In situobservation of monolayer structures of underpotentially deposited Hg on Au(111) with the atomic force microscope

Abstract

The structures of monolayers of Hg atoms underpotentially deposited on Au(l 11) were resolved with the atomic force microscope. In sulfate, nitrate, and perchlorate electrolytes, a hexagonal overlayer with a 0.58 ±0.02-nm spacing was seen. In acetate, a hexagonal lattice exhibiting a 0.74±0.05-nm spacing was observed positive of the underpotential-deposition potential, a rhombic lattice with a 0.43 ± 0.02-nm spacing was seen at intermediate potentials, and another hexagonal lattice with a 0.30±0.03-nm spacing was found just positive of bulk deposition. PACS numbers: 68.55.-a, 82.45.+Z Development of a detailed understanding of the structure of the electrified solid-liquid interface is a central step in understanding electrochemical reactivity. A fundamental example of this reactivity is the deposition of a monolayer of one metal onto another. This phenomenon, known as underpotential deposition (UPD) [1], occurs when metal ions are electrodeposited onto a (different) metal surface at potentials positive from the reversible (Nernst) potential. In this process, only a monolayer or submonolayer of the metal forms on the electrode surface. The structure of these monolayers has attracted considerable attention because of their importance in electrodeposition and electrocatalytic [2] processes. In UHV environments, monolayers of foreign metal adatoms evaporated onto (ill) surfaces exhibit closepacked incommensurate structures [3,4]. These closepacked structures arise from net attractive forces between the adatoms. The demonstration that UPD monolayers of Cu [5,6] and Ag [7] form open adlattice structures in the electrochemical environment has been taken to imply that the force between adatoms has become repulsive, possibly deriving from coadsorption of electrolyte with the metal. Alternatively, other effects could also serve to open the adlattice. In particular, retention of a partial charge on the adatom, complex formation with the electrolyte, or deposition of a non-fcc element onto a (111) surface of an fee material could all lead to open lattices.In this Letter, we report an atomic force microscope ( AFM) [8] structural study of UPD of Hg onto Au(l 11) surfaces in four different electrolytes. The Hg UPD process is particularly important because of the technical significance of Hg amalgams and Hg surfaces in electrochemical and other processes. Underpotential deposition of Hg has been studied extensively with standard electrochemical techniques [9-15], but fundamental insight has been elusive because of the lack of microscopic surface structural information. In addition, the sensitivity of this process to different electrolytes is unknown. This latter point is important because UPD lattices of Cu [5,6] andAg [7] on Au(lll) surfaces exhibit strong electrolyte sensitivity. AFM images were obtained with a Nanoscope II AFM [16] operating in constant force mode. The force, optimized for each image, was nominally 1 x 10"^ N. A cell made from glass held the electrolyte. Au(l 11) electrodes were prepared by evaporation onto ...

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“…In situ observation of mercury monolayer deposition in underpotential conditions on the Au (1 1 1) electrode [16] has shown that the structure of this monolayer is strongly influenced by the experimental conditions, chemical composition of the background electrolyte and the deposition potential. In acetate buffer, the deposited hexagonal mercury lattice has a 0.74 nm atom-atom spacing, which surpasses ca.…”

Interactions and reactivity of Hg(II) on glutathione modified gold electrode studied by EQCN technique

“…The Hg-upd films appear to be partialcharge transfer coadsorption systems that include electrolyte anions in the case of sulfate, chloride, and acetate solutions, with stoichiometry of the film strongly depending on electrolyte anions [20]. For instance, in sulfuric acid media, several 2D structures have been discovered including open hexagonal structure (Chen and Gewirth [9]), commensurate open rectangular 2D lattice (Itaya and coworkers [10]), metastable open incommensurate structure and two other structures (Li and Abruna [11,12], transformed to a final fully discharged liquid Hg 0 monolayer on an amorphous Au(Hg) amalgam [20]. In perchloric acid solutions, upd-Hg was found to conform to Langmuirian isotherm without participation of anions [13].…”

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“…The in-depth structural studies of the Au-Hg-aqueous solution interface performed on Au(h k l) surfaces using AFM [9,16], STM [10], and light synchrotron X-ray techniques [11,12,[17][18][19] including grazing incidence X-ray diffraction (GIXD), extended X-ray absorption near-edge spectroscopy (XANES), and specular crystal truncation rod (CTR) technique, have revealed new details of the atomic structure of upd films which could not be resolved in earlier kinetic studies. The Hg-upd films appear to be partialcharge transfer coadsorption systems that include electrolyte anions in the case of sulfate, chloride, and acetate solutions, with stoichiometry of the film strongly depending on electrolyte anions [20].…”

A new cell design for potentiostatically controlled in situ atomic force microscopy

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