Effect of oxygen partial pressure on the growth of zinc micro and nanostructures

Zinc micro and nanostructures were synthesized in vacuum by condensing evaporated zinc on Si substrate at different gas pressures. The morphology of the grown Zn structures was found to be dependent on the oxygen partial pressure. Depending on oxygen partial pressure it varied from two-dimensional microdisks to one-dimensional nanowire. The morphology and structural properties of the grown micro and nanostructures were studied by scanning electron microscopy (SEM) and X-ray diffraction (XRD). Transmission electron microscopy (TEM) studies on the grown Zn nanowires have shown that they exhibit core/shell-like structures, where a thin ZnO layer forms the shell. A possible growth mechanism behind the formation of different micro and nanostructures has been proposed. In addition, we have synthesized ZnO nanocanal-like structures by annealing Zn nanowires in vacuum at 350 °C for 30 min.     

Synthesis and growth mechanism of ZnO rod‐like nanostructures by a microwave‐assisted low‐temperature aqueous solution route

A zinc oxide (ZnO) nanoarray (rod‐like nanostructure) was successfully synthesized through a low‐temperature aqueous solution and microwave‐assisted synthesis using zinc nitrate hexahydrate (Zn(NO₃)₂·6H₂O) and hexamethylenetetramine (HMTA) as raw materials, and using FTO glass as substrate. The effects of parameters in the preparation process, such as solution concentration, reaction temperature and microwave power, on the morphology and microstructure of ZnO nanoarray were studied. Phase structure and morphology of the products were characterized by X‐ray diffraction (XRD) and scanning electron microscopy (SEM), respectively. The results indicated that hexagonal wurtzite structure ZnO nanoarray with good crystallization could be prepared through a low‐temperature solution method. When the concentration of the mixed solution was 0.05 M, the reaction temperature was 95 °C, and the reaction time was 4 h, high‐density ZnO regular nanorods of 200 nm diameter were obtained. A possible mechanism with different synthesis methods and the influence of microwave processing are also proposed in this paper.     

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.     

Synthesis of zinc oxide crystals with different shapes from zincate aqueous solutions stabilized with triethanolamine

ZnO crystals were synthesized from basic aqueous solutions including zincate ions stabilized with triethanolamine (N(C₂H₄OH)₃, teaH₃) by heating at 60°C. The influence of the basicity of the solutions on the morphology of the ZnO crystals was examined. The aqueous solutions were prepared using ZnSO₄·7H₂O, N(CH₃)₄OH (TMAOH), and teaH₃ as a zinc source, a base, and a stabilizer, respectively, at a zinc concentration of 0.2 M at a teaH₃ / Zn molar ratio of 4. Clear solutions were obtained at a molar ratio of TMAOH / Zn ≥ 3.0. When the clear solutions, in which glass or polyester substrates were placed, were heated at 60°C, agglomerates of ZnO crystals were deposited on the substrates in the TMAOH / Zn range from 3.0 to 3.6. With increasing the TMAOH / Zn ratio, the shape of the resulting ZnO crystals changed from a short asymmetric column with a hexagonal flat edge and a rounded one, through a rocket‐like shape formed by intergrowth, to a hexagonal rod. Although no films of ZnO were formed, ZnO crystals with different shapes were synthesized. When the glass substrates pre‐coated with a ZnO thin film by a sol‐gel method were used, highly oriented, dense ZnO films were formed. The films consisted of rod‐like crystals aligned normal to the substrate surface. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Facile hydrothermal synthesis of uniform 3D γ‐AlOOH architectures assembled by nanosheets

Uniform γ‐AlOOH architectures assembled by nanosheets were successfully synthesized in the mixture of deinonized water and dimethyl sulfoxide (DMSO) at 180 °C. The structure and morphology of products were characterized by X‐ray diffraction analysis (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). The products displayed 3D microstructures with its length of 1 μm and diameter of 400‐500 nm. The obtained γ‐AlOOH structures exhibited large Brunauer‐Emmett‐Teller (BET) surface area of 216.5 m²/g and pore size of 3.7 nm. The formation mechanism of 3D γ‐AlOOH architectures was also discussed based on the experimental results. Furthermore, the γ‐AlOOH architectures exhibited preliminary photoluminescence (PL) phenomenon with a strong peak at 323 nm. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Catalyst‐free thermally‐evaporated growth and optical properties of ZnO nanowires on Si,GaN and sapphire substrates

Vertically well‐aligned zinc oxide nanowires (NWs) with high density were successfully synthesized on Si, sapphire and GaN/sapphire substrates by thermal evaporation of zinc powders without catalysts or additives. The growth behavior of ZnO NWs was strongly dependent on the substrate materials. The effects of the substrate position on the structures and properties of ZnO NWs were primarily discussed. The morphology and crystallinity of the resultant NWs were studied by scanning electron microscope, transmission electronic microscope and X‐ray diffraction. The photoluminescence (PL) characteristics of the ZnO NWs on the different substrates were studied. The results showed that the as‐grown ZnO NWs exhibit a sharp and strong ultraviolet emission at 3.27 eV and a very weak green emission at around 2.48 eV, indicating that the a‐synthesized NWs have excellent PL properties with good crystalline quality and can be an ideal candidate for making luminescent devices. By comparison of PL spectra, we revealed that the green‐to‐UV emission intensity ratios were considerably dependent on the substrate materials, which was explained by the difference in the structural morphology of the produced nanowires.     

Growth,structure and optical characterization of Al‐doped ZnO nanoparticle thin films

Al‐doped ZnO nanoparticle thin films were prepared on glass substrate at the optimum temperature of (410±10) °C by spray pyrolysis technique using zinc nitrate as a precursor solution and aluminium chloride as a dopant. The dopant concentration (Al/Zn at%) was varied from 0 to 2 at%. Structural analysis of the films shows that all the films are of polycrystalline zinc oxide in nature, possessing hexagonal wurtzite structure. The films exhibit variation in peak intensities corresponding to (100), (002) and (101) reflection planes on Al‐doping. The crystallite size calculated by Scherrer formula has been found to be in the range of 35‐65 nm. The optical absorption study shows that the optical band gap in the Al‐doped films varies in the range of 3.11 – 3.22 eV. The width of localized states in the band gap estimated by the Urbach tail analysis has been found to be minimum in case of the 1 at% Al‐doped zinc oxide thin film. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Influence of selenization temperature on synthesis,morphology, and properties of nanostructured CoSe2 films

Nanostructured cobalt selenide (CoSe₂) thin films were deposited on a glass substrate using the selenization of Co films at different selenization temperatures (300 °C, 400 °C, and 500 °C) in a pure Se vapor for two hours. The morphology and structure of the as‐deposited films shows that the film morphology and crystallinity are affected by the selenization temperature. Increasing the selenization temperature from 300 °C to 400 °C and 500 °C results in a change in the surface and cross sectional morphology. At 300 °C, the Co films have an almost amorphous structure, while at temperatures of 400 and 500 °C, the Co films have a crystalline nanostructure with bilayered morphology. Optical analyses of the CoSe₂ films at 500 °C show a large absorption (α > 1.0 × 10⁵ cm⁻¹) and a direct band gap (∼1.0 eV).     

Direct preparation of K0.5Na0.5NbO3 powders

K_0.5Na_0.5NbO₃ powders have been directly synthesized by an alternative solid–state method. Stoichimometric mixture of ammonium niobium oxalate and C₄H₄O₆KNa·4H₂O were calcined in temperature range from 500 to 800 °C for 3 h. The precursor and calcination products were characterized with respect to stoichiometry, purity, crystalline structure, particle size and powder morphology using X–ray diffraction (XRD), X‐ray fluorescence (XRF) spectrometer, scanning electron microscope (SEM), Fourier transform infrared (FTIR) spectra, thermogravimetric (TG) analysis, differential scanning calorimetry (DSC) and UV–Vis diffuse reflectance (UV–Vis) spectroscopy. XRD and XRF results reveal that stoichiometric K_0.5Na_0.5NbO₃ powders could be synthesized by the method. The particle size is about 68 nm for the precursor calcined at 500 °C according to XRD data, which is in good agreement with SEM data. The average band gap energy is estimated to be 3.18 eV by UV–vis diffuse reflectance spectra. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

FTIR,powder XRD,TEM and dielectric studies of pure and zinc doped nano‐hydroxyapatite

Hydroxyapatite, Ca₁₀(PO₄)₆(OH)₂ or HAP, is an important bio‐material, which is having application in bone implants and dentistry. In the present study, zinc doped nano‐hydroxyapatite (Zn‐HAP) was synthesized via chemical precipitation route using surfactant mediated approach. The doping of zinc was confirmed by EDAX. The powder X‐ray diffraction (XRD) pattern revealed the typical hydroxyapatite pattern with broadening and extra peaks were observed for higher concentration. The average crystallite size was calculated by applying the Scherrer's formula to powder XRD pattern and was found in the range of 16 to 33 nm. The morphology of synthesized nano‐particles was also confirmed using TEM. FTIR spectroscopy was used to confirm the presence of various bonds. The dielectric study was carried out at room temperature within the frequency range from 10² Hz to 10⁷ Hz and the variations of dielectric constant with frequency of applied field as well as with the concentration of zinc were studied. It was found that as the concentration of zinc increased the dielectric constant increased. The variations of dielectric loss and a.c. conductivity with frequency of applied field were studied. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Properties of Sm‐doped CaNb2O6 thin films deposited by radio‐frequency magnetron sputtering

Sm‐doped CaNb₂O₆ (CaNb₂O₆:Sm) phosphor thin films were prepared by radio‐frequency magnetron sputtering on sapphire substrates. The thin films were grown at several growth temperatures and subsequently annealed at 800 °C in air. The crystallinity, surface morphology, optical transmittance, and photoluminescence of the thin films were investigated by X‐ray diffraction, scanning electron microscopy, ultraviolet‐visible spectrophotometry, and fluorescence spectrophotometry, respectively. All of the thin films showed a main red emission radiated by the transition from the ⁴G_5/2 excited state to the ⁶H_9/2 ground state of the Sm³⁺ ions and several weak bands under ultraviolet excitation with a 279 nm wavelength. The optimum growth temperature for depositing the high‐quality CaNb₂O₆:Sm thin films, which was determined from the luminescence intensity, was found to be 400 °C, where the thin film exhibited an orthorhombic structure with a thickness of 370 nm, an average grain size of 220 nm, a band gap energy of 3.99 eV, and an average optical transmittance of 85.9%. These results indicate that the growth temperature plays an important role in controlling the emission intensity and optical band gap energy of CaNb₂O₆:Sm thin films.     

Preparation of nanocrystalline lithium niobate powders at low temperature

A facile route to prepare lithium niobate (LiNbO₃) powders was proposed by an alternative solid‐state method. Stoichiometric Li₂C₂O₄ and ammonium niobium oxalate were mixed with small amounts of water and then dried at room temperature. It was demonstrated that Li[NbO(C₂O₄)₂]·n H₂O intermediate was produced by an ion‐exchange reaction. Pure LiNbO₃ powders were successfully synthesized by heating the intermediate at 500, 600 and 700 °C for 3 h. X‐ray diffraction (XRD), scanning electron microscopy (SEM), Fourier‐transform infrared (FTIR) spectroscopy, UV‐Vis diffuse reflectance (UV‐Vis) spectroscopy and thermogravimetric (TG) analysis were used to characterize the precursor compound and as‐prepared samples. XRD results reveal that all the products are identified as hexagonal structure with high relative crystallinity (>87%). The particle size is found to be about 40 nm for the mixture calcined at 500 °C according to XRD data, which is in good agreement with SEM data. The as‐prepared LiNbO₃ powders by this method are high quality according to FTIR spectra. (Li_0.996Nb_0.005)Nb_0.999O₃ phase was formed when the calcination temperature was raised to 800 °C. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Three‐step process improves crystal quality of Cu2ZnSnS4 absorber layer and efficiency of solar cell

Cu₂ZnSnS₄ (CZTS) films are fabricated using the three‐step process, including deposition, preheating and sulfurization of Cu–Zn–Sn (CZT) precursors. The effect of preheating temperature on structures, morphologies, and optical properties of CZTS films is investigated detailedly by X‐ray diffraction, Raman spectra, scanning electron microscopy, and UV spectrophotometer. It is found that the proper preheating temperature can improve the crystal quality of CZTS films. The prepared CZTS film by sulfurizing the preheated precursor at 300 °C presents high crystallinity, uniform surface morphology, and suitable optical properties. Compared with two‐step process, three‐step method can not only improve crystal quality of CZTS films but also decline sulfurization temperature. We also discuss the mechanism of improving the properties of CZTS absorber layer by the preheating route in detail. In addition, the experimental results also indicate that solar cell prepared by three‐step method displays higher conversion efficiency.     

Flux growth and characterization of Ti‐ and Ni‐doped forsterite single crystals

Forsterite monocrystals doped with Ti and Ni were grown by the flux growth technique. A suitable mixture of flux (MoO₃, V₂O₅, Li₂CO₃) and nutrient was slowly cooled down to 750 °C from 1250 °C or 1350 °C. The crystals were then characterized by powder and single‐crystal X‐ray diffraction, scanning electron microscopy and differential scanning calorimetry (DSC). Variations observed in crystal size were attributed by both the varying experimental conditions in which they had been obtained, and to the amount of Ni substituted for Mg in the structure. High abundances of doped forsterite required a cooling rate of 1.8 K h^‐1. These synthetic, well‐characterized Ti and Ni doped forsterite crystals may have potential for exploitation in industrial fields. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Synthesis of Fe‐doped chrysotile and characterization of the resulting chrysotile fibers

This study describes the formation of Fe‐doped chrysotile fibers with partial and total substitution of Mg by Fe. Syntheses were carried out with various starting mixtures (oxides, pure synthetic forsterite) in an externally heated pressure vessel in controlled hydrothermal conditions: temperature, 270 – 400 °C; pressure, 0.5 – 2 kbar; duration of treatment 160 – 480 hours. Pure synthetic forsterite was prepared by the flux growth technique. The starting material and run products were characterized by X‐ray powder diffraction (XRPD), scanning and transmission electron microscopies combined with energy‐dispersive spectrometry (SEM‐EDS and TEM‐EDS), differential scanning calorimetry (DSC) and thermogravimetry (TG). Variations observed in abundance and size of Fe‐doped chrysotile fibers were attributed to different experimental conditions for their synthesis. However, morphological shape turned out to depend on the starting mixtures used. Since natural samples are often difficult to obtain in a sufficiently pure state, these synthetic and well‐characterized Fe‐doped chrysotile fibers can be used for better understanding of the mechanisms involved in asbestos toxicity, as well as of the role of Fe in diseases induced by asbestos phases. These synthetic Fe‐doped chrysotile fibers, together with non‐toxicity testing, may also have potential for exploitation in industrial fields. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Growth and characterisation of Zn:LiNbO3/Mg:LiNbO3 multilayer thin films grown by liquid phase epitaxy

1, 3 and 5 mol% ZnO doped LiNbO₃ film and 2 mol% MgO doped LiNbO₃ multilayer films were grown on the LiNbO₃ (001) substrate by liquid phase epitaxy (LPE) method with a Li₂O‐V₂O₅ system. We examined the optical transmission spectra of the Zn:LiNbO₃ by Fourier Transform‐Infrared Spectrophotometer (FT‐IR). The crystallinity and the lattice mismatch between the Zn:LiNbO₃ film and Mg:LiNbO₃ film was confirmed by x‐ray rocking curve (XRC) and observed the ZnO and MgO distribution in the cross‐section of the multilayer thin films by electron probe micro analyzer (EPMA). Furthermore, the surface morphology of the films was observed using atomic force microscopy (AFM). (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Facile synthesis of low dimensional CuO nanostructures and their gas sensing applications

A simple and cost effective hydrothermal method has been employed to synthesis morphology controlled pure and Cr doped (4 and 8 at. %) CuO nanostructures. Crystalline purity and structure of the nanostructures were validated by X‐ray diffraction and Retvield analyses. Field emission scanning electron microscopy revealed the evolution of rod‐like, sheet‐like and boat‐like morphologies for pure, 4 and 8 at. % Cr doped CuO nanostructures respectively. The optical band gap estimated using the K‐M function plot from diffused reflectance spectroscopy showed a shift in band gap from 1.68 to 1.90 eV with respect to Cr concentration. The synthesized CuO nanostructures were investigated for the efficient room temperature gas sensing of ammonia, ethanol and methanol vapours under different concentrations (100‐600 ppm). The 8 at. % Cr doped CuO nano‐boats showed enhanced gas sensitivity than other CuO nanostructures owing to their typical morphology, larger surface area and related properties.     

Structural and magnetic studies of Mn‐doped ZnO

The bulk samples of Mn‐doped ZnO were synthesized with the nominal compositions Zn_1‐xMn_xO (x = 0.02, 0.05, 0.10, 0.15) by the solid‐state reaction and sol‐gel methods. In both the methods the samples were finally sintered at ∼700 °C in air. The X‐ray diffraction (XRD) studies of the samples synthesized by the solid‐state reaction method exhibit the presence of wurtzite (hexagonal) crystal structure similar to the parent compound (ZnO) in all the samples, suggesting that doped Mn ions sit at the regular Zn sites. However, same studies spread over the samples with Mn content ≥5% and synthesized by the sol‐gel method reveal the occurrence of some secondary phase in addition to the majority wurtzite phase. The magnetic measurements by vibrating sample magnetometer (VSM) clearly indicate ferromagnetic interaction at room temperature in all the samples. The Curie temperatures (T_c) and magnetization vary with concentration of Mn ions in the samples. However, the samples synthesized by sol‐gel method were found to have lower Tc values and also lower magnetization as compared to the corresponding samples synthesized by solid‐state reaction method. It could possibly be due to the presence of antiferromagnetic islands and smaller crystallite sizes in the samples prepared by sol‐gel method. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Microwave hydrothermal synthesis and photocatalytic properties of ZnWO4 nanorods

Cd²⁺‐doped ZnWO₄ nanorods have been synthesized at 200 °C with microwave hydrothermal method, using Zn(NO₃)₂·6H₂O, Na₂WO₄·2H₂O and CdCl₂ as raw materials. The effects of Cd²⁺ doping contents on the structure and morphology of the product were studied. The results show that Cd²⁺ doping into the crystal lattice of ZnWO₄ nanopowder makes the powder orientationally grow along (010), (110) and (200) crystal planes to form the nanorods. The bigger Cd²⁺ doping contents are, the more obviously ZnWO₄ nanorods grow. Meanwhile, the nanopowder is gradually transformed from monoclinic phase into the orthogonal phase. As the charge transference medium between the interfaces, Cd²⁺ restrains the combination of holes and electrons. After doped, the photocatalytic properties of ZnWO₄ nanorods are increased. When Cd²⁺ doping content is 20%, the Cd²⁺‐doped ZnWO₄ nanorods showed the highest degradation rate up to 98% in 2 h.     

Formation of nanostructured tin oxide semiconductors by a simple thermal redox process

Nanostructured tin oxide have been synthesized in one‐step process by a novel simple thermal reduction‐oxidation route. The results show that tin oxide nanowires and nanorods could be synthesized via thermal treatment of a mixture of tin oxide and charcoal powder in air at 1000‐1250°C. At a relatively low temperature, 1000°C, and time, 30 min, the tin oxide structure was found to be a bundle of dense nanorods. By increasing the reaction time to 180 min at the same temperature (1000°C), the bundles separated to form liberated individuals nanowires with almost round cross sectional view. With the increase of temperature and time, the morphology of tin oxide nanostructures varied from nanowires to well‐defined tetragonal rods. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)