Soft templates

Compared to inorganic materials, organic materials such as polymers, surfactant molecules and micelles also act as a capping material or soft template. Figure 5.15 shows TEM images of gold nanorods and nanoparticles synthesized by sonochemical reduction of Au(I) in the presence of cetyltrimethylammonium bromide,... 

Tanaka, S. Katayama, Y. Tate, M. R Hillhouse, H. W. Miyake, Y. 2007. Fabrication of continuous mesoporous carbon films with face-centered orthorhombic symmetry through a soft templating pathway. J. Mater. Chem. 17 3639-3645. 

Guo ZW, Ruegger H, Kissner R, Ishikawa T, Willeke M, Walde P (2009) Vesicles as soft templates for the enzymatic polymerization of aniline. Langmuir 25(19) 11390-11405... 

The kinds of structures adopted by these microorganisms as well as other mineral morphologies, are the subject of increasing study as chemists look at soft templated routes to nanoscale objects. Early work in the 1990s by a team at Mobile used supramolecular micelles, lamellae and bicontinuous phases4 formed by amphiphiles, to assemble inorganic materials, particularly silica and alumina. A range of new... 

Figure 2.15 General concept of the syntheses of mesoporous materials (a) mesoporous silica synthesis with soft template (b) mesoporous carbon synthesis with hard template.

Wan Y Zhao DY, On the controllable soft-templating approach to mesoporous silicates, Chem. Rev., 2007, 107, 2821-2860. 

The utilization of soft templates is helpful for the construction of rare earth hydroxide nanotubes. The s)mthesis of Y(OH)3 nanotubes could be assisted by PEG (Tang et al., 2003) or grafted with PMMA (Li et al., 2004 Mo et al., 2005). Hard templates like A AO are also studied for the fabrication of rare earth hydroxide nanowires (Bocchetta et al., 2007). 

Mesophase structures self-assembled from surfactants (Figure 8.35) provide another class of useful and versatile templates for generating ID nanostructures in relatively large quantities. It is well known that at critical micellar concentration (CMC) surfactant molecules spontaneously organize into rod-shaped micelles [315c]. These anisotropic structures can be used immediately as soft templates to promote the formation of nanorods when coupled with appropriate chemical or electrochemical reaction. The surfactant needs to be selectively removed to collect the nanorods/nanowires as a relatively pure sample. Based on this principle, nanowires of CuS, CuSe, CdS, CdSe, ZnS and ZnSe have been grown selectively by using surfactants such as Na-AOT or Triton X of known concentrations [238, 246]. 

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. 

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. 

The templates used in the synthesis of mesostructured and mesoporous materials can be classified into two categories The first class of templates includes soft templates, which are ordered arrays of self-assembled surfactant micelles, similar to the ones used by the researchers at Mobil. Alternately, mesoporous materials can themselves be used as templates to synthesize new mesostructured materials, and such templates can be termed as rigid templates. In the following sections, we focus on the use of supramolecular assemblies of surfactants as well as on the use of rigid templates as the templates for the synthesis of mesostructured materials. 

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. 

Soft templates, usually molecules and molecular associations such as amines, thermolabile organic polymers, and surfactants, can be removed by heat treatment. In addition, vesicles, ionic liquids, self-assembled colloidal crystals, and air bubbles have been used for soft templating synthesis. 

PbS hollow nanospheres with diameters of 80-250 nm have been synthesized by a surfactant-assisted sonochemical route from Pb(CH3COO)2, thioacetamide, and sodium dodecylbenzenesulfonate [3]. 200-400 nm hollow silica spheres with mesoporous walls were prepared by the application of ultrasound to a mixture of nonionic polyoxyethylene surfactant and tetraethylorthosilicate. The presence of surface-active agents during ultrasonic synthesis was proved to be effective because surfactants can act as soft templates as well as structure directing agents for the assembly and subsequent... 

Successful nanocrystals synthesis has also been carried out employing soft templates such as the water pool in a reverse micelle, the interface of two... 

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. 

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