Temperature-dependent oxidation of pt nanoclusters on a thin film of al₂O₃ on niAl(100)

The temperature-dependent oxidation of Pt nanoclusters on a thin film of Al2O3on NiAl(100), in the absence of a gaseous oxidizing agent, was investigated with various surface probe techniques. The Pt clusters (of mean diameter 2.2 nm and height 0.4 nm) grown from vapor deposition on the thin film of Al2O3on NiAl(100) at 300 K became partially oxidized, as charge transfer from the Pt clusters to the oxide was indicated by a significant negative shift (0.4-0.5 eV) of binding energy (BE) of Alox2p and O 1s states from Al2O3. The oxidation of the cluster proceeded to a further level when the sample was annealed above 450 K; the Pt 4f7/2core level moved positively from BE 72.0 eV with increasing annealing temperature and eventually attained 72.6 eV above 650 K, which indicates a state of Pt2+. Accompanying this further oxidation, signals of both Alox2p and O 1s shifted back to greater BE. The valence spectra indicate that the Pt-Al2O3interaction was sustained whereas a new Pt-O bond was formed. Formation of a PtxAlyOzcomplex is proposed to explain the observations. The new Pt oxide binding was substantially stronger than the initial one, as was evident from the oxidized clusters being resistant to sintering induced in electrochemical processes. The oxidation was associated also with a migration of oxide materials onto the Pt clusters, as both Al2O3(100) and NiAl(001) facets roughened after the annealing, and a probe of methanol adsorption showed no bare Pt clusters exposed but alumina-like structures on the surface.