Flower-like ZnO Microstructures Research Articles

Flexible in-plane thermoelectric modules based on nanostructured layers ZnO and ZnO:In

In this study, with the aim of developing flexible in-plane thermoelectric module we deposited via SILAR method 0.3–1.4 thick nanostructured layers of undoped and indium doped zinc oxide on Kapton polyimide (PI) substrates and thus prepared ZnO:In/PI and ZnO/PI samples. We analyzed surface morphology, crystal structure, chemical composition, optical, electrical and thermoelectric properties of ZnO and ZnO:In thin films grown via SILAR on PI substrates. It was shown that ZnO:In/PI samples are consisted of flower-like ZnO:In microstructures with rod petals having pencil-like ends. ZnO/PI samples have lily flower-like ZnO microstructures with rounded petals. Despite the fact that crystal structure and optical properties of the nanostructured ZnO:In and ZnO films were almost the same, their morphology determined the electrical and thermoelectric properties. More compact 1.4 µm thick ZnO layer in sample ZnO/PI provided the best thermoelectric power factor ≈ 8 µW/(m·K2) due to its lower resistivity at near-room ≈ 0.15 Ω cm. Among four samples of the prepared flexible in-plane thermoelectric modules with single legs in the form of PI strips 3 cm long and 0.5 cm wide, covered with the semiconductor film ZnO or ZnO:In and having thin-film Al ohmic contacts at the edges, the TE module based on ZnO/PI showed the best output characteristics. At the temperature difference 50 К, its output power density reaches 10 µW/cm2, which is commensurate with the up-to-date values for solid miniature and flexible TEGs.

Growth mechanism and photocatalytic evaluation of flower-like ZnO micro-structures prepared with SDBS assistance

Flower-like ZnO microstructures were successfully produced using a hydrothermal method employing ZnSO4/(NH4)2SO4 as a raw material. The effect of the operating parameters of the hydrothermal temperature, OH−/Zn2+ molar ratio, time, and amount of dispersant on the phase structure and micromorphology of the ZnO particles were investigated. The synthesis conditions of the flower-like ZnO microstructures were: hydrothermal temperature of 160°C, OH−/Zn2+ molar ratio of 5:1, reaction time of 4 h, and 4 mL of dispersant. The flower-like ZnO microstructures were comprised of hexagon-shaped ZnO rods arranged in a radiatively. Degradation experiments of Rhodamine B with the flower-like ZnO microstructures demonstrated a degradation efficiency of 97.6% after 4 h of exposure to sunshine, indicating excellent photocatalytic capacity. The growth mechanism of the flower-like ZnO microstructures was presented.

3D porous flower-like ZnO microstructures loaded by large-size Ag and their ultrahigh sensitivity to ethanol

3D porous flower-like ZnO microstructures loaded by large-size Ag were successfully synthesized via a simple two-stage method. XRD and XPS results proved that the synthesized Ag/ZnO was a pure phase. SEM and TEM results showed that the Ag nanoparticles with large size (about 400 nm) were successfully loaded on the porous flower-like ZnO. The sensitivity of Ag/ZnO was 268 to 200 ppm ethanol at 300 °C, which was 6 times higher than that of pure ZnO. In addition, the sensor of Ag/ZnO exhibited a faster response/recovery behavior and a lower detection limit. The enhanced gas sensing performance of Ag/ZnO has been explained from three aspects: the “spillover effect” of Ag, the formation of metal-semiconductor heterojunction and the role of “electronic sensitization”.

Low-temperature synthesis of flower-like ZnO microstructures supported on TiO2 thin films as efficient antifungal coatings for bamboo protection under dark conditions

Bamboo is readily infected with fungi because of its abundant sugar, starch and protein content, which greatly restricts its utilization. To develop a highly effective environmentally sound antifungal agent for bamboo, flower-like ZnO microstructures supported on TiO2 thin films have been synthesized on bamboo surfaces by a facile two-step low-temperature hydrothermal method. According to the results of various analytical techniques, including X-ray diffraction, X-ray photoelectron spectroscopy, Fourier transform infrared–attenuated total reflectance spectroscopy, scanning electron microscopy, and energy-dispersive X-ray spectroscopy, the bamboo substrate combined with anatase TiO2 by a hydrogen bonding or ligand exchange reaction between F― and –OH. The F― ions on the TiO2 surface combined with [Zn(NH3)4]2+ via electrostatic adsorption, contributing to the formation of a flower-like ZnO microstructure, and were then deposited on the TiO2 thin films. Moreover, the antifungal activities of the samples against a hybrid fungi group consisting of Trichoderma viride Pers. ex Fr (T. viride), Penicillium citrinum Thom (P. citrinum), and Aspergillus niger v. Tiegh (A. niger) were investigated during a three-month period under dark conditions. The results showed that pure TiO2 thin films coated on bamboo presented better antifungal activities, with a 21-day inhibition, than that of the pristine bamboo, with a 5-day inhibition. The flower-like ZnO microstructures supported on the TiO2 thin films bamboo exhibited effective antifungal capability, with a 90-day inhibition. The efficient antifungal behavior is due to the increased generation of reactive oxygen species (ROS) from the coatings by a possible synergistic effect between the ZnO and TiO2 nanoparticles in attacking fungi and causing their death.

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.

Shape-controlled synthesis of flower-like ZnO microstructures and their enhanced photocatalytic properties

In this paper, we report a simple one-step method to prepare shape-controlled flower-like ZnO microstructure. The different morphologies of ZnO flowers, including spindle-based, flake-based and rod-based, were synthesized by simply adjusting the Zn2+ in alkaline solution. The photocatalytic activities of the variously structural ZnO were investigated under UV-light irradiation, and the results showed that the flake-based ZnO displayed better in catalytic degradation of dyes than the other two structures. Since the flake-based ZnO featured with small grain size, enormous interface area and abundant mesopores due to its special structures. In brief, the present study provides an efficient method for the enhancement of photocatalytic activity by designing the shape-controlled ZnO flowers, especially the flake-based ZnO. These nanoflakes-assembled ZnO with superior photocatalytic performance are also expected to be used in other application such as gas-sensing.

Facile synthesis and gas sensing properties of the flower-like NiO-decorated ZnO microstructures

In this paper, the pure and NiO-decorated ZnO flower-like structures with uniform sizes were synthesized by a simple two-step process. As the gas sensing materials of oxide semiconductors gas sensors, their sensing properties were investigated systematically. The results manifested that the sensor based on 8.0 at% NiO-decorated ZnO microflowers showed improved gas sensing properties compared with those of pure ZnO microflowers. The introduction of NiO is believed to not only change the quantity of chemisorbed oxygen, but also form the p-n junction with ZnO. Thus, the functionalization of ZnO with NiO may be a promising method for designing and fabricating the high performance gas sensor.

Synthesis and enhanced gas sensing properties of flower-like ZnO nanorods decorated with discrete CuO nanoparticles

Flower-like CuO/ZnO hierarchical structures were prepared via a flexible two-step synthesis method, in which nanorods-assembled flower-like ZnO microstructures with the diameter about 430–1680nm and the length about 3.6–8.8μm were prepared in advance through a chemical solution method, and then a frost-like layer of CuO nanoparticles (NPs) decorated on the surface of ZnO nanorods (NRs) via an ionic-layer adsorption and reaction method. Compared with pure ZnO NRs, the flower-like CuO-decorated ZnO NRs show an obvious improvement in gas sensing performance. The gas sensing mechanism of the CuO-decorated ZnO NRs was discussed in relation to the catalytic effect of CuO and the introduction of the p-n junction between CuO and ZnO.

Morphology-tunable synthesis of ZnO microstructures under microwave irradiation: formation mechanisms and photocatalytic activity

The morphologies and surface conditions of ZnO microstructures have been controlled facilely via a microwave-assisted one-pot synthetic route by varying the ammonia concentration of the reaction mixture. Ammonia affects the nucleation, growth, and hydrolysis kinetics of a zinc glycerolate intermediate to induce shape variation from flower-like ZnO microstructures to various ZnO twin microstructures with a preferred exposure of ± (0001) polar planes. The as-prepared ZnO microstructures are mesoporous, as they have large specific surface areas and high specific pore volumes that have resulted from the microwave-assisted fast hydrolysis of the zinc glycerolate intermediate microstructures. Owing to the novel features of microwaves, the ZnO microstructures have numerous microcracks and wrinkles on their surfaces and show characteristic defect-driven orange emission, whose intensity increases with the specific surface area. The photocatalytic degradation rate constant of rhodamine B via our prepared ZnO microstructures has been found to increase linearly with the specific surface area, the specific pore volume, and the polar-surface exposure. Our simple and rapid microwave-assisted synthetic method is considered to be beneficial to the development of morphology-controlled metal oxides that are applicable for eco-friendly waste-water treatments.

Microstructure, growth process and enhanced photocatalytic activity of flower-like ZnO particles

A simple hydrothermal process was developed to synthesize flower-like ZnO microstructures that exhibited excellent photocatalytic activity.

Highly stabilized and rapid sensing acetone sensor based on Au nanoparticle-decorated flower-like ZnO microstructures

Hierarchical flower-like ZnO microstructures were synthesized by a two-step hydrothermal method. Flower-like ZnO microstructures were made up of ZnO nanorods (NBs) with the diameters of about 100 nm. Then Au nanoparticles (NPs) with the diameter of 10–20 nm were decorated on the surface of flower-like ZnO microstructures by the wet decorating process. The properties of the sensing material were analyzed with X-Ray diffraction (XRD), Scanning electron microscopy (SEM), Energy-dispersive X-ray spectroscopy (EDX), Transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM). Gas sensing device was prepared by thick film technology and tested primarily for acetone gas at different testing temperatures as well as concentrations. At the optimum operating temperature of 280 °C the sensor showed a good response and rapid response-recovery time. Furthermore, because of the stable microstructure the sensor had an excellent stability for six months.

Synthesis and enhanced photocatalytic activity of 3D flowerlike ZnO microstructures on activated carbon fiber

A hydrothermal method was utilized to attach 3D flowerlike ZnO microstructures to the surface of activated carbon fiber (ACF). The phase structure, morphology and optical property of the obtained sample were characterized. SEM demonstrated that the flowerlike structures consisted of multilayered petals, which were wholly comprised of ZnO rods. XRD analysis confirmed patterns for the wurtzite hexagonal phase of ZnO and ACF from the sample. The formation mechanism of the flowerlike ZnO grown on ACF was also discussed. The ZnO/ACF composite showed a superior photocatalytic activity in the degradation of methyl orange as compared with pure ZnO. The enhancement was attributed to a red shift, stronger UV light absorption ability and higher absorption behavior in ZnO/ACF, and the capture and transportation of photogenerated electrons by ACF.

Controlling morphologies and growth mechanism of hexagonal prisms with planar and pyramid tips of ZnO microflowers by microwave radiation

Flower-like ZnO microstructures of hexagonal prisms with planar and hexagonal pyramid tips were successfully synthesized by a microwave radiation method. The precursor solutions were synthesized from zinc nitrate hexahydrate (Zn(NO3)2·6H2O) and hexamethylenetetramine (HMT) at pH 9 and 13. The as-synthesized products were pure hexagonal wurtzite ZnO microstructured flowers of hexagonal prisms with planar tips at pH 9 and of hexagonal prisms with hexagonal pyramid tips at pH 13. Sharp and strong optical phonon Raman-active mode of E2H was detected at 438cm−1. The excitonic characters of the absorption peaks were also detected at 397nm for the first product and 384nm for the second one, and the decolorization efficiency of RhB by Hg-lamp irradiation for 140min was 77% and 87%, respectively.

PEG-assisted hydrothermal synthesis and photoluminescence of flower-like ZnO microstructures

Flower-like ZnO microstructures have been synthesized by polyethylene glycol (PEG)-assisted hydrothermal method using Zn(NO3)2·6H2O and NH3·H2O as raw materials at 120°C for 24h. The products were characterized by X-ray diffraction, scanning electron microscopy, and transmission electron microscopy. The results show that the ZnO samples prepared with PEG4000 exhibit flower-like morphology and hexagonal wurtzite structure with sizes of 5–20μm. The flower-like ZnO microstructures are mainly composed of rod-like crystals with sharp end structures. The photoluminescence spectrum of the flower-like ZnO microstructures at room temperature shows four emission peaks at 362, 392, 421 and 485nm.

Sol−gel-based hydrothermal method for the synthesis of 3D flower-like ZnO microstructures composed of nanosheets for photocatalytic applications

Self-assembled 3D flower-like ZnO microstructures composed of nanosheets have been prepared on a large scale through a sol−gel-assisted hydrothermal method using Zn(NO3)2·6H2O, citric acid, and NaOH as raw materials. The product has been characterized by X-ray powder diffraction (XRD), field-emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), and scanning electron microscopy (SEM). The optical properties of the product have been examined by room temperature photoluminescence (PL) measurements. A possible growth mechanism of the 3D flower-like ZnO is proposed based on the results of experiments carried out for different hydrothermal treatment times. Experiments at different hydrothermal treatment temperatures have also been carried out to investigate their effect on the final morphology of the ZnO. The photocatalytic activities of the as-prepared ZnO have been evaluated by photodegradation of Reactive Blue 14 (KGL) under ultraviolet (UV) irradiation. The experimental results demonstrated that self-assembled 3D flower-like ZnO composed of nanosheets could be obtained over a relatively broad temperature range (90−150°C) after 17h of hydrothermal treatment. All of the products showed good photocatalytic performance, with the degree of degradation of KGL exceeding 82% after 120min. In particular, the sample prepared at 120°C for 17h exhibited superior photocatalytic activity to other ZnO samples and commercial ZnO, and it almost completely degraded a KGL solution within 40min. The relationship between photocatalytic activity and the structure, surface defects, and surface areas of the samples is also discussed.

Growth of flower-like ZnO on polyhedron CuO fabricated by a facile hydrothermal method on Cu substrate

Flower-like ZnO on polyhedron CuO microstructures have been fabricated on Cu substrate via a simple hydrothermal method without any surfactant at low temperature (70°C). The time-dependent experiments indicated that flower-like ZnO and the polyhedron CuO crystallites were obtained when the reaction time is 30min and 1h, respectively. Based on these experiments, possible growth mechanism was proposed to account for the growth of the flower-like ZnO on polyhedron CuO. The different ionic radius and charge density of alkali metal hydroxide used to generate Zn(OH)42− ions have significant influences on the growth of ZnO. The different concentrations of alkali metal hydroxide result in different concentrations of Zn(OH)42−, which leads to the different rates of nucleation and crystal growth. The multinuclei aggregates serve as the sites for the intrinsic growth of one dimension ZnO to form early forms of flower-like ZnO microstructures under hydrothermal conditions. The strong electrolyte neutralizes the surface charges of the CuO, which causes the formation of dispersed CuO.

Sol–gel assisted hydrothermal synthesis of ZnO microstructures: Morphology control and photocatalytic activity

ZnO microstructures of different morphologies were synthesized by the sol–gel assisted hydrothermal method using Zn(NO3)2, citric acid and NaOH as raw materials. Twining-hexagonal prism, twining-hexagonal disk, sphere and flower-like ZnO microstructures could be synthesized only through controlling the pH of the hydrothermal reaction mixture at 11, 12, 13 and 14, respectively. The as-synthesized samples were characterized by X-ray diffraction (XRD), scanning electron microscope (SEM). Optical properties were examined by UV–Vis absorption/diffuse reflectance spectroscopy and room-temperature photoluminescence measurements (PL). Photocatalytic activities of the samples were evaluated by degradation of Reactive Blue 14 (KGL). The results indicated that the flower-like ZnO composed of nanosheets possessed superior photocatalytic activity to other ZnO microstructures and commercial ZnO, which could be attributed to the morphology, surface defects, band gap and surface area. The formation mechanisms of different ZnO morphologies were also investigated based on the experimental results.

Morphological evolution of flower-like ZnO microstructures and their gas sensing properties

Abstract Flower-like ZnO microstructures have been fabricated by a facile microwave hydrothermal method (HM) with the aid of benzoic acid. The obtained products were characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The morphology gradually evolved from flower bud shape to fireworks display with increase of alkalinity in the presence of benzoic acid. The formation mechanism of the flower-like ZnO microstructures was investigated briefly. Furthermore, the gas response of the flower-like ZnO microstructures has been studied to a series of organic vapors. It was found that the gas sensing properties were influenced by the size of the ZnO microstructures. The facile preparation method and the improved gas-sensing properties derived from the flower-like ZnO microstructures demonstrated their potential applications in gas sensor.

Synthesis of flower-like ZnO microstructures for gas sensor applications

In the present work, flower-like ZnO microstructures were used for solid state gas sensor applications. Well-dispersed flower-like ZnO microstructures were synthesized by hydrothermal method. The size of flower-like ZnO microstructures was about 5–7μm in radius and they were made up of dozen of ZnO nanorods building block units. Gas sensing device was prepared by thick film technology and tested primarily for NO2 gas at different testing temperatures as well as concentrations. Response increased with gas concentration as well as temperature (except 400°C). Further, it showed response for 10ppm of NO2 gas at room temperature. The response for NO2 was higher than the CO, ethanol and acetaldehyde at low operating temperatures. These flower-like ZnO microstructures showed some sort of selectivity for NO2 below 300°C.

Hydrothermal conversion of ZnO2to ZnO flowers: A mechanistic investigation and characterization

In this article, we report a novel but simple method for the phase transformation of ZnO 2 to flower-like ZnO microstructures hydrothermally at 90°C with and without the assistance of hexadecylamine as surfactant. The generation of zincate ion Zno 2 2-as a growth unit from the reaction between ZnO 2 and peroxide ion O 2 2-in situ plays a key role in the phase transformation of ZnO 2 to ZnO. The morphology, structure, and composition of the products have been investigated by scanning electron microscopy (SEM), transmission electron microscopy (TEM), Powder X-ray diffraction (PXRD) and energy dispersive X-ray analysis (EDX). It has been demonstrated that the as-fabricated ZnO flowers are composed of self-assembled brooms and rods in the presence and absence of hexadecylamine respectively. On the basis of experimental results, a possible reaction mechanism and the growth processes involved in the formation of flower-like ZnO microstructures are discussed. © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.