Shear stress-induced augmentation of electroactive phases of PVDF for high-power nanogenerator using supersonically sprayed Sr2SnO4 nanorods

Supersonic spraying is a scalable, non-vacuum, rapid coating technique that uses a supersonic gas stream from a de Laval nozzle to deposit precursors under atmospheric conditions. In this study, the supersonic spraying of Sr₂SnO₄ nanorods (SSO-NRs) was found to increase the content of electroactive β- and γ-phases in poly (vinylidene fluoride) (PVDF) by more than twofold. Specifically, shear stress between the PVDF and SSO-NRs, induced by supersonic blowing, amplified the β- and γ-phases, which enhanced the energy-harvesting performance of a flexible piezoelectric nanogenerator (PENG). The swirling of the high-aspect-ratio SSO-NRs intensified the turbulence, thereby magnifying the influence of the shear stress. The supersonically driven shear stress caused multidirectional stretching, elongation, and twisting of PVDF and transformed a large amount of the α-phase into electroactive β- and γ-phases, as evidenced by X-ray diffractometry and infrared spectroscopy. The composite film with a minimal filler content of 2.5 wt.% exhibited a piezopotential of 41 V without additional poling. The optimal SSO/PVDF-based PENG delivered a high power density of 90 µW cm⁻² when subjected to a tapping force. Furthermore, the practical applicability of the PENG was demonstrated using air pressure, vibration, and human body movement. The fabricated PENG device was integrated with a supercapacitor electrode to exhibit a wide application range in wearable and portable electronics.