Physical characterization of electrospun nanofibers

Physical characterization of electrospun nanofibers

9

J. Zhu, Y. Ge, S. Jasper, X. Zhang North Carolina State University, Raleigh, NC, United States

9.1

A brief introduction of electrospun nanofibers

Nanofibers have become increasingly attractive, mainly because of their highly porous structure and large surface-to-volume ratio. In addition, accompanied with the relatively high production rate and simplicity of the setup of electrospinning technique, electrospun nanofibers have been widely used in the fields of electrochemical substances, sensors, biomaterials, filters, etc., due to the aforementioned advantages of nanofibers and electrospinning techniques [1–7]. For instance, degradable polymers, poly(α-hydroxy esters), including poly(glycolic acid) (PGA), poly(lactic acid) (PLA), and their co-polymer poly(lactic-co-glycolic acid) (PLGA) and poly (L-lactic acid) PLLA are normally electrospun for use as the substrate for tissue engineering applications [8]. Ji et al. implanted porous structures in carbon nanofibers (CNFs) to achieve a higher reversible capacity and more stable cycle performance for use in batteries [9]. Ra et al. applied electrospun polyacrylonitrile (PAN)-based CNFs for use as a supercapacitor electrode. High surface area nanofibers are beneficial for high capacitance because they adsorb electric charges on the surface [10]. Compared with the nanostructures of nanoparticles, nanowires, etc., nanofibers are long and continuous, can transport electrons or photons, and have excellent mechanical properties, which is significantly critical in the areas of energy storage, solar cells, etc. [11]. Since electrospun nanofibers exhibit excellent chemical and physical properties for these specific applications. In this chapter, we investigate the physical properties of electrospun nanofibers and study the classification of electrospun nanofibers for their further applications. For example, scanning electron microscopy (SEM), transmission electron microscopy (TEM), and atomic force microscopy (AFM) are basic methods for observing the surface and inner geomorphic structure of nanofibers. Mechanical testing is used for measuring tensile strength and compressive strength of a single nanofiber as well as nanofiber assemblies. According to the starting materials, electrospun nanofibers are divided into four categories: electrospun polymer nanofibers, electrospun metal nanofibers, electrospun CNFs, and electrospun composite nanofibers. Electrospun polymer nanofibers make up the most common classification, where one or two polymers are dissolved into a solvent. At specific concentrations, electrospun polymer nanofibers are fabricated under required electrospinning parameters. Basic methods are normally used for studying the physical properties of electrospun polymer nanofibers. On the other hand, Electrospun Nanofibers. http://dx.doi.org/10.1016/B978-0-08-100907-9.00009-X Copyright © 2017 Elsevier Ltd. All rights reserved.

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electrospun metal nanofibers also exhibit unique properties, such as magnetism. Electrospun CNFs are normally produced by the carbonization of electrospun composite nanofibers. Besides the basic characterizations, the measurement of electronic conductivity is also necessary for electrospun CNFs. Electrospun composite nanofibers are the most complicated type due to their complex composition, morphology, and structure.

9.2

Physical characterization of electrospun polymer nanofibers

It is well known that apart from chemical properties, physical properties also play an important role for the applications of electrospun polymer nanofibers. Polymer nanofiber structures have high surface areas and quantum confinement effects, which can achieve outstanding mechanical, thermal, electrical, and optical properties, and so on [12]. Additionally, through different electrospun fabrication processes and with different polymer phase separations, the resultant polymer nanofiber can have diverse structures, such as core-sheath, porous nanofiber, and so on [13,14]. Overall, physical properties, including mechanical properties, thermal properties, electrical properties, optical properties, etc., are closely related to the morphology and structure of the polymer nanofibers. Therefore, it is essential to conduct detailed physical characterizations (SEM, TEM, and BET) for polymer nanofibers in order to understand the physical morphologic structure and design particular nanofibers for specific applications.

9.2.1 Morphologic characterization SEM is performed in analyzing the surface topography of polymer nanofibers. SEM plays a critical role in pursuing a proper electrospinning precursor and superior electrospinning parameters, including polymer molecular weight, solution concentration, viscosity, and surface tension, as well as voltage, feeding rate, and tip-tocollector distance. Raghavan et al. observed nanofiber morphology via SEM to investigate optimized electrospinning parameters to prepare high-quality PAN nanofibers [15]. Fig. 9.1 shows the bead morphology with increasing PAN solution concentration (8%, 10%, 12%, and 14%). A distinct bead was observed on a single fiber when the PAN concentration was 8 wt%. On the other hand, beads were less likely to form using a concentration of 14 wt%. They proved that fewer beads formed when a more viscous solution was used. In some cases, SEM is utilized to compare the morphology change of samples before and after treatment for modification. For example, Liang et al. prepared a PVDF nanofiber mat to be used as a separator for rechargeable lithium-ion batteries [16]. PVDF was dissolved in a mixed solvent of DMAc/acetone. The nanofiber was produced by electrospinning with the flow rate of 0.4 ml h
1 under a voltage of 16 kV. The distance between the needle tip and the collector was 20 cm. While nanofibers began to stack on the collector, they applied a heat treatment of 150°C, 155°C, and

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Increasing concentration

Fig. 9.1 Bead morphology of electrospun PAN nanofibers with different solution concentrations of 8, 10, 12, and 14 wt%. Reproduced with permission from Raghavan, P., Lim, D.-H., Ahn, J.-H., Nah, C., Sherrington, D.C., Ryu, H.-S., et al., 2012. Electrospun polymer nanofibers: The booming cutting edge technology. React. Funct. Polym. 72(12), 915–930. Copyright 2012, Elsevier.

160°C, respectively on the PVDF nanofiber to give rise to a more stable cycling performance and lower interfacial resistance with the lithium electrode. Herein, SEM was used to observe the change in surface morphology of untreated and modified nanofibers. Fig. 9.2 demonstrates SEM images of PVDF fibrous membranes before and after heat treatment. All four images reflect a continuous fibrous structure of

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Fig. 9.2 SEM images of PVDF fibrous membranes: (A) untreated, (B) heated at 150°C, (C) heated at 155°C, and (D) heated at 160°C. Reproduced with permission from Liang, Y., Cheng, S., Zhao, J., Zhang, C., Sun, S., Zhou, N., et al., 2013. Heat treatment of electrospun Polyvinylidene fluoride fibrous membrane separators for rechargeable lithium-ion batteries. J. Power Sources 240, 204–211. Copyright 2013, Elsevier.

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resultant PVDF samples. As the heat treatment temperature increased, the average fiber diameter increased and more fibers became wider and stuck together. SEM can clearly observe the topological view of a fiber surface and can determine whether it is smooth, rough, or has a porous structure. For example, Bognitzki et al. used volatile solvents such as dichloromethane to produce a porous nanofiber structure [17]. Crystalline poly-L-lactide (PLLA) and the amorphous polymers polycarbonate (PC) and polyvinylcarbazole were used as the polymer composition, and dichloromethane was used as the solvent for the electrospinning precursor. Interestingly, the porous nanofiber structure was observed by SEM, which is shown in Fig. 9.3. Clearly, the nanofiber possessed numerous nanopores on the surface. The average size of the pores was around 100 nm in width and 250 nm in length, which gave a directly perceived sense to understand its construction features. The authors explained that this pore structure was generated from rapid evaporation of dichloromethane in solvent-rich regions, since the pore formation was reduced after using chloroform as the solvent. Also, the pores were stretched due to the uniaxial extension of the jet in the electric field.

Fig. 9.3 SEM of porous PLLA nanofibers electrospun by a solution of PLLA in dichloromethane. Reproduced with permission from Bognitzki, M., Czado, M., Frese, T., Schaper, A., Hellwig, M., Steinhart, M., et al., 2001. Nanostructured fibers via electrospinning. Adv. Mater. 13(1), 70–72. Copyright 2001, Wiley-VCH.

Besides SEM, TEM is another important visualization method to study the morphology of electrospun polymer nanofibers, including fiber diameters, hidden structure inside the nanofiber and so on. Sun et al. designed a core-shell structure by co-electrospinning poly(ethylene oxide) (PEO) as the shell and poly (dodecylthiophee) (PDT) as the core in chloroform (Fig. 9.4A) [18]. The

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Fig. 9.4 (A) Schematic image of electrospinning setup used for core-shell nanofiber, (B) TEM of co-electrospun nanofiber with PEO as shell and PDT as core. Reproduced with permission from Sun, Z., Zussman, E., Yarin, A.L., Wendorff, J.H., Greiner, A., 2003. Compound core-shell polymer nanofibers by co-electrospinning. Adv. Mater. 15(22), 1929–2932. Copyright 2003, Wiley-VCH.

electrospinning setup consisted of two polymer storage baths, a high-voltage power supply, and a copper collector. PEO in chloroform was placed in the outer chamber and PDT in chloroform was placed in the inner chamber. The core-shell structure could be detected by TEM (Fig. 9.4B) because of the strong contrast of PDT and PEO by TEM, according to the amount of sulfur. The diameters of the entire fiber and inner core are around 1000 and 200 nm, respectively. TEM images clearly displayed that the core-shell nanofiber of PEO and PDT were produced. TEM is also used to visualize the cross-section of polymer nanofibers. For example, Ma et al. produced a distinct structure by co-electrospinning poly(styrene-blockisoprene) (SI) and a styrene to form core-shell fibers, and its internal morphology was examined by TEM [19]. Fig. 9.5 shows cross-sectional and longitudinal views of the SI core after annealing at 140°C for 10 days. The images clearly show many spherical isoprene microdomains distributed in the styrene matrix. As the core diameter decreased, the number of spherical microdomains within a cross-section of the fiber core also decreased. The number ranged from approximately 20 domains in a fiber of core diameter 160 nm (Fig. 9.5B) down to a single domain in a fiber of core diameter 60 nm. Fig. 9.5B and C exhibited that the spherical microdomains tended to line up in concentric shells parallel to the core–shell interface. In some of these fibers, a thin surface layer was observed in Fig. 9.5B and C; it was suggested that the SI core debonded with the surrounding shell in some fibers.

9.2.2

Surface area

Since polymer nanofibers produced by electrospinning give rise to large surface areas, which are able to derive other superior properties such as surface adsorption capability, they can be applied for gas or fluid filtration and carriers for catalysts and scaffolds

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Fig. 9.5 (A–C) cross-sectional and (D) longitudinal TEM images of the SI block copolymer in the cores of annealed fibers. All of the images are in the same scale bars of 100 nm. Reproduced with permission from Ma, M., Krikorian, V., Yu, J.H., Thomas, E.L., Rutledge, G.C., 2006. Electrospun polymer nanofibers with internal periodic structure obtained by microphase separation of cylindrically confined block copolymers. Nano Lett. 6(12), 2969–2972. Copyright 2006, American Chemical Society.

for tissue engineering [1,3]. The Brunauer-Emmett-Teller (BET) test is an effective method to quantitatively estimate the surface area of materials. Hussain et al. fabricated PAN nanofibers [20]. In order to determine the internal specific surface area of the fiber mat, they used the BET technique. As shown in Fig. 9.6, red and black data points represented specific surface area scales with the inverse of the fiber diameter and experimental BET results, respectively. When the fiber diameter increased from about 150 nm to 1.3 μm, the specific surface area obtained from BET increased from 40 to 2 m2 g
1. The experimental data points were slightly higher than the calculated results based on geometric arguments, such as the surface of the fibers not being ideally smooth.

9.3

Physical characterization of electrospun metal (oxide) nanofibers

Recently, significant attention has been given to nanoscale materials because of their interesting size-dependent properties, especially magnetic coercivity and electrical conductivity. The electrospinning technique has been widely used to fabricate metallic (oxide) nanofibers due to its relatively low cost and good ability to produce diverse materials with different morphologies. Metal (oxide) nanofibers are commonly used in the field of electrodes for solar cells, electronic devices, and sensors [21–26]. The physical properties of metal (oxide) nanofibers, including their morphologies, magnetic performance, and electrical conductivities, are discussed in this section.

Physical characterization of electrospun nanofibers

Specific surface area of PAN nanofibers by BET methode by calculating with average fiber diameter

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Fig. 9.6 Specific surface area distribution of PAN nanofibers according to fiber diameter. Reproduced with permission from Hussain, D., Loyal, F., Greiner, A., Wendorff, J.H., 2010. Structure property correlations for electrospun nanofiber nonwovens. Polymer 51(17), 3989–3997. Copyright 2010, Elsevier.

9.3.1

Morphology

Smooth and pure nickel metal nanofibers were successfully synthesized by Barakat et al. via the electrospinning technique [22]. The as-spun polyvinyl alcohol (PVA)/ nickel (II) acetate (NiAc) nanofibers were first dried for 24 h at 80°C under vacuum and then calcined at 700°C for 5 h with a heating rate of 2.3°C min
1 in an argon atmosphere. Their surface morphology was examined by SEM, as shown in Fig. 9.7. The randomly oriented images of as-spun PVA/NiAc nanofibers were also taken, shown in Fig. 9.7A and B. It is observed that the surface of as-obtained calcined nanofibers were relatively smooth and circular, which were less crimped as the surface of nanofibers fabricated in a hydrogen atmosphere (Fig. 9.7C–F). The high-resolution TEM (HRTEM) image of the as-obtained nanofibers is presented in Fig. 9.8. The distance between the two adjacent planes was almost the same as the standard value of pristine nickel metal. It is clear that the atoms were arranged uniformly based on the SAED pattern, which demonstrated excellent crystallinity of the synthesized nanofibers. Li et al. synthesized titania (TiO2) nanofibers by electrospinning [24]. Polyvinyl pyrrolidone (PVP), with an average molecular weight of 1,300,000, and titanium tetraisopropoxide were performed as the polymer matrix and titania precursor, respectively. The as-spun fibers were further treated in air at 500°C for 3 h to fabricate anatase nanofibers. The morphologies of the as-spun TiO2/PVP fibers and anatase nanofibers were examined by SEM and TEM, as shown in Figs. 9.9 and 9.10,

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Fig. 9.7 Morphology for the NiAc/PVA nanofibers and the obtained nanofibers after calcination in Ar: (A, B) SEM images for NiAc/PVA nanofiber mats with low and high magnification. (C, D) SEM images of the nanofibers obtained after calcination. (E, F) FE SEM micrographs of the calcined nanofibers. Reproduced with permission from Barakat, N.A.M., Kim, B., Kim, H.Y., 2009. Production of smooth and pure nickel metal nanofibers by the electrospinning technique: nanofibers possess splendid magnetic properties. J. Phys. Chem. C 113(2), 531–536. Copyright 2008, American Chemical Society.

respectively. As shown in Fig. 9.9A, the diameter of the as-spun TiO2/PVP fibers was 79  9 nm. After treatment, the diameter decreased to 53  8 nm (Fig. 9.9B), which was due to the removal of PVP by burning the sample during the heat treatment in air. It is important to note that the TiO2 was uniformly dispersed in the PVP matrix prior to thermal treatment, confirmed by TEM (shown in Fig. 9.10A). Metal nanofibers with highly tunable electrical and magnetic properties via electrospinning have been produced by Hansen et al. [25]. An aqueous solution of PVA and metal acetate was prepared as the electrospinning solution. The prepared

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Fig. 9.8 HRTEM image of nickel nanofibers. The insets are SAED and FTT images. Reproduced with permission from Barakat, N.A.M., Kim, B., Kim, H.Y., 2009. Production of smooth and pure nickel metal nanofibers by the electrospinning technique: nanofibers possess splendid magnetic properties. J. Phys. Chem. C 113(2), 531–536. Copyright 2008, American Chemical Society.

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Fig. 9.9 (A) SEM image of TiO2/PVP nanofibers electrospun from an solution of Ti(OiPr)4 and PVP in ethanol. (B) SEM image of the same sample after calcined in air at 500°C for 3 h. Reproduced with permission from Li, D., Xia, Y., 2003. Fabrication of titania nanofibers by electrospinning. Nano Lett. 3(4), 555–560. Copyright 2003, American Chemical Society.

metallic nanofibers were produced via further thermal treatment. TEM was performed to observe the morphologies of the as-prepared materials. The crystal size, density, and morphology within the nanofiber matrix were controlled by applying heat treatment with various temperatures. Therefore, their electrical and magnetic properties could be optimized. The nanofibers with small, discrete crystalline domains supported

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Fig. 9.10 (A) TEM image of TiO2/PVP composite nanofibers electrospun from an solution of Ti(OiPr)4 and PVP in ethanol. (B) TEM image of the same sample after calcined in air at 500°C for 3 h. Reproduced with permission from Li, D., Xia, Y., 2003. Fabrication of titania nanofibers by electrospinning. Nano Lett. 3(4), 555–560. Copyright 2003, American Chemical Society.

within an amorphous metal nanofiber matrix could be produced at a relatively low temperature (400°C) under an inert gas atmosphere. As seen in Fig. 9.11A and B, it was found that in all cases the crystal domains were uniformly distributed throughout the fibers. If the as-spun nanofibers were first treated at a low temperature (400°C) under air followed by a second low temperature (400°C) treatment under an inert gas atmosphere, the fabricated isotropic crystals connected at narrow regions (Fig. 9.11C). While the as-spun nanofibers were treated at a high temperature of 800°C under an inert atmosphere, purely crystalline nanofibers across the whole fiber diameter could be obtained in the cases of copper and nickel; however, large crystals were found in the cases of iron and cobalt, as shown in Fig. 9.11D. In the study of Kim et al., iridium oxide (IrOx) was synthesized by a combination of electrospinning and a thermal treatment process [27]. The SEM images of the as-spun iridium chloride (IrCl3)/PVP and IrOx nanofibers are shown in Figs. 9.12 and 9.13, respectively. The morphologies of electrospun nanofibers were significantly changed after thermal treatment at 900°C for 2 h in air. As shown in Fig. 9.13, the diameters of these nanofibers decreased as surface roughness increased, which is probably due to the removal of the PVP (Fig. 9.12).

9.3.2

Magnetic performance

It is well known that magnetism is a class of physical phenomenon that is mediated by magnetic fields. Magnetic nanowires have been extensively used for their utilization in high-density magnetic recording, magnetic sensors, and magnetic composites. Generally, the vibrating sample magnetometer (VSM) is used to characterize the magnetic properties of these materials. Although these magnetic nanowires can be synthesized by template assisted electrodeposition, metallization of DNA, solvothermal synthesis,

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(D) Fig. 9.11 TEM images of various metallic nanofibers (copper, nickel, iron, and cobalt) from various thermal treatment procedures. (A) and (B) are the surface and microtomed crosssection of metal nanofibers after heat treatment at 400°C under inert atmosphere, (C) after heat treatment at 400°C under air and then under inert atmosphere, and (D) after heat treatment at 800°C under inert atmosphere. Scale bar is 200 nm. Reproduced with permission from Hansen, N.S., Cho, D., Joo, Y.L., 2012. Metal nanofibers with highly tunable electrical and magnetic properties via highly loaded water-based electrospinning. Small 8(10), 1510–1514. Copyright 2012, WILEY-VCH.

direct electrochemical precipitation, etc., it is hard to obtain uniform nanowires in high yield. In Wu’s et al. study, electrospinning technology was introduced to fabricate uniform metal nanofibers [24]. Polyvinyl acetate with an average molecular weight of 8000 g mol
1 and metal nitrate (metal ¼ Fe, Co, and Ni) were used as a polymer matrix and metal salt, respectively. The as-spun nanofibers were first heated at 500°C for 4 h in air followed by annealing at 400°C for 1 h in a hydrogen atmosphere to prepare Fe, Co, and Ni nanofibers. The room temperature magnetization hysteresis loops of as-prepared metal oxides and their corresponding metallic nanofibers were shown in Fig. 9.14. It can be seen that the hysteresis curves of Fe, Co, and Ni nanofibers showed typical ferromagnetic behaviors, while the curves of their oxide nanofibers displayed nearly a flat line, indicating little magnetization. Similar results can be found at low temperatures, as shown in Fig. 9.15.

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Fig. 9.12 SEM of IrOx/PVP nanofibers electrospun from a solution of hydrated IrCl3 and PVP in ethanol. Reproduced with permission from Kim, S.-J., Kim, Y. L., Yu, A., Lee, J., Lee, S. C., Lee, C., et al., 2014. Electrospun iridium oxide nanofibers for direct selective electrochemical detection of ascorbic acid. Sens. Actuat. B-Chem. 196, 480–488. Copyright 2014, Elsevier.

9.3.3

Electrical conductivity

The electrical conductivities of the as-prepared copper, nickel, iron, and cobalt nanofibers were shown in Fig. 9.16A along with the known bulk electrical conductivity [25]. It can be observed that the second thermal treatment scheme, which generated a nanofiber void of an amorphous regions with isotropic crystals connected to each other, had the highest electrical conductivity of around 106 to 107 S m
1 (which is just an order of magnitude below the electrical conductivity of the bulk material). However, in the cases of copper and nickel, where smaller amorphous regions could achieve high electrical conductivities, the iron and cobalt cases obtained much lower electrical conductivities due to their large crystals connected by amorphous electrically resistive regions. For many applications, such as conducting electrodes, it could be advantageous to have directional electrical conductivity for a directionally charge collection [26]. Anisotropic electrical conductivity could be generated by fabricating anisotropic mats with aligned mats of nanofibers being collected by different devices, such as a rotating collector. The degree of alignment can be controlled by the rotating speed. As shown in Fig. 9.16B, the axial electrical conductivity from the aligned copper nanofibers remained near the value for isotropic electrospun nanofibers. Since the degree of alignment was changed based on the rotating speed, electrical conductivity could be controlled by both crystal structure and nanofiber orientation.

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Fig. 9.13 SEM of IrOx/PVP nanofibers electrospun of a solution of hydrated IrCl3 and PVP in ethanol after calcinated at 900°C for 2 h. Reproduced with permission from Kim, S.-J., Kim, Y. L., Yu, A., Lee, J., Lee, S. C., Lee, C., et al., 2014. Electrospun iridium oxide nanofibers for direct selective electrochemical detection of ascorbic acid. Sens. Actuat. B-Chem. 196, 480–488. Copyright 2014, Elsevier.

9.4

Physical characterization of electrospun CNFs

CNFs have drawn increased attention in applications of catalysts and electrochemically active materials [28,29]. Zhang’s group produced CNFs by electrospinning and subsequent carbonization for use as the electrode materials in lithium-ion batteries and supercapacitors [30,31]. One-dimensional nanostructure framework was aimed to shorten the ion diffusion distance and increase the surface area so that electrochemical performance could be improved accordingly. It is significant to investigate the relationship between the morphology of CNFs and electrochemical properties by carrying on physical characterizations.

9.4.1 Morphologic characterization Zhu et al. investigated nitrogen-doped CNFs derived from PAN for use as the anode material in sodium ion batteries [32]. The morphology and microstructure of N-doped carbon nanofibers (N-CNFs) were observed by FE-SEM, as shown in Fig. 9.17. As the carbonization temperature increased from 700 to 900°C, the diameter of N-CNFs decreased from 250 to 150 nm since larger amounts of small molecules were released at higher temperatures during carbonization. Fig. 9.18 shows the HRTEM images of N-CNFs. A turbostatic structure and rough fiber surface were observable, indicating

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Fig. 9.14 Magnetic properties of the electrospun nanofibers before and after annealed in a hydrogen atmosphere: (A) magnetic hysteresis loops for Fe2O3 and Fe nanofibers; (B) CoO and Co nanofibers; (C) NiO and Ni nanofibers. The measurements were taken at room temperature. The nanofiber film surface was applied with the magnetic field parallel (in-plane). Reproduced with permission from Wu, H., Zhang, R., Liu, X., Lin, D., Pan, W., 2007. Electrospinning of Fe, Co, and Ni nanofibers: synthesis, assembly, and magnetic properties. Chem. Mater. 19(14), 3506–3511. Copyright 2007, American Chemical Society.

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Fig. 9.15 Hysteresis loops at temperature of 300 K and 5 K. (A) Fe, (B) Co, and (C) Ni nanofibers. Reproduced with permission from Wu, H., Zhang, R., Liu, X., Lin, D., Pan, W., 2007. Electrospinning of Fe, Co, and Ni nanofibers: synthesis, assembly, and magnetic properties. Chem. Mater. 19(14), 3506–3511. Copyright 2007, American Chemical Society.

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Fig. 9.16 Electrical conductivities of (A) isotropic copper, nickel, iron, and cobalt nanofibers electrospun from various thermal treatment procedures, (B) the associated axial and perpendicular electrical conductivity of aligned copper nanofibers with three alignment speeds versus the known bulk case. Reproduced with permission from Hansen, N. S., Cho, D., Joo, Y.L., 2012. Metal nanofibers with highly tunable electrical and magnetic properties via highly loaded water-based electrospinning. Small 8(10), 1510–1514. Copyright 2012, WILEYVCH.

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the presence of anamorphous structure and structural defects, which are favorable for Na-ion diffusion from various orientations and can provide sufficient contact area between active materials and the electrolyte. For some specific structures, such as porous CNFs, SEM and TEM are applied to inspect the pore size in the CNF structure. For instance, Kim et al. prepared porous CNFs by using PAN and PMMA as the electrospinning polymer [33]. During

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Fig. 9.17 SEM images of nitrogen-doped carbon nanofibers carbonized at (A) 700°C, (B) 800°C, and (C) 900°C. The insets are high-magnification images. Reproduced with permission from Zhu, J., Chen, C., Lu, Y., Ge, Y., Jiang, H., Fu, K., et al., 2015. Nitrogen-doped carbon nanofibers derived from polyacrylonitrile for use as anode material in sodium-ion batteries. Carbon 94, 189–195. Copyright 2015, Elsevier.

carbonization, PAN was pyrolyzed into carbon while PMMA was completely decomposed by heat. The electrospun continuous fibrous structure with a smooth surface morphology was found in the Fig. 9.19A–C. The fiber diameter was in the range of 200 to 400 nm. Higher mass fractions of PAN resulted in smaller nanofiber diameters. Phase separations occurred during electrospinning, creating the discontinuous and long rodlike PMMA phase and the continuous PAN phase. After carbonization, many hollow cores are created within the resultant carbon fibers. Fig. 9.19D–F shows SEM images of the cross sections of the as-prepared CNFs. Notably, the number of hollow cores increases with an increase in PMMA concentration. Fig. 9.19G shows the TEM images of porous CNFs. It is clearly seen that pores were distinct along the fiber length. In addition, from Fig. 9.20A–C we can see that the hollow cores in a single fiber were stable and that the thickness of pore walls was around 10 nm, which may have consisted of 15 graphene sheets undulated along the fiber length, shown in Fig. 9.20D.

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Fig. 9.18 HRTEM images of nitrogen-doped carbon nanofibers carbonized at (A) 700°C, (B) 800°C, and (C) 900°C. Reproduced with permission from Zhu, J., Chen, C., Lu, Y., Ge, Y., Jiang, H., Fu, K., et al., 2015. Nitrogen-doped carbon nanofibers derived from polyacrylonitrile for use as anode material in sodium-ion batteries. Carbon 94, 189–195. Copyright 2015, Elsevier.

9.4.2

Mechanical property

Some CNFs demonstrate excellent mechanical properties. They can be used in fiber reinforced structural laminates and woven composites, filters, and scaffolds [34]. Basically, the mechanical testing of nanofibers consists of single-fiber testing and fiber assemblies testing. Salman et al. used a microelectromechanical (MEMS)-based nanoscale testing platform (Fig. 9.21A) to test the mechanical properties of various CNFs produced by electrospinning with different voltage values [34]. Fig. 9.21B showed the stressstrain curve of single CNFs carbonized at 1400°C. The fiber tensile strength reached 3.5  0.6 GPa. Fig. 9.22 shows the mechanical performance of CNFs carbonized at different temperatures, with CNFs carbonized at 1400°C exhibiting the highest tensile strength. Although the tensile strength of CNFs carbonized at 1700°C was less, the

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Fig. 9.19 (A–C) Macromorphology of electrospun polymeric nanofibers containing two polymer phases. PAN:PMMA ¼ (A) 5:5, (B) 7:3, and (C) 9:1. (D–F) Cross-sectional fieldemission SEM images of thermally treated nanofibers at 1000°C. PAN:PMMA ¼ (D) 5:5, (E) 7:3, and (F) 9:1. (G) TEM images of sample (D), showing linearly developed hollow cores along the fiber length. The inset is a magnified TEM image. Reproduced with permission from Kim, C., Jeong, Y.I., Ngoc, B.T., Yang, K.S., Kojima, M., Kim, Y.A., et al., 2007. Synthesis and characterization of porous carbon nanofibers with hollow cores through the thermal treatment of electrospun copolymeric nanofiber webs. Small 3(1), 91–95. Copyright 2007, WILEY-VCH.

Young’s modulus reached a maximum average value of 191  58 GPa. Since the increased carbonization temperature led to the presence of randomly oriented turbostratic carbon crystallites the fiber became stiffer, which could lead to early fiber rupture and strength reduction. The mechanical property of individual electrospun PAN-derived CNFs was studied by Zussman et al. [26]. These CNFs were prepared by stabilizing the as-spun PAN nanofibers at 250°C for 30 min in air followed by carbonization under a nitrogen atmosphere at a temperature of 750°C with a heating rate of 5°C min
1 for 1 h. The obtained CNFs were finally heated at 1100°C for another hour. The natural resonance vibration method was performed to determine the bending modulus of the CNFs. As shown in Fig. 9.23, the fiber was bonded to the tip of an AFM cantilever. The attached fiber could be driven into mechanical resonance when an alternating electric potential was to this piezoactuator. The amplitude of vibration could be measured because the resonance frequency was always much larger than the SEM raster scanning rate. The average bending modulus of the CNFs was 63  7 GPa. It should be noted that the calculated value cannot be directly compared to Young’s modulus of CNFs because the scale, shape, and typical microstructure of these CNFs were convoluted in the bending modulus measurement. In addition, the failure stress of these CNFs varied from 0.32 to 0.90 GPa.

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Fig. 9.20 TEM cross-sectional images (A–C) of nanofibers thermally-treated at 2800°C. PAN: PMMA ¼ (A) 5:5, (B) 7:3, and (C) 9:1. TEM image (D) shows how core walls were constructed by graphene sheets after thermal treatment in (A). Reproduced with permission from Kim, C., Jeong, Y.I., Ngoc, B.T., Yang, K.S., Kojima, M., Kim, Y.A., et al., 2007. Synthesis and characterization of porous carbon nanofibers with hollow cores through the thermal treatment of electrospun copolymeric nanofiber webs. Small 3 (1), 91–95. Copyright 2007, WILEY-VCH.

9.4.3

Electrical conductivity

Electrical conductivity of carbonaceous materials is important for applications such as super capacitors and catalyst supports [35–37]. The aligned electrospun PAN-based CNFs were prepared by Zhou et al. [21]. The electrical conductivities of the nanofiber bundles were measured both in parallel and perpendicular fiber directions by a Shanghai-PC9A digital micro-ohmmeter, shown in Fig. 9.24. The equation of electrical conductivity (σ) could be written as σ ¼ L=AR

(9.1)

where A is the cross-sectional area of the nanofiber bundles, R is the electrical resistance of the nanofiber bundle, and L is the distance between the two electrodes. It was

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F Load cell

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Fig. 9.21 (A) Carbon nanofiber loaded on a MEMS device showing how the sample grips. (B) Stress–strain curve of a single nanofiber carbonized at 1400°C. Reproduced with permission from Arshad, S.N., Naraghi, M., Chasiotis, I., 2011. Strong carbon nanofibers from electrospun polyacrylonitrile. Carbon 49(5), 1710–1719. Copyright 2011, Elsevier.

observed that the electrical conductivities of the nanofiber bundle carbonized at 1000°C in the parallel direction and perpendicular directions were (180  6) and (7.7  0.8) S cm
1, respectively. The electrical conductivity in the parallel direction was over 20 times greater than in the perpendicular direction, due to the occasional contact among nanofibers in the bundles. Electrons were insulated if fibers did not come into

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Fig. 9.22 (A) Tensile strength vs. diameter plot of CNFs prepared with four different carbonization temperatures. (B) Average nanofiber strength versus carbonization temperature plot. (C) Elastic modulus versus nanofiber diameter plot of CNFs prepared with four different carbonization temperatures, and (D) average elastic modulus versus carbonization temperature plot. Reproduced with permission from Arshad, S.N., Naraghi, M., Chasiotis, I., 2011. Strong carbon nanofibers from electrospun polyacrylonitrile. Carbon 49(5), 1710–1719. Copyright 2011, Elsevier.

contact. In addition, the nanofiber bundle carbonized at 2200°C possessed greater electrical conductivities in the parallel and perpendicular directions, which resulted from more graphitic and ordered microstructures at 2200°C.

9.5

Physical characterization of electrospun composite nanofibers

In recent years, many different types of nanofiber structures have been produced by electrospinning. Composite nanofibers are one of the most-studied structures, such as nanoparticles imbedded in electrospun CNFs. They exhibit huge potential

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Fig. 9.23 SEM image of a carbonized nanofiber fixed between an AFM tip and a tungsten wire. The background shows a smaller AFM cantilever on the same AFM chip. Reproduced with permission from Zussman, E., Chen, X., Ding, W., Calabri, L., Dikin, D.A., Quintana, J.P., et al., 2005. Mechanical and structural characterization of electrospun PANderived carbon nanofibers. Carbon 43(10), 2175–2185. Copyright 2005, Elsevier.

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Fig. 9.24 Electrical conductivity data of electrospun carbon nanofiber bundles in both parallel and perpendicular directions. Reproduced with permission from Zhou, Z., Lai, C., Zhang, L., Qian, Y., Hou, H., Reneker, D.H., et al., 2009. Development of carbon nanofibers from aligned electrospun polyacrylonitrile nanofiber bundles and characterization of their microstructural, electrical, and mechanical properties. Polymer 50(13), 2999–3006. Copyright 2009, Elsevier.

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applications, owing to good flexibility, free-standing features, and variety [38–44]. In order to understand the ratio and position of each substance in composites nanofibers, physical characterization was usually performed, such as SEM, TEM, TGA, etc.

9.5.1

Morphology

Fu et al. prepared a flexible and self-supporting CNF structure with a vacant chamber to encapsulate Si nanoparticles followed by CVD carbon-coated treatment (vacant Si@CNF@C) for lithium-ion batteries [43]. SEM and TEM were utilized to characterize the morphological features (Fig. 9.25). Si@SiO2 was added into the electrospinning precursor to produce the vacant Si@CNF structure, which aimed to prevent structural failure from Si volume expansion (300%). Fig. 9.25A shows that the as-prepared Si@SiO2 nanoparticles exhibited a clear core–shell structure. The size of the as-prepared Si@SiO2 nanoparticles were around 100–200 nm. SiO2 can be removed during the heat treatment to generate vacant space around Si nanoparticles. Therefore, Fig. 9.25B shows the TEM of vacant Si@CNF@C. Vacant space was found around the Si nanoparticles embedded in the CNF matrix. CVD carbon coating was performed to achieve a more stable vacant Si@CNF@C structure, shown in Fig. 9.25C and D.

Fig. 9.25 TEM images of (A) Si@SiO2 nanoparticles and (B) vacant Si@CNF@C nanofiber, and (C, D) SEM images of a vacant Si@CNF@C nanofiber mat with low and high magnifications. Reproduced with permission from Kun, F., Lu, Y., Dirican, M., Chen, C., Yanilmaz, M., Shi, Q., et al., 2014. Chamber-confined silicon-carbon nanofiber composites for prolonged cycling life of Li-ion batteries. Nanoscale, 6(13), 7489–7495. Copyright 2014, Royal Society of Chemistry.

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Ge et al. prepared a copper-doped Li4Ti5O12/CNFs composite by electrospinning and subsequent heat treatment [45]. They added starting materials, including lithium acetate dihydrate, tetrabutyl titanate, and copper acetate together with poly(vinyl pyrrolidone) (PVP) into the precursor to electrospin the composite nanofiber. During carbonization, the starting materials reacted to form copper-doped Li4Ti5O12 while PVP was pyrolyzed into carbon. It is seen in Fig. 9.26 that the composite exhibited a consistent carbon nanofibrous structure (200–600 nm in diameter) uniformly loaded with inorganic copper-doped Li4Ti5O12 particles (50 nm of grain size). TEM images were also used for detecting changes of composite CNF samples after thermal treatment. For example, Ji et al. prepared α-Fe2O3-PAN nanofibers as the anode for rechargeable lithium-ion batteries [46]. Fig. 9.27 shows TEM images of electrospun FeCl36H2O-PAN before carbonization (Fig. 9.27A and B) and the corresponding α-Fe2O3-CNF composites (Fig. 9.27C–F). It is shown that electrospun FeCl36H2OPAN nanofibers had a rough surface morphology. After carbonization, α-Fe2O3 nanoparticles can be seen with a clearly spherical morphology. It is also seen that the α-Fe2O3 nanoparticles are embedded inside the fiber and are uniformly distributed along the fiber. The average size of these nanoparticles was about 20 nm in Fig. 9.27C–F.

Fig. 9.26 SEM images of copper-doped Li4Ti5O12/CNF composite. Reproduced with permission from Ge, Y., Jiang, H., Fu, K., Zhang, C., Zhu, J., Chen, C., et al., 2014. Copper-doped Li4Ti5O12/carbon nanofiber composites as anode for high-performance sodium-ion batteries. J. Power Sources 272, 860–865. Copyright 2014, Elsevier.

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Fig. 9.27 TEM images of (A, B) bicomponent nanofibers electrospun with 15 wt% FeCl36H2O-PAN and (C–F) the resultant α-Fe2O3-CNFs carbonized at 600°C for 8 h. Reproduced with permission from Ji, L., Toprakci, O., Alcoutlabi, M., Yao, Y., Li, Y., Zhang, S., et al., 2012. α-Fe2O3 nanoparticle-loaded carbon nanofibers as stable and high-capacity anodes for rechargeable lithium-ion batteries. ACS Appl. Mater. Interfaces 4(5), 2672–2679. Copyright 2012, American Chemical Society.

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In the case of nanofiber composites, TEM was also undertaken frequently to determine the crystalline component and distinguish isomorphic isomers. Ge et al. prepared TiO2/CNFs composite as the anode material for sodium-ion batteries [47]. Tetrabutyl titanate was the starting material of TiO2, while PVP was the electrospinning polymer and carbon resource. During carbonization, the starting material hydrolyzed to form TiO2 and PVP was pyrolyzed into carbon. As the temperature increased, the crystal structure (Fig. 9.28) showed the lattice spacings of TiO2/CNFs composite carbonized ˚ and 2.48 A ˚ at 650°C, which was detected by HRTEM. The lattice spacing of 3.52 A could be detected in the composite, corresponding to (101) lattice planes of anatase TiO2 and (101) lattice plane of rutile TiO2. In the image, the amorphous region was detected as the disordered carbon phase. Composite nanofiber sheets with surface-oxidized multiwalled carbon nanotubes (MWNTs) and PAN were prepared by Ge et al. [48]. In the AFM study, Fig. 9.29A and C are the height image and profile of the surface morphology of a

Fig. 9.28 HRTEM images of TiO2/CNFs composite. Reproduced with permission from Ge, Y., Zhu, J., Lu, Y., Chen, C., Qiu, Y., Zhang, X., 2015. The study on structure and electrochemical sodiation of one-dimensional nanocrystalline TiO2@C nanofiber composites. Electrochim. Acta 176, 989–996. Copyright 2015, Elsevier.

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Fig. 9.29 AFM images of a PAN/MWNT composite nanofiber containing 10 wt% modified nanotubes: (A) a height image of the nanofiber cross-section figure, (B) the corresponding phase image, and (C) a height data obtained from (A). Reproduced with permission from Ge, J.J., Hou, H., Li, Q., Graham, M.J., Greiner, A., Reneker, D.H., et al., 2004. Assembly of well-aligned multiwalled carbon nanotubes in confined polyacrylonitrile environments: electrospun composite nanofiber sheets. J. Am. Chem. Soc. 126 (48), 15754–15761. Copyright 2004, American Chemical Society.

PAN/MWNT (90/10) fiber. Fig. 9.29A shows an irregular surface along the fiber axis of the nanofiber. It is attributed to the extension of nanotube ends and the variation in the diameter of the nanofibers. Fig. 9.29C is the corresponding height data of the composite nanofiber along the cross-section of the nanofiber of Fig. 9.29A. The plot showed a round nanofiber with a diameter of 110 nm. The surface was rough with a few nanometers on extruding defects. In Fig. 9.28B, some dark parallel streaks along the fiber axis were observed, which were related to the internal arrangement of the MWNTs within and outside the PAN matrix. The size of these streaks is generally around 20 to 30 nm. When the MWNTs ends were located beyond the composite nanofiber, the tapping mode of AFM would detect them due to different mechanical moduli of the MWNTs and the PAN matrix.

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9.5.2

235

Mechanical property

Dirican et al. prepared flexible Si/SiO2/C nanofiber composite as the anode material for high-energy lithium–ion batteries [44]. They used MTS-30G load frame for tensile testing. Fig. 9.30A shows tensile stress-strain curves of flexible Si/SiO2/C nanofibers and inflexible Si/C nanofibers. First, the strain-at-break of flexible Si/SiO2/C nanofibers (about 4.4%) was higher than that of Si/C nanofibers (about 2.5%). During testing, while observing the fracture behavior of the inflexible Si/C nanofibers, it was observed that the crack on flexible Si/SiO2/C nanofibers spread gradually, which prevented sudden breaking of the entire specimen. Additionally, it was also noticed that flexible Si/SiO2/C nanofibers delivered a larger breaking strength and Young’s modulus (0.54 MPa, 8 MPa) than inflexible Si/C nanofibers (0.43 MPa, 5.5 MPa). In compression tests, when the first visible crack occurred, the strain-at-break and strain magnitude were recorded. Fig. 9.30B showed that the first visible crack occurred at the strain magnitude of 16% for inflexible Si/C nanofibers. In contrast, no crack was observed on the flexible Si/SiO2/C nanofiber sample even when the strain was at 80% (Fig. 9.30C). It is demonstrated that Si/SiO2/C nanofibers possessed mechanically excellent durability and high flexibility.

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Fig. 9.30 Mechanical properties of (A) flexible Si/SiO2/C and Si/C nanofibers; photograph of compression test on (B) inflexible Si/C nanofibers, inset shows the visible crack on the tested sample; (C) flexible Si/SiO2/C nanofibers, inset shows no damaged in the sample after testing. Reproduced with permission from Dirican, M., Yildiz, O., Lu, Y., Fang, X., Jiang, H., Kizil, H., et al., 2015. Flexible binder-free silicon/silica/carbon nanofiber composites as anode for lithium–ion batteries. Electrochim. Acta 169, 52–60. Copyright 2015, Elsevier.

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Conclusions

The electrospinning method is a versatile approach to produce nanofibers of various materials with specific properties. In order to learn about the physical properties of electrospun nanofibers, many physical characterizations are performed according to the end use. Morphologic characterizations such as SEM, TEM, or AFM were discussed for each kind of electrospun nanofibers. BET, mechanical test, and conductivity measurement are also included in this chapter to give an overall view of recent research on electrospun nanofibers.

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