ZnO Nanowire Arrays Research Articles

Tunable Photoluminescence of ZnO Nanowire Arrays via Native Defects

Natural ZnO nanomaterials have abundant oxygen vacancies which has great influence on the physical properties. Here, the ZnO nanowire arrays (ZNAs) have been obtained using microwave heating method. These samples' photoluminescence (PL) properties were studied under different annealing conditions. The field emission scanning electron microscope (SEM) image shows that the vertically aligned ZNAs basically have the same morphology after annealing at different times. X-ray diffraction (XRD) spectra demonstrate the best crystallization when the sample was annealed at 400 °C for 2 hours. Photoluminescence pattern indicates that the near-band-edge (NBE) emission is stronger with increasing annealing times, however, the defect-related emission decreases with the annealing times. This result indicates that crystallization of ZnO nanowires can be improved and the defects can decrease by annealing and the crystallization of the sample is wonderful at 400 °C for 2 h annealing. Meanwhile, adjusting photoluminescence characterizes through annealing can provide the basis for application of ZnO in photoelectric device.

Effect of Hydrothermal Solutions on Optical and Electrical Properties of ZnO Nanowires Grown on a ZnO:Al Seed Layer for Improved Mechanical Performance

ZnO nanowire arrays were fabricated by hydrothermal growth on Al-doped ZnO (ZnO:Al) seed layers coated on soda-lime silicate glass by sol–gel coating. The properties of the ZnO:Al seed layer were evaluated to obtain ZnO nanowires with the optimal size and length and to realize suitable adhesion of the ZnO:Al grains to the substrate. The optimal mechanical performance (adhesion and abrasion resistance) of the ZnO:Al seed layer was obtained at Al 1 at.%. The seed layers annealed between 400 and 500 C exhibited enhanced ZnO nanowire growth. Increasing the annealing temperature within this range improved the electrical and optical properties of the nanowires. Additionally, two chemical compounds, zinc acetate (ZA) and zinc nitrate (ZN), were used to compare the effects of the solution type on the hydrothermal growth. The nanowires grown in the ZA solution were thicker and had higher electrical conductivity compared to the ZN solution. The gamma transmission technique was used to determine the agglomeration of ZnO:Al nanospheres and to examine the crystallite size and density of the ZnO:Al seed layers.

Non-resonant metal-oxide metasurfaces for efficient perovskite solar cells

The short-circuit current density and energy conversion efficiency of single-junction perovskite and perovskite/perovskite tandem solar cells can be increased by photon management. In this study, optical metasurfaces were investigated as potential light trapping structures oppose to commonly used pyramidal surface textures. Herein, metal oxide-based non-resonant metasurfaces were investigated as potential light-trapping structures in perovskite solar cells. The zinc oxide nanowire-based building blocks of the metasurface can be prepared by a templated electrodeposition through a mask of resist. The phase of the incident light can be controlled by the edge length of the subwavelength large zinc oxide nanowires. An array of zinc oxide nanowires was prepared and characterized in the current study. Three-dimensional (3D) finite-difference time-domain (FDTD) optical simulations were used to compare solar cells covered with non-resonant metasurfaces with commonly used light trapping structures. As compared to the solar cells covered with zinc oxide pyramid surface texture, solar cells with the integrated non-resonant metasurfaces exhibit almost identical quantum efficiencies and short-circuit current densities. Investigations of such metasurfaces will not only improve the photon absorption in perovskite solar cells but also reveal a pathway to make high-efficiency next-generation solar cells. Detailed guidelines for the realization of non-resonant metal oxide metasurfaces will be provided.

Hierarchical ZnO@Hybrid Carbon Core-Shell Nanowire Array on a Graphene Fiber Microelectrode for Ultrasensitive Detection of 2,4,6-Trinitrotoluene.

A hierarchical architecture composed of nitrogen (N)-rich carbon@graphitic carbon-coated ZnO nanowire arrays on a graphene fiber (ZnO@C/GF) was fabricated by direct growth of a ZnO@zeolitic imidazolate framework-8 (ZIF-8) core-shell nanowire array on a GF followed by annealing and used as a microelectrode for detection of 2,4,6-trinitrotoluene (TNT). In such a design, ZnO accumulated TNT through a strong nitroxide-zinc interaction and ZIF-8 served as the precursor of the N-rich carbon@graphitic carbon layer that seamlessly connected ZnO with the GF to improve the poor conductivity of ZnO, thus enhancing the sensitivity of the ZnO@C/GF microelectrode. The constructed hierarchical hybrid fiber microsensor exhibited a wide linear response to TNT in a concentration range of 0.1-32.2 μM with a low detection limit of 3.3 nM. This ZnO@C/GF microelectrode was further successfully applied to the detection of TNT in lake and tap water, indicating its promise as a portable sensor for the electrochemical detection of explosive compounds.

Pentagram-Shaped Ag@Pt Core-Shell Nanostructures as High-Performance Catalysts for Formaldehyde Detection.

High-performance HCHO sensors are of great importance in various application fields such as indoor air quality assessments. Herein, bimetallic Ag-Pt nanoparticles are synthesized as high-performance catalysts for ZnO-based gas sensors. Spherical aberration (Cs)-corrected transmission electron microscopy images with atomic resolution clearly indicate that the prepared nanoparticles exhibit a novel Ag@Pt core-shell nanostructure with a pentagram shape. For high-performance HCHO sensor construction, integrated micro-electrodes are first fabricated with the microelectromechanical system (MEMS) technology. Then, the hydrothermal route is used to self-assemble well-aligned ZnO nanowire arrays onto the sensing microregion. After that, the pentagram-shaped Ag@Pt nanoparticles are loaded onto the surface of ZnO nanowires with the inkjet printing technique to form MEMS sensors with Ag@Pt@ZnO as the sensing material. The thoroughly sensing experiments indicate that the Ag@Pt nanoparticles exhibit satisfied catalytic activation to HCHO molecules. The experimental observed detection limit of our sensor to HCHO reaches the parts per billion level. To elucidate the HCHO-sensing mechanism, the online mass spectrum (online MS) is utilized to analyze the components of exhaust gas stream of HCHO flowing through the Ag@Pt@ZnO material. The online MS indicates that with the Ag@Pt catalyst, HCHO molecules are partially oxidized to HCOOH molecules at low temperatures and are completely oxidized to CO2 molecules at high temperatures.

ZnO Nanowire-Anchored Microfluidic Device With Herringbone Structure Fabricated by Maskless Photolithography.

The integration of nanomaterials in microfluidic devices has emerged as a new research paradigm. Microfluidic devices composed of ZnO nanowires have been developed for the collection of urine extracellular vesicles (EVs) at high efficiency and in situ extraction of various microRNAs (miRNAs). The devices can be used for diagnosing various diseases, including kidney diseases and cancers. A major research need for developing micro total analysis systems is to enhance extraction efficiency. This article presents a novel fabrication method for a herringbone-patterned microfluidic device anchored with ZnO nanowire arrays. The substrates with herringbone patterns were created by maskless photolithography. The ZnO nanowire arrays were grown on the substrates by chemical bathing. The patterned design was to introduce turbulent flows as opposed to laminar flow in traditional devices to increase the mixing and contact of the urine sample with ZnO nanowires. The device showed reduced flow rates compared with conventional planar microfluidic channels and successfully extracted urine EV-encapsulated miRNAs.

Facile Fabrication of Plasmonic Enhanced Noble-Metal-Decorated ZnO Nanowire Arrays for Dye-Sensitized Solar Cells

Novel decoration of high aspect ratio zinc oxide nanowires (ZnO NWs) with noble metals such as Ag and Au nanoparticles (NPs) was demonstrated in this work. A facile method of chemical deposition with good controllability, as well as good homogeneity would be a huge advantage towards large scale fabrication. The highlight of this work is the feasibility of multiple component decoration such as a hybrid (co-exist) Ag-Au NPs decorated ZnO NWs formation that could be beneficial towards the development of nanoarchitectured materials with the most desired properties. The local surface plasmon effect (LSPR) of Ag and Au NPs were confirmed using extinction spectra and significant photoelectrochemical conversion efficiency (PCE) enhancement of dye-sensitized solar cells (DSSCs) was achieved. The Ag-NPs and hybrid Ag-Au NPs decorated ZnO NWs marked an impressive 125 and 240% efficiency improvement against pure ZnO NWs. The improved dye light extinction resulted from the LSPR effect that had enabled greater electron generation leading to improved PCE. As the complex design of oxides' nanoarchitectures have reached a point of saturation, this novel method would enable further enhancement in their photoelectrochemical properties through decoration with noble metals via a simple chemical deposition route.

Effect of Process Duration on ZnO Nanowire Growth & Application for Pressure Sensing

This paper presents the synthesis and characterisation of ZnO nanowire array using hydrothermal method. Synthesis of nanowire array on Elgiloy, a metallic alloy substrate was carried out for two different durations (5 hours and 10 hours) and characterised using Scanning Electron Microscope (SEM) to study the effect of growth duration on nanowire morphology. The SEM image indicated tapering of nanowire tip for growth duration of 10 hours. The as-synthesised nanowire array was tested for its performance as a pressure sensor using a laboratory test set-up with a fully blown balloon. This study demonstrates that the hydrothermally grown nanowire array is a good candidate for the development of pressure sensors.

Fabrication and simulation of peroviskite solar cells comparable study of CuO and Nano composite PANI/SWCNTS as HTM

Alternate CuO instead of Novel low-cost Polyaniline (PANI)-single wall carbon nanotubes (SWNT or SWCNT) nanocomposite—based hole transport materials (HTMs) as substitutes by thin film of CuO designed as HTM for the applications in Perovskite solar cells. The HTMs were prepared through a facile steps reaction from cheap starting material. These HTMs exhibit good solubility and charge-transport ability but it has big issue regarding temperature due to degrade the conductive polymer, in the same time using ZnO NW as ETM. The perovskite based on PANI-SWNTs achieved power conversion efficiency (PCE) of 5% under air conditions. For perovskite, we found that the solar cells synthesized based on vertical ZnO nanowire arrays by using perovskite organic materials of CH3NH3PbI3 as active absorber materials, Pani-SWNT enable better photovoltaic performance than using CuO as HTM theoretical study by using simulator SCAPS solar cells. These results show that using CuO instead of PANI-SWNTs-HTMs have great prospect to replace the Pani-SWCNTs which is the most widely used HTM to obtain high—stability devices with temperature.

Systematic study of vertically aligned ZnO nanowire arrays synthesized on p-GaN substrate by hydrothermal method

This study presents a simple and effective way to synthesize well-aligned single-crystalline ZnO nanowires (NWs) by the hydrothermal method (HTM). The influence of growth conditions on the morphology of as-grown ZnO NWs is discussed from the chemical reaction equilibrium perspective. The height and diameter of the NWs demonstrated tunability through the optimization of the growth parameters. The ZnO NWs exhibited optimal alignment along the c-axis perpendicular to the substrate, as observed through scanning electron microscopy and X-ray diffraction. The nucleation and epitaxial growth mechanism of NWs on p-GaN has also been investigated. The conditions to obtain high-density and vertically aligned NWs were optimized as 0.03 mol l−1 concentration, 90 °C temperature and 3 h time. Photoluminescence measurements revealed a strong and narrow UV emission at 380 nm. The formation of a ZnO/p-GaN heterojunction through HTM provides promising optoelectronic device applications riding on the advantages of feasibility, cost-effectiveness and non-toxicity.

Comparative Study of ZnO Nanostructures Grown on Variously Orientated GaN and AlxGa1−xN: The Role of Polarization, and Surface Pits

Through comparing ZnO directly grown on the substrates of a-plane, c-plane, and (11-22) plane GaN and AlxGa1−xN (0.06 ≤ x ≤ 1), the roles of different factors that may influence growth have been studied. Seeded by surface pits, ZnO nanowire (NW) preferentially grew along the polarized direction on top of the nonpolar GaN (laterally aligned), polar GaN and AlGaN (vertically aligned), and semipolar GaN (obliquely upward aligned). Nanosheets were easily formed when the polarized surface of the AlGaN film was not intact. The kinetic effect of polarization must be considered to explain the high aspect ratio of NWs along the polarized direction. It was found that dislocation affected NW growth through the surface pits, which provided excellent nucleation sites. If the surface pits on GaN could be controlled to distribute uniformly, self-organized ZnO NW array could be controllably and directly grown on GaN. Moreover, surface pits could also seed for nanosheet growth in AlN, since Al(OH)4− would presumably bind to the Zn2+ terminated surface and suppress the kinetic effects of polarization.

Tuning Areal Density and Surface Passivation of ZnO Nanowire Array Enable Efficient PbS QDs Solar Cells with Enhanced Current Density

AbstractColloidal PbS quantum dots (QDs) have provoked a revolution in the field of optoelectronic devices owing to their low‐cost fabrication processing and excellent physical properties. Recently, the fabrication of nanostructured PbS QD photovoltaic (PV) devices based on zinc oxide (ZnO) nanowire array appears as an effective strategy for improving the overall device performance. Despite its potentially strong impact on the device performance, the role of nanowire areal density on photon absorption and exciton dynamics has not yet been studied and still remains unexplored. Here, for the first time, the areal density of ZnO nanowires is tuned through controlling the precursor concentration and its impact on PbS QD PV performance is studied. It is found that the device with optimized ZnO nanowire areal density yields significantly increased power conversion efficiency (PCE) (10.1% vs 8.5% of control nanowire‐based device) due to improved antireflection effect and reduced surface recombination states. To further improve the photovoltaic performance, the ZnO nanowire surface is treated with hydrogen plasma. Transient photovoltage (TPV) measurement reveals that this passivation process noticeably reduces the nonradiative charge recombination yielding a champion device with a remarkable PCE of 10.8%.

CdS quantum dots/Au nanoparticles/ZnO nanowire array for self-powered photoelectrochemical detection of Escherichia coli O157:H7

In this paper, the hydrothermally grown ZnO nanowire array (NWs) was modified by Au nanoparticles (NPs) and CdS quantum dots (QDs) to construct a high-performance photoelectrochemical (PEC) electrode. The aligned ZnO NWs, which decorated Au NPs and CdS QDs have the effective light absorption range from UV to visible region. This hybrid structure provided a self-powered PEC electrode with a favorable energy-band configuration for fast charge separation and transportation. Meanwhile, the Au NPs and CdS QDs also made increase of the surface area to improve the immobilization of the analytes. After assembling aptamer as recognition element, this composite nanoarray was further developed as a self-powered PEC biosensor by synergizing above multiple enhancement factors. The PEC aptasensor exhibited a rapid response in a wide linear range of 10–107 CFU/mL with the detection limit as low as 1.125 CFU/mL to Escherichia coli O157:H7 (E. coli O157:H7). This approach would offer an alternative PEC transduction for fast environment monitoring and clinical diagnosis related to pathogenic bacteria.

A synergistic combination of zinc oxide nanowires array with dual-functional zeolitic imidazolate framework-8 for hybrid nanomaterials-based gas sensors

The complementary hybridization of nanomaterials enables to realize newly synergistic properties, which is a central concept in next-generation widespread applications. In particular, it is well-established that the hybridization of ZnO nanostructure with zeolitic imidazolate framework-8 (ZIF-8) allows the improvement of gas selectivity owing to molecular sieving capability caused by inherently size-defined porous structures of the ZIF-8. Here, we focused on a different viewpoint related with the preconcentration effect of ZIF-8 hybridized with ZnO nanowires (NWs) array on NO2, NH3, and H2 gas response. Hydrothermally-grown, structurally optimized ZnO NWs array are rationally employed for the nucleation sites of ZIF-8 nanocrystals as well as gas sensing channel. The chemical conversion of the surface of ZnO NWs to ZIF-8 was systematically implemented for the formation of core-shell hybrid structure, where morphological features of the ZIF-8 nanocrystals hybridized with the ZnO NWs array were optimized by simply adjusting synthetic conditions. The thickness of encapsulated ZIF-8 could be effectively manipulated by altering the concentration of 2-methylimidazole (HmIM), which strongly affects to determining gas response of ZIF-8@ZnO NWs-based gas sensors because of a variation in gas-loading capabilities related with the preconcentration effect. For the gas sensor based on ZIF-8@ZnO NWs synthesized using 8 mM HmIM solution, the gas response of NO2, NH3, and H2 gases were improved compared to those of the pristine ZnO NWs-based gas sensor. In addition, we consolidated the molecular sieving effect of the ZIF-8 by measuring the gas response of CH4 and C3H8 for suggesting the dual functionality of the hybrid system.

Coupling Enhancement of Photo-Thermoelectric Conversion in a Lateral ZnO Nanowire Array

Scavenging sustainable energy from surroundings by the photothermoelectric effect has attracted considerable attention along with the growing crisis of energy security. The cross interaction between heat and light is important for understanding the photothermoelectric mechanism. Herein, we demonstrate a photothermoelectric device based on an undoped ZnO nanowire array with a lateral structure. The output of the ZnO nanowire array depends linearly on the position of the light spot with zero output for the light spot in the center, achieving a power factor of 85 μW m–1 K–2 at room temperature. A coupling enhancement (″1 + 1 > 2″) of photothermoelectric conversion is observed in the ZnO device. To reveal the intrinsic mechanism, the cross interaction between heat and light was investigated under various working conditions. Under a temperature gradient of 12.5 K, the output current and voltage of the lateral ZnO device under 227.4 mW/cm2 light illumination are enhanced by 67.2% and 10.9%, respectively. This s...

Alternating Current‐Driven Oxide Powder Electroluminescent Device Employing Vertically Aligned ZnO Nanowire Array

A novel type of AC‐driven oxide powder electroluminescent (EL) device is demonstrated. The device is based on stable heterogeneous junctions between Mn2+‐activated zinc germanate (Zn2GeO4:Mn) oxide phosphor and a vertically aligned ZnO nanowire array. The ZnO nanowire array is synthesized on an ITO‐coated glass substrate via a hydrothermal growth method. A stable phosphor–nanowire junction is formed by simple drop‐coating of a Zn2GeO4:Mn oxide phosphor suspension on the nanowire layer followed by annealing at 400 °C for 3 h. Bright green light (≈20 cd m−2) with a wavelength of 535 nm from the as‐fabricated EL device is observed, and the as‐fabricated device shows excellent stability in air, maintaining a brightness of ≈10 cd m−2 for 15 days with less than 2.5% intensity loss. The electroluminescence emission mechanism is discussed in detail. This new and facile strategy to build an oxide powder EL device can enable cost‐effective, highly stable, and energy‐efficient devices for lighting or display applications.

Nitrogen plasma treatment of ZnO and TiO2 nanowire arrays for polymer photovoltaic applications

This work reports on a simple, yet unique approach to improving the opto-electronic properties of vertically-aligned arrays of rutile TiO2 and Wurzite ZnO nanowires by means of controlled nitrogen doping during exposure to highly kinetic radio-frequency generated N2 plasma radicals. Morphologically, the plasma treatment causes a distortion of the vertical alignment of the nanowires due to a dissociation of the weak Van der Waals force clustering the nanowires. Optical spectroscopy show that plasma treatment increases the light transmission of TiO2 arrays from 48% to 90%, with the ZnO arrays exhibiting an increase from 70% to 90% in the visible to UV range. The as-synthesized TiO2 array has an indirect band gap of 3.13 eV, which reduces to 3.03 eV after N2 treatment, with the ZnO equivalent decreasing from 3.20 to 3.17 eV post plasma exposure. A study of the 3d transition metal near edge fine structure of both Ti and Zn show that the N2 plasma treatment of the nanowires results in nitrogen doping of both TiO2 and ZnO lattices; this is confirmed by scanning transmission electron microscopy coupled with energy dispersive spectroscopy x-ray maps collected of single nanowires, which show a clear distribution of nitrogen throughout the metal-oxide. Application of these structures in P3HT:PCBM polymer blends shows progressive improvement in the photoluminescence quenching of the photoactive layer when incorporating both undoped and nitrogen-doped nanowires.

Using non-continuous gold film as the seed layer for developing zinc oxide nanowire array

This study aims to discuss the grown morphology and current generation of ZnO nanowire array developed from non-continuous Au thin film as the seed and compare the results with the nanowire array from ZnO thin film as the seed layer. The zinc nitrate hexahydrate and hexamethylenetetramine used as the reaction solution and the hydrothermal process with process temperature less than 95°C are applied. It is revealed that ZnO nanowires developed with non-continuous Au film as the seed appear sparser density, present better crystallization direction (002) and higher collimation. The effect of concentration of reaction solution on the diameter, length and distribution of nanowires is reported. The electron back scattering diffraction technique confirms each grown wire is a single crystal from the bottom to top. The ZnO nanowires with piezoelectric property are measured with the conductive atomic force microscope system. The result shows that single ZnO nanowire could generate more than 800 pA current when being bent.

Superhydrophobic behavior of polymer-coated nanowire-arrays as a function of interfacial affinity and etching

We have investigated the superhydrophobic behavior of coatings of polydimethylsiloxane (PDMS) and polystyrene (PS) on ZnO nanowire-arrays. PDMS produced a conformal coating on the ZnO nanowires due to high interfacial affinity between ZnO and PDMS, resulting in a large water contact angle of 168.0° (compared to 108.8° for planar PDMS surfaces) and droplet roll-off angles that were too small to measure. The superhydrophobicity of these coatings is among the largest reported for any polymeric coating. PS instead preferentially coated the bases of the ZnO nanowires and valleys between nanowires due to low interfacial affinity between ZnO and PS, resulting in a water contact angle of 149.3° (compared to 90.5° for planar PS surface) and droplet roll-off angles of 5°–8°. If, however, the ZnO nanowires were etched away after PS deposition, hollow tube-like nanostructures of PS were left behind, and exhibited a water contact angle of 164.3° and droplet roll-off angles of 1°–2°. These nanostructured PS coatings offer unique potential for food-safe superhydrophobic coatings because they consist only of PS without any inorganic components.

Homogeneous ZnO nanowire arrays p-n junction for blue light-emitting diode applications.

ZnO is a promising short-wavelength light-emitting materials for its wide bandgap (3.37 eV) and large exciton binding energy (∼60 meV), however, practical p-type doped ZnO is the main challenge in this field. Here, Blue light-emitting diodes (LEDs) based on the homogeneous junctions of Sb doped ZnO nanowire arrays grown on Ga doped ZnO single crystal substrate are fabricated. Element analysis, FET and Hall-effect measurements demonstrate that the Sb atom has been successfully doped into ZnO nanowires to from p-type conductivity. On the benefit of high quality of nano-size homojunction, the fabricated LED shows low turn-on voltage turn-on voltage as low as 3.4 V and strong blue emission peak located at 425 nm at room temperature, which originate from interfacial recombination of ZnO nanowire p-n homojunctions. The present blue LED based on ZnO material may have potential applications in short-wavelength optoelectronic devices.