Enhanced visible-light photocatalytic hydrogen evolution using two-dimensional carbon nitride sheets with the removal of amine groups

Two-dimensional (2D) carbon nitride sheets (CNs) with atomically thin structures are regarded as one of the most promising materials for solar energy conversion. However, due to their substantially enlarged bandgap caused by the strong quantum size effect and their incomplete polymerisation with a large number of non-condensed surface amino groups, the practical applicability of CNs in photocatalysis is limited. In this study, CNs with broad visible-light absorption were synthesised using a 5-min fast thermal annealing. The removal of uncondensed amine groups reduces the bandgap of CNs from 3.06 eV to 2.60 eV, increasing their absorption of visible light. Interestingly, the CNs were distorted after annealing, which can differentiate the spatial positions of electrons and holes, enhancing the visible-light absorption efficiency. As a result, when exposed to visible light, the photocatalytic hydrogen production activity of atomically thin 2D CNs rose by 8.38 times. This research presents a dependable and speedy method for creating highly effective visible-light photocatalysts with narrowed bandgaps and improved visible-light absorption.