Emerging WS2/montmorillonite composite nanosheets as an efficient hydrophilic photocatalyst for aqueous phase reactions

Tungsten disulfide (WS2) as one of transition metal dichalcogenides exhibits excellent catalytic activity. However, its catalytic performances in aqueous phase reactions are limited by its hydrophobicity. Here, the natural hydrophilic two-dimensional clay was used to enhance the dispersibility of WS2 in aqueous phase. WS2/montmorillonite (WS2/MMT) composite nanosheets were prepared via hydrothermal synthesis of WS2 on the surface of montmorillonite from WCl6 and CH3CSNH2. The microstructure and morphology show that WS2 nanosheets are assembled parallelly on the montmorillonite with the interface interaction. Through the support of montmorillonite, WS2/MMT possesses higher photocatalytic ability for aqueous phase reactions than WS2, which could be due to the synergistic effect of higher adsorption property, higher hydrophilicity, dispersibility and more catalytic reaction site. The strategy could provide new ideas for obtaining novel hydrophilic photocatalyst with excellent performance.

energy storage matrix 30,31 and drug delivery systems 32 . Based on the special structure and property of MMT, hydrophilic composite nanosheets might be constructed through MMT supporting WS 2 nanosheets, which could be used as an efficient photocatalyst for aqueous phase reactions. Hazardous wastewaters with organic dye are greatly harmful to the environment [33][34][35][36][37] . Photocatalysis technology is a potential approach to treat these waste waters 38 , and design of the efficient photocatalyst is one of the key procedures 39,40 .
Herein, we first designed and prepared the WS 2 /montmorillonite (WS 2 /MMT) composite nanosheets as a hydrophilic photocatalyst for aqueous phase reactions. WS 2 /MMT was successfully fabricated through facile in-situ hydrothermal synthesis of WS 2 on MMT. The microstructure and morphology were characterized, and the photocatalytic ability for aqueous phase reactions was evaluated by degradation of organic dye. The effect of MMT on catalytic ability was studied, and the possible catalysis mechanism for photocatalytic degradation of RhB were explored and illustrated in detail.

Results
As schematically depicted in Fig. 1, the WS 2 /montmorillonite (WS 2 /MMT) composite nanosheets were hydrothermally synthesized with tungsten chloride and thioacetamide at 220 °C for 24 h. Firstly, the montmorillonite (MMT) was added in water solution of WCl 6 and CH 3 CSNH 2 . The surface of MMT is negatively charged for the Si 4+ lattice replaced by Al 3+ . Conventional tungstate precursors such as Na 2 WO 4 and (NH 4 ) 10 W 12 O 41 are unable to adsorb on of MMT. Therefore, WCl 6 was employed as a precursor of tungsten, and W 6+ was adsorbed on MMT by electrostatic interaction. In hydrothermal conditions, the thioacetamide was pyrolyzed and released H 2 S, and WCl 6 was reduced to form WS 2 by sulfurization. Finally, the WS 2 nanosheets were nucleated and grown, and the as-prepared WS 2 was assembled on MMT to prepare WS 2 /MMT nanosheets.
The crystallographic structure was inspected by XRD measurements, and the patterns of MMT, WS 2 , WS 2 / MMT, WS 2 /MMT-0.5 and WS 2 /MMT-2 are presented in Fig. 2a. In the pattern of MMT, the reflection at 7.2° (2θ) is attributed to the (001) reflection of Na-montmorillonite, indicating that the d 001 basal spacing is 1.22 nm. The diffraction peaks at 19.7° and 34.7° (2θ) are corresponding to (100) and (110) planes of MMT. The diffraction peaks in the pattern of hydrothermally synthesized WS 2 can be attributed to 2H-WS 2 phase (JCPDS#08-0237). The reflection at 14.2° due to the (002) diffraction shows the distance of lattice plane along (002) is 0.62 nm for WS 2 multilayer. The reflections assigned to (100) and (110) planes of WS 2 are found at 32.7° and 58.4°, respectively. The main reflections of MMT and WS 2 were observed in XRD pattern of WS 2 /MMT. Compared with WS 2 /  MMT, the intensity of the reflections of WS 2 is relatively lower in the pattern of WS 2 /MMT-0.5, indicating the lower content of WS 2 . The reflection assigned to (100) plane of WS 2 has higher intensity in the WS 2 /MMT-2, while the diffraction peak corresponding to (002) plane is broader and lower, which could be attributed to lower stacking of WS 2 layers.
The interface interaction was studied by FTIR analysis. In the FTIR spectrum of MMT (Fig. 2b), the bands due to vibration of Si-O is found at 471 and 1022 cm −1 , indicating that a silicon-oxygen tetrahedron exists in the layered structure of MMT. The broad band at 3620 cm −1 is due to the aluminum hydroxy stretching vibration. The hydroxy bending vibration and H-O-H stretching vibration corresponding to hydrogen bonding water could be observed at 1637 and 3453 cm −1 . The bands at 1401, 1613 and 3125 cm −1 in the FTIR spectrum of WS 2 are associated with the W-S bending vibration and stretching vibration 41 . For WS 2 /MMT, the bands corresponding to hydroxy decrease significantly, and the band of Si-O shifts to 1037 cm −1 from 1022 cm −1 , which suggests the interface interaction between WS 2 and MMT.
The chemical statuses of the samples were investigated using XPS. Fig. 3a shows the XPS survey spectra of samples in the range 0-700 eV. Compared with the XPS spectrum survey of MMT, the peaks of W and S are observed in the WS 2 /MMT. The binding energy of Si 2p in WS 2 /MMT (Fig. 3b) shifts to a little higher energy state compared with that of MMT, indicating electronic interaction between WS 2 and MMT. The W 4 f peaks of WS 2 / MMT ( Fig. 3c) are located at the binding energies of 32.2, 34.3, 36.1 and 38.2 eV, respectively. The binding energies of W 4f 7/2 and W 4f 5/2 peaks at 32.2 and 34.3 eV are correspond to W 4+ , and the binding energies at 36.1 and 38.2 eV are attributed to W 6+ (oxide states), which might be due to the partial oxidation of tungsten on the surface and the interface interaction between WS 2 and MMT. In the scans of S 2p electrons of WS 2 /MMT (Fig. 3d), the binding energies of S 2p 3/2 and S 2p 1/2 peaks are 161.9 and 163.3 eV, respectively.
The morphologies of samples were characterized with SEM. In the SEM image of MMT (Fig. 4a), the sample presents a lamellar morphology and smooth surface, which is favorable for the supporting of WS 2 . The WS 2 synthesized by hydrothermal method exhibits the agglomerated particles with irregular shapes (Fig. 4c). For WS 2 / MMT (Fig. 4e), WS 2 and MMT exhibit two dimensional morphology and stack with each other to form layer structure. In the WS 2 /MMT composites, MMT could reduce the agglomeration of WS 2 . The energy disperse spectrum of WS 2 /MMT indicates that the mass contents of W and S element are 29.84 Wt% and 10.60 Wt%, which is in accord with the theoretical element ratio of WS 2 .
Further details of the microstructure could be obtained by TEM and HRTEM. The TEM image of MMT ( Fig. 4b) shows dispersed nanosheet. As shown in Fig. 4d, the synthesized WS 2 is irregular nanoparticle. The HRTEM image indicates that the (002) plane WS 2 nanoparticle is 0.62 nm, in accordance with XRD result. From the TEM and HRTEM of WS 2 /MMT (Fig. 4f), the microstructure of sample exhibits composite nanosheets, and The specific surface area was characterized by nitrogen adsorption-desorption isotherms. In the curves of MMT and WS 2 /MMT (Fig. 5a), the type IV adsorption branches are corresponding to the mesoporous structure. The specific surface area of WS 2 /MMT and MMT are calculated to be 16.13 and 38.02 m 2 •g −1 , respectively. No hysteresis loop is observed in the curve of WS 2 , and the specific surface area is 6.56 m 2 •g −1 . Compared with WS 2 , WS 2 /MMT has relatively higher specific surface area, and it might be because MMT could reduce the stack and inhibit the agglomeration of WS 2 nanosheets. The pore size distributions are shown in Fig. 5b. The average pore size of MMT is about 3 nm, while those of WS 2 and WS 2 /MMT are around 40 nm. Due to the support of MMT, WS 2 /MMT composite nanosheets possess larger special surface area than WS 2 , which might provide more reactive sites to enhance photocatalytic activity.
The UV-vis diffuse reflectance spectra of samples are shown in Fig. 6a. The visible light absorption of MMT is very weak. WS 2 and WS 2 /MMT exhibit considerable visible absorption, which is consistent with the black color of www.nature.com/scientificreports www.nature.com/scientificreports/ the samples. It is of great significance for visible light photocatalytic application 42 . The band gap energies of MMT, WS 2 and WS 2 /MMT (Fig. 6b) are 3.86, 1.37 and 1.51 eV, respectively. Compared with WS 2 , WS 2 /MMT possesses larger band gap energy, which could be due to the effects of the few layered WS 2 and MMT.
Compared with the PL peak of WS 2 (Fig. S1), the peak shape and position of WS 2 /MMT are similar, while the peak intensity is higher. It might be related to luminescence-inactive multilayer structure in WS 2 . In the PL spectrum of MMT, the peak at 354 and 398 nm could be due to the intrinsic defects of surplus oxygen on the surface and intrinsic diamagnetic defect center, respectively. The PL peak intensity of WS 2 /MMT is weaker than that of MMT, which is attributed to the less recombination of photo-generated carriers.
Photodegradation of organic dyes was applied to evaluate the photocatalytic abilities of samples for aqueous phase reactions. The variation of decoloration rate with MMT, WS 2 , WS 2 /MMT, WS 2 /MMT-0.5 and WS 2 /MMT-2 as catalysts is shown in Fig. 7a. The RhB aqueous solution is stable with visible light irradiation during 1 h, and the decoloration rate has hardly the change without catalyst. Adsorption equilibrium was reached without light irradiation for 15 min, and physical adsorption of samples was recorded. MMT has the highest adsorption capacity up to 17.0%, and the adsorption rate of WS 2 is lowest. The WS 2 /MMT has higher adsorption rate than WS 2 , and the adsorption rate has an increase tendency with the increasing of MMT content. It might be because the special structure of MMT, and the high specific surface area and abundant surface hydroxyl groups are in favor of the adsorption for RhB 43,44 . WS 2 /MMT has the highest degradation speed, and the overall decoloration rate of RhB is up to 99.8% after visible light irradiation for 45 min. The degradation speeds of WS 2 /MMT-0.5 and WS 2 /MMT-2 are higher than that of WS 2 , but lower than WS 2 /MMT. Photocatalytic ability of WS 2 /MMT composite nanosheets is enhanced via the support of MMT. It might be because the MMT sheets prevent WS 2 nanosheets aggregation, and improve the hydrophilicity and dispersibility of aqueous phase, which could supply composites more reactive sites. As shown in Fig. S4, the photocatalytic ability of WS 2 /MMT does not show obvious change after four cycles, indicating the high stability of WS 2 /MMT in the aqueous phase photocatalytic reaction process. WS 2 /MMT also shows excellent photodegradation performance for Methylene blue (MB), Methyl orange (MO) and Congo red (CR) in the aqueous phase (Fig. S5). The photodegradation of RhB was observed to follow pseudo-first-order kinetics 21 according to the formula (ln(C/C 0 = −kt)), where C and C 0 is the homologous and initial concentration, and k is apparent reaction rate constant. The apparent reaction rate constants of WS 2 /MMT and WS 2 are 0.16 and 0.09 min −1 (Fig. S6), indicating the higher photocatalytic ability of WS 2 /MMT.  www.nature.com/scientificreports www.nature.com/scientificreports/ In Fig. 7b, WS 2 /MMT composite nanosheets uniformly disperse in water, while WS 2 almost floated on the aqueous phase, which could be attributed to the hydrophobicity of hydrothermally synthesized WS 2 . Contact angles of MMT, WS 2 /MMT and WS 2 (Fig. S2) are 18.7°, 32.3° and 54.5°, respectively. With the supporting of MMT, WS 2 /MMT has better hydrophilicity than WS 2 .

Discussion
As shown in Fig. S3, the UV-vis absorption peak of RhB is located at 554 nm, while no obvious absorption peak is observed after RhB is photodegraded. The possible photocatalysis mechanism for the degradation of RhB is shown in Fig. 7c. With the excitation of visible light, the photoinduced electrons (e -) and holes (h + ) are generated in WS 2 , respectively 45 . hydroxyl groups could capture h + to form hydroxyl radicals (OH · ), which could restrain recombination and improve the photocatalytic ability 46 . The photoexcited eelectrons might induce the O 2 with O 2 , and these h + , OH · and O 2 could photo-oxidize organic molecules RhB. In conclusion, WS 2 /MMT nanosheets were prepared by the hydrothermal method, which were utilized as an efficient hydrophilic photocatalyst for aqueous phase reactions. Few-layered WS 2 nanosheets are grown parallelly on MMT, and MMT could reduce the stack and inhibit the agglomeration of WS 2 nanosheets, which supply composites more catalytic reaction sites. With WS 2 /MMT as photocatalyst, the overall decoloration capacity of RhB was up to 99.8%. Through the support of MMT, WS 2 /MMT possesses high hydrophilicity and dispersibility of aqueous phase, which is conducive to the enhancement of catalytic ability. The WS 2 /MMT composite nanosheets have potential to treat organic waste water. The strategy could provide insights for construction of efficient hydrophilic photocatalyst with excellent activity for environmental treatment.  www.nature.com/scientificreports www.nature.com/scientificreports/ thioacetamide (CH 3 CSNH 2 ) and Rhodamine B were purchased from Sinopharm Chemical Reagent Co. Ltd. All reagents were analytical grade and used without further purification 46 . Preparation. The WS 2 /montmorillonite (WS 2 /MMT) composite nanosheets were synthesized by a facile hydrothermal method. In a typical experiment, 1.785 g of tungsten chloride and 3.415 g of thioacetamide were dissolved in 60 mL of deionized water and mechanically stirred for 30 min at room temperature. 1.000 g of MMT was added in the solution, and the mixture suspension was stirred for another 30 min and sonicated for 10 min at room temperature. Then, the suspension was transferred into a 100 mL Teflon-lined stainless steel autoclave, heated up to 220 °C, and kept for 24 h 11 . After cooling naturally, the precipitates were collected by centrifugation, and subsequently washed several times with deionized water. The final products were obtained after drying at 60 °C for 24 h. For comparison, pure WS 2 samples were synthesized by a similar process, without MMT. Other samples were similarly prepared with different additive amounts of tungsten chloride and thioacetamide. A sample prepared with 0.893 g of tungsten chloride and 1.708 g of thioacetamide was labeled as WS 2 /MMT-0.5. A sample with 3.570 g of tungsten chloride and 6.830 g of thioacetamide was labeled as WS 2 /MMT-2 6 . Characterization. Powder X-ray diffraction (XRD) patterns of the samples were obtained on a RIGAKU D/max-2550 PC X-ray diffractometer with Cu Kα radiation (λ = 0.15406 nm) at a scan rate of 0.02°/s 32 . Fourier transform infrared (FTIR) spectra of the samples were obtained between 4000 and 400 cm −1 on a Nicolet Nexus 670 FTIR spectrophotometer using KBr pellets. Scanning electron microscopy (SEM) images were obtained with a JEOL JSM-6360LV scanning electron microscope at an accelerating voltage of 5 kV, which equipped with energy dispersive spectrometer (EDS). Transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) were operated with a JEOL JEM-2100F transmission electron microscope at an acceleration voltage of 200 kV 42 . The N 2 adsorption-desorption isotherms were record at 77 K and analyzed using an ASAP 2020 surface area analyzer. X-ray photoelectron spectroscopy (XPS) measurements were performed using an ESCALAB 250 spectrometer. The UV-vis diffuse reflectance spectra (UV-vis DRS) were obtained with a Shimadzu UV2450 UV-vis spectrophotometer, and barium sulfate was used as reference. The photoluminescence (PL) experiment were conducted on a Hitachi F-4500 fluorescence spectrometer using an excitation wavelength of 254 nm 3 . The contact angles of the samples were measured using the sessile-drop technique using a goniometer (GBX, France).

Materials
Photocatalytic activity evaluation. Photodegradation of Rhodamine B (RhB) was selected as a typical reaction to evaluate the photocatalytic activity of samples for aqueous phase reactions. The light source was a 150 W high pressure mercury lamp with wave length λ > 400 nm. In a typical photocatalytic experiment, 100 mg of catalyst was added in 100 mL RhB aqueous solution (0.02 mmol/L). Firstly, the mixture suspension was magnetically stirred in the dark for 15 min to ensure the establishment of an adsorption-desorption equilibrium between the catalyst and RhB aqueous solution. Then, the reaction vessel was positioned with light irradiation, and 1 mL of 3% H 2 O 2 was added as oxidant to initiate the reaction. About 3 mL of analytical sample was withdrawn from the reaction suspension every 5 min, and the catalyst was removed by a centrifuge. The concentration of the photodegradable compound was monitored by recording the absorbance (A) of the clarified solution at 554 nm with an UV-visible spectrophotometer. The decoloration rate (%) was calculated from the formula: decoloration rate (%) = (A 0 − A)/A 0 × 100%, where A 0 was the initial absorbance, and A was the absorbance at homologous times 25 . The catalyst was centrifuged for the next cycle. Photocatalytic degradation of Methylene blue (MB), Methyl orange (MO) and Congo red (CR) were performed under the similar condition.

Data availability
The data that support the findings of this study are available from the corresponding author on reasonable request.