Exploring the Parameter Space of Colloidal High‐Entropy Alloy Synthesis Methods

High‐entropy alloy (HEA) nanoparticles (NPs) have been subject to increasing interest as prospective catalyst materials. However, highly controllable synthesis methods for HEA NPs remain scarce. To address this issue, we present a systematic study of the synthesis parameter space in a simple colloidal synthesis approach where IrPdPtRuRh NPs were synthesised in triethylene glycol. We show that ultra‐small (1–3 nm), single‐phase HEA NPs can be synthesised through a hot‐injection synthesis approach. In contrast, multiple phases are observed when a one‐pot synthesis is carried out. We also show that the particle sizes and morphologies are affected by the choice of metal precursor counterion, and further investigate the dependence of metal reduction on the reaction temperature. Additionally, we present the synthesis of mixed noble/non‐noble‐metal IrPdPtRuNi and IrPdPtRuCo HEA NPs. By carrying out reduction kinetics experiments, we relate phase separation and compositional gradients of some of the synthesised samples to large differences in the reduction rates of the reacting metals. The individual metal reduction rates are found to vary depending on which other elements are present during particle formation. These results emphasise the importance of understanding the chemical landscape during reaction for the controlled synthesis of HEA NPs.