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

Abstract

Iron oxide nano particles with nominal Fe 2 O 3 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 t 2g /e g peak height ratio of the two leading pre-edge resonances, confirming oxidation of the Fe from Fe 2+ towards Fe 3+ . Transmission electron microscopy (TEM) on the samples treated at temperatures as high as 900°Cshowed 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.

On the improvement of PEC activity of hematite thin films deposited by high-power pulsed magnetron sputtering method

“…Firstly, the crystallinity of hematite is enhanced, meaning that more organized nanocrystals providing boosted charge transfer among the grain boundaries are developed. This assumption is supported by the SEM and AFM investigations and it is also in good agreement with other previously reported studies [4,31,32]. Secondly, less undesirable surface states are generated and these are further sufficiently passivate in a high extent by the ALD coatings.…”

Analysis of fixed bed data for the extraction of a rate mechanism for the reaction of hematite with methane

“…High temperature promotes grain growth with loss of what little surface reactant there is for the carrier [28][29][30]. Callister [28] presents a plot showing the general effect of grains growing with time and temperature where grains can grow in diameter by two orders of magnitude when heated from 550 8C to 850 8C for 10 min -times and conditions comparable to this work.…”

Analysis of hematite re-oxidation in the chemical looping process

“…In other words, increasing temperature decreases the surface reaction contribution. This is attributed to grain growth as observed elsewhere [27][28][29][33][34][35]. The combined functionality of w c can be expressed as…”

“…Morphological changes to the particle during the reaction often have this effect [27][28][29]34]. Since it has been well documented that particle grain grows when heated [33][34][35] and that as the grains grow, the surface area will decrease, Eq. (6) was modified to account for the changes in surface area, S A ln a 1=n…”