Boosting piezoelectric response of KNN-based ceramics with strong visible-light absorption

A series of (1 - x)(K0.48Na0.52)NbO3-x(Bi0.5Na0.5)(Zr0.55Ni0.45)O3-delta (KNN-BNZN) ceramics are designed to achieve excellent piezoelectric response along with narrow bandgap. The ceramics with x = 0.04 exhibit unprecedented piezoelectric coefficient d(33) ~ 318 +/- 10pC N-1 in comparison with all reported narrow bandgap ceramics. A rhombohedral-orthorhombic-tetragonal (R-O-T) phase boundary is observed, indicating the formation of defect dipoles (Ni2+-Vo center dot) at morphotropic phase boundary region is desirable for piezo-/ferroelectric properties. In addition, a narrow bandgap ~2.5 eV along with gap states (~0.9 eV and ~1.6 eV) is obtained from the ceramics when x > 0.02, which can be persuasively explained by the schematic plot of bandgap splitting mechanism proposed in this work, where Ni 3d energy state plays a role as a scaffold in the process of electron transition. More importantly, largely enhanced photovoltaic performance of the ceramics is achieved under AM 1.5 irradiation. The NIR photoresponse property (maximum current density of ~100 nA cm(-2)) indicates such KNN-based ceramics with sub bandgap ~ 0.9 eV may even have potential to be applied in NIR light-activated devices. Our findings might pave way for the further development of piezoelectric/photoresponsive multifunctional devices.