Nucleation, Growth, and Adsorbate-Induced Changes in Composition for Co-Au Bimetallic Clusters on TiO2

The nucleation, growth, and CO-induced changes in composition for Co-Au bimetallic clusters deposited on TiO2(110) have been studied by scanning tunneling microscopy (STM), low, energy ion scattering (LEIS), X-ray photoelectron spectroscopy (XPS), temperature-programmed desorption (TPD), and density functional theory (DFT) calculations. STM experiments show that the mobility of Co atoms on TiO2(110) is significantly lower than of Au atoms; for equivalent or lower coverages of Co, the number of dusters is higher and the average cluster height is smaller than for Au deposition. Consequently, bimetallic clusters are formed by first depositing the less mobile Co atoms, followed by the addition of the more mobile Au atoms. Furthermore, the reverse deposition of Au followed by Co results in clusters of pure Co coexisting with clusters that are Au-rich. For clusters with a total coverage of 0.25 ML, the cluster density increases and average cluster height decreases as the fraction of Co is increased. Annealing to 800 K results in cluster sintering and an increase of similar to 3-5 angstrom in average height for all compositions. LEIS experiments indicate that the surfaces of the bimetallic clusters are 80-100% Au for bulk Au fractions greater than 50% but Co and Au coexist at the surfaces when there are not enough Au atoms available to completely cover the surfaces of the clusters. After heating to 800 K, pure Co clusters become partially encapsulated by titania, and for bimetallic clusters, the Co is selectively encapsulated at the cluster surface. The desorption of CO from the bimetallic clusters demonstrates that the presence of the CO adsorbate induces diffusion of Co to the cluster surface, but the extent of this diffusion is less than what is observed in the Ni-Au and Pt-Au systems. Density functional theory calculations confirm that for a 50% Co/50% Au bimetallic structure: the surface is predominantly Au in the absence of CO; CO induces diffusion of Co to the cluster surface; and this CO induced diffusion is less extensive on Co An than on the Ni-Au and Pt-Au surfaces.