Individual Single-Walled Nanotubes and Hydrogels Made by Oxidative Exfoliation of Carbon Nanotube Ropes

Single-walled carbon nanotubes were oxidized by a technique previously developed for the oxidation of graphite to graphite oxide (GO). This process involves treatment with concentrated H2SO4 containing (NH4)2S2O8 and P2O5, followed by H2SO4 and KMnO4. Oxidation results in complete exfoliation of nanotube ropes to yield individual oxidized tubes that are 40−500 nm long. The C:O:H atomic ratio of vacuum-dried oxidized nanotubes is approximately 2.7:1.0:1.2. XPS and IR spectra show evidence for surface O−H, CO, and COOH groups. The oxidized nanotubes slowly form viscous hydrogels at unusually low concentration (≥0.3 wt %), and this behavior is attributed to the formation of a hydrogen-bonded nanotube network. The oxidized tubes bind readily to amine-coated surfaces, on which they adsorb as smooth and dense monolayer films. Thin films of the oxidized nanotubes show ohmic current−voltage behavior, with resistivities in the range of 0.2−0.5 Ω-cm.