In Situ STM Studies of Electrochemically Polished Cd(0001) Electrode in 1-Ethyl-3-Methylimidazolium Tetrafluoroborate
Authors: Pikma Piret , Selberg Simona , Siinor Liis , Siimenson Carolin , Lust Enn 
Published: February 2020

Recently studies of room temperature ionic liquids (RTIL) have had a lot of attention in modern chemical research. Due to their high stability under applied potential, the wide electrochemical window and dual usability as solvent and electrolyte RTILs have become very attractive in the field of applied electrochemistry and modern energetics [1].
The performance of RTIL based devices, i.e. the electrochemical applications of RTILs depend on the interaction between the RTIL and the geometrically rough and energetically heterogeneous interfaces. Hence, a detailed understanding of the RTIL|solid electrode interface structure, thermodynamics and kinetics is crucial for designing modern electrochemical devices.
The electrochemical kinetics and adsorption properties of various interfacial charge transfer processes at the solid surfaces depend greatly not only on the chemical composition but also on the geometrical characteristics and morphology of the surface studied. According to our knowledge, there are no data for Cd (0001) plane obtained under the electrochemical in situ STM (scanning tunneling microscopy) conditions in RTIL in the literature. Therefore, the main aim of these investigations was to develop the experimental conditions needed to obtain the atomic resolution electrochemical in situ STM data within the wide electrode potential region of the Cd(0001) electrode similarly to our previous works analyzing atomic resolution level structure of the Cd(0001) single crystal electrode in aqueous surface inactive electrolyte solution [2].
The electrochemically polished Cd (0001) plane has been submerged under cathodic polarization (E = −1.1 V vs. Ag|AgCl) into the EMImBF4, previously saturated with Ar (99.999%) inside the glove box (Ar 99.999% atmosphere). The self-made hermetic three-electrode cell with large Pt counter electrode and Ag|AgCl reference electrode, connected to the in situSTM cell through Luggin capillary, has been used.
During the measurements atomically flat areas have been observed on the in situ STM images of cadmium (0001) single crystal electrode surface (Fig. 1). Also in situ STM atomic resolution images were obtained for the electrochemically polished Cd (0001) electrode. According to these data, the regular atomic structure can be observed with interatomic distances d = 2.9 ± 0.1 Å, which are in a good agreement with Cd (0001) crystallographic parameters applying hexagonal close-packed structure model for Cd (0001) plane. It was also determined that the structure of nanometric terraces separated by the steps, is very stable during few hours under the cathodic polarization from –1.6 < E < –0.9 V.

Acknowledgements: This study was partially funded by the Estonian Science Foundation Grants Nos.7791 and 8357, Estonian Energy Technology Program Project SLOKT10209T, Estonian Basic Research Project SF0180002s08 and Estonian Centers of Excellence in Science Project: High-technology Materials for Sustainable Development TK117.

References:

[1] H. Ohno (Ed.), Electrochemical Aspects of Ionic Liquids, John Wiley & Sons, Inc., New Jersey, 2005, p.1.
[2] P. Pikma, V. Grozovski, H. Kasuk, E. Lust, Sur. Sci. 628 (2014) 86.

Figure 1