Electrochemical Characterization of the Self-Assembled 4,4’-Bipyridine Layers at the Sb(111) | Ionic Liquid Interface
Authors: Siinor Liis , Ers Heigo , Pikma Piret , Siimenson Carolin , Lust Enn 
Published: July 2022

Self-assembled monolayers (SAMs) have significant importance in many modern applications1-2. The characteristics of SAMs depend on many aspects, like the characteristics of individual molecules, forming the monolayer; by the interactions between these molecules; and by SAM interactions with the electrolyte; the crystalline surface of the substrate; etc3. Therefore, the fundamental studies of these systems are crucial to determine 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 and compare the results with previous data measured in aqueous electrolyte solutions.
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) | 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., nitrogen-controlled atmosphere.
CV method was used to determine and to evaluate the electrochemical characteristics of the system under investigation within potential region, −0.9 V < E < 0.0 V vs. Ag|AgCl. The current values increase rapidly at E < −0.8 V, indicating that a reduction process started at the Sb(111) | 4,4’-BP+EMImBF4
interface. Thus, unlikely to 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 relation C

s

=−(Z′′2𝜋f)

−1

. Capacitance values calculated depend on the composition of the electrolyte (i.e., on the concentration of 4,4’-BP) as well as on the potential applied. C

s
values are lower in the mixture than those measured in pure EMImBF4 at E > – 0.7 V. This effect is caused by the adsorption and formation of the compact layer of 4,4’-BP molecules at the electrode surface. The C

s

vs.
E dependences of the mixture increase significantly at E < −0.8 V (similarly to CV curves), which is not visible in the case of pure EMImBF4. This is also confirmed by small negative phase angle values at E < −0.8 V, where the shape of the phase angle vs. frequency (f) curves shows that mixed kinetic processes occurring in the whole f range measured (i.e. diffusion and charge transfer steps in addition to adsorption). At E > −0.8 V, phase angle values are close to − 80° for Sb(111)|4,4’-BP + EMImBF4 in a wide range of f. Only at low frequency values the mixed kinetic processes (adsorption, diffusion and partial charge transfer) were observed.
The formation of ordered 4,4’-BP arrangements was most pronounced in the potential range −0.7 V < E < 0.0 V due to the reductive desorption of 4,4’BP at E < −0.8 V.
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).
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