Preparation of Ordered Fe3O4 Nanohoneycombs and Their Application in Capacitors

Fe substrates with a depression pattern were anodized to obtain Fe oxide films with a nanohoneycomb structure and orderly arranged cylindrical pores of uniform size. Crystalline Fe oxide films could be obtained by the heat treatment of amorphous samples obtained by the anodization of Fe substrates, but the atmosphere during heat treatment had a significant effect on the surface structure and crystallinity of the resulting samples. The heat treatment of the anodized samples in air produced a crystalline Fe oxide film consisting of Fe₂O₃ and Fe₃O₄, but the nanohoneycomb structure could not be maintained above 400 °C because the Fe substrate was oxidized during the heat treatment, and its surface structure changed significantly. On the other hand, the heat treatment of the anodized samples in N₂ atmosphere yielded Fe₃O₄ nanohoneycombs, which retained their regular honeycomb structure after heat treatment. The evaluation of the capacitor properties of the heat-treated samples showed that the properties differed markedly owing to the effects of the surface structure and crystallinity, with the sample heat-treated at 400 °C in N₂ atmosphere with the largest specific capacitance. The Fe₃O₄ nanohoneycombs obtained in this study are expected to be useful as electrodes for high-capacity capacitors.