Titanium dioxide (rutile) is the promising raw material for processing photocatalysts for solar water disinfection and photoelectrodes for the generation of solar hydrogen fuel. This work reports defect disorder of TiO_2 in terms of defect diagrams showing the effect of oxygen activity and temperature on the concentration of both ionic and electronic defects and the related semiconducting properties. It is also shown that imposition of the gas/solid equilibrium for the TiO_2-O_2 system should be considered in terms of the equilibration kinetics of both fast and slow defects. In the latter case the time required to reach the equilibrium at 1323 K is up to 4000 h. The effect of indium on surface vs. bulk electrical properties is considered in terms of both thermoelectric power and electrical conductivity determined at 1023 K - 1273 K in the gas phase of controlled oxygen activity [10~(-16) Pa＜p(O_2)＜10~5 Pa]. It is shown that the effect of oxygen activity on these electrical properties is consistent with a dual mechanism of indium incorporation, involving acceptor energy levels in the bulk and donor levels at the interface. The latter process leads to the formation of a quasi-isolated thin interface layer that differs from the bulk phase in the semiconducting properties and the related defect disorder as a result of segregation. This conclusion is supported by surface analysis of In-doped TiO_2 using secondary ion mass spectrometry (SIMS) showing strong indium surface segregation. It is shown that the semiconducting properties may be modified in a controlled manner by defect engineering. It is also shown that segregation may be used as the technology in processing TiO_2 with desired surface vs. bulk semiconducting properties that are required to form high-performance photoelectrodes for the production of solar hydrogen fuel using photoelectrochemical cells and photocatalysts for water purification from toxic contaminants.
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