Purification of Single-Wall Carbon Nanotubes by Electrochemical Oxidation

The electrochemical cyclic voltammetric (CV) oxidation behavior of an arc-derived single-wall carbon nanotube (SWNT) sample in potassium hydroxide solution was investigated. Amorphous carbon in the as-grown SWNT sample was effectively removed by the CV oxidation, as confirmed by analyzing the sp3/sp2 carbon ratio from C1s XPS spectra and HRTEM observations. The removal of the amorphous carbon led to the exposure of metal nanoparticles, hence facilitating the elimination of the metal impurities by subsequent HCl washing. The CV oxidation can be applied as an alternative oxidative treatment for the purification of SWNT samples. Redox peaks were observed during the CV oxidation. The reduction peaks in the range of -0.96 to -1.0 V and the oxidation peaks in the range of -0.61 to -0.49 V were attributed to the electrochemical redox transformations between metallic Fe and Fe(II) oxide, Fe(II) oxide and Fe(III) oxide, as well as Ni and Ni(II) oxide, and the observed reduction peaks in the potential range of 0.29-0.13 V were believed to be caused by the electrochemical reduction of NiOOH into Ni(OH)2. The intensity of all of the redox peaks was dependent on the cycle number because more and more metal nanoparticles could be exposed as a result of the incremental removal or damage of the amorphous carbon coating during the CV oxidation, while the intensity remained almost unchanged after 80 cycles because of the completion of the amorphous carbon removal. Therefore, the redox peaks from the electrochemical redox reactions of Fe and Ni impurities can be considered as a benchmark for the removal extent of the amorphous carbon, and optimal electrochemical oxidation time for the purification of the as-grown SWNT sample can be determined in real time during the CV oxidation treatment. This is a predominant advantage of the CV oxidation over common oxidation methods using air or other oxidizing reagents for SWNT purification.