Nickel sulfide and copper sulfide nanocrystal synthesis and polymorphism

Nickel sulfide and copper sulfide nanocrystals were synthesized by adding elemental sulfur to either dichlorobenzene-solvated (copper sulfide) or oleylamine-solvated metal(II) precursors (nickel sulfide) at relatively high temperature to produce the metal sulfide. Nickel sulfide nanocrystals are cubic Ni3S4 (polydymite) with irregular prismatic shapes, forming by a two-step reduction-sulfidation mechanism where Ni(II) reduces to Ni metal before sulfidation to Ni3S4. Despite extensive efforts to optimize the Ni3S4 nanocrystal size and shape distributions, polydisperse nanocrystals are produced. In contrast, copper sulfide nanocrystals can be obtained with narrow size and shape distributions. The copper sulfide stoichiometry depended on the Cu:S mole ratio used in the reaction: Cu:S mole ratios of 1:2 and 2:1 gave CuS (covellite) and Cu1.8S (digenite), respectively. CuS nanocrystals formed as hexagonal disks that assemble into stacked ribbons when cast from solution onto a substrate. CuS, Cu1.8S, and Ni3S4 differ from the Cu2S and NiS nanocrystals obtained by solventless decomposition of metal thiolate single source precursors, in terms of stoichiometry for copper sulfide, and both stoichiometry and morphology for nickel sulfide.