Critical role of empty in-gap states in the photocatalytic water splitting on carbon nitride nanosheets

Introducing nitrogen vacancies and non-metal dopants is one popular strategy to improve the photocatalytic H2O splitting efficiency of carbon nitride nanosheets. Especially, Nat. Energy 2021, 6, 388–397 reported a fascinating solar-to-hydrogen efficiency of 1.16% for the carbon nitride nanosheets containing nitrogen vacancies and boron dopants. However, effects of these defects on the reaction mechanism remain unknown. Through first-principles calculations we reveal that H2O splitting on perfect melon can only be realized in the excited state under the assistance of charge-transfer excitons, moreover this excited-state reaction is easily destroyed by aqueous solution; the key role of nitrogen vacancies and boron dopants should be that they introduce empty in-gap states which can either make the reaction realized in the ground state or avoid the negative effect of aqueous solution. However, the inter-triazine nitrogen vacancy can be poisoned and the reduction capability can be damaged by boron dopants. These two phenomena do not occur at two-coordinated nitrogen vacancies, but H2O splitting at this defect requires charge-transfer excitons. Above shortcomings may limit further improvement of melon on photocatalytic H2O splitting by doping nitrogen vacancies and boron. Introducing appropriate empty states might be the way to design good photocatalysts for splitting H2O.