Linkage unit modulation of the polymers induces type-I to S-scheme transition in polymer/g-C3N4 heterojunctions for enhanced hydrogen evolution and chromium (VI) reduction.

Building polymer heterojunctions (PHJs) is a promising way to enhance the performance of single-polymer photocatalysts, but it's still challenging to design the ideal structure with well-matched energy levels and strong interface synergy by precisely tuning the molecular structure of polymer. Herein, two triazine-based conjugated porous polymers (CPPs) were synthesized in advance including TB and TR via linkage unit modulation at the molecular level, and then their PHJs with carbon nitride (g-C₃N₄) nanosheet including TB/CN and TR/CN were successfully constructed by the convenient physical ball milling method. Theoretical calculations, electron paramagnetic resonance (EPR), and in situ X-ray absorption near-edge structure (XANES) spectra show that replacing thiophene rings in TR with phenyl rings in TB changes the PHJ structure from type-I (TR/CN) to an S-scheme (TB/CN) heterojunction. Compared to TR/CN, TB/CN exhibits a stronger internal electric field (IEF), better redox ability, longer exciton lifetime, and improved charge separation and transport. As a result, TB (Wang et al., 2023a (20))/CN achieves a much higher hydrogen evolution rate (HER) of 9.11 mmol g^-1 h^-1, which is 1.8 times of TR (Wang et al., 2023a (20))/CN and 6.6 times of pure g-C₃N₄. TB (Wang et al., 2023a (20))/CN also shows superior Cr(VI) reduction efficiency (98.5 % in 60 min), outperforming TR (Wang et al., 2023a (20))/CN (82.0 %) and g-C₃N₄ (21.8 %). This study shows that adjusting the linkage units can effectively tune the interface properties of PHJs, offering a promising strategy for designing efficient polymer-based photocatalysts.