Single-crystal Ni electrodes were investigated by in situ
electrochemical scanning tunneling microscopy (STM)
under potential control in 0.05 M Na2SO4
at pH 3.0. On Ni(111), a Ni(111)-(1×1) structure was
observed
in high-resolution STM images acquired at cathodic potentials.
Under passivation by forming oxide layers,
a hexagonal lattice with an interatomic distance of 0.29 nm was
discerned, which was attributed to oxide
layers of NiO(111) or Ni(OH)2(0001). On
Ni(100), a square lattice with a p(2×2) of oxygen was
observed
even at cathodic potentials. The p(2×2) structure was
converted to a quasi-hexagonal structure in the passive
region due to the formation of bulk oxide films. Sulfur adatoms
chemically attached on Ni(100) formed a
c(2×2) adlattice, which was consistently observed even
under the anodic dissolution of the underlying Ni
substrate. The dissolution of Ni from the S-modified Ni(100)
was found to proceed in the layer-by-layer
mode, forming atomically flat terraces extending over large
areas.
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