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 Fe3C and carbon is proposed. Powders of iron (III) nitrate Fe(NO3)3·9H2O 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 (Tsyn) 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 Tsyn ≈ 400–500 °C, iron oxide γ-Fe2O3 predominates in the synthesis product. At Tsyn ≈ 600–700°C, iron carbide Fe3C is formed, whose content increases and reaches a maximum value (about 75%) with increasing Tsyn to 800–1000°C. At Tsyn ≈ 1000°C, nanocomposites of the core@shell type – Fe3C@C and Fe2O3@C, as well as those with a double shell Fe3C@Fe2O3@C – are formed. In the Fe3C@Fe2O3@C composites, Fe3C is a core, Fe2O3 and carbon C are shells. As Tsyn increases from 500 °C to 1000 °C, the amount of iron oxide γ-Fe2O3 decreases, and at Tsyn ≈ 1000 °C, γ-Fe2O3 oxide completely transforms into α-Fe2O3. In the sample synthesized at Tsyn ≈ 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 Néel/Curie point for bulk Fe3C, Fe3O4, and α-Fe. Such peculiar behavior of magnetization seems to be important, since it can find application in technology.