Facile morphology‐controlled hydrothermal synthesis of flower‐like self‐organized ZnO architectures

Flower‐like self‐organized crystalline ZnO architectures were obtained through a facile and controlled hydrothermal process. As‐synthesized products were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), high‐resolution transmission electron microscopy (HRTEM), X‐ray diffraction (XRD), electron diffraction and UV‐Vis spectroscopy. XRD and electron diffraction results confirmed the obtained materials are pure wurtzite ZnO. The effects of different ratios of starting materials and solvent on the morphologies of ZnO hydrothermal products were also evaluated by SEM observations. It is suggested that the use of water, rather than ethanol as the solvent, as well as employing a precursor of Zn(Ac)₂ and 2NaOH (v/v) in hydrothermal reactions are responsible for the generation of specific flower‐like self‐assembled ZnO structures. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Low temperature hydrothermal growth and optical properties of ZnO nanorods

Well‐faceted hexagonal ZnO nanorods have been synthesized by a simple hydrothermal method at relative low temperature (90°C) without any catalysts or templates. Zinc oxide (ZnO) nanorods were grown in an aqueous solution that contained Zinc chloride (ZnCl₂, Aldrich, purity 98%) and ammonia (25%). Most of the ZnO nanorods show the perfect hexagonal cross section and well‐faceted top and side surfaces. The diameter of ZnO nanorods decreased with the reaction time prolonging. The samples have been characterized by X‐ray powder diffraction (XRD) and scanning electron microscopy (SEM) measurement. XRD pattern confirmed that the as‐prepared ZnO was the single‐phase wurtzite structure formation. SEM results showed that the samples were rod textures. The surface‐related optical properties have been investigated by photoluminescence (PL) spectrum and Raman spectrum. Photoluminescence measurements showed each spectrum consists of a weak band ultraviolet (UV) band and a relatively broad visible light emission peak for the samples grown at different time. It has been found that the green emission in Raman measurement may be related to surface states. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Properties of InAs co‐doped ZnO thin films prepared by pulsed laser deposition

InAs co‐doped ZnO films were grown on sapphire substrates by pulsed laser deposition. The grown films have been characterized using X‐ray diffraction (XRD), Hall effect measurements, Atomic force microscope (AFM) and Field emission scanning electron microscope (FESEM) in order to investigate the structural, electrical, morphological and elemental properties of the films respectively. XRD analysis showed that all the films were highly orientated along the c‐axis. It was observed from Hall effect measurements that InAs co‐doped ZnO films were of n‐type conductivity. In addition, the presence of In and As has been confirmed by Energy dispersive X‐ray analysis. AFM images revealed that the surface roughness of the films was decreased upon the co‐doping. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Structural,Raman, and photoluminescence properties of double‐shelled coaxial nanocables of In2O3 core with ZnO and AZO shells

We synthesized In₂O₃/ZnO/Al‐doped ZnO (AZO) core‐double shell nanowires, in which the inner shell (ZnO) and the outer shell (AZO) have been subsequently deposited on the core In₂O₃ nanowires. With their one‐dimensional morphology being preserved, the X‐ray diffraction (XRD), lattice‐resolved transmission electron microscopy (TEM) image, selected area electron diffraction, and Raman spectrum coincidentally revealed that the shell was comprised of hexagonal ZnO phase. In addition, TEM‐EDX investigation revealed the presence of Al elements in the shell region. The thermal annealing at 700 °C did not significantly change the nanowire morphology, however, the XRD spectrum indicated that the ZnO phase was crystallized by the annealing. PL spectrum of the 700 °C‐annealed In₂O₃/ZnO/AZO core‐double shell nanowires was comprised of three Gaussian bands at approximately 2.1 eV, 2.4 eV, and 3.0 eV, respectively. The integrated intensities of 2.1 eV‐, 2.4 eV‐, and 3.0 eV‐bands were decreased by the thermal annealing. This study will pave the road to the preparation and applicaition of double‐shelled nanowires. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Fabrication and optical properties of two‐dimensional photonic crystal of ZnO pillars

Two‐dimensional (2D) photonic crystal of ZnO pillars was synthesized on silicon substrate by the combination of template method and vapor‐phase transport method. The microstructure and morphology of the ZnO photonic crystal was evaluated by using scanning electron microscope (SEM) and X‐ray diffraction (XRD). Large‐area specular reflectance measurements showed the presence of photonic stop band. The effect of the photonic band gap and the special structure on the photoluminescence (PL) properties of ZnO photonic crystal has been investigated. Both suppression and enhancement in the PL were observed. Raman scattering analyses demonstrated that the defect of ZnO photonic crystal exists in this experiment. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Synthesis and photoluminescence properties of ZnO nanowire arrays

In this paper we report a chemical method named coordination reaction method to synthesize ZnO nanowire arreys. ZnO nanowires with the diameter about 80nm were successfully fabricated in the channels of the porous anodic alumina (PAA) template by the above coordination reaction method. The microstructures of ZnO/PAA assembly were characterized using scanning electron microscopy (SEM), transmission electron microscopy (TEM) and X‐ray diffraction (XRD). The results showed that the ZnO nanowires can be uniformly assembled into the nanochannels of PAA template. The growth mechanism of ZnO nanowires and the conditions of the coordination reaction are discussed. Photoluminescence (PL) measurement shows that the ZnO/PAA assembly system has a blue emission band caused by the various defects of ZnO. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Temperature dependence of growth orientation and surface morphology of Li‐doped ZnO thin films on SiO2/Si substrates

ZnO thin films doped with Li (ZnO:Li) were deposited onto SiO₂/Si (100) substrates by direct‐current sputtering technique in the temperature range from room temperature to 500 °C. The crystalline structure, surface morphology and composition, and optical reflectivity of the deposited films were studied by X‐ray diffraction (XRD), Scanning Electron Microscopy (SEM), X‐ray Photoelectron Spectroscopy (XPS) and optical reflection measurements. Rough surface p‐type ZnO thin film deposition was confirmed. The results indicated that the ZnO:Li films growed at low temperatures show c‐axis orientation, while a‐axis growth direction is preferable at high temperatures. Moreover, the optical reflectivity from the surface of the films matched very well with the obtained results. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Solvothermal growth of highly oriented wurtzite‐structured ZnO nanotube arrays on zinc foil

ZnO nanotube arrays were synthesized on zinc foil by a simple solvothermal approach. In this approach, zinc foil was used not only as a substrate but also as a zinc‐ion source for the direct growth of ZnO nanotube arrays. X‐ray diffraction (XRD) analysis and Scanning electron microscope (SEM) images, indicated that the structure of the ZnO nanotube arrays on the zinc foil substrate was single‐crystalline with a wurtzite structure. The optical properties of the ZnO nanorod arrays were characterized by photoluminescence spectroscopies and Raman. Photoluminescence exhibited strong UV emission and a broad deep‐level (visible) emission emission at with 325 nm excitation. A possible mechanism is also proposed to account for the growth of the ZnO nanotube arrays. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Synthesis of ZnO nanorods by the thermal reduction process of a mixture of ZnO and Al powder

Single‐crystalline Zinc oxide (ZnO) nanorods were firstly synthesized on gold‐coated Si substrate via a simple thermal reduction method from the mixture of ZnO and Al powder. The growth process was carried out in a quartz tube at different temperature (550‐700 °C) and at different oxygen partial pressure. Their structure properties were investigated by X‐ray diffraction (XRD), scanning electron microscope (SEM), X‐ray photoelectron spectroscopy (XPS) and transmission electron microscopy (TEM). The length of the as‐prepared ZnO nanorods was up to several micrometers and their diameters were about 130 nm. The X‐ray diffraction patterns, transmission electron microscopic images, and selective area electron diffraction patterns indicate that the one‐dimensional ZnO nanorods are a pure Single‐crystal and preferentially oriented in the [0001] direction. The reaction mechanism of ZnO nanorods was proposed on the basis of experimental data. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Mixed metal oxide nanocomposites derived from layered double hydroxides as photocatalysts for C.I. Basic Blue 3 degradation under UV light

In this research we report synthesis of the heterostructure Mg‐Al‐Zn mixed metal oxide (ZnO/MMO) nanocomposite photocatalysts derived from Zn(OH)₂/Mg‐Al‐layered double hydroxides (ZLDHs) precursors. The obtained samples were characterized by the X‐ray diffraction (XRD), FT‐IR, BET surface area, ICP and TG/DTG methods. The chemical compositions and morphology of the synthesized materials were investigated by the energy dispersive X‐ray analysis (EDX) and the transmission electron microscopy (TEM). The results reveal that at the reaction time 96 h, ZLDH has the highest crystalinity which was confirmed by the X‐ray diffraction spectra. The calcined samples at 500, 600 and 700 °C for 4 h show that the crystallinity of the nanocomposite improves with the increase of calcination temperature. The photocatalytic activities of synthesized nanocomposites were compared for the degradation of C. I. Basic Blue 3 (BB3) dye under UV illumination in aqueous solution. Among the synthesized nanocomposites, ZnO/MMO calcined at 700 °C shows the highest efficiency towards the removal of dye. The effect of UV illumination on the stability of ZnO in ZnO/MMO nanocomposite and pure ZnO was also investigated. The results showed that the photostability of ZnO in ZnO/MMO nanocomposite is increased compared to the pure ZnO.     

Shape‐controlled synthesis of ZnO architectures

Several ZnO nanoarchitectures have been selectively prepared through hydrothermal method including urchin‐shaped, flower‐shaped, butterfly‐shaped, and cacti‐shaped microstructures. The influence of reaction temperature and the concentration of amphiphilic copolymer on the morphologies and shapes of ZnO samples have been studied. The samples were characterized using XRD, TEM, and SEM. It was found that ZnO nanorods or their assemblies were fabricated at higher temperature, whereas ZnO nanosheet architectures were produced at lower temperature. These flower‐shaped architectures possess high BET surface area of 27.43 m²/g. Room temperature UV‐VIS and PL spectra of the as‐obtained ZnO nanoarchitectures have been examined. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Controlled synthesis of various morphologies of nanostructured zinc oxide: flower,nanoplate, and urchin

Nanoplates, flower‐like nanostructure of ZnO were successfully synthesized by employing ZnSO₄·7H₂O, NaOH as the starting materials at 120°C under hydrothermal condition. Keeping the same parameters, ZnO urchin shape was obtained by addition of vitamin C at 190°C. Characterizations were carried out by X‐ray diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM) at room temperature. Selected area electron diffraction (SAED) pattern confirms that the product is single crystalline nature. The possible formation mechanisms for synthesized ZnO nanosturcture with various morphologies have also been proposed. PL spectrum from the ZnO flower‐like structures reveals weak UV emission and strong green emission. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Synthesis of radial‐like ZnO structure by hydrothermal method with ZnSO4·7H2O and Zn(CH3COO)2·2H2O as zinc sources

Radial‐like ZnO structures were prepared using zinc sulfate (ZnSO₄·7H₂O) and zinc acetate [Zn(CH₃COO)₂·2H₂O] as zinc sources by a facile template‐free hydrothermal method in this paper. Structural and optical properties of radial‐like ZnO structures are characterized by X‐ray diffraction (XRD), field emission scanning electron microscopy (FESEM), UV‐vis spectrophotometer and photoluminescence measurement (PL). It has been found that the distinct surface morphologies of radial‐like ZnO structures grown by different zinc sources. Slim radial‐like ZnO with a hexagonal wurtzite structure is grown by using ZnSO₄·7H₂O as zinc sources, whereas coarse radial‐like ZnO with zincite structure is achieved by zinc acetate. The UV‐vis absorption spectra of them both display an obvious and significant absorption in the ultraviolet region. The room temperature PL spectra of ZnO structures grown by two different zinc sources possess a common feature that consists of a strong ultraviolet (UV) peak and visible emission band.     

Structural and morphological studies on Mn substituted ZnO nanometer‐sized crystals

Mn substituted ZnO nanocrystals synthesized by a co‐precipitation method. X‐ray diffraction (XRD) studies confirms the presence of wurtzite (hexagonal) crystal structure similar to un doped ZnO, suggesting that doped Mn ions go at the regular Zn sites. The lattice parameters a and c are increasing with increasing Mn content. The unit cell volume increases with increasing Mn concentration, indicating the homogeneous substitution of Mn²⁺ for the Zn²⁺. The lattice distortion parameter (ε_v) is evaluated from XRD data and found that it enhances as Mn content increases. Transmission electron microscopy photographs show that the size of the ZnO crystals is in the range of 20‐50 nm. The SAED pattern confirms the hexagonal and crystalline nature of the samples which are in agreement with X‐ray analysis. The chemical groups of the samples have been identified by FTIR studies (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Morphology‐photocatalytic properties‐growth mechanism for ZnO nanostructures via microwave‐assisted hydrothermal synthesis

ZnO nanostructures with various morphologies including rod‐like, sheet‐like, needle‐like and flower‐like structures were successfully synthesized via a fast and facile microwave‐assisted hydrothermal process. Reaction temperature, reaction time and the addition of NaOH were adjusted to obtain ZnO with different morphologies. Scanning electron microscopy(SEM), transmission electron microscope(TEM), X‐ray diffraction (XRD) and ultraviolet spectrophotometer (UV) were used to observe the morphology, crystal structure, ultraviolet absorption and photocatalytic activity of the obtained ZnO. The results indicated that growth rate of ZnO nanostructure along [001] direction was more sensitive to temperature compared with those along [101] and [100] directions. The competition between anionic surfactant and OH⁻ played an important role in the formation of ZnO with various morphologies. Flower‐like ZnO had better ultraviolet absorption property and excellent photocatalytic activity than ZnO in the other morphologies. On the basis of the above results, a possible growth mechanism for the formation of ZnO nanostructures with different morphologies was described.     

Photoluminescent hierarchical Zno micro‐flower and its surface wettability

The flower‐like ZnO with micro‐nano hierarchical structure is successfully obtained by a simple hydrothermal synthesis, using sodium dodecyl benzene sulfonate (SDBS) as a structure direct agent. The resulted ZnO micro‐flowers are very uniform in morphology with particle sizes around 1 µm. A number of techniques, including X‐ray diffraction (XRD), field emission scan electron microscopy (FESEM), energy‐dispersive spectroscopy (EDS), fourier transform infrared (FTIR) spectra and thermogravimetry analysis (TGA), are used to characterize the obtained ZnO. The self‐assemble of ZnO nano‐sheets under the direction of SDBS leads to the formation of ZnO micro‐flowers. The room temperature photoluminescence property of the obtained flower‐like ZnO exhibits a broad visible light emission. The surface of as‐made ZnO shows a very hydrophilic property, while the special micro‐nano hierarchical structure enables the ZnO micro‐flower a superhydrophobic surface after modification of fluoroalkylsilane.     

Structure and magnetism of ZnO/Co multilayers prepared by pulsed laser deposition

ZnO/Co multilayers were fabricated on silicate (100) substrate by a pulsed laser deposition method at room temperature. The x‐ray diffraction (XRD) results reveal that the as‐deposited multilayer film is composed of amorphous phase, which leads to high saturation magnetization and low coercivity. Higher coercivity is observed in the ZnO/Co films annealed at 390 °C due to the formation of crystalline metallic Co and semiconducting ZnO. But much higher annealing temperature leads to the oxidation of metallic Co and the reaction between Co and ZnO, which decreases the saturation magnetization and coercivity obviously.     

Controlled synthesis of ZnO nanostructures with different morphologies in microemulsions

ZnO nanostructures with different morphologies were prepared in microemulsions with ZnSO₄ and ammonia as raw materials. The effects of microemulsion types, concentration of reactants, W values, co‐surfactants, surfactants, oil phases and calcination temperatures were systematically studied. The products were characterized by X‐ray diffraction (XRD), differential scanning calorimetry and thermogravimetry (DSC‐TG), transmission electron microscopy (TEM), high‐resolution TEM (HRTEM), and photoluminescence (PL) spectrum. Results show that ZnO nanoparticles were obtained in water‐in‐oil microemulsions while ZnO nanorods are gained in bicontinuous microemulsions. Water‐in‐oil microemulsions and long carbon chains of surfactants can prevent the preferential growth of ZnO. The particle size of the products increased with the increase of W values, calcination temperatures and the concentration of reactants but decreased with the increase of the carbon chain length of surfactants, co‐surfactants and oil phases. PL spectrums show that the UV emission peak weakened and visible emission peak increased with the decrease of particle size. Meanwhile, the PL spectrums have a little red‐shifted.     

Structural and photoluminescence properties of GaN/ZnO core‐shell nanowires with their shells sputtered

We have reported the preparation of ZnO‐coated GaN nanowires and investigated changes in the structural and photoluminescence (PL) properties by the application of a thermal annealing process. For fabricating the core‐shell nanowires, Zn target was used to sputter ZnO shell onto GaN core nanowires. X‐ray diffraction (XRD) analysis indicated that the annealed core‐shell nanowires clearly exhibited the ZnO as well as GaN phase. The transmissoin electron microscopy (TEM) investigation suggested that annealing has induced the crystallization of ZnO shell layer. We have carried out Gaussian deconvolution analysis for the measured PL spectra, revealing that the core GaN nanowires exhibited broad emission which consist of red, yellow, blue, and ultraviolet peaks. ZnO‐sputtering induced new peaks in the green region. Thermal annealing reduced the relative intensity of the green emission. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Properties of Co‐doped ZnO films prepared by electrochemical deposition

We prepared Co‐doped ZnO films by the electrochemical deposition. X‐ray diffraction (XRD), high resolution transmission microscopy (HRTEM), x‐ray photoelectron spectroscopy (XPS), atomic force microscope (AFM), x‐ray absorption near‐edge structure (XANES), vibrating sample magnetometer (VSM), optical absorption, and photoluminescence (PL) measurements were carried out on the samples. The results showed Co atoms substituted Zn atoms in the ZnO lattice without the formation of the impurity phase. VSM measurements showed the ferromagnetic properties for the Co‐doped ZnO samples. When the Co doping concentration increased, the band gaps were widened and the PL peak positions shifted towards the short wavelength direction. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)