ZnO Nanowire Arrays Research Articles

Role of ZnO nanowire arrays on the impact response of aramid fabrics

Abstract High performance aramid fabrics are extensively used in personal body armor to provide soft, lightweight, yet effective ballistic protection. Grafting nanomaterials on the surface of aramid fibers is one of the promising methods developed for enhancing and tailoring the performance of the fabrics and their composites. Recently, it was shown that growth of ZnO nanowires on aramid fibers drastically improves fabric interyarn friction, as an energy dissipation mechanism. In this work, the influence of ZnO nanowires on the ballistic response of aramid fabrics is evaluated. Intermediate velocity (22–40 m/s) impact tests are conducted on the fabric targets using a gas gun setup. The setup was equipped with a shock accelerometer for measuring contact force and two photoresistors for accurate velocity measurement of the projectile just before impact. As a result of the integration of ZnO nanowires, the maximum impact force of a single aramid fabric is increased approximately 66% in a specific range of impactor velocity. It is postulated that in this velocity range yarn pullout is the dominant failure mechanism of the fabrics for the specific fabric and projectile geometry used in this study. Furthermore, post-test imaging of the fabric targets confirms the efficacy of ZnO nanowires in enhancing impact resistance of aramid fabrics as a result of increased interyarn friction and limiting yarn mobility.

Simulation of wavelength selection using ZnO nanowires array

A new nanometer sized optical device dividing a beam of multi-wavelength light into constituent spectral wavelengths based on ZnO nanowires arrays has been presented, inspired by the diameter dependent energy bandgap of the nanowires. The theoretical validations based on the quantum optics theory have been conducted. It is shown from the simulation results that the output optical spectrum changes upon the energy bandgap of the material, which is determined by the diameter of the wire. The intensity of the optical spectrum is modeled depending on the charge density of the material. Potential applications of the proposed device on pressure sensitive imaging are discussed.

Surface states engineering carbon dots as multi-band light active sensitizers for ZnO nanowire array photoanode to boost solar water splitting

Efficient capture of solar photons is crucial in photo-electrochemical (PEC) water splitting devices for converting solar energy to hydrogen as an energy-dense carrier. Herein, we first reported the unique multi-band light absorption character on carbon dots (CDs) with controllable surface states and built an efficient photoanode by bonding such CDs as favorable solar photosensitizers on one-dimensional (1D) ZnO nanorod arrays (NRAs) for the PEC solar-to-hydrogen (STH) conversion. The multi-transition models related to surface CO, and COH states with different energy levels were identified to dominate the CDs' multi-model optical absorption covering the full-range visible region in the solar spectrum, which renders an excellent advantage of CDs in serving as the solar photosensitizer for photoelectric systems. Moreover, the fabricated ZnO@CDs heterostructure photoanode with the functionalized CDs used to harvest solar photons along with subsequent charge separation at heterointerface and transport along 1D directional channels was demonstrated to boost the photocurrent output and the photoconversion efficiency for solar water splitting.

Enhanced photoelectrochemical performance of n-Si/n-ZnO nanowire arrays using graphene interlayers

We report a powerful approach to improve the photoelectrochemical (PEC) performance of the Si/ZnO nanowire array (NWA) photoanodes via incorporating a graphene layer. The Si/Graphene/ZnO NWAs shows the highest photocurrent, which is, respectively, 1.6 times of that for the Si/ZnO NWAs, and 6.2 times of that for the Si wafers. The introducing of ZnO NWAs and graphene greatly reduces the light reflectance, especially in the UV light region. Carrier recombination at the effective n-Si/n-ZnO junction can compensate the high valence band level of Si and thus enhances the contribution of Si to the photocurrent. The graphene interlayers offer a fast passway for the photogenerated electrons in ZnO to recombine with the photogenerated holes in Si, resulting in enhanced PEC performance of the Si/graphene/ZnO NWAs. This study demonstrates the n/graphene/n heterojunction is a promising configuration for efficient solar water splitting.

Light-Emission Enhancement in a Flexible and Size-Controllable ZnO Nanowire/Organic Light-Emitting Diode Array by the Piezotronic Effect

Enhancing the light emission of LED arrays by the piezotronic effect of ZnO NWs has recently attracted wide attention for the visual mapping of pressure. Here, to overcome deficiencies of ZnO LEDs, such as uncontrollable emission and poor emission efficiency, we designed a flexible, patterned ZnO/organic light-emitting device by combining the advantages of organic light-emitting materials and the piezoelectric effect of ZnO nanowires (NWs). The spatial resolution of the device can be tuned by the pattern size of the ZnO NW array. In addition, the light-emission character is solely determined by the organic light-emitting layer. The light-emission performance of the organic device is enhanced by the piezotronic transistors of the ZnO NW array through the modification of the energy band and reducing the Schottky barrier at the interface of the electrode and the semiconductor. By combining the advantages of the organic material and the piezotronic effect of ZnO NWs, this device shows potential for many appli...

Direct Catalyst‐Free Chemical Vapor Deposition of ZnO Nanowire Array UV Photodetectors with Enhanced Photoresponse Speed

ZnO‐based photodetectors (PDs) have long response times at magnitude of tens to hundreds of seconds, hampering their practical UV detection. The slow oxygen chemisorption/desorption process is the main cause of the slow response of UV PDs based on single ZnO nanowires (NWs). Here, the authors find the response speed of ZnO NW‐based UV PDs could be improved by directly fabricating UV PDs on entangled ZnO NW array grown on SiO2 through a catalyst‐free chemical vapor deposition (CVD) process. Specifically, the interconnections in ZnO NW array creates NW–NW junction barriers, which dominate the inter‐wire charge transport. The switching of UV illumination induces fast tuning of NW–NW junction barrier height, which contributes to the enhanced response speed of the ZnO NW array UV PDs.

Bending-durable colloidal quantum dot solar cell using a ZnO nanowire array as a three-dimensional electron transport layer

The realization of both high efficiency and strong bending-durability is a critical issue for future applications of colloidal quantum dot solar cells (CQDSCs) in flexible devices. Herein, we fabricated flexible CQDSCs with a three-dimensional electron transport layer (ETL) composed of a ZnO nanowire (NW) array for efficient carrier collection and probed the effect of the NW array on the bendability of flexible cells. The good bendability of cells with ZnO NWs was verified from the consistency of photovoltaic performance during mechanical bending treatment with various bending angles and cycles. The efficient release of stress inside the three-dimensional CQDSCs with the ZnO NW array led to the maintenance of 97% of the initial power conversion efficiency at a bending angle of 160°. In contrast, crack formation on the common planar ETL of CQDSCs formed by ZnO nanoparticles resulted in a degradation of device performance to 77% of the initial one at the same angle. Our research demonstrated that the bending stability of the ZnO NW array may be promising for commercial applications of flexible photovoltaic devices.

Analysis on the piezotronic effect in a strained piezo-Schottky junction with AC impedance spectroscopy

Piezotronic effect is always mixed with piezoresistive effect when we investigate the electronic transport behaviors in the study of piezotronic devices. It is essential to separate the two effects, which is beneficial to the data analysis in actual applications. Here, we report a general method, alternative current (AC) impedance spectroscopy, to distinguish the piezotronic and piezoresistive effects in a strained piezo-Schottky junction. The study was performed on a copper (Cu) layer and F:SnO2 ( FTO) glass sandwiched zinc oxide (ZnO) nanowire arrays (Cu/ZnO/FTO). The AC spectra measured at various stress were simulated with appropriate equivalent circuits to reveal the possible change in the device. Our results showed the dominant role of piezotronic effect in the Cu/ZnO/FTO structure instead of piezoresistive effect which has been suspected previously. A capacitive effect of the sputtered ZnO seed layer was also observed. The origin of the capacitance was verified by the bias-independent impedance diagrams. This technique has provided a powerful tool to quantitative analysis on the inner changes of a strained piezotronic device, not only piezo-Schottky junction-based diodes, but also pn junction-based phototronics.

Cytotoxicity of ZnO Nanowire Arrays on Excitable Cells.

Zinc oxide (ZnO) nanowires have been widely studied for their applications in electronics, optics, and catalysts. Their semiconducting, piezoelectric, fluorescent, and antibacterial properties have also attracted broad interest in their biomedical applications. Thus, it is imperative to evaluate the biosafety of ZnO nanowires and their biological effects. In this study, the cellular level biological effects of ZnO nanowire arrays are specifically tested on three types of excitable cells, including NG108-15 neuronal cell line, HL-1 cardiac muscle cell line, and neonatal rat cardiomyocytes. Vertically aligned and densely packed ZnO nanowire arrays are synthesized using a solution-based method and used as a substrate for cell culture. The metabolism levels of all three types of cells cultured on ZnO nanowire arrays are studied using the 3-(4,5-dimethyl-2-thiazolyl)-2,5-diphenyl-2H-tetrazolium bromide (MTT) assays of a full factorial design. Under the studied settings, the results show statistically significant inhibitory effects of ZnO nanowire arrays on the metabolism of NG108-15 and HL-1 cells in comparison to gold, glass, and polystyrene substrates, and on the metabolism of cardiomyocytes in comparison to gold substrate.

High performance hydrogen sensor based on Mn implanted ZnO nanowires array fabricated on ITO substrate

In the present research, we propose a novel approach for the detection of hydrogen gas using Mn implanted ZnO nanowires fabricated onto ITO coated glass substrate by chemical spray pyrolysis deposition. The effect of Mn concentration on the structural, optical and morphological properties of ZnO films were investigated. X-ray diffraction studies showed that the Mn implanted ZnO films were grown as a polycrystalline hexagonal wurtzite phase without any impurities. The (101) peak position of ZnO-Mn films was shifted towards a lower angle with increasing Mn concentration. The optical band gap decreased from 3.45eV to 3.23eV with increasing Mn content. PL spectra, revealed sharp and strong near band edge emission which suggests that ZnO nanowires exhibit high crystalline quality. FE-SEM images of Mn implanted ZnO show perfectly aligned nanowires for all the films fabricated on ITO. The material (Zn, O, Mn) was confirmed by EDX spectra. The hydrogen sensing mechanism of the Mn implanted ZnO nanowire sensor was also discussed. It was found that H2 response was significantly enhanced by more than one order of magnitude with increasing Mn doping concentrations. The studied ZnO-Mn films coated on ITO substrate can be used as a low cost and easy-fabrication hydrogen sensing material.

Fivefold Enhanced Photoelectrochemical Properties of ZnO Nanowire Arrays Modified with C3N4 Quantum Dots

A facile and effective growing strategy of graphite-like carbon nitride quantum dots (CNQDs) modified on ZnO nanowire array composite electrodes has been successfully designed and prepared for the first time. The remarkable quantum enhanced properties were carefully studied by means of scanning electron microscope (SEM), transmission electron microscopy (TEM), X-ray photoelectron spectroscope (XPS), UV-vis diffuse reflectance, PEC performance, and photocatalytic hydrogen production, and the results were in good agreement. Fivefold enhanced photoelectrochemical performances of this novel hierarchical hetero-array prepared in this paper compared with pure ZnO nanowire arrays were obtained under UV-light. The effect was attributed to the remarkable charge separation between CNQDs and ZnO nanowire arrays. Additional investigations revealed that the particular structure of CNQDs/ZnO composites contributed to the separation of a photon-generation carrier and an enhanced photoelectric current. Moreover, the absorption edge of CNQD-modified ZnO nanowire arrays was slightly broadened, and the diameter was reduced as well. The photoelectrochemistry hydrogen evolution splitting water using simulated solar irradiation exhibited the foreground of a possible application of a mechanism of photoelectrochemistry hydrogen evolution over CNQDs/ZnO composite electrodes.

Sonochemical assisted synthesis of RGO/ZnO nanowire arrays for photoelectrochemical water splitting

This article presented the synthesis of a hybrid nanoarchitecture material composed of reduced graphene oxide (RGO) multiple sheets and ZnO nanowire arrays (NWAs) formed on an arbitrary ZnO coated fluorine doped tin oxide (FTO) substrates via pulse sonication and hydrothermal approach. The NWAs have high aspect-ratio, high density, apt positioning and well-ordered formation. FESEM images demonstrated that RGO layers have been effectively intercalated between and on the accessible surfaces of the ZnO NWAs. The diameter of ZnO nanowires is 80–150nm and length about 1–2μm. Raman spectrum of hybrid material exhibited characteristic D and suppressed G peaks for graphene and E2 mode at 437cm−1 for ZnO NWAs. UV–visible spectrum indicated slight red shift towards visible range after formation of RGO/ZnO NWAs heterostructure. The Photoelectrochemical results indicated higher current densities for RGO/ZnO NWAs heterostructure due to water oxidation reaction at the working electrode compared to pristine ZnO NWAs.

Enhanced photoelectrochemical properties of ZnO/ZnSe/CdSe/Cu2-xSe core–shell nanowire arrays fabricated by ion-replacement method

The core-shell structures are designed to take advantages of each material to improve the photoelectrochemical (PEC) performance. Here we report a facile ion-replacement strategy for fabricating ZnO/ZnSe/CdSe/Cu2-xSe core–shell nanowire arrays grown on Fluorine-doped tin oxide (FTO) glass under hydrothermal conditions. Under illumination with AM 1.5G, the designed ZnO/ZnSe/CdSe/Cu2-xSe core–shell nanowire arrays exhibit superior PEC performance with the highest photocurrent density of 20.57mA/cm2, which is 29.4 times higher than that of the ZnO nanowire arrays at 0V versus Ag/AgCl, and achieve the incident photon conversion efficiency (IPCE) of 87.6% at 410nm without applying bias potential. The superior PEC performance of the ZnO/ZnSe/CdSe/Cu2-xSe core–shell nanowire arrays results from the synergistic effects of each material. Vertical aligned ZnO hexagonal prisms provided large specific surface area and electron access along the axial direction. ZnSe layer further extended specific surface area and the range of light absorption. CdSe layer enhanced the visible light absorption vastly and fully utilized the incident light. P-type Cu2-xSe layer produced p-n junctions, which could not only prevent the recombination, but also promote the separation and transmission of photo-generated electron-hole pairs. The synergistic action of each component in ZnO/ZnSe/CdSe/Cu2-xSe core–shell nanowire arrays led an outstanding PEC performance. The synthetic strategy achieved in this work can have promising applications for designing highly efficient electrodes of other materials for water splitting.

Quantitative and simultaneous analysis of the polarity of polycrystalline ZnO seed layers and related nanowires grown by wet chemical deposition

The polarity in ZnO nanowires is an important issue since it strongly affects surface configuration and reactivity, nucleation and growth, electro-optical properties, and nanoscale-engineering device performances. However, measuring statistically the polarity of ZnO nanowire arrays grown by chemical bath deposition and elucidating its correlation with the polarity of the underneath polycrystalline ZnO seed layer grown by the sol–gel process represents a major difficulty. To address that issue, we combine resonant x-ray diffraction (XRD) at Zn K-edge using synchrotron radiation with piezoelectric force microscopy and polarity-sensitive chemical etching to statistically investigate the polarity of more than 107 nano-objects both on the macroscopic and local microscopic scales, respectively. By using high temperature annealing under an argon atmosphere, it is shown that the compact, highly c-axis oriented ZnO seed layer is more than 92% Zn-polar and that only a few small O-polar ZnO grains with an amount less than 8% are formed. Correlatively, the resulting ZnO nanowires are also found to be Zn-polar, indicating that their polarity is transferred from the c-axis oriented ZnO grains acting as nucleation sites in the seed layer. These findings pave the way for the development of new strategies to form unipolar ZnO nanowire arrays as a requirement for a number of nanoscale-engineering devices like piezoelectric nanogenerators. They also highlight the great advantage of resonant XRD as a macroscopic, non-destructive method to simultaneously and statistically measure the polarity of ZnO nanowire arrays and of the underneath ZnO seed layer.

Optimizing the Field Emission Properties of ZnO Nanowire Arrays by Precisely Tuning the Population Density and Application in Large-Area Gated Field Emitter Arrays.

Zinc oxide (ZnO) nanowires are prepared for application in large area gated field emitter arrays (FEAs). By oxidizing Al-coated Zn films, the population density of the ZnO nanowires was tuned precisely by varying the thickness of the Al film. The nanowire density decreased linearly as the thickness of the Al film increased. Optimal field emission properties with a turn-on field of 6.21 V μm-1 and current fluctuations less than 1% are obtained. This can be explained by the minimized screening effect and good electrical conductivity of the back-contact layer. The mechanism responsible for the linear variation in the nanowire density is investigated in detail. Addressable FEAs using the optimal ZnO nanowire cathodes were fabricated and applied in a display device. Good gate-controlled characteristics and the display of video images are realized. The results indicate that ZnO nanowires could be applied in large area FEAs.

Hydrothermal growth of ZnO nanowire arrays: fine tuning by precursor supersaturation

Here we develop a technique that fine tunes the hydrothermal growth of ZnO nanowires to address the difficulties in controlling their growth in a conventional one-pot hydrothermal method. In our technique, precursors are separately and slowly supplied with the assistance of a syringe pump, through the entire course of the growth. Compared to the one-pot method, the significantly lowered supersaturation of precursors helps eliminating competitive homogeneous nucleation and improves the reproducibility. The supersaturation degree can be readily tuned by the precursor quantity and injection rate, thus forming ZnO nanowire arrays of various geometries and packing densities in a highly controllable fashion. The precise control of ZnO nanowire growth enables systematic studies on the correlation between the material's properties and its morphology. In this work, ZnO nanowire arrays of various morphologies are studied as photoelectrochemical (PEC) water splitting photoanodes, in which we establish clear correlations between the water splitting performance and the nanowires' size, shape, and packing density.

Photoluminescence and Growth Mechanism of Oriented Hierarchical Fibrous-Like ZnO Nanowires.

Oriented hierarchical fibrous-like ZnO nanowires with the diameter of about 30–50 nm and the length of about 15–30 um were successfully synthesized on the seed-coated Zn substrates by a simple two-step process. The morphology and structure of the obtained samples were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). It is shown that fibrous-like ZnO nanowires with the aspect ratio of about 500˜1000 present the dense reticular structure, which are grown on ZnO nanowire arrays. But beyond that, as-prepared samples are found to be good single crystalline with hexagonal wurtzite structure and preferential grow along the c-axis. A possible growth mechanism of oriented hierarchical fibrous-like ZnO nanowires is presented in detail, revealing that the synthesis of fibrous-like ZnO nanowires should be attributed to differences in the growth rate of the different crystallographic planes and the two growth ways. The photoluminescence (PL) spectra of oriented hierarchical fibrous-like ZnO nanowires grown at 240 min shows the 5 nm blue-shift and enhanced intensity property in the UV emission.

Evaluation of Effective Elastic, Piezoelectric, and Dielectric Properties of SU8/ZnO Nanocomposite for Vertically Integrated Nanogenerators Using Finite Element Method

A nanogenerator is a nanodevice which converts ambient mechanical energy into electrical energy. A piezoelectric nanocomposite, composed of vertical arrays of piezoelectric zinc oxide (ZnO) nanowires, encapsulated in a compliant polymeric matrix, is one of most common configurations of a nanogenerator. Knowledge of the effective elastic, piezoelectric, and dielectric material properties of the piezoelectric nanocomposite is critical in the design of a nanogenerator. In this work, the effective material properties of a unidirectional, unimodal, continuous piezoelectric composite, consisting of SU8 photoresist as matrix and vertical array of ZnO nanowires as reinforcement, are systematically evaluated using finite element method (FEM). The FEM simulations were carried out on cubic representative volume elements (RVEs). Four different types of arrangements of ZnO nanowires and three sizes of RVEs have been considered. The volume fraction of ZnO nanowires is varied from 0 to a maximum of 0.7. Homogeneous displacement and electric potential are prescribed as boundary conditions. The material properties are evaluated as functions of reinforcement volume fraction. The values obtained through FEM simulations are compared with the results obtained via the Eshelby-Mori-Tanaka micromechanics. The results demonstrate the significant effects of ZnO arrangement, ZnO volume fraction, and size of RVE on the material properties.

ZnO/ZnS Core-Shell Nanowires Arrays on Ni Foam Prepared by Atomic Layer Deposition for High Performance Supercapacitors

Nowadays, a great of attention has been paid to the rational design and fabrication of unique core-shell heterostructures for high performance supercapacitor electrodes. In this work, unique ZnO@ZnS core-shell nanowires (NWs) arrays on nickel foam are successfully prepared by combination of hydrothermal process and atomic layer deposition (ALD). Due to ALD's perfect 3D conformality, ALD ZnS nanofilm can be deposited uniformly and seamlessly on the backbone of ZnO NWs arrays. The ZnO@ZnS/Ni can be directly used as a binder-free supercapacitor electrode with excellent electrochemical performance. This unique electrode exhibits high specific capacitance of 227.0 F g−1 at the current density of 1.28 A g−1, which is much larger than that of commercial ZnS powders. More importantly, this unique core-shell electrode shows great rate capacity. 87.3% of capacitance can be remained when increasing the current density from 1.28 to 12.80 A g−1. Furthermore, this hierarchical structure shows ultrastable cycling ability, as no capacity fading is observed even after 20000 cycles of charge-discharge at high current of 12.80 A g−1. These results indicate that ALD is a powerful and promising technology to design and fabricate unique nanostructured electrode for high-performance supercapacitor applications.

Efficient intracellular delivery of biomacromolecules employing clusters of zinc oxide nanowires.

Zinc oxide (ZnO) nanocomposites have been widely applied in biomedical fields due to their multifunctionality and biocompatibility. However, the physicochemical properties of ZnO nanocomposite involved in nano-bio interactions are poorly defined. To assess the potential applicability of ZnO nanowires for intracellular delivery of biomolecules, we examined the dynamics of cellular activity of cells growing on densely packed ZnO nanowire arrays with two different physical conformations, vertical (VNW) or fan-shaped (FNW) nanowires. Although a fraction of human embryonic kidney cells cultured on VNW or FNW underwent rapid apoptosis, peaking at 6 h after incubation, cells could survive and replicate without significant apoptosis on the foreign substrate after 12 h of lag phase. In addition, the cells formed lamellipodia to wrap FNW, and efficiently took up peptides non-covalently coated on VNW and FNW within 30 min of incubation. Moreover, FNW could mediate intracellular delivery of associated DNAs and their gene expression, suggesting that ZnO nanowires transiently penetrate membranes to mediate intranuclear delivery of exogenous DNA. These results indicate that ZnO nanowire arrays can serve as nanocomposites for manipulating nano-bio interfaces if appropriately modified in a 3-dimensional conformation.