ContentslistsavailableatSciVerseScienceDirect
Sensors and Actuators B: Chemical
j o u r n a l ho me p a g e :w w w . e l s e v i e r . c o m /l o c a t e / s n b
Short communication
Room temperature hydrogen sensors based on metal decorated WO 3
nanowires
Jarmo Kukkola
a,∗, Melinda Mohl
a, Anne-Riikka Leino
a, Jani Mäklin
a, Niina Halonen
a, Andrey Shchukarev
b, Zoltan Konya
c, Heli Jantunen
a, Krisztian Kordas
aaMicroelectronicsandMaterialsPhysicsLaboratories,DepartmentofElectricalEngineering,UniversityofOulu,P.O.Box4500,FI-90014,Finland
bDepartmentofChemistry,Chemical-BiologicalCenter,UmeåUniversity,SE-90187Umeå,Sweden
cDepartmentofAppliedandEnvironmentalChemistryandMTA-SZTEReaction,KineticsandSurfaceChemistryResearchGroup,UniversityofSzeged, Rerrich,B.ter1,H-6720Szeged,Hungary
a r t i c l e i n f o
Articlehistory:
Received17December2012 Receivedinrevisedform10May2013 Accepted24May2013
Available online xxx
Keywords:
Metaloxide Tungstenoxide Gassensor Hydrogensensor Nanowire
a b s t r a c t
Theemerginghydrogeneconomyhascreatedademandforthedevelopmentofimprovedhydrogen sensorsoperatingatroomtemperature.Inthiswork,wepresenthydrogendetectorsbasedonmetal decoratedWO3nanowiresthatwereabletodetect1000ppmofH2,evenatroomtemperature(30◦C), withrelativelyshortrecoverytimeandhighsensitivity.Thenanowiresweresynthesizedbyahydro- thermalprocessanddecoratedwithPdOandPtOxnanoparticlesbydecompositionofPd(acac)2 and Pt(acac)2precursors.ThegasresponsesweretestedforH2,NO,H2SandCOanalytegasesinanairbuffer at150,200and250◦C(H2alsoat30,70and130◦C).
© 2013 Elsevier B.V. All rights reserved.
1. Introduction
Commercialhydrogengassensorsarealreadyutilizedinhydro- genpoweredtransport,firealarms,processandfluegasanalyzers, andleakage detectors. Such devices must meetstrictspecifica- tionsrelatedtoperformance,lifetime,reliabilityandcost.Sensing ofhydrogenisbasedonanumberofdifferentphysical/chemical phenomena related to, e.g. the change in thermal/electrical conductivity,workfunction,mechanicalpropertiesbutevenopti- cal/acousticmethodsmaybeapplied[1–5].
Inresistivegassensingdevices,suchastheonesdiscussedin thiswork,gaseousanalytesarecausinglocalpolarizationinthe solid(i.e.redistributingthelocalchargecarriers)uponadsorption [6].Thechangeofthelocalelectricalfieldisscreenedbythecar- riers,andwillvanishaftersomedistance,calledtheDebyelength [7].Ifthediameterorthicknessoftheparticles,wiresorfilmsis sufficientlylow,theadsorbedmoleculesareinfluencingthewhole volumeofthesolidthusinducingsignificantchangeintheelectri- caltransportbehavior[6].Accordingly,smallerparticlessuggest bettergassensing.However,one shouldalsotakeintoaccount theincreasingnumberofcontactsand thesuperpositionofthe
∗Correspondingauthor.Tel.:+358294487973.
E-mailaddress:jarmo.kukkola@ee.oulu.fi(J.Kukkola).
correspondingcontactresistancesbetweenthenanoparticlesinthe percolationpath[6].Therefore,thereisanoptimalsizeforthepar- ticles.Inordertomaximizethegassensorresponse,atleastone dimensionofthesensingparticlesshouldbeabouttwicetheDebye lengthLD=[ε0εrkBT/(q2n)]1/2,whereε0isthevacuumpermittivity, εristhedielectricpermittivity,kBistheBoltzmannconstant,Tis thetemperature,qistheelementarychargeandnisthecarrier concentration[8].Thecarrierconcentrationisdependentonthe typeandthecrystalstructureofthematerialaswellasonthecon- centrationofdefectsandimpuritiespresentinthelattice.Carrier concentrationintungstenoxideshasbeenevaluatedtobefrom 1023to1025m−3[9–13]withacorrespondingDCdielectricper- mittivityof∼20[14].Basedonthesevalues,theoptimumcrystal size2LDisbetween4and40nm(at500K),whichissimilartothe conclusionofapreviousstudy[15].
Tungstenoxidenanowireswerefirstpreparedabout10years ago[16]andsincethenhavebeenconsideredaspromisingfunc- tionalmaterialsforcatalytic/sensing[17–22],photocatalytic[23], electrocatalytic [24], electrochomic [25], gasochromic [26] and field-emissiondevices[27].Duetotheuniquepropertiescaused bythelargesurfacearea/volumeratio,synthesismethodsofone dimensionaltungstenoxidestructureshavebeenstudiedexten- sively.Solvo-/hydrothermal[28,29],sol–gel[22],template-assisted [30],electrospinning[17],andvariousphysical[31–35]andchem- icalvapordeposition[36]methodshavebeenpresented.
0925-4005/$–seefrontmatter© 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.snb.2013.05.082
Inthiswork,thegassensingcharacteristicsofhydrothermally processedWO3nanowiresdecoratedwithPdOorPtOxnanoparti- clesarediscussed.Mostoftheworkisfocusedonevaluatingthe sensitivityofthepreparedsensors:S=(G−G0)/(G0c)×100%,where GandG0 aretheconductanceofthesensorbeforeandaftergas exposure,respectively,andcisthegasconcentration.Gassensors basedonnanowirenetworksareexpectedtodeliverhighersta- bilityoverdevices withnanoparticlesofasimilardiameter.The morepronouncedtendencyforsinteringofnanoparticlesatele- vatedoperating temperatures is favoring the useof nanowires [37].Inaddition,nanowiresenablepercolationbetweenthemetal electrodeswithfewercontactsthaninthecaseofnanoparticles.
Durability over bulk Pd/Pt based sensors is also expected due tothelackofcontinuousPd/Pt filmsthat crackunder repeated hydrogenexposures.Thepreparednanowireshaverelativelysmall crystallitestooptimizetheresponsestogasexposures.Alsoshort responseandrecoverytimesareexpectedduetoshortdiffusion pathsofgasestothesurfacesoflooselypackednanowires.Thepre- paredsensorswereobservedtocompete[19]andevenoutperform previously studied tungsten oxide nanowire sensors decorated withPdorPtintermsofsensitivitytohydrogenatroomtempera- ture[20,21].
2. Experimental
Pristine tungsten oxide nanowires were synthesized by the hydrothermal method described elsewhere [38,39]. In brief, sodium-tungstate(2.5g)andsodium-sulfate(3.0g)weredissolved indistilledwater(80ml),thenhydrochloricacid(4.5ml,3M)was addeddrop-wiseundercontinuousstirring.After10minofstir- ringthemixturewastransferredintoaTeflon-linedstainlesssteel autoclaveandwaskeptat180◦Cfor48h.Theproduct(i.e.WO3 nanowires)wascollectedbycentrifugation,washedwithdistilled waterand ethanol,and finallydriedat60◦C inair.To decorate thesurfacewithPdOorPtOxnanoparticles,28.6mgPd(acac)2or 20.2mgPt(acac)2wasdissolvedin100mlacetonefollowedbydis- persionof1.0gofthepreparedWO3by3hofultrasonicagitation and6hofmagneticstirring.Thesolventwasevaporatedat80◦C inanN2atmosphere,whilestirringthemixture.Theproductwas
annealedat300◦Cinairfor2h.3.2±0.1mgoftheobtainedWO3, WO3–PdOandWO3–PtOxnanowiresweredispersedindeionized waterof1.0±0.1mlbyultrasonicagitation(5min).Fivedroplets ofthedispersionsweredropcast(∼3lofeach)overthePtelec- trodefingersoftheSi/SiO2substrateswhilekeepingthesubstrate temperatureat90◦Ctoallowforquickdrying.Eachdropletwas driedbeforedepositingthesubsequentone.Asaresult,adense, lightgrayfilmofthenanowiresformedoverandbetweentheelec- trodes.Beforeinsertingintothegaschamber,theprepareddevices wereallowedtodryovernightatroomtemperature.
Themicrostructureofthesampleswasstudiedbyenergyfil- teredtransmissionelectronmicroscopy(EFTEM,Leo912Omega) and field-emission scanning electron microscopy (FESEM, Zeiss ULTRAplus).ThecrystalstructurewasexaminedbyX-raydiffrac- tion(XRD,SiemensD5000,CuK␣radiation).Theanalysisofthe chemical composition and theoxidation states wascarried out byX-rayphotoelectronspectroscopy(XPS,KratosAxisUltraDLD, monochromatedAlK␣source,analysisareaof0.3mm×0.7mm, applyingachargeneutralizer).
Gas response measurements were performed in a Linkam THMS600heatingandfreezingstageconnectedtoanAgilent3458A multimeterandapre-mixinggasblendercontrolledbyacomputer.
Thesensitivitiesofthepreparedgassensorswerestudiedinthefol- lowingorderforNO,H2,CO,andCH4(at150,200and250◦C)and finallyforH2(at30,70and130◦C)analytesinadrysyntheticair bufferat5Vofconstantbias.Beforeeachgasmeasurement,a2-h temperaturepulseof250◦Cinsyntheticairwasappliedtocleanthe sensors’surfaces.Sensorswereheatedandcooledat10◦C/mindur- ingthegasmeasurements.Note:Inthispaper,30◦Cisconsidered astheroomtemperature,becauseitcanbeeasilykeptconstantin ourmeasurementsetup.Therealroomtemperatureisfluctuating andcannotbemaintainedwithoutadditionalcooling.
3. Resultsanddiscussion
Particlesofsize4.9±2.1nm(PdO)and2.5±1.9nm(PtOx)were formedonthesurfaceoftheWO3nanowiresafterthedecompo- sitionoftheiracetylacetonateprecursors(Fig.1).Atleastpartly duetotheirsmallerdiameter,PtOxnanoparticleswereobservedto
Fig.1.(a)EFTEMimagesofpristine,PdOandPtOxdecoratedWO3nanowiresusedforactivesensinglayersinthegassensors.Insetsshowthecorrespondingsizedistribution diagramsofthedecoratingnanoparticles.(b)XRDpatternsand(c)X-rayphotoelectronspectraofpristineanddecoratedWO3nanowires.
Fig.2.FieldemissionscanningelectronimagesofdropcastWO3nanowiresonSi/SiO2substratewithPt-electrodes.Theinsetshowsthemicrostructureofthenanowire networkinthemiddleofthedroplet.
coverthesurfaceoftheWO3nanowiresmoreuniformlythanthe PdOnanoparticles.
BasedonXRDanalysis,thenanowiresarehexagonalWO3with anaveragecrystaldiameterof38±5nmascalculatedfromthe broadeningof(110),(101),(200),(201)and(111)reflections.
AlmostidenticalXRDpatternswereobtainedforthepristineand themetal decoratedsamples(Fig.1b),i.e.theanalysisfailedto uncoverthereduced,ortheoxidephasesofthedecoratingparti- cles(couldbecausedbytheirsmallsize,lowconcentrationand/or amorphousstructure).
ResultsofX-rayphotoelectronspectroscopy(Fig.1c)indicate thenanoparticlesinthePd-modifiedsamplearePdOratherthan metallicPd(bindingenergyofPd3d5/2peakat337.3eV).Inthe caseofthePtmodifiedWO3,boththemetallicandoxidephases arepresent(Pt4f7/2peakpositionsat71.4and72.3eV).
Thepreparedgassensorsarebasedonmodulationoftheresis- tanceoftheWO3nanowiresthatfillthegapsandpercolatebetween thePtelectrodes.DriedWO3nanowiredropletshaveathickouter bordercausedbyacommoncoffeeringeffect[40,41],whileinthe middlesection,atangledbutthinnerlayerofthenanowiresforms (Fig.2).BasedonFESEMandEFTEMmicrographs,thediameterof thenanowireshasrelativelylargevariationfrom∼20to∼200nm.
Relatingthis totheXRDpeak broadeningresults,weconcluded
thatinthedriedpowders,notonlyindividualcrystallitesbutalso bundlesofnanowiresarepresent.
Atalltemperaturesandconcentrations,theresponseofthepris- tinenanowirestohydrogenwasrelativelylowcomparedtothe decoratedsamples(Fig.3),whiletheresponseofallsamplesto NO,COandCH4 wasmoderate(seesupplementarymaterial).At theoptimaloperationtemperatureofthePdOandthePtOxdec- oratednanowirebasedsensors,thesensitivityvaluesafter5min of1000ppmH2exposureareashighas140±70%/ppm(at200◦C) and24±5%/ppm(at250◦C)respectively,asseeninFig.5.Forthe pristinenanowires,thisfigureis0.3±0.3%/ppm(at250◦C),which isconsistentwiththeresultsinourearlierarticleonnanoparticle basedsensors[42].ThehighresponseofthePdOandthePtOxdec- oratedsensorsisrelatedtothepartialreductionoftheparticles andthedissociationofH2 onmetalnanoparticlesfollowedbya spill-overontotheWO3surface[43].Thesupposedsurfacereduc- tionofthePdOandthePtOxnanoparticles,alongwiththeinitial pulsesofH2,seemstobesupportedbythelowersensitivityofthe devicesupontheveryfirstinjectionsof10ppmH2comparedtothe subsequent10ppmH2pulses.
SincethesensorresponsetowardH2wasstillverylarge,even at150◦C,wehaveperformedfurthermeasurementsat30,70and 130◦Ctoseewhetherthedevicescouldbeusedwithoutexternal
Fig.3.Theconductanceofthesensorsasafunctionofhydrogenconcentrationat150,200,and250◦C.Theblackandgraycurvesrepresenttwoidenticallypreparedgas sensingdevices.Pleasenotethecurvesarepartiallyoverlappingthustheotherwisesimilarnoiselevelsarenotclearlyvisiblefortheblackcurves.
Fig.4.Theconductanceofthesensorsasafunctionofhydrogenconcentrationat30,70,and130◦C.Theblackandgraycurvesrepresenttwoidenticallypreparedgassensing devices.Pleasenotethecurvesarepartiallyoverlappingthustheotherwisesimilarnoiselevelsarenotclearlyvisiblefortheblackcurves.
Fig.5.Thesensitivityofthesensorsasafunctionoftemperatureat(a)10ppm,(b)100ppm,and(c)1000ppmconcentrations.Thesevaluesarecalculatedafter5minofgas exposure.Itshouldbenotedthattheconductanceisfurtherincreasing,evenafter1hofgasexposure,asseeninFig.B1ofthesupplementarymaterial.
heating.AlthoughthepristineWO3nanowiresfailedtodetectH2 atroomtemperature(30◦C),boththePdOandthePtOxdecorated samplesshowedexcellentresponsefor1000ppmH2(Fig.4)com- paredtosol–gelprocessedWO3/Pd[44]orWO3/Pt[45]thinfilm hydrogensensorsreportedbyothergroups.Whilsta significant responsecausedbytheH2gaswasseen,theconductancedidnot stabilizeevenduringverylong,(upto1h),gaspulsesat30and 70◦C(Fig.B1).Theconductancewasincreasingalmostlinearlyasa functionoftimeduringthegasexposure.Asconcludedearlierfor porousWO3films[46]hydrogenmoleculesdissociatetohydrogen atomsthatspill-overtoWO3surfaceandbindtosurfaceoxygen ofthelatticethusformingwatermolecules.Desorptionofthese watermoleculesremoveoxygenandhencecreateoxygenvacan- cies,whichdiffusetotheinteriorofthecrystal.Accordingly,the slowresponse time ofoursensors(similar togasochromiccol- orationof WO3 describedinRef. [46])isexplainedbytheslow diffusionofoxygenvacanciesandalsoslowdesorptionofwater fromthetungstenoxidesurface.Therecoverytimeconstantofthe PtandPddecoratedsampleswasobservedtobelessthan10and 15min,respectively,at30◦C.
Even though, many of the nanowires used in this work (Figs.1and2)appeartohavelargerdiameter(20–200nm)thanthe optimum(4–40nm)forresistivegassensingapplications,thesen- sorsshowanextraordinaryincreaseofconductanceuponexposure toH2.Onereasoncanbeagoodpercolationofthinnanowires(i.e.
thosethatareideal)betweentheelectrodes.Inarandomnetwork
ofthin(ideal)andthick(lessideal)nanowires,thechangeofcon- ductanceinthethickernanowiresisnotexpectedtoinfluencethe gassensitivityasmuchasthethinnanowiresprovidedtheselat- teronesarepercolated.Ontheotherhand,accordingtotheEFTEM micrographs(Fig.1),thelargernanowiresseemtobeconsistingof smallercrystals,whichcouldpossiblyalsocontributetothelarge conductivitymodulationcausedbyH2.
4. Conclusions
Hexagonaltungstenoxidenanowiresdecorated withPdOor PtOxnanoparticlesprovedtobeexcellentmaterialsfordetecting H2 gaswithveryhighsensitivityandgoodselectivityincontrast totheNO,COand CH4 analytes.Bothdecoration typesresulted inverysimilarsensorperformances.Theconductanceofthedec- orated nanowiresensordevices wasfound toincreasewithup tofiveordersofmagnitudewhenexposedto1000ppmofH2in air,even atroomtemperature.Bothtypesofdecorated sensors had long response times (conductance is almost a linear func- tionoftime),thoughtherecoveryslopescouldbefittedwellwith exponentialdecaycurvesofa∼10mintimeconstant.Theover- allpowerconsumption of thedevices isvery low.Without the presenceofH2,thereispracticallynocurrentthroughthedevice and thereforethepowerconsumption isalmostzero.WhenH2 moleculesare present,thepeak powerconsumptionat 30◦C is P=U2S=(5V)2×10S=250W.Accordingly,thedevicecouldbe
operatedreasonably well froman ordinary battery-typepower source, and thus the decorated WO3 nanowires reported here areattractivematerialsforgassensingelementsinanumberof differentdevices,suchasportablelowpowerconsumptionhydro- genleakagedetectors,autonomoussensorassembliesandinfire alarms.
Acknowledgements
MariaSzabó(UniversityofSzeged)isacknowledgedforthesyn- thesisofthepristinenanowires.Thefinancial supportofTEKES (Autosys, Prindemo-POC, Imphona projects) and the Academy of Finland (Rocaname) are acknowledged. J.K. acknowledges INFOTECHOuluGraduateSchool,RiittaandJormaJ.Takanen,Tauno TönningandtheEmilAaltonenFoundationforfinancialsupport forthework.Z.K.acknowledgesthesupportoftheTÁMOP-4.2.2.A- 11/1/KONV-2012-0047project.
AppendixA. Supplementarydata
Supplementarydataassociatedwiththisarticlecanbefound,in theonlineversion,athttp://dx.doi.org/10.1016/j.snb.2013.05.082.
References
[1]T.Hübert,L.Boon-Brett,G.Black,U.Banach,Hydrogensensors–areview, SensorsandActuatorsB:Chemical157(2011)329–352.
[2]W.J.Buttner,M.B.Post,R.Burgess,C.Rivkin,Anoverviewofhydrogensafety sensorsandrequirements,InternationalJournalofHydrogenEnergy36(2011) 2462–2470.
[3]C.Christofides,A.Mandelis,Solidstatesensorsfortracehydrogengasdetection, JournalofAppliedPhysics68(1990)R1–R30.
[4]H.Gu,Z.Wang,Y.Hu,Hydrogengassensorsbasedonsemiconductoroxide nanostructures,Sensors12(2012)5517–5550.
[5]K.Gleeson,E.Lewis,Responsechangesofthinfilmpalladiumbasedopticalfibre hydrogensensorsovertime,JournalofPhysics:ConferenceSeries76(2007), 012004(1–6).
[6]N.Barsan,U.Weimar,Conductionmodelofmetaloxidegassensors,Journalof Electroceramics7(2001)143–167.
[7]R.L.Murray,NuclearEnergy:AnIntroductiontotheConcepts,Systems,and ApplicationsofNuclearProcesses,6thed.,ElsevierInc.,Burlington,2009,pp.
201.
[8]H.Czichos,T.Saito,L.Smith(Eds.),SpringerHandbookofMetrologyandTesting, Springer,Heidelberg,Dordrecht,London,NewYork,2001.
[9]K.J.Patel,C.J.Panchal,V.A.Kheral,M.S.Desai,Growth,structural,electricaland opticalpropertiesofthethermallyevaporatedtungstentrioxide(WO3)thin films,MaterialsChemistryandPhysics114(2009)475–478.
[10]P.S.Patil,P.R.Patil,E.A.Ennaoui,Characterizationofultrasonicspraypyrolyzed tungstenoxidethinfilms,ThinSolidFilms370(2000)38–44.
[11]H.Rui,Z.Jing,Y.Rong,Synthesisandelectricalcharacterizationoftungsten oxidenanowires,ChinesePhysicsB18(2009)3024–3030.
[12]M.Regragui,V.Jousseaume,M.Addou,A.Outzourhit,J.C.Bernéde,B.ElIdrissi, ElectricalandopticalpropertiesofWO3thinfilms,ThinSolidFilms397(2001) 238–243.
[13]K.H.Yoon,C.W.Shin,D.H.Kang,PhotoelectrochemicalconversioninaWO3
coatedp-Siphotoelectrode:effectofannealingtemperature,JournalofApplied Physics81(1997)7024–7029.
[14]J.B.Goodenough,A.Hamnett,G.Huber,F.Hulliger,M.Leib,S.K.Ramasesha, H.Werheit,Landolt-Bornstein:NumericalDataandFunctionalRelationships inScienceandTechnology,NewseriesGroupIII,Volume17,Semiconductors Subvolumeg,PhysicsofNon-TetrahedrallyBondedBinaryCompoundsIII, Springer-Verlag,Heidelberg,1984,pp.289.
[15]J.Tamaki,Z.Zhang,K.Fujimori,M.Akiyama,T.Harada,N.Miura,N.Yamazoe, Grain-sizeeffectsintungstenoxide-basedsensorfornitrogenoxides,Journal oftheElectrochemicalSociety141(1994)2207–2210.
[16]G.Gu,B.Zheng,W.Q.Han,S.Roth,J.Liu,Tungstenoxidenanowiresontungsten substrates,NanoLetters8(2002)849–851.
[17]K.M.Sawicka,A.K.Prasad,P.I.Gouma,Metaloxidenanowiresforuseinchem- icalsensingapplications,SensorLetters3(2005)31–35.
[18]D.Meng,N.M.Shaalan,T.Yamazaki,T.Kikuta,Preparationoftungstenoxide nanowiresandtheirapplicationtoNO2sensing,SensorsandActuatorsB:
Chemical169(2012)113–120.
[19]J.Choi,J.Kim,Highlysensitivehydrogensensorbasedonsuspended,function- alizedsingletungstennanowirebridge,SensorsandActuatorsB:Chemical136 (2009)92–98.
[20]O.Garcia-Serrano,O.Goiz,F.Chavez,G.Romero-Parades,R.Pena-Sierra,Pd- decoratedZnOandWO3nanowiresforsensingapplications,in:Proceedingsof theIEEESensorsConference,2011,pp.998–1001.
[21]L.F.Zhu,J.C.She,J.Y.Luo,S.Z.Deng,J.Chen,N.S.Xu,Studyofphysicaland chemicalprocessesofH2sensingofPt-coatedWO3nanowirefilms,Journalof PhysicalChemistryC114(2010)15504–15509.
[22]C.-L.Dai,M.-C.Liu,F.-S.Chen,C.-C.Wu,M.-W.Chang,AnanowireWO3humid- itysensorintegratedwithmicro-heaterandinvertingamplifiercircuitonchip manufacturedusingCMOS-MEMStechnique,SensorsandActuatorsB:Chem- ical123(2007)896–901.
[23]X.Z.Li,F.B.Li,C.L.Yang,W.K.Ge,PhotocatalyticactivityofWOx–TiO2undervis- iblelightirradiation,JournalofPhotochemistryandPhotobiologyA:Chemistry 141(2001)209–217.
[24]X.Cui,L.Guo,F.Cui,Q.He,J.Shi,ElectrocatalyticactivityandCOtoleranceprop- ertiesofmesostructuredPt/WO3compositeasananodecatalystforPEMFCs, JournalofPhysicalChemistryC113(2009)4134–4138.
[25]C.-C.Liao,F.-R.Chen,J.-J.Kai,WO3−xnanowiresbasedelectrochromicdevices, SolarEnergyMaterialsandSolarCells90(2006)1147–1155.
[26]J.Y.Luo,S.Z.Deng,Y.T.Tao,F.L.Zhao,L.F.Zhu,L.Gong,J.Chen,N.S.Xu,Evidence oflocalizedwatermoleculesandtheirroleinthegasochromiceffectofWO3
nanowirefilms,JournalofPhysicalChemistryC113(2009)15877–15881.
[27]Y.Li,Y.Bando,D.Goldberg,Quasi-alignedsingle-crystallineW18O49nanotubes andnanowires,AdvancedMaterials15(2003)1294–1296.
[28]S.J.Yoo,J.W.Lim,Y.-E.Sung,Y.H.Jung,H.G.Choi,D.K.Kim,Fastswitchable electrochromicpropertiesoftungstenoxidenanowirebundles,AppliedPhysics Letters90(2007),173126(1–3).
[29]J.Su,X.Feng,J.D.Sloppy,L.Guo,C.A.Grimes,VerticallyalignedWO3nanowire arraysgrowndirectlyontransparentconductingoxidecoatedglass:synthesis andphotoelectrochemicalproperties,NanoLetters11(2011)203–208.
[30]K.Zhu, H.He, S.Xie, X.Zhang,W.Zhou,S.Jin,B. Yue,CrystallineWO3
nanowiressynthesizedbytemplatingmethod,ChemicalPhysicsLetters377 (2003)317–321.
[31]J.Zhou,Y.Ding,S.Z.Deng,L.Gong,N.S.Xu,Z.L.Wang,Three-dimensionaltung- stenoxidenanowirenetworks,AdvancedMaterials17(2005)2107–2110.
[32]A.Ponzoni,V.Russo,A.Bailini,C.S.Casari,M.Ferroni,A.LiBassi,A.Migliori, V.Morandi,L.Ortolani,G.Sberveglieri,C.E.Bottani,Structuralandgas-sensing characterizationoftungstenoxidenanorodsandnanoparticles,Sensorsand ActuatorsB:Chemical153(2011)340–346.
[33]L.-J.Chou,M.-T.Chang,Y.-L.Chueh,J.J.Kim,H.S.Park,D.Shindo,Electronholog- raphyforimprovedmeasurementofmicrofieldsinnanoelectrodeassemblies, AppliedPhysicsLetters89(2006),023112(1–3).
[34]K.Q. Hong,M.H. Xie, H.S.Wu,Tungsten oxide nanowiressynthesized by a catalyst-free method at low temperature, Nanotechnology 17 (2006) 4830–4833.
[35]Y.B.Li,Y.Bando,D.Goldberg,K.Kurashima,WO3nanorods/nanobeltssyn- thesizedviaphysicalvapordepositionprocess,ChemicalPhysicsLetters367 (2003)214–218.
[36]C.Klinke,J.B.Hannon,L.Gignac,K.Reuter,P.Avouris,Tungstenoxidenanowire growthbychemicallyinducedstrain,JournalofPhysicalChemistryB109 (2005)17787–17790.
[37]G.Korotcenkov,B.K.Cho,Instabilityofmetaloxide-basedconductometricgas sensorsandapproachestostabilityimprovement(shortsurvey),Sensorsand ActuatorsB:Chemical156(2011)527–538.
[38]X.C.Song,Y.F.Zheng,E.Yang,Y.Wang,Large-scalehydrothermalsynthe- sisofWO3nanowiresinthepresenceofK2SO4,MaterialsLetters61(2007) 3904–3908.
[39]M.Szabó,Diplomathesis,UniversityofSzeged,Hungary,2011.
[40]D.Soltman,V.Subramanian,Inkjet-printedlinemorphologiesandtemperature controlofthecoffeeringeffect,Langmuir24(2008)2224–2231.
[41]X.Shen,C.-M.Ho,T.-S.Wong,Minimalsizeofcoffeeringstructure,Journalof PhysicalChemistryB114(2010)5269–5274.
[42]J.Kukkola,etal.,Inkjet-printedgassensors:metaldecoratedWO3nanoparti- clesandtheirgassensingproperties,JournalofMaterialsChemistry22(2012) 17878–17886.
[43]S.J.Ippolito,S.Kandasamy,K.Kalantar-Zadeh,W.Wlodarski,LayeredSAW hydrogensensorwithmodifiedtungstentrioxideselectivelayer,Sensorsand ActuatorsB:Chemical108(2005)553–557.
[44]S.Fardindoost,A.Irajizad,F.Rahimi,R.Ghasempour,PddopedWO3films preparedbysol–gelprocessforhydrogensensing,InternationalJournalof HydrogenEnergy35(2010)854–860.
[45]H.Nakagawa,N.Yamamoto,S.Okazaki,T.Chinzei,S.Asakura,Aroomtemper- atureoperatedhydrogenleaksensor,SensorsandActuatorsB:Chemical93 (2003)468–474.
[46]A.Georg,W.Graf,N.V.Wittwer,Mechanismofthegasochromiccolorationof porousWO3films,SolidStateIonics127(2000)319–328.
Biographies
JarmoKukkolareceivedhisM.Sc.intheoreticalphysicsfromtheUniversityofOulu, Finland,in2008.HeiscurrentlyaPh.D.studentattheMicroelectronicsandMate- rialsPhysicsLaboratories,UniversityofOulu.Hisresearchinterestistheelectrical propertiesofnanostructuredmaterials.
MelindaMohlbornin1982,graduatedwithanM.Sc.inenvironmentalchemistry (2006,UniversityofSzeged,Hungary)andaPh.D.innanotechnology(2011,Univ.
Szeged).SheundertookherinternshipatTheFacultésUniversitairesNotre-Dame delaPaixinNamur(Belgium)in2005–2006.Inautumn2011,shecommencedas
apostdoctoralresearchfellowattheUniversityofOulu,Finland.Previously,she hasalsoworkedonporousmetalnanostructuresandcarbonnanotubebasedsen- sorsatRiceUniversity,USA.Hercurrentresearchinterestisfocusedondeveloping nanostructuresfor(photo)catalysis,sensorsandelectronicdevices.
Anne-RiikkaLeinobornin1985,graduatedwithanM.Sc.inOrganicChemistry (2010,UniversityofOulu,Finland).In2009,shejoinedtheMicroelectronicsand MaterialsPhysicsLaboratoriesasaresearchassistant.Since2010,sheispursuing studiestowardherPh.D.withthetopicofnanoparticleformationandagglomeration overnanostructuredsurfaces.
JaniMäklingraduatedasanelectricalengineerattheDepartmentofElectricaland InformationEngineering,UniversityofOulu,Finland,in2010.Since2006,hehas beenworkingasaresearchassistantinEUandTekesprojectsaimingatsensor applicationsofnanostructuredmaterials.Currently,heispursuingresearchtoward hisdoctoraldegreeinmicroelectronicswithemphasisongassensors.
NiinaHalonenreceivedherM.Sc.inorganicchemistryfromtheUniversityofOulu, Finland,in2006.Inthesameyear,shejoinedtheMicroelectronicsandMaterials PhysicsLaboratories,UniversityofOulu,andstartedpostgraduatestudies.Since 2007,sheisalsoamemberoftheNationalGraduateSchoolinNanoscience.Her expertiseisinthinandthickfilmtechnologies,opticallithographyandcarbon nanotubesynthesis/applications.
AndreyShchukarev,M.Sc.inchemistry(LeningradStateUniversity,Russia,1979), Ph.D.ininorganicchemistry(LeningradStateUniversity,Russia,1983)andDocentin chemistry(UmeåUniversity,Sweden,2009),isworkingasaresearcherandassis- tantprofessorattheEnvironmentalandBiogeochemistryGroup,Departmentof Chemistry,UniversityofUmeå.Hisresearchinterestisthedevelopmentofsurface analysistechniques,inparticularXPS,andtheirapplicationtorealenvironmental, biologicalandbiomedicalobjects.
ZoltanKonyawasborninHungaryin1971,graduatedwithanM.Sc.inchemistry (1994,University of Szeged, Hungary)and Ph.D. inchemistry (1998,Univer- sityofSzeged, Hungary).Heisprofessor ofchemistryand since2010, he is headoftheAppliedandEnvironmentalChemistryDepartmentoftheUniversity ofSzeged,Hungary.Dr.Kónyahaspublished200+papersinrefereedscientific
journalsandco-authored10bookchapters.Heholds10nationalandinternational patents.Hisresearchinterestisthedevelopmentofnewnanostructuredmateri- alsandtheirapplicationtorealchemical,environmental,biologicalandbiomedical objects.
HeliJantunenreceivedherM.Sc.inastronomyandphysics,FacultyofSciences in1982,theDiplomaineducationin1985,andanM.Sc.inelectricalengineering, FacultyofTechnologyin1989,UniversityofOulu,Finland.Afterbeinginindustry for10years,shejoinedtheMicroelectronicsandMaterialsPhysicsLaboratories, DepartmentofElectricalEngineeringin1999andreceivedtheDr.Tech.degreein microelectronics(withhonors)in2001.Sheisprofessoroftechnicalphysicsatthe UniversityofOulu,andheadofthelaboratoryandthedepartment.Sheischairof theScientificAdvisoryBoardofDefence(ElectronicsGroup),theResearchCouncil forNaturalSciencesandEngineering,AcademyofFinland,andtheESFStanding CommitteeforPESC.HerresearchisfocusedonnovelsensorandRFapplicationsby implementationofadvancedmicrowaveandfunctionalmaterials,structuresand nanotechnologyintomultifunctionalmicromodulesandprintedelectronicsdevices.
Shehasbeenaninvitedspeaker,sessionchairand/orscientificcommitteemember of40internationalscientificconferences,andholdsover13nationalandinterna- tionalpatents.Shehas140+scientificjournalpublications(>1100citations).Her currentfocusismainlydevotedtoherERC(EuropeanResearchCouncil)Advanced Grantproject.
Krisztian Kordaswas borninHungary in 1975, graduatedwithan M.Sc.in physicsandchemistry(1998,UniversityofSzeged,Hungary),Dr.Tech.Micro- electronics (2002) and Docent of Nanotechnology (2004, University of Oulu, Finland).Between2004and2009, hewasappointedasanacademyresearch fellow,bytheAcademyofFinland.From2011until2012,hewasanassistant professorattheDepartmentofChemistryoftheUmeåUniverity.Currently,he isaresearchprofessorattheMicroelectronicsandMaterialsPhysicsLaborato- ries,UniversityofOulusupervisingresearchprojects(EU,TEKESandAcademy ofFinland)aswellasdiplomaworksofstudents andpostgraduatestudiesof young researchers.Dr.Kordashaspublished 90+papersinrefereedscientific journalsandco-authored4bookchapters.Hisresearchisfocusedonsynthesis, structuralandelectricalcharacterization,andimplementationofnanostructured materialsforelectronics,sensorsand(photo)catalystapplications.
