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(O₂) 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.