Emerging investigator series: correlating phase composition and geometric structure to the colloidal stability of 2D MoS2 nanomaterials

Abstract

Aggregation significantly influences the transport, transformation and bioavailability of engineered nanomaterials in aquatic environments. As one of the most well-studied two-dimensional transition metal dichalcogenide nanomaterials, the colloidal stability of molybdenum disulfide (MoS₂) has been reported under different environmental conditions. Nonetheless, the intrinsic aggregation behavior of this material as influenced by its physicochemical properties is not well understood. This study investigated the correlation of the phase composition and geometric structure of MoS₂ with the aggregation behavior, aggregate stability and transport behavior of the nanomaterials. The results revealed that soft Lewis acid Pb²⁺ exhibits higher destabilization for metallic 1T MoS₂, whereas the semiconducting 2H phase has a higher tendency to aggregate in the presence of hard Lewis acid Ca²⁺. The different colloidal stabilities in response to cations could be well explained by the inner-sphere complexation of Pb²⁺ and outer-sphere interaction of Ca²⁺ with the surface of MoS₂, respectively. With respect to the influences of geometric structure on the colloidal stability, the suspension of flower-like MoS₂ 3D nanoparticles is more stable compared to its 2D nanosheet counterpart, as reflected by the higher critical coagulation concentrations of cations and lower aggregation rate at the same MoS₂ mass concentration. Finally, redispersion and column experiments were conducted to evaluate the stabilities and remigration behavior of MoS₂ aggregates. Deposition of MoS₂ nanomaterials occurred in the presence of cations and the aggregates of MoS₂ 3D nanoparticles were shown to be unstable and susceptible to redispersion/remigration due to the loose aggregate structure. This study comprehensively evaluated the intrinsic aggregation behaviors of engineered MoS₂ nanomaterials with important implications for environmental fate and risk.