Understanding ordered structure in hematite nanowhiskers synthesized via thermal oxidation of iron-based substrates

Hematite (α-Fe₂O₃) nanowhiskers (NWs) with (001) basal faces synthesized via thermal oxidation of iron-based substrates are known to contain an ordered structure. The ordered structure has been identified to be related to oxygen vacancy ordering. However, the cause of its formation remains a mystery. In this study, with a high-resolution transmission electron microscopy (HR-TEM) investigation based on negative-Cs imaging (NCSI) and atomic-column position analysis, we observed tensile strain in the above-mentioned α-Fe₂O₃ NWs and revealed that the ordered structure was actually periodic interplanar gap expansions induced by oxygen vacancy accumulations. These findings were further confirmed in a monochromated electron energy loss spectroscopy (EELS) analysis of the α-Fe₂O₃ NWs. The EELS data indicated that, in comparison to pristine α-Fe₂O₃, the α-Fe₂O₃ NWs possessed expanded average Fesingle bondO and Osingle bondO interatomic distances and were oxygen-deficient. Clarifying oxygen deficiency in the α-Fe₂O₃ NWs was not attributed to an insufficient oxygen supply during the NW growth, we concluded the ordered structure formed to accommodate tensile strain in the α-Fe₂O₃ NWs. This work demonstrates the applicability of integrating NCSI and monochromated EELS for the examination of strain-induced microstructural and microchemical variations in lightly strained metal oxides