Cracking Shells and Scrambling Eggs: Intermediate Shell Formation and Anion Rearrangement in the Cation Exchange from π‑SnS to Cu1.8S

Cation exchange is used to achieve products of complex morphology, phase, or elemental compositions from host particles with similar features. Understanding which parameters are responsible for preserving or altering these features is key to predicting novel cation exchange products. Here we demonstrate cation exchange of cubic π-SnS to pseudohexagonal roxbyite (Cu1.8S) as confirmed by powder X-ray diffraction, transmission electron microscopy, and energy dispersion spectroscopy mapping. TEM/EDS shows the initial formation of a shell of amorphous Cu-Sn-S. It is only upon elevated temperatures that the shells are breached on one facet and complete cation exchange to a copper sulfide core occurs. Mismatched diffusion rates between the outgoing Sn2+ and the incoming Cu+ cause the formation of voids in the cation exchanged nanocuboids, a documented result of the Kirkendall effect. These mismatched rates are implicated in the change of the sulfide lattice from pseudo-fcc to pseudo-hcp. The prepared nanocrystals were studied as photoabsorbers in quantum dot sensitized solar cells (QDSSCs), and it was found that the shelling of the π-SnS with the amorphous Sn-Cu-S does not greatly change the cell characteristics or performance, indicating that the voltage limiting defect in π-SnS solar cell designs is in the core, not the surface of the material.