Self-Assembly Monolayer of MPS and SPS adsorbed on Au(111) Surface and Their Effect on the Electrochemical Deposition of Copper

Autor: Teng-Yuan Chang, 張騰元
Rok vydání: 2007
Druh dokumentu: 學位論文 ; thesis
Popis: 95
I. Adlayer Structures of MPS and SPS Self-Assembled Monolayers on Au(111) We have used in-situ scanning tunneling microscopy (STM) and cyclic voltammetry (CV) to study the self-assembled monolayers of mercaptopropylsulfonic acid (MPS) and bis(3-sulfopropyl)-disulfide (SPS) on Au(111) electrode. Results of CV profiles and in situ STM images reveal the electrochemical features of Au(111) in 0.1 M H2SO4 and the atomic structure of well-ordered Au(111) surfaces. The herringbone pattern of reconstructed Au(111) (��3×22) was observed by STM imaging. A schematic model is drawn to depict the fcc, hcp, and bridge regions of reconstructed Au(111) surface. On the positive potential excursion, STM imaging revealed two ordered MPS adlattices, (4��3 × 7) and (2��7 × 6). Stepping the potential positively from 0.85 to 0.9 V (vs. RHE) led to formation of pits and expansion of disordered domains on Au(111) surface. Electrochemical studies revealed that around 0.05 V the MPS adlayer was reductively desorbed. Oxidative adsorption of MPS took place at 0.11 V.A precipitous increase of the anodic current emerged at 0.85 V, which arose from irreversible oxidation of the surface-bound MPS molecules. When stepping the potential from 0.75 to 0.8V The SPS adlayer was also found to form a stripped phase with three ordered structures, (6��3 × 2��3)R30°, (10 × ��19), and (6��3 × ��31). These stripped structure resemble those observed previously for disulfide adsorbed at Au(111). The molecular axis of SPS is presumed to lie parallel to the surface. The stripped phase undergoes a structural transformation to a “saturated phase” as stepping potential to 1.05 V. This phase transition can be interpreted as an anodic adsorption of thiolate molecules produced by S-S bond cleavage of SPS on gold, and then molecules change the way of adsorption in order to contain more molecules. Three ordered structures were also seen at saturated phase, corresponding to (6 × 3��7), (3��3 × 3��7), and (7 × 2��7). II. Electrochemical Deposition of Copper onto SPS and MPS Modified Au(111) Electrode Copper electrodeposition from 0.1 M Sulfuric acid solutions onto MPS or SPS- modified Au(111) electrodes was studied by in situ scanning tunneling microscopy and cyclic voltammetry. We first study the electrodeposition of copper onto Au(111) in 0.1 M H2SO4. CV reveals two well-defined features at 0.46 and 0.31 V (vs. RHE), thus, two different adlayer structures would be present; one after the first deposition peak with an intermediate copper coverage of 66.7 % coverage, and one after the second deposition peak with a coverage close to 1. In situ STM images reveal a (��3 × ��3)R30° structure formed in the UPD region. This pattern actually arises from the coadsorbed sulfate anions surrounded by six copper adatoms arranging in a honeycomb pattern. CV profiles for electrodeposition of copper onto MPS or SPS-modified Au(111) are quite similar. As Au(111) was covered with MPS or SPS, the amount of copper deposited in the UPD region decreased, and the potential of bulk copper electrodeposition also shifted to more negative. Adding MPS and SPS alone in the electrolyte are inhibiting reagents for copper electrodeposition. In situ STM images exhibit that electrodeposition of copper onto MPS or SPS modified Au(111) electrode is a two-dimensional growth when potential swept to the UPD region, then a three-dimensional growth mode was observed as stepping potentials negatively to the OPD region. Additionally, in the OPD region, a copper film with distributed grains of different size forming over the MPS or SPS-covered Au(111) surface instead of a copper overlayer consisting of a low number of isolated grains when only 0.1 M H2SO4 only present in the electrolyte. III. Electrochemical Deposition of Copper onto SPS and MPS Modified Au(111) in Electrolyte Contains Chloride Cyclic voltammograms reveal that Cl－ is a catalyst for copper electrodeposition, the presence of Cl－ not only accelerate the copper deposition process, but also increase the amount of copper deposit on Au(111) electrode. In situ STM images reveal that a copper monolayer with a Moiré pattern formed at Au(111) surface in the UPD region. Additionally, large copper isolated grains were seen while stepping potential negatively to the OPD region. Copper deposition from chloride ion containing sulfuric acid solutions onto MPS or SPS-modified Au(111) electrodes was also studied by CV and in situ STM. CV results show the combination of MPS and Cl－ creates a strongly accelerating effect on copper electrodeposition. In situ STM imaging reveals that copper UPD onto MPS or SPS-modified Au(111) by forming a monolayer, and bulk deposition with a low number of isolated grains located on the Au(111) surface. By comparison the surface morphology obtained by STM, we are surprised that the behavior of copper electrodeposition is dominated by Cl－ rather than the addition of MPS or SPS.
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