In situ scanning tunneling microscope study of the passivation of Cu(111)

In situ scanning tunneling microscopy has been used to study the nucleation and growth mechanisms and the structure of passive films formed on Cu(111) surfaces in 0.1 M berate buffer solution (pH 9.3). A surface topography ch aracterized by terraces with monoatomic steps is obtained after potentiodyn amic reduction down to -1.12 V/standard hydrogen electrode (SHE), of the el ectropolished surface exposed to air. The formation of a single Cu2O passiv e layer at 0.03 V/SHE proceeds first by a roughening of the steps assigned to a locally blocked step flow process due to a competition between dissolu tion and preferential nucleation of the oxide at the steps. The observed ox ide nuclei are 2-3 nm wide and about one atomic plane high. This process le ads to the complete coverage of the terraces by a grain-like structure of t he oxide film. The initial terrace topography is completely altered. Thicke ning of this oxide layer leads to unstable scanning tunneling microscope im aging, despite the presence of a subband in the bandgap of the oxide. Duple x passive films, Cu2O\CuO-Cu(OH)(2), produced at 0.98 V/SHE, are characteri zed by a grain-like structure with no detectable evidence of crystallinity. The lateral size of the grains ranges from 2 to 5 nm. The measured height difference is less than or equal to 1.5 nm. Stable: imaging occurs via elec tron tunneling in the conduction band of the passive film. A metal surface topography can be recovered by potentiodynamic reduction of the oxide film down to -0.92 V/SHE. The smallest terraces are eliminated by step coalescen ce in these oxidation/reduction treatments, which leads to steeper surface profiles. (C) 1999 The Electrochemical Society. S0013-4651(98)06-030-5. All rights reserved.