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 Fe ₂O₃/Fe₃O₄/FeO/Fe or Fe ₂O₃/Fe₃O₄/Fe layered structure, followed by hematite nanowire growth on the outer layer of hematite (Fe ₂O₃). 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 Fe₂O ₃/Fe₃O₄ interface reaction stimulate Fe ₂O₃ nanowire formation and that the Fe₂O ₃ nanowire growth is via surface diffusion of Fe cations supplied from the outward grain boundary diffusion through the Fe₂O ₃ 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.