Investigation of synthesis and microstructure of bismuth titanates with TiO<Subscript>2</Subscript> rich compositions

Preliminary examinations regarding formation of bismuth titanates in a part of Bi₂O₃—TiO₂ system rich with TiO₂ have been carried out. Bismuth titanates have been synthesized from mixtures of Bi₂O₃ and TiO₂ (anatase) by the conventional solid-state method at the temperatures ranged from 1273 to 1473 K. Differential thermal analysis (DTA), powder X-ray diffraction (XRD) and scanning electron microscopy (SEM) have been used to study the formation of bismuth titanates. The following compounds have been achieved: Bi₄Ti₃O₁₂, Bi₂Ti₂O₇ and Bi₂Ti₄O₁₁. Existence of controversial bismuth titanate of formula Bi₂Ti₃O₉ in the Bi₂O₃—TiO₂ system has not been confirmed.     

Study of Li<Subscript>4</Subscript>Ti<Subscript>5</Subscript>O<Subscript>12</Subscript> interaction with solid lithium-containing electrolytes

Compatibility of the lithium-titanium spinel Li₄Ti₅O₁₂ in contact with precursors of lithium-conducting solid electrolytes of composition Li_1.3Al_0.3Ti_1.7(PO₄)₃ (LATP), Li_1.5Al_0.5Ge_1.5(PO₄)₃ (LAGP), Li_0.5La_0.5TiO₃ (LLT) was studied. It was found that, in sintering of Li₄Ti₅O₁₂ brought in contact with LATP and LAGP, a solid-phase reaction occurs to give nonconducting phases (TiO₂ and Li₃PO₄). The conductivity of the stable composite Li₄Ti₅O₁₂/LLT (10%) is higher than that of the starting Li₄Ti₅O₁₂, which makes it possible to regard the composite as a promising anode material for lithium-ion batteries.     

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.     

Electrical conductivity studies in the system Li<Subscript>2</Subscript>TiO<Subscript>3</Subscript>-Li<Subscript>1.33</Subscript>Ti<Subscript>1.67</Subscript>O<Subscript>4</Subscript>

Electrical conductivity in the monoclinic Li₂TiO₃, cubic Li_1.33Ti_1.67O₄, and in their mixture has been studied by impedance spectroscopy in the temperature range 20–730 °C. Li₂TiO₃ shows low lithium ion conductivity, σ₃₀₀≈10^–6 S/cm at 300 °C, whereas Li_1.33Ti_1.67O₄ has 3×10^–8 at 20 °C and 3×10^–4 S/cm at 300 °C. Structural properties are used to discuss the observed conductivity features. The conductivity dependences on temperature in the coordinates of 1000/T versus log_e(σT) are not linear, as the conductivity mechanism changes. Extrinsic and intrinsic conductivity regions are observed. The change in the conductivity mechanism in Li₂TiO₃ at around 500–600 °C is observed and considered as an effect of the first-order phase transition, not reported before. Formation of solid solutions of Li_{2– x} Ti_{1+ x} O₃ above 900 °C significantly increases the conductivity. Irradiation by high-energy (5 MeV) electrons causes defects and the conductivity in Li₂TiO₃ increases exponentially. A dose of 144 MGy yields an increase in conductivity of about 100 times at room temperature. Electronic Publication     

Formation and thermal properties of nanocrystalline Bi<Subscript>4</Subscript>Ti<Subscript>3</Subscript>O<Subscript>12</Subscript>

Mechanism by which nanocrystalline Bi₄Ti₃O₁₂ is formed in thermal treatment of coprecipitated hydroxides was studied. It was shown that the onset of the active formation is correlated with the melting point of the surface phase based on bismuth oxide. The technological synthesis parameters of Bi₄Ti₃O₁₂, at which crystallite sizes in the range 35–60 nm are provided, were determined.     

Electrospun Li<Subscript>4</Subscript>Ti<Subscript>5</Subscript>O<Subscript>12</Subscript>/Li<Subscript>2</Subscript>TiO<Subscript>3</Subscript> composite nanofibers for enhanced high-rate lithium ion batteries

Li₄Ti₅O₁₂/Li₂TiO₃ composite nanofibers with the mean diameter of ca. 60 nm have been synthesized via facile electrospinning. When the molar ratio of Li to Ti is 4.8:5, the Li₄Ti₅O₁₂/Li₂TiO₃ composite nanofibers exhibit initial discharge capacity of 216.07 mAh g^?1 at 0.1 C, rate capability of 151 mAh g^?1 after being cycled at 20 C, and cycling stability of 122.93 mAh g^?1 after 1000 cycles at 20 C. Compared with pure Li₄Ti₅O₁₂ nanofibers and Li₂TiO₃ nanofibers, Li₄Ti₅O₁₂/Li₂TiO₃ composite nanofibers show better performance when used as anode materials for lithium ion batteries. The enhanced electrochemical performances are explained by the incorporation of appropriate Li₂TiO₃ which could strengthen the structure stability of the hosted materials and has fast Li⁺-conductor characteristics, and the nanostructure of nanofibers which could offer high specific area between the active materials and electrolyte and shorten diffusion paths for ionic transport and electronic conduction. Our new findings provide an effective synthetic way to produce high-performance Li₄Ti₅O₁₂ anodes for lithium rechargeable batteries.     

Synthesis and thermal decomposition of neodymium(III) peroxotitanate to Nd<Subscript>2</Subscript>TiO<Subscript>5</Subscript>

Neodymium(III) peroxotitanate is used as a precursor for obtaining Nd₂TiO₅. The last one possesses numerous valuable electrophysical properties. TiCl₄, Nd(NO₃)₃·6H₂O and H₂O₂ in mol ratio 1:2:10 were used as starting materials. The reaction ambience was alkalized to pH = 9 with a solution of NH₃. The obtained neodymium(III) peroxotitanate and intermediate compounds of the isothermal heating were proved by the help of quantitative analysis and infrared spectroscopy (IRS). It has Nd₄[Ti₂(O₂)₄(OH)₁₂]·7H₂O composition. The absorption band observed in IRS at 831 cm^?1 relates to a triangular bonding of the peroxo group of Ti, at 1062 cm^?1—terminal groups Ti–OH and at 1491 and 1384 cm^?1—the bridging OH^?-groups Ti–O(H)–Ti. Nd₂TiO₅ was obtained by thermal decomposition of neodymium(III) peroxotitanate. The isothermal conditions for decomposition were determined on the base of differential thermal analysis, thermogravimetric and differential scanning calorimetry results in the temperature range of 20–1000 °C. The mechanism of thermal decomposition of Nd₄[Ti₂(O₂)₄(OH)₁₂]·7H₂O to Nd₂TiO₅ was studied. In the temperature range of 20–208 °C, a simultaneous decomposition of the peroxo groups by the separation of oxygen and hydrate water is conducted and Nd₄[Ti₂O₄(OH)₁₂] is obtained. From 208 to 390 °C, the terminal OH^?-groups are separated and Nd₄[Ti₂O₇(OH)₆] is formed. In the range of 390–824 °C, the bridging OH^?-groups are completely decomposed to Nd₂TiO₅. The optimal conditions for obtaining nanocrystalline Nd₂TiO₅ are 900 °C for 6 h and 20–80 nm.     

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.     

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.     

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.     

Possible ways to obtain materials based on bismuth titanate Bi<Subscript>4</Subscript>Ti<Subscript>3</Subscript>O<Subscript>12</Subscript>

Abietate- and cellulose-based procedures were developed for synthesis of Bi₄Ti₃O₁₂, a promising ferroelectric material.     

Thermal stability of layered perovskite-like oxides NaNdTiO<Subscript>4</Subscript> and Na<Subscript>2</Subscript>Nd<Subscript>2</Subscript>Ti<Subscript>3</Subscript>O<Subscript>10</Subscript>

Thermal stabilities of layered perovskite-like oxides NaNdTiO₄ and Na₂Nd₂Ti₃O₁₀ were studied in the temperature ranges from 780 to 1100°C and from 1100 to 1400°C, respectively. Chemical mechanism of their thermal decomposition was proposed. Higher thermal stability of Na₂Nd₂Ti₃O₁₀ was rationalized on the basis of crystallochemical data.     

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.     

Thermal behaviour and properties of Na<Subscript>2</Subscript>O-TiO<Subscript>2</Subscript>-P<Subscript>2</Subscript>O<Subscript>5</Subscript> glasses

Differential scanning calorimetry (DSC) and thermomechanical analysis (TMA) were used to study the thermal behaviour of (50-x)Na₂O-xTiO₂-50P₂O₅ and 45Na₂O-yTiO₂-(55-y)P₂O₅ glasses. The addition of TiO₂ to the starting glasses (x=0 and y=5 mol% TiO₂) resulted in a nonlinear increase of glass transition temperature and dilatation softening temperature, whereas the thermal expansion coefficient decreased. All prepared glasses crystallize under heating within the temperature range of 300–610°C. The contribution of the surface crystallization mechanism over the internal one increases with increasing TiO₂ content. With increasing TiO₂ content the temperature of maximum nucleation rate is also gradually shifted from a value close to the glass transition temperature towards the crystallization temperature. X-ray diffraction measurements showed that the major compounds formed by glass crystallization were NaPO₃, TiP₂O₇ and NaTi₂(PO₄)₃. The chemical durability of the glasses without titanium oxide is very poor, but with the replacement of Na₂O or P₂O₅ by TiO₂, it increases sharply.     

Synthesis and structure of phosphates M<Subscript>0.5</Subscript>Ti<Subscript>2</Subscript>(PO<Subscript>4</Subscript>)<Subscript>3</Subscript>

Phosphates M_0.5Ti₂(PO₄)₃ (M = Ni, Zn) were synthesized by the sol-gel method and characterized by the methods of X-ray diffraction, IR spectroscopy, and electronic microprobe analysis. Structures of Ni_0.5Ti₂(PO₄)₃ and Zn_0.5Ti₂(PO₄)₃ were studied by Rietveld method using the X-ray powder diffraction data.     

Interactions in the Al<Subscript>2</Subscript>TiO<Subscript>5</Subscript>-Ti<Subscript>2</Subscript>O<Subscript>3</Subscript> System

Interactions in the Al₂TiO₅-Ti₂O₃ system were studied and the regions of existence of Al_2?2xTi _2x ³⁺ Ti⁴⁺O₅ solid solutions with a pseudobrookite structure were determined.     

CaCu<Subscript>3</Subscript>Ti<Subscript>4</Subscript>O<Subscript>12</Subscript>-Based materials with variable copper content

The CaO-3 (1 + x)CuO-4TiO₂ system was studied using powder X-ray diffraction in the concentration region near calcium copper titanate. A single-phase material is formed in this system only when x ～ 0. An excess or deficit of copper gives rise to extra phases: CuO or CaTiO₃ and TiO₂, respectively. Impurities increase the dielectric constant of CaCu₃Ti₄O₁₂-based ceramics. An excess of copper oxide (x ～ 0.08) increases ? more than tenfold.     

Effect of the phase composition of the starting mixture on the formation of the layered perovskite-like compound Bi<Subscript>7</Subscript>Fe<Subscript>3</Subscript>Ti<Subscript>3</Subscript>O<Subscript>21</Subscript>

The effect of the phase composition of the starting mixture on the formation of the layered perovskite-like compound Bi₇Fe₃Ti₃O₂₁ in the Bi₂O₃-TiO₂-Fe₂O₃ system is reported. The rate-limiting process in the formation of this compound is reactant mass transfer to the reaction zone.     

Application of thermal analysis to study of the synthesis of K0.5Bi0.5Tio3 ferroelectric

Synthesis of potassium bismuth titanate ferroelectric by heating of mixtures prepared using oxide precursors, i.e. Bi₂O₃, TiO₂ and K₂CO₃ was investigated. DTA, TG, XRD and SEM methods were used to study the formation of intermediate compounds and the final product. Usage of associated homogenization and grinding of precursors mixture permits to decrease the temperature of formation of K_0.5Bi_0.5TiO₃. This revised version was published online in July 2006 with corrections to the Cover Date.     

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.