Cu, Ag, and Au atoms adsorbed on TiO2(1 1 0): cluster and periodic calculations

The adsorption properties of isolated Cu, Ag, and Au atoms on the regular (1 1 0) surface of rutile have been studied by means of first principles density functional calculations. Cluster models and periodic slabs have been used to simulate the oxide surface. For the metal adsorption four different sites have been considered, two along the bridging oxygens protruding from the surface, and two along the basal planes containing the five-coordinated Ti atoms. Cu and Ag interact strongly with the bridging oxygens and give rise to a transfer of the outer (n+1)s electron to the Ti 3d states; Au forms weaker bonds at these sites with formation of a covalent polar bond. The different behavior of Cu and Ag from that of Au can be understood in terms of the higher ionization potential of the latter element. On the Ti sites only a weak interaction occurs, with no charge transfer. The bonding at these sites is entirely due to metal polarization and results in long metal-surface distances. The Ti rows are therefore channels where rapid diffusion of the metal atoms is expected.