Structure and morphology of nanostructured zinc oxide thin films Prepared by dip-vs. spin-coating methods

In this study, we use dipping and spinning methods to coat glass slides with sol-gel ZnO thin films, composed of zinc acetate dihydrate, monoethanolamine (MEA), de-ionized water and isopropanol. The effect of the annealing temperature on the structural morphology and optical properties of these films is investigated. These ZnO films were preheated at 275 °C for 10 min and annealed either at 350, 450 or 550 °C for 60 min. As-deposited films, formed by amorphous zinc oxide-acetate submicron particles, are transformed into a highly-oriented ZnO after thermal treatment. The surface morphology, phase structure and optical properties of the thin films were investigated by scanning electron microscopy, X-ray diffraction (XRD) and optical transmittance. Both techniques produced nanostructured ZnO thin films with well-defined orientation. The annealed films were transparent in the visible range with an absorption edge at about 375 nm and a transmittance of ca 85–90% with an average diameter of 40 nm. XRD results show the film was composed of polycrystalline wurtzite, with a preferential c-axis orientation of (002) and a single sharp XRD peak at 34.40, corresponding to the hexagonal ZnO. The grain size is increased by the annealing temperature. Both coating techniques create sol-gel ZnO films with the potential for application as transparent electrodes in optic and electronic devices.     

In situ and simultaneous nanostructural and spectroscopic studies of ZnO nanoparticle and Zn-HDS formations from hydrolysis of ethanolic zinc acetate solutions induced by water

The simultaneous formation of nanometer sized zinc oxide (ZnO), and acetate zinc hydroxide double salt (Zn-HDS) is described. These phases, obtained using the sol-gel synthesis route based on zinc acetate salt in alcoholic media, were identified by direct characterization of the reaction products in solution using complementary techniques: nephelometry, in situ Small-Angle X-ray Scattering (SAXS), UV-Vis spectroscopy and Extended X-ray Absorption Fine Structures (EXAFS). In particular, the hydrolytic pathway of ethanolic zinc acetate precursor solutions promoted by addition of water with the molar ratio N=[H₂O]/[Zn²⁺] = 0.05 was investigated in this paper. The aim was to understand the formation mechanism of ZnO colloidal suspension and to reveal the factors responsible for the formation of Zn-HDS in the final precipitates. The growth mechanism of ZnO nanoparticles is based on primary particle (radius ≈ 1.5 nm) rotation inside the primary aggregate (radius <3.5 nm) giving rise to an epitaxial attachment of particles and then subsequent coalescence. The growth of second ZnO aggregates is not associated with the Otswald ripening, and could be associated with changes in equilibrium between solute species induced by the superficial etching of Zn-HDS particles at the advanced stage of kinetic.     

The effect of surface properties on visible luminescence of nanosized colloidal ZnO membranes

Luminescence properties of nanosized zinc oxide (ZnO) colloids depend greatly on their surface properties, which are in turn largely determined by the method of preparation. ZnO nanoparticles in the size range from 3 to 9 nm were prepared by addition of tetramethylammonium hydroxide ((CH3)4NOH) to an ethanolic zinc acetate solution. X-ray diffraction (XRD) indicates nanocrystalline ZnO membranes with polycrystalline hexagonal wurtzite structure. The ZnO membranes have a strong visible-emission intensity and the intensity depends upon hydrolysis time. The infrared spectra imply a variety of forms of zinc acetate complexes present on the surface of ZnO particles. The effect of the ZnO membrane surface properties on photoluminescence is discussed.     

Preparation of ZnO colloids by aggregation of the nanocrystal subunits

Colloidal ZnO particles with narrow size distribution were prepared via a sol-gel process by base-catalyzed hydrolysis of zinc acetate. The morphology of ordered arrays of the particles was recorded by SEM. SEM also reveals that these uniform particles were composed of tiny ZnO subunits (singlets) sized of several nanometers. The size of the singlets, which is confirmed by X-ray diffraction and UV-vis absorption spectra, increases as the aging time is prolonged. The size-selective formation of colloids by aggregation of nanosized subunits is proposed to consist of two-stage growth by nucleation of nanosized crystalline primary particles and their subsequent aggregation into polycrystalline secondary colloids. The aggregates are all spherical because the internal rearrangement processes are fast enough. The ZnO colloids, i.e., the aggregates, tend to self-assemble into well-ordered hexagonal close-packed structures. Room-temperature photoluminescence was characterized for green and aged ZnO.     

Tailoring the particle size of monodispersed colloidal gold

Monodispersed spherical gold particles ranging in modal diameter from 80 nm to 5 μm, were prepared by reducing tetrachloroauric(III) acid with iso-ascorbic acid in aqueous solutions at 20°C. The particle size was altered by changing the pH, which affected the composition of gold(III) solute complexes. The latter controlled the redox potential of the system, essential to the formation of the initial nanosize gold dispersions. Depending on the experimental conditions, the resulting primary particles remained either stable or they aggregated to form much larger uniform spheres. The mechanisms of the precipitation of the precursors (primary) particles and of their mutual interactions to yield the final dispersions are discussed.     

Nanosize precursors as building blocks for monodispersed colloids

It is shown that many monodispersed colloid particles, precipitated in homogeneous solutions, are formed by aggregation of nanosize subunits. A model is described that specifies conditions which may yield such spherical particles of narrow size distribution by interactions of precursor singlets. A good agreement was achieved for size selection of gold and cadmium sulfide dispersions. It is illustrated that particles of other shapes may also formed by the aggregation mechanism, and the challenges facing attempts to quantify such processes are pointed out. Finally, examples are given of consequences caused by particles being composed of nanosubunits. The text was submitted by the author in English.     

Aqueous pathways for the formation of zinc oxide nanoparticles

We examine the effect of reactant concentrations, temperatures and feeding methods on the morphology of ZnO formed when reacting solutions of ZnSO(4) and NaOH. The catalytic effect of hydroxide in excess relative to the stoichiometric ratio is considered. It is shown that, having fixed other reaction conditions, the end-products, particle structures and size strongly depend on the mole ratio of the precursors. The presence of zinc salt hydroxide species was confirmed at sub-stoichiometric ratios in slightly acidic conditions. At the stoichiometric ratio both zinc hydroxide and zinc oxide are formed, while only zinc oxide forms in an excess of hydroxide. The method of feeding the reactants into the reaction vessel also has a strong influence on the end-product properties, as does the reaction temperature. By control of these parameters the specific surface area could be varied from 10 to 33 m(2) g(-1), the particle shape could be varied from equiaxed, through to star-like and needle-like, and the particle size may be varied from 50 to over 300 nm.     

Rapid preparation of uniform colloidal indium hydroxide by the controlled double-jet precipitation

The synthesis of uniform colloidal rod-like In(OH)₃ particles from relatively concentrated solutions of InCl₃ (0.1 mol dm^-3) in short reaction time (<15 min) by the controlled double-jet precipitation (CDJP) technique is described. The effects of the molar ratio of [NH₄OH]/[InCl₃], temperature, concentration of the reactants, and reaction time on the size and shape of the final products are investigated. It is found that such In(OH)₃ particles are formed by aggregation of nanosize subunits. Received: 14 April 1998 Accepted: 15 April 1998     

Controlled organization of ZnO building blocks into complex nanostructures

ZnO complex nanostructure with special mushroom-like morphology was prepared by hydrolysis of zinc acetate dehydrate (Zn(CH3COO)2 2H2O) in water-methanol mixed solvent at 60 degrees C. The formation mechanism was studied using XRD investigation and FE-SEM observation, which showed that the mushroom-like particles were transformed from cauliflower-like layered basic zinc acetate (LBZA), Zn5(OH)8(CH3COO)22H2O, and composed of ZnO subunits with average size less than 10 nm. The introduction of hexamethylenetetramine (HMTA, C6H12N4) to the solution before deposition led to drastic changes in the morphologies of both aggregation particles and ZnO subunits. The novel ZnO microspheres, which were made of regular hexagonal plate-like ZnO with dimensional size 35 x 10 nm, were formed. These hexagonal plate-like ZnO subunits stacked very compactly and aligned regularly. Kinetic study of this unique complex nanostructure using TEM and FE-SEM observation showed the presence of HMTA played an important role on the formation of hexagonal ZnO subunits through different mechanisms related to the different parts of microspheres.     

Non-Basic Solution Routes to Prepare ZnO Nanoparticles

Nanocrystalline ZnO particles were prepared from alcoholic solutions of zinc acetate dihydrate without using base such as NaOH or LiOH through a colloid process carried out at a low temperature of 60°C. A comparative study of chemical reactions from zinc acetate dihydrate to ZnO was made using different types of monool solvents, i.e. methanol, ethanol, and 2-methoxyethanol. It was revealed that layered hydroxide zinc acetate was formed as an intermediate and its transformation into ZnO was a key reaction step in any of the solutions. Reaction time necessary for the precipitation of ZnO was greatly influenced by the solvents used. Methanol was useful for the preparation of the ZnO nanoparticles, which were chemically pure in terms of cation impurities and exhibited green photoluminescence by the ultraviolet excitation.     

Spherical Zinc Oxide Nano Particles from Zinc Acetate in the Precipitation Method

The liquid precipitation method using zinc acetate dihydrate was applied for the synthesis of uniform and spherical ZnO nanoparticles. The ultrafine zinc oxid was prepared in a water‐ethanol mixture solution. The solution containing zinc cation was soluble in water. The surface‐active agent triethanolamine (TEA) was soluble in ethanol. Then alkali precipitated by adding n‐propylamine. The spherical zinc oxide particle morphology was found to be highly dependent on the zinc salt concentration, ethanol‐water ratio, and the surface‐active agent additive. The process can produce white ZnO powder of 50–90 nm in size. The morphology of zinc oxide showed a powder shape by transmission electron microscopy (TEM), the crystallization phase structure of zinc oxide by X‐ray diffraction (XRD), and the zinc oxide remaining by using an organic analysis by infrared spectroscopy (IR).     

海藻酸锌纤维热降解法制备氧化锌纳米结构

采用天然高分子海藻酸钠为原料, 以氯化锌水溶液为凝固浴, 通过湿法纺丝技术成功制备了海藻酸锌(Alg-Zn)纤维.通过在空气中不同温度下对所得海藻酸锌纤维进行热处理, 得到了多种ZnO纳米结构. 利用热失重分析(TG)、X射线衍射(XRD)、电子能量损失谱(EELS)、扫描电子显微镜(SEM)、透射电子显微镜(TEM)和高分辨透射电子显微镜(HRTEM)等手段对产物的组成、形貌和微观结构进行了详细表征. 结果表明, 焙烧温度和时间对所得ZnO纳米结构的尺寸和形貌具有重要影响; 800 ℃下热处理24 h以上可以得到直径约为120 nm的ZnO纳米棒. 通过仔细考察不同热处理时间得到的ZnO纳米结构, 提出了在焙烧条件下ZnO纳米棒的生长机理.     

Synthesis and Characterization of Zinc Oxide Nano-particles by Solid State Chemical Reaction Method

Several important synthetic parameters such as precursor concentration, reaction time are found to determine the growth of ZnO nanostructures. These reaction parameters can be tuned to produce a variety size of nanostructures. In this work we show the importance of these parameters on the size of synthesized zinc oxide nano-powders. ZnO nanoparticles are synthesized by the solid-state reaction using ZnSO₄·7H₂O and NaOH as the reagents. In this method Zn(OH)₂ is the intermediate product of the reaction, we show that by adjusting the molar ratio of the reagents and grinding time, we can be removed this unwanted component in the final product so for obtaining pure ZnO nanostructures the calcinations process is not necessary, also we can tune the size of ZnO nanoparticles. XRD spectra of the nanoparticles demonstrate typical diffraction peaks of a well-crystalline Wurtzite ZnO structure transmission electron microscopic observations show that these nanoparticles are of hexagonal phase ZnO mostly in round shapes and he composition analysis by EDX indicate that final product is pure ZnO. In the optimum conditions by XRD analysis we see that the mean grain size of synthesized zinc oxide nano-particles is about 44 nm.     

Preparation and characterization of nanocomposite ZnO–Ag thin film containing nano-sized Ag particles: influence of preheating,annealing temperature and silver content on characteristics

Nanocomposite ZnO–Ag thin film containing nano-sized Ag particles have been grown on glass substrate by spin-coating technique using zinc acetate dihydrate as starting precursor in 2-propanol as solvent and monoethanolamine as stabilizer. Silver nanoparticles were added in the ZnO sol using silver nitrate dissolved in ethanol-acetonitrile. Their structural, electrical, crystalline size and optical properties were investigated as a function of preheating, annealing temperature and silver content. The results indicated that the crystalline phase was increased with increase of annealing temperature up to 550 °C at optimum preheating temperature of 275 °C. Thermal gravimetric differential thermal analysis results indicated that the decomposition of pure ZnO and nanocomposite ZnO–Ag precursors occurred at 225 and 234 °C, respectively with formation of ZnO wurtzite crystals. The scanning electron microscopy and atomic force microscopy revealed that the surface structure (the porosity and grain size) of the ZnO–Ag thin film (the film thickness is about 379 nm) was changed compared to pure ZnO thin film. The result of transmission electron microscopy showed that Ag particles were about 5 nm and ZnO particles 58 nm with uniform silver nanoclusters. Optical absorption results indicated that optical absorption of ZnO–Ag thin films decreased with increase of annealing temperature. Nanocomposite ZnO–Ag thin films with [Ag] = 0.068 M and [Ag] = 0.110 M showed an intense absorption band, whose maximum signals appear at 430 nm which is not present in pure ZnO thin films. The result of X-ray photoelectron spectroscopy revealed that the binding energy of Ag 3d_5/2 for ZnO–Ag shifts remarkably to the lower binding energy compared to the pure metallic Ag due to the interaction between Ag and ZnO.     

A simple preparation technique for shape-controlled zinc oxide nanoparticles: Formation of narrow size-distributed nanorods using seeds in aqueous solutions

Various shapes of wurtzite-type ZnO nanoparticles were selectively produced in a simple aqueous system prepared by mixing ZnSO₄ and NaOH solutions. Ellipsoidal nanoparticles were obtained by the addition of an alkaline agent into an acidic zinc solution (acidic route), while nanorods were grown by mixing a zinc precursor into an alkaline solution (basic route). The aspect ratio and size distribution of the nanorods grown through the basic routes were controlled by the addition of nanoparticles prepared by the acidic route as seeds. On the other hand, micrometric branching rods were obtained by dilution of the reaction solution in the basic routes. The morphological variation of ZnO particles is ascribed to the balance of the nucleation and crystal growth depending on the degree of the supersaturation. We successfully prepared narrow size-distributed rods with a nanometric width and a submicrometric length using the seed particles, because the presence of the seeds suppressed additional nucleation and then controlled the degree of the supersaturation for steady growth of the crystalline particles.     

碱式硫酸锌转化法制备ZnO多孔片及其光致发光性能

采用化学浴沉积法制备了碱式硫酸锌(ZSH)纳米片, 并经1000 ℃煅烧处理得到了ZnO多孔片. 详细研究了ZSH在Zn²⁺-六亚甲基四胺前驱体溶液体系中的形成机理、ZSH 的热解过程、ZnO的结晶性、微结构以及光致发光性能. 结果表明, 所得ZnO多孔片呈规则六角形状, 其尺寸为10-50 μm, 厚度为200-500 nm, 由于高温固相反应中传质等因素的限制, 构成薄片的ZnO晶粒呈多边形或不规则形貌, 晶粒间的孔为亚微米孔, 尺寸在100-500 nm范围. ZnO多孔薄片结晶性良好, 在388 nm处表现出较强紫外发光, 无可见光区的缺陷发光. 机理分析表明, SO₄^2- 与Zn²⁺的高亲和力是Zn²⁺-六亚甲基四胺体系中ZSH生成的根本原因, 而ZSH的热分解过程对ZnO多孔片的形貌和微结构影响显著. 本研究提出了一种制备高结晶质量ZnO多孔材料的新方法, 所得ZnO多孔片可望在催化、染料敏化太阳能电池、紫外光电器件等领域得到应用.     

Electrospray as a tool for drug micro- and nanoparticle patterning

Carbamazepine (CBZ) microparticles of different sizes and shapes, including spheres, q-tips, elongated spheres, and tear-shaped particles, were formed by electrospraying solutions of different CBZ concentrations. The particle characteristics were determined by the interplay between jet formation, droplet breakup, solvent evaporation, and eventual particle solidification. The average particle size increased with increasing CBZ concentration, with particles of different shapes being observed for different CBZ concentrations. The cascade of sizes and shapes observed was interpreted in terms of Rayleigh instability theory as applied to charged jets and droplets, with the final sizes depending upon the time needed to evaporate the solvent sufficiently for CBZ to solidify; the lower the initial concentration of CBZ, the smaller the final droplets/particles that are formed.     

Zinc sulfide nanoparticles: a mechanism of formation in aqueous solutions and optical properties

The formation of sols and precipitates of zinc sulfide as a result of the exchange reaction in an aqueous solution was studied. The precipitates consist of aggregates of primary particles about 3 nm in size. The primary ZnS particle size in aqueous sols increases with an increase in the concentration of zinc sulfate and sodium sulfide, with the accumulation of the final reaction product, and with temperature. This effect does not exceed an 1.5-fold increase. At the first step, the particles with a considerable fraction of the amorphous phase are formed and undergo intragrain crystallization. The photoluminescence properties of aqueous sols of zinc sulfide were studied. They are caused by defects in the ZnS lattice and by the presence of the lattice oxygen.     

Poly(methyl methacrylate) composites prepared by in situ polymerization using organophillic nano-to-submicrometer zinc oxide particles

Nano- and submicrometer zinc(II) oxide particles were synthesized by the polyol method and were used for the preparation of ZnO/poly(methyl methacrylate) (ZnO/PMMA) composite materials by the chain polymerization of methyl methacrylate (MMA) in bulk. ZnO particles with an organophilic surface layer were homogeneously dispersed in the PMMA matrix. Very low concentrations (0.1 wt.%) of nano zinc oxide absorbed over 98% of UV light as determined by UV-vis spectroscopy. Nano zinc oxide (75 nm) increased the initial decomposition temperature of the PMMA matrix by 30-40 °C at concentrations of 0.1% and above. This was explained by the changes in the termination mechanism of MMA polymerization resulting in a reduced concentration of vinylidene chain ends. Nano ZnO also increased the MMA polymerization reaction rate and reduced the activation energy. Submicrometer ZnO showed lower UV absorption, thermal stabilization and no influence on the reaction kinetics indicating that average particle size is of vital importance for the properties of PMMA nanocomposites and for MMA polymerization.     

Mechanism of Formation of Monodispersed Colloids by Aggregation of Nanosize Precursors

It has been experimentally established in numerous cases that precipitation of monodispersed colloids from homogeneous solutions is a complex process. Specifically, it was found that in many systems nuclei, produced rapidly in a supersaturated solution, grow to nanosize primary particles (singlets), which then coagulate to form much larger final colloids in a process dominated by irreversible capture of these singlets. This paper describes a kinetic model that explains the formation of dispersions of narrow size distribution in such systems. Numerical simulations of the kinetic equations, with experimental model parameter values, are reported. The model was tested for a system involving formation of uniform spherical gold particles by reduction of auric chloride in aqueous solutions. The calculated average size, the width of the particle size distribution, and the time scale of the process agreed reasonably well with the experimental values. Copyright 1999 Academic Press.