Morphology-Dependent Phase Selectivity of Cobalt Sulfide during Nanoparticle Cation Exchange Reactions

Achieving phase selectivity during nanoparticle synthesis is important because crystal structure and composition influence reactivity, growth, and properties. Cation exchange provides a pathway for targeting desired phases by modifying composition while maintaining crystal structure. However, our understanding of how to selectively target different phases in the same system is limited. Here, we demonstrate morphology-dependent phase selectivity for wurtzite (wz) CoS, which is hcp, vs pentlandite Co₉S₈, which is ccp, during Co²⁺ exchange of roxbyite Cu_1.8S plates, spheres, and rods. The plates form wz-CoS, the spheres form both wz-CoS and Co₉S₈, and the rods form Co₉S₈. The plates, spheres, and rods have nearly identical widths but increase in length in the direction that the close-packed planes stack, which influences the ability of the anions to shift from hcp to ccp during cation exchange. This morphology-dependent behavior, which correlates with the number of stacked close-packed planes, relies on an anion sublattice rearrangement that is concomitant with cation exchange, thereby providing a unique pathway by which crystal structure can be controlled and phase selectivity can be achieved during nanocrystal cation exchange.