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.

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Carbon nanotubes (CNTs) photosensitize the production of reactive oxygen species that may damage organisms by biomembrane oxidation or mediate environmental transformations of CNTs. Photosensitization by derivatized carbon nanotubes from various synthetic methods, and thus with different intrinsic characteristics (e.g., diameter and electronic properties), has been investigated under environmentally relevant aquatic conditions. We used the CNT-sensitized photoisomerization of sorbic acid ((2E,4E)-hexa-2,4-dienoic acid) and singlet oxygen formation to quantify the triplet states ((3)CNT*) formed upon irradiation of selected single-walled carbon nanotubes (SWCNTs) and multiwalled carbon nanotubes (MWCNTs). The CNTs used in our studies were derivatized by carboxyl groups to facilitate their dispersion in water. Results indicate that high-defect-density (thus well-stabilized), small-diameter, and semiconducting-rich CNTs have higher-measured excited triplet state formation and therefore singlet oxygen ((1)O2) yield. Derivatized SWCNTs were significantly more photoreactive than derivatized MWCNTs. Moreover, addition of sodium chloride resulted in increased aggregation and small increases in (1)O2 production of CNTs. The most photoreactive CNTs exhibited comparable photoreactivity (in terms of (3)CNT* formation and (1)O2 yield) to reference natural organic matter (NOM) under sunlight irradiation with the same mass-based concentration. Selected reference NOM could therefore be useful in evaluating environmental photoreactivity or intended antibacterial applications of CNTs.