Comparing the SEM images of the Au oxide taken on different regions of the POSC after 30 s HV electrolysis at 300 V in Figs. 5a to d, we observe distinctly different structural properties. First, it seems that the Au oxide film does not form a uniform film on smooth surfaces, namely the Au(111) and the Au(100) facets (Figs. and). In these regions, the Au oxide film contains many cracks and even seems to exfoliate from the surface. Interestingly, the Au oxide film also exfoliates on the Au(111*) facet (see Figs. and, Figs. and and Fig. ). On the Au(100*) facet the Au oxide film is still rather smooth and does not show pronounced crack formation (Figs. and. The region marked with an arrow in Fig. is magnified in Fig. ). For comparison with our previous experiments on polycrystalline (poly) Au wire electrodes, we included SEM images in Figs. and. These electrodes show significant restructuring and crack formation on the entire surface. Intuitively, one could assume that the cracks might be related to grain boundaries present on the polycrystalline samples. In order to elucidate the impact of grain boundaries, we investigated the Au oxide growth on different POSCs containing one or more grain boundaries (see Fig. and Fig. ). Fig. shows a region with a grain boundary after HV electrolysis at 300 V for 30 s. It is apparent that along the grain boundary merely some defects are observed. The cracks only appear on one side of the boundary. Hence the crack formation seems to depend on the adhesion to the substrate and therefore mainly on the crystallographic orientation of the underlying substrate and not the grains itself.