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

Using a coprecipitation route in aqueous medium with different precipitating agents (NH 4 OH and NaOH), iron-oxide nanostructures were prepared with distinct structural and magnetic properties. X-ray diffraction (XRD), 57 Fe Mössbauer spectroscopy and N 2 sorption experiments indicated that the sample synthesized with NaOH had nanocrystalline goethite as the dominant phase, with a specific surface area (SSA) of 80 m 2 /g, whereas the sample synthesized with NH 4 OH was X-ray amorphous, with SSA = 270 m 2 /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. • Nanostructured iron oxides were produced using simple wet chemical routes. • Different materials were produced using NaOH or NH 4 OH as the precipitating agent. • The sample synthesized with NaOH was identified as nanocrystalline goethite. • An amorphous hydrated iron oxide was produced with NH 4 OH as the precipitating agent. • The crystallization of the amorphous material was studied by DSC and in situ XRD.