ZnO Microflowers Research Articles

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.

Facile synthesis of porous ZnO microbelts and analysis of their gas-sensing property

Porous ZnO microbelts were achieved using a facile chemical solution method combined with subsequent calcination. The micro-nanostructures were characterized through X-ray diffraction, field emission scanning electron microscopy, thermogravimetric-differential thermal analysis, and Brunauer–Emmett–Teller N2 adsorption-desorption analyses, among others. The BET surface area of the porous ZnO microbelts was calculated at 23.0 m² g−1. Furthermore, the gas sensing properties of the as-prepared porous ZnO microbelts were investigated using volatile organic compounds. Compared with ZnO microflowers, the porous ZnO microbelts exhibited higher response with certain organic vapors, such as formaldehyde, acetone, and ethanol. The responses to 100 ppm formaldehyde, acetone, and ethanol were 45.7, 40.6, and 38.4, respectively, at a working temperature of 300 °C. The results showed that the porous ZnO microbelts are highly promising candidates for gas sensing applications.

Sensing performance and mechanism of Fe-doped ZnO microflowers

Fe-doped ZnO microflowers have been hydrothermally synthesized without any surfactant at 120°C for 10h. The characteristics of products were examined by XRD, SEM, TG–DTA and XPS. The sensing tests reveal that the response is significantly enhanced by Fe doping, and the 3.0wt%-Fe doped sample exhibits the highest response of 604 to 10ppm NO2 at lower operating temperature of 125°C. The intrinsic sensing characteristic is attributed to be native defects in ZnO, which has been confirmed by room temperature photoluminescence PL and XPS analysis. The response time is reduced disproportionately with the increase in NO2 concentration by modeling transient responses of the sensor using L–H reaction mechanism. The band structures and densities of states for undoped ZnO and two Fe-doped supercells of Zn0.9815Fe0.0185O and Zn0.9583Fe0.0417O have been calculated using the first-principles based on the density functional theory (DFT). The calculated results show that the band gap is significantly narrowed and the conductance is increased by Fe doping, which coincide with that of experimental results of gas sensing.

Application of ZnO micro-flowers as scattering layer for ZnO-based dye-sensitized solar cells with enhanced conversion efficiency

ZnO micro-flowers were synthesized by a simple solution deposition method without the need of seed or keeping fresh deposition solution. The synthesized ZnO micro-flowers were used as scattering layer for dye-sensitized solar cells (DSSCs) fabricated with ZnO nanoparticles photoanode. The UV–vis diffused reflectance absorption spectra (DRS), electrochemical impedance spectroscopy (EIS) and photoinduced open-circuit voltage decay (OCVD) measurements show that the ZnO micro-flowers/nanoparticles bilayer film-based solar cell has much higher light harvesting efficiency, lower resistance and longer electron lifetimes as compared with the ZnO nanoparticles film-based one, and consequently resulting in an improved conversion efficiency due to the scattering effect of ZnO micro-flowers layer. The simple methods of the fabrication of both the ZnO micro-flowers and the ZnO bilayer film-based solar cell, which was fabricated with photoanode consisting of ZnO nanoparticles as underlayer and ZnO micro-flowers as overlayer, are promising for the development of the low-cost, eco-friendly, and durable devices with high light-to-electricity conversion efficiency.

Fabrication of Smart Chemical Sensors Based on Transition-Doped-Semiconductor Nanostructure Materials with µ-Chips

Transition metal doped semiconductor nanostructure materials (Sb2O3 doped ZnO microflowers, MFs) are deposited onto tiny µ-chip (surface area, ∼0.02217 cm2) to fabricate a smart chemical sensor for toxic ethanol in phosphate buffer solution (0.1 M PBS). The fabricated chemi-sensor is also exhibited higher sensitivity, large-dynamic concentration ranges, long-term stability, and improved electrochemical performances towards ethanol. The calibration plot is linear (r2 = 0.9989) over the large ethanol concentration ranges (0.17 mM to 0.85 M). The sensitivity and detection limit is ∼5.845 µAcm−2mM−1 and ∼0.11±0.02 mM (signal-to-noise ratio, at a SNR of 3) respectively. Here, doped MFs are prepared by a wet-chemical process using reducing agents in alkaline medium, which characterized by UV/vis., FT-IR, Raman, X-ray photoelectron spectroscopy (XPS), powder X-ray diffraction (XRD), and field-emission scanning electron microscopy (FE-SEM) etc. The fabricated ethanol chemical sensor using Sb2O3-ZnO MFs is simple, reliable, low-sample volume (<70.0 µL), easy of integration, high sensitivity, and excellent stability for the fabrication of efficient I–V sensors on μ-chips.

Nanosheet‐Assembled ZnO Microflower Photocatalysts

Large scale ZnO microflowers assembled by numerous nanosheets are synthesized through a facile and effective hydrothermal route. The structure and morphology of the resultant products are characterized by X‐ray diffraction (XRD) and scanning electron microscope (SEM). Photocatalytic properties of the as‐synthesized products are also investigated. The results demonstrate that eosin red aqueous solution can be degraded over 97% after 110 min under UV light irradiation. In addition, methyl orange (MO) and Congo red (CR) aqueous solution degradation experiments also are conducted in the same condition, respectively. It showed that nanosheet‐assembled ZnO microflowers represent high photocatalytic activities with a degradation efficiency of 91% for CR with 90 min of irradiation and 90% for MO with 60 min of irradiation. The reported ZnO products may be promising candidates as the photocatalysts in waste water treatment.

ZnO Multi-Branched Microflowers: Fabrication and Temperature-Dependent Photoluminescence Properties

ZnO multi-branched microflowers were synthesized on Si substrate using a simple vapor transport process without any catalysts. X-ray diffractometer (XRD), Raman spectroscopy and field emission scanning electron microscopy (FESEM) were used to characterize the morphologies and structures. The XRD results showed that the as-grown ZnO microflowers possessed hexagonal wurtzite structure with preferred c-axis orientation. The FESEM results indicated that the microflowers had multi-branches, and each branch showed the nanonail morphology with the length of about 1.5 mu m. Their photoluminescence properties were found to be strongly temperature-dependent. The different transitions including free exciton emission, bound exciton emission and the free-to-bound (FB) transition were clearly observed at 86 K. The bound exciton emission and FB emission became weak with the increase of the testing temperature, while the free exciton (FE) emission got relatively strong. The 1-LO phonon replica of FB was rather strong in the multi-branched ZnO microflowers.

Preparation and surface modification of hierarchical nanosheets-based ZnO microstructures for dye-sensitized solar cells

This paper reports a simple one-step hydrothermal route for the preparation of hierarchical nanosheets-based ZnO microstructures and their application to dye-sensitized solar cells. The morphologies of the products were controlled by the dosage of the reactants. Their physical characteristics were detected by X-ray diffraction, a field-emission scanning electron microscope and a surface analyzer. It is proved that the sample of ZnO microspheres with larger surface area and stronger light-trapping capacity since the superiority of their entirely spherical structures exhibits better photoelectrochemical properties than the mixtures of ZnO microspheres and ZnO microflowers. A dye-sensitized solar cell assembled by the ZnO microspheres as photoanode shows an energy conversion efficiency of 2.94% after surface modification by tetrabutyl titanate solution at 90°C. This result is over 1.6 times higher than the non-modified cell fabricated by the ZnO microspheres on the basis of the external improvement and the stability enhancement for the dye-sensitized ZnO photoanode.

Poly(3,4-ethylenedioxythiophene) (PEDOT) hollow microflowers and their application for nitrite sensing

A film of poly(3,4-ethylenedioxythiophene) hollow microflowers (PEDOT-HMF) was electrodeposited on a fluorine-doped tin oxide (FTO) glass substrate by using a film of ZnO microflowers as the template. Each of the PEDOT hollow microflowers shows several 2D nanopetals. Various charge densities were applied for the deposition of the PEDOT film, intending to investigate the charge effect on the film's morphology. The FTO glass with the film of PEDOT-HMF (hereafter PEDOT-HMF modified electrode) was used as a nitrite sensor. A dramatic improvement was observed in the sensitivity of the PEDOT-HMF electrode, with reference to that of a flat PEDOT electrode. Through an amperometric detection, the sensitivity and limit of detection (LOD) of the sensor were found to be 255.2μAmM−1cm−2 and 0.59μM, respectively. The linear range of the PEDOT-HMF electrode was between 50 and 7500μM, and was observed to be larger than that of the corresponding PEDOT electrode. An interference effect was studied, using different kinds of salts that contain anionic interferences in the detection of nitrite. All the ions of the salts, except SO32−, showed negligible effect on the determination of nitrite.

Controlled synthesis and photocatalytic properties of three dimensional hierarchical ZnO microflowers

Controlled synthesis and photocatalytic properties of three dimensional hierarchical ZnO microflowers

Low temperature synthesis of single crystal ZnO microflower composed of hexagonal nanorods

In this letter, a facile synthesis of 2D-flowerlike ZnO was reported. The ZnO was synthesized by a simple addition of zinc salt with potassium hydroxide in water without using any template and surfactant at low temperature. The synthesized ZnO was characterized by X-ray powder diffraction (XRD), Field emission scanning electron microscopy (FE-SEM), and High Resolution Transmission Electron Microscopy (HR-TEM). The XRD pattern indicated the formation of well crystallized Wurtzite ZnO and the FE-SEM results confirmed the formation of flowerlike morphological ZnO. The HR-TEM analysis indicated that the hexagonal nanorods were self-assembled by nanoparticles and microflower morphology was composed of hexagonal rods with the aspect ratios of about 4.2. The self-assembly mechanism was proposed for the formation of hexagonal nanorod and microflower like ZnO.

Enhanced photocatalytic activity of ZnO microflower arrays synthesized by one-step etching approach

A new type of ZnO microflower arrays was successfully synthesized via a one-step etching route based on chemical bath disposition (CBD) method without any assistant reagents. The inner and outer wall surfaces of ZnO microflowers are quite smooth with the average thickness of 50nm. The formation mechanism is proposed that ZnO microflowers were evolved from microrods by preferential etching in reaction solution. ZnO microrod arrays and microcandle arrays were also obtained by varying reaction time. Photoluminescence and photocatalysis properties of ZnO samples were investigated at room temperature. Superior photocatalytic activity of ZnO microflower arrays in the degradation of RhB compared to other samples under identical conditions has been demonstrated, and the origin is mainly ascribed to the increasing surface area and higher oxygen vacancies density of ZnO microflower with partial hollow structure.

Hybrid Ag2O/ZnO Heterostructures

The well‐aligned Ag2O/ZnO microflowers heterostructure was synthesized by a straightforward two‐step procedure. The diameters of the as‐synthesized products were as much as 1.5 μm. The as‐grown Ag2O/ZnO heterostructure was investigated by X‐ray powder diffraction (XRD), scanning electron microscope (SEM), transmission electron microscope (TEM), and photoluminescence (PL) spectroscopy analysis. A possible growth mechanism for flowerlike Ag2O/ZnO heterostructure was proposed based on the experimental results. Compared with pure ZnO microflowers, PL spectrum of the composite with only one strong peak at 383 nm showed good intrinsic emission.

Hollow microflower arrays of PEDOT and their application for the counter electrode of a dye-sensitized solar cell

Hollow microflower arrays (HMFAs) of poly(3,4-ethylenedioxythiophene) (PEDOT) with several two dimensional hollow nanopetals on each microflower are fabricated on a conducting glass by using ZnO microflower arrays as the template. Various charges are applied for the electrodeposition of the film of PEDOT-HMFAs, intending to investigate their effect on the film's morphology. A morphological variation is observed due to the swelling of PEDOT during the removal of the ZnO template. Cyclic voltammetry (CV) is used to optimize the charges for the deposition of the films of both a flat PEDOT and the PEDOT-HMFAs. Long-term stability of the films in an I−/I3− electrolyte is studied by CV. The PEDOT-HMFA film shows a better stability than those of the films of flat PEDOT and sputtered Pt. The PEDOT-HMFA film is employed as the catalytic material on the counter electrode (CE) of a dye-sensitized solar cell (DSSC). A power conversion efficiency of 7.20% is achieved, at 100 mW cm−2 for the DSSC with the film of PEDOT-HMFAs, which is much higher than that of the cell with the flat PEDOT (6.39%) and comparable to that of the cell with a sputtered Pt film on its CE (7.61%). Electrochemical impedance spectroscopy is used to substantiate the photovoltaic parameters. “Hemispherical diffusion of ions” occurs on each PEDOT hollow microflower of the CE with PEDOT-HMFAs in the DSSC, as against the linear diffusion occurring on the CE with flat PEDOT or sputtered Pt. This type of “hemispherical diffusion of ions” is explained to result in a smaller diffusion resistance of ions and thereby in a much higher fill factor for the DSSC using the CE with PEDOT-HMFAs.

ZnO micro-flowers assembled on reduced graphene sheets with high photocatalytic activity for removal of pollutants

In this paper, a simple and efficient strategy was developed for preparing flower-shaped ZnO doped reduced graphene oxide (RGO) composite material by simultaneous crystal growth (of ZnO) and reduction (of graphene oxide) using hydrothermal process. Several analytical techniques, including scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), FT-IR spectroscopy, Raman spectroscopy, and photoluminescence spectroscopy have been used to characterize the resulting ZnO/RGO photocatalyst. Results showed that highly crystalline ZnO nano-flowers were effectively doped on the surface of RGO sheets. The unique properties of RGO sheets not only provide the decreased rate of electron–hole separation in ZnO but also prevent from the loss of photocatalyst during recovery due to the fixed attachment of ZnO NPs on the surface of high aspect ratio graphene sheets. Therefore, as-synthesized composite is an economically and environmentally friendly photocatalyst.

Self-enhanced controllable growth of ZnO micro-flowers from nanospikes and its transformation to nanoparticles by using compositional variation: Essential dielectric switching applications

We are reporting a large scale distribution of zinc oxide nanospikes of length from 745 to 900nm those were grown as a micro-flowers with diameter of 1.2μm. At low temperature, nanocapsules of diameter 186nm and length 576nm were gathered along with nanorods of diameter ranging from 644 to 966nm and those were obtained by the variation of chemical composition of capping reagent hydrazine hydrate. By introducing a surfactant cetyltrimethylammonium bromide instead of hydrazine, we obtained uniform sized nanoparticles of diameter 150nm with well crystallinity by applying high temperature and pressure inside the stainless steel autoclave. A detailed study of optical properties and dielectric spectra revealed a low threshold voltage with 0.45V for nanoparticles in compare to higher threshold voltage 4V for nanospikes having dielectric constant 1.9. We also observed a bi-stable dc switching capability of such nanostructures.

Assembling ZnO Nanorods into Microflowers through a Facile Solution Strategy: Morphology Control and Cathodoluminescence Properties

AbstractIn this work, flowerlike ZnO micro/nanostructures assembled from nanorods are obtained through a facile hydrothermal route. The experimental results indicated that the as-synthesized ZnO microflowers have an average diameter of 2 µm, composed of nanorods of an average diameter of 200 nm and a tapered morphology. ZnO with other morphologies were also obtained by varying the reaction conditions. Systematical condition-dependent experiments were conducted to reveal the growth mechansim of the microflowers. It is suggested that the zinc source types, solution pH value, and reaction temperature, as well as reaction time are responsible for the variations of ZnO morphology. Luminescence properties of ZnO microflowers were investigated through monitoring different parts of nanorods, showing good optical quality.

Controlled Hydrothermal Synthesis of ZnO Nano and Microstructure Materials with Photocatalytic Properties

Various ZnO nano and microstructures, such as nanorods, microflowers composed of nanorods, and microrods were hydrothermally self-assembled. The morphology of the formed ZnO nano and microstructures could be easily tuned by varying the experimental parameters of the NaOH concentration. The crystal structure of samples was investigated by XRD, and the diffraction peaks were indexed to hexagonal wurtzite structure. A possible growth mechanism was proposed on the basis of the morphology evolution of nano and microstructures observed by SEM. Furthermore, the photoluminescence and photocatalytic properties of ZnO nano and microstructures were investigated. The results show that the ZnO microrods and microflowers exhibited higher photodegradation efficiency than that of ZnO nanorods, which maybe due to the exposed polar faces of better crystalline rather than the size of ZnO.

Photocatalytic activity of zinc oxide micro-flowers synthesized via solution method

ZnO micro-flowers composed of hexagonal nanorods were successfully fabricated by soft chemical technique from zinc acetate di-hydrate (Zn(CH 3COO) 2·2H 2O) and sodium hydroxide (NaOH) in 1 h refluxing. The crystallinity of the grown product was analyzed by X-ray diffraction technique revealing the as-prepared micro-flowers have hexagonal wurzite structure. Their morphology was investigated by using FESEM and TEM, which showed that well-dispersed micro-flowers of 2–3 μm in diameter had been formed. Furthermore, the synthesized micro-flowers showed a better photo-catalytic activity in presence of methylene blue (MB) dye. Also, the Zn 2p1/2, Zn 2p3/2, and O 1s peaks in X-ray photoelectron spectroscopy clearly suggest that the grown nanostructures are pure ZnO. Additionally, high performance liquid chromatography (HPLC) and liquid chromatography–mass spectroscopy (LC–MS) were applied to observe the degradated compounds from the photo-catalytic degradation of methylene blue (MB) dye in presence of zinc oxide micro-flowers.

ZnO hierarchical structures for efficient quasi-solid dye-sensitized solar cells

We report a direct precipitation method for mass production of ZnO microflowers (MFs) containing hierarchical structures. The ZnO MFs are constructed by interlaced single crystalline and porous nanosheets which are ideal photoanode material for dye-sensitized solar cells (DSCs) because the MFs can largely improve the energy harvesting performance and the efficiency of DSCs. Compared with other forms of nano-sized structures, the novel hierarchical structures show obvious advantages in DSC application because of their large surface area for dye-loading, good light scattering efficiency and excellent electrical transport property. The quasi-solid state DSCs fabricated with the MF hierarchical structures exhibited an efficiency of 4.12%, much higher than that of ZnO nanoparticle-based DSCs, indicating a great potential for the development of highly-efficient quasi-solid DSCs.