1、2214Article物理化學(xué)學(xué)報(WuliHuaxueXuebao)ActaPhys.-Chim.Sin.2012,28(9),2214-2220doi:10.3866/PKU.WHXB201206122S網(wǎng)狀多級孔結(jié)構(gòu)氧化鐵的制備、合成機(jī)理及其光催化性質(zhì)范海濱張東鳳*郭林*(北京航空航天大學(xué)化學(xué)與環(huán)境學(xué)院,北京100191)摘要:以九水合硝酸鐵為原料,利用改進(jìn)的聚合誘導(dǎo)膠體聚集(PICA)的方法制備出三維網(wǎng)絡(luò)狀多級孔結(jié)構(gòu)氧化鐵(HPH).此結(jié)構(gòu)的制備關(guān)鍵是在合成過程中尿素和甲醛聚合生成脲醛樹脂(UF).脲醛樹脂一方面在鐵的羥基氧化物生長過程中與之雜化形成雜化產(chǎn)物Fe-UF,另一方面則進(jìn)一步

2、聚合,形成脲醛樹脂微球(UFM).脲醛樹脂微球作為模板誘導(dǎo)雜化產(chǎn)物Fe-UF在其表面的聚集.微球與微球之間則由于表面存在的脲醛樹脂間的聚合會相互交聯(lián)形成網(wǎng)絡(luò)狀結(jié)構(gòu).經(jīng)過煅燒處理后,脲醛樹脂及脲醛樹脂微球的分解導(dǎo)致不同尺寸孔結(jié)構(gòu)的生成.光催化研究結(jié)果表明,產(chǎn)物對羅丹明B的降解效率是商用納米氧化鐵的4倍.關(guān)鍵詞:-Fe2O3;多級孔結(jié)構(gòu);聚合誘導(dǎo)膠體聚集法;光催化降解O648中圖分類號:Fabrication,FormationMechanismandthePhotocatalyticPropertiesofHierarchicalPorousHematiteNetworksFANHai-BinZ

3、HANGDong-Feng*GUOLin*(SchoolofChemistryandEnvironment,BeihangUniversity,Beijing100191,P.R.China)Abstract:Hierarchicalporoushematite(HPH)networkstructuresweresuccessfullyconstructedusinganimprovedpolymerizationinducedcolloidaggregationprocesswithFe(NO3)39H2Oastherawmaterial.Thepolymerizationbetweenur

4、eaandformaldehydeintourea-formaldehyde(UF)resinisthekeyfactorforthisconstruction.TheUFresinsappeartobeadvantageousintworespects:theUFoligomerhybridswithferrichydroxide(Fe-UF)andUFpolymerformedmicrocapsules(UFM)actedastemplatestoinducetheaggregationofFe-UFhybridsintomesoporousspheres.Thefurthercrossl

5、inkreactionsamongthehybridspheresgeneratethenetworkstructure.Aftercalcination,thedecompositionoftheUFresinandtheUFMproducesnanoporesinthenanorodsubunitsandmacroporesinthenetworkstructure,respectively.ThephotodegradationactivityoftheuniquestructuredHPHisfourtimesthatofthecommercialhematitenanoparticl

6、eswithrhodamineB(RhB)aspollutant.KeyWords:-Fe2O3;Hierarchicalporousstructure;Polymerizationinducedcolloidaggregationmethod;Photodegradation1IntroductionHierarchicalporousmaterials(HPM),withinterconnectedporesofmultiplelengthscalesasthestructurecharacter,haveattractedspecialresearchinterest.Thecombin

7、edadvantagesofthelargespecificsurfaceareaofmesoporousmaterialsandthehighefficiencyofmasstransportofmacroporousones1-4makethehierarchicalporousmaterialsaspromisingcandi-datesespeciallyinthefieldsrelatedtothediffusion-limitedReceived:May2,2012;Revised:June11,2012;PublishedonWeb:June12,2012.Correspondi

8、ngauthors.ZHANGDong-Feng,Email:dfzhang;Tel:+86-10-82338492.GUOLin,Email:guolin;Tel:+86-10-82338162.TheprojectwassupportedbytheNationalKeyBasicResearchProgramofChina(973)(2010CB934700),NationalNaturalScienceFoundationofChina(21173015),andFundamentalResearchFundsfortheCentralUniversities,China(YWF-11-

9、03-Q-085).國家重點(diǎn)基礎(chǔ)研究發(fā)展規(guī)劃項(xiàng)目(973)(2010CB934700),國家自然科學(xué)基金(21173015)及中央高校基本科研基金(YWF-11-03-Q-085)資助EditorialofficeofActaPhysico-ChimicaSinicaNo.9FANHai-Binetal.:Fabrication,FormationMechanismandthePhotocatalyticPropertiesofHPHNetworks2215processsuchascatalysis,5-7adsorption/separation,8-11solarcells,12supp

10、orts,13-15cathodeandanodematerialsforlithiumionbattery16,17andsoon.Forexample,hierarchicalporousstructuresexhibitenhancedcatalysisandadsorptionactivitieswiththeincreasedexposedactivesitesandtheimprovedmasstransferability.Thesuppressedrecombinationofphotogenerat-edelectronsoriginatedfrommesoporesandt

11、heenhancedlightabsorptionandtheimprovedelectrolytediffusionefficiencyowingtothemacroporesendowHPMwithgreatapplicationpotentialsinsolarcells.12Theinterconnectedporesresultinshorterdiffusionlength,higherelectricalconductivity,andmoreactivesitesforcharge-transferreactions,whichmakeHPMaspromisingcandida

12、tesforbatterymaterials.18Further-more,thehierarchicalporousstructurecouldalsobeeasytobedopedwithhomogeneousorselectiveactivesites,19whichwillextendtheapplicationofthecorrespondingmaterials.Template-basedprocessesdemonstratehighefficiencyfortheconstructionofporousstructures.Mesoporousproductsarealway

13、sachievedbyusingthesurfactantassofttemplate20-27ortheobtainedmesoporousmaterialsashardtemplate;28,29whilethelatexspheres,30-33suchaspolystyrene(PS),poly(methylmethacrylate)(PMMA)orothermaterialswithcertainstruc-tures,34-36areusuallyemployedashardtemplateforthecon-structionofmacroporousstructures.Sin

14、cetheporesizecanbefeasiblytunedbychoosingtemplatewithdifferentstructures,thebi-templatemethodbecomestheprimarychoiceforthepro-ductionofhierarchicalporousstructures.Forexample,Davisetal.34synthesizedsilicafiberswith0.5-m-widechannelsen-closedinwallsofmesoporousMCM-41byusingbacterialsu-perstructurethr

15、eadashardtemplateandcetyltrimethylammo-niumbromide(CTAB)assofttemplate.Intheco-presenceofpolystyrenelatexspheres,surfactants(triblockcopolymers)andcosurfactants(butanolorpentanol),Senetal.37preparedor-deredporoussilicamaterialssimultaneouslypossessingmacro-pores(200-800nm),mesopores(8.2nm),andmicrop

16、ores(2nm).Shietal.38fabricatedhierarchicalporousSiCwithca230nmmacroporesandca4.7nmmesoporesbyusingPSashard-templateandPluronicF127assoft-template.WiththeusingofSiO2ashardtemplateandF127assofttemplate,monolithiccarbonsieveswithca230nmmacroporesandca10nmmeso-poreswereobtained.39HierarchicalporousAl2O3

17、withca320nmmacroporesaswellashexagonallypackedmesoporeswasalsoobtainedbyusingPSandPluronicP123ashardandsofttemplates,respectively.40Generally,thisstrategyinvolvesatime-consumingmulti-stepprocedure:first,thepreparationoftheperiodicthree-di-mensionalarraysofhardtemplate;second,theinfiltrationofthemixe

18、dsolutioncontainingtheinorganicprecursorandsofttemplateintothevoidsofthosearrays;third,thehydrolysisoftheinorganicprecursorandtheremovaloftheexcessivesolu-tionbyvacuumfiltration;fourth,theremovalofthehardtem-platethroughcalcinationorextraction.Forthesuccessfulcon-struction,thereareseveralotherconsid

19、erations,suchasthecompatibilitybetweensolventandthehardtemplatearrays,41theprecisecontrolofthehydrolysisrateoftheprecursorfortheefficientinorganic-organicco-assembly,andthestabilityoftheskeletonaftertheinfiltration.41,42Forthebettercontrolofthehydrolysisrate,metalalkoxidesareusuallyemployedastheinor

20、ganicprecursor,whichisexpensiveanddifficulttopre-pare.Theserequirementslimittheprogressmainlyfocusedonsilica,titania,alumina,andtheir-basedcompounds.Transition-metaloxidesareexpectedtohavegreatapplicationpotentialsinvariousfields,suchasgassensors,batterymaterials,absor-bent,andcatalyst.However,there

21、arefewreports6,8,43,44onthefabricationofhierarchicalporoustransition-metaloxidesow-ingtothelackofafacialmethod.Recently,apolymerizationinducedcolloidaggregation(PICA)strategywasdevelopedforthefabricationofhierarchi-calporousmaterials.45,46Inthisprocedure,thecolloidofthetar-getmaterial(gottenbyhydrol

22、ysisprocessorbysuspendingthepre-preparednanoparticlesinsolution)firstformedinorganic-organichybridwithmonomers/oligomerandthepolymeriza-tionofthemonomer/oligomerinducedtheaggregationofthecolloid.Theremovalofthepolymerproducedhierarchicalpo-rousinorganicmaterials.Theporesizecanbeeasilytunedbychoosing

23、differentmonomersorintroducingporogens.47,48Obviously,thePICAstrategyismorecontrollableincompar-isonwiththebi-templatemethod,10,45,48whichmakesitapoten-tialmethodforthesynthesisofporoustransitionmetaloxides.Jiangetal.10gaveaccesstoporoustitaniamicrospheresbytak-ingusageofthepolymerizationbetweenurea

24、andformalde-hyde.Shietal.45gotthezeolitemicrosphereswithhierarchicalporousstructurethroughanimprovedPICAstrategywiththeuseofurea-formaldehyde(UF)resin.Maoetal.46reportedthefabricationofbimodalmesoporoushematitemicrosphereswiththeaidofpolymerizationofacrylamide-directedaggregation.However,theobtained

25、productsalwaysexhibitedasseparatedmicro-spheres.Thearrayednetworkstructures,expectedtohavemorepotentialapplication,havenotbeenreportedyet.Herein,togethematitewithhierarchicalporousnetworkstructures,animprovedPICAprocesswasproposed,whichfi-nallyverifiedtobeasuccessfulroute.Itwasbelievedthattheenhance

26、dpolymerizationextentbyadoptinglowerpHandlon-geragingtimeisresponsiblefortheformationofthenetworkstructure.Sincethissynthesisrouteishard-templatefree,con-venient,lowcostandtime-saving,itisexpectedtoprovideanewideaforthesynthesisofhierarchicalporousnetworkstruc-turesofothertransition-metaloxides.2Exp

27、erimental2.1MaterialsAllthechemicalswereusedasreceivedwithoutfurtherpuri-fication.Fe(NO3)39H2O(AR,98.5%)andurea(AR,99.0%)werepurchasedfromXilongChemicalIndustryIncorporatedCo.Ltd.NH3H2O(25%-28%,massfraction)andethanol(AR,99.7%)werepurchasedfromBeijingChemicalWorks.2216ActaPhys.-Chim.Sin.2012Vol.28Fo

28、rmaldehydesolution(37.0%-40.0%,massfraction)waspur-chasedfromBeijingYilifinechemicalsCo.Ltd.2.2SynthesisofhierarchicalporoushematiteThetypicalsynthesiswasdividedintothreestagesforbetterunderstandingtheprocess.Inthefirststage,4mmolFe(NO3)39H2Owasdissolvedintoamixedsolutionof10mLethanoland1mLdeionized

29、waterina50mLflaskbyultrasonic.Then,themixturewasheatedat60Cfor40minunderviolentstirring.Toincreasethehydrolysisextent,2LNH3H2Owasaddedintothesolutionandferrichydroxidecolloidformed.Inthesecondstage,afterthecolloidsolutionwascooleddownbyputtingtheflaskinice-waterbathfor10min,1.2gureaand1.2mLformaldeh

30、ydesolutionwereintroducedsequentiallyun-dermildstirring.Afterstirringfor2min,themixturewaskeptstaticfor12h,whichgeneratedyellowprecipitate.Inthelaststage,theyellowprecipitatewasseparatedbycentrifugingat5000rmin-1andwashedwithethanolandwatersuccessivelyforseveraltimes,anddriedat60Cinanoven.Thedriedpr

31、e-cipitatewascalcinatedbyslowlyincreasingtemperaturefromroomtemperatureto400Catarampingrateof1Cmin-1andmaintainedat400Cfor4h.Itproducedhierarchicalpo-roushematite(HPH)network.2.3CharacterizationX-raydiffraction(XRD)patternswererecordedonaRigakuD/max-2200diffractometerwithCuKradiation(=0.15416nm).The

32、morphologyofthesampleswasobservedbyHitachiS-4800withanacceleratingvoltageof10kV.Transmissionelec-tronmicroscopy(TEM)andhigh-resolutionTEM(HRTEM)characterizationswerecarriedoutwithJEOLJEM-2100Fmi-croscopeoperatedat200kV.Nitrogenadsorption-desorptionisothermsweremeasuredonQuantaChrome(Nova2200e)at77K.

33、Allsamplesweredegassedonavacuumlineat300Cfor3hbeforeanalysis.TheBrumauer-Emmett-Teller(BET)methodwasutilizedtocalculatethespecificsurfaceareas.TheporesizedistributionswerederivedfromtheadsorptionbranchesoftheisothermsthroughtheBarrett-Joyner-Halenda(BJH)model.Fouriertransforminfraredspectroscopy(FTI

34、R)analyseswereperformedonanAVATAR360FT-IRspectrome-terintherangeof400-4000cm-1frequencywithascanrateof0.6329cms-1andresolutionof4.0cm-1.2.4PhotocatalysismeasurementThephotocatalyticactivitiesoftheas-preparedHPHmateri-alswereinvestigatedwithrhodamineB(RhB)aspollutant.Typically,30mgofthecatalysiswasdi

35、spersedinto60mLRhBaqueoussolution(110-5molL-1)andmagneticallystirredinthedarkfor2htoensurethethoroughdispersionandfullad-sorption.ThesolutionwasthenexposedtoUVlightirradiationwithadistanceof20cmfroma250-Whigh-pressuremercurylampatroomtemperature.Tomonitorthephotocatalyticpro-cess,4mLmixturesolutionw

36、asaspiratedfromthetesttubewithanintervalof10min.Aftercentrifugation,theUV-Visab-sorptionspectrumofthesupernatantwasrecorded.Theabsorp-tionspectrumofRhBsolutionswasmeasuredbyaLambda950(Perkin-ElmerInstruments)ultraviolet-visiblespectropho-tometer.Toevaluatethestructureeffect,photocatalyticactivi-ties

37、ofcommercialhematitenanoparticleswithaveragediame-tersofca30nm(purchasedfromAladdinInc.,labeledasCNPs)werealsomeasuredunderthesamecondition.3Resultsanddiscussion3.1CharacterizationsofHPHThehierarchicalporoushematite(HPH)wasobtainedbyanimprovedPICAmethod.X-raydiffraction(XRD)wasusedtocharacterizethec

38、rystalstructureoftheas-preparedproducts.AsshowninFig.1b,allthediffractionpatternscanbewellin-dexedto-Fe2O3crystalsofrhombohedralphasewithJCPDSNo.33-0664(Fig.1(a).Thestrongandsharpdiffractionpeakindicatesthehighcrystallinityoftheproduct.Scanningelectronmicroscopy(SEM)images(Fig.2a)showthattheproducti

39、softhree-dimensionalmacroporousnetworkstructurewithaveragemacroporesizeofaround1m.Thehigh-magnifiedSEMimage(Fig.2b)demonstratesthatthemacroporousframeworksarebuiltbytheaggregationofnano-rods.Transmissionelectronmicroscopycharacterizationshowsthattheaggregationproducesmesovoids(insetinFig.2c).High-ma

40、gnifiedTEMimagefurtherrevealsthatthesubunitnanorodsareindeednanoporous(Fig.2c).Fig.2disatypicalHRTEMimagerecordedonanisolatednanorod.Themeasuredinterplanardistanceoftwoadjacentlatticefringeswasabout0.37nm(Fig.2d),correspondingwellwith(012)planeoftherhombohedralhematite(-Fe2O3).Theselectedareaelectro

41、ndiffraction(SAED)patternshownintheinsetofFig.2drevealsthepolycrystallinenatureofthenetworkstructure.Thus,theelectronmicroscopyobservationresultsdemonstratethattheproductsareofhierarchicalporousstructure.Thehierarchicalporouscharacterwasfurtherconfirmedbynitrogenadsorption-desorptionmeasurement.Fig.

42、3showstheN2adsorption-desorptionisothermsoftheHPH.Thesurfacear-eaiscalculatedas37.15m221.8nm.Ascanbeg-1andtheaverageBJHporesizeisaboutseenfromtheisothermpattern,thereisaslowadsorptionofN2startingatlowerrelativepres-sure(p/p0)of0.25,whichindicatestheexistenceofporeswithFig.1XRDpatternsof(a)-Fe2O3with

43、JCPDSNo.33-0664and(b)hierarchicalporoushematiteNo.9FANHai-Binetal.:Fabrication,FormationMechanismandthePhotocatalyticPropertiesofHPHNetworks2217Fig.3Nitrogenadsorption-desorptionisothermsandthecorrespondingBJHporesizedistributioncurve(inset)oftheHPHThepore-sizedistributionwasdeterminedfromtheadsorpt

44、ionbranchoftheisotherm.Fig.2(a,b)SEMand(c)TEMimagesofHPHwithdifferentmagnifications,(d)atypicalHRTEMimagerecordedonananorodInsetin(d)isthecorrespondingSAEDpatternrecordedonanindividualsphere.nanoscaledsize.TheisothermexhibitstypeIVhysteresisloopsatp/p0of0.45-0.98,providingtheevidenceforthemesopo-rou

45、sstructure.Whenthep/p0islargerthan0.89,asharpin-creaseisobservedintheadsorptioncurve.Theadsorptionvol-umecontinuestoincreaseratherthanreachabalance,atypicalcharacteroftheexistenceoflargemicropores.49,50Therefore,thenitrogenadsorption-desorptionresultsfurtherconfirmthehierarchicalporousstructureofthe

46、products.3.2FormationmechanismTogetmoreinformationontheformationmechanismofthehierarchicalporoushematite(-Fe2O3),wetracedthegrowthprocessbycollectingthesamplesatdifferentdurationtime.Sincethecrystallinityofthecolloidsispoor,thecharacteriza-tionswereperformedaftercalcinationat400C.AsshowninFig.4(a,b)

47、,nanorodsaretheexclusiveproductsbeforethein-troductionofureaandformaldehyde.However,incomparisonwiththesubunitnanorodsofthefinalproductsasshowninFig.2c,thereexistsdifferenceintwoaspects.Firstly,theirsizesaresmallerthanthesubunitsofthehollowspheres.Secondly,nointraparticlenanoporesexistinthenanorodso

48、btainedatthefirststage.Whenthemixturewasagedfor6haftertheintro-ductionofureaandformaldehyde,separatedordimer/trimerofhollowspheres(HS)wereobtained(Fig.4(c,d),whichwereaggregatedbynanorods.AscanbeseenfromtheinsetofFig.4d,themorphologyofthenanorodsisthesameasthatofthesubunitinthefinalproducts.Withthea

49、gingtimefurtherFig.4SEM(a,c)andTEM(b,d)imagesoftheproductscollectedatdifferentstagesaftercalcinationinsetin(d)showsamagniedTEMrecordedontheframedareain(d).(a,b)beforetheintroductionofureaandformaldehyde,(c,d)agingfor6haftertheintroductionofureaandformaldehyde;2218ActaPhys.-Chim.Sin.2012Vol.28prolong

50、ed,hierarchicalporousnetworksdominatedtheprod-uctsasshowninFig.2.Onthebasisoftheabovediscussionandcharacterizations,atentativeformationmechanismofthehierarchicalporoushe-matitewasproposedasillustratedinScheme1.Inthefirststage,theweakalkalineNH3H2Oassociatedwithheatingpro-motedthehydrolysisofFe3+into

51、ferrichydroxides.Inthesec-ondstage,thepolymerizationbetweenureaandformaldehydeoccurred,whichproducedUFresins.Itiswell-documentedthatUFoligomeriseasytoadsorbontothesurfaceorevendopedintoinorganiccolloidstoformcomposite.10,51Therefore,theUFresinwouldformhybridswithferrichydroxide(Fe-UF)duringthefurthe

52、rcondensationoftheferrichydrox-ides.Toconfirmthis,theproductwascollectedwhenagingfor3haftertheintroductionofureaandformaldehyde.AscanbeseeninFig.S1a(seeSupportingInformation),nosphereap-pearedatthisstage.Theproductexhibitedasmixturesofnanoparticlesandamorphousmaterial,whichshouldbeUFoligomer.Afterbe

53、ingwashingseveraltimeswithwaterandethanol,theamorphousmaterialswereremovedeffectively.Fig.S2showedtheFTIRspectrumoftheproductsafterbeingwashing.Thedominatedpeakscanbeassignedtosignalsrelat-edtoUFresins.SincethefreeUFwasremovedbywashasdem-onstratedbyTEMobservation,theappearanceoftheFTIRsig-nalsindica

54、tedtheformationofFe-UFcomposite.Aftercalcina-tion,nanorodswithnanovoidsformascanbeseenfromFig.S1c,agreeingwithourproposal.Togetmoreinformationonthegrowthmechanism,wealsocollectedtheproductsobtainedaftertheintroductionofureaandformaldehydeandagedfor6h.TEMobservationdemon-stratedthattheproductsexhibit

55、edassphere-likemorphology.Scheme1SchematicillustrationofatentativemechanismfortheformationofHPHThehigher-magnificationTEMimageshowedthenanoparticleaggragationatthesurfaceofthespheres(Fig.S3c).Thecorre-spondingenergy-dispersivespectrum(EDS)showedthesig-nalsfromFe(Fig.S3c).DuringTEMobservation,thecore

56、be-camevoid(Fig.S3b).InconjunctionwiththeformationofFe2O3hollowstructureaftercalcination(Fig.4(c,d),itisrea-sonabletoconcludethatUFresinfurthercrosslinkedintomi-crocapsuleswithreactionprolonging.TheformedUFmicro-capsules(UFMs)servedassofttemplatefortheaggregationoftheFe-UFhybridnanorods,whichinduced

57、theformationofthehollowspheres.Infact,theformationofUFMs42andtheirtemplatefunctionswerealsoreportedbyothergroups.39Withtheprolongingoftheagingtime,thefurthercrosslink-ingreactionamongtheUFresinonthesurfaceofthehybridsphereswiththeaidofexcessiveformaldehyderesultedintheformationofthenetwork-likestruc

58、tures.Duringcalcination,theUFexperiencedameltingandasubsequentdecomposingprocessasconfirmedbythermogravimetry-differentialthermalanalysis(TG-DTA)measurement(Fig.S4).Themeltandshrink-ageoftheUFMmightinducethemigrationofferricoxyhy-droxidenanoparticles.WiththedecomposingoftheUF,there-leaseofCO2andothergasesontheonehandcausedthecol-lapseofthehollowspheresandthusgeneratedthemacropores,ontheotherhandproducedthenanoporeswithinthenanorodsubunit.3.3PhotocatalyticandelectrochemicalpropertiesPromotedbythehierarchicalporousnetworkstructures,webenchmarkedtheroom-temperaturephotocatalyticactivitiesof