Balancing charge carrier density and exciton recombination in defective g-C3N4 for efficient photocatalytic hydrogen evolution

Abstract

Graphitic carbon nitride (g-C₃N₄) is a promising material for photocatalytic hydrogen production owing to its tunable band structure and high charge carrier density, however, it displays a high carrier recombination rate. Defect engineering can solve this problem by providing active sites within g-C₃N₄ to accelerate the separation of photogenerated electrons and holes and enhance its solar light absorption. Previous studies have implied that there exists a defect concentration threshold for obtaining the maximum photocatalytic hydrogen production performance, but the underlying mechanism remains unclear. In this study, the relationship between charge carrier density and exciton recombination was investigated to address this issue. A series of g-C₃N₄ photocatalysts with different tri-coordinate nitrogen (N_3C) vacancy concentrations were synthesized using in-situ coprecipitation polymerization. The catalytic performance is related to the vacancy concentration, higher vacancy concentration will lead to higher carrier separation efficiency and carrier density of g-C₃N₄, as well as better photocatalytic hydrogen production performance. However, when excessive vacancies act as recombination centers, the charge carrier recombination rate is increased, which will also adversely affect the photocatalytic hydrogen production performance.