Approaching Piezoelectric Response of Pb-Piezoelectrics in Hydrothermally Synthesized Bi0.5(Na1- xK x)0.5TiO3 Nanotubes.

A large piezoelectric coefficient of 76 pm/V along the diameter direction, approaching that of lead-based piezoelectrics, is observed in hydrothermally synthesized Pb-free Bi0.5(Na0.8K0.2)0.5TiO3 nanotubes. The 30-50 nm diameter nanotubes are formed through a scrolling and wrapping mechanism without the need of a surfactant or template. A molar ratio of KOH/NaOH = 0.5 for the mineralizers yields the Na/K ratio of ∼0.8:0.2, corresponding to an orthorhombic-tetragonal (O-T) phase boundary composition. X-ray diffraction patterns along with transmission electron microscopy analysis ascertain the coexistence of orthorhombic and tetragonal phases with (110) and (001) orientations along the nanotube length direction, respectively. 23Na NMR spectroscopy confirms the higher degree of disorder in Bi0.5(Na1- xK x)0.5TiO3 nanotubes with O-T phase coexistence. These findings present a significant advance toward the application of Pb-free piezoelectric materials.