Stabilized Ferroelectric NaNbO3 Nanowires for Lead-Free Piezoelectric Nanocomposite Applications

Ferroelectric nanomaterials often suffer from severe polarization loss compared to their bulk due to a size-induced alteration in their crystalline structure, making them inefficient for piezoelectric applications. Discovering nanomaterials with efficient piezoelectric properties is therefore a challenging task. We report here a direct observation of a single-phase ferroelectric structure with stripe domains stabilized by size-induced thermal residual stress in NaNbO3 nanowires (NWs) and demonstrate their excellent efficiency for lead-free piezoelectric nanocomposites. Polymer composites containing NaNbO3 NWs exhibit piezoelectric coefficients and figure-of-merit values comparable to those of KNbO3 NWs and approximately 9 and 100 times higher, respectively, than those of the reference devices using competing BaTiO3 NWs. The remarkable performance of NaNbO3 NWs compared to BaTiO3 NWs contradicts the ranking of bulk properties, claiming that NaNbO3 ceramics are significantly less active than BaTiO3. However, this counterintuitive behavior can be well understood if we consider structure modifications of these materials at the nanoscale, with a size-induced antiferroelectric-to-ferroelectric transition in NaNbO3 NWs and ferroelectric-to-paraelectric transition in BaTiO3 NWs. These findings are further supported by second harmonic generation characterizations, revealing substantially stronger second harmonic intensities for NaNbO3 and KNbO3 NWs compared to BaTiO3 NWs. Our work confirms the critical role of structural properties in the macroscopic piezoelectric performance of nanomaterials beyond the ranking of the bulk properties. With their scalable synthesis and high aspect ratio, ferroelectric NaNbO3 NWs hold great promise for the large-scale production of efficient, lead-free piezoelectric nanocomposites.