Electrochemical Characterization of the Self-Assembled 4,4’-Bipyridine Layers at the Electrode | Ionic Liquid Interface

Abstract

Self-assembled monolayers (SAMs) have significant importance in many modern applications1-2. The properties of SAMs depend on many aspects, such as the characteristics of individual molecules, forming the monolayer; by the interactions between these molecules; and SAM interactions with the electrolyte; the crystalline surface of the substrate; etc3. Therefore, the fundamental studies of these systems are crucial in determining the function of SAMs in various applications4-5.The aim of the current work was to study the adsorption characteristics of 4,4’- bipyridine from ionic liquid media at different electrodes6.Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) measurements were conducted to characterize the electrochemical behavior of the self-assembled 4,4’-bipyridine (4,4’-BP) layers at the Sb(111) and Cd(0001) | 4,4’-BP+1-ethyl-3-methylimidazolium tetrafluoroborate (EMImBF4) interface. All the experiments were carried out in a three-electrode electrochemical cell in a glove-box, i.e., a nitrogen-controlled atmosphere.CV method was used to examine the electrochemical characteristics of the system under investigation within the potential region −0.9 V < E < 0.0 V for Sb(111) and −2.1 V < E < -0.8 V for Cd(0001) vs. Ag|AgCl. The current values increase rapidly at the more negative end of the studied potential region, indicating that reduction processes started at the Sb(111) and Cd(0001) | 4,4’-BP+EMImBF4 interface. In contrast with the data measured in EMImBF4, where the system shows ideal polarisable behaviour in the whole potential range. Interfacial capacitance (C s) values were calculated using the relation Cs=−(Z′′2\U0001d70bf)−1 . Capacitance values calculated depend on the composition of the electrolyte (i.e., on the concentration of 4,4’-BP), electrode material as well as on applied potential. Cs values are lower in the mixture than those measured in pure EMMImBF4 at a certain potential region in the case of both electrode materials. This effect is caused by the adsorption of 4,4’-BP molecules at the electrode surface. The shape of the phase angle vs. frequency (f) curves shows that mixed kinetic processes occur in the whole f range measured (i.e. diffusion and charge transfer steps in addition to adsorption).Acknowledgments:This work was supported by the Estonian Research Council grant PSG249, and by the EU through the European Regional Development Fund under project TK141 (2014-2020.4.01.15-0011).