A facile solvent-free strategy for g-C 3 N 4 /ZnO Z-scheme heterostructures for enhanced photocatalytic degradation of organic dyes

Introduction: A facile, solvent-free mechanochemical strategy was developed to synthesize g-C 3 N 4 /ZnO heterostructured nanocomposites for the photocatalytic degradation of Rhodamine B (RhB) and Methylene Blue (MB) under UV–visible irradiation, for dye-contaminated water treatment. Materials and methods: g-C 3 N 4 /ZnO nanocomposites containing 0.5, 1.0, and 1.5 wt.% g-C 3 N 4 were prepared by a solvent-free mechanochemical route and characterized using X-ray diffraction (XRD), UV–Vis diffuse reflectance spectroscopy (UV–Vis DRS), field-emission scanning electron microscopy (FE-SEM), energy-dispersive X-ray spectroscopy (EDS), transient photocurrent response, electrochemical impedance spectroscopy (EIS), and Mott–Schottky analysis. Photocatalytic degradation experiments were performed using MB and RhB as model pollutants, together with kinetic, scavenger, reusability, total organic carbon (TOC), and mixed-dye studies. Results: XRD confirmed the formation of hexagonal wurtzite ZnO and its integration with g-C 3 N 4 . UV–Vis DRS indicated enhanced light absorption with an optical band gap of 3.18 eV for the optimized composite. The 1.0 wt.% g-C 3 N 4 /ZnO exhibited the highest photocatalytic activity, achieving 96.16% MB and 99.96% RhB degradation, with apparent rate constants of 0.05264 and 0.08111 min −1 , respectively. A catalyst dosage of 50 mg provided an effective balance between photocatalytic activity and light utilization. Increasing the initial dye concentration decreased the degradation efficiency. Enhanced photocurrent response and reduced charge-transfer resistance confirmed improved charge separation and interfacial carrier transport. Scavenger experiments established the involvement of h + , •O 2 − , and •OH in the degradation process. The catalyst retained 79.69% MB and 80.97% RhB degradation efficiency after seven cycles, while FTIR analysis confirmed preservation of its characteristic chemical structure. TOC analysis showed approximately 69% removal after 60 min, and successful degradation of a mixed MB/RhB solution demonstrated its applicability to multicomponent dye systems. Conclusions: The solvent-free mechanochemical approach provides a simple and environmentally benign route for producing efficient g-C 3 N 4 /ZnO photocatalysts. The optimized heterostructure combines enhanced photocatalytic activity, favourable charge-transfer characteristics, good recyclability, and substantial mineralization capability, highlighting its potential for sustainable wastewater remediation.