Adsorption of Pt and Bimetallic PtAu Clusters on the Partially Reduced Rutile (110) TiO2 Surface: A First-Principles Study

An extensive study of the adsorption of small Pt-n (n = 1-8) and bimetallic Pt2Aum (m = 1-5) clusters on the partially reduced rutile (110) TiO2 surface has been performed via total energy pseudopotential calculations based on density functional theory. Structures, energetics, and electronic properties of adsorbed Pt-n and Pt2Aum clusters have been determined. The surface oxygen vacancy site has been found to be the nucleation center for the growth of Pt clusters. These small Pt clusters strongly interact with the partially reduced surface and prefer to form planar structures for n = 1-6 since the cluster-substrate interaction governs the cluster growth at low Pt coverage. We found a planar-to-three-dimensional structural transition at n = 7 for the formation of Pt,, clusters on the reduced TiO2 surface. GGA+U calculations have also been performed to get a reasonable description of the reduced oxide surface. We observed significant band gap narrowing upon surface-Pt-n cluster interaction which leads to the formation of gap localized Pt states. In the case of bimetallic Pt-Au clusters, Au-m clusters have been grown on the Pt-2-TiO2 surface. The previously adsorbed Pt dimer at the vacancy site of the reduced surface acts as a clustering center for Au atoms. The presence of the Pt dimer remarkably enhances the binding energy and limits the migration of Au atoms on the titania surface. The charge state of both individual atoms and clusters has been obtained from the Bader charge analysis, and it has been found that charge transfer among the Pt-n atoms of Pt clusters and the metal oxide surface is stronger compared to that of Au clusters and the Pt-2-TiO2 system.