Differential Capacitance at Au (111) in 1-Alkyl-3-methylimidazolium

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Differential Capacitance at Au(111) in 1-Alkyl-3-methylimidazolium Tetrafluoroborate Based Room-Temperature Ionic Liquids Muhammad Tanzirul Alam,† Jahangir Masud, Md. Mominul Islam,‡ Takeyoshi Okajima, and Takeo Ohsaka* Department of Electronic Chemistry, Interdisciplinary Graduate School of Science and Engineering, Tokyo Institute of Technology, Mail Box G1-5, 4259 Nagatsuta, Midori-ku, Yokohama 226-8502, Japan ABSTRACT: Interfacial structures at Au(111) electrode in N2saturated 1-alkyl-3-methylimidazolium tetrafluoroborate roomtemperature ionic liquids (RTILs) have been studied by the measurements of differential capacitances and cyclic voltammograms. Capacitance
potential (C
E) curves are found to vary significantly with changing the direction of potential scan and temperature. C
E curves measured by sweeping the potential from negative to positive direction always have lower capacitance values compared to the curves measured by reversing the direction of potential scan, and this difference is larger for the RTIL with longer alkyl group. Special arrangement of the ions at the interface in response to the starting potential, constraint in orientation, and change in the ion electrode and ion
ion interactions are presumed to be the causes for this variation. Temperature has a dramatic effect on the Au(111)|EMIBF4 interface. Capacitance around the potential of zero charge decreases with increasing the temperature at the Au(111)|EMIBF4 interface, whereas at the Au(111)|BMIBF4 and Au(111)|OMIBF4 interfaces, it increases, which is discussed on the basis of the effect of temperature on the so-called solidlike crystallinity of RTILs.

’ INTRODUCTION Room-temperature ionic liquids (RTILs) have attracted significant attention in fundamental and applied research over the past decade due to their unique physicochemical properties, such as wide electrochemical potential window, excellent thermal stability, nonvolatility, relatively inert nature, and good ionic conductivity, which make them exceptionally useful in diverse electrochemical devices.1
4 Electrochemistry is usually done at the interface, and the rate of the reaction is significantly dependent on the structure and dynamics of the interface.5
8 Hence, the study of the interfacial properties of the RTILs is the key to understanding their functional performance in technological applications, for instance in capacitor and solar cell. Although studies of the electrical double layer (EDL) in aqueous electrolyte solutions are numerous, similar studies in RTILs have only recently been undertaken. The analytical formula for the EDL structure at the electrode
RTIL interface was first proposed by Kornyshev.9 On the basis of the mean-field theory, he proposed that the C
E curve for the RTIL with symmetric ion sizes (incompressible, γ > 1/3, γ is the lattice saturation parameter, i.e., the ratio of the average ionic concentration to the maximum possible local concentration) and in the absence of specific adsorption should be bell-shaped with the maximum of the capacitance as the potential of zero charge (PZC), whereas for the RTIL with asymmetric ion size (γ < 1/3) the C
E curve should be camel-shaped with the potential corresponding to the minimum between the humps as the PZC. In addition, capacitance at large potential should decrease with the square root of potential. Molecular dynamic simulation and the treatment of the r 2011 American Chemical Society

EDL with the modified Gouy
Chapman
Stern model also produce the similar outcome.10
12 Very recently it has been shown by Monte Carlo simulations that the C
E curve can also be camel-shaped if the ions are anisotropic in nature (i.e., RTIL containing charged heads and long neutral tails).13,14 Therefore, high compressibility of the RTIL is not a necessary condition for the C
E curve to be camel-shaped as predicted by mean-field theory. The neutral tail in the RTIL plays a role of space fillers or latent void, which allows a potential-induced increase of the counter charge density without a substantial compression of the liquid. Consequently capacitance on both sides of the PZC increases with a small change of potential, producing a camelshaped C
E curve. Lauw et al.15,16 have studied the interfacial structures using a self-consistent mean-field theory. By considering the effective dielectric constant (ε) of RTIL as a function of the local density of the segments of the ions, they found that the polarizability of ions at the interface is another key factor that contributes to the camel-shaped C
E curve. The ε of the RTIL at an electrified interface is considered to be higher due to the accumulation of the polar components of the ions. This trend is the inverse of what is typically found in aqueous electrolytes.8 The same group has also shown that the C
E curve can also be asymmetrically camel-shaped due to the presence of a nonelectrostatic (specific) affinity of ions toward the electrode surface. Besides changing the capacitance, specific adsorption of ions also Received: June 21, 2011 Revised: August 28, 2011 Published: September 02, 2011 19797

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The Journal of Physical Chemistry C shifts the PZC, depending on the type of the charge and the strength of interaction. On the other hand, the variability of the experimental C
E curves is large and can only be partially explained by the variability of the RTILs which were used for studying the interfacial structure at a variety of electrode substrates. In experiment, all the aforementioned parameters (e.g., ion size, adsorption) along with other unknown parameters contribute to the C
E curve and the extent of their contribution changes with RTIL, whereas in theory a few specific parameters are controlled and the others are kept constant. For instance, experimentally Alam et al.17
19 have assigned the minimum of the C
E curve at Hg electrode as the PZC in accordance with the maximum of the corresponding electrocapillary curve (ECC). Specific adsorption of the hydrophobic alkyl group on the Hg surface and the consequent decrease of the value of ε along with the increased effective distance of charge separation were found to be the causes. The same group has also observed U-shaped C
E curves at glassy carbon (GC) and highly oriented pyrolytic graphite (HOPG) electrodes in several RTILs, in contrast with the theoretically expected bell- or camel-shaped C
E curve at metal electrode.20,21 The parabolic nature of the space charge layer capacitance, which generates due to the lower value of the free electron density in GC and HOPE as compared to the metal electrodes, was shown to be the cause. However, Lockett et al.22,23 did not observe a similar sort of U-shaped C
E curve at the GC electrode, which they argued to be the consequence of different characteristics (e.g., density of free electron is different) of the used GC electrodes, because on the basis of the preparation method of the GC used, their properties differ significantly. Spectroscopic studies have revealed that the ions of the RTIL have their preferred orientations at different applied potentials, and they respond to the applied potential by changing their tilt and torsional angle with respect to the normal of the electrode surface.24
28 For instance, in imidazolium-based RTILs at potentials positive to the PZC, anions get adsorbed to the surface and the imidazolium rings are repelled to orient more along the surface normal compared with the potentials negative to the PZC, at which the cations are oriented more parallel to the surface plane and the anions are repelled from the surface. The extent of the change in orientation depends on the applied potential, type of ions, and the length of the alkyl groups. In agreement with our experimental C
E curves at polycrystalline Au electrode,20,29,30 it was also found that at moderate positive potential compared to the PZC, imidazolium cation does not leave the surface completely (due to repulsion of charge), instead adopting a tilted orientation to allow the ring to interact with the Au electrode.27,28 This interaction of the imidazolium ring with the metal electrode was presumed to generate a hump on the positive side of the PZC.20 Experiments were also done at single crystal electrodes. Kolb et al.31 have studied the interfacial structure at Au(111) electrode in 1-butyl-3-methylimidazolium hexafluorophosphate RTIL by using electrochemical impedance spectroscopy and STM techniques. Surprisingly, they did not observe the expected potential dependency of the capacitance. On the other hand, Su et al.,32 in contrast with the theoretical expectation, observed a bell-shaped C
E curve for the RTIL 1-butyl3-methylimidazolium tetrafluoroborate with asymmetric ion size. Therefore, considering the involvement of lots of parameters in a real interface, it may be reasonable to state that no theory alone can capture their effects. However, despite the mentioned advances, our understanding of the structure of RTILs at electrified interfaces is not complete.

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Scheme 1. Structure of the Ions Constituting the RTILs Used in This Study

Here, in this paper we report for the first time the C
E curves measured at single crystal Au(111) electrode in 1-alkyl-3-methylimidazolium tetrafluoroborate-based RTILs (Scheme 1) by systematically changing the length of the alky group at different temperatures. A single crystal electrode was chosen for its atomically smooth surface, which is undoubtedly crucial from a structural point of view and to study how different crystal arrangements of Au affect the interfacial structure. In our upcoming papers we will report our findings on the interfacial structures for the same system at Au(110) and Au(100) electrodes.

’ EXPERIMENTAL SECTION Reagents. All the RTILs of EMIBF4 (1-ethyl-3-methylimidazolium tetrafluoroborate), BMIBF4 (1-butyl-3-methylimidazolium tetrafluoroborate), and OMIBF4 (1-octyl-3-methylimidazolium tetrafluoroborate) with a purity of more than 99% were obtained from Kanto Chemical Co., Inc. The average halide and water contents were less than 10 and 100 ppm, respectively (Kanto Chemical Co., Inc.). RTILs were dried under vacuum for 12 h at a temperature of 100 °C. Au(111) single-crystal disk (purity 99.999%, diameter 5 mm, thickness 3 mm, orientation accuracy < 2°) was purchased from MaTech (J€ulich, Germany). N2 (99.99%) gas was of ultrahigh purity and supplied from Nippon Sanso Co., Inc. Electrochemical Measurements. Electrochemical experiments were performed in a three-electrode cell containing a Au(111) single crystal (diameter = 5 mm) as a working electrode, a spiral platinum wire as a counter electrode, and a homemade silver/silver chloride (Ag/AgCl) wire as a reference electrode. Preparation of this reference electrode and the stability of its potential were described elsewhere.30 Au(111) single-crystal electrode was annealed under hydrogen flame to a slightly red hot state until the characteristic cyclic voltammogram (CV) in 0.09 M NaCIO4 + 0.01 M HCIO4 (taken after quenching in Milli-Q water) was obtained.33 Prior to each experiment, Au(111) single crystal was annealed in a hydrogen flame and cooled down slowly in a nitrogen stream. Experiments were performed by forming a meniscus of the desired electrolyte at the surface of the single crystal electrode. Solarton SI 1260 and SI 1287 were used as impedance/ gain phase analyzer and electrochemical interface, respectively, for the measurement of impedance. Impedance measurements were done at a constant frequency (25 Hz) at a scan rate of 5 mV s
1. From the impedance measurement data, the value of capacitance (C) was derived using the equation
Z00 = 1/2πfC, where Z00 is the imaginary component of impedance and f is the frequency of measurement.8 Capacitance was also derived by fitting the impedance curve (measured at a different potential interval in the frequency range from 5 kHz to 20 Hz) with a simple circuit combination [i.e., electrolyte resistance and constant 19798

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Figure 1. CV measured at Au(111) electrode in N2-saturated EMIBF4 at a scan rate of 50 mV s
1.

Figure 2. C
E curves measured at Au(111) electrode in N2-saturated EMIBF4 starting from different initial potentials.

phase element (CPE) in series]. The latter procedure was timeconsuming and the reproducibility of the data was quite low. We presume it to be related to the gradual change of the surface condition due to adsorption or other phenomenon, and hence we measured the impedance at a constant frequency. Capacitance was measured at 25 Hz as at frequency higher than 50 Hz, C
E curves were found to show hysteresis, and also the measured value of capacitance at 25 Hz was close to the one obtained by the second procedure. RTILs were deoxygenated by purging pure N2 gas for about 30 min, and the gas was kept flowing over the liquid during the electrochemical measurements. A continuous water flowing thermostatic bath was used for measurements at elevated temperatures.

asymmetry and anisotropic nature of the ions, which give the opportunity for the potential-induced polarization of ions, i.e., under the application of potential the charge segments of the ions come close to the electrode surface and the double layer becomes more compact, resulting in an increase of the capacitance on both sides of the minimum, PZC.9,13,14 The maxima on both sides of the PZC are regarded as the saturation of the Helmholtz layer by the respective ions (lattice saturation). At potential beyond these maxima, the capacitance decreases due to the formation of multilayer; i.e., the thickness of the interface continues to increase with increasing the potential. According to Kornyshev,9 the capacitance should decrease with the square root of potential. However, the decrease of the capacitance is not continuous. There are three other peaks at 0.70, 0.47, and
0.48 V. The origins of the peaks are not known, but it could be related to the adsorption, adsorption-related structural change, or partial charge transfer of the ions. Imidazolium cation is known to interact with the metal electrode surface with its π-electron. Spectroscopic investigations reveal that at potential slightly more positive than the PZC imidazolium cation does not leave the surface but instead adopts a tilted orientation, which is considered to be the consequence of the π-electronic interaction of the imidazolium ring with the metal electrode.28 Su et al.32 have reported the etching of the Au(100) surface by the adsorbed BMI+ cation at potential negative to the PZC. This surface etching process was verified by the increased atomic content of gold in the RTIL measured after experiments. Unfortunately, it is difficult to compare our data with their data, as the electrode substrates and the pattern of the C
E curves are different. A through surface study is required in this regard. Temperature Dependence of the C
E Curves at Au(111)| EMIBF4 Interface. Capacitance around the PZC decreases with increasing the temperature in EMIBF4 (Figure 3). Similar phenomenon was also observed by our group at Hg electrode. A maximum that appears at Hg in EMIBF4 at room temperature disappears gradually with increasing temperature, which was substantiated as the partial desorption of imidazolium ring from the electrode surface with concomitant increased interaction of the neutral alkyl group with the electrode.19 The U-shaped C
E curve is the general sign of the presence of the diffuse layer, but it is unlikely that with increasing temperature there develops a diffuse layer as in low concentration electrolytic solution. It has been shown recently that temperature has strong influences on the bulk structure of RTIL.35
40 Alkyl groups in RTIL aggregate

’ RESULTS AND DISCUSSION Differential Capacitance at Au(111)|EMIBF4 Interface. Figure 1 exhibits the CV measured at Au(111) in N2-saturated EMIBF4 at a scan rate of 50 mV s
1. The voltammogram shows several peaks in the potential region measured. In the literature, CVs in RTILs are usually presented in a large current range to demonstrate the wide electrochemical potential window of RTILs, which actually obscure the peaks like those in Figure 1. Unlike aqueous media, we always observed these sorts of peaks at all the electrode substrates studied (i.e., Au, glassy carbon, mercury). Kolb et al.31 and Gore et al.34 have also reported similar sorts of peaks in imidazolium-based RTILs at Au electrodes. None of these studies correlate the peaks with any specific cause. Whatever the case may be, the effects should also be discernible in the corresponding C
E curves. Figure 2 represents the C
E curves measured at Au(111) electrode in N2-saturated EMIBF4 starting from different initial positive potentials. The curves reveal that within the experimental range, the starting potential does not have significant effects on the overall shape of the C
E curves. These C
E curves have five maxima at 0.70, 0.47, 0.05,
0.22, and
0.48 V, which appear close to the peak potentials (0.71, 0.47, 0.12,
0.25, and
0.49 V) of the CVs in Figure 1. Theoretically, one should be able to derive C
E curve from the corresponding CV. Therefore, the peaks that appear in CV should also emerge in the corresponding C
E curve. The curves can be assumed as camel-shaped with a minimum at
0.11 V and two maxima at
0.22 and 0.04 V. In accordance with the theories, the minimum at
0.11 V is assigned as the PZC. The camel-shaped feature of the C
E curve is considered to appear due to the

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Figure 3. (a) C
E curves and (b) CVs measured at Au(111) electrode in N2-saturated EMIBF4 at different temperatures. CVs were measured at a scan rate of 50 mV s
1.

together, forming nonpolar domains that smear out at higher temperature due to thermal vibration giving more room for the ions to move under the application of potential. It may be that due to this relative ease in reorientation at higher temperatures (45 and 75 °C) the electrode potential is screened more effectively by the ions than that at 23 °C; i.e., the ions with opposite sign to the electrode charge can reside more in number in the Helmholtz layer, and consequently the maxima on both sides of the minimum appear at a bit far negative and positive potentials (as more ions need to be dragged in the Helmholtz layer for it to be saturated), producing the U-shaped C
E curves. The maxima are considered as a mark of the saturation of the Helmholtz layer by cations on the left-hand side of the minimum and anions on the right-hand side of the minimum. However, on the whole, the C
E curves at 45 and 75 °C can be considered as camel-shaped with the PZC at the minimum (0.12 V) and two maxima at
0.60 and 0.58 V. The capacitance value at the PZC at 45 and 75 °C is quite lower (13.2 μF cm
2) than that at 23 °C (51.4 μF cm
2). It could be that, due to the relative ease in reorientation at higher temperatures, the neutral alkyl group interacts preferentially with the neutral electrode surface at the PZC, keeping the charged moieties away from the electrode surface, whereas due to the structural constraint, similar preferential interaction of the alkyl group does not occur at 23 °C, and consequently, the capacitance at the PZC becomes higher than that at 45 and 75 °C. The mentioned relative ease in reorientation at higher temperatures allows the charged segments to come closer to the electrode surface under the application of potential than at 23 °C, increasing the local value of ε, which also

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Figure 4. C
E curves measured at Au(111) electrode in N2-saturated EMIBF4 at (a) 23 °C and (b) 23, 45, and 75 °C. Directions of potential scan are indicated in the respective figure.

contributes to the increase in capacitance with potential. Here, it is worthy to mention that the polar segment of the ions in RTIL has a higher value of ε than the nonpolar one, and the overall value of ε decreases with increasing the latter segment.41 It should be noted that C
E curves at higher temperatures also have the camel-shaped features on both wings with the minima at
0.72 and 0.67 V, but as they appear on both sides of the curves, they cannot be correlated with the theories. Analogous effects are also observed in the corresponding CVs. The charging current of the CVs at 45 and 75 °C in the potential region (
0.30 to 0.30 V) of the appearance of minimum in the C
E curves is small as compared to that at 23 °C. Also there appear two peaks on each side of the voltammograms, in compliance with the maxima of the corresponding C
E curves. However, the contribution of the adsorption and related effects on the maxima of the C
E curves at 45 and 75 °C cannot be ignored, as the capacitance was expected to increase more on the positive side of the minimum than on the negative side, due to the smaller size of anion as compared to the cation, but the reverse was observed. Effects of the direction of potential scan on the interfacial structure are presented in Figure 4a. The camel-shaped feature that appeared at 23 °C when the potential was scanned from positive to negative direction gets obscured with reversing the direction of potential scan. Similar observation has also been reported by Gore et al.34 at polycrystalline Au electrode in 1-butyl-3-methylimidazolium trifluoromethanesulfonate. They observed a camel-shaped C
E curve when the potential was 19800

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The Journal of Physical Chemistry C scanned from positive to negative direction and a bell-shaped curve when the direction of potential scan was reversed. No explanation was proposed for this observed variation. It may be that the adsorption pattern of the ions on the electrode surface changes with changing the direction of potential scan, leading to a change in surface structure, local charge density, and surface dipole, which are playing a role in shaping the C
E curves. Lauw et al.15,16 reported that the asymmetry in the C
E curve decreases with decreasing the nonelectrostatic (specific) affinity of ions toward the electrode surface. Su el al.32 observed the loose filmlike adsorption of BMI+ cation on Au(100) at potential negative to the PZC, which almost immediately leads to surface etching, generating defects within the BMI+ film as well as on the underlying substrate. Thus, it is quite possible that the minimum that appeared when the potential was scanned from positive to negative direction gets obscured with reversing the direction of potential scan due to the change in adsorption pattern. The maximum of the C
E curve 2 in Figure 4a cannot be assigned as the PZC as according to the theories the maximum can be assigned as the PZC only for the incompressible RTIL with no anisotropy in ions and in the absence of adsorption.9 Thus, we assign the shallow minimum at
0.02 V as the PZC. On the other hand, a change in the direction of potential scan does not have a significant effect on the C
E curves measured at 45 and 75 °C (Figures 3a and 4b). C
E Curves at Au(111)|BMIBF4 and |OMIBF4 Interfaces. In analogy with the C
E curve at Au(111)|EMIBF4 interface, C
E curves at Au(111)|BMIBF4 and |OMIBF4 in Figures 5 and 6 produce maxima close to the peak potentials of the corresponding CVs, which are marked with staric signs. Here also, the shape of the C
E curves changes significantly with changing the direction of the potential scan. The same reasoning as put forward above, i.e., change in the adsorption pattern with changing the direction of potential scan, can also be considered as a possible cause for this observation in the C
E curves. PZCs for the studied systems are compiled in Table 1. Temperature Dependence of the C
E Curves at Au(111) |BMIBF4 and |OMIBF4 Interfaces. Capacitance at Au(111) electrode in BMIBF4 and OMIBF4 increases with an increase in temperature (Figures 7a and 8a). The same phenomenon has also been observed by our group20 and Lockett et al.22,23 in BMIBF4 at polycrystalline Au and Pt electrodes, respectively. The decrease in ion association with an increase in temperature is presumed to be the cause for this enhancement. It is worthy to mention that the opposite trend was observed in EMIBF4 in this study at Au(111) electrode (Figure 3) and in one of our previous studies at Hg electrode.19 It is well-documented in the literature that RTIL maintains a long-range crystalline structure in its liquid state and this solidlike crystallinity (within the same group of RTIL, e.g., imidazolium tetrafluoroborate based RTIL) increases with increasing the length of the alkyl group.35
40 The inductive force among the alkyl groups which increases with increasing the length of the alkyl group is responsible for this increased crystallinity. However, the crystallinity decreases gradually with a gradual increase in temperate due to thermal vibration. It is quite comprehensible that the increase in temperature will have greater impact on the RTIL with shorter alkyl chain than the RTIL with longer alkyl chain. It may be that an increase in temperature disrupts the so-called crystal structure in EMIBF4 to such an extent that it allows the expected interaction of the neutral alkyl group with the neutral metal surface at the PZC (i.e., ions can move quite freely), and consequently,

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Figure 5. (a) C
E curves and (b) CV measured at Au(111) electrode in N2-saturated BMIBF4 at 23 °C. CV was measured at a scan rate of 50 mV s
1. Directions of potential sweep are indicated in the figure.

capacitance around the PZC decreases with an increase in temperature. For the same reason (i.e., lesser constraint in orientation), the double layer can become more compact at far positive and negative potentials than the PZC, resulting in an increase of capacitance at the wings of the C
E curves (Figure 3). On the contrary, it may be that the increase in temperature does not disrupt the so-called crystallinity in BMIBF4 and OMIBF4 to such an extent that may allow the preferential interaction of the alkyl groups with the electrode surface at the PZC (i.e., the structural constraint for the ions to move is still high), but the ion association decreases for sure, which is presumed by several groups20,23,42 as a probable cause for the overall increase of the capacitance with an increase in temperature. In accordance with the C
E curves, the charging current of the CVs increases with increasing the temperature at Au(111)|BMIBF4 and |OMIBF4 interfaces (Figures 7b and 8b). Common Trends of the Studied C
E Curves. One of the main targets of doing this research is to find out whether the structural change of the interface with changing the length of the alkyl group follows any general trend so that the results obtained in this study can be used in predicting the interfacial structures of other RTIL systems. Therefore, the common features/trends that are of significance are listed below. (1) Capacitance decreases with increasing the length of the alkyl group. For instance, capacitances at the PZC of the C
E curves measured by sweeping the potential from negative to positive direction in EMIBF4, BMIBF4, and OMIBF4 at 23 °C are recorded as 43.4, 18.1, and 15.1 μF cm
2, respectively. A smaller 19801

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Figure 6. (a) C
E curves and (b) CV measured at Au(111) electrode in N2-saturated OMIBF4 at 23 °C. CV was measured at a scan rate of 50 mV s
1. Directions of potential sweep are indicated in the figure.

Table 1. PZCs at Au(111) in Three Different ImidazoliumBased RTILs PZCb vs

direction RTILs EMIBF4

dielectric

Ag/AgCl

at PZC

temp

scan (V)

constant35

(V)

(μF cm
2)

(°C)

12.8 ( 0.6


0.11 0.13

51.4 13.2

23 45

1 to
1


1 to 1

BMIBF4

capacitance

of potential

0.80 to
0.80 11.7 ( 0.6
0.80 to 0.80

OMIBF4
0.80 to 0.80 8.9 ( 0.9a

0.13

13.2

75


0.02

43.4

23

0.20

13.7

45

0.20

15.1

75


0.23

23.7

23

0.04

18.1

23

0.08 0.08

19.0 22.7

45 75

0.10

15.1

23

0.13

20.8

45

0.16

25.9

75

a

Dielectric constant of 1-hexyl-3-methylimidazolium hexafluorophosphate. b The accuracy in the PZC measurements is (0.01 V.

value of ε with increasing the length of alkyl group is deemed to be the cause. (2) In all the three studied RTILs, capacitance of the C
E curves measured by sweeping the potential from negative to

Figure 7. (a) Temperature dependent C
E curves and (b) CVs measured at Au(111) electrode in N2-saturated BMIBF4. CVs were measured at a scan rate of 50 mV s
1. The direction of potential sweep is indicated in the figure.

positive direction is lower compared to the curves measured by reversing the direction of potential scan (Figures 4a, 5a, and 6a). The difference is smaller in EMIBF4 and larger in OMIBF4. At first sight, one may think that this disparity is arising from the nonequilibration of the interface, but this is not the case, as the ending part of one C
E curve matches nicely with the starting part of the other’s, though the curves were measured separately. In agreement with this observation, non-Faradaic current of the corresponding CVs during the anodic scan is found to be smaller compared to that during the cathodic scan (Figures 1, 5b and 6b). It may be that, along with electrostatic interactions, a different extent of specific adsorption (nonelectrostatic interaction) of ions at different starting potentials is playing a significant role in shaping the curve. Experimental evidence shows that the starting potentials that were used in this study (
0.80 e or 0.80 g) are quite sufficient for the specific adsorption of the anion (BF4
) on the anodic side and the cations (EMI+, BMI+, and OMI+) on the cathodic side.27,28,32,43 But for sure the extent and pattern of their adsorption (both electrostatic and nonelectrostatic) will not be the same due to the different structural constraint of the ions in the RTILs used, disparity in ion size, delocalization of charge, and difference in the constituting atoms. Therefore, the local interfacial structures will be different as the density and orientation of the ions that are going to reside close to the electrode surface will largely be determined by the adsorbed ions. For instance, if the anion is adsorbed (which is the case when the potential is swept from positive to negative direction), the imidazolium cation will 19802

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Figure 8. (a) Temperature dependent C
E curves and (b) CVs measured at Au(111) electrode in N2-saturated OMIBF4. CVs were measured at a scan rate of 50 mV s
1. The potential was scanned from negative to positive direction.

Figure 9. Schematic presentation of the interfacial structures at Au(111) electrode at
0.80 (a, c) and 0.80 V (b, d) in N2-saturated OMIBF4 (a, b) and EMIBF4 (c, d).

prefer to interact with the anion via its charged moiety, keeping the alkyl group away from the surface due to the ease of forming hydrogen bond and electrostatic interaction (Figure 9b,d). This assembly of charged moieties adjacent to the electrode surface

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increases the local value of ε, which in turn increases the capacitance. On the contrary, when the cation is adsorbed (which is the case when the potential is swept from negative to positive direction) the proportion of the neutral alkyl group close to the electrode surface increases as compared to the case of the anion adsorption due to the preferential hydrophobic interaction of the alkyl groups (Figure 9a,c), resulting in a decrease of the local value of ε, and consequently, the capacitance of the C
E curve decreases. These particular arrangements at large positive and negative potentials thus bring about a change in ion
electrode and ion
ion interactions, which possibly make the C
E curves potentialdependent, that is, owing to the change in interaction, the ions cannot get an identical arrangement at a particular potential and instead become dependent on the direction of potential scan. The disparity between the C
E curves measured by changing the direction of potential scan is pronounced in OMIBF4 compared to EMIBF4 because depending on the starting potential the separation between the layers of charges and the structural constraint in orientation are quite significantly different in OMIBF4 than in EMIBF4. In the case of OMIBF4 at negative potential, the second layer of charge resides far away from the electrode surface (due to the preferential hydrophobic interaction among the octyl groups), whereas at positive potential it resides close to the first layer. The same phenomenon is also prevailing in EMIBF4, but due to the shorter length of the alkyl group the difference is not as big as it is in OMIBF4. Molecular dynamics simulation of the interfacial structure also supports the above reasoning. Feng et al.44 have shown that the liquid nature of the RTIL and the short-range ion
electrode and ion
ion interactions significantly affect the interfacial structure, particularly at low electrode charge densities. Charge delocalization of the imidazolium ring was found to affect the adsorption, and thus, the mean force experienced by the ions plays an important role in shaping the C
E curve. (3) PZC shifts to the positive direction of potential with increasing the temperature (Table 1) and this shift is significantly large in EMIBF4 compared to OMIBF4. For instance, in EMIBF4 when the potential is swept from negative to positive direction, PZC shifts to the positive direction by 0.22 V for an increase of temperature from 23 to 45 °C,whereas in BMIBF4 and OMIBF4, it shifts by 0.04 and 0.03 V, respectively. It supports our previous argument that an increase in temperature decreases the so-called crystallinity in EMIBF4 to such an extent that the neutral alkyl groups of EMI+ get sufficient room to interact with the neutral electrode surface at the PZC and consequently the interaction of anions decreases and PZC shifts to the positive direction. This preferential interaction of the alkyl group at the PZC is presumed to be the cause of the lower value of capacitance around the PZC at higher temperatures. In support of the above argument, molecular dynamics simulation of the graphite|1-butyl-3-methylimidazolium hexafluorophosphate interface shows that the anion density at the vicinity of the electrode surface decreases with increasing the temperature.45 However, because of a strong inductive force among the long chain alkyl groups, such an effect of temperature is not observed in BMIBF4 and OMIBF4.

’ CONCLUSIONS Interfacial structures at Au(111) electrode in three 1-alkyl-3methylimidazolium tetrafluoroborate based RTILs are studied by the measurements of C
E curves and CVs. C
E curves are found to vary significantly with changing the direction of potential scan and temperature. C
E curve in a particular RTIL 19803

dx.doi.org/10.1021/jp205800x |J. Phys. Chem. C 2011, 115, 19797–19804

The Journal of Physical Chemistry C measured by scanning the potential from positive to negative direction always has higher capacitance value compared to the curve measured by reversing the direction of potential scan, and the difference is greater in OMIBF4 compared to EMIBF4. In analogy with the C
E curves, the charging current of the cathodic part of all the CVs is found to be larger than that of the anodic part. Liquid nature and change in the pattern of ion
electrode and ion
ion interactions with the direction of potential scan are presented as the causes for this observed discrepancy. Capacitance around the PZC decreases with increasing the temperature in EMIBF4, whereas in BMIBF4 and OMIBF4, it increases. At higher temperature, structural constraint in EMIBF4 reduces to such an extent that the neutral alkyl group interacts preferentially with the neutral electrode surface at the PZC, and consequently, the capacitance around the PZC decreases, whereas similar phenomenon does not occur in BMIBF4 and OMIBF4 due to the strong inductive force among the alkyl groups, but the ion association decreases for sure, which is considered as a cause for the increase of capacitance with temperature.

’ AUTHOR INFORMATION Corresponding Author

*Phone: +81-45-9245404. Fax: +81-45-9245489. E-mail: ohsaka@ echem.titech.ac.jp. Present Addresses †

Building M, Room No. 02, Faculty of Science, Health and Education, University of the Sunshine Coast, Sippy Downs, QLD 4556, Australia. ‡ Department of Chemistry, University of Dhaka, Dhaka-1000, Bangladesh.

’ ACKNOWLEDGMENT The present work was financially supported by Grant-in-Aid for Scientific Research (A) (No. 19206079) to T.O., from the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan. M.T.A. gratefully acknowledges the Government of Japan for a MEXT Scholarship. ’ REFERENCES (1) Welton, T. Chem. Rev. 1999, 99, 2071. (2) Binnemans, K. Chem. Rev. 2005, 105, 4148. (3) Tokuda, H.; Hayamizu, K.; Ishii, K.; Susan, M. A. B. H.; Watanabe, M. J. Phys. Chem. B 2005, 109, 6103. (4) Tokuda, H.; Ishii, K.; Susan, M. A. B. H.; Tsuzuki, S.; Hayamizu, K.; Watanabe, M. J. Phys. Chem. B 2006, 110, 2833. (5) Oyama, T.; Okajima, T.; Ohsaka, T. J. Electrochem. Soc. 2007, 154, D322. (6) Belding, S. R.; Rees, N. V.; Aldous, L.; Hardacre, C.; Compton, R. G. J. Phys. Chem. C 2008, 112, 1650. (7) Lynden-Bell, R. M. J. Phys. Chem. B 2007, 111, 10800. (8) Bard, A. J.; Faulkner, L. R. Electrochemical Methods, Fundamental and Application, 2nd ed.; Wiley Inc.: New York, 2001. (9) Kornyshev, A. A. J. Phys. Chem. B 2007, 111, 5545. (10) Fedorov, M. V.; Kornyshev, A. A. Electrochim. Acta 2008, 53, 6835. (11) Fedorov, M. V.; Kornyshev, A. A. J. Phys. Chem. B 2008, 112, 11868. (12) Oldham, K. B. J. Electroanal. Chem. 2008, 613, 131. (13) Georgi, N.; Kornyshev, A. A.; Fedorov, M. V. J. Electroanal. Chem. 2010, 649, 261. (14) Fedorov, M. V.; Georgi, N.; Kornyshev, A. A. Electrochem. Commun. 2010, 12, 296.

ARTICLE

(15) Lauw, Y.; Horne, M. D.; Rodopoulos, T.; Nelson, A.; Leermakers, F. A. M. J. Phys. Chem. B 2010, 114, 11149. (16) Lauw, Y.; Horne, M. D.; Rodopoulos, T.; Leermakers, F. A. M. Phys. Rev. Lett. 2009, 103, 117801. (17) Alam, M. T.; Islam, M. M.; Okajima, T.; Ohsaka, T. J. Phys. Chem. C 2009, 113, 6596. (18) Alam, M. T.; Islam, M. M.; Okajima, T.; Ohsaka, T. J. Phys. Chem. C 2008, 112, 2601. (19) Alam, M. T.; Islam, M. M.; Okajima, T.; Ohsaka, T. J. Phys. Chem. C 2007, 111, 18326. (20) Alam, M. T.; Islam, M. M.; Okajima, T.; Ohsaka, T. J. Phys. Chem. C 2008, 112, 16600. (21) Islam, M. M.; Alam, M. T.; Okajima, T.; Ohsaka, T. J. Phys. Chem. C 2009, 113, 3386. (22) Lockett, V.; Sedev, R.; Ralston, J.; Horne, M.; Rodopoulos, T. J. Phys. Chem. C 2008, 112, 7486. (23) Lockett, V.; Horne, M.; Sedev, R.; Rodopoulosb, T.; Ralston, J. Phys. Chem. Chem. Phys. 2010, 12, 12499. (24) Aliaga, C.; Baldelli, S. J. Phys. Chem. B 2006, 110, 18481. (25) Baldelli, S. J. Phys. Chem. B 2005, 109, 13049. (26) Rivera-Rubero, S.; Baldelli, S. J. Phys. Chem. B 2004, 108, 15133. (27) Baldelli, S. Acc. Chem. Res. 2008, 41, 421–431. (28) Santos, V. O.; Alves, M. B.; Carvalho, M. S.; Paulo, A. Z.; Suarez, P. A. Z.; Rubim, J. C. J. Phys. Chem. B 2006, 110, 20379. (29) Islam, M. M.; Alam, M. T.; Okajima, T.; Ohsaka, T. J. Phys. Chem. C 2008, 112, 16568. (30) Alam, M. T.; Islam, M. M.; Okajima, T.; Ohsaka, T. Electrochem. Commun. 2007, 9, 2370. (31) Gnahm, M.; Pajkossy, T.; Kolb, D. M. Electrochim. Acta 2010, 55, 6212. (32) Su, Y. Z.; Fu, Y. C.; Yan, J. W.; Chen, Z. B.; Mao, B. W. Angew. Chem., Int. Ed. 2009, 48, 5148. (33) Hamelin, A. J. Electroanal. Chem. 1996, 407, 1. (34) Gore, T. R.; Bond, T.; Zhang, W.; Scott, R. W. J.; Burgess, I. J. Electrochem. Commun. 2010, 12, 1340. (35) Wang, Y.; Voth, G. A. J. Am. Chem. Soc. 2005, 127, 12192. (36) Saha, S.; Hayashi, S.; Kobayashi, A.; Hamaguchi, H. Chem. Lett. 2003, 32, 740. (37) Matsumoto, K.; Tsuda, T.; Hagiwara, R.; Ito, Y.; Tamada, O. Solid State Sci. 2002, 4, 23. (38) Hayashi, S.; Ozawa, R.; Hamaguchi, H. Chem. Lett. 2003, 32, 498. (39) Matsumoto, K.; Hagiwara, R.; Mazej, Z.; Benkic, P.; Zemva, B. Solid State Sci. 2006, 8, 1250. (40) Hunt, P. A.; Kirchner, B.; Welton, T. Chem.—Eur. J. 2006, 12, 6762. (41) Wakai, C.; Oleinikova, A.; Ott, M.; Weingartner, H. J. Phys. Chem. B 2005, 109, 17028. (42) Silva, F.; Gomes, C.; Figueiredo, M.; Costa, R.; Martins, A.; Pereira, C. M. J. Electroanal. Chem. 2008, 622, 153. (43) Devanathan, M. A. V.; Tilak, B. V. K. S. R. A. Chem. Rev. 1965, 65, 635. (44) Feng, G.; Zhang, J. S.; Qiao, R. J. Phys. Chem. C 2009, 113, 4549. (45) Kislenko, S. A.; Amirova, R. H.; Samoylova, I. S. Phys. Chem. Chem. Phys. 2010, 12, 11245.

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