High resolution transmission electron microscopy nanostructure of condensed-silica aerogels

High resolution transmission electron microscopy (HRTEM) was used to study the morphology of ultralow-density transparent condensed-silica (CS) aerogels. Silica aerogels were synthesized by two slightly different two-step polymerization processes and they were supercritically dried in a high temperature autoclave. Aerogel CS1 had a density of 9 kg/m3 and a BET surface area of 574 m2/g; aerogel CS2 had a density of 30 kg/m3 and a specific surface area of 630 m2/g. Both samples were fractured, vertically replicated with 0.95 nm Pt-C and backed with approximately 12 nm of rotary evaporated carbon. The silica aerogel was then removed from the replica with dilute HF acid and the replicas were studied by HRTEM. The stereoscopic HRTEM images reveal that connectors in both CS aerogels are extended filaments which resemble bottlebrushes, having microporosity. This morphology results from side-chain formation on a nearly linear CS stem. The slightly different chemistry leads to different morphologies for the two aerogels. For CS1, the connectors between stems have diameters ranging from 1.7 to 14.2 nm with an average of 6.4 ± 0.5 nm and connector lengths averaged 62 ± 21 nm with some as long as 132 nm. Pore sizes ranged from 13 to 240 nm with an average of 74 ± 43 nm. The pores were slightly larger than those in CS2 which ranged from 12 to 277 nm and averaged 61 ± 56 nm. For CS2, the connectors had diameters ranging from 1.5 to 16.5 nm and averaging 9.7 ± 0.5 nm. The connector lengths in the CS2 aerogel averaged 58 ± 27 nm with some as long as 127 nm. The connector diameters in CS2 (9.7 ± 0.5 nm) were the only important aerogel feature significantly different and greater than those in CS1 (6.4 ± 0.5 nm). The CS1 and CS2 connectors had side-chain diameters of 2 ± 0.7 and 0.95 ± 0.5 nm, respectively, and a similar microporosity of approximately 2.0 ± 1 nm.