Structural Investigation of MFe₂O₄ (M = Fe, Co) Magnetic Fluids

Abstract

Ferrites of the type MIIFe2O4 (M = Fe and Co) have been prepared by the traditional coprecipitation method. These ferrites were modified by the adsorption of fatty acids derived from soybean and castor oil and were then dispersed in cyclohexane, providing very stable magnetic fluids, readily usable in nonpolar media. The structural properties of the ferrites and modified ferrites as well as the magnetic fluids were characterized by XRD (X-ray powder diffraction), TEM (transmission electron microscopy), DRIFTS (diffusion reflectance infrared Fourier transform spectroscopy), FTNIR (Fourier transform near-infrared), UV−vis, normal Raman spectroscopy, and surface-enhanced Raman scattering (SERS). XRD and TEM analysis have shown that the magnetic nanoparticles (nonmodified and modified) present diameters in the range of 10−15 nm. DRIFTS measurements have shown that the carboxylate groups of soybean and castor oil fatty acids adsorb on the ferrite surface, forming three different structures: a bridging bidentate, a bridging monodentate, and a bidentate chelate structure. The FTIR and Raman spectra of nonmodified Fe3O4 and CoFe2O4 nanoparticles have shown that the number of observed phonons is not compatible with the expected Oh7 symmetry, since IR-only active phonons were observed in the Raman spectra and vice versa. SERS measurements of a CoFe2O4 thin film on a SERS-active gold electrode at different applied potentials made possible the assignment of the signals near 550 and 630 cm−1 to Co−O motions and the signals near 470 and 680 cm−1 to Fe−O motions.