Interaction of Pb²⁺ ions in water with two-dimensional molybdenum disulfide

The removal of heavy metal contaminants from water is important for public health, and recently many two-dimensional (2D) materials with high specific surface areas are being studied as promising new active components in water purification. In particular, 2D MoS₂ nanosheets have been used for the removal of various heavy metals, but usually in either in complex geometries and composites, or in the chemically exfoliated metallic 1T-MoS₂ phase. However, the interaction of heavy metals dissolved in water with unmodified semiconducting 2H-MoS₂ is not well studied. In this paper, we report a detailed fundamental investigation of how Pb²⁺ ions interact with 2H-MoS₂. We observe small solid clusters that form on the MoS₂ surfaces after exposing them to Pb(NO₃)₂ aqueous solutions as shown by atomic force microscopy and transmission electron microscopy, and for liquid phase exfoliated MoS₂ we observe the nanosheets precipitating out of dispersion along with insoluble solid granules. We use a combination of x-ray photoelectron spectroscopy and x-ray diffraction to identify these solid clusters and granules as primarily PbSO₄ with some PbMoO₄. We put forth an interaction mechanism that involves MoS₂ defects acting as initiation sites for the partial dissolution in aqueous oxygenated conditions which produces MoO₄²⁻ and SO₄²⁻ ions to form the solids with Pb²⁺. These results are an important contribution to our fundamental understanding of how MoS₂ interacts with metal ions and will influence further efforts to exploit MoS₂ for water remediation applications.