Efficient Photocatalytic Hydrogen Evolution and CO₂ Reduction: Enhanced Light Absorption, Charge Separation, and Hydrophilicity by Tailoring Terminal and Linker Units in g-C₃N₄

Although graphitic carbon nitride (g-C3N4) has been identified as a promising photocatalyst, pristine g-C3N4 has a limited light absorption, insolubility, small specific surface area, and rapid electron–hole pair recombination. In this study, hydroxyl-grafted oxygen-linked tri-s-triazine-based polymer (HGONTP) is achieved through the polycondensation of hydrothermally pretreated dicyandiamide (DCDA). The content of C–O–C linkers and terminal OH groups in HGONTP can be regulated by the cyclization and hydrolysis degrees of DCDA through the replacement of the pendant NH2 groups with OH groups. The HGONTP photocatalyst exhibits an outstanding light absorption from UV to near-IR, possessing a narrow band gap of 2.18 eV, a hydrophilic surface, a large specific surface area of 96.1 m2 g–1, and reduced charge recombination. As a result, HGONTP exhibits a hydrogen evolution rate 27.7-fold higher than that for pristine g-C3N4 (6.54 vs 0.236 mmol g–1 h–1). The apparent quantum yield reaches 12.6% at 420 nm and 4.1% at 500 nm. In addition, the photocatalytic conversion efficiency of CO2 to CO reaches as high as 3.3 μmol g–1 h–1 without cocatalysts and sacrificial agents. The selectivity of CO2 to CO achieves 88.4%. The proposed strategy paves a new avenue to design high-performance polymeric photocatalysts used in water.