Facile <i>in situ</i> reductive synthesis of both nitrogen deficient and protonated g-C₃N₄ nanosheets for the synergistic enhancement of visible-light H₂ evolution

Abstract

A new strategy is reported here to synthesize both nitrogen deficient and protonated graphitic carbon nitride (g-C₃N₄) nanosheets by the conjoint use of NH₄Cl as a dynamic gas template together with hypophosphorous acid (H₃PO₂) as a doping agent. The NH₄Cl treatment allows for the scalable production of protonated g-C₃N₄ nanosheets. With the corresponding co-addition of H₃PO₂, nitrogen vacancies, accompanied by both additional protons and interstitially-doped phosphorus, are introduced into the g-C₃N₄ framework, and the electronic bandgap of g-C₃N₄ nanosheets as well as their optical properties and hydrogen-production performance can be precisely tuned by careful adjustment of the H₃PO₂ treatment. This conjoint approach thereby results in improved visible-light absorption, enhanced charge-carrier separation and a high H₂ evolution rate of 881.7 μmol h^-1 achieved over the H₃PO₂ doped g-C₃N₄ nanosheets with a corresponding apparent quantum yield (AQY) of 40.4% (at 420 nm). We illustrate that the synergistic H₃PO₂ doping modifies the layered g-C₃N₄ materials by introducing nitrogen vacancies as well as protonating them, leading to significant photocatalytic H₂ evolution enhancements, while the g-C₃N₄ materials doped with phosphoric acid (H₃PO₄) are simply protonated further, revealing the varied doping effects of phosphorus having different (but accessible) valence states.