Spectroscopic characterization of silica aerogels prepared using several precursors - effect on the formation of molecular clusters

Condensed clusters of hydrolyzed tetramethylorthosilicate (TMOS), methyltrimethoxysilane (MTMS) and vinyltrimethoxysilane (VTMS), as single precursors or in combination, were studied using complementary approaches: (i) Fourier transform infrared (FTIR) spectroscopy; (ii) solid-state NMR spectroscopy, and (iii) density functional theory calculations of energies, structural and vibrational and Si-29 NMR chemical shift data of pertinent TMOS, MTMS and VMTS hydrolysis products, and species with different numbers of silicon atoms and different structures/conformations (linear, cyclic, and cage), performed at the B3LYP/6-311+G(d,p) and B3LYP/IGLO-III (for NMR) levels of theory. The calculated versus experimental (FTIR and solid-state NMR spectra) analysis of the aerogel derived from a single precursor shows that: (i) the TMOS-derived aerogels are mostly made of linear and/or cyclic elements with silicon environments of types Q4, Q3 and Q1, which have four, three and one Si-O-Si bridges, respectively; (ii) the MTMS-derived aerogels are mainly based on cyclic and/or cage structures with silicon of types T3, T2 and T1; (iii) VTMS-based aerogels exhibit the same type of silicon environments as MTMS (T3, T2 and T1), but incorporate structures that can be linear/cyclic/cage-like. The 29Si NMR spectra of the aerogels prepared with the mixtures of TMOS, MTMS and VTMS allowed the quantification of the relative percentages of the constitutive precursor components in the gels, i.e. the final composition of the aerogel is in line with the composition of the prepared solutions. This result is of utmost importance as the composition of the gel affects its final properties. The integrated experimental and computational investigation on the targeted nanostructured materials reported here provides a new understanding of both the microstructures of aerogels and the condensation stage of sol-gel polymerization.