Directional modulation of triazine and heptazine based carbon nitride for efficient photocatalytic H2 evolution

Hollow SPP-MS-CN nanotubes have been developed through a green one-step approach by sodium polyphosphate (SPP) in-situ mediated molten salt (MS) thermal condensation of melamine for the first time. In which, the structure of SPP-MS-CN with triazine and heptazine units can been directionally modulated by the SPP content. The molten salts at high-temperature function as green solvents to facilitate mass transfer, which tends to form triazine based CN. Addition of SPP mediates the fluidity of molten salt system, which in-situ modulates the thermal condensation process, boosting the formation of the extended π-conjugated system of heptazine based CN. The 0.5 wt% SPP-MS-CN exhibit photocatalytic H 2 evolution rate up to 1785 µmol h −1 g −1 , which is 2.16 times higher than that of molten salt synthesis of carbon nitride (MS-CN), even 14.5 times enhancement of photocatalytic H 2 evolution rate over pure CN obtained by directly thermal condensation of melamine. It reveals that the hollow structures endow SPP-MS-CN with increased specific surface area, and the formed heptazine structure of SPP-MS-CN benefit for charge carrier separation and transfer and H 2 reduction ability, which favors for the remarkable photocatalytic activity. Our work opens a new window for in-situ modulation of the structure of CN photocatalysts.