Low temperature solution synthesis and characterization of ZnO nano-flowers

Synthesis of flower-shaped ZnO nanostructures composed of hexagonal ZnO nanorods was achieved by the solution process using zinc acetate dihydrate and sodium hydroxide at very low temperature of 90 °C in 30 min. The individual nanorods are of hexagonal shape with sharp tip, and base diameter of about 300-350 nm. Detailed structural characterizations demonstrate that the synthesized products are single crystalline with the wurtzite hexagonal phase, grown along the [0 0 0 1] direction. The IR spectrum shows the standard peak of zinc oxide at 523 cm⁻¹. Raman scattering exhibits a sharp and strong E₂ mode at 437 cm⁻¹ which further confirms the good crystallinity and wurtzite hexagonal phase of the grown nanostructures. The photoelectron spectroscopic measurement shows the presence of Zn, O, C, zinc acetate and Na. The binding energy ca. 1021.2 eV (Zn 2p_3/2) and 1044.3 eV (Zn 2p_1/2), are found very close to the standard bulk ZnO binding energy values. The O 1s peak is found centered at 531.4 eV with a shoulder at 529.8 eV. Room-temperature photoluminescence (PL) demonstrate a strong and dominated peak at 381 nm with a suppressed and broad green emission at 515 nm, suggests that the flower-shaped ZnO nanostructures have good optical properties with very less structural defects.     

Microscale sphere assembly of ZnO nanotubes

In this paper, we report a facile method to perform microsphere organization of ZnO nanotubes. Study shows ZnO nanotubes with diameters of 80-150 nm and lengths of 1.2-2 μm were assembled into 3-4.5 μm microsphere superstructures, the significance of these assembled ZnO nanotubes is that one-dimensional anisotropic ZnO building blocks are selectively aligned into highly symmetrical three-dimensional isotropic conformations, which may promise us new types of ZnO applications. These microsphere assemblies of ZnO nanotubes have a wide visible yellow emission at 600 cm⁻¹. The assembly mechanism was also proposed, the addition of metallic zinc species into linear poly ethylene glycol (PEG) containing ethanol and water mixed solution led to aggregation of PEG polymer coils to Zn(II)/PEG globules with a diameter of ∼500 nm, the globule turned into tube-like-structured ZnO-PEG microsphere assemblies (∼2 μm) after ultrasonic pretreatment, ZnO nanotubes were formed by the oriented attachment and Ostwald ripening of ZnO growth units onto the tube-like structures during the following hydrothermal process.     

Synthesis of ZnO flowers and their photoluminescence properties

Flower-like ZnO nano/microstructures have been synthesized by thermal treatment of Zn(NH₃)₄²⁺ precursor in aqueous solvent, using ammonia as the structure directing agent. A number of techniques, including X-ray diffraction (XRD), field emission scan electron microscopy (FESEM), transmission electron microscopy (TEM), thermal analysis, and photoluminescence (PL) were used to characterize the obtained ZnO structures. The photoluminescence (PL) measurements indicated that the as-synthesized ZnO structures showed UV (∼375 nm), blue (∼465 nm), and yellow (∼585 nm) emission bands when they were excited by a He-Gd laser using 320 nm as the excitation source. Furthermore, it has been interestingly found that the intensity of light emission at ∼585 nm remarkably decreased when the obtained ZnO nanocrystals were annealed at 600 °C for 3 h in air. The reason might be the possible oxygen vacancies and interstitials in the sample decreased at high temperature.     

Synthesis of Sn-doped ZnO nanorods and their photocatalytic properties

Sn-doped ZnO nanorods were fabricated by a hydrothermal route, and characterized by X-ray diffraction, field emission scanning electron microscope, UV-vis spectroscopy, Raman spectra, solid-state nuclear magnetic resonance (NMR) spectra, and room temperature photoluminescence spectroscopy. Solid-state NMR result confirms that Sn⁴⁺ was successfully incorporated into the crystal lattice of ZnO. Room temperature photoluminescence showed that all the as-synthesized products exhibited a weak UV emission (380 nm) and a strong visible emission (540 nm), but the intensities of the latter emission increased with increase in Sn concentration. The improvement of visible emission at 540 nm in the Sn-doped ZnO samples was suggested to be a result of the lattice defects increased by doping of Sn in zinc oxide. In addition, the photocatalytic studies indicated that Sn-doped ZnO nanorods are a kind of promising photocatalyst in remediation of water polluted by some chemically stable azo dyes.     

Synthesis of novel zinc oxide microphone-like microstructures

Novel microphone-like ZnO microstructures were grown at a very high density via a simple thermal evaporation process using commercially available ZnO powder in ambient air at ∼ 1050 ± 20 °C in 1 h. The unique as-grown microstructures were characterized in detail in terms of their structural and optical properties. The structural properties of the synthesized products confirmed that they were wurtzite hexagonal phase for the as-grown products. Raman-scattering spectra exhibited a strong and dominated Raman-active E₂ (high) mode at 441 cm^− 1, confirming the wurtzite hexagonal phase for the as-grown microphone-like ZnO morphologies. The cathodoluminescence (CL) spectrum shows a suppressed near band edge emission at ∼ 380 nm and strong green emission at ∼ 500 nm.     

Synthesis, characterization and optical properties of sheet-like ZnO

Sheet-like ZnO with regular hexagon shape and uniform diameter has been successfully synthesized through a two-step method without any metal catalyst. First, the sheet-like ZnO precursor was synthesized in a weak alkaline carbamide environment with stirring in a constant temperature water-bath by the homogeneous precipitation method, then sheet-like ZnO was obtained by calcining at 600 °C for 2 h. The structures and optical properties of sheet-like ZnO have been characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), high-resolution transmission electron microscopy (HRTEM), photoluminescence (PL) and UV-vis-NIR spectrophotometer. The results reveal that the product is highly crystalline with hexagonal wurtzite phase and has appearance of hexagon at (0 0 0 1) plane. The HRTEM images confirm that the individual sheet-like ZnO is single crystal. The PL spectrum exhibits a narrow ultraviolet emission at 397 nm and a broad visible emission centering at 502 nm. The band gap of sheet-like ZnO is about 3.15 eV.     

A template-free alcoholthermal route to Ti(Sn)-doped ZnO nanorods

Ti(Sn)-doped single-crystalline ZnO nanorods with an average diameter of 20 nm and length up to nearly 1 μm were synthesized by a facile ultrasonic irradiation-assisted alcoholthermal method without involving any templates. Photoluminescence spectra of the Ti-doped ZnO nanorods were measured at room temperature and three emitting bands, being a violet emission at 400-415 nm, a blue band at 450-470 nm and a green band at around 550 nm, were detected. The emission intensities of the Ti-doped ZnO nanorods enhance gradually with increasing the doping concentrations. As to the Sn-doped ZnO nanorods, the green emission shifts to 540 nm and the emission intensities increase first but decrease later with increasing the doping concentrations.     

Cathodoluminescence and its mapping of flower-like ZnO, ZnO/ZnS core-shell and tube-like ZnS nanostructures

The cathodoluminescence (CL) properties including intensity and distribution of the band to band and defect emission of the flower-like ZnO, ZnO/ZnS core-shell and tube-like ZnS nanostructures have been investigated. It is indicated that the Ultraviolet (UV) emission at 380 nm of the flower-like ZnO nanostructures due to the band to band emission is weaker than their yellow emission at 600 nm induced by interstitial oxygen. Moreover, the UV emission of the ZnO nanorods unevenly distributes from the tip to the end. The UV emission on the tip is stronger than that of others due to the waveguide. On the contrary, the yellow emission at 600 nm is uniform. Furthermore, the UV emission of ZnO has been greatly enhanced and the yellow emission has been inhibited by the formation of ZnO/ZnS core-shell nanostructures in the sulfuration process due to the elimination of interstitial oxygen. However, the polycrystalline tube-like ZnS nanostructures shows the uniform and weak defect emission due to S vacancies.     

Characterization and optical properties of ZnO nanoparticles obtained by oxidation of Zn nanoparticles

Strong UV emission at about 399 nm (3.1 eV) and multiple blue emissions at 440-490 nm (2.70-2.53 eV) by ZnO nanoparticles have been observed. The characterizations of ZnO nanoparticles, obtained by oxidizing Zn nanoparticles prepared by arc-discharging, were investigated. The multiple blue PL becomes stronger as the oxidation temperature and time increase, which is attributed to the existence of various defects, in particular to interstitial zinc at the surface of the ZnO nanoparticles.     

Effect of precursor on epitaxially grown of ZnO thin film on p-GaN/sapphire (0 0 0 1) substrate by hydrothermal technique

Single crystalline ZnO thin film on p-GaN/sapphire (0 0 0 1) substrate, using two different precursors by hydrothermal route at a temperature of 90 °C were successfully grown. The effect of starting precursor on crystalline nature, surface morphology and optical emission of the films were studied. ZnO thin films were grown in aqueous solution of zinc acetate and zinc nitrate. X-ray diffraction analysis revealed that all the thin films were single crystalline in nature and exhibited wurtzite symmetry and c-axis orientation. The thin films obtained with zinc nitrate had a more pitted rough surface morphology compared to the film grown in zinc acetate. However the thickness of the films remained unaffected by the nature of the starting precursor. Sharp luminescence peaks were observed from the thin films almost at identical energies but deep level emission was slightly prominent for the thin film grown in zinc nitrate.     

Photoluminescence and field-emission characteristics of ZnO nanowires synthesized by two-step method

ZnO nanowires with excellent photoluminescence (PL) and field-emission properties were synthesized by a two-step method, and the ZnO nanowires grew along (0 0 2) direction. PL measurements showed that the ZnO nanowires have stronger ultraviolet emission properties at 376 nm and there is 3 nm blue shift after the nanowires were immersed in thiourea (TU) solution compared with those of without immersion. The immersed-ZnO nanowires show a turn-on field of 2.3 V/μm at a current density of 0.1 μA/cm and emission current density up to 1 mA/cm² at an applied field of 6.8 V/μm, which demonstrate that the immersed-ZnO nanowires posses efficient field-emission properties in contrast with those not immersed. The ZnO nanowires may be ideal candidates for making luminescent devices and field-emission displays.     

Facile synthesis, optical and photoconductive properties of novel ZnO nanocones

Large-scale, uniform ZnO nanocones with tips about 200 nm and length about 50 μm have been synthesized by a facile hydrothermal method. The morphology and structures were characterized by X-ray diffraction (XRD), scanning electron microscope (SEM), transmission electron microscope (TEM) and high-resolution transmission electron microscope (HR-TEM). The effects of reaction time and PEG-400 on the morphology of ZnO nanostructures were investigated, also an oriented attachment mechanism has been briefly proposed. The optical properties were investigated by lasing confocal microscope and photoluminescence spectrum, a strong near band edge emission peak centered at 387 nm from the ZnO nanocones was observed in photoluminescence spectrum. Finally, we have fabricated UV photodetector based on single ZnO nanocone, which present good switching properties by turning the UV light on and off.     

Zinc oxide micro- and nanoparticles: Synthesis, structure and optical properties

Zinc oxide (ZnO) spherical nanoparticles (SNPs) and bitter-melon-like (BML) microparticles were synthesized by a hydrothermal route using a zinc (Zn) plate as a source and substrate at various synthesis conditions. The structural analysis confirmed the formation of ZnO with hexagonal wurtzite phase on the hexagonal Zn substrate with growth of the ZnO microparticles along the [1 0 1] direction. The UV-vis absorption spectra of the ZnO microparticles indicated absorption peaks in the UV region which can be attributed to the band gap of ZnO. The room temperature photoluminescence (PL) of the ZnO microparticles exhibited a broad emission band, which is fitted with four Gaussian peaks and were assigned to transitions involving free excitons and various defect centers. The growth model for the formation of ZnO micro- and nanoparticles is presented.     

Rapid preparation, characterization, and photoluminescence of ZnO films by a novel chemical method

A novel and simple chemical route was developed for the deposition of ZnO film from aqueous solution, integrating the merits of successive ionic layer adsorption and reaction and chemical bath deposition. ZnO thin films on glass and Si(1 0 0) substrates were deposited with the precursor of zinc-ammonia complex. As-deposited ZnO film exhibits good crystallinity with the hexagonal wurtzite crystalline structure and the preferential orientation along (0 0 2) plane. With a dense and continuous appearance, the film is composed of ZnO particles in even size of 200-300 nm. Under the excitation of 340 nm, strong and sharp near band gap emission (∼391 nm) dominates the photoluminescence spectra with several weak emission peaks related to the deep level (∼450-500 nm). In addition, the mechanism for the deposition process of ZnO from aqueous solution was preliminarily discussed.     

Large-scale synthesis of hexagonal cone-shaped ZnO nanoparticles with a simple route and their application to photocatalytic degradation

We report the large-scale synthesis of hexagonal cone-shaped ZnO nanoparticles by the esterification between zinc acetate and alcohol. The morphology of the ZnO nanoparticles was investigated by transmission electron microscopy, selected area electron diffraction and scanning electron microscopy measurements. The synthesized ZnO nanoparticles are single-crystalline with hexagonal phase and show a strong UV emission at −378 nm due to the excellent crystallinity of particles. A possible formation mechanism of the hexagonal cone-shape structure is proposed. Furthermore, the as-prepared ZnO particles exhibit high photocatalytic activity for the photocatalytic degradation of Rhodamine B, indicating that the ZnO nanostructure is promising as a semiconductor photocatalyst.     

Photonic stop band effect in ZnO inverse photonic crystal

Three-dimensional photonic crystals are fabricated using inward-growing self-assembly technique from polymethyl methacrylate (PMMA) colloids. Their air voids are infiltrated with zinc oxide (ZnO) by sol-gel method and the PMMA template is removed by the two independent processes of heat treatment and wet chemical method resulting in ZnO inverse photonic crystal. The inversion is confirmed from the structural characterization. In X-ray diffraction (XRD) experiment, the ZnO inverse photonic crystals obtained by both the techniques do not show any signature of single-crystalline ZnO. The inverse photonic crystals obtained by chemical method are further heated at different temperatures and XRD confirms crystalline nature of ZnO for temperature treatment at 400 °C. Laser-induced emission studies on ZnO inverse photonic crystals are carried out at two different excitation wavelengths. Excitation with 355 nm enables the observation of the stop band effect for emission at 45° from the inverse crystal obtained by the inexpensive chemical method.     

Low temperature pH dependent synthesis of flower-like ZnO nanostructures with enhanced photocatalytic activity

In the present study we have synthesized flower-like ZnO nanostructures comprising of nanobelts of 20 nm width by template and surfactant free low-temperature (4 °C) aqueous solution route. The ZnO nanostructures exhibit flower-like morphology, having crystalline hexagonal wurtzite structure with (0 0 1) orientation. The flowers with size between 600 and 700 nm consist of ZnO units having crystallite size of ∼40 nm. Chemical and structural characterization reveals a significant role of precursor:ligand molar ratio, pH, and temperature in the formation of single-step flower-like ZnO at low temperature. Plausible growth mechanism for the formation of flower-like structure has been discussed in detail. Photoluminescence studies confirm formation of ZnO with the defects in crystal structure. The flower-like ZnO nanostructures exhibit enhanced photochemical degradation of methylene blue (MB) with the increased concentration of ligand, indicating attribution of structural features in the photocatalytic properties.     

Facile template-free sonochemical fabrication of hollow ZnO spherical structures

ZnO hollow spherical structures have been synthesized by a facile template-free sonochemical process. The structures and morphologies of products have been characterized by XRD, FESEM and TEM. The results reveal that hollow spherical structures possess a hexagonal wurtzite structure with the in- and out-diameters of about 400 and 500 nm, respectively. The walls of the hollow structures are self-assembled by nanoparticles, partly composed of hexagonal nanoflakes with 40 nm in side lengths. Room temperature photoluminescence (PL) spectrum showed a UV emission at ∼ 384 nm and a broad green emission at the center of 535 nm. A possible formation mechanism was also proposed.     

Synthesis, characterization, UV and dielectric properties of hexagonal disklike ZnO particles embedded in polyimides

A series of novel ZnO/polyimide composite films with different ZnO contents was prepared through incorporation hexagonal disklike ZnO particles into poly(amic acid) of the pre polymer of the polyimide. The hexagonal disklike ZnO particles with a diameter of 300-500 nm were synthesized from zinc acetate and NaOH in water with citric acid. The prepared zinc oxide-polyimide composites were characterized for their structure, morphology, and thermal behavior employing Fourier transform infrared spectroscopy, scanning electron micrograph, X-ray diffraction and thermal analysis techniques. Thermal analyses show that the ZnO particles were successfully incorporated into the polymer matrix and these ZnO/polymer composites have a good thermal stability. Scanning electron microscopy studies indicate the ZnO particles were uniformly dispersed in the polymer and they remained at the original size (300-500 nm) before immobilization. All composite films with ZnO particle contents from 1 to 5 wt% show good transparency in the visible region and luminescent properties.     

Solvothermal synthesis of nanorods of ZnO, N-doped ZnO and CdO

ZnO nanorods with diameters in the 80-800 nm range are readily synthesized by the reaction of zinc acetate, ethanol and ethylenediamine under solvothermal conditions. The best products are obtained at 330 °C with a slow heating rate. Addition of the surfactant Triton^®-X 100 gave nanorods of uniform (300 nm) diameter. By adding a small amount of liquid NH₃ to the reaction mixture, N-doped ZnO nanorods, with distinct spectroscopic features are obtained. CdO nanorods of 80 nm diameter have been prepared under solvothermal conditions using a mixture of cadmium cupferronate, ethylenediamine and ethanol at 330 °C. Similarly, Zn_1−xCd_xO nanorods of a 70 nm diameter are obtained under solvothermal conditions starting with a mixture of zinc acetate, cadmium cupferronate, ethanol and ethylenediamine.