Synthesis of tough nanoporous metals by controlled electrolytic dealloying

Dealloying (selective dissolution) of homogeneous metallic alloys such as Ag-Au generates nanoporous metals with intriguing properties. Until now, it has not been possible routinely to prepare such materials in a form in which they would resist mechanical strains, such as might be encountered in applications as membranes, sensors or actuators. Now a threshold potential has been identified, for dealloying of Ag-Au in aqueous HClO4, above which spontaneous transgranular fracture occurs due to the formation of a monolayer of gold hydroxide. The fracture mechanism involves a reduction in surface diffusivity, preventing relaxation of induced tension in the porous material, and possible more exotic effects. By optimizing the Au content of the alloy, staying below the threshold potential and increasing the temperature of the electrolyte, dealloyed membranes with a high degree of mechanical integrity have been prepared. When prepared in this way, the nanoporous material shows peculiar behaviours, such as grain-boundary sintering under annealing, and reversible stiffening and embrittlement when dried in air.