Sulfur Precursor Conversion Pathways in Phase Control of Cu_2-xS Nanocrystals

In the Cu-S family, it is of great importance to advance the phase control of the Cu_2-xS nanocrystals (NCs) for a given application. In this regard, recently, a large library of sulfur precursors have been explored in the colloidal syntheses. Nevertheless, the links between their conversion pathways and the phase of the resulting Cu_2-xS NCs are rarely established. Herein, we unravel the conversion pathways of thiourea (TU) heated in an oleylamine (OAm) solvent and identify the reactive sulfur species. The pyrolysis of primary TU produced intermediate CS₂, and it converted into ultimate reactive sulfur species H₂S gas and N,N'-disubstituted TU via the reaction with OAm. These two sulfur species are linked to the phase determination of cubic-phase digenite Cu_1.8S NCs and hexagonal-phase djurleite Cu_1.94S NCs, respectively. Furthermore, the above TU-OAm solutions are utilized to the polymorphic synthesis of Cu_2-xS-derivative Cu-In-S and Cu-Sn-S NCs with a cubic zinc blende phase and a hexagonal wurtzite phase. Therefore, our findings in the present study suggest an urgency on unraveling the conversion pathways of primary sulfur precursors in organic solvents, holding great promise in the synthesis of other metal sulfides with phase-related functionalities.