Design and Development of ZnO Nanosheet based Nanogenerators to Drive Self-Powered Systems

Abstract

The never-ending quest for sustainable energy sources has led to the development of novel technology capable of harvesting ambient mechanical energy. Among these, piezoelectric and triboelectric nanogenerators have emerged as attractive opportunities for energy harvesting that are both efficient and environmentally beneficial. The investigation opens with a detailed examination of piezoelectric nanogenerators, clarifying the piezoelectric phenomenon and its application in transforming mechanical vibrations into electrical energy. Various materials are investigated for their applicability in improving the performance of piezoelectric nanogenerators. Following that, the thesis delves into triboelectric nanogenerators, which use the triboelectric effect to generate electricity through material friction. The detailed mechanism of the TENG that drives triboelectric charge generation is investigated, with a focus on the wide range of materials used, such as polymers, metals, and nanostructures. Innovative fabrication processes and surface engineering methodologies are being investigated to improve the output performance, scalability, and adaptability of the triboelectric nanogenerator. The synthesis process involves the direct growth of ZnO nanosheets on an aluminum substrate, and their structural, morphological, compositional, and surface potential analyses are conducted to gain insights into the material's properties. Further, discusses a detailed analysis of the open circuit voltage, short circuit current, and durability of the ZnO nanosheet-based nanogenerators (PENG/TENG). The devices exhibit promising results, showcasing their potential for efficient energy harvesting. Notably, the study explores various applications, demonstrating the device’s ability to charge capacitors and power small electronic devices such as light-emitting diodes (LEDs), digital thermometers, digital watches, etc, owing to the adaptability of PENGs and TENGs used in real-world applications, proving their adaptability in multiple domains. Chapter 1 provides an introduction to the field of energy harvesting and its diverse energy sources. It briefly outlines the various types of nanogenerators and delves into strategies and methods to improve the output performance of TENGs. Additionally, this chapter explores the selection of materials and potential applications for the PENG and TENG devices. Chapter 2 the process of synthesizing zinc oxide nanosheets (ZnO) using the hot plate-assisted hydrothermal method is detailed. This chapter discusses material characterization and justifies the usage of specialized approaches of the synthesized material, utilizing advanced tools for understanding its structural, morphological, compositional, and surface detail. The chapter also (vi) provides insights into the steps involved in creating piezoelectric and triboelectric nanogenerator devices for mechanical energy harvesting. Furthermore, it elucidates the techniques employed for characterizing the devices characterizing devices and using instrumentation to assess their properties. Chapter 3 deals with the synthesis of ZnO nanosheets, covering their characterization, results, and a detailed discussion. The chapter's focus includes optimizing the growth temperature and time for the synthesis of ZnO nanosheets. The study encompassed a range of temperatures from 50°C to 90°C, with a consistent growth duration of 4 hrs. Additionally, at a fixed growth temperature of 85°C, the growth time was varied from 1 hour to 6 hours. Surface morphology, crystalline properties, and microstructure analysis of the ZnO nanosheets were conducted using SEM, XRD, and TEM techniques. TEM revealed the 2D structure of the ZnO nanosheets, while EDAX confirmed their purity. Furthermore, PENG device was fabricated using the ZnO nanosheet film as the active piezoelectric layer, and ITO and aluminum served as electrodes. The chapter includes a comprehensive exploration of the device characterizations. Multiple PENG devices were tested, each employing ZnO nanosheets synthesized at varying growth temperatures and times. The growth conditions of 85°C for 4 hrs were optimized for the PENG device. The chapter also explores the switching polarity test and load characteristics of the PENG device. Notably, the maximum output voltage and power density for the PENG were recorded at 225 mV and 77 µW/m², respectively. The response of the PENG to different finger tapping forces was studied, demonstrating that increased pressure and deformation of the nanosheets resulted in higher output. This finding suggests potential applications in pressure and force sensing. Additionally, the chapter discusses the fabrication of a Triboelectric Nanogenerator (TENG) device utilizing ZnO nanosheets as the tribo-layer, ITO, and aluminum as electrodes. Under hand tapping force, the TENG yielded an output voltage of 2 V, a current of 7 µA, and a power density of 0.1 mW/m². The TENG was also used to charge various capacitors, achieving a maximum stored energy of 2.8 µJ. Notably, the TENG was capable of directly powering one LED under each hand-tapping force. Chapter 4 delves into the synthesis and the characteristic properties of ZnO nanosheets and PET (Polyethylene Terephthalate). X-ray Diffraction (XRD) analysis confirmed the polycrystalline nature of ZnO nanosheets, and their distinctive nanosheet structure was verified through SEM analysis. A notable achievement discussed in this chapter is the fabrication of a simple and cost-effective TENG using ZnO nanosheet film and PET as triboelectric layers for (vii) the first time. This TENG device demonstrated an impressive output, with an output voltage of 4.9 V, a current of 10 µA, and a power density of 1 mW/m². The chapter also explored the TENG's output response at different frequencies of hand tapping, ranging from 1 to 7 Hz, and varied the active areas of the devices for comprehensive study. The TENG's stability was rigorously tested, more than 1000 cycles, demonstrating its durability and reliability over numerous cycles. Furthermore, the chapter delved into the charging characteristics of various capacitors and examined stored voltage, charge, and energy concerning the load capacitor. The TENG achieved a maximum stored charge of 40 µC and stored energy of 16.9 µJ. Further, The TENG successfully powered three LEDs directly, and the stored energy in the capacitor enabled the activation of a digital watch and 24 LEDs. This underscores its potential as a promising solution for self-powered sensors and devices. Chapter 5 focuses shifts to the utilization of ZnO nanosheets in conjunction with Overhead Projector (OHP) sheets for TENG devices in various applications. An innovative addition to this chapter is the introduction of OHP sheets as a novel triboelectric layer. OHP sheets, composed of transparent PET material, possess pre-charged properties on one side, making them efficient at receiving ink particles. The existence of charge on the OHP sheets reduces the surface ion injection step. Comprehensive morphological, elemental, and structural analyses were conducted using SEM, EDX, and XRD techniques. Additionally, Atomic Force Microscopy (AFM) confirmed that the ZnO nanosheet film exhibited greater roughness compared to the OHP sheet. This rougher surface configuration forms a more effective contact area, contributing to the generation of high output power in the TENG. A high-performance TENG device was fabricated, leveraging ZnO nanosheets and OHP sheets as frictional layers. The TENG device exhibited notable characteristics, with an output voltage of 150 V, a current of 34.5 µA, and an impressive power density of 424 mW/m2. The stability of the TENG was rigorously tested, demonstrating its robust performance with over 4,300 cycles under hand tapping and 10,000 cycles under machine tapping. Moreover, the effects of different active areas of TENG devices, varying frequencies of applied forces, and different magnitudes of applied force were thoroughly investigated. Notably, the TENG device achieved remarkable results, with a maximum stored voltage of up to 9 V, a stored charge of 150 µC, and a stored energy of 160 µJ, observed under the optimal load capacitance of 47 µF. In a practical demonstration of its capabilities, the TENG directly powered 135 LEDs and a digital watch when subjected to hand-tapping forces. Additionally, the TENG was employed to power an electroluminescence device, which finds potential applications in e-paper and (viii) digital billboard technologies. Leveraging the TENG's high-output performance, it was integrated with Cholesteric liquid crystal (CLC) devices, opening the door to a range of real time applications. These applications include secure authentication devices, such as facial unlocking in mobile phones and QR code scanning. Furthermore, the TENG was used to drive smart windows and CLC displays, which capitalize on the CLC's ability to control transparency in the planar state and opacity in the focal conic state. This integration allowed for the display of letters and various information on CLC devices, expanding their utility. Chapter 6 A key innovation in this chapter was the surface modification of ZnO nanosheets was introduced to enhance the output of the TENG. We have synthesized ZnO nanosheets on aluminum substrate with any seed layer using hotplate assisted hydrothermal method. The surface of ZnO nanosheets was modified by the direct growth of Zeolitic imidazolate framework-8 (ZIF-8) crystals using the hydrothermal method. ZIF-8, belonging to the Metal Organic Framework (MOF) family, consists of metal ions (Zn2+) and organic linkers (imidazolate). Structural and morphological analyses of both ZnO and ZnO modified with ZIF 8 (ZnO@ZIF-8) were conducted. XRD and Raman spectra confirmed the successful formation of ZIF-8 on ZnO nanosheets. The TENG device employing ZnO@ZIF-8 and PMMA as triboelectric layers exhibited significant improvements, achieving an output voltage of 200 V, a current of 41.5 µA, and a substantial power density of 800 mW/m². This performance represented nearly a twofold increase compared to ZnO-based TENGs. To validate this enhancement, the work function of ZnO and ZnO@ZIF-8 films was studied. The work function difference was notably greater for ZnO@ZIF-8 and PMMA compared to ZnO and PMMA. A larger work function difference enhances electron transfer, contributing to the enhanced output performance of the TENG. Additionally, the chapter explored the characteristics of charging capacitors under hand-tapping forces, examined the TENG's stability over an extended duration, and tested it under machine tapping for approximately 20 minutes, equivalent 4800 cycles. Notably, the ZnO@ZIF-8 TENG demonstrated its ability to directly power various electronic devices, including an electronic watch, calculator, and thermometer. Impressively, it directly powered 180 LEDs without any storage element, highlighting its potential for real time self-powered devices and biomechanical energy harvesting applications. Chapter 7 discusses a high-performance TENG is achieved through a surface engineering method involving emery papers to modify the surface of one of the triboelectric layers. This innovative approach offers several advantages, notably its cost-effectiveness and simplicity. The methodology involves scratching the aluminum substrate surface with different grit sizes (ix) of emery papers, ranging from 80 to 400, in one direction multiple times, followed by the incorporation of ZnO nanosheets using a hydrothermal method. This process not only increases the surface area but also enhances contact points, resulting in improved output. Under an optical microscope, all the samples were examined, and it was observed that the use of P220 grit-size emery paper created optimum roughness and provided more contact points. SEM analysis of the ZnO nanosheets confirmed that the density of the nanosheets increased after the modification of the aluminum substrate, while PDMS exhibited a smoother surface. The work function of various samples was measured, and it was found that ZnO nanosheets incorporated on scratched aluminum with P220 grit size (ZnO-Al220) had a lower work function. Additionally, the work function difference was higher between PDMS and ZnO-Al220(T12). This TENG exhibited the potential for the highest output due to its enhanced electron transfer between the tribo-layers. Multiple TENG devices were fabricated and tested for their output responses, with the T12 TENG producing the highest output voltage, aligning with the work function measurements. The T12 TENG generated the highest output voltage, current, and power density, reaching 1442 V, 155 µA, and 10.8 W/m², respectively. To gain insights into the electrical potential distribution and charge transfer process of the TENG device, finite element simulations were conducted using COMSOL 6.0 software. The stability of the TENG was thoroughly tested through repeated applied forces over 13,000 cycles at a frequency of 3-4 Hz. The chapter also explored various experimental parameters affecting the TENG device (T12), including the active area, frequency of hand-tapping forces, spacing between triboelectric layers, and application of different hand-tapping forces. Impressively, the TENG powered 820 LEDs and activated various electronic gadgets. The TENG device's high sensitivity was further harnessed by attaching it to the human body's chest to monitor output voltage and current signals generated by different breathing patterns. This allowed for the detection of various respiration rates, including normal, deep, and rapid breathing. The TENG device holds significant promise for convenient and accurate breath monitoring, potentially impacting health management and optimizing athletic performance. Chapter 8 describes the general summary, conclusions, and scope for future work