Ligand exchange induced crystal structure and morphology evolution of copper-tin-sulfur binary and ternary compounds

In this manuscript, a simple one-pot heat-up method has been used to prepare multi-component copper-tin-sulfur nanomaterials, including binary Cu_1.94S, ternary Cu₄SnS₄, and Cu_1.94S/Cu₄SnS₄ nanocrystals by varying the reaction temperature, reaction time, and the type of copper source. Post-synthetic ligand exchange (LE) has further been introduced to replace the long-chain ligands originating from 1-dodecanethiol. It has been found that the LE process not only changes the surface ligands but also significantly affects the crystal structure and optical properties of nanocrystals. After LE, the crystal structures of Cu_1.94S and Cu₄SnS₄ transformed to Cu₇S₄ and Cu₃SnS₄, respectively, with the Cu_1.94S/Cu₄SnS₄ nanocrystals showing the same trend. This phenomenon could be ascribed to the loss of Cu⁺ originating from the strong complexation of Cu⁺ and ammonia with the formation of [Cu(NH₃)_n]²⁺ ions under aerobic conditions. Proton nuclear magnetic resonance (¹H NMR) has been used to characterize the ligands on the surface before and after LE, which further demonstrated that the -SH was replaced during LE. Meanwhile, the band gaps of the obtained nanocrystals after LE show an obvious shift in the near-infrared region due to the evolution of crystal structures. This study will provide useful guidance for the LE of nanocrystals and the application of copper-based sulfide nanomaterials in optoelectronics and other fields.