Effect of Hydrogen on Semiconducting Properties of TiO2 Single Crystal. Jonker Analysis

The present work considers the semiconducting properties of TiO2 single crystal in terms of the Jonker formalism (Philips Res. Rep. 1968, 23, 131), which is based on both electrical conductivity and thermoelectric power data. The Jonker analysis is applied for high-purity TiO2 single crystal at elevated temperatures (1073–1323 K), in reducing and oxidizing conditions imposed by the hydrogen–argon mixture (10–13 Pa < p(O2) < 10–5 Pa) and oxygen–argon mixture (10 Pa < p(O2) < 105 Pa), respectively. It is shown that the semiconducting properties of TiO2 equilibrated in oxidizing and reducing conditions are distinctively different in terms of the band gap and charge transport. The experimental data obtained in oxidizing conditions at 1223 K are used for the determination of the band gap according to the hopping and band model, respectively: E g = 3.38 – 1.1 × 10–3 T (eV) and E g = 3.46 – 1.4 × 10–3 T (eV). The semiconducting properties of TiO2 in reducing conditions are considered in terms of the effect of hydrogen on the formation of defect complexes {V Ti ′′′′·4OH•} in the bulk phase, which are responsible for reduction of the band gap by approximately 0.1 eV. The discrepancy between the data observed for the TiO2 single crystal (this work) and the data observed for polycrystalline TiO2 (reported before) is considered in terms of the effect of the local defect disorder, and the related semiconducting properties, of grain boundaries. The data obtained in the present work may be used for engineering of TiO2 with enhanced performance in solar energy conversion.