n-ZnO/p-NiO Core/Shell-Structured Nanorods for Piezoelectric Nanogenerators

Energy-harvesting technologies, which can generate electrical energy from various energy sources, such as solar, thermal, and mechanical movement that are commonly found in the local environment, supply permanent and environmentally friendly energy. In addition, related research has been in the spotlight because it provides power by harvesting and converting a naturally occurring energy source without a charging process through external power. Piezoelectric nanogenerators (PENGs) based on piezoelectric materials have attracted much attention due to their high energy-conversion efficiency, capability for miniaturization, and light weight. Zinc oxide (ZnO), which has a wurtzite crystal structure, is a representative piezoelectric material. The defects that are inevitably present in ZnO nanorods (NRs), however, generate excessive free electrons, which reduce the piezoelectric potential and thus reduce the output characteristics. Herein, ZnO–NiO core–shell structure–based PENGs are designed to enhance their piezoelectric output performance by reducing excess electrons in the ZnO NRs using a p-type semiconducting NiO layer. The thickness and structure of the NiO coated on the ZnO NRs are observed and analyzed by adjusting the molar concentration of solution for the NiO layer coating, and the effects of these on the piezoelectric output are discussed.