High-Coverage Oxygen Structures on Rh(111): Adsorbate Repulsion and Site Preference Is Not Enough

A new O induced structure on Rh(111) displaying a $(2\sqrt{3}\times 2\sqrt{3})R30°$ periodicity with an oxygen coverage of $2/3$ has been studied by high resolution core level spectroscopy, scanning tunneling microscopy, and density functional theory. Although O favors fcc hollow sites in all other known phases, it occupies both fcc and hcp sites in this structure, which cannot be explained by pairwise adsorbate repulsion only. Both the $(2\sqrt{3}\times 2\sqrt{3})R30°$ and $(2\times 2)\mathrm{\text{−}}3\mathrm{O}$ structures also exemplify that density-of-states contrast can lead to oxygen adatoms appearing as protrusions in scanning tunneling microscopy images.

Figure 1

HRCLS of the $(2\sqrt{3}\times 2\sqrt{3})R30°$ structure. The $\mathrm{R}\mathrm{h}3d_{5/2}$ peak can be decomposed into four components with an O coordination of 1O, 2O, and 3O, whereas, the $\mathrm{O}1s$ level exhibits only one component indicating no subsurface or oxide formation. (Inset) LEED pattern of the $(2\sqrt{3}\times 2\sqrt{3})R30°$ structure (electron energy 126 eV).

Figure 2

(a) STM image of the $(2\sqrt{3}\times 2\sqrt{3})R30°$ and $p(2\times 2)\mathrm{\text{−}}3\mathrm{O}$ phase (20 nm wide; $-95\mathrm{m}\mathrm{V}$, 0.46 nA). (b) Simulated STM image of the relaxed structure as obtained by DFT shown in (d). (c) Fourier filtered STM image of the $(2\sqrt{3}\times 2\sqrt{3})R30°$ to be compared to (b). (d) Structure as obtained by DFT. Small balls are O atoms, dark in fcc sites, lighter in hcp sites. In the lower right corner the honeycomb $(\sqrt{3}\times \sqrt{3})R30°\mathrm{\text{−}}2\mathrm{O}$ structure is illustrated. Interaction energies for O pairs and an ${\mathrm{O}}_3$ triangle are also indicated.

Figure 3

Calculated phase diagram for oxygen-on Rh(111). The chemical potential can be related to the temperature and the oxygen partial pressure [28]. The stability regime of the $(2\sqrt{3}\times 2\sqrt{3})R30°$ phase is indicated by small ticks.