Microstructure Optimization of Mos2/Sepiolite Nanocomposites via a Surfactant-Assisted Hydrothermal Strategy for High Efficiency Photocatalysis

The unique structure of two-dimensional molybdenum disulfide (MoS2) with rich active sites makes it a promising catalyst, whereas it also brings structural instability. Surfactant-assisted synthesis of MoS2 can be regarded as a simple way to regulate the microstructure. In this work, the surfactant additives were adopted to optimize the microstructure of MoS2/sepiolite nanocomposite, and the effects of surfactants type and concentration were investigated. For the sample prepared with 1mol/L sodium dodecyl benzene sulfonate (SDBS), it exhibits the highest intensity for the peak of MoS2 at 14.2 °, highly dispersed MoS2 nanosheet on the sepiolite, the lowest absorption intensity of Rhodamine B (RhB) at 553 nm of the wavelength, and the highest photocatalytic activity which is 2.5 times and 4.2 times higher than those prepared with 1mol/L hexadecyl trimethyl ammonium bromide (CTAB) and 1mol/L polyvinyl pyrrolidone (PVP) after a 150-minute irradiation, respectively. The above results suggest SDBS is the optimal surfactant to optimize the microstructure of MoS2/sepiolite nanocomposite. This work could provide new insights into the fabrication of high-quality MoS2-based nanocomposite.


Introduction
As a typical two-dimensional (2D) semiconducting material, MoS 2 with a typical layered structure is formed by stacking of planes. Each plane consists of covalently bonded S-Mo-S atoms in close-packed hexagonal structure, and adjacent planes are held together by van der Waals interactions. MoS 2 has gradually aroused particular interest and enormous attention due to its unique structure which endows MoS 2 with excellent properties for various applications, including photocatalysts [1][2][3], electrocatalysts [4], supercapacitors [5], sensors [6], and lubricants [7]. However, the high specific surface energy makes it easy agglomeration which results in the decrease of active sites and edges [8]. In order to combat these drawbacks, researchers put their effort to increase the dispersion as well as reduce the layer numbers of MoS 2 nano-sheets through different strategies, such as adopting a carrier [9] and taking atomic layer deposition [10]. Nevertheless, the high cost and complex method of preparation restrict them to broad application. In fact, the surfactant-assisted synthesis of MoS 2 has the advantages of low cost, easy operation, and excellent performance, and some surfactants are utilized to optimize MoS 2 -based nanocomposite [11][12][13][14][15]. Sepiolite is a typical clay mineral with the features of environmentfriendly and low cost, which makes it a good carrier for catalyst because the large surface specific area and excellent adsorption are benefit for the high catalytic performance [16][17][18][19]. However, surfactant (cationic, anionic, and nonionic surfactant) with different charges will affect both catalyst and carrier, due to the surface charge of sepiolite [20,21]. To date, only few researches have focused on the influences of surfactant on MoS 2 supported by mineral carriers.
In the previous study, we have found the excellent performance of composites viaadopting minerals [22][23][24]. Subsequently, we succeeded in fabricating natural SEP bulks into nano-sized fibers using high-speed airflow techniques [25] and utilized them to achieve the agglomeration decrease of catalysts [26]. Moreover, we also have prepared MoS 2 /sepiolite nanocomposite via a microwave hydrothermal method [27]. In this study, the surfactant additives were used to optimize the microstructure of MoS 2 /sepiolite nanocomposite, and the effects of surfactants type and concentration were also investigated. This work is believed to offer a new strategy for preparing high-quality MoS 2 -based catalyst.

Experimental
2.1. Materials. The sepiolite was purchased from Hebei Province, China. All chemical reagents were purchased from Tianjin Damao Chemical Co., Ltd., without further purified.     [25] for the natural SEP bulks were added into the solution and kept stirring for another 0.5 h. Next, the mixture was sonicated for 10 min. After that, 50 mL of the above suspension was transferred to a 100 mL Teflon-lined stainless steel autoclave and kept at 220°C for 3 h under microwave heating. After the samples were cooled down to room temperature, the final products were obtained by filtration, washed several times with deionized water, and dried in the vacuum oven at 80°C for 12 h.

Characterization and Performance
Tests. X-ray powder diffraction (XRD) was performed by a D8 ADVANCE X-ray diffractometer with nickel-filtered (V = 40 kV, I = 40 mA) Cu Kα radiation as the X-ray source (λ = 1:54 Å). The morphologies of the as-synthesized samples were observed by SEM (FEI Nano SEM450) under an accelerating voltage of 1.00 kV.
The photocatalytic activity of the as-prepared samples was tested through the photocatalytic degradation of RhB under visible light irradiation. 20 mg of samples were dispersed into 100 mL of RhB solution (20 mg/L), stirring the produced suspension in the dark for 30 min to reach the adsorption/desorption equilibrium. The suspension was subjected to irradiation by a 500 W Xe lamp (λ > 420 nm) under stirring at ambient conditions. After 30 min, 6 mL of the suspension was taken out and centrifuged to remove the photocatalysts. The filtrates were analyzed through recording the  International Journal of Photoenergy UV-Vis spectra by a Shimadzu UV-1800 spectrophotometer. The calculated method of photodegradation efficiency was shown in equation (1) where D represents the photodegradation efficiency of catalyst, A 0 represents the absorbance value at the beginning of illumination, and A t represents the absorbance value at a certain time t of illumination. Figure 1 shows the XRD patterns of the samples prepared with different surfactants. The two main phases are sepiolite and 2H-MoS 2 which are unchanged, indicating the structure of nanocomposite kept stable after adding surfactant [28,29]. Compared with the control sample, surfactant-assisted synthesis of nanocomposite exhibits sharper peaks of MoS 2 (101) and (110) facets at 33.5°and 58.3°, respectively, which suggests the better crystallinity of MoS 2 . Meanwhile, the sample prepared with SDBS shows the highest intensity for the peak of MoS 2 (002) facet at 14.2°, suggesting the increased content of MoS 2 [30]. Besides, the sample prepared by the similar process coupled with PVP or CTAB has almost no influence on the MoS 2 phase in nanocomposite. SEM images directly show the morphology of sample prepared with different surfactants (Figure 2). Sample prepared with PVP suffers from serious agglomeration where MoS 2 nanosheets transferred to a microsphere morphology contrast to the control sample (Figures 2(a) and 2(d)), while MoS 2 in nanocomposite remains the good dispersion when CTAB or SDBS are added (Figures 2(b) and 2(c)). The sample morphology suggests that PVP is not suitable to optimize the MoS 2 supported by sepiolite.

Results and Discussion
To further identify the optimal surfactant, photocatalytic degradation of RhB is adopted as a probe reaction. As shown in Figure 3, the absorbance of RhB for all samples decreased at the maximum absorption wavelength along with the increase of lighting time. However, the sample prepared with SDBS has the maximum descent, compared with those used CTAB and PVP after a 150minute irradiation (Figures 3(a)-3(c)). Moreover, the degradation efficiency of RhB for the sample prepared with SDBS reaches about 93% after irradiation of 150 minutes (Figure 3(d)), which is much higher than those prepared with CTAB (37%) and PVP (22%). Combined with the results of XRD, SEM, and photocatalysis performance, SDBS can be identified as the optimal surfactant.
The most suitable concentration of the SDBS is studied subsequently. As shown in Figure 4, the samples prepared with 1 mol/L and 1.5 mol/L SDBS show the sharper peaks of MoS 2 (101) and (110) facets at 33.5°and 58.3°, respectively, which indicates the appropriate concentration of SDBS can increased crystallinity of MoS 2 in nanocomposite while sample prepared with too high (2 mol/L) or low (0.5 mol/L) concentration of SDBS is unsuitable. Figure 5 shows the morphology of the samples prepared with different concentration of SDBS. It can be seen that MoS 2 in nanocomposite remains the architecture of nanosheet and disperses on the sepiolite surface, but the MoS 2 on the surface of sepiolite disperses sparsely when the concentration is 0.5 mol/L ( Figure 5(a)). When the concentration comes up to 1.5 mol/L and 2 mol/L, a few parts of MoS 2 suffer from slight agglomeration (Figures 5(c) and 5(d)). When the concentration is 1 mol/L, the MoS 2 nanosheets are uniformly dispersed on the sepiolite surface with the high density and no obvious agglomeration ( Figure 5(b)). Consequently, the concentration of SDBS has   The ultraviolet visible absorption spectra and photocatalytic performance of the samples prepared with different concentration of SDBS are also studied to find the optimal condition ( Figure 6). The sample prepared with 1 mol/L SDBS has the maximum descent for the absorbance of RhB compared with those used other concentrations (Figures 6(a)-6(d)). In addition, photocatalytic activity of samples toward RhB degradation can be as an important evidence to obtain the optimal concentration of SDBS, and the RhB degradation efficiency of the sample prepared with 1 mol/L SDBS is the highest (Figure 6(e)). Thus, the optimal concentration of SDBS can be identified as 1 mol/L.

Conclusions
In summary, MoS 2 /sepiolite nanocomposite was optimized via a surfactant-assisted hydrothermal method, and the influence of surfactants type and concentration was also studied. The results show that the photocatalysis activity of the nanocomposite prepared with 1 mol/L SDBS as the optimal condition toward RhB degradation reaches about 93%, which is extremely higher than those used CTAB and PVP with the same concentration. SEM images directly show the microsphere MoS 2 suffering from serious agglomeration when using PVP. Meanwhile, the increased content of MoS 2 is exhibited with the present of SDBS. Among the sample prepared with different concentrations of SDBS, 1 mol/L SDBS as the optimal concentration makes the best crystallinity of MoS 2 in nanocomposite. This work provides a new perspective for regulating the microstructure of MoS 2 -based catalyst.

Data Availability
The manuscript contains all of the data.

Conflicts of Interest
There is no conflict interest to declare.