Amorphous TiO2 nanotube arrays for low-temperature oxygen sensors

Nanotechnology19(2008)405504(7pp)

NANOTECHNOLOGY

doi:10.1088/0957-4484/19/40/405504

AmorphousTiO2nanotubearraysforlow-temperatureoxygensensors

HaoFengLu1,2,FengLi2,GangLiu2,Zhi-GangChen2,

Da-WeiWang2,Hai-TaoFang3,GaoQingLu4,ZhouHuaJiang1andHui-MingCheng2

SchoolofMaterialandMetallurgy,NortheasternUniversity,No.11,Lane3,WenHuaRoad,Shenyang110004,People’sRepublicofChina2

ShenyangNationalLaboratoryforMaterialsScience,InstituteofMetalResearch,ChineseAcademyofSciences,72WenHuaRoad,Shenyang110016,People’sRepublicofChina3

SchoolofMaterialsScienceandEngineering,HarbinInstituteofTechnology,92WestDazhiStreet,Harbin150001,People’sRepublicofChina4

ARCCentreofExcellenceforFunctionalNanomaterials,SchoolofEngineeringandAustralianInstituteofBioengineeringandNanotechnology,UniversityofQueensland,Brisbane,QLD4072,Australia

E-mail:jiangzh@smm.neu.edu.cnandcheng@imr.ac.cn

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Received5June2008,infinalform10July2008Published20August2008

Onlineatstacks.iop.org/Nano/19/405504

Abstract

Titaniananotubearrays(TNTA)weresynthesizedonatitaniumsubstrateusinganodic

oxidationinanelectrolytecontainingammoniumfluorideandevaluatedforlow-temperatureoxygensensing.Theirsensingpropertiesweretestedatdifferenttemperatures(50,100,150,200,250and300◦C)whenexposedtovariousoxygenconcentrations.Theas-preparedTNTAareamorphousandexhibitmuchhighercarrierconcentrationthanthatofannealedTNTA.SuchamorphousTNTAshowmuchhighersensitivitythanthatofannealedTNTA,SrTiO3andGa2O3sensors.Thissampledemonstratesthelowestdetectableoxygenconcentrationof200ppm,excellentrecoveryandgoodlinearcorrelationat100◦C.TheseresultsindicatethatTNTAareindeedveryattractiveoxygen-sensingmaterials.

(Somefiguresinthisarticleareincolouronlyintheelectronicversion)

1.Introduction

Theelectricalconductivityofsemiconductingmetaloxideschangeswhenexposedtoactivegasatmospheres[1–3],whichiswhysemiconductingmetaloxideshavebeenwidelyinvestigatedascandidatesforgassensing.Severalmetaloxides,suchasSnO2,ZnO,TiO2,Ga2O3,WO3,MoO3andIn2O3arefoundtohaveapronouncedsensitivitytogases,suchasH2,NO,NO2,CO,alcoholandotherspecies[4–8].Recently,thedemandsforoxygensensorsarerapidlyincreasing.Oxygenconcentrationisoneofthemostwidelyusedparametersinmanyfields,e.g.tocontroltheair/fuelmixtureinautomobileengines[9,10].Uptillnow,commercializedoxygensensorsemployedthesolidelectrolyteofZrO2[11]basedontheNernstprinciple.Suchsensorshavealsobeenusedtodetecttheoxygencontentinexhausted

0957-4484/08/405504+07$30.00

gasinironandsteelsmelters.However,thistypeofsensorhassomeremarkabledrawbacks:(1)theworkingtemperatureofthesolidelectrolyteisabove1100K;(2)areferenceelectrodeisneededduringtesting,whichisnotsuitabletodetectO2concentrationclosetotheoxygenpartialpressureofthereferenceelectrode;(3)thesensoriscomplexandexpensivetouseandmaintain.Therefore,thedevelopmentofahighlysensitive,portable,lowworkingtemperatureandconvenientsensorishighlydesirableandstillagreatchallenge.

Todevelopbetteroxygensensors,TiO2,SrTiO3andGa2O3withvariousmorphologieshavebeenexplored[12–15].Titaniaasanimportantfunctionalmaterialhasbeenwidelyinvestigatedinbroadareasincludingphoto-watersplitting,dye-sensitizedsolarcells,electronfieldemissionelectrochemicallithiumstorageandgassensing[16–24].However,themaindrawbacksofaTiO2-basedoxygen

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©2008IOPPublishingLtdPrintedintheUK

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Figure1.SEMandTEMimagesofTNTA:(a)and(c)theas-preparedsample,(b)and(d)thesampleafterannealingat450◦Cinairfor2h.

sensorisitslowsensitivityandpoorrecoveryrate[12,15].1DTiO2nanotubearrays(TNTA)areofconsiderableinterestastheyexhibituniquearchitecture,andremarkableandbetterpropertiescomparedtotheirbulkcounterparts.Consideringthenatureofgassensingviatheinteractionofasemiconductingsurfacewithadsorbedgasmolecules[25–27],itisthoughtthatTNTA,grownontitaniumsubstratesbyasimpleandstraightforwardanodizationprocesswiththecharacteristicsofhighlyorderedopentubesandlargespecificsurfacearea,canprovideabundantsitesandchannelsforgasadsorption,diffusionandchemicalreaction.Accordingtoarecentreport,titaniananotubearraysexposedto1000ppmhydrogenexhibitanunprecedentedvariationinresistanceatroomtemperature[28].Becausethesurfaceresistance-controlledmechanismsuitsmostgassensors,itisreasonabletobelievethatTNTAhavethepotentialforhighlysensitiveoxygendetection.Itiswellknownthatoxygenvacanciesexistinn-typesemiconductingtitaniaactingasdonors.Thestructureofamorphoustitaniananotubesismoredisorderedthanthatofcrystallinenanotubes,therebyhavingmanymoredefects(oxygenvacancies)thatcanprovidehighercarrierconcentrationandmoreactivesites.Thismayenhancethesensitivitywhenexposedtooxygen.

Inthiswork,weexploredtheoxygensensingpropertyofamorphousandanataseTNTAatlowtemperaturesoverawiderangeofoxygenconcentrations.TheoxygensensingpropertiesandsensingmechanismofTNTA-basedsensorswerediscussedindetail.

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2.Experimentaldetails

TiO2nanotubearrayswerepreparedfromatitaniumfoilofapproximately250μminthickness(99.5%purefromAlfaAesar).Potentiostaticanodizationwasperformedinatwo-electrodeelectrochemicalcell,connectedtoaDCpowersourceunderaconstantpotentialof20Vusingaplatinumfoilascounterelectrode.Theelectrolyteconsistedof0.5wt%NH4Fand1.0M(NH4)2SO4indeionizedwater.Theanodicoxidationprocesswasconductedatroomtemperaturefor2h.Theas-preparedTNTAwereamorphous,andanataseTNTAwereobtainedbyannealingtheas-preparedsampleat450◦Cinairfor2hwithaheatingrateof5◦Cmin−1.

Afieldemissionscanningelectronmicroscope(FESEM,LEOSUPRA35)wasusedtoobservethemorphologyoftheTNTA.Anx-raydiffractometer(XRD,RINT2200,CuKα)wasusedtodeterminethecrystallinephase.Transmissionelectronmicroscope(TEM)imagesofTNTAwereobtainedbyaJEOL2010at200kV.Mott–SchottkycurvesweremeasuredbyaPrincetonAppliedResearchPARSTAT2273,usingathree-electrodesystem.

Twoplatinumstrips(10mm×2mm)atadistanceof3mmwerefixedontotheTNTAsurface[15]bytwoquartzglassesandfourscrews.Twoplatinumwiresweresolderedonplatinumsheetswiththeohmicconnectiontoacomputer-controlledcurrent–timemeasurementinstrument(CHI630I)tomeasurethecurrentpassingthroughthenanotubearraysunderconstantpotential.Thereisanoxidebarrierlayer(insulating

Nanotechnology19(2008)405504HFLuetal

Figure2.XRDpatternsoftheas-prepared(curvea)andannealed(curveb)TNTA.

layer)betweenmetallicTiandTNTA,andthisbarrierlayercanpreventcurrentgettingacrossmetallicTi[15].Thevariedconcentrationsoftestgas(oxygen)werecontrolledbytwomassflowcontrollers,andthecarriergasemployedwasnitrogen.ThesensitivitytermSisdefinedas

S=

Rgas−R0

R0

(1)

Figure3.(a)Theresistanceresponseand(b)sensitivityvariationoftheamorphousTNTAunderdifferentoxygenconcentrationsat50◦C.

whereRgasandR0representtheresistanceofthesensorsinmixedgasesincludingoxygenandnitrogen,respectively.

3.Resultsanddiscussion

SEMimagesoftheas-preparedTNTAandannealedTNTAfromtopandcrossviewsareshowninfigures1(a)and(b).BothoftheTNTAareverticallyalignedonaTifoilsubstrate.Itisclearthatthesurfacestructureandouterdiameteroftheamorphousandannealedsamplesareverysimilar.Thenanotubeshaveauniformouterandinnerdiameterdistributionaround150and110nm.TheTNTAarecomposedofwell-alignednanotubesofabout2.3μminlength.TEMimagesoftheas-preparednanotubesandannealednanotubesareshowninfigures1(c)and(d).TheinsertedSAEDpattern(figure1(c))provestheamorphousnatureofas-preparedsample,Figure1(d)indicatesthepolycrystallinestructureoftheannealednanotubes.WecanseethatthemorphologyofTNTAremainedthesameastheoriginalafterheattreatment.Thethicknessofthetubewallisapproximately20nm.XRDpatternsoftheas-preparedandannealedTNTAareshowninfigure2.Thepeaksofthelatter,whichwereobtainedbyannealingtheas-preparedsampleat450◦Cinairfor2h,canbeindexedasamainanatasephasecorrespondingto2θ=25.3◦seenforthe(101)crystalorientation.Comparedtotheamorphousnatureoftheas-preparedTNTA,theannealedTNTAshowananatasephaseplusanegligiblerutilephase.

Generally,nanostructuredTiO2usedforgassensing(includingH2,CO,NO2,CH4,etc)shouldbeannealedinairoroxygenatmosphereatacertaintemperaturepriortotestinginordertoformacrystallinestructure[29–32].AtransformationofamorphousTNTAintoanataseand(or)

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rutileTNTAoccursduringtheannealingprocess.CrystallineTiO2ishighlyadvantageousforH2detection,butforoxygencrystallineTiO2exhibitsaverypoorrecoveryproperty[15].Therefore,wemainlyfocusedontheinvestigationofthegassensingresponsesofamorphousTNTAforoxygenatlowtemperatures.

Figure3(a)showstheisothermalresponseoftheelectricalresistanceoftheamorphousTNTAsensorataworkingtemperatureof50◦Cwhentheoxygenconcentrationisvariedfrom1.2%to4.0%.Thesampleholderwasflushedwithnitrogenaftereachexposuretooxygen.TheresistanceofTNTAincreasedinthepresenceofoxygenandrecoveredinnitrogen.Thesensitivityofthissensorintheconcentrationrangestudiedisplottedinfigure3(b)andthereisagoodlinearrelationshipwithoxygenconcentrationsat50◦C.

TocheckthebehaviouroftheamorphousTNTAsensorinawiderconcentrationrangeathighertemperatures,experimentswerecarriedoutat100,150,200,250and300◦C,whiletheconcentrationofoxygenvariedfrom200ppmto20%.Theresistanceresponseatthetemperatureof100◦CoftheamorphousTNTAsensorforoxygenisshowninfigures4(a)–(c).ItcanbeseenthatthereareremarkablechangesintheresistanceofTNTAwithoutobvioushysteresis,andafternitrogenflushingineverycycletheresistanceregainsitsoriginalvalue.ThesensitivityofamorphousTNTAwasstudiedatdifferenttemperaturesasshowninfigure5(a).InthisfigurewecanseethatthesensitivityofamorphousTNTAroughlyincreaseswhentemperatureincreases,butdoes

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Figure4.ElectricalresistancechangeoftheamorphousTNTAwhenexposedtothreedifferentconcentrationrangesat100◦C:(a)200–1980ppm,(b)6.1󰀁–9.5󰀁,(c)1.8%–20.0%.

notexhibitanylinearcorrelationwithoxygenconcentrationsandirregularlyfluctuatesattemperaturesabove180◦C.Furthermore,theresistanceresponsesabove180◦Cshowaverypoorrecoverypropertyaccordingtopresentresults.However,at100◦C,highsensitivity,anexcellentrecoverypropertyandalinearrelationshipwithoxygenconcentrationscanbeobserved.AtthistemperatureanamorphousTNTAsensorexhibitssharpvariationofelectricalresistanceuptotwoordersofmagnitude(∼102),muchhigherthanthatofothermetal-oxidesensorssuchasGa2O3thinfilm(∼1.5)[11],nanoscaleTiO2thickfilm(∼1.5)[12]andSrTiO3thickfilm(∼6.5)[13].Theseperformancesrepresenthighersensitivity,betterrecoveryandlinearcorrelationcomparedtothesemetal-oxidesemiconductorsforoxygensensingreportedintheliterature[12,13,15].

Meanwhile,theannealedTNTAsamplewasalsotestedat100◦C.ThesensitivityofanataseTNTAhasnolinearrelationshipwithoxygenconcentrationsandismuchlowerthanthatoftheamorphoussensor(figure5(b)).ItisobviousthatthesensingpropertyofamorphousTNTAismuchbetterthanthatofannealedTNTAasanoxygensensorat100◦C.Vargheseetal[15]reportedthattheresistanceoftheTiO2nanotubesincreasedwhenexposedto1%oxygenat290◦C,butcouldnotrecovertoitsoriginalelectricalconductivityevenafterseveralhours’exposureinnitrogen.Soitcanbeconcludedthat,atlow(≈100◦C)andrelativelyhigh-temperature(>150◦C),bothamorphousandannealedTNTAexhibitdifferentrecoverybehavioursafterexposuretooxygen.

Ingeneral,thechangesinresistanceofmetal-oxidesemiconductorgassensorswhenexposedtodifferent

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atmospheresareduetoachargecarrierexchangeofadsorbedgaswiththeoxidesurface[32].InordertounderstandthesuperiorityofanamorphousoveranataseTNTAsensorinresponsetooxygenatlowtemperatureandthedissimilarresponsesofTNTAatdifferenttemperatures,weshouldconsidertheuniqueinteractionstowardtheseconditions.AsTiO2isann-typesemiconductor,itselectricalconductivityreliesonfreeelectronssuppliedbythedonorenergylevel,whichisbelowtheconductionbandminimuminthebandgapoftitaniacausedbyoxygenvacanciesorinterstitialtitaniumatoms.WhiletheTNTAareexposedtooxygen-containingatmospheres,oxygenmoleculesareadsorbedontheTiO2surface,thenformingsomeanionswhichactasacceptorsbytrappingelectronsfromthenanotubeconductionbandandcreatingadepletionregiononthenanotubesurfacethatenhancesitselectricalresistance[28].Ithasbeenproposedthattheadsorbedoxygenmoleculestransformtovariousanionspeciesbytransferringelectronsfromthemetaloxidetothechemisorbedoxygenaccordingtothefollowingprocesses[33]:

−

O2(gas)⇔O2(ad)⇔O−2(ad)⇔O(ad)

⇔O2−(ad)⇔O2−(lattice).(2)

Intheseprocesses,thereexistsatransitiontemperaturebelowwhichoxygenadsorbedonthesurfaceismainlyintheformofO−2,whereasabovewhich,chemisorbedoxygendominatesintheformofO−andO2−[34].Thistransitiontemperatureisapproximately450K(≈180◦C)formetal-oxidesurfaces[34].

Nanotechnology19(2008)405504HFLuetal

relationshipbetweencarrierconcentrationandspacechargelayercapacitancetermCSCcanbedefinedas[35,36]

󰀂󰀁

12kT

E−Efb−=(3)2εε0eNdeCSCwherekisBoltzmann’sconstant(1.38×10−23JK−1),ε0the

vacuumpermittivity(8.85×10−14Fcm−1),etheelementarycharge(1.6×10−19C),εthepermittivityofTiO2(foranataseis48),NdthedonordensityandTtheKelvintemperature.FromtheslopesoftheMott–SchottkyplotsintheirlinearrangetherecanbecalculatedNd(donordensityorcarrierconcentration)ofamorphousandanataseTNTA,Nd(amorphous)=1.149×1019cm−3>Nd(anatase)=4.022×1017cm−3.Theamorphousnanotubestructureismoredisorderedthanthatofanatasenanotubes,therebyhavingmanymoredefects(oxygenvacanciesorinterstitialpositions)whichprovideabundantlocaldonorenergylevels.Thedonorenergylevelsenhanceconductivity[34],i.e.concentrationofchargecarriers.Thelargerthebaseconcentrationofchargecarriers(electrons),themoreitchangesafterexposuretooxygen.Thisleadstohighersensitivityforanamorphousthanananatasesensortothesameoxygen-containingatmospheres,andbetterlinearcorrelationwithoxygenchangeoverawiderconcentrationrange.

WhentheTNTAsensorsworkat100◦C,oxygenischemisorbedintheformofO−2,leadingtoanincreaseofresistance.ThereisareasonablemodelinwhichO−2speciesaremainlyboundinthevicinityofvacancies[3].Onremovingtheoxygenatmosphere,oxygendesorptiontakesplaceandelectronstransferbacktonanotubes,recoveringtotheoriginalresistanceofthenanotubes.However,theconditionisdifferentathighertemperatures.Athighertemperatures,O−andO2−ionsexistasprevailingspecies,causingare-oxidationofthecrystalthatleadstorestructuringofthewholesurface[3].BecausetheenergyofO2−inalatticesiteisestimatedtobemuchlower(about20eV)thanthatinanadsorbedstate[37],O2−(ad)isunstableandhastobestabilizedbydiffusingintothelattice.Thischangereducesdefects(oxygenvacancies)whichprovideabundantlocaldonorenergylevelsinamorphousTNTA,i.e.chargecarrierconcentrationisdecreased.Onremovingoxygen,thereductionofthelatticecannotoccurquickly,hencethetimeforasensortoregain

Figure5.(a)ThesensitivityofamorphousTNTAat50,100,150,250and300◦Cfordifferentoxygenconcentrations.(b)ThesensitivityoftheamorphousTNTAandannealedTNTAcorrespondingtodifferentoxygenconcentrationsat100◦C.

AstothedominantmechanismbehindthephenomenathatamorphousTNTAhashighersensitivityandabetterlinearrelationshipwithoxygenconcentrationsthanthoseofanataseTNTA,itisreasonablyconsideredthatthekeypointisthechangeofchargecarrierconcentrationonthenanotubesurface.BasedontheMott–Schottkyprinciple,thecarrierconcentrationcanbeobtainedbyelectrochemicallymeasuringthespacechargelayercapacitance.TheMott–SchottkyplotsofamorphousandanataseTNTAattheACfrequencyof1kHzin0.25MNa2SO4aqueouselectrolyteareshowninfigure6.AccordingtotheMott–Schottkyprinciple,the

Figure6.Mott–Schottkyplotsof(a)amorphousTNTAand(b)anataseTNTAin0.25MNa2SO4,recordedatafrequencyof1kHz.

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itsoriginalconductivityisverylong[15].ThisisaplausiblereasonwhytheTNTAsensorsarerecoverableforcycledchangesbetweenoxygenandnitrogenatlowtemperatureunderthetransitiontemperature,butathighertemperaturetheybecomeunrecoverable.

4.Conclusion

Titaniananotubearrayswerepreparedbyanodizationandinvestigatedfortheirpotentialapplicationinoxygensensors.Theas-preparedamorphousTNTAshowremarkablerecoverableresponsestooxygenatalowtemperatureof50◦C.At100◦Cthesensingproperties(sensitivity,recovery,linearcorrelationwithoxygenconcentrationandresponserange)arethebestandthelowestdetectableconcentrationis200ppm.Interestingly,theoxygensensitivityofTNTAismuchbetterthanthatofothermetal-oxidesensorssuchasnanoscaleTiO2thickfilm,SrTiO3thickfilmandGa2O3thinfilm.TheseresultsdemonstratethatamorphousTNTAstructurescanbeverypromisingcandidatesforoxygensensors,particularlyatlowtemperatures.

Acknowledgments

TheauthorsthankProfessorYingLiandDrLiLiufortheirsupportoftheMott–Schottkycurvemeasurements.ThisworkwassupportedbytheNationalScienceFoundationofChina(50602011).

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