From the SEM images of the entire Au POSC no apparent structural changes were observed, indicating that the Au oxide film grows homogeneous on the entire surface independent from the crystallographic orientation or the electrolysis time (up to 30 s). From a closer look at both the Au(111*) and the Au(100*) (see Figs. and), no clear structural changes of the surface are observed after HV electrolysis for 100 ms at 540 V. After 250 ms, in some of the images of the Au(111*) region, minor changes in the form of elongated ripples at the step edges, are observed (marked by arrows in Fig. ). These features could be considered to be an artifact. Further below, we will indeed show that these features play a role in the NPG film formation. After 500 ms, a slight roughening of the Au(111*) surface is apparent (indicated by an arrow). The Au(100*) sur-Figure : Optical microscope and SEM images of Au POSCs after HV electrolysis for 250 ms, 2 s and 30 s at 540 V in 0.01 M KOH. Optical microscope images after additional electrolysis times after HV electrolysis at 540 V can be found in Fig. and SEM images of the whole POSCs, as well as magnifications of the Au(111*) and Au(100*) facets after the same conditions can be found in Fig. . The size of the beads can be estimated from the wire diameter, which is 1 mm. face seems to change to a smaller extent compared to the Au(111*) surface (see Figs. and). On Au(100*), hardly any changes can be seen up to a HV electrolysis time of 500 ms. A distinct roughening of the surface can be seen after 2 s and longer electrolysis times (up to 30 s) on the Au(111*) and Au(100*) facets (see Figs. S4k-n and Figs. ). The morphology of the Au oxide and the degree of roughening for the different facets observed in the SEM images are hereby similar for all electrolysis times considered.