| Abstract
| - The morphology of gels prepared from (MeO)3Si−X−Ti(OiPr)3 (X = organic spacer) was investigated. Small primary particles (r = 0.5 ± 0.1 nm) are initially formed, which retain their size throughout the gelation and aging process. Condensation proceeds by a slower aggregation of the particles. Additional Si/O from Si(OEt)4 is incorporated between the particles.
- The morphology of gels prepared from a silica/titania single-source precursor (1), obtained by reactionof 3-oxoethyl-6-trimethoxysilyl-hexan-2-one with Ti(OiPr)4, was investigated by small-angle X-rayscattering (SAXS) through all stages of the preparation process, that is, gelation, aging, drying, andcalcination. The same investigations were performed for mixtures of 1 and Si(OEt)4. Immediately afterthe start of the reaction small primary particles are formed, the size of which (r = 0.5 ± 0.1 nm) remainsconstant through the gelation and aging process. Anomalous small-angle X-ray scattering (ASAXS)measurements strongly indicate that the primary particles are formed by hydrolysis and condensation ofthe titanium alkoxide moiety of 1. Condensation proceeds by a slower aggregation of the primary particles.Additional Si/O from Si(OEt)4 is incorporated between the clusters. The size of the mass fractal secondaryparticles formed by aggregation of the primary particles increases approximately exponentially with thetime. Drying by either solvent evaporation or supercritical extraction with CO2 results in a decrease ofthe primary particle size to r = 0.4 nm, and the secondary particles also become smaller. The primaryparticles disappear during calcination in air, that is, when the organic groups tethering the silicon andtitanium atoms are destroyed. However, the general structure of the network is maintained.
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