Helical and Dendritic Unzipping of Carbon Nanotubes: A Route to Nitrogen-Doped Graphene Nanoribbons

Bamboo structured nitrogen-doped multiwalled carbon nanotubes (CNx-MWCNTs) have been successfully unzipped by a chemical oxidation route, resulting in nitrogen-doped graphene nanoribbons (CNx-GNRs) with a multifaceted microstructure. The oxidation of CNx-MWCNTs was carried out using potassium permanganate in the presence of trifluoroacetic acid or phosphoric acid. On the basis of the high resolution transmission electron microscopy studies, the bamboo compartments were unzipped via helical or dendritic mechanisms, which are different from the longitudinal unzipping of open channel MWCNTs. The product graphene oxide nanoribbons were simultaneously reduced and doped with nitrogen by thermal annealing in an ammonia atmosphere. The effects of the annealing temperature, time, and atmosphere on the doping level and types of the nitrogen functional groups have been investigated. X-ray photoelectron spectroscopy results indicate that a wide range of doping levels can be achieved (4-9 at %) simply by changing the annealing conditions. Pyridinic and pyrrolic nitrogen functional groups were the dominant species that were attached to the edges of the CNx-GNRs. The GNRs, with a faceted structure and pyridinic and pyrrolic groups on their edges, have abundant nitrogen sites. These active sites could play a vital role in enhancing the electrocatalytic performance of GNRs.