Effect of Pr, Mn doping on the structure and properties of BiFeO3

The powders of Bi1-xPrxFeO3 (x = 0, 0.05, 0.1) and Bi0.95Pr0.05Fe1-yMnyO3 (y = 0.05, 0.1) were prepared by hydrothermal method. The effects of Pr and Mn doping content on the structure, morphology, magnetic, and photocatalytic properties of BiFeO3 (BFO) have been studied. X-ray diffraction (XRD) demonstrated that the compounds are distorted rhombohedral perovskite structure without any other heterogeneity and structural transition. Field emission scanning electron microscope (FESEM) reflected that the surface of compounds is a dense, agglomerated sphere, and the morphology changes with the addition of Pr, Mn. Energy spectrum analysis (EDS) shows that the Bi0.95Pr0.05Fe0.95Mn0.05O3 sample is mainly composed of 5 elements (Bi, Fe, O, Pr, Mn), and the atomic ratio matches the formula well. Integrating the vibrating sample magnetometer (VSM) into the physical property measurement system (PPMS-9) shows that the introduction of Pr3+and Mn2+ ions can enhance the magnetic properties of BFO at room temperature. In addition, doping with Pr3+ and Mn2+ ions can improve the photocatalytic performance of BFO, and with the increase of Mn2+ concentration, the photocatalytic performance of BFO first rises and then decreases, and its catalytic performance is getting better and better.


Introduction
Multiferroic materials are a typical representative of multifunctional materials, combined electricity, and magnetism. The research of multi-ferrous materials belongs to international hotspots, and it is also a basic frontier scientific issue with application traction, which involves the fields of materials, physics, chemistry, etc. At present, material science and condensed matter physics is a broad new field, which contains a wealth of research topics in material science and physics and has a wide range of application prospects. The research of multi-ferrous BiFeO 3 is one of the most representative single-phase multiiron materials, and the perovskite structure belongs to the R3c space group. It is one of the few materials that has both ferroelectric and antiferromagnetic properties at room temperature, with high ferroelectric Curie temperature (850°C) and ferromagnetic Neel temperature (370°C), as well as narrow forbidden bandwidth and better Chemical stability, etc. These excellent properties make it suitable for applications such as magneto-optical devices [1], information storage [2], sensors [3], nanogenerators [4], and catalysts [5].
However, as BiFeO 3 is caused by the spatial periodic non-uniform spin structure, the properties of weak magnetism limit its practical application. At the same time, BFO is also a typical narrow bandgap semiconductor, which can respond to the visible range that TiO 2 cannot respond to. So that it can exhibit excellent chemical stability in the visible light photocatalysis process [6]. However, pure BFO exhibits poor photocatalytic activity because of the rapid recombination of photogenerated electrons and holes (Table 1).
Over the years, scholars at home and abroad have been working on improving the performance of BFO. Among them, ion doping is the most widely studied method. Ion doping of A, B, and AB was used to improve the performance of BFO. The A site is mainly rare earth elements (La [7,8], Nd [9,10], Sm [11][12][13], Pr [7,12]), because its ionic radius and valence are similar to those of Bi 3? , and it is easy to dope to replace Bi ions and inhibit the volatilization of Bi. The B site is mainly doped with transition metal ions (Cr, Co, Mn) [14]. Doping magnetic ions at the B site can inhibit the valence state of iron and destroy the super-exchange effect in the spin-helical structure of BFO. Therefore, the magnetic performance is enhanced. Meantime, ion doping is an effective method to increase the photocatalytic activity of BFO [15]. Zhang et al. [16] prepared different Gd-doped BFO thin films by sol-gel method. The effect of Gd doping on the catalytic activity of BFO thin films was studied. The research shows that proper Gd doping can increase the photocatalytic activity of the thin films. Gd doping increases light absorption, can effectively separate the migration of photogenerated carriers, and reduce the recombination probability of electron-holes. At the same time, because BFO has weak ferromagnetism, it can be recovered from water contaminants by external magnets for recycling. H. Shokrollahi et al. [17] used traditional ceramic technology to study the structural behavior and magnetic properties of Eu, Ce-YIG garnet. The XPS data confirmed that the Fe 2? and Ce 4? cations are not present in the structure. Moreover, the substituted garnet with 0.25Ce and 0.25Eu increased saturation magnetization by about 21% compared to the pure YIG. The magnetic coercivity increases from 16.41Oe for the pure YIG to 25.52 for the Y 2.75 Ce 0.25 Fe 5 O 12 and then decreases with the Eu substitution.
The AB-doping can be the advantage of combining the two to get a better modification effect [18]. Prdoped BFO and Pr, Ti co-doped BFO were prepared by solid-state reaction. The study found that both the single or double doping properties were obtained and the magnetic properties of the double doping were more pronounced. The reason is that the antiferromagnetic transition temperature (TN) decreases as the doping concentration increases, and the transition temperature is 354°C when x = 0.20. The net magnetization of the doped to sample increased significantly, indicating that non-magnetically active Ti 4? ions were added at the magnetic Fe 3? site, which destroyed the Fe-O-Fe network and formed the AFM-type sublattice spin ring [19].
With the development of science and technology and the needs of society, the demand for small electronic devices is increasing, and the requirements for these materials are becoming more and more strict. Bismuth ferrite is a strong ferroelectric substance with G-type antiferromagnetism. Because BiFeO 3 is composed of non-uniform spin structure with space period, its weak magnetism limits its practical application. Much work has been done to improve the magnetic properties of bismuth ferrite. For example, researchers have done different solutions, such as reducing particle size and doping elements to enhance the magnetization of BFO nanoparticles. At present, the research of doping magnetic elements is more, while the research of doping non-magnetic elements is less. In order to improve the magnetic properties of BiFeO3, a single doping method of Pr, and Mn co-doping was used. Then the structure, magnetic properties, and photocatalytic properties of the prepared samples were studied.

Experimental
The powders of BFO and Bi 1 -x Pr x Fe 1-y Mn y O 3 (x = 0, 0.05, 0.10 y = 0.05, 0.10) were prepared by a hydrothermal method. The raw materials required for the experiment were Bi( MnO 4 , and KOH. First, the proper concentration of KOH solution is prepared, and then the other raw materials are successively added to the KOH solution, and then stirred for 30 min, waiting for the raw materials to completely dissolve. Then the prepared solution was ultrasonic in the ultrasonic machine for 20 min, and then stirred for 40 min, and poured into the reactor. Finally, after heated at 200°C for 24 h, the powders were repeatedly washed with deionized water, and then dried at 60°C for 6 h to get the final powders. The phase compositions of the powders were analyzed by X-ray diffraction (Bruker D8) with CuKa (k = 1.5406 Å ) radiation. The surface morphology of the powders was observed by scanning electron microscopy (Gemini SEM 300, Zeiss, Germany). The ultrastructure of powder was observed by transmission electron microscope (TEM, FEITecnai G2 F300). The chemical state and stoichiometry were determined by the X-ray photo-electron spectroscopy (XPS, Thermo Scientific K-Alpha) with AlKa source (excitation energy 1486.6 eV). The elemental analysis of the powders was observed by energy-dispersive X-ray spectroscope (EDX). The magnetic measurement of the powders was checked by a physical property measurement system (PPMS-9, Quantum Design). The UV-visible absorption spectroscopy of the powders was recorded at room temperature with Agilent Cary-5000. The UV-vis diffuse reflection spectroscopy of the powder was recorded with Carry 5000 UV-Vis-NIR.
The photocatalytic performance of the sample is usually tested by adding the sample to the methylene blue (MB) solution and irradiating it under the xenon lamp. The photocatalytic performance of the sample is determined by measuring the absorbance of the solution at different times. The 0.5 g sample and 100 mL MB solution concentration (0.1 mg/mL) were first placed in a reactor, stirred in the dark for 30 min, and then irradiated under a xenon lamp. Samples were taken every 30 min to measure the absorbance of the solution. The XRD pattern of all the samples confirmed that the rhombohedral perovskite structure has R3C space group. Moreover, When Pr doped BFO alone, with the increase of Pr doping concentration, the XRD diffraction peak shifted to a low angle; when Pr and Mn co-doped BFO, with the increase of Mn doping concentration, the XRD diffraction peak shifted to a higher angle offset. As shown in Fig. 1b, the (104) and (110) peaks of Pr single-doped BFO move to the left, and the peaks of Pr and Mn co-doped shift to the right. This is because the radius of Pr 3? is larger than the radius of Bi 3? , and the radius of Mn 2? is smaller than the radius of Fe 3? . At the same time, the average grain size of all doped and undoped samples was calculated using the Scherrer formula:

Results and discussion
where k = 0.89 is the shape factor, k is the wavelength of the X-ray k = 0.15405 nm, b is the full width at half maximum, h is the diffraction angle, and D is the crystallite size. It is observed from Table 1 that the average grain size of the BFO sample varies from 41 to 28 nm with the increase of Pr single doping and the concentration of Pr and Mn co-doping. With the increase of singledoped Pr concentration, the lattice parameters increase, thus resulting in a larger volume. While with the increase of Mn concentration, the lattice parameters of Pr, Mn co-doped BFO decrease slightly, and the volume shrinks. This may be due to the difference in ionic radius between Pr and Bi, as well as Mn and Fe. These results indicate that Pr and Mn can be effectively introduced into the crystal structure of BFO.  Fig. 2a. It can be seen that the pure phase BFO powder presents a regular polyhedral morphology, which is a condensed spherical morphology composed of many fine crystal grains.  Fig. 2e, it can be seen that the rod-like structure is dominant, the lattice spacing is clear, and the crystallinity is good. In addition, the ratio of cell elongation (C/A) may be the reason for the appearance of rods.
To understand the elemental ratio at the surface and chemical composition of the elements, XPS experiment was carried out in a binding energy range of 0-1200 eV. The typical XPS spectra of Bi 0.95 Pr 0.05-Fe 0.95 Mn 0.05 O 3 sample are depicted in Fig. 3. The resonance peak positions in Fig. 3 represent the characteristic of the bound states of the electrons in the surface atoms. The photoemission peaks at 158.08, 163.48, 709.98, and 723.58 eV correspond to Bi 4f 7/2 , Bi f 5/2 , Fe 2p 3/2 , and Fe 2p 1/2 , respectively, and are shown in Fig. 3a, b. The spin orbit splitting energy of 5.3 eV in the core-level spectra of Bi 4f 7/2 and Bi 4f 5/2 indicates that Bi is in 3 ? oxidation state. Similarly the spin orbit splitting energy of 13.3 eV in the case of Fe 2p 3/2 and Fe 2p 1/2 confirms the existence of Fe in 3 ? oxidation state. Similar result was reported by the other groups [20,21]. The high-resolution XPS spectrum of O1s exhibits two obvious peaks (Fig. 3c). One peak at 528.78 eV is ascribed to the crystal lattice oxygen of BiFeO 3 , and another peak at 530.38 eV demonstrates the presence of chemisorbed oxygen species [22]. For the Pr 3d XPS spectrum of (Fig. 3d), the two stark peaks located at   [25,26]. Moreover, it can be seen that with the increase of the concentration of Pr and Mn ions, the infrared spectrum has a significant red shift, which is mainly due to the high peak of the infrared absorption spectrum caused by the radius of Pr 3? radius larger than Bi 3? . At the same time, the absorption peak of tensile vibration gradually widens until it disappears. There is almost no change in the absorption peak of flexural vibration, indicating that Mn doping causes the bond length to change, but the bond angle does not change much. Figure 5 shows the hysteresis loops of BFO, Bi 1-x-Pr x FeO 3 (x = 0.05, 0.1), and Bi 0.95 Pr 0.05 Fe 1-y Mn y O 3 (y = 0.05, 0.1) samples tested at room temperature by PPMS-9 with a maximum magnetic field of 6T. It can be seen that the pure phase BFO exhibits typical antiferromagnetic behavior, the non-linearity of M (H) is very small, there is no remanence, the coercive force is almost zero, and it shows a linear loop under the action of an external magnetic field. The hysteresis loops of all samples did not reach saturation. In the case of Pr 3? single-doped BFO, the addition of a small amount of Pr 3? does not fundamentally change the direction of the magnetization curve, and the magnetization increases with the doping amount of Pr 3? , except the Pr 3? doping amount is 5%. This shows that Pr doping can improve magnetic properties but requires a minimum doping amount. This may be due to the excessive replacement of Bi 3? by Pr 3? , reducing the oxygen vacancies caused by the volatilization of Bi 3? . The change in the concentration of oxygen vacancies in the ferrite will change the dielectric properties of the sample due to defect polarization. Wu Hongjing et al [27]. used ethylenediamine to synthesize NiCo 2 O 4 absorbent by hydrothermal method. The EMW absorbing properties of the NiCo 2 O 4 mainly originate from remarkable dipole polarization induced by oxygen vacancies and lattice defects, interface polarization stemming from the interfaces of NiCo 2 O 4 fibers, and multiple reflections and scattering in its unique urchin-like introduction of Mn 2? has increased the deformation and surface defects of FeO 6 octahedrons, thereby rapidly recombining the holes and electrons of the corrosion inhibitor, and increasing the concentration of active centers, thereby improving the catalytic performance. Therefore, Pr and Mn co-doped BFO can improve the catalytic performance of BFO by promoting the generation of electron-hole pairs.
In order to explain the photocatalytic mechanism of Pr, Mn-doped BFO, the energy band potential of BFO was determined. Tao Xian et al. [22] prepared AAG/BiFeO 3 composites by hydrothermal and photodeposition methods. In order to illustrate the catalytic mechanism of AuAg/BiFeO 3 composite, the energy band potentials of BiFeO 3 were estimated through the Mott-Schottky (M-S) method. This method proves that BiFeO 3 has n-type semiconductivity. Since there is a negligible gap between the n-type semiconductor and the vfb, the CB potential of the n-type semiconductor is usually very close to the vfb. Therefore, it is estimated that the VB and CB potentials of the prepared BiFeO 3 relative to NHE are 2.51 and 0.36 V, respectively. Figure 8 is a photocatalytic mechanism diagram of Pr, Mn-doped BFO composite material under simulated sunlight. Generally, the photocatalytic material of the photocatalytic reaction is excited by photons with energy greater than its band gap, and photogenerated carriers are generated, and then migrate and recombine. The carriers participate in the redox reaction on the surface of the material. When the energy of the external photon exceeds the band gap (2.15 eV) energy of BFO, the electrons on the BFO VB will be excited and migrate to CB, leaving the corresponding photogenerated holes in VB. Photogenerated electrons and holes migrate to the surface of the BFO through diffusion to participate in the degradation of MB. During the reaction, at the same time, the photogenerated carriers will also recombine a lot in the defect sites inside the BFO. The improvement of the photocatalytic performance of BFO may be due to the co-doping of Pr and Mn, which reduces the optical band gap of BFO, expands the photoresponse range, decreases electron-hole recombination, and increases surface reactive sites.

Conclusion
In this work, the samples of pure phase BFO, Prdoped BFO, and Pr, Mn co-doped BFO were prepared by hydrothermal method The structure is rhombohedral perovskite structure, space group is R3c, and without impurity phase, BFO, Bi 1-x Pr x FeO 3 (x = 0.05, 0.1), and Bi 0.95 Pr 0.05 Fe 1-y Mn y O 3 (y = 0.05, 0.1) samples are all pure phase. When Pr is doped into BFO, the magnetic properties decrease first and then increase with the amount of Pr doping. It is stated that there is a limit on the minimum doping amount for Pr single doping. When Pr, Mn is codoped, the magnetic properties increase with the amount of Mn 2? doping. This is mainly since Mn 2? is a magnetic ion, and its addition contributes to ferromagnetism. The formation of a new network structure can change the spin-helical structure of BFO and improved the magnetic properties. The photocatalytic experiment results show that compared with pure BFO, under visible light irradiation, Pr and Mn codoped BFO can significantly improve the decomposition and photocatalytic activity of organic matter (MB). In addition, with the increase of Mn content, the photocatalytic activity of the sample also increases. In this work, it is found that Pr and Mn co-doped BFO can not only improve the magnetic properties of BFO but also improve its catalytic performance. Therefore, the doping of BFO has a good application prospect for both magnetism and optics. These properties are mainly related to the super exchange.
conflict of interest in connection with the work submitted.