Fabrication and characterization of continuous silver nanofiber/polyvinylpyrrolidone (AgNF/PVP) core–shell nanofibers using the coaxial electrospinning process

Abstract

In the present study, continuous and uniform core–shell silver nanofiber/poly(vinyl) pyrrolidone (AgNF/PVP) nanofibers have been successfully fabricated via an efficient coaxial-spinneret electrospinning method with a vertical configuration using PVP and AgNO₃ as precursor solutions. PVP polymer was primarily used as a guide and provided an appropriate viscoelastic property by surrounding the AgNO₃ (core) solution to fabricate aligned AgNF/PVP core–shell nanofibers. A series of AgNO₃ concentrations were prepared by fixing the concentration of PVP as the shell fluid. The AgNO₃ concentration had a significant influence on the formation of the continuous and uniform AgNF/PVP core–shell nanofiber structure and applied voltage had an effect on the formation of the compound stable Taylor cone. The (AgNF/PVP) core–shell nanofibers were formed via the stretching of the co-electrospinning jet and the reduction temperature for an appropriate time induced the silver nanofiber to be well aligned along the axis of the PVP-template electrospun fiber. The structure and properties of the thus obtained core–shell nanofibers were investigated thoroughly through optical microscopy (OM), transmission electron microscopy (TEM), focused ion beam (FIB), X-ray diffraction (XRD) and selected area electron diffraction (SAED). Energy dispersive spectroscopy (EDS) and X-ray photoelectron spectroscopy (XPS) were also employed to analyze the elemental composition of the core–shell nanofiber surface. On the other hand, UV-vis spectrophotometry was used to test the reduction of silver ions into metal silver. Moreover, electrical measurements were performed on the (AgNF/PVP) core–shell nanofibers, which indicated that the core–shell nanofibers became insulating due to the embedded highly conductive silver nanofibers by insulating the PVP shell. Therefore, coaxial electrospinning is a convenient and cost effective process for the fabrication of continuous and uniform (metal/polymer) core–shell nanostructure fibers.

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Introduction

The demand for novel nanostructures in different fields, such as telecommunications, biotechnology and computing, has increased enormously and has engaged the interests of scientists and engineers to create new and novel nanofiber structures and efficient processes for their production.^1,2 Nanostructures of various morphologies, including nanorods, nanowires, nanofibers, nanotubes, nanocables and nano patterned materials, have been synthesized using different methods. For example, template synthesis,^3,4 self-assembly,⁵ phase separation,⁶ lithography,⁷ and electrostatic spinning.⁸

Electrospinning is a simple and well-established method for the preparation of ultrafine 1D-nanofibers with diameters ranging from the nano to micrometric scale.⁹ A decrease in diameter to the nanometer scale provides an increase in numerous favorable characteristics, including an increase in surface to volume ratio, which enhances the strength of the fibers.^10,11 As compared to other nanofiber fabrication methods, electrospinning has gained increasing attention because this technique is very versatile, productive, cost-effective, and less demanding for processing conditions.^12,13

In recent years, several modifications have been made to the single electrospinning process to improve its functionality and enhance the quality of the resulting nanofiber structures. One such modification that holds great promise and has gained much attention in a variety of applications is the fabrication of core–shell component nanostructure fibers using the coaxial electrospinning process.^13–16 Coaxial electrospinning is an effective and simple method for the fabrication of metal–polymer nanofibers with uniform core–shell structures (micro/nanocapsules and coaxial fibers).¹⁷ In this process, two contrasting materials are delivered independently with a coaxial capillary and drawn to generate nanofibers in a core–shell configuration. There is an increasing need for the fabrication of two material based nanostructures in which one is surrounded by the other at the nano or micro level, and it shows potential for a wide range of applications, such as separating an unstable component and minimizing its chances of decomposition under a reactive environment, supporting a material to improve its mechanical properties and serving as a scaffold for engineering tissues in which a more biocompatible material is surrounded by a less biocompatible material.^18–21 Intense research is being carried out on 1D micro and nanostructures of inorganic/organic core–shell composite fibers via different synthetic methods for several potential applications.^22,23 For example, several publications have been reported on core–shell composite nanofibers, which are composed of a metal and metal oxide in the core (such as, Au,^24–26 Ni,²⁷ Si,^28,29 CuO,³⁰ and TiO₂,³¹) covered by silica and polymers.³² Moreover, metal/polymer core–shell nanostructure fibers, such as silver nanowire/poly(vinyl) pyrrolidone (AgNW/PVP)³³ and copper/poly(vinyl alcohol) (Cu/PVA)²² nanocables, have been fabricated via the single-spinneret electrospinning method. Similarly, different silver (Ag)/poly(vinyl) pyrrolidone (PVP) electrospun nanofibers with different patterns (nonwoven, aligned, and crossed) and palladium/poly(L-lactide) (Pd/PLA) core–shell nanostructured fibers have been fabricated via coaxial electrospinning.^11,34 Moreover, a variety of synthetic methods have been employed to fabricate 1D silver nanostructures with polymers because of their unique properties,^35–39 which include, the hydrothermal method,⁴⁰ photo-reduction processes,⁴¹ sol–gel synthesis⁴² and seed-mediated growth.⁴³ However, these methods have certain limitations such as it is difficult to control the shape and size of the product, complicated synthetic procedures are needed and it is difficult to achieve continuous production. In addition, the widely used single electrospinning method is limited to the synthesis of continuous metal nanofiber/polymer core–shell nanofibers from a metal salt and polymer solutions (i.e. metal salt and polymer such as silver nitrate and polyvinylpyrrolidone).³³ Therefore, to the best of our knowledge, there is no report on the fabrication of continuous silver nanofiber/poly(vinyl) pyrrolidone (AgNF/PVP) core–shell nanofibers using electrospun PVP polymer fiber as a guide and surrounding the core material via the co-axial electrospinning process. To overcome the abovementioned limitations, the coaxial electrospinning process can be used to fabricate continuous and uniform AgNF/PVP core–shell nanofibers from inexpensive materials (i.e. silver nitrate and polyvinylpyrrolidone).

Herein, we describe the fabrication and characterization of continuous and uniform AgNF/PVP core–shell nanofibers from AgNO₃ (which is used as the core solution and a precursor of AgNF) and PVP (used as the shell) via the coaxial electrospinning process. The PVP polymer was primarily used as a guide and it provided an appropriate viscoelastic property by surrounding the core material to fabricate aligned AgNF/PVP core–shell nanofibers. The effect of different AgNO₃ concentrations on the AgNF/PVP core–shell nanofiber formation was investigated by maintaining the same reduction temperature and time. The morphology, uniformity and dimensions of the core–shell AgNF/PVP nanofibers were determined via optical microscopy (OM) and focused ion beam (FIB). The structure of the AgNF/PVP core–shell nanofibers was clearly identified via X-ray diffraction (XRD) and selected-area electron diffraction (SAED) patterns. Transmission electron microscopy (TEM) was used to characterize the size of the final core–shell nanofibers and their morphology. On the other hand, UV-vis spectrophotometry was used to test the reduction of Ag(I) ions to the Ag(0) metal nanostructure. Moreover, the elemental composition of the AgNF/PVP core–shell nanofibers surface was characterized using energy dispersive spectroscopy (EDS) and X-ray photoelectron spectroscopy (XPS).

Experimental section

Materials

Poly(vinyl) pyrrolidone K-90 (PVP, M_w = 1 300 000) was purchased from Sigma-Aldrich (USA). Silver nitrate (AgNO₃, >99%) was purchased from Sigma-Aldrich (UK). Analytical grade ethanol (CH₃CH₂OH, >99.5%) was purchased from Aldrich Co. Ltd (USA). Water was double distilled (deionized water) before use. All chemicals were used without any further purification.

Methods

Preparation of precursor solution. PVP powder and AgNO₃ salts were used to synthesize the nanofibers precursors. In brief, 1 g of PVP powder was dissolved in 11.5 g of ethanol (8% (w/w) of PVP) and vigorously stirred using a magnetic stirrer until a clear solution was obtained. Similarly, different concentrations of AgNO₃ (20%, 30% and 40% w/w) were mixed with deionized (DI) water and stirred gently using a magnetic stirrer until a homogeneous solution was obtained.

Co-axial electrospinning process. The process of coaxial electrospinning is very attractive, simple and an effective top-down approach to prepare core–shell ultrathin fibers with lengths of up to several centimeters on a large scale compared to other methods.⁴⁴ The general experimental setup adopted by most researchers is relatively similar to single electrospinning.¹³ A change is made in the spinneret by inserting an inner (smaller) capillary that fits concentrically inside the outer (bigger) capillary, which results in a coaxial configuration. The inner needle is attached to the reservoir containing the core fluids (a non-viscous liquid) and the outer needle is connected to the reservoir holding the shell fluids (viscous liquid) to form a core–shell compound jet in an electric field, which then solidifies to form core–shell nanofibers.⁴⁵ The solution feeding rates are controlled using metering pumps⁴⁶ and the arrangement is vertical, as illustrated in (Fig. 1).

Fig. 1 Schematic of AgNF/PVP nanofiber formation with a core/sheath structure.

Core–shell AgNF/PVP nanofibers were fabricated from the abovementioned precursor solutions via the co-axial electrospinning process, as displayed in the (Fig. 1). In brief, in the first step, an 8% (w/w) PVP solution was added to a 10 mL syringe, which was connected to the outer capillary with a size of N18 (inner diameter: 0.96 mm and outer diameter: 1.26 mm) and another 10 mL syringe connected to the inner capillary with a size of N25 (inner diameter: 0.26 mm, outer diameter: 0.5 mm) was filled with 20% (w/w) solution of AgNO₃ in deionized water. Second, the two liquids were fed at a fixed rate through the syringe pumps (YSP-101, YSP series syringe pump). All parameters were identical, with a PVP solution flow rate of 0.4 mL h⁻¹, AgNO₃ solution flow rate of 0.1 mL h⁻¹ and the fibers were collected on a glass slide at a tip-to-collector distance of 20 cm under the same ambient conditions (25 ± 2 °C) and relative humidity (20–25%). Then, in the range of 8–10 kV, a voltage was supplied and the electrospinning solution was drawn out from spinneret and it formed a compound Taylor cone with a core–shell nanostructure of the co-axial jet. The fibers with a PVP shell and sliver ion core were produced in the solid form, which were collected on the collector. Third, the collected solid nanofibers were annealed with a hot-stage at 180 °C for 1 h. Subsequently, the fabricated nanofibers were stored in a dry environment and used for further characterization.

Characterization

The morphology of AgNF/PVP core–shell electrospun (ES) nanofiber mats were analyzed using optical microscopy (OM) (Olympus BX51 system microscope (×1000) fitted with a DP-12 digital camera) and a dual beam focused ion beam (FIB) (FEI Quanta 3D FEG) with an energy dispersive spectrometer (EDS). Transmission electron microscopy (TEM) (JEOL JEM-1200EX II Tokyo, Japan) was used for the detailed morphological investigation of the core–shell AgNF/PVP ES nanofiber mats, as well as the measurement of core and shell thicknesses. The elemental composition on the surface of the nanofibers was characterized via X-ray photoelectron spectroscopy (XPS) (VG ESCA spectrometer system with an Al Kα source) and energy dispersive spectroscopy (EDS). The UV-visible spectra of the AgNF/PVP core–shell ES nanofiber mats were obtained on a Jasco V-670 UV-visible-near-infrared (UV-vis-NIR) spectrophotometer over the wavelength range from 200 to 800 nm. The structural properties of the obtained samples were further investigated via X-ray diffraction (XRD) on (a Bruker D8 DISCOVER SSS Multi-Function High Power XRD) X-ray diffractometer using Ni-filtered Cu-Kα radiation (wavelength 1.54 Å) with a scanning rate of 1° min⁻¹ at 40 kV and 200 mA in the 2θ range of 20–80°. Finally, electrical measurements were performed using the standard four-probe technique (Keith Link Technology, Taiwan).

Results and discussion

The main goal of this study was to fabricate and characterize continuous and uniform AgNF/PVP core–shell nanofibers via the coaxial electrospinning process from the inexpensive precursors of silver nitrate and polyvinylpyrrolidone solutions. AgNO₃ solution does not form continuous fiber-like structures upon electrospinning due to its low molecular weight and lack of required viscoelastic properties,¹³ as shown in Fig. 2A. However, obviously, fiber-like structures can be obtained by co-electrospinning with PVP, as shown in Fig. 2B. This is because the viscosity of the PVP (shell) solution required was such that the viscous stress imparted by it on the AgNO₃ (core) solution was strong enough to overcome the interfacial tension between the two solutions and this allowed the formation of a compound Taylor cone and a jet from the latter.⁴⁷ Accordingly, the viscosity of the PVP shell solution is critical and the shell PVP-polymer could be electrospinnable by itself to contribute to the core–shell structure formation.⁴⁶ The particular interest and advantage of the coaxial electrospinning method is that nonelectrospinnable materials such as AgNO₃ solution can be forced into 1D arrangements using electrospinnable materials as shell-like PVP polymer solutions. Therefore, the coaxial electrospinning technique should be very versatile for the processing of a wide variety of systems that are different from those presented here and will certainly foster the design of new materials.

Fig. 2 Optical microscope image of a stable Taylor cone: (A) 20% (w/w) AgNO₃ in DI water on a glass slide and (B) coaxially electrospun fibers of 20% (w/w) AgNO₃ (core) in DI water and 8% (w/w) PVP (shell) in ethanol. Note: inset in A and B shows single and coaxial spinneret electrospinning respectively.

To fabricate continuous and uniform core–shell nanofibers, first we optimized the stable and uniform Taylor cone during the coaxial electrospinning process, because a stabilized compound Taylor cone and initial jet are the requirements for coaxial electrospinning.⁴⁸ Therefore, the effects of applied voltage and different concentrations of AgNO₃ were investigated. The effects of applied voltage (from 5–14 kV) are displayed in the Fig. 3. The result from Fig. 3A, D and E shows that unstable Taylor cones can cause the formation of irregular core–sheath structures and inseparable fibers from the two solutions. The Taylor cone will break when the working voltage is too high (i.e. applied voltage >10 kV); on the other hand, the solution will drop on the collector when the voltage is too low (i.e. applied voltage <8 kV). We observed more stable Taylor cones for applied voltages in the range between 8 and 10 kV, as shown in Fig. 3B and C.

Fig. 3 The shape of Taylor cone is observed via a self-assembled camera with a fixed flow rate of core (0.1 mL h⁻¹) and shell (0.4 mL h⁻¹) solution @ (A) 5 kV, (B) 8 kV, (C) 10 kV, (D) 12 kV and (E) 14 kV.

Similarly, the effects of AgNO₃ were investigated by using various concentrations of AgNO₃ (i.e., 20%, 30% and 40% (w/w)). Among the concentrations used, only 20% (w/w) AgNO₃ revealed good continuity and a uniform core–sheath structure, as displayed in Fig. 4A. On the contrary, as the concentration of AgNO₃ increased, nanofibers with poor continuity and uniformity was observed, as shown in Fig. 4B and C. This is may be due to the highly conductive properties of AgNO₃ after dissociating to Ag⁺ and NO₃⁻, which bring about a devastating effect on the morphology of the core–shell nanofibers such as non-uniformity and breakage of the core-nanofibers.⁴⁹ Therefore, in this study a 20% (w/w) AgNO₃ concentration was used as the basic matrix of the core solution and 8–10 kV of applied voltage was selected throughout the experiment. Therefore, all the results and discussion in this study are based on the abovementioned optimization.

Fig. 4 TEM image of annealed (180 °C/1 h) AgNF/PVP core–shell ES nanofiber mats of 8% (w/w) PVP with (A) 20%, (B) 30% and (C) 40% (w/w) of AgNO₃ solution.

The morphology of the AgNF/PVP nanofibers is summarized in Fig. 5. Optical microscopes were used to observe the formation and size of AgNF/PVP ES nanofiber mats for initial examination. The optical microscope result showed that the nanofibers have a relatively uniform size, as displayed in Fig. 5A. Similarly, the FIB image result of AgNF/PVP core–shell ES nanofiber mats shows a relatively uniform and smooth surface on the glass slide, as illustrated in Fig. 5B. This result reveals that the silver nanofibers are well embedded in the PVP polymer matrix. The TEM image of the annealed coaxial fibers clearly prove that the AgNF/PVP core–shell ES nanofiber mats have a core–shell morphology, as shown in Fig. 5C, with a diameter of 480 nm and 110 nm for the outer and core phase, respectively. The corresponding selected-area electron diffraction pattern in Fig. 5D reveals that the crystalline characteristics are silver-metal, which is consistent with the distinct diffraction rings of the (111), (200), (220), and (311) planes. Moreover, the SAED image of the AgNF/PVP core–shell ES nanofiber mats further reveals that the d-spacing of 0.24 nm is attributed to the (111) plane of Ag, which is similar to the XRD-data in Fig. 6B.

Fig. 5 Morphology of AgNF/PVP nanofibers from 20% (w/w) AgNO₃ and 8% (w/w) PVP at a gap distance of 20 cm with an applied voltage of 8–10 kV. (A) Optical microscopy (OM), (B) FIB, (C) TEM and (D) selected-area electron diffraction (SAED).

Fig. 6 XRD patterns and UV-vis-NIR spectra of PVP and AgNF/PVP core–shell ES nanofiber mats @180 °C for 1 h. (A) XRD pattern of pure PVP, (B) XRD pattern of AgNF/PVP, (C) UV-vis-NIR spectrum of pure PVP and (D) UV-vis-NIR spectrum of AgNF/PVP.

The structural properties of pure PVP and the AgNF/PVP core–shell ES nanofiber mats were also investigated using XRD, as displayed in Fig. 6A and B, respectively. The XRD pattern of pure PVP ES nanofiber mats reveals a broad band located at 2θ = 22°, which clearly indicates the amorphous nature of PVP⁵⁰ (as illustrated in Fig. 5A). Similarly, Fig. 6B shows the XRD pattern of the AgNF/PVP core–shell ES nanofiber mats with diffraction peaks at the 2θ values of 38.12°, 44.23°, 64.44°, and 77.40°, which are indexed to the (111), (200), (220), and (311) planes of the cubic crystal structure of silver, respectively, from the JCPDS card (file no. JCPDS # 04-0783). The XRD data confirms the presence of Ag in the AgNF/PVP core–shell ES nanofiber mats.

Fig. 6C and D show the UV-vis-NIR spectra of pure PVP and the AgNF/PVP core–shell ES nanofiber mats, respectively. The UV-vis-NIR spectrum of pure PVP ES nanofiber mats shows a weak absorption below 300 nm and a strong absorption in the higher wavelength range, whereas the silver nanostructures with different shapes exhibit surface plasmon resonance (SPR) bands at different wavelengths. The three SPR peaks (at ∼350, ∼404 and ∼453 nm) belong to the optical signatures of silver and are clearly observed on the UV-vis-NIR spectrum of the AgNF/PVP core–shell ES nanofiber mats, which proves the reduction of Ag(I) ions into Ag(0) metal nanostructures.^51,52

The elemental composition on the surface of the AgNF/PVP core–shell ES nanofiber mats was also characterized via XPS and EDS, as shown in Fig. 7A and B, respectively. From the XPS spectrum, only peaks related to C, N, and O appear. Very interestingly, the XPS data show that no silver was present on the surface of the AgNF/PVP core–shell ES nanofiber mats. Since XPS elemental analysis can detect at the uppermost ∼100 Å in depth of an analyzed specimen,³⁸ it is reasonable to draw the conclusion that the absence of an Ag signal in the PVP shell surface was because the Ag was deeper than 10 nm at least. The abovementioned results show that the formed AgNF/PVP core–shell ES nanofiber mats correspond with the ideal nanofiber model (the inset of Fig. 1).

Fig. 7 Elemental composition on the surface of the AgNF/PVP core–shell ES nanofiber mats: (A) XPS wide energy survey spectrum and (B) EDS spectrum.

Fig. 7B shows EDS spectrum for the AgNF/PVP core–shell ES nanofiber mats, which shows only peaks related to C, N, O, Si and Pt. From the EDS data, the C, N and O peaks come from the PVP polymer on the surface of the AgNF/PVP core–shell ES nanofiber mats, whereas Si peaks originate from the slide glass that supports the sample and Pt is present due to the sputtering step for sample preparation for FIB imaging. EDS data show that no silver was present on the surface of the AgNF/PVP core–shell ES nanofiber mats, which further supports that the AgNF were successfully incorporated into the PVP polymer matrix. By combining XPS and EDS data, it could be concluded that the AgNFs were completely embedded in the PVP nanofibers, at least deeper than 10 nm, which was the probe depth for XPS. This implies that we have successfully fabricated AgNF/PVP core–shell nanofibers with AgNF embedded inside as the core and PVP outside as the shell, as shown in the TEM image of Fig. 5C.

Furthermore, electrical measurements were performed on the AgNF/PVP core–shell ES nanofiber mats, which indicated that the AgNF/PVP core–shell ES nanofiber mats became insulating due to the embedded highly conductive AgNF by insulating the PVP shell. To summerize, coaxial electrospinning is an attractive, simple and effective top-down approach to fabricate continuous and uniform AgNF/PVP core–shell nanofibers compared with other methods. Therefore, this set up is a promising method for the fabrication of other continuous and uniform metal/polymer core–shell nanostructure fibers with lengths of up to several centimeters on a large scale for several applications such as electronic circuits, optoelectronics, and sensing devices.

Conclusions

In the present study, continuous and uniform AgNF/PVP core–shell nanofibers have been successfully fabricated via an efficient coaxial-spinneret electrospinning method with a vertical configuration using PVP and AgNO₃ as precursor solutions. The silver nanofibers are well aligned along the axis of the PVP electrospun fibers by the stretching of the electrospinning jet and the reduction temperature within an appropriate time. The PVP polymer was primarily used as a guide and provided an appropriate viscoelastic property by surrounding the non-viscous AgNO₃ (core) solution to fabricate aligned AgNF/PVP core–shell nanofibers. The formation of silver nanofibers in the AgNF/PVP core–shell nanofibers was completely confirmed by XRD and UV-vis characterization and FIB images show uniform and smooth PVP nanofibers on the surface. By combining the XRD and EDS results, it is also possible to confirm that the AgNFs were embedded in the PVP matrix, forming a core–shell nanofiber structure. The TEM images clearly assess and display the continuous and uniform core–shell structure of the AgNF/PVP nanofibers. Furthermore, electrical measurements were performed on the AgNF/PVP core–shell nanofiber mats, which indicated that the AgNF/PVP core–shell nanofiber mats became insulating due to the embedded highly conductive Ag-nanofibers by insulating the PVP nanofibers shell. To summarize, coaxial electrospinning is an attractive, simple and cost effective top-down approach to fabricate continuous and uniform AgNF/PVP core–shell nanofibers compared with other methods. Therefore, this setup is a promising method for the fabrication of other continuous and uniform metal nanofiber/polymer core–shell nanostructure fibers with lengths of up to several centimeters on a large scale for several applications such as electronic circuits, optoelectronics, and sensing devices.

Acknowledgements

This study was supported by Ministry of Science and Technology of the Republic of China under Grant MOST 103-2218-E-011-015.

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