Metal complexes in inorganic matrices. 7. nanometer-sized, uniform metal particles in a SiO2 matrix by sol-gel processing of metal complexes
Materials containing uniform, nanometer-sized metal particles that are homogeneously dispersed in an electrically isolating SiO2 matrix are prepared by a three-step prodecure: In the first step an organically substituted alkoxysilane of the type X(CH2)3Si(OR)3 (X=NH2, NHCH2CH2NH2, CN) capable of coordinating metal ions, a metal salt with an organic counterion and, optionally, Si(OR)4 are processed by the sol-gel method. In the second step all organic parts are removed by calcination in air, during which very pure and nanometer-sized metal oxide particles develop. Complexation of the metal ions and anchoring the obtained complexes to the silicate matrix in the first step are the keys to obtain a high dispersion of the metal. The small size of the particles is maintained during the final step, in which the metal oxide particles are reduced by hydrogen. Compared with other methods to prepare metal/ceramic nanocomposites, this modification of the sol-gel route provides smaller and more uni form and homogeneously distributed metal particles with a narrower size distribution in an SiO2 matrix. The metal loading can be varied to a high degree, and even with high metal loading, the metal particles are still small, uniform in size, and well separated from each other. The method is applicable to most transition metals, and the present reuslts show that the advantages of this method can be generalized.