Sunlight-driven photocatalytic degradation of azo and thiazine dyes using sustainable Ag@ZnO heterostructures synthesised via Talinum fruticosum leaf extract.

Growing volumes of dye-laden wastewater have made the development of viable, photoactive catalysts that operate in the visible-light region an urgent priority. In this study, an Ag@ZnO heterostructure was prepared through a green synthesis route using Talinum fruticosum (TF) leaf extract, which acted as a natural reducing, stabilising, and capping agent. Ag content was varied systematically between 2 and 10 wt% to examine its influence on the photophysical and electronic properties of ZnO. Characterisation confirmed the formation of highly crystalline Ag@ZnO heterostructures that retained the hexagonal wurtzite structure. XPS and UV-DRS revealed strong Ag@ZnO interfacial contact, an increase in oxygen-vacancy concentration, enhanced visible-light absorption, and a decrease in band-gap energy from 3.30 to 3.04 eV. Morphological examination showed interconnected nanograin networks favourable for charge transport and separation. Photocatalytic performance, assessed via degradation of methylene blue (MB) and Congo red (CR) under visible light, was highest for the 10 wt% Ag@ZnO heterostructure, which achieved 99.89% MB and 99.97% CR removal. Kinetic evaluation showed that MB degradation was best described by a pseudo-first-order model, whereas CR degradation exhibited pseudo-zero-order behaviour. Scavenging experiments indicated a major contribution from photogenerated holes (h+) and superoxide radicals (˙O2 -) to the observed degradation. This enhancement is attributed to Ag-induced charge separation, localised surface plasmon resonance, oxygen-vacancy-mediated defect engineering, and suppressed electron-hole recombination, underscoring the potential of Ag@ZnO heterostructures for solar-driven wastewater treatment.