It was noted that recent reports had identified bulk defects such as O vacancies as being key factors in visible-light photo-active TiO2. This implied that greater visible light absorption rates might be possible provided effective defect engineering could be achieved. Thus, methods were developed for simulating vacancy formation in bulk TiO2 using ab initio techniques. The initial results of these methods revealed an entropic reduction in the free energy of vacancy formation of 2.3eV over a range of 266K. The use of this result was illustrated by a 'toy' mass-action kinetics model which offered insights into vacancy concentration, rate constants, and enthalpy of reaction.

Ab initio Free Energy of Vacancy Formation and Mass-Action Kinetics in Vis-Active TiO2. J.B.Keith, H.Wang, B.Fultz, J.P.Lewis: Journal of Physics - Condensed Matter, 2008, 20[2], 022202