S3 Zd

Membrane with Initiate polymer linear pores growth within pores attached to an electrode

Polymer growth continues

Figure 13. Assembly of fibrillar ICPs using controlled pore size templates. Reprinted with permission from [4], G. G. Wallace and P. C. Innis, J. Nanosci. Nanotech. 2, 441 (2002). © 2002, American Scientific Publishers.

Figure 14. Scanning electron micrographs of fibrillar structure of PPy/pTS on (a) a flat electrode surface and (b) a PVDF membrane. Reprinted with permission from [188], V Misoska, Ph.D. Thesis, University of Wollongong, 2002.

via a chemical oxidative process. Surface deposited polymer was removed by polishing with alumina powder and ultra-sonication. This membrane was then carbonized in an Ar atmosphere and then etched in 48% HF to remove the alumina template. Metal CNTs impregnated with Pt, Pt-Ru, and Pt-WO3 were prepared by treatment with the appropriate metal salt solutions prior to etching. The resulting CNTs were single wall type with diameters ranging from 1 to 5 nm. The application of these nanostructures as electrode substrates for methanol fuel cell was explored, utilizing the presence of dispersed metals such as Pt0 and Ru0 metal and W6+ in the CNT, providing catalytic regions for methanol oxidation.

Freestanding nanotubes are formed by the selective dissolution of the templating membrane. Hollow tubes of polypyrrole are initially formed in the templating structure followed by the in-filling of these tubes at longer synthesis times. Electrical conductivities of these structures

(e.g., 375 S cm-1 at 20 nm) show an enhancement over that of the bulk material (1.5 S cm-1) grown under similar conditions [110]. A similar conductivity enhancement effect based on tube diameter has also been reported by Cai et al. with conductivities up to 103 S cm-1 [116, 117].

Applications of template-assisted growth of aligned polyaniline nanofibril arrays in the area of field emission have been explored by Wang et al. [118]. Nanofibril arrays were prepared by chemically depositing polyaniline into 20200 nm pores in anodic aluminum oxide film 30 to 60 /m thick that was subsequently etched away with 0.3 M H3PO4 or by ultrasonication [119]. Field emission studies revealed a low electric field threshold of 5-6 V//m, with a minimum emission current density of ca. 0.01 mA/cm2 and a maximum emission current density of 5 mA/cm2. Comparisons to other materials with lower emission current densities, such as diamond and CNTs, were made and the authors noted that aligned nanofibril arrays were attractive due to their high surface area, lower manufacturing cost, and robust mechanical properties.

Hollow ICP cigar-shaped nanotubes with sealed ends, synthesized via the track-etched polycarbonate template route, have also been reported by Mativetsky and Datars [113]. These materials exhibited a small drop in conductivity as the diameter decreased from 400 to 50 nm, contrary to the previous reports [110-112]. The small decrease in conductivity of the order of 50 mS cm-1 is believed to result from an increase in the electron scattering within the nanocylinder walls or from the presence of large impedances in the nano-structure. The authors attributed this different dependence on tube diameter by treating the tube as a solid cylinder, resulting in conductivity trends observed by others, or as reported in this instance as a hollow nanostructure which gave these results.

Others have shown that the electrochemical properties of polyanilines grown in a template such as sol-gel silica or PVDF are greatly improved [120]. This is attributed to improved order at the molecular level.

Other templates used to assist assembly of nanostructured conducting polymers are synthetic opals [121-126] based on polystyrene or silica spheres. The area of synthetic opals is of interest for use as photonic bandgap crystals: materials in which photons of a given energy cannot travel through or propagate in a crystal but rather are reflected by the lattice structure [125]. Of further interest is that opal templates provide a route to establishing high range order in the nanodomain, resulting in high surface-to-volume ratios. Template opal structures (Fig. 15) are preformed from monodisperse spherical colloidal particles that are permitted to self-assemble into close packed arrays via a sedimentation process assisted by either gravity or pressure/microfiltration. After template formation, conducting polymer is formed by infiltration of monomer into the void spaces within the crystal structure, followed by subsequent oxidative polymerization. The final stage is the removal of the templating core to leave an inverse structure from the templating material with essentially the same optical properties as the original host [123] (Fig. 16). Others [121, 122] used a similar templat-ing approach, but prepared self-assembled polystyrene (PS) latex opals onto gold substrates followed by the direct elec-tropolymerization of PPy, PAn, and polybithiophene (PBT)

Monodisperse solution (315nm)

Monodisperse solution (315nm)

Indium-Tin-Oxide glass slide

Room temperature for 90 mins

Indium-Tin-Oxide glass slide

Room temperature for 90 mins

Scanning Electron Microscopy

Synthetic Opal

Figure 15. Self-assembly of opal structures onto electrode substrates via monodisperse colloidal dispersions. Reprinted with permission from [188], V. Misoska, Ph.D. Thesis, University of Wollongong, 2002.

into the interstitial void spaces of the host matrix. Upon removal of the PS, structural shrinkage was observed for PPy and PAn but PBT exhibited no such effect. Using this approach, interchanneling between adjacent template layers was evident, giving evidence for the formation of a three-dimensional (3D) macroporous nanostructure. Yoshino et al. [124] utilized SiO2 opal templates followed by the infiltration of soluble preformed poly(3-alkylthiophene) and poly(2,5-dialkoxy-p-phenylene vinylene) and finally removal of the SiO2 spheres by HF etching. These nanostructured materials were then investigated for photoluminescence (PL) and electroluminescence properties. A spectral narrowing of the PL and an increase in excitation intensity was observed with respect to the solution form of these polymers.

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