Production and characterization of ZnO/Graphene devices for energy harvesting

In this thesis, different types of innovative highly performing piezoelectric nanomaterials and nanocomposites have been synthesized and characterized for energy harvesting application. In order to evaluate the piezoelectric properties of the produced materials, a novel approach to quantitatively evaluate the effective piezoelectric coefficient d33, trough Piezoresponse Force Microscopy (PFM), has been developed. PFM is one of the most widely used techniques for the characterization of piezoelectric materials at nanoscale, since it enables the measurement of the piezo-displacement with picometer resolution. PFM is a non-invasive and easy to use test method; it requires only a bottom electrode (no need of a top-electrode deposition over the material under test), thus considerably simplifying the test structure preparation. In particular, in order to have a quantitative information on the d33 a calibration protocol was developed. To get a macroscale characterization of the piezoelectric coefficient, the PFM signal is averaged over different areas of the sample. The proposed method allows to precisely evaluate the piezoelectric coefficient enabling a proper comparison among the different materials analysed. Two different classes of piezoelectric materials have been synthesized and characterized: zinc oxide nanostructures, in particular zinc oxide nanorods (ZnO-NRs) and zinc oxide nanowalls (ZnO-NWs), polyvinylidene fluoride (PVDF) nanocomposites films. The produced piezoelectric materials were fabricated using process which are cost-effective, time-consuming and easy to scale-up. The ZnO nanostuctures were grown by chemical bath deposition (CBD), that guarantees high deposition rate on a wide variety of substrates. PVDF nanocomposite films were produced with a simple solution casting method, without the need of subsequent electrical poling step. To enhance the piezoelectric properties of PVDF films we investigated different PVDF nanocomposite films: PVDF filled with Graphene nanoplatelets (GNPs) or with ZnO-NRs; PVDF filled with different types of hexahydrate metal-salts (HMS); PVDF filled with HMS in combination with nanofillers, like GNPs or ZnO-NRs. We found that the piezoelectric coefficient of the ZnO-NRs is (7.01±0.33) pm/V and (2.63±0.49) pm/V for ZnO-NWs. The higher piezoelectric response of ZnO-NRs is believed to be due to a better crystallinity and a less defectiveness of the ZnO-NRs if compared to the ZnO-NWs, as it has been confirmed by X-ray diffraction (XRD) spectra and by photoluminescence spectroscopy (PL) measurements. The neat PVDF show a d33 limited to 4.65 pm/V; when the nanofillers are added the d33 increases up to 6 pm/V. This value reaches 8.8 pm/V when a specific hexahydrate metal-salts: [Mg(NO3)2∙6H2O] is dispersed in the PVDF polymer matrix. From the comparative analysis of the synthesized materials we found that the sample produced using the dissolution of HMS in PVDF shows the best piezoelectric response (8.8 pm/V) and the most attractive structural and mechanical properties to fabricate a flexible nanogenerators. Therefore, a porous piezoelectric HMS-PVDF nanocomposite film has been used as active material to fabricate flexible nanogenerator. To build such a device, graphene-gold flexible top electrodes were developed. The bilayer electrode structure avoids short circuits between top and bottom electrodes, observed in the absence of graphene interlayer. The nanogenerator was tested using a commercial mini-shaker and operated successfully. The piezoelectric coefficient determined from the electromechanical tests was 9.00 pm/V, which is in good agreement with the one (8.88±3.14) pm/V measured through PFM on the same PVDF film without top electrode. We also measured the piezoelectric coefficient of PVDF using PFM with and without top electrode and both values were found to be in close agreement. This finding suggests that the local characterization using PFM is also a good representation of the global piezoelectric properties of the samples. The progress on advanced piezoelectric materials reported in this work opens new opportunities to fabricate energy harvesters and sensors for wearable and smart clothing applications.