Hydrothermal synthesis of SnS2/MoS2 Nanospheres for enhanced adsorption capacity of organic dyes

The SnS2/MoS2 nanospheres (NPs) were prepared by a facile hydrothermal route, and microstructure and morphology were investigated by SEM, TEM, XRD and BET. The SnS2/MoS2 NPs exhibited the excellent adsorption properties for organic dyes, and the maximum adsorption capacity of SnS2/MoS2 to Rhodamine B (RhB) is 125 mg g−1 at room temperature. The adsorption process is well fit by the pseudo-second-order adsorption model and Langmuir isotherm model. Moreover, SnS2/MoS2 NPs has outstanding adsorption capacity for both cations and anions organic dyes, and the maximum adsorption capacity for methylene blue (MB), crystal violet (CV) and malachite green (MG) were 202, 165 and 175 mg g−1 respectively. It is attribute to the high specific surface area (101.06 m2 g−1) and small mesopores (3.23 nm) provide numerous adsorption active sites for adsorption of organic dyes. The reusability experiment demonstrated the SnS2/MnS2 NPs could be reused for 5 times. The result show that the SnS2/MoS2 NPs is a potential adsorbent for removal of organic dyes from wastewater.


Introduction
The unreasonable discharge of industrial wastewater contains high concentrations of various organic dyes and their derivatives, which are harmful to human health and eco-environment [1]. Rhodamine B (RhB) as a representative of organic dyes, widely used in the field of textile and leatherwear [2]. In resent years, the removal of RhB has been extensively explored because it is difficult to decompose and harmful to human [3,4]. Previously developed methods for removing organic dyes including chemical precipitation [5], photocatalytic degradation [6], membrane separation [7] and adsorption [8]. Compare with other methods, adsorption is a great potential method due to easy treatment, low cost and high efficiency.
Mesoporous transition metal sulfides have attracted widespread attention the field of wastewater treatment, which attribute to high specific surface area, special microstructure and higher surface activity [9]. MoS 2 as a representative of mesoporous transition metal sulfides that widely used for adsorption of organic dyes [10,11]. Wang et al [12] demonstrated the flower-like MoS 2 used for an effective adsorbent for removal of RhB, and the adsorption capacity reached 49.2 mg g −1 . Song et al [13] synthetized fungus-like MoS 2 nanosheets with ultrafast, and investigated adsorption capacities for organic dyes. The application of traditional MoS 2 is limited because of its low adsorption capacity. The composite of MoS 2 and other similar materials is an effective method to improve the adsorption capacity of materials.
As a potential photocatalyst, SnS 2 has attracted great interest from researchers in recent years [14]. In addition, SnS 2 has potential to be a excellent adsorbent for wastewater treatment due to excellent adsorption properties, chemically stable and simple synthesis [15]. Wang using Sn 4+ and Sn 2+ as raw materials to synthesize the SnS 2 /SnO 2 composites by a green hydrothermal method, and the hierarchical SnS 2 were applied for removel of RhB [16]. Because both MoS 2 and SnS 2 have outstanding adsorption capacity for dye adsorption, the coupling Original content from this work may be used under the terms of the Creative Commons Attribution 3.0 licence.
Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI. structures of MoS 2 and SnS 2 could form a heterostructure to increase the adsorption capacity for organic dyes. In addition, there is no report of SnS 2 /MoS 2 composites used for removal of organic dyes.
In this work, the SnS 2 /MoS 2 NPs were synthesized by a one-pot hydrothermal method, and the adsorption properties of SnS 2 /MoS 2 for organic dyes were discussed for the first time. The adsorption kinetics, adsorption equilibrium, adsorption thermodynamics and adsorption mechanism of RhB adsorption onto SnS 2 /MoS 2 NPs were investigated in detail. In addition, the SnS 2 /MoS 2 NPs exhibited the excellent adsorption capacity and significantly enhanced adsorption performance as compared with pure MoS 2 and SnS 2 composites. The results demonstrated the novel SnS 2 /MoS 2 NPs is a potential adsorbent for removal of organic dyes in wastewater.

Experimental
2.1. Synthesis of SnS 2 /MoS 2 NPs All chemicals were obtained from Sinopharm Chemical Reagent Co., Ltd (Shanghai, China) and were not further purified. The SnS 2 /MoS 2 NPs were synthsized by a hydrothermal method [17]. In a typical procedure, 6 mm thiourea (CN 2 H 4 S), 0.25 mm Ammonium molybdate tetrahydrate ((NH 4 ) 6 Mo 7 O 24 ·4H 2 O) and 2 mm stannous chloride dihydrate (SnCl 2 ·2H 2 O) were added in 30 ml of distilled water. After stiring for 15 min, the mixture was transferred into a 40 ml autoclave and then reacted at 200°C for 12 h. After cooling naturally, the products were obtained and washed with deionized water and ethanol for several times, and dried at 70°C for 12 h in a vacuum oven. Finally, the SnS 2 /MoS 2 was obtained. As a comparison, pure MoS 2 and pure SnS 2 were synthesized.

Material Characterization
The phase structure was tested by powder x-ray diffraction (XRD) on a RINT 2000 diffractometer (Rigaku) with Cu Kα radiation. The shape of the product was tested by a Hitachi S-4800 field emission scanning electron microscope (SEM) and a JEOL JEM 2100 F transmission electron microscope (TEM). The BET surface areas were performed using a micromeritics ASAP 2460. The Raman spectra was performed using a HORIBA HR-800. XPS of the samples were performed by a Thermo ESCALAB 250XI. The ultraviolet-visible (UV-vis) spectrophotometer was performed using a TU-1901.

Adsorption experiments
The adsorption studies was performed in the dark to prevent degradation of RhB. 20 mg of SnS 2 /MoS 2 NPs were disperse to 50 mL RhB aqueous solutions with different concntrate (10-150 mg L −1 ). The solution was magnetically stirred at 1000 rpm for 2 h. 3 ml of RhB solution was collected at different time intervals. The supernatant was collected and centrifuged at 10 000 rpm for 3 min. Then the supernatant was transferred into microcell for the analysis of RhB at 554 nm with a by UV-vis Spectrometry. Blank experiments was carried out under the same conditions. The adsorption rate (a) and adsorption capacity (b) are calculated by the following equations: where C e (mg L −1 ) is equilibrium concentration of RhB solution; C 0 (mg L −1 ) is the initial concentration of RhB solutions; V (L) is the volume of sample solution; m (g) is the mass of the adsorbents.   Figure 2(a) shows that the SnS 2 was composed by irregular and non-uniform flake. It was found that the MoS 2 nanospheres in figure 2(b). As a comparison, the morphology of SnS 2 /MoS 2 is similar with the pure MoS 2 (figure 2(c)).
TEM was used to further study the structure of the SnS 2 /MoS 2 NPs. The image shows that the sample consists of nanosheets, which is consistent with SEM ( figure 3(a)). The HRTEM image (figure 3(b)) indicates two interplanar distance of approximately 0.65 nm and 0.32 nm, that is consistent with the (002) and (100) lattice plane of MoS 2 and SnS 2 . According to the corresponding energy dispersive x-ray spectrum (EDX) results (figure 3(c)), the mass ratio of Mo, S and Sn was to be 0.77: 0.39: 0.66, and the atomic ratio was 32.71%: 32.33%: 34.97%. In addition, energy-dispersive x-ray spectrometry mapping images reveal the elements of Mo, S and Sn were uniformly distributed throughout the surface of SnS 2 / MoS 2 (figures 3(d)-(g)).
The BET surface of the SnS 2 /MoS 2 was performed by N 2 adsorption. The N 2 adsorption-desorption isotherm of SnS 2 /MoS 2 is exhibited in figure 4. The BET surface and average pore size are list in table 1. The N 2 adsorption-desorption curve was assigned as type IV isotherm having H 3 hysteresis loop, suggesting a mesoporous structure. The SnS 2 /MoS 2 has the large surface area of 101.06 m 2 g −1 , which is much larger than SnS 2 and MoS 2 . The large BET surface should add more active sites to the adsorption of Rhodamine B, which facilitated the rapid adsorption and transfer of the adsorbate.

Adsorption properties
The sample pH is a significant parameter for the adsorption properties of RhB by affecting both the existing forms of the RhB molecule and the charge species and density on the surface of SnS 2 /MoS 2 NPs. As the result of zata potential, the surface of SnS 2 /MoS 2 NPs had negative charge with the zeta potential of −40.6 mV. The sample pH was adjusted by B-R buffer solution, and the effect of pH on the adsorption capacity was investigated at pH 3.0 to 11.0. As shown in figure 5, the adsorption capacity of SnS 2 /MoS 2 for RhB was the highest (180 mg g −1 ) at pH 3.0. And the adsorption capacity of SnS 2 /MoS 2 NPs decreases gradually from 125 to 55 mg g −1 in the pH range of 3-11. It could be a reasonable explanation of the finding that, at pH<7, RhB molecule exhibit a cationic form to promoted the electrostatic interactions bettwen RhB and SnS 2 /MoS 2 NPs.
To demonstrate the rapid adsorption properties of MoS 2 /SnS 2 NPs, 20 mg SnS 2 /MoS 2 NPs were added into 10 mg L −1 RhB solutions. Figure 6(a) shown in the adsorption capacity and adsorption rate of RhB at different time intervals (0-15 min), the adsorption efficiency were 70%, 83% and 92% within 5, 10 and 15 min. As shown in the insert image of figure 6(a), the color of the dye solution disappeared after 15 min. The result show that the SnS 2 /MoS 2 NPs quickly adsorbed RhB from dye solution. The adsorption kinetics model is a significant   parameter to illustrate the adsorption mechanism of RhB onto adsorbents. The experimental data were fitted using the pseudo-first-order model (3) and the pseudo-second-order kinetic model (4), which expressed by the following equation: Where k 1 (min −1 ) and k 2 (g mg −1 min −1 ) are the rate constant of pseudo-first-order adsorption and pseudosecond-order adsorption, respectively. Q t is the adsorption capacity at different times and Q e is the adsorption capacity at adsorption equilibrium. The fitting curve of adsorption kinetics models and relevant parameters are shown in figure 7(b) and table 2. It was found that the linearly fitting R 2 of pseudo-second-order adsorption model is 0.9992, and the the calculated adsorption capacity (24.2 mg g −1 ) is similar to the actual adsorption capacity (23.6 mg g −1 ). The pseudo-second-order kinetics fittng curve indicates a better linear relationship for the experimental data. The result show that the adsorption process is controlled by chemisorption, and adsorption process be attribute to the electrostatic attraction between the negative charge on the surface of SnS 2 /MoS 2 NPs and RhB molecule. The adsorption isotherm could explain the adsorbate molecules distribution in solid-liquid adsorption system at the equilibrium. The Langmuir adsorption isotherm model illustrate the adsorbents is monolayer and the active site on the adsorbent surface [18]. The Freundlich isotherm model demonstrates the presence of heterogeneous adsorption surfaces and variable adsorption sites with different adsorption energies [19].   Where K l and K f are the Langmuir and Freundlich isotherm constant, respectively. Figure 8 shows the fitting curve of the adsorption experimental data using Langmuir and Freundlich isotherm models. As shown in table 3, the R 2 of the Langmuir isotherm model is higher than Freundlich isotherm models. And the calculated adsorption capacity of Langmuir isotherm model is 125 mg g −1 , is closer to the actual adsorption capacity (127 mg g −1 ), which illustrated the adsorption process is well fit by the Langmuir isotherm model. The adsorption of RhB onto SnS 2 /MoS 2 NPs conforms to the single-layer adsorption model. The result show that the active sites are located on the surface of SnS 2 /MoS 2 NPs. The thermodynamic studies is an important parameter for provide information on intrinsic energy changes associated with adsorption process. Three thermodynamic parameters were considered to describe the adsorption process; the enthalpy change (ΔH), Gibb's free energy (ΔG) and entropy change (ΔS) were calculated from the following equation: Where R is the universal gas constant (8.314 J mol −1 ·K −1 ); T is the temperature (K); kd is the equilibrium constant. Plots of ln Kd versus 1/T are linear as indicated in figure 7. The values of ΔH and ΔS can be calculated by the slope and intercept of lnKd relative to 1/T. The thermodynamic parameters for the adsorption of RhB on SnS 2 /MoS 2 NPs are shown in table 4. The negative values of ΔG except for RhB adsorption onto SnS 2 /MoS 2 NPs in the range of 293 to 333 K, which demonstrate the adsorption process is spontaneous. The positive value  To illustrate the excellent adsorption properties of SnS 2 /MoS 2 NPs, the adsorption capacity of pure MoS 2 and SnS 2 were discussed respectively. Figure 9(a) show that the adsorption capacity of SnS 2 /MoS 2 to RhB is significantly higher than MoS 2 and SnS 2 . The adsorption for RhB is mainly attributed to the Van der Waals force and electrostatic attraction between adsorbents and RhB. Moreover, the introduction of SnS 2 increases the active center and specific surface area of MoS 2 and further increases the adsorption capacity of MoS 2 for RhB. To prove SnS 2 /MoS 2 has excellent adsorption capacity for different organic dyes, Rhodamine B (RhB), methylene blue (MB), malachite green (MG) and crystal violet (CV) were carried out at room temperature. Figure 9(b) indicate the maximum adsorption capacity of SnS 2 /MoS 2 to RhB, MB, MG, CV were 125 mg g −1 , 202 mg g −1 , 165 mg g −1 and 175 mg g −1 respectively. The results show that SnS 2 /MoS 2 NPs has great potential for wastewater treatment because of its excellent adsorption capacity for both cationic and anionic organic dyes. Table 5 show the adsorption capacity of rhodamine B (RhB) and methylene blue (MB) by developed SnS 2 /MoS 2 NPs and other reported MoS 2 composites, indicating the SnS 2 /MoS 2 NPs has high adsorption capacity for organic dyes. It is mainly attributed to the SnS 2 /MoS 2 NPs has higher specific surface area than other MoS 2 composites reported, resulting in there are more active site exposed on the surface of SnS 2 /MoS 2 NPs.
The reusability of an adsorbent is a important factor for its practical application. According to reported that the adsorption/desorption procedure of RhB could changed by the pH value and the polarity of the solven [20], the sodium hydroxide of ethanol solution (0.5 mol L −1 ) is used to desorb RhB. The adsorbed SnS 2 /MoS 2 NPs were washed three times with 0.5 mol L −1 NaOH solution and distilled water respectively, then SnS 2 /MoS 2 NPs were used for the adsorption of RhB again. The adsorption efficiency of SnS 2 /MoS 2 NPs gradually decreased with increase of repeated times, because the RhB molecule cannot be completely desorbed from SnS 2 /MoS 2 NPs and a little RhB molecule still occupied the active site onto the adsorbent. As shown in figure 10, the adsorption efficiency of SnS 2 /MoS 2 NPs was maintain at 85.8% after repeated use for 5 times. The result illustrated the SnS 2 /MoS 2 NPs has well recycling performance for adsorption of RhB.

Adsorption mechanism
To demostrate the adsorption mechanism of RhB onto the SnS 2 /MoS 2 NPs, the FT-IR spectrum of the SnS 2 /MoS 2 NPs before adsorption ( figure 11(a)) and after adsorption ( figure 11(b)) to RhB was prepared and discussed. Figure 11(a) exhibited the strong characteristic diffraction peaks at 580 cm −1 , which is attribute to the   bending vibrations of Sn-S [8]. After adsorption of RhB, the spectrum of SnS 2 /MoS 2 NPs exhibited the diffraction peaks at 1335 cm −1 and 1585 cm −1 , which are attruibute to the C-C stretching vibrations in the aromatic ring of RhB molecule [24]. The FT-IR spectra of SnS 2 /MoS 2 NPs before and after adsorption show that the RhB is adsorbed on the surface of the SnS 2 /MoS 2 NPs.

Conclusion
In this study, SnS 2 /MoS 2 NPs were synthesized by a facile hydrothermal method, and the adsorption properties for organic dyes was investigated for the first time. SnS 2 /MoS 2 NPs has excellcent adsorption capacity (125 mg g −1 ) for RhB, which much higher than SnS 2 and MoS 2 . The SnS 2 /MoS 2 NPs could adsorb RhB from aqueous solution within 15 min, and adsorption efficiency reached 92%. The adsorption process was fitted to pseudo-second-order kinetic model and Langmuir model. Moreover, the SnS 2 /MoS 2 NPs has excellent adsorption properties for both cationic and anionic organic dyes. The result demonstrated the SnS 2 /MoS 2 NPs as adsorbent to rapidly and effectively removal of different organic dyes from wastewater.