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Magnetic iron oxide/clay composites: effect of the layer silicate support on the microstructure and phase formation of magnetic nanoparticles

Magnetic iron oxide nanoparticles were synthesized on two different clay supports: natural montmorillonite and synthetic laponite. The nanocomposites obtained, characterized by inductively coupled plasma atomic emission spectroscopy (ICP-AES), x-ray diffraction (XRD), transmission electron microscopy (TEM), N2 adsorption, small-angle x-ray scattering (SAXS), vibrating sample magnetometry and Mössbauer spectroscopy, were found to exhibit highly different physicochemical properties despite their similar iron content. The observed size effect of the layered silicate support, resulting in the high abundance of very small particles (diameter of 1–5 nm) on laponite, was explained in terms of the difference between the surface charge densities and the lamellar dimensions of the clay substrates. Moreover, it was revealed that the nature of the layered support greatly affected the nanostructure (fractal dimensions, surface area, porosity) of the formed hybrid solids as well as the phase formation of iron oxide crystals. The high surface area laponite composites, due to the dominance of very small iron oxide particles, exhibited more pronounced superparamagnetic behaviour as compared to the montmorillonite samples prepared under identical conditions. The observed higher saturation magnetization of the laponite composites, attributed to their lower content in the antiferromagnetic hematite and to the onset of superferromagnetism in the aggregated particles, shows their excellent utility for adsorption/magnetic separation.