Segregation at the surfaces of CuxPd1 − x alloys in the presence of adsorbed S

The influence of adsorbed S on surface segregation in CuxPd1 − x alloys (S/CuxPd1 − x) was characterized over a wide range of bulk alloy compositions (x = 0.05 to 0.95) using high-throughput Composition Spread Alloy Film (CSAF) sample libraries. Top-surface and near-surface compositions of the CSAFs were measured as functions of bulk Cu composition, x, and temperature using spatially resolved low energy ion scattering spectroscopy (LEISS) and X-ray photoemission spectroscopy (XPS). Preferential segregation of Cu to the top-surface of the S/CuxPd1 − x CSAF was observed at all bulk compositions, x, but the extent of Cu segregation to the S/CuxPd1 − x surface was lower than the Cu segregation to the surface of a clean CuxPd1 − x CSAF, clear evidence of an S-induced “segregation reversal.” The Langmuir–McLean formulation of the Gibbs isotherm was used to estimate the enthalpy and entropy of Cu segregation to the top-surface, ΔHseg(x) and ΔSseg(x), at saturation sulfur coverages. While Cu segregation to the top-surface of the clean CuxPd1 − x is exothermic (ΔHseg < 0) for all bulk Cu compositions, it is endothermic (ΔHseg > 0) for S/CuxPd1 − x. Segregation to the S/CuxPd1 − x surface is driven by entropy. Changes in segregation patterns that occur upon adsorption of S onto CuxPd1 − x appear to be related to formation of energetically favored PdS bonds at the surface, which counterbalance the enthalpic driving forces for Cu segregation to the clean surface.