ZnO/ZnFe₂O₄ heterostructured nanocomposites: synthesis-driven control of phase composition, defect chemistry, antioxidant, antibiofilm, and dye-specific photocatalytic activity

This study investigates the influence of different synthesis routes on the structure, defect chemistry, and multifunctional properties of ZnO/ZnFe₂O₄ heterostructured nanocomposites. Cold plasma, green synthesis, and co-precipitation methods were employed to control the ZnO/ZnFe₂O₄ phase ratios, crystallite size, and oxygen vacancy density. Comprehensive characterization using X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy/energy dispersive spectroscopy (SEM/EDS), ultraviolet–visible spectroscopy (UV–Vis), Raman spectroscopy, and multivariate statistical analyses (principal component analysis (PCA), 3D-PCA, Pearson correlation, and hierarchical cluster analysis (HCA)) established clear synthesis–structure–function relationships. The cold plasma route yielded the most ZnO-rich and highly crystalline heterostructure (59% ZnO, 38 nm crystallites, 75.8% crystallinity), which exhibited superior antioxidant activity (FRAP: 327.8 µmol AAE/g; TAC: 401.2 µmol AAE/g; β-carotene/linoleic acid bleaching IC₅₀: 0.171 mg/mL) and the strongest inhibition of Staphylococcus aureus and Pseudomonas aeruginosa biofilms. The co-precipitation-derived sample achieved the highest photocatalytic degradation of Rose Bengal (87.4% in 120 min), followed by the cold plasma sample (84.3%) and the green-synthesized sample (81.2%), with the superior performance of the co-precipitated catalyst attributed to its higher grain-boundary density and greater surface active site availability. In contrast, the ferrite-rich co-precipitated sample (57% ZnFe₂O₄) demonstrated superior adsorption and removal of cationic Methylene Blue (88.4%), followed by the green-synthesized sample (87.5%), while the cold plasma sample achieved (82.5%). Statistical modeling revealed that ZnO abundance and crystallite size strongly correlate with biological performance (r > 0.99), while photocatalytic efficiency is governed by dye-specific surface interactions. These findings highlight a synthesis-dependent divergence between biological redox activity and dye-selective photocatalysis, providing a rational design framework for advanced multifunctional oxide-based nanomaterials.