Auger electron spectroscopy investigations of segregation in AuPd and AgPd alloy thin films

Abstract

The surface composition of vapour-deposited thin films of Ag-Pd and of Au-Pd was determined by Auger electron spectroscopy. Some supporting measurements were also made with secondary ion mass spectroscopy. Alloy films were deposited on carbon substrates in situ by simultaneous evaporation of the pure components. At temperatures up to about 323 K (50°C), the Auger compositions nearly matched the bulk compositions, which were determined by X-ray fluorescence analysis. Significant surface enrichment of Ag or Au respectively was found during annealing up to 873 K (600 °C), and this was most pronounced in the range of low to medium bulk concentrations of the segregating metal. The Auger intensity ratios were converted to surface compositions by using a monolayer model of segregation, and taking into account the electron attenuation in subsurface layers. The results were compared with data from the literature. In particular, for Ag-Pd alloys, the converted Auger compositions were found to correspond fairly well to quantitative data on the catalytic activity, which was determined in our laboratory during ongoing studies of the oxidation of C support films by Pd-containing alloy particles at elevated temperature under O. Thus, the results demonstrate that investigations of segregation are valuable for the understanding of catalytic properties of alloys.