Biomedical Engineering Reference
In-Depth Information
little nanotube material can provide ample sites for gas interaction.
A BET study [59] demonstrates that N
adsorption isotherms for the
raw CNTs and a sample purified using a complete two-step process
(DMF solvent-based de-bundling and acid treatment followed by air
oxidation) produce an increase of the surface area from 577 m
2
2
/g to
2
1587 m
/g for raw material and purified material, respectively. This
proves that the purified CNTs exhibit enhanced adsorptive capacity.
In fact, this procedure has yielded purified SWCNTs with a BET area
of 1587 m
2
/g which is the highest value reported for CNTs. Moreover,
Yang
. [60, 61] studied the protocols of purification for CNTs in
order to increase the their surface area. HiPco SWCNTs containing
Fe particles were purified by one-step purification with HCl-washing
(D-method) and two-step purification with HCl-washing after air
oxidation (GD method). The N
et al
adsorption BET isotherms of the
SWCNTs were performed at 77 K. Purification of HiPco-based SWCNTs
was found to increase their total surface area from 524 m
2
2
/g (raw
2
nanotubes) to 587 m
/g (D-method treated nanotubes) and 861
m
/g (GD-method treated nanotubes). Additionally, the purification
protocol applied provides an increase of the micropore volume in the
SWCNTs from 0.15 ml/g (raw nanotubes) to 0.19 ml/g (D-method
treated nanotubes) and 0.27 ml/g (GD-method treated nanotubes);
while it produces a decrease in the average pore width from 3.5 nm
to 1.7 nm and 1.0 nm, respectively. Thus, purification considerably
affects the surface area and pore structures (volume and width) to
enhance the gas adsorption in the CNTs with consequent improved
gas sensitivity and very low limit of gas detection.
Thermogravimetric analysis (TGA) is currently performed on
nanomaterials submitted to purification protocols for the assessment
of their weight loss due to the removal of carbon and/or metal
impurities and/or doping nanoparticles upon thermal treatment
at high temperatures up to 800-1000°C in gas flow (Ar or N
2
). For
2
example, Xu
[62] reported TGA measurements demonstrating
that the purified HiPco Fe-grown SWCNTs samples were stable in
air at temperatures as high as about 450°C and their Fe content was
reduced from 30 wt.% to approximately 1 wt.%. Other TGA studies
devoted to CNTs processed post-growth for their purification are
reported in literature [63-67], mainly for SWCNT structures treated
by acids and air oxidation to remove metal impurities, amorphous
carbon and non-nanotube material. The efficiency of these
purification protocols, assessed by TGA measurements, seems to be
very good.
et al.
