Formation of modulated structures in single-crystalline hexagonal α-Fe₂O₃ nanowires

The microstructures and growth mechanism of Fe₂O₃ nanowires (NWs) synthesized by thermal oxidation of iron are studied in detail by transmission electron microscopy. Three different structures, single-crystalline, bicrystalline, and tricrystalline, are observed in the NWs. It is found that single-crystalline Fe₂O₃ NWs have a hexagonal structure while bicrystalline and tricrystalline NWs possess a cubic one. The differences in the electronic structures of the three Fe₂O₃ NWs are examined by electron energy-loss spectroscopy. A modulated structure with a periodicity of 1.53 nm is observed in single-crystalline Fe₂O₃ NWs, but not in bicrystalline or tricrystalline Fe₂O₃ NWs. The formation of the modulated structure in single-crystalline NWs is attributed to the periodical appearance of stacking faults, because of the shear stress occurred during the growth process. NWs possessing a cubic γ-Fe₂O₃ structure tend to coalesce into the bicrystalline or tricrystalline NWs whereas NWs with the hexagonal α-Fe₂O₃ structure prefer to grow as single-crystalline NWs. The formation mechanism of Fe₂O₃ NWs with the different morphologies is discussed.