Metallic iron (Fe) nanoparticles have been synthesized by reducing SiO2-coated magnetite (Fe3O4) nanoparticles. The Fe3O4 nanoparticles with an average diameter of about 10 nm were prepared by thermal decomposition of the iron-oleate complex, and were subsequently coated with SiO2. The SiO2-coated Fe3O4 nanoparticles were annealed at various temperatures for 2 hr in flowing H2 gas to convert them to Fe nanoparticles. The Fe3O4 nanoparticles with 3- and 5-nm SiO2 coatings were completely reduced to Fe nanoparticles at 800°C. However, under these conditions, coalescence of thus-formed Fe nanoparticles was also observed. The Mössbauer spectrum of the Fe nanoparticles prepared by annealing at 800°C revealed that about 84% of the nanoparticles were ferromagnetic α-Fe at room temperature. Approximately 16% of the Fe nanoparticles were paramagnetic due to superparamagnetic-like behavior at room temperature.
