Abstract
In this study, nanopiezoelectric devices based on ZnO nanorod array/conducting polymers are fabricated for wearable power generation application. To replace the inorganic rigid indium-tin oxide (ITO) conducting coating commonly used in the nanogenerator devices, a series of flexible polyaniline-based conducting copolymers underlying the perpendicularly-oriented ZnO nanorod arrays has been synthesized with improved electric conductivity by the copolymerization of aniline and 3,4-ethylenedioxythiophene (EDOT) monomers in order to optimize the piezoelectric current collection efficiency of the devices. It is found that significantly higher conductivity can be obtained by small addition of EDOT monomer into aniline monomer solution using an in-situ oxidative polymerization method for the synthesis of the copolymer coatings. The highest conductivity of aniline-rich copolymer is 65 S/cm, which is 2.5 times higher than that for homopolymer polyaniline coating. Subsequently, perpendicularly-oriented ZnO nanorod arrays are fabricated on the polyaniline-based copolymer substrates via a ZnO nanoparticle seeded hydrothermal fabrication process. The surface morphology, crystallinity, orientation, and crystal size of the synthesized ZnO nanorod arrays are fully examined with various synthesis parameters for copolymer coatings with different monomer compositions. It is found that piezoelectric current generated from the devices is at least five times better for the device with improved electric conductivity of the copolymer and the dense formation of ZnO nanorod arrays on the coating. Therefore, these results demonstrate the advantage of using flexible π-conjugated copolymer films with enhanced conductivity to further improve piezoelectric performance for future wearable energy harvesting application based on all wet chemical coating processes.
Highlights
The piezoelectric devices investigated in this study has the following structure in the order of metal/piezoelectric ZnO semiconductor nanorod array/conductive copolymer coating /silver paste connected to the atomic force microscopy (AFM) machine
Conductive polymers have the advantage of combining conventional polymer properties with electronically conductive materials, and they have received significant attention due to their low cost, low density, lightweight, and flexible properties for wearable electronic applications. de Leeuw [65] and Ha [66] et al reported that the addition of imidazole as an inhibitor to impede the generation of monomeric and oligomeric radicals during the oxidative polymerization and to increase the overall molecular weight of the synthesized polymers and to enhance the conductivity and transparency of the polymerized PEDOT films
Since aniline monomer acts as an inhibitor for the copolymerization reaction, no additional weak base of imidazole is required for the synthesis of the polyaniline-based copolymer coating
Summary
The research of developing ZnO nanomaterials has recently attracted numerous interests, by the discovery of its particular optoelectronic [1,2,3], piezoelectric [4,5,6], and biocompatible [7,8,9] properties, which grant this nanomaterials great possibility in electronics [10,11,12,13,14,15,16,17,18], biomedical devices [19,20,21], and power generation application [22,23,24,25,26,27]. The displacement of ZnO nanorods generated by external forces results in the lattice deformations for ZnO ionic crystal, which lacks a center of symmetry, inducing the piezoelectric effect. Thereafter, in the field of nanopiezotronics there have been various attempts to implement and utilize the semiconducting/coupled piezoelectric properties of ZnO nanostructure for novel application [60,61,62] It is well-known that the nanopiezotronic devices composed of highly oriented ZnO nanorod arrays exhibit superior performance, as compared to ill-aligned ZnO nanorod structure [63,64]. We present a novel ZnO nanopiezoelectric devices based on a hydrothermal method to synthesize ZnO nanorod arrays on polyaniline-based conducting polymer coatings with different electric conductivities. The surface morphology, crystallinity, orientation, growth rate, rod diameter, and rod length of ZnO nanorod arrays are fully examined with various synthesis parameters for different copolymer coatings with details described below
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