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Evolution of structural properties of iron oxide nano particles during temperature treatment from 250°C - 900°C: X-ray diffraction and Fe K-shell pre-edge X-ray absorption study

Iron oxide nano particles with nominal Fe2O3 stoichiometry were synthesized by a wet, soft chemical method with the heat treatment temperatures from 250°C to 900°C in air. The variation in the structural properties of the nano particles with the heat treatment temperature was studied by X-ray diffraction and Fe K shell X-ray absorption study. X-ray diffractograms show that at lower annealing temperatures nano particle comprises both maghemite and hematite phases. With increasing temperature, the remainder of the maghemite phase transformed completely to hematite. Profile analysis of the leading Bragg reflections reveals that the average crystallite size increases from 50 nm to 150 nm with increasing temperature. The mean primary particle size decreased from 105 nm to 90 nm with increasing heat treatment temperature. The X-ray diffraction results are paralleled by systematic changes in the pre-edge structure of the Fe K-edge X-ray absorption spectra, in particular by a gradual decrease of the t2g/eg peak height ratio of the two leading pre-edge resonances, confirming oxidation of the Fe from Fe2+ towards Fe3+. Transmission electron microscopy (TEM) on the samples treated at temperatures as high as 900°C showed particles with prismatic morphology along with the formation of stacking fault like defects. High resolution TEM with selected area electron diffraction (SAED) of samples heat treated above 350°C showed that the nano particles have well developed lattice fringes corresponding to that of (110) plane of hematite.