<p>Tb³⁺-doped MgO/CuO heterostructured nanocomposites were synthesized via a sustainable green route using <i>Phlogacanthus thyrsiflorus</i> leaf extract as a natural reducing and stabilizing agent. XRD confirmed the crystalline nature and phase purity of the nanocomposites, while Raman spectroscopy indicated defect formation and enhanced interfacial interactions upon Tb³⁺ incorporation. FESEM and AFM analyses revealed interconnected heterogeneous nanostructures with increased surface roughness, contributing to improved catalytic performance. XPS confirmed the oxidation states of Mg, Cu, O, and Tb and evidenced the generation of oxygen vacancies and defect sites induced by Tb³⁺ doping. UV-Vis spectroscopy showed enhanced visible-light absorption and a reduced band-gap, while photoluminescence and electrochemical impedance spectroscopy demonstrated suppressed charge recombination and improved interfacial charge transfer. The optimized Tb³⁺:MgO/CuO photocatalyst exhibited excellent visible-light-driven activity, achieving ~ 94% degradation of malachite green within 120&#xa0;min, following pseudo-first-order kinetics. Adsorption behavior followed the Langmuir isotherm model, indicating monolayer adsorption on homogeneous active sites. Reactive species trapping confirmed •OH and •O₂⁻ radicals as the dominant species. Thermodynamic analysis indicated that the degradation process is spontaneous and endothermic. The catalyst also achieved ~ 72.5% total organic carbon (TOC) removal and retained high efficiency (~ 92%) in real pond water, demonstrating strong stability and practical applicability. Additionally, the nanocomposite exhibited appreciable antioxidant activity.</p> Graphical Abstract <p></p> <p>Green synthesis of Tb³⁺: MgO/CuO heterostructure for visible-light photocatalysis and antioxidant activity.</p>