Effect of oxygen activity on semiconducting properties of TiO2 (rutile)

Intensive research aims to reduce the band gap of TiO2-based semiconductors by the incorporation of a range of cations and anions in order to enhance their performance in solar energy conversion. In this work, we show that the band gap, as well as the related semiconducting properties, may be modified by impositions of variable oxygen activity in the oxide lattice instead of doping with extrinsic ions. It is shown that the band gap may be decreased when TiO2 is annealed in strongly reducing conditions [p(O2)∼10−12 Pa]. The reported data indicate that oxygen activity in the oxide lattice, including TiO2, is the key quantity which must be taken into account in considering compatibility and reproducibility of a range of properties, such as band gap and charge transport.