ZnO Hollow Microspheres Research Articles

One-step synthesis of ZIF-8/ZnO composites based on coordination defect strategy and its derivatives for photocatalysis

Abstract A new approach was proposed for the preparation of metal–organic framework/metal oxide composites in one step using only metal salts. Here, crystalline rhombohedral dodecahedron ZIF-8, hollow ZnO microsphere, and ZIF-8/ZnO composites were obtained in different one-pot solvothermal processes where the growth toward individual ZIF-8 and ZnO phases and toward ZIF-8/ZnO composites was controlled by adjusting the ratio of metal salts to 2-methylimidazole. Subsequently, a series of derived (C and/or N)-doped ZnO nanoparticles was obtained by a two-step calcination process. The absorption kinetics and photocatalytic activity in methyl orange degradation under visible light were investigated. ZIF-8-derived ZnO could maintain their original morphology and displayed excellent photocatalytic activity 10 times higher than that of the original ZIF-8.

TiO2 Passivation Layer on ZnO Hollow Microspheres for Quantum Dots Sensitized Solar Cells with Improved Light Harvesting and Electron Collection.

Light harvesting and electron recombination are essential factors that influence photovoltaic performance of quantum dots sensitized solar cells (QDSSCs). ZnO hollow microspheres (HMS) as architectures in QDSSCs are beneficial in improving light scattering, facilitating the enhancement of light harvesting efficiency. However, this advantage is greatly weakened by defects located at the surface of ZnO HMS. Therefore, we prepared a composite hollow microsphere structure consisting of ZnO HMS coated by TiO2 layer that is obtained by immersing ZnO HMS architectures in TiCl4 aqueous solution. This TiO2-passivated ZnO HMS architecture is designed to yield good light harvesting, reduced charge recombination, and longer electron lifetime. As a result, the power conversion efficiency (PCE) of QDSSC reaches to 3.16% with an optimal thickness of TiO2 passivation layer, which is much higher when compared to 1.54% for QDSSC based on bare ZnO HMS.

Phthalocyanine-sensitized hollow ZnO spheres as an efficient visiblie-light photocatalyst for water treatment

A novel phthalocyanine-sensitized hollow ZnO spheres as an efficient visiblie-light photocatalyst had been prepared successfully. Firstly, a unique hollow ZnO microsphere had been prepared by a facile solvothermal method followed by calcination. Secondly, zinc-tetracarboxyl-phthalocyanine (Pc) impregnated onto the surface of hollow ZnO microsphere. The obtained photocatalyst Pc/ZnO was characterized by XRD, SEM and EDS. The hollow Pc/ZnO hierarchical nanostructure improved the electron-hole separation more effectively and Pc loaded on the surface of ZnO microsphere to enhance photocatalytic activity under visible light. In our photocatalytic experiments, the hollow Pc/ZnO microsphere showed excellent photocatalytic performance under visible light for the removal of Rhodamine B (RhB). As a result, our work provided an effective and green photocatalyst for water treatment.

Insight into charge carrier separation and solar-light utilization: rGO decorated 3D ZnO hollow microspheres for enhanced photocatalytic hydrogen evolution

Improving efficient solar light utilization, facilitating charge transportation and reducing electron-hole recombination, are the three major challenges in photocatalysis, and numerous interests have been devoted into overcoming these issues for obtaining high performance photocatalysts. Herein, ZnO hollow microspheres/reduced graphene oxide (ZnO/rGO) composites were constructed as a high performance photocatalyst for splitting water into H2 via a one-step microwave-assisted solvothermal process. The optimized ZnO/rGO nanocomposite (the mass ratio of GO to ZnO is 1%) reached a maximum H2 evolution rate of 648.1 μmol/h/g without using noble metal as cocatalyst, which exhibiting ~2.3-fold enhancement as compared to that of the bare ZnO. This significant improvement was primarily attributed to great light-harvesting capacity and the efficient charge carrier separation and transfer. The detailed characterization of PL and EIS revealed that, in the ZnO/rGO composite, the rGO nanosheets played important roles in promoting the charge carrier separation and transfer, which therefore resulting in an enhanced activity in H2 evolution. Our present observations provide a valuable methodology for exploring novel high performance photocatalyst, especially in graphene-based inorganic hybrid systems.

Facile Synthesis of Ag/ZnO Hollow Microspheres with Enhanced Photocatalytic Performance under Simulated Sunlight Irradiation

Water pollution caused by intensive use of organic dyes has become an increasingly serious problem recently. Green and efficient processes are desperately needed to remove persistent organic pollutants from waste waters. Herein, Ag nanoparticles loaded ZnO hollow microspheres were synthesized through a simple solvothermal method and used as a photocatalyst for dye degradation. The calculated band gap of Ag/ZnO — 5% (2.97[Formula: see text]eV) is much narrower than that of pure ZnO (3.37[Formula: see text]eV). The obtained Ag/ZnO samples show a remarkable photocatalytic activity in photodegradation of Rhodamine B (RhB) under simulated sunlight irradiation. The degradation efficiency of RhB for Ag/ZnO — 5% is 98.8% after 100[Formula: see text]min irradiation while only 52.8% degradation rate is obtained over pure ZnO. The enhancement is attributed to the exposed active ZnO (001) plane and the surface plasmon resonance (SPR) effect of Ag nanoparticles that promote the separation of photogeneated electrons and holes.

Hollow ZnO microspheres functionalized with electrochemical graphene oxide for the photodegradation of salicylic acid.

Hollow ZnO microspheres were successfully synthesized by a hydrothermal method and then functionalized with graphene oxide (GO) flakes, previously obtained through electrochemical oxidation. Their photocatalytic activity toward the photodegradation of salicylic acid under UV light irradiation was evaluated by UV-Vis spectroscopy. Unfunctionalized microspheres and ZnO functionalized with chemically oxidized graphene were also studied as comparative terms. The hybrid materials of ZnO with both electrochemical and chemical GO gave a similar photodegradation yield of ∼28% against 18% of the non-functionalized microspheres. The similar degradation yields and rate constants obtained with the two GO synthetic methods indicate that electrochemical oxidation of GO represents an eco-friendly option over traditional methods for photocatalytic degradation systems.

ZnO@ZIF-8 core-shell microspheres for improved ethanol gas sensing

The ZnO@ZIF-8 (ZIF-8 = zeolite imidazolate framework-8) core-shell material was synthesized by the self-template method. The positive effects of the structure and morphology of ZIF-8 pre-concentrator on the sensing property of ZnO microsphere were investigated by FTIR, XRD, SEM and TEM characterizations, as well as gas detection measurement. The synthesized ZnO@ZIF-8 core-shell material exhibited much higher sensor response (35.90), selectivity and stability compared with pure hollow ZnO microsphere (14.54) at an optimal temperature of 160°C when exposed to ethanol than other volatile organic compounds (VOCs). The obtained results suggested that ZnO@ZIF-8 core-shell material is efficient in sensing ethanol than pristine ZnO microsphere, showing potential application as a selective ethanol detector.

Synthesis and Optical Properties of Solid and Hollow ZnO Microspheres Prepared by Hydrothermal Method

We successfully synthesized solid and hollow ZnO microspheres by a template-free hydrothermal method on the substrate of porous alumina membranes. The morphology of prepared sample was characterized by scanning electron microscopy and transmission electron microscopy. The growth mechanism of the synthesized ZnO microspheres is discussed in detail. At step 1, solid ZnO microspheres were prepared by the thermal decomposition of the Zn(OH)2 microspheres synthesized under the mixture of Zn(NO3)2·6H2O, C6H12N4, and C6H5Na3O7·2H2O solution. The quantity of C6H5Na3O7·2H2O dopant will greatly affect the morphology of the prepared sample by controlling ZnO nucleation and growth rate. At step 2, hollow ZnO microspheres were prepared by removing the Zn(OH)2 cores in an alkaline solution. The proper pH value also plays a key role in preparing hollow ZnO microspheres with high quality. Compared with hollow nanostructure, the solid ZnO microsphere shows the relatively strong Raman peaks at 437 cm-1, which suggests that the prepared solid spheres should have the better crystal quality. Photoluminescence spectra show that the solid ZnO microspheres have the relatively strong ultraviolet (UV) peak at ~ 380 nm attributed to the recombination of free exciton, and the weak green peak owing to the low surface-to-volume ratio. DOI: http://dx.doi.org/10.5755/j01.ms.24.4.19141

Green synthesis of ZnO hollow microspheres and ZnO/rGO nanocomposite using red rice husk extract and their photocatalytic performance

ZnO hollow microspheres and ZnO hollow microspheres/reduced graphene oxide (ZnO/rGO) nanocomposites were synthesized for the first time by a completely green method based on red rice husk extract as an effective oxidizing agent as well as a chelating agent and water as a solvent. The green ZnO and ZnO/rGO materials were characterized by x-ray diffraction, scanning electron microscope, FTIR spectroscopy, Raman spectroscopy, BET surface area analyzer and TGA. The results showed that the biosynthesized ZnO and ZnO/rGO has a hollow spherical morphology composed of small ZnO particles with the mean particle size of about 15 nm. The Raman and XRD measurements demonstrated that the as-prepared ZnO structures are of wurtzite hexagonal phase with high crystallinity. The synthesized green ZnO microspheres show high photocatalytic activity for the degradation of malachite green dye as a model pollutant. The photoctalytic activity was further increased in the presence of ZnO/rGO nanocomposite evidencing the role of graphene oxide in capturing the photogenerated electrons and decreasing the electron–hole pair recombination.

Soft-templated formation of double-shelled ZnO hollow microspheres for acetone gas sensing at low concentration/near room temperature

The synthesis of well-defined and complex porous hollow structures via a simple method is still a major challenge. In this work, a unique double-shelled ZnO hollow microsphere with a porous surface is successfully synthesized by a facile soft-templated solvothermal method followed by calcination. The presence of ethylene glycol (EG) as soft template leads to the formation of initial single-layered hollow microspheres and then a time-dependent evolution transforms them into uniform ZnO hollow microspheres with tunable shell numbers and void space. When used as sensing materials for detecting acetone, the double-shelled ZnO hollow microsphere sensor exhibits high response toward 100 ppm acetone (101.1) and achieves a rapid response rate and recovery process (within 1/7 s) at 300 °C, which are superior over those for ZnO microparticle and single shelled ZnO hollow microsphere. In addition, this sensor exhibits low detection limit (0.5 ppm), low operating temperature (40 °C), high selectivity to acetone, and long term stability, suggesting their potential applications as advanced gas sensing materials. Such outstanding gas sensing properties of double-shelled ZnO hollow microsphere is due to the larger Brunauer-Emmett-Teller (BET) surface area (76.11 m2 g−1), porous and double-shelled hollow structure, and excellent capabilities of surface adsorbed oxygen.

Synthesis and sensitivity of ZnO and Au-ZnO hollow microspheres

Synthesis and sensitivity of ZnO and Au-ZnO hollow microspheres

Sonochemical synthesis of ZnO hollow microstructure/reduced graphene oxide for enhanced sunlight photocatalytic degradation of organic pollutants

ZnO hollow microspheres and ZnO hollow microspheres/reduced graphene oxide nanocomposites (ZnO-rGO) were synthesized through simple and surfactant free sonochemical method at relatively low temperature. The crystalline structure and morphology of the as-prepared hollow microspheres and their nanocomposites were characterized by X-ray diffraction (XRD), Raman spectroscopy, BET surface area analyzer and electron microscopy (SEM, TEM). The photocatalytic performance of the samples was evaluated for the photodegradation of 2,4-dichlorophenoxy acetic acid (2,4-D) under direct sunlight irradiation. The results showed that the synthesized ZnO have porous hollow microspheres structure, which can act as an antenna system transferring the photogenerated electrons within the network structure as compared to the nonporous ZnO nanoparticles. The introduction of graphene oxide is further enhanced the photocatalytic activity evidencing the role of graphene oxide extending the optical absorption to longer wavelength as well as in decreasing the electron-hole pair recombination. The effect of various parameters such as pH and initial concentration of 2,4-D has been also studied. The kinetic parameters have been determined and the underline reaction mechanism has been proposed.

One-step hydrothermal synthesis of hollow ZnO microspheres with enhanced performance for polyacrylate

In this study, hollow ZnO microspheres were successfully synthesized by a one-step template-free hydrothermal synthetic route straightforwardly just for 30 min at 90 °C. More importantly, hexamethylenetetramine and trisodium citrate played a significant role in the fabrication of hollow ZnO microspheres, and distilled water served as the main solvent without adding any organic solvent and surfactants. The architecture and morphology of as-obtained ZnO microstrutures were estimateded by field-emission scanning electron microscopy (FE-SEM), powder X-ray diffraction (PXRD) and Brunauer-Emmett-Teller (BET). With the morphological evolution apperceived in the time-dependent growth of hollow ZnO microspheres, a possible formation mechanism based on Ostward ripening inside-out was proposed. Then, polyacrylate/hollow ZnO composite latex was obtained by physical blending of hollow ZnO microspheres and polyacrylate emulsion. As expected, hollow ZnO microspheres has a significant impact on water vapor permeability, water resistance and mechanical porperties. Among them, the water vapor permeability and water resistance was relatively increased by 71.18% and 38.42%, respectively.

Flower-like ZnO hollow microspheres loaded with CdO nanoparticles as high performance sensing material for gas sensors

CdO decorated flower-like ZnO hollow microspheres were successfully prepared via a two-step hydrothermal strategy. CdO nanoparticles (∼12nm) equably loaded on the surfaces of ZnO nanosheets, which could be clearly observed from SEM and TEM images. The results of X-ray photoelectron spectroscopy and H2 temperature-programmed reduction (H2-TPR) indicated that the amount of chemisorbed oxygen was increased after the introduction of CdO nanoparticles. Compared with the flower-like ZnO hollow microspheres, the 2.6mol% CdO:ZnO heterostructure composites exhibited highest response (65.5) to 100ppm ethanol at 250°C, which was about 16 folder higher than that of pure ZnO at the same operating temperature of 250°C. Significantly, the detection limit of the 2.6mol% CdO:ZnO heterostructure could reach ppb level (500ppb). The mechanism of the enhanced ethanol sensing was also discussed systematically.

Preparation and gas sensing properties of ZnO hollow microspheres

In this study, zinc oxide hollow microspheres were successfully prepared by a facile one-pot hydrothermal route with the assistance of glucose only using cheap Zn(NO3)3·6H2O as starting materials, followed by a simple calcination treatment. The zinc oxide hollow microspheres were characterized via scanning electron microscopy, transmission electron microscopy and X-ray diffraction. The prepared zinc oxide hollow microspheres were further employed to fabricate a gas sensor to detect volatile organic pollutants. The gas sensing measurement results revealed that the zinc oxide hollow microsphere sensor showed excellent sensing performances to ppm levels of probe gas ethanol vapor including high response, short response–recovery times and good reversibility. The sensor also showed high response to other ppm levels of volatile organic pollutants such as n-butanol, acetone, methanol, toluene and benzene, implying its promising gas sensor application in detecting volatile organic pollutants. Zinc oxide hollow microspheres, prepared by a facile one-pot hydrothermal route, based gas sensor showed excellent sensing performances to ppm levels of volatile organic pollutants including high response, short response–recovery times and good reversibility.

Flower-like ZnO hollow microspheres on ceramic mesh substrate for photocatalytic reduction of Cr(VI) in tannery wastewater

Flower-like ZnO hollow microspheres were synthesized on flexible ceramic mesh substrate using a simple and economical hydrothermal technique and characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The photocatalytic reduction of Cr(VI) in leather tannery wastewater by flower-like ZnO hollow microspheres was evaluated under ultraviolet light irradiation for the first time. The addition of different organics and pH value of the reaction solution played important roles in the photocatalytic reduction of Cr(VI). In addition, UV radiation intensity and catalyst content were critical parameters in the photocatalytic reduction process and their effects were also investigated. Under the optimal conditions, ZnO hollow microspheres exhibited excellent removal efficiency of Cr(VI) (~86%) and good photocatalytic stability. Moreover, more than 24% biochemical oxygen demand (BOD5), 32% chemical oxygen demand (COD) and 30% total organic carbon (TOC) of tannery wastewater were reduced significantly after only 180min of exposure to the UV lamp. It shows much promise to be an attractive photocatalyst for photocatalytic application.

Synthesis and their photocatalytic properties of Ni-doped ZnO hollow microspheres

Abstract

UV activated hollow ZnO microspheres for selective ethanol sensors at low temperatures

UV activated metal oxides for chemiresistive-type gas sensors have been recently studied aiming to lower the working temperature and thus lower the power consumption. In this work, the mesoporous hollow ZnO microspheres were prepared by template-assisted method and examined for VOCs detection with UV LED illumination at lower temperatures than 150°C. The as-synthesized ZnO based sensor indicated an excellent response and selectivity to different concentrations of ethanol (10–1000ppm) with low-powered UV LED (2mW) at 80°C. The response time is only ∼6s while the recovery is a little sluggish (∼94s). The enhanced sensing performance could be attributed to the mesoporous hollow microstructure of the synthesized ZnO and UV activated differential photoctalytic oxidation reactions at the shallow oxide surface regions. The repeatability and long term stability of the sensor and the sensor response were also investigated. A comparison with the conventional thermal-activated gas sensing of the hollow ZnO was also conducted and the photo-activated gas sensing mechanism was discussed.

A two-step hydrothermal route for synthesis hollow urchin-like ZnO microspheres

Three-dimensional hollow urchin-like ZnO microspheres that combine one-dimensional nanorods and zero-dimensional nanoparticles building blocks have been successfully synthesized by two step hydrothermal growth, which is a facile, low-temperature and versatile method and do not need template. By adjusting the annealing temperature of first step and the reaction time of second step, unique and sophisticated urchin-like ZnO superstructures with the ability to control the sphere size in addition to the ZnO nanorod dimensions and morphology can be obtained. A possible growth mechanism for the formation of hollow urchin-like ZnO microspheres was also proposed.

Novel inkjet droplet method generating monodisperse hollow metal oxide micro-spheres

A novel method to synthesize monodisperse hollow TiO2, SnO2 and ZnO microspheres was developed. Water droplets generated from an inkjet nozzle contain clusters of polystyrene latex (PSL) particles covered with metal oxide nanoparticles were continuously dried up forming PSL clusters decorated with metal oxide nanoparticles. By heating the clusters up to 400°C, the PSL particles are evaporated forming hollow spheres. Monodisperse hollow TiO2, SnO2 and ZnO spheres in the size range of 5.1–8.6μm were consequently generated with a narrow particle size distribution. And the hollow TiO2, SnO2 and ZnO spheres have 41.0nm, 30.0nm, and 85.3nm sized pores, respectively. The method enables us to have simpler and faster process to generate hollow TiO2, SnO2 and ZnO spheres and to control the diameter of hollow spheres easily by adjusting waveform applied to a nozzle compared to other processes. A potential applicability to photoelectrochemical cells was demonstrated for the prepared hollow TiO2 grown a Si substrate.