The electrified interfaces of mercury electrodes potentiostatically deposited onto a well-ordered Ir(111) electrode has been examined with cyclic voltammetry and in situ scanning tunneling microscopy (STM). A thickness of 30 monolayer of the mercury film is estimated from the amount of charges passed during the reduction of Hg2+. This filmy Hg electrode exhibits similar electrochemical behavior as that of bulk Hg.
The cyclic voltammograms (CV) obtained in 0.1 M perchloric and sulfuric acid solutions are alike. The CV profile is essentially featureless between –0.5 and 0 V (vs. Ag/AgCl), which is ascribed to be the double-layer charging region. Adsorption of anions, such as perchlorate and sulfate, give rise to a slight increase of current at potential positive of 0 V. In contrast, the cyclic voltammograms, obtained in solutions containing 10 mM potassium halide, exhibit two pairs of features, ascribable to the adsorption/desorption of halides and the formation/stripping of mercury(I) halide (Hg2X2) compounds. The potential regions over which these processes occur vary with the strength of the surface bonding between halide and Hg and the standard potential of Hg2X2. They appear at most negative potential for iodide, followed by bromide and chloride, and finally fluoride. Coulometric results integrated from the reduction wave of the Hg2X2 films (X = I-, Br-, Cl-) indicate the films are 8-9 layers thick, while no such surface film forms in potassium fluoride.
Although it is rather difficult, if not impossible, to image bulk Hg with STM in solutions, we are able to achieve STM atomic resolution of the as-prepared Hg films covered with specifically adsorbed halide. It is remarkable to note that the Hg films exhibit typical terrace-and-step characteristics of a solid, although bulk Hg is known to be a liquid metal at room temperature. It is also surprising to see that the surface morphology of the Hg films bear a strong resemblance to that of the Ir(111) substrate. Because the orientations of steps and breath of the terraces are essentially identical to those of the Ir(111) substrate, it seems that the Hg films, despite being as thick as 30 atomic layers, grow under the guidance of the substrate. On the other hand, the morphological features changes rapidly with time, suggesting that the Hg atoms are rather mobile at the potential of zero charge (PZC) in perchloric acid solution. The high-resolution STM scans are always noisy, we were not able to achieve atomic resolution of the bare Hg surfaces under these conditions.
Time-sequenced STM images are acquired to show the adsorption events of iodide anions, preferentially starting at the lower edges of steps and then creeping onto the terraces. This process proceeds in a nucleation-and-growth, rather than random fashion, suggesting the attractive interaction of the adsorbed halides. STM atomic resolutions are acquired for all the halide, except fluoride because it binds weakly with Hg. Despite adsorbing in ordered superlattices, they are mostly incommensurate. Iodide anions are adsorbed in two unexpected square-like arrays with coverage of 0.52 and 0.61 at potential of –150 and 0 mV, respectively. In contrast, the adlattices of bromide and chloride anions are mostly hexagonal with lattice constants resembling their van der Waals diameters (3.6 and 3.4 Å for Br- and Cl-, respectively). The results of chloride adlayers suggest the possibility of surface hydration, along with the coadsorption of potassium cations, as the anionic character of the chloride increases at more negative potential. STM atomic resolutions were also obtained for the Hg2I2 species formed at potential positive of 200 mV. It grew locally in a layered and crystalline fashion, leading to substantial roughening of the surface morphology.
Oxidation of Hg in 10 mM potassium fluoride does not result in crystalline Hg2F2. Rather, real-time STM imaging of mercury films reveals pronounced dissolution of metallic Hg at potential positive of 0.15 V. Presumably, Hg dissolves as Hg22+ cations, which do not seems to undergo disproportionation to Hg and Hg2+. The Hg22+ cations are likely to be stable in acidic solutions. STM imaging of the Hg electrode after extensive dissolution of Hg reveals still well-defined steps and terraces without noticeable precipitation. Furthermore, we were able to obtained near atomic resolution of the water layers formed at -650 mV more negative than the PZC. Although the images are not clear to decipher the real-space structures of water molecules, tentatively, this is the first direct view of a water layer on metal surfaces.
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