Flexible Piezoelectric Nanogenerators Based on One-Dimensional Neutral Coordination Network Composites

Metal–organic coordination polymers are modular
systems
whose structures can be modified in numerous ways to introduce and
influence non-linear optical and electrical properties. However, their
full potential as piezoelectric nanogenerators for self-powered electronics
is yet to be uncovered. Here, we report a Zn­(II)-based ferroelectric
one-dimensional coordination network {[Zn­(L1)­(bpy)]·(H2O)1.5}∞ (1) derived from a flexible dicarboxylate ligand [PhPO­(NH-(C6H4COOH))2] (L1H2) and 2,2′-bipyridine as a co-ligand. The origin
of polarization in 1, despite its neutral structure,
is due to the polyhedral distortions around the Zn­(II) center as revealed
by ab initio calculations. The presence of polarizable domains was
visualized by piezoresponse force microscopy (PFM) experiments. Also,
from the PFM studies, a sizable converse piezoelectric coefficient
(d33) value of 19.4 pm/V was noticed for 1, which is unprecedentedly high for the class of neutral-network
coordination polymers. Furthermore, flexible composite devices comprising
a thermoplastic polyurethane (TPU) polymer with different weight percentages
(wt %) of 1 were prepared and examined for application
as piezoelectric nanogenerators. Notably, the champion device of this
series (poled 5 wt % 1-TPU composite) exhibits a highest
open-circuit voltage of 5.6 V and power density output of 14.6 μW/cm2.