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Kaerkitcha et al. Nanoscale Research Letters (2016) 11:186 Page 8 of 9 anode material for sodium-ion batteries [40] since it pro- vides high initial Coulombic efficiency, excellent rate capability, and stable cyclability. Among all samples, the HCNFs from normal coaxial nozzle show the higher C(002) peak intensity, 2.11 and 1.21 times than that of the HCNFs from inward and out- ward coaxial nozzle, respectively. The increase of C(002) peak intensity of the HCNFs from normal coaxial nozzle suggests the better orientation of the graphitic plane as compared to the HCNFs from inward and outward co- axial nozzle. Decrease of C(002) peak broadening (FWHM) is reflected by the decrease of Id/Ig ratio and the increase in crystallite size (both Lc and La). Increase of the graphitization degree and crystallite size of the HCNFs from normal coaxial nozzle may arise from the smoother flow of both core and shell polymer solutions, which were pulled out from the same comparative levels of nozzle-end configuration. The electrical conductivity of the obtained HCNFs and CNFs are shown in Table 5. Comparison with the CNFs from single nozzle electrospinning, all HCNFs samples show relatively lower electrical conductivity. Differences of the thickness among the prepared sam- ples may have affected each of the electrical conduct- ivities. Considering the conductance values, the HCNFs produced from the outward coaxial nozzle also show a much higher value than the CNFs pro- duced from the single nozzle. Among the hollow nanofibers, the HCNFs produced from the outward and normal coaxial nozzle show higher electrical conductivity (675 and 648 S m−1, respectively) al- most two orders of magnitude than that of the HCNFs pro- duced from the inward coaxial nozzle system (376 S m−1). The high electrical conductivity of the HCNFs produced from the outward and normal coaxial nozzle may be as- cribed to different reasons. For the HCNFs produced from the outward coaxial nozzle, the major contribution is its high specific surface area to volume ratio (Fig. 6 and Table 3). The hollow morphology and large amount of mesoporous and microporous structure on both inside and outside of the walls play an important role to increase the electrical conductivity [7, 8, 11]. Whereas in case of the normal coaxial nozzle, a high electrical conductivity of the HNCFs may result from a well-ordered structure with a higher graphitization degree [12, 17, 41]. The conductivity of the HNCFs and CNFs from the present work (between 370 and 1100 S m−1) are quite lower as compared to the previously reported results. For examples, Sebastian et al. [41] reported the conductivity of the carbon nanofibers ranged between 470 and 4100 S m−1. The higher electrical conductivity is mainly influenced by its higher orientation of the graphitic planes, which they suggested to be in- creased by an increment of synthesis temperature of the carbon nanofibers. The great effect of graphitic plane orientation also reported by Maita et al. [42] that the con- ductivity value of the composite nanofibers were increased almost two orders of magnitude by an increase of the de- gree of graphitization (Ig/Id). Based on the abovementioned facts, we can imply that an increase of the electrical con- ductivity comes from the mixed contribution of the graph- itic plane orientation and the surface area or the porosity of the HCNFs. Conclusions We have clarified the influence of the applied voltage on the coaxial electrospinning of the PMMA/PAN compos- ite nanofibers with different levels of inner nozzle-end and studied the effect of the levels of inner nozzle-end on the morphologies and physical properties of the ob- tained HCNFs. By using the outward coaxial nozzle sys- tem, we could produce the HCNFs with highest specific surface area of 278.56 m2 g−1, which is 1.69 times larger than that of the CNFs from single nozzle electrospinning. The XRD peaks and Raman spectrum of the HCNFs pro- duced from normal coaxial nozzle electrospinning exhibit much higher crystallinity and graphitization degree as compared to the inward and outward coaxial nozzle sys- tems. The high electrical conductivity of the HCNFs ob- tained from the outward and normal coaxial nozzle systems confirm that the electrical conductivity were af- fected by both of the porosity and the graphitization de- gree of the nanofibers. Abbreviations BET: Brunauer-Emmet-Teller; c/2: interlayer spacing; CNFs: carbon nanofibers; DMF: N,N-dimethylformamide; HCNFs: hollow carbon nanofibers; La: crystallite size along the c-axis; Lc: crystallite size along the c-axis; PAN: polyacrylonitrile; PEG: polyethylene glycol; PMMA: poly(methyl methacrylate); TEM: transmission electron microscopy; XRD: X-ray diffraction. Competing interests The authors declare that they have no competing interests. Authors’ contributions NK designed and performed the experimental work and explained the obtained results and wrote the paper. SC and TS helped in writing of the paper, participated in the experimental work, and gave the advice along the work. All authors read and approved the final manuscript. Acknowledgements The authors would like to thank Dr. T. Yabutsuka and Prof. R. Hagiwara of Graduate School of Energy Science, Kyoto University for utilization of the SEM and the XRD apparatus, respectively. We would like to thank Mr. M. Kagaya and Assoc. Prof. K. Hachiya of Graduate School of Energy Science, Kyoto University for assistance in Raman measurements. We are also grateful to Prof. Y. Tsujii and Assist. Prof. K. Sakakibara of Institute for Chemical Research, Kyoto University for giving an opportunity to use the TEM system. Author details 1Department of Fundamental Energy Science, Graduate School of Energy Science, Kyoto University, Yoshida-Honmachi, Sakyo-ku, Kyoto 606-8501, Japan. 2The Joint Graduate School of Energy and Environment, King Mongkut’s University of Technology Thonburi, 126 Prachauthit Rd., Bangmod, Tungkru, Bangkok 10140, Thailand. 3Centre of Excellence on Energy Technology and Environment, Science and Technology Postgraduate Education and Research Development Office, Bangkok, Thailand.PDF Image | carbon nanofibers obtained from coaxial electrospinning
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