Facile conversion of bulk metal surface to metal oxide single-crystalline nanostructures by microwave irradiation: Formation of pure or Cr-doped hematite nanostructure arrays

We report a method for converting the surfaces of bulk metal substrates (pure iron or stainless steel) to metal oxide (hematite or Cr-doped hematite) nanostructures using microwave irradiation. When microwave radiation (2.45 GHz, single-mode) was applied to a metal substrate under the flow of a gas mixture containing O₂ and Ar, metal oxide nanostructures formed and entirely covered the substrate. The nanostructures were single crystalline, and the atomic ratios of the substrate metals were preserved in the nanostructures. When a pure iron sheet was used as a substrate, hematite nanowires (1000 W microwave radiation) or nanosheets (1800 W microwave radiation) formed on the surface of the substrate. When a SUS410 sheet was used as a substrate, slightly curved rod-like nanostructures were synthesized. The oxidation states of Fe and Cr in these nanorods were Fe³⁺ and Cr³⁺. Quantitative analyses revealed an average Fe/Cr atomic ratio of 9.2, nearly identical to the ratio of the metals in the SUS410 substrate.