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

**Siyi Huang 1, Ziyun You 1, Yanting Jiang 1, Fuxiang Zhang 1, Kaiyang Liu 1, Yifan Liu 1, Xiaochen Chen <sup>1</sup> and Yuancai Lv 1,2,\***


Received: 10 April 2020; Accepted: 24 April 2020; Published: 26 April 2020

**Abstract:** 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 <sup>±</sup> 0.04 mg mL<sup>−</sup>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<sup>−</sup>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−<sup>1</sup> of *qm* for methylene blue and methyl orange, respectively) and heavy metals (185.2, 169.5, and 70.4 mg g−<sup>1</sup> of *qm* for Cd2<sup>+</sup>, 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.

**Keywords:** transition metal dichalcogenides; liquid exfoliation; adsorption; quenching

### **1. 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 (MoS2) 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 MoS2 nanosheets were reported to show excellent prospects in pollution control [3,4]. Therefore, it was urgent to explore an effective method to produce ultrathin MoS2 nanosheets.

To date, a few methods have been reported for efficient preparation of ultrathin MoS2 nanosheets [5–10], 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 MoS2 nanosheets through mechanical methods, the production efficiency was rather low, which severely limited the large-scale applications. Meanwhile, although most of the chemical methods like hydrothermal and solvent thermal routes could produce large-scale few-layer MoS2 nanosheets, they generally needed strict reaction control, such as high temperature and pressure. Instead, due to the controllable operation and high production, liquid-based exfoliation was regarded as the most promising way for the production of ultrathin MoS2 in large scale. According to previous studies [7,8], the solvent showed a significant impact on the exfoliation of MoS2. Among which, pyrrolidone-based solvents like *N*-methyl-2-pyrrolidone displayed excellent MoS2 exfoliation efficiency with 0.3 mg mL−<sup>1</sup> of MoS2 nanosheets concentration, because they showed similar surface energy with MoS2. Nevertheless, considering their significant environmental risk, high toxicity and high-boiling points of the pyrrolidone-based solvents probably limited the large-scale application. To replace these toxic solvents, a number of polar solvents that own low boiling point and molecular weight were tested, such as water, methanol, ethanol, and isopropanol [11,12]. Unfortunately, given the different surface energy between the MoS2 and polar solvents, most of the polar solvents showed dissatisfactory exfoliation efficiency [11]. Interestingly, it was reported that the MoS2 exfoliation efficiency in the mixed solution with two of the polar micromolecular solvents was much better than those in the single solvent [13,14]. Meanwhile, it was worth noting that the MoS2 exfoliation efficiency could be significantly improved when some organic small molecules, surfactants, or polymers were added in the polar micromolecular solvents, such as sodium cholate, Tween 80, Tween 85, sodium naphthalenide, Brij 30, Brij 700, Triton X-100, and so on [15–17]. Nevertheless, the strong van der Waals interaction between the MoS2 layers still limited the MoS2 nanosheets production.

Recently, owing to the effective break of the van derWaals force between the MoS2 layers, quenching was found to be an effective way to exfoliate the graphene analogues [18–21]. Previous investigation showed that the high-quality ultrathin graphene sheets were fabricated by rapidly cooling the hot bulk graphite and pre-expanded graphite in aqueous solutions of NH4HCO3 and hydrazine hydrate, respectively [19,20]. Meanwhile, the boron nitride and MoS2 nanosheets were also synthesized via the rapid quenching of hot bulk boron nitride and MoS2 in the liquid N2 [18,21]. Although the production efficiency of the nanosheets was dissatisfactory, we suspect that if the bulk MoS2 was pretreated with the quenching process, the exfoliation efficiency of MoS2 nanosheets in the polar micromolecular solvents could be significantly improved.

Herein, a hybrid strategy with the combination of quenching process and liquid-based exfoliation was employed to fabricate the ultrathin MoS2 nanosheets. The microstructures, morphology, and optical properties were analyzed. In addition, the adsorption performance of dyes and heavy metals was also discussed.

### **2. Material and Methods**

### *2.1. Materials*

Ammonium tetrathiomolybdate ((NH4)2MoS4) was provided by Sam Chemical Technology Co., Ltd. (Shanghai, China). Hydrazine monohydrate (N2H4·H2O) was provided by Aladdin Reagent Co., Ltd. (Shanghai, China). Sodium hydroxide (NaOH), sulfuric acid (H2SO4), nitric acid (HNO3), methylene blue (MB), methyl orange (MO), AgNO3, CuSO4, and CdCl2 were obtained from Sinopharm Chemical Reagent Co. Ltd. (Shanghai, China). The double-distilled water was prepared with a Milli-Q water purification system (Milli-Q®Reference, Millipore, Billerica, Massachusetts, USA).

### *2.2. Fabrication of the MoS2 Nanosheets*

The MoS2 nanosheets (MoS2-NS) were prepared through a novel combined method including calcination at high temperature, quenching with liquid nitrogen, and ultrasonic-assisted peeling with hydrazine hydrate. Firstly, 4.000 g of (NH4)2MoS4 was calcinated under nitrogen atmosphere at 800 ◦C for 5 h with a rate of 5 ◦C/min and then a black powder (named bulk MoS2) was obtained. After that, the high-temperature bulk MoS2 was quickly transferred into a Dewar bottle containing liquid nitrogen until the liquid nitrogen gasified completely. Subsequently, the pre-expanded bulk

MoS2 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 MoS2 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 MoS2-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.

### *2.3. 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−<sup>1</sup> 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−<sup>1</sup> of the MB and MO concentration and the pH was adjusted to 6.0 ± 0.1 with 1 M H2SO4 solution, while the experiments for heavy metals were conducted under 25 ◦C with metal concentration from 0.5 to 30 mg L−<sup>1</sup> and the pH was adjusted to 5.0 <sup>±</sup> 0.1 with 1 M H2SO4 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 Cu2+/Cd2+/Ag<sup>+</sup> 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 H2SO4 solution) in 300 mL dyes/heavy metals solution (20 mg L−<sup>1</sup> for dyes and 15 mg L−<sup>1</sup> 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−<sup>1</sup> of dyes/heavy metals concentration and 0.10 g L−<sup>1</sup> of adsorbents.

### *2.4. Characterization*

The X-ray powder diffraction (XRD) data of the bulk MoS2 and MoS2-NS were tested with X-ray powder diffractometer (MiniFlex600, Rigaku, Milwaukee, Wisconsin, USA) coupled with a Cu *K*α line at 40 kV and 40 mA.

The microstructural features of prepared bulk MoS2 and MoS2-NS were observed with field emission scanning electron microscope (FESEM, Nova NanoSEM 230, FEI, Hillsboro, Oregon, USA), atomic force microscope (AFM, 5500, Agilent USA), and transmission electron microscope (TEM, TECNAI G2F20, FEI, Hillsboro, Oregon, USA).

The Raman data of bulk MoS2 and MoS2-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 concentrations of MoS2-NS dispersions were analyzed using UV-vis spectrophotometer (UV-2500, Shimadzu, Japan).

The Brunauer–Emmett–Teller (BET) surface areas of the bulk MoS2 and MoS2-NS were obtained from the analysis of N2-adsorption isotherms at 77 K using the N2 physisorption analyzer (ASAP2020, Micromeritics, Norcross, Georgia, USA).

### **3. Results and Discussion**
