PEG-20000 assisted hydrothermal synthesis of hierarchical ZnO flowers: Structure,growth and gas sensor properties

The hierarchically structured materials usually show improved physical and chemical properties. Here, using simple hydrothermal method and reagents of polyethylene glycol (PEG-20000), we synthesize the ZnO crystals and find that they exhibit hierarchical flower-like architectures assembled by nanorods. Further comparative studies demonstrate that PEG provides nucleation sites for the assembling of the nanorods, which play a critical role in producing such unique flower-like architectures. Consequently, the sensor made of ZnO nanoflowers is found to show good gas sensing performance to the hydrothion gas.     

Instability controlled synthesis of tin oxide nanofibers and their gas sensing properties

Instability dependent electrospinning process has been controlled to obtain tin oxide nanofibers with morphological variation. The effect of spinning parameters such as viscosity, conductivity, flow rate, distance and applied voltage on growth rate of different instabilities was simulated and different deposition conditions were defined from the simulation results. The structural morphology was analyzed using X-Ray Diffraction (XRD) and Scanning Electron microscope (SEM). The sensing behavior of different structures was investigated. The branched structure obtained due to axisymmetric instabilities exhibited best sensing performance owing to high surface to volume ratio.     

Low operating temperature of oxygen gas sensor based on undoped and Cr-doped ZnO films

Undoped and doped ZnO with 1 at.% (atomic percentage) chromium (Cr) was synthesized by RF reactive co-sputtering for oxygen gas sensing applications. The prepared films showed a highly c-oriented phase with a dominant (0 0 2) peak at a Bragg angle of around 34.2°. The operating temperature of the prepared ZnO sensor was around 350 °C and shifted to around 250 °C for the doped ZnO sensor which is lower than that of previously reported work. The sensitivity of the sensor toward oxygen gas was enhanced by doping ZnO with 1 at.% Cr. Good stability and repeatability of the sensor were demonstrated when tested under different concentration of oxygen atmosphere.     

The gas response enhancement from ZnO film for H2 gas detection

ZnO thin films were prepared by thermal oxidation of Zn metal at 400 °C for 30 and 60 min. The XRD results showed that the Zn metal was completely converted to ZnO with a polycrystalline structure. The sensors had a maximum response to H₂ at 400 °C and showed stable behavior for detecting H₂ gases in the range of 40 to 160 ppm. The film oxidized for 60 min in oxygen flow exhibited higher response than that of the 30 min oxidation which was approximately 4000 for 160 ppm H₂ gas concentration. The sensing mechanism was modeled according to the oxygen-vacancy model.     

Ultrasound-assisted synthesis of ZnO photocatalysts for gas phase pollutant remediation: Role of the synthetic parameters and of promotion with WO3

The synthesis of ZnO photocatalysts by ultrasound-assisted technique was here investigated. Several experimental parameters including the zinc precursor (acetate, chloride, nitrate), sonication conditions (amplitude, pulse) and post-synthetic thermal treatment (up to 500 °C) were studied. Crystalline ZnO samples were obtained without thermal treatments due to the adopted reactant ratios and synthesis temperature. Sonication plays a major role on the morphological oxide features in terms of particle size and surface area, the latter showing a 20-fold increase with respect to conventional synthesis. Interestingly, 1 and 3 s sonication pulses led to morphological properties similar to continuous sonication. A thermal treatment at moderate temperatures (400–450 °C) promoted the loss of surface hydroxylation and the formation of lattice defects, while higher temperatures were detrimental for the sample morphology. The prepared ZnO was decorated with WO₃ particles comparing an ultrasound-assisted technique using 1 s pulses with a conventional approach, giving rise to composites with promoted visible light absorption. Samples were tested towards the photocatalytic degradation of nitrogen oxides (500–1000 ppb) in humidified air under both UV and visible light. By carefully controlling the synthetic procedure, better performance were observed with respect to the commercial benchmark. Samples from ultrasound-assisted syntheses, also in the case of pulsed sonication, showed consistently better results than conventional references, in particular for ZnO-WO₃ composites. The composite by ultrasound-assisted synthesis showed > 95% degradation in 180 min and doubled NO_x degradation under visible light with respect to the conventional composite.     

Structural and optical properties of ZnO nanopowder prepared by microwave-assisted synthesis

We report the characterization of nano-size zinc oxide (ZnO) powder synthesized via microwave-assisted heating of Zn(CH₃COO)₂·2H₂O and NaHCO₃ solution with deionized water (DI water) as the solvent. The as-synthesized ZnO powder was calcined at temperatures from 400 to 800 °C for 8 h. The X-ray diffraction (XRD) and Fourier transform infrared spectroscopy (FTIR) spectra revealed pure wurtzite structure for the ZnO nanopowder (NP) calcined at 800 °C. Scanning electron microscopy (SEM) images showed increasing size ZnO NP with uniform size distribution with increase in calcination temperature. Significant UV emission at about 373 nm has been observed in the photoluminescence (PL) spectra of the as-synthesized and calcined ZnO NP. Our results showed enhanced PL intensity with a reduced full-width at half-maximum (FWHM) for ZnO NP synthesized at higher calcination temperature.     

Thermal annealing of ZnO substrates

Zinc oxide crystals were grown by Chemical Vapor Transport using Contactless Crystal Growth technique. After X-ray examination the 8 off-axis oriented slices were polished using alumina powder followed by a mixture of oxides. The mechanical polishing was followed by chemo-mechanical polishing using colloidal silica in water and supplemented by thermal annealing in an air atmosphere. The range of temperature was between 770 and 1070 C, time of annealing ranged up to 160 h. The quality of surfaces was studied using atomic force microscopy. A wide spectrum of surface morphology was observed. The morphology was dependent on the annealing conditions and additionally on the quality of chemo-mechanical polishing and crystallographic orientation of the surfaces. It was found possible to obtain the lowest RMS surface roughness factor in extremely different annealing conditions. The best annealing procedure for surface improvement was investigated.     

Solvothermal synthesis, characterization and optical properties of ZnO, ZnO-MgO and ZnO-NiO, mixed oxide nanoparticles

ZnO-MgO and ZnO-NiO mixed oxides nanoparticles were produced from a solution containing Zinc acetate, Mg and Ni nitrate by Solvothermal method. The calcination process of the ZnO-MgO and ZnO-NiO composites nanoparticles brought forth polycrystalline two-phase ZnO-MgO and ZnO-NiO nanoparticles of 40-80 nm in diameters. ZnO, MgO and NiO were crystallized into würtzite and rock salt structures, respectively. The optical properties of ZnO-MgO and ZnO-NiO nanoparticles were obtained by solid state UV and solid state florescent. The XRD, SEM and Raman spectroscopies of these nanoparticles were analyzed.     

The characteristic of photoelectric gas sensing to oxygen and water based on ZnO nanoribbons at room temperature

In this article, ZnO nanoribbons were prepared by multi-components precursor self-assembled method, and the characteristic of photoelectric gas sensing of the nanoribbons was investigated on exposure of oxygen and water environments by surface photocurrent technique. The results showed that surface photocurrent intensity (SPI) was enhanced after oxygen and water adsorption. This increase of SPI was attributed mostly to the improvement of carriers-generating efficiency. And the mechanism was discussed in detail.     

ZnO nanoparticles with controlled shapes and sizes prepared using a simple polyol synthesis

Nanocrystalline zinc oxide (ZnO) particles with controlled shapes and sizes were prepared at 180 ^°C by a simple polyol method. The amount of water and the method of addition played an important role in determining the characteristics of the synthesized particles. Rod-shaped ZnO particles with major axis lengths of ∼114 nm were obtained by heating the precursor solution, while equiaxial particles with average diameters of ∼24 nm were prepared by injecting water into hot precursor solution. Increasing the amount of water added to the precursor solution enlarged the aspect ratio of the rod-shaped particles and increased the particle size of the equiaxial particles due to enhanced hydrolysis and condensation of the Zn ion complex.     

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.     

Catalyst-free synthesis and luminescence of aligned ZnO nanorods

Quasi-aligned undoped ZnO nanorods with diameter in the range 100–300 nm and length of several micrometers have been grown catalyst-free on Si(1 0 0) wafer in a one-step process by direct heating of Zn powders. All nanowires are single crystals and are aligned vertically to the substrate surface with c-axis preferred orientation. XRD, HRTEM and Raman studies revealed that the ZnO nanorods have wurtzite phase, are highly crystalline and well aligned with the lattice parameters a=0.32 nm and c=0.52 nm. The PL spectra measured at different temperatures are dominated by excitonic emission at 380 nm and less intense below band gap emission band centered at 520 nm.     

Chemical synthesis of zinc oxide nanorods for enhanced hydrogen gas sensing

Zinc oxide(ZnO) nanorods are prepared using equimolar solution of zinc nitrate((Zn(NO3)2) and hexamethylenetetramine(C6H12N4) by the hydrothermal technique at 80 C for 12 h. Epitaxial growth is explored by X-ray diffraction(XRD) patterns, revealing that the ZnO nanorods have a hexagonal(wurtzite) structure. Absorption spectra of ZnO are measured by UV–visible spectrometer. The surface morphology is investigated by field emission scanning electron microscopy(FESEM). The synthesized ZnO nanorods are used for detecting the 150 C hydrogen gas with a concentration over 1000 ppm. The obtained results show a reversible response. The influence of operating temperature on hydrogen gas detecting characteristic of ZnO nanorods is also investigated.     

Synthesis and field emission of patterned ZnO nanorods

A simple and self-catalytic method has been developed for synthesizing finely patterned ZnO nanorods on ITO-glass substrates under a low temperature of 500 °C. The patterned ZnO nanorod arrays, a unit area is of 400 × 100 μm², are synthesized via vapor phase transport method. The surface morphology and composition of the as-synthesized ZnO nanorods are characterized by means of scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDX). The mechanism of formation of ZnO nanorods is also discussed. The measurement of field emission (FE) reveals that the as-synthesized ZnO nanorods arrays have a turn-on field of 3.3 V/μm at the current density of 0.1 μA/cm² and a low threshold field of 6.2 V/μm at the current density of 1 mA/cm². So this approach must have a potential application of fabricating micropatterned oxide thin films used in FE-based flat panel displays.     

Hierarchical and Complex ZnO Nanostructures by Microwave-Assisted Synthesis: Morphologies,Growth Mechanism and Classification

Zinc Oxide is an important and multi-purpose material in various industries due to its particular chemical and physical properties. Discovering a cheap, fast, clean, safe, and easy to use method, to synthesize this oxide nanoparticle has attracted a lot of attention in recent applications. The unique properties of the microwave and its special heating capabilities have yielded desirable outcomes by combining different synthesis methods. In the recent years, the vast majority of studies focus on the microwave-assisted synthesis of zinc oxide nanoparticles. This review article attempts to go over the recent advancements on the synthesis of zinc oxide nanoparticles with the aid of microwave, different morphologies and applications obtained by this method. Various microwave-assisted synthesis methods are classified, including the solution-based methods such as hydrothermal, sol-gel, and combustion methods. Morphology of the nanoparticles can affect the properties, and subsequently, applications of these nanoparticles. On the other hand, there is great diversity of morphological and synthesis conditions of zinc oxide nanoparticles. Thus, categorizing the synthesis techniques and providing features of them, facilitates the selection of appropriate method for designing new hierarchical structures with potential properties for future applications. Also it is endeavored to focus on the formation mechanisms of these methods. Finally, the various morphologies obtained under microwave radiation and their formation mechanisms are discussed and the effective factors in the synthesis are analyzed and presented. The potential and suitable fields of development and progress in the future studies are also proposed.     

Solvothermal synthesis and characterization of sandwich-like graphene/ZnO nanocomposites

Graphene-based nanocomposites are emerging as a new class of materials that hold promise for many applications. In this paper, we present a general approach for the preparation of sandwich-like graphene/ZnO nanocomposites in ethylene glycol (EG) medium using graphene oxide as a precursor of graphene and zinc acetylacetonate as a single-source precursor of zinc oxide. The samples were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), Fourier transform infrared (FT-IR) spectroscopy, ultraviolet-visible (UV-vis) spectroscopy and thermogravimetry analysis. It was shown that the as-formed ZnO nanoparticles with a diameter of about 5 nm were densely and uniformly deposited on both surfaces of the graphene sheets to form a sandwich-like composite structure and as a result, the restacking of the as-reduced graphene sheets was effectively prevented. The ZnO-coated graphene nanocomposites can be expected to effectively improve the photocatalysis and sensing properties of ZnO and would be promising for practical applications in future nanotechnology.     

Room temperature soft chemical route for nanofibrous wurtzite ZnO thin film synthesis

Room temperature soft chemical deposition route has been utilized to grow thin films of ZnO on glass substrate. Annealing at 673 K removed zinc hydroxide phase and nanofibrous ZnO films with wurtzite crystal structure were obtained. Decrease in the room temperature electrical resistivity from 10⁷ to 10⁴ Ω cm was observed after annealing. The nanofibrous ZnO thin films were sensitive to the explosive liquefied petroleum gas (LPG) and the maximum response of 17% at 698 K under the exposure of 6500 ppm of LPG was obtained.     

Sn掺杂ZnO薄膜的室温气敏性能及其气敏机理

采用化学气相沉积方法在预制好电极的玻璃基底上制备出Sn掺杂ZnO薄膜和纯ZnO薄膜. 两种样品典型的形貌为四足状ZnO晶须, 其直径约为150-400 nm, 呈疏松状结构. 气敏测试结果显示Sn掺杂ZnO薄膜具有优良的室温气敏性, 并对乙醇具有良好的气敏选择性, 而纯ZnO薄膜在室温条件下对乙醇和丙酮均没有气敏响应. X射线衍射结果表明两种样品均为六方纤锌矿结构. Sn掺杂ZnO样品中没有出现Sn及其氧化物的衍射峰, 其衍射结果与纯ZnO样品对比, 衍射峰向小角度偏移. 光致发光结果表明, Sn掺杂ZnO薄膜与纯ZnO薄膜均出现紫外发光峰和缺陷发光峰, 但是Sn的掺杂使得ZnO的缺陷发光峰明显增强. 将Sn掺杂ZnO样品在空气中退火后, 其室温气敏性消失, 说明Sn掺杂ZnO样品的室温气敏性可能与其缺陷含量高有关. 采用自由电子散射模型解释了Sn掺杂ZnO薄膜的室温气敏机理.     

Facile,high yield ultrasound mediated protocol for ZnO hierarchical structures synthesis: Formation mechanism,optical and photocatalytic properties

Hierarchical flowers-like zinc oxide structures have been successfully obtained by a simple and fast ultrasound-assisted method performed in a ordinary ultrasonic bath using an ammonia solution and zinc acetate, in the absence of any surfactant or template. The composition, structure, crystallinity, morphology and optical properties of the materials obtained at different ultrasound irradiation times were characterized by infrared, UV–Vis and photoluminescence spectroscopy, X-ray diffraction, scanning and transmission electron microscopy investigations. It was proved that the ultrasound irradiation time manipulates both the defect content (implicit the photoluminescent properties) and morphology of the ZnO materials: shorter irradiation times leads to the synthesis of high-defected ZnO structures of flower morphology with triangular-shaped petals, while higher irradiation times favours the formation of low-defected ZnO structures with tipped rod-like petals. A plausible growth mechanism of the architectures that implies aggregation via oriented attachment followed by an Ostwald ripening is advanced based on these results. The ZnO flower-like structures present high photocatalytic activities, a total phenol mineralization being registered in the case of visible light experiments. Electron-spin resonance measurements demonstrate the generation of reactive oxygen species, namely hydroxyl radicals but also C centred radicals adducts derived most probable from the residual acetate adsorbed on ZnO surface.