Morphology-controlled synthesis of TiO₂/MoS₂ nanocomposites with enhanced visible-light photocatalytic activity

Abstract

The development of photocatalysts that utilize sunlight is very important for solving the energy crisis and reducing environmental pollution. In this report, two kinds of TiO₂/MoS₂ cocatalyst with novel morphologies (hollow and yolk–shell structure) are prepared via a polymer assisted targeted-etching method. MoS₂, as a typical two-dimensional (2D) layered transition metal sulfide, can accept electrons and provide active sites for photocatalytic reactions. Besides, with the assistance of MoS₂, the absorption band of the resultant heterostructures becomes broader and covers the entire visible region. Thus the photocatalytic activity of TiO₂ in the visible light region can be improved. Moreover, the hollow and yolk–shell heterostructures of TiO₂/MoS₂ possess a high specific surface area, and the excellent interface of the heterostructures enables the easy separation of holes and electrons, which can enhance the photodegradation of dye. The results demonstrate that the as-prepared TiO₂/MoS₂ nanocomposites with hollow and yolk–shell structures show outstanding catalytic activity which is tested through the degradation of methylene blue and rhodamine B under visible light. Our design provides a new strategy to acquire novel photocatalysts, with various nanostructures, that have remarkable potential applications in environmental protection, e.g. water treatment and dye pollutant degradation.

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Introduction

The development of semiconductor photocatalysts is attracting more attention recently due to their capacity to solve the energy crisis and reduce environmental pollution.¹ To date, a series of semiconductor photocatalysts have been exploited, such as ultraviolet active TiO₂ and SrTiO₃, and visible active C₃N₄, Cu₂O and CdS.² Among these photocatalytic materials, nanostructured TiO₂ has been intensively investigated as an n-type semiconductor photocatalytic material, in self-cleaning and the removal of hazardous compounds, due to its superior photocatalytic activity, chemical stability, low cost and nontoxicity.³ However, a major drawback of TiO₂ is its large band gap (3.2 eV), and thus only UV light, which constitutes merely 2–3% of the solar spectrum, can be utilized, which significantly limits the use of sunlight in photocatalysis.⁴ Besides, the low charge separation rate of pure TiO₂ also reduces its photocatalytic activity. To overcome these shortcomings, many controllable synthesis efforts have already been used to synthesize particles with well-defined sizes and shapes, such as monodispersed nanoparticles, hierarchical microspheres, hollow spheres, nanorods, nanotubes and porous networks, which result in improving the photocatalytic performance.^2a,5 Among these nanostructures, hollow or yolk–shell spheres,⁶ as special core–shell nanostructures, have caught the eye of scientists due to their properties such as low density, large surface area and surface permeability.

Moreover, researchers have reported a lot of work developing materials as cocatalysts with TiO₂-based photocatalysts, such as Ag₂O/TiO₂, C/TiO₂, and nanocluster doped TiO₂,⁷ that could broaden their light harvesting window to the visible range and improve charge separation. Specifically, as a typical two-dimensional (2D) layered transition metal sulfide, molybdenum disulfide (MoS₂), with a structure composed of three stacked atom layers (S–Mo–S) held together by van der Waals forces, has become a focus in photocatalysis research activities as a cocatalyst due to its distinctive electronic, optical and catalytic properties.⁸ Besides, MoS₂ can also accept electrons and provide active sites for photocatalytic reactions.⁹ Recently, various morphological MoS₂ nanomaterials (nanoparticles, hollow microspheres, nanotubes, nanorods, nanowires, nanoflowers) have been prepared by gas-phase synthesis, electrochemical deposition, thermal decomposition, sonochemical synthesis and hydrothermal methods.^2a,e,8a,10 However, irregular aggregates of nanoparticles or stacked multilayers limit the photocatalytic performance of the as-prepared MoS₂.¹¹ Therefore, the challenge still remains to prepare heterostructure photocatalysts based on MoS₂ nanomaterials with enhanced photocatalytic activity. Recently, CdS/MoS₂ and graphene/MoS₂ heterostructures have been prepared.^2e,12 In particular, TiO₂/MoS₂ heterostructures with diverse morphologies such as nanoparticles, nanobelts and nanowires have shown high activities as co-catalysts in the visible-light region.^11b,13 Meanwhile, constructing novel nanostructures of TiO₂/MoS₂ (hollow and yolk–shell structures) turns out to be an effective way of developing photocatalysts with both large surface areas and high numbers of active sites.

Herein, we design a targeted etching route to prepare TiO₂/MoS₂ nanocomposites with novel nanostructures (hollow and yolk–shell structures). The monodispersed TiO₂ microspheres are formed by a typical sol–gel method using tetrabutyl titanate (TBOB) as a precursor.¹⁴ Through a simple solvent-thermal process, the TiO₂/MoS₂ core–shell nanostructure can be synthesized.^11b,15 Then, the novel hollow and yolk–shell structure of TiO₂/MoS₂ can be formed using a polymer assisted targeted-etching method. In this process, the sacrificial core can be easily removed with alkaline and acid solution, or even with water, and the shell forms while the dissolution of the analogue species proceeds. In this case, the problems associated with the coating and removing of templates in the conventional template method do not exist. Thus, the targeted-etching process is highly reproducible and scalable. The resultant nanocomposites show uniform morphologies with few-layer MoS₂, large surface areas and high adsorption abilities. Moreover, as a proof of concept, we demonstrate that the novel TiO₂/MoS₂ heterostructures are excellent photocatalysts for the degradation of organic dyes, e.g. methylene blue (MB) and rhodamine B (RhB). Therefore, this novel photocatalyst may be used in the treatment of polluted water, the degradation of dye pollutants and environmental cleaning, with scope for continuous exploration in the future. Moreover, it can be expected that this design of controllable morphologies will be effective for preparing other nanoparticles.

Experimental

Materials

Tetrabutyl titanate (TBOB), ammonium tetrathiomolybdate ((NH₄)₂MoS₄), methylene blue (MB) and rhodamine B (RhB) were purchased from Sinopharm. Polyvinyl pyrrolidone (PVP, molecular weight of 10 K) and polyethylene glycol (PEG, molecular weight of 2.5 K) were purchased from Sigma-Aldrich. Aqueous ammonia (NH₃·H₂O, 28%), sodium hydroxide (NaOH) and sodium fluoride (NaF) were analytical grade. All reagents were used as received without further purification. De-ionized water was used in all experiments.

The synthesis of TiO₂ microspheres

The monodispersed TiO₂ microspheres (size: ∼180 nm) were synthesized by following the previous procedures.^11b,14,15 In a typical process, NH₃·H₂O (28%, 0.38 mL) solution and H₂O (0.91 mL) were added into a mixed solution (250 mL) containing ethanol and acetonitrile (in a volume ratio of 6 : 4) under magnetic stirring. Then TBOB (3 mL) was promptly injected into the solution under vigorous stirring for 6 h. The resultant TiO₂ microspheres were centrifuged and washed with ethanol and DI water three times in order to remove the unreacted precursors.

The preparation of TiO₂/MoS₂ core–shell nanostructures

The TiO₂/MoS₂ core–shell nanostructures were prepared by a simple solvent-thermal process.^11b In a typical process, 2 mg mL⁻¹ of (NH₄)₂MoS₄ and 2 mg mL⁻¹ of TiO₂ microspheres were dispersed in DMF (10 mL). The mixture was sonicated at room temperature for approximately 30 min until a clear and homogeneous solution was achieved. Then, the solution was transferred to a 20 mL Teflon-lined autoclave and heated in an oven at 180 °C for 24 h. The resultant product was collected by centrifugation at 8000 rpm for 5 min, washed with DI water and recollected by centrifugation.

The synthesis of TiO₂/MoS₂ hollow microspheres

Typically, the TiO₂/MoS₂ core–shell microspheres (20 mg) were re-dispersed in water (10 mL). Then NaF (10 mg) was added into the suspension as the etching agent. After stirring for 1 h, PVP (10 mg) with an average molecular weight (AMW) of 10 K was added. The suspension was stirred for another 1 h before transferring to a Teflon-lined autoclave (20 mL). A hydrothermal reaction was carried out at 120 °C for 2 h to conduct the crystallization and targeted etching processes. The resultant TiO₂/MoS₂ hollow microspheres were collected. After washing with dilute NaOH solution (10 mmol L⁻¹) and water three times, the final samples were obtained by drying at 60 °C for 10 h.

The synthesis of TiO₂/MoS₂ yolk–shell microspheres

TiO₂/MoS₂ core–shell microspheres (20 mg) were re-dispersed in water (10 mL). PEG (AMW, 2.5 K; 10 mg) was added into the solution. The solution was stirred and ultrasonicated to fill PEG into the pores of TiO₂/MoS₂. Then the product was mixed with NaF (10 mg). After stirring for 1 h, PVP (5 mg) was added. The resulting suspension was stirred for a further 1 h before being transferred to a Teflon-lined autoclave (20 mL). A hydrothermal reaction was carried out at 120 °C for 1 h. The white products were collected and washed with dilute NaOH solution (10 mmol L⁻¹) and water three times, then dried at 60 °C for 10 h.

Photocatalytic degradation of methylene blue (MB) under visible light

The photocatalytic activity of the samples was determined by the degradation of MB under visible light irradiation at room temperature. A 500 W Xe-illuminator was used as a light source and set about 10 cm from the reactor. In each run, 3 mg of the photocatalyst was added to the MB solution (100 mL, 5 mg L⁻¹) and the mixture was sonicated for 1 min. Before irradiation, the suspension was stirred for 60 min in the dark to reach the adsorption–desorption equilibrium between the MB and the photocatalyst. During given time intervals, the photo-reacted solution (3 mL) was extracted and then separated by membrane filtration (pore size 0.22 μm) to remove essentially all the photocatalyst particles. Finally, a UV-vis spectrophotometer was used to determine the degradation efficiency: (1 − C/C₀) × 100%.

Characterization methods

UV–Vis absorption spectra of dyes were obtained using a TU-1991 UV–Vis spectrophotometer. UV–vis absorption spectra of the products (solid state) were obtained using a Shimadzu UV-3600 plus PC spectrofluorimeter. X-ray photoelectron spectroscopy (XPS), using Mg K_α excitation (1253.6 eV), was performed using a VG ES-CALAB MKII spectrometer. Binding energy calibration was based on the C 1s peak at 284.6 eV. X-ray diffraction (XRD) data were collected on a Siemens D-5005 X-ray diffractometer with Cu K_α radiation (λ = 1.5418 Å). The morphologies and structures of the samples were characterized by scanning electron microscopy (SEM, Hitachi S4800) and transmission electron microscopy (TEM, JEM-2100).

Results and discussion

The process of preparing TiO₂/MoS₂ nanocomposites with various morphologies is shown in Scheme 1. The monodispersed TiO₂ microspheres were prepared by a typical sol–gel method using tetrabutyl titanate (TBOB) as the precursor.¹⁵ The prepared TiO₂ microspheres showed a good dispersion and spherical morphology with an average diameter of 180 nm (Fig. S1†). Using the resultant TiO₂ microspheres as seeds, TiO₂/MoS₂ core–shell nanostructures were formed through a facile solvent-thermal route, which was similar to that of the previous research.^11b In this process, the (NH₄)₂MoS₄ worked as a Mo and S source simultaneously, and has showed numerous applications in the preparation of MoS₂ nanomaterials.¹⁶ Through adjusting the experimental conditions, uniform TiO₂/MoS₂ core–shell microspheres were formed. Then, with the assistance of PVP and PEG, the TiO₂ core was removed selectively by NaF, and some novel nanostructures of TiO₂/MoS₂ nanocomposites were formed, such as the hollow and yolk–shell structures. Hence, a targeted etching route is developed to control the morphology of TiO₂/MoS₂ nanocomposites, which is an effective way to improve the performance of semiconductor photocatalysts.

Scheme 1 Scheme illustrating the formation of multiple morphologies of MoS₂/TiO₂ nanocomposites including core–shell microspheres, yolk–shell microspheres and hollow microspheres.

Fig. 1a shows the transmission electron microscopy (TEM) image of the as-prepared TiO₂ microspheres, which are 180 nm in diameter and have a smooth surface. After the solvent-thermal process with (NH₄)₂MoS₄, the MoS₂ nanosheets can grow on the surface of TiO₂ and the TiO₂/MoS₂ core–shell structure formed (Fig. 1b). The resultant core–shell structure showed a uniformly spherical morphology and the size was larger than that of the pure TiO₂ microspheres as shown in Fig. 1b. Moreover, such core–shell structures look like flowers with some thin MoS₂ nanosheets coating the surface of the TiO₂ microspheres (inset of Fig. 1b). The high-resolution TEM (HRTEM) image determined the growth direction of the TiO₂ microspheres to be [010], and the lattice fringes perpendicular to the growth direction have a spacing of 0.35 nm,^2a which is equal to the lattice parameter of the (101) facet (Fig. 1c). Besides, the HRTEM image also revealed a highly crystalline structure with continuous lattice fringes between the thin nanosheets separated by about 0.62 nm, corresponding to the (002) faces of the MoS₂ crystals (Fig. 1c).^16b The element mapping suggested that the resultant heterostructure contained four elements: Mo, S, Ti and O, simultaneously (Fig. 1d–f). In particular, considering the features of the spatial distribution of the different elements, the data also confirmed that the TiO₂/MoS₂ microspheres were hierarchical in structure, containing TiO₂ microspheres as the core and MoS₂ nanosheets as the shell (Fig. 1f). It should be noted that the TiO₂/MoS₂ microsphere in the dashed circle was incompletely covered by MoS₂ nanosheets, as shown in Fig. 1d and e, so it possesses a strong Ti and O signal, while the others were weak due to signal interference with the MoS₂ shell (Fig. 1f). After comprehensive consideration of these verified results, we can claim that the TiO₂/MoS₂ core–shell structure was prepared. Moreover, similar to previous mechanisms,^11b this uniform structure was controlled by the ratio of the seeds to (NH₄)₂MoS₄ as shown in Fig. S2.† When the amount of seed (1 mg mL⁻¹) was low, TiO₂/MoS₂ core–shell structures with some irregular MoS₂ nanosheets were synthesized (Fig. S2a†). In contrast, if the amount of seed was high, the MoS₂ nanosheets would not coat the TiO₂ completely to form the uniform core–shell morphology (Fig. S2c and d†). The perfect core–shell structures with uniform size and regular morphology were acquired when the amount of TiO₂ was 2 mg mL⁻¹ (Fig. 1b and Fig. S2b†).

Fig. 1 TEM images of (a) monodispersed TiO₂ microspheres and (b) MoS₂/TiO₂ core–shell microspheres. (c) and inset image of (b) high-resolution TEM images of core–shell microspheres. (d–f) Elemental mapping images of (e) Mo/S and (f) Ti/O.

To obtain hollow (h-TiO₂/MoS₂) and yolk–shell structures (y-TiO₂/MoS₂) of TiO₂/MoS₂, a polymer assisted method was used in the targeted etching process. Polyvinyl pyrrolidone (PVP), which has played an important role in forming novel heterostructures, was introduced in the system.¹⁷ In the targeted etching process, PVP protects the shell structure of MoS₂,¹⁴ and restrains the diffusion of the dissolved Ti species into the solvent. Then, another polymer (polyethylene glycol (PEG)) was added, that filled the inside of the TiO₂/MoS₂ structure and possesses the capability to effectively partially decrease the amount of inner TiO₂. Thus this process was favorable for preparing y-TiO₂/MoS₂. With the help of a hydrothermal reaction, uniform y-TiO₂/MoS₂ was synthesized. As shown in Fig. 2a, after etching, the TiO₂ disappeared and hollow spheres were formed, but the center was a solid sphere with a size of 135 nm. This nanostructure of reported materials is defined as a yolk–shell structure. Moreover, MoS₂ nanosheets were on the surface and their crystal morphology was almost undamaged during the targeted etching process (Fig. 2b). The elemental mappings showed a homogeneous hierarchical distribution of Mo and Ti (Fig. 2c–e). The results confirmed that the inside was TiO₂, forming the yolk, and MoS₂ was on the surface as the shell. Thus uniform y-TiO₂/MoS₂ was formed, but this structure was controlled by the etching time. As shown in Fig. S3,† big yolks start to appear inside the sphere after an etching reaction for 0.5 h (Fig. S3a and c†), and do not have a clear structure like the samples in Fig. 2a. When the etching time was prolonged to 2 h, an irregular yolk–shell structure with a broken MoS₂ shell was acquired (Fig. S3b and d†).

Fig. 2 (a) TEM and (b) high-resolution TEM images of MoS₂/TiO₂ yolk–shell microspheres synthesized by targeted etching for 1 h; (c–e) elemental mapping images of (d) Mo and (e) Ti.

Moreover, PEG contributed to the formation of the yolk–shell structure. Since PEG could fill the inner space of TiO₂/MoS₂, and hinder the penetration of fluoride and water upon soaking in fluoride solution, the inner part could be protected from etching.^14,18 If the experiment was done in the absence of PEG, the h-TiO₂/MoS₂ structure was prepared. As shown in Fig. 3a, after targeted etching for 2 h, the uniform h-TiO₂/MoS₂ structure appeared, with a size of 203 nm and a shell thickness of 11 nm. The outside layer of h-TiO₂/MoS₂ was composed of MoS₂ nanosheets which can be observed in the HRTEM image (Fig. 3b). However, the shell of h-TiO₂/MoS₂ was composed of MoS₂ and TiO₂ nanocomposites, which agreed with the results of the elemental mappings in Fig. 3c–e. In addition, it is easily understood that the extent of the hollow evolution process was time dependent, products with different sizes of hollow structures were synthesized by changing the etching time. At the beginning of the etching (1 h), the hollow structure appeared, but when increasing the etching time up to 3 h, the shell of the hollow structure was damaged (Fig. S4†).

Fig. 3 (a) TEM and (b) high-resolution TEM images of MoS₂/TiO₂ hollow microspheres synthesized by targeted etching for 2 h; (c–e) elemental mapping images of (d) Mo and (e) Ti.

The XRD patterns of the as-prepared TiO₂-anatase, TiO₂/MoS₂ core–shell microspheres, h-TiO₂/MoS₂, y-TiO₂/MoS₂, pure MoS₂ and pure TiO₂ are shown in Fig. 4. TiO₂ microspheres prepared by sol–gel methods were amorphous as shown in Fig. 4f. After hydrothermal treatment, TiO₂-anatase was formed (Fig. 4a), which matched with the standard peaks of a pure anatase phase (JCPDS card no. 21-1272).¹⁹ As for the pure MoS₂ sample (Fig. 4e), the detected peaks can be assigned to the hexagonal phase of MoS₂ (JCPDS card no. 37-1492).²⁰ The weak (002) peak at 14.5° demonstrated that MoS₂ was composed of few layer sheets (the MoS₂ sample was acquired by freeze drying to avoid irregular aggregation).²¹ After forming the TiO₂/MoS₂ composites, the diffraction peaks of TiO₂ were unchanged compared with those of TiO₂-anatase, while the diffraction peaks of MoS₂ were almost not observed (Fig. 4b–d).¹¹ The weak peaks may be attributed to the presence of few layer MoS₂ sheets on the surface of the composites that were too thin to be detected.^2a However, the chemical composition of the resultant samples was confirmed by XPS spectra. The binding energies of the Mo 3d_5/2 and Mo 3d_3/2 peaks at 228.9 and 232.2 eV, and the S 2p_3/2 and S 2p_1/2 peaks at 162.5 and 163.6 eV, indicated that Mo⁴⁺ and S²⁻ were the dominant oxidation states (Fig. S5†).²² The asymmetric peaks and tailing spectra (at about 236 eV) for Mo in the heterostructures were likely due to the existence of a small amount of Mo⁶⁺ (Fig. S5†).^2e Moreover, the amount of Ti in TiO₂/MoS₂, y-TiO₂/MoS₂ and h-TiO₂/MoS₂ decreased during the etching process (Table S1†). These results indicated that the various morphologies of the TiO₂/MoS₂ nanocomposites were synthesized through the polymer assisted targeted-etching method. The composition of the nanocomposites includes TiO₂ and MoS₂ simultaneously. These heterostructures, which are composed of TiO₂ and MoS₂ simultaneously, may provide a large number of interfaces, which favors the separation of electrons and holes when the heterostructures are under visible light, and enhances the photocatalytic performance.

Fig. 4 XRD patterns of (a) TiO₂-anatase; (b) MoS₂/TiO₂ core–shell microspheres; (c) MoS₂/TiO₂ yolk–shell microspheres; (d) MoS₂/TiO₂ hollow microspheres; (e) pure MoS₂ (f) and pure TiO₂.

Moreover, the coexistence of TiO₂ and MoS₂ in the resultant heterostructures enhances the absorption ability of TiO₂ in the visible light region, which might increase the photocatalytic activity. As shown in Fig. S6,† the characteristic absorption band of TiO₂ was in the ultraviolet range (<400 nm), while MoS₂ nanosheets absorb in the range 300–800 nm. After forming heterostructures, the integral areas of TiO₂/MoS₂, y-TiO₂/MoS₂, h-TiO₂/MoS₂ in the ultraviolet range decreased with removal of the inner TiO₂, which is extremely useful for improving the activity of the catalysts under visible light. As in previous reports,²³ with the help of MoS₂, the absorption band of the resultant heterostructures became broader and covered the entire visible region. Additionally, the photoluminescence (PL) spectra of TiO₂, h-TiO₂/MoS₂ and y-TiO₂/MoS₂ are given in Fig. S7.† After forming heterostructures, the PL intensity decreases obviously. Generally, the PL signal is generated from the radioactive recombination of electrons and holes; thus, a low PL emission peak shows a slow recombination rate of charge carriers which means the photocatalytic activity can be improved.²⁴

The photocatalytic activity of the as-prepared samples was evaluated via the photocatalytic degradation of MB in aqueous solution under visible-light (λ > 400 nm) irradiation as shown in Fig. 5, where C and C₀ denote the remnant and initial concentrations of MB, respectively. Control experiments indicated that no degradation of MB was measured in the absence of the photocatalyst (Fig. 5a). Before the light irradiation (0 min), the suspensions were stirred (1 h) in the dark to establish the adsorption/desorption equilibrium between the dye and the catalyst. The adsorption values of pure TiO₂, TiO₂-anatase, pure MoS₂, TiO₂/MoS₂, y-TiO₂/MoS₂ and h-TiO₂/MoS₂ were 189.7, 130.4, 396.6, 299.9, 498.5 and 498.5 mg g⁻¹, respectively (Table S2†). As in previous work,^2e,11b MoS₂ as a two-dimensional layered nanomaterial has an excellent anchoring surface for adsorbing dye molecules, and it possessed a high adsorption value of about 396.6 mg g⁻¹. This suggested that, compared with TiO₂ or TiO₂-anatase, the adsorption values of TiO₂/MoS₂ nanocomposites were improved, which was confirmed by the BET measurement (Table S1†). The BET area of the nanocomposites decreased rapidly. MoS₂, with an anchoring surface for adsorbing dye molecules, filled the pores of TiO₂,^11b forming heterojunctions on the surface and near-surface of TiO₂ under hydrothermal conditions. The resultant heterostructures had highly porous structures (Fig. S8†). However, it should be noted that it is impossible to form heterosjunctions in the TiO₂ core, where the MoS₂ precursors could not diffuse in. With etching, pure TiO₂ in TiO₂/MoS₂ was removed and the proportion of TiO₂/MoS₂ at the interface was improved. Mesoporous structured TiO₂ was the main contribution to the BET area of the composites. But the adsorption values of TiO₂/MoS₂, y-TiO₂/MoS₂ and h-TiO₂/MoS₂ increased with decreasing BET area.

Fig. 5 (a) The photodegradation of MB by different photocatalysts; (b) the photodegradation of MB in the presence of MoS₂/TiO₂ hollow microspheres; the inset is a series of photographs of the photodegradation of MB at various times using MoS₂/TiO₂ hollow microspheres as photocatalysts.

As shown in Fig. 5a, the photocatalytic performances of all the composites was enhanced due to the formation of heterojunctions as catalytic active sites. The degradation rates of TiO₂/MoS₂, y-TiO₂/MoS₂ and h-TiO₂/MoS₂ were measured to be 84.56%, 93.39% and 98.28% under visible-light irradiation for 120 min. Similar to previous reports,^2a,e the photocatalytic activity was controlled by the proportion of heterojunctions. h-TiO₂/MoS₂ showed the highest proportion of heterojunctions obviously, and as a result possessed excellent photocatalytic performance. Fig. 5b shows the corresponding UV-vis spectra of MB in the presence of TiO₂/MoS₂ hollow microspheres as catalysts. The continuous reduction of the absorption intensity at about 650 nm indicates the gradually decreasing concentration of MB with increasing irradiation time; it took about 120 min to completely degrade MB. The change in color of MB can be clearly observed from the corresponding photographs (inset of Fig. 5b). The corresponding UV-vis spectra of MB degraded by other catalysts are shown in Fig. S9.†

In addition, Fig. 6a reveals that the h-TiO₂/MoS₂ was very stable during a five-cycle experiment. In the fifth cycle, the amount of MB degraded in 2 h remains more than 88.4% (Fig. S10a†). Only a slight decrease in the photocatalytic activity occurred, which could partly be caused by a loss of the photocatalyst during each cycle of collection and rinsing. Fig. S10b† shows a TEM image of the TiO₂/MoS₂ hollow microspheres after photocatalytic degradation, and the hollow structure is almost undamaged which contributes to the persistently high photocatalytic activity and efficiency. The degradation of RhB with the same catalyst was also explored (100 mL, 5 mg L⁻¹ RhB solution with 5 mg h-TiO₂/MoS₂ heterostructures) as shown in Fig. 6b. The surprising photocatalytic activity indicates that the as-prepared samples have good potential applications in the degradation of multiple dyes in the future. Moreover, the as-prepared sample showed a high photocatalytic hydrogen evolution activity (Fig. S11†). This result demonstrates that the resultant heterostructures are promising for applications in polluted water treatment, catalysis and energy conversion.

Fig. 6 (a) The cycles of degradation of MB using MoS₂/TiO₂ hollow microspheres as the photocatalysts under visible light irradiation for 2 h; (b) the photodegradation of RhB in the presence of MoS₂/TiO₂ hollow microspheres; the inset is a series of photographs of the photodegradation of RhB after various times, using MoS₂/TiO₂ hollow microspheres as photocatalysts.

The photocatalytic mechanism for the enhanced photocatalytic activity of the heterostructures is shown in Fig. 7. As in previous results,^2a,21b the redox potentials of MoS₂ are changed and its band gap is close to 1.9 eV, when the MoS₂ nanosheets are too thin. Thus, both the conduction band (CB) and valence band (VB) positions of MoS₂ are higher than those of TiO₂ and the energy bands between MoS₂ and TiO₂ match (Fig. 7).²⁵ Under light irradiation, the photoexcited electrons in the CB of the MoS₂ nanosheets are transferred to the CB of TiO₂, and eventually react with oxygen to produce superoxide anion radicals.^11b Simultaneously, the holes left in the VB of the MoS₂ nanosheets, and those transferred from the VB of TiO₂, can react with H₂O or OH⁻ to form hydroxyl radicals. Hydroxyl radicals are powerful oxidants that start a series of oxidation reactions, and can ultimately totally degrade the dyes. Heterojunctions between TiO₂ and MoS₂ accelerated the separation of the photoexcited electrons and holes,^2a and increased the physical distance. Therefore, this kind of heterostructure can degrade dyes effectively under visible light. Moreover h-TiO₂/MoS₂, with the highest proportion of heterojunctions, shows the best photocatalytic activity.

Fig. 7 Schematic illustration of the charge-transfer in TiO₂/MoS₂ induced by light irradiation for photocatalytic activity.

Conclusions

A targeted etching route to prepare TiO₂/MoS₂ nanocomposites with novel nanostructures (hollow structure and yolk–shell structure) has been developed. Through a simple solvent-thermal process, the TiO₂/MoS₂ core–shell nanostructure can be synthesized. Then, the hollow structure and yolk–shell structure of TiO₂/MoS₂ can be obtained by targeted etching using a polymer assisted method. Interestingly, the proportion of interface in TiO₂/MoS₂, y-TiO₂/MoS₂ and h-TiO₂/MoS₂ increases in order, and we achieved the highest photocatalytic activity with h-TiO₂/MoS₂. Therefore, this novel photocatalyst can be used in the treatment of polluted water, the degradation of dye pollutants, and environmental cleaning with scope for continuous exploration in the future. Moreover, it can be expected that this design of controllable morphologies will be a useful method to prepare other nanoparticles with complex structures.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (no. 51503085), open project of state key laboratory of supramolecular structure and materials (sklssm201724), Science Foundation of China University of Petroleum, Beijing (no. 2462017YJRC027).

Notes and references

1. (a) Q. Xiang, J. Yu and M. Jaroniec, J. Am. Chem. Soc., 2012, 134, 6575–6578; (b) Y. Tan, K. Yu, T. Yang, Q. Zhang, W. Cong, H. Yin, Z. Zhang, Y. Chen and Z. Zhu, J. Mater. Chem. C, 2014, 2, 5422–5430; (c) Y. Chao, J. Zheng, J. Chen, Z. Wang, S. Jia, H. Zhang and Z. Zhu, Catal. Sci. Technol., 2017, 7, 2798–2804; (d) S. H. Li, S. Q. Liu, J. C. Colmenares and Y. J. Xu, Green Chem., 2016, 18, 594–607.
2. (a) W. Zhou, Z. Yin, Y. Du, X. Huang, Z. Zeng, Z. Fan, H. Liu, J. Wang and H. Zhang, Small, 2013, 9, 140–147; (b) L. Gao, W. Ren, B. Liu, Z.-S. Wu, C. Jiang and H.-M. Cheng, J. Am. Chem. Soc., 2009, 131, 13934–13936; (c) Q. Xiang, J. Yu and M. Jaroniec, J. Phys. Chem. C, 2011, 115, 7355–7363; (d) J. Y. Ho and M. H. Huang, J. Phys. Chem. C, 2009, 113, 14159–14164; (e) C. X. Wang, H. Lin, Z. Xu, H. Cheng and C. Zhang, RSC Adv., 2015, 5, 15621–15626; (f) J. Matos, S. Miralles-Cuevas, A. Ruiz-Delgado, I. Oller and S. Malato, Carbon, 2017, 122, 361–373.
3. Z. Yin, Z. Wang, Y. Du, X. Qi, Y. Huang, C. Xue and H. Zhang, Adv. Mater., 2012, 24, 5374–5378.
4. X. Chen, L. Liu, Y. Y. Peter and S. S. Mao, Science, 2011, 331, 746–750.
5. W. Zhou, H. Liu, J. Wang, D. Liu, G. Du and J. Cui, ACS Appl. Mater. Interfaces, 2010, 2, 2385–2392.
6. (a) I. Lee, J. B. Joo, Y. Yin and F. Zaera, Angew. Chem., Int. Ed., 2011, 123, 10390–10393; (b) Q. Xiang, J. Yu, B. Cheng and H. Ong, Chem. – Asian J., 2010, 5, 1466–1474.
7. (a) D. Sarkar, C. K. Ghosh, S. Mukherjee and K. K. Chattopadhyay, ACS Appl. Mater. Interfaces, 2012, 5, 331–337; (b) S. M. Oh, J. Y. Hwang, C. Yoon, J. Lu, K. Amine, I. Belharouak and Y. K. Sun, ACS Appl. Mater. Interfaces, 2014, 6, 11295–11301; (c) Y. Shiraishi, S. Shiota, H. Hirakawa, S. Tanaka, S. Ichikawa and T. Hirai, ACS Catal., 2017, 7, 293–300; (d) Y. Z. Gong, C. Yang, H. W. Ji, C. C. Chen, W. H. Ma and J. C. Zhao, Chem. – Asian J., 2016, 11, 3568–3574.
8. (a) D. Gopalakrishnan, D. Damien and M. M. Shaijumon, ACS Nano, 2014, 8, 5297–5303; (b) J. Xie, H. Zhang, S. Li, R. Wang, X. Sun, M. Zhou, J. Zhou, X. W. D. Lou and Y. Xie, Adv. Mater., 2013, 25, 5807–5813; (c) Y. Cheng, S. Lu, F. Liao, L. Liu, Y. Li and M. Shao, Adv. Funct. Mater., 2017, 27, 1700359; (d) S. Zhao, R. Jin, Y. Song, H. Zhang, S. D. House, J. C. Yang and R. Jin, Small, 2017, 13, 1701519.
9. Q. Liu, X. Li, Q. He, A. Khalil, D. Liu, T. Xiang, X. Wu and L. Song, Small, 2015, 11, 5556–5564.
10. S. Kanda, T. Akita, M. Fujishima and H. Tada, J. Colloid Interface Sci., 2011, 354, 607–610.
11. (a) G. Tang, Y. Wang, W. Chen, H. Tang and C. Li, Mater. Lett., 2013, 100, 15–18; (b) C. X. Wang, H. Lin, Z. Liu, J. Wu, Z. Xu and C. Zhang, Part. Part. Syst. Charact., 2016, 33, 221–227.
12. (a) J. Chen, X. J. Wu, L. Yin, B. Li, X. Hong, Z. Fan, B. Chen, C. Xue and H. Zhang, Angew. Chem., Int. Ed., 2015, 54, 1210–1214; (b) Y. Min, G. He, Q. Xu and Y. Chen, J. Mater. Chem. A, 2014, 2, 2578–2584; (c) K. Chang, Z. Mei, T. Wang, Q. Kang, S. Ouyang and J. Ye, ACS Nano, 2014, 8, 7078–7087; (d) Z. Ye, J. Yang, B. Li, L. Shi, H. Ji, L. Song and H. Xu, Small, 2017, 13, 1700111.
13. (a) H. Xia, W. Xiong, C. K. Lim, Q. Yao, Y. Wang and J. Xie, Nano Res., 2014, 7, 1797–1808; (b) H. Li, Y. Wang, G. Chen, Y. Sang, H. Jiang, J. He, X. Li and H. Liu, Nanoscale, 2016, 8, 6101–6109; (c) X. Liu, Z. Xing, H. Zhang, W. Wang, Y. Zhang, Z. Li, X. Wu, X. Yu and W. Zhou, ChemSusChem, 2016, 9, 1118–1124.
14. J. H. Pan, X. Z. Wang, Q. Huang, C. Shen, Z. Y. Koh, Q. Wang, A. Engel and D. W. Bahnemann, Adv. Funct. Mater., 2014, 24, 95–104.
15. R. Asahi, T. Morikawa, T. Ohwaki, K. Aoki and Y. Taga, Science, 2001, 293, 269–271.
16. (a) H. Lin, X. C. Wang, J. Wu, Z. Xu, Y. Huang and C. Zhang, New J. Chem., 2015, 39, 8492–8497; (b) Y. Li, H. Wang, L. Xie, Y. Liang, G. Hong and H. Dai, J. Am. Chem. Soc., 2011, 133, 7296–7299.
17. (a) Q. Zhang, T. Zhang, J. Ge and Y. Yin, Nano Lett., 2008, 8, 2867–2871; (b) Q. Zhang, I. Lee, J. Ge, F. Zaera and Y. Yin, Adv. Funct. Mater., 2010, 20, 2201–2214.
18. J. Liu, S. Z. Qiao, J. S. Chen, X. W. D. Lou, X. Xing and G. Q. M. Lu, Chem. Commun., 2011, 47, 12578–12591.
19. (a) A. L. Linsebigler, G. Lu and J. T. Yates, Chem. Rev., 1995, 95, 735–758; (b) T. L. Thompson and J. T. Yates, Chem. Rev., 2006, 106, 4428–4453.
20. (a) P. Joensen, E. Crozier, N. Alberding and R. Frindt, J. Phys. C: Solid State Phys., 1987, 20, 4043; (b) Y. Peng, Z. Meng, C. Zhong, J. Lu, W. Yu, Y. Jia and Y. Qian, Chem. Lett., 2001, 772–773; (c) H. Ramakrishna Matte, A. Gomathi, A. K. Manna, D. J. Late, R. Datta, S. K. Pati and C. Rao, Angew. Chem., Int. Ed., 2010, 122, 4153–4156.
21. (a) P. Joensen, R. Frindt and S. R. Morrison, Mater. Res. Bull., 1986, 21, 457–461; (b) G. Eda, H. Yamaguchi, D. Voiry, T. Fujita, M. Chen and M. Chhowalla, Nano Lett., 2011, 11, 5111–5116.
22. Y. Wang, J. Z. Ou, S. Balendhran, A. F. Chrimes, M. Mortazavi, D. D. Yao, M. R. Field, K. Latham, V. Bansal and J. R. Friend, ACS Nano, 2013, 7, 10083–10093.
23. (a) X. Zhang, U. Veikko, J. Mao, P. Cai and T. Peng, Chem. – Eur. J., 2012, 18, 12103–12111; (b) J. Tao, J. Chai, L. Guan, J. Pan and S. Wang, Appl. Phys. Lett., 2015, 106, 081602.
24. N. Q. Wu, J. Wang, D. N. Tafen, H. Wang, J. G. Zheng, J. P. Lewis, X. G. Liu, S. S. Leonard and A. Manivannan, J. Am. Chem. Soc., 2010, 132, 6679–6685.
25. W. Ho, J. C. Yu, J. Lin, J. G. Yu and P. S. Li, Langmuir, 2004, 20, 5865.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c7qi00491e

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