Indium-Doped TiO₂ Photocatalysts with High-Temperature Anatase Stability

The thermal stability of anatase titanium dioxide (TiO₂) is a prerequisite to fabricate photocatalyst-coated indoor building materials for use in antimicrobial and self-cleaning applications under normal room light illumination. Metal doping of TiO₂ is an appropriate way to control the anatase to rutile phase transition (ART) at high processing temperatures. In this work, ART of indium (In)-doped TiO₂ (In–TiO₂) was investigated in detail in the range of 500–900 °C. In–TiO₂ (In mol % = 0–16) was synthesized via a modified sol–gel approach. These nanoparticles were further characterized by means of powder X-ray diffraction (XRD), Raman, photoluminescence (PL), transient photocurrent response, and X-ray photoelectron spectroscopy (XPS) techniques. XRD results showed that the anatase phase was maintained up to 64% by 16 mol % of In doping at 800 °C of calcination temperature. XPS results revealed that the binding energies of Ti⁴⁺ (Ti 2p₁/₂ and Ti 2p₃/₂) were red-shifted by In doping. The influence of In doping on the electronic structure and oxygen vacancy formation of anatase TiO₂ was studied using density functional theory corrected for on-site Coulomb interactions (DFT+U). First-principles results showed that the charge-compensating oxygen vacancies form spontaneously at sites adjacent to the In dopant. DFT+U calculations revealed the formation of In - 5s states in the band gap of the anatase host. The formation of In₂O₃ at the anatase surface was also examined using a slab model of the anatase (101) surface modified with a nanocluster of composition In₄O₆. The formation of a reducing oxygen vacancy also has a moderate energy cost and results in charge localization at In ions of the supported nanocluster. PL and photocurrent measurements suggested that the charge carrier recombination process in TiO₂ was reduced in the presence of In dopant. The photocatalytic activity of 2% In–TiO₂ calcined at 700 °C is more comparable with that of pure anatase.