Metal (Fe, Co, Ni) nanoparticles in silica gels:: Preparation and magnetic properties

Metal containing silica gels with an Me/Si molar ratio between 0.01 and 0.2 are submitted to thermal treatment under hydrogen within the temperature range 600 to 1000 degrees C. The changes occurring during these treatments are followed by X-ray diffraction, transmission electron microscopy, Mossbauer spectrometry and static magnetization measurements. During these treatments, nickel is nearly totally reduced to metal particles, the mean size of which increases from 3 to 10 nm with increasing temperature, but does not vary appreciably with nickel content. These particles exhibit a classical superparamagnetic behavior and are stable towards reoxidation when reexposed to air at room temperature. Iron and cobalt behave in a notably different manner. First, in both cases, silicates may be formed during the thermal treatment and consequently the reduction to the metallic state is not complete unless it occurs at high temperature (1000 degrees C and above). Secondly, the particles formed are much less stable toward reoxidation, especially in the case of iron. Accordingly, the evolution of the magnetic behavior is much more complex than in the case of nickel.