Facile One-Step Synthesis of Silica Micro- and Nanotubes and Their Functionalization

Rectangular silica nanotubes (NTs) and microtubes (MTs) were synthesized by a simple one-step sol-gel hydrolysis reaction. Although silica nanotubes are usually prepared in the presence of a template (e.g. metal rods), the proposed procedure eliminates the need for such template insertion and, subsequently, no further etching step is required. The Stober silica nanoparticles (NPs) synthesis employs ammonium hydroxide solution as the base for tetraethyl orthosilicate (TEOS) hydrolysis, leading to the formation of discreet uniform NPs. Replacing ammonium hydroxide with sodium bicarbonate resulted in the formation of silica NTs with unique rectangular shape, both for the tube (external) and the lumen (internal cavity). On the other hand, replacing ammonium hydroxide with ammonium carbonate led to the formation of larger silica MTs with similar geometry. The influence of parameters such as: monomer concentration, base concentration, base: monomer ratio, temperature and solvent, on tube formation was investigated. This procedure was also utilized to prepare mesoporous and functionalized NTs and MTs with unique characteristics as omniphobic coatings. A proposed mechanism for the production of silica rectangular NTs and MTs via sheet folding is offered, based on our findings. (C) 2017 Elsevier Ltd. All rights reserved.