Conference-ISSS-8-Au Atom Diffusions on Reduced and Cl-Adsorbed Rutile TiO 2 ( 110 ) Surfaces : A DFT + U Study

In this study, the effect of Cl-adsorption on the reduced rutile TiO2(110) surface during Au atom diffusion on the surface is investigated using density-functional theory-based calculations. This is done in order to investigate the reason for which the co-existence of Cl during Au/TiO2 catalyst preparation enhances the Au cluster aggregation. The calculation results show that the diffusion barrier along the [1̄10] direction is decreased by the Cl adsorption. Hence, the diffusion rate increases when the surface adsorbs Cl. The enhancement of the Au atom diffusion by Cl is one reason for which Cl-elimination is necessary for preparing Au/TiO2 catalysts with high catalytic activity. [DOI: 10.1380/ejssnt.2018.267]


I. INTRODUCTION
Nanosized Au, such as Au cluster and Au nanoparticles, whose diameter is below 5 nm, shows high catalytic activities [1][2][3][4].Au/TiO 2 , which is nanosized Au deposited on TiO 2 , is a general gold catalyst, and there are various useful catalytic activities, including CO oxidation at low temperature [4,5], the epoxidation of propylene [6,7], selective oxidation of glucose [8], selective hydrogenation of nitro groups [9], among others [1].However, large Au particles do not exhibit catalytic activities because the bulk of Au is inert and chemically stable [2].
No detailed mechanism regarding the enhancement of the Au aggregation has been proposed.To clarify the mechanism, we recently investigated the effects of Cl on interfaces between nanosized Au and stoichiometric, reduced, and Cl-adsorbed rutile TiO 2 (110) surfaces [22][23][24].Recent studies showed that (1) the oxygen defect sites in the reduced TiO 2 surface adsorbs Cl atoms more strongly than Au atoms [22,23], (2) the presence of Cl on the oxygen defect site inhibits the charge transfer from the TiO 2 surface to the Au atom [22][23][24] (the charge transfer is important for Au anchoring [22,25]), (3) a threeor two-dimensional Au cluster interacts with the unsaturated oxygen (O [UC] ) and avoids the Cl in the Cl-adsorbed TiO 2 surface [24], (4) the Au aggregation reactions on the reduced TiO 2 surface are endothermic owing to the destabilization of the Au-Au bond and the strong interaction between Au and the oxygen defect site caused by the charge transfer [24], whereas (5) the Au aggregation reactions on the stoichiometric and the Cl-adsorbed TiO 2 surfaces are exothermic [24].However, the effect of Cl on Au atom diffusion on the TiO 2 surface remains unclear.
In the present study, we carried out DFT + U (density functional theory with onsite Coulomb interaction correlation) [26] calculations of Au atom diffusions on reduced and Cl-adsorbed TiO 2 surfaces.

A. Models
The model surface of the present work is shown in gen (O [2c] ; the oxygen is projecting) is substituted for Cl.
The O [2c] easily desorbs from TiO 2 (110) under vacuum or reduction conditions, and generates an oxygen vacancy.The oxygen vacancy is effective for the high dispersion of the Au cluster [24,27].However, the oxygen defect adsorbs Cl atoms more strongly than the Au cluster [22][23][24].Furthermore, it was observed that Cl functions as a substitute for O [2c]  [28].Therefore, we adopted the slab model shown in Fig. 1(a) as a Cl-adsorbed TiO 2 model.For comparison, Au diffusion on model surface whose oxygen defect site is not occupied by Cl is also calculated [Fig.1(b)].
For the slab model, there are 12 atomic layers, and the vacuum region is ∼1.5-nm thick.During geometric optimization, the lower three atomic layers were fixed to mimic the bulk structure of TiO 2 .The unit size is 1 × 2 (0.660 nm × 0.597 nm); the x direction is TiO 2 [ 110], and the y direction is [001].Chemical interactions between Au atoms and the Au atom in an adjacent unit are negligible [22,24].We calculated the potential energy surface (PES) of Au diffusion on the model surfaces using DFT + U calculations.The mesh used to estimate PES is 17 × 17 (Fig. 2).Considering the symmetry, there is same coordination: namely, and y07 = y09.We then calculated 9 × 9 = 81 points (x = x00-x08, and y = y00-y08).

B. Method
The exchange-correlation functional for the DFT method was PBE functional [29].All electron wave functions were expanded with a plane-wave basis.Core electrons were treated using the projector augmented wave (PAW) method [30,31].The energy cut-off values for the wave function and augmented charge are 400 eV and 2400 eV, respectively.Using the Monkhorst procedure [32], 4 × 4 × 1 k-point sampling was done.The dependency of k-points sampling was investigated in our prior work [22].Optimizations of the electronic structure and Top view.O Ti [5c]   O [2c]   Cl Ti [5c]   Cl O [2c]   O [2c]  Cl Ti [5c]   [001] geometric structure were carried out using the Davidson method and conjugate gradient method, respectively.The pure-DFT approach based on generalized gradient approximation (GGA) is unsuitable for the estimation of the PES of Au atoms because some studies claim that the pure-DFT calculation misestimates the activation barriers and reaction energies (e.g., Ref. [20,33]).While several reasons are considered, it is difficult to correct all errors in the pure-DFT calculations.In the present work, it was assumed that the errors which cause the inaccurate estimation of the band structure of rutile TiO 2 are critical errors of the pure-DFT method.The DFT + U method [26] was then adopted, and the Hubbard potential, U = 2 eV, was added to Ti 3d orbitals in order to obtain more accurate reaction energies relative to those of the pure-DFT method [33].
The program packages employed for first-principle calculations and the visualization of the calculation results were VASP [34][35][36][37] and VESTA [38].The Bader scheme was adopted as the estimation of the atomic charge [39][40][41][42].The mean diffusion rates on the model surfaces were estimated by kinetic Monte-Carlo simulation [43,44] based on the PES estimated by DFT + U ; the details are depicted in Supplementary Material.

A. Results of Au/Cl-adsorbed TiO2
The adsorption energy (E ads ) of each position of Au on the Cl-adsorbed TiO 2 (110) surface is summarized in Fig. 3.The E ads was calculated using Eq. ( 1): where   3(a, b) and Fig. 3(c, d), respectively.The potential-energy curves of the TiO 2 [ 110] direction and TiO 2 [001] direction are shown in Fig. 3(a, c) and Fig. 3(b,  d), respectively.The diffusion barrier (∆E dif ) and the difference in the atomic charges of the Au atom of the most and least stable structures in the diffusion line (∆ρ dif ) in Table I are respectively calculated using Eqs.( 2) and ( 3): where E least and E most are the total energy of the most and least stable structures in the diffusion series, respectively.ρ least (Au) and ρ most (Au) are the Bader atomic charge values of the Au atom for the most and least stable structures in the diffusion series, respectively.As shown in the results of ∆E dif and ∆ρ dif , it is clear that the value of ∆E dif increases as the value of ∆ρ dif increases.The diffusions along the [ 110] direction have higher barriers and a larger value of ∆ρ dif than the diffusions along the [001] direction.In our prior works [23,45], we showed that the orbital of an Au atom is important when it interacts with anions (O 2− and Cl − ) because the orbital is polarized by the anions, while the charge transfer between an Au atom and Ti cation is important when the Au atom interacts with the Ti cation in a reduced surface because of the electrostatic interaction introduced by the charge transfer.Hence, the interaction between an Au atom and Cl-adsorbed TiO 2 changes when an Au atom diffuses along the [ 110] direction; the Au-O (Cl) interaction is polarization, and the Au-Ti interaction is an ionic interaction owing to the electron transfer from the Ti cation.However, the interaction between an Au atom and Cl-adsorbed TiO 2 does not change when the Au atom diffuses along the [001] direction; the interaction is always polarization (or electrostatic) during the diffusion.
Comparing the results of the singlet [Fig.3(a, b)] and triplet [Fig.3(c, d)] states, the singlet states are clearly more stable than the triplet state when the Au atom is adsorbed around the Ti cation, while the differences in the adsorption energies of the singlet and triplet states are small when the Au atom is adsorbed around the O anion or Cl anion.The reasons are also explained by the results of our prior work [23].When the Au atom is adsorbed around the Ti cation in reduced TiO 2 , the Au atom accepts an electron from the Ti cation, and an electrostatic interaction, such as Au − /Ti 4+ , occurs.The electron transfer does not occur in the triplet state; therefore, the singlet state is more stable than the triplet state when the Au atom is adsorbed around the Ti cation.However, the interaction with the O (Cl) anion is a polarization of the d orbital of the Au atom caused by the repulsion with the p z orbital of the O (Cl) anion.Because the polarization occurs in both singlet and triplet states, the energy differences are small.The dependency of the E ads on the distance from Cl (d Cl ) was investigated, and the result is illustrated in Fig. 4. Figure 4 shows that the stability of the Au atom is increasing with increasing d Cl .This result indicates that the Au atom avoids interacting with Cl.This is because the Au-Cl interaction is weaker than Au-O [2c] and Au-Ti [5c] interactions; the polarization by Cl is smaller than that by O [2c] , and the Cl does not donate electrons to the Au atom.

B. Comparison with reduced TiO2 (Cl-free) surface
The PES of Au atom diffusions on Cl-adsorbed and reduced TiO 2 (110) surfaces is shown in Figs. 5 and 6. Figure 5 shows the potential-energy curves of the diffusions along the [ 110] direction, and Fig. 6 shows those along the [001] direction.The results for the Au/reduced TiO 2 system, which are explained below, are consist to prior works [46][47][48][49][50][51] for Au atom adsorption and diffusion on reduced rutile TiO 2 (110).
With respect to the Au atom diffusion along the [ 110] direction, the potential-energy curves on y07-y09 for Cladsorbed TiO 2 [Fig.5(a)] and reduced TiO 2 [Fig.5(b)] are similar because the Au atoms diffuse via the same route, e.g., O B → Ti [5c] → O B , while the difference between the curves of Cl-adsorbed TiO 2 and reduced TiO 2 becomes larger as the Au atom approaches the oxygen defect site.This is because of the strong interaction between the Au atom and oxygen defect site which is due to the charge transfer, ionic interaction, and coordination bond between them [22][23][24][46][47][48][49][50][51].Owing to the difference between interactions with the defect sites (Cl in Cl-adsorbed TiO  TiO 2 [Fig.6(b)] have the same shape as the potentialenergy curve, and this is because both Au atoms on the surfaces diffuse on Ti [5c] atoms.
Table II shows the diffusion barriers for the Au atom diffusion on reduced TiO 2 (110).The range of barriers of the Au atom diffusion on reduced TiO 2 is 0.19-2.11eV, while that of Cl-adsorbed TiO 2 is 0.27-1.25 eV (singlet state).The Au atom will diffuse easily on Cl-adsorbed TiO 2 because the highest barrier of reduced TiO 2 is higher than that of Cl-adsorbed TiO 2 ; however, the lowest barrier of reduced TiO 2 is lower than that of Cl-adsorbed TiO 2 .To confirm the order of ease of diffusion of the Au atom, we carried out the kinetic Monte-Carlo simulation based on the PES estimated using DFT + U calculations, and we estimated the ratio of the average diffusion rates of Au atoms on the surfaces [Eq.( 4)].
Here, v Cl and v R are the average diffusion rates of the Au atoms on the Cl-adsorbed TiO 2 and reduced TiO 2 , respectively.As a result, the ratio R was 1.9 × 10 8 .This result indicates that Au atoms diffuse on Cl-adsorbed TiO 2 more easily than reduced TiO 2 .Finally, we discuss the reason for which Cl-adsorption onto TiO 2 increases the ease of diffusion of Au atoms.Figure 7 shows the dependency of E ads on the difference in the charge of the Au atom before and after the adsorption (ρ Au ).The data can be separated into two regions by atoms that interact with the Au atom.On the right region of Fig. 7, the Au atoms interact with unsaturated Ti atoms (cations).In the right region, E ads and ρ Au correlate; E ads becomes large when ρ Au is large.Comparing the data of Cl-adsorbed TiO 2 and reduced TiO 2 , the plots of Cl-adsorbed TiO 2 shift to the lower left from the plots of reduced TiO 2 .This indicates that Cl-adsorption inhibits the charge transfer between Au and Ti, and destabilizes the Au atom adsorption; this result is consistent with our previous works [22][23][24].Meanwhile, in the left region of Fig. 7, the values of E ads do not correlate to the value of ρ Au , and this is because the Au atom is stabilized by polarization [24,45].Similar to the right region, the E ads value for Cl-adsorbed TiO 2 is smaller than that of reduced TiO 2 .This result indicates that Cl-adsorption decreases the polarization of the Au d orbital.The Cladsorption decreases both Au-Ti and Au-O interactions; however, the effect on the Au-Ti interaction, especially when the Au atom is at the oxygen defect site, is larger than the effect on the Au-O interaction.Therefore, the Au atom diffuses on the Cl-adsorbed TiO 2 surface more easily than the reduced TiO 2 surface.The effect of the co-existence of Cl on the diffusion of Au atoms on the reduced rutile TiO 2 (110) surface was investigated to clarify the mechanism responsible for the enhancement of Au aggregation by Cl during Au/TiO 2 catalyst preparation.The Cl inhibits the interaction between the Au atom and the TiO 2 surface, especially the charge transfer between them.The Au diffusion barriers along the [ 110] direction of the Cl-adsorbed TiO 2 surface are high (∼1.0eV) because the interaction between the Au atom and the TiO 2 surface depends on the position of the Au atom, while those along the [001] direction are low (∼0.3 eV) because the interaction does not depend on the position of the Au atom.On the other hand, Au diffusions along both the [ 110] and [001] directions of the reduced TiO 2 (Cl-free) surface are difficult; the barriers are 1-2 eV.Hence, the diffusion rates of Au atoms on the reduced TiO 2 (Cl-free) surface are much slower compared to those on the Cl-adsorbed TiO 2 surface.In summary, the co-existence of Cl increases the ease of diffusion of Au atoms on reduced TiO 2 .In addition, considering both the results of the present work and our previous works [22][23][24], it is concluded that the co-existence of Cl decreases the positive effects of oxygen defects on Au cluster adsorption onto TiO 2 , on Au cluster aggregation on TiO 2 , and on Au atom diffusion on TiO 2 .These reductions by Cl are reasons for which Cl-elimination is necessary to prepare Au/TiO 2 catalysts with high catalytic activity.

Fig. 1 .
For a Cl-adsorbed model, two-coordinated oxy-ISSN 1348-0391 c ⃝ 2018 The Japan Society of Vacuum and Surface Science Volume 16 (2018) Tada et al.

FIG. 4 .
FIG. 4. Dependency of d Cl (distance between Cl and Au) on E ads (adsorption energy of Au). Green circles indicate the result when the singlet state is more stable than the triplet state, and blue triangles indicate the result when the triplet state is more stable than the singlet state.

FIG. 7 .
FIG. 7. Dependency of E ads (adsorption energy of Au atom) on ρAu (the difference in the charge of Au atom before and after the adsorption).Red indicates the result of Au atom adsorption onto reduced TiO2, and green indicates the result of Au atom adsorption onto Cl-adsorbed TiO2.The circle, cross, and triangle represent singlet, doublet, and triplet states, respectively.

TABLE I .
Diffusion barriers (∆E dif ) and the differences between atomic charges of Au atoms for most stable structures and least stable structures in the diffusion line (∆ρ dif ) of Au atom diffusion on the Cl-adsorbed TiO2(110) surface.

TABLE II .
Diffusion barriers (∆E dif ) and the differences between atomic charges of Au atoms for most stable structures and least stable structures in the diffusion line (∆ρ dif ) for Au atom diffusion on the reduced TiO2(110) surface.