Highly efficient visible-light-induced photocatalytic production of

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Highly efficient visible-light-induced photocatalytic production of hydrogen for magnetically retrievable Fe3O4@SiO2@MoS2/g-C3N4 hierarchical microspheres Dingze Lu, Huiqing Fan, Kiran Kumar Kondamareddy, Huawa Yu, Anxiang Wang, Hongjuan Hao, Min Li, and Junwei Shen ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/acssuschemeng.8b01118 • Publication Date (Web): 10 Jul 2018 Downloaded from http://pubs.acs.org on July 11, 2018

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ACS Sustainable Chemistry & Engineering

Highly efficient visible-light-induced photocatalytic production of

hydrogen

for

magnetically

retrievable

Fe3O4@SiO2@MoS2/g-C3N4 hierarchical microspheres

Dingze Lua, b*, Huiqing Fanb*, Kiran Kumar Kondamareddyc, Huawa Yua, Anxiang Wanga, Hongjuan Haoa, Min Lia, Junwei Shena a

School of Science, Xi’an Polytechnic University, No.19 of Jinhua South Road, Beilin District,

Xi’an 710048, PR China

b

State Key Laboratory of Solidification Processing, School of Materials Science and Engineering,

Northwestern Polytechnical University, Xi’an 710072, PR China

c

Department of Physics, Veltech Dr. RR. & Dr. SR. R&D Institute of science and technology,

Avadi-600062, Chennai, Tamilnadu, India

* Corresponding author. Tel.: +86-29-8233-0277; Fax: +86-29-8233-0277; E-mail address: [email protected] (D.Z. Lu) and [email protected] (H.Q. Fan)

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ACS Paragon Plus Environment

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ABSTRACT A

new

multifunctional

compound

containing

hierarchical

microspheres

of

Fe3O4@SiO2@MoS2/g-C3N4 (FSMG) was created. The microspheres comprised Fe3O4@SiO2 as a magnetic component and a heterostructure of MoS2/g-C3N4 as an outer shell, and both elements are compounded by an effective and applicable method and can be used in photocatalytic applications. Highly efficient separation of the photodriven pairs electron/hole pairs (e−/h+) were exhibited with the as-synthesized FSMG structures under visible light. The photocatalytic activities of Fe3O4@SiO2@MoS2, g-C3N4, and FSMG are assessed by surveying the hydrogen (H2) production and rhodamine B (RhB) photodegradation from water. These contrasting studies show that microspheres of FSMG show promising visible-light-induced photocatalytic activity and exhibit a 1.99-fold and 3.38-fold increased activity over that of Fe3O4@SiO2@MoS2 and g-C3N4 mechanisms, respectively, in RhB degradation and a 4.13-fold and 11.09-fold increase in H2 production from water, respectively. Furthermore, the FSMG microspheres also show good recovery with a magnet. As studied by XPS, TEM, and SEM, photocurrent curves, trapping agent experiments and Nyquist impedance spectroscopy, the extended light response range, intimate contact interface, improved separation speed of carriers and higher photocurrent density resulted in the increased photocatalytic activity of heterostructures of MoS2/g-C3N4. MoS2 trapped electrons to improve the lifespan of classified electron/hole pairs, while the assembled holes located at the surface of g-C3N4 continuously oxidize the dye, which provides a controllable path for photodegradation and H2 production. The improved systems and principles stated here will be great of significance in heterogeneous photocatalysis.

Keywords: MoS2/g-C3N4; Semiconductors; Photocatalysis; Nanocomposites; Magnetic materials.

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INTRODUCTION Semiconductor photocatalysts have received great attention as an ecologically friendly method for pollutant decomposition, hydrogen (H2) generation and conversion of hydrocarbons to carbon dioxide.1-7 TiO2 has been broadly applied as a promising photocatalyst because of its overall good performance, including high photocatalytic activity, stability and nontoxicity, etc.8,9 Nevertheless, its application has been constrained by the dependence of the photoactivity on ultraviolet light due to its broad band gap. Since metal-free graphite carbon nitride (g-C3N4) used for photocatalysis under visible light was reported by Wang et al.,10 it has been widely applied for its high thermal and chemical stability as well as small band gap (~2.7 eV). In addition, purified g-C3N4 shows weaker photocatalytic properties because of its higher combination rate of charge carriers and lower specific surface area.11,12 As visible-light-induced photocatalysts have been continued to be important materials for a variety of applications, scholars and researchers have exhibited interest in MoS2 because of its intrinsic physicochemical and structural properties.13-18 MoS2 has been regarded as a potential photocatalyst for H2 evolution as well as the decomposition of sulfide-containing organic contaminants.13,15-20 Unfortunately, the fast combination of photoinduced e−/h+ pairs stops the photocatalytic activity to a certain degree. Great efforts have been made towards integrating MoS2 with g-C3N4 to lower the possibility of combination, and as a result, enhance the photocatalytic properties of these materials. MoS2/C3N4 heterostructures have been synthesized by Li et al.21 through a facile ultrasonic chemical approach, and the promising heterostructure of MoS2/C3N4 with MoS2 (0.05 wt%) exhibited rate constants as 0.301 min−1, 3.6-fold higher than that of C3N4. Moreover, heterostructures of MoS2/g-C3N4 were further surveyed by Bian et al.22 and other scholars.23-25 Although the combined mechanism of photocatalysis for MoS2/g-C3N4 outperformed the simple individual mechanisms of MoS2 and g-C3N4, it is hard to cycle and classify multicomponent compound from solution. As a result, a method for the effective cycling of coupled MoS2/g-C3N4 photocatalysts should be developed. Herein, a magnetically retrievable MoS2/g-C3N4 photocatalyst coupled with Fe3O4@SiO2 microspheres was synthesized by an efficient and adoptable method. The hierarchical composites of Fe3O4@SiO2@MoS2/g-C3N4 displayed excellent recoverability as well as photocatalytic activity. To our knowledge, to-date, no one has achieved magnetically recoverable MoS2/g-C3N4 composites. Therefore, we were not able to predict from prior reports that the compounds containing magnetic Fe3O4@SiO2 cores and MoS2/g-C3N4 could offer good recoverability and photoactivity as multifunctional photocatalysts. A reliable and novel approach is used to synthesize MoS2/g-C3N4 heterostructures in present study. In addition, the degradation of organic

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pollutants and the production of H2 from water were studied using these materials in order to assess the visible-light-induced photocatalytic activity of different samples.

MATERIALS AND METHODS Synthesis of magnetic Fe3O4@SiO2@MoS2/g-C3N4 heterostructures

Materials: All of the crude materials were obtained from the Sino pharm Chemical Reagent Co. Ltd. The magnetic cores of Fe3O4@SiO2 (FS) were made and enhanced by 3-amino-propyltriethoxysilane (APTES) according to previously procedure.26 In a follow-up step, the magnetic core was dispatched into HNO3 solution for fast-protonation. A stream of positive charges were generated on the surface area.27 Synthesis of magnetic Fe3O4@SiO2@MoS2/g-C3N4 (FSMG) heterostructures: 0.10 g of positively charged Fe3O4@SiO2 cores were aged for 60 mins, and then Na2MoO4·2H2O (3 mM, 100 mL) was added. MoO42− can be absorbed through electroplating on the surface of Fe3O4@SiO2 cores. The precipitate was classified as magnetic, cleaned with deionized water and thus dispatched into 65 mL of aqueous solution with 5 mL of HNO3 (6 M). The solution was then put into a mixed solution containing 30 mL of Na2MoO4·2H2O (3 mM), 0.092 g of g-C3N4, 0.5 g of sodium silicate (Na2SiO3·9H2O) and 30 mL of 9 mM of CH3CSNH2 (TAA). After heating the suspensions at 220 °C for 24 hours, the mixture was sealed in an autoclave using a Teflon-lined process. A black precipitate was received and initially cleaned with NaOH solution (1 M) several times to remove the surplus silicic acid; it was then successively washed with absolute deionized water and ethanol water and dried under vacuum at 60 °C for six hours. Then, the magnetic Fe3O4@SiO2@MoS2/g-C3N4 (FSMG) compounds were obtained. The Fe3O4@SiO2@MoS2 (FSM) hierarchical microspheres for a contrasting study were synthesized using an identical method without the addition of g-C3N4. In addition, the ratio of MoS2/g-C3N4 was regulated according to the photocatalytic performance, as shown in Figure S1. The optimal ratio of MoS2/g-C3N4 is 2.0 at.%.

Characterization

X-ray diffraction (XRD) patterns were gathered by an X-ray diffractometer (D8, Bruker AXS, Germany). The surface area morphologies were observed using transmission electron microscopy (TEM) and high-resolution TEM (JEOL JEM-2010). The multiple-hole structures and the specific surface area (SBET) were researched on a nitrogen absorbing appliance (Gold APP Corp. V-Sorb

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2800P). The X-ray photoelectron spectroscopy measurements were executed by an energy analyzer by Omicron (XPS, Perkin Elmer PHI-5400). A standardized configuration with a 3-electrode mechanism was used, and the surface photocurrent (SPC), as well as the electrochemical impedance spectra, were obtained at room temperature with an electrochemical analyzer (CHI-660C, CH Instruments Co.). The samples were applied as the functional electrode, one platinum wire was used as the counter electrode and a saturated calomel electrode was used as the reference electrode.9,12,26 The photoluminescent spectra were measured on a fluorescence spectrophotometer with a 425 nm excitation wavelength (Hitachi F-4600). An organic carbon analyzer with a variable TOC cube was used to determine the total organic carbon (TOC) in the reaction solution (Elementar Analysensysteme GmbH, Germany). The magnetic conductivity was studied using a vibration sample magnetometer on a physical property testing mechanism (PPMS-9, Quantum Design) with an applied field from −20,000 to 20,000 Oe.

Photocatalysis Study

Assessment of RhB photocatalytic degradation: The photocatalytic degradation was conducted in a homemade reaction vessel with a solution of 100 mL rhodamine B (RhB, 20 ppm). 100 mg of given catalyst in a 350 W Xe-lamp equipped with a cut filter (λ > 420 nm). To ensure adsorption equilibrium, the suspension was blended for one hour in darkness. Then, a 1.6 mL solution was taken and centrifuged for 10 mins with the lamps turned on. After it was diluted 4 times, a Shimadzu UV-2550 spectrophotometer was used to study the clear solution, and the transformation of the absorption band at 554 nm was recorded. After all the photocatalytic testing was completed, the catalysts were taken out with a magnet and then cleaned with deionized water and ethanol for many times. After this, the catalysts were dispersed into the solution of RhB (20 ppm, 100 mL) again and a new round of the photocatalytic cycle started. Testing the activated radical species: During the course of the photoreaction, the main activated species, including hydroxyl radicals (•OH), superoxide radicals (•O2−), and holes (h+) were surveyed by adding tert-butyl alcohol (TBA), 1,4-benzoquinone (BQ), and disodium ethylenediamine tetra-acetate (EDTA). PL with coumarin as a molecular probe was used to test the •OH content. Evaluation of hydrogen production: H2 production tests were conducted at (6 °C) ambient temperature in a photocatalytic reactor from Labsolar-III (AG) linked to a closed gas-circulating mechanism. Then, FSMG (100 mg) was suspended in a triethanolamine (15 vol%, 100 mL). Hydrogen evolution reaction was detected by gas chromatography. Furthermore, the production

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tests for H2 were carried out for 4 hours each and repeated five times to check the stability of the samples.

RESULTS AND DISCUSSION ------------Figure 1-----------Presented in Figure 1A, the synthetic strategy for Fe3O4@SiO2@MoS2/g-C3N4 integrated microspheres was conducted using the following steps: Fe3O4 spheres (Figure 1B) compounded by solvothermal interactions are used as the internal cores for the deposition of a SiO2 shell using a sol-gel approach. Numerous −NH2 groups are revealed on the surface of the inorganic layer of SiO2 when APTES is modified. As shown in Figure 1C, a positively charged layer (-NH3+) forms on the surface after protonation process. While the spheres are in contact with a Na2MoO6 solution, a large amount of MoO62− ions are connected to Fe3O4@SiO2 microspheres via electrostatic adsorption. These preadsorbed MoO62− ions serve as active sites. In the next step of the hydrothermal process, sheets of MoS2 will be used as a growth point, and self-assembly is gradually achieved during the growth process to form a flower-like superstructure. Finally, these lamellar structures are staggered to form Fe3O4@SiO2@MoS2 flower-like microspheres (Figure 1D). During the hydrothermal preparation process of MoS2, g-C3N4 sheets are embedded into nanosheets or deposited onto the surface during the process of the MoS2 self-assembly. It is worth noting that the diameter of the Fe3O4@SiO2@MoS2/g-C3N4 microspheres (~0.426 µm) is smaller than that of Fe3O4@SiO2@MoS2 (~0.481 µm) in Figure S2, which may be due to the presence of g-C3N4, which occupies some of the active sites and inhibits crystallization of the MoS2 sheet structure.

------------Figure 2-----------Figure 2 demonstrates the XRD patterns of the samples at various phases of the synthesis. Sharp and strong diffraction peaks corresponding to Fe3O4 appear at 2θ = 30.4°, 35.7°, 43.4°, 53.8°, 57.4° and 63.0, and are consistent with the structure of Fe3O4 cubic spinels (JCPDS No. 75-1609).28 When a seam of SiO2 covers its surface, Fe3O4@SiO2, the XRD diffraction peaks do not change, suggesting that the SiO2 generated on the surface is amorphous. MoS2 were determined to be orthorhombic (JCPDS Card No. 37-1492).23,25 The diffraction peaks observed for the FSM compound correspond to orthorhombic MoS2 as well as the Fe3O4 cores. Furthermore, peaks at 27.3° and 13.1°corresponding to purified g-C3N4 can be indexed to the (100) and (002)

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reflections, separately.19,21,22 The hierarchical composite of FSMG also shows the same diffraction peaks, but the peaks are broader than those measured for FSM, showing the generation of the heterostructure of MoS2/g-C3N4.20,21 Furthermore, the peak corresponding to the (002) plane of g-C3N4 is also visible.

------------Figure 3-----------Figure 3 shows the microstructures and morphologies of FSMG, FSM, FS and Fe3O4, as indicated in Figure 3A. The pristine products of Fe3O4 show a spherical morphology with a diameter of ca. 400~500 nm. After coating the microspheres with a layer of SiO2 (Figure 3B), there are no large changes in the Fe3O4@SiO2 morphology; however, an apparent core-shell architecture could be identified. The blackened semicircle the supporting core of Fe3O4@SiO2. The Fe3O4@SiO2@MoS2 composites have an average diameter of ~1.3 µm and a flower-shaped microsphere structure (Figures 3D and 3C). Many squared-shaped nanosheets of MoS2 are connected to the same area and are distributed on the microsphere surface area. In Figure 3E, a peculiar morphology of Fe3O4@SiO2@MoS2/g-C3N4 is shown. The g-C3N4 is dispersed on the surface of MoS2 located at the outer shell. It has been described in Figure 3F as the lattice plane of part of the FSMG composite materials. The internal planar distances corresponding to the (002) plane of g-C3N4 and (113) plane of MoS2 are ~ 0.323 nm and ~ 6.155 nm, respectively.23 The pore distribution curves and adsorption/desorption isotherms of nitrogen for the hierarchical FSMG microspheres of are shown in Figure S3. The hierarchical microspheres of Fe3O4@SiO2@MoS2/g-C3N4 showed Type IV isotherms with H3 hysteresis loop, suggesting the appearance of slit-shaped mesopores.29 The FSMG composites have smaller mesopore and average pore sizes of ~21.67 nm. Pore volume and specific surface area are 19.62×10−2 cm3/g and 46.71 m2/g respectively, larger than FSM (29.35 m2/g and 7.34×10−2 cm3/g). It can be deduced that more mesopores were introduced by modifying g-C3N4 into FSMG.

------------Figure 4-----------To progressively study the elemental contents, interaction and valence states of the g-C3N4 and MoS2, high-resolution X-ray photoelectron spectroscopy (XPS) tests were carried out. Due to instrumental limits on the magnetic condition, the MoS2/g-C3N4 samples were synthesized using the same method but without the Fe3O4@SiO2 framework. The interaction between MoS2 and g-C3N4 was analyzed and discussed using the XPS results. Figure S4 shows the survey spectra of MoS2/g-C3N4, which consists of C 1s, Mo 3d, N 1s and S 2p regions. As shown in Figure 4A, C 1s

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spectra consist of two peaks, located at 284.3 eV and 287.8 eV. The 287.8 eV peak is deemed to the sp2-hybridized carbons (i.e., C−N−C) in g-C3N4 skeleton, while 284.3 eV peak can be identified as C−C sites.30-35 From the N 1s spectra, which was shown in Figure 4B, four peaks can be seen at 401.1, 398.9, 398.1, and 395.2 eV. The peak at 401.1 eV can be attributed to ridging nitrogen atoms C−N−H, and the component centered at 398.9 and 398.1 eV is assigned to sp2 N nitrogen atoms in the N−(C)3 bond in melem and ridging nitrogen atoms C=N−C, respectively.32-36 The peak at 395.2 eV corresponds to the C−N−Mo bond.34-36 The Mo 3d spectra is shown in Figure 4C. Two peaks are founded at 228.9 and 232.1 eV, assigning to the Mo 3d5/2 and Mo 3d3/2 components of MoS2, respectively. The peak at 226 eV with low intensity can be rooted in S 2s. As for S 2p spectra (Figure 4D), it can be founded that two peaks are located at 161.8 and 163.1 eV, corresponding to S2− 2p3/2 and S2− 2p1/2.36-38 The above-discussed XPS results suggest the synthesized products exist in the form of the MoS2/g-C3N4 heterojunction with the N−Mo bonds linking the two components.

------------Figure 5-----------The Nyquist impedance spectroscopy (EIS) and surface photocurrent curves (SPC) were measured to explore the relative effectiveness of the photoinduced charge carriers. SPC was taken on the as-synthesized sample in Figure 5. The visible-light-induced photocurrents generated by the Fe3O4@SiO2@MoS2/g-C3N4, Fe3O4@SiO2@MoS2, Fe3O4@SiO2, and Fe3O4 electrodes are 0.383, 0.096, 0.048, and 0.019 µA, respectively. The greatest photocurrent was achieved for FSMG composites, which showed an effective classification and lower combination effectiveness for the photoinduced e−/h+ pairs because of the heterojunction interfaces. Furthermore, changes in the EIS of the current sample are indicated in Figure 6. The diameter of the semicircular of the Nyquist plots symbolizes the surface charge transfer resistance (Rct) of the sample. Lower resistance and smaller radius are preferred for charge transfer.39,40 The relative radii of the semicircles are in the sequence Fe3O4 > FS > MoS2 > g-C3N4 > FSM > FSMG. As a result, the FSMG composites show optimized effectiveness for the transfer and separation of photodriven e−/h+ pairs. The results of this study are in accordance with the SPC testing.

------------Figure 6-----------------------Figure 7-----------The photocatalytic reactions of various samples are tested using the RhB degradation shown in Figure 7. Without catalysts, photolysis is nearly negligible. It can be seen that FSMG maximized

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the photocatalytic reaction. The measured rate constant of FSMG is 0.0477 min−1, 1.99-fold and 3.38-fold higher than FSM (0.0239 min−1) and g-C3N4 (0.0141 min−1), respectively. This shows that the photoactivity is noticeably improved by the heterostructure of MoS2/g-C3N4 as hierarchical microspheres of FSMG. The concentration of the dye solution greatly decreases compared to the solution of pristine RhB, as shown in Figures 7A and 7B. The high-level degradation can be visualized in the photograph in Figure 7B, as the solution becomes transparent. The FSMG composite is recycled using the outer magnetic area and shows good separation performance using the magnet. The enhanced photocatalytic performance is dependent on the generation of the MoS2/g-C3N4 heterostructure. Electron/hole pairs have been stimulated and generated with MoS2 after irradiation with visible light. The FSMG photocatalytic reaction is largely enhanced as the photodriven e−/h+ pairs are degraded by the fast interfacial charge transfer that occurs all over the MoS2/g-C3N4 heterostructure.21,23,25 In addition, the mineralization of the g-C3N4/Ag/MoS2 composite is shown in Figure 7B for RhB, with respect to the removal of TOC. The removal of TOC was slightly slower than the RhB photodegradation, but a removal of ~96.73% TOC was achieved by 1.5 h of irradiation time. This suggests that in the process of degradation, no byproducts are produced. In addition, the simple separation and recovery of the photocatalyst are the most essential advantages. The magnetized saturation value of Fe3O4@SiO2@MoS2/g-C3N4 is tested to be ca. 5.31 emu.g−1 in Figure S5. When the reaction is performed, all the microspheres come towards the magnet in 3 min and attach tightly to the side of the bottle. In sharp contrast to the pristine RhB solution, a transparent and clear liquid phase residue illustrates significant decomposition (Figure 7B). In addition, the recycling experiment was conducted 6 times and showed no evident decrease in the performance (Figure 7C), indicating that the flower-like Fe3O4@SiO2@MoS2/g-C3N4 integrated microspheres are highly stable and recyclable. To interpret the photodegradation mechanism, trapping experiments were carried out, and the main activated species was assessed. Sacrificial agents were adopted during the process of photodegradation, as shown in Figure 7D. Adding BQ (1 mM) efficiently restrained the effectiveness of the FSMG compounds. This suggests that superoxide radicals are the main activating species during the photodegradation. Furthermore, the addition of t-BuOH (1 mM) suppressed the effectiveness of the RhB degradation to 0.0292 min−1 from 0.0477 min−1, which suggests that hydroxyl radicals are playing a fatal role in the process of photocatalytic oxidation. Comparatively, EDTA (1 mM) addition militate the photocatalytic reaction of the FSMG compounds on the minute-scale, showing the passive function of the holes, and as a result, the holes are not the main species. The generation rate of hydroxyl radicals (•OH) can be observing by detecting

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7-hydroxycoumarin. •OH can be generated from H2O molecules or OH− ions by electron-induced, photobased, and multistep degradation of O2− radicals.41,42 Figure S6 shows that the fluorescence intensity is proportional to the amount of generated photoinduced •OH radicals. FSMG show the highest PL intensity, ensuring the presence of many •OH radicals on the surface area in a given period of time (60 min). The EIS and SPC expose a rich number of holes and electrons on the surface area of the FSMG microspheres, and the surface absorption of H2O and O2 could simply lead to the generation of many •OH radicals, further improving the photoactivity. This result also exhibits that radicals of •OH have a principal role in the photocatalytic mechanism, which is consistent with the results of the tapping tests. In addition, the Fe3O4@SiO2@MoS2/g-C3N4 samples have been examined by the XRD, TEM and EDS spectra before and after photocatalytic reaction (after the 6th recycle) to confirm its stability. As shown in Figure S7, the XRD patterns of the Fe3O4@SiO2@MoS2/g-C3N4 before reaction and after reaction were almost same. Comparing the TEM images of the Fe3O4@SiO2@MoS2/g-C3N4 before reaction and after reaction in Figure S8, it can be found that after long-time experiments, the topography of the sample is not destroyed. Furthermore, the EDS spectra of the Fe3O4@SiO2@MoS2/g-C3N4 before reaction and after reaction were measured in Figure S9. The EDS result indicates the Fe3O4@SiO2@MoS2/g-C3N4 composed of Mo, S, Fe, C, N and O elements, and the atom ratio of N:M:S is approximately 4:1:2. After the 6th recycle, almost no change in proportion was found. The results show that the Fe3O4@SiO2@MoS2/g-C3N4 samples process better stability.

------------Figure 8-----------Figure 8 shows the result of hydrogen evolution reaction. In the reaction, triethanolamine (TEOA) was employed as an electron donor. The FSMG and FSM catalysts show much higher photoactivity in contrast with purified MoS2 and g-C3N4. Specifically, an H2 production activity of 110.72 µmol·h−1·g−1 was measured with FSMG under visible light, which is approximately 11.09-fold and 4.13-fold higher than pure g-C3N4 (9.98 µmol·h−1·g−1) and FSM (26.81 µmol·h−1·g−1), respectively. The stability of the hierarchical microspheres of FSMG for H2 production was tested over 5 cycles. After the fifth cycle, there were no obvious declines in evolution rates, suggesting excellent recycling stability of the hierarchical microsphere FSMG for hydrogen evolution from water.

------------Figure 9------------

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The proposed mechanism based on the transformation and separation of the photoformed charges in the compounds containing FSMG hierarchical microspheres is shown in Figure 9. Referencing former reports,21,23,25 the hidden positions of the valence band (VB), as well as the conduction band, are 1.38 eV and −1.33 eV for g-C3N4, and −0.05 eV and 1.70 eV for MoS2. When the visible light has been harvested by g-C3N4, the holes transfer from MoS2 to the VB of g-C3N4 and the electrons from the CB of g-C3N4 enter the CB of MoS2 as the earlier band is higher energy than that of the later band. Therefore, photoinduced e−/h+ pairs are spread across the various semiconductors, showing great improvement in the transfer and separation of the charge carriers. The higher photocatalytic properties might be dependent on the hindrance of the charge carrier combination paths. This transfer system also efficiently describes the RhB photodegradation results. Nevertheless, this system could not explain the results from the FSMG composites in the photocatalytic H2 generation process. It has shown a poor ability to convert H+ into H2, which might be attributed to the function of the band edge of CB (−0.05 eV). In other words, the improved photoactivity for H2 formation could be interpreted based on the transfer system of the Z-scheme where MoS2/g-C3N4 compounds with photogenerated electrons were transferred into the VB of g-C3N4 again from the CB of MoS2. This path could effectively promote the photoinduced e−/h+ pairs classification, and the electrons could still be retained in the g-C3N4 CB, however, the holes remain in the VB of MoS2. Due to the passivize ECB of g-C3N4, the electrons are highly reducible therefore could simply convert H+ to H2, which is in agreement with H2 evolution tests.

CONCLUSION A new hierarchical microsphere structure of Fe3O4@SiO2@MoS2/g-C3N4 exhibited good photocatalytic activity as well as magnetic recoverability. The materials are easily synthesized through an innovative and dependable path. The surface shell of the MoS2/g-C3N4 compound and an important Fe3O4@SiO2 core were used to construct the photocatalysts. The grape-shaped heterostructures of MoS2/g-C3N4 were simply created by functionalizing the Fe3O4@SiO2 support. The Fe3O4@SiO2@MoS2/g-C3N4 photocatalyst shows improved photocatalytic activity via degrading RhB compared to both hierarchical microspheres of Fe3O4@SiO2@MoS2 and g-C3N4. The generation of MoS2/g-C3N4 heterostructure results in the improved photocatalytic performance. The new synthetic method applied to the preparation of multipurpose compounds shows that magnetic photocatalytic activity and recoverability has been enhanced by taking advantage of the magnetic compounds. An alternative plan for enhanced visible-light photocatalytic activity is also proposed. The photocatalyst could potentially be used for the

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removal of pollutants and for splitting water applications.

Associated Content Supporting Information The photocatalytic degradation of RhB by the MoS2/g-C3N4 and Fe3O4@SiO2@MoS2/g-C3N4 with different mass ratios of MoS2 to g-C3N4; The corresponding size distribution of the Fe3O4@SiO2@MoS2 and Fe3O4@SiO2@MoS2/g-C3N4; The pore size distribution curve and the adsorption/desorption isotherms for the samples; The XPS survey spectra of the MoS2/g-C3N4 sample; The hysteresis loop of vavious samples; Formation of 7-hydroxycoumarin (monitored by fluorescence emission) in the suspensions and the formation rate constants of 7-hydroxycoumarin over

various

samples;

XRD

patterns,

TEM

images,

and

EDS

spectra

of

the

Fe3O4@SiO2@MoS2/g-C3N4 before reaction and after reaction. Author Information Corresponding Author Dingze Lu, E-mail: [email protected] Huiqing Fan, E-mail: [email protected] Notes The authors declare no competing financial interest.

ACKNOWLEDGEMENTS This work was supported by the Shaanxi Provincial Association of Science and Technology Youth Talents Lifting Plan (20180418), the China Postdoctoral Science Foundation Funding (2018M631188), the National Natural Science Foundation (51672220), the Natural Science Basic Research Plan in the Shaanxi Province of China (2016JM1018), the Xi’an Science and Technology Foundation (CXY1706-5, 2017086CGRC049-XBGY005), and the Scientific Research Program Funded by the Shaanxi Provincial Education Department (16JK1327).

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Figure 1. (A) Schematic illustration of Fe3O4@SiO2@MoS2/g-C3N4 microspheres synthesized by a novel process; SEM images of Fe3O4 (B), Fe3O4@SiO2 (C), Fe3O4@SiO2@MoS2 (D), and Fe3O4@SiO2@MoS2/gC3N4 (E). 130x169mm (300 x 300 DPI)



Figure 2. XRD patterns of the g-C3N4, MoS2, Fe3O4, Fe3O4@SiO2, Fe3O4@SiO2@MoS2, and Fe3O4@SiO2@MoS2/g-C3N4. 84x66mm (300 x 300 DPI)


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Figure 3. TEM images of Fe3O4 (A), Fe3O4@SiO2 (B), Fe3O4@SiO2@MoS2 (C-D), and Fe3O4@SiO2@MoS2/g-C3N4 (E); High resolution TEM of Fe3O4@SiO2@MoS2/g-C3N4 (F). 182x93mm (300 x 300 DPI)



Figure 4. High-resolution X-ray photoelectron spectroscopy of the MoS2/g-C3N4 composites: (A) C 1s, (B) N 1s, (C) Mo 3d, and (D) S 2p. 171x139mm (300 x 300 DPI)


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Figure 5. Surface photocurrent curves of curves a, b, c, and d: Fe3O4, Fe3O4@SiO2, Fe3O4@SiO2@MoS2, and Fe3O4@SiO2@MoS2/g-C3N4, respectively. 84x61mm (300 x 300 DPI)



Figure 6. Nyquist impedance spectroscopy of curve a-f: Fe3O4, Fe3O4@SiO2, MoS2, g-C3N4, Fe3O4@SiO2@MoS2, and Fe3O4@SiO2@MoS2/g-C3N4, respectively. 84x65mm (300 x 300 DPI)


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Figure 7. (A-B) Visible-light-induced RhB photocatalytic degradation of various samples; (C) Fe3O4@SiO2@MoS2/g-C3N4 cyclic degradation curves; (D) Comparison of photoactivity of RhB for Fe3O4@SiO2@MoS2/g-C3N4 with the addition of EDTA, t-BuOH, BQ and without scavengers. 129x99mm (300 x 300 DPI)



Figure 8. Stable hydrogen evolution from water under visible-light irradiation using (a) MoS2, (b) g-C3N4, (c) Fe3O4@SiO2@MoS2, (d) MoS2/g-C3N4, and (e) Fe3O4@SiO2@MoS2/g-C3N4. 84x65mm (300 x 300 DPI)


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Figure 9. Proposed schematic diagram for the separation and transfer of photogenerated charges in the Fe3O4@SiO2@MoS2/g-C3N4 nanocomposites. 58x38mm (300 x 300 DPI)



Synopsis: A novel synthetic route for magnetically retrievable Fe3O4@SiO2@MoS2/g-C3N4 hierarchical microspheres with highly efficient visible-light-induced photocatalytic degradation of organic pollutant and production of hydrogen is reported. 39x23mm (300 x 300 DPI)


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Highly efficient visible-light-induced photocatalytic production of

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