Microstructural, optical, and magnetic properties and specific absorption rate of bismuth ferrite/SiO2 nanoparticles

In this study, the microstructural, optical, and magnetic properties and specific absorption rate (SAR) of bismuth ferrite/SiO2 nanoparticles were successfully investigated. The coprecipitation method was used to synthesize the nanoparticles. X-ray diffraction patterns showed the presence of sillenite-type Bi25FeO40 with a body-centered cubic structure. The crystallite size of Bi25FeO40 was 35.0 nm, which increased to 41.5 nm after SiO2 encapsulation. Transmission electron microscopy images confirmed that all samples were polycrystalline. The presence of Si–O–Si (siloxane) stretching at 1089 cm−1 in Fourier transform infrared spectra confirmed the encapsulation of SiO2. Magnetic measurements at room temperature indicated weak ferromagnetic properties of the samples. The coercivity of the bismuth ferrite nanoparticles was 78 Oe, which increased after SiO2 encapsulation. In contrast, their maximum magnetization, 0.54 emu g−1, reduced after SiO2 encapsulation. The determined bandgap energy of the bismuth ferrite nanoparticles was approximately 2.1 eV, which increased to 2.7 eV after SiO2 encapsulation. The effect of SiO2 encapsulation on the SAR of the samples was investigated using a calorimetric method. The SAR values of the bismuth ferrite nanoparticles were 49, 61, and 84 mW g−1 under alternating magnetic field (AMF) strengths of 150, 200, and 250 Oe, respectively, which decreased after SiO2 encapsulation. The maximum magnetization and the AMF strength influenced the SAR of the nanoparticles. The results showed that SiO2 has a significant effect in determining the microstructural, optical, and magnetic properties and SAR of the nanoparticles.


Introduction
Because bismuth ferrite possesses both magnetic and photocatalytic properties, it has generated significant interest. It presents many different crystal structures such as perovskite (BiFeO 3 ), sillenite (Bi 25 FeO 40 ), mullite (Bi 2 Fe 4 O 9 ), and composite (Bi 25 FeO 40 /Bi 2 Fe 4 O 9 ) structures [1][2][3]. The Bi 25 FeO 40 and Bi 2 Fe 4 O 9 structures are generally impurity phases in synthesized BiFeO 3 . Bi 25 FeO 40 has a cubic structure with a space group of I23. The lattice parameters of the sillenite structure of bismuth ferrite are a=b=c=1.018 nm [4][5][6]. In the sillenite structure, the Bi-O framework connects isolated oxygen tetrahedra, and the M site (the location of the metal cation) is at the center of each tetrahedron [7]. Four O atoms surround the M atom, which is located at the center of the MO 4 tetrahedra with a distance of 1.65-1.95 Å from the O atoms. This distance is based on the radii of O and M atoms: ∼1.38 Å and 0.27-0.57 Å, respectively [8]. In the sillenite structure of bismuth ferrite, FeO 4 tetrahedra are located at the corners and center of the unit cell, which are linked by a Bi-O polyhedral framework [3]. Bi 25 FeO 40 nanoparticles have been reported to have different magnetic properties, including paramagnetic [9], weak ferromagnetic [10], and superparamagnetic [11] properties. In addition, Bi 25 FeO 40 nanoparticles have a narrow bandgap energy of less than 2.5 eV [2,12,13]. Recently, bismuth ferrite has been most extensively studied for its applications in sensors, photovoltaics, storage devices, spintronics, photocatalysts, and hyperthermia [14][15][16][17][18]. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI.
Hyperthermia is a therapy for cancer treatment. It involves increasing the temperature to a general range of 42°C-47°C to destroy cancer cells in a body [19,20]. The heating mediators commonly used in hyperthermia therapy are magnetic nanoparticles that can produce the heat required to damage cancer cells in the body tissues [21]. In hyperthermia therapy, magnetic nanoparticles are injected into cancerous areas of a body. The heat produced by these magnetic nanoparticles under an alternating magnetic field (AMF) kills the cancer cells without damaging nearby healthy cells. The types of magnetic nanoparticles that provide satisfactory results in hyperthermia applications and have been extensively studied are ferrite-based superparamagnetic nanoparticles, such as MFe 2 O 4 (M=Co, Ni, Mn, Cu, and Mg), and lanthanum strontium manganese oxide [22]. Studies on bismuth ferrite as a heating mediator in hyperthermia are rare. Rajaee et al reported that bismuth ferrite nanoparticles possess unique optical, magnetic, high photoelectric absorption, and inductive heating properties, making them potential candidates for hyperthermia applications [18].
The amount of thermal energy dissipated in hyperthermia applications is determined by measuring the specific absorption rate (SAR). The SAR is the ratio of the thermal power dissipation to the mass of the magnetic nanoparticles, and is influenced by the particle size, saturation magnetization, magnetic anisotropy [23], applied strength, and frequency of the AMF [19,24]. Research on hyperthermia therapy has also generated interest for encapsulating magnetic nanoparticles with nonmagnetic materials to serve as heating mediators. The encapsulation of magnetic nanoparticles with silica has been reported to have potential for hyperthermia therapy because of the high stability of the silica shells and the sufficient heat generation ability of the core nanoparticles [25].
Magnetic nanoparticles possess hydrophobic surfaces. Therefore, they interact to form a cluster and agglomerate [22]. To expand the application of magnetic nanoparticles, their encapsulation is required to improve their biocompatibility, solubility, and stability and prevent agglomeration. An ideal material for encapsulating the surfaces of magnetic nanoparticles is SiO 2 , which is non-toxic. Various cross-linking bonds and an inert external shielding layer are formed by SiO 2 to protect magnetic nanoparticles [26]. SiO 2 encapsulation on the surface of bismuth ferrite can affect its structural, magnetic, electrical, and optical properties [27,28]. In this study, Na 2 SiO 3 was used as the precursor of SiO 2 . Na 2 SiO 3 is inexpensive, non-toxic, environmentally friendly, and water soluble. Sulistiani et al [29] studied the microstructural and magnetic properties of Bi 25 FeO 40 nanoparticles using the coprecipitation method. However, they produced Bi 25 FeO 40 nanoparticles without surface encapsulation. Bi 25 FeO 40 nanoparticles encapsulated with SiO 2 are rarely used. However, surface encapsulation of ferrite-based nanoparticles with SiO 2 has been extensively conducted. SiO 2 can successfully encapsulate the surfaces of ferrite-based nanoparticles, thereby reducing their agglomeration and controlling their shape to become more spherical [30,31].
In this study, Bi 25 FeO 40 nanoparticles were fabricated by the coprecipitation method and encapsulated with SiO 2 . The coprecipitation method was chosen because it has several advantages: ease of use, low temperature, short duration, ease of obtaining raw materials, and ease of obtaining a single phase of the product [2,32,33]. However, SAR studies on SiO 2 -encapsulated Bi 25 FeO 40 nanoparticles have not yet been reported. Therefore, it is challenging to investigate their properties to obtain extensive information for achieving magnetic hyperthermia applications. This study aimed to explore the microstructural, optical, and magnetic properties and SAR of SiO 2 -encapsulated Bi 25 FeO 40 nanoparticles.

Synthesis of bismuth ferrite nanoparticles
The coprecipitation method was used to synthesize bismuth ferrite nanoparticles. Bi 5 O(OH) 9 (NO 3 ) 4 and FeCl 3 .6H 2 O were the primary precursors, with NaOH as the precipitant. The first solution contained Bi 5 O(OH) 9 (NO 3 ) 4 and FeCl 3 .6H 2 O in a stoichiometric ratio of 1:5 dissolved in 33 ml aquades. Subsequently 3.4 ml 37% HCl was added to the solution. This solution was stirred at a constant temperature of 100°C. Simultaneously, the second solution was prepared. Specifically, 6 M NaOH was dissolved in 33 ml aquades and stirred at room temperature. The precipitation process was conducted by gradually dripping the second solution into the first solution and subsequently stirring the mixed solution with pH=13 (basic) at a constant temperature of 100°C for 45 min. The obtained precipitate was washed and dried for 4 h at 90°C. After washing, the pH of the precipitate was 7 (neutral). The bismuth ferrite synthesis process is illustrated in figure 1.

Synthesis of bismuth ferrite/SiO 2 nanoparticles
SiO 2 solutions were prepared by mixing the Na 2 SiO 3 solution into distilled water and stirring using a magnetic stirrer at room temperature for 30 min. The concentrations of the prepared SiO 2 solutions were 20% and 25%. Subsequently 0.5 g bismuth ferrite nanoparticles were added to each solution. Each mixture was stirred at room temperature for 4 h. In the next step, the obtained precipitate was washed. The precipitate was dried at 90°C for 2 h to obtain a nanosized powder. Figure 2 shows the synthesis process of the bismuth ferrite/SiO 2 nanoparticles.

Characterization of bismuth ferrite
The crystal structures of the samples were characterized by x-ray diffraction (XRD) using an x-ray diffractometer (Shimadzu XD-3H) with Cu Kα radiation. The functional group bonds of the samples were analyzed by obtaining their Fourier transform infrared (FTIR) spectra using an infra-red spectrophotometer prestige (Shimadzu-21). Their morphologies and selected area electron diffraction (SAED) were investigated by transmission electron microscopy (TEM, Jeol JEM-1400). The magnetic and optical properties of the samples were characterized using a vibrating sample magnetometer (VSM) (Riken Denshi Co Ltd.) and an ultravioletvisible (UV-Vis) spectrophotometer, respectively. The SAR was calculated by measuring the temperature rise as a function of time using a calorimetric method. Figure 3 shows the XRD patterns of bismuth ferrite nanoparticles before and after SiO 2 encapsulation. To determine the crystallite size, quantitative phase, composition, and lattice parameters of the samples, the XRD patterns were carefully analyzed using Rietveld refinement. Single-phase formation of Bi 25 FeO 40 (ICDD PDF #460416) was observed before encapsulation with SiO 2 , and no secondary phase was observed for this sample. The (220), (310), (222), (321), (530), and (611) planes in the XRD patterns of the Bi 25 FeO 40 nanoparticles at 2 q=20-65°showed the most intense peaks, which is in good agreement with the results reported by Verma et al and Kalikeri et al [5,17]. Bi 25 FeO 40 is a sillenite-type bismuth ferrite with a body-centered cubic cell and I23 space group. The crystallite size of Bi 25 FeO 40 is calculated using the Debye-Scherrer equation:

Structural analysis
where t, k, l, b, and q are the crystallite size, Scherrer's constant with a value of 0.9, wavelength of the x-ray radiation, full width at half maximum value of the x-ray diffraction peak, and half of the diffraction angle, respectively. The crystallite sizes of the Bi 25    crystalline orientation [1].  [9]. Peaks of SiO 2 were not found in the XRD patterns of all encapsulated samples because it was possibly amorphous [25]. TEM images of bismuth ferrite nanoparticles before and after SiO 2 encapsulation are shown in figure 4. The morphologies of the samples agglomerated with an almost spherical shape before and after SiO 2 encapsulation. This agglomeration can be attributed to the coprecipitation method producing particles of very small sizes. Thus, SiO 2 cannot separate these nanoparticles. The SAED patterns of the samples showed that the crystal structures of the bismuth ferrite nanoparticles before and after SiO 2 encapsulation were polycrystalline. Diffraction of the polycrystalline material formed a ring pattern. This can be related the random orientation of the diffracting hkl planes in all possible directions. The diffraction planes of the bismuth ferrite nanoparticles before and after SiO 2 encapsulation were    Figure 6 shows the absorption spectra of bismuth ferrite and bismuth ferrite/SiO 2 with a SiO 2 concentration of 20%. The absorption spectra of the two samples are in the UV and visible region, i.e., in the wavelength of approximately 200-600 nm, which is in good agreement with the results of Ren et al [6]. This shows that the bismuth ferrite nanoparticles present good UV-and visible-light responses. The maximum absorptions of the samples are in the ultraviolet region at approximately 200 nm, suggesting they absorb more strongly in the  ultraviolet region than in the visible light region [38]. The maximum absorption peak of the bismuth ferrite nanoparticles at 221 nm increases and shifts toward a lower wavelength (blue shift) at 218 nm after SiO 2 encapsulation. The shift of the absorption band edge of the sample indicates a change in the electronic transition and bandgap [39]. The calculation of the bandgap energies of the samples is shown in figure 7. The bandgap energy of a sample is obtained by applying the following Tauc plot equation to the UV-Vis absorption spectra:

Optical analysis
where a, h v , ,C, and Eg are the absorption coefficient, Planck's constant, light frequency, independent coefficient, and bandgap energy, respectively. n=½ and 2 are for materials with direct and indirect bandgap, respectively. Bismuth ferrite is a direct-bandgap semiconductor; thus, n=½ [40]. The bandgap energy of Bi 25 FeO 40 was determined as 2.1 eV, which increased to 2.7 eV after SiO 2 encapsulation. This increase in the bandgap energy after SiO 2 encapsulation is possibly due to the insulating nature of SiO 2 . SiO 2 has a bandgap energy of 7.62-9.70 eV and 7.6-8.75 eV from theoretical and experimental data, respectively [41]. An increase in the bandgap energy of bismuth ferrite nanoparticles after SiO 2 encapsulation has been reported by Nayak et al [28]. This suggests that the SiO 2 encapsulation affects the optical properties of the nanoparticles. The bandgap energy of the pure Bi 25 FeO 40 nanoparticles in this study is close to that reported by Jebari et al [42] and smaller than those obtained by Xie et al and Köferstein et al [2,43]. In contrast, it is larger than those reported by Ren et al and Tan et al [6,44]. This variation in the bandgap energy is caused by several factors such as different techniques for determining the bandgap energy and differences in the syntheses and particle size [42].

Magnetic analysis
To investigate the effect of SiO 2 encapsulation on the magnetic performance of bismuth ferrite nanoparticles, M-H hysteresis loops were tested using a VSM and measured at room temperature, and the results are shown in figure 8. On applying a maximum external magnetic field of 15 kOe, saturation magnetization of the sample was not reached. The magnetization increased linearly with the applied magnetic field.   Table 3 lists the measured magnetic parameters of the bismuth ferrite and bismuth ferrite/SiO 2 samples. The coercivity (H C ) increased with the addition of SiO 2 in the bismuth ferrite sample. However, the maximum magnetization (M max ) reduced after SiO 2 encapsulation. These results confirm that all samples present a weak ferromagnetic ordering. Zhou et al showed that sillenite-type bismuth ferrite exhibits a weak ferromagnetic behavior. The H C obtained in their study was higher, whereas M max was lower than those of the nanoparticles in our study [10].
The increase in the coercivity of the magnetic nanoparticles after the nonmagnetic material encapsulation is due to the increase in the interparticle distance between the magnetic cores of the nanoparticles, which weakens the coupling of the magnetic dipole moments [45]. In addition, the increase in the coercivity is possibly caused by the presence of another phase of bismuth ferrite, i.e., Bi 2 Fe 4 O 9 (from the XRD patterns) [46], which forms on the nanoparticles after SiO 2 encapsulation. The reduction in the saturation magnetization of ferrite-based nanoparticles after SiO 2 encapsulation was found in a study reported by Widakdo et al [31]. This reduction was reported to be due to the magnetic properties of SiO 2 that encapsulated the nanoparticles. SiO 2 is a nonmagnetic material; therefore, it can reduce the response of the nanoparticles it encapsulates to an external field.

SAR analysis
The calorimetric method can be used to measure the SAR value to determine the heating efficiency of magnetic nanoparticles. In the calorimetric method, the temperature rise of magnetic nanoparticles was recorded over a specific interval of time under an AMF with a specific strength and frequency [47], and the results are shown in figures 9(a)-(c). The SAR values of all samples are evaluated from the linear part of the slope using the following equation [24] : where C, m , MNP and dT dt are the heat capacity of the nanoparticle dispersion, mass of the magnetic nanoparticles, and initial slope of the temperature rise versus time plot, respectively. The temperature rise of all magnetic nanoparticles recorded over 10 min was very small. In hyperthermia applications, the temperature is controlled in the secure range of 42°C-47°C. This secure temperature range can damage cancer cells while protecting healthy cells [48]. The low temperature rise of the magnetic nanoparticles led to low SAR values. The SAR values of the bismuth ferrite nanoparticles were 49, 61, and 84 mW g −1 under AMF strengths of 150, 200, and 250 Oe, respectively, which correspondingly reduced to 35, 44, and 65 mW g −1 after SiO 2 encapsulation.  Magnetic nanoparticles should possess a high SAR as heating mediators in hyperthermia therapy. However, the SAR values of the magnetic nanoparticles obtained in this study were very small owing to the low AMF frequency [49], i.e., 50 Hz. The heat production mechanism occurred slowly because of the low frequency. Therefore, the amount of energy dissipation was small, resulting in small SAR values. A high frequency is required for hyperthermia applications to acquire a secure temperature range and a high SAR value. Figure 10 shows the temperature rise within 10 min with the higher AMF frequency. A higher frequency of 20 kHz can produce more heat and increases the SAR value to 123 mW g −1 . Therefore, the increase in frequency increases the SAR values of the magnetic nanoparticles, which is in good agreement with the results reported by Narayanaswamy et al [50]. These high SAR values can provide efficient heat generation to destroy cancer cells. Besides the frequency, the external factor that can affect the change in the SAR values of magnetic nanoparticles is the strength of the AMF.
To investigate the effect of the AMF strength on the SAR values of the magnetic nanoparticles in this study, the former was varied as 150, 200, and 250 Oe while keeping the frequency constant. Figure 11 shows the SAR values of the nanoparticles under the influence of various AMF strengths. The SAR values of the nanoparticles increase with increasing AMF strength. The applied AMF strength increases the temperature of the system. Therefore, increasing the AMF strength can generate higher heat energy and higher SAR values of the  nanoparticles. Atkinson et al suggested that the frequency and strength of the AMF in a clinical trial cannot exceed a certain limit (H.f<4.85×10 −8 A ms) −1 for safety reasons [51]. The effects of AMF frequency and strength on the SAR values of nanoparticles in hyperthermia applications have been reported in many studies.
In addition, this study showed the influence of saturation magnetization on the SAR value of the nanoparticles. The SAR values of the magnetic nanoparticles increased with increasing saturation magnetization. The effects of saturation magnetization and AMF strength on SAR values was also reported by Kerroum et al [52]. In hyperthermia applications, the magnetic properties of magnetic nanoparticles are critical factors for increasing the SAR value. Therefore, improving the magnetic properties, particularly of bismuth ferrite nanoparticles, is necessary to obtain optimal results for their applications as heating mediators.

Conclusion
We successfully fabricated a sillenite-type bismuth ferrite nanoparticles (Bi 25 FeO 40 ). The nanoparticles were synthesized by the coprecipitation method and encapsulated in SiO 2 (20 and 25%). SiO 2 encapsulation affected the microstructures and magnetic and optical properties of the nanoparticles. The average crystallite size increased after SiO 2 encapsulation, as confirmed by XRD analysis. The presence of the Si-O-Si stretching vibration in the FTIR spectrum of the bismuth ferrite nanoparticles after SiO 2 encapsulation confirmed that SiO 2 encapsulated the nanoparticle surface. The investigation of the optical properties showed that SiO 2 affected the bandgap energy of the bismuth ferrite nanoparticles, causing it to increase. The coercivity increased and the maximum magnetization decreased after SiO 2 encapsulation. In addition, we measured the SAR value of the nanoparticles, and found it decreased after SiO 2 encapsulation and increased with increasing AMF strength. This study confirmed the effects of the magnetization, frequency, and the AMF strength on the SAR value of the nanoparticles.