Hierarchical ZnO Microstructures Research Articles

Reduced graphene oxide-ZnO hybrid composites as photocatalysts: The role of nature of the molecular target in catalytic performance

Spurred by controversial literature findings, we enwrapped reduced graphene oxide (rGO) in ZnO hierarchical microstructures (rGO loadings spanning from 0.01 to 2 wt%) using an in situ synthetic procedure. The obtained hybrid composites were carefully characterized, aiming at shining light on the possible role of rGO on the claimed increased performance as photocatalysts. Several characterization tools were exploited to unveil the effect exerted by rGO, including steady state and time resolved photoluminescence, electron microscopies and electrochemical techniques, in order to evaluate the physical, optical and electrical features involved in determining the catalytic degradation of rhodamine B and phenol in water.Several properties of native ZnO structures were found changed upon the rGO enwrapping (including optical absorbance, concentration of native defects in the ZnO matrix and double-layer capacitance), which are all involved in determining the photocatalytic performance of the hybrid composites. The findings discussed in the present work highlight the high complexity of the field of application of graphene-derivatives as supporters of semiconducting metal oxides functionality, which has to be analyzed through a multi-parametric approach.

Hierarchical ZnO microstructures decorated with Au nanoparticles for enhanced gas sensing and photocatalytic properties

In this work, hierarchical ZnO (H-ZnO) and Au decorated hierarchical ZnO (Au-H-ZnO) microstructures have been successfully fabricated for improved gas-sensing and photocatalytic properties. Particularly, the H-ZnO microstructures are synthesized by a low temperature (60 °C) aqueous growth followed by a heat treatment process. The growth mechanism of the ZnO architecture is related to the concentration of sodium hydroxide. Gas sensing measurements reveal that the ZnO microstructures decorated with Au nanoparticles exhibited outstanding sensing properties toward acetone and ethanol, including the fast response and recovery rate, high sensitivity and low optimum working temperature, these test results are much better than that of pure ZnO sensor. Also, the UV–vis and photocatalytic measurements of the as-synthesized Au-H-ZnO microstructures displayed remarkable enhancement of photocatalytic properties. The improvement of the gas sensing and optical properties should be ascribed to the reality that the complex structure supplied a high absorption surface area and sufficient oxygen vacancies, and Au nanoparticles play the role as a catalyst to facilitate the absorption and desorption of gas molecules.

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.

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)4 2− 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 2 μm are assembled by large amounts of nanosheets, which have a thickness of ~43 nm. 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 34.86 m2/g. Most significantly, the gas sensing test reveals that the sensor made from 3D flower-like ZnO hierarchical microstructures exhibits outstanding gas sensing performance.

Facile synthesis of hierarchical ZnO microstructures with enhanced photocatalytic activity

AbstractFlower-like ZnO microstructures were successfully synthesized via a facile hydrothermal route without using any surfactants. The morphology of these microstructures can be easily controlled by adjusting the pH of the reaction solution. The possible growth mechanism of ZnO hierarchical microstructures was proposed based on the X-ray powder diffraction (XRD) and scanning electron microscope (SEM) results. The photocatalytic activity studies of ZnO nanocrystals demonstrated their excellent photocatalytic performance in degrading aqueous methylene blue (MB) under UV-A light irradiation. This higher photocatalytic activity of the ZnO nanoplates was mainly attributed to the exposed facets with the higher surface energy.

High response to H2S gas with facile synthesized hierarchical ZnO microstructures

Three different hierarchical ZnO microstructures with high uniformity were synthesized and developed as H2S gas sensors. High response to H2S gas as low as 50ppb was obtained by those microstructures. Particularly, the sphere-like microstructure exhibited extremely high response at the working temperature (220°C). A possible nucleation-growth mechanism was proposed and the structures were carefully investigated. Our studies implied that the morphologies of micro/nanostructures could strongly affect the sensing performance, and hence, the effective design of gas sensors with semiconducting metal oxides could be realized.

Facile fabrication of hierarchical ZnO microstructures assisted with PAMPSA and enhancement of green emission

We report the fabrication of hierarchically microstructured flower-like ZnO by a facile and single-step procedure involving poly(2-acrylamido-2-methyl-1-propanesulfonic acid) (PAMPSA) assisted aqueous chemical method. The shapes and sizes can be controlled just by varying the concentrations of the water-soluble polymer. When a suitable PAMPAS concentration was utilized, uniform well-defined and mono-dispersed chrysanthemum-like ZnO microstructures based on nanorod building blocks were obtained. The formation mechanism of the hierarchical structure was presented. The structured studies using XRD, HRTEM and SAED reveal these ZnO nanorods are composed of a single phase nature with wurtzite structure and grow along with the c-axis. FTIR spectrum indicated the incorporation of a trace of PAMPSA into ZnO crystals. HRTEM, Raman and XPS analyses showed that the hierarchical ZnO microstructures contain high concentration of oxygen vacancies which enable them exhibiting a significant intense deep-level emission centered at green luminescence in its photoluminescence spectra. They also show enhanced photocatalytic efficiency in degradation of methylene blue. It is hoped that the present work may provide a simple method to fabricate ZnO hierarchical microstructures and a positive relationship among polar plane, oxygen vacancy and green emission.

Facile and Surfactant-Free Synthesis of Hierarchical ZnO Microstructures

Hierarchical ZnO crystals with flower-like microstructures were successfully synthesized via a facile hydrothermal route without using any surfactants. The morphology of these microstructures can be easily controlled by adjusting the pH of the reaction solution. The products were characterized by X-ray powder diffraction (XRD) and scanning electron microscope (SEM). Furthermore, a possible growth mechanism of ZnO hierarchical microstructures was proposed.

A novel synthesis and excellent photodegradation of flower-like ZnO hierarchical microspheres

We demonstrate an easy, novel synthesis of nanosheet-based flower-like ZnO microspheres without any catalyst and template. The layered hydroxide zinc acetate Zn5(OH)8(CH3COO)2·2H2O (LHZA) nanostructures, one of important and efficient precursors to synthesize three dimensional (3D) ZnO hierarchical microstructures, are first fabricated in aqueous solution at room temperature. The detailed crystal structure and formation mechanism of the LHZA are also investigated. It is noteworthy that the initial ZnO seed layer is crucial to the growth of the final nano/microarchitectures. We found that the as-synthesized hierarchical ZnO microspheres obtained from the decomposition of LHZA showed excellent photocatalytic activity for the photodegradation of rhodamine 6G. This facile, efficient, and economical work provides a new route to simply and mildly synthesize ZnO 3D hierarchical microspheres, which may be adopted to prepare other metal oxide microstructures.

Ultrasound-Assisted Solution Synthesis and Characterization of Sea Urchin-Like ZnO Hierarchical Microstructures

ZnO hierarchical microstructures with uniform sea urchin-like morphology have been successfully synthesized through a facile and reliable ultrasound irradiation route by using Zn(CH3COO)2•2H2O and KOH as raw materials. The ZnO sample is investigated with XRD, SEM, TEM and HRTEM. Results indicate that the as-prepared sea urchin-like ZnO microstructures are assembled by numerous needle-like nanocrystals with the size about 2µm in length and about 100nm in diameter. The preferred growth orientation of these ZnO nanoneedles are along [101] direction. A blue shift in UV-vis absorption spectrum of the as-prepared ZnO sample is observed.

Hydrothermal Synthesis and Photocatalytic Property of Flower-Like ZnO Hierarchical Microstructures

ZnO hierarchical microstructures with uniform flower-like morphology were successfully prepared on a large scale through a carboxymethylcellulose sodium (CMC)-assisted hydrothermal route. X-ray powder diffraction (XRD) measurement confirmed the formation of wurtzite-structured ZnO phase. Field-emission scanning electron microscopy (FESEM) and transmission electron microscopy (TEM) analysis indicated that the as-prepared ZnO sample was composed of numerous three dimensional flower-like microstructures, each of which was assembled by nanosheets with the thickness of about 40 nm. High-resolution transmission electron microscopy (HRTEM) measurement revealed the good crystallinity nature of the ZnO nanosheets in the flower-like microstructures. The formation mechanism and photocatalytic property of the as-prepared flower-like ZnO hierarchical microstructures were studied.

Synthesis and field-emission properties of novel hierarchical ZnO hexagonal towers

Novel hierarchical ZnO hexagonal towers were fabricated via chemical vapor deposition for the first time by adding trace Fe power in the precursor as a catalyst. The scanning electron microscopy images showed that the outside surface of novel hierarchical ZnO hexagonal towers was layer-by-layer assembly. X-ray diffraction pattern indicated that the hierarchical ZnO hexagonal towers grew preferentially along the [0001] direction. Intense UV emission centered at 380nm was observed at room temperature. Moreover, their field-emission properties were studied. A low turn-on electric field of 2.37Vμm−1 and high field enhancement factor of about 2691 were obtained, due to the novel layer structure and high crystalline quality. This kind of hierarchical ZnO microstructures could be a good candidate for high performance field emission devices.

Controllable synthesis of hierarchical ZnO microstructures via a hydrothermal route

Hierarchical ZnO superstructures assembled by ZnO nanorods have been successfully constructed in mixed solvents of ethylene glycol (EG)–water via a facile hydrothermal method. ZnO nanostructures with dumb-bell-like and microsphere morphologies were obtained by only a change of the precursor concentration. The products were characterised by means of X-ray powder diffraction (XRD), scanning electron microscopy (SEM), Raman scattering and room-temperature photoluminescence spectra. The possible formation mechanism of these hierarchical ZnO nanostructures is also proposed.

Facile Synthesis and Enhanced Photocatalytic Performance of Flower-like ZnO Hierarchical Microstructures

ZnO hierarchical microstructures with uniform flower-like morphology were prepared on a large scale through a template- and surfactant-free low-temperature (80 °C) aqueous solution route. The product was characterized by X-ray diffraction, scanning electron microscopy, transmission electron microscopy, and Brunauer−Emmett−Teller N2 adsorption−desorption analyses. The flower-like ZnO microstructures are assembled by many interleaving nanosheets which have the uniform thickness of about 10 nm and a well-crystalline structure with dominant surfaces as {2110} planes. Control experiments revealed that the formation of the flower-like ZnO was based on the fast nucleation−growth kinetics. The flower-like ZnO sample shows an enhanced photocatalytic performance compared with the other nanostructured ZnO powders of nanoparticles, nanosheets, and nanorods, which can be attributed to the special structural feature with an open and porous nanostructured surface layer that significantly facilitates the diffusion and ...