Linking melem with conjugated Schiff-base bonds to boost photocatalytic efficiency of carbon nitride for overall water splitting

Abstract

Developing an efficient single component photocatalyst for overall water splitting under visible-light irradiation is extremely challenging. Herein, we report a metal-free graphitic carbon nitride (g-C_xN₄)-based nanosheet photocatalyst (x = 3.2, 3.6, or 3.8) with melem rings conjugated by Schiff-base bonds (NC–CN). The presence of the conjugated Schiff-base bond tunes the band gap of g-C_xN₄ and, more importantly, serves as an electron sink to suppress electron–hole pair recombination. The projected density of states (PDOS) calculations suggest that the melem ring and Schiff-base bond act as oxidizing and reducing centers, respectively, for photocatalytic water splitting. As a result, g-C_xN₄, in particular g-C_3.6N₄, can catalyze overall water splitting without the need for any co-catalyst or sacrificial donor. Under visible light (>420 nm wavelength) irradiation, g-C_3.6N₄ catalyzes the overall water splitting with H₂ and O₂ generation rates of 75.0 and 36.3 μmol h⁻¹ g⁻¹, respectively. g-C_3.6N₄ is the most efficient single-component photocatalyst ever reported for overall water splitting. Our studies demonstrate a new approach for tuning the bandgap and the electronic structure of graphitic carbon nitride for maximizing its photocatalytic performance for water splitting, which will be important for hydrogen generation and for energy applications.