Hydrothermal Synthesis and Fe Surface Decoration of TiO₂ for Enhanced Photocatalytic CO₂ Reduction

The photocatalytic conversion of CO₂ into value-added chemicals represents a promising strategy for renewable fuel production and carbon mitigation. This work reports the hydrothermal synthesis of anatase TiO₂ nanoparticles and their surface modification with Fe through a simple impregnation method to investigate the influence of Fe loading on photocatalytic CO₂ reduction. TiO₂ nanoparticles were synthesized from metallic titanium using a peroxide-assisted route followed by hydrothermal treatment at 200 °C for 6 h. The obtained powders were decorated with 0.2, 0.8 and 1.6 wt.% Fe and calcined at 400 °C for 4 h. The photocatalytic activity was evaluated under UV irradiation using aqueous catalyst suspensions. X-ray diffraction confirmed that all samples retained the anatase crystalline phase, indicating that Fe incorporation did not alter the TiO₂ crystal structure. Diffuse reflectance spectroscopy revealed a progressive reduction in the optical band gap from 3.2 eV for pristine TiO₂ to 3.0 and 2.9 eV for the 0.8 and 1.6 wt.% Fe samples, respectively, suggesting enhanced light harvesting. Photocatalytic CO₂ reduction yielded ethanol, methanol, acetic acid/acetate, formate and CO, with liquid products accounting for virtually all of the product selectivity. The catalyst containing 0.8 wt.% Fe exhibited the highest total productivity (3079.7 μmol g^-1), while ethanol remained the predominant product for all catalysts. Formate formation was observed only for the 0.2 wt.% Fe sample. Although increasing Fe loading reduced the band gap, the photocatalytic performance was not directly proportional to this decrease, indicating the existence of an optimal Fe concentration that enhances charge separation while minimizing charge recombination. These findings demonstrate that controlled Fe surface decoration is an effective strategy for improving the activity and selectivity of TiO₂ photocatalysts for solar fuel production from CO₂.