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Preparation of mesoporous polyacrylonitrile and carbon fibers by electrospinning

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CARBON
63(2013562–592
585
alsothethicknessaffectstheintensityoftheresonance,whichisnotfullyconsistentwithanexcitonicresonance.However,anexciton–plasmonstatecouldberesponsiblefortheresonancesinoursystem[9].Graphenesandwichedbe-tweenglassandpolymerhasalsoshowntoposeanexcessbroadbandabsorptionats-polarizationinTIR[10].However,furtherexperimentalandtheoreticalstudiesareneededtoconfirmtheunderlyingmechanismoftheobserveddisper-siveopticalresonance.
Inconclusion,wehaveobservedaclearlydispersiveopti-calresonanceinreflectionspectroscopyofmetallicSWCNTfilms.Itappearsonlyforans-polarizedexcitationsource,andisstrongestatfilmthicknessesaround100nm.ThedependenceofintensityanddispersionoftheresonanceonthethicknessandthesurroundingenvironmentisconsistentwithcreationofmagneticplasmonsorMPPs,andthevicinityofM11andM22transitionssuggeststhatexcitonsmaybein-volvedintheprocess.
ThisworkwassupportedbytheAcademyofFinland(Pro-jectNos.135193and218182.WethankJaakkoKoivistoforacquiringRamanspectra.T.I.thankstheFinnishNationalDoctoralProgrammeinNanoscience.
REFERENCES
AppendixA.Supplementarydata
Supplementarydataassociatedwiththisarticlecanbefound,intheonlineversion,athttp://dx.doi.org/10.1016/j.carbon.2013.07.018.
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Preparationofmesoporouspolyacrylonitrileandcarbonfibersbyelectrospinningandsupercriticaldrying
DandanSuna,GuotongQin
a
a,*
,MiaoLu¨a,WeiWeib,Nu¨Wanga,LeiJiang
a
KeyLaboratoryofBio-InspiredSmartInterfacialScienceandTechnologyofMinistryofEducation,SchoolofChemistryandEnvironment,
BeihangUniversity,37XueyuanRoad,Beijing100191,Chinab
CollegeofArtsandScienceofBeijingUnionUniversity,197BeituchengxiRoad,Beijing100083,China
ARTICLEINFOABSTRACT
Articlehistory:
Received22March2013Accepted4July2013Availableonline13July2013
Mesoporouspolyacrylonitrileandcarbonfibershavebeenpreparedbyelectrospinningandsubsequentsupercriticaldryingandcarbonization.Polyvinylpyrrolidonewasusedasatemplate.Ambientdrying,oxidation,andsupercriticaldryingwereconductedtoinvesti-gatetheeffectsoftreatmentmethodsonthestructureofthefibers.Interestingsurfacemorphologiesofthefibers,includingnanoconvexitiesandnanorods,werefoundwhenthedifferentdryingmethodswereused.Thesurfaceareaofthemesoporouscarbonfiberswasestimatedas602.0m2gÀ1,withanaverageporesizeof3.6nm.
Ó2013ElsevierLtd.Allrightsreserved.
*Correspondingauthor:Fax:+861082338556.
E-mailaddresses:qingt@buaa.edu.cn,guotongqin@gmail.com(G.Qin.0008-6223/$-seefrontmatterÓ2013ElsevierLtd.Allrightsreserved.http://dx.doi.org/10.1016/j.carbon.2013.07.020

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Porousfiberscanbeusedinthermalinsulation,adsorp-tion,filtration,andcatalysis.Bymeansofelectrospinning,thecomposition,characteristics,andradiiofsuchfiberscaneasilybecontrolled.Therearetwomainroutestoporousfi-bers.Thefirstrouteinvolvesadjustmentoftheelectrospin-ningparameters,suchthatphaseseparationprocessesoccurduetoevaporationofthesolvent[1]orthepresenceofvapor[2].Thesecondroutereliesonphaseseparationbe-tweentwosubstancessuchthatoneisdispersedinthema-trixoftheother,andtheninternalporescanbecreatedbyremovalofthedispersedphase[3,4].Inthepresentpaper,wereportanovelapproachforpreparingmesoporouspolyac-rylonitrile(PANandcarbonfiberswithsurfacenano-rough-nessorhighsurfaceareasbymeansofelectrospinninganddifferentdryingmethods.
PAN(Mw=150,000andpolyvinylpyrrolidone(PVP(Mw=1,300,000werepurchasedfromSigma–AldrichCo.(USA.PANandPVPweredissolvedinN,N-dimethylformam-idetogive10and20wt%solutions,respectively.SolutionsforelectrospinningwerepreparedbymixingthesepolymersolutionsinaPANtoPVPmassratioof1:1.Anozzlewithaninnerdiameterof0.8mmservedasanelectrode.Fiberswerecollectedonanaluminumfoil,whichservedasthecounterelectrode.Thenozzleandcollectorwereplaced20cmapartandapotentialof25kVwasappliedbetweenthem.Theas-obtainedelectrospunfibersweretreatedbydif-ferentmethods,specificallyleaching,oxidation,anddrying.Fig.1showsthemicromorphologiesofthePANandcarbonfibers.Theas-obtainedelectrospunPAN/PVPfibersweredensewithsometinycracksandadiameterofabout170nm(Fig.1a.ThesePAN/PVPfiberswerethenleachedwithwatertoremovethePVP,washedwithethanol,anddried.Theobtainedfibersdidnotshowaporousstructure,butnanocon-vexitieswereseenontheirsurfaces(Fig.1b.TheleachingofPVPwasexpectedtoleaveporesinthePANfibers.Thenon-porousnatureoftheobtainedfibersmaybeattributedtoporecollapsingunderdryingstress.ToenhancetherigidityofthePANfibers,soastomakethemmoreresistanttothedryingstress,thePAN/PVPfiberswereoxidizedat250°Cinaflowofairbeforeleaching.ThemorphologyoftheoxidizedPAN/PVPfiberswasseentobealmostthesameasthatoftheas-obtainedelectrospunfibers,thatis,withoutporesbutwithsometinycracks(Fig.1c.Afterleaching,washingwithetha-nol,anddrying,themorphologyoftheoxidizedfibersre-
Fig.1–SEMimagesofthefibersfromdifferentmethods:aselectrospunandambientdryingPAN/PVPcompositefibers(a,PANfibersfromambientdryingafterwaterandalcoholleaching(b,PANfibersfromambientdryingafterpre-oxidizedtreatmentandwater-alcoholleaching(c,PANfibersfromsupercriticaldryingafterpre-oxidizedtreatment(d,PANfibersfromdirectlysupercriticaldrying(e,carbonfibers(f.

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mainedunchanged.Thenon-porousstructureimpliesthatoxidationofthePAN/PVPfibersresultedincross-linkingofeitherPANorPVP,suchthatthePVPwasnotleachable.Themechanicalstrengthoftheskeletonandthedryingmethodarethemainfactorsaffectingthedryingofaporousmaterial.WeemployedasupercriticaldryingmethodwithaviewtoimprovingtheporestructureofthePANfibers.Afterleachingwithwaterandwashingwithethanol,theoxidizedPAN/PVPfiberswereplacedinanautoclavepartiallyfilledwithethanol.Theautoclavewasheatedto250°Cand8MPaandmain-tainedundertheseconditionsfor1h.Itwasthenisother-mallydepressurizedtoambientpressureandthencooledtoroomtemperaturenaturally.Theresultantfibersshowedaninterestingmorphology,withnanorodsgraftedontotheirsur-faces(Fig.1d.Thesenanorodswereofdiameter10–20nm.Thisresultmightberelatedwiththesolvencyofsupercriticalethanol,leachingcross-linkedPVP.However,thesePANfibersdidnotshowaporousstructureaftercarbonization.Thenanorodsdisappearedduringcarbonization.Wealsotreatedtheas-obtainedelectrospunPAN/PVPfiberswithsupercriticalethanolwithoutpre-oxidationorleaching.TheleachingofPVPanddryingofthefibersweresimultaneouslyconductedduringthesupercriticaltreatment.TheresultantPANfiberswereseentobemesoporous(Fig.1e.ThedifferentdryingmethodsresultedinaseriesofPANfibersareschematicallyshowedinFig.S1.ThesemesoporousPANfiberswerecarbon-izedinanN2atmosphereat800°Cfor2h.Theresultantfibersretainedthemesoporousstructure,albeitwithsomeshrink-age(Fig.1f.ThisresultindicatedthatmesoporousPANandcarbonfiberscouldbedirectlyobtainedbysupercriticaldrying.
InordertoinvestigatetheeffectsofPVPcontentinthePAN/PVPprecursoronthestructureofthefibers,fourPANfi-bersampleswithdifferentmassratiosofPAN/PVP(1:1,1:3,1:5,1:7werepreparedandsubjectedtosupercriticaldrying.ThesearedesignatedasS11,S13,S15,andS17,respectively.ThePANfibersdesignatedasS13werecarbonizedat800°CandtheproductisreferredtoasSc13.N2adsorptioniso-thermsandpore-sizedistributionsofthePANandcarbonfi-bersareshowninFig.2.Theisothermsshowhysteresisloopsattributabletothemesoporousstructure.Thecarbonfi-berSc13showedthehighestadsorptionamount.Thepore-sizedistributionsofthevariousPANfibersweremainlyintherange10–50nm.Thepore-sizedistributionofthecarbonfiber,Sc13,wasbroaderthanthatofitsprecursor,S13,cover-ingtherange2–50nm.TheporestructurecharacteristicsofthemesoporousPANfibersareshowninTable1.AlthoughthePANfibersshowedadevelopedporousstructureinscan-
600
dv/dlog(r(cm3g-1
1.00.80.60.40.20.01
10
100
500
Adsorption(cmg
3-1
4003002001000
S11
S13S15S17Sc13
PoreDiameter(nm
0.00.20.40.60.81.0
RelativePressure(P/P0
Fig.2–Nitrogenadsorptionisothermsandporesizedistribution(insetimageofPANandcarbonfibers.
ningelectronmicroscope(SEMimages,theirsurfaceareaswereonly10–70m2gÀ1.S13showedthehighestsurfaceareaamongthePANfibers.ThecarbonfiberSc13(obtainedfromS13showedasurfaceareaof602m2gÀ1.ThenanodomainsofPANandPVPformedinthefibersformedbecauseofincompatibilityofPANandPVPduringelectrospinningin-ducedbyevaporationofsolvent.LeachingthePVPandkeep-ingtheskeletonstructureisimportanttoformporousfibers.Supercriticaldryingisanefficientmethodtoavoidthedryingstress.PVPalsoaffectsthefinalstructureoffibers.WhenPAN/PVPislessthan1/3theporesarenotdevelopedenough(Fig.S2a.WhenPAN/PVPismorethan1/3thepartialPANskeletonscombinetogether,formingthickskeletonandreducingpores(Fig.S2c,d.
MesoporousPANandcarbonfibershavebeenpreparedbyelectrospinningandsupercriticaldrying,withPVPasatem-plate.ThePAN/PVPmassratioanddryingmethodarethemainfactorsaffectingtheporestructureandmorphologyofthefibers.WelldevelopedmesoporousPANandcarbonfiberscouldbeobtainedbydirectlysupercriticaldryingofPAN/PVPfiberswithoutleaching.
Acknowledgements
ProjectSupportedbytheNationalBasicResearchProgramofChina(2010CB934700,2012CB933201andNSFC(51272014.
Table1–SurfaceandporeparametersofPANandcarbonfibers.FibersS11S13S15S17Sc13
Surfacearea(m2gÀ136.670.510.519.3602.0
Porevolume(cm3gÀ10.100.120.040.040.54
Averageporediameter(nm11.06.515.79.23.6

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