Molybdenum Disulfide-Based Nanomaterials for Visible-Light-Induced Photocatalysis.

Visible-light-responsive photocatalytic materials have a multitude of important applications, ranging from energy conversion and storage to industrial waste treatment. Molybdenum disulfide (MoS2) and its variants exhibit high photocatalytic activity under irradiation by visible light as well as good stability and recyclability, which are desirable for all photocatalytic applications. MoS2-based materials have been widely applied in various fields such as wastewater treatment, environmental remediation, and organic transformation reactions because of their excellent physicochemical properties. The present review focuses on the fundamental properties of MoS2, recent developments and remaining challenges, and key strategies for tackling issues related to the utilization of MoS2 in photocatalysis. The application of MoS2-based materials in visible-light-induced catalytic reactions for the treatment of diverse kinds of pollutants including industrial, environmental, pharmaceutical, and agricultural waste are also critically discussed. The review concludes by highlighting the prospects of MoS2 for use in various established and emerging areas of photocatalysis.

Introduction

The use of fossil fuels to meet increasing global energy demands arising from population growth and the rapid industrialization of developing nations is leading to environmental pollution and potentially irreversible anthropogenic climate change. In recent years, photocatalytic technologies have demonstrated great potential for mitigating the energy crisis and controlling environmental pollution.[1] Visible-light-driven photocatalysis, in particular, has the advantage of being able to efficiently utilize the vast amount of energy contained in solar radiation as a clean, cheap, and renewable driving force.[2] Visible-light-active photocatalysts have attracted significant attention because of the ease with which they can be prepared and recycled using simple chemical procedures.[4]Figure shows some of the benefits of using photocatalysts for pollutant removal. The ability of these photocatalysts to harvest light is dependent on their band gap energy.

Figure 1

Advantages of using photocatalysts for the removal of pollutants.

Among the semiconductors used as photocatalysts, metal oxides and metal chalcogenides have drawn the most attention. Metal oxides have been used because of their stability, eco-friendly nature, and availability; however, most metal oxide photocatalysts are only responsive to ultraviolet (UV) light, which accounts for a relatively small fraction (∼8%) of the energy contained in the solar spectrum, while visible light makes up about 43%.[5] In contrast, metal chalcogenides can be used as visible-light-active photocatalysts because they have narrower band gaps than metal oxides, which makes them more responsive to visible light.[6] Other advantageous properties of metal chalcogenides include their tunable band gap energies, controllable morphology, and large surface area, which have led to chalcogenides increasing in popularity for use in photocatalytic systems.[7,8]

Among the chalcogenides, molybdenum disulfide (MoS2) has captivated researchers with its exceptional features such as strong oxidizing activity, high stability and hardness, nontoxicity, large surface area, and a high proportion of catalytically active sites. Some major uses of MoS2 include the photocatalytic evolution of hydrogen,[9,10] oxidative desulfurization,[11] and the photocatalytic degradation of organic pollutants.[12,13] With its hexagonal arrangement of Mo and S atoms, one- or few-layered MoS2 has been considered analogous to graphene;[14] however, MoS2 possesses a narrower band gap than graphene, which enables the generation of electron/hole pairs (e–/h+) upon visible-light excitation and makes MoS2 a strong candidate for visible-light-induced photocatalysis. MoS2 has attracted considerable attention for photocatalysis because of its excellent ability to absorb light and high chemical stability. Previous reports have shown that modification of MoS2 with a second semiconductor or metal or by doping with metal or nonmetal elements are effective approaches for enhancing its photocatalytic activity. For instance, Lu et al.[200] reported that a g-C3N4/1%Ni2P/MoS2 heterojunction exhibited a high H2 production of 532.41 μmol g–1 h–1, which was 2.47 and 5.15 times higher than g-C3N4/1.5%MoS2 and g-C3N4/1%Ni2P, respectively.

Several methods exist for the preparation of MoS2 using different molybdenum precursors and sulfur sources, such as elemental sulfur powder,[15] thiourea,[16] thioacetamide,[17] and l-cysteine.[18] Various interesting morphologies can be prepared by controlling factors such as the reaction solvent, temperature, pH, reaction time, and the use of surfactants or ligands, which are vital for controlling the synthesis to form the desired chalcogenide. Table shows the different methods that have been used to synthesize MoS2 materials alongside the applications of the synthesized materials. The most used methods are solid-state,[19] hydrothermal,[20,21] solvothermal,[22] and hybrid methods.[23]

Table 1

Synthesis Methods and Applications of MoS2

no.	synthesis method	metal precursor used	source of sulfur used	morphology and particle size	applications	ref
1	chemical exfoliation	commercially available MoS2 powder		nanosheets	photocatalytic oxidation of benzyl halides	(24)
2	chemical vapor deposition	ammonium heptamolybdate	sulfur	not mentioned		(25)
3	colloidal	ammonium tetrathiomolybdate [(NH4)2MoS4]	ammonium tetrathiomolybdate [(NH4)2MoS4]	spherical quantum dots with average size ∼5 nm	bioimaging	(26)
4	colloidal	Mo(CO)6	sulfur powder	nanosheets	electrochemical studies	(15)
5	heating	Mo(acac)2	1-dodecanethiol	nanosheet	electrical bistability performance	(27)
6	hot injection	molybdenum(V) chloride (MoCl5)	N,N′-diphenylthiourea	nanosheets		(28)
7	hydrothermal	sodium molybdate dihydrate (Na2MoO4·2H2O)	thioacetamide (CH3CSNH2)	coral-like	lubrication additive; photocatalytic degradation of liquid paraffin	(29)
8	hydrothermal	(NH4)2MoS4	(NH4)2MoS4		fluorescent probe for hyaluronidase detection	(21)
9	hydrothermal	(NH4)6Mo7O24·4H2O	thiourea (H2CSNH2)	layered MoS2 nanoflowers with ∼0.1 μm particle size	photocatalytic degradation of methylene blue and crystal violet dyes	(16)
10	hydrothermal	sodium molybdate dihydrate (Na2MoO4·2H2O)	thioacetamide (C2H5NS)	nanoflowers with average size ∼100 nm	photocatalytic degradation of rhodamine B	(17)
11	hydrothermal	sodium molybdate dehydrate (Na2MoO4·2H2O)	l-cysteine	quantum dots with ∼2.5 nm particle size	detection of methyl parathion (pesticide)	(18)
12	hydrothermal	Na2MoO4·2H2O	cysteine	quantum dots with ∼3.5 nm particle size	fluorescent probe for sensing of hydroquinone and bioimaging	(30)
13	hydrothermal	MoO3	potassium thiocyanate	flowerlike MoS2 spheres with average diameter of 1–2 nm	photocatalytic degradation of methylene blue	(31)
14	hydrothermal	Na2MoO4·2H2O	l-cysteine	microspheres comprising crossed-linked nanorods ∼100 nm in length	photocatalytic degradation of thiocarbamate herbicides	(32)
15	hydrothermal	ammonium tetrathiomolybdate [(NH4)2MoS4]	thiourea (H2CSNH2)	flowerlike microsphere	photocatalytic degradation of rhodamine B and methylene blue	(33)
16	hydrothermal	Na2MoO4·2H2O	thiourea (H2CSNH2)	irregular with average size in the range 12–25 nm	electrochemical studies	(20)
17	hydrothermal	ammonium hepta molybdate tetrahydrate [(NH4)6Mo7O24·4H2O]	• ammonium polysulfide	ammonium polysulfide as the sulfur source: uniform MoS2 nanospheres with average size of ∼100 nm	lubrication additive	(22)
			• thiourea (CH4N2S)	thiourea as the sulfur source: multilayer nanosheets
18	hydrothermal	(NH4)6Mo7O24·4H2O	• thiourea	nanosheets	detection of dopamine	(34)
19	hydrothermal	(NH4)6Mo7O24·4H2O	Na2S	hierarchical porous with the thickness of ∼20–40nm	detection of phenol sulfite oxidase, nicotinamide adenine dinucleotide oxidase and superoxide dismutase mimicking activities	(35)
20	microwave	ammonium tetrathiomolybdate [(NH4)2MoS4]	ammonium tetrathiomolybdate [(NH4)2MoS4]	quantum dots with average diameter of ∼1.72 nm	determination of terramycin	(36)
21	solid state	(NH4)6Mo7O24·4H2O	sulfur	nanosheets, thinner than 5 nm	photocatalytic degradation of rhodamine B	(19)
22	solid state	(NH4)2MoO4	thiourea	sheetlike structure and ultrathin layers	electrochemical measurement	(37)

The present review focuses on the structure, properties, and synthetic methods relevant to MoS2 and MoS2-based materials. Although the number of publications related to the photocatalytic applications of MoS2-based materials has increased exponentially over the last 5 years, a concise description of the problems faced by researchers in the field is still lacking. This review, therefore, aims to summarize the main challenges as well as provide key strategies to tackle the identified problems. Furthermore, the latest advances in the photocatalytic degradation of industrial, pharmaceutical, environmental, and agricultural waste by MoS2-based materials will be discussed. Finally, the future outlook for this rapidly expanding field will be outlined, and some specific suggestions for further development will be made.

Challenges Facing Chalcogenide-Based Photocatalysis

The wide band gap energies of most metal oxides limit their application as photocatalysts under visible-light irradiation. Unlike metal oxides, most metal chalcogenides exhibit narrow band gap energies and are in principle more suitable for visible-light-induced photocatalysis; however, some modifications are required to improve their photocatalytic activities, as will be described in detail below.

Despite years of extensive research into the photocatalytic applications of chalcogenides, certain limitations and drawbacks remain. The first of these is the conditions required to synthesize the materials. Tremendous efforts have been made to develop a large variety of visible-light-active chalcogenide materials, but most of the reported synthesis methods require an inert atmosphere to prevent the formation of oxides. For example, metal sulfides can be oxidized to the corresponding oxides in the presence of atmospheric oxygen, and Dante et al. reported that metal sulfides could be thermally oxidized in the temperature range of 300–600 °C.[38] Moreover, the stability of metal sulfides, selenides, and tellurides is not widely researched, and effective comparison studies should be carried out.

The stability of synthesized chalcogenides under photocatalytic conditions is another challenge faced by researchers. Issues such as photocorrosion and a short excited-state lifetime have been shown to affect the photocatalytic performance of chalcogenides.[39] For instance, Cai et al. reported that when using pure CdS for photocatalytic rhodamine B degradation, photocorrosion led to substantial amounts of Cd2+ being produced in solution.[40] However, photocorrosion is not an issue for MoS2 because it exhibits excellent photostability in solution, which is attributable to the antibonding state formed from the interaction between the molybdenum d2 and sulfur p orbitals at the top of the valence band (VB).[41] Apart from that, recovery of the photocatalyst after completion of the photocatalytic reaction could also be a problem. Lastly, another major problem facing most metal chalcogenides is their limited ability to separate and transfer photogenerated charge carriers to active catalytic sites, which may arise because of short carrier lifetimes, low carrier mobility, or a combination of the two.[42]

Practical Solutions to Overcome the Challenges Facing Chalcogenide-Based Photocatalysis

Even though MoS2 can be considered a visible-light-responsive photocatalyst, it is afflicted by a common drawback of narrow band gap photocatalysts, namely the high recombination efficiency of e–/h+ pairs, which limits the broad application of MoS2 to photocatalysis. Hence, to improve charge carrier separation, MoS2 has been doped and coupled with many varied materials such as metals,[43] metal oxides,[12] and carbon-based materials.[44]Figure shows some of the modifications of MoS2. The doping of MoS2 with metal or nonmetal dopants, as shown in Figure a,b, can generate defects and alter the optical bandgap of MoS2. When coupled with metals, charge separation can be improved because of the presence of the metal–MoS2 interface, as shown in Figure c. For instance, Cheah et al. reported an improved H2 evolution activity of Ag@MoS2 under irradiation by visible light.[45] They reported that the deposition of Ag onto MoS2 assisted e– and h+ separation, prevented charge recombination, and hence, enhanced the overall photocatalytic performance. The formation of a heterojuction aids the rapid separation of e–/h+ pairs by producing a band offset and/or an electric field across the space-charge region at the junction between the two materials. Figure d shows the formation of two types of heterojunctions. A type I heterojunction occurs when the conduction band (CB) of MoS2 is lower than the CB of the second semiconductor, while the VB of MoS2 is higher than the VB of the second semiconductor, so both electrons and holes will move from the second semiconductor to the MoS2. Conversely, a type II heterojunction is formed when the CB of the second semiconductor is below the CB of MoS2, thus serving as an electron sink, and the VB of MoS2 is above the VB of the second semiconductor to act as a hole sink.[46] For example, Hu et al. reported that an optimized Co-doped MoS2/g-C3N4 heterojuction exhibited the highest activity for the photocatalytic reduction of water, with a H2 evolution rate of 0.31 mmol g–1 h–1.[47] These authors reported that the enhanced photocatalytic activity arises because Co-doped MoS2 possesses an edge-enriched 1T phase exhibiting more active sites than compared with pristine MoS2, as well as the presence of a heterojuction between MoS2 and g-C3N4 to accelerate charge transfer and separation. In addition, the incorporation of one or more other elements into binary chalcogenides to produce ternary and quaternary chalcogenides is a potential approach to improve the efficiency of charge separation and interfacial charge transfer through the introduction of multiple interfaces. For example, Lim et al. fabricated a ZnMoS4/ZnO/CuS p–n heterojunction photocatalyst that results in a hydrogen evolution rate 97% higher than an unoptimized ZnMoS4/CuS p–n structure.[48]

Figure 2

Modifications of MoS2 to improve its photocatalytic activity; (a) metal doping of MoS2, (b) nonmetal doping of MoS2, (c) metal deposited on MoS2, and (d) formation of a heterojunction with a second semiconductor.

Furthermore, the deposition of a metal on MoS2 is effective at improving the rate of photocatalysis. Besides increasing the interfacial charge transfer rate and local electric field through Schottky junction formation, metal deposition on MoS2 can broaden the absorption spectrum to include the near-infrared range. The deposition of metals on MoS2, including Au,[49] Pt,[50] Ag,[51] Pd,[52] and Cu,[53] has been studied as a means of producing plasmonic photocatalysts with strong visible light absorption as well as size- and shape-dependent surface plasmon resonance (SPR) effects. Previous studies have shown that metals deposited on MoS2 can function as e– sinks, capturing the photogenerated e– and, thus, speeding up the transfer and separation of e–/h+ pairs. For instance, Hong et al. reported that when nanosized Ag was anchored inside MoS2 nanosheets, the composite showed improved radical scavenging performance with an IC50 of 26 and 46 μg/mL against 1,1-diphenyl-2-picyrl and nitride oxide radicals, respectively, in the presence of low Ag dosage (∼7–20 μg/mL).[54] Furthermore, the nanosized Ag particles also improved the light-harvesting of the photocatalyst because of the SPR effect. These findings revealed that MoS2 composites have higher photocatalytic activity than pristine MoS2, implying that material modification by composite formation is a potential strategy for improving the photocatalytic performance of MoS2.

Structural and Optical Properties of MoS2

As a layered transition metal dichalcogenide, MoS2 is made up of stacked S–Mo–S atomic layers, held together by attractive van der Waals forces.[55] MoS2 is regarded as a multifunctional material because of its ability to show a diverse range of properties as it changes from bulk to nanoscale. MoS2 has two structural features:[56,57] (i) a hexagonal arrangement of S–Mo–S atomic layers with strong covalent bonds between the Mo and S atoms and (ii) constituent layers held together by Van der Waals forces. Figure shows the distinct phases of MoS2, namely, 1T, 2H, and 3R. In nature, crystalline MoS2 exists as either 2H-MoS2 or 3R-MoS2, where “H” and “R” indicate hexagonal and rhombohedral symmetries, respectively. In these structures, each Mo atom is centered in a trigonal prismatic coordination sphere with covalent linkages to six S2– ions. Each S atom has pyramidal coordination and is bonded to three Mo atoms.[58] Intercalating 2H-MoS2 with alkali metals yields a new, metastable metallic phase with trigonal symmetry termed 1T-MoS2, which does not exist in the natural environment.[59]

Figure 3

Metal coordination and crystal structure of MoS2 in its three distinct phases.

X-ray diffractometry (XRD) has been used to investigate the various distinct phases of MoS2. For instance, Saber et al. successfully reported MoS2 materials with different phase compositions synthesized via a hydrothermal method.[60] The authors reported that the powder XRD patterns of the synthesized materials matched the 1T/2H, 3R, and 2H phases, as shown in Figure a. In a different study, Toh et al. reported that they fabricated a single 2H phase MoS2 exhibiting a hexagonal structure, while their 3R-MoS2 was found to be a mixture of rhombohedral 3R phase and hexagonal 2H phase, as shown in Figure b.[61] They also reported that the 3R phase of MoS2 outperforms its 2H phase counterpart in hydrogen evolution reaction catalysis. Veeramalai et al. synthesized single-phase MoS2 nanosheets with a hexagonal crystal structure without any impurities.[62] The authors compared it with the bulk MoS2 standard XRD pattern, as illustrated in Figure c. The characteristic peaks of MoS2 nanosheets were observed at 33.69° and 59.51°, which correspond to the (100) and (110) planes. They also highlighted the absence of the (002) plane peak and the broadness of the other peaks compared with bulk MoS2, which indicated that the obtained MoS2 was a monolayer or comprised only a few layers.

Figure 4

XRD patterns of (a) hybridized MoS2,[60] (b) 2H-MoS2 and 3R-MoS2,[61] and (c) bulk and nanosheets MoS2.[62] Reproduced with permission from refs (60) and (61). Copyright 2018 and 2017, respectively, Royal Society of Chemistry.

Figure a shows the electronic band structures of bulk and monolayer MoS2, calculated using density functional theory.[63,64] The VB maximum occurs at the Γ-point in the bulk material, and the fundamental bandgap transition is indirect. As the number of layers decreases, the bandgap gradually widens until the VB maximum occurs at the K-point and the transition becomes direct in the monolayer. The direct excitonic states at the K-point are essentially unchanged as the number of layers decreases,[65] and the change in the band structure can be attributed to the quantum confinement effect and the resulting change in hybridization between the p orbital of S and the d orbitals of Mo.[64] The calculation results show that the CB states at the K-point mainly arise from localized d orbitals on Mo atoms, which are located in the center of the S–Mo–S layer sandwiches, and are mostly unaffected by coupling between layers. Therefore, as the number of layers changes, the direct excitonic states at the K-point are relatively unaffected, but the transition at the Γ-point shifts from an indirect one to a larger, direct one.[66] Consequently, MoS2 is predicted to undergo an indirect-to-direct bandgap transformation as the number of layers decreases, with a concomitant increase in bandgap energy from 1.2 eV (bulk) to 1.9 eV (monolayer).

Figure 5

(a) Electronic band structures derived from first-principles density functional theory calculations for bulk and monolayer MoS2.[64] Reproduced in part with permission from ref (64). Copyright 2011 American Physical Society. (b) Energy diagrams of the conduction band and valence band edge potentials in differently sized MoS2.[65] Reproduced in part with permission from ref (64). Copyright 2015 John Wiley and Sons. (c) Schematic illustration of the photocatalytic mechanism of MoS2.

The calculated CB and VB edge potentials for MoS2 with different layers are illustrated in Figure b. In the bulk MoS2, the CB edge potential of bulk MoS2 is −0.16 V, which is higher than that of the H+/H2 couple redox (−0.41 V vs SHE, pH 7), while monolayer MoS2 has a lower CB edge potential (−0.53 V) than the H+/H2 couple redox because of the quantum size effect. Multiple studies have demonstrated that the bandgap in MoS2 can be tuned by modifying the number of layers (thickness) from monolayer to multilayer. This strategy could be used to adjust the optical response of MoS2 over a broad spectral range.

The Use of MoS2 and MoS2-Based Materials as Photocatalysts

MoS2 has attracted significant 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 MoS2 is limited by photogenerated e–/h+ recombination, the edge activity effect, and photocorrosion. Past efforts to enhance the photocatalytic properties of MoS2 have involved controlling the morphology, modulation of energy bands through doping, band alignment through heterojunction formation, modification 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 Figure c, upon irradiation the MoS2 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 MoS2 to participate in reduction (e–) and oxidation (h+) processes. Photoexcited e– may react with adsorbed O2 to produce •O2– radicals and, simultaneously, the h+ remaining in the VB can abstract an e– from hydroxyl ions or adsorbed H2O molecules to produce •OH radicals, which are powerful oxidizing agents that can react with harmful organic, inorganic, and biological compounds.

Photocatalytic Treatment of Organic Pollutants, Inorganic Pollutants, and Microbes Using MoS2 and MoS2-Based Materials

Because of the effect of industrialization on the environment, inadequate wastewater treatment remains a major global issue, along with the preservation of the natural ecosystem and human health. The effective treatment of wastewater is crucial because water bodies are polluted with organic/inorganic contaminants including industrial, environmental, pharmaceutical, and agricultural wastes. Figure illustrates the uses of MoS2 and MoS2-based materials under visible-light irradiation.

Figure 6

Applications of MoS2 and MoS2-based materials under visible-light irradiation.

Photocatalytic Treatment of Industrial Waste

Industrial waste effluents are a major cause of water pollution that is harmful to the environment. Treatment techniques including coagulation, adsorption, precipitation, and biodegradation have been used to eliminate contaminants from wastewater. Photocatalysis is another potentially efficient method for the removal of pollutants that cannot be degraded by physical or biological processes. Dyes containing nonbiodegradable and colored pigments can be harmful to living organisms and visible in water even at low concentrations. As a result, the removal of dyes from wastewaters is crucial. A summary of previous work on the photocatalytic degradation of industrial pollutants is tabulated in Table . For example, Sheng et al. successfully demonstrated the photocatalytic degradation of methylene blue dye using hydrothermally synthesized flowerlike MoS2 with a degradation activity of 95.6% within 90 min under visible-light irradiation.[31] The synergistic effects arising from multiple modifications of CdS with MoS2 and Ag2S that showed effective separation of photoinduced e–/h+ pairs and, thus, led to an enhancement in photocatalytic performance, have been reported by Wang et al.[67] Hydrothermally synthesized Ag2S/MoS2/CdS was successfully used to degrade 87% methylene blue within 60 min under visible-light irradiation. In another study, Chen et al. reported that the photocatalytic reaction rate of a MoS2/SrFe12O19 composite was reported to be almost twice that of pure MoS2.[68] Flowerlike MoS2/SrFe12O19 heterojunctions were able to degrade up to 97% of methylene blue under visible-light irradiation. The high photocatalytic activity resulted from MoS2 providing abundant active sites for catalytic reactions, and the exposed interfacial region of the p–n heterojunction formed by MoS2/SrFe12O19 could reduce the recombination rate of the photogenerated e–/h+ pairs.

Table 2

Summary of Previous Work on the Photocatalytic Degradation of Industrial Pollutants Using MoS2-Based Materials

no.	photocatalyst used	pollutants	particle size	band gap energy	light source	degradation efficiency	ref
1	MoS2/CuO	2-mercapto benzothiazole	not mentioned	MoS2: 1.9 eV	300 W Xe lamp	degraded 96.0% of 2-mercaptobenzothiazole within 120 min	(78)
				CuO: 1.7 eV
2	reduced GO/ZnO/MoS2 and CNTs/ZnO/MoS2	aniline	not mentioned	reduced GO/ZnO/MoS2 = 2.24 eV; and CNTs/ZnO/MoS2 = 2.31 eV	two 50 W LED lamps	complete mineralization of aniline after 210 min	(73)
3	rutile-based inorganic hollow microspheres and hydrogenated black TiO2 decorated with MoS2	arsenite	not mentioned	2.18 eV	DH-2000 deuterium tungsten halogen	96.6% within 200 min	(79)
4	MoS2/CoTiO3	bisphenol A	length: ∼100–200 nm and width: ∼50–60 nm	not mentioned	natural sunlight	CoTiO3: 6.1%	(74)
						MoS2: 9.8%
						Different wt % of MoS2/CoTiO3
						2 wt %: 71.3%
						5 wt %: 82.1%
						10 wt %: 77.4%
						20 wt %: 68.1% within 90 min
5	g-C3N4/MoS2- polyaniline	bisphenol A	not mentioned	g-C3N4: 2.21 eV	not mentioned	92.7% within 60 min	(75)
				composite: 2.67 eV
6	MoS2/BiVO4-activated PMS	bisphenol A	pure MoS2: nanoflowerlike structure with a size of 0.5–1 μm	MoS2: 1.67 eV	300 W Xe lamp	93.3% of bisphenol A degraded within 20 min	(76)
				BiVO4: 2.37 eV
			pristine BiVO4: olivelike structure with a length of 2 μm and a width of 1 μm	2-MoS2/BiVO4: 1.82 eV
7	MoS2/ZnS/ZnO	Cr(VI)	4.5 μm	2.8–3.1 eV	300 W Xe lamp	98.7% within 90 min	(80)
8	MoS2	crystal	not mentioned	1.86 eV	natural sunlight	92% within 50 min	(81)
		violet
9	MoS2/ZnS nanoparticles embedded in N/S-doped graphite carbon	dicofol (pesticide)	∼25 nm	not mentioned	350 W Xe lamp	84.5% within 100 min	(82)
10	MoS2-In2S3	hexavalent chromium [Cr(VI)]	MoS2: 2 μm	not mentioned	300 W Xe lamp	complete removal of Cr(VI) within 30 min	(83)
			In2S3: 200–1000 nm
11	MoS2/Ag2CO3	Lanasol Red 5B	MoS2: 0.4–1.5 μm	MoS2: 1.88 eV	500 W Xe lamp	95.0% within 25 min	(84)
			Ag2CO3: 0.5–2.5 μm	Ag2CO3: 2.28 eV
12	BiOI/MoS2	methyl orange	2–3 nm	1.65 eV	not mentioned	methyl orange: 95.6% after 75 min	(85)
13	MoS2/ZnO	methylene blue	not mentioned	ZnO: 3.22 eV	natural sunlight	∼97.0% within 20 min	(12)
				MoS2/ZnO: 3.12 eV
14	MoS2/SrFe12O19	methylene blue	MoS2: 200 nm	MoS2: 1.4 eV	simulated sunlight	the composite achieved up to 97.0% degradation rate	(68)
				SrFe12O19: 1.7 eV
			SrFe12O19: 100 nm	mass ratio of SrFe12O19 to MoS2
				1:2 = 1.37 eV; 1:3 = 1.54 eV
15	MoS2 nanobox embedded g-C3N4@TiO2	methylene blue	not mentioned	g-C3N4: 2.51 eV	350 W Xe lamp	97.5% within 60 min	(86)
				TiO2: 3.18 eV
				g-C3N4@TiO2: 2.76 eV
				MoS2: 1.18 eV
				MoS2@TiO2: 1.25 eV
				g-C3N4@TiO2/MoS2: 1.41 eV
16	carbon-modified MoS2/TiO2	methylene blue	50–100 nm	TiO2: 3.2 eV	150 W Xe arc lamp	99.0% within 60 min	(87)
				carbon-modified MoS2: 1.3 eV
17	MoS2	methylene blue		1 layer MoS2: 1.88 eV	1000 W Halogen lamp	∼95.6% after 120 min	(88)
				10 layer MoS2: 1.35 eV
18	TiO2/MoS2	phenol	600 nm	≤3.25 eV; decreases as TiO2/MoS2 ratio decreases	300 W Xe lamp	78.0% after 150 min	(72)
19	bacterial cellulose/MoS2	pyrocatechol violet	not mentioned	1.66 eV	35 mW/cm2	84.5% within 180 min	(89)
					Xe lamp
20	MoS2	rhodamine b	2.5 μm	2.05 eV	5 W LED	62.1% after 120 min	(33)
21	MoS2 quantum dots/few-layered MoS2 nanosheets coated with Ag3PO4 nanoparticles core@shell heterostructure	rhodamine B	2–14 nm	2.47 eV	300 W Xe lamp	complete degradation of dye in 16 min	(90)
22	BiOI/MoS2	rhodamine B	2 μm	BiOI: 1.73 eV	500 W Xe lamp	complete degradation within 90 min	(91)
				MoS2: 1.42 eV
23	MoS2/ZnO spheres	rhodamine B	1.2–2.1 μm	not mentioned	300 W Xe lamp	completely degrade rhodamine B within 90 min	(92)
24	MoS2/FeVO4	rhodamine B	not mentioned	MoS2: 1.9 eV	250 W Xe lamp	90.0% within 120 min	(93)
				FeVO4: 2.16 eV
25	MoS2@NiFe2O4	rhodamine B	MS nanosheets with thickness of 10–50 nm	MoS2: 1.17 eV	350 W Xe lamp	96.4% within 90 min	(94)
			NiFe2O4 layered nanostructure with thickness of ∼50 nm	NiFe2O4: 2.75 eV
				MoS2@NiFe2O4: 2.01 eV
26	MoS2@rGO	thiophene	14.5 nm	2.02–1.68 eV	125 W Hg lamp	complete removal after 75 min	(77)

In a different study, Zhang et al. successfully prepared MoS2 nanosheet petals with a band gap energy of 2.05 eV via a hydrothermal method using CTAB as the surfactant.[33] The synthesized MoS2 was able to photocatalytically degrade rhodamine B and methylene blue gradually with increasing irradiation time. The effect of light intensity on photocatalytic rhodamine B degradation by MoS2 was investigated by Roy et al.[17] They conducted the experiments under normal exposure to sunlight and used a solar concentrator coupled with an optical fiber bundle to deliver concentrated sunlight to the photocatalytic reactor. They reported that MoS2 nanoflowers with a narrow band gap of 2.2 eV were able to degrade 67.4% and 39.9% of rhodamine B within 120 min under exposure to concentrated sunlight and normal sunlight, respectively. Higher photocatalytic activity was observed for MoS2 nanoflowers irradiated by concentrated sunlight because concentrated sunlight has a higher irradiance and thus contains a greater number of photons to generate more e–/h+ pairs and reactive oxygen species (ROS) species, which leads to faster degradation of rhodamine B dye. Phenol red is a water-soluble dye that is commonly used as a pH indicator. Awasthi et al. reported up to 90% of photocatalytic degradation of phenol red within 80 min under exposure to visible light using flowerlike-shaped ZnO/MoS2 synthesized via a hydrothermal method.[69] The enhanced photocatalytic activity of this material may be due to the narrow band gap of MoS2, which enables the absorption of visible light. Basic red 18 is a cationic azo dye used for coloring textiles. Ugur et al. demonstrated a complete degradation of basic red 18 dye after 180 min under irradiation of visible light using a hydrothermally synthesized ZnO/MoS2/rGO (reduced graphene oxide) composite.[70] This is attributed to the integration of MoS2 and ZnO onto the rGO surface, which helps to reduce the agglomeration of the rGO layers. At the same time, it also increases the catalytic surface area and the active regions for the photocatalytic reactions to take place. Furthermore, when the amount of rGO was increased to 5%, the synthesized materials could fully degrade the dye after 30 min. This may be due to the improved charge separation and acceleration of electron mobility by increasing the amount of rGO.

The photocatalytic degradation of methylene blue by Ag-doped MoS2 nanopetals synthesized via a hydrothermal method was reported by Ikram et al.[43] The authors reported that the band gap energy was reduced from 2.35 eV to values in the range 2.35–1.55 eV, depending on the Ag content. The synthesized Ag-doped MoS2 nanopetals displayed excellent behavior with methylene blue degradation of up to 100% within 50 s. In another study, Sadhanala et al. reported high degradation of crystal violet dye under sunlight using hydrothermally synthesized MoS2.[16] The synthesized materials exhibited a narrow band gap energy of 1.55 eV, which enables it to utilize visible light efficiently, and showed complete degradation within 20 min. Joy et al. achieved 85% degradation of methylene blue within 30 min and up to 95% degradation of para-nitrophenol after 5 h under irradiation of light using a heterostructure comprising MoS2 and g-CNO (oxidized graphitic carbon nitride) as a photocatalyst.[71] Wang et al. successfully prepared multilayered MoS2-coated TiO2 hollow spheres using a hydrothermal method. TiO2/MoS2 exhibited high photocatalytic activities and managed to degrade about 78% of phenol after 150 min.[72] The enhanced photocatalytic activities were attributed to improved light absorption and a faster transfer of photoinduced charge carriers. In another study, Liu et al. effectively used coral-like MoS2 to photocatalytically degrade liquid paraffin.[29] Upon irradiation of light, MoS2 could degrade up to 90.8% of the paraffin within 32 h and it exhibited nearly constant photocatalytic capability after three cycles. This more effective use of visible light may be because of the narrow band gap energy of 2.34 eV, in addition to the less agglomerated and wider layer spacing of particles in coral-like MoS2 that produces abundant active sites for the photocatalytic reaction to occur (Figure ).

Figure 7

Proposed mechanism for the photocatalytic degradation of liquid paraffin using coral-like MoS2.[29] Reproduced with permission from ref (29). Copyright 2017 Centre national de la recherche scientifique (CNRS) and the Royal Society of Chemistry.

Ghasemipour et al. reported an enhanced photocatalytic activity of MoS2 by combining rGO with ZnO for the degradation of aniline under the irradiation of visible light.[73] The full mineralization of aniline was achieved after irradiation for 210 min. The improved photocatalytic activity of the synthesized nanocomposites may be due to the incorporation of rGO, which resulted in an increased surface area and an enhancement in adsorption capacity, thus allowing more pollutant molecules to be adsorbed at adsorption sites. Moreover, rGO caused a delay in the recombination of charge carriers, which could be ascribed to certain characteristics of these carbonaceous materials including their significant e– storage capacity and ability to function as e– sinks, which allows for the accumulation of photoinduced e– in their structure. Bisphenol A is an important industrial chemical that is produced in enormous quantities, primarily for use in the production of epoxy resins and polycarbonate plastics. Unfortunately, depolymerization and subsequent leaching may lead to the contamination of food and water with this harmful material, which may present toxicological risks to humans and animals, even at relatively low levels. Dadigala et al. fabricated flowerlike MoS2/CoTiO3 heterostructures and applied them to the degradation of bisphenol A under simulated solar irradiation.[74] The authors reported up to 82.1% degradation of 10 ppm bisphenol A within 90 min. In a different study, Ahamad et al. successfully fabricated g-C3N4/MoS2-polyaniline to degrade about 92.6% bisphenol A within 60 min under irradiation of visible light.[75] In another study, Zheng et al. reported that hydrothermally synthesized MoS2/BiVO4 activated PMS to exhibit a degradation rate constant of 0.1747 min–1 for bisphenol A, which is 91.9 times higher than that of pure MoS2 and 38.0 times higher than that of pure BiVO4.[76] Thiophene is a common sulfur-containing substance mostly found in gasoline fuels. The photocatalytic removal of thiophene under irradiation of visible light using MoS2@rGO synthesized via a sol–gel method was reported by Alhaddad et al.[77] A complete photocatalytic removal of thiophene was achieved after 75 min of irradiation, and the synthesized material retained 91% of its photocatalytic activity after five independent cycles.

On the basis of the literature, MoS2 and MoS2-based composites have been used in the degradation/removal of several distinct types of pollutants. Other factors, such as pH, temperature, light intensity used/source of light, the ratio of pollutant to solvent, concentration of the photocatalyst used, and the presence or absence of e–/h+ acceptors may also affect the final degradation level, and more research into these factors should be conducted.

Photocatalytic Treatment of Environmental Waste

The removal of heavy metals is a well-known method to avoid environmental issues caused by the direct or indirect release of industrial effluents into the environment. There are several widely used methods for heavy metal removal, including the precipitation method,[95] activated carbon adsorption,[96] and membrane separation.[97] Besides these more established methods, photocatalysis is gaining importance as an approach for the removal of common heavy metal pollutants. The increased use of chromium in modern society, which can result in soil and water contamination, is becoming an increasing concern. Cr can exist in several oxidation states; Cr(VI) is harmful because it is highly mobile, whereas Cr(III) is less toxic as it is less mobile.[98] Zhao et al. successfully demonstrated photocatalytic Cr(VI) reduction using an MoS2/ZnS/ZnO composite to achieve a 98.7% Cr(VI) reduction within 90 min.[80] Such excellent photocatalytic activity can be attributed to the existence of dual type-II heterojunctions that hinder e–/h+ pair recombination and, thus, results in a rapid e– transfer and high carrier separation efficiency. The authors proposed that, upon irradiation of light, photogenerated e– in the MoS2 and ZnS are injected into the ZnO CB because of its lower CB energy compared with MoS2 and ZnS. Meanwhile, h+ generated in the VB of ZnO can be injected into the VB of MoS2 and ZnS on account of their higher VB energies relative to ZnO. In a different study, Wang et al. successfully synthesized Fe-doped g-C3N4/MoS2 with a narrow band gap of 2.63 eV using a hydrothermal approach.[99] The photocatalytic degradation efficiency of Fe-doped g-C3N4/MoS2 toward Cr(VI) is markedly increased to 91.4% in comparison with g-C3N4 (11.4%), g-C3N4/MoS2 (55.6%), and Fe-doped g-C3N4 (83.2%). A quantitative investigation into the reaction kinetics of Cr(VI) reduction was also performed. The value of the rate constant for Fe-doped g-C3N4/MoS2 was 0.021 min–1, which is around 20, 2.5, and 1.5 times higher than that of pure g-C3N4 (0.001 min–1), g-C3N4/MoS2 (0.006 min–1), and Fe-doped g-C3N4 (0.014 min–1), respectively. These results demonstrate that Fe-doped g-C3N4/MoS2 exhibits dramatically enhanced photocatalytic activity compared with the other materials evaluated under identical conditions. Vijayakumar et al. fabricated MoS2/B doped-rGO using a facile and low-cost hydrothermal approach.[100] The band gap energy of MoS2/B doped-rGO was found to be 1.70 eV, and the size ranged from 20 to 100 nm. The nanocomposites achieved up to an 80.1% reduction of Cr(VI) to Cr(III) in 120 min under irradiation of visible light. These authors suggested that the high activity was due to the strong interaction between boron and the metallic phase of MoS2.

Arsenic is another example of a heavy metal that can cause contamination issues. MoS2 has been coupled to metal oxides, such as TiO2, to overcome their ineffectiveness in utilizing visible light. Because of its lower band gap, MoS2 can improve the utilization of visible light, and heterojunction formation can also improve the efficiency of carrier separation and reduce the rate of e–/h+ recombination. For instance, Balati et al. synthesized rutile-based hollow microspheres and hydrogenated black TiO2 decorated with MoS2 heterojunctions using liquid-phase pulsed laser ablation followed by microwave irradiation.[79] The synthesized materials were successful in oxidizing 96.6% of arsenite under irradiation of visible light. The authors reported that this may be due to the formation of photogenerated e–/h+ pairs in MoS2 when it was exposed to visible light. Because the CB and VB energies of MoS2 are higher than those of TiO2, photoexcited e– are readily transferred from the CB of MoS2 to the CB of TiO2. Furthermore, Ti3+ ions present in the TiO2 function as hole traps, thereby hindering e–/h+ recombination.

Uranium is also considered a hazardous metal contaminant because of its chemical and radioactive toxicities. Zhang et al. utilized flowerlike MoS2/g-C3N4 nanosheet heterojunctions to eliminate the radioactive U(VI) pollutant.[101] The synthesized materials exhibited up to 86.8% removal of U(VI) after 75 min of visible-light irradiation. The high photocatalytic activity was attributed to the efficient harvesting of visible light and the promotion of charge transfer, which led to enhanced e–/h+ separation efficiency. Moreover, only a slight decrease in the photocatalytic activity of MoS2/g-C3N4 from 86.80% to 83.07% was observed after five cycles. In another study, MoS2/P-doped g-C3N4 nanocomposites synthesized using an ultrasonic chemical method exhibited up to a 99.75% removal efficiency of uranium (VI) within 60 min.[102] The reported outstanding removal of U(VI) was caused by both photogenerated e– and •O2– radicals reacting with U(VI) to form UO2 on the surface of the synthesized materials. Further reaction of the UO2 with H2O2 led to the formation of (UO2)O2·H2O; thus, U could be extracted from the water in the form of (UO2)O2·H2O following visible-light irradiation in the presence of an MoS2/P-doped g-C3N4 photocatalyst.

Different strategies to improve the photocatalytic activity of MoS2 materials should be further investigated to fabricate Mo-based photocatalysts with high efficiency and ability to maximize the heavy metal treatment effect. Moreover, different processes could be combined for a more effective removal of heavy metals via methods such as adsorption and photocatalysis, electrocatalysis and photocatalysis, etc.

Photocatalytic Treatment of Pharmaceutical Waste

Pharmaceutical compounds including analgesics, antidepressants, antihypertensive, contraceptives, antibiotics, and steroids are designed specifically to resist biological breakdown, and thus, they can preserve their chemical structure to exist in the human body and be discharged in their original state into the environment. Conventional/traditional wastewater treatment methods, such as activated sludge, are insufficient for removing active pharmaceutical compounds completely.[103] The use of heterogeneous photocatalysis is a more effective method for reducing the negative impacts of pharmaceutical wastes, byproducts, and unreacted reactants in the environment.[104]Table shows the reported studies on the photocatalytic degradation of pharmaceutical pollutants using MoS2-based materials. Vaizoğullar reported up to 89% degradation of the antibiotic ofloxacin within 90 min under irradiation of light using a CdS/MoS2/ZnO composite synthesized using a chemical precipitation method.[105] The 89% degradation was possible because of the extended lifetime of the charge carriers. In another study, Guo et al. fabricated 3D nanoflowerlike BiOI/MoS2 microspheres exhibiting a narrow band gap of 1.65 eV, which could improve the charge transfer rate and, hence, suppress the recombination of photogenerated e–/h+ pairs.[106] The prepared materials exhibited up to 91.5% tetracycline removal within 75 min. The presence of MoS2 enabled the effective widening of the light absorption range and enhanced the utilization of light quanta.

Table 3

Summary of Previous Studies on the Photocatalytic Degradation of Pharmaceutical Pollutants Using MoS2-Based Materials

no.	photocatalyst used	target pollutants	concentration of pollutants	irradiation time	source of light	degradation efficiency	ref
1	25 ppm of MoS2/TiO2	acetaminophen	not mentioned	25 min	natural sunlight	40%	(107)
2	CdS-MoS2-coated ZnO	amoxicillin	5 mg/L (50 mL)	60 min	300 W Xe lamp	94.0%	(108)
3	40 mg of MoS2@ZnxCd1–xS	amoxicillin	0.1 g/L (80 mL)	300 min	300 W Xe lamp	highest degradation was 38.3%	(109)
4	C3N4–MoS2	ampicillin	40 ppm (100 mL)	2 h	300 W Xe lamp	74.6%	(110)
5	12 mg of CdSe QDs@MoS2	ceftriaxone sodium	20 mg/mL (50 mL)	180 min	300 W Xe lamp	85.5%	(111)
6	20 mg of MoS2/BiOBr	ciprofloxacin	10 mg/L (100 mL)	6 h	300 W Xe lamp	87.0%	(112)
7	50 mg of MoS2/CoTiO3	ciprofloxacin	20 ppm (50 mL)	90 min	natural sunlight	91.8%	(74)
8	30 mg of CoS2/MoS2/rGO	ciprofloxacin	10 mg/L (100 mL)	75 min	150 W halogen lamp	94.0%	(113)
9	20 mg of pomelo-peel-biochar-decorated MoS2	ciprofloxacin	10 mg/L (100 mL)	90 min	300 W Xe lamp	92.0%	(114)
10	0.7 g/L of In2O3/MoS2/Fe3O4	esomeprazole	35 ppm (50 mL)	50 min	23 W white LED lamp	92.9%	(115)
11	10 mg of MoS2/C	levofloxacin	70 mg/L (100 mL)	180 min	30 mW cm–2 Xe lamp	86.9%	(116)
12	0.5 g/L of CeO2–ZrO2@MoS2	naproxen	10 mg/L (50 mL)	40 min	250 W white LED lamp	96.0%	(117)
13	0.1 g of CdS/MoS2/ZnO	ofloxacin	10 ppm (50 mL)	90 min	400 W Xe lamp	89.0%	(105)
14	20 mg of Ti3C2/MoS2	ranitidine	10 mg/L (20 mL)	60 min	300 W Xe lamp	degradation efficiency = 88.4%	(118)
						mineralization efficiency = 73.58%
15	20 mg of BiOI/MoS2	tetracycline	20 mg/L (65 mL)	75 min	not mentioned	91.6%	(85)
16	0.15 mg of MoS2/BiOBr/carbon fibers	tetracycline	20 mg/L (10 mL)	120 min	300 W Xe lamp	92.4%	(119)
17	10 mg of CoS2/MoS2@Zeolite	tetracycline	0.2 g/L (50 mL)	2 h	300 W Xe lamp	96.7%	(120)
18	0.01 g of MoS2/g-C3N4/ Bi24O31Cl10	tetracycline	20 mg/L (50 mL)	50 min	300 W Xe lamp	97.5%	(121)
19	10 mg of MoS2/Ag/g-C3N4	tetracycline	20 mg/L (50 mL)	50 min	300 W Xe lamp	98.9%	(104)
20	MoS2/Eu/B-g-C3N4	tetracycline	20 mg/L (50 mL)	50 min	300 W Xe lamp	99.0%	(122)
21	CoS2/MoS2/rGO	tetracycline	20 mg/L (100 mL)	10 min	150 W halogen lamp	complete degradation	(113)
22	10 mg of MoS2@zeolite	tetracycline	0.2 g/L (50 mL)	180 min	300 W Xe lamp	87.2%	(123)
23	20 mg of MnFe2O4/MoS2	tetracycline	20 mg/L (100 mL)	60 min	300 W Xe lamp	composite with the mass ratio of MnFe2O4 and MoS2	(124)
						1:10 = 80.9%
						1:50 = 74.3%
						1:1 = 65.3%
						pure MnFe2O4 = 24.8%
						pure MoS2 = 31.7%

CoS2/MoS2@zeolite synthesized using a hybrid method (ultrasonic and hydrothermal methods) showed remarkable removal of tetracycline by combining its ability to adsorb and photocatalytically degrade the pollutant. The presence of zeolite assisted in the adsorption of tetracycline in dark conditions. The synthesized materials were able to degrade the tetracycline by up to 96.7% within 2 h of light irradiation.[120] Interestingly, complete degradation of tetracycline has been reported by Selvamani et al. after 10 min of visible-light irradiation.[113] The authors combined both visible light and ultrasonic irradiation to achieve these remarkable findings.

A ternary hybrid composite, MoS2-decorated CeO2-ZrO2 nanoflower, was prepared using a hydrothermal and liquid self-assembly method, where MoS2 plays a significant role in extending the light absorption toward the visible region because of its narrow band gap energy. Interestingly, Talukdar et al. utilized an ultrasonication-assisted photocatalytic degradation approach to remove naproxen, a nonsteroidal and anti-inflammatory drug, in an aqueous medium.[117] They reported that the sonophotocatalyst was able to degrade naproxen up to 96% within 40 min. Xia et al. prepared MoS2/BiOBr using a microwave-assisted approach, and it exhibited a narrow band gap energy of 2.33 eV.[112] They reported up to 87% photocatalytic degradation of ciprofloxacin using MoS2/BiOBr after 6 h of visible-light irradiation. The CB edge potential of MoS2 (−0.09 V vs SHE) is lower than that of BiOBr (0.29 V vs SHE), whereas the VB of BiOBr (3.06 V vs SHE) is higher than that of MoS2 (1.81 eV). The photogenerated e– are therefore transferred from the CB of MoS2 to the CB of BiOBr because of the energy difference between their CB edges, while the photogenerated h+ are transferred from BiOBr to MoS2. The high photocatalytic activity was attributed to the low recombination rate of the photogenerated e–/h+.

Ceftriaxone sodium is an antibiotic that is used to treat several types of bacterial infections. The photocatalytic degradation of ceftriaxone sodium using CdSe QDs@MoS2 nanocomposites under visible-light irradiation was reported by Zhou et al.[111] After 180 min, the maximum ceftriaxone sodium degradation rate and removal ratio for total organic carbon reached 85.47% and 71.81%, respectively. After the fourth cycle, the authors found the degradation rate only decreased slightly to 78.46%, and this indicates that the prepared nanocomposites are chemically stable under visible-light irradiation and may be utilized multiple times in practical applications.

Esomeprazole is a pharmaceutical drug that is used to treat certain gastroesophageal reflux diseases such as acid reflux and stomach ulcers. Raha et al. investigated the photocatalytic degradation of this drug using a narrowed band gap photocatalyst, In2O3/MoS2/Fe3O4 nanocomposite (2.15 eV).[115] The synthesized In2O3/MoS2/Fe3O4 showed photocatalytic degradation up to 92.9% within 50 min. Carbamazepine is a common mood-stabilizing pharmaceutical compound to treat epilepsy. Zhou et al. developed a MoS2 activator of both persulfate (PS) and peroxymonosulfate (PMS), which were applied in the photocatalytic degradation of carbamazepine.[125] In both the MoS2/PS and MoS2/PMS systems, more than 95% carbamazepine was degraded in 40 min over a pH range from 3 to 9.

In another study, Zeng et al. reported the synthesis of a narrow band gap (1.33 eV), 3D urchinlike MoS2/C nanoparticle composite for the photocatalytic degradation of levofloxacin under visible-light irradiation.[116] Levofloxacin is a member of the broad-spectrum fluoroquinolone antibiotics. Because of its high bactericidal activity and tissue penetration property, it plays a significant role in oral and intravenous formulations. However, it has extensively accumulated in water bodies because of its high solubility and the improper disposal of pharmaceutical industry waste and hospital discharges. Irregularly shaped MoS2 particles ∼80 nm in size adhered onto the surface of g-C3N4 microrods with lengths ranging from 2 to 5 μm have been successfully synthesized using a hydrothermal method. He et al. utilized g-C3N4/MoS2 heterojunctions to degrade levofloxacin up to 75.81% within 140 min.[126] In another study, the urchinlike MoS2/C composite displayed an enhanced degradation of levofloxacin up to 100% after 80 min of visible light exposure when assisted by sonication. It was reported that piperazine ring opening, decarboxylation, and demethylation of the levofloxacin occurred, which led to the formation of less harmful products such as CO2, H2O, F–, etc. Yin et al. fabricated MoS2/PbBiO2Cl nanosheet photocatalysts to increase the rate of interfacial charge transfer and boost the visible-light photocatalytic degradation of ciprofloxacin.[127] The authors reported that the band gap energy of PbBiO2Cl was reduced from 2.39 to 2.02 eV after modification of the material with MoS2. This lowering of the band gap leads to an increased photogeneration of charge carriers and reduced recombination, resulting in the high photocatalytic activity of MoS2/PbBiO2Cl under visible-light irradiation.

In addition to this, proper disposal practices of expired and unused pharmaceutical products should be adopted and strengthened to avoid the growing antimicrobial resistance among microorganisms. Furthermore, only certain antibiotics are examined as model pollutants for photocatalytic degradation. Mixed pharmaceutical pollutants could be further explored to determine the selectivity of MoS2-based photocatalysts and their efficiency.

Photocatalytic Treatment of Agricultural Waste

Agricultural chemicals such as growth regulators, fertilizers, herbicides, pesticides, insecticides, and additives for soil remediation have been widely used to improve the production of crops. Fertilizer application is critical for improving agricultural production since it provides the high concentration of nutrients needed for plant growth.[128] Pesticides are necessary to avoid major agricultural losses, and they are often used to prevent or keep pests from damaging crops.[129] These agricultural chemicals serve crucial roles in the production of crops, and their use has risen dramatically in recent years. However, the excessive use of these chemicals will permanently alter the chemical ecology of soil, and improper usage will result in these materials remaining in the food chain and contaminating the soil, water, and air environments.[130] Pesticides are toxic compounds that are used for killing or repelling pests such as rodents, insects, fungi, and weeds (unwanted plants).[82] Ahamad et al. successfully prepared a novel heterojunction nanocomposite composed of MoS2/ZnS nanoparticles embedded in a N/S-doped graphitic carbon matrix using hydrothermal and postcalcination methods and investigated its application in the photocatalytic degradation of the pesticide dicofol under sunlight irradiation.[82] The synthesized material degraded up to 84.5% of the pesticide and still showed good photocatalytic activity of about 77.2% after five cycles.

Thiobencarb is a carbamate pesticide that is frequently employed in rice fields. The main issue with this pesticide is its persistence in the environment, which can be as long as several years, and the leaching and runoff of its residues from the soil into ground and surface water. Huang et al. reported that MoS2 microspheres synthesized using a hydrothermal method can be used to degrade thiobencarb under visible-light irradiation without requiring the addition of hydrogen peroxide, and the corresponding degradation efficiency could reach up to 95% after 12 h.[32] In another study, 89% of the pesticide fipronil was successfully degraded using a BiOCl/MoS2 composite.[131] Fipronil is a broad-spectrum insecticide made up of phenylpyrazole compounds. The composite exhibited higher photocatalytic degradation of fipronil under visible-light irradiation in comparison with bare BiOCl and MoS2, which possessed about 40.5% and 50.6% degradation capabilites, respectively.

Photocatalytic Treatment of Microbes

MoS2 and MoS2-based materials are potential antimicrobial agents because of their broad-spectrum antibacterial activity, good biocompatibility, and highly efficient harvesting and utilization of light energy. It is generally agreed that the excellent antibacterial activity of these materials is ascribable to ROS-induced bacterial killing mechanisms. Upon irradiation of light, MoS2-based materials become excited, and e–/h+ pairs are generated. The photogenerated charge carriers may either recombine or migrate to the surface of the photocatalyst, where they can react with adsorbed species. Photoexcited e–, in particular, may react with adsorbed O2 molecules to produce ROS. Despite a sizable body of literature focusing on the antimicrobial activity of MoS2 having been reported,[132,133] only a few studies have focused on the effect of light on its antimicrobial properties. Table summarizes recent work on the photocatalytic inactivation of different microbes by MoS2-based antimicrobials.

Table 4

Summary of Previous Studies on the Photocatalytic Inactivation of Different Microbes Using MoS2-Based Materials

no.	photocatalyst used	microbes	method	findings	ref
1	MoS2	E. coli	standard plate counting method (irradiated under 18 W white LED light for 180 min)	disinfection rate of the MoS2 synthesized via: ultrasound = 33%	(134)
				hydrothermal = 62%
				intercalation = 99%
2	MoS2/Ag2CO3	E. coli	colony forming unit method (irradiated under 500 W Xe lamp for 80 min)	complete inactivation of E.coli	(84)
3	Ti/MoS2/MoOx	E. coli	plate counting method (irradiated under 100 W LED lamp for 120 min)	E. coli was completely inactivated	(135)
		E. faecium
4	chitosan/Ag/MoS2	E. coli	spread plate method (irradiated under 18 W white LED light for 20 min)	E. coli = 99.77%	(136)
		S.aureus		S. aureus = 98.66%
5	carbon nanotubes/MoS2/Ag	E. coli	dilution plate method (irradiated under 100 W lamp for 80 min)	composite exhibited better photocatalytic antimicrobial activity against S. aureus than E. coli	(137)
		S. aureus
6	chitosan@MoS2	E. coli	(irradiated under visible light for 10 min)	E. coli = 91.58%	(138)
		S. aureus		S. aureus = 92.52%
7	P-doped MoS2/g-C3N4	E. coli	colony count method (irradiated under 500 W Xe lamp for 180 min)	99.99%	(139)
8	MoS2/Bi2WO6	P. aeruginosa	plate count method (irradiated under 500 W Xe lamp for 60 min)	nearly all P. aeruginosa was inactivated, 99.99%	(140)

A narrow band gap (2.00 eV) MoS2/α-NiMoO4 composite synthesized using a one-step microwave-assisted hydrothermal method has been reported by Ray et al. for the inactivation of Staphylococcus aureus after 150 min of irradiation by visible light.[141] Electron spin resonance measurements and radical trapping experiments revealed that the O2•– radical anion was the principal active species controlling bacterial inactivation under visible-light irradiation. The bacterial inactivation was further confirmed by the observation of distortions to the bacterial cell membrane, DNA leakage, and protein destruction. Figure shows the photoinduced antibacterial activity mechanism of MoS2 and MoS2-based materials.

Figure 8

Mechanism for the antibacterial activity of MoS2 and MoS2-based materials under visible-light irradiation.

Zhang et al. reported the effect of different MoS2 synthesis methods on the photocatalytic antibacterial activity against Escherichia coli (E. coli). MoS2 was synthesized using three different methods, namely ultrasound, hydrothermal, and lithium-ion intercalation, to obtain different morphologies and structures of MoS2.[134] It was observed that MoS2 synthesized using the lithium-ion intercalation method deformed almost all E. coli cells after 180 min of visible-light irradiation. This may be due to the high separation efficiency of photogenerated e– and h+, which will result in the generation of more ROS to promote the photoinduced inactivation of E. coli.

In another study by Zhang et al., Ti/MoS2/MoO photoanodes exhibiting strong solar light absorption were employed for te photoelectrochemical inactivation of bacteria.[135] The in situ generated H2O2 and O•2– were shown to be the main oxidants for E. coli inactivation under irradiation of visible light. Under LED lamp irradiation, the inactivation of E. coli (initial concentration 106 cfu/mL) using Ti/MoS2/MoO reached 99.9999% after 2 h in 0.1 L of water, and 99.99% after 6 h in 3 L. In a different study, Zhang et al. reported that a CuS@MoS2 hydrogel successfully killed 99.3% of E. coli and 99.5% of S. aureus under dual irradiation of visible and near-infrared light within 15 min.[142] The excellent antibacterial properties of the synthesized CuS@MoS2 hydrogel were attributed to the generation of ROS under irradiation of light, which attacks and destroys the bacterial membrane and proteins.

Ray et al. fabricated MoS2/α-NiMoO4 ultrathin nanoneedle composites using a microwave hydrothermal method.[143] Within 150 min, S. aureus was completely inactivated by MoS2/α-NiMoO4 under visible-light irradiation. When exposed to visible light, both α-NiMoO4 and MoS2 were excited, generating e– and h+ in the CB and VB, respectively, of both materials. However, since the VB and CB edges of α-NiMoO4 are higher than those of MoS2, the photogenerated e– were transferred from α-NiMoO4 to MoS2, while the photogenerated h+ of MoS2 migrated to α-NiMoO4 under irradiation of visible light. This enables the efficient separation of photogenerated e– and h+ in this composite material. Furthermore, the reduction of O2 by e– in the CB of MoS2 (including those transferred from α-NiMoO4) produces O2•– radical anions, while h+ from the VB of α- NiMoO4 reacts with H2O/OH– to generate •OH radicals. These photogenerated ROS react with S. aureus to damage its membrane and lead to protein destruction, enzyme inactivation, suppression of ATP production, and ultimately inactivation of the bacteria.

A fundamental understanding of the biological interactions between MoS2 materials and biological entities is vital, and thus further study to explore the underlying mechanisms is required. Apart from that, the choice of evaluation models (i.e., microbes, cells, and tissues), culture system, assay conditions (i.e., temperature and pH), and exposure route should also be considered because these factors may influence the responses of MoS2-based materials and should be optimized. Moreover, the major photocatalytic inactivation mechanism of microbes involves the generation of reactive oxygen species. These species trigger cell membrane breakdowns and promote the internalization of MoS2, ultimately leading to the death of cells. Therefore, improving photogenerated e–/h+ pair separation favors the enhancement of photocatalytic antimicrobial activity.

Other Photocatalytic Applications of MoS2 and MoS2-Based Materials

Selective Organic Transformation Reactions

Photocatalysis is considered a green alternative to traditional synthetic methodologies. It is a promising route for organic synthesis because only mild conditions are required and it also reduces the formation of undesired byproducts.[144] Another widely researched application of MoS2 under visible-light irradiation is photoassisted organic transformation reactions, some of which are tabulated in Table .

Table 5

Summary of Previous Studies on Photocatalytic Transformation Reactions Using MoS2-Based Materials

no.	application	photocatalyst used	morphology	particle size	light source	performance	ref
1	photocatalytic oxidative coupling of thiols	Pd@Cu/MoS2	spherical	64.5 nm	300 W Xe lamp	∼99% conversion under 400–800 nm irradiation	(53)
2	photocatalytic reduction of gold thiosulfate complex	MoS2/ZnS	embroidered balls		under natural light with the intensity of 0.432 kW/m2	1120.56 mg/g reduction of [Au(S2O3)2]3– to Au0	(145)
3	photocatalytic reduction of 4-nitrophenol	TiO2 hollow spheres/crumpled MoS2 nanosheet	hollow sphere	∼200 nm	500 W Xe lamp	99.35% photocatalytic reduction of 4-nitrophenol	(146)
4	photocatalytic conversion of CO2 to methane	MoS2/Cu	MoS2 nanosheets are coated on the surface of Cu nanorods	about 50–700 nm	300 W Xe lamp	maximum yield of methane ∼23 mmol g–1 h–1	(147)
5	photocatalytic reduction of CO2 to methanol	MoS2 grown on hexagonal boron nitride nanoplatelets	MoS2 nanosheets are uniformly grown over the hexagonal boron nitride nanoplatelets.	each MoS2 nanosheet is composed of 2–6 molecular lamellae	20 W white LED lamp	maximum yield of methanol 5994 μmol g–1	(148)
6	photocatalytic reduction of CO2 to methane and CO	In2S3/MoO3@MoS2	distorted hexagonal nanorods		300 W Xe lamp	yield ∼29.4 and ∼35.6 μmol g–1 h–1 for CH4 and CO, respectively	(149)
7	photocatalytic selective oxidation of benzyl alcohols to benzaldehyde	Ag3PO4 nanoparticle@MoS2 quantum dot/few-layered MoS2 nanosheet	nanosheet		300 W Xe lamp	≤92% conversion of benzyl alcohol and ∼87% yield of benzaldehyde after 3 h of irradiation	(150)
8	photocatalytic oxidation of benzyl alcohol to benzaldehyde	Co-doped MoS2/ g-C3N4	2D nanosheet morphology with curly stripes		80 W LED lamp	benzaldehyde production rate of 0.48 mmol g–1 h–1	(47)
9	photocatalytic reduction of 4-nitrophenol to 4-aminophenol	CdS-MoS2/rGO composite	flowerlike morphology		500 W Xe lamp	≤70% reduction of 4-nitrophenol after 60 min of irradiation	(151)
10	photocatalytic reduction of N2 to NH3	C3N4/MoS2/Mn3O4 composite	sandwichlike structure		300 W Xe lamp	reaction yielded 185 μmol g–1 h–1 NH3	(152)
11	photocatalytic reduction of N2 to NH3	P-doped MoS2@N doped-g-C3N4	g-C3N4: layered structure		500 W Xe lamp	composite exhibited up to 689.76 μmol L–1 g–1 h–1 N2 reduction	(153)
			MoS2: flowerlike aggregates

In order to improve photogenerated charge carrier separation, Wan et al. modified Ag3PO4 nanoparticles by coupling them with MoS2 quantum dots and evenly wrapped them with few-layer MoS2 nanosheets to fabricate core@shell heterostructures for the selective photocatalytic oxidation of benzyl alcohol to benzaldehyde.[90] Their reactive-species-trapping experiments revealed that e– transfer from the CB of Ag3PO4 to the few-layered MoS2 nanosheets is more favorable than direct e– transfer from MoS2 to Ag3PO4 because of the suitable energy band matching between the CB of Ag3PO4 and the VB of MoS2. In addition, these authors also reported that the MoS2 quantum dots play the role of e– sinks, which helps to improve the rate of transfer of photogenerated e– from Ag3PO4 because of the excellent e– transport properties of MoS2 quantum dots.

Peng et al. demonstrated the utilization of a CdS-MoS2/rGO composite photocatalyst for the photocatalytic reduction of 4-nitrophenol to 4-aminophenol under irradiation of visible light.[151] Almost complete reduction was achieved by the CdS-MoS2/rGO composite in less than 18 min. This may be because MoS2/rGO functioned well both as charge carriers and e– acceptors to enhance the separation of the photoinduced e–/h+ pairs. Furthermore, the MoS2 could also broaden the light absorption range of the composite into the visible part of the spectrum, allowing more efficient solar energy utilization.

The efficiency of a Pd@Cu/MoS2 composite photocatalyst for oxidative coupling has been demonstrated by Yusuf et al. under visible-light irradiation.[53] 4-chlorobenzenethiol was transformed into bis(4-chlorophenyl) disulfide with high selectivity (>90%). The enhanced photocatalytic activity is attributed to the excellent light absorption of Pd@Cu/MoS2 as well as the synergistic effect of Pd, Cu, and MoS2 in this multifunctional core–shell nanostructure. Ammonia is necessary for the development of modern industry and agriculture. The photocatalytic synthesis of ammonia under irradiation of visible light was reported by Li et al. using C3N4/MoS2/Mn3O4.[152] The composite generated up to 185 μmol g–1 h–1 of NH3. In another study, Liu et al. successfully prepared P-doped MoS2@N-doped-g-C3N4 via a hydrothermal and annealing process for the photocatalytic reduction of N2.[153] The P-doped MoS2@N-doped-g-C3N4 composite exhibited a photocatalytic N2 reduction rate of 689.76 μmol L–1 g–1 h–1 in deionized water without a sacrificial agent under simulated solar irradiation, which is higher than that of pure g-C3N4, MoS2@g-C3N4, MoS2@N-doped-g-C3N4, and P-doped MoS2@g-C3N4. Güy et al. reported the photocatalytic reduction of 4-nitroaniline to p-phenylendiamine with up to 90% yield using a MoS2/Ag/Ag3VO4 nanocomposite under irradiation of visible light.[13] They reported that, upon irradiation, the photogenerated e– in the CB of Ag3VO4 were transferred to the VB of MoS2, and photogenerated e– in the CB of MoS2 caused a rapid reduction of the 4-nitroaniline because of the migration of e– from the photocatalyst surface to the 4-nitroaniline. Regarding the reusability of the prepared material, the reduction yield slightly reduced from 90% to 86% after five cycles. Zheng et al. successfully obtained CH4 from the reduction of CO2 under visible light irradiation using MoO3@MoS2–CuS.[154] The band gap of MoO3@MoS2–CuS (1.91 eV) was found to be lower than that of MoO3@MoS2 (2.28 eV) and MoO3 (2.76 eV). MoO3@MoS2–CuS exhibited better yields of CH4 (44.64 μmol g–1 h–1) and CO (7.59 μmol g–1 h–1) compared with MoO3 (CH4 = 8.53 μmol g–1 h–1; CO = 11.61 μmol g–1 h–1). This could be due to the bare edge sites in edge-rich MoS2, as well as a higher concentration of oxygen vacancies in MoO3, leading to an enhancement of CH4 yield while suppressing CO yield through optimized charge transport and good light-harvesting.

The application of MoS2-based materials as photocatalysts in organic transformation reactions has led to various surprising outcomes, as shown in Table . However, research into the utilization of MoS2-based materials as photocatalysts for organic conversion reactions is still in its infancy, and further study to improve the recovery and reusability of these materials is urgently required.

Photocatalytic Evolution of H2

Hydrogen (H2) is an alternative source of energy because it is nontoxic, clean, eco-friendly, and renewable. Production of H2 employing visible light is among the most promising solutions to various sustainable energy and environmental remediation issues. The evolution of H2 is usually monitored by gas chromatography using either N2 or Ar as the carrier gas and a thermal conductivity detector for H2 detection.[155] For example, the photocorrosion resistant composite material, MoS2/Zn0.5Cd0.5S/g-C3N4, has been successfully synthesized by Tang et al. for the production of H2 under irradiation of visible light.[156] These authors reported that the as-prepared material exhibited a maximum H2 production rate of 4914 μmol g–1 h–1 in Na2S–Na2SO3 solution, which served as a sacrificial agent. The high evolution rate of H2 was attributed to improved charge separation across the interfaces compared with the constituent materials.

In another study, Yuan et al. designed 2D–2D MoS2@Cu-ZnIn2S4 using a solvothermal method for the photocatalytic production of H2.[162] A maximum H2 evolution rate of 5463 μmol g–1 h–1 was observed for 6% MoS2@Cu-ZnIn2S4 under irradiation of visible light, which is 72 times higher than that obtained using pristine Cu-ZnIn2S4. The excellent activity may be ascribed to the strong visible-light absorption and abundant active sites for the H2 evolution reaction to take place. Moreover, after three cycles, the production rate of H2 remained unchanged, which suggests the synthesized MoS2@Cu-ZnIn2S4 is an exceptionally durable photocatalyst for H2 production.

An improved photocatalytic evolution of H2 was performed by Yin et al. with a maximum evolution rate of 872.3 μmol h–1 obtained using MoS2/C composites sensitized with erythrosin B.[164] The improved performance was ascribed to an efficient photogenerated charge transfer and charge separation between erythrosin B and MoS2 or C, as well as a greater number of suitable active sites for H2 evolution. The photocatalytic evolution of hydrogen can also be achieved by employing the photocatalyst material as a photoelectrode in a photoelectrochemical cell. For instance, Hassan et al. reported that optimized MoS2/GaN2 showed significantly higher photoelectrochemical performance under the same visible-light conditions compared with the constituent materials.[172] The as-prepared photoanode exhibited good light harvesting and achieved a photocurrent density of 5.2 mA cm–2, which is 2.6 times higher than that obtained with bare GaN. Moreover, MoS2/GaN2 exhibited a higher applied bias photon-to-current conversion efficiency of 0.91%, while a bare GaN photoelectrode yielded an efficiency of only 0.32%. The authors reported that the decrease in charge transfer resistance between the electrolyte and the semiconductor interface, as well as the enhanced separation of charge carriers in the MoS2/GaN heterostructure, were the reasons for the significant improvement in photocurrent density and overall solar-to-hydrogen conversion efficiency. In a different study, Wu et al. successfully fabricated a dual-defect heterojunction system of TiO2 hierarchical microspheres with oxygen vacancies modified with MoS2 ultrathin nanosheets for efficient visible-light H2 production.[173] The highest H2 production rate was 41.59 μmol g–1 h–1 during enrofloxacin degradation.

MoS2-based nanomaterials have been studied as efficient visible-light responsive materials for H2 production. On the basis of the reported work (Table ), it is revealed that the surface modification of MoS2 can lead to improved photocatalytic activity because of the effective transfer and separation of photogenerated e–/h+ pairs. In addition, the use of different sacrificial agents could also accelerate the rate of H2 production because they could function as e– donors or h+ scavengers to reduce the recombination of photogenerated charge carriers.

Table 6

Summary of Previous Studies on Photocatalytic Evolution of H2 Using MoS2-Based Materials

no.	photocatalyst used	size and morphology	hydrogen production rate	light source	sacrificial reagent	ref
1	MoS2/ Pyrrole/ZnO	ordered porous structure with 100–200 nm macropores	40.22 mmol cm–2 h–1	300 W Xe lamp with cutoff filter (λ = 420 nm)	Na2S/Na2SO3	(157)
2	MoS2/Zn3In2S6	lamellar surface structure with 1–3 μm particle size	74.25 μmol h–1	300 W Xe lamp with cutoff filter (λ = 420 nm)	no sacrificial reagent	(158)
3	MoS2/Carbon QDs/ZnIn2S4	flowerlike microspheres with average size 2.0 μm	150 μmol h–1	300 W Xe lamp with cutoff filter (λ = 420 nm)	triethanolamine	(159)
4	WO3@MoS2/CdS	MoS2 nanosheets uniformly grown on WO3 rods and encapsulated by MoS2 and CdS	8.2 mmol g–1 h–1	300 W Xe lamp	lactic acid	(160)
5	MoS2/Zn0.5Cd0.5S/g-C3N4	MoS2 and g-C3N4 crumpled sheets and Zn0.5Cd0.5S particles	4914 μmol g–1 h–1	300 W Xe lamp	Na2S/Na2SO3	(161)
6	MoS2/Cu-ZnIn2S4	2D flowerlike MoS2 microspheres grown on Cu-ZnIn2S4	5463 μmol g–1 h–1	300 W Xe lamp with cutoff filter (λ = 420 nm)	ascorbic acid	(162)
7	CdS/MoS2/MXene	CdS dispersed on sheet-like MXene and MoS2	9679 μmol g–1 h–1	300 W Xe lamp with cutoff filter (λ = 420 nm)	Na2S/Na2SO3	(163)
8	MoS2/activated carbon composite sensitized by Erythrosin B	Nanosheets (5–50 nm width/length, 3–6 nm thickness)	872.3 μmol h–1	30 W white light LED lamp	Triethanolamine	(164)
10	MoS2/graphene	microball	Na2S = can produce the maximum H2 production rate of 264.9 μmol g–1 h–1	350 W Xe lamp	different sacrificial reagents: Na2S·9H2O	(165)
					Na2SO3
					Na2SO4
					methanol
			formic acid = can produce the maximum hydrogen production rate of 280.5 μmol g–1 h–1		ethanol
					formic acid
					lactic acid
					ethylenediaminetetraacetic acid
11	Co-doped MoS2/g-C3N4	Ultrathin Co (∼4 nm) and MoS2 nanosheets attached to g-C3N4 nanosheets	Lactic acid = 333 μmol g–1 h–1	simulated solar irradiation	lactic acid, methanol and triethanolamine	(166)
			Methanol = 1326 μmol g–1 h–1
			Triethanolamine = 3193 μmol g–1 h–1
12	NiSe2/MoS2	3D NiSe2 nanocrystals are uniformly deposited and tightly attached to the surface of MoS2 microsphere	bare MoS2: 1173.3 μmol g–1 h–1	300 W Xe lamp	Na2S/Na2SO3	(167)
			bare NiSe2: 1065.7 μmol g–1 h–1
			5.4% NiSe2/MoS2: 2473.7 μmol g–1 h–1
13	MoS2@TiO2	2D MoS2 nanosheets coated on 3D TiO2	2985.16 μmol g–1 h–1	300 W Xe lamp	triethanolamine	(168)
14	MoS2/hollow carbon spheres/ZnIn2S4	pure ZnIn2S4: spherical structure with a diameter of 4–5 μm	620.9 μmol g–1 h–1	300 W Xe lamp	triethanolamine	(169)
		MoS2/hollow carbon spheres/ZnIn2S4: spherical
15	MoS2@ZnxCd1-xS	hexagonal	630 μmol g–1 h–1	300 W Xe lamp		(109)
16	CdS nanosheets/MoS2	ultrathin layers of MoS2 with size ∼200 nm well distributed on the surface of CdS nanosheets	1.75 mmol g–1 h–1	300 W Xe lamp	lactic acid formic acid Na2S/Na2SO3	(170)
17	MoS2/CdS	willow-branch-shaped MoS2/CdS composite consisted of rodlike subunits	250.8 μmol h–1	300 W Xe lamp	lactic acid	(171)

Future outlook

Although the synthesis of MoS2 and MoS2-based nanomaterials and their various applications have been addressed in numerous studies, several challenges remain:

MoS2-based materials will be widely used in numerous fields and may potentially achieve commercialization in the future because of their remarkable physical and chemical properties. With this, MoS2 will be released into the environment and applied to different media, potentially affecting environmental safety and human health. Therefore, it is necessary to thoroughly assess the toxicity of MoS2-based materials.

To date, the use of MoS2-based photocatalysts has been restricted to lab-based research; the use of MoS2 under realistic environmental conditions is yet to be explored, and an optimum photocatalyst that is suitable for commercialization and large-scale use is yet to be achieved.

The short life span of charge carriers, fast recombination, and catalyst recovery after use remain major challenges. These problems are intrinsic to MoS2 itself and there is, therefore, a pressing need to seek out a new generation of materials to enhance its photocatalytic activities.

The photocatalytic activity of MoS2 in various model systems has been studied extensively because of its excellent optical properties. However, the development of cost-effective MoS2-based materials capable of targeting actual contaminants present in wastewater remains an important objective.

Certain MoS2 composites are unstable under visible-light irradiation. Therefore, further research is required to develop photostable MoS2-based materials.

There is limited available data about the factors affecting photocatalytic activity such as temperature, pH, and the presence of multiple contaminants. Therefore, photocatalytic degradation should be studied under a wider range of experimental conditions.

The development of practical methods for the preparation of MoS2 in quantities suitable for industrial applications poses a significant challenge. Therefore, more research into the scaling up of synthetic methods is needed for large-scale production.

Further research into optimum dopant concentrations or amounts of other coupled semiconductors is required to reach the full potential of MoS2-based photocatalysts.

It will be necessary to obtain new insights into optimizing the interfacial charge transfer and reaction processes of dual-functional photocatalysts by utilizing multidimensional structures with MoS2 catalysts. Furthermore, the introduction of defects and interface engineering simultaneously will provide a feasible strategy to enable dual-functional photocatalysts for multiple applications such as H2 evolution coupled with the oxidation of various organic substances.

On the basis of the literature, even though MoS2 is considered a light-active material, and its excellent antimicrobial properties are well-known, the effect of light on its antimicrobial properties has not been widely researched. Thus, further research is required to determine the efficiency of microbial inactivation by MoS2 under both dark and light conditions.

Conclusion

MoS2 and MoS2-based nanomaterials are currently under intensive investigation as potential photocatalysts for the degradation of organic and inorganic pollutants, as well as the killing and/or inactivation of microbes. Because of their remarkable physicochemical properties, a wide range of synthetic strategies have been developed for the preparation of MoS2 and MoS2-based materials, and applications have been found in a diverse array of fields. This article reviewed recent progress in the modification of MoS2, including doping with metals and nonmetals, coupling with other semiconductors or metals, and using supports such as carbon-based materials. Moreover, the photocatalytic activities of the synthesized materials toward the degradation of selected pollutants were summarized. Finally, the future outlook for successful visible-light-induced photocatalysis using MoS2-based materials was discussed.