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

Abstract

Recently, nanogenerators (NG) have attracted considerable attention due to the practical applications of converting mechanical energy to electrical energy from various sources. [1-5] Piezoelectric materials generate electricity by generating dielectric polarization when a physical force is applied. A piezoelectric nanogenerator (PENG) based on piezoelectric materials can convert various mechanical energy sources into electrical energy without generating contaminants, and they can be easily miniaturized and are light in weight, so they can be installed where vibration and pressure energy exist. [6,7] Various non-centrosymmetric piezoelectric materials such as lead zirconate titanate (PZT), [8-12] quartz, [13] zinc oxide (ZnO), [3,14-16] barium titanate (BaTiO 3 , BTO), [17-22] zinc sulfide (ZnS), [23-24] polyvinylidene fluoride (PVDF), [25,26] etc. are widely used for PENGs. Among them, wurtzite structured ZnO nanorods (NRs) have undergone numerous studies due to their unique features, such as their piezoelectricity, semiconducting property, transparency, biocompatibility, and low manufacturing cost. [3,13-16] ZnO NRs are an intrinsic n-type semiconductor due to native defects such as zinc (Zn) interstitials and oxygen (O) vacancies. Excessive free electrons from the defects in ZnO NRs limit their harvesting performance due to the screening of piezoelectric potential when mechanical stress is applied. To solve this issue, several approaches have been reported, for example, thermal annealing, [27] O 2 plasma treatment, [28,29] and p-type semiconducting layer coating [30,31] to reduce native defects or modulate carrier concentration. In a previous work, NiO was used as a p-type semiconductor in connection with doping materials research to enhance the output performance of ZnO-based PENGs. [30] The oxidation temperature of Ni for making NiO is very high, however, from 500 to 900 C, so there is a disadvantage in that its use on a