High entropy materials, which contain a large number of randomly distributed elements, have unique catalytic, electrochemical, and mechanical properties. The high configurational entropy of the randomized elements drives the formation of high entropy materials; therefore, high temperatures and quenching are typically required to stabilize them. Because of this, colloidal nanoparticles of high entropy materials are difficult to synthesize and remain rare, despite their desirable high surface areas and solution dispersibilities. Here, we introduce simultaneous multication exchange as an alternative low-temperature pathway to colloidal nanoparticles of high entropy materials. Roxbyite Cu1.8S nanoparticles react with a substoichiometric mixture of Zn2+, Co2+, In3+, and Ga3+ to produce nanoparticles of the high entropy metal sulfide Zn0.25Co0.22Cu0.28In0.16Ga0.11S. The Zn0.25Co0.22Cu0.28In0.16Ga0.11S nanoparticles are thermally stable, and exchange reactions using fewer cations do not produce the high entropy phase. The use of colloidal nanoparticle cation exchange as a synthetic platform provides both entropic and enthalpic driving forces that, in addition to configurational entropy, enable the formation of high entropy phases at solution-accessible temperatures.

Nanocrystal cation exchange is a post-synthetic process that modifies the composition of a nanoparticle while maintaining other important characteristics, including morphology and crystal structure. Partial cation exchange reactions can be used to rationally synthesize heterostructured nanoparticles that contain two or more material segments. Increasingly complex heterostructured nanoparticles are accessible using multiple sequential cation exchange reactions, but achieving targeted structures in high yield requires careful consideration of synthetic parameters and chemical reactivity. Here, we discuss in detail the synthetic protocols used in two distinct partial cation exchange pathways that are differentiated based on the relative amounts of metal salt reagentsexcess vs stoichiometricthat are used during the reaction. Using a model system obtained through Zn 2+ exchange on roxbyite copper sulfide nanorods, we demonstrate how targeted products can be synthesized reproducibly. We show how small deviations in reaction conditions, such as temperature, time, and particle concentration, can significantly impact the outcome of these reactions. We highlight important chemical and physical hazards, issues that can be encountered when characterizing heterostructured nanoparticles, and troubleshooting suggestions for overcoming commonly encountered pitfalls. Clear and detailed descriptions of these aspects of partial cation exchange reactions are important for enabling widespread reproducibly and further development of the field.

Nanoparticles of high entropy alloys (HEAs) have distinct properties that result from their high surface-tovolume ratios coupled with synergistic interactions among their five or more constituent elements, which are randomly distributed throughout a crystalline lattice. Methods to synthesize HEA nanoparticles are emerging, including solution approaches that yield colloidal products. However, the complex multielement compositions of HEA nanoparticles make it challenging to identify and understand their reaction chemistry and the pathways by which they form, which hinders their rational synthesis. Here, we demonstrate the synthesis and elucidate the reaction pathways of seven colloidal HEA nanoparticle systems that contain various combinations of noble metals (Pd, Pt, Rh, Ir), 3d transition metals (Ni, Fe, Co), and a p-block element (Sn). The nanoparticles were synthesized by slowly injecting a solution containing all five constituent metal salts into oleylamine and octadecene at 275 °C. Using NiPdPtRhIr as a lead system, we confirmed the homogeneous colocalization of all five elements and achieved tunable compositions by varying their ratios. We also observed heterogeneities, including Pd-rich regions, in a subpopulation of the NiPdPtRhIr sample. Halting the reaction at early time points and characterizing the isolated products revealed a timedependent composition evolution from Pd-rich NiPd seeds to the final NiPdPtRhIr HEA. Similar reactions applied to FePdPtRhIr, CoPdPtRhIr, NiFePdPtIr, and NiFeCoPdPt, with modified conditions to most efficiently incorporate all five elements into each HEA, also revealed similar Pd-rich seeds with system-dependent differences in the rates and sequences of element uptake into the nanoparticles. When moving to SnPdPtRhIr and NiSnPdPtIr, the time-dependent formation pathway was more consistent with simultaneous coreduction rather than through formation of reactive seeds. These studies reveal important similarities and differences among the pathways by which different colloidal HEA nanoparticles form using the same synthetic method, as well as establish generality. The results provide guidelines for incorporating a range of different elements into HEA nanoparticles, ultimately providing fundamental knowledge about how to define and optimize synthetic protocols, expand into different HEA nanoparticle systems, and achieve high phase purity.

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 Cu1.8S 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.

Nanoparticles of copper sulfide, including roxbyite Cu 1.8 S, are important materials for many applications, and they also serve as versatile templates for cation exchange reactions that transform them into derivative metal sulfide compounds and complex heterostructures. The sizes and shapes of roxbyite nanoparticles are generally determined during synthesis, and their morphologies are retained during postsynthetic modifications such as cation exchange. Here, we demonstrate postsynthetic morphological modification of roxbyite nanoparticles by treating them with 1-dodecanethiol (1-DDT) and tert-dodecanethiol (t-DDT) at temperatures ranging from 90 to 160 °C. These thiols, which are typically used as ligands and/or sulfur reagents in nanoparticle synthesis, induce morphological reshaping while maintaining composition, crystal structure, and particle volume. For example, 56 nm × 21 nm roxbyite nanorods transform to 32 nm spherical particles in the presence of 1-DDT at 130 °C for two or more hours. The nanorods progressively decrease in length and increase in width, forming a series of ellipsoids having tunable aspect ratios at intermediate time points. Control experiments point to a single-crystal-to-single-crystal pathway that involves material diffusion and migration, which can be accelerated by increasing the density of cation vacancies in the nanoparticles. Because of this pathway, the thiol-induced morphology changes are selective to the copper sulfide regions of heterostructured nanorods containing roxbyite and ZnS, Co 9 S 8 , or CuInS 2 made using partial cation exchange reactions, providing access to a library of derivative nanoparticles having otherwise inaccessible morphologies.