Surfactant‐Mediated Fabrication of Silica Nanotubes

Hollow nanotubes are attracting a great deal of attention in both fundamental and industrial studies. They have novel properties, and could be used to study the physical and chemical properties of molecules confined in their inner and outer spaces. They also have potential applications in fields such as electronics, optics, advanced catalysis, and energy storage/conversion, and could be designed to mimic biological channels. Thus, methods must be developed to fabricate hollow nanotubes and modify the properties by filling and coating the tubes for particular applications. Inorganic hollow tubes that have been fabricated so far include those composed of carbon, boron nitride, silica, and vanadium oxide. Apart from vanadium oxide, these inorganic nanotubes have been synthesized under high temperature reaction conditions. For example, carbon nanotubes are produced by arc-discharge evaporation of carbon. On the other hand, recent advances in molecular biology have shown us that nature uses molecular self-assembly to construct microstructures of biomaterials. The bio-inspired method is another important route to the fabrication of nanotubes. The Mobil research group has synthesized ordered nanotubes in condensed forms, i.e., mesoporous materials (the so-called M41S family) using surfactant assemblies as the template. Since then, a variety of mesoporous materials have been synthesized. These materials have condensed forms of unit cylindrical structure. However, single nanotubes or bundles of a few tubes have not yet been synthesized. Nakamura and Matsui obtained silica-gel tubes by the sol±gel method in the absence of template. Lin and Mou synthesized hollow tubes whose wall has the MCM41 structure. Their diameters are all micrometer-size, i.e., much larger than nano-size. The single nanotube is a building unit for fabricating a more complicated hierarchical structure. Also, single nanotubes can be converted to a composite structure by filling the inner cavity with functional molecules and also coating the outside of the tube. Thus, the modification of single nanotubes has great potential for deriving novel properties, which cannot be expected for condensed materials such as the M41S family. Here, we describe the sol±gel method for fabricating single silica nanotubes or bundles of a few tubes by the surfactant-mediated template mechanism and the method for controlling the geometry of the tubes. The strategy of fabricating single nanotubes is as follows: Surfactants such as alkyl ammonium salts self-organize into micelles of various shapes in the equilibrium state. The factor that determines the shape is the packing index, P = Vc/(Slc): [21,22] For spheres, P < 1/3; for cylinders, 1/3 < P < 1/2; and for bilayers, 1/2 < P < 1. Here, Vc and lc represent the volume and effective length, respectively, of the hydrocarbon chain attached to a surfactant polar head, whose area is S. In the sol±gel process under acidic conditions, silicon alkoxide is first hydrolyzed, and then the condensation reaction proceeds to yield silica polymer. When the hydrolysis reaction is fast compared with the condensation reaction, combined molecules composed of the surfactant and hydrolyzed alkoxide are formed. They have an amphiphilic nature and self-organize into cylindrical aggregates if the P value for the combined molecules becomes 1/3 to 1/2. Thereafter, the condensation reaction slowly proceeds on the aggregate surface, resulting in the formation of single cylindrical aggregates covered by silica. The resultant aggregates are converted to nanotubes by calcination. The essential points in the formation of nanotubes are as follows: 1) The combined molecules self-organize into cylindrical assemblies in a quasi-equilibrium state. 2) Deformation of the generated aggregates does not occur during the condensation reaction, i.e., no mismatch of the array occurs between the surfactant molecules and silica units on the aggregate surface during the condensation reaction. We selected a laurylamine hydrochloride (LAHC)/tetraethoxysilane (TEOS) system. The experimental procedure is as follows: TEOS was added to 0.1 M LAHC aqueous solution (pH 4.5), and the reaction was started in a stirred cell at 313 K. The TEOS-to-LAHC molar ratio was adjusted to 4±12. TEOS does not dissolve in water to yield an emulsified solution in the early stages of the reaction. After 2±3 h, TEOS completely dissolved in the aqueous solution due to hydrolysis, and the solution became transparent. After 13±14 h, the solution turned into a homogeneous gel state.