CHINESE JOURNAL OF PHYSICS
VOL. 47, NO. 3
JUNE 2009
Hydrogen and Methane Gas Sensors Synthesis of
Multi-Walled Carbon Nanotubes
P. Samarasekara
Department of Physics, University of Peradeniya, Peradeniya, Sri Lanka
(Received May 23, 2008)
The electrical resistance of multi-walled carbon nanotubes increases or decreases in
methane (CH4 ) or hydrogen (H2 ) gas, respectively. The gas sensitivity was measured in
a mixture of methane and H2 gases with different partial concentrations for the very first
time. The marginal value of the ratio between the methane and H2 gases concentrations in a
gas mixture at which the resistance variation changes sign was studied at several measuring
temperatures. This marginal value depends on the measuring temperature. The best sensitivity, fastest response, and fastest recovery could be observed at 230 or 300 ◦ C in methane
or H2 gas, respectively. These were the highest possible measuring temperatures for each
gas, and samples did not indicate stable properties at operating temperatures above 230 or
300 ◦ C for methane or H2 gas, respectively. Furthermore, the change of resistance in the
H2 partial concentration of 1.3% in air can be inverted by introducing 2.7% of methane into
this mixture of H2 /air at 230 ◦ C.
PACS numbers: 81.15.Cd, 07.07.Df, 73.25.+i
I. INTRODUCTION
Hydrogen (H2 ) is becoming the fuel of the next generation of automobiles. Methane
(CH4 ) is considered to be a natural gas fuel. Both H2 and methane gases were found to be
highly explosive and flammable. Therefore, the detection of the existence of a small amount
of methane or H2 in houses, vehicles, or industrial places is potentially important. Because
methane is a odorless gas, a quick and highly sensitive gas sensor is needed to identify
methane gas. Carbon nanotubes were employed to detect H2 or methane gas individually [1,
2], but not to detect the existence of a mixture of methane and H2 gases with different partial
concentrations. Therefore, the behavior of multi-walled carbon nanotubes in a mixture of
gases will be explained in this report. Single wall and multi-walled carbon nanotubes have
received wide attention due to their potential use in nanotechnology. Multi-walled carbon
nanotubes (MWNTs) find some potential applications in ammonia and water vapor gas
sensor applications [3]. Single wall carbon nanotubes (SWNTs) are widely used to detect
oxidizing gases, such as oxygen and nitrogen dioxide, and reductive gases, such as ammonia,
at room temperature [4–6]. MWNTs are employed in gas sensor applications because of
their larger effective surface area, with many sites allowed to adsorb gas atoms, and their
hollow geometry. Especially the effective surface area of MWNTs is greater than that of
SWNTs. Furthermore, the manufacturing cost of MWNTs is less than that of SWNTs.
http://PSROC.phys.ntu.edu.tw/cjp
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c 2009 THE PHYSICAL SOCIETY
OF THE REPUBLIC OF CHINA
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Previous research experiences with carbon nanotube gas sensors can be summarized
as follows. Carbon nanotubes together with a nanoporous Pd film sensor have been sensitive
to H2 concentrations from 100 ppm to 1.5%. These aligned carbon nanotubes have been
deposited in an anodic aluminum oxide (AAO) template [2]. MWNT films fabricated by
chemical vapor deposition on micro-machined substrates have been employed to detect
nitrogen dioxide (NO2 ) [7]. NH3 gas has been detected at room temperature, using porous
indium oxide nanotubes together with carbon nanotube templates [8]. Carbon nanotubes
synthesized using plasma-enhanced chemical vapor deposition on Si3 N4 /Si substrates have
been employed as resistive gas sensors for NO2 at an operating temperature of 165 ◦ C [9].
MWNTs coated with tin oxide layers have been investigated to detect a trace amount of
oxidizing and reduced gases, such as NO2 . According to the studies, the barrier between tin
oxide and nanocrystals on the MWNTs was the key factor determining the sensor resistance
in different gases [10]. NO2 detection in carbon nanotube field effect transistor chemical
sensors has been investigated. They have revealed that changes at the interface between
the nanotubes and the electrodes are the reason for NO2 detection, not the molecules
adsorbed on the nanotube surface [11]. NH3 and NO2 gases have been detected using
carbon nanotubes synthesized on anodic alumina membranes. 10 V dc voltage applied
for 2 min has been used to shorten the recovery time of these sensors [12]. A dielectric
membrane filled with a mixture of carbon nanotubes has been employed to detect nitrogen
gas. The amplitude and phase change of a microwave signal passing through this sample
have been measured in order to study the gas sensitivity [13].
The electrodeposition technique has been used to prepare carbon nanotubes and
nanofibers on Si(001) for methane gas sensor applications [1]. NH3 gas sensors have been
fabricated from SWNT powder using the screen printing technique, followed by an annealing
pretreatment in open air for 2h at different temperatures [14]. Furthermore, SWNTs of 1.2
nm in diameter synthesized using the same technique have been employed to detect NH3 gas
of 5 ppm [15]. H2 gas sensors have been prepared using SWNT grown on alumina substrates
by means of the airbrushing method. Sensitivity of these SWNTs has been improved
by adding Pd [16]. The dielectrophoretic impedance measurement technique has been
extended to the controllable assembly of carbon nanotube gas sensors. According to the
studies, the relative conductance change of the sensors after NO2 exposure increased almost
proportionally with the initial conductance for a constant NO2 concentration [17]. Chemical
gas sensors sensitive to 10 ppm NH3 have been synthesized using SWNTs electrochemically
functionalized with polyaniline [18]. Electron beam lithography and lift off techniques have
been used to make carbon nanotube gas sensors to detect NH3 . When these sensors are
successively exposed to NH3 gas at room temperature, the conductance of the MWNT
decreases [19].
II. EXPERIMENT
Co film of 5 nm thick was deposited on an SiO2 /Si wafer by means of thermal evaporation, and subsequently annealed at 500 ◦ C in air for 2 hours to form the phase of
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Co2 O3 . Then a gold layer was synthesized using vacuum evaporation. Gold electrodes were
fabricated using photolithography followed by chemical etching of the gold films. These
micro-patterned substrates were preheated in H2 at 450 ◦ C for 2 hours in a chemical vapor deposition (CVD) system. Subsequently the system was heated to the carbon nanotube growth temperature of 950 ◦ C, and CH4 (1%) in H2 was introduced to the system.
The growth period of carbon nanotubes is 90 min. Scanning electron microscopy (SEM)
measurements and Raman spectroscopy were performed to verify the formation of carbon
nanotubes.
The gas sensitivity of the carbon nanotube film sample placed in a glass chamber was
measured with time in methane, H2 , and a gas mixture. The gas sensitivity of this film
sample was determined by measuring the resistance on the film surface between two gold
electrodes, which was measured using a high mega Ohm multimeter. The sample was heated
using a heater coil attached to a dc regulated power supply, in order to study the effect
of temperature on gas sensitivity. The temperature was monitored using a thermocouple
contacting the film sample together with a digital display. In this dynamic process, the ratio
between the particular gas and air inside the glass chamber was controlled by changing the
ratio between gas flow rates passing through the chamber, which were manipulated by
means of needle valves coupled to digital displays.
III. RESULTS AND DISCUSSION
III-1. Variation of gas sensitivity of a MWNT with partial concentration for
one gas
H2 gas sensitivity steps in different H2 partial concentrations at 230 ◦ C without
methane are given in Figure 1. Arrows from the bottom to the top indicate the points,
where H2 percentages of 0.4, 0.7, 0.9, 1.1, 1.4, 1.7, and 2.5% in air, respectively, were
introduced. Sensitivities at these H2 percentages are 0.4, 0.54, 0.58, 0.61, 0.66, 0.71, and
0.8, respectively. The gas sensitivity is defined as the ratio of the difference between the
resistance at a particular point and the initial resistance to the initial resistance. Only the
positive value of gas sensitivity is considered, disregarding negative signs. The resistance
of carbon nanotubes decreases with time in H2 gas. The initial resistance between two gold
electrodes was 80 kΩ in all of these cases.
III-2. Variation of gas sensitivity of a MWNT with operation temperature for
one gas
The gas sensitivity in methane partial pressure of 3% in air at 150 ◦ C (dashed line)
and 230 ◦ C (solid line) without H2 is given in Figure 2. After introducing methane gas,
the resistance gradually increased and reached the peak value. The samples measured at
operating temperatures above 230 ◦ C indicated some unstable properties. The responce
times at 230 and 150 ◦ C are 60 and 90 s, respectively. The recovery times are 90 and 116
s at 230 and 150 ◦ C, respectively. The maximum sensitivities are 0.08 and 0.02 at 230 and
150 ◦ C, respectively. Therefore, the response and recovery times as well as gas sensitivity
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FIG. 1: Gas sensitivity for H2 in different H2 concentrations without methane at 230 ◦ C. Arrows
from bottom to top show the points, where the H2 percentages of 0.4, 0.7, 0.9, 1.1, 1.4, 1.7, and
2.5%, respectively, were introduced.
FIG. 2: Gas sensitivity in methane partial concentration of 3% without H2 : at 150 ◦ C (dashed line)
and 230 ◦ C (solid line).
are better at higher operating temperatures. However, according to our previous studies,
the gas sensitivity of cuprous oxide and tungsten oxide films was higher at the operating
temperatures, at which the response and recovery times were the worst. Therefore, the
behaviors of carbon nanotube gas sensors are different from the behaviors of metal oxide
thin film gas sensors. Although the methane gas sensitivity measured for these MWNT
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365
samples was less than that measured by some other researchers, the main aim of this
project was to investigate the behavior of a MWNT in a mixture of two gases.
FIG. 3: Gas sensitivity in H2 partial concentration of 1% without methane: at 150 ◦ C (thin solid
line), 230 ◦ C (dashed line), and 300 ◦ C (thick solid line)
The gas sensitivities in H2 partial concentration of 1% in air at 150 ◦ C (thin solid line),
230
(dashed line), and 300 ◦ C (thick solid line) without methane are shown in Figure
3. After introducing H2 gas, the resistance gradually decreased and reached the minimum
value. The response times at 150, 230, and 300 ◦ C are 2760, 2500, and 917 s, respectively.
The recovery times are 375, 460, and 215 s at 150, 230, and 300 ◦ C, respectively. The
highest sensitivities at 150, 230, and 300 ◦ C are 0.54, 0.6 and 1.3, respectively. The best
sensitivity, best recovery time, and best response time could be obtained at 300 ◦ C. Similar
to samples measured in methane, the samples measured at operating temperatures above
300 ◦ C indicated some unstable properties. The maximum sensitivity obtained for H2 by
us is really larger than those obtained by some other researchers [2]. But the response and
recovery times reported in that report for H2 are slightly better than those of our samples
measured in H2 . According to the SEM measurements, the outer diameter of our carbon
nanotubes was found to be 6 nm. The diameter of our carbon nanotubes are higher than
that of the carbon nanotubes synthesized by these researchers. The higher the effective
surface area is the higher the gas sensitivity.
When any gas is flowing into the measuring chamber, the concentration of gas inside
the chamber varies with time as [11]
t
.
(1)
N (t) = N0 1 − exp −
t0
◦C
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This can be expanded for small t/t0 values as
t2
t
− 2 .
N (t) = N0
t0 2t0
(2)
The fitting curve equation of the first response part of the curve at 150 ◦ C shown in Figure
3 is s = −2t2 × 10−6 + 0.0214t − 0.1864. This fitting equation is in the form of Equation (2)
with some constant. This reveals that carbon nanotubes absorb gas atoms in an exponential
manner as shown in Equation (1). After comparing the fitting equation with Equation (2),
the constants N0 and t0 were found to be 1.145 × 102 and 5.35 × 103 s, respectively. Because
this t0 value is greater than the highest value of time (t) in this portion of the curve, t/t0 < 1,
and hence the assumption used to obtain Equation (2) is correct.
III-3. Behavior of gas sensitivity of a MWNT in a mixture of two gases
FIG. 4: The first part of the curve indicates the resistance of the MWNT in H2 partial concentration of 1.3% without methane at 230 ◦ C. The arrow indicates the point where methane gas was
introduced.
The initial part of Figure 4 shows the time dependence of resistance in H2 partial
concentration of 1.3% in air without methane at 230 ◦ C. After introducing H2 gas, the
resistance gradually decreased and reached the minimum value. Then the methane gas
was introduced into the measuring chamber together with 1.3% H2 , and the methane gas
flow (methane partial concentration) was gradually increased until the resistance began to
increase. This turning point has been indicated by the arrow. Once the methane gas partial
concentration reached 2.7%, the effect of methane gas dominated the effect of H2 , and the
resistance began to increase. This phenomenon can be explained in terms of the height of
the surface barrier (eVs ), which is a characteristic of adsorbed gas. When the height of the
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surface barrier of one (methane) gas exceeds the surface barrier of the other gas (H2 ), the
change of resistance of the carbon nanotubes alters.
FIG. 5: Maximum sensitivity versus the ratio between the methane and H2 gas partial concentrations: at 150 ◦ C (solid line) and 230 ◦ C (dashed line).
The peak value of gas sensitivity versus the ratio between the methane and H2 partial
concentrations in air at 150 ◦ C (solid line) and 230 ◦ C (dashed line) are given in Figure 5.
The ratio between the methane and H2 gases at the turning points are 0.6 and 0.8 at 150 and
230 ◦ C, respectively. Therefore, this ratio between the methane and H2 gases at the turning
point is a function of the measuring temperature. In methane or H2 only, the resistance
gradually increases or decreases, respectively. Above or below this value corresponding to
the turning point, the effect of methane or H2 gas dominates, respectively. Because the
effect of methane gas is balanced by the effect of H2 gas at the turning point, the sensitivity
reaches a minimum value.
The SEM micrograph of a sample with carbon nanotubes is given in Figure 6. This
picture indicates that the outer diameter of the carbon nanotubes is 6 nm. This micrograph
implies that the sample is very dense and randomly oriented.
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FIG. 6: SEM micrograph of a sample with carbon nanotubes.
IV. CONCLUSION
The sensitivity in H2 is better than that in methane. For these MWNTs, the higher
the sensitivity is, the better the response and recovery times. In methane with a partial
concentration of 3% in air, the maximum sensitivities are 0.08 and 0.02 at 230 and 150
◦ C, respectively. In H with a partial concentration of 1% in air, the highest sensitivities
2
are 0.54, 0.6, and 1.3 at the measuring temperatures 150, 230, and 300 ◦ C, respectively.
The best properties of carbon nanotubes in H2 or methane were observed at the measuring
temperatures of 300 and 230 ◦ C, respectively. Furthermore, the samples indicated some
unstable behavior at higher operating temperatures than at these measuring temperatures
for both gases. In order to study the variation of gas sensitivity at different partial concentrations, the steps were measured as shown in Figure 1. Because the resistance gradually
increases or decreases in methane or H2 gas, respectively, the sensitivity can be minimized
at one particular ratio between methane and H2 partial concentrations in a mixture of H2
and methane gases. This ratio between methane and H2 partial concentrations at the turning points are 0.6 and 0.8 at the measuring temperatures of 150 and 230 ◦ C, respectively.
The gas sensitivity in 1.3% of H2 and air at 230 ◦ C can be inverted by introducing 2.7% of
methane. The effect of methane dominates the effect of H2 gas at this turning point. This
ratio of partial concentrations is a function of the operating temperature as well.
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