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.

Adsorption and desorption of bis-(3-sulfopropyl) disulfide during Cu electrodeposition and stripping at Au electrodes.

The adsorption and desorption of bis-(3-sulfopropyl) disulfide (SPS) on Cu and Au electrodes and its electrochemical effect on Cu deposition and dissolution were examined using cyclic voltammetry stripping (CVS), field-emission scanning electron microscopy (FESEM), and X-ray photoelectron spectroscopy (XPS). SPS dissociates into 3-mercapto-1-propanesulfonate when it is contacted with Au and Cu electrodes, producing Cu(I)- and Au(I)-thiolate species. These thiolates couple with chloride ions and promote not only the reduction of Cu(2+) in Cu deposition but also the oxidation of Cu(0) to Cu(+) in Cu stripping. During Cu electrodeposition on the SPS-modified Au electrode, thiolates transfer from Au onto the Cu underpotential deposition (UPD) layer. The Cu UPD layer stabilizes a large part of the transferred thiolates which subsequently is buried by the Cu overpotential deposition (OPD) layer. The buried thiolates reappear on the Au electrode after the copper deposit is electrochemically stripped off. A much smaller part of thiolates transfers to the top of the Cu OPD layer. In contrast, when SPS preadsorbs on a Cu-coated Au electrode, almost all of the adsorbed SPS leaves the Cu surface during Cu electrochemical stripping and does not return to the uncovered Au surface. A reaction mechanism is proposed to explain these results.

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 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.

Effect of chloride ions on the adsorption of 3-mercapto-1-propanesulfonic acid and bis(3-sulfopropyl)-disulfide on a Au(111) surface.

In situ scanning tunneling microscopy (STM) and cyclic voltammetry (CV) were used to study the adsorption of 3-mercapto-1-propanesulfonic acid (MPS) and bis(3-sulfopropyl)-disulfide (SPS) on Au(111) electrode in a HClO(4) aqueous solution. Chloride ions were introduced into the electrolyte solution, and their effect on the adsorption behavior of MPS and SPS was investigated. The CV results show that SPS and MPS molecules preferentially adsorb on the Au(111) surface compared to chloride ions, and furthermore, chloride ion can induce the adsorption of thiol molecules on the Au(111) surface. In the absence of chloride, no adsorption phase of SPS (or MPS) adlayer can be imaged by STM at low potentials. Raising electrode potential leads to the appearance of disordered adsorption phase at ca. 0.4 V (vs RHE) and ordered adlattices at ca. 0.8 V. In the presence of chloride, ordered adsorption structures of SPS and MPS appear at a lower potential (0.2 V), implying the enhancement effect of chloride to the thiol adsorption. It is inferred that the presence of chloride ions triggers a more positively charged gold surface, enhancing the reaction rate of thiol adsorption. Furthermore, the presence of chloride also leads to a decrease in the thiol-electrolyte interaction, due to the high solvation effect of chloride ions, which promotes the adsorption of SPS and MPS onto the Au surface. With further elevation of electrode potential, electrostatic interaction leads to coadsorption of chloride ions into the adlayer, as well as orientation changes of the ad-molecules. As a result, the ordered adlattice was disrupted and disappeared at ca. 0.5 V.

Revelation of the spatial structures and polymerization of aniline on Au(100) electrode by in situ scanning tunnelling microscopy.

In situ STM imaging of Au(100) electrode in 1 M HClO4 + 0.03 M aniline revealed highly ordered aniline adlattices of (2 radical 2 x 4 radical 2)R45 degrees and (radical 10 x radical 10)R18 degrees and polyaniline molecules exhibiting wiggling conformations at potentials negative and positive of 0.95 V (vs. reversible hydrogen electrode), respectively.

3-Mercapto-1-propanesulfonic acid and Bis(3-sulfopropyl) disulfide adsorbed on Au(111): in situ scanning tunneling microscopy and electrochemical studies.

3-Mercapto-1-propanesulfonic acid (MPS) and bis(3-sulfopropyl) disulfide (SPS) adsorbed on a Au(111) electrode were studied by using in situ scanning tunneling microscopy (STM). Although the adsorptions of MPS and SPS are known to be oxidative and reductive, respectively, on an Au(111) electrode, these two admolecules behave similarly in terms of phase evolution, surface coverage, potential for stripping, and characteristics of cyclic voltammetry. However, different adsorption mechanisms of these molecules result in different structures. Raising electrode potential causes more MPS and SPS molecules to adsorb, yielding ordered adlattices between 0.67 and 0.8 V (vs reversible hydrogen electrode). The ordered adlattices of MPS and SPS appear as striped and netlike structures with molecules adsorbed parallel to the Au(111) surface. Switching potential to 0.9 V or more positive still does not result in upright molecular orientation, possibly inhibited by electrostatic interaction between the end group of -SO(3)(-) and the Au(111) electrode. Lowering the potential to 0.4 V disrupted the ordered adlayer. Stripping voltammetric experiments show that MPS and SPS admolecules are desorbed from Au(111) at the same potential, suggesting that these molecules are both adsorbed via their sulfur headgroups. The S-S bond in SPS is likely broken upon its adsorption on Au(111).