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

Understanding the microscopic electronic structure determines the macroscopic properties of the materials. Sufficient sampling has the same foundational importance in understanding the interactions. The NO₂/MoS₂ interaction is well known, but there are still many inconsistencies in the basic data, and the source of the NO₂ direct dissociation activity has not been revealed. Based on a large-scale sampling density functional theory (DFT) study, the optimal adsorption of the NO₂/MoS₂ monolayer system is determined. The impurity state on the top of the valence band of the S-vacancy monolayer (MoS₂-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 NO₂ on the MoX₂ 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 WS₂ and WSe₂ monolayers. Interestingly, NO₂ dissociation did not occur in the MoTe₂-V_Te monolayer. This may be related to the 6s inert electron pair effect of the Te atom. The double-oriented adsorption behavior of NO₂is also revealed. In contrast to the MoSe₂ and MoTe₂ monolayers, NO2-oriented adsorption on the MoS₂ 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 MoX₂ monolayers followed the order MoTe₂ (64.4%) > MoSe₂ (44.8%) > MoS₂ (42.7%), according to the directed proportion. Renewed insights into the adsorption basic data and the understanding of the electronic structure of NO₂/MoX₂ (X = S, Se, Te) monolayer systems provide a basic understanding of the gas-surface interactions and various future surface-related advanced applications.