New Insights into Crystal Defects, Oxygen Vacancies, and Phase Transition of Ir-TiO2

The impact of iridium (Ir) doping on the oxygen vacancies, relative stability, crystallite size, surface area, and anatase-to-rutile transition of TiO2was comprehensively investigated in this study. Ir-doped TiO2(Ir-TiO2) was synthesized through a sol-gel technique, and the samples were annealed in the temperature range of 400-700 °C. Density functional theory calculations showed that the energy cost of an oxygen vacancy formation for Ir-TiO2was lower, as compared to that of the pristine TiO2, with the formation of Ir3+states in the band gap. Ir could provide more rutile nucleation sites and accelerate the rutile formation through the crystal strain relaxation. The entropy of mixing was reduced by the incorporation of Ir, which could induce the rutile formation with an increase in Gibbs free energy at temperatures below the normal phase transition temperature for pure TiO2. The rutile formation of Ir-TiO2could take place at a low annealing temperature (400 °C) compared to pristine TiO2(600 °C), indicating that the activation energy for phase transition could be decreased by incorporating Ir. XPS revealed the spin-orbit coupling of Ir 4f peaks, Ir 4f7/2(61.96 eV) and Ir 4f5/2(64.77 eV), due to the presence of Ir3+. Raman studies indicated the formation of charge-compensating oxygen vacancies and the presence of d states by Ir doping. It is concluded that the defects originated because the incorporation of Ir could facilitate rutile nucleation sites and thereby accelerate the phase transition through strain relaxation.