Ethylene glycol-assisted solvothermal fabrication of ZnWO4 nanostructures with tunable size,optical properties,and photocatalytic activities

Large scale of uniform small ZnWO₄ nanocrystals and ZnWO₄ nanorods with tunable size have been fabricated in ethylene glycol (EG)-assisted solvothermal process, ZnWO₄ samples ranging in shape from tiny nanocrystals to nanorods were dependent on the volume ratio of EG and water (H₂O). The optical properties of ZnWO₄ nanocrystals and nanorods were investigated by photoluminescence (PL) spectroscopy, showing longer ZnWO₄ nanorods own the increased PL intensity in comparison with that of shorter ones and small ZnWO₄ nanocrystals. The photocatalytic performance of ZnWO₄ nanostructures was studied also, which indicated that the increased size of ZnWO₄ nanorods resulted the degradation of photocatalytic performance in aqueous solution.     

Time-Resolved Spectroscopy in ZnWo4 and ZnWO4:Fe

Time-resolved luminescence and absorption of ZnWO⁴ and ZnWO₄:Fe have been studied. The fast decaying luminescence at ～1.7eV is attributed to either Fe^2? or a Fe^3? related center. The two observed stages in luminescence decay kinetics under ionising radiation are suggested to be due to two types of self-trapped excitons.     

Untersuchung der Lumineszenzabklingung von Wolframatleuchtstoffen nach Anregung mit UV-Licht und Elektronen

The decay of the luminescence of CaWO₄, CdWO₄, MgWO₄, and ZnWO₄ after excitation with short pulses of light and electron beams has been investigated at different temperatures. Excitation with UV-light yields an exponential decay, and from the temperature dependence of the decay time it can be concluded that at high temperatures non-radiative transitions are prevalent. After excitation with electrons, there are additional effects by the heating up of the “excitation channel”. In this case the decay is not purely exponential and thermoluminescence is observed. This indicates the existence of traps, which are assumed to be located in the centers of the luminescence.     

Exited states of the luminescence centers in tungstate crystals

The electronic structures of luminescence centers in ZnWO₄, ZnWO₄:Mo, and ZnWO₄:Cd crystals are calculated by the configuration interaction method using embedded cluster approach. Dependencies of energies of the ground and excited electronic states of the centers on vibration coordinates are computed. The energies and oscillator strengths of radiative transitions in the luminescence centers of regular and doped zinc tungstate crystals are obtained. Formation of emission spectra of ZnWO₄, ZnMoO₄, and CdWO₄ are analyzed using results of the calculations.     

Influence of grain size on luminescence properties of micro- and nanopowder Zn2V2O7 vanadate

Micro- and spongiform nanocrystalline Zn₂V₂O₇ compounds were synthesized by hydrothermal and solid-state reaction techniques, and their morphological features were investigated by scanning electron microscopy (SEM). The grain size ranges of the produced powders were 3–15, 0.5–2 μm, and 50–500 nm. The luminescence spectra of these compounds were measured under pulse cathode beam and photoexcitation (200–400 nm). The luminescence decay properties of Zn₂V₂O₇ were studied.It is found that the intensity, maximum position of luminescence spectra and luminescence decay time of Zn₂V₂O₇ samples depend considerably on the grain size of the synthesized powders. The processes of energy relaxation in Zn₂V₂O₇ and the observed size effect on the luminescence properties are also discussed.     

Hydrothermal preparation and photoluminescence of bundle-like structure of ZnWO4 nanorods

ZnWO₄ nanorods with a bundle-like structure were synthesized at 180°C for 12 h by a hydrothermal technology from Na₂WO₄⋅2H₂O and ZnSO₄⋅7H₂O in the presence of sodium dodecyl sulfate (SDS). The as-synthesized bundle-like structure of ZnWO₄ nanorods was characterized by various techniques: TEM, XRD and EDS. The luminescence properties of the bundle-like structure of the ZnWO₄ nanorods were investigated by photoluminescence (PL) spectroscopy.     

Low-temperature hydrothermal synthesis of highly photoactive mesoporous spherical TiO2 nanocrystalline

TiO₂ microspheres with mesoporous textural microstructures and high photocatalytic activity were prepared by hydrothermal treatment of mixed solution of titanium sulfate and urea with designed time. The prepared samples were characterized by X-ray diffraction, scanning electron microscopy, transmission electron microscopy and N₂ adsorption-desorption measurements. The photocatalytic activity was evaluated via the photocatalytic oxidation of acetone in air at room temperature. The results show that the hydrothermal time significantly influences on the morphology, microstructure and photocatalytic activity of the as-prepared samples. With increasing hydrothermal time, specific surface areas and pore volumes decrease, contrarily, the crystallite size and relative anatase crystallinity increase. The photocatalytic efficiency of the as-prepared samples is obviously higher than that of commercial Degussa P25 (P25) powders. Especially, the as-prepared TiO₂ powders by hydrothermal treatment for 7 h shows the highest photocatalytic activity, which exceeds that of P25 by a factor of more than 2 times.     

Studies of photo-induced charge transfer properties of ZnWO4 photocatalyst

Different sizes of ZnWO₄ photocatalysts were synthesized by a hydrothermal method. The as-prepared sample shows highly efficient photocatalytic activity for the degradation of RhB under UV irradiation, which significantly vary with the increase of the hydrothermal temperatures. Surface photovoltage spectrum (SPS), field-induced surface photovoltage spectrum (FI-SPS) and surface photovoltage transient (TPV) techniques are used to investigate the detailed photoinduced charge transfer behavior. Results indicate that the ZnWO₄ synthesized at 413 K possess the largest BET surface area and the abundant donor surface states which are assumed to inhibit the recombination of the photogenerated electron-hole pairs, and thus a significant enhancement in the reaction rate is observed.     

Effects of microwave drying on the microstructure and photocatalytic activity of bimodal mesoporous TiO2 powders

Bimodal nanocrystalline mesoporous TiO₂ powders with highly photocatalytic activity were prepared by a hydrothermal method using tetrabutylorthotitanate as precursor, and then dried in microwave oven. The prepared samples were characterized by XRD, SEM, TEM, HRTEM and N₂ adsorption-desorption measurement. The photocatalytic activity was evaluated by the photocatalytic degradation of acetone in air under UV light irradiation at room temperature. The effects of microwave drying on the microstructures and photocatalytic activity of the TiO₂ powders were investigated and discussed. The results show that microwave drying not only promotes the growth of the pores but also greatly reduces the state of agglomeration within the powders. All the microwave-dried TiO₂ powders show higher photocatalytic activity than Degussa P-25 (P25) and the TiO₂ powders dried by conventional method.     

Scintillation properties of ZnWO4

Scintillation properties of ZnWO₄, bismuth doped ZnWO₄ and antimon doped ZnWO₄ were studied. Decay time, light yield and their temperature dependence are reported.     

Growth and scintillation properties of doped ZnWO4 crystals

A series of ZnWO₄ crystals with different purities of WO₃ and ZnO were grown by conventional Czochralski method and compared with Fe:ZnWO₄, Nb:ZnWO₄, Ag:ZnWO₄ and Ce:ZnWO₄ crystals grown by the same technology. Scintillation experiments revealed that the light yield of undoped ZnWO₄ crystals varied with the purities of raw materials WO₃ and ZnO. Raising the concentration of iron ions in ZnWO₄ crystals resulted in reduction of the light yield from the lighter appearance. On the contrary, the light yield was enhanced by doping Ag₂O, Nb₂O₅ and CeO₂, equivalent to that observed in ZnWO₄ crystals after oxygen-rich annealing at high temperature. The mechanisms of coloration and decolorization in doped ZnWO₄ crystals were analyzed. And the relationship between enhanced scintillation properties and the color center complex of iron impurity and oxygen vacancy in the crystals was discussed.     

ZnWO4晶体中W原子的配位数

本文依据X射线结构数据和Raman光谱测量,分析了闪烁体ZnWO₄单晶中W原子的配位情况。得出在属黑钨矿结构的ZnWO₄中,认为W的配位数Z为6较4要合适些,即把发光中心看作是WO₆原子基团比WO₄^-2离子更合适些。同时得到四条ZnWO₄晶体的Raman新谱线。 关键词：     

Electronic excitations in ZnWO4 and ZnxNi1?xWO4 (x = 0.1 ? 0.9) using VUV synchrotron radiation

The photoluminescence spectra and luminescence excitation spectra of pure microcrystalline and nano-sized ZnWO₄ as well as the Zn _x Ni_1−x WO₄ solid solutions were studied using vacuum ultraviolet (VUV) synchrotron radiation. The samples were also characterized by x-ray powder diffraction. We found that: (i) the shape of the photoluminescence band at 2.5 eV, being due to radiative electron transitions within the [WO₆]⁶⁻ anions, becomes modulated by the optical absorption of Ni²⁺ ions in the Zn _x Ni_1−x WO₄ solid solutions; and (ii) no significant change in the excitation spectra of Zn_0.9Ni_0.1WO₄ is observed compared to pure ZnWO₄. At the same time, a shift of the excitonic bands to smaller energies and a set of peaks, attributed to the one-electron transitions from the top of the valence band to quasi-localized states, were observed in the excitation spectrum of nano-sized ZnWO₄.     

Immobilized TiO<Subscript>2</Subscript> nanoparticles produced by flame spray for photocatalytic water remediation

Anatase/rutile mixed-phase titanium dioxide (TiO₂) photocatalysts in the form of nanostructured powders with different primary particle size, specific surface area, and rutile content were produced from the gas-phase by flame spray pyrolysis (FSP) starting from an organic solution containing titanium (IV) isopropoxide as Ti precursor. Flame spray-produced TiO₂ powders were characterized by means of X-ray diffraction, Raman spectroscopy, and BET measurements. As-prepared powders were mainly composed of anatase crystallites with size ranging from 7 to 15 nm according to the synthesis conditions. TiO₂ powders were embedded in a multilayered fluoropolymeric matrix to immobilize the nanoparticles into freestanding photocatalytic membranes. The photocatalytic activity of the TiO₂-embedded membranes toward the abatement of hydrosoluble organic pollutants was evaluated employing the photodegradation of rhodamine B in aqueous solution as test reaction. The photoabatement rate of best performing membranes significantly overcomes that of membranes produced by the same method and incorporating commercial P25-TiO₂.     

Raman and photoluminescence spectroscopy of zinc tungstate powders

ZnWO₄ powders, synthesized using co-precipitation technique and annealed in air at different temperatures in the range of 80-, were studied by Raman and photoluminescence spectroscopy. ZnWO₄ single crystal was used for comparison. The interpretation of the observed variations of the Raman spectra and intrinsic photoluminescence band upon annealing is suggested.     

Brightness and decay time of Zn2SiO4:Mn phosphor particles with spherical shape and fine size

Green-light-emitting Zn₂SiO₄:Mn phosphor particles were synthesized by modified large-scale spray pyrolysis using a colloidal solution containing fumed silica particles. The particles had a fine size, filled morphology, spherical shape and non-aggregation characteristics even after post-treatment at 1150 °C. The luminescence characteristics of Zn₂SiO₄:Mn phosphor particles having spherical shape, such as brightness and decay time under VUV excitation light, were investigated as a function of Mn content. The luminescence intensity exhibits a maximum at 5 mol% Mn-doped powder. The decay time increases consistently with decreasing Mn content and more saturated color coordinates are obtained with the composition having 12 mol% Mn. As a consequence, the 10 mol% Mn-doped powder proved to provide a good combination of favorable luminance and acceptable decay time. The influence of post-treatment temperature on the decay time and photoluminescence characteristics of Zn₂SiO₄:Mn particles was also investigated. Particles post-treated at 1155 °C had the maximum photoluminescence intensity, which was identical to 113% that of the commercial product, and a comparable decay time of 5.8 ms. PACS 81.20.Rg; 78. 55.Hx; 78.40.Ha; 81.05.Hd; 81.40.Tv     

Preparation of zinc tungstate nanomaterial and its sonocatalytic degradation of meloxicam as a novel sonocatalyst in aqueous solution

Zinc tungstate (ZnWO₄) was previously used as a photocatalyst. In this paper, for the first time as an sonocatalyst, the performance of ZnWO₄ for sonocatalytic degradation of meloxicam (MEL) under ultrasonic irradiation were studied. Firstly, ZnWO₄ nanomaterials were synthesized at different acidity (pH = 5, 6, 7, 8, 9) via the hydrothermal method. Utilizing SEM, XRD and EDS techniques to characterize composition and morphology of each product, the same crystal forms, but different morphologies (nano-sheet, nano-microspheres or nano-rod) of ZnWO₄ could be obtained. Secondly, the sonocatalytic activities of ZnWO₄ on degradation of MEL were studied. It was found that the degradation ratio varied with the synthetic pH values, with ZnWO₄ under synthetic pH = 6 exhibiting the best sonocatalytic performance (75.7%). While being synthesized at this pH value, ZnWO₄ nano-microspheres had the largest BET surface area (27.068 m²/g), the smallest particle size (40–60 nm) so as to provide more active sites on its surface, which were able to produce more reactive oxygen species (ROS) and holes under ultrasonic irradiation. These ROS and holes had a positive effect on the degradation of MEL into CO₂, H₂O and inorganic. Thirdly, various influential factors including ultrasonic power intensity, ultrasonic time, catalyst addition dosage, initial concentration of MEL solution and reusability of catalyst were also explored. Under the condition of 10 mg/L MEL concentration, 20 mg catalyst dosage, 120 min irradiation time, 0.278 W/cm² ultrasonic power intensity, the degradation ratio on MEL reached 75.7%. Finally, the presence of hydroxyl radical (OH) and singlet molecular oxygen (¹O₂) in the reaction was confirmed by adding ROS scavenger. The experimental results suggested that ZnWO₄ nanoparticle could be used not only as an effective photocatalyst, but also, under the condition of ultrasonic irradiation, a promising sonocatalyst for degradation of organic pollutants in aqueous media.     

Influence of Eu doping content on photoluminescence of Gd2O3:Eu phosphors prepared by liquid-phase reaction method

Europium-doped cubic Gd₂O₃:Eu³⁺ nanoparticles containing various activator content in the range of 5-15 wt% were synthesized by a liquid-phase reaction method to investigate the influence of Eu³⁺ loading on the optical properties of phosphors by using XRD, TEM, BET, spectrometer and fluorometer. The size of Gd₂O₃:Eu³⁺ powders was in the range 21-41 nm. The phosphors showed an initial increase in luminescence and then a subsequent decrease with further doping (above 10 wt%). The decay time was reduced with increasing Eu loading; however, it decreased significantly above the 10% Eu doping. From spectroscopic studies, the Eu³⁺ doping ion distribution was uniform and homogeneous up to the 10 wt% loading because no concentration quenching effect was observed. However, further Eu³⁺ doping above 10 wt% reduced the luminescence due to the concentration quenching effect, as deduced from the shortening of the decay time.     

MBiO2Cl(M=Ca，Sr，Ba)材料的合成、能带计算及发光性能研究

采用固相合成方法制备出CaBiO₂Cl,SrBiO₂Cl和BaBiO₂Cl粉体,研究了该Sillen系列铋基氧卤化合物的光学带隙、电子结构及发光性能.基于密度泛函理论计算表明,SrBiO₂Cl和BaBiO₂Cl均为直接带隙半导体材料,与吸收光谱实验结果相符合.在X射线和紫外光激发下三者均具有宽的可见光发射带(400—550nm),尤其是BaBiO₂Cl粉体的光输出强度约为Bi 关键词： 光致发光 电子结构 Sillen化合物     

Luminescence of CsPbCl3 microcrystals in CsCl:Pb and PbCl2:Cs crystals under synchrotron excitation

This paper reports on a study of the luminescence kinetics in CsPbCl₃ microcrystals dispersed in a CsCl or PbCl₂ host matrix, which are excited by synchrotron radiation at energies E _exc=4–20 eV. The luminescence decay kinetics of CsPbCl₃ microcrystals is found to contain different time components generated by direct excitation of the microcrystals or reabsorption of the luminescence emitted by other centers. It is conjectured that the decreased luminescence decay constant of CsPbCl₃ microcrystals dispersed in these hosts as compared to its counterpart of CsPbCl₃ single crystals is accounted for by size quantization.