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 borate buffer solution (pH 9.3). A surface topography characterized by terraces with monoatomic steps is obtained after potentiodynamic reduction down to -1.12 V/standard hydrogen electrode (SHE), of the electropolished surface exposed to air. The formation of a single Cu2O passive 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 dissolution and preferential nucleation of the oxide at the steps. The observed oxide nuclei are 2-3 nm wide and about one atomic plane high. This process leads to the complete coverage of the terraces by a grain-like structure of the oxide film. The initial terrace topography is completely altered. Thickening of this oxide layer leads to unstable scanning tunneling microscope imaging, despite the presence of a subband in the bandgap of the oxide. Duplex passive films, Cu2O\CuO-Cu(OH)(2), produced at 0.98 V/SHE, are characterized 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 electron 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 coalescence 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.