Tunable Noble Metal Thin Films on Ga Alloys via Galvanic Replacement

Room-temperature liquid metals such as GaInSn or EGaIn present the most attractive properties for soft and highly stretchable electronics. Recently, several methods have been investigated to functionalize the surface of the liquid metal via coatings and encapsulation. However, most can hardly be extended to other samples than droplets. In this study, we focus on the tunability of the process of galvanic replacement of Ga alloys with gold to form thin-film encapsulation. We characterized in-depth the obtainable composition and structure of a noble metal shell formed on the liquid metal via scanning electron microscopy, energy-dispersive X-ray, and topographic laser microscopy and highlighted the change in mechanism of galvanic replacement in different pH ranges. We showed the tunability of the surface morphology selection of different pH ranges, the solutions concentrations, and the reaction time. The adjustment of the pH of KAuBr₄ solution to the preferential Ga₂O₃-free domain led to the successful formation of a sub-micrometer thin uniform coating with more than 60% of Au and reduced level of oxygen from 30% down to 10%. We finally demonstrated the effect of the coating composition on the electrical properties of the liquid metal using a simple and fast phase-drop measurement setup on the droplet and microchannels. A high correlation between the amount of noble metal deposited and the electrical properties of the droplets was demonstrated.