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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] • A sufficient and tunable ohmic junction can be built in allochroic Pt/CN. • The ohmic contact is formed by the photoconversion of semiconducting CN to metalloid. • The reversible photoconversion of CN is accompanied with a color change. • The ohmic contact between Pt and CN significantly enhances the HER activity. • The deactivation and regeneration mechanisms of the allochroic Pt/CN are proposed. The direct hybridization of Pt and g-C 3 N 4 generally forms a Schottky barrier between the two components, which unavoidably hinders the migration of photogenerated electrons from g-C 3 N 4 to the Pt cocatalyst. Herein, we report the first efficient allochroic Pt/g-C 3 N 4 photocatalyst that can form an ohmic contact through photoconversion of semiconducting g-C 3 N 4 to metalloid, accompanied with the charge carrier storage and photocatalyst color change, which is proved experimentally and theoretically. Through intermittent exposure of Pt/g-C 3 N 4 photocatalyst to air for a few minutes during photocatalysis, the photocatalyst shows the highest hydrogen evolution performances. The ohmic contacts greatly promote the electron transfer from the semiconducting g-C 3 N 4 to the Pt cocatalyst driven by the built-in electric field. In addition, the mechanism for the photocatalyst deactivation and activation is presented. The compositional tuning of the allochroic g-C 3 N 4 through light irradiation and exposure to air can control over the photocatalytic activity and long-term stability for hydrogen evolution. This report for the first time unveils the deactivation and regeneration mechanisms of SCN. [ABSTRACT FROM AUTHOR]