Article pubs.acs.org/JPCC
Single Gold Atom Adsorption on the Fe3O4(111) Surface Xiaohu Yu,† Sheng-Guang Wang,‡ Yong-Wang Li,† Jianguo Wang,† and Haijun Jiao*,†,§ †
State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese Academy of Sciences, Taiyuan, Shanxi 030001, People's Republic of China ‡ Synfuels China Co., Ltd., Huairou, Beijing 101407, People's Republic of China § Leibniz-Institut für Katalyse e.V. an der Universität Rostock, Albert-Einstein Strasse 29a, 18059 Rostock, Germany S Supporting Information *
ABSTRACT: For understanding the catalytic activity of Fe3O4-supported gold catalysts, the adsorption structures and energies of a single Au atom on the six terminations of the Fe3O4(111) surface have been computed at the level of density functional theory (GGA+U). For the most stable adsorption configurations, correlation has been found between the surface stability and the Au atom adsorption energy; that is, the more stable the surface, the lower the Au atom adsorption energy. It is also found that the adsorbed Au atom is reduced and has a negative charge on the ironterminated surfaces, whereas it is oxidized and has a positive charge on the oxygen-terminated surfaces, and the latter is in agreement with the experimental observation. No correlation between the transferred charge and the adsorption energy has been found. Regarding the experimentally observed oxidation of gold nanoparticles on the iron oxide surface, it is possible to produce an oxygen-terminated surface for gold adsorption by synthetic tuning.
1. INTRODUCTION Recently, Fe3O4-supported gold catalysts have been regarded as active phases in many reactions, such as CO oxidation1 and the water gas shift reaction.2 The catalytic activity of gold clusters on oxides has been attributed to structural (cluster thickness, shape, and metal oxidation state) and support effects.3 Regarding supports, the most active one is iron oxide (Fe3O4), which is widely used in catalytic systems. Despite the extensive studies, fundamental aspects remain disputed, such as the catalytic roles of gold nanoparticles and the Fe3O4 support, and therefore, detailed information about the structure and stability of gold on Fe3O4 is of great interest and importance. The Au/Fe3O4 catalyst, especially the Au oxidation state, has been studied experimentally. Gatel et al.4 studied Pt, Ag, and Au growth on Fe3O4(001) as well as Pt and Au growth on Fe3O4(111)5 and found different growth modes depending upon deposition temperature and thickness. By using extended X-ray absorption fine structure and X-ray absorption near edge structure, X-ray photoelectron spectroscopy, and Mössbauer spectroscopy, Hutchings et al.6 studied the Au/Fe3O4 catalyst structure and found that the cationic gold plays a crucial role in CO oxidation and crotonaldehyde hydrogenation. Using aberration-corrected scanning transmission electron microscopy to analyze several iron oxide supported catalysts, Herzing et al.7 claimed that the high catalytic activity in CO oxidation correlates with the presence of metallic gold bilayer clusters, which have about 10 atoms with diameters of ∼0.5 nm and close contact with the oxide support. By a scanning tunneling © 2012 American Chemical Society
microscopy and scanning tunneling spectroscopy study of a model catalyst consisting of supported gold nanoparticles on a reduced Fe3O4(111) surface in ultrahigh vacuum, Rim et al.8 found that the gold nanoparticles exhibit a metallic electronic structure and the gold adatom is positively charged and suggested that such a gold atom may play a key role in CO oxidation and the water gas shift reaction. Giordano et al.9 investigated the charging of Au and Pd adatoms on an FeO thin film supported on Pt(111) and found that the Au adatoms become spontaneously positively charged upon deposition via electron transfer from Au to the support thin film. Lee et al.10 studied H2O2 reduction on Au/Fe3O4 and found the enhanced catalytic activity from the polarization effect at the Au/Fe3O4 interface. A recent finding reveals that the oxidized Au (or Pt) species in Fe3O4 is the catalytic site for the low-temperature water gas shift reaction,11 while Au (or Pt) nanoparticles are spectators. On the oxide surfaces, nanoparticles, clusters, and atoms (ions) of Au (or Pt) can coexist, but the chemistry takes place on the atomically dispersed Auδ+ (or Ptδ+) species and involves the neighboring OH groups. Using high-resolution aberration-corrected electron microscopy, Allard et al.11 identified atomic gold decorating step surfaces of iron oxide. Shaikhutdinov et al.12 studied CO adsorption on model gold catalysts and found stronger CO adsorption on small particles than on larger ones. Received: February 9, 2012 Revised: April 18, 2012 Published: April 30, 2012 10632
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There are two distinct Fe sites, the tetrahedral A site and the octahedral B site, and they have magnetic moments oriented in opposite directions. In an ionic picture, the tetrahedral A sites are occupied by Fe3+, and the octahedral B sites are occupied by an equal number of Fe2+ and Fe3+. We set the spin of the octahedral Fe as up and the spin of the tetrahedral Fe as down because Fe3O4 is ferrimagnetic with an anomalously high critical temperature of 860 K and the ferrimagnetic ordering arises from the strong antiferromagnetic coupling between Fe ions on A and B sublattices. The Fe ions couple ferromagnetically within the B sublattice. At room temperature, Fe3O4 is poorly metallic with an electronic conductivity of 4 mΩ cm.21 As the main growth face among the low-index surfaces, the electronic structure and stability of the Fe3O4(111) surface have been reported.13 As shown in Figure 1, Fe3O4(111) has six p(1
The stability of the six terminations of the Fe3O4(111) surface has been reported.13 It is found that Fetet1 and Feoct2 are most stable with similar surface energies (1.056 and 1.184 J/m2, respectively), followed by Ooct2 and Ooct1 with similar surface energies (2.784 and 2.816 J/m2, respectively), and Feoct1 and Fetet2 are the least stable (3.376 and 3.472 J/m2, respectively). The similar surface energies of Fetet1 and Feoct2 reveal their possible coexistence. This agrees with the experiments. For example, the Fe3O4(111) surface is sensitive to the preparation methods, and both Fetet114 and Feoct215 have been observed. Nevertheless, all terminations are used for Au atom adsorption. Despite these efforts to understand the interaction between the catalytically active metal particles and the Fe3O4 support,16 the nature of the defect sites where metals can anchor and be stabilized has not been analyzed. In contrast to the large volume of experimental work, theoretical studies on the interaction between Au and Fe3O4 as well as CO oxidation on Au/Fe3O4 catalysts are rarer. In this paper, we systematically studied the interaction of a single Au atom on the six Fe3O4(111) terminations, and our interests are the preferred adsorption sites along with the surface reconstruction around the adsorbate as well as the nature of the Au and Fe3O4 interaction.
2. METHOD AND SURFACE MODEL The full spin-polarized calculations were performed by using the frozen-core all-electron projector-augmented wave (PAW) method,17 as implemented in the Vienna Ab-initio Simulation Package (VASP).18 The 3p, 4s, and 3d electrons of Fe; the 2s and 2p electrons of O; and the 5d and 6s electrons of Au were treated as valence electrons. The electron exchange and correlation was treated within the generalized gradient approximation (GGA) using the Perdew−Burke−Ernzerhof (PBE) functional.19 The DFT+U method is chosen as our approach, since it can accurately model transition-metal oxide systems.20 The physical idea behind the DFT+U approach comes from the Hubbard Hamiltonian, in which the Hubbard parameter (U) is introduced for Fe 3d electrons to describe the on-site Coulomb interaction. The value of Ueff = U − J was set to 3.8 eV, as suggested in the literature.21 At Ueff = 3.8 eV, the computed magnetic moment of both tetrahedral and octahedral iron sites are 4 μB, in very good agreement with the experiment (4.05 μB).22 The Brillouin zone integrations were performed using Monkhorst−Pack (MP) grids,23 and a Gaussian smearing of σ was 0.2 eV. The number of plane waves was controlled by a 400 eV cutoff energy. For integration within the Brillouin zone, specific k-points were selected using a 7 × 7 × 7 MP grid for the bulk and a 3 × 3 × 1 MP grid for Fe3O4(111). Structure optimizations were performed until the Hellmann−Feynman force on each atom was less than 0.02 eV/Å. These settings were able to generate a lattice constant of 8.405 Å for bulk Fe3O4, which agrees well with the experimental value (8.396 Å),24 and to reproduce the characteristic electronic features, including the energy level of d states and the half-metal property of Fe3O4.25 In a recent publication, Kiejna and Pabisiak26 reported the structure and electronic properties of different terminations of the α-Fe2O3(0001) surface using the spin-polarized DFT method and DFT+U and found that both methods can provide similar surface geometries, but differ very much in the predication of the electronic and magnetic properties, and the surface energies of the clean αFe2O3(0001). Similar results have been reported on the electronic and magnetic properties as well as surface energies of the (111), (110), and (001) surfaces Fe3O4.13
Figure 1. Fe3O4(111) direction and its six terminations (oxygen atom, red ball; iron atom, blue ball).
× 1) terminations by cutting the spinel (111) stacking sequence, the exposed tetrahedral (Fetet1 and Fetet2) and octahedral (Feoct1 and Feoct2) coordinated Fe, and the closely packed oxygen layers (Ooct1 and Ooct2). The surface is represented as slabs, periodically repeated in the z direction perpendicular to the surface. The x and y dimensions of the 10633
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Figure 2. Side and top views of the six terminations of Fe3O4(111) with all possible adsorption sites (t for top site, b for bridge site, and h for hollow site) along with the most stable adsorption configuration and adsorption energy (Fetet1: light blue ball for first-layer Fe atoms, purple ball for secondlayer cap O atoms, brown ball for second uncap O atoms. Feoct2: light blue ball for first-layer Fe atoms, green ball for second-layer Fe atoms, purple ball for third-layer O atoms. Ooct2: purple ball for first-layer single coordinated O atoms, brown ball for first-layer two coordinated O atoms, light blue ball for second-layer Fe atoms, green ball for third-layer Fe atoms. Ooct1: purple ball for first two coordinated O atoms, brown ball for first three coordinated O atoms, light blue ball for second-layer Fe atoms. Feoct1: light blue ball for first-layer Fe atoms, green ball for second-layer Fe atoms, purple ball for third-layer O atoms. Fetet2: light blue ball for first-layer Fe atoms, green ball for second-layer Fe atoms, purple ball for fourth-layer O atoms, blue ball for other Fe atoms, red ball for other O atoms, yellow ball for Au atom).
unit cell were fixed at the calculated equilibrium lattice parameter value (8.405 Å). Fe3O4(111) was modeled by 12 atomic layers in each slab. In structural minimization, the adsorbed Au atom and the top six atomic planes were allowed
to relax and the bottom six atomic planes were fixed. At first, we used 10 Å to separate the slabs for searching the most stable adsorption sites at first, and all of these results are given in the Supporting Information. Subsequently, we tested different 10634
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Au on Ooct2. There are two oxygen atoms exposed on the Ooct2 surface (Figure 2c): one is single coordinated with the Fe atom (O1, brown), and one forms two bonds with iron atoms (O2, purple). Five stable adsorption configurations are found, and the most stable one is the Au atom on the h1 site with an adsorption energy of −3.51 eV. The adsorbed Au atom is oxidized and positively charged (+0.76 e). Au on Ooct1. There are two exposed oxygen atoms on the Ooct1 surface (Figure 2d): the 3-fold coordinated O1 (brown) and 2-fold coordinated O2 (purple). Among the seven possible adsorption sites, five stable adsorption configurations are found. The most stable configuration has the Au atom on the h2 site with and adsorption energy of −4.28 eV and the O1−Au distances of 2.095, 2.095, and 2.095 Å. The adsorbed Au atom is oxidized and positively charged (+0.84 e). Au on Feoct1. The Feoct1 termination has a constructed surface, and three surface iron atoms form a triangle with Fe− Fe distances of about 2.657 Å (bulk value about 2.972 Å). Among the five potential adsorption sites (Figure 2e), only three stable adsorption configurations are found. The most stable configuration has the Au atom on the top of the iron triangle (h1 site) with an adsorption energy of −5.04 eV, and they form a tetrahedron with Au−Fe distances of 2.640, 2.643, and 2.647 Å as well as Fe−Fe distances of 2.777, 2.779, and 2.779 Å. The adsorbed Au atom is reduced and negatively charged (−0.48 e). Au on Fetet2. There are three types of iron atoms exposed on the Fetet2 surface (Figure 2f), the first-layer tetrahedral iron atom (light blue), the second-layer octahedral iron atom (green), and the third-layer tetrahedral iron atom (blue). Among the six potential adsorption sites, five stable adsorption configurations were found. The most stable configuration is the Au atom on the 3-fold hollow site (h1 site) of the three surface iron atoms with an adsorption energy of −5.11 eV, and the distances of Au−Fetet2, Au−Feoct2, and Au−Fetet1 are 2.637, 2.596, and 2.831 Å, respectively. The adsorbed Au atom is reduced and negatively charged (−0.62 e). Analysis. Table 1 lists the computed surface energies, the adsorption of a single Au atom, and the partial charge according
vacuum gaps (12, 14, and 16 Å) for the most stable adsorption sites. It is found that the adsorption energies are very close at 14 and 16 Å for the Fetet1 (−1.74 vs −1.74 eV), Feoct2 (−3.11 vs −3.11 eV), Ooct2 (−3.53 vs −3.51 eV), Feoct1 (−5.01 vs −5.04 eV), and Fetet2 (−5.08 vs −5.11 eV) terminations, whereas the adsorption energies on the Ooct1 termination at 10 (−4.25 eV), 12 (−4.33 eV), 14 (−4.44 eV), and 16 (−4.28 eV) Å show an alternating tendency. Therefore, we reported only these results with a 16 Å vacuum gap, and all test results are given in the Supporting Information. The leading errors induced by the dipole moment in the supercells were corrected by using the methods as implemented in VASP. The adsorption strength is characterized by the adsorption energy (Eads), Eads = E(Au/slab) − [E(Au) + E(slab)], where E(Au/slab), E(slab), and E(Au) are the energies of the Auadsorbed slab (Au/slab), the clean slab (Fe3O4), and a free Au atom, respectively. A negative adsorption energy corresponds to a stabilizing interaction. The Bader-type charge27 for the most stable adsorption Au/Fe3O4 modes was calculated. The possible adsorption sites and the most stable adsorption configuration for all six surfaces are shown in Figure 2.
3. RESULTS AND DISCUSSION Au on Fetet1. There are three types of exposed atoms on the Fetet1 surface (Figure 2a): the tetrahedral Fe atom (light blue); the uncap O atom (brown), which does not bond with the surface tetrahedral iron atom; and the cap O atom (purple), which bonds with the surface tetrahedral iron atom. For the seven possible adsorption sites, four stable adsorption configurations are located. The most stable configuration has the Au atom on the top of the surface tetrahedral iron site (t1) with an adsorption energy of −1.74 eV, and a Au−Fe distance of 2.442 Å, and the adsorbed Au atom is reduced and negatively charged (−0.10 e). In addition to the stoichiometric terminated Fetet1 surface, we also considered the Fetet1 termination with oxygen and iron vacancies. On the stoichiometric termination, deleting one surface oxygen atom forms the reduced Fetet1 termination, while moving one surface Fe atom forms the oxidized Fe tet1 termination. On the reduced Fetet1 termination, only Au adsorption at the vacancy site is considered. In the optimized structure, the gold atom bonds to three third-layer iron atoms with three Au−Fe distances of 2.670, 2.670, and 2.699 Å and the shortest distances between the gold atom and the six nearest-neighboring oxygen atoms are 3.133 Å. On the oxidized Fetet1 termination, the gold atom locates nearly at the center of the three oxygen atoms with Au−O distances of 2.074, 2.074, and 2.083 Å. The calculated adsorption energy is −3.46 eV, much stronger than that on the stoichiometric terminated Fetet1 surface (−1.74 eV) and the reduced Fetet1 termination (−2.35 eV). The adsorbed Au atom is positively charged on the oxidized termination (+0.83 e), while negatively charged on the reduced termination (−0.42 e). Au on Feoct2. There are two exposed iron atoms on the Feoct2 surface (Figure 2b), the octahedral iron (light blue) and the tetrahedral iron (green) atoms. Among the five possible adsorption sites, four stable adsorption configurations are located. The most stable configuration has the Au atom on the bridge site (b1 site) of tetrahedral Fe and octahedral Fe atoms with an adsorption energy of −3.11 eV; the distances of Au− Feoct2 and Au−Fetet1 are 2.535 and 2.769 Å, respectively. The adsorbed Au atom is reduced and negatively charged (−0.53 e).
Table 1. Computed Surface Energies (Esurf, J/m2), Au Atom Adsorption Energies (Eads, eV), and Partial Charges on the Adsorbed Au Atom surface
Esurfa
Eads
δAu
Fetet1 Fetet1−VFe Fetet1−VO Feoct2 Ooct2 Ooct1 Feoct1 Fetet2
0.944
−1.74 −3.46b −2.35c −3.11 −3.51 −4.28 −5.04 −5.11
−0.10 +0.83b −0.42c −0.53 +0.76 +0.84 −0.48 −0.62
1.132 2.729 2.857 3.415 3.581
Esuf = (Eslab − nEbulk)/2A, where Eslab is the total energy for the slab, Ebulk is the bulk total energy per Fe3O4 unit, n is the number of the Fe3O4 units in the slab, and A is the surface area of the slab. bOxidized surface. cReduced surface a
to Bader analysis. The calculated Au adsorption energy correlates well with the computed surface energy among the six terminations. For example, the most stable Fe tet1 termination (0.944 J/m2) has the lowest Au adsorption energy (−1.74 eV), followed by the second most stable termination 10635
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Figure 3. Total and atom-resolved projected density of states (DOS) of the adsorbed Au atom on the Feoct2 (left) and Ooct2 (right) terminations (Fesuf and Osur refer to surface iron atoms and oxygen atoms).
(1.132 J/m2 vs −3.11 eV). The least-stable Feoct1 and Fetet2 terminations have the largest Au adsorption energy (3.415 J/m2 vs −5.04 eV and 3.582 J/m2 vs −5.11 eV), and the oxygenterminated Ooct2 (2.729 J/m2) and Ooct1 (2.857 J/m2) surfaces have a Au adsorption energy of −3.51 and −4.28 eV, respectively. In addition, charge transfer, either from the surface to the adsorbed Au atom or from the adsorbed Au atom to the surface, has been found. On the iron-terminated surfaces, for example, the adsorbed Au atom has a negative charge (−0.10 to −0.62 e), whereas on the oxygen-terminated surfaces, the adsorbed Au atom has a positive charge (0.84 and 0.76 e). This is indeed in line with the proposed electronegativity for both the Fe (1.83) atom and Au (1.92).28 Such an effect can also be seen on the Fetet1 surface with vacancies. For example, on the oxidized surface termination (deleting one surface iron atom), the Au adsorption energy is −3.46 eV and the adsorbed Au atom has a positive charge (+0.83 e); on the reduced surface termination (deleting one surface oxygen atom), the Au adsorption energy is −2.35 eV and the adsorbed Au atom has a negative charge (−0.42 e). However, there is no correlation between the computed Au adsorption energy and the absolute values of the transferred charge. Figure 3 shows the calculated density of states (DOS) of the most stable Au adsorption configuration on the Feoct2 and Ooct2 terminations. Compared to the free Au atom, which has a 6s orbital at the Fermi level and 5d orbitals slightly below the Fermi level, these orbitals of the adsorbed Au atom on the Feoct2 termination are downfield shifted, while the DOS of the total surface and the DOS of the surface iron atoms are upfield shifted, indicating the electron transfer from the surface to the adsorbed Au atom. For the Au atom on the Ooct2 termination, opposite trends have been observed. For example, the DOS of the 5d orbitals of the adsorbed Au atom becomes much broader, whereas the DOS of the surface and the DOS of the surface oxygen atoms are downfield shifted, indicating the electron transfer from the adsorbed Au atom to the surface. These changes are in line with the Bader charge analysis. That the adsorbed Au atom on the oxygen-terminated surface has a
positive charge is in line with the experimentally observed sitespecific adsorption of the gold adatom on oxygen surface atoms and the size-sensitive nature of the electronic structure.8 It is reported that the positively charged gold adatoms on Fe3O4 should be responsible for the catalytic activities of CO oxidation, the water gas shift reaction,8 and H2O2 reduction.10 Since our results show that the gold adatom will have a positive charge only on the oxygen-terminated surfaces, it is reasonable that oxygen-terminated surfaces should be the support surface for the adsorption of gold atoms. It is possible to stabilize the oxygen-terminated surface for gold adsorption by synthetic tuning. Since a single gold atom is too simple to represent gold nanoparticles, it is necessary to study the interaction between small gold clusters and metal oxide surfaces. Similar results have been reported by Kiejna and Pabisiak26 in a very recent work using both DFT and DFT+U methods on the surface properties of a clean α-Fe2O3(0001) surface. They used the most stable single iron-terminated (0001) surface and oxygen-rich termination for Au and Pd atom adsorption and found that Au and Pd atoms have a much stronger adsorption on the oxygen termination than on the iron termination. They29 also studied Au and Pd adsorption on the stable Fe and O terminations of the Fe3O4(111) surface by DFT+U and found that Au and Pd binding on the O-terminated surface is stronger by 1.5−2.5 eV than the Fe-terminated surface. Table 2 also lists the adsorption energies of the Au atom on other metal oxides, but these results are not comparable.
4. CONCLUSION The adsorption of a single Au atom on the six terminations of the Fe3O4(111) surface has been computed with the spinpolarized density functional theory, including the strong on-site Coulomb corrections (GGA+U), for the understanding of the catalytic activity of supported Au nanoparticles on the Fe3O4 surface in CO oxidation and the water gas shift reaction. The most stable adsorption on each termination has been determined, that is, the surface tetrahedral iron atom top site for the Fetet1 termination, the 3-fold hollow site of three surface oxygen atoms for the Ooct1 termination, on the center of trimer 10636
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sites are included. This material is available free of charge via the Internet at http://pubs.acs.org.
Table 2. The Largest Adsorption Energies for a Single Au on Different Surfaces of Different Metal Oxides at Lower Coverage site MgO(001)
O-top
α-Al2O3(0001) θ-Al2O3(001) ZrO2(111) TiO2(110) TiO2(001) TiO2(110) TiO2(110) CeO2(111)
O-top O−Al-bridge O−Zr hollow O-bridge O-bridge Ti-top O-top
PBE
PW91
−0.1330
−1.2031 −0.8932 −0.8133
−0.35 −0.8135 −0.5836 −6.1438 −0.4139
Fe2O3 Fe2O3
O-top O-bridge Fe-top O-hollow
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (No. 21073218), the National Basic Research Program of China (No. 2011CB201406), the Chinese Academy of Science, and Synfuels China Co., Ltd.
−0.6837 −0.5837 −1.3840
−0.2641
−1.6226 −2.1126
−0.7942 −0.9641 −1.1743 −1.1544 −1.1340 −2.0745 −0.9826 −3.8426
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REFERENCES
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iron atoms for the Feoct1 termination, the bridge of surface iron atoms for the Feoct2 termination, the 3-fold hollow site of three surface iron atoms for the Fetet2 termination, and the 3-fold hollow site of three surface oxygen atoms for the Ooct2 termination. It is found that the Au atom adsorption energy correlates well with the computed surface energy; that is, the more stable the surface, the lower the adsorption energy. A net charge transfer has been found between the adsorbed Au atom and the surface. On the iron-terminated surfaces, the adsorbed Au atoms are negatively charged, and this is in line with the proposed electronegativity of iron and gold atoms. On the oxygen-terminated surfaces, the adsorbed Au atoms are positively charged. A much stronger effect can be found on the Fetet1 surface with vacancies. For example, when one surface Fe atom (oxidized termination) is deleted, the Au adsorption energy is −3.46 eV and the adsorbed Au atom has a positive charge (+0.83 e), whereas when one surface O atom (reduced termination) is deleted, the Au adsorption energy is −2.35 eV and the adsorbed Au atom has a negative charge (−0.42 e). No correlation between adsorption energy and the absolute value of the transferred charge is found. On the basis of the experimental finding that the positively charged gold nanoparticles on Fe3O4 should be responsible for the observed catalytic activities and on our results that only the Au atom on the oxygen-terminated surfaces has a positive charge, it should be possible to stabilize the oxygen-terminated surface for Au atoms adsorption by synthetic tuning. Since a single Au atom is too simple to represent Au nanoparticles, we will study the adsorption of small Au clusters on metal oxides, especially on the Fe3O4 surface for the understanding of the catalytic activity of Au/Fe3O4 systems in CO oxidation and the water gas shift reaction.
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AUTHOR INFORMATION
Corresponding Author
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O-bridge CeO2(110)
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GGA+U
ASSOCIATED CONTENT
S Supporting Information *
All stable adsorption sites for Au atom adsorption at 10 Å vacuum gaps and all benchmark calculations on the vacuum gaps from 10, 12, 14, and 16 Å for the most stable adsorption 10637
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The Journal of Physical Chemistry C
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dx.doi.org/10.1021/jp301313u | J. Phys. Chem. C 2012, 116, 10632−10638
