Phase composition, structural and magnetic properties of core-shell nanoparticles based on iron carbide Fe3C synthesized by the sol-gel method

A new one-stage anhydrous sol-gel method for the synthesis of core-shell nanoparticles based on Fe ₃C and carbon is proposed. Powders of iron (III) nitrate Fe(NO ₃) ₃·9 H ₂ O and polyacrylonitrile (PAN) were used as initial reagents, dimethylacetamide (DMAc) was used as a solvent. The synthesis was carried out in an inert argon atmosphere at various temperatures ( T _syn ) up to 1000 °C. The morphology, structure, elemental and phase compositions, magnetic and electronic properties of the obtained core-shell nanocomposites were studied by various methods, such as X-ray diffraction (XRD), transmission electron microscopy (TEM), scanning transmission electron microscopy (STEM), energy dispersive X-ray spectroscopy (EDXS), Raman and Mössbauer spectroscopy, and magnetometry. It was found that at temperatures T _syn ≈ 400–500 °C, iron oxide γ-Fe ₂O ₃ predominates in the synthesis product. At T _syn ≈ 600–700 °C, iron carbide Fe ₃C is formed, whose content increases and reaches a maximum value (about 75%) with increasing T _syn to 800–1000 °C. At T _syn ≈ 1000 °C, nanocomposites of the core@shell type – Fe ₃C@C and Fe ₂O ₃ @C, as well as those with a double shell Fe ₃C@Fe ₂O ₃ @C – are formed. In the Fe ₃C@Fe ₂O ₃ @C composites, Fe ₃C is a core, Fe ₂O ₃ and carbon C are shells. As T _syn increases from 500 °C to 1000 °C, the amount of iron oxide γ-Fe ₂O ₃ decreases, and at T _syn ≈ 1000 °C, γ-Fe ₂O ₃ oxide completely transforms into α-Fe ₂O ₃. In the sample synthesized at T _syn ≈ 1000 °C, particles with a crystallite size of 29.6 nm exhibit magnetic properties with the highest saturation magnetization value of 68 emu/g. The temperature dependence of the magnetization M(T) of this sample has a specific character – there are three sharp drops at temperatures close to the N é el/Curie point for bulk Fe ₃C, Fe ₃O ₄, and α-Fe. Such peculiar behavior of magnetization seems to be important, since it can find application in technology .