Atmospheric O 2 spontaneously reacts with MoS 2 , but little is known about the mechanism. Herein, in-situ Raman spectroscopy during temperature sweeps under controlled environments show that MoS 2 is stable in O 2 -free environments, but O-substitution and MoO 2 formation occur in the presence of parts-per-million levels of O 2 , and oxidation to MoO 3 occurs at higher O 2 -levels. MoO 2 is stable at room temperature, but time-resolved experiments and density functional theory (DFT)-generated phase diagrams reveal it to be a metastable kinetic product that sequentially oxidizes at elevated temperatures, forming Mo 4 O 11 then MoO 3 . DFT calculations reveal that O-substitution makes both S-vacancy formation and O-substitution more favorable, creating a positive feedback loop that destroys MoS 2 . Experimentally, this feedback loop is embodied as formation of MoO 2 at progressively lower temperatures when either O-substitution or O 2 partial pressure in the environment are increased. These findings reveal that MoS 2 is inherently unstable in O 2 -containing environments due to synergistic effects of two defect types.
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