Green Synthesis, Characterization and Process Optimization of a Clerodendrum inerme-Mediated ZnO/g-C₃N₄ Heterojunction for Congo Red Photodegradation

Abstract

The aim of this study was to develop an environmentally sustainable ZnO/g-C₃N₄ heterojunction photocatalyst for visible-light degradation of Congo red using Clerodendrum inerme leaf extract as a green complexing and growth-controlling medium. To address the limitations associated with conventional chemical synthesis and rapid photogenerated charge-carrier recombination, ZnO was coupled with thermally prepared g-C₃N₄ through a plant-assisted route and characterized by XRD, FTIR, SEM–EDX, UV–Vis DRS, and BET analyses. XRD confirmed the coexistence of hexagonal wurtzite ZnO and layered g-C₃N₄ without detectable crystalline impurities, while the ZnO crystallite size decreased from approximately 24.8 nm to 17.6 nm following composite formation. The apparent optical bandgap values were approximately 3.20, 2.68, and 3.13 eV for ZnO, g-C₃N₄, and ZnO/g-C₃N₄, respectively, while the BET surface area increased from 90.0 to 107.0 m² g⁻¹. Under visible-light LED irradiation, ZnO/g-C₃N₄ achieved approximately 87% Congo red degradation within 120 min compared with 65% for g-C₃N₄. The degradation followed pseudo-first-order kinetics, and scavenger experiments identified superoxide radicals as the dominant reactive species, with photogenerated holes and hydroxyl radicals also contributing. These findings demonstrate the potential of Clerodendrum inerme-mediated ZnO/g-C₃N₄ for photocatalytic treatment of dye-contaminated wastewater. Future investigations should evaluate mineralization efficiency, repeated-use stability, catalyst leaching, toxicity, and performance in real industrial wastewater containing mixed contaminants.