Revealing the Distinctive Roles of Nitrogen Vacancies in Generating H₂ and H₂O₂ in Carbon Nitride Photocatalysts

Abstract

Graphitic carbon nitride (GCN) is extensively utilized for photocatalytic H₂ evolution reaction (HER) and H₂O₂ production at scale. However, to overcome the performance bottlenecks, it is critical to establish the role of N-vacancies on the optical properties and the interfacial electron transfer process that are indispensable to generate both H₂ and H₂O₂. In this direction, rationally inducing N-vacancies significantly extends the optical absorption of GCN from 460 to 700 nm. Nevertheless, the HER performance was reduced by a factor of two, attributed to electron trapping by the N-vacancies in GCN. In contrast, H₂O₂ production efficiency increases with N-vacancies. This intriguing observation is validated based on the lower thermodynamic driving energy required for a one-step O₂ reduction reaction to directly form H₂O₂, compared to the reduction potential of H⁺ ions to form H₂. Control experiments with charge scavengers and photophysical and electronic structure analysis via theoretical calculations reveal that the N-vacancies promote O₂ adsorption that facilitates efficient electron transfer to form H₂O₂. Overall, a comprehensive correlation between the effect of N-vacancies on the electron transfer/trapping and their distinctive influence on photocatalytic HER and H₂O₂ production in GCN is elucidated.