Tuning nitrogen functionalities in catalytically grown nitrogen-containing carbon nanotubes

Nitrogen-containing carbon nanotubes (NCNT) were grown from acetonitrile, pyridine or N,N-dimethylformamide over a supported Fe-, Co- or Ni catalyst in the temperature range 823–1123 K. The physico-chemical properties of the obtained NCNT, such as the C/N ratio or the nitrogen type, were related to the synthesis parameters. It was found that the C/N ratio increased with increasing temperature which could be related to the thermodynamic stabilities of the metal-carbides and metal nitrides. Also the type of nitrogen present in the graphene layer changed with increasing temperature from predominantly pyridinic- to quaternary nitrogen. NCNT obtained with the Fe catalyst showed bamboo morphology regardless of the C/N source or growth temperature while straight tubes were obtained with the Co- or Ni catalyst. We propose that this difference in morphology can be explained by the thermodynamic stabilities of the different metal-carbides, leading to a ‘pulsating’ growth in the case of Fe as opposed to a more continuous growth in the case of Co or Ni.