Point defects in metal oxides such as TiO2 were key to their applications in numerous technologies. The investigation of thermally induced non-stoichiometry in TiO2 was complicated by the difficulties in preparing and determining a desired degree of non-stoichiometry. Controlled self-doping of TiO2 was studied by adsorption of 1/8 and 1/16 monolayer Ti at the (110) surface using a combination of experimental and computational approaches to unravel the details of the adsorption process and the oxidation state of Ti. Upon adsorption of Ti, X-ray and ultra-violet photo-emission spectroscopy show formation of reduced Ti. Comparison of pure density functional theory with experiment shows that pure density functional theory provided an inconsistent description of the electronic structure. To surmount this difficulty, density functional theory corrected for on-site Coulomb interaction (DFT+U) was used to describe reduced Ti ions. The optimal value of U was 3eV, determined from comparison of the computed Ti 3d electronic density of states with the photo-emission spectroscopy data. DFT+U and photo-emission spectroscopy show the appearance of a Ti 3d adsorbate-induced state at 1.3eV above the valence band and 1.0eV below the conduction band. The computations show that the adsorbed Ti atom was oxidized to Ti2+ and a fivefold coordinated surface Ti atom was reduced to Ti3+, while the remaining electron was distributed among other surface Ti atoms. The photo-emission spectroscopy data were best fitted with reduced Ti2+ and Ti3+ ions. These results demonstrated that the complexity of doped metal oxides was best understood with a combination of experiment and appropriate computations.

Electronic Structure of Point Defects in Controlled Self-Doping of the TiO2 (110) Surface - Combined Photoemission Spectroscopy and Density Functional Theory Study. Nolan, M., Elliott, S.D., Mulley, J.S., Bennett, R.A., Basham, M., Mulheran, P.: Physical Review B, 2008, 77[23], 235424