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Article

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

by
Mingliang Ma
1,
Yuying Yang
1,
Yan Chen
1,
Fei Wu
1,
Wenting Li
1,
Ping Lyu
1,*,
Yong Ma
2,
Weiqiang Tan
3 and
Weibo Huang
1,*
1
School of Civil Engineering, Qingdao University of Technology, Qingdao 266033, China
2
School of Material Science and Engineering, Shandong University of Science and Technology, Qingdao 266590, China
3
School of Environmental and Municipal Engineering, Qingdao University of Technology, Qingdao 266033, China
*
Authors to whom correspondence should be addressed.
Catalysts 2019, 9(7), 589; https://doi.org/10.3390/catal9070589
Submission received: 14 June 2019 / Revised: 27 June 2019 / Accepted: 1 July 2019 / Published: 5 July 2019
(This article belongs to the Section Catalytic Materials)
Browse Figures
Versions Notes

Abstract

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.

1. Introduction

In recent years, dye wastewater has attracted much attention owing to its high toxicity, strong mutagenicity, and strong carcinogenicity [1,2,3,4]. Methods for removing dyes from wastewater have been developed, including adsorption, coacervation, membrane separation, chemical catalytic oxidation, and more [5,6,7,8]. Among these methods, semiconductor-based photocatalytic technology has been widely used due to its mild reaction conditions, strong oxidizing ability, complete degradation, lack of secondary pollution, and direct use of sunlight. For this method, semiconductor photocatalysts are considered to be key materials for the degradation of organic dyes owing to their low cost, cleanliness, and sustainable use. Therefore, the development of new, high-efficiency and visible-light-responsive photocatalytic materials has become a research hotspot in the field of photocatalysis [9,10,11,12,13,14].
Manganese dioxide (MnO2) 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 MnO2 (δ-MnO2) 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 MnO2 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 (Mn3O4) nanoparticles from pure δ-MnO2 nanosheets and the construction of MnO2/Mn3O4 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 (Fe3O4) 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 MnO2 and Mn3O4 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 Fe3O4 component allows the catalyst to be easily recycled. The synthesis steps are exhibited in Scheme 1. Firstly, the hollow Fe3O4 magnetic core is synthesized by hydrothermal method. Then, a flower-like Fe3O4/MnO2 microsphere having a high specific surface area was prepared through a secondary hydrothermal method. Finally, the hausmannite Mn3O4 nanoparticles were induced by in situ growth of flower-like Fe3O4/MnO2 using a simple reduction method. The hollow flower-like Fe3O4/MnO2/Mn3O4 magnetic catalyst was formed. Fe3O4/MnO2/Mn3O4 composite microspheres with heterojunction showed better catalytic activity than pure Fe3O4/MnO2 microspheres.

2. Results and Discussion

2.1. Characterization of the Photocatalyst

The morphologies of hollow Fe3O4 microspheres, hollow flower-like Fe3O4/MnO2 microspheres, and hollow flower-like Fe3O4/MnO2/Mn3O4 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 Fe3O4 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 Fe3O4 microspheres, in which a distinct gray/dark interface inside each microsphere indicated the existence of hollow structure. When the Fe3O4 microspheres were coated with MnO2, their surface morphology was changed into flower-like hierarchical structure. Figure 1b,c show the SEM images of hollow flower-like Fe3O4/MnO2 microspheres at distant and close view, respectively. One could easily observe that MnO2 shells were assembled by numerous nanosheets. Figure 2b,c shows the TEM images of Fe3O4/MnO2 microspheres, powerfully certifying the core–shell structure and demonstrating the shell thickness of 180 nm. In the SEM image of Figure 1d, hollow flower-like Fe3O4/MnO2/Mn3O4 photocatalysts were displayed with rough edges of MnO2 nanosheets. The subtle morphological change was believed to result from the formation of Mn3O4 nanoparticles. The detailed morphology and composition of Fe3O4/MnO2/Mn3O4 photocatalyst are shown in TEM images in Figure 2d,e. It could be seen that the Mn3O4 nanoparticles with diameters of about 4 nm were evenly distributed on the periphery and inside of the folded MnO2 nanosheets. Figure 2f shows the high-resolution transmission electron microscopy (HRTEM) image of Fe3O4/MnO2/Mn3O4 photocatalyst. The interplanar spacing of 0.352 nm was assigned to the (002) plane of MnO2, and the interplanar spacings of 0.245 and 0.164 nm were attributed to the (202) and (303) planes of Mn3O4. The results presented herein strongly confirmed the successful synthesis of Fe3O4/MnO2/Mn3O4 photocatalyst.
Phase structures of hollow Fe3O4 microspheres, hollow flower-like Fe3O4/MnO2 microspheres and hollow flower-like Fe3O4/MnO2/Mn3O4 photocatalyst were analyzed by X-ray diffraction (XRD). Figure 3a showed that the diffraction peaks located at 30.1°, 35.4°, 43.1°, 53.4°, 57.1°, and 62.5° were successively attributed to (220), (311), (400), (422), (511), and (440) crystal planes. The XRD pattern showed good consistency with that of pure Fe3O4 (JCDPS 85-1436) [37]. Moreover, no other diffraction peaks were observed, which indicated that the preparation of hollow Fe3O4 microspheres was high in purity. As for Fe3O4/MnO2 microspheres, in the XRD pattern of Figure 3b, except for the diffraction peaks of pristine Fe3O4, three new peaks at 12.54°, 25.24°, and 58.32° could be clearly viewed, corresponding to (001), (002), and (203) crystal planes of MnO2 (PDF NO. 80-1098) [38]. With respect to Fe3O4/MnO2/Mn3O4 photocatalyst, the XRD pattern of Figure 3c displayed another extra four new peaks at 18.00°, 36.45°, 56.01°, and 65.40°, which were assigned to (101), (202), (303), and (323) crystal planes of Mn3O4 (PDF NO. 24-0734) [39]. The XRD patterns powerfully demonstrated the successful synthesis of MnO2 and Mn3O4 phases.
For further determining the chemical compositions of Fe3O4/MnO2 microspheres and Fe3O4/MnO2/Mn3O4 photocatalyst, X-ray photoelectron spectroscopy (XPS) measurement was carried out. Figure 4a displayed the characteristic peaks of manganese (Mn), oxygen (O), and adventitious carbon (C). The high-resolution Mn 2p spectra of Fe3O4/MnO2 microspheres were presented in Figure 4b, in which Mn 2p1/2 was at 654.3 eV and Mn 2p3/2 peaks was at 642.5 eV, respectively [40]. The shape and binding energy of Mn 2p3/2 peak was in line with that of δ-MnO2. No other peaks were observed at this binding energy, implying the inexistence of Mn(III) and Mn(II). Hence, the oxidation state of Mn was +4. With regard to Fe3O4/MnO2/Mn3O4 photocatalyst, as shown in Figure 4c, there were two characteristic peaks of Mn(III) and Mn(II) located at 641.4 and 640.3 eV for the Mn 2p3/2 peak. This case demonstrated that the photocatalyst contained multivalent state manganese (Mn(II), Mn(III), and Mn(IV)) of Mn3O4 and MnO2 [41,42]. Such a conclusion was verified by the XPS spectrum of O 1s. By contrast, the high-resolution O 1s spectra of Fe3O4/MnO2 microspheres and Fe3O4/MnO2/Mn3O4 photocatalyst were exhibited in Figure 4d,e. For the former, the binding energies of approximately 529.7, 531.1, and 531.7 eV was separately caused by the bulk oxygen of MnO2, surface hydroxyl bonded to Mn, and absorbed oxygen. Nevertheless, for the latter, another extraordinary peak at 530.2 eV was detected in addition to the three binding energies above, which originated from the existence of lattice oxygen of Mn3O4 [43]. This also demonstrated that the photocatalyst consisted of Mn3O4 and MnO2. The Fe3O4/MnO2/Mn3O4 photocatalyst was rich in oxygen, which played a key role in the next photocatalytic reaction.
The specific surface area and the porosity of samples were obtained through the N2 adsorption–desorption 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 m2/g. At the same time, their pore volume was calculated to be 0.17, 0.27, and 0.34 cm3/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 microspheres, and thus is expected to enhance the catalytic properties. In addition, it could be observed that the formation of Mn3O4 nanoparticles on the flower-like Fe3O4/MnO2 microspheres further increased the specific surface area of the photocatalyst. Fe3O4/MnO2/Mn3O4 photocatalyst possessing noticeably high specific surface area and porosity could effectively provide more active sites, accelerating the photocatalytic process.
The magnetic properties of hollow Fe3O4 microspheres, hollow flower-like Fe3O4/MnO2 microspheres and hollow flower-like Fe3O4/MnO2/Mn3O4 photocatalyst were exhibited by vibrating sample magnetometer (VSM) curves tested at room temperature. From Figure 6, the saturated magnetization values of them were 73.68, 36.61, and 35.34 emu/g, respectively. There was no doubt that the saturated magnetization value decreased considerably along with the formation of MnO2 nanosheets and Mn3O4 nanoparticles, in turn. Such a decrease was a result of a reduction in proportion of the magnetite component. It should be pointed out that Fe3O4/MnO2/Mn3O4 photocatalyst still possessed excellent magnetic responsivity. In the inset image, the separation–redispersion behavior of Fe3O4/MnO2/Mn3O4 photocatalyst, displayed in water, testified that this photocatalyst has remarkable magnetic manipulation ability.

2.2. 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 7. To examine the catalytic effect of Fe3O4/MnO2/Mn3O4 photocatalyst, in Figure 7a–c, the control experiments on pure RhB, RhB+Fe3O4/MnO2 microspheres and RhB + Fe3O4/MnO2/Mn3O4 photocatalyst were conducted under the same experimental conditions. What was noteworthy was that RhB was degraded completely in the presence of Fe3O4/MnO2/Mn3O4 photocatalyst after 130 min, while neither of the other two groups was completely degraded. Figure 7d showed the variation of RhB concentration ratio Ct/C0 with increasing visible light irradiation time, in which C0 was the initial concentration of RhB and Ct 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 Fe3O4/MnO2 microspheres reached 63.5% decomposition, but the RhB solution with the addition of Fe3O4/MnO2/Mn3O4 photocatalyst microspheres amazingly reached 94.5%.
The photocatalytic reaction belonged to a pseudo first-order reaction and could be fitted by the Langmuir–Hinshelwood model of ln(Ct/C0) = −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 Fe3O4/MnO2 microspheres, Fe3O4/MnO2/Mn3O4 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 Fe3O4 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 Fe3O4/MnO2/Mn3O4 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 Fe3O4/MnO2/Mn3O4 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 Fe3O4/MnO2/Mn3O4 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 Fe3O4/MnO2/Mn3O4 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 MnO2 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 Fe3O4/MnO2/Mn3O4 photocatalyst.
The possible photocatalytic mechanism for Fe3O4/MnO2/Mn3O4 photocatalyst was proposed and diagrammatized in Scheme 2. The flower-like morphology of the photocatalyst effectively increased the scattering and reflection of incident light, and the light could reach all the particles thanks to reflection and scattering phenomena in the media, which greatly improved light utilization. In addition, MnO2 and Mn3O4 could constitute a heterojunction structure. Photogenerated carriers could be effectively separated in heterojunctions, which enhanced photocatalytic performance [43]. For the photocatalytic test, Fe3O4/MnO2/Mn3O4 microspheres had better catalytic performance than Fe3O4/MnO2 microspheres, which indicated that the composite of MnO2 and Mn3O4 could significantly enhance the photocatalytic performance. Theoretically, the conduction band and valence band of MnO2 were more positive than those of Mn3O4, which favored the transfer of photogenerated charge in thermodynamic theory. Under visible light irradiation, the electrons (e−) excited into the conduction band in Mn3O4 would be transferred to the conduction band of MnO2. The holes (h+) were left in the valence band of Mn3O4. This effectively avoided the recombination of e− and h+ and prolonged the lifetime of the carriers. Both e− and h+ dominated the active centers to the photocatalysis. In the photocatalytic process, h+ could combine with hydroxyl ions, resulting in the formation of hydroxyl radicals in the valence band of MnO2. In the meantime, the superoxide radical anions appeared in the conduction band of Mn3O4 by interacting with the e− and the dissolved O. Hydroxyl radicals and superoxide radicals could effectively degrade organic pollutants in wastewater. In addition, the RhB could also be directly degraded by h+ in the valence band of Mn3O4 [40,41].

3. Materials and Methods

3.1. Materials

Ferric chloride hexahydrate (FeCl3·6H2O), sodium polyacrylate, urea, sodium citrate, potassium permanganate (KMnO4), ethanol, hydrochloric acid (HCl), RhB, and sodium borohydride (NaBH4) were bought from Shanghai Ebi Chemical Reagent Co., Ltd. All reagents were of analytical grade and applied without further purification. Distilled water was utilized throughout all the preparation procedures.

3.2. Synthesis of Flower-Like Fe3O4/MnO2 Microspheres

Hollow Fe3O4 microspheres were fabricated by solvothermal method [50]. The prepared Fe3O4 hollow microspheres (0.3 g) were added to a KMnO4 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 Fe3O4/MnO2 microspheres.

3.3. Synthesis of Flower-Like Fe3O4/MnO2/Mn3O4 Microspheres

The prepared Fe3O4/MnO2 microspheres (0.2 g) were fully dispersed in a deionized water (50 mL). Then, an aqueous solution of NaBH4 (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 Fe3O4/MnO2/Mn3O4 microspheres.

3.4. 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.

3.5. 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).

4. Conclusions

In summary, the flower-like microspheres composite consisting of Fe3O4 core and MnO2 and Mn3O4 shells were successfully synthesized. First, monodisperse hollow Fe3O4 microspheres were obtained, where after grading MnO2 nanosheets was grown around the Fe3O4 microspheres in a simple hydrothermal system, and then Mn3O4 nanoparticles were in situ fabricated on the MnO2 nanosheets using a simple reduction method. The prepared Fe3O4/MnO2/Mn3O4 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 MnO2 and heterojunction between MnO2 and Mn3O4. 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.

Author Contributions

Conceptualization, M.M. and Y.Y.; investigation, Y.Y., Y.C., F.W. and W.L.; writing—original draft preparation, Y.Y.; writing—review and editing, M.M., Y.M. and Y.Y.; resources, M.M. and Y.M.; supervision, P.L., W.H. and W.T.

Funding

This research was funded by the National Natural Science Foundation of China (51503116 and 51578298) and the Shandong Provincial Natural Science Foundation (ZR2019BB063).

Conflicts of Interest

The authors declare no conflict of interest.

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Scheme 1. The synthesis steps of hollow flower-like Fe3O4/MnO2/Mn3O4 magnetic photocatalyst.
Scheme 1. The synthesis steps of hollow flower-like Fe3O4/MnO2/Mn3O4 magnetic photocatalyst.
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Figure 1. SEM images of hollow Fe3O4 microspheres (a), hollow flower-like Fe3O4/MnO2 microspheres (b,c), and hollow flower-like Fe3O4/MnO2/Mn3O4 photocatalyst (d).
Figure 1. SEM images of hollow Fe3O4 microspheres (a), hollow flower-like Fe3O4/MnO2 microspheres (b,c), and hollow flower-like Fe3O4/MnO2/Mn3O4 photocatalyst (d).
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Figure 2. TEM images of hollow Fe3O4 microspheres (a), hollow flower-like Fe3O4/MnO2 microspheres (b,c), and hollow flower-like Fe3O4/MnO2/Mn3O4 photocatalyst (d–f).
Figure 2. TEM images of hollow Fe3O4 microspheres (a), hollow flower-like Fe3O4/MnO2 microspheres (b,c), and hollow flower-like Fe3O4/MnO2/Mn3O4 photocatalyst (d–f).
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Figure 3. XRD patterns of hollow Fe3O4 microspheres (a), hollow flower-like Fe3O4/MnO2 microspheres (b), and hollow flower-like Fe3O4/MnO2/Mn3O4 photocatalyst (c).
Figure 3. XRD patterns of hollow Fe3O4 microspheres (a), hollow flower-like Fe3O4/MnO2 microspheres (b), and hollow flower-like Fe3O4/MnO2/Mn3O4 photocatalyst (c).
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Figure 4. XPS spectrum of hollow flower-like Fe3O4/MnO2 microspheres and hollow flower-like Fe3O4/MnO2/Mn3O4 photocatalyst (a); Mn 2p spectra of hollow Fe3O4/MnO2 spheres (b) and hollow flower-like Fe3O4/MnO2/Mn3O4 microspheres (c); O 1s spectra of hollow Fe3O4/MnO2 microspheres (d) and hollow flower-like Fe3O4/MnO2/ Mn3O4 microspheres (e).
Figure 4. XPS spectrum of hollow flower-like Fe3O4/MnO2 microspheres and hollow flower-like Fe3O4/MnO2/Mn3O4 photocatalyst (a); Mn 2p spectra of hollow Fe3O4/MnO2 spheres (b) and hollow flower-like Fe3O4/MnO2/Mn3O4 microspheres (c); O 1s spectra of hollow Fe3O4/MnO2 microspheres (d) and hollow flower-like Fe3O4/MnO2/ Mn3O4 microspheres (e).
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Figure 5. N2 adsorption–desorption isotherms of hollow Fe3O4 microspheres (a), hollow flower-like Fe3O4/MnO2 microspheres (b), and hollow flower-like Fe3O4/MnO2/Mn3O4 photocatalyst (c). Insets in (a–c) are the corresponding pore width distribution plots.
Figure 5. N2 adsorption–desorption isotherms of hollow Fe3O4 microspheres (a), hollow flower-like Fe3O4/MnO2 microspheres (b), and hollow flower-like Fe3O4/MnO2/Mn3O4 photocatalyst (c). Insets in (a–c) are the corresponding pore width distribution plots.
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Figure 6. Hysteresis loops of hollow Fe3O4 microspheres (a), hollow flower-like Fe3O4/MnO2 microspheres (b), and hollow flower-like Fe3O4/MnO2/Mn3O4 photocatalyst (c).
Figure 6. Hysteresis loops of hollow Fe3O4 microspheres (a), hollow flower-like Fe3O4/MnO2 microspheres (b), and hollow flower-like Fe3O4/MnO2/Mn3O4 photocatalyst (c).
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Catalysts 09 00589 g007aCatalysts 09 00589 g007b
Figure 7. UV–vis spectra of RhB solutions in different performing times in the presence of pure RhB (a), Fe3O4/MnO2 microspheres (b), and Fe3O4/MnO2/Mn3O4 photocatalyst (c). Conversion of Ct/C0 for the degradation of RhB (d). Linear fit of experimental data by first-order kinetic model (e). Consecutive degradation of RhB with Fe3O4/MnO2/Mn3O4 photocatalyst for four cycles (f). Reaction conditions: RhB = 65 mL of 10 mg/L; Catalyst = 10 mg; initial natural pH = 5.0 ± 0.1; Illuminant = 400 W metal halide lamp; T = 25 °C; Time = 130 min.
Figure 7. UV–vis spectra of RhB solutions in different performing times in the presence of pure RhB (a), Fe3O4/MnO2 microspheres (b), and Fe3O4/MnO2/Mn3O4 photocatalyst (c). Conversion of Ct/C0 for the degradation of RhB (d). Linear fit of experimental data by first-order kinetic model (e). Consecutive degradation of RhB with Fe3O4/MnO2/Mn3O4 photocatalyst for four cycles (f). Reaction conditions: RhB = 65 mL of 10 mg/L; Catalyst = 10 mg; initial natural pH = 5.0 ± 0.1; Illuminant = 400 W metal halide lamp; T = 25 °C; Time = 130 min.
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Figure 8. Effects of the dosage of Fe3O4/MnO2/Mn3O4 photocatalyst on RhB degradation (a). Reaction conditions: RhB = 65 mL of 10 mg/L; Catalyst = variable; initial natural pH = 5.0 ± 0.1; Illuminant = 400 W metal halide lamp; T = 25 °C; Time = 130 min. Effects of the pH on RhB degradation in the presence of Fe3O4/MnO2/Mn3O4 photocatalyst (b). Reaction conditions: RhB = 65 mL of 10 mg/L; Catalyst = 10 mg; pH = variable; Illuminant = 400 W metal halide lamp; T = 25 °C; Time = 130 min.
Figure 8. Effects of the dosage of Fe3O4/MnO2/Mn3O4 photocatalyst on RhB degradation (a). Reaction conditions: RhB = 65 mL of 10 mg/L; Catalyst = variable; initial natural pH = 5.0 ± 0.1; Illuminant = 400 W metal halide lamp; T = 25 °C; Time = 130 min. Effects of the pH on RhB degradation in the presence of Fe3O4/MnO2/Mn3O4 photocatalyst (b). Reaction conditions: RhB = 65 mL of 10 mg/L; Catalyst = 10 mg; pH = variable; Illuminant = 400 W metal halide lamp; T = 25 °C; Time = 130 min.
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Scheme 2. Possible mechanism of Fe3O4/MnO2/Mn3O4 photocatalyst for the improved photocatalytic performance under visible light.
Scheme 2. Possible mechanism of Fe3O4/MnO2/Mn3O4 photocatalyst for the improved photocatalytic performance under visible light.
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Table 1. Physical parameters of the prepared samples.
Table 1. Physical parameters of the prepared samples.
SampleSurface Area (m2/g)Pore Volume (cm3/g)
Fe3O438.470.17
Fe3O4/MnO2117.660.27
Fe3O4/MnO2/Mn3O4143.030.34

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Ma, M.; Yang, Y.; Chen, Y.; Wu, F.; Li, W.; Lyu, P.; Ma, Y.; Tan, W.; Huang, W. Synthesis of Hollow Flower-Like Fe3O4/MnO2/Mn3O4 Magnetically Separable Microspheres with Valence Heterostructure for Dye Degradation. Catalysts 2019, 9, 589. https://doi.org/10.3390/catal9070589

AMA Style

Ma M, Yang Y, Chen Y, Wu F, Li W, Lyu P, Ma Y, Tan W, Huang W. Synthesis of Hollow Flower-Like Fe3O4/MnO2/Mn3O4 Magnetically Separable Microspheres with Valence Heterostructure for Dye Degradation. Catalysts. 2019; 9(7):589. https://doi.org/10.3390/catal9070589

Chicago/Turabian Style

Ma, Mingliang, Yuying Yang, Yan Chen, Fei Wu, Wenting Li, Ping Lyu, Yong Ma, Weiqiang Tan, and Weibo Huang. 2019. "Synthesis of Hollow Flower-Like Fe3O4/MnO2/Mn3O4 Magnetically Separable Microspheres with Valence Heterostructure for Dye Degradation" Catalysts 9, no. 7: 589. https://doi.org/10.3390/catal9070589

APA Style

Ma, M., Yang, Y., Chen, Y., Wu, F., Li, W., Lyu, P., Ma, Y., Tan, W., & Huang, W. (2019). Synthesis of Hollow Flower-Like Fe3O4/MnO2/Mn3O4 Magnetically Separable Microspheres with Valence Heterostructure for Dye Degradation. Catalysts, 9(7), 589. https://doi.org/10.3390/catal9070589

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