Single Water Molecule Adsorption and Decomposition on the Low

Article pubs.acs.org/JPCC

Single Water Molecule Adsorption and Decomposition on the LowIndex Stoichiometric Rutile TiO2 Surfaces Zong-Yan Zhao* Faculty of Materials Science and Engineering, Key Laboratory of Advanced Materials of Yunnan Province, Kunming University of Science and Technology, Kunming 650093, People’s Republic of China ABSTRACT: The adsorption behavior and decomposition process of a single water molecule on the low-index rutile TiO2 surfaces, including (110), (100), and (001), were systematically investigated by using density functional theory calculations. For both molecular and dissociative adsorption, the adsorption energy on these low-index surfaces increases in the order (110) < (100) < (001), while the activation energy of water partial decomposition reaction increases in the order (100) < (110) < (001). In the case of water adsorption on (001) surface, due to its rather flat surface and the stronger interaction with water molecule or its decomposition products, the adsorption energy for water molecule or its decomposition products is the largest; the activation energy of water decomposition on (001) surface is the smallest; and the final decomposition is two surface-adsorbed hydroxyl radicals. Therefore, the photocatalytic water splitting or photocatalytic reaction on rutile (001) surface is more easily accomplished than that of the other two surfaces. The findings in this Article are consistent with experimental observations in published literature, and will be helpful for future development of efficient photocatalysts.

1. INTRODUCTION As one of the most important photocatalytic reactions, the interaction of water with TiO2 has undergone intensive experimental and theoretical studies in recent years. It is becoming a hot research topic because of the following main reasons. First, hydrogen production from photocatalytic water splitting is important in solar energy applications, which is a renewable and environment-friendly energy resource. In these reactions, water molecular is a direct participant. Second, the production of water decomposition, for example, hydroxyl radicals (−OH), plays a key role in most photocatalytic degradation reactions of organic pollutants. In these reactions, molecular water is an indirect participant. The former application can alleviate the energy crisis, while the latter application can contribute to environmental pollution control. In other words, in the photocatalytic technology, water can not only provide a reaction environment (because most photocatalytic reactions occur in aqueous environment), but also be directly or indirectly involved in the photocatalytic reactions. Finally, TiO2 is a typical conventional photocatalyst, and is always used as a prototype for the investigation of photocatalytic mechanism, resulting in the development of novel efficient photocatalysts. Therefore, a better understanding of the adsorption behavior and decomposition process of water on TiO2 surface could help to develop the efficient photocatalysts further. For relevant background and importance of this topic, the reader can be referred to the published reviews.1−3 Because the photocatalytic performance of anatase TiO2 is relatively higher than that of rutile TiO2, so the published research about the interaction of water with TiO2 was more © 2014 American Chemical Society

concentrated on the anatase phase. Thus, only a few efforts were devoted to investigate the interaction of water with rutile TiO2 in the past five years, after early research. In 1989, Kurtz et al. found that water does not dissociate on a perfect rutile TiO2 (110) surface at low coverages.4 However, the subsequent experimental and theoretical studies indicated it is difficult to determine whether water can dissociate on a perfect rutile TiO2 (110) surface, due to the small energy difference between molecular and dissociative adsorption.5−9 Especially, Harris et al. found that the slab thickness of computational models could obviously influence the calculated accuracy, and further extended their results to the infinite-slab limit using the “25% rule”.7 On the other hand, if the (110) surface contains oxygen vacancy, water molecules can spontaneously dissociate at the defect sites and form paired hydroxyls readily.9−11 The same situation also occurs for water adsorb on rutile (100) surface: early experimental works indicated molecular adsorption of water on rutile (100) surface,12,13 but later theoretical studies met considerable controversy. Most theoretical works considered that the dissociative adsorption is favored on a perfect rutile (100) surface,14,15 while other works draw different conclusions.16−18 In the case of rutile (001) surface, experimental studies and theoretical works come to a unanimous conclusion: the dissociative adsorption is favored for water on preface rutile (001) surface.14,19 The abovementioned inconsistent conclusions in published works imply Received: January 7, 2014 Revised: February 2, 2014 Published: February 6, 2014 4287

dx.doi.org/10.1021/jp500177n | J. Phys. Chem. C 2014, 118, 4287−4295

The Journal of Physical Chemistry C

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Figure 1. Clean surface and adsorption configuration of water at different states on the rutile TiO2 (110) surface. The red balls represent oxygen atoms, the gray balls represent titanium atoms, and the blue balls represent hydrogen atoms. The green dash lines represent hydrogen bonds.

atomic nuclei was less than 0.05 GPa, the displacement of the nuclei was less than 1 × 10−3 Å, and the energy change per atom was less than 1 × 10−5 eV. To improve the accuracy of calculated adsorption energies for water on TiO2 surfaces, the dipole corrections were utilized for all models, which can be essential in eliminating nonphysical electrostatic interaction between periodic images.27 The rutile TiO2 (110) surface and (100) surface are simulated by a (2 × 3) periodic slab of 10 O−Ti−O trilayers (60 TiO2 units, 180 atoms), while the rutile TiO2 (001) surface is simulated by a (2 × 2) periodic slab of 11 O−Ti−O trilayers (44 TiO2 units, 132 atoms). All of the slab models also are separated by a 20-Å-thick vacuum layer. The lengths of these models are larger than 8.84 Å, which is enough to avoid the self-interaction effects of the periodic boundary conditions. The bottom four trilayers of the slab are fixed to mimic the bulk effects. Water molecule was placed near the surface, and then optimized to get the most stable molecular adsorption state of water on these surfaces. Near the position of molecular adsorption position, the possible dissociative adsorption configurations were all considered. Because the partial decomposition (H2O → H + OH) of water is easier than the complete decomposition (H2O → 2H + O), and the surface hydroxyl radical is more critical for the photocatalysis, so we only considered partial decomposition of water in this Article. Comparing the adsorption energy of water on different surface locations, we get the most stable molecular adsorption state or dissociative adsorption state of water on these surfaces, that is, the minimum adsorption states. Subsequently, these two states were respectively set as the initial state (reactant) and the final state (product). On the basis of these configurations, the transition state on the minimum reaction energy pathway was identified, using the complete linear synchronous transit (LST) and quadratic synchronous transit (QST) search methods followed by transition-state confirmation through the nudged elastic band (NEB) method.28−34 On the basis of the calculated adsorption energy of these states, we analyzed the reaction

that water adsorption on rutile TiO2 surfaces is very sensitive to both the experimental conditions and the theoretical methods or models. Except for the above controversy, the published works about water absorbed on rutile TiO2 surfaces were independently accomplished by different research groups. So, it is very difficult to directly compare the adsorption behaviors of water on different rutile TiO2 surfaces. In our previous works, we had systematically investigated the adsorption behaviors of water on anatase TiO2 surfaces (including low-index perfect surfaces, modified (101) surface, and water/TiO2 interface).20−23 To further comprehensively understand the interaction of water with TiO2 and solve the above controversies, we used the same calculation method and set up to systematically investigate the adsorption behavior and decomposition process of water on the low-index rutile TiO2 surfaces in this Article. The results could provide a new perspective to understanding this important topic.

2. COMPUTATIONAL METHOD AND MODELS In the present study, all of the density functional theory (DFT) calculations have been carried out by using Cambridge Serial Total Energy Package (CASTEP) codes, employing the ultrasoft pseudopotential.24 Exchange and correlation effects were described by the revised Perdew−Burke−Ernzerhof for solid (PBEsol) of generalized gradient approximation (GGA).25 An energy cutoff of 340 eV has been used for expanding the Kohn−Sham wave functions. The minimization algorithm has been chosen as the Broyden−Fletcher−Goldfarb−Shanno (BFGS) scheme.26 The k-points grid sampling of the Monkhorst−Pack scheme was set as 2 × 2 × 1 in the irreducible Brillouin zone, and the fast Fourier transform grid was set as 60 × 60 × 360. To get accurate results, we optimized atomic coordinates, which were obtained by minimizing the total energy and atomic forces. This was done by performing an iterative process in which the coordinates of the atoms are adjusted so that the total energy of the structure is minimized. The relaxation run was considered converged when the force on the atomic nuclei was less than 0.03 eV/Å, the stress on the 4288



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

dOw−Hw (110)

(100)

(001)

IS TS FS IS TS FS IS TS FS

0.976 0.974 0.979 1.007 1.006 1.003 1.017 1.013 1.008

1.011 1.244 2.092 1.007 1.242 1.625 1.017 1.071 1.606

113.940 125.525 135.983 101.078 97.743 92.712 98.162 97.909 102.517

dObr−Hw 1.739 1.240 0.991 1.785 1.225 1.030 1.713 1.472 1.027

3.699 3.654 3.280 1.785 1.823 2.703 1.713 1.753 1.747

dTi5c−Ow

dOw−S

∠W−S

2.144 1.980 1.816 2.099 1.955 1.847 2.100 2.047 1.858

0.730 0.703 0.834 0.495 0.472 0.495 1.349 1.328 1.230

87.852 87.809 87.988 5.198 5.837 4.823 22.229 19.195 8.889

a

The unit of distance is angstrom, and the unit of angle is degree. The calculated bond length and angle of isolated water molecule are 0.979 Å and 104.42°. In this table, the first column shows the bond lengths of the Ow−Hw bond 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 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 water molecule and the surface is represented by ∠W−S.

Figure 2. (a) The average electron density difference of water at different states on the rutile TiO2 (110) surface along the surface normal direction. The dashed line indicates the surface position that is located at the bridging O2c atom, in which “IS” means initial state (molecular adsorption), “TS” means transition state, and “FS” means final state (dissociative adsorption). (b) The local and partial densities of state of water at different states on the rutile TiO2 (110) surface, in which the arrows indicate the surface states.

relaxed (110) surface, the 2-coordinated bridging O atoms (O2c) and the 5-coordinated Ti atoms (Ti5c) obviously shift downward, while the other saturated atoms (O3c and Ti6c) shift upward, so that the relaxed surface exhibits a wrinkle-like appearance. The surface energy of relaxed (110) surface is 0.544 J/m2, with a work function of 6.908 eV. This calculated result implies that the (110) surface is the most stable surface for rutile TiO2. When water molecule adsorbed onto (110) surface, the most stable adsorption site is located at the top site of the Ti5c atoms, in which the oxygen atom of water molecule is bonded with Ti5c atoms (2.144 Å). In this situation, the molecular plane of water is nearly perpendicular to the surface, with an angle of 87.852°. As compared to the isolated water molecule, the bond angle is increased to 113.94°, and the two O−H bonds are not equal. One hydrogen atom is pointing away from the surface (the O−H bond length is slightly decreased to 0.976 Å), and the other hydrogen atom is pointing

activity and energy for the decomposition reaction of water on rutile TiO2 surfaces. Using the above calculation method, we first optimized the bulk crystal structure of rutile TiO2 and obtained the following lattice constants: a = b = 4.6042 Å, c = 2.9473 Å. This calculation result is well consistent with the experimental measurements:35 a = b = 4.5931 Å, c = 2.9589 Å. We then optimized the structure of perfect TiO2 surfaces using the above calculation method, obtaining the surface configuration that is well consistent with previous reports.36 These calculated results indicate that the calculation models and method in the present study are reasonable.

3. RESULTS AND DISCUSSION 3.1. Water Adsorption and Decomposition on the Rutile TiO2 (110) Surface. As shown in Figure 1, on the 4289



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Figure 3. Clean surface and adsorption configuration of water at different states on the rutile TiO2 (100) surface. The legend is the same as that in Figure 1.

while it is increasing in the region near above surface. Moreover, the electron transfer is more obvious from molecular adsorption, to transition state, and to dissociative adsorption. Reflecting the electronic structure, the empty bands of water (4a2 and 1b1 states) are shifting downward below the Fermi energy level (EF), as shown in Figure 2b. At the same time, the localized electronic states of isolated water molecule are delocalizing, and distribute the whole region of the valence band, due to the interaction with the surface, except for the 1b2 state. The surface states that are related to O2c atom or Ti5c atom are disappearing or weakening, because of the saturation of dangling bonds. The interaction between hydrogen atom and O2c atom is mainly dominated by the hybridization of 1b1 and 1b2 states with O2c-2p states. These interactions are becoming stronger from molecular adsorption, to transition state, and to dissociative adsorption. Their positions are gradually shifting upward. The interaction between the oxygen atom of water and the Ti5c atom is exhibited by the hybridization of 3a1 state with Ti5c-3d states, forming relative isolated and stable band below the valence band. As compared to the electronic structure of different adsorption states, the hybridization states of dissociative adsorption are stronger and closer to EF, resulting in stronger interaction with surface. 3.2. Water Adsorption and Decomposition on the Rutile TiO2 (100) Surface. As shown in Figure 3, the (100) surface is more crinkled than the (110) surface. Along the [010] direction, the surface Ti atoms are moving in the opposite direction with the surface O atoms. The surface energy of the relaxed (100) surface is 0.824 J/m2, with a work function of 7.336 eV. When water adsorbed onto this surface, the most stable adsorption site is the bridge site between Ti5c

to the bridging O2c atom (the O−H bond length is increased to 1.011 Å), forming a hydrogen bond (1.739 Å). In the case of dissociative adsorption, the decomposition production of hydrogen atom is bonded with bridging O2c atom, with a bond length of 0.991 Å, forming a stable surface-terminated hydroxyl radical and another hydrogen bond (2.092 Å) with the oxygen atom of water. Another decomposition production, OH, is still bonded with Ti5c atom (Ow−Ti5c bond length is decreased to 1.816 Å), forming a surface-adsorbed hydroxyl radical. In the present study, the adsorption energies are 0.23, 0.319 eV, for molecular adsorption and dissociative adsorption of water on rutile (110) surface, indicating the dissociative adsorption is the favorable state, which is in line with previous works.5,11,37−39 Yet their difference (reaction energy) is very small, 0.089 eV. Between the molecular adsorption state (initial state) and the dissociative adsorption state (final state), there is a transition state with absorption energy of 0.067 eV. Also, its structural feature is much closer to that of the initial state, so it is an early transition state. The detailed structural parameters of these states are listed in Table 1. According to the definition, the activation energy of the water decomposition reaction is 0.163 eV. On the other hand, the activation energy of the reverse reaction is 0.252 eV. This implies that the water decomposition reaction is more energetically favorable on the rutile (110) surface. Because the Ti5c atom has one dangling bond, there are extra electrons on it in comparison with Ti6c atoms. Thus, when water is bonding contacted on surface, the extra electrons will be transferred from the surface to water molecule or its decomposed products. As shown in Figure 2a, in the region near below surface, the average electron density is decreasing, 4290



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Figure 4. (a) The average electron density difference of water at different states on the rutile TiO2 (100) surface along the surface normal direction. The dashed line indicates the surface position that is located at the bridging O2c atom. (b) The local and partial densities of state of water at different states on the rutile TiO2 (100) surface.

endothermic reaction, with a reaction energy of 0.174 eV. This means that the molecular adsorption is the predominant state for water on rutile (100) surface. As shown in Figure 4, the electrons in the range of 0−2 Å below the surface transfer to water molecule or its decomposition products. This phenomenon is more obvious from the molecular absorption state, to the transition state, and to the dissociative adsorption state. This calculated result indicates that electrons concentrate around the oxygen atom of water, above the surface ∼0.5 Å. In the case of molecular adsorption, the hybridization between 1b1 state and O2c-2p states is overlapping with the valence band, so the interaction between the water molecule and surface is relatively stronger. While the 1b2 state is still isolated implies that the interaction of water molecule with (100) surface is determined by its lone pairs (1b1 and 3a1 states). It is worth noticing that the hybridization of the 1b2 state with the O2c-2p states is very weak in the case of molecular adsorption, but it becomes more obvious and shifts upward to the valence band in the case of the transition state. Finally, it exhibits obvious dominance in the case of dissociative adsorption. However, in the final state, the contribution of water decomposition products is very small for this electronic state. Another obvious phenomenon is that the 3a1 state is separated from the valence band in the transition state, and it is enhanced in the final state. Furthermore, the hybridization of the 1b1 state with O2c-2p states is only partially overlapping over the top of the valence band. These electronic structures lead to the interaction of water with (100) surface being relatively weak in the case of dissociative adsorption. 3.3. Water Adsorption and Decomposition on the Rutile TiO2 (001) Surface. On (001) surface, the coordinated number of Ti atoms is four, while the coordinated number of O atoms is two, indicating the density of dangling bonds is larger than that of (110) or (100) surface. So, the surface energy of the relaxed (001) surface is the largest, 1.398 J/m2, in these three low-index surfaces. Also, its work function is 5.515 eV,

atom and O3c atom. The Ti5c atom that is bonded with Ow atom of water is noticeably moving along the [100] and [010] directions, especially in the case of dissociative adsorption. In the case of molecular adsorption, the two hydrogen atoms are equally pointing to the bridging O2c atoms, with the same hydrogen bonds of 1.785 Å. The bong angle of the water molecule slightly decreased to 101.078°, while the bond length slightly increased to 1.007 Å. The molecular plane of water is almost parallel to the surface, with an angle of 5.198°. At the transition state, one hydrogen atom is moving toward the bridging O2c atom and bonding with it (the bond lengths of Hw−Ow and Hw−O2c are 1.242 and 1.225 Å, respectively), while the other hydrogen atom is slightly moving backward to the Ow atom of water (the bond length of Hw−Ow is 1.006 Å). Finally, to the dissociative adsorption, the former Hw−Ow bond is broken, becoming a hydrogen bond (1.625 Å), and the later Hw−Ow bond is maintained at 1.003 Å. Thus, the former product becomes a surface-terminated hydroxyl radical, while the later product becomes a surface-adsorbed hydroxyl radical. The angle of the ∠Hw−Ow−Hw of decomposition product is noticeabley suppressed to 92.712°. The plane of the H+OH fragment is more parallel to the surface, with an angle of 4.823°. In the present study, the distance between the water molecule or its decomposition products and the (100) surface is the smallest, 0.472−0.495 Å, resulting in them almost lying into the microstep space of (100) surface. Because the structural feature of the transition state is similar to that of the final state (dissociative adsorption), the transition state on (100) belongs to the later transition state. The adsorption energy of these states is 1.332 eV (molecular adsorption), 1.107 eV (transition state), and 1.158 eV (dissociative adsorption), respectively. Therefore, the molecular adsorption is the stable state for water on (100) surface, which is consistent with previous works.16−18,39,40 Also, the water decomposition reaction on (100) surface needs to overcome an activation barrier with 0.225 eV. Moreover, this decomposition reaction is an 4291



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Figure 5. Clean surface and adsorption configuration of water at different states on the rutile TiO2 (001) surface. The legend is the same as that in Figure 1.

Figure 6. (a) The average electron density difference of water at different states on the rutile TiO2 (001) surface along the surface normal direction. The dashed line indicates the surface position that is located at the bridging O2c atom. (b) The local and partial densities of state of water at different states on the rutile TiO2 (001) surface.

which is the smallest in the present study. As shown in Figure 5, (001) surface is rather flatter than (110) or (100) surface, and the displacement of surface atoms is not yet obvious. When water adsorbed onto (001) surface, the most stable adsorption site is the top site of O2c atoms. At the initial state, the two hydrogen atoms are also equally pointing toward bridging O2c atoms, forming two equal hydrogen bonds (1.731 Å). The bond length of Ow−Ti5c is 2.1 Å, so the distance between water or its decomposition products and the surface is the largest

(∼1.349 Å) in the three surfaces. The molecular plane of water is tilting on the surface, with an angle of 22.229°. At the transition state, the two Hw−Ow bonds (1.013 and 1.071 Å) and the two hydrogen bonds (1.472 and 1.753 Å) are not equal. Especially, one bridging oxygen atom does not bond with the Ti6c atom, and thus its number of coordination becomes one. At the same time, it is shifting upward from the surface. Finally, in the case of dissociative adsorption, one Hw−Ow bond is broken, and the product of hydrogen atom is bonded with the 4292



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bridging O1c atom with a bond length of 1.027 Å. The bond lengths of two hydrogen bonds are 1.606 and 1.747 Å, respectively. Moreover, the position of the bridging O1c atom is almost as high as the Ow atom, resulting in that the plane of H +OH segments is almost parallel to the surface with an angle of 8.889°. Thus, at the final state, two surface-adsorbed hydroxyl radicals are formed. Because the structural feature of the transition state is similar to the initial state (molecular adsorption), the transition state on (001) belongs to the early transition state. The adsorption energy of these states is 1.417 eV (molecular adsorption), 1.308 eV (transition state), and 1.707 eV (dissociative adsorption), respectively. Therefore, the dissociative adsorption is the stable state for water on the (001) surface, which is consistent with previous works.14,19,41,42 The water decomposition reaction on (001) surface only needs to overcome an activation barrier with 0.109 eV. Moreover, this decomposition reaction is an exothermic reaction, with reaction energy of 0.29 eV. This means that the dissociative adsorption is the predominant state for water on rutile (001) surface. As shown in Figure 6a, the electrons that transfer from the surface to water or its decomposition product are not obvious on the (001) surface, as compared to the situations of water on (110) surface or (100) surface. Yet electrons above the surface are still concentrated around the Ow atom of water, which means that the electrons in the water molecular or its decomposition products will be redistributed. In the case of molecular adsorption, water mainly interacts with the surface by hybridizing between its lone pair sates and O2c-2p states, which has a certain distance with the Fermi energy level. Because of the electron redistribution, the isolated 1b2 state is more prominent. The electronic structure of the transition state is similar to that of molecular adsorption, except energy bands slightly shift upward. Because of the broken O2c−Ti6c bond and two surface-adsorption hydrogen radicals forming, the 1b1 state strongly interacted with O2c-2p states in the case of dissociative adsorption. Also, the top of the valence band is formed by this hybridized state, with a little distance from the Fermi energy level. At the same time, the hybridized states that are related to 3a1 state and 1b2 state are no longer isolated states as in the case of (110) or (100) surface. These 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 onto the (001) surface. 3.4. Comparison of Water Decomposition Reaction on Low-Index Rutile TiO2 Surfaces. One purpose of this Article is to compare the adsorption behavior and decomposition process of water on different rutile TiO2 surfaces, based on the same computational method and setup. The order of surface energy for these three low-index surfaces (γ(001) > γ(100) > γ(110)) is well in agreement with previous works,36 which means that the calculated results in the present study are reliable. The reaction pathway of water decomposition on different rutile surfaces is plotted in Figure 7, and the corresponding detailed structural parameters are listed in Table 1. The adsorption energy of water molecule on (110) surface is obviously smaller than that on (100) or (001) surface. Although the surface energy of (100) surface is larger than that of (110) surface, its activation energy of water decomposition is also larger than that of (110) surface. Furthermore, the water decomposition is an endothermic reaction on (100) surface. So, for water partial decomposition, the reaction activity of (100) surface is lower than that of (110) surface, and (001) surface

Figure 7. The comparison of decomposition reaction pathways of water on the low-index rutile TiO2 surfaces.

has the highest reaction activity with the highest surface energy. In other words, the reaction activity is not linear with the surface energy, because the surface activity is determined not only by surface energy but also by the surface atomic structure (such as surface space resistance, surface openness, and so on) and surface electronic structure. Exploring the relationship between surface activity and surface structure has been the key task of catalyst research. However, for the practical photocatalytic water splitting reaction, there are many factors to affect the final surface activity. Thus, we can only preliminarily discuss this issue, according to the calculated results of water partial decomposition on these surfaces. As mentioned above, although the surface activity is not linear with the surface energy, another linear relationship could be found. For example, the order of activation energy is just consistent with the surface work function in the present study. The surface work function describes the ability to lose an electron from the surface. It can be seen that the direction of electron transfer is from the surface to the water molecule or its decomposition products, as shown in Figures 2a, 4a, and 6a. So, from this point of view, the surface work function can explain the surface activity. To further analyze the origin of surface activity, the activation energy and the reaction energy are decomposed by the formula as defined in ref 21, and the results are listed in Table 2. From the initial state to the transition state, in the case of (001) surface, the dissociation energy of water (Ediss), the surface deformation energy (Edef), and the difference of interaction between water and surface (ΔEinter) are all the smallest in these three low-index surfaces, so the activation energy is the smallest. Combining the structural parameters in Table 1, it could be found that the structural deformation of water molecule is not obvious in the case of (001) surface, while it is obvious in the case of (100) surface. On the other hand, the (001) surface is rather flat, while the (100) surface is rather corrugated. These reasons lead to the smallest activation energy of (001) surface, and the largest activation energy of (100) surface. From the initial state to the final state, although the dissociation energy of water and the surface deformation energy are not the highest in the case of (100) surface, they have the lowest difference interaction energy, and thus the reaction energy of water decomposition is the highest on (100) surface. 4293



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Table 2. Components of the Activation and Reaction Energies of Water on the Low-Index Rutile TiO2 Surfaces activation energy/eV (110) (100) (001)

reaction energy/eV

Eact

Ediss

Edef

ΔEinter

Ereact

Ediss

Edef

ΔEinter

0.163 0.225 0.109

1.266 0.943 0.125

0.734 0.918 0.513

−1.837 −1.636 −0.529

−0.089 0.174 −0.290

5.457 3.048 2.953

1.947 1.582 1.387

−7.493 −4.456 −4.630

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4. CONCLUSIONS To overall comprehend and compare the adsorption behavior and decomposition process of water on rutile surfaces, single water molecule adsorption and decomposition on the low-index rutile TiO2 surfaces, including (110), (100), and (001), were systematically investigated in the present study, using density functional theory calculations. The calculated results show that the water decomposition reaction on the low-index rutile TiO2 surfaces is a structure-sensitive reaction. For water decomposition, the surface activity is determined not only by surface energy, but also by the surface atomic structure and surface electronic structure. On these surfaces, the dissociative adsorption is the stable state in the case of water adsorbed onto (110) or (001) surface, suggesting that the decomposition reaction of water on these surfaces belongs to the exothermal reaction, which is contrary to the situation of water adsorbed onto (100) surface. Moreover, the activation energy of water decomposition on (001) surface is the smallest, and the final decomposition products are two surface-adsorbed hydroxyl radicals. Therefore, the photocatalytic water splitting or photocatalytic reaction on rutile (001) surface is most easily carried out, ascribing to its rather flat surface and the stronger interaction with the water molecule or its decomposition products.

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +86-871-65109952. Fax: +86-871-65107922. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We would like to acknowledge financial support from the National Natural Science Foundation of China (Grant No. 21263006).

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REFERENCES

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Single Water Molecule Adsorption and Decomposition on the Low

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