Formation of gamma-Fe2O3 isolated nanoparticles in a silica matrix

Isolated nanometric particles (D < 30 nm) of gamma-Fe2O3 in a silica matrix have been prepared by heating at 400 degrees C the gel formed in the hydrolysis of an ethanol solution of Fe(NO3)(3) . 9H(2)O and tetraethylorthosilicate (TEOS). However, when FeCl3 . 6H(2)O 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 gamma-Fe2O3 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 gamma-Fe2O3 nanoparticles in the silica network has been studied as well as the optimum conditions for their preparation. Thus, gamma-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 gamma-Fe2O3 nanocomposites. Similar size effects are also found via the coercivity values at 70 and 5 K.