Study of Adsorption of H₂, CO and NO Gas Molecules on Molybdenum Sulfide and Tungsten Sulfide Monolayers from First-Principles Calculations

Using density functional theory based electronic structure calculations, we study the adsorption of H₂, CO and NO gases on pristine monolayers of two dimensional (2D) monochalcogenides, buckled WS and MoS systems. Preferred orientation, energetic stability, and the degree of charge transfer occurring between the adsorbate and substrate have been predicted by evaluating the adsorption energy (E_ads) and analyzing the Bader charges. We find that both, CO and NO, molecules are chemisorbed on the 2D surface with C/N atoms (with a tilted orientation) pointing towards the surface with appreciable E_ads and charge transfer. On the other hand, the adsorption of H₂ molecules is stabilized via Kubas interaction. Further, Bader analysis and charge transfer theory suggest that all the gas molecules act as electron acceptors. Interestingly, the density of states results show a semi-metallic to semiconductor transition upon H₂ and CO adsorption on the WS monolayer. The studied monochalcogenides exhibit huge sensitivity towards the gases, and this is anticipated due to the presence of transition metal (Mo and W) atoms constituting the top surface of the monochalcogenides. Finally, the present study indicates that these monolayers may serve as promising substrates for the sensing of the studied gas molecules.