Charge transport in polycrystalline titanium dioxide

This work reports semiconducting properties of undoped polycrystalline TiO2 studied using the measurements of the electrical conductivity (EC) and thermopower as a function of oxygen partial pressure and temperature in the ranges of p(O-2) between 10 Pa and 70 kPa and temperature 1173-1273 K. The width of the band gap, determined from the minimum of EC, is equal to 3.055 +/- 0.012 eV. It was found that the apparent concentration of negatively charged defects, involving both acceptor-type aliovalent ions and Ti vacancies, increases with temperature from 0.6 at% at 1173 K to the level of 0.9-1.4 at% at 1273 K. This effect is considered in terms of Schottky-type defects. It was observed that the minimum of EC at the n-p transition is lower than that for TiO2 single crystal thus suggesting that grain boundaries are responsible for the formation of conductivity weak links. (C) 2003 Elsevier Science Ltd. All rights reserved.