ZnO Nanowire Arrays Research Articles

High-Quality, Reproducible ZnO Nanowire Arrays Obtained by a Multiparameter Optimization of Chemical Bath Deposition Growth

ZnO nanowire arrays represent an important multifunctional platform in nanotechnology with a diverse set of emerging applications. Many parameters and conditions are involved in the growth process affecting quality and reproducibility and hence the functionality of the nanowire arrays. Therefore, the fabrication of high-quality arrays is challenging and demands fastidious optimization. A multiparameter optimization of the growth process is presented here using the chemical bath deposition method of ZnO nanowires on seeded substrates. Parameters such as the seed layer morphology, ammonia and the polyelectrolyte capping agent concentrations, the molecular weight of the polyelectrolyte capping agent, the ammonia evaporation, the substrate placement into the reactor vessel, and the growth duration were adjusted to provide optimum nanowire arrays. Criteria used for selecting the optimum growth conditions include high aspect ratio, high nanowire length growth rate, good alignment, and minimum defect density in ...

A comparative study of the gas sensing properties of hierarchical ZnO nanostructures

AbstractTwo types of ZnO nanostructure have been fabricated to make a comparative study on their gas sensing performance: the conventional ZnO nanowire arrays were synthesized by hydrothermal method and the hierarchical ZnO nanowires/nanofibers nanostructures were prepared through a combination of the hydrothermal and electrospinning methods. Field emission scanning electron microscopy study showed a quiet homogeneous morphology both for both nanostructures. Three kinds of commonly used gases, such as ethanol, acetone and ammonia were chosen for ZnO nanostructure gas sensing property study. The UV‐Visible spectroscopy measurements showed a higher detection sensitivity of ZnO NWs for ammonia compared to ethanol and acetone, and an enhanced sensing performance for the hierarchical nano‐ structure, which has a higher surface to volume ratio. On the other hand, the enhancement was more obviously in the case of ethanol sensing. (© 2016 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Self-powered UV–visible photodetectors based on ZnO/Cu2O nanowire/electrolyte heterojunctions

ZnO/Cu2O branched heterojunction arrays (BHAs) were synthesized via hydrothermal and chemical bath deposition process for the application in photoelectrochemical type self-powered UV–visible photodetectors. At zero bias, the photodetectors based on the ZnO/Cu2O nanowire/electrolyte heterojunctions exhibited high responsivities of 19.3 mA W−1 and 8.2 mA W−1 for UV and visible light, respectively, with a rise time of 0.14 s and a decay time of 0.36 s. The spectral response measurements carried out on the self-powered photodetectors based on ZnO/Cu2O BHAs demonstrate a significant increase of responsivity in the visible region compared to ZnO nanowire arrays (NWAs). The excellent photodetecting performance, low cost, non-toxic, and facile fabrication process make the photoelectrochemical type self-powered photodetectors based on metal oxide semiconductors promising candidates for next-generation UV–visible photosensing applications.

Gas Sensor Devices based on CuO- and ZnO- Nanowires directly synthesized on silicon substrate

ABSTRACTIn-situ grown CuO and ZnO nanowire (NW) arrays were evaluated for their gas sensing performance. The metal structures were fabricated by standard e-beam lithography, thermal evaporation and lift-off process onto a silicon substrate with gold electrodes. After integration onto a test structure with resistive heater and thermocouple for temperature control, the samples were thermally oxidized at 400°C. During thermal oxidation, nanowires were grown between the oxidized metal structures. The gas sensing performance of the NW array was tested for carbon monoxide, - and a hydrocarbon-mixture (acetylene, ethane, ethene, and propene) at three relative humidity levels.

Role of Substrate Roughness in ZnO Nanowire Arrays Growth by Hydrothermal Approach

The role of substrate roughness in ZnO nanowire (NW) arrays hydrothermal growth has been systematically studied. Six silicon substrates with different roughness by chemical etching have been selected to grow ZnO NW arrays hydrothermally after sputtering 5-nm-thick ZnO seed layer as catalyst. The as-grown samples reveal that average diameters and number densities of ZnO NW arrays are inversely proportional to the increasing substrate roughness observed by atomic-force microscopy and scanning electron microscopy. Furthermore, the theoretically derived equations based on nucleation with the Gibbs free energy to describe relations of substrate roughness versus average NW diameter and NW number density match well with experimental results. Research results in this paper can be used to control the number density and the average diameter of ZnO NW arrays by alternating substrate roughness.

PbS Nanoparticle Sensitized ZnO Nanowire Arrays to Enhance Photocurrent for Water Splitting

Improving the visible-light absorption is one of the key ways to optimize the photoelectrochemical performance of zinc oxide (ZnO) nanowire arrays (NWs). In this study, as-synthesized PbS nanoparticles (NPs), which are adsorbed onto ZnO NWs through a dip-coating method, are used to enhance the photocurrent of the ZnO NW photoelectrochemical anode for water splitting. The morphology crystalline nature and optical properties of the ZnO NWs and PbS nanoparticles (NPs) were characterized by TEM, HRTEM, XRD, and UV-NIR absorption spectra. The hybrid anode exhibits a significant photocurrent density enhancement which is about ten times larger than that of pristine ZnO NWs. Moreover, we believe through some effective modifications there is ample room for improvement of the photoelectrochemical performance of the PbS NP sensitized ZnO NW photoanode that can be achieved.

Enhancing responsivity of ZnO nanowire based photodetectors by piezo-phototronic effect

A promising approach was proposed to enhance the responsivity of a ZnO nanowires array based photodetector with metal-semiconductor-metal structure through piezo-phototronic effect. The ZnO nanowires arrays were synthesized via hydrothermal method to function as both piezoelectric and light-trapping layer. The photoresponses of the ZnO nanowires based photodetector were tuned by the modulation of local Schottky barrier heights (SBH) at the metal-ZnO interfaces as a result of strain-induced polarization charges. Subjected to a −0.62% compressive strain, the responsivity of the prepared photodetector was increased by as much as 176%. Furthermore, the devices hydrothermally synthesized at 0.10M have a larger photoresponse and higher stress sensitivity compared to the ones prepared at 0.05M and 0.15M. This work not only strengthens the fundamental understanding of piezo-phototronic effect on photodetectors but also provides an efficient means for optimization/improvement of piezoelectric semiconductor nanowires based optoelectronic devices.

Sandwiched assembly of ZnO nanowires between graphene layers for a self-powered and fast responsive ultraviolet photodetector

A heterostructure of graphene and zinc oxide (ZnO) nanowires (NWs) is fabricated by sandwiching an array of ZnO NWs between two graphene layers for an ultraviolet (UV) photodetector. This unique structure allows NWs to be in direct contact with the graphene layers, minimizing the effect of the substrate or metal electrodes. In this device, graphene layers act as highly conducting electrodes with a high mobility of the generated charge carriers. An excellent sensitivity is demonstrated towards UV illumination, with a reversible photoresponse even for a short period of UV illumination. Response and recovery times of a few milliseconds demonstrated a much faster photoresponse than most of the conventional ZnO nanostructure-based photodetectors. It is shown that the generation of a built-in electric field between the interface of graphene and ZnO NWs effectively contributes to the separation of photogenerated electron–hole pairs for photocurrent generation without applying any external bias. Upon application of external bias voltage, the electric field further increases the drift velocity of photogenerated electrons by reducing the charge recombination rates, and results in an enhancement of the photocurrent. Therefore, the graphene-based heterostructure (G/ZnO NW/G) opens avenues to constructing a novel heterostructure with a combination of two functionally dissimilar materials.

ZnO nanowires grown directly on zinc foils by thermal oxidation in air: Wetting and water adhesion properties

Large scale ZnO nanowire arrays were grown directly on zinc foils using the thermal oxidation in air method. The X-ray diffraction and reflectance investigations confirm that the as-grown nanowires properties are typical for ZnO having a hexagonal wurtzite crystalline structure and band-gap values between 3.2 and 3.3eV. The scanning electron microscopy images prove that the density and the dimensions (diameter and length) of the ZnO nanowires can be tuned by controlling the oxidation temperature. Wettability studies reveal in the case of Zn foils a hydrophilic behavior with high water droplet adhesion which is transformed into a superhydrophobic one with low water droplet adhesion after the foils' surfaces are covered with ZnO nanowires. Obtaining functional surfaces with such interesting wetting properties using a simple, inexpensive and highly reproducible thermal oxidation in air technique is very attractive for anticorrosion coatings and self-cleaning applications.

Enhanced photocatalytic activity of ZnO nanostructure for water purification

A study of the ZnO nanowire array photocatalysis performance has been carried out under UV radiation for the degradation of the toxic organic compounds such as methylene blue (MB), methyl orange (MO), and acid red 14 (AR14) dyes, which are commonly used in textile and pharmaceutics industries. UV–Vis spectrometry has been used to follow the dye degradation characterization. The degradation mechanism has been proposed for each dye, which will give a reasonable explanation about different degradation degree under same experimental conditions. MB and AR14 showed a better degradation performance than MO.

CO and C3H8 oxidation activity of Pd/ZnO nanowires/cordierite catalyst

Using nanowires grown on monolith cordierite as catalyst wash-coat is a new concept. ZnO nanowires array has different pore-region diffusion of reactants in the catalyst media because it can have greater effective porosity and average diffusion length scale than traditional catalyst carrier. Also, thickness of ZnO nanowires array is less than conventional wash coat. ZnO nanowires were grown on monolith cordierite channels, and Pd was impregnated on the nanowires. The catalyst structure was characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), and atomic absorption spectrometry (AAS). The activity performance of Pd/ZnO nanowires catalyst for C3H8 and CO was examined under lean, stoichiometric and rich conditions. T50 was achieved under stoichiometric condition of gas mixture for C3H8 and CO at 400 °C and 235 °C, respectively.

Fast photoresponse of zinc ferrite nanotube arrays fabricated by electrodeposition

Although nano-sized zinc ferrite materials exhibit excellent visible light activity, their photoresponse characteristics as photodetectors have rarely been investigated. In this paper, zinc ferrite nanotube arrays were fabricated using electrodeposition with the aid of zinc oxide nanowire arrays as a template. These nanotube arrays showed tens of milliseconds-response photocurrents under the irradiation of a 532 nm solid-state laser, and the magnitudes of the photocurrents were linearly increased with increasing laser power. These features of ZnFe2O4 nanotube arrays revealed their potential applications in high-frequency or high-speed photodevices.

SYNTHESIS OF HIGHLY STABLE SILVER-LOADED VERTICAL ZnO NANOWIRES ARRAY AND ITS ACETYLENE SENSING PROPERTIES

A silver-loaded one-dimensional (1D) vertical ZnO nanowires (NWs) array synthesized by a facile seed mediated hydrothermal-RF magnetron sputtering method has been investigated for the fabrication of a highly stable and reproducible acetylene (C2H2) gas sensor. Successful immobilization of silver nanoparticles (NPs) as a sensitizer on the ZnO NWs array significantly enhanced the C2H2sensing properties and showed a stable sensing performance. The grown structure exhibited high response magnitude (30.8 at 1000[Formula: see text]ppm), short response time (43[Formula: see text]s) and excellent selectivity at 220[Formula: see text]C. The enhanced performance can probably be accounted for the effect of combining the highly orientated ZnO NWs and catalytically active silver-based network. The superior sensing features toward C2H2along with broad detection range (1–1000[Formula: see text]ppm), outstanding stability and excellent reproducibility indicate that the sensor is a promising candidate for practical applications.

Spatially controlled growth of highly crystalline ZnO nanowires by an inkjet-printing catalyst-free method

High-density arrays of uniform ZnO nanowires with a high-crystal quality have been synthesized by a catalyst-free vapor-transport method. First, a thin ZnO film was deposited on a Si substrate as nucleation layer for the ZnO nanowires. Second, spatially selective and mask-less growth of ZnO nanowires was achieved using inkjet-printed patterned islands as the nucleation sites on a SiO2/Si substrate. Raman scattering and low temperature photoluminescence measurements were applied to characterize the structural and optical properties of the ZnO nanowires. The results reveal negligible amounts of strain and defects in the mask-less ZnO nanowires as compared to the ones grown on the ZnO thin film, which underlines the potential of the inkjet-printing approach for the growth of high-crystal quality ZnO nanowires.

Ultrafast and Reversible Gas-Sensing Properties of ZnO Nanowire Arrays Grown by Hydrothermal Technique

High-quality single-crystal ZnO nanowire arrays with diameter ranging from 30 to 110 nm were synthesized using a two-step process: (1) synthesis of ZnO thin film by sol–gel technique, which was used as seed layer, and (2) oriented ZnO nanowire arrays were grown on ZnO seed layer using a hydrothermal reaction process at a low temperature of 90 °C. Experimental results reveal an ultrahigh sensitivity of ∼98% to 100 ppm of H2 gas and 93% to 200 ppm of CO gas, an ultrafast recovery of 1 to 2 ms to CO gas with high repeatability. The current transients demonstrate the reversible type sensing to reducing gas (H2 or CO) detection using as-grown ZnO nanowire arrays sensor. The hydrothermally grown ZnO nanowire-array-based gas sensors may have potential application in industry without much modification.

Synthesis of ZnO nanowire arrays/3D graphene foam and application for determination of levodopa in the presence of uric acid

Three-dimensional (3D) graphene foam (GF) was prepared by chemical vapor deposition (CVD) using nickel foam as the template. ZnO nanowire arrays (ZnO NWAs) were vertically grown on the 3D GF by hydrothermal synthesis to prepare ZnO NWAs/GF. This hybrid combines the properties of ZnO NWAs and 3D GF, which has favorable electrocatalysis and outstanding electrical conductivity. The vertically aligned ZnO NWAs grown on the GF enlarged the electroactive surface area, which was investigated from the Fe(CN)63-4+ redox kinetic study. The ZnO NWAs/GF was used as an electrochemical electrode for the determination of Levodopa (LD) in the presence of uric acid (UA). The electrochemical responses of the ZnO NWAs/GF electrode were investigated by cyclic voltammetry (CV) and differential pulse voltammetry (DPV). The results show that the sensitivity of the electrode for LD is 3.15μAμM−1 in the concentration range of 0.05–20μM and the measured detection limit of the electrode for LD is 50nM. The electrode also shows good selectivity, reproducibility and stability. The proposed electrode is succsefully used to determine LD in human plasma samples and it is potential for use in clinical research.

An optimal thermal evaporation synthesis of c-axis oriented ZnO nanowires with excellent UV sensing and emission characteristics

Well-aligned (c-axis oriented) ZnO nanowire arrays were successfully synthesized on Si (100) substrates through an optimized self-seeding thermal evaporation method. An open-ended chemical vapor deposition (CVD) setup was used in the experiment, with argon and purified air as reaction gases. Epitaxial growth of c-axis oriented ZnO nanowires was observed for 5sccm flow rate of purified air, whereas Zn/Zn suboxide layers and multiple polycrystalline layers of ZnO were obtained for absence and excess of purified air, respectively. Ultraviolet (UV) sensing and emission properties of the as-grown ZnO nanostructures were investigated through the current–voltage (I–V) characteristics of the nanowires under UV (λ=365nm) illumination of 8mW/cm2 and using photoluminescence spectra. Nanowires grown under optimum flow of air emitted four times higher intensity of 380nm UV light as well as exhibited 34 times higher UV radiation sensitivity compared to that of other nanostructures synthesized in this study.

Three Dimensional Sculpturing of Vertical Nanowire Arrays by Conventional Photolithography.

Ordered nanoarchitectures have attracted an intense research interest recently because of their promising device applications. They are always fabricated by self-assembling building blocks such as nanowires, nanodots. This kind of bottom up approaches is limited in poor control over height, lateral resolution, aspect ratio, and patterning. Here, we break these limits and realize 3D sculpturing of vertical ZnO nanowire arrays (NAs) based on the conventional photolithography approach. These are achieved by immersing nanowire NAs in thick photoresist (PR) layers, which enable the cutting and patterning of ZnO NAs as well as the tailoring of NAs. Our strategy of 3D sculpturing of NAs promisingly paves the way for designing novel NAs-based nanoarchitectures.

Complex ZnO/ZnS nanocable and nanotube arrays with high performance photocatalytic activity

The complex ZnO/ZnS nanocable and nanotube arrays have been grown on the silicon or glass substrates with ZnO nanowire arrays by an aqueous chemical growth method at the low reaction temperature. The concentration of ZnS precursor and reaction time can be used to control the different sizes and shell thicknesses of complex ZnO/ZnS nanocable and nanotube arrays. The complex ZnO/ZnS nanocable and nanotube arrays exhibit very prominent blue emission and week UV emission from ZnS defect and ZnO band gap in the cathodoluminescence spectrum, respectively. The photocatalytic activity of the different complex ZnO/ZnS nanocable and nanotube arrays were evaluated in the photocatalytic degradation of methylene blue under 10 W UV light irradiation. The complex ZnO/ZnS nanocable and nanotube arrays revealed much higher photodegradation efficiency than ZnO nanowire arrays. This result is attributed to increase the separation of photogenerated electron–hole pairs on the surface of ZnO/ZnS nanocable and the surface-to-volume ratio on the ZnS nanotube. The photocatalytic mechanism under UV light irradiation was proposed. Furthermore, the reusability test can demonstrated that the complex ZnO/ZnS nanocable and nanotube arrays still maintained high photocatalytic activity over ten cycles. The complex ZnO/ZnS nanocable and nanotube arrays provide a facile, low cost, high surface-to-volume ratio, high photocatalytic efficiency, and high reusability, which shall be also promising in many related areas, such as solar energy conversion, water splitting, and energy storage.

Plasmonic enhancement of Au nanoparticle—embedded single-crystalline ZnO nanowire dye-sensitized solar cells

This paper describes the enhanced efficiency of dye-sensitized solar cells (DSSCs) incorporating Au nanoparticle–embedded single-crystalline ZnO nanowire (Au NPs@ZnO NW) arrays. We fabricated these Au NPs@ZnO NW arrays through sequential hydrothermal ZnO growth, with the Au NPs deposited in between the two ZnO growth processes. Interestingly, despite the presence of embedded Au NPs, the ZnO NWs exhibited continuously matched periodic atomic arrangements across the ZnO NW core and shells, indicating that they were single-crystalline. The surface plasmon resonance peaks of the Au NPs on the ZnO NW arrays and of the Au NPs@ZnO NW arrays appeared near 526 and 574nm, respectively; the red-shift of the latter signal confirmed the embedded geometry of the Au NPs in the ZnO NW arrays. Photovoltaic measurements of the Au NPs@ZnO NW DSSCs revealed enhancements in efficiency that depended on the lengths of the ZnO NWs in the arrays; these efficiencies were all greater than those of corresponding ZnO NW–only DSSCs by at least 20%. In accordance with the geometry of our devices and the improved dye absorption within them, it appears that the near-field localized surface plasmon of the Au NPs in the ZnO NWs was the principal factor governing the enhanced power conversion efficiency.