Self-sacrificed template method for the synthesis of ZnO-tubular nanostructures:reaction kinetics and pathways

Using Zn nanowires as a self-sacrificed template, hierarchical tubes constructed by zinc oxide (ZnO) nanoflakes and ZnO nanotubes have been successfully fabricated by two different thermal-oxidation modes. The products were characterized by the X-ray powder diffraction, transmission electron microscopy and field-emission scanning electron microscopy. The experimental results show that the formation processes of ZnO nanostructures are sensitive to the growth temperature, which is lower or higher the melting point of Zn (419 °C). ZnO nanoflake tubes and ZnO nanotubes can be controlled through the variation of the heat-treatment process of Zn nanowires and their growth pathway can be described by two types of growth mechanism, in terms of Kirkendall effect and the sublimation of the Zn cores, respectively. Our method provides an easy and convenient way to prepare metal oxides tubular nanostructures with different morphologies through self-sacrificed template method via adjusting the heat-treatment process.     

Synthesis,characterization and electrical properties of hybrid Zn2GeO4–ZnO beaded nanowire arrays

We report the syntheses of vertically aligned, beaded zinc germinate (Zn₂GeO₄)/zinc oxide (ZnO) hybrid nanowire arrays via a catalyst-free approach. Vertically aligned ZnO nanowire is used as a lattice matching reactive template for the growth of Zn₂GeO₄/ZnO nanowire. The morphology and structure of the as-prepared samples were characterized using X-ray diffractometry (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and energy-dispersive X-ray spectroscopy (EDS). TEM studies revealed the beaded microstructures of the Zn₂GeO₄/ZnO nanowire. The thickness and microstructures of crystalline beads could be easily controlled by tuning the growth duration and temperature. The photoluminescence spectrum of the Zn₂GeO₄/ZnO nanowires is composed of two peaks, i.e., the ultraviolet (UV) peak and the defect peak. For longer treatment duration of the samples, both the UV and defect peak intensities decrease dramatically. One application of the as-prepared Zn₂GeO₄/ZnO nanowire is to use the nanowire as template for the growth of three-dimensionally (3D) aligned, high-density ZnO nanobranches en route to hierarchical structure. The study of field emission properties of the as-prepared samples revealed the low turn-on voltage and high current density electron emission from the 3D ZnO nanobranches as compared to the ZnO nanowires and Zn₂GeO₄/ZnO nanowires. Furthermore, the electrical transport behavior of single hybrid nanowire device indicates the formation of back-to-back Schottky barriers (SBs) formation at the contacts and its application in white-light response has been demonstrated.     

Structural,Raman, and photoluminescence properties of double‐shelled coaxial nanocables of In2O3 core with ZnO and AZO shells

We synthesized In₂O₃/ZnO/Al‐doped ZnO (AZO) core‐double shell nanowires, in which the inner shell (ZnO) and the outer shell (AZO) have been subsequently deposited on the core In₂O₃ nanowires. With their one‐dimensional morphology being preserved, the X‐ray diffraction (XRD), lattice‐resolved transmission electron microscopy (TEM) image, selected area electron diffraction, and Raman spectrum coincidentally revealed that the shell was comprised of hexagonal ZnO phase. In addition, TEM‐EDX investigation revealed the presence of Al elements in the shell region. The thermal annealing at 700 °C did not significantly change the nanowire morphology, however, the XRD spectrum indicated that the ZnO phase was crystallized by the annealing. PL spectrum of the 700 °C‐annealed In₂O₃/ZnO/AZO core‐double shell nanowires was comprised of three Gaussian bands at approximately 2.1 eV, 2.4 eV, and 3.0 eV, respectively. The integrated intensities of 2.1 eV‐, 2.4 eV‐, and 3.0 eV‐bands were decreased by the thermal annealing. This study will pave the road to the preparation and applicaition of double‐shelled nanowires. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Structural and photoluminescence properties of GaN/ZnO core‐shell nanowires with their shells sputtered

We have reported the preparation of ZnO‐coated GaN nanowires and investigated changes in the structural and photoluminescence (PL) properties by the application of a thermal annealing process. For fabricating the core‐shell nanowires, Zn target was used to sputter ZnO shell onto GaN core nanowires. X‐ray diffraction (XRD) analysis indicated that the annealed core‐shell nanowires clearly exhibited the ZnO as well as GaN phase. The transmissoin electron microscopy (TEM) investigation suggested that annealing has induced the crystallization of ZnO shell layer. We have carried out Gaussian deconvolution analysis for the measured PL spectra, revealing that the core GaN nanowires exhibited broad emission which consist of red, yellow, blue, and ultraviolet peaks. ZnO‐sputtering induced new peaks in the green region. Thermal annealing reduced the relative intensity of the green emission. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

硅基AAO模板内电化学沉积ZnO纳米线及其光电性能研究

本文利用二次阳极氧化法在p型低阻〈100〉晶向的硅衬底上制备了AAO/Si,以硅基AAO为辅助模板,采用电化学沉积的方法以Zn(NO3).6H2O和HMT(C6H12N4)为原料,在80℃的水浴槽中制备了ZnO纳米线结构。采用SEM,XRD和拉曼光谱等手段对ZnO/AAO/Si复合结构进行表征。SEM图表明ZnO纳米线已成功组装到AAO/Si模板里,直径约45 nm,长度约为600 nm。XRD和拉曼光谱表明ZnO具有六角纤锌矿多晶结构。光致发光(PL)谱图表明ZnO/AAO/Si复合结构在565 nm附近有较宽黄绿发射峰,在395 nm附近有微弱的紫外发射峰。场发射测试结果表明,ZnO纳米线的场增强因子的β值为2490,场增强因子很高,具有广泛的应用前景。     

Conversion from ZnO nanospindles into ZnO/ZnS core/shell composites and ZnS microspindles

The formation process of ZnO/ZnS core/shell microcomposites and ZnS microspindles prepared by the reaction of ZnO colloids and thioacetamide under hydrothermal conditions was investigated in detail by X‐ray powder diffraction, field emission scanning electron microscopy, transmission electron microscopy and selected‐area electron diffraction techniques. The precursors of spindlelike ZnO colloids were prepared by a hydrothermal method with the help of a surfactant. A growth mechanism was proposed to account for the formation of ZnO/ZnS core/shell microcomposites and ZnS microspindles. Luminescence measurement revealed that ZnO/ZnS core/shell microcomposites integrated the luminescence effect of ZnO and ZnS. The blue and green emissions were dramatically enhanced, while the orange emission disappeared. The results provide a good approach to tune the visible emission of the ZnO nanostructures by ZnS coating. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

电场辅助电化学法沉积ZnO纳米线

采用电场辅助电化学沉积的方法成功的在阳极氧化铝模板中沉积出ZnO纳米线阵列.透射电子显微镜(TEM)、X射线衍射(XRD)测试结果表明,制备的纳米线是单晶ZnO纳米线,形貌均匀,直径大约为60nm,并且择优于(101)晶面.我们对生长过程中所加辅助电场的作用给出了初步的解释.     

Synthesis of tunable 3D ZnO architectures assemblied with nanoplates

A simple hydrothermal method has been used to synthesize 3D ZnO architectures without any surfactants. The assemblies were composed of thin nanoplates with the thickness of about 35nm. X‐ray diffraction analysis (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and photoluminescence (PL) characterized the structure, morphology and optical property of products. The thickness of plates and assembly pattern can be controlled by adjusting the experimental parameters. Furthermore, these ZnO nanoplate assemblies exhibited enhanced photocatalytic activity in the degradation of methylene blue (MB). (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Synthesis of ZnO nanorods by the thermal reduction process of a mixture of ZnO and Al powder

Single‐crystalline Zinc oxide (ZnO) nanorods were firstly synthesized on gold‐coated Si substrate via a simple thermal reduction method from the mixture of ZnO and Al powder. The growth process was carried out in a quartz tube at different temperature (550‐700 °C) and at different oxygen partial pressure. Their structure properties were investigated by X‐ray diffraction (XRD), scanning electron microscope (SEM), X‐ray photoelectron spectroscopy (XPS) and transmission electron microscopy (TEM). The length of the as‐prepared ZnO nanorods was up to several micrometers and their diameters were about 130 nm. The X‐ray diffraction patterns, transmission electron microscopic images, and selective area electron diffraction patterns indicate that the one‐dimensional ZnO nanorods are a pure Single‐crystal and preferentially oriented in the [0001] direction. The reaction mechanism of ZnO nanorods was proposed on the basis of experimental data. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Fabrication of Zn/ZnO nanocables through thermal oxidation of Zn nanowires grown by RF magnetron sputtering

Fabrication of Zn/ZnO nanocables by thermal oxidation of Zn nanowires grown by RF magnetron sputtering is reported. Single crystalline Zn nanowires could be grown by controlling supersaturation of source material through the adjustment of temperature and Zn RF power. X-ray diffraction and high-resolution transmission electron microscopy showed that surfaces of these Zn nanowires, grown along the [0 1 0] direction, gradually oxidized inward the Zn core to form coaxial Zn/ZnO nanocables in the subsequent oxidation at 200 °C. In the Zn/ZnO nanocable, epitaxial relations of [1 0 0]_Zn//[1 0 0]_ZnO, and (0 0 1)_Zn//(0 0 1)_ZnO existed at the interface between the Zn core and ZnO shell. A number of dislocations were also observed in the interface region of the Zn/ZnO nanocable, which are attributed to large differences in the lattice constants of Zn and ZnO. With further increasing the oxidation temperature over 400 °C, Zn nanowires were completely oxidized to form polycrystalline ZnO nanowires. The results in this study suggest that coaxial Zn/ZnO nanocable can be fabricated through controlled thermal oxidation of Zn nanowires, yielding various cross-sectional areal fractions of Zn core and ZnO shell.     

Preparation,structure and photoluminescence properties of SiO2–coated ZnS nanowires

It is essential to passivate one‐dimensional (1D) nanostructures with insulating materials to avoid crosstalking as well as to protect them from contamination and oxidation. The structure and influence of thermal annealing on the photoluminescence properties of ZnS‐core/SiO₂‐shell nanowires synthesized by the thermal evaporation of ZnS powders followed by the sputter deposition of SiO₂ were investigated. Transmission electron microscopy and X‐ray diffraction analyses revealed that the cores and shells of the core‐shell nanowires were single crystal zinc blende‐type ZnO and amorphous SiO₂, respectively. Photoluminescence (PL) measurement showed that the core‐shell nanowires had a green emission band centered at around 525 nm with a shoulder at around 385 nm. The PL emission of the core‐shell nanowires was enhanced in intensity by annealing in an oxidative atmosphere and further enhanced by subsequently annealing in a reducing atmosphere. Also the origin of the enhancement of the green emission by annealing is discussed based on the energy‐dispersive X‐ray spectroscopy analysis results. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Low temperature synthesis of spindle‐like ZnO nanostructures under microwave irradiation

A simple and general microwave route is developed to synthesize nanostructured ZnO using Zn(acac)₂·H₂O (acac = acetylacetonate) as a single source precursor. The reaction time has a great influence on the morphology of the ZnO nanostructures and an interesting spindle‐like nanostructure is obtained. The microstructure and morphology of the synthesized materials are investigated by X‐ray diffraction (XRD), scanning electron microscopy (SEM), field‐emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), high‐resolution transmission electron microscopy (HRTEM) and selected area electron diffraction (SAED). It is found that all of them with hexagonal wurtzite phase are of single crystalline structure in nature. Ultraviolet–visible (UV‐vis) absorption spectra of these ZnO nanostructures are investigated and a possible formation mechanism for the spindle‐like ZnO nanostructures is also proposed.     

Zn掺杂Z形GaN纳米线的制备及表征

本文以氧化镓、氧化锌和氨气为原料,通过常压化学气相沉积法(APCVD)在Au/Si(100)衬底上成功生长出了Zn掺杂的"Z"形GaN纳米线。利用场发射扫描电镜(FESEM)、X-射线衍射仪(XRD)、透射电子显微镜(TEM)、光致发光谱(PL)等测试方法对样品的形貌、晶体结构及光学性质进行了表征。结果表明:在温度为950℃,氧化镓和氧化锌的质量比为8∶1的条件下,制备出的Zn掺杂Z形GaN单晶纳米线直径为70 nm、长度为数十个微米,生长机理遵循VLS机制。Zn元素的掺杂使GaN纳米线在420 nm处出现了光致发光峰,发光性能有所改善。     

Synthesis of high aspect ratio ZnO nanowires with an inexpensive handcrafted electrochemical setup

In this work, high aspect ratio zinc oxide nanowires are synthesized using templated one-step electrodeposition technique. Electrodeposition of the nanowires is done using a handcrafted electronic system. Nuclear track-etched polycarbonate membrane is used as a template to form the high aspect ratio nanowires. The result of X-ray diffraction and scanning electron microscopy shows that nanowires with a good crystallinity and an aspect ratio of more than 30 can be achieved in a suitable condition. The height of electrodeposited nanowires reaches to about 11 μm. Based on the obtained results, high aspect ratio ZnO nanowires can be formed using inexpensive electrodeposition setup with an acceptable quality.     

Influence of ambient gases on the morphology and photoluminescence of ZnO nanostructures synthesized with nickel oxide catalyst

The morphology and luminescence properties of ZnO nanowires synthesized using NiO catalyst in a chemical vapor deposition system under different growth ambient have been studied. ZnO nanostructures were prepared in nitrogen, ammonia and hydrogen ambient and characterized using X-ray diffraction, scanning electron microscopy, transmission electron microscopy and photoluminescence. Growth in nitrogen ambient yields ZnO nanoneedles while growth with ammonia and hydrogen ambient ends up with ZnO nanowires. Presence of the Ni tip at the end in either morphology indicated the involvement of vapor–liquid–solid growth mechanism. Enhanced green emission in ZnO nanowires implies the presence of a high density of oxygen vacancies. Influence of the ambient gases on the morphology and optical properties of ZnO nanostructures is discussed.     

PMA‐b‐PAA‐controlled synthesis of one‐dimensional CaCO3 superstructures

Crystallization of calcium carbonate (CaCO₃) crystals by a gas‐liquid diffusion method has been carried out in aqueous solution using a double‐hydrophilic block copolymer (DHBC) poly(maleic anhydride)‐b‐poly(acrylic acid) (PMA‐b‐PAA). The as‐prepared products were characterized with X‐ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscope (TEM), selected area electron diffraction (SAED), high‐resolution transmission electron microscopy (HRTEM) and infrared spectroscopic analysis (FT‐IR). Uniform one‐dimensional calcite micro/nanostructures with different morphologies are fabricated through an assembled process. The influence of PMA‐b‐PAA copolymer concentration on the morphology of calcite nano/microwires is investigated, which plays an important role in the morphological control of building blocks composed of one‐dimensional calcite crystals. The possible formation mechanism of one‐dimensional CaCO₃ crystals was discussed. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Facile fabrication of porous ZnO by a template free method

In this paper, a template free method has been employed to fabricate porous ZnO. Brick shaped precursor was first synthesized by a mild hydrothermal process. Accompanied with the decomposition of the precursor during the subsequent annealing treatment, porous ZnO with the inherited morphology of the precursor was obtained. The as‐prepared products were characterized by X‐ray diffraction (XRD) and scanning electron microscopy (SEM). It exhibited that the porous hierarchical frame consists of nano‐sheets with wurtzite‐type. The size of the pores as well as the size of the particles varied with the annealing temperature. Mechanism speculation showed that the crystal‐aggregation in the growth process of the precursor is the key to the establishment of pore structure. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Silicon nanowire growth on poly‐silicon‐on‐quartz substrates formed by aluminum‐induced crystallization

The vertical growth of Si nanowires on non‐monocrystalline substrates is of significant interest for photovoltaics and other energy harvesting applications. In this paper, we present results on using poly‐Si layers formed by aluminum‐induced crystallization (AIC) on fused quartz wafers as an alternative substrate for the vapor‐liquid‐solid (VLS) growth of vertical Si nanowires. Oxidation of the Al surface to Al₂O₃ before the a‐Si deposition was shown to be a key requirement in the formation of the poly‐Si template since it promotes the crystallization of the a‐Si into Si(111) which is required for vertical silicon nanowire growth. The effect of Al deposition technique (DC sputtering versus thermal evaporation) on a‐Si crystallization and Si nanowire growth was investigated. The use of Al thermal evaporation yielded AIC poly‐Si layers with the highest fraction of 〈111〉 grains as measured by orientation imaging microscopy (OIM) which enabled the growth of vertical Si nanowires. Cross‐sectional transmission electron microscopy analysis confirmed that the 〈111〉 Si nanowires grew epitaxially off of {111}poly‐Si grains in the AIC layer. This study demonstrates the potential of using AIC poly‐Si as a template layer for the vertical growth of silicon nanowires on amorphous substrates.     

Synthesis of radial‐like ZnO structure by hydrothermal method with ZnSO4·7H2O and Zn(CH3COO)2·2H2O as zinc sources

Radial‐like ZnO structures were prepared using zinc sulfate (ZnSO₄·7H₂O) and zinc acetate [Zn(CH₃COO)₂·2H₂O] as zinc sources by a facile template‐free hydrothermal method in this paper. Structural and optical properties of radial‐like ZnO structures are characterized by X‐ray diffraction (XRD), field emission scanning electron microscopy (FESEM), UV‐vis spectrophotometer and photoluminescence measurement (PL). It has been found that the distinct surface morphologies of radial‐like ZnO structures grown by different zinc sources. Slim radial‐like ZnO with a hexagonal wurtzite structure is grown by using ZnSO₄·7H₂O as zinc sources, whereas coarse radial‐like ZnO with zincite structure is achieved by zinc acetate. The UV‐vis absorption spectra of them both display an obvious and significant absorption in the ultraviolet region. The room temperature PL spectra of ZnO structures grown by two different zinc sources possess a common feature that consists of a strong ultraviolet (UV) peak and visible emission band.     

Facile morphology‐controlled hydrothermal synthesis of flower‐like self‐organized ZnO architectures

Flower‐like self‐organized crystalline ZnO architectures were obtained through a facile and controlled hydrothermal process. As‐synthesized products were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), high‐resolution transmission electron microscopy (HRTEM), X‐ray diffraction (XRD), electron diffraction and UV‐Vis spectroscopy. XRD and electron diffraction results confirmed the obtained materials are pure wurtzite ZnO. The effects of different ratios of starting materials and solvent on the morphologies of ZnO hydrothermal products were also evaluated by SEM observations. It is suggested that the use of water, rather than ethanol as the solvent, as well as employing a precursor of Zn(Ac)₂ and 2NaOH (v/v) in hydrothermal reactions are responsible for the generation of specific flower‐like self‐assembled ZnO structures. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)