High-performance Flexible Piezoelectric Assisted Triboelectric Hybrid

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High-performance Flexible Piezoelectric Assisted Triboelectric Hybrid Nanogenerator via PDMS Encapsulated Nanoflower-like ZnO Composite Films for Scavenging Energy from Daily Human Activities Dong Hyun Kim, Bhaskar Dudem, and Jae Su Yu ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b00834 • Publication Date (Web): 05 Jun 2018 Downloaded from http://pubs.acs.org on June 5, 2018

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ACS Sustainable Chemistry & Engineering

High-performance Flexible Piezoelectric Assisted Triboelectric Hybrid Nanogenerator via PDMS Encapsulated Nanoflower-like ZnO Composite Films for Scavenging Energy from Daily Human Activities Dong Hyun Kim†, Bhaskar Dudem†, and Jae Su Yu* Department of Electronic Engineering, Kyung Hee University, 1732 Deogyeong-daero, Giheung-gu, Yongin-Si, Gyeonggi-do 446-701, South Korea. *Corresponding author. Email address: [email protected] (J. S. Yu) †

These authors are equally contributed.

ABSTRACT We successfully synthesized flower-like zinc oxide (ZnO) nanoarchitectures by a chemical precipitation method which is a facile, cost-effective, low-temperature, and quick-synthesis process. Furthermore, these nanoarchitectures were used to design an MWCNT (multi-walled carbon

nanotube)/ZnO/PDMS

(polydimethylsiloxane)

composite

film-based

hybrid

nanogenerator (HNG). Here, the ZnO nano-flowers play an important role in enhancing the piezoelectric and triboelectric potentials of HNG, termed as piezoelectric assisted triboelectric (PAT)-HNG. The ZnO nano-flowers were employed as the piezoelectric material as well as to enhance the surface roughness of PDMS, which can increase the triboelectric performance. Besides, the MWCNTs were also utilized to evenly distribute the ZnO nano-flowers and also to reduce the internal resistance of PAT-HNG. To maximize the electrical output power of the device, the concentration of ZnO and the amount of MWCNTs were changed and the electrical output performance of PAT-HNG was investigated. Resultantly, an optimized PAT-HNG with MWCNT/ZnO/PDMS composite film was

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achieved, which consists of ∼4.95 g of PDMS, 4.8 wt% of ZnO, and 0.015 g of MWCNT. The electrical output power of the optimized PAT-HNG was employed to drive twenty commercial light-emitting diodes connected in series. To demonstrate the practical applications of PAT-HNG, it was fixed onto a slipper and efficiently harvested the energy from daily human activities. Consequently, the PAT-HNG device exhibited electrical output voltage/current values of ∼75 V/3.2 µA, ∼150 V/8 µA, and ∼400 V/30 µA, while walking, running, and jumping, respectively. Keywords: Zinc oxide nano-flowers, Multi-walled carbon nanotubes, Hybrid nanogenerator, Piezoelectricity, Triboelectricity

Introduction Nanogenerators have exhibited a promising potential to efficiently harvest different forms of mechanical energies available in our daily activities such as walking, running, bending, clapping, talking, breathing, and eye blinking.1 Over the last few decades, piezoelectric nanogenerators (PENGs) and triboelectric nanogenerators (TENGs) have received significant attention for harvesting these energies. PENGs can generate an electric output power owing to the separation of negative and positive charges across piezoelectric nanomaterials under an external mechanical pressure.2 Moreover, the piezoelectric harvesting method is more advantageous, which can be applied to various places with less influence of external conditions such as humidity and temperature.3 The PENGs using ZnO, PVDF, ZnSnO3, PZT, PMN-PT, KNN, and BaTiO3 have been extensively studied.4-10 However, the ZnO has played an important role in the progress of nanogenerators with various nanowire device configurations (e.g., lateral, vertical, radial, integrated, stacked, and complex nanogenerator configurations).11 Several new routes have been developed for the synthesis of ZnO


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nanoarchitectures such as organometallic precursor process, microemulsion process, sol-gel synthesis, physical vapor deposition, solvothermal method, hydrothermal method, and chemical precipitation.12-18 Among these methods, the chemical precipitation is more convenient for solution chemical routes, is cost-effective, and has advantages of good uniformity and high yield of nanoparticles. On the other hand, TENGs can generate an electrical output power on the basis of contact electrification and electrostatic induction in between two different triboelectric materials, while they brought into contact or rubbed with each other.19,20 Compared to the PENGs, the TENGs yield a relatively high output power and thus extend the application window in powering the portable electronic devices. However, the choice of triboelectric material from triboelectric series and its surface area are more crucial to attain high output power by the TENG.21 Polydimethylsiloxane (PDMS), one of the triboelectric materials from triboelectric series, includes biocompatibility when used in high flexibility, elasticity, and wearable energy harvesters.22 Several studies have been carried out to improve the energy output performance of PDMS-based TENGs by increasing the surface area through micro/nanostructures22-28 and fluorocarbon plasma treatment.29 Another efficient way proposed to achieve this is to add carbon nanotubes (CNTs) or graphite particles to the PDMS.30,31 Nowadays, several researchers are utilizing the hybrid nanogenerators to enhance output performance by combing triboelectricity and piezoelectricity, which are used to boost the electron transfer and induced extra charges.32,33 Most of hybrid nanogenerators were fabricated with a composite film impregnated with piezoelectric nanomaterials (like nanoparticles, nanowires, nanofibers, etc.), and the surface of the composite film can be employed as a triboelectric frictional surface. In this work, we firstly synthesized flower-like ZnO nanoarchitectures by a precipitation method and added the ZnO nano-flowers into PDMS to fabricate a composite



film for a piezoelectric assisted triboelectric hybrid nanogenerator (PAT-HNG). Herein, the ZnO nano-flowers can offer a high surface area as well as the high piezoelectric polarization. In addition, along with ZnO nano-flowers, the multi-walled carbon nanotubes (MWCNTs; as a filler) were incorporated into the composite film to achieve a high-performance flexible PAT-HNG with MWCT/ZnO/PDMS composite film. The fillers play an important role in evenly distributing the ZnO nano-flowers in the composite film. Moreover, they can also provide a conduction path to collect and transfer the charges across the surface of ZnO nanoarchitectures to the Al electrodes, owing to their high conductivity. Finally, the influence of external pushing force and load resistance on the output performance of PAT-HNG device was also examined.

Experimental Procedure Materials. Zinc nitrate (Zn(NO3)2∙6H2O; Aldrich), potassium hydroxide (KOH; Aldrich), ethanol (Aldrich), multi-walled carbon nanotubes (MWCNT; >95%, diameter: 30-50 nm, US Research Nanomaterials, Inc.), Sylgard 184 (Dow Corning Co.), and aluminum tape (Al; Ducksung Hitech Co.) were used. Synthesis of ZnO nano-flowers. Flower-like ZnO nanoarchitectures were synthesized via the chemical precipitation method, which is a facile, cost-effective, low-temperature, and quick-synthesis process. Figure 1a and b depict the schematic procedure to synthesize ZnO nano-flowers. To synthesize ZnO nano-flowers, 0.11 M of zinc nitrate and 1.33 M of KOH solutions were prepared separately at a stirring speed of 300 rpm for 5 min, respectively. Afterward, these solutions were combined together and added into 100 ml of ethanol and stirred for 10 min at room temperature. After the completion of wet-chemical synthesis, the precipitate was collected by the centrifugation and then washed with ethanol. Eventually, the


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flower-like ZnO nanoarchitectures were obtained by drying the precipitate inside the oven at 35 °C for 12 h. Fabrication of ZnO/PDMS and MWCNT/ZnO/PDMS composite films. To fabricate the ZnO/PDMS composite film, 0-9.6 wt% of as-fabricated ZnO powders (which comprise the flower-like ZnO nanoarchitectures) were added into the PDMS, prepared by the mixture of base resin and curing agent (Sylgard 184, Dow Corning Co.) with a weight ratio of 10:1 (i.e., 4.5 g:0.45 g). Subsequently, the mixture of PDMS and ZnO nano-flowers was poured into the mold and subsequently cured at 35 °C for 24 h. After curing, the ZnO/PDMS composite film with the thickness of ∼800 µm and size of 2.5 × 2.5 cm2 was peeled off from the mold. Meanwhile, to fabricate the MWCNT/ZnO/PDMS composite film, the commercially purchased MWCNTs were added to the above-mentioned ZnO and PDMS mixture, and then cured at 35 °C for 24 h (Figure 1c-d). Ultimately, the PAT-HNG devices were fabricated utilizing these composite films. Design of PAT-HNG. Figure 1e depicts the schematic diagram of PAT-HNG device with MWCNT/ZnO/PDMS composite film. For the fabrication of PAT-HNG device, the ZnO/PDMS or MWCNT/ZnO/PDMS composite film was sandwiched between two aluminum (Al) substrates with the size of 2.5 × 2.5 cm2. Here, the bottom Al substrate (i.e., Al adhesive tape) was strongly attached to the composite film without leaving any gap, and it was utilized as a bottom electrode of PAT-HNG. In contrast, the top Al substrate (i.e., Al foil) was simply laminated on the composite film. The gap between the composite film and Al foil was kept constant (i.e., ∼350-450 µm) using a PDMS spacer. Herein, the top Al substrate was employed as a top electrode of PAT-HNG as well as a triboelectric material with a positive tendency. Moreover, the Teflon coated Al wires were connected to these top and bottom Al substrates and employed as electrical connections. Finally, the sandwich-type



PAT-HNG was sealed with a Kapton tape to protect the device from environmental conditions such as temperature and humidity. Figure 1f and g show the photographic images of the MWCNT/ZnO/PDMS composite film with the thickness of ∼800 µm and the PATHNG device, respectively, exhibiting their excellent flexibility. Characterizations. The surface morphology of the as-fabricated ZnO nano-flowers and ZnO/PDMS composite film were analyzed by using a field-emission scanning electron microscope (FE-SEM: LEO SUPRA 55, Carl Zeiss). Furthermore, the transmission electron microscope (TEM: JEM-2100F, JEOL) was employed to analyze the elemental composition and crystallinity of ZnO nano-flowers. The crystalline phase composition of ZnO nanoflowers as well as PDMS and ZnO/PDMS composite films was examined by using an X-ray diffractometer (XRD: M18XHF-SRA, Mac Science) with Cu Kα-radiation (λ= 0.15418 nm) in the 2θ range of 20-80°. The elemental composition and chemical states of ZnO nanoflowers were also analyzed in detail by X-ray photoelectron spectroscopy (XPS; Thermo Multi-Lab 2000). The open-circuit voltage (VOC) of the PAT-HNG device was measured by utilizing a digital phosphor oscilloscope (DPO4104, Tektronix) with an input impedance of 40 M . The output current (ISC) and charge density of the devices were measured by utilizing an electrometer (Keithley, 6514).

Results and Discussion Figure 2a and b show the low- and high-magnification SEM images of the ZnO nanoflowers. As shown in Figure 2a, the synthesized ZnO nano-flowers had a high density and they dispersed uniformly without any aggregation. From the high-magnification SEM images of ZnO nano-flowers (Figure 2b), it can be clearly noted that the size of ZnO nanoflowers was about 600 nm and each flower consisted of about 5-6 nano-horns and rugged


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surface. These features were characterized by widening the surface area. Moreover, the shape of the as-fabricated ZnO nano-flowers appeared almost identical to that of the natural Hoya flowers, as shown in Figure 2c. Furthermore, the morphology and crystalline characteristics of the ZnO nano-flowers were investigated by TEM analysis. The TEM images in Figure 2d and e clearly confirmed that the ZnO nanoparticles are of flower-like shaped architecture with about 5-6 horns. As shown in Figure 2f, the high-resolution TEM (HR-TEM) image of the ZnO nano-flower exhibited a clear lattice fringe with a d-spacing of 0.256 nm, which matches with JCPDS value (# 36-1451) and it can be related to the (002) crystalline plane of ZnO. In addition, the selected area electron diffraction (SAED) pattern of the ZnO nanoflower is shown in the inset of Figure 2f, affirming its crystalline nature. Figure 2g depicts the energy dispersive X-ray (EDAX) spectrum of three ZnO nano-flowers and the corresponding TEM image is also shown in the inset of Figure 2g. As shown in Figure 2g, the EDAX spectrum clearly reveals the existence of constituent zinc (Zn) and oxygen (O), along with C (carbon) and Cu (copper). The Cu and C elements were observed from the grid used for TEM analysis. Moreover, the distinctive elemental mapping images of Zn and O in Figure 2h and i, clearly show the uniform distribution of elements in the ZnO. In addition, the XRD and XPS of ZnO nano-flowers were also investigated (Figure S1 of the Supporting Information), and the results elucidated their elemental compositions. Finally, these ZnO nano-flowers were utilized to design the ZnO/PDMS composite film-based PAT-HNG device, and the cross-sectional view of microscopic image is also shown in Figure S2. As shown in Figure S2, the PAT-HNG device was fabricated by assembling the ZnO/PDMS composite film, Al substrates, and Kapton tape. Moreover, the microscopic image of the PAT-HNG clearly exhibited a gap between the top Al and ZnO/PDMS composite film and noticed ∼350-450 µm (i.e., ∼0.35-0.45 mm). Figure 3a and b shows the top-view SEM image of the ZnO/PDMS composite film, and it is evident that the



surface of the PDMS mixed with ZnO nano-flowers is not flat or smooth. For comparison, the SEM image of bare PDMS without any ZnO nano-flowers is also shown in Figure S3. The randomly distributed ZnO nano-flowers into the PDMS matrix could enhance further the surface roughness compared to that of the flat PDMS. Consequently, the high roughness introduced by the piezoelectric material (i.e., ZnO nano-flowers) on the surface of PDMS can be utilized to enhance the triboelectric potential of HNG, along with the piezoelectric potential.34 However, the above phenomenon is elaborately described and studied experimentally in the succeeding text (Figure 3d and g). The working principle of PAT-HNG device for generating an electrical output is schematically described in Figure 3c. As shown in Figure 3c, during the initial state (i), the electrical output of HNG device is zero owing to the random orientation of dipoles existing in the composite film and also an insufficient friction between the triboelectric materials (i.e., Al and composite films). By applying an external force (ii), the Al electrode brings into the contact with the ZnO/PDMS composite film, and transfers (or generate) the electrons onto the surface of the composite film by leaving behind the positive charges on its surface, owing to the large difference in their electron affinities. Meanwhile, the electrostatic potential difference between the electrodes can result in the electron flow from the top Al to the bottom Al electrode, while the triboelectric current (It) moves in the opposite direction.35 Furthermore, the ZnO nanoflowers in PDMS matrix can undergo compressive deformation due to the persisted external pushing force perpendicular to the device (iii). Consequently, the separation of the static ion charge centers in the flower-like ZnO can generate a piezoelectric potential gradient (or polarization) along with their surfaces. Resultantly, such piezoelectric potential gradient can induce a potential difference on the electrodes, which drives electrons through the external circuit (i.e., Ip ; current due to the piezoelectric effect).35,36 While releasing the external force (iv), the compressive strain on the ZnO nano-flowers can release, and the piezoelectric


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polarization disappears. Consequently, a reverse current (−Ip) can be observed due to the recovery of piezo-induced charges. Finally, when completely removing the force (v), the Al electrode and composite film became separate and drives the triboelectric charges from top Al electrode to the bottom, to reach an electrical equilibrium. Resultantly, an opposite triboelectric current (-It) can be observed. Thus, the continuous electrical output can generate by the PAT-HNG due to combined triboelectric and piezoelectric effect, under a continuous pushing cycles. Initially, we investigated the contribution of the triboelectric and piezoelectric nanogenerators (i.e., TENG and PENG) to the individual output in PAT-HNG. Figure 3 shows the output (d) voltage and (e) current of the device from three different modes. For that, we designed the PENG by sandwich the ZnO/PDMS composite film (added 0.6 wt% of ZnO) in between the two Al electrodes, without leaving any space (or gap). Besides, the TENG was fabricated by using the pure PDMS instead of a composite film. But, for TENG, we maintained a gap of 0.35-0.45 mm in between the Al and PDMS, same as HNG. However, the VOC/ISC values for the piezoelectric, triboelectric, and hybrid operating modes were observed as ∼12.5 V/ 0.48 µA, 22.05 V/ 1.08 µA, and 39.87 V/ 1.61 µA, respectively. In addition, the charge density values (i.e., ∼5.8, 9.7, and 18 µC.m-2) of the piezoelectric, triboelectric, and hybrid operating modes are also following the same trends (Figure S4). These results are clearly concluding that the electrical output of hybrid mode is almost equal to the sum of triboelectric and piezoelectric outputs. In other words, the output of PAT-HNG device is mainly attributed to the combination of triboelectric and piezoelectric phenomena.35 Furthermore, we investigated the effect of the concentration of ZnO nano-flowers on the output performance of the PAT-HNG device. Figure 3f and g shows open circuit voltage (VOC) and short circuit current (ISC) curves of the PAT-HNG devices fabricated with the pure PDMS and ZnO/PDMS composite films at various weight percentages of ZnO (i.e., 0.6, 1.2,



2.4, 4.8, 6.4, 8, and 9.6 wt%). During these measurements, the external pushing force and frequency were maintained constantly at 16 N and 5 Hz, respectively. The photographic images of the composite films with various wt% of ZnO from 0 to 9.6 wt% (inset of Figure 3g) revealed that the transparency of the films gradually decreases with increasing the ZnO concentration. The HNG with pure PDMS exhibits the VOC and ISC values of 22 V and 1 µA. These electrical output values of the pure PDMS-based HNG device were mainly observed due to the triboelectric electric effect only, instead of piezoelectric or combined effect. For this reason, the pure PDMS does not induce any electrical dipoles (no piezoelectric material) and thus results in zero polarization as well as no piezoelectric output power, as discussed above (Figure 3d and e). However, the pure PDMS-based HNG device exhibited a very low electrical signal owing to the friction between the top Al substrate and pure PDMS only. In contrast, the PAT-HNG devices with ZnO/PDMS composites exhibited an electrical output power due to the piezoelectric and triboelectric combined effects (Figure 3d and e). Moreover, the ZnO nano-flowers added into the PDMS matrix were utilized to enhance the surface roughness as well as the electrical polarization of composite film. Consequently, the electrical output power of PAT-HNG owing to the piezoelectric and triboelectric effects was also enhanced (Figure 3f and g). As shown in Figure 3f and g, the VOC and ISC values of the PATHNG devices were increased from ~39.87 V and ~1.61 µA to ~89.06 V and ~3.08 µA, as the concentration of ZnO nano-flowers in the PDMS matrix was increased from 0.6 to 4.8 wt%. By further increasing its concentration from 4.8 wt% to 6.4 and 8 wt%, the electrical output of PAT-HNG device remained constant or almost identical. But, while the concentration of ZnO nano-flowers was increased to 9.6 wt%, the VOC and ISC values of the PAT-HNG were slightly decreased to ~79.32 V and ~2.63 µA, respectively. These studies are clearly suggesting that the electrical dipole orientations or electrical polarizations in the composite film can be enhanced by increasing the concentration of ZnO, thus leading to a high electrical


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output power of PAT-HNG. But, at very high concentration of ZnO in the PDMS matrix, these ZnO nano-flowers can agglomerate together, which forms lumps. Thus, the external force applied on the PAT-HNG device cannot be distributed uniformly throughout the ZnO nano-flowers, and the orientations of the electrical dipoles may partially cancel each other.37 Therefore, the ZnO/PDMS composite film with 4.8 wt% of ZnO nano-flowers was obtained as an optimum to attain a high electrical output performance of PAT-HNG, and the same optimized composite film was utilized in further studies. In order to further enhance the output performance of the ZnO/PDMS composite filmbased PAT-HNG device, we also utilized MWCNTs as a filler material to fabricate the composite film. These MWCNTs play an important role in evenly distributing the ZnO nanoflowers in the composite film. In addition, the MWCNTs dispersed in the ZnO/PDMS composite film can also provide a conduction path to collect and transfer the charges across the surface of ZnO nanoarchitectures to the Al electrodes, owing to their high conductivity and surface area.38 Figure 4a and b depicts the VOC and ISC curves of the PAT-HNG devices with MWCNT/ZnO/PDMS composite films. These composite films were prepared by adding 4.8 wt% of ZnO and different amounts of MWCNTs of 0, 0.005, 0.015, 0.02, and 0.03 g into the PDMS matrix. The inset of Figure 4a shows the top-view SEM images of the MWCNT/ZnO/PDMS composite film, and it is clearly demonstrating that the composite film is composed of PDMS matrix, ZnO, and MWCNT fillers. As shown in Figure 4a, by increasing the amount of MWCNT from 0 to 0.015 g, the electrical output performance of PAT-HNG device was enhanced due to the formation of an effective conductive MWCNT network to provide a channel for the current flow. Meanwhile, further increasing the weight of MWCNT showed a negative effect. The excessive amount of MWCNTs can slightly influence the resistivity, likely because of agglomeration. However, the composite film with the 0.015 g of MWCNT exhibited the highest VOC and ISC values (i.e., 150 V and 6.5 µA,



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respectively) of PAT-HNG, in comparison with those of other composite films. Therefore, the composite film with the 4.8 wt% ZnO nano-flowers and 0.015 g of MWCNTs was noticed as an optimum to achieve a high electrical output power of PAT-HNG device. Furthermore, the influence of external pushing force on the electrical response of the optimized PAT-HNG device was also investigated. Figure 4c and d show the VOC and ISC curves of the PAT-HNG device measured at different external pushing forces from 6 to 16 N. The results indicate that the electrical output power of PAT-HNG device was increased by enhancing the mechanical pressure acting on it. The increment is mainly due to the two reasons; one is the increased contact area in between the Al and composite film and the increased deformation of the composite film under the strongest external pushing force (i.e., triboelectric

effect).

The

other

is

the

strong

electric

polarization

across

the

MWCNT/ZnO/PDMS composite film under a high magnitude of external pushing force (i.e., piezoelectric effect). Furthermore, to investigate the effect of external load resistance on the electrical output of MWCNT/ZnO/PDMS-based PAT-HNG device, its electrical output voltage, current, and power density values were examined at different external load resistances from 100 Ω to 1 GΩ, as can be seen in Figure 5a and b. These measurements were performed under the pushing force and frequency of 16 N and 5 Hz, respectively. As shown in Figure 5a, the VOC value of the PAT-HNG device was increased from 2 to 135 V, as the load resistance was increased from 105 to 108 Ω. In contrast, the ISC value followed a reverse trend. Furthermore, both the VOC and ISC values of the PAT-HNG became saturated at very high resistances (108-109 Ω), exhibiting the highest VOC/lowest ISC value of ∼ 145 V/0.35 µA, respectively. Also, the average power density values were calculated by the equation shown in the inset of Figure 5b. Here, V, I, and S denote the output voltage, output current, and active area (i.e., 2.5 × 2.5 cm2) of the PAT-HNG device, respectively. As shown in Figure 5b,


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the average power density values of HNG was enhanced by increasing the load resistance and became almost zero at very high load resistances. However, the maximum power density value of ∼ 0.26 W cm-2 was achieved at a load resistance of 2 × 107 MΩ. As a result, the output power generated by the PAT-HNG device was sufficient to drive twenty commercial LEDs connected in series, as shown in Figure 5c. For a practical and real-time application, we fixed the PAT-HNG device on a slipper (Figure S5) and the device was also attached on the table to harvest the human activities (or waste biomechanical energies) observed in daily life such as touching, walking, running, and jumping (Figure 6a-h). As shown in Figure 6a and e, an electrical output voltage/current of ∼30 V/1.3 µA was generated, when a human touches the PAT-HNG device fixed on the table. In a similar manner, while a human wears a slipper inbuilt with the PAT-HNG device and has the actions of walking, running, and jumping (Figure 6b-d), the device successfully harvested such human activates. Consequently, it generates the electrical output voltage/current values of ∼75 V/3.2 µA, ∼150 V/8 µA, and ∼400 V/30 µA, while walking, running, and jumping, respectively (Figure 6f-h). Therefore, the above results are elucidating that the proposed PAT-HNG with MWCNT/ZnO/PDMS composite film can be utilized as a wearable device as well as the self-driven physical activity sensor for detecting the human body motions24 or activities. Finally, we also investigated the influence of spacer thickness on the electrical output performance of the PAT-HNG device with the MWCNT/ZnO/PDMS composite film consisting of 4.8 wt% of ZnO and 0.015 g of MWCNT (Figure 7a and b). Figure S6 shows the photographic image of the PAT-HNG device with a spacer thickness of 2.3-2.5 mm. As shown in Figure 7a and b, the VOC/ISC values of the PAT-HNG devices were increased from ∼150 V/6.5 µA to ∼260 V/11.21 µA, as the spacer thickness was increased from 0.35-0.45 to 1.4-1.5 mm, respectively. As the spacer thickness increases, the frictional force or the



triboelectric charge accumulation in between the electrodes of the PAT-HNG device may enhance and result in the improved electrical performance. On the contrary, by further enhancing the spacer thickness to 2.4-2.5 mm, the PAT-HNG device exhibited relatively low VOC/ISC values of ∼210 V/8.2 µA. For the large spacer thickness of 2.3-2.5 mm, it is somewhat difficult to move the electrodes to each other and the contact area is also small between the electrodes. Resultantly, the electrical output performance of HNG device can decrease. Therefore, a spacer thickness of 1.4-1.5 mm was found to be optimum to attaining a high electrical output of PAT-HNG device. Table 1 shows the comparison of the electrical output performance of HNGs, used piezoelectric materials and their synthesis techniques mentioned in previous literatures.35,39-42 From the comparison, it can conclude that the fabrication of ZnO nano-flowers and HNG device is relatively facile compared to the previous reports. In addition, the electrical output observed by the PAT-HNG device is similar or superior to that of the previously reported HNG device. However, the nanogenerators can produce an alternating current (AC) signal, which is not suitable to drive low-power electronic equipment directly. So, a full-wave bridge rectifier is required to convert these AC into direct current (DC). Furthermore, a commercial capacitor needs to store the energy generated from nanogenerators and to supply a continuous output power for low-power electronic equipment. Hence, we first examined the charging process of a capacitor (2.2 µF) by the power generated from an optimized PAT-HNG device. Figure 7c shows the charging process of 2.2 µF capacitor by the rectified voltage generated from an optimized PAT-HNG device at an external pushing frequency and force of 5 Hz and 5 N, respectively. The inset of Figure 7c also depicts the equivalent circuit diagram of the PATHNG device connected to the capacitor through the rectifier. Figure 7c is clearly concluding that the rectified output power produced by the PAT-HNG is sufficient to charge the capacitor continuously. Furthermore, the electrical energy stored in capacitor was supplied to


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drive an LED for few seconds, as shown in the inset of Figure 7c. Therefore, the above results are clearly indicating that the proposed PAT-HNG device can be utilized as a power supply for low-power electronic devices.

Conclusions In summary, we have developed a facile, cost-effective, and flexible MWCNT/ZnO/PDMS composite film-based PAT-HNG to harvest the mechanical energy utilizing the combined piezoelectric and triboelectric effects. The ZnO nano-flowers impregnated into the composite film can act as a piezoelectric material and be also utilized to enhance the surface roughness of PDMS as well as the triboelectric performance. The PAT-HNG device with the 4.8 wt% of ZnO and 0.015 g of MWCNT exhibited the highest electrical output power with the VOC, ISC, and power density values of ∼ 140.81 V, 6.10 µA, and 0.26 W Cm-2, respectively, under an external pushing force and frequency of 16 N and 5Hz. The output power generated by the PAT-HNG device successfully lit up twenty commercial LEDs connected in series. The practical application of PAT-HNG was also verified by fixing it onto the slipper and the electrical output performances by various human activities were observed, verifying that the proposed device can efficiently harvest these energies. These results are clearly concluding that the PAT-HNG can be applied as a self-powered device to power wireless portable electronic devices.

Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: xxxxxxxxx.



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XRD and XPS spectra ZnO nano-flowers, microscopic image of PAT-HNG, SEM image of pure PDMS, charge density of the device in three testing modes, photographic images of the PAT-HNG device fixed onto the slipper and with the spacer thickness of ∼2.5 mm.

Corresponding Author *E-mail: [email protected] (Prof. J. S. Yu)

Acknowledgments This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIP) (2017R1A2B4011998 and 2017H1D8A2031138).

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Table Table 1. Electrical output performance of hybrid nanogenerators and synthesis techniques of active materials used in previous literatures,35,39-42 compared with our work.

Ref. No.

Device structure

[39]

3D-fiber shaped

[40]

Tandem/rectangle

[41]

D-shaped

[42]

Rectangle

Type of integration

Working mechanism

Active materials

Synthesis techniques

Max. voltage & current

external

Triboelectric

ZnO nanorods; PDMS; nylon film

Sputtering and hydrothermal method

NA(not available) /300 nA

ZnSnO3 nanocubes; PDMS; Al

Hydrothermal method

PVDF; silicon rubber; gold; Al

-NA-

BTO NPs; PDMS; Al

-NAPurchased

60 V/1 µA

BTO; PDMS; Copper

-NA(Purchased)

55 V/-NA-

ZnO nanoflowers; PDMS; Al

Chemical precipitation method

260 V/11.21 µA

Piezoelectric

internal

Triboelectric Piezoelectric

external


internal


[35]

Our work

Cantilever resonator

internal

Piezoelectric internal

Tandem/rectangular

Triboelectric



300 V/16 µA

25.8 V/8.82 µA


Figure 1. Schematic diagram for the (a,b) synthesis process of ZnO flowers-like nanoarchitectures by a chemical precipitation method, and (c-e) fabrication steps to design the MWCNTs/ZnO/PDMS composite film-based PAT-HNG. Photographic images of the flexible (f) MWCNTs/ZnO/PDMS composite film and (g) PAT-HNG device with an active area of 2.5 × 2.5 cm2.


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(a)

(b)

(c)

100 nm

300 nm

(d)

Natural Hoya flower

(f)

(e)

2 nm

100 nm

500 nm Zn

(i)

(h)

(g) Zn

O

Zn 300 nm

Zinc

300 nm

Oxygen

Figure 2. (a) Low- and (b) high-magnification FE-SEM images of the flower-like ZnO nanoarchitectures. (c) Photographic image of the natural Hoya flower. (d-e) TEM and (f) HRTEM images of the ZnO nano-flowers. The inset of (f) shows the SAED pattern of a ZnO nanoflower. (g) EDAX spectrum and (h) elemental mapping images of the Zn and O for three ZnO nano-flowers.



Figure 3. (a) Low and (b) high-magnified SEM images of the ZnO/PDMS composite film. (c) Schematic illustration of the working mechanism of the PAT-HNG device. Output (d) voltage (e) current of the device under piezoelectric, triboelectric, and hybrid modes. Measured (f) VOC and (g) ISC curves of the PAT-HNG device at various wt% of ZnO. The inset of (f) depicts the enlarged view of the output voltage of PAT-HNG without any ZnO and the insets of (g) show the digital photographic images of the composite films with different wt% of ZnO nano-flowers.


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Figure 4. (a) VOC and (b) ISC curves of the MWCNT/ZnO/PDMS composite film-based PATHNG device with different amounts of MWCNTs. The inset of (a) depicts the low- and highmagnified view SEM images of the MWCNT/ZnO/PDMS composite film. Measured (c) VOC and (d) ISC curves of the PAT-HNG device under different external pushing forces from 6 to 16 N at a 5 Hz external pushing frequency.



(b)0.30 5

-2

140 120

4

Voltage (V)

100 80

3

Voltage Current

60

2

40 1

20 0 Pushing force & frequency: 16N & 5Hz

-20 102

Power Density (W.Cm )

(a) 160

10

3

10

4

10

5

10

6

10

7

108

10

Current (µA)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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0.25 0.20 0.15 0.10 0.05

0 9

0.00 102

Load Resistance (Ω)

Pushing force & frequency: 16N & 5Hz

10

3

10

4

10

5

106

107

108

109

Load Resistance (Ω)

(c)

Before applying a pushing force

After applying a pushing force

Figure 5. Effect of external load resistance on the output (a) voltage/current and (b) power density values of the MWCT/ZnO/PDMS composite film-based PAT-HNG at an external pushing force and frequency of 16 N and 5 Hz, respectively. (c) Photographic images of the 20 LEDs connected in series, before and after applying a pushing force on the PAT-HNG.


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Figure 6. Demonstration for the practical or real-time applications of the PAT-HNG device to harvest human activities (or waste biomechanical energies) observed in daily life such as (a) touching, (b) walking, (c) running, and (d) jumping, respectively. (e-h) Electrical output voltage and current curves of the PAT-HNG device under the corresponding human activity.



Figure 7. Effect of spacer thickness of PAT-HNG device on the electrical output (a) voltage and (b) current. (c) Charging process of a 2.2 µF capacitor connected through a full-wave rectifier to the PAT-HNG device. The inset shows that the equivalent circuit diagram illustrates the charge process of capacitor by the PAT-HNG device, followed by powering a LED.


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Table of Contents (TOC) Synopsis: PDMS encapsulated nanoflowers-like ZnO composite film was employed to fabricate a flexible hybrid nanogenerator for harvesting daily human activities.


High-performance Flexible Piezoelectric Assisted Triboelectric Hybrid

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