Fabrication of Self-Assembled BiFeO3/CeO2 Nanocatalytic Materials for Efficient Catalytic Dye Degradation

The catalytic treatment of wastewater serves as an effective way to solve the problem of water pollution, in which non-homogeneous Fenton catalysts are widely used. However, the activity enhancement of non-homogeneous Fenton catalysts still remains a great challenge. Herein, self-assembled BiFeO3/CeO2 nanocatalytic materials with different molar ratios were successfully fabricated by a suspension blending method, following which the structure evolution was determined by various characterizations. The catalytic degradation of methylene blue (MB), rhodamine B (RhB), and saffron T (ST) were performed over the BiFeO3/CeO2 nanocatalytic materials. It was found that the 0.2BiFeO3:0.8CeO2 nanocatalytic materials exhibited an 80.8% degradation efficiency for RhB. The 0.6BiFeO3:0.4CeO2 nanocatalytic materials reached 81.1% and 48.7% for ST and MB, respectively. The BiFeO3/CeO2 nanocatalytic materials also showed a good stability during several cycles. The combination of CeO2 with BiFeO3 led to an enhanced activity for dye degradation, probably due to the electron transfer from ≡Fe2+ to ≡Ce4+. This study provides a new approach to dye degradation by using Fenton catalytic systems.


Introduction
With rapid population growth, water consumption by industry, agriculture, and households continues to increase [1], leading to water pollution as a global issue [2]. Synthetic dyes are among the most harmful chemical pollutants in water [3], and water contaminated by them can cause serious health problems [4,5], hence the need for wastewater treatment. Methods of treating water pollutants can be divided into physical, chemical, and biological methods [6]. However, some organic pollutants are toxic and non-degradable, and cannot be treated by conventional physical or biological methods [7]. Still, advanced oxidation processes (AOPs) in chemical methods can solve this problem [8,9]. Studies have shown that the Fenton process is one of the most cost-effective AOPs [10]. However, the homogeneous Fenton reaction has some disadvantages, such as a narrow optimal pH range and the production of iron-containing sludge [11,12]. Therefore, research focused on heterogeneous Fenton catalysis [13]. The non-homogeneous Fenton reaction solves the problems of pH range extension and stability enhancement; so, it is more commonly used in wastewater treatment [14]. H 2 O 2 can be activated by Fe(II) to form a Fenton reaction, and the resulting free radicals (e.g., ·OH − ) are capable of efficiently degrading dye molecules, further advancing the development of wastewater treatment technologies [15]. Iron oxide, zeolite clay, immobilized iron, and carbon materials are widely used non-homogeneous Fenton catalysts, but many show weak catalytic activity [16]. Therefore, efficient catalysts must be prepared to treat dye wastewater [17].
As an important catalytic material, CeO 2 has attracted the attention of researchers in various catalytic applications for its unique structure and redox properties [18][19][20]. Lin Nanomaterials 2023, 13, 2545 2 of 13 et al. prepared PdO/CeO 2 catalysts using the deposition-precipitation method, tested them in the non-homogeneous Fenton degradation of acid orange 7 (AO7), and found that they could remove up to 50% of AO7 [21]. Zhu et al. prepared CuO/CeO 2 catalysts by ultrasonic impregnation and found that the removal rate of diclofenac reached 86.62% by testing [22]. Li et al. prepared LaFeO 3 /3DOM CeO 2 catalyst and found that it maintained effective catalytic degradation of MB after 10 cycles [23]. In addition, CeO 2 of the same purity is cheaper than other oxides; so, CeO 2 was chosen as the loading in this study [24,25]. BiFeO 3 is one of the important perovskite-type oxides that can provide Fe 3 + irrespective of pH [26]. Therefore, many researchers have combined BiFeO 3 with other nanomaterials for dye degradation [27]. Huang et al. successfully prepared CdSe QDs/BiFeO 3 composite catalysts using the stirring impregnation method. The degradation of phenol by 0.5CBFO reached 98.5 as compared to 41.5% for BFO in 60 min [28]. Volnistem et al. synthesized a BiFeO 3 /Fe 3 O 4 catalyst for degradation of methylene blue under visible light. The time required for the complete degradation of methylene blue solution was more than seven times faster for samples containing 20% Fe 3 O 4 than that needed for BiFeO 3 alone [29]. Therefore, CeO 2 and BiFeO 3 composite catalysts may achieve good results in dye degradation.
In this study, we fabricated BiFeO 3 /CeO 2 nanocatalytic materials with different molar ratios for the catalytic degradation of MB, RhB, and ST. The BiFeO 3 /CeO 2 nanocatalytic materials exhibited an enhanced activity, compared with BiFeO 3 or CeO 2 nanoparticles, which might benefit from the electron transfer from ≡Fe 2+ to ≡Ce 4+ . This work offers an effective way for catalytic dye degradation over Fenton catalysts. CeO 2 nanoparticles were obtained using a homogeneous precipitation method. Typically, 10.91 g of Ce(NO) 3 ·6H 2 O was dissolved in 100 mL of aqueous glycol (80 vol%) with vigorous stirring in a 50 • C water bath to obtain a transparent solution. Then, 25 mL of 3.0 M NH 3 ·H 2 O was added dropwise into the above solution, followed by stirring for 24 h. The resulting precipitate was collected by repeatedly washing with water and centrifugation, followed by drying at 80 • C overnight. The dried precipitate was calcined in air at 500 • C for 1 h, following which the CeO 2 nanoparticles were acquired.

Synthesis of BiFeO 3
BiFeO 3 was prepared by a sol-gel method. In a typical synthesis, 3 mmol of Bi(NO) 3 ·5H 2 O and 3 mmol of Fe(NO) 3 ·9H 2 O were dissolved in 50 mL of 0.5 M HNO 3 solution. Afterwards, it was gelatinized by adding 3 mmol of citric acid with vigorous stirring, following which the mixture was dried at 80 • C. The resulting powder was then calcined at 600 • C for 2 h to obtain BiFeO 3 .

Characterization
X-ray diffraction (XRD) data were acquired using a Rigaku Smart Lab diffractometer with Cu Kα radiation. Raman spectra were recorded with a dispersive Horiva Jobin Yvon LabRam HR800 microscope equipped with a He-Ne laser. Nitrogen physisorption was performed with a Micromeritics ASAP 2460 instrument at −196 • C. Scanning electron microscopy (SEM) images and the corresponding element mapping were performed with a field-emission Quanta FEG 250 microscope. Transmission electron microscopy (TEM) images were acquired using a Hitachi HT7700 microscope. X-ray photoelectron spectroscopy (XPS) data were obtained using a Thermofisher ESCALAB 250Xi instrument with monochromated Al Kα radiation as the X-ray source.

Catalytic Tests for Dye Degradation
The catalytic effect of BiFeO 3 /CeO 2 nanocatalytic materials was observed by catalyzing the dyes MB, RhB, and ST. Firstly, 20 mg of BiFeO 3 /CeO 2 nanocatalytic materials with different molar ratios were weighed into 50 mL solutions with concentrations of RhB (12 mg L −1 ), MB (15 mg L −1 ), and ST (15 mg L −1 ), respectively. The mixture solutions were stirred until adsorption and desorption equilibrate in the dark. Subsequently, 30% H 2 O 2 was added dropwise to the mixture to reach a concentration of 0.016 M, followed by a dropwise addition of diluted HCL (2.0 M); the pH of the experimental solution was adjusted around 3, and the centrifugal supernatant was isolated at the same intervals. Finally, the absorbance value at the maximum absorption wavelength of the dye was measured, and the removal rate of the dye was calculated and fitted to the kinetic data according to Equations (1)-(3). The catalytic rates of nanocatalytic materials for different dyes were calculated by Equation (1).
where K represents the catalytic rate, A 0 represents the absorbance concentration of the original dye solution, and A T represents the absorbance at the moment T. We investigated the adsorption properties of these nanomaterials on the dye solutions by a quasi-first-order kinetic model and a quasi-second-order kinetic model.
Equation (2) is a quasi-first-order kinetic equation: Equation (3) is a quasi-second-order kinetic equation: where q e is the adsorption capacity of MB and RhB dyes at equilibrium (mg/g), q t is the adsorption capacity of MB and RhB dyes at time t (mg/g), and the values of k 1 and k 2 are the quasi-first and quasi-second-order kinetic rate constants, respectively. When the catalytic process was finished, the cyclic stability experiments of the BiFeO 3 /CeO 2 nanocatalytic materials were carried out. The composite catalyst with the best catalytic effect was selected, washed with ethanol, and then washed and dried using ultrapure water. The recovered nanocatalytic materials catalysts were then catalyzed for six cycles under the same conditions for the three dyes.

Results and Discussion
Next, we performed various structural characterizations of the prepared CeO 2 nanoparticles, BiFeO 3 nanoparticles, and BiFeO 3 /CeO 2 nanocatalytic materials. Figure 1a shows the XRD patterns of CeO 2 nanoparticles, BiFeO 3 nanoparticles, and four BiFeO 3 /CeO 2 nanocatalytic materials with different molar ratios. There are three prominent absorption peaks of CeO 2 with 2θ of 28.54 • , 47.48 • , and 56.33 • corresponding to the crystallographic planes (111), (220), and (311) in the synthesized nanocatalytic materials. The XRD patterns of CeO 2 nanoparticles do not have peaks corresponding to any secondary or impurity phases. The XRD patterns of BiFeO 3 particles agree with the JCPDS data (Card No. 71-2494) and reveal a rhombic crystal structure of the BiFeO 3 phase with the R3c space group. The well-defined XRD peaks of BiFeO 3 particles indicate their enhanced crystallinity. The characteristic diffraction peaks of BiFeO 3 and CeO 2 crystalline forms can be represented by the XRD patterns of BiFeO 3 /CeO 2 nanocatalytic materials. As shown in the figure, the diffraction pattern of the nanocatalytic materials is very similar to that of pure CeO 2 at a BiFeO 3 molar ratio of 0.2. When increasing the molar ratio of BiFeO 3 in the nanocatalytic material samples, the diffraction patterns are more similar to those of pure BiFeO 3 , and the peaks at the crystalline plane at (101), (012), (110), and (104) are gradually enhanced. This may be due to the high crystallinity of the BiFeO 3 phase and therefore shows as the dominant peak in the XRD spectra of the samples of the nanocatalytic materials.
Nanomaterials 2023, 13, x FOR PEER REVIEW 4 of 13 ultrapure water. The recovered nanocatalytic materials catalysts were then catalyzed for six cycles under the same conditions for the three dyes.

Results and Discussion
Next, we performed various structural characterizations of the prepared CeO2 nanoparticles, BiFeO3 nanoparticles, and BiFeO3/CeO2 nanocatalytic materials. Figure 1a shows the XRD patterns of CeO2 nanoparticles, BiFeO3 nanoparticles, and four BiFeO3/CeO2 nanocatalytic materials with different molar ratios. There are three prominent absorption peaks of CeO2 with 2θ of 28.54°, 47.48°, and 56.33° corresponding to the crystallographic planes (111), (220), and (311) in the synthesized nanocatalytic materials. The XRD patterns of CeO2 nanoparticles do not have peaks corresponding to any secondary or impurity phases. The XRD patterns of BiFeO3 particles agree with the JCPDS data (Card No. 71-2494) and reveal a rhombic crystal structure of the BiFeO3 phase with the R3c space group. The well-defined XRD peaks of BiFeO3 particles indicate their enhanced crystallinity. The characteristic diffraction peaks of BiFeO3 and CeO2 crystalline forms can be represented by the XRD patterns of BiFeO3/CeO2 nanocatalytic materials. As shown in the figure, the diffraction pattern of the nanocatalytic materials is very similar to that of pure CeO2 at a BiFeO3 molar ratio of 0.2. When increasing the molar ratio of BiFeO3 in the nanocatalytic material samples, the diffraction patterns are more similar to those of pure BiFeO3, and the peaks at the crystalline plane at (101), (012), (110), and (104) are gradually enhanced. This may be due to the high crystallinity of the BiFeO3 phase and therefore shows as the dominant peak in the XRD spectra of the samples of the nanocatalytic materials.  Figure 1b shows the N2 adsorption-desorption isotherms of CeO2 nanoparticles, BiFeO3 nanoparticles, and BiFeO3/CeO2 nanocatalytic materials, and the test results are shown in Table 1. As displayed in Figure 1b, the adsorption-desorption of N2 occurred for CeO2 nanoparticles and BiFeO3/CeO2 nanocatalytic materials, but no adsorption-desorption of N2 occurred for BiFeO3 particles. The adsorption and desorption curves overlap at the relative pressure P/P0 < 0.46, mainly due to the reversible monomolecular layer adsorption. As seen in the figure, 0.2BiFeO3:0.8CeO2 adsorbs more N2 than 0.6BiFeO3:0.4CeO2 and the curve of the BiFeO3/CeO2 nanocatalytic materials has a significant hysteresis at higher relative pressures (P/P0 = 0.50-0.8), which implies that the nanocatalytic materials have a mesoporous structure. The CeO2 nanoparticle adsorption belongs to a type IV adsorption, but for the BiFeO3 nanoparticles, N2 adsorption and desorption were not significant. This may be due to the smooth, flaky nature of BiFeO3 nanoparticles.  Figure 1b shows the N 2 adsorption-desorption isotherms of CeO 2 nanoparticles, BiFeO 3 nanoparticles, and BiFeO 3 /CeO 2 nanocatalytic materials, and the test results are shown in Table 1. As displayed in Figure 1b, the adsorption-desorption of N 2 occurred for CeO 2 nanoparticles and BiFeO 3 /CeO 2 nanocatalytic materials, but no adsorptiondesorption of N 2 occurred for BiFeO 3 particles. The adsorption and desorption curves overlap at the relative pressure P/P 0 < 0.46, mainly due to the reversible monomolecular layer adsorption. As seen in the figure, 0.2BiFeO 3 :0.8CeO 2 adsorbs more N 2 than 0.6BiFeO 3 :0.4CeO 2 and the curve of the BiFeO 3 /CeO 2 nanocatalytic materials has a significant hysteresis at higher relative pressures (P/P 0 = 0.50-0.8), which implies that the nanocatalytic materials have a mesoporous structure. The CeO 2 nanoparticle adsorption belongs to a type IV adsorption, but for the BiFeO 3 nanoparticles, N 2 adsorption and desorption were not significant. This may be due to the smooth, flaky nature of BiFeO 3 nanoparticles.  Figure 1c shows the Raman spectra of CeO 2 , BiFeO 3 , and four different molar ratios of BiFeO 3 /CeO 2 nanocatalytic materials. As shown in the figure, the vibration of the Ce-O bond gradually increases, and the vibration of the Fe-O bond gradually decreases with the increase in the BiFeO 3 ratio. The successful preparation of the nanocatalytic materials BiFeO 3 /CeO 2 was further verified.
The SEM images of CeO 2 nanoparticles, BiFeO 3 nanoparticles, and BiFeO 3 /CeO 2 nanocatalytic materials are shown in Figure 2. Figure 2a indicates that the CeO 2 nanoparticles are spherical in shape, small in size, and clustered together. Figure 2b shows the SEM image of the 0.2BiFeO 3 :0.8CeO 2 nanocatalytic materials, which has a large particle shape and attached CeO 2 nanoparticles. Figure 2c shows the SEM image of 0.4BiFeO 3 :0.6CeO 2 nanocatalytic materials, which shows the structure of pores between each large particle and the attachment of CeO 2 nanoparticles. Figure 2d shows the SEM image of 0.6BiFeO 3 :0.4CeO 2 nanocatalytic materials; it can be seen that this nanocatalytic materials appear with many pore structures and different lamellae agglomerated together with a small amount of CeO 2 nanoparticles attached. Figure 2e shows the SEM image of the 0.8BiFeO 3 :0.2CeO 2 nanocatalytic materials, where it can be observed that the morphology is a structure of lamellae superimposed on each other and does not have holes. There is a significant difference in the morphology of BiFeO 3 /CeO 2 nanocatalytic materials with different molar ratios, which may be related to the amount of CeO 2 nanoparticles and the speed and time of stirring. Figure 2f shows the SEM images of the BiFeO 3 particles prepared by the sol-gel method, and it is observed that the morphology is irregularly clustered together in sheets and has a pore-like structure. Figure 2g shows the mapping image of the BiFeO 3 /CeO 2 nanocatalytic materials, and similar elemental distribution results were obtained from different positions in samples. And it can be concluded that the presence of Ce, Fe, and Bi elements proves the successful preparation of the nanocatalytic materials BiFeO 3 /CeO 2 .
The TEM images of CeO 2 nanoparticles, BiFeO 3 nanoparticles, and BiFeO 3 /CeO 2 nanocatalytic materials are shown in Figure 3. Figure 3a shows the TEM image of CeO 2 nanoparticles, which can be observed as CeO 2 nanoparticles are spherical in shape with diameters ranging from 5-10 nm. Figure 3b shows the TEM image of BiFeO 3 particles, which can be observed to be irregularly flaky and with a diameter of about 0.2 µm. Figure 3c-f show the TEM images of BiFeO 3 /CeO 2 nanocatalytic materials with different scales, from which it can be observed that many CeO 2 nanoparticles are attached to BiFeO 3 particles, which, combined with the SEM images, further indicates that BiFeO 3 /CeO 2 nanocatalytic materials are successfully prepared.      Figure 4 shows the XPS spectrum of the 0.6BiFeO 3 :0.4CeO 2 nanocatalytic materials. As shown in Figure 4a, the Ce 3d spectrum is decomposed into eight groups. The Ce 4f orbitals interact with the O 2p hybridization, and the four peaks labeled v (882.4 eV), v (885.3 eV), v (889.0 eV), and v (898.4 eV) belong to Ce 3d 5/2 , and the peaks labeled u (900.7 eV), u (903.6 eV), u (907.3 eV), and u (916.7 eV) are assigned to Ce 3d 3/2 . Of the eight peaks, v and u originate from the Ce 3+ ion (Ce 2 O 3 ), while the remaining six peaks correspond to the Ce 4+ species (CeO 2 ) [30]. The concentration of Ce 3+ can be related to the surface oxygen vacancies; so, the formation of oxygen vacancies is always accompanied by the formation of Ce 3+ . In the BiFeO 3 phase, the stable oxidation state of Fe is +3, which may also have Fe 2+ . This can also be confirmed by the Fe spectrum of its BiFeO 3 nanostructure. The oxygen core spectrum in Figure 4b shows peaks corresponding to lattice oxygen (O L ) and chemisorbed oxygen/other species (O ads ) adsorbed on the surface of the BiFeO 3 /CeO 2 nanostructure. Peaks of element O appear at 529.5 eV and 532.4 eV, indicating lattice (O L ) and chemisorbed oxygen species (Oads), respectively. The XPS spectrum of Bi 4f of the sample is shown in Figure 4c. The characteristic spectrum of Bi4f consists of two characteristic peaks corresponding to the spin double Bi 4f 7/2 and Bi 4f 5/2 , which correspond to binding energy magnitudes of 158.8 eV and 164.1 eV, indicating that the Bi element in the BiFeO 3 /CeO 2 nanocatalytic materials is of +3 valence. As shown in Figure 4d, the peak of the Fe 2p 3/2 line corresponding to Fe 2+ appears at 710.0 eV, and the Fe 2p 3/2 line corresponds to Fe 3+ at 711.0 eV [31]. Figure 4d calculates Fe 3+ :Fe 2+ = 55%:45%, which indicates that BFO has slightly more Fe 3+ ions than Fe 2+ particles. It indicates that the compounded BiFeO 3 particles have multiple oxidation states (Fe 2+ /Fe 3+ ), and Fe ions usually lead to oxygen vacancies in the BiFeO 3 . Nanomaterials 2023, 13, x FOR PEER REVIEW 7 of 13 Figure 4 shows the XPS spectrum of the 0.6BiFeO3:0.4CeO2 nanocatalytic materials. As shown in Figure 4a, the Ce 3d spectrum is decomposed into eight groups. The Ce 4f orbitals interact with the O 2p hybridization, and the four peaks labeled v (882.4 eV), v′ (885.3 eV), v″ (889.0 eV), and v‴ (898.4 eV) belong to Ce 3d5/2, and the peaks labeled u (900.7 eV), u′ (903.6 eV), u″ (907.3 eV), and u‴ (916.7 eV) are assigned to Ce 3d3/2. Of the eight peaks, v′ and u′ originate from the Ce 3+ ion (Ce2O3), while the remaining six peaks correspond to the Ce 4+ species (CeO2) [30]. The concentration of Ce 3+ can be related to the surface oxygen vacancies; so, the formation of oxygen vacancies is always accompanied by the formation of Ce 3+ . In the BiFeO3 phase, the stable oxidation state of Fe is +3, which may also have Fe 2+ . This can also be confirmed by the Fe spectrum of its BiFeO3 nanostructure. The oxygen core spectrum in Figure 4b shows peaks corresponding to lattice oxygen (OL) and chemisorbed oxygen/other species (Oads) adsorbed on the surface of the BiFeO3/CeO2 nanostructure. Peaks of element O appear at 529.5 eV and 532.4 eV, indicating lattice (OL) and chemisorbed oxygen species (Oads), respectively. The XPS spectrum of Bi 4f of the sample is shown in Figure 4c. The characteristic spectrum of Bi4f consists of two characteristic peaks corresponding to the spin double Bi 4f7/2 and Bi 4f5/2, which correspond to binding energy magnitudes of 158.8 eV and 164.1 eV, indicating that the Bi element in the BiFeO3/CeO2 nanocatalytic materials is of +3 valence. As shown in Figure  4d, the peak of the Fe 2p3/2 line corresponding to Fe 2+ appears at 710.0 eV, and the Fe 2p3/2 line corresponds to Fe 3+ at 711.0 eV [31]. Figure 4d calculates Fe 3+ :Fe 2+ = 55%:45%, which indicates that BFO has slightly more Fe 3+ ions than Fe 2+ particles. It indicates that the compounded BiFeO3 particles have multiple oxidation states (Fe 2+ /Fe 3+ ), and Fe ions usually lead to oxygen vacancies in the BiFeO3.  The catalytic properties of CeO 2 nanoparticles, BiFeO 3 nanoparticles, and BiFeO 3 /CeO 2 nanocatalytic materials on different dyes were investigated as follows. Figure 5 shows the adsorption kinetic results curves of CeO 2 , BiFeO 3 , and BiFeO 3 /CeO 2 nanocatalytic materials for the degradation of RhB (a,d,g), MB (b,e,h) and ST (c,f,i). Table 2 shows its fitting according to the quasi-first-order kinetic Equation (2) and quasi-second-order kinetic Equation (3) to make the kinetic curves of each of the three dyes. The 0.6BiFeO3:0.4CeO2 catalysts at the end of the experiment were collected by centrifugation and then washed clean with anhydrous ethanol and ultrapure water before drying and collecting in an oven. The dried catalysts were repeated six times for the dyecatalyzed solution experiments under the same conditions, and the obtained experimental results are shown in Figure 6. The data show that after repeated cycling experiments conducted several times, the catalytic efficiencies of this sample for RhB, MB, and ST dye solutions were 83.5%, 88.7%, and 90.4% of the first time, respectively, indicating that the 0.6BiFeO3:0.4CeO2 nanocatalytic materials have good cyclable stability. Figure 7 shows the reaction mechanism of BiFeO3/CeO2 nanocatalytic materials for the activation of H2O2 under acidic conditions. First, the reaction of Fe 3+ species with H2O2 (Equation (4)) and HO2·(Equation (5)) generates many ≡Fe 2+ species. The standard redox potential of Ce 4+ /Ce 3+ is 1.44 V, and Fe 3+ /Fe 2+ is 0.77 V (Equations (6)- (8)). Therefore, the transfer of electrons from ≡Fe 2+ to ≡Ce 4+ (Equation (6)) is thermodynamically favorable [16]. Based on previous work, we know that cerium can cycle redox in the presence of H2O2 and produce ‧OHads (Equations (7)-(9)), which behaves similarly to iron in Fentonlike reactions [32]. The iron ions above BiFeO3 are dispersed in the native solution and trigger the decomposition of H2O2 by chain reactions (Equations (10)-(12)) to produce ‧OH free radical. A small number of ‧OHads can diffuse from the catalyst surface into the native solution. As shown in Equations (13)- (18), competing reactions may negatively affect oxidation. Finally, the hydroxyl radicals, which are self-carried on the catalyst surface and in the solution, decompose the dye solution.   Figure 5a-c show that the catalytic rate of CeO 2 nanoparticles, BiFeO 3 particles, and BiFeO 3 /CeO 2 nanocatalytic materials in catalyzing the dye solution started fast and then gradually slowed down to a stationary value. The fast catalytic rate at the beginning is because the concentration difference between the dye and the catalyst is relatively apparent, and the amount of H 2 O 2 is sufficient to produce a large amount of ·OH by fully interacting with the catalyst under acidic conditions. As the catalytic reaction proceeds, the amount of H 2 O 2 and the dye concentration gradually decrease; so, the catalytic rate slows. It can also be observed that there is a significant difference in the catalytic effect of CeO 2 , BiFeO 3, and different molar ratios of BiFeO 3 /CeO 2 for other dyes. The catalytic efficiency of each molar ratio catalyst can only reach a maximum of 47.9% for MB dye solution within 120 min, while it can reach more than 80.0% for RhB and ST dye solutions. Among them, the nanocatalytic materials with a molar ratio of 0.2BiFeO 3 :0.8CeO 2 could achieve the maximum catalytic efficiency of 80.8% for RhB within 60 min, while the nanocatalytic materials with a molar ratio of 0.6BiFeO 3 :0.4CeO 2 could reach 81.1% and 48.7% for ST and MB dye degradation, respectively. As also shown in Figure 5, the correlation coefficients R 2 of the fitted quasifirst-order and quasi-second-order models differed very little and were close to 1. Therefore, the quasi-first-order and quasi-second-order models could simulate the process of dye solutions catalyzed by CeO 2 , BiFeO 3, and BiFeO 3 /CeO 2 nanocatalytic materials with different molar ratios more accurately. For RhB dye solutions, the catalytic amount of nanocatalytic materials with a molar ratio of 0.2BiFeO 3 :0.8CeO 2 reaches 32.1 mg/g, larger than the other ratios. For MB and ST dye solutions, the nanocatalytic materials with a molar ratio of 0.6BiFeO 3 :0.4CeO 2 had the highest catalytic amount per unit, reaching 17.1 mg/g and 17.4 mg/g, respectively. The effect of the catalysts with different molar ratios was different for other dyes, possibly due to the pH at the time of catalysis or the amount of H 2 O 2 added.
The 0.6BiFeO 3 :0.4CeO 2 catalysts at the end of the experiment were collected by centrifugation and then washed clean with anhydrous ethanol and ultrapure water before drying and collecting in an oven. The dried catalysts were repeated six times for the dyecatalyzed solution experiments under the same conditions, and the obtained experimental results are shown in Figure 6. The data show that after repeated cycling experiments conducted several times, the catalytic efficiencies of this sample for RhB, MB, and ST dye solutions were 83.5%, 88.7%, and 90.4% of the first time, respectively, indicating that the 0.6BiFeO 3 :0.4CeO 2 nanocatalytic materials have good cyclable stability.    Figure 7 shows the reaction mechanism of BiFeO 3 /CeO 2 nanocatalytic materials for the activation of H 2 O 2 under acidic conditions. First, the reaction of Fe 3+ species with H 2 O 2 (Equation (4)) and HO 2 ·(Equation (5)) generates many ≡Fe 2+ species. The standard redox potential of Ce 4+ /Ce 3+ is 1.44 V, and Fe 3+ /Fe 2+ is 0.77 V (Equations (6)- (8)). Therefore, the transfer of electrons from ≡Fe 2+ to ≡Ce 4+ (Equation (6)) is thermodynamically favorable [16]. Based on previous work, we know that cerium can cycle redox in the presence of H 2 O 2 and produce ·OH ads (Equations (7)-(9)), which behaves similarly to iron in Fenton-like reactions [32]. The iron ions above BiFeO 3 are dispersed in the native solution and trigger the decomposition of H 2 O 2 by chain reactions (Equations (10)-(12)) to produce ·OH free radical. A small number of ·OH ads can diffuse from the catalyst surface into the native solution. As shown in Equations (13)- (18), competing reactions may negatively affect oxidation. Finally, the hydroxyl radicals, which are self-carried on the catalyst surface and in the solution, decompose the dye solution. ·OH +·OH→ H 2 O 2 (18) ·OH + dye→ degraded products (19) . Figure 6. Cycling stability of the 0.6BiFeO3:0.4CeO2 nanocatalytic materials for degradation of RhB, MB, and ST.

Figure 7.
A proposed mechanism for catalytic dye degradation over the BiFeO3/CeO2 nanocatalytic materials.

Conclusions
In conclusion, BiFeO 3 /CeO 2 nanocatalytic materials with different molar ratios were synthesized and characterized using various techniques. The combination of CeO 2 nanoparticles with BiFeO 3 greatly increased the catalytic activity for the degradation of MB, RhB, and ST. The BiFeO 3 /CeO 2 nanocatalytic materials also showed good stability during six consecutive cycles. The electron transfer from ≡Fe 2+ to ≡Ce 4+ within the BiFeO 3 /CeO 2 nanocatalytic materials might dominate the catalytic activity of dye degradation. This finding provides an effective way for activity enhancement of catalytic degradation over Fenton catalytic systems.