Spherical ZnO Research Articles

Synthetic versatility of nanoparticles: A new, rapid, one-pot, single-step synthetic approach to spherical mesoporous (metal) oxide nanoparticles using supercritical alcohols

AbstractA simple, rapid (10 min), one-pot, single-step method for the preparation of solid and hollow spherical porous TiO2 nanoparticles with large surface areas (100–211 m2/g) was developed in supercritical alcohols using carboxylic acids as organic additives. The shell thickness of the hollow TiO2 nanoparticles (20–280 nm) was controlled by adjusting the heating rate (2.0–10.0 °C/min). The preparation of different spherical porous metal oxide nanoparticles, including CeO2, SiO2, TiO2, ZrO2, and ZnO, demonstrated the versatility of the synthetic approach. In addition, several rare earth-doped spherical mesoporous metal oxide nanoparticles, including CeO2:Er, CeO2:Er,Yb, ZrO2:Er, and TiO2:Er, which exhibit energy upconversion emission, were successfully prepared using this one-pot, single-step, supercritical methanol method. The obtained spherical mesoporous CeO2:Er and CeO2:Er,Yb nanoparticles emit green light upon excitation, even when irradiated with a low-power IR laser (980 nm, 10 mW) without calcination. Several other (metal) elements were also easily doped into spherical, mesoporous TiO2 nanoparticles, such as Eu, Ce, Yb, Fe, and N, using a similar procedure. Furthermore, the spherical mesoporous TiO2 nanoparticles were successfully applied as a new material for the transport of DNA via biolistic bombardment.

Simple and rapid preparation of spherical flower-like ZnO and their photocatalytic property

The influences brought about by different microstructure of ZnO on its special chemical and optical properties have been widely discussed. Precipitates from the mixed solution of oleic acid with Zn(NO3)2 and KOH were investigated using SEM, TEM, XRD, Raman, and FTIR. The obtained microstructure of the ZnO precipitates was proved to be a unique hierarchical structure consisted of 5 nm thick nanosheets. The catalytic property of the received flower-like ZnO was explored, and it was found that the photodegradable property of rhodamine B was largely improved when the precipitates were heat treated at 500 °C for 2 h due to a high crystallinity with a heat treatment, and the decomposition of the oleic acid molecules.

Differential toxicity of rod and spherical zinc oxide nanoparticles on human peripheral blood mononuclear cells.

The cytotoxic and genotoxic potential of rod and spherical zinc oxide nanoparticles (ZnO NPs) were evaluated on human peripheral blood mononuclear cells (PBMCs). Their sizes and shapes were determined by Transmission electron microscopy (TEM) and physical characterizations were done by X-ray diffraction (XRD) and Dynamic light scattering (DLS). The cytotoxic potential of rod ZnO NPs was greater than spherical ZnO NPs when applied to PBMCs and was limited to the proliferative lymphocytes. Rod ZnO NPs produced more reactive oxygen species (ROS) compared to spherical ones. Additionally rod ZnO NPs induced significant DNA damage to PBMCs as revealed by cytokinesis block micronucleus (CBMN) assay and comet assay. On the other hand UV absorption property was enhanced in case of spherical ZnO NPs compared to rod ZnO NPs. Thus, present study implicates the shape dependent differential application of ZnO NPs in our daily life.

Single-Crystalline ZnO Spherical Particles by Pulsed Laser Irradiation of Colloidal Nanoparticles for Ultraviolet Photodetection

We report the formation and ultraviolet (UV) photodetection of single-crystalline spherical ZnO particles by pulsed laser irradiation of commercial ZnO nanoparticles in water. The phase and microstructure analysis before and after laser irradiation reveals a crystal size increase and shape transformation from irregular to spherical. Time-dependent laser irradiation confirmed that fusion is the reason for nanoparticle growth up to single-crystalline spherical particles. After rapid cooling, they maintain size and shape and possess unique optical and electrical properties. Because of the single-crystalline feature and smooth surfaces, high and selective sensing of ultraviolet light is observed.

Microstructure optimization and gas sensing improvement of ZnO spherical structure through yttrium doping

ZnO nanofibers has been successfully divided into nanoparticles through adjusting composition of precursor liquid by an electrospinning method and followed by calcination. The ZnO nanoparticles formed spherical structure, the spherical structure was optimized through yttrium (Y) doping and Y doped ZnO (ZnO:Y) had good gas sensing. The as-synthesized nanostructure was investigated by scanning electron microscopy, Brunauer–Emmet–Teller method, transmission electron microscopy, X-ray diffraction, energy dispersive spectroscopy, photoluminescence and X-ray photoelectron spectroscopy. The results showed that the spherical structure was composed of ZnO nanoparticles, ZnO:Y nanoparticles had smaller grain size (−50 to 100nm) and were more dispersed compared with pure ZnO (−100 to 150nm grain size). The numerous oxygen species (O− and O2−) were adsorbed on the surface of ZnO:Y nanoparticles, the specific surface area of ZnO:Y structure (40.2m2/g) was larger than pure ZnO (14.5m2/g). The response of spherical ZnO:Y based sensor was higher than pure ZnO for the same acetic acid concentration. The results show ZnO:Y spherical structure could be a potential material for fabricating acetic acid sensors.

Toxicity of Functional Nano-Micro Zinc Oxide Tetrapods: Impact of Cell Culture Conditions, Cellular Age and Material Properties

With increasing production and applications of nanostructured zinc oxide, e.g., for biomedical and consumer products, the question of safety is getting more and more important. Different morphologies of zinc oxide structures have been synthesized and accordingly investigated. In this study, we have particularly focused on nano-micro ZnO tetrapods (ZnO-T), because their large scale fabrication has been made possible by a newly introduced flame transport synthesis approach which will probably lead to several new applications. Moreover, ZnO-T provide a completely different morphology then classical spherical ZnO nanoparticles. To get a better understanding of parameters that affect the interactions between ZnO-T and mammalian cells, and thus their biocompatibility, we have examined the impact of cell culture conditions as well as of material properties on cytotoxicity. Our results demonstrate that the cell density of fibroblasts in culture along with their age, i.e., the number of preceding cell divisions, strongly affect the cytotoxic potency of ZnO-T. Concerning the material properties, the toxic potency of ZnO-T is found to be significantly lower than that of spherical ZnO nanoparticles. Furthermore, the morphology of the ZnO-T influenced cellular toxicity in contrast to surface charges modified by UV illumination or O2 treatment and to the material age. Finally, we have observed that direct contact between tetrapods and cells increases their toxicity compared to transwell culture models which allow only an indirect effect via released zinc ions. The results reveal several parameters that can be of importance for the assessment of ZnO-T toxicity in cell cultures and for particle development.

CdS quantum dots sensitized ZnO spheres via ZnS overlayer to improve efficiency for quantum dots sensitized solar cells

This paper reports the preparation of three-dimensional ZnO spheres by using a hydrothermal method and their application to quantum dots sensitized solar cells (QDSSCs). After achieving the desired thickness of sensitized CdS quantum dots (QDs) for ZnO spheres, ZnS overlayer was deposited on the surface of CdS/ZnO photo-anodes to further improve the photoelectric properties. CdS QDs and ZnS overlayer were deposited by successive ionic layer adsorption and reaction (SILAR) method. The surface morphology and crystal structure of the samples were verified by field-emission scanning electron microscopy (FE-SEM), Transmission electron microscopy (TEM) and X-ray diffraction (XRD). The CdS QDs sensitized solar cells were ameliorated via using ZnS as a protection-layer between quantum dots and electrolyte. As a result, the power conversion efficiency (η) has been increased from 0.60 to 1.43% after being treated by ZnS overlayer for CdS/ZnO photo-anodes.

Photo-electrochemical Behaviors of Hollow ZnO Spheres Prepared by Zinc in Aqueous Solution

Abstract ZnO with abundant morphologies has been widely applied in photo-electrochemical water splitting. A novel Pt-induced faint electrochemical route is developed to synthesize hollow ZnO spheres from zinc dust in aqueous solution at room temperature. Here, the Zn|ZnO serves as an anode and Pt|H2O|H2 serves as a cathode. The hollow ZnO spheres exhibit enhanced photocurrent intensity due to a multiple light reflection.

A dye-sensitized solar cell based on a boron-doped ZnO (BZO) film with double light-scattering-layers structured photoanode

A double light-scattering-layer boron-doped ZnO (DL-BZO) film based dye-sensitized solar cell (DSSC) is fabricated with a high energy conversion efficiency of 7.2%. The over layer is made of well dispersed sub-micrometer-sized boron-doped ZnO (BZO) sphere arrays (SSA-BZO) to facilitate the fastest movement of electrons towards the electrodes and also function as light scattering centers. The under-layer, consisting of a nanoporous BZO nanoparticulate (NP-BZO) film, increases the dye adsorption and also reduces the injected electrons recombination. The DL-BZO film boosted the cell efficiencies because of its improved light-harvesting ability, larger surface area, photon-to-electron mechanism and doping of the ZnO films. The efficiency of 7.2% achieved for DL-BZO is much larger than the 4.1% obtained for double light-scattering-layers for undoped ZnO (DL-ZnO) films and is also higher than the 2.1% achieved for mono-layer NP-BZO films. Addition of an SSA-BZO layer over the NP-BZO layer significantly improves the transport of electrons and their lifetime when compared to the NP-BZO film only. The DL-BZO film exhibit the slowest recombination rate of changes compared with the other films and is found to be a major factor for the improved cell efficiency. The cells exhibit high stability, in terms of cell performance, even after 35 days. The highest efficiency achieved in the present study is the highest ever reported for a double light-scattering-layer ZnO film-based DSSC.

Size-controlled wurtzite zinc oxide spheres with the characteristics of visible absorption and Mie scattering

Uniform ZnO spheres with tunable size exhibit superior visible absorption and scattering effects.

Synthesis of hierarchical nanoparticles-based ZnO spheres for their application as the light blocking layers in dye-sensitized solar cells

Three-dimensional nanoparticles-based ZnO hierarchical spheres (ZnO-HS) with strong light harvesting and dye loading abilities have been fabricated by a simple hydrothermal method in this paper. These ZnO-HS were designed as the overlayer for light blocking and applied to the dye-sensitized solar cells (DSSCs) based on bare ZnO nanoparticles (ZnO-NP) or TiO2 nanoparticles (TiO2-NP). The results show that the values of the short-circuit current density (Jsc) and the power conversion efficiency (η) have been heightened up to 12.6 mA cm−2 and 3.40 % for the ZnO-NP/ZnO-HS double-layered DSSC, far higher than the bare ZnO-NP DSSC. However, another DSSC assembled by the TiO2-NP/ZnO-HS double-layered film displays an adverse result for the decreasing of Jsc and η even though the ZnO-HS light blocking layer has been established on the TiO2-NP film. According to the electrochemical impedance data compared between the ZnO-NP/ZnO-HS double-layered and TiO2-NP/ZnO-HS double-layered DSSC, it is found that the former possesses less possibility for the occurrence of charge recombination and electronic loss, which is responsible for its better photovoltaic response.

Fast one-step synthesis of ZnO sub-microspheres in PEG200

ZnO sub-microspheres were synthesized via a new, simple, and one-step method by using zinc acetate dihydrate as a precursor and PEG200 as a solvent and modifier. The effect of temperature (160–210 °C) on the crystallization, surface morphology, and luminescence properties of ZnO spheres was investigated using different characterization techniques, including X-ray diffraction, scanning electron microscopy, transmission electron microscopy, Fourier transform infrared spectroscopy, UV–Vis spectroscopy, and room temperature photoluminescence. The results show that the ZnO crystal has a hexagonal wurtzite structure. The products have monodispersed spherical morphology with diameters in the range of 200–600 nm. They have intensive UV emission peaks at ~380 nm and relatively weak and broad green peaks at ~550 nm. The PEG200 molecules adsorb on the surface of ZnO spheres. On the basis of the experimental results, a mechanism was proposed to elucidate the formation of ZnO sub-micron spheres.

Synthesis of Various Sized ZnO Microspheres by Laser Ablation and Their Lasing Characteristics

We have succeeded in synthesizing ZnO nano/microspheres by a simple laser ablation method in the air and demonstrated whispering-gallery-mode (WGM) lasing from the ZnO sphere excited by a pulsed-ultraviolet laser beam. The spheres with diameters of up to 10μm were simply prepared by laser ablation of a ZnO sintered target by a Q-switched Nd:YAG laser in the air. In this study, we succeeded synthesizing large microspheres with diameters of larger than 20μm by the Nd:YAG laser without Q-switching. The mode spacing of the large sphere was in good agreement with the WGM theory.

Ionic liquid-based solvent-induced shape-tunable small-sized ZnO nanostructures with interesting optical properties and photocatalytic activities

We report a simple and one-pot size- and shape-controlled synthesis of small crystalline ZnO nanostructures through the hydrolysis of zinc acetylacetonate precursor using an ionic liquid, tetrabutylphosphonium hydroxide in different organic solvents and solvent mixtures of different polarities. Transmission electron microscopy (TEM) results indeed show the formation of very small (D ∼ 5–7 nm) ZnO spheres and ZnO nanorods of very low diameters (D ∼ 8–11 nm) depending on the solvents. The size and shape of ZnO nanostructures can be easily controlled by simple variation of the solvent and reaction temperature. Protic solvents favor the growth of spherical nanoparticles, whereas the rod-shaped ZnO is formed in aprotic solvents. The spherical ZnO nanostructures exhibit excitonic absorption at lower wavelengths compared to that of rod-shaped ZnO nanostructures. Through adjustment and control of the size and shape of ZnO nanostructures, we can tune the fluorescence emission from blue to green to yellow, when dispersed in the aqueous medium. The obtained ZnO nanostructures show very high photocatalytic activities towards the degradation of different cationic organic dyes (such as rhodamine 6G, crystal violet and methylene blue) as they contain a large number of defect states, as evident from the photoluminescence study. The as-synthesized ZnO nanostructures also show very high stability towards the photocatalytic degradation of organic dyes.

Structural study and phase transition investigation in a simple synthesis of porous architected-ZnO nanopowder

In this work, zinc oxide powder with a rectangular-shaped porous architecture, made of numerous spherical nanometric particles, was obtained. A simple precipitation/decomposition procedure was used comprising a zinc oxalate intermediate, obtained from zinc sulfate and oxalic acid without any additives. Detailed studies on zinc oxalate dehydration, decomposition and zinc oxide formation, were carried out using in-situ temperature X-ray diffraction and thermogravimetric analysis. During the investigation, the temperature dependence of particle sizes, lattice parameters and crystal structures of ZnC2O4·2H2O, ZnC2O4 and ZnO nanopowders were analyzed from room temperature to 450°C. Structural transitions were also discussed. The structure and morphology of the as-prepared ZnO nanopowder were investigated by electron microscopy and compared to the crystalline rectangular shape of ZnC2O4·2H2O. The calcination temperature, counter ion and precipitate agent were found to be related to the product's shape and diameter. Spherical ZnO nanoparticles with diameters of less than 20nm and a maximum specific surface of 53m2/g were obtained using this method.

Cysteine-functionalized zwitterionic ZnO quantum dots

Abstract

Physical and chemical properties of multifunctional ZnO nanostructures prepared by precipitation and hydrothermal methods

Spherical, cabbage-like and flower-like ZnO powders were prepared by precipitation and hydrothermal methods. Zn(CH3COO)2·2H2O, KOH, distilled H2O and C3H9N were used as the zinc source, precipitating agent, solvent and capping agent, respectively. The ZnO powders were characterized by TGA, FTIR, XRD and SEM. The particle shape of the ZnO powders altered when the solution was modified by n-propylamine. The photocatalytic activity of the ZnO powders depended on the particle shape whilst the bactericidal property of the ZnO powders was influenced by both particle shape and defect concentration.

Optical properties and photocatalytic activities of spherical ZnO and flower-like ZnO structures synthesized by facile hydrothermal method

Spherical ZnO and flower-like ZnO were prepared by facile hydrothermal method at 180 °C and 160 °C, respectively. The as-prepared samples were characterized by powder X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), Raman spectroscopy and UV–vis diffuse reflectance spectroscopy (DRS) spectra. The optical properties of as-prepared sample, such as photoluminescence (PL) spectra and Raman spectra were studied. The photocatalytic activities of the as-prepared ZnO particles were investigated by degrading the methyl orange (MO) under UV light irradiation. The photocatalytic studies showed that the organic pollutants have been almost completely degraded and mineralized after irradiation of the UV light. These results indicated that the as-prepared ZnO particles exhibited good optical properties and high photocatalytic activities.

A comparative study on ethanol gas sensing properties of ZnO and Zn0.94Cd0.06O nanoparticles

Nanocrystalline ZnO and Zn0.94Cd0.06O particles were synthesized using sol–gel method and were characterized for their structural and morphological properties by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and energy dispersive X-ray spectroscopy (EDXS). The results confirmed the formation of pure and spherical ZnO and Zn0.94Cd0.06O nanoparticles with wurtzite crystal phase and particle size of 20–25nm obtained by TEM. Additionally, the ethanol sensing properties of ZnO and Zn0.94Cd0.06O based sensors were investigated for different operating temperatures/concentrations and they were compared with commercial ZnO microparticles. Comparative results demonstrated that Zn0.94Cd0.06O nanoparticles exhibit higher and faster response to 250ppm of ethanol at 250°C. Results on response/recovery time, sensing mechanism, conductance variation and thermodynamic/kinetic investigation of ethanol sensing mechanism is also studied and discussed.

Carbonaceous spheres—an unusual template for solid metal oxide mesoscale spheres: Application to ZnO spheres

Abstract A green template route for the synthesis of mesoscale solid ZnO spheres was ascertained. The protocol involves a double coating of the carbonaceous spheres with successive layers of zinc-containing species by alternating a non-ultrasound and ultrasound-assisted deposition, followed by calcination treatments. The composites were characterized by FTIR spectroscopy, thermal analysis, scanning electron microscopy while the obtained ZnO spheres by X-ray diffraction, Raman spectroscopy, scanning and transmission electron microscopy, N 2 adsorption–desorption isotherms and photoluminescence investigations. A growth mechanism of the solid spheres is advanced based on these results. While the spheres' diameters and the mean size values of ZnO are independent on deposition order, the surface area and the external porosity are fairly dependent. The photoluminescence measurements showed interesting emission features, with emission bands in the violet to orange region. The spheres present high photocatalytical activity towards the degradation of phenol under UV irradiation, the main reaction being its mineralization.