Modulating Interfacial Charge Dynamics and O ₂ Activation over Defective SrTiO ₃ via Cation Engineering: Insights from DFT and Experiments

Pharmaceutical residues in wastewater, even in trace amounts, can bring health risks. Sustainable remediation technologies are urgently needed to replace energy-intensive processes. Semiconductor-based photocatalysis is a promising approach to the treatment of pharmaceutical contamination via the strengthening of surface-interface reactions. However, progress is limited by tuning of the electronic band gaps by metal doping, which can negatively influence catalytic efficiency. Herein, a green and precise cation-engineering strategy is developed to fabricate oxygen and Ti³⁺ defect-rich perovskite SrTiO₃ (STO) via a molten-salt-like hydrothermal process using low-toxicity alkali nitrates MNO₃ (M = Li, Na, and K) as mineralizing agents. Crucially, the properties of the alkali metal cations (ionic radius and electronegativity) directly govern the microstructure and defect density. K⁺ induction yielded unique flower-like hierarchical spheres with the highest surface oxygen vacancies (Vos) and Ti³⁺ species concentration. The formation of these defects was correlated with the properties of the introduced mineralizers. The surface Vos significantly enhanced the band structure, interfacial properties, and carrier efficiency (formation, separation, transport, and dynamics) of the STO photocatalysts, which demonstrated comparable visible-light degradation for pharmaceuticals. The optimized defective flower-like STO-K hierarchical spheres achieved degradation rates 2.25 and 2.12 times faster than STO-Li for tetracycline (TC) and ibuprofen (IBP), respectively, alongside and 3.8 times the enhanced turnover frequency (TOF). Using DFT simulations including vacancy formation energy, electronic structure variations, charge analysis, and residue adsorption, the factors responsible for the optimized efficiency were revealed in terms of microstructures, active radicals, surface defects, and carrier dynamics. Results disclosed that the vacancy-induced molecular O₂ activation promoted the robust radical yield, improving the carrier efficiency (4.93 ns carrier lifetime, 5.7 × higher photocurrent) and thereby boosting the performances. Most importantly, the optimized microstructure, stable surface defects, and improved molecule adsorption collectively enhanced performances. Furthermore, DFT-based electrostatic potential, electrophilic index analyses, and radical trapping experiments elucidated the interaction mechanisms at the interface, providing detailed insights into the adsorption behavior and reaction pathways. This work provides a fundamental understanding of how cation-specific defect engineering modulates charge dynamics and reaction pathways, establishing a versatile strategy for designing high-performance perovskite photocatalysts.