This research used an aqueous leaf extract of Allium Caliphalum Wendelbow as an eco-friendly stabilizing and capping agent for the production of ZnO NPs. Different characterization techniques, including SEM, UV-Vis spectroscopy, FTIR, XRD, and EDX, were employed to analyze the morphology, structural, optical, and chemical properties of the ZnO nanoparticles. The SEM analysis revealed that the average particle sizes ranged between 30 and 72 nm depending on pH, but the crystallite sizes obtained from XRD ranged from 15 to 24 nm. UV-Vis spectroscopy indicated a bandgap energy range of 3.14 to 3.25 eV, dependent on the pH of the reaction mixture. The FTIR spectra confirmed the existence of functional groups from the plant extract that facilitate the reduction and stabilization process. The photocatalytic efficacy of ZnO NPs was assessed by decomposing methyl orange dye under ultraviolet irradiation. The nanoparticles produced at an ideal pH of 8 demonstrated the greatest degrading efficiency of 74% in 140 min, owing to their reduced size, enhanced crystallinity, and advantageous bandgap energy (~ 3.25 eV). The findings underscore the substantial impact of pH on the physicochemical and photocatalytic characteristics of biosynthesized ZnO NPs, providing a sustainable method for wastewater treatment applications.