Templates structure preparation

Although random and irregular type GaN nanorods have been prepared by using transition metal nanoparticles, such as Ni, Co, and Fe as catalysts and carbon nanotubes as the template, the preparation of controllable regular array of strai t GaN nanorods has not yet been reported. Fabrication of well-ordered nano-structures with high density is very important for the application of nano-structures to practical devices. 

Electrochemical template-controlled sjmthesis of metallic nanoparticles consists of two steps (i) preparation of template and (ii) electrochemical reduction of metals. The template is prepared as a nano structured insulating mono-layer with homogeneously distributed planar molecules. This is a crucial step in the whole technology. The insulating monolayer has to possess perfect insulating properties while the template has to provide electron transfer between electrode and solution. Probably, the mixed nano-structured monolayer consisting of alkylthiol with cavities which are stabilized by the spreader-bar approach [19] is the only known system which meets these requirements. 

Figure 4.2 Structure of [La(l)4Cp3S03] + [3]. (Reproduced with permission from H. Paul, P.H. Smith and K.N. Raymond, A lanthanide-amine template synthesis. Preparation and molecular structures of Ln(L)(CH3CN)(Cp3S03)3 [L= l,9-bis(2-aminoethyl)-l,4,6,9,12,14-hexaazacyclohexadecane Ln = La, Yb] and La(en)4(CH3CN)(Cp3S03)3, Inorganic Chemistry, 24, 3469, 1985. 1985 American Chemical Society.)...

It is interesting to note that although apparently unaware of the development of molecular imprinting, Pande et al. [28] proposed the use of thermodynamic control for the preparation of synthetic polymer systems with a memory for a template structure. Monte Carlo computer simulations were performed to validate their hypothesis. From these calculations they identified the formation of non-random polymer sequences arising from an evolution-like preferred selection of various monomer components by similar species. These studies have since been expanded upon using statistical mechanics to examine the consequences for protein folding [29]. 

In addition to the one-dimensional templated structure of the MCM-41 materials, two- and three-dimensional systems have also been prepared. A number of papers have used the lamellar structures of amphiphile assemblies to prepare flat, striated metal oxide materials [72,73]. These materials often exhibit enhanced properties over materials that have uncontrolled three-dimensional growth. Vesicles have also been used to engineer spherical imprints into silicates [74,75]. Even more elaborate supramolecular surfactant systems, that yield toroidal and other unusually shaped metal oxides, have also been reported [76,77]. 

Well ordered mesoporous silicate films were prepared in supercritical carbon dioxide.[218] In the synthesis in aqueous or alcoholic solution, film morphology of preorganized surfactants on substrate cannot be fully prescribed before silica-framework formation, because structure evolution is coincident with precursor condensation. The rapid and efficient preparation of mesostructured metal oxides by the in situ condensation of metal oxides within preformed nonionic surfactants can be done in supercritical CCU- The synthesis procedure is as follows. A copolymer template is prepared by spin-coating from a solution containing a suitable acid catalyst. Upon drying and annealing to induce microphase separation and enhance order, the acid partitions into the hydrophilic domain of the template. The template is then exposed to a solution of metal alkoxide in humidified supercritical C02. The precursor diffuses into the template and condenses selectively within the acidic hydrophilic domain of the copolymer to form the incipient metal oxide network. The templates did not go into the C02 phase because their solubility is very low. The alcohol by-product of alkoxide condensation is extracted rapidly from the film into the C02 phase, which promotes rapid and extensive network condensation. Because the template and the metal oxide network form in discrete steps, it is possible to pattern the template via lithography or to orient the copolymer domains before the formation of the metal oxide network. 

New methods are focused on the preparation of solid catalysts without release of gaseous or liquid waste streams. In zeolite synthesis, the chemical compounds generated during combustion of templates (structure-directing agent) are often hazardous and harmful to the environment. A new method teaches disassembling the template within zeolite pore space into small fragments without combustion. ... 

A template-guided synthesis of water-soluble chiral-conducting PAn in the presence of (S)-(—)- and (R)-(+)-2-pyrrolidone-5-carboxylic acid [(S)-PCA and (R)-PCA] has been reported to produce nanotubes.228 The structures prepared have outer diameters of 80-220 nm with an inner tube diameter of 50-130 nm. It was proposed that the tubular structures form as a result of the hydrophobic aniline being templated by the hydrophilic carboxylic acid groups of the PCA in aqueous media during chiral tube formation. The resultant tubes were shown to be optically active, suggesting that the PAn chains possess a preferred helical screw. 

The methods of soft chemistry include sol-gel, electrochemical, hydrothermal, intercalation and ion-exchange processes. Many of these methods are employed routinely for the synthesis of ceramic materials. - There have been recent reviews of the electrochemical methods, intercalation reactions, and the sol-gel technique. The sol-gel method has been particularly effective with wide-ranging applications in ceramics, catalysts, porous solids and composites and has given rise to fine precursor chemistry. Hydrothennal synthesis has been employed for the synthesis of oxidic materials under mild conditions and most of the porous solids and open-framework structures using organic templates are prepared hydro-thermally. The advent of supramolecular chemistry has started to make an impact on synthesis, mesoporous solids being well known examples. ... 

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