Pure Phase BiFeO3 Research Articles

Preparation of amorphous Bi-Fe-O series semiconductor and its crystallization behavior and photocatalytic activity

In that research field of Bi-Fe-O series compounds, the design and development of amorphous Bi-Fe-O semiconductor with high photocatalytic activity and pure phase BiFeO3 with multiferroic properties are two major challenges. Herein, the amorphous Bi-Fe-O series semiconductor is prepared by mechanical alloying method using Fe2O3 and Bi2O3 as the raw materials, and the pure phase nano-BiFeO3 crystalline is subsequently obtained by heat treatment process. Meanwhile the microstructure, band gap structure and photocatalytic or electromagnetic activity of amorphous and crystallized Bi-Fe-O semiconductor are also investigated. The results show that after 25 h of mechanical alloying, the crystalline structure of Bi2O3 and Fe2O3 is destroyed, and the system is transformed into amorphous phase. Moreover, photocatalytic hydrogen production experiment shows that amorphous Bi-Fe-O semiconductor has the highest photocatalytic hydrogen production rate which is 4.03 μmol/g/h, due to more defects and impurity energy levels. In addition, pure phase nano-BiFeO3 crystalline can be obtained when the heat treatment temperature of amorphous Bi-Fe-O semiconductor is 600°C, which have the best ferroelectric properties. This work not only provides new ideas for the research and development of amorphous semiconductor, but also proposes a green method to prepare pure phase nano-BiFeO3 crystalline.

Photocatalytic properties of BiFeO3 powders synthesized by the mixture of CTAB and Glycine

Highly pure BiFeO3 (BFO) powders were prepared by the solution combustion synthesis method using cetyltrimethylammonium bromide (CTAB) and glycine as fuels at various fuel-to-oxidant (φ) ratios. Microstructural characteristics, morphology, optical properties, and thermal analysis were studied by X-ray diffraction (XRD), scanning electron microscopy (SEM), diffuse reflectance spectroscopy (DRS), and differential thermal/thermogravimetry (DTA/TGA), respectively. The combusted powders prepared at different fuel content contained a small amount of impurity phases such as Bi24Fe2O39 and Bi2Fe4O9. During the calcination of BFO powders at 600 °C for 1 h, a nearly pure BFO phase was produced. Combusted powders photodegraded about 80% of methylene blue dye at φ = 2 through 90 min of visible light irradiation.

Hydrothermal Synthesis of Bismuth Ferrite Hollow Spheres with Enhanced Visible-Light Photocatalytic Activity.

In recent years, semiconductor hollow spheres have gained much attention due to their unique combination of morphological, chemical, and physico-chemical properties. In this work, we report for the first time the synthesis of BiFeO3 hollow spheres by a facile hydrothermal treatment method. The mechanism of formation of pure phase BiFeO3 hollow spheres is investigated systematically by variation of synthetic parameters such as temperature and time, ratio and amount of precursors, pressure, and calcination procedures. The samples were characterized by X-ray powder diffraction, scanning electron microscopy, energy dispersive X-ray spectroscopy, and UV-vis diffuse reflectance spectroscopy. We observe that the purity and morphology of the synthesized materials are very sensitive to synthesis parameters. In general, the chemically and morphologically very robust hollow spheres have diameters in the range of 200 nm to 2 μm and a wall thickness of 50-200 nm. The synthesized BiFeO3 hollow spheres were applied as catalysts in the photodegradation of the model pollutant Rhodamine B under visible-light irradiation. Notably, the photocatalyst demonstrated exceptionally high removal efficiencies leading to complete degradation of the dye in less than 150 min at neutral pH. The superior efficiencies of the synthesized material are attributed to the unique features of hollow spheres. The active species in the photocatalytic process have been identified by trapping experiments.

Chelating Agents Assisted Rapid Synthesis of High Purity BiFeO3: Remarkable Optical, Electrical, and Magnetic Characteristics

In this study, we presented two reliable methods, the sol–gel and autocombustion, to synthesis a high purity BiFeO3 single phase with low calcination time using glycine as chelating agent. The glycine-autocombustion method produced a high purity BiFeO3 phase using either low or high concentrations of the reactants (Bi(NO3)3 + Fe(NO3)3), while the glycine-sol–gel method delivered a high purity BiFeO3 phase using low concentrations of reactants. In the case of using tartaric acid and urea as chelating agents, mixtures of BiFeO3 and Bi25FeO40 phases were formed. The morphology, size, and porosity of the particles were obviously changed by varying the synthesis method and chelating agents. The high purity BiFeO3 samples exhibit a visible light band gap of 2.05 eV with long absorption tail extending to the infrared region, suggesting the suitability of the synthesized powders in the solar photocatalytic applications. A weak hysteresis ferromagnetic loop was observed for BiFeO3 (glycine method) and BiFeO3/Bi25FeO40 (urea method) with large contribution from the paramagnetic behavior. On contrast, robust ferromagnetic loops were found for BiFeO3/Bi25FeO40 sample synthesized by tartaric acid with saturation magnetization reaching to 2.5 emu/g. Remarkably, the pure single phase BiFeO3 powders synthesized by sol–gel and auto-combustion methods using glycine possess room temperature dielectric constant values of 622 and 845 respectively at a frequency of 42 Hz. In the case of BiFeO3 powders prepared by using tartaric acid, the dielectric constant exhibits values of 401 and 1118 for sol–gel and auto-combustion assisted samples, at the same frequency, respectively. At low frequency, the values of the real part of the complex permittivity tend to be zero which confirms a negligibly small contribution of the electrode effect.

Structural, dielectric, magnetic, and ferroelectric properties of bismuth ferrite (BiFeO3) synthesized by a solvothermal process using hexamethylenetetramine (HMTA) as precipitating agent

Multiferroic BiFeO3 (BFO) has been synthesized by the solvothermal technique for 4 h at 180 °C using Hexamethylenetetramine (HMTA) as a precipitating agent. Optimizations on HMTA concentration were performed to attain BFO in a narrow possible time using the solvothermal method. The effect of HMTA concentration on structural properties was investigated by XRD, FE-SEM, FT-IR, and Raman techniques. Moreover, the magnetic, ferroelectric, dielectric, and optical properties were studied by VSM, Ferroelectric analyser, Dielectric spectrometer, and UV–Vis–NIR spectrometer, respectively. The XRD data has unveiled the significant role of HMTA, as a precipitating agent, in obtaining the pure phase BFO at a particular concentration. A high concentration of HMTA (6 M) is found to be more favourable for producing pure phase BFO within the short reaction time of 4 h without any impurity phase formations. The FE-SEM images have shown the formation of granular structures that are inhomogeneously distributed throughout the powdered BFO with porosities. In addition, the magnetic measurements confirmed that the BFO synthesized with varying HMTA concentration exhibits a reasonable week ferromagnetic behaviour. However, the pure BFO powder synthesized at 6 M HMTA shows high magnetization value (0.72 emu/g) compared to the other samples synthesized at low concentrations of HMTA. Further, the dielectric measurements of all the synthesized BFO samples have shown a decrease in dielectric loss with an increase in frequency. Whereas the dielectric behaviour exhibited by the pure BFO synthesized at 6 M HMTA has shown a high dielectric constant value with moderate dielectric loss. A ferroelectric analyser studies the ferroelectric behaviour of BFO powder. The study revealed that pure phase BFO synthesized with 6 M HMTA exhibits a high value of remanent polarization with unsaturated P-E loops due to the high leakage current in the sample. The UV–Vis data shows that the BFO samples exhibited an excellent optical absorption in the visible range with narrow optical band gaps. Therefore, all the characterizations related to magnetic, structural, dielectric, ferroelectric, and optical properties of synthesized pure BFO have proven the significance of precipitating agent HMTA in the solvothermal synthesis method. Based on our findings, the synthesized pure phase BFO can be an excellent semiconducting photocatalyst for multiferroic based photocatalysis applications.

XPS Study in BiFeO3 Surface Modified by Argon Etching.

This paper reports an XPS surface study of pure phase BiFeO thin film produced and later etched by pure argon ions. Analysis of high-resolution spectra from Fe , Bi and , O , and the valence band, exhibited mainly Fe and Bi components, but also reveal Fe. High-energy argon etching induces the growth of Fe and Bi and an increment of Fe, as expected. The BiFeO semiconductor character is preserved despite the oxygen loss, an interesting aspect for the study of the photovoltaic effect through oxygen vacancies in some ceramic films. The metal-oxygen bonds in O spectra are related only to one binding energy contrary to the split from bismuth and iron reported in other works. All these data evidence that the low-pressure argon atmosphere is proved to be efficient to produce pure phase BiFeO, even after argon etching.

Outstanding Ferroelectricity in Sol–Gel-Derived Polycrystalline BiFeO3 Films within a Wide Thickness Range

As a promising lead-free ferroelectric, BiFeO3 has a very large intrinsic polarization of ∼100 μC/cm2, enabling its great potential in electronic applications especially in a film format. In this sense, reliable ferroelectric properties are desired; however, pure-phase BiFeO3 films are notorious for their large leakage current, especially of those processed by using the sol-gel method─a facile and industrially scalable method for film preparation. In this study, a protection layer, which can be easily integrated in the sol-gel process, is used to ensure the acquirement of remnant polarization of ∼65 μC/cm2 in ∼200 nm BiFeO3 thin films, whereas O2 annealing can enhance that to ∼120 μC/cm2 in ∼400-700 nm films. Reliable ferroelectricity of BiFeO3 films on Si wafers within a wide thickness range was thus achieved. The obtained ferroelectricity is among the best-achieved properties to date of BiFeO3 films for both thin and intermediate thicknesses, including both chemically and physically derived. These results are helpful to advance potential use of sol-gel-processed BiFeO3 films in electromechanical devices with different desired thicknesses.

Synthesis of bismuth ferrite by sol-gel auto combustion method: Impact of citric acid concentration on its physicochemical properties

In this study, pure phase BiFeO3 was prepared via a facile sol-gel auto-ignition route using water as a solvent. The influence of citric acid to metal nitrates molar ratio on physicochemical properties of BiFeO3 was also investigated through FTIR, XRD, PL, UV, SEM, and BET analysis. The XRD of the prepared samples revealed the rhombohedral perovskite structures belong to space group R3c. In addition, the BFO2:1 sample showed the smallest crystallite size (42 nm) with the highest purity and recombination rate; while the BFO1:2 sample showed the lowest purity and electron charge recombination rate. The surface areas of the BFO1:1, BFO1:2, and BFO2:1 samples were 1.63, 2.13, and 1.84 m2/g, respectively. Moreover, the bandgaps were found to be 2.17, 2.11, and 2.21 eV for the prepared BFO1:1, BFO1:2, and BFO2:1, respectively. In addition, the appearance of a near-IR band around 814 cm−1 confirms the presence of Bi2Fe4O9 byproduct in the BFO1:2 sample. Finally, SEM revealed that BFO2:1 sample was well-distributed and exhibited uniform morphology, referring to citric acid's ability to stabilize and disperse bismuth ferrite NPs. Furthermore, BFO1:2 sample showed enhanced photocatalytic activity in CR dye degradation than BFO1:1 and BFO2:1 samples, attributed by the increase in the surface area and minimized electron-hole recombination. This strategy eliminates the use of conventional organic solvents and provides an effortless and efficient way to prepare pure BiFeO3 and BiFeO3/Bi2Fe4O9 heterojunction nanocomposite that are potentially applicable for photocatalyst and in solar cell production.

Time-temperature-transformation of BiFeO3 phase synthesized by citrate–nitrate route and a synergetic effect for its stabilization

BiFeO3 has been synthesized from the nitrate salts of bismuth and iron at different temperatures at 500, 600 and 700 °C. The time–temperature-transformation (TTT) has been analysed at the annealing time duration of 1, 2, 3, 4, 5, 6 and 7 h. Thermo-gravimetric study of the starting solid has been analysed from room temperature to 800 °C and the reaction kinetics have been investigated from 400 to 700 °C using Flynn-Wall-Ozawa model which has also been correlated with the differential thermal analysis (DTA). Gibbs free energies of the formation of BiFeO3, Bi2Fe4O9 and Bi25FeO39 have been calculated to investigate the reason for the maximum percentage of pure BiFeO3 phase formation at 600 °C. Based on the calculation, though later two compounds are thermodynamically destabilizing secondary phases of BiFeO3, its synergetic stabilization has been supported by the diffusion kinetics and their activation energies.

The Local Structure and Exchange Bias Effect of (Ho, Co)-codoped BiFeO3 investigated by XAFS spectroscopy

Two elements (Ho, Co) co-doped Bi1-xHoxFe0.95Co0.05O3 (x = 0.05, 0.10, 0.15) samples were prepared by sol-gel method. The X-ray diffraction patterns demonstrate that Ho ions doping plays a major role in a systematic structural phase transition. The details of the magnetic properties of the samples are investigated. The magnetic properties of (Ho, Co) co-doped BiFeO3 have been improved greatly, compared with the pure phase BiFeO3. An exchange bias effect is observed in the samples. The local structure of the samples measured by X-ray absorption fine structure is studied to analyze the origin of the enhancement of the magnetic properties and the change of exchange bias effect of the samples. The occurrence of the exchange bias effect is attributed to the changes of the Fe-O bond length and of the local symmetry around Fe atom.

Fabrication, Structure, and Magnetic Properties of Pure-Phase BiFeO₃ and MnFe₂O₄ Nanoparticles and their Nanocomposites

We fabricated pure-phase BiFeO₃ (BFO) and MnFe₂O₄ (MFO) nanoparticles as well as their nanocomposites (BMFO), and then we studied their structures and magnetic properties. Pristine BFO nanoparticles of 93.3 nm average diameter were successfully synthesized using the sol-gel method by varying the solvent condition and the precursor amount. Pristine MFO nanoparticles with a mean diameter of 70.5 nm were synthesized using the co-precipitation method entailing the optimization of the preheating and aging steps. The fabricated MFO nanoparticles showed mostly nanospheres with few nanocubes. The nanocomposite samples of 50 % MFO and 50 % BFO were fabricated through grinding and pelletization, followed by sintering under an inert atmosphere. The crystal structures of the pristine materials in the nanocomposites were well preserved. The magnetization values (Ms) of the BFO, MFO, and BMFO were 4.9, 52, and 33 emu/g, respectively. This latter Ms value was significantly higher than that of BFO, owing to the coexistence of FeSUP2+/SUP and FeSUP3+/SUP in its BFO phase and the incorporation of magnetic MFO. Two synthesis methods and material properties including the structural, morphological, magnetic, and oxidation states of the BFO-MFO nanocomposites were studied in order to achieve a high Ms value of 33 emu/g, which is higher than the bulk values of previously reported BFOMFO composite samples.

Piezoelectric BiFeO3 Thin Films: Optimization of MOCVD Process on Si.

This paper presents a simple and optimized metal organic chemical vapor deposition (MOCVD) protocol for the deposition of perovskite BiFeO3 films on silicon-based substrates, in order to move toward the next generation of lead-free hybrid energy harvesters. A bi-metal mixture that is composed of Bi(phenyl)3, and Fe(tmhd)3 has been used as a precursor source. BiFeO3 films have been grown by MOCVD on IrO2/Si substrates, in which the conductive IrO2 functions as a bottom electrode and a buffer layer. BiFeO3 films have been analyzed by X-ray diffraction (XRD) for structural characterization and by field-emission scanning electron microscopy (FE-SEM) coupled with energy dispersive X-ray (EDX) analysis for the morphological and chemical characterizations, respectively. These studies have shown that the deposited films are polycrystalline, pure BiFeO3 phase highly homogenous in morphology and composition all over the entire substrate surface. Piezoelectric force microscopy (PFM) and Piezoelectric Force Spectroscopy (PFS) checked the piezoelectric and ferroelectric properties of the film.

Enhancement of dielectric, ferromagnetic and electrochemical properties of BiFeO3 nanostructured films through rare earth metal doping

In this present work first assessment of enhanced electrochemical properties of Bismuth Ferrite (BiFeO3)thin films through Samarium(Sm) doping are delivered. Apart from this enhancement of structural, dielectric and magnetic properties with increasing samarium concentration is discussed. The pure phase BiFeO3 films and Sm-doped BiFeO3(Bi1-xSmxFeO3 where x = 0.05 & x = 0.1) films were synthesized using 2methoxy aided sol-gel process and were deposited on platinum substrates through spin coating technique. X-ray diffraction confirmed the formation of pure phase BiFeO3 with Rhombohedral (R3c) structure. Morphological characterization through SEM presented the formation of nanostructures and its structural transformation through doping variant. AFM confirmed the smoothness of the film with a maximum grain size of 172.72 nm for the measured films. The elemental analysis and elemental purity was confirmed through EDAX. Mechanistic aspects of the prepared films were analyzed through Thermogravimetric, Differential Thermal Analysis and Fourier Transform Infrared spectroscopy. The variation of dielectric constant with frequency was measured until 1 MHz and remains almost constant due to the independent nature of polarization with frequency. The magnetic coercivity of the film improved from 77.7G to 240G with samarium doping. The Bi0.9Sm0.1FeO3 films deposited on platinum substrates enhanced the specific capacity to about 184Fg-1 along with its retention capability enabling it to be used as electrode material for supercapacitors.

Size-tunable fabrication of BiFeO3 nanoparticles with enhanced visible-light photocatalytic activity using a facile co-precipitation method

BiFeO3 nanoparticles with controlled particle size were synthesized via a facile co-precipitation method. The samples were characterized by x-ray powder diffraction, transmission electron microscopy, atomic force microscopy and UV–vis diffuse reflectance spectroscopy. By controlling the timing of base addition during the co-precipitation process the particle size can be controlled easily. It was found that longer base addition times, i.e. slower dropping rates, resulted in smaller particle sizes exhibiting narrower band gaps. The smallest BiFeO3 particles prepared by this method are best described as circular flake-like nanoparticles with a diameter of 10–20 nm and a height of approximately 2.5–7 nm. To the best of our knowledge, these particles are the smallest BiFeO3 particles ever prepared by a co-precipitation method. Furthermore, our synthetic protocol allows the facile synthesis of size-tunable, pure-phase BiFeO3 nanoparticles with a low concentration of surface defects and local strain all leading to excellent efficiencies in photodegradation reactions of Rhodamine B in an aqueous solution under visible light irradiation. We show that the different particle sizes affected largely the resulting band gaps as well as the photocatalytic activity of the corresponding samples leading to narrower band gaps and improved photocatalytic activity in smaller particles.

Modulation ferromagnetism in multiferroic BiFeO3 nanocrystals via bandgap engineering

In addition to electric fields and currents, light can also provide an approach to modulate the ferromagnetism with low energy consumption. BiFeO3, with features of relatively small bandgap and large polarization, provides an opportunity for investigating the optical modulation of magnetism. In this work, pure-phase BiFeO3 nanocrystals embedded in Al2O3 films are synthesized. It is demonstrated that the strain generated and accumulated during the growth process of BiFeO3 nanocrystals can lead to the modification of the atomic structure and thus produce a strain engineered bandgap. A distinguished light-modulated ferromagnetism is observed in BiFeO3 nanocrystals. Contributed by the strain engineered bandgap, the ferromagnetism of BiFeO3 nanocrystals can be modulated and enhanced more efficiently by light irradiation. It paves the way for modulating the ferromagnetic properties of BiFeO3 nanocrystals via bandgap engineering, which has promising applications in modern information technology.

Growth and physical properties of BiFeO3 thin films directly on Si substrate

Multiferroic BiFeO3 thin films have been widely studied for their intriguing fundamental physics and exotic functional properties. Integration of the BiFeO3 thin films with semiconductor Si is crucial for the practical development of novel electronic and photosensitive devices. Here, we report the fabrication and physical properties of BiFeO3 thin films grown directly on bare Si substrate by laser-molecular beam epitaxy. It was found that a predeposition process performed at room temperature and high vacuum conditions is effective and necessary for obtaining the pure-phase BiFeO3 thin films. X-ray diffraction measurements indicate that the obtained BiFeO3 thin films are in a single-phase polycrystalline state and show improved crystallinity with increasing thickness, and a transmission electronic microscopy image illustrates an interface layer around 3 nm in thickness existing between the BiFeO3 thin films and Si substrate. Electrical measurements show that the BiFeO3 thin films have good ferroelectricity and low leakage current. Moreover, optical properties investigated by spectroscopic ellipsometry demonstrate that the direct band gap of the thin films increases with decreasing thickness.

The Microstructure, Electric, Optical and Photovoltaic Properties of BiFeO3 Thin Films Prepared by Low Temperature Sol⁻Gel Method.

Ferroelectrics have recently attracted attention as a candidate class of materials for use in photovoltaic devices due to their abnormal photovoltaic effect. However, the current reported efficiency is still low. Hence, it is urgent to develop narrow-band gap ferroelectric materials with strong ferroelectricity by low-temperature synthesis. In this paper, the perovskite bismuth ferrite BiFeO3 (BFO) thin films were fabricated on SnO2: F (FTO) substrates by the sol–gel method and they were rapidly annealed at 450, 500 and 550 °C, respectively. The microstructure and the chemical state’s evolution with annealing temperature were investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM), Raman spectroscopy and X-ray photoelectron spectroscopy (XPS), and the relationship between the microstructure and electric, optical and photovoltaic properties were studied. The XRD, SEM and Raman results show that a pure phase BFO film with good crystallinity is obtained at a low annealing temperature of 450 °C. As the annealing temperature increases, the film becomes more uniform and has an improved crystallinity. The XPS results show that the Fe3+/Fe2+ ratio increases and the ratio of oxygen vacancies/lattice oxygen decreases with increasing annealing temperature, which results in the leakage current gradually being reduced. The band gap is reduced from 2.68 to 2.51 eV due to better crystallinity. An enhanced photovoltaic effect is observed in a 550 °C annealed BFO film with a short circuit current of 4.58 mA/cm2 and an open circuit voltage of 0.15 V, respectively.

Fabrication of dimension controlled BiFeO3 microcrystal

In this research, the effect of different KOH concentrations on the bismuth ferrite (BiFeO3) phase formation and microcrystal size was discussed. Pure phase BiFeO3 microcrystals were successfully synthesized through controlling different KOH concentrations during the hydrothermal process. The diverse sizes of pristine BiFeO3 microcrystals were characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM). Their photocatalytic activity was studied by photocatalytic degradation of methylene blue (MB) in aqueous solution under visible light irradiation. The results reveal that the composition and particle size of BiFeO3 microcrystals phase can be controlled with different mineralizer conditions. This work is able to provide a new perspective on the fabrication research of different dimension BiFeO3 microcrystals.

Influence of diamagnetic Zn on structural, ferroelectric and ferromagnetic properties of BiFe1-xZnxO3 (0% ≤ x ≤ 8%)

Nanocrystallites with the compositional formula BiFe1-xZnxO3 (x = 0.0, 0.01, 0.03, 0.05, 0.08) were synthesised by sol-gel method route. The influence of Zn doping on structural, dielectric, ferroelectric and ferromagnetic properties was discussed in detail. XRD studies confirmed the crystallinity and phase formation of all the samples. Slight lattice distortion has been observed for x = 0.08 sample as compared to the pure phase BiFeO3 (BFO). Microstructural investigations using SEM showed the increase in grain size with the increase in Zn addition. FTIR studies reveal the presence of necessary bonds in the samples. Enhanced dielectric and ferroelectric properties have been observed with Zn doping upto x = 0.05 concentration and then decreases. Room temperature MH loop showed enhanced ferromagnetic properties upto x = 0.03 Zn content and decreases thereafter. Explanation of the above results was discussed extensively. Simultaneous improvement in both ferroelectric and ferromagnetic properties might bring considerable interest in various multiferroic applications.

Thermodynamic Stability and Thermal Expansion of Pure-phase BiFeO3

Thermodynamic Stability and Thermal Expansion of Pure-phase BiFeO3