Open access peer-reviewed chapter
Abstract
Scientists have been paying a special attention for the synthesis of one-dimensional (1D) morphologies to attain new phenomena and novel physicochemical characteristics of materials. Furthermore, 1D nanostructures exhibit long axial ratio, which has a great influence on the physical and chemical properties of materials. It is worth mentioning that electrospinning is one of the most common and efficient techniques used for the preparation of 1D polymer composite nanofibers. Using electrospinning, nanofibers were fabricated by electrostatic stretching of polymer viscous solution by applying a high voltage. This chapter discusses the synthesis of metal oxide nanofibers such as tin oxide (SnO2), zinc oxide (ZnO), titanium oxide (TiO2), and nickel oxide (NiO) using electrospinning process of polymer solution containing metal precursors and followed by annealing procedures to eliminate the polymer galleries, which were chosen as a sacrificial template for the preparation of metal oxide nanofibers. SEM, XRD, and XPS are equipped to characterize the electrospun metal oxide nanofibers and the results settle the formation of homogeneously distributed metal oxide nanofibers.
Keywords
- electrospinning
- metal oxide
- nanofibers
- SEM
- XRD
- XPS
1. Introduction
In the last decades, electrospinning attracted a vast portion of attention from both research and commercial points of view. Scientists paid great interest for electrospinning because of its ability to yield fibers in the submicron scale that are very hard to attain by handling standard mechanical fiber-spinning procedures [1–5]. Electrospinning is a technology that is broadly applied to fabricate polymer fibers with diameters ranging from 2 nm to several micrometers and is also known to have unique properties such as high surface area, controlled porosity, and mechanical strength [6–9]. The pore structure and the diameters of the produced fabrics and nanofibers can be controlled using this technique [10, 11]. Electrospun fibers have been successfully used in varied applications including protective clothing, biomedical, pharmaceutical, security, environmental engineering, chemical sensors, and electrode materials [12–19].
Recently, electrospinning has been used in the research of natural and synthetic polymer nanofibers [20] such as cellulose [21], polyurethanes [22], collagen [23] and hyaluronic acid [24]. Electrospinning can be handled to obtain large quantities of fibers using two-layer electrospinning scheme. This technique consists of a lower layer, which contains a ferromagnetic suspension, and an upper layer, which contains the polymer solution, and the multiple nozzles are arranged in lines or circles. One such example is the bubble electrospinning, which is demonstrated and developed by many researchers [25, 26]. Dosunmu et al. reached high production rates of fibers by using porous hollow tube to obtain multiple jets and by increasing the tube length and the number of holes [27]. In addition, coelectrospinning of different polymers has been reported to control the morphology and tailor the voids volume [28–30].
1.1. Electrospinning process
Figure 1.
Schematic diagram of vertical electrospinning apparatus.
Electrospinning setup consists of a high voltage power supply, spinneret (like pipette tip), and grounded collected plates (like metal screen) and is conducted at room temperature. Polymer solution is charged by applying high voltage and then accelerated toward the collector, which is of opposite polarity [31, 32]. The polymer must be dissolved first before using in electrospinning and then introduced into the capillary tube. At critical voltage, the electrostatic forces counteract the surface tension of the used polymer solution and an electrified jet is produced and ejected from the tip of the Tylor cone and then the solvent will be evaporated and leaving a polymer nanofiber [33, 34]. Currently, there are two standard sets of electrospinning: vertical and horizontal. The vertical electrospinning is represented in Figure 1.
The diameters and morphology of electrospun nanofibers are measured by numerous parameters, which are classified into solution, process, and ambient parameters. These factors are demonstrated clearly in Figure 2.
Figure 2.
Factors affecting on elctrospinning technique.
2. Polymers favored in electrospinning technique
More than 200 polymers (natural, synthetic, and copolymers) were used in electrospinning successfully depending on their applications [35].
2.1. Natural polymers
Nowadays, polymer nanofibers are used in a wide variety of applications in various industries and also in other arenas. Natural polymers are preferred over synthetic polymers in medical and biological uses because of their low immunogenicity and better biocompatibility. Recent researchers have reported the use of electrospinning process of natural polymers such as gelatine, collagen, and silk fibroin [36–41]. It was mentioned that some natural polymers such as collagen lose their properties after electrospinning using fluoro alcohols [42]. To resolve this problem, highly volatile fluoro alcohol such as 2,2,2-trifluoroethanol is used during electrospinning of collagen [43]. Yang et al. [44] found that about 45% of collagen is lost during electrospinning process. In addition, beads formation represents another challenge for the preparation of natural polymer nanofibers. For instance, laminin I nanofibers are fabricated with beads in the mesh structure and this may be related to the presence of the cross-linking agent and these results are reported by Neal et al. [45].
2.2. Synthetic polymers
Synthetic polymers are favored over natural polymers in certain uses because synthetic polymers can be tailored to develop the mechanical and degradation properties [46]. Certain synthetic polymers can be used in biomedical applications with some limitations including polylactide (PLA) [47] and polyglycolide (PGA) [48] to produce electrospun nanofibrous scaffolds.
2.3. Copolymers
Copolymers appear as an attractive opportunity to obtain new structures with desirable characteristics from electrospinning and also have better efficiency than that of homopolymers. The mechanical, thermal, morphological, and biodegradability properties of polymers can be tailored by using copolymers in electrospinning process. Incorporation of the hydrophilic polymer of hydrophobic polyesters enhanced the cell affinity of this sort of polyesters. Moreover, the stiffness of poly (ethylene-co-vinyl alcohol) (PEVA) is improved after the addition of poly (glycolide) (PGA) [49]. Also, copolymerization of methyl methacrylate (MMA) with methacrylic acid (MAA) improved the thermal stability of poly methyl methacrylate [50]. In addition to that, electrospun scaffolds produced from poly (p-dioxanone-co-L-lactide)-block-poly (ethylene glycol) (PPDO/PLLA-B-PEG) have enhanced hydrophilicity and biodegradability due to the addition of PEG to PPDA-PLLA blends [51].
3. Metal oxide nanofibers
Metal oxide materials are critically important from both the academic and industrial point of views. Scientists have been paying a special attention for the synthesis of one-dimensional (1D) morphologies to attain new phenomena and novel physicochemical characteristics of materials.
3.1. Synthesis of crystalline metal oxide nanofibers and their applications
It is worth mentioning that electrospinning is the most common technique used for the preparation of 1D polymer composite nanofibers [52, 43]. Furthermore, 1D nanostructures exhibit long axial ratio, which has a great influence on the physical and chemical properties of materials. Using electrospinning, nanofibers are fabricated by electrostatic stretching of a viscous solution of polymer composites by applying a high voltage [3]. In the recent years, scientists have focused on the fabrication of metal oxide nanofibers such as tin oxide (SnO2), zinc oxide (ZnO), titanium oxide (TiO2), and nickel oxide (NiO) through the electrospinning of polymer solution containing metal precursors and followed by annealing procedures [53–57]. Depending on the thermal degradation of polymers, certain polymers such as polyvinyl alcohol (PVA) and polyacrylonitrile are chosen as a sacrificial template for the fabrication of metal oxide nanofibers. After the annealing process, the organic components will be degraded and the metals will be oxidized by inorganic precursors giving nanofibers the desired metal oxide.
3.1.1. Tin oxide (SnO2) nanofibers
Recently, high interest has been paid to one-dimensional tin oxide (SnO2) nanostructures. There are various techniques designed for the synthesis of different structures of SnO2, such as electrospinning, self-catalytic, thermal evaporation, and thermal oxidation techniques [58–62]. The metal oxide fibers are commonly employed in gas detection application. The gas-sensitive characteristics of the prepared sensors are due to its high conductivity alterations when the prepared sensors adsorb or desorb a very low loading of chemical compounds on its surface [63–72]. High adsorption-desorption gas sensors in one-dimensional nanostructures were developed using metal oxide nanofibers owing to its highly porous networks [73, 74]. The morphological nature of the upper surface layer usually controls the molecular adsorption-desorption process, the response, and the sensitivity of the sensors. Metal oxide nanofibers are employed as a sensitive sheet instead of a thin film deposited by conventional processes such as drop, spinning, and airbrush because of its large surface area and, consequently, a thin film of metal oxide nanofibers enhances the sensitivity and the speed of the reaction.
Santos et al. [75] prepared SnO2 nanofibers by electrospinning process of PVA/SnCl4.5H2O composite followed by annealing at 450°C to eliminate PVA matrix. The fabricated SnO2 electrospun nanofibers are designed as sensitive layers for resistive chemical sensors. The produced fibers exhibit high sensitivity, reproducibility, and quick reaction in the recognition of triacetone triperoxide (TATP) explosive precursors in the ppm scale. In addition, SnO2 hollow nanofibers were fabricated for a high and fast detecting ethanol (C2H5OH) using polyvinylpyrrolidone (PVP) as a template, which was removed by calcination at 600°C for 2 h [76]. The produced SnO2 nanofibers have a large surface area of 26.43 m2g−1 and grain sizes of about 20 nm. Another study by Choi et al. [77] concerns the preparation of Pd-doped SnO2 hollow nanofibers for methane (CH4), H2, carbon monoxide (CO), and C2H5OH detection. They studied the effect of temperature and Pd-doping content on the detection for CH4, H2, CO, and C2H5OH. They noticed a dramatic variance in the selectivity for these compounds between undoped and Pd-doped SnO2 hollow nanofibers. The selective detection of C2H5OH was tested with undoped and 0.08 wt% Pd-doped SnO2 hollow nanofibers. The study showed a significant decline in C2H5OH detection in case of using 0.4 wt% Pd when the temperature is raised from 330 to 440°C, while the responses to CH4 and H2 were improved and, consequently, they adjusted the selectivity of CH4 and H2 at 440°C with minimum interloping to C2H5OH. It was worth studying the enhancement of Pr-doping on the gas sensing performance due to its effective influence on the electronic and structural characteristics of materials. Li et al. [78] discussed the fabrication of Pr-doped SnO2 hollow nanofibers for ethanol detection. They subjected a solution of PVP, SnCl2.2H2O, and Pr(NO)3.6H2O for electrospinning. The prepared nanofibers sample was then annealed at 600°C for 2 h. It was reported that 0.6 wt% Pr-doped SnO2 nanofibers gave the highest sensitivity and response for ethanol at 300°C. This may be related to the high surface area of the porous morphology and the increment in oxygen vacancy leading to enhanced oxygen absorption. Furthermore, Du et al. [79] focused on the fabrication of In2O3/SnO2 nanofibers for formaldehyde gas sensors using bipolar electrospinning setup of double jets with opposite electric fields. Different sensors composition based on SnO2, In2O3, and In2O3/SnO2 nanofibers were tested at 375°C with 0.5–50 ppm of formaldehyde gas. The formaldehyde selectivity of In2O3/SnO2 nanofibers is higher than that of SnO2 and In2O3 nanofibers. The selectivity values for 0.5 and 50 ppm of formaldehyde were 2.2 and 18.9, respectively. In2O3/SnO2 nanofibers have a good detection of formaldehyde in the interfere gasses of methanol, ethanol, acetone, and ammonia. In addition, Tang et al. [80] synthesized hollow hierarchical ZnO/SnO2 nanofibers composite for methanol sensing applications. A solution of PVP, SnCl2.2H2O, and Zn(NO3)2.6H2O is used to prepare electrospun fibers, and then subjected to annealing at 600°C for 3h. This study noted that the nanofibers of ZnO/SnO2 with a molar ratio of 1:1 exhibited an interesting methanol detection response at 350°C in the presence of interfering gasses.
3.1.2. Zinc oxide (ZnO) nanofibers
Another metal oxide of recent interest is ZnO, which is a chemically stable, nontoxic and inexpensive compound that exhibits high binding energy of 60 mV at 30°C and a direct band gap of 3.37 eV [81]. ZnO is industrially applied in a wide range of uses including chemical sensors, ultraviolet light-emitting diodes, dye-sensitized solar cells, functional instruments, piezoelectric materials, and transparent conductors [82–85]. The morphology, crystal structure, quality, and dimensions of ZnO control its electronic characteristics for different applications [86, 87].
The effective structures of 1D ZnO nanofibers have a small quantum restriction of charge carriers and this measure the performance of nanoscale devices [88–91]. Recent articles reported the synthesis of ZnO or ZnO-based nanofibers. Ding et al. [92] addressed the unique characteristics of ZnO nanofibers such as high hydrophobicity, development of nanograins, and electrical properties. The study reported the synthesis of super-hydrophobic ZnO nanofibers through a simple combination of wet chemical and electrospinning techniques. The produced ZnO nanofibers have been coated by fluoroalkyl silane (FAS) to obtain super-hydrophobic ZnO nanofibers. Because of the FAS modification, the fibrous ZnO films were transformed from super-hydrophilic (water contact angle (WCA) of 0°) to super-hydrophobic (WCA of 165°). Electrospinning technique was designed to produce nanofibers with a smooth surface which limits its use in some applications as the cohesion forces will be feeble, and thus rough surface nanofibers are preferred. In this context, Barakat et al. [93] mentioned that the electrospinning of colloidal solutions is useful for synthesizing rough surface ZnO nanofibers. The fibers were produced by electrospinning a colloidal solution of PVA, zinc nanoparticles (ZnNPs), and zinc acetate dehydrate (ZnAc) and then followed by calcination. Furthermore, Zhang et al. [94] addressed the fabrication of a hollow ZnO nanofibers using a ZnNPs-free solution. The study reported the preparation of ZnO hollow nanofibers with diameters of 120–150 nm using an electrospinnable solution of PAN, PVP, and ZnAc composites, then followed by calcination to remove the used polymers. They obtained core-shell structure from elctrospinning process where PAN was the core and PVP/ZnAc composite was the shell. ZnO hollow nanofibers showed interesting results related to sensing characteristics against C2H5OH. Another study by Kanjwal et al. [95] reported the preparation of rough ZnO hierarchical nanofibers as a photocatalyst against methylene blue dehydrate. They electrospun a colloidal solution consisting of ZnNPs, PVA, and ZnAc dehydrate and then followed by a calcination step at 500°C. Moreover, electrospinning a mixture of high molecular weight PVP and ZnAc in dimethylformamide (DMF) synthesized 1D polycrystalline ZnO nanofibers with porous morphologies [96]. The study reported the effect of ZnAc loadings from 10 to 15 wt% after calcination process from 350 to 650°C onto the produced nanofibers. The prepared ZnO nanofibers were utilized for water treatment as a result of its photocatalytic performance.
A comparative study to compare the undoped and Ce-doped ZnO hollow nanofibers for acetone detection was carried by Li et al. [97]. The used electrospinning solution consisted of cerium nitrate, ZnAc, and PVP. The sample was then followed by calcination at 600°C for 1.5 h. The study declared that Ce-doped ZnO nanofibers are very sensitive to acetone at 260°C. This is ascribed to the morphological and electronic modification in the presence of Ce-doping. Moreover, Al-Ga co-doped ZnO nanofibers have attracted high interest because of its excellent cost effectiveness, conductivity improvement, and mechanical flexibility enhancement. Park and Han [98] reported the preparation and the characterization of Al-Ga co-doped ZnO nanofibers by electrospinning a solution of zinc acetate dihydrate, aluminum nitrate nonahydrate, gallium nitrate hydrate, and PVA and then by annealing at 550°C. The study showed a variation of the lattice parameter with the addition of Ga beyond 2 at.% Al-doping owing to smaller gaps in the atomic size. Consequently, more Ga content can be added without extensive strain. Al-Ga co-doped ZnO nanofibers with 2 at.% Al and 1 at.% Ga exhibit the highest conductivity of 9.57 ×10−3 Scm−1. In addition, Ga incorporation to 2 at.% Al-ZnO nanofibers developed the mobility and the degree of crystallinity.
3.1.3. Titanium oxide (TiO2) nanofibers
Wu et al. [99] addressed the usage of TiO2 in many applications including solar, optoelectronic, and catalytic devices. TiO2 exists in different morphologies such as rutile, anatases, brookite, TiO2 B (bronze), and TiO2 R (ramsdellite). It is worth mentioning that the most chosen TiO2 crystal phase for lithium ion batteries is the anatase structure due to its high capacity and low-cost production [100–102].
He et al. [103] reported the fabrication of 100–300 nm conductive TiO2 nanofibers using KOH treatment. In this study, a mixture solution of PVP and titanium tetraisopropanolate was used to produce nanofibers for supercapacitor electrode applications using electrospinning. The as-prepared nanofibers exhibit high conductivity, high cycling stability, and high surface area with a stable specific capacitance even after 10,000 cycles and it is enhanced from 0.02 Fg−1 to 28.94 Fg−1 at 50 mVs−1.
Another study discussed the influence of Ag incorporation to TiO2 nanofibers on H2S detecting characteristics. Pristine and Ag nanoparticles/TiO2 (Ag/TiO2) nanofibers were fabricated by Ma et al. [104]. H2S sensing measurements showed that Ag/TiO2 nanofibers have much higher detection responses and sensitivity than that of pristine Ag/TiO2 nanofibers. It is found that the implementation of AgNPs leads to phase conversion of TiO2 to rutile from anatase phase.
For the catalytic performance of TiO2 nanofibers, Hao et al. [105] reported that mesoporous Au/TiO2 nanofibers were obtained by incorporating Au nanoparticles (AuNPs) to TiO2 nanofibers. The nanofibers were prepared via electrospinning a mixture solution of Ti(OC4H9)4, HAuCl4, and PVP. The effect of AuNPs loading (2, 5, and 10 wt%) toward the reduction of 4-nitrophenol (4-NP) to 4-aminophenol (4-AP) by sodium borohydride was reported. The results reported a decrease in the TOF value of the catalyst and an increase in the reaction rate constant with increasing AuNPs loading and this decline may be attributed to the small size and high surface area in case of 2 wt%, AuNPs. In addition, Tolba et al. [106] discussed the synthesis of ZnO nanobranches attaching TiO2 nanofibers as nonprecious electrocatalyst for ethanol oxidation. The study reported the use of a colloidal solution of titanium isopropoxide, poly (vinyl acetate), and ZnNPs to produce electrospun fibers. Produced nanofibers went through a calcination and hydrothermal treatment. The study concluded that the concentration of ZnNPs has a distinct impact on the electrical conductivity and the electrocatalytic performance of the final nanofibers. For instance, utilizing 0.1 g ZnNPs in the electrospinnable solution produce active and stable nanofibers for ethanol oxidation in alkaline medium. In addition, Li et al. [107] measured the photocatalytic H2-production activity in water splitting of 1D mesoporous NiO/TiO2 composite nanofibers. The composite nanofibers were fabricated by electrospinning process of colloidal solution of tetra-n-butyl titanate (TBT), PVP, and Ni(NO3)2.6H2O, and then the samples were calcined in air at 500°C for 3 h. It was reported that the H2-production activity of TiO2 nanofibers was developed at low NiO content (0–0.5 wt%). In fact, at 0.25 wt% of NiO, the mesoporous composite nanofibers exhibit the highest H2-production activity. This can be related to the behavior of NiO as an active cocatalyst, which hinders the recombination of photogenerated charge carriers and diminishes the overpotential of H2 production.
In another research for solar cells, Zr is doped into TiO2 nanofibers to be utilized as photoanodes of dye-sensitized solar cells (DSCs) [108]. A solution of PVAc, titanium isopropoxy, and zirconium n-propoxy was used to produce nanocomposite electrospun fibers. It was mentioned that the conversion efficiency of DSCs improved after Zr addition. The group tested the effect of Zr-doping amount (0.5, 1,1.5, and 2%) on TiO2 nanofibers formulations for DSCs. The study reported that the photovoltaic efficiency reached 4.51% in case of 1% Zr-doping, while the efficiency was achieved to 1.61% for cells based on pristine TiO2 nanofibers. This improvement can be attributed to the electrons transfer and dye loading. In more details, the presence of Zr developed the band gap through decreasing the charge recombination rate and, consequently, increases the electrons transfer (3.217 eV for 1% Zr-doped TiO2 nanofibers and 3.202 eV for pristine TiO2 nanofibers). Moreover, the dye loading of pristine TiO2 nanofibers is found to be 0.52 ×10−7 molcm−2, which is much lower than that of 1% Zr-doped TiO2 (1.36 ×10−7 molcm−2).
3.1.4. Nickel oxide (NiO) nanofibers
NiO nanofibers exhibit good chemical and thermal stability properties and can be prepared easily using electrospinning. High interest in preparing 3D nickel oxide (NiO) nanofibers technology to be applied in photovoltaic devices has been reported. This is because 3D NiO surfaces exhibit high degree of porosity, which increases the mobility of charge carriers and consequently enhances the efficiency of photovoltaic procedures [109]. Moreover, NiO has been used in batteries and chemical sensors industry because of its attractive characteristics and morphological properties [110, 111].
Macdonald et al. [112] prepared NiO nanofibers for cathode fabrication using electrospinning process of PAN and Ni(AcAc)2. NiO nanofibers were deposited into fluorine-doped tin oxide (FTO) to produce p-type nano photocathodes with a surface area of 0.8 cm2. To remove all organic residues, the electrode was sintered for 30 min at 450°C. The NiO nanofibers diameters were decreased to approximately 100 nm due to the calcination influence at 500°C and the nanofibers became more rough and straw-like structure owing to the elimination of PAN chains. Furthermore, Jian et al. [113] discussed the electrochemical performance of La-doped NiO nanofibers as positive electrode structures. La-doped NiO nanofibers are produced by the calcination procedure at 650°C for 3 h of electrospinning solution of Ni(CH3COO)2.4H2O, La(NO3)3.6H2O and PVA. They designed asymmetric supercapacitor consisting of porous-activated carbon as a negative electrode, as-prepared La-doped nanofibers as a positive electrode, and 2M KOH aqueous solution as an electrolyte. It was seen that the perfect La/Ni loading ratio in the positive electrode was 1.5%. In addition, La-doping was found to develop the specific capacitance and the electrochemical stability. In more details, the specific capacitance became 94.85 F g−1, which is 5.3 times better than the capacitors with undoped NiO nanofibers. Also, the coulomb efficiency of more than 90% is retained even after 1000 cycles.
NiO nanofibers exhibit excellent catalytic behavior. Elzatahry [114] described the use of PAN as a sacrificial template in the electrospinning method to fabricate NiO nanofibers to be designed as an electrocatalyst in methanol oxidation process in alkaline medium. The study reported the use of a solution of PAN and 50 wt% NiO to produce electrospun fibers. Uniform fibers with a diameter of 145–170 nm and low content of submicron and random nanofibers were produced. Porous NiO nanofibers are obtained after the elimination of PAN at 400°C for 3.5 h.
For sensing applications, Choi et al. [115] studied the fabrication of p-type NiO nanofibers and its activity for CO and NO2 detection as a function of the size of the nanograins. The electrospinning solution consisted of 8.3 wt% PVA and 4 wt% nickel II acetate tetrahydrate. The produced nanofibers were subjected to calcination at 400–850°C to adjust the nanograins size. It was found that nanograins’ size has a limited impact on the CO and NO2 detection in case of p-type NiO nanofibers. On the other hand, the responses of n-type NiO nanofibers were highly dependent on the nanograins’ size. Furthermore, Luo et al. [116] addressed the synthesis of Li-doped NiO nanofibers for nonenzymatic glucose sensing. The group reported the use of solution mixture of PVP, Ni(NO3)2.6H2O, and LiNO3 and another solution without LiNO3, then calcined the product at 500°C for 3 h. It was noticed that the average diameter of the formed Li-doped NiO nanofibers was 200 nm, while that of NiO nanofibers was 150 nm. The fabricated Li-doped NiO nanofibers exhibit high surface area, high conductivity, and high electrocatalytic performance through holes and defective sites, which develop the electrons transfer process. In addition, SnO2/NiO composite nanofibers were used in humidity sensing applications [117]. The study reported the use of electrospinning solution of SnCl2.2H2O, NiCl2.6H2O, and PVP and then annealed the produced fibers at 600°C for 3 h. It was noted that the high sensitivity of SnO2/NiO nanofibers to humidity is ascribed to the active surface area that increases the conduction in the presence of water vapor.
3.2. Morphological characterization of metal oxide nanofibers
Morphological measurements of metal oxide nanofibers including fibers orientation, fibers distribution, and fibers diameters can be measured using many analytical instruments such as scanning electron microscope (SEM), X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS).
3.2.1. Scanning electron microscope characterization (SEM)
Scanning electron microscope (SEM), field emission scanning electron microscope (FESEM), transmission electron microscope (TEM), polarized light microscope, and atomic force microscope (AFM) can be utilized to study the nanofibers’ diameters. It is worth mentioning that the high-resolution capability offered by SEM makes it a conventional technique to analyze nanofibers of which their structures are measured by their functionalities. The SEM is also used to study the chemical composition and surface topology and morphology. The nanograph’s resolution is controlled by the interaction between the electron probes with metal oxide nanofibers. In more details, SEM measurements require conductive polymers and a small sample for morphology study. SEM analysis is a quick method used to measure the fiber diameters using a gold coating.
Figure 3 represents the SEM micrograph of electrospun ZnO nanofibers fabricated by Kanjwal et al. [95]. Figures 3(A) and (B) confirms the formation of continuous and smooth nanofibers of ZnAc/PVA. Furthermore, the addition of Zn NPs did not affect the structure of the produced nanofibers without any beads formation as clearly represented in Figures 3(C) and (D). It should be also mentioned that the average diameters of the fabricated nanofibers are decreased after calcination process at 500°C for 90 min and this significant decrease can be attributed to the elimination of polymer matrix due to calcination at high temperatures. The shape of the nanofibers is not influenced by the addition of Zn NPs in general but there is an influence on the nanofibers’ surface. Moreover, the surface of the nanofibers was relatively smooth in the case of Zn/Ac, while the addition of Zn NPs produced nanofibers with a rough surface.
Figure 3.
Low and high magnification SEM images of the powder obtained after calcination of ZnAc/PVA nanofibers mats (A and B), and Zn NPs/ZnAc/PVA (C and D) at 500°C for 90 min [
3.2.2. X-ray diffraction (XRD) characterization
XRD analysis is handled to study the crystal morphologies of various solids and metal fibers, including the lattice constants and geometry, the orientation of the single crystal, crystallite size, and crystal defects. The degree of crystallinity of the produced metal oxide nanofibers can be calculated from X-ray diffraction (XRD), both wide angle and small angle (WAXS and SAXS) and also from differential scanning calorimetry (DSC). A solid sample of metal oxide nanofibers is irradiated with X-rays of fixed wavelength while the angle of the diffracted X-ray beam is controlled as a function of the intensity of the diffracted beam. Moreover, each lattice plane satisfies Bragg’s equation from which the values of the d-spacing between platelet sheets can be determined. XRD analysis was achieved to measure the crystal structures of the calcined ZnO nanofibers prepared by Kanjwal et al. [95] as described in Figure 4.
Figure 4.
XRD patterns of nanofibers after calcination of (A) Zn NPs-free and (B) the hydrothermal product [
XRD results, in this case, confirm the synthesis of ZnO nanofibers. A SPECTRUM A of the annealed ZnO nanofibers confirms the formation of pure ZnO powder. It should be also noticed that the XRD spectra are not influenced by the incorporation of Zn NPs. In addition, the hydrothermal product has the same beaks as represented in spectra B.
3.2.3. X-ray photoelectron spectroscopy (XPS)
Figure 5.
XPS data of the hydrothermally fabricated ZnO nanofibers [
Another important characterization tool to investigate the surface chemistry of electrospun metal oxide nanofibers is X-ray photoelectron spectroscopy (XPS). The surface chemical nature of the metal oxide nanofibers can be estimated by its hydrophilicity which can be characterized by using XPS. XPS is performed to support the obtained XRD results and to measure the oxidation states and the changes in binding energies. The possible changes in binding energies of the prepared ZnO nanofibers have been reported by Kanjwal et al. [95] as represented in Figure 5. The peak of C 1s at 284 eV is related to the graphite tape employed in the sampling. The Zn 2p region in ZnO sample consists of the main 2p3/2 and 2p1/2 spin-orbit components with binding energies of 1020 and 1043 eV, respectively.
4. Conclusions
Electrospinning is an important technique for fabricating polymer and 1D metal oxide nanofibers. The desired characteristics can be measured by nanofibers’ structure, which is controlled by various parameters including electrospinning solution, process, and ambient parameters. Selected polymers are critically used to prepare electrospun fibers’ templates for this approach. The polymers used in this approach can be categorized into three classes including natural, synthetic, and copolymers. Electrospinning technique represents a vital promise with some borders for many applications such as small pore size and lack of cellular infiltration, which can be enhanced through multilayering and using of polymers of altered degradation grades. The technique has been adapted to produce many 1D metal oxide nanostructures for different important applications. This chapter introduced the fabrication of various metal oxide nanofibers which has attracted a significant attention because of their vital applications in the recent years such as tin oxide, zinc oxide, titanium oxide, and nickel oxide. Specific polymers were chosen as a sacrificial template to synthesize metal oxide nanofibers. After the annealing process, the organic specimens will be decomposed and the metal oxide nanofibers will be produced. Scanning electron microscope, X-ray diffraction, and X-ray photoelectron spectroscopy are used to characterize electrospun metal oxide nanofibers. The obtained data confirm the formation of homogeneously distributed metal oxide nanofibers.
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