In Situ STM Study\nof Roughening of Au(111) Single-Crystal\nElectrode in Sulfuric Acid Solution during Oxidation–Reduction\nCycles

Oxidation–reduction cycles (ORCs) on Au(111) in\n0.1 M sulfuric\nacid solution change the electrode morphology due to the formation\nof many new nanosized islands. With increasing the cycle number, the\nroughness of the surface increases due to the formation of multiatomic-step\nadatom islands and pits. The final roughness value is a function of\nthe applied potential window, number of ORCs, scan rate, electrolyte\nconcentration, and any applied delay time. In a first experiment,\nthe roughening was tracked by recording the STM images in 11 steps\nduring 200 ORCs. The results show the formation of pyramidal islands\nand a linear correlation between the roughness amplitude and the cycle\nnumber. In a second experiment, the 200 cycles were studied in 38\nsteps, while after each step, two images were recorded with a 3 min\ndelay by holding the potential in the double-layer window. This leads\nto a lower roughness increase due to the high surface mobility of\nthe Au surface atoms, which smoothens the surface during the delay\ntime. Finally, the oxidation–reduction charge density per cycle\nshows an inverse correlation with surface roughness due to the (111)\nterrace showing a higher surface oxidation charge than the other sites\nand facets. Each delay causes a strong increase in the oxidation charge\nwhich is a consequence of surface smoothening during the delays leading\nto an enhancement of the (111) related oxidation charge.