Unveiling the synergistic role of anatase/brookite homojunctions in TiO2 mixed phases on the enhanced photocatalytic degradation of paracetamol under simulated solar light and the formation of CO2.− radical ion

• Anatase/brookite junctions boost TiO 2 photocatalytic activity under 1 SUN. • SERS enables the direct identification of paracetamol degradation intermediates. • EIS shows anatase/brookite junctions acting as good charge-separating interfaces. • Anatase/brookite junctions stabilize CO 2 .− species in mild conditions. Mixed-phase TiO₂ systems offer unique opportunities for enhancing photocatalytic performance via interpolymorph junctions (homojunctions). While anatase/rutile interfaces have been extensively studied, anatase/brookite junctions remain comparatively underexplored. Here, we demonstrate that homojunctions between anatase and brookite, formed via a template-free, pH-controlled synthesis and low-temperature calcination (200 °C), significantly enhance photocatalytic activity under simulated solar light. High-resolution TEM reveals direct anatase/brookite junctions without isolated brookite crystallites. At the same time, IR spectroscopy detects the formation of CO 2 .− radical ions, suggesting that the homojunctions act as active defect sites, potentially contributing to visible light absorption or increasing photocatalytic performance. Notably, the surface generation of CO 2 .− under mild conditions could open new perspectives for CO₂ activation and solar fuel production, while also positioning this species as a valuable intermediate in organic synthesis for the formation of carboxylic acids. Compared to an anatase/brookite/rutile system obtained through calcination at 600 °C, the sample calcined at low temperature exhibits superior performance in degrading paracetamol, a model emerging contaminant in city water. Importantly, Surface-Enhanced Raman Spectroscopy (SERS) enables direct identification of paracetamol degradation intermediates, revealing a mechanistic pathway similar to that promoted by a commercial anatase/rutile TiO 2 . These findings underscore the potential of anatase/brookite homojunctions as efficient charge-separating interfaces, as further supported by electrochemical impedance spectroscopy.