Electrodeposition of copper on a Pt(111) electrode in sulfuric acid containing poly(ethylene glycol) and chloride ions as probed by in situ STM.

This study employed real-time in situ STM imaging to examine the adsorption of PEG molecules on Pt(111) modified by a monolayer of copper adatoms and the subsequent bulk Cu deposition in 1 M H(2)SO(4) + 1 mM CuSO(4)+ 1 mM KCl + 88 µM PEG. At the end of Cu underpotential deposition (~0.35 V vs Ag/AgCl), a highly ordered Pt(111)-(√3 × âˆš7)-Cu + HSO(4)(-) structure was observed in 1 M H(2)SO(4) + 1 mM CuSO(4). This adlattice restructured upon the introduction of poly(ethylene glycol) (PEG, molecular weight 200) and chloride anions. At the onset potential for bulk Cu deposition (~0 V), a Pt(111)-(√3 × âˆš3)R30°-Cu + Cl(-) structure was imaged with a tunneling current of 0.5 nA and a bias voltage of 100 mV. Lowering the tunneling current to 0.2 nA yielded a (4 × 4) structure, presumably because of adsorbed PEG200 molecules. The subsequent nucleation and deposition processes of Cu in solution containing PEG and Cl(-) were examined, revealing the nucleation of 2- to 3-nm-wide CuCl clusters on an atomically smooth Pt(111) surface at overpotentials of less than 50 mV. With larger overpotential (η > 150 mV), Cu deposition seemed to bypass the production of CuCl species, leading to layered Cu deposition, starting preferentially at step defects, followed by lateral growth to cover the entire Pt electrode surface. These processes were observed with both PEG200 and 4000, although the former tended to produce more CuCl nanoclusters. Raising [H(2)SO(4)] to 1 M substantiates the suppressing effect of PEG on Cu deposition. This STM study provided atomic- or molecular-level insight into the effect of PEG additives on the deposition of Cu.

In situ STM imaging of bis-3-sodiumsulfopropyl-disulfide molecules adsorbed on copper film electrodeposited on Pt(111) single crystal electrode.

The adsorption of bis-3-sodiumsulfopropyldi-sulfide (SPS) on metal electrodes in chloride-containing media has been intensively studied to unveil its accelerating effect on Cu electrodeposition. Molecular resolution scanning tunneling microscopy (STM) imaging technique was used in this study to explore the adsorption and decomposition of SPS molecules concurring with the electrodeposition of copper on an ordered Pt(111) electrode in 0.1 M HClO(4) + 1 mM Cu(ClO(4))(2) + 1 mM KCl. Depending on the potential of Pt(111), SPS molecules could react, adsorb, and decompose at chloride-capped Cu films. A submonolayer of Cu adatoms classified as the underpotential deposition (UPD) layer at 0.4 V (vs Ag/AgCl) was completely displaced by SPS molecules, possibly occurring via RSSR (SPS) + Cl-Cu-Pt → RS(-)-Pt(+) + RS(-) (MPS) + Cu(2+) + Cl(-), where MPS is 3-mercaptopropanesulfonate. By contrast, at 0.2 V, where a full monolayer of Cu was presumed to be deposited, SPS molecules were adsorbed in local (4 × 4) structures at the lower ends of step ledges. Bulk Cu deposition driven by a small overpotential (η < 50 mV) proceeded slowly to yield an atomically smooth Cu deposit at the very beginning (<5 layers). On a bilayer Cu deposit, the chloride adlayer was still adsorbed to afford SPS admolecules arranged in a unique 1D striped phase. SPS molecules could decompose into MPS upon further Cu deposition, as a (2 × 2)-MPS structure was observed with prolonged in situ STM imaging. It was possible to visualize either SPS admolecules in the upper plane or chloride adlayer sitting underneath upon switching the imaging conditions. Overall, this study established a MPS molecular film adsorbed to the chloride adlayer sitting atop the Cu deposit.

In situ STM study of the adsorption and electropolymerization of o-, m-, and p-ethylaniline molecules on Au(111) electrode.

Cyclic voltammetry (CV) and in situ scanning tunneling microscopy (STM) were employed to study the adsorption and polymerization of the geometric isomers of ethylaniline (EA) on a Au(111) single-crystal electrode in 0.5 M H(2)SO(4). All three isomers, namely o-, m-, and p-EA, were adsorbed in highly ordered structures, identified as Au(111)-(4 x 2 square root(3))rect for m- and p-EA and (4 square root(3) x 4 square root(3))R30 degrees for o-EA, at the onset potentials (approximately 0.9 V [vs. reversible hydrogen electrode]) for electropolymerization. Raising the potential in excess of 0.9 V resulted in oxidation and polymerization of m- and o-EA, but decomposition of p-EA. Molecular-resolution STM imaging revealed that poly(m-EA) and poly(o-EA), denoted respectively as m- and o-PEA, exhibited distinctively different molecular shapes. More specifically, m-PEA molecules were predominantly linear and aligned preferentially in the 121 directions of the Au(111) surface; whereas o-PEA molecules were ill-defined in shape and in dimension. These differences in molecular conformation stemmed from unlike arrangements of adsorbed monomers at 0.9 V. Notably, m-EA were adsorbed in zigzags with two nearest neighbors separated by approximately 0.5 nm, which were spatially so similar to the backbones of m-PEA that m-EA molecules coupled readily when the potential was raised high enough to induce the oxidation of m-EA. In contrast, the arrangement of o-EA molecules was so different from the ideal configuration of its polymer that molecules coupled randomly to yield crooked polymer chains less than 20 nm in length. The effect of potential on the structure of m-PEA was examined also, revealing notable branching of linear m-PEA if the electrochemical potential was set at 1.1 V.

In situ STM revelation of the adsorption and polymerization of aniline on Au(111) electrode in perchloric acid and benzenesulfonic acid.

In situ scanning tunneling microscopy (STM) was used to study the adsorption and polymerization of aniline on Au(111) single-crystal electrode in 0.1 M perchloric acid and 0.1 M benzenesulfonic acids (BSA) containing 30 mM aniline, respectively. At the onset potential of aniline's oxidation, approximately 0.8 V [vs reversible hydrogen electrode], aniline molecules were adsorbed in highly ordered arrays, designated as (3 x 2 square root(3)) and (4 x 2 square root(3)) in perchloric acid and BSA, respectively. These structures consisted of intermingled aniline molecules and perchlorate or BSA(-) anions zigzagging in the <110> directions in HClO(4) and in the <121> directions in BSA. The coverage of aniline admolecule on Au(111) was lower in BSA than in HClO(4). Raising the potential to 0.9 V or more positive values triggered the oxidation and polymerization of aniline. With aniline molecules arranging in a way similar to the backbone of PAN in HClO(4), they readily coupled with each other to produce linear polymeric chains aligned predominantly in the 110 directions of the Au(111). Compared with the results observed in H(2)SO(4) (Lee et al. J. Am. Chem. Soc. 2009, 131, 6468), the rate of polymerization was slower in HClO(4) and the produced PAN molecules tended to aggregate on the Au(111) electrode. PAN molecules generated in HClO(4) were anomalously shorter than those formed in H(2)SO(4). In 0.1 M BSA, PAN molecules produced by small overpotential (eta < 100 mV) could assume linear chains or 3D aggregates, depending on [aniline]. These results revealed molecular level details in electropolymerization of aniline, highlighting the important role of anion in controlling the conformation of PAN molecules and the texture of PAN film.