Hierarchical Zinc Oxide Research Articles

Electrodeposition of Hierarchical ZnO Nanorod-Nanosheet Structures and Their Applications in Dye-Sensitized Solar Cells

We present a two-step electrochemical deposition process to synthesize hierarchical zinc oxide (ZnO) nanorod-nanosheet structures on indium tin oxide (ITO) substrate, which involves electrodeposition of ZnO nanosheet arrays on the conductive glass substrate, followed by electrochemical growth of secondary ZnO nanorods on the backbone of the primary ZnO nanosheets. The formation mechanism of the hierarchical nanostructure is discussed. It is demonstrated that annealing treatment of the primary nanosheets synthesized by the first-step deposition process plays a key role in synthesizing the hierarchical nanostructure. Photovoltaic properties of dye-sensitized solar cells (DSSCs) based on hierarchical ZnO nanostructures are investigated. The hierarchical ZnO nanorod-nanosheet DSSC exhibits improved device performance compared to the DSSC constructed using photoelectrode of bare ZnO nanosheet arrays. The improvement can be attributed to the enhanced dye loading, which is caused by the enlargement of internal surface area within the nanostructure photoelectrode. Furthermore, we perform a parametric study to determine the optimum geometric dimensions of the hierarchical ZnO nanorod-nanosheet photoelectrode through adjusting the preparation conditions of the first- and second-step deposition process. By utilizing a hierarchical nanostructure photoelectrode with film thickness of about 7 μm, the DSSC with an open-circuit voltage of 0.74 V and an overall power conversion efficiency of 3.12% is successfully obtained.

Formation of hierarchical ZnO nanostructure on tinfoil substrate and the application on wetting repellency

Hierarchical ZnO (zinc oxide) nanostructures composed with nano-sheet and micro-flower structures (made from the nano-sheet) have been generated on tinfoil substrate via a chemical bath deposition process. Benefiting from an inherent distinct lattice constant compared with commonly used glass or other kinds of substrate, the tinfoil substrate played an important role on the formation of the hierarchical ZnO nanostructures. The resulting hierarchical ZnO surface shows excellent superhydrophobicity and extremely low water rolling angle after being modified with spin coating Teflon. The flexible and superhydrophobic characteristics of such fabricated substrate will be beneficial for applications requiring bendable and lightweight superhydrophobic substrates. In addition, the multifunctional properties of ZnO nanostructures are expected to broaden the applications to electronic and optical applications.

Controllable synthesis of quasi-spherelike ZnO hierarchical nanostructures and performance of their dye-sensitized solar cells

Hierarchical zinc oxide (ZnO) quasi-spheres consisting of nanoparticles with diameter of about 20 nm were synthesized via a one-pot reaction. The size of ZnO quasi-spheres is easily tunable from 80 nm to 3 μm by varying the type of zinc source and its concentration. The three samples 1–3 with the diameter of 80–180 nm, 300–600 nm and 1.2–2.9 μm were selected for fabricating dyesensitized solar cells (DSSCs) and their photovoltaic properties were measured. The results demonstrate that DSSCs fabricated by sample 2 with the diameter within the wavelength of visible light obtain the highest short-circuit current density and over light conversion efficiency, due to resonant scattering increasing the photon absorption.

Growth of hierarchical based ZnO micro/nanostructured films and their tunable wettability behavior

Hierarchical zinc oxide (ZnO) micro/nanostructured thin films were grown onto as-prepared and different annealed ZnO seed layer films by a simple two step chemical process. A cost effective successive ionic layer adsorption and reaction (SILAR) method was employed to grow the seed layer films at optimal temperature (80 °C) and secondly, different hierarchical based ZnO structured thin films were deposited over the seed layered films by chemical bath deposition (CBD). The influence of seed layer on the structural, surface morphological, optical and wettability behavior of the ZnO thin films were systematically investigated. The XRD analysis confirms the high crystalline nature of both the seed layer and corresponding ZnO micro/nanostructured films with a perfect hexagonal structure oriented along (0 0 2) direction. The surface morphology revels a complex and orientated hierarchical based ZnO structured films with diverse shapes from plates to hexagonal rod-like crystal to tube-like structure and even much more complex needle-like shapes during secondary nucleation, by changing the seed layer conditions. The water contact angle (WCA) measurements on hierarchical ZnO structured films are completely examined to study its surface wettability behavior for its suitability in future self-cleaning application. Photoluminescence (PL) spectra of the ZnO structured film exhibit UV and visible emissions in the range of 420–500 nm. The present approach demonstrates its potential for low-temperature, large-scale, controlled synthesis of crystalline hierarchical ZnO nanostructures films.

Microbundles of zinc oxide nanorods: Assembly in ionic liquid [EMIM] +[BF 4] −, photoluminescence and photocatalytic properties

A simple, efficient and low-temperature approach for the assembly of hierarchical Zinc oxide (ZnO) microstructures in ionic liquid [EMIM] +[BF 4] − is reported. The as-obtained ZnO superstructures are composed of microbundles of nanorods from the center points, with the diameter and length in the range of 100–150 nm and 2–4 μm, which have been characterized by X-ray diffraction, scanning and transmission electron microscopy, and photoluminescence spectroscopy. The ZnO microstructures exhibit significant defect-related green-yellow emission and high photodegradation of dye Methyl Orange (5×10 −5 mol/L) under UV excitation within 80 min.

Superhydrophobic surface based on a coral-like hierarchical structure of ZnO.

BackgroundFabrication of superhydrophobic surfaces has attracted much interest in the past decade. The fabrication methods that have been studied are chemical vapour deposition, the sol-gel method, etching technique, electrochemical deposition, the layer-by-layer deposition, and so on. Simple and inexpensive methods for manufacturing environmentally stable superhydrophobic surfaces have also been proposed lately. However, work referring to the influence of special structures on the wettability, such as hierarchical ZnO nanostructures, is rare.MethodologyThis study presents a simple and reproducible method to fabricate a superhydrophobic surface with micro-scale roughness based on zinc oxide (ZnO) hierarchical structure, which is grown by the hydrothermal method with an alkaline aqueous solution. Coral-like structures of ZnO were fabricated on a glass substrate with a micro-scale roughness, while the antennas of the coral formed the nano-scale roughness. The fresh ZnO films exhibited excellent superhydrophilicity (the apparent contact angle for water droplet was about 0°), while the ability to be wet could be changed to superhydrophobicity after spin-coating Teflon (the apparent contact angle greater than 168°). The procedure reported here can be applied to substrates consisting of other materials and having various shapes.ResultsThe new process is convenient and environmentally friendly compared to conventional methods. Furthermore, the hierarchical structure generates the extraordinary solid/gas/liquid three-phase contact interface, which is the essential characteristic for a superhydrophobic surface.

Ascorbate-Assisted Growth of Hierarchical ZnO Nanostructures: Sphere, Spindle, and Flower and Their Catalytic Properties

A simple solution-based method to prepare mainly flowerlike zinc oxide (ZnO) nanostructures using the ascorbate ion as a shape-directing/capping agent at relatively low temperature (ca. 30 and 60 degrees C) was described. However, we observed that different shapes of hierarchical ZnO nanostructures such as flowerlike, spindlelike, and spherical could be obtained with an increase in the synthesis temperature from 60 to 90 degrees C. The effects of other organic capping agents on the shape of hierarchical ZnO nanostructures were also studied. FTIR, FESEM, and XRD characterization were performed on the formed ZnO nanostructures to understand the role of ascorbate in the growth of flowerlike morphology. The nucleation and growth process can regulate by changing the metal precursor and ascorbate ion concentrations. We were able to identify intermediate nanostructures such as spherical/quasi-spherical and spindle that are very much on the pathway of formation of large, flowerlike ZnO nanostructures. Electron microscopy results indicated that these spherical/quasi-spherical ZnO nanoparticles might aggregate through oriented attachment to produce spindlelike and flowerlike nanostructures. On the basis of these results, a possible growth mechanism for the formation of flowerlike ZnO nanostructures was described. The optical properties of these differently shaped ZnO nanostructures were also described. The catalytic activities of the as-synthesized spherical and flowerlike ZnO nanostructures were tested in the Friedel-Crafts acylation reaction of anthracene with benzoyl chloride. The catalysis results indicated that the catalytic activity of flowerlike ZnO nanostructures is slightly higher than the spherical counterpart.

Precursor template synthesis of three-dimensional mesoporous ZnO hierarchical structures and their photocatalytic properties

In this paper, three-dimensional mesoporous Zinc oxide (ZnO) hierarchical structures were fabricated from the thermal-decomposition of layered basic zinc carbonate (LBZC) precursors, which were synthesized by an urea hydrothermal method without a surfactant. X-ray diffraction (XRD) and scanning electron microscopy (SEM) were used to characterize the LBZC precursors and mesoporous ZnO hierarchical structures. The results of N2 adsorption–desorption isotherms indicated that the pore size distribution and specific surface area can be tailored by varying the urea amount and the annealing temperature. Photocatalytic activity of the mesoporous ZnO was evaluated using the degradation of methyl orange. The influence of urea content and annealing temperature on the morphology and surface specific areas and the effect of BET areas on the photocatalytic activity were discussed. Hierarchical ZnO flowers with 0.006 mol urea added annealed at 300 °C showed good photocatalytic activity.

ZnO nanostructures grown on epitaxial GaN

Presented is the growth of zinc oxide nanorod/nanowire arrays on gallium nitride epitaxial layers. A hierarchical zinc oxide morphology comprising of different scale zinc oxide nanostructures was observed. The first tier of the surface comprised of typical zinc oxide nanorods, with most bridging to adjacent nanorods. While the second tier comprised of smaller zinc oxide nanowires approximately 30 nm in width often growing atop the aforementioned bridges. Samples were analysed via scanning electron microscopy, as well as, cross-sectional and high resolution transmission electron microscopy to elucidate the detailed growth and structural elements of the heterostructure.

Fabrication of Hierarchical Zinc Oxide Nanostructures through Multistage Gas-Phase Reaction

A hierarchical nanostructure of wurtzite zinc oxide (ZnO) was synthesized by the thermal evaporation of Zn/ZnO powders. It had a hexagonal prism as the center and six clusters of nanorods as the branches. The formation of this ZnO nanostructure is attributed to three sequential catalyst-free reactions. First, a Zn prism was deposited on the substrate as the result of the condensation of Zn vapor; then, oxygen was introduced in an appropriate temperature range and the Zn prism was oxidized; finally, secondary ZnO nanorods grew on the ZnO surfaces of the central prism and the snowflake-shaped nanostructure was attained. This work has revealed the perspective of achieving hierarchical ZnO nanostructures through simple and controllable gas-phase reactions.

Morphosynthesis of hierarchical ZnO replica using butterfly wing scales as templates

Zinc oxide (ZnO) materials with butterfly wing scales microstructures have been synthesized by using natural butterfly wings as templates. Microstructure characters of original butterfly wing scales are maintained faithfully and intactly in the biomorphic ZnO materials. FESEM and TEM observations show the morphology of the original scales and ZnO replicas. HRTEM reveals that the ZnO replicas are proved to be polycrystalline with the major planes {1 0 0}, {0 0 2} and {1 0 1}.

Synthesis of hierarchical zinc oxide nanotubes

In this paper, we report on the synthesis and structure of hierarchical zinc oxide nanotubes. Hierarchical nanotubes grown by physical vaporization of zinc in the presence of a catalyst were decorated with many secondary zinc oxide nanorods on the outer surface. The axis of these nanotubes with an average diameter of 65 nm was aligned along the c axis of wurtzite zinc oxide. The hierarchical zinc oxide nanotubes, many of which were single crystals, were transparent or opaque, depending on whether they had a zinc layer inside. The opaque nanotubes showed an abrupt change in electronic transmittance during investigation with transmission electron microscopy. The unique structure of the hierarchical ZnO nanotubes and the quantum effect resulting from the reduced dimension will modify the original properties of ZnO, leading to novel applications.