Structure and Reactivity of Methanol Adsorbed on Rutile TiO2(011

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C: Surfaces, Interfaces, Porous Materials, and Catalysis 2

Structure and Reactivity of Methanol Adsorbed on Rutile TiO(011) Surface Yanhong Liang, Xiufeng Lang, Ying Zhang, Jing Ji, TingTing You, Liqin Cao, Hongsheng Zhang, and Woon-Ming Lau J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b08399 • Publication Date (Web): 02 Oct 2018 Downloaded from http://pubs.acs.org on October 6, 2018

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The Journal of Physical Chemistry

Structure and Reactivity of Methanol Adsorbed on Rutile TiO2(011) Surface Yanhong Liang,a Xiufeng Lang,*,a Ying Zhang,a Jing Ji,a Tingting You,b Liqin Cao,c Hongsheng Zhang,c Woon-Ming Lau*,d a

Material Simulation and Computing Laboratory, Department of Physics, Hebei Normal University of Science & Technology, Qinghuangdao, 066004, China b School of Physics and Nuclear Energy Engineering, Beihang University, Beijing 100191, China c College of Environment and Chemical Engineering, Yanshan University, Qinghuangdao, 066004, China d Center for Green Innovation, School of Mathematics and Physcis, University of Science & Technology Beijing, Beijing 10083, China Email: [email protected], [email protected] Abstract: The rutile TiO2(011) suface exhibit a (2×1) reconstruction when prepared by standard techniques in ultrahigh vacuum. However, the reconstructed surface is incomplete and discussed controversially for understanding the enhanced photoactivity occurred in most photocatalytic experiments. Herein, we investigated the reactivity of unreconstructed TiO2(011)-(1×1) and reconstructed TiO2(011)-(2×1) surfaces toward methanol adsorption and oxidation using first-principles calculations. The TiO2(011)-(1×1) surface is preferred energetically over the TiO2(011)-(2×1) surface at high methanol coverage, confirming that the interaction with methanol leads to an inversion of the stabilities of the reconstructed and unreconstructed surfaces. Then, we intensively investigated methanol oxidation into formaldehyde on the TiO2(011)-(1×1) surface and found that the methanol oxidation underwent the same pathways as those on the TiO2(011)-(2×1) and TiO2(110) surfaces. By means of the energy barriers of the rate-determining step (i.e., C-H cleavage), it is demonstrated that the unreconstructed TiO2(011) surface exhibits higher reactivity toward methanol oxidation than the reconstructed TiO2(011) and TiO2(110) surfaces. In addition, the calculated DOS further indicates that the TiO2(011) surface has stronger oxidative ability than TiO2(110) for photocatalysis. Thus, our findings suggest that synergistic effect of surface atomic and electronic structures may plays an important role in the enhanced photocatalytic activity of TiO2(011) observed in aqueous environments typical of most photocatalytic application of TiO2.

1. Introduction TiO2 has become one of the most intensively studied photochemical materials due to its applications for the remediation of pollutants and solar energy conversion.1-3 Its surfaces usually play a key role in the photocatalytic properties and applications of this material.4-6 On rutile TiO2, face-dependent photoactivity has been observed in

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most photocatalytic experiments,7-9 where the TiO2(011) surface exhibited a stronger activity toward oxidation reactions than the TiO2 (110) surface. This lead to numerous studies of the detailed atomic scale geometries of TiO2(011) surface.4, 10 The origin of such face dependence is not fully explained, although the effects of surface atomic and electronic structures on the photoactivity of the TiO2(011) surface has been extensively explored.11-15 For instance, the TiO2(011) was resolved to exhibit (2×1) reconstruction in vacuum by surface x-ray diffraction (XRD), scanning tunneling microscopy (STM) measurements,11-14, 16 and first-principles calculations.13, 17-18 The surface structure, suggested by Torreles et al.,13 was demonstrated theoretically to be the most stable and well reproduce the STM images observed in experiments. However, such surface structure was reported to exhibit the weaker activity toward methanol oxidation than the TiO2(110) surface, supported by an recent experiment carried out in ultrahigh vacuum.9 It has been reported that adsorbate (acetic acid, water, or methanol) induced restructuring of the reconstructed TiO2(011)-(2×1) to form one-dimensional nanocluster,19 and the interaction of the (011) surface with water caused a reverse transformation from TiO2(011)-(2×1) structure to the unreconstructed TiO2(011)-(1×1) structure as posed to liquid water.20-21 Thus, It is essential to intensively investigate the structure and reactivity of the TiO2(011)-(1×1) surface to explain the photoreactivity of the TiO2(011) surface in the aqueous environment typical of most photocatalytic application of TiO2. Methanol is a simple prototype for many organic compounds, and its photo-oxidation is usually chosen to simulate the photo-degradation of organic pollutants.22-24 The Methanol/TiO2(110) system has been investigated in a number of pervious works.24-25 However, to the best of our knowledge, the interaction of methanol of the TiO2(011) was rarely reported. Until recently, the photocatalytic chemistry of methanol on TiO2(011)-(2×1) was just investigated by temperature-programmed desportion (TPD) measurement in vacuum.9, 26 In this work, methanol adsorption and oxidation into formaldehyde were studied systematically on the TiO2(011)-(1×1) and TiO2(011)-(2×1) surfaces by using first-principles calculations. Upon methanol adsorption, an inversion of the stabilities between two surfaces occurs as the methanol coverage increases up to 1/2 monolayer (ML), facilitating the existence of the unreconstructed TiO2(011) surface. Furthermore, we investigated the reaction mechanism of methanol oxidation into formaldehyde on the TiO2(011)-(1×1) surface, and compared the reactive activities of the TiO2(011)-(1×1), TiO2(011)-(2×1) and TiO2(110) surfaces, identifying the enhanced oxidation activity of the TiO2(011)-(1×1) surface.

2. Computational details Electronic structure calculations were performed within the framework of density functional theory (DFT) as implemented in the Vienna Ab Initio Simulation Program (VASP), a plane-wave pseudopotential package.27 The exchange and correlation energies were calculated using the Perdew-Burke-Ernzehof (PBE) functional within the generalized gradient approximation (GGA).28 The projector augmented wave (PAW)29 method was employed to treat valence-core interactions with twelve, six, one


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and four valence electrons for Ti, O, H, and C, respectively. Starting with the experimental lattice parameters a = 4.594Å and c = 2.959 Å, we obtained a = 4.594 Å and c = 2.957 Å after full relaxation of the unit cell. A 400 eV were used for cutoff energy. The (2×2) unit cells were used for examining adsorption of methanol on TiO2(011)-(1×1) and TiO2(011)-(2×1) surfaces. The surface slab contains four O-Ti-O trilayers and a 15Å thick vacuum layer. The relaxation of the atoms was carried out until the force on any atom was less than 0.03 eV·Å-1. The criterion for the total energy was set as 1 × 10-5 eV, and the k-point sampling was restricted to the 2×2×1. The (4×2) unit cells were used for examining the methanol oxidation pathways on the TiO2(011)-(1×1) surfaces in order to compare with the previously reported results on the TiO2(110)25 and TiO2(011)-(2×1) surfaces9. The k-point sampling was restricted to 1×1×1. The methanol and the three top O-Ti-O trilayers were allowed to relax in all directions, with the bottom O-Ti-O trilayer fixed at the optimized bulk position. On the basis of the optimized structures for the initial and final states in the oxidation step of methanol, minimum-energy paths and corresponding activation barriers were calculated using the climbing image nudged elastic band approach (CI-NEB).30-31 The minimum energy path is examined with eight or nine images, including the initial and final states, during the transition state search.

3. Results and Discussions 3.1 Methanol adsorption on the TiO2(011) surfaces We firstly investigate relative stability of unreconstructed TiO2(011)-(1×1) and reconstructed TiO2(011)-(2×1) surfaces in Figure 1. The TiO2(011)-(1×1) surface in Figure 1a shows an undulated profile with exposed twofold oxygens (O2c) at the apices, fivefold Ti atoms (Ti5c) at each of sides, and the threefold O (O3c) at the valleys. As reported before,13 the TiO2(011)-(2×1) surface in Figure 1b is characterized by a corrugated profile with alternating valleys and ridges. There are inequivalent O2c atoms on top of the ridges (O2c-top) and forming ridge-valley bridges (O2c-bridge), and inequivalent Ti5c atoms on the ridges (Ti5c-ridge) and at the bottom of the valleys (Ti5c-valley). The relative energy ∆Erel between two surfaces in Table 1 shows that the reconstructed surface is 0.68 eV lower in total energy than the unreconstructed surface, indicating the better stability of the reconstructed surface. This is in consistent with the previous report that in the ultrahigh vacuum, the TiO2(011)-(2×1) surface had lower surface energy and better stability than the TiO2(011)-(1×1) surface.13, 20



Figure 1. Top views of TiO2(011)-(1×1) and TiO2(011)-(2×1) surfaces are shown in (a) and (b), respectively. The corresponding adsorption structures of 1/2 ML methanol on both surfaces are shown in (c) and (d), respectively. In order to show clearly, the atoms in the surface layers are shown in balls, and the atoms in the lower layers are shown in bonds. H, Ti and O atoms are represented by blue, light blue, and red spheres, respectively. Based on the above surface configurations, we then studied the possible adsorption configurations of 1/2 monolayer (ML) methanol on these surfaces. Herein, 1 ML methanol stands for eight methanol molecules adsorbed at all Ti5c sites on the unreconstructed or reconstructed TiO2(011) surface. To compare thermodynamic stabilities of these adsorption structures, the adsorption energy per methanol molecule is defined as Eads = [E(slab+CH3OH)-E(slab)-n*E(CH3OH)]/n, where E(slab+CH3OH) and E(slab) are the energies of the slab with and without the methanol molecules, respectively, E(CH3OH) is the energy of a gas-phase methanol molecule, and n is the number of the adsorbed methanol. A more negative Eads corresponds to a more stable structure. Figures 1c and 1d show the most stable adsorption structures of 1/2 ML methanol on the TiO2(011)-(1×1) and TiO2(011)-(2×1) surfaces, respectively. All the possible adsorption structures of 1/2 ML methanol on TiO2(011) surfaces are shown in Figure S1 of the Supporting information. In Figure 1c, four methanol molecules adsorb at the Ti5c sites and the hydrogen atoms from the hydroxyls are separately pointing to the nearby O2c atoms. In Figure 1d, all methanol molecules adsorb at the Ti5c-valley sites, and hydrogen atoms from the hydroxyls are pointing to the O2c-top and the O2c-bridge atoms, respectively. Table 1 shows that the TiO2(011)-(1×1) has a more negative Eads than that of the TiO2(011)-(2×1), indicating that the


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TiO2(011)-(1×1) exhibit much higher reactive than the TiO2(011)-(2×1), consistent with its higher surface energy, i.e., lower stability in vacuo. More intriguing is, however, the total energy of the TiO2(011)-(1×1) is about 0.4 eV lower than the TiO2(011)-(2×1) after methanol adsorption, as illustrated by ∆Erel in Table 1. Thus, the interaction with methanol can lead to an inversion of the stabilities of the TiO2(011)-(1×1) and TiO2(011)-(2×1) surfaces. Table 1. Adsorption energies (Eads), relative adsorption energies (∆Eads) and relative energies (∆Erel) of molecular methanol on the unreconstructed TiO2(011)-(1×1) and reconstructed TiO2(011)-(2×1) surfaces. Methanol Coverage

Unreconstructed Eads (eV)

(2×1) reconstructed Eads (eV)

∆Eadsa (eV)

0 ML

∆Erelb (eV) -0.68

1/8 ML

-1.22

-1.21

0.01

-0.69

1/4 ML

-1.17

-0.97

0.20

-0.28

1/2 ML

-1.04

-0.77

0.27

0.42

3/4 ML

-0.86

-0.61

0.25

0.83

1 ML

-0.69

-0.54

0.15

0.53

a

The relative adsorption energy (∆Eads) is defined as difference in the adsorption energies of the reconstructed and unreconstructed surfaces; i.e., ∆Eads=Eads(TiO2(011)-(2×1)) – Eads(TiO2(011)-(1×1)) b The relative energy (∆Erel) is defined as difference in total energies of the reconstructed and unreconstructed surfaces; i.e., ∆Erel = E (TiO2(011)-(2×1)) – E (TiO2(011)-(1×1)).

We further studied the effect of methanol coverage on the absorption ability of TiO2(011)-(1×1) and TiO2(011)-(2×1) surfaces and their relative stability. Figures S2-S4 of the Supporting information shows the possible adsorption structures of 1/8, 1/4, 3/4 and 1 ML methanol on both TiO2(011) surfaces. As shown in Figures S1, a methanol prefers to stay on the Ti5c-ridge site rather than the Ti5c-valley site of the TiO2(011)-(2×1), and this structure exhibits the similar adsorption ability to that on the TiO2(011)-(1×1) surface, as demonstrated by the Eads. This is different from the report about water adsorption on the TiO2(011) surfaces, where the TiO2(011)-(1×1) exhibit much stronger adsorption ability for a water than the TiO2(011)-(2×1).20 With the methanol coverage increasing up to 1/4, 1/2, 3/4 and 1 ML, methanol adsorbed at Ti5c-valley sites become more favorable, and their Eads is more positive than that of the TiO2(011)-(1×1) surface, clarifying the relatively weak adsorption ability of the TiO2(011)-(2×1) surface (see Figures S2 and S4). It is worth mentioning that, it is possible to adsorb a full monolayer of methanol on the TiO2(011)-(1×1) surface, while, only a half-monolayer of methanol binds directly to surface Ti5c sites on the TiO2(011)-(2×1) surface.



Figure 2. Black rectangles shows the relative adsorption energies between the (2×1) reconstructed and unreconstructed (011) surfaces as a function of methanol coverage. Black circles represents the relative energy between (2×1) reconstructed and unreconstructed (011) surfaces with methanol molecules. Figure 2 concretely depicts the relative adsorption energies (∆Eads) and the relative energy (∆Erel) of the most stable adsorption structures on both TiO2(011) surfaces as the function of the methanol coverage and the corresponding data are collected in Table 1. For 1/8 ML methanol coverage, the ∆Eads are close to 0 and the ∆Erel is negative, which indicates that the TiO2(011)-(1×1) surface has similar reactivity and worse stability with respect to the TiO2(011)-(2×1) surface. As the methanol coverage increases up to 1/4 ML, the ∆Eads becomes positive and the ∆Erel is still negative, clarifying better reactivity and worse stability of TiO2(011)-(1×1). When the methanol coverage continues to increase up to 1/2 ML, both the ∆Eads and ∆Erel become positive, indicating the better reactivity and stability of TiO2(011)-(2×1). Similar phenomena also occur in the case of 3/4 ML or 1 ML methanol coverage. These results indicate that the TiO2(011)-(2×1) appears in vacuum and low methanol coverage, while the TiO2(011)-(1×1) surface is energetically preferred in high methanol coverage That is, the relative stability between TiO2(011)-(2×1) and TiO2(011)-(1×1) reverses after methanol adsorption. The experiment carried out in vacuum has demonstrated that TiO2(011) surface could exhibit the (2×1) reconstruction at high methanol coverage.9 This suggests that in vacuum, methanol-induced transformation fromt the (2×1) reconstructed surface into the (1×1) unreconstructed surface possibly needs to overcome a large energy barrier. Such change in the relative stability is closely related to the relative strong reactivity of TiO2(011)-(1×1) surface as compare to the TiO2(011)-(2×1) surface when adsorbing methanol. As shown in Figure 2, ∆Eads is always positive as the methanol coverage increases from 1/8 ML to 1 ML, demonstrating the stronger adsorption ability of TiO2(011)-(1×1) surface than the TiO2(011)-(2×1) surface. The difference in the adsorption ability of two surfaces could be explained by the electronic structures


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of two surfaces with methanol. As shown in Figures S5-S7 of the supporting information, the TiO2(011)-(1×1) always has the valence hand maximum at higher energy level than the TiO2(011)-(2×1) in the absence or presence of methanol. This leads to better reactivity of the TiO2(011)-(1×1) surface to adsorb methanol with respect to the TiO2(011)-(2×1) surface. it is clearly seen from Figure 2 that although the ∆Eads of two surfaces go through a maximum with methanol coverage increasing, the difference in the valence band maximum of two surfaces gradually decrease as the methanol increases, as shown in Figures S5-S7. This indicates that the adsorption ability of the TiO2(011) surfaces may be modulated by both their electronic band and surface atomic structures. In addition, the adsorbed methanol always has higher energy level on the TiO2(011)-(1×1) surface than on the TiO2(011)-(2×1) surface in Figure S5-S7 of the supporting information, which implies that the methanol is more reactive to dissociate on the TiO2(011)-(1×1) surface and the dissociative methanol possibly trap the photo-excited holes more easily to improve the photo-oxidation of methanol on the TiO2(011)-(1×1) surface, as discussed below. 3.2 Methanol oxidation into formaldehyde on the TiO2(011)-(1×1) surfaces It is well established that TiO2(011) surface is more reactive than TiO2(110) surface toward photocatalyzed oxidation reactions in aqueous environments. However, origin of the enhanced photooxidation activity of TiO2(011) surface remains unclear, due to its vague surface structures as exposed to the aqueous surroundings. Very recently, It has been demonstrated that the TiO2(011)-(2×1) surface deconstructed into the TiO2(011)-(1×1) surface as opposed to liquid water.20-21 Herein, we investigate and compare the methanol oxidation on TiO2(011)-(1×1), TiO2(011)-(2×1) and TiO2(011) surfaces to further understand face-dependent photocatalytic activity of TiO2(011) surface. As expected, oxidation pathways of CH3OH on TiO2(011)-(1×1) is the same as those on both TiO2(110) and TiO2(011)-(2×1) surfaces, which includes the pathways of CH3OH dissociation and CH3O oxidation. Figure 3 shows the optimized structures of CH3OH, CH3O, or H2CO adsorbed on the TiO2(011)-(1×1) surface. Two CH3OH molecules adsorb at the Ti5c sites and the hydrogen atoms from the hydroxyls are pointing to the nearby O2c atom (see Figure 3a), resembling the adsorption configurations of 1/4 ML methanol in Figure S3a of the Supporting information. Firstly, one CH3OH molecule transfers one hydrogen atom to one nearby O2c atom to obtain the structure of the mixed CH3OH and CH3O in Figures 3b. Sequently, the other also transfers one hydrogen atom to its nearby O2c atom to obtain the structure of the adsorbed CH3O in Figure 3c. Then, one of the CH3O molecules transfers one hydrogen atom to the surface O2c atom to form the adsorption structures of the mixed CH3O and H2CO in Figure 3d. The calculated Eads value for the CH3OH molecules is -0.98 eV, while the Eads values for mixed CH3OH and CH3O or pure CH3O molecules become -1.03 eV or -1.06 eV. The latter is about 0.04 eV smaller than the former, which indicates that the dissociation of CH3OH on the TiO2(011)-(1×1) surface is energetically favored. This is different from the CH3OH dissociation on the TiO2(110) and TiO2(011)-(2×1) surfaces where the Eads of CH3O is larger than that of CH3OH, demonstrating the thermodynamically infeasible dissociation of CH3OH.9, 25 As the



CH3O is further dissociated into H2CO, the Eads increases up to -0.32 eV for H2CO, demonstrating that the oxidation of CH3O into H2CO is energetically infeasible.

Figure 3. Top and side views of optimized adsorption structures for (a) two CH3OH, (b) mixed CH3OH and CH3O, (c) two CH3O, and (d) mixed CH3O and H2CO on the TiO2(011)-(1×1) surfaces. In order to show clearly, the atoms in the surface layers are shown in balls, and the atoms in the lower layers are shown in bonds. The red, light blue and blue balls represent the O, H and Ti atoms, respectively. Followed by the determination of optimized adsorption configurations, the detailed reaction profiles of CH3OH oxidation into H2CO on the TiO2(011)-(1×1) surface are further explored. Starting from two CH3OH molecules, the calculated potential energy profiles of the elementary reaction steps to generate H2CO are shown in Figure 4. The potential energy profiles of CH3OH oxidation on the TiO2(110) and TiO2(011)-(2×1) surfaces are also replotted in Figure 4 for comparison. The corresponding energy barriers of all elementary steps on three surfaces are collected in Table 2. CH3OH oxidation into H2CO undergoes three elementary steps on TiO2(011)-(1×1) surface, including two dissociation steps from CH3OH to CH3O and an oxidation step from CH3O to H2CO. The hydrogen atoms are firstly transferred from hydroxyls of the CH3OH molecules to the surface O2c sites to produce the CH3O molecules. Both cleavage steps of O-H bond have no energy barriers, indicating that the methanol dissociation can spontaneously proceed on the unreconstructed surface. Then, a methyl hydrogen atom of CH3O is transferred to a surface O2c site to produce H2CO. This is clearly clarified by the transition state (TS) structure in the inset of Figure 4. In this structure the distance between H and O2c is 0.979 Å, much shorter than the distance between H and C (2.243 Å), indicating a transfer trend of the hydrogen atom from the C atom to the O2c atom. This step needs to overcome an energy barrier of 1.67 eV. Thus, C-H scissor is the rate-determining step in the whole oxidation reaction of CH3OH into H2CO on the TiO2(011)-(1×1) surface. Similar rate-determining step was also reported for methanol oxidation on the TiO2(011)-(2×1) and TiO2(110) surfaces, as shown in Figure 4. We also investigated the effect of the adsorbed CH3O on the energy barrier of the C-H scissor step on the TiO2(011)-(1×1)


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surface with one adsorbed CH3O. The calculated energy barrier is 1.80 eV, a bit larger than the aforementioned energy barrier (1.67 eV) on the same surface. Such difference can be attributed to the various distances between the methyl H atom and the bonded O2c atom, 3.105 Å versus 3.057 Å. Thus, the adsorbed CH3O can make the surface O2c atoms up-shift to facilitate the oxidation of other adsorbed CH3O on the TiO2(011)-(1×1) surface.

Figure 4. Calculated potential energy profiles for CH3OH oxidation into H2CO on the TiO2(011)-(1×1) surface. The corresponding structure of the transition state in the cleavage step of C-H is shown as well. The red, light blue, blue balls represent the O, Ti and H atoms, respectively. Note that the potential energy profiles for CH3OH oxidation on the TiO2(110) and TiO2(011)-(2×1) are obtained from our previous work in Refs. [9] and [25] for comparison. The red, blue and black lines represent the energy of each configuration on the TiO2(011)-(1×1), TiO2(110) and TiO2(011)-(2×1) surfaces, respectively. The energy of each configuration on one surface is aligned by the total energy of the initial configuration that two CH3OH molecules adsorb on the surface. Table 2. Calculated energy barriers for reaction pathways for methanol oxidation into formaldehyde on the the unreconstructed TiO2(011)-(1×1), reconstructed TiO2(011)-(2×1) and TiO2(110) surfaces.

CH3OH + Obr → CH3O + ObrH

CH3O + Obr → H2CO + ObrH a

TiO2(011)-(1×1) EBa (eV)

TiO2(011)-(2×1) EBa (eV)

TiO2(011) EBa (eV)

0

0.13

0.35

0

0.15

0.49

1.67

1.96

1.76

The energy barrier (EB) is defined as the difference between the energy of the transition



state (ETS) and the energy of the initial state (Ei); i.e., EB = ETS – Ei.

To clarify the effects of surface structure on the reactive activity of TiO2, we compare the kinetics of CH3O oxidation on the TiO2(011)-(1×1), TiO2(011)-(2×1) and TiO2(110) surface. Table 2 lists the energy barriers of three elementary steps in methanol oxidation on three surfaces. The energy barriers of the C-H scissor step, on the TiO2(011)-(1×1) surface (1.67 eV) is smaller than that on the TiO2(110) surface (1.76 eV) and the reconstructed TiO2(011)-(2×1) surface (1.96 eV). This indicates that the reaction rate on the TiO2(011)-(1×1) surface is faster than that on the TiO2(110) and TiO2(011)-(2×1) surfaces. Such difference in the energy barriers may be attributed to various atomic structures of three surfaces, where the distances between the methyl H atom and the bonded O2c atom are separately 3.057 Å, 3.124 Å and 3.418 Å for the TiO2(011)-(1×1), TiO2(110) and TiO2(011)-(2×1) surfaces. Furthermore, due to the close energy barrier of C-H scissor step on both surfaces, the preference between the TiO2(011)-(1×1) and TiO2(110) for the whole oxidation of CH3OH into H2CO will be dictated by the CH3OH dissociation. As shown in Figure 4, the CH3OH dissociation is exothermic without energy barrier on the TiO2(011)-(1×1) surface and is endoergic along with a large energy barrier on the TiO2(110) surface. Thus, from both thermodynamic and kinetic points of view, the CH3OH oxidation prefers to occur on the TiO2(011)-(1×1) surfaces. Moreover, it has been suggested that methanol thermally dissociates into methoxy on TiO2, and the methoxy, rather molecular methanol, is the effective hole scavenger in photochemical reactions of methanol on TiO2.32 Thus, the barrierless dissociation of methanol into methoxy on the TiO2(011)-(1×1) surface may significantly improve photo-oxidation activity of methanol as compared to the TiO2(110) and TiO2(011)-(2×1) surfaces. It has been suggested that the photocatalytic reactivity of a crystal facet should be related to both its surface atomic structure and surface electronic band structure.5, 33 Herein, we study the electronic properties of the clean TiO2(011)-(1×1), TiO2(110) and the TiO2(011)-(2×1) surfaces to further explore the photo-oxidation activity of three surfaces. Figure 5 shows density of states of these surfaces. It is clearly seen from the figure that the valence band maximums of both the TiO2(011)-(2×1) and the TiO2(011)-(1×1) surfaces are more negative than that on the TiO2(110) surface. This demonstrates that the photo-excited holes on TiO2(011) surface could have stronger oxidation ability than those on the TiO2(110) surface. We may analyze the determining factors for the photo-oxidation order of these surfaces by considering the cooperative effects of surface atomic coordination and electronic band structure. TiO2(011)-(1×1) and TiO2(110) would exhibit a higher photoreactivity than TiO2(011)-(2×1) in terms of activities of surface Ti5c atoms. Conversely, TiO2(011)-(1×1) and TiO2(011)-(2×1) should have superior photoreactivity to TiO2(110) by means of the oxidative activity of photoexcited holes. Consequently, TiO2(011)-(1×1) have both favorable surface atomic and electronic structures, and thus exhibits relatively high photo-oxidation activity as compared to other surfaces. These result further confirm the TiO2(011) possibly exhibits the unreconstructed surface structure and thus show enhanced activity of TiO2(011) surface toward the


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photo-oxidation reactions in the aqueous environment typical of most photocatalytic application of TiO2.

Figure 5. The red, blue and black lines represent the total densities of states of the clean TiO2(011)-(1×1), TiO2(011)-(2×1) and TiO2(011) surfaces, respectively. Energy scales have been aligned at the vacuum level (energy zero).

4. Conclusion In this work, we have studied the structure and oxidation of methanol adsorbed on the reconstructed TiO2(011)-(2×1) and unreconstructed TiO2(011)-(1×1) surfaces to investigate effects of surface structures on the reactivity of the rutile TiO2(011) surfaces. Due to the relatively strong adsorption ability of the TiO2(011)-(1×1) surface, the less stable TiO2(011)-(1×1) occuring in vacuo become more stable than the TiO2(011)-(2×1) as methanol coverage increase up to 1/2 monolayer. This means that interaction with methanol can lead to an inversion of the stabilities of the TiO2(011)-(2×1) and TiO2(011)-(1×1) surfaces. Based on the adsorption structures, we further investigated the mechanism of methanol oxidation into formaldehyde on the TiO2(011)-(1×1) surface and compared the oxidation activities of methanol on TiO2(011)-(1×1), TiO2(011)-(2×1) and TiO2(110) surfaces. Similar elementary steps happen for methanol oxidation on three surfaces. The calculated energy barriers demonstrate that the rate-determining steps on these surfaces are the cleavage of C-H. Moreover, the energy barrier of the cleavage of C-H on the TiO2(011)-(1×1) surface is lower than those on the TiO2(110) and TiO2(011)-(2×1) surface, demonstrating the higher reactivity of the unreconstructed (011) surface. In addition, the density of states of three clean surfaces clearly show the stronger oxidation activity of the photo-excited hole on the TiO2(011) surface with respect to the TiO2(110) surface. By taking atomic and electronic structures of the surfaces into consideration, it is plausible that the unreconstructed TiO2(011) surface has enhanced photo-oxidation ability than the TiO2(110) surface. Our findings may explain the higher photocatalytic



activity of TiO2(011) with respect to the TiO2(110) surface in the most photocatalytic experiments carried in the aqueous environments. We expect that these results give new insight into the adsorbates-induced surface reconstruction and face-dependent photoactivity of TiO2.

Supporting Information Figure S1. Adsorption structures of 1/2 ML methanol on TiO2(011)-(1×1) and TiO2(011)-(2×1) surfaces. Figure S2. Adsorption structures of 1/8 ML methanol on TiO2(011)-(1×1) and TiO2(011)-(2×1) surfaces. Figure S3. Adsorption structures of 1/4 ML methanol on TiO2(011)-(1×1) and TiO2(011)-(2×1) surfaces. Figure S4. Adsorption structures of 3/4 ML and 1 ML methanol on TiO2(011)-(1×1) and TiO2(011)-(2×1) surfaces. Figure S5. Density of states (DOS) of the TiO2(011)-(1×1) and TiO2(011)-(2×1) surfaces with 1/8 ML methanol as well as the clean surfaces. Figure S6. Density of states (DOS) of the TiO2(011)-(1×1) and TiO2(011)-(2×1) surfaces with 1/2 ML methanol as well as the clean surfaces. Figure S7. Density of states (DOS) of the TiO2(011)-(1×1) and TiO2(011)-(2×1) surfaces with 1 ML methanol as well as the clean surfaces.

Acknowledge This work was supported by the National Natural Science Foundation of China (21303006) and Hebei Natural Science foundation of China (B2017407009).

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Structure and Reactivity of Methanol Adsorbed on Rutile TiO2(011

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