Structural characterisation of Fe2O3 nanoparticles

The structure of nano-crystalline Fe2O3 particles, synthesized using the microwave plasma technique, has been analysed using synchrotron based X-ray absorption spectroscopy and X-ray powder diffraction, as well as transmission electron microscopy. Furthermore, magnetic properties, the crystal structure, and the microstructures are compared and the potential model character of the samples for structure simulations is discussed.


Introduction
There is growing interest in the application of monodisperse magnetic nanoparticles in magnetic suspensions, ferrofluids, or for drug delivery. In general, magnetic properties of nanoparticles are observed to be size dependent, independent from the method of synthesis [1]. However, our studies with microwave plasma synthesized Fe 2 O 3 nanoparticles [2] show that the magnetic properties do not always correlate with the particle sizes. Although transmission electron microscopy studies reveal quite similar particle sizes of 5 nm, the saturation magnetization differs significantly (see [3]). The reason for this discrepancy is not yet fully clear. We believe that it is intimately related to the atomic structure of the nanoparticles coupled with possible structural disorder induced by the particle size. In addition to the antiparallel coupling of the spin and the orbital moment in the maghemite structure which has been observed in Fe 2 O 3 /ZrO 2 core/shell particles earlier [4], competing magnetic interactions of disordered crystal sub-lattices, strain as well as surface effects may lead to frustration and disorder of the spin lattices. It is the aim of this paper to study the discrepancy between observed magnetization and particle size in more depth.

Results and discussion
We have determined the magnetic properties and performed an accurate structural and microstructural investigation of two nanocrystalline, microwave plasma synthesized Fe 2 O 3 samples with similar particle size deduced by TEM and one commercial nano-Fe 2 O 3 as a reference. Special emphasis is laid on X-ray absorption fine structure spectroscopy as well as high resolution X-ray powder diffraction.

Magnetic properties and transmission electron microscopy
The magnetic moment  as a function of applied magnetic field  has been determined with a SQUID magnetometer at room temperature in the range from 1 T to -1 T. The investigated Fe 2 O 3 nanoparticles are superparamagnetic and are showing different saturation magnetization. The commercial material in contrast exhibit hysteresis and a significantly higher saturation magnetization (see figure 1). A typical TEM image from microwaveplasma synthesized Fe 2 O 3 is displayed in figure 2.

X-ray powder diffraction
X-ray powder diffraction measurements on samples I and II and on a -Fe 2 O 3 reference sample were performed at room temperature at the ID31 beamline of the ESRF (now ID22). The samples were investigated at =0.400 Å covering a wave-vector Q region up to Q max ~ 25.7 Å -1 . The experimental data have been analysed using: i) the Williamson-Hall (W-H) method [5] in order to establish the crystallite size and strain, ii) the Rietveld method to determine the average crystal structure and iii) the total pair distribution function G(r) to unveil the possible structural disorder. The last one is obtained by Fourier transforming the total scattering functions S(Q) which, in turn, is obtained by the experimental data after suitable corrections and normalizations [6].
Selected results of the Rietveld analysis are displayed in table 1. The most abundant phase in all the investigated -Fe 2 O 3 samples is maghemite with a secondary hematite phase. The -Fe 2 O 3 reference sample had to be refined in a tetragonal space group with a tripling of the c-axis because of ordering of   iron vacancies. Figure 3 (left) shows the G(r) data points on the two samples and reference sample in a large interatomic distance range. The damping of the G(r) functions is related to the finite dimension of the nanoparticles and can be mapped by a spherical envelope function which supplies alternative values of the crystallite dimension (see numbers in Figure 3, left). The damping of G(r) is related to the average volume-weighted diameters (D V ) of the different sample crystallite sizes. On the right are depicted the same G(r) curves. The widths of G(r) peaks pertinent to the nearest-neighbors Fe-O distances at about 2 Å look quite similar for all the samples; conversely, for greater distances a broadening of the G(r) peaks is apparent for samples I and II, as compared to the reference sample. See e.g. the Fe-Fe distances between 3 and 4 Å. This suggests a progressive disordering of interatomic distances, by decreasing the crystallite diameter, i.e. increasing the surface/volume ratio. This finding is in agreement with the high strain parameters detected for samples I and II by the W-H analysis compared to the -Fe 2 O 3 reference sample (see table 1).

X-ray absorption spectroscopy
XAFS spectra have been collected at the Fe-K edge in transmission mode. Experiments were performed at the SUL-X beamline at ANKA. This beamline delivers a focused, monochromatic beam by using a wiggler source, focusing mirrors and a cryogenically cooled Si(111) double crystal monochromator. The data analysis followed a standard procedure using the code WINXAS. For the XAFS analysis, a k-range of 2.4 -14.5 Å -1 was used for calculating the Fourier transform. The near edge structure and the Fourier transform of the XAFS are displayed in figure 4 and 5, respectively.
The XANES region with the pre edge peak assigned to a 1s to 3d transition provides valuable information on the relative occupation of tetrahedral and octahedral sites [7]. In particular, the decrease of the pre-edge peak intensity of the nanomaterials with respect to the reference sample suggests a smaller Fe tetra /Fe oct ratio.     XAFS analysis using (k) k 3 weighting with FEFF theoretical functions [8] have been calculated for the first Fe-O shell and two next nearest Fe-Fe shells using structural data from Shmakov et al. [9] and Petkov et al. [10].
Higher Fe-O coordination shells and multiple scattering paths are not included. The bond lengths R i and coordination numbers CN i for the coordination shells are summarized in table 2.

Conclusions
This investigation reveals major structural characteristics of the synthesized samples. The samples are predominantly maghemite but with considerable fractions of hematite. Lattice parameters are comparable to the bulk; the coordination of Fe ions at the surface appears to be distorted. The site occupation ratio Fe tetra /Fe oct is smaller than for bulk maghemite which is also corroborated by the longer average bond length for the first Fe-O shell determined by XAFS. Although particle sizes seem similar by TEM investigations, the crystallite sizes are significantly different. At decreasing the particle size, disorder increases both allowing atomic relaxations and enhancing the ratio of tetrahedral to octahedral coordinated iron. These differences may explain the differences in magnetic properties.
Still many unanswered questions further remain, i.e. the structural differences of surface and bulk of nanoparticles are to be analysed and discussed in greater detail, whether the surface contains glassy states which would explain magnetic properties, and how particles interact with each other. We hope to reveal such issues by comparing single particle structure simulations with our experimental data. Table 2. Radial distances R i (±0.01 Å) and coordination numbers CN i (error ±10%) determined from the XAFS data (the many body amplitude reduction factor S 0 2 fixed at 0.9). The Debye-Waller factors for the next nearest Fe shells  2 2 and  3 2 have been kept fixed at 0.01 Å 2 during refinement of the three shells.