O2 Adsorption and Dissociation on A Hydrogenated Anatase (101

Article pubs.acs.org/JPCC

O2 Adsorption and Dissociation on A Hydrogenated Anatase (101) Surface Liangliang Liu,† Qin Liu,† Yongping Zheng,d Zhu Wang,† Chunxu Pan,† and Wei Xiao*,‡,§ †

School of Physics and Technology, Wuhan University, Wuhan 430072, P. R. China State Nuclear Power Research Institute, Beijing 100029, P. R. China § National Energy R&D Center of Nuclear Grade Zirconium Materials, Baoji, Shaanxi 721013, P.R.China d Division of WCU Multiscale Mechanical Design, School of Mechanical and Aerospace Engineering, Seoul National University, Seoul 151-744, Republic of Korea ‡

S Supporting Information *

ABSTRACT: O2 adsorption and dissociation on a hydrogenated anatase (101) surface are studied with first-principle calculations coupled with the nudged elastic band (NEB) method. H atoms on the anatase (101) surface or at subsurface sites can increase the absolute values of the O2 adsorption energy. The O2 dissociation barriers on an anatase surface with two H atoms at the subsurface sites or with a H surface adatom and a subsurface atom are much lower than that of the dissociation on a surface with H adatoms on the (101) surface. After the dissociation, OH, H2O, and O adatoms may form on the surface. Because it is not difficult for H adatoms on the surface to diffuse to the subsurface sites, surface H doping atoms are very useful to reduce the O2 dissociation barrier. The anatase particles with hydrogenated (101) surface are efficient catalysts to oxidize the adsorbed toxic gas molecule.

1. INTRODUCTION

Water molecules may be adsorbed on TiO2 surfaces, and O2 molecules can interact with H2O molecules.14−16 In this reaction water molecules dissociate into OH groups and H adatoms.17,18 Hydrogen gas also can generate H adatoms on the anatase surfaces.19,20 H adatoms play an important role in the chemical reaction on the TiO2 surfaces. Tilocca studied O2 reaction and reactivity on a hydroxylated rutile (110) surface.21 H adatoms can improve the adsorption and reactivity of O2 molecule. After O2 adsorption, OOH, OH, H2O, and Oad intermediates form on rutile (110) surfaces.22−25 Furthermore, experimental and computational studies show that H atoms prefer to diffuse into the bulk from the TiO2 surface.19,26,27 We have studied the H adatom effect on the oxygen adsorption and dissociation on an anatase (001) surface,28 but how H atoms on the surface or at subsurface affect the O2 adsorption and dissociation is not clear. In this paper, we study the O2 adsorption and dissociation on an anatase (101) surface with

Due to its unique physical and chemical properties, TiO2 has attracted much scientific interest, for example, it can be used as photocatalyst and dye-sensitizer solar cell materials.1−3 There exist two different phases of TiO2, anatase and rutile. The photocatalytic activity of anatase powders is high and widely studied.4,5 Besides being used as a photocatalyst, TiO2 can be used as a heterogeneous catalyst.2 Surface chemical properties of TiO2 are interesting to studied. O2 molecules adsorbed on the anatase surfaces can be used to oxidize surface adsorbed toxic gas molecule or used as electron scavenger for photocatalytic processes. For example, the chemical reaction of CO oxidized by oxygen adatoms on a rutile TiO2 surface has been investigated.6,7 Because the adsorption of O2 on clean anatase surfaces is unlikely to happen,8,9 the O2 adsorption on the surfaces with defects is an interesting topic. The surface defects include O vacancy, H adatoms, and so on.10−13 Once oxygen molecules are adsorbed on the TiO2 surfaces, they may dissociate, and the oxygen adatoms on the surfaces are active oxidants. © 2014 American Chemical Society

Received: August 16, 2013 Revised: January 16, 2014 Published: January 16, 2014 3471

dx.doi.org/10.1021/jp408221x | J. Phys. Chem. C 2014, 118, 3471−3482

The Journal of Physical Chemistry C

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H adatoms on the surface or H atoms at subsurface sites. On a hydrogenated anatase (101) surface, the adsorption of an O2 molecule is much stronger than that on a clean surface. Especially, subsurface H atoms can decrease the O2 dissociation barrier efficiently.

2. CALCULATION METHODS 2.1. Computational Detail. Density functional theory (DFT) calculations coupled with the nudged elastic band (NEB) method are used to study the oxygen molecule dissociation on an anatase (101) surface. The calculations are performed using the Vienna Ab-initio Simulation Package (VASP).29−31 The valence electronic states are expanded in a set of periodic plane waves, and the interaction between core electrons and the valence electrons is implemented through the projector augmented wave (PAW) approach. Although standard DFT calculations (GGA) are unable to properly describe the localization of excess electrons, the electronic charge transfer is similar to that calculated by the GGA + U method for O2 adsorption on the reduced anatase surface; meanwhile, the O2 relative adsorption strength for different surface adsorption sites are similar for the two methods.32−34 The adsorption and dissociation of an O2 molecule on TiO2 surfaces have been studied by first-principles calculation with the GGA functional.16,28,33,35 In this work, the Perdew−Burke−Ernzerhof (PBE) GGA exchange−correlation functional is used to study the oxygen dissociation on an anatase (101) surface.36−38 In the energy calculations for the structure relaxation and the dissociation process, summations over the Brillouin zone (BZ) are performed with a 2 × 2 × 1 Monkhorst−Pack k-point mesh. The smooth part of the wave functions is expanded in plane waves with a kinetic energy cutoff of 400 eV, and the convergence criteria for the electronic and ionic relaxion are 10−4 eV and 0.05 eV/Å, respectively. Bader charge analysis is done to analyze charge populations in the periodic calculations.39 The adsorption energy of an oxygen molecule (EO2ad) or two oxygen adatoms (E2Oad) on a hydrogenated anatase (101) surface is defined as EO2ad = Esurf + O2 − Esurf − EO2 (1) E2Oad = Esurf + 2O − Esurf − EO2

Figure 1. The side view and top view of the slabs used for anatase (101) surface calculation. The large circles are Ti atoms and the red small ones are the O atoms.

top structure layer, there is one Ti5c layer and one Ti6c layer. A vacuum layer with the thickness of 10 Å on the top of the (101) surface of the super cell is used to eliminate the interaction between the neighbor cells, which is big enough to reduce the interaction between the adsorbed oxygen molecules in the neighbor unit cells due to the periodic boundary condition. In all calculations, the third bottom (TiO2) layer is fixed and the other atoms in the super cell are free to relax.32 The calculated surface energy for the anatase (101) surface is 0.49 J/m2, which is close to Lazzeri et al.’s work.44 The detailed calculation method of the surface energy is in our previous paper.45

3. RESULTS The dissociation of a H2 molecule will generate hydrogen adatoms on an anatase surface, and the dissociation of a H2O molecule can generate both a hydrogen adatom and a OH group on a TiO2 surface.20,46−48 On partially hydroxylated rutile (110) surfaces, O adatoms, OOH, OH groups, and H2O molecules are observed by STM.22,23,25 Similarly, H adatoms may affect the surface chemistry properties of anatase (101) surfaces. In this paper, the hydrogen effect on oxygen dissociation on an anatase (101) surface is studied. Since a higher hydrogen adatom concentration cannot obviously increase the oxygen molecule adsorption strength further,13,34 the hydrogen effect on the oxygen dissociation from one or two hydrogen adatoms (1/6 and 1/3 ML) is studied in our work. Neutral H adatoms are added on an anatase (101) surface or at subsurface sites. Although the electrons of H adatoms can transfer to neighboring Ti ions and will generate Ti3+ states in the band gaps and leave the H+ ions on the anatase surface or at the subsurface sites,49,50 the supercell used for the calculation is electrically neutral. 3.1. Adsorption of an O2 Molecule on an Anatase (101) Surface with Surface H Adatoms. 3.1.1. An O2 Molecule on an Anatase (101) Surface in the Presence of One H Adatom. In Figure 2 (structure A0), a hydrogen adatom is on a surface O2c atom, which is the most stable surface adsorption site on an anatase (101) surface.19,27 The calculated adsorption energy is −2.25 eV, which is close to Aschauer et al.’s result, −2.15 eV.19 The adsorption energy is the system energy minus the energy of a single H atom and a TiO2 slab. There are six Ti5c and six Ti6c ions on the (101) surface, and they are numbered from 1 to 12. Considering the six Ti5c adsorption sites of the calculation supercell, the adsorption energies for the sites 5 and 6 are equivalent and the adsorption energies for sites 10 and 12 are equivalent, too. The adsorption energies for an O2 molecule at different Ti5c sites of the surface are listed in Table 1. Sites 5 and 6 are close to the H adatom

(2)

here Esurf+O2 is the system energy of an O2 molecule on an anatase (101) slab, Esurf+2O is the system energy of two O adatoms on an anatase (101) slab, Esurf is the energy of a slab without any adsorbed oxygen molecule or adatoms, and EO2 is the energy of an O2 molecule in the gas phase. The climb image nudged elastic band (CI-NEB) method40−42 is used to search the minimal energy diffusion paths and the saddle points of the oxygen molecule dissociation on an anatase (101) surface. Between the initial and final configurations, there are four images that are used in the CI-NEB calculations. The vibration frequency calculation is performed to analyze the transition state configurations. 2.2. Configuration of the Computational Model. The lattice parameters for anatase are a = 3.830 Å and c = 9.613 Å, which is close to our previous calculations.43 A 3 × 2 surface supercell (108 atoms in the anatase slab) is used to study the oxygen molecule adsorption and dissociation. There are three stoichiometric (TiO2) structure layers in the super cell used in the anatase (101) surfaces calculations (see Figure 1). In the 3472



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Figure 3. An O2 molecule is at site no. 6 on an anatase (101) surface. A1 structure. Ti, gray; O, red; H, green.

Figure 2. A hydrogen adatom on an anatase (101) surface. The Ti atoms on the surface are numbered from 1 to 12, which are the adsorption sites for oxygen molecule or oxygen adatoms. Ti, gray; O, red; H, green.

and the absolute value of the adsorption energies are the highest. For all Ti5c adsorption sites, the O−O bond length for the adsorbed O2 molecule is about 1.32 Å, which is close to the typical value for a superoxide (O2−: 1.33 Å). For the adsorption configuration A1, an oxygen molecule is on the top of a Ti ion at site 6 and the O2 molecule rotates a little bit to attract the H adatom (see Figure 3). In structure A1, the adsorbed O2 forms two O−Ti5c bonds with a surface Ti5c ion, and the bond lengths are 2.06 Å. The adsorption energy for the O2 on the hydrogenated anatase surface is −0.82 eV. So, the H adatom makes the O2 adsorption on an anatase (101) possible, since the O2 cannot be adsorbed on a clean anatase (101) surface.9 Another adsorption configuration is shown in Figure 4. In this configuration the H adatom is strongly attracted by the O2 molecule and the H−O2c bond is weakened. As a result, an OOH group forms on the anatase (101) surface (see Figure 4). The transformation barrier for A1 to A2 is 0.41 eV by NEB calculation. The −Ti5c−O(1)− bond length is 2.25 Å, and the adsorption energy is calculated to be −0.56 eV. This adsorption configuration is less stable than the structure A1 (Table 1). So, forming an OOH radical on an anatase (101) surface is not energetically favorable. The dissociation of an adsorbed O2 molecule (structure A1) is not energetically favorable. After dissociation, the system energy is about 1 eV higher than that of the structure A1. Consequently, H adatoms on anatase (101) surface will benefit the O2 adsorption process. But the O2 dissociation is still difficult on a hydrogenated anatase (101) surface. 3.1.2. An O2 Molecule on an Anatase (101) Surface in the Presence of Two H Adatoms. Suppose there are two neighboring H adatoms (1/3 ML) on an anatase (101) surface (see Figure 5), which is a relatively stable adsorption configuration.19 The calculated O2 adsorption energies at different Ti5c sites are listed in Table 1. For all adsorption sites, the bond length of the adsorbed O2 molecule is about 1.44 Å. A

Figure 4. An O2 molecule at site no. 6 on an anatase (101) surface. A2 structure. Ti, gray; O, red; H, green.

Figure 5. Two H adatoms on an antase (101) surface. The Ti atoms on the surface are numbered from 1 to 12, which are the adsorption sites for oxygen molecule or oxygen adatoms. Ti, gray; O, red; H, green.

peroxide radical is formed, and most of the electrons are transfer to the adsorbed O2. For the adsorption site B1 (site 5, see Figure 6a,b), the two O−Ti5c bond lengths are 1.91 Å, and the absolute value of the adsorption energy is 1.67 eV, which is about double that of the structure A1. Another adsorption configuration is B2 (see Figure 6c,d). In this configuration, one H atom is attracted by the O2 molecule and the other H adatom is still on the surface O2c atom. The barrier for the transformation process from B1 to B2 is 0.18 eV. As a result, an OOH radical is formed on the surface and the adsorption energy is −1.71 eV, which is lower than that of B1. In B2 configuration, the O(1)−O(2) bond of the O2 molecule is 1.46 Å, the Ti5c−O(2) bond length is 2.22 Å, and the Ti5c−O(1) bond length is 1.93 Å, which is close to the Ti5c−O bond length of structure B1. If the other H adatom also migrates to adsorbed

Table 1. Oxygen Gas or Two Adatoms Adsorption Energy on an Anatase (101) Surface A0

ΔE

B0

ΔE

C0

ΔE

D0

ΔE

4 5, 6(A1) 10, 12 11 A2

−0.71 −0.82 −0.73 −0.78 −0.56

4,6 5(B1) 10 11, 12 B2 B3 B1′ B2′ B3′

−1.35 −1.67 −1.25 −1.45 −1.71 −1.83 −1.55 −2.11 −2.27

4, 11 5(C1) 6, 10 12 C2 C1′ C1″ C2′ C2″

−1.36 −1.43 −1.13 −1.23 −1.51 −1.34 −1.04 −1.92 −1.73

4, 6 5(D1) 10, 11 12 D1′ D1″

−1.03 −1.07 −1.02 −0.93 −0.76 −0.94

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Figure 6. The configurations of an O2 molecule on an anatase (101) surface with two H adatoms. The unit of the bond length is Å.

Figure 7. Configurations after the oxygen dissociation. There are two H adatoms near the oxygen molecule. The unit of the bond length is Å.

O2 molecule, an HOOH (hydrogen peroxide) radical forms on the surface (see structure B3 in Figure 6e,f). The O2 adsorption energy for B3 is −1.83 eV. The transformation barrier from B2 to B3 is 0.32 eV. All of the related energies are listed in Table 1. The adsorption energy for an H2O2 radical on an anatase (101) surface is −0.74 eV, which is close to Mattioli et al.’s data.51 3.1.3. Adsorption Configurations after O2 Dissociation. Since OH, H2O, and O adatoms are found on hydrogenated rutile surfaces,22,23 structures B1′, B2′, and B3′ are considered as the configuration after the dissociation (see Figure 7). Suppose the O2 molecule in structure B1 dissociates and it generates an oxygen adatom on a Ti5c atom with a 1.70 Å O− Ti5c bond length. Meanwhile, the other oxygen atom shares a lattice with a surface O2c. The two H adatoms attract the O adatom (see B1′ in Figure 7a,b). In structure B2′, one H adatom migrates to the O adatom due to the attracting force, and the system energy is lowered. After the dissociation, an OH group is on the surface and the other O atom of the O2 molecule shares a lattice with a surface O2c atom (see Figure 7c,d). The adsorption energy for the two oxygen atoms on the hydrogenated anatase surface is −2.11 eV for the configuration B2′. The barrier for the transformation from B1′ to B2′ is 0.19 eV, as calculated by the NEB method. It suggests that this transformation happens easily. The H atom weaken the O(2)− Ti5c bond, and this bond length in B2′ (1.85 Å) configuration is longer than that for the B1′ configuration (1.70 Å). This OH group can further attract the other H adatom on the surface and form a H2O molecule on the surface. It is the configuration B3′ (see Figure 7e,f). The energy barrier for forming a H2O molecule from the B2′ configuration is 0.32 eV. The adsorption energy for the H2O molecule on the surface in the B3′ structure is −0.80 eV. This value agrees with Vittadini et al.’s calculation, which is −0.74 eV.48 The density of states (DOS) of the adsorption configurations in Figures 6 and 7 are calculated to investigate the charge

transfer and the electron scavenger properties of O2. The DOSs are shown in Figure 8. For a hydrogenated anatase surface with an adsorbed O2 molecule (Figure 6), the extra charges from H adatoms transfer to the adsorbed O2 molecule (Figure 8, B1). Those electrons take the π2p * states of the O2 molecule. One peak of the DOS of the adsorbed O2 is inside the band gap. When a H+ migrates to the adsorbed O2, the localized states of the O2 shift into the valence band and the system energy is lowered (Figure 8, B1→B3). The Bader population analysis shows that the charges on the adsorbed O2 molecule for the structure B1, B2, and B3 are 1.18, 1.57, and 2.0 e−, respectively. The adsorption energies for these three configurations increase correspondingly. The more electrons transferred to the adsorbed O2 molecule, the more adsorption energy that can be released. This agrees with Tilocca’s work.21,52 For the B1′ configuration after dissociation in Figure 7, there is no empty state (see the DOS in Figure 8), and the excess charges mainly tranfer to O(2) adatom, which obtains 1.5 e− by Bader charge analysis. If the two H+ ions bond to the O(2) adatom (from B1′ to B3′ in Figure 7), similarly, the more electrons that transfer to the O(2) adatom, and the higher adsorption energies will be released. 3.2. Dissociation of an O2 Molecule on a Hydrogenated Anatase (101) Surface. For the three different O2 adsorption configurations shown in Figure 6, we first consider the O2 molecule dissociating from structure B1 to B1′. The schematic of this process is shown in Figure 9, and the energy barrier for this process is 1.78 eV. The direction of the O2 dissociation is nearly along [101̅]. At the transition state, the distance between two O adatoms is 2.06 Å, and the distance between O(1) and O2c is 1.78 Å. It suggests that the O−O bond of the oxygen molecule is broken and no new bond forms between O(1) and O2c. The charge density distribution of TS1 3474



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Figure 8. Density of states (DOS) of the adsorption systems, including O2 or O adatom adsorption systems, and transition states (TS) systems. The red lines are the DOS contributions from the adsorbed oxygen molecule or two adatoms.

Figure 9. An O2 molecule dissociates on a hydrogenated anatase (101) surface. There are two H adatoms on the surface. The initial structure B1, transition state TS1, and the final configuration B1′ are shown in the figure. Suppose the system energy of B1 is zero, then the dissociation energy barrier is 1.78 eV. The charge density distribution of the TS1 along the dashed line is in the right figure. It is hard to see the chemical bond between O(1) and O(2), and O(1) and O2c. The blue circles represent the adsorption O atoms and the green ones are H atoms.

Figure 10. An O2 molecule dissociates on a hydrogenated anatase (101) surface from structure B2 to B2′. The initial structure B2, transition state TS2, and final configuration B2′ are shown in the figure. The dissociation energy barrier is 1.75 eV. After dissociation, the system energy is lowered. The charge density distribution of the TS2 along the dashed line is in the right figure.

The second O2 dissociation path is from structure B2 to B2′, with an energy barrier of 1.75 eV (see Figure 10). This barrier is similar to that of the dissociation process from structure B1 to B1′. After the dissociation, the system energy is lowered and 0.4 eV thermal energy is released. The charge density distribution of TS2 shows that the adatom O(1) does not form a chemical bond with O(2) or with the surface O2c. The two empty states in the band gap from the O(1) adatom in the DOS of TS2 (Figure 8) also suggest that the O(1) does not form a chemical bond with other surface oxygen atoms. Bader

shows that the charge density between O(1) and O(2) adatoms, or between O(1) and O2c, is low. Since there is no chemical bond between these two oxygen atom pairs, the p orbitals of the two O adatoms are not fully occupied; we can see this from the DOS of TS1 in Figure 8. Although the system energy after the dissociation (B1′) is 0.13 eV higher than the energy of configuration B1, it only takes 0.19 eV to transfer strucure B1′ to a more stable strucure B2′, the system energy of which is 0.56 eV lower than that of B1′. 3475



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charge analysis shows that the charge number of the two O adatoms are 0.3 e− for O(1) and 1.5 e− for O(2), which means that most excess electrons transfer to the O(2) adatom. Finally, the oxygen dissociation from structure B3 is studied. In this case, the H2O2 becomes two OH radicals on the surface at sites 4 and 5. After the dissociation, system energy increases 1.4 eV. Since the final configuration is unstable compared with the initial one, this process is difficult to achieve. If there is one H adatom on the surface of the anatase (101) supercell, this H adatom can benefit the oxygen molecule adsorption. But the dissociation process is difficult. For the other case, if there are two H adatoms on the surface, two possible reaction paths for the dissociation process are proposed: path 1: +H +

O2 + 2e− → O2 2 −(B1) → Oad 2 − + Oad (B1′)←→ ⎯ +H +

OH− + Oad (B2′) ←→ ⎯ H 2O + Oad (B3′)

(3)

Figure 12. An O2 molecule is adsorbed on an anatase (101) surface with a H adatom on the surface and a H atom at a subsurface site. The unit of the bond lengths is Å.

path 2: +H +

O2 + 2e− → O2 2 −(B1) ←→ ⎯ OOH−(B2)→ +H +

OH− + Oad (B2′) ←→ ⎯ H 2O + Oad (B3′)

(4)

For the two dissociation paths, the dissociation barriers are higher than 1.7 eV. If the O2 dissociation barrier can be decreased, it will be more efficient to generate the O adatoms on the anatase (101) surfaces. Consequently, the toxic gases on the surfaces can be oxidated more efficiently. Next, the hydrogen effect from the H atoms at the subsurface sites will be studied to see how it can affect O2 adsorption and dissociation on the anatase (101) surface. 3.3. Adsorption of an O2 Molecule or O Adatoms on an Anatase (101) Surface with Surface and Subsurface H Adatoms. Since surface H adatoms can diffuse into subsurface sites,19,27 it is interesting to study the synergistic effect from a surface H adatom and a subsurface H atom on the O2 adsorption and dissociation on the surface. In Figure 11, one

Figure 11. One H adatom is on an anatase (101) surface and a H atom is at a subsurface site. The surface Ti atoms are numbered from 1 to 12, which are possible adsorption sites for an oxygen molecule or oxygen adatoms. Ti, gray; O, red; H, green.

H adatom is on a surface O2c and another H atom is at a subsurface site, which is a relatively stable adsorption configuration according to Islam et al.’s work.27 The adsorption energies for an O2 molecule on different Ti5c adsorption sites are listed in Table 1. In the structure C1 (see Figure 12a,b), the O−O bond length is 1.44 Å, and the adsorption energy is −1.43 eV, which is 0.24 eV higher than that of the structure B1. In the DOS of structure C1 (Figure 14), partial excess electrons

Figure 13. The configurations after the oxygen dissociation on an anatase (101) surface with a surface H adatom and a subsurface H atom.

localize on the antibond orbit (π*) of the adsorbed molecule and form two picks. These two picks are similar to that of structure B1, but their energies are relatively higher, and they 3476



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3.4. Dissociation of an O2 Molecule on an Anatase (101) Surface with a Surface H Adatom and a Subsurface H Atom. O2 dissociation on an anatase (101) surface with a surface H adatom and a subsurface H atom is studied in this section. First, suppose the initial oxygen molecule adsorption configuration is C1 and the final structure is C1′ (Figure 15), the dissociation barrier is calculated, which is 1.82 eV. This barrier is close to that for the O2 dissociation on a surface with two H adatoms (see Figure 9). From the charge density distribution of its transition state TS3, the two O adatoms do not form chemical bond with each other, or with surface O2c atoms. The dissociated configuration (C1′) can transfer to configuration C2′. It takes 0.28 eV thermal energy to overcome the barrier and the system energy for C2′ is 0.58 eV lower than that for C1′. The O2 dissociation path from C1 to C1″ is also studied and the energy barrier is 1.25 eV (see Figure 16), which is much lower than the barrier of path in Figure 15. At the transition state, O(1) is on a neighboring Ti6c and O(2) is still on the Ti5c. The bond length of Ti5c−O(2) is 1.75 Å which is shorter than that of C1 and O(2) obtains 1.02 e− electron charges which is higher than that of C1. It suggests that Ti5c−O(2) bond is stronger than that of the initial structure C1. The distance of the two O adatoms is 1.98 Å, and there is no electronic distribution between them at the transition state (see Figure 16). So, the covalent bond of the adsorbed oxygen molecule O(1)−O(2) is broken. Meanwhile, the charge distribution between the O(1) and O2c atoms shows that these two atoms form a chemical bond. The bond length of the O(1)−O2c is 1.69 Å. Accordingly, in the DOS of TS4, most of the electron states of the two O adatoms are occupied (TS4, Figure 14). So in this dissociation process, two O adatoms are strongly bonded on the surface and the dissociation barrier is reduced. Although the system energy increases after the dissociation, it takes 0.22 eV for structure C1″ to transfer to a more stable structure C2″, the system energy of which is 0.69 eV lower than that of C1″. Suppose a H adatom is adsorbed by the O2 and the initial configuration is C2, then the dissociation process is simulated and the energy barrier is calculated. After dissociation, the final configurations are C2′ and C2″. The schematic of the transition from C2 to C2′ is shown in Figure 17 and the dissociation barrier is 1.17 eV. This barrier is lower than that of the transition from structure B2 to B2′ (Figure 10). In the transition state (TS5), the bond length of Ti−O(1) is 1.81 Å and the distance between O(1) and the surface O2c is 1.65 Å. The charge distribution between O(1) and O2c is high, which

Figure 14. Density of states (DOS) of the adsorption status, transition states, and dissociated configurations.

are close to the bottom of the conduction band. As a result, the system energy is higher and the absolute value of adsorption energy is lower than that of B1. Bader charge analysis shows that the number of electrons transferred to the adsorbed O2 is 1.09 e−, while that is 1.18 e− for the structure B1. In the adsorption configuration C2 (see Figure 12c,d), the surface H atom is attracted by the adsorbed O2, and the energy barrier for the transformation from C1 to C2 is 0.24 eV. As a result, an OOH radical is formed on the surface and the O2 adsorption energy is −1.52 eV. Bader charge analysis shows that the O2 gains 1.50 e−, which is 0.43 e− higher than that of C1. Consequently, the localized states of the O2 shift toward to the valence band (C2, Figure 14). In the configuration C2, the bond length of O(1)−O(2) is 1.46 Å, and the bond lengths of Ti5c and two O adatoms are 1.91 and 2.17 Å, which are similar to that of configuration B2. Suppose an oxygen molecule is adsorbed on an anatase (101) surface and the adsorption configuration is C1; after dissociation the configuration may be C1′ or C1″ (see Figure 13). For these two configurations, one oxygen atom is on a surface Ti5c atom and the other oxygen atom shares a lattice with a surface O2c. If the initial O2 adsorption configuration is C2, after dissociation the configuration may become C2′ or C2″, and if the surface H adatom is attracted by O(2), the configurations C1′ and C1″ become the structures C2′ and C2″. The energy barriers for C1′ → C2′ and C1″ → C2″ are 0.28 and 0.22 eV, respectively. The system energies of C2′ and C2″ are lower than the energies of C1′ and C1″ (see Table 1).

Figure 15. An O2 molecule dissociates on a hydrogenated anatase (101) surface from configuration C1 to C1′. TS3 is the transition state of the dissociation process. The side view of the charge density of the transition state along the dashed line is shown in the right figure. The blue circles represent the adsorbed O atoms and the green ones represent H atoms. 3477



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Figure 16. An O2 molecule dissociates on a hydrogenated anatase (101) surface from structure C1 to C1″. The top view of the charge density of the transition state from the surface (101) is shown in the right figure.

Figure 17. An O2 molecule dissociates on a hydrogenated anatase (101) surface from structure C2 to C2′. The initial adsorption configuration, saddle point, and dissociated structure are shown in the figure. The side view of the charge density distribution of the saddle point along the dash line is shown in the right figure.

toward the surface to form a Ti6c−O(1) bond and the bond length is 2.10 Å. The interesting thing is that the O−O bond of the adsorbed O2 molecule does not break and the O−O bond length is still 1.44 Å, which is the same as that of the structure C2. The concerted motion of the Ti6c atom and the adsorbed oxygen molecule makes the dissociation barrier much lower. For this dissociation process, the adsorption energy for C2 is −1.51 eV. The thermal energy released from the adsorption process is large enough to overcome this dissociation barrier. After the dissociation, the system energy becomes lower; the reaction energy for this exothermic process is 0.22 eV. 3.5. Adsorption of an O2 Molecule or O Adatoms on an Anatase (101) Surface with Two Subsurface H Atoms. In this and the following subsection, the hydrogen effect of two H atoms at subsurface sites on the O2 adsorption and dissociation is studied. In Figure 19 two H atoms are at subsurface sites, which is a relatively stable configuration according to Aschauer et al.’s work,19 and this slab is used to investigate the oxygen adsorption and dissociation. The O2 adsorption energies on all possible Ti5c sites are listed in Table 1. In the adsorption configuration D1 (see Figure 20a,b), the O−O bond length is 1.44 Å, and the Ti5c−O bond length is 1.88 Å. In the DOS of structure D1 (Figure 21), a part of excess electrons localize on the adsorbed molecule, and these states (π*2p) are inside the band gap and form two picks. These two picks are similar to that of structure B1 and C1, but their energies are higher. As a result, the system energy is higher and the absolute value of adsorption energy is lower than that of B1 and C1. Bader charge analysis shows that the number of electrons transferred to the adsorbed O2 is 1.01 e−, while it is 1.18 e− and 1.09 e− for the structures B1 and C1, respectively. After dissociation, the oxygen molecule in structure D1

suggests that a covalent bond is formed between these two atoms. Similar to TS4 (Figure 16), the OH group is closer to the surface Ti atom and the other O of the oxygen molecule forms a bond with a surface O2c at TS5. These strong bonds with the surface may lower the dissociation barrier. The dissociation of an O2 molecule on a hydrogenated anatase (101) surface from the structure C2 to C2″ is calculated and the energy barrier for this process is 0.83 eV, which is lower than that of all the paths we discussed before (see Table. 2). In its transition state (see Figure 18), a surface Ti6c (at site 7) above the subsurface H atom moves outward Table 2. Dissociation (Ea) and Recombination (Eb) Energy Barriers for an O2 Molecule on a Hydrogenated Anatase (101) Surfacea start

TS

end

ΔEa (eV)

ΔEb (eV)

Ti5c5−O3c (Å)

Ti6c7−O3c (Å)

Ti6c8−O3c (Å)

B1 B2 C1 C1 C2 C2 D1 D1

TS1 TS2 TS3 TS4 TS5 TS6 TS7 TS8

B1′ B2′ C1′ C1″ C2′ C2″ D1′ D1″

1.78 1.75 1.82 1.25 1.17 0.83 1.19 1.13

1.65 2.15 1.73 0.86 1.58 1.05 0.88 1.00

2.37 2.30 2.50 2.37 2.20 1.96 2.44 2.92

1.92 1.91 2.25 2.62 2.26 2.81 2.61 2.30

1.92 2.04 1.95 1.95 2.10 2.01 2.27 2.32

ΔEa = E2 − E1, ΔEb = E2 − E3; E1 is the energy of the initial adsorption configuration, E2 is the energy of the saddle point, and E3 is the energy of the final configuration after dissociation. The bond lengths (Å) of surface Ti atoms (around the O adatoms) and subsurface O3c are listed. The Ti atoms are at sites 5, 7, and 8, which have been marked in the TSs. a

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Figure 18. An O2 molecule dissociates on a hydrogenated anatase (101) surface from structure C2 to C2″. The initial adsorption state, saddle point, and dissociated structure are shown. The top view of the charge density of the transition state from the surface (101) is shown in the right figure.

Figure 19. There are two H atoms at subsurface sites of an antase (101) surface. The Ti atoms on the surface are numbered from 1 to 12, which are the adsorption sites for an oxygen molecule or oxygen adatoms. Ti, gray; O, red; H, green. Figure 21. Density of states (DOS) of the adsorption system, transition states (TS), and dissociated state are shown in the figure. The red lines are the DOS contributed from the adsorbed oxygen molecule or dissociated oxygen adatoms.

lattice site with a surface O2c atom. The adsorption energies for structures D1, D1′, and D1″ are listed in Table 1. 3.6. Dissociation of an O2 Molecule on an Anatase (101) Surface with Subsurface H Adatoms. The O2 dissociation on a hydrogenated anatase (101) surface with two subsurface H atoms is calculated and the schematic of this process is shown in Figure 22. The initial structure is D1 and the final configuration is D1′. The dissociation path and barrier are similar to that of the process in Figure 16, in which one H adatom is on the surface and a H atom is at a subsurface site. This barrier (1.19 eV) is much lower than the dissociation barriers on a hydrogenated surface with two H adatoms on the surface (see Table 2). However, the reaction is an endothermic process and the system energy increases after the dissociation (0.31 eV higher). After the dissociation, the recombination of an oxygen molecule may happen (see Table 2). If the structure D1″ is the final configuration, the dissociation barrier for this process is 1.13 eV. This process is also an endothermic reaction and the reaction energy is 0.13 eV (see Figure 23). Since the adsorption process for the configuration D1 is an exothermic process and 1.07 eV thermal energy is released, it is close to the dissociation reaction energies. In the TS7, the distance between one adatom O(1) and a surface O2c is 1.67 Å, and the bond length for Ti5c−O(2) atom is 1.72 Å. For the TS8, the bond lengths of Ti5c−O(2) and O(1)−O2c are 1.67 and 1.42 Å. The charge distribution between O(1) and surface O2c in these two dissociation processes (see Figures 22 and 23) is high and it suggests that a chemical bond is formed between these two atoms. Most of the states of the two O adatoms of the adsorbed oxygen molecule are occupied by electrons (TS7 and TS8 in

Figure 20. O2 adsorption state and dissociated configurations on a hydrogenated anatase (101) surface with two subsurface H atoms. The unit of the bond length is Å.

becomes two oxygen adatoms on the anatase (101) surface. Configurations D1′ and D1″ in Figure 20 are two possible configurations after the O2 dissociation from structure D1. One O adatom is still on the surface and the other O adatom share a 3479



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Figure 22. An O2 dissociates on a hydrogenated anatase (101) surface with two subsurface H atoms from structure D1 to D1′. The initial adsorption configuration, saddle point, and dissociated structure are shown in the figure. The top view of the charge density of the transition state from the surface (101) is shown in the right figure.

Figure 23. An O2 dissociates on a hydrogenated anatase (101) surface with two subsurface H atoms from structure D1 to D1″. The initial adsorption configuration, saddle point, and dissociated structure are shown in the figure. The side view of the charge density of the saddle point along the dash line is shown in the right figure.

dissociation process the O−O bond of an oxygen molecule does not break due to the concerted movement of the surface Ti atom, which makes the dissociation barrier even lower. Therefore, the dissociation barrier is determined by the chemical bond formed at the transition state during the dissociation process. Since the location of H atoms results in the different dissociation barriers, it is interesting to see the relation between the location of H atoms and the barriers or the transition states. For the dissociation paths in Figures 9, 15, and 23 (whose transition states are TS1, TS3, and TS8), the initial adsorption configurations, the final dissociated structures, and the reaction energies (0.11 ± 0.02 eV) are similar for these three dissociation paths, but the dissociation barriers are not same. So, the differences are mainly from the corresponding transition states. For example, one O adatom and a surface O2c form a chemical bond in TS8. Meanwhile, the distances between the surface Ti atoms (sites 5, 7, and 8) and their subsurface neighbor O3c in TS8 are much longer than those in the TS1 and TS3 (see Table 2). It shows that the deformation of the surface is more serious in TS8 due to the subsurface H atoms, which facilitates the formation of a new O−O2c bond. Since the subsurface H atoms form OH bonds with subsurface O3c atoms, the subsurface O3c atoms are passivated, and their interaction with the surface Ti atoms is weakened. Although the surface deformation of TS3 is more serious than that of TS1, it is not enough to form a new Oad−O2c bond. As a result, the dissociation barrier does not decrease. Similarly, H atoms weaken the Ti−O bond and introduce structure distortion, which is also found in TiO2 nanotubes.53 For paths in Figures 10 and 17, the initial adsorption configurations, dissociated

Figure 21) and it suggests that the adatoms are bonded with other surface atoms. So, if two H adatoms are at the subsurface sites, then the two O adatoms are strongly bonded with the surface during the O2 dissociation process and the dissociation barriers decrease dramatically.

4. DISCUSSION On a reduced anatase surface, no matter if the H atoms are on the surface or at the subsurface sites, the unpaired electrons are always localized on Ti 3d orbitals inside the band gap.27 Once an O2 molecule is adsorbed on the surface, the excess charge will transfer to the adsorbed O 2 molecule. For the O2 dissociation process on a hydrogenated anatase (101) surface, the dissociation barrier decrease dramatically, especially on a surface with two H adatoms at subsurface sites or on a surface with a surface H adatom and a H atom at subsurface site. It suggests that the location of H atoms affects dissociation barriers. The calculated O2 dissociation paths in this paper can be categorized into three types. In the first category (TS1, TS2, and TS3), the dissociation barriers are close to 1.80 eV (Table 2) and the O−O bond of oxygen molecule is broken at the transition states of these paths. Meanwhile, no new bond is formed between the O adatom and surface O2c at the transition state. In the second case (TS4, TS5, TS7, and TS8), the dissociation barriers are close to 1.20 eV (Table 2) and the O− O bond of an oxygen molecule is broken at the transition states of this category. But one of the O adatoms forms a new chemical bond with a surface O2c atom at transition states, which reduces the barrier significantly. In the last category (TS6), the barrier is only 0.83 eV. At the transition state of this 3480



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structures, and reaction energies are also similar. But the dissociation barriers are not the same, since the configurations of the transition states TS2 and TS5 are different. In TS5, the subsurface H atom again introduces the surface distortion, which facilitates one O adatom to form a chemical bond with a surface O2c. This concerted motion lowers the system energy of the transition state and the dissociation barrier. Similar phenomena can be found in the transition states TS4 and TS7. An extreme transition state configuration is TS6, where the surface O−O bond of the adsorbed O2 molecule does not break during the concerted motion at the transition state and the dissociation barrier is even lower. In general, the subsurface H atoms can passivate the subsurface O3c, and as a result, the bonds between the surface Ti and subsurface O atoms are weakened. This deformation at the transition state keeps a concerted dissociation process, in which chemical bonds are formed between the O adatoms with other O adatom or surface O2c. Consequently, the configuration of the transition state and the dissociation barrier vary with the location of H atoms. Especially, the barrier from structure C2 to C2″ is only 0.83 eV (see Table 2), which is smaller than the O2 dissociation barrier on Co- or Ni-doped graphene (about 1 eV).54 Since the O2 dissociation barrier on a Pt(111) surface is 0.86 eV,55 the TiO2 particles with H atoms at subsurface sites are promising catalysts for O2 dissociation.

ASSOCIATED CONTENT

S Supporting Information *

Additional figures. This material is available free of charge via the Internet at http://pubs.acs.org.

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REFERENCES

(1) Emeline, A.; Ryabchuk, V.; Serpone, N. Photoreactions Occurring on Metal-oxide Surfaces are not all Photocatalytic: Description of Criteria and Conditions for Processes to be Photocatalytic. Catal. Today 2007, 122, 91−100. (2) Fujishima, A.; Zhang, X.; Tryk, D. A. TiO2 Photocatalysis and Related Surface Phenomena. Surf. Sci. Rep. 2008, 63, 515−582. (3) Zhu, Y.; Ding, C. Characterization of Plasma Sprayed Nanotitania Coatings by Impedance Spectroscopy. J. Eur. Ceram. Soc. 2000, 20, 127−132. (4) Linsebigler, A. L.; Lu, G.; Yates, J. T. Photocatalysis on TiO2 Surfaces: Principles, Mechanisms, and Selected Results. Chem. Rev. 1995, 95, 735−758. (5) Xu, M.; Gao, Y.; Moreno, E. M.; Kunst, M.; Muhler, M.; Wang, Y.; Idriss, H.; Wöll, C. Photocatalytic Activity of Bulk TiO2 Anatase and Rutile Single Crystals Using Infrared Absorption Spectroscopy. Phys. Rev. Lett. 2011, 106, 138302. (6) Mitsuhara, K.; Okumura, H.; Visikovskiy, A.; Takizawa, M.; Kido, Y. Reaction of CO with O Adatoms on Rutile TiO2 Surfaces. Chem. Phys. Lett. 2011, 513, 84−87. (7) Wang, Z.; Zhao, Y.; Cui, X.; Tan, S.; Zhao, A.; Wang, B.; Yang, J.; Hou, J. G. Adsorption of CO on Rutile TiO2 (110)-1 × 1 Surface with Preadsorbed O Adatoms. J. Phys. Chem. C 2010, 114, 18222−18227. (8) Filippone, F.; Mattioli, G.; Bonapasta, A. A. Reaction Intermediates and Pathways in the Photoreduction of Oxygen Molecules at the (101) TiO2 (Anatase) Surface. Catal. Today 2007, 129, 169−176. (9) Muhich, C. L.; Zhou, Y.; Holder, A. M.; Weimer, A. W.; Musgrave, C. B. Effect of Surface Deposited Pt on the Photoactivity of TiO2. J. Phys. Chem. C 2012, 116, 10138−10149. (10) 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. (11) Aschauer, U.; Selloni, A. Influence of Subsurface Ti Interstitials on the Reactivity of Anatase (101). Proc. SPIE 2010, 7758, 77580B. (12) Hussain, A.; Gracia, J.; Nieuwenhuys, B. E.; Niemantsverdriet, J. W. (Hans). Chemistry of O- and H-Containing Species on the (001) Surface of Anatase TiO2: A DFT Study. ChemPhysChem 2010, 11, 2375−2382. (13) Deskins, N. A.; Rousseau, R.; Dupuis, M. Defining the Role of Excess Electrons in the Surface Chemistry of TiO2. J. Phys. Chem. C 2010, 114, 5891−5897. (14) Nakamura, R.; Imanishi, A.; Murakoshi, K.; Nakato, Y. In Situ FTIR Studies of Primary Intermediates of Photocatalytic Reactions on Nanocrystalline TiO2 Films in Contact with Aqueous Solutions. J. Am. Chem. Soc. 2003, 125, 7443−7450. (15) Henderson, M. A.; Epling, W. S.; Peden, C. H. F.; Perkins, C. L. Insights into Photoexcited Electron Scavenging Processes on TiO2 Obtained from Studies of the Reaction of O2 with OH Groups Adsorbed at Electronic Defects on TiO2(110). J. Phys. Chem. B 2003, 107, 534−545. (16) Li, Y.-F.; Liu, Z.-P.; Liu, L.; Gao, W. Mechanism and Activity of Photocatalytic Oxygen Evolution on Titania Anatase in Aqueous Surroundings. J. Am. Chem. Soc. 2010, 132, 13008−13015. (17) Tilocca, A.; Selloni, A. Reaction Pathway and Free Energy Barrier for Defect-Induced Water Dissociation on the (101) Surface of TiO2-Anatase. J. Chem. Phys. 2003, 119, 7445−7450. (18) 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. (19) Aschauer, U.; Selloni, A. Hydrogen Interaction with the Anatase TiO2(101) Surface. Phys. Chem. Chem. Phys. 2012, 14, 16595−16602. (20) Jiang, X.; Zhang, Y.; Jiang, J.; Rong, Y.; Wang, Y.; Wu, Y.; Pan, C. Characterization of Oxygen Vacancy Associates within Hydrogenated TiO2: A Positron Annihilation Study. J. Phys. Chem. C 2012, 116, 22619−22624.

5. CONCLUSIONS O2 adsorption and dissociation on a hydrogenated anatase (101) surface are studied with first-principle calculation coupled with the NEB method. H adatoms on the surface or at the subsurface can increase the absolute values of adsorption energy of an O2 on an anatase (101) surface. The O2 dissociation barriers on an anatase surface with a H atom on the surface and a subsurface H atom or two H atoms at the subsurface sites are much lower than the barrier for the dissociation on a surface with H adatoms on the (101) surface. After the dissociation, OH, H2O, and O adatoms may form on the surface. Because it is not difficult for H adatoms on the surface to diffuse to the subsurface sites, surface H doping atoms are very useful to reduce the O2 dissociation barrier. The anatase particles with hydrogenated (101) surface are efficient catalysts to oxidize the adsorbed toxic gas molecule.

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +86-189-1161-8696. Fax: +86-10-5819-7987. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported by the Chinese National Basic Research Program (973 Program) project 2009CB939705, the Chinese National Science Foundation projects 10704058 and 11275142, and the Fundamental Research Fund for the Central Universities (No. 2012202020212). 3481



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(21) Tilocca, A.; Di Valentin, C.; Selloni, A. O2 Interaction and Reactivity on a Model Hydroxylated Rutile(110) Surface. J. Phys. Chem. B 2005, 109, 20963−20967. (22) Zhang, Z.; Du, Y.; Petrik, N. G.; Kimmel, G. A.; Lyubinetsky, I.; Dohnalek, Z. Water as a Catalyst: Imaging Reactions of O2 with Partially and Fully Hydroxylated TiO2(110) Surfaces. J. Phys. Chem. C 2009, 113, 1908−1916. (23) Du, Y.; Deskins, N. A.; Zhang, Z.; Dohnalek, Z.; Dupuis, M.; Lyubinetsky, I. Imaging Consecutive Steps of O2 Reaction with Hydroxylated TiO2(110): Identification of HO2 and Terminal OH Intermediates. J. Phys. Chem. C 2009, 113, 666−671. (24) Matthiesen, J.; Wendt, S.; Hansen, J.; Madsen, G. K. H.; Lira, E.; Galliker, P.; Vestergaard, E. K.; Schaub, R.; Lægsgaard, E.; Hammer, B.; et al. Observation of All the Intermediate Steps of a Chemical Reaction on an Oxide Surface by Scanning Tunneling Microscopy. ACS Nano 2009, 3, 517−526. (25) 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. (26) Enevoldsen, G. H.; Pinto, H. P.; Foster, A. S.; Jensen, Mona C. R.; Hofer, W. A.; Hammer, B.; Lauritsen, J. V.; Besenbacher, F. Imaging of the Hydrogen Subsurface Site in Rutile TiO2. Phys. Rev. Lett. 2009, 102, 136103. (27) Islam, M. M.; Calatayud, M.; Pacchioni, G. Hydrogen Adsorption and Diffusion on the Anatase TiO2(101) Surface: A First-Principles Investigation. J. Phys. Chem. C 2011, 115, 6809−6814. (28) Liu, L.; Wang, Z.; Pan, C.; Xiao, W.; Cho, K. Effect of Hydrogen on O2 Adsorption and Dissociation on a TiO2 Anatase (001) Surface. ChemPhysChem 2013, 14, 996−1002. (29) Kresse, G.; Hafner, J. Ab Initio Molecular Dynamics for Liquid Metals. Phys. Rev. B. 1993, 47, 558−561. (30) Kresse, G.; Hafner, J. Ab Initio Molecular-Dynamics Simulation of the Liquid−Metalamorphous−Semiconductor Transition in Germanium. Phys. Rev. B. 1994, 49, 14251−14269. (31) 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. (32) Aschauer, U.; Chen, J.; Selloni, A. Peroxide and Superoxide States of Adsorbed O2 on Anatase TiO2 (101) with Subsurface Defects. Phys. Chem. Chem. Phys. 2010, 12, 12956−12960. (33) Du, Y.; Deskins, N. A.; Zhang, Z.; Dohnalek, Z.; Dupuis, M.; Lyubinetsky, I. Formation of O Adatom Pairs and Charge Transfer upon O2 Dissociation on Reduced TiO2(110). Phys. Chem. Chem. Phys. 2010, 12, 6337−6344. (34) Liu, L. M.; McAllister, B.; Ye, H. Q.; Hu, P. Identifying an O2 Supply Pathway in CO Oxidation on Au/TiO2(110): A Density Functional Theory Study on the Intrinsic Role of Water. J. Am. Chem. Soc. 2006, 128, 4017−4022. (35) Tan, S.; Ji, Y.; Zhao, Y.; Zhao, A.; Wang, B.; Yang, J.; Hou, J. Molecular Oxygen Adsorption Behaviors on the Rutile TiO2(110)-1′1 Surface: An in Situ Study with Low-Temperature Scanning Tunneling Microscopy. J. Am. Chem. Soc. 2011, 133, 2002−2009. (36) Blöchl, P. E. Projector Augmented-Wave Method. Phys. Rev. B 1994, 50, 17953−17979. (37) Kresse, G.; Joubert, D. From Ultrasoft Pseudopotentials to the Projector Augmented-Wave Method. Phys. Rev. B. 1999, 59, 1758− 1775. (38) Kresse, G.; Hafner, J. Ab Initio Molecular Dynamics for OpenShell Transition Metals. Phys. Rev. B. 1993, 48, 13115−13118. (39) Sanville, E.; Kenny, S. D.; Smith, R.; Henkelman, G. Improved Grid-Based Algorithm for Bader Charge Allocation. J. Comput. Chem. 2007, 28, 899−908. (40) Jósson, H.; Mills, G.; Jacobsen, K. W. Nudged Elastic Band Method for Finding Minimum Energy Paths of Transitions. Classical and Quantum Dynamics in Condensed Phase Simulations; World Scientific: Singapore, 1998; p 385. (41) Henkelman, G.; Uberuaga, B. P.; Jósson, H. A Climbing Image Nudged Elastic Band Method for Finding Saddle Points and Minimum Energy Paths. J. Chem. Phys. 2000, 113, 9901−9904.

(42) Henkelman, G.; Jósson, H. Improved Tangent Estimate in the Nudged Elastic Band Method for Finding Minimum Energy Paths and Saddle Points. J. Chem. Phys. 2000, 113, 9978−9985. (43) Liu, H.; Zhao, M.; Lei, Y.; Pan, C.; Xiao, W. Formaldehyde on TiO2 Anatase (1 0 1): A DFT study. Comput. Mater. Sci. 2012, 51, 389−395. (44) Lazzeri, M.; Vittadini, A.; Selloni, A. Structure and Energetics of Stoichiometric TiO2 Anatase Surfaces. Phys. Rev. B. 2001, 63, 155409. (45) Hu, X.; Tu, R.; Wei, J.; Pan, C.; Guo, J.; Xiao, W. Nitrogen Atom Diffusion into TiO2 Anatase Bulk via Surfaces. Comput. Mater. Sci. 2014, 82, 107−113. (46) Aschauer, U.; He, Y.; Cheng, H.; Li, S.-C.; Diebold, U.; Selloni, A. Influence of Subsurface Defects on the Surface Reactivity of TiO2: Water on Anatase (101). J. Phys. Chem. C 2010, 114, 1278−1284. (47) Chen, X.; Liu, L.; Yu, P. Y.; Mao, S. S. Increasing Solar Absorption for Photocatalysis with Black Hydrogenated Titanium Dioxide Nanocrystals. Science 2011, 331, 746−750. (48) Vittadini, A.; Selloni, A.; Rotzinger, F. P.; Grätzel, M. Structure and Energetics of Water Adsorbed at TiO2 Anatase (101) and (001) Surfaces. Phys. Rev. Lett. 1998, 81, 2954−2957. (49) Di Valentin, C.; Pacchioni, G.; Selloni, A. Electronic Structure of Defect States in Hydroxylated and Reduced Rutile TiO2(110) Surfaces. Phys. Rev. Lett. 2006, 97, 166803. (50) Amore Bonapasta, A.; Filippone, F.; Mattioli, G.; Alippi, P. Oxygen Vacancies and OH Species in Rutile and Anatase TiO2 Polymorphs. Catal. Today 2009, 144, 177−182. (51) Mattioli, G.; Filippone, F.; Amore Bonapasta, A. Reaction Intermediates in the Photoreduction of Oxygen Molecules at the (101) TiO2 (Anatase) Surface. J. Am. Chem. Soc. 2006, 128, 13772− 13780. (52) Tilocca, A.; Selloni, A. O2 and Vacancy Diffusion on Rutile(110): Pathways and Electronic Properties. ChemPhysChem 2005, 6, 1911−1916. (53) Lin, F.; Zhou, G.; Li, Z.; Li, J.; Wu, J.; Duan, W. Molecular and Atomic Adsorption of Hydrogen on TiO2 Nanotubes: An ab Initio Study. Chem. Phys. Lett. 2009, 475, 82−85. (54) Zheng, Y.; Xiao, W.; Cho, M.; Cho, K. Density Functional Theory Calculations for the Oxygen Dissociation on Nitrogen and Transition Metal Doped Graphenes. Chem. Phys. Lett. 2013, 586, 104−107. (55) Eichler, A.; Hafner, J. Molecular Precursors in the Dissociative Adsorption of O2 on Pt(111). Phys. Rev. Lett. 1997, 79, 4481−4484.

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