Surface functionalization of a series of nanosized iron oxide particles (average diameter around 6 nm) with oleic acid was realized in this study. The aim is to suspend the surface-functionalized nanoparticulated materials in insulating mineral oil and evaluate their colloidal stability as a function of time. Nanoparticulated samples presenting stoichiometry close to maghemite were obtained by oxidation of a freshly precipitated magnetite sample. Systematic variations observed in the Fe3+/Fe2+ ratio, average particle size, and lattice constant were attributed to differences in oxidation route and oxidation condition employed. Morphological, compositional, thermal, optical and magnetic characterization techniques were used in the investigation of native (P, PN1, PN2, POX1, POX3, and POX7) and surface-functionalized (POA, PN1OA, PN2OA, POX1OA, POX3OA, and POX7OA) samples. While suspending the oleic-acid-coated nanosized iron oxide particles in insulating mineral oil, the best colloidal stability was achieved at the oxidation profile of Fe3+/Fe2+ = 40 (5.9 nm average core diameter), leading to a surface grafting coefficient of about 75% of a full monolayer coating of chemisorbed species only and resulting in the smallest observed hydrodynamic radius (8.1 nm). Within the range of our investigation, our findings reveal the characteristics and the chemical protocol used to produce a magnetic fluid sample embodying long-term colloidal stability, thus representing a very much promising material for application as a refrigerating fluid in power transformers and related devices.

In this study, we report on how surface-passivated and nonpassivated cobalt ferrite nanoparticles (8 nm diameter), suspended as ionic magnetic fluids and aged under low pH conditions, revealed different behavior as far as the time evolution of the iron/cobalt cation distribution, crystal quality, coercivity, and saturation magnetization are concerned. Different techniques were used to perform a detailed study regarding the chemical stability, structural stability, and surface and magnetic properties of the suspended nanoparticles as a function of the aging time. Properties of surface-passivated and nonpassivated nanoparticles were investigated by transmission electron microscopy, X-ray diffraction, atomic absorption spectrometry, magnetic measurements, Raman spectroscopy, and Mössbauer spectroscopy. Our data showed that the employed nanoparticle surface passivation process, besides the formation of an iron-rich surface layer, modifies the nanoparticle core as well, improving the crystal quality while modifying the Fe/Co cation distribution and the nanoparticle dissolution rate profile. Magnetic data showed that the saturation magnetization increases for surface-passivated nanoparticles in comparison to the nonpassivated ones, though coercivity decreases after passivation. These two observations were associated to changes in the cation distribution among the available tetrahedral and octahedral sites.

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This study reports on the pioneering use of the layer-by-layer (LbL) technique to produce multilayered (1 to 50 bilayers) bifunctional nanocomposite films consisting of negatively charged citrate-coated maghemite nanoparticle (cit-MAG) hosted in positively charged conducting polyaniline (doped-PANI). The aim is to use the LbL assembly to fabricate thin nanocomposite films displaying superparamagnetic and conductivity properties and with fine control of the end properties as a function of the preparation condition. Multilayered cit-MAG/PANI bifunctional nanocomposite films were systematically investigated in order to access information regarding the nanofilm structure, electrical conductivity, and magnetic properties. Using the isothermal adsorption of each individual electrolyte (cit-MAG dispersion and doped-PANI solution) onto solid substrates (silicon and glass) the average time for deposition of a single layer (cit-MAG or doped-PANI) was fixed in 3 min. Independent evaluation using UV−vis spectroscopy and atomic force microscopy indicated a linear correlation between the nominal number of adsorbed cit-MAG/PANI bilayers and the material content (film thickness), even for the smallest number of adsorbed bilayers. Values of electrical conductivity (film thickness) found for the 10-bilayered cit-MAG/PANI nanocomposite films were in the range of 10−2−10−4 Scm−1 (25−63 nm) for γ-Fe2O3 concentration within the employed magnetic fluid suspension in the range of 10−4−10−3 g L−1. Values of the blocking temperature obtained from ZFC/FC curves recorded for the nanofilm produced using the highest γ-Fe2O3 concentrated suspension (2 × 10−3 g L−1) monotonically increase from 30 to 40 K as the number of cit-MAG/PANI bilayers increases from 5 to 50 bilayers. Therefore, we found that the end properties can be easily and precisely modulated by varying the concentration of the magnetic fluid used for film deposition and/or controlling the nominal number of cit-MAG/PANI bilayers in the nanocomposite.

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