Porous nanofibers

DOI: 10.1002/smll.200600243

Synthesis and Characterization of PorousCarbon Nanofibers with Hollow CoresThrough the Thermal Treatment ofElectrospun Copolymeric Nanofiber Webs**

Chan Kim, Young Il Jeong, Bui Thi Nhu Ngoc, KapSeung Yang,* Masahito Kojima, Yoong Ahm Kim,*Morinobu Endo, and Jae-Wook Lee

Porous carbon materials have attracted much attention be-cause of their versatile applications in catalysis, sensors,electronic devices, gas and liquid separation, and memorystorage.[1–6] Conventional processes of producing porouscarbon materials require a pore-creation step (so called acti-vation), which involves complex chemical and physical phe-nomena occurring at multiple time and temperature scalesand is also costly.[7] For these reasons, intensive studies havebeen carried out to find alternative controllable and cost--ACHTUNGTRENNUNGeffective synthetic routes such as template methods and theuse of polymer blends. The template technique has been uti-lized to precisely control pore sizes ranging from the nano-to macroscale,[8–10] although many technical difficulties (e.g.,template design, carbon growth, frame removal, high cost,and scale-up) remain unsolved. The alternative for obtainingporous carbon materials was the melt-spinning of immisci-ble polymer blends and subsequent thermal treatment.[11–13]

Recently, electrospinning has been shown to be a simplebut powerful technique for the preparation of functionalnanofibers with sub-micrometer diameters.[14] Also reported

is the preparation of electrospun-derived carbon nanofibersfrom pure polyacrylonitrile (PAN)[15–22] as PAN is a well-known precursor for the conventional microsized carbonfibers.[23,24] The synthesis of fibrous carbon with single andcontinuous hollow cores has been reported by differentgroups using melt-spinning[25] and co-electrospinning[26] ofimmiscible polymer blends and subsequent thermal treat-ment.

Here, we report the fabrication of porous, fibrous nano-scale carbon materials in the form of a web via the electro-spinning of two immiscible polymer solutions followed bythermal treatment at 1000 8C in an inert atmosphere. Theelectrospinning system enables us to obtain nanosized (ca.400 nm) organic fibers containing two separate phases(“sea” and “islands” when viewed as cross section) in theform of a web (or sheet); the continuous phase (sea) trans-forms into pore walls (or skeletons of nanofibers) and thediscontinuous phase (islands) transforms into many hollowpores developed along the fiber length through thermal de-composition when thermally treated at 1000 8C. The struc-tural features of the resulting porous, fibrous carbon materi-als in the web were carefully characterized using transmis-sion electron microscopy (TEM), scanning electron micros-copy (SEM), ACHTUNGTRENNUNGX-ray powder diffraction (XRD), and Ramanspectroscopy; the surface properties were evaluated by Bru-nauer–Emmet–Teller (BET) techniques. Finally, we con-firmed the dimensional stability of the porous, fibrouscarbon by thermal treatment at 2800 8C in an argon atmos-phere.

The polymer-blend solutions were prepared by dissolv-ing two or more polymers in a solvent; the phase separation,which results in the sea–islands feature, occurs due to the in-trinsic properties (e.g., interfacial tension, viscosity, elastici-ty) of the polymers.[27] As shown in Figure 1, the stableemulsion-like polymer-blend solutions were first preparedby the judicious selection of two types of polymer, theirblend ratios, and a suitable solvent. The continuous phaseconsisted of PAN solution and the homogeneously dispersedphase consisted of poly(methyl methacrylate) (PMMA) so-lution. To understand the resulting morphological changesdue to the blend ratios and the nature of the constituentpolymers, viscosity and surface-tension measurements werecarried out for the polymer blends. All the solutions showedshear-thinning behavior due to the increasing orientation ofdrops along the flow direction at increasing shear rates (seeFigure S1 in the Supporting Information). Also, the viscosityincreased when the volume fraction of PAN in the blend so-lution increased. Interestingly, the surface tension of thePAN solution (5.195 mNm�1) was found to be higher thanthat of the PMMA solution (8.843 mNm�1). Thus, the sur-face tension of the constituent polymers in the blend solu-tion is thought to be the most important factor in determin-ing the cross-sectional morphology of the electrospun organ-ic nanofibers. As a result, the low-surface-tension polymer(PAN) occupies the continuous phase of the solution whilethe high-surface-tension polymer (PMMA) forms the dis-continuous phase.

With optimized electrospinning conditions, homoge-ACHTUNGTRENNUNGneous and long organic nanofibers in the form of a thin

[*] Dr. C. Kim, Dr. Y. I. Jeong, B. T. N. Ngoc, Prof. K. S. YangCenter for Functional Nano Fine ChemicalsChonnam National University300 Youngbong-dong, Buk-gu, Gwangju, 500-757 (Korea)Fax: (+82)625-301-779E-mail: ksyang@chonnam.ac.kr

M. Kojima, Prof. Y. A. Kim, Prof. M. EndoFaculty of EngineeringShinshu University4-17-1 Wakasato, Nagano-shi 380-8553 (Japan)Fax: (+81)262-695-208E-mail: yak@endomoribu.shinshu-u.ac.jp

Prof. J.-W. LeeDepartment of Chemical EngineeringSeonam UniversityNamwon 590-170 (Korea)

[**] This work was supported by the Next Generation Growth Engineof the Ministry of Science and Technology in Korea and the CLUS-TER of Ministry of Education, Culture, Sports, Science, and Tech-nology in Japan. J.W.L. acknowledges the financial support ofgrant No. R-01-2005-000-00414-0 (2005) from the Korea Scienceand Engineering Foundation.

Supporting information for this article is available on the WWWunder http://www.small-journal.com or from the author.

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white web were obtained (Figure S2a in the Supporting In-formation). The conversion of an organic nanofiber into acarbon nanofiber involves stabilization and thermal-treat-ment steps. The air stabilization of electrospun nanofibers,which involves complex chemical reactions such as three-di-mensional cross linking,[23,24] is accompanied by a colorchange from white to reddish brown (Figure S2b). Finally,by thermally treating the air-stabilized fibers, we obtained ablack web (Figure S2c) consisting of fibrous carbon contain-ing hollow cores.

As shown in Figure 2a–c, the electrospun organic nano-fibers exhibit a smooth outer surface, long fibrous morphol-ogy, and homogeneous diameter distributions in the rangeof 200 to 400 nm. The higher the fraction of PAN, the small-er the diameter of the nanofibers; this is due to their highspinability (i.e., the ability to form fibers). It was noted thatthe electrospun single organic nanofiber consisted of twophases: the discontinuous and long rodlike PMMA phaseand the continuous PAN phase. Air stabilization of the elec-trospun nanofiber web was then carried out at 280 8C for 1 hunder controlled air circulation, which is critical to obtaindimensional stability and, hence, a fibrous morphology. Themorphology is not disrupted by thermal treatment at1000 8C in an inert atmosphere as in the case of convention-

al PAN-based carbonfibers.[23,24] Since the elongat-ed PMMA phase decompos-es (or disappears) withoutcarbon residue and the con-tinuous PAN phase is easilytransformed into carbon (ca.40–50%) during thermaltreatment, many hollowcores are created within asingle carbon fiber as aresult. Figure 2d–f showsSEM images of the cross sec-tion of the thermally treatedcarbon fibers; the hollowcores in a single carbon fiber

are clearly visible. Interestingly, the number of hollow coresincreases with an increase in PMMA concentration. In addi-tion, low-resolution TEM images (Figure 2g) indicate thatthe long but discontinuous hollow cores are well-developedalong the fiber length. We are thus able to control the diam-eter, number, and length of the hollow cores in a singlecarbon nanofiber using polymer chemistry. Finally, therugged surface morphology of the thermally treated carbonnanofibers (Figure 2d–f) is believed to be the combinedresult of carbon densification and the abrupt gas evolutionof both polymers. It is therefore expected that the blendratio of the constituent polymers largely affects the patternsof created pores as well as the porosity of the carbon nano-fibers.

In order to track the pore evolution during the thermaltreatment of the electrospun organic nanofibers, the surfaceproperties, including pore size, were studied using N2 ad-sorption isotherms at 77 K. As shown in Figure 3a, all sam-ples followed the hysteresis of type IV isotherms; the largeportion of N2 adsorbed in the low-relative-pressure range(<0.2 P/P0) is associated with micropores, whereas the addi-tional adsorption at high relative pressure (>0.8 P/P0) origi-nates from the mesoporosity developed on the outer surfaceand the hollow cores of the fibers. The porosity parameters

for all samples are summar-ized in Table 1. The totalpore volume, BET specificsurface area, and the fractionand average size of the mes-opores (2–50 nm) increasewith an increase in PMMAconcentration (Figure 3b andTable 1). From these results,we determined that the dis-continuous but rodlikePMMA phase played a keyrole in developing mesoporesduring thermal decomposi-tion. In further work toobtain porous and fibrouscarbons in a more controlledway, a systematic kineticstudy of the pores created as

Figure 1. Schematic diagram of the sequential procedure for producing porous carbon nanofibers withhollow cores. a) Preparation of stable polymer solutions from immiscible copolymers; nanoscale phaseseparation occurs due to their different molecular weights; PMMA forms the discontinuous phase andPAN forms the continuous phase. b) Fiber formation (without change of phase) by electrospinning.c) Removal of the elongated rodlike phase within a single nanofiber through thermal treatment.

Figure 2. a,b,c) Macromorphology of electrospun polymeric nanofibers containing two polymer phases.PAN:PMMA= a) 5:5, b) 7:3, and c) 9:1. d,e, f) Cross-sectional field-emission SEM images of thermallytreated nanofibers at 1000 8C. PAN:PMMA= d) 5:5, e) 7:3, and f) 9:1. g) TEM images of sample (d),showing linearly developed hollow cores along the fiber length. The inset is a magnified TEM image.

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a result of temperature and heating-rate functions duringthermal treatment is critically needed because the complexgas evolution involved (degradation of PMMA) is closelyrelated to pore formation.

In order to understand the structural changes and alsothe dimensional stability of hollow cores, carbon nanofibersthermally treated at 1000 and 2800 8C were investigatedwith XRD and Raman spectroscopy, respectively (see Fig-ure 3S in the Supporting Information). X-ray peaks aroundACHTUNGTRENNUNG2q=258, which correspond to the (002) layers of graphiticcrystallites, sharpen and are shifted up; this is due to thestructural development from disordered to graphitic carbonas a result of thermal treatment.[28] In addition, a large in-crease in the intensity of the G band at 1580 cm�1 (E2

g2

graphitic mode) and a decrease in the intensity of the Dband at 1355 cm�1 (defect mode) also support the structuraldevelopment with the removal of defects.[29, 30] When consid-ering the structural parameters of the nanofibers, as re-vealed by XRD and Raman studies (see Table 1), theporous fibers were found to be typically nongraphitizablecarbon. In addition, the absence of the (100) and (101)peaks indicates that ACHTUNGTRENNUNGPAN:PMMA-derived carbon fiberscould not achieve three-dimensional stacking order eventhough thermally treated at 2800 8C. Based on the detailedTEM studies of the fibers thermally treated at 2800 8C, theouter surface of all samples was transformed from relativelyrugged to largely irregular because of large changes in thereal density due to high-temperature treatment. It is inter-esting to note that the hollow cores in a single fiber are notcollapsed (Figure 4a–c) and that the pore walls (ca. 10 nm)consisting of approximately 15 graphene sheets are crystal-

line on a short range but un-dulated along the fiberlength (Figure 4d).

In summary, we have de-scribed the fabrication of anew type of porous carbon inthe form of a web consistingof linear nanofibers approxi-mately 400 nm in length ach-ieved via the combination offiber formation by electro-spinning two stable, immisci-ble polymer solutions andthermal treatment at 1000 8Cin an inert atmosphere. It isnoteworthy that linear, long,and porous carbon nanofiber

webs were prepared without the need for a cost-consumingactivation step. In addition, we were able to tailor the sur-face area and pore size of the nanofibers simply by changingthe blend ratio of the polymers because the thermal decom-position of the discontinuously elongated phase in a singleorganic nanofiber played a key role in mesopore formationduring thermal treatment. The hollow-core morphology wassustained when the fibers were thermally treated at thehigher temperature of 2800 8C even though the texture ofthe pore walls was changed from small fringes to entangledlinear graphene sheets. These high-temperature-treatednanofiber webs may be utilized as efficient electromagneticshielding materials due to their low density and expectedhigh, selective thermal and electrical conductivities alongthe plane. The results presented here are of considerabletechnological interest as this new type of carbon nanofiber,with controlled porosity and expected high electrical andmechanical properties caused by their interlinked fiber mor-phology, are promising candidates for a wide range of appli-cations in which web (or sheet) morphology and controlledpore structure are strongly required.

Experimental Section

The PAN (average molecular weight Mw=86200) and PMMA(Mw=15000) polymers were dissolved in dimethylformamide(DMF) using a hot plate at 60 8C for 2 h. The PAN and PMMApolymers were selected for use in this study for two principalreasons: 1) they exhibit different thermal behavior: PAN easilytransforms into residual carbon, as proved by our previous

Figure 3. a) Adsorption isotherms and b) micropore size distributions of PAN:PMMA 5:5, 7:3, and 9:1.

Table 1. Surface parameters of the porous carbon nanofibers.

PAN:PMMA blend ratios Specific surface areaACHTUNGTRENNUNG[m2g�1][a]

Total pore volumeACHTUNGTRENNUNG[cm3g�1]

VmesoACHTUNGTRENNUNG[cm3g�1][b]

VmicroACHTUNGTRENNUNG[cm3g�1][c]

Wmeso

[nm][d]Wmicro

[nm][e]

5:5 940 0.82 0.47 0.35 11.89 0.587:3 880 0.60 0.26 0.34 8.53 0.579:1 800 0.50 0.18 0.32 5.68 0.56

[a] Specific surface area calculated by the BET method. [b] Vmeso: volume of mesopore (1.7–300 nm) calculated using the Barret–Joyner–Halenda (BJH) method based on the Kelvin equation. [c] Vmicro : micropore volume calculated using the Horvath–Kawazoe (HK) method.[d] Wmeso: average mesopore width calculated using the BJH method. [e] Wmicro : average micropore width calculated using the HK method.

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study,[17] and PMMA disappears via thermal decomposition;2) both polymers show a stable, emulsion-like phase separationin DMF. The suitably prepared polymer solutions with differentblend ratios (PAN:PMMA=5:5, 7:3, and 9:1) were transferredinto a 20 mL syringe with a capillary tip of 0.5-mm diameter.Using typical apparatus electrospinning was carried out by ap-plying a high positive voltage (20 kV) to the polymer solution viathe tip of the syringe needle. The electrospun fibers were collect-ed as a thin web by rotating a metal drum (ca. 300 rpm)ACHTUNGTRENNUNGwrapped with aluminum foil. The oxidative stabilization wasthen carried out using a conventional muffle furnace at 280 8Cfor 1h in an air atmosphere to induce dimensional stability ofthe carbon nanofibers, which were then thermally treated at1000 and 2800 8C in an inert atmosphere.

The morphologies and structural changes of the obtainedporous fibrous carbon fibers, obtained as a result of thermaltreatment, were analyzed by SEM (Hitachi, S-4700), TEM (JEOLJEM-2010 FEF), XRD (Rigaku RINT 2100, CuKa ACHTUNGTRENNUNGl=1.54056 C),and Raman spectroscopy (Renishaw 1000, 514-nm Ar-ion laser).After preheating the activated carbon nanofibers at 150 8C for2 h to remove adsorbed water, the specific surface area andpore size distribution of the samples were evaluated using theBET equation (ASAP2020, Micromeritics, USA). Pore size distribu-tions were obtained by applying the density functional theorymethod to the nitrogen adsorption isotherms at 77 K.

Keywords:electrospinning ·nanofibers ·polymer blends ·porous materials

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Figure 4. a–c) Cross-sectional images of thermally treated nanofibers at 2800 8CPAN:PMMA= a) 5:5, b) 7:3, and c) 9:1. d) TEM image of sample (a) showing structurally developed corewalls after thermal treatment.

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Received: May 13, 2006Revised: August 22, 2006Published online on October 19, 2006

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