Fabrication of hollow flower-like magnetic Fe3O4/C/MnO2/C3N4 composite with enhanced photocatalytic activity

The serious problems of environmental pollution and energy shortage have pushed the green economy photocatalysis technology to the forefront of research. Therefore, the development of an efficient and environmentally friendly photocatalyst has become a hotpot. In this work, magnetic Fe3O4/C/MnO2/C3N4 composite as photocatalyst was synthesized by combining in situ coating with low-temperature reassembling of CN precursors. Morphology and structure characterization showed that the composite photocatalyst has a hollow core–shell flower-like structure. In the composite, the magnetic Fe3O4 core was convenient for magnetic separation and recovery. The introduction of conductive C layer could avoid recombining photo-generated electrons and holes effectively. Ultra-thin g-C3N4 layer could fully contact with coupled semiconductor. A Z-type heterojunction between g-C3N4 and flower-like MnO2 was constructed to improve photocatalytic performance. Under the simulated visible light, 15 wt% photocatalyst exhibited 94.11% degradation efficiency in 140 min for degrading methyl orange and good recyclability in the cycle experiment.

Preparation of Fe 3 O 4 /C/MnO 2 /C 3 N 4 composite. 6.0 g dicyandiamide was calcined at 550 °C for 4.0 h to produce g-C 3 N 4 . Thereafter, 2.0 g g-C 3 N 4 powder was dispersed in 80 mL deionized water, and then heated at 210 °C for 6 h to form a CN transparent precursor. Fe 3 O 4 /C/MnO 2 microspheres were added to the precursor (5.0, 10, 15, 20, 30 wt%). The solvent was slowly removed through a lyophilized process. Finally, Fe 3 O 4 /C/ MnO 2 /C 3 N 4 flower-like photocatalyst was obtained via annealing at 200 °C for 4.0 h in a tube furnace under N 2 protection.
Characterization. Scanning electron microscope (SEM, JSM-6700F, JEOL Ltd., Japan) was employed to obtain a surface topography image of the samples. Transmission images were gotten by using a high-resolution transmission electron microscope (TEM, JEM-3010, Hitachi Co., Japan). X-ray diffraction patterns of samples were obtained by the use of an X-ray diffractometer (XRD, Shimadzu XRD-7000, Shimadzu Co., Japan). X-ray photoelectron spectrometer (XPS, JPS-9010 MC, JEOL Ltd., Japan) was utilized to obtain the samples' surface elemental composition of the samples. Brunauer-Emmett-Teller (BET, ASAP 2020, Quantachrome, US) means was used to test the pore size and specific surface area of the catalyst. The saturation magnetization of the samples was obtained by employing a vibrating sample magnetometer (VSM, Lake Shore 7307, Lake Shore Ltd., USA). A photochemical reactor (BL-GHX-V, Shanghai Bilang Instruments Co., Ltd., China) was used to simulate the illumination. The ultraviolet-visible absorption spectra were measured on an ultraviolet-visible spectrophotometer (UV-vis, UV-5200PC, YuanXi, China).
Photocatalytic experiment. Firstly, 20 mg Fe 3 O 4 /C/MnO 2 /C 3 N 4 photocatalyst were added to 65 mL, 10 mg/L MO solution. Under dark environment, the mixture was agitated to reach adsorbed-desorbed equilibrium. Secondly, photocatalytic reaction was carried out with simulate light stemming from a 400 W metal halide lamp. The absorbance of the solution at intervals was monitored with the help of UV-visible spectrophotometer. Ultimately, the degradation curves of the MO solution were recorded, followed by the calculation of photocatalytic degradation rate.   Figure 2b shows Fe 3 O 4 /P(MMA-DVB) microspheres prepared by distillation precipitation process. Compared with the former, the surface of the latter becomes much smoother, which proves the successful formation of polymer coating. And these polymer core-shell microspheres have a diameter of 225 nm. To obtain the conductive carbon layer, the polymer microspheres were calcined and carbonized. The SEM image of Fe 3 O 4 /C microspheres is displayed in Fig. 2c. One can see that the original core-shell structure of the material is not destroyed after the calcination treatment. And the agglomeration that originally occurred in Fe 3 O 4 /P(MMA-DVB) polymer microspheres has been slightly weakened due to the carbonization treatment. From Fig. 2d,e, it can be found out that the flower-like morphology of the composite microspheres produced by the hydrothermal method is composed of MnO 2 intersecting sheets. And the overall particle size is about 480 nm. As shown in Fig. 2f, the overall flower-like morphology has not changed, but the thickness of the MnO 2 flower sheets has increased significantly. This case indicates that the ultra-thin C 3 N 4 layer is successfully formed on the surface of MnO 2 to form a flower-like Fe 3 O 4 /C/MnO 2 / C 3 N 4 composite photocatalyst. It can be seen from Fig. 2g that the synthesized magnetic microspheres have a clear hollow structure with a particle size of about 200 nm. Figure 2h shows the TEM image of the Fe 3 O 4 /C microspheres, which have a core-shell structure with 13 nm thickness of C shell. Figure 2i is the TEM image of the flower-like Fe 3 O 4 /C/MnO 2 /C 3 N 4 microspheres. It can be found out that the composite photocatalyst with a complete magnetic core and flower-like shell exhibits the diameter of around 480 nm. According to these results, the composite photocatalyst with a magnetic core and flower-like shell was successfully prepared. The crystal phase composition of the composite was demonstrated by XRD characterization, as shown in Fig. 3I. Figure 3I-a is the diffraction curve of the bulk g-C 3 N 4 obtained by pyrolysis of dicyandiamide. The strong peak near 27.4° belongs to the (002) plane, corresponding to the crystal plane stack of the CN aromatic system 48 . The broad peak at 13.0° belongs to the (100) plane ascribed to the triazine repeat unit 44 . Figure 3I  The XRD patterns cannot verify the existence of the C layer. For further confirming the formation of the C layer, the Raman test was used to characterize the Fe 3 O 4 /C sample. The spectrum in Fig. 3II indicates two different peaks at 1344 cm −1 and 1596 cm −1 , corresponding to D-band and G-band of carbon material, respectively. These results confirm the carbonization of Fe 3 O 4 /P(MMA-DVB) material, and Fe 3 O 4 /C microspheres are successfully obtained. These two bands are related to the A 1g phonon of sp 3 carbon atoms in disordered graphite and the in-plane vibration of sp 2 carbon atoms in the crystalline graphite, respectively 51 . The peak intensity ratio (I D /I G ) can evaluate the carbon material's crystallinity. The smaller the value is, the higher the degree of atomic order is 52 . Herein, the value is 0.79, meaning that the carbon material is graphitized partially. Therefore, the presence of the carbon matrix can improve the electronic conductivity and help avoid the recombination of photo-generated electron holes.
The surface chemical composition and the chemical state of the products were demonstrated by XPS characterization. Figure 4a is the full-scan spectrum of the photocatalyst, presenting the peaks of Mn, O, N, and C elements. From Fig. 4b, as for the Mn 2p spectrum, two peaks at 653.9 eV and 642.3 eV correspond to Mn 2p 1/2 and Mn 2p 3/2 . With respect to the O1s, as illustrated in Fig. 4c, three peaks at 529.7 eV, 531.3 eV, 533.2 eV are fitted, which are separately attributed to the Mn-O-Mn lattice oxygen, surface hydroxyl and surface adsorbed oxygen. The C1s spectrum in Fig. 4d shows the sub-bands centered at 284.8 eV and 288.5 eV, which are ascribed to the C-C coordination of the surface-unstable carbon and N=C-N 2 of g-C 3 N 4 . In addition, there is another peak centering at 286.3 eV, which is assigned to the C-O bond formed between the C of C 3 N 4 and the O of MnO 2 . This result indicates that MnO 2 and g-C 3 N 4 are closely connected and form a solid MnO 2 /g-C 3 N 4 interface, promoting the transfer and separation of photo-generated carriers. In the case of the N1s spectrum (Fig. 4e) Table 1. The former of the Fe 3 O 4 /C/MnO 2 and Fe 3 O 4 /C/MnO 2 /C 3 N 4 products are 119.56 m 2 /g and 120.25 m 2 /g, and the latter of them are 0.35 cm 3 /g and 0.31 cm 3 /g. Since C 3 N 4 does not significantly affect the morphology of the composite structure, these parameters of the two samples are almost similar. The higher values are owing to the flower-like structure of the composite photocatalyst. The increase in specific surface area is conducive to exposing more active sites and increasing more surface adsorption, followed by improving catalytic performance.
To evaluate the saturation magnetization value of Fe 3 O 4 , Fe 3 O 4 /C, Fe 3 O 4 /C/MnO 2 and Fe 3 O 4 /C/MnO 2 /C 3 N 4 , VSM measurement is conducted. It can be seen from Fig. 5a that the magnetization value of the Fe 3 O 4 microspheres is 70.58 emu/g. After the carbon layer is recombined, the value of Fe 3 O 4 /C microspheres decreases to 56.97 emu/g (Fig. 5b). After the flower-like MnO 2 was fabricated, the content of Fe 3 O 4 component is decreasing, which leads to the value of Fe 3 O 4 /C/MnO 2 microspheres decreases obviously to 37.62 emu/g (Fig. 5c). With the further formation of g-C 3 N 4 , the value is 30.02 emu/g (Fig. 5d). This value still meets the needs of magnetic separation. As shown in the illustration, when the magnet is placed next to the Fe 3 O 4 /C/MnO 2 /C 3 N 4 photocatalyst   www.nature.com/scientificreports/ suspension, the photocatalyst can be quickly attracted to the side of the cuvette in a short time. The results show that the photocatalyst has a good magnetic response to the magnetic field, favoring the magnetic separation from the mixed solution.
Determining the adsorption capacity of the photocatalyst in dark reaction, then degrading MO under simulated light is used to investigate the photocatalytic activity of the prepared photocatalyst, and the results are shown in Fig. 6. Figure 6a reveals the mixture reached adsorption-desorption equilibrium within 60 min. And Fe 3 O 4 /C/MnO 2 /C 3 N 4 can adsorb about 22% of MO within 60 min, which is related to its higher specific surface area (120.25 m 2 /g). Figure 6b displays that UV-Vis is employed to monitor the change in the absorbance of the solution during the photocatalytic reaction. In Fig. 6b, one can clearly view that MO was almost completely degraded with adding Fe 3 O 4 /C/MnO 2 /C 3 N 4 composite photocatalyst after 140 min. The photocatalytic degradation MO over Fe 3 O 4 /C/MnO 2 /C 3 N 4 could be described by the following reactions: Figure 6c indicates the change of the MO concentration ratio C t /C 0 with varying the light time, in which C 0 and C t are the initial concentration of MO and the concentration of MO during the reaction, respectively. The degradation rate of MO solution with Fe 3 O 4 /C/MnO 2 /C 3 N 4 photocatalyst reaches 94.11%. From Fig. 6d, this reaction is attributed to a pseudo first-order reaction, which belongs to the Langmuir-Hinshelwood model with ln (C t /C 0 ) = −kt. In the formula, k is the apparent first-order rate constant. The calculated rate constant k of Fe 3 O 4 /C/MnO 2 /C 3 N 4 photocatalyst is 0.022 min −1 . The excellent photocatalytic performance of Fe 3 O 4 /C/MnO 2 / C 3 N 4 composite material benefits from the synergistic effect between the various components.
In order to find the optimal ratio, the effect of amount of g-C 3 N 4 on the photocatalytic efficiency was investigated. Meanwhile, determining the minimum optimal amount of photocatalyst in practical applications is important to reduce the costs. The composite photocatalyst containing different amounts of g-C 3 N 4 (5%, 10%, 15%, 20%, 30%) were used to degrade MO dyes under the same conditions. From Fig. 7a,b, when the amount of g-C 3 N 4 is 15%, the Fe 3 O 4 /C/MnO 2 /C 3 N 4 composite photocatalyst has the highest value. In Fig. 7c, the effect of the amount of photocatalyst on the degradation efficiency is examined. The results show that the photocatalytic efficiency gradually increases when the amount of photocatalyst increases in the range of 0-20 mg, due to the effective reaction area and the reactive site increase. When the amount of photocatalyst continues to increase, the photocatalytic efficiency does not change significantly, which may be caused by the particle agglomeration affecting the increase of active sites. Therefore, the optimal dosage of Fe 3 O 4 /C/MnO 2 /C 3 N 4 photocatalyst is 20 mg. Considering the industrial application of Fe 3 O 4 /C/MnO 2 /C 3 N 4 nanoparticles, it is essential to investigate the recyclability and stability of the photocatalyst. The Fe 3 O 4 /C/MnO 2 /C 3 N 4 was reused four times to examine their www.nature.com/scientificreports/ performances. And Fig. 7d reveals the results that the degradation rates for the four cycles are 94.11%, 90.42%, 88.37% and 79.69%, respectively. There is no doubt that after the photocatalyst is recycled, the conversion rate will decrease, which might result from the loss of sample during the cycle. However, even after four cycles, the value still has 79.69% that might be related with the structure stability of the used photocatalysts, strongly demonstrating that the designed photocatalyst has excellent recyclability. In this study, Fe 3 O 4 /C/MnO 2 /C 3 N 4 photocatalyst was synthesized by compounding g-C 3 N 4 on the surface of MnO 2 . In terms of enhanced photocatalytic activity, it is assumed that the charge transfer in the photocatalyst uses the Z-type mechanism, as shown in Fig. 8. For the individual g-C 3 N 4 or MnO 2 component, due to thermodynamic effects, photo-generated holes in g-C 3 N 4 cannot oxidize OHto form •OH radicals, while photo-generated electrons in MnO 2 cannot generate · O 2 − radicals effectively. Therefore, individual g-C 3 N 4 or MnO 2 material cannot possess good photocatalytic performances. However, after a heterojunction was fabricated between these two components, the photo-generated electrons in the conduction band of MnO 2 can be transferred to the valence band of g-C 3 N 4 and combined with the photo-generated holes there. This configuration of the Z-type scheme makes the utilization of holes from MnO 2 and electrons from g-C 3 N 4 remarkably enhanced. In addition, the conductive C layer can also increase the photo-generated electron-hole pairs' separation in MnO 2 , which effectively prevents the recombination of photo-generated carriers. In the meantime, the higher specific surface area supplies much more active sites for photocatalytic activities. The prepared flower-like Fe 3 O 4 /C/MnO 2 /C 3 N 4 photocatalyst forms a Z-type photocatalytic system, which effectively enhances the separation of carrier, so that the composite material has excellent photocatalytic degradation efficiency.

Conclusions
In summary, a magnetic recyclable flower-like Fe 3 O 4 /C/MnO 2 /C 3 N 4 heterojunction photocatalyst was prepared for degrading organic dyes. The Fe 3 O 4 core was used to facilitate magnetic separation and recovery. The C layer could conduct photo-generated electrons in MnO 2 and protect the core. The thin g-C 3 N 4 layer was compounded on the surface of MnO 2 , which greatly improved the specific surface area and the reactive sites of the material. The obtained Fe 3 O 4 /C/MnO 2 /C 3 N 4 composites exhibited enhanced photocatalytic performance for the degradation of MO solution (65 mL, 10 mg/L) under simulated light irradiation. The maximum photocatalytic degradation efficiency was 94.11% within 140 min. It was assumed that a Z-type heterojunction was fabricated between www.nature.com/scientificreports/ MnO 2 and g-C 3 N 4 , which stimulated the electron transfer from the valence band of MnO 2 to the conduction band of g-C 3 N 4 . This structure promoted the photo-generated electron-hole pairs' separation, inhibited the free charges' recombination, and improved effective use of visible light. In here, an effective method to construct heterostructure nanomaterials was provided for efficient photocatalytic degradation.  www.nature.com/scientificreports/