A One-Pot Synthesis of Monodispersed Iron Cobalt Oxide and Iron Manganese Oxide Nanoparticles from Bimetallic Pivalate Clusters

Monodispersed iron cobalt oxide (Fe2CoO4) and iron manganese oxide (Mn0.43Fe2.57O4) nanoparticles have been synthesized using bimetallic pivalate clusters of [Fe2CoO(O2CtBu)6(HO2CtBu)3] (1), Co4Fe2O2(O2CtBu)10(MeCN)2] (2), and [Fe2MnO(O2CtBu)6(HO2CtBu)3] (3) respectively as single source precursors. The precursors were thermolyzed in a mixture of oleylamine and oleic acid with either diphenyl ether or benzyl ether as solvent at their respective boiling points of 260 or 300 °C. The effect of reaction time, temperature and precursor concentration (0.25 or 0.50 mmol) on the stoichiometry, phases or morphology of the nanoparticles were studied. TEM showed that highly monodispersed spherical nanoparticles of Fe2CoO4 (3.6 ± 0.2 nm) and Mn0.43Fe2.57O4 (3.5 ± 0.2 nm) were obtained from 0.50 mmol of 1 or 3, respectively at 260 °C. The decomposition of the precursors at 0.25 mmol and 300 °C revealed that larger iron cobalt oxide or iron manganese oxide nanoparticles were obtained from 1 and 3, respectively, whereas the opposite was observed for iron cobalt oxide from 2 as smaller nanoparticles appeared. The reaction time was investigated for the three precursors at 0.25 mmol by withdrawing aliquots at 5 min, 15 min, 30 min, 1 h, and 2 h. The results obtained showed that aliquots withdrawn at reaction times of less than 1 h contain traces of iron oxide, whereas only pure cubic iron cobalt oxide or iron manganese oxide was obtained after 1 h. Magnetic measurements revealed that all the nanoparticles are superparamagnetic at room temperature with high saturation magnetization values. XMCD confirmed that in iron cobalt oxide nanoparticles, most of the Co2+ cations are in the octahedral site. There is also evidence in the magnetic measurements for considerable hysteresis (>1T) observed at 5 K. EPMA analysis and ICP-OES measurements performed on iron cobalt oxide nanoparticles obtained from [Fe2CoO(O2CtBu)6(HO2CtBu)3] (1) revealed that stoichiometric Fe2CoO4 was obtained only for 0.50 mmol precursor concentration. All the nanoparticles were characterized by powder X-ray diffraction (p-XRD), transmission electron microscopy (TEM), inductively coupled plasma-optical emission spectroscopy (ICP-OES), electron probe microanalysis (EPMA), X-ray magnetic circular dichroism (XMCD), and superconducting quantum interference device (SQUID) magnetometry.