Journal of Sol-Gel Science and Technology 13, 929–932 (1998) c  1998 Kluwer Academic Publishers. Manufactured in The Netherlands. Metal (Fe, Co, Ni) Nanoparticles in Silica Gels: Preparation and Magnetic Properties T. LUTZ, C. ESTOURN ` ES AND J.L. GUILLE I.P.C.M.S. Groupe des Mat´ eriaux Inorganiques, UMR 46 CNRS, ULP, ECPM, 23, rue du Loess B.P. 20/CR, 67037, Strasbourg Cedex, France Abstract. Metal containing silica gels with an Me/Si molar ratio between 0.01 and 0.2 are submitted to ther- mal treatment under hydrogen within the temperature range 600 to 1000 ◦ C. The changes occurring during these treatments are followed by X-ray diffraction, transmission electron microscopy, M¨ ossbauer 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 ◦ 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. Keywords: metal, nanoparticles, magnetism, silica, gel 1. Introduction Nanosized particles of ferromagnetic metals have been widely studied as they present an interest both for fundamental physics and potential applications such as catalysis and magnetic recording. Various chemi- cal methods have been used to produce these particles [1–6]. The most often used is the reduction of a metal oxide, salt or complex dispersed in a supporting mate- rial. The case where the matrix is porous, typically a silica gel, has been the subject of various studies. The average particle size as well as the particle size distribu- tion are strongly dependent on various parameters such as composition of the initial mixture, concentration of the reducible species, thermal treatment preceding the reduction and reduction treatment itself. The purpose of the present study is to investigate the phase con- stitution and the magnetic properties of composites obtained in similar conditions from silica gels con- taining nickel, iron or cobalt, respectively. These gels were submitted to various thermal treatments and the changes occurring during these treatments were fol- lowed by X-ray diffraction, M¨ ossbauer spectrometry, transmission electron microscopy (TEM) and static magnetic measurements. 2. Experimental 2.1. Sample Preparation Samples were prepared from a mixture of tetraethyl- orthosilicate, formamide, ethanol and 0.1 N nitric acid aqueous solution, the molar ratios being 1 : 1 : 4 : 4.5. The metal was added in the form of nitrate in order to obtain molar Me/Si ratios ranging from 0.01 to 0.20.