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

The influence of adsorbed Son 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, Delta H-seg(x) and Delta S-seg(x), at saturation sulfur coverages. While Cu segregation to the top-surface of the clean CuxPd1 (-) (x) is exothermic (Delta H-seg<0) for all bulk Cu compositions, it is endothermic (Delta H-seg>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 Pd - S bonds at the surface, which counterbalance the enthalpic driving forces for Cu segregation to the clean surface. (C) 2012 Elsevier B.V. All rights reserved.