In general, the amount of Au oxide formed can be determined from the charge which passed during the electrochemical reduction of the Au oxide. The experiment is performed in an electrochemical cell in 0.01 M KOH. The electrode potential is decreased starting from 1.1 V RHE (vs. the reversible hydrogen electrode) with a scan rate of 50 mV s -1 and kept at 0.25 V RHE until the measured current was negligible. A more detailed description can be found in the Experimental Section and elsewhere. Figure shows the evolution of the charge density during electrochemical reduction of the Au oxide formed on the Au POSCs for different HV electrolysis times at 300 V (red curve) and 540 V (blue curve). In both cases, the charge density increases in a linear way with increasing electrolysis time, indicating that the Au oxide film thickness increases with increasing electrolysis time up to 30 s. Note that POSCs modified with the same HV electrolysis treatment but with different bead sizes will show similar charge densities upon electrochemical reduction, although the size distribution of the facets on the crystal may change with the size of the bead. For short electrolysis times, the Au oxide film thickness is similar for electrodes prepared by HV electrolysis at 540 V and 300 V. For long electrolysis times, the Au oxide films are thicker at 300 V than at 540 V. This can be rationalized by the difference in current density for different times and applied voltages. The current density at 540 V is higher for short electrolysis times and lower for long electrolysis times. Moreover, the charge densities obtained after 30 s HV electrolysis are very similar to those reported previously for wire electrodes. Following we discuss the structural properties of the Au oxide films grown during HV electrolysis at both voltages and different electrolysis times, starting with electrodes prepared at 540 V.