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 4f_7/2(61.96 eV) and Ir 4f_5/2(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.