Awakening n → π* electronic transition by breaking hydrogen bonds in graphitic carbon nitride for increased photocatalytic hydrogen generation

Achieving the excitation of lone pairs (n ->pi*) in graphitic carbon nitride (g-C3N4) can not only extend visible light absorption range, but also increase the amount of catalytically active n electrons, thereby leading to enhanced photocatalytic hydrogen (H-2) generation. However, awakening n -> n* electronic transition in g-C3N4 is highly challenging. Here we report on activating n -> n* electronic transition by breaking hydrogen bonds in g-C3N4 through a post thermal annealing process. The g-C3N4 thus obtained shows greatly increased photocatalytic H-2 generation under visible light irradiation. Under optimal conditions, the highest H-2 generation rate reached 33.3 mu mol.h(-1), which is 25 times higher than that of pristine g-C3N4 without n-n* electronic transition (1.3 mu mol.h(-1)). Moreover, the longest wavelength that can photoexcite g-C3N4 to generate H-2 is 510 nm, nearly a 50 nm red shift to the absorption edge of pristine g-C3N4 (-460 nm). This work demonstrates a great potential of utilizing n-n* electronic transition to increase the photocatalytic performance of g-C3N4 via breaking hydrogen bonds.