Pure ZnO Nanostructures Research Articles

Enhanced toluene sensing characteristics of TiO 2-doped flowerlike ZnO nanostructures

The large-scale flowerlike ZnO nanostructures, consisting of many aggregative nanorods with the diameter of about 60 nm, were prepared by a simple and facile hydrothermal route. The toluene sensing properties of the pure ZnO and TiO 2-doped ZnO nanostructures were tested at different operating temperatures from 160 to 390 °C and toluene concentrations ranging from 1 to 3000 ppm. The results indicate that the response of the ZnO nanostructures towards toluene was enhanced significantly by TiO 2 doping, which was deposited onto the ZnO products by evaporation. It is found that the TiO 2-doped ZnO sensor exhibits remarkably enhanced response of 17.1 at the optimal operating temperature of 290 °C to 100 ppm toluene, which has been improved up from 7.4 at 390 °C from pure ZnO nanostructures.

Comparative study of ethanol sensor based on gold nanoparticles: ZnO nanostructure and gold: ZnO nanostructure

Gold colloid:ZnO nanostructures were prepared from Zn powder by using thermal oxidation technique on alumina substrates, then it was impregnated by gold colloid for comparative study. The gold colloid is the solution prepared by chemical reduction technique; it appeared red color for gold nanoparticle solution and yellow color for gold solution. The heating temperature and sintering time of thermal oxidation were 700 °C and 24 h, respectively under oxygen atmosphere. The structural characteristics of gold colloid:ZnO nanostructures and pure ZnO nanostructures were studied using filed emission scanning electron microscope (FE-SEM). From FE-SEM images, the diameter and length of gold colloid:ZnO nanostructures and ZnO nanostructures were in the ranges of 100–500 nm and 2.0–7.0 μm, respectively. The ethanol sensing characteristics of gold colloid:ZnO nanostructures and ZnO nanostructures were observed from the resistance alteration under ethanol vapor atmosphere at concentrations of 50, 100, 200, 500, and 1000 ppm with the operating temperature of 260–360 °C. It was found that the sensitivity of sensor depends on the operating temperature and ethanol vapor concentrations. The sensitivity of gold colloid:ZnO nanostructures were improved with comparative pure ZnO nanostructures, while the optimum operating temperature was 300 °C. The mechanism analysis of sensor revealed that the oxygen species on the surface was O 2−.

Synthesis of Hierarchical Pure ZnO Nanostructures with Controllable Morphology

Hierarchical pure ZnO nanostructures with controllable morphology were synthesized by two-step oxygen-controlled thermal evaporation method without any catalyst. Zn nanowires with little oxidation were deposited firstly on Si substrates located at the low temperature area at step one. The morphology of hierarchical ZnO nanostructures can be effectively modulated at step two by oxygen flow rate, taking advantage of the controllable oxidation rate of Zn and concentration of ZnOx under various oxygen supplies. Transmission electron microscope images and corresponding selected area electron diffraction patterns suggest that low oxygen supplementation causes the epitaxial growth of nanorods from the surface of the Zn nanowire, while high oxygen flow rate will lead to the preferential growth of nanorods along the [001] orientation. Room-temperature photoluminescence measurements demonstrate that the variety of nanostructures can cause the variation of the ultraviolet to green peak intensity ratio, suggesting that the controlled morphologies can be used to modulate the optical properties of ZnO nanostructures.

ZnO:Al nanostructures synthesized on pre-deposited aluminum (Al)/Si template: Formation, photoluminescence and electron field emission

Comb-like aluminum (Al) doped ZnO (ZnO:Al) nanostructures were synthesized on Al/silicon substrate by thermal evaporation at 650 °C. A pre-deposited Al layer on the Si substrate was employed to provide the Al dopant into ZnO nanostructures. High-resolution transmission electron microscopy (HRTEM) images displayed that ZnO:Al nanostructures were grown along the [0001] axis. Energy dispersive X-ray spectroscopy (EDS) mapping showed that the Al element was highly scattered and dispersed throughout the ZnO nanostructures. Photoluminescence (PL) spectrum revealed that the ZnO:Al and pure ZnO nanostructures have a blue band emission at 382 nm and 385 nm, respectively. The Al doping did increase the concentration of the oxygen vacancies, therefore a high intensity of green emission band was obtained in ZnO:Al nanostructures as compared to that of pure ZnO nanostructures. The field emission properties of ZnO:Al and ZnO nanostructures were also investigated in this work. The turn-on field of ZnO:Al and pure ZnO nanostructures were found to be 3.8 and 5 V/μm, respectively; the current density was 1 µA/cm 2. The thresholds field for ZnO:Al and ZnO nanostructures were estimated around 25 and 47 V/μm, respectively; the current density was 1 mA/cm 2.

Investigation of pure and Co2+-doped ZnO quantum dot electronic structures using the density functional theory: choosing the right functional

The electronic structures of pure and Co2+-doped ZnO quantum dots (QDs) with sizes up to 300 atoms were investigated using three different density functional theory approximations: local spin density approximation (LSDA), gradient-corrected Perdew–Burke–Ernzerhof (PBE) and the hybrid PBE1 functionals with LANL2DZ pseudo-potential and associated basis set. Qualitative agreement among the three methods is found for the pure ZnO nanostructures, but only the hybrid functional reproduces the correct bandgap energies quantitatively. For Co2+-doped ZnO QDs, both LSDA and PBE incorrectly model interactions between Co2+ d levels and the valence band of ZnO, which will strongly impair predictions of dopant–carrier magnetic exchange interactions based on such computational results. Experimental observations are reproduced well in calculations at the hybrid PBE1 level of theory, making this the method of choice for exploring the magnetism of transition metal ions in ZnO QDs computationally. The qualitative features of the Co2+ 3d levels do not change appreciably with changes in cluster size over the range examined, leading to size-dependent dopant-band edge energy differences. The results presented here thus provide an experimentally calibrated framework for future ab initio descriptions of dopant–carrier and dopant–dopant magnetic exchange interactions in diluted magnetic semiconductors (DMS) nanocrystals.

Luminescent Properties of La3PO7:Eu3+ Nanoparticles

Hierarchical pure ZnO nanostructures with controllable morphology were synthesized by two-step oxygen-controlled thermal evaporation method without any catalyst. Zn nanowires with little oxidation were deposited firstly on Si substrates located at the low temperature area at step one. The morphology of hierarchical ZnO nanostructures can be effectively modulated at step two by oxygen flow rate, taking advantage of the controllable oxidation rate of Zn and concentration of ZnO(x) under various oxygen supplies. Transmission electron microscope images and corresponding selected area electron diffraction patterns suggest that low oxygen supplementation causes the epitaxial growth of nanorods from the surface of the Zn nanowire, while high oxygen flow rate will lead to the preferential growth of nanorods along the [001] orientation. Room-temperature photoluminescence measurements demonstrate that the variety of nanostructures can cause the variation of the ultraviolet to green peak intensity ratio, suggesting that the controlled morphologies can be used to modulate the optical properties of ZnO nanostructures.

Synthesis and optical properties of hierarchical pure ZnO nanostructures

We report the catalyst-free synthesis of hierarchical pure ZnO nanostructures with 6-fold structural symmetry by two-step thermal evaporation process. At the first step, the hexagonal-shaped nanowires consisting of a great deal of Zn and little oxide were prepared via the layer-by-layer growth mechanism; and at the second step, hierarchical pure ZnO nanostructures were synthesized by evaporating the Zn source on the basis of the step-one made substrate. Scanning electron microscopy, transmission electron microscope images, and the corresponding selected area electron diffraction pattern have been utilized to reveal the screw dislocation growth mechanism, through which the single crystal ZnO nanorods are epitaxially grown from the side-wall of central axial nanowires. Raman and photoluminescence spectra further indicate that, for the hierarchical ZnO nanostructures, the ultraviolet peak is related to the free exciton recombination, while the oxygen vacancies and high surface-to-volume ratio are responsible for the strong green peak emission.

Microwave absorption properties and mechanism of cagelike ZnO∕SiO2 nanocomposites

In this paper, cagelike ZnO∕SiO2 nanocomposites were prepared and their microwave absorption properties were investigated in detail. Dielectric constants and losses of the pure cagelike ZnO nanostructures were measured in a frequency range of 8.2–12.4GHz. The measured results indicate that the cagelike ZnO nanostructures are low-loss material for microwave absorption in X band. However, the cagelike ZnO∕SiO2 nanocomposites exhibit a relatively strong attenuation to microwave in X band. Such strong absorption is related to the unique geometrical morphology of the cagelike ZnO nanostructures in the composites. The microcurrent network can be produced in the cagelike ZnO nanostructures, which contributes to the conductive loss.