Synthesis and characterization of crystalline β-Bi2O3 nanobelts

We synthesized β-Bi₂O₃ nanobelts on silicon substrates without using a metal catalyst. Trimethylbismuth and O₂ were taken as the source of bismuth and oxygen, respectively. X-ray diffraction and transmission electron microscopy studies confirmed the formation of tetragonal Bi₂O₃ phase. The typical width of the β-Bi₂O₃ nanobelts was in the range of 40-400 nm. We suggested that the growth of β-Bi₂O₃ nanobelts was mainly controlled by a vapor-solid mechanism. Photoluminescence measurements at room temperature exhibited a visible light emission band peaking at around 2.81 eV.     

β- and α-Bi2O3 nanoparticles synthesized via microwave-assisted method and their photocatalytic activity towards the degradation of rhodamine B

Amorphous β-Bi₂O₃ nanoparticles were synthesized directly via a liquid phase microwave reaction, and changed gradually into well crystallized sheet-like nanoparticles of β-Bi₂O₃ or α-Bi₂O₃ during the following calcining at lower (300 °C) or higher (350 °C) temperature. Powder X-ray diffraction (XRD), transmission electron microscopy (TEM) and diffuse reflectance spectroscopy (DRS) were used to characterize the samples. The photocatalytic activity of the samples under simulated sunlight was also investigated by taking the degradation of rhodamine B (RB) as model reaction. β-Bi₂O₃ showed lower band gap energy and high absorbance in wider visible light region than α-Bi₂O₃ did, resulting in its higher photocatalytic activity. It was also found that higher crystallinity can improve the activity.     

Synthesis and photoelectrochemical properties of thin bismuth molybdates film with various crystal phases

Bismuth molybdate films with various phase structures including α-Bi₂Mo₃O₁₂, β-Bi₂Mo₂O₉, γ-Bi₂MoO₆, and γ′-Bi₂MoO₆ are fabricated on the indium-tin oxide glass substrates from an amorphous heteronuclear complex via the dip-coating method by appropriate adjustment of the reaction conditions. α-Bi₂Mo₃O₁₂, β-Bi₂Mo₂O₉, and γ-Bi₂MoO₆ film can be obtained at 400 °C, 500 °C, and 500 °C for 1 h, respectively. At 500 °C, γ′-Bi₂MoO₆ can be obtained for 4 h. Film formation process is proposed based on the experimental results. Thin γ-Bi₂MoO₆ films exhibit high photoresponse under visible light irradiation. Incident photon to current conversion efficiency of thin γ-Bi₂MoO₆ film starts to increase near 450 nm. And, it can reach 4.1% at 400 nm. The top of the valence band and bottom of the conduction band are roughly estimated to be − 0.71 and 1.69 eV, respectively. In contrast, γ′-Bi₂MoO₆ generated weak photocurrent; α-Bi₂Mo₃O₁₂ and β-Bi₂Mo₂O₉ film has no photoresponse under visible light irradiation. The reason for the difference in the visible light response was discussed.     

The effect of temperature on the crystallization of α-Fe2O3 particles from dense β-FeOOH suspensions

The effect of temperature on the crystallization of α-Fe₂O₃ particles from dense β-FeOOH suspensions was monitored by ⁵⁷Fe Mössbauer spectroscopy, X-ray powder diffraction, Fourier transform infrared spectroscopy, field emission scanning electron microscopy and energy dispersive spectroscopy. Dense suspensions of very long laterally arranged β-FeOOH fibrils were obtained at 90 °C. Crystallization at 120 °C between 18 and 72 h yielded monodisperse α-Fe₂O₃ particles of a shape close to that of double spheres with ring. The double spheres with ring showed two narrow particle size distributions. In these particles a substructure was detected, i.e., the spheres consisted of the linear chains of interconnected α-Fe₂O₃ subparticles. With further rise in the crystallization temperature the increase in α-Fe₂O₃ particles and porosity became pronounced. Obviously, the aggregation mechanism played an important role in the formation of α-Fe₂O₃ particles.     

Synthesis of nanosized β-BiTaO4 by the polymeric precursor method

β-BiTaO₄ powder was synthesized by the citrate method, using bismuth citrate and TaCl₅ as precursors. The citrate gel was characterized by thermal analyses (TG and DTA), in order to determine the best polymerization temperature. The polymeric precursor is essentially amorphous and after calcination at 400 °C a mixture of tantalates, that are isomorphic to Bi₃NbO₇ and Bi₅Nb₃O₁₅, starts to crystallize. At 600 °C, in addition to those phases, one could observe some peaks related to β-BiTaO₄. Finally, at 800 °C β-BiTaO₄ can be observed as a pure phase, with particle size estimated as 47 nm. The precursor polymeric method allowed obtaining β-BiTaO₄ pure phase at temperatures significantly lower than those found for solid state reaction method.     

Room-temperature synthesis of nanometric α-Bi2O3

Nanometric Bi₂O₃ powder was successfully synthesized by applying the method based on self-propagating room temperature reaction (SPRT) between bismuth nitrates and sodium hydroxide. X-ray powder diffraction (XRPD) and Rietveld's structure refinement method were applied to characterize prepared powder. It revealed that synthesized material is a single phase monoclinic α-Bi₂O₃ (space group P2₁/c with cell parameters a = 5.84605(4)Å, b = 8.16339(6) Å, c = 7.50788(6) Å and β = 112.9883(8)). Powder particles were of nanometric size (about 50 nm). Raman spectral studies conformed that the obtained powder is single phase α-Bi₂O₃. Specific surface area of obtained powder was measured by Brunauer-Emmet-Teller (BET) method.     

New quenched-in fluorite-type materials in the Bi2O3-La2O3-PbO system: Synthesis and complex phase behaviour up to 750 °C

New quenched-in fluorite-type materials with composition (BiO_1.5)_0.94−x(LaO_1.5)_0.06(PbO)_x, x = 0.02, 0.03, 0.04 and 0.05, were synthesised by solid state reaction. The new materials undergo a number of phase transformations during heating between room temperature and 750 °C, as indicated by differential thermal analysis. Variable temperature X-ray diffraction performed on the material (BiO_1.5)_0.92(LaO_1.5)_0.06(PbO)_0.02 revealed that the quenched-in fcc fluorite-type material first undergoes a transformation to a β-Bi₂O₃-type tetragonal phase around 400 °C. In the range 450-700 °C, α-Bi₂O₃-type monoclinic, Bi₁₂PbO₁₉-type bcc and β₁/β₂-type rhombohedral phases, and what appeared to be a ?-type monoclinic phase, were observed, before a single-phase fluorite-type material was regained at 750 °C.     

Optical and magnetic properties of elliptical hematite (α-Fe2O3) nanoparticles coated with uniform continuous layers of silica of different thickness

Elliptical-type α-Fe₂O₃ nanoparticles with/without silica shell have been prepared. The core particles were coated with uniform continuous layers of silica of two different thicknesses by hydrolysis of TEOS. The obtained HCP structure elliptical α-Fe₂O₃ nanoparticles with ∼ 240 nm length and 100 nm width is polycrystalline in nature. The thicknesses of SiO₂ shell coated on α-Fe₂O₃ are about 55 and 30 nm, respectively. The optical and magnetic properties of these nanoparticles have been investigated.     

Properties of screen-printed Ho-Ba-Cu-O films on YSZ substrates

High-T_c screen-printed Ho-Ba-Cu-O films were prepared on YSZ substrates by a melt processing method. The films were fired at T_s = 1000-1050 °C for 5 min and cooled to 450 °C by two steps in flowing O₂. The maximum critical current density J_c (77 K, 0 T) of 2.0 × 10³ A cm^− 2 was only attained under much limited firing conditions; T_s = 1020 °C and cooled to 800 °C at a cooling rate of 400 °C h^− 1.     

Synthesis and characterization of β type solid solution in the binary system of Bi2O3-Eu2O3

We have investigated Bi₂O₃-Eu₂O₃ binary system by doping with Eu₂O₃ in the composition range from 1 to 10 mole% via solid state reactions and succeeded to stabilize β-Bi₂O₃ phase which is metastable when pure. Stability of β-Bi₂O₃ polymorph was influenced by heat treatment temperature. Tetragonal type solid solution was obtained in 3–6 mole% addition range when annealed at 750°C and the range was 2–7 mole% when annealed at 800°C. We have also carried out investigations on lattice parameters, microstructural properties and elemental compositions of this β type solid solution for each doping ratio. Lattice parameters increased with amount of Eu₂O₃ addition. Our experimental observations strongly suggested that oxygen deficiency type non-stoichiometry is present in doped β type solid solutions.     

Orientation-controlled phase transformation of Bi2O3 during oxidation of electrodeposited Bi film

High-temperature fluorite structure Bi₂O₃ is a well-known solid electrolyte owing to its high oxygen ion conductivity. In this study, Bi₂O₃ thin film was prepared by the oxidation process of the electrodeposited metallic Bi film. The crystal structures of the oxidized Bi films varied with the applied voltages during the electroplating process. Pure α-Bi₂O₃ was obtained when the oxidation was conducted for the metallic Bi film electrodeposited at −0.1 V. Only β-Bi₂O₃ was observed as a −0.5 V electrodeposited Bi film was oxidized. The crystal structure of Bi₂O₃ obtained by oxidation of metallic Bi film may dominantly be affected by the orientation of as-electrodeposited Bi film. Such kind of process is favorable to the preparation of functional ceramic with specific crystal structure.     

Effects of Bi2O3 and Cr2Ti3O9 Co-doping on dielectric properties in BaTiO3-based ceramics

A microwave ceramic with general composition (1-x-y) BaTiO₃ + x Cr₂Ti₃O₉ + y Bi₂O₃ has been prepared by solid state synthesis at 1300-1400 °C. The phase composition, perovskite structural parameters and dielectric properties have been obtained by X-ray diffraction and dielectric measurements as a function of chemical composition and temperature. At low doping levels the formation of BaTiO₃-based solid solution has been found. The precipitation of BaCrO₃ has been detected at x = y = 2.0 mol%. A model of the incorporation of Cr³⁺ and Bi³⁺ ions into BaTiO₃-based crystal lattice has been proposed. Diffused phase transition in the temperature range 100-140 °C have been revealed by dielectric measurements for different ceramic composition. As high dielectric constant as 7311 and as low dielectric loss as 0.02 have been found for the composition of 0.98BaTiO₃-0.01Cr₂Ti₃O₉-0.01Bi₂O₃.     

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.     

An investigation of thermal decomposition of β-FeOOH nanowire arrays assembled in AAO templates

β-FeOOH nanowire arrays were assembled into porous anodized aluminum oxide (AAO) templates by electrochemical deposition in the mixture solution of FeCl₃ and (NH₄)₂C₂O₄. In order to obtain well-crystallized α-Fe₂O₃ and other iron oxides nanowires, β-FeOOH nanowire arrays with amorphous crystal structure were heat-treated at different temperatures from 200 to 600 °C. The decomposition products were characterized by DTA, XRD, FTIR, and Mössbauer spectroscopy. When heat-treated at 200 °C, only 65% of β-FeOOH decomposed, whereas, when the temperature was up to 300 °C, it was completely decomposed and formed poorly crystallized β-Fe₂O₃. This transition temperature is higher than the 200 °C obtained on other β-FeOOH materials. However, when heated above 300 °C, the main products are characterized as poorly crystallized α-Fe₂O₃ nanowires, whereas, well-crystallized α-Fe₂O₃ nanowire arrays can be formed when the temperature was up to 600 °C, and this temperature is also higher compared with those temperatures observed on other β-FeOOH materials. From Mössbauer results, the α-Fe₂O₃ nanowires were composed of fine particles in which 66% of the particles are superparamagnetic.     

Synthesis of non-stoichiometric Bi2O4−x by oxidative precipitation

Bi₂O_4−x, a Bi mixed-valence phase was prepared at 95 °C, by a precipitation process, in a basic medium with a highly oxidizing K₂S₂O₈/Na₂S₂O₈. This phase has a low thermal stability as it decomposes below 400 °C in a multiple step process by some O₂ losses prior to finally transforming into γ-Bi₂O₃. The as-prepared powders are 50-60 nm in size with a narrow size distribution. Optical spectra of Bi₂O_4−x exhibit a broad absorption band with a band gap of ∼1.4 eV as compared to 2.61 eV for Bi₂O₃. The composition of this non-stoichiometric phase, which crystallizes in cubic fluorite related structure with a cell parameter of 5.538(3) Å, is Bi₂O_{3.65 ± 0.10}.     

Fabrication of hexagonal GaN on the surface of β-Ga2O3 single crystal by nitridation with NH3

The fabrication of GaN on the surface of a bulk β-Ga₂O₃ single crystal by nitridation with NH₃ was investigated for the purpose of using it as a substrate for GaN epitaxial growth. A β-Ga₂O₃ single crystal was prepared using a floating zone furnace with double ellipsoidal mirrors, and its polished (100) plane was nitridated in NH₃ atmosphere at 850 °C for 5 h. It was found that hexagonal GaN with preferred in-plane orientation was produced on the surface of β-Ga₂O₃, and the thickness of nitride layers was approximately 50 nm. High resolution transmission electron microscopic observation indicated that the synthesized GaN was composed of the aggregation with single crystalline GaN particles, whose size ranged from ∼ 5 nm to ∼ 50 nm, and dislocation or defect was not observed in a GaN particle. This method could be expected as a new route to fabricate a substrate for epitaxial growth of III-nitride materials instead of using a bulk GaN single crystal.     

Synthesis and characterization of Bi2Fe4O9 powders

Bi₂Fe₄O₉ have been successfully prepared using ethylenediaminetetraacetic (EDTA) acid as a chelating agent and ethylene glycol as an esterification agent. Heating of a mixed solution of EDTA, ethylene glycol, and nitrates of iron and bismuth at 140 °C produced a transparent polymeric resin without any precipitation, which after pyrolysis at 250 °C was converted to a powder precursor for Bi₂Fe₄O₉. The precursors were heated at 400–800 °C in air to obtain Bi₂Fe₄O₉ powder and differential scanning calorimetry (DSC), thermogravimetric (TG), Fourier transform infrared spectroscopy (FT-IR), X-ray diffraction (XRD) and field emission scanning electron microscopy (FESEM) techniques were used to characterize the precursors and the derived oxide powders. XRD analysis showed that well-crystallized and single-phase Bi₂Fe₄O₉ with orthorhombic symmetry was obtained at 700 °C for 2 h and BiFeO₃ and Fe₂O₃/FeCO₃ were intermediate phases before the formation of Bi₂Fe₄O₉. Bi₂Fe₄O₉ powders show weak ferromagnetism at room temperature.     

Composition of alumina films grown on Si at low temperature with catalytic CVD

The X-ray photoelectron spectroscopy (XPS) measurements have been used to reveal the compositions of alumina (Al₂O₃) films formed on Si wafers using tri-methyl aluminium (TMA) and molecular oxygen (O₂) with catalytic chemical vapour deposition (Cat-CVD). The atomic ratio (O/Al) for Al₂O₃ samples formed at substrate temperature of 200-400 °C has been obtained to be 1.4 which is close to stoichiometry. The increase of growth rate at substrate temperatures below 200 °C and above 400 °C can be attributed to formation of aluminum oxides with non-stoichiometry and metallic aluminum incorporated in the films resulting from deficient oxygen. Angle resolved XPS measurements have revealed that the alumina/Si interface with no SiO₂ film has been obtained at substrate temperatures below 200 °C.     

Preparation and characterization of hollow α-Fe2O3 irregular microspheres

Hollow α-Fe₂O₃ irregular microspheres were prepared at 160 °C from a hydrolyzing Fe(ClO₄)₃ solution by adding sodium polyanethol sulphonate. The particles were characterized by ⁵⁷Fe Mössbauer, X-ray diffraction, Fourier transform infrared spectroscopy, field emission scanning electron microscopy and energy dispersive X-ray spectroscopy. The walls of these hollow particles consisted of elongated subunits composed of elongated and thin α-Fe₂O₃ rods. The precipitation of hollow α-Fe₂O₃ irregular microspheres was governed by the preferential adsorption of sulphonate/sulphate groups. The lateral aggregation of elongated thin rods and subunits also played an important role in the formation of hollow α-Fe₂O₃ irregular microspheres.     

Preparation and characterization of porous ultrafine Fe2O3 particles

Porous ultrafine Fe₂O₃ particles were prepared by homogeneous precipitation method. Fe³⁺ and urea were chosen as starting materials and anionic surfactant as the template. It is shown that the reaction results in the precipitation of a gelatinous hydrous iron oxide/surfactant mixture, which gives ultrafine Fe₂O₃ particles after drying and calcinations. The products were characterized by XRD, TEM, TG/DTA and BET. Conventional XRD patterns show that the products are mixture of γ-Fe₂O₃ and α-Fe₂O₃ phase after being sintered at 350 °C, and γ-Fe₂O₃ transforms entirely to α-Fe₂O₃ when sintered at 650 °C. The low-angle XRD patterns indicate that the mesostructure can only exist between 350 and 400 °C. TEM results show that the Fe₂O₃ particles have diameters of about 30 nm and lengths ranging from 100 to 120 nm; in each particle, there are several vermiculate-like mesopores with diameter of about 20-25 nm. The BET surface areas in excess of 50 m²/g are obtained after calcinations at 350 °C. The BJH desorption average pore width is around 22 nm, which is in agreement with the TEM results. The results show that anionic surfactant and sintering temperature are important to obtain this special morphology.