JOURNAL OF RAMAN SPECTROSCOPYJ. Raman Spectrosc. 2004; 35: 928–933Published online 25 August 2004 in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/jrs.1233

Raman spectroscopic evaluationof polyacrylonitrile-based carbon nanofibers preparedby electrospinning

Chan Kim, Sang-Hee Park, Jang-Ik Cho, Do-Young Lee, Tae-Jin Park, Wan-Jin Lee andKap-Seung Yang∗

Faculty of Applied Chemical Engineering, Chonnam National University, 300 Youngbong-dong, Buk-gu, Gwangju 500-757, Korea

Received 14 November 2003; Accepted 30 April 2004

Poly(acrylonitrile) (PAN) solutions in N,N-dimethylformamide were electrospun into webs consisting of350 nm ultra-fine fibers. The webs were oxidatively stabilized and followed by heat treatment in the rangeof 700–1000 ◦C. Characterization of the microstructure of PAN-based carbon nanofibers was performedby x-ray diffraction, field-emission scanning electron microscopy, electrical conductivity and Ramanspectroscopy. The Lc.002/ and La.10/ values were calculated to be 1.85–2.15 and 2.23–3.36 nm, respectively. TheLc.002/ and La.10/ values increased by about 86% and 66%, respectively, when the heat treatment temperature(HTT) was increased from 700 to 1000 ◦C. The electrical conductivity of carbonized PAN nanofiber websincreased with increasing carbonization temperature, being 6.8 × 10−3 and 1.96 S cm−1 at 700 and 1000 ◦C,respectively. The D and G bands from Raman scattering were fitted into a Gaussian–Lorentzian hybridizedfunction, and the crystallite sizes in the nanofibers were evaluated from the R-values determined from theratios of the intensity of the G band to that of the D band. The domain size of the graphitic layers was inthe range 1.6–3.2 nm with higher values at higher HTT. Copyright 2004 John Wiley & Sons, Ltd.

KEYWORDS: polyacrylonitrile; electrospinning; x-ray diffraction; electrical conductivity

INTRODUCTION

There are various methods to produce carbon nanofibersor carbon nanotubes, e.g. vapor growth,1 arc discharge,2

laser ablation and chemical vapor deposition.3 However,these are very expensive processes owing to the lowproduct yield and expensive equipment. On the other hand,electrospinning easily generates fibers with diameters in therange 20 nm–10 µm.4 – 8 The electrospinning technology waspatented in 19344 and since than the application area hasexpanded after the development of mass production forhigh-performance filters,9 scaffolds in tissue engineering10

and electrodes for electric double layer capacitors.11

The non-woven fabric obtained from electrospinning hasbeen used to produce carbon nanofibers through stabiliza-tion and carbonization processes. Chun et al.12 reported thatcarbon fibers can be prepared from polyacrylonitrile (PAN)and mesophase-pitch fibers with average diameter of 888 nm.

ŁCorrespondence to: Kap-Seung Yang, Faculty of Applied ChemicalEngineering, Chonnam National University, 300 Youngbong-dong,Buk-gu, Gwangju 500-757, Korea. E-mail: ksyang@chonnam.ac.krContract/grant sponsor: KOSEF; Contract/grant number:R01-2003-000-10100-0.

Kim and Yang13 reported that activated carbon nanofiberselectrospun from PAN showed excellent performance forsupercapacitors.11 Although the importance of having infor-mation of the morphological structure of carbon nanofiberswas understood, it has not yet been reported.

In the present work, characterization of morphologicalstructure was performed with PAN-based carbon nanofibers,heat-treated in the temperature range 700–1000 °C. Themethods applied were mainly Raman spectroscopy, x-raydiffraction (XRD), electrical conductivity and field emissionscanning electron microscopy (FE-SEM).

EXPERIMENTAL

PAN and N,N-dimethylformamide (DMF) were obtainedfrom Aldrich Chemical and were used without furtherpurification. PAN was dissolved in DMF to give 10–20 wt%solutions. The solutions were gently stirred for 24 h at 60 °Cin order to obtain homogeneous solutions.

The experimental setup used for the electrospinningprocess is shown in Fig. 1. A power supply (NT-PS-35K,NTSEE, Korea) with variable high voltage was used for theelectrospinning process. The polymer solution was placed

Copyright 2004 John Wiley & Sons, Ltd.

Raman evaluation of PAN-based carbon nanofibers 929

V

Polymersolution

~35 kV TCD(Tip to Collector

Distance)

Polymer JET

Figure 1. Schematic diagram of electrospinning apparatus.

in a 20 ml syringe with a capillary tip 0.5 mm in diameter.The anode of the high-voltage power supply was clampedto a syringe needle tip and the cathode was connectedto a metal collector. The electrospun fiber was collectedby attaching it to aluminum foil wrapped on a metaldrum rotating at ¾300 rpm. The electrospun fiber web wasstabilized at 280 °C for 1 h in air and then heat treatment wasperformed at 700, 800, 900 and 1000 °C for 1 h under an argonatmosphere.

The thermal stability of the electrospun fibers wasmeasured by thermogravimetric analysis (TGA) (TGA2950,TA Instruments) up to 800 °C at a heating rate of5 °C min�1.

An x-ray diffractometer (Rigaku RINT 2100, 40 kV,30 mA) with a Cu K˛ (� D 0.154056 nm) x-ray source wasused to measure the Bragg diffractions from all samples, and

the lattice parameters Lc of the samples were determinedfrom the width of the (002) XRD lines.

Raman spectra were excited with 514.5 nm radiation froman argon ion laser at 10 mW under ambient conditions andrecorded with a Renishaw Image Microscope System 1000equipped with a charge-coupled device (CCD) multi-channeldetector. The scattered light was collected in the backscat-tering configuration and the first-order (1000–2000 cm�1)Raman spectra were recorded at a 2.0 cm�1 resolution. Thespectra were averaged over three scans to improve the signal-to-noise ratio and the data were corrected for the spectralresponse of the instrument.

The electrical conductivities in the winding direction ofthe webs were measured by the four-point probe method(Model 3387-11, Kotronix, Japan). The cross-sectional areaof the web, A, was calculated by multiplying the measuredwidth by the measured thickness of the sample web. Thenthe electrical conductivity, �, was calculated on the basis ofthe equation

� D L/AR �1�

where R is the electrical resistance in �, A is the cross-sectional area in cm2 and L is the distance between theelectrodes in cm.

0

Num

er o

f obs

erva

tions

50

40

30

20

10

Diameter of fibers (nm)

100 200 300 400 500 600 700

(a) (b)

Figure 2. (a) SEM images of electrospun PAN nanofibers and (b) histogram displaying the statistical distribution of nanofiberdiameters, showing an average diameter of 350 nm. Spinning conditions: concentration, 10 wt%; applied voltage, 20 kV; tip tocollector distance, 15 cm.

Copyright 2004 John Wiley & Sons, Ltd. J. Raman Spectrosc. 2004; 35: 928–933

930 C. Kim et al.

RESULTS AND DISCUSSION

Figure 2 shows (a) SEM images of the electrospun PANnanofiber web and (b) the distribution of fiber diameters,ranging from 200 to 600 nm with an average of 350 nm. Thenanofibers were partially aligned along the winding directionof the drum winder. The average diameter decreased withincrease in carbonization temperature from 350 to 250 nm(Fig. 3) in the HTT range. This behavior could result fromweight loss through carbonization and densification atelevated temperatures.

TGA can be utilized to determine the starting temperatureof oxidative stabilization and carbonization of PAN. Figure 4shows the TGA traces for the electrospun PAN nanofiber web(a) in nitrogen and (b) in air at a heating rate of 5 °C min�1.The weight decreased sharply in the temperature range250–400 °C in a nitrogen atmosphere; the loss slowed above400 °C to give a yield of 40% at 800 °C. In the air atmosphere,the major weight loss occurred in two steps at ca 300 and ca400 °C as previously reported.14 There are indications that theelectrospun web started to react with oxygen at a relativelylow temperature of 200 °C, lost volatiles at 300 °C and thenstarted combustion at 400 °C.

The XRD patterns of the PAN-based carbon nanofiberwebs are shown in Fig. 5. The two XRD peaks near 2� D 24and 44° correspond to the (002) and (10) layers of the graphitestructure.15 The average interlayer spacing d002 and the

crystallite dimensions Lc�002� and La�10� were determined byx-ray measurements using the Bragg and Scherrer equations,respectively:

Lc D 0.91�/ˇ cos �; La D 1.84�/ˇ cos � �2�

where Lc and La are the apparent crystallite sizes along thec- and a-directions, respectively, � is the scattering angle,� is the wavelength of the x-ray used and ˇ is the half-maximum linewidth in radians. The angles 2� D 24 and 42°

were chosen for the determinations of Lc�002� and La�10� andthe results are summarized in Table 1. The Lc�002� and La�10�

values increased but the d002 value decreased with increasingHTT. The Lc�002� and La�10� values at 1000 °C increased byabout 86 and 66%, respectively, compared with an HTT of700 °C. This carbonization behavior could come from thehigh carbonization ability of the fibers caused by molecularmobility and the structural rearrangement that occurred withincreasing HTT.

Figure 6 shows the electrical conductivity of the car-bonized web as a function of carbonization temperature.The electrical conductivity increased with increasing car-bonization temperature. The electrical conductivity at 700 °Cwas 6.8 ð 10�3 S cm�1, whereas that at 1000 °C increased to1.96 S cm�1, i.e. by about three orders of magnitude. Theresults represent that carbonization and ordering of crystalsproceeded at the elevated temperature.

a b

c d

Figure 3. SEM images of PAN-based carbon nanofibers as a function of HTT: (a) 700; (b) 800; (c) 900; (d) 1000 °C.

Copyright 2004 John Wiley & Sons, Ltd. J. Raman Spectrosc. 2004; 35: 928–933

Raman evaluation of PAN-based carbon nanofibers 931

0 200 400 600 8000

20

40

60

80

100

120

Wei

ght /

%

Temperature / °C

(a)

(b)

Figure 4. Thermal analysis of electrospun PAN nanofibers:(a) N2 and (b) air atmosphere.

10 20 30 40 50 60

Inte

nsity

2θ / °, Cu Kα

(10)

(002)

(d)

(c)

(b)

(a)

Figure 5. XRD patterns of PAN-based carbon nanofibers as afunction of HTT: (a) 700; (b) 800; (c) 900; (d) 1000 °C.

Table 1. XRD results for PAN-based carbon nanofibers

HTT/°C 2�/° d�002�/nm Lc�002�/nm La�10�/nm Lc�002�/d002a

700 24.51 0.363 1.85 2.23 5.1800 24.59 0.363 1.94 3.18 5.3900 24.63 0.361 2.11 3.33 5.81000 24.65 0.360 2.15 3.36 6.0

a Number of graphite layer planes.

Raman spectroscopy has been recognized as one of themost sensitive tools for studying the structural properties ofcarbonaceous materials. Raman spectra provide informationon the crystalline perfection of graphite-based materials.16,17

It is well known that the Raman spectra of single-crystalgraphite and highly oriented pyrolytic graphite (HOPG)show a single band at 1582 cm�1 (G mode, E2g2) and

700 750 800 850 900 950

Con

duct

ivity

S c

m-1

2.5

2.0

1.5

1.0

0.5

0.0

HTT / °C1000

Figure 6. Electrical conductivity of PAN-based carbonnanofibers as a function of HTT from 700 to 1000 °C.

another band at 42 cm�1 �E2g1� from in-plane modes withE2g symmetry. Less ordered carbon materials show anadditional strong band at about 1360 cm�1 (D mode) and aweak band at 1620 cm�1 (D0 mode), when excited by 488 nmradiation.

Figure 7 shows the Raman spectra of the PAN-basednovel carbon nanofibers. The line centered near 1350 cm�1 isdue to the disordered portion of the carbons since this lineis absent in HOPG. The second line centered near 1600 cm�1

0

0

1000 1200 1400 1600 1800 2000

D

D"

G

Ram

an in

tens

ity (

arbi

tr. u

nit)

2500

2000

1500

1000

500

1000

(a)

(b)

(c)

(d)

800

600

400

200

0

1000

800

600

400

200

0

800

600

400

200

Wavenumber / cm-1

Figure 7. Raman spectra of PAN-based carbon nanofibers asa function of HTT: (a) 700; (b) 800; (c) 900; (d) 1000 °C.

Copyright 2004 John Wiley & Sons, Ltd. J. Raman Spectrosc. 2004; 35: 928–933

932 C. Kim et al.

is indicative of the ordered graphitic crystallites of thecarbons. When band positions were analyzed using a mixedGaussian–Lorentzian curve-fitting procedure, the best curvefittings were obtained with three bands at 1587–1600 cm�1

(G band), 1345–1350 cm�1 (D band) and 1503–1534 cm�1

(D00 band, associated with amorphous sp2-bonded carbonsor interstitial defects).18 – 22

The dependences of (a) the Raman shift and (b) fullwidth at half-maximum (FWHM) are illustrated in Fig. 8and summarized in Table 2. The D and G band positionsincreased slightly with the HTT and bandwidth decreasedsignificantly with increasing HTT.

It is well known that the R-value, the relative intensityratio of the D-band to the G-band, depends on both the degreeof graphitization and the alignment of the graphitic planes.Also, the R-value is sensitive to the ratio of the concentrationof graphite edge planes and/or crystal boundaries relativeto standard graphite planes, i.e. the lower the R-value,the higher is the amount of sp2 (graphite) clusters thatexist on the sample. The R-values of PNA-based novelcarbon nanofibers decrease with increasing HTT (Fig. 9),representing the transformation of disordered carbon intographitic carbon in the carbonization process.

Knight and White23 developed an empirical formula forthe relationship between the R-value and the crystallitedomain size La of graphite as La D 44/R. Using this equation,La was estimated to range from 1.6 to 3.17 nm with increasingthe HTT, as shown Table 2.

600 700 800 900

Wav

enum

ber

/ cm

-1

1650

1600

1550

1500

1450

1400

1350

1300

(a)

11001000HTT / °C

D-band

G-band

50

100

150

200

250

300

(b)

600 700 800 900 11001000HTT / °C

D-band

G-band

FW

HM

/ cm

-1

Figure 8. Plots of (a) peak position and (b) FWHM of theRaman lines for PAN-based carbon nanofibers as a function ofHTT from 700 to 1000 °C.

Table 2. Wavenumbers (�), bandwidths and relative intensities(R) of the different bands in the first-order Raman spectra ofthe samples

SampleWavenumber,

� cm�1Bandwidth,ω1/2/cm�1

R(ID/IG)

La D 4.4/R/

nm

HTT 700D 1353 167D00 1529 170 2.73 1.6G 1596 92HTT 800D 1348 196D00 1513 154 2.26 1.95G 1595 103HTT 900D 1361 199D00 1535 166 1.95 2.26G 1595 92HTT 1000D 1352 167D00 1494 147 1.39 3.17G 1591 105

HTT / °C600 700 800 900

1.0

1.5

2.0

2.5

3.0

11001000

Rel

ativ

e in

tens

ity (

R =

I D/I G

)

Figure 9. Plot of the relative intensity, R, of PAN-based carbonnanofibers as a function of HTT from 700 to 1000 °C.

CONCLUSIONS

Novel PAN-based carbon nanofibers were successfullyprepared using electrospinning techniques and subsequentlyoxidatively stabilized and carbonized. The carbon nanofibersheat treated at various temperatures were characterized asa function of HTT by XRD, FE-SEM, electrical conductivityand Raman spectroscopy.

The Lc�002� and La�10� values were 1.85–2.15 and2.23–3.36 nm, respectively. The Lc�002� and La�10� valuesincreased by about 86 and 66%, respectively, when HTTincreased from 700 to 1000 °C. Low crystallizations wereobserved in the HTT range studied, with La increasing from1.6 to 3.2 nm with increase in HTT, probably owing to thehigh surface energy of the nanosize fibers.

Copyright 2004 John Wiley & Sons, Ltd. J. Raman Spectrosc. 2004; 35: 928–933

Raman evaluation of PAN-based carbon nanofibers 933

AcknowledgmentsSupport for this work was provided by KOSEF under grant numberR01-2003-000-10100-0. C.K. acknowledges support from the KoreaResearch Foundation (grant KRF-2003-037-D00008).

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