A review on ZnO-based piezoelectric nanogenerators: Synthesis, characterization techniques, performance enhancement and applications

ZnO nanorods (NRs) have received a tremendous amount of attention in the last decade for mechanical energy harvesting devices due to its outstanding semiconducting and piezoelectric properties, low cost, relative abundance, chemical stability towards the air, a biocompatible and in-complex and huge variety of crystal-growth technologies. This review summarizes the development of ZnO-based piezoelectric nanogenerators (PNGs) from synthesis to applications. It was found that most of the researchers used a low-temperature hydrothermal method to grow ZnO nanorods on flexible substrates. Various types of piezoelectric measurements commonly used by researchers for PNGs are compared and discussed in this review. The challenges of ZnO-based PNGs and the corresponding state-of-the-art technologies of piezoelectric output enhancement are presented. The surface treatment and combination techniques between surface treatment and doping are effective ways to elevate the piezo-output. The maximum output voltage of 470 V was achieved from ZnO/polydimethylsiloxane polymer composite flexible PNGs. In the last section of this review, the recent developments of device design and application of ZnO based self-power sensors are reviewed.