Recent atom-probe tomography (APT) and ATEM studies of dealloyed layers in binary AgAu and ternary AgAuPt alloys showed the various ways in which more-noble elements enrich during dealloying, at the atomic scale (1). We have since extended this research in several directions – towards less expensive alloys, lean in more-noble elements, and towards the potential applications of such materials in gas sensing. In this presentation, we will start with a review of published and unpublished data from the APT study, which was done using alloys with 23 at% (Au + Pt). New information will then be shown for alloys with less than 6 at.% (Au + Pt). The dealloyed layers are very Ag-rich, and very interesting conflicts arise between enrichment of the more-noble element(s) on the ligament surfaces and the ultimate supply of those elements. For gas sensing applications, we take advantage of the effect of adsorption on ligament surfaces on electron scattering during conduction in the confined metal phase. Delicate impedance measurements at the milli-ohm level show that two kinds of environmental effect can be distinguished. Adsorption of water or other gases up to several molecular layers induce changes in the measured resistance of the metal phase, while thicker layers induce a double-layer charge separation leading to capacitive changes.
The results of a number of recent investigations into nanoporous metals will be summarized, and some general principles elucidated. The most common nanoporous metal, "nanoporous gold" (NPG) is actually several variants, made from Ag-Au precursors with low (5 at.%), or high (up to 30 at.%) Au, with or without small additions of elements such as Pt.NPG is usually synthesized by dealloyingthe selective electrolytic dissolution of more reactive element(s), in this case Ag, from a solid solution. The operative mass transport process is diffusion of Au (and Pt, where present) at the solidelectrolyte interface, allowing the continual emergence of Ag.One would like to use NPG for various applications, such as membranes, catalysts, sensors, and so on. One drawback of ordinary NPG for such purposes is the continued mobility of Au at the surface, which leads to coarsening of the structure, especially at elevated temperatures. The addition of small amounts of Pt [1] hinders such coarsening, even at low Au contents [2] and also has obvious relevance to catalysis.Atom Probe Tomography (APT) has proved very successful for the nanoscale analysis of NPG [2][3][4]. The essential step was to learn how to electroplate Cu into the pores, achieving complete filling, at least locally. This strengthened the material against the otherwise inevitable fractures that would occur during the APT analysis.Heating of NPG, in air or inert environments, causes surface segregation that can be beneficial, especially for catalysis [4,5]. Much remains to be learned about the operative mass transport processes.Adsorption on to the ligament surfaces in NPG can alter the electrical resistance of the metal itself, leading to potential applications in gas sensing [6].
Copper based catalysts have been shown to reduce CO2 electrochemically into hydrocarbons and oxygenates with high Faradaic efficiencies [1]. The selectivity and the efficiency of CO2 electroreduction could benefit from morphological studies on Cu nanocatalysts. Among several methods of fabricating Cu nanocatalysts, dealloying, i.e., the selective electrolytic dissolution of a less-noble element from an alloy [2], is one of the most flexible, controllable, and economical methods available to date. The product of this dissolution is a metallic nanoporous material: an interconnected ligament-pore structure with nearly zero net curvature. Building on the previous investigations on dezincification of brass [3], the evolution of nanoporous Cu from dealloying Muntz metal (Cu60Zn40) will be presented. The effect of various dealloying parameters, such as anodic potential, temperature, pH, and electrolyte will be resolved via various advanced characterization techniques. [1] C. T. Dinh et al., “CO2 electroreduction to ethylene via hydroxide-mediated copper catalysis at an abrupt interface,” Science, vol. 360, no. 6390, pp. 783–787, 2018. [2] R. C. Newman, S. G. Corcoran, J. Erlebacher, M. J. Aziz, and K. Sieradzki, “Alloy corrosion,” MRS Bull., vol. 24, no. 7, pp. 24–28, 1999. [3] R. C. Newman, T. Shahrabi, and K. Sieradzki, “Direct electrochemical measurement of dezincification including the effect of alloyed arsenic,” Corros. Sci., vol. 28, no. 9, pp. 873–886, 1988.
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