Flexible piezoelectric energy harvester made of vertically-aligned ZnO nanowires hydrothermally-grown by template-assisted synthesis in poled PVDF

The beneficial effect of the hydrothermal method on the morphology and piezoelectric performance of zinc oxide/polyvinylidene fluoride (ZnO/PVDF) composite thin membranes (10 μm-thick) is demonstrated. In this work, high aspect ratio vertically aligned ZnO nanowires were successfully grown on an Au seed layer to create a Schottky-like contact by template-assisted synthesis inside the cylindrical nanopores of a poled β-PVDF. Swift heavy ions irradiation was firstly used to create tracks of damages in this poled β-PVDF thin films. A subsequent chemical etching in alkaline medium revealed these ion-tracks to form dense and statistical arrays of cylindrical nanopores (10⁹ pores cm⁻²) along its thickness. Resulting ZnO/PVDF composites were characterized by infrared spectroscopy, grazing incidence X-ray diffraction, scanning and transmission electron microscopy, atomic force microscopy, reflectance spectroscopy, dielectric measurements and piezoelectric analysis. Vertically aligned ZnO nanowires grown in low-supersaturation conditions exhibited an ideal microstructure for enhancing the piezoelectric performance of β-PVDF, i.e. hexagonal wurtzite structure. From piezoelectric analysis in bending mode, the output power of ZnO/PVDF composites was plotted against 8 resistances ranging from 10⁵ to 10⁶ Ω and fitted up to 10⁸ Ω. A maximum power density of 1.90 μW cm⁻² (i.e. 380 µW cm⁻³ N⁻¹) at 2.10⁶ Ω was found to be 60 ± 10 % higher than of pristine poled β-PVDF. The performance of these ZnO/PVDF composites may be due to the combined effects of Surface Fermi Level Pinning phenomenon, Schottky-like contact and dipole alignment. It thus reveals a very promising transducer in the renewable energy application of electromechanical energy conversion.