Metal Sulfide Ion Exchangers: High Acid Stability of Na_2xMg_2y-xSn_4-yS₈ (NMS) and Topotactic Conversion to 2D Solid Acids with Semiconducting Character

Metal sulfide ion exchange materials (MSIEs) are of interest for nuclear waste remediation applications. We report the high stability of two structurally related metal sulfide ion exchange materials, Na_2xMg_2y-xSn_4-yS₈ (Mg-NMS) and Na₂SnS₃ (Na-NMS), in strongly acid media, in addition to the preparation of Na_2xNi_2y-xSn_4-yS₈ (Ni-NMS). Their formation progress during synthesis is studied with in-situ methods, with the target phases appearing in <15 min, reaction completion in <12 h, and high yields (75-80%). Upon contact with nitric or hydrochloric acid, these materials topotactically exchange Na⁺ for H⁺, proceeding in a stepwise protonation pathway for Na_5.33Sn_2.67S₈. Na-NMS is stable in 2 M HNO₃ and Mg-NMS is stable in 4 M HNO₃ for up to 4 h, while both NMS materials are stable in 6 M HCl for up to 4 days. However, the treatment of Mg-NMS and Na-NMS with 2-6 M H₂SO₄ reveals a much slower protonation process since after 4 h of contact both NMS and HMS are present in the solution. The resultant protonated materials, H_2xMg_2y-xSn_4-yS₈ and H_4x[(H_yNa_y-1)_1.33xSn_4-1.33x]S₈, are themselves solid acids and readily react with and intercalate a variety of organic amines, where the band gap of the resultant adduct is influenced by amine choice and can be tuned within the range of 1.88(5)-2.27(5) eV. The work function energy values for all materials were extracted from photoemission yield spectroscopy in air (PYSA) measurements and range from 5.47 (2) to 5.76 (2) eV, and the relative band alignments of the materials are discussed. DFT calculations suggest that the electronic structure of Na₂MgSn₃S₈ and H₂MgSn₃S₈ makes them indirect gap semiconductors with multi-valley band edges, with carriers confined to the [MgSn₃S₈]^2- layers. Light electron effective masses indicate high electron mobilities.