Synthesis of iron oxide nanoparticles under oxidizing environment and their stabilization in aqueous and non-aqueous media

Synthesis of magnetite (Fe3O4) nanoparticles under oxidizing environment by precipitation from aqueous media is not straightforward because Fe2+ gets oxidized to Fe3+ and thus the ratio of Fe3+ :Fe2+=2:1 is not maintained during the precipitation. A molar ratio of Fe3+:Fe2+ smaller than 2:1 has been used by many to compensate for the oxidation of Fe2+ during the preparation. In this work, we have prepared iron oxide nanoparticles in air environment by the precipitation technique using initial molar ratios Fe3+:Fe2+<= 2:1. The phases of the resulting powders have been determined by several techniques. It is found that the particles consist mainly of maghemite with little or no magnetite phase. The particles have been suspended in non-aqueous and aqueous media by coating the particles with a single layer and a bilayer of oleic acid, respectively. The particle sizes, morphology and the magnetic properties of the particles and the ferrofulids prepared from these particles are reported. The average particle sizes obtained from the TEM micrographs are 14, 10 and 9 nm for the water, kerosene and dodecane-based ferrofluids, respectively, indicating a better dispersion in the non-aqueous media. The specific saturation magnetization (sigma(s)) value of the oleic-acid-coated particles (similar to 53 emu/g) is found to be lower than that for the uncoated particles (similar to 63 emu/g). Magnetization as of the dodecane-based ferrofluid is found to be 10.1 emu/g for a volume fraction of particles phi=0.019. Zero coercivity and zero remanance on the magnetization curves indicate that the particles are superparamagnetic (SPM) in nature. (c) 2006 Elsevier B.V. All rights reserved.