Structure and nitrogen incorporation of carbon nanotubes synthesized by catalytic pyrolysis of dimethylformamide

A systematic study on the effects of reaction and annealing temperatures on the structure and N doping of carbon nanotubes is presented. CNx nanotubes prepared by aerosol-assisted chemical vapour deposition of dimethylformamide exhibit a bamboo-like structure made of numerous tubular compartments for all reaction and annealing temperatures studied. Post-synthesis nanotube annealing in a vacuum lead to a better graphitisation and appearance of a striking structural feature: nanotube tip- and compartment-ends of high crystallinity display either flat or conical morphologies, which remarkably differ from pure carbon nanotubes. The walls of well-structured CNx nanotubes are rigid and easily ruptured, whereas N-free carbon nanotubes are known to be fairly flexible. High N concentrations ( > 10 at.%) within C tubes could be obtained when a reaction temperature is appropriately chosen. This has been explained based on the idea that a pre-existing C-N bond is the key factor to increasing N content in carbon nanotubes. The stability of N atoms within C nanotubes has been investigated by X-ray photoelectron and electron energy loss spectroscopy. The tubes doped with high N contents were found to be chemically unstable: partial loss of N happened when the material was exposed to air. (C) 2004 Elsevier Ltd. All rights reserved.