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8 September 2006 Defect disorder and semiconducting properties of titanium dioxide
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The semiconducting properties of TiO2 single crystal and their changes during oxidation and reduction at elevated temperatures (1073 - 1323 K) under controlled oxygen activity (10-9 - 105 Pa) were monitored using measurements of electrical conductivity and thermoelectric power. The experimental data obtained in equilibrium led to a TiO2 defect disorder model. According to this model, oxygen vacancies are the predominant defect species in TiO2 across a wide range of oxygen activities. This work has discovered the diffusion of Ti vacancies, which are formed during prolonged oxidation at elevated temperatures and in a gas phase of high oxygen activity. Observations indicate that appreciable concentrations of Ti vacancies are formed on the TiO2 surface and then are very slowly incorporated into the bulk. The obtained diffusion data has shown that in the commonly studied temperature range (1000-1400 K) the Ti vacancy concentration is quenched and can be considered as constant. Prolonged oxidation involves two kinetic regimes that are related to the transport of defects of different mobilities. The defect disorder model derived in this work may be beneficial for engineering TiO2 for enhanced water splitting through the selection of optimal processing conditions, including temperature and oxygen activity.
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M. K. Nowotny, T. Bak, and J. Nowotny "Defect disorder and semiconducting properties of titanium dioxide", Proc. SPIE 6340, Solar Hydrogen and Nanotechnology, 634016 (8 September 2006);

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