Clean preparation of nanoparticulate metals in porous supports: a supercritical route

Here we present the synthesis of nanometre sized silver particles which have been trapped within porous substrates; poly( styrene-divinylbenzene) beads and silica aerogels. This is the first time that supercritical carbon dioxide has been used to impregnate such porous materials with silver coordination complexes. In this paper we demonstrate that control over the resultant nanoparticles with respect to size, loading and distribution in the support material has been achieved by simple choice of the precursor complex. The solubility of the precursor complexes in the supercritical solvent is shown to be one of the key parameters in determining the size of the nanoparticles, their distribution and their homogeneity within the support matrix. Moreover, we demonstrate that the same methodology can be applied to two very different substrate materials. In the particular case of aerogels, conventional organic solvents could not be used to prepare nanoparticles because the surface tension of the solvent would lead to fracturing of the aerogel structure. Controlled decomposition of the coordination complexes in situ leads to metallic silver nanoparticles with a narrow size distribution, typically 10-100 nm that are homogeneously dispersed throughout the porous substrate. The whole process is carried out at near ambient temperature and no solvent residues are introduced into the porous media. The silver precursors are specifically designed to be both CO2 soluble and sufficiently labile to ensure facile decomposition to the metal. In-depth characterisation by X-ray diffraction and transmission electron microscopy has been applied to illustrate the homogeneous dispersion of particles throughout the composite material, determine the range and variation in particle size within the solid matrices and fully identify the resultant particles as metallic silver. This enables visualisation of dispersion and concentration, and control over particle size of the fabricated nanocomposite materials.