Effect of diameter and height of electrochemically-deposited ZnO nanorod arrays on the performance of piezoelectric nanogenerators

We investigated the performance of zinc oxide (ZnO) nanorod arrays (NRAs)-based piezoelectric nanogenerators (NGs) by controlling their diameter and height which are closely related to piezoelectric output current. The ZnO NRAs as a nanostructured piezoelectric material were synthesized on flexible indium tin oxide (ITO)-coated polyethylene terephthalate (PET) (i.e., ITO/PET) substrates by a facile electrochemical deposition (ED) method. As the zinc nitrate concentration and growth time increased, the diameter and height of ZnO NRAs also increased. Based on the ZnO NRAs on ITO/PET, piezoelectric NGs were fabricated with an opposite electrode of gold-coated PET film (i.e. Au/PET). At 10 mM of zinc nitrate concentration and 1 h of growth time, the ZnO NRAs exhibited relatively regular and higher output currents, leading to an estimated average value of ∼10.4 nA/cm² under a low external pushing force of 0.98 N. For the samples at 1 and 100 mM, piezoelectric currents were relatively low (∼4.34 and 1.45-5.21 nA/cm², respectively). It was found that the ZnO NRAs with high diameter/height uniformity and good alignment tend to be bent more easily for efficiently generating piezoelectric currents. The bending efficiency of ZnO NRAs was also analysed theoretically by calculating the strain distribution of ZnO NRAs-based NGs with different diameters and heights of the nanorods.