We report in situ scanning tunneling microscopy at atomic-layer resolution of the topographic changes accompanying electrochemical reactions. In dilute perchloric acid, Au(l 11) terraces roughen during the electrochemical formation and reduction of more than a monolayer of gold oxide, but anneal in minutes at moderate potential. Trace chloride enhances step motion, prevents the observation of roughening, and promotes the dissolution of gold during oxidation and rereduction. These observations illustrate the role adsorbates have in determining the surface mobility of substrate atoms.PACS numbers: 61.16.Di, 66.30.Lw, 81.60.Bn, 82.45.+ZThe microscopic details underlying important interfacial electrochemical processes such as electron transfer, plating, passivation, and corrosion are traditionally inferred from in situ measurements of macroscopic quantities: charge, potential, concentrations, surface tension, etc. Recently, various groups have demonstrated the feasibility of using ex situ electron spectroscopies, microscopies, and diffraction techniques 1 " 5 and in situ techniques such as glancing angle x-ray absorption and diffraction 6 " 8 and infrared reflection-absorption spectroscopy 9 to obtain structural information about the electrochemical interface at the atomic level. Scanning tunneling microscopy (STM) could offer a microscopic, realspace view of the electrochemical interface to complement these other tools. 10 One immediate target for investigation with the STM is the growth, dissolution, or rearrangement of crystalline surfaces due to electrochemical reactions. A prototypical example of electrochemically induced change at a crystalline surface is the electrochemical oxidation and rereduction of gold. 11 " 16 Recently, Wiechers et al. have demonstrated resolution of monatomic gold layers at the interface of Au(lll) and aqueous solutions while controlling the electrochemical potential of the gold surface. 17 Here, we report the first in situ STM observations with atomic-layer resolution of the consequences of an electrochemical surface reaction: the roughening, annealing, and dissolution accompanying the oxidation and rereduction of the Au(lll) surface.Gold (111) films on mica were prepared by evaporation of gold onto mica at 225 °C as reported earlier. 18 The samples were clamped to the base plate of a Nanoscope I STM (Digital Electronics) with a Teflon ring, which also formed the electrochemical cell. The counter electrode was a Pt wire, and the reference electrode was an Ag wire in an AgClC>4 solution separated from the cell by a length of Teflon tubing. All electrochemical potentials are reported with respect to the normal hydrogen electrode (NHE). HCIO4 was Ultra grade (Hopkin and Williams Co.), HC1 was reagent grade, water was from a Milli-Q purification train (Millipore Corp.), and oxygen was not removed from the cell. The sample potential was controlled relative to the reference electrode by driving the potential of the counter electrode, and the tip-tosample bias was fixed at -10 mV.The tunnelin...