ZnO Nanowire Arrays Research Articles

Investigation of NOx reduction activity of Rh/ZnO nanowires catalyst

The main pollutant exhaust emissions from gasoline engines are HCs, CO, and NOx. Today, the removal of NOx known to cause the formation of ground-level ozone, acid rain and smog is the most significant issue. The stringent emission regulations require that the emission of NOx is limited. In this study, the catalyst was prepared by impregnating Rh on the ZnO NWs grown on monolith cordierite channels in order to investigate in particular its NOx reduction activity and potential of ZnO nanowire structures to be used as monolith catalyst carrier. ZnO nanowire arrays have different morphology and effective porosity. Furthermore, thickness of ZnO nanowire arrays are less than traditional wash coat. Therefore, the ZnO nanowire arrays can be used as alternative monolith catalyst carrier. The structure of the catalyst was examined by SEM and XRD. Activity tests were performed under lean, stoichiometric and rich conditions in order to examine specifically NOx reduction activity of the finished catalyst. The NO reduction activity of the Rh/ZnO NWs catalyst decreases with the increasing oxygen in the gas mixture. This situation is expected in case of excessive oxygen in TWC reactions.

Improved crystalline silicon solar cells by light harvesting zinc oxide nanowire arrays

In this paper, the performance of crystalline silicon solar cells was improved by integrating ZnO nanowire array antireflection coating by hydrothermal growth method. The average length and diameter of the ZnO nanowires are around 5μm and 50nm. Scanning electron microscope and X-ray diffraction patterns revealed that the ZnO nanowire array are vertically aligned to the substrate and highly crystalline. Optical data confirms that the ZnO nanowire array effectively reduces the reflectance, especially between wavelength from 200 to 100nm. Compared to uncoated devices, the ZnO nanowire array improved the overall conversion by around 25%.

CdS nanoparticles sensitized large-scale patterned ZnO nanowire arrays for enhanced solar water splitting

CdS nanoparticle-sensitized patterned ZnO nanowire arrays (NWAs) were designed and synthesized through two-beam laser interference lithography (2BLIL), hydrothermal synthesis, and successive ion layer adsorption and reaction (SILAR) method. The absorption spectroscopies and two-dimensional finite difference time domain (FDTD) simulations carried out on the patterned ZnO NWAs/CdS samples demonstrate a significant enhancement of light absorption in the visible region compared to the non-patterned ones. The patterned ZnO NWAs/CdS photoanodes have a maximum solar-to-hydrogen conversion efficiency of 0.67 % at −0.4 V vs. Ag/AgCl, which is about 22 times that of the non-patterned ZnO NWA samples and about 3 times that of the CdS film samples. Such enhancement is attributed to that the patterned ZnO NWAs with ordered nanostructures have increased light-harvesting ability due to the light scattering effect, and the ZnO/CdS type II band alignment accelerates separation of photogenerated electron-hole pairs. The results suggest that the narrow band gap semiconductor-sensitized ZnO NWAs with highly ordered nanostructures have promising applications in solar energy conversion devices.

Single-layer MoS2 nanopores as nanopower generators.

Making use of the osmotic pressure difference between fresh water and seawater is an attractive, renewable and clean way to generate power and is known as 'blue energy'. Another electrokinetic phenomenon, called the streaming potential, occurs when an electrolyte is driven through narrow pores either by a pressure gradient or by an osmotic potential resulting from a salt concentration gradient. For this task, membranes made of two-dimensional materials are expected to be the most efficient, because water transport through a membrane scales inversely with membrane thickness. Here we demonstrate the use of single-layer molybdenum disulfide (MoS2) nanopores as osmotic nanopower generators. We observe a large, osmotically induced current produced from a salt gradient with an estimated power density of up to 10(6) watts per square metre--a current that can be attributed mainly to the atomically thin membrane of MoS2. Low power requirements for nanoelectronic and optoelectric devices can be provided by a neighbouring nanogenerator that harvests energy from the local environment--for example, a piezoelectric zinc oxide nanowire array or single-layer MoS2 (ref. 12). We use our MoS2 nanopore generator to power a MoS2 transistor, thus demonstrating a self-powered nanosystem.

Aqueous Phase Synthesis and Enhanced Field Emission Properties of ZnO-Sulfide Heterojunction Nanowires

ZnO-CdS, ZnO-ZnS, and ZnO-Ag2S core-shell heterojunction structures were fabricated using low-temperature, facile and simple aqueous solution approaches. The polycrystalline sulfide shells effectively enhance the field emission (FE) properties of ZnO nanowires arrays (NWAs). This results from the formation of the staggered gap heterointerface (ZnO-sulfide) which could lead to an energy well at the interfaces. Hence, electrons can be collected when an electric field is applied. It is observed that ZnO-ZnS NWAs have the lowest turn-on field (3.0 Vμm−1), compared with ZnO-CdS NWAs (6.3 Vμm−1) and ZnO-Ag2S NWAs (5.0 Vμm−1). This may be associated with the pyramid-like ZnS shell which increases the number of emission nanotips. Moreover, the Fowler-Nordheim (F-N) plot displays a nonlinear relationship in the low and high electric field regions caused by the double well potential effect of the heterojunction structures.

Ultraviolet electroluminescence from Au-ZnO nanowire Schottky type light-emitting diodes

Ultraviolet electroluminescence from Schottky type LED device is demonstrated. The device prototype is based on Schottky junctions formed between Au and the top ends of ZnO nanowire arrays. Rectifying current-voltage characteristics are observed, and three different charge transport mechanisms are discussed in detail. Excitonic electroluminescence at around 380 nm is detected at high forward bias and the linear relationship between intensity and current suggests a LED device performance. The observation of LED signals from the simple Schottky structure provides a potential supplement to the category of ultraviolet LED devices.

Enhanced field emission from ZnO nanowire arrays utilizing MgO buffer between seed layer and silicon substrate

Field emitters based on ZnO nanowires and other nanomaterials are promising high-brightness electron sources for field emission display, microscopy and other applications. The performance of a ZnO nanowire field emitter is linked to the quality, conductivity and alignment of the nanowires on a substrate, therefore requiring ways to improve these parameters. Here, ZnO nanowire arrays were grown on ZnO seed layer on silicon substrate with MgO buffer between the seed layer and Si. The turn-on field and enhancement factor of these nanowire arrays are 3.79V/μm and 3754, respectively. These properties are improved greatly compared to those of ZnO nanowire arrays grown on ZnO seed layer without MgO buffer, which are 5.06V/μm and 1697, respectively. The enhanced field emission properties can be attributed to better electron transport in seed layer, and better nanowire alignment because of MgO buffer.

Controlled Deposition of Li Metal

The increasing demand for higher density of energy storage requires breakthroughs in secondary battery techniques and systems. Metallic lithium (Li0) is attractive because it can be a common anode material for the lithium-sulfur (Li-S), lithium-oxygen (Li-O2 or Li-air) and all-solid lithium batteries due to its low electrode potential (-3.04 V vs. SHE) and high specific capacity (3860 mA h g-1) among the known anode materials. However, it is not viable in commercial secondary batteries at present because of its high activity towards the liquid electrolytes, and safety issues associated with the growth and fracture of Li dendrites during cycling. These issues have been obsessing the scientists and hindering the commercialization of secondary lithium batteries for decades. Herein, controlled deposition of Li metal was realized on specially designed micro-/nano-architectured Si wafer and ZnO nanowires array, on the basis of preferential deposition of Li in the grooves of micro-patterned Ti foil, all showing the confining or guiding effect of the porous structure on Li metal deposition. The preferential deposition (and sometimes the growth) of lithium in the defected grooves of inactive Ti foil, in the micro-wells of active Si, and in the nanopores formed by bunches of active ZnO nanowires, indicate that the Li-storage activity of the material becomes less important in some particular cases but their appropriately designed micro- or nano-architectures become the dominating factor, to determine where the lithium will be deposited. These findings enrich our understanding on the intercalation and deposition properties of lithium and provide important revelations on effectively controlling the intercalation, deposition and growth of lithium, and suppressing the growth of lithium dendrites. The hetero-epitaxial growth of lithium on ZnO nanowires further demonstrates that the crystallographic structure of the electrode material can even affect the growth direction of the lithium nanowires. We expect that the concept of designing micro-/nano-architectured electrodes be a promising strategy to tackle some of the obsessing issues of the Li metal anode and promote the commercialization of the secondary lithium batteries. Acknowledgements This work was financially supported by the National 973 Program of China (No. 2015CB251100). Figure 1

Photovoltaic properties of oriented ZnO nanowires arrays decorated with TiO2 shell layer for dye-sensitized solar cell application

Almost vertically aligned ZnO nanowires have been grown on Silicon substrates via a simple hydrothermal method. In order to improve the photoelectric conversion efficiency for fabricated dye-sensitized solar cells (DSSCs), an easily-operated immersing method was employed to fabricate a TiO2/ZnO nanowires array heterojunction, which has advantage of high aspect ratio, low recombination rate and high absorption of visible light. The structure and surface morphology of the samples were characterized by X-ray powder diffraction (XRD) and scanning electron microscopy (SEM), respectively. The photovoltaic properties of TiO2/ZnO based DSCCs were measured by considering the power efficiency (η), photocurrent density (J sc), open-circuit voltage (V oc), and fill factor (FF). An efficiency of 0.559% is achieved for the composite cell, increasing 0.426 and 0.185% for the ZnO nanowires cell and TiO2 cell, respectively. The short-circuit current and open-circuit voltage are also enhancing. The improvements are because of high surface are of TiO2 shell layer, as well as fast electron transport and light scattering effect of ZnO nanowires.

Synthesis and characterization of ZnO NWAs/graphene composites for enhanced optical and field emission performances

Abstract We demonstrate ZnO nanowire arrays (NWAs) and ZnO NWAs/graphene composites grown on ZnO seed layers on Cu substrate via a hydrothermal method and graphene film layer chemically synthesized at low pressure chemical vapor deposition. In the case of the Cu substrate covered with graphene layer, the uniform and vertically aligned ZnO NWAs/graphene composites with the average diameter of 80 nm grown on ZnO seed layers were obtained and growth mechanism of that has been proposed. The photoluminescence (PL) spectra of ZnO NWAs/graphene composites shows enhanced intensity of UV emission and decreased intensity of visible emission. In addition, field emission measurements reveal that the ZnO NWAs/graphene composites have a lower turn-on field of 1.74 V/μm and higher field enhancement factor of 12130 in comparison to that of graphene film layer and ZnO NWAs, which demonstrate ZnO NWAs/graphene composites are characteristic of having excellent emitting behavior in field emission technology.

Template-Assisted Hydrothermal Growth of Aligned Zinc Oxide Nanowires for Piezoelectric Energy Harvesting Applications.

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.

Facile Fabrication of Network-Like Au/ZnO Nanowire Hetero-Arrays for Improved Photoelectrochemical and Supercapacitor Properties

In this study, we have fabricated network-like Au/ZnO nanowire arrays with special space structure using a simple photo-reduction method. The morphology and photoelectrochemical property of the network-like Au/ZnO nanowire arrays could be controlled by changing the reaction parameters such as the photo-reduction time and concentration of reagents. Moreover, their photoelectrochemical and supercapacitor performances have also been performed, revealing that the network-like Au/ZnO nanowire arrays possesses a much higher photocurrent density than the pure ZnO nanowire arrays. The enhanced photoelectrochemical performances could be originated from the surface plasmon resonance within Au nanostructure and the special space structure of network-like Au/ZnO nanowire arrays. The network-like Au/ZnO nanowire hetero-arrays electrodes have been fabricated by a facile photo-reduction method. Benefiting from the special reticular structure, network-like Au nanorods/ZnO nanowire arrays (Au NR/ZnO NWA) has a much higher photocurrent density and specific capacitance than that of ZnO nanowire arrays (ZnO NWA) and Au nanoparticles/ZnO nanowire arrays (Au NP/ZnO NWA). The enhanced PEC and supercapacitor properties have also been investigated detailedly.

Effects of ZnS layer on the performance improvement of the photosensitive ZnO nanowire arrays solar cells

The impact of ZnS layer as an interface modification on the photosensitive ZnO nanowire arrays solar cells was studied. CdS, CdSe and ZnS were deposited on ZnO nanowire arrays by SILAR method. When a ZnS layer was deposited, the quantum dot barrier was indirectly become in contact with the electrolyte, which thus restrained the flow of electrons. The CdS sensitized solar cells has an efficiency of 0.55% with the deposition of the ZnS(3) layer, that is, with a deposition of three times, whereas the CdS/CdSe co-sensitized solar cells has an efficiency of 2.03% with the deposition of the ZnS(1) layer. It was also noted that as the thickness of the of ZnS layer was increased, Voc, Isc and efficiencies of both the solar cells were first increased and then decreased. In addition, the CdS/N719 solar cells has an efficiency of 0.75% with the deposition of the ZnS(2) layer.

Synthesis of Fe Doped ZnO Nanowire Arrays that Detect Formaldehyde Gas.

Owing to their chemical and thermal stability and doping effects on providing electrons to the conduction band, doped ZnO nanowires have generated interest for use in electronic devices. Here we report hydrothermally grown Fe-doped ZnO nanowires and their gas-sensing properties. The synthesized nanowires have a high crystallinity and are 60 nm in diameter and 1.7 μm in length. Field-emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS) are employed to understand the doping effects on the microstructures and gas sensing properties. When the Fe-doped ZnO nanowire arrays were evaluated for gas sensing, responses were recorded through changes in temperature and gas concentration. Gas sensors consisting of ZnO nanowires doped with 3-5 at.% Fe showed optimum formaldehyde (HCHO) sensing performance at each working temperature.

Low switching-threshold-voltage zinc oxide nanowire array resistive random access memory

We have performed the fabrication and characterization of resistive random access memory devices based on Au/ZnO nanowires/Ag capacitor structure. First, vertically aligned ZnO nanowires were grown on Ag electrode layer. The poly 4-vinyl phenol (PVP) was then spin-coated on nanowires as a support and isolation layer. After appropriate etching of PVP in oxygen plasma to expose ZnO nanowire tips, a layer of Au was sputtered onto the tips of ZnO nanowires forming the top electrode. The resulting ZnO nanowire array devices were found to exhibit bipolar resistive switching characteristics. Switching between high resistance state (HRS) and low resistance state (LRS) could be achieved at a very low threshold voltage of 0.3V for a long conducting channel of 500nm, exhibiting a very low electric field, without electrical forming. The resistance ratios of HRS to LRS were higher than two orders of magnitude. The results indicate the application potentials of Au/ZnO nanowires/Ag devices for nonvolatile memory devices.

ZnO nanowires coated stainless steel meshes as hierarchical photocatalysts for catalytic photodegradation of four kinds of organic pollutants

ZnO nanostructures were grown on the stainless steel mesh substrates using an aqueous chemical growth method. The different additives (such as 1,3-diaminopropane and polyethyleneimine) can be used to control the morphology of ZnO nanostructures. ZnO nanowires exhibit very prominent green emission and week UV emission from defect and band gap in the cathodoluminescence spectrum, respectively. The different morphology of ZnO nanostructures on the stainless steel mesh substrates can be used to irradiate UV light for the photocatalytic degradation of four kinds of organic pollutants, such as methylene blue, rhodamine 6G, methyl orange, and 4-nitrophenol. The ZnO nanowires can provide a higher surface-to-volume ratio and stronger defect emission, resulting in their highest photocatalytic performance in 10 W UV light irradiation. The ZnO nanowire arrays on the stainless steel mesh substrates provide a large-scale, facile, low-cost, high surface area, and high photocatalytic efficiency, which shall be of significant value for practical applications of the decomposition of environment pollutants and reusing of wastewater treatment.

Synthesis of ZnO nanowire arrays on ZnO[sbnd]TiO2 mixed oxide seed layer for dye sensitized solar cell applications

ZnO nanowire arrays (NWAs) were synthesized on ZnOTiO2 mixed oxide seeded FTO conducting glass plate by two-step sol-gel and hydrothermal method, respectively. X-ray diffraction patterns reveal the presence of mixed and hexagonal phases in seed layer and NWAs, respectively. Scanning electron microscope images showed that the FTO glass plate is uniformly covered with grains and a few nanorods in seed layer and dense NWAs are vertically grown on the seed layer. The hexagonal structure and high crystal quality have been confirmed by micro Raman spectra. Photoluminescence spectra also present that NWAs have high crystal quality and less atomic defects. UV spectra indicate that NWAs are absorbed more dye molecules and it has the band gap equal to bulk material. The efficiency of ZnOTiO2 mixed oxide seed layer and ZnO NWAs is found to be 0.56 and 0.84% respectively. Electrochemical impedance spectra reveal that NWAs DSSC has high charge transfer recombination resistance than the seed layer DSSC.

Room temperature optical response of zinc oxide nanowires synthesized by chemical bath deposition to toluene vapors

Zinc oxide (ZnO) nanowires were synthesized by a simple and cost‐effective process, namely chemical bath deposition. X‐ray diffraction (XRD) pattern clearly shows that the nanowires are well crystallized in the wurtzite hexagonal structure. Field‐emission scanning electron microscope (FESEM) images show the formation of perfectly aligned and organized nanowires, with no defects visible on their surface. The diameter and length of nanowires is 65 and 850 nm, respectively. The room temperature UV‐excited photoluminescence spectra show the excitonic band in the UV and a wide emission band centered in the visible. This excitonic and visible emission is studied in the presence of toluene vapors and the results demonstrate the change of the visible photoluminescence of ZnO nanowire array and that the material is able to adsorb toluene. Finally, the mechanism of toluene sensing is discussed.

Fabrication of ZnO Nanowires Arrays by Anodization and High-Vacuum Die Casting Technique, and Their Piezoelectric Properties.

In this investigation, anodic aluminum oxide (AAO) with arrayed and regularly arranged nanopores is used as a template in the high-vacuum die casting of molten zinc metal (Zn) into the nanopores. The proposed technique yields arrayed Zn nanowires with an aspect ratio of over 600. After annealing, arrayed zinc oxide (ZnO) nanowires are obtained. Varying the anodizing time yields AAO templates with thicknesses of approximately 50 μm, 60 μm, and 70 μm that can be used in the fabrication of nanowires of three lengths with high aspect ratios. Experimental results reveal that a longer nanowire generates a greater measured piezoelectric current. The ZnO nanowires that are fabricated using an alumina template are anodized for 7 h and produce higher piezoelectric current of up to 69 pA.

Vertically Well-Aligned ZnO Nanowire Arrays Directly Synthesized from Zn Vapor Deposition Without Catalyst

Vertically well-aligned ZnO nanowire (NW) arrays with high density have been successfully synthesized on sapphire substrate by thermal evaporation of the zinc powders without catalysts or additives. The ZnO NWs were characterized by scanning electron microscopy, transmission electronic microscopy (TEM), x-ray diffraction, ultraviolet–visible, photoluminescence, Raman, and x-ray photoelectron spectroscopy. The results showed that the obtained ZnO NWs had diameters in the range of 100–130 nm, lengths over several micrometers and well aligned in the direction perpendicular to the substrate surface. The as-synthesized ZnO NWs, which were single crystalline in a hexagonal structure, showed uniform morphology, faceted planes at the tips of the NWs, and grown along the [001] direction. The as-synthesized NW arrays had a good crystal quality with excellent optical properties, showing a sharp and strong ultraviolet emission at 380 nm and a weak visible emission at around 500 nm.