Defect-Mediated Electronic Structure Engineering of Green-Synthesized ZnO Nanoparticles Using Citrus sinensis Peel Extract for Enhanced Photocatalytic Degradation of Reactive Red 120

Abstract Zinc oxide (ZnO) is a promising wide-bandgap semiconductor for photocatalytic applications, but its large bandgap limits solar light utilization. In this work, ZnO nanoparticles were synthesized via a green route using Citrus sinensis peel extract as a phytochemical-mediated reducing and stabilizing agent. X-ray diffraction confirmed single-phase hexagonal wurtzite ZnO with an average crystallite size of 17.7 nm, while scanning eelctron microscopy (SEM) and energy-dispersive x-ray spectroscopy (EDS) verified nanoparticle formation without detectable impurities. N $$_2$$ 2 adsorption–desorption analysis showed a specific surface area comparable to commercial ZnO, indicating that surface area differences are unlikely to govern the observed performance gap. Thermogravimetric analysis indicated a small amount of residual phytochemical-derived organic species. Diffuse reflectance spectroscopy revealed a bandgap reduction from approximately 3.1 eV (commercial ZnO) to 2.7 eV (green-synthesized ZnO), with Urbach energy analysis providing quantitative evidence of an elevated density of localized sub-gap states. Photocatalytic degradation of Reactive Red 120 under ultraviolet (UV) irradiation reached approximately 95% removal after 90 min, following pseudo-first-order kinetics ( $$k = 2.0\times 10^{-2}$$ k = 2.0 × 10 - 2 min $$^{-1}$$ - 1 ). Scavenger experiments showed that both hydroxyl and superoxide radicals participate, with superoxide playing the dominant role. Electrochemical impedance spectroscopy revealed a marked decrease in charge-transfer resistance under illumination, while Mott–Schottky analysis showed a substantial negative shift of the flat-band potential under operating conditions, providing an electrochemically grounded explanation for the superoxide dominance. Fourier transform infrared (FTIR) spectroscopy confirmed dye adsorption followed by oxidative degradation during irradiation. The combined structural, optical, textural, electrochemical, and photocatalytic results demonstrate that phytochemical-assisted synthesis effectively tunes the optoelectronic properties of ZnO while providing a sustainable route to high-performance photocatalysts.