Electrodeposited Functional Nanowires for Energy Applications
Nanostructuring functional materials can lead to a variety of enhanced intrinsic material properties. In particular, nanowires (NWs) have large surface-to-volume ratio and large aspect ratio (length / diameter), which makes them sensitive to low-amplitude vibrations and have increased flexibility compared to the bulk form of the material. In this thesis, piezoelectric, ferroelectric, ferromagnetic and magnetoelectric (ME) NWs have been explored in the context of vibrational energy harvesting and magnetic energy harvesting and sensing; because of their increased piezoelectric coefficients and ME coupling compared to bulk. Low-temperature, solution-processable and hence scalable fabrication techniques have been used throughout this work. Electrochemical deposition or electrodeposition (ED) in conjunction with nanoporous templates i.e. template-assisted electrodeposition (TAED) have been used to grow piezoelectric zinc oxide (ZnO) and ferromagnetic nickel (Ni) NWs and three template-wetting based techniques have been used to grow ferroelectric poly(vinylidene fluoride trifluoroethylene) (P(VDF-TrFE)) NWs and nanotubes (NTs). Both techniques have been optimised and subsequently combined to synthesise core-shell or (1-1) Ni - P(VDF-TrFE) composite NWs. The structural and crystalline properties of each type of nanostructure has been studied using a variety of techniques including: scanning electron microscopy (SEM), energy dispersive X-ray analysis (EDX), X-ray diffraction (XRD) and transmission electron microscopy (TEM) and all of the NWs have been shown to be polycrystalline. The energy harvesting performance of vertically aligned ZnO NW arrays embedded in flexible, polycarbonate (PC) templates when incorporated into a flexible nanocomposite nanogenerator (NG), has been tested via periodic impacting and flexing of the NG at different frequencies. The voltage ($V$), current ($I$) and power were recorded during testing and measured across a range of external load resistances. The aligned nature of the embedded NWs ensures good piezoelectric performance across the entire device under impacting, while the PC template ensures mechanical stability and longevity of the device, confirmed by good fatigue performance over 24 hours of continuous testing, which is rarely studied in this field. The power density ($P_\mathrm{d}$) was found to be 151 mW m$^{-3}$ for low-amplitude (0.68 mm) and low-frequency (5 Hz) impacting, resulting in energy conversion efficiencies ($\chi$) and device efficiencies ($\chi$') of $\approx$ 4.2 \% and $\approx$ 3.76 x 10$^{-3}$ \% respectively. The nanoscale or surface piezoelectric charge coefficient ($d_{33}$) was measured to be $\approx$ 12.5 pm V$^{-1}$ on an individual ZnO NW, using a combination of Kelvin probe force microscopy (KPFM) and non--destructive piezoresponse force microscopy (ND-PFM). Both nanoscale and bulk ME measurements have been performed on Ni - P(VDF-TrFE) ME composite (1-1) NWs, nanocomposite (1-3) films and (2-2) laminates. The latter two structures have been fabricated using TAED and ED for the Ni NW and film respectively, in combination with drop-casting and spin-coating for the P(VDF-TrFE) films. The scanning probe microscopy (SPM) measurements used here include atomic force microscopy (AFM), KPFM, magnetic force microscopy (MFM) and piezoresponse force microscopy (PFM) and it has been found that the ME coupling in the (1-1) composites NWs is enhanced compared to the other structures, confirmed by approximating the converse ME coupling coefficient ($\alpha^\mathrm{C}$) of each composite. Additionally, vibrating sample magnetometry (VSM) has been used to confirm the ferromagnetic nature of the Ni phases in the composite structures. ME composite devices based on (2-2) and (1-3) composite materials and have been fabricated and preliminary bulk ME measurements of the ME coupling coefficient ($\alpha^\mathrm{E}$) plus energy harvesting measurements have also been performed as a proof of concept that the nanoscale ME coupling translates to the bulk, to some extent.
nanowires, piezoelectric, magnetoelectric, electrodeposition, functional nanowires, scalable, template-assisted growth of nanostructures, nanotechnology, energy harvesting, sensing, scanning probe microscopy
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