Relationship between carbon nanotube structure and electrochemical behavior: Heterogeneous electron transfer at electrochemically activated carbon nanotubes

The electrochemical activation of multiwalled carbon nanotubes (MWCNTs) (at potentials of 1.5-2.0 V vs Ag/AgCl for 60-360 s) results in significantly increased rate constants (k_obs⁰) for heterogeneous electron-transfer with [Fe(CN)₆]^3-/4- (from 8.34 × 10^-5 cm s^-1 for as-received MWCNTs to 3.67 × 10 ^-3 cm s^-1 for MWCNTs that were electrochemically activated at 2.0 V for 180 s). The increase in the value of k_obs⁰ arises from the introduction of wall defects exposing edge planes of the MWCNTs, as observed by high-resolution TEM. The density of the edge plane defects increases from almost zero (for as-received MWCNTs) to 3.7% (for MWCNTs electrochemically activated at 2.0 V for 180 s). High-resolution X-ray photoelectron spectroscopy (HR-XPS), Raman spectroscopy, and electrochemical impedance spectroscopy were used to gain a better understanding of the phenomena. HR-XPS revealed that the increase in electrochemical activation potential increases the number of oxygen-containing groups on the surface of carbon nanotubes.