Effect of NaBH4 loading and reduction temperature on defect-driven CO2 photoreduction over TiO2

This study investigates the role of defect engineering in enhancing TiO 2 -based photocatalysts for CO 2 photoreduction through a systematically controlled synthesis. In contrast to previous reports focused on Ti 3+ doping of commercial TiO 2 , here we combine sol–gel synthesis with post-synthetic chemical reduction using sodium borohydride (NaBH 4 ) to obtain TiO 2 materials with tunable concentrations of surface defects, specifically oxygen vacancies and Ti 3+ sites. By varying both the reduction temperature and NaBH 4 dosage, we introduce a new level of control over defect formation. The materials were characterized by X-ray diffraction (XRD), Raman spectroscopy, transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), nitrogen physisorption, and photoelectrochemical measurements. Photocatalytic performance was assessed via CO 2 photoreduction under UV–vis irradiation. The sample reduced at 350 °C with 1.5 g NaBH 4 showed the highest activity and selectivity toward CH 4 and CO, clearly surpassing the performance of commercial TiO 2 (P25) and a sol–gel reference without chemical reduction (W-TiO₂_350 °C). The improved performance is attributed to a synergistic balance of Ti 3+ sites, oxygen vacancies, and surface hydroxyls, which enhance charge separation and CO 2 activation. This work introduces new synthesis–structure–activity relationships and demonstrates the potential of defect-tuned TiO 2 materials for efficient and selective CO 2 valorization.