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 x Zn 1– x S 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 Zn 2+ and Co 2+ . Density-functional theory calculations of 401 different prototypical structures and compositions spanning the Co x Zn 1– x S 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 Co 2+ vs Zn 2+ 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 x Zn 1– x S increases; x = 0.25 and x = 0.50 are semiconducting, while x = 0.75 is metallic. The optical properties of the Co x Zn 1– x S solid solutions differ from CoS–ZnS heterostructures, which have similar compositions but different mixing behavior. The Co x Zn 1– x S solid solution can also be integrated into heterostructured nanorods to combine their composition-tunable properties with other materials.