Investigations of carbon nanotube modified electrodes

Abstract

The work presented in this thesis is concerned with electrodes modified with carbon nanotubes. Carbon nanotubes have been characterised with special emphasis on the oxygenated species generated from cutting in acid mixtures. Several different techniques have been used for the analysis, especially infrared spectroscopy (IR) in combination with X-ray spectroscopy (XPS) analysis and transmission electron microscopy (TEM) in combination with atomic force microscopy (AFM). TEM analyses were used to reveal the morphological differences between various nanotube cutting times. The lengths of the nanotube were found to decrease with increasing cutting time. Electrochemical measurements were performed on carbon nanotube modified electrodes using nanotubes of different cutting time. The peak separation of ferricyanide redox reaction was found to depend strongly on the length of nanotube and also on the orientation of nanotube at the interface. Whilst at the randomly dispersed, the peak separation showed a decrease with decreasing nanotube length, vertically aligned nanotubes showed no dependence of the peak separation on the nanotube length. Electrochemical results together with spectroscopy measurements show that the highly electroactive edge planes were located on the carbon nanotubes and the oxygenated species in the ends of the nanotubes from cutting in acid mixtures were responsible for the good electrochemical properties. Bamboo-shaped carbon nanotube is a morphological variation of multi-walled carbon nanotubes where the graphite planes are formed at an angle to the axis of the tube. Glassy carbon electrodes modified with bambootype carbon nanotubes showed greater electrochemical signal compared with electrodes modified with singlewalled carbon nanotubes due to the edge planes of graphite located at regular intervals along the walls of the bamboo-shaped carbon nanotube, thus confirming the importance of the ends of nanotube in controlling the kinetics of electron transfer events. Effect of nanotube orientation was studied using ferrocenemethylamine attached to randomly dispersed and vertically aligned nanotubes. The electron transfer kinetics was found to depend strongly on the orientation of the nanotube with the electron transfer at the randomly dispersed slower than vertically aligned. Results were addressed using the analogy that the ends of the nanotubes are like the ends of the tubes can be described as edge-plane-like whilst the tube walls are basal-plane-like. Difference in electron transfer kinetics suggested that the electron transfer in nanotubes could occur via two different pathways: through the edge plane-like opening of the nanotube or by hopping across the walls of the nanotube. Triton X-100 was used to dialyse the acid cut nanotubes. XPS analysis of dialysed nanotubes was compared with non-dialysed nanotubes. A reduced concentration of sulfate ions was found in the dialysesd sample. Nitrate ion (407 eV) was removed after dialysis. Amino groups (400 ev) and protonated amino-group (402 eV) both seemed to be removed slowly by dialysis. Theses ions could be ascribed to residual ions trapped inside nanotubes from cutting in acid mixtures. The electrochemical response of ferrocenemethylamine was also studied. The electron transfer rate constants were rate constants were higher at dialysed nanotube assembly than non-dialysed, which was attributed to doping effect incurred from cutting. Electron transfer between nanotube and gold electrode surface was studied by attaching nanotubes to linker length of 6, 8, and 11 carbons. The results were exploited to rationalise the role of the chemical structure of the nanotubes in facilitating electron transfer from the redox species to the electrode surface that was otherwise suppressed without the presence of nanotubes. The observed redox activity was a consequence of resonant electron transfer from the LUMO of the acceptor to the HOMO of the donor under the influence of an applied voltage, assuming the nanotube modified electrode behaves similarly to the metal-molecule-metal junction mode.