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

Bulk-to-single layer molybdenum disulfide (MoS 2 ) is widely used as a robust candidate for photodegradation of organic pollutants, hydrogen production, and CO 2 reduction. This material features active edge sites and narrow band gap features, which are useful for generat- ing 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 MoS 2 by tailoring its bulk-to-single layer structure and combining it with other semicon- ductor materials to improve the photocatalytic performance. Different strategies have been successfully applied to enhance the photocatalytic activity of MoS 2 , 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 hetero structured morphologies and study their impact on the photocatalytic efficiency. Finally, the relevance of crystal facets and defects in photocatalysis is outlined. Future applications of MoS 2 with tailoring and tuning physicochemical properties are highlighted.


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 MQ 2 (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 (MoS 2 ) and tungsten disulphide (WS 2 ) 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][3][4][5][6][7][8]. They also have good catalytic properties and resistance to sulfur poisoning [9]. MoS 2 and WS 2 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 MoS 2 and WS 2 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.

Synthesis of MoS 2 nanomaterials
In the reaction stage, the size and shape control during the synthesis of MoS 2 nanomaterials are crucial for obtaining well-defined materials with specific properties. The morphology and size of MoS 2 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 MoS 2 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 MoS 2 nanosheets synthesized by solvothermal approach [6]. These MoS 2 nanosheets were obtained as a few lamellar layers using thiourea as a sulfur source.
The combination of size-and shape-dependent physical properties along with their simple fabrication and processing techniques make MoS 2 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 MoS 2 nanomaterials. However, controlled synthesis with a narrow size distribution and uniform shape remains an important issue in photocatalytic applications.

Synthesis of MoS 2 thin films
Chemical vapor deposition (CVD) is one of the most popular methods for fabricating thin films of few-layered MoS 2 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 MoS 2 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 MoS 2 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 MoS 2 -based layered composite structures have recently been a focus in fundamental semiconductor technology.
The simplest way to develop a monolayer MoS 2 analog to graphene is the Scotch-tape exfoliation method from bulk MoS 2 along the direction of van der Waals interaction. Repeated exfoliation of bulk MoS 2 decreases the number of MoS 2 layers, eventually producing few-to singlelayer MoS 2 . However, this method is difficult for large-scale production due to the poor size controllability. To synthesize MoS 2 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 MoS 2 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 MoS 2 nanosheets. Byproducts and unreacted species are removed by the carrier gas. MoO 3 , Mo(CO) 6 , and MoCl 5 are widely used as Mo sources, while elemental sulfur and H 2 S are used as sulfur sources, and N 2 or Ar is used as a carrier gas. Mo(CO) 6 has recently attracted attention and was successfully applied for large-scale production of MoS 2 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 MoS 2 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 MoS 2 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 MoS 2 nanosheets under reduced pressure or at atmospheric pressure.
Generally, the different shapes of synthesized MoS 2 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 MoS 2 nanosheets grow as a hexagonal shape, which changes to a triangle over time. When the Mo and sulfur sources reach a critical point, the MoS 2 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 MoS 2 nanosheets with suppression of S-zig-zag termination side growth, which results in the formation of a star shape.
Dendritic MoS 2 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 MoS 2 nanosheets with poor crystallinity. Surface treatment of a suitable substrate is very important for obtaining highly crystalline MoS 2 nanosheets. A hydrophilic substrate can be used to obtain few-layer MoS 2 nanosheets, whereas monolayer MoS 2 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.  [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 MoS 2 directly on different dielectric substrates [15]. MoS 2 with photocatalytic properties have become an interesting candidate for the photodegradation of organic dye, hydrogen evolution, and CO 2 reduction, with the advantages of chemical and photostability.

Photocatalytic properties of MoS 2
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 MoS 2 (1.9 eV) photoelectrodes and their considerably lower valence band edge than the water oxidation potential are favorable for water splitting through photoelectro-chemical techniques. The theoretical photocurrent density of chemically exfoliated MoS 2 is ~17.6 mA/cm 2 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 MoS 2 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 MoS 2 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][20][21][22].
Apart from water-splitting applications, the photocatalytic performance of MoS 2 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, The O 2 ·− radicals react with dissociated water (H + ) to form · HO 2 and H 2 O 2 . In addition, hydroxyl groups (OH − ) are formed on the catalyst surface by the reaction with photoinduced holes (h + ) by absorbed water (OH 2 ). Thus, these generated radicals along with intermediate species react with RhB dye and degrade it into nontoxic organic compounds as follows: Rh MoS 2 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 CeO 2 /MoS 2 and attapulgite showed excellent electron transfer within the composite and favor the desulfurization process under solar irradiation [24]. MoS 2 -assisted nanocomposite systems have led to a new era in research and show promise as a highactivity and low-cost photocatalyst for applications such as deep desulfurization. Thurston et al. [28] and Wilcoxon et al. [29] reported on MoS 2 nanoparticles with diameters at 3-4.5 nm as a catalyst for the degradation of phenol, 4-chlorophenol, and pentachlorophenol under visible light irradiation.

Literature reviews on MoS 2 photocatalytic mechanism
We emphasize three different forms of MoS 2 that have been studied. Ongoing research on MoS 2 nanoparticles as a photocatalyst is addressed first, followed by studies associated with MoS 2 composites. This section concludes with a discussion on thin-coated MoS 2 .
Vattikuti et al. synthesized MoS 2 multiwall nanotubes (MWNTs) by a wet chemical method assisted by H 2 O 2 solvent as a growth promotor [41]. The photocatalytic performance of MoS 2 MWNTs was applied to the degradation of RhB. The MoS 2 MWNTs exhibited excellent photocatalytic performance compared to pure MoS 2 . The higher photocatalytic activity of MoS 2 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% MoS 2 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. 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 MoS 2 spheres increased with the S/Mo ratio and enhanced the photocatalytic activity.

Binary MoS 2 photocatalysts
This section reviews the effect of heterogeneous structures or composite forms of MoS 2 on the photocatalytic properties. Combining MoS 2 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 Fe 2 O 3 , TiO 2 , and ZnO.
Thurston et al. [28] demonstrated that MoS 2 nanoparticles with diameter of 8-10 nm could not photodegrade phenol under visible light due to poor light absorption. Hence, they sensitized TiO 2 nanoparticles with MoS 2 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 MoS 2 nanosheets decorated with mesoporous SnO 2 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 SnO 2 nanoparticles significantly suppressed the recombination of electron-hole pairs compared to pure MoS 2 photocatalyst material. The improved photocatalytic performance of the MoS 2 /SnO 2 composite was explained by two mechanisms: (i) the absorption ability of the MoS 2 nanosheets with active edges and (ii) enhanced electron transfer from SnO 2 to the MoS 2 nanosheets. This heterostructured composite facilitated effective electron transfer from the CB of SnO 2 to the MoS 2 nanosheets and suppressed the recombination effect. Therefore, the SnO 2 -decorated MoS 2 nanocomposite showed better photocatalytic performance than pure MoS 2 . Photocorrosion is the main reason for the lower photocatalytic activity of the pure MoS 2 .
Pourabbas et al. [47] synthesized a hybrid MoS 2 /TiO 2 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 Zhang et al. [55] synthesized MoS 2 /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 MoS 2 /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] Figure 2 shows the possible photocatalytic mechanism of both unary and binary photocatalysts.
Hu et al. [62] synthesized MoS 2 /Kaolin composites by calcining a MoS 3 /kaolin precursor in H 2 under strong acidic conditions. The composite had a specific surface area of 16 m 2 g −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 H 2 . The photocatalytic performance of exfoliated MoS 2 was also investigated, and the relationship between the morphology of nano-MoS 2 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.

Summary
Layered MoS 2 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 MoS 2 nanomaterials, three different types of MoS 2 photocatalyst systems were distinguished according to their structural components(single component, heterostructured, and doped MoS 2 ). There is great interest in preparing various MoS 2 photocatalyst systems by novel strategies, as well as hierarchical MoS 2 structures with special functionalities. Therefore, there is ongoing effort to develop new MoS 2 materials with novel structures and their applications.
The importance of MoS 2 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 MoS 2 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 Figure 2. Schematic diagram of photocatalytic mechanism of (a) unary [8] and (b) binary photocatalyst [61]. 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.