In situ imaging of dealloying during nanoporous gold formation by transmission X-ray microscopy Yu-chen Karen Chen-Wiegart a,b,⇑ , Steve Wang b , Wah-Keat Lee b , Ian McNulty b , Peter W. Voorhees a , David C. Dunand a a Department of Materials Science and Engineering, Northwestern University, Evanston, IL 60208, USA b Advanced Photon Source, Argonne National Laboratory, Argonne, IL 60439, USA Received 28 November 2011; received in revised form 27 July 2012; accepted 11 October 2012 Available online 19 November 2012 Abstract The dealloying process is directly imaged, for the first time, by using transmission X-ray microscopy for the case of an Ag–30 at.% Au wire dealloyed under free corrosion in nitric acid. The propagation of a sharp dealloying front separating the alloy from nanoporous Au was observed by two-dimensional real-time in situ imaging at 30 nm resolution and measured in detail in three dimensions by an ex situ nanotomography technique at fixed time intervals. The rate of the dealloying front propagation is independent of the dealloying time up to a 3 lm depth, indicating that the dealloying process to this depth is dominated by interfacial effects (i.e. gold surface diffusion and/or silver dissolution) rather than long-range transport effects (i.e. diffusion of acid and corrosion product in and out of the porous layer). The constant dealloying rate corresponds to a constant silver flux and a constant current density, even though the potential might be fluctuating under free corrosion conditions and the interfacial area is shrinking as a function of time. Free corrosion in this system gen- erates a high current density, implying it is driven by a chemical potential difference that is much higher than the critical potential. Ó 2012 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: TXM; X-ray synchrotron radiation; Metal foam 1. Introduction Dealloying is a simple processing method to fabricate nanoporous metallic structures. By dissolving the less noble component from a binary alloy, a nanoporous (np) struc- ture of the more noble metal with open pores ranging from 3 to 50 nm in size can be fabricated for various geome- tries (nanoscale wires, rods, spheres and core–shell struc- tures, microscale thin films, and macroscale foils and foams) and various metals (Au, Ag, Fe, Co, Ni, Pd, Pt and Ti) [1–8]. Numerous applications – including sensors, actuators, super-capacitor, anodes of Li-ion battery and catalytic substrates – have been reported for np-metals [9–14]. Understanding the mechanism of dealloying is essential to optimize the process and fabricate structures with properties optimized for a particular application. In particular, with an accurate understanding of the variables controlling dealloying front propagation, nanoporous gold could be prepared with the shortest possible dealloying time, thus ensuring the highest possible specific area, since pore/ligament coarsening occurs in freshly dealloyed np-Au in the electrolyte [15,16]. This property affects sensitively the performance of catalysts, sensors, actuators, battery electrode and surface Raman scattering [9–14]. With the accurate measurement of dealloying rate provided here, it is also possible to fabricate partially dealloyed structures where the surface only is nanoporous, combin- ing the functional properties of the dealloyed, nanoporous surface with the enhanced mechanical properties (in partic- ular ductility and toughness) provided by the non-porous, non-dealloyed core of the material. 1359-6454/$36.00 Ó 2012 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.actamat.2012.10.017 ⇑ Corresponding author. Present address: Photon Science, Brookhaven National Laboratory, Upton, NY 11973, USA. Tel.: +1 631 344 3463; fax: +1 631 344 3238. E-mail address: ycchen@bnl.gov (Yu-chen Karen Chen-Wiegart). www.elsevier.com/locate/actamat Available online at www.sciencedirect.com Acta Materialia 61 (2013) 1118–1125