Adsorption of small inorganic molecules on a defective MoS₂ monolayer

We present a theoretical study of molecular adsorption on defects on a MoS₂ monolayer. Based on Density Functional Theory, our calculations confirm that small inorganic molecules, such as CO₂, CO, H₂O, NO, NO₂, H₂ and N₂, remain bonded to the pristine monolayer through weak van der Waals interactions, suggesting that the molecules may easily diffuse over the clean monolayer. On the other hand, the introduction of defects can lead to three different situations, depending on the defect and the molecule considered: physisorption, chemical (strong) bonding to the metallic defects, namely the Mo substitutional atoms on the S vacancies, and dissociation, that can take place spontaneously at 0 K in some specific cases or by the effect of thermal agitation in molecules such as CO₂ or NO₂ on the S vacancy. Our energetic and electronic analyses provide an explanation to such bonding possibilities, showing that in the low interacting situations, the molecules tend to adopt a planar configuration parallel to the monolayer, while a molecular rotation is favored in order to facilitate the bond formation on the reactive sites. Finally, the ab initio based Scanning Tunneling Microscopy (STM) simulations show the fingerprint of each molecule adsorbed on the most reactive site. This work opens the way to the possibility of tuning the catalytic properties of MoS₂ by controlling the creation of specific defects in the MoS₂ monolayer.