Vertically-aligned ZnO Nanorods Research Articles

Synthesis of reduced graphene oxide/ZnO nanorods composites on graphene coated PET flexible substrates

In this work, reduced graphene oxide (rGO)/ZnO nanorods composites were synthesized on graphene coated PET flexible substrates. Both chemical vapor deposition (CVD) graphene and reduced graphene oxide (rGO) films were prepared following by hydrothermal growth of vertical aligned ZnO nanorods. Reduced graphene sheets were then spun coated on the ZnO materials to form a three dimensional (3D) porous nanostructure. The morphologies of the ZnO/CVD graphene and ZnO/rGO were investigated by SEM, which shows that the ZnO nanorods grown on rGO are larger in diameters and have lower density compared with those grown on CVD graphene substrate. As a result of fact, the rough surface of nano-scale ZnO on rGO film allows rGO droplets to seep into the large voids of ZnO nanorods, then to form the rGO/ZnO hierarchical structure. By comparison of the different results, we conclude that rGO/ZnO 3D nanostructure is more desirable for the application of energy storage devices.

Solution growth of vertical aligned ZnO nanorod arrays on ZnO seed layers fabricated by Langmuir–Blodgett method

Abstract In this paper, vertical aligned ZnO nanorod arrays were successfully grown by solution method onto seed layers of ZnO that fabricated by Langmuir–Blodgett (LB) method. The influences of seed layers (numbers of layers, fabricating routes, and with/without seed layers) and growing parameters (temperature, time and solution concentration) on morphology and orientation of ZnO nanorods were observed by SEM analysis results and discussed in details. XRD analysis results revealed that ZnO nanorods were preferred-oriented in (0 0 2) planes with wurtzite structure and highly crystallinity. UV–Vis transmission spectra showed the importance of the microstructures and the array degrees of ZnO nanorods to its optical properties. ZnO nanorods in good orientation led to higher transmittance and improved optical properties.

Substrate dependent growth modes of ZnO nanorods grown by metalorganic chemical vapor deposition

Transmission electron microscopy studies were performed to reveal the underlying growth modes of ZnO nanorods by metalorganic chemical vapor deposition with respect to the type of substrates. On well-matched substrates, such as GaN/Al2O3 (0001) and ZnO (0001) substrates, a 2-dimensional (D) layer-by-layer growth occurs. During further growth after a certain critical thickness of ∼40nm, 1D nanorods start to grow on the 2D layer. In contrast, only vertically-aligned ZnO nanorods grow on a partially-matched substrate, such as Al2O3 (0001), after the coalescence of 3D ZnO islands. On mismatched substrates, such as Si (111) and Si (001), an Si–O amorphous layer of ∼5nm forms first, on which 3D ZnO islands grow. After their coalescence, 1D nanorods start to grow.

Reduced Graphene Oxide-ZnO Nanocomposites for Flexible Supercapacitors

In this paper, we present a novel nanostructure of reduced graphene oxide supported by vertical aligned ZnO nanorods for flexible supercapacitors. Layer-by-layer strategies are adopted to fabricate the electrodes on PET flexible substrates. Cu foil, Al-doped ZnO film and chemical-vapor deposition graphene is introduced on PET to serve as current collectors. ZnO nanorods are synthesized by a hydrothermal method under low temperature. The structural and electrochemical properties of these nanostructured electrodes are systematically investigated.

Construction and Evaluation of High-Quality n-ZnO Nanorod/p-Diamond Heterojunctions

Vertically-aligned ZnO nanorods (NRs) arrays were synthesized by a low-temperature solution method on boron-doped diamond (BDD) films. The morphology, growth direction, and crystallinity of the ZnO NRs were studied by scanning electron microscopy, X-ray diffraction and cathodoluminescence. Electrical characterization of the ZnO NR/BBD heterostructures revealed characteristic p-n junction properties with an on/off ratio of about 50 at +/- 4 V and a small reverse leakage current approximately 1 microA. Moreover, the junctions showed an ideality factor around 1.0 at a low forward voltage from 0 to 0.3 V and about 2.1 for an increased voltage ranging from 1.2 to 3.0 V, being consistent with that of an ideal diode according to the Sah-Noyce-Shockley theory.

Transparent conducting ZnO nanorods for nanoelectrodes as a reverse tunnel junction of GaN light emitting diode applications

We demonstrated transparent vertical aligned ZnO nanorod arrays for nanoelectrodes as a reverse-tunnel junction of GaN LEDs. GaN LEDs with ZnO nanorod arrays for nanoelectrodes demonstrated three times enhanced light output power compared with that of GaN LEDs with tunnel junction ZnO thin film, resulting from increased light extraction efficiency. Tunnel junction ZnO nanorod arrays for nanoelectrodes offer both high transparent current spreading layer for uniform current injection and easy method for high extraction efficiency.

Micromechanism and Kinetic Formulation of Vertically Aligned ZnO Nanorods Grown on Catalytic Bilayers

Vertically aligned ZnO nanorods were grown at 700°C for 2 h on sapphire substrates with catalysts in bilayer configurations of Sn (top)/Ni (bottom) and Sn/In, where the top layer is formed by sputtering and the bottom one is deposited by spin coating. The effects of bilayer catalysts on growth kinetics of nucleation and growth, growth micromechanism, and vertical alignment of growing ZnO nanorods have been investigated. The vertical alignment of the Sn/Ni-catalyzing ZnO nanorods is determined at the initial nucleation stage, where the nuclei are formed as regular candlestick-like platforms. The reason for the formation of the candlestick-like nuclei is due to the contribution of strain energy built in the underlying catalyst bilayers. The variations of axial and radial dimensions with growth duration for the growth of ZnO rods were explained and data fitting with the aids of kinetic growth equations, which are based upon the well-known ledge model for crystal growth from vapor and diffusion kinetics.

Patterned growth of vertically-aligned ZnO nanorods on a flexible platform for feasible transparent and conformable electronics applications

Despite the attractiveness of low temperature hydrothermal processes, the synthesis of vertical ZnO nanostructures has mostly been limited to rigid substrates. Moreover, patterned growth of nanostructures is also commonly carried out on rigid substrates, since conventional optical lithography is not easily applied to polymeric substrates, as focusing and reaction of the substrate with the organic solvent used in the lithography process prove to be a challenge. Here, we demonstrate the limited work on laser writing lithography patterned growth instead of the commonly used soft lithography patterned growth of nanorods on the transparent flexible substrate polyethylene terephthalate (PET) with a practical device demonstration. The visibly-transparent nanorods on the PET platform constitute a superior structural integrity with ohmic electro-conductivity even in a highly bent state. Accordingly, this can pave the way towards integration of vertically-aligned 1D nanostructures on a flexible platform for a transparent, conformable, shock-proof and lightweight product.

Growth of vertically aligned ZnO nanorods using textured ZnO films

A hydrothermal method to grow vertical-aligned ZnO nanorod arrays on ZnO films obtained by atomic layer deposition (ALD) is presented. The growth of ZnO nanorods is studied as function of the crystallographic orientation of the ZnO films deposited on silicon (100) substrates. Different thicknesses of ZnO films around 40 to 180 nm were obtained and characterized before carrying out the growth process by hydrothermal methods. A textured ZnO layer with preferential direction in the normal c-axes is formed on substrates by the decomposition of diethylzinc to provide nucleation sites for vertical nanorod growth. Crystallographic orientation of the ZnO nanorods and ZnO-ALD films was determined by X-ray diffraction analysis. Composition, morphologies, length, size, and diameter of the nanorods were studied using a scanning electron microscope and energy dispersed x-ray spectroscopy analyses. In this work, it is demonstrated that crystallinity of the ZnO-ALD films plays an important role in the vertical-aligned ZnO nanorod growth. The nanorod arrays synthesized in solution had a diameter, length, density, and orientation desirable for a potential application as photosensitive materials in the manufacture of semiconductor-polymer solar cells.PACS61.46.Hk, Nanocrystals; 61.46.Km, Structure of nanowires and nanorods; 81.07.Gf, Nanowires; 81.15.Gh, Chemical vapor deposition (including plasma-enhanced CVD, MOCVD, ALD, etc.)

Structural, Optical and Electrical Properties of Nitrogen Ion Implanted ZnO Nanorods

This study examined the micro-structural and electrical properties of N+-ion-implanted ZnO nanorods. Nitrogen ions with energies of 10-90 keV and beam fluxes of 10(13)-10(16) ions/cm2 were implanted on vertically-aligned ZnO nanorods. Energy dispersive X-ray spectroscopy measurements showed that N+ ions were spread uniformly over the nanorods. Extended X-ray absorption fine structure measurements revealed that the implanted N+s had partially substituted for the oxygen sites. Photoluminescence measurements showed a neutral-donor bound exciton peak at 3.36 eV and a two-electron-satellite peak at 3.33 eV independent of the ion energy and flux. The I-V characteristic curves showed that the current density was not changed by the N+ ion energy and flux much. These results strongly suggested that the N ions substituted for the oxygen sites were neutral.

Synthesis conditions, light intensity and temperature effect on the performance of ZnO nanorods-based dye sensitized solar cells

We present in this work a careful study of the different parameters affecting vertically-aligned ZnO-nanorods (NRs) based dye sensitized solar cells (DSCs). We analyze the effect of synthesis conditions, light intensity, UV light and working temperature, and correlated them to the final photovoltaic properties of the DSC. Although similar studies can be found in the literature for DSCs based on TiO2, this work is, to our knowledge, the first detailed study carried out for DSC based on vertically-aligned ZnO nanorods. The ZnO NRs were grown between 1.6 and 5.2μm long. Electrodes made with 1.6±0.2μm thickness were used to analyze parameters such as synthesis conditions, light intensity (800–1500Wm−2), UV light irradiation and temperature (25–75°C). We have also carried out initial analysis of the solar cell lifetime under continuous light irradiation at 45°C, and analyzed the ZnO electrode before and after testing. The best photovoltaic response was characterized by a power conversion efficiency of 1.02%, with Jsc of 3.72mAcm−2, Voc of 0.603V and 45% FF (at 72°C), for a ZnO NR electrode of 5.2μm thickness. Comparison of our power conversion efficiency values with published data is also presented, as well as a brief discussion on the possible reasons behind the low power conversion efficiency observed for these type of solar cells.

Structural and Optical Properties of ZnO Nanorods Thin Films by Solution-Growth Method

Highly c-axis-oriented ZnO nanorods thin films were obtained on silica glass substrates by a simple solution-growth technique. The most compact and vertically-aligned ZnO nanorods thin film with the thickness of ~800 nm and average hexagonal grain size of ~200 nm exhibits the average visible transmittance 85%, refractive index 1.74, and packing density 0.84. As we demonstrate here, the solution-growth technique was used to produce high-quality and dense ZnO nanorods thin films, and is an easily controlled, low-temperature, low-cost, and large-scale process for the fabrication of optical-grade thin films.

Fabrication of ZnO nanowires and nanorods

In this study, we focused on the fabrication of two types of ZnO nanostructures, nanorods and nanowires, using different techniques. ZnO nanowires were fabricated by cathodically induced sol–gel electrodeposition using an anodic aluminum oxide (AAO) template. ZnO nanowires were approximately 65nm in diameter and 10μm in length. Also, ZnO nanorods were fabricated by a hydrothermal technique using ZnO seed layer coated glass substrate. ZnO nanorods were approximately 30nm in diameter and 1μm in length. Obtained vertical aligned ZnO nanowires and nanorods were characterized by X-ray diffraction (XRD) analysis, scanning electron microscopy (SEM) and UV–vis spectrophotometry.

Structural and electrical properties of nitrogen-ion implanted ZnO nanorods

This study examined the micro-structural and electrical properties of nitrogen-ion implanted ZnO nanorods. Nitrogen ions (N +s) with energy of 50 keV and beam flux of 10 16 particles/cm 2 were implanted on vertically-aligned ZnO nanorods. Energy dispersive X-ray spectroscopy measurements showed that N +s were spread uniformly over the nanorods. Extended X-ray absorption fine structure (EXAFS) measurements suggested that the implanted N +s had partially substituted for the oxygen sites. The I–V characteristic curves showed that the N +-implanted nanorods were n-type. Moreover, annealing at 800 °C enhanced the charge carrier density in the nanorods by 10-fold, compared to the N +-implanted and unannealed ZnO nanorods.

Gas (H2 and NO2) Sensor of ZnO Nanorods in Vacuum

Vertically-aligned resistive-type ZnO nanorod gas sensors have been successfully fabricated on alumina substrate with a patterned Pt-electrode by using sonochemical method. The responses are characterized by measuring resistance at 250 ° in the vacuum chamber where total pressure of 10 torr is maintained and various concentrations of dilute H2 or NO2 are injected. The gas sensors exhibit the responses of 7 % and 1650 % are respectively at 50 ppm of H2 and NO2, and increase as gas concentration rises to 1000 ppm. Compared with previous studies conducted at atmospheric pressure, the response to H2 gas measured at the vacuum is improved, while that to NO2 is similar, promising that ZnO nanorod gas sensors can be developed into high performance gas sensor for vacuum process.

Wettability Control of Micropore-Array Films by Altering the Surface Nanostructures

By controlling the surface nanostructure, the wettability of films with similar pore-array microstructure can be tuned from hydrophilic to nearly superhydrophobic without variation of the chemical composition. PA1 pore-array film consisting of the horizontal ZnO nanosheets was nearly superhydrophobic. PA2 pore-array film consisting of growth-hindered vertically-aligned ZnO nanorods was hydrophilic. The influences of the nanostructure shape, orientation and the micropore size on the contact angle of the PA1 films were studied. This study provides a new approach to control the wettability of films with similar pore-array structure at the micro-scale by changing their surface nanostructure. PA1 films exhibited irradiation induced reversible wettability transition. The feasibility of creating a wetted radial pattern by selective UV irradiation of PA1 film through a mask with radial pattern and water vapor condensation was also evaluated.

Dye sensitized solar cells based on vertically-aligned ZnO nanorods: effect of UV light on power conversion efficiency and lifetime

Electrodes made of vertically-aligned ZnO nanorods (NRs) have been prepared and analyzed in dye sensitized solar cells (DSC). We report a ∼20% power conversion efficiency increase during the first hours of solar cell testing at 1000 W m−2 (AM 1.5). The latter has been attributed to the physisorption/chemisorption of the N-719 dye on the ZnO NRs induced by UV-light irradiation. The ZnO NRs were grown by the hydrothermal method for 6 h obtaining a ZnO layer thickness of about 1.8 μm. The highest solar cell efficiency obtained was 0.69% after UV light irradiation (at 72 °C, 0.63 V, 2.85 mA cm−2, 0.39 FF). The effect of UV light has been monitored by UV-VIS, IV-curves and IPCE analyses with time, and has been related to the solar cell performance.

Effect of testing conditions on the photovoltaic performance of ZnO-based dye sensitized solar cells

Dye-sensitized solar cells based on vertically-aligned ZnO nanorod, were analyzed at different conditions. Stability tests showed an improvement on solar conversion efficiency between ∼20% (1000W/m2) and ∼50% (1800 W/m2). This behavior was ascribed to the physisorption/chemisorption of the N-719 dye on the ZnO due to UV light. Studies at different temperatures proved that the performance of the cells can double when decreasing temperature from 72∘C to room temperature. An increase on the efficiency and decrease in FF was observed when light intensity is increased. IPCE analyses were used to monitor the stability of the solar cells with time.

Solution synthesis of one-dimensional ZnO nanomaterials and their applications

Recently, one-dimensional (1D) ZnO nanomaterials (NMs) have been extensively studied because both their functional properties and highly controllable morphology make them important building blocks for understanding nanoscale phenomena and realizing nanoscale devices. Compared with high temperature (>450 degrees C) vapor phase methods, solution-based synthesis methods can be conducted at low temperatures (25-200 degrees C) allowing for compatibility with many organic substrate materials and offer additional advantages such as straightforward processing, low cost, and ease of scale up. Although there exist several review articles in the literature regarding the synthesis and applications of 1D ZnO NMs, those focusing on solution-based synthesis methods are lacking. Thus, this review focuses mainly on 1D ZnO NMs synthesized by solution-based processing. Firstly, 1D ZnO non-patterned, nanoparticle-seeded synthesis and its associated solution growth kinetics are discussed. Next, synthesis of vertically-aligned ZnO nanorod arrays with controlled pattern and density on various substrates is reviewed. Finally, important applications of 1D ZnO NMs are highlighted including sensors, field emission devices, photodetectors, optical switches, and solar cells.

Compact and vertically-aligned ZnO nanorod thin films by the low-temperature solution method

Highly c-axis-oriented ZnO nanorod thin films were obtained on silica glass substrates by a simple solution-growth technique. The most compact and vertically-aligned ZnO nanorod thin film with the thickness of ∼ 800 nm and average hexagonal grain size of ∼ 200 nm exhibits the average visible transmittance 85%, refractive index 1.74, packing density 0.84, and energy band gap 3.31 eV, and it was fabricated under the optimum parameters: 0.05 M, 75 °C, 6 h, multiple-stepwise, and ZnO seed layer with an average grain size of ∼ 20 nm. The photoluminescence spectrum indicates that the densest ZnO nanorod thin film possesses lots of oxygen vacancies and interstitials. As we demonstrate here, the solution-growth technique was used to produce high-quality and dense ZnO nanorod thin films, and is an easily controlled, low-temperature, low-cost, and large-scale process for the fabrication of optical-grade thin films.