The n–π* electronic transition induced by nitrogen vacancies enhances photocatalytic hydrogen production in carbon nitride

In semiconductor catalysts, long-lived excited states can effectually improve the utilization of photogenerated carriers to enhance photocatalytic performance. Herein, we used supramolecular engineering to synthesize a hollow tubular carbon nitride catalyst with N vacancies and an obvious n–π* transition. The unique hollow tubular structure provides abundant active sites, which are favorable for photocatalytic reaction. The presence of N vacancies expands the π-electron delocalization domains in the conjugated system, which excites the n–π* transition and thus triggers the red-shifted absorption edge at approximately 660 nm. Experiments and DFT calculations demonstrated that the N vacancies are beneficial for narrowing the bandgap and promoting the reduction of H⁺ by photogenerated electrons. Femtosecond transient absorption spectroscopy (fs-TAS) indicated that the n–π* electronic transition in the carbon nitride photocatalyst leads to slower exciton annihilation (lifetime: 38.64 ± 10.6 ps) and extended shallow electron trapping states (lifetime: 325.9 ± 19.3 ps). The appearance of these states adds more photogenerated electrons to the photocatalytic reaction process. The optimal hollow tubular carbon nitride catalyst exhibits a hydrogen production rate of 2664.47 μmol∙g⁻¹∙h⁻¹, which is 31.2 times higher than that of bulk carbon nitride (85.3325 μmol∙g⁻¹∙h⁻¹). This work highlights the ability of the n–π* transition induced by N vacancies to enhance the photocatalytic activity of carbon nitride.