Formation of γ-Fe₂O₃ Isolated Nanoparticles in a Silica Matrix

Abstract

Isolated nanometric particles (D < 30 nm) of γ-Fe2O3 in a silica matrix have been prepared by heating at 400 °C the gel formed in the hydrolysis of an ethanol solution of Fe(NO3)3‚9H2O and tetraethylorthosilicate (TEOS). However, when FeCl3‚6H2O was used as precursor, well-developed hematite particles were obtained in the final composite. This different behavior was already manifest in the initial gels. Thus, the gel obtained from iron nitrate salt shows a compact appearance as a result of its higher degree of network connectivity (polymeric gel) whereas the one from the iron chloride appears more loose and highly hygroscopic (colloidal gel). In addition, small superparamagnetic nuclei are formed during the hydrolysis and condensation of the gel obtained from the iron nitrate salt. The γ-Fe 2O3 nanoparticle formation takes place through a reduction-oxidation reaction which occurs during the burning of the organic species trapped inside the gel pore. The growth mechanism of the γ-Fe2O3 nanoparticles in the silica network has been studied as well as the optimum conditions for their preparation. Thus, γ-Fe2O3 nanocomposites with different particle sizes and distributions can be prepared by adequate modification of the initial gel microstructure through different gelation times, salt concentrations, and mechanical treatment. Superparamagnetic behavior has been found in all nanocomposites at room temperature, meanwhile at 70 K, a transition from superparamagnetic to ferrimagnetic behavior is observed as the particle size increases. In all cases, the variation in particle size observed by X-ray diffraction corresponds well with changes in the saturation magnetization for the γ-Fe 2O3 nanocomposites. Similar size effects are also found via the coercivity values at 70 and 5 K.

Formation of Iron-Containing Clusters in Silica of Predetermined Porosity

“…However, in contrast to other studies, in this work, the colloidal TEOS solution was used in which polymer-like chains O-Si-O-Si have been already formed. For this reason, the final composites could differ from those observed earlier [6] and various iron oxides could be formed. Figure 1 illustrates the magnetic properties of the bulk samples annealed in Ar.…”

“…When precursor with FeCl 3 was used, β-FeOOH was usually formed which transformed to α-Fe 2 O 3 after heat treatment [6]. However, in contrast to other studies, in this work, the colloidal TEOS solution was used in which polymer-like chains O-Si-O-Si have been already formed.…”

Mössbauer Investigation of γ-Fe2O3 Nanocrystals in Silica Matrix Prepared by the Sol-gel Method

“…Moreover, a growing attention has recently been devoted to maghemite nanoparticles dispersed in a transparent matrix for their use in magneto-optical recording systems [7,9,10]. Among the various preparation procedures which can be used to obtain Fe 2 O 3 -SiO 2 nanocomposites, the sol-gel method has proven to offer some interesting features that allow controlling the final properties of the material [11,12]. Several preparation methods have been tested, and some of them have proven to fulfill the requirement of producing selectively the -Fe 2 O 3 phase [7,12].…”

“…Among the various preparation procedures which can be used to obtain Fe 2 O 3 -SiO 2 nanocomposites, the sol-gel method has proven to offer some interesting features that allow controlling the final properties of the material [11,12]. Several preparation methods have been tested, and some of them have proven to fulfill the requirement of producing selectively the -Fe 2 O 3 phase [7,12]. It is difficult, however, to find a way of tailoring the particle size and regulating the connected physical properties.…”

“…Therefore we decided to prepare the Fe 2 O 3 / SiO 2 nanocomposites by a sol-gel method similar to the one successfully used by del Monte et al [12], but varying the evaporation conditions (and as a consequence the gelation time) through a control of the surface / volume ratio of the sol ( ). X-ray diffraction (XRD) and transmission electron microscopy (TEM) analyses of this series of samples indicated that in this way a continuous variation of the particles size can be realised, a progressive decrease of particle size with increasing being observed.…”