Structural-Electronic Coupling in BaAl₂O₄ Drives High-Efficiency Photothermal Water Purification

Designing highly efficient photothermal materials for the degradation of organic pollutants remains a major challenge, particularly with respect to the role of the solution temperature in catalytic performance. This study systematically investigates the interplay between photo- and thermally driven catalysis using barium aluminate (BaAl₂O₄, BAO) and zinc aluminate (ZnAl₂O₄, ZAO) catalysts for the degradation of methylene blue (MB) and crystal violet (CV) dyes over a temperature range of 30-80 °C. Remarkably, BAO achieves more than 99% dye degradation within 10 min of visible-light irradiation when the solution temperature exceeds ∼70 °C, representing one of the highest photothermal efficiencies reported to date for semiconductor oxide catalysts. Temperature-dependent X-ray diffraction (XRD), scavenger experiments, and density functional theory (DFT) analyses reveal that this exceptional performance originates from a temperature-induced structural phase transition in BAO, leading to the formation of a metallic orthorhombic phase with delocalized electronic states at the Fermi level. The emergence of this metallic phase enhances charge-carrier mobility and promotes the generation of superoxide radicals, which are identified as the dominant reactive species responsible for the rapid dye degradation. These findings elucidate how structure-driven metallicity governs photothermal catalytic behavior and provide a mechanistic framework for the rational design of next-generation semiconductor-based photothermal catalysts for sustainable wastewater treatment.