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. Funding: This research was supported by the Large Research Infrastructure ENREGAT (project No. LM2023056), Norway Grants 2014–2021 via the National Centre for Research and Development (grant No. NOR/ POLNORCCS/PhotoRed/0007/2019–00), the project „Waste as an alternative source of energy“, reg. nr. CZ.02.01.01/00/23_021/0008590 under the Programme Johannes Amos Comenius, European Union´s Horizon Europe project SAN4Fuel (Grant No. HORIZON-WIDERA-2021-ACCESS-03–01: 101079384), European Union under the REFRESH - Research Excellence For Region Sustainability and High-tech Industries (project No. CZ.10.03.01/00/22_003/0000048) via the Operational Programme Just Transition, and the grant programme “Support for Science and Research in the Moravia-Silesia Region 2022” (RRC/12/2022).