Molybdenum disulfide (MoS2) based photoredox catalysis in chemical transformations

Abstract

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.

Fig. 1. MoS₂ synthesis techniques with their advantages and disadvantages.

Fig. 2. (a). Visualization of the layered structure of MoS₂ and the forces responsible for its layered structure. (b). Pictorial illustration of the increased interlayer distance due to the presence of impurities. Adapted with permission from ref. , Copyrights 2022 Elsevier. (c). Graphical illustration of the tunable band gap exhibited by MoS₂. (d). Crystal structures of MoS₂. (d₁). 2H, (d₂). 3R and (d₃).1T. Adapted with permission from ref. , Copyrights 2022 Elsevier. (e). Different morphologies portrayed by MoS₂ (through synthesis) (e₁). Nanoflowers at the resolution of 2 μm (e₂). Nanoflowers at the resolution of 500 nm Adapted with permission from ref. , Copyrights 2022 Elsevier (e₃). TEM image of the nanosheets at 10 nm (e₄). TEM image of the nanosheets at 200 nm Adapted with permission from ref. , Copyrights 2022 Elsevier (e₅). Nanotubes Adapted with permission from ref. , Copyrights 2022 Elsevier (e₆). Nanoworms. Adapted with permission from ref. , Copyrights 2022 Elsevier (e₇). Coral-like structure. Adapted with permission from ref. Copyrights 2022 Elsevier (e₈). Quantum dots. Reproduced from ref. with permission from [Elsevier], Copyright [2022].

Fig. 3. Schematic illustration of the photocatalytic mechanism of MoS₂. Reproduced from ref. with permission from [American Chemical Society], copyright [2022].

Fig. 4. 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. Reproduced from ref. with permission from [American Chemical Society], copyright [2022].

Fig. 5. (a) SEM images of CuO, (b) MoS₂, (c) MoS₂/CuO, (d) TEM image of MoS₂/CuO, (e) elemental mapping of MoS₂/CuO, (f) photocatalytic degradation mechanism of MBT over the Z-scheme MoS₂/CuO heterojunction under visible light irradiation. Reproduced from ref. with permission from [Royal Society of Chemistry], copyright [2020].

Fig. 6. (a and b) FE-SEM; and (c and d) HR-TEM images of MCN-500; FE-SEM images of (e) MCN-600; and (f) CN-500; EDS mapping images of (g) mapping area, (g₁) carbon, (g₂) nitrogen, (g₃) sulfur, and (g₄) molybdenum, (h) schematic diagram of band alignment of heterojunction and S-scheme charge transfer on interface of MoS₂ and g-C₃N₄. Reproduced from ref. with permission from [Springer Nature], copyright [2021].

Fig. 7. (a) Photomediated oxidation and reduction cycle of MoS₂ QDs. (b) Proposed mechanism for the catalytic activity of MoS₂ blue QDs in CDC reaction synchronized with HER. Reproduced from ref. with permission from [American Chemical Society], copyright [2022].

Fig. 8. Plausible mechanism for the oxidative coupling of benzylamine to imine photocatalyzed by mixed-phase 2D-MoS₂ nanosheets. Reproduced from ref. with permission from [John Wiley & Sons, Inc.], copyright [2018].

Fig. 9. Schematic illustration of the 2D-MoS₂ photocatalyzed oxidation and reduction cycles in presence of Eosin Y as photosensitizer. Reduction cycle produces H₂ from water as reported earlier, whereas oxidative cycle is responsible for CDC Reaction and formation of additional H₂. Reproduced from ref. with permission from [Royal Society of Chemistry], copyright [2019].

Fig. 10. The Pd-nanodot decorated MoS₂ micro/nanosheet for an efficient visible light induced photocatalytic Suzuki–Miyaura coupling reaction. Reproduced from ref. with permission from [Royal Society of Chemistry], copyright [2017].

Fig. 11. Preparation of MoS₂/CZ300 photocatalyst and the suggested charge transfer mechanism of MoS₂/CZ300 composite (a) under simulated sunlight (b) and visible light irradiation. Reproduced from ref. with permission from [American Chemical Society], copyright [2018].

Fig. 12. (a) Schematic illustration of the electrospun LZO nanofibers, followed by hydrothermal synthesis of MoS₂@ LZO. (b) The band structure demonstrating the type II heterojunction for MoS₂@LZO heterostructures with a reduced Fermi level under applied bias and solar illumination and schematic illustration of the PEC-NRR mechanism following the associative pathway for ammonia production. Reproduced from ref. with permission from [American Chemical Society], copyright [2022].

Fig. 13. Schematic illustration of the energy band structure and electron–hole separation of MoS₂/TiO₂ composites. Reproduced from ref. with permission from [Royal Society of Chemistry], copyright [2021].

Fig. 14. (a) Schematic representation of the band positions and potentials of PPy and MoS₂. (b) Photocatalytic CO₂ conversion into CH₄ and CO gas via Z-scheme mechanism with rGO as redox mediator on the rGO-MoS₂/PPy nanocomposite. (c) The conventional type II electron transfer for H₂ production on the rGO-MoS₂/PPy nanocomposite. Reproduced from ref. with permission from [American Chemical Society], copyright [2020].

Praveen P. Singh

Surabhi Sinha

Geetika Pandey

Vishal Srivastava