a room-temperature operated hydrogen leak sensor

A room-temperature operated hydrogen leak sensor
H. Nakagawa
a,*
, N. Yamamoto
b
, S. Okazaki
b
,
T. Chinzei
a
, S. Asakura
b
a
Research Center for Advanced Science and Technology, University of Tokyo, Komaba, Meguro-ku, Tokyo 153-8904, Japan
b
Graduate School of Engineering, Yokohama National University, Tokiwadai, Hodogaya-ku, Yokohama 240-8501, Japan
Abstract
A new chemi-resister type sensor for hydrogen leak detection is suggested. Tungsten trioxide (WO
3
) with Pt was used as sensing material.
The sensor was fabricated by a sol–gel method. Tungstic acid sol with chloroplatinic acid was spread on a quartz plate with a spinner and
calcined in atmosphere to form a WO
3
film. Pt was expected to act as catalyst for hydrogen reduction of WO
3
.
The conductivity of the sensor was less than 0.001 mS in oxidizing atmosphere, and more than 10
6
times conductivity increase was observed
upon exposure to (1% H
2

/99% N
2
) gas. Transient characteristics of the reduction process and oxidation process were not identical. The
reduction process exhibited super-linear nature, whereas oxidation process may be approximated by a simple exponential decay. The
sensitivity was susceptible to humidity. Not only the response was faster and more sensitive in humid environment than in dry one, it was also
affected by the previous exposure history. Even in dry environment the sensitivity increases if the device was exposed to hydrogen, several
times. A new detection scheme to explain these observations is suggested.
# 2003 Elsevier Science B.V. All rights reserved.
Keywords: Gas sensor; Tungsten trioxide; Hydrogen; Double injection
1. Introduction
With the increasing concern about the global climate
change, more attention is paid to hydrogen as a clean energy
source. Hydrogen burns into water and no global warming
gas is produced. Although pure hydrogen fuel may not be
utilized in near future, fuel cells may be brought in a wide
usage to automobile and home within a few years. Some
precautions are required, however, for the safe use of
hydrogen. Hydrogen has a large diffusion coefficient
(0.61 cm
2
/s in air, methane’s value is 0.15 cm
2
/s) and wide
combustion range (4–75%) and small ignition energy
(0.02 mJ in air, methane’s value is 0.3 mJ). Continuous
monitoring of hydrogen leak at storage or usage sites is
indispensable for safe operation. Two types of sensitive
sensors are widely used for this purpose. A high-temperature
operated oxide–semiconductor gas sensor has high sensitiv-
ity, reliability, as well as maintenance-free nature, but con-

sumes relatively large power for device heating. The other
type, an electrochemical gas sensor consumes very little
power, but electrolyte liquid within the cell has to be
replaced every year for the reliable operation. Development
of a new hydrogen sensor that consumes negligible electrical
energy with negligible maintenance is highly desirable.
Tungsten trioxide (WO
3
) is known to interact with hydro-
gen and other alkaline metal ions in a unique manner. This
interaction seduces the development of hydrogen sensor
[1,2]. In the present work, we report sensing characteristics
of a resistance-sensing hydrogen sensor that is operated at
ambient temperature. WO
3
film with platinum catalyst
derived from a sol–gel method was utilized. The sensor
of this type is particularly suitable for hydrogen leak mon-
itoring because it consumes negligible electrical power due
to the insulating characteristics in air. It had been known that
WO
3
changes its color to blue upon partial reduction. The
reduction may be achieved either by electrochemical reac-
tion in liquid (electrochromism) [3] or by gas-phase reaction
in reducing atmosphere (gasochromism) [4].WO
3
is classi-
fied as oxide semiconductor with a band gap of about 3.2 eV.
Its electrical resistance is very high due to wide band gap in

oxidized state, but the resistance becomes low upon reduc-
tion due to generated free electrons [5].AWO
3
hydrogen
gas sensor that detects the resistance change was reported
more than 30 years ago [1], but the detection mechanism is
still of some controversy. Traditionally, the double injection
model [3], in which both a proton (hydrogen ion) and an
electron are simultaneously supplied to the film, is widely
accepted. Not all the observations were in accordance with
Sensors and Actuators B 93 (2003) 468–474
*
Corresponding author. Tel.: þ81-3-5452-5241; fax: þ81-3-5452-5241.
E-mail address: (H. Nakagawa).
0925-4005/03/$ – see front matter # 2003 Elsevier Science B.V. All rights reserved.
doi:10.1016/S0925-4005(03)00201-6
this model, however, and other models based on the oxygen
deficiency [6,7] were suggested. Our observations may
be interpreted by the double injection/surface oxidation
model.
2. Experimental
AWO
3
sensor was fabricated on a quartz plate by a sol–gel
method and spin coating. An amount equal to 0.5 M sodium
tungstate (Na
2
WO
4
) solution was converted into tungstate

(H
2
WO
4
) sol solution by passing through cationic ion
exchange column (Amberlite IB120, Organo Co.). Appro-
priate amounts of hexachroloplatinum (H
2
PtCl
6
) solution
and ethyl alcohol were added. Addition of ethyl alcohol
prolonged the gelation time. This solution was spread on a
quartz plate and formed a thin film by a home-made spinner
(900 rpm). The film was dried for a few days and calcined
for 1 h at 200 8C in air to remove crystalline water. Final
calcination time was 1 h in air with calcination temperatures
ranging 300–700 8C. The sensor was placed in a flow-
through chamber and the electrical conductance was mea-
sured with a LCR meter (HP 4263B) under various gas
compositions and temperatures. Five hundred millivolts rms
ac signal of 1 kHz was applied during the measurements.
Relatively large applied bias eased the conductance mea-
surement of semi-insulating oxidized WO
3
. Electrical con-
tacts were achieved by physical contact of two parallel
copper electrodes about 0.5 mm apart. Good Ohmic contacts
were ascertained by the linear current–voltage characteris-
tics. Sensitivity measurements were performed at room

temperature unless otherwise stated. Test gases used in
the sensitivity measurements were drawn from a bottle
and the test gases were passed through a water-filled bub-
bling vessel when humid gases were required. The relative
humidity (RH) of humid gas was about 85% at room
temperature.
3. Results and discussions
Knowledge of crystalline structure is essential for the
accurate interpretation of the observed result. Tungsten
trioxides exist in several polymorphic forms, i.e. monoclinic
[8], hexagonal [9], and pyrochlore [10] forms around room
temperature. The basic unit of these three crystals are
octahedral unit in which W atom stays at the center and
oxygen atoms form every corners. Monoclinic crystal struc-
ture may be considered as slightly-warped ReO
3
type, or of
warped perovskite (ABO
3
) with vacant A sites. Corners of
every octahedrons are shared with neighboring octahedrons
and none of the edges are shared in monoclinic form.
Monoclinic WO
3
exhibits six phase transitions with tem-
perature [11]. It is monoclinic below À40 8C and triclinic
between À40 and 20 8C, and another monoclinic between 20
and 325 8C. It then changes to an orthorhombic at 325 8C,
and succession of tetragonal at 725, 900, and 1225 8C.
The actual change associated with these transitions is slight

change of bond length and all crystal phases of monoclinic
family may be considered as a modification of cubic ReO
3
structure in the first order approximation. Monoclinic phase
is the most stable and both hexagonal and pyrochlore
crystals switch to monoclinic phase if it is heated to more
than 500 8C and cooled down to room temperature. Both
hexagonal and pyrochlore are obtained as polycrystalline
powder and no single crystal of appreciable size had not
been obtained. Several crystal forms contain lattice water.
WO
3
Á2H
2
O and WO
3
ÁH
2
O [12] are believed to be mono-
clinic and WO
3
Á(1/3)H
2
O is hexagonal [13]. The initial state
of WO
3
films obtained by sol–gel method or vacuum
deposition was considered to be amorphous with some
coordinated and adsorbed water, but the amorphous state
mainly consists of nanocrystals and short range order of the

crystal structure is conserved [14]. Calcination procedure
coagulates and nanocrystals to form polycrystalline film and
desorbs water.
The effect of calcination temperature on the sensitivity
was investigated. The conductance values of 2 min after
exposure to humid (1% H
2
/99% N
2
gas) were plotted against
calcination temperature in Fig. 1. As will be shown in Fig. 5,
humidity affects the sensing characteristics. The molar ratio
of tungstate and platinum (W/Pt) was chosen to be the best
sensitivity value of 13. Good sensitivity was obtained when
the film was sintered above 400 8C. Although this tempera-
ture coincides with the phase change to the orthorhombic
phase (which changes to monoclinic at room temperature),
the major reason would be the reduction of platinum ions.
Platinum has to be in the form of metal particles to function
as efficient catalysts. If we use K
2
PtCl
4
instead of H
2
PtCl
6
,
then some sensitivity appears even without any sintering.
As the temperature was further increased, the sensitivity

increased further and eventually decreased slightly at
700 8C. As temperature increases, crystal size becomes
larger and quality of crystal improves. But if the crystal
size becomes too large, then surface area would decrease and
s-
e-
Fig. 1. Sensitivity vs. calcination temperature relation. Conductivity of
2 min after exposure to humid (1% H
2
/99% N
2
) gas was plotted. W/Pt ratio
was 13.
H. Nakagawa et al. / Sensors and Actuators B 93 (2003) 468–474 469
nsitivity reduces.
Fig. 2 presents the effect of catalyst amount to the
sensitivity. The conditions of the measurements are as same
as those for Fig. 1. The conductance values of 2 min after
exposure to humid (1% H
2
/99% N
2
) gas were plotted as a
function of W/Pt ratio. The calcinations temperature was
600 8C. The highest sensitivity was achieved when W/Pt
ratio was 13. The sensitivity decreased as W/Pt increases in
the large W/Pt region. This was expected since the con-
centration of the tungstate solution was constant (0.5 M),
larger the W/Pt ratio implies lower amount of catalyst. The
sensitivity also decreased in high Pt region (low W/Pt

range). If the platinum concentration is too high, then the
platinum particles coagulate each other. This coagulation
might have reduced the effective catalyst surface area and
spill-over probability.
The transient responses of the sensor at various tempera-
tures were plotted in Fig. 3. The sensor was exposed to
humid (1% H
2
/99% N
2
) gas for 1200 s and humid 100% O
2
afterwards. Higher temperature resulted faster response. At
13 8C, the sensor did not reached the stable state within
1200 s. But more than 100 times increase in conductance
was obtained within 120 s. The conductance ratios of more
than 10
6
were obtained at every temperature except 13 8C. It
should be mentioned that the transient response to hydrogen
is super-linear, and it is almost exponential at the beginning.
Since ordinate is scaled in logarithm, linear portion of the
curve meant exponential response. Although the rising part
of hydrogen response is super-linear in all temperatures,
they cannot be approximated by a simple power law or a
exponential function. Falling part or oxygen response may
be divided into two distinct regions: a fast decaying portion
at the beginning and a slow decaying portion of the follow-
ing part. Both portions may be crudely approximated by a
simple exponential decay with temperature dependent time

constants. The complicated nature of the reduction and
oxidation processes were confirmed by the separate optical
measurement [15]. To investigate the nature of hydrogen
response further, maximum response speed of response
curve was plotted in Arrhenius format in Fig. 4. The data
point of the oxidation velocity at 50 and 13 8C is likely to be
underestimated than the intrinsic value, since the hydrogen
response at 13 8C did not reached the stable state (Fig. 3).
For the hydrogen response or reduction reaction, the curve
may be divided into two regions. The dominating process
that limits the response may differ in different temperature
range. The Arrhenius energy in the low temperature range
was $75 kJ/mol, whereas that in the high-temperature range
was $23 kJ/mol. Two different mechanisms may be
involved as a rate-determining process. The Arrhenius
energy in the low temperature range is within the range
of reaction limited process, whereas that of the high-tem-
perature range may be in the higher range of the diffusion-
limited processes. Complex rise-time characteristics, how-
ever, suggest the many other possibilities such as simulta-
neous contributions from two sequential processes.
Fig. 2. Sensitivity vs. W/Pt molar ratio relation. Conductivity of 2 min
after exposure to humid (1% H
2
/99% N
2
) gas was plotted. Calcination
temperature was 600 8C.
Fig. 3. Transient characteristics at several temperatures. Humid (1% H
2

/99% N
2
) gas was flown for the initial 1200 s, and humid air was flown
afterwards.
470 H. Nakagawa et al. / Sensors and Actuators B 93 (2003) 468–474
Temperature range for the oxidation reaction may seem to be
divided into two regions, but this could be superficial,
because oxidation process at 13 8C had not started from
the stable states and oxidation velocity at this temperature is
likely to be underestimated. In the high-temperature region
of the oxidation process, Arrhenius energy was $5 kJ/mol.
We attributed this value to the surface diffusion of oxygen.
It has been known that water affects the sensitivity from
the early stage of the sensor development [1,30]. Fig. 5
shows transient response to repeated exposures of dry and
humid (1% H
2
/99% N
2
) gas for 300 s, and dry and humid
(100% O
2
) gas for 300 s. The device was kept in dry or
humid nitrogen atmosphere for several hours before the
measurements. In the dry atmosphere the both reduction
and oxidation responses were weak and slow, but both
responses improved with the repeated exposure to hydrogen.
The improvement in response could be interpreted by the
accumulation of the water in the film generated by hydrogen
exposure. The response to the humid gas is high and fast.

The response slightly decreased in the second exposure, but
decrease was small in comparison to the dry gas response.
The calibration curve for hydrogen in air was plotted in
Fig. 6. The sensitivity is not linear with concentration. The
response in air is roughly three orders of magnitude smaller
than that in nitrogen. The sensor response was obtained as a
result of the competing reactions: hydrogen reduction and
oxygen oxidation.
Faughnan et al. [3] proposed a double injection model for
electrochromic coloration of tungsten trioxide. Protons and
electrons are simultaneously supplied to keep the charge
neutrality, Protons are supposedly intercalated to form a
tungsten bronze. An injected electron reduces a W
6þ
ion to a
W
5þ
ion, and the polaron transition between a W
5þ
ion and
nearby a W
6þ
ion is responsible for the blue coloration [16].
The model nicely explained the coloration and electrical
characteristics. This model may be easily expanded to
reduction of WO
3
by hydrogen if one assumes the double
injection of a proton and an electron from a catalyst metal.
Fig. 4. Arrhenius plots of the reduction and oxidation speeds. Slopes of

the transient response of the data in Fig. 3 and of additional data were
normalized by 1 (mS/s) and their logarithmic value is plotted.
Fig. 5. Transient responses to dry and humid (1% H
2
/99% N
2
) gases with repeated exposures. Relative humidity of humid gas was 85% RH.
Fig. 6. A calibration curve for humid gas. Balance gas was humid air and
conductance values 5 min after the exposure to hydrogen was plotted.
H. Nakagawa et al. / Sensors and Actuators B 93 (2003) 468–474 471
However, there are arguments that hydrogen does not form a
bronze as the other alkaline metals such as lithium and
sodium do [17]. Gerard et al. [18] observed no increase of
hydrogen on coloration on their evaporated film colorization
by the nuclear reaction analysis. They observed some hydro-
gen increase in WO
3
upon colorization in their sputtered
film, but hydrogen content did not decreased upon disco-
loration. Wagner et al. [19] used the same technique and
reported that the hydrogen concentration did not change in
WO
3
films during electrochemical reduction and oxidation.
Their WO
3
film was, however, inserted between ITO elec-
trode and SiO
2
or Ta

2
O
5
material. In their film, hydrogen
might not be able to move in or out easily, and electro-
chemical oxidation or reduction within film might have
taken place. They have observed the increase and decrease
of hydrogen contents by gas reduction/oxidation in their
earlier paper [20]. The accumulation of hydrogen in the film
with repeated exposure was observed in their paper.
Further several works on WO
3
film based on vacuum
deposition reported transparent films despite oxygen defi-
ciency [21,22]. Zhang et al. [23] assumed the existence of
W
4þ
and postulated that the polaron transition takes place
between W
4þ
and W
5þ
ions, not between W
5þ
and W
6þ
as
widely believed. Later, Lee et al. [24] established a new
model in which both W
4þ

–W
5þ
and W
5þ
–W
6þ
transitions
contribute to polaron transitions, but the coloration effi-
ciency of the former is larger based on the Raman spectro-
scopy. This model seems to reconcile previous contradicting
observations in terms of tungsten valency. Apart from these
studies, Georg et al. [7,25,26] published several works based
on oxygen deficiencies instead of hydrogen intercalation.
Lattice oxygen is reduced to water by hydrogen and removed
from the film in their model and effect of water was
explained. Recent work by Lee et al. [27] denied the creation
of oxygen vacancies from their Raman study. Genin et al.
[28] investigated the crystal structures of various structures
and reported that structure changes to cubic symmetry with
increasing lattice constants as amounts of hydrogen is
increased. This observation supports hydrogen bronze
model, rather than oxygen deficiency model.
None of the reported model seemed to explain our
observed data as well as observation of other researchers,
a new scheme, double injection/surface oxidation model, is
suggested to interpret these phenomena in a unified way.
Since the present sensor was fabricated by a sol–gel method
and high-temperature calcination in air, the film was likely
oxidized completely. To exclude possible implications asso-
ciated with oxygen deficiencies, we limit the discussion to

the gas reduction/oxidation of WO
3
. The basic concept is the
formation of tungsten bronze with the intercalation of
hydrogen ions. But the oxygen removal by hydrogen reduc-
tion and water formation on (1 0 0) plain of ReO
3
crystal
structure [29] is included. Further, it was assumed that an
intercalated proton is removed as a form of water from the
film on oxidation. Therefore, reduction and oxidation take
different path and they are not reversible processes. This is in
accordance with our observed data of in Fig. 3 where shapes
of rising transient and falling transient have quite different
nature. Different values of the reduction and oxidation
velocities and their temperature dependence in Fig. 4 further
support this argument.
Two-dimensional view of our model crystal was presented
in Fig. 7. Here, we treat the phenomena within a single crystal
to avoid the implications associated with grain boundaries. It
Fig. 7. A mechanism model for the hydrogen reduction.
472 H. Nakagawa et al. / Sensors and Actuators B 93 (2003) 468–474
should be noted that the surface of the nanocrystal may not be
necessarily a film surface. But most of amorphous WO
3
films
are porous and whether the model surface is real film surface,
or inner crystal grain boundary of the film, is irrelevant.
Arrows schematically guide the reduction processes. They
may be summarized as following three processes:

(1) Dissociation of H
2
on Pt and spill-over to WO
3
surface
(double injection).
(2) Surface Diffusion of a proton and a parallel slow water
formation reaction on (1 0 0) plane.
(3) Internal diffusion of a proton from a surface through
favorable plane or sites.
The first process may not need arguments, since both
double injection model [3] and oxygen deficiency model [7]
accepted spill-over phenomenon. The second process may
need some explanation. It had been known that the existence
of water molecules increased the hydrogen reduction pro-
cess considerably and the increase was interpreted as the
acceleration of hydrogen diffusion by water molecule [30].
Our observation of the sensitivity increase with repeated
exposure to dry (1% H
2
) gas in Fig. 5 suggested the water
generation. Henrich and Cox [29] mentioned that hydrogen
atoms cannot diffuse into the bulk on (1 0 0), but slowly
react with surface oxygen to form water. The water gener-
ated on the surface of internal nanocrystal may stay for
prolonged period since the water has to pass through rela-
tively long crevasse-like narrow grain boundaries before
leaving the film. Their (1 0 0) plane would mean equivalent
(1 0 0) of cubic ReO
3

structure, not of monoclinic. The
accumulation of generated water is responsible for the
sensitivity increase in repeated exposure. Protons that dif-
fused out of (1 0 0) plane may now diffuse into bulk as stated
in the third process. Of course there should be some pre-
ference in surface orientations for the easiness of bulk
diffusion and surface lattice defects or kinks may be a
preferential site for the start of bulk diffusion, but the topic
is out of scope of the present work.
The phenomena may be summarized by a familiar che-
mical equation with explicit valence of double injection
model:
W
6þ
O
3
2À
þ xH
þ
þ xe
À
! H
x
þ
W
1Àx
6þ
W
x
5þ

O
3
2À
: (1)
The surface reaction may be expressed as:
H
þ
þ W
6þ
O
3
2À
þ e
À
!
1
2
H
2
þ
O
2À
þ W
5þ
O
2:5
2À
: (2)
This reaction creates the oxygen lattice defect which might
cause some adverse effects on the sensing characteristics.

The water formation reaction (2), however, is limited at the
outermost layer and the reaction is reported to be slow.
Furthermore, we placed Pt catalyst on (1 0 0) surface for the
explanation purpose, but the tungsten film consists from
many nanocrystals with randomly placed Pt catalyst parti-
cles, and only limited portion of catalyst particles were
placed on (1 0 0) surfaces. Therefore, implication of the
oxygen reaction, except water generation, may be neglected
in first order approximation.
The oxidation mechanism of a reduced WO
3
is different
from the standard double injection model. Instead of hydro-
gen leaving the film as hydrogen molecules, it is oxidized at
the nanocrystal surface. The oxidation processes may be
summarized as:
(1) Dissociation of O
2
on Pt and spill-over.
(2) Surface diffusion of an O
2À
ion and simultaneous bulk
diffusion of a proton to a surface.
(3) Hydrogen oxidation at a surface by a O
2À
ion.
The first process is similar to the hydrogen injection
process. Oxygen molecules are dissociated at Pt surface
and oxygen ion (O
2À

) diffuses to the WO
3
surface with
removal of two electrons from WO
3
bulk. Oxygen ions at the
surface attract the intercalated hydrogen atoms and the
hydrogen atoms diffuse to the surface and form water at
the surface. This water may stayed for prolonged time due to
the crevasse-like grain boundary nature as explained for the
water creation on the (1 0 0) surface associated with the
hydrogen reduction process. We would not deny the possi-
bility of bulk diffusion of oxygen ions and water formation
within bulk, but the probability of this reaction could be
small due to the molecular size difference. There is also
good possibility for water molecules to diffuse into the bulk,
although the diffusion velocity may be small. In any case, the
chemical equation with explicit valence may be expressed:
1
2
xO
2À
þ H
x
þ
W
1Àx
6þ
W
x

5þ
O
3
2À
!
1
2
xH
2
þ
O
2À
þ W
6þ
O
3
2À
þ xe
À
: (3)
There may be various other irregular processes, such as
hydroxyl adsorption at some kinks or steps. But those
discussions are out of scope in the present analysis and it
may be emphasized that the present model explains our
observations and previous data in a unified manner.
4. Conclusion
A sensitive hydrogen sensor was fabricated by a sol–gel
method and characterized. The sensor exhibited high sensi-
tivity with six orders of conductance increase upon 1% H
2

detection. The effect of water was found to be large with
some memory effects. The existing theories failed to inter-
pret the observed data. The new, double injection/surface
oxidation model is suggested to explain the observed data as
well as previously reported data. The reduction mechanism
is based on the double injection model with the addition of
surface oxygen reaction on (1 0 0) crystal surfaces. Oxygen
reaction generates surface water and this accelerates the
sensor response, and super-linear characteristics would be
observed. The oxidation was achieved by the water forma-
tion of dissociated oxygen ions and intercalated hydrogen
ions at the nanocrystal surfaces. The sensitivity increase in
H. Nakagawa et al. / Sensors and Actuators B 93 (2003) 468–474 473
the repeated exposure to dry H
2
gas may be attributed to the
trapped water. The suggested model successfully explained
observed data.
References
[1] P.J. Shaver, Activated tungsten oxide gas detectors, Appl. Phys. Lett.
11 (1967) 255–257.
[2] S. Sekimoto, H. Nakagawa, S. Okazaki, K. Fukuda, S. Asakura, T.
Shigemori, S. Takahashi, A fiber-optic evanescent-wave hydrogen
gas sensor using palladium-supported tungsten oxide, Sens. Actua-
tors B 66 (2000) 142–145.
[3] B.W. Faughnan, R.S. Crandall, P.M. Heyman, Electrochromism in
WO
3
amorphous films, RCA Rev. 36 (1975) 177–197.
[4] M. Zayat, R. Reisfeld, H. Minti, B. Orel, F. Svegl, Gasochromic

effect in platinum doped tungsten trioxide films prepared by the sol–
gel method, J. Sol Gel Sci. Technol. 11 (1998) 161–168.
[5] S.K. Deb, Optical and photoelectric properties and colour centers in
thin films of tungsten oxide, Philos. Mag. 27 (1980) 801–822.
[6] J G. Zhang, D.K. Benson, C.E. Tracy, S.K. Deb, A.W. Czanderna,
C. Bechinger, Chromic mechanism in amorphous WO
3
films, J.
Electrochem. Soc. 144 (1997) 2022–2026.
[7] A. Georg, W. Graf, R. Neumann, V. Wittwer, Mechanism of the
gasochromic coloration of porous WO
3
films, Solid State Ionics 127
(2000) 319–328.
[8] E. Salje, K. Viswanathan, Physical properties and phase transitions in
WO
3
, Acta Crystallogr. A 31 (1975) 356–359.
[9] B. Gerand, G. Nowogrocki, J. Guenot, M. Figlarz, Structure study of
new hexagonal form of tungsten trioxide, J. Solid State Chem. 29
(1979) 429.
[10] A. Coucou, M. Figlarz, A new tungsten oxide with 3D tunnels: WO
3
with the pyrochlore-type structure, Solid State Ionics 28–30 (1988)
1762–1765.
[11] R. Diehl, G. Brandt, E. Salje, The crystal structure of triclinic WO
3
,
Acta Crystallogr. B 34 (1978) 1105–1111.
[12] J.T. Szymanski, A.C. Roberts, The crystal structure of tungstite,

WO
3
ÁH
2
O, Can. Mineral. 22 (1984) 681–688.
[13] M.F. Daniel, B. Desbat, J.G. Lassegues, B. Gerand, M. Figlarz,
Infrared and Raman study of WO
3
tungsten trioxide and WO
3
ÁxH
2
O
tungsten trioxide hydrates, J. Solid State Chem. 67 (1987) 235–247.
[14] N.N. Greenwood, A. Earnshaw, Chemistry of Elements, Butter-
worths–Heinemann, London, 1997, p. 1008.
[15] S. Okazaki, H. Nakagawa, S. Asakura, Y. Tomiuchi, N. Tsuji, H.
Murayama, M. Yashiya, Sensing characteristics of an optical fiber
sensor for hydrogen leak 93 (2003) 142–147.
[16] O.F. Schirmer, V. Wittwer, G. Bauer, G. Brandt, Dependence of WO
3
electrochromic absorption on crystallinity, J. Electrochem. Soc. 124
(1977) 749–753.
[17] R.S. Crandall, B.W. Faughnan, Comment on the cluster model of
alkaline-metal tungsten bronzes, Phys. Rev. B 16 (1977) 1750–1752.
[18] P. Gerard, A. Deneuville, R. Courths, Characterization of a-‘‘WO
3
’’
thin films before and after coloration, Thin Solid Films 71 (1980)
221–236.

[19] W. Wagner, F. Rauch, C. Ottermann, K. Bange, Hydrogen dynamics
in electrochromic multilayer systems investigated by the
15
N
technique, Nucl. Instrum. Methods B 50 (1990) 27–30.
[20] F. Rauch, W. Wagner, K. Bange, Nuclear reaction analysis of optically
active coating on glass, Nucl. Instrum. Methods B 50 (1990);
F. Rauch, W. Wagner, K. Bange, Nuclear reaction analysis of
optically active coating on glass, Nucl. Instrum. Methods B 42 (1989)
264–267.
[21] H. Morita, H. Washida, Electrochromism of atmospheric evaporated
tungsten oxide films (AETOF), Jpn. J. Appl. Phys. 23 (1984) 754–759.
[22] C. Bechinger, M.S. Burdis, J G. Zhang, Comparison between
electrochromic and photochromic coloration efficiency of tungsten
oxide thin films, Solid State Commun. 101 (1997) 753–756.
[23] J G. Zhang, D.K. Benson, C.E. Tracy, S.K. Deb, A.W. Czanderna,
C. Bechinger, Chromic mechanism in amorphous WO
3
films, J.
Electrochem. Soc. 144 (1997) 2022–2026.
[24] S.H. Lee, H.M. Cheong, C.E. Tracy, A. Mascarenhas, D.K. Benson,
S.K. Deb, Raman spectroscopic studies of electrochromic a-WO
3
,
Electrochim. Acta 44 (1999) 3111–3115.
[25] A. Georg, W. Graf, R. Neumann, V. Wittwer, The role of water in
gasochromic WO
3
films, Thin Solid Films 384 (2001) 269–275.
[26] A. Georg, W. Graf, V. Wittwer, The gasochromic coloration of

sputtered WO
3
films with a low water content, Electrochim. Acta 46
(2001) 2001–2005.
[27] S.H. Lee, H.M. Cheong, P. Liu, D. Smith, C.E. Tracy, A. Mascarenhas,
J.R. Pitts, S.K. Deb, Raman spectroscopic studies of gasochromic
a-WO
3
thin films, Electrochim. Acta 46 (2001) 1995–1999.
[28] C. Genin, A. Driouuiche, B. Gerand, M. Figalarz, Hydrogen bronzes
of new oxides of the WO
3
–MoO
3
system with hexagonal, Solid State
Ionics 53–56 (1992) 315–323.
[29] V.E. Henrich, P.A. Cox, The Surface Science of Metal Oxides,
Cambridge University Press, Cambridge, 1994, p. 338.
[30] J.E. Benson, H.W. Kohn, M. Boudart, On the reduction of tungsten
accelerated by platinum and water, J. Catal. 5 (1966) 307–313.
Biographies
Hidemoto Nakagawa obtained his Masters degree in applied science and
PhD degree from University of Toronto in 1974 and 1978, respectively. He
had been a visiting associate professor at Yokohama National University
from 1993 to 2001. He joined the Research Center for Advanced Science
and Technology, University of Tokyo as visiting researcher in 2002. His
research centers on various chemical and biological sensors and their
applications.
Nanako Yamamoto received her BEng in 2000 from Yokohama National
University. And she obtained her MEng degree in 2002. Her research

interest focuses gas sensors as well as medical sensors.
Shinji Okazaki received his BEng and MEng degrees from Yokohama
National University in 1991 and 1993, respectively. He joined Yokohama
National University as a research associate in 1997. His major fields are
electrochemistry and sensor engineering.
Tsuneo Chinzei obtained his MD degree from Faculty of Medicine,
University of Tokyo in 1982. He enrolled at Graduate School of Medicine,
University of Tokyo in 1984 and became a research associate at the
Research Center for Advanced Science and Technology, University of
Tokyo in 1987 and promoted to an associate professor in 1999. He is
specializing in artificial hearts and medical thermography. He is also
interested in micromachining and medical sensors.
Shukuji Asakura received his MEng and PhD degrees from University of
Tokyo in 1965 and 1968, respectively. In 1972, he joined Yokohama
National University, and became a professor in 1988. His fields of interest
are safety engineering, corrosion science and chemical sensors.
474 H. Nakagawa et al. / Sensors and Actuators B 93 (2003) 468–474