Preparation and photoluminescence of ZnO with nanostructure by hollow-cathode discharge

Without the use of a metal catalyst in the process, ZnO with nanostructures was successfully prepared on Si (100) substrate by simple chemical vapor-deposition method. In our work, Ar was used as the plasma forming gas, O₂ was the reactive gas and metal zinc powder (99.99% purity) vaporized by cylinder hollow-cathode discharge (HCD) acted as the zinc source. The crystal structures of the as-synthesized ZnO nanostructures were characterized by X-ray diffraction (XRD); the ZnO sample growing on the wall of the crucible showed a ‘comb-like’ nanostructure, while the other one at the bottom of the crucible showed a ‘rod-like’ structure, which can be attributed to the difference of the oxygen content. The measurement on the photoluminescence (PL) performance of the ZnO nanostructures was carried out at room temperature. The results indicated that the ‘comb-shape’ ZnO nanomaterial possessed a remarkably strong ultraviolet emission peak centered at 388 nm, while ZnO nanorods, except better ultraviolet emission, also had relatively strong blue-green emission ranging from 470 to 600 nm due to the existence of oxygen vacancies. The growth mechanism of ZnO with nanostructures is also discussed in this paper.     

Preparation and photoluminescence of ZnO with nanostructure by hollow-cathode discharge

Without the use of a metal catalyst in the process, ZnO with nanostructures was successfully prepared on Si (100) substrate by simple chemical vapor-deposition method. In our work, Ar was used as the plasma forming gas, O₂ was the reactive gas and metal zinc powder (99.99% purity) vaporized by cylinder hollow-cathode discharge (HCD) acted as the zinc source. The crystal structures of the as-synthesized ZnO nanostructures were characterized by X-ray diffraction (XRD); the ZnO sample growing on the wall of the crucible showed a ‘comb-like’ nanostructure, while the other one at the bottom of the crucible showed a ‘rod-like’ structure, which can be attributed to the difference of the oxygen content. The measurement on the photoluminescence (PL) performance of the ZnO nanostructures was carried out at room temperature. The results indicated that the ‘comb-shape’ ZnO nanomaterial possessed a remarkably strong ultraviolet emission peak centered at 388 nm, while ZnO nanorods, except better ultraviolet emission, also had relatively strong blue-green emission ranging from 470 to 600 nm due to the existence of oxygen vacancies. The growth mechanism of ZnO with nanostructures is also discussed in this paper.     

Fabrication and growth mechanism of hexagonal zinc oxide nanorods via solution process

This article presents, the fabrication of perfectly hexagonal zinc oxide nanorods performed via solution process using zinc nitrate hexahydrate (Zn(NO₃)₂·6H₂O) and hexamethylenetetramine (HMT) at various concentrations of i.e. 1 × 10⁻³ to 10 × 10⁻² M in 50 mL distilled water and refluxed at 100 °C for 1 h. We used HMT because it acts as a template for the nucleation and growth of zinc oxide nanorods, and it also works as a surfactant for the zinc oxide structures. The X-ray diffraction patterns clearly reveal that the grown product is pure zinc oxide. The diameters and lengths of the synthesized nanorods lie in the range of 200–800 nm and 2–4 μm, respectively as observed from the field emission scanning electron microscopy (FESEM). The morphological observation was also confirmed by the transmission electron microscopy (TEM) and clearly consistent with the FESEM observations. The chemical composition was analyzed by the FTIR spectroscopy, and it shows the ZnO band at 405 cm⁻¹. On the basis of these observations, the growth mechanism of ZnO nanostructures was also proposed.     

Facile synthesis of ZnO nanostructures and investigation of structural and optical properties

ZnO nanostructures were synthesized by chemical bath deposition method, using zinc nitrate [Zn(NO₃)₂] and hexa-methylene-tetra-amine [(HMT),C(H₂)₆N₄] as precursors. Controlled size and shape evolution of ZnO nanostructures were achieved by changing the HMT concentration from 0.025 M to 0.1 M, whereas Zn(NO₃)₂ concentration kept constant. X-ray diffraction (XRD) and Raman study confirmed the formation of single crystalline, hexagonal wurtzite ZnO structure. Sharp peaks in Raman spectra, corresponding to E₂(low) and E₂(high) referred to wurtzite structure with higher order of crystallinity. Transmission electron microscopy (TEM) revealed that the shape and size of the nanostructures reduced, with increasing concentration of HMT. Further, effect of structure's size was observed in the band gap (shift). Photoluminescence study showed two peaks at ~ 380 nm and ~ 540 nm corresponding to the band to band transition and defect transitions. Modifications of properties are explained in detail on the basis of shape and size change of the structures and possible mechanism is discussed.     

Effects of ZnO nanowire synthesis parameters on the photovoltaic performance of dye-sensitized solar cells

Determination of the effects of ZnO nanowires on the efficiency of ZnO nanowire-based dye-sensitized solar cells (DSSCs) is important. In this study, we determined the effects of different OH^- precursors, concentrations, the ratio of zinc nitrate to hexamethylene tetramine (HMT), and the hydrothermal synthesis temperature on the physical, crystal, and optical properties of ZnO nanowires and investigated the performance of the resulting DSSCs. We observed that ZnO nanowires synthesized using an equimolar ratio of HMT to zinc nitrate yielded a DSSC with high incident photon-to-current efficiency (IPCE), cell efficiency, short circuit current density (J_sc), and fill factor (FF), and low ZnO-dye-electrolyte interface resistance due to an increased amount of dye and a decreased density of defects. Furthermore, ZnO nanowires made using optimal concentrations and ratios of zinc nitrate to HMT had a high surface area and low defect density. All the photovoltaic performance parameters of DSSCs assessed such as IPCE, cell efficiency, J_sc, open circuit potential (V_oc), and FF increased with synthesis temperature, which was related to a decrease in the resistance at the ZnO-dye-electrolyte interface. We attributed these results to an increased amount of dye facilitated by a large nanowire surface area and fast electron transfer because of the improved crystalline structure of the ZnO nanowires and their low defect density. By optimizing the ZnO nanowires, we increased DSSC efficiency to 0.26% using ZnO nanowires synthesized with 25 mM of both zinc nitrate and HMT at 90 °C, while only a 0.02% increase in efficiency was obtained when NH₄OH was used as OH⁻ precursor.     

Growth of zinc oxide nanorods, tetrapods, and nanobelts without catalyst

Zinc oxide (ZnO) nanostructures with various morphologies have been synthesized without catalyst in a one-step simple redox process. The results show that ZnO nanorods, nanobelts, and tetrapods with hexagonal needled arms could be synthesized via thermal treatment of a mixture of zinc oxide and charcoal powder in a muffle furnace at 1000-1200 degrees C for 240 min. XRD analyses showed that polycrystalline ZnO phase with wurtzite crystal structure was formed. At a relatively low temperature, 1000 degrees C, the ZnO structure was found to be a bundle of denser nanorods. By increasing the reaction temperature to 1100 degrees C, tetrapod-like structures of needle-like arms with pyramidal tips were formed. With the increase of temperature up to 1200 degrees C, the morphology of ZnO nanostructures changed from nanorods and tetrapods to coalescence grains. Reaction temperature was found to be the most important experimental parameter that played an important role in controlling the mode, mechanism of growth, and formation of different ZnO morphologies.     

Controlled morphology of ZnO nanostructures by adjusting the zinc foil heating temperature in an air-filled box furnace

By heating zinc foil in an air-filled box furnace, one-dimensional ZnO nanorods, two-dimensional ZnO nanoplates and three-dimensional ZnO nanotetrapods were prepared by adjusting the temperature in the furnace in the ranges of 500–600, 650–750 and 800–900 °C, respectively. The morphologies, structures and emissions of the synthesized ZnO nanostructures were investigated by scanning electron microscopy, X-ray diffractometry, transmission electron microscopy, selected area electron diffraction and photoluminescence spectroscopy. Mechanisms on the control of the morphology and photoluminescence were discussed in terms of the crystal growth habits combined with the temperature-dependent diffusions of zinc and oxygen atoms in the ZnO lattices.     

Anions effect on the low temperature growth of ZnO nanostructures

Seed mediated aqueous chemical growth (ACG) route was used for the growth of ZnO nanostructures on Si substrate in four different growth mediums. The growth medium has shown to affect the morphology and the size of the different nanostructures. We observed that the medium containing zinc nitrate anions yields the nanorods, in a medium containing zinc acetate anions nano-candles are obtained. While in a medium containing zinc chloride anions ZnO nano-discs were obtained and in a medium containing zinc sulfate anions nano-flakes are achieved. Growth in these different mediums has also shown effect on the optical emission characteristics of the different ZnO nanostructures.     

Growth and properties of ZnO nanowires synthesized by a simple hydrothermal method

Hexagonal ZnO nanowires were synthesized on pre-seeded silicon (100) substrates by a simple hydrothermal method at a relatively low temperature of 95 °C without any catalyst or template. The pre-seeded layer was produced using the sol–gel spin coating technique with 1 M zinc acetate in ethanol and ethanolamine. The structural properties of the nanowires were characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The XRD pattern indicated that the as-grown ZnO nanowires had the single-phase wurtzite structure, formed along the c-axis. SEM revealed that the nanostructure thin film had wire textures and the synthesis processes importantly influence the final size and shape of the ZnO nanowires. High-resolution transmission electron microscopy (HRTEM) provided further insight into the structure of ZnO nanostructures. The obtained HRTEM image was of the tip of an individual nanowire. The ZnO nanowires highly preferentially grew in the (002) crystal plane. The lattice spacing between adjacent (002) lattice planes was calculated to be 0.52 nm. The optical characteristics of the nanowires were determined from cathodoluminescence (CL) spectra. The CL revealed a fairly high surface state density of ZnO nanowires that grew at reaction concentrations of 0.01–0.25 M.     

Fabrication and characterization of tetrapod-like ZnO nanostructures prepared by catalyst-free thermal evaporation

Tetrapod-like ZnO nanostructures were fabricated on ZnO-coated sapphire (001) substrates by two steps: pulsed laser deposition (PLD) and catalyst-free thermal evaporation process. First, the ZnO films were pre-deposited on sapphire (001) substrates by PLD. Then the ZnO nanostructures grew on ZnO-coated sapphire (001) substrate by the simple thermal evaporation of the metallic zinc powder at 900 °C in the air without any catalysts. The pre-deposited ZnO films by PLD on the substrates can provide growing sites for the ZnO nanostructures. The as-synthesized ZnO nanostructures were characterized by using X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM) and Fourier transform infrared spectrum (FTIR). The results show that the tetrapod-like ZnO nanostructures are highly crystalline with the wurtzite hexagonal structure. Photoluminescence (PL) spectrum of as-synthesized nanostructures exhibits a UV emission peak at ～ 389 nm and a broad green emission peak at ～ 513 nm. In addition, the growth mechanism of ZnO nanostructures is also briefly discussed.     

Microwave-assisted hydrothermal synthesis of zinc oxide particles starting from chloride precursor

Zinc oxide (ZnO) was synthesized using a microwave assisted hydrothermal (MAH) process based on chloride/urea/water solution and under 800 W irradiation for 5 min. In the bath, Zn²⁺ ions reacted with the complex carbonate and hydroxide ions to form zinc carbonate hydroxide hydrate (Zn₄CO₃(OH)₆·H₂O), and the conversion from Zn₄CO₃(OH)₆·H₂O to ZnO was synchronously achieved by a MAH process. The as-prepared ZnO has a sponge-like morphology. However, the initial sponge-like morphology of ZnO could change to a net-like structure after thermal treatment, and compact nano-scale ZnO particles were finally obtained when the period of thermal treatment increased to 30 min. Pure ZnO nanoparticles was obtained from calcination of loose sponge-like ZnO particles at 500 °C. The analysis of optical properties of these ZnO nanoparticles showed that the intensity of 393 nm emission increased with the calcination temperature because the defects were reduced and the crystallinity was improved.     

Microwave-assisted synthesis and characterization of flower shaped zinc oxide nanostructures

A microwave-assisted solution-phase approach has been applied for the synthesis of zinc oxide nanostructures. The synthesis procedure was carried out by using two reagents: hydrazine hydrate and ammonia. Flower shaped particles were obtained with hydrazine hydrate whereas mainly spherical agglomerated particles were observed with ammonia. The nanostructures were influenced by microwave irradiation time, reagent concentration and molar ratio of the precursors. High crystalline materials were found without the need of a post-synthesis treatment. The average crystalline size of ZnO nanostructures has been analyzed by X-ray Diffraction (XRD) pattern and estimated to be 18 nm. The presence of flower shaped zinc oxide with nanorods arranged has been confirmed from Scanning Electron Microscopy (SEM) and Transmission Electron Microscopy (TEM) micrographs. The samples were further analyzed by Fourier Transform InfraRed (FT-IR), Thermogravimetric Analysis (TGA) and photoluminescence spectroscopic techniques.     

ZnO准一维纳米结构生长形态的演化机理

利用气相生长系统,通过调控实验参数,制备了多种形貌的ZnO准一维结构,如纳米条带、[011-0]和[21-1-0]取向的单侧生齿的梳状纳米条带、微米尺度的梳状结构,由多节状六角棱柱和八角棱柱组装成的微米条带等.通过X射线衍射、扫描电子显微镜及其所加载的能谱分析和背散射电子衍射仪、高分辨透射电子显微镜等分析技术, 对其中具有代表性的介观结构进行了系统的形貌分析和细致的结构解析.分析出基本的结构单元及其复合体, 揭示了显微尺度下ZnO晶体的外形多样性以及其形态演化中的关联和规律,即ZnO纳米条带、梳状结构和多节状微米条带具有晶体结构上的同一性.     

ZnO nanostructured microspheres and grown structures by thermal treatment

Synthesis of flower-shaped ZnO nanostructures composed of ZnO nanosticks was achieved by the solution process using zinc acetate dihydrate, sodium hydroxide and polyethylene glycol-20000 (PEG-20000) at 180°C for 4 h. The diameter of individual nanosticks was about 100 nm. Detailed structure characterizations demonstrate that the synthesized products are wurtzite hexagonal phase, grown along the [001] direction. The infrared (IR) spectrum shows the standard peak of zinc oxide at 571 cm⁻¹. Raman scattering exhibits a sharp and strong E ₂ mode at 441 cm⁻¹ which further confirms the good crystal and wurtzite hexagonal phase of the grown nanostructures.     

One-step large-scale synthesis of porous ZnO nanofibers and their application in dye-sensitized solar cells

A one-step thermal evaporation and vapor-phase transport method was utilized to synthesize porous ZnO nanofibers in large scale. The synthesized nanofibers are highly porous, with diameters in the range of 200–700 nm and lengths of several micrometers. The addition of the CuCl₂·2H₂O into the Zn precursor powder was proved to be critical for the formation of the porous structures, which were proposed to be resulted from the decomposition of the unstable Cl-containing intermediate products such as zinc hydroxide chloride or zinc oxide chloride hydrate phases. In addition, a demonstration of applying the porous ZnO nanofibers as the photoanode of dye-sensitized solar cells is provided.     

The kinetics of the hydrothermal growth of ZnO nanostructures

The immersion of a material, seeded with ZnO nanoparticles, in an aqueous solution of Zn(NO₃)₂ and hexamethylenetetramine (HMT) at 90 °C yields an extended array of one-dimensional ZnO on the substrate surface. The structure of the ZnO evolves with reaction time. Initially nanorods are formed. At longer times the rods are tipped with nanotubes. Here we report a series of experiments in which both the composition of the reaction solution; concentrations of H⁺, Zn²⁺ and HMT; and the structure of ZnO deposited on the substrate are monitored as a function of reaction time. It was found that the change from ZnO rod to tube growth arises when the solution composition is such that it is no longer thermodynamically favorable to precipitate Zn(OH)₂.     

Controllable synthesis of ZnO with various morphologies by hydrothermal method

ZnO microstructures with various morphologies have been controllably synthesized by hydrothermal route using different precipitant and zinc source in liquid solution. X-ray diffraction (XRD) and scanning electron microscopy (SEM) were used to characterize the ZnO2, Zn(OH)2 and ZnO structures to understand the role of precipitant and precursors in the growth of various morphologies. The nucleation and growth process can regulate by changing the precipitant. When H2O2 was used as precipitant, ZnO particles with a rather uniform particle size of -500 nm and a rather rough surface was obtained. While, ZnO synthesized in this polyvinyl pyrrolidone (PVP) solution has the same granular morphology with particle size of 300-1000 nm. In contrast, ZnO sunflower and polyhedron aggregates composed of several smaller polyhedron were formed, when ammonium hydroxide and NH4HCO3 was applied, respectively. Meanwhile, precursors play an important role in the determination of the morphology of ZnO. Sunflower and dumbbell like ZnO composed of nanosheets were obtained, when different centrifugal component of Zn(OH)2 suspension was applied as zinc source. In contrast, sunflower and dumbbell like ZnO composed of nanorods and ZnO rods were obtained, when different centrifugal components of ZnO2 suspension were used as zinc sources. The growth mechanism of ZnO nanostructures fabricated by the hydrothermal process using different zinc sources was tentatively investigated.     

Effect of microwave power on the morphology and optical property of zinc oxide nano-structures prepared via a microwave-assisted aqueous solution method

The effect of the microwave power on the morphology and optical properties of zinc oxide nanostructures prepared using a microwave-assisted aqueous solution method has been investigated. The ZnO nanostructures were synthesized from zinc chloride and sodium hydroxide mixed aqueous solutions exposed for 5 min to microwave radiation at four different powers, namely 150, 450, 700 and 1000 W. The morphologies of the samples have been characterized by transmission electron microscope (TEM) and scanning electron microscope (SEM). The results showed that the power of microwave radiation influenced the shape and size of the synthesized nanostructures. It is also found that the average particle size of nanostructures decreased with decreasing microwave power. The results of X-ray diffraction (XRD) showed that all the as-prepared ZnO nanostructures are in crystalline form with high purity. The infrared (IR) spectra indicated that the as-prepared nano ZnO product can be used as infrared gas sensors such as an infrared carbon dioxide (CO₂) and/or CO sensor. Optical properties of the as-prepared ZnO nanostructures were investigated by UV–vis spectroscopy and showed that the optical properties of as-synthesized ZnO samples are sensitive to the variation of the power of microwave radiation.     

Role of aqueous ammonia on the growth of ZnO nanostructures and its influence on solid-state dye sensitized solar cells

Synthesis of various ZnO nanostructures for different zinc to ammonia concentration ratios by a single-step aqueous chemical method at relatively low temperature is reported here. These nanostructures show preferential growth along (002) direction as observed in XRD. Morphological characterization by SEM, TEM, HRTEM, and SAED patterns reveal the formation of nanostructures of different shape, size, aspect ratio, and the crystal lattice orientations, which are mainly influenced by zinc to ammonia concentration. These materials when used as photo anodes in dye sensitized solar cells, the sample with the mixture of pyramidal, short nanorods, and spherical nanoparticle exhibits maximum conversion efficiency of 0.123 % due to better dye loading and direct conduction pathway for electron transport.     

Low temperature pH dependent synthesis of flower-like ZnO nanostructures with enhanced photocatalytic activity

In the present study we have synthesized flower-like ZnO nanostructures comprising of nanobelts of 20 nm width by template and surfactant free low-temperature (4 °C) aqueous solution route. The ZnO nanostructures exhibit flower-like morphology, having crystalline hexagonal wurtzite structure with (0 0 1) orientation. The flowers with size between 600 and 700 nm consist of ZnO units having crystallite size of ∼40 nm. Chemical and structural characterization reveals a significant role of precursor:ligand molar ratio, pH, and temperature in the formation of single-step flower-like ZnO at low temperature. Plausible growth mechanism for the formation of flower-like structure has been discussed in detail. Photoluminescence studies confirm formation of ZnO with the defects in crystal structure. The flower-like ZnO nanostructures exhibit enhanced photochemical degradation of methylene blue (MB) with the increased concentration of ligand, indicating attribution of structural features in the photocatalytic properties.