3.7. 2D Metal Dichalcogenides

Inspired by the appealing properties of graphene, researchers have made great efforts in exploring other 2D nanomaterials for gas sensing such as 2D metal dichalcogenides [17]. Han et al. [137] made a MoS₂-SnO₂ heterostructure gas sensor. It was able to detect NO₂ at room temperature, and the response to 5 ppm of NO₂ was 18.7. Compared with other gases, the selectivity was increased (Figure 11). Furthermore, this sensor had reliable long-term stability. Kim et al. [138] synthetized WS₂-SnO₂ C-S nanosheets by ALD. At the optimum shell thickness, this sensor indicated a good selectivity to CO. MoSe₂ was used to manufacture the gas sensor. Abun et al. [139] designed a MoSe₂-ZnO heterostructure gas sensor to detect H₂. Compared with pristine ZnO and MoSe₂, the selectivity and response were greatly increased.

MoS₂-SnO₂ heterostructure gas sensor. Adapted from [137] copyright (2019), with permission from Elsevier. (a) The morphology of MoS₂-SnO₂ heterostructure; (b) the response of MoS₂-SnO₂ heterostructure to NO₂.

The mechanism is due to the formation of heterojunctions. Under the action of heterojunction, response, selectivity, and operation temperature have been improved in varying degrees [137,138,139]. More details about heterojunctions can be found in Section 3.4. The combination of one-dimensional MOS material and two-dimensional metal material is a novel method owing to the large surface area and high surface-to-volume ratio. It is helpful to fabricate this type of heterojunction MOS gas sensor to detect inorganic gases. However, it is difficult to fit them firmly [46]. Therefore, it is necessary to improve the preparation process. Moreover, the unique layered structure of 2D metal dichalcogenides has provided the possibility of fabricating a flexible sensor substrate.