Advances in zinc oxide (ZnO) nanostructure-based nanogenerators: Challenges and opportunities

Zinc oxide (ZnO) has been one of the foremost studied materials over the last three decades. ZnO is an attractive material due to the easy availability of raw materials, low production costs, non-toxic, and easy to synthesize as nanostructures. ZnO nanostructures have created much interest due to their multifunctional applications like energy harvesting, luminescent material, supercapacitors, batteries, solar cells, photocatalysis, biosensors. Energy harvesting with ZnO nanostructures by utilizing its piezoelectric property has established a new research field called nanogenerators. The first nanogenerator was demonstrated way back in 2006 with a single ZnO nanowire with an 8 mV piezoelectric output voltage. Further different nanogenerator design strategies were developed to enhance the output of the ZnO nanostructures-based nanogenerators to meet the practical applications demand in the first phase of the research. In the second phase of the study, new approaches such as doping of ZnO, an increase of areal density of ZnO nanostructures, surface treatment, interfacial modification, and combinational techniques are developed to improve the piezoelectric output of ZnO-based PENGs. This chapter initially discusses various synthesis methods for ZnO nanostructures and the second part discusses the nanogenerator fabrication and testing its performance. The last part of the chapter discusses the developments in the ZnO-based nanogenerators, challenges, and opportunities associated with them.