Engineering a phase transition induced g-C3N5/poly (triazine imide) heterojunction for boosted photocatalytic H2 evolution

• A novel g-C 3 N 5 /PTI heterojunction was synthesized by one-step molten salt method. • The g-C 3 N 5 /PTI heterojunction exhibited ascendant photocatalytic HER activity. • S-scheme heterojunction strategies enhance the separation and migration of light induced carriers. • The hierarchical structure of g-C 3 N 5 /PTI heterojunction provide abundant active sites for photocatalytic H 2 evolution. • The phase transfer process regulated by reaction temperature was discussed in detail. g-C 3 N 5 , as a novel carbon nitride-based photocatalyst, has attracted widespread attention in H 2 production. Constructing g-C 3 N 5 -based heterojunctions has proved to be an effective method to boost the photocatalytic H 2 evolution activity. Nevertheless, the current engineering methods for constructing g-C 3 N 5 -based heterojunctions generally suffer from the cumbersome multi-step pathways. Herein, g-C 3 N 5 /poly (triazine imide) (PTI) heterojunction was successfully constructed by one-step molten-salt approach. This unique molten-salt method can simultaneously induce the formation of g-C 3 N 5 and promote the phase transition from g-C 3 N 5 to PTI. The relative contents of PTI in the g-C 3 N 5 /PTI heterojunction can be regulated readily by changing the calcination temperature. Our results demonstrated that the optimal g-C 3 N 5 /PTI heterojunction exhibits highest photocatalytic H 2 production rate of 2326.8 μmol g -1 h -1 , which is 2.7 and 6.7 times those of pure g-C 3 N 5 and PTI, respectively. The improved photocatalytic activity of g-C 3 N 5 /PTI is mainly attributed to the charge transport pathway provided by S-scheme heterostructure and the construction of the hierarchical structure, which ensure charge separate and migrate sufficiently. This work provided a tractable strategy for the construction of novel metal-free g-C 3 N 5 -based heterojunctions with enhanced photocatalytic performance and may inspire the design and synthesis of other semiconductor heterojunction photocatalysts.