Synthesis, characterization and optical properties of multipod ZnO whiskers

Multipod ZnO whiskers were synthesized successfully by two steps: pulsed laser deposition (PLD) and thermal evaporation process. First, a thin layer of Zn films were deposited on Si(1 1 1) substrates by PLD. Then the whiskers grew on Zn-coated Si(1 1 1) substrate by the simple thermal evaporation oxidation of the metallic zinc powder at 900 °C in the air without any catalysts or additives. The pre-deposited Zn films by PLD on the substrate can promote the growth of ZnO multipod whiskers effectively. The as-synthesized ZnO whiskers were characterized by using X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and high-resolution transmission electron microscopy (HRTEM). The results revealed that the whiskers are highly crystalline with the wurtzite hexagonal structure. Room temperature photoluminescence (PL) spectrum of the whiskers shows a UV emission peak at ∼393 nm and a broad green emission peak at ∼517 nm, which was assigned to the near band-edge emission and the deep-level emission, respectively.     

Growth of nanostructures of Zn/ZnO by thermal evaporation and their application for room-temperature sensing of H<Subscript><Emphasis Type="Bold">2</Emphasis></Subscript>S gas

ZnO micro- and nanostructures were prepared by thermal evaporation of Zn and a mixture of ZnO with graphite. On heating Zn powder in a quartz tube at temperatures between 600 °C to 800 °C, radial growth of nanowires was observed on the source. On increasing the temperature to 900 °C, various interesting micro- and nanostructures of Zn and ZnO were observed to have deposited all over the quartz tube. On the other hand, when ZnO was heated in the presence of graphite, predominant growth of ZnO nanotetrapods was observed. Nanowires and tetrapods of ZnO were characterized by photoluminescence measurements and were found to show significantly improved response for detection of H₂S gas at room temperature when compared with earlier studies. The response was seen to improve with increase in oxygen vacancies in the material. PACS 78.55.Et; 07.07.Df     

Low-temperature growth and ethanol-sensing characteristics of quasi-one-dimensional ZnO nanostructures

ZnO nanorods, nanobelts, nanowires, and tetrapod nanowires were synthesized via thermal evaporation of Zn powder at temperatures in the range 550-600 °C under flow of Ar or Ar/O₂ as carrier gas. Uniform ZnO nanowires with diameter 15-25 nm and tetrapod nanowires with diameter 30-50 nm were obtained by strictly controlling the evaporation process. Our experimental results revealed that the concentration of O₂ in the carrier gas was a key factor to control the morphology of ZnO nanostructures. The gas sensors fabricated from quasi-one-dimensional (Q1D) ZnO nanostructures exhibited a good performance. The sensor response to 500 ppm ethanol was up to about 5.3 at the operating temperature 300 °C. Both response and recovery times were less than 20 s. The gas-sensing mechanism of the ZnO nanostructures is also discussed and their potential application is indicated accordingly.     

Influence of annealing temperature on the properties of ZnO thin films deposited by thermal evaporation

ZnO thin films were deposited by thermal evaporation of a ZnO powder. The as-deposited films are dark brown, rich zinc and present a low transmittance. Then, these films were annealed in air atmosphere at different temperatures between 100 and 400 °C. Their microstructure and composition were studied using XRD and RBS measurements respectively. By increasing the temperature, it was found that film oxidation starts at 250 °C. XRD peaks related to ZnO appear and peaks related to Zn decrease. At 300 °C, zinc was totally oxidised and the films became totally transparent. The electrical conductivity measurement that were carried out in function of the annealing temperature showed the transition from highly conductive Zn thin film to a lower conductive ZnO thin film. The optical gap (E_g) was deduced from the UV-vis transmittance, and its variation was linked to the formation of ZnO.     

Non-catalytic growth of high aspect-ratio ZnO nanowires by thermal evaporation

High-density and high aspect-ratio ZnO nanowires were grown on Si(100) substrates by the thermal evaporation of metallic zinc powder without the use of metal catalysts or additives. The as-grown nanowires had diameters in the range of 60-100 nm with lengths 5-15 μm. Detailed structural characterization indicated that the obtained nanowires are single-crystalline with a perfect hexagonal facet and surfaces. The room temperature PL spectrum exhibited strong UV emission, affirming that the as-grown products have good optical properties. The possible growth mechanism for the formation of hexagonal-faceted and perfect surface ZnO nanowires is also discussed.     

Large-scale synthesis of ZnO nanowires using a low-temperature chemical route and their photoluminescence properties

Single-crystalline zinc oxide (ZnO) nanowires were synthesized from zinc powder and H₂O through a simple chemical route at 730 °C in Ar atmosphere. The potential exists for bulk synthesis of ZnO nanowires at temperatures significantly less than the 200–300 °C of thermal evaporation methods reported formerly. Scanning electron microscopy and transmission electron microscopy observations reveal that the ZnO nanowires are structurally uniform, have lengths up to several hundreds of micrometers and diameters of about 40–60 nm and crystallize in a hexagonal structure. The growth of ZnO nanowires is controlled by the vapor–solid crystal-growth mechanism. Photoluminescence measurements show that the ZnO nanowires have a strong near-band ultraviolet emission at 380 nm and a green light emission at 520 nm caused by oxygen vacancies. PACS 81.05.Ys; 78.55.Et     

Well-aligned ZnO microprism arrays with umbrella-like tips: Low-temperature preparation, structure and UV photoluminescence improvement

Catalyst-free, low-temperature (430 °C), high-density, well-aligned, single-crystalline zinc oxide (ZnO) microprism (ZMP) arrays have been synthesized over the entire c-ZnO/poly-Zn-coated Si substrates by simple thermal evaporation of Zn powder. Specially, the microprisms obtained possess hexagonal umbrella-like tips on which vertical thin ZnO nanowires grow. The growth mechanism of a three-stage thermodynamic process was discussed. Photoluminescence spectra show a strong ultraviolet (UV) emission enhancement of the ZMPs after H⁺ (hydrogen ions) implantation. This kind of special ZnO microstructure may find potential applications in field emission, UV laser emission devices, multifunctional microdevices and highly integrated multichannel nano-optoelectronic devices.     

Ultraviolet detectors based on ZnO films by thermal oxidation of Zn metallic films

Metallic Zn films were deposited on glass substrates by electron-beam evaporation. ZnO films were synthesized by thermal oxidation of Zn metallic films in air. At the annealing temperature of 550 °C, ZnO nanowires appeared on the surface, which mainly result from the decrease of oxidation rate. A ZnO ultraviolet photodetector was fabricated based on a metal-semiconductor-metal planar structure. The detector showed a large UV photoresponse with an increase of two orders of magnitude. It is concluded that promising UV detectors can be obtained on ZnO films by thermal oxidation of Zn metallic films. The ways of performing spectral response measurements for polycrystalline ZnO films are also discussed.      

ZnO-Fe3O4 composite prepared by sol-gel method with H2 deoxidation

Fe doped ZnO powder samples (Fe/Zn=0.05 and 0.1) were prepared by sol-gel method with H₂ deoxidation at 450 ^°C for several hours or just heated in air at the same temperature. It was showed by vibrating sample magnetometer (VSM) that samples heat treated in H₂ could show strong ferromagnetism at room temperature while samples treated in air only show very weak magnetism. XRD using Co kα X-ray revealed that the samples heated in H₂ were not pure phase but like a granular system and the magnetism mainly results from Fe₃O₄ in samples while samples heated in air showed pure ZnO phase. Our work indicated that H₂ deoxidation treatment may be an effective technique to fabricate such magnetic semiconductor-like materials with Curie temperature higher than room temperature.     

Fabrication of highly ordered and stepped ZnO comb-like structures

Highly ordered and stepped ZnO comb-like structures were fabricated using conventional thermal evaporation method. Zn powder covered by a layer of a mixture of ZnO and graphite was employed as the Zn source. The obtained ZnO comb-like structures are several tens of micrometers and some of them are even up to 100 μm. Both the widths of the belts and the lengths of the branches gradually decrease along the growth direction of ZnO comb-like structures. Under the most suitable condition, ZnO nanorods branches have uniform diameters and are evenly distributed on the belt-like stem. Possible growth process of ZnO comb-like structures was discussed. The effect of growth temperature on the morphology of the obtained products was also investigated. Room-temperature photoluminescence spectra from the ZnO comb-like structures and the nanorods film reveal weak UV emission and strong green emission.     

锡催化剂助生长法制备孪晶ZnO纳米线及其结构表征

用SnO和Zn的均匀混合物在高温下共烧通过VLS机制制备出孪晶ZnO纳米线的均匀结构。SEM图像显示孪晶ZnO纳米线的直径在100~200nm之间,长度在几十微米到几百微米之间的范围内,有的甚至达到了毫米级,产率也非常的高。TEM图像中ZnO孪晶纳米线顶端的金属Sn颗粒表明了孪晶结构的Sn催化生长。高分辨电子图谱显示了氧化锌纳米线孪晶结构的特征。电子衍射分析发现孪晶氧化锌的晶带轴的方向是[0110],孪晶面为(1013),并且通过明场像和暗场像分析了孪晶纳米线的晶格关系,确定了孪晶纳米线的汽-液-固(VLS)生长机制。     

Enhanced photocatalytic activity of ultra-high aspect ratio ZnO nanowires due to Cu induced defects

We report the synthesis of ZnO nanowires in ambient air at 650°C by a single-step vapor transport method using two different sources Zn (ZnO nanowires-I) and Zn:Cu (ZnO nanowires-II). The Zn:Cu mixed source co-vaporize Zn with a small amount of Cu at temperatures where elemental Cu source does not vaporize. This method provides us a facile route for Cu doping into ZnO. The aspect ratio of the grown ZnO nanowires-II was found to be higher by more than five times compared ZnO nanowires-I. Photocatalytic activity was measured by using a solar simulator and its ultraviolet-filtered light. The ZnO nanowires-II shows higher catalytic activity due to increased aspect ratio and higher content of surface defects because of incorporation of Cu impurities.     

DRIFTS study of surface reactivity to NO2 by zinc nanoparticle aggregates and zinc hollow nanofibers

Zinc nanostructures synthesized with different morphologies from the same evaporation/condensation technique are studied with concern to surface reactivity to NO₂ by Diffuse Reflectance Infrared Fourier Transformed Spectroscopy (DRIFTS). Synthesis of nanopowders is obtained, according to previous work, by gas flow thermal evaporation at 540 °C of bulk Zn grains. Two types of Zn powders are obtained and studied in experiments. The first one is collected on the cold walls of the reactor as a deposit produced by thermophoretic effect. It is constituted by grains (∼10 μm) originated by the stratification of smaller aggregates (∼200 nm) and isolated primary particles (∼50 nm) born in the gas flow. The second type of powder is grown from the condensation of Zn chemical vapors within the expansion orifice of the quartz reactor after relatively long time (∼1 h) deposition process. It is constituted mainly by hollow Zn nanofibers with external and internal diameter about 100 and 60 nm. Preliminary characterization of the two types of powders is made by SEM, TEM, XRD. Thereafter, the two types of samples are studied by DRIFTS at variable temperature (VT). Comparison is made between the home-synthesized nanopowders with respect to commercial Zn standard dust. The Zn hollow nanofibers when exposed to NO₂ are found to exhibit dramatic reactivity, which is not observed at all either in the case of clustered aggregate zinc or of commercial Zn dust powders. Results indicate that, at increasing temperature from RT to 300 °C, the hollow nanofibers surface reacts distinctively with adsorbant gas NO₂, with contemporary formation of a progressively growing narrow absorption band at 2500 cm⁻¹ and contemporary depression of a doublet (∼1600-1628 cm⁻¹) band. In order to justify this striking spectral feature, we propose the occurring of a possible polymerization process at nanofibers surface where most probably as a consequence of pre-treatment and exposure to gas NO₂ a very thin film of ZnO is formed. The possible role of huge specific surface of hollow nanofibers as inferred by preliminary SEM, TEM, XRD studies is discussed.     

ZnO–SnO2 branch–stem nanowires based on a two-step process: Synthesis and sensing capability

ZnO–SnO₂ branch–stem nanostructures were realized on a basis of a two-step process. In step 1, SnO₂-stem nanowires were synthesized. In step 2, ZnO-branch nanowires were successfully grown on the SnO₂-stem nanowires through a simple evaporation technique. We have pre-deposited thin Au layers on the surface of SnO₂ nanowire stems and subsequently evaporated Zn powders on the nanowires. The ZnO branches, which sprouted from the SnO₂ stems, had diameters in a range of 30–35 nm. As-synthesized branches were of single crystalline hexagonal ZnO structures. Since the branch tips were comprised of Au-containing nanoparticles, the Au-catalyzed vapor–liquid–solid growth mechanism was more likely to control the growth process of the ZnO branches. To test a potential use of ZnO–SnO₂ branch–stem nanostructures in chemical gas sensors, their sensing performances with respect to NO₂ gas were investigated, showing the promising potential in chemical gas sensors.     

Preparation and characterization of ZnO thin films prepared by thermal oxidation of evaporated Zn thin films

In this paper, the experimental results regarding some structural, electrical and optical properties of ZnO thin films prepared by thermal oxidation of metallic Zn thin films are presented.Zn thin films (d=200–400 nm) were deposited by thermal evaporation under vacuum, onto unheated glass substrates, using the quasi-closed volume technique. In order to obtain ZnO films, zinc-coated glass substrates were isochronally heated in air in the 300–660 K temperature range, for thermal oxidation.X-ray diffraction (XRD) studies revealed that the ZnO films obtained present a randomly oriented hexagonal nanocrystalline structure. Depending on the heating temperature of the Zn films, the optical transmittance of the ZnO films in the visible wavelength range varied from 85% to 95%. The optical band gap of the ZnO films was found to be about 3.2 eV. By in situ studying of the temperature dependence of the electrical conductivity during the oxidation process, the value of about 2×10⁻² Ω⁻¹ m⁻¹ was found for the conductivity of completely oxidized ZnO films.     

Catalyst-free growth of well-aligned arsenic-doped ZnO nanowires by chemical vapor deposition method

ZnO nanowires with different arsenic concentration were grown on Si (1 0 0) substrates by chemical vapor deposition method without using catalyst. Zn/GaAs mixed powders were used as Zn and As source, respectively. Oxygen was used as oxidant. The images of scanning electron microscope show that the arsenic-doped ZnO nanowires with preferred c-axial orientation were obtained, which is in well accordance with the X-ray diffraction analysis. The arsenic related acceptor emission was observed in the photoluminescence spectra at 11 K for all arsenic-doped ZnO samples. This method for the preparation of arsenic-doped ZnO nanowires may open the way to realize the ZnO nanowires based light-emitting diode and laser diode.     

Catalyst-free synthesis of novel brush-shaped ZnO particles by a simple oxidation in air

Brush-shaped ZnO particles were synthesized by controlling the growth time in the direct melt oxidation process of Al-Zn mixture in air at atmospheric pressure. Particles with two kinds of structures were formed. One was consisted of nanowires grown along [0 0 0 1] direction at the six corners and the center of (0 0 0 1) basal plane on hexagonal ZnO microrod. The other was constructed by nanobelts between the corner-nanowires as well as nanowires at the corners on ZnO microrod. The structural configuration that the nanowires and the nanobelts have a well coherent orientation alignment with the base microrod implies that the brush-shaped ZnO is single crystal. Room temperature PL spectrum of the brush-shaped ZnO particles displayed predominant green emission with a wavelength of 510 nm.     

单晶ZnO纳米线的合成和生长机理研究

用化学气相输运(CVT)方法合成了直径在20～120nm呈单晶结构的ZnO纳米线.利用场发射扫描电 镜(FESEM)、高分辨透射电镜(HRTEM)以及选区电子衍射(SAED)等技术对ZnO纳米线的生长机理和结构进行 了系统研究,结果表明,纳米线的成核与Au Zn合金催化颗粒的饱和度有直接的关系,先饱和的颗粒上纳米线首 先成核.纳米线顶端合金颗粒组成的变化是导致纳米线生长终止的重要原因,大量纳米线的生长不是同时进行 的.本工作提供了支持纳米线气液固(V L S)生长机理的新实验证据,提出了氧化物纳米线的生长机理.     

Evolution of Zn based high purity phases under NH3 gas atmosphere and their PL properties

We report here the evolution of zinc based high purity phases with novel morphologies such as Zn₃N₂ hollow structures, ZnO nanowires and nanopowders, as well as metallic Zn layered hexagonal microparticles at progressively increased reaction temperature of 600 °C, 700 °C, 800 °C under NH₃ gas atmosphere using Zn powder precursor and keeping all other experimental parameters unchanged. Growth mechanism for Zn₃N₂ obtained by nitridation, ZnO by oxidation and Zn microparticles via thermal evaporation & condensation process are discussed briefly. The as-synthesized products were characterized by X-ray diffraction (XRD), energy dispersive X-ray spectroscopy (EDS) and scanning electron microscopy (SEM). Photoluminescence (PL) studies have revealed very interesting and infrequently observed emission bands at 378 and 661 nm for Zn₃N₂, 359 and 396 nm for ZnO as well as 389 nm for Zn polyhedral microparticles.     

Structural and optical properties of poly-vinylpyrrolidone modified ZnO nanorods synthesized through simple hydrothermal process

The synthesis of nanocrystalline zinc oxide (ZnO) in the presence of poly-vinylpyrrolidone (PVP) as capping agent through hydrothermal process, and their structural and optical properties were reported. PVP modified ZnO nanorods grown hydrothermally involve a heterogeneous chemical reaction in the presence of water as a solvent medium and reaction temperature of 100 °C for 7 h in a hot air oven and calcined in air at 500 °C for 3 h. Crystal structure, phase purity and average crystallite size of ZnO were studied by powder X-ray diffraction (PXRD). The strain associated with the as-prepared samples due to lattice deformation was estimated by Williamson–Hall (W–H) analysis. Structural morphology was investigated using scanning electron microscopy (SEM), which showed the formation of nanorods with PVP capping. The growth mechanism of ZnO nanorods and its capping by poly-vinylpyrrolidone are briefly discussed through FT-IR adsorption spectra. The optical behavior of the samples was analyzed through photoluminescence (PL) spectroscopy with an emission spectra in visible region ∼418 nm indicate the applicability of using it as a transport material in solar cells.