Fabrication and Structural Characterization of Polyacrylonitrile and

13532

J. Phys. Chem. C 2010, 114, 13532–13539

Fabrication and Structural Characterization of Polyacrylonitrile and Carbon Nanofibers Containing Plasma-Modified Carbon Nanotubes by Electrospinning I-Han Chen,† Cheng-Chien Wang,‡ and Chuh-Yung Chen*,† Department of Chemical Engineering, National Cheng Kung UniVersity, Tainan 701, Taiwan, R.O.C., and Department of Chemical and Materials Engineering, Southern Taiwan UniVersity, Tainan 710, Taiwan, R.O.C. ReceiVed: May 02, 2010; ReVised Manuscript ReceiVed: July 09, 2010

Hybrid nanofibers with different concentrations of functionalized carbon nanotubes (CNTs) in polyacrylonitrile (PAN) were fabricated using the electrospinning technique and subsequently carbonized. Acrylonitrile-modified CNTs were dispersed in the PAN before electrospinning. The surface morphologies and structures of the nanofibers were characterized by Raman spectroscopy, and scanning and transmission electron microscopy, which showed that the average diameter of the CNT/PAN nanofibers was 110 nm and that the CNTs were embedded in the nanofibers. Raman results indicate that embedded CNTs in the PAN nanofibers nucleate the growth of carbon crystals during PAN carbonization. The lowest sheet resistance of the carbon nanofiber was 8Ω/sq, and the electromagnetic interference shielding efficiency was about 20 dB. 1. Introduction Recently, carbon nanotubes (CNTs) and carbon nanofibers have attracted significant interest in the scientific community. They possess special properties that are important in the preparation of polymer composites, including a high strength and aspect ratio, good thermal and electrical conductivities, and a low density.1-6 The abilities of traditional vapor growth and plasma-enhanced chemical vapor deposition7-9 to prepare carbon fibers with diameters in the nanometer range have been investigated. However, these methods involve complicated chemical and physical processes, and thus, the associated cost is inevitably high; additionally, the chemical vapor deposition method is only capable of producing relatively short fibers, which are difficult to align, assemble, and process for applications. It is difficult to use conventional methods (for example, wet, gel, dry, and melt spinning)10-14 to spin continuous fibers with diameters below 5 µm. Electrospinning is a simple and efficient technique for the fabrication of nanofibers. The electrospinning technique, invented in the 1930s, has recently gained renewed interest because it can spin a variety of ultrafine polymer fibers at the nanoscale at low cost.15 There are several precursors for the production of carbon fiber, such as polyacrylonitrile (PAN), pitch and cellulose.16-20 PAN is the most widely used precursor for manufacturing high-performance fibers due to its combination of tensile and compressive properties as well as its high carbon yield.21 Electrospinning is no longer applied only to organic materials. Hybrid nanofibers also can be prepared by the electrospinning method. Agarwal et al.22 prepared polyurethane nanofibers, and Li et al.23 prepared zein/silk nanofibers, which have applications for tissue engineering. Liu et al.,24 Hong et al.,25 and Wang et al.26,27 have successfully synthesized ZnO/poly(vinyl alcohol) nanofibers for luminescent applications. In recent years, CNTembedded nanofibers had drawn much attention, due to the many applications.28-34 Nevertheless, the strong van der Waals forces * Corresponding author. E-mail: [email protected]. Telephone: +886-6-2757575-62681-215. Fax: +886-6-2344496. † National Cheng Kung University. ‡ Southern Taiwan University.

that exist between CNTs induce them to become entangled and form cordages and to acquire a low degree of solubility in most organic solvents, causing poor dispersion when mixed with the polymer matrix, which makes the preparation of nanofibers difficult. To solve this problem, surface modification of CNTs has been widely investigated, including living polymerization,35-39 oxidation of acids,40 nitro-oxide-mediated radical polymerization, and γ-ray irradiation.41 The process of grafting polymers and the covalent attachment of polymers onto CNTs are very effective because the grafted polymers on the CNTs surface have good dispersive ability in the organic phase and, thus, can prevent the aggregation of CNTs. Therefore, in light of the aforementioned drawbacks, the present study proposes another strategy to solve the problem, which involves the modification of the plasma surface and then a subsequent grafting of the functional polymers onto the CNTs. This technique not only simplifies the entire manufacturing process but also offers the flexibility of adapting its function to the distinctive needs of the electrospinning experiments. Herein, we report a method that resolves some of the problems

Figure 1. Schematic of the electrospinning apparatus.

10.1021/jp103993b  2010 American Chemical Society Published on Web 07/29/2010

Fabrication of CNT/PAN Nanofibers by Electrospinning

J. Phys. Chem. C, Vol. 114, No. 32, 2010 13533

Figure 2. TEM images of (a) pristine CNTs and (b) CNT-g-AN. (All scale bars are 100 nm.)

Figure 3. Digital photographs of CNT-g-AN (left) and pristine CNTs (right) dispersed in DMF for (a) 0 min, (b) 30 min, (c) 1 h, and (d) one week.

mentioned in the above-referenced reports and demonstrate our ability to fabricate uniformly sized nanofibers. Finally, the embedded CNTs are not only well oriented along the fiber axis in electrospun nanofibers but also lead to higher orientation of PAN chains during the heating process. We expect this process to result in enhanced crystallinity in the fabricated carbon nanofibers. The CNTs-embedded carbon nanofibers will have many potential applications, such as electromagnetic interference (EMI) shielding materials, supercapacitors, and rechargeable batteries.31,42-44

2. Experimental Section 2.1. Preparation of Functionalized CNTs. Functionalized CNTs were grafted with PAN through the process of plasmainduced grafting polymerization. The experiment entailing the treatment given to the plasma was carried out in a parallel plane electrode reactor; the volume of the stainless steel vacuum chamber of the reactor was about 6 × 103 cm3. The electrodes were made of circular copper plates separated by a distance of 3 cm. The CNTs (CTube-100, CNT Co. Ltd., Korea) (0.5 g) were spread regularly on the chamber and exposed to the plasma

13534

J. Phys. Chem. C, Vol. 114, No. 32, 2010

Figure 4. XPS survey (on the left) and deconvolved (on the right) spectra of the C 1s peak of pristine CNTs (top) and CNT-g-AN (bottom).

treatment for 5 min. Prior to activating the plasma reaction, the pressure in the reactor was reduced to 10-5 Torr. Argon gas was then injected into the reactor, and the vacuum pressure of the plasma reactor was controlled to precisely 0.1 Torr with the help of throttle valves;45-49 electrical power of the plasma was supplied by a radiofrequency power generator operating at 50 W. After the plasma treatment, a large number of radicals were generated on the surface of the CNTs by breaking the CdC bonds of the nanotubes, and then the acrylonitrile monomer (AN), with a concentration of 0.1 M dissolved in N,Ndimethylformamide (DMF), was injected into the reactor. The reaction temperature of the grafting polymerization was 50 °C for 3 h. Following this, the CNT grafted AN (CNT-g-AN) was washed three times with DMF by the centrifuging method. The CNT-g-AN were dried overnight in a vacuum oven at a temperature of 50 °C to remove all traces of the solvent.50 2.2. Preparation of CNTs Embedded PAN Nanofibers. The electrospinning technique is based on electrostatic forces drawing out a jet of polymeric solution which experiences high extension due to an electrostatically driven bending instability, resulting in thin nanofibers.33 CNT/PAN nanofibers were prepared by electrospinning using a 10 wt % PAN solution in DMF. CNTs embedded PAN fibers were prepared by electrospinning, using a composite solution of 0.2, 0.5, 0.7, 1.0, and 2.0 wt % functionalized CNTs dispersed into PAN solution in

Chen et al. DMF. A polymer solution was electrospun under the following conditions: an applied positive voltage of 15 kV, a syringe rate of 5 mL/h, a rotation speed of 800 rpm, a distance of 15 cm from the syringe nozzle to the collector, and a syringe needle diameter of 0.8 mm. Figure 1 shows the experimental apparatus used for the electrospinning process. This apparatus is very stable, and the nanofibers can be collected continuously for the mass production. 2.3. Preparation of CNT/Carbon Nanofibers. For carbonization, the CNT/PAN nanofibers produced by electrospinning were placed into a tube furnace and stabilized via heating at 0.5 °C/min up to 250 °C, and then they were incubated at 250 °C for 2 h in an air atmosphere. Carbonization was performed at 750, 900, and 1050 °C for 4 h under an argon atmosphere at a heating rate of 5 °C/min. Three steps are involved in synthesizing carbon fibers from PAN fiber precursors: dehydrogenation, cyclization, and carbonization.51-54 2.4. Characterization. The morphology and structure of the functionalized CNTs and CNT/PAN nanofibers were investigated using a field emission scanning electron microscope (FESEM, JEOL JEM6700, 10 kV) and a transmission electron microscopy (TEM, Hitachi H-7500, 120 kV; Philips Tecnai G2 F20, 200 kV). Samples were collected on 300 mesh copper grids, which were not covered with Formvar film. The CNTg-AN sample for TEM analysis was obtained by placing a drop of the CNT-g-AN diluted in DMF onto a copper grid covered with Formvar film and air drying it at room temperature. The structures of the nanofiber conversions were investigated by Raman spectrometry (JOBIN-YVON T64000 Micro-PL/Raman spectroscopy, excitation wavelength 514 nm). The surface functional groups were measured by X-ray photoelectron spectroscopy (XPS, Kratos Axis ultra DLD; radiation energy 1486.6 eV). The sheet resistance of the nonwoven fabrics was measured using the four-probe method55-57 (Loresta GP lowresistivity meter MCPT600, Mitsubishi Chemical Corp., Japan). The sheet resistance value for the sample reported in this study was the average of sheet resistance measurements at 7 different points (the thickness of the nonwoven fabric measured by micrometer is 150 µm, and the size is 15 × 15 cm2). Finally, the EMI shielding efficiency (SE) of the nonwoven fabric was measured according to the ASTM D4935-99 standards for planar materials using a network analyzer (Agilent, E5071A) over a frequency range of 300 MHz to 3 GHz. 3. Results and Discussion The morphology of the pristine CNTs and the CNT-g-AN are shown in Figure 2. As shown in Figure 2a, the average diameter of the pristine CNTs is ∼10-16 nm. Furthermore, the polymer layers can be clearly seen in Figure 2b, in contrast to Figure 2a, and the average diameter of the CNT-g-AN is ∼15-20 nm. The images reveal that the thickness of the grafted polymer layer is about 5 nm and that there is a fine polymer growth on the surface of the CNTs. Figure 3, however, demonstrates the difference in the solubility levels of the pristine CNTs (right) and the CNT-g-AN (left) in the DMF solvent. It is evident that all pristine CNTs settled down in the dispersion within 1 h. In contrast, the dispersion of the CNT-g-AN in the DMF solvent was indefinitely stable, and due to the chemical affinity between the polar modified groups and the organic solvent, no sedimentation was observed. In addition, Figure 3e presents the dispersion of pristine CNTs and CNT-g-AN in PAN solution in DMF, respectively. It is clear that all pristine CNTs coagulate in the PAN solution. In contrast, the dispersion of CNT-g-AN in PAN solution is infinitely stable and well dispersed.

Fabrication of CNT/PAN Nanofibers by Electrospinning

J. Phys. Chem. C, Vol. 114, No. 32, 2010 13535

Figure 5. SEM images of CNT/PAN nanofibers (a) PAN nanofibers, (b) nanofibers with 0.2 wt % CNTs, (c) 0.5 wt %, (d) 0.7 wt % (e) 1.0 wt % and (f) 2.0 wt %. (All scale bars are 300 nm.)

The presence of CNT-g-AN was further confirmed by XPS, which provides rich information about the functional groups grafted onto the sidewalls of the CNTs. Figure 4 illustrates the comparison of the XPS spectra recorded from the pristine CNTs and the functionalized CNTs.50 Figure 4a presents a broad scan image (from 0 to 500 eV) obtained from the XPS spectrum of the pristine CNTs. The carbon (C 1s, C KLL Auger transition) and nitrogen (N 1s, N KLL Auger transition) peaks represent the major constituents of the sample surface. According to the XPS analysis, the N 1s peak appeared in the CNT-g-AN, which can be attributed to the fact that the PAN was successfully grafted onto the CNTs by the plasma-induced grafting polymerization. Furthermore, detailed analyses of the XPS spectra (carbon) of the pristine CNTs and the CNT-g-AN are shown in Figure 4b. As seen in Figure 4b, the carbon C 1s peak, observed from 281 to 292 eV, is interpreted as the combination of the sp2 CdC and sp3 C-C structures of the pristine CNTs. The C 1s(1) (C-C bonding) of the CNT-g-AN, also shown in Figure 4b, can be observed clearly at 284.3 eV, whereas the C 1s(2) (C-N, C-H bonding) is at 285.5 eV.58,59 The N 1s atomic concentration results from the XPS experimental data reveal that the grafting percentage of the PAN onto the CNTs surface is 40.3 wt %. The morphology of the CNT/PAN nanofibers can be observed directly by SEM. SEM images of the nanofibers, shown in Figure 5, were obtained using an applied positive voltage of 15 kV. The SEM images show that the surface morphology of the nanofibers is smooth at low CNT concentration and rough at

high concentration. Figure 5a shows the PAN nanofibers, which are straight and uniform with a smooth surface. As the CNT concentration is increased, the electrospun nanofibers exhibit rough and uneven surfaces, as shown in Figure 5c-f, which are due to the higher solution viscosity and larger shear thinning behavior.

Figure 6. FT-IR spectra of (a) CNT/PAN nanofibers, (b) stabilized CNT/PAN nanofibers, and (c) CNT/carbon nanofibers.

13536

J. Phys. Chem. C, Vol. 114, No. 32, 2010

Figure 7. Photographs of (a) CNTs/PAN, (b) stabilized CNT/PAN, and (c) CNT/carbon nonwoven fabrics.

Fourier transform infrared spectroscopy (FT-IR) was employed to examine the nanofibers after different heat treatment temperatures (HTTs). The FT-IR spectra of CNT/PAN, stabilized CNT/PAN and CNT/carbon nanofibers are presented in Figure 6. The spectra can be used to analyze the chemical structure of the fibers. As shown in Figure 6a, the peak at 2240 cm-1 can be assigned to the -CtN band, while the peaks at 1450 cm-1 are ascribed to the C-H stretching band. As the HTT is increased to 250 °C (Figure 6b), the most prominent structural changes are decreases in the intensity of the 2240 cm-1 band, attributed to CtN, and the decrease of the intensities of aliphatic C-H bonds, concomitant with the advent and increase of a shoulder-like peak at 1700 cm-1 (due to cyclic CdO), the band at 1590 cm-1 (due to mixed CdN, CdC, and NdH), and the band at 810 cm-1 (due to CdCdH).60,61 Additionally, Figure 6c shows the spectrum of the CNT/carbon nanofibers, in which all of the functional groups should have been eliminated, leaving a structure similar to that of carbon

Chen et al. fibers. Figure 7 shows the electrospun CNT/PAN nonwoven fabrics before and after carbonization. The CNT/PAN nonwoven fabrics (Figure 7a) are initially gray and very dense. After stabilization, the nonwoven fabrics turn dark brown, as shown in Figure 7b. These color changes most likely indicate a polymer backbone containing oxygen-bearing groups, such as those that evolve in the PAN ladder structure,21,50,62 providing greater stability during high-temperature carbonization. Figure 7c shows that the CNT/PAN nonwoven fabrics turn black when carbonized. The dispersion of the CNTs within the PAN matrix was investigated by TEM, as shown in Figure 8. We note that the CNTs are clearly embedded in the PAN nanofibers and the CNTs are well aligned along the axis of the electrospun nanofibers. Due to elongation of the fluid jet, the nanotubes are oriented along the streamlines of the electrospinning solution.63 Figure 8a shows the TEM image of the CNT/PAN nanofibers, in which the average diameters of the nanofibers are 110 nm. Figure 8b shows the HR-TEM image of CNT/carbon nanofibers. This image shows the multilayer wall of the CNT and reveals some initial ordering in the ‘core’ region near the surface of CNT, which indicates that carbonization of PAN at 900 °C can produce order comparable to that of the CNT wall when in their vicinity. Moreover, the images of PAN nanofibers are presented in Figure 8c and Figure 8d, respectively. Figure 8c demonstrates that there are no CNTs embedded in the PAN fibers. As compared with CNT/carbon nanofibers (Figure 8b) the carbon nanofibers of Figure 8d present no ordering region in the nanofibers. The sheet resistance of the CNT/carbon nonwoven fabrics is shown in Table 1 and depends on the carbonization temperature and CNT concentration. Carbonization at high temperature

Figure 8. TEM images of (a) CNT/PAN nanofibers with 2.0 wt % CNTs (scale bar 200 nm), (b) high-resolution image of CNT/carbon nanofibers (scale bar 10 nm), (c) PAN nanofibers (scale bar 200 nm), and (d) high-resolution image of carbon nanofibers (scale bar 5 nm).

Fabrication of CNT/PAN Nanofibers by Electrospinning

J. Phys. Chem. C, Vol. 114, No. 32, 2010 13537

TABLE 1: Sheet Resistance of CNTs/Carbon Nanofibers Nonwoven Mats Depending on the Carbonization Temperature and CNTs Concentrations PAN with PAN with PAN with PAN with PAN with PAN with PAN with 0.0 wt % 0.2 wt % 0.5 wt % 1.0 wt % 2.0 wt % 3.0 wt % 5.0 wt % CNT sheet CNT sheet CNT sheet CNT sheet CNT sheet CNT sheet CNT Sheet temp (°C) resistance (Ω/sq) resistance (Ω/sq) resistance (Ω/sq) resistance (Ω/sq) resistance (Ω/sq) resistance (Ω/sq) resistance (Ω/sq) 750 900