The competition growth of ZnO microrods and nanorods in chemical bath deposition process

Well-aligned ZnO nanorod arrays were synthesized on the c-axis orientated ZnO seed layer by chemical bath deposition (CBD) technique. Randomly distributed ZnO microrods with big diameter and growth speed were also formed simultaneously in the same process. The growth of the microrods followed the expected diffusion-limited Ostwald ripening mechanism reasonably well, while that of the nanorods was suggested to be controlled not only by the nutrition ions available but also by the density of nuclei site and the reaction kinetics on the growing surfaces, etc. The microstructure and optical properties of the well-ordered nanorod were also investigated.     

Multipod structures of ZnO hydrothermally grown in the presence of Zn3P2

Zinc phosphide (Zn₃P₂) is used in the present contribution as an inorganic additive for the surfactant-free hydrothermal synthesis of ZnO structures with controlled morphology. Postulated as a suitable dopant candidate for generating P-related acceptor defects inside the wurtzite structure of ZnO, the obtained results however reveal that a stable P:ZnO solid solution is never formed in the binary ZnO-Zn₃P₂ system. Instead the amorphized phosphide particles may preferentially locate at the surface of ZnO particles, exerting a strong effect on the morphological development of the material and controlling its crystal growth habit: with the incorporation of Zn₃P₂ the system evolves from a situation in which just hexagonal bipods are observed to another one in which the ZnO particles merge to form spherical ball-shaped structures and flower-like architectures. The high propensity of ZnO polar structure to twin is behind the observed growth mechanism.     

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.     

Improved efficiency of the chemical bath deposition method during growth of ZnO thin films

Chemical bath deposition (CBD) is an inexpensive and low temperature method (25-90 °C) that allows to deposit large area semiconductor thin films. However, the extent of the desired heterogeneous reaction upon the substrate surface is limited first by the competing homogeneous reaction, which is responsible for colloidal particles formation in the bulk solution, and second, by the material deposition on the CBD reactor walls. Therefore, the CBD method exhibits low efficiency in terms of profiting the whole amount of starting materials. The present work describes a procedure to deposit ZnO thin films by CBD in an efficient way, since it offers the possibility to minimize both the undesirable homogeneous reaction in the bulk solution and the material deposition on the CBD reactor walls. In a first stage, zinc peroxide (ZnO₂) crystallizing with cubic structure is obtained. This compound shows a good average transparency (90%) and an optical bandgap of 4.2 eV. After an annealing process, the ZnO₂ suffers a transformation toward polycrystalline ZnO with hexagonal structure and 3.25 eV of optical bandgap. The surface morphology of the films, analyzed by atomic force microscope (AFM), reveals three-dimensional growth features as well as no colloidal particles upon the surface, therefore indicating the predominance of the heterogeneous reaction during the growth.     

Self-assembly of ZnO nanosheets into nanoflowers at room temperature

A singularity flower-like ZnO nanostructure was prepared on a large scale through a very simple solution method at room temperature and under ambient pressure in a very short time. The flower-like ZnO nanostructures were self-assembled by thin and uniform nanosheets, with a thickness of around 5 nm. X-ray powder diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM) were used to characterize the structure and morphology. The possible growth mechanism was discussed based on the reaction process. The blue shift in the UV-vis spectra of the ZnO nanostructures was also observed.     

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.     

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.     

Preparation of hollow shells of zinc oxide/bismuth(III) vanadate

The encapsulation of hollow-ZnO microparticles with the BiVO₄ pigment is reported. The crystalline phases present in the composite ZnO/BiVO₄ microparticles were investigated by X-ray powder diffraction (XRD) studies and Raman spectroscopy. The micrometric ZnO/BiVO₄ particles show morphological characteristics related to the hollow-ZnO particles used as templates.     

Tetrapod-shaped ZnO microtubes synthesized from Zn/C mixtures

We report for the first time the growth of tetrapod-shaped ZnO microtubes synthesized via a simple method. The tetrapod-shaped ZnO microtubes were synthesized from mixtures of high-purity Zn powder and activated carbon in a crucible at 930-940 °C in air without the presence of any catalyst. The ZnO microtubes were found to be grown on the arrays of ZnO microcrystals, which were horizontally grown from the inner walls of crucibles. Optical and scanning electron microscopy were utilized to observe the morphologies of the tetrapod-shaped ZnO microtubes, and X-ray diffraction was employed to study the crystal structure. The microtubes were found to be hexagonally shaped with well-defined side faces. A growth model was proposed to explain the formation of the tubular ZnO microcrystals.     

High sensitivity chlorine gas sensors using CdIn2O4 thick film prepared by co-precipitation method

Gas-sensing properties to dilute Cl₂ have been investigated for CdIn₂O₄ thick film sensors prepared by co-precipitation method. Cadmium nitrate and indium nitrate were mixed in de-ionized water. The 0.1 M NaOH was added to the mixed solution. The co-precipitate obtained was washed, filtered, dried, and calcined at 600-900 °C for 4 h. The CdIn₂O₄ sensor prepared using the powder calcined at 600 °C showed high sensitivity (S=R_g/R_a) to dilute Cl₂ at 250 °C. In particular, the CdIn₂O₄ sensor showed the sensitivity as high as 1200 even to 0.2 ppm Cl₂. The crystal structure and surface morphology were examined by means of X-ray diffraction (XRD) and scanning electron microscopy (SEM), respectively.     

A simple route to the synthesis of BaCrO4 microstructures at room temperature

BaCrO₄ with various morphologies such as X-shaped, shuttle, rhombus was produced by using poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) (P123) as a structure-directing agent at room temperature. The effects of experimental parameters on the morphology of BaCrO₄ were investigated. This synthetic route was also extended to the synthesis of BaWO₄ and Pb₂CrO₅. The samples were characterized by X-ray powder diffraction (XRD), scanning electron microscopy (SEM), and Fourier transform-infrared spectroscopy (FT-IR). The photoluminescence (PL) of the sample was investigated.     

Polyol method for the preparation of nanosized Gd2O3, boehmite and other oxides

A modified microwave-assisted polyol method was applied to prepare nanoparticulate ceramic powders of different oxides, i.e. Gd₂O₃, AlO(OH) (boehmite) and TiO₂. Due to the good dielectric properties of the utilised solvents, such as ethylene glycol, diethylene glycol and 1,4 butanediol, a significant decrease in reaction time was achieved under microwave heating. In the case of AlO(OH) and Gd₂O₃, <5 nm primary particle size were obtained. Boehmite was found to be intercalated with the solvent. The general applicability of the process is shown and the advantages in terms of properties and processibility are described. The powders thus prepared were investigated using X-ray diffractometry, electron microscopy and physisorption.     

Monodispersed hollow microsphere of ZnO mesoporous nanopieces: Preparation, growth mechanism and photocatalytic performance

The mesoporous ZnO nanopieces-based nest-like hollow microsphere are successfully synthesized through the thermal decomposition of hydrozincite (Zn₅(CO₃)₂(OH)₆) formed by a facile hydrothermal method. The time-dependent experiments reveal the unique growth process of hydrozincite (Zn₅(CO₃)₂(OH)₆) microsphere. Hydrozincite Zn₅(CO₃)₂(OH)₆ nanopiece clusters were formed via homogenous nucleation process at the initial stage, these hydrozincite nanopieces assemble into the ring-like structures through a complex secondary nucleation with the growth of Zn₅(CO₃)₂(OH)₆ crystalline at the edge of each nanopiece, and further formed the hollow microspheres. Study show ZnO inherits well the size and morphology of hydrozincite crystals after the thermal decomposition. Nest-like hollow microspheres of ZnO mesoporous nanopieces are monodispersed with a uniform size distribution of ∼15 μm. Study also shows ZnO nanopieces interwaved microspheres with high surface area and porosity exhibit excellent photocatalytic activitiy on the photodegradation of methyl orange.     

Fast response ultraviolet Ga-doped ZnO based photoconductive detector

A metal-semiconductor-metal photoconductive detector was fabricated using high quality Ga-doped ZnO film epitaxially grown onto alumina substrate by spray pyrolysis. The photocurrent increases linearly with incident power density for more than two orders of magnitude. Reflectance and photocurrent measurements were carried out to study optoelectronic properties of Ga-doped ZnO thin film. Both spectra are consistent with each other showing good response in UV than visible region. Peak responsivity of about 1187 A/W at 5 V bias for 365 nm light was obtained in UV region.     

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.     

Morphology controllable synthesis of ZnO crystals—pH-dependent growth

ZnO crystals were grown through a solution-based chemical route at ambient pressure and low temperature. It was found that the solution pH is a dominative factor in determining the morphology of crystals: Rod-like ZnO crystals are apt to be formed at near neutral condition, whereas flower-like structured ZnO crystals are preferred to be formed at higher solution pH. By monitoring the reaction intermediates during the ZnO growth process, it was realized that the crucial role of solution pH in determining the morphology of ZnO crystals is to control the structure of the primary reaction intermediates at the early stage of ZnO growth. Moreover, by appreciate controlling the solution pH together with Zn²⁺ concentration, various ZnO crystals of rod-like, dumbbell-like and even more complex flower-like structures were obtained without any template, and a pH-dependent morphology controllable growth mechanism is suggested.     

Zinc oxide nanoparticle growth from homogenous solution: Influence of Zn:OH, water concentration, and surfactant additives

Zinc oxide particle growth from homogeneous solution was monitored using in situ UV-vis spectroscopy. Final particle size and overall growth rate increased with increasing zinc to hydroxide concentration ratio and were both sensitive to the surfactant added. Particle growth was fit using two models: (1) the classic coarsening model and (2) the simultaneous coarsening and oriented aggregation model. Results demonstrate that using adamantane carboxylic acid as a surfactant additive inhibits ZnO nanoparticle growth both by coarsening and oriented aggregation as compared to using other monocarboxylates (e.g., acetate and tribromoacetate). In addition, ZnO nanoparticle growth was independent of water concentration within the range of 40-100 mM for the conditions studied here (1 mM zinc perchlorate, 1.6 mM hydroxide, and 0.38 mM adamantane carboxylic acid). High-resolution transmission electron micrographs confirm inhibited growth by oriented aggregation for ZnO grown with adamantane carboxylic acid. Results are compared to previous work and generally show that ZnO growth by coarsening and oriented aggregation can be selectively inhibited or promoted by judicious selection of the surfactant additive.     

Self-catalytic growth and photoluminescence properties of ZnS nanostructures

Mass production of uniform wurtzite ZnS nanostructures has been achieved by a H₂-assisted thermal evaporation technique. X-ray diffraction (XRD) analyses, scanning electron microscopy (SEM) and high-resolution transmission electron microscopy (HRTEM) observations show that the ZnS nanostructures consist of nanobelts, nanosheets with a hexagonal wurtzite structure. The as-synthesized nanobelts have a length of several tens of micrometers and a width of several hundreds of nanometers. Self-catalytic vapor-liquid-solid (VLS) growth and vapor-solid (VS) growth are proposed for the formation of the ZnS nanostructures because neither a metal catalyst nor a template was introduced in the synthesis process. Room-temperature photoluminescence measurement indicates that the synthesized ZnS nanostructures have a strong emission band at a wavelength of 443 nm, which may be attributed to the presence of various surface states.     

Solvothermal synthesis of copper sulfide semiconductor micro/nanostructures

Covellite copper sulfide (CuS) micro/nanometer crystals in the shape of hierarchical doughnut-shaped, superstructured spheric-shaped and flowerlike architectures congregated from those nanoplates with the thickness of 20-100 nm have been prepared by a solvothermal method. The as-obtained CuS products were characterized by means of scanning electron microscopy (SEM), X-ray diffractometry (XRD) and energy-dispersive X-ray spectroscopy (EDS). A systematic investigation has been carried out to understand the factors influencing the evolution of CuS particle morphology which found to be predominant by solvent, surfactant, sulfur resource and copper salt. The possible formation mechanism for the nanostructure formation was also discussed. These CuS products show potential applications in solar cell, photothermal conversion and chemical sensor.     

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.