Structural characterization and magnetic properties of core-shell Fe@Fe3O4 nanowires

Core-shell Fe@Fe3O4 nanowires (NWs) were successfully prepared on a homemade anodic aluminum oxide (AAO) template by the anodic oxidation of Fe NWs followed by annealing reduction. A field-emission scanning electron microscope was used to observe the morphology of the AAO template, which shows that the diameter of AAO template was 30 nm. TEM results confirm a thin oxide layer was formed on Fe NWs, confirming the core-shell nature of Fe NWs. The TEM analysis shows that with the increase in anodizing time, the oxide layer thickness was increased from 2.4, 4.6, 5.9 nm, respectively. The elemental composition of core-shell Fe NWs was confirmed by energy-dispersive X-ray spectrometer. The oxide phase of Fe could not be easily distinguished by X-ray analysis, because of the line width broadening of nanocrystals; thus, the valence states of Fe in Fe@Fe2O3 and Fe@Fe3O4 NWs were further analyzed by X-ray photoelectron spectroscopy (XPS). XPS results confirm the oxide nature of Fe2O3 and Fe3O4 in core-shell Fe NWs. The influence of oxide layer thickness on the magnetic properties of core-shell Fe@Fe3O4 NWs was investigated by vibrating sample magnetometry. Magnetic measurement revealed that the coercivity and squareness values of core-shell Fe@Fe3O4 NWs gradually increased with the increase in the Fe3O4 shell thickness. Results clearly showed that the size and magnetism of the core-shell Fe@Fe3O4 NWs can be adjusted to a certain extent by this synthetic method, so as to meet different requirements in desired biomedical applications.