PHOTOEMISSION STUDIES OF BIMETALLIC ULTRATHIN FILMS: AU-NI ON YTTRIA-STABILISED ZRO_2(100)

摘要

Ultrathin bimetallic Au-Ni films vapor-deposited on yttria-sta-bilized ZrO_2(100) (YSZ) were characterized by X-ray photoelectron spec-troscopy (XPS) and temperature-programmed desorption (TPD) of CO. The 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 is practically unaffected by the presence of Ni. The binding energies of the Ni XPS peaks were not affected by co-deposited Au for Au/Ni atomic ratios close to unity, but the XPS Au 4 f levels were shifted by 0.2 eV towards lower binding energy with respect to the corresponding values of Au/YSZ for very small Au/Ni atomic ratios in the bimetallic film. CO adsorption/desorption was used as a probe of the reactivity of Ni surface atoms as modified by co-deposited Au. Strong attenuation of the overall TPD curves was observed in all cases, accompanied by a shift of the main peak to lower temperatures for very small Au/Ni atomic ratios in the bimetallic film. Taken together, these results can be understood in terms of a simple picture in which Au atoms always segregate on the surface, leading to the complete covering of the Ni clusters for large Au/Ni atomic ratios, and to a partial intermixing of Au and Ni atoms in the outermost surface layer for small Au/Ni atomic ratios.