First-principles investigations of the structure and stability of oxygen adsorption and surface oxide formation at Au(111)

We perform density-functional theory calculations to investigate the adsorption of oxygen at the Au(111) surface, including on-surface, subsurface, and surface oxide formation. We find that atomic oxygen adsorbs weakly on the surface and is barely stable with respect to molecular oxygen, while pure subsurface adsorption is only metastable. Interestingly, however, we find that the most favorable structure investigated involves a thin surface-oxide-like configuration, where the oxygen atoms are quasithreefold-coordinated to gold atoms, and the gold atoms of the surface layer are twofold, linearly coordinated to oxygen atoms. By including the effect of temperature and oxygen pressure through the description of ab initio atomistic thermodynamics, we find that this configuration is the most stable for realistic catalytic temperatures and pressures, e.g., for low-temperature oxidation reactions, and is predicted to be stable up to temperatures of around $420\mathrm{K}$ at atmospheric pressure. This gives support to the notion that oxidized Au, or surface-oxide-like regions, could play a role in the behavior of oxide-supported nanogold catalysts.

Figure 1

Calculated adsorption energy of oxygen on Au(111) in the fcc-hollow site versus coverages with respect to atomic oxygen. The upper and lower horizontal lines represent the experimental $\left(2.56\mathrm{eV}\right)$ and theoretical $\left(3.14\mathrm{eV}\right)$ values of the ${\mathrm{O}}_2$ binding energy per atom. The solid line is to guide the eyes.

Figure 2

Change in the calculated work function $\Delta \Phi$ (upper) and surface dipole moment $\mu$ (lower) as a function of coverage for O in the fcc-hollow site on Au(111).

Figure 3

Difference electron densities for O in the fcc-hollow site on Au(111) for coverages of 0.06, 0.11, 0.25, and 1.0 ML. The dark isosurfaces indicate an increase in electron density, and the light isosurfaces represent a depletion. The black spheres represent the position of the oxygen and gold atoms. The value of the isospheres is $\pm 7.5\times {10}^{-5}e∕{\mathrm{\AA }}^3$.

Figure 4

Projected density of states for oxygen adsorbed on the surface for various coverages. The dark continuous and dashed lines indicate the $\mathrm{Au}5d$ and $6s$ states, respectively, and the gray continuous and dashed lines represent the $\mathrm{O}2p$ and $2s$ states, respectively.

Figure 5

Sites considered for subsurface adsorption of O under the first Au(111) layer. Dark and light spheres represent oxygen and gold atoms, respectively. The atoms are depicted in the relaxed positions.

Figure 6

Atomic geometry of $\text{on}\text{−}\text{surface}+\text{subsurface}$ structures: (a) oxygen in the fcc site plus subsurface oxygen in the tetra-I site, (b) as for (a) but with additional oxygen in the tetra-II site, and (c) as for (b) but with additional oxygen under the second Au layer in the tetra-I and tetra-II sites. The average adsorption energy, with respect to free oxygen atoms, as well as the corresponding coverage are given. The relative variation of the first and second interlayer spacings, with respect to the value of the corresponding relaxed Au(111) surface, is also given. Large pale gray and small dark circles represent gold and oxygen atoms, respectively.

Figure 7

Atomic geometry of a structure with O in the fcc-hollow site and a surface Au vacancy in a $(2\times 2)$ cell. The surface unit cells are indicated. The oxygen atoms are represented by small dark circles, the uppermost Au atoms by small gray circles, and the intact plane of Au(111) atoms lying below by larger pale gray circles.

Figure 8

The difference electron density $n^{\Delta }\left(\mathbf{r}\right)$ [see Eq. (4)] for the $\text{on}\text{−}\text{surface}+\text{subsurface}$ structure, ${({\mathrm{O}}_{\mathrm{fcc}}∕{\mathrm{O}}_{\text{tetra}\text{−}\mathrm{I}})}_{\theta =0.5}$. Dark and light regions represent electron enhancement and depletion, respectively. The values of the isospheres are $\pm 7.5\times {10}^{-5}e∕{\mathrm{\AA }}^3$. The positions of the O and Au atoms are indicated by black dots.

Figure 9

Projected density of states for the pure subsurface tetra-I structure. The dark lines indicate the Au PDOS of the first metal layer atoms that are bonded to oxygen (top) and the Au PDOS of the second metal layer atoms that are beneath oxygen (bottom). The $\mathrm{Au}6s$ orbital is indicated by the dashed dark line, the $\mathrm{Au}5d$ orbital by the solid dark line, the $\mathrm{O}2s$ orbital by the dashed gray line, and the $\mathrm{O}2p$ orbital by the solid gray line. The Fermi energy is indicated by the vertical line.

Figure 10

Projected density of states for the ${\mathrm{O}}_{\mathrm{fcc}}\text{−}\mathrm{Au}\text{−}{\mathrm{O}}_{\text{tetra}\text{−}\mathrm{I}}$ structure. The Au PDOS of the first metal layer atoms that are bonded to oxygen is indicated by dashed $\left(6s\right)$ and solid $\left(5d\right)$ dark lines, respectively. The gray lines denote the PDOS of O in the fcc site (upper) and tetra-I site (lower). The $\mathrm{O}2s$ and $2p$ orbitals are indicated by the dashed and solid gray lines, respectively. The Fermi energy is represented by the vertical line.

Figure 11

Atomic geometry of surface-oxide-like structures on Au(111) calculated using $(4\times 4)$ surface unit cells (indicated). The oxygen atoms are represented by small dark circles, the uppermost Au atoms by gray circles, and the intact plane of Au(111) atoms lying below by the larger paler gray circles. The lowest-energy structure (g) is framed.

Figure 12

Difference electron density $n^{\Delta }\left(\mathbf{r}\right)$ [see Eq. (4)] for the lowest-energy surface-oxide-like structure [Fig. 11g]. The $(4\times 4)$ surface unit cell is indicated by the line. (a) Top view and (b) side view.

Figure 13

Projected density of states for the lowest-energy surface-oxide-like structure [Fig. 11g]. The Au PDOS of the first metal layer atoms that are bonded to oxygen is indicated by dashed $\left(6s\right)$ and solid $\left(5d\right)$ dark lines, respectively. The gray lines denote the PDOS of O at upper and lower sites. The $\mathrm{O}2s$ and $2p$ orbitals are represented by the dashed and solid gray lines, respectively. The Fermi energy is represented by the vertical line.

Figure 14

Surface free energies for O at Au(111) for the various low-energy structures as a function of the O chemical potential, where $\Delta {\mu }_{\mathrm{O}}$ is defined as ${\mu }_{\mathrm{O}}-(1∕2)E_{{\mathrm{O}}_2}^{\text{total}}$. On-surface chemisorbed oxygen with coverages of 0.06, 0.11, and 0.25 ML, the $(4\times 4)$ surface-oxide structures shown in Figs. 11a, 11e, 11g, (labeled “struc. a,” etc.), and the $(2\times 2)\text{−}\mathrm{O}+\mathrm{vac}$ structure (cf. Fig. 7) are presented. The corresponding temperatures are given for two selected pressures [see Eq. (6)], one corresponding to UHV conditions and the other to atmospheric pressure.

Figure 15

(Color online) The calculated pressure-temperature phase diagram for $\mathrm{O}∕\mathrm{Au}\left(111\right)$ where the ranges of stability of the various phases are indicated.

Figure 16

Vibrational and entropic contributions [cf. Eq. (A1)] to the Gibbs surface free energy for the lowest-energy surface-oxide-like structure [see Fig. 11g] due to the oxygen atoms.