Morphology of silica aerogels obtained from the process catalyzed by NH4F and NH4OH

Twelve samples of silica aerogels were synthesized using different concentrations of catalysts (NH4F, NH4OH), and their morphology was investigated by means of small-angle X-ray scattering (SAXS), N-2 adsorption, and transmission electron microscopy. Both SAXS and the nitrogen adsorption method consistently show a different influence of the catalysts; fluoride anions promote formation of a tenuous branched-polymeric structure, with mass fractal dimension D-m similar or equal to 2.3-2.5, while addition of hydroxyl ions results in a sparser structure (D-m similar or equal to 2.1-2.2) self-similar over a narrower range. The morphology of aerogels obtained from the process catalyzed by fluoride hydroxide resembles that of aerogels produced by the two-step method. This may suggest similar mechanisms of gel formation in both processes, and,consequently, a dramatic acceleration of the hydrolysis reaction caused by NH4F.