Electrochemical sensors based on carbon nanotubes - Electrochemical Sensors, Biosensors and Their Biomedical Applications - page 464

Chemical vapor deposition (CVD) [27] of hydrocarbons over a metal catalyst is a
method that has been used to synthesize carbon fi bers, fi laments, etc. for over 20 years.
Large amounts of CNTs can be formed by catalytic CVD of acetylene over Co and Fe
catalysts on silica or zeolite.
CNTs can also be produced by diffusion fl ame synthesis, electrolysis, use of solar
energy, heat treatment of a polymer, and low temperature solid pyrolysis. In fl ame
synthesis, combustion of a portion of the hydrocarbon gas provides the elevated tem-
perature required, with the remaining fuel conveniently serving as the required hydro-
carbon reagent. Hence, the fl ame constitutes an effi cient source of both energy and
hydrocarbon raw material. Combustion synthesis has been shown to be scalable for a
high volume commercial production.

15.2.4 Purifi cation of carbon nanotubes
In order to obtain the optimal performance of CNTs in various applications, high purity
CNTs will be required. Purifi cation of CNTs generally refers to the separation of CNTs
from other entities, such as carbon nanoparticles, amorphous carbon, residual catalyst,
and other unwanted species. A number of purifi cation methods including acid oxidation,
gas oxidation, fi ltration, and chromatography have been developed to date. In many
cases, various combinations of these methods are used to obtain high quality CNTs.
The acid refl ux procedure was fi rst described by Rinzler et al. [28], in which raw
nanotube materials are refl uxed in nitric acid to oxidize the metals and carbon impuri-
ties. Acid-treated CNTs are considered to have carboxylic acid groups at the tube ends
and, possibly, at defects on the side walls. The functionalized SWNTs have consider-
ably different properties from those of the pristine tubes.
Gas phase oxidation is commonly used for the purifi cation of CNTs. The method
proposed by Ebbesen et al. [20] uses heat treatment of crude CNT products under
a gas containing oxygen. This method has been explored extensively because it can
yield the most highly graphitized tubes without being contaminated by the metal
catalyst.
Filtration is also used to purify CNTs. Bandow [29] have reported a procedure for a
one-step SWNT purifi cation by microfi ltration in an aqueous solution with a cationic
surfactant. Shelimov [30] developed an ultrasonically assisted fi ltration method which
allows the purity of nanotubes to reach

90%.
Many chromatographic methods such as permeation chromatography, column chro-
matography, and size exclusion chromatography have been used to purify CNTs. The
size exclusion chromatography (SEC) is the only carbon nanotube purifi cation method
in the literature that is not subjected to the acid treatments which tend to create the car-
boxylic functionality on CNTs.
Sample purity is documented with SEM, TEM, and electron microprobe elemen-
tal analysis. Raman and UV-vis-near-IR spectra are also useful techniques that can
be used to examine the quality of CNTs at the different stages of the purifi cation
procedure.