**3. Fabrication of nanofibers by electrospinning**

and the design and preparation of electrospun nanofibers with novel secondary structures. Following this, the application of electrospun nanomaterials in constructing electrochemical

The basic setup for electrospinning consists of three major components: a high voltage power supply, a spinneret, and a collector (a counter electrode) (Fig. 1). In the process of electrospin‐ ning, the applied voltage causes a cone-shaped deformation of the drop of polymer solution (Taylor cone). Once the strength of electric field exceeds a threshold value, the electrostatic force on the deformed polymer drop can overcome the surface tension and thus a liquid jet is formed. This electrified jet then moves toward counter electrode, leading to the formation of a long and thin thread. As the liquid jet is continuously elongated and the solvent is evaporated,

solid fibers with diameters as small as tens nanometers are deposited on the colletor.

In spite of the simple setup, there are a number of parameters that can greatly affect the morphology and diameter of electrospun fibers, including: (1) the intrinsic properties of solution such as the type of polymer, concentration, conductivity, and solvent volatility; and (2) the processing parameters such as the strength of the applied electric field, solution flow rate, and the distance between spinneret and collector [2, 10, 11]. In addition, the humidity and temperature of the surroundings may also play an important role in determining the mor‐ phology and diameter of electrospun fibers. Numerous experimental investigations and theoretical models have drawn some general relationships between these parameters and fiber morphology. For example, the higher applied voltage will lead to a larger fiber diameter, but this trend is not monotonic; the higher polymer concentration (higher viscosity) or faster flow rate usually results in the larger nanofiber diameters. In contrast, the increase of solution conductivity can significantly reduce the fiber diameter. These results are instructive to some extent in experiment design and predicting the resultant fiber morphology. However,

sensors and their analytical performance is discussed.

**Figure 1.** Schematic illustration of the basic setup of electrospinning.

**2. General process of electrospinning**

36 Advances in Nanofibers

Electrospinning has been proved to be a versatile method to prepare 1D nanomaterials of polymer, ceramics, metal, and metal oxide. Various functional elements, such as drugs, dyes, DNA, proteins, and nanoparticles, could be incorporated into electrospun nanofibers to form composite nanofibers. Additionally, except for the nanofibers with solid interior and smooth surface, nanofibers with various secondary structures, including core/sheath, hollow, and porous, could be fabricated by electrospinning. In this section, the preparation of electrospun nanofibers with different composition and secondary structures is introduced, and the parameters that control the composition and morphology are highlighted.

#### **3.1. Electrospun nanofibers with different composition**

In principle, almost all natural and synthetic polymers can be electrospun into their 1D nanostructures through judicious selection of solution and processing parameters [1]. Besides itself nanofibers, polymers can also be used as template or host to load nanoparticles or functional molecules. The produced composite nanofibers exhibit various electronic, optical, magnetic, and biological properties.

In order to incorporate nanoparticles into electrospun fibers, pre-synthetic Au [12], Fe3O4 [13], SiO2 nanoparticles [14], CdTe quantum dots [15], and Au nanorods (AuNRs) [16] were introduced in polymer solution and then electrospinning was conducted. For example, AuNRs/poly(vinyl alcohol) (PVA) nanofiber was prepared by electrospinning the mixture solution of AuNRs and PVA [16]. The AuNRs were well aligned along the axis direction of the fibers due to the external fields (Fig. 2A). In a one-step method, silver nitrate was dissolved in poly(vinyl pyrrolidone) (PVP)/N, N-dimethylformamide (DMF) [17], or nylon 6/formic acid [18] solution, where DMF and formic acid acted as both a solvent for polymer and a reducing agent for the Ag+ ion, followed by electrospinning to form Ag nanoparticle-filled composite nanofibers. In addition, the introduction of nanoparticles into polymer nanofibers have also been accomplished by adding appropriate precursors to the electrospinning solution, after that a chemical or physical method was used to reduce the metal precursor. For example, PdCl2 and copolymers of acrylonitrile and acrylic acid (PAN-AA) are dissolved in DMF for electro‐ spinning. And then, the fiber mat was immersed into diluted hydrazine water solution to reduce Pd cations [19]. The as-prepared Pd/PAN-AA composite material showed high catalytic activity toward hydrogenation of dehydrolinalool. Li et al. prepared Ag nanoparticle-loaded PAN nanofibers via electrospinning of PAN/AgNO3-DMF solution followed by UV-irradia‐ tion photoreduction [20].

Carbon nanotubes (CNTs), an actively studied nano-object, can also be incorporated into electrospun fibers. The goal of most studies in this direction is to improve the electrical conductivity and mechanical strength of the fibers [21-25]. Some exciting properties were also observed for CNT-incorporated polymer fibers, such as enhanced thermal stability [21], anisotropic electrical conductivity [24], and the preferential orientation of the CNTs along the fiber axis. These composite fibers can find promising applications in high strength membrane and electronics. Graphene, a single layer of aromatic carbon nanomaterial, has also been used as nanofiller in polymer nanofibers to reinforce the mechanical, electrical, thermal, and optical properties. For example, Bao et al. prepared graphene-poly(vinyl acetate) (PVAc) composite nanofibers by electrospinning [26]. The results indicated that the dispersity of pristine or functionalized graphene greatly influenced the morphology of fibers. When graphene modified by 1-pyrenebutanoic acid succinimidyl ester (G-PBASE) or 4-(2-(pyridin-4-yl)vinlyl) phenyl group (G-dye) was used as nanofillers, uniform and smooth nanofibers were readily obtained (Fig. 2B). In contrast, some micrometer-sized beads were formed when plain graphene oxide (GO) was used due to the poor dispersion of GO in the DMF solvent.

method. In order to reduce the diameter of electrospun fibers, Li and Xia developed a new approach in which polymer was introduced into the sol-gel precursor to control the viscoelastic behavior, at the same time the sol-gel reaction was controlled to take place mainly in the spinning jet rather than in the stock solution [54-56]. In a typical procedure [55], a sol-gel precursor of titanium tetraisopropoxide (Ti(O*i*Pr)4) was mixed with PVP in alcohol solution. After the solution had been electrospun into a thin jet, the metal alkoxide immediately started to hydrolyze by reacting with the moistrure in air to generate a continuous gel network within the polymer matrix. As a result, TiO2/PVP composite nanofibers would be obtained (Fig. 2D). These composite nanofibers could subsequently be converted into TiO2 nanofibers without changing their morphology via calcinations at the elevated temperature (Fig. 2E). The average diameter of these ceramic nanofibers could be controlled in the range of 20-200 nm with relatively narrow size distribution by varying a number of parameters. This method has also been extended to process many other oxide ceramics into nanofibers. Similarly, a great number of metal oxide or sulfide nanofibers have been produced by electrospinning the solutions of appropriate metal precursors and polymers, followed by calcination at elevated temperatures. Electrospun metal oxide nanofibers could be further converted into continuous and thin metal nanofibers in reducing atmosphere, such as Cu [57, 58], Fe, Co, and Ni [59]. Shui and Li prepared long Pt nanowires with a few nanometers in diameter by electrospinning of H2PtCl6/PVP mixture solution and heat treatment [60]. A series of processing parameters were optimized to control the morphology and diameter of the nanowires. Very recently, Greiner's group prepared Au nanowires by electrospinning of highly concentrated aqueous dispersions of gold nanoparticles in the presence of PVA and subsequent annealing at 300-500 ℃ in air [61]. The produced Au nanowires represented solid structures like bulk gold (Fig. 2F). The electro‐ spun metal nanofibers with ultrahigh aspect ratio and ultralow junction resistance are of great interest for foundational research and applications in nanoelectronics, fuel cells, and sensors.

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Carbon fibers or nanofibers, which have many noticeable properties in mechanical strength, electrical conductivity, and special surface area, have been considered as one of the most important materials for modern science and technology. Various electrospun polymer nanofibers could be converted into carbon nanofibers, such as polyacrylonitrile (PAN), polyimide (PI), PVA, poly(vinylidene fluoride) (PVDF) and pitch. Inagaki et al. recently composed a review on the preparation of carbon nanofibers from electrospun polymer nanofibers [62]. Carbon precursors and the control of structure and texture in the resultant

Nanofibers with core/sheath structures have many potential applications in microfluidics, photonics, and energy storage. Electrospinning provides a simple method for the large-scale fabrication of such nanofibers. Up to now, several methods have been developed to prepare core/sheath structured nanofibers by electrospinning. For example, in template-directed method, polymer fibers (template) were produced by ordinary electrospinning, and then the as-prepared fibers were coated with the shell component by various chemical and physical methods [63-67]. With the use of conventional single-nozzle electrospinning, it is also possible

carbon nanofibers were highlighted.

**3.2. Nanofibers with core/sheath structures**

In addition to the nano-objects, drugs, dyes, proteins, DNA, virus, and other compounds can be readily incorporated into electrospinning solutions to produce functional fibers. For example, collagen could be electrospun into fibers from a solution of 1, 1, 1, 3, 3, 3-hexafluoro-2 propanol (HFP) [27, 28], or from a blend with poly(ethylene oxide) (PEO) [29]. Other proteins and enzymes, such as elastin [29], casein [30], α-chymotrypsin [31], bovine serum albumin (BSA) [32, 33], silk fibroin [34], lipase [30, 35], cellulose [36, 37], lysozyme [38, 39], glucose oxidase [40], luciferase [32], alkaline phosphatase and β-galactosidase [41], diisopropylfluor‐ ophosphatase [42], and lactate dehydrogenase [43], could only be processed by electrospinning as blends with synthetic polymers. The catalytic activity of encapsulated enzyme is usually lower than that of free enzyme, but more active than that in the cast membrane due to the higher surface area and porous structures of electrospun fibers. In addition, DNA molecules can also be encapsulated in electrospun fibers from blends with polymers [44, 45]. DNA molecules incorporated into electrospun nanofiber could reserve structurally intact and bioactive. More interestingly, virus could be used to fabricate 1D micro- and nanosized diameter fibers by electrospinning [46]. M13 virus was dispersed in HFP solution to form a homogeneous virus suspension, and then was directly electrospun into fibers (Fig. 2C). Due to the toxicity of HFP to the M13 virus, infectibility of M13 virus in HFP solution was dramat‐ ically decreased, showing no infectibility. In order to improve processing ability and preserve the intact viral structure and infecting ability, the M13 virus suspension was blended with a water soluble polymer PVP. Uniform fibers with the diameter of 100-200 nm could then be obtained.

For inorganic compounds, it is very difficult to directly process by electrospinning due to the strict requirement of solution viscoelasticity. Only a few types of inorganic fibers could be obtained by carefully selecting metallic precursors and solvents [47-49]. Recent studies demonstrated that the combination of electrospinning and sol-gel process could be used for direct producing inorganic fibers, for example TiO2/SiO2 and Al2O3 [50], SiO2 [51], V2O5/SiO2 [52], SiO2/ZrO2 [53]. The key point of this method was to control the hydrolysis rate of sol-gel precursors by adjusting the pH value or aging conditions. However, the fibers prepared via direct electrospinning of inorganic sols are usually several hundred nanometers in diameter with poor monodispersity, and only a limited number of materials can be prepared by this method. In order to reduce the diameter of electrospun fibers, Li and Xia developed a new approach in which polymer was introduced into the sol-gel precursor to control the viscoelastic behavior, at the same time the sol-gel reaction was controlled to take place mainly in the spinning jet rather than in the stock solution [54-56]. In a typical procedure [55], a sol-gel precursor of titanium tetraisopropoxide (Ti(O*i*Pr)4) was mixed with PVP in alcohol solution. After the solution had been electrospun into a thin jet, the metal alkoxide immediately started to hydrolyze by reacting with the moistrure in air to generate a continuous gel network within the polymer matrix. As a result, TiO2/PVP composite nanofibers would be obtained (Fig. 2D). These composite nanofibers could subsequently be converted into TiO2 nanofibers without changing their morphology via calcinations at the elevated temperature (Fig. 2E). The average diameter of these ceramic nanofibers could be controlled in the range of 20-200 nm with relatively narrow size distribution by varying a number of parameters. This method has also been extended to process many other oxide ceramics into nanofibers. Similarly, a great number of metal oxide or sulfide nanofibers have been produced by electrospinning the solutions of appropriate metal precursors and polymers, followed by calcination at elevated temperatures. Electrospun metal oxide nanofibers could be further converted into continuous and thin metal nanofibers in reducing atmosphere, such as Cu [57, 58], Fe, Co, and Ni [59]. Shui and Li prepared long Pt nanowires with a few nanometers in diameter by electrospinning of H2PtCl6/PVP mixture solution and heat treatment [60]. A series of processing parameters were optimized to control the morphology and diameter of the nanowires. Very recently, Greiner's group prepared Au nanowires by electrospinning of highly concentrated aqueous dispersions of gold nanoparticles in the presence of PVA and subsequent annealing at 300-500 ℃ in air [61]. The produced Au nanowires represented solid structures like bulk gold (Fig. 2F). The electro‐ spun metal nanofibers with ultrahigh aspect ratio and ultralow junction resistance are of great interest for foundational research and applications in nanoelectronics, fuel cells, and sensors.

Carbon fibers or nanofibers, which have many noticeable properties in mechanical strength, electrical conductivity, and special surface area, have been considered as one of the most important materials for modern science and technology. Various electrospun polymer nanofibers could be converted into carbon nanofibers, such as polyacrylonitrile (PAN), polyimide (PI), PVA, poly(vinylidene fluoride) (PVDF) and pitch. Inagaki et al. recently composed a review on the preparation of carbon nanofibers from electrospun polymer nanofibers [62]. Carbon precursors and the control of structure and texture in the resultant carbon nanofibers were highlighted.

#### **3.2. Nanofibers with core/sheath structures**

observed for CNT-incorporated polymer fibers, such as enhanced thermal stability [21], anisotropic electrical conductivity [24], and the preferential orientation of the CNTs along the fiber axis. These composite fibers can find promising applications in high strength membrane and electronics. Graphene, a single layer of aromatic carbon nanomaterial, has also been used as nanofiller in polymer nanofibers to reinforce the mechanical, electrical, thermal, and optical properties. For example, Bao et al. prepared graphene-poly(vinyl acetate) (PVAc) composite nanofibers by electrospinning [26]. The results indicated that the dispersity of pristine or functionalized graphene greatly influenced the morphology of fibers. When graphene modified by 1-pyrenebutanoic acid succinimidyl ester (G-PBASE) or 4-(2-(pyridin-4-yl)vinlyl) phenyl group (G-dye) was used as nanofillers, uniform and smooth nanofibers were readily obtained (Fig. 2B). In contrast, some micrometer-sized beads were formed when plain

graphene oxide (GO) was used due to the poor dispersion of GO in the DMF solvent.

obtained.

38 Advances in Nanofibers

In addition to the nano-objects, drugs, dyes, proteins, DNA, virus, and other compounds can be readily incorporated into electrospinning solutions to produce functional fibers. For example, collagen could be electrospun into fibers from a solution of 1, 1, 1, 3, 3, 3-hexafluoro-2 propanol (HFP) [27, 28], or from a blend with poly(ethylene oxide) (PEO) [29]. Other proteins and enzymes, such as elastin [29], casein [30], α-chymotrypsin [31], bovine serum albumin (BSA) [32, 33], silk fibroin [34], lipase [30, 35], cellulose [36, 37], lysozyme [38, 39], glucose oxidase [40], luciferase [32], alkaline phosphatase and β-galactosidase [41], diisopropylfluor‐ ophosphatase [42], and lactate dehydrogenase [43], could only be processed by electrospinning as blends with synthetic polymers. The catalytic activity of encapsulated enzyme is usually lower than that of free enzyme, but more active than that in the cast membrane due to the higher surface area and porous structures of electrospun fibers. In addition, DNA molecules can also be encapsulated in electrospun fibers from blends with polymers [44, 45]. DNA molecules incorporated into electrospun nanofiber could reserve structurally intact and bioactive. More interestingly, virus could be used to fabricate 1D micro- and nanosized diameter fibers by electrospinning [46]. M13 virus was dispersed in HFP solution to form a homogeneous virus suspension, and then was directly electrospun into fibers (Fig. 2C). Due to the toxicity of HFP to the M13 virus, infectibility of M13 virus in HFP solution was dramat‐ ically decreased, showing no infectibility. In order to improve processing ability and preserve the intact viral structure and infecting ability, the M13 virus suspension was blended with a water soluble polymer PVP. Uniform fibers with the diameter of 100-200 nm could then be

For inorganic compounds, it is very difficult to directly process by electrospinning due to the strict requirement of solution viscoelasticity. Only a few types of inorganic fibers could be obtained by carefully selecting metallic precursors and solvents [47-49]. Recent studies demonstrated that the combination of electrospinning and sol-gel process could be used for direct producing inorganic fibers, for example TiO2/SiO2 and Al2O3 [50], SiO2 [51], V2O5/SiO2 [52], SiO2/ZrO2 [53]. The key point of this method was to control the hydrolysis rate of sol-gel precursors by adjusting the pH value or aging conditions. However, the fibers prepared via direct electrospinning of inorganic sols are usually several hundred nanometers in diameter with poor monodispersity, and only a limited number of materials can be prepared by this

Nanofibers with core/sheath structures have many potential applications in microfluidics, photonics, and energy storage. Electrospinning provides a simple method for the large-scale fabrication of such nanofibers. Up to now, several methods have been developed to prepare core/sheath structured nanofibers by electrospinning. For example, in template-directed method, polymer fibers (template) were produced by ordinary electrospinning, and then the as-prepared fibers were coated with the shell component by various chemical and physical methods [63-67]. With the use of conventional single-nozzle electrospinning, it is also possible

**Figure 3.** (A) Schematic mechanism for the formation of core/sheath composite fibers during emulsion electrospin‐ ning. (B) Confocal laser scanning microscope image of core/sheath structured PEO/PEG-PLA nanofiber prepared from

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**Figure 4.** (A) Schematic illustration of the method for producing core/sheath nanofibers that contain an array of col‐ loids in the core. (B) Combination of optical and fluorescent mode images of the core/sheath fiber, consisting of a PCL

Coaxial electrospinning, in which coaxial two spinnerets replaced the single nozzle in the conventional setup for electrospinning, is a more convenient and direct method for the preparation of core/sheath structured nanofibers. Loscertales et al. initially designed a coaxial spinneret to generate steady core/sheath liquid jet from immiscible liquids [78]. However, in their experiment, the liquid jet was broke up to form core/sheath capsules, rather than fibers. Sun and co-workers overcame the instability problem in the coaxial electrospinning process to obtain continuous core/sheath jet, and then core/sheath polymer fibers [79]. The experi‐ mental setup for coaxial electrospinning is shown in Fig. 5A. It was proposed that undesirable mixing of the two polymer solutions could be prevented by the low diffusion coefficients relative to the fast stretching and solidification processes taking place in the electrospinning process. Core/sheath fibers with identical polymers PEO/PEO, or two different polymers polysulfone (PSU)/PEO could be obtained using this method. More importantly, non-spinna‐ ble solutions, such as poly(dodecylthiophene) (PDT) and Pd(OAc)2, could also be used as core

W/O emulsions [71].

sheath and PNIPAm microgel particles in the core [77].

**Figure 2.** Electrospun nanofibers with different composition. (A) Typical backscattering SEM image of the AuNRs/PVA nanofibers [16]. (B) High-magnification TEM image of G-PBASE/PVAc nanofiber. The arrows indicate the graphene flakes inside the nanofiber. The inset shows an enlarged image of G-PBASE embedded in the sidewall of a PVAc nano‐ fiber [26]. (C) SEM image of electrospun M13 virus-only fibers. (Scale bars: 5 μm) [46]. (D) TEM image of TiO2/PVP com‐ posite nanofibers fabricated by electrospinning an ethanol solution that contained 0.03 g/mL PVP and 0.1 g/mL Ti(O*i*Pr)4 [55]. (E) TEM image of TiO<sup>2</sup> nanofibers prepared by calcining (D) sample in air at 500 ℃ for 3 h [55]. (F) Optical microscopy image of gold nanowires on a mica slide, scale bar: 100 μm [61].

to prepare core/sheath nanofibers from emulsion or homogeneous polymer solutions. In the case of emulsion electrospinning, a core/sheath jet was formed in the electrospinning process due to the stretching and collapse of emulsion. This method has been used to prepare poly(methyl methacrylate) (PMMA)/PAN [68, 69], protein-methyl cellulose/poly(D, L-lactide) (MC/PDLLA) [39, 70], and PEO/poly(ethylene glycol)-poly(L- lactic acid) (PEG-PLA) (Fig. 3) [71] core/sheath nanofibers, and has the potential to extend to any pair of water-soluble polymer and hydrophobic (or amphiphilic) polymer. In the case of homogeneous solution electrospinning, the formation of core/sheath structure was mainly attributed to the phase separation of polymer blends, different solubility of the two components, and some other rheological factors [72-76]. The type of polymers, the ratios of components and the additives play key roles in the formation of core/sheath structures, rather than co-continuous morphol‐ ogies. Recently, Jo et al. reported a one-step, single-nozzle electrospinning method for producing core-sheath nanofibers with cross-linked polymeric colloids as core and polymer as sheath (Fig. 4) [77]. Cross-linked PMMA colloids or poly(N-isopropylacrylamide) (PNI‐ PAm) microgels were dispersed in a concentrated polymer solution, e.g. poly(ε-caprolactone) (PCL) in chloroform solution, for electrospinning. In the electrospinning process, fast evapo‐ ration of the solvent from the Taylor cone and following solution jet enhanced the phase separation of colloids from the condensed polymer solution, which resulted in a continuous colloidal packing at the inner region of fibers. If a small amount of colloids was used, the beanpod-like morphology of the nanofibers could be obtained; while a larger amount of colloids would lead to the colloids closely packing at the central area of the fibers, and core/ sheath fibers consisting of a colloidal core could be produced.

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**Figure 3.** (A) Schematic mechanism for the formation of core/sheath composite fibers during emulsion electrospin‐ ning. (B) Confocal laser scanning microscope image of core/sheath structured PEO/PEG-PLA nanofiber prepared from W/O emulsions [71].

to prepare core/sheath nanofibers from emulsion or homogeneous polymer solutions. In the case of emulsion electrospinning, a core/sheath jet was formed in the electrospinning process due to the stretching and collapse of emulsion. This method has been used to prepare poly(methyl methacrylate) (PMMA)/PAN [68, 69], protein-methyl cellulose/poly(D, L-lactide) (MC/PDLLA) [39, 70], and PEO/poly(ethylene glycol)-poly(L- lactic acid) (PEG-PLA) (Fig. 3) [71] core/sheath nanofibers, and has the potential to extend to any pair of water-soluble polymer and hydrophobic (or amphiphilic) polymer. In the case of homogeneous solution electrospinning, the formation of core/sheath structure was mainly attributed to the phase separation of polymer blends, different solubility of the two components, and some other rheological factors [72-76]. The type of polymers, the ratios of components and the additives play key roles in the formation of core/sheath structures, rather than co-continuous morphol‐ ogies. Recently, Jo et al. reported a one-step, single-nozzle electrospinning method for producing core-sheath nanofibers with cross-linked polymeric colloids as core and polymer as sheath (Fig. 4) [77]. Cross-linked PMMA colloids or poly(N-isopropylacrylamide) (PNI‐ PAm) microgels were dispersed in a concentrated polymer solution, e.g. poly(ε-caprolactone) (PCL) in chloroform solution, for electrospinning. In the electrospinning process, fast evapo‐ ration of the solvent from the Taylor cone and following solution jet enhanced the phase separation of colloids from the condensed polymer solution, which resulted in a continuous colloidal packing at the inner region of fibers. If a small amount of colloids was used, the beanpod-like morphology of the nanofibers could be obtained; while a larger amount of colloids would lead to the colloids closely packing at the central area of the fibers, and core/

**Figure 2.** Electrospun nanofibers with different composition. (A) Typical backscattering SEM image of the AuNRs/PVA nanofibers [16]. (B) High-magnification TEM image of G-PBASE/PVAc nanofiber. The arrows indicate the graphene flakes inside the nanofiber. The inset shows an enlarged image of G-PBASE embedded in the sidewall of a PVAc nano‐ fiber [26]. (C) SEM image of electrospun M13 virus-only fibers. (Scale bars: 5 μm) [46]. (D) TEM image of TiO2/PVP com‐ posite nanofibers fabricated by electrospinning an ethanol solution that contained 0.03 g/mL PVP and 0.1 g/mL Ti(O*i*Pr)4 [55]. (E) TEM image of TiO<sup>2</sup> nanofibers prepared by calcining (D) sample in air at 500 ℃ for 3 h [55]. (F) Optical

sheath fibers consisting of a colloidal core could be produced.

microscopy image of gold nanowires on a mica slide, scale bar: 100 μm [61].

40 Advances in Nanofibers

**Figure 4.** (A) Schematic illustration of the method for producing core/sheath nanofibers that contain an array of col‐ loids in the core. (B) Combination of optical and fluorescent mode images of the core/sheath fiber, consisting of a PCL sheath and PNIPAm microgel particles in the core [77].

Coaxial electrospinning, in which coaxial two spinnerets replaced the single nozzle in the conventional setup for electrospinning, is a more convenient and direct method for the preparation of core/sheath structured nanofibers. Loscertales et al. initially designed a coaxial spinneret to generate steady core/sheath liquid jet from immiscible liquids [78]. However, in their experiment, the liquid jet was broke up to form core/sheath capsules, rather than fibers. Sun and co-workers overcame the instability problem in the coaxial electrospinning process to obtain continuous core/sheath jet, and then core/sheath polymer fibers [79]. The experi‐ mental setup for coaxial electrospinning is shown in Fig. 5A. It was proposed that undesirable mixing of the two polymer solutions could be prevented by the low diffusion coefficients relative to the fast stretching and solidification processes taking place in the electrospinning process. Core/sheath fibers with identical polymers PEO/PEO, or two different polymers polysulfone (PSU)/PEO could be obtained using this method. More importantly, non-spinna‐ ble solutions, such as poly(dodecylthiophene) (PDT) and Pd(OAc)2, could also be used as core components to obtain core/sheath structured PDT/PEO (Fig. 5B) and Pd/PLA composite fibers. Yu et al. detailedly studied the coaxial electrospinning process for producing fibers with smaller diameters and core/sheath structure from difficult-to process fluids [80]. They pointed out that the stabilization of the core fluid in the sheath against breakup into droplets were mainly accomplished through two mechanisms: (1) The viscoelastic sheath fluid could delayed or completely suppressed the Rayleigh instability (which resulted in the breakup of fluid jet into droplets) in the core fluid. In the electrospinning process, stretching of the sheath component imparted great elasticity to the interface due to strain hardening, further stabilizing the core fluid. (2) The sheath fluid also reduced the surface forces at the boundary of the core fluid by replacing the relatively high fluid-vapor surface tension typically present in singlefluid electrospinning by a lower fluid-fluid interfacial tension. Additionally, the fast travelling speed of fluids in electrospinning process prevented the two fluids from mixing significantly. Li and Xia also systematiclly investigated the coaxial electrospinning process by using two immiscible liquids of heavy mineral oil and an ethanol solution of PVP and Ti(O*i*Pr)4 as the materials for core and sheath [81]. They argued that rapid stretching of the sheath caused strong viscous stress, which would stretch the oil phase and elongate it along with the sheath solution via the mechanisms of viscous dragging and/or contact friction.

poly(lactic acid) (PLA)/chitosan [94], PEO/chitosan [95], poly(L-lactide-co-caprolactone) (PLLACL)/collagen [96], and poly(hydroxybutyrate-co-hydroxyvalerate) (PHBV)/chitosan [97]. Other functional components, for example, FePt nanoparticles [98], Si nanoparticles [99], multi-walled carbon nanotube (MWNT) [100], O2 indicator (PtOEP) and γ-Fe3O4 [101], proteins [102], and drug molecules [103], have also been used as core components to fabricate core/ sheath fibers. In combination of coaxial electrospinning and sol-gel process, inorganic fibers with core/sheath structures were also prepared, such as LiCoO2/MgO [104], TiN/VN [105],

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Tubular nanostructures with dimensions in the range of submicrometer to a few nanometers are of great interest for applications in catalysis, fluid transportation, drug release, sensing, and gas storage. Various methods have been demonstrated to fabricate such structures from a broad range of materials. Similar to the preparation of core/sheath nanofibers, electrospun nanofibers have been used as sacrificial templates for preparing tubular fibers. For example, Bognitzki and co-workers designed a method termed tubes by fiber templates (TUFT) process for fabricating nano- and mesotubes [108]. They selected electrospun PLA nanofibers as templates. Polymer, polymer-metal hybrid and metal tubes could be obtained after coating and removing the template fibers. In this template method, various coating techniques have been employed, such as chemical vapor deposition [108, 109], physical vapor deposition [108], sol-gel process [110], electrochemical deposition [111], in-situ polymerization [112], layer-bylayer assembly [113-115], vapor deposition polymerization [116], atomic layer deposition [117], and sputtering [118]. After the formation of core/sheath fibers, the templates could be removed by heat treatment [108-110, 117, 118], or solvent extraction [109, 111-116], to obtain tubular structures. Additionally, nanofibers with hollow interior could be prepared by using electro‐ spun nanofibers as sacrificial templates without post-treatment process. For example, Te and BixTe1-x hollow nanofibers were directly synthesized by galvanic displacement reaction of electrospun Ni nanofibers at room temperature [119]. In general, additional coating and etching steps are required in these template methods, and the quality of the resultant tubes is

Nanofibers with hollow structures were also prepared by single-nozzle electrospinning, followed by appropriate post-treatment. For example, ceramics or metal oxide tubes have been fabricated by calcining the composite fibers, which were produced by electrospin‐ ning the mixture solution of polymer and procursors. LiNiO2 [120], CeO2 [121], Y2O3-ZrO2 [122], LaMgAl11O19 [122, 123], ZnO [124], MgO [125], TiO2 [126], BaFe12O19 [127], SiO2 [128], α-Fe3O4 and Co3O4 [129], Fe2O3 [130], CoFe2O4 [131], CuO and Cu [132], and SnO2 [133] tubes have been prepared by this method. Several groups have systematically investigat‐ ed the preparation process and proposed the formation mechanism of hollow fibers [120, 121, 129, 130, 133]. Cheng et al. [130] proposed that: In the electrospinning process, the evaporation of solvent would result in the formation of a gel layer on the surface of composite nanofibers, which has an important function to keep fiber texture during heat treatment. During heating, the gas produced by the decomposition of polymer would

CoFe2O4/Pb(Zr0.52Ti0.48)O3 [106], and SnO2/TiO2 [107].

**3.3. Nanofibers with hollow structures**

strongly dependent on the control of each step.

**Figure 5.** (A) Experimential setup used for coaxial electrospinning of core/sheath nanofibers. (B) TEM image of coelectrospun PEO (shell) and PDT (core) composite nanofibers [79].

With the development of theoretical and experimental aspects, this coaxial electrospinning method has been extended to prepare core/sheath fibers of various composition, such as gelatin/PCL [82, 83], poly(ethylene glycol) (PEG)/PCL [38], PCL/collagen [84], polyurethane/ polycarbonate (PU/PC) [85], PCL/PEG [86], PVP/PDLLA [87], polypyrrole (PPy)/PVP [88], poly(lactide-co-glycolide) (PLGA)/chitosan [89], PVP/poly(L-lactide-co-epsilon-caprolactone) (PLCL) [90], dextran/PLCL [91], Alq3/PVP [92], poly(glycerol sebacate) (PGS)/gelatin [93], poly(lactic acid) (PLA)/chitosan [94], PEO/chitosan [95], poly(L-lactide-co-caprolactone) (PLLACL)/collagen [96], and poly(hydroxybutyrate-co-hydroxyvalerate) (PHBV)/chitosan [97]. Other functional components, for example, FePt nanoparticles [98], Si nanoparticles [99], multi-walled carbon nanotube (MWNT) [100], O2 indicator (PtOEP) and γ-Fe3O4 [101], proteins [102], and drug molecules [103], have also been used as core components to fabricate core/ sheath fibers. In combination of coaxial electrospinning and sol-gel process, inorganic fibers with core/sheath structures were also prepared, such as LiCoO2/MgO [104], TiN/VN [105], CoFe2O4/Pb(Zr0.52Ti0.48)O3 [106], and SnO2/TiO2 [107].

#### **3.3. Nanofibers with hollow structures**

components to obtain core/sheath structured PDT/PEO (Fig. 5B) and Pd/PLA composite fibers. Yu et al. detailedly studied the coaxial electrospinning process for producing fibers with smaller diameters and core/sheath structure from difficult-to process fluids [80]. They pointed out that the stabilization of the core fluid in the sheath against breakup into droplets were mainly accomplished through two mechanisms: (1) The viscoelastic sheath fluid could delayed or completely suppressed the Rayleigh instability (which resulted in the breakup of fluid jet into droplets) in the core fluid. In the electrospinning process, stretching of the sheath component imparted great elasticity to the interface due to strain hardening, further stabilizing the core fluid. (2) The sheath fluid also reduced the surface forces at the boundary of the core fluid by replacing the relatively high fluid-vapor surface tension typically present in singlefluid electrospinning by a lower fluid-fluid interfacial tension. Additionally, the fast travelling speed of fluids in electrospinning process prevented the two fluids from mixing significantly. Li and Xia also systematiclly investigated the coaxial electrospinning process by using two immiscible liquids of heavy mineral oil and an ethanol solution of PVP and Ti(O*i*Pr)4 as the materials for core and sheath [81]. They argued that rapid stretching of the sheath caused strong viscous stress, which would stretch the oil phase and elongate it along with the sheath solution

**Figure 5.** (A) Experimential setup used for coaxial electrospinning of core/sheath nanofibers. (B) TEM image of co-

With the development of theoretical and experimental aspects, this coaxial electrospinning method has been extended to prepare core/sheath fibers of various composition, such as gelatin/PCL [82, 83], poly(ethylene glycol) (PEG)/PCL [38], PCL/collagen [84], polyurethane/ polycarbonate (PU/PC) [85], PCL/PEG [86], PVP/PDLLA [87], polypyrrole (PPy)/PVP [88], poly(lactide-co-glycolide) (PLGA)/chitosan [89], PVP/poly(L-lactide-co-epsilon-caprolactone) (PLCL) [90], dextran/PLCL [91], Alq3/PVP [92], poly(glycerol sebacate) (PGS)/gelatin [93],

via the mechanisms of viscous dragging and/or contact friction.

42 Advances in Nanofibers

electrospun PEO (shell) and PDT (core) composite nanofibers [79].

Tubular nanostructures with dimensions in the range of submicrometer to a few nanometers are of great interest for applications in catalysis, fluid transportation, drug release, sensing, and gas storage. Various methods have been demonstrated to fabricate such structures from a broad range of materials. Similar to the preparation of core/sheath nanofibers, electrospun nanofibers have been used as sacrificial templates for preparing tubular fibers. For example, Bognitzki and co-workers designed a method termed tubes by fiber templates (TUFT) process for fabricating nano- and mesotubes [108]. They selected electrospun PLA nanofibers as templates. Polymer, polymer-metal hybrid and metal tubes could be obtained after coating and removing the template fibers. In this template method, various coating techniques have been employed, such as chemical vapor deposition [108, 109], physical vapor deposition [108], sol-gel process [110], electrochemical deposition [111], in-situ polymerization [112], layer-bylayer assembly [113-115], vapor deposition polymerization [116], atomic layer deposition [117], and sputtering [118]. After the formation of core/sheath fibers, the templates could be removed by heat treatment [108-110, 117, 118], or solvent extraction [109, 111-116], to obtain tubular structures. Additionally, nanofibers with hollow interior could be prepared by using electro‐ spun nanofibers as sacrificial templates without post-treatment process. For example, Te and BixTe1-x hollow nanofibers were directly synthesized by galvanic displacement reaction of electrospun Ni nanofibers at room temperature [119]. In general, additional coating and etching steps are required in these template methods, and the quality of the resultant tubes is strongly dependent on the control of each step.

Nanofibers with hollow structures were also prepared by single-nozzle electrospinning, followed by appropriate post-treatment. For example, ceramics or metal oxide tubes have been fabricated by calcining the composite fibers, which were produced by electrospin‐ ning the mixture solution of polymer and procursors. LiNiO2 [120], CeO2 [121], Y2O3-ZrO2 [122], LaMgAl11O19 [122, 123], ZnO [124], MgO [125], TiO2 [126], BaFe12O19 [127], SiO2 [128], α-Fe3O4 and Co3O4 [129], Fe2O3 [130], CoFe2O4 [131], CuO and Cu [132], and SnO2 [133] tubes have been prepared by this method. Several groups have systematically investigat‐ ed the preparation process and proposed the formation mechanism of hollow fibers [120, 121, 129, 130, 133]. Cheng et al. [130] proposed that: In the electrospinning process, the evaporation of solvent would result in the formation of a gel layer on the surface of composite nanofibers, which has an important function to keep fiber texture during heat treatment. During heating, the gas produced by the decomposition of polymer would diffuse through the fiber surface. Once the rate of gas release was larger than gas diffu‐ sion through the fiber surface, the pressure inside the fibers increased to be larger than that outside of the composite fibers; consequently, hollow fibers could be obtained. However, Xia and co-workers argued that polymer template and Kirkendall effect played an important role to build hollow fibers [133]. Vacancies generated by the diffusion of metal precursors to the fiber surface and the decomposition of polymers finally formed the hollow struc‐ tures. Although the exact mechanism is ambiguous and not consistent, a necessary condition for the formation of tubular structures is that a rigid "skin" must form before the com‐ plete removal of polymer. In this method, the concentration of precursor, the ratio of precursor to polymer, the calcination temperature and heating rate significantly influence the morphology of the final products. In another single-nozzle electrospinning method, tetraethyl orthosilicate (TEOS) [134], PEO [135], or mineral oil [136] was introduced into the electrospinning solution to induce phase separation, and finally hollow fibers were obtained. Yu et al. prepared Sn nanoparticle encapsulated multichannel carbon micro‐ tubes by single-nozzle electrospinning process of tin octoate-PMMA-PAN in DMF emul‐ sion and subsequent calcinations [137]. Because PAN solution is easier to stretch than PMMA/DMF fluid, thus a core-shell jet was formed and the subsequent formation of coreshell fibers. The as-collected electrospun fibers were stabilized in air at 250 ℃, leading to the thermal degradation of the core components to create SnO2 nanoparticles encapsulat‐ ed in porous hollow fibers. After carbonization under an Ar/H2 atmosphere, the fibers were transformed into multichannel hollow porous carbon microtubes and SnO2 was reduced to Sn nanoparticles.

The studies in several groups have demonstrated that electrospinning could be directly utilized to prepare hollow nanofibers. For example, Li and Xia developed a coaxial electrospinning setup to fabricate ceramic hollow fibers by co-electrospinning viscous mineral oil as the core and a mixture ethanol solution of PVP and Ti(O*i*Pr)4 as the shell (Fig. 6A) [81]. The mineral oil was subsequently extracted to form amorphous TiO2/PVP composite tubes (Fig. 6B). After calcination at elevated temperatures in air, hollow TiO2 fibers were obtained (Fig. 6C). The wall thickness and inner diameter of the hollow nanofibers could be varied in the range from tens of nanometers to several hundred nanometers by controlling the processing parameters. The same group also demonstrated that functional nanoparticles (iron oxide, SnO2, Au) or molecular species (dye, octadecyltri‐ chlorosilane) could be directly incorporated into the hollow interiors by pre-dissolving these functional materials into the core liquid [138]. Using a similar setup, Loscertales and coworkers prepared polymer-free SiO2 and ZrO2 tubes by co-electrospinning an aged inorganic sol and an immiscible (or poorly miscible) liquid such as olive oil or glycerin, followed by selective removal of the inner liquid [139]. Turbostratic carbon nanotubes with inner diameter of 500 nm and wall thickness of 200 nm could also be obtained via coelectrospin‐ ning of PAN and PMMA with subsequent thermal degradation of the PMMA core and finally carbonization of the PAN shell [140]. Besides the ceramics and carbon tubes, polymeric microtubes were also fabricated in a single step by using the co-electrospin‐ ning of two polymeric solutions [141]. In this approach, two mechanisms, fast evapora‐ tion of the shell solvent and contact with a nonsolvent, were responsible for the formation and stabilization of the microtubes. Using the coaxial electrospinning, hollow fibers with various composition have been prepared, such as zeolite [142], SiO2 [143, 144], TiO2 [145], LiNiO2 [146], LiCoO2 [147], BaTiO3 [148], LiNi0.8Co0.1Mn0.1O2-MgO [149], PMMA [150, 151], PC [151], poly(3-hydroxy butyrate) (PHB) [152], Sn@carbon nanoparticles encapsulated

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Except for the spinneret with two coaxial capillaries, tri-axial spinneret was also designed to fabricate hollow nanostructures. For example, Lallave et al. [155] prepared Alcell lignin hollow nanofibers by tri-axial spinneret co-electrospinning Alcell lignin solutions at room temperature without any added polymer. The outmost sheath flow of ethanol was used to avoid solidification of the Taylor cone. After stabilization and carbonization of the asspun fibers at elevated temperatures, hollow carbon nanofibers were obtained. Zhao and co-workers developed a multifluidic compound-jet electrospinning technique to fabricate bio-mimic hierarchical multichannel microtubes (Fig. 7a) [156]. They used an ethanol solution of Ti(O*i*Pr)4 and PVP as outer liquid and paraffin oil as inner liquid. After a compound fluidic electrospinning process and removing the organics, TiO2 three-channel tubes were obtained (Fig. 7b, c and d). With the rational design of the spinneret, tubes with two to five channels have been successfully fabricated. Such multichannel structure greatly improved photocatalytic activity of TiO2 for degrading gaseous acetaldehyde due to a cooperative effect of trapping more gaseous molecules inside the channels and multiple

carbon [153], and carbon [154].

reflection of incident light [157].

**Figure 6.** (A) Schematic illustration of the setup for electrospinning nanofibers with a core/sheath structure. The spin‐ neret was fabricated from two coaxial capillaries, through which heavy mineral oil and an ethanol solution containing PVP and Ti(O*i*Pr)4 were simultaneously ejected to form a continuous, coaxial jet. (B) TEM image of two as-spun hollow fibers after the oil cores had been extracted with octane. The walls of these tubes consisted of amorphous TiO2 and PVP. (C) TEM image of TiO2 (anatase) hollow fibers that were obtained by calcining the composite nanotubes in air at 500 ℃ [81].

The studies in several groups have demonstrated that electrospinning could be directly utilized to prepare hollow nanofibers. For example, Li and Xia developed a coaxial electrospinning setup to fabricate ceramic hollow fibers by co-electrospinning viscous mineral oil as the core and a mixture ethanol solution of PVP and Ti(O*i*Pr)4 as the shell (Fig. 6A) [81]. The mineral oil was subsequently extracted to form amorphous TiO2/PVP composite tubes (Fig. 6B). After calcination at elevated temperatures in air, hollow TiO2 fibers were obtained (Fig. 6C). The wall thickness and inner diameter of the hollow nanofibers could be varied in the range from tens of nanometers to several hundred nanometers by controlling the processing parameters. The same group also demonstrated that functional nanoparticles (iron oxide, SnO2, Au) or molecular species (dye, octadecyltri‐ chlorosilane) could be directly incorporated into the hollow interiors by pre-dissolving these functional materials into the core liquid [138]. Using a similar setup, Loscertales and coworkers prepared polymer-free SiO2 and ZrO2 tubes by co-electrospinning an aged inorganic sol and an immiscible (or poorly miscible) liquid such as olive oil or glycerin, followed by selective removal of the inner liquid [139]. Turbostratic carbon nanotubes with inner diameter of 500 nm and wall thickness of 200 nm could also be obtained via coelectrospin‐ ning of PAN and PMMA with subsequent thermal degradation of the PMMA core and finally carbonization of the PAN shell [140]. Besides the ceramics and carbon tubes, polymeric microtubes were also fabricated in a single step by using the co-electrospin‐ ning of two polymeric solutions [141]. In this approach, two mechanisms, fast evapora‐ tion of the shell solvent and contact with a nonsolvent, were responsible for the formation and stabilization of the microtubes. Using the coaxial electrospinning, hollow fibers with various composition have been prepared, such as zeolite [142], SiO2 [143, 144], TiO2 [145], LiNiO2 [146], LiCoO2 [147], BaTiO3 [148], LiNi0.8Co0.1Mn0.1O2-MgO [149], PMMA [150, 151], PC [151], poly(3-hydroxy butyrate) (PHB) [152], Sn@carbon nanoparticles encapsulated carbon [153], and carbon [154].

diffuse through the fiber surface. Once the rate of gas release was larger than gas diffu‐ sion through the fiber surface, the pressure inside the fibers increased to be larger than that outside of the composite fibers; consequently, hollow fibers could be obtained. However, Xia and co-workers argued that polymer template and Kirkendall effect played an important role to build hollow fibers [133]. Vacancies generated by the diffusion of metal precursors to the fiber surface and the decomposition of polymers finally formed the hollow struc‐ tures. Although the exact mechanism is ambiguous and not consistent, a necessary condition for the formation of tubular structures is that a rigid "skin" must form before the com‐ plete removal of polymer. In this method, the concentration of precursor, the ratio of precursor to polymer, the calcination temperature and heating rate significantly influence the morphology of the final products. In another single-nozzle electrospinning method, tetraethyl orthosilicate (TEOS) [134], PEO [135], or mineral oil [136] was introduced into the electrospinning solution to induce phase separation, and finally hollow fibers were obtained. Yu et al. prepared Sn nanoparticle encapsulated multichannel carbon micro‐ tubes by single-nozzle electrospinning process of tin octoate-PMMA-PAN in DMF emul‐ sion and subsequent calcinations [137]. Because PAN solution is easier to stretch than PMMA/DMF fluid, thus a core-shell jet was formed and the subsequent formation of coreshell fibers. The as-collected electrospun fibers were stabilized in air at 250 ℃, leading to the thermal degradation of the core components to create SnO2 nanoparticles encapsulat‐ ed in porous hollow fibers. After carbonization under an Ar/H2 atmosphere, the fibers were transformed into multichannel hollow porous carbon microtubes and SnO2 was reduced to

**Figure 6.** (A) Schematic illustration of the setup for electrospinning nanofibers with a core/sheath structure. The spin‐ neret was fabricated from two coaxial capillaries, through which heavy mineral oil and an ethanol solution containing PVP and Ti(O*i*Pr)4 were simultaneously ejected to form a continuous, coaxial jet. (B) TEM image of two as-spun hollow fibers after the oil cores had been extracted with octane. The walls of these tubes consisted of amorphous TiO2 and PVP. (C) TEM image of TiO2 (anatase) hollow fibers that were obtained by calcining the composite nanotubes in air at

Sn nanoparticles.

44 Advances in Nanofibers

500 ℃ [81].

Except for the spinneret with two coaxial capillaries, tri-axial spinneret was also designed to fabricate hollow nanostructures. For example, Lallave et al. [155] prepared Alcell lignin hollow nanofibers by tri-axial spinneret co-electrospinning Alcell lignin solutions at room temperature without any added polymer. The outmost sheath flow of ethanol was used to avoid solidification of the Taylor cone. After stabilization and carbonization of the asspun fibers at elevated temperatures, hollow carbon nanofibers were obtained. Zhao and co-workers developed a multifluidic compound-jet electrospinning technique to fabricate bio-mimic hierarchical multichannel microtubes (Fig. 7a) [156]. They used an ethanol solution of Ti(O*i*Pr)4 and PVP as outer liquid and paraffin oil as inner liquid. After a compound fluidic electrospinning process and removing the organics, TiO2 three-channel tubes were obtained (Fig. 7b, c and d). With the rational design of the spinneret, tubes with two to five channels have been successfully fabricated. Such multichannel structure greatly improved photocatalytic activity of TiO2 for degrading gaseous acetaldehyde due to a cooperative effect of trapping more gaseous molecules inside the channels and multiple reflection of incident light [157].

In addition to PS, the polymers including PMMA, PC, and PEO were also investigated. In general, electrospun PMMA fibers from CHCl3 and THF exhibited a nanoporous surface texture. PC fibers electrospun from CHCl3 showed elongated nanopores of about 100-250 nm, while those formed from THF exhibited irregular-shaped micropores with diameters of about 20 µm. However, no nanopores were observed on electrospun PEO fibers under any process‐ ing conditions. They also investigated the effect of humidity and molecular weight on the surface morphology of electrospun PS fibers from PS/THF solution [160]. They found that increasing humidity caused an increase in the number, diameter, shape, and distribution of the pores, and increasing the molecular weight of PS resulted in larger, less uniform shaped pores. From these systematic studies, they ascribed the formation of porous surface morphol‐ ogy to the combinative effect of both phase separation and breath figure formation. Dayal and co-workers studied experimentally and theoretically the formation of porous structures from electrospinning of PMMA/CH2Cl2 and PS/THF systems [161]. They proposed that the porous fibers were favored to form if the polymer/solvent system was partially miscible showing an upper critical solution temperature (UCST) envelope at the electrospinning temperature, especially if the solvent utilized were volatile and sensitive to moisture absorption. The pore size depends on various factors such as surface energy and the solvent evaporation rates. With the use of phase separation mechanism, ultrafine porous cellulose triacetate (CTA) fibers were also prepared by electrospinning CTA dissolved in CH2Cl2 or a mixed solvent of CH2Cl2/ ethanol [162]. Similarly, PS fibers with micro- and nanoporous structures both in the core and/ or on the fiber surfaces were prepared in a single process by varying solvent compositions (THF/DMF) and the concentration of PS solutions [163]. Porous polymer fibers of PLLA [164], PAN [165], and cellulose acetate [166] were also prepared by electrospinning with appropriate binary solvent system. The formation of porous structures was mainly due to the spinodal

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decomposition phase separation occurred during the electrospinning process.

**Figure 8.** SEM micrographs of porous PLLA fibers obtained via electrospinning of a solution of PLLA in dichlorome‐

In another method, porous nanofibers could be prepared by the selective removal of a component from nanofibers made of a composite or blend material. For example, the structural changes for fibers consisting of a PLA/PVP blend were investigated when one of the two components was selectively removed [167]. It was found that porous nanofibers were obtained

thane. a) Survey; b) Magnification [158].

**Figure 7.** (a) Schematic illustration of the three-channel tube fabrication system. The immiscible inner and outer fluids were paraffin oil (red), and ethanol solution containing Ti(O*i*Pr)4 and PVP (blue). The inset shows the outlet section of the spinneret. (b) Side-view SEM image of sample after the organics have been removed. (c) Magnified SEM image of tubes in which the channels were divided into three independent flabellate parts by a Y-shape inner ridge. (d) TEM image of a three-channel tube; the individual channels of tube are straight and continuous [156].

#### **3.4. Nanofibers with porous structures**

Nanofibers with porous structures have excited immense interest because of their ultrahigh surface area, and thus potential applications in filtration, absorption, fuel cell, catalysis, tissue engineering, and sensors. Several methods have been reported for fabricating porous electro‐ spun nanofibers. In one method, phase separation was utilized to induce the formation of porous nanostructures in the electrospinning process. For example, Bognitzki et al. prepared porous polymer fibers of poly(L-lactic acid) (PLLA), PC, and polyvinylcarbazole by using dichloromethane as solvent [158]. For PLLA fibers, the average pore size is in the order of 100 nm in width and 250 nm in length with the long axis being oriented along the fiber axis (Fig. 8). The fast evaporation of solvent gave rise to local phase separation, and the solvent-rich regions transformed into pores during the electrospinning process. Rabolt's group systemat‐ ically investigated the influence of polymer/solvent properties on the fiber surface morphology [159]. A variety of solvents (tetrahydrofuran (THF), CS2, toluene, THF/DMF) with different boiling points and vapor pressures were examined to prepare polystyrene (PS) fibers. It was found that a very high density of pores were observed on PS fibers electrospun from THF, while the microtexture and nanopores disappeared as substitution of THF with DMF. This result indicated that the volatility of the solvent significantly influenced the pore formation. In addition to PS, the polymers including PMMA, PC, and PEO were also investigated. In general, electrospun PMMA fibers from CHCl3 and THF exhibited a nanoporous surface texture. PC fibers electrospun from CHCl3 showed elongated nanopores of about 100-250 nm, while those formed from THF exhibited irregular-shaped micropores with diameters of about 20 µm. However, no nanopores were observed on electrospun PEO fibers under any process‐ ing conditions. They also investigated the effect of humidity and molecular weight on the surface morphology of electrospun PS fibers from PS/THF solution [160]. They found that increasing humidity caused an increase in the number, diameter, shape, and distribution of the pores, and increasing the molecular weight of PS resulted in larger, less uniform shaped pores. From these systematic studies, they ascribed the formation of porous surface morphol‐ ogy to the combinative effect of both phase separation and breath figure formation. Dayal and co-workers studied experimentally and theoretically the formation of porous structures from electrospinning of PMMA/CH2Cl2 and PS/THF systems [161]. They proposed that the porous fibers were favored to form if the polymer/solvent system was partially miscible showing an upper critical solution temperature (UCST) envelope at the electrospinning temperature, especially if the solvent utilized were volatile and sensitive to moisture absorption. The pore size depends on various factors such as surface energy and the solvent evaporation rates. With the use of phase separation mechanism, ultrafine porous cellulose triacetate (CTA) fibers were also prepared by electrospinning CTA dissolved in CH2Cl2 or a mixed solvent of CH2Cl2/ ethanol [162]. Similarly, PS fibers with micro- and nanoporous structures both in the core and/ or on the fiber surfaces were prepared in a single process by varying solvent compositions (THF/DMF) and the concentration of PS solutions [163]. Porous polymer fibers of PLLA [164], PAN [165], and cellulose acetate [166] were also prepared by electrospinning with appropriate binary solvent system. The formation of porous structures was mainly due to the spinodal decomposition phase separation occurred during the electrospinning process.

**Figure 7.** (a) Schematic illustration of the three-channel tube fabrication system. The immiscible inner and outer fluids were paraffin oil (red), and ethanol solution containing Ti(O*i*Pr)4 and PVP (blue). The inset shows the outlet section of the spinneret. (b) Side-view SEM image of sample after the organics have been removed. (c) Magnified SEM image of tubes in which the channels were divided into three independent flabellate parts by a Y-shape inner ridge. (d) TEM

Nanofibers with porous structures have excited immense interest because of their ultrahigh surface area, and thus potential applications in filtration, absorption, fuel cell, catalysis, tissue engineering, and sensors. Several methods have been reported for fabricating porous electro‐ spun nanofibers. In one method, phase separation was utilized to induce the formation of porous nanostructures in the electrospinning process. For example, Bognitzki et al. prepared porous polymer fibers of poly(L-lactic acid) (PLLA), PC, and polyvinylcarbazole by using dichloromethane as solvent [158]. For PLLA fibers, the average pore size is in the order of 100 nm in width and 250 nm in length with the long axis being oriented along the fiber axis (Fig. 8). The fast evaporation of solvent gave rise to local phase separation, and the solvent-rich regions transformed into pores during the electrospinning process. Rabolt's group systemat‐ ically investigated the influence of polymer/solvent properties on the fiber surface morphology [159]. A variety of solvents (tetrahydrofuran (THF), CS2, toluene, THF/DMF) with different boiling points and vapor pressures were examined to prepare polystyrene (PS) fibers. It was found that a very high density of pores were observed on PS fibers electrospun from THF, while the microtexture and nanopores disappeared as substitution of THF with DMF. This result indicated that the volatility of the solvent significantly influenced the pore formation.

image of a three-channel tube; the individual channels of tube are straight and continuous [156].

**3.4. Nanofibers with porous structures**

46 Advances in Nanofibers

**Figure 8.** SEM micrographs of porous PLLA fibers obtained via electrospinning of a solution of PLLA in dichlorome‐ thane. a) Survey; b) Magnification [158].

In another method, porous nanofibers could be prepared by the selective removal of a component from nanofibers made of a composite or blend material. For example, the structural changes for fibers consisting of a PLA/PVP blend were investigated when one of the two components was selectively removed [167]. It was found that porous nanofibers were obtained after selective removal of PVP by water extraction or PLA by annealing at elevated tempera‐ tures when equal amount of the two polymers were loaded into the electrospinning solution. However, the fibers remained compact without any alteration of the surface structure after removing the minor component when another component was the major one in the composite fibers. This morphological change was believed to result from the rapid phase separation and rapid solidification in the electrospinning process. Porous inorganic nanofibers of TiO2 [55, 81], SiO2 [168-170], SnO2 [171, 172], NaYF4:Yb3, Er3@silica [173], and ZnCo2O4 [174] have been fabricated by electrospinning the blend solutions of polymer and procursors, followed by selective removal of the polymer component. Porous polymer fibers, such as PEI [175], poly(glycolic acid) (PGA) [176], and PAN [177] were also prepared by electrospinning of a blend solution, followed by thermal degradation or solvent extraction of another component. Salts, such as GaCl3 and NaHCO3, were also introduced into the electospinning solution to induce porous structures after the removal of the salts [178, 179].

Porous carbonaceous materials have been widely used in gas storage, separation, purification, or as catalyst carriers, electrode materials for fuel cells, and electrochemical double-layer supercapacitors, because of their unique mechanical properties, heat resistance, chemical inertness, etc. Porous carbon nanofibers could be prepared by the combination of electrospin‐ ning and post-spun treatment. For example, PAN-based carbon fibers with porous structures have been fabricated by electrospinning the mixture solutions of PAN and other polymers, followed by removal of the polymer and carbonization of the remaining PAN [177, 182, 183]. Kim et al. prepared porous carbon nanofibers by the electrospinning of PAN solution con‐ taining zinc chloride [184]. Zinc chloride trapped in the electrospun PAN nanofibers acted as a dehydrating agent and thus enhanced the oxidation rate, affording a shortened stabilization time. During carbonization process, zinc oxide was formed and acted as the catalyst for creating micropores on the outer surface of carbon nanofibers by etching carbon atoms. Porous structures were also produced on carbon nanofibers during the stabilization and carbonization process by activation using chemical activation regents, such as zinc chloride [185], and KOH

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In addition to the structures aforementioned, beaded, necklace-like, and ribbon nanofibers could be prepared by adjusting the processing and solution parameters for electrospinning. The beads formed in the electrospinning process were usually regarded as by-products, and the formation mechanism was studied by several groups [190, 191]. It was found that the viscoelasticity of the solution, charge density carried by the jet, and the surface tension of the solution were the key factors that influence the formation of the beaded fibers. Jin et al. fabricated necklace-like structure via electrospinning aqueous solution of PVA and SiO2 particles [192]. The results indicated that the diameter of SiO2 particles, the weight ratio of PVA to SiO2, the voltage, and the relative content of PVA/SiO2/H2O greatly influenced the mor‐ phology of electrospun fibers. Especially, the diameter of SiO2 particles greatly influenced the morphology of produced fibers. For example, SiO2 particles with diameter of 143 nm tended to aggregate into bunches in the fibers, while 265 and 910 nm SiO2 particles tended to align along the fibers one by one, resembling necklaces. In addition to round nanofibers, electro‐ spinning a polymer solution can produce thin fibers with a variety of cross-sectional shapes. Koombhongse and co-workers studied a series of polymer solutions, and various shaped fibers were observed, including branched fibers, flat ribbons, ribbons with other shapes, and fibers that were split longitudinally from larger fibers [193]. In the electrospinning process, a thin polymer skin was formed due to the rapid evaporation of the solvent. Following the escape of solvent inside the fibers, tube-like fibers were formed, which collapsed due to atmospheric pressure to create ribbon-like fibers. Branched fibers were formed by the ejection of smaller jets from the surface of the primary jets, while split fibers were obtained by the separation of a primary jet into two smaller jets. They proposed that fluid mechanical effects, electrical charge carried with the jet, and evaporation of the solvent all contributed to the formation of these

[186], or activation using SiO2 nanoparticles [187-189].

**3.5. Nanofibers with other secondary structures**

special shaped fibers.

Xia's group reported a novel method to produce porous nanofibers by modifying the electro‐ spinning setup [180]. In this setup, the collector was immersed in a bath of liquid nitrogen. Porous polymer fibers can be obtained through thermally induced phase separation (TIPS) between the solvent-rich and solvent-poor regions in the fiber during electrospinning, followed by removal of solvent in vacuum. PS fibers with ~1 µm in diameter were obtained by using this method (Fig. 9A). Examination of the end of a broken fiber (inset) indicated that the fiber was porous throughout. It should be noted that the fibers prepared by this method had larger diameters than those prepared without the use of a liquid nitrogen bath. The reason is that the fibers were collected with a smaller distance between the spinneret and the liquid nitrogen (10 cm), which greatly weakened the size reduction caused by whipping and solvent evaporation. This method could be extended to prepare porous fibers from a variety of different polymers, such as PAN, PVDF, and PCL (Fig. 9B). Similarly, Pant et al. developed a water-bath electrospinning setup, and highly porous PCL fibers were prepared by electro‐ spinning from pure PCL, and its blends with methoxy poly(ethylene glycol) (MPEG) [181]. A simultaneous phase separation and dissolution of MPEG from electrospun PCL fibers caused the formation of porous structure during water-bath electrospinning.

**Figure 9.** (A) SEM images of PS porous fibers prepared by electrospinning into liquid nitrogen, followed by drying in vacuum. The inset is a SEM micrograph of the broken end of a fiber at a higher magnification, showing that the fiber was porous throughout. (B) PCL fibers obtained by electrospinning into liquid nitrogen followed by drying in vacuum [180].

Porous carbonaceous materials have been widely used in gas storage, separation, purification, or as catalyst carriers, electrode materials for fuel cells, and electrochemical double-layer supercapacitors, because of their unique mechanical properties, heat resistance, chemical inertness, etc. Porous carbon nanofibers could be prepared by the combination of electrospin‐ ning and post-spun treatment. For example, PAN-based carbon fibers with porous structures have been fabricated by electrospinning the mixture solutions of PAN and other polymers, followed by removal of the polymer and carbonization of the remaining PAN [177, 182, 183]. Kim et al. prepared porous carbon nanofibers by the electrospinning of PAN solution con‐ taining zinc chloride [184]. Zinc chloride trapped in the electrospun PAN nanofibers acted as a dehydrating agent and thus enhanced the oxidation rate, affording a shortened stabilization time. During carbonization process, zinc oxide was formed and acted as the catalyst for creating micropores on the outer surface of carbon nanofibers by etching carbon atoms. Porous structures were also produced on carbon nanofibers during the stabilization and carbonization process by activation using chemical activation regents, such as zinc chloride [185], and KOH [186], or activation using SiO2 nanoparticles [187-189].

#### **3.5. Nanofibers with other secondary structures**

after selective removal of PVP by water extraction or PLA by annealing at elevated tempera‐ tures when equal amount of the two polymers were loaded into the electrospinning solution. However, the fibers remained compact without any alteration of the surface structure after removing the minor component when another component was the major one in the composite fibers. This morphological change was believed to result from the rapid phase separation and rapid solidification in the electrospinning process. Porous inorganic nanofibers of TiO2 [55, 81], SiO2 [168-170], SnO2 [171, 172], NaYF4:Yb3, Er3@silica [173], and ZnCo2O4 [174] have been fabricated by electrospinning the blend solutions of polymer and procursors, followed by selective removal of the polymer component. Porous polymer fibers, such as PEI [175], poly(glycolic acid) (PGA) [176], and PAN [177] were also prepared by electrospinning of a blend solution, followed by thermal degradation or solvent extraction of another component. Salts, such as GaCl3 and NaHCO3, were also introduced into the electospinning solution to

Xia's group reported a novel method to produce porous nanofibers by modifying the electro‐ spinning setup [180]. In this setup, the collector was immersed in a bath of liquid nitrogen. Porous polymer fibers can be obtained through thermally induced phase separation (TIPS) between the solvent-rich and solvent-poor regions in the fiber during electrospinning, followed by removal of solvent in vacuum. PS fibers with ~1 µm in diameter were obtained by using this method (Fig. 9A). Examination of the end of a broken fiber (inset) indicated that the fiber was porous throughout. It should be noted that the fibers prepared by this method had larger diameters than those prepared without the use of a liquid nitrogen bath. The reason is that the fibers were collected with a smaller distance between the spinneret and the liquid nitrogen (10 cm), which greatly weakened the size reduction caused by whipping and solvent evaporation. This method could be extended to prepare porous fibers from a variety of different polymers, such as PAN, PVDF, and PCL (Fig. 9B). Similarly, Pant et al. developed a water-bath electrospinning setup, and highly porous PCL fibers were prepared by electro‐ spinning from pure PCL, and its blends with methoxy poly(ethylene glycol) (MPEG) [181]. A simultaneous phase separation and dissolution of MPEG from electrospun PCL fibers caused

**Figure 9.** (A) SEM images of PS porous fibers prepared by electrospinning into liquid nitrogen, followed by drying in vacuum. The inset is a SEM micrograph of the broken end of a fiber at a higher magnification, showing that the fiber was porous throughout. (B) PCL fibers obtained by electrospinning into liquid nitrogen followed by drying in vacuum

induce porous structures after the removal of the salts [178, 179].

the formation of porous structure during water-bath electrospinning.

[180].

48 Advances in Nanofibers

In addition to the structures aforementioned, beaded, necklace-like, and ribbon nanofibers could be prepared by adjusting the processing and solution parameters for electrospinning. The beads formed in the electrospinning process were usually regarded as by-products, and the formation mechanism was studied by several groups [190, 191]. It was found that the viscoelasticity of the solution, charge density carried by the jet, and the surface tension of the solution were the key factors that influence the formation of the beaded fibers. Jin et al. fabricated necklace-like structure via electrospinning aqueous solution of PVA and SiO2 particles [192]. The results indicated that the diameter of SiO2 particles, the weight ratio of PVA to SiO2, the voltage, and the relative content of PVA/SiO2/H2O greatly influenced the mor‐ phology of electrospun fibers. Especially, the diameter of SiO2 particles greatly influenced the morphology of produced fibers. For example, SiO2 particles with diameter of 143 nm tended to aggregate into bunches in the fibers, while 265 and 910 nm SiO2 particles tended to align along the fibers one by one, resembling necklaces. In addition to round nanofibers, electro‐ spinning a polymer solution can produce thin fibers with a variety of cross-sectional shapes. Koombhongse and co-workers studied a series of polymer solutions, and various shaped fibers were observed, including branched fibers, flat ribbons, ribbons with other shapes, and fibers that were split longitudinally from larger fibers [193]. In the electrospinning process, a thin polymer skin was formed due to the rapid evaporation of the solvent. Following the escape of solvent inside the fibers, tube-like fibers were formed, which collapsed due to atmospheric pressure to create ribbon-like fibers. Branched fibers were formed by the ejection of smaller jets from the surface of the primary jets, while split fibers were obtained by the separation of a primary jet into two smaller jets. They proposed that fluid mechanical effects, electrical charge carried with the jet, and evaporation of the solvent all contributed to the formation of these special shaped fibers.
