Structural and magnetic evolution of FexOy@carbon core-shell nanoparticles synthesized by a one-step thermal pyrolysis

Abstract

Carbon-coated iron oxides core@shell nanoparticles were synthesized by a one-step process of thermal pyrolysis. The influence of the synthesis temperature on the structural and magnetic properties of FexOy@carbon core-shell nanoparticles was investigated by X-ray diffraction, transmission electron microscopy (TEM), Raman and Mössbauer spectroscopy, as well as by magnetic measurements. At reaction temperatures (TR) from 360 to 385 °C, spherical and nearly monodispersed nanoparticles of magnetite with an average diameter of about 40 nm covered with an amorphous carbon shell about 5 nm thick were obtained. When the reaction temperature rises above 385 °C, the nanoparticles of wüstite phase begin to form, and its concentration increases at the expense of the magnetite phase with further TR increasing. In a narrow temperature range from 390 to 400 °C, the wüstite concentration increases from 10 to 40%. At that time magnetite nanoparticles become smaller and superparamagnetic. The Mössbauer spectra indicate that the excess [Fe3+]B ions in octahedral B-sites of magnetite which associated with the particle surface do not participate in the electron hoping [Fe3+ ⇆ Fe2+]B above the Verwey temperature. The wüstite phase appears and grows with a corresponding decrease in the fraction of [Fe3+]B that does not undergo the electronic exchange. This proves that the wüstite phase is generated and formed on the surface of magnetite particles under the influence of carbon. The Mössbauer data allow one to monitor the dynamics of reduction of iron oxide from Fe3O4 to FeO under the influence of carbon.