Surface structures of Au-Pt bimetallic nanoclusters on thin film Al2O3/NiAl(100) probed with CO

With infrared reflection absorption spectra and temperature-programmed desorption of CO as a probe molecule, we investigated the surface structure of Au-Pt bimetallic nanoclusters on thin-film Al2O3/NiAl(100) under ultrahigh vacuum conditions. Vapor Au and Pt (0.0-2.0 ML) were sequentially deposited onto Al2O3/NiAl(100) at 300 K to form bimetallic clusters; under the kinetic constraint, the order of the metal deposition made a variation in the surface structure of the grown clusters. For the deposition of Au onto Pt clusters (first Pt and then Au), the deposited Au decorated preferentially and then aggregated the edge sites of Pt clusters, instead of decorating the rest of Pt surface. For the metal deposition in the reverse order, the deposited Pt covered uniformly the surface of existing Au clusters and no preferential adsorption site was indicated. The infrared absorption for CO on Au sites was significantly enhanced when alloying occurred; this enhancement is primarily a local field effect depending on the coordination of CO-binding Au, and hence provides additional structural characterization of the grown clusters.