Soft-templating approach

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

Traditionally the synthesis of ordered mesoporous silica materials, with pores in the 2-50 nm range, is templated by surfactants, hi this soft-templating approach, quaternary ammonium salts and non-ionic polyether-based surfactants are the most used templates, leading to pore distributions of 2-6 and 4-30 nm respectively [6,7]. Under specific reaction conditions, a mixture of the silica precursor and the surfactant yields a meso-structured solid, which, after removal of the template, becomes an open mesoporous structure. Many synthetic protocols have been developed by varying the surfactant, silicon source or synthesis conditions, but most preparations occur at low or high pH. Extensive reviews on the synthesis of ordered mesoporous materials are available [8-10]. 

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. 

El Hankari, S. and Hesemann, P. (2012) Guanidinium vs. ammonium surfactants in soft-templating approaches nanostructured silica and zwitterionic /-silica from complementary precursor-surfectant ion pairs. Eur. J. Inorg. Chem., 2012, 5288-5298. 

Yamauchi, Y., and Kuroda, K. (2008) Rational design of mesoporous metals and related nanomaterials by a soft-template approach. Chem. Asian /., 3, 664-676. 

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. 

To obtain nanocontainers, Wei et al. employed micelles as soft templates to assist the polymerization [83]. Hollow conical nanostructures are produced by a slow polymerization process (Figure 11.8). Microcontainers could be produced in the open or closed state by changing the polymerization time. However, as far as we are aware, most of the reported actuators were based on bulk materials or synergistic properties of nanostmcture bundles instead of single nanostmcture. Wei et al. realized the manipulation of separated nanocontainers by an electrochemical approach to control of the state of the nanocontainer in situ . Studying the switching of single nanostmctured CPs will open potential... 

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. 

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. 

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. 

One-dimensional polymeric nanostructures or one-dimensional nanostructures derived from polymeric soft templates are being considered as functional building blocks for a broad range of device architectures, and some promising applications have already been reported. Various well-established approaches allow forming virtually any functional material into a tubular or... 

Polymeric cylinders prepared via the self-assembly approach represent a versatile and powerful soft template for the fabrication of 1-D hybrid or inorganic nanomaterials with hierarchical architectures and complex functionaHties. 

By tuning precursor properties formation of covalent bonds takes place by using different templates with required physicochemical properties to enhance the functional behavior via self-assembly based on soft chemical approach. 

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