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Carbon nitride (CN) has attracted intensive attention as a visible light photocatalyst, but the rapid recombination of photogenerated charge carriers limits its photocatalytic activity. Herein, we develop a new strategy to construct both homojunction and ohmic junction into CN via selectively introducing metallized CN (MCN), which leads to rapid separation and transfer of photogenerated charge carriers. The polymerization of urea in the presence of KOH creates CN homojunction with amino and cyano groups. The subsequent molten salt treatment induces a new type of cyano-terminated CN that can be converted to MCN through photodoping, forming homojunction and ohmic contact coexisting CN (HOCN). The formed HOCN photocatalyst exhibits a high photocatalytic H2 evolution rate of 18.5 mmol·g−1·h−1 under visible light irradiation, 45-fold higher than that of bulk CN. This strategy provides a new idea for designing ohmic contact between semiconductor and metal, and realizing efficient photocatalysis by improving charge separation and transfer. [ABSTRACT FROM AUTHOR]

Photocatalytic hydrogen production is considered as an ideal approach to solve global energy crisis and environmental pollution. Graphitic carbon nitride (g-C 3 N 4) has received extensive consideration due to its facile synthesis, stable physicochemical properties, and easy functionalization. However, the pristine g-C 3 N 4 usually shows unsatisfactory photocatalytic activity due to the limited separation efficiency of photogenerated charge carriers. Generally, introducing semiconductors or co-catalysts to construct g–C 3 N 4 –based heterojunction photocatalysts is recognized as an effective method to solve this bottleneck. In this review, the advantages and characteristics of various types of g–C 3 N 4 –based heterojunction are analyzed. Subsequently, the recent progress of highly efficient g–C 3 N 4 –based heterojunction photocatalysts in the field of photocatalytic water splitting is emphatically introduced. Finally, a vision of future perspectives and challenges of g–C 3 N 4 –based heterojunction photocatalysts in hydrogen production are presented. Predictably, this timely review will provide valuable reference for the design of efficient heterojunctions towards photocatalytic water splitting and other photoredox reactions. [Display omitted] • g–C 3 N 4 –based photocatalysts for solar hydrogen production are overviewed. • Heterojunction significantly improved the photocatalytic activity of g-C 3 N 4. • g-C 3 N 4 heterojunction/isotype junction can be synthesized by copolymerization. • The details for the design of g–C 3 N 4 –based heterojunctions are highlighted. • The challenges and prospects of g-C 3 N 4 photocatalysts are presented. [ABSTRACT FROM AUTHOR]

[Display omitted] • The AgBr/HCCN photocatalyst was fabricated using an electric self-assembly approach. • The hybrid photocatalyst features an all-solid-state direct Z-scheme heterojunction. • The Z-scheme heterojunction improves the photogenerated charge carriers' separation. • The contact level of the heterojunction affects the performance of the photocatalyst. • The AgBr/HCCN photocatalyst exhibits a 5.4-fold enhancement in hydrogen evolution. Constructing a Z-scheme heterojunction with enhanced photocatalytic hydrogen evolution for graphitic carbon nitride-based (g-C 3 N 4) composites is challenging because integrating g-C 3 N 4 with other semiconductors, without specific band structure design, typically results in type I or type II heterojunctions. These heterojunctions have lower redox ability and limited enhancement in photocatalysis. Herein, we select highly crystalline carbon nitride (HCCN) as a proof-of-concept substrate. For the first time, we develop a AgBr nanosphere/HCCN composite photocatalyst that features an all-solid-state direct Z-scheme heterojunction for visible-light photocatalytic hydrogen evolution. The electron transfer mechanism is initially studied from the band structures and Fermi levels of HCCN and AgBr. It is subsequently confirmed by X-ray photoelectron spectroscopy (XPS), and electron microscopy. The close heterojunction contact and the built-in electron field of the Z-scheme heterojunction promote the migration and separation of photogenerated electrons and holes in the composite photocatalyst. Due to the redistribution of charge carriers, the photocatalyst shows superior redox capability and a markedly enhanced hydrogen evolution performance compared to its individual components. Combining all the advantages, AgBr nanosphere/HCCN reached an apparent quantum efficiency (AQE) of 6 % under the illumination of 410 nm, which is 4 times higher than that of the single HCCN component. [ABSTRACT FROM AUTHOR]