Sepiolite-Supported WS₂ Nanosheets for Synergistically Promoting Photocatalytic Rhodamine B Degradation

Abstract

Pristine tungsten disulfide (WS₂) nanosheets are extremely prone to agglomeration, leading to blocked active sites and the decrease of catalytic activity. In this work, highly dispersed WS₂ nanosheets were fabricated via a one-step in situ solvothermal method, using sepiolite nanofibers as a functional carrier. The ammonium tetrathiotungstate was adopted as W and S precursors, and N,N-dimethylformamide could provide a neutral reaction environment. The electron microscope analysis revealed that the WS₂ nanosheets were stacked compactly in the shape of irregular plates, while they were uniformly grown on the surface of sepiolite nanofibers. Meanwhile, the BET measurement confirmed that the as-prepared composite has a larger specific surface area and is more mesoporous than the pure WS₂. Due to the improved dispersion of WS₂ and the synergistic effect between WS₂ and the mesoporous sepiolite mineral which significantly facilitated the mass transport, the WS₂/sepiolite composite exhibited ca. 2.6 times the photocatalytic efficiency of the pure WS₂ for rhodamine B degradation. This work provides a potential method for low-cost batch preparation of high-quality 2D materials via assembling on natural materials.

1. Introduction

Since the last century, the environmental pollution caused by excessive discharge of organic pollutants in wastewater has led to the inadequacy of freshwater resources and threatened human health [1,2]. To solve these issues, photocatalysis could be regarded as a green and economical method. In fact, the rapid recombination rate of photogenerated electron-hole pairs and low solar light energy utilization efficiency significantly reduced the photocatalytic efficiency [3]. Compared with the prior photocatalysts containing ZnO and CdS, transition metal dichalcogenides (TMDCs) have received substantial interest due to their sandwich-like layered structure, strong photocatalytic activities, and favorable optical and electronic properties. WS₂ as a typical TMDC has been studied deeply due to its suitable bandgap, excellent stability, and nontoxic features [4,5]. However, pristine WS₂ has high specific surface energy and poor dispersion, and the spontaneous agglomeration of WS₂ with high specific surface energy always leads to a decrease in exposed active sites, which causes an unsatisfactory photocatalytic degradation effect. Thus, there has been little study on photocatalytic degradation of organic wastewater by pure WS₂, which has seriously hindered its practical application.

Fortunately, the development of supported WS₂ photocatalysts has become a feasible and promising approach. The active components dispersed on the surface of the support could increase the specific surface area and improve the catalytic efficiency of the active components per unit mass. A series of materials were combined with pure WS₂ to improve the photocatalytic performance. In previous reports, many carriers have been adopted to support WS₂, such as graphene [6], reduced graphene oxide [7], TiO₂ [8], CdS [9], ZnO [10], Bi₂MoO₆ [11], etc. However, the high fabrication cost and complex process made these carriers unsuitable for broad applications. Natural minerals have the advantages of low cost, environmental friendliness, special morphology, and unique properties, which make them a very promising candidate for carriers. To date, different mineral-based composites with highly dispersed active compounds have been successfully synthesized.

Mineral materials are widely available and have been reported as catalyst carriers. Sepiolite (Sep) is a kind of natural magnesium silicate clay mineral, which is abundant in nature. Ascribed to the 1D fibrous structure, it has a strong adsorption capacity and high specific surface area [12,13,14,15]. As a carrier, sepiolite can improve the dispersion of materials. In addition, the interface effect between sepiolite and the loaded material can also enhance the photocatalytic performance. Sep has attracted much attention in the field of adsorbent and functional carrier for wastewater treatment [16,17,18,19].

In our previous studies, we have successfully synthesized ultrathin MoS₂ nanosheets with the assistance of sepiolite and tourmaline [20,21]. Herein, a novel WS₂/Sep composite with few layered WS₂ nanosheets was fabricated via an environmentally friendly and simple solvothermal route. The ultrathin WS₂ nanosheets that uniformly grow on the surface of the Sep led to the exposure of more surface-active sites and the solution to agglomeration. Furthermore, the WS₂/Sep composite exhibited much higher photocatalytic efficiency toward rhodamine B (RhB) degradation than the pristine WS₂, and the synergistic effect between WS₂ and Sep was also noted. This work offers a new insight for low-cost preparation of dispersed 2D material using natural minerals.

2. Results and Discussion

2.1. Crystal Phase and Groups Analysis

XRD patterns were recorded to examine the phase of the prepared pure WS₂ and WS₂/Sep composite (Figure 1a). All of the diffraction peaks of both the pure WS₂ nanosheets and WS₂/Sep composite matched well with the 2H hexagonal phase (JCPDS Card No.87-2417) [22]. The XRD patterns showed that the main peaks of WS₂ of the WS₂/Sep were located at 2θ = 13.775°, 28.6989°, 34.248°, and 47.105°, corresponding to the (002), (004), (101), and (103) facets of WS₂, and the diffraction peaks at 7.423°, 11.982°, 20.785°, and 26.667° corresponded well to the (011), (031), (131), and (080) facets of the Sep, respectively. The d-spacing value of the composite corresponding to the peak at 13.775° was calculated to be 6.4 Å, which was close to that of the pure WS₂. The few broad bread-like peaks could be attributed to the partial crystallization of the WS₂ nanosheets. After the solvothermal treatment, WS₂ was evenly dispersed on the support, and the diffraction peaks corresponding to WS₂ were sharp and symmetric, further confirming the great crystallization of composite. Furthermore, the FT-IR was used to identify the functional groups of the WS₂/Sep sample (Figure 1b). The broad absorption peaks at around 3567 cm⁻¹ and 1652 cm⁻¹ were ascribed to the stretching vibration of the hydroxyl group of crystal water. The peaks at 1003, 975, 424, and 788 cm⁻¹ were ascribed to the stretching of the Si-O band in the Si-O-Si groups of sepiolite. The peaks at 669, 688, and 645 cm⁻¹ corresponded to the bending vibration of Mg₃OH. The peaks at 464 were attributed to Si-O-Al (octahedral) and Si-O-Si [23,24,25]. The two intensive peaks at around 2358 and 2119 cm⁻¹ originated from the characteristic peaks of hexagonal WS₂. The results of XRD and FT-IR revealed the WS₂/Sep maintained the original crystal structure and surface functional groups of WS₂ and sepiolite.

2.2. Morphology and EDS Analysis

SEM, TEM, and HRTEM were used to observe the microstructure of the pure WS₂ and WS₂/Sep (Figure 2). The pure WS₂ with the shape of irregular plates consisted of WS₂ nanosheets stacked compactly (Figure 2a). In Figure 2b, one can observe the WS₂ nanosheets were uniformly anchored on the surface of Sep nanofibers, and a bark-like structure was formed intimately; the WS₂ nanosheets in the WS₂/Sep composite owned better dispersion. Compared with the pure WS₂ nanosheets, the mineral material (i.e., Sep) as a support of the composite could significantly reduce the agglomeration of the WS₂ nanosheets. Thus, more surface active sites of WS₂ could be exposed outside. The above results provided direct evidence that the WS₂ phase had been uniformly loaded on the Sep nanofibers.

TEM and HRTEM observations were adopted to deeply investigate the microstructure of the WS₂/Sep composite (Figure 2c,d), which further illustrated that the WS₂ phase had been loaded successfully on the surface of the Sep nanofibers. Moreover, the HRTEM images (Figure 2d) indicated a lattice space distance of 0.62 nm, which corresponded to the (002) facet of 2H-WS₂ [26,27]. Lattice spacing consistent with the d-spacing of Sep (031) was also detected. In addition, the elemental mapping images (Figure 3) also confirmed that WS₂ nanosheets were successfully assembled on the surface of Sep mineral.

2.3. Nitrogen Adsorption-Desorption Analysis

Nitrogen adsorption-desorption isotherms and pore size distribution curves were adopted to further compare the surface area and pore structure between the WS₂/Sep composite and pure WS₂ (Figure 4). Both the samples presented Langmuir type IV isotherms with H3 hysteresis type loops, suggesting the existence of slit-like mesoporous structures because of the stacking of sheets. This was consistent with the electron microscopy images. The WS₂/Sep composite possessed a specific surface area of 45.9 m²/g, which was much larger than that of the pure WS₂ (25.3 m²/g). Meanwhile, the pore volume of the WS₂/Sep (0.107 cm³/g) was also much larger than that of the WS₂ (0.048 cm³/g). Consequently, the composite could expose more surface active sites and increase the number of mesopores, which tended toward improving catalytic activity [28].

2.4. Photocatalytic Performance of RhB Degradation

Comparative experiments were performed to evaluate the performance of the WS₂/Sep nanocomposite for RhB degradation, which is shown in Figure 5. It can be clearly seen that the WS₂/Sep composite exhibited much higher photocatalytic efficiency than the pure WS₂. After 150 min catalysis, the RhB degradation rate for the pure WS₂ only reached ~18%, while it achieved ca. 76% for the WS₂/Sep nanocomposites. The 4.2-fold improved activity could be mainly attributed to the better dispersion of the WS₂ nanosheets [20]. In addition, the excellent synergies between the WS₂ nanosheets and Sep nanofibers could also accelerate the photocatalysis, which will be further discussed in the following section.

2.5. The Possible RhB Degradation Mechanism for the WS₂/Sep Composite

Figure 6 shows the possible process of the photocatalytic RhB degradation in which the synergic effect between the Sep and the grown WS₂ in the WS₂/sepiolite nanocomposite is illustrated. Under irradiation, the holes (h⁺) and electrons (e⁻) were produced. Then, the dissolved O₂ would react with e⁻ and OH− would react with h⁺ to generate superoxide anion radical (·O₂⁻) and hydroxyl radical (·OH), respectively [29,30]. Finally, the RhB molecules adsorbed on the surface-active sites of the WS₂ were degraded to CO₂ and H₂O [31,32]. During this process, the excellent adsorption of Sep was conductive to transferring the dissolved O₂ and RhB molecules to the active sites of the WS₂, and the abundant hydroxyl (–OH) groups on the Sep surface also benefited the production of ·OH [33,34,35]. Thus, in addition to the improved dispersion of WS₂, the synergic effects provided by the Sep during photocatalysis also enabled the WS₂/Sep composite to exhibit much higher photocatalytic activity than the pure WS₂ [36]. Furthermore, the presence of mesopores favored multilight scattering, resulting in enhanced harvesting of the exciting light, and accordingly, improved photocatalytic activity [37,38]. These mesopores also facilitated fast mass transport and thus enhanced the performance [39,40].

3. Experiment

3.1. Chemicals and Reagents

The Sep was provided by LB Nanomaterials Technology Co., Ltd. (Henan, China), and the chemical analysis of the Sep was determined as SiO₂ of 53.56 wt%, MgO of 36.79 wt%, CaO of 5.53 wt%, Fe₂O₃ of 1.17 wt%, and other impurities of 2.95 wt%. The ammonium tetrathiotungstate (H₈N₂S₄W) and dimethylformamide (DMF) were supplied by Sigma-Aldrich Co., Ltd (Shanghai, China). The (RhB) was purchased from Kewei Chemical Group Co., Ltd. (Tianjin, China). All chemicals used in the experiments were of analytical grade. These materials were used without further purification. Deionized (DI) water was used in all experiments.

3.2. Synthesis of WS₂/Sep Nanocomposite

The WS₂/Sep nanocomposite was fabricated by a solvothermal method (Figure 7). A total of 30 mg of H₈N₂S₄W was dissolved in 15 mL of N,N-dimethylformamide (DMF) and stirred for 0.5 h. Then, 10 mg of Sep powder was added into the solution and stirred continuously for 0.5 h. Next, the above mixture was sonicated for 10 min. Later, the above suspension was transferred into a 25 mL Teflon-sealed autoclave and heated to 220 °C for 24 h. After cooling down to room temperature, the final product was obtained by filtration, washed several times with DI water, and dried in a vacuum oven at 80 °C for 12 h. The preparation process of the pure WS₂ was similar to that of the WS₂/Sep nanocomposite but without the addition of Sep.

3.3. Physicochemical Characterizations of the Synthesized WS₂/Sep Composite

X-ray powder diffraction (XRD) analysis was performed with the working conditions of Cu K_α radiation (λ = 1.54 Å), 40 mA, and 40 kV on a Smart Lab 9 KW X-ray diffractometer from Rigaku (Tokyo, Japan). Fourier-transform infrared spectroscopy (FTIR) spectra of the samples were recorded in a transmission mode from 400 to 4000 cm⁻¹ on a Tensor II Fourier transform infrared spectrometer manufactured by Bruker (Saarbrucken, Germany). The morphologies of the as-synthesized samples were observed by using a S-4800 scanning electron microscope working at 5 kV, from JEOL (Tokyo, Japan). Transmission electron microscope (TEM) images, high-resolution TEM (HRTEM) images, and energy-dispersive X-ray spectroscopy (EDS) were carried out on a JEM 2100F transmission electron microscope, from JEOL (Tokyo, Japan), with an accelerating voltage of 200 kV. The N₂ adsorption-desorption tests were conducted on Autosorb-iQ2, from Quantachrome (Boynton Beach, FL, USA). All samples were outgassed in nitrogen flow at 200 °C for 4 h before measurements. The specific surface area (SSA) was evaluated using the Brunauer-Emmett-Teller (BET) method, and the total pore volume was calculated at the relative pressure of approximately 0.99. The pore size distribution was computed using the Barrett-Joyner-Halenda (BJH) method.

3.4. Photocatalytic Performance Tests

Photocatalytic activity of the as-prepared samples was assessed by the reduction of RhB in aqueous solutions, using a 500 W Xe lamp equipped with a cut-off filter (λ > 420 nm). In a typical evaluation procedure, 20 mg of sample was added into 100 mL of RhB solution with a concentration of 20 mg/L at room temperature [41]. The suspension solution stirred vigorously in the dark for 0.5 h to reach the equilibrium of adsorption/desorption before visible light irradiation. At an interval of 0.5 h, 5.6 mL of the suspension was collected and centrifuged for absorbance analysis. The concentration of dye solution was analyzed by recording the UV–vis spectra at wavelength of 553 nm using a Shimadzu UV-1800 spectrophotometer from Shimadzu (Shimane, Japan).

4. Conclusions

In summary, a novel WS₂/Sep composite as a photocatalyst was successfully prepared via a facile solvothermal method. The physicochemical characterization results confirmed that the bark-like WS₂ nanosheets uniformly grew on the Sep nanofibers, which led to a larger surface area and more exposed active sites. In a typical photocatalytic application, the as-synthesized WS₂/Sep exhibited considerably improved photocatalytic performance for RhB degradation over the pure WS₂, which could be mainly attributed to the better dispersion of WS₂ nanosheets and the synergistic effect between the WS₂ nanosheets and the Sep nanofibers’ support during the photocatalysis. This work is believed to provide new ideas for the low-cost batch preparation of high-quality two-dimensional materials based on natural minerals. However, further investigation of the as-developed composite photocatalyst with respect to reusability and the versatility for the degradation of other organic pollutants are still needed to see if there are any limitations to its practical applications.