Fe3O4-PEDOT核壳微球的合成及其电磁波吸收性能

RESEARCHARTICLE

SynthesisandElectromagnetic,MicrowaveAbsorbingPropertiesofCoreÀShellFe3O4ÀPoly(3,4-ethylenedioxythiophene)Microspheres

WencaiZhou, , XiujieHu,*, XiaoxiaBai, , ShuyunZhou,*, ChenghuaSun, JunYan, andPingChen

KeyLaboratoryofPhotochemicalConversionandOptoelectronicMaterials,TechnicalInstituteofPhysicsandChemistry,ChineseAcademyofSciences,Beijing100190,China

GraduateUniversityofChineseAcademyofSciences,Beijing100049,China

’INTRODUCTION

Recently,theconjugationofconductingpolymersandinor-ganicmagneticnanoparticleshasattractedmoreattentionbe-causetheresultantmaterialsnotonlyexhibitacombinationoftheconductiveandmagneticpropertiesbutalsotaketheadvantagesofbothnanomaterialsandpolymers.Besides,theinorganicmag-neticnanoparticlescoatedbytheconductingpolymerswillbepreventedfromreunitingcausedbyhighsurfaceactivity.There-fore,theseconductiveandmagneticcompositeshavegreatpotentialapplicationsinthe eldsofelectricalandmagneticshields,mol-ecularelectronics,nonlinearopticsandmicrowaveabsorbingmaterials.1À4

Amongmagneticmetaloxides,Fe3O4withpropertiesofsuperparamagnetisminadditiontoitslowtoxicityandhighbiocompatibilityisthemost-studiedmaterialformagneticnanoparticles5inmagneticstoragemedia,contrastagentsformag-neticresonanceimaging(MRI),6separationofbiomolecules,7environmentalorfoodanalyzes,8immunoassays,9controllingtargeteddrugdelivery/release,10andmicrowaveabsorbing.11

Uptonow,theresearchesonthefabricationofconductiveandmagneticcompositesbaseonFe3O4aremainlyfocusedonFe3O4Àpolypyrrole/polyaniline(Fe3O4ÀPPy/PANI).Liuetal.synthe-sizedelectricandferromagneticFe3O4ÀPPycompositesbya

r2011AmericanChemicalSociety

chemicalmethodusingp-dodecylbenzenesulfonicacidsodiumsalt(NaDS)assurfactantanddopant.12Dengetal.preparedcoreÀshellFe3O4ÀPPynanoparticlesanddemonstratedthatboththeconductivityandthemagnetizationofthecompositesstronglydependedontheFe3O4contentandthedopingdegree.13Luetal.alsosynthesizedhighlyregulatedcoreÀshellFe3O4ÀPPymicrosphereswithlowconductivity.14Ontheotherhand,Reddyetal.synthesizedelectromagneticfunctionalizedFe3O4ÀPANIcompositeswithammoniumperoxydisulfateastheoxidiz-ingagent.15

Poly(3,4-ethylenedioxythiophene)(PEDOT),apolythio-phenederivative,isoneofthemostpromisingconductivepolymerswithexcellentelectrochemicalactivity,highelectricalconductivity,moderatebandgap,lowredoxpotential,andexcellentenvironmentalstability.Andinourrecentstudy,wehaverevealedthemicrowaveabsorbingabilityofPEDOT.16Therefore,thecompositeconsistedofFe3O4andPEDOTwillhaveanattractiveprospect.Reddyetal.havesynthesizedcoreÀshellnanocompositecomposedofFe3O4nanoparticlesand

Received:April18,2011

Accepted:September13,2011Published:September13,2011

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persulfate(APS)(1mmol)wasaddedtoprepare100mLofthemixture.Afterbeingstirredfor24hatroomtemperature,themixturewascentrifugedandwashedthreetimeswithasolventofdeionizedwater/ethanol(1/1,v/v).Theprecipitatewasdriedunderavacuumat60°Cfor24h.

Measurements.ThemorphologyoftheproductswasinvestigatedusingJEM-2100Ftransmissionelectronmicroscope(TEM).Fouriertransforminfrared(FTIR)spectraintherangeof500À2000cmÀ1wereconductedonsamplepelletswithKBrbymeansofaninfraredspectro-photometer(Excalibur3100,America,Varian).ThephaseidentificationofthefinepowdercompositewasperformedusingX-raydiffraction(XRD)analysisonaD8FocusDiffractometer(Germany).ConductivitymeasurementsoftheFe3O4ÀPEDOTsamples(compressedintorec-tangularblock)wereperformedusingaKeithley220SourceMeterfour-pointprobeinstrument.Magneticpropertiesweretestedbyvibratingsamplemagnetometer(VSM,Lakeshore707Series).ThecompositessamplesforelectromagneticparametermeasurementwerepreparedbymixingtheFe3O4ÀPEDOTmicrospheresandparaffinwaxatdifferentvolumefractionoftheFe3O4ÀPEDOTmicrospheres.Themixturewasthenpressedintoatoroidalshapewiththethicknessof2mm.Subsequently,therelativecomplexpermeability(μr)andpermittivity(εr)werecarriedoutbyaHP8722ESnetworkanalyzeratthefrequencyrangeof2À18GHzandthereflectionlosseswerecalculatedusingthemeasuredμrandεr.

Figure1.TEMimagesof(a)pureFe3O4microspheresand(bÀf)Fe3O4ÀPEDOTcoreÀshellmicrospherespreparedwithdi erent(EDOT)/(Fe3O4)ratios:(b)10,(c)15,(d)20,(e)30,and(f)50.

’RESULTSANDDISCUSSION

hollowmicrospheres.TheparticlesaresphericalwithdiametersCharacterizations.Figure1ashowsthemorphologyofFe3O4

rangingfrom200À400nmandawallthicknessofabout50nm.ThedensityofthepreparedFe3O4microspheresis3.56gcmÀ3becauseoftheexistenceofahollowcavity,whichislowerthanthatofthebulkFe3O4.Figure1bÀfshowsthemorphologyoftheobtainedFe3O4ÀPEDOTcoreÀshellmicrospherespreparedwithdifferent(EDOT)/(Fe3O4)ratios.ItisclearthattheFe3O4microspheresarefullycoatedbyPEDOTandtheshellgraduallythickenswiththeincreaseofthe(EDOT)/(Fe3O4)ratio.When(EDOT)/(Fe3O4)ratiois10,theshellthicknessisabout60nm(Figure1b).Theshellsincreasetoabout90and140nmwhen(EDOT)/(Fe3O4)ratiosareat15(Figure1c)and20(Figure1d),respectively.When(EDOT)/(Fe3O4)ratioreaches30,theshellthicknesscanreach250nm(Figure1e).Further,theshellthicknessincreaseslittlewhenthe(EDOT)/(Fe3O4)ratiocontinuestoincreaseto50(Figure1f).Therewillbeasaturationshellthicknessifthe(EDOT)/(Fe3O4)ratiocon-tinuouslyincreases.Theaboveresultsindicatethatthe(EDOT)/(Fe3O4)ratiohasasignificantinfluenceonthestructureoftheFe3O4ÀPEDOTcoreÀshellmicrospheres.

Toidentifythecomponentsofthecomposites,especiallythepolymercomposition,weperformedFTIRanalysesofFe3O4,purePEDOT,andFe3O4ÀPEDOTcompositespreparedwith(EDOT)/(Fe3O4)ratiosof10,20,and50.ThespectraareshowninFigure2.TheFTIRspectrumofFe3O4(Figure2A)

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Scheme1.FormationMechanismofFe3O4ÀPEDOTCoreÀShellMicrospheres

Figure2.FTIRspectraof(A)Fe3O4;(BÀE)Fe3O4ÀPEDOTcom-positespreparedwithdi erent(EDOT)/(Fe3O4)ratios:(B)10,(C)20,(D)50;and(E)purePEDOT.

Figure3.XRDpatternsof(A)Fe3O4microspheresand(BÀF)Fe3O4ÀPEDOTcompositespreparedwithdi erent(EDOT)/(Fe3O4)ratios:(B)10,(C)15,(D)20,(E)30,and(F)50.

showscharacteristicpeakat588cmÀ1,attributedtotheFeÀObondstretching.Thispeakshiftsto576cmÀ1intheFe3O4ÀPEDOTsampleswitha(EDOT)/(Fe3O4)ratioof10andcontinuouslyshiftstolowerwavenumberandoverlapsthepeakofPEDOTwiththeincreaseofthe(EDOT)/(Fe3O4)ratio.Figure2EshowsthespectraofpurePEDOT.Thepeaksat690,845,922,and983cmÀ1areattributedtothedeformationmodesofCÀSÀCinthethiophenering;thepeaksat1091,1147,and1203cmÀ1areassociatedwiththeCÀOÀCbendingvibrationoftheethylenedioxymoiety;thepeakat1357cmÀ1isassignedtoCÀCstretchingofthequinoidalstructure;thepeaksat1473and1517cmÀ1areduetotheCdCstretchingofthequinoidstruc-tureofthethiophenering.ThemainpeaksofPEDOTshiftstohighwavenumberwiththeincreaseofFe3O4inthecompositescomparedtothepurePEDOT,whichisduetosomeinteractionofferriteparticlesandpolymerchains.19Aboveall,theFTIRspectracon rmthecoexistenceofFe3O4andPEDOT.

XRDpatternsofFe3O4microspheresandFe3O4ÀPEDOTcompositespreparedwithdi erent(EDOT)/(Fe3O4)ratioswerealsoobserved(Figure3).Fe3O4(Figure3A)showsdi ractionpeaksat2θ=18.4,30.1,35.6,37.2,43.1,53.5,57.1,and62.7°,whichareinagreementwithliteratures.18,20These

peakscorrespondtothe(111),(220),(311),(222),(400),(422),(511),and(440)latticeplanes.When(EDOT)/(Fe3O4)ratiosare10(Figure3B)and15(Figure3C),thedi ractionpeaksofFe3O4ÀPEDOTcompositesareatthesamepositionastheFe3O4microspheres(Figure3A).However,withthe(EDOT)/(Fe3O4)ratiosincreasingto20(Figure3D)and30(Figure3E),newpeaksat2θ=25.6and11.7°appear.Thesetwopeaksbecomestrongerwithhigher(EDOT)/(Fe3O4)ratios,accompaniedwithdecreasingintensityoftheFe3O4peaks.Thepeaksat2θ=25.6and11.7°provetheexistenceofPEDOTaccordingtoliterature.17Thedi ractionpatternsindicatethatFe3O4ÀPEDOTcompo-sitesarecomposedofpurephasewithnoimpurity.

FormationMechanism.Weexploredtheformationmechan-ismoftheFe3O4ÀPEDOTcoreÀshellstructurebyaseriesofexperiments.ItwasfoundthattheFe3O4ÀPEDOTcoreÀshellstructurecouldnotformintheabsenceofPVAorp-TSAintheexperiments.WithouttheinclusionofPVA,onlyamixtureofFe3O4microspheresandPEDOTwasobtained.MostofPED-OTpresentedinamorphousstatewhereasalittlepolymerwasfoundcoatedontheFe3O4microspherestoformathinlayer.TherewereFe3O4microspheresandalittlepolymerwithoutp-TSA,andthepolymeraggregatedapartfromtheFe3O4micro-spheres.Overall,thecoexistentofPVAandp-TSAisimportantduringtheformationofFe3O4ÀPEDOTcomposites.Inaddi-tion,weusedFe(p-toluenesulfonate)toreplaceAPSandp-TSAinordertoexplorewhethertheoxidantwithSO3Àcouldactasthedualroleofoxidantanddopant.ThesystemdidnotformcoreÀshellstructure,whichindicatedthatSO3Àcouldnottaketheplaceofp-TSA.ThenwesuggestapossiblemechanismoftheFe3O4ÀPEDOTcoreÀshellmicrospheresformationshowninScheme1.

Fe3O4particlesarenaturallyhydrophilicduetoplentifulhydroxylsontheparticlesurface.21ThehydroxylgroupinPVAcanformhydrogenbondswiththehydroxylgrouponFe3O4particles,whichenablesFe3O4particlestobewelldispersed.BecauseoftheweakstaticinteractionsbetweentheSO3Àgroupinp-TSAmoleculesandFe3O4particles,14p-TSAmoleculescanbeabsorbedonthesurfaceofFe3O4particles.AfterEDOTmonomerisadded,themoleculetendstogatheraroundthehydrophobicinPVAbecauseof“similarcompatibility”.Subse-quently,p-TSAservesasthedopanttoenhancetheprotonationofEDOT,thusconnectsEDOTtotheFe3O4particles.OnceAPSoxidantisintroduced,thepolymerizationwilloccurandEDOTmonomerwillbenucleatedonthesurfaceofFe3O4microspheres.AfterEDOTnucleationoccurringonthesurfaceofFe3O4microspheres,thepolymerizationwillcontinuetocarryoutwiththeas-formedPEDOT.Duringtheformation,PVAas

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Figure4.TEMimagesofFe3O4ÀPEDOTcomposite((EDOT)/(Fe3O4)=10)withdi erentpolymerizationtime:(a)24,(b)48,and(c)60h.

Table1.ElectricalandMagneticPropertiesofFe3O4ÀPEDOTcomposites

compositeswithdi erent(EDOT)/(Fe3O4)pureFe3O41015203050

a

conductivity(Scm)5.28Â101.21Â101.06Â102.34Â103.13Â10

b

À4À3À2À1À1À1

Msa(emug)84.858.840.31711.35.85

c

À1

Mrb

À1

Hcc61.964.758.859.161.178.1

(emu3g)(Oe)7.63.62.20.820.560.38

Figure5.Magnetizationcurvesappliedmagnetic eldatroomtem-peratureofFe3O4microspheresandFe3O4ÀPEDOTcompositespre-paredwithdi erent(EDOT)/(Fe3O4)ratios.

Saturationmagnetization.Remnantmagnetization.Coercivity.

thestabilizer,promotesthe“orientedattachment”22tojointheas-formedPEDOTandgivesrisetotheshell.However,inthecaseofFe(p-toluenesulfonate)replacingAPSandp-TSA,thepolymershelldidnotform.ThisisbecausealthoughtheSO3ÀgroupintheFe(p-toluenesulfonate)helpedtheattrac-tiononthesurfaceofFe3O4particles,itcouldnotenhancetheprotonationofEDOT.Thatistosay,thereisnolinkbetweenFe3O4andEDOT,sothesystemcouldnotformthecoreÀshellstructure.

Toprovethemechanismfurther,theotherstabilizeranddopantswereusedtoprepareFe3O4ÀPEDOTcoreÀshellmicro-spheres.Itisfoundthatpolyvinylpyrrolidone(PVP)asatypicalstabiliercancompletelyreplacePVA.Andβ-naphthalenesulfonicacid(β-NSA)insteadofp-TSAinFe3O4ÀPEDOTformationcanformcoreÀshellstructuretoo,thoughthecoreÀshellstructurewasnotasgoodasthatusingp-TSAasthedopant.ThecarbonylgroupinPVPcanformahydrogenbondwiththehydroxylgrouponthesurfaceofFe3O4particles.ThusPVPcanstabilizethepolymersols,andimprovethedispersionoftheparticlesbecauseofsterichindrancee ectingfromPVPadsorptiononparticlesurface.21However,usingoxalicacidasthedopantcannotproduceFe3O4ÀPEDOTcoreÀshellmicrospheres.TheresultsindicatetheinteractionbetweenSO3ÀgroupandFe3O4microspheresplaysanimportantroleintheformationofFe3O4ÀPEDOTcoreÀshellstructure.Insummary,thestabilizerandsulfonicacidgrouptogetherpromotetheformationofcoreÀshellFe3O4ÀPEDOTmicrosphere.

Accordingtothesuggestedmechanism,EDOTmonomeronthesurfaceofFe3O4particleswillincreaseandtheorganiclayerwillthickenasincreaseofthepolymerizationtime.Figure4providestheevidenceforthissuppose.Ata xed(EDOT)/(Fe3O4)ratioof10,theshellofFe3O4ÀPEDOTmicrosphereincreasesfrom60to100nmfollowingthereactiontimein-creasingfrom24h(Figure4a)to48h(Figure4b).Theshell

increasebecomesminorafterareactiontimelongerthan60h(Figure4c).

ElectricandMagneticProperties.TheelectricalpropertiesoftheobtainedFe3O4ÀPEDOTcompositesweremeasuredbyfour-pointprobemethodandtheconductivitiesaredisplayedinTable1.Itisfoundthattheconductivitiesofthecompositesatroomtemperatureareintherangeof1Â10À4to1Â10À1ScmÀ1andincreasewiththe(EDOT)/(Fe3O4)ratioincreasing.Theten-dencyisconsistentwiththeshellthickness(Figure1bÀf)becausetheconductivityismainlydeterminedbythepolymer.Thethickertheshellis,thehighertheconductivityis.

ThemagneticpropertiesofFe3O4andFe3O4ÀPEDOTmicrospherespreparedwithdi erent(EDOT)/(Fe3O4)ratioswereinvestigatedwithaVSMwhichfeaturesasensitivityof1Â10À5emu.AndFigure5showsthehysteresisloopsoftheobtainedsamplesintheappliedmagnetic eldsweepingfromÀ10to10kOeatroomtemperature.ThemagneticparameterscorrespondingtoFigure5areshowninTable1.ThepureFe3O4isatypicalsuperparamagneticmaterial,presentinghighsaturationmagnet-ization(Ms),highremnantmagnetization(Mr),andlowcoercivity(Hc).Withtheincreaseofthe(EDOT)/(Fe3O4)ratio,thesatu-rationmagnetizationandremnantmagnetizationaredecreased,duetothedecreaseofFe3O4contentinthecomposites.Thein-dependenceofcoercivityonthe(EDOT)/(Fe3O4)ratiosuggeststhatthesuperparamagnetismhasthesameorigin,fromFe3O4.ItisclearfromTable1thattheFe3O4ÀPEDOTcompositesexhibitgoodmagneticpropertiesandlowconductivitieswithlower(EDOT)/(Fe3O4)ratios,whilelowmagneticpropertiesandhighconductivitieswithhigher(EDOT)/(Fe3O4)ratios.MicrowaveAbsorbingProperties.Foramicrowave-absorb-inglayerterminatedbyashortcircuit,thenormalizedinputimpedanceisrelatedtotheimpedanceinfreespace,Zin,andreflectionloss(RL)isrelatedtothenormalincidentplanewave,whichcanbegivenbythetheoryoftheabsorbingwall.23

r μr2πp

μrεrfd Zin¼tanh½jcr

ZÀ1

in

RLðdBÞ¼20log Zin3842

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RESEARCHARTICLE

Figure6.ElectromagneticparametersofFe3O4ÀPEDOTcompositeswithdi erent(EDOT)/(Fe3O4)ratiosat50%volumefractioninthe2À18GHzrange:(a)realand(b)imaginarypartsoftherelativecomplexpermittivity;(c)realand(d)imaginarypartsoftherelativecomplexpermeability.

wherecisthevelocityoflightinfreespace,fisthefrequency,anddisthelayerthickness.Therelativecomplexpermittivity(εr)andrelativepermeability(μr)oftheabsorbingmediumareexpressedasεr=ε0Àjε00,μr=μ0Àjμ00.Theimpedancematchingconditionisdeterminedbythecombinationofthesixpara-meters:ε0,ε00,μ0,μ00,f,andd.

Therealpermittivity(ε0)andrealpermeability(μ0)symbolizethestorageabilityofelectromagneticenergy,24whiletheima-ginarypermittivity(ε00)isrelatedtothedissipationofenergyandthemagneticlossisexpressedbyimaginarypermeability(μ00).19Thecurveofε0,ε00,μ0,andμ00ofFe3O4ÀPEDOTcompositeswithdi erent(EDOT)/(Fe3O4)ratiosat50%volumefractionareshownasFigure6.Itisobservedthatthesampleswithhigher(EDOT)/(Fe3O4)ratiosshowhighervaluesofε0andε00(Figure6a,b),whichisrelatedtohigherconductivities.Moreover,theμ0valuesobviouslydecreaseandthenincreasewiththefrequencyincreasinginthe2À18GHzrange(Figure6c).When(EDOT)/(Fe3O4)ratiosare10and15,theμ00valuesexhibitpositiveinthewholerange;whileanegativeμ00valuemeansthemagneticenergyisradiatedoutwithnoabsorption.24Thatistosay,thecompositesmainlyexhibitelectricallosseswhen(EDOT)/(Fe3O4)ratiosare20,30,and50.

Onthebasisofformulas1and2,wecalculatedtheRLofFe3O4ÀPEDOTcompositeswithdi erent(EDOT)/(Fe3O4)ratiosinthefrequencyrangeof2À18GHzat50%volume

Figure7.Re ectionlossesinthethicknessof2mmoftheFe3O4ÀPEDOTcompositespreparedwithdi erent(EDOT)/(Fe3O

4

)ratiosat50%volumefraction.

fraction.Figure7showstheRLvariationwhenthelayerthicknessis2mm.Witha(EDOT)/(Fe3O4)ratioof20,theminimumRLofthecompositeisÀ27.6dB,whichisbetterthanpurePEDOTinourresearchbefore(À24dB).16Theminimum

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RESEARCHARTICLE

Figure8.Re ectionlossesindi erentthicknessofFe3O4ÀPEDOTcompositeswith(a)(EDOT)/(Fe3O4)=20and(b)50at20%volumefraction(c)(EDOT)/(Fe3O4)=20

and(d)50at50%volumefraction.

re ectionlossesofthespecimenwiththe(EDOT)/(Fe3O4)=30and50areÀ4.6dBandÀ3.8dB,paringwiththeconductivitiesinTable1,itcanbefoundthatthecompositeswithhigherconductivitiesat(EDOT)/(Fe3O4)=30and50donotcorrespondtobetterabsorbingparameters.Thisindicatesthatbothhigherconductivityandlowerconductivity((EDOT)/(Fe3O4)=10)arenotbene cialtoimprovingmicrowaveabsorbingandthemaximummicrowaveabsorbingcorrespondstoanintermediateconductivity.25,26

Inaddition,tostudythein uenceofvolumefractiononmicrowaveabsorbingproperty,theelectromagneticparametersofFe3O4ÀPEDOTcompositeswith(EDOT)/(Fe3O4)=20and50at20%volumefractionweremeasuredandthecalculatedre ectionlossesareshownaspanelsaandbinFigure8,res-pectively.Meanwhile,panelscanddinFigure8showthecalculatedre ectionlossesofFe3O4ÀPEDOTcompositeswith(EDOT)/(Fe3O4)=20and50at50%volumefraction,respec-tively.Whenthevolumefractionis20%(Figure8a),thesamplewith(EDOT)/(Fe3O4)=20exhibitsexcellentmicrowaveabsorb-ingpropertyinthelayerthicknessrangeof3-4mmandtheminimumRLisÀ30dBat9.5GHzwithalayerthicknessof4mm;whenthevolumefractionis50%(Figure8c),thiscom-positeexhibitsgoodmicrowaveabsorbingpropertyinthelayerthicknessrangeof2À4mmandtheminimumRLisÀ27.6dBat13GHzwithalayerthicknessof2mm.Besides,theRLoftheFe3O4ÀPEDOTcompositewith(EDOT)/(Fe3O4)=50at20%volumefraction(Figure8b)islargerthanthesampleat50%

volumefraction(Figure8d)andtheminimumRLisÀ22dBat18GHzwithalayerthicknessof2mm.Theresultindicatesthattheconductivity,volumefractionandlayerthicknessallhavegreatimpactsonmicrowaveabsorbingproperty.

’CONCLUSIONS

UniformcoreÀshellFe3O4ÀPEDOTmicrospheresweresuccessfullysynthesizedbyatwo-stepmethod.Theselectionofbothstabilizeranddopantareessentialfortheformationofthecomposites.Thepropertiesofthecompositesaresigni cantlyin uencedbythe(EDOT)/(Fe3O4)ratio.TheFe3O4ÀPEDOTcompositesexhibitedgoodconductivitiesathigh(EDOT)/(Fe3O4)ratiosandexcellentmagneticpropertiesatlow(EDOT)/(Fe3O4)ratios.There ectionlossescalculatedbythetheoryoftheabsorbingwallshowedthattheFe3O4ÀPEDOTcompositewith(EDOT)/(Fe3O4)=20exhibitedthebestmicrowaveabsorbingpropertyintherangeof2À18GHz.TheminimumRLreachedapproximatedÀ30dBatthethicknessof4mm.Insummary,thetwo-stepsynthesisandelectromagneticcoreÀshellFe3O4ÀPEDOTcompositeswillhaveapromisingapplicationinmicrowaveabsorbing eld.’AUTHORINFORMATION

CorrespondingAuthor

*E-mail:huxiujie@(X.H.);zhou_shuyun@(S.Z.).Fax:+86010-82543517;Tel:+86010-82543515.

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’ACKNOWLEDGMENT

ThisworkwassupportedbyNationalNaturalScienceFounda-tionofChina(20874112and60808022).TheauthorsthankX.HuangandM.WangfortheassistanceinTEMcharacterization.’REFERENCES

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