Two-photon photoemission spectroscopy ofTiO2(110)surfaces modified by defects andO2orH2Oadsorbates

Abstract

Two-photon photoemission (2PP) spectra of $\mathrm{Ti}{\mathrm{O}}_{2}(110)$ 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 ${\mathrm{Ar}}^{+}$ sputtering. Nearly perfect or damaged surfaces can be further modified by adsorption of ${\mathrm{O}}_{2}$ or ${\mathrm{H}}_{2}\mathrm{O}$ 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 ${\mathrm{O}}_{2}$ adsorption at $100\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. 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 ${\mathrm{O}}_{2}$ acts as an acceptor molecule on surface defect states whereas ${\mathrm{H}}_{2}\mathrm{O}$ acts as a donor molecule. From simulation of the work function change as a function of dosage, the dipole moment of ${\mathrm{H}}_{2}\mathrm{O}$ adsorbed on $\mathrm{Ti}{\mathrm{O}}_{2}$ surface is derived to be $0.5\phantom{\rule{0.3em}{0ex}}\mathrm{D}$ positive outward. We also find an unoccupied electronic state $2.45\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ above the Fermi level that appears at submonolayer coverage of ${\mathrm{H}}_{2}\mathrm{O}$, which we tentatively assign to charge transfer from surface titanium ions to the surface-adsorbed ${\mathrm{H}}_{2}\mathrm{O}$ molecules or OH ligands.