Influence of La Doping onMagnetic and Optical Properties of Bismuth Ferrite Nanofibers

The influence of La doping on the crystal structure, ferromagnetic, and optical properties of BFO nanofibers was investigated. Bi1−xLaxFeO3 ultrafine nanofibers were synthesized by the electrospinning method. The surface morphology and crystal structure of the as-spun and sintered fibers were not affected by the doping. The impurity phases of the BFO crystals were weakened with the increment of La concentration. The magnetization field curves showed that the magnetization weakened under low La doping proportion, but strengthened with the increase of the doped proportion. The magnetization curves also showed continuous strong enhancement of ferromagnetic behavior. The results of UV-vis and photoabsorption testing revealed little influence of La doping on the optical property.


Introduction
Multiferroicity is a property that enables a material to display both ferroelectric and ferromagnetic properties.Due to its great potential in applications such as spintronics, memory, and data-storage media, multiferroicity has incited great research interest [1][2][3][4].According to the phase-structure relationship, multiferroic materials can be divided into two groups: single phase and multiphase multiferroics, in which the electromagnetic coupling effect is greatly enhanced by the interaction between different phases composed of ferroelectric and ferromagnetic materials [5].Single-phase multiferroics seldom exhibit both ferroelectric and ferromagnetic properties at room temperature.The one exception is BiFeO 3 (BFO), which has a Curie temperature of 1103 K and a Neel temperature of 643 K and shows great merit in research and various applications [2,[6][7][8].
The structure of bulk BFO is perovskite crystal, with G-type ordering antiferromagnetic, in which the magnetic moment of Fe is ferromagnetic in (001) h/(111) c planes and antiferromagnetic in the neighboring two (001) h/(111) c planes [9].A spin-modeling structure with a wavelength of 620 ± 20 Å can be observed, which eliminates the macroscopic magnetic moment of bulk BFO [10].As a result, bulk BFO displays no macroscopic magnetic property.Suppression of the spin-modeling structure must be conducted in order to observe the ferromagnetic property of BFO.According to previous studies, the nanocrystalline effect can considerably enhance the ferromagnetism of BFO of low-dimensional form, where the mean particle size is reduced to below 100 nm [2,[11][12][13].Another way which results in the cancellation of spin modeling is the doping of lanthanum (La) [14].Besides the enhancement of ferromagnetism, there are also reports concerning the enhancement of photocatalytic activity brought by the synthesis of low-dimensional BFO, especially BFO nanofibers, whose fewer grain boundaries and interfaces effectively improve the photocatalytic property.Earlier studies have reported the structure, multiferroic properties of both Ladoped BFO bulk [15] and thin films [14,16,17] and ultrafine BFO nanofibers without doping [18].Certain properties of La-doped BFO nanofibers, together with its optical properties, however, remain unexplored in the literature.

Materials and Methods
La-doped BFO (Bi 1−x La x FeO 3 ) nanofibers with four scales of doping proportions (x = 0.0, 0.05, 0.10, 0.15) are synthesized by sol-gel-based electrospinning.Bismuth nitrate , with an initial ratio of Bi to Fe of 1.05(1 − x) : 1, thus giving four sets of solutions.Lanthanum nitrate (La(NO 3 ) 3 •6H 2 O), also in stoichiometric proportion, was then added to the solution.
After a period of aging, polyvinylpyrrolidone (PVP) was added to the solution, which was then stirred continuously, resulting in a homogeneous 0.3 M Bi 1−x La x FeO 3 solution.
The solution was then loaded into a syringe with a stainless steel needle.High voltage was added to the needle to create a potential difference between the needle and the collector.
The solution was then electrospun, with a feeding rate of 0.8 mL/h and voltage of 13.5 kV.The fibers were collected on a Sn substrate.The as-spun nanofibers were then dried at 200 • C for 1 h, heated at 400 • C for 1 h, and then sintered at 600 • C for 2 h.

Results and Discussion
The scanning electron microscopy (SEM) images of both as-spun and sintered La-doped BFO fibers were taken, as shown in Figures 1(a) and 1(b).The diameter of the asspun nanofibers is approximately 300 nm.The surface of the fibers is smooth, and they are uniform in shape.The sintered fibers are no longer uniform but rough, while the diameter of the fibers is reduced from approximately 300 nm to 100 nm.The crystalline structure of La-doped BFO ultrafine nanofibers is further examined by TEM and HRTEM, as shown in Figures 1(c) and 1(d).It is observed from HRTEM images (Figure 1(d)) that the regular spacings of the observed lattice are 0.397 and 0.280 nm, which are consistent with the (012) and (110) crystal planes of a rhombohedral La-doped BFO phase, respectively.In Figure 2, The EDX pattern of La-doped BFO nanofibers with different doping proportions are also included.From 0 to 0.15 loading content, the EDX pattern exhibits an increase in the La signal at 4.35 KeV, which indicates the formation of a La-doped BFO material.The percentages of atom numbers of La-doped BFO are shown in Table 2. X-ray diffraction (XRD) was carried out to investigate the crystal structures of BFO with different La doping proportions.As shown in Figure 3, it can be observed that the rhombohedral perovskite structure of BFO contributes to most diffraction peaks.Ferromagnetic γ-Fe 2 O 3 , which often constitutes the main impurity phase of BFO, is not detected.The doping of La brings only minor change to the majority of the peaks, which indicates that the lattice constant of the perovskite crystal remains largely unchanged, as the difference between the ion diameters of La 3+ (1.032 Å) and Bi 3+ (1.03 Å) is trivial.It can also be observed that the 28 • impurity peak is weakened with the doping of La.It is possible that the doping of La compensates the volatilization of Bi, which otherwise leads to the existence of the 28 • peak during the preparation of the solution.
Considering the importance of the multiferroic property of BFO, the ferromagnetic properties of La-doped BFO nanofibers are characterized by a vibrating sample magnetometer (VSM), as shown in Figure 4. From each of the four hysteresis loops corresponding to a La doping proportion, we can observe that the doping of La has obvious effect on the ferromagnetic properties of the nanofibers.With the increase of La concentration, the magnetization is at first (x = 0.05) weakened, as the doping of La leads to the reduction of Fe 2+ and oxygen vacancies.The magnetization then increases (x = 0.10, 0.15), as the spin-modeling structure is gradually suppressed by La doping.Saturation magnetization and coercive field, on the other hand, exhibit a continuous and evident increase with the increment of La doping.
Generally, the optical absorption performance of semiconductors is related to the electronic structure feature and their band gaps.The optical properties of samples were studied by measuring their UV-Vis diffuse reflectance absorption spectra.As shown in Figure 5, the absorption spectra of BFO nanofibers with different La doping proportions were measured.The absorption spectra show that BFO nanofibers can absorb considerable amounts of visible light, suggesting their potential applications as visible-light-driven photocatalysts.And compared with the BFO nanofibers with La doping, the pure BFO nanofibers show higher visiblelight absorption.The optical absorption coefficient near the band edge follows the equation αhν = A(hν − E g ) n/2 , where α, h, ν, E g , and A are absorption coefficient, Planck constant, light frequency, band gap, and a constant, respectively.Considering that BFO is a direct band gap material, the value of n for BiFeO 3 is 1.21.The corresponding values of direct band gap of BFO nanofibers can be evaluated by extrapolating the linear portion of (αhν)2 versus (hν); the results are shown in Table 1.The results show that La doping has little effect on the photoabsorbing property of BFO.
To further evaluate the photocatalytic properties of nanofibers, Congo red (CR) with a major absorption peak at 495 nm was chosen as a model organic pollutant.Figure 6 gives the concentration changes of CR at 495 nm by the nanofibers as a function of irradiation time under visible light during the degradation process.With La doping increased, 75% of the CR was decolorized after 3 h, showing better photocatalytic activity than the pure BFO nanofibers.However, the enhanced activity of the photocatalysts does not increase further for more La doping.It can be seen that the photodegradation rates of sample (x = 0.15) were lower than those of other samples.

Conclusion
In summary, the influence of La doping on the crystal structure, ferromagnetic, and optical properties of BFO nanofibers was investigated.Bi 1−x La x FeO 3 ultrafine nanofibers were synthesized by the electrospinning method.The surface morphology of as-spun and sintered fibers, as well as the crystal structure of sintered fibers, was not affected by the doping.The impurity phases of BFO crystals were weakened with the increment of La concentration.The magnetization field curves showed that the magnetization weakened under low La doping proportion, but strengthened with the increase of the doped proportion.The magnetization curves also showed continuous strong enhancement of ferromagnetic behavior.The results of UV-Vis and photoabsorption tests revealed little influence of La doping on the optical property.

Figure 6 :
Figure 6: Photodegradation of CR in the presence of La-doped BFO nanofibers with different doping proportions.

Table 1 :
Band gaps of sintered nanofibers under four La doping proportions.

Table 2 :
The percentages of atom numbers of La-doped BFO nanofibers under four La doping proportions.