In the last years flexible, low-cost, wearable and innovative piezoelectric nanogenerators (NGs), i.e. devices able to convert the mechanical energy into electrical energy at micro- and nano-scale, have attracted a considerable interest to develop energy harvesters and sensors. Among the piezoelectric materials of particular interest are: a) the zinc oxide (ZnO) nanostructures, such as nanorods or nanowalls; b) the Poly(vinylidene fluoride) [PVDF] and its copolymer Poly(vinylidene fluoride-co-trifluoroethylene) [PVDF-TrFE], which are among the most interesting piezoelectric polymers. Recent literature reports how combining the piezoelectric properties of the ZnO nanorods (NRs) and thin film of PVDF make this hybrid nanostructure particularly promising for the production of flexible piezoelectric NGs. The aim of this research project is to develop and characterize flexible NGs for wearable applications, based on PVDF polymer matrix, suitably synthesized and embedding ZnO NRs grown by means of a Chemical Bath Deposition (CBD) (already developed and patented by the research group). In addition, to improve the piezoelectric response of the hybrid nanostructure we propose to fill the PVDF/PVDF-TrFE with ferromagnetic nanoparticles (NP), such as ferrites, and to exploit a magnetic poling. The combination of the hybrid structure ZnO NRs and suitably synthesized PVDF/PVDF-TrFE films with the ferromagnetic NP and magnetic poling will allow to obtain novel flexible and high performance NGs. The fundamental aspects that will be studied and investigated in depth are related to the optimization of the PVDF/ferromagnetic NP nanocomposite, proper choice of ferromagnetic NPs and control of the magnetic poling process.
This project aims to contribute to the field of energy harvesting, a sector that is attracting increasingly interest, thanks to the possibility of powering various wearable and portable devices converting mechanical energy, one of the most reliable and abundant forms of energy, in to electrical energy. The most innovative aspect of this project is the fabrication of low-cost flexible nanogenerators, using non-conventional process techniques that do not require sophisticated and expensive equipments, combining the ZnO NRs and the PVDF or PVDF-TrFE film filled with ferromagnetic nanoparticles and exposed to a magnetic poling process in order to increase the piezoelectric response of the polymer. Furthermore we propose an innovative solution for the top electrode, using a face to face configuration in which an array of ZnO NRs and polymer film is placed on top of another one (¿sandwich¿ structure). One of the most significant challenge in this research field is to improve the piezoelectric response of the PVDF that is linked not only at the presence of the ß-phase but also at its orientation along a specific direction, in our case along the vertical axis (z-axis). One way to increase the piezoelectric response of PVDF is using its copolymer: Poly(vinylidene fluoride-co-trifluoroethylene) [PVDF-TrFE], as we proposed in this project. However, for both PVDF and PVDF-TrFE, one of the most used techniques to obtain an high orientation and a uniform polarization of the ß-phase chains is the electrical poling, i.e. a high electric field (1-4 MV/cm) is applied in a range of temperature of (60-100 °C). The poling process is an effective process but at the same time is not a convenient or cost-effective industrial approach. One of the innovative aspects of the present project is to avoid the electrical poling by filling the PVDF or the PVDF-TRFE film with ferromagnetic nanomaterials, such as ferrites, and exploiting a magnetic poling, that is more simple and cost-effective. This technique is based on the chemical interaction between the ferromagnetic nanoparticles and the polymer chains and on the role that DC magnetic field has in inducing strain in the ferrites nanoparticles. The additional tensile stress at the interfacial areas between the ferrites fillers and the polymer matrix has been shown to increase the ß-phase content of the nanocomposite. This promising technique is still at an early stage of investigation and we plan to fully explore the effectiveness to increase the piezoelectric properties of PVDF. We also plan to investigate the piezoelectric properties trough an advanced technique, the Piezoresponse Force Microscopy (PFM), enabling a quantitative information of the piezoelectric response of the sample and evaluating the d33 not only at the nanoscale but also at the microscale. Another very important aspect of this project is the investigation of different solutions to define the top electrode of the NGs. In particular to further enhance the piezoelectric response of the NGs we propose to use, as a top electrode, a "sandwich" structure in which two layers of PET/ITO-ZnO NRs-PVDF are placed in a face-to-face configuration. The NG-prototype, that represents the final objective of this project, will combine the oriented arrays of ZnO NRs, the PVDF/PVDF-TrFE thin film filled with ferromagnetic nanoparticles magnetically poled and the "sandwich" structure. This prototype is an innovative solution in the field of NGs and we expect that this device will exhibit higher piezoelectric response, if compared to conventional ones.