The electrocatalytic properties of arc-MWCNTs and associated ‘carbon onions’

For the first time we report on the electrochemical characteristics of nanometer sized polyhedral graphite onions dispersed amongst arc-MWCNTs. These are formed during the electric arc discharge method of producing ultrapure MWCNTs (arc-MWCNTs). The carbon onions are randomly dispersed amongst the arc-MWCNTs which are produced with very little amorphous carbon deposits or other unwanted impurities and are formed as closed-ended tubes. By comparison with commercially available open-ended hollow-tube multiwalled carbon nanotubes made using the chemical vapor deposition method (cvd-MWCNTs), a glassy carbon electrode (GCE), an edge-plane pyrolytic graphite electrode (eppg) and basal plane pyrolytic graphite (bppg) electrode, we can speculate that it is the edge-plane-like defect sites that are the electroactive sites responsible for the apparent ‘electrocatalysis’ seen with a wide range of analytes including: ferrocyanide, ruthenium hexaamine(III), nicotinamide adenosine dinucleotide (NADH), epinephrine, norepinephrine, cysteine, and glutathione. The arc-MWCNTs themselves are produced as closed-ended tubes with very few, if any, edge-plane-like defect sites evident in their HRTEM characterization. Therefore we speculate that it is the carbon onions dispersed amongst the arc-MWCNTs which have incomplete graphite shells or a rolled-up ‘Swiss-roll’ structures that posses the edge-plane-like defect sites and are responsible for the observed voltammetric responses. Carbon onions are no more or no less ‘electrocatalytic’ than open-ended MWCNTs which in turn are no more electrocatalytic than an eppg electrode. As the carbon onions are ubiquitous in MWCNTs formed using the arc-discharge method the authors advise that caution should be taken before assigning any electrocatalytic behavior to the MWCNTs themselves as any observed electrocatalysis likely arises from the carbon onion impurities.