Nucleus-Coupled Electron Transfer Mechanism for TiO2-Catalyzed Water Splitting

Using first-principles calculations employing explicit interface of TiO$_2$ crystal and liquid water, we reveal the microscopic mechanism of the oxygen evolution reaction (OER). It is found that, during the formation of an O--O species, such as HO--OH and O--OH, an occupied molecular orbital with anti-bonding character evolves from the valence band and pops up all the way into the conduction band of TiO$_2$. This occupied high-energy orbital results in a high reaction barrier making the OER forbidden in the dark. The presence of photoholes depletes this anti-bonding orbital, which significantly reduces the reaction energy and determines the reaction barrier in the rate-limting step. A novel reaction mechanism, termed necleus-coupled electron transfer (NCET), emerges from this study. In this mechanism, the oxidation of a pair of hydroxyl groups, which is an electron transfer reaction, is enabled by the movement of the nuclei (i.e., the two O atoms moving towards O-O bond formation) that pushes the $reactive$ orbital (the $\sigma^{*}_{2p}$ orbital in the present case) to become the $frontier$ orbital (i.e., above the valence band maximum of TiO). Based on the NCET mechanism, we identify a reaction pathway of the OER that exhibits a kinetic barrier surmountable at room temperature.