Synergistic Effect of Metal Cations and Visible Light on 2D MoS₂ Nanosheet Aggregation

Abstract

Aggregation significantly influences the transport, transformation, and bioavailability of engineered nanomaterials. Two–dimensional MoS2 nanosheets are one of the most well-studied transition-metal dichalcogenide nanomaterials. Nonetheless, the aggregation behavior of this material under environmental conditions is not well understood. Here, we investigated the aggregation of single-layer MoS2 (SL-MoS2) nanosheets under a variety of conditions. Trends in the aggregation of SL-MoS2 are consistent with classical Derjaguin–Landau–Verwey–Overbeek (DLVO) colloidal theory, and the critical coagulation concentrations of cations follow the order of trivalent (Cr3+) < divalent (Ca2+, Mg2+, Cd2+) < monovalent cations (Na+, K+). Notably, Pb2+ and Ag+ destabilize MoS2 nanosheet suspensions much more strongly than do their divalent and monovalent counterparts. This effect is attributable to Lewis soft acid–base interactions of cations with MoS2. Visible light irradiation synergistically promotes the aggregation of SL-MoS2 nanosheets in the presence of cations, which was evident even in the presence of natural organic matter. The light-accelerated aggregation was ascribed to dipole–dipole interactions due to transient surface plasmon oscillation of electrons in the metallic 1T phase, which decrease the aggregation energy barrier. These results reveal the phase-dependent aggregation behaviors of engineered MoS2 nanosheets with important implications for environmental fate and risk.

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

“…The results agree well with the existing reports and could be ascribed to the higher affinity of Pb 2+ than Ca 2+ on the surface of MoS 2 . 18 With respect to the destabilization as a function of MoS 2 phase compositions, the CCCs of Ca 2+ were shown to have a slightly decreasing order of 0.80, 0.76 and 0.74 mM for SL-120, -150 and -200 MoS 2 , respectively, lower than that for SL-pristine (i.e., 0.9 mM) reported in our previous report. 18 The slightly decreased CCCs for the transformed SL-MoS 2 indicated the diminished colloidal stability in the presence of Ca 2+ with the phase transformation from 1T to 2H.…”

“…18 With respect to the destabilization as a function of MoS 2 phase compositions, the CCCs of Ca 2+ were shown to have a slightly decreasing order of 0.80, 0.76 and 0.74 mM for SL-120, -150 and -200 MoS 2 , respectively, lower than that for SL-pristine (i.e., 0.9 mM) reported in our previous report. 18 The slightly decreased CCCs for the transformed SL-MoS 2 indicated the diminished colloidal stability in the presence of Ca 2+ with the phase transformation from 1T to 2H. This result could be ascribed to the diminished zeta potentials and thus decreased electrostatic repulsion between nanosheets.…”

“…, 0.9 mM) reported in our previous report. 18 The slightly decreased CCCs for the transformed SL-MoS 2 indicated the diminished colloidal stability in the presence of Ca 2+ with the phase transformation from 1T to 2H. This result could be ascribed to the diminished zeta potentials and thus decreased electrostatic repulsion between nanosheets.…”

“…The results agree well with the existing reports and could be ascribed to the higher affinity of Pb 2+ than Ca 2+ on the surface of MoS 2 . 18…”

“…[15][16][17] It was demonstrated that the presence of cations or an extremely acidic environment would destabilize MoS 2 suspensions through charge screening or compression of the electronic double layer (EDL), and visible light illumination tends to promote the process of sedimentation. 18 However, correlations of physicochemical properties (such as chemical composition, morphology, etc.) with the colloidal stability of MoS 2 are not well understood.…”