Complex alloys composed of 5 or more equimolar components may be classified as “high-entropy” due an elevated mixing entropy predicted by the Gibbs free energy equation exceeding 1.61R, where R is the universal gas constant. Whereas most metal alloys tend to segregate into intermetallic phases, the high-entropy state permits these materials to exist in unique atomic arrangements such as solid-solution and amorphous microstructures. High-entropy alloys have tunable physicochemical characteristics based on the metals incorporated, suggesting that favorable properties may be obtained if the microstructure can be controlled. Most research into high-entropy materials has focused on bulk metals with a solid-solution microstructure. Conversely, little has been accomplished towards the synthesis high-entropy alloy nanomaterials, which may exhibit markedly different properties than the bulk materials. Here, we demonstrate microstructural tuning of high-entropy materials, which is readily achieved for the conversion of bulk solid-solution phases to bulk intermetallic phases via thermal annealing. Starting with the most disordered microstructure (amorphous) and annealing to produce more ordered structures (solid-solution and intermetallic) is demonstrated to compare and contrast the properties these atomic arrangements exhibit for the first time. This method exploits the sub-femtoliter volumes of water nanodroplets to confine an electrodeposition process whereby stoichiometrically controlled nanoparticles may be formed. By combining this synthetic technique with phase diagram-informed annealing procedures, unique high-entropy nanomaterials may be formed with tunable catalytic activity for next-generation energy generation and harvesting technologies.

Tuning microstructural aspects of high-entropy alloy nanomaterials to elucidate electrocatalytic structure/activity relationships

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