Carbon Microfibers with Hierarchical Porous Structure from Electrospun Fiber-Like Natural Biopolymer.

2.1 MB·English

by  Liang, Yeru

Carbon Microfibers with Hierarchical
Porous Structure from Electrospun
SUBJECTAREAS: Fiber-Like Natural Biopolymer
SYNTHESISAND
PROCESSING
POROUSMATERIALS YeruLiang,DingcaiWu&RuowenFu
SUSTAINABILITY
BIOINSPIREDMATERIALS MaterialsScienceInstitute,PCFMLabandDSAPMLab,SchoolofChemistryandChemicalEngineering,SunYat-senUniversity,
Guangzhou510275,P.R.China.
Received
30October2012 Electrospinningoffersapowerfulrouteforbuildingone-dimensional(1D)micro/nanostructures,buta
commonrequirementfortoxicorcorrosiveorganicsolventsduringthepreparationofprecursorsolution
Accepted haslimitedtheirlargescalesynthesisandbroadapplications.Herewereportafacileandlow-costwayto
19December2012 prepare1Dporouscarbonmicrofibersbyusinganelectrospunfiber-likenaturalproduct,i.e.,silkcocoon,as
precursor.Wesurprisinglyfoundthatbyutilizingasimplecarbonizationtreatment,thecocoonmicrofiber
Published canbedirectlytransformedinto1Dcarbonmicrofiberofca.6 mmdiameterwithauniquethree-dimensional
24January2013 porousnetworkstructurecomposedofinterconnectedcarbonnanoparticlesof10,40nmdiameter.We
furthershowedthattheas-preparedcarbonproductpresentssuperiorelectrochemicalperformanceas
binder-freeelectrodesofsupercapacitorsandgoodadsorptionpropertytowardorganicvapor.
Correspondenceand
requestsformaterials Micro/nanomaterialshaveattractedgreatinterestduetopotentialapplicationsinadsorption,separation,
shouldbeaddressedto catalysis, energy, sensors, nanoelectronic devices, biotechnology and other areas1–8. Therefore, it is
D.W.(wudc@mail. importanttodevelopreliablemethodsforfabricationofmicro/nanomaterials,withdesignedarchitec-
sysu.edu.cn) tureaswellascustomizedframeworkstructure.Developmentofone-dimensional(1D)micro/nanomaterialshas
beenoneofthemostactiveareasinthemicro/nanomaterialsscienceowingtotheiruniquestructurecharacter-
isticsincludinghighlength/diameterratio,largeporevolumeandhighsurfacearea9–17.Amongsuch1Dmicro/
nanomaterials,porouscarbonwith1Dmicro/nanostructurehasattractedsignificantattention,notonlyforits
fundamentalscientificinterest,butalsoformanymodern-daytechnologicalapplications9,18–23.Thiswasmoti-
vatedbyitsuniquephysicochemicalproperties,i.e.,goodelectricalconductivity,highsurfacearea,andexcellent
chemicalstability.Alargenumberofadvancedtechniqueshavebeendevelopedtofabricate1Dporouscarbona-
ceousnanostructurewithwell-controlledmorphologyandchemicalcomposition.Generally,1Dmicro/nano-
materialscanbepreparedbymanymethodsincludingnanocasting,self-assembly,solvothermalsynthesisand
chemical vapor deposition12,15,19,24–26. However, each of these methods has limitations, such as rigid reaction
conditions,highcost,andcomplicatedmultiplesyntheticsteps.
Recently,electrospinning hasbeendemonstrated to beapowerfultechnique tofabricate 1Dmicro/nanos-
tructure.Thismethodfacilitatescontrolovercomposition andtopologyofproducts,enablingenhancedper-
formanceinnumerousapplications18,27–29.Despiterapidadvancesofelectrospinningmethodologiesandtheir
synthesisadvantages(easiness,versatility,rapidity,etc.),currentelectrospinningproceduresofteninvolvetoxic
orcorrosiveorganicsolventsduringthepreparationofsolutionofstartingmaterials,whichisnotinaccordance
withtheurgentrequirementsofenergy-savingandenvironmentalprotection.Hence,establishinganefficient
andfacilesynthesisprotocolusingtheprecursorsobtaineddirectlyfromnatureforpreparationof1Dnanopor-
ouscarbonswithwell-tailoredarchitectureswouldbeofgreatbenefittotheirlargescaleproductionandbroad
applications.
Inthepresentwork,wereportafacileandcost-effectivewaytoprepare1Dporouscarbonmicrofibersbyusing
an electrospun fiber-like natural product, i.e., silk cocoon, as precursor. This biopolymer is obtained by a
silkwormthroughaninartificialelectrospinning-like process,inwhichthesilkwormspinssilkmicrofibersto
form a cocoon around itself. We surprisingly found that by utilizing a simple carbonization treatment, the
electrospunfiber-likenaturalcocoonmicrofiberscanbedirectlytransformedinto1Dcarbonmicrofiberswith
anaveragediameterof6 mm(Figure1).The1Dcarbonmicrofibersconsistofnumerouscarbonnanoparticle
unitsof10,40 nminsize,whichareinterconnectedwitheachothertoformauniquethree-dimensional(3D)
porous network structure. These carbon nanoparticle units are microporous, and their compact and loose
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Figure1|Schematicillustrationoftheapproachtofabricatehierarchicalporouscarbonmicrofibersfromsilkcocoon.
aggregationleadstomesoporesandmacropores,respectively.Inthe Furthermore, interlacement of these 1D carbon microfibers them-
past,fabricationofporousnetworkstructuresusuallyneededfussy selves in the carbonized cocoon forms a great number of micron-
andmultiplesyntheticstepsincludingreaction-inducedphasesepa- scaledlargemacropores(Figure2A).
rationandsupercriticaldrying14.Therefore,directcarbonizationof N adsorptionmethodisusedtofurtherquantitativelyelucidate
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such a natural biopolymer material without any additional harsh the hierarchical porous structure of the as-obtained HPCMF. As
proceduresmakesthepresentapproachmoreattractiveinconstruc- shown in N adsorption-desorption isotherm of Figure 3, the
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tion of micro/nanostructures. Furthermore, interlacement of these HPCMF has a high adsorption uptake at low relative pressure (P/
1Dcarbonmicrofibersthemselvesinthecarbonizedcocoonleadsto P ),indicativeoftheexistenceofplentifulmicropores.Inaddition,
0
formationoflargemacropores. theadsorptionamountincreasesgraduallyandstilldoesnotreacha
Duetogoodporeinterconnectivity,highsurfacearea,largepore plateau neartheP/P of1.0, demonstrating thepresence ofmeso-
0
volumeaswellas highnitrogen content, theas-prepared 1Dhier- poresandmacropores,whichisingoodagreementwiththeobser-
archical porous carbon microfibers (HPCMF) show superior vationfromSEMandTEMimages.Theporesizedistribution(PSD)
electrochemicalperformanceasbinder-freeelectrodesofsupercapa- curveobtainedusingdensityfunctionaltheory(DFT)isdisplayedin
citorsandgoodadsorptionpropertytowardorganicvapor.Wehope Figure3.Threeregionscanbeconfirmed:(1)micropores(,2 nm)
thatourpresentmethodwillopenupnewavenuestoadvancedand with a PSD peak at 1.3 nm; (2) mesopores (2,50 nm) with a
sustainablematerials,consideringthattheemploymentoflow-cost
andenvironmentfriendlysilkcocoonascarbonsourceavoidsmany
chemicalreactionsandphysicalprocessesinnormalproceduresfor
fabricationof1Dporouscarbonfibers.
Results
SEMimageinFigureS1Ademonstratesthatthesilkcocooniscom-
posed of interlaced 1D silk microfibers with about 15 mm in dia-
meter. The silk microfibers are essentially nonporous, judging
fromtheirsmoothframeworksurface(FiguresS1BandS1C)aswell
asverylowBrunauer–Emmett–Teller(BET)surfaceareaof7 m2/g
(TableS1).Afteracarbonizationtreatmentat900uC,theas-prepared
carbon microfibers in the carbonized cocoon preserve the well-
definedfibrousmorphology(Figure2A).Thediameterofthesecar-
bonmicrofibersismeasuredtobeabout6 mm,whichissmallerthan
that of silk microfibers, indicating that the fibrous frameworks
undergo an obvious shrinkage during carbonization because of
burn-off of non-carbon elements and carbon-containing com-
pounds. Surprisingly, it is obviously found from magnified SEM
imageofFigure2Bthatthefibrousframeworksareuniformlyfrac-
tured on the nanometer scale to form numerous interconnected
carbonnanoparticlesof10,40 nmdiameterduringthehightem-
peraturepyrolysistreatment,althoughretainingthefibrousmorpho-
logy (Figures 2A and 2C). As a result, the carbon microfibers
obtainedherehaveauniquehierarchicalporousnetworkstructure:
carbonnanoparticleunitscontainnumerousmicropores(seeTEM
imageinFigure2D),andtheircompactandlooseaggregationleads Figure2|(A,B)SEMand(C,D)TEMimagesofHPCMF. (B)and(D)
to mesopores and macropores, respectively (Figure 2B). correspondtotheareaindicatedbyarectanglein(A)and(C),respectively.
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Figure3|N adsorption-desorptionisothermofHPCMF. TheinsetshowsitsDFTporesizedistribution.
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maximum PSD peak at 9 nm; and (3) macropores (50,400 nm) deconvoluted into four functional components30, i.e., pyridinic N
withamaximumPSDpeakat68 nm.Itshouldbenotedthatlarge (N-6),quaternaryN(N-Q),pyrrolicN(N-5),andpyridine-N-oxide
macropores,i.e.,thespacesbetweeninterlaced1Dcarbonfibersand (N-X), whose percentage is 23.9 at.%, 24.6 at.%, 47.7 at.% and
between very loosely aggregated carbon nanoparticles may be too 3.8 at.%,respectively(TableS2).
largeandtooopentoleadtocapillarycondensationandthustobe Besidestheaboveuniquehierarchicalstructure,anotherimport-
reflected in pore size distribution. As shown in Table S1, a BET antcharacteristicofHPCMFisitsstable,regulableandversatilebulk
calculation gives the BET surface area for the HPCMF equal to form.Figures5Aand5Bdisplay thephotographsofasilkcocoon
796 m2/g; and the t-plot method gives the surface areas of micro- before and after carbonization. It can be found that the elliptical
pores and external large-sized pores equal to 569 and 227 m2/g, shape of cocoon is well retained after pyrolysis, although its color
respectively. Moreover, the total pore volume is calculated to be turnsfromwhitetoblackbecauseofcarbonization.Thecarbonized
0.43 cm3/gaccordingtotheamountadsorbedatarelativepressure cocoon is easily tailored into various forms, e.g., rectangle felt
P/P of0.99. (FigureS4).
0
TheHPCMFfromsilkcocoonisfoundtohavenitrogen-contain- Inthisstudy,totestitsfeasibilityforpracticalapplication,thesilk
ing functional groups by means of thecombustion analysis, X-ray cocoon-basedHPCMFintheformofrectanglefeltisdirectlyusedas
photoelectronspectroscopy(XPS)andelectronenergylossspectro- a binder-free electrode for supercapacitor without any fussy elec-
scopy(EELS).Thenitrogencontent,asmeasuredbythecombustion trodepreparationprocesseswhicharenecessaryfornormalproduc-
analysis,isabout6.5 wt.%(TableS1),and4.5 at.%,accordingtoXPS tionofcarbonelectrodes.Thesupercapacitordeviceisassembledby
(Table S2). The nitrogen element is distributed homogeneously attachingtwopiecesofHPCMFfeltsthatareimmersedinasmallseal
withinthecarbonframeworkofHPCMF,accordingtoEELSmap- bag containing 6 mol/L KOH aqueous electrolytes, as shown in
ping images of carbon and nitrogen elements (Figure 4). The Figure5C.ItisshownthattheHPCMFhasgoodcapacitiveperform-
nitrogen-containingfunctionalgroupsaredeterminedbythedecon- ance(Figures5DandS5).Forexample,atthesweeprateof2 mV/s,
volution of high resolution N 1s XPS spectrum (Figure S2). As HPCMF possesses a mass specific capacitance as high as 215 F/g,
schematically illustrated in Figure S3, the N 1s spectrum can be whichtranslatesintoacapacitancepersurfaceareaupto27 mF/cm2,
Figure4|EELSmappingimagesofHPCMF:(A)bright-fieldimage,(B)carbonand(C)nitrogen.
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Figure5|Photographsofasilkcocoonbefore(A)andafter(B)carbonization,(C)theas-fabricatedbinder-freeHPCMFbasedsupercapacitor,
and(D)specificcapacitancesatvarioussweepratesforbinder-freeHPCMFbasedelectrodes.
a value higher than those of most porous carbons31. With further diameterandahighsurfaceareaofca.800 m2/g.Itisworthmen-
increasing the sweep rate to 50 mV/s, a high mass specific capa- tioningthattheemploymentofnaturalsilkcocoonascarbonsource
citance (154 F/g)isstillobserved, indicativeofexcellentratecap- helps to avoid many chemical reactions and physical processes in
abilityinHPCMFelectrodes.Thegoodcapacitiveperformanceof normal synthetic procedures of 1D porous carbon fibers, such as
HPCMFmayarisefromitsuniquehierarchicalporestructurethat preparationofcarbonsourcesandtemplates,utilizationofabundant
is beneficial to ionic transportation and storage. The interpene- hazardousorganicsolutionduringelectrospinning process,and/or
tratedfibernetworktexturecombinedwiththehierarchicalnano- subsequenttemplateremoval.Furthermore,thecombinationof1D
porous structure in the carbon fiber framework ensures quick fibrousmorphology,hierarchicalporousnetworkstructureandgood
penetration of electrolytes, thus reducing the resistance for the poreinterconnectivityfacilitateseasyaccessofreactionspeciestothe
ions32–34. Moreover, it is believed that the plentiful nitrogen func- innersurfaceofcarbonfiber,andthesesmall-sizednanoporeseffec-
tional groups in the carbon framework can not only improve the tively increase the surface area available for active site dispersion,
wettability of the as-prepared material in the electrolyte but also whichisveryhelpfulinmanyapplications.
provide great pseudocapacitance35,36. Ontheotherhand,thisnaturalbiopolymersilkcocooniscom-
AnotherpotentialuseoftheHPCMFisexploredforadsorption posedoffibroin(72,81%),sericin(19,28%)andverylittleother
towardorganicvapor.Asshowninadsorptionisothermofmethanol materialssuchasfats,wax,sandpigmentsplusminerals.Asshownin
vaporforHPCMFinFigureS6,HPCMFexhibitsanuptakeinthe Table S1, silk cocoon contains considerable nitrogen atoms
rangeoflowrelativepressures,becauseofthepresenceofnumerous (16.2 wt.%), becauseitschemicalstructureismainlycharacterized
micropores. With increasing the relative pressure, the adsorption bythepresenceof18typesofaminoacidssuchasglycine,alanine
capacity continuously increases because of condensation of meth- andserine.Therefore,itisnotsurprisingthattheHPCMFmaterials
anol in mesopores and macropores. Therefore, HPCMF gives the containacertainamountofnitrogen-containingfunctionalgroups.
adsorptioncapacityofmethanolashighas243 mg/g. It is expected that the introduction of these nitrogen-containing
functional groups can not only boost the material performance,
Discussion e.g., in the case of energy storage or catalysis, but also completely
Wehavesuccessfullyfabricatedaclassofnovel1DHPCMFusinga changetheelectronicpropertiesthatareattractiveforawidevariety
simpleone-stepcarbonizationofelectrospunfiber-likenaturalbio- ofapplications37.
polymer, i.e., silk cocoon. As one of the most abundant types of Inaddition,thetailorablebulkformofHPCMFisalsoavaluable
naturalbiomass,silkcocooncanbeproducedviaaninartificialelec- characteristic,whichisderivedfromgoodshapestabilityofsilkcocoon
trospinning-like process. After carbonization, the silk microfibers duringcarbonization.Thisfeatureprovidesanopportunitytocontrol
can be directly transformed into 1D carbon microfibers of ca. thebulkformtocatertotheneedofanytargetedapplication.
6 mm diameter with a unique hierarchical porous structure com- Duetotheseexcellentphysicalandchemicaladvantagesdiscussed
posed of interconnected carbon nanoparticles of 10,40 nm above, the HPCMF materials have been proven to present great
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