Synthesis of monodisperse pancake-like Bi<Subscript>2</Subscript>WO<Subscript>6</Subscript> with prominent photocatalytic performances

Uniform Bi₂WO₆ pancakes were prepared via a solvothermal route in a solvent mixture of glycerol (Gly) and water (V/V = 1). A variety of techniques including scanning electron microscopy, transmission electron micrographs, X-ray powder diffraction, Brunauer–Emmett–Teller, FT-IR spectra, and UV–Vis diffuse reflectance spectra, were employed to characterize the structure and properties of the as-obtained Bi₂WO₆. It was found that Bi₂WO₆ pancakes showed prominent photocatalytic performance for the degradation of rhodamine B (RhB) under visible light (λ ≥ 420 nm) irradiation, which can be attributed to its good crystallization, large surface area, unique morphology and structural features.     

Visible-light-driven photocatalysis of heterostructure Ag/Bi<Subscript>2</Subscript>WO<Subscript>6</Subscript> nanocomposites and their photocatalytic degradation of dye under visible light irradiation

Ag/Bi₂WO₆ nanocomposites were successfully synthesized by a combination of hydrothermal method and ultrasonic vibration. The phases, vibration modes, constituents and morphologies were characterized by X-ray diffraction, Fourier transform infrared spectroscopy, field emission scanning electron microscopy, transmission electron microscopy and X-ray photoelectron spectroscopy. The visible-light-driven photocatalytic activitiy of 0–10 wt% Ag/Bi₂WO₆ samples was studied by determining the photodegradation of rhodamine B under xenon lamp. In this research, 10 wt% Ag/Bi₂WO₆ nanocomposites exhibit the highest efficiency and have the promising photocatalytic properties for waste water treatment.     

Solid-phase synthesis of sodium-bismuth tungstate NaBi(WO<Subscript>4</Subscript>)<Subscript>2</Subscript>

Results of a solid-phase synthesis of polycrystalline sodium-bismuth tungstate NaBi(WO₄)₂ from NaNO₃, Na₂WO₄, Bi₂O₃, and WO₃ are presented. The optimal synthesis conditions are determined and a technological flow chart for synthesis of a single-phase product is suggested.     

Novel La-doped Bi<Subscript>2</Subscript>WO<Subscript>6</Subscript> photocatalysts with enhanced visible-light photocatalytic activity

Novel La-doped Bi₂WO₆ composites were successfully prepared via a facile solvothermal method and well characterized by X-ray diffraction, Brunner?Emmet?Teller measurements, scanning electron microscopy, transmission electron microscopy/high-resolution, energy dispersive spectrometry, X-ray photoelectron spectroscopy, ultraviolet–visible spectroscopy, and Fourier transform infrared spectroscopy. The photocatalytic activity of modified catalysts was evaluated by degrading tetracycline hydrochloride under visible light (450?W Xe lamp irradiation). It was found 5%La-Bi₂WO₆ had the highest light-absorption ability, great morphology, and microstructures. The La dopant enlarged surface area and increased crystal defects, which may enhance the optical absorption activity and inhibit the recombination of the photo-generated charge carrier, respectively. After 150?min illumination, the photocatalysts that 5%La-Bi₂WO₆ and pure Bi₂WO₆ exhibited the best and worst photocatalytic performance, respectively (96.25% vs. 88.92%).

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Effect of oxygen activity in the gaseous phase on the mechanism of reactions in the solid-state synthesis of In<Subscript>2</Subscript>(WO<Subscript>4</Subscript>)<Subscript>3</Subscript> and In<Subscript>6</Subscript>WO<Subscript>12</Subscript>

The effect of oxygen’s activity on the rate of In₂(WO₄)₃ and In₆WO₁₂ formation reactions was studied to determine the reaction mass transfer mechanism. It was established that the formation of In₂(WO₄)₃ in a model reaction cell is due to the transfer of WO ₄ ^2? components and electrons moving in opposite directions through the reaction product. The relation between the diffusion coefficients of the carriers was found. The rate of electron diffusion and the reaction rate were shown to vary according to the law \(K_p \approx D_{\lim } = D_e \sim a_{O_2 }^{ - 1/4} \). We conclude that the formation of electronic conductor In₆WO₁₂ is a two-region process: at the In₂(WO₄)₃ | In₆WO₁₂ interface, the product is formed on the In₂(WO₄)₃ surface due to {WO₃} escaping toward In₂O₃, and at the In₆WO₁₂ | In₂O₃ interface, the product is formed on the In₂O₃ surface via the reaction of diffuse {WO₃} with In₂O₃. The probable relationship between the diffusion coefficients of the In₆WO₁₂ components was obtained. A relation was developed for the process rate. The diffusion coefficients for the limiting component were calculated using the data on the estimated thickness of the product layers.     

Ag<Subscript>3</Subscript>PO<Subscript>4</Subscript>/Bi<Subscript>2</Subscript>MoO<Subscript>6</Subscript> heterostructures with enhanced visible light photocatalytic activity for the degradation of rhodamine B

Highly efficient visible-light-driven Ag₃PO₄/Bi₂MoO₆ hybrid photocatalysts with different mole ratios of Ag₃PO₄ were prepared via sonochemical method and characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The results show that product are cubic Ag₃PO₄ nanoparticles supported on orthorhombic Bi₂MoO₆ nanoplates. Under visible light irradiation (>420 nm), the Ag₃PO₄/Bi₂MoO₆ photocatalysts displayed the higher photocatalytic activity than pure Bi₂MoO₆ for the decolorization of rhodamine B (RhB). Among the hybrid photocatalysts, 10% Ag₃PO₄/Bi₂MoO₆ exhibited the highest photocatalytic activity for the decolorization of RhB due to the efficient separation of electron–hole pairs.     

Mutual spontaneous and electrosurface processes at heterophase interfaces WO<Subscript>3</Subscript>|Me<Subscript>2</Subscript>(WO<Subscript>4</Subscript>)<Subscript>3</Subscript> (Me = In,Eu, Sc)

For the first time ever it is demonstrated in this work that, in spontaneous conditions and following the imposition of an electric field, mutual penetration of components of WO₃ and Me₂(WO₄)₃ occurs at heterophase interfaces WO₃|Me₂(WO₄)₃ where Me = In, Eu, or Sc. Tungstic oxide WO₃ is pulled onto the inner surface of ceramic Me₂(WO₄)₃ and, in turn, components of Me₂(WO₄)₃ penetrate onto the surface of grains of ceramic WO₃, which is confirmed by the method of x-ray—fluorescence analysis. Data concerning the conductivity and transport numbers of Eu₂(WO₄)₃ and a composite on its basis, which was manufactured as a result the electrosurface transport of WO₃, are obtained for the first time ever. With allowance made for the data on the O_2? character of the ionic conduction in MeWO₄ and Eu₂(WO₄)₃ it is concluded that the type of ionic carriers in tungstates (Me ⁿ⁺)_2/n [WO₄] makes no impact on the mechanism of spontaneous and field-induced processes that are developing at the (Me ⁿ⁺)_2/n [WO₄]|WO₃ interfaces.     

Advanced Bi<Subscript>2</Subscript>O<Subscript>2.7</Subscript>/Bi<Subscript>2</Subscript>Ti<Subscript>2</Subscript>O<Subscript>7</Subscript> composite film with enhanced visible-light-driven activity for the degradation of organic dyes

Bi₂O_2.7/Bi₂Ti₂O₇ composite photocatalyst films are synthesized by sol–gel dip-coating. The ratio of adding Bi and Ti precursors can be controlled during the preparation process. The phase structure is confirmed by X-ray diffraction. The UV–visible diffuse reflectance spectrum shows that the composite catalysts present light absorption in the visible region. The obtained Bi₂O_2.7/Bi₂Ti₂O₇ composite films possess superior photocatalytic degradation of rhodamine B, owing to the visible light response of Bi₂O_2.7 and the separation of photogenerated electrons and holes between the two components. As a result, the Bi₂O_2.7/Bi₂Ti₂O₇ (Bi/Ti = 1:1) displays the highest photocatalytic activity under visible light or UV light irradiation for the degradation of different organic dyes, including methyl blue, methyl orange and acid orange 7.     

Bi<Subscript>2</Subscript>O<Subscript>3</Subscript>/BiFeO<Subscript>3</Subscript> heterostructure: preparation,characterization, and photocatalytic activity

Bi₂O₃/BiFeO₃ composite was successfully fabricated by a conventional sol–gel method and structural properties were characterized based on X-ray diffractometer, scanning electron microscope, transmission electron microscope, energy-dispersive X-ray analyzer, nitrogen adsorption–desorption measurement, and UV–visible diffuse reflectance spectroscopy. Bi₂O₃/BiFeO₃ had a good absorption for visible light, which was benefit to photocatalytic activity. The highest degradation efficiency was obtained when the content of Bi₂O₃ in Bi₂O₃/BiFeO₃ was 63.9%. Effect of experimental conditions was investigated, and the highest photocatalytic activity of Bi₂O₃/BiFeO₃ was observed at photocatalyst dosage of 0.5 g/L, initial BPA concentration of 10 mg/L, and solution pH of 6.3. Bi₂O₃/BiFeO₃ photocatalyst exhibited enhanced photocatalytic activity for BPA, and the reaction rate constant over Bi₂O₃/BiFeO₃ composite was 2.23, 3.65, and 8.71 times higher than that of BiFeO₃, Bi₂O₃ and commercial TiO₂ (P25), respectively. Bi₂O₃/BiFeO₃ showed high photocatalytic activity after three cycles, suggesting that it was a stable photocatalyst. The possible photocatalytic mechanism has been discussed on the basis of the theoretical calculation and the experimental results. The hydroxyl and superoxide radicals together with photogenerated holes played significant roles in the photocatalytic reaction.     

Study of cationic substitution in Bi<Subscript>2</Subscript>WO<Subscript>6</Subscript> and derived structures in the framework of the modular approach

The possibilities of substitution of lead, alkaline and rare earth, antimony, and tellurium cations for bismuth ions in the structure of the Bi₂WO₆ ferroelectric and compounds with more complicated derived structures have been studied. The trends in the formation of solid solutions are well described in the framework of the modular approach in which layers are treated as building units of crystal structures. The underlying existence criteria (electroneutrality, geometric and chemical compatibility of layers) formulated for simple (two-layer) structures can be easily extended to more complicated (multilayer) structures. On the basis of the results obtained, the existence of new series of layered bismuth oxohalides was predicted.     

Effect of Bi<Subscript>2</Subscript>O<Subscript>3</Subscript> addition on structure and magnetic properties of Ni<Subscript>0.5</Subscript>Zn<Subscript>0.5</Subscript>Fe<Subscript>2</Subscript>O<Subscript>4</Subscript> nanofibers

Ni_0.5Zn_0.5Fe₂O₄ nanofibers with addition of 0–5 wt% Bi₂O₃ were synthesized by calcination of the electrospun polyvinylpyrrolidone/inorganic composite nanofibers at the temperature below the melting point of Bi₂O₃. The effects of Bi₂O₃ addition on the phase structure, morphology and magnetic properties of the nanofibers were investigated by means of X-ray diffraction, scanning electron microscopy, transmission electron microscopy, energy dispersive X-ray spectroscopy, selected area electron diffraction and vibrating sample magnetometer. It is found that the nanofiber diameter, crystallite size and magnetic parameters can be effectively tuned by simply adjusting the amount of Bi₂O₃ addition. The average diameter of Ni_0.5Zn_0.5Fe₂O₄ nanofibers doped with different contents of Bi₂O₃ ranges from 40 to 63 nm and gradually decreases with increasing Bi₂O₃ content. The addition of Bi₂O₃ does not induce the phase change and all the samples are a single-phase spinel structure. The amorphous Bi₂O₃ tends to concentrate on the nanoparticle surface and/or grain boundary and can retard the particles motion as well as the grain growth, resulting in a considerable reduction in grain size compared to the pristine sample. The specific saturation magnetization and coercivity of the nanofibers gradually decrease with the increase of Bi₂O₃ amount. Such behaviors are explained on the basis of chemical composition, surface effect, domain structure and crystal anisotropy.     

Synthesis of Bi<Subscript>2</Subscript>O<Subscript>4</Subscript>@TiO<Subscript>2</Subscript> Heterojunction with Enhanced Visible Light Photocatalytic Activity

A novel Bi₂O₄@TiO₂ heterojunction was constructed by a simple two-step method. The charges migration between Bi₂O₄ and TiO₂ via the heterojunction improves the electron/hole separation efficiency. Furthermore, Bi₂O₄@TiO₂ heterostructures exhibit better adsorption capability for methyl orange molecular due to their higher specific surface area than pure Bi₂O₄. As a result, Bi₂O₄@TiO₂ hybrids show an improved visible light photocatalytic activity and photostability for the degradation of methyl orange.     

Structural characterization and photocatalytic properties of novel Bi<Subscript>2</Subscript>FeVO<Subscript>7</Subscript>

Bi₂FeVO₇ was prepared by a solid-state reaction technique for the first time and the structural and photocatalytic properties of Bi₂FeVO₇ were studied. The results shows that this compound crystallized in the tetragonal crystal system with space group I4/mmm. Moreover, the band gap of Bi₂FeVO₇ was estimated to be about 2.22(6) eV. For the photocatalytic water splitting reaction, H₂ or O₂ evolution was observed from pure water with Bi₂FeVO₇ as the photocatalyst by ultraviolet light irradiation. Degradation of aqueous methylene blue (MB) dye by photocatalytic way over this compound was further studied under visible light irradiation. Bi₂FeVO₇ shows higher catalytic activity compared to TiO₂ (P-25) for MB photocatalytic degradation under visible light irradiation. Complete removal of aqueous MB was realized after visible light irradiation for 170 min with Bi₂FeVO₇ as the photocatalyst. The reduction of the total organic carbon (TOC) and the formation of inorganic products, SO ₄ ²⁻ and NO ₃ ⁻ revealed the continuous mineralization of aqueous MB during the photocatalytic course.     

NaBO<Subscript>2</Subscript>-Na<Subscript>2</Subscript>CO<Subscript>3</Subscript>-Na<Subscript>2</Subscript>MoO<Subscript>4</Subscript>-Na<Subscript>2</Subscript>WO<Subscript>4</Subscript> quaternary system

This is the first study of the NaBO₂-Na₂CO₃-Na₂MoO₄-Na₂WO₄ quaternary system by differential thermal analysis. Na₂[MoO_4(x)WO_{4(1 − x)}] solid solutions in the quaternary system are found to not decompose.     

Preparation and Characterisation of Nano-Structured WO<Subscript>3</Subscript>-TiO<Subscript>2</Subscript> Layers for Photoelectrochromic Devices

Nano-structured WO₃-TiO₂ layers were prepared by the sol-gel route. To obtain transparent, porous and crack free layers up to 0.8 μ m with a single dipping cycle a templating strategy was used. As a template three-dimensionally network based on organically modified silane was introduced to the WO₃ and TiO₂ sols. The WO₃ layers were dip-coated onto the conductive glass substrate (TCO) and the TiO₂ layers on the top of the WO₃ layer. The morphology and the structure of the layers were determined by Scanning Electron Microscopy (SEM), High Resolution Transmission Electron Microscopy (HR-TEM), Energy Dispersive X-Ray Spectroscopy (EDXS), Auger and Infrared spectroscopy. SEM image of the WO₃-TiO₂ layer confirmed the nano-porosity of the layers and give the size of the particles of about 10 nm for TiO₂ and 30 nm for WO₃ layer. Further analysis indicated that the titanium sol penetrates the WO₃ layer. Particles in the WO₃ layer consist of a crystalline monoclinic WO₃ core surrounded by a 5–10 nm amorphous phase consisting of WO₃, TiO₂ and SiO₂. The WO₃-TiO₂ layers were used to assemble all solid state photoelectrochromic (PE) devices. Under 1 sun irradiation (1000 W/m²) the visible transmittance of the PE device changes from 62% to 1.6%. The colouring and bleaching processes last about 10 minutes.     

Mass spectrometric investigation of vaporization in the Bi<Subscript>2</Subscript>O<Subscript>3</Subscript>-Fe<Subscript>2</Subscript>O<Subscript>3</Subscript> system

The thermodynamics of vaporization in the Bi₂O₃-Fe₂O₃ quasibinary system was studied by high-temperature mass-spectrometry. The partial pressures of the constituents of a saturated vapor over the system at 1100 K were determined. Based on the experimental data, the following parameters were calculated: the activities of the components of the Bi₂O₃-Fe₂O₃ system condensed phase, the standard enthalpies of some heterogeneous reactions, and standard enthalpies of formation and enthalpies of formation for crystalline BiFeO₃ and Bi₂Fe₄O₉ from individual oxides. An optimal temperature for the solid-phase synthesis of bismuth ferrites from simple oxides is recommended.     

Entrapment of Bi<Subscript>2</Subscript>O<Subscript>3</Subscript> nanoparticles in TiO<Subscript>2</Subscript> nanotubes for visible light-driven photocatalysis

In this study, we prepared nanoparticles of the visible light-responsive photocatalyst, Bi₂O₃ entrapped in anatase TiO₂ nanotubes (Bi₂O₃-in-TNTs) via a vacuum-assisted precursor-filling process followed by annealing. Owing to the unique tubular electronic structure of TiO₂ nanotubes, the interior of the nanotube is in an electron-deficient state, which was confirmed by XPS spectra and H₂-TPR. Electrochemical impedance studies showed that the Bi₂O₃-in-TNTs demonstrated a more efficient separation of photogenerated carriers than when Bi₂O₃ nanoparticles were deposited on the outer wall of TiO₂ nanotubes (Bi₂O₃-out-TNTs). Due to the confinement effect of TiO₂ nanotubes, which inhibits photogenerated carriers’ recombination, the Bi₂O₃-in-TNTs exhibited a better photocatalytic performance for the photo-degradation of methyl orange under visible light compared to Bi₂O₃-out-TNTs.     

Composition of the gas phase over Al<Subscript>2</Subscript>O<Subscript>3</Subscript> and the enthalpies of atomization of AlO,Al<Subscript>2</Subscript>O,and Al<Subscript>2</Subscript>O<Subscript>2</Subscript>

The A1, O, AlO, A1₂O, Al₂O₂, WO₂, and WO₃, partial pressures in the vapor over Al₂O₃ in a tungsten Knudsen effusion cell between 2300 and 2600 K were derived from A1⁺, O⁺, AlO⁺, A1₂O⁺, Al₂O₂⁺, WO₂⁺, and WO₃⁺, ion intensities. The mass spectrometer was calibrated against the equilibrium constant of the WO₃(g) = WO₂(g) + O(g) reaction. Refined values of the ionization cross sections of AlO and A1₂O₂ were used in the partial pressure calculations. The enthalpies of atomization of aluminum suboxides were determined to be Δ_at H ^o(AlO, g, 0) = 510.7 ± 3.3 kJ mol⁻¹, Δ_at H ^o(Al₂O, g, 0) = 1067.2 ± 6.9 kJ mol⁻¹, and Δ_at H ^o(Al₂O₂, g, 0) = 1556.7 ± 9.9 kJ mol⁻¹.     

Effect of tungsten oxide loading on metathesis activity of ethene and 2-butene over WO<Subscript>3</Subscript>/SiO<Subscript>2</Subscript> catalysts

The metathesis of ethene and 2-butene to propene was studied over WO₃/SiO₂ catalysts with various WO₃ loadings (2, 4, 8, 12, 16, and 24 wt%). The 2-butene conversion and propene selectivity increased greatly with WO₃ loading increasing from 2 to 8 wt%, reached maximum at 8–12 wt% WO₃ loading, and then decreased when the WO₃ loading was higher than 12 wt%. From the above results and taking the economics into account, the optimal amount of WO₃ loading was ~8 wt%. The catalysts were characterized by physico-chemical and spectroscopic techniques to elucidate the effect of different tungsten oxide loadings on the metathesis reactivity of ethene and 2-butene. The characterization data indicated that three types of tungsten species (i.e., surface tetrahedral tungsten species, surface octahedral polytungstate species, and WO₃ crystallites) were present in the catalysts. It was found that WO₃ was not the active centers, and surface tetrahedral tungsten species might be more active than octahedral polytungstate species in metathesis reaction. The reduced form of tungsten species [W⁺⁴, W⁺⁵, and W^+(6−y) (0 < y < 1)] may be the suitable state of W species acting as metathesis active centers.     

New cobalt and rare earth metal tungstates CoRE<Subscript>2</Subscript>W<Subscript>2</Subscript>O<Subscript>10</Subscript>

A new family of cobalt and rare-earth metal tungstates (CoRE₂W₂O₁₀ where RE=Y, Dy, Ho and Er) were synthesized by heating in the solid-state equimolar CoWO₄/RE₂WO₆ mixtures. The obtained compounds are isostructural and crystallize in the monoclinic system. They melt incongruently in an inert atmosphere at 1494 K (CoY₂W₂O₁₀), 1523 K (CoDy₂W₂O₁₀), 1517 K (CoHo₂W₂O₁₀) and 1493 K (CoEr₂W₂O₁₀). For each CoRE₂W₂O₁₀ compound the solid product of melting is an adequate RE₂WO₆.