GGA+U Study on the Mechanism of Photodecomposition of Water

Dec 29, 2013 - GGA+U Study on the Mechanism of Photodecomposition of Water Adsorbed on Rutile TiO2(110) Surface: Free vs Trapped Hole. Yongfei Ji†, ...
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GGA+U Study on the Mechanism of Photodecomposition of Water Adsorbed on Rutile TiO2(110) Surface: Free vs Trapped Hole Yongfei Ji,† Bing Wang,‡ and Yi Luo*,†,‡ †

Department of Theoretical Chemistry and Biology, School of Biotechnology, Royal Institute of Technology, SE-106 91 Stockholm, Sweden ‡ Hefei National Laboratory for Physical Sciences at the Microscale, University of Science and Technology of China, Anhui 230026, China ABSTRACT: The initial step of O2 evolution reaction on a TiO2 surface is a long-standing puzzle. A recent scanning tunneling microscopy experiment showed that the H2O molecule adsorbed on rutile TiO2(110) surface could decompose under ultraviolet illumination (Tan, S. J.; et al. J. Am. Chem. Soc., 2012, 134, 9978). The underlying reaction mechanism is now examined by our GGA+U study, in which the oxidation of the H2O molecule by both free and trapped holes has been carefully investigated. It is found that the transfer of the hole trapped at the bridge oxygen to the molecule is hindered by the mismatch between the energy and spatial symmetry of the trapped hole orbital and the highest occupied molecule orbital of H2O. The entire oxidation reaction has a high energy barrier and is barely exothermic. In contrast, the oxidation of the molecule by the free hole is energetically more favorable. The free hole is transferred to the H2O molecule via the in-plane oxygen atom when the molecule stays in the transient dissociation state. This mechanism may also be applicable to the photooxidation of other R−OH type molecules adsorbed on the rutile TiO2(110) surface.

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INTRODUCTION The photodecomposition of the H2O on a TiO2 surface has been extensively studied since 1972.1 However, the mechanism for oxygen evolution reaction, especially its initial step, has long been under debate. The reaction was first believed to be initiated via a charge transfer mechanism in which a photogenerated hole was transferred to the H2O molecule or OH anion, resulting in the formation of OH radical.2−4 The photoemission spectroscopy experiment5−7 showed that the occupied states of adsorbed H2O molecule is well below the top of the valence band of the TiO2. According to Marcus’s charge transfer theory,8 this can result in a large reorganization energy which makes the charge transfer rate extremely low.9 Imanishi et al proposed an acid−base type mechanism10,11 in which the reaction was initiated by nucleophilic attack of a H2O molecule to a bridge oxygen, accompanied by the transfer of surfacetrapped hole. Neumann et al.12 pointed out problems associated with this mechanism and suggested that the hole was already trapped at the bridge oxygen before attacking the H2O. This was supported by a recent first-principles study13 which revealed that the bridge oxygen is the most favorite holetrapping site on a TiO2(110) surface, and the H2O molecule is oxidized via the hybridization of trapped hole orbital and the highest occupied molecular orbital (HOMO) of the H2O. But the calculation suggested that the product should be quite different from that in Imaishi’s model. A recent scanning tunneling microscopy (STM) experiment14 observed directly that the H2O molecule adsorbed on the surface can be dissociated under UV illuminations. The © 2013 American Chemical Society

product of the reaction is a bridge OH group and a OH radical diffused away from the initial adsorption site. This is different from the mechanism proposed by Imanishi et al. in that the OH group is inserted into Ti−O−Ti bond on bridge oxygen row. The new experimental evident indicates that there may even be a different mechanism for the oxidation of adsorbed H2O molecule. This is quite normal because the reaction in solution may take place between the H2O molecule above the surface and the hole which has been studied previously.13 Here we perform first-principles calculations to fully describe the reaction mechanism for the adsorbed H2O molecule, which includes the identification of the oxidizing species and the exploration of reactions paths. On a bare TiO2 surface, only two kinds of oxidizing species exist: the free hole and the trapped hole. It was suggested that the photogenerated hole would be trapped within picoseconds,15 and certain photoreactions are believed to be initiated by these trapped holes.9,16−19 Meanwhile, some studies have shown that the free electron and hole may also take part in the photocatalytic reactions.20,21 The free hole may react more efficiently with chemisorbed molecules since it has higher mobility.19 Thus, we have systematically investigated the reactions of both free and trapped holes with the H2O molecule. Received: September 26, 2013 Revised: December 11, 2013 Published: December 29, 2013 1027

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RESULTS AND DISCUSSION Adsorption and Dissociation at the Ground State. There is a long controversy about whether H2O adsorbed on rutile TiO2(110) surface associatively or dissociatively.4,41−48 Generally, it is suggested that the energies of the associative adsorption and dissociative adsorption are nearly degenerate with a difference smaller than 0.1 eV.4 To model the trapped hole, we have to apply the GGA+U method which has not been used frequently to study the adsorption of H2O. In our previous work,13 we found that a U(p) around 5 eV would give a similar magnetic moment at the hole-trapping site as that from HSE hybrid functional.49 Here we perform a test calculation on the adsorption and dissociation of H2O molecule with GGA+U (U(d) = 4.2 eV, U(p) = 5.0 eV) method and compare it to the result from the GGA method, as shown in Figure 1a. The

Due to the self-interaction error (SIE) in DFT method, the hole tends to delocalize in GGA calculations. There are usually two choices to overcome SIE and obtain a localized hole. One is to use hybrid functional,22 but it is often too expensive; the other is to use DFT+U23 methods that only increase the computation effort slightly. U correction to p orbital has been applied in many recent studies for various oxides13,24−30 with values ranging from 4 to 7 eV. Generally, it is believed that there is no single U that can describe all the properties of the material well. Different U has to be applied to get the general picture. In this study, we are going to scan the U from 3 to 7 eV to study the reaction of adsorbed H2O molecule with both free and trapped holes. The results suggest that it is the free hole that is mainly responsible for this oxidation reaction.

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COMPUTATIONAL METHODS We performed spin-polarized GGA+U23 calculations with PBE 31 exchange-correlation functional implemented in VASP32−35 with a plane wave basis cutoff of 400 eV. U correction was applied to both Ti-d and O-p states with U(d) = 4.2 eV36 and U(p) ranging from 3.0 to 7.0 eV. The U(d) applied here is quite typical for TiO2, and it also describes the defect states resulting from the bridge oxygen vacancy on rutile TiO2(110) surface very well.36 It is sufficient to just use this one U(d) in this study13,27 and concentrate on how the results change with U(p). Ion−electron interaction was described by PAW pseudopotential.37 The rutile TiO2(110) surface was presented by a slab model with a vacuum gap of 13 Å. A slab model with one side relaxed and the other side fixed to mimic the bulk has been often used to model the surface. But in our previous studies,13,38 we have shown that such kind of surface model can result in an artificial surface state right on the top of the valence band. Thus it is inappropriate to use it for the study of hole-related properties. Instead, we used a five-layer slab model with only the center layer fixed, and other atoms were allowed to relaxed. Structures were relaxed until all forces on atoms were smaller than 0.02 eV/Å. 3×2 supercell was used, and calculations were performed at Γ point only. We found that for the adsorption and dissociation of H2O molecule at the ground state, its difference from the 4 × 2 supercell is neglectful. The triplet state was first used to mimic the excited state while the slab was kept neutral.13,38 However, we found that this approach would always lead to the formation of the trapped electron and hole pair in GGA+U calculations. In order to study the reaction of the H2O molecule with the free hole, we tried to use a OH group to inject a hole into the system solely (see below for details). The injected hole can remain delocalized (the delocalization was confirmed by checking the magnetic moments of the atoms and the spatial distribution of the hole orbital) or trapped at the surface oxygen atom. Therefore, this method can be used to investigate the reaction with both free and trapped holes. Although the effect of photogenerated electron is not considered in this approach, it is still quite reasonable because the photogenerated electrons are usually conducted to another electrode.1,39 The reaction between the hole and the H2O was calculated with nudged elastic band method with climbing images (CNEB).40 Siteprojected magnetic moment (SPMM) at oxygen atom, which is the integration of spin density over a sphere around the atom (the radius of the sphere is defined with the defaulted value in pseudopotential file), was calculated to track the transfer of the hole.

Figure 1. (a) Adsorption and dissociation of H2O molecule at ground state calculated with GGA and GGA+U methods, U(d) = 4.2 eV and U(p) = 5 eV are applied in the GGA+U calculation; only the structures in GGA+U calculation are shown. (b) Partial density of states for Ti5c and Ob atoms and H2O molecules at associative and dissociative adsorption state; Ev is the energy of the top of the valence band.

associative adsorption is nearly degenerate with (0.03 eV higher than) the dissociative adsorption in our GGA calculations, and the barrier of 0.20 eV also agrees well with other theoretical values.46,47 However, in GGA+U calculations, the dissociative adsorption is now 0.16 eV lower than the associative adsorption, which also agrees with a recent GGA+U work.50 The transition barrier for the dissociation also decreases by 0.12 eV. Thus, in comparison with the GGA method, GGA+U calculations seem to overestimate the dissociative adsorption energy (by 0.13 eV) but underestimate the dissociation barrier. 1028

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make it easy for the hole be trapped at this O atom. It is found that the hole trapping at the Ob-II site is more stable than that at the Ob-I site by 0.12 eV for U(p) = 5 eV (this value changes less than 0.01 eV when changing the U(p) to 4 and 6 eV). This is because the hole trapping at the Ob-I site reduces the negative charge at Ob-I and thus weakens the hydrogen bond between it and H2O molecule. This indicates that the hole has a lower probability to be trapped at Ob-I than at Ob-II. But the distance between Ob-II and H2O molecule is so large that the overlap between the HOMO of H2O and the hole orbital is negligible, and the hole cannot react directly with the molecule. The state A with the hole trapped at Ob-I will be taken as the initial state of the reaction. About the product, the OH group usually desorbs off the surface (or diffuses on it) and finally adsorbs at another Ti site. Sometimes the OH group stays at the Ti atom near the initial adsorption site. Because it is believed that the molecule is oxidized in the reaction, the hole should be transferred to the OH group. So we can consider a OH radical adsorbed at the original site as a product or at least a possible intermediate state. Even adsorbed at the original site, there are still two possible adsorption configurations as shown in Figure 2, marked as structures C and D. In the structure C, the OH group has a hydrogen bond with the H atom, while in the structure D the hydrogen bond is between the Ob in another bridge oxygen row and the H atom in the OH group. The structure D is more stable than the structure C by 0.35 eV (U(p) = 5 eV), which is higher than the energy of a typical hydrogen bond (0.2 eV). In our previous study, we found that the hole trapped at the OH group would be shared with the inplane oxygen (Oip) atom when it is adsorbed on the surface.13 The site-projected magnetic moments at Ow and Oip in structure C are 0.55, 0.37 μB and in structure D 0.74, 0.15 μB, respectively. This means that a larger part of the hole is distributed at Oip in structure C. It has been shown that Oip has a large electrostatic potential which makes it an unfavorable hole-trapping site.52 Nevertheless, we take structure C as the product, because it is quite close to its normal dissociative adsorption configuration shown in Figure 1. And we found that the barrier from the structure C to D is as small as about 0.02 eV (U(p) = 5 eV). The energy profile from structure A to C is calculated with the CNEB method as displayed in Figure 3a. We first discuss the result with U(p) = 5 eV, and then the effects of different U(p) values. Seven images were used in the calculation of the potential energy surface. The SPMMs of Ow, Oip, and Ob-I at each image are plotted in Figure 3c for U(p) = 5 eV. The energy profile indicates that there is an intermediate state (IS) between A and C. The barrier from state A to IS is 0.56 eV, which determines the overall reaction rate. From the SPMMs at the transition state (TS), we can see that a part of the hole has been driven out the bridge oxygen atom. At the state IS, the H2O molecule has dissociated, the Ow, Oip and Ob-I do not have significant SPMMs, and the hole orbital becomes completely delocalized. This suggests that the pathway from A to IS is actually a detrapping of the hole and the dissociation of the H2O molecule, while from IS to C it is the oxidation of OH group by the free hole. The reaction from A to C is endothermic by 0.18 eV, and from C to D will release 0.35 eV energy, so that the overall reaction is exothermic by 0.17 eV. But as we mentioned above, the GGA+U method would overestimate the dissociative adsorption energy by about 0.13

We found that the barrier and the released heat are not sensitive to U(p), so the differences from the GGA results are mainly caused by the U(d) correction. The partial density of states (PDOS) of the H2O molecule are presented in Figure 1b; the first peak of the PDOS of the associative adsorbed H2O molecule is well below the top of the valence band of TiO2. In the dissociative adsorption, the first peak upshifts significantly which agrees well with hybrid functional calculations.51 From the PDOS of the dissociative adsorption, the system is still a semiconductor with fully occupied valence band and empty conductance band. If we remove the H atom adsorbed on the bridge oxygen, a hole will be created on the top of the valence band. This hole should not differ too much from the hole of the bare surface because the electronic states near the top of the valence band barely hybridize with the molecular obitals of H2O. Thus, the use of an OH group to inject a hole into the system should work. The additional OH group is placed at the adjacent Ti row as far as possible from the H2O molecule to minimize the effect on the surface structure. Its effect on the adsorption and dissociation of H2O is found to be negligible. Oxidation by the Trapped Hole. The STM experiments indicate that the H2O dissociates under UV illumination and the OH group is desorbed off or diffused on the surface.14 But details about the reactant and the product are still unknown. If the reaction is initiated by the trapped hole, the first problem about the reactant is the location of the trapped hole. In our previous study, we found that the bridge oxygen (Ob) is the most favored trapping site.13 Because of its localized nature, only the hole trapped near the H2O molecule is possible to react with it. As shown in Figure 2, the hole trapped at either Ob-I (structure A) or Ob-II site (structure B) has been considered. In order to trap the hole at the specific O atom, we elongate the O−Ti bonds that connect to the O atom (upshift it by 0.5 Å); this will reduce the electrostatic potential on this O atom and upshift its PDOS near to the top of valence band, which

Figure 2. Possible initial and final states of the reaction; the hole mainly distributes at the blue oxygen atoms. In A the hole is trapped at bridge oxygen (Ob-I) near to H2O molecule; in B is at the adjacent bridge oxygen (Ob-II); in C and D, the hole is shared by the oxygen in water molecule (Ow) and an in-plane oxygen atom (Oip). 1029

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Figure 3. Oxidation of H2O molecule by trapped hole: (a) is the potential energy surfaces for different U(p); (b−d) SPMM at Ob, Ow, and Oip for different U(p) along the reaction paths; (e) the geometries of the structures labeled in (a) for U(p) = 5.0 eV; blue oxygen is where the hole mainly distributed at the initial state A from Figure 2 with the hole trapped at Ob-I, transition state (TS) with most of the hole remaining at Ob-I, intermediate state (IS) with a delocalized hole, and final state C from Figure 2 with the hole shared by Ow and Oip.

Two trends can be seen as U(p) increases: the barrier increases and the release heat decreases. We found that the barriers for H2O dissociation at the ground state for different U(p)s are almost the same, so that the increase in energy barrier for H2O oxidation is mainly due to the increase of detrapping energy of the hole with U(p). The second trend is actually about the change of the relative energy for hole trapped at Ob-I and the hole trapped at OH group with U(p). A similar trend has already been found in our previous study13 that the relative energy for hole trapped at Ob and subsurface oxygen (Osub) also increases with U(p) The results from different U(p)s all suggest that the oxidation of the H2O molecule needs to detrap the hole and dissociate the molecule first, and then the OH group gets oxidized. Why the H2O cannot be oxidized directly by the hole trapped at Ob-I can be explained by the hybridization mechanism proposed in our previous study.13 First, the occupied states of adsorbed H2O are well below the top of the valence band of TiO2, and the hole orbital is in the band gap. Therefore, the energy mismatch does not favor the formation of the bond. Second, the trapped hole has a character

eV, and the reaction would be barely exothermic after this is taken into account. To see the effect of U(p) on the reaction, we scan the U(p) from 3.0 to 7.0 eV. At U(p) = 3.0 eV, the hole cannot be trapped at the OH group, while for U(p) = 7.0 eV the OH radical undergoes deprotonation spontaneously. Thus only the results with U(p) = 4.0, 5.0, and 6.0 eV are given in Figure 3 for PESs in Figure 3a and SPMMs in Figure 3b−d, respectively. The results for the three U(p) are quite similar; they all point to the existence of the intermediate state. Still they are different in some details. For U(p) = 4.0 eV, the hole is detrapped before reaching the transition state, while for U(p) = 5.0 eV, the hole is completely detrapped at the IS, and for U(p) = 6.0 eV, although the hole is also driven out Ob-I at the IS, we cannot find an image at which the hole is completely delocalized because the large U(p) leads to too strong localization effects. In all three cases the H2O is dissociated at the IS; thus, the energy barrier includes the energy acquired for the dissociation of the molecule and the detrapping of the hole from the bridge oxygen. 1030

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Figure 4. Oxidation of H2O molecule by free hole: (a) is the potential energy surfaces for different U(p); (b−d) SPMM at Ow and Oip for U(p) = 4, 5, and 6 eV along the reaction paths; (e) the geometries of the structures labeled in (a) for U(p) = 5 eV, blue oxygen is where the hole mainly distributed: initial state A′ with a delocalized hole, transition state (TS′) with the hole remains delocalized, intermediate state (IS′) with a delocalized hole and final state C′C from Figure 2 with the hole shared by Ow and Oip.

hole via direct charge transfer mechanism, they cannot rule out the possibility that the hole may take part in the reaction with the H2O molecule at Ob before it is trapped.10 For the reaction of the free hole with the H2O molecule, the initial state is taken as the surface with a delocalized hole and an associatively adsorbed H2O molecule (Figure 4e, structure A′), and the structure C in Figure 2 is still considered as the final state. The calculated energy profiles are shown in Figure 4a. The SPMMs of Ow and Oip along the reaction path for different U(p)s are given in Figure 4b−d. The PESs for all U(p)s also indicate that there is an intermediate state (IS′) for the reaction of the free hole with the H2O molecule. At the IS′, the molecule is dissociated, while the SPMMs suggest that the hole is still delocalized. It implies that from the A′ to IS′ only the dissociation of the H2O molecule takes place. This explains why the calculated energy barriers with three U(p) values are almost the same. From the IS′ to C (or C′), it is again the oxidation of OH group by the hole. The overall barrier seems to be determined by the first step (from A′ to IS′), but as we mentioned above, the GGA+U

of p orbital of oxygen atom, and it is perpendicular to the bridge oxygen row, while the HOMO of H2O is perpendicular to the molecule plane and is nearly parallel to the bridge oxygen row. The symmetry mismatch is another hindering factor for bond formation. Moreover, there is a H atom between Ob-I and Ow, which reduces significantly the spatial overlap between orbitals. The hole cannot be transferred via the hybridization mechanism. These imply that similar adsorbed R−OH type molecules, such as CH3OH,53 probably cannot be oxidized directly by the trapped hole either. It can be concluded that the H2O molecule could not be efficiently oxidized by the trapped hole for several reasons. For instance, the hole has a lower possibility to be trapped at Ob-I than at other Ob atoms; the barrier is high for the detrapping of hole, and the reaction is barely exothermic because the trapping releases too much heat and reduces the hole’s oxidation power. Obviously, the natural question that follows is what the free hole can do. Oxidation by the Free Hole. Although Imanishi et al. suggested that the H2O molecule cannot be oxidized by valence 1031

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molecule is hindered by the mismatch between the energy and spatial symmetry of the hole orbital and the HOMO of the H2O molecule. Moreover, the reaction with the trapped hole has a high energy barrier and is barely exothermic. For the oxidation of the H2O by free hole, the transient dissociation of the H2O is essential and the hole is transferred via the in-plane oxygen nearest to the molecule. This reaction is highly exothermic with relatively low barrier. The obtained results can nicely explain several recent experimental findings, and the proposed mechanism could be a general one for the photooxidation of other R−OH type molecules.

method may underestimate the dissociation barrier and overestimate the dissociative adsorption energy. This may cause the molecule to dissociate before it is oxidized. We still cannot rule out the possibility that the molecule may be oxidized while it is dissociating as we observed for the oxidation of physisorbed H2O molecule in our previous work. But from the PDOS of associatively and dissociatively adsorbed H2O molecule in Figure 1, the dissociatively adsorbed molecule is much more favorable for the hole transfer. As U(p) increases, the trapping energy of hole at OH group increases, so that the relative energy of C relative to A′ and the barrier from IS′ to C decreases. The overall reaction (from A′ to C) is exothermic by 0.14 eV for U(p) = 5.0 eV, including the released heat from C to D, and the oxidation of H2O molecule could be exothermic by 0.49 eV. Even after reducing the error of GGA+U method 0.13 eV, the reaction will still release 0.36 energy, which could convert to the kinetic energy of OH group, resulting in its desorption or diffusion. Another place in common for all U(p) is that when the hole transfers to the OH group it is always shared by the OH group and the Oip. This is because the Oip is the nearest oxygen atom to the OH group and its Pz orbital could hybridize with the HOMO of the OH group with a larger spatial overlap. In other words, the Oip plays an important role in the hole transfer. Interestingly, the hole was proposed to be trapped at the Oip before taking part in the reaction by Imanish et al.10 But we can see that when most of the hole is localized at the Oip, the energy of the system is at or near its local maximum (electrostatic potential at the Oip is too high to accept the hole). Thus, it is unlikely that the hole could be trapped there before the reaction. We can now draw a general picture for the H2O oxidation by the free hole. At the beginning, the adsorbed molecule is in equilibrium between the associative and dissociative adsorption states because their energies are nearly degenerate and the barrier is quite low.47 At its transient dissociative adsorption state, the photogenerated hole transfers to the OH group that leads to its desorption or diffusion. The reason that the OH group is highly mobile could be that the oxidation reaction is highly exothermic and the released heat can be converted to the kinetic energy of the OH group. Another possibility is that the desorption of the OH group may follow a similar mechanism as that for the photodesorption of O2 molecule.21,54 When the hole is suddenly captured by the OH anion to neutralize it, the OH group finds itself in the repulsion part of the potential energy surface and starts to desorb. The transient dissociation of the H2O molecule is essential in the mechanism we proposed. Although the dissociative adsorption has never been observed experimentally, the transient dissociation has been found to be responsible for the phenomena observed in several STM experiments. For example, the transient dissociation of the H2O helps the H atoms to diffuse across the bridge oxygen row,47 and the H2O dissociates into two terminal OH groups when there is a terminal O adatoms present on the other Ti5c row.55 This mechanism may also be responsible for the photooxidation of other R−OH type molecules.56,57

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by NBRP (grants 2010CB923300 and 2011CB921400), NSFC (grant 20925311) of China, and Göran Gustafsson Foundation for Research in Natural Sciences and Medicine. The Swedish National Infrastructure for Computing (SNIC) is acknowledged for computer time.

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REFERENCES

(1) Fujishima, A.; Honda, K. Electrochemical Photolysis of Water at a Semiconductor Electrode. Nature 1972, 238, 37−38. (2) Jaeger, C. D.; Bard, A. J. Spin Trapping and Electron-Spin Resonance Detection of Radical Intermediates in the PhotoDecomposition of Water at TiO2 Particulate Systems. J. Phys. Chem. 1979, 83, 3146−3152. (3) Hoffmann, M. R.; Martin, S. T.; Choi, W. Y.; Bahnemann, D. W. Environmental Applications of Semiconductor Photocatalysis. Chem. Rev. 1995, 95, 69−96. (4) Duncan, D. A.; Allegretti, F.; Woodruff, D. P. Water Does Partially Dissociate on the Perfect TiO2(110) Surface: A Quantitative Structure Determination. Phys. Rev. B 2012, 86, 045411. (5) Brookes, I. M.; Muryn, C. A.; Thornton, G. Imaging Water Dissociation on TiO2(110). Phys. Rev. Lett. 2001, 87, 266103. (6) Muryn, C. A.; Hardman, P. J.; Crouch, J. J.; Raiker, G. N.; Thornton, G.; Law, D. S. L. Step and Point-Defect Effects on TiO2(100) Reactivity. Surf. Sci. 1991, 251, 747−752. (7) Henderson, M. A. The Interaction of Water with Solid Surfaces: Fundamental Aspects Revisited. Surf. Sci. Rep. 2002, 46, 1−308. (8) Marcus, R. A.; Sutin, N. Electron Transfers in Chemistry and Biology. Biochim. Biophys. Acta 1985, 811, 265−322. (9) Salvador, P. On the Nature of Photogenerated Radical Species Active in the Oxidative Degradation of Dissolved Pollutants with TiO2 Aqueous Suspensions: A Revision in the Light of the Electronic Structure of Adsorbed Water. J. Phys. Chem. C 2007, 111, 17038− 17043. (10) Imanishi, A.; Okamura, T.; Ohashi, N.; Nakamura, R.; Nakato, Y. Mechanism of Water Photooxidation Reaction at Atomically Flat TiO2 (Rutile) (110) and (100) Surfaces: Dependence on Solution pH. J. Am. Chem. Soc. 2007, 129, 11569−11578. (11) Nakamura, R.; Okamura, T.; Ohashi, N.; Imanishi, A.; Nakato, Y. Molecular Mechanisms of Photoinduced Oxygen Evolution, PL Emission, and Surface Roughening at Atomically Smooth (110) and (100) N-TiO2 (Rutile) Surfaces in Aqueous Acidic Solutions. J. Am. Chem. Soc. 2005, 127, 12975−12983. (12) Neumann, B.; Bogdanoff, P.; Tributsch, H.; Sakthivel, S.; Kisch, H. Electrochemical Mass Spectroscopic and Surface Photovoltage Studies of Catalytic Water Photooxidation by Undoped and CarbonDoped Titania. J. Phys. Chem. B 2005, 109, 16579−16586.

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CONCLUSION In summary, we have explored the oxidation of the H2O molecule adsorbed on a rutile TiO2(110) surface by both free and trapped holes. We have found that the hole has lower possibility to be trapped at the bridge oxygen near to the molecule, and the direct transfer of the trapped hole to the 1032

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The Journal of Physical Chemistry C

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(56) Panayotov, D. A.; Burrows, S. P.; Morris, J. R. Photooxidation Mechanism of Methanol on Rutile TiO2 Nanoparticles. J. Phys. Chem. C 2012, 116, 6623−6635. (57) Shen, M.; Henderson, M. A. Role of Water in Methanol Photochemistry on Rutile TiO2(110). J. Phys. Chem. C 2012, 116, 18788−18795.

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