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Mechanically robust magnetic Fe3O4 nanoparticle/polyvinylidene fluoride composite nanofiber and its application in a triboelectric nanogenerator Ji-Su Im, and Il-Kyu Park ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b07621 • Publication Date (Web): 06 Jul 2018 Downloaded from http://pubs.acs.org on July 6, 2018

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ACS Applied Materials & Interfaces

Mechanically robust magnetic Fe3O4 nanoparticle/polyvinylidene fluoride composite nanofiber and its application in a triboelectric nanogenerator Ji-Su Im and Il-Kyu Park* Department of Materials Science and Engineering, Seoul National University of Science and Technology, Seoul 139-743, South Korea *Corresponding author. Tel.: +82029706349, fax: +82029736657 e-mail address: [email protected] (I. K. Park)

ABSTRACT Mechanically robust composite nanofibers (NFs) with enhanced magnetic properties were made from polyvinylidene fluoride (PVDF) and Fe3O4 nanoparticles (NPs) by electrospinning method. At up to 11.3 wt%, Fe3O4 NPs were embedded randomly in the PVDF NFs, but when the content exceeded 17 wt%, the NPs aggregated on the NF surfaces. Magnetization of the composite NFs consistently increased with increasing the Fe3O4 NP content. The mechanical strength of the Fe3O4 NP/PVDF composite NF was enhanced by a dispersion strengthening mechanism. A triboelectric nanogenerator was made from the composite, which showed enhanced output performance with Fe3O4 NP content less than 11.3 wt%, but the performance degraded at higher content. These results were attributed to the electret doping effect and surface aggregation of the Fe3O4 NPs on the NFs, respectively.

Keywords: Composite, Electrospinning, Fe3O4, Nanofiber, Nanoparticle, Polyvinylidene fluoride, Triboelectric nanogenerators

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1. INTRODUCTION Wearable electronic devices have attracted much attention for realizing wireless sensor networks.1,2 However, powering the wearable devices has been an obstacle. Flexible, wearable, self-powering energy scavenging technologies have been investigated as possible solution.3,4 Mechanical energy harvesting based on a triboelectric mechanism has received much attention because of the high available power density of up to 1,000 µW/cm2 from the environment, simple fabrication process, and wearable or flexible nature of the active materials.3-5 A wide variety of material systems have been adopted as an active material for the triboelectric nanogenerators (TENGs), such as organic materials, metals, textiles, and papers.6-8 In particular, polyvinylidene fluoride (PVDF) has been investigated widely due to its flexibility and simple fabrication process for making one-dimensional fiber, thin films, or three-dimensional porous structures.9,10 PVDF nanofiber (NF) structures fabricated by electrospinning have been widely investigated to enhance the energy generation performance.9-13 The piezoelectric or triboelectric performance can be enhanced by electrospun PVDF NFs with enlarged surface area and preferential formation of β-phase, which has better piezoelectric and triboelectric properties.9-13 Various methods using additives have been reported to enhance the energy generation performance. Carbon-based


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nanostructures, such as graphene, graphene oxide, and carbon nanotubes have been known to stabilize the electroactive β-phase.14,15 Doping with a rare earth element like Eu or Ce also stimulates the phase transformation from α to β-phase PVDF.16,17 Most research has focused on enhancing the nanogenerator performances by modifying the structural and chemical properties of PVDF NFs. However, the PVDF NFs have intrinsic weak point of poor mechanical strength. In addition, for better compatibility with smart textiles, PVDF NFs also need to have magnetic properties for shielding from electromagnetic interference (EMI) in particular applications.18 In this study, we report on the fabrication of composite PVDF NFs with an embedded magnetic material, Fe3O4 nanoparticle (NP). The composite NFs were obtained by electrospinning to enhance the mechanical strength and control the magnetic properties. The effect of the Fe3O4 NP incorporation on the performance of a TENG was also investigated using a single-electrode mode-based device structure. We considered the feasibility of the composite NF structures for use in mechanically robust TENG devices with EMI shielding.

2. EXPERIMENTAL DETAILS 2.1. Fabrication of Samples The schematic fabrication process of the Fe3O4 NP/PVDF composite NFs by



electrospinning is illustrated in Fig. 1(a). The PVDF source solution was prepared by dissolving 15 wt% PVDF powder (Kynar FLEX 2801, Arkema) in a mixed solution containing N,N-dimethylformamide (DMF, Duksan chemicals) and acetone (Duksan chemicals) with a mass ratio of 3:1 to prevent bead formation by controlling the evaporation rate of the solvents during the electrospinning process. The commercial Fe3O4 NPs (Sigma Aldrich) with average particle size of 47 nm were used without further purification. The particle size was measured from the XRD results by using the Scherrer equation. Mixed solutions were prepared by adding 0, 5.7, 11.3, 17, 22.7, or 28.3 wt% Fe3O4 NPs into 20-ml PVDF solutions. The mixed solutions were moderately stirred at room temperature for 24 hrs and sealed during the stirring process. Once the homogeneously mixed solutions were fabricated, the Fe3O4 NPs did not show any sedimentation but showed stable suspension. The mixed solutions were loaded into 5-ml syringes with 18 gage metal needles of the electro-spinning machine (ESR200R2D, NanoNC Co.). The feed rate of the source was 2.0 ml/hr, and the distance between the syringe needle and the collector was 10 cm. A working voltage of 19 kV was applied with a DC power supply. To collect the electrospun PVDF NFs, Al sheets were wrapped round the rotating drum-shaped collector as shown in Fig. 1(a). The rotation speed of the collector drum was 1,500 rpm to align the NFs along the rotating directions. Figure 1(b) shows the prepared


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electrospun Fe3O4 NP/PVDF composite NF mats (area of 5×5 cm2) and their color changes from white to black with increasing Fe3O4 NP content. This indicates that the Fe3O4 NP/PVDF composite NFs are fabricated successfully with a large area. 2.2. Characterization and Measurement of Samples. The surface morphology of the Fe3O4 NP/PVDF composite NFs was observed by field emission scanning electron microscopy (FE-SEM, Hitachi S-4700). The crystal structure was examined using X-ray diffraction (XRD, Rigaku D/MAX 2500 diffractometer equipped with a Cu Kα source). The magnetic properties were measured at 300 K using a vibrating sample magnetometer (VSM, MicroSence, EV9). The mechanical properties were measured using a micro-fatigue tester (E3000LT, Instron) by exfoliating the composite NF mats with an area of 1×5 cm2 from the Al collector plates. The samples for the mechanical test were cut and elongated along the aligned direction of individual NFs because the mechanical properties of the textile could be affected by the alignment of the fibers. TENG devices that operate in vertical contact-separation mode were fabricated using a triboelectric pair of PVDF NFs and Al thin films on polyethylene terephthalate (PET), as shown in Fig. 1(a). PVDF and Al materials are on the negative and positive sides in a triboelectric series, respectively. In the device structure, the Al plate physically supports the PVDF NFs and acts as a back electrode. The PVDF NFs and Al



surface (contact area: 5×5 cm2) can contact and separate periodically with a variable frequency and generate potential differences. The output voltage and current from the TENGs were measured using a high-speed oscilloscope (TBS1202B, Tektronix) and picoammeter (6485, Keithley).

3. RESULTS AND DISCUSSION Figure 2 shows FE-SEM images of the PVDF NFs with various contents of the Fe3O4 NPs. The PVDF NFs showed no bead defects and randomly oriented shape because of the spiral ejection of the NFs from a Taylor cone of the solutions. As shown in Figs. 2(b)-(e), the Fe3O4 NPs were randomly distributed in the PVDF NFs, and the amount increased with the Fe3O4 NP content in the solution. When the amount of Fe3O4 NPs is less than 11.3 wt%, the NPs are well-embedded in the PVDF NFs, as shown in the inset of Fig. 2(b). As the content increases to 28.3 wt%, however, the NPs aggregate on the PVDF NF surfaces, as shown in the inset of Fig. 2(e). This would be attributed to that the aggregation probability between the Fe3O4 NPs can be increased as the NP content increases because we have not used any surfactant to disperse the NPs in the PVDF solutions. Therefore the aggregation problem is inevitable as the Fe3O4 NPs of NPs increases. Figure 2(f) shows the energy dispersive X-ray spectroscope (EDS) spectra for the PVDF NFs with 11.3 wt% Fe3O4 NPs. Both Fe and F are found due to


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the successful incorporation of the Fe3O4 NPs into the NFs. XRD was used to investigate the phase structures of the composite NFs. The XRD pattern of just the Fe3O4 NPs is consistent with the standard pattern for magnetite (JCPDS Card No. 79-0417) with no trace of other phases, as shown in Fig. 3(a). The pure PVDF NFs showed a broad peak around 20°, which is composed of two peaks for the (020) and (110) planes of the α-phase and the (200) plane of the β-phase of PVDF, as shown in Fig. 3(b).17 As the Fe3O4 NPs are incorporated into the PVDF NFs, the XRD patterns of the composite NFs show all peaks corresponding to the Fe3O4 NPs and PVDF NFs. As the Fe3O4 NP content increased, a diffraction peak for the Fe3O4 gradually developed, indicating an increase of the amount of incorporated Fe3O4 NP. It should be noted that the (200) plane of the β-phase of PVDF is developed as the Fe3O4 NPs are incorporated due to an ion-dipole interaction between CH2 dipoles in the PVDF chain and oxo- or hydroxo-groups on the Fe3O4 NP surfaces.12 To investigate the magnetic properties of the composite NFs, we measured the magnetization with an applied magnetic field at 300 K. Figure 4(a) shows the magnetization versus the magnetic field hysteresis loops for the samples. Due to nanometre dimensions of the magnetic Fe3O4 NPs, the composite NFs exhibit superparamagnetic property, which is maintained even when increasing the Fe3O4 NP content up to 28.3 wt%. Because the Fe3O4 is a ferromagnetic material, the Fe3O4 NPs



can be regarded as a small magnetic domain in the composite NF structures. As the Fe3O4 NP content increases, the saturation magnetization (Ms) increases while maintaining consistent hysteresis loops. Figure 4(b) shows the variation of the Ms and coercive field strength with variation of the Fe3O4 NP content. As the NP content increases from 5.7 to 28.3 wt%, Ms increases linearly from 1.93 to 12.5 emu/g and the coercive field strength increases from 96 to 113 Oe. The coercive field strength shows saturation above the Fe3O4 NP contents of 17 wt%. And a slope of the magnetization curves reflects the magnetic permeability (µ), which increases with increasing the Fe3O4 NP content. Therefore, the modification of the magnetic properties can be directly attributed to the incorporated Fe3O4 NPs. This behaviour can be applicable for shielding the EMI from the environment. The shielding effectiveness (shielding efficiency; SE) of a material by an absorption of the EM wave depends on the structural, electrical, and magnetic properties of the material by19

SE A ≈ 10.33

πµ f RS

t

where µ is the magnetic permeability, f is the frequency of EM wave, t is the thickness of material, and Rs is the sheet resistance. The EMI shielding performance of a material is proportional to its magnetic permeability. Therefore, we can expect that the EMI


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shielding performances of the PVDF NFs would be enhanced by incorporating the magnetic Fe3O4 NPs. To investigate the mechanical properties of the NFs, we measured the stressstrain curve using a micro-fatigue tester as shown in the inset of Fig. 5(a). For the measurement, the composite NF mats were carefully exfoliated from the Al sheet. All of the samples showed typical stress-strain curves, as shown in Fig. 5(a). The PVDF NF mats exhibited two-step elastic behaviour with nonlinear elastic behaviour in the initial stage and linear elastic behaviour until the NFs broke. This two-step break mechanism has been observed for organic NF mats.20,21 The nonlinear elastic behaviour in the initial stage is due to the alignment of nonaligned NFs in the mats along the stress direction. The linear elastic behaviour after the nonlinear region is due to the breaking of the individually aligned NFs in the mats. This indicates that the mechanical strength of the individual NFs is closely related with the second region and the strain at the breaking point. As shown in the Fig. 5(b), the elongation at the breaking point of the composite PVDF NF mats is increased when adding up to 17 wt% Fe3O4 NPs and decreased with higher NP content. The origin of abnormal plastic behaviour observed for the 5.7 wt % sample is not sure at this stage. But we assume that would be due to slipping of NFs or NP-NF interactions when the NPs are well distributed in the NFs. The enhanced mechanical properties of the composite NFs can be explained by the NP dispersion



strengthening mechanism, as shown schematically in the inset of Fig. 5(b). The included NPs can block or deflect crack propagation. These results suggest that the composite NFs have robust mechanical strength and could be applied as promising materials for a wide variety of wearable devices. TENGs with vertical contact-separation mode were fabricated to investigate the charge generation performance of the Fe3O4 NP/PVDF composite NFs. Figure 6 shows the voltage and current outputs of TENGs that consist of an Al electrode and pure PVDF NFs without Fe3O4 NPs, which was used to check the operation of the TENGs. As the PVDF NF and Al surfaces approach and contact each other, electric charges transfer from the Al to the PVDF NF surface because the PVDF has a more negative polarity than the Al in the triboelectric series.17 As they are separated, the electrons flow back through the outer circuit to maintain charge neutrality. In this way, upward and downward peaks are generated as the two surfaces repeat the approach/separation cycles. Figure 6(a) shows the output voltage with increasing frequency of the contactseparation cycles from 3.6 to 5.3 Hz. The output voltage increases as the strain frequency increases. This originates from the charge accumulation during the contact/separation cycles, indicating normal operation of the TENGs. Figures 6(b) and (c) show the voltage and current outputs measured by connecting the electrodes in forward and inverse to the measurement systems to check the inversion of the output


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signals. The voltage and current output signals are inverted when the polarity of the electrode is changed, as shown in Figs. 6(b) and (c). These strain frequency and polarity dependent output results indicate that the TENGs work normally and the output electrical signals are from the TENG devices. Figure 7 shows the TENG performance with variation of the NP content in the PVDF NFs. All devices show similar performance, even after incorporating the NPs. As the NP content increases, the voltage and current increase slightly and then decrease again, as shown in Figs. 7(a) and (b). To investigate the effect of Fe3O4 NP incorporation on the output performance of the TENG with PVDF NFs, the output voltage and current with variation of the Fe3O4 NP content is shown in Fig. 7(c). As the NP content increases up to 11.3 wt%, the average peak output voltage increases from 124 to 138 V and then decreases again to 94 V as the NP content increases further to 28.3 wt%. The output current also varies in a similar way between 3.2 and 5.68 µA. The enhanced triboelectric output performance of the composite NFs with a small amount of NPs could be due to the enhanced formation of polar β-phase PVDF and the electret doping effect resulting from the dielectric properties of the Fe3O4.22 However, further increase of the NP content results in aggregation of the NPs on the PVDF NF surfaces, as shown in Fig. 2(e). The aggregated Fe3O4 NPs can block the contact between the PVDF NFs and Al surface, which eventually results in degradation of the TENG



performance by reducing the effective contact area. Consequently, the performance of the TENG can be affected by both of the beneficial and detrimental effects of Fe3O4 NP incorporation. The results indicate that the mechanical and magnetic properties of the PVDF NFs can be enhanced by incorporating the Fe3O4 NPs. Even though the NPs have beneficial and detrimental effects on the TENG performance, there was no significant degradation of the output voltage and current. Therefore, the Fe3O4 NP/PVDF composite NF structures could be applied in mechanically robust TENG devices with EMI shielding performance.

4. CONCLUSIONS

In summary, Fe3O4 NP/PVDF composite NFs were successfully fabricated by electrospinning, and their structural, mechanical, and magnetic properties were investigated. As the amount of incorporated Fe3O4 NPs increased, the NPs aggregated on the PVDF NF surfaces, while the α- and β-phases of PVDF showed consistent fractions. As the NP content increased from 5.7 to 28.3 wt%, Ms increased linearly from 1.93 to 12.5 emu/g. The mechanical properties of the composite NFs showed a two-step break mechanism, and the tensile strength was enhanced as the Fe3O4 NPs were incorporated due to the dispersion strengthening mechanism. TENG devices made using the composite NFs and Al showed typical output performance. As the NP content


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increased up to 11.3 wt%, the output voltage increased from 124 to 138 V and then decreased to 94 V as the NP content increased further up to 28.3 wt%. The Fe3O4 NPs beneficially affected on the TENG performances by the combined effect of enhanced formation of the polar β-phase of PVDF and the electret doping effect at a small amount of incorporation but detrimentally affected at a large amount due to aggregation. Therefore, the Fe3O4 NP/PVDF composite NF structures can be applied to mechanically robust TENG devices with EMI shielding performance.

Acknowledgement This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science and ICT (NRF2018R1A2B6006968).



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Figure captions Figure 1. (a) Fabrication process of the Fe3O4 NP/PVDF composite NFs by electrospinning method and TENG device structure. (b) Electrospun Fe3O4 NP/PVDF composite NF mats with various Fe3O4 NP contents (area of 5×5 cm2).

Figure 2. FE-SEM images of the electrospun Fe3O4 NP/PVDF composite NFs with (a) 0 wt%, (b) 5.7 wt%, (c) 11.3 wt%, (d) 17 wt%, (e) and 28.3 wt% Fe3O4 NP contents. The inset shows well-embedded Fe3O4 NPs and aggregated Fe3O4 NPs on the surface of the PVDF NFs. (f) EDS spectra of the electrospun PVDF NFs with 11.3 wt% Fe3O4 NPs.

Figure 3. (a) Normalized XRD patterns of pure Fe3O4 NPs, pure PVDF NFs, and Fe3O4 NP/PVDF composite NF with various Fe3O4 NP contents in the solution. (b) Normalized XRD patterns of Fe3O4 NP/PVDF composite NF with variation of Fe3O4 NP content.

Figure 4. (a) Magnetization versus field hysteresis loops for the electrospun Fe3O4 NP/PVDF composite NFs. (b) Saturation magnetization (Ms) and coercive field strength with variation of the Fe3O4 NP contents.


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Figure 5. (a) Stress-strain curves of the electrospun Fe3O4 NP/PVDF composite NF mats. The inset shows a schematic of the microstrain-stress measurement system. (b) Fracture strain of the Fe3O4 NP/PVDF composite NF mats with variation of the Fe3O4 NP contents.

Figure 6. (a) Strain frequency-dependent output voltage measured from a TENG with pure PVDF NFs. Polarity-dependent (b) voltage and (c) current outputs measured from the TENG with pure PVDF NFs.

Figure 7. (a) Current and (b) voltage outputs of TENGs with variation of the Fe3O4 NP contents. (c) Average peak voltage and current outputs of TENGs with variation of the Fe3O4 NP contents.



Figures

Figure 1. J. S. Im et al.


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Table Of Contents (TOC) graphic


polyvinylidene

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