Distortion of charge carrier trapping centers during incipient phase transformations in TiO2 can enhance its photocatalytic performance

Most photocatalytic processes involve physicochemical phenomena occurring at the semiconductor-water interface. The interfacial charge transfer strongly depends on the charge carrier self-trapping or defect-based trapping mechanisms active in the crystal lattice of the photocatalyst. Thus, the crystal lattice distortion is expected to influence the photocatalytic efficiency during polymorphic phase transformations (PPT). A simple synthesis method involving the ultrasound-assisted excess hydrolysis of titanium tetra-isopropoxide (TTIP) (hydrolysis ratio (number of moles of water/number of moles of TTIP) r = 245) was used to obtain multiphase titanium dioxide (TiO₂) nanomaterials with complex defect structures. Electron paramagnetic resonance (EPR) spectroscopy was employed to characterize the paramagnetic centers in the synthesized TiO₂ and their behavior during incipient PPT. The calcined samples showed a complex defect structure comprising three types of paramagnetic centers: F⁺-centers (an electron trapped in an oxygen vacancy (O_v)), V-centers (oxygen ions with trapped holes) and paramagnetic centers involving Ti³⁺ such as Ti³⁺−O_v. The sample obtained at 600 ^∘C, temperature marking the onset of a massive mixed transformation of anatase into rutile and brookite, composed of approximately 81 % anatase, 10 % brookite, and 9 % rutile, exhibited an intense and broadened EPR signal and enhanced photocatalytic activity for hydroxyl radical generation and hydrogen production by water splitting, despite its rather low specific surface area of 34 m²/g. The results revealed the synergistic effects of charge carrier trapping mechanisms in the early stages of PPT, boosting the photocatalytic performance of TiO₂. The present study supports the design of facile synthesis methods for better TiO₂ photocatalysts and promotes the development of further studies regarding lattice defect engineering during phase transformations in nanomaterials.