Hard-Template Method

Martin [21] proposed a mechanism for the growth of the nanostructures prepared by the hard-template method. However, the mechanism is insufficient in explaining the growth of partially filled nanotubes by an electrochemical template synthesis [36]. Recently, Lee and coworkers [37] investigated the electrochemical 

Typically, sealed tube-like CPCs can be prepared by the approach introduced by Son and coworkers [53]. An ITO electrode was used as a substrate electrode, with a conducting 

Martin et al. also introduced another method to seal the open end of the tubes [54,55]. They proved that silica nano-test-tubes could be covalently corked by chemical self-assembly of nanoparticles, which remained attached to the mouths of the nano-test-tubes after liberation from the alumina template. What is more, the caps were present only at the mouth and not within the tubes. Commonly, applications in drug delivery may require modified outer surfaces of tubes carrying specific moieties to direct the carriers to their 

Compared with a conventional template, octahedral CU2O was not only a new type of template (in both shape and quality), but also omitted the need for subsequent template-removal treatment. 

ORDERED POROUS CRYSTALLINE TRANSITION METAL OXIDES 

The steps involved in this method are as follows (l)The template materials are highly ordered two-dimensional hexagonal mesoporous silica such as SBA-15 and three-dimensional caged mesoporous silica. (2) Metal precursors are introduced by capillary force, chelation force, and hydrophilic affinity. (3) A thermal treatment is employed to crystallise the guest materials and to form the mesoporous pattern. (4) The template silica is etched by HF or NaOH to obtain self-standing ordered metal oxide guest frameworks with mesostructure. 

In hexagonal mesoporous SBA-15, straight mesopores are connected three-dimensionally by some microporous bridges. Therefore, a three-dimensional assembly of nanowires with bridges can be obtained from the SBA-15 template. 

Infiltration of a sufficient amount of metal precursors in the mesopores is necessary. The simplest way is to mix mesoporous silica in an ethanol solution of metal salts and then to evaporate the ethanol. The precursor molecules might enter the pores during the evaporation of ethanol by capillary action. The two solvent impregnation method, in which a suspension of mesoporous silica in dry hexane is mixed with an aqueous solution of metal nitrate and then dried, was applied to complete the infiltration. Modification of the mesopore surface to accelerate infiltration was reported. If the melting point of the metal precursor 

Metal precursors can be calcined at a temperature higher than 400 °C, which is the temperature limit for the soft template method. The high temperature changes amorphous materials into crystalline materials. Porous single crystals, in which pores are in a single crystal, are frequently observed. 

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Ordered Mesoporous Carbons The first ordered mesoporous carbons were synthesized by hard templating methods (141). The use of porous inorganic silica templates allowed the preparation of series of ordered mesoporous carbon materials, designated as CMK-x materials (carbon molecular sieves Korean Advanced Institute of Science and Technology). The hrst one, CMK-1, was prepared using MCM-48 as a template and sucrose as the carbon source impregnated in the... 

Ordered mesoporous crystalline metal oxides have been synthesised using template methods, which are generally divided into the soft template method and the hard template method , depending on the nature of the templates. 

The other method for preparing ordered mesoporous materials is the so-called hard template method using hard mesoporous silica or replicated carbon templates. The metal precursors are filled into hard templates. In this method, heat treatment can be performed at a higher temperature without structural collapse and highly crystallised materials can be obtainedl ° ] (Figure 3.6). 

Figure 3.6 Schematic illustration of the hard template method ...

A limitation of the hard template method is that the resulting materials must be stable in HF or NaOH solution and that the precursors must not react with the silica template at a high temperature. For example, in the formation of lithium containing transition metal oxides, it is necessary to first form the transition metal oxide as a mesoporous solid to prevent reaction of the alkali metal Li with the silica template and then to react the mesoporous transition metal solid with a lithium source, such as LiOH. 

Mesoporous crystalline M0O3 and WO3 were obtained in thin films. Ordered mesoporous WO3 was first produced using the soft template method. By changing formation conditions, cubic and hexagonal mesoporous structures were obtained. The hard template method using SBA-15 or KIT-6 produced ordered mesoporous crystalline WO3 materials. With SBA-15 as a template, a porous single crystal was formed, while polycrystalline porous WO3 materials were formed with a KrT-6 template,although reaction conditions were similar. 

Ordered mesoporous crystalline NiO was prepared by using the hard template method. Mesoporous crystalline NiO with a bimodal pore size distribution can be produced. 

Figure 4.1 Schematic illustration of hard template method for the preparation of porous carhon materials from porous inorganic templates. Reprinted with permission from T. Kyotani, Bull. Chem. Soc. Jpn., 79, 1322. Copyright (2006) The Chemical Society of Japan...

The disadvantages of the nanoporous hard-template method are the generally tedious post-synthetic processes, which are required in order to remove the templates, and the destruction of nanostructures during die post-synthetic process, i.e. the formation of undesirable aggregated structures after removal of the templates. 

Nanoporous Hard-Template Methods PANI-NTs have been prepared by the chemical oxidative polymerization of aniline within the pores of PC nanoporous membranes... 

Figure 11.10 A schematic representation of the hard-template method used to prepare CPCs (a) porous membrane, (b) colloidal particles, (c) nanowires...

However, each currently developed method has its own disadvantages. Hard-template methods, for instance, are a universal and controlled approach to obtaining conducting-polymer nanostructures, but the requirement of a template and the post-treatment for template removal not only results in a complex preparation process, but can also destroy the formed structures. Moreover, the size and morphology of available templates is limited. The soft-template method is another relatively simple, cheap, and powerfid approach to obtain CPCs via a self-assembly process. However, the morphology and size control of the self-assembled nanostructures obtained is poor. Therefore, finding a facile, efficient, and controlled route to prepare CPs nanostructures is desirable. 

Djinovic et al. synthesized [42] Cu-Ce02by hard template method and compared that with co-precipitated catalysts. The activity results show that the catalysts synthesized by hard template method show better activity compared to catalysts synthesized by co-precipitation method regardless of Cu loading. 

Park et al. [32] synthesized Cu-Ce-Al catalysts by using one plot templating method. The Cu catalysts synthesized by this method exhibit better activity than impregnation method catalysts. Pintar et al. [33] compared hard templating method with CPs method on Cu/Ce02 catalysts. The catalysts synthesized by hard templating method show better activity than that by CP method. 

Djinovic P, Batista J, Pintar A (2009) WGS reaction over nanostructured CuO-Ce02 catalysts prepared by hard template method characterization, activity and deactivation. Catal Today 147S S191-197... 

Scheme 7.2 Synthesis of MnO -nanoparticles/PEDOT composite nanowires via hard template method. Panel is reproduced with permission [48]. Copyright 2010, ACS.

Soft-template technique offers advantage of scalability [39]. In hard-template method, a porous membrane of inorganic or polymeric material serves as a rigid mold for chemical or electrochemical replication of stracture. This method provides an easy marmer for production of 1-D nanostractures, but with difficulties of scale up. Hard templates such as silica or carbon spheres are also ideal for synthesis of hollow strac-tures (11 Chen et al. 2003). Classical examples where the template enables the control of morphology of a-Fe Oj nanoparticles can be found in literature (Table 1). 

Next section covers extensive discussions of various fabrication methods for conducting polymer nanomaterials in detail. This section is divided by the soft template method, hard template method, and template-free method. 

Hard template method has been used for the 1-D nanostructures such as nanotubes, nanorods and nanofibers of conducting polymers. The commonly used templates are AAO membrane, and track-etched PC membrane, whose pore size ranges from 10 nm to 100 pm. Hard template methods for synthesizing conducting polymer nanomaterials have been extensively reviewed in recent years [156-160]. 

In general, PPy nanotubes have been mainly produced by the hard template method [165,172,225,226,247,248]. For example, PPy nanotube with highly imiform surface and controlled waU thickness was fabricated by one-step VDP using AAO membrane [172]. A template-mediated VDP was foimd to be a facile and effective method to fabricate polymer nanotubes. The vapor phase polymerization provides highly uniform tubular walls as well as easy control over the waU thickness. 

Among the several fabrication methods, hard template method, which was pioneered by Martin et al., has been the most famous route of nanotubes and nanowires. Nanotubes of conducting polymers could be readily prepared by filling the nanopores with polymer or polymer solution using AAO template or track-etched PC membrane. PPV nanotube and nanorod had been fabricated in the pores of alumina or PC filters with pore diameter 10-200 nm by... 

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