Facet-Dependent Formation and Adhesion of Au Oxide and Nanoporous Au on Poly-Oriented Au Single Crystals

Nanoporous Au has different properties compared to bulk Au, making it an interesting material for numerous applications. Depending on the preparation procedure, the porosity, thickness, and homogeneity of the NPG films can be tuned. To modify the structure of NPG films in a targeted manner and thus adapt them to specific applications, a fundamental understanding of the structure formation is essential. In this work, we focus on NPG prepared from Au oxide formed during high voltage electrolysis in an alkaline electrolyte on a poly-oriented Au single crystal electrode. These poly-oriented single crystals consist of a single crystalline metal bead, with faces with different crystallographic orientations. Consequently, these POSCs allow screening of the influence of the crystallographic orientation on the structure formation of the Au oxide formed during high voltage electrolysis and the NPG film formed via electrochemical reduction of the Au oxide for different facets in a single experiment. The high voltage electrolysis is performed at current densities between 2.70 and 3.76 A / cm² (300 V) and between 0.24 and 6.39 A / cm² (540 V) with electrolysis times ranging from 100 ms to 30 s. The amount of Au oxide formed is determined by electrochemical measurements and the structural properties are investigated by scanning electron microscopy and optical microscopy. We show that the Au oxide formation is mostly independent of the crystallographic orientation except for thick Au oxide layers. In turn, the macroscopic structure of the NPG films depend on the experimental parameters, the thickness of the Au oxide precursor thickness, and the crystallographic orientation of the substrate. Possible reasons for the frequently observed exfoliation of the NPG films are discussed.