The reusability of CoFe₂O₄@ZnO core–shell nanoparticles (NPs) for the photocatalytic degradation of methylene blue (MB) under UV radiation was successfully investigated. CoFe₂O₄@ZnO NPs with various CoFe₂O₄–to–ZnO concentration ratios were synthesized as magnetic photocatalysts. The X-ray diffraction spectra showed that the NPs had a cubic spinel ferrite phase structure and a hexagonal wurtzite phase of ZnO. Fourier-transform infrared spectra showed the presence of M_oct-O, M_tet-O, and Zn–O at 593, 347–389, and 410–429 cm⁻¹, respectively. The CoFe₂O₄@ZnO NPs had a saturation magnetization of approximately 30 emu g⁻¹ and a coercivity of approximately 280 Oe. The absorbance spectra showed that the absorbance peak of the CoFe₂O₄@ZnO NPs broadened and shifted to the right (higher wavelength) with increasing ZnO concentration. The CoFe₂O₄@ZnO NPs with higher ZnO concentrations exhibited higher photocatalytic activities and degradation rates. The enhancement of MB degradation can be attributed to the formation of an internal structure between CoFe₂O₄ and ZnO. The degradation rate of CoFe₂O₄@ZnO decreased slightly after each successive recycle. The results indicated that the recycled CoFe₂O₄@ZnO NPs could be reused three times for photocatalytic degradation. As there is no significant decrease in the photocatalytic degradation after four successive recycles, the CoFe₂O₄@ZnO NPs are suitable for application in dye degradation.

In this study, a simple, rapid, and environmentally friendly green approach for synthesizing Fe₃O₄ and Fe₃O₄/chitosan nanoparticles with various concentrations was developed. The nanoparticles had a spherical shape with a cubic inverse spinel structure. The functionalization of the Fe₃O₄ nanoparticles using chitosan increased the crystallite size of the nanoparticles from 7.2 to 7.8 nm. The Fourier transform infrared spectra of the Fe₃O₄/chitosan nanoparticles showed the existence of the characteristic peaks of chitosan in addition to a peak at 578 cm⁻¹, which corresponds to the stretching of the Fe−O group. The UV-visible spectra demonstrated a wide absorption band with the appearance of small peaks of chitosan absorption at 205 and 215 nm. The saturation magnetization of the Fe₃O₄ was 54.1 emu g⁻¹. The surface plasmon resonance (SPR) measurements showed an enhancement in the SPR angle as the ratio of chitosan to Fe₃O₄ increased, wherein the angle shift from 0.17° to 1.91°. The chitosan covering the Fe₃O₄ nanoparticle surface caused the refractive index to change, which increased the SPR angle shift. The obtained results indicated that the SPR properties of the Fe₃O₄ nanoparticles were significantly improved by modification with chitosan. These results also indicated that the use of chitosan in Fe₃O₄ nanoparticles can enhance SPR properties, which has potential for future SPR-based sensor applications.