Modulating ZnO Growth Structures for Maximum Power Output of Hybrid Piezo/Triboelectric Nanogenerator

Hybrid piezo/triboelectric nanogenerators (H-P/TENGs) are developed to compensate the drawbacks of a single nanogenerator by synergizing high triboelectric output voltage and piezoelectric output current in continuous contact-separation cycles. In particular, piezoelectric zinc oxide (ZnO) has been widely used due to its excellent orientation along the c-axis and tunable growth structures. Herein, various growth structures of ZnO, unoriented, oriented, and hierarchical structures, are incorporated in H-P/TENGs, enhancing their power densities from 6 to 17 times. Particularly, oriented ZnO nanorod/PVA (ZnR) achieves a maximum power density of 15.9 W m(-2) (approximate to 17-fold increment compared to pristine PVA). The highly oriented ZnO growth along the c-axis enables large deformation upon a vertical compression, subsequently generating a large piezoelectric polarization. Moreover, the enhancement mechanism via piezoelectric polarization is elucidated using a modified overlapped electron cloud model supported with Kelvin probe force microscopy measurements. The polarization of ZnO nanocomposites enlarges the difference in the highest electron energies (Delta E) between the two triboelectric layers, driving more electrons to transfer during contact electrification, thereby enriching their surface charge densities. This work highlights the significance of growth structure control in maximizing the piezoelectric responses of ZnO, consequently improving the output performances of H-P/TENGs.