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.     

Effect of different pH values on growth solutions for the ZnO nanostructures

In this study, high quality zinc oxide (ZnO) nanostructures were synthesized on glass slide substrates using modified chemical bath deposition (M-CBD) method at low temperature. Through the M-CBD technique the air bubbles will be injected into aqueous growth solution. The RF magnetron sputtering method was utlized to grow ZnO seed layer on the glass substrates. The effect of different pH values of aqueous growth solution on the morphology, elemental chemical composition, crystal structural and the optical properties of ZnO nanostructures have been investigated using field emission-scanning electron microscopy (FE-SEM), Energy dispersive analysis (EDX), X-ray diffraction (XRD), and UV-Visible Spectrometer, respectively. It was observed that altering pH values from acidic to alkaline (basic) by using ammonia solution (NH₃) induced the significant change in morphology from nanorods like ZnO to nano-amber flush rose like ZnO structures. Furthermore, increased pH values had an effect on the influence intensity of the preferred orientation plane (002) and average transmittance spectrum. Whilst the absorption band edge has been shifted to a lower energy region due to the quantum size effect. It was also found that the crystal size fluctuated between 36.30 nm and 84.33 nm with a different values of pH from 6.7 to 12. The ZnO synthesized at 6.7 of pH provided the best results regarding the high aspect ratio,structural and optical properties. At this pH value, ZnO growth revealed the nanorod structure with small diameters, size and a higher energy band gap value.     

Effect of synthesis route on the morphologies of silver nanostructures by galvanic displacement reaction

The present study is concerned with the preparation of Ag nanostructures by reduction of AgNO₃ with zinc foil by galvanic displacement reaction. The results confirm that the synthesis route has a direct influence on the morphologies of Ag nanostructures. In addition, the effect of synthesis conditions, including the concentration of AgNO₃ aqueous solutions and reaction time, are investigated. X-ray diffraction (XRD), filed emission scanning electron microscope (SEM) and UV-vis spectra are used to characterize the obtained products. A reasonable formation process of Ag nanostructures is proposed based on the characterization results.     

微波加热技术的应用与研究进展

文章简述了微波加热的发展概况，阐述了微波加热的介电损耗机理和微波加热的特性．从微波加热与解冻、微波干燥、微波改性、微波烧结、微波杀菌等方面，介绍了微波加热技术在国内的研究与应用情况，指出微波加热技术具有广阔的发展前景，今后应重点加强微波与物料问相互作用理论、微波场中物料的传热和传质机制、微波加热工艺与设备、微波加热技术和其他技术的有机结合等方面的研究．     

Synthesis of morphology-controlled ZnO microstructures via a microwave-assisted hydrothermal method and their gas-sensing property

Controllable ZnO architectures with flower-like and rod-like morphologies were synthesized via a microwave-assisted hydrothermal method. By adjusting the concentration of Zn²⁺ in the aqueous precursors, different morphologies of ZnO microstructures were obtained. The size of ZnO was uniform after ultrasonic treatment. The growth process of ZnO in solution was studied by monitoring the intermediate products, which were extracted at different stages of the reactions: (i) precursor preparation, (ii) microwave irradiation heating, (iii) natural cooling. Studies of the SEM images and XRD data revealed that the formation of ZnO occurred via in situ assembly or dissolution–reprecipitation of zinc hydroxide complexes. The morphology-dependent ethanol sensing performance was observed; the seven-spine ZnO structures exhibit the highest activity.     

Preparation and Dielectric Properties of Nanostructured ZnO Whiskers

By a novel controlled combustion synthesis method, a large number of nanostructured ZnO whiskers with different morphologies, such as tetra-needles, long-leg tetra-needles and multi-needles, are prepared without any additive in open air at high temperature. The morphologies and crystalline structures of the as-prepared ZnO nanostructured whiskers are investigated by SEM and XRD. The possible growth mechanism on the nanostructured ZnO whiskers is proposed. The experimental results indicate that the dielectric constants and losses of the nanostructured ZnO whiskers are very low, demonstrating that the nanostructured ZnO whiskers are low-loss materials for microwave absorption in X-band. However, obvious microwave absorption in nanostructured ZnO whiskers is observed. The quasi-microantenna model may be attributed to the microwave absorption of the ZnO whiskers.     

Dielectric properties of aqueous solutions of α- -glucose at 915 MHz

Using a cavity perturbation technique, dielectric properties of aqueous solutions of α- -glucose at 915 MHz were investigated at concentrations varying from 10 to 70% (w/w) and temperatures ranging from 25–85°C. The dielectric constant increased with temperature but decreased with concentration, whereas the loss factor did the inverse. Dielectric properties for higher concentration glucose solutions showed the greatest variation at higher temperatures. Predictive models of the dielectric properties as functions of concentration and temperature were developed by stepwise regression. Such models are useful in estimating the volumetric heating of these solutions by microwave energy, studying the dielectric behavior of the glucose solutions, and chemical reactions involving glucose in aqueous solutions in a microwave field.     

Effects of morphology on photocatalytic performance of Zinc oxide nanostructures synthesized by rapid microwave irradiation methods

In this study, two different chemical solution methods were used to synthesize Zinc oxide nanostructures via a simple and fast microwave assisted method. Afterwards, the photocatalytic performances of the produced ZnO powders were investigated using methylene blue (MB) photodegradation with UV lamp irradiation. The obtained ZnO nanostructures showed spherical and flower-like morphologies. The average crystallite size of the flower-like and spherical nanostructures were determined to be about 55 nm and 28 nm, respectively. X-ray diffraction (XRD), scanning electronic microscopy (SEM), Brunauer–Emmett–Teller (BET), room temperature photoluminescence (RT-PL) and UV–vis analysis were used for characterization of the synthesized ZnO powders. Using BET N₂-adsorption technique, the specific surface area of the flower-like and spherical ZnO nanostructures were found to be 22.9 m²/gr and 98 m²/gr, respectively. Both morphologies show similar band gap values. Finally, our results depict that the efficiency of photocatalytic performance in the Zinc oxide nanostructures with spherical morphology is greater than that found in the flower-like Zinc oxide nanostructures as well as bulk ZnO.     

Fabrication of ZnO nanostructures of various dimensions using patterned substrates

ZnO nanostructures were grown on silicon, porous silicon, ZnO/Si and AlN/Si substrates by low-temperature aqueous synthesis method. The shape of nanostructures greatly depends on the underlying surface. Scattered ZnO nanorods were observed on silicon substrate, whereas aligned ZnO nanowires were obtained by introducing sputtered ZnO film as a seed layer. Furthermore, both the combination of nanorods and the bunch of nanowires were found on porous silicon substrates, whereas platelet-like morphology was observed on AlN/Si substrates. XRD patterns suggest the crystalline nature of aqueous-grown ZnO nanostructures and high-resolution transmission electron microscopy images confirm the single-crystalline growth of the ZnO nanorods along [0 0 1] direction. Room-temperature photoluminescence characterization clearly shows a band-edge luminescence along with a visible luminescence in the yellow spectral range.     

Enhancement of the light extraction of GaN-based green light emitting diodes via nanohybrid structures

Improvement in the light extraction efficiency (LEE) of GaN-based green light emitting diodes (LEDs) with ZnO nanostructures synthesized by a hydrothermal method is reported. Formation of ZnO nanorods, hemispheres, and cones was controlled by varying the pH of the aqueous synthesis solution. The shape of the ZnO nanostructures integrated onto the LEDs shows a strong relationship with the LEE characteristics of GaN-based green LEDs. The electroluminescence (EL) intensity of LEDs covered by ZnO nanostructures increased compared to conventional LEDs. In terms of LEE, LEDs with surface-textured ZnO hemispheres showed the highest EL intensity, which can be attributed to an increase in the effective critical angle, the escape cone, and multiple scattering. Finite difference time domain (FDTD) simulation was conducted to theoretically confirm the experimental results.     

Green synthesis of ZnO nanoparticles in a room-temperature ionic liquid 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide

Nanoparticles of ZnO with the wurtzite structure have been successfully synthesized via a microwave through the decomposition of zinc acetate dihydrate in an ionic liquid, 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide, as a solvent. Fundamental characterizations including X-ray diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM) were conducted for the ZnO nanostructures.To explore the growth mechanism, the samples have been prepared in different irradiation time and also cetyltrimethylammonium bromide (CTAB) has been used as the capping reagent.     

X-ray analysis of ZnO nanorods grown by microwave irradiation heating on ZnO films

Utilizing microwave irradiation heating, 100-nm-diameter ZnO nanorods were grown from aqueous solution on sputtered ZnO films on glass substrates. Its out-of-plane X-ray diffraction (XRD) measurement indicated that the ZnO nanorods were grown with c-axis orientation, similar with the underlying ZnO films. In the in-plane XRD measurement, intensity of the () diffraction was comparable with that of the () one, suggesting their intensity ratio would contain useful information on nanorods density.     

Terahertz time domain spectroscopy of hydrothermally synthesized boehmite and ammonium dawsonite nanostructures

The frequency dependent optical and dielectric properties of boehmite (AlOOH) and ammonium aluminum carbonate hydroxide (AACH) nanostructures, prepared at different temperatures via hydrothermal synthesis, were studied by using terahertz time domain spectroscopy (THz-TDS). The complex refractive indices, absorption coefficients and complex dielectric constants were determined and compared for different synthesis temperatures. X-ray diffraction (XRD) and Fourier transform infrared spectroscopy (FTIR) were also performed to confirm the formation of boehmite and AACH. Scanning electron microscopy (SEM) was performed to study the morphology of nanostructures. The study reveals the formation of AACH at higher temperatures and decrease in refractive index with increasing synthesis temperature. The higher absorption coefficient and lower refractive index and dielectric constant were observed for AACH than for boehmite.     

One-dimensional and quasi-one-dimensional ZnO nanostructures prepared by spray-pyrolysis-assisted thermal evaporation

One-dimensional (1D) and quasi-1D ZnO nanostructures have been fabricated by a kind of new spray-pyrolysis-assisted thermal evaporation method. Pure ZnO powder serves as an evaporation source. Thus-obtained products have been characterized by X-ray diffraction (XRD) analysis, scanning electron microscope (SEM) equipped with energy dispersive X-ray spectroscopy (EDS), transmission electron microscope (TEM). The room temperature photoluminescence spectrum of these ZnO nanostructures is presented. The results show that as-grown ZnO nanomaterials have a hexagonal wurtzite crystalline structure. Besides nanosaws, nanobelts and nanowires, complex ZnO nanotrees have also been observed in synthesized products. The study provides a new simple route to construct 1D and quasi-1D ZnO nanomaterials, which can probably be extended to fabricate other oxide nanomaterials with high melting point and doped oxide nanomaterials.     

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.     

Effect of source temperature on the morphology and photoluminescence properties of ZnO nanostructures

ZnO nanostructures 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 and at atmospheric argon pressure. The influence of the source temperature on the morphology and luminescence properties of ZnO nanostructures has been investigated. ZnO nanowires, nanoflowres and nanotetrapods have been formed upon the Si(1 0 0) substrates at different source temperatures ranging from 1100 to 1200 °C. Room temperature photoluminescence (PL) spectra showed increase green emission intensity as the source temperature was decreased and ZnO nanowires had the strongest intensity of UV emission compared with other nanostructures. In addition, the growth mechanism of the ZnO nanostructures is discussed based on the reaction conditions.     

Synthesis and properties of CdSe Quantum Dot sensitized ZnO nanocomposites

In this work, zinc oxide nanocrystals with an average particle size of 13–22 nm are readily synthesized in aqueous medium by the wet synthesis method. Different sized nanocrystals obtained with change in calcination temperature are characterized by PL photoluminescence (PL) and UV–vis absorption spectroscopies, X-Ray Diffraction (XRD) and Scanning Electron Microscopy (SEM). The average crystal size of the as prepared ZnO nanopowder is determined by XRD and was found to be in good agreement with the UV–vis absorption analysis. The quality of different ZnO nanopowders is confirmed by XRD spectra. On the basis of different characterizations, ZnO calcined for 1 h (due to its large size and less agglomeration) is chosen for synthesis of ZnO–CdSe nanocomposites with variable sized CdSe QD's (Quantum Dots). Nano-composites are synthesized using bifunctional linker molecule Mercaptopropionic Acid (MPA), and by directly adsorbing CdSe QD's over the surface of ZnO nanocrystals. The difference in charge transfer mechanism in ZnO–CdSe nanocomposites due to different crystallite size of CdSe QD's is studied. Higher crystallinity of ZnO–CdSe nanocomposites can be determined from XRD characterization. Size and mode of attachment in various ZnO–CdSe nanocomposites are determined by SEM studies.     

Effect of heat and time-period on the growth of ZnO nanorods by sol–gel technique

Sol–gel routes to metal oxide nanoparticles in organic solvents under exclusion of water have become a versatile alternative to aqueous methods. We focus on the preparation of well-aligned ZnO nanorod arrays using non-aqueous sol–gel synthesis route, where ZnO nanorods arrays have been grown on glass substrates. This work provides a systematic study of controlled morphology and crystallinity of ZnO nanorod arrays. The investigation demonstrates that the synthesis process conditions of ZnO thin film have strong influences on the morphology and crystallinity of the ZnO nanorod arrays grown thereon, where non-aqueous process offers the possibility of better understanding and controlling the reaction pathways on the molecular level, enabling the synthesis of nanomaterials with high crystallinity and well-defined, uniform particle morphologies. Here the annealing temperature plays an important role on the growth of nanostructures of the ZnO grains and nanorod arrays. The scanning electron microscopy (SEM) image shows that the growth of ZnO nanorod arrays are high-quality single crystals growing along the c-axis perpendicular to the substrates. A detailed analysis of the growth characteristics of ZnO nanostructures as functions of growth time is also reported.     

Growth mechanism and photoluminescence property of flower-like ZnO nanostructures synthesized by starch-assisted sonochemical method

Flower-like ZnO nanostructures have been synthesized by starch-assisted sonochemical method and the effect of starch and ultrasound on the formation of ZnO nanostructure has been investigated. It is observed that starch and ultrasonic wave both plays a vital role on the growth of ZnO nanostructure. X-ray diffraction (XRD) pattern indicated that the synthesized flower-like ZnO nanostructures were hexagonal. FTIR spectrum confirms the presence of starch on the surface of flower-like ZnO nanostructure. The photoluminescence spectrum of flower-like ZnO nanostructure consists of band-edge emission at 393 nm as well as emission peaks due to defects. On the basis of structural information provided by X-ray diffraction (XRD) and morphological information by Scanning Electron Microscopy (SEM), a growth mechanism is proposed for formation of flower-like ZnO nanostructures. Differential Scanning Calorimetry (DSC) of starch in liquid medium confirms that gelatinization is a two step process involving two phases.     

Effect of the reaction conditions on the formation of the ZnO nanostructures

ZnO nanorods were synthesized through a simple chemical method by reacting Zn(C₂H₃O₂)₂·2H₂O and NaOH at low temperature and the effects of changing the order of addition of reactants on the morphological evolution of ZnO nanorods were investigated. The samples were characterized by using XRD, SEM, EDX, TEM, BET and Raman techniques. Optical properties of the ZnO nanostructures were too investigated by UV–Vis spectroscopy at room temperature.The hexagonal wurtzite phase of ZnO was confirmed by X-ray diffraction (XRD) for all the samples. SEM and TEM analysis indicated that different morphologies were obtained by changing the order of addition of reactants.