In-situ green synthesis of Zn/Fe co-doped NiO nanocatalysts using Rheum ribes root extract for band-gap engineering and enhanced solar-light-driven photocatalytic degradation of methylene blue

In-situ green co-doping synthesis offers a sustainable and efficient route for tailoring the structural, optical, and catalytic properties of nanocatalysts, enabling enhanced charge separation, band-gap modulation, and superior photocatalytic performance under solar-light irradiation. This study presents a sustainable phytogenic strategy for the one-pot, in-situ synthesis of pristine NiO and Zn/Fe co-doped NiO nanocatalysts using Rheum ribes root extract, highlighting the significance of green co-doping engineering in tailoring the structural and photocatalytic properties of nanomaterials. Zn and Fe were incorporated to tailor the structural and optical properties of NiO, leading to a reduction in the band gap. Comprehensive characterization using common microscopic and spectroscopic techniques confirmed successful co-doping, high crystallinity, homogeneous morphology, and modified optical behavior. The Zn/Fe co-doped NiO exhibited a reduced optical band gap of approximately 2.8 eV, enabling enhanced visible-light absorption and catalytic degradation under both a white LED lamp and solar light. Photocatalytic performance was evaluated via methylene blue degradation, where the co-doped nanocatalyst achieved 95% degradation within 4 h under sunlight illumination, significantly outperforming pristine NiO. The improved activity is attributed to the synergistic effect of Zn and Fe dopants, which enhance charge carrier separation, suppress the recombination of electron–hole pairs, and increase the creation of reactive oxygen species, particularly hydroxyl radicals (•OH). Furthermore, the nanocatalyst demonstrated outstanding stability and reusability across four repeated cycles without a noticeable decrease in activity. Altogether, this study emphasizes the value of locally sourced Rheum ribes root extract as a green and reproducible platform for developing Zn/Fe@NiO nanocatalysts with enhanced photocatalytic activity for wastewater remediation.