Ambient energy harvesting is a promising route to achieve self-powered electronic devices. A nanogenerator scavenges mechanical energy from surrounding and converts it into electrical energy to supply power to a self-powered system. Using piezoelectric, thermoelectric, and triboelectric effects, several nanogenerators have been developed. Piezoelectric nanogenerators harvest kinetic energy to provide power for portable and small electronics. The kinetic energy generated from human body motions is an excellent energy source to power wearable devices. Biocompatibility, flexibility, high efficiency, and small volume are the main attributes for applications related to the human body. Piezoelectric nanogenerators based on thin films are desirable for their ability to scavenge irregular mechanical energies from bending. The power generation mechanism of a thin film based piezoelectric nanogenerator is determined by the coupled piezoelectric and semiconducting properties of the thin film.
ZnO is an appealing material for piezoelectric nanogenerators thanks to its coupling effect of semiconducting and piezoelectrical properties, extremely high elasticity, high power density, low-cost and controlled growth, and biocompatibility. Herein, a flexible piezoelectric nanogenerator with ZnO nanoflakes-polyethylene terephthalate (PET) is reported. The direct synthesis of ZnO nanoflakes on flexible PET substrate was achieved via a simple, fast, low-temperature, low-cost, highly stable, and reproducible sonochemical method. The synthesized ZnO thin films were characterized in detail. The results show that ZnO nanoflakes were grown with high purity and highly crystallinity along  direction. Our piezoelectric device generated a peak voltage of 62 mV with great reproducibility (p-value of 0.0212). The fabrication of ZnO nanoflakes-PET piezoelectric nanogenerators helps us to develop more flexible and bio-compatible nanogenerators particularly self-powered wearable electronics.