hollow, porous, and yttrium functionalized zno nanospheres with enhanced

c o m / l o c a t e / s n b Weiwei Guoa, Tianmo Liua,∗, Rong Sunb, Yong Chena,c, Wen Zenga, Zhongchang Wangc,∗ a College of Materials Science and Engineering, Chongqing University, Chong

j o u r n al hom e 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

Weiwei Guoa, Tianmo Liua,∗, Rong Sunb, Yong Chena,c, Wen Zenga, Zhongchang Wangc,∗

a College of Materials Science and Engineering, Chongqing University, Chongqing, China

b Institute of Engineering Innovation, The University of Tokyo, 2-11-16 Yayoi, Bunkyo-ku, Tokyo 113-8656, Japan

c WPI Research Center, Advanced Institute for Materials Research, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai 980-8577, Japan

Article history:

Received 20 June 2012

Received in revised form

18 December 2012

Accepted 20 December 2012

Available online 28 December 2012

Keywords:

ZnO

Nanospheres

Gas sensor

Yttrium doping

WereportthesynthesisofahierarchicalnanostructureofhollowandporousZnOnanosphereswitha highspecificsurfaceareaasanovelsensingmaterialtotoxicformaldehydebyasimpletemplate-free hydrothermaltechniqueinorganicsolution.Wedemonstratethattheliquidmixtureratioand hydro-thermaltimeplayapivotalroleinformingsuchuniquemorphologyandproposeagrowthmechanism

ofOstwaldripeningcoupledwithgrainrotationinducedgraincoalescence.Comparisoninvestigations revealthatyttriumallowsresistancereductionofsensorsandenhancessignificantlygas-sensing per-formancesofZnOnanospherestowardtheformaldehydeoverthecommonlyusedundecoratedZnO nanoparticles.Suchhollow,porous,andyttriumfunctionalizedZnOnanospherescouldthereforeserve

ashybridfunctionalmaterialsforchemicalgassensors.Theresultsrepresentanadvanceofhierarchical nanostructuresinenhancingfurtherthefunctionalityofgassensors,andthefacilemethodpresented couldbeapplicabletomanyothersensingmaterials

© 2012 Elsevier B.V All rights reserved

1 Introduction

Inorganicnanomaterialswithhollowandporous

superstruc-turesfindnumeroustechnologicalapplicationswhere

morpholo-gies are known to influence functionality Gas sensors [1–3],

catalysts [4,5], drugdelivery carriers [6,7], and photoelectronic

buildingblocks[8–10]arejustafewsignificantexamples.In

gen-eral,themorphologywithalargespecificsurfaceareaandefficient

porosityisoftenbeneficialforthecatalytic,gas-sensingand

pho-tovoltaic applications due to thelikelihood toenhance surface

reactions.Inthisrespect,theactivesearchofunusualmorphology

iscurrentlythesubjectofintensiveresearchinthenanomaterials

world[11,12].Oneofthemostwell-characterizednanomaterials

intermsofmorphologyisZnO,whichisann-typesemiconductor

withadirectwidebandgap(3.37eV)andalargeexcitationbinding

energy(60meV)[13,14].Todate,asubstantialamountof

exper-imentshave already provided definitiveevidence thatsize and

morphologyofZnOnanomaterialscanaffectgreatlytheir

perform-ances,especiallygas-sensingfunctionality[15–17].Ontheother

hand,dopingZnOwithvariouselements,e.g.,noblemetals[18–20],

rare-earthmetals[21],transitionmetals[22],andmetaloxides[23]

∗ Corresponding authors Tel.: +81 22 217 5933; fax: +81 22 217 5930.

E-mail addresses: tmliu@cqu.edu.cn (T Liu), zcwang@wpi-aimr.tohoku.ac.jp,

wang@cello.t.u-tokyo.ac.jp (Z Wang).

hasbeensuspectedtoenablemodulationofsurfacechargestates

ofZnO,modifyingsignificantlyitsfunctionality

Ageneralapproachtodatetofabricatenanomaterialswiththe hollowandporousmorphologiesaccompaniestheuseof remov-ableorsacrificialtemplates,includingeitherthehardonessuchas monodispersesilica[24],polymerlatexspheres,[25,26]and reduc-ing metalnanoparticles[27],orthesoft onessuchas emulsion micelles[28]andgasbubbles[29].Thedisadvantagesfortheuse

oftemplatesthoughrestwiththehighcostandtedious synthe-sisprocess,posingasignificanthurdletothelarge-scaleindustrial applications.Ideally,one wouldpreferaone-step template-free methodtosynthesizethenanomaterialswithhollowandporous superstructuresina sizetunablemanner.Recentbreakthroughs

in the fabrication of nanomaterials by taking full advantageof knownphysicalphenomena,e.g.,orientedattachment[30,31], Ost-waldripening[32–34],Kirkendalleffect[35,36],andetching-based inside-outevacuation[37,38],hasbroughtsuch“ideal” concept closertoreality.Amongallthefabricationtechniques,theetching processhasbeendemonstratedasafacilechoiceforpreparing hol-lowandporousnanomaterialsbecauseitiseasytodissolveinner nano-crystallitesviaadjusting processingtimeand temperature

[39–41] Here,wereportatechnicallysimpleandflexibleroute:theuse

of a template-freehydrothermal process topreparethehollow andporousZnOnanosphereswithalargespecificareaina con-trollablemanner.Weinvestigateindetailtheeffectoftheliquid mixtureratioandhydrothermaltimeonthemorphologyevolution

0925-4005/$ – see front matter © 2012 Elsevier B.V All rights reserved.

nucleationandself-assemblyofZnObuildingblocks,i.e.,coupling

ofOstwaldripeningwithgrain-rotation-inducedgraincoalescence

(GRIGC).Such a uniquemorphologyis maintainedafterdoping

yttrium(Y)toproducehybridfunctionalityofZnOasagas-sensing

material,which, to thebestof ourknowledge, hasrarelybeen

reported.OurresultsdemonstratethatY-dopedZnOnanospheres

lowerremarkably resistanceandenhance gas-sensing

perform-ances,whichmayopenupanewavenuetodevelopadvancedgas

sensors

2 Experimental

AllZnO nanosphereswere synthesizedby thehydrothermal

method Zinc acetate dehydrate (Zn(CH3COOH)2·2H2O) (4mM)

wasfirstdissolvedintoamixedsolutionofethanol(40mL)and

monoethanolamine(MEA)(30mL)undermechanicalstirringfor

1h.Thesolutionwasthentransferredinautoclaves,whichwere

heated to160◦C for24htoproduce precipitate.The pureZnO

powder was prepared by centrifuging the precipitate, washing

it with distilled water and ethanol to remove unwanted ions,

anddryingat60◦Cinair.Theobtainedpowder(0.03g)was

dis-persedindeionizedwater(20mL),and1.5mLmixedsolutionof

ethanolandyttriumnitratehexahydrate(N3O9Y·6H2O)(0.01M)

wasthenadded.Thesolutionwasstirredthoroughlyfor1hand

driedat80◦C inairbeforeannealingat400◦C for2hto

elimi-nateNO3 ions.TheY-dopedZnOpowderwithamassratioofY

toZnof4%washarvested.Tomakea straightforward

compari-son,theZnOnanoparticleswerealsopreparedbydissolving4mM

Zn(CH3COOH)2·2H2Oand20mMNaOHin70mLdistilledwater,

whichwasthentransferredinautoclavesandheatedat160◦Cfor

20h

Microstructureanalysis wasconductedby theX-ray

diffrac-tion(XRD),scanningelectronmicroscopy(SEM),andtransmission

electron microscopy (TEM) For the XRD, a Rigaku

D/Max-1200X diffractometry with Cu K˛ radiation operated at 40kV

and 200mA was applied Surface morphologies of the

sam-pleswereobservedusingaHitachiS-4300SEM.Microstructures

and chemical composition were analyzed using the JEOL

JEM-2010Felectronmicroscopeoperatedatanacceleratingvoltageof

200kV.Specificsurfaceareawasmeasureduponthemultipoint

Brunauer–Emmett–Teller (BET) analysis of nitrogen adsorption

isotherms, which were recorded on a surface area analyzer

(Micromeritics,ASAP2020M)

Thepowdersupon harvestweremixedwithdiethanolamine

andethanoltoformpastes,whichweresubsequentlycoatedonto

analuminaceramictubepre-loadedwithapairofgoldelectrodes

ateachend Next,thetubewasdriedat400◦C for2hinorder

to eliminate organic binder as well as strengthen the bonding

betweenthepastesandtube.ANi–Crwirewasplacedinsidethe

tubeasaheater.Theheatingwireandtubeweresolderedonthe

pedestalstofabricategassensors.Thesensorswerefinallyaged

at200◦Cfor240hin ordertoimprove stabilityand

repeatabil-ity.Gas-sensingmeasurementswereconductedusingacomputer

controlledmeasurementsystem(HW-30A,HanweiElectronicsCo.,

Ltd.)atroomtemperatureatahumidityof40%.Thesensorwas

firstconnectedtothecircuitboardofmeasurementsystem,and

thenthetestedgaswasintroducedintotheglasschamberthrough

injectingagivenamountofgas.Theoperatingtemperatureof

sen-sorscanbeadjustedpreciselyviaalteringthecurrentflowacross

theNi–Crheater.Resistance(Rs)ofthesensorswasestimatedby

Rs=RL(Vc−Vout)/Vout,wheretheRLwasresistanceofaloadresistor

(RL=47k), and theVc and Vout were circuitand output

volt-age(Vc=6V),respectively.Thesensorresponse(S)wasdefinedas

S=R /R atreductiveatmosphere,whileasS=R /R atoxidative

Fig 1. XRD spectra of Y-free and Y-doped ZnO nanospheres with a series of Y/Zn ratios Textural orientations of detected matters are given as well for easy reference.

atmosphere,whereRaandRgwereresistanceinairandtargetgas, respectively.Theresponseandrecoverytimewasdefinedasthe intervalbetweenwhenresponsereached90%ofitsmaximumand droppedto10%ofitsmaximum

3 Results and discussion

3.1 Chemicalcompositionandmorphology

Toidentifychemicallythepreparedsamples,wefirstconducted XRDanalyses,asshowninFig.1,wheretexturalorientationofthe detectedmattersisshownaswellforeasyreference.FortheZnO, 2%and4%Y-dopedZnOsamples,allofthepeaksareidentifiedas belongingtothewurtzite(hexagonal)structureofZnO(JCPDS (36-1451)).Nosecondaryphaseisdetectedalthoughthelatticeofthe Y-dopedsampleisfoundtobesomewhatexpandedascompared

totheY-freesample.InadditiontoZnO,Y2O3isalsodetectedinthe 6%and8%Y-dopedZnOsamples.ThissuggeststhattheYatomsfill thelatticesitesofZnOatthelowdopingconcentration,buttend

toformanewY2O3phaseathighdopingconcentration(over6%) However,therearenocharacteristicsecondary-phaseXRDpeaks

inthe2%and4%Y-dopedZnOsamples,indicatingthatthe sec-ondaryphase isveryscarceorhighlydispersed.Thisisbecause thereappearwelldefinedXRDpeaksifsizeofthecrystallitesis above1–3nm[42].Thiscaseisalsorecognizedintwo-phase sys-tems,inwhichthesecondaryphasewithasmallconcentrationis highlydispersedonsurfacesofthebasicoxide’sgrains.These indi-catethatthesecondaryoxidephase,ifhave,shouldhaveasmaller grainsizethanthebasicoxideinthesampleswithYdoping con-centrationsof2%and4%.Whenthedopingconcentrationisover 6%,anewY2O3phaseisformedwithagrainsizelargerthan3nm

Table1liststhelatticeconstantsofboththeundopedandY-doped samplesobtainedfromXRDdataandthecrystallitesizecalculated usingtheScherrerformula.Thelatticeconstants(aandc)andgrain sizesincreasewiththeriseoftheamountofY,suggestingthatthe introductionofYdistortsthecrystalstructureofthehostoxide

Table 1

The lattice constants of the Y-doped ZnO sphere and ZnO nanoparticle, and grain sizes calculated using the Scherrer formula.

ThisisbecausetheradiusofY3+ion(0.92 ˚A)islargerthanthatof

Fig.2 SEMimagesofrepresentative regionsin thepristineand

doped nanospheresand the nanoparticles.Asseen in Fig 2(a),

theY-freesampleisindeedcharacterizedasnanospheres,which

areuniformlydistributed.Thesenanospheresarecoarseon

sur-face(Fig.2(b)and(c))andhollowinside,asclearlyverifiedina

brokennanosphere(Fig.2(d)).Interestingly,thenanospheresare

self-assembledtoradiallyalignednanorodsof∼150nminlength

fromtheircoresyettoself-wrappedirregularnanoparticlesatthe

cores.Poresturnuponthenanospheresurfaces,indicatingthatthe

as-synthesizedpristinesamplesarenotonlyhollowbutporous

Suchahierarchicalmorphologyarefurthercorroboratedfromthe

TEMimagesshowingadifferenceintheimagecontrastbetweenthe

marginandcenterofnanospheres,i.e.,thecenterseemsbrighter,

whichindicatestheformationofthewell-definedhollow

nano-structures(Fig.3(a)).Fig.3(b)and(c)givesTEMimagesofedge

regionsofthenanospheres,whichshowunambiguouslythepores

(Fig.3(b)),nanorods,andnanoparticles(Fig.3(c)).Fig.3(d)presents

ahigh-resolutionTEM(HRTEM)imageofanedgeofananosphere

(only theedge is likely for imaging due to thelargethickness

awayfromsurface),fromwhichlatticefringesareclearlyvisible

Thespacingbetweenneighboringlatticeplanesisestimatedtobe

∼0.26nm,inlinewiththatbetweenthe(0001)planesofa

hexag-onalZnO(insetofFig.3(d)),suggestingthattheZnOnanorodsgrow

inthe[0001]direction

Suchinterestinghollowandporousnanospheresarenot

dis-turbedsignificantlybyYdoping(Fig.2(e)–(g)),althoughtheirsize

becomeslargerduetothegrowthduringpost-annealing.Fig.2(h)

showsthemorphologyofnanoparticlesforacomparison,which

areaccumulatedwithameansizeof∼50nm.Fig.3(e)–(g)presents

TEMimagesoftheY-dopedsamples,wheretheyretaintheporous

andhollownature.Likewhatwasseeninthepristinesample,the

nanorodsalsogrowinthe[0001]directionevenwhentheYis

doped.Toidentifychemicallythesamples,weperforman

energy-dispersiveX-rayspectroscopy(EDS)analysisofarepresentative

nanosphereinthepristineand4%Y-dopedsample,asshownin

Fig.3(h).Thenanospheresinthepristinesamplearecomposedof

40.4at%Oand59.6at%Zn,whilethoseinthe4%Y-dopedsample

34.21at%O,63.78at%Znand2.01at%Y,demonstratingthatthe

embeddedadditiveofYisreallypresentintheZnOmatrix.Further

EDSmappingofboththeentiresphereandtheedgerevealsaneven

distributionofO(Fig.3(j))andZn(Fig.3(k)),providingdirect

evi-dencetotheuniformdistributionofYinthedopedsample(Fig.3(l))

andfurthertestifyingthesecondoxidephaseispresentinthe4%

Y-dopedZnOmatrix,inconsistencewiththeXRDresults

3.2 Formationmechanismofhollowandporousnanospheres

To gain insight into formation mechanism of the

hierarchi-calnanostructuresandhowmorphologyevolveswithprocessing

conditions,wefirstinvestigatesystemicallytheroleofsolvent

com-positiononthestructuresofnanomaterials.AsseeninFig.4(a)

and (b), the ZnO nanorodsare clustered when the MEAis not

introduced.OncetheMEAisadded(5mL),thenanorodsare

bun-dledirregularlyandlooselywithameanlengthof500nm(Fig.4(c)

and(d)).FurtherincreaseintheMEAconcentration(15mL)renders

thesebundlesself-assembledtofan-shapedhemispheres(Fig.4(e)

and(f)).Thehollowandporousnanospheresareformedwhenthe

concentrationofMEAisincreasedfurtherto30mL(Fig.4(g)and

(h)).Thenanospheresbecomedenserwithfewerholesonsurfaces

astheMEAconcentrationisincreasedto40mL(Fig.4(i)and(j))

However,thenanospheresarenonporousandsolidwhenthe con-centrationofMEAisincreasedto50mL(Fig.4(k)and(l)),implying thattheprecisecontroloftheMEAconcentrationisessentialto producingahierarchicalsuperstructure

Theformationofnanorodsinthe[0001]directionwithoutMEA

isunderstooduponthestructuralanisotropyandsurfacepolarityof ZnO.The(0001)polarplaneisthemostenergeticallyunfavorable andbearsthehighestgrowthrate,followedby(1011),(1010), (1011),and(0001)planes(insetofFig.3(d))[43,44].Oncethe MEAisintheethanolsolution,thecoordinated[Zn(MEA)m]2+ions (wheremisaninteger)aregenerated,restrainingtheformation

offreeZn2+ionsandtheZn(OH)2,thenucleiofZnOnanomaterial ThechemicalreactionsinpresenceofMEAduringhydrothermal processcanbeexpressedas:

Zn(OOCCH3)2·2H2O+2C2H5OH →Zn(OH)2+2H2O

Asthetemperatureisincreasedintheautoclaves,the[Zn(MEA)m]2+

ionsarereadytobedecomposedtoZn2+ionsandethanolamine molecules (Eq.(1)).Simultaneously,there occursthe esterifica-tion of zinc acetate withethanol toproduce Zn(OH)2 (Eq (2)), whichisultimatelydecomposed toZnOnanomaterials(Eq.(3)) Theethanolaminemolecules,whichareadsorbedonthesurfacesof ZnOnuclei,canserveasassemblingagents,refrainingcrystalsfrom formingnanorodsalongthe[0001]direction[45].Themetastable nanoparticlesareproducedinsteadattheinitialnucleationstage, whichisimportantforthenext-stageOstwaldripeningprocedure Twofactorsareresponsiblefortheevolutionofmorphologyat solvothermalcondition:theinitialnucleationstatusandthe solu-bilityofprecursorsinsolventundersaturationvaporpressure[46] NotethatthesolventMEAislowerthanethanolinthesaturation vaporpressureduetoitshigherboilingpoint(78.29◦Cforethanol while170◦CforMEA).Thisgivesrisetoextensivenucleationof metastablenanoparticles,whichareaggregatedintonanospheres

tolowertheirsurfaceareasandenergies[47].Inadditionto form-ingthesphericalconfiguration,theMEAalsoplaysapivotalrole

inmakingthenanosphereshollowandporous.Incontrasttothe fastmigrationandhighnucleationrateofthereactivespeciesin ethanol,itiskineticallyslowertoformmetastablenanocrystalsin MEAsolutionduetothehigherboilingpointandviscosityofMEA Thisallowsthemixtureofnanocrystalswithvaryinggrowth orien-tationstoassembleintosphericalnanoparticles.Ontheotherhand, theMEAisabletofacilitatetheformationofmetastable nanoparti-cles,theinteriorsofwhicharesusceptibletobedissolved,thereby producingthehollownanospheres

To shed more light ontheformation mechanism of hollow, porousnanospheres,weconductaseriesoftime-dependent inves-tigations,asshowninFig.5.Attheearlystage(4h),solidspherical nanoparticles (Fig 5(a) and (b)) are formed (Fig 5(c)) As the reaction time is increased to 8h, hollowing process starts at the nanosphere cores (Fig 5(d) and (e)), and the surfaces of nanospheres turn rough(Fig.5(f)),indicating that a portionof particlesonsurfacesaredissolved.Furtherextensionofreaction time(16h)enhancesthehollowingeffect(Fig.5(g)and(h)),and thenumerousnanorodswithporesonsurfacesareassembledto nanospheres(Fig.5(h))duetothedissolutionand recrystalliza-tion(Fig.5(i)).Thehollowandporousnanospheresareformedas thereactiontimeis24h.However,thereemergeurchin-like struc-turescomprisingalargeamountofnanorodswithasmallnumber

ofnanoparticles(Fig.5(j))asthereactiontimeis30h(Fig.5(k)and (l)).Interestingly,mostofthenanoparticlesaredissolvedwhenthe

Fig 2. SEM images of the pristine ZnO nanospheres taken at (a) low and (b) high magnification (c) and (d) Magnified SEM images of an open hollow and porous ZnO nanosphere SEM images of the Y-doped nanospheres taken at (e) low and (f) slightly higher magnification (g) Magnified SEM image of an open nanosphere in the Y-doped sample (h) SEM image of the nanoparticles.

reactiontimeisincreasedto35h,leavingbehindslimnanorods

(Fig 5(m)and (o)).Thedisappearanceof thenanospheres

sug-geststheimportantroleofthenanoparticlesinthestabilization

ofnanospheres(Fig.5(n))

Theseimplysuchamechanism:theOstwaldripening[48]

cou-pledwiththegrainrotationinduced graincoalescence(GRIGC)

[49] The Ostwald ripening involves the aggregation of nano-crystallites,followedbyanoutwardmasstransfertoformhollow structures.TheGRIGCprocessoccurswhenparticlescollide,and thegrainrotationtakesplacethereafter.Suchgrainrotation low-ers the energy of system and eliminates the grain boundaries, producing single-phase nanocrystals (i.e., coalescence process)

Fig 3. (a) TEM image of the Y-free ZnO sample, highlighting that the nanospheres are hollow (b) and (c) Enlarged TEM images of the Y-free ZnO nanospheres on edge (d) HRTEM image of a Y-doped ZnO nanosphere (e) TEM image of the Y-doped ZnO (f) and (g) Enlarged TEM images of the doped sample on edge (h) EDS for the pristine (upper) and doped (lower) ZnO nanomaterials The horizontal axis denotes the energy and the vertical one the counts (i.e., intensity) (i) Original area and EDS mapping of (j) O, (k)

Fig 4. SEM image of the ZnO nanospheres prepared under different concentrations of monoethanolamine (MEA): (a) and (b) 0 mL, (c) and (d) 5 mL, (e) and (f) 15 mL, (g) and (h) 30 mL, (i) and (j) 40 mL, (k) and (l) 50 mL The amount of added ethanol is fixed to be 40 mL.

Fig 6 shows schematically formation evolution of the hollow,

porousnanospheres.Attheinitialstage,theZnOnanocrystalsare

generated randomly.Asthereactiontime isincreased,theZnO

colloids are aggregated to form solid nanospheres through the

Ostwaldripeningeffect,whichisdrivenbytheminimizationof sur-faceenergy.Sincecrystalliteshaveahighersurfaceenergyinthe interiorsthanonthesurfaces,theyaremorereadilytobedissolved Oncebeingheated,thenano-crystallitesareeasiertobecollided

Fig 5.SEM image illustrating evolution of morphology of the nanospheres with the reaction time: (a–c) 4 h, (d–f) 8 h, (g–i) 16 h, (j–l) 30 h, and (m–o) 35 h Three images

Fig 6.Schematic illustration of morphology evolution of the ZnO nanospheres.

and rotated,givingrise tocoalescenceofneighboring grains to

formlargesingle-phasegrains.Sucha processlowersthe

inter-facialenergyassociatedwithlargeinterfacialarea.Forthepolar

ZnO,the(0001)planeis mostlikely tobecoalesceddue toits

highestenergyofallplanes,whichexplainstheobservationthat

ZnOcrystallitesaregrowninthe[0001]directiontoproducethe

rod-likeZnOintheshellofhollownanospheres.Meanwhile,the

rotationandmigrationofparticlesinducepores,whichresultsin

thehollowandporousmorphology

3.3 Resistanceasafunctionoftemperature

To gain more insights into surface properties of the

nanospheres,weshowinFig.7nitrogenadsorption–desorption

isothermandsizedistributionoftheporescalculatedusingthe

Barret–Joyner–Halenda(BJH)method.Carefulanalysisoftheplot

identifiestheisothermasatypeIVone,indicatingtheformation

of typicalporousstructure (Fig.7(a)) Although thepore spans

a largerange in size,themajority of poreshave a diameterof

5–15nm,inaccordwiththeaboveTEMobservations(Fig.7(b))

Thespecificsurfaceareaofthenanospheresreaches89.5m2g−1

measuredusingtheBET,confirmingtheporousnature.Itshould

benotedthatthespecificsurfaceareaofthenanoparticlesisonly

24.2m2g−1,whichindicatesthatthemorphologyaffectsgreatly

thespecificsurfacearea

Such a 3D hierarchicalporousnanostructure may hold

sub-stantialpromise for a wide range of applications,especially as

a chemical sensor owingtothe largespecificsurface areathat

cangreatlyenhancegasdiffusionandmasstransport.Extensive

efforthasbeendevotedtodatetoimprovinggas-sensing

prop-ertiesofZnO,includingthefastresponseandrecoveryandhigh

gasresponse Among them, the dopingof rare-earth elements,

e.g.,Y,hasbeendemonstratedasoneofeffectivewaystoactivate

hostmaterials,andmayenablefictionalizationofthehierarchical

nanospheresaswellforadvancedfunctionalgassensors

Totestthisscenario,wefirstpresentinFig.8(a)theresistance

(R)asafunctionoftemperature(T)forthesensorsfabricatedwith

theY-freeandY-dopednanospheresinairtogetherwiththe

sen-sormadeofpristineZnOnanoparticles(Fig.2(g)).Overallfeatureis

differentbetweensamplesandthesampledopedwith4%Yhasthe

lowestresistance.Theresistancedecreaseswithincreasingamount

ofY,butsuchadecreasecomestoahaltwhenthedoping

concen-trationofYisbeyond4%.Carre ˜noetal.reportedtheformationof

asecondphase,Sn2Y2O7,intheSnO2dopedwithasmallamount

ofY[50].Likewise,asimilarsecondphaseofZnYmOnmaybe

pro-ducedinoursampleswhenthedopingconcentrationislowerthan

4%.Thesecondphasehasalowresistance,providingconducting

channelsinthesampleandhenceloweringthecontactresistance

oftheZnOgrains.ThismayreducetheresistanceofZnOsample

However,whenthedopingconcentrationisabove4%,thedoped

YinZnOreachessaturation,formingY2O3precipitatesthatgrow

alongtheZnOgrainboundary.TheY2O3 hasahigherresistance

thanthematrix, thereby increasingthecontact resistance.This

consequentlysuppressesthedroppingofoverallresistance signif-icantly,thatis,theresistanceisincreased

AnotherkeyfeatureinFig.8(a)isthatresistancedropsinaless abruptmannerfortheY-doped(Y/Zn=4%)thanY-freeZnOatthe temperaturerangingfrom300to450◦C,whichcouldbeattributed

tothechemisorbedOonsurfaces.However,thereversiblereactions takeplaceamongOgas(gas),chemisorbedO(ads),andlatticeO (lat)withtheriseoftemperature[51]:

1O2+e−⇔O−ads, (5)

1

2O2+2e−⇔O2−ads, (6)

Theseconcludeintuitivelythatelectrontransferfrom semiconduc-tortoabsorbedOisresponsiblefortheincreaseofresistance.Ithas beenreportedthatpureZnOmaterialsexhibitn-type semiconduc-torcharacteristicsduetotheexistenceofoxygenvacancies[52] FromtheEDSanalysis,wefindthattheO/Znratiodecreasedfrom 67.7%(ZnO)to53.6%(4%Y-dopedZnO).Thefeweramountsof oxy-genandzincintheY-dopedZnOrevealtheincreaseofdefectswith theintroductionofYintheZnOnanospheres.Meanwhilethe asso-ciatedincreaseinlatticeconstantgivesrisetoincreasedintrinsic defects,e.g.,VO•,VO••,andO//i [53].Duringthehydrothermalprocess, defectscanbeproducedandfurtherenhancedbythedopingofY

intheZnOnanospheres.ItisworthnotingthatZnOhasa hexago-nalclose-packedlatticewitharelativelyopenstructureinwhich

Znatomsoccupyhalfofthetetrahedralsitesandallthe octahe-dralsitesareempty.Ingeneral,theoxygenvacancy(VO••)haslower formationenergythanthezincinterstitials(Zn••i ),resultingin Zn-richcompositionsintherealwurtziteZnO[52].Inthissense,the intrinsicdefectsandextrinsicdopantscanbeintroducedduring thefabrication.XualsopointedoutthattheO2moleculesinteract stronglywithoxygenvacanciesonthesurfaceofZnO[54].These implythattheYdopingcanincreasetheconcentrationofOvacancy andhenceabsorbmoreoxygenontheZnOsurface,whichasaresult increasestheconcentrationofO−

3.4 Gas-sensingperformance

Togain insightintogas-sensingpropertiesof theZnO nano-structures,wepresentinFig.8(b)gasresponsetoformaldehyde (HCHO)gasas afunctionof temperatureat50ppm.TheY-free nanospheresshowahighergasresponse(maximumvalueof47.4

at350◦C)thantheundopednanoparticles,indicatingthat mor-phologyiscriticaltotheenhancementofgas-sensingfunctionality Evidently,responseofZnOnanospheresisimprovedwiththe addi-tionofY.However,gasresponseissaturatedtoamaximumvalueof 65.7whentheYconcentrationreaches4%.Furtherincreaseinthe Y-dopingconcentrationresultsinanadverseeffect,i.e.,lowersthe gasresponse.TheY-dopednanosphereshavealoweroptimal tem-perature(300◦C)thantheY-freeones(350◦C).Theenhancement

Fig 7.(a) Nitrogen adsorption–desorption isotherm and (b) corresponding pore size distribution of the ZnO hollow and porous nanospheres.

ofgas-sensingpropertiesbyYdopingcanbeunderstoodas

fol-lows.Inthepristinecase,theOmoleculesareadsorbedonsurfaces

andcaptureefromtheZnOsemiconductor,formingchemisorbed

Ospecies(Eqs.(7)–(10)).Suchprocessgivesrisetosurface

deple-tionlayers,whicheventuallyincreasesresistanceofthesamples

WhenbeingexposedtotheHCHO,theHCHOmoleculesreactwith

theadsorbedOonsurfacesinthefollowingmanner:

HCHO(gas)+2O−ads⇒CO2+H2O+2e− (11)

Thisprocessreleasesthetrappedelectronsbacktoconductionband

ofZnO,increasingtherebyconcentrationofcarriersinthe

semi-conductor[55,56].TheintroductionofYinducesoxygendefects

inZnO,increasesconcentrationofO−adsandhenceimprovesgas

response On the other hand, the low optimal operating

tem-peratureaftertheYdopingcanbeascribedtotheformationof

weaklybondedcomplexesZnYmOn.Thechemisorptionofoxygen

speciesdependsstronglyonthetemperatureandnatureof

mate-rial.TheO2ischemisorbedatlowtemperaturewhileO−andO2−

arechemisorbedathightemperature.SinceZnOisa

semiconduc-tor,oxygenabsorptionandelectrontransferareratherdifficultto

occuratroomtemperature.Thethermalactivationofthe

semicon-ductorisrequiredtoobservegasadsorptiononsurface.Thisiswhy

changeinresistanceisnotobservedwhentheZnOnanospheres

areexposedinthereducedgases.However,thelow-temperature

gasadsorptionbecomespossiblebytheYdopingduetothe

pres-enceoftheweaklybondedcomplexesZnYmOnontheZnOgrain

surface.TheabsorptionofoxygenionscanoccurontheZnO

sur-faceatroomtemperatureduetothehighconductingnatureofthe

ZnYmOn.Inthisrespect,theYactivatesreactionsofHCHOtothe

adsorbedOduetothespillovereffect[57–59],resultinginalower

optimaloperatingtemperature

Fig.9showsresponse–recoverycharacteristicsforthethree

sen-sorsfabricatedwiththepristinenanoparticles,Y-freeandY-doped

nanospheresatdifferentoperatingtemperatures.Six

representa-tivespeciesofvolatileorganiccompound(VOC)gasesarechosen

purposely,includingCH4,NH3,HCHO,CH3OH,CO,and(CH3)2CO

Thegasconcentrationisfixedto50ppm.Thevoltageisincreased

sharplywhen thetestgasis in,yetreturnstoits originalstate whengasisout.Thekeydifferenceamongthethreesamplesis thatthevoltageisincreasedinthemoststrikinglymannerforthe Y-dopedsample,verifyingagainthegas-responseenhancementby morphologyandYdoping.Moreover,theresponseandrecovery transientofthesesensorsissuperiortoHCHOthantotherestof thetestedVOCgases,especiallyintheY-dopedcase(Fig.9(a)–(c)) Uponcloserinspection,wefindthattheresponseandrecoverytime

is∼14and17sforthepristinenanoparticles,while∼10and12s fortheY-freenanospheres.Theyarefurthershortenedto∼4and

6sfortheY-dopednanospheres(Fig.9(d))

ToshedmorelightontheY-dopedsample,wefurthermeasure thegas-sensingpropertiesattheoptimaloperatingtemperatureof

300◦C,asshowninFig.10.Thegasresponseisincreased drasti-callyasthegasconcentrationisincreasedupto250ppm,yetina moregentlefashionastheconcentrationisincreasedfurther.The responseissaturatedat∼800ppm.Interestingly,thegasresponse

is increasedalmost linearlywhenthegas concentrationranges from10to100ppm(insetofFig.10(a)),implyingthattheY-doped ZnOnanomaterialworksevenatlowgasconcentration.Fig.10(b) showsgasresponseoftheY-dopedsensortothesixtypesofVOC gasesat50ppm.Theresultmadeclearisthattheresponsetothe HCHOreachesamaximumvalueof65.7butisnolargerthan16

toothergases.ThisimpliesthattheY-functionalizednanosphere canact asanefficientgas-sensingmaterialforon-site selective detectionof formaldehyde.Sincetheformaldehydehasa single aldehydeandhighreducibilityindetectinggases,theunsaturated

YionstendtoabsorbHCHOmolecules,formingcomplexspecies

ofY–HCHO[59].Simultaneously,theabsorboxygenonthesurface oxidizestheHCHOintoH2OandCO2,resultinginagood selectiv-itytoHCHOforthesensor.Fig.10(c)showsasingle-cycleresponse fortheY-dopedsensoratdifferentHCHOconcentrationsat300◦C Thevoltagesignal(i)isenlargedwiththeriseofHCHO concentra-tion,(ii)isstabilizedin4swhenthesensorisexposedintheHCHO atmosphere,and(iii)returnstooriginalstatein6soncethe sen-sorisexposedinair.Fig.10(d)presentsrepresentativereversible cyclesofthegasresponseinHCHO(50ppm),whereonecansee

Fig 8.(a) Sensor resistance of Y-free nanoparticles, Y-free and Y-doped nanospheres as a function of temperature in air (b) Response of the sensors fabricated with Y-free nanoparticles, Y-free and Y-doped nanospheres with various concentrations to HCHO of 50 ppm measured at temperatures from 200◦C to 500◦C Grain sizes of various ZnO

Fig 9. Response–recovery characteristic for the sensors fabricated with (a) Y-free ZnO nanoparticles, (b) Y-free and (c) Y-doped ZnO nanospheres Six types of VOC gases,

CH 4 , NH 3 , HCHO, CH 3 OH, CO, and (CH 3 ) 2 OH, are chosen purposely The operation temperature for the sensors fabricated with the Y-free nanomaterials is 350◦C and that for the sensor fabricated with Y-doped nanomaterial 300◦C The concentration of the tested gas is fixed to be 50 ppm (d) Single-cycle response and recovery transients of the three sensors to the HCHO gas at 50 ppm.

Fig 10.(a) Gas response of the sensor made of Y-doped ZnO as a function of HCHO concentration operated at 300◦C The inset highlights sensing characteristic at low gas concentration (b) Gas response of the sensor made of Y-doped ZnO to the six types of gases of choice The concentration of each gas is fixed to be 50 ppm and the operating temperature is 300 ◦ C (c) Single-cycle response of the Y-doped ZnO to the HCHO gas at different concentrations at 300 ◦ C (d) Typical response and recovery characteristic of the sensor fabricated with the Y-doped ZnO to the HCHO gas of 50 ppm at 300 ◦ C A few representative cycles are shown only, demonstrating stability of the Y-doped sensor.

thattheresponseandrecoverycharacteristicsarereproducedwell

withnoremarkableattenuation.TheseimplythattheY-dopedZnO

nanospherescanimprovesignificantlygas-sensingperformances

Suchimprovementcannotberealizedforthenanoparticlesand

hencetheY-dopednanospheresshallholdsubstantialpromisefor

thedevelopmentofapracticalsensordevicefortheon-site

detec-tionoftheharmfulHCHOgas

4 Conclusions

WehavefabricatedsuccessfullynovelhollowandporousZnO

nanospheresviathesimpletemplate-freehydrothermaltechnique

inorganicsolution,andinvestigatedthegas-sensingfunctions.We

demonstratethattheratioofMEAinsolutioniscriticalto

mor-phologybecauseitfacilitatesformationofmetastablenanoparticle,

restrains the growth of nanorods, and serves as

fundamen-talbuilding blocks for nanospheres.Systematic microstructural

studies reveal a coupling of the Ostwald ripening with the

grain-rotation-induced grain coalescence growth mechanism whichisresponsiblefortheformationofthehollowandporous nanospheres.Suchhierarchicalnanospherespossessalarge spe-cificsurfaceareaandcanbefunctionalizedwithYforadvanced chemicalgas-sensingapplication.Gas-sensingperformancetothe HCHOisfoundtobeenhancedinthedopedsamplewiththeY con-centrationof4%.ThisworkindicatesthattheY-dopedhierarchical structuresrepresentanimportantstepforwardtoexploringthe novelgassensorsforfutureon-sitedetectionofharmfulgases

Acknowledgements

ThisworkwassupportedinpartbytheNationalNaturalScience

ofChina(51202302)andChinaPostdoctoralScienceFoundation (No.2012M511904).Z.W.appreciatesfinancialsupportsfromthe Grant-in-AidforYoungScientists(A)(grantno.24686069)andthe ChallengingExploratoryResearch(grantno.24656376)

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12 (1987) 425–440.

Weiwei Guois currently a PhD candidate at the College of Materials Science and

Engineering, Chongqing University in China He is now engaged in the synthesis

and characterization of the semiconducting materials and in the investigation of

their gas sensing properties.

Tianmo Liuis a professor of College of Materials Science and Engineering at

Chongqing University in China since 2001 He received Dr Eng from Department

of Solid Mechanics, Chongqing University in 1999 His current research interest

involves functional materials for gas sensors and magnesium alloys He is now also

holding a group leader position at the National Engineering Research Center for

Magnesium Alloys at Chongqing University.

Rong Sunis currently a PhD candidate in the Institute of Engineering Innovation,

The University of Tokyo in Japan Her research interest involves the characterization

of materials using advanced transmission electron microscopy.

Yong Chenis currently a PhD candidate at the College of Materials Science and Engineering, Chongqing University in China, and also an exchange student at Tohoku University in Japan since 2011 He is now engaged in fabricating nanomaterials and

in characterization using advanced transmission electron microscopy.

Wen Zengreceived his PhD degree in material Science from Chongqing University in China He is currently a lecture at the College of Materials Science and Engineering, Chongqing University He is focusing on synthesis of low-dimensional functional materials, on fabrication of semiconducting sensors and on first-principles calcula-tions.

Zhongchang Wangis currently an assistant professor at the WPI Research Center, Advanced Institute for Materials Research, Tohoku University in Japan He received his master degree in 2004 from Chongqing University in China and PhD in 2008 from the University of Tokyo in Japan He is now mainly focusing on gas-sensing materials, interfaces, grain boundaries, dislocations in oxides, and quantum electron transport by combining the state-of-the-art transmission electron microscopy with the first-principles calculations.