Hierarchical Microspheres of MoS2 Nanosheets: Efficient and

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Hierarchical Microspheres of MoS2 Nanosheets: Efficient and Regenerative Adsorbent for Removal of Water-Soluble Dyes Anushka Tina Massey, Rashi Gusain, Sangita Kumari, and Om Prakash Khatri Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.6b01115 • Publication Date (Web): 14 Jun 2016 Downloaded from http://pubs.acs.org on June 15, 2016

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Industrial & Engineering Chemistry Research

Hierarchical Microspheres of MoS2 Nanosheets: Efficient and Regenerative Adsorbent for Removal of Water-Soluble Dyes

Anushka Tina Massey,† Rashi Gusain,† Sangita Kumari and Om P. Khatri*

Chemical Science Division CSIR-Indian Institute of Petroleum, Dehradun – 248005, India *Email: [email protected]

†

These authors contributed equally to this work

1 ACS Paragon Plus Environment


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Abstract This study demonstrates fast and efficient removal of dyes from the wastewater using nanostructural MoS2 as a regenerative adsorbent. The hierarchical microspheres of MoS2 nanosheets were prepared by one-step facile and scalable hydrothermal route using polyethylene glycol as a templating material. Chemical and morphological features of hierarchical microspheres of MoS2 nanosheets were examined by XPS, FTIR, XRD, FESEM and HRTEM. The adsorption of a series of organic dyes using MoS2 nanosheets was systematically investigated. The kinetic study illustrated that adsorption of methylene blue dye on to the MoS2 nanosheets followed the pseudo second order model. The adsorption isotherm at the equilibrium was supported by Freundlich isotherm model. The hierarchical microspheres of MoS2 nanosheets showed adsorption capacity as high as 297, 204, 216, 183, 146 mg.g-1 for methylene blue, malachite green, rhodamine 6G, fuchsin acid and congo red dyes, respectively. The high adsorption capacity was attributed to the hierarchical arrangement of MoS2 nanosheets in the microscopic spheres, where dye molecules have very fast accessibility. Importantly, the hierarchical microspheres of MoS2 nanosheets can be efficiently regenerated and reused for the dye adsorption of subsequent batch without compromising the adsorption capacity.

Keywords: MoS2 nanosheets, organic dyes, regenerative adsorbent, water


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Introduction Environmental pollution has been an issue of great concern across the globe and gaining large attention owing to their harmful and adverse effects to human health and aquatic-to-terrestrial ecosystems.1-3 The industrial effluents having large varieties of toxic and harmful chemicals including organic dyes from textile, printing, leather, distillery, electro-platting, pulp pharmaceuticals and food industries are jeopardising the natural resources. The major risk lies on the water bodies like lakes, ponds, rivers, oceans, etc. and it leads to scarcity of clean and fresh water. The industrial discharges having organic dyes pose several problems, as they impart toxicity to the aquatic life and damage the aesthetic environment nature. Most of dyes are toxic and exhibit carcinogenic, mutagenic and teratogenic properties.4-5

Recently, lot of efforts have been directed to remove the toxic and hazardous chemicals and dyes from the wastewater and industrial effluents, before they discharged into water bodies and terrestrial lands. In this context, numerous approaches including chemical oxidation, membrane separation, filtration, electrochemical treatment, photocatalytic or catalytic degradation, adsorption etc. have been addressed.6-10 Amongst them, adsorption is considered as an efficient and economical process, which can be easily scaled-up for removal of organic dyes at the large scale. A number of adsorbents such as clays, bio-mass, polymeric and carbonaceous materials including activated carbon, graphene-based materials, bio-mass derived carbons etc. have demonstrated for removal of organic pollutants and dyes.7-9 Recently, two-dimensional graphene-based nanomaterials and their three-dimensional nanocomposites / foams are gaining immense interest as adsorbent materials for efficient removal of organic contaminants including various dyes from the wastewater. The high removal efficiency of graphene-based nanomaterials were attributed to their large surface area and fast accessibility of dyes to the adsorption sites.11-16 3 ACS Paragon Plus Environment


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Molybdenum disulfide (MoS2) nanosheet, an inorganic analogue to graphene, is gaining large interest for several applications viz. electronic transistors, lubrication, catalysis, energy storage, sensors etc. and are attributed to its remarkable characteristics including quantum confinement, excellent mechanical strength, low dimension and surface defects.17-18 The nanocomposites of MoS2 with metal oxide and graphitic carbon nitride have been demonstrated as efficient photocatalysts for the dye degradation. The enhanced photocatalytic effect was attributed to the suppression of recombination of photo-induced carriers by the MoS2 nanosheets.19-20 However, the potential of MoS2 nanostructure for environmental applications, particularly for adsorption of organic dyes, has not been explored extensively although it has high surface area and large number of adsorption sites. Recently, Geng et al. synthesized the three-dimensional flower-like MoS2 nanostructure and revealed as an adsorbent for cationic dye Rhodamine B with a maximum adsorption capacity of 49.2 mg.g1 21

.

The two-dimensional MoS2 nanosheets showed potential as an efficient adsorbent for

removal of methylene blue (MB) dye following the pseudo-second order adsorption kinetics.22 The MoS2 nanosheets decorated with magnetite Fe3O4 nanoparticles exhibited good adsorption capacity for the Congo red and can be quickly separated from the suspension owing to their magnetic properties.23

In this paper, we report one-step approach to synthesize the hierarchical microspheres of MoS2 nanosheets and then demonstrated as highly efficient adsorbent for organic dyes. A variety of dyes (both alkaline and acidic) are selected to systematically explore the adsorption capacity using MoS2 as an adsorbent. Furthermore, kinetic study and adsorption mechanism of methylene blue as a representative dye using hierarchical microspheres of MoS2 nanosheets are investigated in detail. More strikingly, this material can be easily regenerated for the subsequent adsorption cycles without compromising the adsorption capacity. The


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MoS2 nanosheets prepared by a facile and scalable procedure revealing the remarkable adsorption capacity for organic dyes with excellent recyclability may provide a better solution for the dye pollutants in water purification applications.

Experimental Section Chemicals and materials Sodium molybdate Na2MoO4.2H2O (99.5%, Sigma Aldrich), thiourea [SC(NH2)2, 99%, Sigma Aldrich), polyethylene glycol 200 (PEG 200, Average molecular weight: 200, Sigma Aldrich) were used for the preparation of hierarchical microspheres of MoS2 nanosheets. Methylene blue (MB, Sigma Aldrich), Rhodamine 6G (Rh, Sigma Aldrich), Malachite Green (MG, Loba Chemie), Fuchsin acid (FA, Lobale Chèmie) and Congo red (CR, Loba Chemie) dyes were used to prepare the simulated wastewater.

Synthesis of hierarchical microspheres of MoS2 nanosheets The hierarchical microspheres of MoS2 nanosheets were prepared by a facile and scalable hydrothermal reaction. A 0.605 g Na2MoO4.2H2O was dissolved in 15 mL distilled water and then thoroughly mixed with 5 mL PEG 200. A 0.78 g SC(NH2)2 was added in to the sodium molybdate solution and stirred for 10 minutes. In order to carry out the hydrothermal reduction, the reaction precursors [Na2MoO4.2H2O and SC(NH2)2] in aqueous medium having PEG 200 as templating material were transferred in to a Teflon-lined stainless steel autoclave. The Teflon container was tightly sealed and kept in the oven at 230 °C. After 24 hours of hydrothermal reaction, Teflon container was allowed to cool down. The developed black color material was collected and washed several times with distilled water until neutral pH of decanted water was attained. Final washing was carried out with ethanol and then material was dried at 70 °C in the oven.



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Material characterization The chemical characterization of MoS2 was carried out by X-ray photoelectron spectroscopy (XPS) and fourier transform infrared (FTIR) spectroscopy. An XPS spectrum of MoS2 was recorded on ESCA 3400 (Kratos Analytical Ltd.) spectrometer by using Mg Kα line as an Xray source. The peak fitting of XPS spectra of Mo 3d and S 2p were carried but by using a Gaussian–Lorentzian function. FTIR spectrum of MoS2 was collected on Thermo Scientific Nicolet 8700 research spectrometer at a resolution of 4 cm-1. The X-ray diffraction pattern (XRD) of hierarchical microspheres of MoS2 nanosheets was collected on Bruker D8 Advanced diffractometer at 40 kV and 40 mA using Cu Kα line of 0.15418 nm wavelength. The microstructural features of hierarchical microspheres of MoS2 nanosheets were analyzed by field emission scanning electron microscopy (FESEM) and high-resolution transmittance electron microscopy (HRTEM).

Adsorption study An aqueous solution having variable concentrations of MB dye was used as simulated wastewater for examining the adsorption characteristics of hierarchical microspheres of MoS2 nanosheets. In a typical experiment, 100 mL aqueous solution having 120 mg.L-1 concentration of MB dye was stirred with 100 mg of MoS2 at room temperature. In parallel set of experiments, aqueous solution of MB dye with variable concentrations viz. 180, 240 and 300 mg.L-1 were used to examined the adsorption characteristics. The adsorption of MB dye on to the MoS2 was examined by scanning the UV-visible spectra (Shimadzu UV-2600 spectrophotometer) of simulated wastewater as a function of time. The concentration of residual dye was analyzed by peak intensity of wastewater at 663 nm. Subsequently, the adsorption capacity (Qe) and percent removal efficiency of MB dye were measured by using following equations:


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= − ×

(1)

% = − / × 100

(2)

where Qe (mg.g-1) is the adsorption capacity at the equilibrium, Co (mg.L-1) is the initial concentration of MB in the simulated wastewater, Ce (mg.L-1) is the concentration of MB at the equilibrium, V (L) is the volume of simulated wastewater and W (g) is the amount of MoS2 as an adsorbent used for the MB dye adsorption. Furthermore, this study was extended to probe the adsorption capability of MoS2 for both the alkaline (Rh, MG) and the acidic (FA, CR) dyes.

Results and Discussion The hierarchical microspheres of MoS2 nanosheets were synthesized by one-step hydrothermal reaction, where the sodium molybdate was reduced with thiourea at high temperature and high pressure. The PEG 200 was used not only as a stabilizing agent to control the growth and crystal facets of MoS2 nanosheets but also functions as templating material to develop the hierarchical structure.24-25 The chemical features of hierarchical microspheres of MoS2 nanosheets were examined by XPS and FTIR analysis. The XPS peaks of Mo and S can be explicitly seen in the survey spectrum of MoS2 nanosheets (Figure 1). The well-resolved doublet peak structure of Mo 3d spectrum (inset of Figure 1) at 229.3 (Mo 3d5/2) and 232.4 eV (Mo 3d3/2) was attributed to the Mo4+ oxidation state of MoS2 nanosheets.20 Furthermore, the characteristic peaks of 2s, 2p1/2 and 2p3/2 orbitals at 226.5, 163.3 and 162.1 eV respectively, were associated to the divalent S in the MoS2 nanosheets. Figure 2 shows vibrational modes at ~3435 and 1635 cm-1 and are assigned to hydroxyl functionalities of adsorbed moisture on the MoS2 nanosheets. The methylene symmetric and asymmetric stretches in the range of 2800-3000 cm-1 were attributed to the PEG molecules



adsorbed on the MoS2 nanosheets. The inset spectrum of Figure 2 exhibits characteristic vibrational mode at 455 cm-1 and was assigned to the Mo-S linkage of MoS2.

Mo 3d3/2 S 2p3/2

Intensity, a.u.

Intensity, a.u.

Intensity, a.u.

Mo 3d5/2

S 2s

240

236

232

228

224

S 2p1/2

Binding Energy, eV 168

166

164

162

160

158

156

Binding Energy, eV

O 1s

Mo 3p

Mo 3d S 2p

1000

800

600

400

200

0

Binding Energy, eV

Fig 1: XPS survey spectrum of hierarchical microspheres of MoS2 nanosheets. Inset graphs show high resolution XPS spectra of Mo 3d and S 2p. 53

50 % Transmittance

52

22.25

40

% Transmittance

%Transmittance

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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30

51

50

22.00

Mo-S

21.50

21.25 3000

49

d+

21.75

48 500

d2950

480

460

440

420

Wavenumber, cm-1

2900

2850

2800

Wavenumber, cm-1

δO-H

20 υO-H

4000

3500

3000

2500

2000

1500

1000

500

-1

Wavenumber, cm

Fig 2: FTIR spectrum of hierarchical microspheres of MoS2 nanosheets. Inset graphs show expanded regions of C-H and Mo-S characteristic peaks.


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The XRD pattern of MoS2 was studied to probe the crystal structure and phase purity. XRD diffractogram of MoS2 (Fig 3) revealed characteristic peaks at 2θ of 14.07°, 33.41°, 39.46°, 50.80° and 59.01° due to diffraction from (002), (100), (103), (105) and (110) planes, respectively. Further, diffraction pattern was found to be fully matched with JCPDS file no. 37-1492 of MoS2. The morphological features of MoS2 nanosheets were investigated by capturing electron microscopic images. The FESEM images (Fig 4a-b) demonstrated the spherical shapes made of several thin sheets of MoS2. The diameter of these microspheres is found to be in the range of 1-5 µm. The high-resolution TEM images (Fig 4c-d) of MoS2 further revealed that each thin sheet of microsphere is composed of 5-30 atomic thick lamellae of MoS2. These atomic lamellae are stacked by van der Waal interaction. The interlayer distance between the MoS2 lamellae was estimated to be ~0.63 nm in (002) plane and it was further supported by the XRD result. (110)

(100) (002)

Intensity, a.u.

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(103) (105)

20

40

60

2θ θ, degree

Fig 3: Powder XRD pattern of hierarchical microspheres of MoS2 nanosheets



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Fig 4: (a, b) FESEM and (c, d) high-resolution TEM images of hierarchical microspheres of MoS2 nanosheets.

The potential of hierarchical microspheres of MoS2 nanosheets was investigated for adsorption of organic dyes from the simulated wastewater using MB as a representing dye. The interaction between adsorbate and adsorbent is an important parameter to control the efficacy and selectivity of adsorbent. High adsorption capacity of nanostructural materials reduces the required quantity of adsorbent and eases the adsorption process. In order to find out the maximum adsorption capacity of the hierarchical microspheres MoS2 nanosheets, the adsorption experiments were performed using 100 mL aqueous solution of MB dye having variable concentrations of 120, 180, 240 and 300 mg.L-1. A 100 mg of MoS2 was used as an adsorbent and added to the dye solution of each concentration. The color of MB starts to fade away in the presence of MoS2 nanosheets and suggested the removal of dye from water by adsorption on to the MoS2 nanosheets. The dye solution samples were drawn as a function of time and concentration of MB dye in each sample was examined by measuring their UV-Vis


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absorption spectra. The changes in concentration of MB dye were determined based on absorbance at 663 nm wavelength using a UV- Vis spectrophotometer.

Figure 5 shows % removal efficiency for MB dye using hierarchical microspheres of MoS2 nanosheets as adsorbent with function of time. Interestingly, the adsorption of MB on MoS2 nanosheets was found to be very fast at all concentrations (120, 180, 240 and 300 mg.L-1) and ~90% MB dye was adsorbed within 10 minutes. The physisorption process is believed to the key driving force for adsorption of organic dyes.9 It is a weak adsorption process, and adsorbate adsorbed on to the surface of adsorbent by either van der Waals or dipole-driven interaction. In the physisorption process, the chemical nature of adsorbing pair remains intact. 94.5

(d) 94.0 93.5

F Removal Efficiency, %

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93.0 92.4

300 mg.L-1

(c)

91.7

240 mg.L-1

91.0

92.4

(b)

91.2

180 mg.L-1

90.0

92.4

(a)

91.7 91.0

120 mg.L-1

90.3 0

100

200

300

400

500

600

Time, Minutes 11 ACS Paragon Plus Environment

1300 1400


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Figure 5: Removal efficiency for (a) 120, (b) 180 (c) 240 and (d) 300 mg.L-1 solutions of MB dye using MoS2 as an adsorbent. The adsorption kinetics and the adsorption mechanism of MB dye on the MoS2 nanosheets were investigated by linear pseudo first order (Eq. 3), linear pseudo second order (Eq. 4) and intraparticle diffusion (Eq. 5) adsorption models using UV-Vis absorption results. The pseudo first order adsorption model is expressed by following equation:

− = − 3 where Qe and Qt are the amount of dye adsorbed (mg.g-1) at equilibrium and at time t (min), respectively. k1 is the pseudo first order rate constant (min-1). The pseudo second order adsorption model is presented by following equation:

1 = + 4 Where k2 is the pseudo second order adsorption process rate contact (g.mg-1.min-1) The intraparticle diffusion model is described using following equation:

= "# .% + 5 where kid is the intraparticle diffusion rate constant (mg.g-1.min-1/2).

Figure 6a represents pseudo first order kinetic model. The R2 values of these plots are found to be in the range of 0.736 to 0.900. The low values of R2 revealed that adsorption process may not be well fitted to the pseudo first order adsorption kinetic model. The intraparticle diffusion plots (Qt vs. t1/2) give comparatively better agreement to the linear fitting (R2 = 0.866-0.961) as shown in Figure 6c. However, the linear fitting of these plots are not passing through the origin, which suggested that intraparticle diffusion is not the rate-controlling step in the adsorption process. The perfect linear fit of t/Qt vs. t (Figure 6b) with R2 of 0.999 explicitly demonstrated the pseudo second order adsorption model for the adsorption of MB


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dye on to the MoS2 nanosheets. The calculated Qe values (Table 1) from the pseudo second order adsorption model were found to be closer to that of experimental Qe and revealed that adsorption kinetic is well fitted by pseudo second order adsorption model. (a)

ln(Qe-Qt)

2.6

2.4

2.2

180 mg/mL 240 mg/mL 300 mg/mL

2.0 0

100

Methylene Blue

200

300

400

500

600

t, min

3.6

(b)

3.0

t/Qt, min.mg/g

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


180 mg/mL 240 mg/mL 300 mg/mL

2.4 1.8 1.2 0.6

Methylene Blue

0.0 0

100

200

300

400

500

600

t, min

289 288 287 286 285

Qt, mg/g

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(c) 300 mg/mL

226 225

240 mg/mL

224 223 165

180 mg/mL

164

Methylene Blue

163 2

4

6

8

10

12

14

16

t1/2, min


18

20


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Figure 6: Comparison of kinetic models for the adsorption of MB dye on to the hierarchical microspheres of MoS2 nanosheets by (a) pseudo first order, (b) pseudo second order and (c) intraparticle diffusion adsorption models.

Table 1: Comparison of kinetic parameters for adsorption of MB on to the MoS2 nanosheets based on the pseudo first order, the pseudo second order and the intraparticle diffusion adsorption models. Models

Parameters

Concentration of MB Dye 180 mg.L-1

240 mg.L-1

300 mg.L-1

Pseudo First Order

R2

0.7359

0.6581

0.9004

Pseudo Second Order

R2

0.9999

0.9999

0.9999

k2 (g.min-1.L-1)

0.0026

0.00078

0.0007

Qe,calc (mg.g-1)

168.1

232.6

303.0

Qe,exp (mg.g-1)

174.6

235.4

297.5

R2

0.8659

0.9172

0.9605

Intraparticle Diffusion

The interaction between the adsorbent and the adsorbate along with the feasibility of different adsorption processes are usually represent by adsorption isotherms at the equilibrium. The dye adsorbed (Qe) by the adsorbent is equal to the dye desorbed at the equilibrium point, as a result the concentration of dye in the medium (Ce) remains constant. Herein, the isotherm results are examined by linear regression analysis of Langmuir and Freundlich isotherms models at the equilibrium. The Langmuir isotherm model demonstrates the monolayer coverage of adsorbate onto the adsorbent surface.26 This model assumes uniform energies of adsorption on to the surface and no trans migration of adsorbate in the plane of the surface.


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Based upon these assumptions, Langmuir isotherm model is represented by following equation:

=

' () 6 1 + ()

Langmuir adsorption parameters were determined by transforming the Langmuir Eq. 6 into linear form.

1 1 1 = + 7 ' ' () 1 = + 8 ' ' () Where Ce is equilibrium concentration of adsorbate (mg.L-1), Qm is the maximum monolayer adsorption capacity (mg.g-1) and KL is the Langmuir isotherm constant (L.mg-1).

Moreover, Freundlich isotherm indicates the presence of heterogeneous adsorption surfaces and variable adsorption sites with different energies of adsorption.27 The Freundlich isotherm model is represented by the following equation:-

⁄ = (- . 9

The adsorption parameters of Freundlich isotherm model are evaluated by converting it in linear form:

= (- +

1 10 -

The Langmuir and Freundlich isotherm models are used to process the adsorption results of MB dye on to the MoS2 nanosheets and are presented in Figure 7. A plot between Ce/Qe and Ce must be linear, if adsorption process is followed by the Langmuir isotherm model. The R2 15 ACS Paragon Plus Environment


extracted by linear fit of adsorption data in Langmuir isotherm model was found to be 0.9589 and the calculated maximum adsorption capacity (Qm: 87.4 mg.g-1) is noted to be significantly lower than that of the experimental values. Further, the adsorption results of MB dye were plotted following the Freundlich isotherm model (Figure 7b). The linear fit of experimental results between lnQe and lnCe with R2 value of 0.9835 indicates that adsorption process of MB dye on to the hierarchical microspheres of MoS2 nanosheets is plausibly controlled by Freundlich isotherm model. These results suggested that MoS2 surface exhibits blends of homogeneous and heterogeneous sites. Initial very fast adsorption of MB could be attributed to the formation of MB monolayer on the homogenous sites of MoS2 lamellae as per the Langmuir isotherm model and is further supported by the good agreement of linear fit (R2 = 0.9589). The presence of several micro channels in the hierarchical microspheres of MoS2 nanosheets as deduced from their high-resolution microscopic images (Figure 4) might function as heterogeneous sites for multilayer adsorption of MB dye as per the Freundlich isotherm model and this is supported by a very good linear fit (R2 = 0.9835). 0.08

(a) 0.06

Ce/Qe

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

0.04

0.02

0.00 2

3

4

5

6

7

Ce, mg/L


8

9

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5.8

(b) 5.6

5.4

ln Qe

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5.2

5.0

4.8

4.6 0.8

1.0

1.2

1.4

1.6

1.8

2.0

2.2

ln Ce

Figure 7: Comparison of (a) Langmuir (b) Freundlich isotherms models for adsorption of MB dye on to the hierarchical microspheres of MoS2 nanosheets. The adsorption of MB dye on the surface of MoS2 nanosheets was further confirmed by FTIR analysis. In this context, aqueous solution of MB dye (240 mg.mL-1) was allowed to adsorb on the hierarchical microspheres of MoS2 nanosheets till the equilibrium adsorption time. The MoS2 collected after adsorption of MB dye was dried and analyzed by FTIR. Figure 8 represents the FTIR spectra of pristine MoS2, recovered MoS2 after adsorption of MB dye. The FTIR spectrum of MB dye was shown for the comparison purpose. The inset graph demonstrates the expanded spectrum of recovered MoS2 after adsorption of MB dye. The presence of characteristics peaks of MB dye at 1594, 1387, 1324, 1178, 1037, 877 cm-1 owing to C=Caromatic, =C-Naromatic, -C-N, -C-S, =C-Haromatic linkages confirmed the adsorption of MB dye on to the hierarchical microspheres of MoS2 nanosheets. Moreover, these vibrational modes are not seen in the pristine MoS2. Furthermore, the Mo-S vibrational mode in recovered MoS2 after adsorption of MB dye remains unchanged.



60 55

Pristine MoS2 Recovered MoS2 after adsoprtion of dye Methylene Blue dye

50 45 40 %, Transmittance

%, Transmittance

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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35 30

Recovered MoS2 after adsoprtion of dye

1600

1400

1200

Wavenumber, cm-1

25 1600

1400

1200

1000

800

Wavenumber, cm-1

Figure 8: FTIR spectra of MB dye, pristine MoS2 nanosheets and recovered MoS2 after adsorption of MB dye. The inset spectrum of recovered MoS2 after adsorption of MB dye is expanded to demonstrate the characteristics peaks of MB dye adsorbed onto the MoS2 nanosheets. Desorption is the process of removal of adsorbed substance on the adsorbent. The MB dye adsorbed on the MoS2 was removed by using an aqueous solution of sodium hydroxide. The residual MoS2 after desorption of MB dye was recovered and then reused for the subsequent cycle of MB dye adsorption. The Figure 9 illustrates that the MoS2 has excellent recyclability and is efficient to adsorb the dye even after five consecutive cycles without degradation of adsorption sites in the MoS2.


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98

Concentration of MB Dye: 240 mg.L-1 Removal Efficiency, %

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


96

94

92

90 1

2

3

4

5

Number of Cycle

Figure 9: Recycling of MoS2 adsorbent for efficient removal of MB dye for five consecutive batches. The scope of hierarchical microspheres of MoS2 nanosheets as adsorbent was further extended to variable dyes viz. malachite green, rhodamine 6G, congo red and fuchsin acid. In this context, 100 mL aqueous solution of each dye (240 mg.mL-1) was mixed with 100 mg of MoS2 nanosheets and adsorption parameters were examined as a function of time. As shown in Figure 10, both MG and Rh alkaline dyes adsorbed very fast on to the MoS2 nanosheets and showed more than 80% removal efficiency within the 50 minutes. The adsorption efficiency of MoS2 nanosheets was noted to be varied with structure of alkaline dye and the removal efficiency was found in the order of MB > Rh > MG. The congo red and fuchsin acid dyes were further selected as acidic dyes and examined their adsorption using hierarchical microsphere of MoS2 nanosheets. Unlike alkaline dyes, both CR and FA showed (Figure 10c-d) comparatively low adsorption on the MoS2 nanosheets. The higher removal efficiency for alkaline dyes might be attributed to the contribution of both the van der Waals and the electrostatic interactions between the MoS2 nanosheets and alkaline dyes. The low



adsorption of acidic dyes compared to that of alkaline dyes on to the MoS2 nanosheets was possibly driven by van der Waals interaction only. Malachite Green

Rhodamine 6G

92 91

84 90 82

D

Removal Efficieny, %

86

89 88

80

(a)

78

(b)

87 86

0

100

200

300

400

1420

1440

0

100

200

300

400

1420

1440

77

Fuchsin Acid

Congo Red 60

76 75

57 74 H

Removal Efficieny, %

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73 72

54

51

(c)

71

(d)

48

70 0

100

200

300

400

0

100

Time, Minutes

200

300

Time, Minutes

Figure 10: Removal efficiency of (a) MG, (b) Rh, (c) FA and (d) CR dyes. The 100 mg hierarchical microsphere of MoS2 nanosheets was used as an adsorbent in 100 mL of each dye solution (240 mg.L-1).

Conclusion In summary, one-step approach is addressed to prepare the MoS2 nanosheets using PEG 200 as a templating material. HRTEM images revealed the hierarchical microspheres of MoS2 nanosheets and each sheet is composed of 5-30 atomic thick lamellae of MoS2. The adsorption performance of MoS2 nanosheets was examined using both alkaline and acidic dyes viz. MB, MG, Rh, FA and CR. The hierarchical microspheres of MoS2 nanosheets showed excellent dye adsorption capacity and were found to be 297, 204, 216, 183, 146 mg.g20 ACS Paragon Plus Environment

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1

for MB, MG, Rh, FA and CR, respectively. The higher removal efficiency for alkaline dyes

was attributed to combined effect of the van der Waals and the electrostatic interactions between the MoS2 nanosheets and alkaline dyes. However, adsorption of acidic dyes onto the MoS2 nanosheets was driven by the van der Waals interaction only, as a result MoS2 nanosheets exhibited lower adsorption capacity for acidic dyes compared to that of alkaline dyes. The experimental results obtained by UV-Visible spectrophotometer using MB as a representative dye were processed with different kinetic models and linear regression data confirmed that adsorption kinetics follows the pseudo-second order adsorption model. Furthermore, the adsorption isotherm at equilibrium was favoured by Freundlich isotherm model. Importantly, the recyclability tests revealed that hierarchical microspheres of MoS2 nanosheets can be regenerated and utilized for subsequent batch without losing the adsorption capacity.

Author Information

Corresponding Author *E-mail: [email protected]

Notes The authors declare no competing financial interest.

Acknowledgements We kindly acknowledge the Director of CSIR-IIP for his kind permission to publish these results. The authors are thankful to CSIR, India for financial support. Analytical support from Analytical Science Division and Refinery Technology Division of CSIR-IIP Dehradun and



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CSIR-CSMCRI Bhavnagar are kindly acknowledged. R.G. thanks CSIR, India for fellowship support.

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ToC Graphic 503x345mm (96 x 96 DPI)

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Hierarchical Microspheres of MoS2 Nanosheets: Efficient and

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