Sensors and Actuators B 140 (2009) 514–519
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Sensors and Actuators B: Chemical
journal homepage: www.elsevier.com/locate/snb
Facile synthesis and NO
2
gas sensing of tungsten oxide nanorods assembled
microspheres
Zhifu Liu
a,∗
, Masashio Miyauchi
a,∗
, Toshinari Yamazaki
b
, Yanbai Shen
b
a
Nanotechnology Research Institute, National Institute of Advanced Industrial Science and Technology (AIST), Central 5, 1-1-1 Higashi, Tsukuba 305-8565, Japan
b
School of Engineering, University of Toyama, 3190 Gofuku, Toyama 930-8555, Japan
article info
Article history:
Received 14 January 2009
Received in revised form 31 March 2009
Accepted 25 April 2009
Available online 7 May 2009
Keywords:
Tungsten oxide
Microsphere
Nanorod
NO

© 2009 Elsevier B.V. All rights reserved.
1. Introduction
Nanostructured materials are considered as good candidates for
gas sensing applications due to their large surface area-to-volume
ratio and the size effect. Since the report of enhanced gas sensing
performance of tin oxide nano-crystallites in 1990s [1], nanomate-
rials based gas sensors attracted more and more attentions [2,3].
Nanostructures of well-established gas sensing materials like tin
oxide [4–6], zinc oxide [7], tungsten oxide [8–10], titanium oxide
[11,12], and indium oxide [13,14] have shown higher sensitivity,
faster response, lower operating temperature, and/or enhanced
capability to detect low concentration gases compared withthe thin
film counterparts.
Tungsten oxides are a class of versatile materials that offer
manifold technological applications including gas sensors [15,16],
opto-electrochromic and optical modulation devices [17,18], pho-
tocatalysis [19], hydrophilic surface design [20], etc. Gas sensors
based on tungsten oxide are sensitive to a variety of gases such as
NO
2
,O
3
,H
2
,H
2
S, andNH
3
[21]. In particular, tungsten oxide showed
superior sensitivity and selectivity in detecting NO

◦
C for 8 h. After the reaction completed, the resulting prod-
uct was centrifuged and washed with deionized water for three
times, and then dried at 60
◦
C overnight. Part of the product thus
treated was annealed at 350, 450, and 550
◦
C for 5 h, respectively,
0925-4005/$ – see front matter © 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.snb.2009.04.059
Z. Liu et al. / Sensors and Actuators B 140 (2009) 514–519 515
for investigating the crystal structure, morphology change, and the
gas sensing properties. For comparison, samples were also synthe-
sized at OA/W ratios of 2:1, 3:1, and 5:1 with a fixed tungsten ion
concentration by the same synthesis process.
2.2. Structural characterization
X-ray diffraction (XRD) measurements were performed on an
X-ray diffractometer (Rigaku, Ultrax 18SF) with an imaging plate
detector using Cu Ka radiation. A Hitachi S-4800 field emission
scanning electron microscope (FESEM) was used to investigate
the morphology of the samples. Transmission electron microscopy
(TEM) characterization was carried out on a Hitachi S-9000
transmission electron microscope. The effective surface area was
measured using physical adsorption/desorption of Kr on a Quan-
tachrome AUTOSORB-1-MP facility.
2.3. Gas sensing measurements
The gas sensors were made by drop-casting method. Briefly,
desired amount of the synthesized powder was dispersed in
methanol with the assist of ultrasonic. Then, the suspension was

3. Results and discussion
3.1. Structure and morphology
All the as-synthesized products are powders with white blue
color. The XRD patterns of the samples dried at 60
◦
C are shown
in Fig. 1. The results indicate that the products synthesized with
OA/W ratio of 2:1, 3:1, 4:1, and 5:1 are all crystallized and have the
same crystalline structure. The peaks of the XRD patterns can match
well with the documented diffraction pattern of orthorhombic
WO
3
·0.33H
2
O (JCPDS card no. 35-0270). Considering the possibility
of the variation of water in the structure during drying and anneal-
ing, we assign the formula of WO
3
·xH
2
O to the samples containing
water in our experiments.
Despite of the same phase composition, the morphology of the
products synthesized with different OA/W ratio is very different.
Fig. 2 presents the FESEM images of the samples synthesized with
OA/W ratio of 2:1, 3:1, 4:1, and 5:1. The sample synthesized with
OA/W ratio of 2:1 shows sphere-like aggregate with nanoplatelet
substructure. The nanoplatelet substructure can still be observed
when the ratio of OA to tungsten is increased to 3:1. However, the
products change to nanorod-like morphology when the ratio of OA

with OA/W ratio of 4:1. These nanorods have an average diameter
less than 100 nm and length in micrometer level. A correspond-
ing diffraction pattern of the nanorods is also presented in Fig. 3.
Diffraction rings can be clearly seen. The diffraction pattern, which
can be indexed to orthorhombic phase, is consistent with the XRD
results.
3.2. Effect of annealing on structural properties
Here we choose the microspheres synthesized with OA/W
ratio of 4:1 to investigate the gas sensing properties. Since gas
sensor requires a material to work continuously at high temper-
ature condition, the microspheres were annealed to stabilize the
microstructure. Fig. 4 represents the XRD patterns of the micro-
spheres annealed at 350, 450, and 550
◦
C, respectively. The sample
lost the crystalline water after annealed at 350
◦
C and transferred
to hexagonal WO
3
(JCPDS card no.33-1387). With the increase of
the annealing temperature, the diffraction peaks at 23–25
◦
and
the peaks at 26–30
◦
separate gradually, indicating that the phase
changed after annealing at higher temperature. The sample com-
pletely transferred to monoclinic WO
3

/g for the samples annealed
at 350, 450, and 550
◦
C, respectively. Crystal growth and the par-
tial collapse of the substructure of the microspheres should be the
main reason of the decrease of effective surface area of the annealed
samples.
Fig. 3. TEM image of the WO
3
·xH
2
O nanorods synthesized with an oxalic
acid/tungsten ratio of 4:1. Inset is the corresponding diffraction pattern.
3.3. Gas sensing properties
The gas sensing properties of the annealed microspheres were
evaluated by exposing the microspheres based gas sensors to NO
2
gas. Fig. 7 shows the typical resistance change profiles of the
microsphere based gas sensors upon exposed to 1 ppm NO
2
gas
at different operating temperatures. The sensor responses quickly
to NO
2
gas at all operating temperatures. The response times (the
time for the resistance increase to 90% of the maximum) are less
than 3 min in all cases, which are much quicker than that of sput-
tered WO
3
thin film sensors measured using the same system [33].

C.
NO
2
gas was turned off. The sensor can recover to initial resistance
only at temperatures above 200
◦
C.
Fig. 8 represents the responses of the sensors based on 350,
450, and 550
◦
C annealed microspheres as a function of operat-
ing temperatures. These sensors exhibit very high response at low
operating temperature. For example, the sensor based on micro-
spheres annealed at 350
◦
C showed a sensor response up to 3000
when operated at 100
◦
C. It is more than 10 times larger than the
sensor response of the thin film counterpart [33]. For all the three
kinds of materials, the sensor response decreases with the increase
of operating temperature. However, it can be noticed that the sen-
Fig. 6. Effective surface area of the WO
3
·xH
2
O microspheres (a) dried at 60
◦
C and
annealed at (b) 350

higher response than the others.
The quick and high response of the sensors should be ascribed to
the distinctive microsphere structure with nanorod substructure. It
is accepted that, upon exposure to NO
2
gas, the NO
2
gas molecules
are directly absorbed on the active sites on tungsten oxide surface.
Charge transfer is likely to occur from WO
3
to absorbed NO
2
because
of the strong electron-withdrawing power of the NO
2
molecules,
which leads to the increase of thickness of the depletion layer [34].
The nanorod substructure in the microspheres can be fully depleted
by exposing to NO
2
gas. As a result, the barrier heights at the bound-
aries between the nanorods increase significantly, resulting in the
large increase in electrical resistance, i.e., the high sensor response.
On the other hand, for a thin film and thick film gas sensor, the
gas diffusion is one of the key factors that determines the sensor
response and response time [35]. In the present work, the sensing
layer made by microspheres is highly porous. The gas can reach the
deep layer of the microspheres based thick film quickly through
the pore network. So the effects of gas diffusion can be ignored

A350
,S
A450
, and S
A550
, respectively) of the sensor response
depends on the annealing temperature and follow the trend:
S
A350
>S
A450
>S
A550
. The higher effective surface area should ben-
efit to the higher response of the sensor based on 350
◦
C annealed
microspheres.
In addition, as shown previously, the phase compositions of the
annealed microspheres are different and the crystal phase changes
from hexagonal to monoclinic structure gradually when the anneal-
ing temperature increases from 350 to 550
◦
C. The gas sensing of
monoclinic tungsten oxide has been extensively studied. There are
also reports on the gas sensing of hexagonal tungsten oxide [36,37].
In our present work, there is no obvious difference in the gas sens-
ing performance among the hexagonal, monoclinic, and the mixed
phase tungsten oxide. This implies that it is possible to obtain higher
sensor response by using the materials annealed at lower tempera-

Acknowledgements
This work is supported by the New Energy and Industrial Tech-
nology Development Organization (NEDO) in Japan and was partly
conducted using the AIST Nano-Processing Facility, which is sup-
ported by the “Nanotechnology Support Project” of the Ministry of
Education, Culture, Sports, Science and Technology of Japan.
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Biographies
Zhifu Liu is a researcher at National Institute of Advanced Industrial Science and
Technology (AIST), Tsukuba, Japan. He received his Ph.D. degree from Shanghai Insti-