Visible Light Photocatalytic Activity of Bismuth Ferrites Tuned by Bi/Fe Ratio

Pure perovskite BiFeO₃, sillenite Bi₂Fe₄O₉, and BiFeO₃/Bi₂Fe₄O₉ were synthesized facilely by controlling the precursor Bi/Fe ion ratio through a hydrothermal method. The phase composition, morphology, optical properties, and photocatalytic activity of as‐prepared samples were investigated in detail. Our results indicate that the morphology and properties of products strongly dependent on the precursor Bi/Fe ion ratio. Photocatalysis of Congo Red reveals that sillenite Bi₂Fe₄O₉ shows superior activity to perovskite BiFeO₃, and BiFeO₃/Bi₂Fe₄O₉ exhibited higher activity with 71.45% degradation rate in 90 min. It provides an easy and efficient way to tune the composition and photocatalytic activity of Bi‐based oxides.     

Reaction pathways in the solid state synthesis of multiferroic BiFeO3

The obtaining of multiferroic BiFeO₃ as a pure single-phase product is particularly complex since the formation of secondary phases seems to be unavoidable. The process by which these secondary impurities are formed is studied by analyzing the diffusion and solid state reactivity of the Bi₂O₃-Fe₂O₃ system. Experimental evidence is reported which indicates that the progressive diffusion of Bi³⁺ ions into the Fe₂O₃ particles governs the solid state synthesis of the perovskite BiFeO₃ phase. However a competition is established between the diffusion process which tends to complete the formation of BiFeO₃, and the crystallization of stable Bi₂Fe₄O₉ mullite crystals, which tend to block that formation reaction.     

Synthesis of Anisotropic Na0.5Bi0.5TiO3 Crystals Using Different Topochemical Microcrystal Conversion Methods

Na_0.5Bi_0.5TiO₃ (NBT) platelets with high aspect ratio were synthesized from Na_0.5Bi_4.5Ti₄O₁₅ (NBIT) precursors via a topochemical microcrystal conversion in molten salt conditions. The effect of the synthesis parameters, such as the molten salt system, synthesis temperature, and the molar ratio of Na₂CO₃ and NBIT, was investigated. The results showed that NaCl–KCl molten salt environment and excess Na₂CO₃ played a positive role in the synthesis, square‐shaped NBT was obtained at 950°C in NaCl–KCl molten salt and a TiO₂‐free environment, and it was a suitable template candidate to achieve NBT‐based textured ceramics using the reactive template grain growth (RTGG) method.     

Effects of precursor chemistry and thermal treatment conditions on obtaining phase pure bismuth ferrite from aqueous gel precursors

Phase pure BiFeO₃ powders are synthesized by an entirely aqueous solution–gel route, starting from water soluble Fe(III) nitrate or citrate, and Bi(III) citrate as precursors. In order to obtain stable solutions, which transform to homogeneous gels upon drying, the pH is adjusted to 7 and a citric acid content equimolar to the metal ions is selected.The presence of nitrate strongly accelerates the thermo-oxidative decomposition step of the precursor gel around 200 °C, and the decomposition is finished at a lower temperature for the nitrate containing precursor (460 °C) than without nitrates (500 °C) in dynamic dry air. An oxidative ambient is required to fully decompose the precursor.The presented synthesis allows very low temperature (400 °C) crystallization of BiFeO₃ together with a secondary phase, as shown by high temperature XRD. This parasitic phase remains up to high temperatures, where decomposition of BiFeO₃ is observed from 750 °C onwards, and Bi₂Fe₄O₉ is formed. However, optimization of the furnace treatment, considering anneal temperatures and heating rates showed that phase pure BiFeO₃ can be obtained, with the heating rate being the crucial factor (5 °C/min). The chemical purity of the powders is confirmed by FTIR, and the antiferromagnetic to paramagnetic phase transition is demonstrated by DSC measurements.     

Low‐Temperature Synthesis of Nanocrystalline NbB2 Powders by Borothermal Reduction in Molten Salt

Nanocrystalline NbB₂ powders were successfully prepared by borothermal reduction in molten salt at 800°C–1000°C. Due to the more homogeneous mixing and more rapid diffusion of species in the liquid state than in the solid state, the synthesis temperature of pure NbB₂ phase was greatly decreased by the presence of molten NaCl/KCl salt. The NbB₂ powders synthesized at 1000°C had the largest specific surface area of 27.09 m²/g and the lowest equivalent average particle size of 32 nm, respectively.     

Electrical properties of bismuth ferrites: Bi2Fe4O9 and Bi25FeO39

Bi₂Fe₄O₉ was prepared by solid-state reaction and the electrical properties measured by impedance spectroscopy. After annealing in O₂ at 900 °C, Bi₂Fe₄O₉ is an electrically-homogeneous insulator. Its high frequency permittivity is constant (∼14.1) over the temperature range 300–400 °C and shows no evidence of incipient ferroelectricity at lower temperatures. On annealing in N₂ at 900 °C, the pellets gradually decompose.Bi₂₅FeO₃₉ was prepared by both solid-state reaction and mechanosynthesis. It showed a modest amount of mixed conduction of both oxide ions and holes. Impedance analysis showed a complex response that best fitted an equivalent circuit consisting of a parallel combination of long-range conduction and short range dielectric relaxation elements.The electrical conductivity of both Bi₂Fe₄O₉ and Bi₂₅FeO₃₉ is less than that of BiFeO₃ prepared by solid-state reaction, which indicates that any leakage conductivity of BiFeO₃ is not due to the possible presence of small amounts of these secondary phases.     

In situ synthesis of Al2O3–SiC powders via molten-salt-assisted aluminum/carbothermal reduction method

Al₂O₃–SiC composite powder (ASCP) was successfully synthesized using a novel molten-salt-assisted aluminum/carbothermal reduction (MS-ACTR) method with silica fume, aluminum powder, and carbon black as raw materials; NaCl–KCl was used as the molten salt medium. The effects of the synthesis temperature and salt-reactant ratio on the phase composition and microstructure were investigated. The results showed that the Al₂O₃–SiC content increased with an increase in molten salt temperature, and the salt–reactant ratio in the range of 1.5:1–2.5:1 had an impact on the fabrication of ASCP. The optimum condition for synthesizing ASCP from NaCl–KCl molten salt consisted of maintaining the temperature at 1573 K for 4 h. The chemical reaction thermodynamics and growth mechanism indicate that the molten salt plays an important role in the formation of SiC whiskers by following the vapor-solid growth mode in the MS-ACTR treatment. This study demonstrates that the addition of molten salt as a reaction medium is a promising approach for synthesizing high-melting-point composite powders at low temperatures.     

Preparation of Giant Dielectric CaCu3Ti4O12 Ceramics via the Molten Salt Method from NaCl Flux

CaCu₃Ti₄O₁₂ (CCTO) powders have been synthesized by the molten salt method using NaCl as the flux. The effects of the synthesis temperature on their crystal structures and micromorphologies were investigated in this study. A new secondary phase, Na₂Ti₆O₁₃, was found in the as‐synthesized powders when the temperature exceeds 800°C. Na₂Ti₆O₁₃ and CuO secondary phases result in a significant increase in the dielectric constant and a certain increase in the dielectric loss. 800°C is a suitable synthesis temperature to prepare CCTO ceramics with relatively good dielectric performance using NaCl as the molten salt.     

Magnetic and morphological characterization of Bi2Fe4O9 nanoparticles synthesized via a new reverse chemical co-precipitation method

Nano-sized Bi₂Fe₄O₉ (BFO) was successfully synthesized using a new reverse chemical co-precipitation method at different pH values of 8–12. These powders were examined by x-ray diffractometery (XRD), thermogravimetrical differential thermal analysis (TG-DTA), field emission scanning electron microscopy (FESEM) and vibrating sample magnetometery (VSM). The XRD analysis showed the formation of pure phase Bi₂Fe₄O₉ at calcination temperature over 700 ℃. The TG-DTA curves indicated the crystallization temperature of 617 ℃ for the Bi₂Fe₄O₉ sample. The FESEM micrographs revealed a precipitates agglomeration, which is related to the nature of the chemical co-precipitation method and free surface energy of nanoparticles. Furthermore, the particle size of the powder samples increased from 43 to 131 nm as the pH value increased from 8 to 12, respectively. Also, the morphological change from nearly cubic to rod-like shape in the nanoparticles was observed by increasing the pH value. The M-H curves of the as-prepared powders confirmed the antiferromagnetic behavior in all samples. Uncompensated spins from the surface led to the appearance of saturation magnetization in the Bi₂Fe₄O₉ nanoparticles. Besides, a decrease in the particles size resulted in more uncompensated spins, thereby improving the saturation and remnant magnetization from M_s = 0.35 emu/g and M_r = 0.010 emu/g for pH = 12 to M_s = 1.15 emu/g and M_r = 0.042 emu/g for pH = 8. Furthermore, as the pH values increase the coercive fields firstly rise up to 196 Oe for pH = 9 and then decrease to 151 for pH = 12.     

The effect of Fe2+ state in electrical property variations of Sn‐doped hematite powders

The structural, electrical, and chemical properties of Sn‐doped Fe₂O₃ powders were investigated. Various quantities of Sn‐doped Fe₂O₃ powders were synthesized using solid‐state reactions. Rietveld analysis for the powders that were doped below 2% revealed a phase‐pure Sn‐doped Fe₂O₃ structure (i.e., identical to Fe₂O₃ structure). Alternatively, the analysis for the powders that were doped more than 3% exhibited secondary phase. The unit cell volume and electrical conductivity of the phase‐pure samples increased with an increase in the doping concentration. X‐ray photoelectron spectroscopy measurements showed an increased Fe²⁺ state with the increase in Sn doping concentration. Therefore, the improved electrical conductivity and unit cell volume with the increase in doping concentration of the phase‐pure powders might be related to the increased Fe²⁺ state.     

Synthesis and optimization of PES‐Fe3O4 mixed matrix nanocomposite membrane: Application studies in water purification

A mixed matrix nanocomposite membrane comprising of polyethersulfone (PES) and nano Fe₃O₄ particles was synthesised with an aim to develop a membrane with superior ability to high water flux while maintaining salt rejection efficiency. The study focused on optimizing the effect of different percentages of nano Fe₃O₄ particles addition on the membrane pure water flux, and rejection of both the NaCl and MgSO₄. The results showed that the PES nanocomposite membrane with 15% Fe₃O₄ exhibited the highest pure water flux, while the highest rejection of NaCl and MgSO₄ belonged to the PES‐10% Fe₃O₄. The rejection sequence of PES‐10% Fe₃O₄ membrane was 68% and 82%, respectively. Also it was observed that nano Fe₃O₄ particles improved membrane hydrophilicity, and helped to construct a membrane with desire surface and cross‐section morphology. POLYM. COMPOS., 34:1870–1877, 2013. © 2013 Society of Plastics Engineers     

A review of the structure,magnetic and electrical properties of bismuth ferrite (Bi2Fe4O9)

Nowadays, investigations on the materials with multiferroic properties are in progress. These materials compromise simultaneous electric and magnetic properties. Ferrite Bismuth (FB) is one of the ceramic materials that enjoy this property and possesses three different crystalline structures (perovskite BiFeO₃, selenite Bi₂₅FeO₄₀ and mullite Bi₂Fe₄O₉). In this review, first, the crystalline structure and the electric and magnetic properties of Bi₂Fe₄O₉ are studied, and then, the effects of adding dopants to the ferrite are discussed. Mullite-type bismuth ferrite (Bi₂Fe₄O₉) as a spin frustrated multiferroic has potential for magnetoelectric coupling, and it might be an appropriate alternative for some of the multiferroics that suffer from a weak magnetoelectric coupling.     

Synthesis of carbon nanotube (CNT)‐BiFeO3 and (CNT)‐Bi2Fe4O9 nanocomposites and its enhanced photocatalytic properties

Pure and carbon nanotube (CNT) attached BiFeO₃ and Bi₂Fe₄O₉ were prepared via hydrothermal route at fixed temperature, with time and mineralizer as variants. Phase purity of products was determined through X‐ray diffraction (XRD) studies. FESEM analyses revealed that synthesized materials exhibited various morphologies, depending on the mineralizer being employed in the synthesis. Optical band gap measurements have been carried out using UV‐Vis spectroscopy analyses. The attachment of CNT reduces the bandgap, and consequently enhances photocatalytic activity due to the electron transfer from BiFeO₃ to CNT. The pseudo‐first order model of reaction kinetics has been used successfully to study the associated mechanism.     

Magnetic and conducting Fe3O4–polypyrrole nanoparticles with core‐shell structure

Magnetic and conducting Fe₃O₄–polypyrrole nanoparticles with core‐shell structure were prepared in the presence of Fe₃O₄ magnetic fluid in aqueous solution containing sodium dodecylbenzenesulfonate (NaDS) as a surfactant and dopant. Both the conductivity and magnetization of the composites depend strongly on the Fe₃O₄ content and the doping degree. With increase of Fe₃O₄ content in the composite, the conductivity at room temperature decreases, but the saturated magnetization and coercive force increase. Transmission electron microscopy (TEM) images of Fe₃O₄ and Fe₃O₄–polypyrrole particles show almost spherical particles with diameters ranging from 20 to 30 and 30 to 40 nm, respectively. The thermal stability of Fe₃O₄–polypyrrole composites is higher than that of pure polypyrrole. Studies of IR, UV–visible and X‐ray photoelectron spectroscopy (XPS) spectra suggest that the increased thermal stability may be due to interactions between Fe₃O₄ particles and polypyrrole backbone. Copyright © 2003 Society of Chemical Industry     

Low temperature synthesis of TaB2 nanorods by molten‐salt assisted borothermal reduction

TaB₂ powders were synthesized by a molten‐salt assisted borothermal reduction method at 900°C‐1000°C in flowing argon using Ta₂O₅ and amorphous B as starting materials. The results indicated that the presence of liquid phase, such as B₂O₃ and NaCl/KCl, accelerated the mass transfer of reactant species and resulted in the complete finish of the reaction at low temperatures. The obtained TaB₂ powders exhibited a flow‐like shape assembled from nanorods grow along [001] direction or c‐axis. The morphology of the synthesized TaB₂ powders could be tailored by the amount of B₂O₃ or NaCl/KCl.     

Effects of Bismuth Oxide Buffer Layer on BiFeO3 Thin Film

Bismuth ferrite (BiFeO₃) thin films with Bi₂O₃ buffer layers were prepared on Si/SiO₂/TiO₂/Pt substrates by sol–gel‐derived spin‐coating method. The structural and electrical properties of BiFeO₃ was effectively improved by adding a Bi₂O₃ buffer layers either at Pt/BiFeO₃ interface or on BiFeO₃ surface, also strongly depending on the positions and the annealing conditions of buffer layers. A 500°C‐annealed Bi₂O₃ buffer layer could act as a Bi source for compensating Bi volatilization and a diffusion barrier for species from BiFeO₃. A near stoichiometric BiFeO₃ with less defects and substrate contamination was obtained by employing a 500°C‐annealed Bi₂O₃ buffer layer in between Pt substrate and BiFeO₃. The structure change in BiFeO₃ led by such a buffer layer should result from the interfacial constraint between buffer layer and BiFeO₃. Furthermore, this crystalline BiFeO₃ specimen exhibited a highly (100)‐textured, where this preferred orientation was attributed to the accumulation of Bi at Pt/BFO interface. Therefore, the Pt/500°C‐annealed Bi₂O₃/BiFeO₃/Pt thin film exhibited the good ferroelectric and magnetic properties. As compared to the usual method for controlling BiFeO₃ composition by adding excess Bi, this study indicates the more advantages using a Bi₂O₃ buffer layer.     

Enhanced Cycle Stability of Magnetite/Carbon Nanoparticles for Li Ion Battery Electrodes

We demonstrate a facile synthesis of monodisperse magnetite (Fe₃O₄) nanoparticles (NPs) via a simple wet chemical route at 180°C using oleylamine (C₁₈H₃₇N), which serves as a solvent, ligand, and surfactant. The particles have a narrow size distribution centered at about 10 nm. To provide better electron conductivity and structural stability, the as‐synthesized particles are given a carbon nanocoating by pyrolysis of the residual surfactant on their surface. This pyrolysis forms a uniform thin nanocoating on each particle, and a core/shell Fe₃O₄/carbon NP network was thus obtained. The core/shell Fe₃O₄/carbon electrode shows better reversible capacity, cycle life, and rate capability than a bare Fe₃O₄ NP electrode because of its efficient electron transport and stress relaxation provided by the thin carbon layer.     

Low‐temperature molten salt synthesis of forsterite powders with controllable morphology

Forsterite powders with controllable morphology were synthesized using oxides as raw materials in NaCl–KCl molten salt media. The effects of MgO/SiO₂ ratio, calcining temperature, and salt/oxide ratio on the phase composition and morphology of the powders are investigated. The results indicate that single‐phase forsterite powders can be synthesized from a mixture of MgO and SiO₂ with a MgO/SiO₂ molar ratio of 2:1.3 at 700°C. With the increase in calcining temperature, the powders obtained changes from an irregular to a columnar morphology. In addition, the morphology of the forsterite powders produced can also be controlled by altering the salt/oxide ratio.     

Influence of post-synthesis NaCl flux treatment on the magnetic properties of jet-milled SrFe12O19 powders

A post-synthesis NaCl flux treatment was carried out on jet-milled strontium hexaferrite, SrFe₁₂O₁₉ powders prepared by conventional high-temperature solidstate reaction starting from SrCO₃ and Fe₂O₃. Microstructural studies reveal that the adverse effects of jet-milling on the particle morphology like jagged edges and ruptured surfaces have been effectively mitigated by annealing at elevated temperatures in the presence of molten NaCl flux. The coercivity values obtained from angle-dependent M versus H measurements revealed that the coercivity mechanism in the jet-milled powders is dominated by reverse nucleation due to the strain induced during milling. Annealing the powders in presence of NaCl flux changes the coercivity mechanism to exclusively domain rotation. This results in a dramatic increase of coercivity from 1.6 kOe for the jet-milled powders to a maximum of 5.3 kOe for the flux-annealed powders. XPS studies show that the NaCl flux treatment has not altered the chemical state of Fe.     

Microstructure and properties of Co-, Ni-, Zn-, Nb- and W-modified multiferroic BiFeO3 ceramics

BiFeO₃ polycrystalline ceramics were prepared by the mixed oxide route and a chemical route, using additions of Co, ZnO, NiO, Nb₂O₅ and WO₃. The powders were calcined at 700 °C and then pressed and sintered at 800–880 °C for 4 h. High density products up to 96% theoretical were obtained by the use of CoO, ZnO or NiO additions. X-ray diffraction, SEM and TEM confirmed the formation of the primary BiFeO₃ and a spinel secondary phase (CoFe₂O₄, ZnFe₂O₄ or NiFe₂O₄ depending on additive). Minor parasitic phases Bi₂Fe₄O₉ and Bi₂₅FeO₃₉ reduced in the presence of CoO, ZnO or NiO. Additions of Nb₂O₅ and WO₃ did not give rise to any grain boundary phases but dissolved in BiFeO₃ lattice. HRTEM revealed the presence of domain structures with stripe configurations having widths of typically 200 nm. In samples prepared with additives the activation energy for conduction was in the range 0.78–0.95 eV compared to 0.72 eV in the undoped specimens. In co-doped specimens (Co/Nb or Co/W) the room temperature relative permittivity was ～110 and the high frequency dielectric loss peaks were suppressed. Undoped ceramics were antiferromagnetic but samples prepared with Co or Ni additions were ferromagnetic; for 1% CoO addition the remanent magnetization (M_R) values were 1.08 and 0.35 emu/g at temperatures of 5 and 300 K, respectively.