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

Au(111) electrodes have been modified with self-assembled rnonolayers (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 mu M), and, optionally, 1.4 mM Cl ions. The deposition process was characterized by cyclic voltarnmetry (CV) and in-situ scanning tunneling microscopy (STM) as a function of the electrode potential. At potentials positive of Cu growth (>= 0.7 V-RHE), freshly modified electrodes are covered by an ordered (5 root 3 x root 21) MPS adlayer (alpha) both in Cl-free and Cl-containing electrolytes. The alpha adlayer becomes disordered at more negative potentials prior to the onset of Cu deposition (<= 0.65 V-RHE). In the potential regime of Cu underpotential deposition (UPD) (approximate to 0.2-0.65 V-RHE), the surface morphology strongly depends on the presence of Cl. In the absence of Cl, a transient, ordered Cu/MPS adlayer phase (delta) forms via 2D growth and covers the entire Au(111) surface. Subsequently, the delta phase transforms into a disordered Cu/MPS phase (sigma c(u)) with small, embedded Cu islands. In Cl-containing electrolyte, a disordered Cu/MPS/Cl phase (gamma) nucleates at Au step edges or surface defects and spreads laterally. Cu islands form simultaneously within the gamma 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 (theta(c) << 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.