Molybdenum disulfide (MoS2) nanosheets have been used as photothermal agents and photocatalysts in water treatment, exploiting their engineerable band structures and broad light absorption over the solar spectrum. Because the bulk form of MoS2 is chemically stable and has low solubility owing to its saturated coordination at the basal surface, MoS2 nanosheets have been considered to be stable in aquatic environments. However, compared to the bulk form, thin MoS2 nanosheets have more defects exposed to aqueous media, which can kinetically promote their oxidative dissolution. Furthermore, under sunlight irradiation, MoS2 nanosheets exhibit different photochemical reactivity from the bulk form of MoS2, owing to their altered band structures. To evaluate the stability of MoS2 nanosheets and dependency of their dissolution on their thickness, we synthesized 18 nm and 46 nm thick MoS2 nanosheets through ultra-sonication. Under simulated sunlight irradiation, MoS2 nanosheets were dissolved faster than those in the dark, with a lower solution pH and higher Mo (VI) composition in the MoS2 particles. Moreover, 18 nm thick MoS2 nanosheets showed higher dissolution rates than 46 nm thick and bulk MoS2, owing to promoted reactive oxygen species generation. These findings highlight the importance of the thickness of MoS2 nanosheets in their photochemical transformation, which influences their physicochemical properties and consequently alters their behavior once they are released into environmental systems.

Photochemically-induced oxidative dissolution of MoS2 nanosheets and its thickness dependency

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