Smooth Thin Film of a CoNiCu Medium-Entropy Alloy Consisting of Single Nanometer-Sized Grains Formed by Electrodeposition in a Water-in-Oil Emulsion

High-entropy and medium-entropy alloys are new classes of alloys that exhibit excellent strength and toughness. These alloys are expected to be used as multifunctional catalysts given that methods for producing nanoparticles of such alloys have been established. By contrast, the production methods to form thin films of such alloys are still very limited. Here, we show that electrodeposition in a chemically stabilized water-in-oil emulsion functions as a general method to form thin films of such alloys. The key to forming thin films of these alloys by electrodeposition is to conduct reduction under diffusion-limited conditions and to confine the distribution of concentration in a droplet. This situation is not achieved in conventional bulk aqueous or nonaqueous single-phase solution but is achieved in a water-in-oil emulsion. Here, we found aerosol OT as a suitable surfactant for the present water-in-oil emulsion to stabilize the emulsion during electrodeposition. It is also clarified that the increase in viscosity of the aqueous solution within water droplets by the addition of glycerol decreases the roughness greatly. The strategy demonstrated here is expected to spur further applied research into thin films of high- and medium-entropy alloys.