Phonon properties of nanosized bismuth layered ferroelectric material—Bi2WO6

Nanosized Bi₂WO₆ was synthesized by a mild hydrothermal crystallization process. This method allowed obtaining plate‐like crystallites of very small thickness (down to 3 nm). The effect of particle size on the structure and properties of Bi₂WO₆ was studied by X‐ray diffraction (XRD), transmission electron microscopy, and Raman and infrared spectroscopies. It has been shown that the orthorhombic distortion decreases with decreasing particle size, but the structure of the smallest crystallites is still orthorhombic. Raman studies have also revealed a very strong intensity decrease for those modes that appear mainly for incident and scattered light polarized perpendicular to the layers. This behavior has been attributed to a decrease in the orthorhombic distortion and a plate‐like shape of the nanocrystallites. Copyright © 2009 John Wiley & Sons, Ltd.     

Raman and IR spectra of the cation‐deficient Aurivillius layered crystal Bi2W2O9

Raman and IR spectra of Bi₂W₂O₉, the m = 2 member of the cation‐deficient Aurivillius phase, have been measured. The obtained spectra are discussed using the factor group approach for the orthorhombic Pna2₁(C_2v⁹) space group. The results are compared with those obtained for Bi₂WO₆ and some m = 2 members of Aurivillius family of bismuth layered ferroelectrics. Copyright © 2009 John Wiley & Sons, Ltd.     

Temperature‐induced phase transformations in Na2WO4 and Na2MoO4 crystals

Temperature‐dependent Raman studies on Na₂WO₄and Na₂MoO₄ crystals were performed in order to obtain information on structural changes induced by temperature evolution. The stability of the cubic phase of Na₂WO₄and Na₂MoO₄crystals was assessed and our results indicate that this phase is stable in the 8–823 K and 15–773 K ranges for Na₂WO₄ and Na₂MoO₄, respectively. The crystal of Na₂WO₄ shows a phase transition occurring at 833 K. Na₂MoO₄undergoes a sequence of three phase transitions, which were observed at 783–803, 823–913 and 943–950 K. In both crystals, a strong first‐order phase transition occurs as indicated by the behavior of the Raman modes: the Raman bands split and new bands appear below 100 cm⁻¹. These transitions are connected with tilting and/or rotations of the WO₄and MoO₄ tetrahedra, which leads to a disorder at the WO₄and MoO₄ sites. Copyright © 2010 John Wiley & Sons, Ltd.     

Luminescence centers in thin films of lead and bismuth tungstates

Luminescence spectra of thin films of PbWO₄ and Bi₂WO₆ are invesigated. The Alentsev-Fock method is used to separate the spectra into elementary components. The emission bands with maxima at 2.8 eV in PbWO₄ and at 2.93 eV in Bi₂WO₆ luminescence spectra are interpreted as the emission of self-localized Frenkel excitons. The bands with maxima at 2.35 and 1.75 eV in PbWO₄ and at 2.35 and 1.9 eV in Bi₂WO₆ are related to oxygen vacancies. L’vov State University, 50, Dragomanov St., L'vov, 290005, Ukraine. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 65, No. 1, pp. 143–145, January–February, 1998.     

Luminescence of Thin Films of Lead and Bismuth Tungstates

The luminescence spectra of thin films of PbWO₄ and Bi₂WO₆ were investigated. It is shown that these spectra are similar and that they consist of three individual bands in the blue (2.80 eV PbWO₄ and 2.93 eV Bi₂WO₆), green (2.35 eV PbWO₄ and Bi₂WO₆), and red (1.75 eV PbWO₄ and 1.90 eV Bi₂WO₆) spectral regions. The differences in the nature of the absorption centers of excitation energy are established. The distinguishing features displayed by the temperature dependences of the individual emission bands in the PbWO₄ films are explained by energy migration between emission centers via transfer of free carriers through the conduction and valence bands.     

Luminescence centers in bismuth-containing oxytungstate ceramics

We have studied the photoexcitation and luminescence spectra of Bi₂WO₆, Y₂WO₆ and Y₂WO₆:Bi ceramics. We used the Alentsev-Fock method to decompose the spectra into elementary components. The emission bands with maximum at 2.93 eV in the luminescence spectrum of Bi₂WO₆, 3.02 eV in the luminescence spectrum of Y₂WO₆, and 2.95 eV in the luminescence spectrum of Y₂WO₆:Bi are assigned to luminescence of self-localized Frenkel excitons. The bands with maxima at 2.35 eV and 1.90 eV in the spectrum of Bi₂WO₆, 2.25 eV and 1.75 eV in the spectrum of Y₂WO₆, and 2.35 eV and 1.85 eV in the spectrum of Y₂WO₆:Bi are connected with oxygen vacancies. __________ Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 74, No. 5, pp. 688–691, September–October, 2007.     

圆盘状Bi2WO6-BiPO4异质结的水热合成及光催化性能 (cited: 1)

采用水热法制备Bi₂WO₆-BiPO₄异质结光催化剂．利用模拟太阳光照射下的罗丹明B降解实验评价了Bi₂WO₆-BiPO₄复合物的光催化性能．结果表明,Bi₂WO₆-BiPO₄光催化活性比Bi₂WO₆和BiPO₄高得多．当Bi₂WO₆与BiPO₄的摩尔比为1:1时复合光催化剂对罗丹明B的降解率最高．Bi₂WO₆-BiPO₄催化活性增强主要归结为两者之间形成了有效的异质结结构,其内建电场能够促进光生载流子的分离．同时,Bi₂WO₆的加入增强了其对可见光的吸收．研究表明O₂· ^-和h⁺在光催化降解过程中是主要的活性物种     

三元异质结构Ag-Bi2MoO6/BiPO4光催化剂高效降解苯酚红

多组分复合体系有利于电荷的有效分离,减少电子空穴对的复合几率。通过低温液相法首次合成Ag-Bi₂MoO₆/BiPO₄三元异质结构光催化剂。利用XRD、SEM、EDX及XPS等技术对样品进行了表征。结果表明,Ag纳米粒子光照积累在Bi₂MoO₆/BiPO₄的表面,通过表面等离子共振增加对可见光的吸收,同时作为电子受体促进了光生电子的转移。Ag、BiPO₄和Bi₂MoO₆形成三元异质结构有效地抑制了光生电子空穴对的复合。Ag-Bi₂MoO₆/BiPO₄表现出优异的光催化性能,其光催化活性较BiPO₄、Bi₂MoO₆和Bi₂MoO₆/BiPO₄样品有较大提高。并且对Ag-Bi₂MoO₆/BiPO₄三元异质结构的光催化机制进行了讨论。光催化过程中反应活性物种捕获实验结果表明h⁺和O₂^·-是主要的活性基团.     

Temperature dependent Raman scattering in polycrystalline Bi4Ti3O12 thin films

Polycrystalline Bi₄Ti₃O₁₂ thin films were prepared on quartz substrates by pulsed laser deposition. The films were crystallized in the orthorhombic layer perovskite structure confirmed by X-ray diffraction and Raman spectroscopy. The Raman spectra are strongly dependent on temperature. A subtle phase transition in the temperature range 473-573 K exists in polycrystalline BTO thin films, which is evidenced by the disappearance of the Raman band at 116 cm⁻¹ and appearance of a new Raman band at 151 cm⁻¹. The two broad Raman bands centered at the 57 and 93 cm⁻¹ at 300 K break up into clusters of several sharp Raman peaks at 77 K, due to monoclinic distortion of orthorhombic structure at low temperature in the as-prepared Bi₄Ti₃O₁₂ thin films.     

Rapid sonochemical synthesis of irregular nanolaminar-like Bi2WO6 as efficient visible-light-active photocatalysts

Irregular Bi₂WO₆ nanolaminars have been successfully synthesized via a rapid sonochemical approach using bismuth nitrate and tungstic acid as precursors in an aqueous solution. The characteristics of them were investigated in detail by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), N₂ adsorption, pore value, PL spectroscopy and UV–vis diffuse reflectance spectroscopy (UV–vis DRS). These irregular nanolaminars are of geometric shapes of orthorhombic Bi₂WO₆ with their basal plane being (0 0 1). They possess high crystallinity, lager surface area and pore value, which means fewer traps and stronger photocatalytic activity. The growth mechanism of such special nanolaminar was related to the sonochemical synthesis route, which played a key role in the formation of Bi₂WO₆ nanolaminar. Simultaneously, it was found that the formation of Bi₂WO₆ nanolaminar is a time dependent process. The Bi₂WO₆ nanolaminar has higher photocatalytic activity than bulk Bi₂WO₆ nanoparticle obtained by refluxing method for rhodamine B (Rh.B) degradation under visible light irradiation (λ > 400 nm).     

Polarised IR and Raman spectra of non‐centrosymmetric Na3Li(MoO4)2· 6H2O crystal—a new Raman laser material

Polarised IR and Raman spectra of Na₃Li(MoO₄)₂· 6H₂O single crystal were measured. Discussion of the results is based on the factor group approach for the trigonal R 3c(C_3v⁶) space group with Z = 2. The assignment of the observed bands was performed on the basis of their polarisation behaviour and literature data. The obtained results for the spontaneous Raman scattering were used in the analysis of the stimulated Raman spectra of the material studied—a new Raman laser crystal. The promoting modes of the stimulated effect were identified. Copyright © 2009 John Wiley & Sons, Ltd.     

The effects of Gd3+ doping on the physical structure and photocatalytic performance of Bi2MoO6 nanoplate crystals

Gd³⁺ doped Bi₂MoO₆ nanoplate crystals were fabricated by solvothermal combined calcination method. The effects of Gd³⁺ doping with different concentrations on the texture, crystal and optical properties of Bi₂MoO₆ were investigated by N₂ physical adsorption, X-ray diffraction (XRD), scanning electron microscope (SEM), transmission electron microscope (TEM), Fourier transform infrared spectroscopy (FT-IR) and ultraviolet–visible diffuse reflection spectrum (UV–vis DRS), photoluminescence (PL) spectroscopy, and X-ray photoelectron spectroscopy (XPS). Under simulated solar light irradiation, the influences of Gd³⁺doping on photocatalytic activity of Bi₂MoO₆ were evaluated by photocatalytic degradation of Rhodamine B. The characterization results showed that with Gd³⁺ doping, a contraction of lattice and a decrease in crystallite size occurred. Meanwhile, an increase in surface area over Gd³⁺ doped Bi₂MoO₆ was observed. Moreover, Gd³⁺ doping could obviously enhance the visible light harvesting of Bi₂MoO₆ and promoted the separation of photogenerated electrons and holes. With optimum Gd³⁺(6 wt%) doping, Gd/Bi₂MoO₆ exhibited the best activity and stability in degradation of Rhodamine B.     

Vibrational spectra of thin films of lead and bismuth tungstates

The IR reflection spectra are investigated for the systems “thin film of PbWO₄ or Bi₂WO₆-molten quartz v-SiO₂ substrate” within the region of 400–1600 cm⁻¹ at T=295 K. Interference in the thin films is considered. Interpretation of the bands belonging to the PbWO₄ and Bi₂WO₆ films is carried out. I. Franko L’vov State University, 50, Dragomanova St., 290005, L’vov, Ukraine. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 65, No. 3, pp. 459–461, May–June, 1998.     

Influence of electron beam irradiation on structural and optical properties of α-Ag2WO4 nanoparticles

The influence of 8 MeV electron beam irradiation on the structural and optical properties of silver tungstate (α-Ag₂WO₄) nanoparticles synthesized by chemical precipitation method was investigated. The dose dependent effect of electron irradiation was investigated by various characterization techniques such as, X-ray diffraction, scanning electron microscopy, UV–vis absorption spectroscopy, photoluminescence and Raman spectroscopy. Systematic studies confirm that electron beam irradiation induces non-stoichiometry, defects and particle size variation on α-Ag₂WO₄, which in turn results changes in optical band gap, photoluminescence spectra and Raman bands.     

A facile preparation strategy for hollow-structured Bi2O3/Bi2WO6 heterojunction with high visible light photocatalytic activity

A hollow-structured heterojunction consisting of Bi₂WO₆ nanoplatelets and Bi₂O₃ nanoparticles was successfully prepared by a facile solvothermal process. Bi₂O₃/Bi₂WO₆ heterojunction is the aggregate of some hollow spheres with diameter ranging from several hundred nanometers to 1.5 μm and is connected to each other by tube-like cavums. On the basis of scanning and transmission electron microscopy observation and X-ray diffraction analysis of the samples synthesized at different reaction stages, a possible growth mechanism was proposed for the growth of hollow-structured Bi₂O₃/Bi₂WO₆ heterojunction. Its photocatalytic activity was evaluated by degradation of rhodamine B under visible-light irradiation (λ>400 nm). The results indicate that the hollow-structured Bi₂O₃/Bi₂WO₆ heterojunction exhibits much higher photocatalytic activity than both pure Bi₂WO₆ and pure Bi₂O₃. The improved photocatalytic performance can be ascribed to the heterojunction of Bi₂O₃ and Bi₂WO₆ in the framework in which the hierarchical hollow structure possesses good permeability and large surface area. More importantly, the hollow-structured Bi₂O₃/Bi₂WO₆ heterojunction is not only highly stable but also easy to be separated by simple sedimentation for recycle.     

Luminescence centers in ceramics of yttrium, scandium, and bismuth oxytungstates

An investigation is made of the luminescence spectra of Me₂WO₆ ceramics (Me=Y, Sc, Bi). The spectra were decomposed into elementary components by the Alentsev—Fock method. Radiation hands with a maximum at 3.02 eV in the Y₂WO₆ luminescence spectrum, at 2.8 eV in the ScWO₆ spectrum, and at 2.93 eV in the Bi₂WO₆ spectrum are assigned to the light emission of self-localized Frenkel excitons. The bands with maxima at 2.25 and 1.75 eV in the Y₂WO₆ spectrum, at 2.36 and 1.9 eV in the Sc₂WO₆ spectrum, and at 2.35 and 1.9 eV in the Bi₂WO₆ spectrum are related to oxygen vacancies. Translated from Zhurnal Prikladnoi Spektroskopii, Vol 67, No. 2, pp. 273–275, March–April, 2000.     

网状钼酸铋的可见光催化活性

采用无助剂、无模板的水热法成功合成网状Bi₂MoO₆. pH值对这一形貌的形成起重要作用. 所制备的网状Bi₂MoO₆样品表现出优异的可见光催化活性,其光催化活性比固相法合成的块状Bi₂MoO₆样品高得多.     

Study of molybdenum coordination state and crystallization behavior in MoO3-La2O3-B2O3 glasses by Raman spectroscopy

The ternary MoO₃-La₂O₃-B₂O₃ glasses containing a large amount of MoO₃ (10-50 mol%) are prepared, and their structure and crystallization behavior are examined from the Raman scattering spectrum measurements and X-ray diffraction analyses. It is found that the glass transition and crystallization temperatures and the thermal stability against crystallization decrease with increasing MoO₃ content. It is suggested that the main coordination state of Mo⁶⁺ ions in the glasses is isolated (MoO₄)²⁻ tetrahedral units giving strong Raman bands at 830-860 and 930 cm⁻¹. It is found that the crystalline phases in the crystallized glasses are mainly LaMoBO₆ and LaB₃O₆, and the main crystallization mechanism in MoO₃-La₂O₃-B₂O₃ glasses is surface crystallization. LaMoBO₆ crystals are found to give strong Raman bands at 810-830 and ∼910 cm⁻¹.     

Bi2MoO6纳米片/TiO2纳米棒阵列复合材料的制备及其可见光响应光电催化性能研究

Novel Bi₂MoO₆/TiO₂ heterojunction was fabricated by growing Bi₂MoO₆ nanosheets arrays on the vertically aligned TiO₂ nanorods arrays via a two-step solvothermal method. The obtained Bi₂MoO₆/TiO₂ hierarchical heterojunction showed excellent visible light photoelectrochemical performance. Compared with the pure TiO₂ and Bi₂MoO₆, the photocurrent density of the heterojunction was increased 57 and 29 times, respectively. Furthermore, the hydrogen generation rate of the Bi₂MoO₆/TiO₂ for photoelectrocatalytic water-splitting was about 6 times higher than that of the pure Bi₂MoO₆. The improved performance can be attributed to the synergistic effects of enhanced absorption of visible light, increase of migration rate and separation efficiency of photo-induced carriers.     

Fabrication and efficient visible light-induced photocatalytic activity of Bi2MoO6/BiPO4 composite

Novel Bi₂MoO₆/BiPO₄ composites with heterojunction structure were fabricated by a one-step hydrothermal method. The photocatalytic properties of Bi₂MoO₆/BiPO₄ composites were evaluated by photocatalytic degradation of rhodamine B (Rh B) under visible light irradiation (λ>420 nm). The results showed that Bi₂MoO₆/BiPO₄ photocatalysts showed much higher photocatalytic activity for the Rh B degradation than the pure BiPO₄ and Bi₂MoO₆ under visible light. The best photocatalytic performance of Bi₂MoO₆/BiPO₄ with about 98.0% Rh B degradation located at molar ratio of 2:1 under visible light illumination for 30 min. The enhanced photocatalytic activity could be mainly ascribed to the formation of heterojunction interface in Bi₂MoO₆/BiPO₄ composites, which is beneficial to the transfer and separation of photogenerated electron–hole pairs, as well as the strong visible light absorption resulting from the sensitization role of Bi₂MoO₆ to BiPO₄. It was also observed that the photodegradation of Rh B is chiefly attributed to the oxidation action of the generated O₂⁻ radicals and the action of h_vb⁺ through direct hole oxidation process.