Highly efficient hydrogen production and selective CO₂ reduction by the C₃N₅ photocatalyst using only visible light

The production of energy sources by metal-free photocatalysts based on graphitic carbon nitride (g-C₃N₄) has garnered substantial attention. In this study, nitrogen-rich carbon nitride (C₃N₅) was successfully synthesized through the thermal polycondensation of 3-amino-1,2,4-triazole. The structural and physical characterization has suggested that a portion of the triazine rings, which constitute the structural framework of g-C₃N₄, may be substituted with five-membered rings in C₃N₅. Furthermore, the polymerization of C₃N₅ proceeded more extensively than that of g-C₃N₄ from melamine precursors. The increased nitrogen content in C₃N₅ resulted in a heightened number of π-electrons and a narrowed energy bandgap, with the potential of the valence band maximum being negatively shifted. Additionally, photocatalytic assessments encompassing nitro blue tetrazolium reduction, H₂ production from triethanolamine aqueous solution, and CO₂ reduction in the liquid phase were performed. All findings demonstrated that C₃N₅ exhibits significantly superior photocatalytic properties compared to g-C₃N₄. It is particularly noteworthy that C₃N₅ selectively generates methanol and H₂ from oversaturated CO₂ solutions under visible light irradiation, while g-C₃N₄ selectively generates formaldehyde. These outcomes strongly indicate that C₃N₅ serves as a metal-free, visible-light-responsive photocatalyst, capable of contributing to both the production of renewable energy sources and the reduction of greenhouse effect gases.