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SensorsandActuatorsB:Chemical

journalhomepage:www.elsevier.com/locate/snb

Enhancedacetonegas-sensingperformanceofLa2O3-dopedflowerlikeZnOstructurecomposedofnanorods

Jian-QunHea,JingYinb,DongLiua,Le-XiZhanga,Feng-ShiCaia,Li-JianBiea,c,∗

a

SchoolofMaterialsScienceandEngineering,TianjinUniversityofTechnology,Tianjin300384,China

SchoolofEnvironmentalScienceandSafetyEngineering,TianjinUniversityofTechnology,Tianjin300384,Chinac

TianjinKeyLabforPhotoelectricMaterials&Devices,TianjinUniversityofTechnology,Tianjin300384,China

b

article

info

abstract

Articlehistory:

Received21February2012

Receivedinrevisedform20February2013Accepted21February2013Available online 1 March 2013

Keywords:Zincoxide

Co-precipitationFlowerlikeLa2O3-dopedGas-sensing

FlowerlikeZnOnanostructurescomposedofnanorodsweresynthesizedbyco-precipitationmethodusingZn(NO3)2·6H2Oand(CH2)6N4asreactants,polyethyleneglycol400assurfactant.La2O3wasdispersedontotheobtainedZnOwithacontentof0.5–2.0wt%toenhancethegas-sensingproperty.Gas-sensingresultsoftheLa2O3-dopedZnOtoethanol,acetone,ammoniaandformaldehyde,respectively,revealthatresponseofLa2O3-dopedZnOtoaspecifiedtestgasishigherthanthatofnoLa2O3-doped.Responseof1.0wt%La2O3-dopedZnOto200ppmacetonereaches54.1attheworkingtemperatureof350◦C,andtheresponsetimeisonly8s,implyingthepotentialfordetectionoflowgasconcentration.

© 2013 Elsevier B.V. All rights reserved.

1.Introduction

Asthemostimportantfunctionaloxideswithadirectwidebandgap(3.37eV)andlargeexcitationbindingenergy(60meV)[1],ZnOhasbeenwidelyusedingas-sensingapplicationduetoitsgoodresponsetoavarietyofreducingoroxidizinggases,lowcost,andbeingfriendlytotheenvironment[2–9].

ApplicationsofZnOorFe2O3semiconductingoxideinacetonegas-sensingwerereportedinliteratures[10,11],butneitherthegas-sensingresponse,northeresponseandrecoverytimeofthesensingdevicewasgoodenoughfordetectinglowgasconcen-tration.Reportsshowedthatthegas-sensingpropertycouldbeimprovedbymodificationofthesemiconductingoxideswithnoblemetals(Au,Pt,Pd,etc.)orrareearthoxide[12–16].

Althoughvarioustechniques,suchasmagnetronsputtering,plasmaenhancedchemicalvapourdeposition,spraypyrolysis,sol–gelprocess,vacuumevaporation,werereportedintheprepa-rationofZnOnanostructureswithanabundantvarietyofshapes[17–20],simpleco-precipitationmethodhasbeenattractingmuchattentioninthesynthesisofnovelZnOnanostructuresrecently.

Inthispaper,facilesynthesisofLa2O3dopedflowerlikeZnOhierarchicalstructurecomposedofnanorodsviaco-precipitationmethodisreported,andalsoarethegas-sensingpropertiesoftheobtainedZnOsamplestoethanol(C2H5OH),acetone(CH3COCH3),ammonia(NH3)andformaldehyde(HCHO).

2.Experimental

2.1.PreparationofLa2O3-dopedflowerlikeZnOnanostructures

Allthereagentsusedintheexperimentwereofanalyticalgradewithoutfurtherpurification.TheflowerlikeZnOnanostructureswerepreparedasfollows:

∗Correspondingauthorat:SchoolofMaterialsScienceandEngineering,TianjinUniversityofTechnology,Tianjin300384,China.Tel.:+862260215285;fax:+862260215285.

E-mailaddresses:ljbie@tjut.edu.cn,ljbie@pku.org.cn(L.-J.Bie).

(1)0.7ghexamethylenetetramine((CH2)6N4)weredissolvedin

100mldistilledwatertogetSolutionA,2.878gzincnitrate(Zn(NO3)2·6H2O)wasdissolvedin100mldistilledwatertogetSolutionB.ThenstoichiometricamountofSolutionAwasmixedwithSolutionBand0.004gpolyethyleneglycol400(PEG400)understirringfor10mintoformatransparentsolu-tion.

(2)Theobtainedsolutionwaskeptat95◦Cfor7h,resultinginthe

formationofwhitepowders.

(3)Thewhitepowderswerecollectedandwashedseveraltimes

withdistilledwaterandethanol,driedat80◦Cfor2h,thencalcinedat400◦Cfor2h,toformtheZnOnanostructures.

0925-4005/$–seefrontmatter© 2013 Elsevier B.V. All rights reserved.http://dx.doi.org/10.1016/j.snb.2013.02.085

J.-Q.Heetal./SensorsandActuatorsB182 (2013) 170–175

171

Fig.1.Schematicillustrationof(a)agassensorand(b)themeasuringelectriccircuitofgas-sensingcharacteristics.

(4)StoichiometricamountofLa2O3wasdispersedontotheas-preparedZnOsamplewithadequateamountofethanolundervigorousstirringinanultrasonicbathfor10min,thenthemix-turewasdriedinairat80◦C,annealedat400◦Cfor2htoobtaintheLa2O3-dopedsample.La2O3contentinZnOsamplewascal-culatedbytheweightratioofLa2O3toZnOintheexperiment.

2.2.Sensorfabricationandgas-sensingpropertymeasurement

Theas-preparedZnOsamplewasgrindedwithseveraldropsofterpineolinanagatemortar,andthentheformedslurrywascoatedontoanaluminatubewithadiameterof1mmandlengthof4mm,positionedwithapairofAuelectrodesandfourPtwiresonbothendsofthetube(Fig.1a).ANi–Cralloycoilinsidethetubewasemployedasaheatertoadjusttheworkingtemperaturebytuningtheheatingvoltage,asalinearcorrelationexistsbetweentheheatingvoltageandtheworkingtemperature.

Astationarystategasdistributionmethodwasusedforthetestofgas-sensingproperties.Gas-sensingtestswereperformedusingaWS-60Asystem(ZhengzhouWinsenElectronicsTechnologyCo.Ltd.,China).Aschematicillustrationofthegas-sensingmeasure-mentisshowninFig.1b.Inthemeasuringelectriccircuitofgassensor,aloadresistorisconnectedinserieswithagassensor.ThecircuitvoltageVcis10V,andtheoutputvoltage(Vout)istheter-minalvoltageoftheloadresistorRl.TheworkingtemperatureofsensorcanbeadjustedbyvaryingtheheatingvoltageVh.Theresis-tanceofasensorinairortestgasismeasuredbymonitoringVout.Inordertoimprovethestabilityandrepeatability,thegas-sensingunitwasagedataheatingvoltageof5Vfor48hinairbeforethemeasurement.

Thesensorresponsetoatestgas,Sr,isdefinedas:

Sr=

RaR(1)

g

whereRaistheresistanceofasensorinair,andRgisthatinatestgas.

Theresponseandrecoverytimearedefinedasthetimeof90%totalresistancechangerequiredataspecifiedworkingtempera-ture.

2.3.Characterizationofthesamples

ThecrystalstructureofsampleswascharacterizedbyX-raydiffractmeter(XRD)(RigakuD/Max2500PC)withmonochroma-tizedCuK␣(󰀁=1.5418A)

˚incidentradiationusingatubevoltageandcurrentof40kVand150mA,respectively.XRDdatawascol-lectedatascanspeedof1◦/minwithastepof0.02◦.

Themorphologyofthesynthesizedsamplewasexaminedusingafield-emissionscanningelectronmicroscope(SEM)(JEOLJSM-6700F).

Thephotoluminescence(PL)measurementswerecarriedoutonaShimadzuRF-5301PCfluorescencespectrophotometerequippedwitha150Wxenonlampastheexcitationsource.

ThespecificsurfaceareasweremeasuredviatheBrunauer–Enmet–Teller(BET)methodusingaN2adsorption

at77Kaftertreatingthesamplesat200◦Cand10−4Pafor2husingaTristar-3000apparatus.

3.Resultsanddiscussion

3.1.XRDcharacterization

Fig.2showstheXRDpatternoftheLa2O3-dopedflowerlikeZnOsample.ThemaindiffractionpeakscanbeindexedasthewurtzitestructureZnOwitha=b=0.3253nm,c=0.5213nm,whichisingoodagreementwiththeJCPDSNo.36-1451.Fourdiffrac-tionpeaksofLa2O3canbeobserved,whichcanbeindexedasahexagonalstructurewitha=b=0.4039nm,c=0.03nm(JCPDSNo.83-1345).

3.2.Structureandmorphology

Fig.3showsthetypicalSEMimagesoftheas-preparedZnOhierarchicalstructuresandthefocusofoneflowerlikeZnOcluster,respectively.Ascanbeseen,themorphologyoftheZnOstruc-turesissimilaronlargescale.ThepreparedZnOstructuresusuallyexhibitsflowerlikeshape,composedofZnOnanorodswithadiam-eterof90nm,whichmightbesuitableforgas-sensingapplicationduetothesterichindrancetoaggregationandeasydiffusionforgasmolecules.

Fig.4showstheSEMimagesofLa2O3-dopedZnOstructureswithvariousLa2O3concentrations.ThemorphologyoftheLa2O3-dopedZnOnanostructuresisflowerlike,similarwithpureZnO,astheLa2O3dopingcontentis0.5wt%and1.0wt%,butagglomerationappearswhentheLa2O3contentreaches2.0wt%.

TheformationofflowerlikeZnOhierarchicalstructurecanbeattributedtoboththeactionofPEG400andthereactionkinetics.PEG400surfactantmoleculeisachainstructurewithhydropho-bicandhydrophilicgroup,whichmightformsphericalcoresinwater[21–23].OxygenatomsonthesurfaceofthesesphericalcoresmayattractZn2+cationstoformtheZnOcrystalseeds.Astheseedcrystalsgrow,ZnOnanorodswillbeformedintheco-precipitationprocess,resultingintheflowerlikenanostructures.AschematicillustrationofthesynthesisprocedureisshowninFig.5.

3.3.Gassensingproperties

Thegas-sensingresponsesofthepureandLa2O3-dopedflower-likeZnOstructuresto100ppmacetoneasafunctionoftheworkingtemperatureareshowninFig.6.ResponseofLa2O3-dopedZnO

Fig.2.TypicalXRDpatternsoftheLa2O3-dopedZnOsamples.

172J.-Q.Heetal./SensorsandActuatorsB182 (2013) 170–175

Fig.3.TypicalSEMimagesof(a)pureZnOhierarchicalstructuresand(b)focusofsingleflowerlikeZnOcluster.

sampleexhibitsarapidincrease,andreachesthemaximumattheworkingtemperatureof350◦C,sotheworkingtemperatureoftheLa2O3-dopedZnOnanostructuresischosenas350◦Cinthefollow-ingmeasurements.AscanbeseenfromFig.5,theresponseofZnOsamplewithLa2O3dopingconcentrationof1.0wt%La2O3hasamuchbiggerresponsethanothersamples,sosamplewith1.0wt%La2O3concentration(La2O3/ZnO)wasfocusedinthefollowingdis-cussion.

Fig.7showsthegas-sensingresponsesofthepureandLa2O3-dopedflowerlikeZnOnanostructuresat350◦Casafunctionofacetoneconcentration.TheresponseoftheLa2O3-dopedZnOsen-sorincreaseswiththeincreaseofacetoneconcentration,andresponseofLa2O3-dopedZnOstructureto200ppmacetoneat350◦Creaches54.1,whichisabout3timesthatofpureZnOsample.Fig.8(a)representsthedynamicvariationofresponsetoace-tonewithconcentrationsvaryingfrom10ppmto200ppm,theresponsetimeandrecoverytimeareabout9sand13swith10ppmacetone,11sand17swith200ppmacetone,respectively,revea-lingthathighandfastgasresponsecanbeachievedindetectinglowconcentrationacetoneusingtheflowerlikeLa2O3-dopedZnOnanostructuresassensingmaterial.

Thegas-sensingselectivityofZnOgassensorhasbeenmea-suredusingethanol(C2H5OH),ammonia(NH3)andformaldehyde(HCHO)withconcentrationof10,100and200ppm,respectively(Fig.9),showingthatLa2O3-dopedflowerlikeZnOstructureshasgoodselectivityforacetone.

3.4.Gas-sensingmechanism

seenfromFig.10,bothweakUVemissionpeak(∼390nm)andstrongvisibleemissionpeak(∼610nm)existinallthesamples.Theformerreferstotheintrinsictransitionbetweenthevalenceband(VB)andtheconductionband(CB),whilethelaterisusuallycausedbysurfacedefects.SupposethattheareaofUVpeak(SB)andvisiblepeak(SD)representpristineZnOandthequantityofsurfacedefects,respectively,thusthecontentsofstructuredefectcouldbecalculatedviatheratiooftheSD/SB.Throughcurvedecon-volution,eachPLspectrumcanbewell-fittedtothesuperpositionoftwoGaussiansub-peaksandareassignedtobandgapandsur-facedefectemission.Interestingly,ZnOdopedwith1.0wt%La2O3showsthehighestratiointhe4samplesasshownintheinsetofFig.10.Thisphenomenonisingoodaccordancewiththegas-sensingresultthat1.0wt%La2O3-dopedZnOexhibitsthelargestresponsevaluetoacetone.Therefore,itisreasonabletoexplaintheeffectofLa2O3dopingongas-sensingenhancementfromtheaspectofsurfacedefects.

Ontheotherhand,oxygensorptionalsoplaysanimportantroleinelectricaltransportpropertiesofZnOintheacetonesensingpro-cess.Theoxygenionosorptionremovesconductionelectrons,andlowerstheconductanceofZnO.

First,reactiveoxygenspeciessuchasO2−,O2−andO−areadsorbedontheZnOsurfaceathightemperatures.Itshouldbenotedthatthechemisorbedoxygenspeciesdependstronglyontemperature.Atlowtemperatures,O2−iscommonlychemisorbed;Athightemperatures,whileO2−disappearrapidly,O−andO2−arecommonlychemisorbed[25].Thereactioncanbedescribedasfollow[26]:

Itiswell-knownthatthegas-sensingresponseofmetaloxidesemiconductorsoriginatesfromthefluctuationsofelectronconcentrationinthecharge-depletionlayerinducedbythecon-sumptionofoxygenadsorbatesbythereactionwithtargetgasesinthetargetgasatmosphereandthenre-formationofchemisorbedoxygeninair.Accordingly,thevariationofacetonevaporresponsecanbeattributedtothedefectformationandchangeinspecificsurfaceareasresultedfromtheLa2O3dopingonZnOsurface.

Ontheonehand,theresponsevalueisnotdirectlyproportionaltotheLa2O3contentloaded,thustheZnOsurfaceregioninfluencedbyLa2O3shouldplayakeyroleinthegas-sensingimprovement.AfterLa2O3doping,someZn2+mightbesubstitutedbyLa3+cations,resultingintheincreaseofelectronsconcentrationinthedopedZnOsamples:

O2(gas)↔O2(adsorbed)

(3)(4)(5)

O2(adsorbed)+e−↔O2−

O2−+e−↔2O−

Asthereducingacetonevaporisintroducedintothetestcham-ber,theconductanceoftheZnOnanorodswillincreaseduetoexchangeofelectronsbetweentheionosorbedspeciesandZnO[27].Thereactionbetweenacetoneandionicoxygenspeciescanbedescribedas[26,28]:

CH3COCH3(gas)+O−→CH3CO+CH2+OH−+e−

(6)(7)

CH3COCH3(gas)+O−→CH3C+O+CH3O−+e−

CH3COCH3+O−(bulk)→CH3COOH+O(vacancies)

(8)(9)(10)

La2O3(s)−→2LaZn+2Ox0

ZnO•

+

1

O(g)+2e󰀈22

(2)

CH3C+O→C+H3+CO

Namely,incrementaloxygenmoleculescanbechemisorbedandthenionizedonZnOsurface,resultinginhigherresponse[24].Asausefultoolforcharacterizingsurfacedefects,roomtemperaturephotoluminescence(PL)measurementwascarriedout.Ascanbe

CO+O−→CO2+e−

ComparedwithZnOsample(b)inFig.8,theoveralldecreaseinsen-sorresistancecanbeobservedintheLa2O3-dopedsample(a).Asthedecreaseinsensorresistancebythebalancecontrolmayresult

J.-Q.Heetal./SensorsandActuatorsB182 (2013) 170–175

173

Fig.4.SEMimagesofLa2O3-dopedflowerlikeZnOhierarchicalstructureswithLa2O3dopingconcentrationof(a)0.5wt%,(b)1.0wt%and(c)2.0wt%.

Fig.5.SchematicillustrationofZnOsynthesisprocedure.

Fig.6.ResponseofthepureandLa2O3-dopedZnOstructuresto100ppmacetoneatdifferentworkingtemperatures.

Fig.7.ResponseofthepureandLa2O3-dopedZnOnanostructurestovariousace-toneconcentrations.

Fig.8.Dynamicresponsesofsamplestodifferentacetoneconcentrations(a)1.0wt%La2O3-dopedZnOand(b)pureZnO.

174J.-Q.Heetal./SensorsandActuatorsB182 (2013) 170–175

Fig.9.ResponseofLa2O3-dopedZnOnanostructuressensorstodifferentgases.

inadecreaseinresponsetoinflammablegasesinn-typesemicon-ductorgassensors,theobservedacetoneresponseenhancementshouldbeattributedtotheincreasedreactivityofacetonebytheLa2O3doping.

Accordingtothesurface-reaction-relatedgas-sensingmecha-nismmentionedabove,theresponsevaluesaredirectlyaffectedbythespecificsurfaceareasofsensingmaterials.Usually,largerspecificsurfaceareasleadtomuchhigherresponsevalues[29].ItisfoundthatthespecificsurfaceareasoftheZnOsampleschangedafterLa2O3doping,althoughtheirnanorodprofileandflowerlikestructurekeptalmostthesameasthatofpureZnO(Figs.3and4).ComparingwiththeBETsurfaceareaofpureZnO(6.40m2/g),thesurfaceareasofLa2O3-dopedZnOsamplesincrease,whichare10.07m2/g(0.5wt%La2O3),9.82m2/g(1.0wt%La2O3)and7.63m2/g(2.0wt%La2O3),respectively.ThemeasuredBETsur-faceareadecreasesasthedopingcontentofLa2O3increases.ItisclearthatallLa2O3-dopedZnOshowenhancedacetoneresponsesthanthatofpureZnOsample,whichisinaccordancewiththeirlargersurfaceareas,althoughanexactlinearrelationshipisnotobservedbetweenthesurfaceareasandtheirresponsevalues.ThisphenomenonrevealsthattheenhancedacetoneresponsecanbebenefitedfromtheincreasedsurfaceareasbyLa2O3-doping,whereas1.0wt%La2O3-dopedZnOdisplaysthehighestresponse

Fig.10.PLspectraofZnOloadedwithdifferentLa2O3contents,theinsetshowsrelationshipbetweenthedefectcontentsandLa2O3dopingcontents.

valueand0.5wt%La2O3-dopedZnOholdthelargestsurfaceareaimpliesthatthesurfaceareaplaysanimportantrolebutnotapreponderantoneontheresponseenhancement.

Inconclusion,theenhancedacetoneresponseofLa2O3-dopedZnOmightbeattributedtothecombinedactionsoftheaforemen-tionedfactors:formationofintrinsicdefectsandincreasedspecificsurfaceareasinducedbythedopingofLa2O3.Additionally,thecat-alyticactivityofrareearthoxide(La2O3inthiswork)mightalsodevotetotheincreasedacetoneresponse,sincetheycanacceleratethedehydrogenationandconsecutiveoxidationofhydrocarbons[30].

4.Conclusion

La2O3-dopedflowerlikeZnOnanostructurescomposedofnanorodscanbefabricatedviaco-precipitationmethodusingPEG400assurfactants.Gas-sensingpropertyoftheLa2O3-dopedZnOtotestgasisgreatlyenhancedcomparedwiththenoLa2O3-doped.Theresponseof1.0wt%La2O3-dopedZnOgassensorto10ppmacetonereaches7.6,andaresponseof54.1to200ppmacetoneisobtainedattheworkingtemperatureof350◦C,witharesponsetimeof8sonly,implyingthepotentialfordetectinglowgasconcentration.

Acknowledgments

ThisworkisfinanciallysupportedbyNationalNaturalSci-enceFoundationofChina(No.21271139),TianjinNaturalScienceFoundation(No.08JCZDJC18700).TheauthorswouldliketothankProfessorWei-PingHuangofNankaiUniversity(China)forhishelp-fuldiscussionregardingthegas-sensingmechanism.

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Biographies

Jian-QunHeisapostgraduateworkingintheareaofgassensorsformasterdegreeatTianjinUniversityofTechnology.HeobtainedhisB.Sc.degreeinPhysicsfromLudongUniversityin2009.

JingYingraduatedfromLiaochengUniversityandreceivedherB.Sc.degreein19.Herresearchinterestsareinthegrowthoffunctionalcrystalmaterialsandthepreparationofnano-materials.

DongLiuisapostgraduateworkingintheareaofgassensorsformasterdegreeatTianjinUniversityofTechnology.HeobtainedhisB.Sc.degreeinEnvironmentalSciencefromNorthChinaUniversityin2010.

Le-XiZhangreceivedhisPh.D.degreeinmaterialogyin2011fromInstituteofCoalChemistry,ChineseAcademyofSciences.Hiscurrentresearchisfocusedonsynthesisofsemiconductornanostructuresandtheirgas-sensingapplications.

Feng-ShiCaireceivedhisPh.D.degreein2012fromNankaiUniversity.Hiscurrentinterestisgas-sensingmaterialsfabricationandproperties.

Li-JianBieobtainedhisMaster’sdegreeinInorganicChemistryfromUniversityofScienceandTechnologyofChinain1991,andPh.D.degreeinInorganicChemistryfromPekingUniversityin2002.HeisnowaprofessorinTianjinUniversityofTech-nology,leadingagroupinresearchforthesynthesisandpropertyofnano-materialsandperovskite-relatedmaterials,includingtheirapplicationinsensors.