Hydrogen Peroxide-Assisted Hydrothermal Synthesis of BiFeO 3 Microspheres and Their Dielectric Behavior

: Despite considerable e ﬀ orts undertaken in a rapidly developing area of multiferroic research, synthesis of phase pure BiFeO 3 is still a matter of intensive research. In this work, we report the shape-controlled synthesis of pure BiFeO 3 microspheres via a facile hydrothermal route. The prepared BiFeO 3 powder has been characterized using powder X-ray Di ﬀ raction (XRD), Di ﬀ erential Thermal analysis (DTA), Scanning Electron microscopy (SEM), and impedance spectroscopy. Powder XRD analysis conﬁrms the formation of pure rhombohedrally distorted perovskite with R 3 c space group. Scanning electron micrograph revealed that the prepared BiFeO 3 microspheres are nearly spherical in shape with uniform size distribution. The BiFeO 3 microspheres exhibit a dielectric constant value of ~110 at 1000 KHz, which is higher than the BiFeO 3 prepared by conventional solid-state reaction and sol–gel method. Variation of dielectric constant with temperature at di ﬀ erent frequencies shows that the BiFeO 3 has a dielectric anomaly of ferroelectric to paraelectric type at 1093 K and this phenomenon is well supported by TGA results.


Introduction
Nowadays, multiferroic (particularly BiFeO 3 ) materials are being widely investigated for spintronics, data-storage, sensors and multiple-state memory applications [1] because of their dual ordering (i.e., both magnetic and polar) characteristics at room temperature. BiFeO 3 crystallizes in a rhombohedral structure with R3c space group, and exhibits ferroelectric and antiferromagnetic properties at~1100 and 630 K, respectively [2]. BiFeO 3 generates spontaneous polarization,~100 µC/cm 2 , from the lone pair of the Bi 3+ ions, while the magnetic ordering originates from the superexchange interaction between the 3d electrons of the Fe 3+ ions [3]. In recent years, BiFeO 3 has attracted significant interest owing to its high Curie temperature (1083 K) [4] and high Neel temperature (625 K) and its potential applications, such as water splitting, organic pollutants degradation (2.2 eV), solar cells and visible light optoelectronic devices [5][6][7][8]. However, BiFeO 3 finds limited application due to the high electric loss via leakage, existence of impurities, high electric coercive field, and weak magnetoelectric coupling due to the significant difference between the Curie temperature (1083 K) and the high Neel temperature (625 K), which leads to inadequate ferroelectric and magnetic properties in bulk BiFeO 3 .
Magnetochemistry 2020, 6, 42 2 of 8 Different methods have been adopted in order to overcome the above limitations [2,3], including but not limited to different synthesis techniques, producing solid solution via rare earth ions doping at both Bi and Fe sites [2]. These methods can improve the stability, multiferroic features and magnetoelectric coupling effect of BiFeO 3 . The magnetoelectric coupling effect can be effectively altered by tuning Curie temperature and Neel temperature. It was reported that the incorporation of rare earth ion at Bi site resulted in the modification of the ferroelectric properties, whereas the rare earth ion at the Fe site alters the magnetic properties [3]. The preparation of doped and un-doped BiFeO 3 via sol-gel and solid-state [1] reactions requires elevated temperature of 600-900 • C and complex/expensive reagents like citric acid and urea [9].
Therefore, there is an urgent need to develop new synthesis processes for obtaining pure BiFeO 3 , which is a challenging task even today due to the appearance of secondary phases in BiFeO 3 . It is very difficult to obtain single phase BiFeO 3 because of non-stoichiometric oxygen in the structure, which leads to the formation of undesirable impurity phases such as BiFe 4 O 9 , Bi 25 FeO 39 and Bi 25 FeO 40 [10]. The presence of BiFe 4 O 9 , Bi 25 FeO 39 and Bi 25 FeO 40 impurities significantly reduces the thermal stability of BiFeO 3 , and they also increase the leakage current [11]. The synthesis of pure BiFeO 3 is quite subtle, because it is necessary to take both kinetic and thermodynamic properties into account. In this view, considerable effort has been made in the past for the controlled synthesis of BiFeO 3 owing to its size-and morphology-dependent magnetic, electrical, and optical properties [5,[12][13][14][15][16]. However, the successive modulation of size and morphology of BiFeO 3 has yet to become a reality.
In the present paper, as the synthesis technique and reaction parameters play a crucial role in controlling the material properties, we successfully applied the hydrothermal synthesis route for the fabrication of single phase BiFeO 3 microspheres without any additional thermal treatment. The hydrothermal synthesis route possessed the potential to produce well-crystallized BiFeO 3 with controlled morphology and narrow distribution of particle size. The BiFeO 3 microspheres prepared by the hydrothermal method show higher dielectric constant value (~110 at 1000 KHz) than the BiFeO 3 prepared by conventional solid-state reaction (~20 at 100 KHz) and sol-gel method (~68 at 1M Hz). Additionally, BiFeO 3 has shown dielectric anomaly of ferroelectric to paraelectric type at 1093 K.

Method
Amounts of 0.01 mol Bi(NO 3 ) 3 ·5H 2 O, 0.01 mol Fe(NO 3 ) 3 ·9H 2 O, 0.051 mol NaOH, 0.049 mol KOH were mixed with 12 mL acetone and stirred for 10 min. The reason for using high concentrations of NaOH and KOH was to sufficiently maintain the basic condition. During this, an exothermic reaction took place, and all the acetone evaporated. After thorough mixing, 4 mL H 2 O 2 was added to provide an oxygen-enriched atmosphere to keep the Fe ion in Fe 3+ form and convert hydroxide into oxide. The obtained resin-like suspension was poured into the Teflon-lined stainless-steel autoclave for hydrothermal treatment. The autoclave was sealed and maintained at 200 • C ± 5 for 24 h. After the completion of the reaction, the product was removed from the autoclave and washed with water and acetone several times and dried in a laboratory oven at 80 • C ± 5 for several hours.

Characterization
The structure and crystallinity of the prepared BiFeO 3 microspheres were characterized by PANalytical X pert X-ray diffractometer (XRD) equipped with graphite monochromatized CuKa radiation (Malvern Panalytical, Lelyweg, The Netherlands). Differential thermal analysis of BiFeO 3 microspheres was carried out from room temperature to 900 • C in pure oxygen atmosphere at a scan rate of 10 • C/min using Q600. Scanning electron microscope (SEM) images were taken with a JEOL. The SEM images were recorded by adding a small amount of sample onto the carbon conductive tape, and then gold was spurred for 9 s. Then, the gold-spurred samples were examined using SEM. The densities of the BiFeO 3 pellets (made with 2% polyvinyl alcohol as a binder) were measured using the Archimedes principle. The sintered BiFeO 3 ceramic specimens were polished and then electroded with high-quality silver paint on both sides of the disk-shaped samples. The dielectric and electrical properties of the samples were measured using a Hewlett Packard 4194A impedance gain phase analyzer (Commonwealth of Massachusetts, Woburn) over wide ranges of frequency 100 Hz-10 MHz and temperature room temperature to 830 • C.

Results and Discussion
The phase composition and structure of the as-prepared BiFeO 3 sintered at different temperatures were examined by powder XRD technique, and the observed XRD patterns are displayed in Figure 1. All the diffraction peaks in the XRD pattern of as-prepared BiFeO 3 (prepared at 200 • C/24 h by hydrothermal method) were consistent with the standard Joint Committee on Powder Diffraction Standards No. 86-1518 of pure BiFeO 3 having a rhombohedral crystal structure and R3c (161) space group. Additionally, the diffraction (012) peaks (2 theta = 22.34) of the as-prepared BiFeO 3 are intense and sharp, indicating the formation of well-crystallized single phase BiFeO 3 . To investigate the effect of heat treatment on the BiFeO 3 samples, the powders were pressed into pellets and heat-treated at different temperatures (at 600, 700 and 800 • C), as shown in Figure 1. It is observed that the sintered BiFeO 3 pellets retain the rhombohedral structure with space group R3c at higher sintering temperature. The secondary phases associated with the BiFeO 3 such as Bi 2 Fe 4 O 9 and Bi 25 FeO 40 are absent in the present study. This suggests the formation of crystalline single phase BiFeO 3 with good thermal stability. The crystallite size (D) of BiFeO 3 samples was calculated by following the Scherrer Formula.
where D is the crystallite size, λ is the wavelength of radiation, β is the full width at half maximum (FWHM) of the diffraction peak in radian and θ is Braggs angle. The crystallite sizes are calculated by considering the prominent diffraction peaks peaked at 2θ = 22.34 and 45.73, corresponding to the (012) and (024) planes, respectively. The average crystallite size of the as-prepared, 600, 700, and 800 • C samples was 34 ± 1.24, 41 ± 0.47, 43 ± 1.67, and 47 ± 0.94 nm, respectively.
Magnetochemistry 2020, 6, x FOR PEER REVIEW 3 of 8 rate of 10 °C/min using Q600. Scanning electron microscope (SEM) images were taken with a JEOL. The SEM images were recorded by adding a small amount of sample onto the carbon conductive tape, and then gold was spurred for 9 s. Then, the gold-spurred samples were examined using SEM. The densities of the BiFeO3 pellets (made with 2% polyvinyl alcohol as a binder) were measured using the Archimedes principle. The sintered BiFeO3 ceramic specimens were polished and then electroded with high-quality silver paint on both sides of the disk-shaped samples. The dielectric and electrical properties of the samples were measured using a Hewlett Packard 4194A impedance gain phase analyzer (Commonwealth of Massachusetts, Woburn) over wide ranges of frequency 100 Hz-10 MHz and temperature room temperature to 830 °C.

Results and Discussion
The phase composition and structure of the as-prepared BiFeO3 sintered at different temperatures were examined by powder XRD technique, and the observed XRD patterns are displayed in Figure 1. All the diffraction peaks in the XRD pattern of as-prepared BiFeO3 (prepared at 200 °C/24 h by hydrothermal method) were consistent with the standard Joint Committee on Powder Diffraction Standards No. 86-1518 of pure BiFeO3 having a rhombohedral crystal structure and R3c (161) space group. Additionally, the diffraction (012) peaks (2 theta = 22.34) of the asprepared BiFeO3 are intense and sharp, indicating the formation of well-crystallized single phase BiFeO3. To investigate the effect of heat treatment on the BiFeO3 samples, the powders were pressed into pellets and heat-treated at different temperatures (at 600, 700 and 800 °C), as shown in Figure 1. It is observed that the sintered BiFeO3 pellets retain the rhombohedral structure with space group R3c at higher sintering temperature. The secondary phases associated with the BiFeO3 such as Bi2Fe4O9 and Bi25FeO40 are absent in the present study. This suggests the formation of crystalline single phase BiFeO3 with good thermal stability. The crystallite size (D) of BiFeO3 samples was calculated by following the Scherrer Formula.
where D is the crystallite size, λ is the wavelength of radiation, β is the full width at half maximum (FWHM) of the diffraction peak in radian and θ is Braggs angle. The crystallite sizes are calculated by considering the prominent diffraction peaks peaked at 2θ = 22.34 and 45.73, corresponding to the (012) and (024) planes, respectively. The average crystallite size of the as-prepared, 600, 700, and 800 °C samples was 34 ± 1.24, 41 ± 0.47, 43 ± 1.67, and 47 ± 0.94 nm, respectively.  The formation of BiFeO 3 under the present experimental conditions can be explained as follows. Bismuth hydroxide and iron hydroxides were formed when bismuth nitrate and iron nitrate were mixed with sodium hydroxide and potassium hydroxide at room temperature (Equations (2) and (3)). Then, under hydrothermal conditions, the formed Fe(OH) 3 and Bi(OH) 3 undergo condensation to form BiFeO 3 microspheres (Equation (4)). Figure 2 presents the DTA curve of the BiFeO 3 microspheres recorded between the temperatures 25 to 850 • C. The endothermic peak appearing at around 1088 K is attributed to the phase transformation ferroelectric to paraelectric, i.e., Curie temperature [17]. This observed Curie temperature is in agreement with the reported values for BiFeO 3 [18,19]. Furthermore, weak transition at 380 • C (653K) corresponds to antiferromagnetic phase transition.
Magnetochemistry 2020, 6, x FOR PEER REVIEW 4 of 8 The formation of BiFeO3 under the present experimental conditions can be explained as follows. Bismuth hydroxide and iron hydroxides were formed when bismuth nitrate and iron nitrate were mixed with sodium hydroxide and potassium hydroxide at room temperature (Equations (2) and (3)). Then, under hydrothermal conditions, the formed Fe(OH)3 and Bi(OH)3 undergo condensation to form BiFeO3 microspheres (Equation (4)). Figure 2 presents the DTA curve of the BiFeO3 microspheres recorded between the temperatures 25 to 850 °C. The endothermic peak appearing at around 1088 K is attributed to the phase transformation ferroelectric to paraelectric, i.e., Curie temperature [17]. This observed Curie temperature is in agreement with the reported values for BiFeO3 [18,19]. Furthermore, weak transition at 380 °C (653K) corresponds to antiferromagnetic phase transition. To study the dielectric permittivity, the density of the BiFeO3 was measured. The BiFeO3 powders were pressed into a pellet of diameter 9 mm. The pressed BiFeO3 pellets were submerged in water. The effective mass under water was determined by subtracting actual mass from the mass of the water displaced. Then, the volume of the BiFeO3 pellets was determined. Finally, the mass was divided by the volume to estimate the average density [20]. The effect of temperature on the density of the pellets can be clearly seen in Figure 3. The density of the prepared BiFeO3 pellets sintered at 500 °C, 600 °C, 750 °C and 800 °C was shown to be 81%, 85%, 83% and 94%, respectively. Therefore, with an increase in the sintering temperature, the density of the BiFeO3 increases, with the maximum being found to be ∼94% at 800 °C. The observed density is h when the sample is sintered at 800 °C. The observed density, ~94%, is close to the BiFeO3 density (96%) reported by spark plasma sintering technique [21]. The lower density compared to the theoretical density may be due to the presence of little porosity in the pellets. To study the dielectric permittivity, the density of the BiFeO 3 was measured. The BiFeO 3 powders were pressed into a pellet of diameter 9 mm. The pressed BiFeO 3 pellets were submerged in water. The effective mass under water was determined by subtracting actual mass from the mass of the water displaced. Then, the volume of the BiFeO 3 pellets was determined. Finally, the mass was divided by the volume to estimate the average density [20]. The effect of temperature on the density of the pellets can be clearly seen in Figure 3. The density of the prepared BiFeO 3 pellets sintered at 500 • C, 600 • C, 750 • C and 800 • C was shown to be 81%, 85%, 83% and 94%, respectively. Therefore, with an increase in the sintering temperature, the density of the BiFeO 3 increases, with the maximum being found to be~94% at 800 • C. The observed density is h when the sample is sintered at 800 • C. The observed density,~94%, is close to the BiFeO 3 density (96%) reported by spark plasma sintering technique [21]. The lower density compared to the theoretical density may be due to the presence of little porosity in the pellets. A typical scanning electron micrograph of the BiFeO3 powder prepared by the present method is shown in Figure 4. It was observed that the BiFeO3 powder exhibited a spherical structure with a rough surface. The average size of the BiFeO3 spherical structures was found to be in the range of 20-30 μm. Each sphere is composed of significantly smaller agglomerated particles. The spherical structures are formed due to the high oxygen partial pressure [22]. The formation of high oxygen partial pressure is due to the decomposition of molten nitrate during heat treatment in a closed container according to Equation (5). Similar results were reported by Xiaobo He et al. for the synthesis of BiFeO3 in presence of potassium and sodium nitrate [23]. The mixture KOH and NaOH was used for precipitation in our synthesis with the aim to carry out the reaction similar to molten metal flux method. Generally, melts can be made more acidic (H2Orich) or basic (O 2-rich) by controlling the water content of the melt, based on the product formation requirement. In our synthesis, both iron nitrate and bismuth nitrate dissolves and heating at 200 °C in air-tight autoclave resulted in slow loss of water as vapors, thereby making the melt more basic and the product insoluble [24]. In addition, the excess added NaOH and KOH maintain a basic pH.
The effect of frequency on the dielectric constant and the dielectric loss factor of BiFeO3 are presented in Figure 5. The dielectric constant is found to decrease slowly when frequency increases A typical scanning electron micrograph of the BiFeO 3 powder prepared by the present method is shown in Figure 4. It was observed that the BiFeO 3 powder exhibited a spherical structure with a rough surface. The average size of the BiFeO 3 spherical structures was found to be in the range of 20-30 µm. Each sphere is composed of significantly smaller agglomerated particles. The spherical structures are formed due to the high oxygen partial pressure [22]. The formation of high oxygen partial pressure is due to the decomposition of molten nitrate during heat treatment in a closed container according to Equation (5). Similar results were reported by Xiaobo He et al. for the synthesis of BiFeO3 in presence of potassium and sodium nitrate [23]. A typical scanning electron micrograph of the BiFeO3 powder prepared by the present method is shown in Figure 4. It was observed that the BiFeO3 powder exhibited a spherical structure with a rough surface. The average size of the BiFeO3 spherical structures was found to be in the range of 20-30 μm. Each sphere is composed of significantly smaller agglomerated particles. The spherical structures are formed due to the high oxygen partial pressure [22]. The formation of high oxygen partial pressure is due to the decomposition of molten nitrate during heat treatment in a closed container according to Equation (5). Similar results were reported by Xiaobo He et al. for the synthesis of BiFeO3 in presence of potassium and sodium nitrate [23]. The mixture KOH and NaOH was used for precipitation in our synthesis with the aim to carry out the reaction similar to molten metal flux method. Generally, melts can be made more acidic (H2Orich) or basic (O 2-rich) by controlling the water content of the melt, based on the product formation requirement. In our synthesis, both iron nitrate and bismuth nitrate dissolves and heating at 200 °C in air-tight autoclave resulted in slow loss of water as vapors, thereby making the melt more basic and the product insoluble [24]. In addition, the excess added NaOH and KOH maintain a basic pH.
The effect of frequency on the dielectric constant and the dielectric loss factor of BiFeO3 are presented in Figure 5. The dielectric constant is found to decrease slowly when frequency increases The mixture KOH and NaOH was used for precipitation in our synthesis with the aim to carry out the reaction similar to molten metal flux method. Generally, melts can be made more acidic (H 2 O-rich) or basic (O 2− rich) by controlling the water content of the melt, based on the product formation requirement. In our synthesis, both iron nitrate and bismuth nitrate dissolves and heating at 200 • C in air-tight autoclave resulted in slow loss of water as vapors, thereby making the melt more basic and the product insoluble [24]. In addition, the excess added NaOH and KOH maintain a basic pH.
The effect of frequency on the dielectric constant and the dielectric loss factor of BiFeO 3 are presented in Figure 5. The dielectric constant is found to decrease slowly when frequency increases from 100 Hz to 100 kHz, and then becomes almost constant, showing the usual dielectric dispersion [25]. It is observed from the figure that the prepared BiFeO 3 microsphere exhibit a high dielectric constant value of~113 ± 4.64 at 1000 KHz, which is significantly higher than the hydrothermally derived Cr-substituted BiFeO 3 (50 at 0.2-1.2 MHz), and BiFeO 3 prepared by conventional solid-state (~20 at 100 KHz) and sol-gel reaction (~68 at 1M Hz) [25][26][27]. Generally, the dielectric properties of the relaxors depend on the microstructural characteristics such as grain size, grain-boundary character and pores. Therefore, microstructural control with an appropriate choice of additive can be critical in the applications. Better powder characteristics, such as a relatively uniform distribution of particle size and less particle agglomeration, obtained by the present method are believed to contribute to the resultant dielectric properties.
Magnetochemistry 2020, 6, x FOR PEER REVIEW 6 of 8 from 100 Hz to 100 kHz, and then becomes almost constant, showing the usual dielectric dispersion [25]. It is observed from the figure that the prepared BiFeO3 microsphere exhibit a high dielectric constant value of ~113 ± 4.64 at 1000 KHz, which is significantly higher than the hydrothermally derived Cr-substituted BiFeO3 (50 at 0.2-1.2 MHz), and BiFeO3 prepared by conventional solid-state (~20 at 100 KHz) and sol-gel reaction (~68 at 1M Hz) [25][26][27]. Generally, the dielectric properties of the relaxors depend on the microstructural characteristics such as grain size, grain-boundary character and pores. Therefore, microstructural control with an appropriate choice of additive can be critical in the applications. Better powder characteristics, such as a relatively uniform distribution of particle size and less particle agglomeration, obtained by the present method are believed to contribute to the resultant dielectric properties. The effect of temperature on the dielectric constant is shown in Figure 6. The dielectric constant of the specimen was measured from room temperature to 850 °C at frequencies of 100, 500, 1000, and 10,000 kHz. It was found that the BiFeO3 has a dielectric anomaly at 1093 K (the Curie temperature, Tc) suggesting the occurrence of ferroelectric-paraelectric phase transition.  The effect of temperature on the dielectric constant is shown in Figure 6. The dielectric constant of the specimen was measured from room temperature to 850 • C at frequencies of 100, 500, 1000, and 10,000 kHz. It was found that the BiFeO 3 has a dielectric anomaly at 1093 K (the Curie temperature, Tc) suggesting the occurrence of ferroelectric-paraelectric phase transition.

Conclusions
Magnetochemistry 2020, 6, x FOR PEER REVIEW 6 of 8 from 100 Hz to 100 kHz, and then becomes almost constant, showing the usual dielectric dispersion [25]. It is observed from the figure that the prepared BiFeO3 microsphere exhibit a high dielectric constant value of ~113 ± 4.64 at 1000 KHz, which is significantly higher than the hydrothermally derived Cr-substituted BiFeO3 (50 at 0.2-1.2 MHz), and BiFeO3 prepared by conventional solid-state (~20 at 100 KHz) and sol-gel reaction (~68 at 1M Hz) [25][26][27]. Generally, the dielectric properties of the relaxors depend on the microstructural characteristics such as grain size, grain-boundary character and pores. Therefore, microstructural control with an appropriate choice of additive can be critical in the applications. Better powder characteristics, such as a relatively uniform distribution of particle size and less particle agglomeration, obtained by the present method are believed to contribute to the resultant dielectric properties. The effect of temperature on the dielectric constant is shown in Figure 6. The dielectric constant of the specimen was measured from room temperature to 850 °C at frequencies of 100, 500, 1000, and 10,000 kHz. It was found that the BiFeO3 has a dielectric anomaly at 1093 K (the Curie temperature, Tc) suggesting the occurrence of ferroelectric-paraelectric phase transition.

Conclusions
BiFeO 3 microspheres were synthesized by direct precipitation followed by hydrothermal route from the mixture of iron nitrate and bismuth nitrate in the molten KOH/NaOH solution at temperature as low as 200 • C. The prepared BiFeO 3 microspheres exhibit higher thermal stability up to 800 • C, and show larger dielectric constant values at different frequencies. The proposed method may be extended for the synthesis of other perovskite materials.