The interface between Au(1 0 0) and 1-butyl-3-methyl-imidazolium-bis(trifluoromethylsulfonyl)imide

doi:10.1016/j.jelechem.2014.06.010

Journal of Electroanalytical Chemistry

Volume 737, 15 January 2015, Pages 218-225

https://doi.org/10.1016/j.jelechem.2014.06.010 Get rights and content

Highlights

•
The interface of Au(1 0 0) and an ionic liquid has been studied.
•
The double layer rearrangement processes are very slow.
•
Ordered adlayers of cations and anions of the ionic liquid have been imaged by STM.

Abstract

The electrochemical interface of Au(1 0 0) and an ionic liquid, BMITf₂N, has been characterized by cyclic voltammetry, electrochemical impedance spectroscopy and in situ STM with the aim to compare this particular electrochemical system with the Au(1 0 0)|BMIPF₆ electrode. It is demonstrated that the two systems share the same electrochemical features: the capacitance spectra around the pztc show a double-arc structure on the complex plane, and the high frequency capacitance limits are practically the same in both systems. The double layer rearrangement processes are very slow; they proceed with time constants of minutes. Ordered adlayers of cations and anions have been imaged by in situ STM.

Introduction

Ionic liquids (ILs) are defined as salts with melting points below 100 °C; many of them are fluid at room temperature. They have been attracting much interest recently in various areas of chemistry, both in academia and in the chemical industry, because of their beneficial properties like non-volatility, high ionic conductivity and nonflammability [1], [2], [3], [4]. Ever since air- and water-stable imidazolium-based ILs have become known [5] and commercially available, ILs are expected to have a profound impact on electrochemistry [6], [7]. From an electrochemical point of view, the most notable feature of ILs is their broad (4–5 volts wide) electrochemical stability window [8], [9], [10], [11], [12] which makes it possible to use them as electrolytes for the deposition of technical metals and other reactive materials [13], [14], [15].

Despite their great potential for new electrochemical applications, the interface structure and properties between a metal electrode and an IL is far from being well understood. This is mainly due to the fact that most of the experimental studies were performed with polycrystalline − hence sometimes ill-defined − metal surfaces and/or with ILs of questionable purity. Therefore, the key points of doing double-layer studies in ILs are the use of single crystal surfaces with well-defined structures and clean ILs. This is why we have studied mostly Au single crystals in a commercially available high-purity imidazolium salt, 1-butyl-3-methyl-imidazolium-hexafluorophosphate (BMIPF₆) [16], [17], [18], [19]. Apart from BMIPF₆, we made experiments also with a home-made guanidinium-based IL [20]; while the results reported in this paper were obtained with an ionic liquid comprising the same imidazolium-based cation as in BMIPF₆, but in combination with the bis(trifluoromethylsulfonyl)imide, Tf₂N⁻ anion (Fig. 1). The first report on the double layer property measurements on BMITf₂N have appeared just recently [21]. Unfortunately, this study was done not on Au, but on other electrode materials, Pt and carbon materials. Hence these results are not comparable with those of the present communication.

The aim of the present study is twofold: (i) to characterize the electrochemical interface between an Au(1 0 0) single crystal and the BMITf₂N IL; and (ii) to make a comparison between this system and a very similar one, the previously studied Au(1 0 0)|BMIPF₆ electrode [17], [18].

The main questions of the title subject are: what kinds of charge structures exist at the interface and how fast their rearrangement processes are. From this point of view, electrochemical impedance spectroscopy (a method to which Professor Tribollet has made considerable contributions [22]) plays a major role in this study.

The measurements presented here are very similar to those with BMIPF₆ summarized in reference [18]: basic characterization is done with cyclic voltammetry (CV); immersion measurements serve for the determination of the potential of zero total charge, pztc; electrochemical impedance spectroscopy (EIS) provides information regarding the double layer structure and relaxation processes therein. Finally, just as with BMIPF₆, in-situ scanning tunnelling microscopy (STM) images give a clue on the adlayer structure [23].

Section snippets

Materials, electrochemical cells and basic electrochemical measurements

In general, the experimental conditions were similar to the ones described in references [18], [20], only with different ILs. The experiments were carried out with the very same Au(1 0 0) single crystal of 12 mm diameter (MaTeck GmbH, Jülich, Germany) that was also used in [18], [20]. BMITf₂N was purchased from Merck KGaA in high quality (purity ⩾ 99.5%, water ⩽ 100 ppm, halides ⩽ 100 ppm). The ionic liquid was vacuum-dried for 24 h at elevated temperatures (80 °C) and purified with a molecular sieve, as

Cyclic voltammetry

Fig. 3 shows the CV of an Au(1 0 0) electrode in contact with BMITf₂N. Significant cathodic and anodic faradic currents (>1 mA/cm²) occur beyond −2 V and +2 V, respectively, thus the potential window of this IL is about 4 V. Just as in the case of BMIPF₆, passing the −2 V negative potential limit causes irreversible changes: the large cathodic current signals the reduction of the BMI⁺ cation; a plausible mechanism suggests the generation of radicals that react with each other to form dimers [27].

Discussion

For the different Au(1 0 0) and Au(1 1 1) electrodes in contact with ILs containing BMI⁺ and guanidinium-cations the double layer rearrangement processes appear to be very slow. This statement is based on “classical” electrochemical measurements, such as CV and EIS, as well as on the time-dependent changes in the corresponding STM images, showing that most of the ordered structures of Fig. 7, Fig. 8, Fig. 9, Fig. 10 are formed after a sufficiently long time (several minutes). The low-frequency

Summary and conclusions

To characterize the interfacial properties of the Au(1 0 0)|BMITf₂N system, electrochemical measurements were done and in situ STM images were taken, with the following results:

By immersion experiments the pztc of the Au(1 0 0)|BMITf₂N system has been determined to be at E = −0.2 V vs. Ag|AgCl. Positive and negative to the pztc ordered adlayers of anions and cations, respectively, exist, as have been revealed by STM images.

The CV and impedance behaviour of the Au(1 0 0)|BMITf₂N system is similar to that

Conflict of interest

There is no conflict of interest.

Acknowledgments

This cooperation was made possible by the exchange project of the Deutsche Akademische Austauschdienst and the Hungarian Fellowship Board, No. 39706. This study has been initiated by the late Professor D.M. Kolb in 2011. We acknowledge the contribution of Dr. Markus Gnahm to some of the electrochemical measurements. Further, support by the Deutsche Forschungsgemeinschaft (DFG) through project KO 576/28-1 is gratefully acknowledged. S. Vesztergom gratefully acknowledges the financial support of

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¹: Permanent address: Eötvös Loránd University, Pázmány Péter sétány 1/A, Budapest H-1117, Hungary. Present address: Department of Chemistry and Biochemistry, University of Bern, Freiestraße 3, Bern CH-3012, Switzerland.

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The interface between Au(1 0 0) and 1-butyl-3-methyl-imidazolium-bis(trifluoromethylsulfonyl)imide

Highlights

Abstract

Introduction

Section snippets

Materials, electrochemical cells and basic electrochemical measurements

Cyclic voltammetry

Discussion

Summary and conclusions

Conflict of interest

Acknowledgments

Electrochim. Acta

Electrochem. Commun.

Electrochim. Acta

Electrochem. Commun.

J. Electroanal. Chem.

Solid State Ionics

J. Electroanal. Chem. Interface Electrochem.

J. Electroanal. Chem.

J. Electroanal. Chem.

Electrochim. Acta

Introduction to Ionic Liquids

Ionic Liquids in Synthesis

J. Chem. Soc. Chem. Commun.

Phys. Chem. Chem. Phys.

J. Am. Chem. Soc.

Chem. Phys. Chem.

J. Phys. Chem. B

Z. Phys. Chem.

Chem. Phys. Chem.

Green Chem.