Well-aligned ZnO Nanorods Research Articles

Growth of Well-Aligned ZnO Nanorod Arrays and Their Application for Photovoltaic Devices

We have fabricated well-aligned ZnO nanorod arrays (ZNRAs) on indium tin oxide-coated glass substrates by a facile chemical bath deposition method. We used field-emission scanning electron microscope, x-ray diffraction and UV–Vis absorption spectroscopy to study the morphology, crystalline structure and optical absorption of the fabricated ZNRAs, respectively. The results showed that ZnO nanorods stood almost perpendicularly on the substrate, were about 30–50 nm in diameter and 800–900 nm in length, and were wurtzite-structured (hexagonal) ZnO. In addition, well-aligned ZNRAs exhibited a weak absorption in the visible region and had an optical band gap value of 3.28 eV. Furthermore, a hybrid ZNRAs/polymer photovoltaic device was made, under 1 sun AM 1.5 illumination (light intensity, ∼100 mW/cm2), and the device showed an open circuit voltage (V oc) of 0.32 V, a short circuit current density (J sc) of 7.67 mA/cm2, and a fill factor (FF) of 0.37, yielding an overall power conversion efficiency of 0.91%. Also, the exciton dissociation and transportation processes of charge carriers in the device under illumination were explained according to its current density–voltage (J–V) curve and the energy level diagram.

Morphology-controllable growth of vertical ZnO nanorod arrays by a polymer soft template method: Growth mechanism and optical properties

Vertically well-aligned ZnO nanorod arrays were synthesized on silicon substrate by a novel polymer soft template method. The ZnO nanorod arrays have been deposited by immersing a silicon substrate into an aqueous solution containing Zn(CH3COO)2·2H2O, Polyvinylpyrrolidone (PVP) and NH3·H2O at 99 °C and then being annealed in oxygen atmosphere. The effects of reaction conditions on the morphology, structure and optical properties of ZnO nanostructures were investigated. The morphology of ZnO nanostructures was demonstrated to be controllable by adjusting the pH values of PVP-Zn2+ complex solution, zinc acetate concentration and growth time, respectively. The nanorod arrays were hexagonal zincite ZnO and the c-axis orientation growth trend of ZnO became more and more obvious with increasing the pH values of PVP-Zn2+ complex solution from 7.5 to 10.5, the zinc acetate concentration from 5 mM to 200 mM, and the growth time from 1 h to 3 h, respectively. The concentration of deep level defects in ZnO was reducing with increasing the pH values of PVP-Zn2+ complex solution, zinc acetate concentration and growth time, respectively. A growth mechanism of vertical ZnO nanorod arrays based on PVP soft template is proposed.

Chemical bath deposition of well-aligned ZnO nanorod arrays on Ag rods for photoelectrocatalytic degradation of rhodamine B

ZnO nanorod arrays (NAs) were prepared by a simple two-step process on the Ag rods. The X-ray diffraction and HRTEM characterizations indicated that ZnO nanorods were hexagonal wurtzite single crystalline structure. Photoelectrocatalytic performances of Ag rods with ZnO NAs (ZnO NAs/Ag rods) were studied systematically for the removal of rhodamine B (RhB). Under the optimal condition, the photoelectrocatalytic efficiency was nearly twice the photocatalytic efficiency, which can be attributed to the more efficient separation of photo-generated carriers. Our results show that ZnO NAs/Ag rods should be a promising photoelectrode for photoelectrocatalysis progress.

Epitaxial growth of ZnO nanorod arrays via a self-assembled microspheres lithography

Through a simple hydrothermal method, well-aligned and periodic honeycomb-like ZnO nanorod arrays were fabricated on a c-plane sapphire with an aluminum-doped ZnO (AZO) seed layer. Vertical and highly ordered ZnO nanostructures with the <0002> orientation were synthesized by employing a self-assembled monolayer of polystyrene (PS) microspheres as a mask. The optimal growth conditions allowed the growth of only one rod in the confined space between the microspheres. A ϕ-scan exhibited six-fold symmetry, which indicates a favorable epitaxial relationship between the ZnO nanorods, seed layer, and c-plane sapphire substrate. The epitaxial relation is as follows: [0001]ZnO∥[0001]AZO∥[0001]c-planesapphire. These results indicate that the AZO seed layer acts as a buffer layer that can relax the strain between ZnO and c-plane sapphire generated by the large lattice mismatch of 18%. The size of the resulting ZnO nanorods of diameter 20–90nm could be tuned by varying the concentration of the solution, pH, and duration of reaction. The large aspect ratio of the ZnO nanorod arrays can serve as a template for high-surface-area applications.

ZnO nanofiber (NFs) growth from ZnO nanowires (NWs) by controlling growth temperature on flexible Teflon substrate by CBD technique for UV photodetector

In this study, ZnO nanofibers (ZnO NFs) were successfully grown for the first time on Teflon substrates using CBD technique. The well-aligned ZnO nanorods (ZnO NRs) were transformed to ZnO nanofibers (NFs) by varying growth temperature and growth time. The high intensity and distinct growth orientation of peaks observed in the XRD spectra of the NFs indicate high crystal quality. The field emission scanning electron microscopy (FESEM) revealed high density of small diameter sized and long ZnO nanofibers (NFs) that are distributed in random directions. Raman analyses revealed a high E2 (high) peak at 436 nm, which indicates the wurtzite structure of ZnO. A flexible ZnO nanofiber (NFs)-based metal-semiconductor-metal UV detector was fabricated and analyzed for photo response and sensitivity under low power illumination (375 nm, 1.5 mW/cm2). The results showed a sensitivity of 4045% which can be considered a relatively high response and baseline recovery for UV detection.

Effect of synthesis conditions on the growth of various ZnO nanostructures and corresponding morphology-dependent photocatalytic activities

Well-aligned ZnO nanorod arrays have been prepared by using seed-assisted hydrothermal method, in which the c-axis oriented ZnO films prepared by a soft-chemical were used as the seed layers on quartz glass. The epitaxial growth mechanisms and corresponding morphology effect of samples are discussed. The results show that the morphology of ZnO nanorod arrays is closely related to the hydrothermal temperature, pH value and initial concentration of Zn source in precursor solution. The as-prepared samples are characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), Brunauer-Emmett-Teller (BET), Raman spectroscopy, photoluminescence (PL) spectra and UV–vis spectra. The well-aligned ZnO nanorod arrays exhibit improved photocatalytic degradation with Rhodamine 6G (R6G) solution under UV radiation, according with the UV–vis spectra investigation and photocatalytic improvement speculation. Noteworthy, the photocatalytic properties of well-aligned ZnO nanorod arrays have obviously improved comparing with other ZnO samples without proper alignment. The oxygen vacancies suppressed recombination process of photon induced electrons and holes, as the oxygen vacancy trapped the charge carriers.

Synthesis of well-aligned ZnO nanorod arrays from mixed sulfide-oxide lead and zinc ore

A novel process of recovery of zinc from mixed sulfide-oxide lead and zinc ore was presented in this paper. ZnO nanorod arrays (ZNRAs) on FTO substrate pre-coated with a layer of ZnO seeds was synthesized successfully from the mixed ore via the oxidative ammoniacal leaching process and a two-step process including precipitation and hydrothermal synthesis. The characterization results from the XRD, SEM and UV–vis spectrophotometer analysis showed that the as-prepared ZNRAs was well-aligned, large-scale, perpendicular to the FTO substrate and had a reduced band gap energy of 3.03eV. XPS analysis indicated that the grown ZNRAs were doped by a small quantity of carbon and did not contain any other impurities particularly Fe and Pb. The total zinc extraction can reach 92.4% after the oxidative ammoniacal leaching process and the precipitation process. The elimination of conventional purification process in zinc production was achieved due to the efficient and selective oxidative ammoniacal leaching process.

Patterned Well-Aligned ZnO Nanorods Assisted with Polystyrene Monolayer by Oxygen Plasma Treatment.

Zinc oxide is known as a promising material for sensing devices due to its piezoelectric properties. In particular, the alignment of ZnO nanostructures into ordered nanoarrays is expected to improve the device sensitivity due to the large surface area which can be utilized to capture significant quantities of gas particles. However, ZnO nanorods are difficult to grow on the quartz substrate with well-ordered shape. So, we investigated nanostructures by adjusting the interval distance of the arranged ZnO nanorods using polystyrene (PS) spheres of various sizes (800 nm, 1300 nm and 1600 nm). In addition, oxygen plasma treatment was used to specify the nucleation site of round, patterned ZnO nanorod growth. Therefore, ZnO nanorods were grown on a quartz substrate with a patterned polystyrene monolayer by the hydrothermal method after oxygen plasma treatment. The obtained ZnO nanostructures were characterized by X-ray diffraction (XRD) and field-emission scanning electron microscope (FE-SEM).

Continuous wet-process growth of ZnO nanoarrays for wire-shaped photoanode of dye-sensitized solar cell

Well-aligned ZnO nanorod arrays have been grown on metal-plated polymer fiber via a mild wet process in a newly-designed continuous reactor, aiming to provide wire-shaped photoanodes for wearable dye-sensitized solar cells. The growth conditions were systematically optimized with the help of computational flow-field simulation. The flow field in the reactor will not only affect the morphology of the ZnO nanorod⧹nanowire but also affect the pattern distribution of nanoarray on the electrode surface. Unlike the sectional structure from the traditional batch-type reactor, ZnO nanorods with finely-controlled length and uniform morphology could be grown from the continuous reactor. After optimization, the wire-shaped ZnO-type photoanode grown from the continuous reactor exhibited better photovoltaic performance than that from the traditional batch-type reactor.

Fabrication of a highly flexible low-cost H2 gas sensor using ZnO nanorods grown on an ultra-thin nylon substrate

A “highly flexible low-cost” H2 gas sensor was fabricated via inclined and vertically well-aligned ZnO nanorods on a “cheap, thin (15 µm), and highly flexible” nylon substrate using the hydrothermal method. Morphological, crystallinity, and optical properties of the prepared ZnO nanorods were studied by field emission scanning electron microscopy, transmission electron microscopy, energy-dispersive X-ray analysis, X-ray diffraction, and photoluminescence measurements. Results revealed the formation of aligned hexagonal-like nanorods with high aspect ratio and density. The results confirmed the formation of wurtzite ZnO phase with a preferred orientation along the (002) direction with high crystallinity, excellent quality, and few defects. The sensitivity and response time behaviors of the ZnO-based gas sensor to hydrogen gas at different operation temperatures and in various hydrogen concentrations were investigated. Under 500 ppm of H2 exposure at different temperatures from room temperature to 180 °C, the sensitivity increased from 109 to 264 %. When the exposed H2 gas increased from 750 to 2000 ppm at a fixed temperature of 75 °C, the sensitivity also sharply increased from 246 to 578 %. Moreover, both the response and recovery time of the device during both tests were enhanced. The hydrogen gas sensing mechanisms of ZnO nanorods in low and high operation temperatures were discussed.

Fabrication of well-aligned ZnO nanorod photoanodes for perovskite solar cells

ZnO nanorods grown on aluminum doped zinc oxide (AZO) substrate and ZnO seed layer coated substrates, including glass slides, fluorine doped tin oxide, and indium tin oxide substrates were studied. ZnO nanorods grown on AZO substrate had the best alignment and highest packing density. The growth rate of ZnO nanorods was regulated by adding polyethyleneimine (PEI) and introducing contact potential. ZnO nanorods co-adjusted by adding PEI and introducing contact-potential-driven growth formed a textured structure that led to a higher transmittance at 600–800 nm. The strong ultraviolet band emission indicated a relatively low concentration of deep level defects in ZnO nanorods. Good crystallization, together with a small amount of aluminum-doping, guaranteed high electron transfer in the ZnO nanorods. The well-aligned ZnO nanorods were incorporated into a perovskite solar cell prepared with carbon counter electrode under completely open-air conditions. A J sc of 15.53 mA cm−2 and an efficiency of 2.71 % were obtained for ZnO nanorods under contact-potential-driven growth. In addition, for ZnO nanorods co-adjusted by adding PEI and introducing contact potential, a J sc of 14.87 mA cm−2 and an efficiency of 3.62 % were obtained. However, fill factors were very low for hybrid solar cells with the structure of AZO/ZnO nanorods-CH3NH3PbIxCl3−x/carbon.

Effects of Growth Duration and Precursor Concentration on the Growth of ZnO Nanorods Synthesized by Chemical Bath Deposition

Well-aligned ZnO nanorods were successfully fabricated on polyethylene naphthalate (PEN) substrates by a simple chemical bath deposition technique at a low temperature. The effects of growth duration (2–8 h) with precursor concentrations of 0.025 and 0.050 M on the optical and structural properties of the prepared ZnO nanorods were systematically studied by the X-ray diffraction (XRD), field emission scanning electron microscopy, transmission electron microscopy, photoluminescence (PL), and UV–visible. The XRD results demonstrated that the ZnO nanorods vertically grew in the (002) direction along the z axis on the substrate. These low compressive strain values revealed that the obtained ZnO nanorods grown on seed-layer ZnO/PEN substrates had a high quality crystal structure. The field emission scanning electron microscopy images showed that the vertical ZnO nanorods were hexagonal shaped, and that the average diameter of the ZnO nanorod arrays on PEN substrate increased from 22 to 55 nm with increased growth duration from 2 to 8 h, respectively. The high-resolution transmission electron microscopy image suggested that [0 0 0 1] is the preferred growth direction for ZnO nanorods. The PL results demonstrated that ZnO nanorods grown for shorter or longer than 5 h exhibited a strong PL peak at around 420 nm (indigo emission).

Color tunable electroluminescence and resistance switching from a ZnO-nanorod–TaOx–p-GaN heterojunction

Well-aligned ZnO nanorods have been prepared on p-GaN–sapphire using a vapor phase transport (VPT) technique. A thin sputtered layer of TaOx is employed as the intermediate layer and an n-ZnO–TaOx–p-GaN heterojunction device has been achieved. The current transport of the heterojunction exhibited a typical resistance switching behavior, which originated from the filament forming and breaking in the TaOx layer. Color controllable electroluminescence (EL) was observed from the biased heterojunction at room temperature. Bluish-white wide band emission is achieved from the forward biased device in both the high resistance and low resistance states, while red emission can only be observed for the reverse biased device in the low resistance state. The correlation between the EL and resistance switching has been analyzed in-depth based on the interface band diagram of the heterojunction.

3D-branched hierarchical 3C-SiC/ZnO heterostructures for high-performance photodetectors.

The ultra-sensitive photodetection of different wavelengths holds promising applications in high-performance optoelectronic devices and it requires an efficient and suitable semiconductor unit. Herein, we demonstrated the designable synthesis of 3D-branched hierarchical 3C-SiC/ZnO heterostructures by a three-step process and their assembling into an ultrasensitive photodetector. Microstructure analyses using high-resolution transmission electron microscopy reveal that the hierarchical 3C-SiC/ZnO heterostructure is composed of single-crystal 3C-SiC nanowires as a central stem and numerous well-aligned single-crystalline ZnO nanorods as branch shells. Optoelectronic tests on the 3C-SiC/ZnO heterostructure photodetector verify the outstanding photo-detection performance with an ultrahigh EQE (1.69 × 108%), a superior photoresponsivity (4.8 × 105 A W-1), a very fast response time (a rise time of 40 ms and a decay time of 60 ms), a high photo-dark current ratio of 187.8 and an excellent photocurrent stability and reproducibility, which is significantly advantageous or comparable to those of ZnO and other inorganic semiconductor nanostructure based photodetectors. To understand the excellent photodetection of hierarchical 3C-SiC/ZnO heterostructures, a band-gap energy diagram describing the photogenerated electron transport process is plotted and the corresponding mechanism is discussed. The strategy proposed in the present work will open up more opportunities for the design and boost of ultra-sensitive photodetectors based on semiconductor heterostructures.

Improvement in the Grain Growth of Plasma-Treated Nano-Sized ZnO Films and Their Characterization.

The well-aligned ZnO nanorods were rapidly grown on an indium tin oxide (ITO)-coated glass substrate using Al-doped ZnO (AZO) thin film as seed layer by the microwave-assisted hydrothermal chemical route. The optimal growth conditions for the well-aligned ZnO nanorods were obtained by modulating H2 plasma pretreatment time for the seed layer and synthesis time for ZnO nanorods. The H2 plasma effect of the seed layer on the alignment, growth rate and crysallinity of ZnO nanods is also demonstrated. The synthesized ZnO nanorods were annealed in atmosphere of N2, O2 and H2 + N2 mixed gas to improve the related physical characteristics, the ZnO nanorods on grapheme/ITO substrate were also investigated. The results show that the alignment and growth rate of ZnO nanorods depends on the physical characteristics and roughness of the seed layer, which can be improved by H2 plasma pretreatment. The average growth rate of ZnO nanorods synthesized by microwave hydrothermal technique is about 2.2 μm/hr which significantly superior to other conventional techniques. After the appropriate N2 annealing treatment, good quality and well-aligned ZnO nanorods, which are single crystal with stacking defects and pyramid or candle shape, were obtained. A fundamental model of the effect of H2 plasma pretreatment on the surface of seed layer and the growth of ZnO nanorods using a microwave-assisted hydrothermal chemical route is also described.

Obtaining a Well-Aligned ZnO Nanotube Array Using the Hydrothermal Growth Method / Labi Sakārtotu Zno Nanocauruļu Kopu Iegūšana, Izmantojot Hidrotermālo Metodi

Abstract Optimal growing parameters have been found using the hydrothermal method to obtain well-aligned vertical ZnO nanorod and nanotube arrays. The influence of different growing factors (such as temperature, growing solution concentration, method of obtaining seed layer and condition) on nanotube morphology and size is described in the paper. Well-structured ZnO nanotubes have been obtained by using a selfselective etching method with lowering temperatures of growth during the hydrothermal process. It is shown that the optical properties of the nanostructure arrays obtained are sensitive to the medium in which they are placed, which is why they can be used as sensors for pure substance detection and in different solutions for impurity determination.

Unusual Air Filters with Ultrahigh Efficiency and Antibacterial Functionality Enabled by ZnO Nanorods.

Porous membranes/filters that can remove airborne fine particulates, for example, PM2.5, with high efficiency at low energy consumption are of significant interest. Herein, we report on the fabrication of a new class of unusual superior air filters with ultrahigh efficiency and an interesting antibacterial functionality. We use atomic layer deposition (ALD) to uniformly seed ZnO on the surface of expanded polytetrafluoroethylene (ePTFE) matrix, and then synthesize well-aligned ZnO nanorods with tunable widths and lengths from the seeds under hydrothermal conditions. The presence of ZnO nanorods reduces the effective pore sizes of the ePTFE filters at little expense of energy consumption. As a consequence, the filters exhibit exceptional dust removal efficiencies greater than 99.9999% with much lower energy consumption than conventional filters. Significantly, the presence of ZnO nanorods strongly inhibits the propagation of both Gram positive and negative bacteria on the filters. Therefore, the functionalized filters can potentially overcome the inherent limitation in the trade-off effect and imply their superiority for controlling indoor air quality.

Electrochemical Synthesis of ZnO Nanorods/Nanotubes/Nanopencils on Transparent Aluminium-Doped Zinc Oxide Thin Films for Photocatalytic Applications.

We report an electrochemical synthesis of homogeneous and well-aligned ZnO nanorods (NRs) on transparent conducting aluminium-doped zinc oxide (AZO) thin films as electrodes. The selected ZnO NRs was then chemically corroded in HCl and KCl aqueous solutions to form nanopencils (NPs), and nanotubes (NTs), respectively. A DC magnetron sputtering was employed to fabricate AZO thin films at various thicknesses. The obtained AZO thin films have a c-direction orientation, transmittance above 80% in visible region, and sheet resistance approximately 40 Ω/sq. They are considered to be relevant as electrodes and seeding layers for electrochemical. The ZnO NRs are directly grown on the AZOs without a need of catalysts or additional seeding layers at temperature as low as 85 degrees C. Their shapes are strongly associated with the AZO thickness that provides a valuable way to control the diameter of ZnO NRs grown atop. With the addition of HCI and KCl aqueous solutions, ZnO NRs were modified their shape to NPs and NTs with the reaction time, respectively. All the ZnO NRs, NPs, and NTs are preferred to grow along c-direction that indicates a lattice matching between AZO thin films and ZnO nanostructrures. Photoluminescence spectra and XRD patterns show that they have good crystallinities. A great photocatalytic activity of ZnO nanostructures promises potential application in environmental treatment and protection. The ZnO NTs exhibits a higher photocatalysis than others possibly due to the oxygen vacancies on the surface and the polarizability of Zn2+ and O2-.

Controllable Growth of Highly Oriented ZnO Nanorod Arrays on Copper by a Two-Step Route

Highly oriented and well-aligned ZnO nanorod arrays were synthesized by low-temperature solution phase method on copper substrate under different conditions. Results illustrated that dense ZnO nanorods were vertically and uniformly distributed on the substrate. The effects of precursor concentration, growth temperature and time on the morphologies of nanorod arrays were investigated systematically. It is demonstrated that the controllable growth of well-aligned ZnO nanorods can be realized by readily adjusting the preparation parameters. The high quality ZnO nanorod arrays could be achieved via the chemical approach at low temperature.

Substrate Effect on Well-Aligned ZnO Nanorods Growth Using a Low Temperature Solution Method

The effect of various substrates on the morphology of ZnO nanorods synthesized by a low temperature solution method has been investigated. ZnO seed layers on the substrates were prepared by radio-frequency sputtering. ZnO nanorods have been characterized by X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM. FESEM results showed that hexagon-shaped ZnO nanorods with a diameter in the range of 30 nm to 80 nm and a length of about 1 μm were formed. The sharp (0002) peak in the XRD spectra indicates that the synthesized nanorods are single crystalline, grown along the [0001] direction. These findings have shown that the morphology of ZnO nanorods strongly depend on the substrate.