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

Abstract

This study investigates the role of defect engineering in enhancing TiO2-based photocatalysts for CO2 photoreduction through a systematically controlled synthesis. In contrast to previous reports focused on Ti3+ doping of commercial TiO2, here we combine sol–gel synthesis with post-synthetic chemical reduction using sodium borohydride (NaBH4) to obtain TiO2 materials with tunable concentrations of surface defects, specifically oxygen vacancies and Ti3+ sites. By varying both the reduction temperature and NaBH4 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 CO2 photoreduction under UV–vis irradiation. The sample reduced at 350 °C with 1.5 g NaBH4 showed the highest activity and selectivity toward CH4 and CO, clearly surpassing the performance of commercial TiO2 (P25) and a sol–gel reference without chemical reduction (W-TiO₂_350 °C). The improved performance is attributed to a synergistic balance of Ti3+ sites, oxygen vacancies, and surface hydroxyls, which enhance charge separation and CO2 activation. This work introduces new synthesis–structure–activity relationships and demonstrates the potential of defect-tuned TiO2 materials for efficient and selective CO2 valorization.