Effect of Ag-Au composition and acid concentration on dealloying front velocity and cracking during nanoporous gold formation

Nanoporous gold has many potential applications in various fields, including energy storage, catalysis, sensing and actuating. Dealloying of Ag-Au alloys under free corrosion conditions is a simple method to fabricate nanoporous gold. Here, we systematically investigate the dealloying rate of Ag-xAu alloy for a range of alloy compositions (x = 20-40 at.%) and nitric acid concentration (7.3-14.9 M) using in situ transmission X-ray microscopy. High-resolution in situ X-ray projections and ex situ tomographic reconstructions allow imaging of the dealloying front position during dealloying. The dealloying front velocity is constant with time, and depends exponentially on the alloy Ag/Au atomic ratio and the acid molar concentration. Only the leanest alloy, Ag-20 Au, shows a large macroscopic shrinkage in sample diameter (∼38%) after dealloying, which leads to crack nucleation and growth observed in real time during dealloying. Finite element modeling is used to estimate dealloying-induced stresses and strains, and sheds light on the cracks created by the diameter shrinkage.