Soft-Template Method

It was difficult to produce ordered mesoporous crystalline transition metal oxides because of the necessity of heat treatment for removing templates and crystallisation of wall transition metal oxides. Under calcination conditions, the mesoporous structure of transition metal oxides is less stable than that of silica. Redox reaction and sintering during the calcination destroy the mesoporous structure. 

The disadvantage of this approach is that the resulting oxides are not highly crystallised because heat treatment cannot be conducted at a temperature above about 400 °C, where structural collapse occurs. This soft method has not been applied for ordered mesoporous crystalline late transition metal (such as Cu, Co, Ni and Fe) oxides. As a rare case, preparation of mesoporous nickel or iron oxide was reported. However, nickel oxide has an amorphous wall and iron oxide (crystalline y-Fe203) has disordered wormhole mesopores.  

Recently, new crystallisation methods for mesoporous amorphous oxides using combined assembly by the soft and hard (CASH) chemistry method and two-step wall reinforcing method have been reported. 

Hard-tcmplatc synthesis can provide micro- and nanocontainers with a controlled geometric shape. However, this approach requires complicated synthetic steps, including the dissolution of the template in corrosive media. Collapse of the hollow structure after template removal is also a critical problem. Therefore, the potential drawback of using hard templates forced scientists to search for more efficient and facile routes to prepare CPCs. Among these newly developed approaches, the soft-template method is considered a powerful tool as an alternative strategy to hard-template synthesis. 

In addition to the above-mentioned soft templates, soup bubbles have been also used as templates to fabricate CPCs. PPy microcontainers with bowl-, cup-, and bottle-like morphologies have been electrochemically generated by direct oxidation of pyrrole in an aqueous solution of /3-naphthalenesulfonic acid (/3-NSA), first reported by Shi et al. [79-82]. The bubbles were produced by the decomposition of water. The gas bubbles and the aqueous solution containing the monomer and /3-NSA led to the formation of soap 

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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. 

Figure 3.5 Schematic illustration of the soft template method ...

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. 

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. 

On the other hand, the soft template method involves cooperative assembly between the structure-directing agents (usually surfactants) and organic precursor species in solution. Therefore, the carbon structures obtained via soft templating are more flexible and their formation is dependent on temperature, type of solvent and ionic strength. However, there are currently only limited examples for the successful fabrication of porous carbon via the soft template method, which were reviewed recently by Wan et alP Soft template and hard template routes have been classified as endotemplate and exotemplate, respectively. 

Several reviews covering the synthesis, properties and applications of porous carbons, especially mesoporous carbon materials, can be found in the literature. In this chapter, we summarise the recent developments in the synthesis and characterisation of templated porous carbon materials. Particular attention is paid to the synthesis of structurally ordered porous carbon materials with narrow pore size distribution via both hard and soft template methods. We especially emphasise those so-called breakthroughs in the preparation of porous carbon materials. The chapter is divided into three sections according to the pore size of carbon materials we first consider the synthesis of microporous carbon materials using zeolites and clays as hard template, then summarise the preparation of mesoporous carbon materials via both hard template and self-assembly... 

The pore size of mesoporous carbon is of importance with respect to practical applications. When mesoporous carbon is synthesised via soft-template methods, the self-assembly of organic-organic species and pore size can be influenced by synthesis conditions, including surfactant type and concentration, and synthesis temperature. For example, Meng et al. observed that the pore size of mesoporous carbon derived from soft-templated mesoporous polymer composites decreased from 7.4 to 5.9 nm when the pyrolysis temperature increased from 400 to 800 How-... 

The soft-template methods are based on the use of structure-directing molecules, such as various soluble oligomers and polymers, as well as surfactants and amphiphilic acids which are able to form, alone or with aniline, aggregates such as cylindrical micelles, and other supramolecular 1-D aggregates. 

Miscellaneous Soft-Template Methods Novel Y-junction PANI-NTs, accompanied with nanorods, have been selectively prepared using in situ self-assembly of water-soluble Fe304 nanoparticles coated with PEG(5)-nonylphenylether and cyclodextrin as templates and pH control in an aqueous medium [373]. A chemical oxidative route to synthesize oriented arrays of conducting PANI-NTs in HCl solution by hydrogen-bonding directionality in the presence of a crown ether derivative (CE-SO3K) has also been reported [374]. 

Figure 11.6 SEM images of polypyrrole microcontainers synthesized electrochemical ly using a soap bubble -assisted soft-template method. (Reprinted with permission from Chemical Communications, Electrochemical synthesis of novel polypyrrole microstructures by L. T. Qu and G. Q. Shi, 2003, 2, 206-207. Copyright (2003) Royal Society of Chemistry)...

Figure 11.11 A schematic representation of the soft-template method used to prepare CPCs (a) micelles, (b) oil droplets, (c) gaseous bubbles...

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. 

For instance, a soft-template method was used to synthesize nanolibers of PEDOT with diameters of 10-50 nm. A high electrical conductivity up to 83 S cm was obtained. Use the nanofibers as counter electrode produced a power conversion efficiency of 9.2%, much higher than its bulk PEDOT counterpart (6.8%) and comparable to that of the platinum electrode (8.6%) (Lee etal., 2012). 

Finally, through electrostatic, hydrophobic and van der Waals interactions between polymer and the surfactant, stable straemres are generated, which serve as ideal soft templates, providing monodisperse nanoparticles. Soft template methods incltrde electrochemical reduction, seeding followed by chemical reduction, redox reactions, selective etching etc. 

Soft template method by using block copolymers was reported for first time by Liang et al. [72] in 2004. After that, a significant progress on the fabrication of carbon with a well ordered mesopores was achieved [32, 73-77]. Zhao and coworkers performed a widespread study of soft template via the triblock poly (ethylene oxide) (PEO) and poly(propylene oxide) (PPO) based systems, PEO-PPO-PEO [65, 78]. Several ordered pore stmctures corresponding to various surfactant liquid crystal phases were synthesized by liquid crystal template pathway, a schematized synthesis procedure is shown in the Pig. 7.11. 

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. 

Recently, soft template method has been used for the fabrication of various morphologies of polymer nanomaterials. There are several soft templates such as surfactant, liquid crystalline polymer, cyclo dextrin, and functionalized polymer [100,101,116-122]. Among them, surfactants, which imply cationic, anionic and non-ionic amphiphiles, are mostly used for the for-... 

Template-free techniques have been extensively studied for the fabrication of conducting polymer nanomaterials fabrication. Compared with hard and soft template methods, these methodologies provide a facile and practical route to produce pure, uniform, and high quality nanofibers. Template-free methods encompass various methods such as electrochemical synthesis, chemical polymerization, aqueous/organic interfacial polymerization, radi-olytic synthesis, and dispersion polymerization. 

Pat. CN101332999 (A) C01G3/02, Method for preparing Cup or CuO hoUow sub-microspheres with particle diameter controllable by water phase soft template method, Yun Fang [CN] Jin Hu [CN] Yueping Ren [CN] Yongmei Xia [CN]. 

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