Silica template, removal

In a melt infiltration-related procedure glassy carbon networks were fabricated by filHng the voids in a sihca opal with a phenol resin, which was subsequently cured, the silica template removed by HF, and the polymer network carbonized by heating to 900-1000 °C in an argon atmosphere [77]. 

FIG. 11 TEM images of (a) a [(Si02/PDADMAC)2]-coated PS particle and hollow silica capsules produced from PS latices coated with (b) one, (c) two, or (d) three Si02 layers. The hollow silica capsules maintain the shape of the original PS particle template. Removal of the core by calcination is confirmed by the reduced electron density in the interior of the capsules (compare b-d with a). The images of the hollow silica capsules show the nanoscale control that can be exerted over the wall thickness and their outer diameter. (From Ref. 106.)... 

The silicalite-alumina membrane was prepared after adding a solution containing the silicalite precursor (i e silica + template) to the above-mentioned porous tube (hereafter called support) and a specific hydrothermal treatment performed [8], under the chosen conditions no material is formed in the absence of the porous support. The tube is then calcined at 673 K for removing the template. 

Recently, the LbL technique has been extended from conventional nonporous substrates to macroporous substrates, such as 3DOM materials [58,59], macroporous membranes [60-63], and porous calcium carbonate microparticles [64,65], to prepare porous PE-based materials. LbL-assembly of polyelectrolytes can also be performed on the surface of MS particles preloaded with enzymes [66,67] or small molecule drugs [68], and, under appropriate solution conditions, within the pores of MS particles to generate polymer-based nanoporous spheres following removal of the silica template [69]. 

Ordered mesoporous materials of compositions other than silica or silica-alumina are also accessible. Employing the micelle templating route, several oxidic mesostructures have been made. Unfortunately, the pores of many such materials collapse upon template removal by calcination. The oxides in the pore walls are often not very well condensed or suffer from reciystallization of the oxides. In some cases, even changes of the oxidation state of the metals may play a role. Stabilization of the pore walls in post-synthesis results in a material that is rather stable toward calcination. By post-synthetic treatment with phosphoric acid, stable alumina, titania, and zirconia mesophases were obtained (see [27] and references therein). The phosphoric acid results in further condensation of the pore walls and the materials can be calcined with preservation of the pore system. Not only mesoporous oxidic materials but also phosphates, sulfides, and selenides can be obtained by surfactant templating. These materials have pore systems similar to OMS materials. 

A mesoporous carbon-CNT hybrid has been prepared using a mesoporous silica template [262]. The template was mechanically mixed with (a) phthalocyanine (Pc) or (b) Ni-phthalocyanine (Ni-Pc) followed by heating to 900 °C in Ar. Upon cooling, the silica template was removed via washing with hydrofluoric acid. SEM images showed... 

Various metal and metal oxide nanoparticles have been prepared on polymer (sacrificial) templates, with the polymers subsequently removed. Synthesis of nanoparticles inside mesoporus materials such as MCM-41 is an illustrative template synthesis route. In this method, ions adsorbed into the pores can subsequently be oxidized or reduced to nanoparticulate materials (oxides or metals). Such composite materials are particularly attractive as supported catalysts. A classical example of the technique is deposition of 10 nm particles of NiO inside the pore structure of MCM-41 by impregnating the mesoporus material with an aqueous solution of nickel citrate followed by calicination of the composite at 450°C in air [68]. Successful synthesis of nanosized perovskites (ABO3) and spinels (AB2O4), such as LaMnOs and CuMn204, of high surface area have been demonstrated using a porous silica template [69]. 

Preformed polymers can also be employed to prepare imprinted core-shell particles [143]. The group of Chang recently prepared a poly(amic acid) bearing oestrone as a template molecule covalently bound to the polymer through a urethane linker (see Fig. 2). A layer of this polymer was subsequently deposited on silica particles (10 pm diameter) prefunctionalised with amino groups at their surface. Thermal imidisation of the polymer yielded finally a polyimide shell (thickness about 100 nm) on the silica particles. Subsequent template removal yielded the imprinted cavities, which exhibited selective rebinding of oestrone in HPLC experiments. 

The principle of the carbon synthesis is shown in Fig. 1. Suitable carbon sources such as sucrose, furfuryl alcohol, phenol-resin monomers and acetylene gas are converted to carbon frameworks inside mesoporous silica template by pyrolysis. An effective method for the restriction of carbonization to inside the template is to incorporate a suitable catalyst such as Al, Sn and Fe onto the silica pore walls prior to the use as template. The template after the carbonization is removed using ethanol-water solution of HF or NaOH. 

Figure 1. Schematics for the synthesis of porous alumina with bimodal pore size distribution. Templates removal steps are followed by dotted-arrow for polystyrene beads and solid-arrow for silica gels as physical templates.

D and 3-D metal nanowire thin films[263] with tunable 3-10 nm wire diameters have been obtained by electrodeposition into mesoporous silica thin-film templates, resulting in nanowire arrays that reflect the pore structure of the template. Removal of silica is achieved via annealing followed by etching to leave mechanically strong freestanding metal nanowire films. 

Nanoporous platinum sponges,[264] which exhibit characteristic X-ray diffraction patterns due to structural order on the mesoscale, have been obtained after removing the silica template with HF following synthesis within the pores of MCM-48 and SBA-15 with Pt(NH3 )4(N03 )2. 

Films are typically allowed to dry immediately after deposition to drive the silica condensation reaction toward completion. However, the film may not be fully condensed and stable after drying, and postsynthesis treatments are often employed to improve film stability, reduce shrinkage upon template removal, and drive the condensation reaction toward completion. 

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