No abstract

Iron nanostructures with morphology ranging from discrete nanoparticles to nearly monodisperse hierarchical nanostructures have been successfully synthesized using solvated metal atom dispersion (SMAD) method. Such a morphological evolution was realized by tuning the molar ratio of ligand to metal. Surface energy minimization in confluence with strong magnetic interactions and ligand-based stabilization results in the formation of nanospheres of iron. The as-prepared amorphous iron nanostructures exhibit remarkably high coercivity in comparison to the discrete nanoparticles and bulk counterpart. Annealing the as-prepared amorphous Fe nanostructures under anaerobic conditions affords air-stable carbon-encapsulated Fe(0) and Fe3C nanostructures with retention of the morphology. The resulting nanostructures were thoroughly analyzed by powder X-ray diffraction (PXRD), thermogravimetric analysis (TGA), transmission electron microscopy (TEM), and Raman spectroscopy. TGA brought out that Fe3C nanostructures are more robust toward oxidation than those of α-Fe. Finally, detailed magnetic studies were carried out by superconducting quantum interference device (SQUID) magnetometer and it was found that the magnetic properties remain conserved even upon exposure of the annealed samples to ambient conditions for months.

Herein, we report a simple solid state synthetic route to prepare carbon‐Fe based magnetic nanoparticles with different compositions and morphologies through annealing of amorphous Fe nanoparticles under appropriate conditions. Tri‐n‐octylphosphine (TOP) capped amorphous Fe nanoparticles with a mean diameter of 3.2 nm were synthesized using solvated metal atom dispersion (SMAD) method. Annealing of as‐prepared Fe nanoparticles at 300 °C produced carbon encapsulated crystalline bcc‐Fe nanoparticles, whereas at higher temperatures i.e., 400 °C and 500 °C, spherical Fe3C/C core‐shell nanoparticles were obtained. Annealing of as‐prepared Fe nanoparticles in the presence of tri‐n‐octylphosphine oxide (TOPO) ligand under optimized conditions yielded rod shaped Fe3C/C core‐shell morphology. The size, composition and particle morphology of these magnetic nanoparticles could be controlled by changing the reaction time, temperature and the concentration of the TOPO ligand. Magnetic measurements show that rod shaped Fe3C nanoparticles exhibit enhanced coercivity (Hc) values compared with spherical Fe3C nanoparticles, which is due to shape anisotropy.