Liquid exfoliation of layered metal sulphide for enhanced photocatalytic activity of TiO₂ nanoclusters and DFT study

Abstract

Layered metal sulphides (LMSs) such as MoS₂, WS₂ and SnS₂ have attracted much attention in the field of photocatalysis due to their excellent properties. Herein, a facile and effective liquid exfoliation solvothermal method for fabricating TiO₂/LMS (LMS = MoS₂, WS₂ or SnS₂) photocatalysts has been developed. The optimum molar ratio of Ti–Mo, Ti–W and Ti–Sn was determined to be 50 : 0.8, 50 : 0.1 and 50 : 0.1, respectively. The optical properties of TiO₂/LMS with a matching solar spectrum contribute to converting the solar energy to chemical energy by photon-driven photocatalytic reactions. The combined effect of liquid exfoliation and solvothermal reforming has been demonstrated as an effective method to obtain high efficiency photocatalysts using bulk metal sulphides as sensitizers. The binding site of TiO₂ and the LMS at the interface of a composite photocatalyst was investigated by the density functional theory (DFT) method at a molecular cluster level, and the calculation results showed that firm structures were formed at the interfaces of TiO₂ nanoparticles and the LMS. The photocatalytic activity evaluation of TiO₂/LMS showed that the LMS played the crucial role in separation of photogenerated e⁻/h⁺ pairs and utilization of photons for enhancing the photocatalytic activity of TiO₂. The study of the electron transfer mechanism indicated that the synergetic effect of superoxide radicals (·O₂⁻) and hydroxyl radicals (·OH) plays the leading role in the dye degradation process.

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Introduction

Semiconductor photocatalysis, as a green technology, has been widely applied in environmental purification (non-selective process),¹ photocatalytic synthesis,² and clean energy.^3,4 Recently, titanium oxide (TiO₂) nanomaterials are generally believed to be the most reliable materials for the decomposition of toxic and hazardous organic pollutants and heavy metals due to their non-toxicity, physical and chemical stability, availability, and unique optical properties.⁵ However, TiO₂ in a pure state can solely absorb UV light (≤380 nm) and has lower capacity for the utilization of solar energy due to its large band gap (3.2 eV).^6–10 Hence, it is highly desirable to develop photocatalysts with high catalytic activities under solar light.^11–15 In recent years, metal sulphide has been extensively studied due to its excellent performance and potential application for solar energy conversion. Certain metal sulphides, especially single- and few-layer 2D transition metal disulphides such as MoS₂,¹⁶ WS₂,¹⁷ and SnS₂,¹⁸ have attracted much attention in photocatalysis due to their excellent properties for utilization of solar light. Thus, using these metal disulphides as photosensitizers will be a highly efficient and low cost way for the modification of TiO₂.^19–21 Numerous research has reported that LMS (MoS₂, WS₂ and SnS₂) nanosheets can be synthesized by a chemical method using the relevant metal salt as raw material. For instance, Liu et al.²² reported layered nano-MoS₂ decorated on TiO₂ nanotube arrays by photocatalytic reduction of (NH₄)₂MoS₄. Nano-MoS₂ can obviously enhance the electrocatalytic activity of TiO₂ nanotube array electrodes in hydrogen evolution reactions. Shao et al.²³ reported ultrathin hexagonal SnS₂ nanosheets assembled onto TiO₂ nanofibers by a hydrothermal process using SnCl₄·5H₂O and thioacetamide as the raw materials. The results showed that the enhancement of photocatalytic activity was mainly due to the photosynergistic effect of TiO₂ and SnS₂, leading to the better separation efficiency of photoinduced electron–hole pairs. Hou et al.²⁴ reported a WS₂/g-C₃N₄ photocatalyst prepared via an impregnation sulfidation approach using (NH₄)₂WS₄ as raw material. The layered WS₂ was loaded on the surface of g-C₃N₄ and significantly enhanced the H₂ production performance. These reviews concentrate on LMS nanosheets synthesized by chemical methods, and surely, a stable heterojunction system could be obtained. However, there are also numerous problems emerging in these methods, such as rigorous processing, lower purity and lower quality, which will limit their industrial applications.

In recent years, the physical method, mechanical exfoliation, has attracted much attention in the preparation of two-dimensional nanosheet materials.^25,26 A simple liquid exfoliation method would allow the formation of novel hybrid and composite materials. The layered compounds, such as MoS₂, WS₂ and SnS₂, can be efficiently dispersed in common solvents and can be deposited as individual flakes or formed into films.^25–29 By blending these materials with the suspensions of other nanomaterials, the hybrid composites could be obtained.^28,30,31 However, the simple blending method may not make the hybrid composites stable and also some effective strategies are needed, such as thermal treatment and coating treatment, to obtain a stable heterojunction system.

Here, we demonstrated exfoliation of bulk LMS crystals (MoS₂, WS₂ and SnS₂) in common solvent (EtOH) to give mono- and few-layer nanosheets. This method is insensitive to air and water and can potentially be scaled up to give large quantities of exfoliated material. Subsequently, using these LMS nanosheets as sensitizers, TiO₂/LMS composites were synthesized by the combined method of ultrasonic exfoliation and solvothermal reforming. In a typical procedure, LMS nanosheets were embedded into the TiO₂ cross-linked structure by the hydrolytic polymerization of TiCl₄, which preferably protected the LMS from the influence of the external system. This special coated structure would be very favourable to the electrons transferred to the surface of the composite, leading to effective separation of photoinduced electron/hole pairs, which significantly enhanced the photocatalytic degradation of organic dye. Moreover, the possible structures were predicted by the density functional theory (DFT) method. The photocatalytic mechanism of the TiO₂/LMS composites was also discussed.

Experimental details

Materials

All of the chemicals used for the synthesis of the catalysts were of analytical grade and were used as-received without any further purification. Tungsten disulphide (WS₂, 99%) was obtained from Aladdin Reagent Co. Tin disulphide (SnS₂, ≥99.5%) was purchased from Changsha Huajing Powder Material Technological Co., China. Titanium tetrachloride (TiCl₄, ≥99%), methyl blue and molybdenum disulphide (MoS₂, ≥98.5%) were purchased from Tianjin Fuchen Chemical Reagent Factory, China. Ethanol was purchased from Nanjing Chemical Reagent Co., China, and both glycerol (C₃H₈O₃, ≥99%) and sodium hexametaphosphate ((NaPO₃)₆) were obtained from Tianjin Kemiou Chemical Reagent Co., China.

Preparation of TiO₂/LMS composite photocatalysts

The preparation procedure of the TiO₂/LMS composite photocatalysts contains two steps: (1) the exfoliation process of the LMS; and (2) the solvothermal process of the TiO₂/LMS precursor. The detailed steps are shown as follows.

0.05 g of LMS particles, 30 mL of ethanol and 1 mL of glycerol were added into the beaker and exfoliated by the ultrasonic method for 2–3 h. The upper dispersion was collected and centrifuged at 9000 rpm for 5 min and washed with ethanol 3 times. The exfoliated LMS nanosheets were obtained after being dried at 60 °C for 15 h.

4 mL of titanium tetrachloride ethanol solution (TiCl₄, 2 mol L⁻¹) and certain exfoliated LMS nanosheets were added into a beaker with 12.5 mL of EtOH at room temperature (30 °C), and the homogeneous dispersion solution was obtained by the ultrasound method for 120 min. Then, 3 mL of glycerol (2%) aqueous solution containing a certain dispersant (SHMP, 0.4%) as the hydrolytic agent was slowly added into the dispersion, and the TiO₂/LMS vitreosol was obtained after ultrasonic treatment for 10 min. Subsequently, the obtained solution vitreosol was transferred into a 25 mL Teflon hydrothermal reactor and kept at 140 °C for 3 h. The as-prepared composite photocatalysts were isolated by centrifugation, washed with absolute ethanol three times and dried at 80 °C for 5 h in an air dry oven.

Characterization

The crystal forms of the composite photocatalyst were analyzed by an X-ray diffractometer (D8 ADVANCE, Bruker) with a Cu tube for generating Cu Kα radiation (k = 1.5418 Å). The morphology of the photocatalysts was measured using scanning electron microscopy (SEM, Hitachi, S-3700N). The morphologies and internal structure were examined by transmission electron microscopy (TEM, JEOL, JEM-2100F, Japan). X-ray photoelectron spectroscopy (XPS) measurements were performed on a Phi X-tool instrument. Chemical compositions and molecular structure of the as-prepared materials were analyzed by a Raman spectrometer (LabRAM Aramis, France). UV-vis diffuse reflectance spectra were recorded on a Hitachi UV-3010 spectrophotometer (BaSO₄ as a reflectance standard).

Evaluation of photocatalytic activities of the TiO₂/LMS composites

The photocatalytic activities of the samples for mineralizing methyl blue (20 mg L⁻¹) were measured as follows: a certain amount of the as-prepared (50 mg) powders was combined with 250 mL of organic substance solution in a water-cooled reactor and stirred for 30 min to reach the absorption–desorption equilibrium in a dark box. Subsequently, the dispersed suspension was irradiated by a XG500 xenon long-arc lamp. At the given irradiation time intervals, 4 mL samples were collected and then filtered with a centrifuge to remove the photocatalyst. The supernatant was analysed by a SHIMADZU UV-2450 spectrophotometer to record the maximum absorbance (599 nm).

Capture of active species (h⁺, e⁻, ·O₂⁻ and ·OH)

Benzoquinone, tert-butyl alcohol, silver nitrate and EDTA-2Na were employed as the scavengers of superoxide radicals (·O₂⁻), hydroxyl radicals (·OH), electrons (e⁻) and holes (h⁺).^32,33 50 mg of the as-prepared sample was dispersed in the MB aqueous solution (250 mL), and then 0.3 mmol scavenger was added into the aqueous solution. The photocatalytic experiment process was similar to the evaluation process of photocatalytic activities.

Results and discussion

The bulk LMS was exfoliated by the liquid exfoliation method in a liquid system of ethanol and glycerol solution for fabricating TiO₂/LMS (LMS = MoS₂, WS₂ or SnS₂). The SEM images before and after exfoliation of the LMS are shown in Fig. 1. A microsheet structure was the main type in the LMS compounds before exfoliation. After exfoliation, the LMS compounds were mainly in the form of nanosheets being mono- or few-layer. This indicated that the ultrasonic exfoliation was a better way to obtain nanoscale LMS sheets, which would provide high quality sensitizers for fabricating TiO₂-based hybrid composites.

Fig. 1 SEM images of LMS (MoS₂, WS₂ and SnS₂). LMS before exfoliation (a₁, b₁ and c₁), and LMS after exfoliation (a₂, b₂ and c₂).

Using these LMS nanosheets as sensitizers, the TiO₂/LMS composites were synthesized by a solvothermal method. From Fig. 2, TEM images were given to investigate the inter-structures of the TiO₂/LMS composites. In Fig. 2a, TiO₂ exists in the form of a cross-linked structure. It is noted that MoS₂ nanosheets are embedded in the TiO₂ cross-linked structure and nanopores were distributed evenly on the composite, which could improve the adsorbing capacity of hazardous substances (Fig. 2b).

Fig. 2 TEM images of pure TiO₂ (a), and the TiO₂/MoS₂ (b), TiO₂/WS₂ (c) and TiO₂/SnS₂ (d) composite photocatalysts.

For the TiO₂/WS₂ composite (Fig. 2c), a clear boundary structure was observed between TiO₂ and WS₂, including 9 nm of a TiO₂ shell and nano-WS₂ core, implying that a stable contact interface was formed between TiO₂ and WS₂. The TiO₂/SnS₂ composite existed in the formation of aggregates and the nano-SnS₂ was coated by TiO₂ nanoparticles (Fig. 2d). These formations of the composites can be explained as follows: the TiCl₄ ethanol fluid system has a lower surface tension, which could help Ti⁴⁺ permeate the interlayer of the LMS. With the hydrolysis of Ti⁴⁺, the Ti–OH sol was uniformly dispersed around the LMS nanosheets, and subsequently, Ti–OH was polymerized to form a TiO₂ cross-linked structure and coated on the surface of the LMS nanosheets (Fig. 3). In this typical process, TiO₂/LMS heterojunctions could be formed at the interfaces of the TiO₂ nanostructure and LMS. These special structures are beneficial for the electron transfer and separation of photogenerated e⁻/h⁺ pairs at the contact interface of the composite photocatalysts.

Fig. 3 Schematic illustration for the fabrication of the TiO₂/LMS (LMS = MoS₂, WS₂ or SnS₂) composite photocatalyst.

The XRD patterns of the as-prepared samples are shown in Fig. 4. The crystal form of TiO₂ in the photocatalyst composites was anatase, since the characteristic diffraction peaks at 25.3°, 37.8°, 48.0°, 53.9°, 55.1°, 62.7°, 68.8°, 70.3° and 75.0° are attributed to the (101), (004), (200), (105), (211), (204), (116), (220) and (215) crystal faces of anatase TiO₂ (PDF#21-1272). For the TiO₂/MoS₂ composite, the modest peaks at 33.0°, 48.1° and 52.1° (PDF#17-0744) corresponded to the (101), (107) and (018) crystal faces of molybdenite-3R-MoS₂. A characteristic peak located at 26.6° corresponded to the (111) crystal faces of MoO₃, which was attributed to part of the MoS₂ being oxidized in the fabricating procedure. For the TiO₂/WS₂ composite, an apparent peak observed at the position of 14.4° was the diffraction peak of tungstenite-3R (PDF#35-0651). For the TiO₂/SnS₂ composite, a diffraction peak at the position of 15.0° is observed, which corresponds to the (001) crystal face of berndtite-2T (PDF#23-0677).

Fig. 4 XRD patterns of pure TiO₂, and the TiO₂/MoS₂, TiO₂/WS₂ and TiO₂/SnS₂ composite photocatalysts.

The crystal sizes of TiO₂ in the composites were calculated by applying Scherrer’s equation to the anatase (101) diffraction peak which represented the highest intensity peak for each pure phase.³⁴

where D is the average crystallite size, k = 0.89, λ is the wavelength of Cu Kα radiation (0.15406 nm), β is the full width at half maximum (FWHM), and θ is the Bragg diffraction angle. The average crystallite size of anatase TiO₂ in the different composites based on eqn (1) are given in Table 1. The effectiveness of the hydrothermal process in the presence of metal sulphide to deform the TiO₂ was evident from the changes to the crystallite size. The results showed that the presence of the LMS in TiO₂/LMS could increase the grain size of TiO₂. This indicated that the LMS could enhance the crystallization degree of TiO₂, leading to the surface energy of the composite being decreased, which could make the composite photocatalysts maintain stability in aqueous solutions.

Table 1 The average anatase TiO₂ crystallite size of TiO₂/LMS

Sample	TiO2	TiO2/MoS2	TiO2/WS2	TiO2/SnS2
TiO2 (d, nm)	9.1	11.4	11.3	11.1

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X-ray photoelectron spectroscopy (XPS) was used to investigate the chemical states of the metal and S in the TiO₂/LMS heterostructures (Fig. 5). The high resolution XPS spectra show that the binding energies of the S 2p_1/2 and S 2p_3/2 peaks in TiO₂/LMS are located at 162.3 and 161.1 eV (TiO₂/MoS₂),³⁵ 163.5 and 161.5 eV (TiO₂/WS₂),³¹ and 163 and 161.5 eV (TiO₂/SnS₂),^23,36 respectively, implying that S²⁻ existed in the TiO₂/LMS composites. Furthermore, the binding energies of Mo 3d_3/2 and Mo 3d_5/2 in TiO₂/MoS₂, W 4f_5/2 and W 4f_7/2, and Sn 3d_3/2 and Sn 3d_5/2 are located at 233.4 and 230.2 eV,^35,37 34.5 and 32.4 eV,³¹ and 495.6 and 486.8 eV,^23,36 respectively, suggesting that Mo⁴⁺, W⁴⁺ and Sn⁴⁺ existed in the TiO₂/MoS₂, TiO₂/WS₂ and TiO₂/SnS₂ composites, respectively. Meanwhile, Mo⁶⁺ and W⁶⁺ were confirmed to exist in the TiO₂/MoS₂ and TiO₂/WS₂ composites, respectively, which was consistent with the XRD results. The high energy component at 166.1 eV can be assigned to S⁴⁺ species in sulphate groups (SO₃²⁻), and these groups could locate at the edges of the SnS₂ layers, which was attributed to the oxidation of S²⁻ at the surface of SnS₂.

Fig. 5 XPS spectra of the TiO₂/LMS composites. S 2p of TiO₂/LMS (a), Mo 3d of TiO₂/MoS₂ (b), W 4f of TiO₂/WS₂ (c), and Sn 3d of TiO₂/SnS₂ (d).

The compositions of the TiO₂/LMS samples were investigated by Raman spectroscopy. The shifting of the Raman band has a close relationship with the particle size and phase structure of the composites. Fig. 6 shows that the TiO₂ of the anatase structure had six modes including the Raman bands of 153.1 cm⁻¹ (E_g), 211.5 cm⁻¹ (E_g), 400.5 cm⁻¹ (B_1g), 515.2 cm⁻¹ (A_1g + B_1g), and 639.2 cm⁻¹ (E_g).³⁸ For the TiO₂/MoS₂ composite, the 2E_g, 2B_1g and A_1g Raman modes of anatase TiO₂ in TiO₂/MoS₂ have a blue shift, and the Raman bands were located at 151.4 cm⁻¹ (E_g), 388.4 cm⁻¹ (B_1g), 506.8 cm⁻¹ (A_1g + B_1g), and 629.7 cm⁻¹ (E_g). This indicated that the vibrational energy was elongated due to the contact interface of the TiO₂/MoS₂ composite formed in the fabrication process.³⁹ The Raman mode near 210 cm⁻¹ was weak and almost disappeared, and instead a new peak at 201.1 cm⁻¹ was observed, which was speculated to be the characteristic peak of MoO₃. For the TiO₂/WS₂ sample, the Raman band at 400.5 cm⁻¹ was divided into three new peak positions of 351.8 cm⁻¹, 399.6 cm⁻¹ and 418.8 cm⁻¹. Among them, the positions of 351.8 cm⁻¹ and 418.8 cm⁻¹ were speculated to be for the A_1g and E¹_2g vibrational modes of WS₂, respectively, which is consistent with previous results.⁴⁰ From the Raman spectrum of the TiO₂/SnS₂ composite, a new peak was observed at 313.4 cm⁻¹, corresponding to the A_1g mode of the SnS₂ hexagonal phase.^31,41 These results displayed the shifting or splitting of the characteristic peak, indicating that a heterostructure was formed at the interface of the TiO₂/LMS composites.

Fig. 6 Raman spectra of pure TiO₂ and the TiO₂/LMS composites.

The absorption spectra of the as-prepared samples were measured using UV-vis diffuse reflectance spectroscopy. As shown in Fig. 7a, the basal absorption edge of TiO₂ occurs at a wavelength shorter than 400 nm, whereas TiO₂/MoS₂, TiO₂/WS₂ and TiO₂/SnS₂ have notable absorption in the visible light region between 400 and 800 nm, which is attributed to the excellent visible light response of LMSs. And the absorption edge of the TiO₂/SnS₂ composite shifts to the visible light area (420 nm) as compared to that of P25. For the TiO₂/WS₂ composite, one obvious absorption band was observed around 626 nm due to the excellent optical property of WS₂ of improving visible light absorption and utilizing photons.⁴² Similarly, for TiO₂ coupled with MoS₂, the optical absorption threshold of the TiO₂/MoS₂ band gap transition has been radically decreased due to the fine matching energy level with solar energy.

Fig. 7 UV-vis absorption spectra (a) and the corresponding Kubelka–Munk function (b) of pure TiO₂ and the TiO₂/LMS composites.

The band gap (E_g) of TiO₂/MoS₂, TiO₂/WS₂, TiO₂/SnS₂ and pure TiO₂ was estimated using the Kubelka–Munk function:^43,44

where F(R) is the Kubelka–Munk function, and R is reflectivity. The curve graph of F(R) ∼ λ can be exported during the test, which can be transformed into a curve graph corresponding to , where E is the energy (E = 1240/λ). Thus, as shown in Fig. 7b, the band gap (E_g) of TiO₂/MoS₂, TiO₂/WS₂, TiO₂/SnS₂ and pure TiO₂ is estimated to be 3.01 eV, 3.06 eV, 3.07 eV and 3.21 eV, respectively. The results showed that the conduction band edge of TiO₂/LMS was about 0.14–0.20 eV more positive than that of pure TiO₂. The results suggested that the existence of LMSs in composites can enlarge the light abstraction width of TiO₂, implying that a stable TiO₂/LMS heterostructure was formed at the interface of TiO₂ and the LMS. Meanwhile, the red shift also indicated that the surface energy of TiO₂ in the TiO₂/LMS composites decreased, which was consistent with the XRD results.

A series of experiments were carried out to further investigate the photocatalytic activities of the TiO₂/LMS composites and the photocatalytic degradation of methyl blue (MB) in the liquid phase, which were conducted under simulated sunlight and the results are shown in Fig. 8. The optimum mole ratios of TiO₂/LMS were determined to be 50 : 0.8 (TiO₂/MoS₂), 50 : 0.1 (TiO₂/WS₂) and 50 : 0.1 (TiO₂/SnS₂). As compared to the TiO₂/MoS₂ composite, the lower molar ratios of SnS₂ and WS₂ were more beneficial for enhancing the photocatalytic activity of TiO₂. The reason for the results was mainly related to the exfoliated-type of LMS nanosheets. The exfoliated-type of the LMSs was mainly MoS₂ nanosheets in the TiO₂/MoS₂ composite, and was mainly particles or few-layered nanosheets in the TiO₂/WS₂ and TiO₂/SnS₂ composites. These typical exfoliated structures of the LMSs made the form of the composites different: an embedded structure of TiO₂/MoS₂, a core–shell structure of TiO₂/WS₂, and a coating structure of TiO₂/SnS₂. Hence, the composite structure of TiO₂/LMS and the exfoliated-type of LMS jointly determined the photocatalytic activity of the composites. However, the coating structure was not more stable than the embedded structure and the core–shell structure, so the photocatalytic activity of TiO₂/SnS₂ was lower than that of TiO₂/WS₂ and TiO₂/MoS₂.

Fig. 8 Photocatalytic degradation of MB using TiO₂/MoS₂ (a), TiO₂/WS₂ (b), TiO₂/SnS₂ (c), and P25, TiO₂ and a physical mixture of TiO₂ and LMS (d) as the photocatalyst.

In addition, the photocatalytic activities for the physical mixture of TiO₂ and the LMS (MoS₂, WS₂ and SnS₂) were also investigated by MB degradation (Fig. 8d). The results showed that the degradation rates of a physical mixture of TiO₂ and MoS₂, a mixture of TiO₂ and WS₂, and a mixture of TiO₂ and SnS₂ were obviously lower than those of the TiO₂/MoS₂, TiO₂/WS₂ and TiO₂/SnS₂ composites obtained by chemical reaction, which suggested that the TiO₂ and LMS in the TiO₂/LMS composites (TiO₂/MoS₂, TiO₂/WS₂ and TiO₂/SnS₂) was not combined by van der Waals forces but by chemical bonds. Also, the photocatalytic activities of pure TiO₂ and P25 were lower than those of the TiO₂/LMS composites, suggesting that the presence of the LMS in the composites could effectively enhance the photocatalytic activity of TiO₂, and the properties and structures of the LMS would be beneficial for electron transfer and reduction of e⁻/h⁺ pair recombination, leading to promotion of the photocatalytic degradation of MB.⁴⁵

To investigate the photostability of the photocatalysts, cycling tests were conducted of MB degradation under simulated sunlight irradiation. For each cycle, the photocatalysts were not recovered and a fresh solution of MB was added into the photoreactor to undergo degradation under identical conditions (the MB initial concentration was 20 mg L⁻¹). As shown in Fig. 9a, the degradation rates of MB (TiO₂/MoS₂, 91.4%; TiO₂/WS₂, 92.0%; TiO₂/SnS₂, 90.0%) slightly decrease after four cycles. The samples of TiO₂/LMS after cycling tests were characterized by XRD and the results are shown in Fig. 9b. From the XRD results, it can be seen that the crystal structure of TiO₂/LMS had not changed significantly. These results demonstrate the better stability and promising application of TiO₂/LMS in the treatment of dye-containing wastewater. This was consistent with the capture test of hydroxyl radicals because the intensity of hydroxyl radicals over time was linear (Fig. S1†).

Fig. 9 Photostability test of the TiO₂/LMS composite photocatalysts for MB degradation (MB initial concentration, 20 mg L⁻¹; irradiation time, 60 min) (a). XRD analysis of the TiO₂/LMS composite photocatalysts after photocatalytic reaction (b).

To deeply understand the formation mechanism of TiO₂/LMS, the DFT method was used to study the possible active site of TiO₂/LMS combination. As shown in Fig. 10, these hexagonal platelets exhibit two kinds of edges, which are named respectively as the metallic and sulphur edge. Generally, the initial form of the Ti–OH sol will couple with the LMS from the two edges. The metallic edges should theoretically be the centres of electron deficiency, which need external electrons to form a structure; meanwhile, the surface of the Ti–OH sol contains vast numbers of negative charges. Under proper conditions, the formation of a chemical bond between the LMS and TiO₂ is possible.

Fig. 10 Representation of the LMS (MoS₂, WS₂ or SnS₂) crystallographic plan (Mo atoms in green, W atoms in blue, Sn atoms in gray and S atoms in yellow).

In Fig. 11, the structures of the global minima of TiO₂ coupled with the LMS are presented. A key feature shown is that the two oxygen atoms of Ti–OH prefer to coordinate with the metallic edges of MoS₂ and WS₂. For SnS₂, instead the oxygen atoms generally will not coordinate with the support, suggesting that the composite formation is closely related to its trigonal crystal system. And yet, MoS₂ and WS₂ have a similar structure of the hexagonal crystal system, leading to the similar results. It is noted that the distances between the preferred sites for Mo and O, W and O, and Sn and O are 2.08 Å, 2.00 Å and 2.36 Å, respectively. Meanwhile, the absorption energies were also calculated from the optimum results, and were −478.89 kJ mol⁻¹ (TiO₂/MoS₂), −640.91 kJ mol⁻¹ (TiO₂/WS₂), and −245.42 kJ mol⁻¹ (TiO₂/SnS₂), respectively. The results indicated that the solid–solid contact interface between TiO₂ and the LMS was connected by chemical bonds, and not van der Waals interaction. From the experimental results, the photocatalytic activities of the TiO₂/MoS₂, TiO₂/WS₂ and TiO₂/SnS₂ composite photocatalysts were higher than that of the TiO₂/LMS physical mixture. Meanwhile, the photostabilities and photocatalytic activities of the TiO₂/MoS₂ and TiO₂/WS₂ composites were higher than that of the TiO₂/SnS₂ composite due to the absorption energies of the TiO₂/MoS₂ and TiO₂/WS₂ composites, which suggested that the experimental and the optimization results are highly consistent with the these theoretical calculation results.

Fig. 11 Global minima of TiO₂ coupled with the LMS (MoS₂, WS₂ or SnS₂) (Mo atoms in cyan, W atoms in blue, Sn atoms in gray, S atoms in yellow, and H, O and Ti atoms of Ti–OH in silver-gray, red and silver, respectively).

The degradation of dye molecules commonly corresponds to the active species such as h⁺, e⁻, ·O₂⁻ and ·OH.^33,46,47 Herein, benzoquinone (BQ), tert-butyl alcohol (TBA), silver nitrate (AgNO₃) and EDTA-2Na were used as scavengers of superoxide radicals (·O₂⁻), hydroxyl radicals (·OH), electrons (e⁻) and holes (h⁺) to investigate the possible mechanism for the degradation of MB. Different scavengers were employed individually to remove the corresponding active species so that the function of different active species in the degradation process was understood. As shown in Fig. 12, the degradation rates are 93.4% (TiO₂/MoS₂), 93.8% (TiO₂/WS₂), and 92.1% (TiO₂/SnS₂), respectively, without a scavenger. When BQ was added into the reaction system, the degradation rates decrease to 34.9% (TiO₂/MoS₂), 33.3% (TiO₂/WS₂) and 29.7% (TiO₂/SnS₂), respectively. When TBA was added into the reaction system, the degradation rates decrease to 71.3% (TiO₂/MoS₂), 70.6% (TiO₂/WS₂) and 68.7% (TiO₂/SnS₂), respectively. The results suggested that ·O₂⁻ significantly influences the photocatalytic activity in the degradation of MB, while ·OH has implications for the degradation of MB. From the capture of the free electrons and h⁺, the influence of free electrons and h⁺ seems to be negligible in the reaction. From Fig. S1,† the formation rate of commercial TiO₂ (P25) is higher than that of TiO₂/LMS, while the photocatalytic activity is lower than that of TiO₂/LMS, which suggests that ·OH was not the main active species for the degradation of dye in the TiO₂/LMS photocatalytic system.

Fig. 12 Effects of scavengers on the degradation of MB (irradiation time = 60 min, scavenger dosage = 0.3 mmol L⁻¹).

To further understand the mechanism of electron transfer, the electron transfer of the photocatalytic reaction in a heterojunction-type photocatalytic system was elaborated at the molecular level. Based on quantum confinement effects,^48,49 the band gap of the bulk LMS can be significantly increased after being transformed into LMS nanosheets, leading to a more negative potential of the conduction band and a more positive potential of the valence band of the LMS. When the LMS nanosheets couple with TiO₂, the higher energy level of the conduction band of the LMS than that of TiO₂ would promote photoinduced electron transfer from the nanoscale LMS to TiO₂. As shown in Fig. 13, the photogenerated electrons in the CB of the LMS migrate to the CB of TiO₂, while the photogenerated holes in the VB of TiO₂ move to the VB of the LMS. Thus, the photogenerated electrons and holes are spatially separated, which greatly reduces the recombination of charge carriers. Due to the negative redox potential in the CB of TiO₂, the photogenerated electrons can reduce O₂ to ·O₂⁻. Moreover, ·O₂⁻ can also oxidize H₂O to produce ·OH, and the formation of ·OH depends on the oxidation of ·O₂⁻, which can explain why ·O₂⁻ was the main active species. Nevertheless, the synergetic effect of ·O₂⁻ and ·OH plays the leading role in the degradation of dye.

Fig. 13 Schematic diagram of the photogenerated charge transfer process on the TiO₂/LMS photocatalysts.

Conclusions

LMS nanosheets were prepared from bulk LMS by a sonication method in a liquid system. The TiO₂/LMS (LMS = MoS₂, WS₂ or SnS₂) photocatalysts were synthesized by a combined method of liquid exfoliation and the solvothermal method. The obtained TiO₂/LMS composite photocatalysts showed high photocatalytic activity under solar irradiation. The calculation results of DFT analysis and the experimental results indicated that the solid–solid contact interface between TiO₂ and the LMS was connected by chemical bonds, and not van der Waals interaction. At present, this approach of using bulk metal sulphide as a direct sensitizer to synthesize TiO₂-based composite photocatalysts has still rarely been previously reported. LMS in the composites acted as an electron donor for the interfacial electron transfer, and thus enhanced the separation of the photogenerated electron–hole pairs. The application of TiO₂/LMS for photocatalytic decontamination of MB demonstrated a higher photocatalytic activity for the TiO₂/metal sulphide composites, which have great potential in the detoxification of harmful pollutants in wastewater.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (Grant No. 21376099, 21546002).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: DFT computational method, and capture tests of hydroxyl radical (·OH). See DOI: 10.1039/c6ra03534e

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