New insights into the structural analysis of maghemite and (MFeO, M = Co, Zn) ferrite nanoparticles synthesized by a microwave-assisted polyol process

The search for more efficient, scalable, reproducible and standardized synthesis methods able to control particle size and crystallinity is still a challenge in nanotechnology. The one-pot microwave-assisted polyol process has been optimized for the synthesis of well-defined ferrite nanoparticles. Highly uniform and crystalline γ-Fe 2 O 3 , CoFe 2 O 4 and ZnFe 2 O 4 nanoparticles, with diameters below 14 nm have been prepared by an easy and reproducible one-pot microwave-assisted heating procedure in a polyol medium. A pure single phase and cubic spinel structure were confirmed by powder X-ray diffraction and Raman spectroscopy. Depending on the metal precursors, nanoparticles display magnetic features from superparamagnetic behaviour (Fe- and Zn-ferrites) to ferrimagnetism (Co ferrite) at room temperature. The iron oxidation state of 3+ and its short-range order coordination were studied by XAS (X-ray absorption spectroscopy). External field Mössbauer spectra, recorded at low temperature, confirmed the ferrimagnetic order with a Fe 3+ and M 2+ partial cationic distribution within both the A and B sites in zinc and cobalt ferrite, respectively. According to these results, the ferrite stoichiometry was (Zn 0.70 Fe 0.30 )[Zn 0.30 Fe 1.70 ]O 4 and (Co 0.28 Fe 0.72 )[Co 0.72 Fe 1.28 ]O 4 . A comprehensive structural analysis was presented on crystalline maghemite and ferrite (MFe 2 O 4 , M = Co, Zn) nanoparticles prepared by the well-known polyol process enhanced by fast, reproducible, easy and efficient microwave heating.