Green hydrothermal synthesis and optical absorption properties of ZnO2 nanocrystals and ZnO nanorods

A green hydrothermal method was proposed for the controllable synthesis of ZnO₂ nanocrystals and ZnO nanorods, using the common and cost-effective 2ZnCO₃·3Zn(OH)₂ powder and 30 mass% H₂O₂ aqueous solution as the raw materials. The characterization results from X-ray diffraction, high resolution transmission electron microscopy, transmission electron microscopy and energy dispersive X-ray spectroscopy indicated that the products synthesized at 100-120 °C for 6 h or at 170 °C for 0 h were cubic phase ZnO₂ nanocrystals; while those synthesized at 170 °C for 3-6 h were hexagonal phase ZnO nanorods. The UV-vis absorption spectra showed that the as-synthesized ZnO₂ nanocrystals and ZnO nanorods had optical band gaps of about 4.1 and 3.3 eV, respectively.     

Preparation of ZnO nanorods through wet chemical method

The different morphologies of nanorods have been obtained via a simple wet chemical method in the present of polyethylene glycol (PEG, M_w = 4000) by using zinc nitrate hexahydrate (Zn(NO₃)₂·6H₂O) and ammonium hydroxide (NH₃·H₂O) as the starting materials. Samples were characterized by XRD, EDS, TEM, SEM, ED and PL. XRD results prove the formation of ZnO with wurtzite structure. The ED and HRTEM reveal that single ZnO nanorod is single crystal and preferentially grows up along the [001] direction. The PL spectra showed that the ZnO nanorods have blue emission at 466 nm and green-yellow emission at 542 nm. The influence of reaction temperature, pH in system and evaporation of ammonia on the morphology has been investigated. A possible growth mechanism of ZnO with various morphologies is discussed.     

Photo-initiated growth of zinc oxide (ZnO) nanorods

A photo-initiated process via femtosecond pulse-induced heterogeneous nucleation in zinc ammine complex (Zn(NH₃)₄²⁺)-based aqueous solution without catalyst and surfactant, followed by hydrothermal treatments for crystal growth into zinc oxide (ZnO) nanorods, was investigated. Flat-top hexagonal ZnO nanorods with smooth planes of diameter ≥ 100 nm and length ≤ 1 μm were grown with laser irradiation, compared to porous rod-like structures without irradiation. The flat-top planes indicate slow growth rate, due to the intermediate step of Zn(NH₃)₄²⁺ decomposition to Zn(OH)₄²⁻, before dehydration to ZnO. Prolonged hydrothermal treatment produced nanotubes and lateral splits due to OH⁻ erosion of the crystal faces. XRD analysis showed a hexagonal crystal structure while photoluminescence study indicated a peak at about 380 nm.     

Temperature induced hierarchical growth of ZnO microcrystal

Unique ZnO microcrystal composed of hierarchical nanorod arrays was successfully prepared by a surfactant-free process in aqueous solution under moderate temperature. The products were characterized by X-ray powder diffraction (XRD) and scanning electron microscopy (SEM). The as-prepared ZnO microstructures are of hexagonal phase and high purity. On the basis of SEM images and parallel experiments, a possible formation mechanism for this ZnO microcrystal with hierarchical nanorod arrays is proposed and the reaction temperatures are found to have great influence on inducing secondary nucleation and the hierarchical growth. In addition, the optical properties of the ZnO samples were also investigated by the photoluminescence (PL) spectrometer.     

Solvothermal synthesis of hexagonal ZnO nanorods and their photoluminescence properties

Pure hexagonal ZnO nanorods were synthesized by low-temperature (90 °C) solvothermal treatment of zinc acetate in 40-80 wt.% hydrazine hydrate aqueous solutions. The products were characterized by means of powder X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electronic microscopy (TEM), selected area electron diffraction (SAED), and room temperature photoluminescence (RTPL) spectra. They show a strong UV emission at around 380 nm upon excitation at 360 nm using a Xe lamp at room temperature. The influence on the quality of the nanorods was investigated while the content of the solvent changed. The as-synthesized ZnO nanorods are promising materials for nanoscale optoelectronic devices due to their excellent UV emission properties.     

Single crystalline ZnO nanorods grown by a simple hydrothermal process

Single crystalline ZnO nanorods with wurtzite structure have been prepared by a simple hydrothermal process. The microstructure and composition of the products were studied by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), high-resolution TEM, energy dispersive X-ray spectrum (EDS) and Raman spectrum. The nanorods have diameters ranging from 100 nm to 800 nm and length of longer than 10 µm. Raman peak at 437.8 cm^− 1 displays the characteristic peak of wurtzite ZnO. Photoluminescence (PL) spectrum shows a blue light emission at 441 nm, which is related to radiative recombination of photo-generated holes with singularly ionized oxygen vacancies.     

Solvothermal synthesis and photoluminescence properties of ZnO nanorods and nanorod assemblies from ZnO2 nanoparticles

Without the use of any extra surfactant or template, hexagonal phase ZnO crystallites consisting of individual nanorods or nanorod assemblies were synthesized simply by solvothermal treatment of several nanometer ZnO₂ nanoparticles in three different solvents (including ethanol, 80 wt.% hydrazine hydrate aqueous solution and ethylenediamine) at 150 °C for 24 h. The structures and optical properties of the resultant products were characterized by means of X-ray powder diffraction (XRD), scanning electron microscope (SEM), and room temperature photoluminescence (RTPL) spectra. The RTPL spectra of the resultant products all showed a much stronger ultraviolet bandgap emission peaking at around 387 nm and a weaker emission associated with the defect level. The as-synthesized ZnO crystallites are promising materials for the optoelectronic devices due to their excellent UV emission properties.     

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.     

Influence of seed layers on the vertical growth of ZnO nanowires

We synthesized vertically aligned ZnO nanowires on SiO₂ wafer <100> using the Au, ZnO and Au/ZnO seed layers through the physical vapor deposition process. The growth direction of ZnO nanowire was controlled by using the three different seed layers. From the XRD results, we observed the highest intensity of the (002) peak on the Au/ZnO seed layer among the three seed layers. The SEM images show that all of the ZnO nanowires have an average diameter of about 100 ~ 200 nm and a length of about 5 μm, and the nanowires grown on the Au/ZnO seed layer are oriented the most perpendicularly to the substrate surface. From the PL analysis, we observed that the intensity of broad emissions at 400-600 nm relating the green emission for the ZnO nanowires on the Au/ZnO seed layer was much weaker than that for the ZnO nanowires on the ZnO seed layer. The experiment results indicate that the selection of seed layers is important to grow nanowires vertically for the application of nanoscale devices.     

Structures and properties of the Al-doped ZnO thin films prepared by radio frequency magnetron sputtering

Al-doped ZnO thin films were deposited by radio frequency magnetron sputtering using a ZnO target with 2 wt.% Al₂O₃. The structures and properties of the films were characterized by the thin film X-ray diffraction, high resolution transmission electron microscopy, Hall system and ultraviolet/visible/near-infrared spectrophotometer. The Al-doped ZnO film with high crystalline quality and good properties was obtained at the sputtering power of 100 W, working pressure of 0.3 Pa and substrate temperature of 250 °C. The results of further structure analysis show that the interplanar spacings d are enlarged in other directions besides the direction perpendicular to the substrate. Apart from the film stress, the doping concentration and the doping site of Al play an important role in the variation of lattice parameters. When the doping concentration of Al is more than 1.5 wt.%, part of Al atoms are incorporated in the interstitial site, which leads to the increase of lattice parameters. This viewpoint is also proved by the first principle calculations.     

Single step synthesis of nanocrystalline ZnO via wet-milling

In this work, we describe the effect of milling speed on the formation, crystallite size, and lattice parameter of nanocrystalline ZnO in a single step process that is based on wet-milling of metallic Zn in distilled water. The samples were characterized by XRD, TEM, and FTIR spectra. The analyses reveal that although the 150 rpm milled sample exhibits imprints of Zn (OH)₂, 200, 250, 300, and 350 rpm milled samples possess the standard hexagonal ZnO wurtzite structure. The crystallite size and lattice parameters of the samples were calculated from the XRD patterns by applying the Maud refinement procedure. According to the results, average crystallite size of the ZnO nanocrystals is in the range of 27.3-31.4 nm depending on the milling speed.     

Highly monodispersed ZnO nanorods: preparation and optical properties

Simple and low cost solution synthesis method was used to synthesise ZnO nanorods. Dodecyl benzene sulfonate was used to control the growth process and monodispersed nanorods with diameters in the range of 25 to 44 nm were obtained. X-ray diffraction, transmission electron microscopy and high resolution transmission electron microscopy measurements demonstrate the structure and morphology of the products. Laser power- and temperature-dependent photoluminescence (PL) experiments confirm the good ultraviolet emission characteristics. Short exciton lifetime feature relevant to thin nanorods was examined by time-resolved PL. Good optical quality and the size characteristics of the obtained products are discussed.     

Control of epitaxial growth orientation in ZnO nanorods on c-plane sapphire substrates

This paper presents a study on low temperature hydrothermal growth of ZnO nanorods (NRs) on pre-seeded (0001) sapphire substrates. Prior to hydrothermal growth of ZnO NRs, epitaxial ZnO seeds were grown by metal-organic chemical vapour deposition under various process conditions. Findings show that the majority of ZnO NRs inclined at a specific angle of about 38° to the direction perpendicular to the substrate surface and exhibited a preferential in-plane alignment, besides other NRs growing vertically from the sapphire surface. X-ray diffraction φ-scan measurements reveal that the ZnO nanorods displayed two distinct epitaxial relationships with sapphire which were (0001)_ZnO//(0001)_sapphire and (0001)_ZnO//(101?4)_sapphire, respectively. Reduced lattice mismatch between ZnO and sapphire is responsible for the inclined ZnO NRs growth. The growth direction of ZnO NRs is remarkably dependent on the growth conditions of ZnO seeds and sapphire substrate pre-treatment. The epitaxial orientations of ZnO seeds grown on the sapphire substrate dominate the subsequent ZnO NRs growth and can be controlled through adjusting growth conditions.     

扩散诱导的锌间隙相关施主缺陷对ZnO纳米棒结构及光电特性的影响

采用射频磁控溅射技术和水浴法在SiO2单晶衬底上生长了Zn纳米颗粒/ZnO纳米棒复合材料(Zn/ZnO)。后期热处理促使Zn/ZnO界面之间发生元素相互扩散,直接向ZnO纳米棒中引入额外锌杂质,从而获得了富锌的ZnO纳米棒材料。借助扫描电子显微镜、X射线衍射仪、霍尔测试仪、分光光度计和拉曼光谱仪研究了富锌ZnO纳米棒的形貌、结构以及光电特性。结果表明,所有ZnO纳米棒均呈整齐的六角纤锌矿结构,相比ZnO纳米棒,富锌纳米棒具有相对较差的结晶质量,较好的导电性,较低的透射率和较窄的禁带宽度。拉曼光谱研究表明,通过扩散法向ZnO纳米棒引入的锌间隙相关施主缺陷,是其拉曼光谱中出现异常的275 cm^-1振动模的来源,也是导致富锌ZnO纳米棒微结构以及光电特性显著变化的主要原因。     

Effect of Si wafer resistivity on the growth of ZnO nanorods

ZnO nanorods were electrochemically synthesized using various resistivities of Si substrates. With the increase of Si wafer resistivity from 0.01-0.02 to 110-150 Ω cm, the average diameter of ZnO nanorods increased, while the density of the nanorods decreased. Initial value of the current density increased with a decrease of the Si resistivity, accelerating nucleation and formation of a ZnO nanorods. The saturated current density was increased with a higher Si wafer resistivity, which may be due to an increased surface area of the ZnO layer exposed to the solution, elevating the surface concentration of electrons.     

Growth of ZnGa2O4 nanowires on a ZnO buffer layer by carbothermal reduction of Ga2O3 powder

The synthesis of pure ZnGa₂O₄ nanowires (NWs) on the ZnO-coated Si substrates could be achieved by carbothermal reduction of Ga₂O₃ powder. The processing parameters such as the weight of Ga₂O₃ powder, the thickness of ZnO buffer layer, and the substrate temperature were explored. The growth of ZnGa₂O₄ NWs followed the vapor-solid process. Surplus ZnO source favored the growth of ZnGa₂O₄ NWs, while higher substrate temperature promoted the growth of Ga₂O₃ nanobelts. The results indicated a window for the growth of abundant and pure ZnGa₂O₄ NWs. The growth mechanism of ZnGa₂O₄ NWs on the ZnO buffer layer via carbothermal reduction of Ga₂O₃ powder was discussed. The ZnGa₂O₄ NWs showed a photoluminescence band centered at 466-475 nm.     

Fabrication of ZnO nanorods using metal nanoparticles as growth nuclei

ZnO nanorods were produced by pulsed laser deposition (PLD). Drops of nanoparticle colloid (gold or silver) were placed on silica substrates to form growth nuclei. All nanoparticles were monocrystalline, with well-defined crystal surfaces and a negative electrical charge. The ZnO nanorods were produced in an off-axis PLD configuration at oxygen pressure of 5 Pa. The growth of the nanorods started from the nanoparticles in different directions, as one nanoparticle could become a nucleus for more than one nanorod. The low substrate temperature used indicates the absence of a catalyst during the growth of the nanorods. The diameters of the fabricated 1-D ZnO nanostructures were in the range of 50-120 nm and their length was determined by the deposition time.     

Uniform ZnO nanorods can be used to improve the response of ZnO gas sensor

Uniform ZnO nanorods were synthesized in high-yield by using metal zinc powder as zinc source via a one-step facile hydrothermal process under mild conditions, in which cetyltrimethylammonium bromide (CTAB) with ordered chain structures acted as the conversion of Zn powder into ZnO nanorods. The characterization results show that the as-synthesized products were structurally uniform and have diameters of 40–80 nm. Gas sensing properties studies show that ZnO nanorods exhibit more excellent response and stability to ethanol than that of ZnO nanoparticles. After working continuously for 50 days, the sensitivity of ZnO nanorods still retained 7.3, whereas, the ZnO nanoparticles showed only 1.0. The facile preparation method and the improved properties derived from typical rods-like nanostructure are significant for the future applications of gas sensing material.     

The effect of seed layer thickness on alignment and morphology of ZnO nanorods

Thin films of ZnO of 20, 40,160 and 320 nm thickness were deposited on Si (100) substrates by rf-magnetron sputtering and then nanorods were grown on the seed layer at 95 °C for 2 h. The ZnO nanorods were synthesized in C₆H₁₂N₄ and Zn (NO₃)₂·6H₂O solution by a hydrothermal method and the effect of seed layer thickness on the alignment, diameter, density and growth rate of nanorods was studied.The results revealed that the alignment of nanorods depended on crystallinity, grain size and roughness frequency of the sputtered seed layer, so that, with increase of seed layer thickness, crystallinity improved. In addition the grain size increased and the roughness frequency decreased and hence alignment and diameter of nanorods increased.Finally, we present a model for the effect of seed layer thickness on the alignment and diameter of the nanorods.