The Hard-Templating Approach

In 1999, almost seven years after the first report of M41S by Mobil scientists, ordered mesoporous carbons were synthesized by Ryoo s and Hyeon s 

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For the oxides that cannot he synthesized by the surfactant self-assembly, the hard-templating approach is a good choice. The key point for the nanocasting method is to increase the loading level of the precursors in the mesopore channels of the templates. Evidently, improving the interactions between template and precursor and between precursors themselves are essential in the hard-templating approach [11]. 

Chiral nematic mesoporous films of Eu " doped Zr02 have been produced via a hard-templating approach using nanoctystalline cellulose-templated silica (Fig. 11). It was found that these chiral nematic nanostructures are capable of modulating the spontaneous emission of the Eu ions. The emission lines of the Eu " at 596 nm, 613 and 625 nm were significantly suppressed, and an increase in the luminescence lifetime is observed. It was suggested that these new chiral luminescent nanomaterials could find potential applications in sensing and new optical nanodevices. 

Fig. 11 A schematic diagram of the fabrication of EDCNMZ-n by a hard-templating approach. Calcination was conducted at 600 °C for 6 h. Reproduced from ref. 87 with permission from the Royal Society of Chemistry.

As a rather unusual application of 1,2,4-triazine, it is necessary to mention the preparation of highly ordered and graphitic mesoporous carbon nitride with an ordered porous structure and a high nitrogen content by a nano-hard-templating approach through a simple polymerization of 3-amino-l,2,4-triazine inside the pore channels of a mesoporous silica template (13JMCA2913). 

The hard-templating growth methods described before should not be confused, on the contrary, with soft-templating approaches which usually are accomplished in solution phase in the presence of free surfactant molecules which severely attend the nucleation of a new particle and the further shape development selectively and dynamically interacting with the evolving particle surface facets. 

Though the hard template method is most efiftcient approach for preparing well controlled and highly oriented nanostructures of conducting polymers, the removal of template is very difficult [18]. 

The chemical methods for the preparation of nanomaterial could be categorized as either template-directed or template-free. The template synthesis methods commonly used for the production of one-dimensional nanostructured PANI are further subdivided into hard template (physical template) synthesis and soft template (chemical template) synthesis approach according to the solubility of the templates in the reaction media. Non-template routes for the synthesis of one-dimensional nanostructured PANI such as rapid-mixing reaction method, radiolytic synthesis, interfacial polymerization, and sonochemical synthesis have also been reported [56], Other approaches like combined soft and hard template synthesis are also known. An overview of hard-template, soft-template, and template-free procedures are presented in the following paragraphs. 

Zeolites were already employed as templates in the synthesis of microporous carbon with ordered structures.[247] The discovery of ordered mesoporous silica materials opened new opportunities in the synthesis of periodic carbon structures using the templating approach. By employing mesoporous silica structures as hard templates, ordered mesoporous carbon replicas have been synthesized from a nanocasting strategy. The synthesis is quite tedious and involves two main steps (i) Preparation and calcination of the silica mesophase, and (ii) filling the silica pore system by a carbon precursor, followed by the carbonization and selective removal of the silica framework. 

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