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.
Electrochemical oxidation of hematite ({alpha}-Fe2O3) nano-particulate films at 600 mV vs. Ag+/AgCl reference in KOH electrolyte forms a species at the hematite surface which causes a new transition in the upper Hubbard band between the Fe(3d)-O(2p)
state region and the Fe(4sp)-O(2p) region, as evidenced by oxygen near edge x-ray absorption fine structure (NEXAFS) spectra. The electrochemical origin of this transition suggests that it is related with a surface state. This transition, not known for pristine {alpha}-Fe2O3 is at about the same x-ray energy, where pristine 1% Si doped Si:Fe2O3 has such transition. Occurrence of this state coincides with the onset of an oxidative dark current wave at around 535 mV - a potential range, where the tunneling exchange current has been previously reported to increase by three orders of magnitude with the valence band and the transfer coefficient by a factor of 10. Oxidation to only 200 mV does not form such extra NEXAFS feature, supporting that a critical electrochemical potential between 200 and 600 mV is necessary to change the electronic structure of the iron oxide at the surface. Decrease of the surface roughness, as suggested by visual inspection, profilometry and x-ray reflectivity, points to faceting as potential structural origin of the surface state.
