Molybdenum disulfide (MoS2) based photoredox catalysis in chemical transformations

Photoredox catalysis has been explored for chemical reactions by irradiation of photoactive catalysts with visible light, under mild and environmentally benign conditions. Furthermore, this methodology permits the activation of abundant chemicals into valuable products through novel mechanisms that are otherwise inaccessible. In this context, MoS2 has drawn attention due to its excellent solar spectral response and its notable electrical, optical, mechanical and magnetic properties. MoS2 has a number of characteristic properties like tunable band gap, enhanced absorption of visible light, a layered structure, efficient photon electron conversion, good photostability, non-toxic nature and quantum confinement effects that make it an ideal photocatalyst and co-catalyst for chemical transformations. Recently, MoS2 has gained synthetic utility in chemical transformations. In this review, we will discuss MoS2 properties, structure, synthesis techniques, and photochemistry along with modifications of MoS2 to enhance its photocatalytic activity with a focus on its applications and future challenges.


Introduction
Molybdenum disulde (MoS 2 ) is an inorganic compound of the transition metal dichalcogenide (TMD) series. It is one of the most used photocatalysts that has a wide range of applications in different elds. It is applicable to degrade pharmaceutically active compounds (PhACs), synthetic dyes (rhodamine B, methylene blue, methyl orange and novacron red Hunts-man), pesticides and herbicides. It also employed for photocatalytic evolution of oxygen, nitrogen xation, ammonia synthesis and water electrolysis. [1][2][3][4][5][6][7] In recent years, MoS 2 has received much attention due to its unique electrical and optical properties. 8a MoS 2 is a twodimensional (2D) transition metal dichalcogenide, a narrow band semiconductor with a band gap of 1.9 eV which can be altered by adjusting the number of layers. Among all 2D TMDs, MoS 2 is one of the few with a natural layered structure, indicating that MoS 2 can be stripped to obtain high-quality 2D MoS 2 without complicated chemical synthesis. 8b Therefore, the cost of preparing 2D MoS 2 is much lower than other 2D TMDs due to the simple synthesis conditions. More importantly, 2D MoS 2 is a 2D semiconductor with a direct band-gap, which has the best electric performance among 2D TMDs. 8c These make 2D MoS 2 get more attention among 2D TMDs.
Several characteristics of MoS 2 such as high absorbance in the visible region, 9 high carrier mobility, 10 stability, 11 preferable catalytic activity, 12 good photostability, 13 outstanding electrochemical properties, 14 strong surface adsorption capacity and higher specic surface area 15 favours its application as a photocatalyst as well as a co-catalyst and also, it is capable of addressing the limitations faced by typical photocatalysts. Additionally, MoS 2 is found to be abundant and cheap. 7 Currently, semiconductor photocatalysts with controllable morphologies are gaining signicant attraction in applications associated with environmental remediation process. 16 Phase engineering, defect engineering, doping of external materials, interlayer engineering, surface modication, and heterojunction 17a,b construction make MoS 2 more exible in terms of tuning its properties during synthesis.
Molybdenum disulde (MoS 2 ) 17c has emerged as one of the most important two-dimensional functional materials, popular new co-catalysts due to their excellent photocatalytic activity, substantial adsorptivity, great value and its non-toxicity. The visible region of electromagnetic spectrum exhibits the absorption spectrum of 2D MoS 2 , providing a plethora of opportunities for widespread applications. 17d Applications for MoS 2 -based materials are said to include energy storage, 17e hydrogen production, 17f pollutant degradation, 17g disinfection, 17h etc. Numerous studies have recently provided a summary of photocatalytic properties of MoS 2 for numerous application areas. 17i-n In continuation of our work on photocatalysed organic synthesis 18,19 this review aims to provide a comprehensive report on the current research, especially the role of MoS 2 in the chemical transformation.

Synthesis of MoS 2
For the most part, two different synthetic procedures are used to create MoS 2 : (1) a top-down strategy that involves exfoliating bulk MoS 2 to create materials with one or two monolayers, and (2) a bottom-up strategy that uses techniques like hydrothermal or chemical vapour deposition. Several methods exist for the preparation of MoS 2 using different molybdenum precursors and sulfur sources, like elemental sulfur powder, 20 thiourea, 21 thioacetamide, 22 and L-cysteine. 23 By regulating variables such the reaction solvent, temperature, pH, reaction time, and the application of surfactants or ligands, which are essential for managing the synthesis to create the desired chalcogenide, a variety of fascinating morphologies can be created. Table 1 exhibits the different methods that have been used to synthesize MoS 2 materials alongside the applications of the synthesized materials. The most used methods are solid-state, 24 hydrothermal, 25,26 solvothermal, 27 and hybrid methods. 28,29 The benets and drawbacks of synthesis techniques 44 such as thermal annealing, plasma-assisted synthesis, magnetron sputtering-based synthesis, atomic layer deposition (ALD), and wet chemical processes are summarised in Fig. 1.

Properties of MoS 2
MoS 2 is a typical and stable 2D transition metal dichalcogenide which reveals that it is a semiconductor of MX 2 type where the transition metals denote M and the chalcogen denotes X 45 with three hexagonal planes (S-Mo-S), which is bonded to each other via ionic-covalent interactions. 46 The sandwiched MoS 2 is brought together to form sheets with an interlayer distance of 0.65 nm by the weak van der Waals force, which assumes responsibility for creating a 2D layered structure. Fig. 2a depicts the MoS 2 structure as described. In addition, Fig. 2b reects the fact that the presence of impurities in MoS 2 alters its interlayer distance. 47 As an n-type semiconductor, MoS 2 has an indirect band gap of 1.2 eV in the bulk. The band gap of this semiconductor is inversely proportional to its number of layers. As a result, a direct band gap of 1.9 eV is obtained by almost reducing the number of layers to just one (Fig. 3c), making it a suitable photocatalyst in the visible region. 11 The narrow band gap facilitates its high absorption of light in the visible region.    55 Due to the 2D nature of MoS 2 , it's intrinsic properties are greatly inuenced by the surrounding environment on the nanoscale. The suspended structures in these systems are hybrid systems with transport performance, 56a-g tunable optoelectronic behaviour, 56h-l and nanomechanical properties, 56m-p all of which are fascinating on their own. The size, surface area, and surface energy of the MoS 2 particles are determined by its morphology, which has a signicant impact on how it executes in photocatalytic applications. Due to their large surface area and narrow band gap, MoS 2 nanoowers are more effective for absorbtion of visible light, which improves their photocatalytic efficacy. The shape of MoS 2 nanoowers determines their surface area, which in turn determines how well they operate as photocatalysts. MoS 2 is anisotropic, piezocatalytic, 56 hydrophobic and low-toxic, which has high specic surface area with high surface energy, 57 high surface to volume ratio, high e − mobility, high optical absorption, high catalytic activity, good tribological properties, 58 good thermal stability and free sulfur groups. 59 Due to its low price and a wealth of earth reserves, it is commercially viable. As a result, MoS 2 's characteristics are largely in favour of its application as a photocatalyst.
Unfortunately, the catalytic activity of MoS 2 is greatly inhibited due to its low dispersion ability which can be attributed to its hydrophobicity and low electrical conductivity. 60 The formation of composites using MoS 2 may be a viable and efficient way to get around this problem and maximise the benets of MoS 2 as a photocatalyst in aqueous bodies because the   properties of other compounds in the composite may enhance the electrical conductivity and dispersion of MoS 2 and thereby facilitate the exhibition of its full potential and surplus advantages.

Photochemistry of MoS 2
MoS 2 has attracted signicant attention, and a wide range of desirable photocatalytic properties have been reported, such as narrow band gap energy, excellent optical absorptivity, and high mobility of charge carriers, in addition to low toxicity and cost. However, the performance of MoS 2 is limited by photogenerated e − /h + recombination, the edge activity effect, and photocorrosion. Past efforts to enhance the photocatalytic properties of MoS 2 have involved controlling the morphology, modulation of energy bands through doping, band alignment through heterojunction formation, modication with carbon nanostructures, and combining with metal particles exhibiting surface plasmon resonance.
The photocatalytic redox processes begin with the production of photoexcited charge carrier pairs (e − /h + ). As illustrated in Fig. 3, upon irradiation the MoS 2 simultaneously generates e − and h + , where the photogenerated e − is promoted to the CB, leaving behind the hole (h + ) in the VB. Subsequently, these photogenerated carriers migrate to the surface of the MoS 2 to participate in reduction (e − ) and oxidation (h + ) processes. Photoexcited e − may react with adsorbed O 2 to produce cO 2 − radicals and, simultaneously, the h + remaining in the VB can abstract an e − from hydroxyl ions or adsorbed H 2 O molecules to produce cOH radicals, which are powerful oxidizing agents that can react with harmful organic, inorganic, and biological compounds.

Modifications of MoS 2 to improve its photocatalytic activity
The high e − /h + pair recombination efficiency of MoS 2 prevents it from being widely used in photocatalysis, despite the fact that it is a visible-light sensitive photocatalyst. This is a frequent problem with narrow band gap photocatalysts. Hence, to improve charge carrier separation, MoS 2 has been doped and coupled with many varied materials such as metals, 61 metal oxides, 62 and carbon-based materials. 63 64 They reported that the deposition of Ag onto MoS 2 assisted e − and h + separation, prevented charge recombination, and hence, enhanced the overall photocatalytic performance. By creating a band offset and/or an electric eld across the space-charge area at the junction between the two materials, a heterojunction enables the separation of e − /h + pairs instantly. Fig. 4d shows the formation of two types of heterojunctions. A type I heterojunction results When the VB of MoS 2 is greater than the VB of the second semiconductor and the conduction band (CB) of MoS 2 is lower than the CB of the second semiconductor,  allowing both electrons and holes to pass from the second semiconductor to the MoS 2 . In contrast, a type II heterojunction is created when the VB of MoS 2 is above the VB of the second semiconductor, acting as a hole sink, while the CB of the second semiconductor is below the VB of MoS 2 , acting as an electron sink. 65 For example, Hu et al. reported that an optimized Co-doped MoS 2 /g-C 3 N 4 heterojuction exhibited the highest activity for the photocatalytic reduction of water, with a H 2 evolution rate of 0.31 mmol g −1 h −1 . 66 According to these authors, Co-doped MoS 2 has an edge-enriched 1T phase with more active sites than pristine MoS 2 , as well as a heterojunction with g-C 3 N 4 to expedite charge transfer and separation and these factors contribute to the enhanced photocatalytic activity. Additionally, the creation of ternary and quaternary chalcogenides by including one or more additional elements into binary chalcogenides is a potential method for increasing the effectiveness of charge separation and interfacial charge transfer through the addition of additional surfaces. For example, Lim et al. fabricated a ZnMoS 4 / ZnO/CuS p-n heterojunction photo-catalyst that results in a hydrogen evolution rate 97% higher than an unoptimized ZnMoS 4 /CuS p-n structure. 67 A hydrothermal and calcination technique is used to successfully create a Z-scheme MoS 2 /CuO photocatalyst that degrades 2-mercaptobenzothiazole (MBT) with excellent activity (96 percent) when exposed to visible light 68 (Fig. 5a-e).
MoS 2 nanosheets and ower-shaped CuO combine to form a Z-scheme heterostructure, which signicantly boosts the separation effectiveness of photogenerated carriers. CuO and MoS 2 's oxidation and reduction characteristics are enhanced by the Z-scheme electron transfer process that gives signicant accumulation of photogenerated electrons and holes.
The authors proposed a possible Z-scheme charge transfer process and the reaction mechanism of MBT degradation by MoS 2 /CuO heterojunction under visible light irradiation and the results are shown in Fig. 5f. Electron-hole pairs are generated by the excitation of both MoS 2 and CuO. The electrons on the CB of CuO migrate to the VB of MoS 2 and combine with holes on the VB of MoS 2 . The photogenerated holes assembled by CuO can degrade MBT molecules in large quantities. In addition, due to the CB position of MoS 2 is much more negative than the potential of O 2 /O 2 , the electrons on the CB of MoS 2 will further undergo a reduction reaction to generate abundant O 2 , which will be further involved in the degradation process. Although CuO has a higher negative VB potential than OH, it is theoretically impossible to form OH; yet, some OH has been found using the ESR technique, which may be the result of further reducing O 2 . The Z-scheme charge transfer mechanism for MoS 2 /CuO signicantly increases the separation efficiency of photogenerated carriers and positively inuences the oxidizability and reducibility of the photocatalysts. Finally, the detailed photocatalytic reaction process of the Z-scheme MoS 2 /CuO heterojunction is in Fig. 5f. A S-scheme MoS 2 /g-C 3 N 4 photocatalyst is developed 69 a one-pot solid-state reaction of thiourea and sodium molybdate as precursors at different temperatures under N 2 gas was applied. The variation in component contents (MoS 2 and gC 3 N 4 ) is determined by the physicochemical characterization of the nal products via an increase in synthesis temperature. The morphology and elemental composition of representatives were investigated using eld emission scanning electron microscopy (FE-SEM), high resolution transmission electron microscopy (HR-TEM), along with energy dispersive X-ray spectroscopy (EDS) mapping (Fig. 6a-g).
The degradation of Rhodamine B in an aqueous solution under visible light was used to evaluate the enhanced photocatalytic activity of the MoS 2 /gC 3 N 4 composites. The best photocatalytic performance there was demonstrated by composites created at 500°C, with a degradation efficiency of 90%, which was signicantly better than that of a single g-C 3 N 4 . The improvement in light harvesting and the extension in the lifetime of photoinduced charge carriers of composites, which are the result of the synergistic interaction between the components, are credited with the signicant improvement in photocatalytic performance. Besides, the photocatalytic mechanism is demonstrated to well-t into the S-scheme pathway with apparent evidences (Fig. 6h).

Applications of MoS 2 based nanophotocatalyst for chemical transformations
The MoS 2 photocatalysis visible-light irradiation is considered a green alternative to traditional synthetic methodologies. As only mild conditions are needed and the formation of undesirable byproducts is decreased, it is a viable path for organic transformation processes. Summary of previous studies on photocatalytic organic transformation reactions using MoS 2based materials are tabulated in Table 2.

Cross-dehydrogenative coupling reaction
De et al. developed 77a a methodology targeted for the utilization of sacricial amine donors for C-H functionalization with MoS 2 quantum dots (QDs) as the catalyst as well as the photosensitizer. Infact, QDs has emerged to be an active participant in the heterogeneous electron transfer process. This approach brings up new opportunities for photomediated organic transformations using nanomaterials without the need for external photosensitizers using a non-toxic, long-lasting procedure. The reduction cycle produces H 2 from water, and the oxidative cycle culminates in the CDC product. (Fig. 7).
The authors proposed a mechanistic pathway for the CDC reaction parallel to the photochemical HER mediated by MoS 2 QDs (Fig. 7). An electron is excited from the valence band (VB) to the conduction band (CB) by radiation. In presence of O 2 , 2-Aryl-1,2,3,4-tetrahydroisoquinoline 1 is oxidized to radical cation 1 + c via transferring an electron to the VB to regenerate the ground state of the QD. There is no need of any external photosensitizer for the oxidation of 1. The radical cation of 1 results the formation of iminium intermediate 1 + by losing a Hc. The nucleophile 2 attacks the iminium intermediate 1 + to give the targeted cross-coupled product 3 with simultaneous evolution of H 2 . Thus, QDs act as a photosensitizer as well as a catalyst that also transfer electron for the water-splitting process and further regenerated in the process. In the proposed mechanism, they have stated that the source of H can be from H 2 O as well as the nucleophile NuH. As their target was to design a methodology for the CDC reaction, they did not carry out any deuterated experiments to conrm the source of H 2 .
De et al. also demonstrated 77b 1T (metallic)-2H (semiconducting) phase boundaries intrinsic to individual sheets of chemically exfoliated 2D-MoS 2 that can serve as heterojunctions for enhanced photocatalysis compare to only semiconducting phase. Due to the abundance of heterojunctions in these multiphasic materials, chemically exfoliated 2D-MoS 2 provides improved stability as well as transfer of photogenerated charges to the reactants, giving better yield. Signicantly, they proved that this simple synthesize material works well as a photocatalyst for the aerobic oxidative coupling of amines to imines when subjected to visible light. Given its broad applications, they believe mixed phase 2D-MoS 2 can be of interest to several industrial synthetic applications related to semiconductorbased photocatalysis. An additional benet is that this heterogeneous photocatalyst can be regenerated up to ve times without signicant loss in the activity (Fig. 8).
Further De et al. 77c have developed an efficient method for Cross Dehydrogenative Coupling (CDC) reaction by using mixed phase 2D-MoS 2 nanosheets and Eosin Y alongside the photochemical Hydrogen Evolution Reaction (HER). The reaction has been carried out at room temperature under visible light irradiation and results in good to excellent yields. It is an example of a CDC reaction using mixed phase 2D-MoS 2 nanosheets as a catalyst regenerator and Eosin Y as photosensitizer. Due to incomplete recovery of the catalyst, the material can be reused with slight loss in the yields. Additionally, the scope of this protocol can be used to create various C-C coupled products. This could be an illustration of how to completely utilise the oxidation and reduction cycles of several semiconductor-based photocatalysts. (Fig. 9).

Suzuki-Miyaura coupling reaction
Lim et al. demonstrated 78 the incorporation of MoS 2 nanosheets with Pd nanodots is a promising way for promoting the visiblelight-induced Suzuki-Miyaura C-C coupling reaction. The MoS 2 nanosheets decorated with Pd nanodots showed excellent photocatalytic activity. The turnover frequency for the biphenyl product was >1600 h −1 even under sunlight illumination at room temperature. Mechanistic researches revealed that the high catalytic activity of MoS 2 originated from the efficient electron-hole pair generation mechanism under visible-light (Fig. 10).

Photocatalytic NH 3 synthesis
He et al. developed 79 the efficient photoreduction of N 2 to NH 3 over ternary MoS 2 /C-ZnO composite, that was prepared via a combination of hydrothermal and photodeposition method (Fig. 11).
Several methods were employed to characterise the produced composite, and the results displayed that carbon was loaded onto the surface of ZnO instead of doped into the lattice in order to prevent charge carrier recombination. By thermal processing, the carbon content of C-ZnO can be altered by which enhances the effectiveness of charge separation. For calcination, the most favourable temperature of 300°C determined. The separation of electron-hole pairs can be further improved by photodepositing MoS 2 nanoparticles on the C-ZnO-300 sample, which traps electrons on the carbon layer. The ideal 1% MoS 2 /C-ZnO-300 composite exhibits the fastest NH 3 generation rate under simulated sunlight irradiation at 245.7 mmol L −1 g −1 h −1 , which is 9.3 and 4.0 times higher than that of ZnO and C-ZnO, respectively. However, when exposed to visible light, C-ZnO performs best, producing NH 3 at a rate of 28.8 mmol L −1 g −1 h −1 , indicating that the composite employs a different process. The carbon layer was thought to act as a photosensitizer by transferring electrons to ZnO or MoS 2 . The processes proposed have been conrmed by EIS and PC evaluations. The addition of a carbon layer and MoS 2 nanoparticles to the ZnO increases the BET area. The increased surface area might be partially responsible for the higher photocatalytic performance. (Fig. 11a and b). There is another way to reduce N 2 to NH 3 via photocatalysis 80,81 .

Photoelectrochemical nitrogen reduction
The photoelectrochemical (PEC) technique, which uses both solar energy as well as electricity efficiently synthesize ammonia under ambient conditions. Han et al. 82 have been synthesized MoS 2 @LZO heterostructures by assembling 2D-MoS 2 nano-akes on La 2 Zr 2 O 7 nanobers via a facile hydrothermal method (Fig. 12a).
Due to the electrical dispersion in LZO, the homogeneous assembly of MoS 2 @LZO prevents the photogenerated electron hole pair from recombining more quickly. Superior PEC-NRR performances can be seen as a result of the interaction between the localised electronic distribution in the MoS 2 @LZO heterostructures, oxygen vacancies in La 2 Zr 2 O 7 , excellent conductivity of the nanobers, and increased exposure of surface sites for electron-hole generation in MoS 2 . The trapped photoelectron during N 2 activation is donated by the reduction of N 2 to NH 3 at the oxygen vacancy, which serves as the      active site for these reactions. The MoS 2 @LZO PEC catalyst exhibits a maximum ammonia yield rate of approximately 10.4 mg h −1 cm −2 and a higher faradaic efficiency of approximately 2.25% compared to pristine MoS 2 and LZO. The heterostructure interface, which is rich in oxygen vacancies and offers a stable design with effective active sites, can be used to conrm this remarkable performance. Compared to the current MoS 2 catalysts, MoS 2 @LZO demonstrates a higher ammonia yield, selective N 2 adsorption, and excellent stability. The 2D-MoS 2 nanoakes, the La 2 Zr 2 O 7 pyrochlore/uorite structured nanobers, and their heterostructure interfaces for enhanced ammonia production are presented with PEC-NRR mechanisms under ambient conditions. For various additional catalysis techniques to efficiently drive photoelectrocatalytic activity, the design strategy of inserting an n-type semiconductor over rich oxygen-vacant stable nanostructures to establish an interface can be used. (Fig. 12b). considerably increases the ability of catalyst ability to absorb visible light, which promotes the production of electrons and holes and ensures the strong photoactivity of catalyst. The composite has an about 2.2 eV band gap, which indicates improved visible light absorption. The titanium substrate is permanently attached to the composite using this straightforward, low-consumption approach, enabling recycling and ecologically responsible implementation possible (Fig. 13).

Photocatalytic conversion of CO 2 and H 2 O to CO, CH 4 and H 2 products
Ray et al. reported, 84 theintegration of photoabsorber MoS 2 and N-containing conducting polymer polypyrrole (PPy) over reduced graphene oxide improved CO 2 photoreduction and H 2 production (rGO). A variety of rGO-MoS/PPy nanocomposites were created and morphologically, structurally, and optically studied using various methods. Under pretend sunlight the optimal rGO-MoS /PPynanocomposite was found to exhibit a remarkable production of CO (3.9 mmol g −1 h −1 ), CH 4 (1.50 mmol g −1 h −1 ), and H 2 (4.19 mmol g −1 h −1 ) in the photocatalytic reduction of CO 2 in an aqueous suspension (Fig. 14). The synergistic effect of the MoS 2 , rGO, and PPy nanostructures, which encouraged the separation and migration of the photogenerated charges through the heterojunction interfaces and decreased electron-hole recombination, was  attributed to the improvement in the photocatalytic performance of the highly active surface material rGO-MoS 2 /PPy-150. The PPy in the rGO-MoS 2 /PPy nanocomposite played multiple roles in improving the photocatalytic reduction of CO due to its superior conductivities, CO 2 adsorption capabilities via its amine groups, and electron storage qualities. Better catalytic performance is possible due to the reduced PPy concentration in nanocomposite. Importantly, the strong recycling performance of rGO-MoS 2 /PPy-150 nanocomposite revealed the robustness and stability of photocatalyst (Fig. 14a-c).

Future outlook
Although many studies have focused on the synthesis of MoS 2 and MoS 2 -based nanomaterials and their different uses, there are still a plethora of challenges: 1. MoS 2 -based materials will be extensively used in a variety of disciplines and could one day become commercially feasible because of their unique physical and chemical qualities. With this, MoS 2 will be applied to various media and released into the environment, potentially posing risks to both human health and the ecosystem. Therefore, a detailed evaluation of the toxicity of MoS 2 -based compounds is required.
2. The application of MoS 2 -based photocatalysts has so far been conned to laboratory-based research; it has not yet been investigated how well MoS 2 performs in real-world environments, and a perfect photocatalyst that can be used on a wide scale and in industry has not yet been developed.
3. Certain MoS 2 composites become unstable when exposed to visible light. Therefore, more investigation is required to design materials based on MoS 2 that are photosensitive. 4. A signicant challenge pertains for the development of practical techniques for the generation of MoS 2 in quantities appropriate for industrial applications. For large-scale production, additional research into the scaling up of synthetic approaches is therefore needed.

Conclusion
In this review, we highlighted the different MoS 2 structures, their synthesising techniques and properties. The primary benets and drawbacks of synthesis approaches were also summarised. MoS 2 is a promising material with a wide range of applications. MoS 2 and MoS 2 -based nanomaterials are currently the topic of extensive research as promising photocatalyst for the photocatalytic degradation and inorganic and organic chemical transformation. MoS 2 and MoS 2 -based materials have exceptional physicochemical features, due to which they exhibit wide range of synthetic applications like coupling reaction, oxidation, reduction as well as organic and inorganic compound production. This review relies on recent developments regarding the application of MoS 2 in chemical transformations. The prospects for successful visible-light-induced photocatalysis using MoS 2 based materials are nally discussed.

Conflicts of interest
There are no conicts to declare.