Phase Composition and Charge Transport in Bismuth Molybdates

The study of structural and electrical properties of three pure bismuth molybdate phases, α-Bi₂Mo₃O₁₂, β-Bi₂Mo₂O₉ and γ-Bi₂MoO₆, prepared by the spray drying technique, is described and discussed. The structure of polycrystalline, layered samples investigated by means of XRD and Raman spectroscopy was found to be monoclinic for the α- and β-phases and orthorhombic for the γ-phase. The microstructure of the as-prepared samples was not sufficiently developed under the given conditions of preparation, however, the thermal treatment can improve it. The high polarizability of Bi³⁺ cations with their lone-pair electrons influences the stability of the disordered oxygen sublattice. All as-prepared phases undergo a slight structural change in the temperature region of 280–430°C resulting in a decrease of the electrical conductivity probably due to an order ⇄ disorder transition in the oxygen arrangement during the sample heating. The change of electrical conductivity observed was found to be reversible in the high-temperature region and irreversible in the low-temperature one. The blocking of oxygen transport by the bismuth lone-pair electrons results in an increase of the activation energy and a decrease of the electrical conductivity in the high-temperature region. Relatively high relative dielectric permittivities ɛ_r, 36–52, were observed in dependence on the investigated phase.__________From Elektrokhimiya, Vol. 41, No. 5, 2005, pp. 523–528.Original English Text Copyright © 2005 by Hartmanova, Le, Driessche, Hoste, Kundracik.This article was submitted by the authors in English.     

209Bi NQR in Mixed Oxides 2Bi2O3· B2O3, 3Bi2O3· 5B2O3, and Bi2O3· 3B2O3

It was earlier found from nuclear quadrupole resonance (NQR) measurements and computer modeling that -Bi₂O₃, Bi₃O₄Br and mixed oxides Bi₂O₃· 2Al₂O₃, Bi₂O₃· 2Ga₂O₃, Bi₂O₃· 3GeO₂, and 2Bi₂O₃· 3GeO₂exhibit local ordered magnetic fields from 30 to 200 G. It thus follows that these compounds are not diamagnets in a conventional sence of the word. With the aim of revealing previously unknown magnetic properties in bismuth(III) oxide-based Main Group element compounds, the mixed bismuth–boron oxides 2Bi₂O₃· B₂O₃, 3Bi₂O₃· 5B₂O₃, and Bi₂O₃· 3B₂O₃were prepared and studied using ²⁰⁹Bi NQR. The quadrupole interactions of the ²⁰⁹Bi nuclei and their electronic environment were studied, the crystallochemical features of the compounds were discussed, and the conformity of the ²⁰⁹Bi results to the X-ray structure data was verified. The preliminary tests in the field of a permanent magnet showed that the resonance intensities increase in external magnetic fields, indicating that a magnetism of unknown nature develops in the titled compounds. It was found reasonable to continue studies of the magnetic properties of these compounds using single-crystal ²⁰⁹Bi NQR in external magnetic fields.     

Visible-light photocatalytic activity of the metastable Bi20TiO32 synthesized by a high-temperature quenching method

Metastable Bi₂₀TiO₃₂ samples were synthesized by a high-temperature quenching method using α-Bi₂O₃ and anatase TiO₂ as raw materials. The photocatalytic activity of the as-prepared samples was measured with the photodegradation of methyl orange at room temperature under visible light irradiation. The Bi₂₀TiO₃₂ samples exhibited good absorption in the visible light region with a band gap of about 2.38 eV and the band structure of Bi₂₀TiO₃₂ was studied. Photodegradation against methyl orange was much better than α-Bi₂O₃ prepared by the same way. The photocatalytic activity of Bi₂₀TiO₃₂ samples is supposed to be associated with the hybridized Bi 6s and O 2p orbitals. In addition, the dispersive characteristic of Bi 6s orbital in the hybridized valence band facilitates the mobility of the photogenerated carriers and hampers their recombination.     

Structures and ionic conductivities in two fluorite type families: Pb5Bi17X5O43 and Pb5Bi18X4O42 (X=P,V and As)

The phases Pb₅Bi₁₇P₅O₄₃ and Pb₅Bi₁₈P₄O₄₂ are among many recent new oxyphosphates discovered in the ternary system PbO–Bi₂O₃–P₂O₅. The syntheses of the vanadates and arsenates led to isostructural compounds. Both series display a distorted 3×3×3 superstructure of the tetragonal δ-Bi₂O₃ polymorph. These types of phases display interesting anion conductivities and measurements were performed from 300 °C to 800 °C on the phosphates, arsenates and vanadates of the two families. The Pb₅Bi₁₈X₄O₄₂ phases have higher conductivity values than those of Pb₅Bi₁₇X₅O₄₃. Of the three homologues the vanadates always have the highest conductivities, i.e., at 800 °C: 1.6×10⁻² Siemens·cm⁻¹ for Pb₅Bi₁₈V₄O₄₂ and 1.6×10⁻³ Siemens·cm⁻¹ for Pb₅Bi₁₇V₅O₄₃. An increase of the volume of the unit cell due to the increasing radius of the pentavalent cations from P, As to V, 0.34 Å, 0.47 Å, 0.59 Å respectively, helps the anion migration through the structure.     

Thin Coatings of α- and β-Bi2O3 by Ultrasonic Spray Coating of a Molecular Bismuth Oxido Cluster and their Application for Photocatalytic Water Purification Under Visible Light

Thin coatings of Bi₂O₃ were deposited on glass substrates by ultrasonic spray coating of THF solutions of the molecular precursor [Bi₃₈O₄₅(OMc)₂₄(DMSO)₉] ⋅ 2DMSO ⋅ 7H₂O (OMc=O₂CC₃H₅) followed by hydrolysis and subsequent annealing. Depending on the synthetic protocol, the bismuth oxido cluster was transformed into either α- or β-Bi₂O₃. The as-synthesized Bi₂O₃ coatings were characterized by powder X-ray diffraction (PXRD), thickness measurements, diffuse reflectance UV-Vis spectroscopy (DRS), photoluminescence (PL) spectroscopy, Raman spectroscopy and scanning electron microscopy (SEM). The thin coatings (thickness: 5–16 μm) were compared with regard to their performance in photocatalytic rhodamine B (RhB) decomposition under visible light irradiation. The β-Bi₂O₃ coatings, that showed the highest photocatalytic activity, were used for the photocatalytic decomposition of other pollutants such as triclosan and ethinyl estradiol. In addition, the interplay between the photooxidation that is induced by the excitation of the catalyst using visible light and the photosensitized decomposition pathway was studied by degradation experiments of aqueous rhodamine B solutions using β-Bi₂O₃ coatings.     

Spectrophotometric study of vanadium(V)-phenylfluorone complex

Spectrophotometric studies of the reaction between vanadium(V) ions and phenylfluorone are presented and used for spectrophotometric determination of vanadium(V). The absorbance at 520 nm obeys Beer's law in the range of 2–15 μg vanadium/10 ml at pH 4. The relative standard deviation is 2% and the molar absorptivity based on vanadium is 2.1 × 10⁴ liters/mol cm. The composition of the complex in solution is of the 1:1 type with stability constant values to 2.5 × 10⁴. Analysis of the solid complex shows that its formula agrees with the formula (C₁₉ H₁₁ O₅)VO₂ · 5H₂O.     

Facile Fabrication of Porous Bi2O3 Microspheres by Thermal Treatment of Bi2O2CO3 Microspheres and its Photocatalysis Properties

β- and α-phase porous Bi₂O₃ microspheres with an average size of around 4 μm had been synthesized by thermal treatment of Bi₂O₂CO₃ microspheres at 350 and 400–500 °C respectively in an air atmosphere. The Bi₂O₂CO₃ microspheres had been synthesized at a temperature of 180 °C by a hydrothermal process using Bi(NO₃)₃ as the bismuth source with the assist of citric acid. By combining the results of X-ray powder diffraction, transmission electron microscope, scanning electron microscopy, and UV–Visible absorption spectra, the structural, morphological and optical properties characterization of the products were performed. The photocatalytic activity of the as-prepared α- and β-phase porous Bi₂O₃ microspheres have been tested by degradation of methylene orange under visible light, indicating that porous β-Bi₂O₃ microspheres showed enhanced photocatalytic performance compared to P25 and α-Bi₂O₃ microspheres.     

Synthesis and characterization of dinuclear vanadium(V) oxido peroxido tartrato complexes

Vanadium(V) oxido peroxido tartrato complexes have been prepared from aqueous-ethanolic media and characterized by spectroscopic methods. Using racemic tartaric acid for the synthesis, the simultaneous crystallization of racemic compounds (racemic phases) and racemic conglomerates (chiral phases) has been observed. The X-ray crystal structure of (NH₄)₄[V₂O₂(O₂)₂((2R,3R)–H₂tart)₂(μ–H₂O)][V₂O₂(O₂)₂((2S,3S)–H₂tart)₂(μ–H₂O)]·8H₂O (tart = C₄H₂O₆ ⁴⁻) revealed that the dinuclear anion is composed of two pentagonal bipyramidal polyhedra about vanadium atoms, which are joined to each other by sharing two oxygen atoms of hydroxyl groups and an oxygen atom from a bridging water ligand. The prepared compounds are not stable in aqueous solution; ⁵¹V NMR spectra exhibit the signals of several peroxido and non-peroxido vanadium(V) complexes.     

V2O5/TiO2 Catalytic Xerogels Raman and EPR Studies

Raman spectroscopy and Electron Paramagnetic Resonance (EPR) studies were performed on a series of V₂O₅/TiO₂ catalysts prepared by a modified sol-gel method in order to identify the vanadium species. Two species of surface vanadium were identified by Raman measurements, monomeric vanadyls and polymeric vanadates. Monomeric vanadyls are characterized by a narrow Raman band at 1030 cm^–1 and polymeric vanadates by two broad bands in the region from 900 to 960 cm^–1 and 770 to 850 cm^–1. The Raman spectra do not exhibit characteristic peaks of crystalline V₂O₅. These results are in agreement with those of X-ray Diffractometry (XRD) and Fourier Transform Infrared (FT-IR) previously reported (C.B. Rodella et al., J. Sol-Gel Sci. Techn., submitted). At least three families of V⁴⁺ ions were identified by EPR investigations. The analysis of the EPR spectra suggests that isolated V⁴⁺ ions are located in sites with octahedral symmetry substituting for Ti⁴⁺ ions in the rutile structure. Magnetically interacting V⁴⁺ ions are also present as pairs or clusters giving rise to a broad and structureless EPR line. At higher concentration of V₂O₅, a partial oxidation of V⁴⁺ to V⁵⁺ is apparent from the EPR results.     

The effect of water-miscible organic solvents on the substitution reaction of Keggin-type heteropolysilicates and -germanates with vanadium(V) ion

The substitution reaction of Keggin-type polyoxometalates, [XM₁₂O₄₀]ⁿ⁻ (X = Si, Ge, P, or As; M = Mo or W), with vanadium(V) ion was investigated with cyclic voltammetry, Raman spectroscopy, and ³¹P NMR. The effect of organic solvents, such as acetonitrile (ACN), tetrahydrofuran (THF), formamide (FA), N-methylformamide (NMF), and N-methylacetoamide (NMA) on their substitution reactions was examined. In aqueous and aqueous-FA, -NMF, and -NMA solutions, the molybdenum or tungsten unit in the Keggin-anions was substituted with vanadium(V) ion to form V(V)-substituted polyoxometalates, while no substitution reaction occurred in aqueous-ACN and -THF solutions. It is suggested that this results from the relationship between the total anion charge and the permittivity of organic solvents.     

Influence of the Mg-content on the cation distribution in cubic MgxFe3−xO4 nanoparticles

The influence of the Mg-content on the structural and magnetic properties of cubic Mg_xFe_3−xO₄ nanoparticles prepared by combustion reaction was investigated using X-ray diffraction, transmission electron microscopy (TEM), Mössbauer spectroscopy, and Raman spectroscopy. Lattice parameter, nanoparticle size, and cation (Mg²⁺, Fe³⁺) distribution were quantified as a function of the Mg-content in the range 0.5≤x≤1.5. We found a mixed-like spinel structure at the smaller x-value end whereas the inverse-like spinel structure dominates samples with larger x-values. Moreover, in the x-value range investigated (0.5≤x≤1.5) we found no change in the quadrupole splitting and isomer shift values, though the hyperfine field decreases as the x-value increases. The splitting of the A_1g Raman mode was used to both quantify the Mg²⁺/Fe³⁺ contents in the tetrahedral site and obtain the cation distribution in the Mg_xFe_3−xO₄ structure. The cation distribution obtained from the Raman data is in very good agreement with the cation distribution obtained from the Mössbauer data.     

Tetrahedral Coordination in the First Structurally Characterized Dichlorooxovanadium(V) Alkoxide

Reaction of VOCl₃ with 2‐phenoxyethanol in n‐hexane in a 1:1 fashion gives dichlorooxo(2‐phenoxyethanolato)vanadium(V). HCl elimination yields the orange vanadium(V) complex, which is the first structurally characterized dichlorooxovanadium(V) alkoxide. The structure analysis reveals an unexpected tetrahedral coordination around the vanadium atom in the monomeric compound. Alcoholysis and hydrolysis reactions of [VOCl₂(OCH₂CH₂OPh)] are monitored by ⁵¹V NMR spectroscopy. Activated with Me₃SiCH₂MgCl or ⁿBu₂Mg the complex catalyses the polymerisation of styrene.     

Characterization of thin metastable vanadium oxide films by Raman spectroscopy

Thin films are potentiodynamically generated on vanadium in Ba²⁺/acetate electrolyte systems at high voltages. The influence of the anodic potential up to 400 V on the composition and structure of the about 500 nm thin anodic conversion films are investigated. Raman spectroscopy indicates that different film types depend on the electrochemical process parameters. The relationship between the Raman laser excitation power and the amorphous or microcrystalline film structure is also discussed. Beside metastable disordered structures the films contain crystalline phases of V₂O₅, V₄O₉ and barium vanadate, respectively. Received: 15 July 1997 / Revised: 16 February 1998 / Accepted: 21 February 1998     

Structural studies of some body-centered cubic phases of mixed oxides involving Bi2O3: The structures of Bi25FeO40 and Bi38ZnO60

The structures of the compounds initially reported to be 7·Bi₂O₃·ZnO and 96·Bi₂O₃·4Fe₂O₃, have been determined by X ray methods. Three dimensional, absorption corrected diffractometer data were used and atomic parameters were refined by least-squares procedures. The structures are isomorphous, cubic witha = 10.194(3)and10.179(3)A?, respectively, and space group I23. Each Bi³⁺ ion is surrounded by five oxygen atoms that form an incomplete octahedral arrangement with BiO distances ranging from 2.07–2.60A?. The6s²inert electron pair completes the octahedron. The Bi³⁺ ions are vibrating anisotropically. Tetrahedral sites in the structures contain 61 and 46 electrons, respectively. These values are consistent with a statistical distribution of Zn²⁺ and Bi⁵⁺ ions or Fe³⁺ and Bi⁵⁺ ions on these sites. Molar ratios are derived that agree with the observed distributions of electron density and give rise to perfectly stoichiometric systems, devoid of cationic or anionic vacancies. The compositions studied correspond to Bi³⁺₂₄Bi⁵⁺Fe³⁺O₄₀ and Bi³⁺₃₆Bi⁵⁺₂ZnO₆₀ and they are optical enantiomorphs.It is proposed that a reduction in the percentage composition of Bi₂O₅ leads to metastable phases, in which all atomic positions remain fully occupied but some tetrahedral sites contain Bi³⁺ ions. The end product of the series is γ-Bi₂O₃ in which 50% of these sites contain Bi³⁺ and the remainder Bi⁵⁺ ions. We believe that γ-Bi₂O₃ is Bi³⁺₂₅Bi⁵⁺O₄₀.     

Ethylenediaminetetraacetato-monoperoxo complexes of vanadium(V)

Summary The monoperoxo complexes, M₂[VO(HEDTA)(O₂)]· 4H₂O, where M is K⁺ or NH ₄ ⁺ and H₄EDTA is ethylene-diaminetetraacetic acid, were prepared and characterized by Raman and i.r. spectra in the solid state and in aqueous solution. The single crystal X-ray study revealed a pentagonal bipyramidal anion structure with a tetradentate HEDTA(3—) ligand. The decomposition of complexes in aqueous solution to blue vanadium(IV) complexes as end products proceeds via a nonperoxo complex of vanadium.     

Multinuclear NMR Spectroscopic Characterization of Vanadium(V) Alkylperoxo Complexes VO(OOtBu)k(OnBu)3-k, where k = 1, 2, 3

Using ⁵¹V, ¹⁷O, ¹³C and ¹H NMR spectroscopy, vanadium(V) alkylperoxo complexes VO(OOtBu)_k(OnBu)_3-k, where k = 1, 2 and 3, were characterized in the reaction of VO(OnBu)₃ with tBuOOH in CH₂Cl₂.     

Vanadium in Italian waters: monitoring and speciation of V(IV) and V(V)

In this work, a highly sensitive method was developed to separate vanadium (IV) from vanadium (V), which are both contained in water at trace levels. A suitable strong anionic exchange column (SAX) loaded with disodium ethylendiaminetetraacetic acid (Na₂EDTA) was used to trap both vanadium species dissolved in 10–100 ml of water at pH 3. The vanadyl ion was selectively eluted by means of 15 ml of an aqueous solution containing Na₂EDTA, tetrabutylammonium hydroxide (TBA⁺OH⁻), and isopropanol (ⁱPr-OH) and was subsequently determined by atomic absorption spectroscopy with electrothermal atomization. The concentration of vanadate ion was calculated by subtracting the vanadyl concentration from the total concentration of vanadium. The optimal conditions for a selective elution were evaluated. The recovery of vanadium (IV) was 95% or better. The proposed method provides a simple procedure for the speciation of vanadium in aqueous matrices. The collection of the two forms could easily be carried out at the sampling site. Therefore, the risk of changing the concentration ratio between vanadium species was widely reduced. The detection limits were 1 μg/l for both species, when a 10-ml sample was eluted through the column. The method was applied successfully to vanadium speciation on different kinds of Italian volcanic water: Mount Etna (Sicily), Lake Bracciano and Castelli Romani (Latium).     

Non-isothermal crystallization kinetics of V2O5-MoO3-Bi2O3 glasses

Characteristic temperatures, such as T _g (glass transition), T _x (crystallization temperature) and T _l (liquidus temperature) of glasses from the V₂O₅-MoO₃-Bi₂O₃ system were determined by means of differential thermal analysis (DTA). The higher content of MoO₃ improved the thermal stability of the glasses as well as the glass forming ability. The non-isothermal crystallization was investigated and following energies of the crystal growth were obtained: glass #1 (80V₂O₅·20Bi₂O₃) E _G=280 kJ mol^-1, glass #2 (40V₂O₅·30MoO₃·30Bi₂O₃) E _G=422 kJ mol^-1 and glass #3 (80MoO₃·10V₂O₅·10Bi₂O₃) E _G=305 kJ mol^-1. The crystallization mechanism of glass #1 (n=3) is bulk, of glass #3 (n=1) is surface. Bulk and surface crystallization was supposed in glass #2. The presence of high content of a vanadium oxide acts as a nucleation agent and facilitates bulk crystallization. This revised version was published online in August 2006 with corrections to the Cover Date.     

Structural and transport characteristics of substituted bismuth niobates

The results of studying solid solutions with the composition of Bi₃Nb_{1 ? y} Zr _y O_{7 ± δ}, Bi_2.95Y_0.05Nb_{1 ? y} Zr _y O_{7 ± δ} (y = 0–0.5; Δy = 0.1), and Bi_6.95Y_0.05Nb_{2 ? y} Zr _y O_15.5 (y = 0.1–1; Δy = 0.1) are presented. XRD and electron microscopy with X-ray microanalysis are used to determine the homogeneity regions of solid solutions; crystallochemical parameters are calculated. It is shown that irrespective of the ratio of Bi: Nb, two cubic phases are formed at an increase in the dopant amount. One of these represents a solid solution based on Bi₃NbO₇ (δ-phase) and the second one is a solid solution based on δ-Bi₂O₃ (δ′-phase). Conductivity of sintered samples is studied using the impedance spectroscopy technique. Introduction of yttrium into the bismuth sublattice results in no increase in conductivity of solid solutions, while in the case of the ratio of Bi: Nb = 3: 1, overall conductivity of solid solutions is somewhat higher at similar dopant concentrations.     

Determination of Vanadium(V) and Vanadium(IV) in Bismuth- and Copper-Containing Compounds by Controlled-Potential Coulometry

It was shown that vanadium(V) and vanadium(IV) can be determined at a large Pt electrode in H₂SO₄ solutions in the presence of copper and bismuth by controlled-potential coulometry with RSD no worse than 0.2%. Compounds of the composition Bi₄V_1.8Cu_0.2O_{10.7 – x} were analyzed.