The ratios of the areas of Fe2O3 to Cr2O3 peaks of all the
samples are shown in Table 3. For the as-received sample, the
ratio is much less than 1, revealing that the surface is mainly
covered by the Cr2O3 film. Figure 2a shows that there is virtually
no CNT on the surface of the as-received stainless steel. This
may be due to the fact that the catalyst iron particles coming
from the gaseous mixture are poisoned by the passive chromium
oxide film present on the substrate, possibly by forming a FeCr-O
compound. The surface is therefore devoid of active iron
particles acting as catalysts for CNT nucleation and growth. This
is analogous to the selective growth mechanism of the CNTs on
Si/SiO2 substrates.27 It has to be noted that, by passing only ethylene carbon source, no nanotubes were observed on the
stainless substrate without a catalyst precursor. The stainless
steel substrate acts as a support for the nanotube growth but is
devoid of any catalyst sources such as free metallic iron particles.