K[Bi_4-MnS₆], Design of a Highly Selective Ion Exchange Material and Direct Gap 2D Semiconductor

Layered sulfides with high selectivity for binding heavy metal ions and radionuclide ions are promising materials in effluent treatment and water purification. Here we present a rationally designed layered sulfide K[Bi_4-MnS₆] (x = 1.28) deriving from the Bi₂Se₃-structure type by targeted substitution to generate quintuple [Bi_4-MnS₆] layers and K⁺ cations between them. The material has dual functionality: it is an attractive semiconductor with a bandgap of 1.40 eV and also an environmental remediation ion-exchange material. The compound is paramagnetic, and optical adsorption spectroscopy and DFT electronic structure calculations reveal that it possesses a direct band gap and a work function of 5.26 eV. The K⁺ ions exchange readily with alkali or alkaline-earth ions (Rb⁺, Cs⁺, and Sr²⁺) or soft ions (Pb²⁺, Cd²⁺, Cr³⁺, and Zn²⁺). Furthermore, when the K⁺ ions are depleted the Mn²⁺ ions in the Bi₂Se₃-type slabs can also be replaced by soft ions, achieving large adsorption capacities. The ion exchange reactions of K[Bi_4-MnS₆] can be used to create new materials of the type M[Bi_4-MnS₆] in a low temperature kinetically controlled manner with significantly different electronic structures. The K[Bi_4-MnS₆] (x = 1.28) exhibits efficient capture of Cd²⁺ and Pb²⁺ ions with high distribution coefficient, K_d (10⁷ mL/g), and exchange capacities of 221.2 and 342.4 mg/g, respectively. The material exhibits excellent capacities even in high concentration of competitive ions and over a broad pH range (2.5-11.0). The results highlight the promise of the K[Bi_4-MnS₆] (x = 1.28) phase to serve not only as a highly selective adsorbent for industrial and nuclear wastewater but also as a magnetic 2D semiconductor for optoelectronic applications.