The SEM images of carbon fiber before and after the growth of
CNTs/CNFs are shown in Fig. 1. Carbon nanotubes possess a hollow
core at the center while this hollow core is either very narrow or
not present at all in case of carbon fibers. SEM image of carbon fiber
after acid cleaning is shown in Fig. 1a, carbon fiber after the
growth of CNTs/CNFs is shown in Fig. 1b–d at different magnifications.
It is evident from these images that the fiber surface is uniformly
covered by CNTs/CNFs. The CNTs grow long enough with
different diameter and length. From the TEM analysis it was observed
that the CNTs grow normal to the fiber substrate as evident
from Fig. 2 and may possess various morphologies [30]. When
these tubes grow very long they assume random direction forming
network like structure as shown in Fig. 1d. The variation in the
tube diameter depends on the catalyst size, distribution and number
density.