We deduce the following from the findings for NPG film formation from electrochemically reducing Au oxides formed during HV electrolysis at 540 V and different times. First, even though the precursor Au oxide layer is homogeneous on the entire bead, it was observed that the resulting NPG film exfoliates for some of the samples from the surface. Since the Au oxide is homogeneous, we suggest that the exfoliation either occurs during the electrochemical reduction or the emersion of the electrodes from the electrolyte after the electrochemical reduction. From our experiment, we can not distinguish which experimental step is at the origin of the exfoliation process. Focusing on a region where the NPG film exfoliated in Fig. , we observed that the underlying Au surface was rather inhomogeneous. We suggest that the inhomogeneity is due to residues or pinning points of the exfoliated NPG film. For short electrolysis times, after 500 ms, exfoliation primarily occurs in regions with Au(111) and Au(111*) orientation and to a certain extent also on the Au(100) facet, but not on the Au(100*) facet. More homogeneous NPG films are formed on the Au(111*) facet for longer electrolysis times, while it still exfoliates from the Au(111) facet. On the Au(100*) and the Au(100) facets, more homogeneous NPG films are formed. Between 2 and 30 s no exfoliation occurs on the Au(100) facet. Nonetheless, for electrolysis times longer than 15 s significant film exfoliation is observed on the entire POSC, making it difficult to assign a facet-dependent growth of the film to any region on the POSC. The reason for the exfoliation can be found in the thickness of the Au oxide film formed on the Au POSC, which increases with increasing electrolysis time (see Fig 2). Assuming that the film thickness is identical on all facets after the HV electrolysis, we suggest that the thin NPG films tend to exfoliate more easily than thick NPG films. The observed facet-dependent exfoliation for short electrolysis times might be related to the adhesion of the NPG film in some regions and/or the lateral interaction of the NPG film. For longer electrolysis times which produce thicker Au oxide films, exfoliation becomes less pro-Figure : SEM images of the Au(111*) (first two rows) and the Au(100*) facet (third and fourth row) after 540 V for 2 s (a-d), 4 s (e-h) and 30 s (i-l) followed by electrochemical reduction. The images in the first and third rows have a size of 30 µm × 20 µm, and those in the second and fourth rows have a size of 3 µm × 2 µm. The contrast and brightness were adjusted in images c, g, and i. nounced due to possible stronger lateral interactions within the formed NPG film. Further experimental and theoretical investigations on single crystals with preferential orientation are required to substantiate these assumptions. 300 V 2-30 s: Next, we focus on NPG films formed by the electrochemical reduction of Au oxide films, which were formed by HV electrolysis at 300 V at different times. Optical microscope images of the resulting POSCs are shown in the first row in Fig. . After 2 s the POSC still appears golden, and the distinctive black color for a NPG film is not observed. Electrochemical measurements (Fig. red curve), however, clearly indicate that a Au oxide film covered the surface. Hence, the film is presumably too thin to change the optical properties of the POSC surface (at least from visual inspection). Electrodes prepared by electrolysis times longer than 5 s clearly show a distinct black color. These electrodes also show already by visual inspection clearly significant exfoliation of the NPG film from the surface. For comparison, the stability of NPG films at 300 V is also shown in Table . Note that grain boundaries also do not influence the NPG formation, which is shown in Fig. (2 s HV electrolysis at 300 V), as in the case of the Au oxide and also for the NPG film formed at 540 V. SEM images of the POSCs are also shown in Fig. . Homogeneous NPG films are observed for electrodes prepared by HV electrolysis at 300 V up to 15 s. After 30 s, the NPG film randomly exfoliates from different surface regions. A closer inspection of, for example, the Au(111*) region shows rather compact NPG structures up to electrolysis times of 15 s (Figs. and), while the NPG film after 30 s HV electrolysis is rather inhomogeneous (Fig. ). This aligns with the fact that the Au oxide film formed at 300 V and 30 s electrolysis time is not smooth (see Figs. and and Figs. and), but shows cracks and regions with exfoliated Au oxide.