Adsorption of gold clusters on metal-supported MgO: Correlation to electron affinity of gold

Adsorption of ${\mathrm{Au}}_N$ clusters $(N=1–6)$ on a bulk MgO(001) surface and on an ultrathin (3 ML) MgO(001) film supported by Mo metal is investigated via density-functional theory calculations. Comparison of the two substrates unambiguously shows that the gold clusters adsorbed on $\mathrm{Mg}\mathrm{O}∕\mathrm{Mo}$ turn into singly charged cluster anions ${\mathrm{Au}}_N^{\delta }$, $\delta \approx -1$. Their structures and internal charging patterns are analogous to singly charged, planar, gas-phase cluster anions ${\mathrm{Au}}_N^{-}$. The adsorption energy has prominent size-dependent odd-even oscillations, correlating fully with the known electron affinity of gas-phase Au clusters in this size range.

Figure 1

(Color online) The adsorption energies and geometries of the optimal clusters on (a) bulk MgO and on (b) $\mathrm{Mg}\mathrm{O}∕\mathrm{Mo}$ as a function of cluster size. In (b), two different planar structures, upright (open circles) and flat (solid circles), are given. Au atoms are yellow (light gray), O atoms are red (dark gray), and Mg atoms are blue (medium gray). Mo atoms are not shown in (b).

Figure 2

(a) The calculated cluster charge as a function of size on MgO (open circles) and $\mathrm{Mg}\mathrm{O}∕\mathrm{Mo}$ (solid circles). (b) The measured (Ref. 39) (open triangles) and calculated (solid triangles) electron affinities of gas-phase Au clusters compared to the enhancement in Au cluster adsorption energy induced by the Mo support below MgO (solid diamonds).

Figure 3

(Color online) The Bader charges of the individual cluster atoms in electrons (numbers on atoms) and the bond lengths in Å (numbers between atoms) for ${\mathrm{Au}}_N∕\mathrm{Mg}\mathrm{O}∕\mathrm{Mo}$ in (a), (c), (e), (g), (i), and (k), and for anionic gas-phase clusters in (b), (d), (f), (h), and (j) with the given symmetries. Yellow (lightest gray), red (darkest gray), blue (medium gray), and gray balls [the bottom most ones in (l)] mark Au, O, Mg, and Mo atoms, respectively. (l) gives a density difference plot of ${\mathrm{Au}}_2$ on $\mathrm{Mg}\mathrm{O}∕\mathrm{Mo}$ shown in (a). Note that the cut plane is (110), which does not coincide with the symmetry planes of $\mathrm{Mo}d$ orbitals, hence the charge depletion from Mo atoms is not clearly visible. Blue or darkest gray (red or second darkest gray) regions indicate charge depletion (accumulation). The MgO-Mo interface is marked with the white line.