Reversible Crystal Phase Interconversion between Covellite CuS and High Chalcocite Cu2S Nanocrystals

Copper deficient copper sulfides (Cu2-xS, 0 <= x <= 1) are earth abundant, nontoxic materials with size-, phase-, and composition-dependent localized surface plasmon resonance (LSPR). Although synthesis of Cu2-xS nanocrystals (NCs) has attracted substantial research attention, understanding of the transformations of copper sulfides between their many possible stoichiometries and crystal phases is still lacking. Here, we develop a reversible transformation between CuS, which has a high density of free charge carriers and strong LSPR, and high chalcocite Cu2S with no LSPR. Initial CuS nanoplatelets (NPls) with a diameter of 55 nm and thickness of 4 rut were transformed into round high chalcocite Cu2S NPls With a diameter of 29.2 +/- 2.0 mu and a thickness of 10.8 +/- 0.7 nm by treatment with 1-dodecanethiol (DDT), which can reduce disulfide bonds in covellite. Treatment with an oleic acid sulfur complex (OA S), which serves as a sulfur source, can restore the hexagonal shape of the original CuS NPlS during the reverse transformation from Cu2S to CuS, producing hexagonal NPls with a diameter of 43.1 +/- 2.0 nm and thickness of 11.2 +/- 0.9 nm. We also treated monodisperse, spherical roxbyite (Cu1.78S) NCs with OA-S and Obtained hexagonal CuS NPls, showing that the tendency of CuS to form hexagonal NPls is an intrinsic result of its crystal structure. For comparison, we used different sulfur precursors to drive the transformation from Cu2S to CuS, illustrating the different reactivities of S-sources with Cu2S. This interconversion not only provides a better understanding of possible transformations in copper sulfide nanostructures but also provides new possibilities for the well-controlled colloidal synthesis of these nanomaterials with combinations of phase, size, shape, and LSPR energy not previously obtainable.