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.