Topotaxial growth of α-Fe₂O₃ nanowires on iron substrate in thermal annealing method

A detail cross-sectional transmission electron microscopy of as-grown α-Fe₂O₃ nanowire sample, synthesized on iron substrate by thermal annealing method, was carried out to understand the mechanism of growth in this system. Iron undergoes sequential oxidation to form a layered structure of Fe/FeO/Fe₃O₄/α-Fe₂O₃. α-Fe₂O₃ nanowires grow on to the top of α-Fe₂O₃ layer. It was found that subsequent oxide layers grow topotaxially on the grains of iron, which results in a direct orientation relationship between the α-Fe₂O₃ nanowire and the parent grain of iron. The results also showed that the grains of α-Fe₂O₃ layer, which were uniquely oriented in [110] direction, undergo highly anisotropic growth to form the nanowire. This anisotropic growth occurs at a twin interface, given by (−11−1), in the α-Fe₂O₃ layer. It was concluded that the growth at twin interface could be the main driving factor for such anisotropic growth. These observations are not only helpful in understanding the growth mechanism of α-Fe₂O₃ nanowires, but it also demonstrates a way of patterning the nanowires by controlling the texture of iron substrate.