On the Origin of the Spectral Bands in the Visible Absorption Spectra

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FEATURE ARTICLE On the Origin of the Spectral Bands in the Visible Absorption Spectra of Visible-Light-Active TiO2 Specimens Analysis and Assignments Vyacheslav N. Kuznetsov† and Nick Serpone*,‡ V. A. Fock Institute of Physics, State UniVersity of St. Petersburg, St. Petersburg, Russia, and the Gruppo Fotochimico, Dipartimento di Chimica Organica, UniVersita di PaVia, Via Taramelli 10, PaVia 27100, Italia ReceiVed: February 4, 2009; ReVised Manuscript ReceiVed: April 2, 2009

This article presents a systematic analysis of the absorption spectral features of various titanium dioxide specimens, whether doped or undoped, in the visible spectral domain reported extensively in the literature; also it briefly examines the origins of such bands in what have become known as visible-light-active TiO2 photocatalysts or second-generation photocatalysts. We have deduced that at energies of hν less than 2.0 eV (λ > ∼600 nm) the three spectral features occurring in the near-infrared and infrared regions originate from Tin+-related (n ) 3, 2) color centers, whereas the three absorption bands seen in the visible spectral region AB1 (2.75-2.95 eV), AB2 (2.50-2.55 eV), and AB3 (2.00-2.30 eV) are associated with oxygen vacancies (F-type color centers) on the basis of recently demonstrated experimental observations. The question also discussed is why reduction of TiO2 that accompanies the process of TiO2 doping results only in the formation of the absorption bands AB1, AB2, and in some cases AB3; that is, the absorption features of Ti-related centers are totally suppressed. Recent studies that have demonstrated the reduction of TiO2 during the doping process were examined to argue on the dominant role of F-type color centers in the visible-light-activity of TiO2 photocatalysts. 1. Introduction The origin of absorption bands in the visible spectral range and the visible-light photoactivity of anion- and/or cation-doped TiO2 specimens remains a hot topic of discussion of so-called second generation photocatalysts,1-3 in spite of numerous (in the hundreds) published studies,4 some of which have recognized that intrinsic defects, including those defects associated with oxygen vacancies, contribute to the absorption of light in the visible spectral region.5-8 However, the actual knowledge of the optical properties of such defects and the conditions needed for the formation of such defects remain experimentally somewhat elusive. As such, the abovementioned recognition is a rather formal generalization. For example, in a recent review article by Fujishima and co-workers,9 the absorption features of oxygen-deficient TiO2 systems in the region hν < 2.5 eV (λ > 496 nm) were derived only from the calculations of Lin et al.10 despite the existence of reported experimental studies (see below). The aim of the present article is to examine the earlier spectral results and to analyze systematically all of the available experimental data on the absorption features of reduced TiO2 specimens (as single crystals or in powdered form) and of some doped TiO2s with the ultimate additional goal of delineating the principal factors that have hampered an unbiased analysis of the origin(s) of the absorption characteristics of visible-lightactive (VLA) TiO2 materials. In this regard, the evolution of absorption spectra of reduced titania specimens, that are comparable to spectra of the yellow/orange colored and doped * To whom correspondence should be addressed. E-mail: nickser@ alcor.concordia.ca; [email protected]. † State University of St. Petersburg. ‡ Universita` di Pavia.

TiO2, was examined in some detail because the majority of absorption spectra of reduced TiO2 systems differ fundamentally from the spectra that evolve from the doping process. Demonstration of the stage of reduction of TiO2 accompanying the doping process is also of great importance as it leads to a more realistic description of processes and sets the stage for understanding the origin(s) of the visible light absorption of VLA materials. 2. Discussion We begin the discussion by the recognition that, despite the large body of literature on titanium dioxide, there exists insufficient information on the optical properties of intrinsic point defects in pristine undoped TiO2 systems. In this regard, it is widely believed that the activity of the [110] surface of TiO2 is critically dependent on the presence of defects (including oxygen vacancies) on the surface and in the subsurface region. These defects have been studied extensively by X-ray photoelectron spectroscopy (XPS), by ultraviolet photoelectron spectroscopy (UPS), by electron energy loss spectroscopy (EELS), by noncontact atomic force microscopy (NCAFM), by scanning tunneling microscopy (STM), by electron paramagnetic resonance spectroscopy (EPR),11 and to an otherwise limited extent by optical spectroscopy. Density functional theory (DFT) calculations have also addressed the issue of point defects (oxygen vacancies, Ti3+ centers, etc.) in TiO2 crystal faces,12 but as otherwise noted by Finazzi and co-workers,13 the DFT findings reveal a limited predictive power of these theoretical methods to describe the electronic structure of reduced titania in the absence of accurate experimental data. Germane to this, recent review articles have failed to bring some clarification to

10.1021/jp901034t CCC: $40.75  2009 American Chemical Society Published on Web 06/22/2009

Feature Article this problem.9,14-16 Thus, it is important to consider first the issues of the red-shifts of the absorption edge of pristine but reduced TiO2 and of anion/cation-doped TiO2 specimens together with the related band gap narrowing claimed by many (see below) to be at the origin of these red-shifts. We then examine the optical spectra of variously doped TiO2 systems and undoped TiO2 materials treated under various conditions, followed by considerations of assigning the absorption bands to certain defects and/or color centers. 2.1. The Issues: Absorption Edge Red-Shifts and Band Gap Narrowing in Visible-Light-Active TiO2 Specimens. A controversy that remains at the forefront in the literature of the past decade concerns the root causes of the red-shifts of the absorption edge in doped TiO2 specimens, in particular aniondoped systems. One school suggests that the red-shifts involve mostly oxygen vacancies in the metal-oxide lattice (surface and bulk) acting as electron traps to yield Ti3+ and/or F-type color centers. However, with regard to band gap narrowing an extensive examination of the experimental and theoretical literature reveals a lack of consensus on whether or not there is band gap narrowing in doped TiO2 materials, a point of view based almost exclusively on DFT calculations of the densities of states and band gap energies. Some studies have deduced that there is a rigid shift of the valence band edge to higher energies, thus narrowing the intrinsic band gap of TiO2 as a consequence of doping, while others have concluded otherwise (see below). The discrepant views are not simply semantic. Anion doping, or for that matter any type of doping, of TiO2 does shift the absorption edge of the doped metal oxide to longer wavelengths, thus affording potentially visible-light photoactive materials in several important applications of surface processes that take place on the TiO2 particle surface. A key question that also keeps coming up in the literature is the chemical nature and the location of species that lead the absorption edge of TiO2 to be red-shifted and consequently to the visible-light photoactivity of TiO2. Species such as NOx, NHx, and N2- have been proposed, as well as hyponitrite, nitrite, and nitrate species that have been confirmed experimentally in N-doped titanias. Another no less important key question regards the electronic structure(s) of (anion)-doped materials and their fate when subjected to UV and/or visible light irradiation. Although, these questions have been addressed in several interesting computational studies, a consensual acceptance of the results has yet to be reached. The first significant reports of anion-doped TiO2 began to appear in the early 1990s, although not until the 2001 study by Asahi and co-workers,17 who doped TiO2 with various anions (e.g., A ) N, C, and S) to produce VLA-TiO2 systems, did such materials attract the attention and the interest of so many aficionados of metal-oxide photocatalysis. In particular, these authors17 observed that doping of TiO2 specimens shifts the absorption edge of TiO2-xAx systems to lower energies and proposed that this result was caused by a narrowing of the TiO2 band gap leading to an increase of their photoactivity in the visible spectral region. Taken literally, band gap narrowing in such doped TiO2 materials would then mean that the intrinsic band gap energy of TiO2 decreases in the presence of dopants. What in fact does change, as seen experimentally, is the energy photothreshold for activating doped titania specimens to carry out surface photoinduced redox processes. In fact, the red-shifted absorption edge reflects the red-limit of TiO2 photocatalysis or the extrinsic band gap(s) of doped TiO2 used in the past to refer to photooxidations and photoreductions that occur on excitation of TiO2s in the visible spectral region. This is in stark contrast

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Vyacheslav N. Kuznetsov is a senior scientific researcher at the V. A. Fock Institute of Physics of the State University of Saint Petersburg, Russia. He obtained his Ph.D. Degree in Physics from Leningrad State University in 1986. His research interests cover the area of photophysics and photochemical reactions on the surface of metal oxides including the degradation of materials under conditions that simulate space environment and materials for the restoration of paints. Currently, he is interested in the true understanding of the nature of the photoactivity of TO2-based and ZnObased photocatalysts in the visible spectral region.

Nick Serpone received a B.Sc. (Honors Chemistry, 1964) from Sir George Williams University (Montreal) and a Ph.D. from Cornell University (Physical-Inorganic Chemistry, 1968), after which he joined Concordia University in Montreal as Assistant Professor (1968-73), Associate Professor (1973-80), and Professor (1980-98). He spent 4-year leaves in Bologna (1975-76), Lausanne (1983-84), Lyon (1990-91), and Ferrara (1997-98) and was a consultant to 3M’s Imaging Sector. He took early retirement from Concordia University as a University Research Professor (1998-2004) and Professor Emeritus (2000 to present). He was Program Director at NSF (1998-2001) and has been a Visiting Professor at the University of Pavia since 2002. In July-August 2008 he was a Visiting Professor at the Tokyo University of Science, Noda Campus. His research interests are currently in the photophysics and photochemistry of semiconductor metal oxides, heterogeneous photocatalysis, environmental photochemistry, photochemistry of sunscreen active agents, and application of microwaves to nanomaterials and to environmental remediation. He has co-authored over 390 articles and has co-edited four monographs. He was the recipient of the “Best Paper Award” from the Society of Imaging Science & Technology (1997; with Mel Sahyun and Boris Levy). He maintains an active collaboration with researchers at the Institute of Physics of the State University of St. Petersburg, Russia (Dr. Kuznetsov, Dr. Emeline, and Dr. Ryabchuk) and with Prof. Hidaka (Meisei University, Tokyo), Dr. Horikoshi (Tokyo University of Science), and Prof. Albini (Pavia, Italy).

to the intrinsic band gap of TiO2 (anatase, 3.2 eV; rutile, 3.0 eV) that is used to describe the electronic structure of pristine undoped titania. In their 2001 seminal report in Science, Asahi and coworkers17 reported DFT calculations of densities of states in anatase TiO2 for substitutional doping of oxygen in the lattice by C, N, F, P, and S species using the full-potential linearized augmented plane wave (FLAPW) formalism in the framework

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of the local density approximation (LDA). The calculations were carried out without geometry optimization for the five aniondopings because the resulting atomic forces were apparently too large to obtain reasonable positions in the unit cell (8 TiO2 units per cell). It was noted that to the extent that S2- has a larger formation energy for substitution (4.1 eV) than is required for N (1.6 eV), and that because of its larger ionic radius it would be difficult to incorporate S2- into the TiO2 lattice. In the case of C- and P-doping, the dopants introduce states too deep in the band gap of TiO2 to satisfy the condition that states within the band gap should overlap sufficiently with band states of TiO2 to transfer photoexcited carriers to reactive sites at the catalyst surface within their lifetime. For N-doped titanias three types of doping with N were considered: (a) substitutional N doping (NS), (b) interstitial N doping (NI), and (c) both types of doping (NS+I) in the anatase TiO2 architecture. Substitutional doping of N proved the most effective because its 2p states presumably contribute to band gap narrowing by mixing with the O 2p states in the valence band of TiO2. The calculated imaginary parts of the dielectric functions of TiO2-xNx showed a red-shift of the absorption edge by the N doping, with the dominant transitions being N 2pπ f Ti 3dxy rather than O 2pπ f Ti 3dxy as in pristine TiO2. However, the calculated band gap energies were considerably underestimated relative to the experimental value (calculated Ebg ) 2.0 eV versus experimental Ebg ) 3.2 eV for anatase). This was attributed, in part, to the well-known shortcomings of the LDA approach. A scissors operator, which displaces the empty and occupied bands relative to each other by a rigid shift of 1.14 eV, was then used so that the minimum band gap was in line with experiment (corrected Ebg ) 3.14 eV) for the band gap of anatase TiO2.17 The band gaps of N-doped TiO2-xNx systems were also adjusted by 1.14 eV on the assumption that the amount of band gap underestimation in the LDA approach was not affected by N-doping, because long-range screening properties in TiO2-xNx were likely similar to those in TiO2. The absorption edge of N-substituted TiO2 photoactive films (denoted N-TiO2(X)), prepared by a RF-MS deposition method with various N2/Ar mixtures as the sputtering gas (X ) 2, 4, 10, 40) and a calcined TiO2 plate as the source material, shifted smoothly to 550 nm with the extent of the shift depending on the concentration of N (X) in the range of 2.0-16.5% substituted within the TiO2 lattice.18 XPS and XRD measurements showed significant substitution of lattice O atoms of TiO2 by N atoms, which according to the authors play a crucial role in the band gap narrowing of the TiO2 thin films (from 2.58 to 2.25 eV depending on X compared to 3.2 eV for anatase) and display good visible-light photoresponse. XRD patterns of pale yellow, yellow, and dark green TiO2-xNx powders (x ) 0, 0.0050, 0.011, and 0.019) prepared by annealing anatase TiO2 powder (ST-01) in a flow of NH3 at 550, 575, and 600 °C, respectively, showed that the samples retained the anatase structure and that no TiN phase formed; the band gap energy remained at 3.2 eV.19 Degradation of isopropanol with UV light and with visible radiation resulted in different quantum yields for the evolution of CO2, indicating that N-doping formed a narrow N 2p band above the valence band of TiO2 since the occurrence of any band gap narrowing in TiO2-xNx, as believed by Asahi et al.,17 would have required identical quantum yields. In addition, irradiating with visible light led to a decrease of the quantum yields with increase in the quantity (x) of N dopant because of the increase in the number of oxygen vacancies (VO) with increase of x in TiO2-xNx. In this case, the VOs act as recombination centers for

Kuznetsov and Serpone the photogenerated charge carriers, as do the doping sites since under UV irradiation the quantum yields also decreased with increase in x.19 Sakthivel and Kisch20 prepared slightly yellow N-doped anatase TiO2 with various N loadings. The procedure involved hydrolysis of TiCl4 with a N-containing base (e.g., aqueous NH3, (NH4)2CO3 or NH4HCO3) followed by calcination at 400 °C. However, contrary to the assertions of Asahi and co-workers,17 the valence band edge did not change upon N-doping, despite the red shift of the TiO2 absorption edge to ∼520 nm. Also contrasting the inferences of Asahi et al.17 a mechanistic study into the photooxidation of water (evolution of O2) through measurements of anodic photocurrents at N-doped TiO2 film electrodes led Nakamura and co-workers21 to deduce that the visible-light responses of N-doped TiO2 originates from an N-induced midgap level located slightly above the upper level of the O 2p valence band. These authors also concluded that photooxidations of organic compounds under visible light illumination proceed mainly by reactions with surface intermediates of water oxidation (e.g., •OH radicals) or oxygen reduction (e.g., O2•-) and not by direct reactions with holes that may be trapped at the N-induced midgap level. Indeed, visible-light-active TiO2 systems doped with C, S, or N possess, in most cases, good attributes toward the photooxidation of organic and inorganic (e.g., NOx) substrates, particularly N-doped TiO2 materials prepared by various but otherwise simple methods. Although they all display absorption features and red-shifted absorption edges (at least to 550 nm), their photoactivity under visible light has not always correlated with such absorption features. Tachikawa and co-workers22 addressed these issues and described some mechanisms (Figure 1) of the photoactivity of VLA-TiO2 specimens. They also echoed the conclusions of Nakamura et al.21 in that photooxidations by VLA TiO2-xNx systems proceed by O2•- and/or •OH radical oxidation and not by direct reaction with h+ trapped at the N-induced midgap level. The exact cause(s) of the absorption edge red-shifts in various N-doped TiO2 (anatase) powders became confused with the report from Yates’ group23 that the absorption edge of a N-doped TiO2 rutile single crystal shifts (in fact) to higher energy (i.e., the band edge is blue-shifted). Related to these observations, spin-polarized DFT calculations by Di Valentin and co-workers24 have shown that the localized N 2p states in anatase are located just above the O 2p states of the valence band and red-shift the absorption edge to lower energy, whereas in rutile the tendency to red-shift the absorption edge is offset by the concomitant contraction of the O 2p band, resulting in an overall increase in the optical transition energy by 0.08 eV. Experimentally the blue-shift was ca. 0.20 eV. DFT-calculated band gaps for pure undoped anatase and rutile TiO2 were nonetheless also underestimated at 2.19 and 1.81 eV versus the experimental 3.2 and 3.0 eV, respectively. These results confirm the serious shortcomings of the DFT method in this regard. In any case, the simultaneous presence of N dopants and VO vacancies can lead to charge transfer states that also contribute to the visible-light photoactivity of the doped specimens (reaction 1). Subsequent DFT calculations performed using the plane-wave-pseudopotential approach together with the Perdew-Burke-Ernzerhof exchange correlation functional and ultrasoft pseudopotentials deduced, among others, that N-doping leads to a substantive reduction of the energy of formation of the oxygen vacancies VOs (4.3 to 0.6 eV for anatase) with important consequences in the generation of F-type and Ti3+ color centers.25 Substitutional N-doping is stabilized by the presence of oxygen vacancies

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Figure 1. Schematic illustrations of the photocatalytic reaction processes involving the substrate Sub adsorbed on the surfaces of pure, N-, S-, and C-doped TiO2 nanoparticles. In the upper cartoon of pure TiO2 the photogenerated hole (h) can, in principle, react directly with the Sub or else it is first trapped and then is scavenged by the substrate. The photogenerated electrons are scavenged by the ubiquitous adsorbed O2 at the surface which then reacts with Sub. Similar events occur for doped TiO2 specimens under UV irradiation (middle cartoon). In the bottom cartoon for doped TiO2 having a midgap level of the dopant but irradiated with visible light wavelengths, no reaction can take place with the photogenerated holes produced at the midgao level; only the reaction with the scavenged electrons through the O2•- radical anion is thermodynamically possible. Adapted from ref 22.

(NS-O + VO) under oxygen-poor experimental conditions, whereas under oxygen-rich conditions interstitial N species (NI) are favored apparently.

VO••(F) + N f VO•(F+) + N-

(1)

The question of the blue-shift of the absorption edge of single crystals of N-doped TiO2 rutile, which contrasts the red-shifts in N-doped TiO2 powders, was taken up in a DFT study by Yang and co-workers26 using the plane-wave method. Results confirmed those of Di Valentin et al.24 in that some N 2p states lie above the O 2p valence band when N substitutes O in the

TiO2 lattice and when N is located at interstitial positions. No band gap narrowing was predicted by the calculations.26 The significant visible-light activity displayed by S-doped TiO2 has been attributed by Ohno and co-workers27 to the presence of S4+ species substituting Ti4+ in the lattice. Matsushima and co-workers28 re-examined the electronic structure through first-principles DFT band calculations and concluded that (i) the S atom is located at either Ti or O sites in the anatase structure depending on the preparative conditions, and that (ii) S atoms located at Ti sites lead to lower visible-light activity. These predictions contrast the earlier results of Umebayashi et al.29 who noted that as-prepared S-doped TiO2 was 2-fold more visible-light active than as-prepared N-doped specimens30 under visible light irradiation. The effects of S-doping have been further examined by Tian and Liu31 using the plane-wave-based pseudopotential DFT method to characterize the electronic structure when S atoms substitute O atoms in anatase TiO2. Evidently, S-doped anatase TiO2 is converted into a direct band gap semiconductor (Γ position) in line with results for S-doped rutile TiO2 caused by S 3p states localized above the upper edge of the valence band.32 The DFT analysis also showed that the band gap energies are concentration-dependent (Table 1).31 That is, the band gap (calculated and corrected) decreases with an increase in S doping, but most interesting are the data in column 6 of Table 1 which show that the S midgap level merges toward the O 2p valence band of TiO2 with increase in S concentration. The latter would lead to the inescapable conclusion (see also below) that the greater the quantity of dopant in TiO2 the more likely the doped TiO2 turns into a totally different material such as, e.g., a titanium oxysulfide in this case. DFT calculations of C-doped TiO2 at low C concentrations under oxygen-poor conditions indicate that substitutional (for oxygen) carbon and oxygen vacancies are favored, contrary to oxygen-rich conditions in which both interstitial and substitutional (for Ti) C dopings are preferred.34 Although the calculations indicate that C-doping favors the formation of oxygen vacancies in bulk TiO2, they further underestimate the band gap energies of both anatase and rutile TiO2 systems. Thus far, all the DFT-based calculations have failed to calculate the experimentally observed band gap energies for undoped anatase and rutile TiO2, and consequently also the band gaps of all anion-doped TiO2 systems, unless one applies a scissor operator correction done with no convincing reasons. However, in a comprehensive theoretical investigation of substitutional anion doping in C-, N-, and S-doped TiO2 materials using an ab initio tight-binding method (FIREBALL) based on density functional theory with a nonlocal pseudopotential scheme, Wang and Lewis33 estimated a direct band gap of 3.05 eV for the rutile structure in accord with the experimental gap of 3.06 eV as determined by Pascual and co-workers35 Table 2. Earlier we alluded to the weakness of the local density approximation (LDA) approach as generally underestimating the experimental band gap for insulators and semiconductors; the band gap obtained from ab initio plane-wave calculations for TiO2 was around 2.0 eV.36 In their theoretical treatment, Wang and Lewis33 compensated for the shortcomings of the LDA approach by the use of a local orbital basis set. For anatase, the direct band gap was thus estimated as 3.26 eV in close agreement with the experimentally observed band gap of 3.20 eV.37 Clearly there is no literature consensus about whether or not there is band gap narrowing in doped TiO2 materials. When photoactivated by visible light absorption, the C-, S-, and N-doped TiO2 systems also generate charge carriers. The

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TABLE 1: DFT Calculated Properties of S-Doped TiO2 Specimens31 concentration of S (%)

calculated band gap energy (eV)

corrected band gap energy (+1.4 eV)

∆a (eV)

absorption edge (nm)

Db (eV)

0.00 1.39 2.08 4.17

1.80 1.35 1.27 1.15

3.20 2.75 2.67 2.55

0.45 0.53 0.66

387 451 465 488

0.21 0.066 0.002

a Lowering of band gap relative to undoped TiO2. b Energy difference between the lower energy levels of the S states and the upper edge states of the valence band.

TABLE 2: Comparison of Calculated Values33 for the Electronic Properties of Undoped Rutile and Anatase TiO2 with Experimental and Calculated Values Reported by Others (where EBG is the Direct Band Gap) Wang and Lewis

EBG (rutile), eV

EBG (anatase), eV

refs

3.05 3.06

3.26

33

experiments 3.20 3.42 other calcns

2.00 1.78

2.22 2.00

35 37 38 35 39 40

intrinsic absorption edge of the metal oxide per se is not changed by the doping. In other words, the valence and conduction bands of TiO2 are not affected by the doping for low dopant concentrations. However, if they were indeed affected, as might occur for large dopant concentrations (see above and Table 1) then one must consider the material as no longer being TiO2 but some titanium oxynitride, or titanium oxycarbide, or titanium oxysulfide with entirely different reactivities and thermodynamic properties, not least of which are the new electronic structures of the valence and conduction bands. Accordingly, we must seek alternative causes that lead the absorption edge of doped TiO2 specimens to be red-shifted by the presence of dopants. We seek these from an analysis of the optical spectra of various doped and undoped TiO2 systems, which is the central theme of this discussion, and with the assistance of electron paramagnetic resonance spectroscopy (EPR) that we discuss next. 2.2. Optical Spectra of Visible-Light-Active TiO2 Specimens. Germane to the present discussion, the original studies by Cronemeyer41 and by Johnson and co-workers42 on the absorption spectra of reduced rutile single crystals are particularly significant. The spectra of strongly reduced TiO2 crystals displayed absorption maxima in the near-infrared region at 0.75 and 1.18 eV (1653 and 1051 nm)41 or at 0.8 eV (1550 nm; Figure 2, curve 1).42 No spectral features were seen in the visible region. The absorption spectrum of neutron-irradiated TiO2 single crystals also showed only one spectral band at 1.2 eV (1033 nm; Figure 2, curve 3).43 Accordingly, the absorption spectra of reduced TiO2 crystals are incommensurate with the absorption spectra of the yellow- or orange-colored doped TiO2 photocatalysts. To the best of our knowledge, only the study reported in the past decade by Khomenko and co-workers44 showed clearly that the absorption spectra of reduced rutile crystals with a spectral band at 0.86 eV (1442 nm) also display absorption features at 1.49, 2.29, and 2.91 eV (832, 541, and 426 nm; Figure 2, curve 2). Starting from the above results, data on the evolution (transformation) of the absorption spectra of reduced TiO2 specimens (crystals, powders) under additional oxidative treatments are of fundamental importance. With such treatments, the absorption spectrum of reduced TiO2 with spectral maxima in the infrared region evolves into a spectrum that is similar to

Figure 2. Absorption spectra of reduced rutile single crystals reported by Johnson et al.42 (curve 1) and by Khomenko and co-workers44 (curve 2), neutron-irradiated rutile TiO2 single crystal43 (curve 3), neutronirradiated and partially oxidized rutile crystal45 (curve 4), reduced and then partially (curve 5) and fully oxidized (curve 5a) rutile crystal.46 Curve 6 is the spectral dependence of photocurrent in TiO2 crystalselectrolyte systems.47

the spectra of yellow/orange doped photocatalysts. Unfortunately there is a dearth of literature that deals with such treatments. This is one of the main factors that hampers progress into an impartial analysis of the absorption spectra of visible-light-active TiO2 systems. For example, the oxidative annealing of a neutronirradiated rutile TiO2 crystal at 1270 K45 caused the intensities of the featureless absorption spectrum (plateau; see spectrum 2 in Figure 2 of ref 45) in the range 3.0-0.2 eV (413-6200 nm), induced by neutron irradiation prior to the annealing, to decrease yielding a broad absorption envelope consisting, however, of several overlapping absorption bands (compare for example curve 4 with curve 3 in Figure 2 herein). Quantitative and qualitative changes of the absorption spectra of the strongly reduced rutile crystal under additional oxidative treatment have also been demonstrated by Lu and co-workers.46 Initially, the spectrum in the range 3.0-1.38 eV (413-900 nm, the range of optical measurements) was obtained by reduction of TiO2 in an oxy-hydrogen flame at ca.1470 K for 60 s (spectrum not shown in Figure 2). During subsequent oxidation, the intensity of the spectral features decreased, and the shape of the spectrum was transformed to the point that the absorption features in the visible region became dominant with maximum at 2.9 eV (428 nm) and a small band at 1.7 eV (730 nm) (Figure 2, curve 5).

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Figure 3. Polarized absorption spectra of a reduced anatase single crystal with different colors:50,51 pale blue, E⊥c (curve 1), dark green, E ⊥ c (curve 2), dark green, E|c (curve 2a), yellow, E⊥c (curve 3). Absorption spectra of N-doped TiO2 after preirradiation in vacuum at λ ) 436 nm (curve 4) and λ ) 546 nm (curve 4a).3 Absorption spectra of P-25 TiO2 reduced in H2/CO and bleached by O2 adsorption (curve 5)48 and P-25 TiO2/polymer molecular compositions irradiated in air (curve 6) and under a N2 atmosphere (curve 6a).52 Spectral dependence of the photocurrent for a TiO2 thin film - electrolyte system (curve 7).53 Curves 2b, 4b, 6b, 7a, and 7b are bands from the Gaussian fitting of the corresponding experimental absorption bands.

The absorption band at 2.75 eV (450 nm) in the visible region of the absorption spectrum of reduced TiO2 (Degussa P-25) was delineated by adsorption of molecular oxygen in a study by Kuznetsov and Krutiskaya.48 Subsequent to reduction of P-25 TiO2 in a H2 or CO atmosphere at T g 700 K, the metal-oxide specimen displayed very broad absorption characteristics (difference diffuse reflectance spectra, ∆DRS) in the range from ca. 3.0 to 0.5 eV (413 to 2480 nm) with a single maximum at 1.27 eV (976 nm). Adsorption of NO or N2O at ambient temperature partly decreased the absorption in the near-infrared region and blue-shifted the band maximum to 1.64 eV (756 nm). However, adsorption of O2 caused extensive bleaching of the sample in the near-infrared region and made the absorption band at 2.75 eV (450 nm) the dominant feature in the spectrum (Figure 3, curve 5).48 It is relevant to note that the absorption envelope consisting of the initial spectrum of reduced P-25 TiO2 and the spectra after the bleaching in the presence of various gases could be deconvoluted into a sum of absorption bands of different intensities with maxima at 2.81, 2.55, 2.00, and 1.17 eV (441, 486, 620, and 1060 nm).49 The phenomenon of bleaching of powdered reduced TiO2 has been confirmed in several studies. An immediate color change from blue to gray-white with an increase of the infrared transmission of a P-25 TiO2 specimen reduced under a H2 atmosphere at T > 470 K occurs on introducing oxygen at ambient temperature.54 No absorption spectra were reported in the study by Haerudin and co-workers, however.54 Related to this, the grayish-yellow color and the absorption features in the visible spectral region of a H2 plasma-treated ST-01 TiO2 specimen disappeared immediately on exposure of the sample to air.55

J. Phys. Chem. C, Vol. 113, No. 34, 2009 15115 In several studies48,49,54,55 reduction was relatively “soft”. This led the color centers to be concentrated on the surface and in a small volume segment beneath the surface. Note that the color and the absorption spectrum of the H2 plasma heat-treated (470 K < T < 670 K) ST-01 sample remained stable not only after contact with atmospheric air at ambient temperature and pressure, but also after exposure to sunlight for more than a week.55 A careful detailed study by Sekiya and co-workers50,51 examined the evolution of the absorption spectrum of an anatase TiO2 single crystal on which repetitive heat treatment in temperature steps of 50-100 degrees under a H2 atmosphere and then under an O2 atmosphere revealed a wide change of crystal colors together with a variety of absorption spectra (Figure 3, curves 1, 2, 2a, and 3). Prominent absorption bands at 2.85 eV (435 nm; E|c polarization) (Figure 3, curve 2a) and at 3.0 eV (413 nm) (Figure 3, curve 3; E⊥c polarization) dominate in the visible spectral region (2.5 < hν e 3.3 eV; 496 > λ g 375 nm). The former absorption band at 2.85 eV has a half-bandwidth of 0.4 eV and a Gaussian-type shape, parameters typical of local defects in metal oxides which are associated with oxygen vacancies (F-type color centers, see below).56 Other studies have examined optical properties of undoped titanium dioxide systems characterized by prominent absorption bands. In this regard, two well-resolved bands at 2.10 and 2.53 eV (590 and 490 nm) and a poorly resolved band in the range 3.0-2.75 eV (413-450 nm) were observed in the spectral dependence of the photocurrent for an n-type (reduced) TiO2 thin film electrode immersed in an electrolyte system (Figure 3, curve 7).53 The electrode was fabricated by oxidation of a titanium sheet in air at 870 K. Another study reported photocurrent peaks at 1.24 and 0.95 eV (1000 and 1305 nm) for a TiO2 crystal-electrolyte system irradiated with near-infrared light (Figure 2, curve 6).47 The existence of a prominent absorption band at 2.0-2.1 eV (620-590 nm) was recently demonstrated in a study by Emeline and co-workers for doped TiO2 (Figure 3, curves 4 and 4a).3 In this case, although the titanium dioxide specimen was N-doped, the absorption band was induced by irradiation in vacuum in the presence of either O2 or H2. Data on the positions of spectral maxima in the absorption spectra of various reduced and reduced/oxidized TiO2 specimens are collected in Table 3 in chronological order. Note that Table 3 reports only the experimental results. No results of calculations or deconvolution of experimental spectra are given. Comparison of hνmax values for a set of absorption spectra allows us to distinguish a few regions where values of hνmax differ slightly. As a result, Table 3 displays six columns denoted AB1 to AB6 with each column corresponding to a single dominating absorption band. The data of Table 3 also infer that absorption bands have the same spectral positions in all the absorption spectra, including the poorly resolved spectra composed of strongly overlapping absorption bands. Perusal of the data in Figures 2 and 3 reveals that the relative intensities of the bands differ strongly and depend on the contribution of the reductive and oxidative treatments. Comparison of the absorption spectra collected in Figures 2 and 3, and the numerical hνmax values reported in Table 3 show that the absorption bands in the visible region (AB1, AB2, and AB3) are characteristic of the reduced and reduced/oxidized TiO2 specimens as demonstrated experimentally in the last three decades. It is quite evident that only the intrinsic defects in TiO2 are responsible for these AB bands. It is also rather clear that the AB bands are independent of the nature of the anion or the

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Kuznetsov and Serpone

TABLE 3: Positions of Spectral Maxima in the Absorption Spectra of TiO2a row AB1 hνmax (eV) AB2 hνmax (eV) AB3 hνmax (eV) AB4 hνmax (eV) AB5 hνmax (eV) AB6 hνmax (eV) sample figure /curve 1 2 3 4 5 6 7 8 9 10 11 12 13

x + + x 2.75d 2.91 2.93 2.88 + x 2.85 2.93 2.90 -

x + 2.53 x + + + 2.50 + (2.55) -

+ + 2.10 x + 2.29 2.1-2.2 + + + 2.2-2.3 + 2.0-2.1

+ + x 1.67 1.64c 1.49 + 1.70 1.63 + + 1.74 x x +

1.18 + x 1.24/0.95 1.27b + x x x + 1.20 x x x x x

0.75 0.80 x x + 0.86 x x x 0.73 + x x x x x

s/c, R s/c, R f, R/el s/c,R/el P25 s/c, R s/c, A s/c, R s/c, R s/c, R s/c, A P25 TP-S

1/1 2/7 1/6 2/5 1/2 1/5 1/4 1/3 2/1 2/2a 2/3 2/6,6a 2/4,4a

refs 41 42 53 47 48,49 44 57 46 45 43 50,51 52 3

a Rutile single crystal (s/c, R), anatase single crystal (s/c, A), powders “Degussa P-25” (P25), irradiated N-doped TP-S201 (TP-S) and spectral positions of photocurrent peaks for rutile thin film - electrolyte (f, R/el) and rutile crystal - electrolyte (s/c, R/el) systems (rows 3 and 4). Sign “+” means presence of unresolved absorption bands, sign “-” denotes absence of absorption, sign “x” means absence of spectral data in that spectral region. In row 12 the absorption band AB2 was obtained as the difference between curves 6a and 6 in Figure 3. b Corresponds to spectrum after reduction in H2/CO. c Spectrum after reduction and NO addition. d After reduction and O2 addition.

cation dopant. Neglect and/or lack of knowledge of these results and observations are one of the principal factors that have hampered an unbiased analysis of the origin of the absorption characteristics of visible-light-active TiO2 specimens. 2.3. Analysis of the Absorption Spectra of VLA TiO2 Samples. In earlier articles we simplified the analysis of the absorption spectra of visible-light-active TiO2 specimens by selecting and examining only relatively narrow spectra.52,58,59 We showed that several such spectra were similar and independent of the doping method and of the type and nature of the dopants as illustrated in Figure 4. This afforded an analysis of the absorption spectra averaged for variously doped VLA titania samples. A typical averaged narrow absorption spectrum displays one maximum at 3.0 eV (413 nm) and a low-energy tail, which can extend down to 1.8 eV (690 nm; curve 1 in the inset of Figure 4). For some VLA TiO2 systems, the absorption spectra with maxima at 3.0 eV also showed a clear shoulder at ca. 2.5 eV (496 nm). The spectra of both types could be fitted using the absorption bands AB1 and AB2.52,58,59 The result of deconvolution of a narrow absorption spectrum using two bands is illustrated in the inset of Figure 4. For fitting spectrum 1 no particular computer program was used. Curve 1a in the inset was calculated as the sum of two Gaussian bands. The hνmax values are close to those reported in Table 3 and equal to 2.98 eV for AB1 and 2.59 eV for AB2; the halfbandwidths are 0.50 eV for AB1 and 0.55 eV for AB2. To minimize the differences between experimental and calculated curves in the range of energies hν < 3.0 eV the above parameters for the AB bands varied slightly. The noticeable difference in the range of intrinsic absorption in Figure 4 is not surprising because the samples differed in phase composition (see for example ref 50) and thickness. Moreover, authors often choose any available TiO2 system such as, for example, the Degussa P25 TiO2 as the nonabsorbing reference sample in the visible region. The present section focuses attention on an analysis of the changes in the absorption spectra of visible-light-active TiO2 specimens resulting from differences in the treatment conditions and the type of dopants. In addition, the aim of this part of the study is to examine additional reasons for considering the spectra in the visible region as the sum of the absorption bands.

Figure 4. Absorption spectra of anion-doped TiO2 specimens before averaging and DRS of Degussa P25 TiO2. Selected absorption spectra of (i) mechanochemically activated N-doped TiO2,60 (ii) N-doped oxygen-deficient TiO2,61 (iii) N,F-doped TiO2 sample prepared by a spray pyrolytic method,62 (iv) N-doped anatase TiO2 specimen prepared by a solvothermal process,63 (v) N-doped rutile TiO2 sample also prepared by a solvothermal process,63 (vi) yellow N-doped TiO2 system synthesized in short time at ambient temperatures using a nanoscale exclusive direct nitridation of TiO2 nanocolloids with alkyl ammonium compounds,64 and (vii, viii) N-doped TiO2 samples prepared by evaporation of the sol-gel with N-doping performed under a stream of ammonia gas at different temperatures,65 (ix) N-doped TiO2 prepared via sol-gel, mixing a solution of titanium(IV) isopropoxide in isopropyl alcohol with an NH4Cl solution.66 The inset shows the absorption spectrum averaged for various doped TiO2s (curve 1) and the result of the fitting (curve 1a) using two absorption bands (see text).

A large set of diffuse reflectance spectra of undoped and N-doped TiO2 nanoparticles sintered in air or in a N2 atmosphere at different temperatures was recently reported in a study by Zhao and co-workers.67 The set of spectra permits a comparison of the shape (form) of the absorption spectra {∆DRSs, ∆F(hν)} as a function of the heat treatment conditions. The analysis included digitization of the DRS spectra, calculation of the absorption spectra by taking the difference between the DRS spectra to give the ∆(DRS) {or ∆F(hV)} followed by the normalization of ∆(DRS) by the ∆Fmax factor. Figure 5 illustrates the changes in the absorption spectra resulting from an increase of the heating (sintering) temperature of a TiO2 specimen

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Figure 5. Differences of normalized DRSs between pure and N-doped TiO2 before sintering (heating) (curve 1), and after heating in N2 at 420 (curve 2), 470 (curve 3), and 520 K (curve 4). The inset shows differences between the absorption spectra ∆(∆Fnorm) (hν) shown in the figure. Spectrum 1i was obtained as spectrum 2 minus spectrum 1, spectrum 2i is spectrum 3 minus spectrum 1, 3i is spectrum 4 minus spectrum 1, spectrum 4i is spectrum 3 minus spectrum 2. The inset shows also the differences between the absorption spectra of untreated N-doped TiO2 and the spectrum after heating in air at 470 K (curve 5i) and between the absorption spectra of N-doped TiO2 heated in air at 470 and 420 K (curve 6i).

N-doped in a N2 atmosphere in the range 420-520 K. The absorption spectra, which are presented in a normalized form, clearly show that the maxima of the absorption spectra lie at 3.0 eV, i.e., close to the maximum of the absorption band AB1 and shift slightly to lower energy by less than 0.05 eV when the temperature of the heat treatment increases. The main evolution of the spectra (which are often regarded as “absorption red-shifts”) in reality consists in the growth of the absorption bands reported in the inset of Figure 5, where curves 1i-4i represent the differences in the absorption spectra displayed in Figure 5 (curves 1-4). Curves 5i and 6i in the inset were obtained using the same calculation procedure for the absorption spectra induced by sintering in air at different temperatures67 (absorption spectra not shown in Figure 5). To a first approximation, all of the curves in the inset of Figure 5 show striking similarities and display a set of absorption bands with maxima in the range 2.11-2.25 eV (588-550 nm), that is in the range of the AB3 band (see Table 3) with halfbandwidths of about 0.9 eV. However, the shift of the maxima of curves 1i-3i relative to curves 4i-6i and the fairly large half-bandwidths indicate that these absorption bands in fact consist of two poorly resolved components, namely AB2 and AB3. The absorption spectra of S-, N-, and C-doped TiO2 nanoparticles have been investigated in recent studies by Chen and co-workers.68,69 Figure 6 shows the difference between the absorption spectra of doped TiO2 and pure rutile68 (note that the calculations of ∆F(hV) and ∆(∆Fnorm)(hV) throughout were done in an identical manner). Spectral differences in the near band-edge absorption region (i.e., at 3.0-3.2 eV) can be attributed to differences in crystal phase69 and are not discussed here. The principal band maxima of all the spectra are in the range 2.9-2.8 eV, i.e., close to the maximum of the absorption band AB1. However, at energies hν e 2.8 eV (λ g 442 nm) the spectra differ significantly. The inset to Figure 6 displays the origin of the differences at long wavelengths. It is evident that the same absorption bands with maxima at about 2.18 eV (569 nm) and half-bandwidths of about 0.67 eV are responsible for

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Figure 6. Difference between the normalized absorption spectra of TiO2-zSz (curve 1) and pure TiO2, TiO2-xNx, and pure TiO2 (curve 2), and TiO2-yCy and pure TiO2 (curve 3). The inset shows differences between the spectra shown in Figure 6: spectrum 1i was obtained as spectrum 3 minus spectrum 1, spectrum 2i is spectrum 3 minus spectrum 2 (but magnified by a factor of 1.4).

the differences between the absorption spectra of C- and S-doped and also between C- and N-doped TiO2 systems. Accordingly, in keeping with our interpretation, only the relative contribution of the absorption band AB3 determines the differences between the absorption spectra examined, which as a whole are the sum of the above-mentioned bands AB1, AB2, and AB3 that are characteristic of undoped tatania. From their studies, Chen and co-workers69 concluded that the observed visible-light absorptions of main-group element-doped TiO2 nanomaterials originate from optical transitions between intragap dopant-induced levels and Ti 3d orbitals in the conduction band. 2.4. Assignment of Absorption Bands in the Spectra of Reduced and VLA TiO2 Specimens. Assignment of the absorption bands AB1 to AB6 and in particular the absorption bands in the visible region (AB1, AB2, and AB3) to specific intrinsic defects is a very challenging problem. Only optical spectroscopy combined with other techniques, such as electron paramagnetic resonance (EPR), ultraviolet photoelectron spectroscopy (UPS), and photoconductivity methods can provide conclusive assignments. Unfortunately, only a few studies have reported such data. Moreover, for an explanation of the visible light absorption and the ensuing photoactivity in this spectral region, it is of critical importance to propose a scheme of optical transitions that illustrates the stable (both chemical and electronic) state of the light-absorbing defects during the photoinduced surface reactions. Sufficient evidence of such stability of states in N-doped3,70 and reduced (H2 plasma heat-treated55) visible-light-active TiO2 photocatalysts exists to infer such a scheme. We begin by noting that the principal types of TiO2 intrinsic defects associated with oxygen vacancies, i.e., F-type and defects related to Ti3+centers, have been described in earlier articles.2,58 At present, it is less of a challenge and more reasonable to assign the absorption of reduced TiO2 crystals in the range of energies hν less than 2.0 eV (i.e., at wavelengths λ > ca. 600 nm); viz., the absorption bands AB4, AB5 and AB6 in Table 3 to electronic transitions involving the various defects related to Ti3+ ions (and possibly also Ti2+ ions) in different lattice sites of the TiO2 architecture. These Ti-related defects include: (i) Ti3+ in substitutional sites (Tisubs), i.e., Ti3+ states associated with oxygen vacancies, (ii) Ti3+ in interstitial sites (Tiinter) as evidenced by optical and EPR data on a rutile crystal reported by Li and co-workers,71 (iii) Ti3+ (3d1) states in substitutional sites (at low temperatures) in the bulk of an anatase

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Figure 7. Scheme illustrating the otherwise facile trapping of an electron(s) in an oxygen vacancy to form F centers or Ti3+ color centers in a pristine undoped TiO2 sample: left-hand side, octahedral site symmetry maintained, whereas on right-hand side the oxygen vacancy site is of lower symmetry.

crystal from optical and EPR data reported by Sekiya and coworkers,50 (iv) absorption originating from a Ti3+ T Ti4+ intervalence charge transfer whereby an electron is shared between the two Tin+ cations on adjacent interstitial and octahedral sites in the rutile structure (optical and EPR data from the study of Khomenko and co-workers44), and (v) electron transitions involving {Ti3+-VO} clusters in a reduced rutile crystal as suggested by Lu et al.46 on the basis of optical spectra and calculations. Thus, it seems rather likely for electrons trapped in an oxygen vacancy VO to reduce an adjacent Ti4+ ion to form a Ti3+ (and/or Ti2+) color center (Figure 7); suffices only a break in the octahedral site symmetry of the oxygen vacancy as a result of crystal packing forces and the presence of vacancy sites. In contrast to the near-infrared and infrared absorption bands attributable to Tin+-related color centers, unambiguous assignment of the absorption bands AB1 and AB2 (the main spectral features of VLA TiO2 specimens (see the inset of Figure 4)) are less evident, and thus clearly absent in the past literature, despite certain inferences. The ensuing discussion below rationalizes assignments of the visible absorption bands to F-type color centers. The EPR spectra of the dark blue and dark green anatase crystals, whose corresponding absorption spectra are displayed in Figure 3 (curves 2 and 2a), revealed the existence of conduction band electrons that can be trapped partly by oxygen vacancies.50 The EPR spectra of the yellow crystal (absorption spectrum corresponding to curve 3 in Figure 3) at ambient temperature is rather weak and different from the EPR spectra of crystals displaying other colors (absorption spectra). The absence of prominent EPR signals of an electron trapped in oxygen vacancies (F+ centers) infers that oxygen vacancies are present mainly in the form of nonparamagnetic F centers and F2+ centers. However, to the extent that the F2+ centers are optically silent they can be ruled out from any further discussion. Thus, from the above hypotheses, we deduce that F centers occurring in more than one lattice site (structural position) are the principal defects responsible for the AB1 absorption bands seen at about 2.9 eV (428 nm) and the AB2 bands at ca. 2.55 eV (486 nm). At the same time it would seem reasonable to expect that more pronounced assignment could have F+ centers in both reduced and doped TiO2 specimens since these centers should be optically and EPR observable. Several studies have reported experimental data on the formation of paramagnetic centers in both reduced and doped TiO2 specimens that have been attributed to F+ centers. The EPR signal with g ) 2.0034 is characteristic of these centers. For example, a symmetrical sharp signal at g ) 2.004 was detected by Nakamura and co-workers72 for the H2-plasma

Kuznetsov and Serpone treated ST-01 TiO2 sample at 673 K. Diffuse reflectance spectra clearly showed absorption features in the region 375-550 nm that resulted from such a treatment. The main specificity of the EPR signal observation in this study was the need to use visible light irradiation of the sample during the EPR measurements. Moreover, the intensity of the g ) 2.004 signal increased 12fold with irradiation time at various visible wavelengths ultimately reaching a plateau (>420, > 475, > 515, > 570 nm). However, changes in the absorption spectral features were not reported. The EPR signal at g ) 2.004 in this and in many other studies was assigned to F+ centers based on literature data. Anatase titania nanoparticles (size, 10 nm; specific surface area, ca. 180 m2 g-1) obtained by a metal-organic chemical vapor deposition technique using titanium tetrabutoxide as the starting precursor in oxygen-containing atmospheres73 displayed a single EPR signal at g ) 2.0034 in ambient air at room temperature. The line width of this EPR signal increased from 4.2 G at T < 353 K to 4.98 G at T > 413 K. The g value () 2.0034) of the EPR signal remained invariable in the whole temperature range investigated. Results of the analysis showed that F+ centers occurred mainly on the surface of the nanoparticles and desorption of water influenced the line width of the paramagnetic centers.73 EPR signals of paramagnetic centers with g ) 2.002-2.004 and Ti3+-centers (g ) 1.955) are characteristic of H2 heat-treated TiO2 powders (anatase, 30 nm in size).74 It is noteworthy that the intensity of the Ti3+ EPR signal increased only at T > 700 K (when consumption of H2 occurred), whereas the intensity of the signal with g ) 2.002 was significant prior to the H2heat treatment. Liu and co-workers concluded that the latter signal is likely due to adsorbed O2- species when the H2 treatment temperature was below 673 K. To the extent that the H2-heat treatment temperature vas greater than 670 K, the peaks with g ) 2.003 are then attributable to oxygen vacancies by analogy with ZrO2. The strong EPR lone signal at g ) 2.0034 under dark conditions is also characteristic of the composites of anatase and brookite TiO2 prepared by dehydration of titanic acid nanotubes.75 The signal was assigned to a single electron trapped in an oxygen vacancy (F+ centers in our terms). An analysis of the photoluminescence spectra revealed the existence of a subband within the band gap induced by F+ centers. The width of the sub-band was estimated to be ca. 0.48 eV, whereas the bottom of the sub-band was positioned at 1.79 eV above the upper level of the valence band. The absorption spectrum, which could be derived from the diffuse reflectance spectra of dehydrated titanic acid nanotubes and that could have been indicative of F+-centers, is rather featureless extending well into the infrared (>800 nm) region. Suitable changes in the absorption and photoluminescense spectra on irradiation of the specimen in the absorption band of F+ centers were not reported.75 Other studies have reported EPR signals of F+-centers in variously cation-doped TiO2 systems. For instance, Co-doped TiO2 nanocrystals prepared by hydrothermal hydrolysis of titanium n-butoxide after annealing at 630 K displayed an EPR signal at g ) 2.0033.76 Anisotropic surface F+ centers were unstable being destroyed at high temperature, while isotropic lattice F+ centers that formed by high-temperature annealing were rather stable even at 700 K. Pan and co-workers76 correlated the ferromagnetism of the Co-doped specimens with F+ centers, thereby providing direct experimental evidence for F+-center-mediated coupling in diluted magnetic metal oxides. In summary, the expected pronounced assignment of EPR signal(s) at g ) 2.003-2.004 to F+ centers and the concurrent

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Figure 8. (a) Scheme illustrating the positining of the F and F* centers, and (b) the electron reduction of the N level in N-doped TiO2 specimens.

demonstration of prominent absorption band(s), corresponding to electron excitation from the ground state to the exited state of F+ centers, are confronted with systematic experimental difficulties. At present, it seems most reasonable to assign the absorption of TiO2 in the visible spectral region to F centers. 2.4. Simplified Electronic Structure of TiO2 and Midgap States of VLA TiO2. If the ground state of an F center lies within the band gap and near the valence band (Figure 8a), then its exited state F* (hνAB eV distant from the ground state) should lie within or close to the conduction band because the energy difference [Ebg - hνAB] is only about 0.2-0.3 eV for the rutile/ anatase AB1 feature and 0.55-0.65 eV for the rutile/anatase AB2 band. In this case, the optical absorption (electronic transition from the ground state to the exited sate) should ultimately produce an electron in the conduction band (eCB-; reaction 2) and a hole in the valence band (O-)VB (reaction 4). If the level of the excited F* center is located lower than the bottom of the conduction band, a thermally stimulated electron transition from the F* state to the conduction band should occur (reaction 3b). In either case, hole formation supposes capture of an electron by the F+ center from the valence band (reaction 4).

F + hν f {F*} f F+ + eCB-

(2)

F + hν f F*

(3a)

F* + ∆ f F+ + eCB-

(3b)

F+ + (O2-)VB f F + (O-)VB

(4)

If the F centers are positioned within the bulk of the TiO2 particles, reactions 2, 3a, and 4 ensure the photogeneration of both conduction band electrons and valence band holes under visible light irradiation, as well as the above-mentioned stability of these processes during the photoexitation events and the photoinduced surface reactions. In the EPR and optical studies of Li and co-workers71 and in several others, oxygen vacancies were considered only as the origin of electrons trapped as Ti3+ or trapped on impurity ions, but not as the centers capable of absorbing light. At present the former viewpoint predominates excessively.

The intriguing question worth asking is why reduction of TiO2, that accompanies the process of TiO2 doping (see e.g. next section), results in the formation of an absorption spectrum (absorption bands AB1 and AB2 and, in some cases, AB3) that can be obtained only on partial oxidation of a TiO2 specimen previously reduced in a H2 or CO atmosphere, or otherwise by neutron irradiation (curve 5 in Figure 2, and curves 2, 3, and 5 in Figure 3). In other words, why does the introduction of dopants into the TiO2 lattice suppress the absorption features in the infrared region, i.e., suppress the absorption of Ti-related centers? An interesting proposal in this regard was recently suggested by Batzill and co-workers77 to explain the principal changes in the ultraviolet photoemission spectra of a rutile crystal arising from N implantation. They proposed that electrons occupying Ti3+ centers (electrons resulting from the formation of an O vacancy, VO) are transferred to the N impurity dopant, i.e., to N-induced 2p states lying within the band gap approximately 0.4 eV from the upper edge of the valence band (Figure 8b). Accordingly, N-related species suppress the formation of Tirelated centers and act in a manner similar to other effective electron scavengers (see below). It would appear, therefore, that N dopants are more effective electron scavengers than are the Ti-related defects and can suppress Ti 3d states located at the upper-end of the band gap. With regard to N-induced 2p states lying near the top of the valence band, Lisachenko and Mikhailov78 noted that vacuum heating or UV irradiation of undoped P-25 TiO2 produces surface F-type centers. UPS spectral features also showed a level near the top (ca. 0.5 eV) of the valence band (VB). It means that the O vacancy can also be an effective electron acceptor forming F-type centers. The infrared absorption features of slightly reduced P-25 TiO2 are suppressed by adsorption of such molecules as O2, NO, and N2O at ambient temperature48,49 through electron capture by these species. The same feature at 410 K in the temperatureprogrammed desorption spectra is characteristic of molecular oxygen adsorbed (i) on the surface of P-25 TiO2 that was previously reduced in a H2 or CO atmosphere,48 (ii) on a vacuum-annealed rutile crystal,79 (iii) on a vacuum-annealed P-25 TiO2 surface,49 and (iv) on P-25 TiO2 with O2 adsorbed upon UV irradiation.49 In all cases, desorption of O2 molecules at 410 K resulted from the removal of electrons (oxidation) from the adsorbed O2- species. The above considerations lead to the conclusion that the occurrence of absorption bands in the visible spectral region during the doping of TiO2 results from the reduction of titania (formation of oxygen vacancies) in the presence of an effective electron acceptor(s) that competes with Ti-related sites in the capture of electrons. Germane to the above discussion, a N-doped TiO2 (anatase) material displaying high visible-light photoactivity was reported recently by Feng and co-workers.8 The specimen was prepared by thermally treating nanotube titanic acid in a flow of ammonia and was subsequently examined by XRD, transmission electronic microscopy (TEM), diffuse reflectance spectroscopy (DRS), XPS, ESR, and photoluminescence (PL) techniques. Nitrogen atoms were incorporated into the anatase TiO2 polymorph as NO species (XPS results), whereas EPR data on the N-doped specimens produced in the temperature range 770-970 K indicated an EPR triplet at g ) 2.004, 2.023, and 1.987 with the first signal apparently originating from a single electron trapped in an oxygen vacancy (VO•, also referred to as an F+ center). The latter two signals apparently originate from the NO species located at an interstitial site adjacent to the oxygen

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Figure 9. Cartoon illustrating the formation of the F+-NO-Ti4+ lattice complex upon nitrogen doping a TiO2 sample;8 note that an electron trapped in the oxygen vacancy is equivalent to an F+ center.

vacancy (Figure 9). Interestingly, the visible light photoactivity increased with the intensity of the major peak at g ) 2.004 from which Feng et al.8 deduced that the F+ defect was formed in a well crystallized TiO2 surface layer and accompanied by chemisorbed NO (i.e., complexes F+-NO-Ti4+) should be a key structure for the appearance of visible-light photoactivity. Formation of the latter complex quenched the photoluminescence that originates from the F+ center on interaction with a valence band hole. Earlier we reported that N-doping stabilizes the color centers as a result of a defect charge compensation effect.3 Reduction of the NO in the lattice complex yields the TiN phase that effectively destroys the photoactive center and decreases the visible-light photoactivity of the TiO2 specimen.8 The nature of the N species in N-doped titania specimens seems to depend on the preparative route.1 In a series of interesting articles, the Giamello group80 prepared N-doped specimens by mixing a solution of the titanium(IV) isopropoxide precursor in isopropanol with an ammonium ion aqueous solution, followed by removal of solvent and calcinations in air. Their EPR results also revealed a triplet that was attributed to the neutral molecular NO species permanently trapped into (so-called) microvoids in the particle, and to a nitrogen-based paramagnetic center identified as NO22-. A subsequent EPR and DRS experimental study combined with DFT calculations by Livraghi et al.66 on a similarly prepared sample re-examined the earlier assignment and attributed the EPR triplet to a unique signal of a Nb• species split into three peaks by the hyperfine interaction of the unpaired electron with the nucleus of the N atom. This Nb• species, of unknown nature at the time, was proposed as being at the origin of photoactivity of N-doped TiO2. Note that EPR parameters of Nb• species differ only slightly for anatase (A) and rutile (R) and are very close to the g factors of F+ centers: g1 ) 2.007/2.005 (A/R), g2 ) 2.005/ 2.004 (A/R), and g3 ) 2.004/2.003 (A/R).81 Accordingly, great care must be exercised when making assignments of EPR signals to certain entities. In a more recent study, the Giamello group6 re-examined the issue of the nature of the Nb• species by preparing N-doped specimens via three different preparative routes (sol-gel, mechano-chemistry, and oxidation of TiN) that were subsequently characterized by X-ray diffraction, electron microscopy, and by various other spectroscopic techniques (EPR, XPS, NMR, etc.), together with refined DFT calculations. All the samples displayed a visible light response with the specimen prepared by the sol-gel method displaying the greater visiblelight photoactivity in the oxidative degradation of some organic substrates. Various nitrogeneous species were identified: for example, NH4+ (by NMR) and CN- (by FTIR) ions together with molecular NO that formed in TiO2 samples possessing a closed porosity. The previously reported bulk radical species Nb• (by EPR under irradiation) was again present in all the

Kuznetsov and Serpone samples and is apparently the principal source of visible-light photoactivity with promotion of electrons from band gap localized states to the conduction band or to surface-adsorbed species. Their results6 confirm those of Feng et al.8 in that molecular NO is produced in the metal-oxide lattice upon nitrogen doping with suitable precursors. However, they describe the Nb• species as being the rather unusual two-electron reduced NO2- anion radical species formed in an oxygen vacancy of the TiO2 lattice.6 Apparently, such a species is formed in such small amounts that it often escapes detection with other techniques except by the sensitive EPR method. The above findings point to the strong likelihood that the high sensitivity of the EPR technique may well be the principal reason which has, heretofore, precluded a clarification and a classification of the optical properties of F+-type centers in such metal oxides as TiO2. 2.5. Reduction of TiO2 Specimens during the Doping Process. Demonstrating (revealing) the stage of the reduction of TiO2 during the doping process constitutes separate complex problems, especially if one ignores the possibility of such a stage. In their study, Batzill et al.77 acknowledged that implantation of N impurity ions at energies of 0.6 to 1 keV and a fluence of 1015 ions cm-2 creates more oxygen vacancies than the sputtering process. XPS and UPS data on polycrystalline titania films N-implanted by ions at an energy of 4 KeV (fluence ∼1015 ions cm-2) have shown that films became not only N-doped but also strongly reduced.82 Along these lines, Orlov and coworkers82 suggested that visible light activity of N-doped TiO2 cannot be attributed exclusively to the effect of nitrogen-doping and that other effects, such as creation of oxygen vacancies, may be a significant consequence of the doping process. To argue for TiO2 reduction during the N-doping process over other methods, we now examine such doping that occurs through NH3 treatment. High-temperature (870 K) treatments of titania were used in the early study by Asahi et al.17 for doping anatase powder and in the study by Diwald and co-workers83 for doping rutile crystals. Germane to such treatments, pyrolysis of gaseous NH3 at T g 800 K and at pressures p g 12 Torr84 yields •NH2 radicals as evidenced by laser absorption spectroscopy. As a whole, the proposed NH3 pyrolysis mechanism also includes formation of H atoms and H2 molecules. Diwald et al.83 deduced that the broad optical absorption of doped TiO2 in the range 0.5-2.0 eV (2480-620 nm) was associated with defects which resulted from the reduction of the crystal originating from the thermal decomposition of NH3 and yielded molecular H2. At the same time, the absorption band near the band-edge at 3.0 eV (413 nm) was atrtributed to N-H codoping of the TiO2 specimen.83 That TiO2 catalyzes the decomposition of NH3 is highly possible since light absorption in the range 400-500 nm can be induced by heating TiO2 powder in an atmosphere of NH3 beginning at low (570 K) temperatures.85 Moreover, visible light absorption results from the milling of anatase powder in gaseous NH3.86 Under such conditions, N-doping of TiO2 occurs as evidenced by the appearance of the N 1s peak in the XPS spectrum at 396 eV and by the experimental formation of significant quantities of H2 in the gas phase. Some processes are also initiated by electron transfer from the O2- species of the fresh oxygen-rich surface to the adsorbed NH3 molecule. Decomposition of ammonia and formation of oxygen vacancies, which accelerate the anatase to rutile transformation, contribute partially to the optical absorption. Revealing the stage of TiO2 reduction during the synthesis and/or the doping process becomes a rather complex task if

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various uncontrollable impurities and residuals are determinant. Nonetheless, experimental evidence of such reduction has been obtained in some cases. The best example of impurities-assisted reduction of an anatase crystal was reported by Sekiya and coworkers.57 The absorption spectrum identical to the one shown in Figure 3 (curve 3) was obtained simply by the heat-treatment of an anatase crystal grown by a chemical vapor transport reaction. The absorption band at 3.0 eV (413 nm) was attributed to the formation of two oxygen vacancies formed during the removal of molecular Cl2 (or H2O) from the lattice originating from the presence of Cl- (or OH-) impurity ions. Desorption of neutral molecules produces electrons that can be trapped by oxygen vacancies which remained behind the desorbed impurity molecules giving rise to F-type centers (reactions 5 and 6).

VA + e- f F+(VO•)

calcination at 973 K is 1.7 for the S-doped material and 1.89 for the undoped samples, it was deduced that carbon residuals were responsible for the formation of oxygen vacancies in the TiO2 specimens. In support of the proposed reduction of TiO2 (oxidized titanium foil), Lu and co-workers90 treated TiO2 in the presence of carbon powder at temperatures g873 K for 0.5 h. The carrier density and the nonstoichiometric number (2 - x) of TiO(2-x) was estimated using the weight increment during the reoxidizing of TiO2-x in air at 1273 K for about 5 h. Values of (2 - x) for the carbothermally reduced titania were 1.99 and 1.95 for temperatures 873 and 1073 K, respectively, causing the light yellow colored metal-oxide specimen to darken. Unfortunately, the authors90 reported no absorption spectra that would have confirmed such event.

(5) 3. Concluding Remarks

VA+2 e- f F(VO••)

(6)

An intensive study of the preparative stage of a series of nanosized N-doped TiO2-based materials was reported recently by Belver and co-workers.87,88 N-doped titanium isopropoxide precursors were synthesized using three different amine-type ligands by a reverse micelle microemulsion method. Evolution of gaseous products during the thermal decomposition of the solid precursors and formation of the final nanoparticles were monitored by a thermogravimetric analysis in combination with mass spectrometry and infrared spectroscopy. Different N- and C-containing fragments were detected in both gas phase and in the catalyst solids. Within the context of the present discussion, formation of oxygen containing products such as CO and CO2 is an essential result. The authors argued that oxygen species of the solid precursor (O2-; •OH, etc.) participate in the decomposition process leaving not only C and N impurities but also oxygen vacancies. Accordingly, under adequate experimental conditions, reduction of TiO2 specimens during the synthesis and/or doping process appears to be a controllable process. Other experimental evidence of TiO2 reduction during calcination in air of S-doped specimens was reported by Colon and co-workers.89 The TiO2 specimen was prepared by a sol-gel method using the titanium tetraisopropoxide precursor in isopropanol. Sulfation was performed by dispersing the powder sample in a 1 M H2SO4 aqueous solution. Surface characterization by XPS, LEIS and 1H MAS NMR of the samples calcined at different temperatures showed complete loss of the sulfate for materials treated at 973 K in air. A low XPS O/Ti-atomic ratio (about 1.7) was characteristic, indicative of the existence of oxygen vacancies in the TiO2 specimen that displayed a broad absorption band at 400-600 nm in the diffuse reflectance spectrum. The spectrum was fairly typical of those being displayed by other visible-light-active TiO2 specimens. Data on C(1s) photoemission obtained in the study of Colon and co-workers89 are of great importance within the present context. Different types of carbon species were detected. The main XPS peak at 283.9 eV was associated with the organic residues from the alkoxide precursor, whereas a second peak at 285.8 eV was ascribed to slightly oxidized organics that were removed after calcination at 973 K. Some graphitic carbon (peak at 284.6 eV) and some carbonate-like species nonetheless remained in the material. Note that the content of temperaturestable C species amounted to a level g4 at% in both S-doped and undoped samples.89 Because the O/Ti atomic ratio after

In summary, it is rather surprising that there are only a scant number of studies that have demonstrated the existence of prominent absorption bands in the spectra of undoped titanium dioxide in the visible region (3.1 < hV < 1.8 eV; 400 > λ > 690 nm). Nonetheless, the limited available data reveal that such absorption bands become observable in the visible spectral region only when reduced TiO2 is treated under special oxidizing conditions. The temperature, time and other conditions of the oxidative treatment depend strongly on the type of material (single crystal, powder) and the degree of reduction of the metaloxide specimen. Simple addition of molecular oxygen at room temperature to a prereduced TiO2 sample (Figure 3, curve 5)48 and prolonged oxygen annealing of a TiO2 crystal at T > 770 K (Figure 3, curve 3)50,51 yield similar spectra. The various experimental observations reported thus far strongly infer that the absorption bands AB1, AB2, and AB3 that induce the visible-light photoactivity to several TiO2 specimens originate from F-type color centers associated with oxygen vacancies, whereas the spectral features at wavelengths greater than ca. 600 nm (i.e., in the near-infrared, hν < 2.0 eV) are attributable to Ti-related color centers. Acknowledgment. One of us (N.S.) is grateful to Prof. Angelo Albini and his group at the University of Pavia, Italy, for their continued warm hospitality through the winter semesters of 2002-2009 and to Prof. Abe of the Tokyo University of Science for a visiting professorship (to N.S.) for the period July-August 2008. Note Added in Proof. In a recent study that appeared during the proofreading of our proofs (June 25, 2009 issue of J. Photochem. Photobiol. A: Chem.) and relevant to the present discussion, Livraghi and co-workers91 reported an analysis of two series of N-doped and N,F-co-doped TiO2 samples with impurity loadings from 1% to 30% prepared using a sol-gel synthesis and successive calcinations in air using NH4Cl and NH4F as the source of dopants for N-TiO2 and N,F-TiO2, respectively. The analysis based on the combined use of EPR spectroscopy and DFT calculations led them to deduce that in the N-doped TiO2 specimens a bulk nitrogen paramagnetic species (initially labeled Nb•, i.e., NO2-) formed whose energy level lay some tenth of an eV above the upper levels of the valence band. The species could be excited selectively at 440 nm (2.82 eV), a wavelength that corresponds to the AB1 absorption band in the visible spectral region (Table 3). The species responsible for such a feature was ascribed to an interstitial N atom in close proximity to a lattice O2- ion forming

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NO2- (see Figure 9) in equilibrium with the diamagnetic species NO3–, both of which were said to be responsible for the optical absorption of the yellow colored specimens. In the N,F-codoped TiO2 systems, the additional electron is seemingly localized at Ti4+ sites yielding shallow electron states (Ti3+) whose energy level resides just below the conduction band. The co-doped system is apparently stabilized by electron transfer from the upper Ti3+ states to the lower N-related levels (Nb•), see Figure 8b, occurring spontaneously thereby increasing the concentration of Nb- (i.e., NO3–) species and decreasing the number of Ti3+ color centers. References and Notes (1) Serpone, N.; Emeline, A. V.; Kuznetsov, V. N.; Ryabchuk, V. K. Second Generation Visible-Light-Active Photocatalysts: Preparation, Optical Properties and Consequences of Dopants on the Band Gap Energy of TiO2. In EnVironmentally Benign Photocatalysis - Applications of Titanium-Oxide Based Catalysts; Anpo, M., Kamat, P. V., Eds.; Springer: New York, in press 2009. (2) Emeline, A. V.; Kuznetsov, V. N.; Rybchuk, V. K.; Serpone, N. Intern. J. Photoenergy 2008. doi) 10.1155/2008/258394; see http:// www.hindawi.com/GetArticle.aspx?doi)10.1155/2008/258394. (3) Emeline, A. V.; Sheremetyeva, N. V.; Khomchenko, N. V.; Ryabchuk, V. K.; Serpone, N. J. Phys. Chem. C 2007, 111, 11456. (4) See for example: (a) Yang, X.; Cao, C.; Erickson, L.; Hohn, K.; Maghirang, R.; Klabunde, K. J. Catal. 2008, 260, 128. (b) Fang, J.; Wang, F.; Qian, K.; Bao, H.; Jiang, Z.; Huang, W. J. Phys. Chem. C 2008, 112, 18150. (c) Zhang, G.; Ding, X.; Hu, Y.; Huang, B.; Zhang, X.; Qin, X.; Zhou, J.; Xie, J. J. Phys. Chem. C 2008, 112, 17994. (d) Rengifo-Herrera, J. A.; Mielczarski, E.; Mielczarski, J.; Castillo, N. C.; Kiwi, J.; Pulgarin, C. Appl. Catal. B: EnViron. 2008, 84, 448. (e) Li, X.; Xiong, R.; Wei, G. Catal. Lett. 2008, 125, 104. (f) Ohno, T.; Murakami, N.; Tsubota, T.; Nishimura, H. Appl. Catal. A:General 2008, 349, 70. (g) Tojo, S.; Tachikawa, T.; Fujitsuka, M.; Majima, T. J. Phys. Chem. C 2008, 112, 14948. (5) See for example: (a) Shen, Y.; Xiong, T.; Li, T.; Yang, K. Appl. Catal. B: EnViron. 2008, 83, 177. (b) Yamada, K.; Yamane, H.; Matsushima, S.; Nakamura, H.; Sonoda, T.; Miura, S.; Kumada, K. Thin Solid Films 2008, 516, 7560. (c) Gao, J.-C.; Tan, X.-W.; Zou, J.; Xin, R.-L.; Wang, Y. Gongneng Cailiao 2008, 39, 1367. (d) Gao, J.-C.; Tan, X.-W.; Wang, Y.; Zou, J. Gongneng Cailiao Yu Qijian Xuebao 2007, 13, 485. (e) Yamada, K.; Yamane, H.; Matsushima, S.; Nakamura, H.; Ohira, K.; Kouya, M.; Kumada, K. Thin Solid Films 2008, 516, 7482. (f) Jing, L.; Li, S.; Song, S.; Xue, L.; Fu, H. Solar Energy Mater.Solar Cells 2008, 92, 1030. (g) Li, D.; Ohashi, N.; Hishita, S.; Kolodiazhnyi, T.; Haneda, H. Mater. Res. Soc. Symp. Proc. 2006. 900E (Nanoparticles and Nanostructures in Sensors and Catalysis), Paper No.: 0900-O09-04. Materials Research Society; see http:// www.mrs.org/s_mrs/ bin.asp?CID)6225&DID) 170684&DOC)FILE.PDF. (h) Wang, Y.; Meng, Y.; Ding, H.; Shan, Y.; Zhao, X.; Tang, X. J. Phys. Chem. C 2008, 112, 6620. (i) Iijima, K.; Goto, M.; Enomoto, S.; Kunugita, H.; Ema, K.; Tsukamoto, M.; Ichikawa, N.; Sakama, H. J. Lumin. 2008, 128, 911. (j) Yang, J.; Bai, H.; Jiang, Q.; Lian, J. Thin Solid Films 2008, 516, 1736. (k) Huang, C.-M.; Chen, L.-C.; Cheng, K.-W.; Pan, G.-T. J. Mol. Catal. A:Chem. 2007, 261, 218. (l) Wang, Z.; Gong, W.; Hong, X.; Cai, W.; Jiang, J.; Zhou, B. J. Wuhan UniV. Technol. Mater. Sci. Edn. 2006, 21, 71. (m) Rane, K. S.; Mhalsiker, R.; Yin, S.; Sato, T.; Cho, K.; Dunbar, E.; Biswas, P. J. Solid State Chem. 2006, 179, 3033. (6) Livraghi, S.; Chierotti, M. R.; Giamello, E.; Magnacca, G.; Paganini, M. C.; Cappelletti, G.; Bianchi, C. L. J. Phys. Chem. C 2008, 112, 17244. (7) Xiao, Q.; Zhang, J.; Xiao, C.; Si, Z.; Tan, X. Solar Energy 2008, 82, 706. (8) Feng, C.; Wang, Y.; Jin, Z.; Zhang, J.; Zhang, S.; Wu, Z.; Zhang, Z. New J. Chem. 2008, 32, 1038. (9) Fujishima, A.; Zhang, X.; Tryk, D. A. Surf. Sci. Rep. 2008, 63, 515. (10) Lin, Z.; Orlov, A.; Lambert, R. M.; Payne, M. C. J. Phys. Chem. B 2005, 109, 20948. (11) See for example: (a) Zhao, Y.; Li, C.; Liu, X.; Gu, F.; Du, H. L.; Shi, L. Mater. Chem. Phys. 2008, 107, 344. (b) Namai, Y.; Matsuoka, O. J. Phys. Chem. B 2005, 109, 23948. (c) Nakajima, N.; Kato, H.; Okazaki, T.; Sakisaka, Y. Surf. Sci. 2004, 561, 79. (d) Henderson, M. A.; Epling, W. S.; Peden, C. H. F.; Perkins, C. L. J. Phys. Chem. B 2003, 107, 534. (e) Jiang, H.; Song, H.; Zhou, Z.; Liu, X.; Meng, G. Mater. Res. Bull. 2008, 43, 3037. (f) Yamada, K.; Yamane, H.; Matsushima, S.; Nakamura, H.; Sonoda, T.; Miura, S.; Kumada, K. Thin Solid Films 2008, 516, 7560. (g) Cui, X.; Wang, B.; Wang, Z.; Huang, T.; Zhao, Y.; Yang, J.; Hou, J. G. J. Chem. Phys. 2008, 129, 044703/1. (h) Gopal, N. O.; Lo, H.-H.; Ke, S.C. J. Am. Chem. Soc. 2008, 130, 2760.

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