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 (TiO 2 ) nanomaterials with complex defect structures. Electron paramagnetic resonance (EPR) spectroscopy was employed to characterize the paramagnetic centers in the synthesized TiO 2 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 Ti 3+ such as Ti 3+ − 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 m 2 /g. The results revealed the synergistic effects of charge carrier trapping mechanisms in the early stages of PPT, boosting the photocatalytic performance of TiO 2 . The present study supports the design of facile synthesis methods for better TiO 2 photocatalysts and promotes the development of further studies regarding lattice defect engineering during phase transformations in nanomaterials. • Excess hydrolysis of TTIP led to nanosized TiO 2 with paramagnetic F + and V centers • Synergic charge carrier trapping occurred during incipient phase transitions (PT) • TiO 2 obtained at the onset of massive PT showed enhanced photocatalytic activity • Hydrolysis of TTIP in ultrasonicated water led to nanoparticles with inner pores