Synthesis, phase transition, and magnetic property of iron oxide materials: effect of sodium hydroxide concentrations

In this paper, a class of novel iron oxide particles has been fabricated through surfactant-directed structure approach in hydrothermal reaction. The obtained iron oxide nanostructures with distinct morphologies, such as rhombohedra, octahedral, plate-like, as well as dendritic, can be obtained by gradually increasing the concentrations of NaOH. The as-prepared iron oxide particles were characterized utilizing scanning electronic microscopy (SEM), X-ray diffraction (XRD), and transmission electron microscopy (TEM). Results reveal that the as-made particles, such as rhombohedra and octahedral, can be indexed as pure rhombohedral phase of hematite and show better single crystalline feature. In contrast, when NaOH concentrations are 1.05 and 6 M, respectively, we accordingly can obtain novel flake-like and dendrtic superstructure that possess a mean arm length of ~1.5 μm; both the as-made two samples can be easily indexed as a mixture of hematite and maghemite based on their XRD and TEM results. Additionally, it was found that the obtained iron oxide samples at different NaOH concentrations show obviously morphologies-dependent feature. Namely, the as-made samples can undergo transition from typical ferromagnetic to ferrimagnetic behavior when the NaOH concentrations are gradually increased. In general, the presented synthesis approach could be extended to prepare other metal oxides with specific morphology and structure.