3.1.2. 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 UV-vis 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 (*R2* = 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 as-prepared MoS2 NS dispersions was 0.65 <sup>±</sup> 0.04 mg mL<sup>−</sup>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<sup>−</sup>1, and the corresponding few-layer MoS2 NS yield was calculated to be as high as 25.9% in water.

**Figure 5.** Photographs (**a**), UV-vis absorption spectra (**b**), absorbance change with standing time (**c**) of the prepared MoS2 NS dispersions and photographs (**d**), UV-vis absorption spectra (**e**) and standard curve (**f**) of MoS2 NS dispersion in different concentrations.

### *3.2. Adsorption Behavior of MoS2-NS Towards Dyes and Heavy Metals*

### 3.2.1. 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 (Cu2<sup>+</sup>, Cd2+, 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].

**Figure 6.** Adsorption isotherms (**a**) and kinetics (**c**,**e**) of MB, MO for bulk MoS2 and MoS2 NS at 20 mg L<sup>−</sup>1; adsorption isotherms (**b**) and kinetics (**d**,**f**) of Cu2+, Cd2<sup>+</sup> and Ag<sup>+</sup> for bulk MoS2 and MoS2 NS at 15 mg L<sup>−</sup>1.

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<sup>2</sup>* values suggested that the Langmuir model better described the adsorption of dyes and heavy metals onto MoS2 NS and bulk MoS2 than the Freundlich model. Based on the Langmuir model fitting, the relative parameters like the maximum adsorption capacity (*qm*) and affinity constant (*KL*) for dyes and heavy metals were obtained and listed in Table 1. The *qm* values of MB and MO for MoS2 NS were 344.8 and 123.5 mg g<sup>−</sup>1, respectively, which were 12.77 and 6.94 larger than those (27.0 and 17.8 mg g−<sup>1</sup> for MB and MO, respectively) of bulk MoS2. Meanwhile, the similar results were observed in the heavy metal adsorption, indicating that the MoS2 NS exhibited much more excellent adsorption performance than the bulk MoS2. In addition, for the dyes, the higher *qm* and *KL* values of MB implied that MoS2 materials exhibited better adsorption capacity and affinity to MB. For heavy metals, the highest *qm* value occurred to Cd2<sup>+</sup> (185.2 mg g<sup>−</sup>1), following Cu2<sup>+</sup> (169.5 mg g<sup>−</sup>1) and Ag<sup>+</sup> (70.4 mg g<sup>−</sup>1), indicating that MoS2 NS were more beneficial to Cd2<sup>+</sup> and Cu2<sup>+</sup> adsorption than Ag+.


**Table 1.** Fitted parameters for the adsorption of dyes and heavy metals on MoS2 NS and bulk MoS2.

Figure 6c,d displayed adsorption kinetics data for dyes and heavy metals over MoS2 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 MoS2 NS (Figure 6e,f). As shown in Table S1, the higher *R2* 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 MoS2 NS and dye molecule or metal ions played a controlling role during the adsorption [38].

### 3.2.2. Adsorption Mechanism

Based on the results, the MoS2 NS showed much better dye or metal adsorption performance than bulk MoS2. 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 [41–43]. 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 adsorption mechanisms [41]. According to above-mentioned results, the obtained MoS2 NS owned

much larger specific surface area than bulk MoS2, while the MoS2 NS also exhibited more excellent adsorption performance on dyes and heavy metals. Thus, it could be inferred that the 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<sup>−</sup>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−<sup>1</sup> for Cu2<sup>+</sup>, Cd2<sup>+</sup>, 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−<sup>1</sup> for Cu2<sup>+</sup>, Cd2<sup>+</sup>, and Ag+, respectively). After standardization (Figure 7b,d), all of the *qe* ratios between the MoS2 NS and bulk MoS2 significantly decreased from 12.89 (MB), 5.61 (MO), 10.12 (Cu2<sup>+</sup>), 10.68 (Cd2<sup>+</sup>), 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.

**Figure 7.** Equilibrium adsorption capacity (**a**,**c**) and standardized equilibrium adsorption capacity (**b**,**d**) of dyes (MO and MB) and heavy metals (Cu2<sup>+</sup>, Cd2+, and Ag<sup>+</sup>) for bulk MoS2 and MoS2 NS.

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 *q*<sup>e</sup> values for MO suggested that the MB adsorption over MoS2 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*<sup>e</sup> values increased with the pH value and reached a peak (186.2 mg g<sup>−</sup>1) at pH = 6.0, and then gradually declined when pH > 6. Meanwhile, Zeta potential results (Figure 8c) showed that the isoelectric point of MoS2 NS was about 3.8. This meant that the surface of MoS2 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 MoS2 NS through the electrostatic interaction once the surface charge of MoS2 NS turned to negative, leading to an increasing of the *q*e values.

**Figure 8.** Effects of pH on dyes (**a**) and heavy metals (**b**) over MoS2 NS, and the Zeta potential (**c**) of MoS2 NS at different pH values. For dyes: 20 mg L−<sup>1</sup> of the initial concentration, for heavy metals: 15 mg L−<sup>1</sup> of the initial concentration.

Similarly, the pH also markedly influenced the adsorption of heavy metals over MoS2 NS (Figure 8b). The *q*<sup>e</sup> values of Cu2<sup>+</sup>, Cd2<sup>+</sup>, and Ag<sup>+</sup> 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 MoS2 NS turned to negative and the values gradually increased, which meant that stronger electrostatic interaction occurred between the heavy metal ions and MoS2 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*<sup>e</sup> value of Ag<sup>+</sup> was much lower than those of Cu2<sup>+</sup> and Cd2<sup>+</sup>, which was ascribed into the weaker electrostatic interaction between Ag<sup>+</sup> and MoS2 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 Cd2<sup>+</sup> and Cu2<sup>+</sup> were 0.97 Å and 0.73 Å, respectively. If the ion exchange was the main adsorption mechanism, the number of the adsorbed Cd2<sup>+</sup> should be larger than that of Cu2+. Actually, in the system of 15 mg−<sup>1</sup> L (Figure 8b), the molar adsorption capacity of Cd2<sup>+</sup> (1.04 mmol g<sup>−</sup>1, 117.0 mg g−1) was visibly lower than that of Cu2<sup>+</sup> (1.75 mmol g<sup>−</sup>1,112.4 mg g−1), which indicated that the ion exchange was not the main mechanism during the heavy metals over MoS2 NS. Similarly, Nguyen et al. also found that the ion exchange played a negligible role during the Cd2<sup>+</sup> and Cu2<sup>+</sup> adsorption over the activated carbon [43].

### **4. Conclusions**

In summary, the ultrathin 2H-MoS2 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-MoS2 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<sup>−</sup>1. Besides, the obtained 2H-MoS2 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.

**Author Contributions:** S.H. and Y.L. (Yifan Liu) conceived the study, designed the experiments and wrote the manuscript. S.H., Z.Y. and Y.J. performed the experiments. F.Z. and K.L. finished the characterization and data analysis. Y.L. (Yuancai Lv) and X.C. edited the manuscript. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by by the Open Project Program of National Engineering Research Center for Environmental Photocatalysis (Grant No. 201904), Fuzhou University.

**Conflicts of Interest:** The authors declare no conflict of interest.

### **References**


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