Microwave-Assisted Solvothermal Route for One-Step Synthesis of Pure Phase Bismuth Ferrite Microflowers with Improved Magnetic and Dielectric Properties.

The prototypical plum-free, one-phase multiferric ferrite BiFeO3 (BFO) is solid, parallel, with a high ferroelectric Curie temperature and Neel temperature and antiferromagnetic and ferroelectric propagation. This work aims to synthesize pure-phase BFO in the quickest possible way. We followed the microwave-assisted solvothermal (MWAST) method to achieve pure-phase BFO in the shortest duration of 3 min. The experiment involves simple optimizations with KOH concentration and microwave power levels. The surface morphology along with magnetic properties of BFO synthesized via the MWAST method are altered with varying KOH concentrations and microwave (MW) power levels. Our X-ray diffraction findings reveal that the pure-phase BFO is formed at 800 W MW power, and the structural characterizations like transmission electron microscopy, field emission scanning electron microscopy with energy-dispersive X-ray analysis have displayed the formation of uniformly distributed spherical microflowers of pure-phase BFO exhibiting a single-crystalline nature. Besides, the magnetic measurements affirmed a reliable weak ferromagnetic behavior (magnetization ∼1.25 emu/g) in BFO synthesized at 800 W MW power. In addition, good dielectric behavior with low dielectric loss was accompanied by frequency-dependent dielectric studies indicating an excellent frequency response of the material, and also the room-temperature ferroelectric properties were studied using a ferroelectric analyzer. The polarization of pure-phase BFO increases with the applied electric field and exhibits unsaturated polarization-electric field loops due to leakage current. Moreover, the Fourier transform infrared spectrum of the synthesized material has indicated the pure-phase BFO, and the Raman data have elucidated the vibrational modes of BFO. Further, the analysis of X-ray photoelectron spectroscopy data has confirmed the presence of fewer Fe2+ ions and oxygen vacancies in the pure-phase BFO. Therefore, the collective characterizations and detailed analysis of BFO material have revealed the uniqueness of the MWAST method in producing the pure-phase BFO in 3 min with improved magnetic and dielectric properties, and hence the BFO synthesized via the MWAST method can be a potential candidate for multiferroic applications.

Introduction

A room-temperature magnetoelectric (ME) multiferroic bismuth ferrite has attracted much attention due to the coexistence of ferromagnetism and antiferromagnetism in a single-phase compound with Neel temperature and predominant ferroelectric transition temperatures. This excellent multiferroic behavior can be used develop novel functional materials in the field of spintronics, information storage, multiple-state memories, and sensors.[1−4] Bismuth ferrite (BFO) shows a G-type antimagnetism with a uniform magnetic field spin configuration at Neel temperature (650 K) and high Curie temperature (TC = 1100 K). In BFO, the spontaneous magnetization originates from the partially filled orbits of Fe, and electric polarization arises from the 6s2 lone-pair electrons present on Bi, which might be integrated with a unique phase. However, its future excellent potential applications in satellite communications, optical filters, sensors, cameras, memory, and smart devices are hindered due to the presence of secondary bismuth-rich phases, poor spontaneous polarization, and weak magnetic properties. Moreover, the presence of its minor insulation sensitivity caused due to the reduction from Fe3+ to Fe2+ and oxygen vacancies has restricted its applications.[5,6]

In recent decades, many researchers have made appreciable efforts to produce pure BFO crystals and ceramics using several synthesis methods such as solid-state, soft-chemical, wet-chemical, microemulsion, and so on.[7−9] Since most of these methods involve a common sintering step (typically at 500–900 °C), the possibility of impurity additions is more due to high temperatures and pressures, where the stability of the pure-phase BFO is disrupted sensitively during the synthesis process. In addition, the solid-state routes use nitric acid as a leaching agent for removing the secondary impure phases (Bi2Fe4O9 and Bi25FeO40) that are formed during the synthesis process. However, the intermixing of bismuth and iron oxide followed by calcination in this method results in obtaining a low-quality powder with poor reproducibility. Therefore, considering all the involving effects during the synthesis process, the requisite pure-phase BFO is challenging. Recently, Ghosh et al.[12] have obtained pure-phase BFO nanopowder using a tartaric acid-based sol–gel route with an additional sintering process. Wang et al.[10] and Pradhan et al.[11] have prepared pure BFO ceramics with a rapid fluid-phase sintering process. In such approaches, the BFO crystallization process requires high temperatures above the ferroelectric Tc, making it challenging to prevent bismuth condensation.

Currently, two synthesis methods have been found that are more favorable for obtaining single-phase BFO, namely, liquid-phase sintering and percolation methods. The methods are employed to remove the unwanted phases by diluting the precursors using nitric acid. However, samples produced using these two methods have a high leakage current, limiting the prepared sample’s practical device applications. Therefore, it is essential to synthesize and explore more on novel synthesis methods for obtaining pure single-phase BFO. The soft-chemical routes have been introduced for synthesizing microcrystalline BFO even at low temperatures. Among all the chemical methods, the hydrothermal and solvothermal methods are more attractive and gained tremendous attention due to their simple and controllable nature over the particle size, shape, and degree of crystallinity during the synthesis process.[13−21] Nevertheless, this method takes hours to days for a reaction to be completed, which can limit the reliable applications on large-scale industrial productions. Hence, the necessity of increasing the crystallization rate in the above-mentioned methods is essential to reduce the reaction time and obtain pure-phase BFO with particle size uniformity and bulk homogeneity at a quick pace. Recently, the microwave (MW) heating method has been fascinating due to its rapid reaction rate, sample homogeneity, lower processing temperature, and uniform nucleation growth of the powders.[24−26] Various research groups have synthesized single-phase nanomaterials with high-purity perovskites in a relatively short time at moderate temperatures avoiding all other steps involved, such as sintering, agglomeration, gross powders growth, and so on.[22,23] Li et al.[27] reported pure degree BFO nanoplates using NH3·H2O as a leaching agent to control the formation of secondary phases. The synthesis of pure-phase BFO by the microwave-assisted solvothermal (MWAST) process is rarely reported. Moreover, the ferroelectric and dielectric behaviors of BFO synthesized by this method have not been studied much.

In our study, by utilizing the advantages of both solvothermal and microwave methods, we synthesized BFO flowers by using ethanol as a solvent to enable us to have a rapid reaction rate considering the high boiling point and high loss tangent, unlike other solvents.[28] To the best of our knowledge of the literature related to the quickest synthesis methods (MW-assisted) of pure-phase BFO, very few studies have reported the reaction times ranging from 15 to 60 min.[29−33] For the first time here, we tried to endorse our work on the MWAST method to designate it as the quickest way to attain pure-phase BFO in just 3 min without using any harmful acids and studied the magnetic, ferroelectric, and dielectric properties of as-formed pure-phase BFO flowers at room temperature.

Experimental Section

Materials

This method of synthesis uses bismuth chloride (BiCl3, purity 98%), ferric chloride (FeCl3·6H2O, purity 98%), potassium hydroxide (KOH, purity 98%), ethanol, and double-distilled water (DDW). All these chemical reagents are commercially purchased from the SRL company.

Synthesis of BFO

Pure-phase BFO is synthesized by the MWAST method in a domestic solo-microwave oven with variable power from 200 to 800 W and operating at 2.45 GHz. We prepared the BFO in powder form with two metal precursors (BiCl3, FeCl3·6H2O) and a mineralizer (KOH). A solution of equimolar concentrations (0.05M) of BiCl3 and FeCl3·6H2O was dissolved in ethanol and mechanically stirred for 1 h. The original solution was altered by the dropwise addition of 8 M KOH solution. A hydroxide precipitate of bismuth and iron took place after 30 min of magnetic stirring. Later, 10 mL of this mixture was poured into a customized sealed microwave acid digestion vessel (model 4782 from Parr) of 45 mL capacity and then placed in a homemade microwave oven. The chemical reactions were performed by varying microwave power levels from 360 to 800 W at a constant microwave heating time of 3 min and allowed the reaction vessel to cool to room temperature. The final reaction precipitate was washed in DDW several times and dried at 80 °C for 4 h. We prepared three sets of samples at three different MW power levels (i.e., 360, 700, and 800 W) for 3 min with 8 M KOH. The samples were named as [BFO:360W, KOH(8M)], [BFO:700W, KOH(8M)], and [BFO:800W, KOH(8M)]. The pure-phase BFO nanoflowers were formed at 800 W and 8 M KOH concentrations in 3 min. So further, we optimized the reactions by varying the KOH concentrations from 6 to 12 M to examine the effect of KOH on the morphology of as-formed BFO flowers.

To compare the microwave technique utilized in our experiments with the commercially available microwave hydrothermal system (model: Anton Paar Rotor 16), we also synthesized BFO at 650 W for 15 min using four vessels (maximum capacity of the system is 16 vessels) of the commercial system after optimizing the conditions. Later, the X-ray diffraction (XRD) results of the samples obtained from both techniques were compared to check for versatility in obtaining pure-phase BFO.

Characterization Techniques

We confirmed the formation of the pure-phase BFO by an XRD analysis (PANalytical, X’Pert Powder Diffractometer) and Rietveld refinement (Xpert High score plus-ver. 4.9). The morphology of BFO flowers was investigated by field-emission scanning electron microscopy (FESEM, Carl Zeiss Smart Sem) and transmission electron microscopy (TEM, JEM-F200 kV). The energy-dispersive X-ray (EDX) spectra along with X-ray photoelectron microscopy (XPS) (model: Axis Supra) data have confirmed the elemental (Bi, Fe, and O) composition of pure-phase BFO microflowers. The magnetic, dielectric, and ferroelectric measurements of pure-phase BFO were performed using a vibrating sample magnetometer (VSM, LakeShore), dielectric spectrometer (Novocontrol GmBH), and ferroelectric analyzer (Trek model 609B), respectively, at room temperature. To understand the composition of molecular mixtures and vibrational modes of the synthesized BFO material, Fourier transform infrared (FTIR) spectroscopy (model: PerkinElmer-L1600400) and Raman spectroscopy (model: HORIBA Micro-Raman) measurements were performed.

Results and Discussions

Structural Analysis

Figure S1 displays the XRD patterns of as-synthesized BFO using a commercial microwave system (model: Anton Paar Rotor 16, Figure S2) under the power conditions of 650 W for 15 min. From the obtained XRD data, we observed the formation of many impurity peaks corresponding to Bi- and Fe-rich phases that are more prominent than the pure-phase BFO peaks (Figure S1). This implies an inadequate reaction time for the reaction to complete in a commercial microwave system. The undesirable impurity formations might be due to the system configurations and operating conditions. Since the system demands a minimum of four vessels to run, we believe the power distribution for all the vessels might not be uniform, which eventually left the reactants unreacted even for longer durations of MW heating. These unreacted chemical species are giving impurity peaks in the XRD data. Hence, to avoid these experimentally obtained limitations of a commercial microwave system and to fasten the chemical reaction we need a sophisticated, simple method for obtaining pure-phase BFO.

Therefore, we tried a simple microwave system that uses a domestic microwave oven and acid digestion vessel (Parr, model 4782, Figure S3) to produce a pure-phase BFO. Using this microwave system, the pure-phase BFO is achieved in a short reaction time of 3 min. Since this microwave system requires a single heat container, the possibility of nonuniform microwave energy distribution is minimum, and the effective cross-sectional area is maximum, which can collectively enhance the reaction rate. Hence, for all our experiments on achieving the pure phase BFO, we used this rapid method (MWAST) to avoid most of the limitations offered by other methods/techniques. To begin with, the microwave heating time was varied from 1 to 4 min at different microwave power levels (800, 700, and 360 W) with a fixed concentration of KOH (8 M) to set an optimized parameter for the reaction time to form pure-phase BFO. The XRD results of BFO synthesized at 800, 700, and 360 W for different MW heating times (1–4 min) are presented in Figures S4–S6, respectively. From the XRD data obtained, it was observed that the pure-phase BFO with a rhombohedral crystal structure is produced at 800 W in 3 min of microwave heating. Hence, the reaction time of 3 min under microwave heating was set as a constant parameter for obtaining pure-phase BFO using the MWAST method. Figure shows the XRD patterns of the BFO synthesized by the MWAST method by utilizing ethanol as a solvent and by varying the microwave power from 360 to 800 W under a constant microwave heating of 3 min. Our findings based on XRD data [Figure a] reveal that, in just 3 min of microwave heating at 360 W, a high percentage of impurity peaks were observed. These impurity peaks may be identified as Bi22Fe2O36 (ICSD: 98-024-8805), Bi2Fe4O9 (ICSD: 98-002-6808), Bi25FeO40 (ICSD: 98-004-1937), Fe2O3 (ICSD: 98-005-6372), Bi2O3 (98-042-1856), and anorthic BFO (ICSD: 98-023-7537) along with existing rhombohedral pure-phase BFO peaks. The presence of these impurity peaks may indicate the unreacted chemical species. With increasing the microwave power from 360 to 700 W, the intensity of all impurity peaks and their number decreased significantly in the second sample synthesized at 700 W as shown in Figure b. A further increase in the microwave power from 700 to 800 W resulted in the removal of all the impurity peaks that were present before in Figure a,b. The obtained XRD [Figure c] results were well-matched with the rhombohedral distorted perovskite structure of BFO in the R3C space group (ICSD 98-019-1940) with the average crystallite size of ∼20 nm. Hence, the microwave power optimizations (360, 700, 800 W) at a fixed concentration of KOH (8M) lead to selecting a single MW power level (i.e., 800 W) to synthesize pure-phase BFO in just 3 min using the MWAST method.

Figure 1

XRD patterns of BFO synthesized at different MW powers. (a) 360, (b) 700, and (c) 800 W for 3 min with 8 M KOH.

It is worth noting that the reduction in reaction time in our experiment with the MWAST technique is due to various experimental parameters and conditions. In the microwave oven heating experiments, we believe multiple factors may affect the sensitivity of the reaction rate. The uniform and swift heating, solvent type, volume, vessel size, reaction mixture, and microwave power levels are important factors that strongly govern and affect the reaction rates and final product formations. Ethanol, being a solvent that has a high boiling point and high loss tangent, can initiate a positive environment for inorganic material synthesis by microwave technology and supply sufficient heat for the reactants in the reaction medium.[34] The volume of the solvent/reaction vessel chosen has its impact on heating the reaction mixture uniformly over the entire volume, which eventually fastens the reaction rate leaving no unreacted remnants. Lower volumes can be used to make the quickest reactions (in laboratories) by gaining uniform heat supplied by microwave energy. We can also further increase the microwave energy/power for greater volumes of reaction mixtures to obtain the products in bulk for potential industrial applications. Hence, microwave power is an undeniable factor that greatly impacts obtaining rapid reactions sensitively and selectively.[35] Following the XRD analysis for the sample (prepared at 800 W, 8 M KOH, 3 min), we performed a Rietveld refinement for the obtained data. Figure displays the results obtained from the Rietveld refinement performed by utilizing the distorted perovskite rhombohedral crystal structure belonging to the R3c space group. By imposing this structural system, we could replicate all the noticed reflections. On the basis of the Rietveld refinement profile, the attained lattice cell parameters were shown in Table , and it has been observed that the experimental values obtained are in good agreement with the values reported in the literature.[36]

Figure 2

Rietveld refinement of XRD of pure-phase BFO synthesized with 8 M KOH under 3 min of MW irradiation at 800 W output power.

Table 1

Crystallographic Unit Cell Parameters Obtained from Rietveld Refinement for Pure-Phase BFO As Synthesized at 800 W with 8 M KOH

parameters	experimental data	literature values[36]	reference (ICSD card No.98-019-1940)
a (Å)	5.577 66	5.572	5.5820
b (Å)	5.577 66	5.572	5.5820
c (Å)	13.86787	13.847	13.8780
α (deg)	90.000	90.000	90.000
β (deg)	90.000	90.000	90.000
γ (deg)	120.000	120.000	120.000
volume (106 pm3)	373.63	372.326	374.49
c/a ratio	2.4863	2.4851	2.4862
agreement parameters indices	Rexp = 4.2142, Rp = 4.3124, Rwp = 5.0806, χ2 = 1.4520

Further, we also investigated the effect of KOH concentration on morphology and structural modifications of as-synthesized pure-phase BFO at 800 W microwave heating for 3 min. Figure displays the XRD patterns of BFO synthesized at different KOH concentrations (6, 8, 10, and 12 M), keeping all other parameters constant. At 6 M KOH concentration, we noticed the presence of unreacted remnants as α-Bi2O3 along with significant pure BFO formations, as shown in Figure a. This may be because of the lower solubility of precursors at low KOH concentrations. The increase in KOH concentration from 6 to 8 M has altered the existing impurity phases (at 6 M KOH) and turned all the unreacted chemical species, leaving no remnants for possible impurity peaks in XRD data implying pure-phase BFO as observed in Figure b. Next, an increase in the KOH concentration (from 8 to 10 M) led us to remark that an interesting observation of back-dropped impurities (α-Bi2O3, Bi2Fe4O9, and Bi25FeO49) at 10 M KOH concentration as shown in Figure c may be due to the decomposition of pure-phase BFO into previous precursor phases.[37,38]

Figure 3

XRD patterns of pure-phase BFO synthesized at different molar concentrations of KOH. (a) 6, (b) 8, (c) 10, and (d) 12 M under 3 min of MW irradiation at 800 W output power.

Further, at highly alkaline conditions (12 M KOH), we observed that the pure-phase BFO was driven back with no impurity phases observed as shown in Figure d. This indicates that, at higher alkaline conditions, all the impurity phases were converted to single-phase BFO. Henceforth, our findings based on an XRD data analysis reveal that the concentration of mineralizer (KOH) is an essential factor in obtaining the single pure-phase BFO.

Microstructural Analysis

The unique properties of multiferroic materials depend significantly on their microstructures of pure composites.[39−42] In recent times, Chybczyńska et al. have studied the effect of mineralizer (KOH) on the formation of BFO microflowers and showed the strong dependence of KOH concentration on the size and shape of BFO microflowers to improve the magnetic and dielectric properties.[43] The structural investigations were performed by FESEM for the multiferroic material BFO synthesized at 800 W microwave heating for 3 min by varying the KOH concentration (6, 8,10, and 12 M). It can be observed from Figure a that the synthesized BFO (800 W, 6 M KOH, 3 min) in powder form majorly comprises microflowers with nanopetals having a structural resemblance with a miniature rose. The structural morphology of an isolated miniature rose has multiple openly exposed petals where four among them are connected at the center, forming a rectangular cage as shown in Figure a.

Figure 4

FESEM micrographs of BFO synthesized in 3 min at 800 W for (a) 6, (b) 8, (c) 10, and (d) 12 M KOH concentrations.

Following the same but for 8 M KOH, similar-sized uniform spherical BFO flowers were developed, as shown in Figure b. The magnified single spherical BFO ball has been seen with numerous magnificent petals with rectangular cages on its surface indicating a similar cage-like pattern as observed in Figure a but with closed/covered petals on the spherical surface. Further, by increasing the KOH concentration from 8 to 10 M, we observed the disappearance of floral cages due to the increased size and growth of the petals as shown in Figure c. Similarly, for 12 M KOH concentration, the floral cages were completely disappeared, which may be due to masking from largely grown petals and turning the microflowers into perfectly spherical balls [Figure d] indicating the nucleation of petals at higher KOH concentrations. Therefore, from the FESEM microstructural analysis, we say that the mineralizer (KOH) has played a significant role in posturing the shape and size of BFO microflowers, and the fully developed microflowers with a large number of crystalline petals were formed at 8 M KOH (800 W). We believe that the inhomogeneous petal positioning on the surface of microflowers can contribute to enhancement in the dielectric property of the pure-phase BFO synthesized at 8 M KOH demonstrating a high dielectric constant value [Figure a].

Figure 10

Frequency response room temperature. (a) Dielectric constant and (b) dielectric loss of pure-phase BFO synthesized with 8 M KOH in 3 min at 800 W.

The elemental composition of BFO synthesized via the MWAST method was obtained from an EDX analysis as shown in Figure . The spectral peaks quantitatively represent the constituent elements present in the synthesized material sample. It is shown and confirmed from Figure that the prepared sample is of pure BFO with no extra impurities/contaminants. From the EDX quantitative analysis, the elemental composition (%) of Bi, Fe, and O was found out to be 65.55%, 23.93%, and 10.52%, respectively. These elemental mole (%) values are very close to pure bismuth ferrite’s empirical formula of BFO, that is, 66.80%, 17.85%, and 15.34% of Bi, Fe, and O, respectively.[44]

Figure 5

FESEM micrograph (a) and EDX spectra (b) of pure-phase BFO microflowers synthesized with 8 M KOH in 3 min at 800 W.

Transmission Electron Microscopy (TEM)

In-depth structural characteristics of synthesized pure-phase BFO are obtained from TEM, high-resolution TEM (HRTEM), and selected area electron diffraction (SAED) analyses. The TEM sample preparation was done by drop-casting a BiFeO3 solution onto a copper grid (coated with carbon). The BiFeO3 solution was prepared by dispersing BiFeO3 powder in ethanol using ultrasonication. Figure a displays TEM images of nanostructures of petals of pure-phase BFO microflowers synthesized at optimized conditions (800 W, 8 M KOH, 3 min) using the MWAST method. It has been observed from Figure a that the petals of pure-phase BFO are composed of nanocrystalline BFO blocks with a size exceeding 50 nm. To examine the crystallinity in nanostructures of petals, we performed SAED and HRTEM analyses. The SAED patterns of pure-phase BFO [Figure b] display a regular arrangement of sharp diffraction spots indicating a perfect crystallization of pure-phase BFO, which is assigned to a perovskite rhombohedral structure with R3c space group having no amorphous phases. Figure c illustrates the HRTEM images of nanostructures of floral petals of pure-phase BFO synthesized at optimized parameters (800 W, 8 M KOH, 3 min). The interplanar spacing of crystal planes was observed to be 3.9, 2.2 Å, which corresponds to the respective crystal planes (012), (202) of rhombohedral pure BFO. Unlike the aggregations and amorphous phases of BFO in the previously reported works,[45] our TEM analysis for structural characterizations of BFO has shown remarkable results about the crystallinity of pure-phase BFO microflowers and no particle–particle aggregations that can enhance the multiferroic properties of BFO.

Figure 6

HRTEM images of BFO microflowers (a) TEM image, (b) SAED patterns, and (c) HRTEM image of BFO microflowers synthesized at 800 W with 8 M KOH in 3 min.

Fourier Transform Infrared Spectroscopy (FTIR)

The FTIR analysis is an essential characterization tool that acquires qualitative structural details about the functional groups and their associations in multiferroic materials. Here, we analyzed the obtained data to extrapolate the information from elemental absorption bands of BFO synthesized at 800 W, 8 M KOH, 3 min. Figure represents the FTIR spectrum whose spectral region was taken from 400 to 4000 cm–1. The broad absorption band from 3000 to 3500 cm–1 is associated with the antisymmetric and symmetric stretching modes of water and OH– bond factions. The peak at 1631 cm–1 is attributed to the vibration bending mode of water molecules. The high-intensity absorption peak at 561 cm–1 is related to the bending vibration mode of the Fe–O bond, while the absorption peak at 442 cm–1 corresponds to the stretching vibration mode of the Fe–O bond, which can be interpreted as the characteristic feature of octahedra [FeO6] in the BFO perovskite structure.[46,47] It has also been observed that there were no nitrates or any other intermittent impurities, indicating our material is in a close match with the available literature, particularly with the pure phase of BFO.[48,49]

Figure 7

FTIR spectrum of BFO synthesized at 800 W in 3 min with 8 M KOH.

Raman Analysis

Raman scattering is a molecular vibrational spectroscopic technique that can determine the unique fingerprint of any molecular system and is used to study the lattice dynamics of various materials. Figure represents the Raman spectra obtained from a laser excitation of 632 nm at room temperature. It is known from the literature that there are 13 Raman modes of rhombohedral distorted pure-phase BFO material that correspond to the R3C space grouping.[50] The pure-phase BFO synthesized at 800 W in 3 min has 3A1 and 9E transverse optical (TO) vibrational modes. On the basis of a literature review, the assignments of the observed vibrational modes were made. We observed 10 transverse vibrational modes (one A1(TO) + nine E(TO)) and two longitudinal (two A1(L.O.)) for pure BFO in the obtained Raman spectrum. Raman peaks at 73, 131, 169, 219, 268, 279, 346, 379, 437, 476, 497, and 522 cm–1 correspond to E(T01), E(T02), A1(L01), A1(L02), E(T04), E(T05), E(T06), E(T07), E(T08), E(T08), E(T09), and A1(T04) modes, respectively. At lower wavenumber regions (i.e., below 170 cm–1), the vibrational modes and longitudinal modes give information about Bi atoms, while at higher wavenumbers (i.e., above 270 cm–1), the oxygen motion is predominant.[51,52]

Figure 8

Room-temperature Raman spectra of pure-phase BFO synthesized with 8 M KOH at 800 W MW power for 3 min of MW irradiation.

Most of the Fe atom modes are seen in the 152–268 cm–1 region, and a few are at higher wavenumbers. The covalent bond of Bi is represented by the E(TO1) and E(TO2) modes that are present in the Raman spectrum of our pure-phase BFO sample. Therefore, from the Raman analysis, it is worth saying that the multiferroic material prepared via the MWAST method has given a unique fingerprint about all bonds and vibrational modes, confirming the formation of pure-phase BFO.

X-ray Photoelectron Spectroscopy (XPS)

A study of electron binding energies of the constituent elements (Bi, Fe, and O) was achieved by XPS and confirms the phase purity of the single-crystalline BFO. In the pure-phase BFO, the presence of oxygen vacancies and Fe2+ ions are carefully monitored while the XPS data are analyzed, as these components have an enormous impact on the multiferroic properties.[53,54] A stoichiometric BFO may exhibit a variety of structural defects that may affect the magnetic and electrical properties. Figure a–c illustrates typical XPS spectra for the Bi, Fe, and O elements at the core level.

Figure 9

XPS core-level spectra for (a) Bi, (b) Fe, and (c) O elements of pure-phase BFO microflowers synthesized at 800 W with 8 M KOH in 3 min.

Figure a represents the core-level spectra of Bi 4f, which confirms the +3 oxidation state of bismuth ion as its binding energy peaks are at 161.62 and 156.31 eV. The corresponding spin–orbit splitting energies are found to be 5.31 eV for 5/2 and 7/2 spin–orbit doublet components. All the obtained values of Bi 4f are closely matched with the reported ones.[52]Figure b displays the deconvolution of XPS core-level spectra of Fe 2p. For Fe 2p3/2, it is well-known that the binding energy peaks are located at 709 eV for the +2 oxidation state and 710.5 eV for the +3 oxidation state.[55−57] It has been observed that the broad asymmetric scan of pure Fe 2p exhibits two wide doublet peaks positioned at 724.21, 711.03 eV for Fe 2p1/2, 2p3/2, respectively, with the +3 oxidation state and the spin–orbit splitting energy of 13.18 eV, which are in good agreement with previous reports.[58] Two sub-bands in Fe 2p3/2 located at 711.03 and 709.49 eV indicate the presence of +3 and +2 oxidation states, respectively. The XPS peak at 717 eV corresponds to the satellite peak of Fe3+, and the intensity of Fe2+ to an absolute peak is calculated by the relative integral of the Fe2+ to the total peak, which gives the percentage of Fe2+(43%) and Fe3+(57%) ions. Therefore, it is understood that the synthesized pure-phase BFO via the MWAST method has more Fe3+ ions than Fe2+. Further, a detailed analysis of the O 1s XPS spectrum of pure BFO (800 W, 8 M KOH, 3 min) is performed to investigate the oxygen vacancies. From Figure c, it can be seen that O 1s peaks are fitted with two components, as oxygen vacancies often introduce an additional component to the lattice oxygen.[59] The O 1s peaks are characterized as a superposition of two nearby peaks that are located at 529.27 and 531.1 eV and denoted by “O1” and “O2”, respectively. The presence of a dual O 1s peak is very common for oxide materials that contain cations in the multiple valence states. This indicates that the O12– are present in oxygen-rich neighboring atoms, whereas O22– ions exist in oxygen-deficient areas.[60] Therefore, the O1 peak corresponds to the lattice oxygen, and the O2 peak can be assigned to the oxygen vacancies.[61] We calculated the percentage of O1 and O2 by integrating the intensity of the O2 peak relative to the total peak intensity and estimated the value to be 36% for O2 and 64% for O1. Hence, a detailed analysis of the XPS data demonstrated the oxidation values of Fe and O to suggest the presence of Fe-based impurities along with abundant oxygen vacancies at room temperature.

Dielectric Analysis

The room-temperature dielectric constant and dielectric loss as a function of frequency was measured for our BFO sample prepared at 8 M KOH, 800 W for 3 min. The frequency response of the dielectric constant and dielectric loss was measured in the range from 100 Hz to 20 MHz, and the response curve was plotted as shown in Figure . It can be observed from Figure a that the sample BFO has a high dielectric constant value throughout the frequency range with moderate dielectric loss. However, the strong frequency-dependent dielectric constant was observed at lower frequencies and tends to decrease rapidly with a rise in frequency. Decreasing the rate of dielectric constant value with frequency is observed to be very slow at higher frequencies, and the value reached to the lowest stable value at the highest frequency point, which is a very usual behavior for all dielectric materials. To date, many researchers have reported a similar dielectric behavior for multiferroic materials,[62,63] and our results are in good agreement with reported works. At low-frequency regions (∼100 Hz), the synthesized single-phase BFO exhibits a high value of dielectric constant (4000), and at higher-frequency regions (∼1 MHz) it exhibits a low dielectric constant value (136), which indicates that pure BFO is an excellent dielectric material at higher frequencies. It has been noted from the literature[64,65] that the constituent materials having thin and inhomogeneous grain boundaries are majorly responsible for a high dielectric constant value at low frequencies because of the inverse relationship between dielectric constant and grain boundary thickness. Hence, as we have observed, the formation of thin crystalline floral petals that are homogeneously oriented in different directions in Figure b with inhomogeneous grain boundaries of crystalline petals are majorly responsible for the material’s dielectric strength in a low-frequency regime. In addition, at higher-frequencies regions, the material reaches an extremum where all the electric dipoles neither rotate nor align due to a rapidly accelerating field, and eventually, the electric polarization cannot hold up its saturation leading to a drop in the dielectric constant.[66,67] The dropped value of the dielectric constant at higher frequencies remains constant as saturation because of the inability of electric dipoles to follow a rapid electric-field variation.[68] The corresponding dielectric dissipation (tan δ) can be observed from Figure b. The summary of the noticed moderate dielectric loss of the pure-phase BFO can be described as follows. The frequency dependence of dielectric loss is related to the structural homogeneity, stoichiometry, sintering temperature, and compositions of the material sample.[69] In general, any dielectric material exhibits high electrical resistance at low frequencies because of the restricted electron motion from Fe2+ to Fe3+ ions demanding a large amount of energy for the electronic exchange, which eventually exhibits high dielectric loss. At the low-frequency regime, pure-phase BFO (800 W, 8 M KOH, 3 min) exhibits high electrical resistance due to the existence of a large number of inhomogeneous grain boundaries, which demands a huge amount of energy for electron exchange between Fe2+ and Fe3+ and, thus, exhibits high dielectric loss at the low-frequency regime. Similarly, at higher frequencies, the material (BFO) exhibits low electrical resistance due to the existence of numerous crystalline petals and, hence, requires a small amount of energy for electron exchange implying minimal restrictions on the flow of electrons/ions for a reduced stable dielectric loss in the sample as shown in Figure b.

Magnetic Properties

The room-temperature magnetic behavior of the BFO sample synthesized at different microwave powers (300, 700, and 800 W), 8 M KOH, and at a constant reaction time of 3 min was studied here. As displayed in Figure , we studied the room-temperature magnetic behavior of BFO samples as a function of the applied magnetic field. The sample synthesized at 360 W showed a high magnetic order (∼1.51emu/g) compared to the pure-phase BFO synthesized at 800 W due to the high percentage of impurity phases such as Bi2Fe4O9 and Bi22Fe2O36, Bi25FeO40, and Fe2O3(contributed to magnetic order enhancement) in this sample. In contrast, the sample synthesized at 700 W exhibited low magnetic order (∼0.54emu/g) when compared with pure-phase BFO synthesized at 800 W, which might be due to the reduced impurity phases such as Fe2O3, Bi2Fe4O9, Bi25FeO40, and the presence of Bi oxide phases, which majorly affected the magnetic behavior and lowered the magnetic order.

Figure 11

Magnetic moment as a function of applied magnetic field for BFO synthesized (8 M KOH, 3 min) at different MW power levels (360, 700, and 800 W).

In general, single-phase BFO primarily exhibits antiferromagnetic behavior with a long-range spin cycloid structure (62 nm). The sample prepared at 800 W of pure-phase BFO is believed to follow usual ferromagnetic behavior with complete saturated magnetic properties (M-H) loops. From the hysteresis data obtained, we witnessed a weak ferromagnetic behavior due to acquired irregular antiparallel spins of existing antiferromagnetic behavior. It has been studied and noted from the literature[70−72] that bulk BFO exhibits straight line M-H curves, which are in contrast to our witnessed M-H curves of pure-phase BFO. The saturation of the M-H curve widely depends on the applied field. The field from −1.5 to +1.5 T has been applied for our BFO sample to study the magnetization effect. The M-H curve in Figure represents the unsaturated curve behavior due to limited applied field (i.e., −1.5 to +1.5 T). We believe a further increase in the applied field for our BFO sample may form saturated magnetic loops. However, the range of applied fields using our VSM instrument has limited our further investigations on magnetic behavior at higher fields. The pure-phase BFO has a magnetic value of 1.25 emu/g, which is slightly greater than the reported values,[73,74] and a remnant magnetization value of 0.04 emu/g with a low critical field of +171.01 and −159.06 Oe. The slight variations in two required fields indicate both ferromagnetic and antiferromagnetic order in the single-phase BFO sample. This variation has a guide to set up the phenomenon of an exchange bias to promote the reposition unidirectional anisotropy and coercivity. Lastly, the particle size effect has an impact on improving the magnetic behavior of the BFO sample. We used Debye–Scherrer’s formula to calculate the average crystallite size (26.33 nm), which is found to be smaller than the spin cycloid structure (62 nm). We believe that the high surface-to-volume ratio of tiny floral petals of BFO microflowers may be responsible for improved magnetic properties at room temperature and that the magnetization developed because these microflowers can be associated with size due to the uncompensated spin closures on the surface.[75]

Influence of KOH Concentration on Magnetic Properties of BFO

The room-temperature magnetic properties (M-H loops) of synthesized BFO material (800 W, 3 min) were studied as a function of the magnetic field by varying KOH concentrations from 6 to 12 M (Figure ). The magnetization M(H) curves obtained from different KOH concentrations (6, 8, 10, and 12 M) are similar in shape, but the magnetization values are distinctly showing a weak ferromagnetic behavior at room temperature. It was observed that the BFO synthesized (800 W, 6 M KOH, 3 min) showed a high magnetic moment (0.82 emu/g) when compared with the BFO synthesized at 10 and 12 M KOH, which may be due to the presence of Bi25FeO40, Bi2Fe4O9, and Fe2O3 magnetic phases at 6 M KOH concentration [Figure a]. The increase in KOH concentration from 6 to 8 M improved the magnetic moment (1.25 emu/g), which can be attributed to the size variations and uniformity in the crystalline petals to improve the magnetic moment value of the BFO material.[76] As the KOH concentration further increased from 8 to 10 M, the magnetic moment of the sample is decreased to 0.36 emu/g. This decrease in magnetic value can be attributed to impurity phases such as Bi2O3 (weak magnetic phase) along with the pure BFO crystal phase [Figure c]. Moreover, the effect of BFO (800 W, 10 M, 3 min) microstructures may also contribute to the reduction in the magnetic moment due to the presence of thick floral petals on the surface of spherical microflowers [Figure c]. Further, at 12 M KOH, the value of the magnetic moment is found to be 0.6 emu/g, which is lower than that of pure-phase BFO synthesized at 8 M KOH. This reduction in the magnetic moment with a retained pure phase of BFO at 12 M KOH can be attributed to the nucleation and growth of crystalline petals, which eventually closed the existing open cages of BFO microflowers [Figure b,d]. Hence, there will not be any contribution from each petal to the magnetic property.

Figure 12

Room-temperature magnetic behavior of the BFO microflowers synthesized with different KOH concentrations (6, 8, 10, and 12 M) at 800 W for 3 min of MW heating.

Ferroelectric Properties

The ferroelectric properties of BFO synthesized at 8 M KOH, 800 W, 3 min were studied at room temperature. We obtained polarization–electric field (P-E) hysteresis loops for our BFO material sample at a constant frequency of 100 Hz by varying electric fields from 2 to 25 kV/cm, as shown in Figure . The remnant polarization increases with the applied field, and the maximum remnant polarization (∼15.67 μC/cm2) is obtained at 25 kV/cm. The lossy behavior and unsaturated P-E loops are associated with the considerable leakage in the sample, which may be understood from the ionic moments of Fe3+ to Fe2+ ions enabling the formation of oxygen vacancies.[77−80] Since the XPS data of our pure-phase BFO revealed the existence of Fe2+ ions and oxygen vacancies, the BFO material may result in exhibiting a high leakage current, which might affect its ferroelectric property at room temperature. Hence, this high leakage current in the pure-phase BFO can contribute to poor ferroelectric and unsaturated P-E loops as shown in Figure .

Figure 13

P-E Hysteresis loops at a constant frequency (100 Hz) with varying fields of BFO synthesized with 8 M KOH in 3 min at 800 W.

In the previous works associated with the microwave-assisted hydrothermal synthesis of BFO, to the best of our knowledge, no group has focused on investigating the ferroelectric behavior of BFO.[81] As we can see from Figure , the polarization value of the material increases with the applied electric field, so we attempted to use large electric fields in our experiments to investigate the ferroelectric property of the sample BFO in a broad range of electric fields to obtain the complete well-saturated P-E loops. However, because of the high leakage current in our sample, we could not go beyond 25 kV/cm in order to observe the saturation. Hence, only unsaturated P-E loops were obtained.

Conclusions

In summary, we have demonstrated the simple and quickest way to synthesize the multiferroic material (BFO) using a collaborative technique from solvothermal and microwave heating. The pure phase of BFO is attained in just 3 min of microwave irradiation of chemical reactants in an ethanol medium. The microwave power variations (360, 700, and 800 W) with fixed KOH concentration (8 M) and KOH concentration variations (6, 8, 10,12 M) with fixed microwave power (800 W) were the overall optimizations undergone to investigate the optimized favorable condition for obtaining desired pure-phase BFO. The results from all the characterizations have affirmed the formation of the same with barely any impurity phases. The morphology and structural analyses have shown interesting pure-phase BFO microflowers with magnificent petals on the surface with no agglomerations at the high microwave power of 800 W for 3 min of irradiation with 8 M KOH. Our optimizations on alkaline conditions conclude that the surface morphology of microflowers is much sensitive to KOH concentrations. After the phase and structural confirmations from respective XRD, FESEM, and TEM characterizations, the dielectric properties were qualitatively investigated by dielectric spectroscopy at room temperature. The pure-phase BFO has shown a good frequency response of dielectric constant with moderate loss. Further, the effect of microwave power and KOH concentration on magnetic properties was investigated from a VSM analysis (magnetic hysteresis), and we observed the existence of a weak ferromagnetic order for pure-phase BFO (800 W, 8 M KOH, 3 min) with a magnetization value of 1.25 emu/g. The XPS analysis of pure-phase BFO revealed the existence of the Fe2+ oxidation state and the formation of oxygen vacancies, which led to the high leakage current in the pure-phase BFO material. We then studied the ferroelectric behavior of pure-phase BFO from P-E hysteresis and confirmed the moderate ferroelectric behavior with the applied electric field. Hence, as a whole, we have produced the potential multiferroic candidate (BFO) with fascinating microstructures and improved magnetic and ferroelectric properties in just 3 min using the MWAST method.