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 (Formula presented) low-energy electron diffraction pattern is explored and its alloy nature is established. The (Formula presented) 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 (Formula presented) This surface alloy is buckled with the Sn atoms displaced upward from the Rh surface plane by (Formula presented) 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 (Formula presented) and -0.6 eV for the Rh (Formula presented) and Sn (Formula presented) core levels, respectively, were observed for the Rh-Sn alloy compared to the pure elements. A shift of (Formula presented) 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.