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[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]

• The S + K-codoped g-C 3 N 4 presents excellent H 2 production (8.78 mmol g−1 h−1) under visible light. • The S + K-codoped g-C 3 N 4 inhibits the recombination of photogenerated carriers. • The S + K-codoped g-C 3 N 4 photocatalyst shows apparent quantum efficiency of 70 % at 420 nm for H 2 production. Modifying the structure of a photocatalyst to tailor its electronic and physicochemical properties is an effective approach for efficient photocatalysis. Herein, we demonstrate that co-doping non-metal (S) and metal (K) atoms into graphitic carbon nitride (g-C 3 N 4) provides excellent visible-light photocatalytic hydrogen production activity of 8.78 mmol g−1 h−1, which is 98 times higher than that of pristine g-C 3 N 4 (0.09 mmol g−1 h−1). The apparent quantum efficiency of the S+K-co-doped g-C 3 N 4 reaches 70 % at 420 nm. This outstanding photocatalytic performance attributed to an increased specific surface area from 6.78 to 74.23 cm3 g−1, which reduced the recombination of photogenerated charge carriers and enhanced conductivity. Various characterizations are undertaken to elucidate the S+K-co-doped g-C 3 N 4 photocatalytic mechanism. Our work not only demonstrates a facile, eco-friendly and scalable strategy for the synthesis of S+K-co-doped g-C 3 N 4 photocatalysts, but also opens a new avenue for the design of co-doped photocatalysts. [ABSTRACT FROM AUTHOR]