In Situ STM Studies on the Underpotential Deposition of Antimony on Au(111) and Au(100) in a BMIBF4 Ionic Liquid

In this article, the underpotential deposition (UPD) of Sb on Au(111) and Au(100) over a wide potential
range in a BMIBF4 ionic liquid containing SbCl3 is investigated by cyclic voltammetry and in situ scanning
tunneling microscopy (STM). The cyclic voltammograms of the two surfaces show similar features of Sb
UPD, which is kinetically sluggish at high underpotential shift but rapid at low underpotential shift with a
large and broad reduction peak. The total charge flux associated with the UPD suggests an equivalent of
nearly 1 ML quantity of Sb deposit. In situ STM results showed precursor adsorption and several stages of
the UPD. The prior UPD reversible adsorption of SbCl3 precursor occurs only on Au(111) with an ordered
hexagonal array of six- and seven-membered clusters in (x31 × x31)R8.9° registration with the Au(111)
surface. On both Au(111) and Au(100), the initial stage of Sb UPD (from -0.2 to -0.4 V) proceeds with the
formation of similar short atomic strips of Sb atoms with a length of 2-2.5 nm and a width of two atoms.
The average coverage of the Sb atoms is ca. 0.24 ML at -0.4 V with an electrosorption valency of close to
3. In the later stage of Sb UPD (from -0.45 to -0.65 V), dramatically different behaviors were observed on
Au(111) and Au(100). On Au(111), a multilevel feature of (x3 × x3)R30° structure was observed at a
fractional coverage with a small quantity of Sb aggregates at the top level. This was followed by a two-layer
surface alloying at potentials close to that of Sb bulk deposition. On Au(100), however, the formation of a
distinct second layer composed of islands of ∼4 nm was the sole characteristic at this stage. No surface
alloying appeared on Au(100). These features contrast those observed in aqueous solutions containing
oxygeneous SbO+ species and display profound medium effects on the UPD of Sb. Discussions of these
observations, especially the possible origin of the clusters on Au(111) and the structural invariance of the
atomic strips on Au(111) and Au(100), is provided in view of mutual interactions at the interface that are
modified by the environment of the ionic liquid.