Study on carbon quantum dots/BiFeO<Subscript>3</Subscript> heterostructures and their enhanced photocatalytic activities under visible light irradiation

Carbon quantum dots/Bismuth ferrite (CQDs/BiFeO₃) composite materials were successfully synthesized by a facile hydrothermal treatment of Fe(NO₃)₃·9H₂O, Bi(NO₃)₃·5H₂O and CQDs solutions. The structural and optical characteristics of the composite materials were characterized by X-ray diffraction, Fourier transform infra-red spectroscopy, transmission electron microscopy and ultraviolet–visible absorption, respectively. The photocatalytic activities of pure BiFeO₃, CQDs and CQDs/BiFeO₃ composite materials had also been carried out by using Rhodamine B as test stuff. The experimental results indicated that for QDs/BiFeO₃ composite materials, the CQDs were attached to the surfaces of BiFeO₃ materials, CQDs and BiFeO₃ belong to different phase. Owing to the heterojunction formed at the interface between CQDs and BiFeO₃ materials together with CQDs as an electron reservoir, the photocatalytic activities of CQDs/BiFeO₃ composite materials were significantly improved. Especially, the CQDs/BiFeO₃ composite sample with 3.3 wt% CQDs has the highest degradation rate, which was about 7.3, 3.7 times higher than those of pure BiFeO₃ and CQDs, respectively. Moreover, the mechanism of RhB degradation catalyzed by CQDs/BiFeO₃ composite materials was also thoroughly explained.     

Enhanced magnetic properties of chemical solution deposited BiFeO3 thin film with ZnO buffer layer

Magnetic properties of BiFeO₃ films deposited on Si substrates with and without ZnO buffer layer have been studied in this work. We adopted the chemical solution deposition method for the deposition of BiFeO₃ as well as ZnO films. The x-ray diffraction measurements on the deposited films confirm the formation of crystalline phase of BiFeO₃ and ZnO films, while our electron microscopy measurements help to understand the morphology of few micrometers thick films. It is found that the deposited ZnO film exhibit a hexagonal particulate surface morphology, whereas BiFeO₃ film fully covers the ZnO surface. Our magnetic measurements reveal that the magnetization of BiFeO₃ has increased by more than ten times in BiFeO₃/ZnO/Si film compared to BiFeO₃/Si film, indicating the major role played by ZnO buffer layer in enhancing the magnetic properties of BiFeO₃, a technologically important multiferroic material.     

Large-scale growth and shape evolution of bismuth ferrite particles with a hydrothermal method

Large-scale polyhedral bismuth ferrite (BiFeO₃) particles were synthesized with a hydrothermal method under a series of experimental conditions. X-ray diffraction revealed that the BiFeO₃ powders had a perovskite structure. Scanning electron microscopy images showed different BiFeO₃ particles were formed, including sphere, octahedron, truncated octahedron, cubo-octahedron and truncated cube. The experimental results showed that the concentration of KOH, reaction time, heating and cooling rates had important impacts on the size and morphology of the BiFeO₃ particles. The formation mechanism and change process of the large-scale polyhedral BiFeO₃ particles were discussed in detail. The obtained BiFeO₃ showed ferroelectric behavior and magnetic response, which approved the multiferroic property of the BiFeO₃ crystallization. The optical behaviors of BiFeO₃ particles revealed the band gap energy of about 2 eV, which is smaller than the BiFeO₃ bulk due to the nano-crystallites.     

Structural, dielectric and magnetic characterization of large scale template synthesized Gd doped BiFeO3 nanowires

Pure and 10 % Gd doped BiFeO₃ nanowires of 100-nm diameter have been synthesized by sol–gel template-assisted technique. Phase-dependent structural, dielectric and magnetic properties of pure and Gd doped BiFeO₃ nanowires have been investigated. X-ray diffraction study reveals that pure BiFeO₃ nanowires possess rhombohedral structure while 10 % Gd doped BiFeO₃ nanowires are orthorhombic in nature. Magnetic study confirms that the value of saturation magnetization, increased with structural change via doping of Gd in host BiFeO₃.     

Low-temperature acetone-assisted hydrothermal synthesis and characterization of BiFeO3 powders

Well-crystallized pure perovskite bismuth ferrite (BiFeO₃) powders have been synthesized by a facile hydrothermal route at the temperature as low as 130 °C with the aid of acetone. In the synthesis, acetone played important roles in the low-temperature synthesis of pure BiFeO₃. The as-prepared BiFeO₃ powders mainly consisted of cubic particles with the size range from 50 to 200 nm. zero-field-cooled and field-cooled magnetization measurements indicated that pure BiFeO₃ powders showed a spin-glass transition below the freezing temperature. The as-prepared pure BiFeO₃ powders showed weak ferromagnetism and ferroelectricity simultaneously at room temperature. Moreover, the bismuth ferrite BiFeO₃ exhibit efficient photocatalytic activity under visible light irradiation.     

Low-temperature synthesis of BiFeO3 nanoparticles by ethylenediaminetetraacetic acid complexing sol-gel process

This paper reported a novel approach to synthesize pure BiFeO₃ nanoparticles through an ethylenediaminetetraacetic acid complexing sol-gel process at low temperature. The pure BiFeO₃ nanoparticles were attained at much lower temperature as 600 °C by this process, in contrast to above 800 °C for the traditional solid-state sintering process. The SEM results showed that the prepared BiFeO₃ nanoparticles had a better homogeneity and fine grain morphology. The BiFeO₃ nanoparticles show a weak ferromagnetic order at room temperature, which is quite different from the linear M-H relationship in bulk BiFeO₃. The origin of the weak magnetic property in our samples should be attributed to the size-confinement effects of the BiFeO₃ nanostructures.     

Synthesis and thermodynamic stability of multiferroic BiFeO3

Ceramic BiFeO₃ samples were prepared by the sol gel combustion method using urea as fuel. The obtain powders were thermal treated at different temperatures (300-840  °C) and times (1-64  h) to investigate the best synthesis conditions of the material. The resulting materials were analysed by TGA, FTIR, SEM/EDS and XRD. Rietveld analysis was applied to the diffraction data. The temperature and time of the heat treatment are critical for a high BiFeO₃ phase content. Thermal treatment of 1  h at 600  °C yielded 99% molar of the BiFeO₃ phase with a mean particle size of 120  nm. Upper or lower calcinations temperatures yielded higher content of the secondary phases Bi₂Fe₄O₉ and Bi₂₅FeO₃₉. Further heat treatment in air or in argon, up to 64  h, induces a decomposition of the BiFeO₃ phase according to the reaction 49 BiFeO₃ BiFeO₃ → 12 Bi₂Fe₄O₉ + Bi₂₅FeO₃₉ pointing out that BiFeO₃ is not thermodynamically stable at 600  °C. The BiFeO₃ decomposition follows Avrami-Erofeev law with a slope of 1 indicating a one-dimensional kinetics.     

Structural, Optical and Multiferroic Properties of BiFeO3 Nanoparticles Synthesized by Soft Chemical Route

BiFeO₃ nanoparticles were prepared via a soft chemical method using citric acid and tartaric acid routes followed by calcination at low temperature. Structural characterization showed remarkably different conditions for pure phase formation from both routes. The tartaric acid route was effective in obtaining pure phase BiFeO₃ nanoparticles while citric acid route required leaching in dilute nitric acid to remove impurity phases. Further optical, magnetic, and dielectric characterizations of pure phase BiFeO₃ nanoparticles obtained by tartaric acid route were done. X-ray diffraction and Raman spectroscopy confirmed the distorted rhombohedral structure of BiFeO₃ nanoparticles. The average crystallite size of BiFeO₃ nanoparticles was found to vary in the range 30–50 nm. Fourier Transformed Infrared spectra of BiFeO₃ samples calcined at different temperatures were studied in order to analyze various bond formations in the samples. UV-Visible diffuse absorption showed that BFO nanoparticles strongly absorb visible light in the wavelength region of 400–580 nm with absorption cut-off wavelength of 571 nm. The band gap of BiFeO₃ nanoparticles was found to be 2.17 eV as calculated from absorption coefficient spectra. Magnetic measurement showed saturated hysteresis loop indicating ferromagnetic behavior of BiFeO₃ nanoparticles at room temperature. Temperature dependent dielectric constant showed anomaly well below the antiferromagnetic Néel temperature indicating decrease in antiferromagnetic Néel temperature of BiFeO₃ nanoparticles.     

Decomposition behavior and dielectric properties of Ti-doped BiFeO3 ceramics derived from molten salt method

The effects of Ti-doping on the decomposition behavior, crystal structure, sintering behavior and dielectric properties have been investigated for the Ti-doped BiFeO₃ ceramics derived from molten salt method. XRD reveals the almost pure phase BiFeO₃ is synthesized in the Ti-doped BiFeO₃ powders. And the particle size of Ti-doped BiFeO₃ powers obviously decreases with the increase of Ti content. However, the sintering temperature elevates significantly after Ti-doping. The DC resistivity can enhance by up to four orders of magnitude (from 10⁴ to 10⁸?Ωm) with only 5 at% Ti doping. But the dielectric constant is suppressed from 10⁴ to 10², and dielectric loss obviously reduces with a small amount of Ti doping. The variation of dielectric properties has been discussed from the decomposition of BiFeO₃ phase. The Ti-doping can effectively suppress the decomposition reaction in Ti-doped BiFeO₃ ceramics.     

Effect of rare earth dopants on the morphologies and photocatalytic activities of BiFeO3 microcrystallites

Pure and rare earth elements (La, Yb)-doped BiFeO₃ microcrystallites were synthesized by a typical hydrothermal process. The crystal structure, morphologies and photocatalytic activities of these microcrystallites were investigated. The results and analysis revealed that substitution of La and Yb not only changed the energy band gap of BiFeO₃ system, but also affected the morphologies and dimensions of BiFeO₃ microcrystallites. Despite much smaller particle size compared with pure BiFeO₃, the Yb-doped BiFeO₃ microcrystallites exhibited lower photocatalytic efficiency due to much larger energy band gap, which suggests the energy band configuration intensely influences the photocatalytic activity rather than the particle size.     

The study on Ni and Ti-co-doped BiFeO3 nanorods: quenching of magnetism

The investigation has been made on the effect in nanorod-structured BiFeO₃ thin film on its structural, morphological, optical, and magnetic properties on co-doping with Ni–Ti. BiFeO₃ and Ni–Ti-co-doped BiFeO₃ nanorods with various compositions were synthesized by a hot-wall-assisted spray pyrolysis system. The effect of Ni–Ti co-doping in BiFeO₃ nanorods has been found to have an impact in lattice parameter evident from the room-temperature XRD. The positions of the Raman peaks are found to change significantly to higher frequency depending on Ni–Ti co-doping. The field-emission scanning electron microscopy imaging of Ni–Ti co-doping was found to have a slight modification in size and shape of BiFeO₃. An interesting blue shift in the bandgap emission is observed in the UV–Vis absorption spectra of the NRs as a result of co-doping. In Ni and Ti-co-doped BiFeO₃, the magnetism originated due to the breaking of the cycloid-canted spin structure and charge compensation. The incorporation of Ni in the Ti-rich BiFeO₃ confirms that the anisotropic energy barrier is decreased and results in quenching of magnetism.

     

Butterfly-shaped multiferroic BiFeO3@BaTiO3 core–shell nanotubes: the interesting structural, multiferroic, and optical properties

Highly pure butterfly-shaped BiFeO₃@BaTiO₃ nanotubes have been synthesized through a sol–gel method. An obvious ferromagnetic behavior was obtained at room temperature, with a large coercive field (5,403 Oe) and high Mr/Ms value (0.52). The dielectric permittivity of BiFeO₃@BaTiO₃ nanotubes was found to be 1919, which is much higher than that of pure BFO nanostructure. The dielectric loss of BiFeO₃@BaTiO₃ nanotubes is low in the frequency range from 10 Hz to 1 MHz. The maximum magnetoelectric coefficient of BiFeO₃@BaTiO₃ nanotube is 0.272 μV/cm Oe. Moreover, the BiFeO₃@BaTiO₃ nanotubes have exhibited a better ferroelectric property with a large band gap of 3.2 eV which demonstrates the core–shell nanostructure.     

Synthesis and dielectric properties of BiFeO<Subscript>3</Subscript> derived from molten salt method

BiFeO₃ powder was synthesized in NaCl media at temperature range from 700 to 800 °C, using Bi₂O₃ and Fe₂O₃ as raw materials. Effects of calcining temperature and salt ratios on the synthesis of BiFeO₃ powder had been investigated. It was found that NaCl effectively promoted the formation of BiFeO₃. Almost pure BiFeO₃ phase with a very small amount of Bi₂Fe₄O₉ phase was synthesized at 750 °C with salt weight ratios of 1:1. A large amount of BiFeO₃ phase decomposed to Bi₂Fe₄O₉ and Bi₂₅FeO₃₉ phase when the temperature was up to 800 °C. In the present method, the calcining temperature played an important role in the formation of BiFeO₃ phase. BiFeO₃ ceramics derived from molten salt method were prepared and exhibited the higher dielectric constant.     

Self-assembly growth of flower-like BiFeO3 powders at low temperature

In this work, flower-like BiFeO₃ powders were synthesized by a hydrothermal method at low temperature. The effects of mineralizer, reaction temperature and soak time on the crystal phase structure and morphology of the BiFeO₃ powders have been investigated. Perovskite BiFeO₃ powders were obtained as the concentration of KOH is 1–6?mol/L. The SEM images of BiFeO₃ powders which synthesized at 150?°C for 1?h with 2?mol/L KOH exhibit cubic morphology. Whereas, the powders prepared for 6?h show flower-like morphology.     

Magnetic and optical properties of multiferroic bismuth ferrite nanoparticles by tartaric acid-assisted sol-gel strategy

Pure BiFeO₃ nanoparticles have been successfully synthesized through the tartaric acid-assisted sol-gel method at relatively low temperature. The as-prepared nanoparticles were characterized by a variety of techniques. The success in preparing pure BiFeO₃ may be attributed to the formation of heterometallic polynuclear complexes in the tartaric acid system. The ferroelectric phase transition (T_C = 851 °C) was determined, revealing the ferroelectric nature of the as-prepared BiFeO₃ nanoparticles. The result of magnetic measurement indicates the weak ferromagnetic order of BiFeO₃ nanoparticles at room temperature, which may be ascribed to the size confinement effect. The observed strong absorption in the UV region will enable BiFeO₃ nanoparticles to be potentially used as promising photocatalytic decomposition material.     

Effects of addition of BiFeO<Subscript>3</Subscript> on phase transition and dielectric properties of BaTiO<Subscript>3</Subscript> ceramics

Samples of xBiFeO₃–(1 − x)BaTiO₃ (x = 0, 0.02, 0.04, 0.06, 0.07 and 0.08) were synthesized by solid state reaction technique and sintered in air in the temperature range 1,220–1,280 °C for 4 h. X-ray diffraction data showed that 2–8 mol% BiFeO₃ can dissolve into the lattice of BaTiO₃ and form single perovskite phase. The crystal structure changes from tetragonal to cubic phase at room temperature when 8 mol% of BiFeO₃ was added into BaTiO₃. Scanning electron microscope images indicated that the ceramics have compact and uniform microstructures, and the grain size of the ceramics decreases with the increase of BiFeO₃ content. Dielectric constants were measured as functions of temperatures (25–200 °C). With rising addition of BiFeO₃, the Curie temperature decreases. For the sample with x = 0.08, the phase transition occurred below room temperature. The boundary between tetragonal and cubic phase of the BiFeO₃–BaTiO₃ system at room temperature locates at a composition between 7 and 8 mol% of BiFeO₃. The diffusivity parameter γ for compositions x = 0.02 and x = 0.07 is 1.21 and 1.29, respectively. The relaxor-like behaviour is enhanced by the BiFeO₃ addition.     

Novel report on single phase BiFeO<Subscript>3</Subscript> nanorod layer synthesised rapidly by novel hot-wall spray pyrolysis system: evidence of high magnetization due to surface spins

This is the novel report on the rapid synthesis of single phase BiFeO₃ nanorods by novel hot wall assisted spray pyrolysis system. The deposition has been carried out in an indigenously fabricated system and the entire process is completed in 4 s time duration. The structural, morphological, optical and magnetic properties of BiFeO₃ nanorods have been studied in the work. The mechanism of growth of BiFeO₃ nanorods has been explained extensively. The magnetic studies carried out with SQUID-VSM results BiFeO₃ nanorods showing higher saturated magnetization in comparing with previous reports.     

Room-Temperature Multiferroic Properties and Local Structures of the Mn-Doped and (Pb, Mn)-Codoped BiFeO3

The Mn-doped and the (Pb, Mn)-codoped BiFeO₃ polycrystalline samples are prepared by a sol–gel method. The X-ray diffraction patterns manifest that all samples are in single phase and a lattice structural transformation appears during doping process. Compared with Mn-doped BiFeO₃, the magnetic property of (Pb, Mn)-codoped BiFeO₃ is enhanced and ferroelectric polarization decreases. The results of XAFS indicate that the bond length of Fe–O shortens due to doping Mn ions into BiFeO₃, which results in the increase of magnetic property. Pb ion doping makes the valence state of Mn ion change, rather than Fe ion, and induces the occurrence of Mn⁴⁺–O–Fe³⁺ antiferromagnetic coupling, which decreases the magnetic property of the Bi_0.95Pb_0.05Fe_0.95Mn_0.05O₃ sample.     

Enhanced photocatalytic activity of BiFeO3 particles by surface decoration with Ag nanoparticles

BiFeO₃ particles with different morphologies and sizes were synthesized via a hydrothermal process, where the morphology and size was tailored by using different KOH concentrations in precursor solution. The samples prepared at n(KOH) = 3, 4.5, 6, and 7.5 M are composed, respectively, of octahedron-shaped particles (500–600 nm), cube-like particles (200–500 nm), irregular spherical agglomerates (9–16 μm) formed from disk-like grains with diameter of 1.4–2.8 μm and thickness of 0.2 μm, and cuboid-shaped particles with length-to-width ratio of 1.4:1–3.5:1 and width size ranging from 80 to 280 nm. Ag nanoparticles were deposited on the surface of BiFeO₃ particles by a chemical reduction method to produce Ag@BiFeO₃ nanocomposites. The photocatalytic activity of prepared samples was evaluated by degrading rhodamine B under simulated sunlight irradiation. It is demonstrated that Ag-decorated BiFeO₃ particles exhibit an enhanced photocatalytic activity compared to bare BiFeO₃ particles. This can be explained by the effective transfer of photogenerated electrons from the conduction band of BiFeO₃ to Ag nanoparticles and hence increased availability of holes for the photocatalytic reaction. Hydroxyl radicals were detected by the photoluminescence technique using terephthalic acid as a probe molecule and are found to be produced over the irradiated BiFeO₃ and Ag@BiFeO₃ photocatalysts; especially, an enhanced yield is observed for the latter.     

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.