New Insights into Gas–Solid Interactions of NO₂/MoS₂ Monolayers: a Comparative Study with MoSe₂ and MoTe₂ Monolayers

Abstract

Understanding the microscopic electronic structure determines the macroscopic properties of the materials. Sufficient sampling has the same foundational importance in understanding the interactions. The NO 2 /MoS 2 interaction is well known, but there are still many inconsistencies in the basic data, and the source of the NO 2 direct dissociation activity has not been revealed. Based on a large-scale sampling density functional theory (DFT) study, the optimal adsorption of the NO 2 /MoS 2 monolayer system is determined. The impurity state on the top of the valence band of the S-vacancy monolayer (MoS 2 −V S ) was determined by cross-analysis of the band structure and density of states, which has been neglected for a long time. This provides a reasonable explanation for the direct dissociation of NO 2 on the MoX 2 monolayers. Further atomic structure analysis reveals that the impurity state originates from the not-fully occupied valence orbitals. This also corroborates the fact that the Mo material has dissociation activity, while the W material does not. There is no impurity state on the top of the valence band of the X-vacancy WS 2 and WSe 2 monolayers. Interestingly, NO 2 dissociation did not occur in the MoTe 2 −V Te monolayer. This may be related to the 6s inert electron pair effect of the Te atom. The double-oriented adsorption behavior of NO 2 is also revealed. In contrast to the MoSe 2 and MoTe 2 monolayers, NO2-oriented adsorption on the MoS 2 perfect monolayer deviates obviously, which is speculated to be related to space limitation and larger electronegativity of the S atom. The oriented adsorption ability of the MoX 2 monolayers followed the order MoTe 2 (64.4%) > MoSe 2 (44.8%) > MoS 2 (42.7%), according to the directed proportion. Renewed insights into the adsorption basic data and the understanding of the electronic structure of NO 2 /MoX 2 (X = S, Se, Te) monolayer systems provide a basic understanding of the gas−surface interactions and various future surface-related advanced applications.