ORIGINAL PAPER

Hydrogen gas-sensing properties of Pt/WO3 thin filmin various measurement conditions

Yuki Yamaguchi & Yukari Emoto & Tohru Kineri &Masakatsu Fujimoto & Hideo Mae & Atsuo Yasumori &Keishi Nishio

Received: 15 August 2011 /Accepted: 12 February 2012 /Published online: 7 March 2012# Springer-Verlag 2012

Abstract A well-known gasochromic material is Ptparticle-dispersed tungsten trioxide (Pt/WO3). Its opticalproperties could make it effective as a hydrogen gas sensor.In this study, Pt nanoparticle-dispersed WO3 thin films wereprepared using the sol–gel process, and their optical andelectrical properties dependent on the working environment(i.e., temperature, hydrogen gas concentration, oxygen par-tial pressure, etc.) were investigated. The Pt/WO3 thin filmsprepared at 400 °C showed the largest change in opticaltransmittance and electrical conductivity when exposed tohydrogen gas compared with the films prepared at othertemperatures. The optical absorbance and electrical conduc-tivity were found to be dependent on the hydrogen andoxygen gas concentration in the atmosphere because gener-ation and disappearance of W5+ in the thin films dependon the equilibrium reaction between injection and rejectionof H+ into and from the thin films. In addition, the equilib-rium reaction depends on the hydrogen and oxygen gasconcentrations.

Keywords Tungsten oxide . Thin film . Hydrogen sensor .

Gasochromism . Sol–gel

Introduction

Hydrogen energy generation systems are known to be cleanbecause the only by-product is water, and they are highlyefficient compared with other energy systems. Fuel cells areused as a portable power source and for home electricitygeneration, and they will be in the near future. Hydrogen gascan be stored, so only the exact quantity needed is used.However, it is quite a dangerous gas because of its largeflame propagation (3.42 m/s in air) and wide combustionrange (4.0–74.2 vol.% in air). A high-performance sensorthat can quickly and sensitively detect hydrogen gas leaks isthus necessary. Many types of hydrogen gas sensors havebeen studied, e.g., metal or metal oxide semiconductors [1],catalytic combustion [2], thermoelectric materials [3], opti-cal materials [4], and so on. These sensors show a goodability to detect hydrogen gas, but they have problems suchas a limited range of detectable gas concentration.

In this study, we utilized the gasochromism of platinumnanoparticle-dispersed tungsten trioxide (Pt/WO3) thin filmsas a hydrogen gas leakage sensor. WO3 thin film has beenwidely studied as a chromic material since its property ofchanging from transparent to blue through an electricalredox reaction was discovered by Deb [5, 6]. Pt particles,as a catalyst, were dispersed on WO3 thin films that becomecolored by a reduction action with hydrogen gas. Thisphenomenon is called gasochromism. In a hydrogen gasatmosphere, H2 molecules are dissociated by the Pt catalystson WO3 into H atoms and ionized into protons and elec-trons, which are injected into the WO3 structure. This iscalled the spillover effect. The protons become stable invoids made by WO6 octahedral clusters, and the electronstrapped in the WO3 reduce the tungsten ions. This leads tointervalence charge transfer (IVCT) [7, 8] and charge trans-fer from the valence band to a split-off W5+ state [9]. Optical

Y. Yamaguchi (*) :A. Yasumori :K. NishioDepartment of Materials Science and Technology,Tokyo University of Science,Noda, Tokyo, Japane-mail: j8210703@ed.tus.ac.jp

Y. Emoto : T. KineriDepartment of Applied Chemistry,Tokyo University of Science, Yamaguchi,Sanyo-Onoda, Yamaguchi, Japan

M. Fujimoto :H. MaeYamaguchi Prefectural Industrial Technology Institute,Ube, Yamaguchi, Japan

Ionics (2012) 18:449–453DOI 10.1007/s11581-012-0683-2

absorption can then be observed from the long wavelengthrange of visible light to the near infrared area, and the WO3

changes to blue. Therefore, coloration of Pt/WO3 thin filmindicates the presence of hydrogen gas, which is colorlessand odorless. In this reaction, the electrical conductivity ofPt/WO3 increases under the hydrogen gas atmosphere be-cause the protons and electrons work as electrical carriers.Pt/WO3 thin film can also be used to detect hydrogen gas bythe change in its electrical conductivity [10].

Hydrogen sensors consisting of noble metal catalyst dis-persed WO3 thin film have been widely studied. Most WO3

thin films are prepared using physical methods, such as evap-oration [11] or sputtering [12], and chemical processes such asthe sol–gel process [13, 14]. Some of these physical methodsare costly, and fabricating large films with them is difficult. Inthis study, Pt/WO3 thin films were fabricated on glass sub-strates by using the sol–gel process and then heat treated atdifferent temperatures in air. Large films and large quantitiesof the films can easily be prepared using the sol–gel process.The hydrogen gas-sensing properties of gasochromic WO3

thin films are often evaluated by measuring the optical trans-mittance change as a function of time with exposure to variousconcentrations of hydrogen gas. In many studies, hydrogengas mixed with nitrogen gas or another inactive gas is used[15–17]. In this study, we investigated the effect of oxygen inthe gas mixture and of various measurement temperatures forhydrogen-sensing properties.

Methods and procedures

Sample preparation

Pt/WO3 thin films were prepared by using the sol–gel pro-cess. Tungsten chloride as a starting material was dissolvedinto ethanol, and the resultant solution was stirred until itcooled. This solution had good viscosity for coating glasssubstrates and an ideal tungsten alkoxide concentration. Inmost conventional studies, a Pt catalyst is deposited on thesurfaces of WO3 thin films by sputtering [15, 18]. Thismethod is quite simple and homogeneous, and the Pt par-ticles are dispersed in the WO3 thin film matrix in a randommanner. Hydrogen hexachloroplatinate(IV) hydrates andtungsten hexachloride were dissolved at a ratio of Pt toW01:13. This solution was spin coated on nonalkaline glasssubstrates and dried in air at 100 °C. The coating processwas repeated five times, and the resultant thin films werefinally heat treated at several temperatures in air.

Measurement conditions

The internal structural properties of the Pt/WO3 thin filmswere analyzed using X-ray diffraction (XRD). The phase

and crystallinity of the thin films were evaluated using XRDmeasurement (Ultima IV, Rigaku, Japan) with a scan rate of2.00°/min. The optical response properties of the Pt/WO3

thin films to hydrogen gas were evaluated using ultraviolet–visible spectroscopy (UV-630, Jasco, Japan). All the sam-ples were measured in transparent plastic cells, and anempty cell was used as a reference. The transmission spectraof Pt/WO3 thin films between 300 and 800 nm were mea-sured in air, and after 100%, H2 gas was flowed into thecells. The transmittance changes of the thin films at 800 nmwere measured to evaluate the hydrogen gas-sensing prop-erty. The transmittance was normalized in this measurement.The responses of the films when the atmosphere waschanged by flowing large amounts of H2 gas or air intothe cells were also evaluated. The electrical response prop-erties were evaluated by conductivity measurement. Comb-shaped electrode was made on the films by evaporation ofgold, and the electric conductivity of Pt/WO3 thin films inseveral hydrogen gas concentrations was measured by atwo-terminal method.

Results and discussion

The XRD patterns of Pt/WO3 thin films heat treated atseveral temperatures are shown in Fig. 1. Clear diffractionpeaks assigned to WO3 were observed for the films heattreated at temperatures above 350 °C. The crystallinity of Pt/WO3 thin films prepared using the sol–gel process increasedas the heat treatment temperature increased up to 450 °C.This indicates that the crystallinity of the thin film can becontrolled by the heat treatment temperature. All diffractionpeaks could be assigned to monoclinic WO3 (JCPDS 89-4477). A cross-sectional TEM image of Pt/WO3 thin filmheat treated at 400 °C is shown in Fig. 2. The TEM and anenergy-dispersive X-ray spectrometry investigation revealed

Fig. 1 XRD patterns of Pt/WO3 thin films heat treated at 350 (i), 400(ii), 450 (iii), and 500 °C (iv)

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that the film was about 300-nm thick and the Pt particles(5–20 nm in size) dispersed on and in the thin filmhomogeneously.

The optical and electrical properties of the Pt/WO3 thinfilms due to the gasochromic phenomenon were evaluatedbecause the optical transmittance and electrical conductivitychange when the films are exposed to hydrogen gas. Theas-deposited thin films prepared at several temperaturesshowed high transmittance in the visible light range. Whenthe thin film was exposed to 100% H2 gas, its color changedfrom transparent to blue. The optical absorption bandwas widely above 500 nm in the visible light range, andthe absorbance of the thin film increased with wavelength.In a comparison of the absorbance of these films in thevisible light range, that of the thin film prepared at 350 °Cwas slightly weaker than those of films prepared at above400 °C. The XRD investigation results and the opticalmeasurements suggest that in WO3 thin film prepared at atemperature lower than 350 °C, the WO6 octahedron struc-ture is not well constructed and thus the film does notchange color. When protons and electrons are inserted, theyare trapped at tungsten-ion sites as change balances and thetungsten ions are reduced (changing W6+ to W5+). Electronstrapped at tungsten-ion sites lead to IVCT and d–d transfer

with optical absorption. The protons and electrons exist onlyin crystallized states, and such states are needed for colora-tion of Pt/WO3.

The normalized transmittance at 800 nm of Pt/WO3 thinfilms as a function of time is shown in Fig. 3. The WO3 thinfilms were exposed to 100% H2 gas, and the response timeto the gas was evaluated by measuring transmittance as afunction of time. The thin films prepared at 400 and 450 °Cresponded faster than those prepared at 350 and 500 °C: ittook less than 0.2 s for a 50% transmittance change. Figure 4shows the normalized transmittance at 800 nm of the sampleheat treated at 400 °C for an atmosphere of 1% H2 gasrespectively mixed with 100% N2 gas and with a 20% O2–80% N2 mixture. When the sample was exposed to 1%hydrogen gas mixed with N2 gas, its transmittance decreasedgreatly. When the sample was exposed to 1% hydrogen gasdiluted by a 20% O2–80% N2 mixture gas, the normalizedtransmittance of the thin film hardly changed.

Fig. 2 TEM image of Pt/WO3 thin film prepared by sol–gel processand heat treated at 400 °C

Fig. 3 Normalized transmittance changes of Pt/WO3 thin films heattreated at 350 (i), 400 (ii), 450 (iii), and 500 °C (iv)

Fig. 4 Normalized transmittance changes of Pt/WO3 thin films, heattreated at 400 °C, caused by exposure to 1% H2 diluted with 100% N2

gas (i)and 1% H2 diluted with 20% O2–80% N2 gas (ii)

Fig. 5 Electrical conductivity changes of Pt/WO3 thin films heattreated at 350 (i), 400 (ii), 450 (iii), and 500 °C (iv); measured at200 °C

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The gasochromic reaction of Pt/WO3 is shown by thethree following equations:

H2 ! 2H ð1Þ

2H ! 2Hþ þ 2e� ð2Þ

xHþ xe� þWO3 ! HxWO3 ð3ÞEquation 1 is the decomposition of a hydrogen molecule tohydrogen atoms at a Pt surface. Equations 2 and 3 arerespectively the diffusion of hydrogen atoms into the WO3

structure, and the ionization of hydrogen atoms and inser-tion of protons and electrons into the WO3 lattice.

Protons and electrons are removed from the WO3 struc-ture as shown in Eq. 4.

2Hþ 2e� þ 1=2 O2 ! H2O ð4ÞThe existence of oxygen is predicted to affect this reaction.

The electrical conductivity of Pt/WO3 thin films preparedat several temperatures was measured in air and in anatmosphere of 1% H2 gas mixed with air at 200 °C(Fig. 5). Although the deposited Pt/WO3 was an insulator,the thin film exhibited increased electrical conductivitieswhen it was exposed to H2 gas. Notably, the thin filmprepared at 400 °C showed high electrical conductivity in1% H2 gas. This result was in near agreement with theoptical properties. Figure 6 shows the electrical conductivityof the sample heat treated at 400 °C for an atmosphere of 1%H2 gas respectively mixed with 100% N2 and with a 20%O2–80% N2 mixture, at a measurement temperature of 200 °C. The electrical conductivity of the thin film was greatlyincreased by exposure to the H2 and N2 mixture gas, com-pared with exposure to H2 gas mixed with the N2–O2

mixture. Moreover, when the thin film was exposed to amixture gas including oxygen, a sharp peak appeared at thegas flowing point. This peak is due to equilibrium of thereactions between carrier generation (Eqs. 1, 2, and 3) andelimination (Eq. 4). Figure 7 plots the dependence of theelectrical conductivity of the Pt/WO3 thin film prepared at

Fig. 6 Electrical conductivity changes of Pt/WO3 thin films, heattreated at 400 °C, caused by exposure to 1% H2 diluted with 100%N2 gas (i) and 1% H2 diluted with 20% O2–80% N2 gas (ii); measuredat 200 °C

1% H2 gas 100 ppm H2 gas

a bFig. 7 Electrical conductivitychanges of Pt/WO3 thin films,heat treated at 400 °C, causedby exposure to 1% (a) and100 ppm (b) H2 gas, measuredat 100, 200, and 300 °C

Fig. 8 Electrical conductivity of Pt/WO3 thin films, heat treated at400 °C, with hydrogen gas concentration measured in severaltemperatures

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400 °C on the H2 concentration measured at 100, 200, and300 °C. The thin film succeeded in detecting H2 gas in aconcentration range from 100 ppm to 1% at all measurementtemperatures. The electrical conductivity of the thin film,measured at 200 and 300 °C, increased drastically with H2

gas flow and then immediately achieved equilibrium. At100 °C, the reactions of Eqs. 1, 2, and 3 did not progressin this way. The reaction for injection of protons and elec-trons into WO3 was superior to the reaction for removingprotons and electrons from WO3. Thus, immediately afterthe exposure to hydrogen gas, the electrical conductivityincreased rapidly and then decreased to the equilibrium stateof electrical conductivity. This is because the electricalconductivity in the equilibrium state is dependent on thehydrogen concentration in the atmosphere since carriersconsisting of protons and electrons are dependent on hydro-gen partial pressure. In contrast, at 300 °C, the reactionshown by Eq. 4 was more dominant than those shown byEqs. 1, 2, and 3. Consequently, the electrical conductivity ofPt/WO3 thin film at 200 °C was superior to that at 300 °C.Figure 8 shows electrical conductivity dependent on hydro-gen gas concentrations, the electrical conductivity measuredin the synthetic air at a several temperatures. The electricalconductivity increased with increasing hydrogen gas con-centration linearly between 100 and 100,000 ppm. Thisresult revealed that the Pt/WO3 thin films prepared by sol–gel process would be useful for applications working in airinvolving hydrogen gas leakage sensor and quantitativeanalysis [10].

Conclusion

Pt nanoparticle-dispersed tungsten trioxide (Pt/WO3) thinfilms were prepared using the sol–gel process. These filmswere heat treated at several temperatures, and their struc-tures were measured through XRD and TEM observation.The hydrogen gas-sensing properties of the Pt/WO3 thinfilms were evaluated by measuring the optical transmittanceand change in electrical conductivity when samples of thefilm were exposed to various concentrations of hydrogengas. Films having low crystallization and density werefound to show good response to hydrogen gas. When

oxygen was present, an elimination reaction of the protonsand electrons in the WO3 occurred, and sensitivity wasremarkably decreased by the presence of 20% oxygen inthe atmosphere, which is like air. The conductivity changeshowed a sharp peak at the hydrogen gas flowing point. Thispeak explains the equilibrium behavior between carrier gen-eration and elimination. This equilibrium of gasochromismstrongly depended on temperature. In a high-temperaturecondition, the conductivity achieved the equilibrium stateimmediately but the conductivity change decreased becausecarrier elimination was dominant. These results show thatPt/WO3 thin films could be effectively used for detectinghydrogen gas leaks.

Acknowledgment This work was partly supported by a Grant-in-Aidfor Research (C) by the Japanese Ministry of Education, Science,Sports, and Culture.

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