Influence of synthesis procedure on the morphology of bismuth oxide particles

A modified sol-gel procedure, based on the esterification reaction, was used to prepare the Bi₂O₃ precursor, which was then heated to 400 or 500 °C. β-Bi₂O₃ obtained at 400 °C showed well-shaped plate-like particles. The mixture β-Bi₂O₃/α-Bi₂O₃, obtained by prolonged heating at 400 °C, yielded pseudospherical particles having about 100 nm in size and much larger particles, as found by FE SEM. α-Bi₂O₃ obtained at 500 °C consisted of particles of varying shapes and sizes. Vitrification of α-Bi₂O₃ was also observed. XRD showed a small fraction of unidentified phase(s) in these samples. Different microstructures were obtained when the precipitation from aqueous Bi(NO₃)₃ solution with tetramethylammonium hydroxide at pH ∼ 14 was used. The precipitation at pH ∼ 3.5 yielded cloverleaf-like particles of good uniformity, which were assigned to BiOOH (isomorphous with (La_0.26Bi_0.24)Bi_0.5OOH. It was found that these particles were made up of much smaller primary BiOOH particles.     

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.     

Facile preparation of heterostructured Bi2O3/Bi2MoO6 hollow microspheres with enhanced visible-light-driven photocatalytic and antimicrobial activity

Hierarchical β-Bi₂O₃/Bi₂MoO₆ heterostructured flower-like microspheres assembled from nanoplates with different β-Bi₂O₃ loadings (0–26.5 mol%) were synthesized through a one-step template-free solvothermal route. Under visible-light illumination (λ > 420 nm), over 99% of rhodamine B was degraded within 90 min on the 21.9 mol% of β-Bi₂O₃ loading Bi₂O₃/Bi₂MoO₆ microspheres. The remarkable enhancement of photocatalytic activity of the hierarchical Bi₂O₃/Bi₂MoO₆ micro/nanostructures can be attributed to the effective separation of the photoinduced charge carriers at the interfaces and in the semiconductors. The electrons (e⁻) are the main active species in aqueous solution under visible-light irradiation. The Bi₂O₃/Bi₂MoO₆ also displays visible-light photocatalytic activity for the destruction of E. coli. In addition, the β-Bi₂O₃ in the hierarchical Bi₂O₃/Bi₂MoO₆ microspheres is very stable and the composite can be easily recycled by a simple filtration step, thus the second pollution can be effectively avoided. A possible photocatalytic mechanism was proposed based on the experimental results.     

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.     

A thermal reduction route to nanocrystalline transition metal carbides from waste polytetrafluoroethylene and metal oxides

Starting from waste polytetrafluoroethylene (PTFE), metal oxides (TiO₂, V₂O₅, Nb₂O₅, and MoO₃) and metal sodium, several nanocrystalline transition metal carbides (TiC, VC, NbC, and Mo₂C) have been prepared through a thermal reduction route in an autoclave at 600 °C. It is found that the obtained NbC nanocrystallines have a superconducting transition temperature at 11.6 K.     

Surface and catalytic properties of MoO3/Al2O3 system doped with Co3O4

Thermal solid-solid interactions in cobalt treated MoO₃/Al₂O₃ system were investigated using X-ray powder diffraction. The solids were prepared by wet impregnation method using Al(OH)₃, ammonium molybdate and cobalt nitrate solutions, drying at 100 °C then calcination at 300, 500, 750 and 1000 °C. The amount of MoO₃, was fixed at 16.67 mol% and those of cobalt oxide were varied between 2.04 and 14.29 mol% Co₃O₄. Surface and catalytic properties of various solid samples precalcined at 300 and 500 °C were studied using nitrogen adsorption at −196 °C, conversion of isopropanol at 200-500 °C and decomposition of H₂O₂ at 30-50 °C.The results obtained revealed that pure mixed solids precalcined at 300 °C consisted of AlOOH and MoO₃ phases. Cobalt oxide-doped samples calcined at the same temperature consisted also of AlOOH, MoO₃ and CoMoO₄ compounds. The rise in calcination temperature to 500 °C resulted in complete conversion of AlOOH into very poorly crystalline γ-Al₂O₃. The further increase in precalcination temperature to 750 °C led to the formation of Al₂(MoO₄)₃, κ-Al₂O₃ besides CoMoO₄ and un-reacted portion of Co₃O₄ in the samples rich in cobalt oxide. Pure MoO₃/Al₂O₃ preheated at 1000 °C composed of MoO₃-αAl₂O₃ solid solution (acquired grey colour). The doped samples consisted of the same solid solution together with CoMoO₄ and CoAl₂O₄ compounds.The increase in calcination temperature of pure and variously doped solids from 300 to 500 °C increased their specific surface areas and total pore volume which suffered a drastic decrease upon heating at 750 °C. Doping the investigated system with small amounts of cobalt oxide (2.04 and 4 mol%) followed by heating at 300 and 500 °C increased its catalytic activity in H₂O₂ decomposition. This increase, measured at 300 °C, attained 25.4- and 12.9-fold for the solids precalcined at 300 and 500 °C, respectively. The increase in the amount of dopant added above this limit decreased the catalytic activity which remained bigger than those of un-treated catalysts. On the other hand, the doping process decreased the catalytic activity of treated solids in isopropanol conversion especially the catalysts precalcined at 300 °C. This treatment modified the selectivities of treated solids towards dehydration and dehydrogenation of reacted alcohol.The activation energies of H₂O₂ decomposition were determined for pure and variously doped solids. The results obtained were discussed in light of induced changes in chemical composition and surface properties of the investigated system due to doping with cobalt oxide.     

Solution combustion synthesis of γ(L)-Bi2MoO6 and photocatalytic activity under solar radiation

The facile method of solution combustion was used to synthesize γ(L)-Bi₂MoO₆. The material was crystallized in a purely crystalline orthorhombic phase with sizes varying from 300 to 500 nm. Because the band gap was 2.51 eV, the degradation of wide variety of cationic and anionic dyes was investigated under solar radiation. Despite the low surface area (<1 m²/g) of the synthesized material, γ(L)-Bi₂MoO₆ showed high photocatalytic activity under solar radiation due to its electronic and morphological properties.     

Redetermination of the crystal structure of α-Bi2O3.3MoO3 by neutron diffraction and the catalytic oxidation of propene

The powder profile obtained by neutron diffraction was analysed by the Rietveld method. The catalytic oxidation of propene over α-Bi₂O₃.3MoO₃ is examined. The high activity is related to the fact that the principal cleavage plane contains together Bi and Mo ions. On the contrary γ-Bi₂O₃.MoO₃ is less active, probably because only small faces are active. The lone pairs disposition is not favorable to oxygen ionic conductivity; this rather low conductivity may favour a rather high selectivity in acrolein.     

Synthesis and characterization of porous platelet-shaped α-Bi<Subscript>2</Subscript>O<Subscript>3</Subscript> with enhanced photocatalytic activity for 17α-ethynylestradiol

The porous platelet-shaped α-Bi₂O₃ photocatalyst was successfully synthesized by a novel hydrothermal–calcination method assisted with ethylenediamine and polyvinylpyrrolidone. The physical and chemical properties of α-Bi₂O₃ photocatalyst were characterized based on XRD, XPS, SEM, TEM, EDS, UV–Vis DRS, and PL techniques. The influence of preparation conditions on the formation of α-Bi₂O₃ photocatalyst was investigated, and the effect of catalyst dosage and pH value on the EE2 removal rate was also investigated. The synthesized porous platelet-shaped α-Bi₂O₃ photocatalyst exhibited excellent photocatalytic activity for 17α-ethynylestradiol (EE2), and 97.8% of EE2 was removed after 75 min of visible light irradiation using α-Bi₂O₃ as photocatalyst. The reaction rate constant over the porous platelet-shaped α-Bi₂O₃ photocatalyst was 11.6 and 11.4 times of that of traditional α-Bi₂O₃ and N-TiO₂, respectively. The possible photocatalytic mechanism has been discussed on the basis of the theoretical calculation and the experimental results. The porous platelet-shaped α-Bi₂O₃ was a stable and efficient photocatalyst, proving that it is a promising photocatalyst.     

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.     

PH-induced structural evolution,photodegradation mechanism and application of bismuth molybdate photocatalyst

In this work, we have elucidated the pH-induced structural evolution of bismuth molybdate photocatalyst based on a hydrothermal synthesis route. With increasing the pH value of precursor solution, pure Bi₂MoO₆ was synthesized at pH 2–5, Bi₂MoO₆-Bi₄MoO₉ mixture was obtained at pH 7–9, pure Bi₄MoO₉ was obtained at pH 11, and pure α-Bi₂O₃ was derived at pH 13. The as-derived samples mainly present particle-like shapes but with different particle sizes (except the observation of Bi₂MoO₆ nanowires in sample S-pH9). The photocatalytic performances between the samples were compared via the degradation of methylene blue (MB) under irradiation of simulated sunlight. The Bi₂MoO₆ sample synthesized at pH 2 exhibited the highest photodegradation performance (η(30 min) = 89.8 %, k_app = 0.05007 min^?1) among the samples. The underlying photocatalytic mechanism and degradation pathways of MB were systematically analyzed. Moreover, the photodegradation performance of the Bi₂MoO₆ photocatalyst was further evaluated at different acidic-alkaline environments as well as in degrading various color and colorless organic pollutants, which provides an important insight into its practical application.     

An investigation of phase equilibria in the system MoO3V2Ox (4 < x < 5) by analytical electron microscopy

The β-phase in the system MoO₃V₂O_x (4 < x < 5) has been prepared at temperatures between 500 and 600°C, and the products characterized by analytical electron microscopy and powder X-ray diffraction. The M₃O₈ stoichiometry is retained at all temperatures, but the cation ratio varies from approximately (V_0.5Mo_0.5)₃O₈ at 500°C to (V_0.6Mo_0.4)₃O₈ at 600°C.     

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 and laser patterning of ferroelastic β′-RE2(MoO4)3 crystals (RE: Sm,Gd, Tb,Dy) in rare-earth molybdenum borate glasses

The glasses with the compositions of 22.5RE₂O₃–47.5MoO₃–30B₂O₃ (RE: Sm, Gd, Tb, and Dy) (mol%) were prepared by a conventional melt quenching method, and the formation and laser patterning of ferroelastic β′-RE₂(MoO₄)₃ crystals were examined. The initial crystalline phase was β′-RE₂(MoO₄)₃ in all the glasses, and the second crystalline phases were rare-earth borates such as Gd(BO₂)₃, Tb(BO₂)₃, and DyBO₃, and α-RE₂(MoO₄)₃ in the glass with Sm₂O₃. It was confirmed from polarized optical microscope observations and micro-Raman scattering spectra that β′-RE₂(MoO₄)₃ crystal lines having periodic refractive index changes were patterned by irradiations of continuous wave Yb:YVO₄ fiber laser (wavelength:1080 nm, power: 0.5–2.0 W, scanning speed: 8–20 μm s⁻¹). It was found that the periodic degree of refractive index changes depends on the kind of RE³⁺ ions. The present study demonstrates that the appearance of periodic domain structures is an intrinsic feature in the laser patterning of ferroelastic β′-RE₂(MoO₄)₃ crystals.     

Thermal expansion studies of Gd2Mo3O12 and Gd2W3O12

Simultaneous thermogravimetric/differential thermal analysis of Gd₂Mo₃O₁₂ showed an irreversible phase transition at 1178 K where as Gd₂W₃O₁₂ showed reversible phase transition at 1433 K, which were confirmed by powder X-ray diffraction. The thermal expansion behavior of α-Gd₂Mo₃O₁₂ (room temperature phase), β-Gd₂Mo₃O₁₂ (phase obtained by heating Gd₂Mo₃O₁₂ at 1223 K) and Gd₂W₃O₁₂ have been investigated using high temperature X-ray diffractometer. The cell volume of α-Gd₂Mo₃O₁₂, β-Gd₂Mo₃O₁₂ and Gd₂W₃O₁₂, fit into polynomial expression with respect to temperature, showed positive thermal expansion up to 1073, 1173 and 1173 K, respectively. The average volume expansion coefficients for α-Gd₂Mo₃O₁₂, β-Gd₂Mo₃O₁₂ and Gd₂W₃O₁₂ are 39.52 × 10⁻⁶, 21.23 × 10⁻⁶ and 37.96 × 10⁻⁶ K⁻¹, respectively.     

Synthesis and photocatalytic activity of cobalt oxide doped ZnFe2O4-Fe2O3-ZnO mixed oxides

Novel cobalt oxide doped ZnFe₂O₄-Fe₂O₃-ZnO mixed oxides with the Zn/Fe molar ratio of 1/2 were synthesized with a citric acid complex method. The effects of cobalt oxide and calcination temperature on phase composition and photocatalytic activity of the mixed oxides were investigated. X-ray diffraction (XRD) analysis revealed that there were mainly ZnFe₂O₄, α-Fe₂O₃, amorphous ZnO and Fe₂O₃ in the 6 mol% cobalt oxide doped products calcined at 500 °C. 5-10 mol% cobalt oxide doping could significantly enhance the formation of ZnFe₂O₄ and altered the phase composition of the mixed oxides. Experimental results showed that cobalt oxide doping could remarkably improve the photocatalytic activity of the mixed oxides for phenol degradation. The 6 mol% cobalt oxide doped mixed oxides calcined at 500 °C exhibited better photocatalytic activity as compared with other samples.     

Graphite oxide-assisted sonochemical preparation of α-Bi2O3 nanosheets and their high-efficiency visible light photocatalytic activity

Novel α-Bi₂O₃ nanosheets have been successfully synthesized by graphite oxide (GO)-assisted sonochemical route and subsequent calcining treatment. The morphology of the final products can be controlled by tuning GO nanosheets in the synthesis process. Porous and nanosheet-covered α-Bi₂O₃ microrods have also been obtained, and the growth mechanism has been discussed. Scanning electron microscopy and transmission electron microscopy analyses reveal that the α-Bi₂O₃ nanosheets are 10–20 nm in thickness with suppressed growth along the [2 1 ?4] direction. Owing to the unique structure, the α-Bi₂O₃ nanosheets show broader optical absorption and narrower band gap (2.25 eV) as compared to other two forms of α-Bi₂O₃ (~2.8 eV). Photocatalytic experiments demonstrated that the α-Bi₂O₃ nanosheets exhibited excellent activity much higher than the porous and nanosheet-covered α-Bi₂O₃ microrods in degrading organic pollutants under visible light. This study opens up a new route for the preparation of α-Bi₂O₃ nanosheets with superior photocatalytic performance.     

Improved structural stability of titanium-doped β-Bi2O3 during visible-light-activated photocatalytic processes

Due to its strong absorption to visible light and intrinsic polarizability, β-Bi₂O₃ could be a promising candidate for the visible-light-activated photocatalysis. However, its structural instability during a photocatalytic process prevents it from being used practically. In this work, titanium-doped β-Bi₂O₃ was synthesized by a hydrothermal method with subsequent calcination under 400 °C. Its crystal structure, photophysical property, and structural stability were investigated by using powder X-ray diffraction, Raman, infrared and diffuse reflectance UV–vis spectroscopies. The crystal structure of the titanium-doped β-Bi₂O₃ is analogous to β-Bi₂O₃. These two oxides exhibited comparable photocatalytic activities on the photodegradation of indigo carmine, rhodamine B, and methylene blue under visible-light irradiation. However, unlike β-Bi₂O₃, the titanium-doped β-Bi₂O₃ was quite stable during these photocatalytic reactions. The improvement in structural stability was attributable to the substitution of titanium species in the host crystal lattice. The current investigation results point toward the possibility of metal ion-doped bismuth oxides as efficient visible-light-activated photocatalysts.     

Thermal oxidation of single crystal aluminum nitride — A high resolution transmission electron microscopy study

The impact of the oxidation time on the structures of thermal oxides formed on AlN was determined by high resolution transmission electron microscopy (HRTEM). Oxidation of AlN single crystals was performed for 2 to 6 h at 1000 °C. Oxidation for 2 h produced mostly amorphous oxide layers whereas oxidation for both 4 and 6 h produced partially crystalline oxide layers. The oxide layer thickness varied from 205 to 600 nm for oxidation times of 2 and 6 h respectively. The crystalline oxide was mostly single phase α-Al₂O₃ except at the surface where it was a mixture of γ-Al₂O₃ and α-Al₂O₃. Based on the different structures produced for different oxidation times, we speculate that the oxide formed changes with thickness: first an amorphous oxide, then γ-Al₂O₃, and finally α-Al₂O₃ as the oxide thickness increases. The AlN crystal was nearly defect- and oxygen-free for oxidation at 1000 °C. This could be due to the rapid diffusion of the nitrogen and aluminum interstitials at a high temperature leading to a point-defect equilibrium throughout the nitride. A faceted interface between Al₂O₃ and AlN could be attributed to the surface diffusion to minimize energy.     

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.