Monalisa Pal, Rupali Rakshit, and Kalyan Mandal
Citation: Applied Physics Letters 104, 233110 (2014); doi: 10.1063/1.4882904 View online: http://dx.doi.org/10.1063/1.4882904
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FacilefunctionalizationofFe2O3nanoparticlestoinduceinherentphotoluminescenceandexcellentphotocatalyticactivity
MonalisaPal,a)RupaliRakshit,andKalyanMandal
S.N.BoseNationalCenterforBasicSciences,Block-JD,Sector-3,SaltLake,Kolkata700098,India
(Received18April2014;accepted30May2014;publishedonline11June2014)
Herein,wereporttheemergenceofintrinsicmulticolorphotoluminescenceinFe2O3nanoparticles(NPs)rangingfromblue,cyan,togreen,uponfacilefunctionalizationandfurthersurfacemodificationwithasmallorganicligand,Na-tartrate.Moreover,wehavefoundunprecedentedphotocatalyticpropertyofthefunctionalizedFe2O3NPsinthedegradationofamodelwater-contaminant.MeticulousinvestigationthroughUV-visibleabsorptionandfluorescencestudyalongwiththeoreticalsupportfromliteratureunfoldsthatligand-to-metalcharge-transfertransitionfromthetartrateligandtothelowestunoccupiedenergylevelofFe3þoftheNPsanddÀdtransitionscenteredoverFe3þionsintheNPsplaythekeyroleintheemergenceofmultiplephotoluminescencefromtheligandfunctionalizedFe2O3NPs.Moreover,vibratingsamplemagnetometrymeasurementsdemonstratethatthesurfacemodificationchangesthemagneticbehaviourofFe2O3NPsuponfunctionalization.Webelievethatthegreatpotentialofourversatile,ferromagnetic,multicolorphotoluminescentFe2O3NPswouldstimulatethedevelopmentofnumerousopportunitiestowardtheirbiologicaland
C2014AIPPublishingLLC.[http://dx.doi.org/10.1063/1.4882904]technologicalapplications.V
Developmentofmultifunctionalnanoprobecombining
variousbeneficialpropertieswithinasingleentityhasbeenapivotalresearchareasincethepastdecade.Inthisrespect,transitionmetaloxidenanoparticles(NPs)haveattractedutmostinterestinrecentyears,becauseoftheiruniqueopti-cal,electronic,magnetic,andcatalyticproperties.Moreover,nanotechnologyandbiologyhavesynergizedindynamicde-velopmentofapromisingemergingresearchareacallednanobiotechnology.Inparticular,properlyfunctionalizedmagneticNPs(MNPs)possessmanifoldadvantages1whichgiverisetonumerousexcitingopportunitiesinthefieldofbiomedicalapplicationssuchasmagnetictweezersinmag-neticseparationofproteinsorcells,2extractionofDNAmol-ecules,3andtherapeuticapplicationsincludingACmagneticfield-assistedhyperthermia.4Noticeably,MNPscanbecon-trolledbyanexternalfield,whichpromotestargeteddeliveryofradioactiveisotopesanddrugsforradiotherapyandchem-otherapyaswellasgenetargetingandinnoninvasivediag-nosis,enhancingcontrastinmagneticresonanceimaging(MRI),5andfluorescenceimaging.6–8Additionally,tunablesizeofMNPsrangingfromafewnanometerstotensofnanometersfacilitatestheirinteractionprobabilitywithdifferentbiologicalentities.Ontheotherhand,MNPshavebeenprovedtobeapromisingcandidateincatalysisforselectivechemicaltransformationshavingbotheconomicandenvironmentalbenefits,consideringtheirhighactivity,lowcost,facilepreparationmethod,adequatestability,andcontrolledseparationbyanexternalmagneticfield.9–13Sofar,MNPsforfluorescenceimaginghavebeenpre-paredeitherbymolecularattachmentwithfluorescentdyesorpolymersorformingnanocompositeswithquantumdots(QDs).7,8Poorphotostabilityoffluorescentdyesandinherent
a)Authortowhomcorrespondenceshouldbeaddressed.Electronicaddresses:monalisa.pal6@gmail.comandmonalisa12@bose.res.in
toxicityofQDsimposesevereconcerntotheirbio-imagingapplications.14So,tosolvetheseproblems,thedevelopmentofbiocompatibleMNPs,havingintrinsicphotoluminescencepropertyandphotostability,ishighlydesirable.AmongtheMNPs,Fe2O3nanomaterialshaveattractedsignificantatten-tionoftheresearchersbecauseoftheirinnateenvironmen-tallybenigncharacterandoutstandingthermalstabilityinpracticalapplicationssuchasintargeteddrugdelivery,15magneticdatastorage,gassensing,Li-ionbattery,16,17fabri-cationofphotoanodesforphoto-assistedelectrolysis,pig-ments,andcatalysis.18–23Butthedevelopmentofintrinsicphotoluminescence,playingwiththesurfaceelectronicstructureoffunctionalizedFe2O3NPsissparseinliterature.Inspiteofenormouseffortsofdifferentgroups,aqueousphaseinsolubilityandtheabsenceofanyinherentphotolu-minescencepropertiesoftheNPsinhibittheirdirectbiomed-icalandseveraltechnologicalapplications.Thus,fabricationofappropriatelysurfacemodifiedFe2O3NPshavingintrinsicphotoluminescencetoexploreitsdiversebiologicalandtech-nologicalapplicabilityishighlydesirable.
OurpresentworkreportsthedevelopmentofFe2O3NPsasamultifunctionalnanoprobehavinginherentmulticolorphotoluminescence,ferromagnetism,andexcellentphotoca-talyticactivity,simultaneously.Utilizingthereactivityoftartrateligands,wehavesolubilizedtheas-preparedNPsintothewatermedium.Uponfurthersurfacemodificationofwater-solubilizedtartrate-functionalizedFe2O3NPs,wehavediscoveredtheemergenceofmulticolorphotolumines-cence,startingfromblue,cyan,togreen.Mysteryofthegen-erationofthismulticolorphotoluminescencepropertycanbesatisfactorilysolvedwithligandfieldtheory.Ithasbeenfoundthattheligand-to-metalcharge-transfer(LMCT)fromtartrateligandtothelowestunoccupiedenergylevelsofFe3þmetalionsintheNPsanddÀdtransitionsplaythekeyrole.Finally,wehaveintendedtoexploitthebroadphotoexcitation(coveringalargeportionofUV-visregion)ofthe
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functionalizedFe2O3NPsinphotocatalysis.Interestingly,thefunctionalizedFe2O3NPsexhibitexcellentphotocata-lyticpropertyinthedegradationofmethyleneblue(MB),acommonlyusedorganicdyeintextileindustriesandamodelwater-contaminant.
WehavesynthesizedFe2O3NPsfollowingawetchemi-calmethodpreviouslyreportedbySunetal.,withslightmodification24andfunctionalizedwithtartrateligands(describedinsupplementarymaterial38indetail).TEM(Transmissionelectronmicroscopy)studywasperformedtocharacterizethesizeandmorphologyoftheas-preparedNPsshowninFig.1(a).TEMimageofFe2O3NPsdemonstratesthataveragesizeoftheparticleswas$50nm.XRD(X-raydiffraction)patternoftheas-preparedNPs,showninFig.1(b)exactlymatcheswiththeaphaseofFe2O3(hematite,JCPDS,fileno.33-0664).Thepresenceofaverylowintensitypeakcorrespondingto(400)planeofc-Fe2O3(maghemite,JCPDS,fileno.39-1346)intheXRDpatternofas-preparedNPsindicatesthattheas-preparedNPswerepre-dominantlyinaphase.EDX(Energy-dispersiveX-ray)spec-trumoftheNPsasshowninFig.1(c)confirmsthepresenceofbothFeandO.Fig.1(d)showstheSAED(Selectedarea(electron)diffraction)pattern,indicatinghighcrystallinityoftheas-preparedFe2O3NPs.
TosolubilizeFe2O3NPsinanaqueousenvironment,wefunctionalizedtheas-preparedNPswithabiocompatibleorganicligand,Na-tartrate.Aftersurfacefunctionalization,sizeofthehighlywater-solubilizedNPsremainedalmostunchangedasevi-dentfromFig.1(f).TheHRTEM(High-resolutiontransmissionelectronmicroscopy)imageoftartrate-functionalizedFe2O3NPs(asshowninFig.1(e))clearlyexhibitshighlycrystallinenatureoftheNPs.Thecalculatedinterplanardistancebetweenthe
fringeshasbeenfoundtobe0.28nmcorrespondingto(104)planeofthea-Fe2O3crystallattice.
FTIR(Fouriertransforminfraredspectroscopy)studywascarriedoutforFe2O3NPsbeforeandafterfunctionaliza-tionalongwiththeligandalone,toconfirmtheattachmentofligandmoleculeswithNPs’surface.AsshowninFig.2(a),thecharacteristicpeaksofFe2O3NPsat547and470cmÀ1canbeassignedtoFe-Ostretchingandbendingvibrationmode,respectively.25Thosepeaksarenotdis-tinctlyvisibleafterfunctionalization(i.e.,inthecaseofthefunctionalizedFe2O3NPs),whichsuggeststhateffectivesur-facemodificationoftheNPshastakenplaceuponinteractionwithtartrateligands.Whereas,incaseoftartrate,twosharppeaksarisingat1066and1112cmÀ1areduetotheC–OHstretchingmodes,26andpeaksat1411and1621cmÀ1areattributedtosymmetricandasymmetricstretchingmodesofthecarboxylategroups(COOÀ)oftartrate,respectively.27UponinteractionwiththeNPs’surface(i.e.,incaseoffunc-tionalizedFe2O3),allthesedifferentbandsareperturbedsignificantlyalongwiththebandat3399cmÀ1,generatedduetothestretchingvibrationalmodesofhydroxylgroup(O-H),26clearlyindicatesthatboth–COOÀand–OHgroupsareinvolvedinthefunctionalizationprocess.
Fig.2(b)showsthemagneticstudyofbothas-preparedandfunctionalizedFe2O3NPs(inset)atroomtemperature.Itisnoteworthythattheshapeofthehysteresisloopiscon-stricted.Constrictedloopsaretypicallyobservedinasystemhavingmixtureofsoftandhardmagneticphases.Thus,atroomtemperaturetheobservedresponsecouldbedueto
FIG.1.(a)TEMimageoftheas-preparedbareFe2O3NPs.(b)XRDpatternofas-preparedFe2O3NPs.Alldiffractionpeaksinthefigureareperfectlyindexedintheliteraturetotherhombohedralcorundumstructureofa-Fe2O3NPsalongwithonlyoneimpuritypeakcorrespondingto(400)planeofc-Fe2O3.(c)EDXspectrumoftheNPsindicatesthepresenceofbothFeandO.(d)SAEDpatternofas-preparedFe2O3NPsindicateshighcrystallinity.(e)HRTEMimageoftartrate-functionalizedFe2O3NPsindicateshighcrys-tallinityremainsunalteredaftersurfacemodificationandshowslatticefringes.(f)TEMimageofthetartrate-functionalizedFe2O3NPs.
FIG.2.(a)FTIRspectraofas-preparedFe2O3andfunctionalizedFe2O3NPsalongwithNa-tartratealone.(b)Magnetizationversusappliedmagneticfieldplotfortheas-preparedbareFe2O3NPs.IntheinsethysteresisloopoffunctionalizedFe2O3NPsindicatesferromagneticnatureatroomtempera-ture(300K).
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233110-3Pal,Rakshit,andMandalAppl.Phys.Lett.104,233110(2014)
FIG.3.UV-visabsorptionspectraoffunctionalizedFe2O3NPsandNa-tartrate.
combinationofaandverysmallamountofcphases(asevi-dentfromtheXRD).Thecphaseisasoftphasewithhighermoment,whereastheaphasehavehighercoercivitybutlowermoment.Combinationofthesetwomagneticproper-tiesleadstoaconstrictedhysteresisloop.Coercivityofas-preparedsamplewasfoundtobe174.34Oeandthemag-netizationcurvedidnotsaturateuptothemaximumappliedmagneticfieldof1.6T.Afterfunctionalization,themagneticbehaviourofFe2O3NPsconvertedtotypicalferromagneticpatternwithslightdecreaseincoercivity(145.46Oe)andsaturationmagnetization.Thechangeinmagneticbehaviouruponfunctionalizationcanbeexplainedbyligandfieldtheory.28AlthoughtheoxygencoordinationforFe3þionsinbulkorcoreFe2O3,havingcorundumstructure,isquitesym-metricalintheformsofoctahedronortetrahedron,thecoor-dinationsymmetryisgreatlyreducedformetalcationsatthesurfaceduetomissingofsomecoordinationoxygenatoms.Consequently,themagneticstructureatthesurfacelayercouldbedrasticallydifferentthanthecore.Surfaceusuallyexhibitssomedegreeofspindisorderandpinning.Qualitatively,adsorbedligandscanbeviewedaseffectivelytakingthepositionsofthemissingoxygenatoms,whichmakesthesymmetryandcrystalfieldofthesurfacemetalionmorecloselyresemblingthatofthecore,andtherefore
reducesthespindisorderandpinning.SuchchangescertainlyaffectthesurfaceanisotropyandconsequentlythecoercivityofNPs.Tartrateligand,containingbothther-donor(ÀOHÀ)andp-donor(ÀCOOÀ)functionalgroupfavoursthequench-ingofmagneticmomentsofFeþ3ionsinthesurfaceoffunc-tionalizedFe2O3NPs,resultingadecreaseinthesaturationmagnetization.29Ontheotherhand,quenchingofthemag-neticmomentsreducesitsspin-orbitcoupling,leadingtodecreaseinmagnetocrystallineanisotropy,whichresultsinreductionofcoercivity28infunctionalizedFe2O3NPsascom-paredtotheas-preparedNPs.
UV-visabsorbancespectrumoffunctionalizedFe2O3NPsasdepictedinFig.3consistsofabroadbandhavingmaximumataround320nm.Interestingly,uponexcitingthesampleat320nm,weobservedphotoluminescenceat410nm,althoughwithalowintensity.Toincreasethephoto-luminescenceintensity,weperformedfurthersurfacemodifi-cationbyheatingafterhighpHtreatment,whichresultedingenerationoftwomorephotoluminescencepeakswithmul-tiplefoldincreaseinoverallintensity,uponexcitationatproperwavelengths.Thereasonforenhancementofthepho-toluminescenceintensityaswellasgenerationoftwomoreopticalbandsuponsurfacemodificationcouldbeduetoincreasedcoordinationbetweentheligandfunctionalgroups(carboxylateandhydroxylmoieties)andFe3þcentersattheNPsurface.Fig.4(a)showsnormalizedsteady-statephotolu-minescenceemissionspectraobtainedfromsurfacemodifiedFe2O3NPs.Uponexcitationatwavelengthsof320,365,and410nm,theNPssolutiongaverisetointensephotolumines-cencepeaksat410,460,and490nm,respectively.Except320nm,otherobservedexcitationbandsataround365and410nmwerenotobservedintheabsorptionspectrum(Fig.3),presumablybecausethesebandsweremaskedbythemoreintense320nmabsorption,however,weredis-tinctlyvisibleintheexcitationspectrumasshowninFig.4(b).ThephotoluminescencemicrographsoffunctionalizedFe2O3NPs,demonstratethattheblackpowderoffunctional-izedNPsunderbrightfield(Fig.4(c)),givesrisetophotolu-minescentcolorslikecyan(Fig.4(d))andgreen(Fig.4(e))uponexcitationat365and436nm,respectively,byusing
FIG.4.(a)Normalizedsteady-statephotoluminescenceemissionspectraobtainedfromtartrate-functionalizedFe2O3NPswiththreedifferentexcitationwavelengthsof320,365,and410nm,respectively.(b)Photoluminescenceex-citationspectraoffunctionalizedFe2O3NPsatdifferentemissionmaximaof405,460,and500nm,respectively.Photoluminescencemicrographsofpow-derfunctionalizedFe2O3NPsunder(c)brightfield,(d)UV(365nm),and(e)blue(436nm)lightirradiation.
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233110-4Pal,Rakshit,andMandalAppl.Phys.Lett.104,233110(2014)
properfilters.Noticeably,thephotoluminescencemicroscopicimagesofas-preparedbareFe2O3NPsunderidenticalcondi-tions(asshowninFig.S1ofthesupplementarymaterial38)havenosuchcoloration.Photoluminescencequantumyields(QY)ofthefunctionalizedFe2O3NPshavebeencalculatedbyfollowingtherelativemethodofWilliamsetal.,30whichinvolvestheuseofwellcharacterizedstandardfluorescentcompoundswithknownQYvalues.PhotoluminescenceQYsof8.958%(for410nmband),0.2%(for460nmband),and0.5%(for490nmband)wereobtainedrelativetothestandardfluorescentcompoundssuchas2-aminopurine(2AP),40,6-diamidino-2-phenylindole(DAPI),andHoechst33258,respectively.Thus,theemergenceofmulticolorphoto-luminescenceinFe2O3NPswasactivatedbytartratefunction-alizationandthenreinforcedbyfurthersurfacemodification.
Themysteryofthegenerationofintrinsicphotolumines-cencecanalsobesolvedbyligandfieldtheory.28,31Ontheba-sisofligandfieldtheory,theligandcoordinationprovidesthecrystalfieldsplittingenergy(CFSE)Dgeneratedfromdorbi-talssplittingwithamagnitudedeterminedbytheligandsatagivencoordinationsymmetry.Asaligandbecomesmoreba-sic,thestrengthofthemetal-ligandrbondincreases,andcon-sequentlyCFSEDassociatedwiththeligandincreases,whichultimatelyleadtothesplittingbetweenthedegeneratedorbi-tals.Ontheotherhand,duetothestrongLMCTfromHOMO(Highestoccupiedmolecularorbital,centeredintheligand)toLUMO(Lowestunoccupiedmolecularorbital,centeredinmetalions),bondinginteractionbetweenthemetalandligandincreasessignificantly.Photoluminescencepeakarisingat410nmcanbeattributedtoLMCTinvolvingHOMOoftar-trateligandandLUMOcenteredovermetalionFeþ3.32Theotheremissionpeaksat460and490nmuponexcitationat365and410nmcanbeattributedto6A1g!4T1gand6A1g!4T2gtransitionsinvolvingd-dorbitalsofFe3þions,respec-tively,duetothecrystalfieldsplittingofaFeO6octahedronwithOhsymmetryasthefirstapproximation.Boththed-dtransitionsareformallydipoleandspinforbiddentransitions,however,theycanhaveconsiderablestrengthsduetotherelaxationofselectionrulesbyoctahedraldistortionandspin-orbitcouplingasdiscussedinotherFe3þ(3d5)containingmaterials.33,34Consideringtherecentremarkablegrowthofnanocatal-ysisbyengineeringandmanipulatingvariousmaterialsatthenanoscaletoacceleratetherateofseveralbeneficialreac-tions,weintendedtoutilizethestrongbroadexcitationofthefunctionalizedFe2O3NPsthroughouttheUV-visregion,inphotocatalysisforwaste-watertreatment.FunctionalizedFe2O3NPsshowedunprecedentedphotocatalyticproperty(asshowninFig.5(a))towardsthedegradationofmethyleneblue,acommonlyuseddyeintextileindustriesandamodelwater-contaminent,uponUVlightirradiation.WehavefoundthatthephotodegradationofMBinpresenceoffunc-tionalizedFe2O3NPstakesplaceexponentiallywithtimefollowingfirst-orderrateequationwithakineticratecon-stant(k)of27.55Â10À2minÀ1.
Wealsocheckedthereusabilityofthecatalyst,inevery64minofinterval.WeaddedsamedoseofMBintothereac-tionmixtureupto10dose,keepingthecatalystconcentra-tionfixed(withoutadditionofextracatalystafterthe1stcycle),MBdecompositionratesofdifferentcycleswere
FIG.5.(a)UV-visspectralchangesofaqueoussolutionofMBwithtimeinpresenceoffunctionalizedFe2O3NPs,underUVirradiation.(b)TheplotsofrelativeconcentrationofMBmonitoredat660nmversustimeforconsec-utive10cycles,showingreusabilityoffunctionalizedFe2O3NPsinMBdegradationunderUVlight.(c)TherateofphotocatalyticdegradationofMB(monitoredat660nm)inabsenceandpresenceoffunctionalizedFe2O3NPsundervisiblelight.InsetshowsthefullabsorptionspectraofMBinpresenceoffunctionalizedFe2O3NPswithtime,undervisible-lightirradiation.
measuredbymonitoringthedecreaseofMBabsorbanceat660nmusingUV-visspectroscopy.Fig.5(b)showstheplotsofrelativeconcentrationofMBversustime,upto10consec-utivecycles,affirmingthereusabilityoffunctionalizedFe2O3NPscatalystwithalmostconsistentdegradationrate.Havingevidencefromourrecentfindingsincaseofphotode-gradationbyligandfunctionalizedMnFe2O4andMn3O4NPs,weproposethatthephotodegradationprocessmayfol-lowradicalpathwayinvolvingreactiveoxygenspecies(ROS).35–37Furthermore,weinvestigatedwhethervisiblelightcanalsoactivatethephotocatalyticefficiencyoffunc-tionalizedFe2O3NPs.AsshowninFig.5(c),wefoundthatfunctionalizedFe2O3NPscandegradeMBinpresenceof
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visiblelightalso,following1storderreactionkineticshavingrateconstant(k)of54Â10À4minÀ1.DecreaseinrateofphotocatalysisinpresenceofvisiblelightascomparedtoUVlightcanbeattributedtohigherenergyofUVlightthanvisiblelightaswellaslowerabsorbanceoffunctionalizedFe2O3NPsinvisibleregionincomparisonwithUVregion.
Inconclusion,developmentofsurfacemodifiedFe2O3NPsasmultifunctionalnanoprobehavingsimultaneouslyintrinsicmultiplephotoluminescencecoveringabroadrangeofUV-visregionfromblue,cyan,togreen,inherentferro-magnetism,andexcellentphotocatalyticpropertieshavebeenachievedbyaveryfacileligandfunctionalizationandsubsequentsurfacemodificationstrategy.Moreover,corre-latingwiththereportedtheoreticalaspect,wehaverationallyexplainedtheemergenceoftheinterestingopticalproperties.Wehopethatthemagneto-photoluminescentFe2O3NPswillhavegreatrelevancetodiversefieldofbiologicalandtech-nologicalapplicationsrangingfrombioimaging,drugdeliv-ery,tonanocatalysis.
WethankCSIRforthefinancialgrants.AuthorswouldliketothankMr.MaheshAgarwalandDr.DeepakKumarSinhafromIACS,Kolkataforprovidingfluorescencemi-croscopyfacilityusedinthisstudy.
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