Temperature dependence of the polarized Raman spectra of ZnWO4 single crystals

The growth of a large GdPO4 crystal guided by theoretical simulation and the study of its phonon properties

It has remained a big challenge to grow large crystals of compounds at very anisotropic crystal growth rates. GdPO₄ is such an example, which usually grows into a needle-like shape. This work solved this problem by using Li⁺ ions, predicted by first-principles calculations, to adjust the crystal morphology of GdPO₄. Based on the calculated surface energies and the polar properties of the surfaces, we identified two crystal planes, i.e. (-102) and (-111), which can be used to adjust the morphology of the GdPO₄ crystal by using extrinsic cations in the flux system. With a predicted Li containing flux system, Li₂O-MoO₃-B₂O₃, a large crystal with a record size, 10.0 × 9.0 × 18.2 mm³, was grown by using the top-seeded solution growth (TSSG) method. Based on the grown large crystal and the polarized Raman measurements, four new Raman bands missing in early studies and other bands were unambiguously assigned. By introducing a new formula, the phonon-phonon interactions were shown to be weak in the temperature range from 83 to 803 K. Our study indicates that the crystal of GdPO₄ can be used as a promising laser host material working under relatively high temperature conditions.

The stability and electronic and photocatalytic properties of the ZnWO4 (010) surface determined from first-principles and thermodynamic calculations

We present the results of the generalized-gradient approximation of Perdew, Burke and Ernzerhof (GGA-PBE) and the Heyd-Scuseria-Ernzerhof (HSE06) hybrid functional calculations of the atomic and the electronic structures of ZnWO₄ (010) surfaces. The total energies obtained from these calculations are used to analyze the thermodynamic stability of the surfaces. The surface phase diagrams are constructed by surface Gibbs free energies obtained as a function of temperature and oxygen partial pressure. Our results suggested that the stable area of the surface terminations of ZnWO₄ (010) has little correlation with the functional selected. The stability phase diagram shows that O-Zn, DL-W, and DL-Zn terminations of ZnWO₄ (010) can be stabilized under certain thermodynamic equilibrium conditions. Based on the HSE06 hybrid functional, we calculate the electronic structures for three possible stability surface terminations. It is found that there is a fat band of the surface states in DL-W termination, which shows a delocalized feature. This fat band acts as an electron transition bridge between the valence band (VB) and conduction band (CB). It contributes to visible-light absorption by two-step optical transition with the first transition from the VB to the fat band and the second from the fat band to the CB. Significantly, the conduction band minimum (CBM) band edge position of DL-W termination is favourable for H₂ evolution as the CBM edge is located above the water reduction level (H⁺/H₂). Simultaneously, DL-W termination's valence band maximum (VBM) potential shows a strong potential for O₂ generation from water oxidation because of the higher VBM edge with respect to the water oxidation level (H₂O/O₂). These results may help explore ZnWO₄ (010) surfaces' intrinsic properties, providing a helpful strategy for experimental studies of ZnWO₄-based photocatalysts in the future.

Reactive Dual Magnetron Sputtering: A Fast Method for Preparing Stoichiometric Microcrystalline ZnWO4 Thin Films

Thin films of ZnWO4, a promising photocatalytic and scintillator material, were deposited for the first time using a reactive dual magnetron sputtering procedure. A ZnO target was operated using an RF signal, and a W target was operated using a DC signal. The power on the ZnO target was changed so that it would match the sputtering rate of the W target operated at 25 W. The effects of the process parameters were characterized using optical spectroscopy, X-ray diffraction, and scanning electron microscopy, including energy dispersive X-ray spectroscopy as well as X-ray photoelectron spectroscopy. It was found that stoichiometric microcrystalline ZnWO4 thin films could be obtained, by operating the ZnO target during the sputtering procedure at a power of 55 W and by post-annealing the resulting thin films for at least 10 h at 600 °C. As FTO coated glass substrates were used, annealing led as well to the incorporation of Na, resulting in n+ doped ZnWO4 thin films.

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%). Whilebeing synthesized at this pH value, ZnWO₄nano-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.

Direct Z-Scheme g-C3N4/FeWO4 Nanocomposite for Enhanced and Selective Photocatalytic CO2 Reduction under Visible Light

Photocatalytic reduction of CO₂ to renewable solar fuels is considered to be a promising strategy to simultaneously solve both global warming and energy crises. However, development of a superior photocatalytic system with high product selectivity for CO₂ reduction under solar light is the prime requisite. Herein, a series of nature-inspired Z-scheme g C₃N₄/FeWO₄ composites are prepared for higher performance and selective CO₂ reduction to CO as solar fuel under solar light. The novel direct Z-scheme coupling of the visible light-active FeWO₄ nanoparticles with C₃N₄ nanosheets is seen to exhibit excellent performance for CO production with a rate of 6 μmol/g/h at an ambient temperature, almost 6 times higher compared to pristine C₃N₄ and 15 times higher than pristine FeWO₄. More importantly, selectivity for CO is 100% over other carbon products from CO₂ reduction and more than 90% over H₂ products from water splitting. Our results clearly demonstrate that the staggered band structure between FeWO₄ and C₃N₄ reflecting the nature-inspired Z-scheme system not only favors superior spatial separation of the electron-hole pair in g-C₃N₄/FeWO₄ but also shows good reusability. The present work provides unprecedented insights for constructing the direct Z-scheme by mimicking the nature for high performance and selective photocatalytic CO₂ reduction into solar fuels under solar light.

Photoluminescence Properties of Dy3+ Activated CaWO4 Nanophosphors: a Potential Single Phase near White Light Emitter

A multicolor tunable CaWO₄:xDy³⁺ nanophosphors have been synthesized via hydrothermal route. X-Ray Diffraction and Fourier transform infrared confirm the formation of CaWO₄:Dy³⁺ nanophosphors. Transmission electron microscopy image and selected area electron diffraction (SAED) reveal the formation of nanosize and crystalline CaWO₄:Dy³⁺. Dependence of energy transfer rate from WO₄^2- to the activator (Dy³⁺) is observed from the photoluminescence studies. An enhancement of energy transfer efficiency from 36% to 90% is observed after annealing the as-prepared samples at 800 °C. The exchange type energy transfer mechanism is observed to be dominant in as-prepared samples while the electric dipole-dipole interaction is dominant in annealed samples. Variation in energy transfer rate from the host to Dy³⁺ activator ions leads to the tuning of color emission from this nanophosphor. A near white light emission could be achieved with 6 at.% Dy³⁺ doped CaWO₄ annealed at 800 °C with x = 0.310 and y = 0.327.

Evaluation of photocatalytic and supercapacitor potential of nickel tungstate nanoparticles synthesized by electrochemical method

Nickel tungstate nanoparticles (NiWO₄ NPs) that were synthesized by an electrochemical method under various reaction conditions, namely, different tungstate ion concentrations, voltages, temperatures, and stirring rates, were studied here.

Crystal growth, optical spectroscopy and laser action of Tm3+-doped monoclinic magnesium tungstate

We report on the crystal growth, spectroscopic investigation and laser performance of Tm³⁺-doped monoclinic magnesium tungstate (Tm:MgWO₄), for the first time, to the best of our knowledge. A high-quality crystal has been grown by the top seeded solution growth method. The relevant spectroscopic properties are characterized in terms of absorption, luminescence and Raman spectroscopy. Judd-Ofelt (J-O) analysis is performed to evaluate the spontaneous emission probabilities and the radiative lifetimes. The absorption, stimulated-emission and gain cross-section spectra are determined for the principal light polarizations. The first laser action in the 2 μm spectral range is demonstrated in the regime of continuous-wave operation with a maximum output power of 775 mW and a slope efficiency of 39%.

Temperature-dependent optical phonon behaviour of a spinel Zn2TiO4 single crystal grown by the optical floating zone method in argon atmosphere

The spinel Zn₂TiO₄ single crystals were grown via optical floating zone technology in an argon atmosphere for the first time. And temperature dependent Raman spectra were presented.

Synthesis and photoluminescence of novel red-emitting ZnWO₄: Pr³⁺, Li⁺ phosphors

Zn0.997WO4: Pr(3+)(0.003) and different concentrations (0.1 mol% to 0.9 mol%) of Pr, Li co-doped ZnWO4 red phosphors were prepared by means of solid-state reaction process. The crystalline, surface morphology and luminescent properties of Zn0.997WO4: Pr(3+)(0.003) and Zn(1-x-y)WO4: xPr(3+), yLi(+) phosphors were investigated by the X-ray diffraction patterns (XRD), scanning electron microscope (SEM) and fluorescent measurements. From powder XRD analysis, the formation of monoclinic structure with C(2/h) point-group symmetry and P(2/c) space group of the as-synthesized samples is confirmed. The SEM image showed that surface morphology of the phosphor powder is irregular cylindricality. The luminescent spectra are dominated by the red emission peaks at 607, 621 and 643 nm, respectively, radiated from the (1)D2→(3)H4, (3)P0→(3)H6 and (3)P0→(3)F2 transitions of Pr(3+) ions. The concentrations of the highest luminescent intensity is determined at 0.3 mol% Pr(3+) and 0.3 mol% Li co-doped ZnWO4 powder crystal, and the peak intensity is improved more than 3 times in comparison with that of 0.3 mol% Pr(3+) single-doped ZnWO4. The enhanced luminescence comes from the improved crystalline and from the charge compensation of Li(+) ions. The decay curve and CIE chromaticity coordinates of as-prepared samples are also studied in detail.

Identifying and rationalizing the morphological, structural, and optical properties of [Formula: see text]-Ag2MoO4 microcrystals, and the formation process of Ag nanoparticles on their surfaces: combining experimental data and first-principles calculations

We present a combined theoretical and experimental study on the morphological, structural, and optical properties of β-Ag2MoO4 microcrystals. β-Ag2MoO4 samples were prepared by a co-precipitation method. The nucleation and formation of Ag nanoparticles on β-Ag2MoO4 during electron beam irradiation were also analyzed as a function of electron beam dose. These events were directly monitored in real-time using in situ field emission scanning electron microscopy (FE-SEM). The thermodynamic equilibrium shape of the β-Ag2MoO4 crystals was built with low-index surfaces (001), (011), and (111) through a Wulff construction. This shape suggests that the (011) face is the dominating surface in the ideal morphology. A significant increase in the values of the surface energy for the (011) face versus those of the other surfaces was observed, which allowed us to find agreement between the experimental and theoretical morphologies. Our investigation of the different morphologies and structures of the β-Ag2MoO4 crystals provided insight into how the crystal morphology can be controlled so that the surface chemistry of β-Ag2MoO4 can be tuned for specific applications. The presence of structural disorder in the tetrahedral [MoO4] and octahedral [AgO6] clusters, the building blocks of β-Ag2MoO4, was used to explain the experimentally measured optical properties.

Floating zone growth and optical phonon behavior of corundum Mg4Ta2O9 single crystals

We demonstrate that corundum Mg₄Ta₂O₉ crystals, free of low-angle grain boundaries and bubbles, were prepared for the first time by the infrared heating optical floating zone method.

Studies on Polyaniline/Silver Molybdate Nanocomposites

Silver molybdate nanoparticles were successfully prepared by the hydrothermal process. Polyaniline–silver molybdate nanocomposites were prepared by in situ chemical oxidative polymerization technique. Silver molybdate nanoparticles and the polymer samples were characterized by conductivity studies, Fourier transform infrared spectra (FT-IR), UV-visible spectra, photoluminescence spectra, X-ray diffraction (XRD), scanning electron microscopy (SEM) and thermogravimetric analysis (TGA). The electrical conductivity of PANI- Cl - increases when doped with silver molybdate nanoparticles and follows the percolation threshold.

Electrospinning-derived Tb2(WO4)3:Eu(3+) nanowires: energy transfer and tunable luminescence properties

One-dimensional Tb(2)(WO(4))(3) and Tb(2)(WO(4))(3):Eu(3+) nanowires have been prepared by a combination method of sol-gel process and electrospinning. X-Ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), photoluminescence (PL), low voltage cathodoluminescence (CL) and time-resolved emission spectra as well as kinetic decays were used to characterize the resulting samples. The as-obtained precursor samples present fiber-like morphology with uniform size, and Tb(2)(WO(4))(3) and Tb(2)(WO(4))(3):Eu(3+) nanowires were formed after annealing. Under ultraviolet excitation and low-voltage electron beams excitation into WO(4)(2-) and the f-f transition of Tb(3+), the Tb(2)(WO(4))(3) samples show the characteristic emission of Tb(3+) corresponding to (5)D(4)-(7)F(6, 5, 4, 3) transitions due to an efficient energy transfer from WO(4)(2-) to Tb(3+), while Tb(2)(WO(4))(3):Eu(3+) samples mainly exhibit the characteristic emission of Eu(3+) corresponding to (5)D(0)-(7)F(0, 1, 2) transitions due to an energy transfer occurs from WO(4)(2-) and Tb(3+) to Eu(3+). The increase of Eu(3+) concentration leads to the increase of the energy transfer efficiency from Tb(3+) to Eu(3+). The PL color of Tb(2)(WO(4))(3):x mol% Eu(3+) phosphors can be tuned from green to red easily by changing the doping concentration (x) of Eu(3+), making the materials have potential applications in fluorescent lamps and color display fields.

Optical properties of planar waveguides on ZnWO₄ formed by carbon and helium ion implantation and effects of annealing

We report on the optical properties of ZnWO(4) planar waveguides created by ion implantation, and the effect annealing has on these structures. Planar optical waveguides in ZnWO(4) crystals are fabricated by 5.0 MeV carbon ion implantation with a fluence of 1 × 10(15) ions/cm(2) or 500 keV helium ion implantation with the a fluence of 1 × 10(16) ions/cm(2). The thermal stability was investigated by 60 minute annealing cycles at different temperatures ranging from 260°C to 550°C in air. The guided modes were measured by a model 2010 prism coupler at wavelengths of 633 nm and 1539 nm. The reflectivity calculation method (RCM) was applied to simulate the refractive index profile in these waveguides. The near-field light intensity profiles were measured using the end-face coupling method. The absorption spectra show that the implantation processes have almost no influence on the visible band absorption.

Microwave-Assisted Synthesis of MWO4 and MMoO4 (M = Ca, Ni) Nano-Powders Using Citrate Complex Precursor

Nano-sized MWO4 and MMoO4 (M = Ca, Ni) powders, which have scheelite and wolframite type structure, were successfully synthesized at low temperatures by a modified citrate complex method assisted by microwave irradiation. The citrate complex precursors were heattreated at temperatures from 300 to 600 °C for 3 h. Crystallization of the MWO4 and MMoO4 precursors was detected at 400 °C and completed at a temperature of 500 °C. Most of the MWO4 and MMoO4 powders heat-treated between 300 and 600 °C showed primarily spherical and homogeneous morphology. The average crystallite sizes of MWO4 (M = Ca, Ni) were between 22 and 39 nm, and those for MMoO4 (M = Ca, Ni) were between 19 and 35 nm respectively, showing an ordinary tendency to grow with temperature.