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

Abstract

Iron oxide nano particles with nominal Fe2O3 stoichiometry were synthesized
by a wet, soft chemical method with the heat treatment temperatures from
250{\deg}C to 900{\deg}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{\deg}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{\deg}C showed that the
nano particles have well developed lattice fringes corresponding to that of
(110) plane of hematite.