Synthesis of nanostructured iron oxides and study of the thermal crystallization process using DSC and in situ XRD experiments

Abstract

Using a coprecipitation route in aqueous medium with different precipitating agents (NH4OH and NaOH), iron-oxide nanostructures were prepared with distinct structural and magnetic properties. X-ray diffraction (XRD), 57Fe Mössbauer spectroscopy and N2 sorption experiments indicated that the sample synthesized with NaOH had nanocrystalline goethite as the dominant phase, with a specific surface area (SSA) of 80 m2/g, whereas the sample synthesized with NH4OH was X-ray amorphous, with SSA = 270 m2/g. The crystallization process of the amorphous material was investigated in detail, using in situ XRD experiments combined with differential scanning calorimetry (DSC). The X-ray diffractograms recorded at high temperatures for this sample showed first the formation of maghemite, which was then converted into hematite as the heat treatment temperature increased. DSC curves recorded at different heating rates allowed the assessment of the kinetics of the crystallization process, with identification of two consecutive steps (formation of maghemite and conversion of maghemite into hematite) and the determination of the corresponding activation energies (166 and 273 kJ/mol, respectively). From the magnetic viewpoint, the nanostructured iron oxide samples showed non zero magnetization values, indicating a strong contribution form the uncompensated iron spins at the particle surfaces, leading to a superparamagnetic regime (i.e., magnetically non-blocked state) observed for temperatures above 50 K for both samples. These results evidenced how the chemical composition and the structural features of nanostructured iron oxides can be controlled by the conditions employed in the synthesis and the posterior thermal treatments performed on these materials.