(Invited) Electrochemical Deposition of Ag- or Au-Based Alloy Films

Abstract

Precious metal-based alloy coatings are of interest for both decorative and technological applications, the latter including electrocatalysis, electrical contacts, and plasmonic devices among others [1,2]. The ternary system Au-Ag-Cu in particular provides a playground to finely tune optical and other functional properties. Synthesis of these alloys by electrodeposition further enhances the ability to tune these properties, since metastable phase configurations can be realized by varying the deposition kinetics. Au-Cu alloys have been deposited by simple and complexing electrolytes; alloy composition could be predicted based on the alloy enthalpy of mixing and the microstructure could be controlled through electrolyte chemistry. In particular, the relative nobility of the two components could be switched via selective complexation, resulting in distinct atomic configurations. Ag-Cu alloys were also electrodeposited from a simple and a thiourea complexing electrolyte. Deposition from the non-complexing solution shows a lack of luster; the two metals are reduced independently, resulting in phase separation, with limited or no mutual solubility. The thiourea electrolyte on the other hand is capable to form shiny films. The crystal structure in this case shows a gradual variation of lattice constant, suggesting the formation of a continuous series of solid solutions. Electron microscopy however was unable to distinguish between a solid solution or eutectic formation. Metal complexation and tuning of the deposition rate in this case results in a kinetically induced metastable structure. Finally, Au or Ag alloys with Ni exhibit in the bulk a miscibility gap; electrodeposited films in contrast form apparent solid solutions that are generated by various kinetic phenomena, including kinetic trapping, spinodal decomposition and growth instabilities [3].