Mixed silica Titania materials prepared from a single-source sol-gel precursor:: A time-resolved SAXS study of the gelation, aging, supercritical drying, and calcination processes

The morphology of gels prepared from a silica/titania single-source precursor (1), obtained by reaction of 3-oxoethyl-6-trimethoxysilyl-hexan-2-one with Ti((OPr)-Pr-i)(4), was investigated by small-angle X-ray scattering (SAXS) through all stages of the preparation process, that is, gelation, aging, drying, and calcination. The same investigations were performed for mixtures of 1 and Si(OEt)(4). Immediately after the start of the reaction small primary particles are formed, the size of which (r = 0.5 +/- 0.1 nm) remains constant 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 of the 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 secondary particles formed by aggregation of the primary particles increases approximately exponentially with the time. Drying by either solvent evaporation or supercritical extraction with CO2 results in a decrease of the primary particle size to r = 0.4 nm, and the secondary particles also become smaller. The primary particles disappear during calcination in air, that is, when the organic groups tethering the silicon and titanium atoms are destroyed. However, the general structure of the network is maintained.