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Surface Science Reports 63 (2008) 515–582 Contents lists available at ScienceDirect Surface Science Reports journal homepage: www.elsevier.com/locate/surfrep TiO 2 photocatalysis and related surface phenomena Akira Fujishima a,* , Xintong Zhang b , Donald A. Tryk c a Kanagawa Academy of Science and Technology, 3-2-1 Sakado, Takatsu-ku, Kawasaki-shi, Kanagawa 213-0012, Japan b Center for Advanced Optoelectronic Functional Materials Research, Northeast Normal University, 5268 Renmin Street, Changchun 130024, China c Fuel Cell Nanomaterials Center, University of Yamanashi, Takeda 4-3-11, Koufu, Yamanashi 400-8510, Japan article info Article history: Accepted 1 October 2008 editor: Y. Murata Keywords: Titanium dioxide Titania TiO 2 Self-cleaning surfaces Superhydrophilic effect Anion doping Water splitting Environmental cleaning abstract The field of photocatalysis can be traced back more than 80 years to early observations of the chalking of titania-based paints and to studies of the darkening of metal oxides in contact with organic compounds in sunlight. During the past 20 years, it has become an extremely well researched field due to practical interest in air and water remediation, self-cleaning surfaces, and self-sterilizing surfaces. During the same period, there has also been a strong effort to use photocatalysis for light-assisted production of hydrogen. The fundamental aspects of photocatalysis on the most studied photocatalyst, titania, are still being actively researched and have recently become quite well understood. The mechanisms by which certain types of organic compounds are decomposed completely to carbon dioxide and water, for example, have been delineated. However, certain aspects, such as the photo-induced wetting phenomenon, remain controversial, with some groups maintaining that the effect is a simple one in which organic contaminants are decomposed, while other groups maintain that there are additional effects in which the intrinsic surface properties are modified by light. During the past several years, powerful tools such as surface spectroscopic techniques and scanning probe techniques performed on single crystals in ultra- high vacuum, and ultrafast pulsed laser spectroscopic techniques have been brought to bear on these problems, and new insights have become possible. Quantum chemical calculations have also provided new insights. New materials have recently been developed based on titania, and the sensitivity to visible light has improved. The new information available is staggering, but we hope to offer an overview of some of the recent highlights, as well as to review some of the origins and indicate some possible new directions. © 2008 Elsevier B.V. All rights reserved. Contents 1. Introduction........................................................................................................................................................................................................................ 516 2. Historical overview ............................................................................................................................................................................................................ 516 3. Properties of TiO 2 materials .............................................................................................................................................................................................. 519 3.1. Crystal structures ................................................................................................................................................................................................... 519 3.2. Electronic properties ............................................................................................................................................................................................. 520 3.3. Surface structure studies ....................................................................................................................................................................................... 523 3.4. Surface chemical studies: Interactions with water ............................................................................................................................................. 523 3.5. Surface chemical studies: Interactions with dioxygen and other species ......................................................................................................... 527 3.6. Bulk chemistry—Hydrogen.................................................................................................................................................................................... 527 3.7. Electrochemical properties ................................................................................................................................................................................... 529 3.8. Photoelectrochemical properties.......................................................................................................................................................................... 534 4. Fundamentals of photocatalysis........................................................................................................................................................................................ 534 4.1. Mechanisms of photocatalysis .............................................................................................................................................................................. 534 4.1.1. Photoelectrochemical basis of photocatalysis ...................................................................................................................................... 534 4.1.2. Time scales .............................................................................................................................................................................................. 538 4.1.3. Trapping of electrons and holes............................................................................................................................................................. 541 * Corresponding author. Tel.: +81 (0)44 819 2020; fax: +81 (0)44 819 2038. E-mail address: [email protected] (A. Fujishima). 0167-5729/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.surfrep.2008.10.001

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Surface Science Reports 63 (2008) 515582

Contents lists available at ScienceDirect

Surface Science Reportsjournal homepage: www.elsevier.com/locate/surfrep

TiO2 photocatalysis and related surface phenomenaAkira Fujishima a, , Xintong Zhang b , Donald A. Tryk ca b c

Kanagawa Academy of Science and Technology, 3-2-1 Sakado, Takatsu-ku, Kawasaki-shi, Kanagawa 213-0012, Japan Center for Advanced Optoelectronic Functional Materials Research, Northeast Normal University, 5268 Renmin Street, Changchun 130024, China Fuel Cell Nanomaterials Center, University of Yamanashi, Takeda 4-3-11, Koufu, Yamanashi 400-8510, Japan

article

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a b s t r a c tThe field of photocatalysis can be traced back more than 80 years to early observations of the chalking of titania-based paints and to studies of the darkening of metal oxides in contact with organic compounds in sunlight. During the past 20 years, it has become an extremely well researched field due to practical interest in air and water remediation, self-cleaning surfaces, and self-sterilizing surfaces. During the same period, there has also been a strong effort to use photocatalysis for light-assisted production of hydrogen. The fundamental aspects of photocatalysis on the most studied photocatalyst, titania, are still being actively researched and have recently become quite well understood. The mechanisms by which certain types of organic compounds are decomposed completely to carbon dioxide and water, for example, have been delineated. However, certain aspects, such as the photo-induced wetting phenomenon, remain controversial, with some groups maintaining that the effect is a simple one in which organic contaminants are decomposed, while other groups maintain that there are additional effects in which the intrinsic surface properties are modified by light. During the past several years, powerful tools such as surface spectroscopic techniques and scanning probe techniques performed on single crystals in ultrahigh vacuum, and ultrafast pulsed laser spectroscopic techniques have been brought to bear on these problems, and new insights have become possible. Quantum chemical calculations have also provided new insights. New materials have recently been developed based on titania, and the sensitivity to visible light has improved. The new information available is staggering, but we hope to offer an overview of some of the recent highlights, as well as to review some of the origins and indicate some possible new directions. 2008 Elsevier B.V. All rights reserved.

Article history: Accepted 1 October 2008 editor: Y. Murata Keywords: Titanium dioxide Titania TiO2 Self-cleaning surfaces Superhydrophilic effect Anion doping Water splitting Environmental cleaning

Contents 1. 2. 3. Introduction........................................................................................................................................................................................................................516 Historical overview ............................................................................................................................................................................................................516 Properties of TiO2 materials ..............................................................................................................................................................................................519 3.1. Crystal structures ...................................................................................................................................................................................................519 3.2. Electronic properties .............................................................................................................................................................................................520 3.3. Surface structure studies.......................................................................................................................................................................................523 3.4. Surface chemical studies: Interactions with water .............................................................................................................................................523 3.5. Surface chemical studies: Interactions with dioxygen and other species .........................................................................................................527 3.6. Bulk chemistryHydrogen....................................................................................................................................................................................527 3.7. Electrochemical properties ...................................................................................................................................................................................529 3.8. Photoelectrochemical properties..........................................................................................................................................................................534 Fundamentals of photocatalysis........................................................................................................................................................................................534 4.1. Mechanisms of photocatalysis ..............................................................................................................................................................................534 4.1.1. Photoelectrochemical basis of photocatalysis ......................................................................................................................................534 4.1.2. Time scales ..............................................................................................................................................................................................538 4.1.3. Trapping of electrons and holes.............................................................................................................................................................541

4.

Corresponding author. Tel.: +81 (0)44 819 2020; fax: +81 (0)44 819 2038.E-mail address: [email protected] (A. Fujishima). 0167-5729/$ see front matter 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.surfrep.2008.10.001

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5.

6.

7.

4.1.4. Oxidizing species at the TiO2 surface ....................................................................................................................................................542 4.1.5. Role of molecular oxygen .......................................................................................................................................................................545 4.1.6. Effect of crystal face................................................................................................................................................................................547 4.1.7. Remote photocatalysis ...........................................................................................................................................................................549 4.2. Photocatalytic reactions ........................................................................................................................................................................................551 4.2.1. Decomposition of gaseous pollutants ...................................................................................................................................................551 4.2.2. Decomposition of aqueous pollutants...................................................................................................................................................552 4.2.3. Decomposition of liquid and solid films ...............................................................................................................................................553 4.2.4. Photocatalytic sterilization ....................................................................................................................................................................553 4.3. Visible-light-induced photocatalysis....................................................................................................................................................................555 4.3.1. Non-metal doping...................................................................................................................................................................................555 4.3.2. Origin of visible light photoactivity.......................................................................................................................................................556 4.3.3. Activity and stability of N-doped TiO2 photocatalysts .........................................................................................................................558 Fundamentals of the photo-induced hydrophilic (PIH) effect ........................................................................................................................................560 5.1. Overview ................................................................................................................................................................................................................560 5.2. Mechanisms of the PIH effect................................................................................................................................................................................562 5.2.1. Decomposition of organic films .............................................................................................................................................................562 5.2.2. Reductive mechanism ............................................................................................................................................................................563 5.2.3. Oxidative mechanism.............................................................................................................................................................................563 5.2.4. Combined redox mechanism .................................................................................................................................................................563 5.2.5. Visible-light-induced PIH effect.............................................................................................................................................................564 Brief review of applications...............................................................................................................................................................................................565 6.1. Self-cleaning surfaces ............................................................................................................................................................................................565 6.2. Water purification .................................................................................................................................................................................................567 6.3. Air purification .......................................................................................................................................................................................................568 6.4. Self-sterilizing surfaces .........................................................................................................................................................................................569 6.5. Anti-fogging surfaces.............................................................................................................................................................................................570 6.6. Heat transfer and heat dissipation........................................................................................................................................................................571 6.7. Anticorrosion applications ....................................................................................................................................................................................571 6.8. Environmentally friendly surface treatment .......................................................................................................................................................572 6.9. Photocatalytic lithography ....................................................................................................................................................................................572 6.10. Photochromism......................................................................................................................................................................................................574 6.11. Microchemical systems .........................................................................................................................................................................................574 Summary ............................................................................................................................................................................................................................575 Appendix. TiO2 film preparation methods ..................................................................................................................................................................576 References...........................................................................................................................................................................................................................576

1. Introduction Photocatalysis is generally thought of as the catalysis of a photochemical reaction at a solid surface, usually a semiconductor [116]. This simple definition, while correct and useful, however, conceals the fact that there must be at least two reactions occurring simultaneously, the first involving oxidation, from photogenerated holes, and the second involving reduction, from photogenerated electrons. Both processes must be balanced precisely in order for the photocatalyst itself not to undergo change, which is, after all, one of the basic requirements for a catalyst. It will be seen in this review of the fundamentals and selected applications of photocatalysis, principally on titanium dioxide, that there is a host of possible photochemical, chemical and electrochemical reactions that can occur on the photocatalyst surface. The types of reactions occurring, their extent and their rates depend upon a host of factors that are still in the process of being unraveled. Furthermore, there can indeed be changes that occur, involving the surface and bulk structure and even decomposition of the photocatalyst, a fact that appears to stretch the definition of the term. This topic started its early history as mostly a nuisance involving the chalking of titania-based paints [17,18] and then gradually transformed into a highly useful approach to the remediation of water and air and then into an approach to maintain surfaces clean and sterile. Along the way, it has also transformed into an approach to photolytically split water into hydrogen and oxygen [1921] and also an approach to perform selective oxidation reactions in organic chemistry [22]. Clearly, with so many varied aspects, photocatalysis is nearly impossible to review comprehensively. In the present review, we

have tried to put together an overview of some of the more fundamental aspects, which are in their own right extremely scientifically interesting and which also need to be better understood in order to make significant progress with applications. The review will be divided into several sections: 2. Historical overview; 3. Properties of TiO2 materials; 4. Fundamentals of photocatalysis; 5. Fundamentals of the photo-induced hydrophilic effect; 6. Brief review of applications; 7. Summary, and Appendix (film preparation methods). 2. Historical overview We will give a brief overview of the early history of photocatalysis, which will be based just on papers that we have been able to access, which means that we will almost certainly be ignoring some important papers. The earliest work that we have been able to find is that of Renz, at the University of Lugano (Switzerland), who reported in 1921 [17] that titania is partially reduced during illumination with sunlight in the presence of an organic compound such as glycerol, the oxide turning from white to a dark color, such as grey, blue or even black; he also found similar phenomena with CeO2 , Nb2 O5 and Ta2 O5 . For TiO2 , the reaction proposed was: TiO2 + light Ti2 O3 or TiO. (2.1)

Baur and Perret, at the Swiss Federal Institute of Technology, were the first to report, in 1924, the photocatalytic deposition of a silver salt on zinc oxide to produce metallic silver [23]. Even at this early date, the authors suspected that both oxidation and reduction

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were occurring simultaneously. The reaction pathway proposed was this: ZnO + h h+ + e h+ + OH 1 4 O2 + 1 2 H2 O (2.2) (2.3) (2.4)

e + Ag+ Ag0 .

Three years later, Baur and Neuweiler proposed simultaneous oxidation and reduction to explain the production of hydrogen peroxide on zinc oxide [24]. 2h+ + CH2 O + 2OH CO + 2H2 O 2e + 2H + O2 H2 O2 . +

(2.5) (2.6)

It was not until many years later that this was absolutely confirmed, however. In 1932, Renz reported the photocatalytic reduction of silver nitrate to metallic silver and gold chloride to metallic gold on a number of illuminated oxides, including TiO2 and Nb2 O5 , [25] and discussed the results in terms of the Baur redox mechanism. It has been recognized for quite a long time that titania-based exterior paints tend to undergo chalking in strong sunlight. This means that a non-adherent, white powdery substance tends to form on the surface, similar to the chalk on a blackboard. This effect was recognized to result from the actual removal of part of the organic component of the paint, leaving the titania itself exposed. With this background, Goodeve and Kitchener, at University College, London, carried out an excellent study on the photocatalytic decomposition of a dye on titania powder in air in 1938, including absorption spectra and determination of quantum yields (Fig. 2.1) [26]. These authors proposed that titania acts as a catalyst to accelerate the photochemical oxidation and also studied a number of other oxides and speculated on the precise mechanism [27]. In 1949, Jacobsen, at the National Lead Company (USA), also attempted to explain the paint chalking phenomenon in terms of a redox mechanism. He found a correlation between the tendency of a number of different titania powders to undergo photo-induced reduction in the presence of organic compounds to their chalking tendency [18]. The photo-induced reduction was measured as a loss of reflectivity, due to the discoloration of the powder upon reduction, presumably to various oxygen-deficient forms, all the way to Ti2 O3 . The author proposed a cyclic redox process in which the titania was reduced while the organic paint components were oxidized, followed by re-oxidation of the titania by oxygen from the air. The changes experienced by the titania were recognized to be completely reversible, while those experienced by the organic paint were recognized to be irreversible, leading to the formation of water-soluble organic acids and CO2 . Even though Jacobsen was apparently unaware of the work of Baur on the redox mechanism, he referred to the 1921 paper of Renz on the photo-reduction of metal oxides and proposed the same basic mechanism that had been proposed by Baur; thus, a foundation was laid for later work on the redox mechanism. During the 1950s, the development of photocatalysis shifted to zinc oxide. In 1953, two studies appeared in which the puzzling phenomenon of hydrogen peroxide production on zinc oxide illuminated with UV light was studied [28,29], followed by a series of follow-up studies in ensuing years [3034]. In these studies, the overall reactions and mechanisms were completely clarified, and it became apparent that an organic compound was oxidized while atmospheric oxygen was reduced. Even in the earliest study, an overall reaction with phenol to produce catechol was proposed, and the involvement of radical species such as the hydroxyl radical (OH) was also speculated upon [28]. Thus, the original proposal

Fig. 2.1. Original data of Goodeve and Kitchener showing the photocatalytic decomposition of a dye (chlorazol sky blue) adsorbed on anatase powder under UV illumination at 365 nm [26]. 1938, Royal Society of Chemistry.

of Baur and Neuweiler was finally confirmed, with the overall reaction: RHOH + H2 O + O2 H2 O2 + R(OH)2 . (2.7)

Markham, first at the Catholic University of America and later at St. Josephs College (USA), continued to study photocatalytic reactions on ZnO, and her papers constitute an impressive, yet underappreciated, body of work [28,30,31,3539]. This work culminated in a highly intriguing study in which Markham and co-worker Upreti constructed and studied a number of different types of photo-assisted fuel cells, using illuminated ZnO as the photo-anode with formamide or several alcohols as the organic substrates [39]. At the dark cathode (platinum), several different redox mediators were examined, with atmospheric oxygen ultimately being the electron acceptor. The authors may have been discouraged by the inevitable problem of ZnO photocorrosion, which prevented this system from reaching practical application. It was not until years later that the same basic ideas were reexamined with TiO2 . Unfortunately, Markham and Laidler, in their initial study in 1953, examined TiO2 but subsequently abandoned it, since it did not produce measurable amounts of hydrogen peroxide [28]. It is also interesting to note that Stephens et al. (Wayne State University), in their study in 1955 of hydrogen peroxide production on a large assortment of illuminated semiconductors, but, unfortunately, not TiO2 , remarked that zinc oxide and the other catalytic solids should not be abandoned as devices for capturing solar energy in a form capable of transfer to some chemical system [32]. These authors found that CdS was the most active photocatalyst, exceeding ZnO in activity. In a study reported in 1956 in Nature, Hindson and Kelly (Defense Standards Laboratory) reported on the effects of various rot-inhibitors on tent fabrics for use in Australia. They examined the effects of fabric strength after one year of exposure to sunlight. They stated: The effect of anatase is startling. Fabrics containing 3% of this pigment lost 90% in strength. In 1958, Kennedy et al., at the University of Edinburgh, studied the photo-adsorption of O2 on TiO2 in order to try to more fully understand the photocatalytic process [40]. They concluded that electrons were transferred to O2 as a result of photoexcitation, and the resulting reduced form of O2 adsorbed on the TiO2 surface. These authors found a correlation between the ability of the TiO2 sample to photocatalytically decompose chlorazol sky blue (the same dye used earlier by Goodeve and Kitchener) and the ability

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to photo-adsorb O2 . This phenomenon is certainly important for photocatalysis and will be commented upon later. During this period, researchers in Russia were active. Photoadsorption of O2 on illuminated ZnO was studied by Terenin and Solonitzin at the University of Leningrad (now University of St. Petersburg) [41]. In a very interesting early work, Filimonov, at the same institution, compared the photocatalytic oxidation of isopropanol to acetone on ZnO and TiO2 [42] and concluded that the mechanism on TiO2 involved an overall reduction of O2 to H2 O, while the reduction of O2 on ZnO only went as far as H2 O2 . On TiO2 , the surface reactions were proposed to be: TiO + (CH3 )2 CHOH (CH3 )2 CO + TiO + H2 O 2 TiO + 1 2 O2 TiO2 . (2.8) (2.9)

Thus, this mechanism is a more detailed version of the Baur cyclic mechanism. It involves the removal of a surface lattice oxygen atom, which would be a kind of reduction process. This mechanism will be discussed later also, in Section 3.3. In Japan, at the Kyoto Institute of Technology, an early study (1964) by Kato and Mashio also found that various types of titania powders had different photocatalytic activities, specifically to oxidize hydrocarbons and alcohols, simultaneously producing hydrogen peroxide [43]. Interestingly, these authors found that anatase powders were more active than the rutile ones. In further work at the University of Edinburgh, McLintock and Ritchie, using gas-phase adsorption measurements, studied the photocatalytic oxidation of ethylene and propylene at TiO2 [44]. This study is one of the first that we have found that shows that it is possible to oxidize organic compounds completely to CO2 and H2 O: C2 H4 + 3O2 2CO2 + 2H2 O. (2.10)

Fig. 2.2. Photocurrent vs. potential for illuminated rutile single crystal. SCE refers to the saturated calomel electrode, which is 0.241 V vs. the standard hydrogen electrode (SHE) (taken from Fujishima et al. [214]). 1971, Chemical Society of Japan.

The mechanism was proposed to involve the production of superoxide from oxygen: O2 + e O . 2 (2.11)

Markham et al. had already proposed this reaction to take place on illuminated ZnO [31]. Work on similar photo-reactions has continued into more recent years [45]. In an important study for the relationship between photoelectrochemistry and photocatalysis, which we will come back to later, in Section 3.2, Lohmann, at the Cyanamid European Research Institute, in 1966 published a highly detailed study of the photoelectrochemical (PEC) behavior of ZnO, both in the presence and absence of redox couples, including ferro/ferricyanide and methylene blue [46]. He clearly showed that the overall current at the ZnO electrode under illumination is the sum of anodic and cathodic currents, the anodic current being a combination of the dissolution of the ZnO itself and the oxidation of any redox species present. The cathodic process was the reduction of O2 to H2 O2 . This same approach had been introduced in 1938 by Wagner and Traud, at the Technical University of Darmstadt, to help explain the corrosion of metals, coupled with either hydrogen evolution or oxygen reduction [47,48]. Another PEC study that we will mention in this overview is that of Morrison and Freund, of the Stanford Research Institute, who also studied ZnO [49]. These authors also demonstrated in detail the various situations that arise in the presence and absence of redox couples. They also showed that oxidation products of some organic compounds are different in the case of the PEC electrode poised at the open circuit potential, i.e., with both oxidation and reduction currents balanced, compared to the case of a purely electrochemical oxidation. This difference was proposed to be due to the presence of cathodically generated superoxide. This is one of

the key points in understanding photocatalysis, and we will return to it later. During the late 1960s, one of the present authors, at the University of Tokyo, began to study the photoelectrochemistry of titania and found that oxygen gas was evolved at potentials very much shifted from the thermodynamic expectation, for example, with an onset of ca. 0.25 V vs. the standard hydrogen electrode (SHE), compared to the standard potential of +0.95 V in pH 4.7 aqueous buffer (Fig. 2.2) [50,51]. At first, there was skepticism of this result, but then it slowly became accepted. One reason that this result was difficult to understand is that the photoexcitation process converts the photon energy to chemical energy with little loss, and thus the photogenerated hole has a very high reactivity, so that it can react directly with either water or quite robust organic and inorganic compounds. Subsequently, a number of studies were carried out in which the photoelectrochemical oxidation process on TiO2 was examined for the competitive oxidation of water to O2 with the oxidation of a variety of inorganic and organic substrates [52,53]. Both types of reactions, of course, involve the use of light energy to get over an energy barrier, either an overall uphill process, as in the case of O2 evolution, or an overall downhill process, as in the case of organic oxidations. With the report of the ability to simultaneously generate hydrogen gas in 1972 (see Fig. 2.3) [19], the PEC field started to receive much wider attention, due to its implications for solar energy conversion [54,55]. From this point, also, photoelectrochemistry became closely associated with photocatalysis. We shall return to this topic later, in Section 3.2, and more carefully describe the detailed relationships. In this overview, we briefly mention some of the early work of Bard and co-workers at the University of Texas. Frank and Bard were the first ones to propose that illuminated TiO2 could be used for the purification of water via the photocatalytic decomposition of pollutants [56,57]. They suggested that cyanide and sulfite could be photocatalytically oxidized to cyanate and sulfate, respectively. In one of these studies, they found that photocatalytic oxidations could also occur at other illuminated semiconductors, such as ZnO, CdS, Fe2 O3 and WO3 . The most active semiconductor was found to be ZnO [57]. These authors expanded this study to a long list of inorganic and organic species [58] and speculated that photocatalysis could be a useful approach to both environmental cleanup and photo-assisted organic synthesis. The Bard group also suggested that each small illuminated semiconductor particle could be considered as a PEC cell, with both photo-assisted

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Fig. 2.3. Photoelectrochemical cell used in the photolysis of water [19].

1972, Nature Publishing Group.

oxidation and dark reduction reactions taking place [59]. The Bard group also proposed photocatalysis as a way to remove toxic metals from wastewater [60]. For a period of several years, the photocatalysis area continued to expand as a technology for both the selective oxidation of organic compounds [22] and the unselective oxidation of organic compounds for purposes of water purification [1,2,6164] and, to some extent, also air purification [6569]. There have also been reviews and listings of references of work on both air and water purification [4,7072]. For these technologies, it is typically necessary to use powerful ultraviolet (UV) light sources. For passive purification, without special light sources, it became apparent in the early 1990s that the amount of light present in either natural sunlight or artificial light was insufficient to process large amounts of organic compounds. Therefore, attention was turned to applications in which a relatively small number of UV photons could be used to carry out reactions at the TiO2 surface, for example, to decompose thin organic films on solid surfaces or to kill bacteria on surfaces [5,6,7376]. Thus, the focus turned from water purification to passive, self-cleaning, self-sterilizing solid surfaces, which, with sometimes only slight modification, could also be used to purify air. For these types of applications, it was necessary to develop ways to coat various materials with TiO2 films. Such applications included the self-cleaning glass cover for highway tunnel lamps, as well as a number of others, which have been reviewed previously and which will also be reviewed briefly later in this article. The large number of applications has also generated a renewed scientific interest in photocatalysis, and indeed on photo-assisted reactions on semiconducting metal oxides in general. One of the ways that we have tracked this activity is by looking at the number of citations of the 1972 Nature paper on water photolysis (Fig. 2.4(a)). This number of yearly citations has been climbing steadily over the past ten years or so and of course is correlated with the number of publications appearing on photocatalysis (Fig. 2.4(b)).

Fig. 2.4. (a) Citations per year of the 1972 Nature paper: Electrochemical photolysis of water at a semiconductor electrode [19]; (b) Numbers of research articles appearing on photocatalysis per year: search results in the period of 19722007 with the Web of Science (a) by the keyword photocataly (blue bars) and (b) the keywords TiO2 AND photocataly* (green bars). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

3. Properties of TiO2 materials 3.1. Crystal structures As often described, there are three main types of TiO2 structures: rutile, anatase and brookite. The size dependence of the stability of various TiO2 phases has recently been reported [77,78]. Rutile is the most stable phase for particles above 35 nm in size [77]. Anatase is the most stable phase for nanoparticles below 11 nm. Brookite has been found to be the most stable for nanoparticles in the 1135 nm range, although the Grtzel group finds that anatase is the only phase obtained for their nanocrystalline samples [79,80]. These have different activities for photocatalytic reactions, as summarized later, but the precise reasons for differing activities have not been elucidated in detail. Since most practical work has been carried out with either rutile or anatase, we will focus more attention on these. Rutile has three main crystal faces, two that are quite low in energy and are thus considered to be important for practical polycrystalline or powder materials [81]. These are: (110) and (100) (Fig. 3.1a, b). The most thermally stable is (110), and therefore it has been the most studied. It has rows of bridging oxygens (connected to just two Ti atoms). The corresponding Ti atoms are 6-coordinate. In contrast, there are rows of 5-coordinate Ti atoms running parallel to the rows of bridging oxygens and alternating with these. As discussed later, the exposed Ti atoms are low in electron density (Lewis acid sites). The (100) (Fig. 3.1b) surface also has alternating rows of bridging oxygens and 5coordinate Ti atoms, but these exist in a different geometric

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Fig. 3.1. Schematic representations of selected low-index faces of rutile: (a) (110); (b) (100); and (C) (001).

relationship with each other. The (001) face (Fig. 3.1c) is thermally less stable, restructuring above 475 C [81]. There are double rows of bridging oxygens alternating with single rows of exposed Ti atoms, which are of the equatorial type rather than the axial type. Anatase has two low energy surfaces, (101) and (001) (Fig. 3.2a, b), which are common for natural crystals [80,82]. The (101) surface, which is the most prevalent face for anatase nanocrystals [79], is corrugated, also with alternating rows of 5-coordinate Ti atoms and bridging oxygen, which are at the edges of the corrugations. The (001) (Fig. 3.2b) surface is rather flat but can undergo a (1 4) reconstruction [82,83]. The (100) surface is less common on typical nanocrystals but is observed on rod-like anatase grown hydrothermally under basic conditions (Fig. 3.2c) [80]. This surface has double rows of 5-coordinate Ti atoms alternating with double rows of bridging oxygens. It can undergo a (1 2) reconstruction [84]. Recently, the brookite phase, which is rarer and more difficult to prepare, has also been studied as a photocatalyst (see later). The order of stability of the crystal faces is (010) < (110) < (100) (Fig. 3.3) [85]. Recently also, the discovery of high-pressure phases of TiO2 was made [86]. These are expected to have smaller bandgaps but similar chemical characteristics [87]. Their existence was theoretically predicted and then experimentally proven; specifically, a form of TiO2 with the cotunnite structure was prepared at high temperature and pressure and then quenched in liquid nitrogen. It is the hardest known oxide. There are actually quite a variety of different structures for compounds with compositions close to TiO2 , including those with

excess titanium, such as the Magneli phases, Tn O2n1 , where n can range from 4 up to about 12 and the titanium oxide layered compounds, in which there can be as much as several percent excess oxygen. The oxygen-deficient Magneli phases, which also exist for V, Nb, Mo, Re and W, have been known for many years [8891]. In these compounds, oxygen vacancies are ordered and lead to the slippage of crystallographic planes with respect to each other; this leads to formation of planes in which, instead of corner or edge-shared TiO6 octahedra, there are now face-shared octahedra. Fig. 3.4 shows a schematic diagram of this situation. The corresponding Ti atoms are then unusually close and can interact electronically [92]. It has been found recently that laser ablation of a TiO2 rutile target can produce Magneli-phase nanoparticles [93]. There are also quite a number of layered titanate compounds in which there is an apparent excess of oxygen. For example, the layered protonic titanate Hx Ti x/4 x/4 O4 H2 O has been prepared 2 and exfoliated into single sheets, termed titania nanosheets. Fig. 3.5 shows (a) a diagram of the layered structure and (b) TEM and AFM images of single sheets. 3.2. Electronic properties It was reported in 1942 by Earle that rutile and anatase TiO2 in the form of powders are n-type semiconductors and that the conductivity decreases with increasing O2 partial pressure at temperatures above 600 C [94]. The effect of O2 was explained on the basis of an equilibrium involving thermal release of O2 from the lattice. We recognize today that this leads to the creation of Ti3+

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Fig. 3.2. Schematic representations of selected low-index faces of anatase: (a) (101); (b) (100); and (C) (001).

sites, which are responsible for the electronic conductivity. The activation energy for the electronic conductivity was found to be 1.75 eV for unsintered rutile powder and 1.7 eV for sintered rutile powder. No evidence for ionic conduction was found. Cronemeyer and Gilleo reported in 1951 that rutile single crystals exhibit a band-gap energy of 3.05 eV [95]. Absorption spectra were reported for both normal and slightly reduced crystals. For the latter, the blue color was based on a very broad absorption that peaked at 1.8 m. In the following year, Cronemeyer published a very extensive study of the electronic properties of single crystal rutile in which the preliminary findings were substantiated [96]. Detailed photoconductivity measurements were made. Dark conductivity and photoconductivity measurements were also made on a slightly reduced sample (reduction in H2 at 600 C). Interestingly, there was found to be a marked hysteresis in the dark conductivity when the sample was raised from room temperature to 250 C and then cooled back to room temperature. After cooling with a high applied electric field, the blue color was found to be concentrated at the negative electrode; the author ascribed this to movement of oxygen vacancies, but this is not confirmed. It is also

possible that the migration involved interstitial hydrogen. This is an ambiguity that has persisted for many years. Strong reduction of various types of samples was examined, at various temperatures between 300 and 1150 C. The strong reduction turns the samples blueblack. The activation energy for electronic conduction had already been reported to be 0.07 eV at room temperature, to produce a conductivity of ca. 1 1 cm1 . The conductivities were found to increase with increasing reduction time. A ceramic sheet sample heated in hydrogen at 800 C was found to experience a weight loss of 0.1%, corresponding to a release of oxygen that would provide 3 1020 electrons cm3 . Hall effect measurements showed a close agreement between the numbers of carriers and those calculated on the basis of the weight loss, indicating that all of the electrons were electrically active. Breckenridge and Hosler also published extensive work on the electrical properties of rutile [97]. The effective electron mass was found to be anomalously large, 30100 times greater than that of the free electron. These authors presented convincing arguments that the source of electronic conductivity in rutile is Ti3+ , which

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Fig. 3.3. Schematic representation of the brookite structure (taken from Beltran et al. [85]). 2006, American Chemical Society.

Fig. 3.5. Layered lepidocrocite-like protonic titanate: (a) schematic representation; (b) AFM image (taken from Shibata et al. [633] and Sasaki [634], respectively). 2007, Royal Society of Chemistry; 2007, Ceramic Society of Japan.

the neutral (fully reduced) vacancy being the lowest, followed by the singly reduced and finally the unreduced vacancy. The observed optical absorption edge was proposed to correspond to the transition between the valence band (based on O2 ) to the neutral oxygen vacancy, which was considered to be a narrow impurity band, with a high effective electron mass. The band at 3.67 eV above the valence band was proposed to be due to Ti4+ . The observed temperature and oxygen partial pressure dependences were fully explained by the equilibrium: O2 = 1 2 O2 + O2+ + 2e . v (3.1)

Fig. 3.4. Schematic representation of the crystallographic shear process to form Magneli phases from rutile (taken from Marezio et al. [632]). 2000, Elsevier Science.

results from the loss of oxygen, which produces oxygen vacancies Ov . It was proposed that these vacancies (valence +2) can have 0, 1 or 2 electrons associated with them, with distinct energies,

The electronic properties of rutile and anatase thin films were studied by Tang et al. [98]. There were large differences in the electronic conductivities of the two types of films after reduction by heating in vacuum at either 400 or 450 C. The anatase films became essentially metallic, with no change in conductivity with temperature. The rutile films, in contrast, retained measurable activation energies, 0.076 eV for 400 C and 0.06 eV for 450 C. The difference in behavior was considered to be due to the following properties for rutile: the average static dielectric constant of ca. 100, the effective electron mass of 20 m0 , and the donor state radius of ca. 2.6 . Since the latter is similar to the distance between Ti4+ sites, there is little overlap between donor wave functions. Anatase has the following properties: static dielectric coefficient of ca. 30, and reduced effective mass of ca. 1 m0 , based on an estimated donor state radius of ca. 15 . Based on optical absorption spectra, the band-gap energies were estimated to be 3.0 eV for rutile and 3.2 eV for anatase. Forro et al. reported on the electronic properties of

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high purity anatase single crystals and found an activation energy for electronic conduction of 0.004 eV [99]. Recently, Hendry et al. pointed out the problem that the exact nature of electron transport had not been solved, given the very wide range of values of Hall mobilities (0.0110 cm2 V1 s1 ) and polaron (electron + accompanying lattice distortion) effective masses (8me 190me , where me is the free electron mass) [100]. Part of the problem might involve the presence or absence of dopants, which were found to decrease the mobility. These workers, using THz spectroscopy with undoped rutile single crystals, based on Feynmans analysis [101], found intermediate size polarons and a mobility of 1 cm2 V1 s1 . Thus, they concluded that, in TiO2 films such as those used in dye-sensitized nanocrystalline solar cells, the main limiting factor might be interparticle contacts. However, this assumes a zero dopant level, which is unrealistic. Clearly, even now, further work needs to be done to clarify this basic issue. Other recent studies have also focused on the electronic properties from the standpoint of the dye-sensitized solar cell application (see, e.g., work of Aduda et al. [102]). For example, the effect of the morphology of porous titania films on the electron drift mobility was studied. Other studies have been focused on the gas-sensing properties, e.g., for hydrogen gas [103105]. In nanostructured form, titania has very high sensitivity for H2 , increasing greatly in electronic conductivity in the presence of low concentrations, for example, an increase of three orders of magnitude upon introduction of 1000 ppm H2 [103]. The mechanism proposed for the increased conductivity was thought to involve the adsorption of hydrogen on the titania surface, rather than incorporation into the bulk. The platinum electrodes that were used to contact the surface in this study were also possibly involved, acting to dissociate the H2 molecule, so that H atoms could be produced and adsorbed more easily. The subject of hydrogen interactions with titania has been well studied and will be treated in more detail in Section 3.5. 3.3. Surface structure studies It is quite difficult to separate work that has been carried out on the surface structure of titania from work that has been carried out on surface science in general and also on surface chemistry. The latter two subjects will be taken up in the two sections that follow. In this section, we will briefly treat the stoichiometric rutile and anatase surfaces. A detailed treatment has been given as part of Diebolds extensive review on the surface science [106]. Diebold shows how rutile can be cleaved to produce the commonly shown (110) surface, which is the most stable rutile surface (see Fig. 3.6). Ramamoorthy et al. carried out theoretical calculations on the rutile structure and found the (110) surface to be the most stable, based on the fact that it has the least dangling bonds [81]. The structure shown in Fig. 3.1a is that of the unrelaxed bulk and is rather flat, but these authors predicted that this structure should pucker slightly upon relaxation, with the five-fold-coordinated Ti atoms depressed by 0.32 a.u. (0.169 ), and the bridging oxygens also depressed, by 0.15 a.u. (0.079 ). Vogtenhuber et al., using similar calculations, found that the fivefold Ti atoms were depressed by 0.180 , the bridging O atoms by 0.156, the planar O atoms by 0.115 and the six-fold Ti atoms by 0.049 [107]. An experimental study that made use of surface X-ray diffraction found that the five-coordinate Ti was depressed by 0.16 , while the six-coordinate Ti was pushed out by 0.12 , and the bridging O was depressed by 0.27 . A total of seven theoretical studies were compared with the experimental surface X-ray diffraction results in the review of Diebold [106]. Most of these studies have agreed on the depression of the five-coordinate Ti, on the pushing out of the six-coordinate Ti and the depression

Fig. 3.6. Schematic diagram of the cleavage of rutile along the (110) plane (taken from Diebold [106]). 2003, Elsevier Science.

of the bridging O. However, a more recent experimental study that used LEED found that the bridging O is pushed out by 0.12 , in contrast to the earlier experimental study [108]. New theoretical calculations were mostly in agreement with the LEED study, except that the bridging O position was almost unchanged from the unrelaxed structure [109]. The titania surface may undergo significant structural changes when it is exposed to water. Onishi and co-workers have shown that single crystal rutile surfaces that have been prepared by standard methods used for UHV can actually erode and roughen when exposed to an aqueous electrolyte [110]. Subsequently, Nakato and co-workers reported on a method by which atomically flat single crystal surfaces could be prepared that were stable in aqueous electrolyte [111]; this method involved etching in 20% HF, followed by air-annealing at 600 C. The X-ray crystal truncation rod (CTR) technique has been used by Zhang et al. to examine the rutile (110) surface in the presence of pure water and of 1 molal RbOH aqueous solution [112]. Interestingly, the five-coordinate Ti, which had been found to be significantly depressed in UHV, was found to be depressed to a much smaller degree in pure water (0.051 ) and only slightly depressed in 1 m Rb+ . This is because the terminal position, which is empty in UHV, is occupied by a water molecule in aqueous solution and by a hydroxide ion in the alkaline Rb+ solution. The six-coordinate Ti, which had been found to be pushed out in UHV, was found to be depressed to a small degree (0.002 in water and 0.019 in Rb+ solution). The bridging O, which had been previously been observed to be depressed in the X-ray study and pushed out in the LEED study, was found to be pushed out, by 0.004 in water and 0.010 in Rb+ solution. All of the displacements were smaller than those found in vacuum, which the authors propose to be due the fact that either water molecules or Rb+ ions occupy positions that would be occupied in the bulk lattice. Further conclusions from this study will be discussed in Section 3.4, in which we treat interactions of titania with water. Other predictions from the Ramamoorthy work were in regard to the relative stabilities of the other rutile single crystal surfaces. The order of stability was found to be (110) > (100) > (011) > (001). This calculation is strictly only valid for 0 K and is for vacuum. Based on the results of the X-ray CTR study for aqueous solution, this ordering might be modified slightly, since the stabilities are based in part on the presence of dangling bonds, which would of course not be present any longer in the presence of water. 3.4. Surface chemical studies: Interactions with water The interactions of titania surfaces with water have been studied extensively and have been reviewed [106,113]. The earlier work involved conventional surface science methods. The studies

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Fig. 3.7. High-resolution electron energy loss spectra of rutile (110) at various dosages of water, starting from the clean surface at bottom (taken from Henderson et al. [114]). 1996, Elsevier Science.

reported during the past five years on this subject have involved both theoretical and experimental studies, the latter including a large number of scanning tunneling microscopy studies. The interactions with water are important to understand, because water, either liquid or vapor, is almost always present in photocatalytic reactions. These interactions are especially important for the later discussion of the photo-induced hydrophilic effect. Much of the work that has appeared over the past decade has been targeted at the question of whether water is adsorbed molecularly or dissociatively. Going back to one of the pioneering works on this subject, Henderson reported a high-resolution electron energy loss spectroscopy (HREELS)-temperature-programmed desorption (TPD) study that concluded that the adsorption of water on rutile (110) is molecular on the stoichiometric surface and dissociative on the reduced surface, which is conventionally produced by heat treatment, presumably forming oxygen defects [114]. The progression of HREELS spectra as a function of water coverage is shown in Fig. 3.7. In the background spectrum at the bottom, there is a very small peak at 3690 cm1 . This is due to the OH stretch for OH groups that are not hydrogen-bonded, often called isolated OH groups. This vibrational frequency is close to that for OH groups that stick out from the surface of liquid water, without being hydrogen-bonded to any neighbors, as observed with sumfrequency generation (SFG) spectroscopy [115]. At higher coverages, the peaks that appear are shifted to lower wavenumbers, indicating hydrogen bonding. There is no longer any evidence of the high wavenumber peak, except at the highest coverage. There is also a peak that appears at 1605 cm1 , which is due to the HOH bending mode of liquid water. Thus, it is certain that there are water molecules adsorbed. However, it is not certain whether there are also dissociated water molecules present that are hydrogen bonded to neighboring water molecules or to bridging oxygens. There is not much doubt that water dissociates at oxygen vacancies that are produced by heating in vacuum [113]. The

Fig. 3.8. Schematic diagram of a mixed molecular water-dissociated water monolayer on the rutile (110) surface (taken from Lindan [127]). 2003, American Institute of Physics.

main question is whether or not this is true for the non-reduced, stoichiometric surface. Theoretical studies have been divided into those that predict molecular adsorption [116122], those that predict dissociative adsorption [123129], and those that also find stability for mixed molecular-dissociative adsorption [122,125 127]. This is a particularly difficult problem, since the energy differences are rather small. One of the studies that has predicted dissociative adsorption also predicts a mixed layer of molecular and dissociated water at higher coverages [127]. Fig. 3.8 is a schematic diagram taken from this paper that shows how the mixed monolayer is arranged; the two structures both include hydrogen bonding between a water molecule adsorbed at a fivecoordinate Ti site and an OH group adsorbed at an adjacent 5coordinate Ti site. There is also a weak interaction of the water molecule with the bridging oxygen. For reference, the diagrams for purely dissociative and purely molecular adsorption are shown. Zhang and Lindan have also calculated a theoretical vibrational spectrum, which we show along with one of the HREELS spectra from Hendersons work (Fig. 3.9). Even though the simulated spectrum is significantly shifted upward in wavenumber, the two

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Fig. 3.9. (a) A HREELS spectrum for water adsorbed on rutile (111), taken from Henderson [114]; (b), (c) and (d) show simulated vibrational spectra for the various types of submonolayers and monolayers studied by Zhang and Lindan [127]. 2003, American Institute of Physics.

peaks have an appearance that is similar to the experimental spectrum. The strong peak at high wavenumber is due to an almost completely non-hydrogen-bonded OH, presumably the terminal hydroxyl group, and the weaker, lower wavenumber peak is due to the OH group of the water molecule that is bonded to the hydroxyl group. This type of double-peak structure is rather commonly observed in experimental infrared spectra for both rutile and anatase powders (Fig. 3.10). The quite sharp peak or peaks at high wavenumber are due to isolated OH groups, and the broader, lower wavenumber peak or peaks are due to hydrogen-bonded OH groups. In all of the spectra shown, there is some fine structure. Although not certain, this could be due to the existence of different crystal faces, with slightly different geometries for adsorption. Thus, it appears likely that, at least on powders, with coverages on the order of a monolayer, there could be mixed monolayers. Certainly, there is molecular water, and there must also be hydroxyl groups, with the OH group pointing up, normal to the surface, so that there is little opportunity for hydrogen bonding. However, for powders, there are, of course, a variety of crystal faces exposed, and distinct situations might be found on each. This is expected from the work of Henderson in a comparison of the (110) and (100) surfaces [130]. The latter was found to support dissociative, while the former was found to support molecular adsorption. Direct evidence for molecular adsorption on stoichiometric rutile (110) can also be found in STM work that has been targeted at interactions of water with oxygen vacancies. This general topic will be discussed next. The background of the work on the interaction of water with oxygen vacancies on rutile (110) has been given by Henderson

Fig. 3.10. A series of three sets of infrared spectra for (a) anatase [635] and (b) [636], (c) [637]) rutile powders acquired at various temperatures and water coverages. In (a), the water coverage decreases with spectrum number, and in (b) and (c), with spectrum letter. In (b) and (c), the original spectra were obtained in the transmission mode; all spectra have also been replotted with increasing wavenumber. In (b), the main peaks are listed. 1988, Royal Society of Chemistry; 1987, American Chemical Society; 1971, Royal Society of Chemistry.

[113]. One of the interesting aspects is that Ti3+ sites by themselves do not have special reactivity; for example, such sites on rutile (100) and on Ti2 O3 are not reactive. Only on the (110) surface are they reactive. The background of the STM work has also been discussed in the thorough reviews of Henderson [113] and Diebold [106]. For example, the latter discusses the problems of distinguishing between oxygen vacancies and hydroxyl groups that have been produced as a result of a water molecule reacting with an oxygen vacancy. A number of authors concluded that the mediumbrightness spots that they observed in STM between bright rows were due to oxygen vacancies. Diebold et al. pointed out that there appeared to be two types of defects that were observable on rutile (110), which they termed A and B [131]. Between the rows of 5-coordinate Ti atoms, which appear bright due to their high electron density, there are darker rows that are due to the bridging oxygens. The A type were observed to be significantly brighter than the B type and were proposed to be oxygen vacancies. The A defects were removed by scanning the tip at a voltage of +3 V, while the B type remained. The A-type defects were also found to be quite mobile. Suzuki et al. subsequently reported similar images and also found that the brighter spots were removable with a scan at +3 V [132]. These authors found that the spots were also removable by electron-stimulated desorption. They were also able to produce additional spots by dosing with atomic hydrogen. Thus, they proposed that the bright spots were due to hydroxyl groups formed by hydrogen adsorption on bridging oxygens. Brookes et al. also carried out STM measurements on rutile (110) and found that,

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Fig. 3.11. STM images of rutile (110) showing (a) oxygen vacancies and (b) bridging hydroxyl groups (taken from Wendt et al. [129]). 2005, Elsevier Science.

when they purposefully dosed the surface with water, it adsorbed without dissociation at 150 K and then dissociated at 290 K. Significantly, no terminal features were observed, i.e., there was no evidence for oxygens or hydroxyls adsorbed on 5-coordinate Ti sites, which would have resulted if the water had dissociated on the stoichiometric surface. Features were only observed on the dark, bridging oxygen rows. Thus, the authors concluded that the water had dissociated at oxygen vacancies. Schaub et al. carried out further work and found that there were two types of A defects, a smaller type and a larger, brighter one [117]. The latter was assigned to an oxygen vacancy and the former to a hydroxyl group, based on theoretical calculations. These authors continued to support the idea of water being dissociatively adsorbed only at oxygen vacancies, in line with their theoretical calculations. In further work from this group, however, the assignments were modified [129]. In this work, it was recognized that the surfaces that had been examined earlier were mostly hydroxylated. Special care was taken to achieve extremely low levels of background water, and thus it became clear that the darker features were the actual oxygen vacancies and the mediumbright features were individual hydroxyl groups that had been formed via water dissociation (Fig. 3.11). Subsequent work showed even more clearly how a water molecule moves along a row of 5-coordinate Ti sites and then reacts with a vacancy, first producing a pair of hydroxyl groups on neighboring bridging oxygens [133]. A second water molecule can then further catalyze the splitting of the hydroxyl pair in an energetic reaction that can result in one of the protons jumping several rows away. The initial water-dissociation process is shown in Fig. 3.12. Both the dissociation and splitting processes are also available as movies [134,135]. This paper also corrects the assignments that had been given in work focused on the interaction of oxygen (O2 ) with oxygen defects [136]. A paper by Bikondoa et al. appeared in early 2006 [137], apparently written without the knowledge of the one by Wendt et al., which appeared in 2005 [129]; this paper also clearly showed the whole situation regarding previously published assignments by the various groups. These authors can be credited with having recognized, from the beginning, the fact that the bright spots that were being observed by various groups were in fact due to hydroxyl groups, either single or double, that had been formed via water reaction at oxygen vacancies. Additional work has recently appeared on the STM observation of the reaction of water molecules with oxygen vacancies. Zhang et al. reported that the two bridging OH groups that are produced from the reaction are actually not identical [138]. The one that is produced at the site of the original oxygen vacancy is not observed

Fig. 3.12. STM images of rutile (110) showing the dissociation of a water molecule at an oxygen vacancy (taken from [133]). 2006, American Physical Society.

to move, whereas the one that is produced at an adjacent bridging oxygen due to the addition of a proton, is quite mobile. This result appears to be consistent with the report of Wendt et al., in which it was shown that proton can jump several rows away [133]. The implication is that the electrons associated with the original vacancy are rather localized. All the theoretical results have not been in agreement with this picture. The work just described on the interactions of water with vacancies on rutile (110) has a more general implication, in addition to the obvious one. Some of the studies that have been carried out over the years that have discussed reactions of oxygen vacancies may have been actually dealing with bridging hydroxyl groups. Henderson had already pointed out this effect in 1996 [114]. We note also the work of Mezhenny et al., which was focused on the question of whether or not UV light produces oxygen vacancies on the rutile (110) surface [139]. This work showed little effect of ordinary intensity levels of UV light, i.e., similar to those that are present in sunlight, in producing oxygen vacancies on the surface. It is likely that this work might have also suffered from unrecognized background levels of water, since they report STM images that are characterized by the brighter spots that have been assigned by later workers to bridging hydroxyls. Nevertheless, if oxygen vacancies had indeed been produced, there would have

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been an increase in the number of such hydroxyls, whereas no increase was observed. This result is in contrast to earlier results obtained with second harmonic generation and X-ray photoelectron spectroscopy that concluded that large numbers of defects were in fact produced and were reactive toward dioxygen [140]. These issues will be discussed at greater length in the next section and in Section 4, which deals with the photo-induced hydrophilic effect. Work on the interaction of water with titania surfaces has also been carried out with other techniques. For example, anatase powder was examined with gas-chromatographymass spectrometry (GCMS) and quantum chemical calculations. Experimental evidence was found for water dissociation, which was consistent with the theoretical calculations; the latter showed that the adsorption is molecular on the anatase (101) surface and dissociative on the (001) surface. The previously mentioned X-ray CTR study of Zhang et al. provides rather clear evidence for molecular adsorption for pure water on the stoichiometric rutile (110) surface and dissociative adsorption in alkaline aqueous solution (pH 12) [112]. 3.5. Surface chemical studies: Interactions with dioxygen and other species It was realized in 1998 that dioxygen does not only react with oxygen vacancies (or possibly, as discussed above, bridging hydroxyls) to produce a near-stoichiometric surface at temperatures above 600 K, but also, at temperatures below 600 K, it can leave behind an oxygen atom adsorbed at a 5-coordinate Ti site [141]. This realization led to doubts concerning previously published work that had found dissociative water adsorption at rutile (110). It also led to a reassessment of what had been an accepted procedure for the preparation of high quality, clean surfaces. The scheme that Epling et al. proposed to explain the interactions of O2 with oxygen vacancies and subsequent reaction with water is shown in Fig. 3.13. In the same paper, these authors also found evidence to support a similar end product that resulted when water was present initially, so that bridging hydroxyls had already been formed. Henderson et al. reported later that O2 can adsorb at a reduced, i.e., vacancy-containing, rutile (110) surface without dissociation at temperatures below 150 K [142]. One of the more interesting aspects was the observation that O2 can adsorb, probably as O , 2 at a ratio of up to three molecules per oxygen vacancy, which necessarily means that it does not have to interact directly with the vacancy but can reside on an adjacent cation site. Another paper from the same group appeared more recently exploring the reaction of O2 with bridging hydroxyl groups in more detail [143]. These authors conclude that the role played by O2 in photocatalysis involves specifically this reaction. They also found, in agreement with their earlier work, that a second monolayer of water blocks the access of O2 to the bridging OH groups, effectively impeding the electron transfer. On the basis of these results and other studies in which superoxide was generated both on thermally reduced titania and on UV-illuminated titania, the authors proposed that bridging hydroxyls are a key intermediate in the photocatalytic process. We agree with this proposal and also propose (see later) that such bridging hydroxyls can be generated electrochemically. The effect of gas-phase O2 on nanocrystalline titania films has been studied in terms of the gas-sensing application [144]. It was found that the film conductivity decreased in the presence of O2 . This effect could also be related to the effect discussed above but is more likely to involve the scavenging of bulk trapped electrons, as discussed in the next section. The authors also observed photo-induced adsorption of O2 , a phenomenon that had been described by Kennedy et al. in 1958, as mentioned in the historical overview [40].

Fig. 3.13. Schematic diagram of an O2 molecule reacting at an oxygen vacancy on rutile (110) and dissociating, with further reaction with a water molecule (taken from Epling et al. [141]). 1998, Elsevier Science.

We include here a brief mention of surface photochemical reactions involving O2 . The work of Thompson and Yates has employed the photodesorption of O2 as a means of monitoring the arrival of photogenerated holes to the surface of a rutile single crystal with exposed (110) face [145]. This process is essentially the reverse of the photo-adsorption process just alluded to, which requires a trapped electron, creating a partially or fully reduced O2 , i.e., O . The presence of methanol as a hole trapping agent 2 significantly decreased the photodesorption. In further work by the same authors, they proposed a fractal rate law to fit the observed kinetics of the reaction of trapped electrons with trapped holes [146]. It was assumed that the electrons were associated with oxygen vacancies, but this picture may be in some doubt, based on the ability of trace water to convert these to bridging hydroxyls. 3.6. Bulk chemistryHydrogen In this section, we briefly review the literature on the bulk chemistry of titania. This subject is mostly limited to the incorporation of elemental hydrogen. It also can include the incorporation of lithium or sodium, but this is beyond the present scope. The subject of hydrogen incorporation can also include electrochemically-induced processes; these will be treated in the next section. The characteristics of hydrogen as a bulk impurity in titania are central to the understanding of its electrical, electrochemical and photoelectrochemical behavior, which is, in turn central to the understanding of the photocatalytic behavior. An early paper on the optical and infrared absorption spectra of rutile single crystals by Soffer showed evidence for the incorporation of H, already present in the as-received crystal,

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Fig. 3.14. (a) H binding site within the rutile structure and (b) proposed diffusion path within the solid structure, along a c-channel (taken from Bates and Perkins [154]). 1979, American Physical Society.

and D, which was introduced by heating at 900 C in D2 ; these were evidenced by the appearance of IR bands at 3277 (main) and 3322 cm1 due to an OH stretch and at 2442 cm1 for the corresponding OD stretch [147]. The author remarked that the bands are unusually narrow for a solid-state OH stretch and also discussed the possibility of a hydrogen-bonding-related shift. Prior to the work of Hill in 1968 [148], a number of different studies had suggested that non-stoichiometry in rutile TiO2 was associated with increased electrical conductivity, but the mechanism was not clear. It had also been suggested that hydrogen acts as a dopant in rutile. In Hills work, heating rutile crystals in hydrogen below 600 C led to increases in the bulk hydrogen concentration, measured by IR. Heating above 650 C in vacuum led to decreases in hydrogen concentration, coupled with oxygen loss to produce water. This led to increased conductivity, probably due to the formation of faults involving Magneli phases. For the heated crystals, the electrical behavior was modeled involving a series network of two parallel RC circuits, with one being associated with an exhaustion layer, i.e. either a depletion layer or layer that is very low in carriers, as discussed in the next section. Johnson et al. reported further detailed work on the optical and infrared spectra for H and D incorporated in rutile single crystals [149]. This paper referred to the earlier work of von Hippel et al. that provided presumably more accurate values for the absorption maxima: 3276 and 3317 cm1 for OH and 2435 and 2463 cm1 for OD. That paper had proposed that the peak splitting was due to slightly differing OTi distances. Johnson et al., however, denied this possibility due to the symmetry of the structure. H and D doping was carried out by heating in an atmosphere of H2 O or D2 O, plus O2 at 850 C, or in some cases, H2 or D2 below 550 C. Conduction-band electrons produced a broad absorption band at 1.5 m. This was remarked to not be due to conventional free electron behavior, which should produce no peak. These authors carried out a detailed analysis of the various possible binding sites for H or D within the crystal. The same group reported the use of the IR absorption band in the precise determination of H and D concentrations in rutile [150]. DeFord and Johnson studied the H/rutile system in detail from the viewpoint of theoretical semiconductor and thermodynamic properties [151]. Later, they made measurements of H and D diffusion using the isotope exchange technique in order to avoid internal electric fields [152]. The diffusion was carried out simply by heating the samples in the appropriate atmosphere (see above) for various times and then measuring the H or D concentrations via the IR absorption. The diffusion coefficients for H varied from ca. 3 108 cm2 s1 at 350 C to ca. 1.7 106 cm2 s1 at 700 C along the c-axis,

compared with 8 108 cm2 s1 at 698 C along the a-axis. The diffusion coefficient estimated for room temperature was 1.8 1013 cm2 s1 , which was in reasonable agreement with the value very roughly estimated by Chester and Bradhurst, in the range 1011 1013 cm2 s1 , based on electrochemical insertion. Bates and Perkins measured the infrared frequencies for H, D and T in rutile TiO2 and carried out a detailed structural analysis, comparing the results with theory for anharmonic oscillators and also with that of hydrogen bonding [153]. The agreement with the latter was poor. Bates et al. later published a much more detailed study, including a review of the literature up to 1979 [154]. They carried out a detailed analysis of the mechanism of diffusion of H in rutile. The binding site within the lattice for the proton is shown in Fig. 3.14a, and the proposed path for diffusion in Fig. 3.14b. The binding site was later confirmed by Klauer and Whlecke using polarized Raman [155]. The understanding of this system achieved in this work is excellent. It was concluded that the wavenumber shift of the IR absorption was not due to hydrogen bonding, which is consistent with the observed sharpness of the band. Instead, it was proposed to be due to the electrostatic environment within the lattice (however, see below). Further work was also published on tritium diffusion [156]. Peacock and Robertson have carried out quantum chemical calculations for H in a variety of oxides that are considered as high dielectric constant oxide gate materials [157,158]. They find that the H0 energy level lies above the conduction band in ZnO, TiO2 and SrTiO3 , consistent with the fact that H is a shallow donor in all the three. Work of Park et al. also confirmed these results for rutile [159]. Koudriachova et al. have also carried out a recent ab initio quantum chemical calculation on H incorporation in rutile [160]. These authors sought to recheck the binding site, due to the difficulty already mentioned, i.e., the OHO bond distances were not consistent with established rules relating them to vibrational frequencies. The new calculation found a distortion in the cage surrounding the H atom such that the distances became highly consistent with the correlation. They found that the H atoms are most favorably located at ordered positions, as shown in Fig. 3.15a. The lattice expands linearly with increasing H incorporation (Fig. 3.15b). It is also appropriate to mention here theoretical calculations that were carried out on the surface adsorption of hydrogen as H2 [161]. In that work, it was found that up to one monolayer is adsorbed, with all of the bridging oxygens becoming hydroxylated and the underlying Ti4+ ions being reduced to Ti3+ . We might note that this type of surface could in principle be produced thermally by removing half of the bridging oxygens, followed by exposure to

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Panayotov and Yates recently reported on experiments in which they reduced titania pellet samples (Degussa P25) with a source of H atoms [164]. They were able to observe the broad, featureless background visible-IR spectrum expected for conduction-band electrons. In addition, there were increases in electronic conductivity. However, there was no discernable IR absorption at ca. 3280 cm1 for the internally bound OH stretch already discussed above. The activation energy for the diffusion of H into the solid was estimated to be 0.09 eV. 3.7. Electrochemical properties The intrinsic electrochemistry of TiO2 has been studied continuously for a long period, since the first report of Boddy in 1968 on the oxygen evolution reaction [165]. This work made use of single crystal rutile electrodes. Interestingly, this work contains a figure in which the photocurrent vs. potential behavior is given, one year prior to the reports of one of the present authors [50,166]. This will be discussed further, in the next section. Many aspects of the behavior of TiO2 can be explained on the basis of a semiconductor model, as already discussed. One of the ways of characterizing a semiconductor is to measure its flat-band potential electrochemically, for example, with capacitance. In our original work reported in 1969, we described such measurements and concluded that the flat-band potential EFB for rutile (001) is pH-dependent, with a relationship similar to the following [167]: TiO(O) + H+ + e = TiO(OH) with the resulting Nernst relationship: EFB = 0.00 + F RT ln[H+ ] = 0.0591pH at 25 C. (3.3) (3.2)

Fig. 3.15. (a) Illustration of a high stability ordered arrangement of interstitial hydrogen atoms in the rutile structure, with stoichiometry H1/4 TiO2 (taken from Koudriachova et al. [160]). (b) Plot of lattice volume for various numbers of interstitial H (open circles) and Li atoms (filled circles) (taken from Koudriachova et al. [160]). 1994, American Physical Society.

water. It could also be produced in principle electrochemically (see Section 3.7). It has been found that titania nanotubes respond to the presence of hydrogen in the gas phase, as already discussed in the section on electronic properties. In that work, it was not considered that hydrogen could actually be absorbed. However, work of Lim et al. showed that for nanotubes that were prepared hydrothermally, there was a reversible uptake of ca. 2% [162]. The incorporation led to an increase in the IR absorption (3427 cm1 ), which is significantly higher than that for single crystal rutile. Only 75% of the uptake was reversible at room temperature with the remaining 25% requiring temperatures up to 130 C to desorb. A relatively detailed study has been carried out on small rutile crystals by the use of optical and IR absorption and Raman scattering [163]. The effect of neutron irradiation was also studied. The incorporation of H in minerals is of interest to geologists, because it affects the macroscopic properties and can be a way by which water is incorporated in minerals that normally do not absorb water.

This result has essentially been confirmed by subsequent workers, with the value at pH 0 being +0.01 0.05 V vs. SHE for rutile (001) [169,171,175]. The pH dependence is typical of the behavior of most oxide semiconductors and has generally been considered to be due to a surface acidbase equilibrium for these oxides. The value for anatase is more negative: 0.20 V vs. SHE [168]. There has been a certain amount of discussion devoted to the question of exactly how the capacitance measurements should be conducted and how the results should be interpreted, even for single crystals. This discussion is interesting, because it displays the convergence of an ideal, simple theoretical model with a real, non-ideal complicated material. The model, already discussed, involves an ideal semiconductor with a space charge region whose thickness is dependent upon the potential difference between the Fermi level, to which we can assign an electrochemical equivalent EFL and the conduction-band-edge energy, again given on the electrochemical scale, ECB . The energy difference between ECB and EFL deep within the bulk of the material is dependent upon the carrier concentration. The space charge capacitance CSC based on this simple model is given by 12 CSC

=

2(E EFB )

0 eND

(3.4)

where ND is the bulk concentration of donors, with the consequence that a plot (MottSchottky) of C 2 vs. potential yields the carrier concentration from the slope and the flat-band potential from the intercept with the potential axis. Many workers have found that these plots are either non-linear or that the intercept gives a result that is not reasonable, for example, more negative than that given in Eq. (3.3). Such results have been explained in various ways, including (1) non-uniform depth profile of carriers and (2) deep trap levels. There appears to be some consensus on the merits of the first explanation.

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Fig. 3.16. MottSchottky plots for a rutile (001) surface in pH 4.7 buffer: (a) raw data for a more lightly doped sample (circles), with a solid line showing the fit to Eq. (3.5) (including both space charge CSC and passive layer CPL contributions); (b) calculated intrinsic CSC behavior, after removing the effect of CPL ; (c) raw data for a more heavily doped sample (squares), with a solid line showing the fit to Eq. (3.5); (d) calculated intrinsic CSC behavior, after removing the effect of CPL . SCE refers to the saturated calomel electrode, which is 0.241 V vs. the standard hydrogen electrode (SHE) (based on [167]).

A model that can explain the appearance of a steep slope near the intercept and a shallower slope at more positive potentials is that of a thin layer near the surface that has a lower carrier concentration. This model is reasonable, because, as has been reported often, the effect can arise as a result of oxidizing etching treatments, which could remove electrons from such a surface layer, especially if the treatment is conducted for a relatively short time. In this case, the overall capacitance behaves as if there is a smaller, potential-independent capacitance due to the passive layer CPL , in series with the potential-dependent space charge capacitance. The slope of the upper portion of the curve is somewhat increased from that which is characteristic of the carrier concentration in the bulk of the material, and the intercept is shifted to the negative. The overall behavior is described by 12 CTOT

=

2(E EFB )

0 eND1 CSC 1 CPL

+

2(E EFB )

1/2

1 CPL

0 eND

+

12 CPL

(3.5)

which is derived by substituting 1 CTOT

=

+

(3.6)

into Eq. (3.4). In our original paper, we reported curves with this type of double slope (see Fig. 3.16) and made a mathematical correction of this type in order to estimate the true flat-band potential [167]. Our explanation at that time invoked a Helmholtz layer capacitance, which was later correctly disputed by Dutoit et al. [169]. Based on our nitric acid etching procedure used in that work, however, it is quite reasonable to invoke the passive layer model just described. DeGryse et al. also found fault with the involvement of the Helmholtz layer and proposed an inhomogeneous carrier concentration [170]. Tomkiewicz proposed a model based on surface states with energies in the middle of the band-gap [171]; this model was criticized by Ullman, who proposed a surface passive layer [172]. It should be noted that this same model has been discussed by Schoonman et al. [173], but the equation given in that paper was different, in that the cross-term was not included, leading to the erroneous result that the curve is simply raised but has the same slope. If the MottSchottky behavior does not fit this simple model, it is still possible that it can be fitted with a more complicated model, in which the slope at any given point in the curve can be related to the carrier concentration at a certain depth; thus a complete concentration profile can be estimated [174].

Finklea has given an excellent summary of the practical methods of obtaining linear MottSchottky plots with correct intercepts [175]. The opposite situation can also arise if a surface layer exists with a higher concentration of carriers compared to the bulk. This leads to a shallow slope near the flat-band potential, followed by a steeper slope at more positive potentials. In certain cases, if the carrier concentration is sufficiently high, the space charge capacitance can approach that of the electrical double layer, and the Helmholtz capacitance must also be considered. It can be argued in general that the behavior of TiO2 is simply not ideal, and, in each individual case, a full impedance treatment is necessary to extract the space charge capacitance or even to evaluate whether or not it exists. Similar situations also exist for oxide films; the situation is particularly complicated for oxide films grown on metals, for example, iron and zirconium [176]. For certain types of TiO2 electrodes, it may be doubtful that the behavior can be strictly described with a semiconductor model. For example, the MottSchottky plots may exhibit a significant frequency dispersion, either with or without non-linearities. This can signal the fact that a different type of model might be more appropriate. In most cases, it is advisable to examine the full impedance spectrum over a range of potentials in order to determine whether or not the behavior does in fact follow that expected for a semiconductor. One specific type of behavior that has been observed is that of an electrochromic film, in which all of the charge injected into the film is associated with increased absorption of visible light (see Refs. [177181] and later discussion). The electrochemical impedance spectral (EIS) behavior can be quite powerful in elucidating the behavior of electroactive materials, i.e., those that can undergo electron transfer reactions within pores and even within the solid material itself. Nogami examined the EIS behavior of several single crystal rutile samples and obtained sets of straight lines, with varying slopes in Bode amplitude (log |Z | vs. log frequency) plots [182]. He explained this behavior on the basis of a disordered layer on the surface of the single crystal. The Bode plots show straight lines for the logarithm of the magnitude of the impedance (absolute value of the complex impedance) vs. the logarithm of the frequency. At more positive potentials, the slopes of the lines are close to 0.7, while, at more negative potentials, the slopes are close to 0.5. For a pure capacitance, the slope should be 1.0, while, for a pure resistance, it should be 0.0. For intermediate cases, there are both resistance and capacitance distributed through the material. The simplest electrical case is that of the uniform semi-infinite transmission line, in which there is a ladder of constant resistances and capacitances (Ref. [183] and references therein). This type of behavior has been shown to be followed by a porous medium with cylindrical pores. A rate-limiting diffusion process in which an electroactive species diffuses through a uniform medium behaves electrically in the same way, and thus there is some ambiguity as to which process is actually operating. In either case, the slope should be 0.5, as observed by Nogami for more negative potentials. For the general case, in which the slope of the Bode plot can have an arbitrary value between 0.5 and 1, the same type of RC ladder is also valid, and the physical picture is also similar to that for the uniform network, but with pores that are noncylindrical. For example, Wang and Bates have shown that hornshaped pores can give rise to Bode slopes in this range [184]. Thus, the results of Nogami could be explained by a situation in which horn-shaped pores of larger diameter (lower roughness factor) exist at higher potentials; at intermediate potentials, the diameters become smaller (higher roughness factor); finally, at the most negative potentials, the pores behave as if they are cylindrical and longer than the penetration length of even the lowest frequencies

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Fig. 3.17. Electrochemical impedance spectrum for a nanocrystalline anatase film on a conductive support: (upper left) experimental results from Cao et al. [187]; (upper right, lower right) simulated log of the impedance amplitude vs. log frequency and phase angle vs. log frequency, respectively. 1995, American Chemical Society.

measured. It should be noted that impedances with arbitrary slopes are often referred to as constant phase elements, meaning simply that the phase angle, which is 0 for a pure resistance, 90 for a pure capacitance and 45 for the uniform transmission line, is constant but is not one of these standard ones. In any case, it seems clear that Nogamis conclusion, i.e., that the frequency dispersion of the MottSchottky plots is due to the presence of a disordered layer on the single crystal surface, might be valid. Frequency dispersion is a result of the mixing of resistive (i.e., electronically conducting) character with capacitive (i.e.,