Photocatalytic Reduction of CO₂ in Aqueous Solution on Surface-Fluorinated Anatase TiO₂ Nanosheets with Exposed {001} Facets

Photocatalytic reduction of carbon dioxide can activate chemically inert carbon dioxide by the use of renewable energy. In the present work, the main products of photocatalytic reduction of CO₂ in aqueous TiO₂ suspensions were found to be methane, methanol, formaldehyde, carbon monoxide, and H₂. Anatase TiO₂ catalysts with various morphologies, such as nanoparticle, nanotube, and nanosheet, were synthesized through a hydrothermal method. The TiO₂ nanosheets were more active than the nanotubes or nanoparticles in the reduction of CO₂ in aqueous solution. This is because the photogenerated carriers prefer to flow to the specific facets. The TiO₂ sheet with high-energy exposed {001} facets facilitates the oxidative dissolution of H₂O with photogenerated holes, leaving more photogenerated electrons available for the reduction of CO₂ on {101} facets. Moreover, surface fluorination promotes the formation of Ti³⁺ species, which is helpful in the reduction of CO₂ to CO₂^– and in extending the lifetime of photogenerated electron–hole pairs. The optimum ratio of exposed {001} to {101} facets for surface-fluorinated TiO₂ nanosheets was found to be ∼72:28, which corresponds to an initial F/Ti ratio of 1. From our analysis of the effect of adding of known intermediates on the photocatalytic reduction of CO₂, we propose that the photocatalytic reduction of CO₂ with H₂O on surface-fluorinated TiO₂ nanosheets proceeds via a mechanism involving generation of hydrogen radicals and carbon radicals.