Traditional Raman or X-ray analysis of the CNT
crystallinity is not straightforward against the background of the
underlying carbon fibre, but these CNTs can be expected to be
tubular and moderately defective, giving rise to the observed curvature.
The diameters were between 20 and 50 nm (mean value
around 30 nm), and lengths were a few micrometres (Fig. 1e–f).
As expected, the catalyst particles were a similar diameter to the
CNTs grown from them [31]. The initial oxidation of the carbon fibres
before catalyst deposition is crucial; the oxygen-containing
groups present after acid treatment encourage the wetting of fibres
by the aqueous salt solutions and help to stabilise small iron particles
on the surface during the initial catalyst deposition [18].