Optical and bio-sensing characteristics of ZnO nanotubes grown by hydrothermal method

ZnO nanostructures were fabricated on copper substrates by hydrothermal method at an optimized growth temperature of -95 degrees C. Structural properties were investigated by field emission scanning electron and transmission electron microscopy. Distinct morphologies were found to be formed at different growth times. The formation of nanotubes mainly involved the initial nucleation followed by the growth of nanorods at 95 degrees C, and then with the increase of dissolution time at room temperature, the preferential chemical dissolution of the metastable Zn-rich [0001] polar surfaces resulted in removing the atoms from the surfaces, thus leading to the thinning of the wall of the nanostructures. Completely hollow ZnO nanotubes could be obtained at a high dissolution time. The room temperature photoluminescence and optical absorption properties of ZnO nanotubes have been studied as a function of dissolution time. The efficacy of ZnO nanotubes for glucose sensing applications has been studied.     

Shape and size control of ZnO nanostructures via a simple solution route

ZnO nanostructures with different morphologies were synthesized by condensing the Zn(OH)4(2-) precursors under hydrothermal conditions in the presence of a surfactant, cetyltrimethylammonium bromide (CTAB). Shape and size control of ZnO nanostructures was achieved by relatively simple variations of molar ratio of CTAB to Zn(OH)4(2-). With a higher molar ratio, ZnO nanotubes were obtained, whereas with a lower molar ratio, ZnO nanorods were formed; furthermore, with a moderate w value, the coexistence of ZnO nanorods and nanotubes was also observed. The photoluminescence of ZnO nanorods and nanotubes was also investigated.     

The shape control of ZnO based nanostructures

Tetrapod-shape ZnO nanostructures are formed on Si substrates by vapor phase transportation method. The effects of two important growth parameters, growth temperature and VI/II ratio, are investigated. The growth temperature is varied in the range from 600 degrees C to 900 degrees C to control the vapor pressure of group II-element and the formation process of nanostructures. VI/II ratio was changed by adjusting the flux of carrier gas which affects indirectly the supplying rate of group VI-element. From the scanning electron microscopy (SEM), systematic variation of shape including cluster, rod, wire and tetrapod was observed. ZnO tetrapods, formed at 800 degrees C under the carrier gas flux of 0.5 cc/mm2 min, show considerably uniform shape with 100 nm thick and 1-1.5 microm long legs. Also stoichiometric composition (O/Zn - 1) was observed without any second phase structures. While, the decrease of growth temperature and the increase of carrier gas flux, results in the irregular shaped nanostructures with non-stoichiometric composition. The excellent luminescence properties, strong excitonic UV emission at 3.25 eV without deep level emission, indicate that the high crystalline quality tetrapod structures can be formed at the optimized growth conditions.     

Controllable synthesis of undoped/Cd-doped ZnO nanostructures

Various undoped/Cd-doped ZnO nanostructures have been synthesized through a simple evaporating method. The gold particles-filled anodic aluminum oxide templates and catalyst-free graphite sheet were used as substrates. The morphologies of the products can be controlled by simply tuning the evaporation temperature of zinc and gases flow rates. Moreover, morphological difference between the undoped and Cd-doped ZnO nanostructures is presented. These as-grown ZnO nanostructures could be nanowires, nanobelts, nanoneedles, nanocombs, and saw-like structures, wherein in the saw-like ZnO structure was firstly found to originate nanowires on its rough lateral side. Room-temperature photoluminescence measurement shows strong green emissions from the ZnO nanostructures.     

Growth parameter dependent structural and optical properties of ZnO nanostructures on Si substrate by a two-zone thermal CVD

We investigated the effect of growth parameters on the structural and optical properties of the ZnO nanostructures (NSs) grown on Au-coated Si substrate by a two-zone thermal chemical vapor deposition. The morphologies of ZnO NSs were controlled by various growth parameters, such as growth temperature, O2 flow rate, and working pressure, for different thicknesses of Au layer. The nanorod-like ZnO NSs were formed at 915 degrees C and the growth of two-dimensional structures, i.e., nanosheets, was enhanced with the increase of growth temperature up to 965 degrees C. It was found that the low working pressure contributed to improvement in vertical alignment and uniformity of ZnO NSs. The Zn/O atomic % ratio, which plays a key role in the growth mechanism of ZnO NSs, was changed by the growth parameters. The Zn/O atomic % ratio was increased with increasing the growth temperature, while it was decreased with increasing the working pressure. Under proper O2 flow rate, the ZnO nanorods with good crystallinity were fabricated with a Zn/O atomic % ratio of -0.9. For various growth parameters, the photoluminescence emission was slightly shifted with the ultraviolet emission related to the near band edge transition.     

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.     

Controlled synthesis and field emission properties of ZnO nanostructures with different morphologies

By simply controlling atmosphere, rods, tetraleg-rods, and arrays of ZnO nanostructures have been fabricated respectively through pure zinc powder evaporation without catalyst at temperature of 650 - 700 degrees C. Investigations through HRTEM and XRD showed that the growth of the synthesized ZnO nanostructures was controlled by vapor-solid mechanism. Field emission measurements revealed that all of the structures, owing to their very low turn-on voltage, sufficient emission current and proper linearity of 1/V - Ln(l/V2), are likely to be potential candidates as a field emitter. The results also indicated that field emission properties are relative to morphology and size of the tips of ZnO nanostructures, and the nanorods with sharp tips possess the first-class FE property.     

Synthesis of well dispersed, regular shape ZnO nanorods: effect of pH, time and temperature

In this paper we presented a systematic study on the morphological variation of ZnO nanostructure by varying the pH of precursor solution, reaction time and reaction temperature via cetyl trimethylammonium bromide-assisted hydrothermal method. The phase and structural analysis was carried out by X-ray diffraction, showed the formation of single phase ZnO with hexagonal wurtzite structure in all the specimens. Morphological and structural analysis was carried out by scanning electron microscopy and transmission electron microscopy showed that the shape of ZnO nanorods were greatly influenced by pH of precursor precipitate while size was affected by reaction time as well as temperature. The selected area diffraction pattern showed that the as synthesized ZnO nanorods were single crystalline in nature and preferentially grow along [0001] direction. A plausible growth mechanism of as prepared ZnO nanostructures was discussed in detail. Furthermore, the optical property of as prepared ZnO nanostructures was studied by photoluminescence spectroscopy.     

两步法实现ZnO纳米结构的集成及光学性质研究

通过简单的两步热蒸发方法成功地实现了ZnO纳米管和纳米棒的集成.SEM结果表明,大量的纳米线以层层生长的机理从约200℃低温Si基片表面生长出来.EDS和XRD结果进一步表明第一步所制备的样品主要是由大量Zn和少量Zn的氧化物组成.第二步所制备样品的SEM和TME图像证实了在高温下以第一步所制备的样品作为第二步的基片,可实现纳米管和纳米棒的集成.TEM图像表明,纳米管的表面所生长的纳米棒是单晶的.通过改变工作气压,可调控纳米管表面纳米棒的尺寸和形貌,实现ZnO纳米结构可控生长.室温光致发光谱表明,样品的光学性质可通过可控的形貌来调制.     

Effects of surface potential on growth of ZnO nanorod arrays investigated by electric force microscopy

The electric and Kelvin force probe microscopy were used to investigate the surface potentials on the ZnO seed layer, which shows a remarkable dependence on the annealing temperature. The optimum temperature for the growth of nanorod arrays normal to the surface was found to be at 600 degrees C, which is in the range of right surface potentials and energy measured between 500 degrees C and 700 degrees C. We demonstrated from both electric and Kelvin force probe microscopy studies that surface potential controls the growth of ZnO nanorods, illustrating the fact that this is a promising technique to visualize the control of ZnO nanorod arrays by studying their surface potentials. This study will provide important understanding of growth of other nanostructures.     

Mesoporous nanocrystalline ZnO microspheres by ethylene glycol mediated thermal decomposition

Zinc oxide (ZnO) nanostructures with various morphologies have been fabricated in literature owing to their potential applications in various emerging fields. In this study, we report a facile, one-step gram-scale synthesis of nanocrystalline mesoporous ZnO microspheres by thermal decomposition of zinc acetate dihydrate in ethylene glycol at 250?°C for 12?h. The average size of the hollow microspheres is found to be 3.01?±?0.52?µm, which are formed by loosely bonded nanocrystallites with average sizes of 17?±?4?nm. We propose a formation mechanism for the mesoporous microspheres, Ostwald ripening of spherical-like nanocrystallites, on the basis of the results obtained by different synthesis durations. We also report the possibility of tuning the morphologies of the obtained ZnO by simply modifying the thermal decomposition solution, where porous ZnO nanoplates are obtained when a mixture of ethylene glycol and water is used and ZnO nanorods with aspect ratios of ～3 are synthesized by using diethylene glycol. ZnO nanowires with lengths up to several microns are fabricated when no solvent is used, i.e. thermal decomposition in air atmosphere. Microstructural and phase characterizations of the samples are conducted by using a field-emission gun scanning electron microscope and X-ray diffractometer. Performances of the obtained nanocrystalline mesoporous ZnO microspheres in photocatalytic degradation of Rhodamine B and as active anode materials in lithium-ion batteries are also presented.     

Growth and characterization of ZnO nanoflakes by hydrothermal method: effect of hexamine concentration

Flake-like ZnO nanostructures, which have potential applications in fields of photo catalysts, biosensors, solar cells, et al., were fabricated on indium doped tin oxide substrates through hydrothermal method at 90 °C. Zinc chloride hydrate and hexamine (HMT) were used as zinc source and the alkali source, respectively, no other surfactants were involved. The morphology evolution of ZnO from nanoflakes to the occurrence of nanorods can be attributed to the competition between the absorption effect of chloride ions and HMT. With the adsorption of chloride ions onto the polar faces instead of growth units, ZnO nanoflakes were formed by restraining the crystal growth along <0001> direction. Controllable growth of different ZnO nanostructures was achieved by tuning the corresponding stoichiometric ratios of reactants. ZnO nanoflakes were formed with equivalent concentrations of zinc chloride hydrate and HMT. Increase in the concentration of HMT resulted in the fabrication of the rod-on-flake structure, or even multilevel ZnO nanoflakes. Scanning electron microscopy results revealed that all these ZnO nanoflakes are with typical wurtzite hexagonal phase. The crystal and optical properties of the as-synthesized samples were characterized by X-ray diffraction spectrum and room temperature photoluminescence spectrometer, which showed close relationship with structure variations.     

Low temperature growth and optical properties of ZnO nanowires using an aqueous solution method

ZnO nanowires were grown on indium tin oxide (ITO) coated glass substrates at a low temperature of 90 degrees C using an aqueous solution method. The ZnO seeds were coated on the ITO thin films by using a spin coater. ZnO nanowires were formed in an aqueous solution containing zinc nitrate hexahydrate (Zn(NO3)2 x 6H2O) and hexamethylenetetramine (C6H12N4). The pH value and concentration of the solution play an important role in the growth and morphologies of ZnO nanowires. The size of ZnO naonowires increased as the concentration of the solution increased. It was formed with a top surface of hexagonal and tapered shape at low and high pH values respectively. Additionally, the single crystalline structure and optical property of the ZnO nanowires were investigated using high-resolution transmission electron microscopy and photoluminescence spectroscopy.     

One-pot synthesis of hexagonal ZnO nanosheets with a novel structure-directing agent and their application in cell imaging

Recently, ZnO nanostructures with various morphologies have been synthesized with different structure directing agents. Among these works, little reports have been focused on the syntheses of the ZnO nanostructures at nearly neutral pH and relatively low temperature. This work explores a novel structure-directing agent, tris(hydroxymethyl)aminomethane (tris), for one-pot synthesis of hexagonal ZnO nanosheets at nearly neutral pH (i.e., 7.3) and low temperature (i.e., 80 degrees C). From the XRD and TEM results, the obtained ZnO nanosheets possess the perfect single crystalline structures. The tris molecule acts not only as the hydroxide anion generator, but also as the surface modification agent, as evidenced by the FT-IR spectrum. The yield of the products can be up to about 1.0 g per pot, indicating the industrial prospects for its large-scale and low-cost synthesis. The mechanism of nucleation and growth of the ZnO nanosheets has been proposed. The obtained ZnO nanosheets own the sharp strong fluorescence at 590 nm, which can be attributed to the defects on the surface of the ZnO nanostructures. In virtue of the fluorescent properties, the ZnO nanosheets have been successfully used for cell imaging, suggesting their promising clinic and biomedical applications.     

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.     

Synthesis of zinc oxide nanotetrapods and nanorods by thermal evaporation without catalysis

ZnO nanotetrapods and nanorods have been synthesized by a simple thermal evaporation of Zn powder (300 mesh, 99.99% purity) under simultaneous flow of oxygen and argon gases in two-zone furnace in two different temperature regions. These ZnO nanostructures have hexagonal structure, which grow along the [001] direction in the form of nanotetrapods (diameter approximately 60-150 nm, length approximately 1-4 microm) and nanorods (diameter approximately 30-60 nm, length approximately 2-5 microm). The morphologies of these ZnO nanostructures have been investigated by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). It has been found that growth parameters like temperature, gas flow rate etc., control the diameter of the nanotetrapods and nanorods. These novel structures of ZnO nanorods and nanotetrapods may be attractive for optical and other nanodevices.     

Enhancement of Ethanol Sensing Properties by Impregnating Platinum on Surface of ZnO Tetrapods

ZnO tetrapods with a cross-sectional size of about 200–1000 nm were synthesized via an oxidation reaction technique. The sensors based on ZnO tetrapods and platinum impregnated ZnO tetrapods were fabricated and investigated for ethanol sensing properties. The gas sensing properties of the sensors were investigated for ethanol concentration of 50–1000 ppm at different operating temperatures. It was found that the sensitivities of platinum impregnated ZnO tetrapod sensors were higher than that of pure ZnO tetrapod sensors. The enhancement of sensitivity due to platinum impregnation to ZnO tetrapods may be explained either by an increase of adsorbed oxygen density or an increase of reaction rate coefficient in a rate equation for an ethanol adsorption reaction on the ZnO surface. Also, the slope value of the plot between $log(S-1)$ and $log C$ suggested that adsorbed oxygen ion species at the surface of the platinum impregnated ZnO tetrapods was $O^{2-}$ which was the same as pure ZnO tetrapods. Finally, these results have an important implication for a development of ethanol sensors based on metal oxide semiconductors for alcohol breath analyzers.      

Controllable synthesis of vertically aligned ZnO nanorod arrays in aqueous solution

High-density well-aligned ZnO nanorod array was successfully synthesized on a large-area magnetron sputtering deposited Al doped ZnO film-coated Si (AZO/Si) substrate via a convenient solution method. X-ray diffraction and scanning electron microscopy show that the nanorods are well-oriented perpendicular to the substrate. The influences of the reaction temperature, time, on the size and shapes of the as-prepared ZnO nanorods (ZNs) samples have been studied. The length and diameter of the nanorods became bigger when a longer reaction time was used. When the temperature is elevated to 130 degrees C, a new conical ZNs was synthesized. Room-temperature photoluminescence (PL) spectra of all the ZnO products showed a strong ultraviolet (UV) emission. The photoluminescence from free excitons of the ZNs synthesized at higher temperature reflects the high purity and nearly defect free structure of nanorods. The well-aligned feature of the nanorod array is attributed to the nanorods' epitaxial growth from the AZO films.     

CdS microflowers and interpenetrated nanorods grown on Si substrate: Structural,optical properties and growth mechanism

CdS semiconductor with different morphologies have been achieved by simple thermal evaporation of CdS powder at 1050 °C in a flowing Ar atmosphere. The products were characterized by X-ray diffraction, Scanning electron microscopy, Transmission electron microscopy and Photoluminescence. microflowers and interpenetrative nanorods of CdS were formed on catalyst free Si wafers at a temperature of 700 °C and 600 °C respectively. The flower like structures are composed of many interleaving nanorods which have the uniform diameter of about 700 nm and a well crystalline structure with [0001] as growth direction. The interpenetrative nanorods are found to be bounded with six side facets. X-ray diffraction studies revealed the hexagonal structure in both the products. The formation mechanism of microflowers and interpenetrated nanorods was discussed on the basis of nucleation growth kinetics. Room temperature photoluminescence spectra showed a strong green emission band (at ∼510 nm) from the CdS flower like structures, but on the other hand a red emission shoulder along with strong green emission band was observed for interpenetrative nanorods. These CdS micro/nanostructures with abundant morphologies may find applications in various micro/nanodevices, and the kinetics-driven morphology might be exploited to synthesize similar structures of other functional II–VI semiconductors.     

ZnO纳米四脚状阵列电极染料敏化太阳能电池

采用化学气相沉积(CVD)法制备了不同尺寸的四脚状纳米ZnO和ZnO纳米棒。采用X射线粉末衍射(XRD)、扫描电子显微镜(SEM)对纳米ZnO的晶型结构和形貌进行表征,研究结果表明CVD法制备的四脚状纳米ZnO具有三维空间结构,其最小平均臂宽约为70nm,臂长约300nm,制备的纳米棒直径约为84nm,长约2μm,且都为六方纤锌矿晶型结构。将ZnO纳米四脚状及纳米棒利用滚涂法在FTO导电玻璃上形成ZnO光阳极,经N719染料敏化后组装成染料敏化太阳能电池,光电性能结果表明,染料敏化小尺寸的四脚状纳米ZnO太阳能电池光电转换效率(η=1.88%)高于染料敏化大尺寸的四脚状纳米ZnO太阳能电池光电转换效率(η=1.18%),远高于染料敏化ZnO纳米棒太阳能电池的光电转换效率(η=0.7%)。