g-C3N4 has drawn lots of attention due to its photocatalytic activity, low-cost and facile synthesis, and interesting layered structure. However, to improve some of the properties of g-C3N4, such as photochemical stability, electrical band structure, and to decrease charge recombination rate, and towards effective light-harvesting, g-C3N4–metal oxide-based heterojunctions have been introduced. In this review, we initially discussed the preparation, modification, and physical properties of the g-C3N4 and then, we discussed the combination of g-C3N4 with various metal oxides such as TiO2, ZnO, FeO, Fe2O3, Fe3O4, WO3, SnO, SnO2, etc. We summarized some of their characteristic properties of these heterojunctions, their optical features, photocatalytic performance, and electrical band edge positions. This review covers recent advances, including applications in water splitting, CO2 reduction, and photodegradation of organic pollutants, sensors, bacterial disinfection, and supercapacitors. We show that metal oxides can improve the efficiency of the bare g-C3N4 to make the composites suitable for a wide range of applications. Finally, this review provides some perspectives, limitations, and challenges in investigation of g-C3N4–metal-oxide-based heterojunctions.

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A Comprehensive Review of Graphitic Carbon Nitride (g-C3N4)–Metal Oxide-Based Nanocomposites: Potential for Photocatalysis and Sensing

• Review of recent advances in g-C 3 N 4 -based S-scheme heterojunction photocatalysts. • The basic theory and preparation strategy of g-C 3 N 4 -based S-scheme heterojunctions are discussed. • The potential applications of g-C 3 N 4 S-scheme heterojunctions are reviewed. • The possible tendency of g-C 3 N 4 -based S-scheme heterojunctions is proposed. With the rapid development of economy and modern industry, serious environmental pollution and energy shortage have become major urgent challenges to the human society. Photocatalysis is a promising technology to provide green energy. As a typical metal-free polymer photocatalyst, g-C 3 N 4 has attracted more and more attention due to its excellent performance. Unfortunately, the fast recombination of photo-induced charges, limited light response range as well as weak oxidation ability are still the key drawback that restrict the photocatalytic performance of g-C 3 N 4 . These problems can be effectively addressed by constructing g-C 3 N 4 -based heterojunctions with two or more semiconductor materials, during which the respective advantages can be integrated. Up to now, the various oxidation semiconductor photocatalysts have been tried to construct the novel S-scheme heterojunction photocatalysts with g-C 3 N 4 . Thus, this review provides a comprehensive introduction of g-C 3 N 4 -based S-scheme heterojunctions, including the main characteristics of the S-scheme heterojunction, photocatalytic mechanisms, design rules and preparation methods of g-C 3 N 4 -based S-scheme heterojunction photocatalysts. Moreover, this review summarizes recently reported works on the potential applications of g-C 3 N 4 -based S-scheme photocatalysts in various important photocatalytic reactions, including photocatalytic hydrogen production, photocatalytic degradation of contaminants, photo-reduction of CO 2 into fuels, and photocatalytic sterilization. Finally, based on the current research progress, we propose some shortages in the preparation methods and applications of g-C 3 N 4 -based S-scheme heterojunctions, which are to be further investigated and resolved in this promising and creative research field. 

Review on g-C3N4-based S-scheme Heterojunction Photocatalysts

• Hierarchial CoTiO 3 /Cd 9.51 Zn 0.49 S 10 heterostructures were fabricated by growing Cd 9.51 Zn 0.49 S 10 nanowires on CoTiO 3 microprism in situ via a hydrothermal method. • The transfer path of photogenerated charge carriers in CoTiO 3 /Cd 9.51 Zn 0.49 S 10 heterostructure follows a S-scheme mechanism. • The CoTiO 3 /Cd 9.51 Zn 0.49 S 10 hybrids manifest high activity and good stability for photocatalytic CO 2 reduction, achieving an AQE of 7.27% at 420 nm. Cd 1− x Zn x S solid solutions with strong light absorption are promising materials for solar-driven CO 2 reduction; however, their relatively weak redox ability and intrinsic photo-corrosion limit their further development as a photocatalyst. The addition of a second photocatalyst with a suitable band structure to construct a S-scheme photocatalytic system can solve both problems simultaneously. Here, we report a S-scheme photocatalyst based on the heterostructure of CoTiO 3 /Cd 9.51 Zn 0.49 S 10 (abbreviated as CoTiO 3 /CdZnS) that enables the efficient photocatalytic reduction of CO 2 . Detailed physicochemical characterization resolves the S-scheme charge transfer mechanism in this composite photocatalyst. With the well-designed structure of particles and desirable band offsets, this hybrid system offers visible light absorption in a broad spectral region, large surface area, strong redox ability, and fast carrier separation and transportation. Under visible-light illumination, the CoTiO 3 /CdZnS hybrid system displays a CO formation rate of about 11 mmol h −1 g −1 combined with a long-term operational stability. Besides, a high apparent quantum efficiency (AQE) of 7.27% is realized for the CO 2 -to-CO reduction reaction by the optimized CoTiO 3 /CdZnS hybrid under 420 nm monochromatic light irradiation. 

S-scheme CoTiO3/Cd9.51Zn0.49S10 heterostructures for visible-light driven photocatalytic CO2 reduction

The implication of photocatalytic reactions based on semiconductor heterojunction are critical for impacting both environment and energy. Scientists are passionate for developing their photocatalytic competence. The high recombination rates of photoinduced charge carriers beside their limited oxidation and reduction abilities for solely semiconductor lead to the construction of heterojunctions by several approaches. Ranging from type-I, type-II, Schottky, and Z-scheme heterojunctions, the Step-Scheme (S-Scheme) heterojunction composed of two independent semiconductors with different band energies was emerged. A brief discussion on the difference between previously studied heterojunction types compared to the newly proposed S-Scheme is presented in this review in terms of charge-transfer mechanism and key characterization tools are presented. The current design strategies and perspective applications of S-scheme heterojunction photocatalysts in fuel production, carbon cycling (namely oxidation of organic compounds and reduction of carbon dioxide), as well as other important redox reactions are selectively highlighted as emerging green approach for clean energy and environmental remediation. Moreover, another related environmental reactions such as oxidation of nitrous oxide to green products and reduction of heavy metal ions to less toxic chemicals are discussed including our current research. The current challenges and perspectives for S-Scheme photocatalysis are also summarized. 

Wensheng Yang

Papers

Fabrication of 0D/2D TiO2 Nanodots/g-C3N4 S-scheme heterojunction photocatalyst for efficient photocatalytic overall water splitting

Accumulation of localized charge on the surface of polymeric carbon nitride boosts the photocatalytic activity