Hydrophobic SiO2 Electret Enhances the Performance of Poly

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Hydrophobic SiO Electret Enhances the Performance of Polyvinyl Fluoride Nanofiber Based Triboelectric Nanogenerator Tao Huang, Hao Yu, Hongzhi Wang, Qinghong Zhang, and Meifang Zhu J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b07382 • Publication Date (Web): 07 Nov 2016 Downloaded from http://pubs.acs.org on November 12, 2016

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The Journal of Physical Chemistry

Hydrophobic SiO2 Electret Enhances the Performance of Polyvinyl Fluoride Nanofiber Based Triboelectric Nanogenerator Tao Huanga, Hao Yu*a, Hongzhi Wangb, Qinghong Zhangc, Meifang Zhua a

State Key Laboratory for Modification of Chemical Fibers & Polymer Materials, College of Materials

Science and Engineering, Donghua University, Shanghai, 201620, P. R. China. b

Shanghai Key Laboratory of Functional Hybrid Materials, College of Materials Science and

Engineering, Donghua University, Shanghai, 201620, P. R. China. c

Engineering Research Center of Advanced Glasses Manufacturing Technology, College of Materials

Science and Engineering, Donghua University, Shanghai 201620, P. R. China.

ABSTRACT: Mechanical energy harvesting is a green technology with great potential for applications in self-powered portable electronics, wireless sensing, implanted devices, and security systems. We have previously reported a triboelectric nanogenerator (TENG) fabricated from polyvinylidene fluoride (PVDF) electrospun nanofibers. This device enabled harvesting of mechanical energy and exhibited an output voltage of 210 V. Here we report doping of the PVDF nanofibers with silica (SiO2) nanoparticles, which increased the output performance of the resultant TENG devices. The output peak to peak voltage was increased to 370 V as the SiO2 nanoparticles content was increased to 0.6 wt%, and then declined with further increases in SiO2 content. Hydrophobic SiO2 nanoparticles (mSiO2) were prepared by an octanol treatment and markedly increased the performances of the resulting TENG devices. An output peak to peak voltage of up to 430 V, was achieved with 0.8 wt% mSiO2 ACS Paragon Plus Environment


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representing a 48% increase over the value obtained from aPVDF-0.8 wt% SiO2 nanofiber based TENG. On the basis of our high performance TENG devices, we developed a self-powered digital thermometer for temperature measurement without a battery.

INTRODUCTION

Over the last few decades, deepening concerns about fossil fuel depletion and environmental pollution have motivated research into green and renewable energy sources. 1-3 A triboelectric nanogenerator (TENG) that can convert mechanical energy into electricity has recently been shown to be a promising strategy for establishing a brand new energy harvesting method.4 These devices have developed rapidly in terms of materials selection 5-7 and surface morphology.8, 9 Furthermore, a greater understanding of device operating principles,10, 11 has allowed optimization of device structures to improve power densities and enable various applications.12-19 The working principle of TENG devices is based on a combination of triboelectrification and electrostatic induction effects. A typical TENG consists of two flexible substrates covered by polymers with different electron affinities. Triboelectric charges with the opposite signs are generated at the two dissimilar surfaces (friction layers) upon contact and release movement of the device. Electrons are induced to flow back and forth through an external circuit under the driving force of the triboelectric potential between the two friction layers. These charge generation and separation processes represent basic mechanical to electrical energy conversion. Therefore, enhancing the charge density on the triboelectric surfaces is an essential strategy to improve the performance of TENGs. To date, many efforts have been made to maximize the surface charge densities. The majority of approaches have focused on designing novel device structures15, 20-28 and surface modification of the triboelectric polymer. For example, morphologies such as nanowire/fiber2, 29, 30, nanoparticle31, and other nanoscale patterns8, 9, 32-34 have been used to

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increase the surface area of the triboelectric materials and thus provide more charge carrying sites. In addition, surface functionalization35 and ion treatments36-38 have also been used to improve the charge density on the polymer surfaces. However, in practical use, TENGs require intimate contact and sometimes friction between the triboelectric materials causes the surface of the triboelectric materials to wear. In this regard, surface modification/engineering has certain limitations for improving the long-term operation of TENGs. Therefore, it will be useful to extend the property engineering from the surface to the bulk of the material.

We have previously found that GO dielectric added to tribo-materials (PVDF) could enhance the surface charge density as well as the output performance of TENG devices.39 These improvements were attributed to specific GO structures dispersed in the polymers, which increased the interfacial charge storage. On the basis of these findings, we have investigated new potential charge storage materials, SiO2 electrets. These materials feature a high resistance, the ability to retain electrical charges over a long period of time, and an external quasi static electric field that can promote charge trapping.40-42 Good electret performance requires strong dipole orientation and large charge density; thus, silica (SiO2) electrets have also been widely used as energy harvesters.43,44 Furthermore, nanocomposites based on PVDF and SiO2 nanoparticles, have been widely investigated and used in various fields. Li et al.45 fabricated superhydrophobic PVDF/SiO2 composite nanofibers for use in membrane distillation. Kim et al.46 found that SiO2-dispersed PVDF nanofiber membranes had a morphology suitable for applications in high-performance lithium batteries, providing a large surface areas, high porosity, and high ionic conductivity. Here, we report doping of hydrophobic SiO2 nanoparticles, prepared by a treatment with octanol, into PVDF electrospun nanofibers. These materials were combined with ACS Paragon Plus Environment


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poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV) nanofibers to fabricate a book-shaped TENG, which showed high performance. This inorganic electret doping approach provides a new route to the modifying the bulk electrical properties of polymers, and is a promising strategy for improving the performance of functional polymer based TENGs. Finally, a self-powered thermometer was fabricated with an S-shaped TENG (two parallel connected book-shaped TENGs). Self-powering capabilities will allow electronic device packages to operate without bulky energy storage components such as batteries. EXPERIMENTAL Materials The PVDF (FR904) was obtained from Shanghai 3F New Material Co., Ltd. N, N-dimethylformamide (DMF, AR grade), trichloromethane, octanol, p-toluenesulfonic acid, and acetone (AR grade) were purchased from the Shanghai Chemical Reagent Plant. PHBV (Y1000P, Mw = 2.67 × 105, HV = 1.10 mol%) was purchased from Zhengjiang Tianan Biological Material Co. Ltd. SiO2 nanoparticles (99%, average size 15 nm) was purchased from Meryer (Shanghai) Chemical Technology Co., Ltd. All the materials were used without any further purification. Hydrophobic surface modification of SiO2 nanoparticles Hydrophobic SiO2 nanoparticles were prepared by a surface modification reaction with octanol, which replaced hydroxyl groups on the surface of SiO2 particles. In a typical experiment, SiO2 (5 g), octanol (30 mL) and p-toluenesulfonic acid (0.2 g) as the catalyst were added to a flask. The reaction was completed with heating at 120°C for about 6 h. Electrospinning Different weight ratios (0.2 wt%, 0.4 wt%, 0.6 wt%, 0.8 wt%) of the SiO2 and octanol modified SiO2 (mSiO2) were first ultrasonically dispersed in DMF/acetone (3/2/w/w) mixtures for 3 h. The


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PVDF powder was dissolved in the resulting suspensions at a polymer/solvent weight ratio of 1:9 (10%, w/w). Each mixture was stirred for 12 h to form a homogenous transparent dispersion, which remained stably dispersed for at least for 3 days. PHBV was dissolved in trichloromethane at 60°C to prepare the PHBV solution (8%, w/w). The electrospinning process was conducted in a homemade set-up, as shown in Fig. S1, with a DC power supply (JG50-1 Model HV, Shanghai Shenfa Detection Instrument, China); in this experiment, the applied voltage was 18 kV. The spinning solution was drawn into a hypodermic syringe and delivered through a blunt needle tip at 1 mL h−1 with a micro-syringe pump (KDS101, USA) with a fixed collection distance (15 cm) between the syringe tip and the roller collector. The spinning time was maintained at 5 h for each sample to control the final weight of the PVDF/SiO2 nanofibers. The spinning solution remained transparent and clear during the electrospinning process, which ensured that the SiO2 contents of the PVDF/SiO2 nanofibers were consistent with those of the PVDF/SiO2 dispersions. The relative humidity remained below 30%. Fabrication of the book-shaped TENG A rectangular PET film (10 cm × 6 cm × 100 µm) was folded in half lengthways (Fig. 1a (1) and (2)). Pieces of PVDF or PHBV nanofiber mat (4 × 5 cm) were pasted onto the top and bottom of the PET film, respectively (Fig.1a (3)). The aluminum foil that was used to collect the nanofibers during electrospinning acted as the electrode for the TENG. Characterization The morphology of the electrospun fiber structure and SiO2 nanoparticles were examined with a field emission scanning electron microscope (FE-SEM, S-4800, Hitachi, Japan). A thin platinum layer was sputtered onto the sample surface before FE-SEM examination. PVDF/SiO2 (mSiO2) nanofibers were observed with a transmission electron microscope (TEM; JEM-2100, JEOL, Japan) after being directly electrospun onto a copper grid coated with a holey carbon film. Fourier transform infrared ACS Paragon Plus Environment


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(FTIR) spectra of the (m)SiO2 nanoparticles were measured with a NEXUS-670 spectrophotometer (Nicolet, USA). Thermogravimetric curves of the (m)SiO2 were measured with a TGA/DSC simultaneous thermalanalyzer (Netzsch STA409PC, Germany) at a heating rate of 10°C/min from 50–800°C under a N2 atmosphere. Water contact angle measurements were measured with a contact angle goniometer (OCA40Micro, Germany) that had a CCD camera equipped for image capture. An oscillometer (LeCroy, Wavesurfer 104MXs-B) was used to measure the electrical signals generated by the book-shaped TENGs under the bending force of a stage with a frequency of 1.8 Hz. A Keithley 2400 meter was used to measure the voltage across the capacitors. The surface potential of the PVDF/mSiO2 nanofibers was detected with an electrostatic voltmeter (Trek, 541A). RESULTS AND DISCUSSION The fabrication process of the book-shaped TENG, shown in Figure 1a, was similar to that of our previous reports.39 A PVDF nanofiber (top side) and a PHBV nanofiber (top layer) were used to induce triboelectric charging based on the nanofibers’ electron affinities. The book-shaped structure of the TENG promoted effective charge separation and contact with the bent elastic PET substrates. The working area of the TENG was approximately 5 cm × 4 cm as shown in Figure 1b (1). Operation of the book-shaped TENG was performed by applying a cycled compressive force to the whole device area with a pressing stage, as illustrated in Figure 1b (2). The top and bottom of the PET substrate were fixed with PVDF and PHBV nanofibers, respectively. The two layers were periodically pressed into close contact with each other (Figure 1b (3)). Once released, the top and bottom substrates separated and automatically reverted back to their original state because of stored elastic energy in the PET substrate (Figure 1b (4)). The book-shaped structure and operation process are illustrated in Figure S1a-e.


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Figure 1. Illustration of the fabrication process of a book-shaped TENG and photograph of the device. (a) Schematic diagram of a TENG on a folded PET substrate. (b) Photographs of a working TENG in its (1) initial, (2) compressed, and (3) released states. Figure 2a-f show SEM images of the electrospun PHBV and PVDF/SiO2 nanofibers with different SiO2 concentrations (0 wt%, 0.2 wt%, 0.4 wt%, 0.6 wt%, and 0.8 wt%). The images show that smooth and bead-free solid fibers were formed a randomly-oriented fiber web for all the samples. As the SiO2 content was increased there was no noticeable influence on the shape of the PVDF/SiO2 nanofibers. However, a slight decrease in the diameter and an increase in the size distribution were found at high SiO2 contents. This could be due to an increase in the charge density in the solution caused by the charge retention properties of the SiO2 electret. This effect in turn caused greater electrostatic repulsion and bending instabilities of the solution jet during electrospinning.



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Figure 2. SEM images of the nanofibers used in the book-shaped TENGs. Morphology and size distributions (inset) of PHBV and PVDF nanofibers with different weight ratios of SiO2: (a) PHBV nanofiber, (b) PVDF with 0% SiO2, (c) PVDF with 0.2% SiO2, (d) PVDF with 0.4% SiO2, (e) PVDF with 0.6% SiO2, and (f) PVDF with 0.8% SiO2. We measured the output voltage of the fabricated book-shaped TENG under an external force from the pressing stage operating at a frequency of 1.8 Hz. As shown in Figure 3a, under continual pressing and release cycles, the TENG composed of PHBV and PVDF nanofibers without SiO2 nanoparticles generated a peak to peak voltage of 190 V (Figure 3a). Notably, the voltage increased sharply when a small amount of SiO2 was included in the PVDF nanofibers. The generated peak to peak voltage reached 270 V for the sample containing 0.2 wt% SiO2 in PVDF (Figure 3b). The performance of the TENG was further improved to 370 V by increasing the content of SiO2 up to 0.6 wt% (Figure 3d). This increase in peak to peak voltage represents a 95% improvement compared with the results of the TENG that contained bare PVDF nanofibers. This shows that SiO2 modified electrets can trap more charges at the surface and bulk of the PVDF nanofibers compared with bare PVDF owing to the orientation of the permanent dipole in the modified electrets41. Furthermore, the SiO2 dispersed in the ACS Paragon Plus Environment

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PVDF nanofibers also acted as charge trapping sites, which increased the interface for charge storage. However, the peak to peak voltage decreased to 290 V when the content of SiO2 in the PVDF nanofibers was further increased to 0.8 wt% (Figure 3e, f). This decrease may be caused by the strong hydrophilic nature of the silanol groups on the surface of the SiO2 nanoparticles. The excess hydrophilic SiO2 particles noticeably increased the surface hydrophilicity of the PVDF nanofibers (Figure S3). Our previous research has indicated that an adsorbed water layer plays an important role in charge generation and the overall performance of TENGs.47 A thick water layer (greater than 2 nm) increases the surface conductivity and causes the surface to discharge48, which decreases the output voltage. On the basis of our previous work, we suspected that the output performance of the TENGs could be further improved by turning the surface of the SiO2 nanoparticles from hydrophilic to hydrophobic.



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Figure 3. The output voltage of a book-shaped TENG fabricated by PVDF nanofibers with different weight ratios of SiO2. (a) PVDF-0% SiO2, (b) PVDF-0.2% SiO2, (c) PVDF-0.4% SiO2, (d) PVDF-0.6% SiO2, (e) PVDF- 0.8% SiO2. (f) Output peak to peak voltage of the TENG containing PVDF nanofibers with different weight ratios of SiO2. The reaction used to modify the SiO2 nanoparticles with octanol is shown in Figure S4. In this reaction, the hydroxy groups on the surface of the SiO2 nanoparticles were substituted for alkyl groups. The XRD patterns of the SiO2 nanoparticles, shown in Figure S5, confirmed a typical amorphous


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structure, which remained the same after chemical modification. Figure 4a shows FTIR spectra of the original and modified SiO2 (mSiO2) nanoparticles. The bands seen at 3430 cm−1, 1630 cm−1 are attributed to the O-H stretching vibration and bending mode of O-H groups, respectively. These signals sharply decreased in the spectra of the mSiO2. Simultaneously, characteristic absorption bands appeared at 2963, 2933, and 2878 cm−1, which were attributed to C-H, -CH2 and C-CH3 stretching vibrations, respectively. These results confirmed that the alkyl chain was successfully grafted onto the surface of the SiO2. The relative content of grafted alkyl chains on the mSiO2 was measured by thermal gravity analysis (TGA) through thermal decomposition of the organic components. The TGA curve of a sample of the unmodified SiO2 nanoparticles showed only a slight weight loss of 2% at 800°C due to desorption of chemisorbed water on the surface of the unmodified nanoparticles. However, a clear weight loss appeared for the mSiO2 sample in the temperature range of 250–600°C, which indicating decomposition of the alkyl chains. The difference in the weight loss between the SiO2 and mSiO2 nanoparticles revealed that grafted alkyl chains accounted for about 8% of the nanoparticles mass (Figure 4b). The hydrophobic nature of the alkyl chains increased the surface contact angle of the SiO2 nanoparticles from 15° (Figure 4c) to 143° (Figure 4d, mSiO2). This confirmed that the surface of the SiO2 nanoparticles was modified from hydrophilic to hydrophobic after the octanol treatment.



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Figure 4. Evaluation of the unmodified and modified SiO2 nanoparticles. (a) FTIR spectra and (b) TGA curves of the unmodified and modified SiO2 (mSiO2) nanoparticles. Surface contact angle measurements of the (c) unmodified and (d) modified SiO2 nanoparticles. Therefore, we added mSiO2 instead of SiO2 into PVDF nanofibers to fabricate the TENG devices and studied the output performance. When the surface of the mSiO2 was grafted with alkyl chains, we found that better dispersion was achieved in the mixture used to electrospin the sample with PVDF-0.8 wt% mSiO2 (Figure 5a) compared with that of PVDF-0.8 wt% SiO2 (Figure 5b). This result was also confirmed from SEM images of the SiO2 and modified SiO2 nanoparticles.(Figure S6). As expected, the output voltages of the TENGs fabricated with PVDF/mSiO2-PHBV were much higher than those of PVDF/SiO2-PHBV, for the same SiO2 content. Notably, the output voltage of the PVDF-0.8 wt% SiO2 TENG declined compared with devices based on lower SiO2 contents. However, the TENG with ACS Paragon Plus Environment

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modified SiO2, PVDF-0.8 wt% mSiO2, showed a higher maximum peak to peak voltage of 430 V than those of devices based on lower modified SiO2 (Figure S7 and Figure 5c). This represented an increase of 48%, compared with the PVDF-0.8 wt% SiO2. The hygroscopicity of SiO2 is believed to play an important role in the output performance. The moisture content of the unmodified SiO2 nanoparticles was 21.4 wt%, while that of the mSiO2 nanoparticles was 6.6 wt%, as calculated at a relative humidity of 40% (Table S1). This large difference in the SiO2 moisture content likely explains the difference in the surface resistivity of PVDF/SiO2 nanofibers for SiO2 contents up to 0.8 wt%. Figure 5d shows the surface resistivity of PVDF, PVDF-0.8 wt% SiO2 and PVDF-0.8 wt% mSiO2 nanofibers. We found that the surface resistivity of the PVDF-0.8 wt% SiO2 decreased by an order of magnitude compared with bare PVDF nanofibers, while the value of PVDF-0.8 wt% mSiO2 remained almost the same as that of the bare PVDF nanofibers. Thus, this experiment suggested that at ambient conditions, water adsorbed to the fiber surfaces increased the surface conductivity of polymers and dissipating generated charges, having a negative effect on TENG performance.



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Figure 5 TEM images of (a) PVDF-0.8% mSiO2 and (b) PVDF-0.8% SiO2 nanofibers. (c) Bar chart showing the difference in peak to peak voltage of TENGs fabricated from PVDF/SiO2 and PVDF/mSiO2 nanofibers. (d) The surface resistivity of PVDF, PVDF-0.8% SiO2 and PVDF-0.8% mSiO2 nanofibers.

The output current of the TENGs fabricated with PVDF nanofibers, having different weight ratios of mSiO2, showed a similar nanoparticle content dependence to that of the voltage. As shown in Figure 6a-f, the output peak to peak current of the TENGs increased with mSiO2 concentration. A maximum output peak to peak current of 85 µA was recorded for the PVDF-0.8% mSiO2 device, which represented a 204% increase compared with the value of the bare PVDF nanofibers.


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Figure 6. Output current of book-shaped TENG fabricated with PVDF nanofibers having different weight ratios of mSiO2 under an external load of 100 kΩ. (a) PVDF-0% mSiO2, (b) PVDF-0.2% mSiO2, (c) PVDF-0.4% mSiO2, (d) PVDF-0.6% mSiO2, (e) PVDF-0.8% mSiO2. (f) Bar chart showing the increase of output peak to peak current of TENGs with higher mSiO2 content.



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The energy generated by the TENGs fabricated with PVDF nanofibers having various weight ratios of mSiO2 could be stored by connecting the device to a capacitor (2.2 µF) through a bridge circuit (see Figure S7). This setup can be used to reflect the charging powers of the TENGs. The accumulated charges and energies measured across the capacitor after 5 min of powering by different TENGs are shown in Figure 7a and b. It can be seen that the charging power of the TENG increased with the mSiO2 content in the PVDF nanofibers, in line with the increases in output voltage and current. The charge and energy generated by the bare PVDF nanofibers after 5 min were 174 µC and 6.9 mJ, respectively. These results gave an average charging current of 0.581 µA and an average power of 0.023 mW, respectively. In contrast, the charge and energy generated by the PVDF-0.8% mSiO2 nanofiber TENGs were 438 µC and 43.5 mJ, corresponding to an average charging current of 1.46 µA and an average power of 0.145 mW. These values represent increases of the average charging current and average power by 151.7% and 530.4%, respectively.


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Figure 7 (a) Accumulated charges and (b) stored energy in a 2.2-µF capacitor over 5 min. (c) Surface potential decay and (d) the normalized surface potential decay of PVDF/mSiO2 nanofibers with different concentrations of mSiO2. To elucidate the influence of the SiO2 electrets on the output performance of the PVDF/mSiO2-PHBV nanofiber based TENGs, we studied the surface charge retention ability of the PVDF/mSiO2 nanofibers. The surface potential of the PVDF/mSiO2 nanofibers increased as the content of mSiO2 increased, indicating an increase in the surface charge densities, as shown in Figure 7c. Furthermore, the surface potential decayed slower for PVDF nanofibers with higher mSiO2 contents (Figure 7d). These results indicate that the addition of the mSiO2 electrets improved the permanent dipole orientation and provided more charge trapping sites in the PVDF nanofibers. This improvement in turn allowed for higher surface charge densities, which was a key factor influencing the TENG performance.



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To demonstrate the ability of our PVDF/mSiO2-PHBV nanofiber based TENG to function as a direct power source, a self-powered digital thermometer was designed. To enhance the output current, two book-shaped TENGs were connected in parallel to form an S-shaped structure, for use as the power source (Figure 8a). A simple full wave rectifier circuit and voltage-stabilizing circuit were used to supply the working voltage for a digital thermometer (Figure 8b). Without any battery, the thermometer could measure the temperature of the environment in real-time after continuous pressing of the TENG for several seconds (Figure 8c-e, Supporting Movie S8). This result clearly shows the practical applicability of our TENG to power small conventional sensor systems.


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Figure 8. S-shaped TENG used to drive a digital thermometer. (a) Diagram showing the structure of the S-shaped TENG, which was applied to drive a small thermometer; (b) Schematic diagram of the prototype thermometer-powering circuit; (c–e) photographs of the thermometer operating while pressing the S-shaped TENG.

CONCLUSIONS In summary, we demonstrated enhanced performance of a TENG by doping PVDF electrospun nanofibers with SiO2 nanoparticles. Although the SiO2 nanoparticles increased the output voltage of the TENG, owing to the excellent charge retention ability of the SiO2 electret, a thick water layer ACS Paragon Plus Environment


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formed on the surface of the PVDF nanofibers. This water layer reduced the surface charge density due to its high conductivity and decreased output performance. However, the surface hydrophobicity of the SiO2 nanoparticles could be modified with octanol to offset this drawback. Using this strategy we produced TENGs with output peak-to-peak voltages and currents as high as 430 V and 85 µA, respectively. The corresponding maximum average charging current and power for charging a 2.2 µF capacitor were measured as 1.46 µA and 0.145 mW, respectively. Furthermore, as a power source, two parallel connected TENG were successfully used to drive a digital thermometer without any battery or capacitor. This simple but effective electret doping technique offers new opportunities for tuning the performance of polymer-based TENGs for effective harnessing of mechanical energy.

ASSOCIATED CONTENT

Supporting Information: Homemade electrospinning apparatus; power generation mechanism of the TENG device; surface contact angle of bare PVDF and PVDF-0.8% SiO2 nanofibers; reaction equation of hydrophobic surface modification of SiO2 nanoparticles; XRD patterns of SiO2 and mSiO2 nanoparticles; SEM images of SiO2 and mSiO2 nanoparticles; output voltage of book-shaped TENGs; hygroscopicity of SiO2 and mSiO2 nanoparticles; charging power circuit for the book-shaped TENG; movie for the S-shaped TENG driven a digital thermometer. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *Phone: +86-21-67792853. E-mail: [email protected] Notes The authors declare no competing financial interest.


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44x27mm (300 x 300 DPI)


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Hydrophobic SiO2 Electret Enhances the Performance of Poly

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