Self-assembly montmorillonite nanosheets supported hierarchical MoS2 as enhanced catalyst toward methyl orange degradation

Materials Chemistry and Physics 246 (2020) 122829

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Self-assembly montmorillonite nanosheets supported hierarchical MoS2 as enhanced catalyst toward methyl orange degradation Lang Yang a, b, Qingmiao Wang c, Jose Rene Rangel-Mendez d, Feifei Jia a, e, *, Shaoxian Song a, b, e, Bingqiao Yang f a

Hubei Key Laboratory of Mineral Resources Processing and Environment, Wuhan University of Technology, Luoshi Road 122, Wuhan, Hubei, 430070, China Doctorado Institucional de Ingeniería y Ciencia de Materiales, Universidad Autonoma de San Luis Potosi, Av. Sierra Leona 530, San Luis Potosi, 78210, Mexico Department of Mines, Metallurgy and Geology Engineering, University of Guanajuato, Av. Benito Ju� arez 77, Zona Centro, Guanajuato, 36000, Mexico d Instituto Potosino de Investigaci�on Científica y Tecnol� ogica, A.C. Divisi� on de Ciencias Ambientales, Camino a la Presa San Jos�e No. 2055, Lomas 4a Secci� on, 78216, San Luis Potosí, S. L. P, Mexico e School of Resources and Environmental Engineering, Wuhan University of Technology, Luoshi Road 122, Wuhan, Hubei, 430070, China f Xingfa School of Mining Engineering, Wuhan Institute of Technology, Wuhan, Hubei, 430073, China b c

H I G H L I G H T S

� The hierarchical MoS2 supported by self-assembly MMT nanosheets was first proposed. � The MoS2/MMT hybrid displayed good hydrophilia, much stronger than that of pure MoS2. � The catalytic activity of MoS2/MMT hybrid was strongly enhanced, as much as 60 times of MoS2. A R T I C L E I N F O

A B S T R A C T

Keywords: MoS2 Montmorillonite nanosheets Self-assembly Catalysis Methyl orange

Layered MoS2 and montmorillonite nanosheets were hybridized through a simple in-situ hydrothermal synthesis to develop an efficient catalyst (MoS2/MMT) for better exposure of the active sites on MoS2 toward Methyl Orange (MO) degradation, enable MoS2/MMT hybrid to achieve an excellent catalytic performance on MO removal. The characterizations of MoS2/MMT, based on XRD, FTIR, XPS, SEM, TEM, TGA, UV, contact angle detector and laser particle size analyzer, suggested that the montmorillonite nanosheets self-assemble into a cross-linked structure first, then the MoS2 nanosheets grew along the montmorillonite nanosheets surface, formed a cross-link atypical grid construction. The hybrid exhibited an excellent catalytic decomposition (98.6%) of MO into 4-amino-dimethylaniline and sulfanilic acid, much higher than that (48.6%) of pure MoS2, and the corresponding reaction rate of MoS2/MMT is about 8 times of MoS2. The superb decomposition capacity was found to be attributed to the more available of active sites and the high hydrophilicity of MoS2/MMT. The former greatly improved the hydrolysis of NaBH4 reductant, while the latter brought the catalyst a good dispersion in aqueous solutions. Furthermore, the hybrid performed a good catalytic reusability and stability. It is demon­ strated that the MoS2/MMT hybrid might be an outstanding material in catalytic filed and water treatment.

1. Introduction MoS2 and analogous MoS2 materials have attracted much physical and chemical attention due to its two dimensional graphene-like layer structure [1,2]. Compared with the bulk MoS2, MoS2 nanosheets with single or few layers exhibit much more active sites, larger surface area, bigger surface energy, as well as stronger electronic and physical

properties, and thus leading to more remarkable applications in energy storage and conversion, environmental treatment, catalysis and flame retardant fields, etc [3–10]. Numerous methods have been developed to obtain MoS2 thin layers, such as mechanical and chemical exfoliation [11,12], liquid exfoliation [13,14], laser thinning [15], solvothermal and hydrothermal synthesis [16]. The former top-to-down exfoliation methods usually suffer from low efficiency, as a comparison, the

* Corresponding author. Hubei Key Laboratory of Mineral Resources Processing and Environment, Wuhan University of Technology, Luoshi Road 122, Wuhan, Hubei, 430070, China. E-mail address: [email protected] (F. Jia). https://doi.org/10.1016/j.matchemphys.2020.122829 Received 28 May 2019; Received in revised form 6 February 2020; Accepted 16 February 2020 Available online 19 February 2020 0254-0584/© 2020 Published by Elsevier B.V.

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down-to-up synthesis is widely used because of its high efficiency and easy operation. While the MoS2 nanosheets tend to agglomerate into stacked multilayers or flower-like clusters during the synthesis due to the high surface energy and strong hydrophobicity of MoS2 nanosheets [17–19], which results in a serious deterioration on the performance. Synthesis of MoS2 on supported materials might be an effective way out of the problem. Mesoporous graphene [20], carbon nanotubes [21], Fe3O4 nanospheres [22], nanoporous metals [23] and metal-organic frameworks [24] were reported to be used as supports in the synthesis of MoS2 and these hybrids exhibited more exposed edges and better performance in hydrogen evolution reaction [25], lithium ion storage [26], catalysis [17] than the pure MoS2. However, most of the supported materials were expensive or the obtained hybrids were subjected to complicated processes. In addition, some of the synthesized hybrids were unstable in acid/alkali solutions or at high temperature. Thus, a low cost and easy operated synthesis of stable MoS2 hybrid is eagerly needed. Montmorillonite (MMT) is one of the most abundant and cheapest nature layered silicates. It features at the excellent dispersibility in water, good intercalation by other ions and easy exfoliation under minor mechanical force [27,28]. MMT has been widely used as a matrix for synergistic toughening of artificial nacre [29], catalyst carriers for photocatalytic degradation [30], and constituents of advanced drug-delivery systems [31]. As reported, MMT nanosheets (MMTNS) were negative charged in the platelet surfaces while positive charged on the edges [32–36], thus they could self-assemble into a so-called “house of cards” structure in aqueous solution through the electrostatic bonding between the platelet surfaces and edges of MMTNS, and finally formed a porous three dimensional “house of cards” structure (MMTHC). If MoS2 nanosheets are synthesized onto MMTHC, the active sites on MoS2 nanosheets would be well preserved. In addition, MoS2 was hydrophobic and could not be dispersed well in aqueous solutions, while the loading of MoS2 on strongly hydrophilic MMTHC would avoid its hydrophobic agglomeration when being applied in aqueous phase. It is therefore, MMTHC may be a promising support for MoS2 nanosheets, which may greatly improve the catalytic performance of MoS2 in aqueous solution. In this paper, an attempt was made to investigate the synthesis of MoS2/MMT hybrid and its catalytic performance. The MoS2/MMT hybrid was firstly fabricated through a facile in-situ hydrothermal syn­ thesis MoS2 on self-assembly montmorillonite nanosheets, followed by studying its catalytic performance with methyl orange (MO) as a model. The microstructure, morphology and surface property of MoS2/MMT hybrid were characterized by the measurements of XRD, SEM, TEM, FTIR, XPS, etc. The catalytic activity of the hybrid was evaluated by the decomposition of methyl orange with UV. The objective is to synthesize a MoS2/MMT hybrid for a better exposure of the active sites on MoS2 and therefore enable MoS2/MMT hybrid to achieve an excellent performance.

Fig. 1. XRD patterns of MMTNS, MoS2 and MoS2/MMT hybrid.

Firstly, MMT nanosheets were prepared by ultrasonic exfoliation as the exfoliation of graphene oxide [37]. 100 g MMT was added into 2 L deionized water and stirred at 1200 rpm for 2 h, then the suspension was centrifuged by a Sorvall ST16 centrifuge (Thermo Fisher Scientific, U. S. A.) at 1000 rpm for 1 min to remove the coarse particles. The supernate was collected for a further ultrasonic exfoliation by CP505 ultrasonic dispersion instrument (Vernon Hills, U. S. A.) at a strength of 450 w for 5 min, and subsequently centrifuged at 4000 rpm for 2 min. The ob­ tained supernate was the exfoliated MMTNS, the concentration of which was about 11.6 mg/ml. After that, the MoS2/MMT hybrid was synthe­ sized through a simple hydrothermal reaction. 9.212 g (NH4)6Mo7O24⋅4H2O and 16.940 g CH4N2S were dissolved in 40 ml water, then added into 360 ml of the MMT nanosheets suspension. The pH of the mixture was kept at 6.5 with 1 M HCl or NaOH, followed by stirring it at 500 rpm for 10 min. Then, the suspension was added into 100 ml Teflon-lined autoclave and thermally treated at 220 � C for 6 h. The obtained precipitate was collected via centrifugation and succes­ sively washed with deionized water. Subsequently, the sample was freeze dried, and the obtained product was MoS2/MMT hybrid. For a comparison, pure MoS2 sample was synthesized by the similar method without the adding of MMT nanosheets. 2.2.2. Catalysis and catalytic stability of MoS2/MMT hybrid for methyl orange The reduction of MO was chosen to evaluate the catalytic activity of the samples, which was common for most of the catalysts [38,39]. 0.600 g NaBH4 was added into 200 ml MO solution with concentration of 100 mg/L and pH of 9.0 at room temperature. 0.040 g of the sample was then added into the solution to initiate the reduction reaction. After a desired time, 2 ml of the solution was sucked and filtered through a 0.22 μm filter membrane, followed by the determination of MO concentration with UV. The catalytic efficiency was calculated by the following formula:

2. Experimental section 2.1. Materials The MMT was purchased from Chifeng Ningcheng MMT Co. Ltd. (Inner Mongolia, China). XRD pattern of MMT in Fig. 1 exhibited that all the diffraction peaks corresponded to MMT, suggesting the high purity of MMT. Ammonium molybdate tetrahydrate ((NH4)6Mo7O24⋅4H2O), thiourea (CH4N2S), Sodium borohydride (NaBH4), methyl orange (MO), sodium hydroxide (NaOH) and hydrochloric acid (HCl) obtained from Sinopharm Chemical Reagent Co. Ltd. (China) were of analytical grade. The deionized water used in the experiments was from Milli-Q direct 16.

E¼

A0 A A0

where E is the decomposition efficiency (%), A0 is the initial absorbance and A is the absorbance at set time. It is well established fact that catalyzed reduction reactions of MO in the presence of excess NaBH4 proceed via pseudo first order kinetics [40, 41]. The apparent rate constant of pseudo first order kinetics is usually used to evaluate the catalytic activity and determined from the following rate equation:

2.2. Methods 2.2.1. Preparation of MoS2/MMT hybrid A facile hydrothermal method was applied to prepare the hybrid. 2

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ln

Ct ¼ C0

Materials Chemistry and Physics 246 (2020) 122829

3. Results and discussion

kt

3.1. XRD

where C0 (mg/L) is the initial concentration of MO, Ct (mg/L) is the concentration of MO at homologous time t (min), and k (min 1) is the apparent reaction rate constant. The catalytic stability of MoS2/MMT hybrid was performed as following: when the catalytic reaction was over, the MoS2/MMT hybrid used in the experiment was collected from the beaker, and washed with 50 ml water. Then, the washed MoS2/MMT hybrid was added into the fresh MO solution again under the same operations for the next cycle.

The XRD was always used to detach the structures of catalysts [42, 43], and the XRD analysis of MMTNS, MoS2 and MoS2/MMT hybrid was illustrated in Fig. 1. The peaks located at 6.72� , 20.9� , 26.5� , 29.1� , 35.2� and 62.3� on the XRD pattern of MMTNS corresponded to the charac­ teristic (100), (103), (005), (110) and (060) planes of MMTNS, respec­ tively (PDF card No. 47–0197) [44]. For the pure MoS2, the peaks at 32.6� , 39.1� , 48.5� and 58.3� on the pattern were related to the (100), (103), (105) and (110) planes of 2H–MoS2, respectively (PDF card No. 37–1492) [45]. While for the MoS2/MMT hybrid, it exhibited combined diffraction peaks of MMTNS and MoS2, demonstrating the good composition of MoS2/MMT hybrid. In addition, the combined peaks became broader and their intensity decreased after the composition, indicating the decrease of their crystallinity on MoS2/MMT hybrid, which should be attributed to the formation of atypical grid construction.

2.3. Measurements The structures of the samples were obtained from X-ray diffraction (XRD, D8 Advance) using Cu Kα radiation (λ ¼ 0.154 nm) at a scan rate of 0.02� /s. The morphologies were measured by scanning electronic microscopy (SEM, Zeiss Ultra Plus) at an accelerating rate of 5 kV. Transmission electronic microscopy (TEM) images were obtained on a JEM-2100F operating at 200 kV. Fourier transform infrared spectra (FTIR) were measured by a Nicolet 6700 FT-IR spectrophotometer with KBr pellets. X-ray photoelectron spectroscopy (XPS) measurements were performed on an ESCALAB 250Xi spectrometer at 1486.6 eV Al K exci­ tation source. The contact angles of the samples was obtained on a contact angle detector (JC2000C1) using the sessile drop technique. The MO concentration was measured on an ultraviolet spectrophotometry (UV, Orion Aquamate 8000) at 466 nm. The thermogravimetric analysis (TGA) was used to evaluate the stability of the samples from room temperature to 700 � C under air atmosphere with a heating rate of 10 � C/min. Particle size distribution determined by laser particle size analyzer (Malvern 2000).

3.2. FT-IR and XPS FT-IR spectra of MMTNS, MoS2 and MoS2/MMT hybrid were shown in Fig. 2(a). In the FT-IR spectrum of MMTNS, the bands at 3627 cm 1, 3441 cm 1, 1638 cm 1, 1030 cm 1 and 466 cm 1 were mainly ascribed to the –OH stretching vibration of Al–O–H group, the asymmetric and symmetric H–O–H stretching vibrations of H-bonded water, the O–H blending vibration of water, the stretching vibration of Si–O–Si, and the blending vibration of Si–O, respectively [46]. No characteristic bands were detected on MoS2 expect for the two water bands at 3441 cm 1 and 1635 cm 1, which was in good agreement with the previous work [47]. In the FT-IR spectrum of MoS2/MMT hybrid, the band at 3627 cm 1 disappeared and the bands in the range of 3441 to 1623 cm 1 red shifted, due to the disappeared hydroxyl group. While the characteristic

Fig. 2. (a) FTIR spectra of MMTNS, MoS2 and MoS2/MMT hybrid, (b) XPS survey of MMTNS, MoS2 and MoS2/MMT hybrid, (c) high-resolution scan of Mo 3d and (d) high-resolution scan of S 2p of MoS2 and MoS2/MMT hybrid. 3

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band of Si–O–Si shifted to 1015 cm 1 from 1030 cm 1, which could be related to the interaction between MMTNS and MoS2 in the synthesized progress. The XPS survey spectra of MMTNS, MoS2 and MoS2/MMT hybrid were presented in Fig. 2(b). O, Si and Al peaks were observed on the spectrum of MMTNS, and Mo, S and O peaks arose in the pure MoS2. After the composition of the two materials, MoS2/MMT hybrid exhibited both the characteristic peaks of MMTNS and MoS2, giving another evi­ dence for the successful synthesis of the hybrid. The percentage of Mo and S atoms in MoS2/MMT hybrid were 5.95% and 14.98%, respec­ tively, the ratio of which was close to the theoretical value in MoS2, suggesting that Mo and S were more likely presented in the form of MoS2. High resolution spectra of Mo 3d and S 2p in MoS2 and MoS2/ MMT hybrid were illustrated in Fig. 2(c) and (d), respectively. The Mo 3d3/2, Mo 3d5/2, S 2p1/2 and S 2p3/2 peaks at binding energy of 231.68, 228.48, 162.78 and 161.45 eV corresponded to MoS2, further proving the presence of Mo and S as a form of MoS2 [48]. It was interesting to find that the Mo and S peaks in Fig. 2(c) and (d) had a right shift on MoS2/MMT hybrid compared that on the pure MoS2, demonstrating the interaction existed between MoS2 and MMTNS in MoS2/MMT hybrid.

much more exposed active sites [50], demonstrating MMTHC would be an outstanding support for the growth and dispersion of MoS2. The EDS spectrum inset in Fig. 3(c) showed that MoS2/MMT hybrid was mainly comprised of O, Al, Si, S, Mo, and a small amount of Mg, further demonstrating the successful hybridization of MoS2 and MMTNS. TEM was employed to further observe the structure of MMTNS, pure MoS2 and MoS2/MMT. Fig. 4(a) exhibited the characteristic of MMTNS, and the MMT displayed in nano-platelets and stacked in a wrinkled form due to the dehydration during drying process. As shown in Fig. 4(b), the pure MoS2 presented in small microspheres and trended to link or stack together because of mechanical mixing in the synthesis and drying progress, and the microspheres were quite consistent with the clustering of MoS2 in the SEM image. A TEM image of the typical MoS2/MMT hybrid illustrated in Fig. 4(c) showed that MoS2/MMT had a cross linked skeleton with some nanosheets parallel to the view vision while the others vertical, which can effectively hinder the severe agglomeration and enable the well dispersion of MoS2 on MMT. The EDS elemental mappings of combined and single Mo, S, Si, Al and O on the surface of MoS2/MMT hybrid were showed in Fig. 4(d) and Fig. S1, respectively. It could be clearly observed that the main elements of MoS2 and MMT distributed evenly on the surface of MoS2/MMT hybrid, further demonstrating the successful preparation of MoS2/MMT hybrid and the good dispersion of MoS2 with MMTHC as supported material. The facile hydrothermal synthesis process of MoS2/MMT hybrid was schematized in Fig. 5. The pH of pure MMTNS suspension was about 9.7, both the planes and edges of MMTNS were negatively charged, resulting in the repulsion among MMTNS and good dispersion of MMTNS in so­ lution. When added with (NH4)6Mo7O24⋅4H2O and CH4N2S, the pH of the suspension changed to around 6.5. It was reported that the edges of MMTNS changed to positive charged while the planes remained in negative under pH 6.3, and the presentation of extra electrolyte could make the edges of MMTNS moved to positive and the planes remained in negative even in a higher pH [51]. Thus the MMTNS could easily self-assemble to a “house of cards” structure by electrostatic bonding between edges and planes. The formation of the special structure pro­ vided more surfaces for the loading of MoS2 nanosheets and prevented the aggregation of MoS2 during drying. After the formation of MMTHC, CH4N2S was adsorbed on the planes of MMTHC by amidogen, while (NH4)6Mo7O24⋅4H2O dispersed in the suspension. When the tempera­ ture of the mixture rose to 220 � C, CH4N2S decomposed and reacted with (NH4)6Mo7O24⋅4H2O to anchor on the planes of MMTHC through nucleation [52], thus the growth of MoS2 nanosheets proceeded in situ

3.3. SEM and TEM SEM was employed to investigate the general morphologies of MMTNS, MoS2 and MoS2/MMT hybrid, as presented in Fig. 3. Fig. 3(a) showed that almost all the layered MMTNS had lateral size less than 1 μm, and the sheets tended to lay in disorder and aggregated to form a flat surface. The as obtained pure MoS2 (Fig. 3(b)) exhibited a fullerene-like microsphere structure, and the microspheres trended to cluster and stack together. Notably, when MMTNS and MoS2 hybridized together, the morphology of MoS2/MMT hybrid (Fig. 3(c)) was quite different from that of individual MMTNS or MoS2. The sheets in MoS2/MMT cross linked together, through the interaction between MMTNS and MoS2 nanosheets, with some ones laying vertically while others standing horizontally. The cross linked sheets seemed like atypical grid structure, which were similar to the “house of cards” structure of MMTHC in previous work [49]. Thus, it could be inferred that MMTNS self-assemble into “house of cards” structure, and act as a supporting substrate for the synthesis of MoS2 on the surface of each MMTNS to form the hybrid of MoS2/MMT hybrid. The drastic morphological dif­ ference among MMTNS, MoS2 and MoS2/MMT hybrid indicated that MMTHC in the MoS2/MMT hybrid could not only well block the ag­ gregation of MMTNS but also effectively eliminate the clustering and stacking of MoS2 microspheres. The dispersive nanosheets enabled MoS2

Fig. 4. TEM images of (a) MMTNS, (b) MoS2, (c) MoS2/MMT hybrid, (d) EDS elemental mapping of Mo, S, Si, Al and O on MoS2/MMT hybrid.

Fig. 3. SEM morphologies of (a) MMTNS, (b) MoS2, (c) MoS2/MMT hybrid. 4

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from aggregation, the particle size of MoS2/MMT at cumulative fraction of 50% and 90% were 13.2 μm and 26.3 μm, respectively, much smaller than that of the pure MoS2. Thus, more active sites could be available on the smaller MoS2/MMT particles, the enhanced catalytic efficiency could be obtained. 3.5. Catalytic activity and mechanism The catalytic activity of the samples was estimated via the decom­ position of MO in aqueous solution with NaBH4 as reductant. To get an intuitive understanding of the catalytic progress, successive UV adsorption spectra and digital photographs of MO solution with MoS2/ MMT hybrid as catalyst at different times were shown in Fig. 7. As shown in the successive UV adsorption spectra (Fig. 7(a1)), the MO so­ lution displayed a strong absorbance peak at 464 nm and a weak one at 272 nm before catalysis (t ¼ 0 min). The strong absorbance peak was attributed to conjugated structure formed by the azo bond (-N– N-) (Fig. 7 (b1)) under the strong influence of the electron-donating dime­ thylamino groups, while the weak one was ascribed to the π→π* tran­ sition in aromatic rings (Fig. 7(b2)) [53–56]. In the presence of catalyst MoS2/MMT hybrid (t ¼ 1–120 min), the two peaks at 464 nm and 272 nm decreased drastically, meanwhile, new sharp peaks at around 225–260 nm raised. As magnifying the new peaks in Fig. 7(a2), two peaks at 244 nm and 248 nm were found, which were resulted from the amino group (-NH2) in 4-amino-dimethylaniline (Fig. 7(b3)) and sulfa­ nilic acid (Fig. 7 (b4)), respectively, demonstrating the decomposition of azo bond (-N– N-) into amino group (-NH2) and, MO molecules into 4-amino-dimethylaniline and sulfanilic acid [53,57,58]. About 60 min later, the peaks at 464 nm and 272 nm almost disappeared and only the new peaks at 244 nm and 248 nm existed, indicating the complete decomposition MO molecules. As the decomposition proceed, the color of the MO solution changed a lot (Fig. 7(c)). The MO solution was in high concentration with a croci color before the decomposition, while it turned into yellow quickly after the addition of MoS2/MMT hybrid, then faint yellow, and finally became colorless at around 60 min, further demonstrating the complete decomposition of MO. The decomposition efficiency of MMTNS, MoS2 and MoS2/MMT hybrid was given in Fig. 8. It was seen that the pure NaBH4 had a circa 10% effect on the decomposition efficiency due to its hydrolyzation in the solution. When MMTNS added, the decomposition efficiency remained 10%, indicating the MMTNS did nothing on the decomposi­ tion of MO. While the decomposition efficiency had a big increased change with the addition of pure MoS2, and finally reached a plateau at about 48.6% in 120 min, suggesting the good catalysis of MoS2 for MO. Notably it was surprised to find that MoS2/MMT hybrid exhibited a pretty good 98.6% decomposition efficiency, which was much better than that of pure MoS2. Furthermore, it costed MoS2/MMT hybrid approximately only 1/60 time of pure MoS2 to achieve the decomposi­ tion efficiency of pure MoS2, while the real content of MoS2 in MoS2/ MMT hybrid was 1/2 of MoS2/MMT hybrid. The more exposed active sites and the better dispersion of MoS2/MMT hybrid should be accounted for the much better decomposition performance. The former provided more sites for the reaction, resulting in higher decomposition efficiency, while the later supplied more chances to reaction in a time which made the faster decomposition efficiency. Fig. 9 showed the reusability of MoS2/MMT hybrid for the catalytic decomposition of MO. The hybrid presented a relatively high and stable catalytic performance after five regeneration cycles, demonstrating that MoS2/MMT hybrid could be well regenerated and reused. The thermal stability of the catalyst was also analyzed (Fig. S2). A slight weight loss occurred on the MoS2/MMT hybrid before 150 � C due to desorption of physisorbed water. When being heated to 300 � C, the weight loss became obvious because of the decomposition of MoS2 to MoO3 and SO2. Only 10.62% weight loss occurred on MoS2/MMT when being heated to 700 � C, implying the good stability of MoS2/MMT. Fig. S4 showed the kinetic linear simulation curves of MoS2 and

Fig. 5. Synthesis route of MoS2/MMT hybrid. (a) MMTNS suspension, (b) formation of MMTHC, (c) synthesis of MoS2/MMT hybrid through hydrother­ mal synthesis.

with the planes of MMTHC as matrix. 6 h later, the black MoS2/MMT hybrid was obtained. Previous research demonstrated that the “house of cards” structure of MMTHC was easy to collapse during drying, while an obvious cross-linked atypical grid structure was observed in MoS2/MMT hybrid, which indicated that the growth of MoS2 on the planes of MMTHC strengthened the special structure, making the MMTHC free from collapse. The SEM and TEM results displayed that MoS2 nanosheets grew along the planes of MMTHC and presented in the form of nano­ sheets instead of fullerene-like microsphere construction, which effec­ tively eliminated the clustering and aggregation of MoS2 nanosheets, enabling MoS2 more exposed active sites. 3.4. Dispersity Contact angles of MoS2, MMTNS and MoS2/MMT hybrid were pre­ sented in Fig. 6 to investigate their hydrophility and potential dispersion in aqueous solutions. As for pure MoS2, the contact angle was 53.5� (Fig. 6(a)), while it was 17.1� for the MMTNS (Fig. 6(b)). After the hy­ bridization of the two materials, MoS2/MMT hybrid exhibited a 35.1� contact angle, much lower than that of pure MoS2. According to the contact angle theory, the smaller the contact angle, the more hydrophilic the surface, as well as the better dispersibility in solution. Therefore, the MoS2/MMT hybrid had a much better dispersion in water than the pure MoS2, which may greatly improve its efficiency in catalytic progress. Fig. S3 exhibited the particle size distribution of MoS2 and MoS2/ MMT. For the pure MoS2, the particle size at cumulative fraction of 50% is 25.5 μm, while it increased a lot to 78.4 μm at cumulative fraction of 90%, due to the serious aggregation of MoS2 particles. As for the MoS2/ MMT, the addition of MMT significantly prevented the MoS2 particles

Fig. 6. Contact angles of (a) MoS2 (b) MMTNS (c) MoS2/MMT hybrid. 5

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Fig. 7. (a) Successive UV adsorption spectra ((a2) is magnified view of the shadow in (a1)) and (b) the chemical structures of materials corresponding to the peaks in successive UV spectra (c) digital photographs of MO solution with MoS2/MMT hybrid as catalyst at different times.

Fig. 9. The reusability of MoS2/MMT hybrid for the catalytic reduction of MO.

Fig. 8. Decomposition efficiency of MMTNS, MoS2 and MoS2/MMT hybrid.

performance. The catalysis mechanism of MO with MoS2/MMT hybrid as catalyst was schematically illustrated in Fig. 10. When the pure NaBH4 was added into MO solution, a few NaBH4 molecules interacted with water molecules and hydrolyzed to release a small amount of hydrogen atoms. When the azo and quinone imine structures of MO encountered the hydrogen atoms, the MO would be decomposed. While the

MoS2/MMT hybrid for MO degradation. The apparent rate constants (k, kt, displayed min 1) of MoS2/MMT, calculated from the ln(Ct/C0) ¼ a much higher value (k ¼ 0.0328 min 1) than that of MoS2 (k¼0.0045 min 1), confirming the excellent catalytic performance of MoS2/MMT. Compared to the other catalysts listed in Table 1 [41,59–64], MoS2/MMT hybrid also exhibited strong competitiveness in consider­ ation of the low cost, simple preparation method and favorable 6

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reusability and stability, performing high-potential applications in catalysis and waste water treatment.

Table 1 Comparison of catalytic performance of MoS2/MMT and catalysts reported in literatures. Catalyst

Catalyst dosage (mg)

MO

k (min 1)

Ref.

TiO2 film

–

0.0098

[41]

ZnO/GO

5

0.0300

[59]

Bi12TiO20/ BiFeO3 Er3þ:YAlO3/TiO2 Bi2O3

100

0.0240

[60]

0.0179 0.0074

[61] [62]

ZnO

100

0.0223

[63]

BiOI

50

0.0280

[64]

MoS2

40

0.0045

MoS2/MMT

40

950 ml, 0.016 mM 100 ml, 0.078 mM 100 ml, 0.025 mM 50 ml, 3.1 mM 100 ml, 0.019 mM 100 ml, 0.07 mM 100 ml, 0.062 mM 200 ml, 0.31 mM 200 ml, 0.31 mM

This work This work

1000 80

0.0328

Declaration of competing interest We declare that we have no financial and personal relationships with other people or organizations that can inappropriately influence our work, there is no professional or other personal interest of any nature or kind in any product, service and/or company that could be construed as influencing the position presented in, or the review of, the manuscript entitled “Self-assembly Montmorillonite Nanosheets Supported Hierar­ chical MoS2 as Enhanced Catalyst toward Methyl Orange Degradation”. Acknowledgements The financial supports for this work from the National Natural Sci­ ence Foundation of China (No. 51704220 and 51674183), the Natural Science Foundation of Hubei Province (No. 2016CFA013), and the Research Fund Program of Key Laboratory of Rare Mineral, Ministry of Land and Resources (KLRM-F201802) were gratefully acknowledged. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.matchemphys.2020.122829. References [1] M.B. Tahir, A.M. Asiri, G. Nabi, M. Rafique, M. Sagir, Fabrication of heterogeneous photocatalysts for insight role of carbon nanofibre in hierarchical WO3/MoSe2 composite for enhanced photocatalytic hydrogen generation, Ceram. Int. 45 (2019) 5547–5552. [2] M.B. Tahir, G. Nabi, T. Iqbal, M. Sagir, M. Rafique, Role of MoSe2 on nanostructures WO3-CNT performance for photocatalytic hydrogen evolution, Ceram. Int. 44 (2018) 6686–6690. [3] M. Pumera, Z. Sofer, A. Ambrosi, Layered transition metal dichalcogenides for electrochemical energy generation and storage, J. Mater. Chem. A. 2 (2014) 8981–8987. [4] Z. Wang, B. Mi, Environmental applications of 2D molybdenum disulfide (MoS2) nanosheets, Environ. Sci. Technol. 51 (2017) 8229–8244. [5] F. Jia, Q. Wang, J. Wu, Y. Li, S. Song, Two-dimensional molybdenum disulfide as a superb adsorbent for removing Hg2þ from water, ACS Sustain. Chem. Eng. 5 (2017) 7410–7419. [6] X.Q. Qiao, Z.W. Zhang, F.Y. Tian, D.F. Hou, Z.F. Tian, D.S. Li, Q. Zhang, Enhanced catalytic reduction of p-nitrophenol on ultrathin MoS2 nanosheets decorated with noble metal nanoparticles, Cryst. Growth Des. 17 (2017) 3538–3547. [7] F. Jia, C. Liu, B. Yang, S. Song, Microscale control of edge defect and oxidation on molybdenum disulfide through thermal treatment in air and nitrogen atmospheres, Appl. Surf. Sci. 462 (2018) 471–479. [8] F. Jia, K. Sun, B. Yang, X. Zhang, Q. Wang, S. Song, Defect-rich molybdenum disulfide as electrode for enhanced capacitive deionization from water, Desalination 446 (2018) 21–30. [9] M.B. Tahir, M. Sagir, Carbon nanodots and rare metals (RM¼ La, Gd, Er) doped tungsten oxide nanostructures for photocatalytic dyes degradation and hydrogen production, Separ. Purif. Technol. 209 (2019) 94–102. [10] M.B. Tahir, Construction of MoS2/CND-WO3 ternary composite for photocatalytic hydrogen evolution, J. Inorg. Organomet. Polym. Mater. 28 (2018) 2160–2168. [11] J. Brivio, D.T. Alexander, A. Kis, Ripples and layers in ultrathin MoS2 membranes, Nano Lett. 11 (2011) 5148–5153. [12] K.G. Zhou, N.N. Mao, H.X. Wang, Y. Peng, H.L. Zhang, A mixed-solvent strategy for efficient exfoliation of inorganic graphene analogues, Angew. Chem. 50 (2011) 10839–10842. [13] S. Bai, L. Wang, X. Chen, J. Du, Y. Xiong, Chemically exfoliated metallic MoS2 nanosheets: a promising supporting co-catalyst for enhancing the photocatalytic performance of TiO2 nanocrystals, Nano Res 8 (2015) 175–183. [14] L. Guardia, J.I. Paredes, J.M. Munuera, S. Villarrodil, M. Ay� anvarela, A. Martínezalonso, J.M. Tasc� on, Chemically exfoliated MoS2 nanosheets as an efficient catalyst for reduction reactions in the aqueous phase, ACS Appl. Mater. Interfaces 6 (2014) 21702–21710. [15] A. Castellanosgomez, M. Barkelid, A.M. Goossens, V.E. Calado, V.D.Z. Hs, G. A. Steele, Laser-thinning of MoS2: on demand generation of a single-layer semiconductor, Nano Lett. 12 (2012) 3187–3192. [16] C. Altavilla, M. Sarno, P. Ciambelli, A novel wet chemistry approach for the synthesis of hybrid 2D free-floating single or multilayer nanosheets of MS2@ –Mo,W), Chem. Mater. 23 (2011) 3879–3885. oleylamine (M–

Fig. 10. Schematic illustration of the catalytic mechanism of MoS2/ MMT hybrid.

decomposition of MO was restricted because of the limited hydrogen atoms generated from NaBH4. When MoS2 was added, the hydrolysis of NaBH4 was enhanced by the MoS2 catalytic active sites, abundant of hydrogen atoms were quickly released into the solution, leading to a rapid decomposition of MO. Due to its better hydrophilic and dispersion in solution, there were more chances for MoS2/MMT to meet NaBH4 molecules at one moment, thus more hydrogen atoms could be released quickly to reduce the MO molecules, enhancing the degradation of MO. In addition, benefited from its smaller particles, much more active sites on MoS2/MMT could be available for NaBH4 to release more hydrogen atoms to reduce the MO molecules, further enhancing the degradation of MO. Thus, the MoS2/MMT displayed a much better catalytic effect for MO decomposition. 4. Conclusion MoS2 was successfully synthesized on self-assembly montmorillonite nanosheets to obtain a MoS2/MMT hybrid through in situ hydrothermal synthesis. In the synthesis, montmorillonite nanosheets self-assembled into a “house of cards” structure first, and then MoS2 nanosheets were synthesized along the surface of montmorillonite nanosheets. The MoS2/ MMT hybrid displayed a cross-link atypical grid structure, effectively hindering the aggregation of montmorillonite nanosheets and the for­ mation of MoS2 microspheres, resulting in much smaller particle size which enabled MoS2 exposed abundant active sites. The composition of MoS2 with montmorillonite nanosheets also made the hybrid exhibit a much strong hydrophilicity, which brought MoS2/MMT a better dis­ persibility in aqueous solution, resulting in an enhanced catalytic decomposition of MO in water. Furthermore, the hybrid owned excellent 7

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Materials Chemistry and Physics 246 (2020) 122829

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