dc.description.abstract | In situ scanning tunneling microscopy (STM) and cyclic voltammetry (CV) were used to examine (I) the spatial structure of adsorbed 3-Mercapto-1-propanesulfonic acid (MPS) molecules and its effect on the electrodeposition of copper on a Pt(111) electrode in 0.1 M KClO4 + 1 mM HCl + 10-7 M MPS (pH 3). Two ordered MPS structures, Pt(111) - (2 × 2) (θ = 0.25) and (√3 × √3)R30° (θ = 0.33) structures were observed between -0.3 V ~ 0.1 V (vs Ag/AgCl). These MPS structures were no longer present at E > 0.1 V. Shifting the potential negatively could restore the ordered structures of (√3 × √3)R30° and (2 × 2). By contrast, the MPS adlayer seen in 0.1 M HClO4 was always disordered, regardless of the potential of Pt(111) electrode or adjusting the concentration of MPS. It is reasonable to state that pH, potential control, and/or countercations to the sulfonate group of the MPS admolecule could be important in guiding the adsorption of MPS molecules on Pt(111) electrode. Real-time STM imaging revealed random nucleation of copper adatoms on MPS-modified Pt(111), whch contrasts markedly with copper deposition on Pt(111). This difference implies that copper adatoms could have different mobility on the substrates, thereby rendering unlike nucleation and growth processes. Segregated domains of (√3 × √3)R30° - MPS and chloride, were observed atop a monolayer of Cu deposit. By contrast, the Cu deposit on MPS-modified Pt(111) in 0.1 M HClO4 was decidedly rough. With a small overpotential (η < 20 mV), multilayer copper was electroplated on Pt(111) in a layered manner, producing atomically smooth Cu deposit capped by patches of (3 × 3) MPS. pH and the thickness of Cu film could affect the coverage and spatial structure of MPS. Strongly adsorbed MPS molecules on the Pt(111) electrode could impede the rate of Cu2+ reduction, thereby inhibiting rather than accelerating electrodeposition of copper under the present conditions. The UPD peaks still appeared at the same potentials, but broadened noticeably and decreased in intensity. Meanwhile, a weak reduction feature emerged at 0.1 V, which is associated with Cu UPD at MPS-occupied domains on Pt(111). MPS and chloride anions could compete for surface sites in the potential region of Cu UPD. These CV results indicate that chloride and MPS adspecies segregated into different domains, rather than mixed homogeneously to form a different structure.
(II) Nickel and cobalt thin film electrodeposited onto a Pt(111) – supported copper film in 0.1 M KClO4 + 1 mM HCl (pH 3) + 0.06 M NiCl2/0.04 M CoCl2. The role of boric acid, a buffer used frequently in the deposition of Ni and Co, was investigated. Both Ni and Co deposit were grown in 2D mode on Pt(111). The Pt(111) – supported copper film was atomically smooth up to 10 layers, onto which a highly ordered chloride adlattice was formed. By a small (< 20 mV) overpotential triggered random nucleation at surface defects of pit and step. Cobalt clusters measured several nanometer in diameter and 1-2 atoms in height were stable against prolonged STM imaging. For nickel, it’s used to deposit prior to the defects in a monolayer in height. The morphology of copper substrate was unchanged by nucleation, suggesting a minimal surface mixing at the Co or Ni/Cu interface. Deposition of cobalt/nickel proceeded subsequently in layers, whose atomic structures were deduced from the atomic-resolution STM images obtained with the chloride adlayer. On the first layer the chloride adlayer produced a pair-lined pattern, followed by a hollow ring pattern seen with Cl adsorbed on the second layer of metal film. Starting from the third layer and up to the 10th layer, the chloride adlayer looked alike, exhibiting a prominent moiré pattern along with minor triangular depressions. Cobalt/nickel atoms would have to occupy different sites on the Cu substrate, yielding unlike corrugation heights of the adsorbed chloride adatoms. As the film thickened, the Pt substrate exerted less influence on the structure of Ni. Raising the potential resulted in stripping of Ni, which appeared to start from step edges and proceeded in a layered mode. Adding boric acid in the electrolyte could shift the potential for Ni deposition to more positive values and promoted the reduction of Ni2+. However, boric acid had little effect on Co deposition; it clearly inhibited hydrogen evolution. It appears that boric acid acted not only as a buffer but also as a catalyst toward the reduction of Ni and Co.
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