mixed sno2 tio2 included with carbon nanotubes for gas-sensing application

Mixed SnO
2
/TiO
2
included with carbon nanotubes for gas-sensing application
Nguyen Van Duy
a
, Nguyen Van Hieu
b,c,
Ã
, Pham Thanh Huy
b,c
, Nguyen Duc Chien
c,d
,
M. Thamilselvan
a
, Junsin Yi
a
a
School of Information and Communication Engineering, Sungkyunkwan University, 300 Chunchun-dong, Jangan-gu, Suwon 440746, South Korea
b
International Training Institute for Materials Science (ITIMS), Hanoi University of Technology (HUT), No. 1 Dai Co Viet Road, Hanoi, Vietnam
c
Hanoi Advanced School of Science and Technology (HAST), Hanoi University of Technology (HUT), Vietnam
d
Institute of Engineering Physics (IEP), Hanoi University of Technology (HUT), Vietnam
article info
Article history:
Received 21 April 2008
Received in revised form

9 July 2008
Accepted 9 July 2008
Available online 25 September 2008
PACS:
61.48.De
07.07.df
81.07.De
Keywords:
Mixed SnO
2
–TiO
2
Carbon nanotubes
Gas sensor
abstract
TiO
2
and SnO
2
are the well-known sensing m aterials with a good thermal stability of the former and a
high sensitivity of the latter. Carbon nanotubes (CNTs) have also gas sensing ability at room
temperature. CNTs-included SnO
2
/TiO
2
material was a new exploration to combine the advantages of
three kinds of materials for gas-sensing property. In this work, a uniform SnO
2
/TiO
2

solution was
prepared by the sol–gel process with the ratio 3:7 in mole. The CNTs with contents in the range of
0.001–0.5 wt% were dispersed in a mixed SnO
2
/TiO
2
matrix by using an immersion-probe ultrasonic.
The SnO
2
–TiO
2
and the CNTs-included SnO
2
–TiO
2
thin films were fabricated by the sol–gel spin-coating
method over Pt-interdigitated electrode for gas-sensor device fabrication and they were heat treated at
500 1C for 30 min.
FE-SEM and XRD characterizations indicated that the inclusion of CNTs did not affect the particle
size as well as the morphology of the thin film. The sensing properties of all as-fabricated sensors were
investigated with different ethanol concentrations and operating temperatures. An interesting sensing
characteristic of mixed SnO
2
/TiO
2
sensors was that there was a two-peak shape in the sensitivity versus
operating temperature curve. At the region of low operating temperature (below 2801C), the hybrid
sensors show improvement of sensing property. This result gives a prospect of the stable gas sensors
with working temperatures below 250 1C.
& 2008 Elsevier B.V. All rights reserved.

1. Introduction
Semiconductor metal oxide gas sensors have been investigated
extensively since the past decades owing to their advantages of
high sensitivity to pollutant gases, fast response and recovery, low
cost, easy implementation, and small size [1,2]. Gas sensors based
on SnO
2
materials have been commercially available [3,4]. Thin-
film gas sensors have improved the gas-sensing properties from
bulk or thick- film ones. They not only give a high sensitivity but
also have very fast response and recovery times. However, there
still exist great disadvantages of SnO
2
and TiO
2
materials. SnO
2
is
thermally unstable and its electrical properties can be degener-
ated upon prolonged thermal treatment in reducing the gas
atmosphere [1]. On the other hand, in spite of the high thermal
and chemical stability, the gas sensors based on TiO
2
materials
require high operating temperatures (normally up to 400 1C). This
would result in high power consumption and difficulty of
packaging. Mixed oxide has been studied to combine the
advantages of the sensing property of each oxide component
[4–6]. The formation of mixed oxide is classified into three types
as follows:

(1) Chemical compound.
(2) Solid solution.
(3) Mix of (1) and (2) types.
SnO
2
–TiO
2
falls into the second category. The use of mixed
oxides in gas detection has been tried successfully in some
systems such as SnO
2
–WO
3
[5], TiO
2
–WO
3
[5–7], TiO
2
–SnO
2
[5,8].
Among these mixed oxides, the SnO
2
–TiO
2
system has been
investigated more extensively for gas-sensing applications
[5,8–12].
Carbon nanotubes (CNTs) have been the most actively studied

material in recent years due to their unique electrical, mechanical
and chemical properties, and much attention has been paid to
their application in various fields of nanotechnology [13,14].
Moreover, they have nanoscale size and large surface area that can
ARTICLE IN PRESS
Contents lists available at ScienceDirect
journal homepage: www.elsevier.com/locate/physe
Physica E
1386-9477/$ - see front matter & 2008 Elsevier B.V. All rights reserved.
doi:10.1016/j.physe.2008.07.007
Ã
Corresponding author at: International Training Institute for Materials Science
(ITIMS), Hanoi University of Technology (HUT), No. 1 Dai Co Viet Road, Hanoi,
Vietnam. Tel.: +84 4 8680787; fax: +84 4 8692963.
E-mail addresses: ,
(N. Van Hieu).
Physica E 41 (2008) 258–263
provide excellent gas absorption properties. These extreme
absorption properties make CNTs advantageous for use in many
areas of applications. For example, the gas absorption of CNTs at
room temperature will change its electric properties with fast
response time, which can enable CNTs to be a good candidate of
gas-sensing applications [15–17]. For the gas-sensing materials,
there are various approaches using CNTs as the solution such as
CNTs for dispersion, CNTs for composite, CNTs for filling, CNTs for
coating, etc [18–26]. It has been recently reported in the literature
that single-wall CNTs (SWCNTs) doping on SnO
2
can significantly
improve SnO

2
gas-sensor performance, and especially the sensor
can function at room temperature with sufficient sensitivity [27].
Some other endeavors on including CNTs into tungsten tri-oxide
(WO
3
) [25], polymethylmethacrylate [28], polypyrrole [29], etc.
have been published. In our previous work, we have demonstrated
the improvement of performance of the TiO
2
-based sensor by
including CNT s [30] and high performance of the room-temperature
NH
3
gas sensor b y u sing SnO
2
/CNT s composites [31].Inthis
work, w e e xplor e possibilities to impr ov e the performance and t o
reduce the operating temperature of the SnO
2
–TiO
2
ethanol sensors
by adding CNTs.
2. Experimental
2.1. Materials synthesis and characterizations
SnO
2
–TiO
2

sol was prepared by the sol–gel method. The
precursors used to fabricate the solutions were tetra propylortho
titanate Ti(OC
3
H
7
)
4
(99%), tin ethylhexanoate Sn(OOCC
7
H
15
)
2
and
isopropanol C
3
H
7
OH (99.5%). To synthesize the hybrid SWCNT/
SnO
2
–TiO
2
material, the SWCNTs with a diameter lower than 2 nm
and multi-wall CNTs (MWCNTs) with diameters ranging from 20
to 40 nm purchased from Shenzhen Nanotech Port Ltd. Co.
(Shenzhen China) were introduced in the SnO
2
–TiO

2
sol solution
by an ultrasonic shaker at a power of 100 W for 10 min. The CNTs
content was varied in the range of 0.001–0.5 wt%. The film was
deposited by spin coating on silica substrate at 4000 rpm for 20 s
and a film thickness of around 320 nm was obtained. The sensors
realized with different SWCNTs contents were signed as S0–S7.
Meanwhile, the sensors with various MWCNTs contents were
signed as M0–M7. As-deposited films were dried for 30min at
60 1C and then they were annealed at 500 1C for 30 min. The
morphology and the crystalline phase of the films were char-
acterized using a field emission scanning electron microscope (FE-
SEM; 4800 Hitachi, Japan). The microstructure of the sintered film
was characterized by X-ray diffraction (XRD), using a Bruker-AXS
D5005.
2.2. Gas sensor fabrication and testing
The fabrication of the gas sensor was carried out in the
following manner: (i) the inter-digitated electrode was fabricated
using a conventional photolithographic method with a finger
width of 100
m
m and a gap size of 70
m
m. The fingers of the inter-
digitated electrode were made by sputtering 10nm Ti and 200 nm
Pt on a layer of silicon dioxide (SiO
2
) with a thickness of about
100 nm thermally grown on top of a silicon wafer; (ii) the sensing
layers were deposited on top of the electrode with subsequent

heat treatment at 500 1C for 30 min.
The sensor under test was placed on top of a hot plate and held
by two tungsten needles. Then they were loaded in a glass
chamber with a volume of 4 L as shown in Fig. 1. More details
about the measurement set-up can be found elsewhere [31]. The
desired ethanol concentrations were obtained by mixing ethanol
gas with air using a mass flow control system with computer
control (AALBORG model GFC17S-VALD2-A0200) and subse-
quently injected into the chamber. The chamber was purged with
air and the experiment was repeated. The electrical resistance
response during testing was monitored by a precision semicon-
ductor parameter analyzer HP4156A, which can be used to detect
a very low electrical current (around 10
À12
A). This allows us to
measure the high resistance of the mixed oxide films. The
resistance responses of the sensor in air ambient and upon
exposure of ethanol pulses were monitored. The sensor response
(S) was defined as the ratio of the sensor resistance in air (R
a
) and
in ethanol gas (R
g
).
3. Results and discussion
3.1. Microstructure characterizations
The formation of the SnO
2
–TiO
2

solid solution can be seen by
an XRD pattern in Fig. 2. With the mole ratio of SnO
2
:TiO
2
at 3:7, it
shows that the diffraction peaks of the oxide solution follow
Vegard’s law. A similar result has been seen for SnO
2
–TiO
2
deposited by sol–gel [11] and sputtering methods [5]. The solution
is formed by mixing SnO
2
and TiO
2
lattices in the rutile phase in
which both the materials are in the tetragonal structure. From
XRD peaks, we get the inter-planar spacing values of SnO
2
–TiO
2
-
mixed oxide as shown in Table 1. The peak shift is explained by the
substitution of Sn
4+
for Ti
4+
in the TiO
2

crystal structure. Because
of the larger radii of Sn
4+
, the lattice spacing increases when the
substitution occurs. In the sol–gel process, the chemical reaction
controlled at low speed gives the possibility of a homogenous
mixed solution. Sn–O and Ti–O bonding disperse uniformly during
stirring and hydrolysis reaction. From the peak broadening, the
crystallite size estimated by the Scherrer equation was found to be
about 5.5 nm. XRD was carried out with the highest SWCNTs
content of 0.5 wt% (sample S7); it is understandable that the
SWCNTs peaks were not detected in the XRD pattern.
FE-SEM images show the surface morphology of the thin films
after heat treatment. They exhibit that the particle size is around
10 nm. These results may be caused by the impeding of the
polycrystalline aggregate process of each other SnO
2
and TiO
2
.
This grain size is approximately two times the Debye length for
the depletion layer on the surface. It implies that the surface-
sensing mechanism is more effective in these films. Another result
is that all the films’ surfaces are highly porous and uniform in
granular shape. The high porosity of the thin film makes it more
easy to adsorb and desorb gas molecules. All these characteristics
promise good gas sensing properties of the material. The FE-SEM
ARTICLE IN PRESS
H P 4 1 5 6 A
Delta Electronic

ES30-5 Power
Supply
Exhaust
Target Gas
Rotary Pump
MFC
Mass Flow Controller
Fig. 1. Apparatus for gas-sensor testing.
N. Van Duy et al. / Physica E 41 (2008) 258–263 259
images of S0 and S4 samples as shown in Fig. 3a and b indicate
that the film morphology was not clearly different between the
undoped and the SWCNTs-doped samples. CNTs trace cannot be
seen in the FE-SEM image of 0.1wt% SWCNTs/SnO
2
–TiO
2
(S6) after
annealing at 500 1C for 30 min. We suggest that at low content of
CNTs, they are embedded in the oxide matrix. In addition,
SWCNTs–TiO
2
and SWCNTs–SnO
2
bondings can be formed
naturally through some physicochemical interactions such as
Van der Waals force, H bonding and other bondings. The
interaction between –OH groups in the course of the hydrolysis
reaction of Sn(OC
7
H

15
)
2
, Ti(OC
3
H
7
)
4
and –COOH, –OH groups on
SWCNTs formed by the purification process can be a case for
explanation. This indicated that the crystallites would grow up
and enclose SWCNTs during the heat treatment. Therefore, it is
very difficult to find CNTs on the film surface. In general, the trace
of CNTs on the film surface could be seen in the composite
material in which the CNTs’ content would normally be higher
than 5 wt%.
3.2. Ethanol sensing properties
We have measured the responses of all sensors to ethanol at
different concentrations ranging from 125 to 1000 ppm and at
operating temperature in a range from 210 to 40 0 1C to investigate
the gas-sensing properties. The sensor responses at various
operating temperatures are shown in Fig. 4. It was found that
the response and recovery times of the sensors are less than 10 s.
We have observed that the metal oxide thin-film sensor show a
relatively low response-recovery time, and the hybrid CNTs/metal
oxide thin-film sensor show even lower values. This observation
was also previously reported [20,24,26,32].
ARTICLE IN PRESS
Table 1

The interplanar spacing values of SnO
2
–TiO
2
mixed oxide calculated by Vegard’s
law are close to the measured values
d(110) (A
˚
) d(101) (A
˚
) d(2 0 0) (A
˚
) d(211) (A
˚
)
SnO
2
3.35 2.64 2.37 1.76
TiO
2
3.24 2.48 2.30 1.68
SnO
2
–TiO
2
3.28 2.51 2.32 1.70
Calculation with
Vegard law
3.27 2.53 2.32 1.71
Fig. 3. FESEM images depict the uniform and highly porous surface of blank (a)

and hybrid (b) 0.1% SWCNTs/SnO
2
–TiO
2
samples.
1M
0 50 100 150 200 250 300
350
10M
100M
1G
Air
Air
500 ppm
1000 ppm
375 ppm
250 ppm
125 ppm
305
°
C
335
°
C
365
°
C
400
°
C

R (
Ω
)
t (s)
Fig. 4. Ethanol response characteristics of sensor S4 at different temperatures
show fast response and recovery times less than 10s.
SnO
2
TiO
2
2-Theta - Scale
Intensity (Counts)
20 25 30 35 40 45 50 55
60
Fig. 2. X-ray diffraction pattern of SnO
2
–TiO
2
(at ratio 3:7 in mole) shows the
diffraction peaks of solid solution following Vegard’s law. Dot lines indicate SnO
2
rutile peaks and dash lines indicate TiO
2
rutile peaks.
N. Van Duy et al. / Physica E 41 (2008) 258–263260
The stepwise decrease in electrical resistance obtained with
increasing ethanol concentration from air to 1000 ppm ethanol
gas in air, and after several cycles of the gas injection, the
resistance turns back to the original value when the sensor is
exposed to air. These characteristics indicated that the hybrid

sensor has relatively stable response. However, the high resistance
of around 10
9
O
at an operating temperature below 300 1Cisa
drawback of the hybrid material.
Working temperature is one of the most important parameters
for gas sensors. The conventional gas sensors based on SnO
2
and
TiO
2
materials operate at the temperature region from 300 to
400 1C. The response versus operating temperature (S–T) curves of
our sensors at 100 0 ppm ethanol depict the two-peak shape
characteristic. The first maximum in response appears at an
operating temperature of around 260 1C and the second peak is
around 380 1C. This can be seen clearly with S1 and S4 sensors
based on the SWCNTs/SnO
2
–TiO
2
material, as shown in Fig. 5. For
the SWCNTs content of 0.001 wt% (S1) and 0.025 wt% (S4), we get
the response to 1000 ppm ethanol of 11.1 and 9.6 at operating
temperature of 2601C, 32 and 41 at an operating temperature of
380 1C, respectively. Meanwhile, at higher content of SWCNTs,
there is a strong degradation in the response. This observation
cannot be clearly explained yet. A plausible explanation for the
observed effect is that the addition of SWCNTs considerably

increases the surface adsorption area of the mixed oxide and
added more p/n junction of SWCNTs/SnO
2
–TiO
2
as discussed
below. However, when the CNT content is sufficiently high, the
SWCNTs begin to connect together and results in a shorter
resistance path that shunts the gas-sensing current of the mixed
oxide layer. Thus, the gas sensitivity is reduced for a very high
SWCNT content.
The dependence of the response on ethanol concentration at
operating temperatures of 260 and 380 1C is given in Fig. 6. It can
be seen that all the sensors present more or less linear
characteristic in the investigated range from 125 to 1000 ppm
ethanol, which makes their use more convenient. Once again, S1
and S4 dedicate the best in slope than the others. The slope values
of fit lines are given in Table 2. We have also surveyed the
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200
0
5
10
15
20
25
30
35
40
45

S (R
air
/R
ethanol
)
S0
S1
S3
S4
S7
0
5
10
15
20
25
30
35
T (
o
C)
S (R
kk
/R
Ethanol
)
M0
M2
M3
M4

M7
420
400
380360
T (°C)
340320
300
280260240
220
200
420
400
380360340320
300
280260240
220
Fig. 5. The dependence of response on operating temperature depicts the two
maximum characteristics on both SWCNTs/SnO
2
–TiO
2
(a) and MWCNTs/SnO
2
–TiO
2
(b) systems. The first peak is around 260 1C and the second one is around
380 1C.
5
10
15

20
25
30
35
40
45
S0
S1
S3
S4
S7
C
ethanol
(ppm)
S (R
air
/R
ethanol
)
0
2
4
6
8
10
C
Ethanol
(ppm)
(R
Air

/R
Ethanol
)
S0
S1
S3
S4
S7
1000800600400200
0 1000800600400200
Fig. 6. Response versus on ethanol concentration characteristics in the range from
125 to 1000 ppm at operating temperatures of 240 (a) and 380 1C (b).
Table 2
Fitting slope of S–C curves at operating temperatures of 240 and 380 1C
S0 S1 S2 S3 S4 S5 S6 S7
At 240 1C (/100 ppm) 0.28 0.69 0.34 0.20 0.84 0.38 0.27 0.24
At 380 1C (/100 ppm) 1.91 3.78 2.12 2.78 2.74 2.07 2.07 1.09
N. Van Duy et al. / Physica E 41 (2008) 258–263 261
influence MWCNTs inclusion on the sensing properties of
the mixed oxide material. The sensing properties of this hybrid
material are quite similar to that of SWCNTs/SnO
2
–TiO
2
(see
Fig. 5b). From the response values at operating temperatures of
240 and 380 1C given in Table 3, we can see that the best
improvement in ethanol sensing is obtained for 0.01 and
0.025 wt% MWCNTs. However, the effect of MWCNTs on the
ethanol sensing property of mixed oxide is not as high as SWCNTs.

To summarize all the results, we plotted the maximum
sensitivities versus the CNTs doping content, as seen in Fig. 7.It
is easy to see how better when the CNTs-doped mixed SnO
2
–TiO
2
sensors are working at the low temperature. The best improve-
ment for operating temperatures of 380 and 260 1C is achieved at
SWCNTs contents of 0.001% and 0.025%, respectively. These
observations are the same as in the case of MWCNTs inclusion.
These results of the sensing properties at a working temperature
below 250 1C even give ethanol detectability that is 20–25 times
smaller compared to the CNTs/SnO
2
composite sensor prepared by
electron beam evaporation [26].
3.3. Gas sensing mechanism
At first, one needs to discuss the two-peak shape of response-
operating temperature curves. For the sensors based on tin oxide
and titanium oxide, such results have never been seen before. We
assumed that the presence of both SnO
2
and TiO
2
makes the
mixed oxide material with combined properties. At the operating
temperature below 500 1C, the surface sensing mechanism plays a
dominant role. Ethanol vapor adsorbs on the surface grain
boundaries and reacts with the adsorbed oxygen ions on the
surface. It should be noted that the adsorbed oxygen ions trap

electrons, inducing a surface depletion layer between the grains.
This means the surface density of the negatively charged oxygen
decreases by the ethanol vapor absorption, so the barrier height in
the grain boundary is reduced. The reduced barrier height
decreases sensor resistance. We propose that these processes
take place more easily for SnO
2
than for TiO
2
due to the lower
working temperature of SnO
2
[9]. The presence of both SnO
2
and
TiO
2
has two effective working temperature regions. At an
operating temperature of around 250 1C, the sensing properties
of the mixed oxide are due to SnO
2
, while TiO
2
is more sensitive at
a temperature around 380 1C.
As described in the previous section, the CNTs inclusion has
caused no obvious differences in surface morphology as well as
particle size. Consequently, the porosity and particle size cannot
result in a remarkable improvement of the hybrid CNTs/SnO
2

–-
TiO
2
gas-sensor performance. The improvement of the SnO
2
–TiO
2
gas-sensor performance by including SWCNTs has not been well
understood so far and not much work has been published on the
subject. The model proposed by Wei et al. [27] seems to be
reasonable for the explanation. This model was applied for
SWCNTs-doped SnO
2
and somehow we can apply for our case.
The model has been hypothesized that CNTs-doped SnO
2
–TiO
2
materials can build up p/n hetero junctions, which was formed by
(n-oxide)/(p-CNT)/(n-oxide). Fig. 8 schematically depicts the
changes in the electronic energy bands for two depletion layers,
one is on the surface of mixed-oxide particles and the other is at
the interface between CNT and mixed oxide. When the mixed
oxide is exposed to ethanol gas, the gas molecules will react with
oxygen ions previously adsorbed on the surface of mixed oxide.
This can simply be described as [33]
2C
2
H
5

OH þ O
À
2
¼ 2CH
3
CHO
þ
þ 2H
2
O þ e
The electrons released from the surface reaction transfer back
into the conductance bands, which increase the conductivity of
ARTICLE IN PRESS
1E-3
0
5
10
15
20
25
30
35
40
45
50
CNTs content (%)
SWCNTs, T=240-260°C
SWCNTs, T=360-880°C
MWCNTs, T=240-260°C
MWCNTs, T=360-880°C

S (R
air
/R
ethanol
)
0.50.050.01
Fig. 7. Maximum response of two sensor systems at low and high operating
temperature regions, ethanol concentration of 1000 ppm.
CNT
CNT
n
d
1
d
2
d
3
d
4
n
E
e
E
f
E
v
p
Depletion layer
Distance
Potential

In reducing gas
In air
Grain boundary
TiO
2
/SnO
2
CH
3
/CHO
CH
3
/CHO
CH
3
/CHO
CH
3
/CHO
TiO
2
/SnO
2
TiO
2
/SnO
2
O
2
O

2
O
2
O
O
O
Fig. 8. Schematic of potential barriers to electronic conduction at grain boundaries
and at p–n heterojunctions for CNTs/mixed oxide; d
1
and d
3
are depletion layer
widths when exposed to ethanol; d
2
and d
4
are depletion layer widths in air.
Table 3
Two maximum values in response of MWCNTs/SnO
2
–TiO
2
to 1000 ppm ethanol:
S
m1
, S
m2
T
m1
(1C) S

m1
T
m1
(1C) S
m2
M0 240 5.3 380 23.5
M2 240 5.6 365 13.8
M3 240 9.7 365 28
M4 240 9.3 365 31.4
M6 240 9.7 365 18
M7 240 6.9 380 21.9
N. Van Duy et al. / Physica E 41 (2008) 258–263262
the sensing material. It is noted that the adsorption of the ethanol
gas may change the two depletions layers as described above.
Before the ethanol gas is adsorbed, the widths of the depletion
layers at the interface between mixed oxide grains and mixed
oxide/CNT are given as d
2
and d
4
, respectively. After the
adsorption, the widths of these depletion layers are d
1
and d
3
,
respectively. The change in both the depletion layers at the oxide
grain boundaries and the n/p junction contributed to the
improved sensitivity of the sensing materials. In other words,
n-type mixed oxide and p-type CNT form a heterostructure. Like

the working principle of an n–p–n amplifier, the CNT works as a
base, blocked electrons transfer from n (emitter) to n (collector),
and thus lowering the barrier a little bit allows a large amount of
electrons to pass from the emitter to the collector [24]. This
amplification effect may explain the fact that the hybrid materials
(SnO
2
/SWCNTs) can detect NO
2
at room temperature [27]. So the
improvement of the gas sensor performance and the shift of
operation temperature toward the lower temperature region in
our work can be attributed to the amplification effect of the p–n
junctions in addition to the effect of the grain boundaries.
Meanwhile, the fact that the contribution of MWCNTs
(20odo40 nm) is not as much as SWCNTs (do2 nm) can be
explained based on the quantum effect as follows. The space
charge layer thickness (Debye length) is around 3 nm for the metal
oxides (for example SnO
2
). So the largest distance between
adjacent boundaries accessing gas molecules should be less than
6nm [34]. However, mixed oxide (SnO
2
/TiO
2
) grains are much
larger than 6 nm so that not all metal oxides can participate in the
reaction when gas absorbs on it. Therefore, the mixed-oxide/
SWCNT material structure formed by inclusion of the SWCNTs

with diameter lower than 2 nm will produce quantum effects
between SWCNTs and mixed oxide nanoparticles. The SWCNTs
with a diameter of o2 nm reduce the distance between two
adjacent gas-assessing and reaction surface to be less than the
space charge layer thickness.
4. Conclusion
SnO
2
–TiO
2
mixed oxide has been studied at the ratio 3:7 in
mole for ethanol-sensing properties. At appropriate annealing
conditions, it has shown the formation of the solid solution from
two components by the XRD pattern. All the film surfaces were
uniform and highly porous. In addition, the grain size around
10 nm gave a high specific surface. The new explorer in the two-
peak shape of the response versus operating temperature
characteristics has proved the combined behavior of the mixed-
oxide material. SnO
2
and TiO
2
are complementary to each other
for gas-sensing properties. The inclusion of CNTs at specific
contents into the mixed oxide system improved the response of
the sensor in the low operating temperature region. Further
studies on this type of material would make it a promising
candidate for gas sensing application that can work at around
250 1C with a high stability.
Acknowledgements

This work was financially supported by HAST Project no. 01.
The authors also acknowledge Grant no. 405006 (2006) from the
Basic Research Program of the Ministry of Science and Technology
(MOST) and for the financial support from Third Italian-Vietnamese
Executive Programme of Co-operation in S&T for 2006–2008
under the project ‘‘Synthesis and Processing of Nanomaterials for
Sensing, Optoelectronics and Photonic Applications’’.
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