Nanocasting processes

Two kinds of template, viz. hard template and soft template, are usually available for nanocasting processes. The true liquid crystal templating synthesis can be considered a soft-template process. In general, the hard template means an inorganic solid. For example, mesoporous silica as a template to replicate other materials, such as carbon or metal oxides, by which the pore structure of the parent can be transferred to the generated porous materials. A 3-D pore network in the template is necessary to create a stable replica. Mesoporous silica and carbon are commonly used templates for nanocasting synthesis. 

Zhao[266] demonstrated the successful synthesis of highly ordered mesoporous silicon carbides with unusually high surface areas (430-720 m2/g), uniform pore sizes (<3.5 nm), and extremely high thermal stabilities (up to 1400 °C) replicated by mesoporous silica hard templates via a one-step nanocasting process. Highly ordered 2-D hexagonal (p6m) and bicontinuous cubic (Ia3d) SiC nanowire arrays have been cast from the hard templates, mesoporous silica SBA-15 and KIT-6, respectively. 

Figure 3.2 Schematic representation of the nanocasting process using ordered mesoporous SBA-15 siiica as the hard tempiate.

In Attard s approach, tetramethylorthosilicate (TMOS) was hydrolyzed and condensed in the aqneons domain of the liqnid crystal phase at pH of abont 2, leading to mesostmctured hexagonal, cubic, or lamellar sihca. Methanol from the hydrolysis of TMOS destroys the long-range order of the liquid crystal however, upon the removal of methanol, the lyotropic liquid crystal is restored and serves as the template phase for the further condensation of silicates. The resnlting pore system replicates the shape of the lyotropic mesophase, so this process is also termed nanocasting . 

Fig. 5 The three-step process of nanocasting . First, a condensed LLC phase is formed, with the inorganic precursors occupying the liquid (aqueous) phase. Second, the inorganic molecules are solidified by some chemical or electrochemical means. Third, the LLC template is removed, leaving a mesoporous structure. Partially reproduced with permission from [32], 2002 by the Royal Society of Chemistry...

The applicability of nanocasting as an analytical tool has been demonstrated [38] by comparing the silica structures obtained from the lyotropic phase, which has been crosslinked using y-rays, in order to provide sufficient mechanical stability to allow thin-sectioning, with those of a silica nanocast obtained from a lyotropic phase of the same composition (Fig. 7). The similarity between the structures is striking. A reference sample was prepared by filHng the pore system of the crosslinked polymer gel with sihca and subsequent calcination. The pictures prove without doubt that the sol-gel process indeed does not have any structurally disrupting effect on the hquid crystalhne phase [38]. 

It was indicated in the previous section that the promise of nanocasting is that the template morphology will fully determine the structure of the porous system. However. certain conditions have to be fulfilled n the template-network combinations in order to make the nano-casting process work. 

Rg. 1 It is shown how ideas from the ancient bell-casting technique (a) are transferred to obtain nanoporous materials (b-c). This process, accordingly named nanocasting, uses nanoseale template structures (b) ranging from molecules (i) to colloidal crystals (vi). This template structure is imprinted into a solid hybrid material (c). Finally, after removal cf the template, one ideally achieves a pore resembling the template. (View this art ia color at www.dekker.com.)... 

Nanocasting can be considered as the best method to obtain a narrow pore size distribution. This is a very important feature because carbon supports with tunable properties allow the modeling of the system, in order to optimize the processes of methanol oxidation (adsorption and re-dissolution) and mass transport in fuel cells. 

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