Xiao Qu

Hierarchical flower-like ZnO microstructures: preparation, formation mechanism and application in gas sensor

Three-dimensional (3D) flower-like ZnO hierarchical microstructures with high uniformity were fabricated from the decomposition of Zn(OH)42− precursor, which were synthesized by a facile hydrothermal reaction. XRD, FESEM and TEM were employed to characterize the morphology and structure of the products. The as-prepared flower-like ZnO microstructures with an average diameter of about 2xa0μm are assembled by large amounts of nanosheets, which have a thickness of ~43xa0nm. The morphology of the ZnO architectures can be tailored by changing hydrothermal conditions, e.g., hydrothermal temperature, reaction time, and concentration of Zn2+. On the basis of experimental results, the possible formation mechanism of the flower-like ZnO microstructures was also proposed, and the Zn2+ concentration was found to be a vital role in the growth and crystallzation of the flower-like microstructures. The Brunauer–Emmett–Teller measurement shows that the 3D flower-like ZnO hierarchical microstructures possess a specific surface area of about 34.86xa0m2/g. Most significantly, the gas sensing test reveals that the sensor made from 3D flower-like ZnO hierarchical microstructures exhibits outstanding gas sensing performance.

A controllable hydrothermal fabrication of hierarchical ZnO microstructures and its gas sensing properties

In this paper, rod-flower like ZnO hierarchical microstructures with high uniformity are synthesized from the thermal decomposition of Zn(NH3)42+ precursors, which are prepared via a surfactant-assisted hydrothermal process. The as-synthesized hierarchical ZnO microstructure is assembled from columnar nanorods, and the measured length to diameter ratio of the nanorod is about 20. The morphology of the hierarchical ZnO microstructure can be tailored by varying hydrothermal conditions, e.g., hydrothermal temperature, reaction time, concentration of Zn2+ and zinc salts. Moreover, based on the experimental results, the possible reaction mechanism for the growth of the as-synthesized hierarchical ZnO microstructures is also discussed in detail,and the Zn2+ concentration was found to be a crucial role in the formation and nucleation of the rod flower-like microstructures. In addition, the gas sensing test demonstrates that the sensors based on hierarchical ZnO microstructures exhibits excellent gas sensing properties due to its unique architecture.

Synthesis of SiO2-Coated Core–Shell ZnO Composites for Preparing High-Voltage Varistors

Monodispersed ZnO composite microspheres were successfully prepared by a facile ultrasound irradiation method. Then, the uniform core–shell structured composites were synthesized through the hydrolysis of tetraethyl orthosilicate on the surface of the ZnO composite microspheres. Microstructural studies of the as-obtained powders were carried out using the techniques of the x-ray powder diffraction, field emission scanning electron microscopy and transmission electron microscopy with energy dispersive x-ray spectroscopy. The results show that the pink ZnO composite powders as the core were spherical structures with the size of approximately 100xa0nm, and the SiO2 shell was fully coated on the surface of the core. On the basis of these results, the effect of SiO2 content on the thickness of the synthesized composites and microstructure, as well as the electrical properties of the ZnO varistors sintered in air at 1150°C for 2xa0h, were fully studied. In particular, the ZnO varistor prepared with the appropriate amount of the SiO2 coating (∼40xa0nm) leads to a superior electrical performance with the high breakdown voltage of 418xa0Vxa0mm−1 and an excellent nonlinear coefficient of 70.7, compared with the varistors obtained without the SiO2 coating. The high performance is attributed to the smaller and more homogeneous ZnO grains obtained via the SiO2 coating.

Microstructure and varistor properties of Pr–Co co-doped ZnO ceramics obtained by sol–gel method

Zn0.99−xCo0.01PrxO (xu2009=u20090, 0.005, 0.015 and 0.020) nanoparticles for varistor application have been synthesized by sol–gel method. The phase and microstructure of the as-prepared samples were characterized by X-ray diffraction, transmission electron microscopy, scanning electron microscopy and energy dispersive X-ray spectra. The results reveal that the samples exhibited hexagonal zinc oxide (ZnO) lattice structure without secondary phases and were well crystallized with preferential orientation towards (101) direction. The calculated average crystalline size decreases from 39.9312 to 39.3605xa0nm when Pr3+ content increased from 0 to 2 at.%. The microstructure of Zn0.99−xCo0.01PrxO (xu2009=u20090.02) varistors ceramics consisted of ZnO grain as a primary phase and Pr-rich phase (Pr6O11) and spinel phase (Pr2CoO4) as minor secondary phase. The breakdown field increased over a wide range from 2332 to 7845xa0V/cm with the Pr3+ content increasing. The varistors doped with 1.5 at.% Pr3+ exhibited the highest nonlinear coefficient of 17.9. Further increase caused nonlinear coefficient decreased to 15.4 at 2xa0at.%. The current–voltage (E–J) characteristics curves indicated the Pr–Co co-doped ZnO varistors showed good electrical properties and could be a promising material to replace ZnO–Bi2O3 varistors which is appreciable for the fabrication of voltage switching devices in the near future.

Microstructures and electrical properties of ZnO–V2O5–MnO2 varistors with low-temperature sintering

The microstructures and electrical properties of 95xa0at.%ZnOxa0+xa0xV2O5xa0+xa0(5−x)MnO2 varistors (xxa0=xa01, 2, 3, 4, 5xa0at.%) sintering at 900xa0°C were investigated. The microstructure of all the samples consisted of ZnO grain with β-Zn3(VO4)2 and γ-Zn3(VO4)2 as minority secondary phases. The average grain size decreased from 10.4 to 4.5xa0μm with the decrease of V2O5xa0mol fraction and it is attributed that ZnO grains could probably grow rapidly in the presence of a rich liquid phases related to V2O5 concentrations. The breakdown voltage increases from 6444 to 1446xa0V/cm with increase of V2O5xa0mol fraction due to the decrease of the average grain size. The samples containing 2xa0at.% V2O5 and 3xa0at.% MnO2 varistors exhibited the best nonlinear properties, with present nonlinear coefficient (20.8).