ZnO Flowers Research Articles

Solution-derived ZnO nanoflowers based photoelectrodes for dye-sensitized solar cells

Flower-like ZnO nanostructures, composed of hexagonal nanorods were manipulated, via a low temperature hydrothermal method from an aqueous solution using polyethyleneimine (PEI) as a surfactant in the seed sole. It was found that the PEI in the seed sole not only affected the geometrical shape of ZnO flowers but also changed the length and spacing of the petals. These flower-like nanostructures were used as photoelectrodes in dye-sensitized solar cells. Compared to rod-like flowers (sample X1), the blade-like flowers (sample X2) nanostructures demonstrate increased power conversion efficiency (η) of about 106%. Meanwhile, short-circuits current density (JSC), open-circuit voltage (VOC), and fill factor (FF) are all substantially improved. The enhancement of η, VOC and FF in the blade-like-flower photoelectrodes were mainly ascribed to the enlargement of internal surface area between the blades for higher dye loading and the improved light harvesting from efficient light scattering. The band gap energies were 3.31eV and 3.27eV for rod-like flowers and blade-like flowers respectively. The decrease in the optical band gap with the increase of film roughness was due to decrease of lattice defects.

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.

Morphology Controlling of ZnO Sub-Micron- and Micro-Structures from Sub-Micron Zinc Citrate Precursor.

A wet-chemical route has been demonstrated to investigate the morphology evolution of high crystalline ZnO sub-micron- and micro-structures created from a zinc citrate precursor consisted of zinc citrate nanoparticles. The concentrations of precursor zinc citrate and the addition of trisodium citrate were key factors in the controlling of ZnO micro-morphology. Assembled growth resulted in the formation of ZnO sub-micron- and micro-structure with twin-cone and flower-like morphologies. The ZnO flower was consisted of cone petals. The shape of ZnO microstructures was further adjusted using trisodium citrate to created thin and thick hexagonal-plates. In the case of a high Zn concentration, thick hexagonal-plates were split into a flower-like morphology. The investigation of morphological evolution indicated that trisodium citrate is critical to control the growth rate of polar (0001) plane. The formation of a flower-like structure is ascribed to the assembly of crystal units with a high zinc citrate concentration.

An impedance match method used to tune the electromagnetic wave absorption properties of hierarchical ZnO assembled by porous nanosheets

In this study, three hierarchical porous ZnO flowers were prepared via calcination of the precursor at different temperatures (400 °C, 500 °C and 600 °C). It can be found that the calcination temperature influences the porous structure and size of the ZnO nanoparticles. The porous structure can effectively tune the permittivity to improve the impedance match. The porous ZnO flowers obtained at 400 °C show a large surface area and small size of ZnO nanoparticles and thus, hold good impedance match. Moreover, the porous ZnO flowers self-assembled by nanosheets can induce multiple reflection and scattering, which prolong the microwave travel path and consume more electromagnetic energy. Therefore, the porous ZnO flowers obtained at 400 °C exhibit outstanding microwave absorption properties. The minimal reflection loss is −46.1 dB at 13.6 GHz with only a thickness of 1.9 mm. The absorption bandwidth of RL less than −10 dB can reach up to 11.5 GHz (6.5–18.0 GHz) by adjusting the absorber thickness (1.5–4.0 mm). The lightweight, small thickness, and strong and tunable absorbing properties of the porous ZnO flowers make it attractive for developing a high-performance electromagnetic wave absorber.

Regeneration of an efficient, solar active hierarchical ZnO flower photocatalyst for repeatable usage: controlled desorption of poisoned species from active catalytic sites

Regeneration of hierarchical ZnO flower photocatalyst after repeatable usage by desorption of poisoned species from surface active site.

Highly selective detection of dimethyl methylphosphonate (DMMP) using CuO nanoparticles /ZnO flowers heterojunction

Here we report the fabrication of high surface area CuO nanoparticles (NPs) on micron-scale ZnO (CuO/ZnO) “flowers” with dimethyl methylphosphonate (DMMP) gas sensing capabilities. The formation of CuO NPs/ZnO heterojunction structures was confirmed by PXRD and TEM analyses. The gas sensing properties of the CuO NPs/ZnO structures showed a faster response time (26.2s) compared to the exclusively ZnO-based sensor (330s). The heterojunction sensors demonstrated the highest selectivity in 10ppm DMMP, reaching the high value of 626.21 at 350°C. This CuO NPs/ZnO heterojunction structure provides an extension of the depletion layer and an increase of the resistance (Ra) in air, leading to a reduction of the depletion layer and resistance (Rg) when exposed to reducing DMMP gas. The higher surface area (6.0m2/g) of the CuO/ZnO heterojunction structure with a 0.5h synthesis time of the ZnO flowers further promoted the adsorption kinetics for the reaction between C3H9O3P and O2− when exposed to DMMP, thus enhancing its sensing properties.

Surfactant free, simple, morphological and defect engineered ZnO nanocatalyst: Effective study on sunlight driven and reusable photocatalytic properties

Abstract We demonstrate the facile synthesis of solution grown ZnO structures (rods, buds, brooms, spindles, stars, flowers, multipods) without using any surfactant and capping agents. Morphology of the nanostructures was simply controlled by varying the Zn supersaturation in the growth solution which transforms the growth mode from anisotropic to isotropic and then to hierarchical nature. All photocatalysts were characterized by X-ray diffraction, UV–vis absorption, photoluminescence, scanning electron microscopy, high resolution transmission electron microscopy analysis. ZnO nanoflowers show excellent photocatalytic (PC) degradation efficiency above 95% against Methyl orange, Rodamine B and Methylene blue dyes in 120 min under natural solar irradiation compared to the other nanostructures. It is noteworthy that the PC performance of the ZnO nanostructures correlates well with the defect states and the production of photo induced reactive oxide species (ROS). Investigations show that the enhanced PC performance of ZnO nanoflowers was due to the synergy of larger surface area and oxygen related defects assisted ROS production. The photocatalytic recycling ability of ZnO nanoflowers was investigated. ZnO flowers showed excellent recyclability with a slight decrease in photocatalytic efficiency after being subjected to ten consecutive cycles.

ZnO flower/PEDOT:PSS thermoelectric composite films

ZnO flower/poly(3,4-ethylenedioxythiophene):poly(4-styrenesulfonate) (PEDOT:PSS) composite films were prepared by spin-coating dimethyl sulfoxide doped PEDOT:PSS on the ZnO flowers grown on glass substrate. The thermoelectric properties of the ZnO flower/PEDOT:PSS composite films were measured at room temperature. As the number of spin coated PEDOT:PSS layer increased, the electrical conductivity of the ZnO flower/PEDOT:PSS composite films increases dramatically from 1-layer (177.3 S/m) to 4-layer (910.4 S/m), however, all the composite films have almost the same Seebeck coefficient (~20–22 μV/K). A maximum power factor of ~0.4 μWm−1 K−2 at room temperature was obtained from the composite film with 4-layer PEDOT:PSS.

Synthesis, characterization and photocatalytic activity of ZnO flower and pseudo-sphere: Nonylphenol ethoxylate degradation under UV and solar irradiation

ZnO particles (flower and pseudo sphere) were synthesized via precipitation route and characterized using X-ray diffraction (XRD), Scanning electron microscopy (SEM), Energy dispersive X-ray analysis (EDX), Fourier transform infra-red (FTIR) spectroscopy, Atomic force microscopy (AFM), Particle size analyzer and UV–visible techniques. The photocatalytic activities (PCA) of ZnO flower (uncalcined) and pseudo-sphere (calcined) were evaluated by degrading nonylphenol ethoxylate-9 (NP9EO) under UV and solar irradiation. The process variables i.e., catalyst dose, calcination temperature, H2O2 concentration, pH and UV/solar light exposure were investigated and under optimum conditions of process variables, paper, textile and leather industries effluents were also treated. Calcination at high temperature affected the morphology of ZnO particles. Both ZnO flower and pseudo-sphere degraded NP9EO and pollutants in industrial wastewater efficiently under both UV and solar irradiation. Maximum NP9EO degradation was achieved at 2.5 g/L catalyst dose, high calcination temperature, 4% H2O2 concentration, 6 pH, 110 UV exposure and 12 h solar light exposure. Results reveal that ZnO is efficient photo-catalyst and could be used under solar irradiation for photocatalytic application by tuning the band gap.

ZnO flower: Self-assembly growth from nanosheets with exposed [formula omitted] facet, white emission, and enhanced photocatalysis

ZnO flowers consisting of single crystal nanosheets with exposed {1 1¯ 0 0} facets were fabricated from trisodium citrate via hydrothermal at 180°C. The single crystal nanosheet has the thickness of about 70nm and a well-crystalline structure with dominant surfaces as {1 1¯ 0 0} planes. The site-specific nucleation-growth process contributes to the formation of hierarchical flower-like ZnO structures. The ZnO flowers exhibited white emission. The visible emission gradually decreased with time and the UV emission suggests that the recombined rate of photogenerated electrons and holes of samples varied with the synthesis parameters. The ZnO flowers displayed an enhanced photocatalytic performance compared with ZnO microspheres. The maximized exposure of the reactive {1 1¯ 0 0} facets also favors the enhanced photocatalytic performance. Additionally, the special loose structural feature with an open microstructure has more important influences on the photocatalytic activity than specific surface area.

Synthesis and characterization of ZnO flower-like structures

The flower-like ZnO nanorods have been successfully synthesized by a surfactant-assisted hydrothermal method. The nanostructures formed by ZnO nanorods were synthesized and deposited without seeding in glass flask by a hexamethylenetetramine (HMTA)-assisted hydrothermal method at low temperature with NaOH as surfactant and catalyst. The synthesized ZnO flowers comprise of several spike structures that have hexagonal cross section and taper toward the end. The structures are investigated using X-ray diffraction, scanning electron microscope and transmission microscope. The optical properties are studied with UV-VIS, FTIR, and Raman spectroscopy. The process of synthesis is simple and highly reproducible. The synthesized flower-like structures are suitable for use as sensors applications.

Photochemical performance of ZnO nanostructures in dye sensitized solar cells

In this work, the photoconversion efficiencies of ZnO having diverse microstructures and structural defects have been investigated. A conversion efficiency of 1.38% was achieved for the DSSCs fabricated with as prepared ZnO nanorods having minimum vacancy defects and a favourable one dimensional directional pathway for electron conduction. The DSSCs fabricated with ZnO nanoparticles exhibited relatively low conversion efficiency of 1.004% probably due to multiple trapping/detrapping phenomena within the grain boundaries and ZnO flowers though exhibited a high dye adsorption capability exhibited the lowest conversion efficiency of 0.59% due to a high concentration of structural defects. Based on the experimental evidences, we believe that the type of defects and their concentrations are more important than shape in controlling the overall performance of ZnO based DSSCs.

PEG-20000 assisted hydrothermal synthesis of hierarchical ZnO flowers: Structure, growth and gas sensor properties

The hierarchically structured materials usually show improved physical and chemical properties. Here, using simple hydrothermal method and reagents of polyethylene glycol (PEG-20000), we synthesize the ZnO crystals and find that they exhibit hierarchical flower-like architectures assembled by nanorods. Further comparative studies demonstrate that PEG provides nucleation sites for the assembling of the nanorods, which play a critical role in producing such unique flower-like architectures. Consequently, the sensor made of ZnO nanoflowers is found to show good gas sensing performance to the hydrothion gas.

Effect of electrodeposition duration on the morphological and structural modification of the flower-like nanostructured ZnO

Flower-like morphology of ZnO thin films were successfully electrodeposited via a one-step route only by changing electrodeposition duration and potential at a fixed concentration on n-Si (100) in 78.10−4 M Zn (NO3)2, 6H2O and 0.1 M KNO3 solution at 70 °C without using any catalyst, additives or seed layer. The as electrodeposited products were characterized by X-ray diffraction, scanning electronic microscopy and photoluminescence. The effects of the electrodeposition potential and duration on the morphology, structure and photoluminescence properties were studied. The results show that the product morphology changes with a change in the applied voltage and electrodeposition duration. Well defined shape of flower-like ZnO morphology was generated under the limited conditions of −1.2 V for 60 min, with (002) preferred orientation. Strong UV emission is obtained in the case of ZnO flower like morphology electrodeposited at −1.2 V for 60 min. The band-gap energy value of ZnO flower-like, determined from optical reflectance spectra, is 3.37 eV.

Hierarchical ZnO Nanostructures with Blooming Flowers Driven by Screw Dislocations

Hierarchical ZnO nanostructures with a large yield were fabricated by a simple thermal evaporation method. For the first time, novel ZnO flowers were observed blooming at certain sites of a variety of spines, identified as Zn-terminated polar (0001) planes or tips. The spines for as-synthesized hierarchical structures can be nanowires, nanobelts, nanodendrites, nanobrushes, etc. This growth phenomenon determines the key role of polar sites in the fabrication of hierarchical structures. The spiral feature of ZnO flowers indicates an unusual screw dislocation driven growth mechanism, which is attributed to a high concentration of Zn vapor.

Fabrication of porous 3D flower-like Ag/ZnO heterostructure composites with enhanced photocatalytic performance

Porous 3D flower-like Ag/ZnO heterostructural composites were fabricated by hydrothermal and photochemical deposition methods, without using any pore-directing reagents and surfactants. The obtained samples were characterized by XRD, SEM, TEM, XPS, BJH, DRS, and PL spectrum. The experiment results show that the silver nanoparticles successfully load on the surface of assembled ZnO flowers. The TEM and SEM morphologies demonstrated unique porous 3D flower-like structure of Ag/ZnO. Such special structure makes larger surface area and more active sites exposed during the reaction, facilitating the transportation of reactants and products and increasing the reaction rate. The photocatalytic degradation experiments under UV irradiation using Rhodamine B (RhB) as a model dye were executed. The relative results demonstrate that the photocatalytic activity of Ag/ZnO is obviously improved compared with the pure ZnO and the commercial TiO2 (Degussa P25), the AZ-15 sample has the highest photocatalytic activity. The Ag/ZnO heterostructure composites promoted the separation of photo-induced electrons and holes, which was proved by photoluminescence spectra (PL).

Preparation of ZnO flower/reduced graphene oxide composite with enhanced photocatalytic performance under sunlight

Reduced graphene oxide (RGO) coated ZnO flower was synthesized by a simple one-pot hydrothermal method. The morphology and properties of ZnO/RGO composites were characterized by UV–vis spectroscopy, Raman spectroscopy, SEM, TEM, EDX, XRD, photoluminescence spectroscopy and photocurrent measurement. It was found that the ZnO flowers were effectively covered with RGO sheets and formed a network structure. The photocatalytic activities of the as-prepared composites were investigated by photodegrading methylene blue under both UV light and sunlight. Results show that the ZnO/RGO composites exhibited a remarkably enhanced photocatalytic efficiency compared to pure ZnO flowers. The cause of the enhanced photocatalytic performance could be ascribed to the synergistic effect between ZnO flowers and RGO sheets. Moreover, we found that the content of graphene oxide introduced into composite material was a crucial factor for its improved photocatalytic performance.

Hydrothermal Synthesis of ZnO Crystals from Zn(OH)2 Metastable Phases at Room to Supercritical Conditions

The originality of this work is to highlight the effect of temperature and pressure on the size and morphology of hydrothermal ZnO particles from ambient to supercritical conditions (T > 374 °C and P > 221 bar) using a unique continuous one-step process. Experiments were carried out from zinc nitrate (Zn(NO3)2) and potassium hydroxide (KOH) solutions in the ranges of 1–300 bar and 30–400 °C. The as-prepared particles of ZnO (flower, ellipsoid, and sphere) and ε-Zn(OH)2 (polyhedral) sized from nano to micrometers were characterized by X-ray diffraction and electronic microscopy. The wulfingite phase (ε-Zn(OH)2) was detected inside some powders especially at room temperature for higher pressures. The proportion of each phase was determined by the quantitative Rietveld analysis. On the basis of Loüer’s method, ZnO crystals exhibiting a hexagonal structure were considered as cylinders with a diameter D and a height H. The D/H parameter, also known as the aspect ratio, was correlated with electronic microscopy observations to investigate the ZnO nanoparticle morphology evolution according to the temperature and pressure. Thus, a schematic synoptic summarizing these results is presented.

Light extraction enhancement of GaN based light emitting diodes by ZnO nanorod arrays.

This study examined the effects of the alignment of ZnO nanorod arrays (NRAs) on the light extraction enhancement of GaN-based light emitting diodes (LEDs), where ZnO NRAs were synthesized hydrothermally. The shape of the ZnO NRAs was controlled using seed layers for flower and vertical structures. Numerical analysis based on the two-dimensional (2D) finite difference of time domain (FDTD) method showed that the extraction efficiency of LED with bare (without ZnO NRA), vertical ZnO NRA and flower shaped ZnO NRA was 37%, 60% and 49%, respectively. The optical output power of the LEDs with vertical ZnO NRA and flower ZnO NRA was improved by 50% and 30% compared to that of the LED without ZnO NRA at an input current of 100 mA. These results suggest that the vertical alignment of ZnO NRA is important for enhancing the light extraction of GaN based LEDs.

Naturally oxidized synthesis of ZnO dahlia-flower nanoarchitecture

Three dimensional ZnO dahlia-flowers have been engineered at room temperature relying on natural oxidation based aqueous chemical synthetic approach. Glycine abetted multicomponent isotropic morphology has been synthesized through the conglomeration of thin nanopetals as building blocks with highly large surface area to volume ratio. Highly controllable morphology of thin nanopetals is achieved by stabilizing their polar faces through the adsorption of reactive hydroxyl and amide functions of glycine. Fourier transform infrared conclusions also exemplify good corroboration towards the use of organic additives. Moreover, the synthesized ZnO flowers have been utilized to fabricate cholesterol biosensor and biosensing measurements which have been performed over the cholesterol concentration range of 1×10−6M to 1×10−3M.