How to tailor maghemite particle size in gamma-Fe2O3-SiO2 nanocomposites

A sol gel procedure for the tailoring of maghemite particle size in silica-based nanocomposites is proposed. The preparation method allows control of the gelation time, which is varied from 2 to 16 days in samples having the same concentration of iron oxide (25%). Superparamagnetic particles of gamma-Fe2O3 with sizes in the 2.5-5.6 nm range were found in all the samples, as confirmed by TEM, Mossbauer spectroscopy and magnetic measurements. The particle size was independent of the porosity of the silica host matrix, but strongly dependent on the amount of solvent trapped inside the gels. The solvent plays an important role, favouring the formation of Fe3O4 nanoparticles as an intermediate step before the final oxidation to maghemite.