O2 induced Cu surface segregation in Au-Cu alloys studied by angle resolved XPS and DFT modelling

Surface segregation effects on polycrystalline Au-Cu alloys (Au0.80Cu0.20, Au0.85Cu0.15 and Au0.90Cu0.10) were studied at room temperature by angle resolved XPS (ARXPS) and density functional theory (DFT) before and after exposure to O-2. Au surface enrichment was found as predicted from calculations showing that this process is energetically favourable, with a segregation energy for Au in a Cu matrix of -0.37 eV atom(-1). Surface enrichment with Cu was observed after exposure to O-2 due to its dissociative adsorption, in agreement with DFT calculations that predicted an energy gain of -1.80 eV atom(-1) for the transfer of Cu atoms to a surface containing adsorbed oxygen atoms, thus leading to an inversion in surface population.