Surface modification of multiwall carbon nanotubes by electrochemical anodic oxidation

The surface modification of multiwall carbon nanotubes (MWCTNs) was achieved by electrochemical anodic oxidation in NaOH and H2SO4 electrolytes. Their defect structures, functional groups, morphology and dispersibility in aqueous solutions were characterized by SEM, TEM, XPS, FTIR, Raman spectroscopy, zeta potential analysis and a stability test of their suspensions. Results indicate that anodic oxidation with the NaOH electrolyte removes more amorphous carbon, introduces fewer defects and more oxygen-containing functional groups (mainly -OH), and produces shorter nanotubes, as a result of which the resulting nanotubes are more stable when dispersed in aqueous solutions compared with those oxidized by the H2SO4 electrolyte. MWCTNs are more easily oxidized in the NaOH electrolyte, and the numbers of oxygen-containing functional groups and hydroxyl groups increase continuously with the oxidation degree. In the H2SO4 electrolyte, however, the numbers of oxygen-containing functional groups and hydroxyl groups increase and level off with the oxidation degree.