Transformation of the Anion Sublattice in the Cation-Exchange Synthesis of Au₂S from Cu_2–xS Nanocrystals

Cation exchange is a versatile postsynthetic technique that has been exploited in the synthesis of metastable nanocrystals through preservation of the anion sublattice. Here, we report on the mechanistic details of the synthesis of metastable Au₂S via cation exchange with Cu_2–xS nanocrystals. This conversion requires a transformation of the anion sublattice, from hexagonal close-packed in Cu_2–xS to body-centered cubic in Au₂S, accompanied by an expansion of the unit cell. The ligand environment plays a key role in the driving force of the reaction as the presence of oleylamine allows the conversion to proceed at room temperature, whereas the addition of trioctylphosphine hinders the reaction. By employing transmission electron microscopy (TEM) on faceted nanocrystals and partial cation exchange of nanocrystals, it was demonstrated that the reaction proceeds in a highly directional manner through the pyramidal facets. Since cation exchange produces high-quality nanocrystals as seen through X-ray diffraction and TEM, UV–vis and Raman spectroscopy were used to characterize the optoelectronic properties of the metastable Au₂S nanocrsytals. A Tauc plot analysis revealed a band gap of 2.6 eV, whereas two intrinsic Raman modes were identified at 265 and 329 cm^–1. Density functional theory calculations of structures, energy bands, optical spectra, and phonon spectra were performed and combined with the experimental data to provide additional insights into the characterization of Au₂S nanocrystals.