Bi2SiO5 nanosheets prepared by molten salt method

Bi₂SiO₅ nanosheets were successfully synthesized by the molten salt method in NaCl–Na₂SO₄ flux using Bi₂O₃ and SiO₂ as raw materials. XRD analysis and SEM observation show that the incorporation of salt medium can greatly lower the formation temperature of Bi₂SiO₅ phase, and promote their crystallization. The results also demonstrate that the mass ratio of salts to reactants, the reaction time, and the calcination temperature have significant effects on the phase formation and morphology of the Bi₂SiO₅ powders. The UV–vis spectra result shows that the absorption of the Bi₂SiO₅ powders located at 380 nm, and the corresponding band gap energy (Eg) calculated is 3.26 eV.     

Preparation of needle-like TiZrO4 and PZT powders

Needle-like TiZrO₄ powder was prepared by reacting TiO₂ and ZrO₂ in the presence of molten chloride (NaCI or KCI) or sulphate (Na₂SO₄ or Li₂SO₄). Either a single-phase TiZrO₄ or a three-phase mixture (TiZrO₄, ZrO₂ and amorphous TiO₂) was obtained from the chloride or sulphate system. Particle morphology was dependent on heating temperature and duration as well as salt species. Needle-like PZT powder was obtained by reacting needle-like TiZrO₄ powder with PbO above 750 °C. The PZT powder obtained at 750°C was composed of both tetragonal and rhombohedral phases, indicating a wide compositional variation. An increase in heating temperature reduced the degree of compositional variation, but enhanced the deformation of needle-like particles.     

Hydrothermal synthesis of bismuth sodium titanate particles with different morphologies

Nanostructured Na_0.5Bi_0.5TiO₃ particles have been synthesized by a hydrothermal synthesis method using a layered titanate H_1.07Ti_1.73O₄·nH₂O as a Ti precursor. The obtained Na_0.5Bi_0.5TiO₃ particles showed different morphologies including plate-like, wire-like, and cubic-like structures in different hydrothermal conditions. The effect of the NaOH concentration on the growth and morphology evolution of hydrothermally derived Na_0.5Bi_0.5TiO₃ powders were investigated. It was found that alkaline concentration had a great effect on the phase and morphology of the resultant powders. The dissolution–recrystallization and in situ topotactic transformation mechanisms were suggested in different alkaline concentrations according to the evolution process.     

Homogeneous (co)precipitation of inorganic salts for synthesis of monodispersed barium titanate particles

Various processes of coprecipitation or crystallization of inorganicsalts of barium and titanium from homogeneous solutions were studiedin this work. In particular, barium hydroxide and barium chloridesalt as well as titanium tetrachloride were used as the startingmaterials for dielectric-tuning homogeneous precipitation in mixedsolvents of isopropanol and water. Hydroxypropylcellulose was used asa steric dispersant. Evaluations of size, shape, and composition ofsynthesized particles were made using scanning electron microscopy,high-temperature X-ray diffractometry, and differential thermalanalysis. Results show that salt concentration, pH, and reaction timeare important in determining the morphology and composition of thefinal powder. The titania particles from dielectric-tuningprecipitation are perfect microspheres with narrow size distribution(near monodispersed), while the particles from barium salts areflake-like, irregular in shape and size. Instead of particlescontaining uniform compositions of barium and titanium compounds,dielectric-tuning coprecipitation yielded powders of two separatedphases, i.e., monodispersed titania microspheres (1 m) coated onbarium chloride salt flakes. Titanium-rich barium titanate wasobtained after calcination of coprecipitated powders. However,preliminary results show that the titania particles obtained bydielectric-tuning precipitation can be hydrothermally converted toBaTiO₃ particles that are fully crystallized after calcination above950°C.     

Comparison Between γ-Bi2MoO6 and Bi2WO6 Catalysts in the CO Oxidation

Bismuth molybdate (-Bi₂MoO₆) and bismuth tungstate (Bi₂WO₆) catalysts were prepared by solid-state reaction and their catalytic properties evaluated in the CO oxidation reaction. We characterize their structure by X-ray diffraction, scanning electron microscopy, Auger electron spectroscopy, and BET nitrogen absorption. X-ray diffraction analysis shows that both -Bi₂MoO₆ and Bi₂WO₆ are structural analogs (SG P2₁ ab). Auger analysis shows that Bi₂WO₆ catalysts have more bismuth on the surface than -Bi₂MoO₆, although both samples are bismuth deficient as compared to the stoichiometric compound. The results regarding catalytic activity show that Bi₂WO₆ prepared at 1073 K reaches total conversion of CO (100%) at a lower temperature when compared to -Bi₂MoO₆. It indicates that Bi₂WO₆ is a potential candidate to be used as catalyst in the CO to CO₂ oxidation.     

Synthesis and characterization of novel nanorod superstructures and twin octahedral morphologies of WO3 by hydrothermal treatment

Controlled WO₃ morphologies, such as nanorods and octahedral structures, were synthesized by the hydrothermal technique using sulfate salts based structure directing agents (SDAs). The role of the sulfate salts’ cation in controlling the shape, size and phase of WO₃ nanomaterials was investigated by choosing sulfate salts whose cations are from d-bloc elements (FeSO₄, (NH₄)₂Fe(SO₄)₂, CoSO₄, CuSO₄, ZnSO₄), an alkaline earth metal (MgSO₄) and a non-metal ((NH₄)₂SO₄). In addition chloride (MnCl₂) and acetate (Zn(CH₃CO₂)₂) anion based SDAs were also used in order to clarify the role of sulfate ions in the growth of WO₃ nanostructures. We controlled the pH of the reaction medium with oxalic acid. The obtained WO₃ samples were investigated by SEM, micro-Raman, and XRD. At pH = 1, the WO₃ samples exhibit novel superstructures consisting of aligned hexagonal nanorods, whereas at pH = 5.25, novel twin octahedral morphology with a cubic structure is obtained. The results demonstrate that the phase and morphology change is influenced by the pH and both the anion and the cation of the SDA. A growth mechanism for the obtained novel WO₃ morphologies is presented.     

The enhanced visible light photocatalytic activity of nanosheet-like Bi2WO6 obtained by acid treatment for the degradation of rhodamine B

Bi₂WO₆ samples were fabricated by chemical solution decomposition (CSD) method and nanosheet-like Bi₂WO₆ samples could be obtained by concentrated nitric acid treatment at 70 °C for 20 min. The products were characterized by X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), and UV-vis diffuse reflectance spectra. Photocatalytic activities of the samples were evaluated by the degradation of rhodamine B (RhB) under visible light irradiation. The temperature of acid treatment obviously influenced morphology and the visible light photocatalytic activity of the Bi₂WO₆ samples. The nanosheet-like Bi₂WO₆ photocatalysts obtained by acid treatment exhibited the highest photocatalytic activity under visible light irradiation.     

Thermoelectric properties of the hot-pressed (Bi0.2Sb0.8)2Te3 alloy with addition of BN and WO3 powders

Thermoelectric properties of the (Bi_0.2Sb_0.8)₂Te₃ alloy, fabricated by mechanical alloying and hot-pressing, were investigated with addition of BN and WO₃ powders as phonon scattering centers. The Seebeck coefficient and the electrical resistivity of the hot-pressed (Bi_0.2Sb_0.8)₂Te₃ alloy increased with increasing the volume fraction of BN and WO₃. Although the thermal conductivity of the hot-pressed (Bi_0.2Sb_0.8)₂Te₃ alloy decreased with increasing the volume fraction of BN and WO₃ due to the reduction of _el, the lattice thermal conductivity was not lowered. The figure-of-merit of the hot-pressed (Bi_0.2Sb_0.8)₂Te₃ alloy, 3.05 × 10^–3/K without addition of BN and WO₃, decreased with increasing the volume fraction of BN and WO₃, because the increment of the electrical resistivity was much larger than the decrement of the thermal conductivity due to the grain refinement.     

Large-scale hydrothermal synthesis of WO3 nanowires in the presence of K2SO4

WO₃ nanowires were fabricated by a hydrothermal method in the presence of K₂SO₄. The nanowires exhibit a well crystallized one-dimensional structure with 10 nm in diameter and several microns in length. Effects of other alkali salts (KNO₃, NaNO₃ and Na₂SO₄) on the morphologies of WO₃ nanocrystals were also investigated. The important role of K₂SO₄ salt in the WO₃ nanowires synthesis has been demonstrated.     

Molten salt synthesis and luminescence properties of Bi4Si3O12 powders

In this paper, Bismuth silicate (Bi₄Si₃O₁₂, BSO) powders were prepared by reaction in molten salts (NaCl and KCl) using Bi₂O₃ and SiO₂ powders as raw materials. The effects of salt contents and excessive contents of Bi₂O₃ and SiO₂ were characterized by X-ray diffraction and scanning electron microscopy. The properties of Bi₄Si₃O₁₂ powders were analyzed via fluorescence spectroscopy. Forming mechanism of platelets was discussed. The experimental results suggest that Bi₄Si₃O₁₂ powders are produced at 780 °C for 4 h with 40 wt% salts and 5 wt% excessive content of Bi₂O₃. The peaks of excitation spectrum and emission spectrum for BSO powders are separately located at 266 and 457.6 nm. Compared with crystal materials, the excitation spectrum and emission spectrum of Bi₄Si₃O₁₂ powders indicate blue shift. Interface reacting mechanism can be used to explain the coarsening process of bismuth silicate platelets produced in molten salt synthesis. In addition, dissolution–precipitation mechanism also plays an important role in the synthesis of bismuth silicate platelets.     

Controlled synthesis of three-dimensional hierarchical Bi2WO6 microspheres with optimum photocatalytic activity

Three-dimensional (3D) hierarchical Bi₂WO₆ microsphere and octahedral Bi₂WO₆ have been synthesized by a facile hydrothermal method using KNO₃ solution and distilled water as solvent, respectively. The obtained products were characterized by X-ray diffraction, scanning electron microscopy, N₂ adsorption/desorption, and UV–vis diffuse reflectance spectroscopy in detail. The concentration of KNO₃ played a key role in the formation of 3D hierarchical Bi₂WO₆ microspheres. A possible formation mechanism of Bi₂WO₆ microsphere was proposed. The photocatalytic activity of the as-synthesized products was evaluated by monitoring the degradation of MB solution under sunlight irradiation. It was found that the photocatalytic activity of the 3D hierarchical Bi₂WO₆ microsphere was superior to the octahedral Bi₂WO₆, which was attributed to the larger surface area and special hierarchical structure of Bi₂WO₆ microsphere.     

Preparation and characteristics of Ni-ferrite powders obtained in the presence of fused salts

This paper describes Ni-ferrite formation in the presence of Li₂SO₄-Na₂SO₄ molten salts, and in particular the effects of the raw materials, amount of salts, heating temperature and time on the size and shape of the ferrite powders, as well as on the rate of ferrite formation. The molten salts accelerate ferrite formation and complete ferrite formation is attained at lower temperatures than in solid state reactions. Ferrite powders with two types of shapes are obtained from NiO with different particle sizes and aggregation states; one is similar to the starting Fe₂O₃ particles and the other has an octahedral crystal habit. The difference follows from the different dissolution rates of Fe₂O₃ and NiO. Growing particles in molten salts have a crystal habit, but the most stable particle shape is rounded.     

Stereochemical influence of the Bi3+ lone pair of electrons on polymorphism in Bi2WO6

This paper discusses how the Bi³⁺ lone pair is exerting the stereo-chemical effects on the structures of three Bi₂WO₆ polymorphs, i.e. the low-temperature form (Bi₂WO₆(1), orthorhombic), the intermediate form (Bi₂WO₆(i), orthorhombic), and the high-temperature modification (Bi₂WO₆(h), monoclinic); all their structures consist of alternating Bi₂O₂ layers and WO₄ layers. The monoclinic deformation of Bi₂WO₆(h) was explained by assuming appreciable repulsion between two facing lone pairs, each of which belongs to neighboring Bi₂O₂ layers, in a hypothetical transient state with the La₂MoO₆-related structure under the Bi₂WO₆(i) → Bi₂WO₆(h) transition. The polymorphism of Bi₂MoO₆, which has a Bi₂WO₆(1)-related structure in a koechlinite phase, is also discussed on the basis of the stereochemistry of the Bi³⁺ lone pair.     

Anisotropic grain growth of Bi4Ti3O12 in molten salt fluxes

Bismuth titanate (Bi₄Ti₃O₁₂) platelets were obtained by the molten salt synthesis method in NaCl-KCl and Na₂SO₄-K₂SO₄ fluxes, using an amorphous Bi₄Ti₃O₁₂ precursor and a mechanically mixed Bi₂O₃-TiO₂ mixture as the starting materials. It was found that the synthesizing temperature, salt species and starting materials could significantly affect the crystallization habit and morphology of Bi₄Ti₃O₁₂ platelets. Under the same processing conditions (i.e. in Na₂SO₄-K₂SO₄ flux at 1000 °C), the Bi₄Ti₃O₁₂ platelets synthesized from the Bi₂O₃-TiO₂ mixture generally showed a smaller particle size than those synthesized using the amorphous Bi₄Ti₃O₁₂ precursor. The Bi₄Ti₃O₁₂ platelets prepared in the chloride flux were faceted along either (0 0 1) or (1 1 2) planes, while those prepared in the sulfate flux were solely (0 0 1) faceted. The former system also had smaller particle size than the latter. Furthermore, the salt content and the addition of plate-like Bi₄Ti₃O₁₂ seeds in the amorphous Bi₄Ti₃O₁₂ precursor showed a strong influence on the particle size of the synthesized Bi₄Ti₃O₁₂ platelets. The particle size of Bi₄Ti₃O₁₂ platelets prepared in the sulfate flux decreased as the mole ratio of the sulfate salts to Bi₄Ti₃O₁₂ was increased from 7.9 to 23.7. And the addition of 10 wt.% plate-like Bi₄Ti₃O₁₂ seeds into the amorphous Bi₄Ti₃O₁₂ precursor led to a significant increase in the particle size of the resulting Bi₄Ti₃O₁₂ platelets.     

Preparation and properties of uniform mixed and coated colloidal particles

Colloidal dispersions of uniform spherical particles of zirconium basic sulphate and zirconium oxy-basic carbonate were prepared by ageing zirconium sulphate solutions at elevated temperatures in the presence of urea. Different chemical compositions of the above products resulted when the ageing temperature was altered. Depending on the nature of the original solids, calcination at 800° C resulted in the formation of tetragonal or monoclinic zirconia. Under certain conditions a mixed phase, including cubic zirconia, has also been identified. The particle morphology was retained during these transformations. Coprecipitation in mixed solutions of zirconium and yttrium salts aged at 80° C yielded composite spherical particles of basic carbonate with a zirconium to yttrium ratio of the solid similar to that used in the initial solution. Zirconium basic sulphate particles coated with yttrium basic carbonate were prepared by ageing, in the presence of urea, a Zr₂(OH)₆SO₄ dispersion containing yttrium nitrate.     

Effect of melt-spun powder additions on the thermoelectric properties of bismuth and antimony chalcogenides

We have studied the thermoelectric properties of p-type Bi_0.5Sb_1.5Te₃ and n-type Bi₂Te_2.4Se_0.6 solid solutions prepared by vacuum hot pressing of mixtures of powders differing in particle size composition. The powders were prepared by mechanically grinding ingots and by ultrarapid melt cooling (melt spinning). Fracture surfaces of samples were examined by scanning electron microscopy. The samples consisted of large, layered particles of the major component (up to hundreds of microns in size) and small flakes (a few to tens of microns in size) of the component prepared by melt spinning. The microstructure of the materials was examined under an optical microscope. On grain boundaries in the p-type materials, we observed telluriumbased eutectic precipitates in the form of a white phase. In some of the n-type samples, the Bi₂Te_2.4Se_0.6 solid solution was found to undergo ordering, resulting in the formation of the ternary compound Bi₂Te₂Se. The Seebeck coefficient, electrical conductivity, and thermal conductivity of the materials were measured in the temperature range 100–700 K. The addition of 40 wt % powder prepared by melt spinning to hot-pressed p-type samples was shown to have no effect on their thermoelectric figure of merit (ZT)_max, which was 1.0 at 350 K. On the addition of 20 wt % powder prepared by melt spinning, we obtained (ZT)_max = 1.1 at 550 K in a hot-pressed n-type sample.     

Molten salt synthesis of anisotropic BaFe12O19 powders

The formation mechanism, morphology, and magnetic properties of anisotropic BaFe₁₂O₁₉ powders prepared by the 0.8 Na₂SO₄-0.2 K₂SO₄ molten salt synthesis have been investigated. The reaction can be completed at temperatures lower than 845 °C which is the melting point of the 0.8 Na₂SO₄-0.2 K₂SO₄ salt mixture, due to the presence of the low melting phase related to the formation of BaSO₄ and Na₆CO₃(SO₄)₂. Even at temperatures lower than 845 °C, with the aid of the low melting phase, the morphology of ferrites was thin platelets. Magnetic properties such as coercivity, saturation magnetization, and remanent force are comparable to those found in other processing techniques. The orientation degree of the sintered body was very high due to anisotropic morphology; thus powders from molten salt synthesis are suitable for grain oriented ceramics.     

Facile fabrication of flower-shaped Bi2WO6 superstructures and visible-light-driven photocatalytic performance

Flower-shaped porous Bi₂WO₆ superstructures assembled from squared nanoplates have been successfully fabricated by a facile hydrothermal method without any surfactants or additives. Parameter-dependent experiments show that the initial strong acidic condition of the hydrothermal precursor is essential for the construction of tiny nanoplates into hierarchical Bi₂WO₆ flowers. The sample exhibits improved visible-light-driven photocatalytic activity in the degradation of Rhodamine-B, compared with Bi₂WO₆ multilayered disks, isolated platelets prepared at elevated pH values and bulk Bi₂WO₆ obtained by solid state method, respectively. It is believed that the higher BET surface area and nano/mesopore structure can afford more reactive sites and abundant transport paths for the degradation reaction. Moreover, the sample shows excellent circulation stability, suggesting its potential application in dealing with enviromental pollutions.     

Effect of Y(NO3)3 additive on morphologies and size of metallic W particles produced by hydrogen reduction

Tungsten, one of the refractory metals, has lots of excellent properties, making tungsten powders become a choice of raw material for many applications. This work reported a simple method (the salt-assisted hydrogen reduction of WO₃ powders) for preparing tungsten powders with desired morphologies and size. By adjusting the amount of additive (Y(NO₃)₃) and reduction temperature, metallic tungsten particles with different morphologies and size were obtained. It was found that both temperature and additive had apparent effects on the morphology and size of metallic W particles, and the large parallelepiped-shaped powders disappeared and large irregular blocky-shaped powders were degraded under the influence of additive. With the increase of temperature and amount of salt, particles became more dispersed and the size increased at higher temperatures, and the shape of particles was changed from spherical to polyhedral as well. On the contrary, the size of sub-particles became smaller as increasing amount of Y(NO₃)₃ at lower temperatures. Based on the results, the reaction and refining mechanisms for preparing tungsten were proposed as well.