The effect of adsorbed sulfur on surface segregation in a polycrystalline Pd70Cu30 alloy

Surface segregation in alloys can be influenced by the presence of adsorbed species. In this work, the effect of adsorbed sulfur on surface segregation in a Pd70Cu30 alloy was studied for sulfur coverages from zero through saturation and for temperatures from 400 to 1000 K. X-ray photoemission spectroscopy (XPS) was used to determine the alloy composition in the near-surface region (∼7 atomic layers) and low energy ion scattering spectroscopy (LEISS) was used to probe the composition of the topmost atomic layer of the alloy. Surface segregation was observed to depend on both the presence of adsorbed sulfur and heat-treatment history. The near-surface region of the clean alloy was enriched in Pd relative to the bulk, but the topmost atomic layer was enriched in Cu. Adsorbed sulfur caused a reversal of the Cu enrichment of the topmost surface, resulting in a top layer that contained only Pd and S atoms. Segregation reversal may be driven by the formation of thermodynamically favored Pd-S bonds at the terminating surface of the alloy.