TiO2 by XPS

The surfaces of titanium oxide belong to the most-studied oxide systems in the surface science literature. This is in part because TiO2 surfaces and interfaces play a major role in several technological applications, e.g., as promoters in catalysis, as photocatalysts, and as gas sensors. TiO2 is a reducible oxide, i.e., several phases with different stoichiometries exist. Because Ti is highly reactive towards oxygen, titanium oxides are formed readily when Ti is exposed to an atmosphere containing water or oxygen. The oxidation behavior of the metal is of interest for the properties of protective coatings. Although accurate knowledge of the XPS binding energies of different oxidation states is necessary for XPS investigations of titanium oxides, a recent review of the 16 literature data of the binding energy of Ti 2p3/2 from Ti4+ showed wide scatter of the reported values with a mean of 458.7 eV and a standard deviation of 1.3 eV [J. Mayer, E. Garfunkel, T. E. Madey, and U. Diebold, J. Electron Spectrosc. Relat. Phenom. 73, 1 (1995)]. TiO2 is easy to handle experimentally. Although it has a bulk band gap of 3 eV, no charging problems occur during surface spectroscopies after single-crystalline samples are reduced by heating in UHV (1000 K, 45 min). This treatment causes loss of bulk oxygen and results in n-type doping. A stochiometric TiO2 surface can reproducibly be prepared through sputtering and annealing in oxygen (2 × 10−4 Pa, 900 K). Our XPS core level spectra are measured from a bulk-reduced titanium dioxide (rutile) (110) surface using a VSW hemispherical analyzer. The binding energy of Ti 2p3/2 is determined as 459.3 eV, and the binding energy of O 1s as 530.4 eV.