Crystalline Zinc Oxide Research Articles

Finite size effect in ZnO nanowires

To clarify the size effect in semiconductor nanowires with decreasing diameters but not yet reaching the quantum confinement region, single crystalline zinc oxide nanowires with diameters around 10nm are synthesized. Electrical transport measurements of these thin nanowires show significant increase in conductivity accompanied by diminished gate modulation and reduced mobility. This phenomenon is a result of the enrichment of surface states owing to the increased surface-to-volume ratio. The enhanced surface effect is confirmed by the temperature dependent photoluminescence measurements and contributes to the “anomalous” blueshift. This study shows that surface states play a dominant role in the electrical and optical properties of quasi-one-dimensional materials.

Synthesis of single crystalline europium-doped ZnO nanowires

Single crystalline Eu 3+-doped wurtzite zinc oxide (ZnO) nanowires were synthesized by a vapor deposition method under a controlled oxidative atmosphere. The nanowires were deposited on a Si substrate coated with 5 nm gold nanoparticles. The reaction was carried out for 30 min which resulted in high yield of vertically grown nanowires of diameter 40–150 nm and length up to several microns. The nanowires were grown along ±[0 0 0 1] direction. The concentration of dopant Eu 3+ in the synthesized nanowires was below 1 atomic %. The crystal structure and the microstructures of the doped nanostructures were studied and compared with undoped ZnO.

ZnO nanoparticles obtained by mechanochemical technique and the optical properties

Crystalline ZnO nanoparticles were synthesized by mechanochemical method. Mechanochemical processing involves the mechanical activation of solid-state displacement reactions at low temperatures in a ball mill. Statistical design was used to investigate the effect of main parameters (i.e. time, milling rate and calcination temperature) on ZnO crystallite size and morphology. After annealing at 400 °C in air, zinc oxide (ZnO) nanoparticles were obtained. The milled powders are analyzed by X-ray diffraction (XRD), TG/DTA and transmission electron microscope (TEM). The crystallite size of ZnO samples calculated from XRD is consistent with the TEM images and estimated to be less than 20 nm. The optical properties of the samples were studied by UV–vis spectrophotometer.

Structural and thermal interpretation of the synergy and interactions between the fire retardants magnesium hydroxide and zinc borate

The mechanism of cooperative action of commercial fire retardants is interpreted as resulting from specific chemical reaction and phase changes. This investigation focuses on the thermally initiated interactions between two forms of commercially available fire retardant compounds. The fire performance of a polyolefin with a metal hydroxide fire retardant, magnesium hydroxide, can significantly reduce the heat release rate through absorption of heat during conversion to its metal oxide. Formation of water, followed by vaporisation, decreases heat and dilutes volatiles from polymer degradation. The second form of fire retardant compounds are zinc borates (2ZnO·3B 2O 3·3H 2O and 4ZnO·B 2O 3·H 2O), that undergo dehydration with increasing temperature. Differential thermal analysis and wide-angle X-ray spectroscopy indicated that various structural changes occurred during heating. Endothermic transitions were observed for all components, while zinc borate (2ZnO·3B 2O 3·3H 2O) showed an exothermic crystallisation transition at relatively high temperature. The exotherm was modified by the development of a new crystalline phase, magnesium orthoborate (3MgO·B 2O 3) that formed on reaction with magnesium oxide (MgO) at temperatures greater than 500 °C. Formation of crystalline zinc oxide (ZnO) was also detected. From zinc borate (4ZnO·B 2O 3·H 2O), ZnO was primarily formed. No new crystalline phases were observed in the presence of MgO over the temperature range investigated.

ZnO Nanorods Synthesized by Self-Catalytic Method of Metal in Atmospheric Microwave Plasma Torch Flame

Zinc oxide (ZnO) nanorods synthesized by an explosive oxidation reaction in a microwave plasma torch flame were presented. The structural characteristics and morphology of the as-synthesized ZnO nanorods were investigated by X-ray diffraction (XRD) analysis, field-emission scanning electron microscopy (FESEM), field-emission transmission electron microscopy (FETEM), and photoluminescence (PL) spectroscopy. It was revealed by FETEM that most of the ZnO nanorods were pure and had an average diameter of 80 nm and a length of up to 300 nm, and some ZnO nanorods had a hexagonal tip, tripod-like shape with a length of up to 1 µm. PL spectra show only strong UV emission at approximately 380 nm without any defect-related emissions, indicating good crystalline ZnO. Also, a possible mechanism of the growth of the ZnO nanorods is discussed.

Low‐Temperature Synthesis of Zinc Oxide Nanoparticles

Crystalline zinc oxide nanoparticles have been prepared by mixing aqueous solutions of zinc nitrate and hexamethylenetetramine (HMT) at 60°C and 80°C. Transmission electron microscopy and X‐ray diffraction show that the ZnO nanoparticles of diameters ranging from 15–33 nm and 25–43 nm long are formed. Aspect ratio is observed to range from 1.18 to 1.74 at 60°C and 1.22 to 1.70 at 80°C as the HMT to zinc nitrate concentration ratio increases from 10 to 150. Nanoparticle size decreases as the concentration of HMT increases. Much larger ZnO particles are formed with ammonium hydroxide as a hydrolysis agent without HMT. In summary, HMT is an ammonium‐hydroxide source in the reaction, a surfactant for retaining nanosize, and not necessarily a template for ZnO nucleation.

Hydrothermal synthesis of highly crystalline ZnO nanoparticles: A competitive sensor for LPG and EtOH

Highly crystalline zinc oxide (ZnO) nanoparticles have been synthesized by hydrothermal route at 120 °C over a range of different time periods. The synthesized nanoparticles are characterized for their phase and morphology by X-ray diffraction (XRD) and transmission electron microscopy (TEM). Thermogravimetric-differential thermal analysis (TG-DTA) follows the thermal changes accompanying the heat treatment and ultra-violet diffuse reflectance spectroscopic studies (UV-DRS) give a measure of the optical properties. Electrical properties of the synthesized nanoparticles are studied by AC impedance and DC conductivity measurement. The gas sensing properties were studied towards reducing gases like liquefied petroleum gas (LPG), ammonia, hydrogen, ethanol (EtOH), etc., and it is observed that the nanoparticles show high sensitivity to LPG and ethanol at relatively low operating temperatures. Pd incorporation results in a decrease in operating temperature by more than 100 °C, and improves the sensing characteristics in terms of response and recovery times.

Humidity controlled thermal analysis

The low temperature formation of crystalline zinc oxide via thermal decomposition of zinc acetylacetonate monohydrate C10H14O4Zn·H2O was studied by humidity controlled thermal analysis. The thermal decomposition was investigated by sample-controlled thermogravimetry (SCTG), thermogravimety combined with evolved gas analysis by mass spectrometry (TG-MS) and simultaneous differential scanning calorimetry and X-ray diffractometry (XRD-DSC). Decomposition of C10H14O4Zn·H2O in dry gas by linear heating began with dehydration around 60°C, followed by sublimation and decomposition above 100°C. SCTG was useful because the high-temperature parallel decompositions were inhibited. The decomposition changed with water vapor in the atmosphere. Formation of ZnO was promoted by increasing water vapor and could be synthesized at temperatures below 100°C. XRD-DSC equipped with a humidity generator revealed that C10H14O4Zn·H2O decomposed directly to the crystalline ZnO by reacting with water vapor.

Nanosprings Take Shape

Nanosprings made of piezoelectric materials that change shape in response to applied electric voltage would be of great value in nanotechnology as actuators and sensors. In his Perspective, [Korgel][1] discusses results reported in the same issue by [ Gao et al. ][2] in which crystalline zinc oxide nanosprings have been fabricated with appreciable yield. An unusual mechanism appears to create the coiling: A superlattice defect structure within the material causes the lattice to rotate into a helical shape. The mechanical properties were measured, and future studies will probe the piezoelectric properties of these structures. Moreover, the superlattice rotation mechanism may lend itself to nanospring formation in other materials. [1]: http://www.sciencemag.org/cgi/content/full/309/5741/1683 [2]: http://www.sciencemag.org/cgi/content/short/309/5741/1700

Hybrid Solar Cells Using a Zinc Oxide Precursor and a Conjugated Polymer

AbstractWe describe a new method towards bulk‐heterojunction hybrid polymer solar cells based on composite films of zinc oxide (ZnO) and a conjugated polymer poly[2‐methoxy‐5‐(3′,7′‐dimethyloctyloxy)‐1,4‐phenylene vinylene] (MDMO‐PPV). Spin‐coating diethylzinc as a ZnO precursor and MDMO‐PPV from a common solvent at 40 % humidity and annealing at 110 °C provides films in which crystalline ZnO is found to be intimately mixed with MDMO‐PPV. Photoluminescence and photoinduced spectroscopy demonstrate that photoexcitation of these hybrid composite films results in a fast and long‐lived charge transfer from the polymer as a donor to ZnO as ato be obtained n acceptor. Using the ZnO‐precursor method, hybrid polymer solar cells have been made with an estimated air‐mass of 1.5 (AM 1.5) energy conversion efficiency of 1.1 %. This new method represents a fivefold improved performance compared to similar hybrid polymer solar cells based on amorphous TiO2.

Microwave-hydrothermal synthesis and photoluminescence characteristics of zinc oxide powders

A microwave-hydrothermal process for the synthesis of crystalline zinc oxide powders has been developed in this study. Well-crystallized zinc oxide powders exhibiting different morphology, crystallinity, and particle size have been successfully prepared by controlling the process temperature and molarity of NH4OH in the starting solution. With increasing process temperature and NH4OH molarity during synthesis, the morphology of ZnO powders changes from flowerlike agglomeration to a well-developed rodlike shape. The band gap of ZnO powders increases with a decrease in the molarity of NH4OH during synthesis. Vacuum ultraviolet radiation (VUV) excited luminescence studies for ZnO powders reveal an excitation band at 161 nm possibly due to the absorption of O2− 2p electrons in the valence band. The VUV excitation band of ZnO powders observed at 161 nm will be useful for excitation of gas-discharged plasma display devices.

Morphology Control of ZnO Nanomaterials using Double Hydrophilic Block Copolymers

AbstractHighly crystalline zinc oxide (ZnO) nanomaterials are synthesized using a seeded growth sol-gel method. In order to control the morphology and the organization of the ZnO nanomaterials, a double hydrophilic block copolymer has been introduced during the seeded growth synthesis: the Polyacrylic acid-Polyacrylamide (PAA-PAM). Depending on the amount of PAA-PAM copolymers, different morphologies were obtained, such as ZnO nanostructured spheres or flat hexagonal crystals. Thus, systematic studies have been done to investigate the influence of the copolymer addition on ZnO nanomaterial morphologies and explain the mechanisms of the morphological modifications.

Hybrid thin films of ZnO with porphyrins and phthalocyanines prepared by one-step electrodeposition

Hybrid thin films of crystalline zinc oxide and the zinc complex of 5,10,15,20-tetrakis-(4-sulfonatophenyl)porphyrin (ZnTPPS) have been prepared by cathodic electrodeposition from different aqueous zinc salt solutions. Films from a solution containing zinc nitrate and the porphyrin derivate show a rough surface and porosity, whereas films prepared from an oxygen-saturated zinc chloride solution were very smooth. The presence of the porphyrin derivative in the deposition solution has a clear influence on the morphology of the hybrid films compared with pure zinc oxide films. In aqueous zinc nitrate solution the addition of the porphyrin derivative hinders the growth of zinc oxide. In contrast, the addition of the dye to the oxygen-saturated zinc chloride solution leads to an increase of the growth rate of ZnO /porphyrin hybrid films. An increased spectral absorbance was reached when two dyes (zinc complex of tetrasulfonated phthalocyanine and porphyrin) were added to the zinc nitrate solution since the electrodeposited hybrid thin films contained both dyes. UV-vis spectra revealed the presence of both macrocyclic metal complexes in the ZnO films.

Organometallic chemical vapor deposition of compound semiconductors

Nano-sized zinc oxide (ZnO) was prepared by metal organic chemical vapor deposition (MOCVD) in a sulfonated polystyrene matrix. The effects of ultraviolet light irradiation on the growth of ZnO was examined. Scanning electron microscopy (SEM) analysis reveal that the ZnO nanoparticles have polycrystalline hexagonal wurtzite structure. The photodeposited films showed lower resistivities relatively to the non-irradiated films at 150 °C. Quantum chemistry was used to investigate the possible reactions of diethylzinc (DEZ) on PS-SO 3H surface. The semiempirical simulations at PM3 level indicates strong interactions of DEZ with the polymeric surface and may be able to explain the growth of the highly crystalline ZnO on the polymeric substrate, as observed experimentally.

Preparation of c-axis-oriented zinc-oxide thin films and the study of their microstructure and optical properties

c-axis-oriented zinc-oxide (ZnO) thin films were prepared on microscope glass substrates by sol–gel deposition. A homogeneous and stable solution was prepared by dissolving zinc-acetate 2-hydrate (ZnAc) in 2-propanol and diethanolamine. Crystalline ZnO thin films were obtained following an annealing process at temperatures between 450°C and 550°C for 1 h. Increasing annealing temperature increased the grain size and the c-axis orientation. The X-ray diffraction analysis revealed single-phase ZnO with hexagonal zincite structure. The absorption edge analysis revealed that the optical band-gap energy for the films were between 3.26 and 3.28 eV and electronic transition was direct transition type.

Synthesis of zinc oxide nanorods using carbon nanotubes as templates

The filling of carbon nanotubes with zinc oxide via wet chemical techniques is presented. Crystalline zinc oxide was introduced into carbon nanotubes by immersing empty and dried opened tubes in zinc nitrate solution followed by filteration and calcination, which was characterized by means of energy-dispersive X-ray spectroscopy and transmission electron microscopy (TEM). After burning off the carbon nanotubes at 750°C in air for 2 h, crystalline zinc oxide nanorods, with diameters in the range of ca. 20–40 nm and length of up to 1 μm, were formed, and characterized by powder X-ray diffraction (XRD), TEM and HRTEM. XRD indicated that the nanorods were hexagonal ZnO.

Nanocolumnar ZnO films for photovoltaic applications

Zinc oxide (ZnO) is a wide gap semiconductor that if grown under nanostructured form, is suitable for photovoltaic applications as a way to enlarge the active part of a solar cell. Voids of enlarged surfaces of textured substrates can be filled with a thin absorber. The main advantage of such light-collecting devices is the short electronic transport distance needed which reduces the requirement of large diffusion lengths. Crystalline ZnO nanocolumns were obtained by electrodeposition on polycrystalline conductive glass and on monocrystalline GaN substrates. Sizes and surface–volume ratios of the ZnO columns depend mainly on the growth parameters such as current density, deposition time and temperature of the bath. The intense ultraviolet photoluminescence emission observed in ZnO columns grown under different conditions depends mainly on post-annealing treatment.

Influence of DC magnetron sputtering parameters on the properties of amorphous indium zinc oxide thin film

Amorphous or crystalline indium zinc oxide (IZO) thin films, which are highly transparent and conducting, were deposited by DC magnetron sputtering. X-Ray diffraction technique was used for analyzing microstructures of the films, and also differential thermal analysis was performed for observing their crystallization behavior. The IZO thin films prepared were crystallized at much higher temperature than ITO films were. The crystallized samples showed (222) preferred orientations. By varying process parameters, the optimum conditions for the highest electrical conductivity and optical transmittance, and the lowest surface roughness were found. The resistivity of IZO films decreased as the deposition temperature increased until 250 °C, but sharp rise occurred at or above 300 °C. The extinction coefficients diminished in the films prepared with the conditions of higher deposition temperature, sputtering gas of light mass, and heat treatment. However, excessive amount of oxygen flow during deposition brought about the increase of the extinction coefficients. The variations of extinction coefficients mainly influenced the transmittance of the samples. On the basis of X-ray photoelectron spectroscopy analysis, atomic force microscopy measurement, spectroscopic ellipsometry and spectrophotometer measurement, several characteristics of IZO thin films were discussed comparing with those of ITO thin films. Very low surface roughness of IZO thin films could satisfy the requirement for organic light-emitting diode.

One-step electrochemical synthesis of ZnO/Ru(dcbpy) 2(NCS) 2 hybrid thin films and their photoelectrochemical properties

Hybrid thin films of crystalline zinc oxide (ZnO) and Ru(dcbpy) 2(NCS) 2 (dcbpy=4,4′-dicarboxy-2,2′-bipyridine) have been synthesized by one-step cathodic electrodeposition from oxygen-saturated aqueous solutions containing zinc chloride and the Ru complex. While the pure ZnO thin films are oriented with the c-axis perpendicular to the substrate, the ZnO crystallites in the hybrid thin films are oriented with the c-axis parallel to the substrate as a consequence of the adsorption of the Ru complex onto the growing surface of ZnO. Thin hybrid films (up to about 0.5 μm) consist of grains with approximately 300 nm diameter, which are composed of ZnO crystallites of approximately 20 nm. For thicker films, the formation of an overlayer in a totally different morphology has been observed. The hybrid thin films have a composition of ZnO:Ru(dcbpy) 2(NCS) 2=30:1 in molar ratio, corresponding to a volume ratio of ZnO:Ru(dcbpy) 2(NCS) 2=1:1.7, indicating a very high amount of the Ru complex in the film. The loaded Ru complex can be mostly extracted by dipping the film in a dilute alkaline solution, indicating the porous nature of the film and the presence of the dye mainly on the surface of ZnO. The hybrid thin film electrodes exhibit sensitized photoanodic currents upon white light illumination in organic electrolyte solutions containing iodide which are analyzed in terms of wavelength dependence, I–V characteristics and by time-resolved photocurrent measurements. The thinner films performed considerably better than the thicker films in which dye aggregation seemed to decrease the electron injection efficiency.

Hydrothermal synthesis of zinc oxide powders with controllable morphology

Crystalline zinc oxide powders with various morphologies have been prepared by hydrothermal treatment of zinc acetate in pure water, KOH or ammonia aqueous solution. It was found that the selected solvents play a different role in controlling the morphologies of the obtained powders. The variation of morphology of the obtained ZnO powder with solvents mainly depends on the different zinc species and their environment during the hydrothermal processing.