Model inverse catalysts in which metal oxide nanostructures are deposited onto metal supports have been 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 and XPS measurements were used to investigate the morphology and thermal stability of mass-selected niobia clusters (Nb_xO_y) deposited on Au(111) surface. The clusters were prepared by in a magnetron cluster source by reactive sputtering of a Nb target and single cluster ions were selected by a down-stream quadrupole mass filter prior to deposition onto the Au(111) surface. STM images show that the Nb₃O₇ and Nb₃O₆ clusters assemble into dendritic structures with fractal dimensions of 1.70±0.05 and 1.74±0.04, respectively, which are close to that expected for diffusion limited aggregation. The fractal assemblies preferentially nucleate at the step edges, but the Nb₃O₇ assemblies are significantly larger than that for Nb₃O₆ at similar coverages. The Nb₃O₅ clusters form even smaller assemblies that show no preference for binding to the steps. Annealing the surfaces to temperatures up to 550K leads to growth of the dendritic structures for Nb₃O₇ and Nb₃O₆, while the smaller Nb₃O₅ assemblies lose their internal structure and ultimately appear as uniform nanoparticles. Niobium 3d XPS spectra versus temperature exhibit increases in the average Nb oxidation state that suggests increased inter-cluster bonding at higher temperatures. DFT calculations for single clusters and dimers on Au(111) are used to help explain the trends in assembly sizes and thermal stability versus cluster stoichiometry.

Jindong Kang

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