Advancing the n → π* electron transition of carbon nitride nanotubes for H2 photosynthesis

Melon-based carbon nitride (g-C3N4) is a promising metal-free and sustainable material for photocatalytic water splitting. In principle, pristine carbon nitride only exhibits moderate activity due to insufficient visible light absorption and fast charge recombination. Enhancement of the solar-to-energy conversion efficiency of g-C3N4 depends on the rational design of its morphology and electronic structure. Herein, we report the self-assembly of g-C3N4 nanotubes by co-polycondensation of urea and oxamide with their similar structure and reactivity to optimize the textural and electronic properties. Unlike pristine g-C3N4, the obtained copolymers exhibit clear optical absorption above 465 nm, which is ascribed to the n -> p* electron transition involving lone pairs of the edge nitrogen atoms of the heptazine units. Besides, the charge carrier mobility was also optimized in the spatially separated nanotube structure, which contributes to the generation of more hot electrons. The optimized copolymers show dramatically enhanced H-2 evolution activities especially with green light. The achieved apparent quantum yield (AQY) of optimal CN-OA-0.05 for H-2 evolution with a green LED (lambda = 525 nm) reaches 1.3%, which is about 10 times higher than that of pure CN with state-of-the-art activity in this wavelength region.