Formation of highly crystalline maghemite nanoparticles from ferrihydrite in the liquid phase

Fe₅O₇(OH)·4H₂O ferrihydrite is a low-crystallinity antiferromagnetic material. γ-Fe₂O₃ (maghemite) magnetic nanoparticles were prepared from a ferrihydrite precursor, by chemically induced transformation in FeCl₂/NaOH solution. The magnetization, morphology, crystal structure and chemical composition of the products were determined by vibrating sample magnetometry, transmission electron microscopy, X-ray diffraction (XRD), energy-dispersive X-ray spectroscopy and X-ray photoelectron spectroscopy (XPS). Ferrihydrite underwent aggregation growth and transformed into α-FeO(OH) (goethite) particles, which subsequently transformed into γ-Fe₂O₃ nanoparticles, that became coated with NaCl. The γ-Fe₂O₃ particles had a flake-like morphology, when prepared from 0.01 mol/L FeCl₂ and a FeCl₂:NaOH molar ratio of 0.4. The γ-Fe₂O₃ particles were more spherical, when prepared from a FeCl₂:NaOH molar ratio of 0.6. The Fe content of the flake-like particles was lower than that of the spherical particles. Their magnetizations were similar, and the coercivity of the flake-like particles was larger. The differences in morphology and magnetization were attributed to the surface effect, and the difference in coercivity to the shape effect.