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Contents lists available at ScienceDirect
Sensors and Actuators B: Chemical
journal homepage: www.elsevier.com/locate/snb
Gas-sensing properties of tin oxide doped with metal oxides and carbon
nanotubes: A competitive sensor for ethanol and liquid petroleum gas
Nguyen Van Hieu
a,∗
, Nguyen Anh Phuc Duc
b
, Tran Trung
c
,
Mai Anh Tuan
a
, Nguyen Duc Chien
b
a
International Training Institute for Materials Science (ITIMS), Hanoi University of Technology (HUT), No. 1 Dai Co Viet Road, Hanoi, Viet Nam
b
Institute of Engineering Physics, Hanoi University of Technology, Hanoi, Viet Nam
c
Faculty of Environment and Chemistry, Hung Yen University of Technology and Education, Hung Yen, Viet Nam
article info
Article history:
Available online xxx

2
sensor
exhibits higher sensitivity to ethanol gas and LPG than the sensors doped with the other dopants. Espe-
cially, the (1 wt%) PtO
2
-doped SnO
2
sensor shows higher selectivity to ethanol gas over LPG, while the
(0.1wt%, 20 <d < 40 nm)-doped SnO
2
shows higher selectivity to LPG over ethanol gas in the same testing
conditions.
© 2009 Elsevier B.V. All rights reserved.
1. Introduction
The hybrid materials made of semiconductor metal oxides
(SMO) such as TiO
2
, SnO
2
and WO
3
and carbon nanotubes (CNTs)
have been given much attention in recent year for their various
applications such as photocatalysis, anode material for lithium-
ion batteries and gas sensor [1–14]. The nanoarchitectures forming
hybrid materials between SMO and CNTs have been conducted in
different ways such as SMO/CNTs composite [1–5], SMO-coated
CNTs [6–8], SMO-filled CNTs [9] and CNTs-doped SMO [10–14]. The
special geometries and properties of the hybrid materials facilitate
their great potential applications as high-performance gas sensors.

hybrid materials in comparison
with the sensorsbased on the separated materials was attributed to
additional nanochannel for gas diffusion and p/n junctions formed
by CNTs and SnO
2
. These mechanisms were previously represented
in [4,5,10].
Ethanol gas sensors are extensively used for the control of
drunken driving and monitoring of fermentation and other pro-
cesses inchemical industries, whileLPG sensorsare frequently used
in the detection of the gas leakages to prevent accidental explo-
sion. The development of ethanol gas and LPG sensors based on
SnO
2
thin film technology offers great advantages such as high
sensitivity, fast response, and low cost. Therefore, much effort has
been devoted to improve its sensitivity and selectivity by intro-
ducing various dopants such as PtO
2
, CdO, La
2
O
3
, PdO, SiO
2
, and
RuO
2
[16–31] or by mixing with other metal oxides such as Nb
2

0925-4005/$ – see front matter © 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.snb.2009.03.043
Please cite this article in press as: N. Van Hieu, et al., Gas-sensing properties of tin oxide doped with metal oxides and carbon nanotubes:
A competitive sensor for ethanol and liquid petroleum gas, Sens. Actuators B: Chem. (2009), doi:10.1016/j.snb.2009.03.043
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Fig. 1. XRD pattern of typical SnO
2
thin film heat-treated at 500 (a), 600 (b), and
700
◦
C (c).
2. Experimental
Stannic acid gel was synthesized by hydrolyzing 0.2 M solution
of tin chloride (SnCl
4
) with ammonia, using the following reaction
at room temperature:
SnCl
4
+ 4NH
4
OH → SnO
2
·nH
2
O + 4NH
4

2
-99 wt% SnO
2
sols were prepared by mix-
ing the required amount of dissolutions of Cu(NO
3
)
2
, Fe(NO
3
)
3
,
La(NO
3
)
3
and PtCl
4
(0.1M) to the pure SnO
2
sol.
Functionalized MWCNTs with different diameters (d <10nm,
d =20–40 nm, and d =60–100 nm) were used for the fabrication of
the MWCNTs-doped SnO
2
sensors with a calculated amount of the
MWCNTs in order to achieve 0.1 equivalent wt% MWCNTs–99.9 wt%
SnO
2

nanoparticles (a); FE-SEM image of a SnO
2
thin film heat-treated at 500
◦
C (b); TEM image of an MWCNTs-doped SnO
2
sol (c); FE-SEM image of
an MWCNTs-doped SnO
2
(d).
Please cite this article in press as: N. Van Hieu, et al., Gas-sensing properties of tin oxide doped with metal oxides and carbon nanotubes:
A competitive sensor for ethanol and liquid petroleum gas, Sens. Actuators B: Chem. (2009), doi:10.1016/j.snb.2009.03.043
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Fig. 3. Response of PtO
2
,Fe
2
O
3
,La
2
O
3
, and CuO-doped SnO
2
sensors to 2500 ppm (0.25%) LPG in air (a) and 250 ppm ethanol gas in air (b); the response to ethanol gas
(250 ppm) and LPG (2500 ppm) of (1 wt%) a PtO

tent of MWCNT and the well-embedded MWCNTs in SnO
2
matrix,
which have already been reported in the literature [1,3,4,7].Itcan
be seen that the heat-treated samples are well crystallized with all
diffraction peaks which can be well indexed to the tetragonal rutile
structure of SnO
2
. The broad and well-defined reflections were
observed at 2Â =26.51, 33.67 and 51.78 corresponding to (11 0),
(1 01) and (21 1) planes, respectively, in the XRD spectrum of the
annealed SnO
2
thin films, which are in good agreement with the
previously reported [20,21], confirmingthe formation ofa polycrys-
talline SnO
2
thin film. The estimated value of the lattice constants
were found to be a = b = 4.734Å and c = 3.185 Å (JCPDS 21-1250). The
value of the crystallite size of the heat-treated SnO
2
thin film was
estimated by fitting the width of (1 1 0) reflection using Scherrer
formula d=K/ˇcos Â, where K is 0.94,  is the X-ray wavelength, ˇ
the peak full width half maxima (FWHM) and, Â is the diffraction
peak position.
The roughly estimated values of crystallite size of the sam-
ples heat-treated at 500, 600, and 700
◦
C are found to be about

4 N. Van Hieu et al. / Sensors and Actuators B xxx (2009) xxx–xxx
Fig. 5. Response of (0.1wt%) MWCNTs (with d < 10 nm; 20 nm < d < 40 nm; 60 nm < d < 100 nm) -doped SnO
2
sensors to 2500 ppm (0.25%) LPG in air (a) and 250 ppm ethanol
gas in air (b); response to ethanol gas (250 ppm) and LPG (2500 ppm) of (0.1 wt%, 10 nm < d < 20 nm) MWCNTs-doped SnO
2
sensor (c); step wise decrease in electrical resistance
obtained with an increase in LPG concentration from air to 10000ppm (1%) LPG in air for (0.1wt%, 10 nm < d< 20 nm) MWCNTs-doped SnO
2
sensor operating at 240
◦
C; (e)
the sensor response versus LPG concentration.
The particle size and morphology of the SnO
2
thin film charac-
terized further by TEM and FE-SEM are shown in Fig. 2. The TEM
image (Fig. 2a) shows that the particle size is quite homogenous in
the range of 4–8 nm. The FE-SEM image (Fig. 2b) shows the mor-
phology of the SnO
2
thin film treated at 500
◦
C. It is shown that
the particle size is smaller than 10 nm. The MWCNTs-doped SnO
2
sample was characterized by TEM and FE-SEM as shown in Fig. 2c
and d, respectively. Fig. 2c shows the TEM image of the MWCNTs-
dispersed SnO
2

film after heat-treatment at 500
◦
C. It can be seen that
the MWCNTs are well encapsulated with a SnO
2
matrix and is still
present after the heat-treatment at 500
◦
C.
The sensing characteristics of (1 wt%) metal oxides (PtO
2
,Fe
2
O
3
,
CuO, La
2
O
3
)-doped sensors to ethanol gas and LPG have indicated
in Fig. 3. The sensor responses as a function of operating tempera-
ture to LPG and ethanol gas are respectively shown in Fig. 3a and b.
It seems that the optimized operating temperatures of the sensor
to ethanol gas and LPG are around 350 and 250
◦
C, respectively.
It can be recognized that all the metal oxides-doped SnO
2
sen-

be seen that the sensor has relatively good selectivity to ethanol gas
over LPG. Fig. 3d shows the electrical resistance variations obtained
with several steps of different ethanol concentration from air to
1000 ppm ethanol in air for the (1 wt%) PtO
2
-doped SnO
2
sensor at
an operating temperature of 240
◦
C. As can be seen, upon switching
on ethanol gas, the film reaches the saturated resistance R
g
in 50 s
and at the end of the injection cycle, when dry air is introduced, its
electrical resistance returns to the original value (R
a
). This fact is a
proof of the reversibilityof theprocess. Thestepwise decrease of the
electrical resistance of the film is very consistent with an increas-
ing amount of ethanol oxidation. Greater ethanol oxidation caused
the introduction of more electrons into the SnO
2
surface and the
film became less resistive. Fig. 3e depicts the correlation between
the ethanol gas concentration and the response of the (1 wt%) PtO
2
-
doped SnO
2

ing characteristics of these sensors were measured with 250 ppm
ethanol at different operating temperatures, and the results are
shown in Fig. 4. It can be seen that the (1 wt%) PtO
2
-doped SnO
2
sensor shows a higher response. From this, it can be concluded that
the PtO
2
doping content of 1 wt% is the optimal value.
The 90% response time for gas exposure (t
90%(air-to-gas)
) and that
for recovery (t
90%(gas-to-air)
) were calculated from the resistance-
time data shown in Fig. 3d. The t
90%(air-to-gas)
values are around 23 s,
while the t
90%(gas-to-air)
value is around 46 s. These results are quite
comparable with those of SnO
2
-based sensors reported previously
[17–20].
Recently, hybrid CNTs/SnO
2
sensors have been extensively
investigated. Therefore, for comparison with metal oxides-doped

(n-type)/MWCNTs (p-type) can
not functionalize well at a temperature higher than 350
◦
C due to
the transition from semiconductor behavior to metallic one of the
CNTs. More detail on this mechanism can be found further in recent
worksby us andothers [4,5,10,14]. Additionally,it also observed that
the effect of MWCNTs on the response of the MWCNTs-doped SnO
2
sensors is not significant in the detection of LPG and ethanol gas. It
seems that (d = 10–20 nm) MWCNTs-doped SnO
2
sensors have bet-
ter performance to LPG and ethanolgas atan operatingtemperature
range of 280–350
◦
C.
The specific surface area (SSA) of MWCNTs with diameter of <10,
20-40 nm and 60–100 nm were 242.2, 112.2, and 45.2m
2
/g, respec-
tively. In principle,the material with ahigher SSA would have better
gas response. However, we have observed that the doping content
is so small that it could not affect the SSA of the MWCNTs-doped
SnO
2
materials. Thus, the SSA factor cannot be a piece of evidence
on the difference in the sensor response. The observed effect can be
explained by the fact that the MWCNTs embedded in SnO
2

electrical resistance variations obtained with several steps of dif-
ferent LPG concentration from air to 1% LPG in air for the (0.1wt%,
20 < d < 40 nm) MWCNTs-doped SnO
2
sensor operating at 320
◦
C.
Similar to the PtO
2
-doped SnO
2
sensors in the detection of ethanol,
the MWCNTs-doped SnO
2
sensors shows a good reversibility in the
detection of LPG and the stepwise decrease of electrical resistiv-
ity of the MWCNTs-doped SnO
2
film is very consistent with the
increasing amount of LPG oxidation. More LPG oxidation caused
the introduction of more electrons into the SnO
2
surface and the
film became less resistive. Fig. 5e depicts the variation of response
with LPG concentration in air for the MWCNTs-doped SnO
2
sensors
at an operating temperature of 320
◦
C. It can be observed that the

2
materials by doping CNTs that can enhance the diffusion in and out
of the gas molecules.
To study the effect of MWCNTs doping on the sensing proper-
ties to ethanol gas and LPG, we plotted the response of undoped
SnO
2
sensor to 250 ppm ethanol gas and 2500 ppm (0.25%) LPG as
shown in Fig. 6. It is indicated that the response of undoped SnO
2
sensors to 250 ppm ethanol gas is higher than that to 2500 ppm
(0.25%) LPG over an operating temperature range of 190–360
◦
C.
Therefore, this points out that the higher response of the MWCNTs-
doped SnO
2
sensors to LPG than to ethanol can be attributed to the
MWCNTs doping. This is an interesting finding that cannot yet be
clearly explained as of now. The pure SnO
2
sensor is more sensitive
to ethanol than LPG even though the ethanol gas concentration is
about 10 times lower than the LPG concentration. This has alsobeen
explored in previous works [15]. The sensing mechanism of the
ethanol and LPG has long been known and widely adopted in pre-
vious reports [24–30]. However, to explain why the ethanol is more
sensitive than LPG, even though the former has a lower concentra-
tion than the later, is still unclear. It has long been known that there
Please cite this article in press as: N. Van Hieu, et al., Gas-sensing properties of tin oxide doped with metal oxides and carbon nanotubes:

+ O
2
−
→ 2CH
3
CH
2
CHO + H
2
O + e (3)
2CH
4
CH
2
CH
2
CH
3
+ O
2
−
→ 2CH
4
CH
2
CH
2
CHO + H
2
O + e (4)

20 <d < 40 nm) MWCNTs-doped SnO
2
sensors (∼12.3 for 250ppm
at 260
◦
C, see Fig. 5b) is about three times higher than that of the
undoped SnO
2
sensor (∼3.4 for 250 ppm at 260
◦
C, see Fig. 5b).
The reason for this was previously explained in detail [4,5,10,14].
The question to raise here is that why the MWCNTs-doped SnO
2
is more sensitive to 2500 ppm LPG than to 250 ppm ethanol as
depicted in Fig. 5c. Further intensiveinvestigationshould be done to
understand this phenomenonmore comprehensively.The plausible
explanation for the observed effect can be based are as follows: (i)
the MWCNTs are hollow nanotubes that gas absorption could occur
in the inside andoutside of theMWCNTs[34], (ii) themethane (CH
4
)
molecules (e.g. propane and butane) can be physically adsorbed
on the outgassed nanotubes (i.e., nanotube after oxygen exposure)
[35], and (iii) the oxygen molecules are strongly adsorbed on the
defective sites of MWCNTs (adsorption energy is about 0.32 eV) [36]
that can serve as a reactive gas for the oxidation reactions of LPG.
These reasons can enhance Reactions (3) and (4). Additionally, the
consumption of the adsorbed oxygen can affect the electrical prop-
erties of the MWCNTs [36,37], and the electrical resistance of the

shows good selectivity to LPG over ethanol gas, at the same testing
conditions. The gas-sensing mechanism of the hybrid sensor has
been discussed. However, further study is needed to understand
better the selectivity of the hybrid sensor to ethanol gas and LPG.
Acknowledgments
The work was supported by the National Foundation for Sci-
ence and Technology Development (NAFOSTED) of Vietnam (for
Basic Research Project: 2009-2012), the National Key Research Pro-
gram for Materials Technology (Project No. KC 02-05/06-10), the
research projects of Vietnam Ministry of Education and Training
(Code B2008-01-217 and B2008-21-09) and Key basic research pro-
gram for application orientation (2009-2012).
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Biographies
Nguyen Van Hieu received his MSc degree from the International Training Institute
for Materials Science (ITIMS), Hanoi University of Technology (HUT) in 1997 and PhD
degree from the Department of Electrical Engineering, University of Twente, Nether-
lands in 2004. Since 2004, he has been a research lecturer at the ITIMS. In 2007, he
worked as a post-doctoral fellow at Korea University. His current research inter-
ests include nanomaterials nanofabrications, characterizations and applications to
electronic devices, gas sensors and biosensors.
Nguyen Anh Phuc Duc received the BS degree in Engineering Physics from Institute
of Engineering in Physics, Hanoi University of Technology, Vietnam in 2005, his MSc
degree in Materials Science from the International Training Institute for Materials
Science (ITIMS), Hanoi University of Technology (HUT) in 2007, and he is currently
working toward his PhD degree at Leuven University, Belgium. His current research
interests include oxide semiconductors nanoparticle for gas-sensing applications.
Tran Trung received his MSc degree in 1994 and his PhD degree in 1998 from the
Department of Electrochemistry, Hanoi University of Technology. In 2000 and 2001,
he worked as a post-doctoral fellow in Pusan National University, Korea. At present
he is working as an Associate Professor at the Faculty of Environment and Chemistry,
Hung-Yen University of Technology and Education. His research activities are related