In recent years, the material characteristics and properties of transition metal dichalcogenide (TMDCs) have gained research interest in various fields, such as electronics, catalytic, energy storage. In particular, many researchers have been focusing on the applications of TMDCs in dealing with environmental pollution. TMDCs provide a unique opportunity to develop higher-value applications related to environmental matters. This work highlights the applications of TMDCs contributing to pollution reduction in (i) gas sensing technology, (ii) gas adsorption and removal, (iii) wastewater treatment, (iv) fuel cleaning, and (v) carbon dioxide valorisation and conversion. Overall, the applications of TMDCs have successfully demonstrated the advantages of contributing to environmental conversation due to their special properties. The challenges and bottlenecks of implementing TMDCs in the actual industry are also highlighted. More efforts need to be devoted to overcoming the hurdles to maximize the potential of TMDCs implementation in the industry.
Transition metal dichalcogenides (TMDCs) are a large family of two-dimensional (2D) layered materials, which are scientifically interesting and industrially important. These materials have attracted tremendous attention because of the unique structural features and interesting properties, such as optoelectronics, electronics, mechanical, optical, catalytical, energy-storage, thermal, and superconductivity properties . TMDCs are the compounds of the chemical formula MX2, where M is a transition metal element of groups IV-VII B.(Mo, W, V, Nb, Ta, Ti, Zr, Hf, Tc, Re) and X is a chalcogen element(S, Se, Te). The X-M-X unit layer consists of three atomic layers, in which one centre atom layer (M) is sandwiched between two chalcogen atom layers(X). TMDCs occupy the layered structures, which resemble that of graphite. The interlayers are stacked by weak van der Waals force, leading to the formation of monolayers or nanolayers from the bulk materials via exfoliation. Different stacking of the layers along c-axis determines polymorphic crystal structures in TMDCs, and the common phases are 1T, 2H, 3R, and Td phases(T -trigonal, H -hexagonal, and R -rhombohedral, Td - distorted octahedral).
There are more than 40 different TMDC types, including metals (such as TiS2, VSe2), superconductors (such as TaS2, NbS2), semimetals (such as MoTe2, WTe2), and semiconductors (MoS2, MoSe2, WS2, WSe2). TMDCs exhibit interesting band structures with tunable bandgaps. The bandgap is one of the most important factors in 2D materials for determining the properties and applications. For instance, graphene is a semimetal with zero bandgap, which limits its applications in electronics and photo-electronics. TMDCs exhibit variable bandgaps from 0 eV to 3 eV, which can be tuned by thickness, defects, dopants. and mechanical deformations (by applying the tensile strain or compressive strain). The most studied semiconducting TMDCs (e.g., MoS2, MoSe2, WS2, WSe2) have shown typical features in electronic structures. The bandgap increases with the decreasing thickness and it possesses the transition from indirect in the bulk crystals to direct in the monolayers. For instance, the indirect bandgap of -1.29 eV will be changed to a direct bandgap of -1.8 eV when bulk MoS2 is down to a monolayer.
Benefiting from their unique crystal structures and electronic structures, TMDCs have shown great potential in various fields, including electronics/optoelectronics , catalysis , energy storage and conversion , sensing and so on. The application of TMDCs in pollution reduction is a compelling research topic. The increasing environmental pollution issue has been one of the serious problems in the earth. Enormous efforts have been made to search the efficient and low-cost methods for addressing the environmental pollution issue. TMDCs may be a kind of promising materials for tacking these problems with several advantages. Firstly, TMDCs have a high surface-to-volume ratio. They offer more effective active sites on the surface, as well as abundant unsaturated surface sites. Thus, the layered TMDCs are excellent platforms for the anchor of semiconductor nanoparticles in various photocatalytic applications . Due to their high surface-to-volume ratio, TMDCs are extremely sensitive to the surrounding atmosphere and can be utilized in toxic gas sensing and adsorption. Secondly, TMDCs have tunable bandgaps, which enhances the photocatalytic performance in nanocomposite by offering appropriate bandgap and band alignment . Thirdly, defect engineering can be easy to implement in 2D materials, which have been confirmed to be an efficient method for intensifying the catalytic activities in TMDCs. Lastly but importantly, there is a large variety for TMDCs (about 40 kinds) and they have an abundant amount in nature or can be synthesized . So far, MoS2, WS2, MoSe2 and ReS2 have been naturally found . Specifically, MoS2 exists as molybdenite in nature and is the main source of molybdenum with a large amount . The main metals (W and Mo) in TMDCs are both abundant, cheap and widely used in industry. TMDCs can be prepared by using various techniques, such as chemical vapour deposition (CVD) , chemical vapour transport(CVT) , flux growth method, hydrothermal synthesis, Langmuir−Schaefer deposition, etc. In addition, the top-down exfoliated method can be also used to fabricate few-layer TMDCs from bulk crystals, e.g. mechanical exfoliations, liquid phase exfoliations. With increasing interests in TMDCs for applications, we aim to prepare an overview of the recent progress of TMDCs in reducing the environmental pollution. We will summarize the representative efforts, including gas adsorption and removal, gas sensing, wastewater treatment, fuel cleaning, CO2 valorisation and conversion.
In summary, this work reviews the recent advanced work of TMDCs in applications of pollution reduction. The unique and exclusive features of TMDCs (e.g., layered structure, tunable bandgap, unique optical, thermal and electrical properties, etc.) have been the main driving force that drives the researchers’ attention on exploring the potential of the 2D materials in pollution reduction applications. This work summarises the state-of-the-art applications of various TMDCs under the context of pollution mitigation (include (i) gas adsorption and removal, (ii) gas sensing, (iii) wastewater treatment, (iv) flue cleaning and (v) CO2 valorisation and conversion). In addition to the up-to-date progress of TMDCs research, this article also discusses some of the key challenges for the future commercialisation of TMDC materials. Apparently, many of the reviewed research works have authenticated their substantial potential to substitute the existing pollution mitigation media. Nevertheless, the current applications are still restricted to lab basis, where the deviation of the actual performance of TMDCs under larger-scale production remains as the research gap. To usher TMDCs into the next level of utilization (i.e., from lab scale to industrial scale), the following three research directions should be followed up, (i) techno-economic analysis (TEA) study; (ii) experimentation under more rigorous and realistic conditions and (iii) experimental optimization for application purpose. The authors sincerely hope this review can serve as an insightful guideline that inspires more researchers to venture into this new and exciting cutting-edge field.