Electronic excitations at oxygen deficient TiO 2(110) surfaces: A study by EELS

Abstract

TiO 2(110) surfaces with controlled oxygen deficiency introduced by 160 eV electron bombardment have been studied by XPS and EELS. Stoichiometry was monitored by the growth of core peaks due to Ti 3+ states in XPS. Oxygen desorption is characterised by an initial cross section of 3 × 10 −21 cm 2 that decreases with increasing oxygen loss, tending toward a limiting composition Ti 4O 7. The oxygen deficient surfaces display sub-bandgap excitations in electronic EELS, whilst in the vibrational region there is a selective downward shift and attenuation of the highest energy phonon loss. This is attributed to modification of the effective background dielectric constant by the defect excitations. Quantitative consideration of the changes in HREELS leads to an estimate of 0.1 for the oscillator strength of the defect electronic excitations. The high value supports the idea that electrons at oxygen deficient TiO 2 surfaces occupy states that are pulled down below the conduction band by polaronic self trapping.