Mixed hydro-solvothermal synthesis process: An unconventional route to obtain mixed iron oxide systems

The present study explores a new mixed hydro-solvothermal protocol for the synthesis of iron oxide-based materials at relatively mild conditions (150 °C), by selecting different water/2-propanol mixtures as reaction environments, and NaOH as a precipitating agent. Syntheses were performed in the absence of any structure directing agents and reducing agents, employing directly a mixture of Fe(II) and Fe(III) salts as iron oxide precursors. Morphological, structural, and magnetic variations of the resulting materials were monitored by means of scanning electron microscopy (SEM), X-ray powder diffraction (XRPD), and vibrating sample magnetometry (VSM) characterizations. Experimental results demonstrated the influence of the composition of the reaction medium on the crystal phase composition, morphology, and magnetic response. In particular, syntheses carried out at a high water/2-propanol volume ratio observed the predominant formation of the magnetite (Fe3O4)/maghemite (γ-Fe2O3) crystal phase, whereas the low water/2-propanol volume ratio favored the formation of the hematite (α-Fe2O3) crystal phase, thus outlining the important role played by the solvent mixture composition in directing the particles’ nucleation, growth, and agglomeration. Furthermore, magnetic measurements demonstrated a direct correlation existing between the synthesis conditions (and consequently the crystal phase compositions) and the observed magnetic properties, recording an increase of the magnetization along with the content of the magnetite/maghemite crystal phase. These results demonstrated that with this mixed hydro-solvothermal protocol it is possible to finely tailor both structural and functional properties of iron oxide-based materials by simply varying the synthesis conditions, thus providing valuable insights for the rational design of mixed phase materials.