De-alloying, i.e. selective dissolution of alloys, is currently studied to produce nanoporous gold items suited for use in catalysis, electrochemical applications, sensors and actuators. Both crystalline and amorphous alloys can be selectively etched. In the former, less noble atoms are removed from surface terraces of grains layer by layer, while noble ones form mounds. These evolve by undercutting and electrolytepercolation to form a ligament network. The mechanism of ligament development by de-alloying amorphous alloys is unknown. Here we show that for de-alloying a Au-based glass, in this case Au 40 Cu 28 Ag 7 Pd 5 Si 20 , percolation of the electrolyte through cracks of the native surface oxide initiates the formation of protuberances which are soon undercut. An interlayer develops, where Au crystals germinate, grow to nanometer size by diffusion and impinge. This is how ligaments start to coarsen. The interlayer is found at all stages between coarsened ligaments and amorphous phase. The ligaments are defective polycrystals, as opposed to single crystals obtained from crystalline alloys.

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enhanced Raman scattering from the NPG was investigated using pyridine and 4,4′-bi-pyridine as probe molecules. It was found that the activity is at maximum when the ribbon is fully de-alloyed although the ligaments then have a larger size. The enhancement is attributed to the small size of crystals in the ligaments, to their morphology and to trapped atoms.

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