Year

2007

Degree Name

Doctor of Philosophy

Department

Department of Chemistry - Faculty of Science

Abstract

Nanotechnology provides an effective and direct way to create novel properties and phenomena through the reduction in material sizes without changing the materials’ chemical composition. A number of routes to the preparation of novel nanostructured electrodes were investigated in this thesis. These involve the formation of nanoporous opaline electrodes, three dimensional nanofibrous networks and the synthesis of flexible nanoelectrodes based on highly dense ordered aligned carbon nanotubes and conducting polymers. Excellent improvements with the use of nanostructures in a wide range of application areas such as methanol oxidation, photoelectrochemical cells, enzyme biosensors, cell culturing and energy storage are presented in this research work. Nanoporous opaline structures including inverse opals and opals were prepared by either electrodepositing Pt or sputter coating ITO onto self-assembled polystyrene (PS) synthetic opals, followed by the removal of the PS opal templates. A highly ordered dense nanoporous structure with the porous structure on the top (so-called Pt inverse opal) or with the porous structure on the bottom (so-called ITO opal) was consequently obtained after the removal of PS templates. The improvement in electrochemical area with the use of nanostructures was observed during electrochemical characterisation. The resultant nanostructured Pt inverse opal electrodes were employed in electro-oxidation of methanol. Compared with the Pt film electrode, the nanostructured Pt inverse opal electrode showed a higher catalytic performance and good stability with a 100 mV negative shift of the potential of methanol oxidation. The mesoporous ITO opal electrode was used as the substrate for the electrodeposition of polyterthiophene and the resultant structure was subsequently utilized in photoelectrochemical cells. An excellent power-conversion efficiency of 0.109% and an outstanding short circuit current density of 1470 μA•cm-2 for polyterthiophene deposited at room temperature were obtained; dramatically improved from the previous published work. Nanofibrous electrodes were fabricated from biomaterials (such as DNA and poly(styrene-β-isobutylene-β-styrene) (SIBS)) and single-walled carbon nanotubes (SWNTs) using the electrospinning technique. Initial studies quantitatively determined the influence of solution properties (such as the solution ionic conductivity, surface tension and viscosity) and process parameters (e.g. tip-to-collector distance, applied potential and the feed rate) on the electrospinning results. Results showed that good electrospun fibrous networks could be obtained from the solution with comparatively high conductivity and viscosity with low surface tension. It was also found that the average diameter of the electrospun fibers decreased with decreased feed rates, increased tip-to-collector distance and increase in the potential employed. With the addition of SWNT, both biomaterial nanofiber electrodes exhibited enhanced electrochemical properties. The resulting DNA based electrospun fiber electrode showed a broad linearity range and high sensitivity in enzyme biosensors. The SIBS/SWNT nanofibrous electrode demonstrated excellent biocompatibility and suitability for the growth of L-929 cells. Flexible, light and highly conductive nanostructured electrodes were prepared from aligned carbon nanotubes (ACNTs) and conducting polymers by coating with Pt coated poly(vinylidene fluoride) (PVDF) or poly(3,4-ethylenedioxythiophene) (PEDOT)/PVDF. Pt nanoparticles were subsequently electrodeposited on the ACNT/Pt/PVDF structure. The utilization of the nanostructured ACNT/conducting polymer electrodes in anodic methanol oxidation and as anodic materials in Lithium-ion batteries was demonstrated. Pt nanoparticles coated ACNT/Pt/PVDF electrode exhibited an outstanding electrochemical capacity (133 Fg-1) and amazing electrochemical surface area (143 m2g-1 for Pt nanoparticles). The Pt nanoparticles-ACNT/Pt/PVDF electrode also showed a 2.5 times higher steady current density for methanol oxidation when compared with the ACNT/Pt/PVDF electrode. A stable current density over a long period (more than 12 hours) was obtained. A 50% improvement in capacity during Lithium-ion battery tests when compared with a SWNT paper was obtained with the ACNT/PEDOT/PVDF electrode. Nanostructured flexible and conductive electrodes were also obtained from ACNTs and biomaterials (such as SIBS and poly(lactide-co-glycolide)). SWNTs or Pt were introduced to improve the conductivity. A significant improvement in electrochemical properties with the addition of Pt or SWNT was obtained. The biocompatibility of ACNTs, SWNTs and Pt was confirmed during cell culturing experiments.

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