Chemistry Reference
In-Depth Information
Fig. 5 Depiction of the CVD-based synthesis of carbon nanotubes
of growth (length per minute) [
22
]. It has been observed that nanotubes grow faster
and better aligned on porous silicon substrates compared to flat silicon or alumina
supports.
Depending on the parameter setting (temperature, operation pressure, active
particle nature and size, supports, reaction time) the morphology of the product is
widely tunable, giving selective SWCNT or MWCNT growth in powder or forest
form. CVD can afford yields higher than 99% with remarkable purity and low
amorphous carbon amounts.
For all these reasons, the majority of the methods currently employed for
nanotube production are reinterpretations of the CVD technique. One of the most
common is high-pressure carbon monoxide (HiPco) synthesis, renowned for the
unique capacity to make SWCNTs on a kilogram per day scale [
23
].
2.3.1 HiPco Synthesis
This new synthetic approach uses a volatile organometallic precursor for the
generation of the catalyst inside the reaction chamber. More precisely, a high-
pressure carbon monoxide stream (30-50 atm) is introduced continuously into the
furnace (900-1,100
C) together with Fe(CO)
5
or Ni(CO)
4
. Under such conditions
of temperature and pressure, carbon monoxide disproportionates, acting as a carbon
feedstock, while the organometallic species decompose, giving the catalytic
clusters on which nanotubes can nucleate and elongate (Fig.
6
).
Average diameters of the nanotubes produced vary between 0.7 and 1.1 nm,
depending on the pressure applied in the reaction chamber and the catalyst nature.
The synthesis product yield is up to 97% with purity exceeding 70%. This
method is particularly suitable for large-scale synthesis since the reaction can be
carried out continuously and allows the reuse of carbon monoxide [
24
].
