Plant-Mediated Green Synthesis of Iron Oxide Nanoparticles for Visible-Light-Driven Photocatalytic Dye Degradation: Mechanistic Insights, Structure PropertyActivity Relationships, and Future Directions (Review)

Abstract

The escalating discharge of synthetic dyes from textile, pharmaceutical, and printing industries poses severe environmental and public health risks due to their persistence, toxicity, and resistance to conventional treatment processes. Photocatalysis has emerged as a promising advanced oxidation strategy, particularly using iron oxide nanoparticles (FeOx), owing to their visible-light responsiveness, magnetic recoverability, chemical stability, and redox versatility. In recent years, plant-mediated green synthesis has gained attention as a sustainable alternative to conventional chemical routes, utilizing phytochemicals such as polyphenols, flavonoids, terpenoids, and alkaloids as reducing and stabilizing agents. This review critically evaluates recent progress in the plant-assisted synthesis of Fe₃O₄, α-Fe₂O₃, and γ-Fe₂O₃ nanoparticles for photocatalytic dye degradation. Emphasis is placed on structure–property–activity relationships, including the influence of particle size, crystallinity, defect density, band gap modulation, surface area, and magnetic characteristics on photocatalytic efficiency. Mechanistic insights into charge carrier dynamics, reactive oxygen species (•OH, O₂•⁻, h⁺) generation, and photo-Fenton synergism are systematically discussed. Comparative kinetic analyses reveal that nanoparticles within the 10–20 nm range, exhibiting moderate oxygen vacancy concentration and surface area above 70 m²/g, demonstrate superior degradation rate constants under visible light. However, reproducibility challenges, inconsistent band gap reporting, limited real wastewater validation, and scale-up barriers remain significant constraints. The review concludes by outlining research priorities including standardized green synthesis protocols, hybrid nanocomposite engineering, pilot-scale solar reactors, and life-cycle assessment to advance commercialization potential.