a room-temperature operated hydrogen leak sensor

Keywords: Gas sensor; Tungsten trioxide; Hydrogen; Double injection 1.. Traditionally, the double injection model [3], in which both a proton hydrogen ion and an electron are simultaneou

A room-temperature operated hydrogen leak sensor

H Nakagawaa,*, N Yamamotob, S Okazakib,

T Chinzeia, S Asakurab

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 (WO3) 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 WO3film Pt was expected to act as catalyst for hydrogen reduction of WO3

The conductivity of the sensor was less than 0.001 mS in oxidizing atmosphere, and more than 106times conductivity increase was observed upon exposure to (1% H2/99% N2) 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 cm2/s in air, methane’s value is 0.15 cm2/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, reliabilsensitiv-ity, 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 (WO3) 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 WO3 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

WO3changes 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] WO3is 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] A WO3hydrogen 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

*

Corresponding author Tel.: þ81-3-5452-5241; fax: þ81-3-5452-5241.

E-mail address: nakagawa@bme.rcast.u-tokyo.ac.jp (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

A WO3sensor was fabricated on a quartz plate by a sol–gel

method and spin coating An amount equal to 0.5 M sodium

tungstate (Na2WO4) solution was converted into tungstate

(H2WO4) sol solution by passing through cationic ion

exchange column (Amberlite IB120, Organo Co.)

Appro-priate amounts of hexachroloplatinum (H2PtCl6) 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 WO3 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 ReO3type, or of

warped perovskite (ABO3) with vacant A sites Corners of

every octahedrons are shared with neighboring octahedrons

and none of the edges are shared in monoclinic form

Monoclinic WO3 exhibits six phase transitions with

tem-perature[11] It is monoclinic below 40 8C and triclinic

between40 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 ReO3 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

WO32H2O and WO3H2O [12]are believed to be mono-clinic and WO3(1/3)H2O is hexagonal[13] The initial state

of WO3 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% H2/99% N2gas) were plotted against calcination temperature inFig 1 As will be shown inFig 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 K2PtCl4instead of H2PtCl6, 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.

nsitivity reduces.

Fig 2 presents the effect of catalyst amount to the

sensitivity The conditions of the measurements are as same

as those forFig 1 The conductance values of 2 min after

exposure to humid (1% H2/99% N2) 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% H2/99% N2) gas for 1200 s and humid 100% O2

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 106were 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 was75 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.

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 was5 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% H2/99% N2) gas for 300 s, and dry and humid

(100% O2) 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 W6þion to a

W5þion, and the polaron transition between a W5þion and nearby a W6þion 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 WO3by 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 /99% N ) 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.

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 WO3 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

WO3films during electrochemical reduction and oxidation

Their WO3film was, however, inserted between ITO

elec-trode and SiO2or Ta2O5material 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 WO3 film based on vacuum

deposition reported transparent films despite oxygen

defi-ciency[21,22] Zhang et al.[23]assumed the existence of

W4þand postulated that the polaron transition takes place

between W4þand W5þions, not between W5þand W6þas

widely believed Later, Lee et al [24] established a new

model in which both W4þ–W5þand W5þ–W6þtransitions

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 WO3 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 formareduc-tion on (1 0 0) plain of ReO3crystal 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 inFig 3where shapes

of rising transient and falling transient have quite different nature Different values of the reduction and oxidation velocities and their temperature dependence inFig 4further support this argument

Two-dimensional view of our model crystal was presented

inFig 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.

should be noted that the surface of the nanocrystal may not be

necessarily a film surface But most of amorphous WO3films

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 H2on Pt and spill-over to WO3surface

(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% H2) gas inFig 5suggested 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 ReO3 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:

W6þO3 þ xHþþ xe! Hx þW1x6þWx þO3 : (1)

The surface reaction may be expressed as:

Hþþ W6þO3 þ e!1

2H2 þO2þ W5þO2:5 2: (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 WO3is different from the standard double injection model Instead of hydro-gen leaving the film as hydrohydro-gen molecules, it is oxidized at the nanocrystal surface The oxidation processes may be summarized as:

(1) Dissociation of O2on Pt and spill-over

(2) Surface diffusion of an O2ion and simultaneous bulk diffusion of a proton to a surface

(3) Hydrogen oxidation at a surface by a O2ion The first process is similar to the hydrogen injection process Oxygen molecules are dissociated at Pt surface and oxygen ion (O2) diffuses to the WO3 surface with removal of two electrons from WO3bulk 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

2xO2þ Hx þW1x6þWxþO3 

!1

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% H2 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

the repeated exposure to dry H2gas 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 H 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 xH 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.