Defect-controlled growth of ZnO nanostructures using its different zinc precursors and their application for effective photodegradation

Various zinc precursors, such as zinc acetate, zinc nitrate, zinc sulfate, and zinc chloride, have been used to control the formation of zinc oxide (ZnO) nanostructures onto aluminum substrate by chemical means. FESEM images of the ZnO nanostructures showed the formation of different morphologies, such as flakes, nanowalls, nanopetals, and nanodisks, when the nanostructures were synthesized using zinc acetate, zinc nitrate, zinc sulfate, and zinc chloride precursors, respectively. The TEM image of disk-like ZnO nanostructures formed using zinc chloride as a precursor revealed hexagonally shaped particles with an average diameter of 0.5 μm. Room-temperature photoluminescence (PL) spectra revealed a large quantity of surface oxygen defects in ZnO nanodisks grown from zinc chloride compared with those using other precursors. Furthermore, the ZnO nanostructures were evaluated for photocatalytic activity under ultraviolet (UV) light illumination. Nanostructures having a disk-like shape exhibited the highest photocatalytic performance (k = 0.027 min⁻¹) for all the ZnO nanostructures studied. Improved photocatalytic activity of ZnO nanodisks was attributed to their large specific surface area (4.83 m² g⁻¹), surface oxygen defects, and super-hydrophilic nature of their surface, which is particularly suitable for dye adsorption.     

Synthesis of ZnO compound nanostructures via a chemical route for photovoltaic applications

A facile, low-temperature, and low-cost chemical route has been developed to prepare ZnO nanowire and nanosphere compound structures. The morphology, structure, and composition of the yielded products have been examined by field-emission scanning electron microscopy, transmission electron microscopy, and X-ray diffraction measurements. We have systematically investigated the optical properties of the ZnO nanostructures by micro-Raman, photoluminescence, and transmission spectroscopy. The results demonstrate that the yielded ZnO nanostructures possess good optical quality with high light absorption. We have further successfully employed the obtained ZnO compound nanostructures in dye-sensitized solar cells. The light-to-electricity conversion results show that the compound nanostructure exhibits a significant enhancement of short-circuit current density due to the increased surface area and light scattering in the compound nanostructures. The present chemical route provides a simple way to synthesize various compound nanostructures with high surface area for nanodevice applications.     

Synthesis and field-emission properties of roselike ZnO nanostructures

Self-assembled roselike ZnO nanostructures were synthesized via thermal evaporation of zinc powders without catalytic assistance at the relatively low temperature of 550 °C. The roselike structures consist of a large number of ZnO nanorods that uniformly arrange into hexagonal multilayers. The spontaneous nanoindentation effects under geometric constraints can be used to explain the structures. The cathodoluminescence spectra show a wide visible emission band related to Zn interstitials and oxygen vacancies. Field emission measurements demonstrate that the roselike ZnO nanostructures possess good electron emission characteristics with a turn-on field of 4.3 V/μm. PACS 68.70.+w; 78.55.Cr; 81.05.Cy     

Synthesis and ultraviolet emission of aligned ZnO rod-on-rod nanostructures

Aligned ZnO rod-on-rod nanostructures were synthesized on silicon substrate via a simple thermal evaporation process at low temperature without catalysts. Pictures taken with the use of the scanning electron microscope demonstrate that the well-ordered ZnO rod-on-rod nanostructures grow on the Si substrate, and the single nanostructure consists of two parts. Transmission electron microscopy image and the selected area electron diffraction pattern indicate that the single-crystal nanorod grows along [0001] direction. The X-ray diffraction pattern proves that the samples have good crystal quality. The detailed nanorod growth mechanism is proposed and discussed. The room-temperature photoluminescence (PL) spectrum shows the dominant ultraviolet emission, which indicates their potential application in ultraviolet optoelectronic devices. The temperature-dependent PL spectra reveal that the strong ultraviolet emission should originate from the longitudinal optical phonon replicas of free exciton.     

Synthesis and characterization of undoped and tin-doped ZnO nanostructures

In this paper, undoped and tin-doped ZnO nanostructures were grown onto non-conductive substrates by a simple solution method. Structural, morphological, optical and electrical properties of the structures were investigated with respect to tin concentration. From XRD studies, all the ZnO nanostructures were found as hexagonal wurtzite type structures growing preponderantly oriented with c-axis normal to the substrate. An increase in tin content resulted in a decrease in grain size, whereas the dislocation density increases. SEM observations indicated that all the structures were textured throughout the substrates without any cracks or pores. The influence of incorporation of tin on surface morphology of the samples was clearly seen. Average diameter of the nanostructures decreased with increasing tin content. Absorption spectra of the structures revealed that the band gap of the films increases with increasing tin concentration. It is found that the tin-doped samples have higher average transmittance than the undoped one. The 1?% tin-doped sample exhibited ??80?% average transparency, which was the best transparency among the doped samples. Electrical measurements showed that resistivity of the structures increased with increasing dopant concentration. This increasing was attributed due to a decrease in carrier concentration caused by carrier traps at the grain boundaries.     

Effect of zinc sources on the morphology of ZnO nanostructures and their photoluminescence properties

The morphology and photoluminescence properties of ZnO nanostructures synthesized from deferent zinc sources by a vapor deposition process were investigated. The zinc sources involved pure zinc, ZnO, and ZnCO₃ powders, respectively. It was found that the zinc sources have a strong effect on the morphology of the ZnO nanostructures. For the pure zinc and ZnO sources, uniform ZnO nanowires and tetrapods are obtained, respectively. However, in the case of the ZnCO₃ source, the products are nanowire–tetrapod combined nanostructures, in which ZnO nanowires grow from the ends of tetrapod arms. The morphology differences of these products may be mainly concerned with the yield and constituents of the corresponding zinc vapor. Photoluminescence measurements show that the nanowires have a relatively stronger near-band UV emission than the other products. The strongest green-light emission from the tetrapods implies that more defects exist in the tetrapods. An evident peak at 430 nm is found in the spectrum of the nanowire–tetrapod combined nanostructures, which may be caused by oxygen-depletion interface traps. PACS 73.61.Tm; 81.10.Bk; 78.55.Et     

Effect of substrates and anions of zinc salts on the morphology of ZnO nanostructures

The ZnO nanostructures were hydrothermally synthesized on glass and Al substrates, respectively, using zinc chloride, zinc nitrate, and zinc acetate as precursor. The as-prepared products were characterized by scanning electron microscopy (SEM) and X-ray powder diffraction (XRD). Different ZnO nanostructures were obtained, such as nanorods, nanosheets, flower-like nanostructures and so on. The effects of the substrates and anions of zinc salts on the morphologies of the resulting products have been investigated.     

Effect of zinc nitrate concentration on the structural and the optical properties of ZnO nanostructures

ZnO nanorods and nanodisks were formed on indium-tin-oxide-coated glass substrates by using an electrochemical deposition method. Scanning electron microscopy images showed that the ZnO nanorods were transformed into nanodisks with increasing Zn(NO₃)₂ concentration. X-ray diffraction patterns showed that the ZnO nanostructures had wurzite structures. The full widths at half maxima of the near band-edge emission peak of photoluminescence spectra at 300 K for ZnO nanorods were small, indicative of the high quality of the nanorods. These results indicate that the structural and the optical properties of ZnO nanostructures vary by changing Zn(NO₃)₂ concentration.     

Synthesis, morphology and random laser action of ZnO nanostructures

Three-dimensional (3-D) ZnO random-wall nanostructures and one-dimensional (1-D) ZnO nanorods were prepared on silicon substrates by a simple solid-vapour phase thermal sublimation technique. Optical pumped random lasing has been observed in the ZnO random-wall arrays with a threshold intensity of 0.38 MW/cm² in the emission wavelength from 380 to 395 nm. The optical gain was attributed to the closed-loop scattering and light amplification of the ZnO random-wall. The experimental result suggests that the morphology of nanostructure is the key factor to effect random lasing.     

Field emission from lotiform-like ZnO nanostructures synthesized via thermal evaporation method

Novel lotiform ZnO nanostructures were synthesized on silicon substrate via simple thermal evaporation. The average diameter of the ZnO nanostructures is ∼1.5 μm. The lotiform-like ZnO structures were formed by nanorods arrays with the average diameter of 70 nm. The as-grown lotiform ZnO nanostructures have excellent field-emission properties such as the low turn-on field of 3.4 V/μm, and very high emission current density of 12.4 mA/cm² at the field of 9.6 V/μm. These features make the lotiform-like ZnO nanostructures competitive candidates for field-emission-based displays. PACS 61.46.-w; 61.82.Rx; 78.67.-n; 73.63.Bd; 74.78.Na     

Synthesis of nanocrystalline ZnO nanobelts via pyrolytic decomposition of zinc acetate nanobelts and their gas sensing behavior

The pyrolytic decomposition of layered basic zinc acetate (LBZA) nanobelts (NBs) into nanocrystalline ZnO NBs is investigated using scanning electron microscopy (SEM), atomic force microscopy (AFM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and photoluminescence (PL). We also report on the gas sensing response of the resulting ZnO nanomaterial to CO. The LBZA NBs are grown at 65 °C in an aqueous solution of zinc acetate dihydrate. AFM and SEM results show as-grown products possess the characteristic layered structure of the LBZA crystals. XRD and XPS results show that annealing as-grown products at 210 °C in air causes a transformation from zinc acetate to nanocrystalline ZnO NBs via thermal decomposition. The ZnO crystalline domain size increases with temperature from 9.2 nm at 200 °C to 94 nm at 1000 °C, as measured from XRD. SEM shows evidence of sintering at 600 °C. The thickness of the NBs, determined via AFM, ranges from 10 to 50 nm and remains approximately constant with annealing temperature. XPS confirmed the chemical transformation from zinc acetate to ZnO and showed a significant remaining zinc hydroxide component for the ZnO NBs consistent with published results. PL measurements at room temperature show a blue shift in peak emission as the nanobelts change from LBZA to ZnO at 200 °C. Above this transition temperature, the ZnO nanobelts possess strong band edge emission at 390 nm and little broad band emission in the visible region. The AFM and SEM images reveal that the crystallites within the nanobelts orientate in rows along the long axis during annealing. This structure provides a high surface area to volume ratio of aligned nanoparticles which is beneficial for gas sensing applications. Gas sensors fabricated from 400 °C annealed nanobelts showed a response of 1.62 when exposed to 200 ppm of CO in dry air at 400 °C, as defined by the ratio of resistance before and during exposure. This indicates that ZnO nanostructures obtained by thermal decomposition of LBZA NBs could provide a cost effective route to high sensitivity gas sensors.     

Morphology engineering of ZnO nanostructures

Nanosized ZnO structures were grown by atmospheric pressure metalorganic chemical vapor deposition (APMOCVD) in the temperature range 200–500 °C at variable precursor pressure. Temperature induced evolution of the ZnO microstructure was observed, resulting in regular transformation of the material from conventional polycrystalline layers to hierarchically arranged sheaves of ZnO nanowires. The structures obtained were uniformly planarly located over the substrate and possessed as low nanowires diameter as 30–45 nm at the tips. The observed growth evolution is explained in terms of ZnO crystal planes free energy difference and growth kinetics. For comparison, the convenient growth at constant precursor pressure on Si and SiC substrates has been performed, resulting in island-type grown ZnO nanostructures. The demonstrated nanosized ZnO structures may have unique possible areas of application, which are listed here.     

Synthesis of zinc oxide nanostructures catalytically active in the optical range without irradiation

A technique for synthesizing catalytically active zinc oxide-based nanostructures is described. The synthesis product represents a blue-gray powder. When it is introduced into a water solution of methyl orange the latter bleaches out both under daylight and in the dark. The activity of the material is measured by the Kubelka-Munk formula in the wavelength range 0.3–1.8 μm. As follows from X-ray phase analysis data, the synthesized material is a composite consisting of hexagonal structures with the parameters ZnO [a: 3.2491, c: 5.2046] and Zn [a: 2.6639, c: 4.9397]. A mechanism underlying the formation of a ZnO-Zn nanocomposite is suggested.     

Synthesis of ZnO nanostructures with controlled morphology and size in ionic liquids

Nanostructured ZnO has been synthesized by a hydrothermal route, using different ionic liquids (ILs) as the morphology templates. The morphology of ZnO changes from rod-like to star-like and flower-like in different ILs. A 3D nano/micro structure ZnO with unique flower-like morphology has been synthesized via the assembly of dicationic IL and [Zn(OH)₄]²⁻. The flower-like pattern was obtained in the presence of IL 1. The flower-like ZnO structure has a hexagonal prism, with a hexagonal pyramid on the tip, and diameter of ~444 nm. While the ZnO prepared in IL 2, shows uniform rod-like shape with a diameter of 91 nm, star-like morphology consisting of nanorods with diameter of ~109 nm was formed in IL 3. The XRD, SEM, and PL spectra have been employed for characterization of the synthesized ZnO nano structures.     

Growth of zinc oxide nanostructures

Zinc oxide (ZnO) nanowhiskers have been prepared using a multilayer ZnO(50 nm)/Zn(20 nm)/ZnO(2μm) structure on a polished stainless steel (SS) substrate by high rate magnetron sputtering. The formation of uniformly distributed ZnO nanowhiskers with about 20 nm dia. and 2 to 5 μm length was observed after a postdeposition annealing of the prepared structure at 300–400° C. An array of highlyc-axis oriented ZnO columns (70–300 nm in dia. and up to 10 μm long) were grown on Si substrates by pulsed laser deposition (PLD) at a high pressure (1 Torr), and Raman studies showed the activation of surface phonon modes. The nanosized powder (15–20 nm) and nanoparticle ZnO films on glass substrate were also prepared by a chemical route. Nanowhiskers showed enhanced UV light detection characteristics, and the chemically prepared ZnO nanoparticle films exhibited good sensing properties for alcohol     

Sonochemical synthesis of hierarchical ZnO nanostructures

This work is about fabrication of ZnO nanostructures (ZnO-NS) via a simple sonochemical method. The chemicals used for the synthesis of various shaped ZnO are Zn salt, sodium hydroxide and ammonia solution without other structure directing agent or surfactant needed. This method is feasible and green, as it does not require high temperature and/or highly toxic chemicals. The shape of the ZnO-NS can be tuned by adjusting the ultrasound energy dissipated via varying the ultrasonication time from 5 to 60 min. It was found that uniform ZnO nanorods with diameter around 50 nm were formed after 15 min of ultrasonication while flowerlike ZnO-NS was formed after 30 min. This method produces high quality ZnO-NS with controllable shapes, uniformity, and purity.     

Synthesis of flower-like cobalt nanostructures: optimization by Taguchi design

In this research, flower-like cobalt nanostructures were synthesized in a solution of water and ethylene glycol under microwave radiation without using metal foils or templates for self-assembly. The synthesized nanoparticles were completely stable in free air. Size distribution and phase of nanoparticles can be controlled through the synthetic process. X-Ray Diffraction (XRD), Fourier Transform IR (FTIR), and Scanning Electron Microscopy (SEM) measurements were carried out to investigate the structural properties and to interpret the formation process of flower-like cobalt nanostructures. Size distribution and average size of particles were determined by Dynamic Laser Light Scattering (DLLS) measurements. Thermal Gravimetric Analysis (TGA) was used to investigate the thermal stability of cobalt nanoparticles. All experiments were designed to fulfill the L₈ Taguchi design and data analysis was performed using MINITAB software.