Phase and morphology evolution of bismuth ferrites via hydrothermal reaction route

Phase-controlled synthesis of bismuth ferrites has been achieved via hydrothermal route by adjusting the KOH concentration. The as-prepared powders were characterized by X-ray diffraction, field-emission scanning electron microscopy, transmission electron microscopy, high-resolution transmission electron microscopy, and selected area electron diffraction. The particle morphologies of the as-prepared powders evolve from nanoflakes, to self-assembled particles, and microparticles when the concentration of KOH was changed from 1.5 M, 2.5 M, to 3.5 M, and 5 M. Correspondingly, the main phase of the samples changed from orthorhombic Bi₂Fe₄O₉, both Bi₂Fe₄O₉ and BiFeO₃, to pure rhombohedral BiFeO₃. On the basis of these experiments, the phase formation and morphology evolution mechanism of the samples are discussed. Furthermore, the photocatalytic activity of the as-prepared samples was investigated by the photo-degradation of rhodamine-B solution.     

Boosting electron population in δ-Bi2O3 through iron doping for improved photocatalytic activity

We successfully stabilize the delta phase of bismuth oxide (δ-Bi₂O₃) down to room temperature through a simple hydrothermal method. The prepared δ-Bi₂O₃, which is composed of ultrathin nanosheets, possesses excellent crystallinity. When δ-Bi₂O₃ is doped with Fe³⁺, the photocatalytic activity for the mineralization of a number of refractory organic compounds is improved. The improved photocatalytic activity is caused by an increased electron population upon the Fe³⁺ doping. Additional thermal treatment through a post-calcination at 500?°C under air atmosphere results in further increases of photocatalytic activity due to the improved crystallinity, extended light absorption, and better incorporation of Fe³⁺ into the host lattice.     

Synthesis of Bi2Fe4O9/reduced graphene oxide composite by one-step hydrothermal method and its high photocatalytic performance

Visible-light-driven degraded organic pollutant with high efficiency is crucial in the current photocatalysis research. A new kind composite photocatalyst with high visible-light photocatalytic activity which consists of Bi₂Fe₄O₉ and reduced graphene oxide (RGO) has been synthesized through one-step hydrothermal method at low temperature. Pure Bi₂Fe₄O₉ was formed with the addition of graphene oxide (GO) when the concentration of NaOH is 12 mol/L (M) at 180 °C for 72 h hydrothermal reaction. At the same time, the GO was reduced to RGO and adsorbed on the surface of Bi₂Fe₄O₉. The resultant composite photocatalyst showed higher absorption not only in the UV range but also in the visible light than pure Bi₂Fe₄O₉ indicating more electron–hole pairs generated. The band gap of photocatalysis was reduced from 1.91 to 1.69 eV and recombination of photo-generated electron–hole pairs in composites were decreased through marrying RGO with Bi₂Fe₄O₉. As a result, the Bi₂Fe₄O₉/RGO composite photocatalyst displayed higher catalytic activity for the degradation of methyl violet under visible light irradiation than rare Bi₂Fe₄O₉, promising the use of the Bi₂Fe₄O₉/RGO composite in visible-light photocatalysis.     

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.     

A Highly Solvent‐Stable Metal–Organic Framework Nanosheet: Morphology Control,Exfoliation, and Luminescent Property

Compared to bulk metal–organic framework (MOF), 2D MOF nanosheets have gained intensive research attention due to their ultrathin thickness and large surface area with highly accessible active sites. However, structural deterioration and morphological damage have impeded producing high‐quality MOF nanosheets during exfoliation. Here, first a new layered bulk MOF ZSB‐1 is synthesized and several solvents such as isopropanol, methanol, n‐hexyl alcohol, and N,N‐dimethylformamide are surveyed to examine their performance for the exfoliation of layered ZSB‐1. As a result, a highly solvent‐stable metal–organic framework rectangular nanosheet retaining undamaged morphology is obtained by the soft‐physical method in n‐hexyl alcohol. Theoretical simulations reveal that the strong interaction energy between n‐hexyl alcohol and MOF layers is responsible for the best exfoliation performance of making the bulk MOF into nanosheets. In addition, ZSB‐1 shows a tunable fluorescence peak position, fluorescent lifetime, and quantum yield by simply changing the solvent and morphology. Besides, the ZSB‐1 was selected as a fluorescence sensor to detect metal ions, and ZSB‐1 nanosheet exhibits excellent sensing ability for Fe³⁺. It is worth noting that the ZSB‐1 nanosheet has better detection limit performance of 0.054 × 10^?6 m than that of its bulk counterpart.     

A study of sintering and magnetic parameters of spinel lithium ferrite: II. Lithium ferrite containing Bi2O3

High-dispersity powders of spinel lithium ferrite, Li_0.5Fe_2.5O₄, containing different amounts of Bi₂O₃ were prepared by thermal treatment of mixtures of /Fe₃(HCOO)₆(OH)₂/HCOO.4H₂O, LiHCOO.H₂O and Bi(HCOO)₃ obtained by spray drying. It was found that sintering of lithium ferrite in the presence of Bi₂O₃ leads to very high densities of the products at 900 – 1000°C. This is due to the formation, by Li_0.5Fe_2.5O₄ and Bi₂O₃, of an easily melting eutectic. The study of the magnetic properties showed that the presence of Bi₂O₃ ensured the formation of lithium ferrites with good characteristics at considerably lower temperatures than those usually observed.     

Optimizing phase and microstructure of chemical solution-deposited bismuth ferrite (BiFeO3) thin films to reduce DC leakage

Polycrystalline bismuth ferrite (BiFeO₃ or BFO) thin films were prepared by chemical solution deposition to explore the impact of processing conditions including annealing temperature, percent excess bismuth, and gel drying temperature on film microstructure and properties. Incorporating 0–5 % excess Bi and annealing at 550 °C in air produced stoichiometric single-phase BiFeO₃ films. Deviation from this temperature yielded the bismuth-rich Bi₃₆Fe₂O₅₇ phase at temperatures below 550 °C or the bismuth-deficient Bi₂Fe₄O₉ phase at temperatures above 550 °C, both of which contributed to higher DC leakage. However, even single-phase BiFeO₃ films produced at 550 °C show high DC leakage (~1.2 × 10^?1 A/cm² at 140 kV/cm) due to a porous microstructure. We have thus investigated unconventional thermal treatments that significantly increase film densification while maintaining phase purity. Under these revised thermal treatment conditions, room temperature leakage current values are reduced by three orders of magnitude to ~1.0 × 10^?4 A/cm² at 140 kV/cm.     

Effect of processing temperature on characteristics of metal-ferroelectric (BiFeO3)-insulator (HfLaO)-silicon capacitors

The effect of temperature in rapid thermal annealing (RTA) process on the physical and electrical properties of bismuth ferrite ceramic thin films on HfLaO/p-Si substrates has been investigated. In metal-ferroelectric-insulator-silicon (MFIS) capacitors, the high-k HfLaO dielectric layer was prepared as the insulator layer. On HfLaO/Si substrates the bismuth ferrite thin film was fabricated via sputtering process with a BiFeO₃ (BFO) target at room temperature followed by RTA. The RTA temperature ranged from 500 to 700 °C. It is found that the root mean square roughness of ceramic films increases for high-temperature process. The maximum ferroelectric memory window is 1.6 V obtained from a sweep voltage of ± 4 V at the lowest RTA temperature of 500 °C. This good ferroelectric memory performance can be attributed to the low leakage current as a result of smooth surface of nanocrystalline ferroelectric BFO and Bi₂Fe₄O₉ thin films.     

Effects of alkaline-earth dopants on structural,optical and magnetic properties of Bi<Subscript>2</Subscript>Fe<Subscript>4</Subscript>O<Subscript>9</Subscript> powders

The pure Bi₂Fe₄O₉ and Bi_{2 (1?x)} A_2xFe₄O₉ (A?=?Ca, Ba, x?=?0.03) powders are synthesized via a modified solid-state reaction method to study the effects of alkaline-earth metal ions doping on crystal structural, optical and magnetic properties. Both X-ray diffraction and Raman spectroscopy data reveal that all the powders are Mullite-type Bi₂Fe₄O₉ orthorhombic single phase without any impurities. Much greater structural distortion in Bi_1.94Ba_0.06Fe₄O₉ than that of Bi_1.94Ca_0.06Fe₄O₉ is observed. The chemical compositions of Ba²⁺ and Ca²⁺ doped powders have been investigated with energy dispersive X-ray spectroscopy (EDS). X-ray photoelectron spectroscopy results indicate that oxygen vacancies could be found in all doped powders. The ratio of Fe²⁺ in the total Fe ions is almost unchanged by Ca doping and increases a little with Ba substitution. Compared with that of pure Bi₂Fe₄O_9, the band gap values decrease slightly in Bi_1.94Ca_0.06Fe₄O₉ but drop dramatically in Bi_1.94Ba_0.06Fe₄O₉. A clear and obvious ferromagnetic behavior is found in Bi_1.94Ba_0.06Fe₄O₉ at 10 K. However, Bi_1.94Ca_0.06Fe₄O₉ shows a weak ferromagnetism with enhanced magnetization and Bi₂Fe₄O₉ exhibits antiferromagnetism with a linear M–H relationship. The varied bandgap and magnetization resulting from the alkaline-earth metal ionic species are discussed in terms of structural distortion due to the ionic radius size effect.     

Self-Sacrificing Template Synthesis of Carbon Nanosheets Assembled Hollow Spheres with Abundant Active Fe–N4O1 Moieties for Electrocatalytic Oxygen Reduction

Single-atom Fe–N–C (Fe₁–N–C) materials represent the benchmarked electrocatalysts for oxygen reduction reaction (ORR). However, single Fe atoms in the carbon skeletons cannot be fully utilized due to the mass transfer limitation, severely restricting their intrinsic ORR properties. Herein, a self-sacrificing template strategy is developed to fabricate ultrathin nanosheets assembled Fe₁–N–C hollow microspheres (denoted as Fe₁/N-HCMs) by rational carbonization of Fe³⁺ chelating polydopamine coated melamine cyanuric acid complex. The shell of Fe₁/N-HCMs is constructed by ultrathin nanosheets with thickness of only 2 nm, which is supposed to be an ideal platform to isolate and fully expose single metal atoms. Benefiting from unique hierarchical hollow architecture with highly open porous structure, 2 nm-thick ultrathin nanosheet subunits and abundant Fe–N₄O₁ active sites revealed by X-ray absorption fine structure analysis, the Fe₁/N-HCMs exhibit high ORR performance with a positive half-wave potential of 0.88 V versus the reversible hydrogen electrode and robust stability. When served as air-cathode catalysts with ultralow loading mass of 0.25 mg cm⁻², Fe₁/N-HCMs based Zn–air batteries present a maximum power density of 187 mW cm⁻² and discharge specific capacity of 806 mA h g_Zn⁻¹ in primary Zn–air batteries, all exceeding those of commercial Pt/C.     

Assembly of WO<Subscript>3</Subscript> nanosheets/Bi<Subscript>24</Subscript>O<Subscript>31</Subscript>Br<Subscript>10</Subscript> nanosheets composites with superior photocatalytic activity for degradation of tetracycline hydrochloride

In this work, a high-performance composite photocatalyst composed of WO₃ nanosheets and Bi₂₄O₃₁Br₁₀ nanosheets was successfully synthesized. The photocatalytic activity of the obtained samples was studied by the degradation of tetracycline hydrochloride under visible light irradiation. The results showed that Bi₂₄O₃₁Br₁₀ modified with the appropriate amount of WO₃ nanosheet exhibits higher catalytic activity and stability during the photocatalytic processes, and the holes (h⁺) is involved in the photolysis reaction as the main active species. The crystallization, morphology, optical and electrochemical properties of the as-prepared composite photocatalyst were characterized, and the mechanism of high photocatalytic activity was also explored. The optimal sample (10%-WO₃/Bi₂₄O₃₁Br₁₀) exhibited the best performance for tetracycline hydrochloride (TC) degradation, and more than 80% of the TC was degraded after 60 min under light irradiation. The degradation rate constant k was about 3.34-fold and 1.54-fold higher than pure WO₃ and Bi₂₄O₃₁Br₁₀, respectively. Its high photocatalytic performance can be attributed to the following reasons: the appropriate conduction band and valence band positions between WO₃ and Bi₂₄O₃₁Br₁₀, the close contact between the two visible light-driven photocatalysts, and the effective separation of the spatial charge. Our work may help to further expand the potential application of oxygen-rich bismuth oxyhalides photocatalyst in wastewater treatment, and provide a new strategy for the modification of nanostructured photocatalysts.     

Bi4Ti3O12–(Ni0.5Zn0.5)Fe2O4 dielectric–ferromagnetic ceramic composites synthesized with nanopowders

Bi₄Ti₃O₁₂ and (Ni_0.5Zn_0.5)Fe₂O₄ nanopowders were prepared by co-precipitation and hydrothermal methods, respectively. It was found that ethanolamine is effective as a precipitating agent in the synthesis of Bi₄Ti₃O₁₂ nanopowders via co-precipitation, and it also plays an important role in the synthesis of (Ni_0.5Zn_0.5)Fe₂O₄ nanopowders. Bi₄Ti₃O₁₂–(Ni_0.5Zn_0.5)Fe₂O₄ multiferroic ceramics were obtained by sintering the as-prepared nanopowders. Lower sintering temperatures (800–900 °C) were available when compared with the traditional solid state method and ceramic composites with higher density and limited interfacial reaction were obtained. The ceramics also showed lower dielectric loss and higher magnetic moments. Both the ferroelectric and magnetic phases preserve their individual properties in bulk composite form and thus, these types of composite ceramics appear to be good candidate multiferroic materials.     

Mechanochemical activation of starting oxide mixtures for solid-state synthesis of BiFeO3

We have studied the effect of mechanochemical activation on the reactivity of Bi₂O₃ + Fe₂O₃ oxide mixtures, assessed the temperature effect on the kinetics of BiFeO₃ formation in the oxide mixtures before and after mechanochemical activation, and optimized the conditions of bismuth ferrite synthesis by solid-state reaction. A material with a BiFeO₃ content of at least 98.3 wt % has been obtained, and its magnetic properties have been investigated.     

Effect of chromium substitution on structure and magnetic properties of Bi2Fe4O9

Orthorhombic Bi₂Fe_4 − xCr_xO₉ (x = 0.0, 0.25, and 0.75) nanoplatelets were synthesized by a simple hydrothermal method. The structure, morphology, and magnetic properties of the obtained powders have been characterized. Calculation of the lattice parameters of Bi₂Fe_4 − xCr_xO₉, as well as bond lengths and angles, was carried out by X-ray diffraction Rietveld refinement. The volumes of the metal-oxygen tetrahedra and octahedra were calculated to be sequentially increasing as the Cr doping level increases. The samples undergo an antiferromagnetic transition at 250 ± 5 K. The magnetic moments of the samples increase with higher Cr doping level. The 3d electron spin state for Fe³⁺ in the as-prepared samples is different, which is possibly due to the distortion of Fe-O tetrahedra and octahedra in the crystal structure after chromium substitution.     

Synthesis and characterization of manganese ferrite nanostructure by co-precipitation,sol-gel,and hydrothermal methods

ABSTRACT

In this research work, manganese ferrite nanoparticles (MnFe₂O₄) were synthesized by three different methods including the co-precipitation, sol-gel, and hydrothermal route. Structure, size, morphology, and magnetic properties of nanostructures were determined and compared using X-ray diffraction, Fourier-transform infrared spectroscopy, field emission scanning electron microscopy analysis (FESEM), and the vibration sample magnetometer (VSM). X-ray diffraction analysis from Debye–Scherrer’s formula with the (2θ?=?35.08°) peak indicated that the mean size of the synthesized manganese ferrite nanocrystallites were obtained to be 36, 45, and 16?nm for co-precipitation, sol-gel, and hydrothermal, respectively. Also, the sample prepared by the hydrothermal method has the lowest crystal sizes, which it is approved by FESEM analysis. Field emission scanning electron microscopy analysis images confirmed the existence of three types of basic morphology of MnFe₂O₄ nanoparticles: spherical shape, multi-walled hollow nanosheets, and reticular structure. In addition, Based on VSM data magnetization saturation (M_s) was 41.89?emu/g for hydrothermal synthesized samples, 38.76?emu/g for co-precipitation samples, and 9.52?emu/g for sol-gel samples. These findings show that various methods of nanoparticle synthesis can lead to different particle sizes and magnetic properties.     

Preparation and structural characterization of bismuth ferrite crystals of different morphological types

We have studied conditions for bismuth ferrite, BiFeO₃, crystallization from off-stoichiometric Bi₂O₃-Fe₂O₃ melts, obtained crystals of different morphological types (dendritic and faceted pseudocubic) up to 3 mm in size, and characterized them by a variety of techniques (X-ray diffraction, IR spectroscopy, differential thermal analysis, X-ray structure analysis, and X-ray microanalysis).     

Hydrothermal formation of Ni-Zn ferrite from heavy metal co-precipitates

The hydrothermal synthesis of Ni-Zn ferrite from simulated wastewater containing Ni²⁺ and Zn²⁺ ions has been studied. The influence of co-precipitation order, the existence of Na⁺ in suspension, the hydrothermal reaction time and temperature on the composition, morphology and saturation magnetization (_s) of the hydrothermal products is reported. Adding the simulated wastewater to the NaOH solution can prevent the formation of -Fe₂O₃ in the Ni-Zn ferrite. Increasing the hydrothermal reaction time improved the magnetization of the Ni-Zn ferrite, while the influence of temperature, stirring intensity and Na⁺ in suspension on the hydrothermal formation of ferrite were not obvious. Thermodynamic calculation indicated that under hydrothermal conditions (180–240°C), the order of chemical stability is as follows: NiFe₂O₄ > Fe₂O₃ > Na₂Fe₂O₄. The high Gibbs formation energy of Na₂Fe₂O₄ prevented the incorporation of Na⁺ into the ferrite lattice.     

Comparative X-ray diffraction and Mössbauer spectroscopy studies of BiFeO3 ceramics prepared by conventional solid-state reaction and mechanical activation

The aim of this work was to prepare BiFeO₃ by modified solid-state sintering and mechanical activation processes and to investigate the structure and hyperfine interactions of the material. X-ray diffraction and Mössbauer spectroscopy were applied as complementary methods. In the case of sintering, BiFeO₃ phase was obtained from the mixture of precursors with 3 and 5 % excess of Bi₂O₃ during heating at 1023 K. Small amounts of impurities such as Bi₂Fe₄O₉ and sillenite were recognized. In the case of mechanical activation, the milling of stoichiometric amounts of Bi₂O₃ and Fe₂O₃ followed by isothermal annealing at 973 K resulted in formation of the mixture of BiFeO₃, Bi₂Fe₄O₉, sillenite and hematite. After separate milling of individual Bi₂O₃ and Fe₂O₃ powders, mixing, further milling and thermal processing, the amount of desired BiFeO₃ pure phase was significantly increased (from 70 to 90 %, as roughly estimated). From Mössbauer spectra, the hyperfine interaction parameters of the desired BiFeO₃ compound, paramagnetic impurities of Bi₂Fe₄O₉ and sillenite were determined. The main conclusion is that the lowest amount of impurities was obtained for BiFeO₃ with 3 % excess of Bi₂O₃, which was sintered at 1023 K. However, in the case of mechanical activation, the pure phase formed at a temperature by 50 K lower as compared to solid-state sintering temperature. X-ray diffraction and Mössbauer spectroscopy revealed that for both sintered and mechanically activated BiFeO₃ compounds, thermal treatment at elevated temperature led to a partial eliminating of the paramagnetic impurities.     

Multifunctional Co0.85Se‐Fe3O4 Nanocomposites: Controlled Synthesis and Their Enhanced Performances for Efficient Hydrogenation of p‐Nitrophenol and Adsorbents

A new kind of multifunctional Co_0.85Se‐Fe₃O₄ nanocomposites is synthesized by loading Fe₃O₄ nanoparticles (NPs) with a size of about 5 nm on the surface of Co_0.85Se nanosheets under hydrothermal conditions without using any surfactant or structure‐directing agents. The Co_0.85Se‐Fe₃O₄ nanocomposite exhibits remarkable catalytic performance for hydrogenation of p‐nitrophenol (4‐NP) at room temperature and good adsorption behavior for methylene blue trihydrate in water. This nanocomposite also shows a high specific surface area and magnetic separation capability for recyclable utilization. The enhanced performances both in catalysis and adsorption are better than either individual component of Co_0.85Se nanosheets or Fe₃O₄ nanoparticles, demonstrating the possibility for designing new multifunctional nanocomposites with improved performances for catalysis, adsorbents, and other applications.     

Hydrothermal synthesis and magnetic properties of Co0.2Cu0.03Fe2.77O4 nanoparticles

Co_0.2Cu_0.03Fe_2.77O₄ nanoparticles with different morphologies have been synthesized directly via a simple hydrothermal method. The effects of pH value, precursor concentration, reaction temperature and surfactant on the particle size were discussed. X-ray diffraction analyses showed that the as-synthesized Co_0.2Cu_0.03Fe_2.77O₄ nanoparticles possessed typical spinel structure. Scanning electron microscope images showed different morphologies of the particles, including truncated octahedron and octahedron. It was indicated that well-dispersed Co_0.2Cu_0.03Fe_2.77O₄ nanoparticles can be synthesized at pH values ranging from 11 to 13, and reaction temperature of 160 °C. The particle size decreased from 18 to 10 nm after the addition of sodium dodecyl sulphate at the pH value of 9. The magnetic measurement showed that the as-prepared Co-Cu spinel ferrite nanoparticles possessed hard magnetic property.