Stoichiometry of “titanium suboxide” Part 2. Electric properties

Abstract

Reduction of titania (TiO₂–P25) with hydrogen leads to the formation of oxygen vacancies and Ti³⁺ ions, most likely located on the titania surface, and electrons that occupy donor sites in the bulk of titania. The number of these defects is controlled by equilibrium and is therefore quenched by the presence of water. The electrons located at the donor sites are responsible for the high conductance of hydrogen treated titania. From analysis of the activation energies for conduction it is deduced that the electrons are located at donor sites about 0.1 to 0.2 eV below the conduction band. In titania supported noble metal catalysts a similar increase of the electric conductance with increasing reduction temperature is observed. The higher density of charge carriers causes a narrowing of the space charge region at the metal support phase boundary. Thus, the charge transfer necessary to align the Fermi levels of the two phases becomes larger which most likely effects the catalytic properties of these catalysts. Either the charge transferred or the resulting high electric field working at the interface may be one additional reason for the well-known SMSI (strong-metal-support-interaction) effect.