Adsorbate Isotherm Analysis by Reflection Anisotropy Spectroscopy on Copper (110) in Hydrochloric Acid

Abstract

Reflectance anisotropy spectroscopy (RAS) is a powerful optical probe that works on a polarization contrast basis. It can be operated in any environment, ranging from ultrahigh vacuum to vapor phases and liquids. The measured optical anisotropies are caused by several symmetry breaking effects and are exclusively assigned to the surface for otherwise bulk isotropic materials. In this work, we present a systematic study comprising in situ RAS-transient to assess the surface thermodynamics of the chloride adsorption on Cu(110) upon systematic variations of the applied electrode potentials in comparison to cyclic voltammetry (CV). Numerical time-derivatives of the measured RAS-transients are shown to be exclusively associated with electrical currents of those electrochemical reactions, which change the properties of the electrode surface. The recorded transient line-shapes track the Frumkin type isotherm properties related to chloride coverage. Both connections are theoretically discussed. Owing to the surface and interface specificity, RAS is shown to exhibit a high surface sensitivity. In particular, processes taking place in parallel, namely, the hydrogen evolution reaction (HER) as well as the copper dissolution as Cu+ and Cu2+, do not contribute to the RAS response.

Chloride-Induced Surface States in Cu(110)/Liquid Interfaces

“…The impedance was measured at electrode potentials between −550 and −100 mV versus Ag/AgCl in 3 M/L NaCl. The boundaries of the process are at the cathodic side, the onset of the hydrogen evolution reaction (HER) and at the anodic side, the Cu ++ dissolution . The frequency range was selected on the basis of the observed relaxation times between 1 Hz and 10 kHz.…”

“…It is around this potential that EC-STM has shown a faceting of the surface . In ref , these two processes have been denoted as “A” and “B”, and both are superimposed by the onset of the so-called “constant current” region associated to Cu + dissolution . The respective area is marked with olive lines in Figure .…”

“…Additionally, the formation of a CuCl aggregate layer is reported. In CV experiments, the Cu + dissolution shows up in a peak “C”, which depends strongly on the scan speed. ,, At even higher potentials, the α-value further decreases because of the Cu ++ dissolution. It is important to note that ECM is not sufficient to reproduce the diffusion-limited dissolution in a correct manner.…”

“…CV scans (not shown here), measured before and after the experiments, and after every RAS spectrum, were used as reliable indicators for the successful preparation of the sample and the protection of the electrolyte and the sample against oxygen as well. In this way, we ensure that our experiment meets the conditions of a well-defined Cu(110) surface as previously measured with the EC-STM. , …”

“…Whether these findings share the same origin remains an open question despite both structures being obviously induced by an adsorption of anions. As a matter of fact, quasi static experiments show that anions (Cl – ) adsorb with an attractive lateral interaction, which is seemingly responsible for the chain-like structures …”