Influence of the ZnO Nanostructures Shape on Piezoelectric Energy Harvesters Performance

Due to the small size and ability to supply energy to remote devices, metal-oxide-based piezoelectric energy harvesters have recently gained an enormous amount of attention. The nanostructure of the piezoelectric metal-oxide profoundly influences the operational characteristics of these devices. Here, the effect of geometrical parameters of various ZnO nanostructures, i.e., nanowire (NW), nanotube (NT), and nanoneedle(NN), on their mechanical resonance frequency, piezoelectric voltage, and output power is investigated at different load resistances through COMSOL simulations. The geometrical parameters considered in the simulations are in the range of those obtained from experimental works: 1-3-μm height for all nanostructures, 50-150 nm for NWs cross section, and a few tens of nanometer for NT hollow cores and NN tips. Results show as the nanostructures are made thinner and longer, better piezoelectric performance is observed. At a constant pressure of 10 N/m
²
, open-circuit piezoelectric voltages of 37.88, 154.56, and 599.23 mV are achieved from the champion NWs, NTs, and NNs, respectively. The respective maximum output powers of these nanostructures are 0.58, 1.93, and 2.6 mW/cm
²
. The analytical equations of the moment of inertia (I) for all of these nanostructures are extracted, and the superior performance of NNs is attributed to a lower moment of inertia. The load resistance significantly impacts the mechanical resonance frequency, piezoelectric voltage, and output power of these nanostructures; the effect is described by deriving and analyzing the Mason equivalent circuits.