Silica nanotubes template synthesis

Figure 1.24 Silica nanotubes are formed by template synthesis using anisotropic materials (V307 H2Q, in this case.)...

Figure 1.26 Template synthesis using organic gels expands the versatility of the methodology, affording largely different silica nanotubes. (Reproduced from ref. 51, with permission.)...

The advantage of template synthesis is that organo or hydrogelator templates can direct the shape-controlled synthesis of oxide nanotubes. Recent reports describe the use of carbon nanofibers as a template for the shape-controlled synthesis of zirconia, alumina and silica nanotubes [78]. The shape of vapor grown carbon nanofiber... 

In case of silica nanotubes formed from TEOS and with [Pt(NH3)4](HC03)2 as templating salt, the synthesis products consist almost exclusively of nanotubes. As deduced from TEM-micrographs (Fig. 1), the portion of non-structured silica formed can be neglected. The lengths of the tubes vary between 50 nm and several pm and the iimer diameters range from 10 nm up to 300 nm with a maximum of frequency aroimd 50 nm. The silica walls are X-ray amorphous with a thickness of about 30-50 nm. 

FIGURE 24.2 Scanning electron micrographs. (A) The surface and cross section of a typical nanopore alumina template membrane prepared in the authors lab. Pores with monodisperse diameters that run like tunnels through the thickness of the membrane are obtained. (B) Silica nanotubes prepared by solgel template synthesis within the pores of a template like that shown in (A). After solgel synthesis of the nanotubes, the template was dissolved and the nanotubes were collected by filtration. (From Lee, S.B., Mitchell, D.T., Trofin, L., Li, N., Nevanen, T.K., Sbderlund, H., and Martin, C.R., Science, 296, 2198, 2002. With permission.)... 

Nanotubes of oxides of several transition metals, as well as of other metals, have been synthesized by employing different methodologies [24, 216-220]. Silica nanotubes were first produced as a spin-off product during the synthesis of spherical silica particles by the hydrolysis of tetraethylorthosilicate (TEOS) in a mixture of water, ammonia, ethanol and D,L-tartaric acid [216]. Since self-assembly reactions are not straightforward with respect to the desired product, particularly its morphology, templated reactions have been employed using carbon nanotubes to... 

Curved structures are not only limited to carbon and the dichalcogenides of Mo and W. Perhaps the most well-known example of a tube-like structure with diameters in the nm range is formed by the asbestos mineral (chrysotil) whose fibrous characteristics are determined by the tubular structure of the fused tetrahedral and octahedral layers. The synthesis of meso-porous silica with well-defined pores in the 2-20 nm range was reported by Beck and Kresge.6 The synthetic strategy involved the self-assembly of liquid crystalline templates. The pore size in zeolitic and other inorganic porous solids is varied by a suitable choice of the template. However, in contrast to the synthesis of porous compounds, the synthesis of nanotubes is somewhat more difficult. 

In the Pt-doped hexagonal mesophase formed from CPCI (cetyl pyridinium chloride), platinum ions are adsorbed at the surface of the surfactant cylinders. They are reduced radiolytically into a metal layer as a nanotube of around 10 nm diameter and a few hundred nm long (Fig. 3f). Extraction of all these nanostructures is achieved by dissolution of the soft template using alcohol. This possible easy extraction constitutes a marked advantage over the synthesis in hard templates, such as mesoporous silica or carbon nanotubes, the dissolution of which is more hazardous for the metal nanostructures. 

Another recent report describes the large scale synthesis of ahgned carbon nanotubes, of uniform length and diameter, by passage of acetylene over iron nanoparticles embedded in mesoporous silica [107]. The latter two methods, based on the pyrolysis of organic precursors over templated/catalysts supports, are by far superior by comparison with plasma arcs, since other graphitic structures such as polyhedral particles, encapsulated particles and amorphous carbon are notably absent (Fig. 16). 

Yoon etal)- reported the synthesis of mesoporous carbon using as-synthesised MCM-48 silica/surfactant mesophase as the template, followed by introduction of carbon precursor (divinylbenzene), carbonisation and removal of silica. Hyeon and co-workers " reported the synthesis of mesoporous carbon by the carbonisation of composites containing silica, P123 triblock copolymer and phenol resin, followed by removal of silica. The synthesis was achieved by treating the as-synthesised silica/triblock copolymer nanocomposite with sulfuric acid to crosslink the triblock copolymers followed by carbonisation. " Kim et al. reported the synthesis of carbon nanotubes using PI 23 surfactant inside mesoporous silica, although ordered arrays of carbon nanotubes were not observed.Kawashima et al. synthesised mesoporous carbon via copolymerisation of tetraethoxysilane (TEOS) and furfuryl alcohol. 

---