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

Photo catalytic 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 H-2. 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 similar to 72:28, which corresponds to an initial F/Ti ratio of I. 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.