A Theoretical Study of Water Adsorption and Decomposition on the

Article pubs.acs.org/JPCC

A Theoretical Study of Water Adsorption and Decomposition on the Low-Index Stoichiometric Anatase TiO2 Surfaces Zongyan Zhao,†,‡ Zhaosheng Li,*,†,§ and Zhigang Zou*,† †

Ecomaterials and Renewable Energy Research Center (ERERC), National Laboratory of Solid State Microstructures, and Department of Physics, Nanjing University, Nanjing 210093, People’s Republic of China ‡ School of Materials Science and Engineering, Kunming University of Science and Technology, Kunming 650093, People’s Republic of China § Department of Materials Science and Engineering, Nanjing University, Nanjing 210093, People’s Republic of China

ABSTRACT: Water adsorption and decomposition on the low-index anatase TiO2 surfaces were investigated by the periodic density functional theory calculations. The reaction pathway between water molecule adsorption and dissociative adsorption was determined by using the method of complete linear synchronous transit/quadratic synchronous transit. The existence of Brønzed−Evans−Polanyi relationships for the decomposition reaction of water on TiO2 surfaces was revealed. The results showed that the water decomposition reaction is a surface-structure-sensitive process, and it is determined not only by the surface energy but also by the surface properties. On these low-index surfaces, the chemical activity for the water decomposition reaction decreases in the order (001) > (103)f > (100) > (110) > (103)s > (101). The surfaces can be divided into two groups, according to the magnitude of additional surface dipole moment, that obey the Brønzed−Evans− Polanyi relationship.

1. INTRODUCTION Titanium dioxide (TiO2) has been extensively investigated because it is one of the most promising functional materials to help resolve the energy crisis and address environmental pollution. Most of its applications are related to its interaction with water.1 In the case of photocatalysis, water not only provides the reaction environment but also is the key participant, as it can decompose to produce hydrogen, oxygen, and hydroxyl radicals.2−4 In the applications of self-cleaning and antifogging, it shows amphiphilic properties under UV-light irradiation, owing to the particular interaction of water with the TiO2 surface.5−7 In the application of renewable energy, water can decompose on the surface of TiO2 photoelectrodes to produce hydrogen and oxygen,8 or it can participate in the generation and transfer of photoexcited carriers in dye-sensitized solar cells.9 Furthermore, in the field of surface catalytic science, water is the ideal probe molecule, due to its simple molecular structure and important properties. In this way, the properties of catalysts, such as adsorption capacity, corrosion resistance, catalytic activity, and redox processes, and the role of surface defects can be investigated. © 2012 American Chemical Society

Previous studies have focused on the adsorption behavior of water on TiO2 surfaces, such as whether there is molecular or dissociation adsorption, and whether there is multilayer adsorption.10−17 This has greatly enhanced the understanding of the interaction of water with TiO2 surfaces. However, questions still remain: How does water decompose on the TiO2 surfaces? What are the differences between water adsorption and decomposition on different TiO2 surfaces? What factors influence the process of water decomposition on TiO2 surfaces? Water decomposition is the rate-limiting step in many photocatalysis reactions. Thus, a better understanding of the decomposition process of water on TiO2 surfaces could aid in the development of photocatalytic materials in the future. It has been widely reported that anatase TiO2 has a higher activity than rutile TiO2.18 Specifically, the anatase phase is more stable than the rutile phase at the nanoscale.19,20 As a consequence, there has been more research into the surface Received: December 22, 2011 Revised: March 5, 2012 Published: March 6, 2012 7430

dx.doi.org/10.1021/jp212407s | J. Phys. Chem. C 2012, 116, 7430−7441

The Journal of Physical Chemistry C

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adsorption configuration. On the basis of this stable molecular adsorption, we compared the possible dissociative adsorption configurations and chose the minimum energy configuration as our objective. To find the reaction pathway of water decomposition on the anatase TiO2 surfaces, the transition state that connects the stable molecular adsorption and dissociative adsorption through a minimum energy pathway was identified using complete linear synchronous transit (LST) and quadratic synchronous transit (QST) search methods followed by transition-state confirmation through the nudged elastic band (NEB) method.32−38 On the basis of the calculated adsorption energy of these states, we analyzed the reaction activity and energy for the decomposition reaction of water on the anatase TiO2 surfaces.

behavior of anatase TiO2. On the basis of the literature, among the low-index stoichiometric anatase TiO2 surfaces, the (101) surface is the most stable, and the (001) surface is the second most stable surface.20,21 However, the (001) surface has a higher photocatalytic activity than the (101) surface.22−25 For TiO2 nanomaterials, the exposed facets are not only (101) surfaces but also high energy surfaces, such as the (100) and (001) surfaces. Therefore, it is necessary to compare and analyze the behavior of water on different anatase TiO2 surfaces. To attempt to answer the aforementioned questions, we have used density functional theory (DFT) to investigate the behavior of water adsorption and decomposition on the six low-index stoichiometric anatase TiO2 surfaces. We have compared the adsorption configurations, adsorption energy, electron transfer, and the electronic structure of water on these surfaces. In addition, we obtained the reaction pathway between molecular adsorption and dissociation adsorption, to explore the decomposition mechanism of water on these surfaces.

3. RESULTS AND DISCUSSION 3.1. Water Adsorption and Decomposition on the Anatase TiO2(101) Surface. The adsorption of a water molecule on the (101) surface has been a hot topic in recent years.11,39,40 In our previous work, we thoroughly investigated the interaction of water with this surface.17 However, we did not analyze the kinetic processes of the water decomposition reaction. In this article, we obtained the similar adsorption configurations for both molecular adsorption and dissociative adsorption. The decomposition reaction pathways and the energy profile are illustrated in Figure 1. In agreement with

2. CALCULATION METHOD AND MODELS All of the DFT calculations were carried out using the Cambridge Serial Total Energy Package (CASTEP) codes.26 An ultrasoft pseudopotential was chosen to deal with the interaction between ion cores and valence electrons. The exchangecorrelation effects between valence electrons were described by the Perdew−Burke−Ernzerhof (PBE) functional using the generalized gradient approximation (GGA).27 The Kohn− Sham wave functions for the valence electrons were expanded using a plane-wave basis set within a specified energy cutoff that was chosen to be 380 eV. The Monkhorst−Pack scheme K-points grid sampling was set as 7 × 7 × 1 (or 5 × 3 × 1; different surface has different setting in this region) for the irreducible Brillouin zone. A 30 × 30 × 432 (or 40 × 72 × 300; different surface has different setting in this region) mesh was used for fast Fourier transformation. The Broyden−Fletcher− Goldfarb−Shanno (BFGS) scheme was chosen as the minimization algorithm.28 Its iterative process was considered to be converged when the force on the atoms was less than 0.01 eV/Å, the stress on the atoms was less than 0.02 GPa, the atomic displacement was less than 5 × 10−4 Å, and the energy change per atom was less than 5 × 10−6 eV. We first calculated the bulk crystal structure of anatase TiO2, which gave the following lattice parameters: a = b = 3.7748 Å, c = 9.5990 Å, dap = 1.9883 Å, deq = 1.9318 Å, and 2θ = 155.403°. This is in good agreement with the experimental parameters:29 a = b = 3.7848 Å, c = 9.5124 Å, dap = 1.9799 Å, deq = 1.9338 Å, and 2θ = 156.230°. We then calculated the surface structure of the different low-index anatase TiO2 surfaces, which gave similar results to other theoretical studies.20,21 This indicated that our calculation methods in the present work were reliable. The anatase TiO2 surfaces were simulated by the periodic (2 × 2)-(3 × 3) slab models. These models have eight O−Ti−O trilayers (32−72 units of TiO2), and the total number of atoms range from 96 to 216. The slabs were separated by a 20 Å thick vacuum. Thus, the lengths of the three directions of the models are larger than 10 Å, which is sufficient to screen the selfinteraction effects of the periodic boundary conditions. Using the thicknesses of the slab or vacuum of these models, we can obtain the accurate results, as previous theoretical works.21,30,31 The atomic positions of the bottom four O−Ti−O trilayers were fixed to simulate the bulk effects. A water molecule was placed on all possible adsorption sites on the surface and then optimized using the BFGS method to obtain the most stable

Figure 1. Adsorption configurations and decomposition reaction pathway of water on the anatase TiO2(101) surface. “SS” denotes the separate state, “IS” denotes the initial state, “TS” denotes the transition state, and “FS” denotes the final state. The reference energy is that of the separate states. The red balls represent the oxygen atoms, the gray balls represent titanium atoms, and the blue balls represent hydrogen atoms.

previous works, water prefers molecular adsorption to dissociative adsorption on this surface, which is not only observed by theoretical calculations but also verified by the temperature-programmed desorption measurement.11,39−41 The adsorption energies are 0.948 and 0.658 eV, for molecular and dissociative adsorption, respectively. The order of energy for these two absorption states and their adsorption configurations are consistent with published theoretical results.39,42,43 The most stable site for molecular adsorption is the bridge site between the 5c-Ti and 3c-O atoms, and the molecular plane of water is approximately parallel to the surface (7.16°) when water is adsorbed at this site. In the case of dissociative adsorption, one of the water decomposition products, the OH group, is adsorbed onto the 5c-Ti atom with its O atom bonded to the Ti atom, thus forming a surface-adsorbed hydroxyl 7431



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Table 1. Structural Parameters of Water Adsorption and Decomposition on Different Anatase TiO2 Surfacesa,b ∠Hw−Ow−Hw

dOw−Hw (101)

(100)

(001)

(103)f

(103)s

(110)

IS TS FS IS TS FS IS TS FS IS TS FS IS TS FS IS TS FS

0.995 1.032 0.983 0.989 0.974 0.978 0.994 0.993 0.978 0.984 0.992 0.982 1.007 0.996 0.977 0.982 0.984 0.981

0.995 1.209 2.734 0.987 1.300 2.609 0.994 0.986 1.505 1.016 1.003 1.492 1.007 2.795 4.607 1.015 1.218 3.216

102.585 160.525 165.820 104.962 119.628 160.955 101.768 102.062 117.815 105.735 107.237 109.836 101.570 87.679 81.231 110.054 127.331 157.376

dObr−Hw 2.101 3.371 3.028 2.144 3.414 3.047 1.810 2.358 3.153 1.743 1.977 3.005 1.855 2.736 3.445 1.811 3.415 2.678

2.101 1.118 1.039 2.230 1.177 0.980 1.810 1.740 1.033 2.481 2.159 1.033 1.855 1.002 0.976 2.877 1.267 1.028

dTi5c−Ow

dOw−S

∠W−S

2.236 1.960 1.816 2.236 2.010 1.818 3.656 3.198 1.877 2.149 2.126 1.871 2.241 1.825 1.833 2.145 2.003 1.824

1.153 1.316 1.259 1.946 1.717 1.864 2.043 2.055 2.105 0.383 0.500 0.442 0.819 1.196 1.739 0.954 1.423 1.502

7.16 48.61 88.28 27.59 40.31 90.00 73.30 50.78 90.00 46.60 33.46 87.10 40.74 47.21 57.76 15.47 31.84 34.34

a

The unit of distance is Å; and the unit of angle is degree. bThe calculated bond length and angle of an isolated water molecule are 0.979 Å, and 104.503°. In this table, the first column of the bond lengths of the Ow−Hw bond is those in water or the dissociated hydroxyls radical. The second column is either the second Ow−Hw bond length, in the case of water, or the distance between Ow and the dissociated hydrogen atom, in the case of dissociated water. Obr−Hw is the distance between surface bridging oxygen atoms and the hydrogen atoms of water or its decomposed products. The distance between water and the surface, dOw−S, is the perpendicular distance between the oxygen atom of water and the upmost atom on the surface. The angle between the plane of the water molecule and the surface is represented by ∠W−S.

Figure 2. (a) Averaged difference electron density of water adsorbed on the anatase TiO2(101) surface along the surface normal direction. The vertical dashed line denotes the surface position. (b) Local density of states for the molecular and dissociative adsorption.

radical. The other water decomposition product, the H atom, is adsorbed onto the 2c-O atom, which is directly bonded with a 5c-Ti atom. Thus, it becomes a stable surface-terminated hydroxyl radical. The plane of the dissociation fragments is approximately perpendicular to the surface, with an angle of 88.28°. The detailed structural parameters are listed in Table 1. The saddle point (transition state) between these two

adsorption states on the potential energy surface is shown in Figure 1. It has an adsorption energy of −0.651 eV. From the viewpoint of energy, it is the least energetically favorable state. Because its structure is closer to that of the final state (dissociative adsorption), it is a later transition state of the water decomposition reaction. Starting from the initial state (molecular adsorption), the activation energy of the water decomposition reaction is 1.60 eV, 7432



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molecular adsorption, the oxygen atom of water interacts with a 5c-Ti atom, with the two hydrogen atoms pointing toward neighboring 2c-O and 3c-O atoms. The angle between the molecular plane of water and the surface is 27.59°. During the decomposition process, the molecular plane of water gradually tilts up. The hydrogen atom that originally pointed toward a 3c-O atom now points away from the surface, and the other hydrogen atom dissociates from the oxygen atom. At the final dissociative adsorption stage, the plane of the H+OH segments is perpendicular to the (100) surface. The adsorption energies of these two states are very close, 0.743 and 0.777 eV, respectively. The decomposition reaction of water on this surface is an exothermic reaction with a reaction energy of 0.034 eV. The transition state is similar to the initial state, so it is an early transition state. Its adsorption energy is 0.402 eV. Thus, the activation energy of water decomposition is 0.341 eV, while that of its reverse reaction is 0.375 eV. Therefore, molecular adsorption is slightly more energetically favorable than dissociative adsorption on the (100) surface. However, their adsorption energies and activation energies are similar, so the probabilities of these two states being observed on the (100) surface are approximately the same. As shown in Figure 4a, after water adsorbs to the (100) surface, the electron density of the surface near the oxygen atom of the water increases, while that in the region between water and the surface decreases. This indicates that the electrons of the water molecule partially transfer to the surface and also redistribute around the oxygen atom. Of further note is that the changes in amplitudes of these two states are similar. Moreover, the interaction of water with the (100) surface in both states is relatively weak, because the lone pair states of water only partially overlap with the electronic states of unsaturated surface atoms (Figure 4b). The other electronic states of water retain the features of their separate states. Because of these reasons, the adsorption energies of water in both molecular and dissociative adsorption states are relatively low. 3.3. Water Adsorption and Decomposition on the Anatase TiO2(001) Surface. The (001) surface, which has the highest photocatalytic activity of the anatase TiO2 surfaces, has been the focus of considerable experimental and theoretical research in recent years. Using specific chemical synthesis approaches, researchers have obtained a large exposed area of the (001) surface, which is dominant in anatase TiO2 nanomaterials. These nanomaterials have strong photocatalytic redox activities for organic degradation.22,24,47−51 The behavior of water on this surface has been widely studied. Vittadini et al. compared the adsorption of water on the (101) and (001) surfaces and found that water favors dissociative adsorption on the (001) surface.39 Sumita et al. adopted first-principles molecular dynamic methods to study the interfacial structures of water with the (101) and (001) surfaces.52 Our result is completely consistent with these observations. As shown in Figure 5, the most stable adsorbed site for molecular adsorption is the hollow between the bridging 2c-O atoms on the (001) surface. The two hydrogen atoms point toward the 2c-O atoms, forming two equivalent hydrogen bonds, while the oxygen atom points away from the surface. The molecular plane of water is approximately perpendicular to the surface, with an angle of 73.3°. During water decomposition, the oxygen atom of water tilts toward a 5c-Ti atom. One hydrogen atom is directed toward a 2c-O atom, while the other hydrogen atom is directed away from the surface. The final dissociative adsorption configuration is shown in Figure 5 and is consistent

and its reaction energy is 0.29 eV. This means that water decomposition is an endothermic reaction on this surface. On the other hand, the activation energy of the reverse reaction is 1.31 eV. This implies that the reverse reaction of the water decomposition reaction is more energetically favorable on the (101) surface. Therefore, molecular adsorption is the predominant state on this surface. For molecular and dissociative adsorption, the difference electron density and local density of states are shown in Figure 2. From Figure 2a, it can be seen that the electron density of the surface decreases when water is adsorbed onto it. This indicates that there is electron transfer from the surface to the water molecule or the water decomposition products. For molecular adsorption, the electron density increases in the region between the surface and the water, whereas the electron density decreases in the same region for dissociative adsorption. In addition, the electron density for dissociative adsorption decreases between 2 and 3 Å from the surface. These results indicate that electrons concentrate between the surface and the water molecule due to a strong interaction for molecular adsorption. That is, a strong covalent bond is formed between the water molecule and the surface. In contrast, for dissociative adsorption, electron density is concentrated on the oxygen atom of the OH group. Thus, the water decomposition products mainly form ionic bonds with the surface. These results are reflected in the characteristics of the local density of states in Figure 2b. For molecular adsorption, the interaction of water with the (101) surface is mainly determined by the hybridization of its lone pairs of electrons, which include 1b1 and 3a1 states, with 2c-O atom 2p states and 5c-Ti atom 3d states. Their overlap is obvious and complete, whereas for dissociative adsorption, their overlapping is unobvious and partial. The states in the range of −7 to −6 eV in Figure 2b arise from the covalent interaction of the surface-terminated hydroxyl radical in dissociative adsorption. 3.2. Water Adsorption and Decomposition on the Anatase TiO2(100) Surface. Recently, the properties of the (100) surface have attracted considerable interest due to the strong reduction activity of this surface.44 The adsorption behavior of water on this surface has also been widely studied.45 For example, Wahab et al. adopted the cluster model to study this behavior.46 Their results indicated that dissociative adsorption of water is energetically more favorable than molecular adsorption on the (100) surface, with a hydrogen bond between water and the Ti atom being the dominant interaction. In the present work, we obtained completely similar results for the (100) surface, as shown in Figure 3. For

Figure 3. Adsorption configurations and decomposition reaction pathway of water on the anatase TiO2(100) surface. The legend is the same as that in Figure 1. 7433



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Figure 4. (a) Averaged difference electron density of water adsorbed on the anatase TiO2(100) surface along the surface normal direction. (b) Local density of states for the molecular and dissociative adsorption.

decompose on the (001) surface and the reaction is irreversible. Moreover, the decomposition products are two surfaceadsorbed hydroxyl radicals that are weakly bonded to the surface and are, therefore, active species for photocatalysis reactions. This result could explain why the (001) surface has highly photocatalytic activity. As shown in Figure 6a, for molecular adsorption, the electron density of the surface does not greatly change. The main change is electron redistribution in the water molecule, which is now concentrated around the oxygen atom. In contrast, for dissociative adsorption, there is electron transfer from the surface to the decomposition products, in particular, to the oxygen atom of the OH group. From the local density of states in Figure 6a, it can be seen that water mainly interacts with the (001) surface by hybridizing between the bonded state (3a1) of its lone pair states with the 2p state of a bridging 2c-O atom. The 1b2 and 1b1 states maintain their isolated features, and the 1b1 state still contains empty orbitals. In the dissociative adsorption state, the decomposition products strongly interact with the unsaturated surface atoms, and the hybridization effect is more obvious. The corresponding states completely overlap, and their positions and shapes change. The changes in the electronic structure indicate that the interaction of water with the (001) surface is stronger in the dissociative adsorption state. Therefore, dissociative adsorption is more favorable than molecular adsorption to the (001) surface. 3.4. Water Adsorption and Decomposition on the Anatase TiO2(103)f Surface. In our previous works, we analyzed the atomic structure of the (103)f surface. It can be considered as a stepped surface consisting of (100) (terrace) and (001) (step) surfaces.21 Because of the large terraces between the steps, the water molecule has relatively little space resistance and partially adsorbs onto the inside of the (103)f surface, as shown in Figure 7. From the adsorbed structure, the

Figure 5. Adsorption configurations and decomposition reaction pathway of water on the anatase TiO2(001) surface. The legend is the same as that in Figure 1.

with experimental observations.53 The angle between the plane of H+OH segments and the surface is 90°. In the case of dissociative adsorption, the bridging 2c-O atom that is bonded to the hydrogen atom of water is drawn away from the surface. It forms a surface-adsorbed hydroxyl radical, in preference to a surface-terminated hydroxyl radical. The transition state is close to the initial state and is thus an early transition state. The adsorption energy of molecular adsorption and dissociative adsorption is 1.153, and 2.835 eV, respectively. Therefore, the water decomposition reaction on the (001) surface is an exothermic reaction and has a reaction energy of 1.683 eV. The adsorption energy of the transition state is 1.05 eV. Thus, starting from the molecular adsorption state, a water molecule only needs to overcome an activation energy of 0.103 eV to decompose, while the activation energy of its reverse reaction is 1.785 eV. This result indicates that a water molecule can easily 7434



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respectively. In other words, the decomposition reaction of water on this surface is exothermic, with a reaction energy of 1.137 eV. Its transition state is an early transition state, because the structure and energy are similar to the initial state, and the adsorption energy of the transition state is 0.814 eV. Thus, the water molecule only needs to overcome a small activation energy (0.155 eV). The activation energy of the reverse reaction is 1.292 eV. Therefore, water favors the dissociative adsorption on the (103)f surface. The adsorption structures and the decomposition process of water on the (103)f surface are similar to that on the (001) surface (section 3.3). Because of the particular surface structure of the (103)f surface, the water molecule and its decomposed products can move into the surface. Hence, the electronic structure of the surface changed. As shown in Figure 8a, the electron density of the surface increases in the case of molecular adsorption, while it decreases in the case of dissociative adsorption. Above the surface, the electron density first increases, and then decreases, which is more obvious in the case of dissociative adsorption than in the molecular adsorption. The density of states is shown in Figure 8b. The water molecule and its decomposition products interact with the unsaturated surface atoms by their lone pair states. The corresponding peaks of the density of states completely overlap. In the case of dissociative adsorption, hybridization is more obvious, owing to the stronger interaction with the surface, and the position of hybridized states is closer to the Fermi level. There are two distinct peaks of the density of sates below the valence band, due to the existence of surfaceadsorbed hydroxyl radicals in dissociative adsorption. 3.5. Water Adsorption and Decomposition on the Anatase TiO2(103)s Surface. Although both (103)f and (103)s surfaces are stepped surfaces, and are composed of (100) and (001) surfaces, the atomic structure of the (103)s surface is smoother and more complex than the (103)f surface.

Figure 7. Adsorption configurations and decomposition reaction pathway of water on the anatase TiO2(103)f surface. The legend is the same as that in Figure 1.

behavior of water on the (103)f surface can be considered as a combination of water adsorbed on the (100) and (001) surfaces. The most stable adsorption site is atop a 5c-Ti atom on the terrace. One of the hydrogen atoms is pointing toward the 3c-O atom of the step, while the other hydrogen atom is pointing away from the surface. The angle between the molecular plane of water and the surface is 46.6°. In the dissociative adsorption state, the hydrogen atom that was pointing away from the surface moves forward the 2c-O atom located between two 5c-Ti atoms, forming a surface-terminated hydroxyl radical. The oxygen atom of the water molecule moves from the 5c-Ti site and away from the surface. The hydrogen atom that was pointing toward a 3c-O atoms moves from the surface and forms a surface-adsorbed hydroxyl radical with the oxygen atom. The angle between the plane of H+OH segments and the surface is almost perpendicular (87.1°). The adsorption energies of these two states are 0.969 and 2.106 eV, 7435



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Figure 8. (a) Averaged difference electron density of water adsorbed on the anatase TiO2(103)f surface along the surface normal direction. (b) Local density of states for the molecular and dissociative adsorption.

As shown in Figure 9, for molecular adsorption, the most favorable adsorbed site is in the hollow near the 4c-Ti atom.

surface is smaller than the corresponding energy on the (103)f surface. The angle between the plane of H+OH segments and the surface is 57.76°. On the basis of these results, the decomposition reaction of water on the (103)s surface is exothermic, with a reaction energy of 0.508 eV. The transition state is located between the initial and final states. Its structure is more similar to the final state; thus, it is classified as a later transition state. The corresponding adsorption energy is 0.462 eV. Therefore, a water molecule needs to overcome an activation energy of 0.727 eV to decompose. The activation energy of the reversed reaction is 1.235 eV. Thus, from the viewpoint of energy, the dominant state of water on the (103)s surface is dissociative adsorption. As shown in Figure 10a, the interaction of water with the surface induces an increase in electron density of the surface for the molecular and dissociative adsorption. This indicates that there is some electron transfer from water or its decomposed products to the surface. In addition, for dissociative adsorption, electron density is redistributed in the decomposed products. Electron density decreases on the side nearest the surface, while it is increases on the side furthest from the surface. The density of states of water and its decomposed products on the (103)s surface are shown in Figure 10b. Water mainly interacts with the surface by hybridization between the lone pair states of water and the electronic states of the unsaturated surface atoms. However, this effect is not obvious in the case of molecular adsorption. The electronic states of the adsorbed water molecule maintain features of the isolated molecule. On the other hand, the position of these electronic states decreases, compared to those of the isolated molecule. In contrast, the interaction of the decomposed products of water with the surface is relatively strong. The corresponding electronic states are completely overlapped with the surface states, and their position is closer to the Fermi energy level.

Figure 9. Adsorption configurations and decomposition reaction pathway of water on the anatase TiO2(103)s surface. The legend is the same as that in Figure 1.

The oxygen atom interacts with the 4c-Ti atom, and the two hydrogen atoms interact with 2c-O atoms in the fourth atomic layer via hydrogen bonding. Because all of the atoms of the water molecule interact with the surface, the corresponding adsorption energy (1.189 eV) of water on this surface is greater than that on the (103)f surface. The angle between the plane of the water molecule and the surface is 40.74°. For dissociative adsorption, the oxygen atom moves toward the 4c-Ti atom, which is slightly pulled out of the surface. The surface oxygen atom that bonds with one of the hydrogen atoms is also pulled out of the surface, thus forming a surface-adsorbed hydroxyl radical. The other hydrogen atom moves toward the 2c-O atom, forming a surface-terminated hydroxyl radical. Because the surface-adsorbed hydroxyl radical is above the surface, the absorption energy (1.697 eV) of dissociated water on this 7436



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Figure 10. (a) Averaged difference electron density of water adsorbed on the anatase TiO2(103)s surface along the surface normal direction. (b) Local density of states for the molecular and dissociative adsorption.

3.6. Water Adsorption and Decomposition on the Anatase TiO2(110) Surface. The surface energy of the relaxed (110) surface is the highest of the low-index anatase TiO2 surfaces.21 For molecular adsorption, the most stable adsorption site is in the hollow near the 4c-Ti atom, as shown in Figure 11. The two hydrogen atoms bonded to 2c-O atoms.

water decomposition reaction on the (110) surface is 0.484 eV, which is an exothermic reaction. The transition state of this reaction pathway is similar to the final state, which belongs to a later transition state. The corresponding adsorption energy is 0.768 eV. The activation energies of this reaction are 0.443 eV. These results indicate that the dominant state of water on the (110) surface is dissociative adsorption. The change in electron density is shown in Figure 12a. The electronic redistribution induced by water adsorption is not obvious. This indicates that there is not considerable electron transfer between water and the surface. The main electronic redistribution occurs on the side of the water molecule nearest the surface. Because the distance between the water molecule and the surface is less in molecular adsorption, the electron density in the region between the molecule and the surface increases. Consequently, on the side furthest from the surface, the electron density decreases. For dissociative adsorption, the electron density first increases and then decreases nearer the surface. As shown in Figure 12b, the shape and position of the lone pair states of water have changed for molecular adsorption. The hybridization effect mainly occurs in the range from −4 to −2 eV. The electronic states near the Fermi energy level do not show the complete hybridization. In contrast, for dissociative adsorption, the lone pair states of the decomposed products of water are completely hybridized with the electronic states of the unsaturated surface atoms. Furthermore, the hybridization effect mainly occurs near the Fermi energy level. This indicates that there is a stronger interaction between water decomposed products and the surface than with molecular water. 3.7. Mechanism of Water Decomposition on the Anatase TiO2 Surfaces. To find the relationship between the physical properties of a materials' surface, such as electronic structure and surface atomic structure, and its surface reactivity is a long-term goal of research in heterogeneous catalysis. In the

Figure 11. Adsorption configurations and decomposition reaction pathway of water on the anatase TiO2(110) surface. The legend is the same as that in Figure 1.

The angle between the plane of the water molecule and the surface is 15.47°. For dissociative adsorption, the oxygen atom moves toward a 4c-Ti atom, and one of the hydrogen atoms is pointing away from the surface. These two atoms form a surface-adsorbed hydroxyl radical. The other hydrogen atom interacts with the 2c-O atom that is bonded with the 4c-Ti atom. The 2c-atom is drawn into the surface, resulting in the formation of a surface-terminated hydroxyl radical. The angle between the plane of the H+OH segments and the surface is 34.34°. The adsorption energies of these two states are 1.211 and 1.695 eV, respectively. Thus, the reaction energy of the 7437



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above discussions, we analyzed the changes of the structure and energy of water adsorption and decomposition on the different anatase TiO2 surfaces in detail. The corresponding structural parameters are listed in Table 1. On the (101) and (001) surfaces, the molecular structure retains the symmetrical features for molecular adsorption. Apart from the transition state on the (001) surface and dissociative adsorption on the (103)s surface, the bond length of Ow−Hw lengthens. For dissociative adsorption, the bond length of the cleaved Ow−Hw bond is in the range of 1.492−4.607 Å. The highest absorption energies of decomposed water are on the (103)f and (001) surfaces, in which the bond lengths of the cleaved Ow−Hw bond are the shortest. Dissociated adsorption on the (103)s and (110) surfaces are the next most energetically favorable, where the bond lengths of the cleaved Ow−Hw bond are the longest. For molecular adsorption, the bond lengths of Ti5c−Ow between the oxygen atom of the water molecule and the 5c-Ti atom on the surface are longer than the bond lengths of the Ti−O bond in the bulk anatase TiO2 crystal structure (1.932 and 1.988 Å). In contrast, this bond length is shorter for dissociative adsorption. This indicates that the interaction of water with the surface in dissociative adsorption is stronger than in molecular adsorption, and there is more electron transfer. In the case of water dissociative adsorption on the (103)f, the distance between water and the surface is the shortest (0.442 Å). The longest distance is seen for dissociative water adsorption on the (001) surface (2.105 Å). The smallest angle between the molecular plane of water and the surface is water molecular adsorption on the (101) surface and is almost parallel to the surface with an angle of 7.16°. The largest angles are seen for dissociative adsorption on the (100) and (001) surfaces, with the plane of H+OH segments being almost perpendicular to the surface. Surface energy is considered to be approximately proportional to the density of dangling bonds at the surface. The more

dangling bonds on the surface, the greater the opportunity that the surface has to participate in surface reactions. Thus, in most previous studies, it has been considered that, if the surface has a higher surface energy, its surface reactivity is also higher. However, we have found that this is not completely true. In Figure 13, we have plotted the relationship between the surface

Figure 13. The relationship between surface energies of the low-index anatase TiO2 surfaces and various energy forms of water on these surfaces.

energy of different anatase TiO2 surfaces against the various energy forms of water on the surface. This figure shows that the surface activity of water on anatase TiO2 surfaces, including adsorption energy, activation energy, and reaction energy, is not simply linearly related to surface energy. The surface energy of the (110) surface is the greatest, but its surface activity is not. If surface activation energy is used to represent the surface 7438



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activity, the order of surface activity for these low-index anatase TiO2 surfaces would be (001) > (103)f > (100) > (110) > (103)s > (101). The difference arises because surface activity is determined not only by surface energy but also by the surface atomic structure. The main factors that affect the surface activity are the surface space resistance, surface openness or the degree of atomic density, and the surface electronic structure. In surface heterogeneous catalysis research, the fundamental law is the Brønzed−Evans−Polanyi (BEP) relationship, which states that there is a linear relationship between adsorption energy, activation energy, and reaction heat.54,55 In Figure 14, we have plotted these relationships and fitted curves. From these figures, it can be seen that they better obey the BEP relationship if the six low-index surfaces are divided into two groups: the first group contains the (101), (103)s, and (110) surfaces; and the second group contains (100), (103)f, and (001) surfaces. In our previous work, we found that the surfaces of the first group are polar surfaces that have large additional surface dipole moments.21 The surfaces of the second group are nonpolar surfaces that have small additional surface dipole moments. We have also determined that the dipole interaction is the pivotal factor for water interacting with the anatase TiO2 surface.17 On the basis of these observations, it is easier to understand why the decomposition reaction of water on the low-index anatase TiO2 surfaces needs to be divided into two groups to obey the BEP relationship. Moreover, the results of the present work further confirm that the dipole moment is an important factor for the interaction of water with anatase TiO2 surfaces. To further analyze the origin of the activation and reaction energies, we broke down the energies into their components. First, the definition of the adsorption energy of different states is dry

gas

IS = E IS Eads surface + E H2O − E H2O−surface dry

gas

FS = E FS Eads surface + E H2O − EH···OH−surface dry

(1) (2)

gas

TS = E TS Eads surface + E H2O − EH···OH−surface

(3)

The definitions of activation energy and reaction energy are TS − E IS Eact = Eads ads

(4)

FS − E IS Ereact = Eads ads

(5)

and the components of water adsorbed on the surface are IS IS IS IS EH = EH + Esurface + E interaction 2O 2O−surface

(6) Figure 14. Basic law of the decomposition of water on the low-index anatase TiO2 surfaces: (a) adsorption energy plotted against activation energy; (b) adsorption energy plotted against reaction energy; (c) reaction energy plotted against activation energy. The parameter R is the correlation coefficient for the fitted data.

TS TS TS TS EH ···OH−surface = EH···OH + Esurface + E interaction (7)

Combining eqs 1−7, we can get the components of the activation energy

where Ediss represents the energy of water decomposition, Edef represents the energy of surface deformation, and ΔEinter represents the change of the interaction of water with the surface. By the same method, we also get the components of the reaction energy

IS TS Eact = E H − EH ···OH−surface 2O−surface IS TS IS TS = (E H − EH ···OH) + (Esurface − Esurface) 2O IS TS + (E interaction − E interaction )

= Ediss + Edef + ΔE inter

Ereact = Ediss + Edef + ΔE inter

(8) 7439

(9)



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Table 2. Components of the Activation and Reaction Energies of Water on the Low-Index Anatase TiO2 Surface (101) (100) (001) (103)f (103)s (110)

Eact

Ediss

Edef

ΔEinter

Ereact

Ediss

Edef

ΔEinter

1.600 0.342 0.103 0.155 0.728 0.454

2.247 1.550 −0.008 −0.009 6.627 1.194

1.272 0.565 0.205 0.204 2.266 0.123

−1.919 −1.773 −0.095 −0.040 −8.165 −0.863

0.290 −0.034 −1.682 −1.138 −0.508 −0.484

7.946 7.113 2.072 2.466 7.331 6.790

1.349 1.601 1.086 1.006 1.522 1.199

−9.005 −8.748 −5.470 −4.610 −9.366 −8.473

On the basis of these formulas, we determined the components of the activation and reaction energies for the decomposition reaction of water on the low-index anatase TiO2 surfaces, and the results are listed in Table 2. On the highly active surfaces, such as (001), (103)f, and (100) surfaces, the water molecule only needs a small energy to dissociate. This process is even exothermic on the (001) and (103)f surfaces. The surface deformation energy and its change when water interacts with the surface are small. This is the reason for the small activation energy of the water decomposition reaction on these three surfaces. This is in contrast to when water adsorbed on the (110), (103)s, and (101) surfaces. In these cases, the water molecule needs a relatively large energy to dissociate. Apart from the (100) surface, the reaction energies are relatively large on the high activity the (001) and (103)f surfaces. This can be attributed to the small bond lengths of the cleaved Ow−Hw bonds on these two surfaces (1.505, 1.492 Å, respectively), and the water molecule only needs a relatively small energy to dissociate. On the basis of the same reasoning, the bond length of the cleaved Ow−Hw bond on the (101) surface is the largest, 2.743 Å, so the water molecule needs a relatively large energy to dissociate, resulting in the water decomposition reaction on the (101) being endothermic.

anatase TiO2 surfaces can be divided into two groups to obey the BEP relationship. The first group contains the (101), (103)s, and (110) surfaces, which have relatively small surface additional dipole moments. The second group contains the (100), (103)f, and (001) surfaces, which have relatively large surface additional dipole moments.

4. CONCLUSIONS In this article, we have systematically investigated the change of atomic structure and adsorption energy of water molecules decomposed on the low-index anatase TiO2 surfaces using density functional theory calculations. The results show that the water decomposition reaction on the low-index anatase TiO2 surfaces is a structure-sensitive reaction. The reaction parameters, such as activation and reaction energies, are determined not only by the surface energy but also by the surface atomic structure and surface electronic structure. If we used the activation energy to determine the surface activity, the order of the surface activity of these six low-index surface is (001) > (103)f > (100) > (110) > (103)s > (101). The decomposed products of water on these surfaces give two kinds of surface hydroxyl radicals. One is the surface-adsorbed hydroxyl radical, which comes from the decomposed OH group, in which the oxygen atom bonds with an unsaturated 5c-Ti or 4c-Ti atom of the surface, and its hydrogen atom is directed away from the surface. The other is the stable surfaceterminated hydroxyl radical, which comes from the decomposed hydrogen atom adsorbed on an unsaturated 2c-O atom of the surface. The interaction of water with the low-index anatase TiO2 surfaces is more complicated than with a pure metal surface, due to the acid−base amphiprotic character of anatase surfaces. On the surface Ti sites, the surface loses electrons, whereas on the surface O sites, the surface gains electrons. However, in general, the surface loses electron density upon water adsorption, while there is electronic redistribution inside the water molecule and its products. The decomposition reaction of water on the low-index

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AUTHOR INFORMATION

Corresponding Author

*Tel: +86-25-83686603. Fax: +86-25-83686632. E-mail: zsli@ nju.edu.cn (Z.L.), [email protected] (Z.Z.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors would like to acknowledge financial support from the National Basic Research Program of China (973 Program, Grant Nos. 2007CB613301 and 2007CB613305), the National Natural Science Foundation of China (50732004 and 21073090), and the Jiangsu Provincial Science and Technology Research Program (BK2008028 and BE2009140).

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A Theoretical Study of Water Adsorption and Decomposition on the

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