Contents lists available at ScienceDirect Sensors and Actuators B: Chemical ELSEVIE journal homepage: www.elsevier.com/locate/snb Facile synthesis and NO2 gas sensing of tungst
Trang 1
Contents lists available at ScienceDirect
Sensors and Actuators B: Chemical
ELSEVIE journal homepage: www.elsevier.com/locate/snb
Facile synthesis and NO2 gas sensing of tungsten oxide nanorods assembled
microspheres
Zhifu Liu**, Masashio Miyauchi**, Toshinari Yamazaki”, Yanbai Shen?
4 Nanotechnology Research Institute, National Institute of Advanced Industrial Science and Technology (AIST), Central 5, 1-1-1 Higashi, Tsukuba 305-8565, Japan
5 School of Engineering, University of Toyama, 3190 Gofuku, Toyama 930-8555, Japan
Article history:
Received 14 January 2009
Received in revised form 31 March 2009
Accepted 25 April 2009
Available online 7 May 2009
Tungsten oxide nanorods assembled microspheres were synthesized by a facile hydrothermal process
at 180°C using ammonium metatungstate and oxalic acid as starting materials The morphology and structural properties were investigated using scanning electron microscopy, powder X-ray diffraction, and transmission electron microscopy The as-synthesized microspheres are composed of orthorhombic WO3-xH20 nanorods with diameter less than 100nm These microspheres lose water gradually during annealing and transfer to monoclinic WO3 when annealed at 550°C The gas sensing properties of the microspheres annealed at different temperatures were studied by exposing the gas sensors made from Microsphere microspheres to NO2 gas The results indicated that the crystalline phase of the microspheres has no
NO; highest response to NO2 gas due to the three-dimensional network based on the nanorods and the high
Keywords:
Tungsten oxide
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 with the 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, 03, Hz, H2S, and NH3 [21] In particular, tungsten oxide showed
superior sensitivity and selectivity in detecting NO gas [22,23] On
the other hand, nanostructural tungsten oxide such as nanorods
[24], nanowires [25], nanotubes [26], nanoflakes [27], nanodisks
* Corresponding authors
E-mail addresses: zhifu_liu@yahoo.com (Z Liu),
m-miyauchi@aist.go.jp (M Miyauchi)
0925-4005/$ - see front matter © 2009 Elsevier B.V All rights reserved
doi:10.1016/j.snb.2009.04.059
[28], and nanotrees [29] have been synthesized using high tem- perature evaporation, precipitation, hydrothermal reaction, and electrochemical or template assisted methods These nanostruc- tures provide good blocks for developing high performance gas sensors Herein, we report the synthesis of tungsten oxide nanorods assembled microspheres by a facile hydrothermal method To our knowledge, there is no report on the gas sensing of the microsphere- like tungsten oxide nanostructures We expect that this kind of microsphere with nanorod substructure will benefit to the gas sens- ing performance of their based gas sensors
2 Experimental 2.1, Synthesis
A facile hydrothermal process was employed to synthesize the samples Ammonium metatungstate and oxalic acid (99.9%, Wako Pure Chemicals Co.) were used as starting materials In a typical experiment, 0.53 g ammonium metatungstate and 0.72 g oxalic acid (OA) (the mole ratio of OA/W is 4:1) were dissolved in 50 ml deion- ized water Clear solution was obtained after stirring for 30 min Then, the mixture solution was transferred into a 100 ml Teflon- lined stainless autoclave The autoclave was sealed and maintained
at 180°C for 8h 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 5h, respectively,
Trang 2for 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
dispensed dropwise onto the oxidized Si substrate with a pair of
interdigitated Pt electrodes The gas sensors were ready for char-
acterization after dried and then aged at 350°C for 2h The gas
sensing properties were measured in a tube system with a coil
resistance heater The carrier gas (dry synthetic air) mixed with a
desired concentration of NO>2 gas was flowed at 200 ml/min through
the quartz tube (45 mm in diameter and 400 mm in length) kept
at a set temperature The electrical measurement was performed
by a voltamperometric method at a constant bias of 10V, and a
multimeter (Agilent 34970A) was used to monitor the change of
electrical resistance upon turning the target gas on and off The
sensor response is defined as (Ra —Ro)/Ro, where Ro is the resis-
tance in air and R, is the maximum resistance after the NO» gas
was introduced
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
WOs.0.33H+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 WO3-xH20 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
to tungsten is 4:1 These nanorods assemble to microspheres with
average diameter of around 3 ym For the sample synthesized at an
OA/W ratio of 5:1, radial nanorod aggregates are obtained Since no
other templates or assistant agents were used in our experiments,
the formation of various microstructures at different OA/W ratios
should be ascribed to the interaction between tungsten ions and
OA It is known that OA can stabilize the hydrolyzed tungsten oxide
So
>4
S
a
o _
"0 © N — © t
|
‘ 1 + L 4 ˆ L nm ˆ
20 (degree)
Fig 1 XRD patterns of the products synthesized with oxalic acid/tungsten mole ratios of (a) 2:1; (b) 3:1; (c) 4:1; and (d) 5:1
nanocrystals in aqueous solution by forming coordination com- plex [30] The OA ligand would affect the growth direction of the nanocrystals by binding to specific surface of the nanocrystals The WO3:XH20 nanocrystals can grow in the platelet habit in the pres- ence of a small amount of OA However, in the presence of large amount of OA, for example when the OA/W ratio is 4, the crystal- lization habit is changed by the surrounding OA ligands, leading
to the formation of rods like morphology On the other hand, the intermolecular force among the OA molecules may contribute to the formation of the microsphere morphology [31] The NH4* ions
in the solution may also affect the microstructure formation [32] More detailed work should be done to clarify the self-assembly mechanism of the microspheres
Fig 3 shows a typical TEM image of the nanorods obtained with OA/W ratio of 4:1 These nanorods have an average diameter less than 100nm 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 WO3 (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 WO3 (JCPDS card no 43-1035) after annealed at 550°C for 5h
The FESEM images of the samples annealed at different temper- atures are shown in Fig 5 For the samples annealed at 350 and 450°C, the nanorods on the microsphere surface were damaged to some extent However, the nanorods inside the microspheres can still be clearly seen The nanorod substructure was totally destroyed for the sample annealed at 550°C and these nanorods changed
to nanoparticles However, it was noticed that the microsphere morphology still exists for the samples annealed at all conditions
Trang 3
Hy > “4
| Oe UIẾN
Fig 2 SEM images of the products synthesized with oxalic acid/tungsten mole ratios of (a) 2:1; (b) 3:1; (c) 4:1; and (d) 5:1
These microspheres are very helpful for forming porous sensing
layer
The effective surface areas of the samples were evaluated by
isothermal Kr gas physical adsorption/desorption measurements
and are shown in Fig 6 The sample dried at 60°C has an effec-
tive surface area of 36 m2/g The effective surface area decreases
after annealing and is 20, 13, and 6 m2/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 WO3-xH20 nanorods synthesized with an oxalic
3.3 Gas sensing properties The gas sensing properties of the annealed microspheres were evaluated by exposing the microspheres based gas sensors to NO2 gas Fig 7 shows the typical resistance change profiles of the microsphere based gas sensors upon exposed to 1 ppm NO, gas
at different operating temperatures The sensor responses quickly
to NO2 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 WO3 thin film sensors measured using the same system [33] However, we also notice that the sensor cannot recover to initial resistance at low operating temperatures of 100 and 150 °C after the
e
« Monoclinic WO,
- *
3 e H
2
(b) (a)
1 1 s 1 1 1 * 1 1 1 1
20 (degree)
Fig 4 XRD patterns of the WO3-xH20 microspheres (a) dried at 60°C and annealed
Trang 4
Fig 5 SEM images of the WO3-xH20 microspheres annealed at (a) 350°C; (b) 450°C;
and (c) 550°C
NO2 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-
+ o
nN a '
= o 0 T 1 1 i 1 L * :
Fig 6 Effective surface area of the WO3-xH20 microspheres (a) dried at 60°C and annealed at (b) 350°C; (c) 450°C; and (d) 550°C
NO, gas off
7
— 3 EE
$ 10°
”
8 10 i :
10°
oe a & ¡ tttrrrrrrrrxrxuarertrrxrrrrro
Operating time (min)
Fig 7 The resistance change profile of the microspheres (annealed at 350°C) based gas sensor to 1 ppm NO, gas at different operating temperatures
10°F
a i
oF , |
Na 10 Ẹ
a :
a R
2
o 10 F
@ 7
o i
2 10}
® :
Operating temperature (°C)
Fig 8 The sensor response of the microspheres based gas sensor to 1 ppm NO; gas
as a function of operating temperatures.
Trang 5
20 ppm
10 F
F = |
a
0 20 40 60 80 100 120 140 160
Operating time (min)
(c)
x 8
S am
G6 a a
~ s
@
wz
+
i + L iL
Operating time (min)
4 + L + L + i
120 140 160
5 ppm
407 4 1 + L 4 1 + L 4 L 4 1 1 1 4 L
Operating time (min)
500}-
400}
300}
200
NO, gas concentration (ppm)
Fig 9 The dynamic response of the gas sensors based on microspheres annealed at (a) 350°C, (b) 450°C, and (c) 550°C to 1, 3, 5, 10, and 20 ppm NO; gas pulses The sensor response as a function of gas concentration is shown in (d)
sor based on microspheres annealed at 350°C showed a relatively
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> gas, the NO» gas molecules
are directly absorbed on the active sites on tungsten oxide surface
Charge transfer is likely to occur from WO3 to absorbed NOz because
of the strong electron-withdrawing power of the NOz 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 NO2 gas Asa 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
and the surface phenomena such as adsorption/desorption of NOz
molecules should be the dominating factor of the sensor perfor-
mance This is supported by the fast response of the sensors to NO
gas
In order to investigate the relation between the sensor response
and gas concentration, the sensors were exposed to NO2 gas
with concentrations of 1, 3, 5, 10, and 20 ppm Fig 9a—c indicates
the dynamic response of the sensors based on the microspheres
annealed at 350, 450, and 550°C at an operating tempera-
ture of 200°C All the three kinds of sensors exhibit good
response/recovery cycle to the NO» gas pulses and the sensor
responses increase with the increase of gas concentrations Fig 9d
shows the profiles of the sensor responses as a function of NO2 gas concentrations The sensor responses increase nearly linearly with the increase of NOz gas concentration It can also be noticed that the concentration coefficient (the slop of the lines, assigned
as Sa350, Saaso, and Sasso, respectively) of the sensor response depends on the annealing temperature and follow the trend: Sa350 > Saaso > Sasso 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- ture such as 350 and 450°C under which the nanorod substructure can be well kept
4 Conclusions
In conclusion, the microspheres composed of WOs.xH;O nanorod were synthesized by a facile hydrothermal process The microsphere morphology and the nanorod substructure can be reserved when annealed at temperatures lower than 450°C Gas sensing properties of the microspheres annealed at different tem- peratures were investigated by exposing the microspheres based gas sensors to NO» gas The gas sensor based on 350°C annealed
Trang 6microspheres showed relative higher response to NO» gas than oth-
ers Phase composition of the microspheres had no obvious effect
on the gas sensing performance This kind of microspheres with
nanorod substructure provides a new block for developing high
performance gas sensors
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
References
[1] J Tamaki, C Xu, N Miura, N Yamazoe, Grain size effects on gas sensitivity of
porous SnO2-based elements, Sens Actuators B 3 (2) (1991) 147-155
[2] E Comini, Metal oxide nano-crystals for gas sensing, Anal Chim Acta 568 (1-2)
(2006) 28-40
[3] A Kolmakov, M Moskovits, Chemical sensing and catalysis by one-dimensional
metal-oxide nanostructures, Ann Rev Mater Res 34 (2004) 151-180
[4] A Kolmakov, Y.X Zhang, G.S Cheng, M Moskovits, Detection of CO and O2 using
tin oxide nanowire sensors, Adv Mater 15 (12) (2003) 997-1000
[5] E Comini, G Faglia, G Sberveglieri, D Calestani, L Zanotti, M Zha, Tin oxide
nanobelts electrical and sensing properties, Sens Actuators B 111 (2005) 2-6
[6] Y.B Shen, T Yamazaki, Z.F Liu, D Meng, T Kikuta, N Nakatani, M Saito, M
Mori, Microstructure and Hz gas sensing properties undoped and Pd-doped
SnO2 nanowires, Sens Actuators B 135 (2009) 524-529
[7] Q Wan, Q.H Li, YJ Chen, T.H Wang, X.L He, J.P Li, C.L Lin, Fabrication and
ethanol sensing characteristics of ZnO nanowire gas sensors, Appl Phys Lett
84 (18) (2004) 3654-3656
[8] A Ponzoni, E Comini, G Sberveglieri, J Zhou, S.Z Deng, N.S Xu, Y Ding, Z.L
Wang, Ultrasensitive and highly selective gas sensors using three-dimensional
tungsten oxide nanowire networks, Appl Phys Lett 88 (20) (2006) 203101
[9] K.M Sawicka, A.K Prasad, P.I Gouma, Metal oxide nanowires for use in chemical
sensing applications, Sens Lett 3 (1) (2005) 31-35
[10] C.S Rout, A Govindaraj, C.N.R Rao, High-sensitivity hydrocarbon sensors based
on tungsten oxide nanowires, J Mater Chem 16 (40) (2006) 3936-3941
[11] O.K Varghese, C.A Grimes, Metal oxide nanoarchitectures for environmental
sensing, J Nanosci Nanotechnol 3 (4) (2003) 277-293
[12] A.S Zuruzi, A Kolmakov, N.C Macdonald, M Moskovits, Highly sensitive gas
sensor based on integrated titania nanosponge arrays, Appl Phys Lett 88 (10)
(2006) 102904
[13] C Li, D.H Zhang, X.L Liu, S Han, T Tang, J Han, C.W Zhou, Inz03 nanowires as
chemical sensors, Appl Phys Lett 82 (10) (2003) 1613-1615
[14] C Li, DJ Zhang, X.L Liu, S Han, T Tang, C.W Zhou, Doping dependent NH3
sensing of indium oxide nanowires, Appl Phys Lett 83 (9) (2003) 1845-1847
[15] H.T Sun, C Cantalini, L Lozzi, M Passacantando, S Santucci, M Pelino,
Microstructural effect on NO2 sensitivity of WO; thin film gas sensors Part 1
Thin film devices, sensors and actuators, Thin Solid Films 287 (1996) 258-265
[16] C Cantalini, W Wlodarski, Y Li, M Passacantando, S Santucci, E Comini, G
Faglia, G Sberveglieri, Investigation on the O3 sensitivity properties of WO; thin
films prepared by sol-gel, thermal evaporation and r.f sputtering techniques,
Sens Actuators B 64 (1-3) (2000) 182-188
[17] T Oi, Electrochromic materials, Ann Rev Mater Sci 16 (1986) 185-201
[18] S.K Deb, Opportunities and challenges in science and technology of WO; for
electrochromic and related applications, Solar Energy Mater Solar Cells 92
(2008) 245-258
[19] R Abe, H Takami, N Murakami, B Ohtani, Pristine simple oxides as visible light
driven photocatalysts: highly efficient decomposition of organic compounds
over platinum-loaded tungsten oxide, J Am Chem Soc 130 (2008) 7780-7781
[20] M Miyauchi, A Nakajima, T Watanabe, K Hashimoto, Photocatalysis and pho-
toinduced hydrophilicity of various metal oxide thin films, Chem Mater 14
(2002) 2812-2816
[21] G Eranna, B.C Joshi, D.P Runthala, R.P Gupta, Oxide materials for development
of integrated gas sensors—a comprehensive review, Crit Rev Solid State Mater
Sci 29 (2004) 111-188
[22] M Akiyama, J Tamaki, N Miura, N Yamanoe, Tungsten oxide based semi- conductor sensor highly sensitive to NO and NOz, Chem Lett 20 (1991) 1611-1614
[23] M Akiyama, Z Zhang, J Tamaki, N Miura, N Yamanoe, Tungsten oxide based semiconductor sensor for detection of nitrogen oxides in combustion exhaust, Sens Actuators B 14 (1993) 619-620
[24] Y.S Kim, S.C Ha, K Kim, H Yang, S.Y Choi, Y.T Kim, Room-temperature semi- conductor gas sensor based on nonstoichiometric tungsten oxide nanorod film, Appl Phys Lett 86 (2005) 213105
[25] J Polleux, A Gurlo, N Barsan, U Weimar, M Antonietti, M Niederberger, Template-free synthesis and assembly of single-crystalline tungsten oxide nanowires and their gas-sensing properties, Angew Chem Int Ed 118 (2005) 267-271
[26] Z.G Zhao, M Miyauchi, Nanoporous-walled tungsten oxide nanotubes as highly active visible-light-driven photocatalysts, Angew Chem Int Ed 47 (2008) 7051-7055
[27] Z.F Liu, T Yamazaki, Y.B Shen, D Meng, T Kikuta, N Nakatani, Fabrication
of WO3 nanoflakes by a dealloying-based approach, Chem Lett 37 (2008) 296-297
[28] Y.G Choi, G Sakai, K Shimanoe, Y Teraoka, N Miura, N Yamazoe, Preparation
of size and habit-controlled nano crystallites of tungsten oxide, Sens Actuators
B 93 (2003) 486-494
[29] M Shibuya, M Miyauchi, Site-selective deposition of metal nanoparticles on aligned WO3 nano-trees for super-hydrophilic thin film, Adv Mater 21 (2009) 1373-1376
[30] E Lassner, W Schubert, Tungsten: Properties, Chemistry, Technology of the Element, Alloys, and Chemical Compounds, Kluwer Academic, New York, 1999 [31] Z.Gu, T Zhai, B Gao, X Sheng, Y Wang, H Fu, Y Ma, J Yao, Controllable assembly
of WO3 nanorods/nanowires into hierarchical nanostructures, J Phys Chem B
110 (2006) 23829-23836
[32] J.-H Ha, P Muralidharan, D.K Kim, Hydrothermal synthesis and characteriza- tion of self-assembled h-WO3 nanowires/nanorods using EDTA salts, J Alloys Compd 475 (2009) 446-451
[33] Z.F Liu, T Yamazaki, Y.B Shen, T Kikuta, N Nakatani, Influence of annealing on microstructure and NOz-sensing properties of sputtered WO; thin films, Sens Actuators B 128 (2008) 173-178
[34] J Tamaki, Z Zhang, K Fujimori, M Akiyama, T Harada, N Miura, N Yama- noe, Grain-size effects in tungsten oxide-based sensor for nitrogen oxides, J Electrochem Soc 141 (1994) 2207-2210
[35] N Yamazoe, Toward innovation of gas sensor technology, Sens Actuators B 108
(2005) 2-14
[36] C Balazsi, L Wang, E.O Zayim, I.M Szilagyi, K Sedlackova, J Pfeifer, A.L T6th,
PJ Gouma, Nanosize hexagonal tungsten oxide for gas sensing applications, J Eur Ceram Soc 28 (2008) 913-917
[37] L Wang, J Pfeifer, C Balazsi, PI Gouma, Synthesis and sensing properties to NH3 of hexagonal WO; metasatble nanopowders, Mater Manufact Proc 22 (6) (2007) 773-776
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- tute of Ceramics, Chinese Academy of Sciences, in 2004 His current research interest
is nanostructured semiconductor materials and their based devices for environmen- tal and clean energy applications
Masahiro Miyauchi is a senior research scientist at National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba, Japan He received his Ph.D degree from the University of Tokyo in 2002 His research interests are in the areas
of nanostructured materials for energy and environmental issues
Toshinari Yamazaki received his Ph.D degree from Nagoya University, Nagoya, Japan, in 1983 He is currently an associate professor at University of Toyama His research interests are in the areas of semiconducting oxide gas sensors and the deposition process of sputtered films
Yanbai Shen is a Ph.D student at University of Toyama, Japan He received his MS degree at Northeastern University, China, in 2004 His current research is focused on the microstructural, electrical, and gas sensing properties of oxide semiconductor thin films.