Morphology of Silica Aerogels Obtained from the Process Catalyzed by NH₄F and NH₄OH

Abstract

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), N2 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 Dm = 2.3-2.5, while addition of hydroxyl ions results in a sparser structure (Dm = 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 NH 4F.

Surface effects on the degradation mechanism of bioactive PDMS-SiO2-CaO-P2O5 hybrid materials intended for bone regeneration

“…The calculated surface fractal dimensions of the hybrid materials soaked in SBF presented similar D s values, corresponding to sol-gelderived materials [31,32] as shown in Fig. 5.…”

LDH and TiO2/LDH-type nanocomposite systems: A systematic study on structural characteristics

“…The mean size of the clusters n can be defined by 2p/q G. From q p the size of the elementary particles a is 2p/q p. When from a ln-ln plot of I(q)/q versus q can be made and the slope of the line Àa (attenuation index, if 1 < a < 3), D m is equal to a. From the same plot, if 3 < a < 4 the surface fractal dimension D s equals 6 À a. SAXS investigations on this type of nanocomposites are very limited [29,30]. In general, three characteristics can be observed: the mean size of the clusters (n), which can be connected to the form of the network, the mean size of the primary particles (a), which form the cluster, and the fractal dimension (D with D s and D m represented, respectively mass distribution and surface coarseness/structure compactness).…”

Catalytic applications of composite aerogels

“…Alumina-based alcogels were prepared by using the fast sol-gel synthetic route developed by our laboratory [5,6]. Silica-based alcogels were obtained from the accelerated sol-gel process catalyzed by NH 4 OH and NH 4 F [7]. Magnesia-based alcogels were obtained by a controlled sol-gel process using ethylene glycol and acetic acid.…”

“…Silica aerogels were obtained from the accelerated sol-gel process catalyzed by NH 4 OH and NH 4 F [7]. Using NH 4 F drastically accelerated the base-catalyzed gelation of TEOS.…”