Defect Chemistry of Titanium Dioxide: Evidence of the n-p Transition during Cooling

This work reports on the semiconducting properties of TiO2 (rutile) during cooling from 1273 K to room temperature in the gas phase of controlled oxygen activity. The studies are based on simultaneous in situ monitoring of electrical conductivity and thermoelectric power. The changes in semiconducting properties during cooling are preferentially related to surface properties, and they depend on oxygen activity during prior annealing and the cooling procedure. We show that annealing in strongly oxidizing conditions, at p(O2) = 100 kPa, results in the imposition of p-type properties, and cooling results in an enhancement of p-type properties. However, annealing at 1273 K in mildly oxidizing conditions, at p(O2) = 10 Pa, results in the imposition of n-type properties and subsequent cooling results in the n-p transition at 950 K. The observed surprising shift toward p-type properties during cooling is considered in terms of preferential ionization of the predominant acceptor-type ionic defects, titanium vacancies. TiO2 reduced in the argon-hydrogen mixture exhibits n-type semiconducting properties during annealing at 1273 K and subsequent cooling. The latter n-type behavior is induced by decreased oxygen activity leading to the formation of intrinsic donor-type ionic defects (oxygen vacancies and titanium interstitials) and the formation of donor-type hydrogen-related extrinsic defects.