Transition metals vs. chalcogens: the impact on NOx adsorption on MoS2, MoSe2 and WS2 transition-metal dichalcogenides

The widely developed industry of today generates significant amounts of harmful gases, which prompts the search for modern materials allowing for their efficient and reliable detection. Transition-metal dichalcogenides (TMD) constitute well-known example of such, with particularly high potential for excellent sensing of NO2. It is known, that the adsorption of this hazardous molecule varies on the TMD composition, however the importance of transition metal and chalcogen types were never previously contrasted. Moreover, the other NOx compounds, namely NO and N2, interact much less with TMD sheets, the reason for which is not yet well understood. This work utilizes density functional theory (DFT) approach to untangle these problems by examining the adsorption processes of NO2, NO, and N2 on the monolayers of WS2, MoS2, and MoSe2. The calculations allowed to establish two important conclusions: (i) the chalcogen is significantly more important than transition metal, allowing for much greater increase in adsorption of NO2 on MoSe2 than on WS2, as compared to that on MoS2, (ii) only molecules acting as an acceptor with respect to the TMD sheet can benefit from the enhancement coming from the composition of the latter. The gained insight can likely contribute to the informed design of devices allowing selective detection, the lack of which is a recognized problem among semiconductor sensors.