<i>In Situ</i> X‑ray Diffraction Study of the Formation, Growth, and Phase Transition of Colloidal Cu_2–<i>x</i>S Nanocrystals

The formation, growth, and phase transition of colloidal monodisperse spherical copper sulfide nanocrystals synthesized in dodecane­thiol have been followed by <i>in situ</i> synchrotron powder X-ray diffraction (PXRD). The formation of nanocrystals involves a thermal decomposition of the crystalline precursor [CuSC₁₂H₂₅], which upon heating forms an isotropic liquid that subsequently turns into colloidal β-chalcocite phase Cu₂S nanocrystals. The redox reaction step in the precursor solution has been studied by proton NMR. Upon heating, high digenite phase nanocrystals are formed through a solid-state rearrangement phase transition of the β-chalcocite phase nanocrystals at temperatures above 260 °C. TEM and PXRD reveal that the nanocrystal size is independent of synthesis temperature and stabilizes after the phase transition has completed. Spherical monodisperse nanocrystals are obtained in all experiments, with the nanocrystals in the β-chalcocite phase (7 nm) being smaller than those in high digenite phase (11 nm)