Templating membranes

The first part of this section focuses on the main characteristics and fabrication techniques used for obtaining templating membranes and depositing metal nanostructures by suitable electroless and elecuochemical procedures. Methods such as sol-gel (10-12) or chemical vapor deposition (10, 13), which have been used primarily for the template deposition of carbon, oxides, or semiconducting-based materials, will not be considered here in detail. The second part of the section focuses on the electrochemical properties of the fabricated nanomaterials with emphasis on the characteristics and applications of nanoelectrode ensembles (NEEs). 

Aluminum oxide films with regular pore distribution can be formed by electrooxidation of high-purity aluminum substrates in acidic electrolytes (14, 15). The sttucture of the oxide film consists of a uniform array of parallel alumina cells packed hexagonally, each containing a nearly cylindrical pore. The uniform diameter of the pores is a consequence of the 

The electrolysis of aluminum is carried out in a two-electrode cell. The anode is usually a high-purity (99.9% or even 99.99%) aluminum sheet. The metal surface must be carefully cleaned via chemical or electrochemical polishing (17). After degreasing in 1 2 1 ethanol-dichloromethane-acetone solution (17), the aluminum surface is cleaned by immersion in NaOH (1 M (18) or 0.05 M (17)), followed by rinsing in distilled water, and then electropolished using one of the following typical experimental conditions  

According to Homyak et al. (17), the electropolished aluminum is rinsed immediately in distilled water (often with the aid of a strong stream of distilled water to remove the tenacious gelatinous oxide layer), immersed in concentrated nitric acid for 10 min, rinsed and left to dry in air. Well-polished aluminum should present a shiny/mirror-like surface accurate polishing is crucial for obtaining high-quality aluminum oxide membranes. 

The pre-treated aluminum is then anodized potentiostatically or galvanostatically in a thermostatically controlled bath (14,16,21). Potentials from 10 to 160 V and current densities from 10 to 30 mA cm are usually applied however, for high pore spacing, voltages as high as 700 V have been used (22). The electrolyte is typically 15% sulfuric acid (10 °C), 4% phosphoric acid (24 °C), 2% oxalic acid (24 C), or 3% chromic acid (38 °C) (14) the temperature ranges from 38 to 0 °C and even lower (18, 23). 

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It is important to point out that if plating is terminated before solid Au nanowires are obtained, Au nanotubules that span the complete thickness of the template membrane are deposited within the pores. We have shown that these nanotubule membranes have interesting ion [71] and molecular [72] transport properties. This will be subject of the following section. 

FIG. 3. Transmission electron micrograph of a microtomed section of a polycarbonate template membrane after deposition of An tubules within the pores of the membrane. Pore diameter was 50 mn. 

The Pt current collector was first used to deposit short ( 2 pm) Pt nanoposts [37,73] into the template membrane (Fig. 21A). These Pt nanoposts anchor the alumina membrane to the Pt surface and will serve to make electrical contact to the LiMu204 nanotubes. After Pt deposition, the pores in the membrane were filled with an aqueous solution that was 0.5 M in LiNOs and 1 M in Mn(N03)2 (Fig. 21B). The excess solution was wiped from the membrane surface, and the solvent (water) was removed by heating (50°C) in vacuum for 1 hour. The assembly was then heated at 500°C in air for 5 hours. This burns away the plastic tape and also causes tubules of LiMu204 to form within the pores (Figs. 21C, 22). 

FIG. 26. Scanning electron micrographs (A) the template-synthesized gold tubule ensemble obtained after dissolution of the polyester template membrane (B) as per A, but after CVD synthesis of TiSj outer tubes on the Au inner tubes. These tubular microstructures contained 0.86 mg of TiS2 cm of substrate A1 surface area (C) as per B, but with a larger quantity (2.04 mg cm ) of TiSj (D) CVD TiSj film. 

The properties of ordered structures in block copolymer melts have yet to be fully exploited, but the structural and rheological anisotropy is likely to lead to applications not all of which can be envisaged yet. The precision self-assembly of block copolymers into ordered structures for thin film and interfacial applications has enormous potential. Other applications such as nanoscale templates, membranes and filters could exploit the self-assembly of block copolymers into domains with periods 10-100 nm. The possibilities are limited only by the molecular engineer s imagination. 

Nano-materials in lithium ion battery electrode design, presentation of a plasma-assisted method to create a carbon replica of an alumina template membrane... 

The electrophoretic sol-gel template method could overcome the pore size limitation to certain extent (down to a few tens of nanometers), but it is still limited by the size of the sol particles which were preformed prior to being subjected to the electric field. To address this problem, Miao et al. (2002) reported an electrochemical sol-gel template method in which the sol particles were generated within the pores of the AAO template membrane, as shown in Fig. 18.10. 

Metal microtubules were prepared in a similar marmer, with lower internal diameters of 0.8 nm, and the authors could demonstrate that ion transport through such tubules can be switched from positive to negative ions with external potentials applied to the membrane. 2 Gold nanowires (of as little as 10 nm diameter) deposited in nanoporous filtration disks can also act as nanoelectrodes with sensitive cyclic voltammetric detection limits. Finally, the template membrane approach has also been extended to arrays of cadmium chalcogenide phases. Thus, 200 nm-diameter CdSe or graded CdSe/CdTe cylinders were electrodeposited in the pores of Anopore membranes. A Ni-CdSe array was found to be rectifying.i ... 

West GD, Diamond GG, Holland D, Smith ME, and Lewis MH. Gas transport mechanisms through sol-gel derived templated membranes. J. Membr. Sci. 2002 203 53-69. 

FIGURE 24.2 Scanning electron micrographs. (A) The surface and cross section of a typical nanopore alumina template membrane prepared in the authors lab. Pores with monodisperse diameters that run like tunnels through the thickness of the membrane are obtained. (B) Silica nanotubes prepared by solgel template synthesis within the pores of a template like that shown in (A). After solgel synthesis of the nanotubes, the template was dissolved and the nanotubes were collected by filtration. (From Lee, S.B., Mitchell, D.T., Trofin, L., Li, N., Nevanen, T.K., Sbderlund, H., and Martin, C.R., Science, 296, 2198, 2002. With permission.)... 

We have also developed a chemistry that allows us to attach the Fab to only the inner surfaces of the nanotubes. While still within the pores of the template membrane, the inner surfaces were treated with 3-aminopropyltrimethoxysilane. The template membrane was then dissolved and the amino sites on the inner surfaces were coupled to free amino groups on the Fab fragment using the well-known glutaraldehyde coupling reaction [4]. When 18 mg of these interior-only Fab-modified nanotubes were incubated with 1 mL of a 10 pM racemic mixture of the drug, 80% of the RS (and none of the SR) enantiomer was extracted. 

The gold nanotube membranes were prepared via the template synthesis [21,22] method by electroless deposition of gold along the pore walls of a polycarbonate template membrane [19,43]. The template was a commercially available filter (Osmonics), 6 p.m thick, with cylindrical 30 nm diameter pores and 6 x 10 pores per square centimeters of membrane surface area. 

Most of the time, metal/dielectric nanocomposites are studied in the form of solutions or thin solid films on a substrate Colloids, doped and annealed glasses, sol-gels, surfactant-stabilized nanoparticles, micelles, two- or three-dimension self-assembled nanocomposites, self-organized mesoporous oxides filled with metals, electrochemically-loaded template membranes, metal-ion implanted crystals, nanocomposite films elaborated by laser ablation, cluster-beam deposition, radio-frequency sputtering, or nanolithography. 

Figure 3.12 SEM (a) and TEM (b) micrographs of 100-nm-diameter polypyrrole nanowires after the removal of alumina template membranes. (Reprinted with permission from Reference [54]. Copyright 2007 American Chemical Society.)...

Parthasarathy and Martin [104] synthesized PAn microtubules within the pores of polycarbonate template membranes and reported that the conductivity increases drastically as the tubule diameter decreases. Polarized infrared absorption spectroscopy (PIRAS) data showed that the polymer deposited directly on the pore wall is highly ordered, which is believed to be responsible for the enhancement in conductivity. The PIRAS data also showed that the polymer chains are preferentially aligned perpendicular to the tubule axis. 

FIGURE 20.2 (a) The surface and cross section of a typical nanopore alumina template membrane prepared in the authors lab. The mono-... 

A copolymer of pyrrole and thiophene nano-fibrUs was electrochemically polymerized within the pores of microporous, anodic, aluminum oxide template membranes [105]. The copolymer nucleated and grew on the pore wall of the membrane since the polymers were cationic and the membrane had anionic sites on the pore wall. The length, thickness, and diameter of the copolymer nanofibrils could be controlled and with higher applied potential, more thiophene units were incorporated into the copolymer nanofibrUs [105]. Copolymer nanofibrils of pyrrole and aniline were also electrochemically polymerized within the pores of microporous, anodic, aluminum oxide template membranes [106]. Copolymer nanofibrils of PPy and poly(3-methylthiophene) prepared chemically in the microporous aluminum oxide template showed higher conductivity than the homopolymers did [107]. 

To prepare nanostructures with monodispersed nanoscopic fibrils and tubules, the pores in the membranes have been used as templates in the template-synthesis method. With the pores of nanoporous PC membrane filters as templates, PPy nanotubules and nanofibrils were chemically synthesized with oxidizing agent, FeClg [98,100]. Figure 8.54 shows transmission electron microscope (TEM) images of the PPy nanostructmes after dissolution of the PC template membrane. The tubule wall thickness increased with increasing polymerization time, but the outer diameter did not change with polymer-... 

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