Surface electronic structure of corundum transition-metal oxides:Ti2O3

The surface electronic structure of ${\mathrm{Ti}}_2$ ${\mathrm{O}}_3$ has been studied on single-crystal samples cleaved in ultrahigh vacuum. Ultraviolet photoelectron spectroscopy, electron-energy-loss spectroscopy, etc., have been used to study nearly perfect surfaces, surfaces containing defects, and ${\mathrm{O}}_2$ chemisorption on both types of surfaces. Cleavage surfaces have a band structure essentially the same as the bulk, with a (1.4±0.1)-eV-wide Ti $3d$ $a_{1g}$ band separated from the (5.5—6)-eV-wide O $2p$ valence band by a (2.5±0.1)-eV band gap. No measurable band bending was seen on nearly perfect surfaces. Surface defects are associated with surface O vacancies and an accompanying transfer of charge to surface Ti $3d$ levels. The chemisorption of ${\mathrm{O}}_2$ on nearly perfect ${\mathrm{Ti}}_2$ ${\mathrm{O}}_3$ surfaces removes charge from the surface Ti ions, creating a negative adsorbed species (probably ${\mathrm{O}}^{2-}$). The resulting surface dipole layer causes the surface O bands to shift in energy much more than either the filled or empty Ti $3d$ bands. Similar effects are seen for ${\mathrm{O}}_2$ chemisorption on defect surfaces.