Gold/nickel ultrathin bimetallic overlayers on yttria-stabilized ZrO2 (100)

Ultrathin bimetallic Au/Ni films vapor-deposited on yttria-stabilized ZrO2 (100) (YSZ) were characterized by X-ray and UV photoelectron spectroscopy (XPS, UPS) at 300 and 570 K and by temperature-programmed desorption of CO (TPD). Photoemission results indicate that, upon co-deposition of the two metals, the Ni signal intensity is significantly lower than that from pure Ni/YSZ, whereas the corresponding Au signal intensity is practically unaffected by the presence of Ni. The binding energies of the Ni XPS peaks were not affected by co-deposited An, for Au/Ni atomic ratios close to unity, however, the XPS Au 4f levels, were shifted by 0.2 eV toward lower binding energy with respect to the corresponding values of Au/YSZ for very small Au/Ni atomic ratios in the bimetallic film. Carbon monoxide adsorption/desorption was used as a probe of the reactivity of Ni surface atoms, modified by co-deposited An. Strong attenuation of the overall TPD curves was observed in all cases, accompanied by a shift of the main peak to lower temperature for very small Au/Ni atomic ratios in the bimetallic film. The combination of the spectroscopic and CO desorption results can be understood in terms of a simple picture, in which Au atoms always segregate on the surface, thus leading to complete covering of the Ni clusters for large Au/Ni atomic ratios and to a partial intermixing of An and Ni atoms in the outermost surface layer for small Au/Ni atomic ratios.