Strong D–A Interactions and π–π Interfaces Enable Efficient Hydrogen Evolution in S -Scheme D–A type Conjugated Polymer/g-C3N4 Heterojunctions

Polymer heterojunctions (PHJs) have emerged as a promising approach for solar-to-hydrogen conversion, owing to their tunable electronic structures and potential for efficient charge separation. A critical factor in achieving high photocatalytic performance lies in the rational design of photocatalysts with well-matched energy levels and effective carrier separation pathways. In this study, we designed and synthesized two S-scheme heterojunctions composed of donor–acceptor (D–A) type linear conjugated polymers integrated with g-C3N4 nanosheets for photocatalytic hydrogen evolution (PHE). The polymers were constructed using 10-methyl-10H-phenothiazine as a constant donor unit, while two different electron acceptors, dibenzothiophene-S,S-dioxide (BTDO) and dithieno[3,2-b:2′,3′-d]thiophene-S,S-dioxide (DTTDO), were employed to yield the polymers SOPTZ and DTPTZ, respectively. Among them, SOPTZ, incorporating the BTDO acceptor with higher electron affinity, provided a stronger driving force for intramolecular charge transfer (ICT) and separation. When composited with g-C3N4 nanosheets, the phenothiazine-based polymer formed a strong π–π conjugated heterointerface. The resulting S-scheme heterojunction induced band bending and an internal electric field (IEF), which synergistically enhanced charge separation while maintaining strong redox capability. As a result, the SOPTZ/g-C3N4-30 composite achieved a remarkable hydrogen evolution rate (HER) of 56.34 mmol g–1 h–1 under visible-light irradiation without any additional cocatalyst, significantly outperforming the DTPTZ/g-C3N4-10 catalyst. This work provides valuable insight into the rational construction of high-performance S-scheme PHJs for efficient PHE activity.