Polarization orientation, piezoelectricity, and energy harvesting performance of ferroelectric PVDF-TrFE nanotubes synthesized by nanoconfinement

1D nanostructures of soft ferroelectric materials exert promising potential in the fields of energy harvesting and flexible and printed nanoelectronics. Here, improved piezoelectric properties, energy-harvesting performance, lower coercive fields, and the polarization orientation of poly(vinylidene fluoride-trifluoroethylene) (PVDF-TrFE) nanotubes synthesized with nanoconfinement effect are reported. X-ray diffraction (XRD) patterns of the nanotubes show the peak corresponding to the planes of (110)/(200), which is a signature of ferroelectric beta phase formation. Piezoforce spectroscopy measurements on the free-standing horizontal nanotubes bundles reveal that the effective polarization direction is oriented at an inclination to the long axis of the nanotubes. The nanotubes exhibit a coercive field of 18.6 MV m^-1 along the long axis and 40 MV m^-1 (13.2 MV m^-1 considering the air gap) in a direction perpendicular to the long axis, which is lower than the film counterpart of 50 MV m^-1. The poled 200 nm nanotubes, with 40% reduction in poling field, give larger piezoelectric d₃₃ coefficient values of 44 pm V^-1, compared to poled films (≈20 pm V^-1). The ferroelectric nanotubes deliver superior energy harvesting performance with an output voltage of ≈4.8 V and power of 2.2 μW cm^-2, under a dynamic compression pressure of 0.075 MPa at 1 Hz. PVDF-TrFE nanotubes are synthesized by melt wetting of an anodic aluminum oxide (AAO) template. The synthesized nanotubes show improved piezoelectric coefficients and reduced coercive fields compared to the films counterpart. The nanotubes polarization direction is at an inclination to the long axis. The energy harvesting performance of these nanotubes is superior to the film's counterpart, with a maximum output voltage of 4.8 V and a power output of 2.2 μW cm^-2 in response to the compression pressure of 0.075 MPa.