Chemical etching, porous templates

Chemical etching, porous templates, 171-172 Chemical modifications, implant materials, 147-148... 

The formation of nanostructured arrays of conjugated polymers by the utilization of nanoporous templates has been reported. The deposition of the polymer inside the pores can be achieved by filling the pores with a solution of polymer and evaporation of the solvent or by the direct synthesis of conjugated polymer inside the pores by chemical or electrochemical approaches. Porous templates were based on track-etched polycarbonate membranes [106-108] or alumina that is obtained by anodic aluminum oxidation (AAO) [109-lllj. Thus, periodic vertical channels with diameters between 20 and 120 nm are formed by first electrochemical oxidation and etching and then subsequent etching for pore widening (Figure 13.16). 

Porous silicon template could be carefully removed via selective chemical etching in tetramethyl ammonium hydroxide (TMAH) solution at 40-90 °C. Such dissolution process is crucial and not so easy to control. The etch rate is not the same for different Si templates. It depends on the Si dopant type and level as well as on the pore depth and diameter. The removal of template could be done using KOH solution, but the etching behavior is completely different from the case of TMAH. Ethylene diamine pyrocatechol is also reported as a chemical etch solution at 115 °C to successfully... 

Table 1 summarizes some microstructural and electrochemical properties of porous Si anode materials, as pertaining to the second approach mentioned above, collected from the literature published since 2005. Several synthesis methods have been identified for preparing the porous Si anode materials (column 1, Table 1). One of the two most adopted methods is known as the metal-assisted chemical etching (MACE denoted as E in Table 1). The fundamental principle of this method can be found in the handbook chapter Porous Silicon Formation by Metal Nanoparticle Assisted Etching. Figure 2 shows an example of the MACE-derived porous Si particle. The other most adopted method is magnesiothermic reduction (denoted as M in Table 1). In this method (see handbook chapter Porous Silicon Formation by Porous Silica Reduction ), porous Si oxide materials are reduced by magnesium vapor under high-temperature thermal treatment. The porous Si oxide precursors may be synthesized via the conventional sol-gel processes. Porous Si particles with unique pore structures, such as hollow interior and ordered mesoporosity, may be obtained from Si oxides having the same pore structures which are achieved by using proper templates.

Both acid anodized aluminum (alumite membrane) (141, 142) and nuclear track-etched membranes have been used as a template. The nuclear track-etched membranes are prepared by irradiation/chemical etching (143) and contain linear, cylindrical pores of nearly uniform pore diameter. This ensures that the holes in the template are circular and approximately of the same diameter. More recently, porous nano-channel glass (144), and etched wafers of micro-channel plate glass have served as templates (125). The advantage of using porous membranes as templates can be found in their function as arrays of microelectrodes. They exhibit small potential drops, which make the electrochemical measurements possible when... 

Polymeric templates are also widely described. For example, a porous polymeric membrane can be obtained by the bombardment (irradiation) of a polymeric film with high energy heavy ions, followed by chemical etching. The pore density (number of pores per square unit) depends on the intensity and duration of the irradiation and the diameter of the pores is related to the intensity of the etching process... 

In the template synthesis method, nanofibers are produced in hollow channels of porous ceramic or polymer templates. Figure 13.12 shows the nanofiber formation process using the template synthesis method. The porous template first is filled with monomers. Polymer nanofibers are synthesized from the monomers chemically or electrochemically in the hollow chaimels of the porous template. Separated nanofibers are obtained after the removal of the template by dissolving or etching. Figure 13.13 shows a SEM image of polypynole (PPy) nanofibers prepared by the template synthesis method. Nanofibers produced by template synthesis often have a hollow stmcture because the synthesized polymer tends to precipitate onto the inner surface of the hollow channels. 

Among the porous templates for preparing nanotubes, the AAO membrane is the most frequently used. Yuan et al. synthesized Pt nanotube arrays by using this template via electrochemical deposition [103]. Here, the inner wall of the AAO pores was modified with 3-aminopropyltrimethoxysilane to allow the easy transpart of ionic precursors and release of the nanotubes. The electrodeposited Pt nuclei grew along the modified wall to form tubes. The Pt nanotubes were obtained after the removal the template, using chemical etching. 

Fig. 8.1 Refilling of the voided nano-channels of an organic DG template via atomic layer deposition of titania, which is illustrated in the inset. 1 Chemical modification of the styrenic polymer scaffold to introduce functional surface groups and improve the thermal stability. This enables the uniform nucleation of the ALD growth 2 Ideally, the nano-channels are gradually filled until a non-porous layer is formed at the free-surface. 3 This layer is removed by reactive ion etching. 4 Finally, the combined organic/inorganic deposit is calcinated to remove the template and ideally, crystallize the titania...

The third method is the template process (T), where Si is deposited, by either chemical vapor deposition or solution chemistry, on Si02 or other template materials, and the templates are subsequently removed to produce porous Si (see handbook chapter Routes of Formation for Porous Silicon ). Anodic etching method (AE) has been adopted to produce porous Si surface layers/film on Si wafer (chapter Porous Silicon Formation by Anodization ). Finally, electrospraying (ES) is used for making Si-C porous composite particles. 

Template-based synthesis involves the fabrication of the desired material within the pores or channels of a nanoporous template. A template may be defined as a central structure within which a network forms in such a way that removal of the template creates a filled cavity with morphological and/or stereochemical features related to those of the template. Track-etch membranes, porous alumina, and other nanoporous structures have been characterized as templates. Electrochemical and electroless depositions, chemical polymerization, sol-gel deposition, and chemical vapor deposition have been presented as major template synthetic strategies. Template-based synthesis can be used to prepare nanostructures of conductive polymers, metals, metal oxides, semiconductors, carbons, and other solid matter... 

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