Inﬂuences of MgO(001) and TiO 2 (101) Supports on the Structures and Properties of Au Nanoclusters

: Due to the unique structures, photoelectric properties, good catalytic activity, and broad potential applications, gold nanoclusters (Au n ) received extensive attention in catalysis, bioengineering, environmental engineering, and so on. In the present work, the structures and properties of Au n adsorbed on the MgO(001) and TiO 2 (101) surfaces were investigated by density functional theory. The results showed that the catalytic properties of Au n will be enhanced when Au n is adsorbed on certain supports. Because the difference of the outer electronic structure of metals in supports, the direction of the charge transfer was different, thus inducing the different charge distribution on Au n . When Au n was adsorbed on MgO(001) [TiO 2 (101)] surface, Au n will have negative [positive] charges and thus higher catalytic activity in oxidation [reduction] reaction. The variation of surface charges caused by the support makes Au n possess different catalytic activity in different systems. Moreover, the electronic structure of the support will make an obvious inﬂuence on the s and d density of states of Au n , which should be the intrinsic reason that induces the variations of its structure and properties. These results should be an important theoretical reference for designing Au n as the photocatalyst applied to the different oxidation and reduction reactions.


Introduction
The research on gold nanoclusters (Au n ) started to attract extensive attention when Haruta et al. [1] found that Au n on TiO 2 have a good catalytic activity in CO low-temperature oxidation reaction. Further deep explorations of Au n uncovered that Au n not only exhibited a variety of physical and chemical features [2][3][4], but also had broad applications in many fields, such as catalysis, biological engineering, nanotechnology, and so on [5][6][7][8][9][10]. More importantly, the study on the Au n is still the research focus even now.
Among the factors of determining the Au n physical and chemical features, the structures of Au n are found to play a crucial role. In the theoretical aspect, Häkkinen et al. [11] and Min et al. [12] found that the neutral Au n will prefer the two-dimensional structures when the atom number n is less than 13. Research of the Au n (2 ≤ n ≤9) structures carried out by Mao et al. [13] indicates that with the increasing of n the average binding energy per atom will increase gradually, and an odd-even oscillation will appear in the Fermi energy, the electron affinity energy, and the ionization potential energy. Moreover, Bulusu et al. [14] investigated the Au n with n = 15∼19 and revealed that the Au n structure will be transformed to the hollow structure at n = 17 and the pyramid structure at n = 19. Also, Fa et al. [15] found that Au n will be the two-dimensional planar structures in the case of n ≤ 12 and the transition from the planar to the three-dimensional structures will take place at n = 13∼15. Zhao et al. [16] found that the ground-state geometry of Au n (n = 19∼22) is tetrahedron, and Au 24 is a tube-like structure. All the researches indicate that the Au n structures are closely dependent on the Au n size.
On the other hand, great attention has also been paid to the study of the Au n adsorbed on metal oxide surface, especially MgO and TiO 2 . Yoon et al. [17] found that Au 8 on the MgO surface will show a good catalytic activity in the CO oxidation reaction because the charges are transferred from MgO surface to Au n . Liu et al. [18] reported that the structure of Au n will be a three-dimensional structure at n = 7 in the study of Au n (n = 1∼8) vertically adsorbed on the surface of MgO(001). Roldan et al. [19] studied the adsorption activity of the O 2 activation on Au 5 /MgO, and Stamatakis et al. [20] investigated the CO oxidation on Au n /MgO. Both these two works demonstrate that the charge transfer occurring between the clusters and the O 2 promotes the adsorption of oxygen. Meanwhile, many researches on Au n /TiO 2 have also begun. Kim et al. [21] found that the charge transfer between support materials and the Au n will increase the catalytic activity of the clusters. The study of the stability of Au n on anatase TiO 2 (110) surface carried out by Pabisiak et al. [22] showed that the finite linear Au n clusters will be formed on the highly defected surface. Asakura [23] found that Au anion is very important for the stability of Au n adsorbed on TiO 2 (110) surface. At the same time, Li et al. [24] explored the influence of Au n /TiO 2 on CO oxidation and showed that the reaction rate is dependent on the size and shape of the Au n . Recently, Cao et al. [25] found that the charge transfer between Au n and TiO 2 (110) depends on the number of oxygen free radicals and the size of Au n .
All the researches indicate that the chemical and physical properties of Au n are closely related to its structure, its size, and the surfaces of the supports. Although many researches of Au n adsorbed on metal oxide surfaces have been done, the intrinsic mechanism and the meso-scale control mechanism combining the macroscopic properties with the microscopic electronic structures are still not quite clear. So a further systematic research of Au n on MgO(001) and anatase TiO 2 (101) surfaces is required. In this paper, Au n , Au n /MgO(001), and Au n /TiO 2 (101) will be studied by using the density functional theory (DFT) [26,27]. The transition of the geometric structure, the variations of the average bond lengths and the partial density of states (PDOS), and the charge transfer will be discussed. Furthermore, the reasons why these variations appear and the possible intrinsic mechanisms will be analyzed.

Geometric Structures of Au n
To find the steadiest adsorption site of Au atom on the supports, four types of initial structures ( Figure 1) will be considered including O atom top, Mg atom top, Mg-O bridge, and Mg-O hole. After optimization, Au atoms initially adsorbed on Mg-O bridge or Mg-O hole will be moved to the site of O atom top. Also, the adsorption energy of the Au atom on the top site of O atoms is found to be less than that on Mg atoms. Therefore, we can draw a conclusion that the steadiest adsorption site should be the top site of O atom, which is in accordance with the results reported in the literature [28][29][30]. For each Au n with a certain atom number n, its steadiest configuration on the supports will be determined by comparing lots of optimized structures obtained from different initial configurations.   (001) surface, and Au n on TiO 2 (101) surface, respectively. By comparing the corresponding stable structures of Au n , Au n /MgO(001) and Au n /TiO 2 (101), we can see that Au n structures are distorted obviously when they are adsorbed on the MgO(001) and TiO 2 (101) surfaces. In the case of Au n /MgO(001), the bond length of Au 2 on MgO(001) almost does not change and Au 2 is askew adsorbed on the surface, as shown in Figure 3. Au 3 on MgO(001), retaining a planar structure similar to that in vacuum, is adsorbed with the plane perpendicular to MgO(001) surface. In the corresponding side view, the middle Au atom is far away from the surface of MgO(001), but still on the top of oxygen atoms, which is in accordance with the references [28,29]. The structures of Au n (n = 4∼13) on MgO(001) surface show partial distortion. In the top view, they seem to maintain the same basic structures as in vacuum, while in the side view obvious distortions can be observed. Au atoms located on the sites of the top and bridge of O atom are moved far away from the MgO(001) surface, but Au atoms on other sites are moved close to the MgO(001) surface, which makes the flat structures distort. Au 14 shows a three dimensional flat cage structure in vacuum, but distorts seriously when it is adsorbed on MgO(001) with the atoms near MgO surface almost in one plane. For another three-dimensional flat cage structure, Au 15 will change little when it is adsorbed on MgO(001). On TiO 2 (101) surface, Au 3 structure does not distort obviously, and the bond length is only slightly stretched. For other Au n , roughly speaking, they show similar distortion with its bond lengths longer than those in vacuum, and with these variations less than those on MgO(001) surface. The changes of the structures of Au n on MgO(001) and TiO 2 (101) surfaces show that the adsorption properties of Au n (n 4) are different from Au n (n 5). Moreover, these results show that when Au n are adsorbed on metal oxide surface, some atoms in the Au n plane will move up and others will move down relative to the original Au n plane, making the bonds stretched and the flat structure become a quasi-flat structure, which will be beneficial for improving the catalytic activity of Au n . Furthermore, we analyze the adsorption energy E a , the binding energy E b , the internal binding energy E c and the average bond length for Au n on MgO(001) and TiO 2 (101) with different n in Figure 5. In Figure 5a, it can be found that the adsorption energy of some Au n is very large, such as 1.51 eV (0.78 eV) for Au (Au 2 ) on the MgO(001), and 0.62 eV for Au 2 on the TiO 2 (101), possibly implying that the adsorption between Au n (n 4) and supports should be the chemisorption, while other Au n (n 5) may be adsorption. Meanwhile, the average adsorption energy of Au n tends to decrease with the increase of the number n of Au n on MgO(001) surface, and approaches zero. This means that Au n in enough large size is difficult to be adsorbed on metal oxide surface. Except Au 5 /TiO 2 (101), all the adsorption energy values of other Au n on TiO 2 (101) are smaller than the counterparts of Au n on MgO(001). This demonstrates that Au n on Mg(001) surface may possess stronger stability. For Au n on TiO 2 (101) surface, Au 5 is almost the steadiest adsorption structure except the chemisorption of Au 2 . Figure 5b gives the variations of E b of Au n adsorbed on MgO(001) and TiO 2 (101). Either on MgO(001) or TiO 2 (101) surface, the E b increases with the increase of Au atom number n. The E b of Au n on MgO(001) becomes nearly invariant when n 6, while on TiO 2 surface, it will become invariant when n 7. This indicates that Au n of larger size is steadier than the smaller one, and the smaller Au n cluster may incline to merge to a larger one. In Figure 5c, the E c of Au n also increases with the atom number n of Au n . The corresponding E c will be reduced when Au n is adsorbed on metal oxide surface, especially on TiO 2 (101) surface. The interaction between TiO 2 and Au n is more beneficial for polymerizing. By comparing the values of E a with E c for Au n on the same metal support in the same size in Figure 5a,c, we can see that the E c within cluster (n ≥ 5) is always larger than the E a . This demonstrates that the interaction of Au-Au in clusters is stronger than that of Au-O and Au-Mg, likewise for Au-O and Au-Ti, further indicating that either adsorbed on TiO 2 (101) or MgO(001), Au atoms prefer to exist in the form of clusters. The bond length of Au n corresponding to different n is given in Figure 5d. For clarity, the corresponding structure data before and after Au n adsorption are also shown in Table 1. It can be found that all the adsorption-induced variations of the structure data for Au-Au, Mg-O, and Ti-O are in the range −0.081 0.081 Å . For Au n of the same atom number, except Au 2 , all the average Au-Au bond lengths of Au n adsorbed on metal oxide is longer than that in vacuum, indicating the activity will be improved. In addition, the variation of Au-Au bond length in Au n /MgO(001) is larger than that in Au n /TiO 2 (101), meaning that the activity of Au n on MgO(001) is higher, which agrees about the analysis of adsorption energy. As presented in references [31][32][33], the bond length variation in cluster is closely dependent on the competition between the cluster-support interaction and the intrinsic interaction within the cluster. Here, since E c is greater than E a for Au n (n 5), the increase of the average Au-Au bond length is very tiny.     c. d.

PDOS
To study the effect of the adsorption-induced variation of the internal electronic structures of Au n on the catalytic activity, we present all the related PDOS in Figure 6. First, we show the PDOS of O and Mg atoms in the surface of pure MgO(001) in Figure 6a. Clearly, it is the PDOS of p state of O atom that plays the dominant contribution. Therefore, we further only show the PDOS of p state of the O atom in the surface layer of the pure MgO(001), MgO(001) adsorbed with Au 6 , and Mg(001) adsorbed with Au 8 in Figure 6b. Obviously, a whole shift of the PDOS toward the low energy direction will appear due to the adsorption of Au 6 or Au 8 . This will cause the decrease of the corresponding PDOS value at Fermi energy, which also indicates that the adsorption of Au 6 or Au 8 will lead to the variation of the electron structure of the O atom in MgO(001) surface. In Figure 6c, we plot the PDOS of Au 2 , Au 2 /MgO(001), and Au 2 /TiO 2 (101). It can be found that in Au 2 the PDOS of s and d states of Au 2 will play the dominant contribution. Also, the peaks become broader in width and lower in height, and eventually three peaks incline to merge into a wider peak. Furthermore, we can see that the adsorption will push the three peaks below the Fermi energy to the low energy direction. In addition, the two peaks above the Fermi energy disappear for Au 2 /TiO 2 (101), and the two peaks will approach each other for Au 2 /MgO(001). These imply that the different supports are able to induce the different variations of the PDOS of the s and d states in Au n . Thus, the electronic structures of Au 2 on MgO(001) and TiO 2 (101) surface will be changed due to adsorption. Eventually, this should be beneficial to improving the Au n catalytic activity. c.

Charge Transfer
As demonstrated in many works [34][35][36], charge transfer is a key factor in improving the catalytic activity of Au n . Therefore, we intend to investigate the charge transfer occuring in Au n /MgO(001) and Au n /TiO 2 (101). As examples, the electron density differences of Au 8 /MgO(001) and Au 5 /TiO 2 (101) are shown in Figure 7a,b, respectively. Also, in Figure 8 we show the Mülliken charge distribution of Au 5 , TiO 2 , and Au 5 /TiO 2 (101). Meanwhile, for clarity, the corresponding charge transfer data are listed in Table 2. In Figure 7a, the negative charges accumulate around atoms of Au 8 , while the negative charges near the O and Mg atoms connected with Au n atoms decrease. This demonstrates that the electrons transferring from Mg-O to Au atoms, makes Au 8 have negative charges. On the other hand, from the Mülliken charge distribution of Au 8 /MgO(001) (see Table 2), the same conclusion can be drawn. Obviously, the whole charges of each Au n and the charges of every Au atom in Au n are all negative when Au n is adsorbed on MgO(001). This indicates that the negative charges transfer from MgO surface to Au n , which is consistent with the studies of Roldan [19] and Stamatakis [20]. Different from the case of MgO(001) surface, charges will accumulate near O atom of the TiO 2 (101) which is close to Au atoms when Au 5 is adsorbed on TiO 2 (101), as shown in Figures 7b and 8, and Table 2. The charges of the two Au atoms directly connected to TiO 2 surface change from −0.06 to 0.06, and the charges of the central Au atom change from 0.09 to 0.03. Consequently, the whole charges of Au 5 change from 0.00 to 0.23. These phenomena suggest that the negative charges will transfer from Au atoms to TiO 2 surface, inducing that Au n has positive charges. Therefore we conclude that the charge transfer direction in Au 8 /MgO(001) is different from that in Au 5 /TiO 2 (101).   To understand the direction of the electron transfer between Au n and supports, we should resort to the outer electron structure of the metal as reported in reference [33]. The outer electron structure of Au, Mg, and Ti are 5d 10 6s 1 , 3s 2 , and 3d 2 4s 2 , respectively. Obviously, Au has an almost filled 5d orbit and half-filled s orbit, and Mg has no d orbit, while Ti has unfilled 3d orbits, inducing that the charge can flow from Mg-O to Au or from Au to Ti-O. Fundamentally speaking, the different direction of charge flow is determined by the difference of the outer electron structure of the metal d orbit. Therefore, the negative charges flow from Mg-O in MgO(001) to Au and finally Au has negative charges, which makes the Au n possess high catalytic activity in oxidation reaction. However, the negative charges flow from Au to Ti-O in TiO 2 (101) and finally Au has positive charges, which makes the Au n possess high catalytic activity in reduction reaction. Therefore, catalytic activity of Au n can be controlled by choosing suitable metal oxide supports.

Materials and Methods
All calculations are performed by the plane-wave ultrasoft pseudo potential method based on DFT with the CASTEP [37] program of Material Studio package. The GGA-PBE [38,39] version of exchange-correlation energy function is applied, which was proved to be reasonable [40]. The cutoff energy for the plane wave basis set is 340 eV and the total energy convergence criterion for the self-consistent field (SCF) is 1.0 × 10 −5 eV/atom. Based on the literature [41][42][43][44][45][46] and our experience, the MgO(001) and TiO 2 (101) should be the absorption surfaces. Calculations of the two systems are performed for a two layer slab [41] with 6 × 6 surface periodicity and 3 Å [47,48] with 3 × 3 surface periodicity respectively, and both separated from their replicas by vacuum region of 15 Å width.
First of all, the stablest structures can be determined by optimizing Au n structures in vacuum with the atom number ranging from 1 to 15. Then the chosen structures are adsorbed on MgO(001) and TiO 2 (101) surfaces, respectively, which will be optimized further. Finally, the steadiest structures of the two systems can be determined according to the adsorption energy, the binding energy, and the average bond length. The adsorption energy per Au atom is calculated by the formula E a = (E s + E n − E t )/n.
Here E t stands for the total energy of the Au n /sub system while E s and E n represent the energies of the isolated oxide substrate and the Au n clusters, respectively. Therefore, a larger adsorption energy will indicate a stronger adsorption between cluster and support. The binding energy (per Au atom) of Au atom and oxide substrate is calculated via the formula where E 0 is the energy of an isolate Au atom calculated in a large cell. In addition, the internal binding energy E c and the average bond length R of Au n are calculated according to the following formulae: Here R ij denotes the distance between Au atoms i and j in Au n and m represents the number of bonds in Au n .