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Kaerkitcha et al. Nanoscale Research Letters (2016) 11:186 Page 4 of 9 Table 2 Diameters and wall thickness of the HCNFs before and after carbonization process Nozzle-end level Inward Normal Outward Methods Before carbonization Overall diameter (μm) 1.42 1.38 1.88 Core diameter (μm) 0.82 0.57 1.04 Wall thickness (μm) 0.26 0.39 0.36 After carbonization Overall diameter (μm) 0.72 0.53 0.47 Characterization Core diameter (μm) 0.31 0.29 0.27 Wall thickness (μm) 0.22 0.11 0.14 Shrinkage (%) 49.30 61.59 75.03 Preparation of the HCNFs The morphologies of the samples were observed by field emission scanning electron microscopy (FESEM; Hitachi SU-6600). High-resolution images were obtained from transmission electron microscopy (TEM; JEOL JEM- 2100) operated at 200 kV. The physical adsorption properties were determined by N2 adsorption/desorption measurements (BEL Japan, BELSORP 18). The Brunauer-Emmet-Teller (BET) method was used to de- termine the specific surface area (SSA), the pore volume and the pore size of the HCNFs and CNFs. The crystal- linity and the degree of graphitization were investigated by X-ray diffraction analysis (XRD; Rigaku, Smartlab) and Raman spectroscopy (Ocean Optics, QE Pro High Performance Spectrophotometer, λ = 785 nm). From C(002) peak of XRD spectra, we can calculate the inter- layer spacing (c/2) of the carbon nanofiber by Bragg’s law and the crystallite size along the c-axis (Lc) by Scher- rer’s equation. The graphitization degree of the carbon nanofiber was determined by Id/Ig ratio, which is the ra- tio between the peak intensity of the disordered carbon (1355 cm−1) and graphitic carbon (1575 cm−1) from Ra- man spectrum [38], the crystallite size along a-axis (La) then were calculated from Id/Ig ratio. The conductivity measurement was carried out using two-electrode cells. A paste of grinded carbon nanofi- bers were prepared by mixing 0.1 g of HCNFs and 0.04 g of polyethylene glycol (PEG; Mw: 7300∼9300, Wako) into 0.6 mL of ethanol/water. The HCNF paste was spread on an ITO-coated glass substrate (Geoma- tech, 10 Ω/sq, thickness 1.1 mm) by the doctor-blade technique using adhesive tape masking to control the thickness. The concise layer thickness was measured by Poly(methyl methacrylate) (PMMA; Mw: 996,000 g mol−1, Sigma-Aldrich) and poly(acrylonitrile) (PAN; Mw: 150,000 g mol−1, Sigma-Aldrich) were used as thermally degradable core precursor and carbonizing shell precursor, respectively. The polymers were dissolved separately in N,N-dimethylformamide (DMF; 99.5 %, Wako), where the concentration of PMMA and PAN were 10 and 12 wt%, respectively. Composite nanofibers of PMMA/PAN were prepared by in-house designed coaxial nozzle electrospin- ning which adjustable inner nozzle-end for 1 mm inward and outward direction compared to the outer nozzle. To study the influence of inner/outer nozzle-end level, the inner nozzle was set to three levels as shown in Fig. 1. The outer and inner diameters of the nozzles are 1.20 and 0.58 mm, respectively. The nozzle-end-to-collector dis- tance was 20 cm, and the flow rates of inner (PMMA) and outer (PAN) precursor solutions were 1.0 and 2.0 mL h−1, respectively. The applied voltage was varied from 10 to 25 kV to study its effect on the morphologies of the ob- tained PMMA/PAN composite nanofibers, as spun PMMA/PAN composite nanofibers were thermally treated for oxidative stabilization for 30 min after increasing temperature to 250 °C at a rate of 5 °C min−1 in air, car- bonized for 1 h at 800 °C in nitrogen, and finally heated at 1000 °C in nitrogen for another hour to obtain the HCNFs. For comparison, the normal CNFs were also prepared by electrospinning of PAN solution (12 wt%) using single nozzle (20 G, 0.9 mm) with a flow rate of 1.0 mL h−1 under the same electrospinning conditions. The stabilization and carbonization process was also carried out under the same conditions as the HCNFs. Fig. 4 SEM images of the carbon nanofibers after carbonization. a CNFs from single nozzle and b HCNFs from normal coaxial electrospinningPDF Image | carbon nanofibers obtained from coaxial electrospinning
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