Formation processes of silica nanotubes and integrated ordered microstructures

The formation processes of both silica nanotubes and integrated ordered microstructures were investigated. The formation processes of the silica nanotubes by a surfactant-assisted templating mechanism were elucidated in a laurylamine hydrochloride (LAHC)/tetraethoxysilane (TEOS) system by measuring the evolution of the shape and size of surfactant/silicate molecular assemblies with small-angle X-ray scattering, together with transmission electron microscopy, scanning electron microscopy, nitrogen adsorption isotherms and Si-29 NMR. The formation processes in dilute LAHC solution at pH 4.5 were elucidated. Partially hydrolyzed TEOS combines with surfactant molecules, and the packing parameter of the complex molecule decreased in the rod-formation region, resulting in the formation of cylindrical aggregates. The polymerization of the hydrolyzed TEOS then proceeds on the surface of the aggregates, and silica nanotubes form. Various morphologies of the microstructure were synthesized by the variation of the pH in the silica nanostructure formation in the LAHC/TEOS system. Integrated ordered structures, such as a hexagonal array or bicontinuous cubic phases, were formed only in a narrow pH region from 10 to 11.5. A liquid-liquid interface between the organic and the aqueous phases played an important role in the formation of the integrated ordered microstructure.