Enhancement of photoelectrochemical performance of BiFeO3 by Sm3+ doping

Bismuth ferrite (BiFeO₃) holds great potentials in the photoelectrocatalysis due to the advantages of low cost, narrow band gap and good chemical stability. However, the photoelectrochemical performance of BiFeO₃ is usually inhibited by the poor charge carrier transport. Here, we report the flame annealing synthesized Sm³⁺ doped BiFeO₃ photocathodes for efficient hydrogen production. Greatly enhanced water reduction activity was found in doped samples. The Bi_0.95Sm_0.05FeO₃ composition exhibited largest photocurrent density of 0.1061 mA cm^?2 at 0 V vs. RHE in 0.5 M Na₂SO₄, which was 5.6 times higher than that of the pristine BiFeO₃. Mott-Schottky analysis and electrochemical impedance spectra proved that the Bi³⁺substitution increased the charge carrier concentration and facilitated the charge migration. Incident photon-to-electron conversion efficiency (IPCE) value for Bi_0.95Sm_0.05FeO₃ film (～6.39% at 325 nm) was approximately ten times higher than that of undoped sample. The high performance can be ascribed to the rational Sm³⁺ doping, which can improve visible light absorption ability, facilitate the charge carrier transport kinetics and hinder the recombination of photogenerated carriers. Our work provides a facile cation doping with rare earth ions to improve the photoelectrochemical performances.     

Enhanced photocatalytic performance through the ferroelectric synergistic effect of p-n heterojunction BiFeO3/TiO2 under visible-light irradiation

A BiFeO₃/TiO₂ p-n heterojunction photocatalyst with ferroelectric synergistic effect under visible-light irradiation was developed through facile hydrolysis and precipitation by forming nanospheres of TiO₂ on BiFeO₃ nanocube to improve the photocatalytic efficiency. Analyses of the microstructure, optical properties, and photoelectrochemical performance indicate the formation of a core–shell heterostructure of BiFeO₃/TiO₂ with excellent energy band matching. The BiFeO₃/TiO₂ p-n heterojunction has enlarged specific surface area, higher sensitivity to visible-light, and improved separation and transfer efficiency of photoelectron-hole pairs than single TiO₂ and BiFeO₃. Moreover, the composite exhibits superior photocatalytic degradation performance for methylene blue (MB) and common antibiotic tetracycline (TC) under UV and visible-light irradiation. The MB degradation rate within 180 min reaches 78.4% and 90.4% under UV and visible-light irradiation, respectively. Furthermore, the enhanced photocatalytic mechanism of BiFeO₃/TiO₂ is explored by photoluminescence (PL), electrochemical impedance spectroscopy (EIS), transient photocurrent analysis, radical quenching, and band structure characterization.     

Structure and multiferroic properties of BiFeO3 powders

Bismuth ferrite powders were synthesized by a simple sol–gel method at the temperature as low as 450 °C. Single phase BiFeO₃ powders with a rhombohedral perovskite structure were fabricated after Bi–Fe gels were calcined at 450–650 °C. Atomic ratio of Bi to Fe is approximately 1:1 for BiFeO₃ powders, as determined by energy dispersive X-ray spectrometer. BiFeO₃ powders show weak ferromagnetism at room temperature and strong size-dependent magnetic properties, which is different from the linear M–H relationship in BiFeO₃ ceramics. Dielectric anomaly at round 330 °C near the magnetic transition point corresponds to the antiferromagnetic to paramagnetic phase transition, indicating the coupling between polarization and magnetization in BiFeO₃ powders. A reversible ferroelectric phase transformation of BiFeO₃ powders has been detected at 827 °C by a differential thermal analysis.     

A visible-light-active BiFeO<Subscript>3</Subscript>/ZnS nanocomposite for photocatalytic conversion of greenhouse gases

Given the changes in environmental conditions in the world, photocatalytic conversion of greenhouse gases is of great interest today. Our aim was to increase the photocatalytic efficiency of BiFeO₃/ZnS (p-n heterojunction photocatalyst) by varying the molar ratio of ZnS to perovskite structure of BiFeO₃ using hydrothermal synthesis. The results of X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), FT-IR spectroscopy showed the small crystal size and suitable distribution of ZnS particles on the BiFeO₃ structure. The results of UV-visible, and photoluminescence (PL) spectroscopy analyses showed the good behavior of p-n heterostructure in absorption of visible light and lowering electron-hole recombination. The best visible light photocatalytic efficiency of CO₂ reduction, 24.8%, was obtained by an equimolar ratio of BiFeO₃/ZnS.     

First principle study on structural,elastic and electronic properties of cubic BiFeO3

We present the first principle studies of the structural and electronic properties in high temperature cubic phase (Pm3m) of BiFeO₃ based on Density Functional Theory (DFT). All calculations are performed within Local Density Approximation (LDA) functional and Generalized Gradient Approximation (GGA) functional with Ultrasoft Pseudopotentials (USP). It shows that the calculated structural parameters of cubic BiFeO₃ are in a good agreement with previous literatures. Based on the calculated of elastic properties of cubic BiFeO₃, this material shows a stable mechanical structure. In electronic band structure, the electron wave propagates through Brillouin zone X–R–M–G–R points where the highest valence band overlap with the lowest conduction band to give zero energy band gaps. The Density of States (DOS) demonstrated the significant hybridization between Bi6p, Fe3d and O2p in the range of ?5 eV–5 eV. Thus, it can be implied that multiferroic BiFeO₃ are metallic at cubic phase and the metal–insulator transition in this material obeys the band theory.     

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.     

The synthesis of Ag/AgCl/BiFeO3 photocatalyst with enhanced visible photocatalytic activity

A novel AgCl/Ag/BiFeO₃ photocatalyst was synthesized via an ultrasonic-assisted precipitation-photoreduction method. X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), High Resolution Transmission Electron Microscopy (HRTEM), UV–vis diffuse reflectance spectroscopy (DRS) and photoluminescence emission spectra (PL) analysis were implemented to characterize the composition, morphology, structure, and optical property of the as-synthesized photocatalyst. For the decomposition of methyl orange (MO) and other organic dyes, AgCl/Ag/BiFeO₃ photocatalyst manifested much superior visible-light catalytic activity than pure BiFeO₃ and AgCl/Ag. Based on the trapping experiments and band structure analysis, a probable Z-scheme light catalytic mechanism was proposed.     

Synergistic flame retardant effect of BiFeO3 in intumescent flame‐retardant polypropylene composites

The BiFeO₃ was used to intumescent flame retardant (IFR) polypropylene (PP) composites as a synergist. The limiting oxygen index (LOI) and UL‐94 tests indicated that there is an optimum synergistic concentration of BiFeO₃ in the PP/IFR composites. Thermogravimetric analysis (TG) results of flame retardant PP showed that the moderate of BiFeO₃ can reduce the decomposition rate of sample at high temperatures. TG of APP/PER/BiFeO₃ showed that BiFeO₃ main affects the third mass loss stage of APP/PER. So the morphology and composition of the char residue of APP/PER/BiFeO₃ composites were characterized by scanning electron microscopy (SEM), X‐ray diffraction (XRD), X‐ray photoelectron spectroscopy (XPS), and laser Raman spectroscopy (LRS). An appropriate amount of BiFeO₃ can react with APP/PER forming Bi O P and Fe O P bond, and so more P elements was involved in a crosslinking reaction to form more stable char residue, which can effectively increase the flame retardant properties of PP. POLYM. COMPOS., 38:2771–2778, 2017. © 2015 Society of Plastics Engineers     

Hydrothermal synthesis of Co3O4 nanosheets and its application in photoelectrochemical water splitting

In this study, Co₃O₄ nanosheets were synthesized through hydrothermal method using cobalt nitrate hexahydrate. X-ray diffraction, diffuse reflectance spectra, Fourier transform infrared spectroscopy, energy-dispersive X-ray spectroscopy, and field emission scanning electron microscopy were applied to investigate the properties of as-synthesized samples. Ultimately, the electrochemical and photoelectrochemical properties were evaluated by Mott–Schottky analysis and measuring photoconversion efficiency of Co₃O₄ nanosheets. The results indicated that Co₃O₄ nanosheets exhibited a maximum efficiency of 0.92% for water electrolysis under simulated 1.5 global sunlight air mass, which further suggests the excellent potential of Co₃O₄ nanosheets for application in hydrogen generation through photocatalytic water splitting.     

Structural,magnetic and optical properties of BiFeO3 synthesized by the solvent-deficient method

Bismuth ferrite (BiFeO₃, BFO) powders were synthesized by a simple and cost-effective solvent-deficient method using bismuth nitrate, iron nitrate, and ammonium bicarbonate as the only precursors. Single phase BiFeO₃ powder was fabricated after the Bi:Fe ratio was adjusted and after the precursor mixture was calcined for one hour at 600 °C. We investigated the formation reactions, crystal structure, particle size distribution, magnetic and optical properties of synthesized BiFeO₃. X-ray diffraction revealed the formation of well-crystallized BFO nanocrystallites starting at a temperature of 450 °C. BiFeO₃ powder calcined at 600 °C showed very weak ferromagnetism at room temperature which is different from the linear M–H relationship in bulk BiFeO₃ ceramics. The remnant magnetization value (Mr) was found to be 5 × 10⁻³ emu g⁻¹ and a coercive field value (Hc) nearly 500 Oe. The UV–visible spectra showed maximum adsorption at ∼464 nm with a derived bandgap value of 1.85 (1.8449 ± 0.0013) eV for BFO nanocrystallites supporting their potential application as visible light-response photocatalysts. Direct bandgap value obtained from reflectance measurement is determined to be 2.25 (2.2464 ± 0.0065) eV.     

Synthesis of pure phase BiFeO3 powders by direct thermal decomposition of metal nitrates

Pure phase BiFeO₃ powders were successfully synthesized by direct thermal decomposition of metal nitrates at 500 °C. The as-prepared BiFeO₃ had a perovskite structure which was studied using X-ray diffraction. The porous structures were investigated through scanning electron microscopy. Morphology of BiFeO₃ changed from micron-sized porous structures to nano-sized particles as NH₃HCO₃ was added. Furthermore, the mechanism of formation of BiFeO₃ was also discussed through X-ray diffraction, thermogravimetry and differential scanning calorimetry. The optical absorption band gap of the micro-sized porous structure BiFeO₃ is 1.8 eV.     

An in situ resonant photoemission and x-ray absorption study of the BiFeO3 thin film

Multiferroic bismuth ferrite (BiFeO₃) thin films were prepared using the pulsed laser deposition (PLD) technique. The electronic structure of the film was studied in situ using photoemission spectroscopy (PES) and x-ray absorption spectroscopy (XAS). Both the Fe 2p PES and XAS spectra show that the Fe ions were initially in a+3 valence state. The Fe 2p and O K edge XAS spectra indicate that the oxygen octahedral crystal ligand field divides the unoccupied Fe 3d state into t_2g↓and e_g↓states. Valence band Fe 2p–3d resonant photoemission results indicate that hybridization between Fe 3d and O 2p plays an important role in the multiferroic properties of BiFeO₃ thin films.     

Investigation of transport behavior in Ba doped BiFeO3

Bi_1?xBa_xFeO₃ (x = 0.00–0.25) samples were prepared by conventional solid state reaction method. X-ray diffraction revealed the rhombohedrally distorted perovskite structure for undoped BiFeO₃ with a phase transition from rhombohedral to pseudo cubic on Ba substitution. The leakage current density of 10% Ba substituted sample is found to be four orders of magnitude less than that of the pure BiFeO₃. Grain boundary limited conduction and space charge limited conduction mechanisms are involved in low and high electric field regions respectively for all the samples except 10% Ba doped BFeO₃ which obeys grain boundary limited conduction mechanism in whole of the electric field range. Dielectric measurements showed that the dielectric constant and dielectric loss attained their minimum values at 10% Ba substitution. Thus 10% Ba is found to be optimum concentration to have better multiferroic properties. Undoped BiFeO₃ and 5% Ba doped samples have very large values of dielectric constants and leakage current densities which can be attributed to a large number of oxygen vacancies in these samples, indicating an extrinsic response of these compositions.     

Facile Synthesis,Characterization, Catalytic and Photocatalytic Activity of Multiferroic BiFeO3 Perovskite Nanoparticles

We report the synthesis of multiferroic BiFeO₃ perovskite nanoparticles using the microwave combustion technique. Phase evolution is investigated by XRD, which confirms that the formation of a secondary α-Bi₂O₃ phase with a monoclinic structure along with the existing rhombohedral (BiFeO₃) structure. The average crystalline size has been found at 50 nm. The optical band gap was calculated from the Tauc’s plot it has been found 2.18 eV. The appearances of FT-IR spectra revealed bands at 550 and 444 cm^?1 were correlated to the rhombohedral stretching modes of BiFeO₃ nanostructure. The surface morphology showed the formation of nanosized grains with pores. The magnetization-Field (M-H) hysteresis curves revealed the appearance of ferrimagnetic behavior at room temperature. The BET surface area of BiFeO₃ perovskite nanoparticles was found 44.86 m²/g. The as-fabricated BiFeO₃ perovskite nanoparticles were investigated for their superior catalytic activity in two applications, which include (i) Glycerol to formic acid oxidation in the liquid phase with a high efficiency of over 98 percent, (ii) Under visible light, the photocatalytic breakdown of rhodamine B achieved maximal efficiency (almost 99 percent). Finally, we concluded that the BiFeO₃ perovskite nanoparticles exhibit high performance in future multifunctional devices is demonstrated by the simultaneous enhancement of catalytic and photocatalytic activities.

     

Facile green synthesis of ytrium-doped BiFeO3 with highly efficient photocatalytic degradation towards methylene blue

Bismuth ferrite (BiFeO₃) is considered as one of the most promising materials in the field of multiferroics with great potentials in photocatalysis due to their excellent properties of relatively small band gap, stable structures, and low cost. In this work, a facile green route was successfully used for the fabrication of high-purity yttrium-doped and undoped bismuth ferrite (BiFeO₃) nanoparticles. κ-carrageenan seaweed was used as a biotemplate for the construction of the material. The obtained products were characterized and the photocatalytic effect of doped and undoped BiFeO₃ were evaluated on the degradation of methylene blue (MB) under direct sunlight. The formed particles are in the range of 80–90 nm that exhibited morphology of rhombohedral perovskite structure as confirmed by FESEM and HRTEM analysis. Decreasing of band gap energy from 2.07 eV to 2.05 eV as the concentration of yttrium dopant increased significantly affected their photocatalytic behaviour. There was a remarkable improvement in the photocatalytic activity of 1% of yttrium-doped BiFeO₃ towards the decomposition of methylene blue (MB) under direct sunlight irradiation. This was attributed to the strong absorption of visible light and the effective separation of photoinduced e− and h + pair, as compared to the pristine BiFeO₃. In addition, the influence of operational parameters on the removal efficiency of MB, such as catalyst dosage and initial dye concentrations, was optimized as a function of time. The kinetics of the photocatalytic MB removal was later found to follow Langmuir Hinshelwood model.     

Ethanol‐Assisted Hydrothermal Synthesis and Characterization of BiFeO3 Nanopowders

Well‐crystallized pure BiFeO₃ nanopowders were successfully synthesized at the temperature as low as 120°C by an ethanol‐assisted hydrothermal process. In this synthesis, the composition of the solvent played important roles in the formation of pure BiFeO₃. The BiFeO₃ nanopowders synthesized with 4:3 ethanol/water ratio mainly consists of cubic structures with size from 50 to 150 nm. Zero‐field‐cooled (ZFC) and field‐cooled (FC) magnetization measurements indicated that pure BiFeO₃ nanopowders showed a spin‐glass transition below the freezing temperature. Moreover, the BiFeO₃ nanopowders exhibited ferromagnetic order at room temperature.     

Influence of Eu and Sr co-substitution on multiferroic properties of BiFeO3

Pure BiFeO₃ (BFO), and Eu-Sr co-substituted BFO samples were prepared by a sol–gel method. The effects of Eu and Sr codoped on the structural, morphological, magnetic and ferroelectric properties were systematically investigated. The X-ray diffraction and Fourier transform infrared spectroscopy reveal that substitution of Eu and Sr at the Bi site results in structural change and single phase formation. The maximum remnant magnetization of 0.287 emu/g and coercive field of 10.305 kOe are observed in the Bi_0.85Eu_0.05Sr_0.10FeO₃ sample. The suppression of spin cycloid caused from the structural distortion can play an important role in the improvement of magnetic properties. The Eu and Sr co-doped samples also exhibit good ferroelectric properties, which may be attributed to suppressing the formation of oxygen vacancies by Eu substitution.     

Enhancement of ferromagnetic and ferroelectric properties in calcium doped BiFeO3 by chemical synthesis

Calcium (Ca)-doped bismuth ferrite (BiFeO₃) thin films prepared by using the polymeric precursor method (PPM) were characterized by X-ray diffraction (XRD), atomic force microscopy (AFM), polarization and magnetic measurements. Structural studies by XRD and Rietveld refinement reveal the co-existence of distorted rhombohedral and tetragonal phases in the highest doped BiFeO₃ (BFO) where enhanced ferroelectric and magnetic properties are produced by internal strain. A high coercive field in the hysteresis loop is observed for the BiFeO₃ film. Fatigue and retention free characteristics are improved in the highest Ca-doped sample due to changes in the crystal structure of BFO for a primitive cubic perovskite lattice with four-fold symmetry and a large tetragonal distortion within the crystal domain.     

3D/2D ZnIn2S4/BiFeO3 as S-scheme heterojunction photocatalyst for boosted visible-light hydrogen evolution

Photocatalytic H₂ evolution technique has been proved to be one of the promising approaches to overcome the present energy and environmental issues caused by the combustion of fossil fuel. Constructing heterojunction can realize the efficient separation and migration of charges and thus achieve enhanced H₂ evolution performance. Herein, we designed and prepared a ZnIn₂S₄/BiFeO₃ heterojunction photocatalyst with a 3D/2D structure via an ultrasonic self-assembly process. The typical 3D/2D structure with intimate interface was obtained, which not only provided more active sites but also boosted the migration of photogenerated charges. The optimal mass ratio of BiFeO₃ in ZnIn₂S₄/BiFeO₃ was determined to be 10%, and a 10.5-fold increase in H₂ evolution rate in comparison with of pure ZnIn₂S₄ was achieved. Furthermore, the ZnIn₂S₄/BiFeO₃ composite exhibited excellent recyclability and structural stability based on cycling experiment. A S-scheme heterojunction mechanism was revealed according to the experimental results of photocatalytic H₂ evolution and electrochemical tests.     

Intensifying the photocatalytic degradation of methylene blue by the formation of BiFeO3/Fe3O4 nanointerfaces

A novel highly efficient photocatalyst composite BiFeO₃/Fe₃O₄ has been synthesized by mechanosynthesis and applied to the degradation of Methylene Blue under visible light. Structural, optical and photocatalytic properties of the proposed photocatalyst composites are carefully investigated. The nanointerfaces, associated to ferrous Fe²⁺ ions of the Fe₃O₄ nanoparticles, improve the photocatalytic efficiency when compared with pure BiFeO₃ or Fe₃O₄. The time required to the complete degradation of Methylene Blue solution is 40 min for the sample with 20% of Fe₃O₄ which is more than 7 times faster than the time required using BiFeO₃ alone. Moreover, with the addition of H₂O₂ a complete degradation is achieved just after 10 min, which is faster than any other photocatalytic reaction reported for BiFeO₃-based materials. This enhancement is assumed to be related to an electron drain process due to the difference between energy levels of the conduction bands of BiFeO₃ and Fe₃O₄ combined with the direct Fenton-like process associated with the Fe²⁺ ions of the composites.