Insights into the carbonate effect on water oxidation over metal oxide photocatalysts/photoanodes.

Photocatalytic/photoelectrochemical water splitting using metal oxide semiconductors is a promising technology for direct and simple solar-energy conversion. The addition of carbonate salts to an aqueous reaction solution has been known to promote stoichiometric O 2 evolution and H 2 O 2 production via H 2 O oxidation. To elucidate the effect of carbonates, density functional theory calculations are performed to study the photoinduced H 2 O and H 2 CO 3 oxidation mechanisms on TiO 2 and BiVO 4 . The oxidation reactions proceeded via peroxide intermediates, such as H 2 O 2 for H 2 O, H 2 C 2 O 6 for H 2 CO 3 , and H 2 CO 4 for the coexistence of H 2 O and H 2 CO 3 molecules. Regardless of the reactant and metal oxide, the free energy changes in the four proton-coupled electron-transfer (PCET) steps of the oxidation mechanism indicate that the first PCET requires the highest energy input and is the rate-limiting step. All PCET steps of the H 2 O oxidation, except the second one, are more endergonic than those of the H 2 CO 3 oxidation. The H 2 O reactant requires a larger energy barrier at the highest energy profile, as well as at the final state, than the H 2 CO 3 reactant. The computational results verify that the adsorbed H 2 CO 3 molecule is easily photo-oxidized compared with the adsorbed H 2 O molecule, facilitating the formation of the peroxide intermediate and improving O 2 evolution and H 2 O 2 production.