Formation of modulated structures induced by oxygen vacancies in α-Fe2O3 nanowires

Two modulated structures caused by long-range ordering of oxygen vacancies have been observed in α-Fe2O3 nanowires (NWs) produced after oxidation of Fe, one being ten times (303¯0) interplanar spacing and the other being six times (112¯0) interplanar spacing. Both types of oxygen vacancy ordering structures have a similar modulation periodicity of 1.45 or 1.50 nm with corresponding atomic ratios of Fe and O (Fe/O) of 0.7407 and 0.7273, respectively. The Fe/O ratios in the NWs with oxygen-vacancy ordering are very close to that of Fe3O4 (0.7500). The similar Fe/O ratio between NWs and Fe3O4 may explain the similar modulation periodicity of different oxygen-vacancy orderings. Electron energy-loss spectroscopy studies show that the Fe/O ratio of NWs is close to that of Fe3O4 when oxygen atoms are not sufficient, which makes the NWs energetically favorable. The elucidation of the mechanism governing the formation of the modulated structures in α-Fe2O3 NWs is critical for controlling the microstructure and correspondingly physicochemical properties of NWs.