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 r ange 600 to 1000 degrees C. The changes occurring during these treatments a re followed by X-ray diffraction, transmission electron microscopy, Mossbau er spectrometry and static magnetization measurements. During these treatments, nickel is nearly totally reduced to metal particle s, the mean size of which increases from 3 to 10 nm with increasing tempera ture, but does not vary appreciably with nickel content. These particles ex hibit a classical superparamagnetic behavior and are stable towards reoxida tion 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 te mperature (1000 degrees C and above). Secondly, the particles formed are mu ch less stable toward reoxidation, especially in the case of iron. Accordin gly, the evolution of the magnetic behavior is much more complex than in th e case of nickel.