Metal Oxide Aerogels with Controlled Crystallinity and Faceting from the Epoxide-Driven Cross-Linking of Colloidal Nanocrystals.

We present a novel method to produce crystalline oxide aerogels which is based on the cross-linking of preformed colloidal nanocrystals (NCs) triggered by propylene oxide (PO). Ceria and titania were used to illustrate this new approach. Ceria and titania colloidal NCs with tuned geometry and crystal facets were produced in solution from the decomposition of a suitable salt in the presence of oleylamine (OAm). The native surface ligands were replaced by amino acids, rendering the NCs colloidally stable in polar solvents. The NC colloidal solution was then gelled by adding PO, which gradually stripped the ligands from the NC surface, triggering a slow NC aggregation. NC-based metal oxide aerogels displayed both high surface areas and excellent crystallinity associated with the crystalline nature of the constituent building blocks, even without any annealing step. Such NC-based metal oxide aerogels showed higher thermal stability compared with aerogels directly produced from ionic precursors using conventional sol-gel chemistry strategies.