≡FeII redox cycles for enhanced Fenton-like activity

Colloids and Surfaces A 578 (2019) 123607

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BiFeO3/MoS2 nanocomposites with the synergistic effect between ≡MoVI/ ≡MoIV and ≡FeIII/≡FeII redox cycles for enhanced Fenton-like activity ⁎

Kaifeng Yua, Lehang Lia, Chengjie Zanga, Bin Zhaob, , Feng Chena,

T

⁎

a

Key Laboratory for Advanced Materials and Institute of Fine Chemicals, School of Chemistry and Molecular Engineering, East China University of Science and Technology, 130 Meilong Road, Shanghai 200237, China b College of Materials Science and Engineering, Shenzhen University, 1066 Xueyuan Avenue, Shenzhen 518055, Guangdong Province, China

G R A P H I C A L A B S T R A C T

A R T I C LE I N FO

A B S T R A C T

Keywords: BiFeO3 MoS2 Nanocomposite Fenton-like reaction Redox cycles

BiFeO3/MoS2 nanocomposites with different composition ratios were synthesized under hydrothermal treatment and employed as heterogeneous Fenton-like catalysts. The nanocomposite sample with 80% MoS2 content exhibited the highest Fenton-like activity by degrading acid orange 7, which was 406.0 and 8.8 times higher than that of pristine BiFeO3 and MoS2, respectively. Instrumental characterizations including Raman and XPS were employed to deeply investigate the interfacial interaction between BiFeO3 and MoS2. The results indicated that the strong interfacial coupling of Bi-S bond was formed as a fast electron transfer channel where BiFeO3 could accept electrons from MoS2. A proposed mechanism suggested that the Fenton-like performance of BiFeO3/MoS2 was promoted by the synergistic effect between ≡MoVI/≡MoIV and ≡FeIII/≡FeII redox cycles.

1. Introduction Molybdenum disulfide (MoS2), a graphene-like two-dimensional material, has been recently reported for the environmental applications in water-related fields [1–3], owing to its outstanding electronic, optical, and chemical properties [4]. In 2014, Lin et al. disclosed that MoS2 nanosheets possessed instinct Fenton-like catalytic activity, which were thus applied in H2O2 detection with a colorimetric method [5]. ⁎

MoS2 facilitates the electron transfer behavior for decomposing H2O2 into %OH [5], and the Mo6+/Mo4+ redox couple is responsible for the Fenton-like activity [6]. Furthermore, defects can act as catalytic sites in MoS2 [7–9], resulting in the faster electron transfer and higher charge carrier density for redox processes [10,11], which is beneficial for the Fenton-like catalytic activity [12]. In order to further enhance catalytic activity, heterogenous Fentonlike reaction is optimized by two major strategies including introducing

Corresponding authors. E-mail addresses: [email protected] (B. Zhao), [email protected] (F. Chen).

https://doi.org/10.1016/j.colsurfa.2019.123607 Received 6 May 2019; Received in revised form 15 June 2019; Accepted 24 June 2019 Available online 28 June 2019 0927-7757/ © 2019 Elsevier B.V. All rights reserved.

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hydrothermal method. Typically, 0.125 g BiFeO3 and 0.500 g MoS2 were dispersed in 30 mL distilled water by vigorous stirring. Then aqueous suspension was added into a 50 mL Teflon-lined stainless steel autoclave and submitted to hydrothermal treated at 220 °C for 6.0 h. The product was then washed several times with distilled water and absolute ethanol, and then dried in a thermostat drying oven at 80 °C for 12 h to obtain the BiFeO3/MoS2 nanocomposite.

extra power (irradiation, electricity, microwave) and forming composite materials [13–17]. Extra power can enhance regeneration rate of Fe2+ but increase the economic costs for wastewater treatment. Compositing materials get attention because they can accelerate redox cycle by the synergistic effect and achieve high catalytic activity, while some of them have complicated synthesis routes. MoS2-contain materials possibly have enhanced Fenton-like performance with low costs. A large number of studies have shown that MoS2-contain materials are efficient photocatalysts [18,19]. Compared with other advanced oxidation process such as photocatalysis process, Fenton process has some superiority like simple operation and quick degradation [13,16]. However, few studies focus on the Fenton-like performance of MoS2contain materials. In the classic Fenton reaction, MoS2 can serve as a cocatalyst as such redox properties of Mo atom would accelerate the Fe3+/Fe2+ redox cycle [20]. We anticipate that compositing MoS2 with iron-based materials is a workable strategy to design heterogenous Fenton-like catalysts, and the similar synergistic effect would enhance the Fenton-like activity of MoS2. Bismuth ferrite (BiFeO3), a perovskite-type mixed oxides (ABO3), can be used as photocatalyst [21] and Fenton-like catalyst [22]. BiFeO3 can catalytically decompose H2O2 into %OH via a ≡FeIII/≡FeII redox cycle [22]. Furthermore, previous studies showed that the strong interface coupling between MoS2 and iron oxide could enable a facile charge transfer at the interface [23], and thus benefited the catalysis of the nanocomposites [24,25]. Bi species can also be used to modulate the interfacial property of MoS2 [26,27], and it is worth investigating the interfacial behavior between BiFeO3 and MoS2. In this work, BiFeO3/MoS2 nanocomposites are fabricated by a facile hydrothermal method. The Fenton-like activity of as-synthesized nanocomposites is evaluated by the degradation of acid orange 7 (AO7), in which the factors of MoS2 content and H2O2 concentration are investigated. Instrumental characterizations (Raman, XPS, etc.) are carried out to further investigate the interfacial interaction between BiFeO3 and MoS2, and a Fenton-like reaction mechanism of BiFeO3/ MoS2 nanocomposites is put forward with regard to the synergistic effect between ≡MoVI/≡MoIV and ≡FeIII/≡FeII redox cycles.

2.2. Fenton-like degradation of acid orange 7 (AO7) The activity of different catalysts was evaluated with the oxidation degradation of AO7. Generally, 0.025 g catalyst powder was added into a quartz tube containing 50 mL AO7 (20 mg/L) aqueous solution. The suspension was stirred in the dark for 1.0 h to ensure the establishment of an adsorption-desorption equilibrium. Then H2O2 solution was added into the solution to reach a H2O2 concentration of 20 mmol/L. At the given time intervals, samples (2.0 mL) were taken from the mixture and immediately filtered with a micro-filtration membrane (0.22 μm). The concentration of dye in supernatant was analyzed by recording its variation of the absorbance at 484 nm by utilizing a UV–vis spectrophotometer (Shimadzu UV-2600). 2.3. Determination of H2O2 The concentration of H2O2 was determined by an iodometric method with sodium thiosulfate (Na2S2O3). Sample solution (1.0 mL) was added into iodine flask, and then H2SO4 (0.1 mL, 3 mol/L) and KI (1 mL, 20.0 g/L) solutions were added. Two to three drops of 3.0% ammonium molybdate ((NH4)6Mo7O24·4H2O) was added as catalyst to catalyze the generation of I2. The solution was diluted with H2O (20 mL). After reaction of 10 min in the dark, the produced I2 was titrated with Na2S2O3. When the color of solution turned yellowish, starch solution (1.0 mL, 10.0 g/L) was added for coloration, and then the titration went on until the blue disappeared and no longer appeared within 1 min. 2.4. Methods of analysis

2. Experimental Raman measurements were performed at room temperature using a Renishaw Via + Reflex Raman spectrometer with the excitation light of 532 nm. XRD analysis of the catalysts was carried out with Rigaku Ultima IV apparatus using Cu Kα radiation (0.15406 nm) and a graphite monochromator at room temperature, operated at 40 kV and 40 mA. The morphologies of catalysts were obtained by a transmission electron microscope (TEM, JEOL JEM-F2100) equipped with a Gatan Ultrascan CCD camera and Oxford INCA (Aztec) EDS facility. The elements on the Chemical composition of the samples was determined by inductively coupled plasma spectroscopy (ICP, Agilent 725ES). A Perkin-Elmer PHI 5000C ESCA system with Al Kα radiation operated at 250 W was utilized for X-ray photoelectron spectroscopy (XPS) measurement to study the surface chemical constitution of the samples. Electron Paramagnetic Resonance (EPR) measurements were performed in a 100G-18KG/EMX8/2.7 spectrometer at X band microwave bridge (9.3–9.9 GHz) at room temperature.

2.1. Materials synthesis 2.1.1. BiFeO3 synthesis 5.0 mmol Bi(NO3)3·5H2O and 5.0 mmol Fe(NO3)3·5H2O were dissolved in 30 mL deionized water. Then 50 mL 12.8 M KOH solution was added into the solution under constant stirring. The suspension was ultrasonically treated for 10 min and stirred vigorously for 30 min to form a uniform mixture. The resulting mixture was then sealed in a Teflon-lined stainless steel autoclave of 100 mL capacity and hydrothermally treated at 180 °C for 12 h. Then the autoclave was cooled naturally to room temperature. The as-obtained brown precipitate was collected and washed several times with distilled water and absolute ethanol, and then dried in a thermostat drying oven at 80 °C for 12 h to obtain the final product, BiFeO3. 2.1.2. MoS2 synthesis MoS2 was synthesized with a hydrothermal method [8]. 1.0 mmol (NH4)6Mo7O24·4H2O and 30 mmol thiourea were dissolved in deionized water (30 mL) under vigorous stirring to form a homogeneous solution. Then the solution was transferred into a 50 mL Teflon-lined stainless steel autoclave and maintained at 220 °C for 20 h. After being cooled to room temperature, the final product was washed with water and absolute ethanol for several times, dried in a thermostat drying oven at 80 °C for 12 h to obtain the final product.

3. Results and discussion 3.1. Characterization of the catalysts Fig. 1 shows the XRD patterns of the pristine BiFeO3, MoS2 and BiFeO3/MoS2 nanocomposites. BiFeO3 exhibits a rhombohedral structure with R3c space group (JCPDS Card No.86-1518) and MoS2 agrees well with the standard pattern of hexagonal MoS2 (JCPDS Card No.731508), where the (002) planes vertically stack to compose lamellar structure by van der Waals interactions. By calculating the value of the (110) diffraction peak of BiFeO3 and the (002) diffraction peak of MoS2

2.1.3. BiFeO3/MoS2 synthesis BiFeO3/MoS2 nanocomposite was synthesized with a simple 2

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displacements of Mo and S atoms, and peak at 402 cm−1 (A1g) is related to out of layer symmetric displacements of S atoms along c axis [33]. As expected, there are new peaks in BiFeO3/MoS2. The peaks at 818 cm−1 (Ag, B1g) originate from the symmetric stretch of the terminal oxygen atoms and the peaks at 990 cm−1 (Ag, B1g) represent the terminal oxygen atoms [34]. The new interface Mo-O band is ascribe to surface-oxidized MoS2 during second hydrothermal step [35]. The peak at 925 cm−1 is assigned to Bi2S3 vibration mode of Bi-S bond, and the peak at 884 cm−1 comes into being in heterojunction of BiFeO3 and MoS2 [18]. Besides, the peaks of BieO and FeeO disappear, possibly because of the coverage of Bi2S3 and MoS2. It is noted that the ratio of peak intensity of MoS2 at 402 cm−1 to that at 379 cm−1 decreases from 2.06 (in MoS2) to 1.91 (in BiFeO3/MoS2), suggesting that the number of S out of layer decreases and more Mo atoms would be exposed. The Raman analysis further proves the strong interfacial coupling (BieS bond) is generated. From the combined analysis of XRD, TEM and Raman, heterojunction of BiFeO3/MoS2 nanocomposites can be determined.

Fig. 1. XRD patterns of BiFeO3, MoS2 and BiFeO3/MoS2.

according to the Scherrer Formula, the average size of BiFeO3 nanoparticles is 86 nm and the thickness of MoS2 is approximately 10.1 nm. Besides the characteristic XRD signals of BiFeO3 and MoS2, several new peaks appear in the XRD pattern of BiFeO3/MoS2 composites, which can be assigned to the XRD signals of Bi2S3 (JCPDS Card No. 170320). The formation of Bi2S3 ascribes to strong coupling between the BiFeO3 and MoS2. Due to the exceedingly low solubility of Bi2S3 (Ksp = 1 × 10−97), the chemical transformation from BiFeO3 to Bi2S3 is thermodynamically favored. As shown in Fig. S1 (the Supplementary Material), as temperature goes on, the peaks for BiFeO3 become weaker, while those for Bi2S3 become stronger, suggesting that the coupling can be smoothly carried out at hydrothermal conditions [28]. However, the characteristic peaks of BiFeO3 and MoS2 in BiFeO3/MoS2 are just at the same positions as those of the pristine materials, suggesting that newly formed Bi2S3 has little effect on lattice distortion of BiFeO3 and MoS2. The weaker peaks intensity of BiFeO3 indicates that the BiFeO3 is etched and the crystallinity decreases. Fig. 2 shows the morphologies of the MoS2, defects of MoS2, BiFeO3 and BiFeO3/MoS2. Fig. 2a clearly shows the ultrathin nanosheet morphology of MoS2. In Fig. 2b, the breakpoints represent the dislocations and distortions in the MoS2 crystals, suggesting MoS2 nanosheet is a defective structure. The peak signal at g = 2.03 in EPR measurement (Fig. S2, the Supplementary Material) further indicates S vacancies exist in defected MoS2 [29,30]. Besides, the interlayer spacing of MoS2 is 0.63 nm. Combining with thickness and interlayer spacing of MoS2, the average number of S-Mo-S layers is 16 by calculation. In Fig. 2c, it is observed that the size of BiFeO3 nanoparticles is within the scope from 60 nm to 100 nm. The morphologies of BiFeO3/MoS2 nanocomposites are showed in Fig. 2d. The lattice spacing of 0.28 nm and 0.63 nm correspond to the (110) plane of BiFeO3 and (002) plane of MoS2, respectively. The EDS elemental mappings conducted on both TEM (STEM mode) and FESEM (Fig. S3 and S4, the Supplementary Material) indicate the existence of Mo, S, Bi, Fe and O elements in BiFeO3/MoS2. In addition, MoS2 still keeps defective structure (Fig. S5a, the Supplementary Material) and a part of MoS2 becomes more defective (Fig. S5b, the Supplementary Material). On the basis of XRD and TEM results, the strong coupling between BiFeO3 and MoS2 decreases the crystallinity of BiFeO3 and intensifies defects in MoS2. To further identify whether other new species on the interface of BiFeO3 and MoS2, we use Raman spectroscopy which is widely used to distinguish the chemical bonds in materials characterization [31], shown in Fig. 3. In the case of BiFeO3, the peak at 136 cm−1 (E(TO2)) is due to the displacement of Bi atoms. The peaks at 169 cm-1 (A1(TO1)), 209 cm−1 A1(TO2) and 268 cm−1 (E(TO4)) are related to the FeeO bond. The peaks above 900–1400 cm−1 are overtones of the first-order phonon mode [32]. In the Raman spectra of MoS2, the peak at 379 cm−1 (E12g) associates with in-layer

3.2. Fenton-like activity Fig. 4a shows Fenton-like activity of the as-obtained samples. Single H2O2 can hardly remove AO7 in 80 min. In present of BiFeO3 and H2O2, the AO7 removal is slight (< 5% over 80 min). The weak Fenton-like activity of BiFeO3 is possible related to exposed facets in different preparation condition, leading significant difference in Fenton-like activity [36,37]. In addition, in the neutral condition, the Fenton-like activity of iron-based materials is often weak due to the low dissolved fraction of iron ions [15,16,38]. With MoS2 and H2O2, the AO7 is adsorbed approximately 41% after adsorption equilibrium and 80% AO7 removal is achieved within 80 min, showing MoS2 possesses intrinsic Fenton-like activity. The XRD pattern and Raman spectra of MoS2 before and after reaction are nearly unchanged (Fig. S6, the Supplementary Material), indicating that MoS2 is stable in 20 mmol/L H2O2. With BiFeO3/MoS2 and H2O2, the AO7 removal can reach up 99.4% in 10 min. The BiFeO3/MoS2 shows superior Fenton-like activity and the k value of BiFeO3/MoS2 is 406.0 and 8.8 times higher than that of pristine BiFeO3 and MoS2, respectively (Fig. S7 and Table S1, the Supplementary Material). The results indicate that the assistance of BiFeO3 remarkably enhances the Fenton-like activity of MoS2. Fig. 4b shows the H2O2 concentration change during AO7 removal. There is a positive correlation between H2O2 spending rate and AO7 removal efficiency. The H2O2 concentration decreases slightly with BiFeO3 in 80 min. As a contrast, the H2O2 mostly exhausts at 20 and 60 min with BiFeO3/MoS2 and MoS2, respectively. The efficient AO7 removal of BiFeO3/MoS2 is ascribed to quick H2O2 decomposition. In the present of MoS2, the AO7 removal curve begins to flatten after 30 min while the H2O2 consumption is less than half initial H2O2 concentration. Replenishing H2O2 to 20 mmol/L at 60 min slightly increases the AO7 removal (Fig. S8, the Supplementary Material) and the recycled MoS2 can hardly adsorb AO7 (Fig. S9, the Supplementary Material), supposedly indicating the degradation intermediates occupy the active sites and adsorption sites. To verify the which specie promotes the Fenton-like activity of MoS2, the Fe/MoS2 and Bi/MoS2 are synthesized by using the equal mole metal ion (Fe3+ or Bi3+) instead of BiFeO3, and 2nd-MoS2 is synthesized by second hydrothermal treatment of MoS2, which represents the surface oxidized MoS2, details in SI. The k values of BiFeO3/MoS2, Fe/MoS2, Bi/MoS2, MoS2 and 2nd-MoS2 are 0.272, 0.138, 0.048 and 0.027 min−1, respectively (Fig. S10b and the Table S2, the Supplementary Material). The BiFeO3/MoS2 still has the highest activity among above catalysts. The k values of Fe/MoS2 and Bi/MoS2 are 4.5 and 1.5 times higher than that of MoS2, respectively, suggesting that Fe species have main effect and Bi species have subordinate effect on enhancing the Fenton-like activity of MoS2. After introducing Bi3+, the ratio of S to Mo decreases from 1.845 (in MoS2) to 1.531 (in Bi/ 3

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Fig. 2. TEM images of (a) MoS2, (b) defects of MoS2, (c) BiFeO3 and (d) BiFeO3/MoS2.

Supplementary Material), indicating O2%− is mainly from H2O2 instead of O2. The recyclability of BiFeO3/MoS2 is evaluated by successive batches of AO7 removal, shown in Fig. S12 (the Supplementary Material). The Fenton-like activity of BiFeO3/MoS2 gradually decreases during three successive runs, probably due to the MoS2 consumption which decreases the synergistic effect. The Fe leaching is 2.2 mg/L at 30 min measured by ICP (Fig. S13, the Supplementary Material). In addition, the XRD pattern (Fig. S14, the Supplementary Material) indicates that part MoS2 in BiFeO3/MoS2 is consumed after Fenton-like reaction. 3.3. Effects of MoS2 content and H2O2 concentration on the AO7 removal The AO7 adsorption monotonically increases with increasing MoS2 content in BiFeO3/MoS2 ranging from 20% to 90% (Fig. S15a, the Supplementary Material), suggesting the MoS2 plays a leading role in AO7 adsorption. The k value reaches a maximum 0.262 min−1 at 80% MoS2 content (Fig. 6a and Table S3, the Supplementary Material). On the one hand, increasing MoS2 content can enhance the AO7 adsorption, which is benefit for heterogeneous catalytic reaction. On the other hand, higher MoS2 content decreases the sum of ≡FeIII sites and the synergistic effect between BiFeO3 and MoS2 is weaker. The proper quantity of MoS2 would modulate the Fenton-like activity of BiFeO3/ MoS2. The effect of initial H2O2 concentration on AO7 removal is investigated by varying H2O2 concentration from 5 mmol/L to 50 mmol/L (Fig. 6b and Table S4, the Supplementary Material). When H2O2 concentration is under 20 mmol/L, there are desorption behavior of AO7 (Fig. S15b, the Supplementary Material). The k value increases to 0.450 min−1 when H2O2 concentration increases from 5 to 40 mmol/L. And further increasing H2O2 concentration to 50 mmol/L, the k values slight decrease to 0.446 min−1. At higher H2O2 concentration, the disproportion reaction of excessive O2%- can be accelerated, which

Fig. 3. Raman spectra of BiFeO3, MoS2 and BiFeO3/MoS2.

MoS2), analyzed by ICP. The lower ratio of S to Mo means Bi/MoS2 exposes more active Mo atoms than MoS2. The k value of 2nd-MoS2 is close to that of MoS2, indicating that the surface oxidation has negligible effect on the Fenton-like activity of MoS2. The above discussion shows that introduction of both iron and bismuth species can enhance the Fenton-like activity of MoS2. In order to identify the reactive oxygen species (ROS) in the Fentonlike reaction of BiFeO3/MoS2, isopropyl alcohol (IPA) and benzoquinone (BQ) are used as a scavenger of %OH and O2%−, respectively [39], shown in Fig. 5. With the presence of BQ, the AO7 removal is obviously inhibited. In contrast, the presence of IPA has little influence on AO7 removal. The results indicate that O2%− is the main ROS on AO7 removal in BiFeO3/MoS2. Moreover, in the absence of H2O2, continuously insufflating air has almost no effect on the AO7 removal (Fig. S11, the 4

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Fig. 6. Pseudo-first-order kinetics fitting to AO7 removal in different effect parameters: (a) MoS2 content (initial H2O2 concentration 20 mmol/L), (b) H2O2 concentration (80% MoS2 content in BiFeO3/MoS2).

Fig. 4. (a) AO7 removal with different catalysts; (b) Corresponding changes of H2O2 concentrations. Reaction conditions: initial H2O2 concentration 20 mmol/ L, AO7 concentration 20 mg/L, catalyst load 0.5 g/L and initial pH = 7.0. (80% MoS2 content in BiFeO3/MoS2).

respectively. The weak peaks at 226.9 eV is ascribed to S 2 s, demonstrating the existence of S2−. After reaction, an additional high binding energy peak at 235.9 eV is assigned to the Mo 3d of Mo6+ species [40]. The results suggest that Mo4+ on the surface of BiFeO3/MoS2 takes part in Fenton-like reaction. In the high-resolution of Fe 2p spectrum (Fig. 7b), the existence of Fe3+ is characterized by the two main peaks with the band energy of 711.7 and 725.2 eV assigning to Fe 2p3/2 and Fe 2p1/2, respectively. The low peaks intensity of Fe 2p is due to the low Fe content (2.6% measured with ICP). The XPS spectra of Bi 4f are shown in Fig. 7c. The binding energies of Bi 4f5/2, Bi 4f7/2, S 2p1/2 and S 2p3/2 in fresh BiFeO3/MoS2 are 166.0, 160.8, 163.6 and 162.5 eV respectively, which suggest the existence of Bi3+ and S2− in the sample. The Bi 4f5/2 and Bi 4f7/2 peaks of the used BiFeO3/MoS2 negatively shift to 165.1 and 159.7 eV, respectively. In BiFeO3-H2O2 system, electron transfer between Bi3+ and H2O2 does not happen and the binding energies of Bi 4f are stable before and after Fenton-like reaction [22,41]. Hence, the negative shit of Bi 4f should be induced by the reductive Mo4+ and electron transfer from MoS2 to BiFeO3 happens. In addition, there are no peaks of high valence state sulfur at high binding energy after reaction, suggesting the S atoms do not participate in redox reaction. The peak intensity of S 2p become weaker in used BiFeO3/MoS2, because abundant O2%- generating from Fenton-like reaction can attack Mo-S bonds in the highly activity sites (defects) and etch MoS2 [42,43], which account for the MoS2 consumption during the Fenton-like reaction of BiFeO3/MoS2. In the O 1s spectra (Fig. 7d), three types of oxygen species can be distinguished. The binding energies of 530.9, 532.2, 533.2 eV are assigned to lattice oxygen, hydroxyl oxygen and physically adsorbed oxygen, respectively. To investigate the effect of dissolved ions, the reaction liquid at the given time intervals is filtered with a micro-filtration membrane and

Fig. 5. Comparison of the Fenton-like activity of BiFeO3/MoS2 for AO7 removal without scavengers or with isopropyl alcohol (IPA) and benzoquinone (BQ). (80% MoS2 content in BiFeO3/MoS2).

reduces the utilization of H2O2 in AO7 removal.

3.4. Fenton-like reaction mechanism XPS is used to confirm the composition and the chemical state of the BiFeO3/MoS2 before and after the reaction. The Mo 3d spectrum is shown in Fig. 7a. Two strong peaks at 232.7 and 229.6 eV are attributable to Mo 3d3/2 and Mo 3d5/2 of Mo4+ in fresh BiFeO3/MoS2, 5

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Fig. 7. XPS spectra of fresh BiFeO3/MoS2 and used BiFeO3/MoS2.

literature [6,45], the ≡MoVI/≡MoIV redox cycle is responsible for the intrinsic Fenton-like activity of MoS2, and the Fenton-like reaction of MoS2 can be described as Eqs. (1) and (2). BiFeO3 can catalytically decompose H2O2 via a ≡FeIII/≡FeII redox cycle [22] (Eqs. (3) and (4)), and HO2%/O2%− can achieve mutual transform via Eq. (5). The synergistic effect between two catalytic centers can facilitate the redox cycle [15,46], which is strongly associated with the Fenton-like activity. Similar to the co-catalysis of MoS2 in the classic Fenton reaction, coupling ≡MoVI/≡MoIV and ≡FeIII/≡FeII plays a vital role in enhancing the Fenton-like activity of MoS2, as shown in Eq. (6).

then is stored for 24 h at room temperature, shown in Fig. S16 (the Supplementary Material). Based on the previous experiment results of H2O2 concentration, H2O2 are not exhausted within 20 min with BiFeO3/MoS2. However, no remarkable difference is observed in reaction liquid between after reaction and after 24 h stored, suggesting the dissolved ions have little effect on AO7 removal. This is because the dissolved Mo species inhibit catalytic activity of dissolved Fe species and accelerate H2O2 disproportionation [44]. Therefore, the Fentonlike activity mainly ascribes to the heterogeneous catalysis of BiFeO3/ MoS2 instead of dissolved ions. Based on the information obtained above, a Fenton-like reaction mechanism of BiFeO3/MoS2 is put forward (Fig. 8). According to the

≡MoIV + H2O2 → ≡MoVI + OH− + %OH

Fig. 8. The proposed Fenton-like reaction mechanism of BiFeO3/MoS2. 6

(1)

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≡MoVI + H2O2 → ≡MoIV + HO2% + H+ ≡Fe + H2O2 → ≡Fe II

≡Fe

III

III

+ OH

+ H2O2 → ≡Fe + II

−

HO2%

%

+ OH

≡Mo

+ ≡Fe

III

(3) [8]

+

+H

HO2%↔ O2%− + H+ IV

(2)

(4) [9]

(5)

→ ≡Mo

VI

+ ≡Fe

II

(6)

[10]

In Fenton-like reaction of BiFeO3/MoS2, BiFeO3 as an electron acceptor can speed up the redox cycle of ≡MoVI/≡MoIV (Fig. 8). The ≡FeIII in BiFeO3 accepts the electrons from MoS2 to regenerate ≡FeII, and consequently the ≡MoIV is oxidized to ≡MoVI. The semblable reduction process has been reported that Fe3O4 can achieve the regeneration of ≡FeII during the Fenton-like reaction due to the partial reduction in graphene oxide (GO) [47,48]. After the Fenton-like reaction of regenerative ≡FeII, the ≡FeIII goes on promoting the redox cycle of ≡MoVI/≡MoIV. Furthermore, the strong interfacial coupling (BieS bond) between BiFeO3 and MoS2 can build a bridge to facilitate the charge transfer on interface [49], acting as a fast electron transfer channel to stimulate the synergistic effect between ≡MoVI/≡MoIV and ≡FeIII/≡FeII redox cycles, which is critical for highly promoting the Fenton-like activity of BiFeO3/MoS2 nanocomposites.

[11]

[12]

[13]

[14]

[15]

[16] [17]

4. Conclusions [18]

In conclusion, BiFeO3/MoS2 nanocomposites with different composition ratios were synthesized under hydrothermal treatment. The nanocomposite sample with 80% MoS2 content exhibited the highest Fenton-like activity to quickly decompose H2O2 into O2%− and achieved 99.4% AO7 removal in 10 min, which was 406.0 and 8.8 times higher than that of pristine BiFeO3 and MoS2, respectively. The interfacial interaction between BiFeO3 and MoS2 was further investigated by Raman and XPS. The results indicated that the formation of strong interfacial coupling (BieS bond) acted as a fast electron transfer channel where BiFeO3 could accept electrons from MoS2 in order to stimulate the synergistic effect between ≡MoVI/≡MoIV and ≡FeIII/≡FeII redox cycles, which was critical for highly promoting the Fenton-like activity of BiFeO3/MoS2 nanocomposites.

[19]

[20]

[21]

[22]

[23]

Acknowledgement

[24]

This work was supported by the National Natural Science Foundation of China (21876051, 21677049).

[25]

[26]

Appendix A. Supplementary data

[27]

Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.colsurfa.2019.123607.

[28]

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