Silver-modified ZnO/CaO/NiO ternary oxide nanocomposites prepared through a green sol–gel method for pH-responsive photocatalytic removal of methylene blue

Silver-loaded ZnO/CaO/NiO ternary nanocomposites were successfully synthesized through an environmentally friendly sol–gel process utilizing Malva sylvestris extract as a natural reducing and capping medium. The photocatalytic capability of the resulting materials was investigated through the degradation of methylene blue (MB) in aqueous solutions with varying pH values. Structural characterization and microscopic analyses confirmed the formation of well-crystallized heterogeneous oxide networks in which Ag nanoparticles were effectively dispersed throughout the matrix. The photocatalytic performance was found to be highly influenced by the reaction medium, with markedly improved dye removal observed under neutral and basic conditions. UV–Vis analysis revealed a gradual decline in the characteristic absorption band of MB during light exposure, confirming progressive decomposition of the organic contaminant. Kinetic studies showed that the degradation process followed pseudo-first-order reaction kinetics, while the calculated rate constants increased as the solution became more alkaline. The enhanced activity at higher pH was attributed to stronger interactions between the catalyst surface and dye molecules, increased generation of reactive oxidative species, and more efficient suppression of electron–hole recombination. Overall, the synthesized Ag-incorporated ZnO/CaO/NiO nanocomposites demonstrate considerable potential as efficient photocatalysts for the treatment of dye-polluted wastewater and other environmental cleanup applications.