Mechanism of Photocatalytic Water Oxidation by Graphitic Carbon Nitride.

Carbon nitride materials are of great interest for photocatalytic water splitting. Herein, we report results from first-principles simulations of the specific electron- and proton-transfer processes that are involved in the photochemical oxidation of liquid water with heptazine-based molecular photocatalysts. The heptazine chromophore and the solvent molecules have been described strictly at the same level of electronic structure theory. We demonstrate the critical role of solvent molecules for the absorption properties of the chromophore and the overall photocatalytic cycle. A simple model is developed to describe the photochemical water oxidation mechanism. Our results reveal that heptazine possesses energy levels that are suitable for the water oxidation reaction. We suggest design principles for molecular photocatalysts which can be used as descriptors in future experimental and computational screening studies.