Iron oxide–silica aerogel and xerogel nanocomposite materials
Introduction
Maghemite (γ-Fe2O3) is a material of great technological importance for its use in magnetic recording systems and in catalysis whose properties are particularly enhanced when the size of the particles reaches the nanometer range [1], [2], [3], [4]. Nanosized γ-Fe2O3 transforms into α-Fe2O3 (hematite) at rather low temperatures (∼350 °C) [5], [6] but can be stabilized through the incorporation of the nanoparticles into a polymeric, glassy or ceramic matrix [6], [7], [8].
Fe2O3–SiO2 nanocomposite materials can be prepared by a variety of methods among which the sol–gel process offers some interesting features [9]. In fact, the method involves many parameters which can be finely modulated in order to lead the process towards the final results which are desired. Previous investigations on the sol–gel preparation of Fe2O3–SiO2 nanocomposites have shown that maghemite can be stabilized into the amorphous silica matrix up to an oxide content of about 23 wt% [10]. On the other hand, the presence of additional iron oxide phases (α-Fe2O3 and ε-Fe2O3) together with maghemite was observed when the iron oxide content is higher than 23 wt% [10].
The stabilization of maghemite in Fe2O3–SiO2 nanocomposite materials may be related to the porous structure of the host matrix which is influenced by many sol–gel parameters among which a key role is played by the drying step. In fact, when the solvent is removed from the alcogel by usual heating, the capillary forces at the liquid/vapor interface produce shrinkage and cracking so that the original porous structure is lost and dense xerogels are obtained. On the other hand, when the solvent is removed above its critical parameters, Tc and Pc, aerogels with high surface areas and pore volumes are obtained since the skeletal alcogel structure can be preserved [9].
In this paper, we present a study of the influence of different drying procedures on the structural and morphological properties of iron oxide–silica nanocomposite materials prepared by the sol–gel method using TEOS and iron nitrate as precursors. To this end thermal analysis, N2-physisorption, X-ray diffraction (XRD) and transmission electron microscopy (TEM) techniques were used.
Section snippets
Experimental
Iron oxide–silica gels were prepared using iron nitrate nonahydrate (Aldrich, 98%) and tetraethoxysilicate (TEOS, Aldrich, 98%) as metal and silica precursors, respectively and ethanol (Carlo Erba, 95%) as mutual solvent following the procedure reported in [10]. The starting sol was prepared mixing an alcoholic solution of TEOS to an alcoholic solution of the iron salt. The molar ratio of TEOS/EtOH was equal to 18. After vigorous stirring for 1 h, the sol was poured into a teflon vessel and
Results
In Fig. 1(a) the TGA curves and in Fig. 1(b) the corresponding DTA curves are reported for the fresh Fe23 alcogel and for the nanocomposite aerogels A1-Fe23 and A2-Fe23. Very similar results were obtained for the Fe33 alcogel and aerogel samples.
The fresh alcogel shows a remarkable weight loss up to 200 °C which corresponds to a broad endothermic peak in the DTA curve. The weight loss is due to solvent removal and to decomposition of iron nitrate and of organics. For higher temperatures the
Discussion
The thermal analysis results show that most of the solvent is removed during supercritical extraction. A2 method which uses 99% EtOH to fill the autoclave is more effective in removing the solvent since the weight loss at low temperature of A2 samples is lower than A1 ones, as already observed in NiO–SiO2 aerogels [19]. The exothermic peaks between 200 °C and 500 °C observed in the DTA curves of the aerogel samples are ascribed to the combustion of organics which mainly come from the
Conclusion
The different drying conditions give rise to a variation in the porous structure of the final nanocomposites. Xerogels, in which the original pore structure of the alcogels is lost, are obtained by slow heating, while aerogels with high surface areas and pore volumes are obtained when the drying step is performed under supercritical conditions in an autoclave. Depending on the supercritical drying conditions, aerogels with a different porous structure can be obtained.
The stabilization of
Acknowledgements
Authors would like to thank MURST for financial support.
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