Synthesis of Hollow Flower-Like Fe3O4/MnO2/Mn3O4 Magnetically Separable Microspheres with Valence Heterostructure for Dye Degradation

In this manuscript, hollow flower-like ferric oxide/manganese dioxide/trimanganese tetraoxide (Fe3O4/MnO2/Mn3O4) magnetically separable microspheres were prepared by combining a simple hydrothermal method and reduction method. As the MnO2 nanoflower working as precursor was partially reduced, Mn3O4 nanoparticles were in situ grown from the MnO2 nanosheet. The composite microspheres were characterized in detail by employing scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), Brunauer–Emmett–Teller (BET), vibration sample magnetometer (VSM) and UV–visible spectrophotometer (UV–vis). Under visible light conditions, the test for degrading rhodamine B (RhB) was used to verify the photocatalytic activity of the photocatalyst. The results showed that the efficiency of the Fe3O4/MnO2/Mn3O4 photocatalyst in visible light for 130 min is 94.5%. The catalytic activity of photocatalyst far exceeded that of the Fe3O4/MnO2 component, and after four cycles, the catalytic performance of the catalyst remained at 78.4%. The superior properties of the photocatalyst came from improved surface area, enhanced light absorption, and efficient charge separation of the MnO2/Mn3O4 heterostructure. This study constructed a green and efficient valence heterostructure composite that created a promising photocatalyst for degrading organic contaminants in aqueous environments.

Manganese dioxide (MnO 2 ) has already been used as catalyst due to its abundant sources, low cost, good redox, and environmental friendliness [15][16][17][18][19][20][21][22]. The degradation of organic pollutants by active metal oxides strongly depends on the specific surface area of the catalyst. Hence, the design of a layered birnessite-type MnO 2 (δ-MnO 2 ) into a flower-like microsphere having a high specific surface area is expected to display greater adsorption and catalytic ability. However, rapid recombination of photogenerated electron-hole pairs in photocatalysts will greatly reduce photocatalytic efficiency. Therefore, how to modify MnO 2 and improve its catalytic performance has become an urgent unsolved scientific challenge.
The construction of a heterojunction photocatalytic system has been demonstrated to be a promising green method to degrade organic dyes. The heterojunction can effectively separate the charge, thereby greatly improving the photocatalytic efficiency [23][24][25][26]. As a novel heterojunction, the valence heterojunction can readily realize the transfer of photogenerated carriers and energy without obvious loss, which results in significantly improving the photocatalytic activity. For this case, the presence of mixed valence states plays a decisive role in electron transport during chemical reactions [27][28][29][30]. Therefore, the induction of hausmannite trimanganese tetraoxide (Mn 3 O 4 ) nanoparticles from pure δ-MnO 2 nanosheets and the construction of MnO 2 /Mn 3 O 4 composites with valence heterojunctions hope to be an ideal high-efficiency photocatalyst.
Additionally, it is well known that heterogeneous catalysts have the advantage of being easier to separate and recycled than homogeneous catalysts. At the same time, the recycling of catalyst resources meets the goals of sustainable economic development and environmental protection. However, it is not convenient to separate the heterogeneous catalyst from the reaction system by conventional separation methods, such as centrifugation and filtration. On the basis of our preliminary work [31][32][33][34], the hollow magnetic ferric oxide (Fe 3 O 4 ) support was combined with the catalytically active component to prepare a hollow magnetic composite catalyst. The magnetic catalyst can be conveniently separated from the reaction medium when a suitable magnetic field is applied [35,36]. The introduction of the magnetic component greatly facilitates the recovery of the heterogeneous catalyst.
In this work, a hollow flower-like magnetic manganese-based photocatalyst is synthesized. To the best of our knowledge, this is the first time that a flower-like MnO 2 and Mn 3 O 4 nanoparticles are combined to form a heterogeneous photocatalyst for dye degradation. The increased specific surface area and the presence of the valence heterojunction increase the active sites and achieve efficient charge separation, which greatly improves the catalytic performance of the catalyst. In addition, the magnetic hollow Fe 3

Characterization of the Photocatalyst
The morphologies of hollow Fe 3 O 4 microspheres, hollow flower-like Fe 3 O 4 /MnO 2 microspheres, and hollow flower-like Fe 3 O 4 /MnO 2 /Mn 3 O 4 photocatalyst were determined by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). From the SEM image of Figure 1a, it could be found that as-synthesized Fe 3 O 4 microspheres were spherical in shape and well-monodispersed. Meanwhile, it was clear that their diameters were mainly about 200 nm. Figure 2a shows the TEM image of Fe 3 O 4 microspheres, in which a distinct gray/dark interface inside each microsphere indicated the existence of hollow structure. When the Fe 3 O 4 microspheres were coated with MnO 2 , their surface morphology was changed into flower-like hierarchical structure.      The specific surface area and the porosity of samples were obtained through the N2 adsorptiondesorption isotherms [44]. As shown in Figure 5a, the pore width of Fe3O4 microspheres was mainly ca. 4.0 nm. In Figure 5b,c, a typical IV isotherm of Fe3O4/MnO2 microspheres and Fe3O4/MnO2/Mn3O4 photocatalyst was observed, indicating the formation of pore structures resulting from the accumulation of MnO2 nanosheets. In Table 1, the specific surface area of Fe3O4 microspheres, Fe3O4/MnO2 microspheres, and Fe3O4/MnO2/Mn3O4 photocatalyst were respectively calculated to be 71.8, 117.3, and 143.03 m 2 /g. At the same time, their pore volume was calculated to be 0.17, 0.27, and 0.34 cm 3 /g, respectively. Therefore, the introduction of MnO2 nanosheets to form a flower-like structure is advantageous for increasing the porosity and specific surface area of the composite The specific surface area and the porosity of samples were obtained through the N 2 adsorption-desorption isotherms [44]. As shown in Figure 5a, the pore width of Fe 3 O 4 microspheres was mainly ca. 4.0 nm. In Figure 5b, 117.3, and 143.03 m 2 /g. At the same time, their pore volume was calculated to be 0.17, 0.27, and 0.34 cm 3 /g, respectively. Therefore, the introduction of MnO 2 nanosheets to form a flower-like structure is advantageous for increasing the porosity and specific surface area of the composite microspheres, and thus is expected to enhance the catalytic properties. In addition, it could be observed that the formation of

Photocatalytic Tests
The test of degrading rhodamine B (RhB) under simulated visible light was employed to verify the photocatalytic activity of the photocatalyst, as seen in  Figure 7d showed the variation of RhB concentration ratio C t /C 0 with increasing visible light irradiation time, in which C 0 was the initial concentration of RhB and C t was the concentration of RhB during the reaction. After 130 min illumination, it could be seen intuitively that pure RhB only had 2.3% self-decomposition through the dye sensitization pathway [45], RhB solution with adding   The photocatalytic reaction belonged to a pseudo first-order reaction and could be fitted by the Langmuir-Hinshelwood model of ln(C t /C 0 ) = −kt, where k was the apparent first-order rate constant [46][47][48][49]. In Figure 7e, compared with the rate constant k = 0.009 min −1 of Fe 3 O 4 /MnO 2 microspheres, Fe 3 O 4 /MnO 2 /Mn 3 O 4 photocatalyst exhibited the rate constant k = 0.025 min −1 , confirming that the elaborately fabricated photocatalyst had excellent photocatalytic activity. It was very important that for the photocatalyst, on the one hand, the quality and the chemical properties remained unchanged and, on the other hand, extraordinary reusability was maintained. Herein, the introduced magnetic Fe 3 O 4 microspheres could readily favor the recycling of the catalyst through magnetic separation. Figure 7f displayed the results of the photocatalyst being subjected to recycling experiments. There was no doubt that the conversion rate was reduced after the photocatalyst had been recycled. However, even after four cycles, the Fe 3 O 4 /MnO 2 /Mn 3 O 4 photocatalyst still 78.4% decomposition. This fact strongly demonstrates that the prepared photocatalyst had outstanding recyclability.
Minimizing the amount of catalyst was the key to reducing the cost of the catalytic reaction, which was also important for guiding the practical application of the catalyst. Hence, the effect of the dosage of Fe 3 O 4 /MnO 2 /Mn 3 O 4 photocatalyst on RhB degradation was investigated. In Figure 8a, it can be seen that the degradation efficiency of RhB was greatly improved by increasing the dosage of photocatalyst from 1 to 10 mg. This was because as the dosage increased, the total active surface increased correspondingly, thereby enhancing catalytic performance. Once the dosage exceeded 10 mg, the degradation efficiency was almost constant. This situation meant that after the maximum dosage was applied, the increase of the dosage had no effect on promoting degradation efficiency, which might be ascribed to an agglomeration of photocatalyst under high concentrations. As a result, for RhB degradation (65 mL of 10 mg/L), the dosage of Fe 3 O 4 /MnO 2 /Mn 3 O 4 photocatalyst was fixed at 10 mg for subsequent studies.
The effect of the pH on RhB degradation was investigated to determine the optimal pH range for the Fe 3 O 4 /MnO 2 /Mn 3 O 4 photocatalyst. In Figure 8b, it was seen that RhB degradation was obviously influenced by the pH of the reaction system. It was measured that the isoelectric point of the prepared catalyst in water was about pH = 3. When the pH was below 3, the periphery of the photocatalyst was positively charged, resulting in a repulsion between the photocatalyst and RhB cations. When the pH was beyond 3, the surface of the photocatalyst was negatively charged, giving rise to an increased electrostatic interaction between the photocatalyst and RhB cations. In this case, the favorable adsorption process accelerated RhB degradation. As the pH was in the range from 7 to 9, the degradation decreased. Due to the high pH, the MnO 2 component of the photocatalyst could interact with hydroxyl ions to form hydrated manganese oxides, thereby deteriorating the degradation efficiency. As a conclusion, the relatively acidic environment was more advantageous for the degradation of cationic dyes in the case of using Fe 3 O 4 /MnO 2 /Mn 3 O 4 photocatalyst.

Synthesis of Flower-Like Fe 3 O 4 /MnO 2 Microspheres
Hollow Fe 3 O 4 microspheres were fabricated by solvothermal method [50]. The prepared Fe 3 O 4 hollow microspheres (0.3 g) were added to a KMnO 4 aqueous solution (0.055 M, 80 mL), followed by the ultrasonication for 30 min. Then, HCl (37%, 1.0 mL) was dropwise added to the above solution. Thereafter, the homogeneous solution was transferred to a Teflon-lined stainless-steel autoclave and reacted at 100 • C for 6 h. The product was isolated by an external magnet after cooling to room temperature. The product was then separately washed 3 times with deionized water and ethanol and lyophilized to obtain flower-like Fe 3 O 4 /MnO 2 microspheres.

Synthesis of Flower-Like Fe 3 O 4 /MnO 2 /Mn 3 O 4 Microspheres
The prepared Fe 3 O 4 /MnO 2 microspheres (0.2 g) were fully dispersed in a deionized water (50 mL). Then, an aqueous solution of NaBH 4 (0.75 M, 20 mL) was poured into the suspension. The suspension was stirred for half an hour at room temperature. The product was washed with deionized water several times and lyophilized to obtain Fe 3 O 4 /MnO 2 /Mn 3 O 4 microspheres.

Photocatalytic Tests
Photocatalyst (10.0 mg) was dispersed into a RhB solution (10.0 mg/L, 65 mL). The solution was stirred for 60 min under dark conditions to achieve an equilibrium of adsorption and desorption. Then, the visible light was simulated with a 400 W metal halide lamp to carry out a photocatalytic reaction. The absorbance change of the solution was tested by UV-vis at intervals, the degradation curves of RhB solution with time were recorded, and the rate of photocatalytic degradation was calculated.

Characterization
Transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM) images were conducted on a JEM-3010 instrument (Hitachi Co., Tokyo, Japan). Scanning electron microscopy (SEM) images were conducted on a JSM-6700F instrument (JEOL Ltd., Tokyo, Japan). X-ray diffraction (XRD) patterns were obtained by a Shimadzu XRD-7000 X-ray diffraction meter (Shimadzu Co., Kyoto, Japan). X-ray photoelectron spectroscopy (XPS) characterization was measured on a JPS-9010 MC spectrometer (JEOL Ltd., Tokyo, Japan). The specific surface area and the average pore diameter of the catalysts were tested by Brunauer-Emmett-Teller (BET) method respectively with an ASAP 2020 system (Quantachrome, Boynton Beach, FL, USA). Vibrating sample magnetometer (VSM) curves were conducted on a Lake Shore 7307 instrument (Lake Shore Ltd., Columbus, OH, USA). Visible light irradiation was simulated on a BL-GHX-V photochemical reactor (Shanghai Bilang Instruments Co., Ltd., Shanghai, China). UV-vis absorption spectra were measured on a UV-5200PC UV-vis spectrophotometer (YuanXi, Shanghai, China).

Conclusions
In summary, the flower-like microspheres composite consisting of Fe 3 O 4 core and MnO 2 and Mn 3 O 4 shells were successfully synthesized. First, monodisperse hollow Fe 3 O 4 microspheres were obtained, where after grading MnO 2 nanosheets was grown around the Fe 3 O 4 microspheres in a simple hydrothermal system, and then Mn 3 O 4 nanoparticles were in situ fabricated on the MnO 2 nanosheets using a simple reduction method. The prepared Fe 3 O 4 /MnO 2 /Mn 3 O 4 photocatalyst exhibited impressive photocatalytic activity under visible light. Specifically, with the aid of this photocatalyst, the degradation rate of RhB could be as high as 94.5% under visible light irradiation for 130 min. Moreover, after four cycles of testing, the catalytic performance of the catalyst remained at 78.4%. The presentation of excellent photocatalytic performance was attributed to the flower-like morphology of MnO 2 and heterojunction between MnO 2 and Mn 3 O 4 . The former greatly improved the light utilization efficiency, and the latter effectively achieved spatial separation of photoinduced carriers. This strategy established an efficient and green binary heterojunction which enriches the way for the preparation of valence isomeric photocatalysts.