Article pubs.acs.org/JPCC
Effect of Surface Structure on the Photoreactivity of TiO2 Xinchun Mao,†,# Zhiqiang Wang,†,# Xiufeng Lang,‡,∥,# Qunqing Hao,† Bo Wen,‡,§ Dongxu Dai,† Chuanyao Zhou,*,† Li-Min Liu,*,‡ and Xueming Yang*,† †
State Key Laboratory of Molecular Reaction Dynamics, Dalian Institute of Chemical Physics, Chinese Academy of Science, 457 Zhongshan Road, Dalian, 116023, Liaoning, P. R. China ‡ Beijing Computational Science Research Center, Beijing, 100084, P. R. China § International Center for Quantum Materials (ICQM) and School of Physics, Peking University, Beijing, 100871, P. R. China ∥ Department of Physics, Hebei Normal University of Science & Technology, Qinhuangdao, 066004, Hebei, P. R. China S Supporting Information *
ABSTRACT: Although it has been widely accepted that the crystal phase, morphology, and facet significantly influence the catalytic and photocatalytic activity of TiO2, establishing the correlation between structure and activity of heterogeneous reactions is very difficult because of the complexity of the structure. Utilizing ultrahigh vacuum (UHV) based temperature-programmed desorption (TPD) and density functional theory (DFT) calculations, we have successfully assessed the photoreactivity of two well characterized rutile surfaces ((011)-(2×1) and (110)-(1×1)) through examining the photocatalyzed oxidation of methanol. The photocatalytic products, such as formaldehyde and methyl formate, are the same on both surfaces under UV illumination. However, the reaction rate on (011)-(2×1) is only 42% of that on (110)-(1×1), which contradicts previous reports in aqueous environments where characterization of TiO2 structure is difficult. The discrepancy probably comes from the differences of the TiO2 structure in these studies. Our DFT calculations reveal that the rate-determining step of methanol dissociation on both surfaces is C−H scission,; however, the barrier of this elementary step on (011)-(2×1) is about 0.2 eV higher than that on (110)-(1×1) because of their distinct surface atomic configurations. The present work not only demonstrates the importance of surface structure in the photoreactivity of TiO2, but also provides an example for building the correlation between structure and activity using surface science techniques and DFT calculations.
1. INTRODUCTION Titanium dioxide (TiO2) has attracted extensive attention due to its wide variety of applications, for example, solar energy conversion, gas sensors, lithium ion batteries, catalysis, and photocatalysis, which are closely linked to energy and the environment. 1−4 A prerequisite for all the functional applications mentioned above is the interaction between molecules/ions and the TiO2 surfaces, which is determined by the electronic structure as well as the atomic structure of TiO2. The anisotropic chemical reactivity on TiO2 surfaces has been investigated.5−7 Many experiments have been carried out to fabricate different TiO2 nanostructures with specific facets in the past few years.8,9 Usually, those surfaces with a higher percentage of undercoordinated surface atoms are regarded as more reactive. In addition, constructing TiO2 heterojunctions that exhibit distinctive properties at different facets has also become a promising field.10,11 Therefore, it is of great significance to establish the correlation between structure and activity of heterogeneous reactions, which is very difficult because of the complexity of the structure. Owing to the well characterized structure of single crystals in ultrahigh vacuum © XXXX American Chemical Society
(UHV) conditions, studying heterogeneous reactions on these substrates using the combination of surface science techniques and density functional theory (DFT) calculations will considerably help build the structure−activity correlation. Rutile is the most abundant crystal phase of TiO2, and the surface dependence of its photoreactivity has also been observed.11−19 Among the low Miller index surfaces, (011) has been reported to be more reactive than (110) toward photocatalyzed oxidation reactions. For example, photooxidation of Pb2+ occurred on the (011) surface, whereas the reduction of Pt2+ is on (110).11 In addition, (011) is more efficient in the photocatalyzed degradation of methylene blue than (110).18 Several interpretations have been proposed to account for the enhanced photocatalytic activity of rutile (011) based on the surface atomic and electronic structures in vacuum.20−22 It is worth noting the photoreactivity tested above is in aqueous environment which often alters the surface Received: January 17, 2015 Revised: February 26, 2015
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12, 4, and 6 valence electrons for Ti, C, and O, respectively. During examining properties of the TiO2(011)-(2×1) surface, (2×2) unit cells were used. The corresponding surface slab contains 4 TiO2 trilayers and a 15 Å thick vacuum layer. Since our models are quite large, the k-points for all configurations are sampled on a single gamma point. During structure optimization, the adsorbates (CH3OH, CH3O, and H2CO) and the three top TiO2 trilayers were allowed to relax in all directions, while the bottom TiO2 trilayer was fixed at the optimized bulk position. A 400 eV were used for cutoff energy. The force and the total energy were converged to 0.03 eV Å−1 and 1 × 10−5 eV, respectively. The minimum energy paths and corresponding activation barriers were calculated using the climbing image nudged elastic band approach (CI-NEB).36,37 The minimum energy path is examined with eight images, including the initial and final states, during the transition state search. In order to check whether eigtht images are enough, we also examined nine images for the TS4 (see Figure 5), and the calculated energy barrier is the same as the one with eight images. With the same computational setups, we also examined the adsorption energy of CH3O + H2CO, which is the initial configuration of the TS4 with energy cutoff of 500 eV. The difference in adsorption energies with 400 and 500 eV is within 0.01 eV, suggesting an energy cutoff of 400 eV is good enough to describe this system.
structure significantly, making it difficult to establish the correlation between activity and surface structure at an atomic level.23 To avoid such complexity, photocatalysis studies in UHV condition with well characterized surface structure are desirable. In order to study the effect of surface structure, we have assessed the photoreactivity of rutile TiO2(011)-(2×1) and (110)-(1×1) surfaces quantitatively by monitoring the photocatalyzed oxidation of methanol using UHV based temperatureprogrammed desorption (TPD) and DFT calculations. The reaction products of photocatalyzed oxidation of methanol on rutile TiO2(011)-(2×1) are the same as those on (110).24−30 The most prominent difference comes from the reaction rate. Contradicting previous studies where (011) is more reactive than (110) toward photocatalyzed oxidation reactions,11,18 the reaction rate of photocatalyzed oxidation of methanol on TiO2(011)-(2×1) is only 42% of that on (110)-(1×1) in the present work. The discrepancy probably comes from the differences of the TiO2 structure in these studies. In the present work, our DFT calculations exhibit that the rate-determining step is the cleavage of C−H bond in methanol dissociation on both (011) and (110) surfaces, while the barrier of this elementary step on (011)-(2×1) is about 0.2 eV higher due to the different surface configuration. This work demonstrates the importance of surface structure in the photoreactivity of TiO2. On the other hand, it also provides an example for building the structure−activity correlation using the combination of single crystals, surface science techniques, and DFT calculations.
3. RESULTS AND DISCUSSION TiO2(110)-(1×1) (Figure 1A) is a prototype of metal oxides. 5fold coordinated titanium (Ti5c) and 2-fold coordinated
2. EXPERIMENTAL AND THEORETICAL METHODS 2.1. Experimental Details. TPD experiments were conducted in a UHV chamber, which has been described in detail previously.31 A mass spectrometer (SRS, RGA 200) was shielded by a glass enclosure and differentially pumped for the TPD measurements.32 The temperature was ramped at 2 K/s during all the TPD experiments. To ensure accurate temperature reading, a K type thermocouple was directly glued to the TiO2 sample. The second harmonic (SH, 400 nm) of an amplifier (Coherent, Legend-HE-USP, 1 kHz) was delivered in pulses of 50 fs duration and 0.4-mJ energy to induce photocatalytic reactions. The diameter of the laser spot on TiO2 surfaces was ∼6 mm. TiO2 single crystals (10 × 10 × 1 mm3, Princeton Scientific Corp.) were cleaned by cycles of Ar+ bombardment and UHV annealing between 850 and 950 K. After preparation, no contaminations have been detected by Auger electron spectroscopy (AES) (Figure S1, Supporting Information) and single phase 1 × 1 and 2 × 1 terminations have been observed on (110) and (011) surfaces (Figure S2, Supporting Information), respectively, by low energy electron diffraction (LEED). The preparation history of these two surface studied in the present work were similar, which is reflected by the similar work function in 2PPE spectra. Methanol (Sigma-Aldridge) was purified by freeze−pump− thaw cycles and introduced onto the TiO2 surfaces through a home-built, calibrated effusive molecular beam doser at 120 K. 2.2. Theoretical Methods. DFT calculations were carried out with plane waves and the plane augmented waves (PAW) potentials, as implanted in VASP (Vienna ab initio simulation package).33 The Perdew, Burke, and Ernzerhof (PBE) functional within the frame of the generalized gradient approximation (GGA) was chosen to deal with the exchangecorrelation energy.34 The projector augmented-wave method (PAW)35 was employed to treat valence-core interactions with
Figure 1. Structure of rutile TiO2(110)-(1×1) (A) and (011)-(2×1) (B) surfaces. Oxygen and Ti atoms are represented as red and gray spheres, respectively.
bridging oxygen (O2c) atoms run alternatively in the [001] direction.4 The most stable phase of rutile TiO2(011) is reconstructured by (2×1). The atomic structure of (011)(2×1) as suggested by X-ray diffraction and DFT calculations21,38 are shown in Figure 1B. Different from TiO2(110), inequivalent types of under-coordinated Ti and O atoms exit, namely, the valley Ti5c, ridge Ti5c, top O2c, and bridge O2c. The B
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The Journal of Physical Chemistry C top O2c atoms display in a zigzag style, which shade the ridge Ti5c sites severely. Before the photocatalyzed oxidation of methanol on TiO2 surfaces was investigated, the adsorption behavior of methanol was first examined. Figure S3, Supporting Information shows the TPD spectra of methanol (m/z = 31) as a function of coverage with respect to the corresponding density of Ti5c sites. Here, monolayer (ML) corresponds to 5.2 × 1014 molecules/ cm2 on (110)-(1×1) while, this value is 4.0 × 1014 molecules/ cm2 on (011)-(2×1).39 The adsorption behavior of methanol on TiO 2 (110) has been well established. 40 TPD of chemisorbed methanol (bounded to Ti5c) yields a peak at 296 K, and the adsorption of the second layer does not occur until the saturation of the chemisorbed layer (0.67 ML).40 Because of the corrugated surface structure, the desorption of methanol from TiO2(011)-(2×1) is much more complicated. With the lowest coverage of 0.09 ML, the desorption signal spreads from 300 to 430 K. As the coverage increases, the desorption feature around 340 K becomes more and more intense. In the meanwhile, a new desorption feature develops at ∼220 K and shifts downward to 200 K. In analogy with the TPD results of methanol/TiO2(110)-(1×1)40 and the water/ TiO2(011)-(2×1),41 the features with desorption temperature above 250 K should origin mainly from methanol adsorbed on valley Ti5c sites. To compare the photoreactivity of (110)-(1×1) and (011)(2×1), photocatalyzed oxidation of methanol on both surfaces has been systematically studied using identical experimental conditions. All possible masses have been measured, and no products with m/z > 60 has been detected. The illumination dependent TPD spectra of methanol, formaldehyde, water, and methyl formate from 0.67 ML methanol covered TiO2(110)(1×1) surface are shown in Figure S4, Supporting Information. Photocatalytic oxidation of methanol on rutile (110)-(1×1) produces formaldehyde and bridging hydrogen atoms (Hb), which recombine as water by abstracting a bridging oxygen at moderate temperature.26 Formaldehyde further couples with methoxy into methyl formate.27−29 Figure 2a shows the TPD spectra of 0.67 ML methanol (m/z = 31) on (011)-(2×1) as a function of illumination. The signal around 350 K declines obviously with the UV irradiation, suggesting methanol on Ti5c sites is consumed. To determine the photochemical products, all possible carbon containing species are measured by TPD. An illumination dependent signal at 176 K for mass 30 (Figure 2b) is detected. The desorption profiles of mass 28, 29, and 30 at the proximity of 176 K are the same, indicating they are the fragments from formaldehyde decomposition in the ionizer region. This result unambiguously suggests that methanol is oxidized into formaldehyde on TiO2(011)-(2×1). The consumed methanol signal is at around 350 K, while the produced formaldehyde signal is at ∼176 K. Owing to the very intense signal of methanol in the >200 K range, it is not easy to extract formaldehyde signal in this region. The low desorption temperature of formaldehyde is possibly due to the significant steric hindrance on this surface. Formaldehyde signal at 176 K increases within the first 40 min and then shrinks. The decrease can be accounted for by the photoinduced desorption of formaldehyde, which has been reported on TiO2(110).6,26,42 Another possible reason may be that formaldehyde further reacts to produce methyl formate, and this will be discussed later.
Figure 2. TPD spectrum for (a) m/z = 31, (b) m/z = 30 and (c) m/z = 18 as a function of light illumination (400 nm, flux 2.24 × 1018 photons/cm2/s) after 0.67 ML methanol was dosed onto the TiO2(011)-(2×1) surface at 120 K.
To produce formaldehyde, a methanol molecule has to lose both a hydroxyl and a methyl hydrogen. The released hydrogen atoms are expected to transfer to the surface oxygen atoms. There are three types of oxygen on rutile (011)-(2×1). The trough O3c is fully coordinated, and thus it is not a possible residence. Both the top and bridge oxygen atoms are undercoordinated; however, the formation of hydroxyl is more readily on the top O2c owing to the stronger Bronsted acidity.39 The adsorption of H on this surface due to photocatalyzed oxidation of methanol is explored by TPD measurements of the possible H related products, i.e., H2 and H2O. No change of H2 signal is observed. However, desorption signal of mass 18 is unambiguously detected, which suggests the abstraction of lattice oxygen by forming H2O occurs. Figure 2c shows the TPD spectra of mass 18 with various illumination durations. The magnitude of mass 18 signal increases with the UV irradiation, and the peak shifts downward. This line-shape progression together with the desorption temperature region resembles that of recombination desorption of bridging hydroxyls on rutile (110)-(1×1) with different densities.43−45 In analogy with the results on (110)-(1×1), the conclusion that adjacent surface hydroxyls (O2cH) on TiO2(011)-(2×1) recombine into water at elevated temperature can be drawn. This means that produced hydrogens during photocatalyzed oxidation of methanol on TiO2(011)-(2×1) transfer to the O2c sites, probably top O2c. Another two reaction channels of the hydrogen atoms adsorbed on (011)-(2×1), i.e., diffusion into the bulk at elevated temperature and recombination as molecular hydrogen under UV illumination, have also been proposed.39 However, identifying these two channels is not possible using the experimental methods in the present work. C
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reactions.11,18 The discrepancy probably comes from the differences of the TiO2 structure in these studies. Measurements in ref 11 and 18 were performed in solution where the characterization of TiO2 structure is difficult. In the present work where the structure of TiO2 are well unraveled, what are the underlying factors of the photoreactivity difference on these two TiO2 surfaces? Defects have been reported to accelerate the photocatalyzed methanol dissociation on TiO2.46 To compare the defect density on rutile (110)-(1×1) and (011)-(2×1) used in the above TPD experiments, two-photon photoemission spectroscopy (2PPE) for these two clean TiO2 surfaces were acquired (Figure S5, Supporting Information). The work function of both surfaces are similar according to the 2PPE spectra, suggesting the same reduction extent of these two TiO2 substrates.47 This means the two substrates contains similar concentration of defects. Therefore, the effect of defects could be ruled out. Conventionally, the effect of surface atomic structure on the activity of a crystal in heterogeneous reactions is greatly related to the percentage of under-coordinated atoms. The higher the percentage of undercoordinated surface atoms, the more reactive the crystal is. In TiO2 photocatalysis, the active centers are Ti5c sites. On (011)-(2×1) surface, all the titanium atoms are 5-fold coordinated, whereas on (110)-(1×1), only half of them are undercoordinated. If the number of the 5-fold coordinated titanium atoms is the key factor, the (011)-(2×1) surface should be more reactive than (110)-(1×1). However, this is obviously not the case here. In fact, the violation of the conventionally accepted surface atomic structure dependence of the photoreactivity has also been reported on anatase surfaces.48 Electronic structure might be another factor, which affects the photoreactivity of TiO2.48 In an ultraviolet photoemission spectroscopy (UPS) study of the valence and band gap structure of TiO2(011)-(2×1) and (110)-(1×1), binding energy of the band gap states of the former is 0.34 eV higher than the latter. It is thus expected that the electron trapping and electron−hole separation on (011)-(2×1) surface is more efficient.22 And this postulation has been tentatively used to account for the higher photoreactivity of (011) surface toward oxidation reactions. It should be noted these photocatalyic reactions were performed in aqueous environment which could even reverse the stability of the restructured and unrestructured surfaces.23 Therefore, it is never too careful to explain the photoreactivity in solutions according to the surface structure determined in ultrahigh vacuum (UHV). By contrast, adsorption of submonolayer of methanol on TiO2 surface will not change the surface structure much.49,50 As a result, trying to
Further reaction products have also been measured. Very weak TPD signal of methyl formate has been detected with UV illumination for more than 60 min. This is similar to the case on rutile (110)-(1×1), where further coupling of formaldehyde and methoxy to produce methyl formate has been reported.27−29 Therefore, the photocatalyzed oxidation of methanol on rutile (011)-(2×1) is similar to that on (110)(1×1). Although the chemical products of photocatalyzed oxidation of methanol on TiO2(011)-(2×1) and (110)-(1×1) are the same, the reaction rate on these two surfaces differs from each other. Figure 3 shows the normalized methanol signal on these
Figure 3. Normalized methanol desorption signal as a function of illumination time at the coverage of 0.67 ML methanol on TiO2(011)(2×1) (top) and (110)-(1×1) (bottom), respectively. A single exponential function was used to fit the experimental data. The decay time constant (τ) reflects the reaction rate of photocatalyzed oxidation of methanol these two surfaces.
two surfaces as a function of illumination time. Single exponential fitting of the data yields the decay time constants of 79.7 and 33.3 min on (011)-(2×1) and (110)-(1×1), respectively. The reaction on (110)-(1×1) is about 2.4 times of that on (011)-(2×1), suggesting (110)-(1×1) is relatively reactive toward photocatalyzed oxidation of methanol. This result is at variance with previous reports that TiO2(011) is more reactive than TiO2(110) toward photocatalyzed oxidation
Figure 4. Top and side views of optimized adsorption structures for (a) two CH3OH, (b) CH3OH and CH3O, (c) two CH3O, (d) CH3O and H2CO with the dissociated methyl hydrogen on a bridge O2c site and (e) CH3O and H2CO with all the dissociated hydrogens on top O2c sites on the TiO2(011)-(2×1) surface. In order to show clearly, the atoms in the surface layer are shown in balls, and the atoms in the lower layers are shown in bonds. The red, pink, green, and white balls represent the O, Ti, C, and H atoms, respectively. D
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The Journal of Physical Chemistry C establish the relationship between TiO2 photoreactivity in the present work and the electronic structure measured in UHV22 is reasonable. According to the ability of electron trapping, (011)-(2×1) should be more reactive than (110)-(1×1). Nevertheless, this is clearly not the case either. Besides the percentage of undercoordinated surface atoms and electronic structure, surface atomic configuration is also a very important factor that affects the reactivity of a crystal surface. To unravel the role of surface structure in TiO2 photoreactivity, potential energy profiles of methanol oxidation into formaldehyde on both TiO2(110)-(1×1) and (011)-(2×1) have been investigated by first-principle calculations. The oxidation of methanol into formaldehyde comprises of two steps, i.e., the cleavage of O−H and C−H bonds. In a most recent study on (110) surface, the barrier of these two cleavage processes are calculated to be 0.35 and 1.76 eV, respectively, suggesting dissociation of the C−H bond is the ratedetermining step.51 This is consistent with the previous TPD and DFT study.26 In order to understand why (011)-(2×1) exhibits a lower photoreactivity than TiO2(110)-(1×1), identical DFT calculations were performed on (011)-(2×1) to compare with those reported on (110)-(1×1).51 We have investigated all the possible configurations, and the optimized structures of methanol, methoxy, and formaldehyde adsorb on TiO2 (011)-(2×1) are shown in Figure 4. Two methanol molecules adsorb at the valley Ti5c sites and the hydrogen atoms from the hydroxyls are pointing to the nearby top O2c (Figure 4a). Adsorption geometries of methoxy are quite similar to those of methanol (Figure 4b,c). Formaldehyde also adsorbs at the valley Ti5c site, and its CH2 group points diagonally to the bridging O2c rows (Figure 4d). After the cleavage of O−H bonds of methanol and C−H bonds of methoxy, the hydrogen atoms are transferred to the nearby top O2c and bridge O2c sites, producing surface bridging hydroxyls. Followed by the determination of optimized adsorption configurations, the detailed reaction profiles of CH3OH oxidation into H2CO are further explored. Starting from two methanol molecules, the calculated potential energy profiles of the elementary reaction steps to generate formaldehyde are shown in Figure 5. The cleavage of O−H bond of the first CH3OH has to overcome an energy barrier of 0.13 eV. The atomic configuration of transition state (TS1) possesses a strong O2c−H bond (1.155 Å) and a weak O−H bond of CH3OH (1.307 Å), which is close to the product of CH3O. The reaction barrier of the O−H dissociation of the second methanol is 0.15 eV, and the configuration of TS2 is similar to TS1. The low barrier of O−H dissociation suggests that the production of CH3O from CH3OH should not be too difficult. When CH3O is further oxidized into H2CO, a methyl hydrogen atom is transferred to a bridge O2c site. The C−H scissor step needs to overcome an energy barrier of 1.96 eV. As has been reported, the hydroxyl at top O2c sites is the most stable, and the calculated barrier for a hydrogen atom to diffuse from a bridge O2c to a top O2c is about 0.57 eV (TS4, Figure 5), which agrees well with the previous value of 0.61 eV.39 This means the hydrogen atoms at bridge O2c might diffuse to top O2c (Figure 4e) during the heating process of the TPD experiments. The energy barrier calculations suggest C−H scission is the rate-determining step in the oxidation of methanol into formaldehyde on TiO2(011)-(2×1), as reported on TiO2(110)-(1×1).51 Interestingly, the reaction barrier of this elementary step, i.e., the dissociation of C−H bond of
Figure 5. Calculated potential energy profiles for CH3OH oxidation into H2CO on the TiO2(011)-(2×1) surface. The corresponding structures of the transition states in each step are shown as well. Note that the energy of each configuration is relative to the total energy of the initial configuration that two CH3OH molecules adsorb on the TiO2(011)-(2×1) surface. It should be noted that the TS4 corresponds to the diffusion process of one hydrogen from a bridge O2c to a top O2c. The red, pink, green, and white balls represent the O, Ti, C, and H atoms, respectively.
methanol, on (011)-(2×1) is by 0.2 eV higher than that on (110)-(1×1). By examining the optimized structure of methoxy on both TiO2 surfaces, the distance between the methyl hydrogen and nearest O2c is 3.418 Å on (011)-(2×1) and 3.124 Å on (110)-(1×1), respectively. This should be the reason why the reaction barrier of this rate-determining step on (011)(2×1) is higher than that on (110)-(1×1). This result suggests surface atomic configuration plays an essential role in the photocatalysis on TiO2 surfaces. Recently, the role of bulk charge transportation in the photoreactivity of TiO2 has been proposed.19 However, the charge mobility data along the (011) and (110) directions are not immediately available. Future efforts are needed to study the anisotropic charge transportation of TiO2 and other photocatalysts. Despite the uncertainties, our work demonstrates the importance of surface atomic configuration in the photoreactivity of TiO2.
4. CONCLUSIONS In summary, UHV based TPD and DFT calculations have been used to assess the photoreactivity of well characterized rutile (011)-(2×1) and (110)-(1×1) surfaces through photocatalyzed oxidation of methanol. Despite the same photocatalytic products on both surfaces, the reaction rate on (011)-(2×1) is only 42% of that on (110)-(1×1), which is at variance with previous studies where TiO2(011) was found to be more reactive than TiO2(110) toward photocatalyzed oxidation reactions in solution. The discrepancy probably comes from the differences of the TiO2 structure in these studies. Our DFT calculations suggest the rate-determining step on both surfaces is the cleavage of C−H, and the corresponding energy barrier on (011)-(2×1) is 0.2 eV higher than that on (110)-(1×1) due to the longer distance between methyl hydrogen and surface O2c. The present work not only demonstrates the importance of surface structure in the photoreactivity of TiO2, but also provides an example for building the correlation between E
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(11) Ohno, T.; Sarukawa, K.; Matsumura, M. Crystal Faces of Rutile and Anatase TiO2 Particles and Their Roles in Photocatalytic Reactions. New J. Chem. 2002, 26, 1167−1170. (12) Hotsenpiller, P. A. M.; Bolt, J. D.; Farneth, W. E.; Lowekamp, J. B.; Rohrer, G. S. Orientation Dependence of Photochemical Reactions on TiO2 Surfaces. J. Phys. Chem. B 1998, 102, 3216−3226. (13) Lowekamp, J. B.; Rohrer, G. S.; Hotsenpiller, P. A. M.; Bolt, J. D.; Farneth, W. E. Anisotropic Photochemical Reactivity of Bulk TiO2 Crystals. J. Phys. Chem. B 1998, 102, 7323−7327. (14) Sugiura, T.; Itoh, S.; Ooi, T.; Yoshida, T.; Kuroda, K.; Minoura, H. Evolution of a Skeleton Structured TiO2 Surface Consisting of Grain Boundaries. J. Electroanal. Chem. 1999, 473, 204−208. (15) Tsujiko, A.; Kisumi, T.; Magari, Y.; Murakoshi, K.; Nakato, Y. Selective Formation of Nanoholes with (100)-Face Walls by Photoetching of N-TiO2 (Rutile) Electrodes, Accompanied by Increases in Water-Oxidation Photocurrent. J. Phys. Chem. B 2000, 104, 4873−4879. (16) Ahmed, A. Y.; Kandiel, T. A.; Oekermann, T.; Bahnemann, D. Photocatalytic Activities of Different Well-Defined Single Crystal TiO2 Surfaces: Anatase Versus Rutile. J. Phys. Chem. Lett. 2011, 2, 2461− 2465. (17) Nakabayashi, Y.; Nosaka, Y. OH Radical Formation at Distinct Faces of Rutile TiO2 Crystal in the Procedure of Photoelectrochemical Water Oxidation. J. Phys. Chem. C 2013, 117, 23832−23839. (18) Takahashi, H.; Watanabe, R.; Miyauchi, Y.; Mizutani, G. Discovery of Deep and Shallow Trap States from Step Structures of Rutile TiO2 Vicinal Surfaces by Second Harmonic and Sum Frequency Generation Spectroscopy. J. Chem. Phys. 2011, 134, 154704. (19) Luttrell, T.; Halpegamage, S.; Tao, J.; Kramer, A.; Sutter, E.; Batzill, M. Why Is Anatase a Better Photocatalyst Than Rutile? Model Studies on Epitaxial TiO2 Films. Sci. Rep. 2014, 4, 4043. (20) Beck, T. J.; Klust, A.; Batzill, M.; Diebold, U.; Di Valentin, C.; Selloni, A. Surface Structure of TiO2(011)-(2×1). Phys. Rev. Lett. 2004, 93, 036104(1)−036104(4). (21) Torrelles, X.; Cabailh, G.; Lindsay, R.; Bikondoa, O.; Roy, J.; Zegenhagen, J.; Teobaldi, G.; Hofer, W. A.; Thornton, G. Geometric Structure of TiO2(011)-(2×1). Phys. Rev. Lett. 2008, 101, 185501. (22) Tao, J. G.; Batzill, M. Role of Surface Structure on the Charge Trapping in Tio2 Photocatalysts. J. Phys. Chem. Lett. 2010, 1, 3200− 3206. (23) Aschauer, U.; Selloni, A. Structure of the Rutile TiO2(011) Surface in an Aqueous Environment. Phys. Rev. Lett. 2011, 106, 166102. (24) Zhou, C. Y.; et al. Site-Specific Photocatalytic Splitting of Methanol on TiO2(110). Chem. Sci. 2010, 1, 575−580. (25) 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. (26) 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. (27) 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. (28) 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. (29) Yuan, Q.; Wu, Z.; Jin, Y.; Xu, L.; Xiong, F.; Ma, Y.; Huang, W. Photocatalytic Cross-Coupling of Methanol and Formaldehyde on a Rutile TiO2(110) Surface. J. Am. Chem. Soc. 2013, 135, 5212−5219. (30) Zhou, C.; Ma, Z.; Ren, Z.; Wodtke, A. M.; Yang, X. Surface Photochemistry Probed by Two-Photon Photoemission Spectroscopy. Energy Environ. Sci. 2012, 5, 6833−6844. (31) Ren, Z. F.; Zhou, C. Y.; Ma, Z. B.; Xiao, C. L.; Mao, X. C.; Dai, D. X.; LaRue, J.; Cooper, R.; Wodtke, A. M.; Yang, X. M. A Surface Femtosecond Two-Photon Photoemission Spectrometer for Excited
structure and activity using the combination of surface science techniques and DFT calculations.
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ASSOCIATED CONTENT
S Supporting Information *
AES and LEED pattern of rutile TiO2(110)-(1×1) and (011)(2×1) surfaces. Adsorption behavior of methanol on rutile TiO2(110)-(1×1) and (011)-(2×1) surfaces. The illumination dependent TPD spectra of methanol, formaldehyde, water and methyl formate from 0.67 ML methanol covered TiO2(110)(1×1) surface. 2PPE spectra of rutile TiO2(110)-(1×1) and (011)-(2×1) surfaces. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Authors
*(C.Z.) E-mail:
[email protected]. *(L.-M.L.) E-mail:
[email protected]. *(X.Y.) E-mail:
[email protected]. Tel: 86-411-84695174. Fax: 86-411-84675584. Author Contributions #
These authors (X.M., Z.W., X.L.) contributed equally to this work. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We acknowledge the financial support from the National Natural Science Foundation of China (Grant No. 21203189, 51222212, 21303006 and 21321091), National Basic Research Program of China (Grant No. 2013CB834605) and the Key Research Program of the Chinese Academy of Science (KGZDEW-T05).
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