Dipole-directed assembly of Fe3O4 nanoparticles into nanorings via oriented attachment

We have developed a novel hydrothermal method to fabricate Fe3O4 ring-like nanostructures via oriented attachment of Fe3O4 nanoparticles through intermediate platelets with incorporation of inorganic salts. The obtained samples were characterized by X-ray diffraction (XRD) analysis, X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), transmission electron microscopy (TEM), selective-area electron diffraction (SAED), high-resolution TEM (HRTEM) and vibrating-sample magnetometer (VSM). A reasonable mechanism based on the self-assembly of magnetic Fe3O4 nanoparticles on graphene nanosheets using inorganic ions as crystal growth modifiers is proposed. The intrinsic dipole-dipole interactions between the individual magnetic Fe3O4 nanoparticles act as the driving force for nanoring formation. Saturation moment of the superparamagnetic Fe3O4 nanorings is much higher than that of the corresponding disassembled nanoparticles. The enhancement of saturation moment is due to the oriented ring-like assembly with synergistic magnetism. This study could provide an additional tool for fabricating other nanostructures via oriented attachment.