Controllable synthesis and magnetism of iron oxides nanorings.

Hematite (alpha-Fe2O3) nanorings were prepared via a facile hydrothermal route without using any template. The products were characterized by X-ray powder diffraction (XRD), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). On the basis of these characterizations and condition experiments, an "oriented dissolution and recrystallization" mechanism under the effect of H2PO4- ions was proposed to explain the formation process of nanoring structure. Magnetite (Fe3O4) nanorings were obtained by reducing alpha-Fe2O3 nanorings, and then maghemite (gamma-Fe2O3) nanorings were obtained by reoxidizing Fe3O4 nanorings. The magnetic properties of these nanorings were investigated, and it was found that these nanorings have higher coercivity and lower saturation magnetization than many other nanostructures of iron oxides. The adsorbed phosphate on the surface and the nanoring morphology might be responsible for this phenomenon. Furthermore, it is interesting to find that the coercivity of the nanorings increased with the increase of d(in)/d(out) (d(in) and d(out) are the inner and outer diameters of the rings, respectively), and a rapid increase was observed at the value of d(in)/d(out) around 0.5.