Dealloying Silver/Gold Alloys in Neutral Silver Nitrate Solution: Porosity Evolution, Surface Composition, and Surface Oxides

The electrochemistry of dealloying silver/gold alloys in neutral pH silver nitrate solution to form nanoporous gold (NPG) is discussed. At pH 7, porosity evolution was observed to occur at high potentials, above that required for oxygen evolution, and within the nominal domain of the Pourbaix diagram where silver would be expected to form a passivating oxide. Electron microscopy shows that a small pore (∼5 nm) NPG is formed over a potential regime of 1.3-2.0 V vs normal hydrogen electrode, but electrochemical measurements show that the specific capacitance of samples over the same voltage range rises nearly threefold. The observations are explained in terms of residual surface oxides passivating the pores behind the dissolution front, which is itself acidified (and thus corrosive) due to an accumulation of protons associated with oxide formation and water dissociation. A model is proposed that is consistent with the electrochemical and microscopy results. This method of fabricating NPG has advantages of simplicity and safety, and the porosity formation mechanism may be extended to other systems.