Ultra-thin films of In on Pd(111) characterized by X-ray photoelectron diffraction

Metallic surfaces involving transition metals are widely used in the preparation of catalysts. The conversion rates and the selectivity can be improved, in many instances, by a surface alloy formed by a transition metal with a less reactive sp metal. Bimetallic systems based on PdIn have been shown to be efficient catalysts for methanol steam reforming and nitrate reduction in water. The catalytic reaction depends on both the electronic structure and the geometric arrangement of the surface alloy atoms. In and Pd have very limited miscibility in the bulk but can form ordered surface alloys. We have investigated the growth and the surface alloy formation of an ultra-thin film of In deposited on a Pd(111) single-crystal substrate surface. The electronic structure and the surface composition have been characterized by X-ray photoelectron spectroscopy (XPS), and the atomic surface structure has been determined by a combination of low energy electron diffraction (LEED) and X-ray photoelectron diffraction (XPD). Ultra-thin films of In having approximately 1 to 5 monolayers (ML) were deposited at room temperature, and then annealed at 500 K. Pd 3d and In 3d intensities obtained by XPS indicated the Stranski-Krastanov growth mode, i.e. flat bi-dimensional overlayers in the first layer followed by three-dimensional islands. For annealed In films having 0.33 to 2.5 ML, a (3x3)R30° LEED pattern was observed. For a non-annealed In film having approximately 3 ML, a p(2 × 2) LEED pattern was observed, that changed to (3x3)R30° upon annealing at 500 K. For a slightly higher coverage, about 3.6 ML, a (1 × 1) pattern was obtained after annealing. XPS results for the annealed ~3.6 ML film indicate the diffusion of In atoms into the outermost layers of the Pd(111) single-crystal. For the same In film, XPD indicated the formation of bidimensional In islands, with some bare patches of the substrate.