Quasi One-dimensional ZnO Nanostructures Fabricated without Catalyst at Lower Temperature

One- or quasi one-dimensional zinc oxide nanostructures possess plenty of morphologies. Only by controlling the gas flow rates, and partial pressures of argon, oxygen and zinc vapor, can various types of high-quality ZnO nanomaterials (such as wires, belts, arrays, saws or combs, tetraleg rods, nails, and pins) be synthesized through pure zinc powder evaporation without a catalyst at the temperature range of 600–700°C. In this study, deposited nanostructures were characterized by means of scanning electron microscopy, X-ray diffraction and high-resolution transmission electron microscopy. The authors propose and discuss the growth mechanisms of various ZnO. In addition, properties of room temperature photoluminescence and field emission of several typical ZnO nanostructures are measured and investigated.     

Study of ZnO-coated SnO2 nanostructures synthesized by a two-step process

We investigated the influence of the ZnO coating on the properties of one-dimensional (1D) nanostructures of SnO₂. We have employed X-ray diffraction, scanning electron microscope, transmission electron microscope and photoluminescence (PL) spectroscopy to characterize both as-synthesized and ZnO-coated products. We observed that deposition process of ZnO by using an atomic layer deposition technique resulted in the SnO₂ core/ZnO shell structure. The photoluminescence of the ZnO-coated products exhibited broad bands in the UV and green region, suggesting a possible contribution of the emission from the ZnO outlayers.     

Synthesis of ZnO nanoplates decorated rhombus-shaped ZnO nanorods and their application in solar cells

Novel nanostructures of ZnF(OH) nanoplates decorated rhombus-shaped ZnF(OH) nanorods were fabricated. The obtained precursors were transformed by calcination to porous hierarchical ZnO nanostructures with the original morphologies retained. Field emission scanning electron microscope images exhibit that the nanoplates are grown in the interstices between the nanorods and on the top of the nanorods. The structure and composition of the obtained products have been confirmed by transmission electron microscope and X-ray diffraction measurements. The obtained ZnO nanostructures have been successfully used in solar cells. The light-to-electricity conversion results show that the complex nanostructures exhibit a power conversion efficiency of 1.36% with a photoelectrode thickness of 4.2 µm, which is comparable to those based on 40 µm vertically aligned hexagonal-shaped ZnO nanowire array photoelectrodes. These results indicate that the synthesized ZnO nanoplate decorated rhombus-shaped ZnO nanorod nanostructures are more suitable for application as a photoelectrode in solar cells.     

具有四角锥状纳米ZnO的制备及强蓝光发射特性

采用热蒸发锌(Zn)粉的方法在不同的氩气流量下制备了大量的四角锥状ZnO纳米材料。用扫描电子显微镜(SEM)、透射电子显微镜(TEM)和光致发光(PL)谱对样品进行了形貌和发光特性分析。结果表明:合成的ZnO纳米材料受氩气流量影响较大,随着流量的增大,形貌由杂乱无章的结构(线状、块状、树枝状等)变成了均匀一致的四角锥结构,其发光特性随着生长过程中氧相对含量的减少,紫外光发射减弱、蓝光发射明显增强,说明氧空位是引起蓝绿光发射的主要原因。经700℃在空气中氧化处理后蓝绿光消失和氧化后的样品在700℃再经氢气还原处理后蓝绿光又重新出现的实验结果进一步证实了这个结论。本实验结果提供了支持纳米ZnO蓝光发射来自氧空位的实验证据,并初步探讨了四角锥状ZnO纳米结构的生长机理和强蓝光发射机制。     

氧流量对热蒸发CVD法生长β-Ga2O3纳米材料的结构及发光特性的影响

用热蒸发CVD法制备了β-Ga₂O₃纳米材料, 并用光致发光(PL)方法研究了其发光特性. Ｘ射线衍射分析显示, 产物为单斜结构的β-Ga₂O₃. 扫描电子显微镜测试表明: 在较小氧流量条件下制备的产物为β-Ga₂O₃纳米带, 宽度小于100nm, 长度有几微米; 较大氧流量时制备出β-Ga₂O₃纳米晶粒结构, 晶粒尺度在80—15 关键词： 光致发光 氧流量 纳米结构 2O₃')" href="#">Ga₂O₃     

Spontaneous formation of single crystal ZnO nanohelices

This paper reports a novel helix-like ZnO nanostructure with several tens of nanometres in thickness synthesized on a gold-coated Si substrate by thermal evaporation of zinc sulfide powder at 1020°C. Transmission electron microscope characterization shows that as-synthesized ZnO nanohelices extend along [01\bar 11] direction and the axial direction of the helix is along [0001] direction. A catalyst-intervened dislocation-induced growth mechanism has been suggested to explain the formation of the helix-like ZnO nanostructures. This study opens a new route to construct helix-like ZnO nanostructures by different evaporation sources.     

水热法制备Co掺杂ZnO纳米棒及其光学性能

采用水热法在石英衬底上以Zn(CH3COO)2.2H2O和Co(NO3)2.6H2O水溶液为源溶液,以C6H12N4(HMT)溶液作为催化剂,在较低温度下制备了Co掺杂的ZnO纳米棒。采用X射线衍射(XRD)和扫描电子显微镜(SEM)对所生长ZnO纳米棒的晶体结构和表面形貌进行了表征,考察了Co掺杂对ZnO纳米棒微观结构和对发光性能影响的机制。结果表明:Co掺杂的ZnO纳米棒呈六方纤锌矿结构,具有沿(002)面择优生长特性,Co掺杂使ZnO纳米棒的直径变细;同时室温光致发光(PL)谱检测显示Co掺杂ZnO纳米棒具有很强的近带边紫外发光峰,而与深能级相关的缺陷发光峰则很弱。本研究采用水热法在石英衬底上于较低温度下生长出了具有较高光学质量的Co掺杂ZnO纳米棒。     

TEM investigations on ZnO nanobelts synthesized via a vapor phase growth

Unusual ZnO nanostructures have been successfully synthesized via selenium-controlled chemical vapor phase growth on Si (111) substrates at about 500 degrees C. The microstructure and chemical compositional characteristics of the ZnO nanomaterials have been systematically investigated by means of analytical transmission electron microscopy (TEM), including energy-dispersive X-ray spectroscopy and electron energy-loss spectroscopy. Most of the nanostructures have a belt-like morphology with typical widths of approximately 150 nm and lengths up to several micrometers. All the investigated materials are found to be stoichiometric ZnO with a hexagonal crystal structure. The growth directions for the nanobelts are found to be [1010] and [2110] respectively. Regular-triangle and needle-like heads with diameters only approximately 25-35 nm have been found in the straight nanobelts. High-resolution TEM images indicate that all the nanostructures are single crystals and free of defects. The growth mechanisms of such interesting and unique morphologies are briefly discussed.     

Synthesis and optical properties of flower-like ZnO nanorods by thermal evaporation method

Flower-like ZnO nanorods have been synthesized by heating a mixture of ZnO/graphite powders using the thermal evaporation and vapor transport on Si (1 0 0) substrates without any catalyst. The structures, morphologies and optical properties of the products were characterized in detail by using X-ray diffraction (XRD), scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), photoluminescence (PL) and Raman spectroscopy. The synthesized products consisted of large quantities of flower-like ZnO nanostructures in the form of uniform nanorods. The flower-like ZnO nanorods had high purity and well crystallized wurtzite structure, whose high crystalline quality was proved by Raman spectroscopy. The as-synthesized flower-like ZnO nanorods showed a strong ultraviolet emission at 386 nm and a weak and broad yellow-green emission in visible spectrum in its room temperature photoluminescence (PL) spectrum. In addition, the growth mechanism of the flower-like ZnO nanorods was discussed based on the reaction conditions.     

Surface enhanced Raman scattering and photoluminescence properties of catalytic grown ZnO nanostructures

Sword-like (diameter ranging from 40 nm to 300 nm) and needle-like zinc oxide (ZnO) nanostructures (average tip diameter ∼40 nm) were synthesized on annealed silver template over silicon substrate and directly on silicon wafer, respectively via thermal evaporation of metallic zinc followed by a thermal annealing in air. The surface morphology, microstructure, chemical analysis and optical properties of the grown samples were investigated by field emission scanning electron microscopy, X-ray diffraction, energy dispersive X-ray analysis, room temperature photoluminescence and Raman spectroscopy. The sword-like ZnO nanostructures grown on annealed silver template are of high optical quality compared to needle-like ZnO nanorods for UV emission and show enhanced Raman scattering.     

Fabrication and characterization of ZnO micro and nanostructures prepared by thermal evaporation

A simple method of thermal evaporation to fabricate micro and nanostructures of zinc oxide was presented. ZnO micro and nanostructures, prepared under different quantity of O₂, were characterized by techniques such as scanning electron microscopy (SEM), X-ray diffraction (XRD), Raman spectroscopy and analytical transmission electron Microscope. The SEM images indicated that the products prepared under the condition of sufficient O₂ were needle-like microrods and the samples synthesized under the condition of deficient O₂ were nanorods and nanowires with very high aspect ratio. The results of XRD and Raman shifts revealed that the ZnO micro and nanostructures synthesized under different quantity of O₂ were both single crystalline with the hexagonal wurtzite structure. The HRTEM images indicated that the ZnO nanowire prepared under the condition of deficient O₂ was single crystalline and grown along the direction of [0 0 1]. Photoluminescence measurement was carried out and it showed that the spectra of ZnO micro and nanostructures prepared under different quantity of O₂ exhibited similar emission features. In addition, the growth mechanism of ZnO micro and nanostructures was preliminarily discussed.     

Ultrasound-assisted green synthesis of nanocrystalline ZnO in the ionic liquid [hmim][NTf2

For the first time, zinc oxide nanoparticles have been synthesized by the sonochemical method in an ionic liquid, 1-hexyl-3-methylimidazolium bis (trifluoromethylsulfonyl) imide, liquid [hmim][NTf(2)] as a solvent. The morphology and structure of ZnO nanoparticles have been characterized using X-ray diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). A possible mechanism is proposed to explain the formation of ZnO nanostructures.     

Growth of ZnO nanodisk, nanospindles and nanoflowers for gas sensor: pH dependency

Suitable morphology for fast electron transportation is a crucial requirement for the fabrication of gas sensor application. Highly oriented and well defined zinc oxide (ZnO) nano/micro-scale structures are grown on the glass substrates using aqueous chemical route. The grown nanostructures have been characterized by X-ray diffraction pattern (XRD), scanning electron microscope (SEM) and optical absorption techniques. The SEM micrographs revealed the formation of disk, rod, spindle and flower-like morphologies at different pH values ranging from 5 to 10. The grown nanostructures were employed for acetone gas-sensing measurement. It is observed that the sensors based on nanoflowers showed higher response (95%) for acetone gas at 325 °C. The high acetone gas sensitivity of ZnO nanoflowers can be attributed to the surface morphology. Moreover, nanoflower-like structure exhibits the fast response and recovery.     

Synthesis and ultraviolet emission of aligned ZnO rod-on-rod nanostructures

Aligned ZnO rod-on-rod nanostructures were synthesized on silicon substrate via a simple thermal evaporation process at low temperature without catalysts. Pictures taken with the use of the scanning electron microscope demonstrate that the well-ordered ZnO rod-on-rod nanostructures grow on the Si substrate, and the single nanostructure consists of two parts. Transmission electron microscopy image and the selected area electron diffraction pattern indicate that the single-crystal nanorod grows along [0001] direction. The X-ray diffraction pattern proves that the samples have good crystal quality. The detailed nanorod growth mechanism is proposed and discussed. The room-temperature photoluminescence (PL) spectrum shows the dominant ultraviolet emission, which indicates their potential application in ultraviolet optoelectronic devices. The temperature-dependent PL spectra reveal that the strong ultraviolet emission should originate from the longitudinal optical phonon replicas of free exciton.     

Photoluminescence and Raman analysis of novel ZnO tetrapod and multipod nanostructures

Novel ZnO tetrapod and multipod nanostructures were successfully synthesized in bulk quantity through thermal evaporation method. The morphologies and structures of the ZnO nanostructures were characterized by scanning electron microscopy, X-ray diffraction and transmission electron microscopy. The results revealed that the ZnO nanostructures consisted of tetrapods and multipods with tower-like legs. The ZnO nanostructures were of high purity and were well crystallized with wurtzite structure. The preferred growth direction of legs was found to be the [0 0 0 1] direction. Possible growth mechanisms were proposed for the formation of the ZnO nanostructures. Room temperature photoluminescence (PL) spectra showed that the as-synthesized ZnO nanostructures had a strong green emission centered at 495 nm and a weak ultraviolet emission at 383 nm. Raman spectroscopy was also adopted to explore the structural quality of the ZnO nanostructures.     

Field emission and room temperature ferromagnetism properties of triangle-like ZnO nanosheets

Triangle-like ZnO nanosheets have been synthesized via conventional thermal evaporation method at a low temperature of 550 °C using CuO as catalyst. The obtained samples were investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM), high-resolution transmission electron microscopy (HRTEM), energy dispersive X-ray spectroscopy (EDX) and photoluminescence (PL) spectra. The great influences of Cu catalyst on the morphology of the obtained ZnO nanostructures were investigated. The field emission measurements confirmed that the ZnO nanosheets possessed good performance with a turn-on field of 3.1 V μm⁻¹ and a field enhancement factor of 3250, which have promising application as a competitive cathode material in FE microelectronic devices. Room temperatures ferromagnetism has been observed in the triangle-like ZnO nanosheets, although the products consist of only nonmagnetic elements.     

Synthesis and Raman analysis of 1D-ZnO nanostructure via vapor phase growth

1D-nanostructural zinc oxide (ZnO) with different shapes have been synthesized on p-type Si(1 0 0) and glass substrates via vapor phase growth by heating pure zinc powder at temperatures between 480 and 570 °C. The different ZnO nanostructures depend on the substrates and the growth temperatures. Scanning electron microscopy and X-ray diffraction revealed that a well-aligned nanowires array, which are vertical to the substrate of Si(1 0 0) with 18 sides on their heads, but six sides on their stems, has been formed at 480 °C. Raman study on the ZnO nanostructures shows that the coupling strength between electron and phonon determined by the ratio of the second- to the first-order Raman scattering cross-sections declines with decreasing diameter of the nanowires. However, a little changes of the coupling strength in terms of the width of the nanobelts have been observed.     

Effect of substrates and anions of zinc salts on the morphology of ZnO nanostructures

The ZnO nanostructures were hydrothermally synthesized on glass and Al substrates, respectively, using zinc chloride, zinc nitrate, and zinc acetate as precursor. The as-prepared products were characterized by scanning electron microscopy (SEM) and X-ray powder diffraction (XRD). Different ZnO nanostructures were obtained, such as nanorods, nanosheets, flower-like nanostructures and so on. The effects of the substrates and anions of zinc salts on the morphologies of the resulting products have been investigated.     

Synthesis of ZnO compound nanostructures via a chemical route for photovoltaic applications

A facile, low-temperature, and low-cost chemical route has been developed to prepare ZnO nanowire and nanosphere compound structures. The morphology, structure, and composition of the yielded products have been examined by field-emission scanning electron microscopy, transmission electron microscopy, and X-ray diffraction measurements. We have systematically investigated the optical properties of the ZnO nanostructures by micro-Raman, photoluminescence, and transmission spectroscopy. The results demonstrate that the yielded ZnO nanostructures possess good optical quality with high light absorption. We have further successfully employed the obtained ZnO compound nanostructures in dye-sensitized solar cells. The light-to-electricity conversion results show that the compound nanostructure exhibits a significant enhancement of short-circuit current density due to the increased surface area and light scattering in the compound nanostructures. The present chemical route provides a simple way to synthesize various compound nanostructures with high surface area for nanodevice applications.     

Ultrasonic induced photoluminescence decay in sonochemically obtained cauliflower-like ZnO nanostructures with surface 1D nanoarrays

Cauliflower-like ZnO nanostructures with average crystallite size of about 55 nm which have surface one dimensional (1D) nanoarrays with 10 nm diameter were successfully fabricated through a simple sonochemical route. X-ray diffractometry (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and room temperature photoluminescence (PL) characterizations were performed to investigate the morphological and structural properties of the obtained nanostructures. It has been shown that the synthesized cauliflower-like ZnO nanostructures irradiated UV luminescence and a green peak in visible band. Ultrasonic post-treatment of the particles for about 2 h increased the density of surface defects resulted in an increase in the green emission intensity.