The stainless steel samples with different acid treatment times
of 1, 5, and 10 min along with the as-received sample (as the
control sample) were considered to elucidate the effect of surface
properties on the CNT growth. Figure 1 shows the scanning
electron microscope (SEM) micrographs of MWNTs synthesized
on the stainless steel foils treated for different times. It is
observed that the density and alignment of the CNTs strongly
depend on the duration of the acid etching time. The as-received
stainless steel (Figure 1a) barely had any CNTs on its surface,
whereas the sample etched for 1 min (Figure 1b) had randomly
scattered CNTs. The density of the CNTs gradually increased
on the substrates as the etching time increased, and highly aligned
CNTs were obtained on the substrate etched for 10 min (Figure
1d).