The anodic dissolution process of Au(111) in a 0.1 M perchloric acid (HClO4) solution containing chloride anion (Cl-) was investigated using an in situ scanning tunneling microscope. The initial dissolution of gold was observed at the step sites when the electrode potentials became more positive than +1.0 V. The rate of the anodic dissolution increased as the potential became more positive. When the potential became more positive than +1.35 V, dissolution on the terraces was also observed. The dissolution of Au(111) anisotropically proceeds in a layer-by-layer mode. Step lines along the  direction, which were found in the double-layer region, disappeared, and ones along the  direction were newly formed during the dissolution process. The gold surface became rougher when the electrode potential became more positive than +1.45 V, where anodic dissolution and oxide formation simultaneously took place. The gold surface was completely passivated at +1.7 V. The mechanism for the anisotropic dissolution is discussed in relation to the structure of the chloride adlayer on the Au(111) electrode surface.
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