dc.description.abstract | In situ scanning tunneling microscopy (STM) and cyclic voltammetry (CV) have been used to examine the reconstruction, molecular adsorption, and metal deposition at the interfaces of well-ordered Au(111), Au(100), and Pt(100) electrodes.
The atomic structures of well-ordered Au(111) and Au(100) surfaces were examined meticulously in 0.l M HClO4, aiming to unraveling the potential-controlled phase transition from the reconstruction to the unreconstruction. In the case of Au(111) in situ STM results reveal typical herringbone structures with pairwise protruded stripes repeating in every 6.3 nm at the negative potential (0 V vs. RHE). This characteristic spacing of 6.3 nm increased with more positive potential (0.4 V) to 6.83 nm before each pair of stripe was diminished to produce nanometer size gold islands on the surface of Au(111). The spacing between the paired lines, however, did not change with potential, which implied that the lifting of reconstruction occurred via the expansion of the face-centered-cubic (fcc) domain with no change in the hexagonal-close-packing (hcp) domain. However, this expansion of atomic lattice of two corrugated “ropes” did not appear on the Au(100) surface. On both Au(111) and Au(100), the number of protruded islands increased with more positive potential. On the other hand, the herringbone structures on Au(111) and “waves” on Au(100) decreased from the reconstruction to the unreconstruction.
In situ STM imaging of Au(111) and Au(100) revealed anisotropy in surface dissolution and deposition at -0.6 V in pH11, 0.1 M KClO4 containing 50 ?M KCN. This anisotropic phenomenon largely originated from the intrinsic anisotropy in the surface energy of the reconstructed surface structures of both electrodes. Cyanide species were predominant anions on the surfaces of Au(111) and Au(100), even in acidic KCN solutions. The predominant species in acidic cyanide solution, HCN, is proposed to undergo oxidative adsorption resulting in dehydrogenation in this process. Cyanide ad-species were found to adsorb in (6 ? 6), (?7 ? ?7)R19.1°, and oblique(?7 ? ?21) on Au(111), and (3?2 ? 3?2)R45° on Au(100) in 0.1 M HClO4 containing 50 ?M KCN. All molecular features in the STM images of Au(100) produced the identical corrugation height, implying that CN- ad-species resided at atop sites, which was consistent with the results of spectroscopic studies. The 3-fold fcc and hcp sites were the most likely coordination sites of CN- ad-species on Au(111). At 0.7~0.8 V, a compound phase of Au+CN- adlayer was formed on both electrodes. Presumably, the production of this phase is driven by the positive potential which pulls cyanide into the gold lattice. Gold dissolves universally once the potential is made more positive than 0.8 V. Surfaces of Au(111) and Au(100) appeared to be rough under this condition, which contrasted markedly with the atomically flat pseudo layer-by-layer dissolution observed at negative potentials.
Immersion of Au(111) or Au(100) electrodes under potential control at 0.1~0.7 V in a CO-saturated perchloric acid generated several ordered structures, but they gradually transformed into a more stable (9 ? ?3) - 5CO structure on Au(111) and a hitherto undetermined structure on Au(100). Molecular-resolution STM revealed CO molecules of different corrugation heights, implying CO molecules were adsorbed on different symmetric sites. Dosing Au(111) with a CO-saturated HClO4 at 0.1 V caused lifting of the Au(111) - (?3 ? 22) reconstruction, but adjusting the potential from 0.1 to -0.1 V rendered the reconstructed Au(111) to reform, with increasing the periodicity of the herringbone feature from 6.36 to 7.5 nm. This phenomenon is similar to the wavy features seen on Au(100) from 1.49 to 1.7 nm. Molecular-resolution STM revealed that not all CO molecules have the same corrugation heights, unlike implies CO molecules are adsorbed on different sites. Raising the potential from 0.7 to 0.8 V resulted in a slow phase transition from (9 ? ?3) - 5CO to (7 ? ?7) - 3CO on Au(111), as CO molecules were partially desorbed or electrooxidized. The coverage decreased from 0.28 to 0.14 as a consequence of these restructuring events. These results suggest that electrooxidation of CO proceeded via the Langmuir-Hinshelwood mechanism on Au electrodes.
Finally, high-resolution STM and CV techniques were employed to examine the self-assembled monolayers (SAMs) of organosulfur (benzenethiol, BT) admolecules, the deposited bilayer of silver, and tip-induced of copper deposition on Pt(100) electrodes in electrochemical environment. The BT admolecules are adsorbed in a highly ordered adlattice on Pt(100), identified as (?2 ? ?2)R45° by molecular-resolution STM imaging. Two neighboring admolecules are separated by only 0.39 nm within this ordered array, necessitating vertically molecular configuration with its sulfur-headgroup directly bonded to 4-fold hollow sites on Pt(100).
Prior to the commencement of bulk deposition, underpotential deposition (UPD) of Ag proceeded stepwise depositing bilayer of Ag metal on Pt(100). The first step resulted in a sharp peak at 1.15 V in the CV with a charge of 189 ?C/cm2 which is close to that needed for the deposition of a monolayer of Ag. STM imaging results also supported this reasoning, as a pseudo-morphic Ag adlayer was observed. The second and third Ag UPD steps are close in potential, both commencing near 0.4 V more negative than the first one. They contain 116 and 40 ?C/cm2 charges, which added up to account for the deposition of another monolayer of Ag. STM discerns a (?2 ? ?2)R45? structure, which transformed to a pseudo-hexagonal lattice before bulk deposition.
The overlap of electric double layers between the STM tip and Pt electrodes is illustrated by using Cu deposition as a probe. It is shown that the local potential at an Pt(100) electrode under the tip differed from the remaining area whose potential was pinned mainly by the potentiostate. The dimension of the tip-dominated area varied with the scan size, indicating that only the very end of the tip was influential to the electrode potential. This tip-and-substrate interaction is likely to be electrostatic. In addition, since the Pt(100) surface was coated with the iodine adatoms, the deposited Cu adatoms would selectively go to those iodine sites of the weakest bonding. | en_US |