Parametric Analysis of NO2 Gas Sensor Based on Carbon Nanotubes
PHOTONIC SENSORS /
Parametric Analysis of NO2 Gas Sensor Based on Carbon Nanotubes
Asama N. NAJE 0
Russul R. IBRAHEEM 0
Fuad T. IBRAHIM 0
0 University of Baghdad, Collage of Science, Department of Physics , Baghdad , Iraq
Two types of carbon nanotubes [single walled nanotubes (SWCNTs) and multi walled carbon nanotubes (MWCNTs)] are deposited on porous silicon by the drop casting technique. Upon exposure to test gas mixing ratio 3% NO2, the sensitivity response results show that the SWCNTs' sensitivity reaches to 79.8%, where MWCNTs' is 59.6%. The study shows that sensitivity response of the films increases with an increase in the operating temperature up to 200◦C and 150℃ for MWCNTs and SWCNTs. The response and recovery time is about 19 s and 54 s at 200℃ for MWCNTs, respectively, and 20 s and 56 s at 150℃ for SWCNTs.
SWCNTs; MWCNTs; NO2 gas; sensitivity; response time
Detecting gas molecules is basic to
environmental monitoring, control of chemical
processes, space mission, agricultural and medical
applications. The sensing of NO2 is important to
monitor environmental pollution resulting from
combustion or automotive emissions [
dioxide (NO2) is flammable, colorless, and
dangerous, even at very low concentration.
Moreover, the interaction of NO2 and CO in sunlight
tends to produce O3, which is believed to be harmful
to plants and the respiratory system of human beings
and animals because of its strongly oxidizing
behavior. Therefore, from the public health and
environmental protection viewpoint, the sensitive
detection of nitrogen dioxide is of great scientific
A gas sensor is a device which detects the
presence of different gases in a region, particularly
those gases which might be harmful to humans or
animals. The development of the gas sensor
technology has received considerable attention in
recent years for monitoring environmental pollution.
It is well known that chemical gas sensor
performance features, such as sensitivity, selectivity,
time response, stability, durability, reproducibility,
and reversibility, are largely influenced by the
properties of the sensing materials used [
basic principle behind gas detection is a change in
an electrical property of the detecting material upon
exposure to the gas. For example, in the case of
carbon nanotubes, the resistance changes with
exposure to different gases like NO2 and NH3. Other
electrical properties of nanotubes like thermoelectric
power and dielectric properties also change upon gas
]. Common gas sensors are metal oxide
semiconductor such as tin oxide, zinc oxide,
titanium oxide, and aluminum oxide. Problems
encountered with these sensors are lack of flexibility
and poor response time, and it is operated at the
elevated temperature [
]. But nowadays,
researchers’ goal is to develop the sensors, which
can operate at room temperature and consume low
power. Carbon nanotubes (CNTs) based sensors
have been recently studied due to their excellent
electrical, mechanical, and sensing properties.
Because of the high surface area, CNTs can adsorb
large amount of gases, which make them a probable
contender for a gas sensor with very high sensitivity
and low response time. Particularly, gas adsorption
in CNTs is an important issue for both fundamental
research and technical applications [
]. There are
two types of carbon nanotubes’ morphology.
Single-walled nanotubes (SWNTs) consist of a
honeycomb network of carbon atoms and can be
visualized as a cylinder rolled from a graphitic sheet.
The other is multi-walled nanotubes (MWNTs) that
are a coaxial assembly of graphitic cylinders
generally separated by the plane space of graphite
The present study focuses on the synthesis and
chemi-resistive characteristics of carbon nanotubes
(CNTs) thin film sensor. The material is prepared by
a simple chemical solution method and the thin
films are fabricated by drop-coating method.
2.1 Preparation of the samples
In this work, 22 cm2 dimensions primary n-type
silicon wafer substrates were thoroughly cleaned to
de-contaminate their surface from any available
stains and dirt. A porous silicon layer (PS) was
prepared via photochemical wet etching. This
process was carried out by using ordinary light
source. Its main apparatus consisted of a Quartz
Tungsten Halogen lamp (250 W), a focusing lens
(3.8 cm) with focal length, and the diluted etching
HF acid of 50% concentration mixed with ethanol in
(1:1) ratio in a Teflon container. To prepare CNT
sample, 0.01 g of CNT was dispersed in
Dimethylformamide (DMF). A magnetic stirrer was
incorporated for this purpose for 15 minutes,
followed by 1 hour sonication. The obtained solution
was used for film deposition on porous silicon by
the drop casting method.
2.2 Gas sensor testing system
The detail of the gas sensor testing unit, which
was used in the current tests, was described
]. A steel cylindrical test chamber of
diameter 163 mm and of height 200 mm with the
bottom base made removable and of O-ring sealed.
The effective volume of the chamber was 4173.49 cc,
which had an inlet for allowing the test gas to flow
in and an air admittance valve allowing atmospheric
air after evacuation. Another third port was provided
for the vacuum gauge connection. A multi-pin feed
through at the base of the chamber allowed for the
electrical connections to be established to the sensor
and the heater assembly. The heater assembly
consisted of a hot plate and a k-type thermocouple
inside the chamber in order to control and set the
desired operating temperature of the sensor. The
thermocouple sensed the temperature at the surface
of the film exposed to the analyte gas. The
PC-interfaced multi meter, of type UNI-T UT81B,
was used to register the variation of the sensor
conductance (reciprocal of resistance) exposed to
predetermined air – NO2 gas mixing ratio. The
chamber can be evacuated by using a rotary pump to
a rough vacuum of 2×102 bar. A gas mixing
manifold was incorporated to control the mixing
ratios of the test and carrier gases prior to being
injected into the test chamber. The mixing gas
manifold was fed by zero air and test gas through a
flow meter and needle valve arrangement. This
arrangement of mixing scheme was done to ensure
that the gas mixture entering the test chamber was
premixed thereby giving the real sensitivity.
3. Results and discussion
The response of a sensor upon the introduction
of a particular gas species is called the sensitivity (S).
The most general definition of sensitivity applied to
solid state chemi-resistive gas sensors is a change in
the electrical resistance (or conductance) relative to
the initial state upon exposure to a reducing or
oxidizing gas component. It is calculated by using
the equations below [
S R 100
Rgas Rair 100
S G 100
Ggas Gair 100
where R is the electrical resistance, G is the
electrical conductance, and the subscript “air”
indicates that background is the initial dry air state
and the subscript “gas” indicates the analyte gas has
Figure 1 shows the scanning electron microscope
(SEM) images for the MWCNT and SWCNTs
deposited on PS. In these figures, long nanotubes
with large agglomerates and closely packed CNTs
Fig. 1 SEM images for the CNT on PS: (a) MWCNTs and (b)
The atomic force microscopic (AFM) of the
PSi/CNTs is shown in Fig. 2. Results of surface
morphology of the CNTs film had a good uniform
surface homegensity and good indication for having
nanoporous with a regular distribution of the CNTs.
The roughness average (Sa) for this layer of
PSi/CNTs was 1.04 nm while the root mean square
roughness (Sq) was 1.24 nm and ten point height (Sz)
was 5.86 nm.
Fig. 2 AFM images for the CNTs deposited on PS.
Figure 3 shows the response of the MWCNTs
and SWCNTs thin films upon exposure to NO2 gas
for mixing ratio 3% and at varying operating
temperatures. It can be seen that the response of the
films increases as the operating temperature
increases up to 200℃ and 150℃, for MWCNTs and
SWCNTs, respectively, and then decreases. The
gas-sensing response increases with temperature in
the 50℃ ‒ 200℃ and 50℃ ‒ 150 ℃ ranges for
MWCNTs and SWCNTs, respectively, because
thermal energy helps the reactions involved
overcome their respective activation energy barriers.
However, if the operating temperature becomes too
high (i.e., >150 ℃ for SWCNTs or 200◦C for
MWCNTs), the adsorbed oxygen species at the
sensing sites on the film surface will be diminished
and less available to react with NO2 molecules,
thereby limiting the film’s response.
As shown in Fig. 4, the sensitivity S% increases
with increasing the operating temperature T, where
the maximum sensitivity of MWCNTs is 59.61%, at
200 ℃ of the testing temperature after which it
begins to drop with increasing T and the test is
While the sensitivity of SWCNTs is 79.81% at
150℃ of testing temperature after which it begins
to drop with increasing T and the test is terminated.
0 200 400 600 800
Fig. 4 Transient response of CNTs thin film at various testing
temperatures upon exposure to NO2 gas, 3% maxing ratio, at 5 V
bias voltages for (a) MWCNTs and (b) SWCNTs.
At any operating temperature, the sensor
response of the SWCNTs thin film is higher than
that of the MWCNTs thin film. The results show that
SWCNTs have a better performance than MWCNTs,
and these results strongly depend on the structure of
CNTs. When the structure of atoms in a carbon
nanotube minimizes the collisions between
conduction electrons and atoms, a carbon nanotube
is highly conductive. The strong bonds between
carbon atoms also allow carbon nanotubes to
withstand higher electric currents than material. So
when the diameter of tubs decreases the current
increases. Since SWCNTs have small diameter
(1 nm‒2 nm), it will have the higher conductivity
and better performances [
Fig. 5 Current-time variation of CNTs sensor time at 25℃,
50 ℃ , 100◦C, 150 ℃ and 200 ℃ testing temperature upon
exposure to NO2 gas and 5 V bias voltage for (a) MWCNTs and
The observed current appears in Fig. 5 increases
MWCNTs and SWCNTs. The response time was 19
(i.e resistance decreases) when exposing the CNT
s and 20 s for MWCNTs and SWCNTs, respectively.
networks to NO2, which can be due to that the
electron transfer occurs from CNTs to NO2 which
has highly oxidizing natures. The variation of device
resistance (or current) in the presence of oxidizing
gas can be explained as follows: when oxidizing
species like NO2 are adsorbed on the surface of
p-type CNTs, the Fermi levels are shifted towards
valance band, generating
more holes and thus
decreasing the electrical resistance.
The response time of the CNTs gas sensor
decreases with increasing the operating temperature,
and with the shortest response and recovery time
being at about 19 s and 54 s at 200℃ for MWCNTs,
respectively, and 20 s and 56 s at 150℃ for SWCNTs.
In order to explain the results, the chemical
nature of the NO2 molecule is considered. Recent
conductance of an individual semiconducting carbon
nanotube strongly increases upon NO2 gas exposure
and the NO2 is identified as an electron acceptor. In
light of the present work, it is reasonable to propose
that this behavior in the nanotube film is also due to
adsorbed NO2 in the tube wall. According to recent
theoretical calculations, a possible interpretation of
the electrical response of CNT films to NO2 gas
could be explained in terms of the physical
absorption of this molecule. NO2 has an unpaired
electron and is known as a strong oxidizer. Upon
NO2 adsorption, a charge transfer is likely to occur
from the CNTs to the NO2 due to the
electronacceptor character of NO2 molecules. The electrical
response of the CNTs indicates that there is a charge
transfer between the test gas and sensing element,
and hence, the physisorption of gases in the
nanotubes is the dominant sensing mechanism.
Thin films of two types of carbon nanotubes
(SWCNTs and MWCNTs) were prepared by the
drop casting method. The maximum variation of
resistances to NO2 was found at an operating
temperature of around 200 ℃
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