(Invited) Characterization and Functional Improvement of Nanoporous Metals
Authors: El-Zoka Ayman A. , Langelier Brian , Newman Roger C. 
Published: February 2020

Dealloying of binary AgAu and ternary AgAuPt alloys has been proven to yield nanoporous metals (NPG and NPG-Pt) that are promising candidates as catalysts and smart materials. The alloy is exposed to electrolytic conditions at which it is favorable for one metal to dissolve, leaving behind a nanoporous network that is enriched in the other more noble metal(s) [1-2]. The structural stability, and catalytic activity of nanoporous metals are mainly affected by aspects including structural evolution, surface area stability and nanoscale composition. Achieving a deeper realization of the mechanisms that govern the nanoscale behavior in this material at such small scale is vital for pushing forward this class of materials to industrial applications. Thus, high-resolution characterization could be used to improve the already existing insights into the formation and tunability of nanoporous metals.
Atom probe tomography (APT) of as-dealloyed and thermally coarsened NPG/NPG-Pt was enabled by inner-pore potentiostatic deposition of Cu (Fig 1) [3]. The recently demonstrated ability of complete infiltration of NPG/NPG-Pt with Cu brings attention to the peculiar electrochemical properties of NPG that are worth further consideration. A previous investigation by Lee et al. [4] on the electrodeposition of Cu onto nanoporous Pt, made by dealloying of CuPt alloys, revealed a sub-potential deposition regime that lies between the UPD (underpotential deposition) and the equilibrium deposition regimes. The sub-potential deposition regime was attributed to the curvature of the nanoporous Pt surface.
We report a deeper understanding of the complete infiltration of NPG and NPG-Pt with potentiostatically electrodeposited Cu from acidic solutions of copper sulfate. The main parameters controlling the extent of filling are shown to include ligament /pore sizes, the concentration of Cu2+ ions in solution and the electrochemical parameters of the deposition. The crystallographic structure of Cu and its relation to that of the NPG substrate is also studied at high resolution. The infiltration of NPG and NPG-Pt at significant depths serves to highlight their interesting electroactive properties, and the great prospects that lie in using NPG as a template for growth of 3D nanostructures.
Fig 1. Schematic of the electrochemical process by which NPG samples are formed and prepared for APT sample fabrication. From bulk AgAuPt alloy (a) selective dissolution of Ag creates a dealloyed layer of NPG-Pt (b). Consolidation of this open-cell network for APT sample preparation is then achieved by electrodeposition of Cu (c).
References
[1] R.C. Newman, “Dealloying,” in Shreir’s Corrosion (4th ed.), Elsevier, vol. 2, pp. 801– 809, 2010.
[2] J. Erlebacher, M.J. Aziz, A. Karma, N. Dimitrov, and Karl Sieradzki, “Evolution of nanoporosity in dealloying,” Nat., vol. 410, pp. 450-453, 2001.
[3] A.A. El-Zoka, B. Langelier, G.A. Botton, and R.C. Newman, “Enhanced analysis of nanoporous gold by atom probe tomography,” Mater. Charact., vol. 128, pp. 269-277, 2017.
[4] L. Lee, D. He, A. G. Carcea, and R. C. Newman, “Exploring the reactivity and nanoscale morphology of de-alloyed layers,” Corros. Sci., vol. 49, pp. 72–80, 2007.

Figure 1