2010 International Conference on Nanotechnology and Biosensors
IPCBEE vol.2 (2011) © (2011) IACSIT Press, Singapore
Fabrication of Gas Sensors based on Carbon Nanotube for CH4 and CO2 Detection
Amin Firouzi, Shafreeza Sobri, Faizah Mohd Yasin, Fakhru’l-razi b. Ahmadun
Department of Chemical and Environmental Engineering
Universiti Putra Malaysia
Serdang, Malaysia
[email protected], [email protected], [email protected], [email protected]
Abstract—This study was carried out to investigate the
adsorption effect of CH4 and CO2 on carbon nanotubes (CNTs)
based gas sensors by measuring the variation of their electrical
resistance. The CNTs were synthesized by Floating Catalyst
Chemical Vapor Deposition (FC-CVD) method on quartz
substrate under benzene bubble at temperature of 700˚C. Then,
they were tested for gas detection. Upon exposure to gaseous
molecules, the electrical resistance of CNTs dramatically
increased for both CH4 and CO2 gases with short response time
and high sensitivity. In addition, the recovery of the sensors
and mechanism of gas sensing procedure are discussed.
Keywords-carbon nanotubes; chemical vapor deposition; gas
sensor; quartz substrate
I.
INTRODUCTION
Discovered in 1991 by Iijima [1] carbon nanotubes
(CNTs) are being globally as a new dream material in the
21st century and broadening their applications to almost all
the scientific areas, such as electrochemical actuators [2]
hydrogen storage [3], emission display [4] and chemical
sensors [5]. To utilize them in fundamental investigation and
industrial application, synthesizing high quality CNTs on a
large scale is necessary. Considering the production cost and
application requirement, Chemical vapor deposition (CVD)
technique is no doubt the most comprehensive one [6].
In the present report, CNTs were synthesized directly on
a substrate by using a liquid reactants and the vapor of
catalyst precursor that are bubbled into the reaction zone
with carrier gas such as H2 and/or Argon [7].
Recently, CNTs have been successfully applied as
promising candidates for fabricating gas and chemical
sensors, due to their high surface area, size and hollow
geometry [5, 8]. The fabrication of sensors using CNTs was
first reported by Dai and coworkers [9]. These authors
demonstrated that small concentration of NO2 are capable of
producing large changes in the sensor conductance, shifting
the Fermi level to the Valence band and generating hole
enhanced conductance [9]. Also, CNTs based sensors for
detection of gases such as H2, NH3, CO2 or CH4 have already
been successfully demonstrated [8, 10, 11].
In this research, we present that the grown CNTs could
be used in the fabrication of fast response gas sensor
operating at room temperature.
II.
EXPERIMENTAL
A. Synthesis of CNTs
Carbon nanotubes were synthesized in a conventional
CVD apparatus consisted of double-stage electric furnace, a
ceramic tube located horizontally inside both furnaces (50
mm OD, 40 mm ID and 1 m length) and gas flow controlling
units [12].
In a typical experiment, we put a quartz substrate (1 cm ×
1 cm) that was placed in the ceramic boat and 200 mg of
Ferrocene (catalyst precursor) in furnace 2 and 1,
respectively, as depicted in Fig. 1. At first, the ceramic tube
was flushed with argon in order to eliminate oxygen from the
reaction chamber during heating up the reactor [13].
When the second furnace reached to a temperature of
700˚C, argon flow was stopped. Then the temperature of first
furnace was raised to 120˚C to sublimate the catalyst [14].
After 5 minutes, hydrogen carrier flow (150 ml/min) was
bubbled into the benzene (200 ml) that used as a
hydrocarbon source before mixing with Fe nanoparticles.
Then, benzene decomposed in the presence of Fe
nanoparticles in the second furnace to form CNTs. The
reaction is over after 45 minutes and furnace was cooled
down with argon flow.
Figure 1. Schematic diagram of CVD apparatus.
165
B. Gas Sensing Set up
Two types of gas (CH4 and CO2) were employed for gas
sensing application. Electrical contacts were made to the
sensors by connecting two copper wires using silver paste.
Fig. 2 Shows that the silver paste contact were made directly
to the top of the nanotubes grown over quartz. Also, the
wires from the CNTs film were connected to the digital
Multimeter( ESCORT, 3145A) used to obtain and
monitoring the values of resistance every second on the
computer.
The sensors were placed in a sealed plexiglas chamber of
0.027 m3 volume with electrical feedthrough. Argon gas was
used continuously as the carrier gas throughout the work.
After purging of chamber with pure argon when the
resistance of CNTs gave constant results, the tested gas, CH4
or CO2, was injected into the chamber with concentration of
369 ppm.
The resistance of CNTs was recorded before and after
injection at every one second. The experiments have been
repeated three times with four times injection for each gas to
get the precise results. All the measurements were taken at
room temperature of 25˚C.
III.
RESULTS AND DISCUSSION
A. Morphology Observations
CNTs grown on quartz substrate were examined by
Scanning Electron Microscopy (SEM) and Transmission
Electron Microscope (TEM) to get the general view of the
product formed such as morphology (alignment), topology
(uniformity) and diameter.
Fig. 3a shows the SEM micrograph of synthesized CNTs
on quartz. From SEM image, we can clearly see that the
CNTs are mostly entangled and exhibit high accumulation
with the shape of matchstick.
TEM image (Fig. 3b) indicates the presence of multiwalled CNTs type at reaction condition. Also, the image
shows that the grown CNTs are long and uniform with
relatively thick diameters of about 25-65 nm. Long reaction
time of 45 minutes can be caused to synthesize large
diameters in CNTs due to the fact that catalyst particles have
sufficient time for more collisions, resulting in the increase
of the Fe nanoparticles size and CNTs diameter [12].
Figure 3. Synthesized CNTs on quartz (a) SEM image (b) TEM image.
B. Sensor Characteristics
In general, these sensors were reliable and exhibited good
performance in terms of response time and sensitivity in the
overall resistance.
Fig. 4 and Fig. 5 show the electrical resistance variations
of p-type semiconductor CNTs [15] upon exposure to CH4
and CO2 filling and pumping environment, respectively.
For both gases, sensors showed a fast response of ca. 30
second. This indicates that the sensors are highly sensitive
towards CO2 and CH4 adsorption at room temperature of
25˚C. Also, it can be seen that the resistance of CNTs
increases at the first injection of target gas. The same
increment can be observed for the second, third and fourth
injection.
Figure 2. Connected copper wires directly to the top of the grown
nanotubes by using silver paste.
166
In the case of CO2, the recovery of sensor is not complete.
It can be as a result of high adsorption energy between CO2
molecules and CNTs molecules resulting in the incomplete
CO2 desorption in the short time [16]. Fabricated sensors
have been highly electrical resistance, in the range of 1200
ohm. The increasing of resistance can be described as
follows:
As we know CH4 and CO2 are reducing gases [17].
Exposure to these gases, impressively causes to move the
valence band of CNTs away from the Fermi level thereby,
the positive hole carrier of p-type semiconductor depleted
and the electrical resistance increases [9].
The sensor sensitivity (S) is calculated using (1) [15]:
S (%) = (( Rg – R0 )/R0 )*100
(1)
Where, Rg and R0 are the resistance values of the sensor
with and without gas exposure, respectively. Sensitivity of
sensors to gas adsorption is summarized in Table 1.
IV.
CONCLUSION
We have obtained high density of uniform CNTs having
diameters in the range of 25-65 nm by FC-CVD method of
benzene at 700˚C. The CNTs patterned on quartz substrate
have been used for gas sensing application. These sensors are
highly sensitive with fast response to CH4 and CO2
molecules and the recovery process is complete only for CH4.
Our results have shown that CNTs have potential to be an
excellent gas sensor at room temperature. Thus, future work
will concentrate on further enhancing the performance of
these CNTs-based gas sensors.
ACKNOWLEDGMENT
This work was supported by the research grant provided
by the Malaysian Ministry of Science, Technology and
Innovation.
REFRENCES
Figure 4. Electrical resistance variations of CNTs-based sensor upon
injection of CH4 gas.
[1]
[2]
[3]
[4]
[5]
[6]
Figure 5. Electrical resistance variations of CNTs-based sensor upon
injection of CO2 gas.
TABLE I.
[7]
SENSITIVITY, S (%), OF SENSORS TO CH4 AND CO2
EXPOSURE
[8]
167
Iijima, S., "Helical microtubules of graphitic carbon," Nature, Vol.
354, pp. 56-58, 1991.
Baughman, R.H., C. Cui, A.A. Zakhidov, Z. Iqbal, J.N. Barisci, G.M.
Spinks, G.G. Wallace, A. Mazzoldi, D. De Rossi, A.G. Rinzler, O.
Jaschinski, S. Roth and M. Kertesz, "Carbon nanotube actuators,"
Science, Vol. 284, pp. 1340-1344, 1999.
Tibbetts, G.G., G.P. Meisner and C.H. Olk, "Hydrogen storage
capacity of carbon nanotubes, filaments, and vapor-grown fibers,"
Carbon, Vol. 39, pp. 2291-2301, 2001.
Rinzler, A.G., J.H. Hafner, P. Nikolaev, L. Lou, S.G. Kim, D.
Tomanek, P. Nordlander, D.T. Colbert and R.E. Smalley, "Unraveling
nanotubes: Field emission from an atomic wire," Science, Vol. 269,
pp. 1550-1553, 1995.
Modi, A., N. Koratkar, E. Lass, B. Wei and P.M. Ajayan,
"Miniaturized gas ionization sensors using carbon nanotubes," Nature,
Vol. 424, pp. 171-174, 2003.
Chen, P., X. Wu, J. Lin and K.L. Tan, "High H2 uptake by alkalidoped carbon nanotubes under ambient pressure and moderate
temperatures," Science, Vol. 285, pp. 91-93, 1999.
Maruyama, S., R. Kojima, Y. Miyauchi, S. Chiashi and M. Kohno,
"Low-temperature synthesis of high-purity single-walled carbon
nanotubes from alcohol," Chemical Physics Letters, Vol. 360, pp.
229-234, 2002.
Sayago, I., E. Terrado, E. Lafuente, M.C. Horrillo, W.K. Maser, A.M.
Benito, R. Navarro, E.P. Urriolabeitia, M.T. Martinez and J. Gutierrez,
"Hydrogen sensors based on carbon nanotubes thin films," Synthetic
Metals, Vol. 148, pp. 15-19, 2005.
[9] Kong, J., N.R. Franklin, C. Zhou, M.G. Chapline, S. Peng, K. Cho
and H. Dai, "Nanotube molecular wires as chemical sensors," Science,
Vol. 287, pp. 622-625, 2000.
[10] Urita, K., S. Seki, S. Utsumi, D. Noguchi, H. Kanoh, H. Tanaka, Y.
Hattori, Y. Ochiai, N. Aoki, M. Yudasaka, S. Iijima and K. Kaneko,
"Effects of gas adsorption on the electrical conductivity of single-wall
carbon nanohorns," Nano Letters, Vol. 6, pp. 1325-1328, 2006.
[11] Star, A., V. Joshi, S. Skarupo, D. Thomas and J.C.P. Gabriel, "Gas
sensor array based on metal-decorated carbon nanotubes," Journal of
Physical Chemistry B, Vol. 110, pp. 21014-21020, 2006.
[12] Izawati, R.N., A. Fakhru’l-Razi, M.A. Atieh, S. Rashid and S. Ali.
Effects of hydrogen flow rate on CNT structure synthesized by CVD
method. in 14th annual scientific Malaysia Conference. 2005.
Malaysia.
[13] Niu, J.J., J.N. Wang, Y. Jiang, L.F. Su and J. Ma, "An approach to
carbon nanotubes with high surface area and large pore volume,"
Microporous and Mesoporous Materials, Vol. 100, pp. 1-5, 2007.
[14] Cao, A., L. Ci, G. Wu, B. Wei, C. Xu, J. Liang and D. Wu, "An
effective way to lower catalyst content in well-aligned carbon
nanotube films," Carbon, Vol. 39, pp. 152-155, 2001.
[15] Varghese, O.K., P.D. Kichambre, D. Gong, K.G. Ong, E.C. Dickey
and C.A. Grimes, "Gas sensing characteristics of multi-wall carbon
nanotubes," Sensors and Actuators, B: Chemical, Vol. 81, pp. 32-41,
2001.
[16] Li, J., Y. Lu, Q. Ye, M. Cinke, J. Han and M. Meyyappan, "Carbon
nanotube sensors for gas and organic vapor detection," Nano Letters,
Vol. 3, pp. 929-933, 2003.
[17] Roy, R.K., M.P. Chowdhury and A.K. Pal, "Room temperature sensor
based on carbon nanotubes and nanofibres for methane detection,"
Vacuum, Vol. 77, pp. 223-229, 2005.
168