THE PREPARATION AND CHARACTERIZATION OF SILICA AEROGELS AND XEROGELS DOPED WITH TRANSITION-METAL SPECIES

Silica alcogels, aerogels and xerogels doped with some transition metals (e.g. M(C2H5OH)6-x(H2O)x, M = Ni2+, Cu2+, Co2+; Cu(NH3)6-x(H2O)x and Cu(C5H5N)6-x(H2O)x) have been prepared and examined by X-ray diffraction, transmission electron microscopy, nitrogen adsorption and electron spin resonance (ESR). The different doping techniques of 'polymerisation' and 'impregnation' give no differences in the properties of the resultant gels. ESR spectroscopy shows that, in the silica alcogels doped with [Cu(C2H5OH)6-x(H2O)x]2+ complexes, the complex resides in the gel pores in solution, whereas in the gels doped with [Cu(NH3)6-x(H2O)x]2+ and [Cu(C5H5N)6-x(H2O)x]2+, the complexes are immobilised on the gel pore surface. ESR also shows that xerogels derived from all these types of alcogel contain Cu2+ ions in a similar six-coordinate oxygen environment. The final morphologies of the doped xerogels depend upon the dopant used. The morphologies and surface areas of silica aerogels are significantly altered on doping. New particles appear in the Ni2+ and Co2+-doped aerogels for doping concentrations greater than M/Si = 1/100 (M = Ni2+, Co2+).