In situ scanning tunneling microscopy measurements of Cu(111) in 0.1 M NaOH
in the underpotential range of Cu2O formation are reported. Adsorption and
desorption processes are observed at the reversible potential of -0.675+/-
0.02 V/SHE. These processes are initiated preferentially at the step edges
on the upper terrace side. The adsorption decreases the mobility of the Cu
atoms along the step edges. It also induces the lateral growth of the terra
ces, which is thought to result from the accumulation, at the step edges, o
f Cu atoms ejected from the adlayer superstructure due to reconstruction of
the outermost Cu plane of the substrate. The same mechanism leads, in the
last stages of the adsorption process, to the formation of Cu adislands on
the terraces when the adlayer grow in adsorbate-free areas surrounded by th
e superstructure. The adlayer forms an ordered superstructure with an hexag
onal lattice having a unit vector of 0.6+/-0.02 nm and one adspecies per un
it cell (coverage of ca. 0.2 ML). The lattice parameters suggest that the o
utermost Cu plane is reconstructed according to the Cu plane in Cu2O (111),
i.e., with Cu-Cu distances of ca. 0.3 nm, with the adlayer forming a (2 x
2) lattice. The comparison between the STM data and the charge transfer mea
surements indicates that the adspecies are hydroxide or hydroxyl groups rat
her than adsorbed oxygen. (C) 2000 Elsevier Science B.V. All rights reserve
d.