Synthesis and characterization of manganese dioxide spontaneously coated on carbon nanotubes

Manganese dioxide (MnO₂) was coated on carbon nanotubes (CNTs) by simple immersion of the CNTs into a KMnO₄ aqueous solution. The synthesis mechanism was investigated by in situ monitoring of the reduction potential and pH of the solution and supplementary UV-VIS analysis of MnO₄^- ions in the solution. CNTs were found to act as a reducing agent and substrate for the heterogeneous nucleation of MnO₂ in an aqueous KMnO₄ solution. The morphology of the CNTs before and after MnO₂ deposition was examined using scanning electron microscopy, which showed MnO₂ deposited as a thin and uniform layer on the CNTs at an initial pH of 7, but as nano-rods of MnO₂ at an initial pH 1. The MnO₂ was shown to be a Birnessite-type MnO₂ by X-ray powder diffraction and Raman spectroscopy. The thermal stability of the CNTs, examined by thermogravimetric analysis, was improved by the thin, uniform and continuous coating of MnO₂.