An heterogeneous system which consisted of Pt nanoclusters on the (001)-(1x4) surface was investigated using scanning tunnelling microscopy. It was found that the Pt formed 3-dimensional clusters with a narrow size distribution. During annealing in a vacuum, the clusters migrated to step edges (via a cluster diffusion mechanism); mainly along terrace atomic rows. The cluster size distribution and the average surface cluster density remained constant during annealing; thus implying that no cluster coalescence took place under the annealing conditions considered. A 1-dimensional model was used to study cluster diffusion on terraces and to deduce diffusion coefficients from the experimental data. The model was applied to 2 data sets, and the results showed that cluster diffusion depended upon the cluster size and exhibited an Arrhenius temperature dependence; with the pre-exponential factor and the activation energy depending upon the cluster size. The difference in cluster binding energy to a terrace or step edge was found by treating the cluster system as being a lattice gas on an heterogeneous surface, and imposing chemical equilibrium.

Binding and Diffusion of Pt Nanoclusters on Anatase TiO2(001)-(1x4) Surface. A.El-Azab, S.Gan, Y.Liang: Surface Science, 2002, 506[1-2], 93-104