Star-Shaped Surface Cracking during Inhibited Dealloying Controlled by Grain Crystallography

Nanoporous gold (npg) produced by dealloying of Au alloys has been recently proposed for many different applications from catalysis to bio-sensors. Historically, Au alloys constitute an important model system in the context of corrosion, dealloying, and stress corrosion cracking. While electrochemical dealloying in sulfuric acid produces homogeneous nanoporosity a thiol-modified starting surface results in surface cracks showing a nanoporous core with very fine ligament size [1]. The intact surrounding area exhibits the knowm ultrathin Au layer on top of the alloy stabilized by the self-assembled thiol film [2]. Adsorbed self-assembled monolayers of thiol molecules inhibit thus the dealloying reaction. On thiol-modified Cu-Au alloys dealloying finally proceeds in a localized reaction instead of a homogeneous process. Volume shrinkage during dealloying has been reported for Ag-Au alloys [3]. The forming cracks within the nanoporous defect regions indicate a volume shrinkage and stress accumulation also for Cu-Au alloys. Thiols can be also removed from Au surfaces and thus hierarchical structures may be formed by combining different potential treatments [4].

The crystallographic orientation of the surface influences the number density of the formed cracks as well as the crack morphology. The crack density is on some grains of special surface orientations especially high, while others exhibit only few cracks. The morphology reveals a clearly crystallographic influence on the crack morphology. Especially on (001) and (111) oriented surfaces clear 4-fold (Fig.1) and 3-fold symmetries are observed, respectively. The crystallographic nature of the cracks formed under near-isotropic hydrostatic stress (Fig.2) is in contrast to uni-directional compression behavior, e.g. during nanoindentation  [5]. With the possibility to control the initiation of dealloying by a controlled introduction of defects [6] in the self-assembled thiol film the here presented system offers an interesting approach to address the mechanical behavior of nanoporous material during a corrosion process.

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