Synthesis of Pr3+-doped WO3 particles: correlation between photoluminescent and photocatalytic properties

The WO3 and WO3:Pr3+ particles were successfully synthesized by the co-precipitation method. The XRD analysis with Rietveld refinement revealed the formation of a monoclinic phase for WO3 and for doped samples, this result was later confirmed by Raman spectroscopy studies. The presence of Pr3+ in the WO3 crystalline lattice induced structural and optical changes in the particles, increasing the crystallite size, distorting the clusters (shortening of the W-O bonds), favoring the crystallinity and changing the optical gap. The predominant morphology of the particles of WO3 and WO3:Pr3+ obtained was nanocubes constituted by the superposition of plates of nanometric thicknesses. The photoluminescence of WO3 and WO3:Pr3+ was produced by the existence of surface defects in the particles. The increase in the concentration of Pr3+ promoted an increase in the intensity of PL, due to the increase in the rate of recombination of electron/hole charges. The WO3 sample exhibited emission in the white region due to the adjustment of simultaneous electronic transitions in the blue, green and red regions, characteristic of the broadband spectrum. The interval of the 2.65 eV gap band and the high efficiency in the separation of the photogenerated charges (e-/h+) with the low recombination rate contributed to the photocatalytic degradation of Crystal Violet (CV) by the catalyst. The WO3:4% Pr3+ sample showed the best photocatalytic efficiency, degrading 73% of the CV dye in 80 minutes. This result was associated with a reduction in particle size and density of oxygen vacancies on the material surface.

Photocatalytic degradation of methylene blue and dye mixture using indium-doped CaWO4 synthesized by sonochemical and microwave-assisted hydrothermal methods

Synthesis methods and characterization of indium-doped calcium tungstate particles.

Structural and optical characterization of Er-doped CaMoO4 down-converting phosphors

xEr₂O₃-(1-x)CaMoO₄ (x = 1, 3, 5, 7 and 10 mol%) nanoparticles were synthesized by solid-state sintering at 800°C. X-ray diffraction studies confirmed the tetragonal crystal structure of CaMoO₄, while the doped samples show the co-existence of cubic Er₂O₃ and tetragonal CaMoO₄ and rule out the replacement of Ca²⁺ by Er³⁺ in the structure. The crystal unit-cell dimensions, phase concentration and atomic position coordinates were determined by Rietveld refinement. The short-range structure of CaMoO₄ consists of tetrahedral MoO₄ and snub disphenoid deltahedral CaO₈ units, while the unit cell of Er₂O₃ consists of two types of ErO₆ octahedral units. All MoO₄ units contain Mo-O bonds of equal lengths, whereas two types of slightly different Ca-O bond lengths exist in CaO₈. Raman spectra of the doped samples show only Mo-O vibrational modes and the Raman peaks of Er₂O₃ are masked by Mo-O bond vibrations. CaMoO₄ shows bluish-green emission at 500 nm, while Er-doped samples show strong green emission under UV excitation. UV irradiation (380 nm) induces down-conversion green emission at 531 nm and 552 nm and good color purity in 1 mol% Er₂O₃-CaMoO₄ sample which makes it a potential candidate for applications in optical devices.

Salt-assisted size-controlled synthesis and luminescence studies of single phase CaWO4:Dy3+: an insight into its morphological evolution, energy transfer and colour evaluation

The size of the particles of CaWO₄:Dy³⁺ increases with the addition of NH₄Cl/KCl electrolyte and enhances its luminescence.

Investigation on optical band gap, photoluminescence properties and concentration quenching mechanism of Pb1-x Tb3+xWO4 green-emitting phosphors

A series of monophasic Tb³⁺ (2, 5, 7, 10 and 15at%) doped PbWO₄ phosphors were successfully prepared via hydrothermal method. X-ray diffraction patterns revealed that the prepared samples possess a high crystallinity with tetragonal scheelite-type structure. FT-IR and Raman analysis exhibited a WO stretching peak of WO₄^2- group, which is also related to the scheelite structure. UV-visible diffuse reflectance spectra indicated a reduction in the optical band gap with the replacement of Pb²⁺ by Tb³⁺ ions. The presence of strong and intense emission peaks characteristic of Tb³⁺ with the dominant peak at 545nm (green, ⁵D₄→⁷F₅ transition) under UV irradiation at 320nm demonstrated an efficient energy transfer from the host to Tb³⁺ ions. Using Van Uitert's model, the concentration quenching mechanism between Tb³⁺ ions in PbWO₄:Tb³⁺ phosphor was attributed to a dipole-dipole interaction and the critical distance was determined to be ~12Å. The decay lifetimes and CIE chromaticity co-ordinates of PbWO₄:Tb³⁺ phosphors were also investigated in detail. These prepared materials might serve as a potential phosphor for LED applications.

Controllable synthesis of lanthanide Yb3+ and Er3+ co-doped AWO4 (A = Ca, Sr, Ba) micro-structured materials: phase, morphology and up-conversion luminescence enhancement

The combination of CDs can mainly maintain the morphologies of AWO₄:Yb³⁺,Er³⁺ but prominently increase their up-conversion luminescence performance.

Structural evolution, growth mechanism and photoluminescence properties of CuWO4 nanocrystals

Copper tungstate (CuWO₄) crystals were synthesized by the sonochemistry (SC) method, and then, heat treated in a conventional furnace at different temperatures for 1h. The structural evolution, growth mechanism and photoluminescence (PL) properties of these crystals were thoroughly investigated. X-ray diffraction patterns, micro-Raman spectra and Fourier transformed infrared spectra indicated that crystals heat treated and 100°C and 200°C have water molecules in their lattice (copper tungstate dihydrate (CuWO₄·2H₂O) with monoclinic structure), when the crystals are calcinated at 300°C have the presence of two phase (CuWO₄·2H₂O and CuWO₄), while the others heat treated at 400°C and 500°C have a single CuWO₄ triclinic structure. Field emission scanning electron microscopy revealed a change in the morphological features of these crystals with the increase of the heat treatment temperature. Transmission electron microscopy (TEM), high resolution-TEM images and selected area electron diffraction were employed to examine the shape, size and structure of these crystals. Ultraviolet-Visible spectra evidenced a decrease of band gap values with the increase of the temperature, which were correlated with the reduction of intermediary energy levels within the band gap. The intense photoluminescence (PL) emission was detected for the sample heat treat at 300°C for 1h, which have a mixture of CuWO₄·2H₂O and CuWO₄ phases. Therefore, there is a synergic effect between the intermediary energy levels arising from these two phases during the electronic transitions responsible for PL emissions.

Unexpected formation of scheelite-structured Ca1-xCdxWO4 (0 ≤ x ≤ 1) continuous solid solutions with tunable photoluminescent and electronic properties

Design of a solid solution with tunable functionality is an attractive strategy toward realizing novel devices with multi-functionalities. In this work, a series of Ca_1-xCd_xWO₄ solid solutions in the entire range 0 ≤ x ≤ 1 with tetragonal scheelite structure have been successfully prepared for the first time. X-ray diffraction (XRD), Fourier transform infrared (FT-IR) and Fourier transform Raman (FT-Raman) spectroscopies indicated that all the nanocrystals have a tetragonal scheelite structure without wolframite phase. Structural refinement data revealed that the lattice volume decreased with the replacement of Ca²⁺ by Cd²⁺ ions. UV-Vis diffuse reflectance spectra indicated that optical band gap reduced with the replacement of Ca²⁺ by Cd²⁺ ions. Scanning electron microscopic (SEM) images showed that morphologies of the nanocrystals changed with the chemical compositions. The structure evolution of the solid solutions was further investigated by high-resolution transmission electron microscopy (HRTEM). Moreover, the influence of chemical compositions on the photoluminescent and electric performance has been performed and discussed. The reported synthetic approach and findings reported here are important to understand the structure and structure-property relation of scheelite-structured tungstate and molybdate compounds, which has potential applications in the design of other kinds of novel functional materials.

Morphological and structural investigation of SrWO4 microcrystals in relationship with the electrical impedance properties

Synthesis of SrWO₄ crystal with spindles and spheres morphologies at ambient temperature. A structuring in tetragonal scheelite favors mixed conduction properties based on order–disorder transition.

Facile hydrothermal synthesis and pulsed laser deposition of Ca0.5Y1−x(WO4)2:xEu3+ phosphors: investigations on the luminescence, Judd–Ofelt analysis and charge compensation mechanism

PL emission spectra of Ca_0.5Y_1−x(WO₄)₂:xEu³⁺ for powder and thin film phosphors, and PL spectra of Ca_0.5Y_1−x(WO₄)₂:xEu³⁺ co-doped with alkali metal ions.

Novel red-emitting LiGd(WO4)2:Eu3+phosphor with high thermal stability and high color purity for application in white light-emitting diodes

Novel red-emitting LiGd(WO₄)₂:Eu³⁺phosphors with high thermal stability and high color purity have been developed for ultraviolet-excited white light-emitting diodes.

Lanthanide-activated scheelite nanocrystal phosphors prepared by the low-temperature vapor diffusion sol–gel method

The high degree of synthetic flexibility inherent to the vapor diffusion sol–gel method has enabled the synthesis of a CaWO₄:(Eu,Tb) dual-sensitized white light emitting nanocrystal phosphor.

Effects of chemical substitution on the structural and optical properties of α-Ag2−2xNixWO4(0 ≤ x ≤ 0.08) solid solutions

In this work, we investigated the effects of chemical substitution of α-Ag_2−2xNi_xWO₄(0 ≤x≤ 0.08) solid solutions prepared by a facile microwave-assisted hydrothermal method.

Structural and photoluminescence properties of Eu3+ doped α-Ag2WO4 synthesized by the green coprecipitation methodology

Europium doped silver tungstates α-Ag_2−3xEu_xWO₄ (x = 0, 0.0025, 0.005, 0.0075 and 0.01 mol) were synthesized by the coprecipitation method at 90 °C for 30 minutes. PL spectra showed a broad band related to the [WO₆] group and characteristic narrow peaks due to the f–f transitions of Eu³⁺ as a result of efficient energy transfer from the matrix.

A new single-phase white-light-emitting CaWO4:Dy3+ phosphor: synthesis, luminescence and energy transfer

A new single-phase white-emitting CaWO₄:Dy³⁺ phosphor has been successfully synthesized and the photoluminescence properties and energy-transfer mechanism have been carefully investigated.