Construction of K-Doped g-C3N4-Based Heterojunctions for Efficient Light-Driven Hydrogen Evolution reaction

• Potassium-doped porous g-C 3 N 4 was constructed and coupled with ZnIn 2 S 4 and NiCo 2 O 4 to form ZIS/KCN and NCO/KCN heterojunctions. • ZIS/KCN delivered a photocurrent density about 2.7 times higher than that of pristine KCN. • A hydrogen evolution yield of 735.42 μmol·g -1 within 5 h was achieved by ZIS/KCN, demonstrating a strong synergistic effect. Doping and heterojunction construction are efficient strategies to boost visible-light absorption efficiency and retard the fast recombination of photogenerated charge carriers of g-C 3 N 4 . In this work, potassium-doped g-C 3 N 4 (KCN) was prepared and subsequently coupled with ZnIn 2 S 4 (ZIS) and NiCo 2 O 4 (NCO) to construct ZIS/KCN and NCO/KCN heterojunction photocatalysts. The successful integration of ZIS nanosheets and NCO nanospheres with the KCN matrix was identified by X-ray diffractometry (XRD), electron microscopy, and X-ray photoelectron spectroscopy (XPS). Photoluminescence (PL), transient photocurrent (I-t), and electrochemical impedance spectroscopy (EIS) measurements consistently demonstrated that ZIS/KCN exhibited superior photoelectrochemical behavior. ZIS/KCN exhibited the lowest PL emission intensity, the smallest Nyquist semicircle radius, and the highest photocurrent density, which was approximately 2.7 times higher than that of KCN. ZIS/KCN achieved a hydrogen evolution yield of 735.42 μmol·g -1 within 5 h, which was 67.34 times higher than that of pristine KCN and 4.02 times higher than that of NCO/KCN. Based on the band structure analysis together with the PL, I-t, and EIS results, a possible S-scheme charge-transfer pathway is proposed for the ZIS/KCN heterojunction, which facilitates charge separation and transfer while preserving favorable redox ability. The synergistic combination of potassium doping and heterojunction construction provides an effective approach to improving charge-transfer dynamics and photocatalytic hydrogen evolution performance in g-C 3 N 4 -based systems.