Morphological control of flower-like ZnO nanostructures

Flower-like ZnO nanostructures composed of different building blocks, such as hexagonal pyramids, hexagonal prisms, and cones, have been synthesized on a large scale by a simple hydrothermal method in the absence of surfactants or organic solvents. The effects of the concentration of NaOH, reaction temperature, and reaction time on the morphologies of the resulting products have been investigated. The morphologies and the crystal structures of flower-like ZnO nanostructures were characterized by X-ray powder diffraction (XRD), field-emission scanning electron microscopy (FE-SEM), and transmission electron microscopy (TEM).     

Electrochemical synthesis and shape control of Al-Sb nanostructures

Al-Sb intermetallic compounds are a kind of important semiconductors for a great deal of well-established commercial technologies. Here Al-Sb nanostructures were successfully synthesized by electrodeposition at room temperature. By changing the synthetic parameters, such as the concentrations of AlCl₃ and SbCl₃, current densities, and deposition time, the densities and sizes of the nanoparticles and surface morphologies of the deposits can be well controlled. The prepared deposits were characterized by SEM, XRD, and EDS. The result of XRD suggested that the film was amorphous. The formation process of Al-Sb nanoparticle chains was investigated, and a possible formation mechanism based on a diffusion-limited electrodeposition was represented.     

One-dimensional ZnO nanostructures

The wide-gap semiconductor ZnO with nanostructures such as nanoparticle, nanorod, nanowire, nanobelt, nanotube has high potential for a variety of applications. This article reviews the fundamentals of one-dimensional ZnO nanostructures, including processing, structure, property, application and their processing-microstructure-property correlation. Various fabrication methods of the ZnO nanostructures including vapor-liquid-solid process, vapor-solid growth, solution growth, solvothermal growth, template-assisted growth and self-assembly are introduced. The characterization and properties of the ZnO nanostructures are described. The possible applications of these nanostructures are also discussed.     

Multicenter uric acid biosensor based on tetrapod-shaped ZnO nanostructures

In this work, we developed a tetrapod-shaped ZnO nanostructure (T-ZnO) biosensor to determine uric acid (UA), which is the primary end product of purine metabolism. The as-fabricated UA sensor presents a higher performance than that of the reported biosensors based on ZnO nanorods and ZnS quantum dots, etc. High-quality ZnO nanotetrapods were characterized by field emission scanning electron microscopy (FESEM), energy dispersive X-ray spectra (EDX), X-ray diffraction (XRD) and Raman spectroscopy, respectively. A high affinity of uricase/ZnO to UA was revealed by cyclic voltammograms. The biosensor performance has been systematically investigated by amperometric response measurements. A fast current response time is within 9 s. It was also found that the uricase/T-ZnO biosensor presented a high and reproducible sensitivity of 80.0 microA cm(-2) mM(-1) and an experiment limit of detection of 0.8 microM. This study provides an insight utilizing the unique ZnO nanostructure to develop the highly sensitive and rapidly responsive nano-bio devices.     

Optical properties of ZnO nanostructures

We present a review of current research on the optical properties of ZnO nanostructures. We provide a brief introduction to different fabrication methods for various ZnO nanostructures and some general guidelines on how fabrication parameters (temperature, vapor-phase versus solution-phase deposition, etc.) affect their properties. A detailed discussion of photoluminescence, both in the UV region and in the visible spectral range, is provided. In addition, different gain (excitonic versus electron hole plasma) and feedback (random lasing versus individual nanostructures functioning as Fabry-Perot resonators) mechanisms for achieving stimulated emission are described. The factors affecting the achievement of stimulated emission are discussed, and the results of time-resolved studies of stimulated emission are summarized. Then, results of nonlinear optical studies, such as second-harmonic generation, are presented. Optical properties of doped ZnO nanostructures are also discussed, along with a concluding outlook for research into the optical properties of ZnO.     

Hydrothermal growth of ZnO nanostructures

Abstract

One-dimensional nanostructures exhibit interesting electronic and optical properties due to their low dimensionality leading to quantum confinement effects. ZnO has received lot of attention as a nanostructured material because of unique properties rendering it suitable for various applications. Amongst the different methods of synthesis of ZnO nanostructures, the hydrothermal method is attractive for its simplicity and environment friendly conditions. This review summarizes the conditions leading to the growth of different ZnO nanostructures using hydrothermal technique. Doping of ZnO nanostructures through hydrothermal method are also highlighted.     

Hydrothermal treatment of ZnO nanostructures

Hydrothermal treatment of ZnO nanostructures involves low temperatures (150-200 °C) and elevated water vapor pressure for the purpose of the improvement in the material properties. Under such moderate conditions, no significant changes in the morphology would be expected. Nevertheless, such treatment results in a significant change of nanostructured morphologies of ZnO. The observed changes are dependent on the starting material properties and the substrate used for the growth. In the case of Si substrate, hydrothermal treatment results in significant Si contamination of the samples. In terms of the optical properties, improvements are observed only in some cases, while samples with excellent starting optical properties are degraded by the treatment. Mechanisms responsible for the observed changes are discussed.     

Hydrothermal growth of ZnO nanostructures

One-dimensional nanostructures exhibit interesting electronic and optical properties due to their low dimensionality leading to quantum confinement effects. ZnO has received lot of attention as a nanostructured material because of unique properties rendering it suitable for various applications. Amongst the different methods of synthesis of ZnO nanostructures, the hydrothermal method is attractive for its simplicity and environment friendly conditions. This review summarizes the conditions leading to the growth of different ZnO nanostructures using hydrothermal technique. Doping of ZnO nanostructures through hydrothermal method are also highlighted.     

Shape and size control of ZnO nanostructures via a simple solution route

ZnO nanostructures with different morphologies were synthesized by condensing the Zn(OH)4(2-) precursors under hydrothermal conditions in the presence of a surfactant, cetyltrimethylammonium bromide (CTAB). Shape and size control of ZnO nanostructures was achieved by relatively simple variations of molar ratio of CTAB to Zn(OH)4(2-). With a higher molar ratio, ZnO nanotubes were obtained, whereas with a lower molar ratio, ZnO nanorods were formed; furthermore, with a moderate w value, the coexistence of ZnO nanorods and nanotubes was also observed. The photoluminescence of ZnO nanorods and nanotubes was also investigated.     

The kinetics of the hydrothermal growth of ZnO nanostructures

The immersion of a material, seeded with ZnO nanoparticles, in an aqueous solution of Zn(NO₃)₂ and hexamethylenetetramine (HMT) at 90 °C yields an extended array of one-dimensional ZnO on the substrate surface. The structure of the ZnO evolves with reaction time. Initially nanorods are formed. At longer times the rods are tipped with nanotubes. Here we report a series of experiments in which both the composition of the reaction solution; concentrations of H⁺, Zn²⁺ and HMT; and the structure of ZnO deposited on the substrate are monitored as a function of reaction time. It was found that the change from ZnO rod to tube growth arises when the solution composition is such that it is no longer thermodynamically favorable to precipitate Zn(OH)₂.     

Complex and oriented ZnO nanostructures

Extended and oriented nanostructures are desirable for many applications, but direct fabrication of complex nanostructures with controlled crystalline morphology, orientation and surface architectures remains a significant challenge. Here we report a low-temperature, environmentally benign, solution-based approach for the preparation of complex and oriented ZnO nanostructures, and the systematic modification of their crystal morphology. Using controlled seeded growth and citrate anions that selectively adsorb on ZnO basal planes as the structure-directing agent, we prepared large arrays of oriented ZnO nanorods with controlled aspect ratios, complex film morphologies made of oriented nanocolumns and nanoplates (remarkably similar to biomineral structures in red abalone shells) and complex bilayers showing in situ column-to-rod morphological transitions. The advantages of some of these ZnO structures for photocatalytic decompositions of volatile organic compounds were demonstrated. The novel ZnO nanostructures are expected to have great potential for sensing, catalysis, optical emission, piezoelectric transduction, and actuations.     

Hybrid nanostructures of titanium-decorated ZnO nanowires

Hybrid nanostructures of titanium (Ti)-decorated zinc oxide (ZnO) nanowire were synthesized. Various thick Ti films (6 nm, 10 nm, and 20 nm) were coated to form a titanium oxide (TiO) coating layer around ZnO nanowires. Transmission electron microscope analysis was performed to verify the crystallinity and phases of the TiO layers according to the Ti-coating thickness. Under UV illumination, a bare ZnO nanowire showed a conventional n-type conducting performances. With a Ti coating on a ZnO nanowire, it was converted to a p-type conductor due to the existence of electron-captured oxygen molecules. It discusses the fabrication of Ti-decorated ZnO nanowires including the working mechanisms with respect to UV light.     

Rapid synthesis of flower-like ZnO nanostructures

Flower-like ZnO nanostructures were prepared via microwave assisted heating in the presence and absence of ionic liquid (IL). X-ray diffraction analysis (XRD), Scanning electron microscopy SEM and room temperature photoluminescence (PL) spectra have been employed for characterization of the products. The SEM image illustrates the surface of flower-like ZnO prepared in the presence of IL is not smooth and consists of nanoparticles with grain size of about 48 nm. PL spectra of flower-like ZnO in absence and presence IL reveal similar photoluminescence features: a strong UV, weak blue and green-yellow emissions peak at a bout 393 nm, 448 nm and 583 nm respectively. The strong UV photoluminescence and the weak green emission indicate the good crystallization quality of the flower-like nanostructure. The results show that imidazolium-based IL can be used as template for achieving very high level control over the size and shape of nanostructures. The approach developed in this work can potentially be used as a viable method for making various other uniform nanostructures in the presence of IL. This method is simple, fast, low-cost and suitable for large-scale production of ZnO nanostructures.     

Formation of ZnO three-side teethed nanostructures

Three-side teethed feather-like nanocomb structures of ZnO were produced based on a vapor-phase transport process. ZnS powder was used as source material and Si substrate as a collector, at temperature ∼ 1100 °C. The morphology of the product showed a ribbon-like stem and nanoteeth array aligned evenly along three sides of the nanoribbon. It was found that the nanoribbon grew mainly along the [011¯¯0] direction, and the self-assembled branching nanoteeth grew epitaxially along the [0001], [0001¯] and [211¯¯0] orientations.     

Morphology control of 1D ZnO nanostructures grown by metal-organic chemical vapor deposition

The morphology of ZnO nanostructures grown by metal-organic chemical vapor deposition under various growth conditions was examined by scanning electron microscopy. An increase in the growth temperature resulted in the formation of 1D nanostructures with diameters decreasing with increasing temperature. Vertically aligned nanorods with a needle-like tip shape were grown on a previously deposited homo-seed layer. Also the seed layer reduced the growth temperature of 1D nanorods. Low-temperature growth posterior to nanorod growth resulted in the formation of nanorods with an inverted graded diameter.     

High-yield self-assembly of flower-like ZnO nanostructures

High-yield three-dimensional (3D) flower-like nanostructures self-assembled from 1 D ZnO nanorods and nanotubes are experimentally demonstrated. The Zn and O terminated crystal planes of ZnO nanorods results in positively and negatively charged top (001) and bottom (00-1) surfaces, respectively. The nanorods self-assembled into 3D nanostructures via the electrostatic interaction between the crystal planes with opposite charges. Moreover, on the basis of the different stability of polar and nonpolar planes in wurtzite-type ZnO, the nanorods based 3D nanostructures transformed into nanotubes based ones spontaneously. This provides a new approach to prepare multi-dimensional materials without the necessity to employ any external intervention.     

掺杂氧化锌一维纳米结构的研究进展

氧化锌（ZnO）纳米结构的形态是已知纳米结构中最为多样的多功能材料之一，由于一维ZnO纳米结构在电子与光电子装置中如表面声波、光子晶体、光发射二极管、光电探测器、紫外激光器、生物／化学传感器、场效应晶体管等诸多领域均展现出其独特的物理化学性能，已经引起人们的极大研究热情．而选择不同元素掺杂为调节其电、光和磁性质提供了一种有效方法，对实际应用至关重要，因此一维纳米结构的掺杂日益成为研究和应用的焦点。按照掺杂方法和掺杂元素的不同，对目前ZnO一维纳米结构的掺杂进展进行了回顾，提出了掺杂工艺中存在的问题，并对其发展趋势及前景进行了展望。     

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.