highly reproducible synthesis of very large-scale tin oxide nanowires

Please cite this article in press as: N. Van Hieu, Highly reproducible synthesis of very large-scale tin oxide nanowires used for screen-
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Highly reproducible synthesis of very large-scale tin oxide nanowires used for
screen-printed gas sensor
Nguyen Van Hieu
∗
International Training Institute for Materials Science (ITIMS), Hanoi University of Technology (HUT), No. 1 Dai Co Viet Road, Hanoi, Viet Nam
article info
Article history:
Available online xxx
Keywords:
Gas sensor
Nanowires sensor
Tin oxide
abstract
A truly simple procedure was presented for highly reproducible synthesis of very large-scale SnO
2
nanowires (NWs) on silicon and alumina substrates. The growth involves thermally evaporating SnO
powder in a tube furnace with temperature, pressure, and O
2
gas-flow controlled to 960
◦
C, 0.5–5 Torr,
and 0.4–0.6sccm, respectively. The scanning- and transmission-electron-microscopic studies show that

the diameter and length of the nanowires vary from 50 to 150 nm and 1 to 10 ␮m, respectively.
As-synthesized SnO
2
NWs on alumina substrates were used to fabricate gas sensor by screen-printing
method. A good ohmic contact of the screen-printed NWs sensor was obtained. Randomly selected gas-
sensor devices were tested with various gases such as C
2
H
5
OH, CH
3
COCH
3
,C
3
H
8
, CO, and H
2
for studying
gas-sensing properties. The results reveal that as-fabricated sensors exhibit relatively reproducible and
good response to ethanol gas. Typically, the response to 100 ppmethanol in airis around 11.8, andresponse
and recovery times are around 4 and 30 s, respectively.
© 2009 Elsevier B.V. All rights reserved.
1. Introduction
In recent years, there have been extensive efforts in the syn-
thesis, characterization, and application of a new generation
of semiconductor metal oxides (SMOs) nanostructures such as
nanowires, nanorods, nanobelts, and nanotubes [1,2]. These struc-
tures with a high aspect ratio (i.e., size confinement in two

coordinates) offer better crystallinity, higher integration density,
and lower power consumption [1]. In addition, they demonstrate
superior sensitivity to surface chemical processes due to the large
surface-to-volume ratio and small diameter comparable to the
Debye length (a measure of the field penetration into the bulk)
[2,3]. Although many different quasi-one-dimension (Q1D) nanos-
tructures of SMO such as SnO
2
, ZnO, In
2
O
3
, and TiO
2
have been
investigated for gas-sensing applications, researchers have paid
greater attention to SnO
2
nanowires (NWs)-based sensors because
its counterparts such as a thick film, porous pellets, and thin films
are versatile in being able to sense a variety of gases [4] and are
commercially available.
Presently, various synthesis methods have been reported for
producing SnO
2
NWs such as hydrothermal methods [5,6], thermal
decomposition of precursor powders Sn, SnO, and SnO
2
followed by
vapor–solid (VS) [7,8] or vapor–liquid–solid (VLS) growth [9–11].

Although there were a large number of reports on the synthesis
∗
Tel.: +84 4 8680787; fax: +84 4 8692963.
E-mail addresses: ,
of SnO
2
NWs by the thermal decomposition using SnO as a source
material, we found that it is rather difficult to synthesize the SnO
2
NWs based on the previously reported procedures [1–3,11–13].We
also found experimentally that the nature of evaporation apparatus
plays a very important role in the selection of the gown condi-
tions such as temperature, pressure, flow-rate of carrier gas, and
flow-rate of oxygen gas to successfully synthesize the SnO
2
NWs.
Hence, the development of a simple and reproducible procedure
to synthesize SnO
2
NWs is significantly meaning for gas-sensing
application. The fabrication of the SnO
2
NWs-based gas sensors
has been demonstrated by using various methods such as dielec-
trophoretic assembly to align on metal electrodes [12], making
electrical contacts formed field effect transistor (FET) [13,14], dis-
persal of the NWs on prefabricated electrodes [5,15,16], deposited
metal electrode on the top of the NWs [17,18], and directly grown
the NWs on the electrodes [19]. In summary, these techniques are
used either expensive equipments such as electron-beam lithogra-

phy, focus ion beam, sputtering system to fabricate the electrical
contacts or a series of uncontrollable processes such as sonifica-
tion and dispersal of NWs on prefabricated electrodes. Due to the
difficulties in synthesis and fabrication of the SnO
2
NWs-based
gas sensor, the practical application of the NWs sensor is still in
question. In this work, the thermal evaporation method was intro-
duced to synthesize the SnO
2
NWs. A truly facile procedure cable
of highly producing a very large-scale of SnO
2
NWs is presented.
As-obtained SnO
2
NWs on alumina substrates are used to fabricate
gas sensor by screen-printing method, which is much simpler com-
pared with previously reported methods. Electrical properties and
0925-4005/$ – see front matter © 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.snb.2009.02.043
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Fig. 1. Thermal evaporation apparatus.
gas-sensing properties are characterized with randomly selected
devices.

2. Experimental
2.1. Material synthesis and characterizations
The SnO
2
NWs were grown in a quartz tube located in a horizon-
tal furnace with a sharp temperature gradient (Lingdberg/Blue M,
Model: TF55030A, USA). Both ends of the quartz tube were sealed
with rubber O-rings. The ultimate vacuum for this configuration
was ∼5 × 10
−3
Torr. The carrier gas-line (Ar) and O
2
gas-line were
connected to the left-end of a quartz tube and their flow-rate was
modulated by a digital mass-flow-control system (Aalborg, Model:
GFC17S-VALD2-A0200, USA). The right end of the quartz tube was
connected to a rotary pump through a needle valve in order tomain-
tain a desired pressure in the tube. High purity SnO powder (Merck,
99.9%) was placed in an alumina boat as an evaporation source.
Substrates with a previously deposited Au catalyst layer (thickness:
∼10 nm) was placed approximately 2–3 cm from the source on both
sides from the source (up-stream and down-stream) as indicated in
Fig. 1. The growth process was divided into two steps. Initially, the
quartz tube was evacuated to 10
−2
Torr and purged several times
with Ar gas (99.99%). Subsequently, the quartz tube was evacuated
to 10
−2
Torr again and the furnace temperature was increased to

960
◦
C for 25 min. It should be noted that the Ar gas-flow did not
introduced during this step. This is completely different from many
previous reports on synthesizing SnO
2
NWs by thermal evapora-
tion. After 2–4min, thefurnace temperature reached 960
◦
C, oxygen
gas was added to the quartz tube at a flow rate of 0.4–0.6 sccm, and
the process was maintaine d for 30 min during the growth of the
SnO
2
NWs. During the O
2
addition step, the pressure inside the
tube with controlling is in the range of 0.5–5 Torr by the needle
valve. The as-synthesized SnO
2
NWs were characterized by scan-
ning electron microscopy (FE-SEM, Hitachi S4800), transmission
electron microscopy (TEM, JEM-100CX), energy dispersive X-ray
analysis (EDX, HORIBA EX-420 attached to the FE-SEM), and X-ray
diffraction (XRD, Philips Xpert Pro) with Cu K␣ radiation generated
at a voltage of 40 kV as a source. Additionally, Nikon microscope
L200 attached with Olympus digital camera was used to observe
the large-scale of SnO
2
NWs on the substrates.

2.2. Gas-sensor fabrication and testing
Fig. 2 shows a schematic diagram of gas sensor fabrication.
A patterned Au catalyst layer was deposited on the Al
2
O
3
sub-
strate by ion sputtering through a shadow mask (with mesh size
of 100 ␮m). Then this substrate was used to grown SnO
2
NWs by
previously indicated procedures. Comb-shape Au electrodes were
screen-printed on the top of the SnO
2
NWs grown on the alumina
substrate with size of 5 mm × 5 mm, followed heat-treatment at
600
◦
C for 5 h. We fabricated a quite large numbers of gas sen-
sors by this technique. However, randomly selected sensors were
tested. For gas sensor characterization, the flow-through tech-
nique was employed. The sensor characteristics were measured
at a temperature of 400
◦
C using horizontal tube furnace and at
various ethanol gas concentrations (10, 50, and 100 ppm). Oth-
erwise, the sensors were also tested with other gases such as
100 ppm CH
3
COCH

3
, 100 ppm C
3
H
8
, 100 ppm CO, and 100 ppm H
2
.
The gas response (S = R
a
/R
g
) was measured at 400
◦
C by compar-
ing the resistance of the sensor in high-purity air (R
a
) with that
in the target gases (R
g
). Electrical characteristics (I–V curve) were
Fig. 2. Gas-sensor fabrication process steps.
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Fig. 3. Optical microscopes image of SnO
2

NWs on the Si and Al
2
O
3
substrates from the up-stream sample (a) and the down-stream sample (e); FE-SEM images of SnO
2
NWs
at different magnification on the samples from the up-stream (b and c) and the down-stream (f and g); TEM images of SnO
2
NWs on the substrates from the up-stream (d)
and the down-stream (h); FE-SEM image of SnO
2
nanowires with Au catalyst cap on the substrates from up-stream (i) and down-stream (l); EDX spectrum measured at the
catalyst cap of the up-stream sample (k) and the down-stream sample (m).
measured by using a Precision Semiconductor Parameter Analyzer
(HP4156A).
3. Results and discussion
3.1. Morphology and microstructure characterizations
Fig. 3 shows the morphology of the as-synthesized SnO
2
NWs
on Si and Al
2
O
3
substrates that was characterized by optical micro-
scope, FE-SEM, and TEM. Uniform SnO
2
NWs with homogeneous
entanglement were produced on a very large area (1 cm× 10 cm)

on the substrates placed at up-stream and down-stream from the
source, respectively, shown in Fig. 3a and e by optical microscope
and Fig. 3b and f by FE-SEM. Fig. 3c and g shows FE-SEM images of
the samples placed at the up-stream and down-stream at higher
magnification, respectively. It can be seen that the morphologies
of as-synthesized SnO
2
NWs on the both sides are very similar.
The diameter of the SnO
2
NWs ranged from 50 to 150 nm (see
Fig. 3d and h) and the lengths ranged from 50 to 150 ␮m. All the
NWs were smooth and uniform along the fiber axis. Actually, we
have intensively investigated the NWs morphology obtained from
the both sides for various synthesis runs by FE-SEM and TEM, and
the results reveal that their morphology are not much different.
We have also tried to synthesize the NW with the same synthesis
process by using three dif ferent evaporation apparatuses, and very
similar results were obtained (not shown). This suggests that the
synthesized process proposed in the present work is very simple
and highly reproducible. In other words, a very large scale of SnO
2
NWs can be obtained.
Fig. 3i and lobtained from theup-stream anddown-stream show
a SnO
2
NWs with a catalyst particle on its tip. These catalyst par-
ticles are not easily found in the FE-SEM image, because the NWs
are too long. The growth mechanism of SnO
2

NWs in the present
work could be explained on the basis of the vapor–liquid–solid
(VLS) mechanism that has been reported by Wagner and Ellis for
the first time [24] and many researchers lately [1,5,6,8,10].Inour
experiment, EDX (see Fig. 3k and m for up-stream and down-stream
samples) reveals that the catalyst particles are composed of Au, Sn
and O, which indicates Au particles also play an important role in
the growth of SnO
2
NWs. Briefly, the NWs growth mechanism in
our experiment can be described as follows. Sn vapors, as coming
from the SnO source after the decomposition in SnO
2
(solid) and
Sn (liquid), are naturally spread out by thermal diffusion over the
both substrates placed at the up-stream and the down-stream and
condensed again on the substrates forming Sn–Au alloyed droplets
by reacting with the Au particles. At the same time, these alloyed
droplets can provide the energetically favored sites for adsorption of
Sn vapor. Subsequently, the oxygen-flow, which is introduced in the
reaction chamber, reacts with the liquid Sn in the droplets to form
SnO
2
. The peak of Sifrom the EDX is attributed to thecontamination
come from the Si substrate. Fig. 4a and b shows the XRD patterns of
the commercial SnO
2
powders and the as-synthesized SnO
2
NWs

and their magnified patterns, respectively. The XRD pattern of the
SnO
2
powders is indexed to the tetragonal rutile structure, which
agrees well with the reported data from JCPDS card (77-0450). The
representative XRD pattern of the SnO
2
NWs is identical to that
of the SnO
2
powders, indicating that these NWs are indeed a pure
rutile phase SnO
2
. In addition, a careful comparison between the
magnified XRD patterns in Fig. 4b reveals that three XRD peaks
for the SnO
2
NWs are relatively broadened and shifted to the lower
diffraction angle, as compared withthe SnO
2
powders. Theseobser-
vations may attribute to the small size effect and tensile stress of
SnO
2
NWs [5,6].
The thermal evaporation procedure, which was used to synthe-
size the SnO
2
NWs have shown some advantages in comparison
with previous reports [5,8,10,13]. In general, Ar gas-flow is used to

transport the Sn vapor from the source to the substrate. To obtain a
large-scale of SnO
2
NWs with high reproducibility, the Ar flow rate
is greatly needed to optimize ourselves that cannot be used from
the literature data. It should be noted that the optimized Ar flow
rate is strongly effected by various factors of evaporation apparatus
such as diameter of the reacted tubes, the temperature gradient of
the furnace, the nature of the boat and substrate, the positions of
the substrates and source, speed of rotary pump, directions of gas-
lines in and out (vertical or horizontal), and source materials (Sn,
SnO
2
powders or foils). Furthermore, withthe system withoutusing
automatic reactive pressure control unit is difficult to control the
pressure in the reacted tube. Consequently, the oxygen flow is also
needed to optimize correspond to the optimized Ar flow rate. These
matters indicate that it is rather difficult to reproducibly synthesize
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the SnO
2
NWs on large-scale by using Ar flow for transportation of
the Sn vapor. Our synthesized method is very simple. The carrier
gas was not used in the NWs growth so the transportation of Sn
vapor would take place only by flow caused by thermal diffusion.

The oxygen flow rate (lower than 1 sccm) was used during grow-
ing the SnO
2
NWs. Hence, the pressure in the reacted tube is quite
easy to control. We have found that this synthesized method can
be used to grow SnO
2
NWs with any thermal evaporation appara-
tus. Recently, we have very successfully synthesized SnO
2
NWs at
low temperature (∼700
◦
C) from Sn powder source by using this
method that will be published in another paper.
3.2. Electrical and gas-sensing properties
The screen-printing method for gas sensor device fabrication
proposed in this work is very much simple and this method is more
efficient compared to thatadopted by previous works. Hencea large
number of sensorswere obtained as shown in Fig. 5a. FE-SEMimage
of thefabricated sensorat a higher magnification is shown inFig. 5b.
The patterns of the SnO
2
NWs growth are shown in Fig. 5c. Fig. 5d
represents current–voltage (I–V) characteristics of the gas sensor in
air atdifferent temperatures.The (I–V) curveof theas-fabricated gas
sensor device shows a good ohmic behavior. This points out that not
only metal–semiconductor junction between the Au contact layer
and SnO
2

NWs but also the semiconductor–semiconductor junc-
tion between the SnO
2
NWs are ohmic. The ohmic behavior is very
Fig. 4. XRD patterns of SnO
2
powders and as-synthesized SnO
2
NWs (a) and their
magnified pattern (b).
Fig. 5. As-fabricated gas sensors imaged by optical microscope (a); FE-SEM of the
sensor at higher magnification(b andc); I–V characteristic of the sensors at different
temperatures (d).
important to the gas-sensing properties, because the sensitivity of
the gas sensor is affected by contact resistance. We have measured
the I–V characteristics at temperature up to 400
◦
C and found that
there is no difference in the I–V curve. Hence, it could point out
that the combining of the synthesis and fabrication methods in the
present works is a prospective platform for large-scale fabrication
of the gas sensor, which are relatively good reliability and capable
of working in real-world environments.
The gas-sensor testing by using set-up at our laboratory, which
can only measure with single device each time, is time-consuming
with testing a relatively large number of the sensor. Therefore, only
randomly selective devices were tested. Fig. 6a shows the responses
of the SnO
2
NWs sensors under exposure to 10, 50, and 100 ppm of

ethanol gas at 400
◦
C. It can be seen that the resistance of the sen-
sors in dry air is relatively large variation. This can be attributed to
slightly difference in the NWs density and could be a disadvantage
of the sensor fabrication method. However, the responses of the
sensors are not much different as shown in Fig. 6b. The latter issue is
much more important for practical application than the former one.
As also shown in Fig. 6b, the responses of all the measured sensors
are increased linearly with increasing of concentration of ethanol
gas with a small fluctuation. The linear dependence of the response
to ethanol gas of Q1D SnO
2
nanostructures was already investigated
in previous reports [6,20]. This could offer a suitable application of
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Fig. 6. Response characteristics of randomly tested sensors to various ethanol con-
centrations at a temperature of 400
◦
C (a) and response as a function of ethanol
concentration (b).
the SnO
2
NWs sensor for detecting ethanol gas. The sensitivity of
our sensors to ethanol gas is comparable with the SnO

2
NWs-based
ethanol sensors fabricated by other methods [6,18]. The sensitivity
and selectivity of our sensor can be greatly improved by function-
Fig. 7. Transient response of randomly selected sensors (named as S1–S6) to various
gases (C
2
H
5
OH, CH
3
COOCH
3
,C
3
H
8
, CO, and H
2
) with concentration of 100 ppm.
alizing with catalytic nanoparticles as reported in our previous and
other works [21–23].
As-fabricated sensors were also tested with different gases such
as CH
3
COCH
3
,C
3
H

8
, CO and H
2
. It can be seen that their response
characteristics are very similar, and the results are shown in Fig. 7.
Table 1
The SnO
2
NWs sensor response comparison between this work and previous works.
Sensor description Measured gases Reference
C
2
H
5
OH CH
3
COCH
3
CO H
2
Screen-printed SnO
2
NWs sensor 100 ppm 100 ppm R
a
/R
g
∼10.8
100 ppm R
a
/R

g
∼2.9 100 ppm R
a
/R
g
∼3.4
a
SnO
2
nanorods sensor 100 ppm R
a
/R
g
∼10, 10.8 [6,27]
Sb-doped SnO
2
NWs 100 ppm R
a
/R
g
∼2.2 [5]
In-doped SnO
2
NWs 200 ppm R
a
/R
g
∼40 [25]
SnO
2

nanobelts sensors 10 ppm R
a
/R
g
∼25, 50 ppm
R
a
/R
g
∼18.3
20 ppm R
a
/R
g
∼2 [18,26]
Single SnO
2
NWs sensor 100 ppm R
a
/R
g
∼2,
500 ppm, R
a
/R
g
∼1.2,
100 ppm Gg/G
a
∼1.9

20,000 ppm (in N
2
)
(G
g
− G
a
)/G
a
∼0.6
[22,27–29]
Pd-doped SnO
2
NWs sensor 50 ppm R
a
/R
g
∼15 50 ppm R
a
/R
g
∼10 50 ppm R
a
/R
g
∼33 [30]
Network SnO
2
NWs sensor 100 ppm (G
g

− G
a
)/G
a
∼0.5
[31]
Percolating effect SnO
2
NWs sensor 50 ppm (R
a
− R
g
)/R
a
∼3.2 10 ppm (R
a
− R
g
)/R
a
∼4.1
[32]
a
From this work.
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Fig. 8. The estimation of response–recovery time from transient response.
for the selected sensors. This is to suggest further that the sensor
fabrication method in the present work is quite reproducible. Addi-
tionally, the responses to the measured gases of the sensors in the
present work were used to extensively compare with the previ-
ous works. The responses (R
a
/R
g
)toC
2
H
5
OH (100 ppm), CH
3
COCH
3
(100 ppm), CO (100 ppm), and H
2
(100 ppm) are round 11.8, 10.8,
2.9, and 3.4, respectively. These obtained values are comparable
with most of the previous works (see Table 1 and Fig. 7).
It can be also seen that there are various SnO
2
NWs-like sen-
sors showed a relatively higher response, but the SnO
2
-doped or
functionalized with catalytic materials have been used for the NWs
gas sensor. For instance, the response to ethanol of our sensors can

be increased with about 6 times with functionalizing with La
2
O
3
as reported [21]. This suggests that the synthesis and fabrication
methods can be easily used to develop semiconductor oxides NWs
gas-sensor and the gas-sensor array for detection of multi-gases
application by functionalizing with different catalytic materials.
The dynamic response transients were obtained for the SnO
2
NWs sensors. 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. 8. The t
90%(air-to-gas)
values in
the sensing of 10, 50, and 10 0 ppm C
2
H
5
OH ranged from 4 to 6 s,
while the t
90%(gas-to-air)
value ranged from 20 to 40 s. These results
are quite comparable with the NWs-based sensor of the most of the
literature [6,8,15,17,18,21].
4. Conclusion

We demonstrated that single-crystalline SnO
2
NWs were suc-
cessfully prepared on silicon and alumina substrates through
simple thermal evaporation of SnO powder at 960
◦
C under con-
trolling of pressure (0.5–5 Torr) and oxygen gas flow (0.4–0.6 sccm).
It was used to synthesize in different evaporation apparatuses
with very high reproducibility, and a very large-scale of the NWs
was obtained. The as-synthesis NWs were used to fabricate gas
sensor by screen-printing method. The fabrication process does
not involve any tedious and time-consuming steps such as photo
or electron-beam lithography. As-fabricated SnO
2
NWs sensors
exhibit relatively good performance to ethanol gas. However, the
sensitivity and selectivity can be improved further by surface cat-
alytic doping or functionalizing or plasma treatment.
Acknowledgments
The work has been supported by the National Foundation for
Science & Technology Development (NAFOSTED) of Vietnam (for
Basic Research Project: 2009–2011), the National Key Research Pro-
gram for Materials Technology (Project No. KC 02-05/06-10), and
the research project of Vietnam Ministry of Education and Training
(Code B2008-01-217).
Appendix A. Supplementary data
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.snb.2009.02.043.
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printed gas sensor, Sens. Actuators B: Chem. (2009), doi:10.1016/j.snb.2009.02.043
ARTICLE IN PRESS
G Model
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N. Van Hieu / Sensors and Actuators B xxx (2009) xxx–xxx 7
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Biography
Nguyen Van Hieu received his MSc degree from the International Training Institute
for Material 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, Korea University. His current research inter-
ests include nanomaterials, nanofabrications, characterizations and applications to
electronic devices, gas sensors and biosensors.