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In the current commercial market, the majority of the carbon fibers are thermally converted from stabilized polyacrylonitrile fibers. Conventional carbonization is carried out using standard furnaces without complicated electrical fields involved. In this paper, we utilized microwave plasma enhanced chemical vapor deposition (MPECVD) technique as an alternative method to carbonize the stabilized PAN fibers in order to form carbon fibers. Scanning electron microscopy, Raman spectroscopy and Fourier transform infrared spectroscopy have been utilized to systematically study the evolution process of PAN fibers via various treatments. It was found that MPECVD carbonized PAN fibers does not exhibit any significant change in the fiber diameter, whilst conventionally carbonized PAN fibers have shown 33% reduction in the fiber diameter. More interestingly, an additional coating of carbon nanowalls (CNWs) was formed on the surface of MPECVD carbonized PAN fibers during the MPECVD process without the assistance of any metallic catalysts. The result presented here will have a potential to develop a novel, economical and straightforward approach towards mass production of carbon fibers based composites containing CNWs, which will provide exciting opportunities for applications ranging from high power microwave sources to field emission display.
Carbon nanowall (CNW), a new type of carbon material, was reported firstly by Wu et al . It is now widely known that CNWs consist of nano-graphite domains with a few tens of graphene layers, which stand on a substrate and can be described as vertically aligned carbon sheets with an average thickness of several nanometers . CNWs have been considered to have high aspect ratio, large surface area, chemical stability and mechanical strength. This unique shape and structure offered by CNWs has stimulated not only fundamental studies of transport properties and gas absorption, but also various applications in electron field emitters, catalyst supports in fuel cells and electrodes in lithium ion batteries .
Since the discovery of CNWs, various groups have reported the synthesis of these remarkable structures and performed systematical characterisation using electron microscopy and Raman spectroscopy. Wu et al. reported that the formation of CNWs on the various catalyzed substrates, including stainless steel, Cu, Si, GaAs, SiO2/Si, and sapphire in CH4 and H2 system using microwave plasma enhanced chemical vapor deposition (MPECVD) . Kurita et al. documented that the CNWs were grown on the Ni, or Ni/Cr catalyzed substrate at substrate temperature of 550-800Â°C by using direct current (DC) plasma enhanced CVD . Furthermore, fabrication of CNWs on two-dimensional substrates without using catalysts by CVD was also reported by many other authors [4-6]. Shang et al. synthesized uniform carbon flakes with a thickness of less than 20nm on Si substrates at substrate temperature of 400-700Â°C by hot-filament (HF) CVD using mixture of C2H2 and H2 . Wang et al. produced free-standing carbon nanosheets on substrates, such as Si, W, Mo, Zr, Ti, Hf, Nb, Ta, Cr, 304 stainless steel, SiO2, and Al2O3 without catalyst or special substrate treatment, by using radio-frequency (RF) plasma enhanced CVD . Lisi et al. deposited CNWs on carbon paper at growth temperature of 700 Â°C in CH4/He system using HF-CVD . To the best of our knowledge, most of the work reported so far is limited to two-dimensional substrates and the use of a catalyst is essential in the case of the MPECVD process. In this paper, we report the growth of CNWs on carbonized PAN fibers without any catalysts during MPECVD process for the first time.
In the current commercial market, 90% of the CFs are thermally converted from stabilized polyacrylonitrile (PAN) fiber precursors via high temperature carbonization . Our original motivation was to convert PAN fibers into carbon fibers using the MPECVD technique. Therefore we implemented a novel method of utilizing a MPECVD technology coupled with electromagnetic radiation to produce carbon fibers from stabilized PAN fiber precursors, in order to avoid the high costs associated with the conventional carbonization process . To this end, we have compared the MPECVD induced carbonization process with the conventional furnace carbonization process. The stabilized PAN fibers have been carbonized into carbon fibers during the MPECVD process. In addition, a coating of CNWs was formed on the surface of carbon fibers during the MPECVD process without the assistance of any metallic catalyst. The changes in the morphology, functional groups and chemical structure of PAN fibers produced by electrospinning technique after various treatments were investigated.
PAN powders with average molecular weight of 150000 and N, N- dimethylformamide (DMF) solvent were purchased from Scientific Polymer Products Inc, and Fisher Scientific, respectively. All chemicals were used without further purification. A predetermined amount of PAN powders was added into DMF at room temperature. After 3-hour ultrasonication in air a homogeneous PAN solution with a concentration of 15% (w/v) was formed (ultrasonic processor: Hielscher UP400S).
2.2. Fabrication of electrospun PAN fibers
Electrospinning technique appears to be a highly flexible approach to produce fiber precursors from various materials, and is suitable for both academic research as well as mass-production for industrial purpose . It has been used here for the PAN fiber precursor fabrication. As-prepared PAN solution was transferred into a 20ml syringe and connected to blunt metallic needle (21G) with inner diameter of 0.495mm via a plastic tubing. The non-woven electrospun PAN fiber was collected on Al foil with the following conditions: applied voltage of 15kV, tip-to-collector distance of 20cm, and flow rate of 1.5ml/h, respectively. In Fig 1, sample (a) is the as-electrospun PAN fiber collected on Al foil.
2.3. Treatments of electrospun PAN fiber
In order to retain fiber morphology in the subsequent heat-treatments, the electrospun PAN fiber was stabilized at 270Â°C in air for 1h using a tube furnace (Carbolite, Sheffield, UK). In Fig 1, sample (b) is the stabilized PAN fiber. For the comparison purpose, the stabilized PAN fiber was further carbonized by two different methods: (1) conventional furnace carbonization, and (2) MPECVD carbonization. In the conventional approach, the stabilized PAN fiber was heated from room temperature to 1000Â°C with a heating rate of 10Â°C/min, and then held at 1000Â°C for 1h in a N2 atmosphere. In the latter case, the stabilized PAN fiber was placed in the ASTeX5010 microwave plasma assisted chemical vapour deposition (MPECVD) chamber (Seki Technotron Corp., 2.45GHz, 1.5 kW). Initially a hydrogen plasma was ignited at the following conditions: microwave power of 800 W, gas pressure of 10 Torr and H2 flow rate of 90 s.c.c.m (standard cubic centimeter per minute). After 5 mins, CH4 gas at the flow rate of 10 s.c.c.m was fed into the hydrogen plasma to form CH4/H2 gas mixtures. Stabilized microwave plasma was formed at an optimized condition as shown below: microwave power of 800 W, gas pressure of 40 Torr, and CH4/H2 flow ratio of 10 s.c.c.m/90 s.c.c.m. The carbonization was processed at this condition for 2 hours. The substrate temperature was about 750Â°C, which was recorded by an infrared thermometer (model: Williamson Pro 92-40-C, range: 475-1475Â°C). In Fig 1, samples (c) and (d) were carbonized PAN fibers by the furnace and the MPECVD process, respectively.
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Fig. 1 - The evolution process of PAN fibers via various treatments.
The morphology of the four samples was examined by scanning electron microscopy (SEM, JEOL, JSM-7000F). Due to the nonconductive nature, electrospun PAN fibers (sample a) and the stabilized PAN fibers (sample b) were gold coated before SEM characterization, using sputter coater (emscope, SC 500). The carbonized PAN fibers (sample c and sample d) are conductive and therefore were used directly without an additional gold coating for SEM imaging. The functional groups of PAN at various states was inspected by Fourier Transform Infrared Spectroscopy (FTIR, Nicolet iS5). The chemical structure of the PAN fibers evolved with various treatments was investigated by Raman spectroscopy. The excitation laser wavelength of 532nm (Thermo Scientific DXR Raman spectrometer) was utilized.
3. Results and discussion
3.1. SEM observation
Fig. 2 shows SEM images of PAN fibers at different evolution stages. Fig 2 (a) shows the SEM image of the PAN fibers as-produced by electrospinning demonstrating high uniformity with an average fiber diameter of 644 Â± 76nm. Fig 2 (b) shows the SEM image of the PAN fibers after the stabilization process. It can be seen that the fiber shown in Fig 2 (a) and Fig 2 (b) are similar in size. The only notable difference in the present study is that the color of fibers changed from white (for sample a) to dark brown associated with a slight mass loss (for sample b). The above observations are in agreement with other reports, reflecting that the infusible ladder like polymer structure is established after the fiber stabilization in air [7, 10, 11]. Figs 2 (c) and (d) show the SEM images of furnace carbonized PAN fibers and MPECVD carbonized fibers, respectively. It can be seen that no sign of fiber melting is found in either sample. In the case of furnace carbonized PAN fibers, the average fiber diameter is significantly reduced to 433 Â± 63nm as shown in Fig 2 (c). The dramatic reduction in fiber dimension is likely to be associated with the reaction and evolution of various gases, such as H2O, N2, and HCN, which ultimately lead to the formation of carbon fibers . In contrast, little change in the fiber diameter has been found in the case of the MPECVD carbonized PAN fiber as shown in Fig 2 (d). According to literature, higher carbonization temperature is likely to promote the shrinkage of fiber diameter. Panapoy et al. reported that the fiber diameter was decreased from 275 nm to 208 nm by increasing carbonization temperature from 800Â°C to 1000Â°C . Zhou et al. also concluded that average fiber diameter was reduced from 250 nm to 220 nm as carbonization temperature increased from 1000Â°C to 1800Â°C . In the case of our present study, the MPECVD carbonization process was carried out at the temperature around 750oC, which is significantly lower than the conventional furnace carbonization process normally carried out at 1000oC. This might be able to explain the difference of the fiber diameters between samples c and d. However, temperature difference may not be the only reason responsible for the observed phenomena. The different physical and chemical nature of these two carbonization processes can also be considered as alternative possibility causing the fiber diameter discrepancy.
In the case of sample d, apart from the successful MPECVD fiber conversion from the stabilized PAN fiber to carbon fibers (CFs), interestingly, we have found a layer of "carbon" species on the top of the carbonized PAN fiber as shown in Fig 2 (e)-(f). Such carbon species, known as Carbon Nanowalls (CNWs), have been previously reported to grow on thin films substrates by PECVD methods [1, 3-6]. Fig 2 (e) shows that CNWs were uniformly grown on the carbonized PAN fibers simultaneously during the MPECVD carbonization process. Fig 2 (f) shows SEM image of CNWs at higher magnification. It can be seen that the as-grown CNWs consist of multiple nanosheets with thickness of approximately 20nm, grown in the direction perpendicular to the fibers.
Fig. 2 - SEM images PAN fibers at various stages: (a) electrospun PAN fibers; (b) stabilized PAN fibers; (c) furnace carbonized PAN fibers; (d) MPECVD carbonized PAN fibers; (e) MPECVD carbonized PAN fibers with CNWs coating; and (f) CNWs at higher magnification.
3.2. FTIR studies
In order to study the functional groups, FTIR spectroscopy was used to characterize a series of PAN based samples. Fig. 3 shows the FTIR spectra of PAN at six different stages: as-received PAN powders, electrospun PAN fibers, stabilized PAN fibers, furnace carbonized PAN fibers, and MPECVD carbonized PAN fibers both with and without CNWs coating. Fig 3 (a) shows the characteristic FTIR peaks of PAN powders. It can been seen that there is a strong peak located at 2242 cm-1 which corresponds to nitrile group (Câ‰¡N) [13, 14]. Other peaks located at 2938 cm-1, 1453 cm-1, 1357 cm-1, 1249 cm-1 are corresponding to the vibrations of the aliphatic CH groups (CH, CH2 and CH3)  . A weak peak located at 1623 cm-1 was assigned to amide group . Fig 3 (b) shows the FTIR spectrum of the electrospun PAN fiber. In comparison with Fig 3 (a), it is found that a new peak at 1667 cm-1 appears, which is associated with C=O bond . The presence of this C=O peak implies that the residual DMF exists in the PAN fibers , due to the incomplete DMF evaporation during the electrospinning process.
Fig 3 (c) shows the FTIR spectrum of the stabilized PAN fibers. It can be seen that after the fiber stabilization, the intensity of the peak associated with nitrile group and aliphatic CH groups has reduced significantly. In addition, some other new FTIR peaks appear, which are located at 1700 cm-1 (C=O), 1582 cm-1 (a mix of C=N, C=C and N-H groups), and 805 cm-1 (C=C-H) . These dramatic changes in the FTIR spectra imply that the chemical structure of PAN fibers have undergone cyclization, cross-linking, dehydrogenation, as well as oxidation. The result shows an excellent agreement with the published literatures [10, 15].
Fig. 3 (d)-(f) shows the FTIR spectra of the carbonized PAN fibers by both conventional furnace and MPECVD, respectively. It can be seen that there are few functional groups present in the FTIR spectra. This is due to the very high absorbance of carbon products .
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Fig. 3 - FTIR spectra of PAN at various stages: (a) as-received PAN powders; (b) electrospun PAN fibers; (c) stabilized PAN fibers; (d) furnace carbonized PAN fibers; (e) MPECVD carbonized PAN fibers; and (f) MPECVD carbonized PAN fibers with CNWs coating.
3.3. Raman investigation
Raman spectroscopy is one of the most powerful tools for characterizing the structural properties of carbonaceous materials. Fig 4 shows Raman spectra of PAN fibers at four different stages. Fig 4. (a) shows Raman spectrum of as-electrospun PAN fibers. It can be seen that there is a broad Raman peak centered at 1552 cm-1. The broadness of the Raman band suggests that as-electrospun PAN fibers lack ordered arrangements of PAN molecules. Similar results were reported by Sutasinpromprae et al., who attributed low crystallinity of electrospun PAN fiber to (1) the insufficient crystallization time of PAN molecules before solvent vaporization and (2) fiber structure frozen-in caused by very short in-flight time of charged jet (i.e. ~ 1ms) . It is generally believed that the as-spun PAN fibers without any treatment exhibit an almost featureless Raman spectrum .
Fig 4. (b) shows the Raman spectrum of stabilized PAN fibers. It can be seen that there are two main bands observed at 1374 cm-1, and 1567 cm-1 respectively. The former corresponds to D band which is associated with disordered turbostratic structures; whereas the latter corresponds to G band which is attributed to the ordered graphitic structures . The Raman peak position of D band and G band observed in our study was in agreement with the results by Zhang et al., who reported the D band and G band positioned at 1370 cm-1 and 1580 cm-1 in the stabilized PAN fiber under the conditions of 300Â°C for 30min . It is well known that the ratio of peak intensity of D band (ID) and G band (IG), denoted as R-value, can be used to correlate the balance among the disorder and crystalline character in the carbon materials (i.e. higher R-value indicates lower crystallinity) [12, 14, 19]. As shown in Fig 4. (b) and summarized in Table 1, the R-value calculated for the present stabilized PAN fibers is 1.6 (sample b).
Fig 4. (c) shows the Raman spectrum of furnace carbonized PAN fibers. It can be seen that both D band and G band are present, although the relative peak intensity has changed, which results in the decrease of R-value to 1 approximately. The decrease in R-value implies that the increased crystallinity of the carbon structures has been developed through the carbonization process. In addition, there is a broad band centered at 2835 cm-1.
Fig 4. (d) shows the Raman spectrum of MPECVD carbonized PAN fibers. By comparing the Raman spectrum of sample c and sample d, it is observed that, the Raman peaks assigned to D band (1353 cm-1) and G band (1581 cm-1) are much shaper, with several additional weak peaks appearing at 1621 cm-1, 2453 cm-1, 2711 cm-1, 2943 cm-1, and 3246 cm-1. The weak peak at 1621 cm-1 corresponds to D' band and is induced by disorder . Such disorder could be resulted from the low crystallinity of carbonized PAN fibers (i.e. sample (d)), due to the relatively low carbonization temperature [12, 14, 19, 20]. As we discussed before, sample d is carbonized at ~ 750ËšC, a temperature which is significantly lower than the carbonization temperature (at 1000ËšC) for sample c when using conventional furnace method. Therefore, samples (d) shows the presence of D' band representing the higher level of disorder than sample c.
Other peaks at 2453 cm-1, 2711 cm-1, 2943 cm-1, and 3246 cm-1 are attributed to the second-order Raman features and have been assigned to the contribution from highly ordered carbon structure [21, 22]. These are unlikely to be derived from the disordered low crystalline carbonized PAN fibers for sample d, but could originate from the CNWs. Zhu et al. have identified similar Raman peak positions for their carbon nanosheets as shown in sample d . In combination with SEM images, it can be concluded that sample d is a composite material, containing both low crystalline carbonized PAN fibers and CNWs. Therefore the Raman spectrum shows a characteristic combination of both PAN fiber and CNWs.
The ratio of peak intensity of D band (ID) and G band (IG), denoted as R-value for all samples, is shown in Table 1. It can be seen that R-value is dropped from 1.0 for sample c to 0.4 for sample d. The decrease of R value indicates that the overall quality of sample d (i.e. include both carbonized PAN fibers and CNWs) has good crystallinity, and is believed to be related with the presence of H2 plasma during the MPECVD process . The H2 plasma has proven to be an effective method in promoting the crystallinity of the fabricated materials, as explained by the following factors: (1) atomic hydrogen can preferentially etch amorphous phase, and (2) atomic hydrogen can induce the crystallization .
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Fig. 4 - Raman spectra of PAN fibers at four different stages: (a) electrospun PAN fibers, (b) stabilized PAN fibers, (c) furnace carbonized PAN fibers, and (d) MPECVD carbonized PAN fibers with CNW coatings.
Table 1. Assignment of main Raman peaks of PAN fibers derived from various treatments. vD, vG, vD' are the Raman peak position of D band, G band, and D' band, respectively. ID/IG represents R-value, is the intensity ratio of D band to G band.
We have reported for the first time of the utilization of MPECVD technique as an alternative method to carbonize the stabilized PAN fibers in order to fabricate carbon composites, consisting of both carbonized PAN fibers and CNWs. SEM, Raman and FTIR have been utilized to systematically characterize the fibers at different stages of evolution. It was found that the MPECVD carbonized PAN fibers did not result any significant change in the fiber diameter, whilst the furnace carbonized PAN fibers showed a 33% reduction in the fiber diameter. More interestingly, an additional coating of CNWs was formed on the surface of MPECVD carbonized PAN fibers during the MPECVD process without the assistance of any metallic catalysts. The result presented here will have a potential to develop a novel, economical and straightforward approach towards mass production of carbon fibrous composites containing CNWs, which will provide exciting opportunities for many applications. More importantly, the concept of this paper can be used as an efficient way to fabricate other PAN derived CFs composites, such as diamond coated CFs, and CNTs coated CFs.
The authors wish to acknowledge the funding for this research provided by the Science Research Investment Fund (SRIF) via Aston University. The School of Engineering and Applied Science at Aston University is acknowledged for a PhD studentship to Mr S. Su and an ORS award to Miss J. Li.