PNNL

Abstract

A novel heterogeneous system consisting of Pt nanoclusters on TiO2(001)-(1x4) surface has been investigated by scanning tunneling microscopy (STM). It has been found that Pt forms three-dimensional clusters with a narrow size distribution and that, upon annealing in vacuum, the clusters migrate to step edges via a cluster diffusion mechanism. It has also been found that the cluster size distribution and the average surface cluster density remian constant during annealing, implying that no cluster coalescence has taken place. Inspection of STM images and autocorrelation ananlysis have shown that clusters diffuse on terraces primarily along the atomic rows. A one-dimensional model has been used to study cluster diffusion on terraces and recover the cluster diffusion coefficient from the experimental data. The model has been applied to two data sets and the results show that cluster diffusion on the terraces exhibits an Arrhenius temperature dependence in both cases. By considering the cluster system to be a lattice gas on a heterogeneous surface, enforcing the principles of chemical equilibrium coupled with analysis of the cluster population on both terraces and step edges yielded the difference in cluster binding energy on terraces and step edges.