Hydrogen-Bond Bridged Water Oxidation on {001} Surfaces of

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Hydrogen-Bond Bridged Water Oxidation on {001} Surfaces of Anatase TiO Hongna Zhang, Peng Zhou, Zuofeng Chen, Wenjing Song, Hongwei Ji, Wanhong Ma, Chuncheng Chen, and Jincai Zhao J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b11900 • Publication Date (Web): 10 Jan 2017 Downloaded from http://pubs.acs.org on January 10, 2017

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The Journal of Physical Chemistry

Hydrogen-Bond Bridged Water Oxidation on {001} Surfaces of Anatase TiO2 Hongna Zhang,a,c Peng Zhou,a,c Zuofeng Chen,*b Wenjing Song,a Hongwei Ji,a Wanhong Ma,a Chuncheng Chen,*a and Jincai Zhao*a a.

Key Laboratory of Photochemistry National Laboratory for Molecular Sciences Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190 (China)

b.

Shanghai Key Lab of Chemical Assessment and Sustainability, School of Chemical Science and Engineering, Tongji University, Shanghai 200092 (China)

c.

University of Chinese Academy of Sciences, Beijing 100049 (China)

ABSTRACT: To gain an atomic-level understanding of the relationship among the surface structure, the interfacial interaction and the water oxidation activity on TiO2, we studied the adsorption of water and its photocatalytic oxidation on anatase TiO2 with {101} and {001} exposed surfaces by in situ infrared spectroscopy, kinetic isotope effect studies and density functional theory (DFT)-based molecular dynamics calculations. Our experimental results demonstrate that the oxidation reaction occurs exclusively on hydrogen bonded water molecules (via surface hydroxyls) over {001} surface, while water molecules coordinated on the {101} surface, which are conventionally assigned to the reactive target for hole transfer, remain unchanged during the irradiation. The theoretical calculations reveal that the selective oxidation of water adsorbed on the {001} surfaces is primarily attributed to the formation of hydrogen bonds, which provides a channel to the rapid hole transfer and facilitates the O-H bond cleavage during water oxidation.

Introduction The TiO2-based photocatalytic redox reaction has received increasing attention due to its promising applications in solar energy conversion and pollutants elimination.1 Oxidation of water molecules on TiO2 by photoinduced holes represents a fundamental photocatalytic reaction because of its importance to both photocatalytic water splitting and photodegradation of pollutants.2 The understanding of the relationship between the surface structure and the photocatalytic activity of water oxidation and the process of interfacial holes transfer at atomic-level is critical to improve the photocatalytic efficiency through surface design. Anatase, the most active crystalline phase of the TiO2 photocatalyst, usually has two specific exposed surfaces: the {101} and {001} facets. Numerous experimental and theoretical studies have been performed on the adsorption of water over different TiO2 facets. It is commonly accepted that water molecules non-dissociatively adsorb on the {101} surface of TiO2 and dissociatively adsorb on the {001} surface of TiO2.3,4 However, the facet-dependent photocatalytic activities toward water oxidation are still controversial. The experimental investigation showed that TiO2 with enriched {001} facets exhibit more pronounced hydroxylation reaction than that of the benchmark Degussa P25 TiO2 (more than 5 times),5 and those with dominant {101} or {100} facets.6 In contrast, the theoretical calculations suggested that water oxidation activity is not

sensitive to the exposed facet, because both the maximum values of the valence band (VBM) and the overpotential for water oxidation are quite similar for the anatase {001} and {101} facets, suggesting that oxidation of water could be realized on both anatase {001} and {101} facets.7 The atomic-scale characterization and an understanding of the relationship between the surface structure and the photocatalytic activity of water oxidation remain a great challenge. Although atomic images for the adsorption mode of water can be obtained by electron microscope techniques on single crystal surface under vacuum conditions,3 it is difficult to gather structural and mechanical information on TiO2 powders in humid or aqueous environment, which are more relevant to practical application of photocatalysis. The recent super-resolution imaging experiments on modified TiO2 nanorods indicated that the most active sites for water oxidation are also the most important sites for reduction and for chargecarrier recombination.8 Using electron spin resonance spectroscopy, D’Arienzo et al. revealed that under vacuum conditions, the concentration of trapped holes (O‒ centers) increases with increasing {001} surface area.9 The ability of the trapped holes to oxidize water is unknown on water-adsorbed surface because the interaction between water molecules with the surface would greatly affect the band-edge potentials and the interfacial stability of the electron and hole.10,11

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Infrared (IR) spectroscopy serves as a powerful tool to distinguish the facet-dependent adsorption and photocatalytic behaviors, because the characteristic vibrations of the adsorbates are sensitive to the interaction modes,12 which in turn, are largely determined by the different atomic arrangements on the different crystal facets. Herein, by using in situ IR spectroscopy, we studied the adsorption of water molecule and its photocatalytic oxidation on anatase TiO2 with different {101} and {001} exposed surfaces. We for the first time provide the direct experimental evidence that the water oxidation reaction exclusively occurs on the {001} facet where the water molecules adsorbed on TiO2 {001} surfaces via hydrogen bond is the reactive water species. Additionally, combined with density functional theory (DFT)-based molecular dynamics calculations, we reveal that the selective oxidation of water adsorbed on the {001} surfaces is primarily attributed to the formation of hydrogen bonds between water molecules and the surface, which provides a channel to the rapid hole transfer and facilitates the O-H bond cleavage during water oxidation. Experimental details Materials. Hydrofluoric acid (40 wt%) was purchased from J&K Scientific. Titanium tetrafluoride (TiF4) was purchased from Aldrich Chemical. 2-propanol (PrOH, HPLC grade) was purchased from B&J. Ethanol was purchased from Beijing chemical work. Deuterium oxide (D, 99.9%) and oxygen 18O-labelled water (98%) were purchased from Cambridge Isotope Laboratories. All reagents were of HPLC grade and were used without further purification. Deionized water (Millipore, Milli-Q, resistivity 18.2 MΩ cm-1) was used throughout this study. Sample synthesis. Anatase TiO2 samples were synthesized by a traditional hydrothermal method.13,14 Hydrochloric acid (HCl, 1.5 M) was used to adjust the pH of deionized water (1.0 L) to 2.0. Titanium tetrafluoride was then dissolved in this solution under vigorous stirring to give a concentration of 0.04 M. For a typical experiment, 4.5 mL of the above TiF4 aqueous solution was added into the mixed solution with 42 ml of deionized water and 40 mL of 2-propanol. Then different amount of HF aqueous solutions (1.2 mL, 1.5 mL, 1.8 mL and 2.1 mL) were slowly dropped into the above mixture solution under vigorous stirring. Finally, the mixture was transferred to a Teflonlined stainless steel autoclave and kept at 453 K for 12 h in an electric oven. After reaction, the white precipitates were collected by centrifugation and washed with ethanol and deionized water for 3 times, respectively. Then the obtained precipitates were calcined at 723 K for 3 h in a tube furnace with air flow to remove fluoride ions. Characterization. The morphology of the samples was determined by field emission scanning electron microscope (FESEM) on a Hitachi S4800 with an accelerating voltage of 10 kV. The surface F- in samples was characterized by X-ray photoelectron spectroscopy (XPS), using an ESCA laboratory 220i-XL spectrometer with an Al KR (1486.6 eV) X-ray source and a charge neutralizer. All the

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spectra were calibrated to the C 1s peak at 284.8 eV. X-ray diffraction (XRD) measurements were performed on a Regaku D/Max-2500 diffractometer with the Cu KR radiation (1.5406 Å). Diffraction patterns were recorded from 10 to 80° 2θ with a step size of 0.04° at 4° min-1. DRIFTS measurements. A Thermo Nicolet 6700 FT-IR spectrometer equipped with a mercury cadmium telluride (MCT) detector was used for the diffuse reflectance FTIR measurements (DRIFTS). A praying mantis diffuse reflectance accessory and a reaction cell equipped with a heater (Harrick Scientific) formed the reaction system reported in our previous work.15 20 mg of TiO2 samples were housed at a sample cup inside the reaction cell. A cover dome contained three windows: two were made of ZnSe to permit the entry and exit of detection infrared beam and the third (quartz) was for the transmission of UVlight beam during in situ reactions. Before IR measurement, the samples were heated at 723 K for 0.5 h under O2 flow (50 mL min-1) to clear surface and heal the Ovacancies possibly existing on the surface and subsequently cooled to 298 K under O2 flow. After the cooling process, the samples were maintained under Ar flow (50 mL min-1) for 0.5 h to completely replace the adsorbed O2 and then subjected to IR measurements and photocatalytic reactions. The above process was called the dehydrated process in the following discussion. After that, all of the dehydrated samples were flushed by keeping the online water-saturated Ar flow (15mL min-1) for 30 min at 298 K and then maintained at 423 K under a flowing dry Ar flow (15 mL min-1) for 30 min. Finally, the samples were cooled to 298 K and simultaneously sealed the cell under 1atm Ar atmosphere at 298 K for 30 min. This process was called the pretreatment process. After such pretreatment, UV irradiation was performed with a 365 nm LED (5 W). IR spectra ranging from 4000 to 1000 cm-1 were recorded by averaging 32 scans with a resolution of 4 cm-1. KubelkaMunk (K-M) units which were used specifically for reflectance spectra were used to measure the spectral intensity. Mass spectrometry detection. The product of the photoreaction for the H218O-adsorbed H33 and H48 was detected by mass spectrometry. In this experiment, 60 mg the H33 and H48 were adsorbed H218O by dispersing in 150μL H218O respectively. After 2 hrs of irradiation by LED (W) under argon atomosphere, the formation of 18O2 (m/z=36) was detected. Theoretical calculations. Anatase TiO2 {101} and {001} facets were investigated by the density functional theory (DFT) calculations based on the Vienna Ab-initio Simulation Package (VASP).16-19 The Perdew-Burke-Ernzerhof (PBE) of the generalized gradient approximation (GGA) was used as the exchange-correlation function.20,21 The interaction between valence electrons and the ionic core was described by the PAW pseudo-potential. The interaction between adjacent atom slabs was eliminated by the vacuum slab with thickness of 20 Å. 2 × 2 supercell and 2 × 1 supercell with eight O-Ti-O layers were used to simulate TiO2 {001} and {101} surfaces, respectively. A layer of

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water molecules is located on TiO2 {001} and {101} surfaces. The cutoff energy of 450 eV and the Monkhorst-Pack k-point mesh of 4 × 4 × 1 are adopted to conduct the geometry optimization calculations. The energy convergence was set to 2.0 × 10-5 eV. After geometry optimization calculations, the obtained structures were used to the molecular dynamics calculation (Figure S1). One k-point at Γ was used for sampling the Brillouin zone. The molecular dynamics calculation was performed at 300 K, similar to the common experiment condition. The time step was set to 1 fs and 5000 steps was required. The IR adsorption spectrum of model is obtained from the trajectory from molecular dynamics calculations via nMOLDYN Packages.10 The projected density of states (PDOS) of H2Oadsorbed TiO2 {101} and {001} surfaces were also calculated based on the structure obtained from molecular dynamics calculation. Further, the more O-Ti-O layers, the higher cutoff energy and the wider Monkhorst-Pack kpoint mesh were test, respectively. The obtained results showed little difference, indicating high accuracy of present calculations.

results show that the surface fluorine on TiO2 was completely removed after the calcination treatment (Figure S3). Water adsorption on TiO2 {001} and {101} facets: hydrogen bonded vs. coordinated. The Fourier transform infrared (FTIR) spectra of adsorbed water were recorded by carefully controlling the water content on TiO2. Because the interfacial hole transfer is expected to occur from TiO2 surface to the directly-adsorbed water molecules. To prevent the outer layers of water from shielding the IR information of directly-adsorbed water molecules, the IR measurement was performed on the samples after being treated at 423 K under an Ar atmosphere to remove the outer layers water molecule. After such a treatment, the water adsorbed on TiO2 should be less than two layers. This argument is supported by the presence of isolated hydroxyl groups with vibration of 3740 cm-1 (Figure S7 below) since the wavenumber of IR absorption for the isolated hydroxyl was reported to be larger than 3700 cm-1 when less than two layers of water molecules adsorbed on TiO2.12,22

The adsorption energy (Eads) of the H2O molecule on the TiO2 surface was calculated by the following formula: Eads = (Eslab + n * Ewater - Etotal)/n Where Eslab is the free energy of the TiO2 surface, n is the number of adsorbed H2O molecules on the TiO2 surface, Ewater is the free energy of one water molecule under vacuum and Etotal is the total free energy of the H2O-adsorbed TiO2 surface. Results and discussion Structure and morphology.

Figure 2. (a) IR spectra of samples H33, H48, H79 and H90 in the δ(H2O) region. The background was collected on corresponding dehydrated samples. (b) Dependence of the relative peak intensity at 1647 cm-1 on the ratio of the {001} surface. (c) Dependence of the relative peak intensity over sample H48 on the dehydration temperature.

Figure 1. FESEM images of H33 (a), H48 (b), H79 (c) and H90 (d). According to the XRD patterns in Figure S2, nanocrystals with different exposed {101} and {001} facets are pure anatase TiO2. As shown in Figure 1, all TiO2 samples are truncated octahedral bipyramids with eight {101} facets on the sides and two {001} facets on the top and bottom. The uniform morphologies of the nanocrystals facilitate the estimation of the percentage of {001} facets in each sample, which is 33%, 48%, 79% and 90% on average, and marked as H33, H48, H79 and H90, respectively. The XPS

Figure 2a shows the IR spectra of different samples in the bending vibration δ(H2O) region (the full spectra are provided in Figure S4). The absorption band in this region is evidently asymmetric, with the dominant peak at 1615 cm-1, and a shoulder peak at a higher wavenumber of 1647 cm-1, which implies more than one IR adsorption modes present in all these samples with varying ratios of {101} and {001} facet exposure. In addition, the deconvolved relative intensity of the shoulder peak at 1647 cm-1 increased with the increase in the ratio of the {001} facet (Figures 2b). Actually, at the dehydration temperatures from 323 K to 473 K, the correlation between the peak intensity and the ratio of {001} surface was also observed (Figures S5 and S6a). Such correlation leads to the hypothesis that the two peaks at 1647 and 1615 cm-1 arise

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from the water molecules adsorbed on the TiO2 {001} and {101} surfaces, respectively. Figures 2c and S6b show that the intensity of the peak at 1647 cm-1 decreases more rapidly than that at 1615 cm-1 with the increase of temperature for all samples, suggesting that water molecules on TiO2 {001} surface are more readily to desorb than those on TiO2 {101} surface. Moreover, the peak at 3740 cm-1, which corresponds to the terminal hydroxyls,22,23 became more and more pronounced from samples H33 to H90 (Figure S7). The increase in terminal hydroxyls with {001} surface originates from the dissociation of the first-layer adsorbed water molecules on the {001} facet, which would lead to the formation of terminal hydroxyl groups.22,23 On basis of these observations, we assign the IR peak at 1647 cm-1 to the δ(H2O) absorption of water molecules adsorbed on terminal hydroxyls via hydrogen bonds on TiO2 {001}, which is easy to desorb because of the weakness of hydrogen bond. On the other hand, the peak at 1615 cm-1 can be attributed to the water molecules adsorbed directly on Ti atom on {101} surface. These assignments are further verified by theoretical calculation (Figure S12a). Water oxidation determined by adsorption mode. To test the facet-dependent activity toward photocatalytic water oxidation on TiO2, the samples (after pretreatment at 423 K) were irradiated at 298 K under an Ar atmosphere, and IR spectra were in situ recorded by using the samples before irradiation as a background. A monotonic IR absorption band ranging from 3000 to 1000 cm-1, characteristics of the absorption of the accumulated conduction-band electrons (ecb-),24 was observed for all the samples during UV irradiation (Figures 3a and S8). Concomitantly, two negative-going peaks for ν(OH) (3700 to 3000 cm-1) and δ(H2O) (1647 cm-1) of the water molecules appeared, indicating the loss of a considerable amount of surface-bound water molecules. Moreover, the peak intensity of the terminal-hydroxls ν(OH) (3740 cm-1) was enhanced. It is expected that the proton released from water oxidation would react with the surface oxygen sites of TiO2 to form surface hydroxyl groups, which should have the identical IR absorption to those from dissociative adsorption of water. As a result, the amount of the surface hydroxyl groups and consequently the intensity of hydroxyl absorption at 3740 cm-1 would increase during the water oxidation. In addition, both the increase in the peak intensity at 3740 cm-1 and the decrease at 1647 cm-1 show excellent linear relationships with the increase in the absorbance at 2000 cm-1 (I2000) (Figure 3b), indicating that the loss of water molecules and the production of terminal hydroxyl groups are directly related to the accumulation of ecb-. A pronouced deuterium kinetic isotope effect (KIE) (kH2O/kD2O = 3.9) for the decrease of water molecules was observed (Figures 4a and S9), indicating that the breakage of the O-H bond in water molecules during the photoreaction is the rate-determining step for the loss of water.25 This also excludes the possibilities that the photoinduced electron accumulation in TiO2 is caused by

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the residual organic hole scavengers or by the oxidation of the lattice oxygen of TiO2,25,26 which would no lead to a obvious KIE value.

Figure 3. (a) The time-dependent IR spectra of H48 upon UV irradiation under an Ar atmosphere. (b) The area of the negative peak at 1647 cm-1 (δ(H2O)) (left axis) and the area of the peak at ~3740 cm-1 (ν(OH)) (right axis) as a function of the absorbance at 2000 cm-1 (I2000). Oxidation of water to molecular oxygen was verified by detcting 36O2 on mass spectroscopy during H218O-over H33 and H48 (Figure S10). H48 gave higher amount of molecular oxygen, consistent with the amount of accumulated electrons at the equilibrium state and the area of peak at 1647 cm-1 (Figures 4b and S8).

Figure 4. (a) Kinetics of the integrated δ(H2O), δ(HDO) and δ(D2O) bands during the photocatalytic oxidation of H2O/D2O (1:1, v/v) mixture on H48. (b) Influence of irradiation time on the absorption peak intensity at 2000 cm-1. Inset: The absorption peak intensity at 2000 cm-1 for different samples after the photoreaction reaches equilibrium. The peak intensity at 2000 cm-1 has been normalized by specific surface area of TiO2 particles. It is notable that only the absorption at 1647 cm-1 decreased during irradiation, whereas the one at 1615 cm-1, the dominant peak for the water molecules adsorbed on all the samples (Figure 2a), remained unchanged. By contrast, the direct thermal desorption of water on different facets led to a full decrease in the absorption (including that at 1615 cm-1) of the water molecules, no increase in the ν(OH) (Figure S11). These different behaviors suggest that the photoinduced decrease in the absorption of adsorbed water is not a simple thermal desorption-like process. In addition, the increase in the absorption of the terminal hydroxyl excludes the possibility that the photooxidation of terminal hydroxyl is responsible for the electron accumulation. Hence, the photoinduced decrease in the IR absorption at at 1647 cm1 is caused by oxidation of water adsorbed on TiO2 {001} surface by H-bond, which has characteristic bending vibrations at 1647 cm-1. As noted above, this process is

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Figure 5. (a) Adsorption modes of H2O molecules on TiO2 {101} and {001} surfaces and the spin-density distribution in the presence of one extra hole. (b) The corresponding plots of projected density of state (PDOS) of different potential hole trapping sites on TiO2 {101} and {001} surfaces. (c) The relative energetic levels of the bulk layer, surface layer and adsorbed water molecules around the valence band top on TiO2 {101} and {001}. accompanied with production of terminated hydroxyls and accumulation of conduction-band electron. With increasing UV irradiation time, the absorption intensity of the accumulated ecb- gradually leveled off and reached equilibrium after 3 min irradiation (Figure 4b) because of the accelerated electron-hole recombination with the increasing amount of ecb-. Additionally, the amount of accumulated electrons at the equilibrium state increases with the increase in {001} facet at the low ratio of the {001} surface, but undergoes decrease with further increase of {001} ratio, which leads to the highest water oxidation activity being obtained on H48 (inset of Figure 4b). In the earlier studies, an optimum facet ratio has also been observed in the facet-dependent CO2 reduction27 and the photodegradation of methyl orange.28 In nanocrystals with two exposed facets, the spatial separation of electrons and holes on different facets can be enhanced by the preferential ﬂow of photogenerated carriers to different surfaces.27,29 As suggested by our electronic structures calculations in Figure S12b, the photoinduced electrons and holes tend to migrate to {101} and {001} surface respectively. With the increase of {001} facet, the oxidation of water molecule on {001} facet was increased, which leads to more electron accumulation at {101} facet. The accumulation of electron would in turn accelerate the charge recombination and inhibit the general photocatalytic efficiency. These two opposite effects can be responsible for the presence of the optimum facet ratio. Theoretical calculations. To consolidate these experimental observations, we simulated the models of water adsorption and the distribution of hole on TiO2 {101} and {001} surfaces using a DFT-based molecular dynamics method. As shown in Figure 5a, all of the water molecules are nondissociatively adsorbed onto the TiO2 {101} surface by forming a Ti-O bond with the surface Ti5c

atoms. By contrast, on {001} surface, after the adsorption of water molecules occurred, the first layer of water was dissociated into terminal hydroxyl groups, which leads to the protonation of half of O2c to the terminal hydroxyl (Ti–OH). These results are consistent with the earlier theoretical calculations5 and supported by our experimental observation that more terminal hydroxyl groups exist on the TiO2 {001} surface than on {101} (Figure S7). The only possible adsorption mode for the molecular water on the {001} surface is through the formation of a hydrogen bond with the formed terminal hydroxyl groups (Figures 5 and S1). The calculated adsorption energy for the H2O molecule on the TiO2 {101} surface (0.89 eV) is much larger than that on the hydroxylated TiO2 {001} surface (0.29 eV), which is consistent with the experimental observation that the water molecules on the TiO2 {001} surface can be removed more easily than those on the TiO2 {101} surface (Figures 2c and S6). It is notable that the calculated bending vibration frequency was 1639 cm-1 for the water molecules adsorbed via hydrogen bonding on the {001} surfaces and 1593 cm-1 for those coordinated by the Ti-O bond on the {101} surfaces (Figure S12a), which are in good agreement with the experimental values of 1647 cm-1 and 1615 cm-1 (Figures 2a), respectively. The calculated PDOS plots show that on the TiO2 {101} facet, the 2p states of the O atom in the water molecule adsorbed on the TiO2 {101} surface by a Ti-O bond (OH2O) are represented lower than the top of the valence band, which is by the 2p states of O atoms in bulk (Obulk). This finding suggests that the hole transfer from the TiO2 {101} surface to the adsorbed water molecules is thermodynamically unfavorable. On the contrary, the 2p states of the O atom in the H2O molecules adsorbed via hydrogen bonding on the TiO2 {001} surface are located above the top of the valence band (Figure 5b). The spin

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distribution analysis of the slabs with an extra hole indicates that spin densities are dominantly localized on the oxygen atoms of the water molecules adsorbed on the {001} surface by hydrogen bonds (Figure 5a). In contrast, on the {101} surface, the spin densities are primarily distributed over the surface oxygen atoms of TiO2, and much fewer holes are on the Ti-coordinated water. The calculation also indicates that on the {001} surface the 2p state of an O atom of the terminal hydroxyl has a similar energy level to that of the top of the valence band (Figure 5b), and there is a large hole distribution over these hydroxyl groups. These results imply that the surface hydroxyl groups can capture the photogenerated hole from the valence band and transfer the hole to the Hbonded adsorbed water molecule. Therefore, the terminal hydroxyl can act as an electron transfer bridge and can provide a channel for hole transfer from the valence band to water molecules (Figure 5c). The water molecules by acting as H-bond donors would increase the electron density on the oxygen atoms of water, and are more easily oxidized.15 In addition, the H-bond adsorption would facilitate the O-H bond cleavage during the hole transfer to the water molecules through a proton coupled electron transfer pathway, which would greatly accelerate the water oxidation process from a kinetics perspective. For the {101} surface, it is known that the second-layer water molecules would adsorb onto the first-layer water molecules or onto the two-fold coordinated O2c via H-bonds. The electronic states of these H-bonded water molecules might be located above the top of the valence band. However, both the PDOS and the hole distribution indicates that the two-fold coordinated O2c are not a good hole trapping sites. Instead, the holes are most likely to be trapped on the three-fold coordinated O3c. The long distance between the hole-trapping sites (O3c) and the second layer of water molecules adsorbed via hydrogen bonds would hinder the hole transfer to the water, even if this transfer is thermodynamically possible. At the valence band top, the PDOS of the surface layer of the {101} surface is slightly lower than that of the bulk layer (Figure S12b), indicating that the hole transfer to the {101} surface is energetically unfavorable. However, the lower edge of PDOS on the {001} surface around the conduction band bottom is higher than that of the bulk layer, and the electron trapping on this surface is avoided, which is consistent with the earlier theoretical calculations.11,27 This surface band edge arrangement avoids the occurrence of photoinduced holes and trapped electrons on the same surface, which would enhance the charge separation spatially and hinder their recombination. As a result, the particles with a appropriate combination of the {101} and {001} surfaces can oxidize water, even in the absence of any extra electron scavengers. On the contrary, water oxidation cannot occur on TiO2 particles without specific facet exposure.15 It is notable that the surface protonation/deprotonation of TiO2 was found to greatly influence the surface atomic

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configuration of different TiO2 facets, and consequently the relative redox preference of the facets.30 In addition, the hydrogen-bond structure and the adsorption mode of the water are also expected to be largely determined by the surface protonation/deprotonation. Therefore, surface protonation/deprotonation or the pH in aqueous solution would have greatly influence on the activity for water oxidation. For example, the above results indicate that the hydrogen bond between the surface hydroxyl and the adsorbed water molecule plays an important role in the water oxidation. A surface deprotonation under alkaline conditions might enhance the hydrogen-bond interaction between surface and water, which would accelerate the water oxidation. Conclusions In conclusion, the facet-dependent water adsorption on TiO2 and the selective photocatalytic water oxidation on {001} surface have been investigated by in situ FTIR spectroscopy, kinetic isotope effect studies and DFT-based molecular dynamics calculations. On {101} surface, water molecules adsorb onto surface Ti sites by forming Ti-O bonds, while on the {001} surface, the first layer of water dissociatively adsorbs, and the second layer adsorbs on the first layer via hydrogen bonds. In addition, the second layer water molecules adsorbed on the TiO2 {001} surfaces via hydrogen bonds are exclusively oxidized. The experimental and theoretical results suggest that the hydrogen bonds provide a direct hole-transfer channel from the TiO2 {001} surface to the water molecule, which accounts for the photocatalytic water oxidation. This study provides new insights into water oxidation on TiO2. It also deepens our understanding of the structureactivity relationship of photocatalytic reactions.

ASSOCIATED CONTENT Supporting Information. Characterization of the samples; the additional IR data of adsorption and oxidation of water on TiO2; mass spectra of products from photocatalytic oxida-

tion of water and the additional data of theoretical calculation. This material is available free of charge via the Internet at http://pubs.acs.org

AUTHOR INFORMATION Corresponding Authors [email protected] [email protected] [email protected] Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT The authors are grateful for the financial supports of this work from 973 Project (2013CB632405) and NSFC (Nos. 21525729, 21590811, 21377134, 21521062) and the “Strategic Priority Research Program” of CAS (No. XDA09030200).

REFERENCES 6


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