Sonochemical synthesis of amorphous nanoscopic iron(III) oxide from Fe(acac)3

Amorphous nanoscopic iron(III) oxide with interesting magnetic properties was prepared by sonolysis of Fe(acac)3 under Ar in tetraglyme with a small amount of added water. The organics content and the surface area of the Fe2O3 nanoparticles can be controlled with an amount of water in the reaction mixture and it increases from 48 m2 g−1 for dry solvent up to 260 m2 g−1 when wet Ar is employed. For further monitoring of the particle size and morphology and for the study of the surface, magnetic and thermal properties, the sample with 2 vol.% of H2O was chosen. SEM showed nanoscopic composite particles of a uniform size distribution and nearly spherical shapes with an estimated diameter of 20 nm. Such composites are built from amorphous iron(III) oxide nanoparticles (3 nm) embedded in an acetate matrix as proved by TEM and IR spectroscopy. Temperature-dependent Mössbauer spectra demonstrate a very narrow magnetic transition with an unusually low transition temperature around 25 K reflecting the system of magnetically non-interacting ultrasmall particles with a narrow size distribution. The in-field (5 T) Mössbauer spectrum recorded at 5 K shows a minimum change compared to the zero-field spectrum indicating an absence of the long-range magnetic ordering. The composite particles are thermally stable up to 150 °C, which is confirmed by DSC, TG, and by the constant surface area. At higher temperatures, acetate groups are removed from the particle surface, which is documented by the increased surface area and disappearance of their IR bands.