The origin of hematite nanowire growth during the thermal oxidation of iron

The oxidation of Fe in pure oxygen between 400 °C and 600 °C has been investigated in order to obtain insight into the mechanism of the spontaneous formation of α-Fe2O3 nanowires. By varying the oxidation temperature, Fe can be oxidized to form Fe2O3/Fe3O4/FeO/Fe or Fe2O3/Fe3O4/Fe layered structure, followed by hematite nanowire growth on the outer layer of hematite (Fe2O3). It is observed that Fe2O3 nanowires have a bicrystal structure and form directly on the top of the underlying Fe2O3 grains. It is shown that the compressive stresses generated by the volume change accompanying the Fe2O3/Fe3O4 interface reaction stimulate Fe2O3 nanowire formation and that the Fe2O3 nanowire growth is via surface diffusion of Fe cations supplied from the outward grain boundary diffusion through the Fe2O3 layer. This principle of nanowire formation may have broader applicability in layered systems, where the stress gradient in thin layers can be introduced via solid-state interfacial reaction or other means.