Controlled synthesis and photoluminescence properties of hierarchical BaXO4 (X = Mo, W) nanostructures at room temperature

Controlled synthesis of hierarchical Barium molybdate (BaMoO₄) nanostructures with different morphologies, such as peanut-like, cube-like and flower-like, was successfully achieved in aqueous solution at room temperature. The obtained products were characterized by a scanning electron microscope (SEM) and an X-ray power diffractometer (XRD). The morphologies of the obtained products were found to be greatly dependent on reaction time, EDTA concentration and the [Ba²⁺]/[MoO₄²⁻] ratio. This controllable method could be readily extended to produce hierarchical Barium tungstate (BaWO₄) nanostructures with peanut-like, dumbbell-like, sphere-like and flower-like morphologies. The photoluminescence (PL) properties of the obtained BaMoO₄ and BaWO₄ nanostructures exhibited strong dependence on the morphologies and sizes, respectively.     

Facile synthesis of BaMoO4 and BaMoO4/BaWO4 heterostructures with type -I band arrangement and enhanced photoluminescence properties

A series of BaMoO₄ and BaMoO₄/BaWO₄ phosphors were successfully prepared via a polyacrylamide gel method and low temperature calcination technology. The effects of sintering temperature and mass percentage of BaMoO₄/BaWO₄ on the phase purity, functional group, surface morphology, charge state, photoluminescence properties and photocatalytic activity of the prepared products were studied in detail. The results indicate that the BaMoO₄ phosphor is a scheelite tetragonal structure with high crystallinity. The photoluminescence spectra indicates that the phosphors have a strong blue emission peak at 440 nm with excitation wavelength of 282 nm for the BaMoO₄ phosphor, and three emission peaks at 400, 440 and 460 nm with excitation wavelength of 284 nm for the BaMoO₄/BaWO₄ phosphors. These photoluminescence behaviors can be ascribed to the ¹T₂→¹A₁ transition, Jahn–Teller distorted tetrahedral symmetry of [MoO₄]^2- and surface defect. Photocatalytic experiments further confirmed that the BaMoO₄/BaWO₄ phosphors exhibit a high recombination rate of electron hole pairs. The result further indicates that the type-I band arrangement structure of BaMoO₄/BaWO₄ phosphors is beneficial to enhance the photoluminescent properties of single-phase phosphors. This study provides a novel route for preparing the type-I band arrangement structure composite phosphors with high photoluminescent properties and potential applications in light emitting devices, optoelectronic devices, laser devices and white pigments.     

Controlled synthesis and photoluminescence properties of BaXO4 (X = W, Mo) hierarchical nanostructures via a facile solution route

Shape-controlled synthesis of BaWO₄ hierarchical nanostructures has been achieved in a mixed solvent of water and ethanol at room temperature. By simply adjusting the volume ratio of C₂H₅OH and H₂O (R ratio), the size and shape of BaWO₄ nanostructures, such as shuttle-like and ellipsoid-like, are successfully controlled. This simple method has been extended to synthesize BaMoO₄ hierarchical nanostructures. Both BaWO₄ and BaMoO₄ hierarchical nanostructures exhibited new green emission peaks at 558 and 560 nm, respectively.     

Phase equilibria diagrams,crystal growth peculiarities and Raman investigations of lead and sodium-bismuth tungstate–molybdate solid solutions

In this paper a comprehensive study of lead and sodium-bismuth tungstate–molybdate solid solutions was carried out, including the clarification of their structural peculiarities and phase diagrams of PbMoO₄–PbWO₄ and NaBi(MoO₄)₂–NaBi(WO₄)₂ systems, the study of spontaneous Raman spectra of these compounds, as well as preliminary experiments on single crystals growth of lead tungstate–molybdate. The linewidths, peak and integral intensities of the totally symmetric Raman vibrations of solid solutions were estimated in comparison with known SRS-active crystals. The conditions of the Czochralski growth of optically transparent lead tungstate–molybdate mixed crystals were found and SRS effect was observed in these crystals when pumping by 12 ns 1064 nm laser pulses.     

Growth of BaMoO4 and BaCO3 crystals in silica gel media

Barium molybdate, BaMoO₄, and barium carbonate, BaCO₃, crystals have been grown in sodium metasilicate gels. The influence of concentration of reactant solutions and pH of gels on the nucleation and growth, and on the penetration depth of the growth front of BaMoO₄ crystals in the test tubes were primarily studied. Combinations of pH 6 gel mixture with the 0.4 m and 0.6 m supernatant BaCl₂ and Na₂MoO₄ solutions resulted in the best size of crystals, inter-crystalline separation and clarity of the gel media. Two best test tube experimental combinations of hybrid growth were repeated in the U-shaped glass tubes to obtain further information. Depending on the growth sites in the tubes, different morphologies of the crystals were observed. The bulk morphologies of these crystals were studied by scanning electron microscopy. Growth of BaCO₃ crystals was performed only as exploratory work in the test tubes.At Universidad Central de Venzuela only.     

Recombination luminescence in alkali metal sulfates

In this paper we present the results of investigation of the nature of intrinsic luminescence caused by photon excitation in a wide spectral range from 10.3 eV to 4 eV at a temperature of 15–300 K. It is shown that in alkali metal sulfates the main emission band formed after excitation by X-rays and photons with an energy of 9–11 eV and 4–7.5 eV at 15–300 K is located in the spectral range of 3.65–3.9 eV. When the sulfates are excited by 4–7.75 eV photons, in addition to the emission band at 3.65–3.9 eV the other effective long-wave band at 3.1–2.5 eV appears. It is assumed that the 3.65–3.9 eV radiation results from the recombination of electrons with unevenly located holes of SO₄⁻ type. The long-wave emission bands in the alkali metal sulfates may be connected with the formation of electron-hole trapping centers after irradiation by photons with energies above 4.4 eV.     

Synthesis of BaWO<Subscript>4</Subscript>/NRGO–g-C<Subscript>3</Subscript>N<Subscript>4</Subscript> nanocomposites with excellent multifunctional catalytic performance via microwave approach

Novel barium tungstate/nitrogen-doped reduced graphene oxide-graphitic carbon nitride (BaWO₄/NRGO-g-C₃N₄) nanocomposite has been synthesized by a simple one-pot microwave technique. The synthesized nanocomposites are well characterized by diffraction, microscopic and spectroscopic techniques to study its crystal structure, elemental composition, morphological features and optical properties. The material prepared is tested for its performance as an electrocatalyst, photocatalyst and reduction catalyst. The nanocomposite catalyzed the photodegradation of methylene blue (MB) dye in 120 min, reduction of 4-nitro phenol (4-NP) to 4-amino phenol (4-AP) in 60 s, showed an impressive Tafel slope of 62 mV/dec for hydrogen evolution reaction (HER). The observed results suggest that the nanocomposite acts as an efficient multifunctional catalyst. The reported approach provides fundamental insights which can be extended to other metal tungstate-based ternary composites for applications in the field of clean energy and environment in the future.     

Comment on “Molten salt synthesis of barium molybdate and tungstate microcrystals”

In this comment we discuss recent results presented by P. Afanasiev on the optical properties of microcrystalline BaMoO₄ and BaWO₄ [Materials Letters 61 (2007) 4622]. Its aim is to show that the band-gap energy reported in that work for BaWO₄ is not reliable and largely underestimate the correct value. As a consequence of it, the challenge made in Ref. [P. Afanasiev, Materials Letters 61 (2007) 4622] to the previous understanding of the electronic structure of scheelite-type compounds is not valid.     

Cathodoluminescence study of h- and c-GaN single crystals grown by a Na or K flux

Luminescence properties of hexagonal (h-) and cubic (c-) GaN freestanding single crystals were studied by means of cathodoluminescence spectroscopy. The h-GaN crystals of about 0.2–2 mm in dimension were synthesized at 750 °C by the reaction of Ga and N₂ in a Na flux, while c-GaN crystals of about 0.3 mm or less in a K flux. The h-GaN showed rather strong band edge emission at room temperature compared with the crystal grown by using NaN₃ as a nitrogen source. At 20 K, the band edge emission of h-GaN was split into four peaks. The main energy peak position was 3.478 eV, which was assigned as the A-free exciton emission. The energy position of the main peak of c-GaN was 3.268 eV. Assuming the binding energies of excitons in h- and c-GaN as 25 and 26 meV, respectively, the energy difference of bandgap between h- and c-GaN is estimated to be 209 meV. Since these crystals are free from strain from the substrates, the peak energies are reliable for the intrinsic GaN crystals. The full widths at half maximum of the emission of c-GaN were much broader than those of h-GaN, suggesting that the cubic phase is much defective compared with the hexagonal one.     

Synthesis of BaMoO4:Er/Yb particles by an MAM method and their upconversion photoluminescence properties

Er³⁺-doped BaMoO₄ (BaMoO₄:Er³⁺) and Er³⁺/Yb³⁺ co-doped BaMoO₄ (BaMoO₄:Er³⁺/Yb³⁺) particles were successfully synthesized by a cyclic microwave-assisted metathetic (MAM) method, and show fine and homogeneous morphology with particle sizes of 0.5–1 μm. At 980-nm excitation, BaMoO₄:Er³⁺ and BaMoO₄:Er³⁺/Yb³⁺ particles exhibited a strong 525-nm emission band and a weak 550-nm emission band in the green region. The Raman spectrum of BaMoO₄:Er³⁺/Yb³⁺ particles indicated the appearance of additional peaks at higher frequencies (390 and 505 cm⁻¹) and at lower frequencies (218 and 255 cm⁻¹).     

Unique photoluminescence properties of highly crystallized BaMoO4 film prepared by chemical reaction

Highly crystallized luminescent BaMoO₄ films have been prepared directly on molybdenum substrates at room temperature (25 °C) in barium hydroxide aqueous solution by a simple chemical reaction. In order to form the desired surface morphology and crystal structure, a mild oxidizing agent, NaClO, seems to be the key to the high crystallinity. In alkaline solution, the molybdenum substrate can gradually convert into MoO₄²⁻ ions, combined with Ba²⁺ ions which have a poor migration just depending on thermal motion to form a film. XRD and SEM results indicate that the films have single phase and uniform surfaces. FA results reveal that the films appear to have scintillating luminescent character when they are under different excitations at room temperature. Further, the excitation and emission wavelengths comply with a linear relationship which will be a potential technological application of BaMoO₄ in a variety of optical devices.     

MgWO4–A new crystal scintillator

Magnesium tungstate (MgWO₄) crystals of ~1 cm³ volume were obtained for the first time using a flux growth technique. The crystal was subjected to comprehensive characterisation that included room-temperature measurements of the transmittance, X-ray luminescence spectra, afterglow under X-ray excitation, relative photoelectron output, energy resolution, non-proportionality of scintillation response to γ-quanta, response to α-particles, and pulse shape for γ-quanta and α-particles. The light output and decay kinetics of MgWO₄ were studied over the temperature range 7–305 K. Under X-ray excitation the crystal exhibits an intense luminescence band peaking at a wavelength of 470 nm; the intensity of afterglow after 20 ms is 0.035%. An energy resolution of 9.1% for 662 keV γ-quanta of ¹³⁷Cs was measured with a small (≈0.9 g) sample of the MgWO₄ crystal. The photoelectron output of the MgWO₄ crystal scintillator is 35% that of CdWO₄ and 27% that of NaI(Tl). The detector showed pulse-shape discrimination ability in measurements with α-particles and γ-quanta, which enabled us to assess the radioactive contamination of the scintillator. The results of these studies demonstrate the prospect of this material for a variety of scintillation applications, including rare event searches.     

Synthesis of hierarchical barium tungstate corns and their shape evolution process

Barium tungstate nanocorns with lengths of 200-800 nm and with diameters of 20-50 nm in the middle section were synthesized by a facile stepwise solution-phase method. Various comparison experiments showed that several experimental parameters, such as the volume ratio of DMF/H₂O, and the quantity of urea and CTAB, played important roles for the morphological control of BaWO₄ nanostructures. A possible mechanism is offered for the formation of the hierarchical nanostructures. The obtained samples are characterized by means of X-ray diffraction, energy dispersion X-ray spectrometry, scanning electron microscopy, and transmission electron microscopy.     

Radiative recombination in Cu2ZnSnSe4 monograins studied by photoluminescence spectroscopy

In this study we investigated the optical properties of Cu₂ZnSnSe₄ monograin powders that were synthesized from binary compounds in the liquid phase of flux material (KI) in evacuated quartz ampoules. The monograin powder had p-type conductivity. Radiative recombination processes in Cu₂ZnSnSe₄ monograins were studied using photoluminescence spectroscopy. The detected low-temperature (T = 10 K) photoluminescence band at 0.946 eV results from band-to-impurity recombination in Cu₂ZnSnSe₄. The ionization energy of the corresponding acceptor defect was found to be 69 ± 4 meV. Additional photoluminescence bands detected at 0.765 eV, 0.810 eV and 0.860 eV are proposed to result from Cu₂SnSe₃ phase whose presence in the as-grown monograins was detected by Raman spectroscopy and SEM analysis. Considering photoluminescence results, it is proposed that the optical bandgap energy of Cu₂ZnSnSe₄ is around 1.02 eV at 10 K.     

Microwave-assisted synthesis of BaWO4 nanoparticles and its photoluminescence properties

Single-crystal BaWO₄ nanoparticles have been successfully synthesized under microwave irradiation. The results show that nearly monodisperse BaWO₄ nanoparticles have been successfully prepared without using surfactants. The products were characterized by X-ray powder diffraction (XRD), transmission electron microscopy (TEM), and photoluminescence (PL). The XRD results indicated that the BaWO₄ nanoparticles obtained had a tetragonal unit cell (a = 0.5612, c = 1.2706 nm). The TEM images show that the as-prepared BaWO₄ have good narrow particle-sized distributions containing a number of nanoparticles with uniform sizes. The products show a strong photoluminescence peak at 432–436 nm with the excitation at 365 nm.     

Electrical conductivity of single crystal nickel tungstate

Measurements of a.c. and d.c. electrical conductivity in crystals of nickel tungstate, in the temperature range 300 to 1100 K, are presented. NiWO₄ is found to be a semiconductor with a band gap of 2.10 eV. The nature of the electrical conduction is discussed in the light of various conduction models.     

Hydrothermal synthesis,characterization, and luminescence of BaWO4 nanorods

Barium tungstate (BaWO₄) nanorods were synthesized by a hydrothermal process with the assistant of cetyltrimethyl ammonium bromide (CTAB). The samples were analyzed by X-ray diffraction (XRD), Fourier transform infrared (FT-IR) spectroscopy, scanning electron microscopy (SEM), and photoluminescence measurements. The XRD and FT-IR results show that BaWO₄ samples can be indexed as a pure tetragonal scheelite structure. The SEM results show that the morphologies are nanorods with a diameter about 45 nm and a length exceeding 1 μm. The CTAB and “oriented attachment” play key roles in the growth of BaWO₄ nanorods in the hydrothermal process. When excited at 265 nm, BaWO₄ nanorods show the intrinsic emission band centered at 467 nm. The calculated luminescence lifetime of WO₄ ^2? in BaWO₄ is 8.9 μs.     

Melt growth and characterization of Mg2Si bulk crystals

We have grown Mg₂Si bulk crystals by the vertical Bridgman method using a high-purity Mg (6N-up) source. The grown crystals were single-phase Mg₂Si and had well-developed grains (1-5 mm³). Laue observations and SEM-EDX observations confirmed that crystalline quality in the grains was single crystal with stoichiometric composition. Electron concentration of the single crystalline specimens grown from 6N-up-Mg was 4.0 × 10¹⁵ cm^− 3 at room temperature (RT). This value is more than one order of magnitude lower than that of specimens grown from 4N-Mg [(5-7) × 10¹⁶ cm^− 3]. The Hall mobility of 14,500 cm²/Vs was observed at 45 K in the crystals grown from 6N-up-Mg. We also found that Al impurity plays an important role in the crystals grown from a low-purity Mg source. From the optical absorption measurement, we estimated that the indirect energy gap was about 0.66 eV at 300 K and about 0.74 eV at 4 K.     

Microstructures and microwave dielectric properties of Ba4LiNb3O12–BaWO4 composite ceramics

Composite ceramics of (1 ? x)Ba₄LiNb₃O₁₂–xBaWO₄ (x = 0.36–0.69) had been synthesized by co-firing the mixtures of Ba₄LiNb₃O₁₂ and BaWO₄ powders. The structures and microwave dielectric properties of the ceramics were studied by XRD, SEM, and TEM. These ceramics consisted of hexagonal Ba₄LiNb₃O₁₂ and tetragonal BaWO₄. The two phases co-existed well in the ceramics, and there was not obvious reaction at interfacial areas among grains. These ceramics had low sintering temperatures and excellent microwave dielectric properties, especially the small temperature coefficients of resonant frequency (τ_f) and high quality factor (Q × f) values. For composition at x = 0.69, the ceramic sintered at 1070 °C had Q × f value of 75,500 GHz and τ_f value of +8.7 ppm/°C.     

Pr3+ doped BaMoO4 octahedron to shuttle-like microcrystals: synthesis and luminescence properties

Octahedron to shuttle-like microcrystals of highly ordered BaMoO₄:Pr³⁺ have been successfully prepared through a facile hydrothermal method assisted with PVP (K30) (polyvinylpyrrolidone). Comprehensive structural, morphological studies like X-ray diffraction, scanning and transmission electron microscopy were employed to characterize the as synthesized microcrystals. In the hydrothermal process, a very small amount of PVP was added which not only acts as a surfactant to facilitate the formation of BaMoO₄:Pr³⁺, but also acts as a surface capping agent. By varying the reaction time, octahedron to shuttle-like morphologies has been obtained and rest of the parameters like, molar ratio and temperature are kept as constant have crucial influence on the shape evolution and microstructures. Photoluminescence (PL) studies on BaMoO₄:Pr³⁺ showed strong red emission upon UV illumination, and this implied potential application in the luminescent field.