Fundamental electrochemical properties of carbon nanotube electrodes

We report on the fundamental electrochemical properties of inherently hydrophobic Multi-walled Carbon Nanotubes (MWNT) electrodes and acid-treated MWNTs. The electrochemical response of the electrodes was investigated by cyclic voltammetry and AC impedance spectroscopy using the redox probe of (Fe(CN)₆^3- / Fe(CN)₆^4-, which has well-known kinetic behaviour on carbon surfaces. Comparisons were made between the electrochemical behaviour observed at MWNT electrodes and electrochemically polished glassy carbon, GCE. Confirmation of carbon oxygen functionalities on the surface of acid-treated nanotubes was accomplished by x-ray photoelectron spectroscopy. Slow electron transfer kinetics with a peak separation larger than 200 eV, and an electron transfer resistance of approximately 37011 were observed on pristine MWNTs. However, acid treatment was found to significantly improve the electron transfer kinetics of Fe(CN)₆^-3/-4 redox couple, approaching almost reversible ET kinetics. Acid treatment modifies the charge carrier density at the surface by introducing surface states and serves to facilitate electron transfer. The better wetting properties of functionalised carbon nanotubes and the higher local density of states as compared to untreated MWNT could be responsible for favouring faster ET kinetics.