forms of carbon are essentially the same for hydrogen molecules.
516
Close-ended SWNTs and open
SWNTs have a relatively low accessible surface areas due to bundling of the tubes.
MWNTs can be used as additives to metal adsorbents.
517
Mg–5 wt% MWNTs composite absorbed
4.86 wt% (80% maximum hydrogen storage capacity) within 1 min at 553 K, under 2.0 MPa.
The problem has been discussed in the reviews.
518–527
One can trace the change from rather
optimistic estimations of the prospects to use NTs as effective storage media in early publications,
to more cautious conclusions and even to a pessimistic point of view in the latest ones. Today, it is
still unclear whether NTs will have real practical applications in the hydrogen storage area. Addition
of some metals and salts to the NTs, chemical activation, and aligning of NTs leads to an increase
in adsorption capacity. However, some other problems which need to be solved include achievement
of fast kinetics of discharge, high stability during cycling, low self-discharge level, etc.
2.7.2 CARBON NANOTUBE GAS SENSORS
The adsorption of different gases and vapors in NT materials as reported in publications up to year
2000 has been briefly reviewed earlier.
8
Apart from hydrogen, adsorption of noble gases,
528–532
nitrogen,
531–534
oxygen,
531,532,534–536
carbon monoxide,
534
carbon dioxide,
531,533,537–539
nitrogen diox-
ide,
531,540
ammonia,
531,540
water vapor,
531,534
methane,
528,531,533,541–544
acetylene,
545
acetone,
546
carbon
tetrachloride,
547
carbon tetrafluoride,
530
sulfur hexafluoride,
530
methanol,
548
ethanol,
548,549
linear-
chain alkanes, and fatty acids
550
has been characterized in the last 5 years. The gas transport prop-
erties of NTs has been measured.
551,552
NTs have been found to be an effective separation media for removing some admixtures from
flue gases; they can store gases having relatively high molecular mass (e.g., methane). However, the
most promising field of their application is the development of new sensing devices. Sensing
gaseous molecules is important in environmental monitoring, control of chemical processes and
agriculture, and biological and medical applications. NT-based sensors have huge prospects in outer
planetary exploration and for incorporation into yarns and fabrics.
Chemical doping of NTs, particularly of SWNTs, changes their electronic properties and can
induce strong changes in conductance
48,553,554
and in thermoelectric power.
555–557
Chemical sensors
are measurement devices that convert a chemical or physical property of a specific analyte into a
measurable signal whose magnitude is usually proportional to the concentration of the analyte. For
sensing applications, NTs have advantages, such as small size with larger surface, high sensitivity,
fast response, and good reversibility.
The first sensor based on SWNTs was reported by Dai and coworkers,
558
who demonstrated that
small concentrations of NO
2
produced large changes in sensor conduction. To clarify the reason for
the discrepancy of recovery time between theoretical results and experimental data, the formation
of adsorbed NO
3
has been postulated.
559
Recent works
560,561
show the possibility of using MWNT
thin films for measuring sub-ppm NO
2
concentrations (10 to 100 ppb) in dry air, with the maximum
response at 165°C. The gas-sensing properties of NT thin films depend on the nature of the defect
and concentration.
562
Gas-sensing characteristics of MWNTs as applied to humidity, NH
3
, CO, and CO
2
partial pres-
sure have been studied.
563
A practical gas sensor for ammonia and water vapor, based on measuring
the variation of the electrical conductivity of MWNT ropes, has been proposed.
564
In particular, the
absorption of different gases in the MWNT/SiO
2
layer changes the permittivity and conductivity of
the material, consequently altering the resonant frequency of the sensor, which is tracked remotely,
using a loop antenna.
564
Detecting properties of MWNTs for NH
3
allow the exploitation of the sen-
sor at room temperature.
565
The construction of gas sensors based on MWNT thin films to measure NO
2
, CO, NH
3
, H
2
O,
and C
2
H
5
OH concentrations has been described.
561
The thermoelectric response due to the interaction of adsorbed molecules with NT walls can be
used to detect gases such as He, N
2
, and H
2
.
566
Chemistry of Carbon Nanotubes 69