Synthesis and characterization of nanometric magnetite coated by oleic acid and the surfactant CTAB

Abstract

Nanometric magnetite (nm-Fe₃O₄) particles were prepared by the reverse co-precipitation synthesis method, obtaining particle sizes that ranged from 4 to 8.5 nm. In their synthesis, the concentration of iron salts of ferric nitrate, Fe(NO₃)₃⋅9H₂O, and ferrous sulfate, FeSO₄⋅7H₂O, were varied relative to the chemical reaction volume and by using different surfactants such as oleic acid (OA) and hexadecyltrimethylammonium bromide (CTAB). The nm-Fe₃O₄ particles were characterized by transmission electron microscopy (TEM), Mössbauer spectroscopy (MS), magnetic and X-ray diffraction (XRD) measurements. Typical asymmetrical and/or broad lines shapes appeared in all Mössbauer spectra of the as prepared samples suggesting strong magnetic inter-particle interactions, reducing these interactions to some extent by gentle mechanical grinding. For the smallest particles, maghemite instead of magnetite was the main preparation product as low temperature Mössbauer and magnetic measurements indicated. For the intermediate and largest particles a mixture of magnetite and maghemite phases were produced as the saturation magnetization values of M_S ∼ 60 emu/g indicated; these values were measured for most samples, independently of the coating surfactant concentration, and according to the ZFC-FC curves the blocking temperatures were 225K and 275K for the smallest and largest magnetite nanoparticles, respectively. The synthesis method was highly reproducible.