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ZnO-Polystyrene Composite as Efficient Energy Harvest for Self-Powered Triboelectric Nanogenerator

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Published 26 August 2020 © 2020 The Author(s). Published on behalf of The Electrochemical Society by IOP Publishing Limited
, , Citation Akhilesh Kumar Gupta et al 2020 ECS J. Solid State Sci. Technol. 9 115019 DOI 10.1149/2162-8777/aba7fa

2162-8777/9/11/115019

Abstract

Energy harvest systems are a scientific key and an economic driver for global industries in the near future with applications in health care, environmental monitoring, and more. Among them, the solution-processed Triboelectric Nanogenerator (TENG) substrate has fascinated important attention in the past decades and increasingly becomes the most suitable and promising prototype for healthcare/environmental protection as no battery is needed to power the devices. In this paper, we proposed a solution-processed ZnO-NR & ZnO-Polystyrene (ZnO-PS) composite for the development of the TENG model for the future self-powered medical device applications. Morphology of nanostructure shown via FE-SEM images, an improvement of ZnO-PS composite NRs due to the diffusion of polystyrene in ZnO-NR at higher seeding temperature. Furthermore, samples were characterized and analyzed by Raman spectrum & UV-visible absorption, which verify the improvement of the morphology. The power density of the ZnO-PS composite (2.30 × 10−4 W m−2) was 71% higher than that of the TENG with ZnO Nanorods (1.65 × 10−4 W m−2) due to the surface improvement. Thus, we present a new perspective of the ZnO-PS composite TENGs model for developing a new technology which is vitally important in the future application for self-powered healthcare monitors.

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Nowadays, energy is one of the most significant resources that command the quality of our life. However, electronics are typically linked with human activities for the purpose of health, safety, and communication.1 Numerous novel methodologies have been tried to harvest mechanical energy, such as vibration energy to produce electrical energy by utilizing piezoelectric,2 thermoelectric,3 and contact electrification.4 Among them, triboelectric Nanogenerator (TENGs) shows the potential of higher power density, in which the choice of contact material plays an important role. The first step is to choose high-performance TENGs material for appropriate frictional contact and pairs in the triboelectric series.5 Moreover, the fast progress of these kinds of energy-based material with low-cost, high stability, and high efficiency are urgently required. Recently, work has also been carried out on TENGs based 2D materials such as graphene6 and MoS27 monolayers to increase the potential for power generation. However, TENG's importance and capacity for various devices have always been essential to propose new methods for improving the efficiency of TENG-based systems, and different directions are taken into account to achieve this goal. The use of zinc oxide (ZnO) as an optical semiconductor and piezo material is superior in its wide direct bandgap (3.7 ev) and high exciton binding energy (60 meV).8 Previously, ZnO also began its use in TENG and self-power sensors due to the rapid growth of triboelectric technology4,9,10 as well as piezoelectric nano-generator.11 On the other side, polymer composite nanostructure-based piezoelectric & triboelectric nanogenerator plays an important role in producing voltage and current differences. TENG research has worked as a sensor and has also been shown to improve its usage and energy collection performance based on a three-dimensional nanostructure. Recently, our group published Au@ZnO-NR based PC-SET plasmonic model in the form of light-harvesting optical devices for optoelectronic applications.12 Further, the researcher reported metal and semiconductor-based TENG model as Photo-stimulated charge transfer in contact electrification coupled with plasmonic excitations.13 Other groups have reported composite PVDF-TrFE/ZrO2 NPs as a negative friction surface and Nylon-11/PMMA as a positive layer. Later by exploiting the surface charge density of the composite structures will subsequently improve by proper polarization of the electric dipoles inside.14 Further, the researcher also reported the triboelectric effect of ZnO Nanorods/PAN in a flexible Nanogenerator by adding TiO2 nanoparticle and discuss the improvement of output performance.15 Some other groups also reported, ZnO-PDF, human skin-PDMS, and FF-Kapton composites for various kind of self-powered as PH sensor, smart skin sensor, and hybrid piezo-triboelectric NGs.1618 Also, microsphere lithography is the most promising because of the self-assembly property of 2D colloidal microspheres. A template in form of vacant spaces could be made utilizing a close-packed structure of polystyrene (PS) nanosphere, where particular materials can be added to the template using several approaches such as physical adsorption, electro-deposition, hydrothermal and chemical bath deposition.19 Moreover, morphology can be controlled by varying the self-assembly conditions such as temperature, solvent composition, and humidity with composite materials. We are the first group to provide comprehensive relation between ZnO-NR and ZnO-PS composite nanostructure via thermal treatment over the surface for the development of a triboelectric nanogenerator as self-powered medical devices in the future. Further studies on flexible matrix integration and optimization of piezoelectric and triboelectric material composition will be done in order to enhance the system output and improve the economic efficiency of the industry. Finally, in view of imminent advances and harvesting applications, potential technological trends evolving product architectures and new materials will be studied.

In this study, a simple method, low cost, and highly enhanced power density of ZnO & ZnO-PS composite nanostructure via the TENG effect were introduced. However, the piezoelectric property cannot be investigated due to a non-flexible substrate. We compared surface charge density as well as the power density of ZnO-NR & ZnO-PS as a positive layer and Kapton/Cu as a negative friction layer. We found that the power density of ZnO-PS composite increases because of polystyrene diffuse in ZnO at 300 °C seeding temperature to fulfill the oxygen vacancy over the surface as well as the improved morphology of composite nanostructure. The improvement of the ZnO-PS composite morphology was proven effective by thermal annealing treatment. In this manuscript, we present a new perception of the ZnO-PS composite TENG model to generate energy to be used as power sources for electronic devices in the future.

Experimental

Materials

Zinc acetate dihydrate (Zn (OAc)2. 2H2O) and microparticle size polystyrene (diameter = 500 nm) were acquired from Sigma Aldrich (St. Louis, USA). Hexamethylenetetramine (HMTA) [(CH2)6N4] and Dodecyl sodium sulfate were purchased from Merck (Darmstadt, Germany). Acetone and isopropyl alcohol (IPA) were acquired from Avantor (Pennsylvania, USA). ITO on a glass substrate (sheet resistance; 7Ω/square) was purchased from Uni-Onward (Taipei, Taiwan). The experimental solutions were prepared using deionized water (resistivity of 18.2 M Ωcm at 25 °C) generated by a MilliQ system by Merck (Darmstadt, Germany).

Methods

ZnO-NR was grown on an indium-tin-oxide (ITO)-coated glass substrate followed by the ZnO seed layer solution based low-cost hydrothermal method.8 For the preparation of the ZnO-PS composite, O2 plasma treatment was done to the ITO substrate for 30 min to make it a more hydrophilic nature. Firstly, a solution of 2% wt. PS was made in DI water. Then, 10 μl of mixed solution poured onto the ITO upper surface which dipped in ultrapure water. Further, 5 μl of 2% dodecyl sodium sulfate solution was applied to adjust the water surface tension, pushing the monolayer of PS nanospheres toward the targeted ITO substrate. Thus, the pickup of the sample by gentle and smooth tilting with a tweezer which shows a single layer of 90% PS attached to the ITO substrate, which was verified by FE-SEM images. To reduce the PS nanosphere clustering, the substrate has then been sequentially dried at room temperature, in an 80 °C incubator, and on a hot plate of 100 °C for 30 min and 2 min, respectively.20 Further, we poured zinc acetate precursor solution of the same concentration (ZnO-NR growth conc.) over PS/ITO substrate to cover all PS nanosphere. Then, we used thermal annealing on the zinc acetate precursor based PS/ITO at 300 °C for 30 min to obtain the composite seeding template of ZnO-PS. Further, the NRs were grown via the hydrothermal process at 95 °C for 8 h. The whole process of device fabrication is described in Fig. 1. Further, ZnO-NR & ZnO-PS composite NR samples were characterized by Raman-spectrum and UV-visible apparatus.

Figure 1.

Figure 1. Schematic diagram and stepwise fabrication of the ZnO-PS composite TENG device (ZnO-NR as a control sample).

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Apparatus

Surface morphological images and elemental analysis were taken from FE-SEM combined with energy-dispersive spectroscopy (EDS) (FE-SEM, JEOL JSM-7500F). Raman analysis was done with 473 nm laser by UniDRON Co. Ltd. (New Taipei City, Taiwan). Oxygen plasma treatment was done from Harrick Plasma (New York, USA). The output short-circuits current and open-circuit voltage of the TENG were measured by linear motor contact separation mode using a low-noise current preamplifier (Stanford Research System Model SR570) and a Keithley 6514 programmable electrometer.

Results and Discussion

Surface morphology and EDX measurement

Figs. 2A, 2B shows the FE-SEM images of the uniform PS nano ball onto the bare ITO, the ring template was created after 300 °C seeding temperature over ZnO-PS/ITO substrate. In Figs. 2C, 2D, the presence of carbon and oxygen content over the ZnO-PS surface was confirmed by the EDX measurement. An increasing trend of the atomic content of carbon and oxygen was showed after the pour of Zinc acetate precursor over the surface. Atomic content of carbon decreased with a percentage of 66.37% to 5.27% after deposition of zinc precursor solution and the atomic content of oxygen increases by 17.85% to 67.51%. Moreover, after hydrothermal growth of both samples, the morphology of ZnO & ZnO-PS composite NRs changes concerning size and density due to polystyrene diffuse in ZnO to fulfill oxygen vacancy and reduce defects over its surface as shown in Figs. 2E, 2F. Also, polymer matrix acting as a surface passivator to fill the defects in the ZnO-NR surface.21 The density of NRs increases with and without polystyrene diffusion at 300 °C seeding temperature as shown in Fig. 2F inset.

Figure 2.

Figure 2. FE-SEM images (A) PS/ITO arrays; (B) Zinc acetate precursor over PS/ITO and annealed at 300 °C temperature; (C)–(D) EDX of PS/ITO and zinc acetate precursor over PS/ITO (atomic percentage of oxygen increases due to partial fulfill of oxygen vacancy via diffusion over ZnO surface); (E) ZnO-NR grown via the hydrothermal method (sample used 300 °C seeding temperature); (F) ZnO-PS NRs grown via the hydrothermal method (sample used 300 °C seeding temperature) [inset: compare of NRs density].

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Characterization of ZnO & ZnO-PS composite

Raman analysis was performed using Raman micro spectrometers. Spectra peaks of polystyrene were present at 1002 cm−1 and 1030 cm−1, which corresponds to aromatic breathing and C–H bending. Moreover, peaks at 1602 cm−1 and 620 cm−1 assigned as C=C aromatic ring stretching and ring deformation respectively. Further, the ZnO-NR peak shows 437 cm−1 assigned as E2 high modes and 580 cm−1assigned to LO mode attributed to oxygen deficiency defects in ZnO. In this case, peaks at 1150 cm−1 appeared due to the multiple-phonon scattering process.22 After compositing with PS, a strong peak generated at 437 cm−1 attributes to improve the morphology of hexagonal ZnO-PS composite NRs. Figure 3B shows UV-Visible absorbance in which ZnO-blank sample exhibited an exciton absorption peak located at 373 nm, corresponding to the bandgap of ZnO (3.3 eV). The slight changes in the absorbance after the incorporation of the ZnO with the polymer films show that more photons were efficiently absorbed by the doped polystyrene.23

Figure 3.

Figure 3. (A) Raman analysis of PS, ZnO-NR, and ZnO-PS composite; (B) UV-Visible absorbance of PS, ZnO-NR, and ZnO-PS composite NRs.

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TENG mechanism

The schematic diagram of the TENG-based on ZnO & ZnO-PS composite is shown in Fig. 4A, its operation is based on the contact-separation of the positive and negative triboelectric layer. The TENG fundamental structure composed of the ZnO & ZnO-PS, which functions as a triboelectric positive layer on the ITO (∼4.5 eV) and sandwiched between a new negative triboelectric strand (the Kapton on the Cu electrode (∼4.65 eV)).24 The conduction between the positive and the negative layer occurred due to the negative layer Kapton/Cu structure was forced onto the positive ZnO-PS layer. Here, we used a sample with a size 2.5 × 2.5 cm and a 30 mm gap between both layers together with a 1-second time cycle of contact and separation mode. The generated charges occurred due to contact between both the triboelectric layer and charges move to conduct electrodes due to the separation of triboelectric electrodes as shown in Figs. 4C–4D.25

Figure 4.

Figure 4. (A) Measurement setup for positive and negative triboelectric layers; (B) ZnO-NR TENG mechanism; (C)–(D) ZnO-PS composite mechanism with Kapton and direction of flow of current.

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Output voltage and current

The output performance of ZnO-NR and ZnO-PS composite NRs based TENG in the form of voltage and current as shown in Fig. 5. The output voltage of the ZnO-PS TENG was (∼7 V) higher than that of the ZnO TENG without the PS nanoball (ZnO ∼ 5.8 V) as shown in Figs. 5A & 5C. In other words, ZnO-PS composite TENG shows having a higher triboelectric power generation ability. Under a short-circuit condition, a higher output current of the ZnO-PS layer was obtained (∼9.9 × 10−8 A) than ZnO-NR (∼8 × 10−8A) as shown in Figs. 5B & 5D. However, the enhancement performance of the TENGs with the ZnO-PS layer could be endorsed to an increase in the static electricity on the surface due to the existence of the PS nanoball, yielding electricity increment that generated by the TENGs system.26,27

Figure 5.

Figure 5. (A)–(B) Output voltage & current of ZnO-NR; (C)–(D) Output voltage & current of ZnO-PS composite NRs (comparison of output voltage and current).

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Tribo-charge density and power density

The values of the tribo-charge density (σ) of the ZnO-NR & ZnO-PS NRs based TENGs system were determined from their output voltage values. The following equation gives the relationship between Vout and σ for TENGs.28

Equation (1)

where d is the distance between two tribo-electrodes and ε0 is the electric constant. With the increase in voltage output, the tribo-charge density can be increased linearly and the distance between the two electrodes used in the experiment was 30 mm. TENGs based on ZnO and ZnO-PS composite NRs had a maximum tribo-charge density of 8.19 × 10−4 μC m−2 and 9.57 × 10−4 μC m−2 that corresponds to triboelectric charge over surfaces by TENG operation without transfer of charges as shown in Fig. 6A. We can see that ZnO-PS NRs had the most efficient performance of TENG with a ZnO-NRs. Moreover, the maximum current values of the ZnO & ZnO-PS based TENGs were ∼8 × 10−8A and ∼9.9 × 10−8A, respectively, due to charges were transferred in short circuit condition from one electrode to another electrode. The power densities of ZnO and ZnO-PS composite TENGs were calculated based on the maximum values of the output voltage & current in the positive side of the output as ∼1.65 × 10−4 W m−2 and ∼2.30 × 10−4 W m−2 as shown in Fig. 6B. From the above results, we conclude that ZnO-PS is an efficient energy harvest material that has a 71% higher power density than ZnO-NR. To evaluate the reproducibility, we consider the voltage in positive side potential. From the reproducibility study as illustrated in Fig. 6C, it is proven that our proposed structure has high reproducibility and stability as indicated by the coefficient of variation (CV); ZnO = 1.2% & ZnO-PS = 2% for three different substrates. While the error bar on the graph in Fig. 6C describes the measurement repeatability of each sample. In the future, this kind of efficient power can be used as self-powered devices as well as energy-harvesting applications as shown in Fig. 6D. Thus, TENGs based on ZnO-NR & composites provide a great opportunity for grasping large-scale applications in the future as self-powered electronic sensor devices. A comparison table of the output performance of different NGs can be shown in Table I.

Figure 6.

Figure 6. (A) Tribo charge density of ZnO-NR & ZnO-PS composite NRs; (B) Power densities for the contact-separation mode of the triboelectric Nanogenerator based on ZnO-NR & ZnO-PS composite NRs; (C) Reproducibility study; (D) Power density utilized as a power source in near future for self-powered medical devices.

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Table I.  Comparison table of the output performance of different NGs.

S.N. Materials/Substrate Electrode Output voltage Output current Nanogenerator References
1. ZnO- PVDF composite/PDMS Au 0.33 V 62 nA Piezoelectric 16
2. ZnO NRs/Paper Ag 10 mV 10 nA Piezoelectric 11
3. Human skin and PDMS/PET Ag-NW 5 V Triboelectric 17
4. FF/Kapton/PET Al-foil 1.55 V 23 nA Triboelectric 18
5. Sb doped ZnO-NR/PDMS Au 1.7 V 2.2 nA Triboelectric 10
6. ZnO-NR & ZnO-PS composite/ITO Cu 7 V 0.99 nA Triboelectric Present work

Conclusions

In summary, this study reports the ZnO-NR & ZnO-PS composite nanostructure as positive and Kapton as the negative layer of contact electrification to generate efficient energy harvester-based TENG. We found that ZnO-PS composite nanostructure produced a more enhanced surface charge due to modified work function by (PS) polymer diffusion via thermal annealing. Improved morphology of composite NRs was verified by FE-SEM & EDX measurement. The output of surface charge density increased as well as the power density of the ZnO-PS composite which was 71% higher than ZnO-NR. This study provides an important insight into the ZnO & ZnO-PS composite and concept related to contact-electrification properties of materials so that in future this kind of polymer composite ZnO devices act as self-powered devices and can be optimized for future application. Thus, TENGs based on ZnO-NR & composites should provide a pronounced opportunity for large-scale applications and future applications in self-powered devices.

Acknowledgments

This research is financially supported by the Ministry of Science and Technology, Taiwan under a project number of MOST 108-2218-E-182-002,109-2221-E-182-013-MY3 and Chang Gung Memorial Hospital Research Project under a grant number of CMRPD2K0051 and CORPD2J0071.

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