Hydrothermal green synthesis of MoS2 nanosheets for pollution abatement and antifungal applications

In this study, we report a green synthesis of MoS2 nanosheets (NSs) using a facile hydrothermal technique in the presence of l-cysteine. l-Cysteine can serve as a greener source of sulfur as well as a capping agent to help the growth of MoS2 nanosheets. The prepared materials were characterized by X-ray powder diffraction (XRD), scanning electron microscopy (SEM) with energy dispersive spectroscopy (EDS), electron transmission microscopy (TEM), X-ray photoelectron microscopy (XPS), and Brunauer, Emmett, and Teller (BET) analysis. The results showed that MoS2 NSs are of high crystallinity with a lattice spacing of 0.61 nm. The optical bandgap of MoS2 NSs nanosheets prepared using l-cysteine as a source of sulfur was found to be 1.79 eV. The photocatalytic degradation of MoS2 NSs towards methylene orange (MO) and rhodamine blue (RB) dyes under sunlight was found to be promising for practical applications. The fast kinetics of degradation of MO and RhB was observed over a wide range of pH range. Moreover, MoS2 NSs showed excellent antifungal activities against Trichophyton mentagrophytes and Penicillium chrysogenum fungus.


Introduction
During the last decades, the rapid increase in industrialization and human population has brought a multitude of contaminants to the aquatic environment. Synthetic organic dyes from dyeing industries make a substantial contribution. 1 Due to this millions of people have been dying every year because of water pollution. 2 Therefore, it is a prime concern to reduce or remove pollutants from their point source before releasing them into the environment. Adsorption, Fenton-oxidation, occulation, and photocatalysis techniques have been widely used to abate toxic effluents. [3][4][5] Photocatalytic degradation due to their efficiency can degrade pollutants without leaving unwanted residues. 6 TiO 2 and ZnO in this regard play a signicant role in the harvesting of light for the photocatalytic degradation of organic compounds. 7 However, inefficient utilization of light in the visible region and high electron-hole recombination rate prohibit its practical applications. Searching or modication of photocatalysts that working efficiently in the visible region is the prime concern in this topic. 8 Composite with different sensitizers, doping of different elements and morphology optimization are common strategies for the modication of photocatalysts. 9 More recently, the combination of photocatalyst with persistent luminescent materials as inner secondary light sources is introduced to enhance the photocatalytic efficiency of semiconductors. [10][11][12] Two-dimensional (2D) functional nanostructured materials, such as graphene, hexagonal boron nitride (hBN), graphitic carbon nitride (g-C 3 N 4 ) and metal dichalcogenides (MX 2 ), owing to their inherent physicochemical properties, including high specic surface-to-volume ratio, anisotropy, chemical inertness, better charge carrier separation, the rich option of host-guest species, and excellent tribological efficiency, have gotten a lot of attention for photocatalytic applications. 13,14 Moreover, they are widely used in a variety of elds, such as lubricants, energy storage, electrocatalysis, magnetoresistance, organic catalysis, sensing, and eld transit. 14,15 Recently, MX 2 -based transition metal chalcogenides (TMDs) where M (¼ Mo, W, etc.) is a transition metal of group VI and X (¼ S, Se, etc.) is a chalcogen that has been received a lot of attention owing to unique electrical, optical and mechanical properties. Among 2D nanomaterials, MoS 2 due to intriguing physicochemical properties shows an application in antibacterial, 16,17 biomedical, 18,19 transistors, 20 photodetectors, 21 and, solar cells. 22 Bulk MoS 2 is an indirect bandgap semiconductor. However, as the number of layers is reduced to a few, bilayers, and even monolayers, the bandgap becomes direct. 23 Such direct bandgap semiconductor materials are suitable for harvesting light. Hence, the synthesis of MoS 2 with a few layered structures is a prime concern for nanosheets for photocatalytic applications such as dye degradation and solar energy conversion. 24,25 MoS 2 NSs with enhanced properties have been synthesised by various techniques, including chemical vapour deposition (CVD), 26 thermal reductions, 27 hydrothermal, 28 laser ablation, 29 Liquid Phase Exfoliation (LPE), 30,31 and sol-gel methods. 32,33 Despite the quality of MoS 2 NSs synthesized by the aforementioned techniques, but they require highly sophisticated equipment and commonly used environmental malignant reagents. Most of the synthesis methods were used organic solvents, and surfactants during the synthesis of MoS 2 NSs, which introduce toxicity to the environment and human health. Therefore, searching for kind reagents for the preparation of MoS 2 NSs should require considerable attention. Biomoleculeassisted synthetic pathways have been a promising technique in recent years because they are greener and having suitable chemistry for the preparation of nanomaterials.
Green synthesised nanostructured materials play a signicant role in practical applications, including medicinal and environmental. Biological ways of synthesizing nanoparticles using microorganisms, enzymes, fungi, and plants or plant extracts are eco-friendly green synthesis alternatives to chemical and physical methods. 18,19,[34][35][36] In this regard, amino acids due to interesting aqueous chemistry are widely applicable as a reducing agent, capping agent and a source of dopant atoms during the synthesis of nanomaterials. 37 Among these, Lcysteine is a sulphur-containing derivative resulting from the oxidation of the side chains of cysteine amino acid thiol. It functions as an antioxidant, shielding tissues from radiation and toxins and thereby slowing the ageing process. L-Cysteine also reported as a sulphur source for the preparation of MoS 2 . For example, 1D CNTs-MoS 2 hybrid materials synthesised by hydrothermal method using L-cysteine as sulphur soured for anodized materials in lithium-ion batteries. 38 Similarly, Veeramalai et al. 39 reported MoS 2 layered structure by the reduction of MoO 3 using L-cysteine as a sulphur source. The synthesised MoS 2 showed high eld performance applications. Despite the potential of L-cysteine as a source of sulphur and capping agent to protect an irreversible agglomeration, there is no comprehensive report on the facile hydrothermal synthesis and photocatalytic investigation towards dye degradation of MoS 2 NSs.
Hence, we aimed to synthesis MoS 2 NSs using a facile hydrothermal method. (NH 4 ) 6 Mo 7 O 24 $4H 2 O salt and L-cysteine are the only precursors for the preparation of MoS 2 NSs. The electronic, crystal phase formation, composition and surface morphology of the synthesised MoS 2 nanosheet powder was investigated by UV-Vis, UV-DRS, XRD, FT-IR, XPS, TEM/SAED, and FE-SEM/EDX. The Brunauer-Emmett-Teller (BET) method used to determine the surface area analysis. The photocatalytic activities MoS 2 NSs was investigated by the degradation of the two stable organic dyes, such as rhodamine B (RhB) and methylene orange. These two dyes commonly found in industrial wastewater and are extremely toxic to human health due to their solubility, carcinogenicity, and teratogenicity. Moreover, the antifungal activity of MoS 2 was studied against Penicillium chrysogenum and Trichophyton mentagrophytes fungus.

Materials
All of the chemicals used in this analysis were analytical grade levels from Sigma-Aldrich, and Himedia. All chemicals were directly used without further purication. Specically, ammonium molybdate tetrahydrate [(NH 4 ) 6 Mo 7 O 24 $4H 2 O], and Lcysteine (C 3 H 7 NO 2 S) were obtained from Himedia, India. Methylene orange (MO), and rhodamine blue (RhB) were supplied from Sigma-Aldrich, India. The ultrapure water (Milli-Q) was used as a solvent throughout the whole experiment.

Nanosheet MoS 2 synthesis
MoS 2 NSs were synthesized by a facile and green hydrothermal synthesis approach using L-cysteine as a sulphur source. Particularly, 2.0 g (NH 4 ) 6 Mo 7 O 24 $4H 2 O and 4.0 g L-cysteine were mixed in 50 mL deionized water. The solution was then sonicated for 30 minutes to create a clear solution. In a muffle furnace, the solution was poured into a 100 mL stainless steel autoclave and heated at 200 C for 12 hours (Fig. 1). The autoclave was cooled to room temperature aer the reaction had completed. Next, the black precipitate was centrifuged and thoroughly washed with water to remove unreacted residue and dried in the oven at 70 C for 18 hours. The black powder, the assynthesised MoS 2 nanosheet, was kept for further use (ESI, Fig. 1 †).

Characterizations
PANalytical Xpert Pro diffractometer with, Cu K 1 ¼ 0.154056 nm as a radiation source was used to record the X-ray diffraction (XRD) of MoS 2 NSs. Transmission electron microscopy (TEM) images were recorded from FEI TECNAI G2 S-Twin, equipped with a 200 kV eld emission gun (FEG). The eld emission scanning electron microscopy (FE-SEM) images were obtained from JEOL, JSM-7600F SEM at an accelerated voltage of 0.1 to 30 kV equipped with X-ray dispersive spectroscopy (EDX). The chemical composition of the MoS 2 NSs was detected by X-ray photoelectron spectroscopy (XPS, ESCALAB 250Xi). The Brunauer-Emmett-Teller (BET) (BELSORB-28SA/18SA/18PLUS) analyser studied the surface area and pore size from the N 2 adsorption data. 40,41

RhB and MO photocatalytic degradation
The photocatalytic activity of the MoS 2 NSs towards the degradation of MO and RhB was investigated under natural sunlight irradiation. The adsorption-desorption equilibrium of dye was carried in the dark for 30 min before illumination. Specically, 30 mg of MoS 2 NSs was taken for 50 mL of 20 ppm dye solution. The sample was in dark to attain the adsorption-desorption and illuminate at midday (11 am to 3 pm) sunlight. At different time intervals, 3 mL of the solution took from the reaction vessel and centrifuged at 5000 rpm for 5 min to separate the catalyst. The residual concentration of MO and RhB in the solution was determined using a UV-visible spectrophotometer at l max ¼ 500 nm and 553 nm, respectively. The quantitative explanation of adsorption in the dark and degradation under sunlight was calculated by eqn (1) and (2) respectively.
where q (mg g À1 ) the amount of MO adsorbed on the surface of MoS 2 nanosheet; V (L) is the volume of the aqueous solution, m (g) is the mass of MoS 2 nanosheet, C 0 (mg L À1 ), the initial concentration of MO/RhB dye; C t (mg L À1 ) the concentration of MO/RhB dye aer adsorption/degradation. Furthermore, the antifungal activities of MoS 2 NSs using Agar-Well diffusion method was further investigated against the T. mentagrophytes and P. chrysogenum fungus. The detailed protocol presents in ESI 1. †

Characterizations of MoS 2 nanosheets
The UV-Vis absorption spectrum of MoS 2 NS was recorded to investigate the optical property. As shown in Fig. 2, the absorption edge of MoS 2 NS extends to the visible region.
Moreover, the optical bandgap energy of MoS 2 NS was calculated from the UV-DRS spectrum (Fig. 3).  42 The broadened of the (002) peak and diffraction at a low angle (2q: 13.6 ) than bulk MoS 2 (14.37 ) shows the formation of few or single layers of MoS 2 like graphene. 43 The FTIR spectrum of the MoS 2 NSs was recorded to investigate the composition of samples based on the vibrational properties (Fig. 5). The spectrum of MoS 2 NSs shows the broad bands between 3748 and 3400 cm À1 attribute to the characteristics of O-H stretching of the intermolecular and intramolecular hydrogen bonds. 44,45 The characteristic vibrational   bands at 3300-2800 cm À1 and 1488 cm À1 can be assigned to the stretching of the C-H alkyl stretching band in L-cysteine. 46 Thus, the FTIR results further affirmed the presence of residual amino acid on the surface of MoS 2 . The medium absorption bands observed at 756 cm À1 , 1091 cm À1 , and 1400 cm À1 are due to MoS 2 . 47,48 Moreover, the band between 540 cm À1 and 400 cm À1 credit to the S-S bond vibrations. 47 The composition and chemical state of MoS 2 NSs was studied by X-ray photoelectron spectroscopy (XPS).  Fig. 6b are deconvoluted with only one function, indicating the presence of only one molybdenum chemical species at the surface. 49,50 Moreover, the binding energy shows a shi as compared to the elemental Mo peaks affirms the formation of Mo 4+ chemical state. 39,51 Likewise, a similar analysis was done on sulfur, obtained characteristic peaks at the binding energy of 161.58 eV and 162.78 eV due to the S 2p 3/2 , and assigned to S 2p 1/2 , respectively. 52,53 SEM and TEM microscopic techniques were used to investigate the surface morphology of MoS 2 NSs. Fig. 7a-c shows the SEM image of the synthesized MoS 2 NSs. The image shows an aggregated and rough surface morphology of MoS 2 NSs. Interesting sheet-like morphology observed under TEM analysis (Fig. 8). As it is shown in Fig. 8a and b, a few layers and aggregated MoS 2 were synthesized via a facile hydrothermal method. The selected area electron diffraction (SAED) pattern (Fig. 8c) of the MoS 2 sheet shows the smoothness of the concentric circle, which indicates the poor crystallinity of the sheet. Moreover, the (002), (100), and (001) planes keep stable, indicating the good stability of MoS 2 along this plane.

RhB and MO dye degradation via photocatalysis
The pH of the solution an important factor that affects the surface charge of the photocatalyst and the charge of the    organic dye. As a result, the electrostatic interaction between the dye molecule and catalyst surface is highly affected by the pH of the solution. Methyl orange (MO) and rhodamine B (RhB) dyes were selected as representative anionic and cationic dyes respectively, for the photocatalytic investigation of MoS 2 NSs. The acidic (pH ¼ 4), neutral (pH ¼ 7) and basic (pH ¼ 9) medium were optimized to study the pH effect. Fig. 9 shows the effect of pH on the photocatalytic degradation of MO. As can be seen in Fig. 9, MoS 2 NSs (30 mg) show a strong potential to degrade MO (20 mg L À1 ) at a given pH of the solution. Still, degradation of MO shows fast kinetics and almost complete degradations at pH ¼ 4 due to the protonated surface at low pH enhance the electrostatic interaction with the anionic dye. Alkaline pH (pH ¼ 9) also suitable to get a complete degradation of MO slightly at a longer time (Fig. 9c) than acidic. The increase in hydroxyl ions concentration in the solution facilitated the formation of further hydroxyl radical; this could be plausible for an enhanced reaction rate of degradation under alkaline conditions. The neutral pH (pH ¼ 7) shows a relatively poor rate of degradation and efficiency (72%) (Fig. 9b). Hence, the acid or alkaline medium is suggested to get a better photocatalytic degradation efficiency of MO.
A similar investigation on the degradation of RhB performed at various pH of the solution. The potential of MoS 2 NSs also observes for the degradation of a stable RhB dye. As shown in Fig. 10, complete degradation of 50 mL RhB (20 mg L À1 ) using 80 mg MoS 2 NSs at acid and neutral solutions. Similar justication on the effect of pH for the degradation of RhB is applied to as MO. RhB was successfully removed with a high rate of degradation was observed at neutral pH (Fig. 10b). Concomitantly, the absorption maxima of RhB at l max ¼ 553 nm showed a shi to a lower wavelength region during the degradation process, which attributes to the formation of intermediate products in the meanwhile.
The photocatalytic activity of MoS 2 NSs was further investigated at various doses of the catalyst. At a xed amount of dye concentration (50 mL, 20 mg L À1 ), at neutral pH and room temperature. Fig. 11 shows the effect catalyst dose for the removal of MO at the various dose of MoS 2 (10 mg, 20 mg, and     However, yet there is no signicant difference between 20 mg and 30 mg dosages, and therefore 20 mg of could be considered as optimum for 50 mL MO (20 mg L À1 ). The same analogy was observed for RhB degradation. As shown in Fig. 12, the degradation of RhB (50 mL, 20 mg L À1 ) was investigated at various doses of MoS 2 NSs (40 mg, 60 mg, and 80 mg). The degradation efficiency increases with an increasing dose of MoS 2 NSs. The maximum value was obtained at 80 mg MoS 2 NSs dose, considered as optimum. Fig. 13 shows the photocatalytic degradation mechanism of organic dye using catalysts. Firstly, the dye adsorbed on the surface of the MoS 2 NSs in the dark because of the interaction between the shade and MoS 2 NSs. Owing to the low bandgap of the MoS 2 nanosheets, electrons and holes are generated. Electrons in the conduction band react with the available oxygen to form superoxide radicals (O 2 c). Similarly, holes in the valence band react with water to form hydroxyl (OHc) radicals. The formation of the OHc and O 2 c radicals has appeared. As a result, the generated OH radicals degrade the adsorbed dyes efficiently at room temperature by simple magnetic stirring. The production of radicals results in the mineralization of dyes into CO 2 , H 2 O, and other gaseous products.

Anti-fungal activity of MoS 2 nanosheets
The antimicrobial activity of MoS 2 NSs against standard strains of Trichophyton mentagrophytes and Penicillium chrysogenum presented in Fig. 14. The higher inhibition zone is observed in Penicillium chrysogenum with higher concentrations of the synthesized material, i.e., 5 mg mL À1 of MoS 2 nanosheet solutions. Mainly due to the increase in concentrations of MoS 2 nanosheets, the diameter of the inhibition zone increased, i.e. 10 mm at 1 mg mL À1 , 14 mm at 3 mg mL À1 and 16 mm at 5 mg mL À1 . At higher concentrations, the diameter of the inhibition zone was more than the same concentrations of the standard drug inhibition (uconazole (5 mg mL À1 )). This result shows that a newly prepared nanosheet shows promising and requires further study for antifungal-related applications.

Conclusions
In conclusion, MoS 2 NSs are synthesised successfully via a simple and green hydrothermal method using ammonium molybdenum hydrate and L-cysteine. In this process, to help the growth of MoS 2 NSs, L-cysteine can be used as a sulfur source and a capping agent. The synthesized MoS 2 NSS exhibited excellent photocatalytic degradation efficiency towards MO and RhB dyes under sunlight irradiation. It was achieved >99% degradation efficiency relatively fast rate reaction. The narrow bandgap (1.79 eV), and large surface area (6.46 m 2 g À1 ) of the MoS 2 NSs are responsible for the degradation of the dyes under sunlight. MoS 2 NSs shows the toxic effect against fungi growth.

Conflicts of interest
There are no conicts to declare. Fig. 13 The plausible degradation mechanism of organic dye.