Chemical Stability and Transformation of Molybdenum Disulfide Nanosheets in Environmental Media

Layered transition metal dichalcogenides, including molybdenum disulfide (MoS₂), have previously been considered stable in the ambient environment due to the absence of dangling bonds in the electron-filled shells of the end chalcogen atoms. Here, we evaluate the chemical stability of MoS₂ nanosheets fabricated by chemical exfoliation (ceMoS₂) and surfactant dispersion (sMoS₂). The results demonstrate that sMoS₂ exhibits greater long-term persistence. Contrarily, ceMoS₂ underwent progressive deterioration, in which preferential oxidation of the 1T of a mixture of 1T and 2H phases was observed. The oxidative degradation of ceMoS₂ was retarded in the presence of natural organic matter (NOM), including Suwannee River natural organic matter (SRNOM) and Aldrich humic acid (ALHA), in the dark ambient condition, while the aging process of MoS₂ with co-occurring ALHA was accelerated under sunlight exposure. The observed inhibition effect on the deterioration of ceMoS₂ by NOM was mainly attributed to slower dissolution kinetics with rapid initial oxidation (i.e., forming Mo-O bonding) or carbon grafting, rather than prevention of the formation of secondary small suspended Mo-containing particles. The compiled results highlight that the environmental fate of MoS₂ nanosheets will be regulated by the combined effects of exfoliating agents and environmentally relevant factors including organic macromolecules and sunlight exposure.