Organic Crystal Templating of Hollow Silica Fibers

Abstract

The template-directed sol-gel synthesis of organized inorganic matter offers a new and wide-ranging approach to useful materials with controlled architecture and porosity across a range of length scales. 1 Organic templates with extended long-range structure, such as block copolymer lyotropic mesophases, 2,3 colloidal crystals, 4,5 and bacterial superstructures 6 have been used to prepare monolithic forms of porous silica. In contrast, the direct synthesis of discrete inorganic architectures necessitates the use of dispersed organic supramolecular structures with commensurate dimensionality; for example, hollow fibers of amorphous silica have been prepared by template-directed processes using the external surface of self-assembled phospholipid fibers, 7 viroid cylinders, 8 or organic-gel filaments. 9 The use of such specialized molecules, however, has the potential drawback that the costs associated with scale-up are likely to be highly prohibitive. Thus, the recent report by Nakamura and Matsui 10 on the formation of silica tubes from ethanol/water/NH 4 OH/tetraethyl orthosilicate (TEOS) mixtures that contained small amounts of a simple organic acid (for example, racemic dl-tartaric acid), is particularly interesting. The authors speculate that chains of hydrogen-bonded dl-tartaric acid molecules act as a template for the deposition of the silica tube, but this seems highly unlikely considering the submicrometer dimension of the central lumen, and the unusual square-shaped cross section of the channel. For these reasons, we have undertaken further studies to elucidate the template mechanism. Here we show that the incipient crystallization of ammonium dl-tartrate filaments is responsible for patterning the tubular structure through specific interactions involving the {010} and {001} crystal faces. We use a similar mechanism to synthesize hollow silica fibers with triangular or rectangular-shaped channels by coupling TEOS hydrolysis/condensation reactions to the in situ crystallization of ammonium oxalate.Silica tubes were prepared according to previous work 10 with several modifications. 11 In general, the method involved the addition of concentrated NH 4 OH to unstirred ethanol-rich solutions of TEOS and dltartaric acid at respective molar ratios of 1:0.038. Washing and filtration of the resulting white precipitate gave a sample that consisted predominantly of flexible, open-ended hollow silica fibers with smooth continuous surface textures (Figure 1). Typically, the tubes were 200-300 µm in length, 0.1-1 µm in width, and contained a single 200-800-nm-wide pseudo-rectangular channel with uniform walls of amorphous silica, 30-300 nm in thickness. Larger tubes, up to 5 µm in width, but with similar wall thickness could be produced by adding TEOS after the addition of NH 4 OH to an ethanol/water solution of dl-tartaric acid (Figure 1c). In many cases, SEM images indicated that the rectangular shape of the central channel was structurally preserved, even in fibers with 50-nm-thick walls and micrometersized channels...

Hollow Inorganic Nanospheres and Nanotubes with Tunable Wall Thicknesses by Atomic Layer Deposition on Self‐Assembled Polymeric Templates

“…Some examples of rodlike templates for hollow nanotubes are nanowires, [15] tobacco mosaic viruses, [11] metal-salt crystals, [16] organic crystals, [17] and membranes with straight pores. [18] It is worth noting that the synthesis of rodlike templates typically requires different materials and methodologies compared to spherical templates.…”

Synthesis of Hybrid Silica-Carbon Tubular Structures by Chemical Vapor Deposition with Methane or Ethene

“…When the water content is lower (r w/TEOS = 0.5; STw-), smaller structures are obtained, probably due to the longer time required for the formation of the oligomers, and the more controlled deposition over the surface of the ammonium tartrate crystals. Miyaji et al proposed that the crystals of DL-ammonium tartrate are C 2017, 4, 1 3 of 11 responsible for the tube's growth [26,27], so the solubility of the tartrate in the water/ethanol/ammonia mixture is critical. Hence, tartaric acid and ethanol content were changed as to ensure template availability.…”

“…Despite the fact that some authors have used carbon nanotubes as templates for the growth of the silica structure [30,31], the cost of this template is significantly higher than the others. One of the cheapest and simplest processes for obtaining the silica tubes is the one proposed by Nakamura and Matsui [23], in which in-situ formed DL-ammonium tartrate needle-like structures are used as templates [26]. Through this process, silica submicron tubes (0.8-1.0 µm) can be easily obtained alongside spherical silica particles as a byproduct.…”

Ceramics in Nanotech Revolution

“…The fundamental challenges in nanostructured materials are the ability to control size of the system, ability to obtain required composition, control of interfaces, distribution of nanocomponents within fully formed materials and the ability to scale-up synthesis for development of industrial scale methods for nanomaterials. [10] Development of nanoscale composites may provide potential new systems for a wide range of materials applications. Another particularly challenging aspect of fostering research in the nanomaterials is its highly interdisciplinary characteristic.…”