CuS Nanosheet Composites as an Efficient and Ultrafast Adsorbent for

Article Cite This: J. Chem. Eng. Data XXXX, XXX, XXX−XXX

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Facile Synthesis of MoS2/CuS Nanosheet Composites as an Efficient and Ultrafast Adsorbent for Water-Soluble Dyes Chengxiang Tian,† Xia Xiang,*,† Juwei Wu,† Bo Li,† Chao Cai,‡ Bilawal Khan,† Hua Chen,§ Yonggang Yuan,*,§ and Xiaotao Zu‡ †

School of Physics, University of Electronic Science and Technology of China, Chengdu, 610054, China Institute of Fundamental and Frontier Science, University of Electronic Science and Technology of China, Chengdu, 610054, China § Institute of Nuclear Physics and Chemistry, China Academy of Engineering Physics, Mianyang, 621900, China Downloaded via KAOHSIUNG MEDICAL UNIV on September 29, 2018 at 23:27:58 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

‡

ABSTRACT: This study demonstrates quick and efficient removal of different dyes from wastewater by using MoS2/ CuS nanosheet composites (NCs) as adsorbent. The MoS2/ CuS NCs are prepared by a facile hydrothermal route, and the composites exhibit high adsorption capacity with 273.23, 432.68, 98.78, and 211.18 mg/g for rhodamine B (RhB), methylene blue (MB), methyl orange (MO), and rhodamine 6G dyes (RhB 6G), respectively. This is ascribed to its high specific surface area (106.27 m2/g) and small mesopores (2.299 nm) which provide numerous adsorption sites and uniform coverage for dye molecules. High adsorption efficiency is obtained for RhB (93.8%), MB (100%), and RhB 6G (84.73%), except for MO (48.9%) at the adsorption equilibrium time at the solution concentration of 80 mg/L. The adsorption of MoS2/CuS NCs can be well described by the pseudo-second-order kinetic model, and the adsorption isotherm at the equilibrium fits well with the Langmuir model. The rapid and efficient adsorption ensures MoS2/CuS NCs to be a broadspectrum adsorbent for different dye contaminants in water.

1. INTRODUCTION Today, dyes play a critical role in textile, paint, and pigment manufacturing industries, and at least 100 000 different dye types are currently commercially available.1,2 Dye contamination has become an inevitable issue for human development.3 For example, rhodamine B (RhB) is representative of dyes used extensively in the textile, food, and photographic industries.4 Methyl blue (MB) has wide applications in the coloring paper, temporary hair colorant, dyeing cottons, and coating for paper stock.5 However, some dyes are stable to light and heat and hence not easily degradable, and are major water pollutants.6,7 Their discharge into water can cause many environment problems related to their toxicity to aquatic life and human carcinogenicity, etc. So the removal of synthetic dyes from water has become a big challenge.8,9 Therefore, the development of techniques for the removal of dyes from wastewater is necessary and urgent.10 In recent years, the removal of dye pollutants in wastewater has also aroused concern.11 At present, several physical and chemical techniques such as chemical oxidation,12 electrochemical oxidation,13 photocatalytic oxidation,14 biodegradation,15 coagulation16 and so on, have been developed to treat dye-contaminated wastewater. Among these methods, adsorption technology is considered as a very promising method due to its easy operation and high efficiency, and it has been developed to remove different dyes from the aquatic environment.17 © XXXX American Chemical Society

Many kinds of conventional adsorbents have been reported, such as activated carbon,18 natural minerals,19 zeolites,20 and nanocomposites.21,22 Nevertheless, there are still some limitations in the complex procedures involved in the high cost and low adsorption capacity for some adsorbents. Therefore, to develop a simple and effective adsorbent is still a tremendous challenge.23 Recently, mesoporous nanomaterial has attracted wide attention and achieved good results21 in wastewater treatment owing to its high surface area, porosity, and chemically adjustable properties.24−26 As a result, nanomaterial is in demand for efficient and cost-effective techniques to remove dyes. MoS2 is a layered graphene-like transition metal chalcogenide and has been widely used in the fields of catalysis, energy,27 environmental pollution, and so on.28,29 MoS2 has a high specific surface area and numerous adsorption active sites, and exhibits immense potential for environmental applications. Meanwhile, the adsorption properties of MoS2 with different morphologies were studied.9,17,30−32 For example, the functional MF@ MoS2 sponges showed high discoloration efficiency of 98% methyl orange within 10 min.33 Wang et al. found that the flower-like MoS2 had a maximum adsorption capacity of about 49.2 mg/g for RhB in aqueous solutions.17 Li Received: July 9, 2018 Accepted: September 18, 2018

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DOI: 10.1021/acs.jced.8b00593 J. Chem. Eng. Data XXXX, XXX, XXX−XXX

Journal of Chemical & Engineering Data

Article

X-ray photoelectron spectra (XPS) of the composites were measured using a Kratos XSAM 800 system with an Al Kα Xray photoelectron spectrometer. Fourier transform infrared spectra (FTIR) were recorded via a Nicolet 6700 spectrometer. The ultraviolet−visible (UV−vis) diffuse reflectance spectra were recorded on a UV−visible spectrophotometer (TU-1901). 2.3. Adsorption Behavior Measurements of Dyes. In this work, the adsorption experiments were carried out in a beaker at room temperature. The adsorption behavior measurement of dyes was conducted according to the following procedure: 0.02 g of as-prepared MoS2/CuS nanocomposites was dispersed into 50 mL of RhB solution with different concentrations, that is, 30, 40, 50, 60, 70, 80, 100, 110, 130, 160, 180, 200, and 220 mg/L, respectively. Then the solution was stirred vigorously with a magnetic stirrer. The RhB solutions were taken out at different time intervals (10, 20, 30, 60, 120, 240 min) and immediately centrifuged at 12000 rpm for 3 min. Finally the suspension was collected. The concentrations of RhB were analyzed at 553 nm with a UV−vis spectrometer. The adsorption capacity and mechanism are calculated by the following equations:

et al. synthesized porous MoS2 nanosheets for RhB dye removal, and the maximum adsorption capacity reached 163.0 mg/g.32 Mesoporous transition metal sulfides with hollow structure and easy to control morphology have also attracted much attention in adsorption and catalysis. Consequently, the adsorption properties of CuS with nanoparticles and microspheres were studied.34−36 For instance, Hamed and his coworkers synthesized CuS nanoparticles loaded on the activated carbon, and the adsorption capacity for MB is 208.3 mg/g.36 Liu et al. synthesized CuS microspheres for removal of MB, and the maximum adsorption capacity was 19.23 mg/g.35 Since both MoS2 and CuS have wide applications in the field of dye adsorption, can the two materials be combined together to enhance the adsorption capacity in the premise that the structure and morphology are not broken? Perhaps the synergistic effect between MoS2 and CuS will lead to better adsorption capacity if the original high specific surface area can be retained. To our knowledge, there has no report on the adsorption performance of MoS2 and CuS composite. In this work, we first report the MoS2/CuS nanosheet composites (NCs) synthesized by facile one-pot hydrothermal method, and discuss the adsorption kinetics and adsorption mechanism of MoS2/CuS NCs for RhB dyes in detail. To prove high efficiency and capacity to dyes adsorption, representative anions and cationic dyes RhB, MB, MO, and RhB 6G have been studied, and the results show the excellent adsorption capacity of MoS2/CuS NCs. The results show that the composites are a promising adsorbent to deal with dye sewage.

Q t = (C0 − Ct )V /W

(1)

Er = 100 × (C0 − Ct )C0

(2)

where Qt (mg/g) is the adsorption capacity at different times, C0 (mg/L) is the initial concentration of the dye solution, Ct (mg/L) is the concentration of the dye solution at time t of adsorption, V (L) is the volume of the dye solution, and W (g) is the weight of the adsorbent. Er is the dye adsorption efficiency. The Langmuir isotherm model is represented by the following equation:

2. EXPERIMENT 2.1. Synthesis of MoS2/CuS NCs. The MoS2/CuS NC samples were prepared by a simple hydrothermal method. In a typical experiment, 2 g of hexaammonium heptamolybdate tetrahydrate ((NH4)6Mo7O24·4H2O), and 1.4 g of copper nitrate (Cu(NO3)2) were dissolved in 20 mL of deionized water. Then 1 g of sublimed sulfur was added to the above solution and stirred for 30 min followed by ultrasonic dispersion for 1 h. After that, 15 mL of hydrated hydrazine (N2H4·H2O) was slowly dropwise added, and the solution was transferred to the ultrasound machine for 1 h. Finally, the solution was transferred into a 50 mL Teflon-lined autoclave, which was kept in an oven at 200 °C for 24 h. After cooling to room temperature in an oven naturally, the black precipitate was collected by centrifugation and washed in sequence with diluted hydrochloric acid, water, and ethanol for three times, respectively, and finally dried in air at 60 °C for 12 h. To compare the adsorption capacity of the MoS2/CuS NCs with the individual MoS2 and CuS samples, the individual MoS2 and CuS samples were synthesized using the same conditions. The only difference for MoS2 and CuS is the dosage of 2 g (NH4)6Mo7O24·4H2O and 0.81 g of sublimed sulfur, and 1.4 g of Cu(NO3)2, and 0.89 g of sublimed sulfur, respectively. 2.2. Characterization. A powder X-ray diffraction (XRD) was conducted to determine the phase of the as-synthesized composites, with Cu Kα radiation operated at 40 kV and 30 mA. The morphologies and microstructures of the composites were characterized by employing a field emission scanning electron microscope (SEM, Hitachi S-4800) and a transmission electron microscope (TEM, ZISSLibra 200). Nitrogen adsorption−desorption isotherm measurements were conducted at 77 K using a micromeritics system (JW-BK132F).

Q e = Q mKLCe/(1 + KLCe)

(3)

The adsorption parameters of the Langmuir equation are determined by converting to a linear equation: Ce C 1 = e + Qe Qm KLQ m

(4)

The Freundlich isotherm model is represented by the following equation: Q e = K f Ce1/ n

(5)

The adsorption parameters of the Freundlich equation are determined by converting to a linear equation: ln Q e = ln K f +

1 ln Ce nf

(6)

Here, Ce is equilibrium concentration of adsorbate (mg/L), Qm is the maximum monolayer adsorption capacity (mg/g). KL and Kf are the Langmuir and Freundlich isotherm constant (L/ mg), respectively. The pseudo-first-order and pseudo-second-order adsorption models are expressed by following equations: Q t = Q e(1 − e−k1t )

ij yz 1 zz Q t = Q ejjjj1 − zz 1 + k Q t 2 e { k B

(7)

(8) DOI: 10.1021/acs.jced.8b00593 J. Chem. Eng. Data XXXX, XXX, XXX−XXX


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where Qe and Qt are the adsorption amounts of the dye at equilibrium time and at time t (min), respectively; k1 and k2 are the pseudo-first and the pseudo-second-order adsorption rate constant (g mg−1 min−1), respectively.

of MoS2 and CuS in Figure 1c and 1d clearly reveal the individual morphology of MoS2 and CuS, respectively. The low magnification TEM image (Figure 1e) shows the intercrossed nanosheets with dense and smooth surfaces of MoS2/CuS NCs, and indicates the evidence of the structure formed by thin nanosheets. Figure 1f shows the high resolution TEM images of MoS2/CuS, where the stripes appear in the micrographs to allow identification of CuS and MoS2, indicating the CuS particles or clusters randomly dispersed on the MoS2 nanosheets. The interplanar spacing of 0.271 nm corresponds to the (006) plane of CuS, while the spacing of 0.227 nm corresponds to (103) plane of MoS2, which is consistent with the XRD results. Furthermore, as shown in Figure 2a, the STEM (scanning transmission electron microscopy) image and the correspond-

3. RESULTS AND DISCUSSION 3.1. Material Characterization. The crystal structure of the obtained composites was investigated by X-ray diffraction (XRD). Figure 1a shows the XRD pattern of the MoS2/CuS

Figure 2. (a) The DF-STEM image corresponding to STEM-EDS mapping images reflects S, Cu, and Mo atom distributions. XPS spectra of (b) Mo 3d, S 2s, (c) S 2p, (d) Cu 2p of the MoS2/CuS NCs.

ing energy dispersive spectrometer (EDS) mapping images indicate the homogeneous distribution of Cu, Mo, and S elements on the whole ultrathin nanosheets. The uniform distribution of each element reveals the successful synthesis of MoS2/CuS NCs heterogeneous structure. The chemical composition of the MoS2/CuS NCs was analyzed in detail via X-ray photoelectron spectroscopy (XPS). XPS is a technique which is versatile to surface analysis and can be used for compositional and chemical state analysis. It can be seen in Figure 2b that the Mo 3d spectrum consists of peaks at around 229.17 and 232.41 eV that correspond to the Mo4+ 3d5/2 and Mo4+ 3d3/2 components of 2H-MoS2, respectively (dark cyan line). The binding energy is in good agreement with the reported value for the MoS2 crystal.39 However, XPS spectra show additional peaks located at 228.49 and 231.74 eV (blue line in Figure 2b). These peaks reveal additional peaks shifted to lower binding energies by ∼0.7 eV with respect to the position of the 2H-MoS2 peaks. Moreover, there are only two composites, MoS2 and CuS, in the samples deduced from the results of XPS because the curve of S 2p can be only decomposed into two peaks, as shown in Figure 2c. In addition, peaks at 161.2 and 162.3 eV are found besides the known doublet peaks of 2H-MoS2, which appear at 163.2 and 161.9 eV, respectively. This is similar to those reported for single crystal MoS2 progressively intercalated by Li where

Figure 1. (a) XRD pattern of MoS2/CuS NCs, MoS2 and CuS, (b, c, and d) SEM image of the MoS2/CuS NCs, MoS2, and CuS, respectively, and (e and f) HRTEM images of MoS2/CuS NCs.

NCs, MoS2, and CuS. The peaks at ∼14.3°, 39.5°, 49.7°, and 32.6° (red vertical line) are clearly indexed to the MoS2 (JCPDS card no. 37-1492), and correspond to the (002), (103), (105), and (100) crystal planes of the MoS2, respectively. Four peaks located at ∼31.7°, 47.9°, 29.2°, and 32.8° (black vertical line) correspond to the diffraction from the (103), (110), (102), (006) planes of CuS (JCPDS card No. 06-0464). Also, the crystallization of MoS2 and CuS is similar to the previous reports.34,37,38 The slight offset of the (002) peak position of the MoS2 crystal plane indicates a slight increase in the spacing of the layers, which is possibly caused by the introduction of copper sulfide. The strongest peak at ∼32.8° is superimposed by the (103) plane of CuS and (100) plane of MoS2. The microstructure of the MoS2/CuS NCs is explored by SEM and TEM analysis. The SEM image in Figure 1b clearly reveals the ultrathin nanosheet morphology. Meanwhile, the MoS2/ CuS NCs possess hierarchically porous vertically aligned structure, composed of small nanosheets. The SEM images C



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Figure 3. (a) FTIR spectra of MoS2/CuS NCs before and after adsorption of RhB, and origin RhB given for reference; (b) enlarged image of the zone marked in panel a.

additional peaks appear due to formation of 1T phase.40,41 The binding energies of the S 2p region show a presence of two peaks at 162.45 and 163.58 eV of 2H-MoS2, and these peak positions are well consistent with literature.42,43 The intensity ratios are taken into account in the peak fitting, where the intensity ratios of 1T to 2H peak are ∼1.19:1 for the highresolution Mo 3d and S 2p spectra, which indicates that the ratios of two different phases of MoS2 (1T/2H ≈ 1.19/1) are the same. The experimental binding energies for Cu 2p3/2 and Cu 2p1/2 are 932.49 and 952.47 eV, respectively, which are well consistent with the values (932.1−932.9 eV for Cu 2p3/2 and 952.4−952.8 eV for Cu 2p1/2) for monovalent CuS, Cu2S, and Cu from the NIST XPS database and recent reports.44−49 Because the difference in binding energy is small between Cu(I), Cu(II), and Cu, the peak position and peak shape are usually considered to make a comprehensive analysis. The values of Cu 2p3/2 and Cu 2p1/2 are clearly separated by 19.98 eV, and there are shakeup satellite peaks in the region from 936 to 946 eV, indicating the absence of Cu(I) and their oxides. In addition, the symmetrical shapes of the two Cu 2p XPS peaks and the “shakeup” satellite peaks in the higher binding-energy ranges imply the presence of pure CuS. The presence of two strong peaks separated by 20.0 eV are essentially identical binding energies for the Cu 2p orbital in accord with Cu(II), and the XPS spectrum of S 2p in Figure 2c is characterized by a pair of peaks at 162.45 and 163.58 eV, which are attributed to the S2− species, and all these peak positions are well matched with literature.43,50−55 Moreover, the symmetrical shape of the two Cu 2p XPS peaks also indicates the presence of only CuS.56 Therefore, we believe that the two peaks of the composite material are CuS. Further, the analysis of XRD and TEM further confirmed the analysis results of XPS. FTIR spectroscopic analysis is performed to study the adsorption mechanism of MoS2/CuS NCs. The infrared spectra of MoS2/CuS NCs before and after adsorption and RhB are shown in Figure 3. The new peaks at about 1588 and 1336 cm−1 can be observed in the spectra of the MoS2/CuS NCs after adsorption of RhB, which are characteristic of in-ring C−C and C−H stretching vibration in the aromatic ring of RhB, respectively.17,57 The peak at 1336 cm−1 has a slight blue shift compared to the peak of RhB (Figure 3b) due to introduction of the MoS2/CuS NCs. The characteristic peaks of raw RhB can easily be found in the composites after

adsorption, which directly proves that adsorption plays a conclusive role in this reaction rather than decomposition. 3.2. Adsorption Mechanism. To illustrate the rapid adsorption properties of MoS2/CuS NCs, 20 mg composites were added into 50 mL of RhB solutions with concentrations of 80, 70, 60, 50 mg/L, respectively, and the adsorption efficiency of RhB at different time intervals was recorded in Figure 4a. The adsorption efficiency reaches 79.84%, 81.36%, 87.58%, and 95.09% within 10 min for concentrations of 80, 70, 60, and 50 mg/L, respectively.

Figure 4. (a) Adsorption efficiency curves of 20 as-prepared MoS2/ CuS NCs in 50, 60, 70, 80 mg/L and 50 mL of RhB solutions. The inset photo is the comparison before and after adsorption indicating the solution becomes clear within 2 min. (b and c) Experimental data and red fitting curves according to Langmuir and Freundlich models, respectively. (d) Adsorption kinetic curves of RhB at 50, 60, 70, and 80 mg/L, and the green and red curves are drawn according to pseudo-first- and pseudo-second-order models, respectively.

The adsorption efficiency is an important parameter that characterizes the performance of the adsorbent. Hence, 20 mg composites were added to 50 mL of RhB solution at an initial concentration of 30 mg/L in order to directly observe the rapid adsorption efficiency. After 2 min, the RhB was rapidly removed from the solution and the red color of the solution disappeared completely, as shown in the inset of Figure 4a. The time of adsorption equilibrium can be achieved in ∼30 min, which is significantly quicker than previously reported in D



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which the tungstate oxide nanourchins reached adsorption equilibrium at ∼100 min.58 To describe the adsorption process and investigate the mechanism of adsorption, two adsorption isotherm models, namely, Langmuir and Freundlich isotherms, were applied to fit the experimental data.59 The Langmuir isotherm model demonstrates the monolayer coverage of adsorbate onto the adsorbent surface,60,61 while the Freundlich isotherm indicates the presence of heterogeneous adsorption surfaces and variable adsorption sites with different energies of adsorption.61,62 Figure 4 panels b and c show the fitting curve of the adsorption amount of RhB by using the Langmuir model and Freundlich model, respectively, indicating that the Langmuir model can better fit the experimental data than the Freundlich model. The detailed calculated results are listed in Table 1

According to eqs 7 and 8, the kinetic parameters determined by the least-squares fitting are listed in Table 2 and shown in Figure 4d. When the RhB concentration is relatively low, the adsorption rate of the dye is very fast. Figure 4a shows fast adsorption efficiency of the composites. The time parameter is a major influence factor for the first-order and second-order fitting formula. The RhB is almost completely removed in a short period of time at lower concentrations. The time parameters have little effect on the fitting results, so the difference is hardly shown for the low concentration fitting. This also explains why the difference between the fitting results becomes more pronounced as the concentration increases. It is noteworthy that the second-order fitting coefficients are greater than the first-order fitting coefficients with the increasing concentration. As shown in Table 1, the deviation between the first-order fitting results and the experimental data is significantly higher than that of the second-order fitting coefficient. The second-order kinetic fitting results of different concentrations are closest to the experimental data, with deviation mostly less than 1. The results reveal that adsorption kinetics is well fit by the pseudo-second-order adsorption model, and the fitting result is similar to the reference.31,36 As described in the literature,61,63 the pseudo-second-order model is more suitable to describe the adsorption process, which indicates a rate-limiting adsorption through sharing or exchanging electrons between the adsorbent and adsorbate. To compare the adsorption capacity of the MoS2/CuS NCs with the individual MoS2 and CuS samples, the experiments are completed. The adsorption of dyes is mainly contributed by the specific adsorption sites, specific surface area, and morphology of the adsorbents. It was found that the adsorption amount of MoS2 on various dyes was significantly higher than that of CuS, as shown in Figure 5a. This result shows that the adsorption of dyes is mainly attributed to MoS2 nanosheets. Because of the noncentrosymmetric Mo-2S structure on the (100) plane, the edge active sites of MoS2 nanosheets possess an inherent polarization and then can interact with some polar molecules.64−66 Meanwhile, the introduction of the CuS leads to the production of some 1T phase of the MoS2 and expansion of the interlayer space between the MoS2 sheets, which also produces the active sites, further improving the adsorption capacity of the MoS2/CuS NCs. Therefore, the MoS2 nanosheets possess an excellent adsorption capacity and a fast adsorption rate toward dyes with the abundant edge active sites and a multilayer adsorption process. Moreover, these sheets provide sufficient pores between the neighboring nanosheets, which can facilitate rapid diffusion of molecules leading to outstanding adsorption performance.

Table 1. Langmuir and Freundlich Isotherm Parameters for Adsorption of RhBa parameter 2

R K Qec (mg/g) Qexpm (mg/g)

Langmuir

Freundlich

0.999 0.661 276.24 273.23

0.979 10.416 176.37

a

Qec is the maximum adsorption amount of the dye at equilibrium time calculated by eqs 6 and (8), respectively. Qexpm is the maximum experimental adsorption amount of the dye. The R2 is the fitting coefficient corresponding eqs 6 and (8), respectively.

based on the two above models. The calculated adsorption results of RhB, adsorption amount Qec (276.24 mg/g) and linearly fitting R2 (0.999), are well consistent with the experimental maximum adsorption amount Qexpm (273.23 mg/g) when the Langmuir isotherm model is applied. However, the calculated adsorption amount Qec (176.37 mg/ g) is significantly lower than the experimental data with linearly fitting R2 (0.979) when the Freundlich isothermal model is applied. The rapid adsorption at initial time is attributed to the fact that the dye is well and uniformly attached to the active site exposed to the adsorbent. Subsequently, the slower adsorption efficiency is because that adsorption and desorption reach a relative dynamic equilibrium between RhB molecules and MoS2/CuS NCs. The adsorption process of most nanomaterials follows the second order kinetic model and the Langmuir mechanism. To study the mechanism of adsorption, the adsorption is found to attain equilibrium within 240 min for 80, 70, 60, and 50 mg/L RhB solution. In fact, the adsorption−desorption equilibrium has almost arrived as early as in 30 min, which is fully acceptable for application of wastewater treatment as reported by Xiao et al.61

Table 2. Kinetic Parameters for Adsorption of RhB onto the MoS2/CuS NCsa pseudo-first-order

pseudo-second-order

RhB (mg/L)

Qec1 (mg/g)

K1 (g mg−1 min−1)

R21

Qec2 (mg/g)

K2 (g mg−1 min−1)

R22

Qexp (mg/g)

80 70 60 50

181.96 165.23 143.69 122.82

0.1934 0.1856 0.2320 0.3404

0.989 0.993 0.994 0.999

189.29 171.73 147.89 124.18

0.0025 0.0027 0.0048 0.0166

0.999 0.999 0.999 0.999

188.96 170.50 147.83 124.32

a

Qec1 and Qec2 are the adsorption amounts of the dye at equilibrium time calculated by eqs 3 and 4, respectively. Qexp is the experimental adsorption amount of the dye. R21 and R22 are the fitting coefficients corresponding eqs 3 and 4, respectively. E



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Table 3. Comparison of Adsorption Capacity of Different Adsorbents for RhB and MB Dyes dye

adsorbent

RhB

Fe3O4/MoS2 nanocomposites flower-like MoS2 MoS2/MIL-101 hybrid MoS2 nanosheets MoS2 nanosheets MoS2 sponges MoS2-glue sponge MoS2/CuS nanosheets (this work) CuS microspheres tomato waste CuS nanoparticles activated carbons PKN2 MoS2 nanosheets activated carbon by activation with sulfuric acid MoS2 nanosheets MoS2/CuS nanosheets (this work)

MB

Figure 5. Adsorption amount of RhB, MB, MO, RhB 6G by (a) MoS2 and CuS (b) MoS2/CuS NCs; (inset photo) comparison before and after adsorption of MB. (c) N2 adsorption−desorption isotherm of MoS2/CuS NCs; (inset photo) pore size distribution curves. (d) Removal efficiency of RhB after repeated adsorption−desorption cycles at C0 = 150 mg/L.

To further confirm the adsorption ability of MoS2/CuS NCs, the adsorption of RhB, MB, RhB 6G and MO were also conducted in the same condition. Figure 5b show the specific adsorption to the four dyes, RhB, MB, RhB 6G, and MO, respectively. Their maximum adsorption values were 276.24 432.68, 211.18, and 98.78 mg/g, respectively. It is found that 50 mL of 80 mg/L MB can be completely removed by 20 mg composites within 10 min, showing the rapid adsorption of MB on the MoS2/CuS NCs. The detailed adsorption results are shown in the inset of Figure 5b. The pore size distribution and specific surface area of MoS2/ CuS NCs are shown in Figure 5c, and a type-IV isotherm with a type-H3 hysteresis loop in the relative pressure range of 0− 1.0 P/P0 indicates the presence of mesoporous structure. Accordingly, the specific surface area is calculated to be 106.27 m2/g via the Brunauer−Emmett−Teller (BET) method. Meanwhile, the pore volume calculated by the Barret-Joyner Halenda (BJH) method is 0.324 cm3/g. The pore size distribution is derived from the BJH method as shown in the inset of Figure 5c. The mesoporous of MoS2/CuS NCs have an average pore size of 7.654 nm with the most possible pore size distribution at 2.299 nm. The overall distribution of pore size indicates that the mesoporous is homogeneous with a narrow pore size distribution. To investigate the recycle performance of MoS2/CuS NCs as a kind of adsorbent, the RhB solution of C0 = 150 mg/L was chosen to perform the adsorption−desorption cycle experiment. After adsorbing RhB, MoS2/CuS NCs were washed with deionized water and alcohol, respectively, and the regenerated samples were used to adsorb the dyes again. As shown in Figure 5d, the removal efficiency of RhB decreases slightly from almost 100% to 93% after six cycles. These results indicate that MoS2/CuS NCs is a reusable adsorbent. Table 3 lists the adsorption capacity of RhB and MB by different adsorbents, indicating that MoS2/CuS NCs can be effectively used for the removal of dyes from aqueous solution, in which its adsorption capacity is higher than those reported for many of the known adsorbents. In particular, the adsorption capacity of MoS2/CuS NCs to MO and RhB 6G is also higher than many materials, indicating that MoS2/CuS

specific surface area (m2/g)

adsorption capacity Qexpm(mg/g)

72.07

249

18.68 2903 76.85 106.98 26.38

49.217 344.864 16032 7630 213.8466 127.3965 276.24

106.27

925 106.98 1200

19.2335 40078 1534 76579 7930 43580

106.27

29731 432.68

1093

NCs could be used as an exceptional adsorbent toward various dyes. The Qe (RhB 6G) value is slightly lower than that reported for titanium phosphate (217.39 mg/g)67 and is considerably higher than those (16−105 mg/g) reported for dyes adsorbed by several elaborated adsorbents.68−72 Meanwhile, the adsorption capacity of MO is much higher than that of a hyper-cross-linked polymeric adsorbent functionalized with formaldehyde carbonyl groups73 (76 mg/g), multiwalled carbon nanotubes74 (42−78 mg/g), and λ-Fe2O3/multiwalled carbon nanotubes (MWCNTs)/chitosan composite (60.5− 66.1 mg/g).75 The specific surface area of MoS2 is basically higher than that of other kinds of MoS2 or its composites, except that of activated carbon materials. When the CuS nanoparticles are loaded on the MoS2 nanosheets and CuS and MoS2 keep their original structure and morphology as shown in Figure 1, the adsorption capacity is better than those of separate CuS naonparticles and MoS2 nanosheets. This is because that the introduction of CuS increases the active sites of MoS2. Therefore, MoS2/CuS NCs with a high specific surface area will provide more adsorption pores and channels for adsorption. The results promise our composite a huge commercial potential in wastewater treatment due to its excellent adsorption efficiency to different dyes. The interaction between adsorbate and adsorbent is important to control the efficacy and selectivity of adsorbent. As reported in the literature,31,76,77 the higher adsorption capacity for cationic dyes is attributed to the combined effect of the van der Waals and the electrostatic interactions between the MoS2/CuS NCs and cationic dyes. Moreover, lower molecular weight combined with smaller ionic size may be another reason for the higher adsorption capacity of MB compared with other cationic dyes. It is well-known that the surface structure of adsorbent as well as the intermolecular force between adsorbent surface and dye species is responsible for the dye removal. Hence, the relatively low maximum adsorption value F



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of MO in this work may be due to the weak van der Waals force, which lacks electrostatic interactions. To visually observe the adsorption efficiency of various dyes, the adsorption efficiency was studied for the four dyes with a concentration of 60 mg/L. Obvious fading can be observed after MoS2/CuS NCs are added into dye aqueous solutions (Figure 6a). The results in Figure 6b show that the adsorption

Figure 6. (a) Photograph of different dyes at the initial time and equilibrium time during adsorption, and (b) adsorption efficiency for several anionic and cationic dyes at C0 = 60 mg/L.

efficiency of dyes except methyl orange could reach more than 96%. The rapid and efficient adsorption ensures that the MoS2/CuS NCs are a broad-spectrum adsorbent for different water-soluble dyes.

4. CONCLUSION In summary, a facile synthesis approach is conducted to prepare the MoS2/CuS NCs. The adsorption performance of MoS2/CuS NCs is determined using both cationic and anionic dyes, and the composites exhibit excellent dye adsorption capacity, that is, 276.24, 432.68, 98.78, and 211.18 mg/g for RhB, MB, MO, and RhB 6G, respectively. The adsorption results of RhB show the adsorption process can be well described by pseudo-second-order kinetic model, and the adsorption isotherm at the equilibrium fits well with the Langmuir model by linear regression data analysis. Both MoS2 and CuS are lamellar-like structures, which provide high contact area and uniformity coverage for adsorption of dye molecules. On the basis of the high adsorption of several common dyes, the MoS2/CuS NCs are suitable as a broadspectrum adsorbent for different dyes.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. Tel.: 86-28-8320-2130. *E-mail: [email protected]. ORCID

Xia Xiang: 0000-0002-4042-1016 Funding

This work is financially supported by the NSAF Joint Foundation of China (Grant No. U1630126). Notes

The authors declare no competing financial interest.

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REFERENCES

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CuS Nanosheet Composites as an Efficient and Ultrafast Adsorbent for

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