Electronic Interactions of Size-Selected Oxide Clusters on Metallic and Thin Film Oxide Supports

The interfacial electronic structure of various size-selected metal oxide nanoclusters (M₃O_x; M = Mo, Nb, Ti) on Cu(111) and a thin film of Cu₂O supports were investigated by a combination of experimental methods and density functional theory (DFT). These systems explore electron transfer at the metal-metal oxide interface which can modify surface structure, metal oxidation states, and catalytic activity. Electron transfer was probed by measurements of surface dipoles derived from coverage dependent work function measurements using two-photon photoemission (2PPE) and metal core level binding energy spectra from X-ray photoelectron spectroscopy (XPS). The measured surface dipoles are negative for all clusters on Cu(111) and Cu₂O/Cu(111), but those on the Cu₂O surface are much larger in magnitude. In addition, sub-stoichiometric or "reduced" clusters exhibit smaller surface dipoles on both the Cu(111) and Cu₂O surfaces. Negative surface dipoles for clusters on Cu(111) suggest Cu → cluster electron transfer, which is generally supported by DFT-calculated Bader charge distributions. For Cu₂O/Cu(111), calculations of the surface electrostatic potentials show that the charge distributions associated with cluster adsorption structures or distortions at the cluster-Cu₂O-Cu(111) interface are largely responsible for the observed negative surface dipoles. Changes observed in the XPS spectra for the Mo 3d, Nb 3d, and Ti 2p core levels of the clusters on Cu(111) and Cu₂O/Cu(111) are interpreted with help from the calculated Bader charges and cluster adsorption structures, the latter providing information about the presence of inequivalent cation sites. The results presented in this work illustrate how the combined use of different experimental probes along with theoretical calculations can result in a more realistic picture of cluster-support interactions and bonding.