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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]