Nanocrystalline TiO2 for Photocatalysis

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Encyclopedia of Nanoscience and Nanotechnology

www.aspbs.com/enn

Nanocrystalline TiO2 for Photocatalysis Hubert Gnaser, Bernd Huber, Christiane Ziegler Universität Kaiserslautern, Kaiserslautern, Germany

CONTENTS 1. Introduction 2. Electronic and Charge-Transfer Processes in Photocatalysis 3. Preparation of Nanostructured Materials and Thin Films 4. Structural Properties of Nanocrystalline TiO2 Films 5. Electrical Properties of Nanocrystalline TiO2 Films 6. Photocatalytic Properties of Nanocrystalline TiO2 7. Photocatalytic Applications of Nanocrystalline TiO2 Glossary References

1. INTRODUCTION The development of novel materials and the assessment of their potential application constitutes a major fraction of today’s scientiﬁc reasearch efforts. In fact, there exist various major governmental research and development programs related to nanostructured materials. Furthermore, it is estimated that nanotechnology has grown into a multibillion dollar industry and may become the most dominant single technology of the twenty-ﬁrst century. To allow for this fact, this encyclopedia [1] encompasses a series of contributions devoted to a very prominent ﬁeld of current materials research activities, namely, nanoscience and nanotechnology. The importance of these developments is reﬂected also in a number of recent books and articles reviewing this rapidly evolving ﬁeld [2–10]. This article focuses on a speciﬁc class of such novel nano-scaled materials, nanocrystalline TiO2 , and its photocatalytic properties. The title of this article encompasses three main terms (“(photo)catalysis,” “nanocrystalline,” and “TiO2 ”) which, individually, stand for very important areas of scientiﬁc research and of, perhaps even more important, technological applications. Their synergistic combination, as

ISBN: 1-58883-062-4/$35.00 Copyright © 2004 by American Scientiﬁc Publishers All rights of reproduction in any form reserved.

indicated by the present theme, has stimulated great hopes in accomplishing thereby achievements with paramount beneﬁts for human beings and the global environment. To outline the present state of that quest is the major goal of this article. “Catalysis” is probably the most familiar of the three terms mentioned. A catalyst is incorporated in essentially everybody’s automobile, with the goal of reducing or even eliminating the engine’s toxic gaseous components by converting them into less harmful (albeit not necessarily benign) substances. As is the case in all catalytic reactions, the catalyst itself is not part of the reaction, but is expected to enhance its rate, that is, the velocity of the transformation from the original components (the “educts” in the chemist’s terminology) into the ﬁnal ones (the “products”). Hence, a catalyst is an entity that accelerates a chemical reaction without being consumed itself in the process. Without catalysts, various chemical reactions of great importance would proceed too slowly [11]. The economic signiﬁcance of catalysis is enormous. In the U.S. alone, the annual value of products manufactured with the use of catalysts is roughly in the vicinity of one trillion dollars [12]. Indeed, more than 80% of the industrial chemical processes in use nowadays rely on one or more catalytic reactions [13]. A number of those, including oil reﬁning, petrochemical processing, and the manufacturing of commodity chemicals (oleﬁns, methanol, ethylene glycol, etc.), are already well established. But many others, as will be seen in this contribution, represent challenges requiring the development of entirely new approaches. But apart from their industrial importance, catalytic phenomena effect virtually all aspects of our lives. They are crucial in many processes occurring in living things, where enzymes are the catalysts. They are important in the processing of foods and the production of medicines. The reader may have noticed that we have as yet refrained from specifying the meaning of photocatalysis; which will be one of the major topics of this article. This term refers to a catalytic process that is triggered by illuminating the system by visible light or ultraviolet irradiation. Ideally, that light ﬂux would be the sun’s radiance. Next we shall consider the meaning of “nanocrystalline.” First, it is noted that in today’s science world rather inﬂationary used, the preﬁx “nano” refers to a fraction of

Encyclopedia of Nanoscience and Nanotechnology Edited by H. S. Nalwa Volume 6: Pages (505–535)

506 one part in one billion (109 and, hence, its correct usage would require it being connected to some kind of unit (e.g., of length, time, energy, mass, etc.). In the present context (and in that of “nanotechnology”), “nano” most often relates to the dimension, that is, the size of an object. Therefore, nanocrystalline in the ensuing discussions will designate particles (of crystalline structure and, primarily, with the chemical composition of titanium dioxide) whose typical sizes are in the range of a few to several nanometers (nm), that is, of the order of the one billionth part of one meter. Obviously, these are extremely tiny objects and can be “seen” and studied only with the help of sophisticated analytical instruments like an electron microscope. At ﬁrst glance, it may appear that such tiny particles are a rather modern contrivance, but this is probably a premature conclusion. In fact, it is quite ﬁrmly established that nm-sized particles (mostly very refractory ones like corundum, diamond, or silicon carbide) are ubiquitous in the universe [14] and that they were already present at the time and the location of the formation of the solar system. This “stardust” originated from stellar outﬂows and supernova ejecta, which may have occurred eons before the gas and dust condensed into what is now the sun, the earth, and the planets. In fact, this dust has intrigued astronomers since the days of William Herschel who noted, in the 1780’s, the existence of small regions in the sky where there appeared to be a complete absence of stars [15]. These regions are most easily seen against the rich star-ﬁelds of the Milky Way. Evidence of the presolar origin of these nanocrystalline particles comes primarily from their isotopic abundance pattern [16], which deviates typically to such an extent from any other known matter that a terrestrial or solar origin is virtually impossible. (Most of these particles that have been investigated were extracted from primitive meteorites in which they were incorporated during the formation stage of the solar system; these did not experience any later modiﬁcation and, hence, preserved the presolar dust particles unaltered [17].) Only now, some billions of years later, mankind has initiated the manufacture and application of such nanocrystalline materials. Nanostructured materials with crystal sizes in the range of 5–50 nm of a variety of materials, including metals and ceramics, have been artiﬁcially synthesized by many different techniques in the past couple of years [2, 3, 5–7]. Such new ultraﬁne-grained materials have properties that are often signiﬁcantly different and greatly enhanced as compared to coarser-grained or bulk substances. These favorable changes in properties result generally from their small grain sizes, the large percentage of atoms in grain boundaries and at surfaces, the large surface-to-bulk ratio, and the interaction between individual crystallites. Since these features can be tailored to a considerable extent, during synthesis and processing, such nanophase materials are thought to have great technological potential even beyond their current applications. Let us ﬁnally turn to a brief discussion of the third term, “TiO2 ” ( i.e., titanium dioxide). TiO2 has three different crystal structures [18]: rutile, anatase, and brookite; only the former two of them are commonly used in photocatalysis. Like for many other metal oxides (also for titanium oxide) have the respective structural, optical, and electronic properties

Nanocrystalline TiO2 for Photocatalysis

been elucidated through several decades of intense scientiﬁc research (for a review see, e.g., [19]); some of them will be referred to in the course of the present overview. The feature probably most important in the present context is the fact that TiO2 is a semiconductor with a bandgap of ∼ 3.2 eV. On the other hand, TiO2 , in its nanocrystalline form, constitutes an enormously important commercial product. In fact, the world production of titanium dioxide white pigments amounts to some 4.5 million tons per annum and the global consumption may be considered a distinct economic indicator. White pigments of TiO2 have average particle sizes of around 200–300 nm, optimized for the scatter of white light, resulting, thereby, in a hiding power. Reducing the crystallite size (to ≤ 100 nm), the reﬂectance of visible light (vis) decreases and the material becomes more transparent; it is widely employed, for example, in paints, plastics, paper, or pharmaceuticals. Nanocrystalline TiO2 exhibits, in addition, a pronounced absorption of ultraviolet (UV) radiation. Because of this high UV absorption and the concurrent high transparency for visible light, TiO2 particles with a size of <100 nm have found widespread use in such diverse areas as sun cosmetics, packaging materials, or wood protection coatings. Hence, although perhaps not generally realized, TiO2 is ubiquitous in our everyday life. Apart from this wellestablished range of usage, an increasing number of catalytic applications of nanocrystalline TiO2 have emerged in recent years. At this point, it appears appropriate to return to a more general view of the present topic. Nanostructured materials have generally the potential [3] for incorporating and taking advantage of a number of size-related effects in condensed matter ranging from electronic effects (so-called “quantum size effects”) caused by spatial conﬁnement of delocalized valence electrons and altered cooperative (“many-body”) atom phenomena, like lattice vibrations or melting, to the suppression of such lattice-defect mechanisms as dislocation generation and migration in conﬁned grain sizes. The possibilities to assemble size-selected atom clusters into new materials with unique or improved properties will likely create a revolution in our ability to engineer materials with controlled optical, electronic, magnetic, mechanical, and chemical properties for many pending future technological applications. Among those nanocrystalline materials, semiconductors appear to play a pivotal role in such distinct ﬁelds as [20]: (i) heterogeneous photocatalysis; (ii) photoelectrochemistry, including electrochemical photovoltaic cells; (iii) photochemistry in zeolites, intercalated materials, thin ﬁlms, and membranes (like self-assembled structures); (iv) supramolecular photochemistry. This diversity is thought to be largely due to the fact that heterogeneously dispersed semiconductor surfaces provide both a ﬁxed environment to inﬂuence the chemical reactivity of a wide range of adsorbates and, in addition, a means to initiate light-induced redox reactivity in these weakly associated molecules [21]. Upon photoexcitation of semiconductor nanoparticles, either in solutions or ﬁxed to a suitable substrate, simultaneous oxidation and reduction reactions may

507

Nanocrystalline TiO2 for Photocatalysis

occur; molecular oxygen is often assumed to serve as the oxidizing agent. Such semiconductor-mediated redox reactions are commonly grouped under the rubric of heterogeneous photocatalysis [21]. In a heterogeneous photocatalytic system, photo-induced molecular reactions take place at the surface of the catalyst. Depending on where the initial excitation occurs, photocatalysis can be generally divided into two classes of processes [22]: In the case that the initial photoexcitation occurs in the adsorbate molecule, which then interacts with the ground state catalyst substrate, the process is referred to as a catalyzed photoreaction. On the other hand, when the initial excitation takes place in the catalyst substrate and the excited catalyst transfers an electron or energy into a ground state molecule, this process is referred to as a sensitized photoreaction [22]. Apparently, a considerable degree of synergism may be crucial when, for example, a transition metal oxide photocatalysts is combined with supramolecular spectral sensitizing ligand complexes used to harvest light and vectorially transfer photo-generated electrons and holes along selected energetic pathways. In 1972, Fujishima and Honda reported [23] the photocatalytic splitting (i.e., the simultaneous oxidizing and reducing) of water upon illumination of a TiO2 single-crystal electrode; in analogy to natural photosynthesis, they demonstrated the photoelectrolysis of water making use of a photoexcited semiconductor in what was essentially a photochemical battery. In that system, an n-type TiO2 semiconductor electrode, which was connected through an electrical load to a platinum counter electrode, was exposed to near-UV light (cf. Fig. 1). When the surface of the TiO2 electrode was illuminated with light of wavelength shorter than ∼415 nm, photocurrent was observed to ﬂow [23, 24]. Furthermore, oxygen evolution (i.e., an oxidation reaction) on the TiO2 and hydrogen evolution (a reduction) on the Pt electrode was observed. The photoexcitation of TiO2 injects electrons from its valence band into its conduction band; the electrons ﬂow through the external circuit to the Pt cathode where water molecules are reduced to hydrogen gas and the holes remain in the TiO2 anode where water molecules are oxidized to oxygen. These observations indicate that water can be decomposed by means of UV-VIS light according to the

e-

load

e-

e-

H2O

eCB

-

hν

VB

+

H2O

e- Pt TiO2

H2

O2

Figure 1. Schematic arrangement for the photosplitting of water in an electrochemical cell (in the actual setup, both electrodes are immersed in an aqueous solution and the chambers are separated by an ionically conducting porous material). When the TiO2 photoanode is irradiated with light, O2 evolves from it, whereas H2 evolves from the Pt counterelectrode, while electrons will ﬂow through an external load.

following scheme

TiO2 + 2h → 2e− + 2h+

electron-hole pair formation in TiO2

H2 O + 2h+ → 1/2O2 + 2H+

reaction at the TiO2 electrode

2H+ + 2e− → H2

reaction at the Pt electrode

H2 O + 2h → 1/2O2 + H2

overall reaction

It appears to be generally accepted that this discovery boosted extensive research efforts in the era of heterogeneous photocatalysis [21, 25–27]. These studies, often carried out in an interdisciplinary fashion with the participation of physicists, chemists, and chemical engineers, focused on issues that are of great relevance both economically as well as ecologically like energy renewal and storage [28–30], the decomposition of organic compounds in polluted air and wastewaters [31–33], chemical energy generation [34, 35], and photovoltaic devices [36, 37]. Most of these either already proven or envisaged applications are intimately linked to the extraordinary properties of nanocrystalline TiO2 . In fact, nanocrystalline metal-oxide semiconductors such as TiO2 have been applied successfully in modern technologies including solar energy conversion, gas sensors, catalysis, and photocatalysis [38–42]. Following the ﬁrst examination in 1977, using TiO2 to decompose cyanide in water [43, 44], a great deal of effort has been devoted in recent years to developing heterogeneous photocatalysts with high photocatalytic activities for their applications in solving environmental cleanup and pollution remediation problems [31, 32, 45, 46]. Photocatalytic reactions on TiO2 surfaces are very important in such environmental processes, as the oxidation of organic materials and the reduction of heavy metal ions in industrial waste waters. Apart from the utilization for water and air puriﬁcation, TiO2 photocatalysis has been shown useful for the destruction of microorganisms such as bacteria [47] and viruses [48], for the inactivation of cancer cells [46, 49], the clean-up of oil spills [50, 51], and other applications [45]; a more detailed account will be given later in this article. As mentioned, TiO2 is a semiconductor with a bandgap of 3.2 eV; when excited by light of energy equal to or exceeding that value, electrons are promoted from the valence band to the conduction band leaving positive holes in the valence band. These electrons and holes are capable of, respectively, reducing and oxidizing compounds at the TiO2 surface. If the electrons and holes do not recombine (and produce heat), they can follow various reaction pathways; it is commonly accepted that the hole is quickly converted to the hydroxyl radical (• OH) upon oxidation of surface-adsorbed water and that the hydroxyl radical is the major reactant responsible for the oxidation of organic compounds. Typical reductive and oxidative reactions could be

508

Nanocrystalline TiO2 for Photocatalysis

the following [25, 52–54] TiO2 + h → TiO2 e− + h+

electron-hole pair formation

e− + Mn+ → Mn−1+

reduction reaction

+

•

h + H2 Oads → OH + H •

+

OH + Rads → • Rads + H2 O

oxidation of adsorbed water oxidation of organic species

where Mn+ is the oxidized compound and Rads the adsorbed organic species. Because the production of the hydroxyl radical is considered the decisive step, the determination and optimization of the corresponding quantum yield in illuminated TiO2 is an important task [55, 56]. According to those concepts, TiO2 nanoparticles are expected to show a unique surface chemistry due to their larger surface area [57]. The origin of the distinct photocatalytic activities exhibited by nanoparticles of TiO2 is crucial in understanding the reaction mechanisms, for example, if they are purely due to the increased surface area or if they have to be addressed to the presence of distorted sites on the surface or to the whole lattice of the particles. In order to commercialize these treatment techniques, it is of great importance to improve the preparative methods of titania, because the photocatalytic activity and phase transition behavior of TiO2 are signiﬁcantly inﬂuenced by the preparative conditions [58–63]. As previously mentioned, these catalytic processes constitute a major issue of this work and will be outlined in the following sections. The photocatalytic activity of TiO2 is very useful not only in environmental puriﬁcation by decomposition of organic substances, but also in the materials industry such as mirrors and glasses due to its self-cleaning [64] and antifogging effects. The latter has been attributed to the photoinduced hydrophilic nature of the surface [65, 66]. A further enhancement of the photocatalytic activity could be effected by means of composite TiO2 materials; examples would be metal doping, mixing with insulating substances like SiO2 or Al2 O3 [67], and monolayer coverage by SiO2 [68]. Another prominent future application of nanocrystalline semiconductors is thought [69–71] to lie in photovoltaics, that is, the conversion of sunlight into electrical power. The limited reserves of fossil fuels and the increasing concern of global warming due to the greenhouse effect caused by the combustion of those fuels has triggered, in the last decades, widespread efforts into the development of photovoltaic devices. With an energy supply from the sun to the earth of 3 × 1024 Joule per year (about 10,000 times the global annual energy consumption), this enterprise appears all but unreasonable. In such solar cells, photon incident on a semiconductor can create electron-hole pairs, basically a result of the photoelectric effect, discovered by Becquerel already in 1839 [72]. At a junction between two different materials, this may establish an electrical potential difference across this interface. Until now, the material of choice for manufacturing such junctions has been (doped) silicon (crystalline or amorphous), with compound semiconductors also being considered more recently. While this traditional

approach clearly has room for further improvements, it may ultimately face limitations in terms of cost efﬁciency (manufacturing costs per unit of energy produced). Novel materials and fabrication schemes are therefore explored. A promising approach consists of electrochemical photovoltaic cells utilizing nanoporous semiconducting electrodes fabricated by lightly sintering nanosized TiO2 particulates, followed by spectral sensitization with an appropriate dye. Metal oxide particles with diameters of some 10 nm can be prepared as paste and spread out over a surface of ﬂuorine-doped SnO2 conducting glass to form a threedimensional network of interconnected nanoparticles. These nanostructured metal oxide layers, and in particular those constructed from anatase titanium dioxide (TiO2 , have aroused great interest because of their unprecedented properties as electrodes. They ﬁnd application in dye-sensitized solar cells, which nowadays show light-to-electricity conversion efﬁciencies of 10% [69, 73–77]. The nature of electron migration in these electrodes has been debated in past years as the experimental results and their interpretation does not converge to a generally accepted model. Instead, the exact role of electron trapping and the concomitant screening of the electric ﬁeld remain unclear. It is reported that soon after electrons are injected into the conduction band of TiO2 a large fraction of them get trapped in surface states. Migration of these electrons must then proceed with a hoppingtype process in which the electrons remain most of the time in localized states [78–83]. On the other hand, it is also reported that the injection of electrons into the conduction band shows the so-called “free-electron” absorption, which extends over a wide spectral range from the visible to the infrared [84–88]. This suggests that not all injected electrons become trapped but that a substantial fraction of them remain in the conduction band. This article is organized in the following way: Section 2 outlines the electronic and charge-transfer processes as relevant for photocatalysis, with a special emphasis towards nanocrystalline TiO2 . Since in photocatalysis and related applications the respective nanostructured materials are employed either in colloidal solutions or attached to a suitable support (e.g., as electrodes or thin ﬁlms), both of these preparation techniques are discussed (Section 3). Furthermore, various approaches for surface and thin-ﬁlm modiﬁcation are described, as well as novel deposition methods and structures. The structural and electronic properties of nanocrystalline TiO2 ﬁlms are examined in Sections 4 and 5, respectively. The photocatalytic properties of nanocrystalline TiO2 constitute the central theme of Section 6, highlighting the dependence of the photocatalytic activity on different parameters like ﬁlm structure and phase, surface morphology, electronic properties, and the effects induced by various surface modiﬁcations. Representative examples of photocatalytic applications utilizing nanocrystalline TiO2 materials are presented in Section 7. Finally, an extensive set of references is provided that should be useful for further study: although the number is substantial, no attempt was made, however, to be comprehensive; any such attempt might be bound to fail due to the rapidity with which this ﬁeld is evolving.

509

Nanocrystalline TiO2 for Photocatalysis

2. ELECTRONIC AND CHARGE-TRANSFER PROCESSES IN PHOTOCATALYSIS

hν

2.1. Electronic Excitations and Charge-Carrier Trapping A photocatalytic process is initiated by the absorption of photons by a molecule or the substrate to produce highly reactive electronically excited states. The efﬁciency is controlled by the system’s light absorption properties. Three fundamental steps are of relevance: (1) the electronic excitation of a molecule upon photon absorption, (2) the band-gap excitation of the semiconductor substrate, and (3) the interfacial electron transfer. Since a detailed account of these processes has been given in a lucid treatise by Linsebigler et al. [22], we shall brieﬂy summarize here only the more important points, referring thereby partly to that work. The probability of an electronic transition can be calculated from quantum mechanical perturbation theory and is proportional to the square of the amplitude of the radiation ﬁeld and the square of the transition dipole moment [89, 90]. The latter may be computed via the molecular wave function which, in turn, depends on the product of the electronic spatial wave function, the electronic spin wave function, and the nuclear wave function. Further arguments [89] lead to some general selection rules in terms of which electronic transitions are allowed and which might be forbidden. Typically, the excitation of a weak transition will not effectively induce a photochemical reaction, because few of the incident photons will be absorbed (low cross-section); however, an overall high reaction rate may still be possible in the case of a high quantum yield, that is, if the probability of product molecule formation per absorbed photon is high. The photochemical efﬁciency will also be determined by which deexcitation channels are dominant. In particular, the pertinent lifetimes of the involved processes are to be considered. Whereas the absorption of a photon occurs very rapidly on the order of 10−15 s, deexcitation events are much slower, favoring the decay channel which minimizes the lifetime of the excited state. The initial process for heterogeneous photocatalysis of organic and inorganic compounds by semiconductors is the generation of electron-hole pairs in the semiconductor particles. Once excitation across the bandgap has occurred, the lifetime is sufﬁcient (in the nanosecond regime [91]) for the created electron-hole pair to undergo charge transfer to adsorbed species on the semiconductor surface from solution or gas phase. Figure 2 exempliﬁes these processes. The enlarged section shows the excitation of an electron from the valence band to the conduction band initiated by light absorption with energy equal or greater than the bandgap of the semiconductor. The ﬁgure also illustrates several deexcitation pathways for the electrons and holes. The electron transfer to adsorbed species or to the solvent results from migration of electrons or holes to the surface. At the surface, the semiconductor can donate electrons to reduce an electron acceptor (often oxygen in an aerated solution), corresponding to pathway c in Figure 2; conversely, a hole can migrate to the surface where an electron from a donor species can combine with the surface hole

-

a

+

b

+

A-

-

c

CB

+

VB

hν

d

+

-

+

+

+

D+

D

A Figure 2. Schematic illustration of the photoexcitation in a semiconductor particle followed by deexcitation events. CB and VB designate the conduction and valence band, respectively. For further details see text.

oxidizing the donor (pathway d). Competing with charge transfer to adsorbed species is electron and hole recombination, occurring either in the volume (pathway b) or on the surface of the semiconductor (pathway a; in both cases heat will be released. Naturally, electron and hole recombination is detrimental to the efﬁciency of a semiconductor photocatalyst. Modiﬁcations to semiconductor surfaces, such as metal addition, dopants, or combination with materials, can be beneﬁcial in decreasing the recombination and concurrently increasing the quantum yield of the process. An efﬁcient means to retard the recombination of photoexcited electron-hole pairs may proceed via the trapping of charge carriers. The occurrence of surface and bulk irregularities resulting from the preparation process is associated with surface electron states; these may serve as charge carrier traps and can suppress the recombination of electrons and holes. The charge carriers trapped in such states are localized to a speciﬁc site on the surface or in the bulk; their population is dependent on the energy difference between the trap and the bottom of the conduction band. Experimental observations of such trapped states in TiO2 will be reported later. On the basis of laser ﬂash photolysis measurements [45, 92], the characteristic times for the individual steps occurring during heterogeneous photocatalysis on TiO2 have been derived. Whereas the primary process of charge-carrier generation is extremely fast (∼fs), charge-carrier recombination may occur on time scales of 10–100 ns. Charge-carrier trapping can be very fast (100 ps) for the (reversible) trapping of a conduction-band electron in a shallow trap, but moderately fast (∼10 ns) for a deep trap or for the surface trapping of a valence-band hole, resulting in a surface-bound hydroxyl radical. Finally, interfacial charge transfer can be slow (∼100 ns) for the oxidation of an electron donor or very slow (∼ms) for the reduction of an electron acceptor. In general, the valence-band holes are powerful oxidants (+10 to +35 V versus NHE depending on the type of semiconductor and pH), while the conduction-band electrons are good reductants (+0.5 to −1.5 V vs NHE) [93]; most organic photodegradation reactions utilize the oxidizing power of these photo-generated holes.

510

Nanocrystalline TiO2 for Photocatalysis

2.2. Band-Edge Position and Band Bending

semiconductor

The ability of a semiconductor to undergo photo-induced electron transfer to adsorbed species on its surface is governed by the band energy position of the semiconductor and the redox potentials of the adsorbate. The relevant potential of the acceptor must be below (more positive than) the conduction band potential of the semiconductor. By contrast, the potential of the donor needs to be above (more negative than) the valence-band position of the semiconductor in order to donate an electron to the vacant hole. The bandedge positions of several semiconductors are depicted in Figure 3; the internal energy scale is given both with respect to the vacuum level (left scale) and for comparison to normal hydrogen electrode (NHE) in a solution of an aqueous electrolyte at pH = 1. When a semiconductor is brought into contact with another phase (e.g., liquid, gas, or metal), the transfer of mobile charges across this junction occurs until electronic equilibrium is reached. This redistribution of charges involves the formation of a space-charge layer, that is, the charge distribution on each side of the interface differs from the bulk material (cf. Fig. 4). Depending on whether the electrons accumulate or deplete at the semiconductor side, an accumulation or depletion layer is formed, causing concurrently a shift in the electrostatic potential and a downward (or upward) bending of the bands in the semiconductor. The depletion of electrons may reach such an extent that their concentration at the surface decreases below the intrinsic level. As a consequence, the Fermi level is closer to the valence than to the conduction band; this situation is called an inversion layer, as the semiconductor is p-type at the surface and n-type in the bulk. These features are well documented [94] and will not be further discussed here. Of some interest in the present context is the situation encountered ENHE [eV]

E [eV] vacuum

0.0

-2.5 -3.0 -3.5 -4.0

SiC (n,p)

GaAsP GaP (n,p) (n,p) GaAs (n,p)

CdSe (n)

CdS (n)

WO3 SnO2 (n) (n)

TiO2 (n)

-5.0

-6.0 -6.5 -7.0 -7.5 -8.0

-1.5 -1.0

ZnO (n)

-4.5

-5.5

-2.0

-0.5

Eu2+/3+

0.0

H2/H+

0.5

[Fe(CN)6]3-/4Fe2+/Fe3+

1.0

∆E= 1.4 2.25 2.25 eV eV eV 1.7 eV

1.5 3.0 eV

2.5 eV

Ru(bipy)2+/3+

2.0

3.0 3.2 eV

3.2 eV

-8.5

+

- + - + - + + - + + + -

+

E CB

E

CB

Ec EF

Ec EF

Eref

EV

Eref

EV VB

VB

(a)

-

(b)

- +

+ -

+

-

+

+ + -

-

-

+

+ +

+

+ +

+

+ + -

-

-

E E CB

Eref

CB

Eref

Ec EF

Ec EF

EV

EV VB

VB

(c)

(d)

Figure 4. Space-charge layer of an n-type semiconductor in contact with another phase (e.g., an electrolyte or gas): (a) ﬂat band situation, (b) accumulation layer, (c) depletion layer, and (d) inversion layer.

when an n-type semiconductor like TiO2 is in contact with an electrolyte as in a photoelectrochemical cell [71]; such devices are thought to have great potential both in photovoltaics for producing electric current and as fuel cells for the generation of hydrogen via photo-cleavage of water. Because of these potential applications, the characteristics of the semiconductor-electrolyte interface have been investigated in great details [93, 95, 96]. In particular, the potential distribution within a spherical semiconductor particle could be derived [97] using a linearized Poisson–Boltzmann equation. As discussed in [69], two limiting cases are of special importance for light-induced electron transfer in semiconductor dispersions. For large particles, the total potential drop within the particle is =

kT 2e

w LD

2 (1)

Ce4+/3+

2.5 3.2 eV

electrolyte

+ - + + + - + - +

3.5 4.0

3.8 eV

Figure 3. Bandgap energies of various semiconductors in an aqueous electrolyte at pH = 1.

where w is the width of the space charge layer and LD = 0 kT /e2 Nd 05 is the Debye screening length [94], which depends on the dielctric constant, , of the material and on the number density of ionized dopants, Nd . This potential variation is identical with that of a planar Schottky depletion layer [98]. For very small semiconductor particles (with radius R) the potential drop within the particle becomes =

kT 6e

R LD

2 (2)

511

Nanocrystalline TiO2 for Photocatalysis

From the latter equation, it is found that the electrical ﬁeld in nano-sized semiconductors will usually be small and that high dopant levels are required to produce a signiﬁcant potential difference between the center and the surface. For example [69], in order to obtain a 50 meV potential drop in a nanocrystalline TiO2 particle with R = 6 nm, a concentration of 5 × 1019 cm−3 of ionized donor impurities is necessary. Undoped TiO2 nanocrystallites have a much lower carrier concentration and the band bending within the particles is therefore negligibly small.

2.3. Photo-Induced Charge-Transfer Processes on the Catalyst Surface The principle of electron and hole transfer at the photoexcited semiconductor particle has already been alluded to previously in Fig. 2. Both free and trapped charge carriers participate in these interfacial redox reactions. Due to the quantization effects, by decreasing the particle’s size, it is possible to shift the conduction band to more negative potentials and the valence band to more positive values. It was concluded [99] that the shift of the bandgap is proportional to 1/R2 , R being the particle size. Therefore, redox processes that cannot occur in bulk materials can be facilitated in quantized semiconductor particles. Figure 5 shows schematically such possible transfer reactions for an adsorbate at the surface of a semiconductor. When there are accessible energy levels in the substrate, an electron may be transferred from the donor (D) into a substrate level and then into the acceptor orbital as shown in Figure 5(a). This scheme operates in the photosensitization of semiconductor particles by dye molecules. An electron is injected from the excited state dye molecule into the semiconductor, which then reduces another adsorbate particle. Early experimental conﬁrmation [100, 101] of these processes used the reduction of methyl viologen in colloidal semiconductor systems and the water splitting process. Later, such reductive processes have been investigated for many different systems (see, e.g., [102]); some illustrative examples will be presented in Section 6. For an initial excitation on the semiconductor substrate (Fig. 5(b)), a positively charged hole is created at the bandedge of the valence band. An electron is transferred into hν CB

CB

CB

D

D*

A

A

VB

VB

D+

-

AVB

(a) CB hν VB

CB

-

+

-

e-

CB

A

A

A-

D

D

D+

VB

+

e

-

VB

(b) Figure 5. Sensitized photoreaction with (a) an initial excitation of the adsorbate, or (b) an initial excitation of the solid.

the empty acceptor orbital and, simultaneously, an electron is donated from the ﬁlled donor level to recombine with the original hole. This is a very general process for wide bandgap oxide semiconductors like TiO2 and others. For example, the oxidation of many organic substances in colloidal suspensions has been investigated [102]. The energetics of the semiconductor valence band and the oxidation potential of the redox couple inﬂuence this photocatalytic oxidation. For example, the enhancement in the efﬁciency of halide oxidation at TiO2 follows the sequence Cl− < Br− < I− , correlating with the decrease in the oxidation potential. Again, some recent examples will be presented in Section 6. The kinetic analysis [103] of electron transfer in colloidal semiconductor systems is often complex. Apart from the energetics of the conduction band of the semiconductor and the redox potential of the acceptor, factors such as the surface charges of the colloids, adsorption of the substrates, participation of surface states, and competition with charge recombination inﬂuence the rate of charge transfer at the semiconductor interface [102]. This fact is evident from the widely differing rates of experimentally observed charge transfer rates, with time scales ranging from picoseconds to milliseconds for different experimental conditions and various semiconductor systems.

2.4. Quantum-Size Effects Size quantization effects in metals or semiconductors have attracted considerable attention in the past decade [104–107]. Semiconductor nanoparticles may experience a transition in terms of electronic properties from those typical for a solid to that of a molecule, in which a complete electron delocalization has not yet occurred. These quantum-size effects arise when the Bohr radius of the ﬁrst exciton (an interacting electron-hole pair) and the semiconductor becomes comparable with or larger than that of the particle; the Bohr radius [94] rB = 40 2 /e2 m∗

(3)

depends on the dielectric constant and the effective mass m∗ of the charge carriers (electrons and holes). The latter is frequently radically different for holes and electrons and, in some cases, m∗ is more than an order of magnitude smaller than the free-electron mass me . Hence, such quantum-size effects play a role for crystallites of approximately 1–10 nm in diameter. Under these conditions, the energy levels available for the electrons and holes in the conduction and valence bands become discrete and the effective bandgap of the semiconductor is increased. To a ﬁrst approximation, the energy spacing between quantized levels is inversely proportional to the effective mass and the square of the particle diameter. A schematic energy diagram resulting from such conﬁnement effects is shown in Figure 6. Several attempts have been carried out to compute the electronic energy levels in such quantum dots [99, 108–110]. According to these concepts, the energy of the lowest excited state of a semiconductor particle with radius R is given approximately by 2 2 1 1 18e2 + (4) − ER = Eg + 2R2 m∗e m∗h R

512

Nanocrystalline TiO2 for Photocatalysis bulk semiconductor

cluster LUMO

CB Eg

shallow trap

deep trap

deep trap surface state

VB HOMO Figure 6. Schematic correlation diagram relating bulk-semiconductor electronic states to quantum crystalline states. Adapted from [110], M. G. Bawendi et al., Annu. Rev. Phys. Chem. 41, 477 (1990). © 1990, Annual Reviews.

Here Eg is the bandgap of the bulk semiconductor, the second term is the quantum energy of localization, increasing as R−2 for both electron and hole, and the third term is the Coulomb attraction [99]; whereas the Coulomb term shifts ER to smaller energy as R, the quantum conﬁnement contribution increases ER as R2 . As a result, the apparent bandgap will always increase for small enough R. But while the shift can be appreciable (∼1 eV) for small band gap materials like InSb, it might be considerably smaller (∼0.2 eV) for semiconductors with a large bandgap like TiO2 or ZnO [99]. Apart from a large effect on the optical properties (e.g., a change in color of the material due to the blue shift of the absorption), size quantization may also lead to major changes in the photocatalytic properties. While these effects have been studied in great detail for several compound semiconductor materials like CdS, ZnO, or PbS, related data for TiO2 appear to be still rather limited.

2.5. Optical Properties Semiconductors absorb light below a threshold wavelength g which is related to the bandgap energy Eg by [93] g nm = 1240/Eg eV

(5)

Within the solid, the extinction of light follows an exponential law I = I0 exp− z

(6)

where z is the penetration depth and is the reciprocal absorption length. For TiO2 , for example, = 26 × 104 cm−1 at a wavelength of 320 nm; this implies that such light is extinguished to 90% after passing a distance of 390 nm in the semiconductor. Near threshold, the value of increases with increasing photon energy. Frequently, a proportionality of the type

h = Ch − Eg n

(7)

provides a satisfactory description of the absorption where h is the photon energy. C is a constant scaling with the effective masses of the charge carriers, but is independent of the photon frequency. The exponent n has a value of 0.5 for a direct semiconductor and 2 for an indirect one [94]. Experimental data [111, 112] obtained on both anatase and rutile TiO2 thin ﬁlms indicated, however, that the actual situation might be more complex.

In colloidal solution, semiconductor particles reduce the light intensity of the incident beam both by scattering and absorption. In the absence of quantum-size effects the extinction spectrum is described by the Mie theory [113, 114]. This theoretical approach can be applied to an assembly of spherical particles if the interparticle distance is larger than the wavelength of the incident light (i.e., the particles scatter independently); if, furthermore, the particle size is much smaller than the wavelength, the energy-dependent absorption cross-section for the irradiation of a solution containing the particles can be derived [115–117]: = 9Vp

s3/2 2 c 1 + 2s 2 + 22

(8)

where = 1 + i2 is the complex, frequency-dependent dielectric constant of the semiconductor particle, s is the dielectric constant of the embedding medium (the solvent), Vp is the volume of a particle, is the frequency of the incident light, and c is the velocity of light. For the case of a dilute system of particles with the number density n, can be related to the reciprocal absorption length

= n . It follows that the imaginary part of the dielectric constant is a direct measure of the light absorption by the particles; it increases steeply near the absorption edge, that is, for close to the threshold frequency. As noted in [69], Mie’s theory has been widely employed to interpret the extinction spectra of colloidal systems [118]. For example, the brilliant ruby or yellow colors caused, respectively, by colloidal gold or silver particles are distinctly explained by this theory.

3. PREPARATION OF NANOSTRUCTURED MATERIALS AND THIN FILMS Several distinct techniques have been utilized in recent years to synthesize nanocrystalline TiO2 thin ﬁlms by chemical, electrochemical, and organized assembly methods [69, 119– 121]. A simple approach involves casting of the thin ﬁlm directly from colloidal suspensions [122]. This method of preparation is relatively simple and inexpensive compared with other existing methods such as chemical vapor deposition or molecular beam epitaxy. Preparation of nanoclusters in polymer ﬁlms and Langmuir–Blodgett ﬁlms has also been considered. The sol–gel technique has been found to be useful in developing nanostructured semiconductor membranes with either a two-dimensional or three-dimensional conﬁguration. Organic-template-mediated synthesis has been employed to develop nanoporous materials. The nanostructured materials are highly porous and can easily be surface modiﬁed with sensitizers, redox couples, and/or other nanostructured ﬁlms.

3.1. Preparation from Colloidal Suspensions Nanostructured semiconductor ﬁlms of TiO2 have been prepared frequently from colloidal suspensions [123–133]. By controlling the preparative conditions of the precursor colloids, it is possible to tailor the properties of these ﬁlms.

Nanocrystalline TiO2 for Photocatalysis

Typically, these thin TiO2 ﬁlms exhibit interesting photochromic, electrochromic, photocatalytic, and photoelectrochemical properties that are inherited from the native colloids. The synthetic procedure involves the preparation of ultrasmall particles (diameter 2–10 nm) in aqueous or ethanolic solutions by controlled hydrolysis. The colloidal suspension is coated onto a conducting glass plate (an optically transparent electrode (OTE)) and dried; ﬁnally, the ﬁlm is annealed at 200–400 C in air for some 1–2 h. The conducting surface facilitates direct electrical contact of the nanostructured thin ﬁlm. This simple approach of coating preformed colloids onto a surface and annealing generally produces an oxide ﬁlm, which is robust and exhibits an excellent stability in both acidic and alkaline media, a feature very important in several potential applications. Generally, some optimization is required for thicker ﬁlms and for higher colloidal concentrations in order to avoid cracking of the ﬁlms upon drying. Further details of the methodology of preparation can be found in [134–136]. Transmission electron micrographs of nanostructured ﬁlms prepared from colloidal suspensions show a three-dimensional network of oxide nanocrystals of particle diameter as small as a few nanometers. No signiﬁcant aggregation or sintering effects are observed during the annealing process. X-ray diffraction analysis also conﬁrms the crystallinity of these nanostructured ﬁlms. Composite ﬁlms, which in some cases may exhibit improved properties as compared to singlecomponent ﬁlms, can be manufactured by mixing two or more components prior to casting of the ﬁlm. Titania sol and gel prepared through the hydrolysis of tetrabutyl titanate in acid water solution are sensitive to ultraviolet (UV) irradiation and turn into blue color [137]. Electron spin-resonance measurement showed that the photochromism was ascribed to the presence of titanium (III) ions in the Ti-O-Ti network. The titanium (III) ions could be oxidized by the oxygen in the atmosphere, and then the sol and gel faded slowly. The absorption peaks in the optical absorption spectra of the titania gel were attributed to the transition of 3-dimensional electrons of a trivalent titanium in certain environments. The morphology of TiO2 particles affects their catalytic activity and electrical properties. In recent years, many methods for preparing TiO2 nanoparticles and thin ﬁlms have been studied [138]. TiO2 nanocrystals prepared by the sol–gel method often have fully hydroxylated surfaces and these hydroxyl groups have a strong inﬂuence on the catalytic and photocatalytic properties such as electron-transfer rates and reducing properties [139]. In order to develop photocatalysts with high activities, it is very important to prepare porous anatase nanoparticles with a high speciﬁc surface area. Furthermore, the preparation method should be simple and should be a low temperature process. Mixtures of rutile and anatase precipitates could be obtained by the hydrothermal treatment of an alcohol solution of Ti alkoxide at 573 K [140], while anatase nanoparticles were prepared by heat treatment of a H2 O–EtOH solution of TiOSO4 at 373 K [141]. Thus, the anatase and rutile particles were usually formed in a solution by conventional soft chemical synthesis methods. The preparation of monodispersed oxide particles by the “sol–gel method” enables the manufacture of oxide particles through a gel state with a regulated

513 particle growth rate [142–144]. In a recent study [145], anatase nanoparticles were prepared in a Ti-peroxy gel without the collapse of the gel during the particle formation process. The gel was made from titanium tetraisopropoxide (Ti(O iPr)4 and H2 O2 . The crystallization rate of titania gels was found to be much higher in water than in methanol or n-hexane [146]; also, the crystallite size of anatase prepared in water was larger than those in organic solvents. Processing parameters very often control the crystallite size and phase. Nonagglomerated, ultraﬁne anatase particles have been generated by hydrothermally treating sol–gel-derived hydrous oxides [147]. The degree of crystallinity and purity of the synthesized materials may affect their structural evolution during any heat treatment. TiO2 thin ﬁlms with different surface structures were prepared from alkoxide solutions containing polyethylene glycol (PEG) via the sol–gel method [148]. The larger the amount of PEG added to the precursor solution, the larger the size and number of pores produced in the resultant ﬁlms. When PEG was added to the gel, the ﬁlms decomposed completely during heat treatment. The adsorbed hydroxyl content of such porous thin ﬁlms is found to increase with increasing amount of PEG. However, the transmittance of the ﬁlms decreases due to the scattering of light by pores of larger size and a higher number in the ﬁlms. Photocatalytic degradation experiments show that methyl orange is efﬁciently decolorized in the presence of the TiO2 thin ﬁlms by exposing its aqueous solution to ultraviolet light. However, in ﬁlms deposited on soda-lime glass [149], diffusion of sodium and calcium ions from the glass into the nascent TiO2 ﬁlms was found to be detrimental to the photocatalytic activity of the resulting ﬁlms. Sodium and calcium diffusion into nascent TiO2 ﬁlms was effectively retarded by a 03 m SiO2 interfacial layer formed on the soda-lime glass [149, 150]. TiO2 thin ﬁlm photocatalysts coated onto glass plates were prepared [151] by thermal decomposition of tetraisopropyl orthotitanate with a dip-coating process using alphaterpineol as a highly viscous solvent. Two types of ligands— polyethylene glycol 600 and (ethoxyethoxy)ethanol—were added to the dip-coating solution as the stabilizer of titanium alkoxide and thin ﬁlms were obtained after calcination at 450 C for 1 h. The ﬁlm thickness obtained with a single dipping was proportional to the viscosity of the dip-coating solutions. The thin ﬁlms obtained were transparent with a thickness of 1 m. The crystal form of the ﬁlms was anatase alone. The thin ﬁlms were formed with aggregated nanosized TiO2 single crystals (7–15 nm), and the size of the TiO2 crystals became smaller for the polymer-added systems. Transparent anatase TiO2 -based multilayered photocatalytic ﬁlms synthesized via a sol–gel process on porous alumina and glass substrates showed a sponge-like microstructure and a mean crystallite dimension of ca. 8 nm [152]. Doping such ﬁlms with iron(III) impeded the photocatalytic activity. The effects of calcination on the microstructures of nanosized titanium dioxide powders prepared by vapor hydrolysis was investigated in detail [153–155]: Among the factors examined [153], large surface area and good dispersion of the powders in the reaction mixture are favorable to photoactivity. Conversely, prolonged calcination at high

514 temperatures is detrimental to photoactivity. Powders produced at higher temperatures are predominantly anatase and are generally more photoactive. The formation, structure, and photophysical properties of functional mixed ﬁlms of 5,10,15,20-tetra-4-(2-decanoic acid)phenyl porphyrin (TDPP) with TiO2 nanoparticles formed from the two-dimensional sol–gel process of tetrabutoxyltitanium (TBT) at the air/water interface was reported [156]. The composite multilayer ﬁlms were assembled by transferring the mixed monolayer onto quartz plates. The sensitization of TDPP upon TiO2 nanoparticles was conﬁrmed by the spectral changes of UV-visible absorption and ﬂuorescence of TDPP in the composite ﬁlms. Furthermore the photosensitization greatly affected the photocatalytic activity of TiO2 particles with respect to the degradation of methylene blue. Crystalline titania thin ﬁlms were obtained [157] on glass and various kinds of organic substrates at 40–70 C by deposition from aqueous solutions of titanium tetraﬂuoride. Transparent ﬁlms consisting of small anatase particles (∼20 nm) exhibited excellent adhesion to relatively hydrophilic surfaces. Uniform coatings were successfully prepared on substrates with complex shapes such as cotton and felt ﬁber. Growth rate and particle size were controlled by both the deposition conditions and the addition of an organic surfactant. Organic dyes were incorporated into the anatase ﬁlms using organic-dye dissolving solutions and a surfactant.

3.2. Chemical Precipitation Rutile-phase nano-sized TiO2 powders having a high speciﬁc surface area of 180 m2 /g were prepared by homogeneous precipitation at ambient or very low temperatures (<100 C) [158]. Ultraﬁne SnO2 TiO2 -coupled particles could be synthesized [159] by homogeneous precipitation; they were employed for photocatalytic degradation of azo dye active red X-3B in aerated solution. The results show that a very rapid and complete decolorization of the azo dye can be achieved, and the photoactivity of the coupled particles is higher than that of pure ultraﬁne TiO2 , and the optimum loading of SnO2 on TiO2 is 18.4%. The enhanced degradation rate of X-3B using coupled photocatalysts is attributed to increased charge separation in these systems.

3.3. Gas Condensation and Consolidation Another method to synthesize nanostructured materials is by way of gas condensation followed by the in-situ consolidation under high-vacuum conditions [2]. This approach can produce ultraﬁne-grained materials which may exhibit size-related effects. The basic aspects of the generation of nanometer-sized materials via gas condensation are conceptually rather simple [2]: A precursor material, either an element or a compound, is evaporated in a gas maintained at a low pressure, usually well below one atmosphere. The evaporated atoms or molecules lose energy via collisions with the gas atoms and undergo a homogeneous condensation to form atomic or molecular clusters in the highly supersaturated vicinity of the precursor source. In order to maintain small cluster sizes, by minimizing further atom/molecule accretion and cluster-cluster coalescence, the clusters once

Nanocrystalline TiO2 for Photocatalysis

nucleated must be removed from the region of high supersaturation. Since the aggregates are already entrained in the condensing gas, this is readily accomplished by setting up conditions for moving this gas. Typically, there are three fundamental rates that essentially control the formation of the clusters in the gascondensation process [160]. These are (i) the rate of supply of atoms to the region of supersaturation where condensation occurs, (ii) the rate of energy removal from the hot atoms via the condensing medium, the gas, (iii) the rate of removal of the cluster upon nucleation from the supersaturated region. The clusters that are collected via thermophoresis on the surface of a cold ﬁnger usually form very open, fractal structures. The clusters are held on the collector surface rather weakly, via Van-der-Waals forces, and are easily removed by means of a scraper. The material removed is consolidated in a compaction unit at typical pressures of 1–2 GPa; the scraping and consolidation are carried out under ultra-highvacuum conditions in-situ after the removal of the gas from the chamber, in order to maximize the cleanliness of the particle surfaces and the interfaces that are formed.

3.4. Film Deposition by Sputtering and Vacuum-Based Techniques Crystalline titanium dioxide ﬁlms are often deposited by various techniques employing vacuum conditions, using, for example, RF magnetron [161–166], DC sputtering [167–171], chemical vapor deposition [172, 173], plasma spraying [174, 175], or related techniques [176, 177]. Rapid electroplating of photocatalytically highly active TiO2 -Zn nanocomposite ﬁlms on steel was achieved [178] and the gas-phase oxidation of CH3 CHO was employed as an indicator of the photocatalytic activity. Not surprisingly, the ﬁlm and surface morphologies and the crystallinity are strongly dependent on the total and the oxygen partial pressure, the deposition rate, and the phase composition; the resulting photocatalytic properties can be modiﬁed over a wide range by those parameters. Such ﬁlms may show good uniformity of thickness over large areas, high optical transmittance (∼80%) in the visible region, and considerable mechanical durability [169]. Transmission electron microscopy was used to study [179] the structure, morphology, and orientation of thin TiO2 ﬁlms prepared by reactive magnetron sputtering on glass slides at different substrate temperatures (100 to 400 C). The microstructure and photocatalytic reactivity of TiO2 ﬁlms have been shown to be functions of deposition temperature. In the temperature range examined, all ﬁlm samples have a porous nanostructure and the dimension of particles grew with increasing deposition temperature. Films are amorphous at temperatures of 100 C and only the anatase phase forms at 200 C and above. Films deposited between 200 to 300 C show a preferred orientation, while ﬁlms at 400 C change into complete random orientation. Deposition at 250 C yields high efﬁciency in photocatalytic degradation owing to the high degree of preferred orientation and nanocrystalline/nanoporous anatase phase.

Nanocrystalline TiO2 for Photocatalysis

Another frequently employed and convenient way for the preparation of TiO2 thin ﬁlms is pulsed laser deposition [180], although this technique does not produce nanocrystalline structures.

3.5. Surface and Film Modiﬁcations The surface of as-deposited ﬁlms has frequently been modiﬁed in the quest to enhance the catalytic activity. For example, Fe [181] or Sn [182] ions have been implanted into transparent and colorless TiO2 thin ﬁlms fabricated on microscope glass slides by DC magnetron reactive sputtering using Ar and O2 as working gases. The efﬁciency of this procedure could not as yet be proven unambiguously. Apart from grain size, the presence of reactive species on the surface [183] may inﬂuence the photocatalytic activity. Implanted metal ions (V or Cr) were observed [184] located at the lattice positions of Ti4+ in TiO2 and were stabilized after calcination of the samples in an O2 ambient at around 775 K. Spectroscopic studies showed that the presence of these substitutional metal ion species are, in fact, responsible for a large shift in the absorption spectra of these catalysts toward visible light regions. Porous anatase TiO2 ﬁlms were densiﬁed by Zn+ ion implantation up to the ion penetration depth [185]. After the subsequent annealing at 800 C, the phase transformation from anatase to rutile accompanied with grain growth up to the ﬁlm thickness was observed. In addition, the phase transformation was not induced by the annealing up to 800 C with or without preceding Ar+ ion implantation. Thus, the implanted impurity Zn assisted the phase transformation. Annealing in O2 tends to reduce the rate of phase transformation and create ZnTiO3 . Optical absorption above the photon energy of 2.9 eV was increased remarkably by the Zn+ or Zn+ and O+ ion implantation and subsequent annealing and is due to the phase transformation. The presence of active species such as Ti3+ and hydoxyl on the surface of ultraﬁne TiO2 particles, prepared by a colloidal chemical approach and subjected to different heating treatments, was inferred [186] from optical absorption and photoelectron spectra; these species may enhance the photocatalytic activity of particles. Treating TiO2 powder by a hydrogen plasma resulted in a reduction of the oxide particles and electrons trapped at the oxygen-defect sites were found [187]. Also laser ablation of TiO2 photocatalysts aiming at the enhancement of the activity was reported [188]. Noble metal particles of Au, Pt, and Ir were deposited on nanostructured TiO2 ﬁlms using an electrophoretic approach [189]. The improved photoelectrochemical performance of the semiconductor-metal composite ﬁlm was attributed to the shift in the quasi-Fermi level of the composite to more negative potentials. Continuous irradiation of the composite ﬁlms over a long period causes the photocurrent to decrease as the semiconductor-metal interface undergoes chemical changes. Doping of nanostructured TiO2 both as particles and in thin ﬁlms with a variety of metals has been reported, some common examples being Pt [190, 191], Pb [192], Au [193], or others [194]. A shift of the UV-vis absorption towards longer wavelengths was observed upon Pb doping, which indicates a decrease in the bandgap of TiO2 . In TiO2 ﬁlms embedding Au nanoparticles [193], the speciﬁc resistance of

515 the ﬁlms experienced a rising phase, followed by a dramatic drop with an increasing number of Au particles. Ultraﬁne Pt particles [191] were embedded into rutile TiO2 particles by decomposing a colloidal organic-Pt complex, resulting in very narrow size distribution with a mean diameter of 3 nm. These nm-sized Pt particles were found to grow epitaxially on the TiO2 crystallites with a well-deﬁned crystallographic relationship. Doping a nano-structured TiO2 electrode sensitized with tetrasulfonated gallium phthalocyanine with tetrasulfonated zinc porphyrin (ZnTsPP) greatly enhances the photoelectric conversion at long wavelengths, with 20- and 60fold improvement of the quantum efﬁciency at 680 and 700 nm [195]. Semiconductor/metal composite nanoparticles have been synthesized [196] by chemically reducing HAuCl4 on the surface of preformed TiO2 nanoparticles. These gold-capped TiO2 particles (diameter 10–40 nm) were stable in acidic (pH 2–4) aqueous solutions. The role of the gold layer in promoting the photocatalytic charge transfer has been probed using thiocyanate oxidation at the semiconductor interface. More than 40% enhancement in the oxidation efﬁciency was found with TiO2 /Au nanoparticles capped with low concentration of the noble metal. Magnetic photocatalysts were synthesized by coating titanium dioxide particles onto colloidal magnetite and nanomagnetite particles [197]. The photoactivity of the prepared coated particles was lower than that of single-phase TiO2 and was found to decrease with an increase in the heat treatment. These observations were explained in terms of an unfavorable heterojunction between the titanium dioxide and the iron oxide core, leading to an increase in electronhole recombination. TiO2 -based powders, doped with a small amount of SiO2 , were prepared by a sol–gel method and were subsequently heated to precipitate ﬁne anatase crystals [198]. The obtained powders have large speciﬁc surface areas (∼200 m2 /g) and upon treating them chemically with aqueous NaOH, the photocatalytic property of the powders was extremely improved and the powders showed higher activity than the undoped TiO2 powders. In composite TiO2 -SiO2 thin ﬁlms prepared by a sol–gel process, the refractive index and the photocatalytic activity decrease with increasing SiO2 content in these ﬁlms [199]. Alkoxide sol–gel processing has been investigated for the synthesis of stable SiO2 -TiO2 high-permeability catalytic membranes to be used in alkene isomerization [200]. Nanocrystalline TiO2 was prepared on mesoporous silica both by sol–gel processes [201] and by an impregnation method with titanium complexes featuring different ligands [202]. Binary mixed oxide of Fe/Ti (1:1 composition) with homogeneous distribution of iron into the TiO2 has been prepared by sol–gel impregnation using metal alkoxide precursors and ﬁring at different temperatures (500, 700, and 900 C) [203]. The mixed oxide exhibits excellent absorption in the visible spectral region (570–600 nm). The photocatalytic activity of the Fe/Ti oxide reduces to a large extent at a high sintering temperature of the sample due to the presence of a increasing amount of the inactive (Fe2 /TiO5 pseudobrookite phase. Nanostructured TiO2 /SnO2 binary oxides were prepared by

516 combustion of stearic acid precursors [204], with metal precursors being dispersed in the stearic acid at the molecular level. It was found that preparative methods affected the crystalline structure of the powders and the anatase phase of TiO2 was stabilized by the addition of SnO2 . Spray “painted” (spray deposited) titanium dioxide coatings were sensitized [205] with chemically deposited cadmium selenide thin ﬁlms; the structural, optical, and photoelectrochemical characterization of these composite ﬁlms indicate the importance of thermal treatments in improving the photocurrent quantum yields. Up to 400 C, the effect of air annealing is to shift the onset of absorption to longer wavelengths and improve the photocurrent substantially. Organic compounds may play a crucial role in chemical processes for ceramic coatings [206]. Organic compounds remained in a ﬁxed position in the coating, which was prepared from the chemically modiﬁed titanium tetraisopropoxide solution and heated at temperatures as high as 673 K. It was not until the organic compounds decomposed to carbon dioxide and the gas phase was left from the coating that the nanostructure, consisting of nano-sized pores and anatase crystallites with preferred orientation, developed at 723 K. Cobalt(II) 4,4 ,4 ,4 -tetrasulfophthalocyanine, covalently linked to the surface of titanium dioxide particles, TiO2 CoTSP, was shown [207] to be an effective photocatalyst for the oxidation of sulfur (IV) to sulfur (VI) in aqueous suspensions. Upon bandgap illumination of the semiconductor, conduction-band electrons and valence-band holes are separated; the electrons are channeled to the bound CoTSP complex resulting in the reduction of dioxygen, while the holes react with adsorbed S(IV) to produce S(VI) in the form of sulfate. The photoactivity of the Pt/TiO2 system in the visible region was improved [208] by the addition of the sensitizer ([Ru(dcbpy)2 (dpq)]2+ [where dcbpy = 4,4 -dicarboxy 2,2 bipyridine and dpq = 2,3-bis-(2 -pyridyl)-quinoxaline] leading to an efﬁcient water reduction. Photocatalytic properties for hydrogen production were investigated [209] on layered titanium compounds intercalating CdS in the interlayer, which were prepared by direct cation exchange reactions and sulfurization processes. The photocatalytic activity of the compounds intercalating CdS was superior to those of simple CdS and the physical mixture of CdS and metal oxides. The improvement might be attributed to the formation of microheterojunctions between the CdS nanoparticles and the layers of oxides.

3.6. Novel Deposition Methods and Structures In recent years, there has been increased interest in studying and manufacturing nanoscaled TiO2 materials as nanoparticles [210], nanowires [211], nanorods [212], whiskers [213], and nanotubes [214–217]. There are many synthetic routes for the creation of nanocrystals of oxides and the controlled hydrolysis of metal alkoxides is the most generalized solution-phase synthetic strategy [218]. Increased photocatalytic activity was reported recently for TiO2 prepared by ultrasonic irradiation and glycothermal methods. This novel method [219] for preparing highly photoactive nanometer-sized TiO2 photocatalysts

Nanocrystalline TiO2 for Photocatalysis

with anatase and brookite phases has been developed by hydrolysis of titanium tetraisopropoxide in pure water or a 1 + 1 EtOH–H2 O solution under ultrasonic irradiation; the photocatalytic activity of TiO2 particles prepared by this method exceeded that of Degussa P25 and was the ﬁrst report that showed high photocatalytic activity of a photocatalyst containing an 80% anatase and 20% brookite phase. A novel and convenient nonhydrolytic approach to the preparation of uniform, quantum conﬁned TiO2 nanocrystals, using an intramolecular adduct stabilized alkoxide precursor, was described recently [220]. In contrast to established aqueous sol–gel-techniques, the processing in hydrocarbon solvents at high temperatures allows access to very small free-standing crystallites, and opens up new possibilities for control over size distribution, surface chemistry, and particle agglomeration. It has been reported that the columnar morphologies in sputtered TiO2 ﬁlms enhances the photocatalytic [221] and photovoltaic [222] efﬁciency. Following these studies, enhanced surface-reaction efﬁciency has been demonstrated in the photocatalysis of sculptured thin ﬁlms of TiO2 [223]. In obliquely deposited ﬁlms with variously shaped columns such as zigzag, cylinder, and helix, the columnar thickness and spacing play an important role in the enhancement of the effective surface area, while the columnar shape is less important. The optimum morphology for a surface reaction has been obtained at the deposition angle of 70 , where the photocatalytic activity is 2.5 times larger than that at 0 . The morphology of these obliquely deposited thin ﬁlms appears well suited for application as solar cells, electro- and photochromic devices, and photocatalysts. The template method for synthesizing nanostructures involves the synthesis of the desired material within the pores of a nanoporous membrane or other solid. This approach has been used in several experiments [224–229] for the preparation of TiO2 nanotubes and nanorods; typically, porous aluminum oxide (PAO) nano-templates were used. Compact, continuous, and uniform anatase nanotubules with diameters in the range 50–70 nm were produced inside PAO nano-templates by pressure impregnating the PAO pores with titanium isopropoxide and then oxidatively decomposing the reagent at 500 C [230]. Cleaning the surface of the template and repeating the process several times produced titania nanotubules with a wall thickness of ∼3 nm per impregnation. The tube exteriors appeared to be faithful replicas of the pores in which they were formed. Nano-TiO2 whiskers were prepared by various techniques [213, 231]; using, for example, controlled hydrolysis of titanium butoxide [231] it was found that the nano-TiO2 whiskers obtained were anatase and grew selectively in the [001] direction with a diameter of about 4 nm and a length of about 40 nm. Acetic acid played an important role in the oriented growth of nano-TiO2 whiskers. Highly ordered TiO2 nanowire (TN) arrays were prepared [211] in anodic alumina membranes by a sol–gel method. The TNs are single crystalline anatase phase with uniform diameters around 60 nm. At room temperature, photoluminescence measurements of the TN arrays show a visible broadband with three peaks, which are located at about 425, 465, and 525 nm that are attributed to self-trapped excitons, F , and F + centers, respectively.

517

Nanocrystalline TiO2 for Photocatalysis

4. STRUCTURAL PROPERTIES OF NANOCRYSTALLINE TiO2 FILMS 4.1. The Lattice Structure of Rutile and Anatase Three different crystal structures of TiO2 exist [18]: rutile, anatase, and brookite; only the former two of them are commonly used in photocatalysis, with anatase typically exhibiting the higher photocatalytic activity [232]. The structure of rutile and anatase can be described in terms of chains of TiO6 octahedra. The two structures differ by the distortion of each octahedron and the actual pattern of the chains. Figure 7 depicts the unit cell structures of rutile and anatase crystals [233–235]. Each Ti4+ ion is surrounded by an octahedron of six O2− ions. The octahedron in rutile shows a slightly orthorhombic distortion, whereas the respective octahedra in anatase are signiﬁcantly distorted, resulting in a symmetry that is lower than orthorhombic. The Ti-Ti distances in anatase are greater (0.379 and 0.304 nm as compared to 0.357 and 0.296 nm in rutile) while the Ti-O distances are shorter than in rutile (0.1934 and 0.1980 nm in anatase versus 0.1949 and 0.1980 nm in rutile). In the rutile structure, each octahedron is in contact with 10 neighboring ones (two sharing edge oxygen pairs and eight sharing corner oxygen atoms), whereas in the anatase crystal each octahedron is in contact with eight neighbors (four sharing an edge and four sharing a corner). These differences in lattice structure result in different mass densities ( = 4250 g/cm3 for rutile and = 3894 g/cm3 for anatase) and electronic band structures for the two forms of TiO2 . Synthetic titanium oxide crystallizes in two polymorphs: anatase and rutile. Anatase is metastable and transforms exothermally and irreversibly to rutile. Some properties of TiO2 may strongly depend on its polymorphic phase. The anatase-rutile transformation is strongly inﬂuenced by the synthesis method, atmosphere, grain growth, and impurities. Some additives, such as ZrO2 and Al2 O3 , retard the anatase-rutile transformation, whereas others, such as CoO and ZnO, accelerate such a process [236]. The anatase-torutile phase transformation of doped nanostructured titania was studied [237] using differential thermal analysis (DTA) and X-ray diffraction (XRD). The presence of Cu and Ni was found to enhance transformation as well as sintering. Titanium

90˚ 81.21˚

On the other hand, La retarded both transformation and densiﬁcation. Transmission electron microscopy (TEM) is typically used to investigate the crystal size distribution, grain-boundary disorder, and defect structure in nanocrystalline TiO2 materials prepared by the various techniques outlined in the previous section. In a recent study of ﬁlms with an average grain size of 15 nm prepared by reactive sputtering [238], evidence of both ordered and disordered grain-boundary regions was found and planar defects were observed in grain interiors identiﬁed as (011) deformation twins. Also, crystallographic shear defects can occur as a result of aggregation of oxygen vacancies in understoichiometric titanium oxide. Figure 8 shows results of TEM investigations [239] of nanocrystalline TiO2 ﬁlms prepared from colloidal suspensions; the TiO2 crystallites were nominally pure anatase phase with a size of 16 nm.

4.2. Structure of Crystalline TiO2 Surfaces The geometric structure of crystalline surfaces of TiO2 has been studied predominantly on (macroscopically) large single crystals; in particular, the (110) surface of rutile, being thermodynamically the most stable one [240], has been investigated extensively by a broad variety of surface science tools [241–243]. Among the different questions addressed therein, a prominent one was related to the possible types of oxygen defects at the surface; in fact, three distinct oxygen vacancy sites were tentatively identiﬁed: lattice, singlebridging, and double-bridging vacancies [244]. With the widespread use of scanning-tunneling microscopy (STM), atomically resolved investigations of TiO2 surfaces became feasible and a more detailed picture of the rutile (110) surface structure emerged [245–248]. Consistently, these studies corroborate the longstanding notion about the prominent importance of surface defects as the active sites for various types of chemical reactions [249–251], for example, for

Oxygen

78.12˚ 92.43˚

Figure 7. Crystal structures of rutile (left) and anatase (right) TiO2 .

Figure 8. Cross-section transmission electron microscopy image of a nanocrystalline TiO2 anatase ﬁlm. The nominal crystallite size is 16 nm.

518 the dissociation of water [252, 253]. The number of studies on the structural properties of anatase single crystals is considerably smaller, which appears to be largely due to the difﬁculties encountered in preparing such surfaces in a defect-free state. Nevertheless, a rather distinct picture of the anatase TiO2 surface structure [254–257] and its properties in terms of adsorption/desorption reactivity [258–260] has been achieved. The structure and composition of a nanocrystalline surface may have a particular importance in terms of chemical and physical properties because of their small size. For instance, nanocrystal growth and manipulation relies heavily on surface chemistry [261]. The thermodynamic phase diagrams of nanocrystals are strongly modiﬁed from those of the bulk materials by the surface energies [262]. Moreover, the electronic structure of semiconductor nanocrystals is inﬂuenced by the surface states that lie within the bandgap but are thought to be affected by the surface reconstruction process [263]. Thus, a picture of the physical properties of nanocrystals is complete only when the structure of the surface is determined. To understand and improve the applications of titaniumoxide nanoparticles, it is extremely important to perform a detailed investigation of the surface and the interior structural properties of nanocrystalline materials, such as rutile and anatase with diameter of a few nanometers. Detailed experiments using X-ray absorption near-edge structure spectroscopy (XANES) demonstrated [264] that the presence of both defects and surface states strongly inﬂuence the X-ray absorption spectra, even though the ﬁrst nearestneighbor geometrical arrangement around the central Ti atom in both rutile and anatase is quite similar: the differences in the XANES spectra arise from the outer-lying atomic shells, indicating that “medium” to “long-range” effects play an important role to the near-edge features. In another study of this kind [265], a shorter Ti-O distance for surface TiO2 , resulting from Ti-OH bonding was observed together with a minor disorder of the lattice in smaller nanoparticles. Nevertheless, the Ti sites largely remain octrahedral even in particles with diameters of 3 nm. Because the interfacial electron/hole transfer occurs via surface Ti or O atoms, the observed structural changes around the surface Ti atoms in small TiO2 particles could be responsible [265] for the unique photocatalytic properties. A qualitative analysis of opaque nanostructured glasssupported TiO2 ﬁlms was carried out [266] using scanning force microscopy (SFM), and surface parameters such as average grain diameter, roughness exponent, and fractal dimension [267] were determined. The TiO2 surfaces exhibit distinct roughness due to the large aggregates formed by the interconnected TiO2 particles. Fractal dimension was found to range between 2.10 and 2.45, depending on the scanned range and the preparation method. The surface morphology of nanocrystalline materials prepared by compacting nanometer-sized clusters was investigated by SFM [268]; these materials had a grain size of 5–15 nm and contained about 1019 interfaces per cubic centimeter. Upon heat treatment, grains were found to fuse together forming bamboolike structures and then lined up as tubular structures. The inﬂuence of the iron concentration in mixedoxide (TiO2 and Fe2 O3 thin ﬁlms prepared by reactive

Nanocrystalline TiO2 for Photocatalysis

radio-frequency sputtering on the structural properties of the layers has been studied [269]. This characterization allowed the correlation of the inhibition of the grain growth of titania to the presence of iron oxide and its segregation at grain boundaries. This behavior could be ascribed to a superﬁcial-tension phenomenon. As a possible application of these thin ﬁlms, it was observed that they were able to sense CO down to the level requested for environmental monitoring. A study [270] of the structure and morphology of a titanium dioxide photocatalyst (Degussa P25) reveals multiphasic material consisting of an amorphous state, together with the crystalline phases anatase and rutile in the approximate proportions 80/20. Transmission electron microscopy provides evidence that some individual particles are a mixture of the amorphous state with either the anatase phase or the rutile phase, and that some particles, which are mostly anatase, are covered by a thin overlayer of rutile that manifests its presence by the appearance of Moiré fringes. The photocatalytic activity of this form of titanium dioxide is reported as being greater than the activities of either of the pure crystalline phases, and an interpretation of this observation has been given in terms of the enhancement in the magnitude of the space-charge potential, which is created by contact between the different phases present and by the presence of localized electronic states from the amorphous phase.

5. ELECTRICAL PROPERTIES OF NANOCRYSTALLINE TiO2 FILMS Most studies into the carrier transport in nanocrystalline TiO2 were carried out with the ﬁlms in contact with electrolytes, mostly due to their use in highly efﬁcient electrochemical solar cells, often called “Grätzel cells” [38, 71]. In this application, the pore surface is covered with an ultrathin organic dye layer and contacted with an electrolytic solution that penetrates the pore structure. The experimental work indicates [271–274] that in this conﬁguration the electrolyte screens any electric ﬁeld within the porous structure and establishes diffusion conditions for the carrier propagation. On the other hand, investigations in which the nanopores are in contact with an insulating medium (a gas or vacuum) may allow one to obtain quantitative insight into the electronic properties of the material and the basic feature of carrier transport. In a series of measurements [275–277] on the electron transport in nanoporous TiO2 ﬁlms with gas-ﬁlled, insulating pores employing Pt/TiO2 , Schottky barrier structures indicate a barrier height of 1.7 eV, compatible with an electron afﬁnity of 3.9 eV for the TiO2 ﬁlms. Below ∼300 K, tunneling transport through the barrier occurs, resulting in barrier lowering effects. Carrier drift mobilities, recombination lifetimes, and their dependence on injection level in TiO2 are reported. It is found that the mobility-lifetime product is independent of injection level, while drift mobility and recombination lifetime change strongly with injection. A trap-ﬁlling model appears as the most plausible model compatible with the experimental ﬁndings [277]. Comparison with recent experiments on nanoporous ﬁlms in contact with electrolytes indicate that the transport and recombination mechanism is qualitatively similar for the two cases.

Nanocrystalline TiO2 for Photocatalysis

Various observations indicate that electron transport in the nanocrystalline TiO2 dominates the transient response of the system. Transient photocurrent measurements reveal a very slow (∼millisecond), multiphasic response to both continuous wave [278, 279] and pulsed [280–282] illumination. The characteristic rise or decay time of the response is dependent upon the intensity of the light source [278, 281–283]. Comparison with the transient response without electrolyte indicates that it is the TiO2 , and not the electrolyte, which is responsible for the very long tail [284, 275]. A slow and multiphasic time dependence has also been observed in the rereduction of oxidized dye molecules in a redox inactive environment [285]; the same work indicates that the rate of dye reduction is controlled by the concentration of electrons introduced into the TiO2 by externally applied bias. The wide range of time scales is consistent with the assumption that electron transport within the TiO2 is the rate limiting step. The slow processes are attributed to the trapping of electrons by a high density of localized states in the TiO2 . Since the TiO2 grains are normally crystalline [286], the localized states are believed to be concentrated at the grain boundaries and on the very large surface. There is evidence for intraband-gap states in bias-dependent optical absorbance spectra [287] and surface photovoltage spectra. It would, therefore, be extremely useful to correlate the density and nature of those states with the electronic transport properties of the material. Investigating electron migration in nanostructured anatase TiO2 ﬁlms with intensity-modulated photocurrent spectroscopy [288], it was found that, upon illumination, a fraction of the electrons accumulated in the nanostructured ﬁlm is stored in deep surface states, whereas another fraction resides in the conduction band and is free to move. These data indicate that the average concentration of the excess conduction band electrons equals about one electron per nanoparticle, irrespective of the type of electrode, the ﬁlm thickness, or the irradiation intensity. The photocurrent in thin ﬁlm TiO2 electrodes prepared by sol–gel methods was studied in [289] as a function of ﬁlm thickness. Films with thickness smaller than the space charge layer were found to show a larger photocurrent than ﬁlms with thickness larger than the space charge layer. It was concluded that the increase in photocurrent is due to the effective electron-hole separation throughout the whole ﬁlm thickness and the reduction of bulk recombination. The use of TiO2 thin-ﬁlm electrodes for photocatalytic devices might therefore be useful to gain high device performance. In a series of papers, Dittrich et al. carried out extensive investigations of the electrical conductivity in nanoporous TiO2 ﬁlms [290–296]. Studying the temperature- and oxygen partial pressure-dependent conductivity of rutile and anatase, they noted [292] that is thermally activated with EA = 085 eV, independent of the absolute value of and depends on p(O2 by a power law for p(O2 < 1–10 mbar. The electrical properties of reduced nanoporous TiO2 are determined by surface chemical reactions which lead to the formation of shallow donor and deep trap states. Furthermore, this group examined in detail the photovoltage in nanocrystalline TiO2 [293, 295] and the injection currents in these porous specimens [294, 296].

519 Such a power dependence of on the oxygen partial pressure was noted also in recent work [297] investigating electrical and defect thermodynamic properties of nanocrystalline titanium oxide. At high O2 pressures, p(O2 > 1 mbar, the conductivity is constant, whereas at values p(O2 < 10−14 mbar a steep increase of with decreasing pressure was found, following a power dependence ∝ p(O2 n with n = −1/2 [297]. The plateau of conductivity at high oxygen pressures can be interpreted as being a domain of ionic conductivity, an unexpected behavior for titanium dioxide. In a coarse-grained material, dominant hole conductivity is observed in this partial pressure range. This difference may be due to the high density of grain boundaries in nanocrystalline ceramics, which can be preferred paths for diffusion at reduced temperatures. Furthermore, an increase in ionic conductivity is also expected due to enhanced defect formation in the space charge regions adjacent to grain boundaries [298]. At low oxygen partial pressure, nanocrystalline TiO2 exhibits enhanced electronic conductivity as compared to coarse-grained TiO2 . The power exponent n = −1/2 can be explained under the assumption that doubly charged titanium interstitials are formed. The intrinsic disorder of titanium dioxide is reputedly of the cationic Frenkeltype [299–302], although alternative defect models based on Schottky disorder are also described in the literature [303, 304]. In the domain of ionic conductivity, the activation energy of conduction is ∼10 ± 01 eV [297], a value typical of migration enthalpies for ionic defects. By contrast, the activation energy in the reduction-controlled regime was found to be ∼39 ± 02 eV. In titanium oxide thin ﬁlms prepared by a d.c. sputtering technique onto glass substrates with average grain sizes of 100–200 nm, the surface structure and phase morphology of the ﬁlms was found [305] to depend on the deposition conditions. The current-voltage characteristics of these ﬁlms are ohmic for values of applied voltage lower than 0.5 V. For higher values, the mechanism of electrical conduction is determined by space-charge-limited currents [306]; then, a power-law dependence was observed with n ∼ 2.3–2.9. In much thicker Ti oxide ﬁlms (1.9–8 m) deposited by sputtering [307], both the surface roughness and the internal surface area increased with ﬁlm thickness; this resulted in an enhancement of the incident photon-to-current efﬁciency. Electrical and optical spectroscopic studies of TiO2 anatase thin ﬁlms deposited by sputtering showed [308, 309] that the metastable phase anatase differs in electronic properties from the well-known, stable phase rutile. (From the broadening of the X-ray diffraction peaks, the average grain size of the ﬁlms is estimated to be in the range of 30–40 nm [308].) Resistivity and Hall-effect measurements revealed an insulator-metal transition in a donor band in anatase thin ﬁlms with high donor concentrations. Such a transition is not observed in rutile thin ﬁlms with similar donor concentrations. This indicates a larger effective Bohr radius of donor electrons in anatase than in rutile, which in turn suggests a smaller electron effective mass in anatase. The smaller effective mass in anatase is consistent with the high-mobility, bandlike conduction observed in anatase crystals. It is also responsible for the very shallow donor energies in anatase. Luminescence of self-trapped excitons was

520 observed in anatase thin ﬁlms, which implies a strong lattice relaxation and a small exciton bandwidth in anatase. Optical absorption and photoconductivity spectra show that anatase thin ﬁlms have a wider optical absorption gap than rutile thin ﬁlms. The extrapolated optical absorption gaps of anatase and rutile ﬁlms were found to be 3.2 and 3.0 eV, respectively, at room temperature. The observation of space charge limited currents (SCLC) in nanoscaled pure and chromium-doped titania was reported [310] and both the free-charge carrier density and the trapped-charge carrier density were given. Photoconductivity was also studied in compound systems; for example, in a TiO2 -C60 bilayer system the conductivity increases signiﬁcantly in the fullerene upon irradiation at wavelengths <300 nm [311]. Although being an efﬁcient photocatalyst for the detoxiﬁcation of organically charged waste water, titanium dioxide suffers from the drawback of poor absorption properties because of a bandgap of 3.2 eV. Thus, wavelengths shorter than 400 nm are needed for light-induced generation of electron-hole pairs. That is the reason why doping with transition metal ions is interesting for inducing a reduction of the bandgap. However, this doping changes other physical properties such as lifetime of electron-hole pairs and adsorption characteristics [312].

6. PHOTOCATALYTIC PROPERTIES OF NANOCRYSTALLINE TiO2 Most experimental investigations reported a higher photocatalytic efﬁciency in the anatase TiO2 phase; as a possible reason it was suggested that the recombination of the electron-hole pairs produced by UV irradiation occurs more rapidly on the surface of the rutile phase, and the amount of reactants and hydroxides attached to this surface is smaller than on the surface of the anatase phase. The study of the photocatalytic activity of nanocrystalline TiO2 materials is a longstanding research effort [313–315]; in most lab-scale experiments it was evaluated by means of the degradation observed for typical substances (e.g., aqueous methyl orange, methylene blue, etc.) upon exposure of the specimen to UV irradiation. In such a way, the possible inﬂuence of light intensity, structural properties, surface morphology, phase and chemical composition, resulting from various deposition or preparation methods, could conveniently be explored [316–320]. Furthermore, any correlation with the optical or electrical properties of the nanocrystalline ﬁlms could thereby be investigated. In addition, alternative approaches have come under scrutiny. A new simple method for characterizing photocatalytic activity by measuring photo-generated transient charge separation at the surface of semiconductor photocatalysts was proposed [321]. In this technique, the charge separation generated by a pulse dye laser is obtained as a function of the incident laser energy; thereby, the photocatalytic activity and the type of surface reaction (reduction or oxidation) in titanium dioxide ﬁlms were rapidly determined. In the following sections some examples of such studies will be given.

Nanocrystalline TiO2 for Photocatalysis

6.1. Dependence of Photocatalytic Activity on Film Structure and Phase The photoactivities of ultraﬁne TiO2 nanoparticles in anatase, rutile, or mixed phases were tested in the photocatalytic degradation of phenol [322]. For TiO2 nanoparticles, mainly in the anatase phase and mixed-phases, the photocatalytic activities increased signiﬁcantly with the content of the amorphous part decreasing. The completely crystallized rutile nanoparticles exhibited size effects in this photocatalytic reaction and the photocatalytic activity of rutile-type TiO2 nanoparticles with a size of 7.2 nm was much higher than that with 18.5 nm or 40.8 nm and was comparable to that of anatase nanoparticles. A modiﬁed sol–gel process was used [323] to prepare nano-structured TiO2 catalysts of controlled particle size (i.e., 6, 11, 16, and 20 nm). The effect of TiO2 particle size on gas-phase photocatalytic oxidation of toluene was examined under dry and humid conditions. Main reaction products were CO2 and water, although small amounts of benzaldehyde were also detected. The smaller particle size (i.e., 6 nm) led to higher conversion and complete mineralization of toluene into CO2 and H2 O. Electronic and structural effects (i.e., size and ensemble effects) were responsible for the excellent performance of a 6 nm TiO2 catalyst for toluene photodegradation. The dependency of the photoactivity on the crystallite size of anatase titania for the decomposition of trichloroethylene (TCE) was investigated [324]. It was found that the photoactivity of all titania samples was linearly increased as the crystallite size of the anatase phase increased, regardless of the preparation method, as long as there was no signiﬁcant rutile phase. The enhancement of the photocatalytic activity of TiO2 was investigated as a function of added amount of other oxides to promote desired oxidation or reduction reactions [325]. Mixed oxides of Nb2 O5 or Li2 O with TiO2 were prepared by the sol–gel process. The target material of dichloroacetic acid (DCA) was chosen for oxidation reactions and K2 Cr2 O7 for reduction. While the Nb-oxide had a deleterious effect on the decomposition rate of DCA, the excess electrons due to the doping of Nb2 O5 into TiO2 promoted the reduction process for Cr6+ . Li2 O (1 wt%) with TiO2 was found to be the most efﬁcient photocatalyst for DCA oxidation, resulting in photocatalytic activity of 50%. A highly sensitive biochemical oxygen demand (BOD) sensor using a commercial TiO2 semiconductor and photocatalytic pretreatment was developed to evaluate low BOD levels in river waters [326]. The photocatalytic oxidation was investigated as a function of irradiation times, TiO2 concentrations, and pH. The optimal irradiation time was 4 min. The sensor response was increased with increasing pH and the responses obtained by photocatalysis to river samples were higher than those obtained without photocatalysis.

6.2. Inﬂuence of Surface Morphology and Defects Photocatalytic reduction of CO2 to organic compounds was carried out [327] in a semiconductor suspension system under simulated solar power using a TiO2 catalyst. Experimental results show that the photocatalytic activity can be

Nanocrystalline TiO2 for Photocatalysis

improved by depositing Pd or Ru on the TiO2 surface. Films of TiO2 dispersed or coated with platinum were deposited on glass and Pt-buffered polyamide substrates, respectively, by magnetron sputtering [328]. The photocatalytic activity of the ﬁlms was evaluated through the decomposition of acetic acid under UV irradiation. The Pt-dispersed TiO2 ﬁlm with approximately 1.5 wt% platinum shows a maximum activity due to the formation of anatase phase with a ﬁne grain size. Platinum particles ∼2 nm in thickness coated on anatase ﬁlm greatly improves activity. The activity shows a steplike dependence of ﬁlm thickness, where the critical thickness varies between 150 and 200 nm depending on the deposition temperatures. The correlation between defects and activity was veriﬁed by measuring either the temperature dependence of electric resistance or the shift of binding energy from X-ray photoelectron spectroscopy (XPS). In crystalline TiO2 ﬁlms deposited by reactive RF magnetron sputtering on glass substrates without additional external heating, the photocatalytic activity was evaluated by the measurement of the decomposition of methylene blue under UV irradiation [162]. The results showed that crystalline anatase, anatase/rutile, or rutile ﬁlms can be successfully deposited on unheated substrates. Anatase TiO2 ﬁlms with a more open surface, a higher surface roughness, and a larger surface area, formed at higher total pressures, exhibit the best photocatalytic activity. The photocatalytic activity of polycrystalline anatase TiO2 ﬁlms was found [329] to be affected by the crystalline orientation that depends on the deposition temperature; it was greater on the (112)oriented than on the (001)-oriented ﬁlm. The former ﬁlm exhibited a columnar structure resulting in a larger surface area for photocatalytic reaction than the ﬁlms with the (001)-preferred orientation. Furthermore, the introduction of structural defects associated with oxygen vacancies was found [169] to create some energy levels around the mid-gap, indicating that they could work as recombination centers of photo-induced holes and electrons, causing the decrease in photocatalytic activity. Therefore, the decrease in the structural defects associated with oxygen vacancies appears to be important for improving the photocatalytic activity of the ﬁlms. A marked difference of the photocatalytic activity between the TiO2 ﬁlms coated on quartz and glass substrates was conﬁrmed [330], which would be interpreted in terms of the difference in the photocarrier’s diffusion length induced by impurity Na+ ions. These results lead to a conclusion that the crystallinity and defects of TiO2 as well as the ﬁlm thickness and surface area have a great inﬂuence on the photocatalytic activity. An enhanced photocatalytic activity of TiO2 could be achieved also by deposition of the ﬁlms on sulfonated glass substrates [331] or by using special support materials [332, 333]. Photo-oxidative self-cleaning and antifogging effects of transparent titanium dioxide ﬁlms has attracted considerable attention for the past decade [334, 335]. In order to understand the photo-induced hydrophilic conversion on titanium dioxide coatings in details, it is inevitably necessary to understand the relationship between the photo reaction and the surface crystal structure; this can be done, for example, by an evaluation of the photo-induced hydrophilic conversion on the different crystal faces of rutile single crystals and also

521 polycrystalline anatase titanium dioxide [336]. Self-cleaning and antifogging effects of TiO2 ﬁlms prepared by magnetron sputtering were investigated [161, 163] in terms of the photocatalytic behavior by measuring the decomposition of methylene blue and the reduction of the contact angle between water and TiO2 under ultraviolet irradiation. The phase conversion from the rutile to the anatase TiO2 ﬁlm leads to an enhancement of the activity; the anatase ﬁlms with the best photocatalytic behavior are prepared at higher total pressures (>1.50 Pa) and characterized by a high decomposition efﬁciency, a contact angle about 10 after irradiation, and a good stability in darkness. Titanium dioxide thin ﬁlms prepared with various surface morphologies by metalorganic chemical vapor deposition were found to exhibit reversible wettability control by light irradiation [337]. These TiO2 surfaces became highly hydrophilic by UV irradiation, and returned to the initial relatively hydrophobic state by visible-light (VIS) irradiation. The hydrophobic-hydrophilic conversion induced by UV light was ascribed to the increase in dissociated water adsorption on the ﬁlm surface. By contrast, the conversion from hydrophilic to hydrophobic by VIS irradiation was caused by the elimination of water adsorbed on the surface due to the heat generated. Changes of the water contact angle between hydrophilic states and hydrophobic ones strongly depended on the roughness of the ﬁlm surface. The self-cleaning property of thin transparent TiO2 coatings on glass under solar UV light was studied [338] for two compounds: palmitic (hexadecanoic) acid and ﬂuoranthene, which are both present in the atmospheric solid particles. The removal rates of layers of these compounds sprayed on the self-cleaning glass were found to be sufﬁcient for the expected application. The identiﬁed intermediates (about 40 for each compound) show the gradual splitting of the palmitic acid chain and the oxidative openings of the aromatic rings of ﬂuoranthene. In the case of palmitic acid, the products give some indications about the photocatalytic mechanism. About 20% of the organic carbon contained in the initial compounds was transformed into volatile carbonyl products. An extreme photo-induced hydrophilicity was achieved [339, 340] when TiO2 ﬁlms were covered by SiO2 overlayers (with 10–20 nm in thickness). These multilayer ﬁlms exhibited much more extreme hydrophilicity than a TiO2 ﬁlm without overlayer. The surface analyses revealed that the enhanced photo-induced hydrophilic surface of the multilayer ﬁlms exhibited an improved photocatalytic activity towards decomposition of organic substances on their surfaces. An extreme light-induced superhydrophilicity was also reported [341] for mesoporous TiO2 thin ﬁlms (crystallite size ∼15 nm, surface area ∼50 m2 /g, pore size ∼3.6 nm). For such ﬁlms, the water contact angle was found to be reduced essentially to zero upon UV-irradation for a duration of about 60 min. In addition, the photocatalytic activity of those ﬁlms could be enhanced by treating the substrate surfaces with an H2 SO4 solution [342]. In order to investigate the cathodic photoprotection of the steel from corrosion, stainless steel was coated with TiO2 thin ﬁlms, applied by a spray pyrolysis [343]. It was concluded that these coatings exhibit both a cathodic

522

Nanocrystalline TiO2 for Photocatalysis

photo-protection effect against corrosion and the frequently reported photocatalytic self-cleaning effect.

hν CB

-

VB

+

hν

6.3. Inﬂuence of Electronic Properties Detailed spectroscopic investigations of the processes occurring upon bandgap irradiation in colloidal aqueous TiO2 suspensions in the absence of any hole scavengers showed [344] that while electrons are trapped instantaneously, that is, within the duration of the laser ﬂash (20 ns), at least two different types of traps have to be considered for the remaining holes. Deeply trapped holes are rather long-lived and unreactive, that is, they are not transferred to the ions of model compounds for photocatalytic oxidation like dichloroacetate or thiocyanate. Shallowly trapped holes, on the other hand, are in a thermally activated equilibrium with free holes which exhibit a very high oxidation potential. The overall yield of trapped holes can be considerably increased when small platinum islands are present on the TiO2 surface, which act as efﬁcient electron scavengers competing with the undesired e− /h+ recombination. While molecular oxygen, O2 , reacts in a relatively slow process with trapped electrons, the adsorption of the model compounds on the TiO2 surface prior to the bandgap excitation appears to be a prerequisite for an efﬁcient hole scavenging.

6.4. Enhanced Photocatalytic Activity Via Surface Modiﬁcations A driving force for research in heterogeneous photocatalysis using TiO2 (and semiconductors in general) is the creation and application of systems capable of using natural sunlight to degrade a variety of organic and inorganic contaminants in wastewater or polluted air. As mentioned, the photocatalytic activity depends strongly, among other factors, on the wavelength range response. Since the bandgap of TiO2 is ∼3.2 eV, it is active only in the ultraviolet region which amounts to <10% of the overall solar intensity. Principally, there are several remedies to circumvent (at least partially) this limitation: (i) Deposition of metals on the semiconductor; (ii) using multicomponent semiconductors; (iii) surface modiﬁcation with sensitizing dyes. The merits of these options will be outlined brieﬂy in the following.

-

+

Schottky Barrier metal

semiconductor

Figure 9. Metal-modiﬁed semiconductor photocatalyst particle.

activity. The Pt/TiO2 system is probably the most frequently studied metal-semiconductor pair (see, e.g., [350, 351]); Figure 10 exempliﬁes the enhancement of the photocatalytic activity of nanocrystalline TiO2 by platinization [350]: three commercially available TiO2 -catalysts, namely, Degussa P25, Sachtleben Hombikat UV100, and Millennium TiONA PC50, were platinized by a photochemical impregnation method with two ratios of platinum deposits (0.5 and 1 wt%). The photocatalytic activities of these samples were determined using three different model compounds, EDTA, 4-chlorophenol (4-CP), and dichloroacetic acid (DCA). Platinization resulted in all cases in an enhancement of the activity; Figure 10 shows the degradation of DCA as a function of illumination time (light intensity 23 W/m2 at a wavelength of 320–400 nm) for pure TiO2 and the two platinized specimens. After 2 h of illumination, the initial concentration of 120 ppm total organic carbon (TOC) was reduced to 2.3 ppm at pH 3 employing the best photocatalyst, in this case, Hombikat UV100 with 0.5 wt% Pt. For this system, an initial photonic efﬁciency (i.e., number of degraded molecules per number of incident photons) of ∼43% was obtained [350]. Apart from platinum, an enhanced photocatalytic activity has been also noted for other metals and semiconductors. Their inﬂuence on the photocatalytic activities has been studied in detail, for example, utilizing

1.0

without Pt 0.5 wt% Pt 1 wt% Pt

0.8

TOC/TOC0

6.4.1. Deposition of Metals on the Surface The selectivity and efﬁciency of a photochemical reaction can be improved by modifying the surface with a noble metal. The deposition of metal particles on oxide surfaces has been the subject of several recent reviews [345–348] and therefore, there is, no need to duplicate it here. In terms of photocatalytic activity, a drastic enhancement has, for the ﬁrst time, been observed for the photocatalytic conversion of water into hydrogen and oxygen upon a fractional coverage of the TiO2 surface with platinum [349]. After excitation the electron migrates to the metal where it becomes trapped and electron-hole recombination is suppressed. The hole is then free to diffuse to the semiconductor surface where oxidation of organic species can occur. These processes are schematically depicted in Figure 9. The presence of the metal can be beneﬁcial also because of its own catalytic

+

0.6

0.4

0.2

0.0

0

20

40

60

80

100

120

illumination time (min) Figure 10. Degradation of dichloroacetic acid (expressed as the relative change of TOC) with platinized TiO2 in comparison to pure TiO2 as a function of illumination at pH 3. Data from [350], D. Hufschmidt et al., J. Photochem. Photobiol. A: Chem. 148, 223 (2002).

523

Nanocrystalline TiO2 for Photocatalysis

the following systems: Au-modiﬁed TiO2 [352, 353], silvermodiﬁed titanium particles [354], transition-metal doped TiO2 photocatalysts [355–357], or rare-earth-doped TiO2 nanoparticles [358]. It should be noted in this context that various analytical techniques like transmission electron microscopy or scanning force microscopy are often very useful in determining the size of the particles and their distribution in the bulk and at the surface of these nanocrystalline materials.

6.4.2. Composite Semiconductors The advantage of composite semiconductors is usually twofold: ﬁrst, to extend the photo-response by coupling a large bandgap semiconductor with another featuring a smaller one and, second, to retard the recombination of photo-generated charge carriers by injecting electrons into the lower lying conduction band of the large bandgap material. Two types of geometries have been employed: capping the nanocrystallites of one semiconductor with those of the second or bringing the nanocrystalline particles of the two materials into intimate contact. The principle of charge exchange and separation for both arrangements is illustrated in Figure 11. Let us consider the case of coupling CdS with TiO2 ; the energy of the excitation light is too small to directly excite the TiO2 particle, but it is large enough to excite an electron from the valence band across the bandgap of CdS (Eg = 25 eV) to the conduction band. According to the energetics, the hole produced in the CdS valence band remains in the CdS particle, whereas the electron transfers to the conduction band of the TiO2 particle; this increases the charge separation and efﬁciency of the photocatalytic process. The separated electrons and holes are then free to undergo electron transfer with adsorbates at the surface. While the mechanisms of charge separation in a capped system are similar, in a capped semiconductor only one of the charge carriers is accessible at the surface for catalytic reactions. Several semiconductors have been studied CB TiO2

-

CB SnO2

hν

VB

hν'

VB +

+

A A+

(a)

TiO2

CdS hν

CB -

thoroughly in combination with TiO2 : TiO2 CdS [359–361], TiO2 CdSe [362], TiO2 coupled SnO2 [363], TiO2 capped SiO2 [364], and some mixed-oxide systems like Fe2 O3 TiO2 [365] or ZrO2 TiO2 [366].

6.4.3. Surface Modiﬁcation with Sensitizing Dyes Surface sensitization of a wide bandgap semiconductor photocatalyst like TiO2 via adsorbed dyes can increase the efﬁciency of the excitation step and, as a consequence, the activity. This process can also expand the wavelength range of excitation. Some common dyes that are used as sensitizers include erythrosin B, eosin, rhodamines, cresyl violet, thionine, porphyrins, [Ru(bpy)3 ]2+ and its analogues, and many others (see, e.g., [102] for a more comprehensive list). The individual charge-transfer and excitation steps involved in the dye sensitizer surface process are exempliﬁed in Figure 12. The primary excitation of an electron in the dye molecule occurs in either the singlet or triplet excited state of the molecule; if the oxidative energy level of the excited state of the dye molecule, with respect to the conduction band energy level of the semiconductor, is more negative, then the dye molecule can transfer the electron to the conduction band of the semiconductor. The surface accepts electrons from the dye molecules which, in turn, can be transferred to reduce an organic acceptor molecule adsorbed on the surface. The dye-sensitized semiconductor can also be used in oxidative degradation of the dye molecule itself after charge transfer; this appears to be an important process in view of the large quantities of dye substances in wastewater from the textile industries and others. In passing, it can be noted that the process of utilizing subbandgap excitation with dyes that absorb strongly in the visible for photosensitization is frequently employed in color photography and other imaging science applications. This approach of light-energy conversion is also similar to plant photosynthesis, in which chlorophyll molecules act as light harvesting antenna molecules. Generally, the high porosity and strong bonding character of nanostructured TiO2 (and other) semiconductor ﬁlms facilitate the surface modiﬁcation with organic dyes or organometallic complexes. For example, photoconversion efﬁciencies in the range 10–15% in diffused daylight have been reported [367] for nanostructured TiO2 ﬁlms modiﬁed with a ruthenium complex. The charge injection from a singlet excited sensitizer into the conduction band of a large bandgap semiconductor is thought to be an ultrafast process

B -

CdS CB

S*

B-

-

CB

hν S

A

hν'

VB +

A

VB

A+

CB

S+

CB

-

A VB

S+

AVB

+

VB (b)

Figure 11. Photo-induced charge separation in composite semiconductor particles: (a) capped and (b) coupled semiconductor nanocrystallites. Photo-generated charge carriers move in opposite directions.

Figure 12. Sequence of excitation and charge-transfer steps using a dyemolecule sensitizer. In the ﬁrst step, the sensitizer (S) is excited by an incident photon of energy h and an electron is transferred into the conduction band of the semiconductor particle; the electron then can be transferred to reduce an organic acceptor molecule (A) adsorbed on the surface.

524

Nanocrystalline TiO2 for Photocatalysis

occurring at a picosecond time scale; in the case of different organic dyes, it has been shown [368, 369] that charge injection takes place within 20 ps. A similar fast electron transfer has also been noted for [Ru(H2 O)2 ]2− on a TiO2 surface at very low coverage [370]. Progress in femtosecond laser spectroscopy opened a venue for investigations on even much shorter time scales. In fact, in recent studies charge carrier injection times in the range of 20–200 fs were reported [371–375] for various dyes on nanocrystalline TiO2 particles. Signiﬁcant enhancement effects of electron acceptors (additives) such as hydrogen peroxide, ammonium persulfate, potassium bromate, and potassium peroxymonosulfate (oxone) on the TiO2 photocatalytic degradation of various organic pollutants were observed already in early investigations [376]. The results showed that these additives markedly improved the degradation rate of 2,4dichlorophenol. The enhanced photocatalytic oxidation of sulﬁde ions on phthalocyanine modiﬁed titania was ascribed [377] to the additional formation of superoxide radicals. Sensitization of wide bandgap semiconductor electrodes by dyes absorbing visible light are routinely used also in dyesensitized photoelectrochemical cells, in order to achieve high photon-to-current conversion efﬁciencies [378, 379]. The preparation and dynamics of interfacial photosensitized charge separation in metal oxides such as TiO2 ﬁlms has been reviewed [70, 71, 380]. Principally, the photophysical reactions occurring in those dye-sensitized injection solar cells, which are based on a dye adsorbed onto a porous TiO2 layer, are very similar to those relevant to photocatalysis. Because of their importance as an energy source, Figure 13 presents a schematic drawing [69] of such TCO glass E

3

dye e-

S+/ S*

TCO glass with Pt hν

2

1

4 e-

S+/ S

e-

I- / I3-

∆V

e

-

5 e-

por-TiO2 electrolyte I- / I3e-

load

e-

Figure 13. Schematic outline of a dye-sensitized photovoltaic cell, showing the electron energy levels in the different phases. The system consists of a semiconducting nanocrystalline TiO2 ﬁlm onto which a Ru-complex is adsorbed as a dye and a conductive counterelectrode, while the electrolyte contains an I− /I−3 redox couple. The cell voltage observed under illumination corresponds to the difference, V , between the quasi-Fermi level of TiO2 and the electrochemical potential of the electrolyte. S, S∗ , and S+ designate, respectively, the sensitizer, the electronically excited sensitizer, and the oxidized sensitizer. See text for details. Adapted from [69], A. Hagfeldt and M. Grätzel, Chem Rev. 95, 49 (1995). © 1995, American Chemical Society.

a dye-sensitized nanocrystalline TiO2 solar cell, depicting the relevant energy levels and the pathway for the photoexcited electrons. In this speciﬁc example, a ruthenium complex [367] was adsorbed as a dye onto the TiO2 and an I− /I− 3 redox couple was used in the electrolyte. Contrary to conventional semiconductor devices, in the dye-sensitized cells the function of light absorption is separated from the charge-carrier transport. The Ru-complex has to absorb the incident sunlight and to effect, via this energy, the electron1 and 2 in Fig. 13). Apart from transfer reaction (numbers supporting the dye, the TiO2 ﬁlm acts as an electron acceptor and electronic conductor: the electrons injected into the TiO2 conduction band drift across the nanocrystalline ﬁlm to the conducting glass support which functions as current 3 in Fig. 13). At the counterelectrode, the eleccollector ( 4 ) tron is transferred to the redox couple in the electrolyte ( 5 ). The cell which, in turn, serves to regenerate the dye ( voltage observed under illumination, V , is determined by the difference between the Fermi-level of TiO2 and the electrochemical potential of the electrolyte (cf. Fig. 13 [69]).

7. PHOTOCATALYTIC APPLICATIONS OF NANOCRYSTALLINE TiO2 While many examples of the photocatalytic activity of nanocrystalline TiO2 have been presented already in the foregoing sections, the following discussions will focus on novel and important (and sometimes large-scale) applications. A very prominent area appears to be environmental catalysis [31, 32, 45] which, in recent years, has expanded considerably beyond the traditional ﬁelds like NOx removal from stationary and mobile sources, or the conversion of volatile organic compounds (VOC). According to [381], these potential new areas include: (i) catalytic technologies for liquid or solid waste reduction or puriﬁcation; (ii) use of catalysts in energy-efﬁcient catalytic technologies and processes; (iii) reduction of the environmental impact in the use or disposal of catalysts; (iv) new ecocompatible reﬁnery, chemical or nonchemical catalytic processes; (v) catalysis for greenhouse gas control; (vi) use of catalysts for user-friendly technologies and reduction of indoor pollution; (vii) catalytic processes for sustainable chemistry; (viii) reduction of the environmental impact of transport. Hence, (photo)catalysis in environmental applications can be instrumental in promoting the quality of life and environment, in promoting a more efﬁcient use of resources, and in promoting sustainable processes and products. Because of the tremendous importance of those environmentally related areas, the use of nanocrystalline TiO2 for such photocatalytic applications is illustrated in this section by means of selected examples. Before doing so, it is stressed that there exists at least one other rather important issue in this context. In fact, hydrogen production from aqueous solutions using semiconductor particles such as, CdS, TiO2 , WO3 as photocatalysts is envisaged to become a potential

525

Nanocrystalline TiO2 for Photocatalysis

major application of these materials, and new concepts and approaches are developed continuously. For example, a new photocatalytic reaction that splits water into H2 and O2 was designed [382] by a two-step photo-excitation system − composed of a IO− 3 /I shuttle redox mediator and two different TiO2 photocatalysts, Pt-loaded TiO2 -anatase for H2 evolution and TiO2 -rutile for O2 evolution. Simultaneous gas evolution of H2 (180 mol/h) and O2 (90 mol/h) was observed from a basic (pH = 11) NaI aqueous suspension of two different TiO2 photocatalysts under UV irradiation ( > 300 nm, 400 W high-pressure Hg lamp). An extensive review [383] assesses photocatalytic efﬁciencies with reference to hydrogen production by means of light energy in the presence and absence of loaded metals, electrondonors/acceptors, and hole scavengers.

whereas it inhibited that of acetone. As for the effect of photon ﬂux, it was found that photocatalytic degradation occurs in two regimes with respect to photon ﬂux: for illumination levels distinctly blow 1000–2000 W/cm2 , the photocatalytic degradation rate increased linearly with photon ﬂux, whereas for power densities above that value, the rate was found to scale with the square root of the ﬂux. Figure 14 shows some of those data [388], depicting in panel (a), the degradation of methanol as a function of UV illumination time for ﬁve different initial concentrations. (Using TiO2 anatase nanocrystallites with 7 nm diameter in a solution, in this work photocatalytic TiO2 ﬁlms were deposited onto glass substrates by dip-coating.) The reaction kinetics were found to follow the L-H model, in which the reaction rate r varies proportionally with the surface coverage according to

7.1. Reduction/Removal of Toxic Gases r = k =

kKc 1 + Kc

(9)

concentration (10–3 mol/m3)

where c is the concentration of the VOC and k and K are, respectively, the reaction rate constant and the adsorption equilibrium constant. Figure 14(b) exempliﬁes this ﬁnding, showing the initial reaction rates r0 derived from data like those in Figure 14(a) as a function of the respective initial methanol concentrations c0 . The solid line in Figure 14(b) is a ﬁt to the data according to Eq. (9).

initial reaction rate r0 (10–3 mol/m3min)

The conversion of nitrogen oxides to less toxic compounds is important both because of their toxicity and the global atmospheric pollution. NOx can be converted to N2 and other nitrogen compounds by reduction. TiO2 -loaded zeolites and the vanadium silicate-1 were found [384] to decompose NO under irradiation, in particular, TiO2 included in zeolite cavities results in complete decomposition into N2 and O2 . Titanium oxide catalysts prepared within the Y-zeolite cavities via an ion-exchange method exhibit [385] high and unique photocatalytic reactivities for the decomposition of NO into N2 and O2 , as well as the reduction of CO2 with H2 O showing a high selectivity for the formation of CH3 OH. It was also found that the charge transfer excited state of the titanium oxide species, (T3+ -O− )∗ , plays a vital role in these unique photocatalytic reactions. In yet another approach, an efﬁcient catalytic reduction of NO at low temperature by means of NH3 could be achieved using Mn-, Cr-, or Cuoxides on a nanocrystalline TiO2 support [386]. The NOx removal process was studied experimentally in a pulsed corona discharge combined with the TiO2 photocatalytic reaction [387]. NO2 was found to adsorb easily on the photocatalyst surface, whereas NO was hardly adsorbed. Addition of water vapor enhanced the NO2 adsorption. It was concluded that the main role of the plasma-chemical reaction in this system is the oxidation of NO into NO2 . A considerable part of NO2 is adsorbed on the photocatalyst surface, and is transformed to HNO3 through photocatalytic reaction with OH. The photocatalytic degradation of VOCs in the gas phase constitutes another very important example in this range of applications. Utilizing variously prepared TiO2 photocatalysts (e.g., deposited on glass ﬁber cloth, as pellets or as thin ﬁlms), the photo-induced reactions of trichloroethylene, acetone, methanol, and toluene were investigated [388–390]. The photocatalytic degradation rate was observed [388] to increase with increasing initial concentration of the VOCs, but remained almost constant beyond a certain concentration. It matched well with the Langmuir–Hinshelwood (L-H) kinetic model [11]. For the inﬂuence of water vapor in a gas-phase photocatalytic degradation rate, there was an optimum concentration of water vapor in the degradation of trichloroethylene and methanol. Furthermore, water vapor enhanced the photocatalytic degradation rate of toluene,

20 (a)

methanol 15 10 5 0

0

2

4

6

8

10

illumination time (min)

2.0 1.5 1.0 0.5 (b)

0.0

0

5

10

15

20

25

initial concentration c0 (10–3 mol/m3)

Figure 14. (a) Photocatalytic degradation of methanol with different initial concentrations as a function of UV illumination time (light intensity 2095 W/cm2 at a wavelength of 254 nm) at a H2 O concentration of 0.38 mol/m3 and a reaction temperature of 45 C. (b) Initial reaction rates r0 as derived from the data in (a) versus the initial methanol concentrations; the solid line is a ﬁt according Eq. (9). Data from [388], S. B. Kim and S. C. Hong, Appl. Catal. B: Environ. 35, 305 (2002).

526

The degradation of organic compounds is probably the most widely used photocatalytic application of nanocrystalline TiO2 and other semiconductor materials. In an aqueous environment, the holes created under UV irradiation are scavenged by surface hydroxyl groups to generate • OH radicals that then promote the oxidation of organics. This radical-mediated oxidation has been successfully employed in the mineralization of several hazardous chemical contaminants such as hydrocarbons, haloaromatics, phenols, halogenated biphenyls, surfactants, and textile and other dyes [102]. The possible photocatalytic decomposition of a broad range of organic compounds has been investigated using nanocrystalline TiO2 particles. Detailed studies reported the oxidation of dissolved cyanide [391], the degradation of various kinds of acids [392–398], and of several herbicides [399–402], for the photocatalytic oxidation of toluene, benzene, cyclohexene, and benzhydrol [403–406] or for the 1,1 -dimethyl-4,4 -bipyridium dichloride decomposition [407]. In another application, a titanium oxide photocatalyst of ultra-high activity has been employed for the selective N-cyclization of an amino acid in aqueous suspensions [408]. Anatase crystallites of average diameter of ∼15 nm were platinized by impregnation from aqueous chloroplatinic acid solution followed by hydrogen reduction. The catalyst was suspended in an aqueous L-lysine (Lys) solution and photoirradiated under argon at ambient temperature to obtain L-pipecolinic acid. The photocatalytic degradation and oxidation of phenol and phenol-based compounds has been examined quite frequently [409–414]. The decomposition of aqueous phenol solutions to carbon dioxide have been studied using natural sunlight in geometries simulating shallow ponds [415]. The photocatalyst was titanium dioxide freely suspended in the solution or immobilized on sand or silica gel. Photodegradation rates were approximately three times faster with the free suspension than with the immobilized catalyst under the same conditions, and were dependent on the time of year and the time of day. The seasonal variation correlated roughly with seasonal solar irradiance tabulations for the UV component of the spectrum. For 10 ppm of phenol, the maximum rate of solar degradation resulted in a decrease in concentration to 10 ppb in less than 80 min with total mineralization in 110 min. An efﬁcient degradation of aqueous phenol was achieved [416] by a new rotating-drum reactor coated with a TiO2 photocatalyst, in which TiO2 powders loaded with Pt are immobilized on the outer surface of a glass drum. The reactor can receive solar light and oxygen from the atmosphere effectively. It was shown experimentally that phenol can be decomposed rapidly by this reactor under solar light: with the used experimental conditions, phenol with an initial concentration of 22.0 mg/dm3 was decomposed within 60 min and was completely mineralized through intermediate products within 100 min. The photocatalytic degradation of various types of dyes appears to be another prominent and extensively explored application of nanocrystalline TiO2 in environmental catalysis [417–423].

In a recent study [424], the photocatalytic degradation of ﬁve dyes in TiO2 aqueous suspensions under UV irradiation has been investigated; it was attempted to determine the individual steps of such a degradation process by varying the aromatic structures, using either anthraquinonic (Alizarin S (AS)), or azoic (Crocein Orange G (OG), Methyl Red (MR), Congo Red (CR)) or heteropolyaromatic (Methylene Blue (MB)) dyes. Figure 15 exempliﬁes the photocatalytic degradation of three of these dyes (CR, OG, and MR) as a function of UV irradiation. The initial reaction rates were found to fall in the range from 1.9 mol/l min (for CR) to 3.6 mol/l min (for MR). In addition to a prompt removal of the colors, TiO2 /UV-based photocatalysis was simultaneously able to fully oxidize the dyes, with a complete mineralization of carbon into CO2 . Sulfur heteroatoms were converted into innocuous SO2− 4 ions. The mineralization of nitrogen was more complex. Nitrogen atoms in the 3-oxidation state, such as in amino groups, remain at this reduction degree and produced NH+ 4 cations, subsequently and very slowly converted into NO− 3 ions. For azo-dye (OG, MR, CR) degradation, the complete mass balance in nitrogen indicated that the central N N azo group was converted into gaseous dinitrogen, which is the ideal issue for the elimination of nitrogen-containing pollutants. The aromatic rings were submitted to successive attacks by photogenerated • OH radicals leading to hydroxylated metabolites before the ring opening and the ﬁnal evolution of CO2 induced by repeated reactions with carboxylic intermediates. The photocatalytic degradation of acid derived azo dyes in aqueous TiO2 suspensions follows apparently ﬁrst-order kinetics [425, 426]. The site near the azo bond (C N Nbond) is the attacked area in the photocatalytic degradation process, while the TiO2 photocatalytic destruction of the C N( ) bond and N N bonds leads to fading of the dyes. The pH effect on the TiO2 photocatalytic degradation of the acid-derived azo dyes varies with dye structure. Hydroxyl radicals play an essential role in the ﬁssion of the C N N conjugated system in azo dyes in TiO2 photocatalytic degradation. Metalized azo dyes were studied 80

concentration (µmol/l)

7.2. Degradation of Organic Compounds

Nanocrystalline TiO2 for Photocatalysis

CR OG MR

60

40

20

0

0

50

100

150

200

illumination time (min) Figure 15. Photocatalytic degradation of three different dyes, Congo Red (CR), Crocein Orange (OG), and Methyl Red (MR), given in terms of the concentration versus the time of illumination. Data from [424], H. Lachheb et al., Appl. Catal. B: Environ. 39, 75 (2002).

Nanocrystalline TiO2 for Photocatalysis

[427] under TiO2 photocatalytic and photosensitized conditions in aqueous buffering solutions. The size and strength of intramolecular conjugation determines apparently the lightfastness of the dyes; the more powerful OH radicals in TiO2 photocatalytic process are highly reactive towards the azo dyes.

7.3. Wastewater and Soil Remediation The major causes [428] of surface water and groundwater contamination are industrial discharges, excess use of pesticides, fertilizers (agrochemicals), and landﬁlling domestic wastes. Typically, the wastewater treatment is based upon various mechanical, biological, physical, and chemical processes. After ﬁltration and elimination of particles in suspension, the biological treatment is the ideal process (natural decontamination). Unfortunately, organic pollutants are not always biodegradable; a promising approach then relies on the formation of highly reactive chemical species, which degraded the more recalcitrant molecules into biodegradable compounds. These are called the advanced oxidation processes (AOPs). Although there exist differences in their detailed reaction schemes, their common feature is the production of OH radicals (• OH); these radicals are extraordinarily reactive species (oxidation potential 2.8 V). They are also characterized by a low selectivity of attack, which is a useful attribute for an oxidant used in wastewater treatment and for solving pollution problems. These photocatalytic degradation of wastewater employing nanocrystalline TiO2 has been examined in various set-ups [429] and pilot-plant scale solar photocatalytic experiments have been realized [428]. Several recent studies reported on the removal or reduction of metals or metal-containing contaminants in wastewater, based on the principles outlined in the foregoing paragraph. Those investigations examined, for example, the removal of cadmium and mercury from water using modiﬁed TiO2 nanoparticles [430, 431], the radical, mediated photo-reduction of manganese ions in UV-irradiated titania suspensions [432], the simultaneous photocatalytic Cr(VI) reduction and dye oxidation in a TiO2 slurry reactor [433], or the removal of iron(III) cyanocomplexes [434]. While the efﬁcient use of a photocatalytic process in the presence of TiO2 to degrade many different types of pollutants in wastewater has been conﬁrmed repeatedly, the question of how to efﬁciently separate and reuse TiO2 from treated wastewater became a notable problem in the application of a TiO2 photo-oxidation process. A recent study [435] aimed to develop an advanced process for dyeing wastewater treatment, in which dyeing wastewater was initially treated by an intermittently decanted extended aeration (IDEA) reactor to initially remove biodegradable matters and further treated in a TiO2 photocatalytic reactor for complete decolorization and high chemical oxygen demand (COD) removal. Suspended TiO2 powder used in the photo-oxidation was separated from slurry by a membrane ﬁlter and recycled to the photo reactor continuously. Photocatalytic destruction of chlorinated solvents in water with solar energy was investigated [436] using a nearcommercial scale, single-axis tracking parabolic trough system with a glass pipe reactor mounted at its focus. In

527 the photocatalytic degradation of industrial residual waters, the use of peroxydisulfate (S2 O2− 8 ) as an additional oxidant (electron scavenger) was observed to have an outstanding effect, producing an important increase in the degradation rate [437]. The impact of pH and the presence of inorganic ions and organic acids commonly found in natural waters on rates of TiO2 photocatalyzed trinitrotoluene (TNT) transformation and mineralization was examined [438]. Raising the pH slightly increased the rate of TNT transformation, primarily as a result of an increased rate of TNT photolysis, but signiﬁcantly reduced rates of mineralization due to increased electrostatic repulsion between the catalyst surface and anionic TNT intermediates. The presence of inorganic anions did not substantially hinder TNT transformation at alkaline pH, but mineralization rates were diminished when the anion either adsorbed strongly to the photocatalyst or was an effective hydroxyl radical scavenger. Immobilized TiO2 photocatalysts were used to sterilize and reclaim the wastewater of bean sprout cultivation from a continuous hydrocirculation system [439]. The photocatalysts effectively killed bacteria and degraded organic pollutants in the wastewater. Stimulation of bean sprout growth and suppression of decaying pathogens were also induced by the TiO2 photocatalytic activity. Photocatalytic decomposition of seawater-soluble crude oil fractions using high surface area colloid nanoparticles of TiO2 under UV irradiation was explored [440]; although no mineralization occurred due to photolysis, important chemical changes were observed in the presence of TiO2 , with the degradation reaching 90% (measured as dissolved organic carbon, (DOC)) in waters containing 9–45 mg C/l of seawater-soluble crude oil compounds after 7 days of artiﬁcial light exposure. During light exposure, transient intermediates that showed higher toxicity than the initial compounds were observed, but were subsequently destroyed. Heterogeneous photocatalysis using TiO2 was considered to be a promising process to minimize the impact of crude oil compounds on contaminated waters. TiO2 -photocatalytic degradation of a cellulose efﬂuent was evaluated [441] using multivariate experimental design. The efﬂuent was characterized by general parameters such as adsorbable organic halogens (AOX), TOC, COD, color, total phenols, acute toxicity, and by the analysis of chlorinated low molecular weight compounds using GC/MS. The optimal concentration of TiO2 was found to be around 1 g/l. After 30 min of reaction more than 60% of the toxicity was removed and after 420 min of reaction, none of the initial chlorinated low molecular weight compounds were detected, suggesting an extensive mineralization. Photocatalysts, based on titanium dioxide, were used for the puriﬁcation of contaminated soil polluted by oil [442]. Commercially produced slurry of titanium dioxide was modiﬁed with barium, potassium, and calcium. The experiments were performed under natural conditions in summer months (July and August) applying direct solar-light irradiation. The most active photocatalyst for soil puriﬁcation was titanium dioxide modiﬁed with calcium. Two different photocatalysts, namely, Hombikat UV100 (Sachtleben Chemie) and P25 (Degussa) have been used in batch experiments [443] to compare their ability to degrade the toxic components of a biologically pretreated landﬁll

528

Nanocrystalline TiO2 for Photocatalysis

leachate. A strong adsorption of the pollutant molecules was observed for both TiO2 -powders, with a maximum of almost 70% TOC reduction for Hombikat UV100.

7.4. Puriﬁcation of Drinking Water

1000

100

800

80

600

60

400

40

200

20

0

0

20

40

60

protein phosphatase inhibitor (%)

concentration microcystin-LR (µg/ml)

Pathogens in drinking water supplies can be removed by sand ﬁltration followed by chlorine or ozone disinfection. These processes reduce the possibility of any pathogens entering the drinking water distribution network. However, there is doubt about the ability of these methods to remove chlorine-resistant microorganisms including protozoan oocysts. Titanium dioxide (TiO2 ) photocatalysis is a possible alternative/complementary drinking water treatment method and several studies [444, 445] reported a strong and swift photocatalytic inactivation of bacteria and bacteriophages in aqueous solutions. For example, the rate of disinfection was explored using TiO2 electrodes prepared by the electrophoretic immobilization of TiO2 powders with different crystallinity. These electrodes were tested for their photocatalytic bactericidal efﬁciency with E. coli K12 as a model test organism [446]. Similar studies were reported for natural water from a river [447]. Cyanobacterial toxins produced and released by cyanobacteria in freshwater around the world pose a considerable threat to human health if present in drinking water sources. Therefore, various treatments have been applied to remove these toxins. The effectiveness of TiO2 photocatalysis for the removal of microcystin-LR from water has been established [448]. Not only does the process rapidly remove the toxin but also the by-products appear to be nontoxic. The photocatalytic process has also signiﬁcantly reduced the protein phosphatase 1 (PP1) inhibition. Protein phosphatase 1 inhibition is potentially one of the most serious harmful effects to humans who may consume water contaminated by microcystins. Figure 16 shows some of these data, namely, the reduction of the microcystin-LR concentration and the PP1 inhibition as a function of the illumination time. The results indicate that about 86% of

0

illumination time (min) Figure 16. Destruction and protein phosphatase (PP1) inhibition of microcystine-LR via TiO2 photocatalysis as a function of the duration of UV illumination (xenon lamp with 480 W at a wavelength of 330– 450 nm). Data from [448], I. Liu et al., J. Photochem. Photobiol. A: Chem. 148, 349 (2002).

microcystin-LR was destroyed within the ﬁrst 5 min of photocatalysis, with 97% of the toxin removed in 20 min. The addition of 0.1% H2 O2 to the photocatalytic system was found [448] to further enhance the degradation rate: 99.6% of microcystin-LR was destroyed within 5 min and no toxin was left after 10 min of photocatalysis. Photocatalytic inactivation of different bacteria and bacteriophages in drinking water at different TiO2 concentrations with or without concurrent exposure to O2 was studied in [449] using UV irradiation (5.5 mW/cm2 at 365 nm). For example, for this light intensity, the most effective inactivation of Escherichia coli CN13 was obtained at 1 g/l suspension of TiO2 , resulting in a reduction by ﬁve orders of magnitude in 5 min. Under the same experimental conditions, MS2 bacteriophage was reduced by four orders of magnitude, also in 5 min. The addition of O2 into the experimental environment increased the inactivation of Deinococcus radiophilus by four orders of magnitude in 60 min.

7.5. Miscellaneous Photocatalytic Applications It may have become apparent from the foregoing discussions and examples that the solution of environmental problems constitutes one of the (if not the) major driving forces in research and development in photocatalysis using nanometer-sized TiO2 (and other semiconductor) particles. Another one, of course, is the production of hydrogen from water splitting. Apart from these main applications, there exist, on the other hand, many attempts to explore novel areas for the photocatalytic use of nanocrystalline TiO2 materials. To give a ﬂavor of the diversity of these efforts, some selected (and mostly recent) examples follow. Nano-sized titanium oxide (TiO2 thin ﬁlms have been explored for alcohol-sensing applications. TiO2 thin ﬁlms with different doping concentrations were prepared on alumina substrates [450] using the sol–gel process using the spin-coating technique for ethanol and methanol alcohol. Experimental results indicated that the sensor is able to monitor alcohols selectively at ppm levels; the ﬁlms are stoichiometric with carbon as the dominant impurity on the surface. The morphologies and crystalline structures of the ﬁlms were studied by scanning electron microscopy (SEM) and XRD. X-ray diffraction patterns showed that the ﬁlms are pure anatase phase up to an annealing temperature of 600 C. As the annealing temperature increased to 800 C, a small amount of rutile phase formed along with the anatase phase. Optical waveguides were prepared by depositing a sol–gelderived titania ﬁlm onto a silica substrate [451]. The titania ﬁlm is mesoporous, with pore sizes ranging from 3 to 8 nm. Deposition of the titania does not change the critical angle of total internal reﬂection. Thus, the titania-coated waveguides propagate light in an attenuated total reﬂection mode, despite the relatively high refractive index (n = 1.8 in air) of the titania ﬁlm relative to the silica substrate (n = 05). The light output of electric lighting gradually decreases due to stain buildup on lamps and covers during operation. Roadway, and especially tunnel lighting, experiences a large amount of contamination due to dust, carbon particles found in vehicle engine exhausts and other airborne contaminants,

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which results in the rapid deterioration of the light output. Photocatalytic reactions caused by TiO2 are known to decompose such stains. This reaction is caused by the absorption of UV light ( ≤ 400 nm, corresponding to the bandgap of ∼3 eV) irradiated from lamps or the sun, and followed by oxidation. Extensive ﬁeld tests revealed [452] that a ﬁne ﬁlm coating of TiO2 on lamps and luminaires can effectively decompose various organic compounds such as vehicle exhaust gases, oil, nicotine, etc. This leads to an improvement of the luminous performance of installed lighting systems and reduces the cost of maintenance by approximately one-half. It has been recently found [453] that photocatalytic TiO2 coated with polycarbonate (PC) releases a huge amount of exothermic energy in the temperature range between 200 and 400 C (ca. 1.85 kJ/g). The strong interaction between oxygen-deﬁcient sites in TiO2 and carbonyl groups of PC mediated by a good PC solvent is found to be a prerequisite for a release of the enormous amount of exothermic energy. This ﬁnding suggests that PC-coated TiO2 powders or related oxides work as a combustion-assisting agent in a relatively lower temperature range and can be utilized for incineration applications in order to suppress the formation of extremely toxic dioxins. A somewhat unusual application reported [454] the photocatalytic deposition of a gold particle onto the top of a SiN cantilever tip, employing the photocatalytic effect of titanium dioxide. When the titanium dioxide immersed in a solution including gold ions is subject to optical exposure, the excited electrons in the conduction band reduce gold ions into gold metal. Illumination by an evanescent wave generated with a total reﬂection conﬁguration limits the deposition region to the very tip. In the experiments, 100–300 nm gold particles on SiN cantilever tips for atomic force microscopes were obtained. In a related vein, photoinduced deposition of copper on nanocrystalline TiO2 ﬁlms was proposed [455]. Solar photocatalytic oxidation processes (PCO) for degradation of water and air pollutants have received increasing attention. In fact, some ﬁeld-scale experiments have demonstrated the feasibility of using a semiconductor (TiO2 in solar collectors and concentrators to completely mineralize organic contaminants in water and air [456]. Although successful preindustrial solar tests have been carried out, there are still discrepancies and doubt concerning process fundamentals such as the roles of active components, appropriate modelling of reaction kinetics, or quantiﬁcation of photo-efﬁciency. Challenges to development are catalyst deactivation, slow kinetics, low photo-efﬁciency and unpredictable mechanisms. The development of speciﬁc nonconcentrating collectors for detoxiﬁcation and the use of additives such as peroxydisulfate have made competitive use of solar PCO possible.

GLOSSARY Charge transfer The transfer of a charge carrier (electron or hole) from an excited semiconductor to an adsorbed species on its surface. This transfer may initiate a reaction (oxidation or reduction) in the adsorbed molecule. Dye-sensitized semiconductor Adsorbing a suitable dye on the surface of a wide band gap semiconductor (like TiO2 ) can

enhance the efﬁciency of the excitation step and, hence, the catalytic activity. Electron-hole pair The absorption of a photon of sufﬁcient energy may excite in a semiconductor an electron from the valence band to the conduction band, thereby creating a hole in the valence band. Nanocrystalline A material composed of individual crystallites which have a size in the range of nanometer (nm); 1 nm = 10−9 m. Photocatalysis A catalytic reaction triggered or enhanced by illuminating the system with visible or ultraviolet irradiation. This reaction involves normally the electronic excitation of the catalyst via the absorption of photons and an interfacial charge transfer to an adsorbed species. Typically, the photocatalyst is not consumed in the reaction. Photocatalytic degradation The removal or reduction of (usually unwanted) substances via a photocatalytic reaction. Quantum yield The probability of product formation per adsorbed photon in a photocatalytic reaction. Titanium oxide Titanium dioxide with the nominal composition TiO2 is a semiconductor with a band gap of ∼3.2 eV; it exists in three different crystalline modiﬁcations, two of which (anatase and rutile) are commonly employed in photocatalysis.

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