Model inverse catalysts in which metal oxide nanostructures are deposited onto metal supports have proven to be especially useful for studying the properties of metal oxide-metal interfaces that play important roles in heterogeneous catalysis. In this work, STM measurements were used to investigate the morphology and thermal stability of mass-selected titania clusters (TixOy) deposited on a Cu(111) surface as a comparison with our recent studies of NbxOy and TixOy clusters on Au(111). In that prior work, we found that the oxide clusters form assemblies on the Au(111) surface whose preferred binding sites (terraces and step edges), and assembly size and morphology depend sensitively on the metal-to-oxygen stoichiometry. Several of the clusters were observed (Ti3O5, Nb3O7, Nb3O6) to form large dendritic fractal-like structures which implies that the clusters are highly mobile on the Au(111) surface. Although we expected stronger O-Cu interactions to limit growth of dendritic structures on the Cu(111) surface, STM images show that both Ti3O5 and Ti3O6 clusters assemble into dendritic structures on Cu(111). For Ti3O5. nucleation occurs predominantly at step edges and cluster assemblies are only found on the terraces when the step edges are completely occupied. The formation of clusters assemblies is also accompanied by significant restructuring of the Cu surface, with many irregularly shaped step edges and pits which are lined with clusters and small assemblies. Moreover, the Cu surface around the clusters exhibits different STM contrast compared to bare regions which we attribute to local oxidation of the Cu surface, the extent of which increases with cluster coverage. The Ti3O5 surface structures are found to be stable after annealing the surface to 700K. Overall, the observed surface morphology of the Ti3O5,6 clusters and thermal stability on Cu(111) suggest strong cluster-cluster and cluster-Cu interactions, with the latter leading to Cu atom dislocations and surface oxidation.

Effects of stoichiometry on the morphology and thermal stability of metal oxide clusters on metal surfaces

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