nanowires, [ 7 ] PbZr x Ti 1-x O 3 nanowires [ 8 ] and nanoribbons [ 9,10 ] and poly(vinylidene fl uoride) (PVDF) nanofi bers [ 11 ] have all revealed promising energy harvesting performance. There has since been an ongoing concerted effort in developing this relatively new research fi eld, connecting nanotechnology with the fi eld of energy. [ 12 ] Piezoelectric nanowires are particularly attractive for energy harvesting due to their robust mechanical properties and high sensitivity to typically small ambient vibrations. [ 13 ] The implications of these properties, in fact, go beyond energy harvesting, as nanowirebased nanogenerators have recently been shown to function as bio mechanical sensors, [ 14 ] sensitive pressure sensors [ 15 ] and precision accelerometers. [ 16 ] The challenge lies in the large scale production of low-cost piezoelectric nanowires that can offer reproducible and reliable energy harvesting and/or sensor performance.The polymer PVDF [(CH2-CF2) n ] exhibits good piezoelectric and mechanical properties with excellent chemical stability and resilient weathering characteristics. [ 17,18 ] PVDF thin fi lms are thus commonly used as sensors and actuators. [ 19 ] However, the piezoelectric performance of PVDF is dependent on the nature of the crystalline phase present. Typically, PVDF occurs in the α, β and γ crystalline phases [20][21][22] and needs to be electrically poled (using an electric fi eld of the order of 100 MV m −1 ) and/ or mechanically stretched [ 20,22 ] to achieve the polar β-phase that shows the strongest piezoelectric behavior. [ 21 ] is a co-polymer that crystallizes more easily into the β-phase due to steric factors, [ 22 ] an advantage that we exploit in this work. Nanowires of PVDF and its copolymers have been previously incorporated into piezoelectric nanogenerators [ 11 ] but the relatively complex electrospinning fabrication process employed requires high voltages (5-50 kV) and specialized equipment. The associated high electric fi elds and stretching forces result in poled nanowires, however this fabrication process often suffers from poor control over nanowire size-distribution and alignment, and is yet to be conveniently and cost-effectively scaled up. [ 23 ] Here we report the growth of aligned P(VDF-TrFE) nanowires with a narrow size distribution using a simple, cost-effective and easily scalable template-wetting method, [ 24,25 ] where the template-induced space confi nement promotes high crystallinity and preferential orientation of the lamellar crystals in the polymer nanowires. [ 26,27 ] This results in the enhancement of piezoelectric properties, even without the need for electrical poling. A nanogenerator fabricated using template-grown, self-poled P(VDF-TrFE) nanowires is shown to have excellent electrical output when subjected to periodic vibrations. Using a circuit comprising a rectifi er to convert its AC output to DC, and a bank of capacitors to store the harvested energy, the nanogenerator is shown to be capable of lighting a commercial light emitting ...
Energy harvesting from ambient vibrations originating from sources such as moving parts of machines, fluid flow and even body movement, has enormous potential for small power applications, such as wireless sensors, flexible, portable and wearable electronics, and bio-medical implants, to name a few. Nanoscale piezoelectric energy harvesters, also known as nanogenerators (NGs), can directly convert small scale ambient vibrations into electrical energy. Scavenging power from ubiquitous vibrations in this way offers an attractive route to provide power to small devices, which would otherwise require direct or indirect connection to electrical power infrastructure. Ceramics such as lead zirconium titanate and semiconductors such as zinc oxide are the most widely used piezoelectric energy harvesting materials. This review focuses on a different class of piezoelectric materials, namely, ferroelectric polymers, such as polyvinlyidene fluoride (PVDF) and its copolymers. These are potentially superior energy harvesting materials as they are flexible, robust, lightweight, easy and cheap to fabricate, as well as being lead free and biocompatible. We review some of the theoretical and experimental aspects of piezoelectric energy recovery using Polymer-based NGs with a novel emphasis on coupling to mechanical resonance, which is relevant for efficient energy harvesting from typically low frequency (<1 kHz) ambient vibrations. The realisation of highly efficient and low cost piezoelectric polymer NGs with reliable energy harvesting performance could lead to wide ranging energy solutions for the next generation of autonomous electronic and wireless devices.
Despite exhibiting weaker piezoelectric properties than commonly used ferroelectric ceramics [ 1 ] (such as barium titanate and lead zirconium titanate) their piezoelectric properties are still technologically viable [ 2 ] while simultaneously possessing a range of advantages over ceramics including being fl exible, low-temperature and solution-processable, light weight, nontoxic and biocompatible, chemically robust, and mechanically stable. [ 3 ] The recent surge in interest has arisen from their suitability in a range of developing technologies such as sensing, [ 4 ] actuation, [ 5 ] nonvolatile memory, [6][7][8] and vibrational energy harvesting applications. [ 2,3 ] For piezoelectric/ pyroelectric applications in particular, there is a requirement for the material to be poled, i.e., to have orientated dipoles, in order for the piezoelectricity/pyroelectricity to manifest. This is typically achieved through externally applied electric fi elds, high temperatures, and/or mechanical stretching.Ferroelectric polymer nanowires grown using a template-wetting method are shown to achieve an orientated "self-poled" structure resulting from the confi ned growth process. Self-poling is highly desirable as it negates the need for high electric fi elds, mechanical stretching, and/ or high temperatures typically associated with poling treatments in ferroelectric polymers, as required for piezoelectric and/or pyroelectric applications. Here, differential scanning calorimetry, infrared spectroscopy, and dielectric permittivity measurements have been presented on as-fabricated template-grown polyvinylidene fl uoride-trifl uoroethylene nanowires, and quantitatively compared with spin-cast fi lms of the same composition that have been electrically poled, both before and after subsequent depoling temperature treatment. The measurements reveal remarkably similar trends between the physical properties of the as-grown nanowires and the electrically poled fi lm samples, providing insight into the material structure of the "self-poled" nano wires. In addition, piezoresponse force microscopy data are presented that allow for unambiguous identifi cation of self-poling in ferroelectric polymer nanostructures. Our results indicate the suitability of the template-wetting approach in fabricating nanowires that can be used directly for piezoelectric/pyroelectric applications, without the need for post-deposition poling/processing.
A flexible and robust piezoelectric nanogenerator (NG) based on a polymer-ceramic nanocomposite structure has been successfully fabricated via a cost-effective and scalable template-assisted hydrothermal synthesis method. Vertically aligned arrays of dense and uniform zinc oxide (ZnO) nanowires (NWs) with high aspect ratio (diameter ∼250 nm, length ∼12 μm) were grown within nanoporous polycarbonate (PC) templates. The energy conversion efficiency was found to be ∼4.2%, which is comparable to previously reported values for ZnO NWs. The resulting NG is found to have excellent fatigue performance, being relatively immune to detrimental environmental factors and mechanical failure, as the constituent ZnO NWs remain embedded and protected inside the polymer matrix.
A piezoelectric nanogenerator has been fabricated using a simple, fast and scalable template-assisted electrodeposition process, by which vertically aligned zinc oxide (ZnO) nanowires were directly grown within a nanoporous polycarbonate (PC) template. The nanowires, having average diameter 184 nm and length 12 μm, are polycrystalline and have a preferred orientation of the [100] axis parallel to the long axis. The output power density of a nanogenerator fabricated from the as-grown ZnO nanowires still embedded within the PC template was found to be 151 ± 25 mW m−3 at an impedance-matched load, when subjected to a low-level periodic (5 Hz) impacting force akin to gentle finger tapping. An energy conversion efficiency of ∼4.2% was evaluated for the electrodeposited ZnO nanowires, and the ZnO–PC composite nanogenerator was found to maintain good energy harvesting performance through 24 h of continuous fatigue testing. This is particularly significant given that ZnO-based nanostructures typically suffer from mechanical and/or environmental degradation that otherwise limits their applicability in vibrational energy harvesting. Our template-assisted synthesis of ZnO nanowires embedded within a protective polymer matrix through a single growth process is thus attractive for the fabrication of low-cost, robust and stable nanogenerators.
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