The effects of carrier gas and liquid feed flow rates on longitudinal patterns of CNT growth

We report detailed longitudinal profiles of four samples of carbon nanotubes (CNT) arrays synthesized using ferrocene-xylene in 850 deg. C. Point-to-point analysis of the CNTs grown in 'growth windows' (GWs) were performed using optical microscopy, Raman spectroscopy, FESEM, and high-resolution TEM techniques. All samples showed a maximum array height around the middle of their GWs. The height profiles comprised a secondary peak/shoulder, attributed to evolution of C2 byproducts. The results showed that pyrolytic deposition of amorphous carbon as well as catalytic growth of CNTs strongly depend on the local temperature along the GW. It was also observed that the CNT grew longer at the higher flow rate of the carrier gas while the GWs became wider and extended towards the end of the reactor. On the other hand, the higher liquid flow rate reduced the average diameter and length of the CNTs. While residence time was revealed to be important, no significant analogy was found between samples with the same feed dilution. The termination of GWs was attributed to the inhibition of amorphous carbon as well as temperature drop below ∼815 deg. C.