Chemistry Reference
In-Depth Information
Fig. 6 Schematic representation of a HiPco furnace
2.4 Solar Energy
The solar energy method emerged originally for the production of fullerenes, until
1998, when Laplaze et al. brought some variations to the process in order to launch
carbon nanotubes synthesis [
25
].
The peculiarity of this technique is to employ solar rays to mimic the laser
ablation process, obtaining gram scale production of nanotubes relatively inexpen-
sively and in a more controllable manner.
The principle of the synthesis is based on the sublimation of a graphite com-
posite rod induced by sunlight, which is collected and reflected by a parabolic
mirror onto the target surface.
The high temperature (2,600-2,700
C) causes both the carbon and the catalyst to
vaporize and afterwards to condense onto the walls of a water-cooled screen.
MWCNTs and SWCNTs grow preferentially in relation to both catalyst compo-
sition and pressure: using Ni/Co mixtures, at low pressure, the sample collected
mainly contains MWCNTs, whereas at higher pressures only bundles of SWCNTs
are obtained. Luxembourg et al. reported the mass production of SWCNTs using a
50-kW solar reactor, able to synthesize 0.1-100 g/h of highly pure material [
26
].
It has also been observed that the best quality product resulted from using 15 cm
long graphitic rods with 2% of Ni/Co catalyst, under He atmosphere.
2.5 Electrolysis
Another novel and low-consumption method for nanotube production is electroly-
sis, developed by Hsu et al. in 1995 [
27
]. The novelty of this approach consists in
the generation of nanotubes by passing a current through an ionic salt medium
between two electrodes. Alkali metals such as Li, Na, or K dissociate from their
chloride salts on a graphite cathode, which is consumed during the electrolysis with
subsequent nucleation of nanotubes. The material so formed, containing a mixture
of CNTs and nanoparticles, is separated from the ionic salt by dissolution in
distilled water followed by filtration.
