrations (mM)' (a) 0; (b) 50; (c) 100; (d) 150; (e) 200; and (£) 250. The dependence of the peak current density on the Ag(l) concentration is shown in the inset. Reprinted with permission from- M. Panizza, 1. Duo, P.-A. Michaud, G. Gerisola, and Ch. Comninellis. Electrochemistry and Solid-State Letters 3(12) 550 (2000); Copyright 2001, The Electrochemical Society, Inc.

The diffusion coefficient of Ag(I) in 10 M HNOs w a s calculated from t h e slope of t h e s t r a i g h t line in t h e Fig. 20.3 inset, yielding a value of 8.51.10"^ cm^ s^, using t h e Randles-Sevcik equation. This value is closed to those given in t h e l i t e r a t u r e .

457

F r o m the comparison of t h e v o l t a m m o g r a m s in t h e presence and absence of Ag (I), we can predict t h a t Ag(II) can be produced with high c u r r e n t efficiency by oxidation of Ag(I) at a BDD anode under

potentiostatic

conditions

at

2.2 V vs. S H E .

In

fact,

p r e p a r a t i v e electrolysis in a solution of 10 M HNO3 + 100 mM AgNOs, applying a constant potential of 2.2 V vs. SHE, r e s u l t s in 11% conversion of Ag(I) to Ag(II) after two hours of electrolysis, with a c u r r e n t efficiency of 81% [8].

20.2.2.2. Oxidation of sulfate to peroxodisulfate The efficiency of the electrochemical production of peroxodisulfate (eq. 20.7) strongly depends on the electrode material. High oxygen overpotential anodes m u s t be used to minimize t h e side reaction of oxygen evolution. The conventional electrochemical process for peroxodisulfate synthesis uses smooth p l a t i n u m anodes. 2S04-2->S208-2+2e-

(20.7)

The m a i n problems in the peroxodisulfate production process using the P t anode are- the high corrosion r a t e of Pt, t h e necessity of t h e

use of additives

(thiocynates),

and

the

necessity

for

purification of the electrolyte from the Pt corrosion product and from t h e additives before recycling. Preparative

electrolysis h a s been carried

out in a two-

c o m p a r t m e n t electrolytic flow cell u n d e r galvanostatic conditions. During electrolysis the m a i n side reaction is the anodic oxygen evolution (eq. 20.8) 2 H2O ^ O2 + 4 H^ + 4 e

458

(20.8)

20. Application of Synthetic Boron-Doped Diamond Electrodes in Electrooxidation Processes

and the chemical decomposition of peroxodisulfate to O2 (eq. 20.7) to monopersulfate (eq. 3.10), which is further decomposed to H2O2 (eq. 3.11) S2O82

+

H2O -> 2HS04-

+ ^2 02

S208-2

+

H2O -> SO52

+ SO42

SO52

+

H2O -^

+ SO42

H2O2

(20.9) +

2H^

(20.10) (20.11)

In order to find the optimal conditions for peroxodisulfate formation on BDD, the influence of the operating conditions (temperature, H2SO4 concentration) on the current efficiency of peroxodisulfate formation has been investigated [9]. Fig. 20.4 shows the influence of H2SO4 concentration on the current efficiency of peroxodisulfate formation. Peroxodisulhjric acid production

H2S04lmolL']

Fig. 20.4. Current efficiency of peroxodisulfate formation versus H2SO4 concentration.

At low H2SO4 concentration (< 0.5 M) the main side reaction is the discharge of water to 02(eq. 20.8). The chemical decomposition

459

of peroxodisulfate (eq. 20.9 - 20.11) also takes place at this low H2SO4 concentration. At high H2SO4 concentration (> 2.0 M) the main anodic reaction

is

the

electrochemical

oxidation

of

sulfate

to

peroxodisulfate (eq. 20.7). Small amounts of monopersulfate (eq. 20.10) and H2O2 (eq. 20.11) are also formed by the chemical decomposition of peroxodisulfate. Fig. 20.5 shows the influence of the temperature on the current efficiency of peroxodisulfate formation in 1 M H2SO4 under galvanostatic conditions (23 mA cm 2).

80^

1

•

70 i • 60 J

1

v>

^50 J 1 0

1 1 0

•

\

20.

10 -{

1 0

10

20

30

40

50

60

70

Temperature [°C]

Fig. 20.5. Influence of temperature on the ciurent efficiency of peroxodisulfate formation in 1 M H2SO4, on a BDD anode,* current density, 23 mA cm"2; H2SO4 conversion, 5 %. The decrease of current efficiency with temperature is due to the chemical decomposition of peroxodisulfate to oxygen (eq. 20.9). We speculate

460

that

hydroxyl

radicals

are

involved

in

the

20. Application of Synthetic Boron-Doped Diamond Electrodes in Electrooxidation Processes

electrochemical oxidation of sulfate to peroxodisulfate according to eq. 20.12 [9]. 2HS04" +

20H*^

S2O82

+ 2H2O

(20.12)

20.2.2.3. Oxidation of Mn2+ to Mn04 The electrochemical oxidation of the manganous ion (Mn2+) to permanganate is an important subject from fundamental and practical points of view. The electrochemical oxidation of Mn^^ was shown to produce manganese oxyhydroxide [MnOOH, Mn(III)], manganese dioxide [Mn02, (IV)], and permanganate [Mn04", Mn(VII)] at lead dioxide (Pb02) electrodes by earlier investigators, but the Pb02 electrode can be leached into the solution, depending on the experimental conditions, as pointed out early in this chapter. Both Mn(III) and Mn(VII) are important as strong oxidants, which have been used for both analytical and synthetic purposes as well as for the destruction of organic pollutants. The electrochemical oxidation of Mn2+ at BDD electrodes does not proceed without problems? all three high valence states, i.e., Mn(III), Mn(IV), and Mn(VII), are produced, depending on experimental

conditions

[lO].

Figure

20.6

shows

cyclic

voltammograms for Mn2+ oxidation in a 1 M HCIO4 solution containing Mn^^ of various concentrations. A few points may be summarized from this figure* l) anodic peaks in the potential range of 1.4-1.8 V are observed due to oxidation to Mn(IV) at higher Mn2+ concentrations, 2) the most anodic peak responsible for the generation of Mn(VII) at about 2.2-^2.3 V is not directly proportional to the Mn2+ concentration, and 3) the cathodic peak is observed for reduction of the Mn02 film back to Mn2+ at about 1.2

461

V. The oxidation of Mn2+ to Mn(VII) takes place at a potential significantly more positive than its thermodynamic potential of 1.51 V vs. SHE or 1.70 V vs. Ag/AgCl (in saturated KCl). This is attributed to the lack of capabilities of the BDD electrode for efficient oxygen transfer. The results of spectroelectrochemical experiments led to the product assignments described above and also to a conclusion that Mn(VII) is a major product at a concentration lower than about 20 mM, whereas Mn(III) is a primary product at higher concentrations. Also, the thin Mn02 films formed were found to impede the formation of both Mn(III) and Mn(VII) by passivating the electrode surface.

< E

E, V vs. Ag/AgCI

Fig. 20.6. CycUc voltammograms of Mn(II) oxidation at 10 mV s"i in 1 M HCIO4 for Mn(II) concentrations of (a) 10, (b) 25, (c) 50, and (d) 100 mM. When, however, Bi^+ was added as an electron transfer mediator, the Mn02 films were oxidized by electrogenerated bismuthate [BiOs, Bi(V)] and the overall current efficiencies for

462

20. Application of Synthetic Boron-Doped Diamond Electrodes in Electrooxidation Processes

Mn(VII) generation were improved significantly as can be seen from Fig. 20.7. Bismuthate is a well known oxidant for the oxidation of Mn(II) to Mn(VII) for quantitative analysis of manganese by spectrophotometric methods. The mechanism for this electrocatalytic reaction is shown to beHBi03 + 5H+ + 2e

Bi3+ + 3 H 2 O

(20.13)

4 Mn02 + 2H2O + 3/2Bi(V) ^ Mn04 + 4H+ I 3/2Bi3+i (20.14) Mn2+ + 4H2O + 5/2Bi(V) -> Mn04 + 8H+ +;5/2Bi3+ .(20.15)

50 H 45 40 ^

35

>. ^ 30 CD

125 UJ

I 20 I 15 10

g__,0^^^*

5

1

0 1.6

1.7

1

1

1.8

.

1

1.9

1

1

2.0

1

1

1^

2.1

1

2.2

1

1

2.3

1

1

2.4

r

2.5

E, V VS. Ag/AgCI

Fig. 20.7. Effects of potentials on the current efficiencies for Mn04 generation in solutions containing 10 mM MnS04 (—•—) only and 10 mM MnS04 + 2 mM Bi(III) (—o—).

One important observation made in this work was that the direct oxidation of Bi3+ to Bi(V), which had not been reported in the literature, was observed at 2.2 V vs. Ag/AgCl in the absence of Mn2+. Although the observed redox potentials for Mn2+/Mn(VII)

463

a n d Bi3+/Bi(V) p a i r s are about t h e same at ~2.2 V in t h i s work, electrogenerated Bi(V) is capable of oxidizing both Mn2+ a n d Mn(IV) to Mn(VII), acting a s a n electrocatalyst, because it is in a higher t h e r m o d y n a m i c state t h a n t h e thermodynamic potentials of Mn2+/Mn(VII)

and

Mn(IV)/Mn(VII)

pairs.

For

some

reason,

however, the BDD electrode requires a large overpotential for the oxidation of Mn2+ to higher oxidation s t a t e s .

20.2.2.4. Oxidation of Fe(III) to Fe(VI) While oxidation reactions described in previous sections h a s been demonstrated

to

occur

at

electrodes

other

than

BDD,

the

electrochemical generation of ferrate [Fe04^, Fe(VI)] would have been impossible in acidic aqueous media h a d it not for a BDD electrode [ l l ] . This is due not only to its high oxidation potential compared to t h a t of w a t e r oxidation b u t also to its high reactivity with its environment. The electrochemical generation of Fe(VI) h a s been shown to be obtained by a direct oxidation of metallic iron rods in strongly alkaline media, where the ferrate salt is stable. Figure 20.8 shows a series of cyclic v o l t a m m o g r a m s recorded at various scan r a t e s for t h e oxidation of Fe(II) to Fe(VI) via Fe(III). The first anodic p e a k at about +1.0 V is due to the oxidation of Fe2+ to Fe3+ whose cathodic counter p a r t is observed below about 0.60 V. The sluggish electron transfer r a t e of this reaction m a k e s t h e p e a k separation vary to a large extent depending on the voltage scan r a t e . The second anodic p e a k observed above about 2.3 V, which is 8~10 times of t h e first anodic peak, is assigned to the oxidation of Fe3+ to ferrate according to the following reactionFe3+ + 4H2O -> Fe042 -f 8H+ + 3e"

464

(20.16)

20. Application of Synthetic Boron-Doped Diamond Electrodes in Electrooxidation Processes

/g )

20m-

15m-

#d\(,7

10m-

/: , 1/ I

5m-

,.----^-.^^.._ ^ 0-

-^v:v.=.JJL^iIl!!!jj^^ ""'"^

1

•

1

'

i...i1iiBi.-iM

1

'

/ -riBWw

1

1

1

—1

1

r-

E (V vs. Ag/AgCI)

Fig. 20.8. Cyclic voltammograms for oxidation of 6.0 mM FeS04 at a BDD electrode in (a) 0.10 M HCIO4, and at scan rates of (b) 10, (c) 50, (d) 100, (e) 250, (£) 500, and (g) 1000 mV s'l. The oxidation potential observed here is consistent with the thermodynamic potential of 2.20 ± 0.03 V, theoretically estimated for this redox pair in the literature [12]J however, the number of electrons transferred

(napp) estimated from the ratio of the

respective cyclic voltammetric peak currents is much larger than 3.0, which is expected from the stoichiometry. The stoichiometry shown by reaction 20.16 indicates that it is a three electron process requiring water as a reactant. In other words, the reaction would not proceed in a rigorously dry nonaqueous medium, which has been shown to be true in dry acetonitrile. Only after a certain amount of water is added, the reaction proceeds in a similar way as observed

in water. Also, the

calculation

of napp from

chronoamperometric data as a function of time shows that it is 3.0

465

at the extrapolated time of 0, increasing to as large as 40 in about 1000 s. This indicates that the initial napp"value is 3.0, which increases to a larger value due to a fast E C (electron transfer followed by catalytic regeneration) reaction mechanism, i.e., Fe'^ -^4H.O -^ FeO/^ + 8 / / " +3c^'

t 2FeO/~ ^SHp^lFe''

+ f Q, +10//' (20.17)

Hence, Fe^^ is rapidly regenerated, resulting in an increase in the value. It is important to point out here that the observation of the direct electrochemical oxidation of Bi^^ to BiOs" and Fe^^ to Fe042 has not been reported at any other electrodes studied thus far. While the BDD electrodes appear to require large overpotentials for electrochemical reactions, in which oxygen atoms need to be incorporated into their reaction products, due to the lack of oxide layers such as on platinum, gold, and/or ruthenium

electrodes,

they certainly offer a solution to problems arising from the high thermodynamic

redox

potentials

thanks

to

such

a

large

overpotential for oxygen evolution.

20.3. Application of BDD in the Electrochemical Combustion of Organic Pollutants Biological treatment of polluted water is the most economical process and is used for the elimination of "readily degradable" organics present in wastewater. The situation is completely

466

20. Application of Synthetic Boron-Doped Diamond Electrodes in Electrooxidation Processes

different when the wastewater contains refractory (resistant to biological treatment) organic pollutants or if their concentration is high and/or very variable. In this case, another type of treatment must be used. Many treatment technologies are already in use or have been proposed for the recovery or destruction of pollutants. These technologies include activated carbon adsorption and solvent extraction (for recovery) or oxidation (for destruction). Several applications of chemical oxidation using hydrogen peroxide and ozone have been reported. The electrochemical method for the treatment of wastewater containing organic pollutants has attracted a great deal of attention recently. Major advantages are the ease of control and increased efficiencies. Another advantage is the possibility of building compact bipolar electrochemical reactors. The aim of the present work was to investigate the anodic oxidation of some model organic pollutants at BDD anodes to examine the reaction mechanism and to elucidate the possibilities of the electrochemical method for wastewater treatment.

20.3.1. Mechanism of the anodic oxidation of organics Two mechanisms can be distinguished for the electrochemical oxidation of organic compounds- direct oxidation and indirect oxidation via electrogenerated intermediates formed at the anode surface. Cyclic voltammetry has been used to investigate the mechanism of the electrochemical oxidation of two classes of organic compounds on BDDl) simple carboxylic acids (formic, oxalic and acetic acids);

467

2) phenolic compounds (phenol, chlorophenol a n d p-naphthol).

20.3.1.1. Cyclic voltammetry of carboxylic acids on BDD The decomposition behavior of carboxylic acids w a s determined by cyclic voltammography in 1 M H2SO4 at 25°C containing various concentrations of the organic acids [13]. The only difference in the presence of t h e investigated carboxylic acids (formic, oxalic and acetic acids) w a s a decrease in the s t a r t i n g potential of w a t e r discharge and/or decomposition of t h e supporting electrolyte. Fig. 20.9 shows typical v o l t a m m o g r a m s obtained with oxalic acid [13].

14 1 12 -

5

10 %

8-

0

4

< ^

//

64 2 0 -11

J

3

/// 2

1

1

1

1.2

1.4

1.6

1—

1.8

I

2

2.2

2.4

1

1 2.6

potential [V vs. SHE]

Fig. 20.9. Cychc voltammograms of BDD (l) in 1 M H2SO4, (2) in 1 M H2SO4 + 0.05 M oxaUc acid, (3) in 1 M H2SO4 + 0.1 M oxahc acid, (4) in 1 M H2SO4 + 0.2 M oxahc acid, and (5) in 1 M H2SO4 + 0.5 M oxahc acid; scan rate, 50 mV s"i; temperature, 25°C.

The decrease in t h e onset potential of w a t e r discharge in the presence of carboxylic acids may indicate t h a t the p a t h w a y for t h e oxidation of these compounds involves i n t e r m e d i a t e s t h a t

468

are

20. Application of Synthetic Boron-Doped Diamond Electrodes in Electrooxidation Processes

formed during the decomposition of water and/or the supporting electrolyte (indirect mechanism). The following reaction schema can be proposed for the oxidation of the carboxylic acids (oxalic acid) on the BDD anode1) formation of hydroxyl radicals (OH) on the BDD surface by water discharge (eq. 20.18)* BDD(H20) -> BDD(-OH) + H^ + e

(20.18)

2) oxidation of carboxylic (oxalic) acid by the electrogenerated hydroxyl radicals at the BDD electrode (eq. 20.19)^ BDD(-OH) + (C00H)2-> BDD + 2C02 + H2O + H+ + e (20.19) The main side reactions during the anodic oxidation of organics in H2SO4 are oxygen evolution, and H2O2 and H2S2O8 formation. 20.3.1.2. CycUc voltammetry of phenolic compounds on BDD Voltammetric measurements of phenolic compounds (phenol, chlorophenol and |3-naphthol) have shown that, in the potential region less positive than oxygen evolution, an anodic peak is obtained due to oxidation of the phenolic compound to the corresponding phenoxy radical [6,14-15]. This anodic reaction can induce polymerization, resulting in the deposition of an adherent polymeric material on the electrode surface. The formation of this polymeric material results in electrode deactivation [6,14-15]. Washing

with

organic

solvents

(isopropanol)

does

not

reactivate the electrode. However, the electrode surface can be restored to its initial activity by an anodic polarization in the same

469

electrolyte in the potential region of water decomposition (E > 2.3 V vs. SHE). In fact, this potential is in the region of water discharge.

On

BDD, it

involves

the

production

of

active

intermediates, probably hydroxyl radicals, which oxidize the polymeric film present on the electrode surface. The electrode

deactivation

by polymeric

materials

and

reactivation at high anodic potentials can be illustrated using phenol as a model phenolic compound. Fig. 20.10 shows t5T)ical cyclic voltammetric curves for BDD electrodes obtained in a solution containing 2.5 mM of phenol in 1 M HCIO4 at a scan rate of lOOmVs-i.

(a)=(e) (d) (c)

1 -\

(b)

1.4

1.6

E [V v s . S H E ]

Fig. 20.10. Cyclic voltammograms on BDD for a 2.5 mM phenol solution in 1 M HC104- (a) first cycle,* (b) after 5 cycles,* (c) after reactivation at +2.84 V vs. SHE for 10 s,' (d) after reactivation at +2.84 V vs. SHE for 20 s,* and (e) after reactivation at +2.84 V vs. SHE for 40 sJscan rate, 100 mV s'^; temperature, 25**C. The inset shows the dependence of the normalized current peak (ipeak / iVak , where iVak is the current peak during the first scan) during the reactivation. Reprinted with permission from' M.A. Rodrigo, P.-A. Michaud, I. Duo, M. Panizza, G. Gerisola and Ch. ComnineUis. J. Electrochem. Soc. 148 (5) D60, 2001; Copyright 2001, The Electrochemical Society, Inc. 470

20. Application of Synthetic Boron-Doped Diamond Electrodes in Electrooxidation Processes

In the first scan (Fig. 20.10, curve a) an anodic current peak corresponding to the oxidation of phenol is observed at about 1.65 V. As the number of cycles increases, the anodic current peak decreases to almost zero after about five cycles (Fig. 20.10, curve b). The same figure shows the voltammetric responses obtained after electrode reactivation at the fixed anode potential of 2.84 V vs. SHE for 10 and 20 and 40 seconds (Fig. 20.10, curves c, d, and e). The trend of the normalized current peaks (ipeak/iVak, where 1 peak I S

the current peak during the first scan) as a function of

polarization time at 2.84 V vs. SHE is given in the inset in Fig. 20.10. Fig. 20.10 shows clearly that when the polarization time during electrode reactivation exceeds 40 s, the phenol oxidation peak comes back to its initial position, meaning that the electrode is restored to its initial activity [14].

20.3.2. Oxidation of organic compounds on BDD at high anodic potential The electrochemical oxidation of a large number of organic compounds (Table 20.1) at high anodic potentials (close to the potential region of supporting electrolyte/water decomposition) on BDD has shown that the oxidation can be achieved at high current efficiency without any indication of electrode deactivation (this was the case for phenolic compounds at low anodic potentials) [6,14-15]. Furthermore,

the

oxidation

products

depend

on

the

experimental conditions. In fact, it has been found that either the partial oxidation of the organic compound, for electroorganic synthesis, or the complete oxidation, for wastewater treatment.

471

can be obtained. Table 20.1. Organic compounds investigated on the BDD anode Carboxylic acids Acetic, Formic, Maleic and Oxalic Alcohols and ketones Methanol, Ethanol, Isopropanol, Acetone Phenolic

compounds

Phenol, p-Chlorophenol, |3-Naphthol Aromatic acids Benzoic acid, Benzenesulfonic acid, Nicotinic acid

In particular, when working at high current densities (above the limiting current for the complete combustion given by eq. 20.20), complete oxidation of the organic compound can be achieved. ium(t) = 4FkmC0D(t)

(20.20)

where iiim(t) = limiting current density (A m'^) at a given time t, 4 = number of exchanged electrons, F = Faraday's constant (C mol'O, km = average mass transport coefficient (m s'O, COD(t) = chemical oxygen demand (mol O2 m^) at time t. A theoretical model has been developed permitting prediction of the chemical oxygen demand (COD) and instantaneous current efficiency (ICE) during the electrochemical oxidation of organic 472

20. Application of Synthetic Boron-Doped Diamond Electrodes in Electrooxidation Processes

pollutants on BDD electrodes in a batch recirculation system under galvanostatic conditions. The model assumes that the rate of the electrochemical oxidation of the organic compounds (main reaction) is a fast reaction in relation to the oxygen evolution reaction (side reaction). Depending on the applied current density and the limiting current density (eq. 20.20), two different operating regimes have been identified1) iappi. < iiim* the electrolysis is under current control?* the current efficiency is 100 % and the COD decreases linearly with time. 2) iappi > iiim- the electrolysis is under mass-transport control; side reactions (such as oxygen evolution) are involved, resulting in a decrease of ICE. In this regime COD removal, due to masstransport limitation, follows an exponential trend. The equations that describe the temporal trends of COD and ICE in both regimes are summarized in Table 20.2. The model has been tested for different classes of organic compounds (Table 20.1). For almost all of the organic compounds investigated, there is a good agreement between the model and the experimental data. The instantaneous current efficiency (ICE has been obtained through the measurement of COD using relation (20.21)*

UCOD) -(COD)

ICE= 4FV-L^

-^-^ I At

1

-^^^,

^

,

(20.21)

where {COD\ = chemical oxygen demand at time t (mol O2 dm"^); {C0D\^^

= chemical oxygen demand at time t+At (mol O2 dm'^);

473

I = c u r r e n t (A); F - F a r a d a y ' s constant (26.8 Ah); V = volume of electrolyte (dm^); a n d At = time interval of COD m e a s u r e m e n t (h).

Table 20.2.: Equations describing COD and ICE evolution during oxidation at a BDD electrode. V R = reservoir volume (m^), k m — mass" transfer coefficient (m s"0, A= electrode area (m^), COD^^ initial chemical oxygen demand (mol O2 m^), a = i / i ^ . Instantaneous Current Efficiency ICE (-)

Chemical Oxygen Demand COD (mol O2 m 3)

COD(t) =

lappl. ^ Him

ICE= 1

under current-

COD° 1

limited control lappl. ^

Him

\

ICE = /

under masstransport

exp

=^t V„

COD(t) = Ak^ !2-t + V„

1-a a

aCOD^ exp

-i + a

control

Reprinted with permission from'- M A . Rodrigo, P.-A. Michaud, I. Duo, M. Panizza, G. Gerisola and Ch. Comninellis, J. Electrochem. Soc, 148 (5), D60 2001; Copyright 2001, The Electrochemical Society, Inc.

474

20. Application of Synthetic Boron-Doped Diamond Electrodes in Electrooxidation Processes

50 n 45 ^

\

^ ""^^ °A

40 35

\ \

UJ 0.6

\ \

1 ^.30

o 1 25 8 20

V \

O

0

^V

5

15 -

10 Q [Ah.dm-']

15

20

10 X

^^sw

^•^*"^*^^

5 0 -1

1

1

0

5

10

u

•

f

15 Q [Ah.dm"']

20

25

30

Fig.20.11. Influence of 4-CP concentration on the evolution of COD and ICE (inset) with the specific electrical charge passed during electrolyses on a boron-doped diamond anode. The experimental conditions were* electroljrte, sulfuric acid (l M),* temperature, 25**C; applied current density, 30 mA cm"2; initial 4-CP concentration^ (n) 3.9 mM; (x) 7.8 mM; and (•) 15.6 mM. The soUd lines represent model predictions. Reprinted with permission from'- M.A. Rodrigo, P.-A. Michaud, I. Duo, M. Panizza, G. Gerisola and Ch. Comninellis. J. Electrochem. Soc, 148 (5) D60 (2001); Copyright 2001, The Electrochemical Society, Inc. A typical example is shown in Fig. 20.11. Both theoretical and experimental COD and ICE trends are given for the anodic oxidation of 4-chlorophenol at a BDD anode. As can be seen, the model is in good agreement with the experimental data. Similar results were obtained for almost all of the organic compounds investigated (Table 20.1).

475

References 1.

G.M. Swain, J. Electrochem.

2.

J.C. Angus and C.C. Hayman, Science, 241 (1988) 913

3.

G.M.

Swain,

BunetinlSeptemher

A.B.

Soc, 141 (1994) 3382

Anderson

and

J.C.

Angus,

MRS

56-60 (1998)

4.

Yu. V. Pleskov, Russ. Chem. Rev., 68 (1999) 381

5.

B. Marselli, J. Garcia-Gomez, P-A. Michaud, M.A. Rodrigo and Ch. Comninellis, J. Electrochem.

6.

J. Iniesta, P.-A. Michaud, M. Panizza G. Cerizola, A. Aldaz and Ch. Comninelhs, Electrochim.

7.

Commun., 3 (2001) 346.

M. Panizza, I. Duo, P.-A. Michaud, G. Cerisola and Comninelhs, Electrochem.

9.

Ch.

Solid State Lett, 3 (2000) 550.

P.-A. Michaud, E. Mahe, W. Haenni, A. Perret and Comninellis, Electrochem.

10.

Acta, 46 (2001) 3573

J. Iniesta, P.-A. Michaud, M. Panizza and Ch. Comninelhs, Electrochem.

8.

Soc, 150(3) (2003) D79.

Solid State Lett,

Ch.

3 (2000) 77.

J. Lee, Y. Einaga, A. Fujishima, and S.-M. Park, J.

Electrochem.

Soc, 151 (2004) E265. 11.

J. Lee, D. A. Tryk, A. Fujishima, and S.-M. Park, Chem.

Comm.,

(2002) 486. 12.

R. Wood, J. Am. Chem. Soc, 80 (1958) 2038.

13.

D. Gandini, E. Mahe, P.-A. Michaud, W. Haenni, A. Perret, Ch. Comninellis, J. Appl. Electrochem,

14.

30 (2000) 1345

M.A. Rodrigo, P.-A. Michaud, I. Duo, M. Panizza, G. Cerisola and Ch. Comninelhs, J. Electrochem.

Soc, 148 (5) (2001) D60

15. M. Panizza, P.-A. Michaud ,G. Cerisola and Ch. Comninellis, J. Electroanal

476

Chem., 507 (2001) 206

21. Oxidant Production on BDD Anodes and Advanced Oxidation Processes Nicolaos Vatistas, Christos Comninellis, Roberto M. Serikawa and Gabriele Prosperi

Effluents containing low concentrations of biorefractory organic contaminants require specific treatments to transform selectively the biorefractory organic species into biodegradable ones or into fully inorganic species like CO2. The common characteristic of these specific

treatment

methods,

known

as Advanced

Oxidation

Processes (AOPs), is the production of the highly active hydroxyl radical, which oxidizes efficiently these organic species [l]. Even the process of electrochemical oxidation with boron-doped diamond (BDD) anodes is due to the hydroxyl radicals produced on its surface [2], and thus this method also has the characteristic that is crucial for an AOP [3]. Industrial biorefractory

wastewaters

with

low

organic species derive from

concentrations

of

the production of

pharmaceuticals, pesticides, pigments, dyes, wood preservatives and rubber [4]. Wash effluents derive from the washing of multipurpose reactors. Scrubber effluents derive from solutions used to eliminate organic species from gaseous phasestreams. Wastewaters Nicolaos Vatistas e-mail: [email protected] 477

that derive from two-phase reactions involve an organic phase that contains the products of a reaction and an aqueous phase that contains small concentrations of biorefractory organic species. AOPs include two consecutive steps. In the initial step, chemical, photochemical or electrochemical energy is transformed into a higher-level chemical energy by forming highly reactive hydroxyl radicals [5]. In the subsequent step, these highly active radicals oxidize efficiently the biorefractory organic species to biodegradable ones or to fully inorganic species. Active hydroxyl radicals have been detected on the surface of BDD anodes, and their action explains the efficient elimination of organic species [l]. The elimination of the organic species occurs on the surface of the BDD anode, and thus it has the characteristic typical of a heterogeneous AOP. This chapter considers the effluent treatment with BDD anodes

under

the

wider

point

of view

of

an

advanced

electrochemical oxidation process in order to point out the possibilities and limits of this anode in the wastewater treatment field. In fact, a new process is described in this work according to which hydroxyl radicals produced on the BDD surface are trapped by an oxidizable species like sulfate or carbonate to form the corresponding peroxide. These peroxides are relatively stable and can be produced at high concentration in the electrolyte without any problem of mass transport limitations. The treatment of the wastewater can take place in a separate chemical reactor?* in this reactor the peroxide is activated thermally or with UV radiation to 478

21. Oxidant Production on BDD Anodes and Advanced Oxidation Processes

produce hydroxyl radicals. These hydroxyl radicals oxidize t h e organic p o l l u t a n t s in a n AOP. A second possibility is to introduce in t h e electrolyte a n oxidizable species (like sulfate)

during

the

electrochemical t r e a t m e n t of t h e w a s t e w a t e r . In this case t h e peroxide formation avoids t h e side reaction of oxygen evolution a n d can act as a mediator in the oxidation of the organic pollutants.

21.1. Mass Transfer Limitation in the Direct Electrochemical Wastewater Treatment Process Boron-doped diamond h a s a high overpotential for oxygen evolution, in contrast to t r a d i t i o n a l anodes. This high overpotential can allow t h e formation of t h e active hydroxyl radical (0H°) by w a t e r discharge, according to the following reaction (eq. 21.1)* (21.1)

H2O ^ H 0 ° + H+ + eRadicals layer COD

Fig. 21.1. Heterogeneous advanced oxidation process on the BDD anode. As Fig. 21.1 indicates, only organic species t h a t reach t h e anodic

surface

can be oxidized by electrogenerated

hydroxyl 479

radicals. The degradation rate of organics by these hydroxyl radicals is very fast, and the reaction take place in a thin film close to the anode surface. This process is heterogeneous in nature, and consequently it is subject to mass transfer limitations. As the oxidation of the organic species on the BDD anode surface involves hydroxyl radicals, the treatment

can be considered

as

an

electrochemical AOP. In previous work (see Chapter 20 in this book) a model has been developed permitting prediction of the chemical oxygen demand {COD) during the electrochemical oxidation of organic pollutants on BDD under galvanostatic conditions, as shown in Fig. 21.2.

Fig. 21.2. Schematic diagram of a direct batch electrochemical wastewater treatment process. The model assumes that the rate of organic oxidation at the anode surface is fast and that the reaction is limited by mass transfer. The proposed relation for COD estimation during anodic

480

21. Oxidant Production on BDD Anodes and Advanced Oxidation Processes

oxidation under galvanostatic conditions (ikppi>iim) is given by eq. 21.2:

COD{t) = aCOD' e x p [ - ^ f +

^—^

(21.2)

where COIJ^ is the initial COD value, COlXi) is that after the treatment time t, A is the surface area of the anode, VR is the volume of the solution, and a is the ratio of the applied current density

iappl,

to the limiting current density iim.

The limiting current density decreases during the treatment, and it is related to the CODhy eq. 21.5*

i,M = ^FkSOD(t) An efficient

operation mode during the

(21.3) electrochemical

oxidation process is to modulate the applied current density in order to operate always at the limiting current density. This can avoid the side reaction of oxygen evolution and allow operation with a current efficiency of 100%. Under these conditions, the parameter a of the model assumes a constant value (a =l), and eq. 21.2 can be written as*

COZ)(0 = a C O D ' e x p [ - ^ H

(21.4)

From this relation, the required anodic surface area A, in order to decrease the chemical oxygen demand from COLP to COLk, after an electrolysis time t, can be calculated using the equation481

V^ , CODf ... „

kj

(21.5)

COD°

The value of the required anodic surface area A vs. the final COi? value is shown in Fig. 21.3, when VR-\

m^, t - Ih, km- 2

xlO-5 m s i and COLK = 3000 ppm and /^V = 3 V). The depicted anodic surface area value vs. CODr, shows that, in order to reach the required low COD values, high surface areas of BDD anode must be used.

o

500

1000

1500

2000

2500

Final COD Concentration, ppm

Fig. 21.3. Anodic surface area and electrical energy vs. final COD concentration for the treatment of 1 m^/h of a wastewater with an initial CODi of 3000 mg dm 3. The required electrical energy {E) for the treatment of 1 m^ of the wastewater in order to decrease the chemical oxygen demand from COD to CODi, is*. E = 4F(COD.

- COD^

where A K is the applied electrical potential. 482

)AV

(21.6)

21. Oxidant Production on BDD Anodes and Advanced Oxidation Processes

The results depicted in Fig. 21.3 indicate that the direct oxidation with BDD anodes allows one to use efficiently the fiirnished electric energy, but the BDD anodes are not efficiently utilized. The mean value of the applied current density imean, indicates the degree of utilization of BDD anode in this treatment, and its value is related to the logarithmic mean COD concentration (eq. 21.7):

coa-coD,

= 4kF-

In

COD.

(21.7)

COD, Figure 21.4 shows the values of mean current density vs. final COD concentration for a given Jcm (2 x 10"^ m s O and initial chemical oxygen demand COD (3000 ppm):

500

1000

1500

2000

2500

Final COD Concentration, ppm

Fig. 21.4. Mean current density vs. final COD concentration (Am^ 2 x 10 5 m s 1, COIK = 3000 ppm). In conclusion, the low mean values of the applied current density obtained indicate a low utilization of the rather expensive 483

BDD

anodes

during

the

direct

electrochemical

wastewater

treatment.

21.2. Peroxide Production on BDD Anodes Followed by Advanced Oxidation Processes in a Separate Chemical Reactor As

h a s been shown previously, the low concentrations of t h e

organic species in t h e w a s t e w a t e r limit t h e efficient use of BDD anodes in the direct electrochemical t r e a t m e n t . In this work, a n alternative

method

is proposed

according to which

hydroxyl

radicals produced on t h e BDD surface are t r a p p e d by a n oxidizable species like sulfate to form the corresponding peroxide (eq. 21.7)* 2 H 0 * + 2HSO4" -> S2O82 + H2O

(21.7)

_^ B D D anode

I Oxidant

I Wastewater AOP Fig. 21.5. Combination of oxidant production on a BDD anode and an advanced oxidation process (AOP). These peroxides are relatively stable a n d can be produced a t high concentration in the electrolyte without any problem of m a s s

484

21. Oxidant Production on BDD Anodes and Advanced Oxidation Processes

transport limitations. The treatment of the wastewater occurs in a separate chemical reactor, as shown in Fig. 21.5. In the reactor, the peroxide is activated thermally or with UV radiation to produce hydroxyl radicals that oxidize the organic pollutants; in other words, an advanced oxidation process actually occurs in the reactor. In the reactor, the peroxide is well mixed with the wastewater before its activation in order to maximize the contact between the oxidant and the organic species. The above combined method of the local production of the peroxide and the subsequent AOP step avoids the mass transfer limitation of the direct electrochemical wastewater treatment. An efficient

wastewater

treatment

of

low

concentrations

of

biorefractory organic species can be reached with this combined method. Peroxides like hydrogen peroxide, ozone, percarbonate and peroxodisulfate can be produced efficiently with the use of the BDD anode. The first two oxidants are normally used in AOPs, while peroxodisulfate, despite its superior characteristics, has not been sufficiently considered for this kind of process. Experimental tests have indicated that, with the use of a nonelectroactive supporting electrolyte (HCIO4), hydrogen peroxide [6], ozone [7,8] and oxygen are easily produced on the BDD anode. The hydrogen peroxide production is due to the recombination of two hydroxyl radicals (eq. 21.8) that are just formed by water discharge, according to eq. 21.12HO*-^H202

(21.8)

while the ozone production is due to the following reactions485

H O * - ^ 0 * + H^+e-

(21.9)

20*-^0 2

(21.10)

0* + 0 2 ^ 0 3

(21.11)

The experimental results indicate that the concentrations of both ozone and hydrogen peroxide in the electrolyte increase linearly with the applied current density [9]. Recently, it has been reported that using concentrated sulfuric acic solutions ([H2SO4] > 2 mol dm 3) and low temperature {t < 21 °C) the peroxodisulfate is efficiently produced on BDD anodes {rj > 90%): 2H0* + 2HSO4' -> S2O82 + H2O

(21.12)

A small quantity of hydrogen peroxide and ozone are also produced during this process [6,9]. These results show that the innovative BDD anode can be used for the in situ production of strong oxidants, which can be activated in a separated chemical reactor in order to produce active hydroxyl radicals for the oxidation of organic pollutants. The BED anode facilitates the application of the advanced oxidation process.

21.3. Homogeneous and Heterogeneous Advanced Oxidation Processes The efficiency of AOPs in wastewater treatment is due to the high activity of hydroxyl radicals that are formed during the process. On the

BDD anode, hydroxyl radicals

during

the

electrochemical wastewater treatment, and consequently

this

treatment can be classified as an AOP.

486

are

formed

21. Oxidant Production on BDD Anodes and Advanced Oxidation Processes

Hydroxyl radicals are formed when UV radiation impinges upon the surface of titanium dioxide, or when it impinges upon solutions that contain hydrogen peroxide or ozone. Hydroxyl radicals are formed in a solution when hydrogen peroxide is mixed with ferrous ion (Fenton reactant), as well as when peroxodisulfate is mixed with silver ion or when a peroxodisulfate solution is heated. Fig. 21.6 depicts hydroxyl radical formation on surfaces, as in the case of the BDD anode, Ti02/UV and OsGn air)/UV systems. In this case, the AOPs are heterogeneous, and thus they are subject to mass transfer limitations, especially when the concentration of the organic species is low.

(a)

UV

/

431

I

v

<^) ^ u v

Is

re

o

O

Fig. 21.6. Heterogeneous advanced oxidation processes' (a) BDD anode, (b) Ti02/UV process and (c) O3/UV process. The mass transfer limitation can be avoided by efficient mixing of the peroxide with the wastewater before activation. A typical example is the case of the H2O2/UV AOP- a more homogeneous activation is obtained if the hydrogen peroxide is well

487

mixed with t h e w a s t e w a t e r before UV radiation, a s indicated in Fig. 21.7a.

vl/

1\

Steam H2O2

•fl

UV

11

Mixer

Heater

jy

Reactor (a)

KU

sioi"

Reactor

(b)

Fig. 21.7. Homogeneous advanced oxidation processes* (a) H2O2/UV process and (b) S2082Vheat process. W h e n t h e peroxodisulfate/heat AOP is used, the scheme of Fig. 21.7b is suggested. The pre-heating of the w a s t e w a t e r a s s u r e s a uniform t e m p e r a t u r e of the wastewater, a n d t h e

subsequently

introduced peroxodisulfate is t h e n more homogeneously activated.

21.4. Peroxodisulfate/Heat Advanced Oxidation Processes The s t a n d a r d redox potential E"" of peroxodisulfate in aqueous solution is 2.01 V, which is comparable to those of other AOP oxidants- ozone ( ^ ^ = 2.07 V) a n d hydrogen peroxide ( ^ ^ = 1.78 V). Peroxodisulfate is not a n active oxidant a t a m b i e n t t e m p e r a t u r e , b u t it is activated with UV radiation or heating. The effect of h e a t i n g or UV radiation is formation of hydroxyl radicals (eq. 21.12-21.13) t h a t oxidize m a n y organic species to carbon dioxideS2O82 + heat/UV -> 2 S 0 4 488

(21.12)

21. Oxidant Production on BDD Anodes and Advanced Oxidation Processes

S O r + H2O -> HO* + HS04'

(21.13)

The efficient oxidation of many organic species with heat/ peroxodisulfate or UV/peroxodisulfate has led to the use of this process as a standard for the determination of total organic carbon {TOO, in wastewater [ l l ] . The innovative BDD anodes reduce the complexity of the actual peroxodisulfate production [12], and consequently simplify the AOPs that use this oxidant. Figures 21.8a and 21.8b depict two alternative methods for wastewater treatment that use BDD anodes for the oxidation of biorefractory organic species. In the first treatment (Fig. 21.8a), peroxodisulfate is produced with a high current efficiency from a concentrated

sulfate

solution.

The

oxidant

(peroxodisulfate)

produced is subsequently mixed with the heated wastewater in order to achieve its activation, i.e., the production of hydroxyl radicals. The AOP occurs efficiently in the bulk of the wastewater, and thus the mass transfer limitation is avoided.

BDD anode steam

^

Steam

S2OI Cell

Heater

804'

BDD anode JJJ anode

SO4"

Heater

D-

vi/ Pump

Reactor

(a)

Reactor

(b)

Fig. 21.8. Alternative wastewater treatment using BDD anodes- (a) homogeneous peroxodisulfate/heat process and (b) heterogeneous and homogeneous peroxodisulfate/heat process.

489

In the second treatment (Fig. 21.8b), the wastewater is initially heated, as before, but the electrochemical method is applied in the wastewater using the BDD anode initially without sulfate, and sulfate is added in the wastewater in order to produce peroxodisulfate when the organic species reach low concentration values and the applied current density is higher than the limiting current. In this electrochemical process the organic species are subjected

to

two

distinct

oxidation

mechanisms-

first,

a

heterogeneous AOP due to the hydroxyl radicals produced on the surface of BDD anode, and second, a homogeneous AOP that is due to the combination of the formed peroxodisulfate and the heating of the solution. Experimental studies are in progress in order to point out the characteristics of the two alternative methods previously reported, and some results of these studies are reported below.

21.5. The Peroxodisulfate/Heat Homogeneous Process The effect of temperature on peroxodisulfate solution reactivity is due to the formation of active hydroxyl radicals. These radicals can oxidize both organic species and water. The water oxidation is a parasitic reaction, and it is usually described as a decomposition of peroxodisulfate solutions. The efficiency rj of the homogeneous peroxodisulfate/heat process is given by the following* Rate of organic species oxidation

n=

490

Rate of water oxidation

(21.14)

21. Oxidant Production on BDD Anodes and Advanced Oxidation Processes

This ratio

needs to be optimized

for

the

wastewater

investigated in order to obtain the maximum efficiency.

21.5.1. The peroxodisulfate decomposition reaction in aqueous solutions The temperature has a strong effect on the peroxodisulfate aqueous solution stability.

^ . . T = 50 °C

o

11 \ : 80 \ 90 0

Xeo \70

500

1000

1500

2000

2500

3000

3500

4000

Time, min Fig. 21.9. Peroxodisulfate concentration ratio vs. time of peroxodisulfate decomposition (initial peroxodisulfate concentration, 12gdm3;/?iy=i). Figure

21.9

depicts

log(C/Co)

values

(peroxodisulfate

concentration at a given time t relative to the initial concentration) vs. the reaction time at various temperatures. This figure indicates that the reaction is first order with respect to peroxodisulfate. The values of the first order rate constant kaq, for the decomposition of peroxodisulfate have been estimated at various temperatures, and Arrhenius behavior has been assumed*

491

^.,(n = < „ , e x p | ^ | ^ j The

frequency

factor

and

the

(21.15)

activation

energy

for

the

decomposition of peroxodisulfate have been estimated by fitting the above equation (Ao.aq= 5.64x101^ min" \Ea.aq^

llSkJmol-i).

21.5.2. Formic acid oxidation Formic acid has been oxidized with peroxodisulfate at various temperatures. An initial series of experiments indicates that the rate of the formic acid oxidation follows first order kinetics with respect to peroxodisulfate and does not depend on formic acid concentration The

peroxodisulfate

concentration

was

measured,

and

log(C/Co) values vs. time are shown at various temperatures (Fig. 21.10). These results have been used in order to estimate the pseudo-first"order rate constant kor for formic acid oxidation at various temperatures.

492

21. Oxidant Production on BDD Anodes and Advanced Oxidation Processes

200

300

400

Time, min

Fig. 21.10. Peroxodisulfate concentration ratio vs. time of formic acid oxidation (initial peroxodisulfate concentration, 8 g dm"^; pH= l). The increase of the rate constant with temperature follows the Arrhenius equation, and its parameters have been estimated for formic acid oxidation with peroxodisulfate {Ao,or- 1.85 x lO^^ min'i, Ea,or^ l i e k J m o l i ) .

21.5.3. Efficiency of the peroxodisulfate/heat homogeneous process The efficiency of the peroxodisulfate/heat homogeneous AOP is related to the following factors- (i) the oxidation rate of the organic species; and (ii) the selectivity of the peroxodisulfate for the oxididation of the organic versus the rate of peroxodisulfate decomposition.The oxidation rate of the organic species is related to the rate constant kor, while the selectivity is related to the ratio koiJkaq. Figure 21.11 shows Arrhenius plots for both formic acid oxidation {kor) and peroxodisulfate decomposition {kaq).

493

Fig. 21.11. The kinetic rate constants for peroxodisnlfate degradation kaq and formic acid oxidation korvs. \IT. The strong effect of the temperature on the rate constant kor indicates that a high oxidation rate can be reached by increasing the reaction temperature. The observed large difference between kor with respect to kaq assures a high selectivity for the oxidation of the organic species versus the degradation of peroxodisnlfate. These results indicate that the peroxodisulfate/heat AOP oxidizes efficiently formic acid at moderately high temperatures.

21.6. The Combined Heterogeneous-Homogeneous Peroxodisulfate/Heat Process One of the major problems in the direct electrochemical wastewater treatment process with BDD electrodes is the large anode surface needed, especially if low concentrations of organic pollutants have to be treated (see §21.1). In fact, the maximum operating current density (limiting current density) is dictated by the COD value of 494

2L Oxidant Production on BDD Anodes and Advanced Oxidation Processes

the wastewater. Working above this Hmiting current can result in efficiency losses due to the side reaction of oxygen evolution. The objective of the proposed combined process is to avoid the side reaction of oxygen evolution. This can be achieved by introducing in the wastewater a suitable amount of an inorganic compound (for example, sulfate) which can be oxidized by trapping the

electrogenerated

hydroxyl

radicals

to

produce

the

corresponding peroxo compound (for example, peroxodisulfate).

-R I OH°

^^ Heterogeneous AOP CO2+ H2O

•lliii

2S2O4

lllii jl Jill j iOH°

(b) Homogeneous AOP . SzOi" ^^^^> 0H° + R

> CO2+ H2O

Fig. 21.12. Homogeneous and heterogeneous AOPs during the electrochemical treatment of wastewater with the BDD anode. As Fig. 21.12 shows, the oxidation of sulfate

ions to

peroxodisulfate replaces the side reaction of oxygen evolution, and at a sufficiently high temperature, the AOP is activated in the bulk of the wastewater. The organic species are oxidized by the hydroxyl radicals formed at first on the BBD anode and by those formed subsequently in the solution. An increase of the total efficiency of the process can be reached with the use of this method. In order to

495

observe if the combined effect of heterogeneous and homogeneous AOP occurs, salicyHc acid solutions with and without sulfate have been treated with BDD anodes at various temperatures.

o 1 mol dm"^ of sulfuric acid + 0 mol dm"^ of sulfuric acid

20

40

60

80

100

120

140

160

180

200

Time, min Fig. 21.13. COD values vs. time of electrochemical treatment of salicylic acid solution with and without sulfuric acid (current density, 158 A m 2; temperatiu-e, 70°C). Fig. 21.13 depicts the electrochemical treatment results for two salicylic acid solutions with and without sulfuric acid at 70°C at the same value of current density Uappi- 158 A m"2). Under these conditions, a more efficient elimination of COD was observed when the sulfuric acid was present. The comparison of the results obtained indicates that both homogeneous and heterogeneous AOPs occur when the solution contains sulfate ions.

496

21. Oxidant Production on BDD Anodes and Advanced Oxidation Processes

Figure 21.14 depicts the temporal evolution of COD during formic acid treatments at 25°C and 70°C; sulfuric acid was added in both cases (l mol dm'^).

1500-

+ T = 25°C o T = 70°C

40

60

100

120

40

160

180

200

Time, min

Fig. 21.14. COi7 values obtained during the electrochemical treatment of salicylic acid at low and high temperatures (25 and TO^'C) with sulfuric acid (l mol dm 3). The

lower

temperature

treatment

exhibited

a

lower

elimination rate compared to that obtained at 70°C. The results obtained indicate that at low temperatures the peroxodisulfate produced on the BDD surface is not activated and the organic species are not oxidized by peroxodisulfate. The concentration of peroxodisulfate has been analyzed during the treatments. Fig. 21.15 depicts the temporal evolution of peroxodisulfate during treatments at 25 and 60°C using the same

497

current density {iappi- 158 A m 2) and sulfuric acid concentration (1 mol dm 3).

12000

100

150

Time, min Fig. 21.15. Peroxodisulfate concentration vs. time at two different temperatures during formic acid oxidation (sulfuric acid concentration, 1 mol dm"^; current density, 158 A m*2). At the lower temperature

(T = 25°C), relatively

high

concentrations of peroxodisulfate are formed; this is certainly due to the fact that at this temperature peroxodisulfate is not activated. The estimated current efficiency for peroxodisulfate formation at the low temperature (T - 25^0, is about 30%. The observed lower concentration of peroxodisulfate at the higher temperature (T = 60^0 in Fig. 21.15) indicates the activation of peroxodisulfate to hydroxyl radicals, which further oxidize formic acid. The reported results show that the peroxodisulfate is produced during the

electrochemical

treatment

with BDD anodes of

wastewater containing sulfate. When the treatment occurs at low 498

21. Oxidant Production on BDD Anodes and Advanced Oxidation Processes

temperature (T = 25^0 the peroxodisulfate produced is inactive and is accumulated in the wastewater, while at high temperature (T = 70^0, the peroxodisulfate produced is activated to generate hydroxyl radicals, which oxidize the organic species.

21.7. Conclusions The production of numerous active oxidants- hydroxyl radicals, hydrogen peroxide, ozone, peroxodisulfate, etc., has been simplified with the use of the BDD anodes. The AOPs use these oxidants to destroy low concentrations of biorefractory organic species. Some of these oxidants are unstable, and thus the innovative BDD anodes allow an easier use of the AOPs in the field of wastewater treatment. The capacity of the BDD anode to oxidize organic species is due to its ability to produce hydroxyl radicals on its surface. The mass transfer of the organic species to the anode surface limits their efficient use, particularly when the concentration of the organic species reaches low values during the treatment. This aspect, which is common for all of the heterogeneous AOPs, has been considered, and alternative homogeneous AOPs have been proposed. In fact, a new combined two-step process is described in this work, according to which hydroxyl radicals produced on the BDD surface are trapped by an oxidizable species like sulfate to form the corresponding peroxide, e.g., peroxodisulfate. The peroxodisulfate is relatively stable and can be produced at high concentration in 499

t h e electrolyte without any problem of m a s s t r a n s p o r t limitations. The t r e a t m e n t

of t h e w a s t e w a t e r

t a k e s place in a

separate

chemical reactor. In t h e chemical reactor, t h e peroxodisulfate is activated t h e r m a l l y to produce hydroxyl radicals. These hydroxyl radicals oxidize the organic pollutants in a n AOP. An efficient w a s t e w a t e r t r e a t m e n t is obtained by use of

suitable operating

conditions for t h e two successive steps. The BDD anode easily produces active oxidants used in t h e AOPs, a n d t h u s this innovative anode open up new possibilities in the field of w a s t e w a t e r t r e a t m e n t , particularly for w a s t e w a t e r t h a t contains biorefractory organic species at low concentrations.

References 1.

P. R. Gogate and A. B. Pandit, Adv. Environ. Res., 8 (2004) 553.

2.

B. Marselh, J. Garcia-Gomez. P A . Michaud, M.A. Rodrigo and Ch. Comninellis, J. Electrochem.

3.

Soc, 150 (2003) D79.

A. Kraft, M. Stadelmann and M. Blaschke, J. Hazard.

Mater.,

B103 (2003) 247. 4.

V. Uberoi and S.K. Bhattacharya, Water Environ. Res., 692 (1997) 146.

5.

M. Pera-Titus, V. Garcia-Molina, M. A. Bafios, J. Gimenez and S. Esplugas, Applied Catalysis B-'Environmental,

6.

Ullmann's

Encyclopedia

of Industrial

47 (2004) 219.

Chemistry,

John Wiley &

Sons Inc (1997), Vol A19, 190. 7.

500

P.A. Michaud, M. Panizza, L. Ouattara, T. Diaco, G. Foti and Ch.

21. Oxidant Production on BDD Anodes and Advanced Oxidation Processes

Comninellis, J. Appl Electrochem.,

33 (2003) 151.

8. A. Perret, W. Haenni, P. Niedermann, N. Skinner, C. Comminellis and D. Gandi, , Proc. Electrochem. 9.

Soc, (1997) 97.

N. Katsuki, E. Takahashi, M. Toyoda, T. Kurosu, M. Lida, S. Wakita, Y. Nishiki and T. Shimamune, J. Electrochem.

Soc, 145,

(1998) 2358. 10. K.

Serrano,

Electrochimica

P.A. Michaud,

C. Comninellis

and

A.

Savall,

Acta, 48 (2002) 431.

11. L.M. da Silva, M.H.P. Santana, J.F.C. Boodts, Quim. Nova, 26, (2003) 880. 12. L.S. Clesceri, A.E. Greenberg and A.D. Eaton, 1998. Methods

for the Examination

of Water and Wastewater,

Standard 20^^ ed.

APHA, AWWA, and WEF.

501

22. Ozone Generation with Boron-Doped Diamond Electrodes and Its Applications Eun-In Cho, Gyu-Sik Kim, Jong-Eun Park and Soo-Gil Park

22.1. Introduction Ozone is a strong oxidant, which is widely used to supplement or replace chlorine in a variety of processes associated with water treatment. [1-2] Oxidants kill microorganisms and precipitate various chemicals. However, there is still a need for a safe, inexpensive process for the production of ozone for water treatment in swimming pools and small drinking water plants. Ozone is greatly preferable to chlorine as a disinfectant, but the low ozone concentration available using an electric discharge in the gaseous phase (corona process) or UV light absorption (photochemical process) has prevented ozone from being applied in several green chemical processes, for example, the decomposition of

resistant

organic

pollutants,

where

a

higher

ozone

concentration is necessary. Electrochemical ozone production is a promising technology due to the possibility of producing ozone in higher concentrations than in conventional methods. There

are

certain

limitations

that

make

the

commercialization of this technology very slow. Although the economic aspect is not a major limitation, considering the rigid

502

22. Ozone Generation with Boron-Doped Diamond Electrodes and Its Applications

e n v i r o n m e n t a l legislation, t h e lack of availability of a stable electrode with a high oxygen overpotential h a s been t h e m a i n obstacle

for

the

application

of

the

electrochemical

ozone

generation process. The electrode reactions relevant to ozone generation are as follows3H2O - ^ 0 3 + 6H+ + 6e-

Eo= +1.6V

(l)

3H2O ^ 0 2 + 4H+ + 4e

Eo= +1.23V

(2)

2H+ +2e- -^ H2

Eo= O.OV

(3)

Due to the lower s t a n d a r d evolution potential of oxygen (1.23V vs. RHE) t h a n t h a t of ozone (1.51V vs. RHE), it occurs simultaneously d u r i n g the ozone production, as shown in Eqs. 1-3. There are basically t h r e e r e q u i r e m e n t s for electrochemical ozone production-

(l) t h e

electrode

should

present

a

high

anodic

overpotential for oxygen evolution; (2) anions a n d cations from the electrolyte should not interfere with ozone/oxygen evolution a n d hydrogen evolution respectively; and (3) t h e electrode (anode) should p r e s e n t m a x i m u m dimensional stability. Usually, t h e P b 0 2 electrode is the preferred electrode for ozone generation due to its reasonable cost a n d s t r u c t u r a l stability in comparison to other electrodes such as P t a n d glassy carbon. However, it is still not t h e best electrode due to its s t r u c t u r a l degradation d u r i n g long-term use, a n d t h e presence of P b 0 2 itself is a n e n v i r o n m e n t a l concern. Boron-doped

diamond

(BDD)

electrodes

have

been

studied

extensively due to t h e i r u n i q u e physicochemical characteristics, e.g., large potential range a n d small background current, which have let to diverse applications such as selective electrochemical detection of biologically active molecules, trace m e t a l analysis

503

with stripping voltammetry, and waste water treatment (O3). [3-4] Diamond is the electrode material best suited for ozone generation due to its large overpotential for oxygen evolution, its high electrochemical stability and its environmental compatibility. [5" 12]

22.2. Ozone Generation with Diamond Electrodes- Experimental Silicone rubber-shielded

electrodes

(diamond anode and

Pt

cathode) were used in order to avoid electrolyte leaks. The diamond and Pt electrodes were attached by with silver paste (Dotite D-550, Fujikura Co., Ltd., Japan) on the back surfaces of each electrode. A constant current power supply (Jungdo Co. TR" 200, Korea) was used in all operations. The electrolyte was circulated by a rotary pump (Manostat Co. "Vera", USA). The sandwich-type test cell of was fabricated with the BDD electrode as the anode, the Pt electrode as the cathode, and aqueous H2SO4 solution as the electrolyte. The electrolysis cell is shown in Figure 22.1b. Figure 22.1a shows the flow system of the experimental setup. The H2SO4 solution was circulated in a closed loop on the anode side by a polytetrafluoroethylene (PTFE) circulation pump. The current density was controlled from 0 to 2.5 A cm-^, and the electrolyte temperature was maintained at 20 °C . A constant circulation flow rate was maintained throughout all experiments at

160 mL min'^ The generated ozone concentration

was

measured by use of iodometry after reaching a steady state of ozone evolution (30 min after of the onset of ozone evolution).

504

22. Ozone Generation with Boron-Doped Diamond Electrodes and Its Applications

(a)

KI Solution

Electrolyte

E lee tro ly te »ron-doped d ia m o n d

(c)

•

Out

-j;::^^^

P t plat.

^^^

s^^^

Silicone rubber

Silicone rubber Lead wire Boron-doped diamond electrode Silicone rubber

Fig. 22.1. Schematic diagram of (a) ozone generation system, electrolysis cell, and (c) electrolysis flow cell.

505

22.3. Factors Influencing Ozone Generation 22.3.1. Effect of current density on ozone generation The current density is expected to have a strong influence on ozone generation, because it enhances the rate of ozone generation along with the oxygen generation. Figure 22.2 shows the ozone generation

characteristics

concentrations

with

obtained

different

current

for

various

electrolyte

densities. The

ozone

concentration increased with increasing current density. Initially, the ozone generation was slow at low current density. However, the ozone concentration gradually increased with increasing current density. At a current density of 1.3 A cm 2, the amount of ozone generation reached 5000 ppm in 1.0 M H2SO4 solution at room temperature. The amount of ozone increased with increasing sulfuric acid concentration, reached a maximum at 0.1 M sulfuric acid and decreased at higher concentrations.

Because sulfuric

acid

with

acts

as

the

supporting

electrolyte,

increasing

concentration, the ohmic drop in the cell is minimized, and the ozone generation efficiency increases. However, at higher sulfuric acid concentrations, there is interference from sulfate oxidation, which dominates the generation of ozone. As a result, the ozone yield decreases beyond of a sulfuric acid concentration of 0.1 M. Therefore, the optimum concentration for ozone generation is 0.1 M sulfuric acid. Figure 22.3 shows the ozone concentration as a function of electrolyte temperature in 0.1 M sulfuric acid at a current density of 1.3 A cm'^. The ozone concentration decreased with increasing electrolyte temperature. This shows that

506

lower

22. Ozone Generation with Boron-Doped Diamond Electrodes and Its Applications

electrolyte

temperatures

can

be

used

to

obtain

higher

concentrations of ozone.

0.5

1.0

1.5

2.0

2.5

Electrolyte concentration (M)

Fig. 22.2. Effect of electrolyte concentration and current density on ozone generation characteristics.

Electrolyte temperature ("C)

Fig. 22.3. Effect of electrolyte temperature on ozone generation in 0.1 M H2SO4 solution at a current density of 1.3 A cm'^.

507

22.3.2. Ozone generation in a flowcell system Figure 22.4 shows the amount of ozone production as a function of various flow rates and operation temperatures for a flowcell system. H2SO4 solution was circulated in a closed anode loop by a polytetrafluoroethylene (PTFE) circulation pump. The current density was fixed at 1.3 A cm^. The circulation flow rate was controlled from 60 mL to 350 mL min 1. The ozone yield increased almost

linearly

with

increasing

flow

rate

and

operation

temperature. This indicates that the reaction is controlled by mass transport of the electrolyte. The reaction originates from water discharge* the mass-transport effect is due to the removal of gas generated at the surface of the boron-doped diamond electrode, which becomes more efficient with increasing flow rate.

2

3

4

5

Flow rate (mL S"^) Fig. 22.4. Effect of flow rate and operation temperature on ozone generation in 0.1 M H2SO4 solution.

508

22. Ozone Generation with Boron-Doped Diamond Electrodes and Its Applications

22 A. Durability Testing Generally, the Pb02 electrode is unstable under strongly oxidizing conditions. The problem with Pb02 anodes is that,

under

conditions of ozone evolution in strong acids, the anodes slowly disintegrate, leaving an extremely fine precipitate identified as Pb02. Thus, the stability of the electrode is a very important parameter in ozone evolution experiments. In order to assess the stability of diamond electrodes, ozone was generated during 4000 hours continuously. The generated amount of ozone was almost constant. However, in a Pb02 electrode system, the amount of generated ozone gradually decreased, resulting from structural degradation of the electrode, as shown in Figure 22.5.

This

phenomenon was caused by application of a high potential to the structurally weak electrode. Bakchisaraits'yan et al. studied the anodic stability of electrolytically deposited a-Pb02 in sulfuric acid solutions and found the anode weight loss to increase linearly with applied current density. They found the erosion to increase markedly with acid concentration, yet were unable to offer an explanation beyond the observation of a transfer of fine particles into the solution. Pavlov believes the maintenance of a high concentration of O atoms on the oxide surface at high potentials leads to internal stress, leading to cracks, and parts of the oxide peel off the surface. The precipitation from Pb02 anodes can be plausibly explained through a flaking of the anode surface due to stress in the brittle material caused by local temperature differences set up by uneven current distribution under conditions of rapid gas evolution. In

509

contrast, it has been confirmed that the diamond electrode is very stable in generating ozone by electrolysis. After durability testing, SEM images of diamond and Pb02 electrodes were examined to assess the degree of structural and morphological degradation for each electrode, as shown in Figure 22.6. In the case of the diamond electrode, only a slight degree of edge rounding-off was observed, i.e., structural and morphological degradation were almost negligible. Some researchers studied the anodic stability of electrochemically deposited a- Pb02 in sulfuric acid solutionsfds and found anode weight loss to increase linearly with applied current density. They found the erosion to increase markedly with acid concentration and applied potential. These results show that the diamond electrode is more suitable for ozone generation by electrolysis method than the Pb02 electrode.

9000

6000

0

800

1600 2400 Time (h)

3200

4000

Fig. 22. 5. Durability test of (a) a boron-doped diamond electrode and (b) a Pb02 electrode in 1 mol dm*3 H2SO4 solution at a current density of2.5Acm-2. 510

22. Ozone Generation with Boron-Doped Diamond Electrodes and Its Applications

ini

VhO.

iUiUinh*

( h ) IHiifiKMid t l i H l n M l r

Itiirbfl

\rier|4i^H> liiNj

Fig. 22.6. SEM images of Pb02 and BDD electrodes: (a) before and (b) after 4000 h ozone generation. Raman

spectra

for

BDD

electrodes

before

and

after

electrolysis also support the stability, as shown in Figure 22.7. Figure 22.7(a) shows the surface Raman spectrum of a BDD electrode before electrolysis. The spectrum exhibited a sharp peak at 1332 cm'i, which provides strong evidence for a high degree of sp3 bonding in the BDD film, i.e., high-quality diamond, with no other apparent peaks related to any non-diamond phase. [13] The spectrum of the BDD electrode after 4000 hours of electrolysis shows almost the same intensity at 1332 cm"i. The Raman 511

spectrum suggests that the carbon surface microstructure of BDD electrode is affected by absorption of hydrogen ion or oxygen on the surface of diamond electrode.

<*uu

1334 cm**

. ._ . (a) Before

(b) After

300

^ a>

200

1

•H

1

100

l\

iV

0 ¥1^

\j*fe«^^a^^»«^

^^:;MW^iiip»h

9(K)

1050

1200

1350

1500

1650

Wave number / cm"^ Fig. 22.7. Raman spectra of BDD electrodes obtained (a) before; and (b) after electrolysis.

22.5. Applications of Ozone Ozone is playing an important role as a clean and powerful oxidant in water treatment, in the pulp and food industry and in the medical industry, because ozone, unlike chlorine, does not generate harmful residues such as haloform, etc. during the reactions and is six times as strong as chlorine in oxidizing power. Disinfection methods are divided into four categories- hightemperature disinfection, UV disinfection, iodine disinfection, and chlorine disinfection.

512

22. Ozone Generation

with Boron-Doped

Diamond Electrodes

and Its

Applications

UV disinfection Energy contents is large, and there can be chemical and physiological effects

High-temperature disinfection Sterilize all bacteria Destroy nutrients in food

265-nm light is most effective (^ Harmful to human body

Disinfecting method

V. Iodine disinfection

/ Chloride disinfection

Reactivity is strong

Cause bacteriocide via oxidation

Sterilizing power is large at low pH

Hurtful to human body (poisonous gas)

Used mainly as a skin disinfectant

22.5.1. Sterilization Ozone is a strong oxidant, reacting readily with a wide range of organics and biological species. The bleaching effect produced by ozone on indigo was used as the basis of a method to qualitatively determine ozone concentrations. Ozone has also been used as a selective disinfectant (E. coli) in brewing and so on.

E. coli

Figure 22.8 shows the disinfection effect of ^ . (?6>7i cultivated for 2 days. The ozone treatment was carried out for 60 s. The

513

concentration of ozone w a s controlled from 0 to 20 ppm. E. coli cells

were

completely

eliminated

after

60

s

at

an

ozone

concentration of 20 ppm. The disinfection w a s much more rapid with ozone t h a n with chlorine. The disinfecting power

also

depends on ozone concentration. We could see t h i s effect with bacteria cultivated from different vegetables.

J.4K 10^ H» (>|)in

3-4 X IIP 15 n n m

2 0 utiiii

h-A - ill

Fig. 22.8. Disinfection concentrations.

H t niiuoliMiv

effect

for

E.

coli

at

various

ozone

Figure 22.9 shows the disinfection effect of a 60-s t r e a t m e n t after a 2-day cultivation for various media, for example, celery cabbage, grapes, lettuce, a n d perilla leaf. We obtained t h e result t h a t the disinfection power w a s highest for ozone and decreased in the order ozone>chloride>water a n d also depended on t h e ozone concentration. Complete disinfection by ozone at 20 p p m w a s 514

22. Ozone Generation with Boron-Doped Diamond Electrodes and Its Applications

obtained consistently. The sterilizing power was 90% for chlorine and 100% for ozone at 60 s in 20 ppm solutions of these oxidants. We were able to obtain the same effect with other foods and vegetables, i.e., complete sterilization of ^ . (?6>7i and other bacteria within 1 min at an ozone concentration of 20 ppm. H-U

2i\ ppm O^

3H p^mj ( I

^m^ ^r

^r \A

r : ' I'i

"^r

ir

i"' > # '^0

Fig. 22. 9. Microbicidal effect on various medial (a) celery cabbage; (b) grapes; (c) lettuce; and (d) perilla leaf in water, 20-ppm CI2 solution, and 20 ppm O3 solution. 22.5.2. Bleaching experiments Ozone acts as a strong bleaching agent, similar to chlorine. Figure 10 shows the bleaching effect of ozone on methyl orange, red, blue.

515

and black inks dissolved in water. Complete decolorization was accomplished within 15 min at an ozone concentration of 500 ppm. Figure 22.11 shows the decolorization of soup solution, urine, and wastewater from a dye industry.

Fig. 22.10. Decolorization of various colors of inks

1ii»ti;U

Sdiip sciitttton

2«>fiiiii

I riiie

Fig. 22. 11. Decoloration of soup solution, urine, whitening of papers, and wastewater from the dye industry

516

22. Ozone Generation with Boron-Doped Diamond Electrodes and Its Applications

22.5.3. Air purification Because gaseous contaminants such as odorous molecules spread easily in the air due to rapid mass transport, indoor pollution, whether in the workplace or in the home, is a common problem everywhere! Most air purifiers do not reduce mold and sources of indoor pollution that contribute to allergies, asthma, bacterial or viral infections, hay fever and home respiratory problems. Some only trap the mold or bacteria, allowing them to grow more rapidly, and your health declines even faster. The strong oxidizing power of ozone can eliminate these contaminants easily. In particular, odors that are objectionable to humans are due to compounds whose concentrations are on the order of 10 ppm by volume; at these concentrations, the presence of small amounts of ozone can eliminate these odors completely.

Figure 22.12 shows the

decomposition of cigarette smoke by ozone. Decomposition was complete within 15 min at 500 ppm ozone concentration.

Itiitiiil

I0miii

ISmin

Fig. 22.12. Purification of cigarette smoke with 500 ppm of ozone. Table 22.1 shows several detailed examples of deodorization reactions involving ozone as an oxidizer, and Table 22.2 shows a more extensive list of deodorization reactions. Table 22.3 also

517

shows a n extensive list of compounds and the ozone deodorization t r e a t m e n t conditions . On t h e other hand, t h e reaction steps for t h e decomposition of phenol u n d e r ozone t r e a t m e n t can be predicted, as shown in Scheme

22.1. Moreover,

the

decomposition

reactions can

be

catalyzed in the presence of UV light. The reaction can t a k e two competitive p a t h w a y s - one is the formation of catechol; t h e other is t h e formation of hydroquinone. The decomposition of phenol w a s accelerated in the presence of UV light at a wavelength of 270 nm.

Table 22.1. Deodorization reactions involving ozone

(1) H^S (Hydrogen Sulfide) H.S + 0 3 - ^ SO3 + H.O H3S + O3 -> S + H3O + O3 (2) CH2SH (Methyl Mercaptan) CH3SH + O3 -> CH3-SO3H + O. (3) (CH3)2S (Dimethyl Sulfide) CH3-S-CH3 + 0 3 ^ CH3-S-CH3 + O2

II O (4) (CH3)2S2 (Dimethyl Disulfide) CH3-S-S-CH3 + 0 3 - ^ CH3-SO3 H +O3 + O2 (5) R3N (Tertiary Amines) R3N + O3 -> R3NO +O2

518

22. Ozone Generation with Boron-Doped Diamond Electrodes and Its Applications

(6) Unsaturated hydrocarbons

X=C( R^

+0,

^

^ C = 0 + R-C02H

^R

(7) Olefins CnHjii + O3 -^ Aldehyde, H j O , CO3 etc. (8) Formalin H C H O + O3 -^ Peroxy acid

Table 22.2. More complete list of deodorization reactions of ozone

Malodorous component

Molecular weight

NH3

17.0

H2S

34.1

CH3SH

48.1

(CH3)2S

62.1

(CH3)2S2

94.2

(CH3)3N

59.1

CH3CHO

44.1

Chemical absorption & oxidation Chemical absorption

C5H5CHCH2

104.1

Chemical absorption

Deodorization principle Chemical absorption

Reaction product

addition stoich. no.

Deoderization %

NH4^

2

84%

SO4 ^ S

4

99%

CH3SO3H

3

99%

(CH3)2SO

1

99%

CH3SO3H

5

99%

(CH3)3NO

1

95%

-

2

95%

-

2

95%

Oxidation Oxidation Oxidation Oxidation

519

The ozonolysis process is very important for understanding the decomposition mechanism. When ozone reacts with ethylene, the primary ozonide is formed as an intermediate product. This product decomposes into a carbonyl fragment and a carbonyl oxide. Also, possible ozonolysis product groups are shown in Scheme 22.2. Catechol H

<9-benzoquinone 0

Organic Acid CO9 + H2O

1 + 0^ CO.

Phenol

OH O Hydroquinone /7-benzoquinone

Scheme 22.1. Decomposition mechanism of phenol by ozone in the presence of UV light

520

22. Ozone Generation with Boron-Doped

Diamond Electrodes and Its

Applications

Polyaromatic System

O3/H (CH2)n Cyclic Olefines Cephalosporines Unsaturated Heterocycles Compounds ^n

Linear Olefines ^ 'n Fatty acids

Scheme 22.2. Possible ozonolysis product groups Table 22.3. Deodorization t r e a t m e n t of ozone

Ozone concentration for 100% deodorization treatment

Compound

Chemical formula

Ammonia

NH3

167,300

55 ppm

Hydrogen sulfide

H2S

17,000,000

1 ppm

Methylmercaptan

CH3SH

53,300,000

35ppb

Dimethyl sulfide

(CH3)2S

2,760,000

0.1 ppm

Dimethyldisulfide

CH3SSCH3

-

7.4 ppb

Trimethylamine

(CH3)3N

493,500

4 ppm

Acetaldehyde

CH3CHO

4,300,300

0.3 ppm

Styrene

C5H5CHCH2

44,400

8.3 ppb

Acetone

CH3COCH3

720

300 ppm

0. L*

521

Acrolein

CH2CHCHO

Amyl alcohol

CH3(CH2)2CHOHCH3

Benzene

19,300

20ppm

368

lOppm

300

300 ppm

3

1.33 ppm

Ethoxyethanol

C2H5OCH2CH2OH

Dibutyl ketone

CH3(CH2)7CH3

9,800

46.44 ppm

Ethyl acetate

CH2COOC2H5

1,900

50 ppm

Ethylene

C2H4

57,100

100 ppm

Formaldehyde

HCHO

5,000,000

1 ppm

Hydrazine

H2NNH2

5,300

1 ppm

Butanol

CH3(CH2)2CH20H

120

5000 ppm

Methyethyl ketone

CH3COC2H5

3,800

30 ppm

Toluene

C6H5CH3

720

40 ppm

Xylene

QH4(CH3)4

136,000

2 ppb

*O.I.=olfactory index

22.6. Conclusions Electrochemical ozone generation using conductive boron doped diamond electrodes has been found to be very attractive due to their high overpotential for oxygen evolution and excellent dimensional stability. Electrochemical ozone generation has two advantages. First, the production efficiency of ozone is higher that that for other methods, with yields as high as 10 - 15%. As the solubility of ozone in water is higher than that of oxygen and the lifetime of ozone is sufficiently long (several minutes), water containing ozone acts as an effective oxidant, in addition to the high concentrations of ozone in the oxygen stream liberated at the anode.

522

22 Ozone Generation with Boron-Doped Diamond Electrodes and Its Applications

The technology of electrochemical ozone generation h a s been h a m p e r e d due to a lack of suitable electrodes, in t e r m s of cost and environmental compatibility. Diamond t h i n film electrodes, in this respect, are highly promising, as t h e high dimensional stability compensates t h e cost, while t h e m a t e r i a l itself is environmentally friendly. One can look forward to a commercial ozone generator using this promising electrode in t h e n e a r future.

References 1. M. R. Khan, M. W. Khan, Environmental Botany, 42 2.

(1999)

3659.

Biodeterioration

& Biodegradation,

44

(1999)

4.

D. Grandini, E. Mahe, P. A. Michaud, W. Haenmi, A. Perret, Ch. Comniellis, J. AppL Electrochem.,

4.

Experimental

R. Viera, P. S. Guiamet, M. F. L. de Mele, H. A. Videla, International

3.

and

30

(2000)

1345.

G. Foti, D. Gandini, Ch. Comniellis, A. Perret, W. Haenmi, Electrochem.

SolidState

Lett, 2 (1999) 228.

5. W. B. Wilson, Phys. Rev., 127 (1962) 1549. 6.

H. B. Martin, A. Argoitia, A. Anderson, U. Landau, J. C. Angus, J. Electrochem.

7.

Soc, 143 (1996) 3382.

S. Jolly, M. Koppang, T. Jackson, G. M. Swain, Anal. Chem., 69 (1997) 4099.

8. J. Xu, G. M. Swain, Anal Chem., 70 (1998) 1502. 9.

M. Yanagisawa, L. Jiang, D. A. Tryk, K. Hashimoto, A. Fujishima, DiamondRelat.

Mater,

8 (1999) 2059.

10. J. W. Lindsay, J. M. Larson, S. L. Grishick, Diamond

Relat.

523

Mater., 6(1997) 481. 11. G. M. Swain, R. Ramesham, Anal. Chem., 65 (1993) 345. 12. J. W. Strojek, M. C. Granger, G. M. Swain, T. Dallas, M. W. Holtz, Anal. Chem., 68 (1996) 2031. 13. T. Yano, D. A. Tryk, K. Hashimoto, A. Fujishima, J. Phys. B, 102 (1998) 4933.

524

Chem.

23. Application of Diamond Electrodes for Water Disinfection Tsuneto Furuta, Phylippe Rychen, Hozumi Tanaka,

Laurent

Pupunat, Werner Haenni and Yoshinori Nishiki

With regard to traditional water treatment technologies and their markets, new solutions should be found in order to settle current and future problems, as well as to improve the economical and ecological

situations

of

several

leading-edge

processes.

Electrochemical processes, such as electrodialysis, electrowinning, electrodeionization, electroflotation, electroflocculation and others, appear to provide solutions in certain ways to several problems. Conductive

boron-doped

polycrystalline

CVD

diamond,

identified as BDD (boron-doped diamond) electrodes, exhibit good electrochemical stability, together with the largest overpotentials for water electrolysis among well-known electrodes. In particular, this combination of properties is the reason for increasing efforts with the aim of developing highly efficient processes with BDD electrodes

electrochemical

[1,2]. Applications such

as

disinfection [3,4] and electrolytic oxidation [5-9] are areas where electrolysis based on BDD electrodes can offer new or improved solutions. In this chapter, the production capacity of oxidants possible with the BDD electrode in aqueous solutions and an experimental Tsuneto Furuta e-mail: [email protected] 525

result focusing on Legionella

inactivation for domestic

water

t r e a t m e n t purposes or i n d u s t r i a l cooling w a t e r systems, in order to avoid the propagation of Legionnaire's disease, are

hereafter

described.

23.1. Oxidant Production Capacity of BDD Electrodes 23.1.1. Oxidant production in pure water Since hardly any oxygen is electrogenerated via w a t e r oxidation on BDD electrodes, even w h e n a n anodic potential of more t h a n 2 V vs. N H E is applied, various forms of active oxygen, such as the hydroxyl radical, hydrogen peroxide a n d ozone, can therefore be produced. Indeed, t h e BDD electrode shows a high capability to produce active oxygen, as shown in Table 23.1. Table 23.1. Current efficiency (%) to produce ozone and hydrogen peroxide in pure water by various anodes in a two-compartment cell at a current density of 0.1 A cm 2.

Anode

Pt

O3 H2O2

0.2 0.005

DSA 0.05 0.001

Unit; % BDD 2 5 0.15 0.001 Pb02

In addition to ozone and hydrogen peroxide, radical species such as the hydroxyl radical are detected in p u r e w a t e r electrolyzed at BDD

electrodes,

as

shown

in

Fig.

23.1. Even

though

the

m e c h a n i s m for active oxygen generation on t h e BDD electrode h a s not necessarily been clarified to date, the BDD electrode h a s t h e

526

23. Application of Diamond Electrodes for Water Disinfection

highest capability to produce active oxygen. The elementary processes are considered to be as follows [lO]1)

The hydroxyl radical is adsorbed on the BDD surface as a consequence of water discharge.

2)

Ozone and hydrogen peroxide are generated by the reaction of adsorbed hydroxyl radicals with solution-phase hydroxyl radical and/or water.

3)

Simultaneously, the hydroxyl radical is produced by the reaction of ozone with hydrogen peroxide.

3310

3360

3410

Magnetic field (G)

Fig. 23.1. ESR spectra of pure water electrolyzed at various electrodes in a two-compartment cell at a current density of 0.1 A cm'^, obtained 90 minutes after addition of DMPO into the anolyte. 23.1.2. Oxidant production in sulfate solution Fig. 23.2 shows a comparison in production capacity for peroxides, e.g., peroxo'disulfate, between a BDD electrode [ll] and a platinum electrode. The use of BDD electrodes allows the efficient

527

production of peroxide over a wide p H range. In sulfate media, it w a s also observed t h a t BDD electrodes generated ozone at a concentration of approximately

1% in the anode gas. Peroxo"

disulfate generation by t h e oxidation of t h e sulfate ion with t h e hydroxyl radical is proposed as a possible reaction step, in addition to the electrochemical oxidation of t h e sulfate ion. NaHS04

Na2S04

0.4 0.8 1.2 1.6 Sodium concentration (mol L'^)

2.0

Fig. 23.2. Current efficiency for peroxide production in 1 M 804^" at 25°C and 23 mA cm-2: (A) BDD electrode; and ( T ) Pt electrode.

23.1.3. Oxidant production in chloride solution Fig. 23.3 [12] shows differences a m o n g various electrode m a t e r i a l s for hypochlorite production in chloride solutions. Hypochlorite production is much more efficient with the use of t h e BDD electrode t h a n with the use of t h e DSA (dimensionally stable anode) or the p l a t i n u m

electrode. Figure 23.4 shows

anodic

polarization curves for BDD and p l a t i n u m electrodes in sodium chloride a n d sodium sulfate solutions. Concerning t h e p l a t i n u m

528

23. Application of Diamond Electrodes for Water Disinfection

electrode, the anodic potentials in sodium chloride solution are approximately 300 mV more positive a t given potentials t h a n those in sodium sulfate solution.

0

400 800 1200 1600 2000 Chloride concentration (mg L'^)

Fig. 23.3 Faradaic efficiency of hypochlorite production in sodium chloride solution: (A) BDD electrode; (•) DSA; and ( T ) Pt electrode.

OH

L

AT

> > B o

J

[

u

r

2

A

i

L

A -^

E

^'^

f'^^

< ^

0.01

1

1 1 11 I I I

0.1

1

1 1 1 1 III

1

1 Current density (A cm"^)

1 1 1 1 III

10

Fig. 23.4. Anodic polarization curves^ (A) BDD; and ( T ) P t electrodes in (soUd Une) 3 wt% NaCl and (dashed hne) 3 wt% Na2S04.

529

On the contrary, the anodic potentials in sodium chloride solution for the BDD electrode are approximately 800 mV less positive than those in sodium sulfate solution. This could be responsible for the difference in hypochlorite production capability between the BDD and platinum electrodes. 23.1.4. Oxidant production in carbonate solution Fig. 23.5 [13] shows current efficiencies for the production of peroxides, i.e., peroxo-carbonate, in sodium carbonate solution obtained on BDD and platinum electrodes. The efficiency for the former is 80%, which is approximately three times higher than that for the latter. 100

Current density (A cm" ) Fig. 23.5. Current efficiency for peroxide production* (A) BDD; and (T) Pt electrodes in 1 M Na2C03 at 0**C. As is the case with the generation of peroxo'disulfate, peroxocarbonate generation by the oxidation of carbonate ion with

530

23. Application of Diamond Electrodes for Water Disinfection

the

hydroxyl

radical

is

considered,

in

addition

to

the

electrochemical oxidation of carbonate ion.

23.2. Legionella Inactivation with BDD Electrodes BDD electrodes are applied in a typical closed, pressurized electrolyzer (DiaCell®), and its technology has been successfully tested for Legionella inactivation in several water compositions and under various working conditions.

23.2.1. Experimental conditions Fig. 23.6 shows a schematic diagram of the test unit with the DiaCell® to be used for preparing electrolyzed waters from tap water or deionized (DI) water with a conductivity of approximately IjiS cm'i.

Electrolyzed water sample ready for Legionella injection

Fig. 23.6. Schematic representation of the unit with the DiaCell® to be used for producing electrolyzed water. Tap water is used for tests without any additives, with sodium chloride injection and with sodium hypochlorite injection. On the other hand, DI water was used with various dissolved salts. 531

Several current densities were applied to the DiaCell® with an active electrode surface of 65 cm^. The hydraulic flow was set at 160 L h i , and the water temperature was maintained between 22 and 26°C. Legionella solution (5 mL) was injected into each water sample without any treatment, and into samples of electrolyzed water. No test was performed passing Legionella through the DiaCell® electrolyzer. Table 23.2. Tap water quality in Switzerland

Parameter Conductivity pH Chloride, CI" Nitrate, NO3" Sulfate, S04^" Bicarbonate, HCO3' Calcium, Ca^"^ Magnesium, Mg^"^ Sodium, Na"^

Value 476 mS cm'^ 7.8 3.5 ppm 2.1 ppm 5.4 ppm 324 ppm 74 ppm 21 ppm 3.7 ppm

Before each test of inactivation, Legionella was injected into the tap water sample, whose quality is shown in Table 23.2, and it was confirmed that the Legionella viability remained stable in tap water after a 1-hour test, as shown in Fig. 23.7, which shows the Legionella viability in hypochlorite solution as well. The minimal injection of hypochlorite into tap water in order to guarantee immediate inactivation requires a chlorine concentration range between 0.67 and 1.12 ppm. The total oxidant concentration was measured with the DPD standard method (value DPD-3) and expressed as total chlorine (ppm) even when the electrolytes contained no chloride. 532

23. Application of Diamond Electrodes for Water Disinfection

•

M 0

10

i I

^

• ^ - ^

20 30 40 50 60 Contact time (minutes)

70

Fig. 23.7 Legionella viability in tap water and chemical hypochlorite solution- (•)tap water; (o) 0.18 ppm oxidant as CI2 (tap water + NaOCl); (n) 0.67 ppm oxidant as CI2 (tap water + NaOCl); and (O) 1.12 ppm oxidant as CI2 (tap water + NaOCl).

23.2.2. Inactivation with electrolyzed tap water Tap w a t e r containing low concentration of chloride (3.5 p p m CI) w a s electrolyzed. Fig. 23.8 shows t h e inactivation oi Legionella

as

a function of contact time a n d c u r r e n t density. It is i n t e r e s t i n g t h a t t h e r e w a s no major difference in Legionella

deactivation

between t h e two lower c u r r e n t densities, which were coupled with lower oxidant levels (50 mA cm"2 - 0.11 p p m oxidant as CI2, 100 mA cm"2 - 0.13 p p m oxidant a s CI2), b u t these were m u c h smaller in comparison with the higher c u r r e n t density (150 mA cm'^) where the observed inactivation w a s more t h a n t h r e e times larger. The major inactivation increase seemed to occur a t t h e beginning (<5 minutes). The highest inactivation (89%) w a s reached after 60 m i n u t e s at 150 mA cm'2, which w a s coupled with t h e high (0.19 ppm) oxidant level. 533

loa

10

20 30 40 50 Contact time (minutes)

60

70

Fig. 23.8. Legionella inactivation in electrolyzed tap water: ( T ) 50 mA cm 2 - 0.11 ppm oxidant as CI2; (•) 100 mA cm 2 - 0.13 ppm oxidant as Ck; and (A) 150 mA cm"2 - 0.19 ppm oxidant as CI2.

0

10 20 30 40 50 Contact time (minutes)

60

70

Fig. 23.9. Legionella inactivation with tap water with added sodium hypochlorite and electrolyzed tap water^ (A) 0.18 ppm oxidant as CI2 (tap water + NaOCl); and ( T ) 0.19 ppm oxidant as CI2 (tap water 150mAcm-2).

534

23. Application of Diamond Electrodes for Water Disinfection

Fig. 23.9 shows the difference between

dosing

tap

water

with

in Legionella chemical

inactivation

hypochlorite

and

electrolyzing chloride-containing tap w a t e r to produce a similar total oxidant concentration. This non-negligible difference may be a t t r i b u t e d to t h e chemical n a t u r e of t h e disinfectants. Table 23.3. Legionella inactivation in electrolyzed DI water with injected sodium chloride (injected NaCl, 330 ppm; intrinsic CI", 200 ppm).

Current density (mA cm') 0 25

50 100

Contact time (minutes) 60 <1 5 60 <1 5 <1 5

Elimination (%)

0 99.58 > 99.98 > 99.99 > 99.99

Total chlorine (ppm) 0 0.96 0.55 0.19 1.51 1.04 2.7 1.72

Table 23.4. Legionella inactivation in electrolyzed tap water with injected sodium chloride (injected NaCl, 128 ppm; intrinsic CI', 79 ppm; intrinsic HCO3 , 324 ppm).

Current density (mA cm'^) 0 50 100 150

Contact time (minutes) 60 <1 5 <1 5 <1 5

Elimination (%)

0 > 99.90 > 99.99 > 99.90 > 99.99 > 99.90 >99.99

Oxidant (ppm as CI2) 0 0.71 0.5 1.16 1.01 1.45 1.25

535

Tables 23.3 and 23.4 show Legionella

inactivation with

electrolyzed DI water and electrolyzed tap water, both containing injected sodium chloride. Comparing these data, the inactivation behavior was similar, despite chloride concentrations that differed by more than a factor of two. With a comparison at a current density of 50 mA cm 2, the oxidant levels were also differed by more than a factor of two. This suggests that not only hypochlorite is active but also one or more other disinfectants are generated in tap water electrolysis. lOOi

30 40 50 60 70 Contact time (minutes) Fig. 23.10. Legionella inactivation in electrolyzed DI water with sodium bicarbonate- (T) 50 mA cm"2 - 0.11 ppm oxidant as CI2J (•) 100 mA cm"2 - 0.17 ppm oxidant as CI2; and (A) 150 mA cm"2 - 0.21 ppm oxidant as CI2.

23.2.3. Inactivation with electrolyzed bicarbonatecontaining solutions Because most types of water, starting from drinking water or surface water, contain some bicarbonates, as shown in Table 23.2, by passing water through a DiaCell® with BDD anodes, it is

536

23. Application of Diamond Electrodes for Water Disinfection

expected to obtain peroxo-carbonates and their derivatives, as described in section 23.1. DI water containing sodium bicarbonate (345 ppm HCO3) enabled us to evaluate Legionella inactivation at an ionic strength similar to that of the tested tap water. Figure 23.10 summarizes the inactivation efficiencies at several current densities. It clearly appears that there is remarkable inactivation, independent of both current density (between 50 and 150 mA cm 2) and oxidant concentration. IOO1

80o

> 60-

40-

0

10

20 30 40 50 60 Contact time (minutes)

70

Fig. 23.11. Legionella inactivation in electrolyzed DI water with sodium sulfate: (A) 25 mA cm"2 - 0.11 ppm oxidant as CI2J (•) 50 mA cm-2 - 0.16 ppm oxidant as ChJ and (T) 100 mA cm"2 - 0.22 ppm oxidant as CI2. 23.2.4. Inactivation w i t h electrolyzed sulfate-containing solutions In an approach similar to that for electrolyzed bicarbonatecontaining solutions, it is expected that peroxo'disulfate and its derivatives should work as disinfectants. DI water containing

537

sodium sulfate (298 p p m SO42") w a s evaluated at a n ionic s t r e n g t h similar to t h a t of the tested t a p water. Fig. 23.11 s u m m a r i z e s the inactivation efficiencies a t several c u r r e n t densities. In practice, t h e r e w a s absolutely no inactivation with peroxo-disulfate and its derivatives, even w h e n oxidants were generated up to 0.22 p p m as CI2.

23.2.5. Electrolytically produced disinfectants Fig. 23.12 shows Legionella

inactivation a s a function of contact

time.

0

10

20 30 40 50 60 Contact time (minutes)

70

Fig. 23.12. Legionella inactivation versus contact time after injection(0) 0.71 ppm oxidant as CI2 (tap water + NaCl - 50 mA cm-2); (a) 0.67 ppm oxidant as CI2 (tap water + NaOCl)J (o) 0.18 ppm oxidant as CI2 (tap water + NaOCl); (A) 0.19 ppm oxidant as CI2 (tap water - 150 mA cm-2); (•) 0.13 ppm oxidant as CI2 (tap water - 100 mA cm'^); and ( T ) O.llppm oxidant as CI2 (tap water - 50 mA cm'2).

This w a s not necessarily a fair comparison, because t h e applied c u r r e n t densities and produced oxidants were not identical, b u t it 538

23. Application of Diamond Electrodes for Water Disinfection

was observed that the ranking of disinfection capabilities was as followshypochlorite > peroxide from carbonate » peroxide from sulfate When sufficient contact time is possible, for example, in situations

of

loop-electrochemical

treatment,

Legionella

inactivation is possible with tap water without any additives, or with water containing at least bicarbonates. If immediate disinfection is required, very small additions of sodium chloride can help, i.e., approximately 80 ppm of chloride is enough. The main characteristics are summarized as follows1)

Total Legionella

inactivation of more than 80% can be

reached with the DiaCell® when tap water is electrolyzed at more than 150 mA cm'^ and the contact time is sufficiently long, i.e., more than 1 hour. 2)

Peroxide from carbonate is the most powerful disinfectant in electrolyzed tap water. Bicarbonates definitely have many advantages, as there is no hypochlorite production, i.e., no chlorine-related

drawbacks,

and

small

total

oxidant

production is sufficient for good inactivation, even at small current densities. Since bicarbonates are always present in tap water, tap water electrolysis can also result in good Legionella inactivation. 3)

The more chloride is contained in electrolyzed water, the faster is the inactivation, even at low current densities, i.e., Legionella

can

be

completely

inactivated

through

the

DiaCell® with current densities as small as 50 mA cm'2 and contact times of 1 minute, when sodium chloride is added up to approximately 80 ppm chloride.

539

4)

Electrolyzed water containing only sulfate has no impact on Legionella inactivation.

23.3. Concluding Remarks As is already well known, BDD is a very promising electrode material for water treatment technologies and their markets due to its outstanding features. This may be associated with the production of the hydroxyl radical, which may also be responsible for the production of ozone, peroxo'disulfate, peroxo'carbonate, hydrogen peroxide and their derivatives, which are powerful oxidants in naturally mineralized water. Since most surface waters contain some bicarbonates and sulfates, which can be transformed into peroxide compounds, and chlorides, which can be transformed into hypochlorite, using the BDD electrode, water itself can help industrial water treatments. Concerning disinfection applications, Legionella inactivation, in particular, is possible with tap water without additives or with water containing at least bicarbonates, which is mostly the case. These

types

of

solutions

provide

the

advantage

of

low

concentrations of chloride and thus chlorine. This provides fairly easy

operation

disinfection.

540

and

low

environmental

impact

for

water

23. Application of Diamond Electrodes for Water Disinfection

References 1.

New Diamond

Front

Carbon Techno!., 12 No.2 (2002), Special

Issue on the 4th International Workshop on Diamond Electrodes. 2.

New Diamond Front

Carbon Technol,

13, No.2, (2003), Special

Issue on the 5th International Workshop on Diamond Electrodes. 3.

W. Haenni, J. Gobet, A. Perret, L. Pupunat, Ph. Rychen, C. Comninellis and B. Correa, New Diamond

and Frontier

Carbon

Technology, 12 (2002) 83. 4.

Ph. Rychen, L. Pupunat, W. Haenni and E. Santoli,

New

Diamond and Frontier Carbon Technology, 13 (2002) 109. 5.

J.J. Carey, C.S. Christ and S.N. Lowery, US Patent b, 399, 247 (1995).

6.

M. Fryda, A. Dietz, D. Hermann, A. Hampel, L. Schafer, C.P. Klages, A. Perret, W. Haenni, C. Comninellis and D. Gandini, Abstract

of 6th Int. Symposium

on Diamond

Materials,

(1999)

Abstract No. 834. 7.

G. Foti, D. Gandini, C. Comninellis, A. Perret and W. Haenni, Electrochem.

8.

SolidState

Lett,

2 (1999) 228.

S. Hattori, M. Doi, E. Takahashi, T. Kurosu, M. Nara, S. Nakamatsu, Y. Nishiki, T. Furuta Electrochem.,

9.

Appl.

33 (2003) 85.

A.M. Polcaro, A. Vacca, Electrochem.,

and M. lida, J.

S. Palmas, M. Mascia,

J.

Appl.

33 (2003) 885.

10. P. Michaud, M. Panizza, L. Ouattara, T. Diaco, G. Foti and C. Comninellis, J. Appl. Electrochem.,

33 (2003) 151.

11. W. Haenni, J. Gobet, A. Perret, L. Pupunat, Ph. Rychen, C. Comninellis and B. Correa, Proceedings

of the 4th

International

541

Workshop on Diamond Electrodes (2001). 12. S. Ferro, A. Di Battisti, I. Duo, C. Comninellis, W. Haenni and A. Perret, J. Electrochem.Soc,

147 (2000) 2614.

13. M. S. Saha, T. Furuta and Y. Nishiki, Electrochem. Lett, 6 (2003) D5.

542

SolidState

24. Direct Ozone-Water Generation by Electrolysis: Novel Application of SelfStanding Diamond Electrodes Kazuki Arihara and Akira Fujishima

24.1. Introduction 24.1.1. Applications of ozone-water Ozone is an extremely strong oxidant, close to fluorine in strength, and has been applied for the purposes of sterilization, deodorization, and decolorization.

Because the excess ozone spontaneovisly

reconverts to oxygen in these processes, the applications of ozone are environmentally friendly. Ozone dissolved in water further improves its bactericidal activity toward viruses and bacteria thanks to the generation of reactive oxygen species such as OH , HO2 and O2 [l]. Commercial ozone-water generators have already been introduced into food processing factories, kitchens, sanitary facilities, and medical centers, among others.

A post-washing process is unnecessary,

resulting in the elimination of workloads and costs. Ozone-water can

be

reasonably

applied

to

the

washing

processes

of

semiconductor substrates and electronic parts, where high purity water, without additives other than ozone and oxygen, is absolutely necessary. Kazuki Arihara e-mail: [email protected] 543

24.1.2. Previous electrodes for electrolytic ozone generation Ozone is usually produced by UV light absorption, silent electric discharge and water electrolysis. Although each method for ozone generation has both merits and demerits, in terms of system size, electrical efficiency, generation rate and concentration, only the electrolysis method can produce ozone-water directly, without any additional equipment other than the generator. Generally, lead dioxide (Pb02) and platinum (Ft) electrodes are used as electrocatalysts for ozone generation [2-4]. The electrolytic cell consists of a porous anode, a porous cathode and a solid'state polymer electrolyte membrane instead of an electrolyte solution^ these are stacked, as shown schematically in Fig. 24.1(a).

Pure

water, or tap water without additives as an electrolyte, is directly supplied to the anode compartment, the electrolysis of water occurs, and the electrolyzed water containing dissolved ozone is directly drained. Electrolytic ozonizers based on this system have already become available on the market. p-Pb02 has been considered to the most efficient electrocatalyst for electrolytic ozone generation [2]. The crystalline state of p-Pb02 rather quickly converts to the a-state when the electric power to the electrolytic cell is cut off, leading to the elimination of electrocatalytic activity for ozone generation. In preparation for an unexpected power cutoff, a backup power source must be available to supply electric power continuously and preserve the crystalline state of the p-Pb02 electrode. The electrolyzed water containing dissolved ozone must not be used as is, because it also contains lead compounds originating from the gradual dissolution of the Pb02 electrode. Commercial products include a gas separator to separate

544

24. Direct Ozone-Water Generation by Electrolysis: Novel Application of Self-Standing Diamond Electrodes

the ozone gas from the electrolyzed water and a dissolution tower to dissolve the ozone gas into pure water or tap water, producing ozone-water. The total ozonizer system depicted in Fig. 24.1 (b) is difficult to scale down due to these attachments. In any case, the lead usage should preferably be restrained, in view of the negative environmental aspects.

^Q^

ozone hydrogen water water

(b)

cathode gas separator

anode gas separator

^

L dissolution tower

porous anode

, porous cathode

hydrogen decomposer

(±H h3

rr

iri/

Fig. 24.1. Schematic diagrams of (a) an electrolytic ceU for ozone generation and (b) a flow system to generate ozone-water using the electrolytic cell with a Pb02 electrode. Although Pt is a relatively stable material and is commonly used

as an

electrode

for electrochemical

measurements,

it

gradually erodes as the electrolysis continues under high current conditions. Since the dissolved Pt particles do not adversely affect the human body, ozone-water that originates from electrolysis with Pt electrodes can be used directly for sterilization and cleansing. However, the electrolyzed water, since it contains particles, cannot be applied to the wash-processing of semiconductors and electronic components. In addition, platinum itself serves as a catalyst for ozone decomposition, probably leading to low current efficiency.

545

24.1.3. Merits of diamond electrode usage Diamond is a promising electrode m a t e r i a l for electrochemical ozone generation because of its mechanical durability and chemical inertness.

Diamond electrodes are suitable for w a t e r electrolysis

u n d e r quite high c u r r e n t conditions, with no need for concern regarding dissolution. Electrolytic w a t e r decomposition reactions proceed as follows2H2O -> O2 + 4H+ + 4e

Eo = +1.23 V

(l)

3H2O -> O3 + 6H^ + 6e

EO = +1.51 V

(2)

H2O + 0 2 ^ 0 3 + 2H+ + 2e

EO = +2.07 V

(3)

Thermodynamically, t h e oxygen evolution reaction 1 is preferred. Conversely,

because

the

diamond

electrode

presents

a

large

overpotential for oxygen evolution as a parasitic reaction, ozone generation would occur more efficiently. The continuous evolution of bubbles accompanying electrolysis conventional

can

lead

to

electrode

considerable

mechanical

water

damage

m a t e r i a l s . Mechanical durability

is

to an

additional r e q u i r e m e n t for t h e ozone-generating anode, which is sufficiently satisfied by diamond electrodes. The application of diamond electrodes to ozone generation h a s already been reported, for work in which a conventional onec o m p a r t m e n t electrolytic cell w a s used with sulfuric acid solutions as the electrolyte [5,6]. Recently, a n ozone generation system with a diamond electrode set in a thin-layer electrolytic cell w a s developed; in t h i s cell, t h e anode a n d cathode lie in parallel, the electrolyte solutions flow between them, and the electrogenerated ozone gas is collected [5]. For the sake of developing a simpler system for direct ozone-

546

24. Direct Ozone-Water Generation by Electrolysis: Novel Application of Self-Standing Diamond Electrodes

water generation with the use of an electrolytic cell with a polymer membrane, a diamond electrode formed on a mesh or a porous substrate can be reasonably applied [8].

However, diamond

deposition onto complex substrates is a quite difficult task. Such areas as curved surfaces, hole interiors, and vertical edges are usually covered with either lower quality diamond or non-diamond layers (i.e., diamond-like carbon or graphite). The thinner areas of the diamond film tend to contain pinholes, leading to the erosion of the substrate after long-term usage under high current conditions.

24.2. New Forms of Diamond Electrodes for Ozone Generation 24.2.1. Self-standing diamond electrodes Ordinarily, by means of hot filament-assisted and plasma-assisted chemical vapor deposition (CVD) methods, diamond electrodes are synthesized as thin films on various substrates, including titanium, niobium, tungsten, graphite and silicon, and used as deposited. We can obtain diamond electrodes with dimensions up to 500 x 1000 mm at relatively high deposition rates [9]. However, the formation of ideal, uniform diamond layers on complex substrates is very difficult, as referred to above. To avoid problems arising from pinholes and film non-uniformities, nonsupported, i.e., free-standing, boron-doped diamond electrodes can be an ideal solution for electrolysis under high power conditions. Because the electrode itself consists of diamond

only,

the

mechanical strength and chemical inertness are considerably superior. Commercially, we can obtain electrodes up to 140 mm in

547

diameter, with thickness greater t h a n 0.4 mm. The

electrical

resistivity of t h e electrode is sufficiently low, from 0.04 up to 0.1 Q cm.

Various types of geometric modifications, including cuts,

excavations and perforations of the self-standing diamond plates are easily performed

by m e a n s of laser or

electric-discharge

machining. A photograph and a scanning electron microscopic (SEM) image of a self-standing diamond electrode (purchased from E l e m e n t Six, UK) are shown in Fig. 24.2. The electrode, with dimensions of 15 x 50 X 0.8 mm, w a s cut from a 140-mm diameter diamond plate.

(b)

Fig. 24.2. (a) Photograph and (b) SEM image of the as-grown side of a self-standing diamond electrode. The electrode dimensions were 15 x 50 mm X 0.8 mm. Brilliant crystals of diamond are recognized at a glance on the photograph, whose average size is ca. 100 ^im, judging from the S E M image.

A R a m a n spectrum obtained from the

flat

side

(originally adjacent to the s u b s t r a t e surface. Fig. 24.3(a)) exhibits a peak

centered

at

1330

cm i characteristic

of

the

diamond

crystalline structure, accompanying with a p e a k around 1550 cm i due to diamond-like carbon. 548

Ideal R a m a n spectra for diamond

24. Direct Ozone-Water Generation by Electrolysis: Novel Application of Self-Standing Diamond Electrodes

electrodes were obtained not only on t h e as-grown surface (Fig. 24.3(b)) b u t also on the laser-cut faces (Fig. 24.3(c)). Interestingly, the

laser b e a m

treatment

had

no discernable

effect

on

the

crystallinity of the diamond sp3 s t r u c t u r e .

12000

1200

1400

1600 •1

Raman shift I cm'

Fig. 24.3. Raman spectra for a self-standing boron-doped diamond electrode. The spectra were obtained at (a) the side attached to the substrate surface, (b) the as-grown surface and (c) the laser-cut face.

24.2.2. Self-standing diamond electrodes with holes The perforation of t h e self-standing diamond electrodes w i t h m a n y holes w a s performed by m e a n s of laser b e a m processing. The basic p a t t e r n of the hole a r r a n g e m e n t w a s properly chosen so t h a t the electrode s t r e n g t h w a s m a i n t a i n e d d u r i n g the processing. Fig. 24.4 shows a representative self-standing perforated diamond electrode, for which the hole d i a m e t e r w a s 1 m m and the interval between holes w a s 2 mm. 549

Fig. 24.4. Photograph of the as-grown side of a self-standing perforated diamond electrode. The electrode dimensions were 15 x 50 mm x 0.8 mm. The hole diameter was 1 mm, and the interval between holes was 2 mm.

24.3. Direct Ozone-Water Generation with Selfstanding Perforated Diamond Electrodes 24.3.1. Electrolysis of ultrapure water For

water

electrolysis,

the

self-standing

perforated

diamond

electrode w a s used as the anode, being set as shown in Fig. 24.1 (a). P l a t i n u m mesh (55 mesh, Nilaco Co., J a p a n ) w a s used as the cathode. Nafion* films (DuPont, USA) were used a s the solid-state polymer electrolyte m e m b r a n e to s e p a r a t e the anode and cathode c o m p a r t m e n t s , to which t h e anode and cathode adhered firmly and uniformly. Millipore

Ultrapure Japan,

water

Ltd.)

was

(purified

by

continuously

c o m p a r t m e n t at a flow r a t e of 0.1 L min^. performed

by

the

constant

current

a Milli-Q supplied

system,

into

each

The electrolysis w a s method.

The

ozone

concentration w a s checked with a n ozone m e t e r (03-2Z, K a s a h a r a Chemical I n s t r u m e n t s Co., J a p a n ) . The electrolyzed ozone-water, whose concentration w a s up to

550

24. Direct Ozone-Water Generation by Electrolysis: Novel Application of Self-Standing Diamond Electrodes

ca. 3 p p m at a n applied current of 0.5 A, w a s produced continuously. As shown in Fig. 24.5, the production r a t e of ozone dissolved in w a t e r w a s not linear t h r o u g h the origin with respect to t h e applied current. The c u r r e n t efficiency improved at larger applied currents, where ozone generation t h r o u g h reaction 2 is favored. In addition, the successive reactions, 1 and 3, occur simultaneously at the high cell voltage, resulting in efficient ozone generation.

0.1

0.2

0.3

0.4

Applied Current I PK

Fig. 24.5. Plots of ozone production rate and current efficiency as a function of apphed current. Pure water was suppUed to the electrolytic cell at a rate of 0.1 L min'i.

24.3.2. Durability test of diamond electrodes Tap w a t e r w a s directly introduced into the electrolytic cell with t h e self-standing perforated diamond electrode. W a t e r electrolysis w a s performed for a full day, a n d the ozone concentration w a s checked with t h e ozone meter. The flow rate of t a p w a t e r w a s m a i n t a i n e d between 3.0 to 3.2 L min'i, which was checked continuously with a flow meter.

551

Fig. 24.6 r e p r e s e n t s the r e s u l t s of the durability test of this electrolytic

ozone-water

electrode.

Even

generation

though

data

system

points

with

are

the

not

diamond

represented

continuously in the figure, the w a t e r electrolysis w a s performed continuously.

The concentrations of ozone-water were ca. 1.0, ca.

1.8 a n d ca. 3.0 p p m a t applied c u r r e n t s of 6, 10 a n d respectively.

15 A,

Practically usable electrolyzed ozone-water

with

sufficient concentration a n d volume w a s continuously produced with this system.

200

300

Operation Time / hr

Fig. 24.6. Plots of ozone production rate and current efficiency as a function of operation time. Tap water was supplied to the electrolytic cell at a rate of 3.0-3.2 L min'i. The apphed current values were 6 A (•), 1 0 A ( A ) a n d l 5 A ( B ) . After a long-term durability test, over 500 hours, S E M images of the electrode surface were measured, which were compared with the ones obtained before the durability test.

Surface topography

and the hole d i a m e t e r hardly changed before

and after

the

electrolysis (Fig. 24.7 (a) a n d (c)). However, t h e edges of the holes a p p e a r to be slightly shaved off a n d rounded (Figure 24.7 (b) and 552

24. Direct Ozone-Water Diamond Electrodes

Generation

by Electrolysis:

Novel Application

of

Self-Standing

(d)). The laser processing probably gives rise to superficial stress on the hole edge, fi^om which microcrystallites are vulnerable to detachment. Although about 10 hours of operation brought about the rounding of the edges, the morphology remained unchanged thereafter.

The electrode structure should be improved in the

future to protect against hole edge damage. (a)

(c)

A0£'mk

(d)

Fig. 24.7. SEM images of the self-standing perforated diamond electrode measured before (a and b) and after (c and d) the long-term water electrolysis. Enlarged micrographs of the hole edge (b and d) were taken form an oblique angle.

24.4. Conclusions and Future Development The application of self-standing perforated diamond electrodes to an ozone generation system is a very promising technique.

The

steady production of ozone-water and the excellent electrode durability, in spite of the considerable applied current, are

553

attractive aspects in terms of developing an ideally usable electrolytic ozonizer with low maintenance.

Fig. 24.8 shows the

world's first prototype of an ozone-water generator with the selfstanding perforated diamond electrode.

Fig. 24.8. Photograph of an ozone-water generator with a self-standing perforated diamond electrode.

References 1.

J. Weiss, Trans. Faraday Soc, 31 (1935) 668.

2.

P. C. Poller and C. W. Tobias, J. Electrochem. Soc, 129 (1982) 506.

3.

S. Stucki, G. Theis, R. Kotz, H. Devantay and H. J. Christen, J. Electrochem. Soc, 132 (1985) 367.

4.

P. Tatapudi and J. M. Fenton, J. Electrochem. Soc, 141 (1994) 1174.

5. 554

N. Katuki, S. Wakita, Y. Nishiki, T. Shimamune, Y. Akiba and M.

24. Direct Ozone-Water Generation by Electrolysis: Novel Application of Self-Standing Diamond Electrodes

lida, Jpn. J. AppL Phys., 36 (1997) L260. 6.

N. Katuki, E. Takahashi, M. Toyoda, T. Kurosu, M. lida, S. Wakita, Y. Nishiki and T. Shimamune, J. Electrochem.

Soc, 145

(1998) 2358. 7.

S.-G Park, T. Ohsaka, Y. Einaga, A. Fujishima, Abstract International

Mini Symposium

on Diamond

of 7th

Electrochemistry,

(2004)18. 8.

M. lida, Y. Nishiki, T. Shimamune, S. Ogata, M. Tanaka, S. Wakita, S. Takahashi, t«'P5/^e72/^ 5, 900, 127(1999).

9.

N. Katuki, S. Wakita, Y. Nishiki, T. Shimamune, Y. Akiba and M. lida, Jpn. J. Appl. Phys., 36 (1997) L260.

10.

I Troster, M. Fryda, D. Herrmann, L. Schafer, W. Hanni, A. Perret, M. Blaschke, A. Kraff and M. Stadelmann, Mater,

DiamondRelat.

11(2002)640.

555

25. Fundamental and Applied Aspects of Diamond Electrodes Akira Fujishima, Yasuaki Einaga, Tata N. Rao and Donald A. Tryk

25.1. Introduction Diamond

electrochemistry

outstanding

is advancing rapidly

electrochemical

properties,

which

due to its have

been

described in the preceding chapters. Boron-doped diamond has found its place as an electrode material in various fields, including, bioanalytical, environmental, and synthetic chemistry, which we will review here briefly. However, the fiindamental research on diamond electrochemistry has not progressed as rapidly as the applications-oriented

research. Although various

applications

based on the unique properties have been realized, the reasons reported to justify these properties are not completely convincing. There are several fijndamental issues to be resolved in order to expand the possible applications of these electrodes. In the following sections, some of the investigations that have been aimed at gaining an understanding of the unique properties of these electrodes are summarized.

Akira Fujishima e-mail: [email protected] 556

25. Fundamental and Applied Aspects of Diamond Electrodes

25.2. Wide Working Potential Window A clean poly crystalline diamond film with negligible sp^ carbon impurities can exhibit an electrochemical potential window as large as 2.5 V, covering wide regions on both the negative and positive potential sides in aqueous

solutions due to high

overpotentials for the oxygen and hydrogen evolution reactions [l]. The high overpotentials for oxygen and hydrogen are not clearly understood yet, although some reasonable explanations have been suggested [2]. This topic has also already been treated to some extent in Chapter 3. One of the main difficulties in attaining a complete understanding is due to the presence of sp^ carbon impurities on the diamond film. Previous studies on diamond electrodes suggested that the presence of significant amounts of sp2 carbon could drastically narrow the potential window [1,3] It has also been pointed out that other factors, such as the boron doping level (see Chapter 5), the crystalline order (see Chapter 8) and the surface termination, e.g., oxygen termination produced via anodic treatment, can also have a marked effect on the width of the potential window [4,5]. Both of these effects are presumbably due to an effect of the number of charge carriers in the nearsurface region of the diamond electrode. A high quality, heavily boron-doped, as-deposited diamond electrode was shown to exhibit a potential window from -1.25 V to + 2.3 V vs. SHE, with a very low exchange current density for the hydrogen evolution reaction, on the order of lO'io A cm'^, which is 10 to 100 times smaller than those for Ti and Nb, and half of that for basal plane HOPG [l].

557

The slow kinetics for hydrogen evolution at the diamond electrode can be explained based on the lack of adsorption sites on the

hydrogen-terminated

diamond

surface

for the

reaction

intermediate. However, the surface is not completely inert to adsorption, as

hydrogen evolution is still possible, with weak

interaction, as is evident from the fact that hydrogen evolution can be observed at a potential of -1.25 V, which is much lower than the thermodynamic value (-2.11 V) for direct production of atomic hydrogen [6]. This suggests that the weak adsorption of hydrogen is the main reason for the slow kinetics of hydrogen evolution on the diamond surface. Weak adsorption of hydrogen on Hg is known to be the reason for the high overpotential for hydrogen evolution at this electrode [7]. It is known from the volcano plot (M-H bond energy vs. exchange current density) for hydrogen evolution that the stronger is the M-H bond, the weaker is the hydrogen atom adsorption. As the C-H bond energy is approximately 81 kcal, which is greater than the Ti'H or Nb-H bond energies, diamond shows higher overpotentials for hydrogen evolution. Anderson and Kang used ab initio methods to calculate the characteristics of proton reduction on a diamond-like cluster [6]. With recent advances in calculation methods, it has been possible to increase the sizes of clusters used in calculations, so that they become increasingly realistic. treated

this

topic using

Recently, Ohwaki et al. have

density

functional

theory

(DFT),

comparing the diamond and graphite surfaces and have found definite differences in the potential dependences of the overlap in

558

25. Fundamental and Applied Aspects of Diamond Electrodes

orbitals of the electrode surface and a solution-phase proton, comparing these two surfaces [2]. There are also correlations with the above considerations and the results for other redox-active species, for example, halogen evolution, and other species that undergo multiple electron transfers, possibly also involving chemical reactions interspersed between the electron transfers. Even in the case of simple electron transfers, in general, diamond shows sluggish kinetics for inner-sphere electron transfer reactions [8,9]. LevyClement has treated the effect of boron doping level on the kinetics for various redox couples in Chapter 5.

25. 3. Low Double Layer Capacitance Low background current within the double layer region is another unique property of the diamond electrode; a clean, high quality (with negligible sp^ carbon content), highly boron-doped, asdeposited, poly crystalline film can exhibit a capacitance as low as 3 ^iF cm"2, which is about one order of magnitude lower than that usually observed at clean glassy carbon electrodes but nearly the same as that for highly ordered pyrolytic graphite (HOPG) [lO]. On the other hand, the capacitance of a high quality single crystal-like homoepitaxial surface, particularly the (lOO) surface, can be even lower, as shown in Chapter 8. Pleskov has treated the question of space charge capacitance for semiconducting diamond in Chapter 4.

Generally speaking, for the highly doped films,

there are several hypotheses that have been proposed to explain the low background currents observed for these electrodes

559

The first hypothesis is that diamond behaves analogously to HOPG, in the sense that the low density of electronic states (DOS) at the Fermi level is responsible for the low capacitance and resulting low background current at the diamond electrode [ll](see also Chapters 4 and 5). This idea is reasonable for semiconducting (lightly boron-doped) diamond, but it is not clear as to how high in doping level it may be extended; in particular, there has been discussion as to whether it applies precisely to highly boron-doped (B/C, ca. 0.01) diamond; however, we should keep in mind the fact that, even for highly doped material, the DOS is still at least an order of magnitude lower than that for a metal such as gold. Another important consideration to take note of at this stage is that poly crystalline diamond films consist of various crystallites with different crystal orientations. It has recently

become

understood

from

Raman

[12]

and

electroluminescence studies [13] that the surface structure of polycrystalline diamond film is inhomogeneous in terms of conductivity, as a surface such as (lOO) is doped to a lesser extent and is thus less conducting than the ( i l l ) surface. Due to this inhomogeneity, the conducting ( i l l ) faces appear to be embedded in

a

semiconducting

microelectrode array.

matrix,

behaving

somewhat

like

a

This is also somewhat similar to the

chlorofluorocarbon resin-graphite ("Kel'graf) composite electrode [13], which contains well-dispersed graphitic particles in an insulating matrix, giving rise to microelectrode array behavior. This

effect

is

also

involved

in

the

polycrystalline diamond thin film electrodes.

560

low

capacitance

of

25. Fundamental and Applied Aspects of Diamond Electrodes

0

0.2

0.4

0.6

Potential A^ vs SCE Fig. 25. 1. Cyclic voltammogram for 1 mM K4FeCN6 in water (without supporting electrolyte) at a highly doped, as-deposited polycrystalline diamond electrode; potential sweep rate, 10 mV s"i.

If the Kel-graf type behavior is a reliable model for the polycrystalline

diamond

electrode,

it

should

operate

as

a

microelectrode a r r a y . Although t h e r e is no experimental report available, Rao et al. [15] have recently found interesting evidence for this. They have carried out cyclic voltammetric experiments using

as-deposited

diamond

electrodes

(usually

hydrogen-

t e r m i n a t e d ) in w a t e r containing only 1 m M K4Fe(CN)6 , i.e., without a supporting electrolyte. A resulting

voltammogram,

shown in Fig. 25.1, is well defined, with a p e a k separation (AEp) of 130 mV. Although this value is relatively high for a reversible couple, it is far less t h a n expected for a p l a n a r electrode in a poorly conducting medium. Only a microelectrode is expected to produce a reasonable v o l t a m m o g r a m in such a m e d i u m .

The

absence of a sigmoidal shape (expected for a microelectrode) in Fig. 1 indicates t h a t t h e diamond electrode acts as a n a r r a y with very

561

closely spaced microelectrodes, for which the diffusion profiles of t h e individual elements overlap a n d result in a voltammogram expected for a p l a n a r electrode. Although these r e s u l t s

are

preliminary, they provide evidence for t h e expected Kel-graf-type electrode behavior. F u r t h e r studies in t h i s direction are necessary to justify this conclusion.

-0.2

0

0.2

0.4

0.6

Potential (V vs. SCE) Fig. 25. 2. Cyclic voltammograms for serotonin in phosphate buffer (pH 7) at (A) as-deposited diamond and (B) an anodically oxidized (+1.8 V vs. SCE, 10 min) diamond electrode.

Another reason for t h e low background c u r r e n t h a s been suggested to be t h e hydrogen t e r m i n a t i o n of t h e diamond surface, which does not contain surface carbon-oxygen functional groups. For example, t h e etching of as-deposited diamond

562

(hydrogen

25. Fundamental and Applied Aspects of Diamond Electrodes

terminated) with an oxygen plasma for a short period (l min) causes an increase in the double layer capacitance from 13 to 238 \iF cm^, indicating the possibility of the role of oxygen groups produced on the surface [ l l ] . However, a mild electrochemical anodic oxidative treatment,

which

essentially

converts

the

termination completely to oxygen, similar to the oxygen plasma treatment, did not show any notable effect on the voltammetric behavior (compare the flat double layer region in Fig. 25.2), indicating that the oxygen-containing groups do not influence the double layer capacitance to a great extent. The reason for the drastic increase in the double layer capacitance in the case of oxygen plasma treatment [16] may be surface damage at a microscale (not observed in SEM), which increases the defect density, especially in the semiconducting crystals. This idea is also supported by work of Kondo et al., described in Chapter 6, which shows XPS evidence for the formation of surface graphite as a result of oxygen plasma treatment.

It is also possible that the

difference in the type of treatment may introduce different types of oxygen functional groups, which contribute to the observed differences. Whatever the reason, the low double layer capacitance of diamond makes it very attractive for electrochemical sensor applications. Diamond exhibits background currents that are typically one order of magnitude lower than those of metal electrodes and several orders of magnitude lower than those for glassy carbon electrodes. Occasionally, it is possible to obtain low background currents even with glassy carbon (GC) with careful

surface

563

treatment.

However,

long-term

operation

causes

drastic

fluctuations of the background current.

140 ; 120 1

-

inn L <

L

80 _-

GC-20Tokai ^^ ~

qi; S

*

"^Sj:-^^^^

^

^^^ ^"^-A^^^

GC-GLtokai^A

40 -

U

0

-

100

200

300

400

500

600

700

Time/ min Fig. 25. 3. Current vs. time profiles for diamond and commercial glassy carbon electrodes (GC-20, GCGL) in flow injection analysis. In analytical applications such as high performance

liquid

chromatography (HPLC), what is important is the stability and reproducibility of the background response.

The interesting

aspect of diamond is that it attains low, stable values of background current very quickly after the application of the operational potential for measurements, while GC requires a relatively long time to attain a stable response (Fig. 25. 3) [17]. Furthermore, even after this initial period, the GC surface is prone

to

contamination

(adsorption

of

impurities)

electrochemical corrosion (at high operating potentials) [18].

564

and

25. Fundamental and Applied Aspects of Diamond Electrodes

25. 4. Inertness to Adsorption The electrochemical response of diamond, unlike that for other (sp2-based) carbon electrodes, is usually stable from weeks to months. This is due to the unique nature of the diamond surface, which contains closely packed sp^ carbon atoms with well ordered surface-terminated functional groups. There is a tremendous amount

of experimental

evidence

for

the

role of

surface

termination in controlling adsorption properties [19]. Hydrogen termination has been shown to be responsible for the absence of adsorption

of

polar

anthraquinone-2,6-disulfonate

(AQDS)

molecules, which tend to adsorb strongly on GC and defective HOPG. It is has been well documented in the literature that the oxygen functional groups on GC or defective HOPG are important for AQDS adsorption. As such, one might expect strong adsorption of AQDS surprisingly,

on

the even

oxygenated

diamond

oxygen-terminated

surface.

However,

(anodically

treated)

diamond shows a negligible tendency for adsorption [20]. This suggests that, even though oxygen-containing groups are present on the diamond surface, their close proximity, due to the closely packed carbon atoms in the diamond structure, causes the formation of a negative polar field on the surface, which blocks the partially charged carbon atoms from interacting with the AQDS molecules.

Furthermore,

the

negative

polar

surface

electrostatically repels the AQDS molecules, as shown in Fig. 4. This property makes diamond more interesting for

sensor

applications. For example, while hydrogen-terminated diamond acts as an excellent electrode for the detection of negatively

565

charged

DNA

[21], the

oxygen-terminated

electrode

works

exceptionally well for the detection of the positively charged oxidized form of glutathione [22], due to the operation of strong electrostatic interactions. AQDS

•^

o

J

^ 1^ ^

o

-^

o

u^ X o ''•

Anodized diamond surface Fig. 25. 4. Schematic diagram showing the electrostatic repulsions at the surface. Although the electrostatic interactions can be beneficial in improving the selectivity and sensitivity for the detection of certain types of chemical species, these may be a nuisance in other cases. For example, the diamond electrode requires cleaning after a few measurements, by applying a high anodic potential (~3 V vs. SCE), to remove adsorbed organic layers in the analysis of compounds such as phenols. Application of such a high potential also introduces oxygen on the surface, which causes variations in analytical response for negatively charged molecules such as DNA. In such cases, it is desirable to arrest these interactions. One simple way to arrest such electrostatic interaction is to increase

'5(^6

25. Fundamental

and Applied Aspects of Diamond

Electrodes

the supporting electrolyte concentration, which facilitates the masking of the negative surface charge of oxygen-terminated diamond by increasing the number of positive counter-ions in the solution, resulting in a relatively neutral surface. This has been demonstrated in the case of DNA analysis.

0

0.2

0.4

0.6

0.8

1

1.2

O-terminated BDD

1.4

1.6

3M

0.4 1

u 0.3

1I

0.2

1 \y / \

0.1

0.1 M / 0

•.^••••l. y i y , ^ , ^ . Ill ^1.^1,

0

0.2

0.4

1 1 1 1 |l 1 |>^<«f * ; ' • • " — "•'",

0.6

0.8

1

1.2

, , 1 . ,

1.4

,-

1.6

Ionic Strength /M Fig. 25. 5. Background-subtracted square-wave voltammograms for ss-DNA at two different concentrations of acetate buffer solution (pH 5).

567

Fig. 25.5 shows the effect of ionic strength of the supporting electrolyte on the voltammetric behavior for ss'DNA at hydrogen terminated (A) and oxygen terminated (B) diamond electrodes. For the hydrogen-terminated surface, being relatively neutral, the electrostatic interactions at the surface are minimal; hence the ionic strength does not affect the voltammogram to a great extent. However, the repulsive interactions operating at the oxygenterminated diamond surface (Fig. 25.5) give rise to a suppressed voltammogram at low supporting electrolyte concentrations, while at high concentrations, the masking of the surface charge by the increased number of positive counter-ions in the solution makes the surface, as well as the DNA molecule, neutral and arrests the electrostatic repulsions, resulting in an improved voltammogram, with high peak current.

25. 5. Dimensional Stability Diamond is well known for its chemical stability, due to its rigid crystalline network. However, it cannot withstand high-energy ions, which cause damage to diamond in terms of graphitization or etching. While graphitization of diamond is known to occur during ion-implantation [23], the etching process takes place during the exposure to an oxygen plasma [24], by combustion of the carbon to CO2. The term electrochemical dimensional stability is used for conditions that are very different from those for etching or implantation and refers to the stability of the electrode during the application of high current densities in an electrochemical process such as electrosynthesis or the electrochemical treatment of 568

25. Fundamental and Applied Aspects of Diamond Electrodes

wastewater.

In the case of electrochemical treatment, the

electrode is usually subjected to high anodic potentials, which for conventional materials would result in the oxidation of the electrode itself or the corrosion of the electrode due to an electrode reaction with the electrochemically produced oxidants during anodic discharge. Diamond, due to its rigid crystalline network, is expected to be more stable than existing dimensionally stable anodes (DSAs).

There

are

several reports on the

stability

and

electrochemical treatment applications of diamond electrodes. Swain and coworkers have studied the morphological structural stability of diamond electrodes in both acidic and basic media [25,26]. The diamond films in their study were found to be dimensionally stable in both media, even at a current density of 0.5 A cm^ for 20 h. However, when low quality diamond films, with significant sp^ impurities, were used, this harsh treatment resulted in the formation of pits at the grain boundaries. Ramesham and Rose [27] demonstrated the high corrosion resistance of diamond films in comparison to molybdenum, as well as noble metals such as Pt and Au. While diamond, no doubt, is a promising dimensionally stable anode material, challenges still remain. One notable point is that large area (for high current densities) diamond films are required for treatment applications. However, it is challenging to produce diamond films with large areas while maintaining high quality. CONDIAS, a German company, is now producing hot filament CVD-grown films on metal mesh with areas as high as 0.5 m2, which has generated hope in terms of industrial

569

applications. However, efforts are still under way to improve the quality of these films by reducing the graphitic impurities. A challenging task now is to make these films as thin as possible in order to reduce the material costs, while maintaining the high diamond quality.

26. 6. Applications We will now go on to summarize some of the applications that have already been treated in the preceding chapters. One of the most apparent of these has been in electroanalytical chemistry. In Chapter 3, this topic has been introduced, and it is shown graphically how the response of a boron-doped electrode (BDD) enables one to carry out the analysis of electroplating bath additives via linear sweep voltammetry, while for a Pt electrode, it does not, due to the high catalytic activity of the latter for reactions such as water oxidation (O2 evolution). Another aspect of analytical chemistry that is introduced in Chapter 3 is the effect of the gaseous atmosphere on the surface conductivity. This effect can be used to sense either the oxidizing or reducing character of a gas mixture or the acid-base character. Although this effect has received much attention in the past several years, it remains somewhat controversial in terms of the mechanisms involved.

It appears, however, that there is some

transfer of charge from the near-surface region of the diamond to a surface adsorbate, which could be molecular oxygen, for example, with the formation of a surface peroxide, and the resulting holes

570

25. Fundamental and Applied Aspects of Diamond Electrodes

that are left in the diamond actually contribute to an increased carrier concentration, with increased surface conductivity. Many of the chapters in this book have been devoted to various

aspects

of electroanalytical

chemistry

at

diamond

electrodes, for example, the preparation and characterization methods for BDD (Chapter 2), the question of the large potential working range (Chapter 3 for aqueous solution, and Chapter 6 for a wide variety of non-aqueous solutions), the influence of the boron doping levels on the kinetics for redox reactions (Chapters 4 and

5), the characterization

and influence

of the

surface

termination (Chapter 7 for hydrogen termination and Chapter 10 for oxygen termination), chemically modified surfaces in general (Chapter 9), metal-modified and ultrasmooth surfaces (Chapter 11),

and

the

use

of

microelectrodes

(Chapter

18)

and

nanostructured diamond electrode surfaces (Chapter 19). The analytical

intervening applications,

chapters for

have

example,

dealt biological

with

specific

compounds,

including DNA (Chapter 12), neurotransmitters (with capillary electrophoresis.

Chapter

14),

sulfur-containing

compounds

(Chapter 15), trace toxic metals (Chapter 16), and a variety of industrially important compounds (Chapter 17). The second main topic is that of water purification, which is an extremely important one for diamond, because it is one of the only electrode materials that can be used continuously for many months at the high potentials necessary to produce the highly oxidizing species that can be used for water purification.

These

species can include ionic iron species, ionic manganese species and hydroxyl radicals (Chapter 20), species such as peroxodisulfate

571

and peroxocarbate, which can be electrogenerated directly from naturally occurring anions (sulfate and bicarbonate, respectively, Chapters 21 and 23), and ozone (Chapters 22 and 24). In conclusion, diamond electrochemistry has come a very long way over the past decade or so, with this area making increasingly important contributions to the overall science and technology of diamond materials. standing

of

diamond

The fundamental

electrochemistry

remains

a

underhighly

interesting, fruitful one for further investigation; the applications areas, already vibrant and reaching commercialization, are very likely to expand and diversify.

References 1.

H. B. Martin, A. Argoitia, U. Landau, A. B. Anderson and J. C. Angus, J. Electrochem. Soc, 143 (1996) L133.

2.

T. Ohwaki, T. Murai and K. Yamashia, Bull Chem. Soc. Jpn., 75 (2002) 45.

3.

J. A. Bennett, J. Wang, Y. Show and G. M. Swain, J. Electrochem. Soc, 151 (2004) E306.

4.

D. A Tryk, K. Tsunozaki, Tata N. Rao and A. Fujishima, DiamondRelat. Mater., 10 (2001) 1804.

5.

T. Kondo, K. Honda, D. A. Tryk and A. Fujishima, Electrochim. Acta, 48 (2003) 2739.

6.

A. B. Anderson and D. B. Rang, J. Phys. Chem. A, 102 (1998) 5993.

7. 572

N. Vinokur, B. Miller, Y. Avyigal and R. Kahsh,

J.

25. Fundamental and Applied Aspects of Diamond Electrodes

Electrochem. 8.

Soc, 143 (1996) L238.

M. C. Granger, M. Witek, J. Xu, J. Wang, M. Hupert, A. Hanks, M. D. Koppang, J. E. Butler, G. Lucazeau, M. Mermoux, J. W. Strojek and G. M. Swain, Anal

Chem., 72

(2000) 3793. 9.

I. Duo, P. A. Michaud, W. Haenni, A. Perret and Ch. Comninellis, Electrochem.

10.

Solid-State

Lett,

3 (2000) 325.

M. S. Hossain, D. Tryk and E. Yeager, Electrochim.

Acta, 34

(1989) 1733. 11.

G. M. Swain and R. Ramesham, Anal

Chem., 65 (1993) 345.

12.

C. Levy-Clement, N. A. Ndao, A. Katty, M. Bernard, A. Deneuville, C. Comninellis and A. Fujishima, Diamond

Relat

Mater., 12 (2003) 606. 13.

K. Honda, M. Yoshimura, T. N. Rao and A. Fujishima, J. Phys. Chem. B, 107 (2003) 1653.

14.

J. E. Vitt, D. C. Johnson, J. Appl Electrochem.,

15.

T. N. Rao, B.V. Sarada and A. Fujishima, unpublished results.

16.

K. Honda, T. N. Rao, D. A. Tryk, A. Fujishima, M. Watanabe, K. Yasui and H. Masuda, J. Electrochem.

17.

Soc, 147 (2000) 659.

T. N. Rao, B. V. Sarada, D. A. Tryk and A. Fujishima, J. Electroanal

18.

24 (1994) 107.

Chem., 491 (2000) 175.

T. A. Ivandini, B. V. Sarada, C. Terashima, T. N. Rao, D. A. Tryk, H. Ishiguro, Y. Kubota and A. Fujishima, ibid, 521 (2002) 117.

19.

J. Xu, Q. Chen and G. M. Swain, Anal

Chem., 70 (1998) 3146.

20.

T. N. Rao, T. A. Ivandini, C. Terashima, B. V. Sarada and A. Fujishima, New Diamond Front. Carbon Technol,

13 (2003)

79.

573

21.

T. A. Ivandini, B. V. Sarada, T. N. Rao and A. Fujishima, Analyst

22.

(Cambridge,

U.K.), 128 (2003) 924.

C. Terashima, T. N. Rao, B. V. Sarada and A. Fujishima,

Anal

Chem., 74 (2002) 895. 23.

C. U. Saguy, C. Cytermann, R. Brener, V. Richter, M. Shaanan and R. Kalish, Appl Phys. Lett, 61 (1995) 1194.

24.

K. Honda, M. Yoshimura, R. Uchikado, T. Kondo, T. N. Rao, D. A. Tryk, and A. Fujishima, M. Watanabe, K. Yasui, H. Masuda, Electrochim.

Acta, 47 (2002) 4373.

25.

G. M. Swain, J. Electrochem.

Soc, 141 (1994) 3382.

26.

R. DeClements and G. M. Swain, J. Electrochem.

Soa, 144

(1997) 856. 27.

R. Ramesham and M. F. Rose, Diamond (1997) 17.

574

Relat

Mater.,

6

Index anodic polarization, 33, 34, acceptor concentration, 71, 74, 75

181, 233, 234, 300-302, 469,

activation energy, 40, 86, 494

528, 529

adsorption and desorption of hydrogen, 432 advanced oxidation process, 477, 479, 485, 486 ammonia, 3, 27, 107 AMN (2-acetyl-6-methoxy naphthalene), 381, 382, 384 amperometric detector, 246, 313, 376 analysis of aromatic amines neurotransmitters, 270, 312, 313, 571

anodic stripping, 342, 344, 346, 347, 351, 358-362, 366-370 anodic treatment, 34, 162, 167, 186, 187, 198, 199, 202, 225, 303, 557 APTES, 186, 187, 190, 194, 196, 197, 200, 202-204, 231, 232, 235, 236 ascorbic acid, 122, 183, 234, 239, 242, 243, 274, 276, 281 atomic force microscopy (AFM), 152, 164, 253

b

nitroaromatic explosives, 314, 315 organophosphate nerve agents, 314, 315

bacteria, 514-516, 543 biocompatibihty, 261, 398 biorefractory 477, 478, 485, 489,

phenols, 272, 280, 281, 314, 315, 452, 566 purines, 315, 317, 318 anode (ic), anodic alumina, 123, 415, 416 anodic oxidation, 69, 122, 191, 253, 254, 296, 297, 384, 456, 467, 469, 475, 480

499, 500 biosensor, 403 boron, actual boron concentration, 152 boron doping, 12, 183, 203, 263, 557, 559, 571 boron impurity band, 87, 103, 575

Ill

graphitic, 22, 80, 82, 99, 110, 111, 198, 206, 560, 570

boron oxide, 12, 84 diborane, 12, 17, 20, 84, 101

screen-printed carbon, 314,

trimethylborate, 84, 100 trimethylboron, 84, 101

315-318 sp2 carbon, 23, 28, 30, 34, 115,

c

206, 224, 300, 327, 557,

caffeine, 268, 269, 290, 292 capacitance,

559 sp3 carbon, 123, 129, 224, 253,

reduced capacitance, 397 differential capacitance, 59, 60,

565 cathodic stripping, 365, 366, 368,

62, 67, m, 73 frequency dependence, 60, 64 double layer capacitance, 120-123, 125, 127-129, 399, 400, 405, 422, 425, 427, 441, 442, 563 Capillary Electrophoresis (CE-ED), 309, 312, 336 carbamate pesticides, 271, 272, 290

370, 386, 387 charge, charge density, 256, 406 charge transfer resistances, 441, 442, 444 charging current, 397, 400 space charge layer, 42, 103 Chemical Oxygen Demand (COD), 480-483

carbohydrates, 244

Chlorophenols, 184, 280, 281, 290

carbon

conductivity,

amorphous, 22, 23, 27, 30, 68, 88, 90, 110 carbon fiber, 1,310,313

428, 443 metallic conductivity, 80, 82,

carbon nanotube, 1, 123

87, 95, 100, 106-108, 111,

glassy carbon, 1-3, 5, 27, 28, 118,

176

184, 185, 198, 261, 262, 263,

surface conductivity, 26, 40, 41,

274, 316, 317, 321, 343, 352,

43,162,164,165,167,218,

356, 364, 377, 403, 502, 559,

254, 570, 571

563, 564 576

electrolyte conductivity, 426,

contactangle, 220, 231

Index

contaminants, 83, 152, 153, 310, 398, 477, 517 crystal,

Microwave Plasma-Assisted (MPACVD), 12, 13, 81, 82, 100, 101, 375, 399

d

crystal face, 69, 73-75, 152, 154, 156, 157, 162, 163, 197, 204 individual crystal faces, 69

decomposition, 12, 19, 101, 109, 111, 116, 120, 138, 303,

crystal plane, 346

331, 452, 459, 460, 468-471,

crystallinity 23, 92, 549

490-493, 502, 517, 518, 520,

intercrystaUine boundaries, 68, 69

545, 546

monocrystalline diamond film, 82, 98 polycrystalline film, 21, 23, 69, 74, 84, 415, 559 single crystal, 28, 33, 34, 54, 59,

detection, azide detection, 268 detection of plating additives, 30 DNA, 163, 168, 174, 177, 181,

61, 65, 68, 69, 71-74, 82, 143,

185, 186, 272, 273,

146, 149, 150, 152-155,

566-568, 571

157-162, 164, 168, 559

metal detection, 6 deuterium, 134, 136, 138, 139,

current, current efficiency, 142, 456,

143

458-460, 471-474, 481, 489,

DiaCeU, 531, 532, 536, 539

498

Diachem electrodes, 20

current mapping, 164, 165 CVD (chemical vapor deposition), 11-16, 19-23, 63, 68-70,

diamond diamond crystaUites, 21, 23, 69

73-75, 81, 83, 149, 150-152,

diamond doping level, 57, 74

218, 238, 263, 311, 312, 525,

diamond microelectrode, 312,

547, 569 combustion flames (CFCVD), 12 hot filament-assisted (HFACVD), 12, 19, 81-83, 101, 547

313, 396, 400, 401, 403, 405, 406 diamond microfiber, 313, 398, 400, 403, 405, 407-409 577

diamond-like carbon, 68, 547, 548 homoepitaxial diamond, 77, 79, 149, 153, 164, 165, 187, 206, 400 single-crystal homoepitaxial, 149, 164 HTHP diamond, 69, 70, 73

disulfides, 278, 332, 333 disodium (bis(3-sulfopropyl)) disulfide, 30, 32 DNPH, 186-187, 190, 192-195, 198-200, 228-230 DOPAC, 242, 243 DSA, 20, 526, 528, 529 durabiUty, 282, 509, 546, 551-553

e

hydrogen-terminated, 30, 37-39, 43, 70, 104, 155, 161,

E. coU, 513-515

164-167, 176, 179, 181, 183,

effect of molecular size, 439

187, 188, 191, 198-200,

electrocatalytical reduction of

202-204, 220, 235, 252, 253, 263, 273, 274, 276, 283, 289, 558, 561, 565, 568 nanostructured diamond electrode, 414, 571 nano-porous diamond, 123, 124, 431 nanohoneycomb films, 419, 420, 429, 431, 439 oxygen-terminated, 70, 161, 162, 165, 176, 191, 198, 201, 229, 252, 253, 263, 272, 273, 276, 565-568 poUshed diamond, 123, 420 sulfur-treated diamond, 52, 53, 62 discharge curve, 126-128 disinfection, 512-514, 525, 539, 540 578

oxygen, 391 electrochemical electrochemical activity, 28, 81, 103, 104, 107, 111, 112, 116, 157, 263, 408, 450 electrochemical capacitor, 207, 418, 426 electrochemical combustion, 456, 466 electrochemical detection, 158, 242, 244, 502 electrochemical process, 82, 83, 110,289,296,368,382, 458, 490, 568 electrochemical transfer doping, 40, 42 electrochemical treatment, 2, 5, 6, 479, 484, 495-498,

Index

539, 568, 569 electrode electrode deactivation, 261, 314, 451, 453, 454, 469-471 modified electrode, 6, 163, 249, 252, 263, 277, 292, 336 platinum electrode, 291, 302,

429, 450, 451, 568 end-column, 311, 312 energy densiti(es), 425, 426 environmental monitoring, 319 ESR spectra, 527 exchange current, 55, 57, 58, 557, 558

527-530 rotating disk electrode(s) (RDE), 352, 364, 365, 392 transparent electrode, 34 electron, electron gun (e-gun), 134, 135, 141, 142, 144 electron stimulated desorption (ESD), 134, 135 electron stimulated desorption (ESD):TOF-ESD, 132-144 electron transfer rate, 104, 156,

/ Fano effect, 93, 95, 103 Fermilevel, 37-41, 183, 560 Ferrocyanide, 34, 104, 161, 179, 183, 241, 242, 328, 401-403 flat-band potential, 3, 37, 38, 62, 67, 68, 70 flow-injection analysis (FIA), 245, 266, 268, 273, 281, 329, 333, 336, 337, 379, 380 formic acid, 492-494, 497, 498

157, 222, 223, 227, 256, 257

fouUng, 1, 26, 43, 184, 262, 295,

multi-electron transfer processes,

298, 314, 316, 318, 343,

405

365, 398, 451

electrooxidation phenol, 234, 235, 271, 272,

g Gaussian, 97

280-283, 314-317, 449,

Glucose, 206, 244, 248-251, 405

451-454, 468-471, 475, 517,

grain boundaries, 34, 83, 85, 90,

519 electrostatic interaction, 161, 218, 224, 227, 232, 233, 566, 568 electrosynthesis, 43, 51, 262, 375,

99, 110, 154, 156, 266, 346, 390,431,569 graphite, 1, 12, 17, 22, 28, 29, 82, 88, 89, 90, 100, 102, 110, 579

157, 185, 190, 206, 271, 288,

228, 231, 235, 290, 303,

290, 291, 295, 352, 547, 558,

449-451, 454-456, 460,

559, 560, 563

469, 470, 477, 480,

graphitic impurities, 80, 99, 110, 111,

484-490, 495, 498-500, 526-528, 531, 540, 571

570 growth sectors, 73, 84

h heat, 16, 19, 82, 136-138, 141, 142, 368, 487-490, 493, 494 Heterogeneous, 13, 104, 145, 222,

hydroxyl radical(s), 2, 5, 6, 219, 290, 303, 449, 452, 454, 460, 469, 470, 477-480, 484-490, 495, 498-500, 526-528, 531, 540, 571

256, 478, 479, 480, 486, 487,

hydroxylamine, 108

489, 490, 494-496, 499

hypochlorite, 528-536, 539, 540

hexagonal pattern of cyUndrical

I

pores, 415, 419, 421, 423,

imipramine, 270

425-428, 431, 437, 439,

impedance, 27, 120, 126, 392, 419,

441-445

421, 423-428, 439-445

histamine, 265, 266, 296

impedance measurement, 186,

HOMO, 116-119

204, 419, 421, 423, 426,

Homogeneous, 13, 101, 156, 157,

439

329, 431, 486-490, 493-496, 499 HPLC, 269, 271, 331, 564 hydrodynamic voltammetry, 379 hydrogen evolution reaction (HER), 132 hydrogen peroxide (H202), 467, 485-488, 499, 526, 527, 540 hydrophobicity 167, 168, 235, 375 hydroxyl, 34, 163, 175-177, 182, 186, 189, 191, 194, 196-198, 219, 580

impedance plots, 423, 424, 427, 440-443, 445 complex plane representation, 423 hnear impedance, 64 nonlinear impedance, 64 Warburg impedance, 441, 442 in vivo, 183,261, 396 industrial, 13, 83, 280, 316, 389, 450, 451, 526, 540, 569 inner-sphere

Index

inner-sphere reaction, 54

472-474, 478, 485, 500,

outer-sphere mechanism, 103

508, 517

outer-sphere reaction, 54 instantaneous current efficiency,

coefficient, 457, 472, 474 mass transport Umitations, 435, 478, 485, 500

472-474 Insulating(semiconducting)/metaUic transition, 94

mass transport-controlled kinetics, 442

intercalation, 33, 124-127, 206 intermediate, 14, 59, 106, 110, 132, 133, 323, 354, 382, 451, 452, 455, 467, 468, 470, 520, 558 ion implantation, 3, 174, 206, 244, 245, 250, 252 IR drop, 127, 239, 397

k,l Koutecky-Levich (K-L) equation,

metal metal-modffied, 244, 250, 252, 274, 405, 571 cadmium, 342, 343, 344, 369 copper, 30, 206, 244, 343, 361-363, 399 gold, 136, 167, 181, 184, 207, 288, 322, 352, 355, 356, 358, 364, 367, 376, 377, 390-392, 397, 456, 466,

392 laser wavelength, 89, 90 Legionella, 526, 531-540

560 lead, 1, 26, 36, 70, 83, 152, 159,

Li+ battery 126

161, 183, 189, 207,

Umiting current, 241, 242, 379, 392,

342-347, 350, 351,

402, 403, 481, 490, 494, 495

361-365, 369, 377, 387,

Lorentzian peak, 98

388, 393, 451, 461,

low-pressure synthesis method, 11

544-546

LUMO, 116, 117, 120

m Marcus theory, 156

manganese, 461, 463, 571 mercury 6, 288, 290, 298, 322, 327, 336, 399

masking agent, 362

metal detection, 6, 344, 363

mass transport, 239, 353, 364, 368,

metal nanoparticles-

397, 404, 435, 437, 442, 445,

Au nanoparticle 581

Pt nanoparticles, 408, 429, 431, 435, 438

NiOOH, 386 Nitrite, 108, 109, 302

nickel, 206, 243-246, 250 number of exposed surface Pt atoms, 408

nitrogen, 3, 30, 109, 110, 194, 230, 250, 302 non-aqueous electrolyte, 115, 117,

Pt active area, 406

119-123, 125, 126, 129,

Pt oxide formation, 432

376, 398

o

silver, 157, 288, 364, 366, 399,

off-axis angle, 151

487, 503 microchannel plates (MCPs), 136

onset potential, 68, 468

microchipCE, 309, 311

overpotential, 6, 27, 40, 107, 298,

microelectrode(s), 238, 239, 241-243, 313, 562, 571 microarray(s), 397 microband(s), 397 microdisk(s), 396-399, 401 microdisk array (MDA) electrode(s), 238, 239

450, 456, 458, 464, 466, 479, 503, 504, 522, 546, 558 oxaUc acid, 274, 283, 416, 468,469 oxidant(s), 2, 5, 6, 249, 290, 450, 451, 456, 461, 463, 484-486, 488-500, 502,

molecular volume, 120

512, 513, 515, 522,

Mott—Schottky plot, 62-65, 67, 68,

525-528, 530, 532-540

70, 71, 73 multi-electron transfer processes, 405

2-propanol oxidation, 436, 437 ethanol oxidation, 436-438,

n NADH, 263-265 NanoUthography, 150, 164, 167 Naproxen

441, 444 methanol oxidation, 435, 438, 439, 441, 443, 444 oxidation mechanism

[(S)-6-methoxy-a-methyl-2-na

ofCo(II),292

phthalene acetic acid], 381

ofL-cysteine, 296, 328

Ni(0H)2, 386 582

oxidation,

of nitrite, 302

Index

oxidize, 2, 205, 244, 253, 290, 303,

ozone, 6, 467, 485-489, 499, 502,

470, 477-479, 485, 488, 490,

503-523, 526-528, 540,

494, 498-500

543-547, 550-554

oxygen

direct ozone-water generation,

active oxygen, 526, 527 oxygen evolution, 2, 6, 28, 30, 31,

543, 546, 550 ozone generation, 502-508, 510,

69, 132, 134, 143-145, 153,

511, 522, 523, 544-547, 551,

191, 207, 266, 271, 272, 289,

553

291-293, 300, 458, 466, 469, 473, 479, 481, 495, 503, 504, 522, 546 oxygen evolution reaction, 134,

ozone-water, 543-545, 550, 552-554 ozone-water generator, 543, 554

143, 191, 266, 271, 291, 405, 473 oxygen plasma, 30, 33, 38, 123, 187, 188, 190, 204, 219, 220-225, 228-230, 233, 234, 276, 415, 417, 419, 422, 563, 568 oxygen reduction, 364, 415, 417, 419, 422 oxygen-containing surface groups carbonyl group, 175, 193, 194, 198-200, 228, 229, 231 carboxyl group, 226-228, 234 hydroxyl group, 34, 163, 175-177,

P parasitic phases, 90-92, 110 peak current ratios, 437, 438 penetration depth, 67, 126, 425, 439, 441, 444 Peroxides, 180, 478, 484, 485, 527, 530 peroxo-carbonate, 530, 537, 540 peroxodisulfate, 456, 458-461, 485-495, 497-500, 571 phase element, 60, 62 phenoHc compounds, 281, 468, 469, 471 phonon of diamond, 90

182, 186, 189, 196, 197, 228,

phonon-electron couphng, 93

231, 232, 235

photochemical, 174, 175, 179-181, 185, 478, 502 photocurrent, 66-68 583

photoelectrochemistry, 3, 26, 66

rf-GDOES, 252

photographic waste effluents, 381

roughness factor, 126, 154, 390,

photopotential, 66, 68

399, 400, 405, 408, 419,

polyalkylene glycol, 30, 32, 33

421, 425, 441

s

porous alumina membranes, 415 Porphyrin, 363

saUcyUc acid, 451, 455, 496, 497

preparation

scanning electron microscopy

benzoquinone, 235, 451-453

(SEM), 21, 152, 345, 419,

nicotinic acid, 451, 454, 455

429

protoscope, 133-135, 137, 141, 143,

semiconductor, 543

145 r radical, 6, 14, 118, 122, 133, 143, 175,

p-type semiconductor, 11, 54, 59, 82, 133

180, 181, 185, 219, 249, 298,

sensor, 26-28, 30, 31, 43, 261-265,

323, 324, 382, 450, 469, 477,

304, 403, 563, 565

479, 487, 526-528, 531, 540

serotonin, 159, 160, 265, 270, 296,

Raman spectroscopy Raman scattering, 22, 88, 89, 93, 96 Raman spectra(um), 81, 88, 92,

562 signal-to-background, 268, 291, 311, 314, 322 sihcon substrate, 19, 85, 238-240

94, 101, 246, 247, 253, 400,

SIMS, 84, 85, 88, 99, 152

510, 511, 548, 549

size-selective electrocatalytic

Randies equivalent circuit, 441, 442

properties, 429 sodium thiosulphate (Hypo),

redox Fe(CN)63-/4-, 57, 72, 74, 200, 203 reduction hydrogen reduction, 107 nitrate reduction, 106, 108, 109 resistivity, 18, 35, 36, 55, 57, 82, 85-87, 221, 548 584

self-ordering, 416

375-378 space charge layer, 42, 103 spherical diffusion, 242, 402, 408, 409 standard addition method, 298, 357

Index

steady state response, 397, 404

218-222, 228, 232, 234,

step-flow growth, 151

236, 301, 311, 398

sulfa drugs Captopril, 335-337

surface passivation, 316 surface termination, 3, 6, 26,

Sulfadiazine, 333, 334

161, 164, 168, 198, 256,

Sulfamerazine, 333

263, 283, 557, 565, 571

Sulfamethazine, 333 Tiopronin, 335-337 sulfur-containing compounds

hydrogen-terminated, 104, 155, 161, 164-167, 176, 179, 181, 183, 187, 188,

2-Mercapto ethanesulfonic acid

191, 198-200, 202-204,

cephalexin, 323-325

263, 273, 274, 276, 283,

glutathione, 278, 296, 323, 325,

289, 345, 558, 561, 565,

331-333, 566 homocysteine, 296, 322-325, 329-331 oxidized glutathione, 278, 332

568 oxygen-terminated, 161, 162, 165, 176, 191, 198, 201, 263, 272, 273, 276 , 565

t

sulfide, 322, 331, 518, 521 super capacitor, 124 surface

template materials, 415 template synthesis, 414

smoothed surface, 251, 253, 256 surface functional group, 157, 163, 167, 168, 176, 186, 189, 190, 204, 218, 226 surface modification, 150, 157, 161, 164, 185, 187, 197, 204, 219, 244, 251, 327 surface chemical modification, 163

termination hydrogen termination, 28, 29, 175, 176, 562, 565, 571 oxygen termination, 176, 204-205, 256, 283, 557, 571 theophyUine, 268, 269, 290-292 Thiols, 278, 279, 333 topographic image, 165

surface orientation, 146

Total Organic Carbon, 453, 489

surface oxidation, 162, 192, 203,

transfer coefficient, 58, 59, 68, 474 585

transmission line model, 424

52-54, 75, 116, 177, 125,

triton, 369

153, 198, 241, 243, 248,

tyrosinase, 235

264, 266, 274, 275, 277-279, 325, 326, 328, 329, 401, 403, 406, 409,

uric acid, 158, 183, 274, 276, 277,

421, 432-437, 461, 462,

281, 283 utilization, 296, 304, 483 voltammetry(ic)

464, 465, 468, 470 linear sweep voltammogram,

anodic voltammetry

158, 191

ofaniHne, 294, 295, 303 ofCo(II), 293 ofL-cysteine, 296, 328

cooling water, 526

of xanthines, 296

ozone-water, 543-545, 550,

cathodic striping voltammetry cycUc voltammetry, 104, 186, 204,

552-554 wastewater, 2, 6, 13, 110, 111,

268, 287, 304, 351, 354, 397,

450, 456, 466, 467, 471,

419, 422, 432, 433

478-480, 482, 484-491,

differential pulse voltammetry (DPV), 382 linear sweep voltammetry, 348, 362, 364, 404 voltammetric curves, 55, 74, 351, 401, 457, 470 voltammetric determination ofL-cysteine, 296, 328 of nitrite, 302 of xanthines, 292 square wave voltammetry voltammogram cycUc voltammogram, 31, 32, 586

water

494, 495, 498-501, 504, 516, 569 water disinfection, 525, 540 water electrolysis, 2, 28, 29, 31, 525, 536, 539, 544, 546, 550-553 water quaUty, 532 water treatment, 375, 501, 503, 511, 525, 526, 540 XPS, 137, 139, 163, 186-190, 192-194, 196, 200, 204, 225-227, 230, 232, 247, 249, 253, 254, 563

Recommend Documents