Electrochemical Cu Growth on MPS-Modified Au(111) Electrodes

Au­(111) electrodes have been modified with self-assembled
monolayers
(SAM) of 3-mercapto-1-propanesulfonic acid (MPS) and used as a substrate
for Cu electrodeposition. Aqueous plating solutions contained 0.1
M H2SO4, low Cu concentrations (≤80 μM),
and, optionally, 1.4 mM Cl ions. The deposition process was characterized
by cyclic voltammetry (CV) and in-situ scanning tunneling microscopy
(STM) as a function of the electrode potential. At potentials positive
of Cu growth (≥0.7 VRHE), freshly modified electrodes
are covered by an ordered (5√3 × √21) MPS adlayer
(α) both in Cl-free and Cl-containing electrolytes. The α
adlayer becomes disordered at more negative potentials prior to the
onset of Cu deposition (≤0.65 VRHE). In the potential
regime of Cu underpotential deposition (UPD) (≈0.2–0.65
VRHE), the surface morphology strongly depends on the presence
of Cl. In the absence of Cl, a transient, ordered Cu/MPS adlayer phase
(δ) forms via 2D growth and covers the entire Au(111) surface.
Subsequently, the δ phase transforms into a disordered Cu/MPS
phase (σCu) with small, embedded Cu islands. In Cl-containing
electrolyte, a disordered Cu/MPS/Cl phase (γ) nucleates at Au
step edges or surface defects and spreads laterally. Cu islands form
simultaneously within the γ phase. Two-dimensional growth of
these islands results in a pure Cu-UPD layer. Overpotential deposition
(OPD) proceeds via layer-by-layer mode with second layer nucleations
at surprisingly small critical coverages (θC ≪
0.5). Our observations differ significantly from those in previous
studies, demonstrating that the Cu growth behavior critically depends
on the concentrations of MPS, Cu, and Cl at the interface.