ATOMIC STRUCTURES OF A Cu(111) SURFACE UNDER ELECTROCHEMICAL CONDITIONS: AN IN-SITU STM STUDY

Abstract

The atomic structures of a Cu(111) electrode in dilute sulfuric electrolyte have been studied using in-situ STM. At anodic potentials near the copper dissolution the adsorbed sulfate anions form a characteristic anisotropic Moiré pattern. The appearance of the long range Moiré modulation is explained by a sulfateinduced reconstruction (expansion) of the topmost copper layer and the resulting misfit between the first and the second copper layer. For the first time it was possible to image not only the sulfate adsorbate but also the underlying reconstructed copper substrate at the same anodic working potential by a systematical variation of the tunneling parameters. On an atomic scale the observed sulfate structure on Cu (111) is very similar to those found for other fcc(111) surfaces (Au, Pt and Rh). On these electrodes sulfate anions form a regular ( √ 3 × √ 7) superstructure. For the Cu(111) surface a ( √ 3 × √ 7)-like unit cell is found which is slightly distorted. Also for the first time it was possible to image the atomic structure of an electrode during a massive hydrogen evolution current at cathodic potentials. Even under such extreme electrochemical conditions far from the thermodynamic equilibrium an ordered superstructure of hydronium cations is found. At the anodic end of the cyclic voltammogram a pit-etching mechanism is induced by a fast copper corrosion. ExperimentalAll STM measurements shown in this paper were carried out using a homebuilt STM based on the "beetle type" which was modified for the electrochemical 907 Surf. Rev. Lett. 1999.06:907-916. Downloaded from www.worldscientific.com by FLINDERS UNIVERSITY LIBRARY on 02/07/15. For personal use only. Surf. Rev. Lett. 1999.06:907-916. Downloaded from www.worldscientific.com by FLINDERS UNIVERSITY LIBRARY on 02/07/15. For personal use only.

In-situ Characterization of Metal/Electrolyte Interfaces: Sulfate Adsorption on Cu(111)

“…The main spots in Fig. 2 are attributed to adsorbed sulfate molecules whereas the smaller spots arranged in zigzag rows between the sulfate particles are assigned to coadsorbed water molecules [13]. This structure is very similar to those sulfate adlayers observed on other fcc(111) electrode surfaces (Pt, Rh, Au, Pd) [14][15][16][17][18][19][20][21].…”

“…By means of bias potential dependent (i.e. spectroscopic-like) STM studies it is shown that the appearance of this Moiré pattern is caused by a reconstruction of the topmost copper layer [13]. The long range modulation arises from a misfit between the reconstructed (expanded) first and the underlying unreconstructed second copper layer.…”

“…All these adlayers are based on a 2 1 1 1 2 unit cell. Note, however, that the additional long range modulation of this structure (darker regions) is only observed on copper [13,22]. By means of bias potential dependent (i.e.…”

“…The long range modulation arises from a misfit between the reconstructed (expanded) first and the underlying unreconstructed second copper layer. A detailed analysis of this reconstruction by means of spectroscopic-like STM measurements, which allow a simultaneous imaging of the sulfate adlayer and the underlying reconstructed copper layer in one and the same STM picture, can be found elsewhere [13,23]. The measurements provide direct evidence for a 6% expansion of the sulfate covered first Cu layer as well as full registry of the sulfate overlayer with this expanded Cu surface lattice.…”

Adsorption of Dodecanethiol on Cu(110): Structural Ordering upon Thiolate Formation

“…The lack of complete agreement between the STM image and a simple hard-ball stacking model on an unperturbed substrate suggests that the observed Moiré pattern may result from a more complicated mechanism, perhaps also involving a lateral relaxation of the Cu substrate, similar to that observed for a sulfate-covered Cu(111) surface. 31 No evidence for a pronounced reconstruction of the Cu(110) substrate underneath the dodecanethiol layer was found in the present STM and LEED data. This is in contrast to the complete rearrangement of the top layer that has been observed following the adsorption of thiols on Cu(111).…”