In recent years, owing to the magnetoelectric coupling effect, multiferroic materials have been widely applied in the fields of information storage, electronic devices, and sensors, and thus they have attracted extensive attention from researchers worldwide. Bismuth ferrite (BiFeO₃, or BFO) stands out among multiferroic material because of its ferromagnetism and ferroelectricity at room temperature [1, 2]. BFO can also be used as a photocatalyst to degrade organic pollutants and decompose water because it is a semiconductor material with a narrow band gap (2.2 eV) and excellent chemical stability [3, 4].

At present, there are two popular ways to decompose water: photocatalytic decomposition and electrocatalytic decomposition. The electrocatalytic decomposition of water involves two reactions: the anodic oxygen evolution reaction (OER) and the cathodic hydrogen evolution reaction (HER), both of which are related to electrocatalysts. OER is the key reaction in energy storage processes, such as water cracking and metal–air batteries [5]. However, it is difficult to carry out because it involves four proton-coupled electron transfer processes, and so it requires a very high overpotential to complete the process [6]. At present, IRO _x /RuO _x and Pt/C are the benchmark electrocatalysts for OER and HER, respectively. Although these electrocatalysts exhibit high electrocatalytic activity and kinetics, their high cost, scarcity, and unsatisfactory electrochemical stability prevent them from being widely used. In recent years, many efforts have been made to design low-cost, high-catalytic, stable, and environmentally friendly non-noble metal or carbon-based electrocatalysts. Perovskite oxides (ABO₃−δ) have been widely studied as electrocatalysts owing to their composition and structural flexibility [7–10]. Perovskite oxides (ABO₃) also have a variety of structures and physical and chemical properties, which have attracted considerable interest for use in OER. The perovskite oxide BFO is a multiferroic functional material, as well as a ferroelectric semiconductor that facilitates the separation of electrons and holes, and thus BFO is increasingly being investigated for applications in solar energy conversion and for designing desired electrocatalysts.

BFO has made many representative achievements in catalytic performance and energy storage of fuel cells [11–13]. The application and development of BFO as a supercapacitor has also achieved a lot of fruitful work [14–18]. Bi_0.15Sr_0.85Co_0.8Fe_0.2O_3-δ presents a low OER overpotential (354 mV at 10 mA cm⁻²) and good reaction kinetics for both OER and ORR (94.21 mV dec⁻¹ and 72.5 mV dec⁻¹, respectively) [19]. The current density for the OER of Bi_0.6Ca_0.4FeO₃ showed a significantly high value of 6.93 mA/cm² at a fixed overpotential of 0.42 V (1.65 V vs. RHE) [20]. A 15% Ca-doped Bi_0.85Ca_0.15FeO₃ catalyst showed an increased kinetic current density of 54.562 mA/cm² at an onset potential of 2.122 (V vs. RHE) with an overpotential of 0.892 V in an alkaline electrolyte [21]. All of these tests are performed on glassy carbon, but surface chemistry is an important prerequisite for advanced water electrolysis. The excellent specific surface area of nickel foam has not been fully utilized to demonstrate the OER performance of BFO. The porous structure of the Ni foam can expose more active sites of the material.

In this work, Sm and Eu are chosen for co-doping into the BFO host materials. Because Sm doped BiFeO₃ can improve magnetic and electric properties [12]. Eu substitution for the Fe-site is expected to modulate structure and band gap width to a greater extent resulting in a good water decomposition and electrocatalytic performance [22, 23]. Herein, we report a simple and feasible method for the fabrication of perovskite BiFeO₃ photoanodes. Sm and Eu co-doped BiFeO₃ nanoparticles were fabricated by a double sol–gel method. With increasing Eu content, the BFO structure changed from perovskite to orthorhombic, as demonstrated by both X-ray diffraction (XRD) and Raman patterns. Interestingly, the Sm and 15% Eu co-doped BiFeO₃ had a strong adsorption performance, and the adsorption rate of methyl orange reached 95% in 5 min. The electrocatalytic performance was also superior, such that at an onset potential of 1.48 V vs. RHE, the overpotential was merely 294 and 310 mV to support current densities of 100 mA/cm² and 400 mA/cm², respectively. BFO is a multifunctional composite material. It is therefore hoped that the electrochemical properties of BFO can be further studied using its ferroelectric and ferromagnetic properties.

Figure 1 shows the XRD patterns of the Bi_0.95Sm_0.05Fe_1−x Eu _x O₃ (x = 0.0, 0.05, 0.10, 0.15) nanoparticles and the distorted rhombohedral structures with an R3c space group. The doubly split peaks of BFO in the 2θ range of 31–33° merge to a single peak upon doping with Eu, and two smaller peaks formed on each side, which became higher as the doping proportion increased. These analyses indicate that the crystallinity of the BFO powder was considerably affected by doping with Eu. In addition, at x = 0.15 and 0.2, the diffraction peaks in the 2θ range of 38–40° split, and there were some changes in other peaks. Previous research has shown that an orthorhombic structure can be transformed during element substitution [24, 25], thus the unit-cell symmetry of BSFEO shifted from R3c to Pbnm with increasing Eu content.

For a more detailed view of the structural changes, the Raman spectra, which confirm the XRD results, are shown in Figure 2. It is well known that perovskite BFO has nine vibration modes, namely, four strong peaks (A₁-1, A₁-2, A₁-3, and A₁-4 modes) and six weak peaks (E-2, E-3, E-4, E-5, E-6, and E-7 modes) [26]. It is obvious from the Raman results that with the addition of Eu, the peak of the main vibration mode of the perovskite structure gradually weakened. In contrast, a new mode at 310 cm⁻¹ became gradually more pronounced, which was caused by the A-site vibration and the oxygen sloping of the Pbnm orthorhombic structure [26, 27]. The Raman results again proved that the Sm and Eu co-doped BFO system evolved from an R3c to a Pbnm structure. It is noted that for BSFEO nanoparticles with doping contents of 0.10 and 0.15, the peak at 310 cm⁻¹ was very pronounced.

To fully understand the properties and morphological characteristics of the Eu and Sm co-doped BFO nanoparticles, SEM images of BSFEO powders with various doping contents are shown in Figure 3. It can be seen that the characteristics of agglomeration and surface structure are becoming more conspicuous and homogeneous with increasing Eu substitution concentration. Upon further increasing the Eu content, the grain size of the samples continued to increase. The average grain size increased from 30 to 50 μm at x = 0 to 50–70 μm at x = 0.15. More importantly, the distance between the particles decreased with increasing doping content. The excellent magnetic properties of BFO are related to its surface structure. The tighter the BFO lattice system, the more difficult it is for the O and Bi elements to escape. The elevation in the potential magnetic properties of BFO can be explained by the reduction in the number of oxygen vacancies and the helical structure owing to the dense surface structure. TEM image of BSFO–15Eu nanoparticles are shown in Figures 3E,F. It is evident from TEM images that the crystallinity of the sample was very good and clear diffraction spots can be seen, and the diffraction patterns of these spots agree well with the orthorhombic phase Pbnm of BFO, which is also consistent with the XRD results. The axial direction of the diffraction spot is [ 1 ¯   00 ] .

Figure 4 shows the UV-vis peaks of BSFEO nanoparticles with various doping contents. It is clear that a spectrum shift occurred when BSFO was doped with Eu, which increased the band gap. According to the data in Table 1, the cut-off absorption wavelength of BSFEO nanoparticles in this experiment was approximately 1.90 nm and the minimum was 1.80 eV. By comparing the data, it was found that as the doping ratio increased, the band gap width decreased. This increase in band gap may result in improved magnetic and electrocatalytic performance of the BSFEO nanoparticles. Our results indicate that the BSFEO band gap can be tuned by increasing the doping ratio to increase the range of electrocatalytic and magnetic properties.

As an important structural parameter, the surface area may affect the OER. The specific surface areas of the prepared samples were determined by N₂ adsorption–desorption isotherms. The isotherms and pore-size distribution curves of the BSFO-5Eu, BSFO-10Eu, and BSFO-15Eu samples are shown in Figure 5. The Brunauer–Emmett–Teller (BET) results demonstrated that the specific surface areas for the powder samples were 2.47, 3.26, and 0.90 m²/g for the BSFO–5Eu, BSFO–10Eu, and BSFO–15Eu samples, respectively, indicating that the surface area did not directly affect the OER performance. It therefore seems that the specific surface area did not play a decisive role in OER. However, the average pore size of the three samples were 6.8, 24, and 31 nm for the BSFO–5Eu, BSFO–10Eu, and BSFO–15Eu samples. Generally speaking, large pores are conducive to the transmission of media in the absence of stirring. The larger the pore, the more conducive to mass transfer results in the improvement of OER performance in electrocatalysis.

The OER performance of the Sm and Eu co-doped BFO series was measured via LSV. As shown in Figure 6A, the BSFEO samples all exhibited favorable electrocatalytic activity toward OER superior to that of NF. It is worth noting that the onset potential was small at 1.46 V vs. RHE (Figure 6A). To achieve a current density of 100 mA cm², the corresponding overpotentials were 295, 306, 322, and 332 mV for BSFO–15Eu, BSFO–10Eu, BSFO–5Eu and NF, respectively. To achieve a current density of 400 mA cm², the corresponding required overpotentials were 316, 328, 352, and 366 mV for BSFO–15Eu, BSFO–10Eu, BSFO–5Eu, and NF, respectively. Figure 6B shows the corresponding Tafel plots, wherein all BSFEO samples exhibit fast OER kinetics with extremely small Tafel slopes. In particular, the slope of BSFO–15Eu is 38 mV dec⁻¹ and relatively small, indicating that its OER dynamic process was the fastest and most powerful. Detailed data on OER are shown in Table 2. These results suggest that BSFEO possesses excellent OER performance, comparable to those of LaCoO₃ series [8, 10], Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3-δ [28], NiFe-LDH [29], NiCo₂O₄ [30], Mn₃N₂ [31], CoFe₂O₄ [32] and CoFe-LDH [33] nanosheets, as shown in Table 3.

There are two theories about the OER origin of perovskite ABO3. One is that an oxygen vacancy is the active site, and the other is B-site transition metal ions. In our system, because both Sm and Eu elements are trivalent, there is no tendency to introduce oxygen vacancies according to the principle of defect chemical equilibrium. In other words, all samples contain the same amount of oxygen vacancies. Therefore, the transition metal at the B site serves as the active site in our system, and it can be seen from the above experimental results that Eu or Fe is the active site. The specific OER reaction process is as follows: O H − + M ⇄ M O H + e − M O H + O H − ⇄ M O + H 2 O + e − M O + O H − ⇄ M O O H + e − M O O H + O H − ⇄ O 2 + H 2 O + e − + M

The magnetic hysteresis (M-H) loops of the samples are shown in Figure 7, where both the remanent magnetization (M _r ) and saturation magnetization (M _s ) increased significantly with increasing Eu substitution content. The values of M _s for BSFO–5Eu, BSFO–10Eu, and BSFO–15Eu were 0.29, 0.31, and 0.308 emu/g, respectively. The values of M _r for BSFO, BSFO–5Eu, BSFO–10Eu, and BSFO–15Eu were 0.004, 0.021, and 0.0256 emu/g, respectively. This comes from the substitution-driven modification of the spiral G-type antiferromagnetic ordering, which is characteristic of BFO, to the collinear G-type antiferromagnetic structure, in which the canted component of the antiferromagnetically ordered spins becomes measurable. That is to say, this enhancement of the ferromagnetism was attributed to the structural distortion, which modulates the canted spin structure and change the Fe–O–Fe angle. It was expected that this structural distortion could lead to suppression of the spin spiral and hence enhanced the magnetization [12].

Interestingly, double ferromagnetic hysteresis loops were observed in the Sm and Eu co-doped samples. In particular, the BSFO–10Eu sample had the most obvious double ferromagnetic hysteresis loop, which is located at the coexistence phase of the R3C and Pnma phases. The double ferromagnetic hysteresis loop of BFO systems has rarely been reported or observed. In general, double hysteresis loops are caused by the coexistence of hard/permanent magnets and soft magnets or antiferromagnetic and ferromagnetic phases [34–36]. In the Eu and Sm co-doped BFO system, the two different magnetic phases may be associated with the ferromagnetic Pnma phase and antiferromagnetic R3C phase. Increasing the response efficiency of OER using a magnetic field as a new regulatory approach has recently attracted widespread attention. Ren et al. showed the spin-polarized kinetics of the OER [37, 38].

In summary, BSFEO powders with various doping contents were synthesized using sol–gel technology. With increasing Eu content, the XRD spectra showed that the structure transitioned from the R3C phase to the Pnma phase. The diameters of the Eu-doped particles increased and the distances between them significantly decreased. The peak of the Raman spectra at 136 cm⁻¹ clearly decreased, and the peak at 310 cm⁻¹ significantly increased. The band gap width was in the range of 1.80–2.07 eV. A comparison of the data showed that the M _s and M _r values for the Eu-doped samples improved. Simultaneously, all the samples exhibited superior OER performance, with a desirable onset potential of approximately 1.50 V vs. RHE. For the sample in which x = 0.15, to support a current density of 100 mA/cm², an overpotential of only 294 mV was required, which is much better than that of pure NF (330 mV). The reason and mechanism of OER performance are being systematically studied.