Oxidative etching of S-vacancy defective MoS2 monolayer upon reaction with O2

Abstract

The reactions of O₂ with S vacancy sites within a MoS₂ monolayer were investigated using density functional theory calculations. We considered the following defects: single S vacancy, double S vacancy, two adjacent S vacancies and two S vacancies separated by a sulphur atom. We found that the surface distribution of S vacancy sites plays a key role in determining the surface reactivity towards O₂. We observed the desorption of SO₂ only for the last vacancy distribution. For the other cases, the surface becomes passivated with very stable structures having O atoms on the original vacancy sites and in some cases an SO group in an adjacent position. The ab initio molecular dynamics simulations showed that the impingement of the O₂ molecule on an S vacancy site produces a stable chemisorbed O₂ molecule with an upright configuration. The surface reactions initiate after the O₂ molecule switches to the lying-down configuration which favours the breakage of the O–O bond and the concurrent formation of S–O bonds. In the most reactive vacancy site configuration, the dissociation of the first O₂ molecule produces an SO intermediate which finally leads to desorption of SO₂ after oxygen abstraction from the other adjacent O₂ molecule. The formation of a MoO₃ moiety within the monolayer was also observed in the molecular dynamic simulations at higher oxidation levels.