Solar light-based photocatalysis for organic pollutant degradation: Mechanistic understanding of double Z-scheme dynamics in ZnO/Fe₂O₃-doped NiCo₂O₄

The double Z-scheme photocatalysts provide an effective strategy for enhancing the separation and transfer of photo-induced charge carriers. In this study, pure NiCo₂O₄ (NC) and ZnO/Fe₂O₃-doped NiCo₂O₄ (ZNFC1-3) nanoparticles (NPs) were synthesized using the co-precipitation method. The impact of doping on the structural and photocatalytic properties was thoroughly investigated. The NPs were characterized through X-ray diffraction (XRD), Fourier-transform infrared (FTIR) spectroscopy, Raman spectroscopy, and UV–Visible analysis. XRD confirmed that the ZNFC NPs possess a face-centered cubic structure, with particle sizes ranging from 19 to 29 nm. FTIR analysis identified metal–oxygen bonds, validating the successful incorporation of dopants. The optical bandgap of the NPs decreased from 2.68 eV (undoped) to 1.64 eV (highly doped), demonstrating enhanced light absorption. Photoluminescence (PL) intensity decreased with increasing dopant concentrations, indicating improved inhibition of charge carrier recombination. The photocatalytic performance of the pure and doped ZNFC3 NPs was evaluated for the degradation of Crystal Violet (CV) dye and salicylic acid (SA) under visible light irradiation. The ZNFC3 photocatalyst exhibited superior degradation efficiency, achieving 91% removal of CV and 90% of SA under optimized conditions, including pH, pollutant concentration, and catalyst dosage. Trapping experiments confirmed that hydroxyl radicals (OH^•) were the primary reactive species responsible for pollutant degradation, followed by photo-induced holes and electrons. Recyclability tests over multiple cycles demonstrated the excellent stability and reusability of the ZNFC3 photocatalyst, making it a promising candidate for removing organic pollutants from wastewater effluents.