Temperature-Dependent Selection of Reaction Pathways, Reactive Species, and Products during Postsynthetic Selenization of Copper Sulfide Nanoparticles

Rational design of elaborate, multicomponent nanomaterials is important for the development of many technologies such as optoelectronic devices, photocatalysts, and ion batteries. Combination of metal chalcogenides with different anions, such as in CdS/CdSe structures, is particularly effective for creating heterojunctions with valence band offsets. Seeded growth, often coupled with cation exchange, is commonly used to create various core/shell, dot-in-rod, or multipod geometries. To augment this library of multichalcogenide structures with new geometries, we have developed a method for postsynthetic transformation of copper sulfide nanorods into several different classes of nanoheterostructures containing both copper sulfide and copper selenide. Two distinct temperature-dependent pathways allow us to select from several outcomes─rectangular, faceted Cu_2-xS/Cu_2-xSe core/shell structures, nanorhombuses with a Cu_2-xS core, and triangular deposits of Cu_2-xSe or Cu_2-x(S,Se) solid solutions. These different outcomes arise due to the evolution of the molecular components in solution. At lower temperatures, slow Cu_2-xS dissolution leads to concerted morphology change and Cu_2-xSe deposition, while Se-anion exchange dominates at higher temperatures. We present detailed characterization of these Cu_2-xS-Cu_2-xSe nanoheterostructures by transmission electron microscopy (TEM), powder X-ray diffraction, energy-dispersive X-ray spectroscopy, and scanning TEM-energy-dispersive spectroscopy. Furthermore, we correlate the selenium species present in solution with the roles they play in the temperature dependence of nanoheterostructure formation by comparing the outcomes of the established reaction conditions to use of didecyl diselenide as a transformation precursor.