How to determine whether an electron transfer channel is type-II or S-scheme in g–C₃N₄–based photocatalysts? A critical review

The formation of heterojunctions has been identified as the most promising strategy for circumventing the constraints of the g-C₃N₄ photocatalyst, including sluggish surface kinetics and rapid recombination loss of photoexcited electron and hole pairs. Combining g-C₃N₄ with other semiconductors to form S-scheme heterojunctions can boost charge carrier surface migration, reduce recombination loss, and maintain high redox potentials, which synergistically improve the photocatalytic performance of g–C₃N₄–based systems. However, as the development and understanding of g–C₃N₄–based S-scheme catalysts has advanced, several conflicts and confusions have emerged, which must be clarified to widen the scope applicability of g–C₃N₄–based photocatalysts. To understand whether the charge migration route in g–C₃N₄–based heterojunctions is type-II or S-scheme, this review includes a systematic discussion with the support of theoretical models and advanced experimental techniques that illustrate the requirements for building type-II and S-scheme charge migration. Furthermore, in contrast to the conventional role, the actual function of Fermi levels bending at equalization in g–C₃N₄–based S-scheme heterojunctions to provide the potential difference for maintaining the interfacial built-in electric field, the persistence of driving, and strategies to strengthen the driving force are explained in depth. The current review offers fresh insights into the criteria used to establish the prerequisites for the creation of g–C₃N₄–based type-II and S-scheme heterojunctions.