Synthesis and Magnetic Properties of Nearly Monodisperse CoFe2O4 Nanoparticles

NANOEXPRESS

SynthesisandMagneticPropertiesofNearlyMonodisperseCoFe2O4NanoparticlesThroughaSimpleHydrothermalCondition

Xing-HuaLi•Cai-LingXu•Xiang-HuaHanLiangQiao•TaoWang•Fa-ShenLi

•

Received:5January2010/Accepted:31March2010/Publishedonline:16April2010ÓTheAuthor(s)2010.ThisarticleispublishedwithopenaccessatSpringerlink.com

AbstractNearlymonodispersecobaltferrite(CoFe2O4)nanoparticleswithoutanysize-selectionprocesshavebeenpreparedthroughanalluringmethodinanoleylamine/ethanol/watersystem.Well-definednanosphereswithanaveragesizeof5.5nmhavebeensynthesizedusingmetalchlorideasthelawmaterialsandoleicamineasthecap-pingagent,throughageneralliquid–solid-solution(LSS)process.Magneticmeasurementindicatesthattheparticlesexhibitaveryhighcoercivityat10Kandperformsuper-paramagnetismatroomtemperaturewhichisfurtherillu-minatedbyZFC/FCcurves.Thesesuperparamagneticcobaltferritenanomaterialsareconsideredtohavepoten-tialapplicationinthefieldsofbiomedicine.Thesynthesismethodispossibletobeageneralapproachfortheprep-arationofotherpurebinaryandternarycompounds.KeywordsMonodisperseÁCobaltferriteÁSuperparamagneticÁNanoparticlesÁMagneticÁBiomedcine

Introduction

CoFe2O4,asatypeofmagneticmaterials,haslongbeenofintensiveimportanceinthefundamentalsciencesandtechnologicalapplicationsinvariousfieldsofelectronics[1],photomagnetism[2],catalysis[3],ferrofluids[4],hyperthermia[5],cancertherapy[6],andmolecularimagingagentsinmagneticresonanceimaging(MRI)[7].TheapplicationsofCoFe2O4arestronglyinfluencedbyitsmagneticproperties.Forbiomedicalapplications,CoFe2O4nanoparticlesarerequiredtohaveanarrowsizedistribu-tion,highmagnetizationvalues,auniformsphericalshape,andsuperparamagneticbehavioratroomtemperature.Sofar,varioussyntheticrouteshavebeenexploredforthepreparationofCoFe2O4nanoparticles,suchashydrother-mal[8],coprecipitation[9,10],microemulsion[11],forcedhydrolysis[12],reduction–oxidationroute[13].However,themaindifficultyofthesetraditionalmethodsisthattheas-preparednanoparticlesareseverelyagglomeratedwithpoorcontrolofsizeandshapeinmostcases,whichgreatlyrestricttheirapplications[14].Inordertosolvetheaboveproblems,thermaldecompositionoforganometallicpre-cursorsinhigh-boilingorganicsolutionhasbeenexplored[15,16]forthepreparationofsize-andshape-controlledmonodisperseCoFe2O4nanoparticles[14,17–19].How-ever,themajordisadvantagesofthismethodaretheneedoftoxicandexpensivereagents,highreactiontemperature,andcomplexoperations.Toaddresstheseconcerns,Lietal.adoptedageneralliquid–solid-solution(LSS)phasetransferandseparationmethod[20].Thisstrategyisbasedonageneralphasetransfermechanismoccurringattheinterfacesoftheliquid,solid,andsolutionphasespresentduringthesynthesis.Throughthisgeneralmethod,Lietal.successfullysynthesizedFe3O4dopedwithCo,whichhasacoercivityabout250Oeatroomtemperature[21].

X.-H.LiÁC.-L.XuÁX.-H.HanÁL.QiaoÁT.Wang(&)ÁF.-S.Li(&)

InstituteofAppliedMagnetics,KeyLaboratoryofMagnetismandMagneticMaterialsofMinistryofEducation,LanzhouUniversity,730000Lanzhou,People’sRepublicofChinae-mail:wtao@lzu.edu.cnF.-S.Li

e-mail:lifs@lzu.edu.cn

C.-L.Xu

KeyLaboratoryofNonferrousMetalChemistryandResourcesUtilizationofGansuProvince,LanzhouUniversity,730000Lanzhou,People’sRepublicofChina

123

1040However,thesynthesisofCoFe2O4nanoparticleswithasuperparamagneticbehavioratroomtemperaturehasnotbeenreported.Inthisletter,wereportasignificantimprovementofthemethodofLietal.[20]tosynthesizenearlymonodispersedCoFe2O4nanoparticlesandsystem-aticallyinvestigatethemagneticpropertiesoftheas-pre-parednanomaterials.Atroomtemperature,theseas-preparednanoparticleswerefoundtohavehighsaturationmagnetizationvaluesof50emu/gandsuperparamagneticbehaviorwithnegligiblecoercivity,whichisexpectedtohavepotentialapplicationinbiomedicine.

Experimental

SynthesisofCoFe2O4SphericalNanoparticles

TheprocessforsynthesizingnearlymonodisperseCoFe2O4withsuperparamagneticbehavioratroomtemperaturewascarriedoutasfollows:Inatypicalsynthesis,1.6g(6mmol)ofFeCl3Á6H2Oand0.7gof(3mmol)CoCl2Á6H2Oweredissolvedinthesolventcomposedof80mlofwaterand40mlofethanol.Afterthat,7.3g(24mmol)ofsodiumoleateand7mlofoleicaminewereaddedintotheabovesolutionwithstronglystirringatroomtemperaturefor2h.Then,thereactionprecursorwastransferredintoaTeflon-linedstainlessautoclavewithacapacityof150ml.Inordertocrystallizetheparticles,thereactiontemperatureoftheautoclavewasincreasedandmaintainedat180°Cfor12h.Then,thesystemwascooleddowntoroomtemperaturenaturally.Theproductswereseparatedfromthefinalreactionsolutionbytheadditionofhexane.TheredsupernatantliquorcontainingCoFe2O4nanoparticleswasseparatedbyaseparatingfunnel.Theas-preparedcobaltferritecouldbedepositedbyaddingetha-nolandobtainedbycentrifugatingatahighspeed(10,000rpm)withoutanysize-selectingprocess.Theas-preparedsamplescouldbewellredispersedinahexanesolventandstoredforseveralmonthswithoutdelamination.Characterization

Propertiesoftheas-synthesizedsampleswerecharacteredthroughseveraltechniques.ThephasecontentsandcrystalstructuresofthesampleswereanalyzedbyX-raydiffrac-tion(XRD)withCuKaradiationonaPhilipsX’pertdif-fractometer.ElementalanalysisformetalironwasmeasuredbyanIRISER/Sinductivelycoupledplasmaemissionspectrometer(ICP-ES).High-resolutionTEM(HRTEM)analysiswascarriedoutonaJEM-2010trans-missionelectronmicroscopewithanacceleratingvoltageof200kV.OnedropletofhexanedispersionofCoFe2O4123

NanoscaleResLett(2010)5:1039–1044

nanoparticleswasdroppedonacarbon-coatedcoppergridandthendriednaturallybeforerecordingthemicrographs.FTIRspectraofthesamplescappedwitholeicaminewereperformedona170SXspectrometerintherangeof500–4,000cm-1.MagneticpropertiesoftheproductswerecharacterizedatroomtemperaturewithaLakeShore7,304vibratingsamplemagnetometer(VSM).TemperatureandfielddependencesofthesampleswererecordedonaQuantumDesignMPMS-XLsuperconductingquantuminterferencedevice(SQUID).ZFC/FCmeasurementswerecarriedoutinthetemperaturerangeof10–330Kwithanappliedfieldof100Oe.

ResultsandDiscussion

TheX-raypatternoftheas-synthesizedsamplesisdepictedinFig.1.Thepositionsandrelativeintensitiesofallthepeaksindicatethatthecrystallinestructureoftheproductsfavorstheformationofcubicspinelphaseonly,whichisaccordanttoJCPDScardNO.22-1086.Nootherimpurityphasesareobserved.Additionally,itclearlyshowsthattheas-synthesizedCoFe2O4samplesrevealbroadeningdif-fractionpeak,whichisduetothereducedparticlesize.Theaveragegrainsizeoftheas-synthesizednanoparticlescal-culatedbyScherer’sformula[10]is6nm.Basedofthehighestintensitypeakof(311),thecalculatedlatticeparameteris0.8456nm,whichislargerthanthebulkCoFe2O4valueof0.8391nm.Theenhancementofthecalculatedlatticeparameterprobablyresultsfromdifferentdistributionofmetalcationscomparedwiththebulkspinelcobaltferriteandthesurfacedistortionofparticlesinducedbythesizeeffectofnanoparticles[13].

Thechemicalcompositionoftheas-synthesizedprod-uctsisfurtheranalyzedbytheinductivelycoupledplasma

Fig.1XRDpatternoftheas-synthesizedCoFe2O4nanoparticles

NanoscaleResLett(2010)5:1039–1044atomicemissionspectroscopy(ICP-AES).TheresultrevealsthatthemolarratioofCoandFeis1:2.05,whichisnearlyconsistentwiththeexpectedstoichiometryofCoFe2O4.

Figure2showsTEMimagesoftheCoFe2O4nanopar-ticlesobtainedwithoutanysize-sortingprocess.Itrevealsthattheas-synthesizednanoparticleswerenearlymono-dispersewithsphericalshape.TheparticlesizewithanarrowdistributionisgivenintheinsetofFig.2a.Theaverageparticlessizeis5.5nm,whichisingoodagree-mentwiththeparticlesizesestimatedbyScherer’sfor-mula.Thissuggeststhateachindividualparticleisasinglecrystal[19].Figure2bperformsthehigh-resolution(HR)TEMcharacterizationsoftheparticles,andthehighlycrystallinenatureofthesamplesisrevealedintheinsetofFig.2b.

FTIRspectraofthesamplescappedwitholeicaminewereperformedintherangeof500–4,000cm-1(Fig.3).Thewidepeakaround3,374cm-1isascribedtothecomplexationbetween-NH2and-OHonthesurfaceofCoFe2O4.Thepeakat3,007cm-1isassignedtothe

Fig.2TEMimageoftheas-synthesizedCoFe2O4nanoparticles

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Fig.3FTIRspectraoftheas-synthesizedCoFe2O4nanoparticles

stretchingofthevinylgroup.Thepeaksat2,921and2,850cm-1areattributedtotheasymmetricandsymmet-ricstretchingoftheCH2groups,respectively.Thesharppeaksareduetothelonghydrocarbonchainofoleicamine.Thepeaksat1,409and1,307cm-1correspondtoC–HbendingofCH2group.Thepeakat965cm-1isattributedtotheO–Houtplanevibration.Thepeakat593cm-1isowingtothepresenceofferritenanoparticles.TheFTIRspectrumconfirmsthattheas-synthesizednanoparticlesarecoatedbyoleicamine,whichcanproviderepulsive(elec-trostaticrepulsionandstericrepulsion)forcestobalancetheattractiveforces(dipole–dipoleinteraction,exchangeinteraction,andvanderWaalsforce.)betweenthenano-particles.Thus,onaccountoftherepulsion,theas-preparedCoFe2O4samplesareeasilydispersedinthenonpolarsolventsandstabilizedinthesuspensionwithoutagglomeration.

Thefielddependenceofthemagnetizationofas-syn-thesizedparticlesmeasuredat300and10KisshowninFig.4.Magneticmeasurementsindicatethattheas-pre-paredparticlesexhibitsuperparamagneticbehaviorwithnegligiblecoercivity(about11Oe)andremanenceatroomtemperature.

Thesaturationmagnetizationvalueis50emu/gatroomtemperature,whichislessthanthebulkvalueof74emu/g[10].Fornanoscalednanoparticles,thelossofthesatura-tionmagnetizationisduetosurfacespincantingeffect[22]andthepresenceofamagneticdeadorantiferromagneticlayeronthesurface[13,23],whichiscausedbyfinite-sizeeffectofthesmallmagneticnanoparticles.Additionally,themagneticperformanceoftheferrite-structurednanomaterialsisalsoinfluencedbythedistributionofmetalcations,whichisdifferentfromthebulkferrite.Asummaryofthemagneticpropertiesbetweentheas-syn-thesizedproductsandthereportedCoFe2O4isgiveninTable1.Inourbestknowledge,CoFe2O4nanoparticles

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1042Fig.4Hysteresisloopoftheas-synthesizedCoFe2O4nanoparticlesmeasuredata300K,b10K

preparedinthisworkhaveahighersaturationmagnetiza-tionvaluecomparedwiththereportedsampleswithsu-perparamagneticbehaviorsintheappliedfieldof12,000Oeatroomtemperature.Thesaturationmagneti-zationvalue(73.8emu/g)measuredat10KisclosetothevalueofbulkCoFe2O4(74emu/g).

Theparticlesexhibitsuperparamagneticbehaviorwithnegligiblecoercivity(about11Oe)atroomtemperature,whichismuchlowercomparedwiththevalue(250Oe)reportedbyLietal.[21].Themagneticpropertiesofsamplesaregreatlyrelatedtomanyfactors,suchasshape,size,andstructure,whichareinfluencedbythesyntheticmethodandexperimentalparameters.Thisgreatlyreducedcoercivityisunderstoodasfollows:Theas-synthesizedCoFe2O4nanoparticlesaresphericalinshape,well-iso-lated,andtheparticlesizeoftheproductisfoundintherangeofthecriticalsizeofCoFe2O4forsuperparamagneticlimitreportedinliterature[24],whichisabout4–9nm.Additionally,thedecreaseofcoercivityinoursamplesilluminatesthatthecoercivityhasaparticle-size-dependentcharacter[29].Whereas,thecoercivityofthesamplesas-synthesizedreaches14.55kOe,muchlargerthanthevalueofbulkCoFe2O4(about5kOeat5K).Thecomparisonsof

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Table1Comparisonofmagneticpropertiesoftheas-synthesizedcobaltferritesandthereportedCoFe2O4measuredatroomtemperatureReferenceParticlesize(nm)Hc(Oe)Ms(emu/g)Thiswork5.51150[9]14.624337[10]20–30519About55[21]15.7250About50[24]83936304.2[12]2–60About40[25]40040[13]9030[26]30Negligible30[27]4–10018[28]

6

0

9

Thesaturationmagnetizationsarecomparedatanappliedmagneticfieldof12,000Oe

Table2Themagneticpropertiesoftheas-synthesizedCoFe2O4measuredatdifferenttemperature

Temperature(K)Ms(emu/g)Hc(Oe)Mr(emu/g)R(=Mr/Ms)30050120.50.0110

73.8

14,550

50.7

0.69

themagneticpropertiesmeasuredat300and10KforoursamplesaresummarizedinTable2.

Figure5showsthezero-field-cooledandfield-cooled(ZFC/FC)curvesoftheas-preparedCoFe2O4samplesmeasuredattemperaturesbetween10and330Kwithanappliedfieldof100Oe.Asthetemperaturerisesfrom10to330K,theZFCmagnetizationincreasesfirstandthendecreasesafterreachingamaximumat240K,whichiscorrespondtotheblockingtemperature(TB).ThisresultfurtherprovesthattheCoFe2O4samplesas-prepareddis-playasuperparamagneticbehavioratroomtemperature.WhereastheFCmagnetizationdecreasedendlesslyasthetemperatureincreased.ItisimaginedthatthedifferencebetweenZFCmagnetizationandFCmagnetizationbelowTBiscausedbyenergybarriersofthemagneticanisotropy[30].ThemagneticanisotropyconstantKofthesamplesas-preparedcanbecalculatedbythefollowedformula[30,31]:

K¼25kBTBVÀ1

ð1Þ

wherekBistheBoltzmanconstant,TBistheblockingtemperatureofthesamples,andVisthevolumeofasingleparticle.ThecalculatedmagneticanisotropyconstantKofoursamplesis3.89106ergs/cm3,whichisslightlylarger

NanoscaleResLett(2010)5:1039–1044Fig.5Zero-field-cooled(ZFC)andfield-cooled(FC)curvesfortheas-synthesizedCoFe2O4nanoparticlesunderanappliedmagneticfieldof100Oe

thanthatoftheKvaluesofbulkCoFe2O4[(1.8–3.0)9106ergs/cm3].

Thedistributionfunctionofthemagneticanisotropyenergybarriersf(T)canbeobtainedthroughthefollowingequation[13,30]:

fðTÞ¼d󰀂MZFC

󰀃dTMð2Þ

FCwhereMFC(FCmagnetization)involvesthetotalmagne-tizationfromthecontributionofallparticles,MZFC(ZFCmagnetization)isdeterminedbythemagnetizationfromonlythecontributionofthenanoparticleswhoseenergybarriersareovercomedbythethermalenergy(kBT)atthemeasuringtemperature,andf(T)reflectsaquantitativecharacterizationforsuperparamagnetismofthemagneticnanoparticles.

Figure6revealsthecalculatedanisotropyenergydis-tributionfortheas-synthesizedCoFe2O4nanoparticles.Generally,thevolumeandshapedistributionofthe

Fig.6Energybarrierdistributionsofmagneticanisotropyfortheas-synthesizedCoFe2O4nanoparticles

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samplesdeterminethemagneticanisotropyenergydistri-bution.Therefore,theresultimpliesthatthethermalenergiesofmostparticleshaveexceededtheenergybar-riersbeyondTB(about240K).Sotheas-synthesizedsamplesdisplaysuperparamagneticbehavioratroomtemperature.Inaddition,thenarrowmagneticanisotropyenergydistributionrevealsthattheas-preparedCoFe2O4nanoparticlespossessuniformsizes[13,30].Thesuper-paramagneticbehaviorandnarrowsizedistributionimplythatthesamplepreparedinthisworkisagoodcandidateforthepossiblebiomedicalapplications.

Conclusions

Inconclusion,nearlymonodispersedCoFe2O4nanoparti-cleswerepreparedunderasimplehydrothermalcondition.Theas-synthesizedsamplesareconsideredtohavepoten-tialapplicationsinbiomedicineforitsnarrowparticlesizedistribution,highsaturationmagnetizations,andsuper-paramagnetizationatroomtemperature.Thesimplesyn-thesisrouteusedinthisworkisexpectedtobeageneralapproachforthepreparationofbinaryandternarymetaloxide,especiallyspinelferrite.

AcknowledgmentsThisworkissupportedbyChinaPostdoctoralScienceFoundationFundedProjectandtheNationalNaturalScienceFoundationofChinaunderGrantNos.50602020.

OpenAccessThisarticleisdistributedunderthetermsoftheCreativeCommonsAttributionNoncommercialLicensewhichper-mitsanynoncommercialuse,distribution,andreproductioninanymedium,providedtheoriginalauthor(s)andsourcearecredited.

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