Tuning the properties of CVD-grown multiwalled carbon nanotubes by ex situ codoping with boron and nitrogen heteroatoms

The electronic, optical and thermal properties of multiwalled carbon nanotubes (MWCNTs), synthesised using an atmospheric pressure chemical vapour deposition technique, were tuned by ex situ codoping with boron and nitrogen (B/N) atoms at different temperatures (700-900 degrees C). High magnification TEM micrographs showed that the morphology of the as-grown B/N-MWCNTs was affected by the doping temperature. For instance, low temperatures (700-800 degrees C) led to growth of B/N-MWCNTs exhibiting the two-compartment morphology, comprising a bamboo-like tubular inner compartment and a coiled flower-like outer compartment. At 900 degrees C, an alignment and restructuring of the coiled flower-like outer compartment occurred and resulted in growth of tubular and thicker B/N-MWCNTs. XPS data analysis corroborated this finding by indicating the incorporation of stable boron and nitrogen configurations, such as h-BN, BCN and graphitic-N, into the carbon lattice with increasing temperature. The optical analyses of B/N-MWCNTs were in good agreement with the XPS data, by depicting that the optical properties were governed by the type and amount of bonding states within the carbon lattice. Finally, thermal analysis studies revealed that the B/N-MWCNTs were more thermally stable owing to the incorporation of B atoms. Thus, the study illustrated that by adjusting the doping temperature, the properties, such as the optical properties, which are usually difficult to adjust for MWCNTs, could be tuned.