Abstract
Electrochemical dealloying has become a standard technique to produce nanoporous network structures of various noble metals, exploiting the selective dissolution of one component from an alloy. While achieving nanoporosity during dealloying has been intensively studied for the prime example of nanoporous Au from a AgAu alloy, dealloying from other noble-metal alloys has been rarely investigated in the scientific literature. Here, we study the evolution of nanoporosity in the electrochemical dealloying process for both CoPd and AgAu alloys using a combination of in situ grazing-incidence small-angle X-ray scattering (GISAXS), kinetic Monte Carlo (KMC) simulations, and scanning transmission electron microscopy (STEM). When comparing dealloying kinetics, we find a more rapid progression of the dealloying front for CoPd and also a considerably slower coarsening of the nanoporous structure for Pd in relation to Au. We argue that our findings are natural consequences of the effectively higher dealloying potential and the higher interatomic binding energy for the CoPd alloy. Our results corroborate the understanding of electrochemical dealloying on the basis of two rate equations for dissolution and surface diffusion and suggest the general applicability of this dealloying mechanism to binary alloys. The present study contributes to the future tailoring of structural size in nanoporous metals for improved chemical surface activity.
Highlights
Owing to their high surface-to-volume ratios, nanoporous metals are employed for applications in catalysis[1−3] and energy.[4−7] Among various routes for the synthesis of such nanoporous metals,[8] electrochemical dealloying is a versatile yet simple approach
For our comparative study of the dealloying processes for CoPd and AgAu alloys, in a first step, scanning transmission electron microscopy (STEM) images of dealloyed nanoporous Pd and Au structures are shown in Figure 2a,b in late stages of the dealloying process, i.e., after complete conversion of the alloy into the nanoporous structure in the topmost region (∼100 nm), which is the region investigated experimentally
In analogy to the argument of lower annealing temperatures being responsible for faceting, we argue that higher binding energy for CoPd compared to AgAu induces a ligament faceting during electrolytic coarsening in a similar way
Summary
Owing to their high surface-to-volume ratios, nanoporous metals are employed for applications in catalysis[1−3] and energy.[4−7] Among various routes for the synthesis of such nanoporous metals,[8] electrochemical dealloying is a versatile yet simple approach. Several interesting aspects of dealloyed materials are still subject of contemporary research, such as the structural stability and coarsening behavior of the ensuing nanoporous structures[17] or their mechanical properties.[18] A broad variety of in situ methods have been utilized to study the dealloying process itself, ranging from dilatometry,[19] resistometry,[20] magnetometry,[21] UV−vis spectroscopy,[22] transmission X-ray microscopy (XTM),[23] X-ray diffraction (XRD),[24−26] and Raman spectroscopy[27] to real-space imaging techniques such as scanning tunneling microscopy[28] and transmission electron microscopy (TEM).[29] While microscopic techniques, in general, allow the extraction of size information for the nanoporous structures, they suffer from the disadvantage of averaging over a limited sample area only. Experimental scattering data contain all relevant information about the nanoporous
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