A high performance flexible two dimensional vertically aligned ZnO nanodisc based piezoelectric nanogenerator via surface passivation

Review of Zinc Oxide Piezoelectric Nanogenerators: Piezoelectric Properties, Composite Structures and Power Output

Lead-containing piezoelectric materials typically show the highest energy conversion efficiencies, but due to their toxicity they will be limited in future applications. In their bulk form, the piezoelectric properties of lead-free piezoelectric materials are significantly lower than lead-containing materials. However, the piezoelectric properties of lead-free piezoelectric materials at the nano scale can be significantly larger than the bulk scale. This review looks at the suitability of ZnO nanostructures as candidate lead-free piezoelectric materials for use in piezoelectric nanogenerators (PENGs) based on their piezoelectric properties. Of the papers reviewed, Neodymium-doped ZnO nanorods (NRs) have a comparable piezoelectric strain constant to bulk lead-based piezoelectric materials and hence are good candidates for PENGs. Piezoelectric energy harvesters typically have low power outputs and an improvement in their power density is needed. This review systematically reviews the different composite structures of ZnO PENGs to determine the effect of composite structure on power output. State-of-the-art techniques to increase the power output of PENGs are presented. Of the PENGs reviewed, the highest power output belonged to a vertically aligned ZnO nanowire (NWs) PENG (1-3 nanowire composite) with a power output of 45.87 μW/cm² under finger tapping. Future directions of research and challenges are discussed.

Extremely Reduced Dielectric Constant and Band Gap Enhancement in Few-Layered Tungsten Disulfide Nanosheets

Highly crystalline few-layered tungsten disulfide (WS₂) nanosheets were synthesized via a cost-effective, low-temperature hydrothermal route. X-ray diffraction and HR-TEM analysis confirmed the formation of hexagonal nanosheets with thickness of ∼6-8 nm. Raman analysis and AFM results confirmed the few-layered 2H phase of WS₂ nanosheets. The UV-vis study shows absorption peaks at 219 and 271 nm with large band gap value of ∼3.12 eV for WS₂ nanosheets. Surprisingly, WS₂ nanosheets show a dielectric constant of approximately ε' ≈ 5245, whereas bulk WS₂ material exhibits a dielectric constant of 7482373. An almost 1426-fold decrease in the value of dielectric constant for the WS₂ nanosheet is observed. Such an extreme reduction in dielectric constant and observance of large band gap in WS₂ nanosheet were observed for the first time. The present study reveals the excellent and unusual optical and dielectric properties for their potential application in optoelectronic, dielectric, solar, phosphor, and various nanoelectronic devices.

The Deformation Behavior and Bending Emissions of ZnO Microwire Affected by Deformation-Induced Defects and Thermal Tunneling Effect

The realization of electrically pumped emitters at micro and nanoscale, especially with flexibility or special shapes is still a goal for prospective fundamental research and application. Herein, zinc oxide (ZnO) microwires were produced to investigate the luminescent properties affected by stress. To exploit the initial stress, room temperature in situ elastic bending stress was applied on the microwires by squeezing between the two approaching electrodes. A novel unrecoverable deformation phenomenon was observed by applying a large enough voltage, resulting in the formation of additional defects at bent regions. The electrical characteristics of the microwire changed with the applied bending deformation due to the introduction of defects by stress. When the injection current exceeded certain values, bright emission was observed at bent regions, ZnO microwires showed illumination at the bent region priority to straight region. The bent emission can be attributed to the effect of thermal tunneling electroluminescence appeared primarily at bent regions. The physical mechanism of the observed thermoluminescence phenomena was analyzed using theoretical simulations. The realization of electrically induced deformation and the related bending emissions in single microwires shows the possibility to fabricate special-shaped light sources and offer a method to develop photoelectronic devices.

Boosting Performance of Self-Polarized Fully Printed Piezoelectric Nanogenerators via Modulated Strength of Hydrogen Bonding Interactions

Self-polarized piezoelectric devices have attracted significant interest owing to their fabrication processes with low energy consumption. Herein, novel poling-free piezoelectric nanogenerators (PENGs) based on self-polarized polyvinylidene difluoride (PVDF) induced by the incorporation of different surface-modified barium titanate nanoparticles (BTO NPs) were prepared via a fully printing process. To reveal the effect of intermolecular interactions between PVDF and NP surface groups, BTO NPs were modified with hydrophilic polydopamine (PDA) and hydrophobic 1H,1H,2H,2H-perfluorodecyltriethoxysilane (PFDTES) to yield PDA-BTO and PFD-BTO, respectively. This study demonstrates that the stronger hydrogen bonding interactions existed in PFD-BTO/PVDF composite film comparative to the PDA-BTO/PVDF composite film induced the higher β-phase formation (90%), which was evidenced by the XRD, FTIR and DSC results, as well as led to a better dispersion of NPs and improved mechanical properties of composite films. Consequently, PFD-BTO/PVDF-based PENGs without electric poling exhibited a significantly improved output voltage of 5.9 V and power density of 102 μW cm-3, which was 1.8 and 2.9 times higher than that of PDA-BTO/PVDF-based PENGs, respectively. This study provides a promising approach for advancing the search for high-performance, self-polarized PENGs in next-generation electric and electronic industries.

Humidity Sustainable Hydrophobic Poly(vinylidene fluoride)-Carbon Nanotubes Foam Based Piezoelectric Nanogenerator

Light weight lead free, polymer, and carbon nanotubes based flexible piezoelectric nanogenerators have prompted widespread concern for harvesting mechanical energy and powering next generation electronics devices. Herein, lightweight polyvinylidene fluoride (PVDF)-carbon nanotube (CNT) foam was prepared to fabricate humid resistant hydrophobic flexible piezoelectric nanogenerator to converts mechanical energy into electricity for the first time. Hydrophobic piezoelectric PVDF-CNT foam with density of 0.15 g/cm³ was prepared by solution route. PVDF-CNT foam exhibited crystalline and a well-defined chain likes structure with 65% fraction of β-phase. Self-poled PVDF-CNT foam shows piezoelectric charge coefficient (d₃₃) of 9.4 pC/N. High d₃₃ of PVDF-CNT foam is caused by dipole alignment induced by local electric field of CNT in the microcellular structure of PVDF. The developed foam exhibits ultrahigh dielectric constant (ε') ∼ 3048 at 150 Hz. Flexible piezoelectric PVDF-CNT foam based nanogenerator was fabricated, which generates high output voltage ∼12 V and current density of 30 nA/cm² at small compressive pressure of 0.02 kgf. Piezoelectric output performance was measured under different humid condition and an output voltage up to 8 V was achieved even under 60% RH condition. PVDF-CNT foam exhibited hydrophobic behavior and high surface water contact angle of 139°. Such high output voltage even under small pressure, without applying electrical poling and under humid condition was originated though CNT induced self-alignment of electric dipoles in PVDF polymer. These excellent performances of developed foam based device confirmed its potential application in organic based ultrasensitive self-powered nanosensors and nanosystems.

Polysaccharides and proteins-based nanogenerator for energy harvesting and sensing: A review

Nanogenerator is a promising energy harvesting device that can scavenge tiny mechanical energy from the surrounding environment, and then convert it into electricity. Natural bio-polymers are the potential candidates for the design of nanogenerators due to their excellent characteristics like piezoelectricity, triboelectricity, non-toxicity, biocompatibility and biodegradability. Especially, nanogenerators using bio-sourced polymers as the core raw materials are suitable for wearable and implantable devices. In this review, major advancements in the sensing field of nanogenerators based on natural polysaccharides and proteins (cellulose, chitosan, alginate, agarose, starch, lignin, silk fibroin, collagen, gelatin, keratin, peptide, M13 bacteriophage, β-cyclodextrin, spider silk, etc.) are summarized. Also, challenges in the improvement of electric output performance, flexibility, anti-humidity and energy management for natural polymers based-nanogenerators are proposed. In the future, they will be applied in daily life as an alternative for traditional power source after addressing issues mentioned above.

A review of low-temperature H2S gas sensors: fabrication and mechanism

Reduced detection temperature of hazardous gases such as H2S can lower power consumption and increase the long-term stability. The decreased operating temperature can be achieved via physical and chemical modification of the sensing layer.