Fabrication of Ultrathin MoS2 Nanosheets and Application on Adsorption of Organic Pollutants and Heavy Metals

Owing to their peculiar structural characteristics and potential applications in various fields, the ultrathin MoS2 nanosheets, a typical two-dimensional material, have attracted numerous attentions. In this paper, a hybrid strategy with combination of quenching process and liquid-based exfoliation was employed to fabricate the ultrathin MoS2 nanosheets (MoS2 NS). The obtained MoS2 NS still maintained hexagonal phase (2H-MoS2) and exhibited evident thin layer-structure (1–2 layers) with inconspicuous wrinkle. Besides, the MoS2 NS dispersion showed excellent stability (over 60 days) and high concentration (0.65 ± 0.04 mg mL−1). The MoS2 NS dispersion also displayed evident optical properties, with two characteristic peaks at 615 and 670 nm, and could be quantitatively analyzed with the absorbance at 615 nm in the range of 0.01–0.5 mg mL−1. The adsorption experiments showed that the as-prepared MoS2 NS also exhibited remarkable adsorption performance on the dyes (344.8 and 123.5 mg g−1 of qm for methylene blue and methyl orange, respectively) and heavy metals (185.2, 169.5, and 70.4 mg g−1 of qm for Cd2+, Cu2+, and Ag+). During the adsorption, the main adsorption mechanisms involved the synergism of physical hole-filling effects and electrostatic interactions. This work provided an effective way for the large-scale fabrication of the two-dimensional nanosheets of transition metal dichalcogenides (TMDs) by liquid exfoliation.


Introduction
Given the special structure and potential applications, two-dimensional materials have drawn plenty of concerns [1,2], such as graphene, boron nitride, and molybdenum disulfide. Among them, the ultrathin molybdenum disulfide (MoS 2 ) nanosheets, which exhibit an evident layered structure, have attracted ample attentions because of their excellent performance on several fields, such as catalysis, sensors, and pollution remediation [1,3]. Recently, the ultrathin MoS 2 nanosheets were reported to show excellent prospects in pollution control [3,4]. Therefore, it was urgent to explore an effective method to produce ultrathin MoS 2 nanosheets.
To date, a few methods have been reported for efficient preparation of ultrathin MoS 2 nanosheets [5][6][7][8][9][10], for example, mechanical exfoliation, sputtering, atomic layer deposition, chemical methods, and liquid-based exfoliation. In spite of the excellent performance of the prepared monolayer or few-layer MoS 2 nanosheets through mechanical methods, the production efficiency was rather low, which severely limited the large-scale applications. Meanwhile, although most of the chemical MoS 2 was transferred into a serum bottle with 100 mL of hydrazine hydrate and the bottle was sonicated at a frequency of 40 kHz for 24 h. After centrifugation, the residual MoS 2 powders were added into another serum bottle with deionized water and sonicated for 12 h (In addition, the recycled hydrazine hydrate can be reused in a new procedure.). Finally, the resulting suspensions were centrifuged at 3000 rpm for 2 h and then the dark green MoS 2 -NS dispersions were obtained. After dialyzed with dialysis tubing with 3000 dalton of molecular weight cut off, the obtained ultimate green dispersions were close to 7 of pH and stored in the fridge at 4 • C.

Adsorption Batch Experiments
Adsorption isotherm batch experiments were carried out in a 40-mL serum bottle containing 10 mL liquid with 0.1 g L −1 of the adsorbent concentration. The adsorption isotherm for MB and MO was conducted under 25 • C in the range of 0.5 to 50 mg L −1 of the MB and MO concentration and the pH was adjusted to 6.0 ± 0.1 with 1 M H 2 SO 4 solution, while the experiments for heavy metals were conducted under 25 • C with metal concentration from 0.5 to 30 mg L −1 and the pH was adjusted to 5.0 ± 0.1 with 1 M H 2 SO 4 solution. After sealed with polytetrafluoroethylene (PTFE) caps, all the bottles were shaken at 250 rpm for 6 h. At sampling points, one bottle was taken out. After filtered with 0.22 µm glass fiber filters (Tianjin Branch billion Lung Experimental Equipment Co., Ltd., Tianjin, China), the MB/MO concentrations were determined by UV-vis spectroscopy (UV-1780, SHIMADZU, Japan) at 664/464 nm, while the residual Cu 2+ /Cd 2+ /Ag + concentrations were analyzed with ICP-MS (XSERIES 2, Thermo). The adsorption isotherms data were treated with Langmuir and Freundlich models [22,23]. The experiments for the adsorption kinetics study were operated at 25 • C and 6.0 ± 0.1/5.0 ± 0.1 of pH (adjusted with 1 M H 2 SO 4 solution) in 300 mL dyes/heavy metals solution (20 mg L −1 for dyes and 15 mg L −1 for heavy metals). All the samples were shaken at 250 rpm for 6 h. At sampling points, 1.5 mL of the solution was taken out and then filtered through the filters. The residual dyes/heavy metals concentrations were determined with ICP-MS. The adsorption kinetics data were treated with the pseudo-first order kinetic and pseudo-second-order non-linear kinetic models.
To study the effects of pH values (2-10)/(3-7) on dyes/heavy metals adsorption, the batch experiments were conducted at 25 • C in a serum bottle with 20 mg L −1 /15 mg L −1 of dyes/heavy metals concentration and 0.10 g L −1 of adsorbents.

Characterization
The X-ray powder diffraction (XRD) data of the bulk MoS 2 and MoS 2 -NS were tested with X-ray powder diffractometer (MiniFlex600, Rigaku, Milwaukee, Wisconsin, USA) coupled with a Cu Kα line at 40 kV and 40 mA.
The Raman data of bulk MoS 2 and MoS 2 -NS were recorded by a confocal laser Raman microscopy (Invia Reflex, Renishaw, UK) with 532 nm of laser wavelength and 0.6 mW of laser energy.
The X-ray photoelectron spectroscopy (XPS) data were recorded with X-ray photoelectron spectrometer (ESCALAB 250, Thermo Scientific, Waltham, Massachusetts, USA) coupled with the Al Ka radiation at 15 kV and 51 W. The binding energies were confirmed by using the C1s component as the reference and the binding energy of C-C/H bonds were set at 284.5 eV.
The Brunauer-Emmett-Teller (BET) surface areas of the bulk MoS 2 and MoS 2 -NS were obtained from the analysis of N 2 -adsorption isotherms at 77 K using the N 2 physisorption analyzer (ASAP2020, Micromeritics, Norcross, Georgia, USA).

Microstructures and Morphology
To study the variation of the crystal structure during the MoS 2 -NS preparation, the precursor ((NH 4 ) 2 MoS 4 ), bulk MoS 2 and MoS 2 -NS were analyzed by XRD and the results were illustrated in Figure 1. As shown in Figure 1, after calcination under N 2 , the characteristic peaks of (NH 4 [24,25]. In addition, after exfoliation by sonication, the resulting MoS 2 -NS still kept the same peaks with bulk MoS 2 , manifesting that the 2H-MoS 2 -NS was successfully obtained. However, compared to the bulk MoS 2 , the (002) plane peak of MoS 2 -NS became broadened and lower, suggesting an increase of the d spacing between MoS 2 layers [26].  [26,27], which convincingly proved the successful exfoliation of MoS 2 -NS. Among which, the E 1 2g mode peak at 381.6 cm −1 involved the in-layer displacements of Mo and S atoms, whereas the A 1g mode peak at 407.9 cm −1 represented the out-of-layer symmetric displacements of S atoms along the c-axis [28]. Noticeably, compared to the bulk MoS 2 , the E 1 2g (377.8 cm −1 ) and A 1g (402.2 cm −1 ) mode peaks displayed evident blue shift, and the interval (∆ = 24.4 cm −1 ) between E 1 2g and A 1g peaks was lower than that of bulk MoS 2 (∆ = 26.3 cm −1 ), which was ascribed to the decrease of the MoS 2 thickness. According to the previous literature [4,25,29], it was found that both E 1 2g and A 1g peaks were the characteristic peaks of MoS 2 and their frequencies would vary with the layer number. When the layer number increases, the interlayer van der Waals force in MoS 2 suppressed atom vibration, resulting in higher force constants [30]. On the contrast, the force constants between the layers would weaken with the layer number decreases. Thus, both E 1 2g and A 1g modes were supposed to stiffen (blue-shift) along with the reduction of MoS 2 layers. Figure 2a-d presented the FESEM images of the bulk MoS 2 and MoS 2 -NS. As seen in Figure 2a,b, the bulk MoS 2 displayed varisized particle-like morphology but clearly thick layer-structure. After exfoliation, the MoS 2 -NS exhibited evident thin layer-structure with inconspicuous wrinkle (Figure 2c (Figure 2j) from the center of the MoS 2 -NS evidently exhibited hexagonally symmetric structure, which was consistent with the results of XRD analysis. All the above results manifested that the obtained MoS 2 -NS still retained hexagonal single crystalline nature during pre-expansion and sonication treatments, which agreed with the previous findings [25,31,32]. However, the BET analysis ( Figure 3) showed that the specific surface areas of bulk MoS 2 and MoS 2 -NS were 5.6 and 26.6 m 2 g −1 , respectively, indicating that the exfoliation greatly changed the specific surface area of the MoS 2 materials. With the decreasing of the layers, more and more MoS 2 was exposed, resulting in a promotion of the BET surface.   The chemical composition and element valence on the surface of MoS 2 NS were analyzed with XPS ( Figure 4). As depicted in Figure 4a, the survey spectra clearly confirmed the presentence of C, O, NS, and Mo elements. The weak C1s (~284 eV) peak was attributed into the calibration of binding energy with carbon, while the N1s (~400 eV) peak was ascribed into the adsorbed hydrazine hydrate during the sonication. In Mo3d core-level spectra (Figure 4b), the appearance of Mo3d5/2 (233.5 eV) and Mo3d3/2 (232.6 eV) peaks for Mo3d doublet indicated the characteristic +4 oxidation state [1]. Besides, two weak Mo 6+ 3d peaks (3d5/2 peak at 233.5 eV and 3d3/2 peak at 235.9 eV) were ascribed to the slight oxidation of MoS 2 NS edge during the MoS 2 transfer under high temperature [25]. In the high-resolution scans of S2p (Figure 4c), two feature peaks (S2p1/2 and S2p3/2) were observed at 162.0 and 163.3 eV, respectively, which greatly matched the binding energy of S 2− ions in 2H-MoS 2 [2]. In addition, the appearance of O1s also confirmed the oxidation of MoS 2 . In the high-resolution spectra of O1s (Figure 4d), the peak of O 2− species located at 532.0 eV and the peak at 533. 5  results of XRD analysis. All the above results manifested that the obtained MoS2-NS still retained hexagonal single crystalline nature during pre-expansion and sonication treatments, which agreed with the previous findings [25,31,32]. However, the BET analysis ( Figure 3) showed that the specific surface areas of bulk MoS2 and MoS2-NS were 5.6 and 26.6 m 2 g −1 , respectively, indicating that the exfoliation greatly changed the specific surface area of the MoS2 materials. With the decreasing of the layers, more and more MoS2 was exposed, resulting in a promotion of the BET surface.  The chemical composition and element valence on the surface of MoS2 NS were analyzed with XPS ( Figure 4). As depicted in Figure 4a, the survey spectra clearly confirmed the presentence of C, O, NS, and Mo elements. The weak C1s (~284 eV) peak was attributed into the calibration of binding energy with carbon, while the N1s (~400 eV) peak was ascribed into the adsorbed hydrazine hydrate during the sonication. In Mo3d core-level spectra (Figure 4b), the appearance of Mo3d5/2 (233.5 eV) and Mo3d3/2 (232.6 eV) peaks for Mo3d doublet indicated the characteristic +4 oxidation state [1]. Besides, two weak Mo 6+ 3d peaks (3d5/2 peak at 233.5 eV and 3d3/2 peak at 235.9 eV) were ascribed to the slight oxidation of MoS2 NS edge during the MoS2 transfer under high temperature [25]. In the high-resolution scans of S2p (Figure 4c), two feature peaks (S2p1/2 and S2p3/2) were observed at 162.0 and 163.3 eV, respectively, which greatly matched the binding energy of S 2− ions in 2H-MoS2 [2]. In addition, the appearance of O1s also confirmed the oxidation of MoS2. In the high-resolution spectra of O1s (Figure 4d

Optical Properties of MoS2 Nanosheets Dispersion
To obtain pure few-layer MoS2 NS, the suspensions were first centrifuged at 3000 rpm for 2 h to remove the no exfoliated precipitate. Figure 5a displayed the photographs of the as-prepared MoS2 NS in water. As shown in the Figure 5a, the evident Tyndall phenomenon was observed both of the fresh MoS2 NS dispersions and the dispersions after 60 days. Meanwhile, the UV-vis absorption spectra (Figure 5b,c) also exhibited no evident change during 60 days. All the results suggested the excellent stability (stable for over 60 days) of as-prepared MoS2 NS dispersions. In addition, the UVvis absorption spectra (Figure 5e) of the resulting MoS2 NS dispersions with different concentrations (Figure 5d) displayed two distinctly characteristic peaks for 2H-MoS2 [33]. The two peaks located at 615 (B-exciton) and 670 nm (A-exciton) were attributed to the direct excitonic transitions of MoS2 at the K point of the Brillouin zone [34,35]. According to Hai et al.'s study [25], the relationship between the concentrations of MoS2 NS dispersions and the measured absorbance at a given wavelength (615 or 670 nm) were estimated by using the Beer-Lambert law. The fitting results (Figure 5f) proved that the concentrations of the dispersions showed good linear relationship (R 2 = 0.9996) with the absorbance at 615 nm in the range of 0.01-0.5 mg L −1 , which meant that the quantitative analysis of the MoS2 NS dispersions was available. Based on the above relationship, the concentration of the asprepared MoS2 NS dispersions was 0.65 ± 0.04 mg mL −1 , which was much higher than previous findings [7,25]. The initial concentration of the bulk MoS2 (2.510 g of bulk MoS2 were obtained after the calcination of (NH4)2MoS4) was 2.51 mg mL −1 , and the corresponding few-layer MoS2 NS yield

Optical Properties of MoS 2 Nanosheets Dispersion
To obtain pure few-layer MoS 2 NS, the suspensions were first centrifuged at 3000 rpm for 2 h to remove the no exfoliated precipitate. Figure 5a displayed the photographs of the as-prepared MoS 2 NS in water. As shown in the Figure 5a, the evident Tyndall phenomenon was observed both of the fresh MoS 2 NS dispersions and the dispersions after 60 days. Meanwhile, the UV-vis absorption spectra (Figure 5b,c) also exhibited no evident change during 60 days. All the results suggested the excellent stability (stable for over 60 days) of as-prepared MoS 2 NS dispersions. In addition, the UV-vis absorption spectra (Figure 5e [34,35]. According to Hai et al.'s study [25], the relationship between the concentrations of MoS 2 NS dispersions and the measured absorbance at a given wavelength (615 or 670 nm) were estimated by using the Beer-Lambert law. The fitting results (Figure 5f) proved that the concentrations of the dispersions showed good linear relationship (R 2 = 0.9996) with the absorbance at 615 nm in the range of 0.01-0.5 mg L −1 , which meant that the quantitative analysis of the MoS 2 NS dispersions was available. Based on the above relationship, the concentration of the as-prepared MoS 2 NS dispersions was 0.65 ± 0.04 mg mL −1 , which was much higher than previous findings [7,25]. The initial concentration of the bulk MoS 2 (2.510 g of bulk MoS 2 were obtained after the calcination of (NH 4 ) 2 MoS 4 ) was 2.51 mg mL −1 , and the corresponding few-layer MoS 2 NS yield was calculated to be as high as 25.9% in water.

Adsorption Isotherms and Kinetics
The adsorption performance of the MoS 2 NS was tested by selecting two dyes (methylene blue, MB and methyl orange, MO) and three heavy metal ions (Cu 2+ , Cd 2+ , and Ag + ) as the targets. As seen in Figure 6a, in the bulk MoS 2 systems, the equilibrium adsorption capacities of the two dyes only slightly increased with the increasing of the dye concentrations, manifesting that the bulk MoS 2 exhibited unsatisfactory adsorption performance of MB and MO. Instead, the equilibrium adsorption capacities of MoS 2 NS for MB and MO significantly increased under high concentration of dyes, which were much larger than those of bulk MoS 2 . Meanwhile, the as-prepared MoS 2 NS also displayed more excellent adsorption performance on heavy metals than the bulk MoS 2 . All the results indicated that the exfoliation was beneficial to improve the adsorption performance of MoS 2 , which was in accordance with previous studies [3, 36,37].

Adsorption Isotherms and Kinetics
The adsorption performance of the MoS2 NS was tested by selecting two dyes (methylene blue, MB and methyl orange, MO) and three heavy metal ions (Cu 2+ , Cd 2+ , and Ag + ) as the targets. As seen in Figure 6a, in the bulk MoS2 systems, the equilibrium adsorption capacities of the two dyes only slightly increased with the increasing of the dye concentrations, manifesting that the bulk MoS2 exhibited unsatisfactory adsorption performance of MB and MO. Instead, the equilibrium adsorption capacities of MoS2 NS for MB and MO significantly increased under high concentration of dyes, which were much larger than those of bulk MoS2. Meanwhile, the as-prepared MoS2 NS also displayed more excellent adsorption performance on heavy metals than the bulk MoS2. All the results indicated that the exfoliation was beneficial to improve the adsorption performance of MoS2, which was in accordance with previous studies [3, 36,37].  In addition, to well study the adsorption behavior, the Langmuir and Freundlich models were employed to fit the experimental data (Figure 6a,b, Figure S1). As listed in Table 1, the high R 2 values suggested that the Langmuir model better described the adsorption of dyes and heavy metals onto MoS 2 NS and bulk MoS 2 than the Freundlich model. Based on the Langmuir model fitting, the relative parameters like the maximum adsorption capacity (q m ) and affinity constant (K L ) for dyes and heavy metals were obtained and listed in Table 1. The q m values of MB and MO for MoS 2 NS were 344.8 and 123.5 mg g −1 , respectively, which were 12.77 and 6.94 larger than those (27.0 and 17.8 mg g −1 for MB and MO, respectively) of bulk MoS 2 . Meanwhile, the similar results were observed in the heavy metal adsorption, indicating that the MoS 2 NS exhibited much more excellent adsorption performance than the bulk MoS 2 . In addition, for the dyes, the higher q m and K L values of MB implied that MoS 2 materials exhibited better adsorption capacity and affinity to MB. For heavy metals, the highest q m value occurred to Cd 2+ (185.2 mg g −1 ), following Cu 2+ (169.5 mg g −1 ) and Ag + (70.4 mg g −1 ), indicating that MoS 2 NS were more beneficial to Cd 2+ and Cu 2+ adsorption than Ag + .  Figure 6c,d displayed adsorption kinetics data for dyes and heavy metals over MoS 2 NS. As revealed in Figure 6c, both MO and MB adsorption increased rapidly at the beginning, then proceeded at a slower rate, and tended to equilibrium at the end. The similar results occurred to the adsorption of heavy metals. Besides, to further analyze the time-dependent variation during the adsorption process, pseudo-first-order and pseudo-second-order kinetic models were employed to fit the dyes and heavy metals adsorption on MoS 2 NS (Figure 6e,f). As shown in Table S1, the higher R 2 values suggested that the pseudo-second-order model better described both dyes and heavy metals adsorption than the pseudo-first-order model, suggesting that the electron transfer between MoS 2 NS and dye molecule or metal ions played a controlling role during the adsorption [38].

Adsorption Mechanism
Based on the results, the MoS 2 NS showed much better dye or metal adsorption performance than bulk MoS 2 . According to the previous studies [3, 37,39,40], the main mechanisms reported during the adsorption of dyes or metal by the inorganic materials involved physical hole-filling effects, electrostatic interactions, and ion exchange.

Physical Hole-Filling Effects
The specific surface area often displayed significant effect on the adsorption of the pollutants [41][42][43]. The adsorbents with large specific surface area usually owned abundant pores, which greatly provided a sufficient adsorption site to capture the pollutants, resulting in the promotion of their adsorption performance. For nano materials, the physical hole-filling effect was considered as one of the important physical hole-filling effect probably played a vital role in the promotion of dyes or heavy metal adsorption. Herein, to verify the role of specific surface area during the dyes or heavy metal adsorption over MoS2 NS and bulk MoS2, the obtained qe data were standardized with the BET surface area and the results were showed in Figure 7. As shown in Figure 7a, for dyes, the equilibrium adsorption capacities of MoS2 NS for MB and MO were 312.0 and 92.6 mg g −1 , which were 12.89 and 5.61 times larger than those of bulk MoS2, respectively. Meanwhile, the as-prepared MoS2 NS also displayed excellent adsorption performance on heavy metals (Figure 7c), with 141.0, 152.8, and 64.2 mg g −1 for Cu 2+ , Cd 2+ , and Ag + , respectively, which were 10.68, 10.12, and 6.42 folds larger than those of bulk MoS2 (13.2, 15.1, and 10.0 mg g −1 for Cu 2+ , Cd 2+ , and Ag + , respectively). After standardization (Figure 7b and d), all of the qe ratios between the MoS2 NS and bulk MoS2 significantly decreased from 12.89 (MB), 5.61 (MO), 10.12 (Cu 2+ ), 10.68 (Cd 2+ ), and 6.42 (Ag + ) to 2.72, 1.12, 2.24, 2.11, and 1.33, respectively, suggesting that the physical hole-filling effect played positive role in the promotion of dyes or heavy metal adsorption over MoS2.
In addition, no evident variation was observed between the standardized qe values of MoS2 NS and bulk MoS2 (Figure 7b), meaning that the physical hole-filling effect was the sole mechanism during MO adsorption over MoS2 NS. However, the significant enhancement between the standardized qe values of MoS2 NS and bulk MoS2 (Figure 7b,d) suggested that besides the physical hole-filling effect, some other mechanisms were involved during the adsorption of MB and heavy metals over MoS2 NS.

Electrostatic Interactions
Electrostatic interaction was often considered as a possible mechanism to explain the adsorption of dyes and heavy metals [37,40,44]. To confirm the role of electrostatic interaction during dyes and heavy metals adsorption over MoS2 NS, the adsorption efficiency in various pH values were conducted. As depicted in Figure 8a, the slight fluctuation among the qe values for MO suggested that the MB adsorption over MoS2 NS was not controlled by the pH values. Instead, the MB adsorption In addition, no evident variation was observed between the standardized q e values of MoS 2 NS and bulk MoS 2 (Figure 7b), meaning that the physical hole-filling effect was the sole mechanism during MO adsorption over MoS 2 NS. However, the significant enhancement between the standardized q e values of MoS 2 NS and bulk MoS 2 (Figure 7b,d) suggested that besides the physical hole-filling effect, some other mechanisms were involved during the adsorption of MB and heavy metals over MoS 2 NS.

Electrostatic Interactions
Electrostatic interaction was often considered as a possible mechanism to explain the adsorption of dyes and heavy metals [37,40,44]. To confirm the role of electrostatic interaction during dyes and heavy metals adsorption over MoS 2 NS, the adsorption efficiency in various pH values were conducted. As depicted in Figure 8a, the slight fluctuation among the q e values for MO suggested that the MB adsorption over MoS 2 NS was not controlled by the pH values. Instead, the MB adsorption was notably influenced by the pH values. At low pH (<6) conditions, the q e values increased with the pH value and reached a peak (186.2 mg g −1 ) at pH = 6.0, and then gradually declined when pH > 6. Meanwhile, Zeta potential results (Figure 8c) showed that the isoelectric point of MoS 2 NS was about 3.8. This meant that the surface of MoS 2 NS displayed a positive charge when the pH value was below 3.8, while a negative charge above 3.8. As a typical cationic dye, MB molecules could strongly adhere to the MoS 2 NS through the electrostatic interaction once the surface charge of MoS 2 NS turned to negative, leading to an increasing of the q e values.  Similarly, the pH also markedly influenced the adsorption of heavy metals over MoS 2 NS (Figure 8b). The q e values of Cu 2+ , Cd 2+ , and Ag + evidently increased with an increasing pH, and stabilized at about 112.4, 117.0, and 64.4 mg g −1 , respectively. When the pH increased, the surface charge of MoS 2 NS turned to negative and the values gradually increased, which meant that stronger electrostatic interaction occurred between the heavy metal ions and MoS 2 NS at higher pH, resulting in improvement of the adsorption performance. In addition, the charge values of the heavy metal ions also showed visible effects on the adsorption capacity. Due to the lower value of the charge for Ag + , the q e value of Ag + was much lower than those of Cu 2+ and Cd 2+ , which was ascribed into the weaker electrostatic interaction between Ag + and MoS 2 NS. According to the Coulomb law, electrostatic interaction was in direct proportion to the value of the surface charge. The similar results were also found in Yang at al.'s studies [45].

Ion Exchange
According to previous studies [43,46], the ion exchange only occurred with heavy metals adsorption. It was well known that the affinity to the metal ions in the ion exchange process increased with the ion radius and the ion radius of Cd 2+ and Cu 2+ were 0.97 Å and 0.73 Å, respectively. If the ion exchange was the main adsorption mechanism, the number of the adsorbed Cd 2+ should be larger than that of Cu 2+ . Actually, in the system of 15 mg −1 L (Figure 8b), the molar adsorption capacity of Cd 2+ (1.04 mmol g −1 , 117.0 mg g −1 ) was visibly lower than that of Cu 2+ (1.75 mmol g −1 ,112.4 mg g −1 ), which indicated that the ion exchange was not the main mechanism during the heavy metals over MoS 2 NS. Similarly, Nguyen et al. also found that the ion exchange played a negligible role during the Cd 2+ and Cu 2+ adsorption over the activated carbon [43].

Conclusions
In summary, the ultrathin 2H-MoS 2 nanosheets with 1-2 layers were successfully obtained via a hybrid stagey with combination of quenching process and liquid-based exfoliation. The as-prepared 2H-MoS 2 nanosheets exhibited evident optical properties and could be accurately quantified with the absorbance at 615 nm in the range of 0.01-0.5 mg L −1 . Besides, the obtained 2H-MoS 2 nanosheets also showed a promising application in pollution control. It could be a candidate absorbent for the removal of dyes and heavy metals. This work provided an effective way for the large-scale fabrication of the two-dimensional nanosheets of transition metal dichalcogenides (TMDs) by liquid exfoliation.
Supplementary Materials: The following are available online at http://www.mdpi.com/2227-9717/8/5/504/s1, Figure S1. Linear fittings of dyes adsorption (a) and heavy metals (b and c) over bulk MoS2 and MoS2 NS with the Freundlich model, Table S1 Adsorption kinetics parameters of dyes and heavy metals adsorption over MoS2 NS.