Simple one-pot synthesis of Cu₄SnS₄ nanoplates and temperature-induced phase transformation mechanism

A simple one-pot heat-up method has been developed to synthesize Cu₄SnS₄ nanoplates with highly exposed (002) surfaces. Temperature-dependent phase transformation from monoclinic Cu_1.94S to orthorhombic Cu₄SnS₄, and eventually to partially triclinic Cu₂SnS₃ has been observed. It is revealed that the Sn⁴⁺ existing in the laminar structure of Cu_1.94S nanoplates starts being incorporated into the Cu_1.94S crystal structure at a temperature of around 220 °C, and form pure orthorhombic Cu₄SnS₄ nanoplates at 260 °C. As the orthorhombic structure of Cu₄SnS₄ is metastable, it undergoes a phase transformation to the more stable Cu₂SnS₃ phase at a higher reaction temperature of 270 °C. Furthermore, the size and thickness of Cu₄SnS₄ nanoplates can be well tuned by the amount of Sn content in the initial reaction mixture to form Cu_1.94S nanoplates. Additionally, the formation of Cu_1.94S nanoplates is essential for the formation of pure Cu₄SnS₄ nanocrystals at a fixed reaction temperature. UV-vis and Mott-Schottky analysis show that the as-prepared orthorhombic Cu₄SnS₄ nanoplates exhibit n-type semiconducting behavior with a band gap of about 1.65 eV. The valence and conduction band potentials have also been determined via theoretical calculations. We believe that our synthetic method and findings will be attractive for the synthesis of other copper-based chalcogenide nanoplates as well as their potential catalytic applications.