Adsorbate-induced dissociation of metal clusters: TiO2(1 1 0)-supported Cu and Ni clusters exposed to oxygen gas

Scanning tunneling microscopy (STM) investigations of Cu clusters grown on TiO2(1 1 0)-(1 × 1) have shown that exposure to oxygen causes small three dimensional (3D) clusters to dissociate into two dimensional (2D) structures at room temperature. Larger Cu clusters do not completely dissociate upon oxygen exposure, but the clusters become smaller. Although Ni is more easily oxidized than Cu, the oxygen-induced dissociation of Ni clusters is much slower than that of Cu for similar oxygen exposures. The formation of 2D clusters is thermodynamically favorable for Cu and Ni based on the lower surface energy of the oxygen-covered metal compared to that of the pure metal. The reduction in surface energy upon oxidation is smaller for Ni than for Cu, resulting in less driving force for the Ni clusters to become 2D. As expected, exposure of Cu clusters to oxygen at 500 K increases the rate at which the 3D clusters disappear. However, oxidation of the Ni clusters at 500 K causes significant changes in the surface morphology due to the oxidation of the TiO2 surface itself. The new titania layers preferentially grow around the Ni clusters; since molecular oxygen is readily dissociated on Ni, the oxygen atoms can then spillover to the titania surface and promote the growth of titania islands in the vicinity of the Ni clusters.