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Influence of Water on the Electrified Ionic Liquid/Solid Interface: A Direct Observation of the Transition From a Multilayered Structure to a Double Layer Structure Tong Cui, Abhishek Lahiri, Timo Carstens, Natalia Borisenko, Giridhar Pulletikurthi, Chantal Kuhl, and Frank Endres J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b02549 • Publication Date (Web): 07 Apr 2016 Downloaded from http://pubs.acs.org on April 10, 2016

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The Journal of Physical Chemistry

Influence of Water on the Electrified Ionic Liquid/Solid Interface: a Direct Observation of the Transition from a Multilayered Structure to a Double Layer Structure Tong Cui1, Abhishek Lahiri1*, Timo Carstens1, Natalia Borisenko1*, Giridhar Pulletikurthi1, Chantal Kuhl1, Frank Endres1* 1

Institute of Electrochemistry, Clausthal University of Technology, Arnold-Sommerfeld-Str. 6, 38678 Clausthal-Zellerfeld, Germany

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Abstract Ionic liquids are potential designer electrolytes for energy storage devices such as batteries and capacitors wherein by changing the cation and anion of the ionic liquid (IL) the solid/liquid interface can be tuned, thereby influencing the charge and mass transfer processes. In this paper, we show the influence of water on the electrified ionic liquid 1-ethyl3-methylimidazolium trifluoromethylsulfonate ([Emim]TfO)/Au(111) interface using in situ atomic force microscopy (AFM) and spectroscopy. A clear ‘water in IL’ to ‘IL in water’ transition could be observed in the range of 20 to 30 vol% of water using vibrational spectroscopy. Above 30 vol% of water the cation-anion interaction in the ionic liquid drastically reduced which was ascertained both by spectroscopy and interfacial studies using in situ AFM. In situ AFM results further revealed that the structure of the innermost (Stern) layer depends both on the applied electrode potential and the amount of added water. A transition from a multilayered structure to a classical double layered structure occurred at -1.0 V on changing the water concentration from 30 vol% to 50 vol%. Furthermore, the morphology of the electrodeposited Zn could be altered with addition of water to the electrolyte which has some potential for Zn-based batteries.


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Introduction Ionic liquids (ILs) consist entirely of ions and have melting points usually below 100 °C. Due to their physical properties, e.g. high electrical conductivities, low vapor pressures, high thermal stability and wide electrochemical windows, they have a broad range of possible applications from energy storage devices, electroplating and biological sensors.1-4 Furthermore, the physicochemical properties of ionic liquids can be changed by varying the cation and anion combination making them ideal solvents for task-specific purposes especially in the fields of electrochemistry, catalysis, separation technology to mention a few.5-8 Electrochemistry is governed by reactions at the solid/electrolyte interface. Varying the combination of cations and anions in ILs can alter the interface strongly. ILs have been shown to form a layered structure at the electrode/electrolyte interface, which depending on the substrate can extend up to 10 nm. The electrode/electrolyte interface has been explored by various techniques such as scanning probe microscopy (SPM), X-ray reflectivity, Sum frequency generation (SFG), X-ray photoelectron spectroscopy (XPS) and surface force measurement (SFA).9-14 From these techniques, the IL interface can be classified into three zones: the innermost (Stern) layer defined as the ion layer in direct contact with the electrode surface; the bulk phase, wherein the degree of ion amphiphilicity plays a decisive role for the configuration of the bulk liquid region, and the transition zone between the innermost layer and the bulk phase.15 These solvation layers can influence electrochemical reactions, the morphology of electrodeposits and also material properties.16-19 Recent theoretical studies have progressed towards a fundamental understanding of IL/solid interfaces both at neutral and at charged surfaces.20-23 The standard textbook models are usually not applicable for ILs systems. The conventional models describe that the electrolytes consist of small spherical ions with uniform charge density on their surface which are dissolved in a large amount of solvent and interact with each other via long-range Coulomb forces. These models are applicable for diluted electrolytes where dissolved salt ions are surrounded by a large number of solvent molecules. In contrast, ILs exhibit a much higher ion charge density compared to aqueous electrolytes. Neat ILs are comprised of only ions (without any molecular solvent), which are generally large and asymmetric in nature. Therefore, the classical Gouy-Chapman-Stern theory for aqueous electrolytes is not valid for ILs systems. Theoretical studies have shown that the ion-surface interaction is strong in ILs 3 ACS Paragon Plus Environment


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which is mainly due to electrostatic attraction and van-der-Waals forces.20-22 The IL completely screens the charged surfaces within several bound ion layers by electrostatic interactions. Bazant et al.23 showed that when the innermost (Stern) layer is insufficient to neutralize a surface electric potential (e.g. at higher potentials), a crowding of ions occur at the surface, while an overscreening of ions was predicted if the charge of the Stern layer is greater than that of the potential of the electrode (e.g. at lower potentials), which is compensated by ions in the successive layers. In general, both theoretical and experimental studies in ILs show that strong ion-surface interactions can induce an ion ordering at the IL/solid interface leading to the formation of an innermost (Stern) layer. This ion ordering further extends into the successive layers (i.e. templating the surface-induced ordering in subsequent near surface structure), that extends up to several nanometers in the solution. The reordering of the Stern layer structure compensates the interfacial charge induced by the surface potential (e. g. by applying the electrode potential). In the case of addition of solutes (e.g. water) in ILs, very little is known in terms of electrochemical reactions. Recently, IL-water mixtures were explored for protein separation, biopolymers, possible electrolytes for electrodeposition, and for metal-air batteries.24-26 Water in pure ILs has been studied theoretically and spectroscopically. For example, molecular dynamics simulations showed that a high concentration of water leads to the disruption of polar networks, reduces viscosity, and increases the self-diffusion of constituent ions.27-30 Furthermore, it has been shown by means of Infrared and Raman spectroscopy that the anion of the IL is mainly affected by the presence of water due to hydrogen bonding.31-34 However, the length of the alkyl chain in the cation has also shown to affect the IL ionic structure in the presence of water.35 In comparison to the interaction of IL’s with water in the bulk phase, little has been known for the IL-water/electrode interface. Horn et al.36 investigated the influence of water in ethylammonium nitrate (EAN) on a mica substrate using SFA and found that as the water concentration was increased to 50 %, the strength of the oscillation force decreased. However, with very high concentration of water, they found that EAN behaves as a simple electrolyte which follows the Derjaguin-Landau-Verway-Overbeek (DLVO) theory wherein the solidliquid interaction is the sum of electrical double layer repulsion and a Van-der-Waals attraction. From Sum-Frequency-Vibrational spectroscopy, the IL-water interface was explored on SiO2 substrate from which both ‘ice-like’ and ‘water-like’ features were found on 4 ACS Paragon Plus Environment

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the SiO2 substrate.37 Similar to Horn et al.’s observation, Smith et al.38 showed using AFM that on addition of water, the number of solvation layers decreased in EAN. Using SFA, Espinosa-Marzal et al.39, 40 analyzed the confinement of ionic liquids on mica. They found that water modifies the ionic liquid orientation resulting in an increased adhesion and a reduction of the lubrication performance. Recent molecular dynamics simulation studies have shown that the preferential distribution of water at an electrode surface depends on many factors such as the electric field, the association with the surrounding ions and the availability of free space. The authors also showed that at neutral and charged electrodes, the water changes from a random structure to an oriented structure.41 Cheng et al.42 studied using both SFA and atomic force microscopy (AFM) the effect of water in 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([Emim]TFSA) on both mica and gold electrodes. They observed that the influence of water is limited to interactions within the IL layers. However, the electrosorption of water on the polarized electrodes was not observed. AFM studies of both [Emim]TFSA and 1-ethyl3-methylimidazolium tetrafluoroborate ([Emim]BF4) on silica and on mica were reported. On addition of water to [Emim]TFSA, a disruption occurred on both surfaces.43 In this paper we report on spectroscopic and in situ AFM studies of water in 1-ethyl3-methylimidazolium trifluoromethylsulfonate ([Emim]TfO) ionic liquid. In contrast to literature, where interfacial studies have been performed mainly with low concentrations of water in IL, here the water concentration was changed from 10 vol% to 70 vol%. In situ AFM studies reveal that the IL/Au(111) interface is strongly altered by addition of water, and a clear transition occurs at -1.0 V from a multilayered to a classical double layer structure for the IL-water mixtures between 30 vol% and 50 vol%. The results show that the structure of the innermost (Stern) layer depends both on the applied electrode potential and the added amount of solute (water). Finally we show that the morphology of the electrodeposited Zn is altered by the addition of water to Zn(TfO)2-[Emim]TfO, which has some potential application in Zn-based batteries.

Experimental 1-ethyl-3-methylimidazolium trifluoromethylsulfonate ([Emim]TfO) was purchased from Io-Li-Tec, Germany. Before use, the liquid was dried under vacuum at 120 °C to water 5 ACS Paragon Plus Environment


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contents well below 2 ppm and stored in a closed bottle in an argon-filled glovebox with water and oxygen contents below 2 ppm (OMNILAB from Vacuum Atmospheres). The electrochemical measurements were carried out by using a PARSTAT 2263 potentiostat/galvanostat controlled by Power CV and PowerStep software. The scan rate during cyclic voltammetry was 10 mV s-1. The working electrode in all CV experiments were Au on glass. Platinum wires were used as counter and quasi-reference electrodes. The quasireference electrode was found to be electrochemically stable during the experiments. The electrochemical cell was made of Teflon and clamped over a Teflon-covered Viton O-ring onto the substrate, yielding a geometric surface area of 0.3 cm2. Prior to the experiments, the Teflon cell and the O-ring were cleaned in a boiling mixture of 50:50 vol% of ultrapure water and H2O2 (35 %) followed by refluxing in ultrapure water. Raman Spectra were recorded with a VERTEX 70 V, RAM 2, Bruker Optics GmbH with a (Nd: YAG 1064 nm) Ge detector. The ionic liquid with water was sealed in a glass capillary and the spectra were obtained at an average of 250 scans with a resolution of 2 cm-1. Fourier transform infrared spectroscopy was also obtained using VERTEX 70 V, Bruker Optics GmbH. Force-distance curves were collected using a Molecular Imaging PicoPlus AFM in contact mode at 22 °C. The substrate for AFM experiments was Au(111) (gold on mica), purchased from Molecular Imaging. A silicon SPM-sensor from Nano World was employed for all the experiments. The spring constant was 6 N/m. Two thin Pt wires were used as the CE and RE, respectively. Before each experiment, the electrochemical cell and Pt wires were cleaned with isopropanol in an ultrasonic bath for 5 min and then annealed in hydrogen flame to red glow in order to remove any possible contaminations. All force curves were acquired at room temperature in an argon-filled glovebox.

Results and Discussion Prior to understanding the structure of the interface, the bulk structure and the influence of water in the ionic liquid needs to be clarified as the bulk liquid structure will influence the interfacial structure. Vibrational spectroscopy was used to evaluate the effect of water in [Emim]TfO.44 In IR spectra, the symmetric and antisymmetric stretching modes of water were found in the range of 3000-3700 cm-1 45 and the stretching modes of [Emim]+ occur between 2800 and 3300 cm-1.31, 32, 46 Therefore, the IR spectra between 2800 and 3700 cm-1 6 ACS Paragon Plus Environment

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provide information on the interactions of water with cations. Figure 1a compares the IR spectra of pure [Emim]TfO and its mixtures with water. The black curve shows the pure IL where the CH stretchings of the [Emim]+ cation were seen in the regions from 2900 to 3200 cm-1. On addition of 10 vol% of water, a new peak at 3540 cm-1 and two new shoulders at ~ 3420 and ~ 3260 cm-1 are developed. The 3540 cm-1 signal corresponds to the antisymmetric stretching mode of H2O.47 The broad shoulders at ~ 3420 and ~ 3260 cm-1 are due to the hydrogen-bonded water wherein the former is attributed to strained hydrogen bonds and the later to unstrained hydrogen bonds.33, 48-50 As the concentration of water is increased from 10 to 70 vol%, these peaks form a broad peak indicating a strained interaction between [Emim]+ and water. The alkyl group stretching/asymmetric stretching vibrations marked in Figure 1a increase with an increase in water concentration and can still be seen at 50% water. Consequently, the typical C-H stretching in the [Emim]+ cation diminishes with increase in water content and finally merges at 70 vol% of water. To further understand the interaction of water with the [Emim]+ cation, Raman spectroscopy is shown between 2700 and 3200 cm-1 (Figure 1b) which probes the C-H stretching modes of the [Emim]+ cation.51 On increasing the water concentration, a decrease in the peak intensities corresponding to the C-H stretching modes of methyl and ethyl groups and the imidazolium ring signal are observed between 2800 and 3100 cm-1. Two weak bands at 3114 and 3168 cm-1 corresponding to C(2) and C(4,5) positions of the imidazolium ring, respectively are also seen which have been shown to be sensitive to hydrogen bonding.51, 52 In particular for the C(4,5) position, it was found that there is a 4 cm-1 shift on addition of 20 vol% water, whereas a shift of ~ 7 cm-1 occurred on addition of 30 vol% water (inset in Figure 1b). On further increasing the water concentration from 30 to 70 vol% only an additional shift of 1 cm-1 is observed. In summary, it can be concluded that water preferentially interacts with the IL cation at C(2)H of imidazolium ring thereby weakening the C(2)H bond, which consequently affects the vibration modes at C(4,5)H for spectra above 50 vol% of water.



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Figure 1: (a) Comparison of IR spectra of [Emim]TfO with various concentrations of water. (b) Raman spectra of [Emim]+ region with various concentrations of water. Inset shows the changes in the C(4,5) positions (c) Changes in the Raman Spectra of CF3 bending mode in pure [Emim]TfO and on addition of water. (d) IR spectra between 1000 and 1300 cm-1 wavenumbers in pure [Emim]TfO and various concentrations of water.

To assess the interaction of water with the TfO- anion, Raman and IR spectra were analysed from 740 to 780 cm-1 and from 1000 to 1300 cm-1 which corresponds to the CF3 bending and SO3 stretching modes in the TfO- anion, respectively. Figure 1c shows the changes in the Raman spectra of the CF3 bending mode. A shift of about 9 cm-1 occurs on addition of 70 vol% of water. This shift can be attributed to the interaction of water with the triflate anion. Such a shift was also observed when triflic acid was added to [Emim]TfO.52 Furthermore, if this Raman peak position is plotted against the water concentration, Figure S1 shows that the slope changes sharply until 20 vol% of water and above 40 vol% the change in the Raman shift is not significant. The symmetric and antisymmetric peaks of SO3 and CF3 stretching of TfO- occur between 1000 and 1300 cm-1 in both Raman and IR spectra (Figure S2 and Figure 1d, respectively). The symmetric stretching of SO3 and CF3 do not shift, but the 8 ACS Paragon Plus Environment

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intensity decreases with increasing water amount (see Figure S2). However, the changes in the asymmetric stretching’s are not clear in the Raman spectra. The IR spectra in Figure 1d reveal that the symmetric stretching’s of SO3 blue shift by 5 cm-1 (Figure S3), whereas shifts in the asymmetric stretching’s were observed. The νasSO3 at 1252 cm-1 shows the formation of a shoulder at 1276 cm-1 on addition of water which might be attributed to the formation of triflic acid. Figure S3 shows a plot of the changes in wavenumber for νasCF3 with change in concentration of water. It is evident from the plot that there is a shift in the νasCF3 with change in the water concentration. The wavenumber shifts in the asymmetric stretching suggest an interaction of water with triflate. These spectroscopic studies reveal that up to 40 vol% of water results in the formation of hydrogen bonded complexes ([Emim+…H2O] and [TfO…H2O] or [TfO-…H+….H2O].53, 54 On increasing the concentration of water above 40 vol%, the hydrogen bond interaction between [Emim]+ and TfO- considerably changed probably due to an increase in the formation of [[Emim]+-H2O] and [TfO--H2O]/[TfO-…H+….H2O] species. Thus, the IR and Raman results suggest that water interaction with cation and anion occurs in a different fashion. The cyclic voltammograms (CVs) of [Emim]TfO along with various concentrations of water is presented in Figure 2. Scans were initially swept negatively from the open circuit potential (OCP) with a scan rate of 10 mV s-1. The pure ionic liquid exhibits an electrochemical window of around 4.4 V, which is limited by the irreversible reduction of the organic cation (C3) and gold oxidation and/or oxidation of the anion (Figure 2a). The broad oxidation peak between -1.0 V and +0.5 V is due to the oxidation of the cation decomposition products as this peak is obtained only when the electrode potential is reversed at potentials below -2.1 V for pure IL and for 10 vol% water in the IL as shown in Figure 2a. On addition of water, the electrochemical window of the pure IL narrows significantly from 4.4 V to ∼ 3.3 V. In the cathodic regime below -1.5 V, hydrogen evolution and the decomposition of the organic cation might occur for IL-water mixtures. Furthermore, on addition of water, a new oxidation process (A3) occurs which corresponds to either oxygen evolution and to the decomposition of solvated anion or to the oxidation of gold. Figure 2b shows the CVs where the interfacial processes occur. The cathodic processes C1 and C2 and the anodic processes A1 and A2 in Figure 2b are related to the adsorption/desorption of the ionic liquid ions on the gold surface and/or the formation of solvation layers, as shown previously by in situ STM investigations.55 Furthermore, upon addition of water two new cathodic peaks (C* and C’) and an anodic peak (A*) appear (Figure 2b, red CV). C* and A* are most likely related with the adsorption of species like [[Emim]+-H2O] and [TfO--H2O], respectively. At C’ the reduction of gold that 9 ACS Paragon Plus Environment


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was dissolved at A3 occurs. The appearance of new peaks correlate with the adsorption/desorption of [Emim]+ and TfO-, which varies with the amount of water as evident in the CVs (Figure 2b). These processes can be correlated with the presence of water which influences the IL/electrode interface. Furthermore, upon addition of water, both peaks A1 and A2 disappear at water concentrations above 30 vol%. In addition, peak A3 occurs at more positive potentials (ca. +1.2 V for 50 and 70 vol% of IL-water mixture). Quite recently, the interaction of [Emim]TfO with Pt(111) was investigated by IR spectroscopy and the study revealed that the adsorption processes depend on the electrode potential.56 To understand this behaviour, we measured the pH of the IL-water mixtures (Supporting Information, table S1). The pH of mixtures varies from 6.4 to 5.8 upon increasing the water from 10 vol% to 70 vol%, respectively. The increase in acidity might be responsible for narrowing the electrochemical window. Furthermore, a qualitative analysis of the oxidation processes observed at more positive potentials to OCP in the CVs is difficult due to the changes in the composition and concentration of IL-water mixtures.

Figure 2: CVs of pure [Emim]TfO and [Emim]TfO with 10 vol%, 30 vol%, 50 vol% and 70 vol% H2O on Au(111) at 22 °C (a) between -2.5 V and +2.5 V; (b) between -1.5 V and +1.5 V. Scan rate: 10 mV s-1.


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Changes in the interfacial structure of [Emim]TfO with water were evaluated using in situ AFM using force-distance measurements. Figure 3 shows the force-separation profiles for an AFM tip approaching the Au(111) surface at -0.2 V, -0.5 V and -1.0 V in pure [Emim]TfO and [Emim]TfO with 10 vol% water, respectively. At -0.2 V the force-separation curves recorded for the pure IL and IL with 10 vol% water are quite similar (Figure 3a and Figure 3b, respectively). In both systems four solvation layers can be detected at 0.68 mn, 1.4 nm, 2.1 nm and 2.9 nm. Beyond 4 nm no force signal is recorded as the tip does not experience any significant resistance when moving through the bulk of the liquid.

Figure 3: Typical force-separation curves for an AFM tip approaching the Au(111) surface in [Emim]TfO and [Emim]TfO with 10 vol% water at (a, b) -0.2 V (c, d) -0.5 V and (e, f) -1.0 V.

The force required to rupture each layer increases as the tip approaches the Au(111) surface and in the case of the pure IL about 5 nN is needed to push through the innermost layer 11 ACS Paragon Plus Environment


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at 0.68 nm. Since the [Emim]TfO ion pair has a diameter of 0.68 nm, which is estimated by assuming a cubic packing geometry, the width of each separation layer in Figure 3a and Figure 3b corresponds to the presence of the IL ion pair. The number of solvation layers increases by changing the electrode potential in the cathodic regime. At -0.5 V and at -1.0 V five solvation layers can be observed (Figure 3c, Figure 3d, Figure 3e and Figure 3f) compared to four layers observed at OCP. There is no significant change in the width of the separation steps and each solvation layer corresponds to the [Emim]TfO ion pair. However, the force required to rupture the layers (especially the innermost layer) increases with decreasing the electrode potential to -0.5 V with addition of water. A force of almost 18 nN is needed to rupture the innermost layer in the IL with 10 vol% water (Figure 3d), which could be related to the hydrogen bond between [Emim]TfO and water, and might have led to a better ordering. Molecular dynamic simulations have predicted a better ordering of water-IL molecules at the interface on applying a potential whereas random ordering was revealed at neutral electrodes,41 consistent with the experimental observations in Figure 3d. The force decreases slightly by further reducing the electrode potential to -1.0 V. This can be associated with the reorientation of hydrogen bonded IL ion pair as some surface reduction processes has been seen between -0.5 V and -1.0 V in the CV. In general, at negative electrode potentials the force required to rupture the layers increases due to a better ordering of the innermost layer and stronger near surface structure.57 Upon addition of water the larger forces could be related to the hydrogen bond between [Emim]TfO and water, which might have a better ordering. On changing the potential to -1.0 V, the width of the innermost layer is reduced to 0.59 nm (Figure 3f). The complete change in the innermost layer could be due to a reorientation of the cation-H2O and anion-H2O as a slight decrease in the current is seen in the CV (Figure 2). Figure 4 shows the force-separation profiles for an AFM tip approaching the Au(111) surface at -0.2 V, -0.5 V and -1.0 V in [Emim]TfO with 30 vol% and 50 vol% water, respectively. The profile of the force-separation curves do not change significantly on increasing the concentration of water to 30 vol% (Figure 4a, Figure 4c and Figure 4e). Similar to the pure IL and IL with 10 vol% water, four (at -0.2 V) and five (at -0.5 V and at -1.0 V) solvation layers corresponding each to the IL ion pair are observed at the IL/Au(111) interface. However, the forces required to rupture each solvation layer increase compared to the pure IL and IL+10 vol% water systems. In Raman and IR spectroscopy, it was found that only above 30 vol%, the interaction of cation-anion diminished. Therefore the addition of 30 vol% of water 12 ACS Paragon Plus Environment

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could be considered as the transition zone wherein the presence of strong H-bonded IL requires a higher force to rupture the solvation layer. Interestingly, by further increasing the concentration of water to 50 vol% the number of solvation layers decreases drastically (Figure 4b, Figure 4d and Figure 4f). At -0.2 V and -0.5 V only two solvation layers at 0.41 nm and 1.4 nm can be detected at the IL/Au(111) interface and the second solvation layer is hardly visible (Figure 4b and Figure 4d). When the electrode potential is further reduced to -1.0 V, only one layer is seen (Figure 4f). The width of this layer is 0.41 nm, which is considerably lower compared to the pure IL. It has been shown by force spectroscopy and molecular dynamics simulation that the imidazolium cation has a separation of 0.3 nm on HOPG.58 Therefore, with the error in our AFM measurements of ±0.05 nm, the width of the layer of 0.41 nm for 50 vol% water in the IL may correspond to the dimensions of the hydrated cationic layer ([Emim]+-H2O) along with solvated water.



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Figure 4: Typical force-separation curves for an AFM tip approaching the Au(111) surface in [Emim]TfO with 30 vol% and 50 vol% water at (a, b) -0.2 V, (c, d) -0.5 V and (e, f) -1.0 V.

Figure 5 shows the changes in the IL/Au(111) interfacial structure at -1.0 V with increasing of the water concentration from 30 vol% to 50 vol% in the IL. In the case of 30 vol% water five solvation layers are obtained at -1.0 V (Figure 5a). The width of the innermost layer is 0.6 nm which correlates well with the diameter of the IL ion pair. Both the number of the solvation layers and the width of the innermost layer decrease if more water is added into the IL. Thus, at 40 vol% water three layers with the innermost layer at 0.56 nm are present (Figure 5b), while at 50 vol% only one layer at 0.41 nm corresponding to the hydrated cationic layer can be detected (Figure 5c).

Figure 5: Typical force-separation curves for an AFM tip approaching the Au(111) surface in [Emim]TfO with (a) 30 vol% , (b) 40 % and (c) 50 vol% water at -1.0 V.

It is evident that up to 30 vol% water, the innermost structure has a dimension between 0.5 and 0.6 nm and does not change much with varying potentials. A drastic change in the innermost layer is seen on increasing the water concentration to 50 vol%. This change agrees with the spectroscopic observation wherein the cation-anion interactions diminished above 30 vol% water. At 50 vol% the stretching peak of the pure IL’s cation is not observed (Figure 1a). This suggests that at 50 vol%, the interface is determined by cations bound to water. Thus, between 30 vol% and 50 vol% of water in the [Emim]TfO there is a clear transition from a multilayered interfacial structure typical for the pure IL to a classical double layer structure well-known for aqueous electrolytes. We can conclude from in situ AFM and spectroscopic measurements that up to 30 vol% of water the typical multilayered IL/electrode interfacial structure exists. The IL’s cations and anions interact with the electrode surface to produce an innermost (Stern) layer that templates 14 ACS Paragon Plus Environment

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the subsequent layers into the bulk of the liquid. Above 50 vol% of water only one wellordered (Stern) layer exists at the interface and is dominated by the hydrated [Emim]+. This layer is similar to the case of diluted aqueous solutions with metal salts, where a classical double layer exists. The region between 30 vol% and 50 vol% of water can be considered as a transition zone, where ‘water in IL’ interfacial behavior transforms into ‘IL in water’ interfacial structure. This in turn should affect the reactions that occur at the IL/electrode interface (e. g. metal deposition). Finally to manifest, how a change in the interface affects the electrodeposition of Zn, we focused on two zones, i.e. on addition of 30 vol% water (‘water in IL’) and 50 vol% water (‘IL in water’). Figure 6 shows the morphology of Zn electrodeposited at -1.5 V from 0.2 M Zn(TfO)2 in [Emim]TfO + 30 vol% H2O and [Emim]TfO + 50 vol% H2O for 2 hours, respectively.

Figure 6: Microstructure of Zn electrodeposited for 2 h at -1.5 V from 0.2 M Zn(TfO)2 in (a) [Emim]TfO + 30 vol% H2O and (b) [Emim]TfO + 50 vol% H2O. Insets show higher magnification images. From the morphology of the electrodeposited Zn, it is clear that with 30 vol% water, a porous network structure of Zn is obtained with an average spherical size of 15 µm, whereas on addition of 50 vol% water, the formation of a more dense structure with an increased amount of deposit is obtained. This clearly shows that the change of the interface by addition of water affects the deposition characteristics significantly, which ultimately results in an increased rate of Zn deposition.

Conclusions



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In conclusion, we have shown the influence of water in [Emim]TfO/H2O mixtures wherein hydrogen bonds are formed between water and the IL. Above 30 vol% of water, the interaction between [Emim]+ and TfO- is considerably weakened or destroyed due to the ongoing formation of [Emim]+-H2O and TfO--H2O solvation sheaths. In situ AFM results reveal the transition region wherein below 30 vol% water, multiple solvation layers consisting of the IL ion pairs are observed, whereas above 50 vol% of water, only one solvation layer of the IL cations is obtained. To summarize the observations, the classical Gouy-Chapman-Stern theory, which explains the double layer formation for aqueous electrolytes, cannot sufficiently explain the electrode/electrolyte interface. This is due to the fact that the ions of ILs adopt a multilayer structure at the interface. However, with adding a defined quantity of water to the IL we can modify the interface, which exclusively forms a Stern type double layer as observed in aqueous electrolytes. This study shows that the addition of water changes the interface significantly, which can alter electrodeposition processes. Thus, with 30 vol% water, a porous Zn structure was obtained electrochemically, while with 50 vol% water a dense zinc structure is formed.

Supporting Information Supporting Information (SI) shows the changes in the Raman and IR spectra and pH values of the IL-water mixtures.

Author Information Corresponding Authors: *Phone: (49) 5323 723734;

e-mail:

[email protected]

(N.B.)

Phone:

(49) 5323 722885; e-mail: [email protected] (A.L.) Phone: (49) 5323 723134; e-mail: [email protected] (F.E.)

Acknowledgement Financial support by the BMBF project LUZI (BMBF: 03SF0499A) is gratefully acknowledged. The authors would also like to thank Mrs. Karin Bode, Institute of Inorganic Chemistry (Prof. A. Adam) for help with Raman and IR measurements.

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Graphical Abstract (TOC)


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Solid Interface: A

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