Surface Reconstruction-Induced Site-Specific Charge Separation and

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Surface Reconstruction-Induced Site-Specific Charge Separation and Photocatalytic Reaction on Anatase TiO(001) Surface 2

Feng Xiong, lili yin, Zhengming Wang, Yuekang Jin, Guanghui Sun, Xueqing Gong, and Weixin Huang J. Phys. Chem. C, Just Accepted Manuscript • Publication Date (Web): 27 Apr 2017 Downloaded from http://pubs.acs.org on May 2, 2017

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The Journal of Physical Chemistry

Surface Reconstruction-Induced Site-Specific Charge Separation and Photocatalytic Reaction on Anatase TiO2(001) Surface Feng Xiong,1 Li-Li Yin,2 Zhengming Wang,1 Yuekang Jin,1 Guanghui Sun,1 Xue-Qing Gong2 and Weixin Huang1* 1 Hefei National Laboratory for Physical Sciences at the Microscale, CAS Key Laboratory of Materials for Energy Conversion and Department of Chemical Physics, University of Science and Technology of China, Hefei 230026, P. R. China. 2 Key Laboratory for Advanced Materials, Centre for Computational Chemistry and Research Institute of Industrial Catalysis, East China University of Science and Technology, Meilong Road 130, Shanghai 200237, P. R. China. *Corresponding authors: E-mail: [email protected] (WH)

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ABSTRACT：：Photocatalytic reaction of methanol on an anatase TiO2(001)-(1×4) reconstructed surface, a prototype reaction for photocatalysis, was studied by means of x-ray photoelectron spectroscopy, thermal desorption spectrum and density functional theory calculations. Photocatalytic oxidation reaction was observed to exclusively occur at the Ti4C sites of the (1×4) added row but not at the Ti5C sites of the (1×1) basal surface. The accompanying density functional theory calculation results demonstrate that the valence band maximum is localized at the oxygen atoms of (1×4) added row and the methoxy species bonded to the Ti4C sites respectively for the clean and methanol-covered anatase TiO2(001)-(1×4) surfaces. This leads to a Ti4C site-specific oxidation of the methoxy species by photogenerated holes. These results reveal a concept of surface reconstruction-induced site-specific charge separation and photocatalytic reaction on oxide photocatalysts that will greatly deepens the understanding of the vital role of oxide surface structure in photocatalytic reactions.


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1. INTRODUCTION Driven by the sustainable solar energy, photocatalysis is emerging as an attractive clean technology to produce chemical fuels1 and fine chemicals,2 but the efficiency must be significantly increased before it comes into application. Photocatalysis is very complex and involves photo-absorption and charge generation, charge separation and migration to surfaces, and charge-participated surface reactions, thus photocatalytic activity of semiconductor photocatalysts is related to many different issues such as direct/indirect band gaps, bulk/surface defects, and surface adsorbate structures.3-5 Great efforts have been therefore devoted to increasing the charge separation efficiency of semiconductor photocatalysts during photocatalytic reactions, and the optimization of charge transfer processes at semiconductor surfaces by manipulation of the energy bands is a theme of broad generality and current applicability.6,7 Band bending in semiconductors has been demonstrated as an effective approach, but it is affected by a variety of structural factors including metal/semiconductor contact, field-effect, surface or subsurface-state (oxygen vacancies) and adsorbates.8-10 For example, stoichiometric TiO2 single crystal surfaces were demonstrated with flat bands,11 but TiO2 is actually non-stoichiometric in which TiO2 surface inevitably contains certain amounts of defects, generally in the form of surface oxygen vacancies (Ov), and thus TiO2 exhibits the characteristic of an n-type semiconductor.12,13 It was reported that a charge accumulation layer is developed in the near-surface region of TiO2 with electrons in the oxygen vacancies acting as donor-like states, leading to a downward surface band bending.14 The direction of band bending is also strongly affected by the adsorption of molecules on semiconductor surfaces with the occurrence of charge transfer between surfaces and adsorbates. An upward band bending generally occurs with adsorbates acting as electron-acceptors while a downward band bending


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generally occurs with adsorbates acting as electron-donors.15-17 Therefore, surface structures of semiconductor photocatalysts that play the decisive role in the surface-adsorbate interaction and band bending can be expected to exert great influences on the photocatalytic performance, but the relevant fundamental understanding is not well-established due to the structural complexity of powder photocatalysts. Surface reconstruction, an often-observed phenomenon of oxides, alters the atomic arrangement of the top-most layer and should strongly affect the surfaceadsorbate interaction and band bending, but its effect on photocatalytic reactions over oxide surfaces has been seldom unambiguously identified. The use of single crystal model catalysts with well-defined surface structures is an effective approach to fundamentally study surface structure-catalytic property relations. The methanol/TiO2 system is a representative system for fundamental investigations of photocatalysis on oxide surfaces. Photocatalytic reactions of methanol on various TiO2 single crystals have been studied. Early works mainly focus on rutile TiO2(110)-(1×1) surface, in which the methoxy species (CH3O(a)) formed by methanol dissociation at surface oxygen vacancy sites was proposed to be photoactive but the molecularly-adsorbed methanol species (CH3OH(a)) is not.1820

However, recent results demonstrated both CH3O(a) and CH3OH(a) species on rutile

TiO2(110)-(1×1) surface are photoactive21-23 and complex cross-coupling reactions initiated by the photocatalytic reactions of methanol occur to produce methyl formate.23-25 Similar photocatalytic reactions were also observed for methanol on other TiO2 single crystals including rutile TiO2(011)-(2×1) surface and anatase TiO2(101) surface with different photocatalytic efficiencies.26-28 TiO2 single crystal surfaces are known to exhibit rich surface reconstruction phenomena. Wilson and Idriss previously observed that rutile TiO2(001)-{011} faceted surface was far more active in photoreaction of acetic acid than rutile TiO2(001)-{114} faceted surface


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and proposed surface reconstruction-induced collective changes of the electric field to be responsible for the large difference in reactivity.29 Wang et al. studied photocatalytic reactions of CH3OH on rutile TiO2(011)-(2×1) reconstructed surface and mainly observed similar behaviors to those on rutile TiO2(110)-(1×1) surface.26 The anatase TiO2(001) facet is one of the dominant facets exposed on anatase TiO2 polymorph.30 The anatase TiO2(001) single crystal surface exhibits a TiO2(001)-(1×4) reconstructed surface that can be described by an added row model in which the (1×4) added row and (1×1) basal surface respectively expose fourfold-coordinated (Ti4c) and fivefold-coordinated (Ti5c) Ti4+ sites (Scheme 1).31,32 Our recent results demonstrate that the Ti4C sites is very reactive to dissociate CH3OH molecules to form CH3O(a) and even minor CHx and C species while the Ti5C sites molecularly adsorb CH3OH(a).33 Herein, we report a combined x-ray photoelectron spectroscopy (XPS), thermal desorption spectrum (TDS) and density functional theory (DFT) calculation study of photocatalytic reactions of methanol on an anatase TiO2(001)-(1×4) reconstructed surface, in which a surface reconstruction-induced Ti4c sites-specific transport of photogenerated holes in anatase TiO2(001) and the subsequent Ti4c sites-specific oxidation reaction are demonstrated for the first time (Scheme 1). 2. EXPERIMENTAL SECTION AND CALCULATION DETAILS All experiments were performed in a Leybold stainless-steel ultrahigh vacuum (UHV) chamber with a base pressure of 1.2×10−10 Torr. The UHV chamber was equipped with facilities for X-ray photo-electron spectroscopy with a XR 50 X-ray source (SPECS GmbH) and a PHBIOS 100 MCD hemispherical energy analyzer (SPECS GmbH, calibrated with a clean Au(110) surface), ultraviolet photoelectron spectroscopy, low energy electron diffraction, ion


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scattering spectroscopy (ISS) and differential-pumped thermal desorption spectroscopy. An anatase TiO2(001) single crystal (5 mm×5 mm×0.5 mm) purchased from MaTeck was mounted onto a Ta support plate (1 mm thick and of the same dimensions as the crystal) with mixed high temperature alumina-based inorganic adhesive (Aremco 503) and graphite powder (99.9995%, Alfa Aesar China Co., Ltd.). The Ta support was cooled and resistively heated by two Ta wires spot-welded to its back side. The sample temperature could be controlled between 100 and 1273 K and was measured by a chromel-alumel thermocouple spot-welded to the backside of the sample. Prior to experiments, the anatase TiO2(001) sample was cleaned by repeated cycles of Ar ion sputtering at 0.6 kV for 10 min, annealing in oxygen for 10 min at 800 K and annealing at 900 K for 10 min until LEED gave a sharp (1×4) reconstructed diffraction pattern and no contaminants could be detected by XPS. Methanol (99.8%, Sinopharm Chemical) was purified by repeated freeze−pump−thaw cycles. Formaldehyde was generated via thermal decomposition of paraformaldehyde (95%, Sinopharm Chemical) in a glass tube connected to the UHV apparatus. Prior to experiments, paraformaldehyde was thoroughly degassed by overnight pumping at 60 °C. The purity of all reactants was checked by QMS prior to experiments. A line-of-sight stainless steel doser (diameter: 8 mm) positioned ∼ 2 cm in front of the anatase TiO2(001)-(1×4) surface was used for the exposures to keep the chamber pressure below 5×10−10 Torr. The doser could be retracted 50 mm after the exposure. All exposures were reported in Langmuir (1 L=1.0×10−6 Torr⋅s) without corrections for the gauge sensitivity. During TDS measurements, the sample was positioned ∼1 mm away from a collecting tube of a differential-pumped QMS, and heated from exposure temperatures to 900 K at a heating rate was 2 K/s. XPS spectra were recorded at exposure temperatures using Mg Kα radiation (hν = 1253.6 eV) with a pass energy of 20 eV.


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The UV light irradiation was accomplished using a 100 W high-pressure Hg arc lamp (Oriel 6281), which provides a pressure-broadened emission spectrum from gaseous Hg in the UV-light region. When the light wavelength is below 250 nm, the light irradiance of this source decreases rapidly and is only 0.05 mW/m2 for the 200 nm light at a distance of 0.5 m. The absorption of methanol in the UV region below 200 nm can thus be neglected under our experimental condition. A water filter was used to remove the IR portion of the emission spectrum. The UVlight was focused onto the tip of a single strand, 0.6 mm diameter fused silica fiber optic cable that directed the light through a UHV-compatible feedthrough onto the anatase TiO2(001)-(1×4) face without exposure to extraneous surfaces. Exposure of the anatase TiO2(001)-(1×4) crystal at 115 K to the UV-light resulted in the rising of crystal temperature no more than 3 K. All DFT calculations were carried out with the GGA-PBE functional34 using the Vienna ab initio simulation package (VASP).35,36 The valence electronic states of Ti (3s, 3p, 3d, 4s) and C, O (2s, 2p) were described by plane-wave basis sets with a cut-off energy of 400 eV, and the core-valence interactions were treated with the projector-augmented wave method (PAW).37 The Brillouin zone was sampled with 1×1×1 Monkhorst-Pack grid. On-site Coulomb interaction correction38 with U = 4.2 eV to describe the electronic properties of Ti 3d states and long-range dispersion interactions (DFT-D)39,40 were also taken into account. The anatase TiO2(001)-(1×4) surface was modeled as a periodic slab involving six O-Ti-O trilayers of oxide with one added TiO2 row on each side of the slab,41 and the adsorptions were also calculated at both sides of the slab in a symmetric way. A vacuum layer (>15 Å) was employed to avoid interactions between the repeated slabs. In the calculation of the reconstructed TiO2(001)-(1×4) surface slab, all the atoms were allowed to relax until atomic forces reached below 0.02 eV/Å. 3. RESULTS AND DISCUSSION


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The clean anatase TiO2(001) single crystal shows a sharp (1×4) reconstructed low-energy electron diffraction (LEED) pattern (Figure S1), demonstrating the formation of anatase TiO2(001)-(1×4) reconstructed surface31,32 that can be described by an added row model in which the (1×4) added row and (1×1) basal surface respectively expose fourfold-coordinated (Ti4c) and fivefold-coordinated (Ti5c) Ti4+ sites (Scheme 1). Our previous results demonstrate that CH3OH molecules preferentially dissociate at Ti4C sites to form methoxy species (CH3O(a)) and even minor CHx and C species while molecularly adsorb at Ti5C sites (CH3OH(a)).33 A recent firstprinciples calculations study also demonstrated that methanol could dissociate facilely at surface oxygen vacancy sites, i.e. Ti4C sites, on anatase TiO2(101) surface.42 Following previous recipes,33 the anatase TiO2(001)-(1×4) surface covered with 1.0, 0.35, 0.28 and 0.11 ML adsorbed methanol could be reproducibly by flashing a surface covered with physisorbed CH3OH at 115 K to 170, 230, 250, and 320 K, respectively. Herein 1 ML refers to the total number of surface Ti and O sites on the anatase TiO2(001)-(1×4) surface (3.3×1015 sites·cm−2). Saturating CH3OH molecules adsorbed at the Ti and O sites of the anatase TiO2(001)-(1×4) surface was assumed to be 1 ML whose TDS spectrum was shown in Figure S2. Figure 1 compares TDS spectra of anatase TiO2(001)-(1×4) surfaces covered with 0.11 and 0.35 ML CH3OH without and with UV light illumination at 115 K. Agreeing with our previous results,33 CH3OH desorption peaks arising from the recombinative desorption of CH3O(a) at 370 K and from CH3OH(a) at 280 K (only for 0.35 ML CH3OH), simultaneously desorption peaks of HCHO and CH3OH resulted from the disproportionation reaction between CH3O(a) appears at 780 K, and dimethyl ether (DME) desorption peak resulting from the dehydration coupling of CH3O(a) appears at 710 K were observed from the surfaces without UV light illumination. After a 20 min UV light illumination, the methanol desorption traces at low temperatures seldom


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changes while those of HCHO, CH3OH and DME resulted from the reactions of CH3O(a) at high temperatures attenuate; meanwhile, a new HCHO desorption peak appears at 315 K. Enhanced water desorption was also observed (Figure S3), and other products were not detected. These TDS results demonstrate the photocatalytic oxidation of methanol into HCHO(a) and hydroxyl groups on anatase TiO2(001)-(1×4) surface at 115 K. Such photocatalytic reactions were well established for methanol on other TiO2 single crystal surfaces.23-28 In addition to desorption, HCHO(a) also undergoes photocatalytic cross-coupling reaction with CH3O(a) to produce methyl formate on other TiO2 single crystal surfaces.23-28,43 However, only molecular desorption of HCHO(a) occurs upon heating for anatase TiO2(001)-(1×4) surface. Thus the reaction behavior of co-adsorbed HCHO(a) and CH3O(a) on anatase TiO2(001)-(1×4) surface differs from those on previous-reported other TiO2 single crystal surfaces, demonstrating the vital role of surface structure in the reactivity of adsorbates. Figure 2 shows C 1s XPS spectra of CH3OH-covered anatase TiO2(001)-(1×4) surfaces with UV light illumination at 115 K for different times. All C 1s XPS spectra were peak-fitted with the XPSPEAK software (Version 4.1) employing a line shape of 30% Gaussian and 70% Lorentzian and a Shirley-type background. The peak position and full-width at half-maximum (FWHM) for the same species were fixed during the peak-fitting process. On the surface covered with 0.11 ML CH3OH (Figure 2A and 2D), CH3O(a) with C 1s binding energy at 286.9 eV

33

dominates, and minor CHx and carbon species with C 1s binding energy respectively at 285.7 and 284.8 eV

33

also appear. Increasing the CH3OH coverage to 0.35 ML results the growth of

CH3O(a) component (Figure 2B and 2E), the decrease of CHx component, and the appearance of strong CH3OH(a) component. These observations demonstrate that CH3OH dissociates at the Ti4c sites at the coverage of 0.11 ML and both dissociates at the Ti4c sites and molecularly


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adsorbs at the Ti5c sites at the coverage of 0.35 ML, in consistence with our previous results.33 The C 1s XPS spectra change obviously upon UV light illuminations. After an 1 min UV light illumination on the surface covered with 0.11 ML CH3OH, the CH3O(a) component obviously weakens while two new components with C 1s binding energy at 288.4 and 290.4 eV emerge; meanwhile, the carbon component slightly grows at the expense of the CHx component. On the basis of previous results,23,43 the components with C 1s binding energy at 288.4 and 290.4 eV can be respectively assigned to adsorbed HCHO(a) and HCOO species on TiO2 surface. This confirms the above TDS results of photocatalytic oxidation of CH3O(a) into HCHO(a). With the prolonging of UV light illumination, the photocatalytic oxidation reactions of CH3O(a) into HCHO(a) and HCOO and of CHx into carbon keep proceeding. When anatase TiO2(001)-(1×4) surface covered with 0.35 ML CH3OH was illuminated by UV light at 115 K, the photocatalytic oxidation of CH3O(a) into HCHO(a) and HCOO occurs, resulting the increasing coverages of HCHO(a) and HCOO at the expense of CH3O(a); and the photocatalytic oxidation of CHx species slightly happens. However, the C 1s component of CH3OH(a) does not change. These observation suggest that CH3O(a) and CHx species adsorbed at the Ti4C sites of anatase TiO2(001)-(1×4) surface are photoactive while CH3OH(a) adsorbed at the Ti5C sites is not. In order to check this, another anatase TiO2(001)-(1×4) surface covered with 0.28 ML CH3OH was prepared by flashing a surface covered with physisorbed CH3OH to 250 K and then illuminated by UV light at 115 K. As shown in the C 1s XPS spectra results (Figure 2C and 2F), the CH3OH(a) coverage is much smaller on the 0.28 ML CH3OH-covered anatase TiO2(001)-(1×4) surface than on the 0.35 ML CH3OH-covered anatase TiO2(001)-(1×4) surface while the CHx(a) coverage is much larger, which can be explained by the vacant surface sitesdependent dissociation of CH3OH on anatase TiO2(001)-(1×4) surface33. During the subsequent


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UV light illumination at 115 K, CH3O(a) and CHx species were observed to be are photoactive while CH3OH(a) was not, similar to the case of 0.35 ML CH3OH-covered anatase TiO2(001)(1×4) surface. These observations are distinctly different from previous results21-23 that both CH3O(a) and CH3OH(a) on rutile TiO2(110) surface are photoactive. The C 1s XPS spectra of anatase TiO2(001)-(1×4) surface covered with 0.35 ML CH3OH with a 20 min’s UV light illumination at 115 K followed by annealing at elevated temperatures are shown in Figure 3A and the corresponding intensity variations of different components are shown in Figure 3B. CH3OH(a), CH3O(a), HCHO(a), HCOO, CHx are present on the surface after UV light illumination. In combination with corresponding TDS results (Figure 1B), the decrease of CH3OH(a) component at 230 K and disappearance at 320 K are due to both decomposition and desorption; the initial increase of CH3O(a) component till 320 K results from the decomposition of CH3OH(a), and the subsequent decrease at 420 K and disappearance at 900 K are due to the recombination to produce CH3OH, the dehydration coupling reaction to produce DME and the disproportionation reaction to produce HCHO and CH3OH; the decrease and disappearance up to 320 K of HCHO(a) component are due to the desorption; the decrease and disappearance up to 680 K of HCOO component are due to the decomposition; the initial increase of CHx component up to 280 K results from the decomposition of HCOO, and the subsequent decrease and disappearance up to 680 K are due to the decomposition; and the emergence of the carbon component at 320 K and continuous growth are due to the decomposition of CHx species. The surface reactions of CH3OH(a) and CH3O(a) upon annealing agree with our previous results.33 The above results demonstrate adsorbed HCHO(a) as the product of photocatalytic reaction of methanol on anatase TiO2(001)-(1×4) surface. Thus adsorption and photocatalytic reactions of


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HCHO on clean anatase TiO2(001)-(1×4) surface were studied. Only the desorption traces of HCHO was observed in the TDS spectra following HCHO exposures at 115 K (Figure 4A). With the HCHO exposure increasing, a HCHO desorption peak appears at 315 K with a tail up to about 500 K, grows and saturates at an exposure of 1 L HCHO; and then another HCHO desorption peak appears at 135 K and increases without saturation. The HCHO desorption peaks at 315 and 135 K could be attributed to the desorption of chemisorbed and physisorbed HCHO(a) from the surface, respectively. The corresponding C 1s XPS spectra exhibit complex patterns (Figure 4B and 4C). The C 1s components arising from HCHO(a) (288.4 eV), HCOO (290.4 eV), CH3O(a) (286.9 eV), CHx (285.7 eV) and carbon (284.8 eV) species appear following an exposure of 0.1 L HCHO at 115 K, demonstrating the occurrence of molecular adsorption, decomposition into CHx and even carbon, and even Cannizzaro-type reaction of adsorbed HCHO(a) into CH3O(a) and HCO(a) that subsequently transforms into more stable HCOO species. The decomposition and Cannizzaro-type reaction should be associated with the presence of highly-reactive Ti4C sites, as in the case of methanol adsorption. With the HCHO exposure increasing, the decomposition and Cannizzaro-type reaction get gradually suppressed due to the decrease in the available surface sites, and molecularly adsorbed HCHO(a) grows quickly and dominates on the surface. Figure 5 shows C 1s XPS spectra after an exposure of 5 L HCHO on clean anatase TiO2(001)-(1×4) surface at 115 K followed by annealing at elevated temperatures. Molecularlyadsorbed HCHO(a) dominates at 115 K. Annealing the surface at 280 K results in a great decrease of the HCHO(a) component, mainly corresponding to the HCHO desorption peak at 135 K; meanwhile, the components of HCOO, CH3O(a), CHx and carbon species grows, suggesting the occurrence of the decomposition and Cannizzaro-type reaction of chemisorbed


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HCHO(a) upon heating. Further annealing the surface at 360 K leads to the disappearance of the HCHO(a) feature, corresponding to the desorption of the HCHO desorption peak at 315 K, and the features of other components do not change much. Increasing the annealing temperature to 650 K results in the growth of the carbon feature at the expenses of the HCOO, CH3O(a) and CHx features. This suggests the decomposition of HCOO, CH3O(a) and CHx species into carbon since no gas-phase products other than HCHO could be observed in the TDS spectra; however, the growth of the carbon feature is much smaller than the total reduction of the HCOO, CH3O(a) and CHx features. We proposed the occurrence of reverse Cannizzaro-type reaction between CH3O(a) and HCOO to produce HCHO, giving the shoulder desorption peak of HCHO up to 500 K in the HCHO TDS spectra (Figure 3A). After annealing at 900 K, only carbon remains on the surface with an enhanced C 1s feature. Figure 6A shows TDS spectra after an exposure of 2 L HCHO on anatase TiO2(001)-(1×4) surface at 115 K with UV light illumination at 115 K for 20 min and Figure 6 B and C show corresponding C 1s XPS spectra. No obvious changes could be observed between the TDS spectra without and with UV light illumination. As demonstrated in the C 1s XPS spectra, the HCOO feature increases upon the UV light illumination while the CH3O(a) feature decreases and the HCHO(a) feature slightly decreases. This suggests the occurrence of photocatalytic conversions of CH3O(a) and HCHO(a) respectively into HCHO(a) and HCOO. The formation of HCHO(a) by the photocatalytic conversion of CH3O(a) can counteract the consumption of HCHO(a) due to its photocatalytic conversion into HCOO. The evolutions of various surface species upon subsequent annealing at elevated temperatures are similar to those without UV light illumination.


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The above results demonstrate two unique characteristics of photocatalytic reactions of CH3OH on anatase TiO2(001)-(1×4) surface different from those on previously reported TiO2 surfaces23-28: one is that no methyl formate is produced during the photocatalytic oxidation of CH3O(a); the other is that CH3O(a) species at the Ti4C sites is photoactive but CH3OH(a) at the Ti5C sites is not. It is noteworthy that previously reported TiO2 surfaces,23-28 including rutile TiO2(110), rutile TiO2(011)-(2×1) and anatase TiO2(101), all expose Ti5C sites on the perfect surfaces. Formed by the photocatalytic oxidation of CH3O(a) at the Ti4C sites, HCHO(a) and HCOO should locate at the Ti4C sites. It is likely that the strong adsorption of HCHO(a) and HCOO with the Ti4C site favourites single-molecular decomposition reaction over bi-molecular cross-coupling reaction with CH3O(a) to produce methyl formate. CH3OH(a) adsorbed at the Ti5C sites of rutile TiO2(110) surface has been proved photoactive,21-23 thus our observation suggests that the photoexcited holes in anatase TiO2(001)(1×4) should be exclusively transported to the (1×4) added row but seldom to the (1×1) basal surface so that CH3O(a) species at the Ti4C sites can be photocatalycally oxidized into HCHO(a) and HCOO but CH3OH(a) species at the Ti5C sites can not (Scheme 1). The results demonstrate an interesting site-specific charge separation and photocatalytic oxidation of methanol on anatase TiO2(001)-(1×4) reconstructed

surface.

The

co-adsorbed

hydroxyl

groups

or water

accompanying the formation of CH3O(a) at Ti4C sites could influence the photo-reactivity of CH3O(a), but the effect should not be so strong to induce the Ti4C site-specific photocatalytic oxidation. CH3O(a) formed at the surface oxygen vacancies of rutile TiO2(110) surface is also accompanied with hydroxyl groups, but both CH3OH(a) and CH3O(a) species are photoactive.2123

N-type TiO2 exhibits an upward surface band bending that facilitates the migration of

photoexcited holes to the surface. We propose that a surface with stoichiometrically deficient


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oxygen formed by the (1×4) surface reconstruction of anatase TiO2(001) should strengthen such an upward bend bending from the (1×1) basal surface to the (1×4) reconstructed surface and greatly promote the migration of photoexcited holes to the (1×4) added row. DFT calculations were further performed to analyze the band structures of clean and methanol-covered anatase TiO2(001)-(1×4) surfaces (Figure 7). The optimized structure of clean anatase TiO2(001)-(1×4) surface agrees with previous results,31,32 and the PDOS analysis results show that the valence band maximum and the conduction band minimum are respectively localized at the surface oxygen atoms on the added row and the Ti atoms in the bulk. This indicates that the photogenerated holes and electrons in anatase TiO2(001) should be thermodynamically most stable respectively at the surface oxygen atoms on the added row and the Ti atoms in the bulk. CH3OH is molecularly adsorbed at the Ti5C sites of basal surface but dissociates at the Ti4C sites of the added row of reconstructed (1×4) surface, forming CH3O(a) at Ti4C sites and H(a) at neighbouring O sites with the breaking of involved Ti4C-O surface bonds.33 The PDOS analysis results show that the conduction band minimum is still localized at the Ti atoms in the bulk while the valence band maximum is localized mainly to the O atoms in the CH3O(a) species. This indicates that the photogenerated holes and electrons in methanol-covered anatase TiO2(001) should be thermodynamically most stable respectively at the oxygen atoms of CH3O(a) species bonding to the Ti4C of the added row and the Ti atoms in the bulk. As the subsequence, the CH3O(a) species at the Ti4C sites should be preferentially oxidized by the photogenerated holes in reconstructed anatase TiO2(001) while the CH3OH(a) species at the Ti5C sites of (1×1) basal surface is not.


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The DFT calculation results also indicate that the photo-oxidation of CH3O(a) species at the Ti4C sites of TiO2(001)-(1×4) surface should involve the hole transfer from the O atom of CH3O(a) to the C-H bond, i.e., a intramolecular transfer mechanism. This is different from previous DFT calculation results of photo-oxidation of methanol molecularly adsorbed on rutileTiO2(110) surface44 in which an interfacial hole transfer from the O atom of TiO2(110) surface to adsorbed CH3OH molecule is proposed. Our results also demonstrate that the PDOS maximum of O atom of the anatase TiO2(001) (1×1) basal surface is more positive than that of O atom of molecularly-adsorbed CH3OH at the Ti5C sites of the (1×1) basal surface. This suggests that the photogenerated holes within the anatase TiO2(001) (1×1) basal surface should be more stable at O atom of the surface than at O atom of molecularly-adsorbed CH3OH and that the photooxidation of molecularly-adsorbed CH3OH at the Ti5C sites of the (1×1) basal surface should involve an interfacial hole transfer from the O atom of TiO2 surface to adsorbed CH3OH molecule, similar to the photo-oxidation of methanol molecularly adsorbed on rutile-TiO2(110) surface.44 These results reveal the important role of surface structure of oxide photocatalysts in determining the surface charge transfer process and subsequent photocatalytic reaction. Surface reconstruction commonly occurring on oxides significantly induces the structure of oxide surfaces, and thus should strongly influence the charge transfer and photocatalytic reaction of oxide photocatalysts. Our results can also rationalize Henderson et al.’s experimental results of photochemical inactive CH3OH(a) on rutile TiO2(110)18-20 to high concentration surface oxygen vacancies that likely result in surface oxygen vacancy-specific transport of photogenerated holes and photocatalytic oxidation of CH3O(a) therein. 4. CONCLUSIONS


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In summary, by studying the photocatalytic reaction of methanol on a reconstructed anatase TiO2(001)-(1×4) surface, we successfully demonstrate a concept of surface reconstructioninduced site-specific charge separation and photocatalytic reaction of oxide photocatalysts. Such a concept greatly deepens fundamental understandings of the vital role of oxide surface structure in photocatalytic reactions and provides a strategy to engineer surface structures of oxide photocatalysts to optimize charge transfer processes and increasing charge separation and photocatalytic reaction efficiencies. Supporting Information. LEED pattern, and TDS spectra. Notes The authors declare no competing financial interest. Acknowledgment This work was financially supported by National Basic Research Program of China (2013CB933104), National Natural Science Foundation of China (21525313), Chinese Academy of Sciences (KJZD-EW-M03), MOE Fundamental Research Funds for the Central Universities (WK2060030017), Hefei Science Center of Chinese Academy of Sciences (2015HSC-UP014) and Collaborative Innovation Center of Suzhou Nano Science and Technology. REFERENCES (1) Fujishima, A.; Zhang, X. T.; Tryk, D. A. TiO2 Photocatalysis and Related Surface Phenomena. Surf. Sci. Rep. 2008, 63, 515-582. (2) Lang, X.; Ma, W.; Chen, C.; Ji, H.; Zhao, J. Selective Aerobic Oxidation Mediated by TiO2 Photocatalysis. Acc. Chem. Res. 2014, 47, 355-363. (3) Linsebigler, A. L.; Lu, G.; Yates, J. T. Photocatalysis on TiO2 Surfaces: Principles, Mechanisms, and Selected Results. Chem. Rev. 1995, 95, 735-758.


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Page 18 of 29

(4) Henderson, M. A. A Surface Science Perspective on Photocatalysis. Surf. Sci. Rep. 2011, 66, 185-297. (5) Xu, M.; Gao, Y.; Moreno, E. M.; Kunst, M.; Muhler, M.; Wang, Y.; Idriss, H.; Wöll, C. Photocatalytic Activity of Bulk TiO2 Anatase and Rutile Single Crystals Using Infrared Absorption Spectroscopy. Phys. Rev. Lett. 2011, 106, 138302. (6) Yates, J. T.; Petek, H. Introduction: Photochemistry and Photophysics on Surfaces. Chem. Rev. 2006, 106, 4113-4115. (7) Cai, Y.; Feng, Y. P. Review on Charge Transfer and Chemical Activity of TiO2: Mechanism and Applications. Prog. Surf. Sci. 2016, 91, 183-202. (8) Zhang, Z.; Yates, J. T. Band Bending in Semiconductors: Chemical and Physical Consequences at Surfaces and Interfaces. Chem. Rev. 2012, 112, 5520-5551. (9) Ma, Y.; Wang, X.; Jia, Y.; Chen, X.; Han, H.; Li, C. Titanium Dioxide-Based Nanomaterials for Photocatalytic Fuel Generations. Chem. Rev. 2014, 114, 9987-10043. (10) Petrik, N. G.; Kimmel, G. A.; Shen, M.; Henderson, M. A. Quenching of Electron Transfer Reactions through Coadsorption: A Study of Oxygen Photodesorption from TiO2(110). Surf. Sci. 2016, 652, 183-188. (11) Hardman, P.J.; Raikar, G.N.; Muryn, C. A.; Van Der Laan, G.; Wincott, P. L.; Thornton, G.; Bullett, D. W.; Dale, P. A. D. M. A. Valence-Band Structure of TiO2 along the Γ-∆-X and Γ-ΣM Directions. Phys Rev B 1994, 49, 7170. (12) Henderson, M. A. A HREELS and TPD Study of Water on TiO2(110): The Extent of Molecular versus Dissociative Adsorption. Surf. Sci. 1996, 355, 151-166. (13) Diebold, U.; Lehman, J.; Mahmoud, T.; Kuhn, M.; Leonardelli, G.; Hebenstreit, W.; Schmid, M.; Varga, P. Intrinsic Defects on a TiO2(110)-(1×1) Surface and Their Reaction with Oxygen: A Scanning Tunneling Microscopy Study. Surf. Sci. 1998, 411, 137-153. (14) Diebold, U. The Surface Science of Titanium Dioxide. Surf. Sci. Rep. 2003, 48, 53-229. (15) Martinez, U.; Hammer, B. Adsorption Properties versus Oxidation States of Rutile TiO2(110). J. Chem. Phys. 2011, 134, 194703. (16) Deskins, N A.; Rousseau, R.; Dupuis, M. Defining the Role of Excess Electrons in the Surface Chemistry of TiO2. J. Phys. Chem. C 2010, 114, 5891-5897. (17) Zhang, Z.; Yates, J. T. Effect of Adsorbed Donor and Acceptor Molecules on Electron Stimulated Desorption: O2/TiO2(110). J. Phys. Chem. Lett. 2010, 1, 2185-2188. (18) Shen, M.; Henderson, M. A. Identification of the Active Species in Photochemical Hole Scavenging Reactions of Methanol on TiO2. J. Phys. Chem. Lett. 2011, 2, 2707-2710.


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Page 19 of 29 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(19) Shen, M.; Henderson, M. A. Role of Water in Methanol Photochemistry on Rutile TiO2(110). J. Phys. Chem. C 2012, 116, 18788-18795. (20) Shen, M.; Acharya, D. P.; Dohnálek, Z.; Henderson, M. A. Importance of Diffusion in Methanol Photochemistry on TiO2(110). J. Phys. Chem. C 2012, 116, 25465-25469. (21) Zhou, C.; Ren, Z.; Tan, S.; Ma, Z.; Mao, X.; Dai, D.; Fan, H.; Yang, X.; LaRue, J.; Cooper, R.; et al. Site-Specific Photocatalytic Splitting of Methanol on TiO2(110). Chem. Sci. 2010, 1, 575-580. (22) Guo, Q.; Xu, C.; Ren, Z.; Yang, W.; Ma, Z.; Dai, D.; Fan, H.; Minton, T. K.; Yang, X. Stepwise Photocatalytic Dissociation of Methanol and Water on TiO2(110). J. Am. Chem. Soc. 2012, 134, 13366-13373. (23) Yuan, Q.; Wu, Z.; Jin, Y.; Xu, L.; Xiong, F.; Ma, Y.; Huang, W. Photocatalytic CrossCoupling of Methanol and Formaldehyde on a Rutile TiO2(110) Surface. J. Am. Chem. Soc. 2013, 135, 5212− 5219. (24) Phillips, K.R.; Jensen, S.C.; Baron, M.; Li, S. C.; Friend, C. M. Sequential Photo-Oxidation of Methanol to Methyl Formate on TiO2(110). J. Am. Chem. Soc. 2013, 135, 574–577. (25) Guo, Q.; Xu, C.; Yang, W.; Ren, Z.; Ma, Z.; Dai, D.; Minton, T. K.; Yang, X. Methyl Formate Production on TiO2(110), Initiated by Methanol Photocatalysis at 400 nm. J. Phys. Chem. C 2013, 117, 5293 −5300. (26) Wang, Z.; Hao, Q.; Mao, X.; Zhou, C.; Dai, D.; Yang, X. Photocatalytic Chemistry of Methanol on Rutile TiO2(011)-(2×1). Phys. Chem. Chem. Phys. 2016, 18, 10224-10231. (27) Mao, X.; Wang, Z.; Lang, X.; Hao, Q.; Wen, B.; Dai, D.; Zhou, C.; Liu, L.-M.; Yang, X. Effect of Surface Structure on the Photoreactivity of TiO2. J. Phys. Chem. C 2015, 119, 61216127. (28) Xu, C.; Yang, W.; Guo, Q.; Dai, D.; Chen, M.; Yang, X. Molecular Hydrogen Formation from Photocatalysis of Methanol on Anatase-TiO2(101). J. Am. Chem. Soc. 2014, 136, 602-605. (29) Wilson, J. N.; Idriss, H. Effect of Surface Reconstruction of TiO2(001) Single Crystal on the Photoreaction of Acetic Acid. J. Catal. 2003, 214, 46-52. (30) Liu, G.; Yang, H. G.; Pan, J.; Yang, Y. Q.; Lu, G. Q.; Cheng, H.-M. Titanium Dioxide Crystals with Tailored Facets. Chem. Rev. 2014, 114, 9559-9612. (31) Herman, G.S.; Sievers, M.R.; Gao, Y. Structure Determination of the Two-Domain (1×4) Anatase TiO2(001) Surface. Phys. Rev. Lett. 2000, 84, 3354-3357. (32) Liang, Y.; Gan, S.; Chambers, S. A; Altman, E. I. Surface Structure of Anatase TiO2(001): Reconstruction, Atomic Steps, and Domains. Phys. Rev. B 2001, 63, 235402.


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Page 20 of 29

(33) Xiong, F.; Yu, Y.-Y.; Wu, Z.; Sun, G.; Ding, L.; Jin, Y.; Gong, X.-Q.; Huang, W. Methanol Conversion into Dimethyl Ether on the Anatase TiO2(001) Surface. Angew. Chem. Int. Ed. 2016, 55, 623-628. (34) Perdew, J. P.; Burke, K.; Ernzerhof, M. Generalized Gradient Approximation Made Simple. Phys. Rev. Lett. 1996, 77, 3865. (35) Kresse, G.; Furthmüller, J. Efficiency of Ab-Initio Total Energy Calculations for Metals and Semiconductors Using a Plane-Wave Basis Set. Comput. Mater. Sci. 1996, 6, 15-50. (36) Kresse, G.; Furthmüller, J. Efficient Iterative Schemes for Ab Initio Total-Energy Calculations Using a Plane-Wave Basis Set. Phys. Rev. B 1996, 54, 11169-11186. (37) Blöchl, P. E. Projector Augmented-Wave Method. Phys. Rev. B 1994, 50, 17953-17979. (38) Dudarev, S.L.; Botton, G.A.; Savrasov, S.Y.; Humphreys, C.J.; Sutton, A.P. ElectronEnergy-Loss Spectra and the Structural Stability of Nickel Oxide: An Lsda+ U Study. Phys. Rev. B 1998, 57, 1505-1509. (39) Grimme, S. Accurate Description of Van Der Waals Complexes by Density Functional Theory Including Empirical Corrections. J. Comput. Chem. 2004, 25, 1463-1473. (40) Grimme, S. Semiempirical GGA‐Type Density Functional Constructed with a Long‐ Range Dispersion Correction. J. Comput. Chem. 2006, 27, 1787-1799. (41) Lazzeri, M.; Selloni, A. Stress-Driven Reconstruction of an Oxide Surface: The Anatase TiO2(001)−(1×4) Surface. Phys. Rev. Lett. 2001, 87, 266105. (42) Lang, X.; Liang, Y.; Sun, L.; Zhou, S.; Lau, W.-M. Interplay between Methanol and Anatase TiO2(101) Surface: The Effect of Subsurface Oxygen Vacancy. J. Phys. Chem. C 2017, 121, 6072–6080. (43) Yuan, Q.; Wu, Z.; Jin, Y.; Xiong, F.; Huang, W. Surface Chemistry of Formaldehyde on Rutile TiO2(110) Surface: Photocatalysis Vs Thermal-Catalysis. J. Phys. Chem. C 2014, 118, 20420-20428. (44) Migani, A.; Blancafort, L. Excitonic Interfacial Proton-Coupled Electron Transfer Mechanism in the Photocatalytic Oxidation of Methanol to Formaldehyde on TiO2(110). J. Am. Chem. Soc. 2016, 138, 16165-16173.


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Scheme 1. Schematic illustration of surface reconstruction-induced band bending and (1×4) surface-specific transport of photogenerated holes that leads to Ti4C sites-specific photocatalytic oxidation of CH3OH on anatase TiO2(001)-(1×4) surface. Red, grey, dark grey, green and white balls respectively represent O and Ti of TiO2, and C, O and H of CH3OH.


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Figure 1. TDS spectra of anatase TiO2(001)-(1×4) surfaces covered with (A) 0.11 and (B) 0.35 ML CH3OH without and with UV light illumination at 115 K.


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Figure 2. C 1s XPS spectra and corresponding component intensities of anatase TiO2(001)-(1×4) surfaces covered by (A and D) 0.11 ML, (B and E) 0.35 ML and (C and F) 0.28 ML CH3OH with UV light illumination at 115 K for different times.


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Figure 3. (A) C 1s XPS spectra and (B) corresponding component intensities of 0.35 ML CH3OH-covered anatase TiO2(001)-(1×4) surfaces with UV light illumination at 115 K for 20 min followed by annealing at elevated temperatures.


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Figure 4. (A) TDS and (B) C 1s XPS spectra and (C) corresponding component intensities following HCHO adsorption on anatase TiO2(001)-(1×4) surface at 115 K.


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Figure 5. (A) C 1s XPS spectra and (B) corresponding component intensities after an exposure of 5 L HCHO on anatase TiO2(001)-(1×4) surface at 115 K followed by annealing at elevated temperatures.


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Figure 6. (A) TDS spectra, (B) C 1s XPS spectra, and (C) corresponding component intensities after an exposure of 2 L HCHO on anatase TiO2(001)-(1×4) surface at 115 K with UV light illumination at 115 K for 20 min followed by annealing at elevated temperatures.


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Figure 7. Calculated structures (left) and corresponding DOS and PDOS (right) of (A) anatase TiO2(001)-(1×4) surface and (B) anatase TiO2(001)-(1×4) surface with dissociatively and molecularly adsorbed methanol respectively at Ti4C site of added row and Ti5C site of (1×1) basal surface. The inset in the left panel of B shows another side-view of the dissociatively adsorbed methanol at Ti4C site.


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Surface Reconstruction-Induced Site-Specific Charge Separation and

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