Effect of Dialysis on the Electrochemical Properties of Acid-Oxidized Single-Walled Carbon Nanotubes

The impact of residual acid moieties adsorbed on single-walled carbon nanotubes (SWNTs), as a result of the purification and shortening of the SWNTs in a concentrated nitric and sulfuric acid mixture (1:3 ratio, v/v), on the electrochemical properties of carbon-nanotube-modified electrodes was assessed. Two different nanotube samples were prepared: acid-cut SWNTs that were rinsed with water until the washings had a neutral pH to remove excess acid (nondialyzed SWNTs) and acid-cut SWNTs that were dialyzed against a solution of Triton X-100 for a period of 18 h (dialyzed SWNTs). X-ray photoelectron spectroscopic analysis of dialyzed SWNTs showed a reduced concentration of sulfur-containing ions and nitrogen-containing species when compared with that of the nondialyzed SWNTs. These ions can be ascribed to residual acid moieties adsorbed onto the nanotube surfaces during the purification process using acid mixtures. The impact of these residual acidic species on the conductivities of the carbon nanotubes was assessed via electrochemical measurements using self-assembled ferrocene-functionalized nanotube monolayers on a gold electrode. Our measurements showed that the nondialyzed nanotubes exhibited a slower rate of electron transfer compared with the dialyzed nanotubes. We postulate that the adsorbed nitrogen-containing and, to a lesser extent, the sulfur-containing species in the nondialyzed samples become desorbed by the dialysis process, and the vacant sites thus created are subsequently replaced by more electron-withdrawing species such as the perchlorate used as the counterion in the electrochemical experiments. Hence, the concentration of holes formed by charge transfer from the π-electrons to the adsorbed species will be higher, and an electrode constructed with dialyzed carbon nanotubes will exhibit a higher conductivity than that constructed with nondialyzed nanotubes.