Electronic Signatures of a Model Pollutant–Particle System: Chemisorbed Phenol on TiO₂(110)

Environmentally persistent free radicals (EPFRs) are a class of composite organic/metal oxide pollutants that have recently been discovered to form from a wide variety of substituted benzenes chemisorbed to commonly encountered oxides. Although a qualitative understanding of EPFR formation on particulate metal oxides has been achieved, a detailed understanding of the charge transfer mechanism that must accompany the creation of an unpaired radical electron is lacking. In this study, we perform photoelectron spectroscopy and electron energy loss spectroscopy on a well-defined model system–phenol chemisorbed on TiO₂(110) to directly observe changes in the electronic structure of the oxide and chemisorbed phenol as a function of adsorption temperature. We show strong evidence that, upon exposure at high temperature, empty states in the TiO₂ are filled and the phenol HOMO is depopulated, as has been proposed in a conceptual model of EPFR formation. This experimental evidence of charge transfer provides a deeper understanding of the EPFR formation mechanism to guide future experimental and computational studies as well as potential environmental remediation strategies

Growth and Structure of Cu and Au on the Nonpolar ZnO(101̅0) Surface: STM, XPS, and DFT Studies

The morphology and electronic structure of Cu and Au clusters deposited via thermal evaporation onto ZnO(101̅0) substrates have been studied via scanning tunneling microscopy (STM) and X-ray photoelectron spectroscopy (XPS). The initial stages of nucleation and growth (∼0.2 ML) of both Cu and Au are compared with density functional theory (DFT) calculations, which show an excellent agreement with the cluster morphologies observed by STM, with Cu nucleating three-dimensional (3D) islands even at small coverage while Au nucleates single-layer islands that grow layer by layer. DFT also gives insight into the diffusion behavior of Cu and Au adatoms on the ZnO substrate, showing strongly anisotropic diffusion barriers for Cu atoms which results in the experimentally observed preferential cluster nucleation along [0001] step edges, whereas Au shows no such anisotropy and Au clusters are observed to have no preferred nucleation sites. XPS results show a slight positive charging of the small Cu clusters at 0.2 ML coverage, which disappears at higher coverage. The single-layer Au islands formed at low coverage show some evidence of positive charging as well, which likewise disappears with increasing cluster size. Additionally, the Au clusters show a trend of increasing metallicity as the clusters grow and transition from single-layer islands to 3D structures, demonstrated by the increasing asymmetry in the Au 4f line shape as a function of Au coverage. In general, the observed charge transfer trends are supported by Bader charge analysis