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...
