Molybdenum Disulfide-Based Photocatalysis:Bulk-to-Single Layer Structure and Related Photomechansim for Environmental Applications

Abstract

Bulk-to-single layer molybdenum disulfide (MoS2) is widely used as a robust candidate for photodegradation of organic pollutants, hydrogen production, and CO2 reduction. This material features active edge sites and narrow band gap features, which are useful for generating reactive species in aqueous suspensions. However, the high-charge carrier recombination, photocorrosion, unstable sulfide state, and formation of Mo-S-O links during photocatalytic reactions limit its applicability. Thus, research has focused on improving the performance of MoS2 by tailoring its bulk-to-single layer structure and combining it with other semiconductor materials to improve the photocatalytic performance. Different strategies have been successfully applied to enhance the photocatalytic activity of MoS2, including tailoring of the surface morphology, formation of heterojunctions with other semiconductors, doping, and modification with excess sulfur or carbon nanostructures. This review describes the influence of starting precursors, sulfur sources, and synthetic methods to obtain heterostructured morphologies and study their impact on the photocatalytic efficiency. Finally, the relevance of crystal facets and defects in photocatalysis is outlined. Future applications of MoS2 with tailoring and tuning physicochemical properties are highlighted.

1. Introduction

The environment continues to become more polluted due to industrialization. However, traditional chemical methods that deal with environmental pollution have been unable to meet the requirements of saving energy and environmental protection. Environmental problems induced by toxic and organic pollutants that are hard to degrade (such as halides, dioxins, pesticides, and dyes) are important issues for human well-being and development. The sun is an abundant source of energy and sustains life on earth; and photocatalysis has been studied extensively for waste water recycling in various industries to remove organic pollutants using photocatalysts. The use of photocatalysts for waste water treatment is promising for meeting increasing water recycling demands without compromising the quality of our environment.

Although photocatalysis is successful in laboratory studies, there are some technological problems that hamper the extensive commercial applicability of this technique. Optimum utilization and commercial viability of photocatalysis could possibly be achieved by replacing the expensive and technically complex artificial light sources with low cost and renewable energy from sunlight as a natural excitation source. In recent decades, photocatalysts with high activities based on transition metal dichalcogenides (TMDs) have been used extensively for environmental applications such as air purification, water disinfection, hazardous waste remediation, and water purification. TMDs materials such as MQ₂ (M = Mo, Nb, Re, V, W and Q = S, Se) have gained much attention due to their unique properties for a wide range of applications, since the nanoscale form of these inorganic materials was discovered.

Tenne et al. [1] discovered spherical fullerene-like nanoparticles of molybdenum disulfide (MoS₂) and tungsten disulphide (WS₂) nanotubes in 1992. Since then, research on these materials and their tribological properties has intensified. One-dimensional (1D) or two-dimensional (2D) structures of TMDs materials have remarkable properties, such as chemical inertness, anisotropy, photocorrosion resistance, electronic properties, tribological properties, and photocatalytic behavior [2–8]. They also have good catalytic properties and resistance to sulfur poisoning [9]. MoS₂ and WS₂ are the most prominent family of TMDs materials and are most commonly used in layered structured forms. They have a layered, close-packed hexagonal crystal structure confined in vertically stacked monolayers that bond together by weak van der Waals forces. However, the structural and morphological features of these materials widely depend on the synthesis strategies. The starting materials, surfactants, sulfur sources, and solvents play crucial roles in the structural and morphological features.

The different morphologies of MoS₂ and WS₂ include nanospheres, few-layered nanosheets, nanofibers, nanotubes, and nanorods. These forms have attracted extensive interest owing to their intriguing physical properties and prospects for applications in nanoelectronics, electrochemistry, catalysis, and lubrication. However, the performance characteristic of these materials depends on the particle size, shape, and structure. These materials are synthesized by various approaches, including chemical vapor deposition, photothermal, sonochemical, solvothermal, hydrothermal, and two-step electrochemical synthesis methods.

2. Synthetic methods

2.1. Synthesis of MoS₂ nanomaterials

In the reaction stage, the size and shape control during the synthesis of MoS₂ nanomaterials are crucial for obtaining well-defined materials with specific properties. The morphology and size of MoS₂ play a significant role in catalysis, sensors, and other applications. Shape control is also important for applications in photochemistry and fuel cell catalysis. Other factors include monodispersity, avoiding agglomeration, and surface functionalization.

The wide variety of synthetic methods for obtaining MoS₂ nanomaterials can be divided into three main groups: (i) mechanical methods (e.g., grinding, ultrasonic cracking, or milling), (ii) liquid phase methods (e.g., sol-gel, hydrothermal, or wet chemical methods), and (iii) gas phase techniques (e.g., chemical vapor deposition or laser ablation deposition). Liquid phase techniques are used most often due to their simplicity, low cost, and the wide variety of different sizes, shapes, and surface functionalities that can be obtained. Also, the size and agglomeration are effectively controlled by functionalization of the nanomaterial surface with surfactants. In some cases, these surfactants are used to control the shape and to promote growth in a specific direction by selective binding to some crystalline faces. Figure 1 shows the SEM and TEM images of MoS₂ nanosheets synthesized by solvothermal approach [6]. These MoS₂ nanosheets were obtained as a few lamellar layers using thiourea as a sulfur source.

Figure 1.

SEM (a,b) and TEM (c-e) images of solvothermally- synthesized MoS₂ nanosheets [6].

The combination of size- and shape-dependent physical properties along with their simple fabrication and processing techniques make MoS₂ nanomaterials, a promising candidate for a wide range of applications. The properties of the individual particles and their mutual interactions determine important features of nanomaterial systems. For example, optical properties are highly dependent on the size, shape, and crystallinity of the MoS₂ nanomaterials. However, controlled synthesis with a narrow size distribution and uniform shape remains an important issue in photocatalytic applications.

2.2. Synthesis of MoS₂ thin films

Chemical vapor deposition (CVD) is one of the most popular methods for fabricating thin films of few-layered MoS₂ nanosheets [10, 11]. In addition, impurity-assisted methods are presently gaining much attention for increasing the grain size and decreasing the growth temperature. The growth conditions of single-layer MoS₂ in a CVD system depend on the nature of the substrate and the surface treatments used. Kinetic effects and on-off stoichiometric growth conditions can be used to produce different shapes in MoS₂ nanosheets, including star and dendrite shapes. The controllability and reproducibility of shape control are still being improved. In photocatalyst applications, the crystallinity and morphology strongly affect the device performance. Heterostructured MoS₂-based layered composite structures have recently been a focus in fundamental semiconductor technology.

The simplest way to develop a monolayer MoS₂ analog to graphene is the Scotch-tape exfoliation method from bulk MoS₂ along the direction of van der Waals interaction. Repeated exfoliation of bulk MoS₂ decreases the number of MoS₂ layers, eventually producing few- to single-layer MoS₂. However, this method is difficult for large-scale production due to the poor size controllability. To synthesize MoS₂ thin films on a large scale, three important techniques are mostly applied by researchers: thermal vapor sulfurization (TVS), dip coating, and CVD. CVD is the most prominent technique and is used with gasified Mo- and S-containing species that react and deposit on the surface of a substrate. The major difference between the CVD and TVS approaches is the state of Mo source: a solid Mo source is used in TVS, whereas a gasified Mo source is used in a CVD system. CVD systems can be classified according to energy source as (i) hot-walled thermal CVD, (ii) plasma CVD, and (iii) metal organic CVD, among others.

Hot-walled thermal CVD systems are widely used to synthesize MoS₂ nanosheets. Both sources of Mo and sulfur are gasified and transported to the surface of a substrate, adsorbed, and decomposed into reactive atoms, which results in the formation of covalent bonds and the growth of MoS₂ nanosheets. Byproducts and unreacted species are removed by the carrier gas. MoO₃, Mo(CO)₆, and MoCl₅ are widely used as Mo sources, while elemental sulfur and H₂S are used as sulfur sources, and N₂ or Ar is used as a carrier gas. Mo(CO)₆ has recently attracted attention and was successfully applied for large-scale production of MoS₂ thin film.

CVD systems can also be classified according to the design and features into four categories: (i) single-zone, (ii) two-zone, (iii) three-zone, and (iv) two-flow CVD systems. Two-flow CVD systems are the most sophisticated and are designed with three-zone CVD systems and a one-zone CVD system. The main features of a two flow-system are (i) independent control of the temperatures of the Mo and sulfur sources and the flow rates of each carrier gas for both sources, and (ii) the Mo source can be completely adjusted or stopped during when ramping and lowering the temperatures. Therefore, the growth rate of thin film can be controlled easily with a two-flow CVD system compared to conventional ones. However, careful attention is needed to obtain high-quality MoS₂ nanosheets.

Generally, the growth rate depends on various parameters, including the nature of the source materials, the temperature ranges of the sources and substrates, the system pressure, the vacuum levels, the type of substrates, and the type of carrier gas. However, unified conditions for the synthesis of MoS₂ nanosheets have not been developed due to the lack of a common practice and a unique CVD system design. The temperature ranges of the Mo and sulfur sources are 500–800°C and 130–300°C, respectively. The reaction temperature range of CVD systems is 650–1000°C, and the range of low rate of the carrier gas is 1–800 sccm. The duration ranges from 30 s to 60 min. Some researchers synthesize MoS₂ nanosheets under reduced pressure or at atmospheric pressure.

Generally, the different shapes of synthesized MoS₂ nanosheets reflect the crystal structure. Hexagonal nanosheets consist of S-zig-zag and Mo-zig-zag termination sides and form triangle shapes that can be grown under different Mo and S source ratios. If either a small excess of the Mo or sulfur source is used, the shape becomes a truncated triangle instead of a perfect triangle. Initially, the MoS₂ nanosheets grow as a hexagonal shape, which changes to a triangle over time. When the Mo and sulfur sources reach a critical point, the MoS₂ nanosheets become star-shaped. For instance, a large excess amount of the sulfur source can facilitate the growth of only Mo-zig-zag termination sides in hexagonal MoS₂ nanosheets with suppression of S-zig-zag termination side growth, which results in the formation of a star shape.

Dendritic MoS₂ nanosheets form in the case of large flow rates of Mo and sulfur sources. These large flow rates create a thinner boundary layer, which results in the formation of dendritic shapes. Nonoptimal growth conditions cause round-shaped MoS₂ nanosheets with poor crystallinity. Surface treatment of a suitable substrate is very important for obtaining highly crystalline MoS₂ nanosheets. A hydrophilic substrate can be used to obtain few-layer MoS₂ nanosheets, whereas monolayer MoS₂ nanosheets can be obtained by using a superhydrophobic substrate. However, the exact mechanism of forming a monolayer on a superhydrophobic substrate is still not clear.

Park et al. [12] fabricated a thin film transistor (TFT) based on CVD-grown single-layer MoS₂, and the photoresponsive current and voltage characteristics of the TFT were measured with varying intensities of incident light. The photocurrent and mobility increased with increasing light intensity due to the contribution of photoinduced charge carriers from the valance band and trap states of the single-layered MoS₂. An exfoliated single-layer MoS₂-based TFT by Lin et al. exhibited higher mobility than the one based on CVD-grown MoS₂ [13]. However, the main advantage of CVD is the relatively large area of samples with homogenous qualities [14]. The CVD method has been used to grow MoS₂ directly on different dielectric substrates [15]. MoS₂ with photocatalytic properties have become an interesting candidate for the photodegradation of organic dye, hydrogen evolution, and CO₂ reduction, with the advantages of chemical and photostability.

3. Photocatalytic properties of MoS₂

The photocatalysis process involves the conversion of solar energy into chemical energy. The main goal for researchers is maximum utilization of the solar energy to enable the practical use of photocatalysts. However, photocatalytic ability has still been limited due to the fast recombination effect of electron-hole pairs and an insufficient absorption coefficient. Therefore, enhancing the efficiency of photocatalysts under visible light still remains a challenge for practical applications.

Many semiconductor oxides, sulfides, and nitrides have been used as photocatalysts for various applications. However, most sulfides and nitrides have lower band gaps, which limit usage due to stability issues of these materials in an aqueous medium. This issue is an important one to solve to promote the use of these materials. The band gap of the oxides is higher than that of sulfides, and the absorption edge is only in the UV region. This limits the usage in the solar spectrum, although the material has good stability in an aqueous medium. The main characteristics of an ideal photocatalyst are maximum absorption in the visible solar spectrum, favorable band edges for promoting reactions, environmental friendliness, low cost, good stability, and reusability [16].

The band gap value of MoS₂ (1.9 eV) photoelectrodes and their considerably lower valence band edge than the water oxidation potential are favorable for water splitting through photo-electro-chemical techniques. The theoretical photocurrent density of chemically exfoliated MoS₂ is ~17.6 mA/cm² at 0.0 V vs. RHE under solar irradiation [17], and the solar energy conversion efficiency is ~18.7% for an ideal PEC cell. However, the photocatalytic activity of MoS₂ is limited by the factors mentioned thus far, which lead to low efficiencies and require larger potential to promote the photo-assisted water oxidation process [18]. Many researchers have tried to overcome these drawbacks of MoS₂ by decreasing the recombination rate through forming a composite or heterogeneous structure, as well as enhancing the conductivity by doping with metal and promoting the charge carrier transferability [19–22].

Apart from water-splitting applications, the photocatalytic performance of MoS₂ can be used for the degradation of organic compounds in waste water treatment applications. Vattikuti et al. [23] reported the mechanism of the degradation of rhodamine B (RhB) dye. The photosensitization of the RhB dye first takes place when charge transfer occurs from the valence band of the dye to the conduction band of the photocatalyst. This is followed by the initial photocatalytic reaction, where MoS₂ generates electron-hole pairs under photoirradiation. The electrons transfer from the valence band of MoS₂ to the conduction band and settle down holes within the valence band. The photoinduced electrons of MoS₂ produce intermediate superoxide radicals (${\mathrm{O}}_2^{·-}$) by responding with chemisorbed oxygen on the photocatalyst surface and oxygen in the aqueous solution. The ${\mathrm{O}}_2^{·-}$ radicals react with dissociated water (H⁺) to form · HO₂ and H₂O₂. In addition, hydroxyl groups (OH⁻) are formed on the catalyst surface by the reaction with photoinduced holes (h⁺) by absorbed water (OH₂). Thus, these generated radicals along with intermediate species react with RhB dye and degrade it into nontoxic organic compounds as follows:

$\mathrm{R}\mathrm{h}\mathrm{B}+\mathrm{h}\mathrm{\upsilon }\to \mathrm{R}\mathrm{h}{\mathrm{B}}^{·}$E1

$\mathrm{R}\mathrm{h}{\mathrm{B}}^{·}+\mathrm{M}\mathrm{o}{\mathrm{S}}_2\to \mathrm{R}\mathrm{h}{\mathrm{B}}^{·}+\mathrm{M}\mathrm{o}{\mathrm{S}}_2\left({{\mathrm{e}}_{\mathrm{C}\mathrm{B}}}^{-}\right)$E2

$\mathrm{M}\mathrm{o}{\mathrm{S}}_2+\mathrm{h}\mathrm{\upsilon }\to \mathrm{M}\mathrm{o}{\mathrm{S}}_2\left({{\mathrm{e}}_{\mathrm{C}\mathrm{B}}}^{-}+{{\mathrm{h}}_{\mathrm{V}\mathrm{B}}}^{+}\right)$E3

${\mathrm{H}}_2\mathrm{O}\to {\mathrm{H}}^{+}+\mathrm{O}{\mathrm{H}}^{-}$E4

${\displaystyle {{\mathrm{e}}_{\mathrm{C}\mathrm{B}}}^{-}}+{\mathrm{O}}_2\to {\mathrm{O}}_2^{•-}$E5

${\mathrm{O}}_2^{•-}+{{\mathrm{H}}^{+}}_{\mathrm{a}\mathrm{q}}\to {\mathrm{H}{\mathrm{O}}_2}^{·}$E6

$\mathrm{H}{{\mathrm{O}}_2}^{·}+\mathrm{H}{{\mathrm{O}}_2}^{·}\leftrightarrow {\mathrm{H}}_2{\mathrm{O}}_2+{\mathrm{O}}_2$E7

${{\mathrm{h}}_{\mathrm{V}\mathrm{B}}}^{+}+\mathrm{O}{{\mathrm{H}}^{-}}_{\mathrm{a}\mathrm{q}}\to \mathrm{O}{\mathrm{H}}^{·-}$E8

$\mathrm{R}\mathrm{h}{\mathrm{B}}^{·}/\mathrm{R}\mathrm{h}{\mathrm{B}}^{·+}+\left({\mathrm{O}}_2^{•-},\mathrm{H}{\mathrm{O}}_2^{·},{\mathrm{H}}_2{\mathrm{O}}_2,\mathrm{O}{\mathrm{H}}^{·-}\right)\to \mathrm{Degradation}\mathrm{products}\dots \text{.}$E9

MoS₂ is also a good photocatalyst for photocatalytic oxidative desulfurization [24]. Sulfur compounds in fuel convert into SOx, which causes air pollution and acid rain. Generally, the hydrodesulfurization (HDS) process is widely employed commercially to remove sulfur species at high temperature (350°C) and pressure (7 MPa). Recently, photooxidative desulfurization has been popular because it is economical and has high efficiency [25, 26], which can provide cleaner and more efficient removal of sulfur species from petroleum fuel oils [27]. Lia et al. reported that CeO₂/MoS₂ and attapulgite showed excellent electron transfer within the composite and favor the desulfurization process under solar irradiation [24]. MoS₂-assisted nanocomposite systems have led to a new era in research and show promise as a high-activity and low-cost photocatalyst for applications such as deep desulfurization. Thurston et al. [28] and Wilcoxon et al. [29] reported on MoS₂ nanoparticles with diameters at 3–4.5 nm as a catalyst for the degradation of phenol, 4-chlorophenol, and pentachlorophenol under visible light irradiation.

4. Literature reviews on MoS₂ photocatalytic mechanism

We emphasize three different forms of MoS₂ that have been studied. Ongoing research on MoS₂ nanoparticles as a photocatalyst is addressed first, followed by studies associated with MoS₂ composites. This section concludes with a discussion on thin-coated MoS₂.

4.1. Unary MoS₂ photocatalyst

A high aspect ratio plays a key role in the photocatalytic activity of materials, and researchers have concentrated on reducing the size of the photocatalyst and improving the photocatalytic activities of these materials by making nanoscale MoS₂. Many approaches have been used to synthesize nanocrystalline MoS₂ with different morphologies, including ultrasonic cracking [30], hydrothermal methods [31–33], chemical synthesis [34], combustion methods [35–38], wet chemical methods, and coprecipitation methods [23, 38–40].

Vattikuti et al. synthesized MoS₂ multiwall nanotubes (MWNTs) by a wet chemical method assisted by H₂O₂ solvent as a growth promotor [41]. The photocatalytic performance of MoS₂ MWNTs was applied to the degradation of RhB. The MoS₂ MWNTs exhibited excellent photocatalytic performance compared to pure MoS₂. The higher photocatalytic activity of MoS₂ MWNTs was ascribed to the large number of active sites with a high specific surface area. The performance of the optimal amount of 0.5 wt% MoS₂ MWNTs was attributed to the higher transfer of electrons and holes during the photoreaction, which effectively suppressed the recombination of the electron-hole pairs and enhanced the degradation efficiency.

Zhou et al. [42] hydrothermally synthesized porous MoS₂ without any sacrificial template using sodium molybdate and thioacetamide as Mo and S sources. Porous MoS₂ showed 89.2% degradation efficiency of MB under 150 min of visible light irradiation. MB photodegradation in the presence of porous MoS₂ was obtained with a pseudo-first-order kinetic reaction rate of 0.01484 min⁻¹. Polycrystalline porous MoS₂ shows attractive photocatalytic activities that are ascribed to the active edge sites. Sheng et al. [43] synthesized flower-like MoS₂ spheres via the hydrothermal method and studied the effects of excess sulfur source on the flower-like MoS₂ structure. To obtain the flower-like MoS₂ spheres, MoO₃, and potassium thiocyanate (KSCN) were used as Mo and S sources with different S/Mo ratios. The optimal S/Mo ratio of 2.75 resulted in the highest degradation rate of MB with a degradation rate of 0.03833 min⁻¹ under 90 min of visible light irradiation. The increased photocatalytic performance was ascribed to the increased exposed area of the [43] facets with the optimal S/Mo ratio in the hydrothermal synthesis environment. The sheet thickness of the MoS₂ spheres increased with the S/Mo ratio and enhanced the photocatalytic activity.

Liu et al. [44] produced MoS₂ nanosheets by a hydrothermal method with H₂SiO₃ (silicic acid) hydrogel containing ammonium molybdate hydrate and thiourea precursors. MoS₂ nanosheets were obtained by removing the H₂SiO₃. These MoS₂ nanosheets have a high specific surface area (S_BET) of 37.8 m²g⁻¹ and present notable absorption of MO under visible light rather than ultraviolet light in 70 min of irradiation. Different shapes of MoS₂ nanosheets were obtained by varying the concentration of silicic acid with MoS₂ molar ratios of 2.5 and 0.8, such as leaf-shaped and flower-shaped MoS₂ nanosheets. These provide steric hindrance for MoS₂ nanosheet growth. The amount of hydroxyl radicals was highest at pH 2 and decreased when increasing to pH 9. The OH group plays a major role in MO photodegradation in the catalytic system. The reaction time, initial concentration, catalyst dosage, and local structures are also key factors that affect the photocatalytic performance of the materials.

4.2. Binary MoS₂ photocatalysts

This section reviews the effect of heterogeneous structures or composite forms of MoS₂ onthe photocatalytic properties. Combining MoS₂ with metals or nonmetals and semiconductor materials is a common practice for enhancing photocatalytic performance by facilitating and promoting efficient charge transfer at the interfaces. Similar attempts have been made for other classes of materials to improve photocatalytic activity, including Fe₂O₃, TiO₂, and ZnO.

Thurston et al. [28] demonstrated that MoS₂ nanoparticles with diameter of 8–10 nm could not photodegrade phenol under visible light due to poor light absorption. Hence, they sensitized TiO₂ nanoparticles with MoS₂ nanoparticles, which enabled photodegradation under visible light irradiation. This composite structure showed a blue shift in absorbance due to quantum confinement of the charge carriers [28, 45]. We recently reported improved photocatalytic performance of MoS₂ nanosheets decorated with mesoporous SnO₂ nanospheres by a facile two-step method [46]. We also observed the photocatalytic effect in the degradation of RhB with less than 50 min of UV light irradiation. The supported mesoporous SnO₂ nanoparticles significantly suppressed the recombination of electron-hole pairs compared to pure MoS₂ photocatalyst material. The improved photocatalytic performance of the MoS₂/SnO₂ composite was explained by two mechanisms: (i) the absorption ability of the MoS₂ nanosheets with active edges and (ii) enhanced electron transfer from SnO₂ to the MoS₂ nanosheets. This heterostructured composite facilitated effective electron transfer from the CB of SnO₂ to the MoS₂ nanosheets and suppressed the recombination effect. Therefore, the SnO₂-decorated MoS₂ nanocomposite showed better photocatalytic performance than pure MoS₂. Photocorrosion is the main reason for the lower photocatalytic activity of the pure MoS₂.

Pourabbas et al. [47] synthesized a hybrid MoS₂/TiO₂ composite using a modified hydrothermal method. The changes from the normal hydrothermal method included using sodium lauryl sulfate as a surface-active agent with 1-octanol as a cosurfactant and varying reaction temperature. The hybrid composite was used as a photocatalyst for the photo-oxidative removal of phenol. The composite showed enhanced photocatalytic performance in the phenol degradation under both UV (70 min) and visible light (24 min) compared to pure TiO₂ and MoS₂. The complete mineralization of phenol during the photo-oxidation reaction in 145 min of UV irradiation was indicated by HPLC chromatograms. Zhou et al. [48] and Bai et al. [49] did similar work and evaluated the photocatalytic performance of the MoS₂/TiO₂ composite for photodegradation of MB under visible light irradiation.

MoS₂-coated TiO₂ nanobelt composites showed excellent photocatalyst properties for RhB degradation under 33 min of visible light irradiation. The matched energies of the TiO₂@MoS₂ composite are favorable for the charge transfer and suppress the recombination of electron-hole pairs. The photocatalytic hydrogen production was also enhanced. Liu et al. [50] synthesized a composite of TiO₂ nanobelts decorated with MoS₂ nanoparticles using a two-step hydrothermal method. The photocatalytic degradation of the TiO₂/MoS₂ composite was evaluated with RhB under 90 min of visible light irradiation. The sample with 40 wt% MoS₂ nanoparticles decorated on TiO₂ nanobelts showed the best photocatalytic performance, which was attributed to the prevented recombination of photoinduced electron-hole pairs. This sample showed a high photocatalytic reaction rate constant that is about 4.78 times that of pure TiO₂.

Cao et al. [51] synthesized MoS₂/TiO₂ hybrid composites by a two-step hydrothermal route. The MoS₂/TiO₂ hybrid composite showed excellent photocatalytic performance in the degradation of RhB in 100 min of visible-light irradiation in comparison to pure forms. The improvement in photocatalytic activity of the composite was mainly ascribed to the properly matching CB and VB energy levels and the enhanced separation efficiency of photoinduced electron-hole pairs at interfacial contacts of the composite. Wang et al. [52, 53] reported the in situ deposition of Ag₃PO₄ on graphene-like MoS₂ nanosheets via a wet chemical route. The goal was to improve the photocatalytic performance for the degradation of RhB in 20 min of visible light irradiation (>400 nm). The improved photocatalytic performance of the heterostructure of Ag₃PO₄/MoS₂ composite is ascribed to the efficient separation of photoinduced electron-hole pairs within the photocatalyst.

Ding et al. [54] synthesized a MoS₂-GO hydrogel composite using a hydrothermal method for MB degradation under 60 min of solar light irradiation. This composite showed enhanced photocatalytic performance in the degradation of MB with a maximum degradation rate of 99% for 60 min under solar light irradiation. The improvement was attributed to the increased light absorption and suppressed recombination effect of semiconductor photocatalysis.

Zhang et al. [55] synthesized MoS₂/rGO photocatalyst for the fluorescence detection of glutathione in a ·OH radical elimination system based on the reducing ability of glutathione under visible light irradiation. The MoS₂/rGO composite efficiently generated ·OH radicals and reduced ·OH radicals by the absorption of glutathione under visible light, which is reflected by a reduction of the fluorescence intensity due to the elimination of ·OH radicals. This kind of photocatalyst can be effectively implemented for the identification of glutathione in commercial drugs and human serum.

Wang et al. [56] synthesized MoS₂/Bi₂O₂CO₃ composites for RhB photodegradation under 150 min of visible light irradiation by a simple hydrothermal method. The effect of photocatalyst concentration on the photocatalytic efficiency was observed. This composite has more active sites of MoS₂ on Bi₂O₂CO₃, which promoted the photocatalytic performance by absorbing and decomposing more RhB pollutant than pure Bi₂O₂CO₃. The remarkable enhancement in the photocatalytic activity could be ascribed to the synergistic effect between the MoS₂ and Bi₂O₂CO₃ in the heterostructured composite. Li et al. [57] reported MoS₂/BiVo₄ hetero-nanoflower composites as an excellent photocatalyst for MB degradation with less than 120 min of sunlight irradiation.

Li et al. [58] successfully synthesized a 2D heterojunction photocatalyst of g-C₃N₄ coupled with MoS₂ nanosheets using a simple impregnation and calcination method. The g-C₃N₄/MoS₂ composite promoted the charge transfer and improved the separation efficiency of photo-induced electron–hole pairs in RhB and MO degradation under 180 min of visible light irradiation. Jo et al. [59] synthesized MoS₂ nanosheets loaded with ZnO-gC₃N₄ ternary photocatalyst for MB photodegradation under 60 min of UV-visible light irradiation. The ternary nanocomposite significantly improved the lifetime of charge carriers and facilitated effective migration and charge separation at the interface.

Zhang et al. [60] synthesized a ternary composite system of TiO₂/MoS₂@zeolite using a facile ultrasonic-hydrothermal synthesis method with TiCl₄ as a Ti source and zeolite as a carrier. The photocatalytic performance was investigated for MO degradation for 60 min under xenon long-arc lamps as a visible light source. The photoinduced electrons and holes are collected in the CB of MoS₂ and the VB of TiO₂. The more negative bottom CB energy of MoS₂ and more positive top CB energy of TiO₂ allow the photoinduced electrons in the CB of MoS₂ to reduce the absorbed O₂ into ·O₂⁻. ·OH can be produced easily in the VB of TiO₂, and the ·O₂⁻ and ·OH active species lead to MO degradation. Figure 2 shows the possible photocatalytic mechanism of both unary and binary photocatalysts.

Figure 2.

Schematic diagram of photocatalytic mechanism of (a) unary [8] and (b) binary photocatalyst [61].

Hu et al. [62] synthesized MoS₂/Kaolin composites by calcining a MoS₃/kaolin precursor in H₂ under strong acidic conditions. The composite had a specific surface area of 16 m²g⁻¹ and showed a positive photocatalytic effect on MO degradation under 105 min of visible light irradiation. This performance was attributed to the good absorption capacity in the visible light region. The photocatalyst has remarkable stability and can be regenerated and reused via filtration. The deactivating photocatalyst could be reactivated even after photocatalytic reaction at 450°C for 30 min under H₂. The photocatalytic performance of exfoliated MoS₂ was also investigated, and the relationship between the morphology of nano-MoS₂ and the photocatalytic properties was discussed [63]. The photocatalytic performance of this and other heterostructured composites are influenced by the quantity of photocatalyst, initial concentration of pollutant or dye, pH, irradiation time, type of light source, and degradation temperature.

5. Summary

Layered MoS₂ materials have attracted continuously increasing interest and demand, and preparation techniques have been successfully developed. We have a provided a detailed overview of the photocatalytic performance of MoS₂ nanomaterials, three different types of MoS₂ photocatalyst systems were distinguished according to their structural components(single component, heterostructured, and doped MoS₂). There is great interest in preparing various MoS₂ photocatalyst systems by novel strategies, as well as hierarchical MoS₂ structures with special functionalities. Therefore, there is ongoing effort to develop new MoS₂ materials with novel structures and their applications.

The importance of MoS₂ photocatalysts has been highlighted for the degradation of pollutant from contaminated waste water through solar light irradiation. There have been a number of advances in this field, including the development of materials with lower band gap, low cost, and increased stability and reusability. These developments make MoS₂ photocatalyst a promising candidate for further practical advances in the future. However, degradation rates are still generally low, the materials are somewhat unstable over repeated usage, and there is great variability in the reported reduction rate and efficiencies of these systems. It is of great importance that reduction rates be reproduced from one lab to another, and repeatability and reusability are currently some of the significant deficiencies in the field. In future, scientists should focus on material design and the realization of practical applications.

Acknowledgments

This work was conducted under the framework of the National Research Foundation of Korea (NRF) and funded by the Ministry of Science, ICT, and Future Planning (2014R1A2A2A01007081).