g-C₃N₄ Photocatalysts: Utilizing Electron–Hole Pairs for Boosted Redox Capability in Water Splitting

Since the first discovery of solar-driven water splitting catalyzed by TiO₂ semiconductors, extensive research works have been devoted over the decades. Currently, the design of a photocatalyst with dual redox potential is of prominent interest to fully utilize both photogenerated electrons and holes in the redox reactions. Among all, the coproduction of H₂ and O₂ from water using metal-free carbon nitride (g-C₃N₄) has been viewed as a rising star in this field. However, the hole-mediated oxidation reaction is commonly recognized as the rate-determining step, which drastically leads to poor overall water splitting efficiency. On top of that, rapid recombination and undesirable back reaction appeared as one of the challenging parts in overall water splitting. In this review, the up-to-date advances in modified g-C₃N₄-based photocatalysts toward efficient overall water splitting are summarized, which are mainly classified into structural and defect engineering, single-atom catalysis, cocatalyst loading, and heterojunction construction. This review also addresses the underlying idea and concept to tackle the aforementioned problem with the use of emerging modification strategies, hence serving as the guiding star for future research. Despite the outstanding breakthrough thus far, critical recommendations related to g-C₃N₄ photocatalytic systems are prospected to pave the way toward the implementation in the practical energy production process.