The Au oxide films prepared by HV electrolysis on the Au POSC were reduced electrochemically in a separate glass cell (diameter: 6 cm) to avoid the autocatalytic reduction of the Au oxide by H 2 O 2 formed during electrolysis. The electrochemical cell was filled with 150 mL 0.01 M electrolyte. The Au POSC was used as electrochemical working electrode, a Pt-sheet (10 × 7.5 mm) was used as electrochemical counter electrode, and a homemade reversible hydrogen electrode (RHE) served as the reference electrode. A FHI ELAB potentiostat was used to control the potential during the electrochemical reduction experiments. To reduce the Au oxide, the potential was lowered from an initial potential of 1.1 V RHE to approximately 0.25 V RHE with a scan rate of 50 mV s -1 . The potential was held until the current was almost zero. Subsequently the Au POSC was removed and the structural properties were characterized by optical microscopy and SEM. For most measurements, the electrolyte was not deaerated, since the reductive current resulting from the reduction of Au oxide is much higher compared to the current resulting from the reduction of residual oxygen dissolved in the electrolyte. The electrolyte was not stirred.