Study of the Phase Composition of Fe2O3 Nanoparticles

The changes of the phase composition of iron oxide samples prepared by solgel method using single precursor both for nanoparticles and the matrix were studied by x-ray diffraction. Obtained data were analyzed by an approach using the Debye formula which is suitable for the size of particles up to about 10 nm. The phase composition of the nanoparticles was described by a core-shell model corresponding to the assumed inward movement of the phase interface between two phases. First results of the phase composition of Fe2O3 nanoparticles have already been obtained. Introduction Important physical properties of nanoparticles are determined mainly by their atomic structure, especially by their phase composition and the presence of structure defects. X-ray diffraction is a good tool for studying the structure of the nanoparticles, its application for very small particles is however limited by very small intensity of the scattered wave. For this reason special experimental setups, like e.g. diffraction with small incidence angle, are used and many experiments have to be done at synchrotrons. Standard methods of the measured data analysis based on the description of the diffraction using instrumental functions and functions of physical broadening of the lines fail in the case of very small particles. An ab-initio calculation method (based on the Debye formula [Cervellino et al., 2004; Cervellino et al., 2003]) has to be used instead. In this work the Debye formula is used for the description of the diffraction of iron oxide samples measured at ANKA synchrotron in Karlsruhe. Using this approach we determine basic parameters of the particles such as lattice parameters and the size of the particles, as well as the presence of different phases. During annealing, subsequent phase transitions from γ-Fe2O3 to ε-Fe2O3 and to α-Fe2O3 take place. New phases nucleate probably at the surface of the nanoparticles and the phase transformation proceeds towards the particle center ([Woo et al., 2008; Gich et al., 2006]), so that the structure of the nanoparticles can be described by a core-shell model; this model was used in the Debye-formula based simulation. From the analysis of the experimental data we determined the kinetic parameters of the phase transitions and their dependence on the nanoparticle sizes. Measured samples The great interest in Fe2O3 nanoparticles is caused mainly by magnetic properties of these particles, namely extremely high room temperature coercivity of epsilon phase of these iron oxide nanoparticles. The samples were prepared by ex-situ annealing of organic precursors and then measured at ANKA synchrotron in Karlsruhe with incidence angle 5o and the wavelength of 0.95007 Å. The primary beam was monochromatized by a 2x111Si monochromator, the diffracted radiation was measured by a point detector equipped with a narrow entrance slit and a filter suppressing the Fefluorescence. The series of samples was prepared with the final annealing temperature from 900 oC to 1150 oC with the step of 50 oC. To the temperature of 900 oC all the samples were heated at the speed 1 oC per minute and stayed at this temperature for 4 hours. As for the sample with the final temperature 900 oC this was the whole procedure. The other samples were then with the same speed heated to their final annealing temperature with the 4-hour waiting each 50 oC up to their final heating temperature. This procedure causes the creation of Fe2O3 nanoparticles in the amorphous SiO2 matrix. From the literature [Brázda et al., 2009] it is known that the particles created at the lowest final temperature should be in the form of maghemite and with increasing final temperature the phase of Fe2O3 particles should change to ε and hematite. 208 WDS'09 Proceedings of Contributed Papers, Part III, 208–212, 2009. ISBN 978-80-7378-103-3 © MATFYZPRESS