Bimetallic Pt−Au Clusters on TiO₂(110): Growth, Surface Composition, and Metal−Support Interactions

Abstract

Au, Pt, and Au−Pt clusters were grown on TiO2(110) at room temperature and studied by scanning tunneling microscopy. For the same metal coverages, the deposition of pure Pt produces smaller clusters and higher cluster densities compared to pure Au because of the greater mobility of Au on the surface. Heating the surface causes greater sintering of the Au clusters compared to Pt; this behavior is explained by the stronger metal−metal bonds for Pt and the fact that atom detachment is the rate-limiting step in cluster sintering. For the deposition of 0.024 ML of Pt followed by 0.072 ML of Au, bimetallic clusters are formed from the nucleation of Au at existing Pt clusters, whereas the reverse order of deposition results in pure Pt clusters and pure Au clusters coexisting on the surface. The presence of Pt in the bimetallic Pt−Au clusters inhibits sintering, and the average size of the clusters after annealing decreases with increasing Pt composition. Low energy ion scattering experiments demonstrate that the deposition of Au on Pt does not produce core−shell structures with Au on top. Bulk thermodynamics predicts that the cluster surfaces should be pure Au, given that the Au surface free energy is lower than that of Pt, and Au and Pt are immiscible at the compositions studied here. However, surface compositions of the Au−Pt clusters are 10−30% richer in Pt compared to the overall compositions for total coverages of 0.10 ML and 25−75% Pt. These results demonstrate that Au and Pt atoms can intermix at room temperature and the surface properties of Au−Pt nanoclusters are different from those of the bulk. Grazing angle X-ray photoelectron spectroscopy experiments show that annealed Au−Pt clusters are covered by reduced titania. Annealing the Au−Pt clusters to temperatures above 600 K induces encapsulation of the clusters, but the presence of Au at the cluster surface decreases the extent of encapsulation compared to that of pure Pt clusters.