Formation of 2,2′-bipyridine adlayers at Sb(111)|ionic liquid + 2,2′-bipyridine solution interface

doi:10.1016/j.elecom.2015.08.012

Electrochemistry Communications

Volume 61, December 2015, Pages 61-65

https://doi.org/10.1016/j.elecom.2015.08.012 Get rights and content

Highlights

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2,2′-Bipyridine adsorption from ionic liquids
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Good agreement between modeled and measured EIS and in situ STM data
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Influence of potential and polarization time on 2,2′-bipyridine adsorption layer structure

Abstract

In situ scanning tunneling microscopy (STM), cyclic voltammetry and electrochemical impedance spectroscopy methods have been applied to study the adsorption of 2,2′-bipyridine (2,2′-BP) on Sb(111) single crystal electrode surface from 1-ethyl-3-methylimidazolium tetrafluoroborate + 1% 2,2′-BP solution. Influence of the Sb(111) electrode potential on the adsorption characteristics of 2,2′-BP has been demonstrated. The in situ STM data revealed that 2,2′-BP forms highly ordered patterns on the Sb(111) single crystal electrode surface. The formed dense organic adlayer decreases the dielectrical permittivity of the interface, acting as a dielectric capacitor with significantly lower characteristic relaxation time than the pure ionic liquid system.

Introduction

In recent years room temperature ionic liquids (RTILs) and the adsorption of organic compounds on metal substrate surfaces have attracted a considerable amount of attention [1], [2], [3]. The RTILs' high stability under applied electrode potential, low vapor pressure, excellent thermal stability, high ionic conductivity, non-flammability and tunable counterpart ions have made the RTIL|electrode interface a very attractive research subject for modern technological applications, since it determines the characteristics of various electrochemical power sources, including batteries, supercapacitors, fuel cells, etc [1], [2], [3], [4], [5], [6], [7]. In particular, the wide ideal polarizability region and dual usability as solvent and electrolyte have opened interesting possibilities in the field of fundamental and applied electrochemical studies [1], [2], [3], [4], [5], [6], [7]. The molecular self-assembly of organic molecules on solid surfaces has been in the center of interest, because the formation of the nanostructures with well-controlled properties and functionality reveals fascinating perspectives in nanoscale science and nanotechnology [8], [9], [10], [11], [12], [13], [14], [15], [16]. For instance, the smart tailoring of the structural properties of the functionalized electrodes attracts a lot of attention due to an ability to trigger specific electrochemical processes. The effect of organic molecule functionalization of electrodes in ionic liquid media has been studied in very few research papers [17], [18], [19], [20] that detail the effect of the concentrated ionic atmosphere on adlayer stability and charge transfer properties.

Although these two major research subjects have been disserted in many papers, to our knowledge no data analyzing the strong specific adsorption process of organic compounds from RTILs can be found in the literature.

The main aim of this study was to investigate the adsorption of 2,2′-bipyridine (2,2′-BP) from RTIL on single crystal metal electrodes and to compare the adsorption characteristics with aqueous electrolyte interface.

2,2′-Bipyridine is a well-known bidentate chelating agent for transition metal ions. The 2,2′-BP molecule is an interesting compound for surface coordination studies, since the molecule may assume different conformations when adsorbed on a metal surface. Therefore the interaction of 2,2′-BP with metal surfaces has been extensively studied. STM results show that 2,2′-BP forms well-ordered molecular stacks of perpendicularly oriented molecules when adsorbed on the positively charged Au (111) surface [8], [21], [22], [23], [24], [25].

To conduct this study 1-ethyl-3-methylimidazolium tetrafluoroborate (EMImBF₄) was chosen as the RTIL, since it has been examined thoroughly in various papers and has been proved to be very stable at various electrodes [4], [5], [6], [7]. Sb(111) single crystal electrode was chosen as the substrate, since the unique electronic properties of Sb, such as small Fermi energy combined with highly anisotropic electron and hole masses and charge carrier concentrations several orders of magnitude lower than that for noble metals are the basis of many possible applications, including thermoelectric converters [26].

Section snippets

Experimental

The surface of Sb(111) has been prepared by electrochemical polishing in saturated aqueous KI + 0.5% HCl aqueous solution at a current density 0.5 < j < 1.5 A × cm^− 2 [27]. After electrochemical polishing the Sb(111) was submerged under small positive surface potential (E = − 0.6 V vs. Ag|AgCl|EMImBF₄) into the EMImBF₄ (Fluka Analytical, for electrochemistry, ⩾99.0%, H₂O < 200 ppm) + 1% 2,2′-BP (by mass) solution (Aldrich, 99 + %). The hermetic three-electrode cell with large Pt counter electrode and Ag|AgCl|EMImBF

Results and discussion

Current density (j) vs. electrode potential (E) dependencies (CVs, Fig. 1a) were measured within a potential region from − 0.9 to 0.1 V for Sb(111)|EMImBF₄ interface (curve 1, corresponding to the region of ideal polarizability) and from − 1.0 to 0.0 V for Sb(111)| EMImBF₄ + 2,2′-BP (vs. Ag|AgCl|RTIL) interface. Based on the data in Fig. 1a, there are no surface oxidation or reduction peaks in the CVs at Sb(111)|EMImBF₄ interface. The values of j are somewhat higher for the EMImBF₄ + 2,2′-BP solution

Conclusions

Adsorption characteristics of 2,2′-bipyridine (2,2′-BP) at Sb(111) electrode surface from 1-ethyl-3-methylimidazolium tetrafluoroborate + 1% 2,2′-BP solution has been studied using electrochemical and in situ scanning tunneling microscopy (STM) methods. Analysis of cyclic voltammetry and impedance results reveals that 2,2′-BP indeed adsorbs at the Sb(111) interface, forming a thin dielectric layer at the electrode surface decreasing the differential capacitance nearly two times. This formed layer

Conflict of interest

The authors declare that there are no conflicts of interest.

Acknowledgments

This study was partially funded by Estonian Energy Technology Program Project SLOKT10209T, Tartu University Institutional Research Project IUT20-13 and Estonian Centers of Excellence in Science Project: High-technology Materials for Sustainable Development TK117.

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The organised layer of molecules on a solid surface is at the centre of numerous molecular devices. Although the adsorption of molecules from aqueous electrolytes has been thoroughly investigated, only a limited number of studies have focused on the adsorption from ionic liquid electrolytes. This paper characterises the 4,4′-bipyridine adsorption at the Bi(111) | ionic liquid interface. We combined electrochemical impedance spectroscopy and scanning tunnelling microscopy with density functional theory calculations to gain a comprehensive overview of the interfacial processes. The results show a notable influence of electrode potential and the electrolyte on the 4,4′-bipyridine adsorption behaviour. The formation of organic adlayer on the electrode's surface was demonstrated, while highly organised striated adlayer structure was visualised only in a narrow potential range.
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In this study, electrochemical impedance spectroscopy and in situ STM measurements have been used to investigate the interface between a Cd(0001) single-crystal electrode and an ionic liquid electrolyte containing an organic additive. The addition of 4,4ʹ-bipyridine clearly influenced the interfacial structure and properties by forming an adlayer on the surface of the electrode during the experiment. Furthermore, in the potential range under study, the organic adlayer of adsorbed 4,4ʹ-bipyridine undergoes structural changes, as seen in both cyclic voltammograms and capacitance–potential curves. The processes occurring on the interface were further visualised using in situ STM, and the formation of 4,4ʹ-bipyridine agglomerates on the electrode’s surface was observed. The results underline the impact of the electrolyte characteristics (water vs. ionic liquid) on the adsorption of organic additives and the formation of adlayers.
Order beyond a monolayer: The story of two self-assembled 4,4′-bipyridine layers on the Sb(111) | ionic liquid interface
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Citation Excerpt :
The most distinct formation of ordered structure (Layer A) is observed at E = −0.85 V and −0.75 V, shown in Figs. 3b and 3c, respectively. These rows we attribute to the adsorbed 4,4′-BP molecules, as in a previous in situ STM study [35], which investigated Sb(111) | EMImBF4 interface, at similar E values, the atomic resolution of Sb(111) surface was imaged instead of adsorbed EMImBF4 ions. The ordered surface structure disappeared in the STM images at E > −0.75 V (Fig. 3d), most probably due to changes in the interfacial structure, related to adsorption of 4,4′-BP together with less strongly adsorbing and more mobile EMImBF4 ions on top of the ordered layers on the Sb(111) surface.
In this study, it has been shown that the ordering of organic molecules is not limited to a highly organised self-assembled monolayer on the electrode's surface. At the interface between semi-metallic Sb(111) electrode and ionic liquid with 4,4′-bipyridine addition, two distinctive stacked adsorbed layers upon each other have been identified by applying in situ scanning tunnelling microscopy and electrochemical impedance spectroscopy methods. The first adlayer in contact with the electrode consists of tightly packed ordered rows. The second adlayer, on top of the first, is characterised by low surface concentration, and its unit cell was obtained experimentally. In addition to experimental data, the likely distribution of 4,4′-bipyridine molecules in the first adlayer has been described using density functional theory calculations combined with machine learning. The gained insights of organic's adsorption from ionic liquids are of fundamental and practical importance in the development of nanoelectronic devices.
In situ scanning tunneling microscopy study of bipyridine adsorption at semi-metallic Sb(111) plane
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Thus, the surface coverage of the adlayer may be manipulated by modifying the pH of the solution, changing the applied potential or the base electrolyte concentration [2,17]. Despite numerous publications detailing the parameters that govern the organic molecule adsoption/phase reorientation process at metallic electrodes [1,15], there is little understanding of the corresponding parameters for semi-metallic interfaces [2,18,19]. The purpose of this work is to expand our knowledge of this topic using in situ scanning tunnelling microscopy (in situ STM), cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS).
The adsorption characteristics and organic adlayer formation properties of bipyridine molecules at semi-metallic Sb(111) electrodes in an aqueous electrolyte solution are studied by means of in situ scanning tunnelling microscopy (STM), cyclic voltammetry and electrochemical impedance spectroscopy. The capacitance–potential response shows a small capacitance hump for 4,4′-bipyridine (BP) adsorption at the Sb(111) interface, the position of which is slightly dependent on the electrolyte pH. No such capacitance vs. potential curve feature was observed for the adsorption of 2,2′-BP at Sb(111). STM measurements show a clean initial surface structure for the 4,4′-BP system and a higher level of noise interference in the case of the 2,2′-BP│Sb(111) system. The atomic resolution of the Sb(111) structure can be measured for both 2,2′-BP and 4,4′-BP additives and the formation of adsorbed molecular rows can be observed for the 4,4′-BP additive at the Sb(111) surface. The surface coverage of the molecules and the gaps between them suggest that 4,4′-BP is adsorbed in bi-protonated form with solvated anions stabilizing the adlayer structure. The reduction of BP molecules starts negative polarization and is significantly both pH- and potential-dependent. The formation of non-structured deposits is seen as a result of this process, which is deemed to be irreversible.
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Specific adsorption from an ionic liquid: impedance study of iodide ion adsorption from a pure halide ionic liquid at bismuth single crystal planes
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Electrical double layer (EDL) formation and adsorption of ions from ionic liquids is important for the fundamental understanding of interfacial processes and their dependence on both the electronic and ionic surface structure. Electrochemical impedance spectroscopy coupled with electrical equivalent circuit modelling has been applied to analyse the interfacial processes that govern the interface between three single-crystal bismuth electrodes with a pure iodide ionic liquid. Major differences in the double layer capacitance are observed for the three bismuth planes in the region of iodide specific adsorption, dependent on both the electronic structure and metallic characteristics of the electrode interface, also observed in case of aqueous iodide solutions. It is shown that this is due to the strong influence of both the electrode material and specifically adsorbed ion layer on the capacitive behaviour, rather than the bulk electrolyte composition. These results suggest a new way of describing the EDL in ionic liquids as well as show how the reactivity of the interface is determined.

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Formation of 2,2′-bipyridine adlayers at Sb(111)|ionic liquid + 2,2′-bipyridine solution interface

Highlights

Abstract

Introduction

Section snippets

Experimental

Results and discussion

Conclusions

Conflict of interest

Acknowledgments

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