The gradual depletion of fossil fuel reserves that contribute to $85% of global energy production and release of toxic effluents urges the transformation toward renewable fuels. Thus, the sustainable utilization of sunlight for water splitting and CO 2 reduction with heterogeneous photocatalysts has come to light. As a semiconductor photocatalyst, ZnIn 2 S 4 has hit the limelight owing to its narrow bandgap and visible-light-responsive properties. However, the limitations of ZnIn 2 S 4 include limited active sites, fast charge-carrier recombination, and low photoconversion efficiency. Beginning from the fundamental photocatalytic mechanism, this review then provides in-depth insights into several modification strategies of ZnIn 2 S 4 , extending from defect engineering, facet engineering, cocatalyst loading to junction engineering, enabling the synergistic construction of high-performance ZnIn 2 S 4 -based systems. Subsequently, the structure-performance relation of ZnIn 2 S 4 -based photocatalysts for hydrogen evolution (HER), overall water splitting (OWS), and CO 2 reduction applications in the last 4 years will be discussed and concluded by the future perspectives of this frontier.

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Since the first discovery of solar-driven water splitting catalyzed by TiO 2 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 H 2 and O 2 from water using metal-free carbon nitride (g-C 3 N 4 ) 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-C 3 N 4 -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-C 3 N 4 photocatalytic systems are prospected to pave the way toward the implementation in the practical energy production process.