Recrystallisation-accompanied phase separation in Ag-Fe and Ag-Ni nanocomposites: a route to structure tailoring of nanoporous silver

We present, for the first time, a preparation method of nanoporous silver by phase separation in nanocomposite Ag-Fe and Ag-Ni systems, which are metal pairs immiscible in both liquid and solid states. Conveniently using high-energy mechanical milling, we achieve uniform mixing of the metal grains at the nanoscale in the mixtures containing 50 vol.% Ag. When Fe and Ni grains dissolve in hydrochloric acid, they leave behind a nanoporous silver skeleton, in which the size of coherently scattering domains is larger than that in the starting nanocomposite before the phase separation, while the microstrain is reduced. We explain these effects by recrystallisation of silver through the dissolution-precipitation mechanism enabled by oxidation of silver by oxygen of ambient air and the concomitant formation of HAgCl2 in the solution followed by its reduction by metallic Fe or Ni in a galvanic displacement reaction. The originality of the proposed synthesis is in the dual functionality of the Fe and Ni sacrificial phases acting also as reducing agents and opening up a new mechanism of microstructure tailoring of nanoporous silver. Selective dissolution of Spark Plasma Sintered Ag-Fe and Ag-Ni compacts retaining silver nanograins can be used to produce nanoporous silver membranes.