Solid Solution Formation from Sequential Interfacial Reactions during Nanoparticle Cation Exchange

Nanoparticles that consist of solid solutions formed between two or more end members often exhibit tunable and synergistic properties. The synthesis of solid-solution nanoparticles typically requires simultaneous delivery of multiple reagents with carefully balanced reactivities so that all constituent elements combine uniformly at the same time. Cation exchange reactions provide an alternative postsynthetic strategy for introducing solid solutions into nanoparticles; these solid-solution formation reactions also are assumed to require simultaneous delivery of reagents. Here, we show that solid solution formation during nanoparticle cation exchange can emerge from a series of in situ sequential reactions that occur in a stepwise manner throughout a reaction where reagents are delivered simultaneously. We demonstrate that during simultaneous coexchange of Cu_1.8S nanorods with Ni²⁺ and Co²⁺, a pre-existing interface between roxbyite Cu_1.8S and either nickel sulfide, cobalt sulfide, or a nickel–cobalt sulfide is required to form the nickel–cobalt sulfide solid solution Ni_xCo_9–xS₈. Our data suggest that during simultaneous coexchange of Ni²⁺ and Co²⁺, Ni²⁺ incorporates into a transient and metastable cobalt sulfide phase to stabilize it as Ni_xCo_9–xS₈. We then leverage these insights to selectively synthesize Ni_xCo_9–xS₈ nanorods having a composition gradient and Ni_xCo_9–xS₈ nanorods having uniform compositions throughout. Interfacial reactivity and structural stability can therefore influence cation migration during simultaneous coexchange and ultimately facilitate solid solution formation, leading to the introduction of complex features, including composition gradients, into colloidal nanoparticles.