Chemical Insights into the Formation of Metastable Zinc Cobalt Sulfide Solid-Solution Nanoparticles through Simultaneous Multi-Cation Exchange

Nanoparticle materials that consist of a solid solution between two end member compounds often have composition-dependent physical properties. Their synthesis can be challenging, as it requires balancing the competing reactivities of many different reagents to favor the formation of a single-phase product rather than a phase-segregated mixture of its end members. Here, we provide chemical insights into the synthesis of wurtzite Co_xZn_1-xS nanoparticle spheres, rods, and plates for x = 0.25, 0.50, and 0.75, which represent solid solutions of CoS and ZnS, by simultaneously exchanging the Cu⁺ cations in roxbyite copper sulfide for Zn²⁺ and Co²⁺. Density-functional theory calculations of 401 different prototypical structures and compositions spanning the Co_xZn_1-xS solid solution space confirm that they are metastable with positive mixing enthalpies and formation energies that are 100-150 meV per formula unit above the convex hull. Competition experiments reveal preferential exchange of Co²⁺ vs Zn²⁺ when both are present in excess. We balance their reactivities by controlling the ratio of total cations to copper sulfide, thereby avoiding the formation of cobalt sulfide byproducts. UV-vis-NIR absorption spectra reveal a decrease in band gap as x in Co_xZn_1-xS increases; x = 0.25 and x = 0.50 are semiconducting, while x = 0.75 is metallic. The optical properties of the Co_xZn_1-xS solid solutions differ from CoS-ZnS heterostructures, which have similar compositions but different mixing behavior. The Co_xZn_1-xS solid solution can also be integrated into heterostructured nanorods to combine their composition-tunable properties with other materials.