Atomic-Scale Surface Local Structure of TiO2 and Its Influence on the

May 29, 2014 - This Perspective focuses on the atomic-scale surface local structure dependence of those four kinds of competitive reactions on a TiO2 ...
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Atomic-Scale Surface Local Structure of TiO2 and Its Influence on the Water Photooxidation Process Akihito Imanishi* and Ken-ichi Fukui Division of Chemistry, Graduate School of Engineering Science, Osaka University, Toyonaka, Osaka 560-8531, Japan ABSTRACT: The water photooxidation reaction on TiO2 and related metal oxides has been attracting strong attention from the point of view of solar water splitting. The water photooxidation reaction (i.e., oxygen evolution reaction) accompanies three other kinds of side reactions (photoluminescence (PL), surface roughening, and nonradiative recombination). These reactions are competitive with each other, and the ratio of their quantum efficiencies strongly depends on the atomic-scale surface local structure. This Perspective focuses on the atomic-scale surface local structure dependence of those four kinds of competitive reactions on a TiO2 (rutile) singlecrystal electrode on which not only a terrace structure but also step structures were strictly controlled. The experimental results are discussed based on the reaction model of water photooxidation that we previously proposed. The photocatalytic activity of the TiO2 surface roughened by the photoinduced roughening process is also focused on.

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consideration of stabilization by electronic polarization of solvent water.43 The effective O 2p levels for OHaq and H2Oaq were also determined by the photoelectron emission spectra. From these estimations, we found that those surface O species have the O 2p levels below the top of the valence band at the surface and cannot be oxidized by the valence band holes by the electrontransfer mechanism, indicating the nonvalidity of the conventional electron-transfer mechanism.43

he oxygen photoevolution (or water photooxidation) reaction on TiO2 and related metal oxides1−17 has been attracting strong attention from the point of view of solar water splitting. Actually, many kinds of materials and strategies for water photooxidation have been suggested by many researchers in the previous Perspectives.17−20 On the other hand, for exploring new materials for photocatalysts, it is of key importance to elucidate molecular mechanisms of the reactions, which should strongly depend on the morphological, chemical, and electronic structures of materials at the surface as well as the reactivity (or energy) of photogenerated holes. Such studies will also serve for research to lower high overvoltages in the oxygen evolution at metallic electrodes. It has long been assumed21−35 that the water photooxidation reaction at the TiO2 surface is initiated by oxidation of surface Ti−OH group (or OH− ions in an aqueous solution) by photogenerated holes, h+, by an electron-transfer mechanism.36,37 Actually, the formation of •OH radicals in UV-irradiated TiO2 systems was reported by a spin-trapping ESR23,33 or lowtemperature ESR27 method, diffuse reflection FTIR spectroscopy,33 and gas-phase emission spectroscopy.38 However, it was reported39 that the water photooxidation reaction at the TiO2 surface did not produce any free •OH radicals but adsorbed •OH (or Ti−O•) radicals, the latter of which gave ESR signals similar to those reported by the spin-trapping method. It was also reported40 that •OH radicals were produced from hydrogen peroxide (H2O2) formed via reduction of molecular oxygen by electrons in the conduction band. Recently, it was experimentally elucidated that the contribution of •OH radicals to the O2 production is negligibly small.41 Moreover, theoretical calculation has shown42 that surface Ti(OH) groups can act as electron traps but cannot act as hole traps by formation of Ti4+(OH)• radicals. We estimated the enerygy levels of occupied O 2p orbitals for surface oxygen species (Ti−OH−Ti and Ti−OH) with © 2014 American Chemical Society

The investigation of a detailed oxidation mechanism taking all of the competitive side reactions into consideration is necessary for comprehensive understanding of the mechanism of the photooxidation of water on a TiO2 surface. On the other hand, we have proposed43−45 that the water photooxidation reaction is not initiated by the electron-transfer oxidation of Ti−OH but by a nucleophilic attack of a H2O molecule (Lewis base) to a surface-trapped hole (STH, Lewis acid). The details of the mechanism are shown in Figure 1.43 In this mechanism, the parts of the photogenerated holes are first trapped by triply coordinated oxygen on the surface and are consumed by electron−hole recombination, resulting in photoluminescence (PL). Other parts of them are consumed by Received: March 6, 2014 Accepted: May 29, 2014 Published: May 29, 2014 2108

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Figure 1. Mechanism for the water photooxidation reaction and side reactions on a rutile TiO2 surface.

room temperature. Thus, the investigation under the practical condition (i.e., in an aqueous solution) is necessary to reveal the photooxidation mechanism of water. Therefore, we have investigated the photooxidation mechanism of water using essentially atomically smooth TiO2 (rutile) (110) and (100) surfaces prepared by an alternative method of chemical etching and thermal annealing under the atmosphere.50,51 The TiO2 surfaces prepared by this method were stable in aqueous solutions of pH 1−13 (under the dark condition, with no applied potential), suggesting that they were free from damage and defects that may induce etching reactions (note that some researchers reported that the flat TiO2 (rutile) (110) surface prepared by Ar+ ion sputtering and thermal annealing under UHV became morphologically rough after exposition to neutral or alkaline aqueous solutions52). The investigations of crystalface dependences of surface band edges and hole reactivity became possible by this success in preparation of such atomically smooth and stable TiO2 surfaces.43,45,53 PL Process. PL is the most-studied process among the three side reactions. Many researchers have investigated PL emitted from TiO2 under UV irradiation because PL provides insight into the photooxidation dynamics on TiO2 in relation to thermalization and trapping. It is known that the energy and intensity of the PL strongly depend on the local structure of the emission site. In addition, the distortion of the TiO2 lattice affects the PL intensity. Actually, the distorted Ti−O octahedral structure in anatase TiO2 facilitates exciton binding as opposed to the less distorted structure in the rutile TiO2 lattice that favors free excitons.54−58 The flexibility of the local structure also affects the properties of PL because the trapping of the carrier induces the distortion of the lattice, which leads to the energy shift of PL.43 Chemical groups on the surface have also been shown to exert unique influences on the PL emission.59−61 Of course, bulk defects or dopants in TiO2 have been shown to have an effect on the self-trapped luminescence state.56,62,63

nonradiative recombination at different sites (probably a step site, as described later). On the other hand, the rest diffuse to the bridging oxygen at step, kink, or terrace sites, where the STH is easily nucleophilically attacked by H2O molecules (of course, it cannot be excluded that a photogenerated hole before relaxation directly reacts with water at the bridging oxygen). H2O attack induces the bond scission of the TiO2 lattice, resulting in the surface roughing of TiO2. Finally, oxygen gas is generated via peroxide species formed on the surface. We should note that, in this model, not only an O2 evolution reaction but also three other side reactions were considered (PL, surface roughening (i.e., photocorrosion), and nonradiative recombination). Although many researchers have known that these processes (other than surface roughening) occurred during water oxidation, few studies were reported in which the photooxidation mechanism was discussed taking all of the competitive reactions into consideration. However, we should note that those four reactions are competitive with each other. Thus, the investigation of a detailed oxidation mechanism taking all of the competitive side reactions into consideration is necessary for comprehensive understanding of the mechanism of the photooxidation of water on a TiO2 surface and would be a prerequisite to improve the efficiency of O2 evolution. In this model, the triply coordinated O where the hole is trapped is located only at terrace sites. On the other hand, the bridging O, where water molecules attack STH, is located at terrace and step sites. The bridging O at a step site may be more active than those at a terrace site because such sites are easily distorted, which is favorable for the formation of intermediate species such as Ti−O− O−Ti species. Thus, it is quite possible that the ratio of quantum efficiency of those four competitive reactions strongly depends on the atomic-level surface local structure. A single-crystal surface is suitable for the investigation of the surface local structure dependence of the photooxidation process. Actually, up to now, many researchers have investigated the correlation between the surface structure and photocatalytic activity using the single-crystal surface. However, most of them have been carried out under the vacuum condition (the atomically well-defined TiO2 surfaces have been prepared by a method of Ar+ ion sputtering and thermal annealing under ultrahigh vacuum (UHV) conditions46,47), under which the surface properties (surface chemical states, surface structure, surface potential, and so on) are largely different from those in an aqueous solution and/or air. For the TiO2 surfaces in contact with aqueous solutions, significant reconstruction, including hydrolysis and ion adsorption, should occur. The same holds for the surfaces placed in air because it is reported48,49 that more than several monolayers of water are adsorbed on them at

The ratio of quantum efficiency of those four competitive reactions strongly depends on the atomiclevel surface local structure. On the other hand, unfortunately, PL emitted from rutile TiO2 is not as well-studied as it is for anatase because exciton quenching is more frequently radiative in anatase and almost exclusively nonradiative in rutile.58 Thus, defects, surfaces, and impurities are the likely sources of PL, especially in rutile.43,45,59,64,65 Therefore, the ideal material without bulk defects such as 2109

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high-quality single-crystal TiO2 is necessary for the precise investigation of PL emitted from surface sites. Ef fect of Surface Faces of TiO2 on the PL Process. Figure 2 shows j versus U and the PL intensity versus U for a 0.05 wt %

Figure 2. j versus U for an n-TiO2 rutile electrode with the atomically flat (110) surface in 0.1 M HClO4 (pH 1.1) compared with the PL intensity versus U. The PL intensity versus U for an atomically nonflat surface is also included for reference. The illumination was carried out by the 365 nm band from a 500 W high-pressure mercury lamp, obtained by use of band-pass filters and metal nets for adjusting the UV intensity (0.2 mW/cm2). Ufb: flat band potential. (Reprinted from refs 43, 45, and53.)

Figure 3. PL spectra from the (110) and (100) surfaces of n-TiO2 (rutile) in 0.1 M HClO4 (pH = 1.1) and 0.1 M NaOH (pH = 13.0). The illumination was carried out by the 365 nm band from a 500 W high-pressure mercury lamp. The spectra were obtained under the UV irradiation intensity of 1.0 mW/cm2 at potentials at which the PL intensity took a maximum. The PL intensity at the maximum wavelength is normalized to the same level between pH 1.1 and 13.0 to make clear the difference in the spectral position and shape. (Reprinted from ref 43.)

Nb-doped n-TiO2 rutile electrode with the atomically flat (110) surface in 0.1 M HClO4 (pH 1.1). The flat band potential (Ufb) for the n-TiO2(110) surface (−0.25 (0.01 V versus Ag/AgCl/ sat. KCl at pH 1.1)43,45,53 is also indicated for reference. A large positive deviation of the onset potential of the photocurrent, Uon, from the Ufb is attributed to surface carrier recombination via surface states and/or a large activation energy, as mentioned later. The PL intensity took a maximum near the Uon, indicating that the PL is really arising from surface carrier recombination (i.e., radiative recombination between conduction band electrons, e-CB, and the STHs). The disappearance of the PL at positive potentials is simply attributed to increased band bending in the n-TiO2 electrode, whereas that at negative potentials can be attributed to formation of reduced surface species, such as Ti3+,43,45,53,65 which can trap efficiently the valence band holes nonradiatively. We should note that no PL was observed for atomically nonflat TiO2 surfaces (Figure 2).43,45,53 This result is consistent with our model, in which the PL emission site (triply coordinated oxygen) is loacted only at terrace sites because the numbered of step and kink sites is relatively large at roughend surface. Figure 3 compares the PL spectra in 0.1 M HClO4 (pH 1.1) and 0.1 M NaOH (pH 13.0) for both the (110) and (100) surfaces, obtained under the UV irradiation intensity of 1.0 mW/cm2 at potentials at which the PL intensity took a maximum (the PL intensity at the maximum wavelength is normalized to the same level between pH 1.1 and 13.0). The PL bands from the (110) and (100) surfaces are peaked at about 810 and 840 nm, respectively, indicating that the energy level of the PL-emitting species (STH) is slightly different between the (100) and (110) surfaces.43,45,53 From this reslut, the energy levels of the PL emission site were estimated by taking account that the Ufb for the (100) face

Figure 4. Schematic energy level diagrams for the atomically smooth (100) and (110) surfaces of n-TiO2 (rutile) at pH 0, estimated from the flat band potential and the PL spectra. (Reprinted from ref 45.)

is about 0.09 V more negative than that for the (110) face53,66 (Figure 4). The energy level for the STH at the (100) face, E(STH)-(100), is estimated to be about 0.14 eV above that at the (110) face, E(STH)-(110). This can be explained by the difference in the location of the PL emmistion site (i.e., triply coordinated oxygen site) between (110) and (100) surfaces. In other words, the difference in the E(STH)-(100) and E(STH)(110) thus comes from a difference in the stabilization energy 2110

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of the PL intensity at the peaked potential for the (110) and (100) TiO2 surfaces, obtained under the low-intensity UV irradiation of 0.2 mW/cm2, are shown in Figure 6, where the PL intensity at pH 1.1 is normalized to unity in both surfaces. The PL intensity was the highest in an acidic solution (pH 1.1) and decreased stepwise with increasing pH; namely, the PL intensity sharply decreased at around pH 4 and at about pH 13. It is to be noted that pH 4.0 is close to the point of zero charge (pzc) of rutile TiO2 (about 5.0) reported.67−69 This result can be explained by the change of the surface charge (formal charge) of TiO2. In the interior of the TiO2 crystal, a Ti4+ ion is coordinated with six O2− ions, and an O2− ion is coordinated with three Ti4+ ions. Accordingly, the surface oxygen vacancies or five-coordinated Ti atoms (species b in Figure 5) should have a formal charge of +(2/3)e (e: elementary charge), whereas the bridging oxygen atoms (species c in Figure 5) should have a formal charge of −(2/3)e. Thus, in an acidic solution (pH 1.1), the TiO2 surface is mainly covered with Ti−OH 2(2/3)+ and Ti−OH−Ti(1/3)+. The nucleophilic attack of a H2O molecule to a protonated bridging oxygen, Ti−OH−Ti(1/3)+, will hardly occur because the positively charged STH cannot come close to such a positively charged species (Ti−OH−Ti(1/3)+). Thus, the STH remains at the terrace for a while without causing any reaction, resulting in effective emission of the PL. The reaction of the STH may occur at the nonprotonated bridging oxygen (Ti−O−Ti(2/3)−) present at a very low density in terraces or kinks or steps. It is also highly probable that the accumulation of the STH with positive charges at the surface causes the downward shift of the surface band energies (or Ufb) of the n-TiO2 electrode under anodic bias,70 which accelerates deprotonation at Ti−OH−Ti(1/3)+ and hence the nucleophilic attack of H2O. With increasing pH, the surface density of Ti−O−Ti(2/3)− (species c in Figure 5) will increase by deprotonation. Thus, the rate of water oxidation via bridigng oxygen increases, resulting in the decrease in the density of the STH and hence in the lowering of the PL intensity. This explanation is supported by the fact that the sharp decrease in the PL intensity occurs at around pH 4, near the pzc of TiO2 of about 568,69 (Figure 6). In the case of the (110) surface, the pH dependence of the PL intensity shows the second sharp decrease near pH 13 (Figure 6) (there is a possibility that a similar drop exsits in the case of the (100) surface; however, we could not differentiate it due to small PL intensity). This might tentatively be attributed to formation of readily oxidized species, Ti−O(4/3)−, by deprotonation of Ti−OH(1/3)− (species d′), which can cause an electron-transfer reaction.

Figure 5. Crystal lattice models for the (110) and (100) TiO2 (rutile) surfaces: white circle, O2−; black circle, Ti4+; large gray circle, O atom of adsorbed water or OH; small gray circle, H atom. (Reprinted from ref 43.)

of STH between the (100) and (110) faces. For the (110) terrace, the triply coordinated O atom plane (species a in Figure 5), where the STH exists, is placed parallel to the surface and hence rigidly bound to the surface crystal lattice on all sides, as seen from Figure 5. On the other hand, the triply coordinated O atom plane at the (100) terrace is placed slantwise to the surface and hence is only partially bound to the surface crystal lattice, with the electrolyte side of the plane being left open (Figure 5). This means that the triply coordinated O atom plane at the (100) terrace can be more easily distorted than that at the (110) face, or in other words, the STH at the (100) terrace can be more stabilized by the surface-lattice relaxation than that at the (110) terrace. Thus, the E(STH)-(100) is slightly above the E(STH)-(110). On the other hand, such a difference in the location of the PL emmision site affects the quenching efficiency. It was reported that the quenching efficiency of PL by hydroquinone at (100) was lower than that at the (110) surface.43 This can be explained by the fact that the hydroquinone molecule cannot easily come close to the triply coordinated oxygen site at the (100) surface because the site is placed inside of the topmost layer, whereas the site at the (110) surface is directly exposed to the electrolyte. The pH of the solution also affects the behavior of PL emission because the surface chemical species (a kind of surface local structure) depends on the pH of the solution by protonation and deprotonation of surface species.67 The detailed pH dependences

Figure 6. pH dependence of the PL intensity for the n-TiO2(110) and (100) surfaces. (The illumination was carried out by the 365 nm band from a 500 W high-pressure mercury lamp. UV intensity: 0.2 mW/cm2.) (Reprinted from ref 43.) 2111

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The formation of Ti−O(4/3)− in very high pH was suggested71 from an increased cathodic current due to increased adsorbed oxygen molecules in a form of Ti−O−···O2. The occurrence of reaction 1 in strongly alkaline solutions is also supported by AFM inspection of the TiO2 surface, which showed that atomic-level surface roughening was largely suppressed in pH 13 (Figure 8). As reaction 1 does not accompany any bond breaking at the surface crystal lattice, it is quite reasonable that the water oxidation reaction initiated by reaction 1 does not lead to the surface roughening.

terrace (note that in our model, PL was emitted from a triply coordinated oxygen site, which is located only at a terrace). The former result indicates that the efficiency of PL emission depends on the lattice structure of step edges. Considering the fact that PL is emitted from terrace site, the difference in the PL intensity originated from the difference in the efficiency of other competitive side reactions such as surface roughening or nonradiative recombination, which may depend on the lattice structure of step edges. The other factor that may explain this result is “anisotropic diffusion of carriers”. The (110) face has two-fold symmetry, and it is known that the carriers preferenticially diffuse along the ⟨001⟩ direction in the TiO2 crystal. If the hole preferentially diffused along the ⟨001⟩ direction, the probabillity that the diffusing holes are consumed by other competitive reactions at the step edges became larger on the surface having the ⟨1̅10⟩ step. Surface Roughening Process. Surafce roughning is a kind of photoinduced anodic etching reaction (i.e., photoinduced selfdissolution reaction) that is frequently oberved for the semiconductor electrode. Although, in general, TiO2 has been believed to be stable even during the photocatalytic reaction, it is quite possible that atomic-level etching occurs on the surface. Actually, it is known that photoinduced etching is drastically enhanced in H2SO4 aqueous solution.66,73,74 (Note that adsorbed SO42− species probably play a partial role in the case of H2SO4 solution, whereas it was confirmed that no chemical species other than the water molecule was adsorbed in a 0.1 M HClO4 aqueous solution. Thus, the observed roughening is purely due to bond scission of the TiO2 lattice induced by H2O attack, as shown in Figure 1.) Figure 8a and b shows the AFM images of the rutile TiO2(110) surface before (a) and after (b) the photooxidation reaction under UV irradiation in 0.1 M HClO4 (pH 1.1). We can see the ideal step−terrace structure on the surfaces before the photooxidation reaction, whereas the surface has many steps and kinks after the photooxidation reaction, indicating that the surface was roughed with the progress of the photooxidation reaction. The estimated roughness factor (Rq) of the roughened surface was about 0.2 nm, whereas that for the surface before the photooxidation reaction were 0.1 nm, indicating that the degree of roughness was quite small and roughening occurred on the atomic scale. On the other hand, as mentioned before, PL is observed only for the atomically smooth surfaces and not for rough surfaces. Actually, the PL intensity was decreased with the progress of the surface roughening reaction.45 We evaluated the rate of the surface roughening reaction by using this phenomenon. Figure 9 shows PL intensities plotted against the irradiation time of UV light (5.1 mW/cm2). The PL intensities for both the (100) and (110) surfaces show substantial decays with the illumination time under the high-intensity illumination (note that those under the low-intensity illumination, under which the surface roughening hardly occurred, remained nearly constant45). It is important to note in Figure 9 that the PL intensity from the (110) face decays much faster than that from the (100) face. This implies that the photoinduced surface roughening for the (110) face occurs faster than that for the (100) face and indicates that the reaction rate of surface roughening depends on the surface local structure such as surface faces. On the other hand, it is quite possible that the reaction rate of surface roughening also depends on the surface defects such as step and kink sites. We estimated the reaction rate of surface roughening on the aforementioned four kinds of vicinal surfaces.

It is known that surface defects such as step and kink sites sometimes play an important role in the surface reaction. Actually, it is quite possible that those sites became a key site even in the case of an actual photocatalyst. Ef fect of the Step−Terrace Structure on the PL Process. It is known that surface defects such as step and kink sites sometimes play an important role in the surface reaction. Actually, it is quite possible that those sites became a key site even in the case of an actual photocatalyst. Vicinal single-crystal surfaces are often used for the purpose of controlling the step−terrace structrue. We investigated how the step−terrace structure affects the four kinds of competitive reactions (including oxygen photoevolution) using four kinds of vicinal single-crystal TiO2 (rutile) (110) as substrates. Two of them have the steps running along the ⟨001⟩ direction, and their terrace widths are 40 and 10 nm. The other ones have the steps running along the ⟨11̅ 0⟩ direction, which is perpendicular to the ⟨001⟩ direction, and their terrace widths are also 40 and 10 nm. Figure 7 shows the PL inteinsty emitted from the surfaces having the same terrace width. The PL intensity from the

Figure 7. PL intensity versus applied potential for vicinal n-TiO2(110) (rutile) surfaces. The illumination was carried out by the 365 nm band from a 500 W high-pressure mercury lamp (UV intensity: 3.0 mW/cm2).

surface having the step running along the ⟨001⟩ direction was larger than those emitted from the surface having the step running along the ⟨1̅10⟩ direction.72 When we used the PL inteinsty emitted from the surfaces having the same step direction, the PL intensity from the surface having a larger terrace width was larger than those emitted from the surface having the smaller terrace width (data not shown). The latter result indicates that the PL emission site is located at the 2112

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Figure 8. AFM images of the atomically flat n-TiO2(110) (rutile) surface (a) before and (b,c) after the UV irradiation (365 nm, 0.57 mW/cm2), during which the TiO2 surface was immersed in (b) 0.1 M HClO4 (pH 1.1) and (c) 0.1 M NaOH (pH 13.0). The irradiation was carried out at the potential of 1.5 V versus Ag/AgCl/sat. KCl, and the electricity passing across the electrode surface was regulated to be 0.5 C/cm2 in both the solutions. (Reprinted from ref 43.)

reactions and irradiated photointensity. Although the details have not been clarified yet, some interesting results have been obtained. Both efficiencies of the oxygen evolution and the nonradiative recombination strongly depend on the step direction and step density. These results strongly indicate that there is a strong correlation between four kinds of reactions. In addition, these results indicate that one can control the quantum efficiency of oxygen evolution by controlling the efficiency of side reactions (i.e., by controlling the surface local structure).

If the surface roughening occurred, the surface local structure would be changed, which may induce the change of the reaction efficiency of four kinds of competitive reactions. Photoactivity of Roughened Surfaces. We should note that there is a possibility that the above explanation is available only for the initial stage of the photooxidation reaction. If the surface roughening occurred, the surface local structure would be changed, which may induce the change of the reaction efficiency of four kinds of competitive reactions. Therefore, it is an important subject to investigate the photooxidation reactions on the photoinduced roughened surface. However, unfortunately, although many researchers have discussed the surface face dependence of photocatalytic activity based on the lattice models of ideal surface faces, few researchers consider such an effect. Figure 10 shows the photocurrent density (j) versus potential (U) curves of flat (a) and roughened (b) TiO2(110) rutile electrodes measured in 0.1 M HClO4 aqueous solution under UV irradiation (0.65 mW/cm2). Those measured in 0.1 M NaOH aqueous solution are denoted by (a′) and (b′). The large photocurrent was mainly attributed to the oxygen evolution at TiO2 surfaces, though the surface roughening process also contributes to the photocurrent. The onset potential (Uonset) values of all of the samples are shown in Table 1. The large difference in the Uonset between HClO4 solution and NaOH solution comes from the well-known pH dependence of the flat band potential (Ufb).43 The most important feature is the difference in the Uonset for oxygen photoevolution between flat and roughened TiO2 electrodes, which was observed only for the acidic solution (HClO4 aqueous solution). The result suggests that, in the case of HClO4 aqueous solution, the overvoltage for oxygen photoevolution at the roughened TiO2 electrode was

Figure 9. PL intensity versus the illumination time for the atomically flat (upper) (100) and (lower) (110) n-TiO2 (rutile) surfaces in 0.1 M HClO4 under continuous UV illumination at an intensity of 5.1 mW/cm2. The applied potential was stepped alternately from 0.0 to 1.5 V versus Ag/AgCl and in the inverse direction with constant intervals. The inset represents the changes in the PL intensity on an expanded time scale. (Reprinted from ref 45.)

We found that the terrace width and step direction also affect the rate of surface roughening (data not shown). This result suggests the possibility that the surface roughening reaction occurs mainly at the step edge. When we take into account this result, it is quite possible that the aforementioned difference in the surface roughening rate between the (110) and (100) surfaces came from the difference in the step structure between these surfaces. Interestingly, the tendency of the step−terrace structure dependence of the roughening rate is opposite to that of the PL emission, indicating that those side reactions (PL and surface roughening) are competitive with each other. Nonradiative Recombination and Oxygen Evolusion Processes. The efficiencies of two other reactions, oxygen evolution and nonradiative recombination, should be estimated for comprehensive understanding of all of the competitive reactions. We are investigating the efficiency of generated oxygen gas by measuring the dissolved oxygen in the airtight container and also estimating the nonradiative efficiency from the difference between the sum of the efficiencies of other three kinds of 2113

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surface local structures of TiO2 electrodes. This result suggests that oxygen photoevolution at the TiO2 surface in the alkaline solution was proceeded by another mechanism in which the photocatalytic activities for oxygen photoevolution are less affected by the surface local structure. As mentioned before, one of the possible mechanisms in alkaline solution is the electron-transfer mechanism denoted by eq 1. We should note that in this mechanism, the reaction proceeds without the formation of any intermediates, which induces the distortion of the lattice; thus, the activation energy of the oxygen evolution reaction was less affected by the change of the surface local structures. Therefore, in the case of the j−U curves obtained in NaOH aqueous solution, the Uonset values of roughened and flat TiO2 were almost the same. Up to now, many researchers have explored new materials for photocatalysts based on the properties of their band structure such as the energy level of the conduction band and valence band and the band gap energy (i.e., band engineering). However, we should note that the photocatalytic activity is not always determined only by the band structure of the photocatalyst because, as mentioned in this Perspective, the atomiclevel surface local structure of the photocatalyst has a strong influence on the photocatalytic activity. In this Perspective, we have summarized the mechanism of photooxidation of water and the other three side reactions, with a focus being placed on the surface local structure dependence of those competitive reactions. It was revealed that the ratio of the quantum efficiency of four kinds of reactions induced by photogenerated holes, including oxygen photoevolution, strongly depends on the surface local structure such as facet faces and/or the step− terrace structure. On the other hand, the surface roughening process, which is one of the four competitive reactions, induced the drastic change of the surface local structure and also induced the change in the photocatalytic activity for the oxygen photoevolution reaction. These investigations also contribute to the more comprehensive understanding of the photocatalytic activity of the actual catalyst. Recently, L. Zhang et al. reported the morphological change of the surface of the TiO2 particle (anatase) during a water splitting reaction.76 When TiO2 is exposed to light and water vapor, the initial crystalline surface converts to an amorphous phase 1−2 monolayers thick. The amorphous layer is stable and does not increase in thickness with time and is heavily hydroxylated. On the other hand, K. Yoshida et al. also reported the changes in the crystal structure and grain modifications in TiO2 thin films during the photocatalytic oxidation of hydrocarbons.77 When the hydrocarbon and collodion films were irradiated, single-crystalline TiO2 transformed into polycrystals. They claimed that the changes were associated with the loss of oxygen atoms in the TiO2 crystal lattice. Although these

Figure 10. Photocurrent density (j) versus potential (U) curves of flat (a) and roughened (b) n-TiO2(110) (rutile) electrodes measured in 0.1 M HClO4 aqueous solution under UV irradiation (0.65 mW/cm2). Those measured in 0.1 M NaOH aqueous solution are denoted by (a′) and (b′). All of the j−U curves were measured with the potential sweep rate of 50 mV s−1. (Reprinted from ref 75.)

smaller than that at the flat TiO2 electrode. On the other hand, in the case of the j−U curves measured in NaOH aqueous solution, the Uonset values of flat and roughened TiO2 were almost the same as each other, indicating that the overvoltage for oxygen photoevolution at the TiO2 in NaOH aqueous solution was not affected by the surface local structure of the TiO2 electrode. We should note that the Uonset of the semiconductor is affected by three factors, (i) Ufb, (ii) the surface recombination rate of the photogenerated carriers, and (iii) the activation energy of the active sites for the oxygen photoevolution reaction. By the detailed consideration, it was concluded that the surface roughing induced the decrease of the activation energy at the active sites for oxygen photoevolution. The oxygen photoevolution mechanism at TiO2 shown in Figure 1 supported this explanation. In this mechanism, the hole trapped at the bridging oxygen site is attacked by a H2O molecule, followed by the formation of peroxide species (intermediate species). We should note that the rate of the water photooxidation reaction strongly depends on the relaxation energy against the distortion of the surface lattice, which is induced by the formation of the intermediates (i.e., TiOOTi) for oxygen evolution. Thus, the surface local structure is of key importance for catalytic activity for oxygen photoevolution at the TiO2 surface because the relaxation energy of the lattice depends on the surface local structure. In the case of the j−U curves obtained in NaOH aqueous solution, the Uonset of the TiO2(110) surface for oxygen photoevolution did not change after the surface roughening, indicating that the photocatalytic activity for oxygen photoevolution in NaOH aqueous solution was less affected by the

Table 1. Onset Potential of Oxygen Photoevolution (Uonset) and Flat Band Potential (Ufb) of (110) n-TiO2 (rutile) Electrodesa (110) sample electrolyte 0.1 M NaOH

0.1 M HClO4 aq

aq

surface morphology U U a

(V vs Ag/AgCl) onset (V vs Ag/AgCl) fb

flat −0.30 ± 0.01 0.12 ± 0.03

rough −0.31 ± 0.00 −0.08 ± 0.03

flat −1.15 ± 0.01 −0.73 ± 0.01

rough −1.08 ± 0.02 −0.73 ± 0.02

Reprinted from ref 75. 2114

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phenomena are not necessarily the same as the phenomena that we observed (i.e., surface roughening in an aqueous solution), these reports strongly suggest that the change in the surface morphology induced by the photooxidation reaction is very important to understand the detailed mechanism of the water photooxidation reaction even in the case of actual catalysis. In addition, we should note that such a change in the surface morphology strongly depends on the local structure of the original surface. For example, we have already found that the efficiency of the surface roughening strongly depends on the step−terrace structure on the original TiO2 surface, and the surface roughening reaction is effectively suppressed on the surface having the steps running along the specific direction. Therefore, there is a possibility that we can fabricate the surface on which the surface local structure changes during the photooxidation reaction so that photocatalytic activity increases by controlling the surface local structure of the original surface. We should also focus on the reaction mechanism on the lattice distorted surface and the amorphous surface. As mentioned in this Perspective, the surface lattice distortion strongly affects the stability of the STH and/or reaction intermediates. For example, we reported that the onset potentials of the amorphous RuO2 films for oxygen evolution were shifted toward the negative side by 0.06−0.03 V from those for the rutile crystalline samples.78 The shift of the onset potential is probably attributed to the structural flexibility, which is characteristic of the amorphous surface (note that the relaxation energy against the structural distortion on the amorphous surface is largely different from that on the surface having rigid structure). On the other hand, it was recently reported that the hydrogenated TiO2 nanocrystal, which has a disordered layer on its surface, shows high photoconversion efficiency for water splitting.79−81 Although most of the researchers focus on the change of its band structure induced by the lattice distortion, it is quite possible that the stability of the photogenerated hole and/or reaction intermediate on the distorted surface layer plays an important role in the highly efficient photocatalytic activity. In any case, comprehensive understanding of the consumption process of the photogenerated hole and its surface local structure dependence is essential to control the photocatalytic activity. By development of this research, we will be able to not only prepare the photocatalyst having ideal surface structure but also design the spatial arrangement of the co-catalyst based on the knowledge of the diffusion behavior of the photogenerated hole. We believe that these challenges will facilitate progress in the fabrication of efficient photocatalysts.

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Ken-ichi Fukui has been a Professor in Chemistry at Osaka University since 2008. He received his Ph.D from the University of Tokyo in 1994. His present research interests cover the microscopic mechanism of catalysis at solid surfaces, the molecular view of an electric double layer for novel energy conversion particularly at ionic liquid/electrode interfaces, and the function of edge states of graphene. (http://www. surf.chem.es.osaka-u.ac.jp/index-e.html)

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ACKNOWLEDGMENTS The authors are thankful to Mr. T. Sakao, Dr. R. Nakamura, Dr. E. Tsuji, and Prof. Y. Nakato for their contributions to our research. This work was financially supported by a Grant-in-Aid for Scientific Research (B) (No. 23350065) from the Japan Society for the Promotion of Science (JSPS).

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: +81-6-68506237. Fax: +81-6-6850-6237. Notes

The authors declare no competing financial interest. Biographies Akihito Imanishi is an Associate Professor of the Graduate School of Engineering Science at Osaka University. He received his Ph.D. from The University of Tokyo in 1998. His present research activities are studies on the photoelectrochemical reaction on well-defined surfaces and quantum beam induced reactions in ionic liquid. His research aims include understanding interface structure dependence of electrontransfer mechanisms at solid/liquid interfaces. (http://www.dma.jim. osaka-u.ac.jp/view?l=en=2749) 2115

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