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Peashell-likenanostructure—anewkindofone-dimensionalnanostructure:thecaseofmagnesiumoxidew

JunLiu,abWenjunWang,aZaipingGuo,bRongZeng,bShixueDoubandXiaolongChen*a

Received1stMarch2010,Accepted12thApril2010

FirstpublishedasanAdvanceArticleontheweb6thMay2010DOI:10.1039/c0cc00167h

Anoveltypeof1Dnanostructurewithcharacteristicsbetweenthoseofasolidnanowireandahollownanotube,i.e.apeashell-likenanostructurecomprisinga1Dwirewithperiodicallyembeddedhollownanobubbles,ispresentedforthe rsttime;asanexample,MgOpeashell-likenanostructuresexhibitextraordinarydielectricandmagnetizationproperties.Sincecarbonnanotubeswerediscovered,1alargenumberoftraditionalbulkmaterialshavebeenfoundtoformone-dimensional(1D)nanostructures(suchassolidnanowiresandhollownanotubes),andtoexhibitinteresting1D-nanostructure-inducedpropertieswhichhaveledto1Dnanostructureswithawiderangeofapplicationsinnanoelectronics,optoelectronics,plasmonics,medicaldiagnostics,catalysis,drugdelivery,therapeutics,separation,andchemicalsensing.2–5However,mostofthese1Dnanostructuresaresolidnanowiresorhollownano-tubes.Therefore,itisstillnecessarytoseeknewkindsof1Dnanostructuresandexplorenewuniquepropertiesandapplications.Herein,wepresentanoveltypeof1Dnanostructurethatisdi erentfromsolidnanowiresandhollownanotubes,i.e.apeashell-likenanostructure,whichisa1Dwirewithperiodicallyembeddedhollownanobubbles.Thecombinationofsuchultra- nehollownanobubblesandthe1Dnanowiremayimpartsomenovelpropertiestothepeashell-likenanostructure.

Inthiscommunication,animportantdielectricmaterial,magnesiumoxide(MgO),6hasbeenselectedasanexampletosynthesizeapeashell-likenanostructureandshowsomeintriguingpropertiesofsuchanovelnanostructure.Thesynthesisstrategyinvolvesaself-catalysisvapor–liquid–solid(VLS)processinconjunctionwiththeKirkendallprocess.Duetotheirultra- nediametersandperiodicallyembeddedhollownanobubbles,thelargelysynthesized1Dpeashell-likeMgOnanostructuresshowgreatlyenhanceddielectricfunction,30timeshigherthanthatofMgOmicropowdersinthelow-frequencyrange.Moreover,suchultra- ne,embedded,hollownanobubblesalsogivethepeashell-likeMgOnanostructuresa9timesstrongersaturationmagnetizationthanthatofMgOnanoparticles(22nm).Therefore,webelievethatthisnewtypeof1Dpeashell-likenanostructurenotonlyconfersthe

BeijingNationalLaboratoryforCondensedMatterPhysics,InstituteofPhysics,ChineseAcademyofSciences,Beijing100190,China.E-mail:chenx29@http://www.77cn.com.cn;Fax:+86-010-82649646;Tel:+86-010-82649036b

InstituteforSuperconductingandElectronicMaterials,UniversityofWollongong,NSW2522,Australia

wElectronicsupplementaryinformation(ESI)available:XRDpattern,TEMimagesofthetipmorphology,bright eldanddark eldTEMimagesofultra- ne1Dpeashell-likeMgOnanostructures,and eld-cooledandzero- eld-cooledhysteresisloopsof1Dpeashell-likeMgOnanostructures.SeeDOI:10.1039/c0cc00167h

capabilitiesofamulti-functionalnanomaterialonMgO,butcanalsoa ecttheproperties(andmayintroducenovelproperties)ofothermaterials,suchastransitionmetaloxides.TheX-raydi raction(XRD)patternofas-synthesizedproductscanbewellindexedtoaface-centeredcubiccellwith

,ingoodagreementwiththestandarddataa=4.2102A

(ICDD-PDF65-0476),con rmingthatthisproductisMgOwithaface-centeredcubicstructure,asshowninaSupportingInformation(SI)Figure(Fig.SI-1w).Fig.1demonstratesthattheproductcontainsalargeamountofuniformultra- ne1Dnanostructures.Eachnanostructurehasabigwhitetipwitharectangularshape(Fig.SI-2w).Thetransmissionelectronmicroscope(TEM)imageinFig.SI-3(a)windicatesthattheseultra- ne1Dnanostructuresare10–30nmindiameter.Eachnanostructurehasperiodicalbrightspotswhenimagedunderbright eldTEMconditions[Fig.SI-3(b)w].Thebrightspotsinvertcontrastandappeardarkwhenimagedunderdark eldTEMconditions[Fig.SI-3(c)w].TheTEMwascon guredtocollectallofthedi ractedelectronintensityfordark eldimaging,asshownbyalloftheMgOnanowiresappearingwhite.Thedark eldimageshowsthatthebrightspotsfromthebright eldimageappeardark,meaningthattheydonotgenerateanydi ractedelectronsintensity.Thiscon rmsthatthebrightspotsareduetoabsorptioncontrastandanincreaseintransmittedelectronintensity.TheincreaseintransmittedelectronintensitycanbeduetoamaterialimpuritylighterinatomicnumberthanMgO,orreducedthicknessoftheMgOnanowire.ConsideringtheabsenceofimpuritiesintheXRDpattern,itisproposedthatthebrightpointsareduetoreducedthickness.Todeterminewhetherthereducedthicknessisduetosurfacedepressionsorinternalpores(nanobubbles),aTEMimageclearlyshowingthenanowirestructureandahighresolutionTEM(HRTEM)areshowninFig.2(a)and(b),respectively.Itcanbeseenthattheatomarrayinthebrightpointsisthesameastheatomarrayinthedarkersurroundingareas.Nosunkenlayerstepscanbeseenaroundthewhitepoints,whichindicatesthatthesewhitepointsarenotumbilicationsonsurfacesofthenanowiresbutarenanobubblesinthenanowires.Suchnanobubblesincreasethetransmittedelectronintensityandappearasbrightpointsunderbright eldimagingconditions.

Inthepastdecade,theVLStechniquehasbeenwidelyappliedtosynthesize1Dnanowiresandnanotubes,inwhichliquidmetallicnanodropletsactascatalyststoadsorbanddissolvereactantspeciestoinducethenucleationandgrowthofnanostructures.7,8TheVLSmechanismhasanobviousfeature,i.e.eachas-prepared1Dnanostructurehasabigtip.DuetothebigtipsobservedintheTEMimagesoftheas-prepared1Dpeashell-likenanostructuresandtheabsence

Fig.1SEMofpeashell-likeMgO

nanostructures.

ofothermetallicdroplets,itislikelythatMgdropletshavefunctionedasacatalyst,inducingtheformationof1DMgOnanostructuresduringourexperiment.Inthe rststep,MgpowderisheatedtogenerateMgvaporsat9501Cunderargonatmosphere,andsomeMgvaporsarecondensedtoformliquidMgdropletsinthelow-temperatureregion.Then,oxygengasisintroducedtoreactwiththeMgvapors,sothatthereactantMgO,andtheunreactedMgvaporsandO2gascanbeadsorbedanddissolvedintheMgdroplets,asisshowninScheme1(A).UponsupersaturationintheliquidMgdroplets,MgOwillprecipitatefromtheedgeoftheMgdroplets(Scheme1(B)).Duetothefastoxidationrate,thegrowthofMgOisrelativelyfast,sothatpartoftheMgdropletdeformsintoacylindricalshape.Oncethiscylinderexceedsacertainlength,itbecomesunstable,andtheMgdropletspincho periodicallywhiletheoxygensupplyismaintained,untiltheMgvaporsourcesareconsumed(Scheme1(C)–(E)).Itisworthmentioningthatnotallnano-wireshaveperiodicallyembeddeddropletsduetotemperature uctuationinthegrowthprocess.WhentheMgvaporsourcesareconsumed,theMgdroplettipswillbeoxidizedintoMgOtipswitharectangularshape.Afterthat,1DMgOnanostructureswithperiodicallyembeddedMgdroplets,Mg-in-MgOpeapods,areformed(Scheme1(F)).Thisissimilartocasesofother1Dmetaloxidenanowires,suchasAu-in-SiO2,9Au-in-Ga2O3,10andAu-in-MgO11peapodswithperiodicallyembeddedgoldcatalystparticles,whichhavealsobeensynthesizedbytheVLSprocess.Insteadofusinggolddropletsascatalyst,hereweusedMgdropletsascatalystinourexperiment.

Inthesecondstep,theinnerperiodicallyembeddedMgdropletswillout-di usethroughtheMgOwalltoreactwiththeoxygengasinthetubefurnace(Scheme1(G)).Becausethedi usionrateoftheMgatomsisfasterthanthatofOatoms,vacancy-inwarddi usionwilloccurtocompensatefortheunequalmaterial owandeventuallyresultsintheformationofhollownanobubbles(Scheme1(H)).Thisprocessistheso

Scheme1Schematicillustrationoftheformationofpeashell-likeMgOnanostructuresbyself-catalysisvapor–liquid–solid(VLS)processandKirkendalldi usionprocess.

Fig.2TEMimage(a)ofpeashell-likeMgOnanostructuresand(b)HRTEMimageofsingle

nanostructure.

calledKirkendalldi usionprocess,12,http://www.77cn.com.cningtothenanoscale,sincethesynthesisofhollownanocrystalswas rstreportedbyAlivisatosetal.,14extensionstohollownanostructures,suchashollownanospheres15,16andnanotubes,17haverecentlybeenreportedforthise ectivestrategy.

Thedielectricpropertiesoftheultra- ne1Dpeashell-likeMgOnanostructuresweremeasuredinthefrequencyrangefrom40Hzto10MHzatroomtemperature.Fig.3showstheroom-temperaturefrequencyspectraoftherelativedielectricconstant,er,forsamplesofultra- nepeashell-likeMgOnanostructuresandMgOmicropowders.Itcanbeseenthaterofthepeashell-likeMgOsampleincreaseswithdecreasingmeasurementfrequency,whichissimilartopreviousreportsforothernanomaterials.18,19Amazingly,theeroftheultra- nepeashell-likeMgOnanostructuresisenhancedby30timesoverthatoftheMgOmicropowders,especiallyatlowfrequencies.Fangetal.alsoreportedthattheerofMgOnano owersis4timeshigherthanthatofMgOmicropowdersatlowfrequencies.20Accordingtothetheoryofdielectricpolarization,21itisthespacechargepolarizationthatmainlycontributestotheenhanceddielectricbehaviorofultra- nepeashell-likeMgOnanostructures.Innanomaterials,thesmallersize,largerinterfacearea,andhigherdensityofdefectscaninducemorepositiveandnegativespacecharges.Whenthechargesaretrappedbythesedefects,spacechargepolarizationwilloccur.Thatistosay,nanostructuredmaterialswithsmallsizesandlargeinterfaceareascanbene tintermsofthespacechargepolarization,andanabundanceofdefects,suchasvacancies,dislocations,anddanglingbondsinnano-structuredmaterials,canalsoresultinenhancementofthespacechargepolarization.Therefore,itisnotdi culttounderstandthatourultra- nepeashell-likeMgOnanostructures,whichareonly10–30nmindiameterwithperiodicallyembeddedhollownanobubbles,havegreatenhancementofercomparedtothebulkMgOmicropowders.TheerenhancementoftheMgOnanostructuresoccursatlowfrequenciesduetothespacechargepolarizationappearinginthelow-frequencyrange.

FigSI-4(a)wshowstheresultsofmagnetizationmeasurementsperformedat305Konthepeashell-likeMgOnanostructures.ItcanbeseenthattheseMgOnanostructuresshowa

much

Fig.3Dielectricpropertiesofthepeashell-likeMgOnanostructuresandmicropowdersatroomtemperature.

strongersaturationmagnetization(0.01817emu/g),9timeshigherthanthatofthe22nmMgOnanoparticles(0.00204emu/g)whichwerereportedbyKumar.22Itiswellknownthatmagnetismcanarisefromsurfacedefects,whichdecreasewithincreasingparticlesizeduetothedecreaseinthesurfacetovolumeratio.MgvacancieshavealsobeenreportedtoresultinferromagnetismofMgOnanoparticles.23However,ourpeashell-likeMgOnanostructuresare10-30nmindiameterwithasimilarorlowersurfacetovolumeratiocomparedtothatoftheMgOnanoparticlesreportedbyKumar(22nm),andtheyshownoobviousenhancementofMgvacanciesduetothericherMgvaporduringsynthesis.Thissuggeststhatthestrongmagneticmomentsofthepeashell-likeMgOnano-structuresmaynotoriginatefromthesurfaceofthenano-structuresorfromMgvacancies,butoriginatefromtheperiodicallyembeddedhollownanobubbles,whichincludenanobubbleswithregularshapes:rhombohedral,cubic,andspherical(Fig.SI-2w).Ithasalsobeenreportedthatthecornersofsuchregularshapescanimpartstrongmagneticmomentstootherwisenon-magneticmaterials.24Ontheotherhand, eld-cooled(FC)andzero- eld-cooled(ZFC)hysteresisloopsweremeasuredat5K,respectively(Fig.SI-4(a)and(b)w).Thereisnoobviousshiftofthehysteresislooptowardnegativemagnetic eldsandnoobviousenhancedcoercivity,theso-calledexchangebias,indicatingthatthestrongmagneticmomentsmostlyoriginatefrominnernanostructures,i.e.hollowbubbleswhichincluderegularshapes,notfromthesurface.Thetwophenomenaindicatethattheperiodicallyembeddedhollownanobubblesarelikelytobemainlyresponsibleforthestrongmagneticmoments.

Inconclusion,anoveltypeof1Dnanostructurecombiningfeaturesofasolidnanowireandahollownanotube,apeashell-likenanostructurethatconsistsofa1Dwirewithperiodicallyembeddedhollownanobubbles,isreportedforthe rsttime.Takinganimportantdielectricmaterialasanexample,largeamountsofultra- ne1DMgOpeashell-likenanostructureshavebeensuccessfullysynthesizedbyaself-catalysisVLSprocessinconjunctionwiththeKirkendallprocess.Theultra- ne1Dnanostructureandtheperiodicallyembeddedhollownanobubblesimpartgreatlyenhanceddielectricresponses,30timeshigherthanthatofMgO

micropowders,tothesepeashell-likeMgOnanostructures.Moreover,theperiodicallyembeddedhollownanobubblesalsogivethesepeashell-likeMgOnanostructuresa9timesstrongersaturationmagnetizationthanthatofMgOnano-particles.Webelievethatsuchanewkindof1Dnanostructure,thepeashell-likenanostructure,notonlyconfersthepropertiesofamulti-functionalnanomaterialonMgO,butcouldalsoimpartsomenovelpropertiestoothermaterials.Therefore,thesenovelpeashell-likenanostructuresarelikelytoinspirealargeamountoffutureworkthatwillbeimportantforthedevelopmentofmodernscienceandtechnology.

ThisworkissupportedbyTheNationalBasicResearchProgramofChina(973Program)grantNo.2007CB936300,theNationalHighTechnologyResearchandDevelopmentProgramofChina(863Program)grantNo.2006AA03A107andtheNationalNaturalScienceFoundationofChina(grantNo.50702073).

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