Effect of prolonged oxidation on semiconducting properties of titanium dioxide

The present work reports the effects of prolonged oxidation of titanium dioxide on its semiconducting properties at elevated temperatures. The studies were performed by using simultaneous measurements of both electrical conductivity and thermoelectric power during oxidation at 1123 and 1323 K in the gas phase of controlled oxygen activity, p(O-2) = 75 kPa. The effects of oxidation on these electrical properties are considered in terms of a chance in the concentration of titanium vacancies, which are formed during oxidation. It is shown that the increase in the concentration of titanium vacancies results in a change of semiconducting properties, including the following: (1) The shift of the n-p transition point toward lower values of oxygen activity. This shift results in an increase of the oxygen activity regime in which TiO2 exhibits p-type properties. (2) Increase of the ionic electrical conductivity component. (3) Increase of the mobility terms for electrons and decrease of the mobility of electron holes. It is shown that the band gap of TiO2 is 3.1 eV and remains independent of the concentration of titanium vacancies. The data reported in this work may be used for processing TiO2 with controlled semiconducting properties. These data also pave the way for the processing of undoped TiO2 that exhibits p-type properties without the need to incorporate acceptor-type foreign ions.