Semi-crystalline graphitic carbon nitride with midgap states for efficient photocatalytic nitrogen fixation

Photocatalytic nitrogen fixation is considered as a promising candidate for artificial green NH3 production. Searching an environment-friendly photocatalyst with high activity is necessary for the development of photocatalytic nitrogen fixation. Here, a metal-free semi-crystalline g-C3N4 with midgap states (CNV-550) was prepared by heating melamine in a sealed environment. The CNV-550 presented a photocatalytic nitrogen fixation rate of 91.6 mu mol L-1 h(-1) under visible light without any sacrificial reagent, which was 14.8 times as high as that of pristine g-C3N4. The midgap states from the interaction between g-C3N4 and melem extended the light response range of CNV-550 to 700 nm. The recombination of photogenerated electron-hole pairs was inhibited by the midgap states and the order-disorder interfaces of the semi-crystalline g-C3N4; accordingly, the apparent quantum efficiency reached 1.89% at 420 nm. This work not only present a metal-free high efficient photocatalyst for nitrogen fixation, but also introduces a new method to modify g-C3N4 material by controlling the reaction pressure.