Probing the structures of bimetallic Sn/Rh(111) surfaces: Alkali-ion scattering and x-ray photoelectron diffraction studies

We have investigated the bimetallic Sn/Rh(111) system formed by vapor deposition of Sn on a Rh(111) single-crystal surface by applying two powerful structural probes: alkali-ion scattering spectroscopy (ALISS) and x-ray photoelectron diffraction. For initial submonolayer exposures, the surface structure responsible for the observed $(\sqrt{3}\times \sqrt{3})R30\mathrm{}°-\mathrm{S}\mathrm{n}/\mathrm{R}\mathrm{h}\left(111\right)\mathrm{}$ low-energy electron diffraction pattern is explored and its alloy nature is established. The ${\mathrm{Na}}^{+}$ ALISS results indicate that the formation of this surface alloy is produced by the replacement of top-layer Rh atoms by Sn atoms. The resultant alloy surface can be strictly two dimensional if the annealing temperature is high enough $(>~1300\mathrm{K}).\mathrm{}$ This surface alloy is buckled with the Sn atoms displaced upward from the Rh surface plane by $0.29\pm 0.05\AA .$ In addition, x-ray photoemission spectroscopy core-level measurements have been performed on this surface alloy at grazing exit angles and these are compared with results on the analogous Sn/Pt(111) surface alloy. Binding energy shifts of $-0.4$ and -0.6 eV for the Rh $3d$ and Sn $3d$ core levels, respectively, were observed for the Rh-Sn alloy compared to the pure elements. A shift of $-0.3\mathrm{eV}$ was also seen for the valence-band centroid upon alloying. From temperature-programmed desorption studies it was determined that CO adsorption is decreased on the Sn/Rh surface alloy, but with only a small (4 kcal/mol) decrease in the adsorption energy. The growth mechanism of the Sn film in the Sn/Rh(111) bimetallic system was also probed. The vapor deposition of Sn on Rh(111) at 300 K does not form epitaxial clean Sn films or pure Sn clusters but rather forms a random alloy of increasing thickness.