Crystalline Zinc Oxide Research Articles

Atomistic Investigation of the Solid–Liquid Interface between the Crystalline Zinc Oxide Surface and the Liquid Tetrahydrofuran Solvent

Zinc oxide is typically used as an electron acceptor in the preparation of hybrid solar cells. Such hybrids are, in many cases, synthesized from solutions. In this work, we investigate the possible...

Photodissolution Suppression and Photocatalytic Activity Improvement through Crystallinity Improvement of ZnO Nanocrystalline Thin Films

Zinc oxide (ZnO) is an attractive candidate for semiconductor photocatalysis studies because of its strong oxdizability and wide bandgap. However, ZnO has the problem of photodissolution in the aqueous phase and photocatalytic activity decrease due to dissolution. In this paper, we report the photocatalytic property of nanocrystalline ZnO thin films fabricated by laser ablation. The films grown at different annealing temperatures and times have been characterized using X-ray diffraction and a scanning electron microscope. The photocatalytic activity of ZnO thin films was determined by the decoloration of azo dye OrangeII solution and hydrogen generation. It was shown that the photocatalytic activity of ZnO thin films depends on the annealing temperature and time, and photodissolution of the sample can be inhibited by increasing the crystallinity of ZnO. It was also shown that the ZnO thin layer with high crystallinity generates hydrogen from NaOH solution without applying bias voltage.

Effect of rapid thermal annealing on the structural and electrical properties of solid ZnO/NiO heterojunctions prepared by a chemical solution process

Transparent pn-heterojunction electrodes composed of n-type zinc oxide (ZnO) nanorod arrays and p-type NiO are fabricated by a chemical solution route combined with rapid thermal annealing (RTA). The effects of RTA treatment (450 °C, 1 min) in reducing atmosphere on the structural properties and alignment of the ZnO nanorods are investigated. SEM images and XRD patterns show that the average diameter and length are increased, and the alignment and crystal quality of the nanorod arrays are significantly improved. The RTA-treated ZnO shows uniform and well-aligned nanorod arrays with a high aspect ratio of ∼12.4. The structural improvements associated with ZnO crystalline and defect reduction are further verified from the enhanced UV to visible emission ratio by a photoluminescence analysis. Moreover, the effect of RTA on the I–V characteristics of the ZnO/NiO heterojunction electrodes is also investigated; the electrical parameters of the junction are determined from the forward-bias I–V characteristics using Cheung's method. The results reveal that the series resistance is significantly reduced from 784.4 to 114.8 Ω, which leads to better rectifying behaviour with rectifying slope increased to ∼6.67 mA V−1. The RTA-processed ZnO/NiO heterojunctions with excellent optoelectronic properties may find wide applications in sensors and solid-state oxide junction-based devices.

A study of nanosized zinc oxide and its nanofluid

The synthesis and characterization of nanosized zinc oxide and its nanofluid in a polyvinyl alcohol (PVA) matrix have been done in the present investigation. Crystalline zinc oxide nanoparticles are synthesized using single-step chemical method while the nanofluids are prepared by the dispersion of nanoparticles in PVA solution using an ultrasonicator. The prepared nanoparticles are characterized using X-ray diffraction, SEM–EDX and UV–visible spectrum. The particle size distribution measurement is carried out by acoustic particle sizer. The ultrasonic velocities are measured in the synthesized nanofluid under different physical conditions using an ultrasonic interferometer. It is found that the degree of crystallinity of nanoparticles depends on the evaporation rate during its synthesis and ultrasonic velocity has non-linear relation with temperature for the present nanofluid.

Growth of ZnO nanostructures on Si by means of plasma immersion ion implantation and deposition

Crystalline zinc oxide (ZnO) nanostructures have been grown on Si substrates by means of Plasma Based Ion Implantation and Deposition (PIII&D) at a temperature of about 300 °C and in the presence of an argon glow discharge. In the process a crucible filled with small pieces of metallic zinc plays the role of the anode of the discharge itself, being polarized by positive DC voltage of about 400 V. Electrons produced by thermionic emission by an oxide cathode (Ba, Sr, Ca)O impact this crucible, causing its heating and vaporization of Zn. Partial ionization of Zn atoms takes place due to collisions with plasma particles. High negative voltage pulses (7 kV/40 μs/250 Hz) applied to the sample holder causes the implantation of metallic zinc into Si surface, while Zn deposition happens between pulses. After annealing at 700 °C, strong UV and various visible photoluminescence bands are observed at room temperature, as well as the presence of ZnO nanoparticles. The coated surface was characterized in detail using X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), energy dispersive spectroscopy (EDS), scanning electron microscopy (SEM), atomic force microscopy (AFM) and photoluminescence (PL) spectroscopy. XRD indicated the presence of only ZnO peaks after annealing. The composition analysis by EDS revealed distinct Zn/O stoichiometry relation depending on the conditions of the process. AFM images showed the formation of columns in the nanoscale range. Topography viewed by SEM showed the formation of structures similar to cactus with nanothorns. Depth analysis performed by XPS indicated an increase of concentration of metallic Zn with increasing depth and the exclusive presence of ZnO for outer regions. PIII&D allowed to growing nanostructures of ZnO on Si without the need of a buffer layer.

Sonochemical approach for rapid growth of zinc oxide nanowalls

The sonochemical process drives chemical reactions with sound fields by creating extraordinarily high density of energy, pressure and temperatures. The process resulted in a number of unexpected chemical species and thought-provoking results in the recent past. In this paper, we present a new sonochemical approach to synthesize ZnO (zinc oxide) nanowalls (NWalls) on aluminum and alumina coated substrates at room ambient conditions. We achieved highly dense and uniform ZnO NWalls in areas that are coated with Al or Al2O3 (alumina). The synthesis process was shown not to occur on Si, SiO2, Cr, or Ag surfaces. A series of experiments on understanding the growth kinetics offers detailed insight into the growth dynamics over time. Photoluminescence (PL) measurements, UV Vis spectroscopy, and SEM-EDS results confirm NWalls composed of crystalline ZnO that are formed via Al assisted growth induced by phase transformations under extraordinary pressure, temperature, and chemical growth kinetics. The chemical growth method as reported here, is applicable to arbitrary substrates coated with an Al thin film. We demonstrate the applications of the as-formed NWalls in UV photoconductors and gas sensors.

Self-aligned nanocrystalline ZnO hexagons by facile solid-state and co-precipitation route

In this study, we report the synthesis of well-aligned nanocrystalline hexagonal zinc oxide (ZnO) nanoparticles by facile solid-state and co-precipitation method. The co-precipitation reactions were performed using aqueous and ethylene glycol (EG) medium using zinc acetate and adipic acid to obtain zinc adipate and further decomposition at 450 °C to confer nanocrystalline ZnO hexagons. XRD shows the hexagonal wurtzite structure of the ZnO. Thermal study reveals complete formation of ZnO at 430 °C in case of solid-state method, whereas in case of co-precipitation method complete formation was observed at 400 °C. Field emission scanning electron microscope shows spherical morphology for ZnO synthesized by solid-state method. The aqueous-mediated ZnO by co-precipitation method shows rod-like morphology. These rods are formed via self assembling of spherical nanoparticles, however, uniformly dispersed spherical crystallites were seen in EG-mediated ZnO. Transmission electron microscope (TEM) investigations clearly show well aligned and highly crystalline transparent and thin hexagonal ZnO. The particle size was measured using TEM and was observed to be 50–60 nm in case of solid-state method and aqueous-mediated co-precipitation method, while 25–50 nm in case of EG-mediated co-precipitation method. UV absorption spectra showed sharp absorption peaks with a blue shift for EG-mediated ZnO, which demonstrate the mono-dispersed lower particle size. The band gap of the ZnO was observed to be 3.4 eV which is higher than the bulk, implies nanocrystalline nature of the ZnO. The photoluminescence studies clearly indicate the strong violet and weak blue emission in ZnO nanoparticles which is quite unique. The process investigated may be useful to synthesize other oxide semiconductors and transition metal oxides.

Synthesis and Characterization of Nano Structured Zinc(II) Cysteine Complex under Ultrasound Irradiation

Cysteine and zinc oxides are important materials that have a wide range of applications especially in biomedical sciences. In this study, synthesis and characterization of nano structured Zn(II) complex of cysteine, (cysteine = 2-amino-3-mercaptopropanoic acid), in two different particle sizes, has been investigated. The reaction of Zinc(II) acetate and KI with cysteine ligand under ultrasonic irradiation, leads to the formation of nano sized Zn(II) cysteine complex. Particle sizes of the product have been decreased after thermal treatments in an autoclave. The hexagonal micro crystalline zinc oxide, has been prepared using Zn(II) cysteine complex as precursor. Characterization of the Zn(II) complex, has been performed using elemental analysis, FTIR spectroscopy, energy dispersed X-ray analysis (EDAX), scanning electron microscopy (SEM), thermal analysis (TG/DTA) and gel-permeation chromatography (GPC). Based on the results, chemical formula for the Zn(II) cysteine complex is proposed as [ZnI2(C3H5NO2S)]n. Scanning electron microscopy (SEM) analysis has demonstrated nano particles Zn(II) complex with average diameter of about 38-54 nm. Hexagonal ZnO micro structures were characterized by FTIR, X-ray powder diffraction (XRD), scanning electron microscopy (SEM) and energy dispersive Xray (EDAX) techniques. Research Article

A Catalyst-Free Growth of ZnO Nanowires on Si (100) Substrates: Effect of Substrate Position on Morphological, Structural and Optical Properties

An investigation into the catalyst-free growth of highly crystalline zinc oxide nanowires from metallic zinc powder on Si (100) substrate was investigated through the vapor solid (VS) process at 900°C in a tube furnace. The effect of substrate position on the morphology, structure and photoluminescence (PL) properties of the fabricated ZnO nanowires was evaluated. The diameter of the resulted nanowires varies from 50 nm to 300 nm upon increasing the separation distance, between the substrate and the Zn powder source, from 12 cm to 16 cm. The EDX and XRD results showed that the zinc to oxygen ratio and the crystallinity of the fabricated nanowires are dependent on the substrate position. Also, the ratio of the UV to visible emission peaks of the fabricated ZnO nanowires was found to be strongly dependent on the substrate position. The PL spectra showed the enhancement in UV emissions for the samples fabricated on the substrate farther away from the Zn source with enhanced near band-edge emission for thicker nanowires. A possible mechanism for the growth of ZnO nanowires is discussed.

Functional one, two, and three-dimensional ZnO structures by solvothermal processing

Crystalline zinc oxide (ZnO) has been fabricated by solvothermal techniques under quite different temperature and pressure environment stretching from sub- to supercritical conditions. Among them, one-step processing of inorganic–inorganic hybrid ZnO structures based on ZnO substrate with attached ZnO nanocrystals has been challenged. Combination of doping ions was made to yield ZnO structures with luminescent properties of high potential for application in fast-decay devices. The room temperature decay of antimony, lithium (Sb, Li) codoped homoepitaxial films is reported.

A study on the photoluminescence properties of electrospray deposited amorphous and crystalline nanostructured ZnO thin films

Transparent amorphous and crystalline nanostructured zinc oxide (ZnO) thin films were prepared by electrospray deposition technique on a glass substrate at 250°C using zinc nitrate and ammonia as the precursors. The structural morphology and optical properties of the thin films were evaluated and also the effect of formation rate of basic generating species on the morphology of the thin films was discussed. Optical studies indicated that all of the thin films have a near band edge emission at 374nm and a band gap of 3.32eV. In addition, the films have an emission at 393nm corresponding to ultra violet (UV) transmittance. However, the amorphous thin films have oxygen trap peak at 430nm and two green transmittance peaks at 485nm and 530nm which are absent in the 0.1 molar nanostructured sample. The crystallite size of the thin films which calculated from XRD patterns via Debye–Scherrer's formula was around 30nm.

The effect of complexing agent on the crystallization of ZnO nanoparticles

In this work, some structural and optical properties of the zinc oxide (ZnO) nanoparticles were studied. The highly crystalline ZnO nanoparticles were produced by the hydrothermal and sol–gel methods. The analyses of the XRD patterns, STEM images and UV spectroscopy showed that the size of the nanoparticles prepared by oxalic acid was smaller than the ones by urea. The properties of oxalic acid and urea were also investigated to determine the most effective crystallization process of ZnO nanoparticles. It has been shown that pH, decomposition temperature and activity coefficient of the complexing agent have certain effects on crystallization process.

ETHANOL AND TRIMETHYL AMINE SENSING BYZnO-BASED NANOSTRUCTURED THIN FILMS

Undoped and fluorine (F)-doped nanostructured zinc oxide ( ZnO ) thin films were deposited over glass substrates by spray pyrolysis technique using zinc acetate dihydrate with and without ammonium fluoride as dopant in precursor solution respectively. The deposition conditions and the concentration of the dopant were optimized to obtain nanostructured ZnO thin films. The film obtained from 0.05 M of zinc acetate aqueous solution and 20% F sprayed at the rate of 3 ml/min on preheated substrate kept at 503 K yielded spherical shape well-connected grains, which has large surface to volume ratio. The structural and morphological studies of the films were investigated by using X-ray diffraction (XRD) and scanning electron microscope (SEM) respectively. The diffraction peak positions in XRD confirmed the formation of highly crystalline ZnO film with hexagonal wurtzite phase. Further sensing behavior of the films towards various concentrations of volatile organic compounds (VOCs) such as ethanol and trimethyl amine (TMA) has been investigated at an optimized operating temperature of 373 K and reported.

Effects of growth duration on the structural and optical properties of ZnO nanorods grown on seed-layer ZnO/polyethylene terephthalate substrates

Well-aligned single crystalline zinc oxide (ZnO) nanorods were successfully grown, by hydrothermal synthesis at a low temperature, on flexible polyethylene terephthalate (PET) substrates with a seed layer. Photoluminescence (PL), field-emission scanning electron microscopy (FE-SEM), X-ray diffraction (XRD) and high-resolution transmission electron microscopy (HRTEM) measurements were used to analyze the optical and structural properties of ZnO nanorods grown for various durations from 0.5 h to 10 h. Regular and well-aligned ZnO nanorods with diameters ranging from 62 nm to 127 nm and lengths from 0.3 μm to 1.65 μm were formed after almost 5 h of growth. The growth rate of ZnO grown on PET substrates is lower than that grown on Si (1 0 0) substrates. Enlarged TEM images show that the tips of the ZnO nanorods grown for 6 h have a round shape, whereas the tips grown for 10 h are sharpened. The crystal properties of ZnO nanorods can be tuned by using the growth duration as a growth condition. The XRD and PL results indicate that the structural and optical properties of the ZnO nanorods are most improved after 5 h and 6 h of growth, respectively.

Interfacial mixing during annealing of zinc oxide nanoparticle junctions

The process of forming a junction between crystalline zinc oxide (ZnO) nanoparticles during pulsed thermal annealing in liquid tetradecane is studied using molecular dynamics simulation. Pairs of equal and unequal size particles are considered with emphasis on neck growth and atom mixing. The contact area and interface width of the junction are found to increase with heat pulse power albeit at different rates. The results suggest that it is possible to increase the junction area without significant mixing of atoms across the junction interface by tailoring the heat pulse power.

Effect of silica doping on the densification and grain growth in zinc oxide

The ability of silica (SiO 2) in controlling the densification and grain growth behavior of nano crystalline zinc oxide (ZnO) has been systematically studied. It has been observed that SiO 2 acts as a sintering inhibitor in the ZnO–SiO 2 system up to 4 wt.% limiting value beyond which densification behavior of the system remains almost unchanged, especially above 1100 °C. The addition of SiO 2 to ZnO retards grain growth which in turn results a finer ultimate grain size as compared to the undoped ZnO. However, stabilization in grain size occurs at ≥4 wt.% SiO 2 addition. It has been observed that SiO 2 incorporation changes the grain growth mechanism up to 4 wt.% addition, beyond which no remarkable changes was noticed. The grain growth ( n) shows distinctly different slopes as a function of sintering time for the SiO 2 doped ZnO systems than undoped ZnO. The different slopes tend to indicate that different diffusion mechanisms and probably the formation of a secondary phase (Zn–Si–O) at the grain boundary control the densification and grain growth. The thermal expansion coefficient of the system has been found to decrease substantially beyond 4 wt.% SiO 2 addition to ZnO.

The Effect of Oxygen Partial Pressure on the Formation of Nano Crystalline Zinc Oxide Films

The Effect of Oxygen Partial Pressure on the Formation of Nano Crystalline Zinc Oxide Films

Influence of oxygen gas content on the structural and optical properties of ZnO thin films deposited by RF magnetron sputtering at room temperature

Zinc oxide (ZnO) thin films were deposited on sapphire (0001) substrates at room temperature by radiofrequency (RF) magnetron sputtering at oxygen gas contents of 0%, 25%, 50% and 75%, respectively. The influence of oxygen gas content on the structural and optical properties of ZnO thin films was studied by a surface profile measuring system, X-ray diffraction analysis, atomic force microscopy, and UV spectrophotometry. It is found that the size of ZnO crystalline grains increases first and then decreases with the increase of oxygen gas content, and the maximum grain size locates at the 25% oxygen gas content. The crystalline quality and average optical transmittance (>90%) in the visible-light region of the ZnO film prepared at an oxygen gas content of 25% are better than those of ZnO films at the other contents. The obtained results can be attributed to the resputtering by energetic oxygen anions in the growing process.

Controlled Synthesis of ZnO Nanospheres Using Hydrothermal Process

A novel, versatile, and template-free method has been developed to synthesize solid crystalline zinc oxide (ZnO) spheres via hydrothermal process at a temperature of 100oC within 2h. Using an aqueous solution of zinc acetate dihydrate, ammonium hydroxide, ethanol, and urea, the effects of pH values and structure directing agents on the morphology of ZnO crystals were systematically investigated. We found that the key parameters affecting ZnO morphologies are the pH value of the solution as well as the structure directing agents. When the structure directing agents such as ethanol and urea were used, at a pH value of 12, isotropic growth of ZnO spheres was developed, while at pH value less than 12, anisotropic growth of ZnO structures was observed. Furthermore, by modulating the volume ratio of ethanol in the solution, the size of the ZnO spheres could be controlled from nanometer to micrometer range.

A Sonochemical Approach to the Fabrication of Laterally Aligned ZnO Nanorod Field Emitter Arrays on a Planar Substrate

A simple and novel sonochemical route has been demonstrated to produce the lateral growth of zinc oxide (ZnO) nanorod arrays on a quartz wafer without the use of metal catalysts. Highly crystalline ZnO nanorods grew laterally on the edge of a Zn thin film seed layer, with preferential growth along the [0001] direction. The average diameter and length of the ZnO nanorods, grown laterally from the edge of the Zn thin film, were 156 and 933 nm, respectively. The vertical growth of ZnO nanorods on the top surface of the seed layer was suppressed by a Si thin film growth barrier. The lateral ZnO nanorod arrays showed excellent field emission properties, 60 mA/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> at 6.5 V/μm, with a low turn-on field in the range of 3.9-4.0 V/μm. Because of the compatibility of this process with current standard semiconductor microfabrication technologies, this sonochemical approach constitutes a practical technique for the design of state-of-the-art nanodevices based on laterally grown ZnO nanorod arrays on planar substrates.