Dealloying of Ag-Au Alloys in Halide-Containing Electrolytes

Abstract

We present the results of dealloying studies of Ag 0.7 Au 0.3 and Ag 0.65 Au 0.35 alloys in 0.1 M HClO 4 with the addition of either 0.1 M KCl, 0.1 M KBr, or 0.1 M KI. The critical overpotential decreases with the addition of halides with KI having the largest potential reduction of almost 50%. This decrease is discussed with respect to a competition between the rates of increase of Au surface diffusivity and Ag exchange current density with halide additions. The size scale of porosity produced during the dealloying of Ag 0.65 Au 0.35 in the above electrolytes was found to increase with the addition of halides. Without the addition of halides, a pore size of approximately 8 nm is produced while 17, 16, and 67 nm is measured in the KCl-, KBr-, and KI-containing electrolytes. The value of surface diffusivity required to coarsen the dealloyed structure to these size scales has been calculated to be 2 ϫ 10 Ϫ16 cm 2 s Ϫ1 ͑0.1 M HClO 4 ), 3 ϫ 10 Ϫ15 cm 2 s Ϫ1 ͑KBr or KCl͒, and 8 ϫ 10 Ϫ13 cm 2 s Ϫ1 ͑KI͒.Dealloying is a corrosion process in which one or more elements are selectively removed leaving behind a bicontinuous porous residue of the remaining element͑s͒. This bicontinuous metal-void structure is highly brittle in nature and has been linked to stress corrosion cracking in many alloy systems ͑see, for example Ref. 1-6͒. Dealloying has been observed in numerous systems including the systems Cu-and even during the reduction of titanium dioxide in molten calcium chloride. 24,25 This is an illustrative rather than an exhaustive list as it appears that dealloying can be electrochemically driven in any system where a large electrochemical potential difference exists between the alloying elements. Beyond its direct relevance to stress corrosion cracking, dealloying has been linked to the accelerated corrosion of Al2024-T3, 26,27 the corrosion of austenitic stainless steel in acidified chloride-containing electrolyte, 2,28 and the production of Raney metal catalysts. 29,30 New Pt-based alloy systems have been investigated by the authors with the motivation of producing nanoporous metals for various highsurface area electrode applications particularly in the biomedical area. 31 Not until 1980 was the first detailed micromorphological study of alloy dissolution reported. 18 This study was conducted by Forty et al. for the dealloying of Ag from Ag-Au alloys in nitric acid. It is now well recognized that the morphology of dealloyed structures consist of bicontinuous metal-void phases in what is loosely referred to as a sponge-like structure ͑see, for example, Fig. 5͒. The smallangle neutron scattering from structures obtained through leaching of one phase of a spinodally decomposed system ͑e.g., porous Vycor͒ is quantitatively similar to that of dealloyed Au. 32-34 Interestingly, one recent theory views the formation of porosity as a spinodal decomposition process occurring at the dealloyed surface. 35 The as-dealloyed structure typically consists of pore diameters on the nanometer length scale; the smallest size that we are ...

25th Anniversary Article: Galvanic Replacement: A Simple and Versatile Route to Hollow Nanostructures with Tunable and Well‐Controlled Properties

“…Variations in temperature and concentrations of relevant ions, as well as the involvement of other non-standard conditions can all affect the actual value of reduction potential. [ 38,39 ] These changes may reverse the direction of a replacement reaction and result in the termination or prevention of galvanic replacement (see Section 5). …”

Plasmonic nanosponges

“…Alternatively, an anodic potential can be applied for the electrochemical dealloying. Here, the important thermodynamic parameter is the critical potential which is the threshold potential for dealloying and dependent on the composition of the alloy, the electrolyte and the further additives [32]. Pore/ligament size can be tuned by changing the dealloying conditions (such as solution concentration, dealloying time, and temperature), and vary from around 10 nm to over micron [12,33].…”

Nanoporous Plasmonic Metamaterials

“…This conclusion is consistent with the observation that the feature size in np Au can be controlled by the composition of the electrolyte which in turn controls the diffusion length. [21] For plasmonic applications it is important to note that the process can be easily extended to two-dimensional films by using commercially available white-gold leave with a thickness of a few hundred nanometers. [22] An equally important aspect of the dealloying process is that the feature size in nanoporous gold can be controlled over a wide range from 10 nm to the micron length scale through a simple annealing procedure.…”