Two-photon photoemission spectroscopy of TiO_2(110) surfaces modified by defects and O_2 or H_2O adsorbates

摘要

Two-photon photoemission (2PP) spectra of TiO_2(110) surfaces are measured for the nearly perfect surface, and surfaces modified by introduction of defects and adsorbed molecules. Defects are generated on nearly perfect surfaces by three methods: electron irradiation, annealing in vacuum, and Ar~+ sputtering. Nearly perfect or damaged surfaces can be further modified by adsorption of O_2 or H_2O molecules. 2PP spectroscopy is used to systematically investigate the work function change due to the presence of defects or adsorbates. 2PP spectroscopy detects both surface and bulk oxygen vacancy defects. We find from the results on oxygen adsorption that oxygen vacancies created by electron irradiation are localized on the surface and may be removed by O_2 adsorption at 100 K. The surface defects are substantially different from those created by annealing or by ion sputtering where vacancies in the subsurface region are proposed. We find that O_2 acts as an acceptor molecule on surface defect states whereas H_2O acts as a donor molecule. From simulation of the work function change as a function of dosage, the dipole moment of H_2O adsorbed on TiO_2 surface is derived to be 0.5 D positive outward. We also find an unoccupied electronic state 2.45 eV above the Fermi level that appears at submonolayer coverage of H_2O, which we tentatively assign to charge transfer from surface titanium ions to the surface-adsorbed H_2O molecules or OH ligands.