Biomedical Engineering Reference
In-Depth Information
material sometimes hinders the advantageous CNTs properties
and limits the optimal performance of the CNTs applications to
new functional devices with enhanced gas-sensing characteristics.
The modification of the chemical composition at the surface of the
CNTs has been proved efficient at overcoming these drawbacks.
Purification methods attempt to remove the unwanted carbon and
metal impurities without damaging the CNTs nanostructure.
A number of purification methods have been developed to
date [59-66, 104, 182-185]. They can be classified into four
major methods: gas oxidation, acid treatment, filtration, and
chromatography. A common approach has been to use the gas
phase oxidation by a thermal annealing in presence of oxidizing gas
followed by an acid treatment. The oxidative treatment of pristine
CNTs material is effective in removing non-nanotube carbon from
bundles and promotes also the removal of the carbon coating from
metal catalysts; while the acid treatment removes the metallic
catalytic impurities. The annealed and acid-treated nanotubes are
thought to have functional groups at the tube ends and at sidewall
defects giving rise to chemical, structural and electrical changes
of the CNTs. Such modifications are likely to alter the physical
properties and in particular the gas adsorption behavior of the CNTs.
Thus, the relationships between the adsorption properties and such
purification-induced changes are critical for enhanced gas sensing
applications. The purified CNTs have been found to exhibit higher
surface area and improved gas adsorption capacity [61, 185]. Hence,
they used as promising sensor nanomaterials promote enhanced
gas sensitivity. In other terms, the possible effects of the purification
of the CNTs on gas sensitivity should be better outlined by sensing
dynamics achieved using specific transducers.
Penza
. [186] developed a nanocomposite layer based on filler
of HiPco SWCNTs and matrix of CdA by LB technique for SAW gas
sensor applications at room temperature. HiPco SWCNTs containing
Fe particles were purified in a two-step purification process by
thermal annealing in oxygen environment and post-treatment in
HCl acid. Nanocomposite LB films of pristine and purified SWCNTs
embedded in an organic matrix of CdA were prepared with a fixed
weight filler content of 75 wt% onto a SAW 433 MHz quartz two-
port resonator. Figure 9.21 shows the molecular structure of the
CdA-SWCNT nanocomposite film.
et al
