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The surface area of nanoporous gold films of thickness up to 930 nm and pores tens of nanometers in size was characterized by various electrochemical methods, including Cu underpotential deposition (UPD), surface Au oxidation/reduction reaction, and electrochemical impedance spectroscopy (EIS). The different approaches provide surface area values that are comparable and, in all cases, linearly increasing with thickness. However, whereas Cu UPD and Au oxidation/reduction methods yielded surface area values in good quantitative agreement, estimates based on EIS measurements were found to be 1.5-1.75 times larger. This discrepancy is tentatively attributed to a higher specific capacitance of nanoporous gold, possibly due both to the presence of residual surface Ag and to the high curvature of the nanoporous structures, that may have different interfacial properties as compared to planar surfaces.

Gold sponges consisting of bicontinuous, 3D networks of branched nanowires and tortuous pores are prepared by anodic dissolution of silver from Ag 75 Au 25 alloy sheets in 1 mol L -1 HClO 4 , an electrochemical process occurring under mixed charge transfer and mass transport control. Samples resulting from dissolution are characterized by SEM, which reveals different surface and bulk morphologies, and EIS that allows an estimate of the Au sponge surface area through the measurement of its double layer capacity. This capacity depends linearly on the dissolution charge and attains values of 3-10 F g -1 of gold, increasing for increasing dissolution potentials in the explored range. The relation between typical size of the nanostructures and measured capacity is discussed by reference to a simple geometrical model of the internal wire-like structure. Aging causes some capacity decay, to be attributed to slow coarsening processes.

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