Electronic Structure Of Point Defects In Controlled Self-doping Of The Tio_2 (110) Surface: Combined Photoemission Spectroscopy And Density Functional Theory Study

摘要

Point defects in metal oxides such as TiO_2 are key to their applications in numerous technologies. The investigation of thermally induced nonstoichiometry in TiO_2 is complicated by the difficulties in preparing and determining a desired degree of nonstoichiometry. We study controlled self-doping of TiO_2 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 ultraviolet photoemission spectroscopy (XPS and UPS) show formation of reduced Ti. Comparison of pure density functional theory (DFT) with experiment shows that pure DFT provides an inconsistent description of the electronic structure. To surmount this difficulty, we apply DFT corrected for on-site Coulomb interaction (DFT+ U) to describe reduced Ti ions. The optimal value of U is 3 eV, determined from comparison of the computed Ti 3d electronic density of states with the UPS data. DFT+ U and UPS show the appearance of a Ti 3d adsorbate-induced state at 1.3 eV above the valence band and 1.0 eV below the conduction band. The computations show that the adsorbed Ti atom is oxidized to Ti~(2+) and a fivefold coordinated surface Ti atom is reduced to Ti~(3+), while the remaining electron is distributed among other surface Ti atoms. The UPS data are best fitted with reduced Ti~(2+) and Ti~(3+) ions. These results demonstrate that the complexity of doped metal oxides is best understood with a combination of experiment and appropriate computations.