Reactivity of the Defective Rutile TiO2 (110) - American Chemical

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Reactivity of the Defective Rutile TiO2 (110) Surfaces with Two Bridging-Oxygen Vacancies: Water Molecule as a Probe Hui Shi, Ying-Chun Liu, Zhi-Jian Zhao,† Meng Miao, Tao Wu,* and Qi Wang* Department of Chemistry and Soft Matter Research Center, Zhejiang University, 38 ZheDa Road, YuQuan Campus , Hangzhou 310027, P. R. China S Supporting Information *

ABSTRACT: Defective rutile TiO2 (110) surfaces with one bridging-oxygen vacancy pair (OVP) and two next nearest neighbored bridging-oxygen vacancies belonging to the same row (NNN-OVs, i.e., two bridging-oxygen vacancies separated by a single oxygen atom) were studied using density functional theory (DFT) calculations. The results of a perfect surface and a defective surface with single bridging-oxygen vacancy (OV) were also shown. The reactivity of these surfaces was investigated by studying their interaction with a water molecule. Results show the NNN-OVs site is the most favorable site for water adsorption of two modes, molecular and dissociated adsorption, especially for dissociated adsorption. Upon dissociated adsorption on the NNN-OVs site, the whole system would release energy of 2.07 eV, much more than the energy released in any other site. It indicates the high reactivity of NNN-OVs as the best trap center. The 5-fold Ti sites show similar behaviors despite the existence of different defects. Adsorption on this site is the least stable, and molecular adsorption is favored. A water molecule needs to overcome energy barriers of 0.25−0.27 eV to dissociate on 5-fold Ti atoms. However, the recombination barrier is even lower, and the fragments would recombine and exist stably in the molecular mode. Slightly higher barriers are observed on the defective sites. sites,10−14 while a recent study carried out by Zhou et al.6 showed that 5-fold Ti was an essential element of bond dissociation. A later study by Duncan et al.15 argued that water partially dissociated on the perfect surface. Another experimental work implemented by Wesolowski et al.16 showed pHdependent levels of water dissociation. For the defective surface with single OVs, there was little dispute that water preferred to adsorb dissociatively on the OV site;17−22 the single OVs were proved to be a good trap center for water molecules. Single OVs were not immobile on the surface,23 and the migration rate increased evidently with the increase of the temperature.24 During the diffusion, they could form other types of defects, for instance, bridging-oxygen vacancy pairs (OVPs) and two next nearest neighbored OVs belonging to the same bridging-oxygen row (NNN-OVs, i.e., two OVs separated by a single oxygen atom).25 The OVPs could survive for a few minutes to hours even at room temperature after being produced, and meanwhile, NNN-OVs could exist during the process of OV diffusion.25 These two cases are supposed to influence the physical and chemical properties of the surface. For example,

1. INTRODUCTION TiO2 surfaces have attracted considerable attention due to their important roles in the technological field, especially in heterogeneous catalysis, solar cells, and semiconductor devices.1,2 Among different types of the surfaces, the rutile TiO2 (110) surface emerged as the model system for fundamental surface studies.2−4 Varieties of defects, e.g., oxygen vacancies and interstitial Ti, would be formed on the surface. These defects would largely influence the properties and reactivity of the surface. Therefore, a fundamental understanding of the roles of defects is highly desirable. Previous study5 showed that the most predominant surface defect was single bridging-oxygen vacancies (OVs). In many cases, small molecules, e.g., alcohols,6−9 were used to probe the reactivity of the surface, which showed a single OV defect on the surface assisted the adsorption and dissociation of alcohols. In addition, water as probe molecule is very attractive owing to its producing green energy of hydrogen under proper conditions. The interaction between water and TiO2 surfaces has been extensively studied experimentally and theoretically. For perfect rutile TiO2 (110) surface, there is still a dispute about the adsorption modes, molecular or dissociated. Most experimental and theoretical studies held the view that water absorbed predominantly in molecular mode on the 5-fold Ti © 2014 American Chemical Society

Received: January 21, 2014 Revised: July 31, 2014 Published: August 14, 2014 20257

dx.doi.org/10.1021/jp500721z | J. Phys. Chem. C 2014, 118, 20257−20263

The Journal of Physical Chemistry C

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Rasmussen et al.26 found the presence of OVP strongly affects the ability of O2 to dissociate. Cui et al.25 also found the OVP could enhance the dissociation of O2 molecule. To further explore the role of single OVs and OVP, theoretical studies have been implemented. The results show the reactivity for methanol dissociation on OVP was weaker than single OVs.27 Surface defects changed the electronic structure and could have a remarkable effect on chemical reactivity of the surface.2,28,29 Studies of a single OV defect show its important roles in the chemical process on a rutile TiO2 (110) surface. Single OVs were proved to be a good trap and reactive center for molecules. However, few studies concern the influence caused by different arrangement of OVs, such as OVPs and NNNOVs. Due to the importance of defects on the rutile TiO2 (110) surface for catalytic and energy application, the present work, using density functional theory (DFT) method, focuses on the reactivity of two types of defects on the rutile TiO2 (110) surface, OVPs and NNN-OVs, with a water molecule as a probe. To clearly demonstrate the effects caused by defects, the cases of the perfect and single OV surfaces are investigated first. Then, the OVP and NNN-OV surfaces are discussed, respectively. Besides the sites of defects, whether or not the behavior of 5-fold Ti atoms is affected by these defects is also discussed. The adsorption structures of two modes, molecular and dissociated adsorption, on possible sites are discussed to find the most stable site. Dissociation paths and energy barriers are studied to further explore the reactivity of different sites.

fold Ti atoms, bridging-oxygen atoms, and in-plane oxygen atoms. The test for slab thickness of three, four, and five layers of 2 × 1 surface unit cell shows that the four layer model could achieve reliable results balancing the accuracy and time cost. In this study, a four layer periodic slab model of 4 × 2 surface unit cells (Supporting Information Figure S1) with one water molecule adsorbed on one side was adopted. A vacuum of 10 Å was chosen. In order to form defective surfaces, certain oxygen atoms were removed (Figure 1a) as follows: single OV, removing O atom 2; OVP, removing O atom 2 and 3; NNNOVs, removing O atom 2 and 4. Possible adsorption sites on the defective surface with OVP and NNN-OVs were labeled in Figure 1b,c. The lattice constants were a = b = 4.663 Å and c = 2.968 Å from optimization of the bulk structure, in accordance with the experimental values.30 All the calculations were carried out using Vienna ab initio simulation package (VASP).31−34 The generalized gradient approximation (GGA) with the spin-polarized Perdew−Burke− Ernzerhof (PBE) functional35 and projector-augmented wave (PAW) potential36 were used. A plane-wave cutoff value of 450 eV was chosen on the basis of convergence testing for the bulk while the Ti 1s to 3p and O 1s electrons are treated as core states. A Monkhorst−Pack grid37 of 4 × 4 × 6 (9 k-points) was used for the bulk optimization. Gamma point was used for optimizing the large slabs, and a Monkhorst−Pack grid of 3 × 3 × 1 was used for static calculations of the electronic structure. The bottom two layers were frozen to mimic the bulk, while the positions of the remaining atoms were allowed to relax. The force acting on each atom was converged to 0.03 eV/Å. The reaction pathways and transition states with the energy barriers were found by the climbing image nudged elastic band (CINEB)38,39 method. Five images between the initial state (molecular adsorption structure) and final state (dissociated adsorption structure) were linear interpolated. The adsorption energy Eads is defined as Eads = −[Etotal − (ETiO2 + EH2O)]. Etotal is the total energy of surface with one water molecule, ETiO2 is the total energy of clean surface, and EH2O is the energy of one water molecule. The energy difference between molecular and dissociated adsorption is defined by ΔE = Eadsdiss − Eadsmole. Eadsdiss and Eadsmole represent the adsorption energy of dissociated and molecular adsorption, respectively. A positive ΔE value illustrates that the dissociated adsorption structure is more favorable, while a negative value illustrates that the molecular adsorption is more favorable. For energy profiles of dissociation reactions, the energy of initial state is set as 0. For an exothermic reaction, the relative energy of the final state will be below 0, indicating the dissociation is energetically favored, whereas for an endothermic reaction, the relative energy of the final state will be higher than 0, indicating the dissociation is not energetically favored. A lower energy barrier indicates an

2. COMPUTATIONAL METHOD The side and top views for the structure of a perfect TiO2 (110) surface are shown in Figure 1a. It contains 5-fold Ti atoms, 6-

Figure 1. Top and side views of (a) perfect TiO2 (110) surface, (b) defective TiO2 (110) surface with OVP, and (c) defective TiO2 (110) surface with NNN-OVs (only the first layer is shown). Atoms 1−4 in part a label bridging-oxygen atoms. Sites 1−3 in parts b and c represent adsorption sites. Blue represents Ti, and red represents O.

Figure 2. Structures (a, b) on 5-fold Ti site of perfect surface, and (c) energy profile for water dissociation. m and d denote molecular and dissociated adsorption, respectively. Blue represents Ti, red represents O, and pink represents H. 20258



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in agreement with previous experimental observations that the diffusion of water along the 5-fold Ti row through dissociation and recombination.22 Meanwhile, due to the energetically favored molecular adsorption, the water molecule would largely exist in the form of molecular adsorption. 3.2. Reactivity of Single OV Defect. On the defective TiO2 (110) surface with single OV, the most stable adsorption site for water is OV itself. The adsorption energies for molecular water and dissociated fragments are 0.73 and 1.14 eV (Table 1), respectively, indicating that the dissociated adsorption of water is energetically favorable by 0.41 eV. Similar results were shown in Wendt and co-workers’ calculation.40 For molecular adsorption (Figure 3a), the water is trapped in the OV, with two same Ti−Ow bonds of 2.36 Å. To elucidate the dissociation paths, the direct mechanism (one hydrogen atom of the water molecule directly transfers to a neighbor Ob in the same row) is calculated. In the transition state structure (Figure 3c), the whole water molecule tilted to the neighbor Ob where the H atom is transferred, with H atom in the middle of Ow and Ob at similar distances of 1.27 and 1.21 Å. Through dissociation, one H atom is transferred to its adjacent Ob in the same row, to form two identical hydroxyls (Figure 3b). The Ti−Ow bonds were shortened by about 0.3 Å compared to the molecular structure. The energy barrier for dissociation is 0.49 eV, consistent with the work of Oviedo et al.42 Once the water molecule is dissociated on the OV site, it would stably exist. There are smaller chances for the dissociated fragments to recombine because of the high energy barrier for recombination (0.90 eV). Upon dissociation of the water molecules, the surface has OH groups, which could further exhibit interesting properties. Liu et al.43 showed that the water and oxygen molecule could adsorb and diffuse easily on OH/ TiO2 (110) surface. 3.3. Surface with Paired OV Defect. Two types of sites were discussed (Figure 1b) on the surface with paired OV (OVP). Site 1 is OV of the OVP. Sites 2 and 3 are 5-fold Ti atoms, with site 2 close to the vacancy and site 3 a little further. The exposed 4-fold Ti formed by OVP is not discussed here since it is not a stationary point in the adsorption of methanol molecule.27 For water adsorbed on the OVP site, the molecule would be trapped in one OV of the OVP. Two possible modes of its adsorption could occur, as shown in Figure 4a,b. One is the molecular structure (Figure 4a, A1-m), and the other is the dissociated structure (Figure 4b, A1-d). The A1-m molecular adsorption structure has two Ti−Ow bonds of 2.47 Å (Ti1− Ow) and 2.26 Å (Ti2−Ow), with Ti2−Ow bond length a little shorter. The two Ow−H bonds are not disturbed by the adsorption. The adsorption energy of the A1-m structure is 0.88 eV (Table 1). GGA+U calculations with U values of 3 and 5 give adsorption energies of 0.85 and 0.88 eV, respectively.

easier reaction. When referring to different types of O atoms, the following notation is used, for convenience: Ow is the O atom of water molecule, and Ob is the bridging O atom on TiO2 surface.

3. RESULTS AND DISCUSSION 3.1. Reactivity of Perfect Surface. First, the case of perfect TiO2 (110) surface was investigated. The water molecule tends to molecularly adsorb on a 5-fold Ti site (Figure 2a). The Ti−Ow bond length is 2.24 Å, consistent with the experimental value obtained by Allegretti and co-workers.12 After adsorption, the whole system would release energy of 0.72 eV (Table 1), which is in agreement with the results of Table 1. Adsorption Energy of Water Molecule Adsorbed on the Surfaces configurationa

Eadsmole (eV)

Eadsdiss (eV)

ΔE (eV)

perfect OV A1 A2 A3 B1 B2 B3

0.72 0.73 0.88 0.67 0.69 1.08 0.65 0.68

0.54 1.14 1.35 0.49 0.52 2.07 0.46 0.51

−0.18 0.41 0.47 −0.18 −0.17 0.99 −0.19 −0.17

a

Perfect, OV, A, and B represent the perfect, single OV, OVP, and NNN-OV surfaces, respectively. Numbers 1, 2, 3 represent different adsorption sites. Eadsmole and Eadsdiss represent the adsorption energy of molecular and dissociated adsorption. ΔE is defined as ΔE = Eadsdiss − Eadsmole.

Wendt et al.40 One of the H atoms is directed to a neighbor Ob, and the distance between H and Ob is about 1.90 Å, close to the theoretical value in the work of Hammer and co-workers.22 The Ob atom protrudes from the row where it is (Figure 2a), indicating the existence of an attractive interaction between them. Once the water is dissociated, the H atom that directed to Ob is transferred to this Ob to form ObH, leaving OwH on 5fold Ti (Figure 2b). The adsorption energy of these fragments is less than the molecular adsorption by 0.18 eV (Table 1), implying the dissociated adsorption is less stable. To elucidate the dissociation process, the reaction path and transition state were located. Distances between OwH and ObH in the transition state structure are 1.31 and 1.16 Å, respectively. The values indicate the H atom is transferring from Ow to Ob. The remaining OwH does not show any changes. The energy barrier for water dissociation on this site is 0.27 eV (Figure 2c), consistent with the result of Guo et al.,41 while it only needs to overcome a barrier of 0.09 eV to recombine and form molecularly adsorbed water. These calculated low barriers are

Figure 3. Structures (a, b) on OV site of OV defective surface, and (c) energy profile for water dissociation. m and d denote molecular and dissociated adsorption, respectively. Blue represents Ti, red represents O, and pink represents H. 20259



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Figure 4. Structures (a, b) on site 1 of OVP defective surface, (c) energy profile for water dissociation, and (d, e) projection of density of states (DOS) on water molecule. A denotes OVP defective surface. 1 denotes site 1. m and d denote molecular and dissociated adsorption, respectively. Reference energy is the valence-band maximum in parts d and e. Blue represents Ti, red represents O, and pink represents H.

These results indicate that the values of adsorption energy are not obviously changed whether or not the U parameter is set. In addition, Miao et al.44 showed that it does not play a relevant role in the energy barrier calculation for water dissociation on Ti/TiO2 surface. Calculations in this study are done without U parameter applied. For hybrid functionals such as HSE, it has been argued that varying the amount of HF exchange could result in wrongfully predicted defect properties.45 Besides, Guo et al.41 showed that GGA with the PBE functional predicts reasonable values and trends. Balancing the accuracy and computing resource, pure DFT methods were used. The A1-m structure is more stable than the cases of molecular adsorption on the perfect and OV surfaces (0.72 and 0.73 eV, respectively). In the transition state (Figure 4c), the structure does not show significant difference compared with the case of OV defect. Distances of Ow−H and Ob−H are both 1.25 Å, implying the weakening of OwH and the forming of ObH. After dissociation, the A1-d structure (Figure 4b) has one OwH and one ObH in the same Ob row. The two bonds are both 0.97 Å long. In this case, two Ti−Ow bonds are both shorter than the molecular structure, indicating stronger binding between the water molecule and the surface. Upon dissociated adsorption, the system would release energy of 1.35 eV (Table 1), allowing it to be the most stable structure among all the studied structures on the OVP surface. The strong adsorption could be verified by the projection of density of states (DOS) on the adsorbed water molecule (Figure 4e). Compared with the DOS of the single water molecule (Figure 4d), the positions of the molecular orbital moved after adsorption. In the dissociated adsorption (A1-d), the highest-occupied molecular orbital (HOMO) and sub-HOMO of the molecule have strong hybridization with the surface. In the molecular adsorption (A1m), only the HOMO has slight difference. For the dissociation process, the water molecule needs to overcome an energy barrier of 0.52 eV (Figure 4c) to form H and OH. Once it is dissociated, the system would release extra energy of 0.47 eV, similar to the case of a single OV site (dissociated adsorption is favored by 0.41 eV compared to molecular adsorption). However, both modes, molecular water and dissociated

fragments, are more stable on OVP compared with the case of adsorption on single OV. Results imply that the OVP could be a better trap center for molecules than single OV, consistent with the study of Cui et al.25 For water adsorbed on 5-fold Ti sites, two types of circumstance, sites 2 and 3 (Figure 1b), were studied. On the site close to the vacancy, namely site 2, the molecular configuration (Figure 5a, A2-m) is more stable than the

Figure 5. Structures (a, b) on site 2 of OVP defective surface, (c) energy profile for water dissociation, and (d) projection of density of states (DOS) on water molecule. A denotes OVP defective surface. 2 denotes site 2. m and d denote molecular and dissociated adsorption, respectively. Reference energy is the valence-band maximum in part d. Blue represents Ti, red represents O, and pink represents H.

dissociated fragments (Figure 5b, A2-d). In the A2-m structure, one H atom points to an Ob to gain an additional attraction between the two atoms. The Ow atom is bonded to the surface 5-fold Ti with Ow−Ti distance of 2.27 Å, similar to the case on the perfect surface. Moreover, the transition state structure (Figure 5c) is not obviously affected by the existence of OVP 20260



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Among all the sites studied in this work, the NNN-OVs site is the best site for water adsorption. After the water molecularly adsorbed on this site, the system could release energy of about 1.08 eV (Table 1). For dissociated adsorption, more energy is released, about 2.07 eV (Table 1). In addition to the large adsorption energy, the projection of DOS on the water molecule (Figure 6d) also demonstrates the strong adsorption.

because it does not have large difference compared to the case on the perfect surface (Figure 2c). The distances of Ow−H and Ob−H in A2-ts structure are 1.31 and 1.16 Å, respectively. It indicates that the Ow−H bond is weakened and the Ob−H bond is forming. On the basis of the molecular configuration A2-m, the dissociated configuration A2-d is formed by the breakage of Ow−H together with the formation of Ob−H. Ti− Ow bond is shortened to 1.90 Å in the dissociated structure, close to the bulk value. Small difference in bond length was observed between Ow−H and Ob−H. The system would be endothermic for 0.18 eV (Figure 5c) upon dissociation, indicating that the dissociated adsorption is energetically less favorable. Slight shift of the molecular orbitals after adsorption (Figure 5d) by comparison to the single water molecule (Figure 4d) indicates the hybridization between the molecule and surface, but the interaction between them in this case is weaker than that in the case adsorbed on the OVP. The dissociation energy barrier is 0.26 eV (Figure 5c), similar to the value of 0.27 eV on the perfect surface. Likewise, although the energy barrier for water dissociation in this case is low, the barrier for recombination is even lower. It suggests that the water molecule prefers molecular adsorption rather than dissociated adsorption on 5-fold Ti atoms. The behaviors of a water molecule on site 3 are consistent with the cases on site 2 and perfect surface. In the molecular adsorption structure (Supporting Information Figure S2a, A3m), the Ti−Ow bond length is 2.25 Å, and one of the hydrogen atoms points to Ob. Besides, the H atom could point to the Ob either on the left or on the right, which causes a negligible difference. After dissociation, the H atom which pointed to Ob in the A3-m structure is transferred to the Ob (Supporting Information Figure S2b, A3-d), forming two identical OHs with Ti−Ow bond shortened. The dissociation process requires overcoming a low energy barrier of 0.25 eV (Supporting Information Figure S2c), similar to the cases of site 2 and perfect surface. In addition, a transition state structure similar to that in the cases of site 2 and perfect surface was observed. On site 3, the molecular structure is 0.17 eV (Table 1) lower compared with the dissociated structure, indicating the molecular adsorption is more favorable. The results imply that the molecular adsorption of water is preferred on 5-fold Ti sites. Although the energy barrier for water dissociation is low, the dissociated fragments are less stable energetically, and they would recombine easily. The low barrier could assist the diffusion of water through dissociation and recombination. However, water would energetically exist as molecular mode. During the diffusion, once the water molecule is trapped in OV or OVP, it would dissociate to fragments and stay stable. Furthermore, the 5-fold Ti sites on the defective OVP surface show similar behaviors as in the case of a perfect surface, indicating that the existence of OVP does not obviously affect the surface Ti atoms. 3.4. Surface with NNN-OV Defect. Cui et al. 25 investigated the stability of one OVP (Figure 1b) and NNNOVs (Figure 1c). Although the total energy of one OVP is lower than NNN-OVs, there is still an energy barrier of about 0.7 eV for the NNN-OVs to overcome to combine into one OVP. Therefore, NNN-OVs could exist to some extent. This section explores how the NNN-OVs could affect the chemical properties and the behaviors of an adsorbed water molecule. The top view of the surface is shown in Figure 1c. Site 1 is OV of the NNN-OVs. Sites 2 and 3 are 5-fold Ti sites, with site 2 close to the vacancy and site 3 a little further.

Figure 6. Structures (a, b) on site 1 of NNN-OV defective surface, (c) energy profile for water dissociation, and (d) projection of density of states (DOS) on water molecule. B denotes NNN-OV defective surface. 1 denotes site 1. m and d denote molecular and dissociated adsorption, respectively. Reference energy is the valence-band maximum in part d. Blue represents Ti, red represents O, and pink represents H.

Meanwhile, the dissociated adsorption is more favored. Large changes of the HOMO and sub-HOMO of the dissociative adsorbed water molecule (Figure 6d B1-d) indicate strong hybridization between adsorbed water molecule and the surface. The interaction between them is stronger than that in the case of molecular adsorption (Figure 6d, B1-m). Results imply that the NNN-OVs could be a favorable trap center for molecules. Indeed, as reported in the work by Cui et al.,25 they argued that redissociation of OVP into NNN-OVs is energetically unfavorable, and under certain conditions, OVP could redissociate into OVs in different Ob rows, whereas adsorbed water molecule on the NNN-OVs may stabilize this defect. In our study, the high adsorption energy of both of the two modes on this site implies strong interaction between water molecule and the surface. The molecular adsorption structure has one water molecule trapped in one OV of the NNN-OVs, with two symmetrically identical Ti−Ow bonds, namely Ti2− Ow and Ti3−Ow with bond length of 2.31 Å (Figure 6a, B1-m). After dissociation, one of the H atoms is transferred to neighbor Ob in the same row (Figure 6b, B1-d). The two hydroxyls Ow−H and Ob−H are not significantly different. The bonding between Ti and Ow in the dissociated adsorption is stronger than that in the molecular adsorption, with shorter bond lengths of 2.04 Å (Ti2−Ow) and 2.01 Å (Ti3−Ow). Meanwhile, the adsorption has affected the binding between Ob(H) and 6-fold Ti atoms (Ti1−Ob and Ti2−Ob in B1-d). On the perfect surface, the distances of two Ti−Ob bonds are both 1.85 Å. They are elongated to 2.02 Å (Ti1−Ob) and 2.10 Å (Ti2−Ob) after dissociated fragments adsorbed. This binding information further indicates the strong interaction between the 20261



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on NNN-OVs site, 0.49 eV on single OV site, and 0.52 eV on OVP site. The 5-fold Ti sites, in spite of different types of surfaces, i.e., perfect or defective, present similar behavior: the water molecule favors the molecular adsorption mode, and the dissociated adsorptions are less stable energetically. Energy barriers for water dissociation on 5-fold Ti atoms are 0.25−0.27 eV, the lowest of the calculated sites, but the energy barriers of recombination are even lower than that of the dissociation on 5-fold Ti atoms, so the dissociated fragments could recombine to molecular water more easily. Current study indicates that surface defects are crucial for adsorption of molecules and reaction processes on the surface. We hope the work could give insights into the study of physical and chemical properties of metal oxide.

water molecule and the surface. The water molecule needs to overcome a barrier of about 0.45 eV to dissociate. The transition state structure (Figure 6c) is not significantly different from the ones on single OV (Figure 3c) and OVP (Figure 4c), while the energy barrier for water dissociation in this case is slightly lower. Taken into consideration that the adsorption of both of the two modes on the NNN-OVs is more stable, the dissociation process would occur a little more easily in this case. Results imply the NNN-OVs site could be an active site for molecule adsorption and dissociation. Sites 2 and 3 (Figure 1c) are both 5-fold Ti sites on the NNN-OV surface. The only difference lies in that only one Ob atoms is on the right of site 2, while two Ob atoms are on both the right and left of site 3. Therefore, the water molecule would have only one orientation after adsorption on site 2, and two orientations on site 3. Nonetheless, this difference does not affect the behaviors of an adsorbed molecule, like the cases of sites 2 and 3 on the surface of OVP. The configurations are consistent with the structures discussed in the foregoing sections about 5-fold Ti sites on perfect and OVP surfaces. These 5-fold Ti sites show similar behaviors. When adsorbed as a water molecule, as shown in Supporting Information Figures S3a, B2-m, and S4a, B3-m,, the Ow atom is bonded to 5-fold Ti, with distances of 2.27 and 2.26 Å, respectively. One of the H atoms is directed to neighbor Ob which obviously protrudes from the row where it is. This indicates the attractive interaction between them. After dissociation, this Ob captured the H atom to form new ObH, leaving OwH where it is (Supporting Information Figures S3b, B2-d, and S4b, B3-d). On sites 2 and 3, the molecular adsorption modes gained 0.19 and 0.17 eV in energy (Table 1) compared to the dissociated modes. The results show that, in spite of different types of studied surfaces in this work, the water molecule favors molecular adsorption mode on 5-fold sites with similar structures. The energy barriers for the dissociation process on the two sites are both 0.26 eV. The value is nearly identical with the cases of perfect surface and 5-fold sites on OVP surface. Results also imply that the dissociation processes are very similar on 5-fold Ti sites. The transition state structures (Supporting Information Figures S3c and S4c) do not have significant difference as well. This further illustrates that 5-fold Ti atoms have similar behaviors and reactivity despite the existence of different defects. Although the energy barrier for water dissociation on these 5-fold Ti sites is low, the fragments would recombine easily. In addition, the molecular adsorption is energetically more favorable.

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ASSOCIATED CONTENT

S Supporting Information *

Additional figures showing structures discussed in this work. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. Phone: +86-571-87952424. Fax: +86-571-87951895. *E-mail: [email protected]. Present Address

† School of Chemical Engineering, Purdue University, West Lafayette, Indiana 47907−2100, United States.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by National Natural Science Foundation of China (21074115, 21273200), and funding from Ministry of Education of China (J20091551). Calculations were carried out at Beijing Computing Center.

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REFERENCES

(1) Sun, C.; Liu, L.-M.; Selloni, A.; Lu, G. Q.; Smith, S. C. TitaniaWater Interactions: A Review of Theoretical Studies. J. Mater. Chem. 2010, 20, 10319−10334. (2) Diebold, U. The Surface Science of Titanium Dioxide. Surf. Sci. Rep. 2003, 48, 53−229. (3) Pang, C. L.; Lindsay, R.; Thornton, G. Chemical Reactions on Rutile TiO2(110). Chem. Soc. Rev. 2008, 37, 2328−2353. (4) Dohnálek, Z.; Lyubinetsky, I.; Rousseau, R. Thermally-Driven Processes on Rutile TiO2(110)-(1 × 1): A Direct View at the Atomic Scale. Prog. Surf. Sci. 2010, 85, 161−205. (5) Schaub, R.; Wahlströ m, E.; Rønnau, A.; Lægsgaard, E.; Stensgaard, I.; Besenbacher, F. Oxygen-mediated Diffusion of Oxygen Vacancies on the TiO2(110). Surf. Sci. 2003, 299, 377−379. (6) 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. (7) Zhang, Z.; Rousseau, R.; Gong, J.; Kay, B. D.; Dohnálek, Z. Imaging Hindered Rotations of Alkoxy Species on TiO2(110). J. Am. Chem. Soc. 2009, 131, 17926−17932. (8) Hansen, J. Ø.; Huo, P.; Martinez, U.; Lira, E.; Wei, Y. Y.; Streber, R.; Lægsgaard, E.; Hammer, B.; Wendt, S.; Besenbacher, F. Direct Evidence for Ethanol Dissociation on Rutile TiO2(110). Phys. Rev. Lett. 2011, 107, 136102.

4. CONCLUSION A water molecule was used to probe two different types of defects, OVP and NNN-OVs on the TiO2 (110) surface, combined with their influence on surface Ti atoms by the DFT method. The perfect surface and single OV defect surface are also presented. The calculations show that, on OV, OVP, and NNN-OVs sites, the water molecule tends to adsorb as dissociated fragments as ObH and OwH. The NNN-OVs site has the largest adsorption energy of all the sites studied, especially for the dissociated adsorption. After a water molecule dissociated on the NNN-OVs site, the whole system would release energy of 2.07 eV, much more than any other site calculated, indicating this site is the most favorable site for water adsorption. For the dissociation process of water molecule among the OV, OVP, and NNN-OVs sites, the NNN-OVs site shows a slightly lower energy barrier, 0.45 eV 20262



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1 Surface: An in Situ Study with Low-Temperature Scanning Tunneling Microscopy. J. Am. Chem. Soc. 2011, 133, 2002−2009. (30) Burdett, J. K.; Hughbanks, T.; Miller, G. J.; Richardson, J. W.; Smith, J. V. Structural-Electronic Relationships in Inorganic Solids: Powder Neutron Diffraction Studies of the Rutile and Anatase Polymorphs of Titanium Dioxide at 15 and 295 K. J. Am. Chem. Soc. 1987, 109, 3639−3646. (31) Kresse, G.; Hafner, J. Ab Initio Molecular Dynamics for Liquid Metals. Phys. Rev. B 1993, 47, 558−561. (32) Kresse, G.; Hafner, J. Ab Initio Molecular-Dynamics Simulation of the Liquid-Metal-Amorphous-Semiconductor Transition in Germanium. Phys. Rev. B 1994, 49, 14251−14269. (33) 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. (34) 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. (35) Perdew, J. P.; Burke, K.; Ernzerhof, M. Generalized Gradient Approximation Made Simple. Phys. Rev. Lett. 1996, 77, 3865−3868. (36) Kresse, G.; Joubert, D. From Ultrasoft Pseudopotentials to the Projector Augmented-Wave Method. Phys. Rev. B 1999, 59, 1758− 1775. (37) Monkhorst, H. J.; Pack, J. D. Special Points for Brillouin-Zone Integrations. Phys. Rev. B 1976, 13, 5188−5192. (38) Mills, G.; Jónsson, H.; Schenter, G. K. Reversible Work Transition State Theory: Application to Dissociative Adsorption of Hydrogen. Surf. Sci. 1995, 324, 305−337. (39) Henkelman, G.; Uberuaga, B. P.; Jónsson, H. A Climbing Image Nudged Elastic Band Method for Finding Saddle Points and Minimum Energy Paths. J. Chem. Phys. 2000, 113, 9901−9904. (40) Wendt, S.; Schaub, R.; Matthiesen, J.; Vestergaard, E. K.; Wahlström, E.; Rasmussen, M. D.; Thostrup, P.; Molina, L. M.; Lægsgaard, E.; Stensgaard, I.; et al. Oxygen Vacancies on TiO2(110) and Their Interaction with H2O and O2: A Combined HighResolution STM and DFT Study. Surf. Sci. 2005, 598, 226−245. (41) 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. (42) Oviedo, J.; Sánchez-de-Armas, R.; San Miguel, M. Á .; Sanz, J. F. Methanol and Water Dissociation on TiO2(110): The Role of Surface Oxygen. J. Phys. Chem. C 2008, 112, 17737−17740. (43) Liu, L.-M.; Crawford, P.; Hu, P. The Interaction between Adsorbed OH and O2 on TiO2 Surfaces. Prog. Surf. Sci. 2009, 84, 155− 176. (44) Miao, M.; Liu, Y.; Wang, Q.; Wu, T.; Huang, L.; Gubbins, K. E.; Nardelli, M. B. Activation of Water on the TiO2(110) Surface: The Case of Ti Adatoms. J. Chem. Phys. 2012, 136, 064703. (45) Deak, P.; Aradi, B.; Frauenheim, T. Polaronic Effects in TiO2 Calculated by the HSE06 Hybrid Functional: Dopant Passivation by Carrier Self-Trapping. Phys. Rev. B 2011, 83, 155207.

(9) Xu, C.; Yang, W.; Guo, Q.; Dai, D.; Chen, M.; Yang, X. Molecular Hydrogen Formation from Photocatalysis of Methanol on TiO2(110). J. Am. Chem. Soc. 2013, 135, 10206−10209. (10) Liu, L.-M.; Zhang, C.; Thornton, G.; Michaelides, A. Structure and Dynamics of Liquid Water on Rutile TiO2(110). Phys. Rev. B 2010, 82, 161415. (11) Brinkley, D.; Dietrich, M.; Engel, T.; Farrall, P.; Gantner, G.; Schafer, A.; Szuchmacher, A. A Modulated Molecular Beam Study of the Extent of H2O Dissociation on TiO2(110). Surf. Sci. 1998, 395, 292−306. (12) Allegretti, F.; O’Brien, S.; Polcik, M.; Sayago, D. I.; Woodruff, D. P. Adsorption Bond Length for H2O on TiO2(110): A Key Parameter for Theoretical Understanding. Phys. Rev. Lett. 2005, 95, 226104. (13) Liu, L.-M.; Zhang, C.; Thornton, G.; Michaelides, A. Reply to “Comment on ‘Structure and Dynamics of Liquid Water on Rutile TiO2(110)’”. Phys. Rev. B 2012, 85, 167402. (14) Wendt, S.; Matthiesen, J.; Schaub, R.; Vestergaard, E. K.; Lægsgaard, E.; Besenbacher, F.; Hammer, B. Formation and Splitting of Paired Hydroxyl Groups on Reduced TiO2(110). Phys. Rev. Lett. 2006, 96, 066107. (15) Duncan, D. A.; Allegretti, F.; Woodruff, D. P. Water Does Partially Dissociate on the Perfect TiO2(110) Surface: A Quantitative Structure Determination. Phys. Rev. B 2012, 86, 045411. (16) Wesolowski, D. J.; Sofo, J. O.; Bandura, A. V.; Zhang, Z.; Mamontov, E.; Předota, M.; Kumar, N.; Kubicki, J. D.; Kent, P. R. C.; Vlcek, L.; et al. Comment on “Structure and Dynamics of Liquid Water on Rutile TiO2(110)”. Phys. Rev. B 2012, 85, 167401. (17) Schaub, R.; Thostrup, P.; Lopez, N.; Lægsgaard, E.; Stensgaard, I.; Nørskov, J. K.; Besenbacher, F. Oxygen Vacancies as Active Sites for Water Dissociation on Rutile TiO2(110). Phys. Rev. Lett. 2001, 87, 266104. (18) Bikondoa, O.; Pang, C. L.; Ithnin, R.; Muryn, C. A.; Onishi, H.; Thornton, G. Direct Visualization of Defect-Mediated Dissociation of Water on TiO2(110). Nat. Mater. 2006, 5, 189−192. (19) Bjørheim, T. S.; Kuwabara, A.; Mohn, C. E.; Norby, T. Defects at the (110) Surface of Rutile TiO2 from ab Initio Calculations. Int. J. Hydrogen Energy 2012, 37, 8110−8117. (20) Henderson, M. A. An HREELS and TPD Study of Water on TiO2(110): The Extent of Molecular versus Dissociative Adsorption. Surf. Sci. 1996, 355, 151−166. (21) Zhang, Z.; Bondarchuk, O.; Kay, B. D.; White, J. M.; Dohnálek, Z. Imaging Water Dissociation on TiO2(110): Evidence for Inequivalent Geminate OH Groups. J. Phys. Chem. B 2006, 110, 21840−21845. (22) Hammer, B.; Wendt, S.; Besenbacher, F. Water Adsorption on TiO2. Top. Catal. 2010, 53, 423−430. (23) Tilocca, A.; Selloni, A. O2 and Vacancy Diffusion on Rutile(110): Pathways and Electronic Properties. ChemPhysChem 2005, 6, 1911−1916. (24) Zhang, Z.; Ge, Q.; Li, S.-C.; Kay, B. D.; White, J. M.; Dohnálek, Z. Imaging Intrinsic Diffusion of Bridge-Bonded Oxygen Vacancies on TiO2(110). Phys. Rev. Lett. 2007, 99, 126105. (25) Cui, X.; Wang, B.; Wang, Z.; Huang, T.; Zhao, Y.; Yang, J.; Hou, J. G. Formation and Diffusion of Oxygen-Vacancy Pairs on TiO2(110)-(1 × 1). J. Chem. Phys. 2008, 129, 044703. (26) Rasmussen, M. D.; Molina, L. M.; Hammer, B. Adsorption, Diffusion, and Dissociation of Molecular Oxygen at Defected TiO2(110): A Density Functional Theory Study. J. Chem. Phys. 2004, 120, 988−997. (27) Shi, H.; Liu, Y.-C.; Miao, M.; Wu, T.; Wang, Q. Adsorption and Dissociation of Methanol on Defective Rutile TiO2(110) Surface with Bridging Oxygen-Vacancy Pairs. Chem. Phys. Lett. 2013, 584, 98−102. (28) Petrik, N. G.; Zhang, Z.; Du, Y.; Dohnalek, Z.; Lyubinetsky, I.; Kimmel, G. A. Chemical Reactivity of Reduced TiO2(110): The Dominant Role of Surface Defects in Oxygen Chemisorption. J. Phys. Chem. C 2009, 113, 12407−12411. (29) Tan, S.; Ji, Y.; Zhao, Y.; Zhao, A.; Wang, B.; Yang, J.; Hou, J. G. Molecular Oxygen Adsorption Behaviors on the Rutile TiO2(110)-1 × 20263


Reactivity of the Defective Rutile TiO2 (110) - American Chemical

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