Near- and supercritical water is an attractive medium for the synthesis of metal oxide nanoparticles. The continuous hydrothermal synthesis is based on the mixing of a cold metal salt stream with hot compressed water. Here, it is exploited that the properties of water –in particular the permittivity– strongly change around the critical point (p = 22,1 MPa, T = 647,1 K). Mass transport related characteristics are similar to those of gases, whereas the density and the dissolving power are in the range of fluids. Thus, the formation of oxide nanoparticle dispersions without the addition of any additives is enabled.
Iron oxide nanoparticles were produced with this method by varying the mixing ratio, the flow rate and the concentration of the reactant. The size-dependent physical and chemical properties are of great importance for technical applications. Structure and properties of iron oxide nanoparticles strongly depend on the particle size, their history of formation and the surrounding medium.
The crystal structure and the mean crystallite size of the synthesised particles were investigated by powder x-ray diffraction (PXRD). A Bragg-Brentano goniometer D8 (Bruker AXS) with Cu-radiation and scintillation counter was used. The system was additionally equipped either with a secondary monochromator or two symmetrically arranged Göbel-mirrors (parallel beam geometry). The identification of the obtained material was carried out with the database Powder Diffraction Files (PDF) of the International Centre for Diffraction Data (ICDD); the evaluation of the crystallite size with the software tool TOPAS (Bruker AXS).
Particles synthesised from aqueous iron nitrate solutions could be identified as alpha-Fe2O3 (hematite) with crystallite sizes of about 35 nm. Increasing the flow rate leads to the formation of particles of less than 20 nm (confirmed by TEM-images). These samples show no or weak reflections at the positions for hematite. However, the determination of the crystallite size failed.
Magnetite (Fe3O4) was synthesised from aqueous iron acetate solutions. The crystal structure was clearly identified by PXRD with a mean crystallite size of 48 nm. TEM-images show the existence of two particle fractions, one of 50 nm – 200 nm and the other with particles below 10 nm. Stable dispersions were produced by adding nitric acid to the iron acetate solutions. Particles of about 10 nm in diameter with a narrow particle size distribution were obtained (confirmed by TEM). Again, a clear determination of the structure was not possible.
[1] M. Daschner de Tercero, J. Schubert, U. Fehrenbacher, M. Türk, U. Teipel, Kontinuierliche Herstellung von Eisenoxidnanopartikeln in überkritischem Wasser, 4. Symposium Produktgestaltung in der Partikeltechnologie, Pfinztal, 2008.
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