Growth and optical properties of faceted hexagonal ZnO nanotubes

Well-faceted hexagonal ZnO nanotubes were synthesized by a simple hydrothermal method and the subsequent aging process without any catalysts or templates. The formation of the tubular structure is closely linked to the polarity of ZnO and the selective adsorption behavior of Zn2+ amino complexes. The surface-related optical properties were studied with use of Raman and photoluminescence spectra. It was found that the oxygen vacancy-related visible emission intensity decreased while surface defect-related visible emission intensity increased when the nanotubes were annealed in oxygen ambient. The anomalous enhancement of PL integrated intensity with the temperature shows fairly high surface state density existing in ZnO nanotubes.     

Comparative structure and optical properties of Ga-, In-, and Sn-doped ZnO nanowires synthesized via thermal evaporation

ZnO nanowires doped with a high concentration Ga, In, and Sn were synthesized via thermal evaporation. The doping content defined as X/(Zn + X) atomic ratio, where X is the doped element, is about 15% for all nanowires. The nanowires consist of single-crystalline wurtzite ZnO crystal, and the average diameter is 80 nm. The growth direction of vertically aligned Ga-doped nanowires is [001], while that of randomly tilted In- and Sn-doped nanowires is [010]. A correlation between the growth direction and the vertical alignment has been suggested. The broaden X-ray diffraction peaks indicate the lattice distortion caused by the doping, and the broadening is most significant in the case of Sn doping. The absorption and photoluminescence of Sn-doped ZnO nanowires shift to the lower energy region than those of In- and Ga-doped nanowires, probably due to the larger charge density of Sn.     

Polyvinylpyrrolidone-assisted growth and optical properties of ZnO hexagonal bilayer disk-like microstructures

ZnO hexagonal bilayer disk-like microstructures are successfully prepared assisted with polyvinylpyrrolidone (PVP), and the photoluminescence (PL) spectrum of as-prepared ZnO samples showed a very strong ultraviolet (UV) emission at the UV region.     

Growth mechanism and characterization of ZnO nano-tubes synthesized using the hydrothermal-etching method

ZnO nano-tubes (ZNTs) have been successfully synthesized via a simple hydrothermal-etching method, and characterized by X-ray diffraction, field emission scanning electron microscopy and room temperature photoluminescence measurement. The as-synthesized ZNTs have a diameter of 500 nm, wall thickness of 20–30 nm, and length of 5 μm. Intensity of the plane (0002) diffraction peak, compared with that of plane (10$ \bar 1 $ \bar 1 0) of ZNTs, is obviously lower than that of ZnO nano-rods. This phenomenon can be caused by the smaller cross section of plane (0002) of the nano-tubes compared with that of other morphologies. On basis of the morphological analysis, the formation process of nano-tubes can be proposed in two stages: hydrothermal growth and reaction etching process.     

Growth and characterization of Cl-doped ZnO hexagonal nanodisks

Cl-doped ZnO nanodisks were grown on a Si(111) substrate using a thermal evaporation method. The prepared nanodisks exhibited a hexagonal shape with an average thickness of 50 nm and average diagonal of 270 nm. In addition, undoped ZnO disks with hexagonal shape were grown under the same conditions, but the sizes of these undoped ZnO disks were on the micrometer order. A possible mechanism was proposed for the growth of the Cl-doped ZnO nanodisks, and it was shown that the Cl¹⁻ anions play a crucial role in controlling the size. X-ray diffraction and Raman spectroscopy clearly showed an extension in the crystal lattice of ZnO because of the presence of chlorine. In addition, these nanodisks produced a strong photoluminescence emission peak in the ultraviolet (UV) region and a weak peak in the green region of the electromagnetic spectrum. Furthermore, the UV peak of the Cl-doped ZnO nanodisks was blueshifted with respect to that of the undoped ZnO disks.     

Growth of self-organized hierarchical ZnO nanoarchitectures by a simple In/In2S3 controlled thermal evaporation process

Novel hierarchical ZnO nanoarchitectures, such as microtrepangs, microbelts, nanoflowers, nanocombs, nanowheels, and nanofans assembled by ZnO nanocones, nanobowling pins, nanobottles, nanoarrows, and nanonails, have had their growth controlled by the thermal evaporation of Zn and a mixture of In and In2S3. Both the morphologies of the products and their construction units could be efficiently controlled by simple adjustment of the weight ratio of In/In2S3. The phase structure, morphologies, and photoluminescence properties of the ZnO products were investigated by X-ray diffraction, scanning electron microscopy, high-resolution transmission electron microscopy, and photoluminescence spectroscopy. These novel hierarchical ZnO nanoarchitectures may be attractive building blocks for creating optical or other nanodevices.     

Influences of Co doping on the structural,optical and magnetic properties of ZnO nanorods synthesized by hydrothermal route

Pure and Co-doped ZnO nanorods have been synthesized by a hydrothermal process. The structure, morphology and properties of as-prepared samples have been studied using X-ray diffraction (XRD), transmission electron microscopy (TEM), energy dispersive spectroscopy (EDS), X-ray photoelectron spectroscopy (XPS), photoluminescence (PL) spectrometer as well as by superconducting quantum interference device (SQUID). The structure and morphology analyses show that Co doping can slightly impede the ZnO crystallinity, influence the nanorods morphology, but cannot change the preferred growth orientation of ZnO nanorods. The amount of Co doping contents is about 3.0 at% in ZnO nanorods and dopant Co²⁺ ions substitute Zn²⁺ ions sites in ZnO nanocrystal without forming any secondary phase. The optical measurements show that the Co doping can effectively tune energy band structure and enrich surface states in both UV and VL regions, which lead to novel PL properties of ZnO nanorods. In addition, ferromagnetic ordering of the as-synthesized Zn_1?xCo_xO nanorod arrays has been observed at room temperature, which should be ascribed to sp–d and d–d carrier exchange interactions and presence of abundant defects and oxygen vacancies.     

On the synthesis and characterization of ZnO/MgO nanocomposite by thermal evaporation technique

ZnO/MgO nanocomposites have been synthesized by an easy and cost effective thermal evaporation technique. Various growth temperatures ranging from 800 to 900 °C were tried. It is observed that the process temperature plays a key role in the formation of ZnO/MgO nanocomposite and the proper formation of ZnO/MgO nanocomposite occurs at 875 °C temperature as confirmed by X-ray diffraction studies. Scanning electron microscopic images indicate that the ZnO/MgO nanocomposite is formed as agglomerated nanoparticles distributed over a large area. Energy dispersive X-ray analyses also reveal that the Mg composition in the synthesized nanocomposite strongly depends on the process temperature. Photoluminescence (PL) spectrum exhibits a blue shift for the ZnO/MgO nanocomposite synthesized at 875 °C indicating the incorporation of Mg into the ZnO crystal lattice. A higher PL intensity ratio of band-edge to deep band emission has been observed for this sample indicating the presence of low crystalline defects.     

Synthesis and evolution of novel hollow ZnO urchins by a simple thermal evaporation process

Delicate hollow ZnO urchins have been fabricated by thermal evaporation of metallic zinc powders in a tube furnace without the use of additive, high temperature, or low pressure. The phase transformation, morphologies, and photoluminescence evolution of the ZnO products were carefully studied and investigated with X-ray diffraction (XRD), scanning electron microscopy (SEM), high-resolution transmission electron microscopy (HRTEM), selected area electron diffraction (SAED), and photoluminescence (PL) spectra. These studies indicated that the growth of hollow ZnO urchins involves the vaporization of Zn powder, solidification of liquid droplets, surface oxidation, sublimation, and self-catalytic growth of one-dimensional nanowires.     

Growth, structural, optical and thermal properties of gamma-glycine crystal

Single crystals of gamma-glycine were grown from a mixture of glycine, water and lithium bromide. Single crystal X-ray diffraction analysis confirmed the growth of gamma-glycine phase. Presence of various functional groups of gamma-glycine was identified by FTIR spectrum. Optical absorbance spectrum recorded in the wavelength range of UV-vis-NIR revealed that this crystal has good optical transparency in the range 250-1500 nm. Vickers microhardness values were estimated on the prominent (100) face. Thermogravimetric and differential scanning calorimetric analyses were carried out to study the thermal properties of gamma-glycine. Second harmonic generation efficiency of the crystal measured by Kurtz's powder method using Nd:YAG laser is about three times that of KDP.     

Growth mechanism, photoluminescence, and field-emission properties of ZnO nanoneedle arrays

ZnO nanoneedle arrays have been grown on a large scale with a chemical vapor deposition method at 680 degrees C. Zn powder and O(2) gas are employed as source materials, and catalyst-free Si plates are used as substrates. Energy-dispersive X-ray and X-ray diffraction analyses show that the nanoneedles are almost pure ZnO and preferentially aligned in the c-axis direction of the wurtzite structure. The growth mechanism of ZnO nanoneedle arrays is discussed with the thermodynamic theory and concluded to be the result of the co-effect of the surface tension and diffusion. Photoluminescence spectrum of the as-grown products shows a strong emission band centering at about 484 nm, which originates from oxygen vacancies. Field-emission examination exhibits that the ZnO nanoneedle arrays have a turn-on voltage at about 5.3 V/microm.     

Photocatalytic properties of ZnO powders synthesized by conventional and microwave-assisted solution combustion method

Zinc oxide (ZnO) powders have been prepared by solution combustion synthesis method using conventional and microwave ignition routes. The effects of starting solution acidity on the combustion behavior, phase evolution, microstructure, optical properties and photocatalytic performance were investigated by thermal analysis, X-ray diffractometry, electron microscopy and diffuse reflectance spectrometry techniques. The chelated species in dried gels were predicted by theoretical calculations and confirmed by Fourier transform infrared spectroscopy. The combustion reaction rate increased with the increase of pH values. Single phase and well-crystalline ZnO powders were achieved by both of ignition methods regardless of pH values. The hexagonal particles (200–80?nm) formed by microwave ignition were larger than the spherical particles (60–40?nm) in conventional heating. Conventionally combusted ZnO powders exhibited higher photocatalytic activity under ultraviolet irradiation, due to their narrower band gap and smaller particle size.

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C/C0 vs. irradiation time for photodegradation of MB dye under ultraviolet light irradiation by the as-combusted ZnO powders (filled symbols present conventional combusted powders and open symbols are for the microwave combusted powders)

     

Synthesis and effect of post-deposition thermal annealing on morphological and optical properties of ZnO thin film

Zinc oxide thin films have been deposited on glass substrates by the chemical bath deposition method; a surfactant, cetyltrimethylammonium bromide (CTAB); was used as capping agent. The films were annealed at two different temperatures: 200 and 300 °C. The structural features were investigated by X-ray diffraction analysis which exhibited hexagonal wurtzite structures along with c-axis orientations. Crystallite size was estimated and found to be around 33–41 nm. The effect of post-deposition thermal annealing on the morphological and optical properties has been investigated by scanning electron microscopy and photoluminescence spectra at room temperature. The band gap energies of uncapped and CTAB-capped ZnO films were found to be 3.28 and 3.48 eV, respectively.     

Growth mechanism and photocatalytic properties of flower-like Bi2WO6 powders synthesized by a microwave hydrothermal method

Pure orthorhombic phase Bi₂WO₆ powders were synthesized by a microwave hydrothermal method in the absence of surfactants and templates, using Bi(NO₃)₃·5H₂O and Na₂WO₄·2H₂O as raw materials. Photocatalytic properties of the samples prepared at different reaction temperatures were also studied with Rhodamine B (RhB) solution as the target catabolite under visible light. The results indicate that flower-like Bi₂WO₆ powders can be obtained by controlling the microwave reaction temperatures in the absence of any additives. The growth of flower-like Bi₂WO₆ powders is a multistage layer assembly process, in which the flower-like Bi₂WO₆ self-assembling with the uniform size about 2 μm is synthesized at 180 °C. At the same time, the photocatalytic reaction rate constant (k) gets up to 0.04167/min and the degradation rate of RhB solution is more than 96 % after being irradiated under visible light for 70 min.     

Growth, thermal, and optical properties of l-asparagine monohydrate NLO single crystal

Single crystal of l-asparagine monohydrate (LAM), organic nonlinear optical material has been grown by slow evaporation solution growth technique. Single crystal X-ray diffraction analysis was employed to identify the cell parameters. FT-IR analysis was used to estimate the qualitative presence of the amino acids in the grown crystal. Anisotropy properties like thermal and dielectric properties were studied on the grown crystal. The sample was thermally stable up to 125 °C. Also various thermodynamic properties were calculated and reported for the first time. Optical properties such as optical absorption, second harmonic generation, photoluminescence and photoconductivity analysis were also studied on the grown crystal. Optical absorption studies showed a lower UV cut-off of 225 nm. The SHG efficiency of the sample was seven times higher than that of KDP. Photoluminescence study confirms the suitability of the material for the generation of blue radiation. Multilayer plate-like pattern of growth was observed by scanning electron microscopy. The photoconductivity study confirms that the LAM crystal has negative photoconducting nature.     

Characterization and field emission properties of ZnMgO nanowires fabricated by thermal evaporation process

In this paper, we investigate the roles of gold catalyst using modified thermal evaporation set-up in the growth process of ZnMgO nanowires. ZnMgO nanowires are fabricated on silicon substrates using different thickness of gold catalyst. A simple horizontal double-tube system along with chemical vapor diffusion of the precursors, based on Fick’s first law, is used to grow the ZnMgO nanowires. Field emission scanning electron microscopy images show that the ZnMgO nanowires are tapered. The optical properties of the ZnMgO nanowires are characterized by room temperature photoluminescence (PL) measurements. The PL studies demonstrate that the ZnMgO nanowires grown using this method have good crystallinity with excellent optical properties and have a larger band-gap in comparison to the pure ZnO nanowires. Field emission characterization shows that the turn-on field for the nanowires grown on the thinner gold film is lower than those grown on the thicker gold film.     

Optical properties of sol-gel synthesized calcium doped ZnO nanostructures

Optical properties of Ca doped ZnO nanoparticles prepared at room temperature through wet chemical method have been investigated. X-ray diffraction studies show that particles are crystalline in nature and doping did not induce impurity phases. Optical absorption measurements show an absorption peak at ∼372 nm which is due to excitonic absorption of the ZnO. Photoluminescence studies reveal a broad emission at an excitation wavelength of 335 nm and the bands are attributed to near band edge emission, oxygen vacancies, surface dangling bonds and zinc interstitials. Incorporating Ca²⁺ induces reduction in near band edge emission and there is an enhancement in the oxygen vacancy peaks which are attributed to the shape changes in the nanoparticles.     

Electrical,optical and photocatalytic properties of Ti-loaded ZnO/ZnO and Ti-loaded ZnO nanospheres

Ti-loaded ZnO and Ti-loaded ZnO/ZnO nanoparticles have been synthesized by sol–gel method and analyzed for photocatalyst application. The phase confirmation was analyzed by powder XRD and surface morphology with HR-SEM and EDAX spectrum. The particle size measured using HR-TEM and SAED pattern confirms the crystalline nature of Ti-loaded ZnO and Ti-loaded ZnO/ZnO nanoparticles. The optical properties were studied with UV–visible diffuse reflectance spectra. The DRS of Ti-loaded ZnO/ZnO nanoparticles are similar to those of pristine ZnO nanoparticles. The KM plots show both the synthesized Ti-loaded ZnO/ZnO and Ti-loaded ZnO exhibit in UV-A region. The electric properties are studied with impedance analyzer, and the results show the charge-transfer resistance of Ti-loaded ZnO/ZnO is larger than that of Ti-loaded ZnO nanoparticles. The photocatalytic activity was studied with methylene blue dye and phenol degradation by Ti-loaded ZnO/ZnO, Ti-loaded ZnO, TiO₂ and ZnO nanoparticles. The photocatalytic activity of Ti-loaded ZnO/ZnO nanospheres is slightly higher than that of other nanoparticles, which shows that they have excellent application as photocatalyst.     

Growth mechanism of long aligned multiwall carbon nanotube arrays by water-assisted chemical vapor deposition

Highly aligned arrays of multiwalled carbon nanotube (MWCNT) on layered Si substrates have been synthesized by chemical vapor deposition (CVD). The effect of the substrate design and the process parameters on the growth mechanism were studied. Adding water vapor to the reaction gas mixture of hydrogen and ethylene enhanced the growth which led to synthesis of longer CNT arrays with high density. Environmental scanning electron microscopy (ESEM), energy-dispersive spectroscopy (EDS), and atomic force microscopy (AFM) were used to analyze the CNT morphology and composition. Quadrupole mass spectroscopy (QMS) provided in-situ information on the gas spices within the reaction zone. On the basis of results, we verified the top growth mechanism and evaluated the reason of decline and stoppage of the CNT growth after extended period of deposition. Multilayered Si substrates with a top film of Al2O3, having appropriate roughness, provide favorable conditions to form catalyst islands with uniform distribution and size. Using water-assisted CVD process and optimized substrate design, our group succeeded to grow vertically aligned, patterned MWCNT up to 4-mm long. The arrays were of high purity and weak adhesion which allowed to be peeled off easily from the substrate.