Inherent Size Effects on XANES of Nanometer Metal Clusters: Size Selected Platinum Clusters on Silica

X-ray absorption near-edge structure (XANES) is commonly used to probe the oxidation state of metal-containing nanomaterials; however, as the particle size in the material drops below a few nanometers, it becomes important to consider inherent size effects on the electronic structure of the materials. In this paper, we analyze a series of size-selected Pt-n/SiO2 samples, using X-ray photoelectron spectroscopy (XPS), low energy ion scattering, grazing-incidence small-angle X-ray scattering, and XANES. The oxidation state and morphology are characterized both as-deposited in UHV, and after air/O-2 exposure and annealing in H2. The clusters are found to be stable during deposition and upon air exposure, but sinter if heated above similar to 150 degrees C. XANES shows shifts in the Pt L-3 edge, relative to bulk Pt, that increase with decreasing cluster size, and the cluster samples show high white line intensity. Reference to bulk standards would suggest that the clusters are oxidized; however, XPS shows that they are not. Instead, the XANES effects are attributable to development of a band gap and localization of empty state wave functions in small clusters.