Electrical conductivity and defect disorder of tantalum‐doped TiO₂

Abstract

The present work reports the electrical properties of polycrystalline Ta-doped TiO 2 (0.39 at.% Ta) determined in situ at elevated temperatures (1173-1323 K) in the gas phase of controlled oxygen activity (10 À12 Pa to 10 5 Pa). The effect of oxygen activity on the electrical conductivity and thermoelectric power of TiO 2 is discussed in terms of defect disorder, including (1) the intrinsic electronic disorder that is governed by electronic compensation in the strongly reducing regime, (2) the extrinsic electronic disorder that is governed by electronic charge compensation in the reducing regime, and (3) the extrinsic ionic disorder that is governed by ionic compensation in the oxidizing regime. It is shown that tantalum ions are incorporated into the titanium sublattice of TiO 2 leading to the formation of donor-type energy levels. The Arrhenius-type plot of the electrical conductivity data leads to the determination of the formation enthalpy terms. The obtained results are considered in terms of the effect of tantalum and oxygen activity on the defect disorder and the associated key performance-related properties in the light-induced partial water oxidation. K E Y W O R D SPoint defects, electrical properties, oxides, titanium dioxide | INTRODUCTIONTitanium dioxide is the promising material for water oxidation using solar energy. [1][2][3][4][5] This process, leading either to partial or total water oxidation, may be applied in water purification and the generation of solar fuel, respectively. Intensive research aims at processing TiO 2 with enhanced performance in these applications.The most commonly applied research strategy in processing TiO 2 with enhanced performance is based on the incorporation of a range of extrinsic ions into the TiO 2 lattice.2-5 The influence of the extrinsic ions on the performance in solar energy conversion by TiO 2 has been commonly considered to be determined by reduction of the band gap, which leads to enhanced absorption of sunlight. While the effect of the band gap on light absorption is obvious, the recent studies show that the solar-to-chemical energy conversion by oxide semiconductors and the related charge transfer is profoundly influenced by a range of the key performance-related properties, KPPs, such as the concentration of surface active sites, charge transport, Fermi level and the band gap. [5][6][7] It is shown that the KPPs are defect related. For example, titanium vacancies are the anodic active sites for water oxidation. 5-7 Therefore, these can be modified by defect engineering. 5-7The aim of the present work, which is part of a larger research program on the effect of KPPs of TiO 2 on the solar-to-chemical energy conversion, is the determination of the effect of tantalum on the semiconducting properties

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