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 gamma-Fe2O3 structure tend to coalesce into the bicrystalline or tricrystalline NWs whereas NWs with the hexagonal alpha-Fe2O3 structure prefer to grow as single-crystalline NWs. The formation mechanism of Fe2O3 NWs with the different morphologies is discussed.