Green synthesis of ZnO nanoparticles using <i>Justicia adhatoda</i> for photocatalytic degradation of malachite green and reduction of 4-nitrophenol

Achieving the smallest crystallite/particle size of zinc oxide nanoparticles (ZnO NPs) reported to date, measuring 5.2/12.41 nm with <i>Justicia adhatoda</i> (<i>J. adhatoda</i>) leaf extract, this study introduces a facile green synthesis. Utilizing aqueous <i>J. adhatoda</i> leaf extract as both a reducing and stabilizing agent, the method leverages the plant's rich phytochemical composition to produce highly crystalline and morphologically controlled ZnO NPs. This precise particle size control highlights the effectiveness of the synthesis process in morphological tuning. The synthesized NPs were thoroughly characterized using XRD, UV-vis spectroscopy, FTIR, FESEM, and HRTEM, which collectively revealed superior crystallinity, controlled morphology, and unique surface properties conferred by phytochemical bio-capping. The photocatalytic performance of these biogenic ZnO NPs was evaluated for the degradation of two model pollutants: malachite green (MG), a synthetic dye, and 4-nitrophenol (4-NP), a toxic organic compound. The NPs exhibited exceptional photocatalytic efficiency, achieving 99.8% MG degradation within 180 minutes and demonstrating a rapid photocatalytic reduction of 4-NP to 4-aminophenol with a reaction rate constant of 0.245 min^-1 under UV and sunlight irradiation. Mechanistic studies attributed this high performance to reactive oxygen species (ROS) generation and electron-hole pair interactions, supported by improved charge separation and high surface area. This work not only establishes the potential of <i>J. adhatoda</i>-mediated ZnO NPs in addressing persistent organic pollutants but also sets a benchmark for size-controlled NPs synthesis. By delivering scalable and eco-friendly water remediation technologies, this study advances green nanotechnology.