Hierarchical MoS intercalated clay hybrid nanosheets with

Nano Research 2017, 10(2): 570–583 DOI 10.1007/s12274-016-1315-3

Hierarchical MoS2 intercalated clay hybrid nanosheets with enhanced catalytic activity Kang Peng1,2,§, Liangjie Fu1,2,§, Huaming Yang1,2,3 (), Jing Ouyang1,2, and Aidong Tang4 () 1

Centre for Mineral Materials, School of Minerals Processing and Bioengineering, Central South University, Changsha 410083, China Hunan Key Lab of Mineral Materials & Application, Central South University, Changsha 410083, China 3 State Key Lab of Powder Metallurgy, Central South University, Changsha 410083, China 4 School of Chemistry and Chemical Engineering, Central South University, Changsha 410083, China § These authors contributed equally to this work. 2

Received: 13 August 2016

ABSTRACT

Revised: 25 September 2016

Emerging hierarchical MoS2/pillared-montmorillonite (MoS2/PMMT) hybrid nanosheets were successfully prepared through facile in-situ hydrothermal synthesis of MoS2 within the interlayer of cetyltrimethylammonium bromide PMMT, and their catalytic performance was evaluated by the reduction reaction of 4-nitrophenol (4-NP) using NaBH4 as a reductant. Microstructure and morphology characterization indicated that MoS2/PMMT exhibited hybrid-stacked layered structures with an interlayer spacing of 1.29 nm, and the MoS2 nanosheets were intercalated within the montmorillonite (MMT) layers, with most of the edges exposed to the outside. The catalytic activity and stability of MoS2/PMMT were both enhanced by the MMT. With the MoS2/PMMT as the catalyst, the apparent reaction rate constant of the 4-NP reduction was 0.723 min−1 and was maintained at ~0.679 min−1 after five reaction cycles. The structural evolution of MoS2/PMMT and the possible catalysis mechanism for the reduction reaction of 4-NP were investigated. The as-prepared MoS2/PMMT hybrid nanosheets are promising candidates for catalytic application in the water-treatment and biomedical fields. The strategy developed in this study can provide insights for designing hybrid nanosheets with diverse heterogeneous two-dimensional (2D) nanomaterials.

Accepted: 7 October 2016 © Tsinghua University Press and Springer-Verlag Berlin Heidelberg 2016

KEYWORDS pillared montmorillonite, MoS2, hybrid nanosheets, hydrothermal synthesis, heterogeneous catalysis

1

Introduction

Since the initial exfoliation and identification of graphene, layered ultrathin two-dimensional (2D) nanomaterials have rapidly become some of the most

promising materials over the last decade [1, 2]. 2D nanomaterials have unconventional physical and chemical properties as well as potential applications in catalysis [3, 4], electronics [5], optoelectronics [6], environment remediation [7], and energy storage [8].

Address correspondence to Huaming Yang, [email protected]; Aidong Tang, [email protected]

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In addition to graphene, new 2D nanomaterials such as transition-metal dichalcogenides (TMDs) [9], transitionmetal oxide nanosheets [10], layered double hydroxides [11], and graphene analogues [12] have attracted worldwide attention and extensive research in recent years. Multifarious 2D hybrid nanosheets with novel structures and exceptional properties have also been designed and synthesized by various methods. As one of the typical layered TMDs, molybdenum disulfide (MoS2) has a sandwich interlayer structure formed by stacked nanosheets. A single nanosheet of MoS2 consists of covalently bonded three-atom (S–Mo–S) layers, and adjacent nanosheets are held together by weak van der Waals interactions. The 2D nanostructure of MoS2 crystal provides abundant active sites for catalysis, which originate from the under-coordinated sulfur atoms at the edges. Therefore, mono- or fewlayered MoS2 sheets exhibit higher catalytic activity than bulk MoS2, as they have more exposed edges. Various methods have been used to obtain few-layered MoS2 sheets, such as hydrothermal and solvothermal synthesis, modified liquid exfoliation [13, 14], chemical routes, and chemical vapor deposition. However, MoS2 nanosheets easily aggregate owing to their high surface energy, which results in deteriorated catalytic activity and stability. One effective solution is to load MoS2 nanosheets with support materials [15] such as carbon nanotubes [16], graphene [17], porous metals [18], or TiO2 nanofibers [19]. Montmorillonite (MMT) is a layered aluminosilicate mineral with a 2D sheet-like morphology. Monolayer MMT is composed of a central AlO4(OH)2 octahedral sheet and two SiO4 tetrahedral sheets connected via shared O atoms, having a total thickness of ~1 nm between the top and bottom O atoms. MMT typically consists of stacked sheets connected via positive ions with a unique combination of swelling, ion exchange, and intercalation, which makes it valuable as a building block for composite materials [20], catalyst supports [21, 22], adsorbents of inorganic and organic pollutants [23], templates in organic synthesis [24], and constituents of modified drug-delivery systems [25]. To improve the properties of MMT, the pillaring process is often performed, which involves exchanging the interlayer cations with organic or inorganic cations. Various mineral composite materials are used widely

in catalysis, energy storage, and wastewater treatment together with supporting minerals such as kaolinite [26–28], halloysite [29–34], attapulgite [35–37], talc [38], expanded perlite [39], and tailings [40–42]. MoS2 and most MoS2 composites are hydrophobic and cannot be dispersed in water. MMT—a hydrophilic mineral— might improve the dispersion of MoS2 in water, which is significant for its catalytic performance in the aqueous phase. Nitroaromatic compounds are widely used as synthetic intermediates in the manufacture of pharmaceuticals, fungicides, pesticides, plasticizers, and dyes and are regarded as water pollutants owing to their presence in industrial effluents [43]. 4-Nitrophenol (4-NP)—one of the most important nitroaromatics— has been listed as a “priority pollutant” by the US Environmental Protection Agency because it is highly soluble, has good chemical stability, and is not easily removed by natural degradation. Water polluted with 4-NP poses significant environmental and health risks, including eye and skin irritation, toxicity, and carcinogenicity. Therefore, it is increasingly important to effectively treat the 4-NP in waste water before the waste water is discharged into the environment. Among various treatment technologies, the catalytic reduction of 4-NP to 4-aminophenol (4-AP) has been most extensively studied [44, 45]. The reduction product 4-AP is less poisonous and an important intermediate for the synthesis of analgesic and antipyretic drugs such as paracetamol, phenacetin, and acetanilide. Owing to its easy measurement and complete conversion without byproducts [46], the catalytic reduction reaction of 4-NP to 4-AP with NaBH4 as a reductant is widely used as a model reaction for the evaluation of catalytic activity [47, 48]. In our previous study, MoS2 was synthesized on the surface of MMT by a facile hydrothermal method, and the composites exhibited high catalytic activity for a reduction reaction [49]. However, loading the catalyst on the surface of the matrix may introduce a risk of separation, which affects the stability of the catalyst. The stability can be enhanced by intercalating the catalyst into the layers. Therefore, in this study, we performed in-situ hydrothermal synthesis of MoS2/ pillared-MMT (MoS2/PMMT) hybrid nanosheets by intercalating MoS2 within the interlayer of MMT. The

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microstructure and morphology of the samples were characterized, and the catalytic activity and stability were evaluated by the reduction reaction of 4-NP using NaBH4 as a reductant. The influences of the MoS2/ PMMT concentration and MoS2 ratio on the catalytic performance were investigated. The structural evolution of MoS2/PMMT and the catalysis mechanism for the reduction reaction of 4-NP were explored and illustrated in detail.

2 2.1

Experimental Material preparation

MMT obtained from Zhejiang, China was purified to produce ultrafine clay powders (>97% Na-MMT). Cetyltrimethylammonium bromide (CTAB, C16H33(CH3)3NBr), sodium molybdate (Na2MoO4·2H2O), thioacetamide (CH3CSNH2), sodium borohydride (NaBH4), 4-NP (NO2C6H4OH), 4-nitroaniline (4-NA, NO2C6H4NH2), methyl orange (MO, C14H14N3SO3Na), and K3[Fe(CN)6] were purchased from Sinopharm Chemical Reagent Co., Ltd. MoS2/PMMT was prepared via in-situ hydrothermal synthesis of MoS2 within the interlayer of CTAB PMMT. In a typical preparation process, 1.000 g of MMT and 0.350 g of CTAB were added to 60 mL of deionized water, and the mixture suspension was mechanically stirred for 30 min at 60 °C. A PMMT suspension was obtained after the mixture suspension aged for 24 h at 60 °C. 1.210 g of sodium molybdate and 1.503 g of thioacetamide were added to the PMMT suspension. Then, the suspension was sonicated for 30 min at room temperature and stirred for 24 h at 50 °C. Finally, it was transferred into a 100-mL Teflon-lined autoclave and heated at 220 °C for 24 h. The precipitates were collected by centrifugation and subsequently washed with distilled water at least three times. After drying at 60 °C for 24 h, the final products were obtained. For comparison, pure MoS2 samples were prepared by a similar process, without MMT. Other samples were similarly prepared with different additive amounts of sodium molybdate and thioacetamide. A sample prepared with 0.605 g of sodium molybdate and 0.751 g of thioacetamide was labeled as MoS2/ PMMT-0.5. A sample with 2.420 g of sodium molybdate

and 3.006 g of thioacetamide was labeled as MoS2/ PMMT-2. 2.2 Characterization Powder X-ray diffraction (XRD) measurements of the samples were performed with a DX-2700 X-ray diffractometer using Cu Kα radiation (λ = 0.15406 nm) at a scan rate of 0.02 °/s. The Raman spectra of the samples were obtained using a Renishaw Micro-Raman System 2000 spectrometer at a spectral resolution of 2 cm−1. Scanning electron microscopy (SEM) was performed using a JEOL JSM-6360LV scanning electron microscope at an accelerating voltage of 5 kV, which was equipped with an energy-dispersive detector. High-resolution transmission electron microscopy (HRTEM), high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM), and elemental-mapping images were obtained using a JEOL JEM-2100F HRTEM system operating at 200 kV. N2 adsorption–desorption isotherms were recorded at 77 K and analyzed using an ASAP 2020 surfacearea analyzer. The Fourier transform infrared (FTIR) spectra of the samples were obtained using a Nicolet Nexus 670 FTIR spectrophotometer with KBr pellets. X-ray photoelectron spectroscopy (XPS) measurements were taken using an ESCALAB 250 spectrometer. Thermogravimetry (TG) and differential scanning calorimetry (DSC) analysis was conducted using a NETZSCH STA449C thermal analyzer at a heating rate of 10 °C/min in an air atmosphere. The ultraviolet– visible (UV–vis) diffuse-reflectance spectra were measured using a Shimadzu UV2450 UV–vis spectrophotometer, with barium sulfate as the reference. 2.3 Catalytic-activity evaluation The catalytic activity of the samples was evaluated with the reduction of 4-NP as a model reaction. In a typical catalytic experiment, 1 mL of catalyst was mixed with 50 mL of 4-NP solution (0.12 mmol/L), and then 0.136 g of NaBH4 was added to trigger the reduction reaction. The concentration of NaBH4 (72 mmol/L) was considered to be constant throughout the reaction. The reaction progress was monitored by measuring the absorbance of the peak located at 400 nm. The decoloration rate (%) was calculated according to the

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following formula: decoloration rate (%) = (A0 − A)/A0, where A0 is the initial absorbance, and A is the absorbance at homologous time. The stability of the catalyst was evaluated by a catalytic cycle test. At the end of each cycle, the suspension was centrifuged, and the catalyst was tested in the next cycle. The reduction reactions of 4-NA, MO, and K3[Fe(CN)6] were conducted under similar reaction conditions, and the MoS2/PMMT concentration was 1.0 g/L.

3

Results and discussion

The crystallographic structures of MMT, PMMT, MoS2, and MoS2/PMMT were inspected by XRD measurements, and their patterns are presented in Fig. 1(a). The XRD analysis revealed the layer distance of the samples, which is important information about the microscopic status of the intercalated molecules in the clay layer structure. The XRD pattern of MMT clearly displays a typical diffraction peak at 2θ = 7.26°, indicating that the d001 spacing of Na-MMT was 1.22 nm, as calculated by Bragg’s equation. For PMMT, the diffraction peak of (001) shifted to 4.08°, indicating that the d001 spacing of CTAB PMMT increased to 2.17 nm compared with Na-MMT. The interlayer spacing of the clay expanded along the c axis because of the intercalation of CTAB in the layer structure. The TG–DSC curves of MMT and PMMT (Fig. S1 in the Electronic Supplementary Material (ESM)) indicate that the mass content of CTAB in PMMT was ~15%. The diffraction peak of the layered MoS2 (002) plane

at 14.38° indicates a d002 spacing of 0.62 nm, and the characteristic diffraction peaks at 32.68° and 58.33° correspond to the (100) and (110) planes of 2H-MoS2. In the XRD pattern of the MoS2/PMMT, the diffraction peaks corresponding to the (100) and (110) planes of 2H-MoS2 are observed. However, the diffraction peak of the layered MoS2 (002) plane is not observed, indicating that the restacking of the MoS2 plane along the c-axis hardly occurred. The diffraction peak of the MMT (001) plane at 6.75° indicates a d001 spacing of 1.29 nm. This might be because under hydrothermal conditions, the CTAB was removed from the clay layer, and MoS2 was synthesized in the layer structure. Raman spectroscopy was used to gain further insights into the phase structure of the samples. The Raman spectra of MoS2 and MoS2/PMMT are shown in Fig. 1(b). The two dominant peaks of pure MoS2 at 380 and 406 cm−1 correspond to the in-plane E12g and out-of-plane A1g vibration modes of hexagonal MoS2, respectively. The Raman spectrum of MoS2/PMMT indicates that MoS2 was successfully obtained on account of the characteristic peaks located at 378 and 402 cm−1 due to the E12g and A1g vibrations of the Mo–S bonds, respectively. Generally, the interlayer van der Waals force in MoS2 suppresses atom vibration as the layer number increases, which leads to the blue shift of the A1g vibration mode. The peak for the A1g vibration mode of MoS2/PMMT was redshifted compared with that of bulk MoS2, indicating a far lower stacking height [50]. In addition, the peak spacing between E12g and A1g of MoS2/PMMT was smaller than that of bulk MoS2,

Figure 1 (a) XRD patterns of MMT, PMMT, MoS2, and MoS2/PMMT; (b) Raman spectra of MoS2 and MoS2/PMMT. www.theNanoResearch.com∣www.Springer.com/journal/12274 | Nano

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which suggests a decreasing number of MoS2 layers. As shown in Fig. S2 in the ESM, the peak of amorphous carbon is observed in the Raman spectrum of MoS2/ PMMT. The amorphous carbon might have partly transformed from CTAB under hydrothermal conditions. The general morphologies of the samples were investigated by SEM. The cross-section morphology of MMT (Fig. 2(a)) was a typical layered structure, and the MMT sheets 15–20 μm in width and 0.5–1 μm thick had smooth surfaces without contamination. The morphology of PMMT (Fig. 2(c)) was similar to that of MMT, comprising small stacked sheets. After the intercalation with CTAB, the sheets of PMMT were more separated, and the surface was smoother. The organic-intercalated modification affected both the interlayer space and the surface of MMT, which reduced the hydrophilicity of the material. As shown in Fig. S3(a) in the ESM, the pure MoS2 prepared by the hydrothermal route exhibited a three-dimensional (3D) sphere-like structure with a diameter of ~4 μm, which was comprised of nanosheets. As shown in the SEM image of MoS2/PMMT (Fig. 2(e)), the MoS2 nanosheets were intercalated in the layer structure of MMT, and most of the edges were exposed to the outside, which significantly increased the exposure of the active edge sites. The hydrothermal synthesis of MoS2 in the layer of MMT reduced the stacking of the MoS2 nanosheets. The drastic morphological diﬀerence indicates the important role of MMT for the nucleation and growth of MoS2. Energy-dispersive spectroscopy (EDS) (inset in Fig. 2(e)) was adopted to identify the elemental compositions of MoS2/PMMT. The EDS spectrum reveals that MoS2/PMMT is mainly comprised of Si K (18.28 wt.%), Al K (4.94 wt.%), Na K (0.78 wt.%), O K (30.03 wt.%), S K (16.72 wt.%), and Mo L (29.24 wt.%). TEM and HRTEM characterization was adopted to obtain further structural insights regarding the samples, which had been dispersed fully in ethyl alcohol and sonicated for 30 min. The dispersed MMT (Fig. 2(b)) exhibited a layered structure ~500 nm in width. The TEM image of PMMT (Fig. 2(d)) indicates that CTAB was intercalated into the layer of MMT and that part of the CTAB chain was exposed to the outside. As shown in Fig. S3(b) in the ESM, the pure MoS2 exhibited a small nanosheet, which is the basic unit of 3D

Figure 2 SEM images of (a) MMT, (c) PMMT, and (e) MoS2/ PMMT, the inset shows the EDS spectrum of MoS2/PMMT. TEM images of (b) MMT, (d) PMMT, and (f) MoS2/PMMT, the inset shows the HRTEM image of MoS2/PMMT.

sphere-like particles in the SEM image. The HRTEM image of MoS2 (inset in Fig. S3(b) in the ESM) clearly depicts the well-stacked layered structure of MoS2, with a lattice spacing of 0.62 nm, which corresponds to the (002) plane of hexagonal MoS2. As shown in Fig. 2(f), MoS2/PMMT exhibited 2D hybrid nanosheets. The MoS2 nanosheets had improved dispersibility due to the intercalation in the layer of MMT. The particle sizes of MMT and PMMT were obviously larger than that of MoS2/PMMT because the MMT particles in MMT and PMMT usually show mutual aggregation but were partly dispersed and delaminated upon the hydrothermal reaction. The HRTEM image of MoS2/ PMMT (inset in Fig. 2(f)) exhibits hybrid-stacked layered structures with an interlayer spacing of 1.29 nm. The HAADF-STEM image and elemental-mapping images of MoS2/PMMT show the alternant distribution of


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Si and S elements (Fig. S4 in the ESM), which is attributed to the hybrid stack of MMT and MoS2 hybrid nanosheets. To analyze the textural properties of MMT, PMMT, MoS2, and MoS2/PMMT, the specific surface area and pore-size distribution of the samples were measured using the N2 adsorption–desorption isotherms. The N2 adsorption/desorption isotherm curves of MMT, MoS2, and MoS2/PMMT (Fig. 3(a)) exhibit a type IV adsorption branch with a H3 hysteresis loop, which is characteristic of the mesoporous structure. However, the N2 adsorption/desorption isotherm curve of PMMT has almost no hysteresis loop, which may be because the interlayer space of MMT was filled with CTAB. The Brunauer–Emmett–Teller (BET) specific surface areas of MMT, PMMT, MoS2, and MoS2/PMMT were calculated to be 38.02, 39.70, 6.29, and 20.66 m2/g, respectively. Obviously, MoS2/PMMT had a higher specific surface area than pure MoS2. This may be because the MMT reduced the stack of the MoS2 nanosheets and prevented the self-assembly of nanosheets into a sphere. The pore-size distributions of MMT, PMMT, MoS2, and MoS2/PMMT were calculated using the Barrett–Joyner–Halenda (BJH) method (Fig. 3(b)). The average pore diameters of MMT, PMMT, MoS2, and MoS2/PMMT were 6.69, 6.73, 25.35, and 18.26 nm, respectively. Interestingly, there was a sharp peak at ~1 nm in the pore-size distribution of MoS2/ PMMT, which could be due to the hybrid stack of MoS2 and MMT. The vibrational bands and the specific interactions

of MMT, PMMT, MoS2, and MoS2/PMMT were investigated by FTIR analysis, as shown in Fig. 4(a). In the FTIR spectrum of MMT, the bending-vibration band of Si–O at 471 cm−1 and the stretching-vibration band of O–Si–O at 1,021 cm−1 indicate that the layered structure of MMT consisted of a silicon-oxygen tetrahedron. The obvious band at 3,620 cm−1 is ascribed to the –OH stretching vibration corresponding to the Al–O–H group. For the PMMT sample, the sharp bands at 2,919 and 2,850 cm−1 are ascribed to the C–H stretching vibrations of –CH2 and –CH3 from CTAB, respectively. The other vibration band in the FTIR spectrum of PMMT is similar to that of MMT. For pure MoS2, the band at 445 cm−1 corresponds to the stretching vibration of Mo–S. In the FTIR spectrum of MoS2/ PMMT, the main vibration bands of MMT and MoS2 are observed. The bands of the C–H stretching vibrations are obviously weakened, which might be because most of the CTAB transformed under hydrothermal conditions. XPS measurements were conducted to further investigate the surface elemental composition and the chemical status of MMT, PMMT, MoS2, and MoS2/ PMMT. As shown in Fig. 4(b), a wide survey scan of XPS spectra was performed in the range of 0–700 eV. The peaks of O, Al, and Si in MMT, the peaks of C, O, Al, and Si in PMMT, and the peaks of S and Mo in MoS2 were also clearly observed. In the XPS spectrum of MoS2/PMMT, the peaks of C, O, Al, Si, S, and Mo are observed. The atomic percentages of Mo and S on the surface of MoS2/PMMT were 5.64% and 9.95%,

Figure 3 (a) N2 adsorption/desorption isotherm curves and (b) BJH pore-size distributions of MMT, PMMT, MoS2, and MoS2/PMMT.


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Figure 4 (a) FTIR spectra and (b) XPS survey spectra of MMT, PMMT, MoS2, and MoS2/PMMT. High-resolution scans for (c) Mo 3d and (d) S 2p electrons of MoS2 and MoS2/PMMT.

respectively, which are almost consistent with the theoretical values for MoS2. As shown in the high-resolution spectra of Mo 3d (Fig. 4(c)) and S 2p (Fig. 4(d)), the binding energies of the Mo 3d3/2, Mo 3d5/2, S 2p1/2, and S 2p3/2 peaks in pure MoS2 were located at 233.1, 229.9, 163.9, and 162.8 eV, respectively. After the MoS2 was intercalated in the layer of MMT, the Mo 3d3/2, Mo 3d5/2, S 2p1/2, and S 2p3/2 peaks shifted to 232.3, 229.1, 163.4, and 161.9 eV, respectively, which are lower than the corresponding values for pure MoS2. The binding-energy shifts indicate electronic interaction between MoS2 and MMT. The interaction between MMT and MoS2 can enhance the structural stability of MoS2/PMMT and thus improve its catalytic stability. As shown in the high-resolution spectra of Si 2p (Fig. S5(a) in the ESM) and Al 2p (Fig. S5(b) in the ESM), the corresponding binding energies of PMMT shifted to a lower energy state com-

pared with those of MMT, and then the corresponding binding energies of MoS2/PMMT shifted back. This might be a result of the interlayer-spacing change of MMT [51]. As shown in Fig. S6(a) in the ESM, the MoS2 and MoS2/PMMT particles absorbed considerable amounts of visible light, whereas the MMT exhibited weak absorption of visible light. The bandgap energy (Eg) was estimated according to a plot of (αhν)2 versus the photon energy (hν), where α, h, and ν are the absorption coefficient, the Planck constant, and the frequency of the light, respectively. The bandgap energies of MMT, MoS2, and MoS2/PMMT were estimated to be 3.86, 1.21, and 1.57 eV, respectively (Fig. S6(b) in the ESM). MoS2/PMMT had a larger bandgap energy than MoS2, which might be because the layer space of MMT limited the increase of the layer numbers of the MoS2 nanosheets. The bandgap


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of the single-layer MoS2 nanosheet was ~1.85 eV [52]. The bandgap of MoS2/PMMT was between those of bulk MoS2 and single-layer MoS2, which is ascribed to the MoS2 in the interlayer and on the surface of the MMT. The reduction of 4-NP to 4-AP using NaBH4 as a reductant was employed to estimate the catalytic performance of the samples. As is well-known, the reduction of 4-NP with NaBH4 is thermodynamically feasible but kinetically hindered by the high activation barrier. It has become a model reaction for the evaluation of catalytic activity. The reaction is also a key step in the synthesis of analgesic and antipyretic drugs such as paracetamol. The progress of the reaction can be readily monitored by examining the variation of the reaction solution with UV–vis absorption spectroscopy. Figure 5(a) shows the decoloration performance of the samples with a catalyst

concentration of 1.0 g/L. The 4-NP was hardly discolored with the presence of MMT and PMMT, indicating that only physical adsorption of 4-NP occurred for MMT and PMMT. The absorption performance of PMMT was slightly higher than that of MMT, which is mainly attributed to the organic modification of MMT. In the presence of pure MoS2, the reduction reaction reached equilibrium after ~8 min, and the decoloration rate was 89.37% after 10 min. With MoS2/PMMT as the catalyst, the reduction reaction reached equilibrium after 5 min, and the decoloration rate reached 97.79% after 10 min. The MoS2/PMMT had higher catalytic activity than pure MoS2, which is attributed to the intercalation of MoS2 in the layer structure of MMT. The MMT reduced the stacking and aggregation of MoS2 nanosheets and enhanced the dispersibility of MoS2, which caused the MoS2/PMMT to have a large surface area and more reactive sites.

Figure 5 (a) Decoloration of 4-NP with MMT, PMMT, MoS2, and MoS2/PMMT. (b) UV–vis absorption spectra of the 4-NP solution during the reaction with MoS2/PMMT. (c) The decoloration of 4-NP. (d) The apparent reaction rate constant at different MoS2/PMMT concentrations. www.theNanoResearch.com∣www.Springer.com/journal/12274 | Nano

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Figure 5(b) shows the successive UV–vis absorption spectra of the 4-NP solution in the presence of excess NaBH4 with MoS2/PMMT as the catalyst. The UV–vis absorption spectrum of an aqueous 4-NP solution without NaBH4 exhibits a strong absorption band at 318 nm. After NaBH4 was added to the solution, the absorption band was redshifted to 400 nm, which is attributed to the deprotonation of 4-NP to the 4nitrophenolate anion with the increase in pH. The disappearance of the absorption band at 400 nm, accompanied by the appearance of a new absorption band at 298 nm, was monitored during the reduction reaction and suggests that the reduction of 4-nitrophenolate with NaBH4 yields 4-aminophenolate anions. Therefore, the time evolution of the absorbance at 400 nm can be measured to follow the reaction kinetics. To investigate the influence of the MoS2/PMMT concentration on the catalytic performance, decoloration tests were performed on 4-NP at different catalyst concentrations. As shown in Fig. 5(c), the catalytic performance improved with the increase of the MoS2/ PMMT concentration. The catalytic performance for a MoS2/PMMT concentration greater than 1.0 g/L is significantly higher than that for a concentration less than 1.0 g/L. The reduction reaction of 4-NP was observed to follow pseudo-first-order kinetics according to the following formula ln

Table 1

a

Ct   kt C0

(1)

where C0 is the initial concentration of 4-NP, Ct is the concentration of 4-NP at homologous time, and k is the apparent reaction rate constant. The time evolution of absorbance measured at 400 nm reflects the concentration of 4-NP. The calculated apparent reaction rate constants are shown in Fig. 5(d). The apparent reaction rate constant increased with the MoS2/PMMT concentration, and the typical k values obtained for the reduction reaction of 4-NP with MoS2/PMMT were on the order of 10−1 min−1. The value of k for a MoS2/ PMMT concentration of 1.0 g/L (0.723 min−1) was remarkably higher than that for a concentration of 0.5 g/L (0.334 min−1). The catalytic performances of MoS2/PMMT were compared with those of other catalysts reported in Refs. [53–61] (Table 1). MoS2/ PMMT showed excellent catalytic activity with the advantages of a low cost and a facile synthetic method. MoS2/PMMT exhibits excellent catalytic performance for the reduction of 4-NA, MO, and K3[Fe(CN)6] (Fig. S7 in the ESM), indicating that it is an efficient catalyst for reduction reactions in the aqueous phase. The stability and recyclability of MoS2/PMMT were evaluated by monitoring the reactivity of MoS2/ PMMT during five reaction cycles. As shown in Fig. 6(a), the catalytic performance of MoS2/PMMT hardly changed, and the decoloration rate of 4-NP reached 97.28% after five successive cycles. Figure 6(b) shows that the apparent reaction rate constant decreased little during the five reaction cycles, remaining at

Comparison of the catalytic performances of MoS2/PMMT with those of other catalysts reported in literature Catalysta

k (min−1)

Catalyst loading

Initial 4-NP

NaBH4

References

Co0.85Se-Fe3O4

0.393

1.0 mg

0.1 mL, 5 mM

1.0 mL, 0.02 M

[53]

MCA-Pd/Au

0.280

0.0875 mg

0.25 mL, 0.536 mM

0.25 mL, 47.1 mM

[54]

Ag/boron nitride

0.163

0.03 mL, 0.1 g/L

0.75 mL, 0.4 mM

0.75 mL, 0.4 M

[55]

Pt-Au/RGO

0.575

30 μL, 2g/L

2.7 mL, 0.1 mM

0.3 mL, 0.1 M

[56]

MoS2-Fe3O4/Pt

0.942

9 μg

20 μL, 10 mM

0.1 mL, 0.1 M

[57]

Fe3O4@MoS2

0.732

1 mL, 0.25 g/L

100 μL,1 mM

100 μL,100 mM

[58]

Graphene

0.057

2.5 mg

5 mL, 0.1 mM

14.5 mg

[59]

Gold nanorods

1.02

10 μL, 3 nM

60 μL, 5 mM

3 mL, 0.05 M

[60]

Ni/carbon black

0.139

2 mL, 0.5 g/L

50 mL, 0.5 mM

0.1 g

[61]

MoS2

0.235

1 mL, 1 g/L

50 mL, 0.12 mM

0.136 g (72 mM)

This work

MoS2/PMMT

0.723

1 mL, 1 g/L

50 mL, 0.12 mM

0.136 g (72 mM)

This work

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~0.679 min−1. MoS2/PMMT exhibited excellent catalytic stability, which might be because the MMT protected the MoS2 nanosheets within the interlayer from corrosion. MoS2/PMMT-0.5 and MoS2/PMMT-2 were synthesized to investigate the optimal ratio of MoS2, and the catalytic activities of the samples were evaluated via decoloration of 4-NP. In the XRD patterns of the samples (Fig. S8 in the ESM), the diffraction peaks corresponding to the MoS2 (100) and (110) planes of MoS2/PMMT-2 are higher than those of MoS2/PMMT-

0.5. The diffraction peaks corresponding to the MMT (001) plane of the three samples had no obvious shift. As shown in Fig. S9(a) in the ESM, MoS2/PMMT had the highest catalytic activity among the three samples. Figure S9(b) (in the ESM) shows that the apparent reaction rate constant of MoS2/PMMT (0.723 min−1) was far higher than that of pure MoS2 (0.235 min−1). Therefore, considering the catalytic activity and material cost, MoS2/PMMT was chosen as the preferred catalyst. As illustrated in Fig. 7(a), MoS2/PMMT was prepared

Figure 6 (a) Cycling runs for the decoloration of 4-NP with MoS2/PMMT, and (b) apparent reaction rate constants of cycling runs with MoS2/PMMT.

Figure 7 Schematic for (a) the synthesis of MoS2/PMMT and (b) the catalytic-reaction mechanism. www.theNanoResearch.com∣www.Springer.com/journal/12274 | Nano

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through in-situ hydrothermal synthesis of MoS2 within the interlayer of PMMT. First, Na-MMT was dispersed in an aqueous solution, and CTAB was added with mechanical stirring. Cetyltrimethylammonium (CTA+) cations replaced Na+ within the MMT layers by cationic exchange after aging for 24 h. Then, the obtained PMMT was fed with sodium molybdate and thioacetamide as the MoS2 initial material, which was adsorbed within the interlayer of PMMT through stirring for 24 h. Under hydrothermal conditions, most of the CTAB was decomposed, and the nucleation and growth of the MoS2 nanosheets proceeded in-situ within the interlayer of MMT. Finally, MoS2/PMMT hybrid nanosheets were successfully obtained with MoS2 intercalation in MMT. The interlayer of PMMT was modified by CTAB, which easily adsorbs MoO42−; therefore, a small amount of MoS2 was very likely to grow on the surface of the MMT. If the interlayer space is totally occupied by MoS2, the theoretical maximum mass ratio of MoS2/MMT is 1.96 (Table S1 in the ESM). The possible catalysis mechanism for the reduction reaction of 4-NP with MoS2/PMMT as a catalyst is schematically illustrated in Fig. 7(b). The BH4− hydrolyzed to produce active hydrogen atoms with the catalysis of MoS2, and the nitro of 4-NP was hydrogenated to amidogen by the active hydrogen atoms. As a result, the 4-NP was catalyzed to form 4-AP. As previously reported, the catalytic active sites of MoS2 were mainly located in the exposed edges. In the MoS2/PMMT hybrid nanosheets, the edge of the MoS2 nanosheets was exposed to the outside, and the basal plane of MoS2 was protected by MMT. Therefore, MMT enhanced both the catalytic activity and stability of MoS2/PMMT. The high adsorption property of MMT can form a higher apparent concentration of 4-NP around the catalyst, which further synergistically enhances the catalytic activity.

4 Conclusions MoS2/PMMT hybrid nanosheets were successfully prepared through in-situ hydrothermal synthesis of MoS2 in an interlayer of PMMT. The nucleation and growth of few-layered MoS2 nanosheets proceeded within the interlayer of MMT, and MoS2/PMMT exhibited hybrid-stacked layered structures with an

interlayer spacing of 1.29 nm. The MoS2 nanosheets were intercalated within the MMT layers, with most of the edges exposed to the outside. MoS2/PMMT had a large surface area and an interesting pore distribution at ~1 nm. The decoloration rate of 4-NP reached 97.79% after 10 min of reaction with MoS2/PMMT as the catalyst, and the apparent reaction rate constant reached 0.723 min−1. MoS2/PMMT exhibited excellent catalytic stability and recyclability after five reaction cycles. MMT enhanced both the catalytic activity and stability of MoS2/PMMT by dispersing MoS2 with exposed edges and protecting the MoS2 nanosheets from corrosion. The as-prepared MoS2/PMMT hybrid nanosheets are promising candidates for catalytic applications in the water-treatment and biomedical fields.

Acknowledgements This work was supported by the National Science Fund for Distinguished Young Scholars (No. 51225403), the National Natural Science Foundation of China (No. 41572036), the State Key Laboratory of Powder Metallurgy, Central South University (2015-19) and the Hunan Provincial Science and Technology Project (No. 2015TP1006). Electronic Supplementary Material: Supplementary material (calculation of mass ratio between MoS2 and motmorillnite, TG–DSC curves of MMT and PMMT, Raman spectrum of MoS2/PMMT, SEM image and TEM image of MoS2, HAADF-STEM image and elemental mapping images of MoS2/PMMT, high-resolution scans for Si 2p and Al 2p electrons of MMT, PMMT and MoS2/PMMT, UV–vis diffuse reflectance spectra of MMT, MoS2 and MoS2/PMMT, decoloration of 4-NA, MO and K3[Fe(CN)6] with MoS2/PMMT, XRD patterns, decoloration and the apparent reaction rate constants of MoS2/PMMT-0.5 and MoS2/PMMT-2) is available in the online version of this article at http://dx.doi.org/ 10.1007/s12274-016-1315-3.

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