Retrosynthetic Design of Core@Shell Nanoparticles for Their Thermal Conversion to High Entropy Alloy Nanoparticles

In recent years, high-entropy alloy (HEA) nanoparticles (NPs) have attracted a lot of attention as a new class of nanomaterials in the field of catalysis and energy applications due to their tunable compositions, high hardness and strength, high-temperature resistance, and remarkable ductility. The synthesis of HEAs as monodisperse NPs is of high interest to understand how their structure and composition contribute to their function. However, downsizing HEAs to the nanoscale has been synthetically challenging as most colloidal routes to metal NPs rely on the reduction of metal salts, which will occur at different rates depending on the metal precursor. Past work has shown that colloidally prepared monodisperse core@shell NPs can be converted to monodisperse HEA NPs by annealing them at high temperatures. This approach requires fewer metal precursors in each step compared to direct coreduction, facilitating the formation of monodisperse samples with much more control. The initial system considered core@shell PdCu@PtNiM NPs as precursor NPs to HEA NPs, where M = Co, Ir, Rh, Ru, or Fe. These core@shell NPs were prepared by seed-mediated co-reduction (SMCR), and while quite versatile, some metals (e.g., Au and Sn) could not be used in the SMCR process with PdCu seeds. The conversion chemistry not only bounded to PdCu but other bimetallic seeds (e.g. AuCu, PdAg, Pd₃Sn) can also be used. However, consideration must be taken as to which metals go in the core and which metals go in the shell. Herein we propose that the metal precursors for the core and shell of the core@shell NPs synthesis should be judicially chosen according to their relative redox potentials with all the participating metals or metal precursors. We also show that metal precursors with a fast reduction rate that can generate homogenous nucleation in the SMCR step should go in the core of the core@shell NPs. Retrosynthesis is the technique of employing chemical reactions to "deconstruct" a target molecule into easily accessible starting ingredients. Similarly, considering different variables, we may design a retrosynthetic reaction pathway that results in HEA NPs with the targeted metal compositions. More generally, obtaining control over monodisperse HEA NP composition can pave the way for long-lasting catalysts for a variety of processes.