Hybrid Composite as Visible Light Responsive Photocatalyst with Enhanced Photocatalytic Performance

Magnetically Recyclable MoS

₂

/Fe

₃

O

₄

Hybrid Composite as Visible Light Responsive Photocatalyst with Enhanced Photocatalytic Performance

Xiaozi Lin,

^†,§

Xi Wang,

^§

Qingwei Zhou,

^∥

Chengyan Wen,

^⊥

Shaoqiang Su,

^†

Jie Xiang,

^†

Pengfei Cheng,

^†

Xianbiao Hu,

^†

Ye Li,

^†

Xin Wang,

^‡

Xingsen Gao,

^†

Richard Nözel,

^‡

Guofu Zhou,

^‡

Zhang Zhang,*

^,†,‡

and Junming Liu

^†,∥

†Institute for Advanced Materials, South China Academy of Advanced Optoelectronics, South China Normal University, Guangzhou 510006, China

‡National Center for International Research on Green Optoelectronics, South China Academy of Advanced Optoelectronics, South China Normal University, Guangzhou 510006, China

§Guangdong Provincial Key Lab of Functional Materials for Environmental Protection, Guangdong Provincial Engineering Research Center for Drinking Water Safety, School of Chemistry & Environment, South China Normal University, Guangzhou 510006, China

∥Laboratory of Solid State Microstructures and Innovation Center of Advanced Microstructures, Nanjing University, Nanjing 210093, China

⊥CAS Key Laboratory of Renewable Energy, Guangzhou Institute of Energy Conversion, Chinese Academy of Sciences, Guangzhou 510640, China

*^S Supporting Information

ABSTRACT: Photocatalysis is one of the most promising technologies in wastewater treatment. However, the inactivity to visible light and the inconvenience to recycle severely limit its practical application. In this work, via a facile hydrothermal method, Fe₃O₄NPs were integrated onto the surfaces of 3D ball-flower-like MoS₂microspheres as efficiently visible light responsive and magnetically recyclable photocatalysts. Experimental results indicate that, an optimal loading amount (20 wt %) of Fe₃O₄NPs can not only effectively enhance the photocatalytic ability of the MoS₂/Fe₃O₄ (MF) hybrid composite with approximately 2 times better than pure MoS₂, but also make it conveniently recycle from water by an external magnetic field. The photoelectrochemical studies also reveal that the incorporation of Fe₃O₄NPs can effectively enhance the charge transfer rate and accelerate separation of photoinduced charge carriers.

The surface catalytic mechanism of MF hybrid composite was also explored through XPS spectra. With both the excellent photocatalytic performance and

magnetical recyclability, the 20 wt %-MF hybrid composite is considered to be a promising and competitive photocatalyst for wastewater treatment utilizing solar energy.

KEYWORDS: Fe₃O₄NPs, MoS₂, photocatalyst, magnetical recyclability, visible light responsive

■

INTRODUCTION

A rapid economic development, however, has caused severe environmental pollutions, restricting the sustainable develop- ment and the health of human.¹The photocatalysis based on semiconductor materials, regarded as one of the most promising technologies to degrade the extremely toxic, carcinogenic and persistent organic molecules, has been paid a lot of attentions.²⁻⁴Hence, exploiting the cost-effective and environment-friendly photocatalysts is quite necessary for environmental remediation. A variety of semiconductors that can degrade environmental pollutants have been developed, such as TiO₂, g-C₃N₄, BiOCl, CdS, etc.⁵⁻⁸ Among these photocatalysts, molybdenum disulfide (MoS₂), a transition

metal chalcogenide with two-dimensional (2D) layered nanostructures, was found to possess excellent characteristics, including large specific surface areas, relatively high carrier mobility, good conductivity, high chemical reactivity, and adjustable bandgap energy (1.2−1.9 eV).⁹ As a result, MoS₂ becomes a promising photocatalytic material in hydrogen evolution reaction (HER), energy conversion/storage, super- capacitor, and pollutant degradation.^1,10−15However, there are still some limitations of MoS₂ that restricted its practical

Received: October 21, 2018 Revised: November 27, 2018 Published: December 12, 2018

pubs.acs.org/journal/ascecg Cite This:ACS Sustainable Chem. Eng.2019, 7, 1673−1682

Downloaded via NANJING UNIV on February 23, 2019 at 11:24:28 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

application in wastewater treatment using solar energy, such as the low-density of active site and the high recombination rate of photoinduced charge carriers.¹⁶Therefore, modified design of MoS₂is required to meet the needs in practical application of environmental protection. For example, the incorporation of cocatalysts onto the surface of MoS₂can evidently facilitate the efficient charge separation process.¹⁷Another feasible strategy is to combine MoS₂ with transition metals to improve its conductivity and catalytic performance.¹⁸However, in practical wastewater purification, most of these nanoscaled photo- catalysts are inconvenient to recycle, which may lead to loss of catalysts and even create new wastes.

As a traditional transition metal oxide, iron ferrite (Fe₃O₄) nanoparticles (NPs) have attracted much interests for their unique properties.¹⁹⁻²³ The introduction of Fe₃O₄NPs into photocatalysts can effectively remove or recycle nano- and microparticles of photocatalysts and prevent further water pollution by semiconductor materials.²⁴⁻²⁶ Meanwhile, both the high conductivity of Fe₃O₄ (1.9 × 10⁶ Sm⁻¹) and the matching energy band structure make it possible for enhancing the photoinduced charge separation and transport rates, and thus effectively improving the photocatalytic perform- ance.^25,26,27 Besides, Fe₃O₄ NPs act as a photoinduced electron-trapping site and can produce the active species after a series of reactions.²⁹ Hybrid catalysts with the introduction of Fe₃O₄ including TiO₂/Fe₃O₄, Bi₂O₃/Fe₃O₄, g-C₃N₄/Fe₃O₄, all have shown improved performance that single-component catalysts cannot achieve.^19,24,28

Although MoS₂@Fe₃O₄ nanocomposite could work as a remarkable photocatalyst, its surface catalytic mechanism and photoelectrochemical properties are still quite limited.³⁰In this work, we integrate superparamagnetic Fe₃O₄ NPs and 3D MoS₂microspheres into a single hybrid composite as a visible light responsive high-performance photocatalyst. The mor- phologies, structures and properties of as-prepared MoS₂/ Fe₃O₄ (MF) composites were systematically characterized.

And the results demonstrate a successful integration of Fe₃O₄ NPs onto the surfaces of 3D MoS₂ microspheres, with significantly improved visible-light responsive photocatalytic performance in the optimized Fe₃O₄loading amount. Besides, the integration of Fe₃O₄NPs onto MoS₂microspheres was an effective method to recover most catalysts from aqueous solutions with an external magneticfield. The comprehensive electrochemical studies reveal the efficient separation of photoinduced charge carriers in heterostructured catalysts and its potentials for supercapacitor applications. To the best of our knowledge, it is thefirst time to confirm the alternation between the H₂O molecules, Fe³⁺ions and Fe²⁺ ions and the formation of FeOOH to produce the active species during the photocatalytic degradation. Our work may provide a new insight into the surface catalytic mechanism concerning Fe₃O₄ coated photocatalytic materials.

■

MATERIAL AND METHODS

Materials. The following chemical reagents were used in this study: ferric chloride hexahydrate (FeCl₃·6H2O, AR, Shanghai Aladdin Bio-Chem Technology Co., LTD), polyvinylpyrrolidone (PVP-K30, AR, Shanghai Macklin Biochemical Co., Ltd.), sodium acetate anhydrous (NaAc, AR, Shanghai Aladdin Bio-Chem Technology Co., LTD), ethylene glycol (C₂H₅O₂, AR, Shanghai Macklin Biochemical Co., Ltd.), ethylenediamine (C₂H₈N₂, AR, Shanghai Macklin Biochemical Co., Ltd.), thiourea (CH₄N₂S, AR, Chengdu Kelong Chemical Co., Ltd.), sodiummolybdate dehydrate (Na₂MoO₄·2H2O, AR, Shanghai Macklin Biochemical Co., Ltd.),

methyl orange (C₁₄H₁₄N₃SO₃Na, Ind, Sinopharm Chemical Reagent Co., Ltd.), potassium hydroxide (KOH, AR, Chengdu Kelong Chemical Co., Ltd.), sodium sulfate (Na2SO4, AR, Tianjin Zhiyuan Reagent Co., Ltd._.), ethyl alcohol absolute (C₂H₅OH, AR, Chengdu Kelong Chemical Co., Ltd.).

Synthesis of MoS₂/Fe₃O₄ Photocatalyst and Character- izations.First, Fe3O4NPs were synthesized by the method of Tu et al.³¹Next, Na₂MoO₄·2H₂O (2 mmol) and thiourea (4.5 mmol) were added in 30 mL deionized water and stirred for 10 min to be dissolved completely. Then, a certain loading amount of Fe₃O₄NPs was added and the mixture after 15 min sonication was brought into a stainless-steel autoclave and maintained for a whole day at 220°C.

Finally, the as-prepared product was separated and washed with deionized water for several times and dried in vacuum at 60°C for 8 h. Samples synthesized with different loading amounts of 5, 10, 15, 20, and 25 wt % Fe₃O₄in the composites were labeled as 5, 10, 15, 20, and 25 wt %-MF, respectively.

The morphology, structure, composition, surface area, magnetic and optical properties of as-prepare samples were characterized.

Details for characterizations are inSupporting Information (SI) Text S1.

Photocatalytic Activity. Typically, 50 mg as-prepared powder catalysts were suspended in 50 mL 10 mg/L methyl orange (MO) aqueous solution in a 100 mL water-cooled reactor with reflux water, being stirred for 30 min in the dark. Next, the solution was exposed under the xenon lamp (300 W) with a 400 nm UV cutoff filter. At each selected time interval, a 3∼5 mL solution was collected and filtered through a 0.22μm menbrance. The concentration of methyl orange solution was measured by a spectrophotometer (JINGHUA UV-1800) employing 464 nm.

In the recyclability study, four consecutive cycles of the MO photodegradation were tested. After each cycle, the catalysts were separated from the MO solution and washed thoroughly to remove residual pollutants and dried in a vacuum oven at 60°C, and were used in the succeeding cycle.

Electrode Preparation and Electrochemical Measurement.

The Linear Sweep Voltammetry (LSV), surface photocurrent (SPC) response, Cyclic Voltammograms (CV), EIS curves, and Galvano- static CD were conducted to investigate the electrochemical and photoelectrochemical performances. Details for electrode preparation and electrochemical measurement are inSI Text S2.

■

RESULTS AND DISCUSSION

Morphological, Structural, and Compositional Stud- ies. The morphologies of MF hybrid composite at different synthesis steps are characterized by scanning electron microscopy (SEM).Figure 1a is SEM image of the prepared Fe₃O₄ NPs, which reveals that small Fe₃O₄ NPs tightly agglomerate into bigger clusters. Typically, the cluster being consisted of several NPs is less than 200 nm in diameter. A bundle of as-prepared MoS₂microspheres are shown inFigure 1b. Obviously, the 3D microspheres are consisted of randomly arranged 2D MoS₂ nanosheets, which are tightly aggregated like petals of ball-flowers. After integration of the two components, Figure 1c reveals that the hybrid composite consists of both the Fe₃O₄NPs clusters and the 3D ball-flower- like MoS₂microspheres. Obviously, a large amount of Fe₃O₄ NPs are integrated onto the 3D MoS₂. Figure 1d is a high- magnification SEM image of a single MF microsphere. It is confirmed that clusters of Fe₃O₄NPs are massively absorbed on the ball-flower-like MoS₂ microsphere. Specifically, the clusters can locate on the edges even into the interspaces of 2D MoS₂ nanosheets. Basically, such a complex hybrid structure should lead to the increased specific surface area.

The X-ray diffraction (XRD) patterns of Fe₃O₄NPs, MoS₂, and MF are plotted in Figure 2a. The diffraction peaks at scattering angles (2θ) of 30.2, 35.5, 43.2, 53.6, 57.2, and 62.7° ACS Sustainable Chemistry & Engineering

can be attributed to the face-centered cubic Fe₃O₄with (220), (311), (400), (422), (511), and (440) planes, respectively.³² The XRD peaks of MoS₂at 2θ of 14.0, 33.3, 39.6, 49.3, and 59.0°correspond well with the (002), (100), (103), (105), and (110) planes, respectively. It is noticeable that the MF composite exhibits a coexistence of both MoS₂ and Fe₃O₄, with peaks at 2θof 14.0, 30.1, 33.0, 35.4, 39.5, 43.1, 49.4, 53.6, 57.1, 58.5, and 62.6°. XRD patterns of MF composites with different Fe₃O₄loading amounts were compared (seeSI Figure S1), with strengthening Fe₃O₄ peaks due to the increase of Fe₃O₄ content. However, scattering angles of Fe₃O₄ component in MF composite were less than those for pure Fe₃O₄ NPs. The subtle deviations could relate to the interaction between Fe₃O₄and MoS₂, suggesting that a stable heterostructure of MoS₂/Fe₃O₄was formed.³³

Figure 2b is the corresponding Raman spectra obtained from Fe₃O₄NPs, MoS₂, and MF. The Raman peaks of Fe₃O₄NPs at 215, 279, 393, and 587 cm⁻¹ can be attributed to the A_1g vibration mode of Fe₃O₄ NPs.³² For the MoS₂, the Raman peaks at 377 and 400 cm⁻¹can be indexed to the E_12gand A_1g vibration modes of MoS₂, respectively.³⁴ In Raman spectros- copy of the MF hybrid composite, the peaks at 336, 776, 816, 920, 970 cm⁻¹are also observed, which might correspond to Mo−O₃, Mo−O₂, and Mo−O vibrations of MoO₃bonds.^35,36

Typically, the Raman peaks for Mo−O_xbonds also reveal the formation of heterostructure and strong coupling between MoS₂and Fe₃O₄.^36,37

X-ray photoelectron spectroscopy (XPS) was carried out to examine the surface composition and electronic interaction between MoS₂and Fe₃O₄in the hybrid composite.Figure 3a displays two characteristic XPS peaks locate at 228.66 and 231.82 eV, which were attributed to the Mo 3d_5/2 and Mo 3d_3/2.^35,38A weak peak corresponds to the Mo⁶⁺is obtained at 234.98 eV, which should be derived from MoO₃or MoO₄²⁻, suggesting the partial oxidation of sample in air.^35,39In the S 2p XPS spectrum (Figure 3b), the peaks at 161.48 and 162.67 eV can be related to the S 2p_3/2and S 2p_1/2, respectively.^32,35 Figure 3c displays the typical Fe 2p XPS spectrum. The peaks observed at 724.05 and 711.36 eV were attributed to Fe 2p_3/2 and Fe 2p_1/2,^40,41corresponding to the Fe²⁺(FeO) and Fe³⁺

(Fe₂O₃) in Fe₃O₄.³⁰As shown inFigure 3d, the peaks at 530.0 eV was the O 1s XPS spectrum, which is typically convoluted into three spectral bands at 529.81, 531.02, and 532.05 eV, corresponding to the lattice oxygen (O₂−), hydroxide (OH⁻) and physically adsorbed H₂O, respectively.⁴²Two main peaks in pure MoS₂at 232.40 and 229.24 eV correspond to Mo 3d_5/2 and Mo 3d_3/2, respectively, whereas peaks at 162.05 and 163.23 eV are indexed as S 2p_1/2and S 2p_3/2 (seeSI Figure S2). Thus, the XPS peaks of the hybrid composite shift toward the lower binding energy direction compared with pure MoS₂, which originates from the electronic interaction between MoS₂ and Fe₃O₄.⁴³

Brunauer−Emmet−Teller (BET) Surface Area and Magnetic Properties. Photocatalysts with higher surface areas and larger pore volumes are beneficial for enhancement of their photocatalytic activities, because of the increased active sites, the improved adsorption ability of pollutants and the fast transportation of reactant molecules and products.⁴⁴As plotted in Figure 4a, both MoS₂ and 20 wt %-MF have type IV isotherms and type H3 hysteresis loops, indicating mesoporous structures in the MF composites.⁴⁴ The decoration of Fe₃O₄ NPs on MoS₂ surface gives rise to an increase of the surface area (7.16 m²g⁻¹) in comparison with the pure MoS₂ (4.31 m²g⁻¹). Compared with the MoS₂, apparently, the pore sizes distribute in a broader range from 21 to 74 nm in 20 wt %-MF (see Figure 4a inset), which suggests a larger amount of mesopores existed in the 20 wt %-MF (SI Table S1).

Figure 4b is the magnetization versus magneticfield (M-H) curves of Fe₃O₄ NPs, 10 wt %- and 20 wt %- MF hybrid composites, in thefield range of−20 kOe < H < +20 kOe at room temperature (300 K). All the M-H curves exhibit an Figure 1. SEM images of (a) Fe₃O₄ NPs, (b) 3D ball-flower-like

MoS₂ microspheres, (c) low-, and (d) high-magnification of MF hybrid composite.

Figure 2.(a) X-ray diffraction patterns of Fe3O₄NPs, MoS₂, and MoS₂/Fe₃O₄hybrid composite, and (b) the corresponding Raman spectra.

ACS Sustainable Chemistry & Engineering

Figure 3.High-resolution XPS spectra: (a) Mo 3d, (b) S 2p, (c) Fe 2p, and (d) O 1s.

Figure 4.(a) N₂ adsorption−desorption isotherms of pure MoS2and 20 wt %-MF; the inset is the Barrett−Joyner−Halenda (BJH) pore size distribution plots of pure MoS₂and 20 wt %-MF. (b) M-H curves of MF hybrid composites with different Fe3O₄component ratios, and inset photo of the actual separation effect of 20 wt %-MF from solution by an external magnet.

Figure 5.(a) UV−vis diffusion reflectance spectra of the Fe₃O₄NPs, MoS₂, and hybrid composites of 5 wt %-MF, 10 wt %-MF, 15 wt %-MF, 20 wt

%-MF, 25 wt %-MF, (b) the magnified region of 550−700 nm.

ACS Sustainable Chemistry & Engineering

obvious symmetric hysteresis loop, being with magnetization saturation (Ms) values of 76.03, 8.91, 19.90 emug⁻¹ from Fe₃O₄, 10 wt %-MF, 20 wt %-MF, respectively. Normally, the Ms of the hybrid composites gradually increases with the increasing loading ratio of magnetic Fe₃O₄ NPs. Despite the big magnetism reduction compared to the pure Fe₃O₄NPs, as demonstrated in the inset photo, the 20 wt %-MF can be effectively separated from aqueous solution by an external magnet.

UV−Vis Diffuse−Reflectance Spectroscopy. Figure 5a illustrates the UV−Vis diffusion reflectance spectra of the Fe₃O₄ NPs, MoS₂, and MF hybrid composites with different loading amounts of Fe₃O₄. All the results demonstrate a strong absorption across the whole wavelength range. And the absorption edges of the MF hybrid composites demonstrate a red shift compared with that of MoS₂ (Figure 5b), while Fe₃O₄ NPs exhibits high absorbance from 400 to 700 nm without any obvious absorption peak. The absorption edge shift of the MF composites is mainly due to the interaction

between the MoS₂and the Fe₃O₄NPs with a narrow bandgap (0.1 eV) in the hybrid system. According to the Tauc/David− Mott model,⁴⁵ since the bulk MoS₂ is a indirect bandgap semiconductor,⁴⁶the bandgap of MoS₂and 20 wt %-MF was evaluated to be 2.12 and 1.80 eV, respectively (seeSI Figure S3).

Visible Light Responsive Photocatalysis to Dyes.As plotted inFigure 6a, after 100 min visible light irradiation, the degradation rate of MO with MoS₂, 5 wt %-MF, 10 wt %-MF, 15 wt %-MF, 20 wt %-MF, 25 wt %-MF, and 30 wt %-MF and Fe₃O₄ reached to 36.25%, 17.78%, 29.24%, 20.44%, 79.53%, 58.95%, 7.20%, and 3.11%, respectively. Obviously, photo- catalytic activities of the MF composites can be adjusted by the different loading amounts of Fe₃O₄. Especially, in 20 wt % Fe₃O₄, we have the best degradation rate of almost 80%, which is approximately 2 times better than that from pure MoS₂. Besides, the degradation kinetics of MO by Fe₃O₄NPs, MoS₂, and MF hybrid composites were investigated to describe the degradation of MO. The experimental data fit well with the Figure 6. (a) Degradation rates of MO with different loading amounts of Fe3O₄ under visible light irradiation (λ > 420 nm), and (b) the corresponding kinetic plots of In (C₀/C) versus irradiation time for the degradation of MO.

Figure 7.(a) SPC responses of the MoS2, 20 wt %-MF, and 25 wt %-MF under visible light irradiation. (b) LSV and (c) EIS curves of the MoS2

and 20 wt %-MF in dark and under visible light illumination, the inset is the magnified EIS curves of 20 wt %-MF. (d) CV curves of MoS2and 20 wt %-MF.

ACS Sustainable Chemistry & Engineering

pseudo first-order kinetics equation (Figure 5b), and the apparent rate constant of 20 wt %-MF (0.01401 min⁻¹) is 3 times higher than that of pure MoS₂ (0.00443 min⁻¹). The detailed information can be seen inSI Table S2. Generally, the introduction of Fe₃O₄ NPs could improve the catalytic performance, which was attributed to the formation of heterojunctions.^39,45However, with the weight ratio of Fe₃O₄ to MF below 20 wt % or above 25 wt %, the corresponding photocatalytic activities are even lower than pure MoS₂. Due to the lower conduction band and a higher valence band of Fe₃O₄, such a band structure can increase electron−hole recombination rates.⁴⁷ We assumed that the relative quantity of Fe₃O₄ to MoS₂ had vital effects on the photocatalytic performances, which overwhelmed the possible effects from Fe₃O₄ NPs.^30,48 Generally, the formation of MoS₂/Fe₃O₄ heterojunction with an optimal loading amount of Fe₃O₄can facilitate the separation of photogenerated charge carriers and thus improve the carrier transport. However, with an excessive amount of Fe₃O₄compound with MoS₂, the Fe₃O₄NPs might cover the reactive sites and introduce more recombination centers for photoinduced charge carriers, which would lead to the attenuation of photocatalytic activities.^28,30

Electrochemical Studies.Accordingly, the enhancement of photocatalytic performances may derive from the efficient separation of photogenerated electron−hole pairs. Therefore, the surface photocurrent (SPC) response and the photo- luminescence (PL) were conducted to further investigate the photoinduced charge separation performance. Figure 7a

illustrates that the SPC responses for each switch-on and -off operation of the MoS₂, 20 wt %-MF, and 25 wt %-MF electrodes are 0.001166, 0.01147, and 0.008513 mA, respectively. It is worthwhile to note that the photocurrent of the 20 wt %-MF and 25 wt %-MF electrodes is about 9.84- fold, and 7.32-fold that of MoS₂ electrode, respectively.

Furthermore, the PL spectra of the MoS₂, 5 wt %-MF, 10 wt

%-MF, 15 wt %-MF, 20 wt %-MF, and 25 wt %-MF were also investigated (seeSI Figure S4). There is a significant decrease of PL intensity in the 20 wt %-MF compared to that of MoS₂. The increase in photocurrent and decrease of PL intensity demonstrated that the recombination of photoinduced electrons and holes were suppressed by the synergistic effects of MoS₂and Fe₃O₄in the hybrid composites.⁴⁹

To further evaluate the visible-light responsive photo- catalytic performances, LSV was conducted in darkness and under visible light irradiation at a scan rate of 50 mV s⁻¹with the potential ranging from 0 to 0.9 V (vs Ag/AgCl). Obviously, the photocurrent of 20 wt %-MF is much higher than that of MoS₂ with applied potential larger than 0.3 V (Figure 7b).

Besides, the photocurrent difference between dark and light of MoS₂is weaker than the one of 20 wt %-MF with the larger applied potential. The 20 wt %-MF can be excited easier by visible light, and more photoinduced charge carriers were generated, and thus obtain a higher photocatalytic perform- ance.^50,51

Typically, to investigate the transport properties, EIS was tested in darkness and under visible light irradiation with the Figure 8.(a) Reusability of the 20 wt %-MF. (b) O 1s XPS envelop of the fresh and used 20 wt %-MF catalyst, (c) Fe 2p XPS envelop of the fresh and used 20 wt %-MF catalyst.

ACS Sustainable Chemistry & Engineering

frequency range of 1−10⁶ Hz.⁵² Figure 7c displays the EIS Nyquist plots of MoS₂ and 20 wt %-MF. In general, the Nyquist plots include semicircular arcs with the diameter along theZ_real’axis, and the diameter of semicircular arcs is related to charge transfer resistance. Compared with the MoS₂, 20 wt

%-MF electrode exhibited a semicircular arc with a much smaller radius, suggesting a higher transfer rate and a lower recombination rate of photogenerated charges at the interfaces.⁵² These measurements confirm that the incorpo- ration of Fe₃O₄ NPs and the interfacial interaction between Fe₃O₄ and MoS₂ can effectively reduce the electron/ion transport resistance and thus enhance the visible light induced charge transfer efficiency.

The CV curves were also conducted to evaluate the interactions between Fe₃O₄ and MoS₂ at the potential from 0 to 0.4 V (vs Ag/AgCl) with a scan rate of 5 mV/s.

Significantly, the CV curve of 20 wt %-MF electrode exhibits the larger integrated area and the higher current than those of the MoS₂electrode (Figure 7d). These results suggest that the 20 wt %-MF electrode has the higher electron transfer rate, which may derive from the coating of Fe₃O₄ NPs effectively preventing the agglomeration of MoS₂ microspheres and reducing the electron/ion transport resistance.^52,53In addition, the CV curves of 20 wt %-MF at different scan rates from 5 to 100 mV/s and Galvanostatic CD curves at current loads of 1− 10 Ag⁻¹were presented in SI Figure S5 and S6, respectively.

The specific capacitances obtained for 20 wt %-MF at a current of 1, 2, 3, 5, 7, and 10 Ag⁻¹are 343.5, 315.7, 291.6, 261.58, 239.19, and 221 Fg⁻¹ (see SI Figure S7), respectively. It demonstrates that the 20 wt %-MF electrode with an enhanced capacitive performance can be a promising material in supercapacitor applications.

Reusability.To evaluate the reusability of the 20 wt %-MF photocatalyst, as plotted inFigure 8a, recycle experiments of MO decomposition were performed under visible light irradiation. The decomposition efficiency was nearly 80% in the first cycle and remains 74.9% even after four cycles, demonstrating the high stability of the photocatalyst. The 20 wt %-MF photocatalyst retained nearly 94% of its first degradating performance after four successive degradation experiments, which is essential for future practical application.

To better understand the surface catalytic mechanism during the photocatalytic reaction, XPS analysis of fresh and used 20 wt %-MF was performed. As shown inFigure 8b of the oxygen spectra, the binding energy values increased slightly with 530.5 eV for O₂−, 531.9 eV for OH−, and 533.0 eV for H₂O in comparison with fresh sample, suggesting that the ratios between O₂−, OH−, and H₂O were changed after the photodegradation. The ratios of O₂−, OH−, and H₂O on the catalyst surface accounted for 36.9%, 37.9%, and 25.1%

before reaction, respectively, whereas it accounted for 33.6%, 52.3%, and 13.9% after degradation, suggesting the fraction of OH− increased with the H₂O fraction decreased during the degradation process. It can be concluded that the H₂O molecules attached on the surface of catalyst participated in the reactions and produced OH−.⁴² While for the Fe 2p XPS spectra before and after utilization, the Fe 2p_3/2binding energy values of used catalyst shifted positively to 711.67 eV (711.36 eV for the fresh catalyst), indicating the alternation of ratios of Fe(II) and Fe(III) after photocatalytic reactions.⁵⁴ Based on the deconvolution of Fe(II) and Fe(III) envelop, Fe(II) and Fe(III) accounted for 58.8% and 41.2% before utilization, respectively, and for 32.4% and 67.6% afterward. The

alternation indicates that the Fe³⁺ ions react with photo- induced electrons and Fe²⁺ ions were formed, then the Fe²⁺

ions react with dissolved O₂to produce the active species·O²⁻

and Fe³⁺ions.^29,55Afterward, Fe³⁺ ions could react with H₂O molecules attaching on the catalyst and form FeOOH and H⁺.⁵⁶The structural stability of recycled 20 wt %-MF was also investigated by XRD and SEM after the four successive degradations (see SI Figure S8−S10). Compared with the initial 20 wt %-MF, no obvious change could be observed for the recycled one.

Mechanism of Enhanced Photocatalytic Perform- ance. The synergistic effect in the MF hybrid composites under visible-light irradiation is schematically illustrated in Scheme 1. First, the photogenerated electrons transfer from the

MoS₂ CB to the Fe₃O₄ CB (eq 1), resulting in the enhancements to the transfer and separation of photoinduced electrons and holes.^26,28 Afterward, electrons excited from MoS₂react with Fe³⁺ion in Fe₃O₄to produce Fe²⁺ion (eq 2), and the Fe²⁺ion sequentially react with the O₂to produce Fe³⁺

ions and active species·O²⁻(eq 3).^29,55Meanwhile, h⁺ from the VB of MoS₂could oxidize the Fe²⁺ion and form Fe³⁺ion (eq 4) .⁵⁵Fe³⁺ions can react with H₂O molecules attached on the catalyst and form FeOOH and H⁺(eq 5).⁵⁶·O²⁻react with H⁺in aqueous solutions and produce·OH (eq 6).⁵⁷The MO can also be oxidized by the holes generated from MoS₂ (eq 7).^29,57In other words, the introduction of Fe₃O₄could reduce photogenerated charge recombination, enhance charge transfer rate as well as acting as photoexcited electron-trapping site to produce more active species, resulting in an increase of photocatalytic reaction centers to enhance photocatalytic activity. The degradation and photoinduced electrons and holes transferring processes are described as follows:

+ ν→ ⁻+ ⁺

MoS₂ h MoS (e₂ h ) (1)

+ →

− + +

e Fe³ Fe² (2)

+ → · +

+ − +

Fe² O₂ O₂ Fe³ (3)

+ →

+ + +

Fe² MoS (h )₂ Fe³ (4)

+ → +

+ +

Fe³ 2H O₂ FeOOH 3H ₍₅₎

·O²⁻ +2H⁺→ ·2 OH ⁽⁶⁾ Scheme 1. Schematic Illustration of the Mechanism for Photoinduced Charge Carrier Transfer in the MF System under Visible Light Irradiation

ACS Sustainable Chemistry & Engineering

·O²⁻ + ·OH+h⁺+MO→small molecules/ions (7)

■

^CONCLUSION

In summary, through a hydrothermal route, a hybrid composite of 3D ball-flower-like MoS₂ microspheres with Fe₃O₄ NPs decorated on the surfaces has been successfully synthesized. The characterizations reveal the formation of MoS₂/Fe₃O₄heterostructure with the stable interface between MoS₂and Fe₃O₄. The photocatalytic degradation rate of MO by the 20 wt %-MF is approximately 2 times that by MoS₂, under a 100 min visible light irradiation. By studies, the enhanced photocatalytic performance is derived from the more efficient separation, less recombination of photoinduced electron−hole pairs and the ratio alternation between the H₂O molecules, Fe³⁺ions and Fe²⁺ions to produce the active species. More importantly, the superparamagnetic properties of MF microspheres enable their easy separation and recyclability from aqueous solutions. The 20 wt %-MF hybrid composite is proved to be a promising photocatalyst for environmental remediation applications.

■

ASSOCIATED CONTENT

*^S Supporting Information

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssusche- meng.8b05440.

Characterizations (Text S1); Electrode preparation and electrochemical measurement (Text S2); XRD patterns of MF composites with different Fe₃O₄loading amounts (Figure S1) and recycled 20 wt %-MF (Figure S8);

High-resolution XPS spectra of pure MoS₂ and Fe₃O₄ (Figure S2) and recycled 20 wt %-MF (Figure S9); The corresponding tauc plots of MoS₂ and 20 wt %-MF (Figure S3); PL spectra (Figure S4); Cyclic voltammo- gram (Figure S5); Galvanostatic CD curves (Figure S6);

Specific capacitance of 20 wt %-MF at different current load (Figure S7); SEM image (Figure S10); Detailed surface area and pore information (Table S1); Details of percentage degradation and kinetic data (Table S2) (PDF)

■

AUTHOR INFORMATION Corresponding Author

*E-mail:zzhang@scnu.edu.cn.

ORCID

Xiaozi Lin:0000-0002-4410-558X

Xingsen Gao:0000-0002-2725-0785

Zhang Zhang: 0000-0001-6287-502X

Junming Liu:0000-0001-8988-8429 Notes

The authors declare no competingfinancial interest.

■

ACKNOWLEDGMENTS

This work was supported by the National Key R&D Program of China (2016YFB0401501), Program for Chang Jiang Scholars and Innovative Research Teams in Universities (No.

IRT_17R40), Cultivation project of National Engineering Technology Center (grant no. 2017B090903008), Xijiang R&D Team (X.W.). The Guangdong National Science Foundation (No. 2018A030313377).

■

(1) Li, Q.; Zhang, N.; Yang, Y.; Wang, G.; Ng, D. H. L. High^REFERENCES Efficiency Photocatalysis for Pollutant Degradation with MoS₂/C₃N₄ Heterostructures.Langmuir2014,30(29), 8965−8972.

(2) Ye, L.; Wang, D.; Chen, S. Fabrication and Enhanced Photoelectrochemical Performance of MoS₂/S-Doped G-C₃N₄ Heterojunction Film. ACS Appl. Mater. Interfaces 2016, 8 (8), 5280−5289.

(3) Kumar, A.; Khan, M.; Fang, L.; Lo, I. M. C. Visible-Light-Driven N-TiO2@SiO2@Fe3O4 Magnetic Nanophotocatalysts: Synthesis, Characterization, and Photocatalytic Degradation of PPCPs. J.

Hazard. Mater.2017,DOI: 10.1016/j.jhazmat.2017.07.048.

(4) Seifollahi Bazarjani, M.; Hojamberdiev, M.; Morita, K.; Zhu, G.;

Cherkashinin, G.; Fasel, C.; Herrmann, T.; Breitzke, H.; Gurlo, A.;

Riedel, R. Visible Light Photocatalysis with c-WO_3−X/WO₃×H2O Nanoheterostructures In Situ Formed in Mesoporous Polycarbosi- lane-Siloxane Polymer.J. Am. Chem. Soc.2013,135(11), 4467−4475.

(5) Yan, S. C.; Li, Z. S.; Zou, Z. G. Photodegradation Performance of G-C₃N₄Fabricated by Directly Heating Melamine.Langmuir2009, 25(17), 10397−10401.

(6) Huang, W.-C.; Lyu, L.-M.; Yang, Y.-C.; Huang, M. H. Synthesis of Cu₂O Nanocrystals from Cubic to Rhombic Dodecahedral Structures and Their Comparative Photocatalytic Activity. J. Am.

Chem. Soc.2012,134(2), 1261−1267.

(7) Zhu, Y.; Wang, Y.; Chen, Z.; Qin, L.; Yang, L.; Zhu, L.; Tang, P.;

Gao, T.; Huang, Y.; Sha, Z.; et al. Visible Light Induced Photocatalysis on CdS Quantum Dots Decorated TiO2 Nanotube Arrays. Appl.

Catal., A2015,498, 159−166.

(8) Zhang, K.; Liu, C.; Huang, F.; Zheng, C.; Wang, W. Study of the Electronic Structure and Photocatalytic Activity of the BiOCl Photocatalyst.Appl. Catal., B2006,68(3−4), 125−129.

(9) Radisavljevic, B.; Radenovic, A.; Brivio, J.; Giacometti, V.; Kis, A.

Single-Layer MoS₂Transistors.Nat. Nanotechnol.2011,6(3), 147−

150.

(10) Li, Y.; Wang, H.; Xie, L.; Liang, Y.; Hong, G.; Dai, H. MoS₂ Nanoparticles Grown on Graphene: An Advanced Catalyst for the Hydrogen Evolution Reaction. J. Am. Chem. Soc. 2011, 133 (19), 7296−7299.

(11) Jaramillo, T. F.; Jorgensen, K. P.; Bonde, J.; Nielsen, J. H.;

Horch, S.; Chorkendorff, I. Identification of Active Edge Sites for Electrochemical H2 Evolution from MoS2 Nanocatalysts. Science 2007,317(5834), 100−102.

(12) Yuan, G.; Wang, G.; Wang, H.; Bai, J. Half-Cell and Full-Cell Investigations of 3D Hierarchical MoS₂/Graphene Composite on Anode Performance in Lithium-Ion Batteries.J. Alloys Compd.2016, 660, 62−72.

(13) Ren, L.; Zhang, G.; Yan, Z.; Kang, L.; Xu, H.; Shi, F.; Lei, Z.;

Liu, Z.-H. Three-Dimensional Tubular MoS₂/PANI Hybrid Electrode for High Rate Performance Supercapacitor. ACS Appl. Mater.

Interfaces2015,7(51), 28294−28302.

(14) Swain, G.; Sultana, S.; Naik, B.; Parida, K. Coupling of Crumpled-Type Novel MoS₂ with CeO₂ Nanoparticles: A Noble- Metal-Free P−n Heterojunction Composite for Visible Light Photocatalytic H2Production.ACS Omega2017,2(7), 3745−3753.

(15) Swain, G.; Sultana, S.; Moma, J.; Parida, K. Fabrication of Hierarchical Two-Dimensional MoS₂Nanoflowers Decorated upon Cubic CaIn₂S₄ Microflowers: Facile Approach To Construct Novel Metal-Free P−n Heterojunction Semiconductors with Superior Charge Separation Efficiency.Inorg. Chem. 2018, 57(16), 10059−

10071.

(16) Li, J.; Liu, X.; Pan, L.; Qin, W.; Chen, T.; Sun, Z. MoS₂− reduced Graphene Oxide Composites Synthesized via a Microwave- Assisted Method for Visible-Light Photocatalytic Degradation of Methylene Blue.RSC Adv.2014,4(19), 9647.

(17) Li, H.; Yu, K.; Lei, X.; Guo, B.; Fu, H.; Zhu, Z. Hydrothermal Synthesis of Novel MoS₂/BiVO₄Hetero-Nanoflowers with Enhanced Photocatalytic Activity and a Mechanism Investigation.J. Phys. Chem.

C2015,119(39), 22681−22689.

ACS Sustainable Chemistry & Engineering

(18) Cheah, A. J.; Chiu, W. S.; Khiew, P. S.; Nakajima, H.; Saisopa, T.; Songsiriritthigul, P.; Radiman, S.; Hamid, M. A. A. Facile Synthesis of a Ag/MoS₂ Nanocomposite Photocatalyst for Enhanced Visible- Light Driven Hydrogen Gas Evolution.Catal. Sci. Technol.2015,5 (8), 4133−4143.

(19) Xuan, S.; Jiang, W.; Gong, X.; Hu, Y.; Chen, Z. Magnetically Separable Fe₃O₄ /TiO₂ Hollow Spheres: Fabrication and Photo- catalytic Activity.J. Phys. Chem. C2009,113(2), 553−558.

(20) Du, X.; Wang, C.; Chen, M.; Jiao, Y.; Wang, J. Electrochemical Performances of Nanoparticle Fe₃O₄ /Activated Carbon Super- capacitor Using KOH Electrolyte Solution.J. Phys. Chem. C 2009, 113(6), 2643−2646.

(21) Wu, Z.-S.; Yang, S.; Sun, Y.; Parvez, K.; Feng, X.; Müllen, K. 3D Nitrogen-Doped Graphene Aerogel-Supported Fe3O4 Nanoparticles as Efficient Electrocatalysts for the Oxygen Reduction Reaction.J. Am.

Chem. Soc.2012,134(22), 9082−9085.

(22) Padhi, D. K.; Panigrahi, T. K.; Parida, K.; Singh, S. K.; Mishra, P. M. Green Synthesis of Fe₃O₄/RGO Nanocomposite with Enhanced Photocatalytic Performance for Cr(VI) Reduction, Phenol Degradation, and Antibacterial Activity.ACS Sustainable Chem. Eng.

2017,5(11), 10551−10562.

(23) Mishra, P. M.; Naik, G. K.; Nayak, A.; Parida, K. M. Facile Synthesis of Nano-Structured Magnetite in Presence of Natural Surfactant for Enhanced Photocatalytic Activity for Water Decom- position and Cr (VI) Reduction.Chem. Eng. J.2016,299, 227−235.

(24) Wang, Y.; Li, S.; Xing, X.; Huang, F.; Shen, Y.; Xie, A.; Wang, X.; Zhang, J. Self-Assembled 3D Flowerlike Hierarchical Fe₃O₄@ Bi2O3 Core-Shell Architectures and Their Enhanced Photocatalytic Activity under Visible Light.Chem. - Eur. J. 2011, 17(17), 4802− 4808.

(25) Wang, Q.; Dong, S.; Zhang, D.; Yu, C.; Lu, J.; Wang, D.; Sun, J.

Magnetically Recyclable Visible-Light-Responsive MoS₂@Fe₃O₄ Photocatalysts Targeting Efficient Wastewater Treatment. J. Mater.

Sci.2018,53(2), 1135−1147.

(26) Xi, G.; Yue, B.; Cao, J.; Ye, J. Fe₃O₄/WO₃Hierarchical Core- Shell Structure: High-Performance and Recyclable Visible-Light Photocatalysis.Chem. - Eur. J.2011,17(18), 5145−5154.

(27) Zhu, Z.; Lu, Z.; Wang, D.; Tang, X.; Yan, Y.; Shi, W.; Wang, Y.;

Gao, N.; Yao, X.; Dong, H. Construction of High-Dispersed Ag/

Fe₃O₄/G-C₃N₄ Photocatalyst by Selective Photo-Deposition and Improved Photocatalytic Activity.Appl. Catal., B2016,182, 115−122.

(28) Kumar, S.; T, S.; Kumar, B.; Baruah, A.; Shanker, V. Synthesis of Magnetically Separable and Recyclable G-C3N4−Fe3O4 Hybrid Nanocomposites with Enhanced Photocatalytic Performance under Visible-Light Irradiation.J. Phys. Chem. C2013,117(49), 26135−

26143.

(29) Gao, N.; Lu, Z.; Zhao, X.; Zhu, Z.; Wang, Y.; Wang, D.; Hua, Z.; Li, C.; Huo, P.; Song, M. Enhanced Photocatalytic Activity of a Double Conductive C/Fe₃O₄/Bi₂O₃Composite Photocatalyst Based on Biomass.Chem. Eng. J.2016,304, 351−361.

(30) Wang, Q.; Dong, S.; Zhang, D.; Yu, C.; Lu, J.; Wang, D.; Sun, J.

Magnetically Recyclable Visible-Light-Responsive MoS₂@Fe₃O₄ Photocatalysts Targeting Efficient Wastewater Treatment. J. Mater.

Sci.2018,53(2), 1135−1147.

(31) Tu, J.; Ding, M.; Zhang, Y.; Li, Y.; Wang, T.; Ma, L.; Wang, C.;

Li, X. Synthesis of Fe3O4-Nanocatalysts with Different Morphologies and Its Promotion on Shifting C5+ Hydrocarbons for Fischer−

Tropsch Synthesis.Catal. Commun.2015,59, 211−215.

(32) Krishna Kumar, A. S.; Jiang, S.-J.; Warchoł, J. K. Synthesis and Characterization of Two-Dimensional Transition Metal Dichalcoge- nide Magnetic MoS2@Fe3O4 Nanoparticles for Adsorption of Cr(VI)/Cr(III).ACS Omega2017,2(9), 6187−6200.

(33) Song, H. J.; You, S.; Jia, X. H.; Yang, J. MoS₂ Nanosheets Decorated with Magnetic Fe3O4 Nanoparticles and Their Ultrafast Adsorption for Wastewater Treatment. Ceram. Int.2015, 41 (10), 13896−13902.

(34) Song, H. J.; You, S.; Jia, X. H.; Yang, J. MoS₂ Nanosheets Decorated with Magnetic Fe3O4 Nanoparticles and Their Ultrafast

Adsorption for Wastewater Treatment. Ceram. Int. 2015, 41 (10), 13896−13902.

(35) Song, H. J.; You, S.; Jia, X. H.; Yang, J. MoS₂ Nanosheets Decorated with Magnetic Fe3O4 Nanoparticles and Their Ultrafast Adsorption for Wastewater Treatment. Ceram. Int. 2015, 41 (10), 13896−13902.

(36) Song, H. J.; You, S.; Jia, X. H.; Yang, J. MoS₂ Nanosheets Decorated with Magnetic Fe3O4 Nanoparticles and Their Ultrafast Adsorption for Wastewater Treatment. Ceram. Int. 2015, 41 (10), 13896−13902.

(37) Song, H. J.; You, S.; Jia, X. H.; Yang, J. MoS₂ Nanosheets Decorated with Magnetic Fe3O4 Nanoparticles and Their Ultrafast Adsorption for Wastewater Treatment. Ceram. Int. 2015, 41 (10), 13896−13902.

(38) Yin, X.-L.; He, G.-Y.; Sun, B.; Jiang, W.-J.; Xue, D.-J.; Xia, A.- D.; Wan, L.-J.; Hu, J.-S. Rational Design and Electron Transfer Kinetics of MoS2/CdS Nanodots-on-Nanorods for Efficient Visible- Light-Driven Hydrogen Generation.Nano Energy2016,28, 319−329.

(39) Zheng, X.; Xu, J.; Yan, K.; Wang, H.; Wang, Z.; Yang, S. Space- Confined Growth of MoS2Nanosheets within Graphite: The Layered Hybrid of MoS₂and Graphene as an Active Catalyst for Hydrogen Evolution Reaction.Chem. Mater.2014,26(7), 2344−2353.

(40) Zhu, M.; Diao, G. Synthesis of Porous Fe3O4Nanospheres and Its Application for the Catalytic Degradation of Xylenol Orange.J.

Phys. Chem. C2011,115(39), 18923−18934.

(41) Zhang, W.; Shen, F.; Hong, R. Solvothermal Synthesis of Magnetic Fe₃O₄ Microparticles via Self-Assembly of Fe3O4 Nano- particles.Particuology2011,9(2), 179−186.

(42) Tan, C.; Gao, N.; Deng, Y.; Deng, J.; Zhou, S.; Li, J.; Xin, X.

Radical Induced Degradation of Acetaminophen with Fe₃O₄Magnetic Nanoparticles as Heterogeneous Activator of Peroxymonosulfate. J.

Hazard. Mater.2014,276, 452−460.

(43) Liu, C.; Wang, L.; Tang, Y.; Luo, S.; Liu, Y.; Zhang, S.; Zeng, Y.; Xu, Y. Vertical Single or Few-Layer MoS₂ Nanosheets Rooting into TiO₂ Nanofibers for Highly Efficient Photocatalytic Hydrogen Evolution.Appl. Catal. B Environ.2015,164, 1−9.

(44) Li, C.; Zhang, S.; Zhou, Y.; Li, J. A Situ Hydrothermal Synthesis of a Two-Dimensional MoS₂/TiO₂Heterostructure Composite with Exposed (001) Facets and Its Visible-Light Photocatalytic Activity.J.

Mater. Sci. Mater. Electron.2017,28(12), 9003−9010.

(45) Li, X.; Zhu, H.; Wei, J.; Wang, K.; Xu, E.; Li, Z.; Wu, D.

Determination of Band Gaps of Self-Assembled Carbon Nanotube Films Using Tauc/Davis−Mott Model. Appl. Phys. A: Mater. Sci.

Process.2009,97(2), 341−344.

(46) Ke, J.; Liu, J.; Sun, H.; Zhang, H.; Duan, X.; Liang, P.; Li, X.;

Tade, M. O.; Liu, S.; Wang, S. Facile Assembly of Bi 2 O 3 /Bi 2 S 3 /MoS 2 N-P Heterojunction with Layered N -Bi₂O₃and P -MoS₂for Enhanced Photocatalytic Water Oxidation and Pollutant Degradation.

Appl. Catal. B Environ.2017,200, 47−55.

(47) Beydoun, D.; Amal, R.; Low, G. K.-C.; McEvoy, S. Novel Photocatalyst: Titania-Coated Magnetite. Activity and Photodissolu- tion.J. Phys. Chem. B2000,104(18), 4387−4396.

(48) Jing, J.; Li, J.; Feng, J.; Li, W.; Yu, W. W. Photodegradation of Quinoline in Water over Magnetically Separable Fe3O4/TiO2

Composite Photocatalysts.Chem. Eng. J.2013,219, 355−360.

(49) Han, W.; Zang, C.; Huang, Z.; Zhang, H.; Ren, L.; Qi, X.;

Zhong, J. Enhanced Photocatalytic Activities of Three-Dimensional Graphene-Based Aerogel Embedding TiO₂ Nanoparticles and Loading MoS 2 Nanosheets as Co-Catalyst. Int. J. Hydrog. Energy 2014,39(34), 19502−19512.

(50) Ansari, S. A.; Khan, M. M.; Ansari, M. O.; Lee, J.; Cho, M. H.

Visible Light-Driven Photocatalytic and Photoelectrochemical Studies of Ag−SnO2 Nanocomposites Synthesized Using an Electrochemi- cally Active Biofilm.RSC Adv.2014,4(49), 26013−26021.

(51) Khan, M. M.; Ansari, S. A.; Pradhan, D.; Ansari, M. O.; Han, D.

H.; Lee, J.; Cho, M. H. Band Gap Engineered TiO₂Nanoparticles for Visible Light Induced Photoelectrochemical and Photocatalytic Studies.J. Mater. Chem. A2014,2(3), 637−644.

ACS Sustainable Chemistry & Engineering

(52) Liu, C.; Zhang, L.; Liu, R.; Gao, Z.; Yang, X.; Tu, Z.; Yang, F.;

Ye, Z.; Cui, L.; Xu, C.; et al. Hydrothermal Synthesis of N-Doped TiO₂ Nanowires and N-Doped Graphene Heterostructures with Enhanced Photocatalytic Properties.J. Alloys Compd.2016,656, 24−

32.

(53) Li, Z.; Zhang, Y.; Zhang, W. Controlled Synthesis of CNTs/

MoS₂/Fe₃O₄ for High-Performance Supercapacitors. Mater. Res.

Express2017,4(5), No. 055018.

(54) Tan, C.; Dong, Y.; Fu, D.; Gao, N.; Ma, J.; Liu, X.

Chloramphenicol Removal by Zero Valent Iron Activated Perox- ymonosulfate System: Kinetics and Mechanism of Radical Gen- eration.Chem. Eng. J.2018,334, 1006−1015.

(55) Zhu, Z.; Huo, P.; Lu, Z.; Yan, Y.; Liu, Z.; Shi, W.; Li, C.; Dong, H. Fabrication of Magnetically Recoverable Photocatalysts Using G- C₃N₄ for Effective Separation of Charge Carriers through like-Z- Scheme Mechanism with Fe₃O₄Mediator. Chem. Eng. J.2018,331, 615−625.

(56) Peng, J.; Yan, J.; Chen, Q.; Jiang, X.; Yao, G.; Lai, B. Natural Mackinawite Catalytic Ozonation for N, N-Dimethylacetamide (DMAC) Degradation in Aqueous Solution: Kinetic, Performance, Biotoxicity and Mechanism.Chemosphere2018,210, 831−842.

(57) Zhai, Y.; Yin, Y.; Liu, X.; Li, Y.; Wang, J.; Liu, C.; Bian, G.

Novel Magnetically Separable BiVO₄/Fe₃O₄Photocatalyst: Synthesis and Photocatalytic Performance under Visible-Light Irradiation.

Mater. Res. Bull.2017,89, 297−306.

ACS Sustainable Chemistry & Engineering