Soft templates surfactants

The difficulty in direct synthesis of mesoporous transition metal oxides by soft templating (surfactant micelles) arises from their air- and moisture-sensitive sol-gel chemistry [4,10,11]. On the other hand, mesoporous silica materials can be synthesized in nimierous different solvent systems (i.e., water or water-alcohol mixtures), various synthetic conditions (Le., acidic or basic, various concentration and temperature ranges), and in the presence of organic (Le., TMB) and inorganic additives (e.g., CT, SO, and NOs ) [12-15]. The flexibility in synthesis conditions allows one to synthesize mesoporous silica materials with tunable pore sizes (2-50 nm), mesostructures (Le., 2D Hexagonal, FCC, and BCC), bimodal porosity, and morphologies (Le., spheres, rods, ropes, and cubes) [12,14,16-19]. Such a control on the physicochemical parameters of mesoporous TM oxides is desired for enhanced catalytic, electronic, magnetic, and optical properties. Therefore, use... 

Surface/Template Derivatized method Template techniques are common to some of the previous mentioned methods and use two types of equipments hard-templates (porous solids as carbon or silica) and soft-templates (surfactants). Surface-mediated and template-nanoparticles precursors have been used to synthesize self-assembled systems [47]. 

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,... 

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 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... 

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. 

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. 

As mentioned above, metal/CPs with core-sheath structure can be prepared by the template method. However, the approach based on the template technique is complicated and non-economical because of the need to remove the templates. In fact, metal/CPs with core-sheath structure can be fabricated via a one-step chemical polymerization [83-87]. Niu and co-workers demonstrated that Au/PANI coaxial nanocables could also be fabricated by the redox reaction between chloroauric acid and aniline in the presence of d-CSA [85]. In that case, CSA acted not only as a dopant, but also as a surfactant or a soft template. In addition to Ag/PPy and Au/PANI nanocables, cable-like Au/poly(3,4-ethylenedioxythiophene) (PEDOT) nanostructures have been synthesized in the absence of any surfactant or stabilizer through one-step interfadal polymerization of EDOT dissolved in dichloromethane solvent and HAuCl dissolved in water [86]. Microscopy studies showed (Figure 13.6) that the outer and inner diameters of Au/PEDOT nanocables were aroimd 50 and 30 nm, respectively. 

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. 

Mesoporous carbon by soft template Carbons with mesostructures have been synthesized by using amphiphilic block copolymer as direct template, self-assembly surfactants or polyelectrolytes in the polymerization media of the carbon precursor. 

A soft template route involves an organic compound, such as polymers or surfactants, which is used as a direct mold in order to obtain a structured carbon precursor. Then, the soft template is eliminated during the carbonization stage. Nowadays, there are a large number of preparation methods reporting materials with a wide variety of structures and pore size distributions. In general, these methods are more versatile and cheaper than nanocasting ones. Moreover, the... 

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. 

Using functional molecules as structural directors in the chemical polymerization bath can also produce polyaniline nanostructures. Such structural directors include surfactants [16-18], liquid crystals [19], polyelectrolytes (including DNA) [20,21], or complex bulky dopants [22-24]. It is believed that functional molecules can promote the formation of nanostructured soft condensed phase materials (e.g., micelles and emulsions) that can serve as soft templates for aniline polymerization (Figure 7.3). Polyelectrolytes such as polyacrylic acid, polystyrenesulfonic acid, and DNA can bind aniline monomer molecules, which can be polymerized in situ forming polyaniline nanowires along the polyelectrolyte molecules. Compared to templated syntheses, self-assembly routes are more scalable but they rely on the structural director molecules. It is also difficult to make nanostructures with small diameters (e.g., <50 nm). For example, in the dopant induced self-assembly route, very complex dopants with bulky side groups are needed to obtain nanotubes with diameters smaller than 100 nm, such as sulfonated naphthalene derivatives [23-25], fidlerenes [26], or dendrimers [27,28]. 

In general, template method is classified by soft and hard templates. Whereas anodic aluminum oxide (AAO) membrane, track-etched polycarbonate (PC) and zeolite can be used as hard templates, soft templates include surfactant, cyclodextrin, liquid crystal, etc. Compared with soft and hard templates, template-free method represents the fabrication technique of conducting polymer nanomaterials without the template, which is discussed in this section [115]. 

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