(A) study on biomechanical energy harvesting capability of ZnO-based composite materials

Abstract

This dissertation presents the studies on zinc oxide (ZnO)-based piezoelectric nanogenerators such as the highly cost-effective design for an aluminum (Al) foil-based ZnO/Ag/ZnO-stacked piezoelectric nanogenerator (ZAZ-NG), a symmetrically stacked highly durable and energy-efficient sandwich-type ZnO/carbon tape/ZnO piezoelectric nanogenerator (ZCZ-NG), and an eco-friendly stretchable flexoelectricity-enhanced piezoelectric nanogenerator (F-PENG) based on zinc-aluminum layered double hydroxide nanosheets (ZnAl:LDH Ns)-ZnO heterostructure. Both Al foil sheets and a silver (Ag) paste layer were utilized to make a ZAZ-NG composed of an Ag paste layer sandwiched between two ZnO layers. The output voltages of the ZAZ-NGs with various ZnO thicknesses are measured for three different bending strains. As a result, the devices could generate a relatively high peak-to-peak output voltage (V$_{pp}$) of up to 2.5 V, which is 28 times higher than that of the single ZnO layered device. In addition, the device performance shows a strong dependence on the thickness of the ZnO layer. Moreover, the ZAZ-NG is structurally stable and can be fabricated using cost-effective methods. The sandwich-type ZnO/carbon tape/ZnO nanogenerators (ZCZ-NGs) were fabricated in a cost-effective way by depositing ZnO layers on indium tin oxide (ITO)-coated polyethylene naphthalate (PEN) substrates in a radio frequency (RF) magnetron sputtering system to form ZnO/ITO/PEN-stacked blocks as well as using a conductive double-sided adhesive carbon tape to bond two ZnO/ITO/PEN blocks together, appreciably reducing the overall fabrication time and processing steps. The output voltage and current of the fabricated ZCZ-NG devices were measured for various bending strain rates, device sizes, and thicknesses of ZnO layers, generating up to about 30 V in terms of the peak-to-peak output voltage, which is much higher than those of other similar sandwich-type piezoelectric nanogenerators. Moreover, the output voltage variations of various ZCZ-NG devices due to their ZnO thicknesses, bending strain rates, and device sizes were investigated through the observation of their output voltages. An eco-friendly and stretchable flexoelectricity-enhanced piezoelectric nanogenerator (F-PENG) based on zinc-aluminum layered double hydroxide nanosheets (ZnAl:LDH Ns)-ZnO heterostructure is demonstrated on the stretchable polydimethylsiloxane (PDMS) substrates as the fabrication of a high-performance piezoelectric nanogenerator (PENG) with high stretchability and durability is desirable for the next-generation of stretchable and wearable electronics. The vertically-oriented eco-friendly ZnAl:LDH Ns were facilely synthesized by dipping the 10 wt% aluminum-doped zinc oxide (AZO) thin films in deionized (DI) water at room temperature. The enhanced output performance of the F-PENG is demonstrated under tapping, bending, and stretching modes, and is attributed to the synergistic flexoelectric and piezoelectric effects. The achieved maximum output power density of F-PENG under tapping is ~2.7 μW/cm$^2$. The pressure-sensing capability of the F-PENG is demonstrated by the generated outputs under the three applied modes. In addition, the biomechanical energy harvesting capability of the F-PENG is demonstrated by subjecting it to various biomechanical motions. The F-PENG exhibits excellent mechanical durability in all three modes of operation. The present study not only paves the way toward the facile fabrication of stretchable and high-performance F-PENG with combined flexoelectric and piezoelectric effects but also validates a wide range of applications in the next generation of stretchable and wearable electronics.