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Preparation and surface structures of carbon nanofibers produced from electrospun PAN precursors
NEW CARBON MATERIALS Volume 23, Issue 2, March 2008 Online English edition of the Chinese language journal Cite this article as: New Carbon Materials, 2008, 23(2): 171–176.
RESEARCH PAPER
Preparation and surface structures of carbon nanofibers produced from electrospun PAN precursors GU Shu-ying1, WU Qi-lin2*, REN Jie1 1
Institute of Nano- and Bio- Polymeric Materials, School of Materials Science and Engineering, Tongji University, Shanghai 200092, China
2
State Key Lab for Modification of Synthetic Fibers and Polymer Materials, Donghua University, Shanghai 200051, China
Abstract: Carbon nanofibers with diameters in the range of 100 nm to 300 nm were obtained by stabilizing and carbonizing electrospun polyacrylonitrile (PAN) precursors. The morphologies and structures of the nanofibers and PAN precursors were investigated by scanning electron microscopy, scanning tunneling microscopy, and differential scanning calorimetry. The diameters of the PAN precursors and carbon nanofibers showed a log-normal distribution. The cyclization exothermic peak shifted to a lower temperature for electrospun fibers, which suggested that cyclization could be more easily initiated. Pits 10 nm in length and 5 nm in width formed on the surface of the carbon nanofibers, caused by the rough surface of the electrospun precursors and their shrinkage during heat treatment. Key Words: Carbon nanofibers, Polyacrylonitrile (PAN), Electrospinning, Morphology
1
Introduction
Carbon fibers are industrially important and have gained great attention in applications, from sports equipment to the aerospace industry. Most (90%) of the carbon fibers produced worldwide are obtained from polyacrylonitrile (PAN) and the rest from other raw materials, such as phenolic, rayon, or pitch fibers. Although PAN fibers are more expensive than rayon fibers, they are used extensively as a source of carbon fibers because their carbon yield is almost twice that of rayon[1]. Pitch carbon fibers have poor mechanical properties or poor reproducibility in their properties. Carbon nanofibers, such as other one-dimensional (1D) nanostructured materials, for example, nanowires, nanotubes, and molecular wires, are receiving increasing attention because of their large length to diameter ratio. They can be potentially used in nanocomposites[2], hydrogen storage[3], templates for nanotubes[4], filters[5], supercapacitors[6,7], bottom-up assembly of nanoelectronics, photonics[8], and so on. Carbon nanofibers can be produced by traditional vapor growth method[9-11] or plasma enhanced chemical vapor depositing method, which was developed at the beginning of this century[12]. However vapor growth or plasma enhanced chemical vapor depositing methods involve a complicated process and high cost. Carbon nanofibers can also be produced by stabilizing, carbonizing, and activating electrospun precursors. Electrospinning is a simple and efficient technique for the fabrication of nano to micro scale fibers. In a typical process, an electrical potential is applied between a droplet of
polymer solution or melt held at the end of a capillary and a grounded collector. When the applied electric field overcomes the surface tension of the droplet, a charged jet of polymer solution or melt is ejected. The jet becomes longer and thinner, because of the bending instability or splitting[13], until it solidifies or collects on the collector. The nanometer’s diameter size promises a high specific surface area and durable physical properties on the compression process[14,15]. Reneker[5] et al. produced carbon nanofibers with diameters in the range from 100 nm to a few microns, from electrospun polyacrylonitrile and mesophase pitch precursor fibers. Wang[16-18] et al. produced carbon nanofibers from carbonizing electrospun PAN nanofibers and studied their structures and conductivity. Hou[4] et al. reported a method to use the carbonized electrospun PAN nanofibers as substrates for the formation of multi-walled carbon nanotubes. Kim[6] et al. produced activated carbon nanofibers from PAN-based or pitch-based electrospun fibers and studied the electrochemical properties of the carbon nanofibers web as an electrode for a supercapacitor. Park[14] et al. successfully prepared carbon fibers by the electrospinning technique and overcame the problems of the brittle properties of pitch-based carbon fibers. The electrical conductivities of the carbonized web were measured to be 6.3 and 8.3 S/cm at 1000 and 1200 °C, respectively, which indicated that the web was suitable to make electrodes of electrical double layer capacitors (EDLCs). However, the diameter of fibers from pitch-based electrospun fibers was in the range of several microns. In this study, carbon nanofibers with diameters in the range
Received date: 15 September 2007; Revised date: 26 May 2008 *Corresponding author. Tel: +86-21-67792939, E-mail: [email protected] Copyright©2008, Institute of Coal Chemistry, Chinese Academy of Sciences. Published by Elsevier Limited. All rights reserved.
GU Shu-ying et al. / New Carbon Materials, 2008, 23(2): 171–176
The nonwoven fabric was peeled off the foil for carbonization and analysis. As-spun fibers were stabilized at 300 °C for 30 min in air, and then carbonized at 1 000 °C for 1 h in N2. 2.5
Fig.1 Schematic set up of electrospinning
of 100 to 300 nm were obtained by stabilizing and carbonizing electrospun PAN nanofibers. The morphologies and structures of as-spun nanofibers and carbon nanofibers were studied.
2 2.1
Experimental Raw materials
PAN (cat 18,131-5) was obtained from Aldrich and used without further purification. 99.5% N, N-dimethylformamide (DMF) was purchased from Merck. The mass fraction of 10% PAN/DMF homogeneous solution was prepared by mixing and stirring at room temperature for 30 to 50 h. The PAN film was cast from mass fraction of 10 % PAN/DMF solution and dried in N2 for 30 h. 2.2
Electrospinning of PAN nanofibers
Fibers were electrospun as reported by Bergshoef[19,20]. The schematic electrospinning setup used for the electrospinning process is shown in Fig.1. The polymer solution was held in a vertical glass pipette with a tip opening around 1 mm in diameter. A stainless steel electrode was immersed in the solution and connected to a high voltage power supply (Bertan series 230-30R), which could generate DC voltage up to 30 kV. A flat metal plate with aluminum foil placed below served as a grounded counter electrode. The voltage between the electrode and the counter electrode could be controlled by high voltage power supply. The air pressure above the solution was controlled with an air pump so that a stable drop of the solution was suspended at the tip of the capillary before the power was supplied. The applied voltages between the tip and collector were set at 20 kV with a tip-to-collector distance of 10 cm. The fibers dried in flight and were collected on the aluminum foil in the form of nonwoven fabric. 2.3
Thermal behaviors of electrospun PAN precursors
DSC curves of cast film and electrospun fibers were obtained using a DSC Q100 by heating from 30 to 350 °C in N2 at a heating rate of 10 K/min. 2.4
Preparation of carbon nanofibers
Morphologies of carbon nanofibers and precursors
The morphologies of electrospun PAN fibers were observed by scanning electron microscopy (SEM, JEOL JSM-5600LV) after being gold-coated. Carbon nanofibers were examined by SEM without coating. The diameters of electrospun precursors and carbon nanofibers were analyzed with an image analyzer (SemAfore 5.0, JEOL). The surface structures of the electrospun precursor were examined by field emission scanning electron microscopy (FESEM, Quanta 200 FEG). A NanoScope III Multimode AFM (Digital Instruments, Santa Barbara, CA, USA) was used to evaluate the surface structures of carbon nanofibers in scanning tunneling microscope (STM) mode.
3
Results and discussion
3.1 Morphologies of as-spun PAN nanofibers and carbon nanofibers SEM images of electrospun PAN nanofibers obtained from mass fracton of 10% PAN/DMF solution at 20 kV with a tip-to-collector distance of 10 cm and their derived carbon nanofibers are shown in Fig.2. Morphologies of carbon nanofibers could be observed without coating because of the high conductivity of carbon nanofibers. PAN nanofibers with an average diameter of 206 nm were obtained. After carbonization, carbon nanofibers with an average diameter of 155 nm were obtained as shown in Fig.2 and Fig.3. The average diameter of electrospun PAN fibers shrank by about 25% after carbonization. Moreover, the distribution of both electrospun nanofibers and carbon nanofibers could be well fitted as log-normal distribution rather than normal distribution (as shown in Fig.4), which was consistent with the results reported by Chun[5]. 3.2
Thermal behavior of as-spun PAN nanofibers
Generally PAN begins to degrade when heated near its melting point. The degradation reaction of PAN is so exothermic that it tends to obscure its melting endotherm in ordinary DSC traces. Therefore, the melting endotherm is normally not observed in PAN. In this study, DSC was conducted in N2 atmosphere as shown in Fig.5. There is one sharp exothermic peak at 287.67 °C and 293.62 °C for electrospun fibers and cast film, respectively. It has been reported that an exothermic reaction ranging between 200 and 350 °C in an inert atmosphere is typical of PAN. The peak is attributed to the cyclization of the nitrile groups of PAN[21,22]. However, the peak shifts to lower temperature for electrospun fibers. The shift of exothermic onset peak to low temperature suggests that cyclization is more easily initiated at low temperature for electrospun fibers. It has been reported by Mathur and
GU Shu-ying et al. / New Carbon Materials, 2008, 23(2): 171–176
Fig.2 SEM images of (a) as spun PAN nanofibers and (b) carbon nanofibers
Fig.3 Distribution of diameters of (a) as spun PAN nanofibers and (b) carbon nanofibers
Fig.4 Log-normal base e probability plot for (a) as spun PAN nanofibers and (b) carbon nanofibers
Jung et al.[23,24] that low cyclization temperature may be because of the improvement in the orientation of molecular chains. Molecular chains were oriented within the electrospun fibers during the electrospinning process, which was confirmed by the results from wide angle X-ray diffraction and infra-red spectrum as mentioned in a previous study[25]. On the other hand, the shift may be attributed to the large area to volume ratio of electrospun fibers. The detailed mechanism of the shift will be studied further. 3.3
Surface structures of as-spun PAN nanofibers and
carbon nanofibers Fig.6 shows the surface structures of as-spun PAN nanofibers and carbon nanofibers. The electrospun PAN nanofibers have a rough surface, which is consistent with the results in the electrospun pitch fibers[26], polystyrene fibers[27], and so on. The formation of a rough surface is ascribed to the vapor pressure of the solvent. Pits 10 nm in length and 5 nm in width have been observed on the surface of the carbon nanofibers as shown in Fig.6 (b). On the one hand, the formation of the pits may be attributed to the rough surface of the electrospun
GU Shu-ying et al. / New Carbon Materials, 2008, 23(2): 171–176
Fig.5 DSC curves of electrospun fibers and cast film
Fig.6 Surface structures of (a) as-spun nanofibers (FESEM) and (b) carbon nanofibers (STM)
precursors. On the other hand, the shrinkage of fibers during heat treatment can also lead to the formation of pits. The pits will increase the surface area of carbon nanofibers and increase the chemically accessible surface area of the carbon as the microporous structures of PAN-based activated carbon fibers[28]. The atomic scale STM images have not been obtained because of the poor integrality of the graphitic structure. To obtain the atomic scale STM images of carbon nanofibers, fibers must be graphitized at higher temperature, which will be studied in the future .
4
Conclusions
(1) Carbon nanofibers with diameters in the range of 100 nm to 300 nm were obtained by stabilizing and carbonizing electrospun PAN precursors. (2) Diameters of PAN precursors and carbon nanofibers showed a log-normal distribution. (3) The cyclization exothermic peak shifted to low temperature for electrospun fibers, which suggested that cyclization could be more easily initiated at low temperature for electrospun fibers. (4) Pits 10 nm in length and 5 nm in width were formed on the surface of the carbon nanofibers owing to the rough surface of electrospun precursors and the shrinkage of electrospun fibers during the heat treatment.
References [1] HAN Shu-Peng, XU Liang-Hua, CAO Wei-Yu, et al. Aggregate states of PAN in the process of fiber formation. New Carbon Materials, 2006, 21: 54-58. [2] Shofner M L, Rodriguez-Macias F J, Vaidyanathan R. Single wall nanotube and vapor grown carbon fiber reinforced polymers processed by extrusion freeform fabrication. Composites, 2003, 34A: 1207-1217. [3] MAO Zong-Qiang, XU Cai-Lu, YAN Jun, et al. Preliminary study on hydrogen storage in carbon nanofibers. New Carbon Materials, 2000, 15: 64-67. [4] Hou H, Reneker D H. Carbon nanotubes on carbon nanofibers: A novel structure based on electrospun polymer nanofibers. Advanced Materials, 2004, 16: 69-73. [5] Chun I, Reneker D H, Fong H, et al. Carbon nanofibers from polyacrylonitrile and mesophase pitch. Journal of Advance Materials, 1999, 31: 36-41. [6] Kim C, Yang K S. Electrochemical properties of carbon nanofiber web as an electrode for supercapacitor prepared by electrospinning. Applied Physics Letters, 2003, 83: 1216-1218. [7] Arbizzani C, Mastragostino M, Meneghello L, et al. Electronically conducting polymers and activated carbon: electrode materials in supercapacitor technology. Advanced Materials, 1996, 8: 331-335. [8] Duan X, Huang Y, Wang J, et al. Indium phosphide nanowires as building blocks for nanoscale electronic and optoelectronic de-
GU Shu-ying et al. / New Carbon Materials, 2008, 23(2): 171–176
vices. Nature, 2001, 409: 66-68.
[19] Jaeger R, Bergshoef M M, Battle C M I, et al. Electrospinning
[9] Endo M, Takeuchi K, Igarashi S. The production and structure of pyrolytic carbon nanotubes. Journal of Physics & Chemistry of Solids, 1993, 54: 1841-1847.
of ultra-thin polymer fibers. Macromolecular Symposium, 1998, 127: 141-150. [20] Bergshoef M M, Vancso G J. Transparent nanocomposites with
[10] HOU Peng-Xiang, BAI Shuo, FAN Yue-Ying. Effect of gas flowing state on the preparation of carbon nanofibers. New Carbon Materials, 2000, 15: 17-20.
ultrathin, electrospun nylon-4,6 fiber reinforcement. Advanced Materials, 1999, 11: 1362-1365. [21] Kim H S. Preparation and electrochemical properties of
[11] DU Jin-Hong, SU Ge, BAI Shuo, et al. Electroless nickel deposition on the surfaces of vapor-grown carbon nanofibers. New Carbon Materials, 2000,15: 49-53.
electrically conductive polyetherimide- polypyrrole composites. Journal of Polymer Science, 1996, B34: 1181-1189. [22] Liu Jie, Wang Lei, Zhang Wang-Xi, et al. Relationships between
[12] Merkulov V I, Melechko A V, Guillorn M A. Controlled
thermal stress and the thermo-chemical reaction of PAN fibers
alignment of carbon nanofibers in a large-scale synthesis
during thermal stabilization. New Carbon Materials, 2005，20:
process. Applied Physics Letters, 2002, 80: 4816-4818.
343-349.
[13] Reneker D H, Yarin A L, Fong H, et al. Bending instability of electrically charged liquid jets of polymer solutions. Journal of [14] Park S H, Kim C, Choi Y O, et al. Preparations of pitch-based webs
by
electrospinning.
stabilisation of PAN fibres. Carbon, 1992, 30: 657-660. [24] Jung K T, Hwang D K, Shul Y G, et al. The preparation of
Applied Physics, 2000, 87: 4531-4547. CF/ACF
[23] Mathur R B, Bahl O P, Mittal J. A new approach to thermal
Carbon,
2003,
41:
2655-2661. [15] Yang K S, Edie D D, Lim D Y, et al. Preparation of carbon fiber web from electrostatic spinning PMDA-ODA poly(amic acid) solution. Carbon, 2003, 41: 2039-2046. [16] Wang Y, Serrano S, Santiago-Aviles J J. Raman characterization of carbon nanofibers prepared using electrospinning. Synthetic Metals, 2003, 38: 423-427. [17] Wang Y, Santiago-Aviles J J. Large negative magnetoresistance and two-dimensional weak localization in carbon nanofiber fabricated using electrospinning. Journal of Applied Physics, 2003, 94:1 721-1 727. [18] Wang Y, Serrano S, Santiago-Aviles J J. Conductivity measurement of electrospun PAN-based carbon nanofibers. Journal of Materials Science Letters, 2002, 21: 1055-1057.
isotactic polyacrylonitrile using zeolite. Materials Letter, 2002, 53: 180-184. [25] Gu S Y, Ren J, Wu Q L. Preparation and structures of electrospun PAN nanofibers as a precursor of carbon nanofibers. Synthetic Metals, 2005, 155: 157-161. [26] Park S H, Kim C, Yang K S. Preparation of carbonized fiber web from electrospinning of isotropic pitch. Synthetic Metals, 2004, 143: 175-179. [27] Megelski S, Stephens J S, Chase D B, et al. Micro- and nano-structured surface morphology on electrospun polymer fibers. Macromolecules, 2002, 35: 8456-8466. [28] XU Bin, WU Feng, CAO Gao-Ping. Effect of carbonization temperature on microstructure of PAN-based activated carbon fibers prepared by CO2 activation. New Carbon Materials, 2006, 21: 14-18.
RESEARCH PAPER
Preparation and surface structures of carbon nanofibers produced from electrospun PAN precursors GU Shu-ying1, WU Qi-lin2*, REN Jie1 1
Institute of Nano- and Bio- Polymeric Materials, School of Materials Science and Engineering, Tongji University, Shanghai 200092, China
2
State Key Lab for Modification of Synthetic Fibers and Polymer Materials, Donghua University, Shanghai 200051, China
Abstract: Carbon nanofibers with diameters in the range of 100 nm to 300 nm were obtained by stabilizing and carbonizing electrospun polyacrylonitrile (PAN) precursors. The morphologies and structures of the nanofibers and PAN precursors were investigated by scanning electron microscopy, scanning tunneling microscopy, and differential scanning calorimetry. The diameters of the PAN precursors and carbon nanofibers showed a log-normal distribution. The cyclization exothermic peak shifted to a lower temperature for electrospun fibers, which suggested that cyclization could be more easily initiated. Pits 10 nm in length and 5 nm in width formed on the surface of the carbon nanofibers, caused by the rough surface of the electrospun precursors and their shrinkage during heat treatment. Key Words: Carbon nanofibers, Polyacrylonitrile (PAN), Electrospinning, Morphology
1
Introduction
Carbon fibers are industrially important and have gained great attention in applications, from sports equipment to the aerospace industry. Most (90%) of the carbon fibers produced worldwide are obtained from polyacrylonitrile (PAN) and the rest from other raw materials, such as phenolic, rayon, or pitch fibers. Although PAN fibers are more expensive than rayon fibers, they are used extensively as a source of carbon fibers because their carbon yield is almost twice that of rayon[1]. Pitch carbon fibers have poor mechanical properties or poor reproducibility in their properties. Carbon nanofibers, such as other one-dimensional (1D) nanostructured materials, for example, nanowires, nanotubes, and molecular wires, are receiving increasing attention because of their large length to diameter ratio. They can be potentially used in nanocomposites[2], hydrogen storage[3], templates for nanotubes[4], filters[5], supercapacitors[6,7], bottom-up assembly of nanoelectronics, photonics[8], and so on. Carbon nanofibers can be produced by traditional vapor growth method[9-11] or plasma enhanced chemical vapor depositing method, which was developed at the beginning of this century[12]. However vapor growth or plasma enhanced chemical vapor depositing methods involve a complicated process and high cost. Carbon nanofibers can also be produced by stabilizing, carbonizing, and activating electrospun precursors. Electrospinning is a simple and efficient technique for the fabrication of nano to micro scale fibers. In a typical process, an electrical potential is applied between a droplet of
polymer solution or melt held at the end of a capillary and a grounded collector. When the applied electric field overcomes the surface tension of the droplet, a charged jet of polymer solution or melt is ejected. The jet becomes longer and thinner, because of the bending instability or splitting[13], until it solidifies or collects on the collector. The nanometer’s diameter size promises a high specific surface area and durable physical properties on the compression process[14,15]. Reneker[5] et al. produced carbon nanofibers with diameters in the range from 100 nm to a few microns, from electrospun polyacrylonitrile and mesophase pitch precursor fibers. Wang[16-18] et al. produced carbon nanofibers from carbonizing electrospun PAN nanofibers and studied their structures and conductivity. Hou[4] et al. reported a method to use the carbonized electrospun PAN nanofibers as substrates for the formation of multi-walled carbon nanotubes. Kim[6] et al. produced activated carbon nanofibers from PAN-based or pitch-based electrospun fibers and studied the electrochemical properties of the carbon nanofibers web as an electrode for a supercapacitor. Park[14] et al. successfully prepared carbon fibers by the electrospinning technique and overcame the problems of the brittle properties of pitch-based carbon fibers. The electrical conductivities of the carbonized web were measured to be 6.3 and 8.3 S/cm at 1000 and 1200 °C, respectively, which indicated that the web was suitable to make electrodes of electrical double layer capacitors (EDLCs). However, the diameter of fibers from pitch-based electrospun fibers was in the range of several microns. In this study, carbon nanofibers with diameters in the range
Received date: 15 September 2007; Revised date: 26 May 2008 *Corresponding author. Tel: +86-21-67792939, E-mail: [email protected] Copyright©2008, Institute of Coal Chemistry, Chinese Academy of Sciences. Published by Elsevier Limited. All rights reserved.
GU Shu-ying et al. / New Carbon Materials, 2008, 23(2): 171–176
The nonwoven fabric was peeled off the foil for carbonization and analysis. As-spun fibers were stabilized at 300 °C for 30 min in air, and then carbonized at 1 000 °C for 1 h in N2. 2.5
Fig.1 Schematic set up of electrospinning
of 100 to 300 nm were obtained by stabilizing and carbonizing electrospun PAN nanofibers. The morphologies and structures of as-spun nanofibers and carbon nanofibers were studied.
2 2.1
Experimental Raw materials
PAN (cat 18,131-5) was obtained from Aldrich and used without further purification. 99.5% N, N-dimethylformamide (DMF) was purchased from Merck. The mass fraction of 10% PAN/DMF homogeneous solution was prepared by mixing and stirring at room temperature for 30 to 50 h. The PAN film was cast from mass fraction of 10 % PAN/DMF solution and dried in N2 for 30 h. 2.2
Electrospinning of PAN nanofibers
Fibers were electrospun as reported by Bergshoef[19,20]. The schematic electrospinning setup used for the electrospinning process is shown in Fig.1. The polymer solution was held in a vertical glass pipette with a tip opening around 1 mm in diameter. A stainless steel electrode was immersed in the solution and connected to a high voltage power supply (Bertan series 230-30R), which could generate DC voltage up to 30 kV. A flat metal plate with aluminum foil placed below served as a grounded counter electrode. The voltage between the electrode and the counter electrode could be controlled by high voltage power supply. The air pressure above the solution was controlled with an air pump so that a stable drop of the solution was suspended at the tip of the capillary before the power was supplied. The applied voltages between the tip and collector were set at 20 kV with a tip-to-collector distance of 10 cm. The fibers dried in flight and were collected on the aluminum foil in the form of nonwoven fabric. 2.3
Thermal behaviors of electrospun PAN precursors
DSC curves of cast film and electrospun fibers were obtained using a DSC Q100 by heating from 30 to 350 °C in N2 at a heating rate of 10 K/min. 2.4
Preparation of carbon nanofibers
Morphologies of carbon nanofibers and precursors
The morphologies of electrospun PAN fibers were observed by scanning electron microscopy (SEM, JEOL JSM-5600LV) after being gold-coated. Carbon nanofibers were examined by SEM without coating. The diameters of electrospun precursors and carbon nanofibers were analyzed with an image analyzer (SemAfore 5.0, JEOL). The surface structures of the electrospun precursor were examined by field emission scanning electron microscopy (FESEM, Quanta 200 FEG). A NanoScope III Multimode AFM (Digital Instruments, Santa Barbara, CA, USA) was used to evaluate the surface structures of carbon nanofibers in scanning tunneling microscope (STM) mode.
3
Results and discussion
3.1 Morphologies of as-spun PAN nanofibers and carbon nanofibers SEM images of electrospun PAN nanofibers obtained from mass fracton of 10% PAN/DMF solution at 20 kV with a tip-to-collector distance of 10 cm and their derived carbon nanofibers are shown in Fig.2. Morphologies of carbon nanofibers could be observed without coating because of the high conductivity of carbon nanofibers. PAN nanofibers with an average diameter of 206 nm were obtained. After carbonization, carbon nanofibers with an average diameter of 155 nm were obtained as shown in Fig.2 and Fig.3. The average diameter of electrospun PAN fibers shrank by about 25% after carbonization. Moreover, the distribution of both electrospun nanofibers and carbon nanofibers could be well fitted as log-normal distribution rather than normal distribution (as shown in Fig.4), which was consistent with the results reported by Chun[5]. 3.2
Thermal behavior of as-spun PAN nanofibers
Generally PAN begins to degrade when heated near its melting point. The degradation reaction of PAN is so exothermic that it tends to obscure its melting endotherm in ordinary DSC traces. Therefore, the melting endotherm is normally not observed in PAN. In this study, DSC was conducted in N2 atmosphere as shown in Fig.5. There is one sharp exothermic peak at 287.67 °C and 293.62 °C for electrospun fibers and cast film, respectively. It has been reported that an exothermic reaction ranging between 200 and 350 °C in an inert atmosphere is typical of PAN. The peak is attributed to the cyclization of the nitrile groups of PAN[21,22]. However, the peak shifts to lower temperature for electrospun fibers. The shift of exothermic onset peak to low temperature suggests that cyclization is more easily initiated at low temperature for electrospun fibers. It has been reported by Mathur and
GU Shu-ying et al. / New Carbon Materials, 2008, 23(2): 171–176
Fig.2 SEM images of (a) as spun PAN nanofibers and (b) carbon nanofibers
Fig.3 Distribution of diameters of (a) as spun PAN nanofibers and (b) carbon nanofibers
Fig.4 Log-normal base e probability plot for (a) as spun PAN nanofibers and (b) carbon nanofibers
Jung et al.[23,24] that low cyclization temperature may be because of the improvement in the orientation of molecular chains. Molecular chains were oriented within the electrospun fibers during the electrospinning process, which was confirmed by the results from wide angle X-ray diffraction and infra-red spectrum as mentioned in a previous study[25]. On the other hand, the shift may be attributed to the large area to volume ratio of electrospun fibers. The detailed mechanism of the shift will be studied further. 3.3
Surface structures of as-spun PAN nanofibers and
carbon nanofibers Fig.6 shows the surface structures of as-spun PAN nanofibers and carbon nanofibers. The electrospun PAN nanofibers have a rough surface, which is consistent with the results in the electrospun pitch fibers[26], polystyrene fibers[27], and so on. The formation of a rough surface is ascribed to the vapor pressure of the solvent. Pits 10 nm in length and 5 nm in width have been observed on the surface of the carbon nanofibers as shown in Fig.6 (b). On the one hand, the formation of the pits may be attributed to the rough surface of the electrospun
GU Shu-ying et al. / New Carbon Materials, 2008, 23(2): 171–176
Fig.5 DSC curves of electrospun fibers and cast film
Fig.6 Surface structures of (a) as-spun nanofibers (FESEM) and (b) carbon nanofibers (STM)
precursors. On the other hand, the shrinkage of fibers during heat treatment can also lead to the formation of pits. The pits will increase the surface area of carbon nanofibers and increase the chemically accessible surface area of the carbon as the microporous structures of PAN-based activated carbon fibers[28]. The atomic scale STM images have not been obtained because of the poor integrality of the graphitic structure. To obtain the atomic scale STM images of carbon nanofibers, fibers must be graphitized at higher temperature, which will be studied in the future .
4
Conclusions
(1) Carbon nanofibers with diameters in the range of 100 nm to 300 nm were obtained by stabilizing and carbonizing electrospun PAN precursors. (2) Diameters of PAN precursors and carbon nanofibers showed a log-normal distribution. (3) The cyclization exothermic peak shifted to low temperature for electrospun fibers, which suggested that cyclization could be more easily initiated at low temperature for electrospun fibers. (4) Pits 10 nm in length and 5 nm in width were formed on the surface of the carbon nanofibers owing to the rough surface of electrospun precursors and the shrinkage of electrospun fibers during the heat treatment.
References [1] HAN Shu-Peng, XU Liang-Hua, CAO Wei-Yu, et al. Aggregate states of PAN in the process of fiber formation. New Carbon Materials, 2006, 21: 54-58. [2] Shofner M L, Rodriguez-Macias F J, Vaidyanathan R. Single wall nanotube and vapor grown carbon fiber reinforced polymers processed by extrusion freeform fabrication. Composites, 2003, 34A: 1207-1217. [3] MAO Zong-Qiang, XU Cai-Lu, YAN Jun, et al. Preliminary study on hydrogen storage in carbon nanofibers. New Carbon Materials, 2000, 15: 64-67. [4] Hou H, Reneker D H. Carbon nanotubes on carbon nanofibers: A novel structure based on electrospun polymer nanofibers. Advanced Materials, 2004, 16: 69-73. [5] Chun I, Reneker D H, Fong H, et al. Carbon nanofibers from polyacrylonitrile and mesophase pitch. Journal of Advance Materials, 1999, 31: 36-41. [6] Kim C, Yang K S. Electrochemical properties of carbon nanofiber web as an electrode for supercapacitor prepared by electrospinning. Applied Physics Letters, 2003, 83: 1216-1218. [7] Arbizzani C, Mastragostino M, Meneghello L, et al. Electronically conducting polymers and activated carbon: electrode materials in supercapacitor technology. Advanced Materials, 1996, 8: 331-335. [8] Duan X, Huang Y, Wang J, et al. Indium phosphide nanowires as building blocks for nanoscale electronic and optoelectronic de-
GU Shu-ying et al. / New Carbon Materials, 2008, 23(2): 171–176
vices. Nature, 2001, 409: 66-68.
[19] Jaeger R, Bergshoef M M, Battle C M I, et al. Electrospinning
[9] Endo M, Takeuchi K, Igarashi S. The production and structure of pyrolytic carbon nanotubes. Journal of Physics & Chemistry of Solids, 1993, 54: 1841-1847.
of ultra-thin polymer fibers. Macromolecular Symposium, 1998, 127: 141-150. [20] Bergshoef M M, Vancso G J. Transparent nanocomposites with
[10] HOU Peng-Xiang, BAI Shuo, FAN Yue-Ying. Effect of gas flowing state on the preparation of carbon nanofibers. New Carbon Materials, 2000, 15: 17-20.
ultrathin, electrospun nylon-4,6 fiber reinforcement. Advanced Materials, 1999, 11: 1362-1365. [21] Kim H S. Preparation and electrochemical properties of
[11] DU Jin-Hong, SU Ge, BAI Shuo, et al. Electroless nickel deposition on the surfaces of vapor-grown carbon nanofibers. New Carbon Materials, 2000,15: 49-53.
electrically conductive polyetherimide- polypyrrole composites. Journal of Polymer Science, 1996, B34: 1181-1189. [22] Liu Jie, Wang Lei, Zhang Wang-Xi, et al. Relationships between
[12] Merkulov V I, Melechko A V, Guillorn M A. Controlled
thermal stress and the thermo-chemical reaction of PAN fibers
alignment of carbon nanofibers in a large-scale synthesis
during thermal stabilization. New Carbon Materials, 2005，20:
process. Applied Physics Letters, 2002, 80: 4816-4818.
343-349.
[13] Reneker D H, Yarin A L, Fong H, et al. Bending instability of electrically charged liquid jets of polymer solutions. Journal of [14] Park S H, Kim C, Choi Y O, et al. Preparations of pitch-based webs
by
electrospinning.
stabilisation of PAN fibres. Carbon, 1992, 30: 657-660. [24] Jung K T, Hwang D K, Shul Y G, et al. The preparation of
Applied Physics, 2000, 87: 4531-4547. CF/ACF
[23] Mathur R B, Bahl O P, Mittal J. A new approach to thermal
Carbon,
2003,
41:
2655-2661. [15] Yang K S, Edie D D, Lim D Y, et al. Preparation of carbon fiber web from electrostatic spinning PMDA-ODA poly(amic acid) solution. Carbon, 2003, 41: 2039-2046. [16] Wang Y, Serrano S, Santiago-Aviles J J. Raman characterization of carbon nanofibers prepared using electrospinning. Synthetic Metals, 2003, 38: 423-427. [17] Wang Y, Santiago-Aviles J J. Large negative magnetoresistance and two-dimensional weak localization in carbon nanofiber fabricated using electrospinning. Journal of Applied Physics, 2003, 94:1 721-1 727. [18] Wang Y, Serrano S, Santiago-Aviles J J. Conductivity measurement of electrospun PAN-based carbon nanofibers. Journal of Materials Science Letters, 2002, 21: 1055-1057.
isotactic polyacrylonitrile using zeolite. Materials Letter, 2002, 53: 180-184. [25] Gu S Y, Ren J, Wu Q L. Preparation and structures of electrospun PAN nanofibers as a precursor of carbon nanofibers. Synthetic Metals, 2005, 155: 157-161. [26] Park S H, Kim C, Yang K S. Preparation of carbonized fiber web from electrospinning of isotropic pitch. Synthetic Metals, 2004, 143: 175-179. [27] Megelski S, Stephens J S, Chase D B, et al. Micro- and nano-structured surface morphology on electrospun polymer fibers. Macromolecules, 2002, 35: 8456-8466. [28] XU Bin, WU Feng, CAO Gao-Ping. Effect of carbonization temperature on microstructure of PAN-based activated carbon fibers prepared by CO2 activation. New Carbon Materials, 2006, 21: 14-18.