Interfacial Charge-Transfer Excitons Help the Photoreduction of CO2 on TiO2

Photoreduction of CO2 into organic molecules under the catalysis of anatase TiO2 has aroused great attention. Its reaction mechanism is still far from understood. It is generally thought that the redox reactions take place after exciton dissociation, and hence previous investigations only consider the role of isolated photoelectrons in the reduction reactions. Our first-principles calculations identify that interfacial charge-transfer (CT) excitons might play a crucial role in the reduction reactions. CT excitons can not only simplify the previously proposed two-electron processes in the reduction of CO2 -> HCOOH and HCOOH -> HCO to one-electron processes but also cut down substantially the reaction barriers that have been predicted to be insurmountable. Stability of the key intermediate radical CO2 center dot- can also be greatly enhanced with the help of the CT exciton. We also discover that the remaining holes after the reduction of CO2 to HCOO can promote the dissociation and the oxidation of H2O at the bridging oxygen atom with an extremely low barrier. This can not only provide a timely supply of protons for the reduction reactions but also lead to the formation of the bridging oxygen vacancy. The oxygen vacancy could be the active site for either further H2O dissociation or the reduction of CO2 to CO. Our work reveals the synergism between the reduction and the oxidation half-reactions and demonstrates that the role of excitons before their dissociation might require attention in designing photocatalysts for CO2 reduction. Depositing hole acceptors with specific features onto the anatase surface might be beneficial for the utilization of excitons and therefore improving the photocatalytic efficiency of CO2 reduction.