Growth and properties of thin Ag films on Pt(100) surfaces

Abstract

Adsorption of Ag on Pt(100) surfaces including the multilayer regime has been studied using the low energy electron diffraction, X-ray photoelectron spectroscopy, ultraviolet electron spectroscopy, and thermal desorption spectroscopy methods. Deposition of Ag sub-monolayers on Pt(100) below 300 K leads to the formation of three-dimensional crystallites, which decompose into a pseudomorphic layer upon annealing above ≈ 350 K. Deposition of Ag multilayers proceeds along the same growth mode. Adsorbed Ag lifts the hex reconstruction of the Pt(100) surface. Up to an Ag coverage of ≈ 0.8, Ag desorbs according to a zero-order desorption rate law, E des = 322 kJ mol −1. This desorption order is interpreted by the presence of a mixed Pt/Ag layer on top of the square Pt(100) substrate into which all excess Pt atoms from the hex surface are expelled upon lifting of the reconstruction. Therefore, Ag desorption reflects the decomposition of a monolayer-confined Ag-Pt alloy. Between Ag coverages of Θ ≈ 0.7 and Θ = 1, first-order desorption is observed, while above Θ = 1 zero-order desorption is again observed ( E des = 288 kJ mol −1) A 760 K annealed Ag monolayer suffices to suppress CO adsorption. XP spectra reveal a binding energy shift of the Ag 3d 5 2 level from 367.5 eV (monolayer) to 368.1 eV (thick multilayer). In UP spectra an Ag/Pt(100) interface state at 1.5 eV below the Fermi level is observed, which is interpreted to stem from Pt 5d electrons. Low-temperature deposited Ag sub-monolayers exhibit in UP spectra a broad 4d band structure, which collapses upon annealing into two extremely narrow Ag 4d emission peaks, Δ E ≈ 0.2 eV, at 4.6 eV and 4.8 eV. They are interpreted as emission from chain-like structures of Ag atoms in the mixed Pt/Ag layer structures which are formed in the annealing process.