dc.description.abstract | In situ scanning tunneling microscopy (STM), cyclic voltammetry (CV) and reflection high energy electron diffraction (RHEED) have been used to examine mercury films electrochemically deposited onto well-ordered single crystalline electrodes of Pt(111) and Ir(111). In situ STM imaging reveals atomically flat surface morphology of mercury films deposited on ordered Pt(111) and Ir(111) electrodes, implying layer-by-layer growth mode of Hg on these electrodes in 0.1 M HClO4 + 1 mM Hg(ClO4)2. Depending on the amount of Hg deposit on Pt(111) electrode, in situ STM reveals a number of ordered arrays, which could derive from the production of intermetallic compounds with unidentified chemical compositions. These structures could be associated with PtHg2 and PtHg4 characterized by X-ray diffraction methods in previous studies. These ordered arrays of PtHg could be transformed reversibly by the modulation of potential, which could reflect surface alloy and dealloy as a function of potential.
Reflection high energy electron diffraction (RHEED) was used to characterize mercury film electrodeposited Pt(111) electrode at room temperature. Depending on the amount of Hg deposit, two different growth modes were observed. At low Hg coverage, crystalline (0001)Hg adlayer accompanied by 30◦-rotated (111)-Pt patches was found on Pt(111). Deposition of multilayer Hg resulted in layered PtHg2 and PtHg4 amalgams, which grew epitaxially by aligning their (201) and (1-10) planes, respectively, parallel to the Pt(111) substrate. The preference of these epitaxial relationships for the electrochemically formed Pt–Hg intermetallic compounds on Pt(111) could result from minimization of the surface energy.
In situ STM was also used to examine multilayered Hg film prepared by electrochemical deposition onto well-ordered Ir(111) single crystal electrode in 0.1 M HClO4 + 1 mM Hg(ClO4)2. Topography STM scans discerned the surface morphology of the as-prepared Hg film, but it was difficult to achieve atomic resolution. In potassium iodide solution atomic resolution STM images were readily obtained on the Hg film, revealing the structures of an iodine monolayer adsorbed on this Hg film. Iodine adatoms formed an incommensurate hexagonal array with a nearest neighbor spacing of 0.43 nm at -0.35 V. Shifting potential positively transformed the iodine adlattice into a commensurate (8 ? 2?3)rect structure, which is further converted into another incommensurate structure before the Hg film was oxidized to Hg2I2-like species. The formation of highly ordered iodine adlayers on Hg film indicates that the Hg substrate had to be crystalline at room temperature, which highlights the most intriguing finding of this study. The lattice constant of the Hg deposit is estimated to be 0.31 nm from the structure of iodine adlattice. This is consistent with separate electron diffraction measurements.
To gain insights into the electrodeposition of heavy metal on Hg film, we used in situ STM to examine electrodeposition of Cd and Cu on Ir-supported Hg films. The as-produced Cd and Cu films were atomically smooth up to a thickness of 3 - 4 layers. The surface morphology of the Hg film appeared to become roughened after the Cd deposit, but remained mostly unchanged after the deposition and removal of Cu. We found that the surface roughness of Hg film hinges on how much Cd was deposit and how long the Cd deposit was allowed to stay on the Hg film. To further substantiate this issue, we obtained STM results with Hg film electrode coated with sub-monolayer of Cd and Cu. Cu is deposited on Hg film in a layered fashion, producing crystalline Cu film, as revealed by STM atomic resolution. It did not amalgamate with the Hg substrate. Cd is deposited in a layered fashion, producing Cd film on Hg film. It amalgamates with Hg film easily, which is responsible for the broader stripping peak, as compared to Cu. These STM results are useful in improving the understanding of the use Hg as the electrode for stripping analysis. | en_US |