Hierarchical microfibrillar gels from evaporation-induced anisotropic self-assembly of in situ-generated nanocrystals

Whilst nanocrystal gels may be formed via destabilization of pre-functionalized nanocrystal dispersions, gelation via assembly of unfunctionalized nanocrystals into fibrillar networks remains a significant challenge. Here, we show that gels with hierarchical microfibrillar networks are formed from anisotropic self-assembly of in situ-generated mesolamellar nanocrystals upon evaporation of ZnO nanofluids. The obtained gels display the thermo-reversible behavior characteristic of a non-covalent physical gel. We elucidate a three-stage gelation mechanism. In the pre-nucleation stage, the cloudy ZnO nanofluid transforms into a transparent stable suspension, comprising multi-branched networks of aggregates self-assembled from in situ-generated layered zinc hydroxide (LZH) nanocrystals upon solvent evaporation. In the subsequent nucleation and anisotropic 1D fibre growth stage, further evaporation triggers nucleation and growth of 1D nanofibers through reorganization of the nanocrystal aggregates, before rapid nanofibre bundling leading to microfibrillar networks in the ultimate gelation stage. Our results provide mechanistic insights for hierarchical self-assembly of nanocrystals into fibrillar gels and open up facile fabrication routes using reactive transition metal-oxide nanofluids for new functional fibres and gels.