Surface Structure and Composition of Au-Rh Bimetallic Nanoclusters on TiO2(110): A LEIS and STM Study

Gold, rhodium, and their coadsorbed layers were prepared on a nearly stoichiometric titania surface by physical vapor deposition (PVD) and were characterized by low-energy ion scattering (LEIS) and scanning tunneling microscopy (STM). It was found that because of the strong tendency of Au segregation in the Au-Rh bimetallic system, Rh atoms that impinged onto Au clusters pregrown on TiO2(110) became covered by gold atoms by place exchange or surface diffusion even at room temperature. The incorporation of rhodium led to a slight enlargement of gold clusters indicated by STM and to an increase in the number of Au atoms on the outermost layer of metal clusters evidenced by LEIS. At the same time, separate Rh clusters were also formed on the free oxide surface. The observed effect on the bimetallic nanoclusters is attributed to the different surface free energies of metals. For Au, this value is much smaller than for Rh, which provides a driving force for the bimetallic clusters to be covered by gold atoms. Annealing experiments revealed that monometallic Rh clusters are encapsulated at similar to 750 K by the oxide. Encapsulation by titania is negligible up to 900 K for the bimetallic nanoparticles almost completely covered by Au.