Fig. 4.

Ion-functional group interactions. (A) The effect of different metal cations on the IR absorption spectra of the carboxylate group in the MoS₂-COOH membranes. The dashed lines are the corresponding negative second-order derivatives of the absorption spectra (solid lines) for peak pinpointing. (B) The peak frequency of the coordinated COO⁻--Mⁿ⁺ complexes extracted from A. (C) Schematic illustrations of the monodentate and bidentate coordination mode between metal cations and carboxylate group.