Imidazolium-Based Ionic Liquids

Dec 10, 2012 - ... cations on Au(100) at potential around the peaks of the bell-shaped feature, ... Song Li , Katherine L. Van Aken , John K. McDonoug...
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Electric Double Layer of Au(100)/Imidazolium-Based Ionic Liquids Interface: Effect of Cation Size Yuzhuan Su, Jiawei Yan,* Miangang Li, Meng Zhang, and Bingwei Mao* State Key Laboratory for Physical Chemistry of Solid Surfaces, and Department of Chemistry, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen, Fujian 361005, P. R. China S Supporting Information *

ABSTRACT: We have carried out differential capacitance measurements and in-situ scanning tunneling microscope (STM) characterizations to investigate the effect of the length of alkyl side chains on an electric double layer of Au(100)/ imidazolium-based ionic liquids interface. In ionic liquids consisting of BMI+ cation (1-butyl-3-methylimidazolium), differential capacitance curves present an obvious bell-shaped feature. In ionic liquids with PMI+ (1-methyl-3-propylimidazolium) or OMI+ (1-methyl-3-octylimidazolium) cations, the rising of capacitance from about −0.5 V disturbs the bellshaped feature. In-situ STM characterizations reveal the generality of surface etching and micelle-like adsorption of imidazolium cations on Au(100) at potential around the peaks of the bell-shaped feature, demonstrating that the potential of zero charge (PZC) should locate at the potential close to the peaks. Because of the longer side chain length and stronger interaction with Au(100) substrate, an extra capacitance peak appears at the potential as negative as −1.65 V in OMIPF6 and a corresponding order−disorder transformation of OMI+ cation adlayer is revealed by STM, indicating a correlation between differential capacitance curve and STM.

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the latent voids resulting from the neutral “tails” of ions, which could be replaced by charged groups via rotations and translations of ions. The above studies reveal the complexities of the structure of the electric double layer in ionic liquids. In the aspect of experiments, differential capacitance as a function of potential can be derived from data acquired by electrochemical impedance spectroscopy (EIS)14 or measured directly at a fixed frequency, i.e., single-frequency measurement. Differential capacitance measurements in ionic liquids have been extensively carried out by Ohsaka’s group. A series of single-frequency measurements in the alkylimidazolium-based ionic liquids with tetrafluoroborate anions at GC (glassy carbon),15 Hg,16 and polycrystalline Au17 electrodes present parabolic-shaped differential capacitance curves. Minor modifications of the curves such as humps are attributed to interaction between the ions and the surface. The capacitance minima are assigned to the PZCs. Interestingly, in other ionic liquids consisting of bis(trifluoromethanesulfonyl)imide anions which have a size similar to that of cation, the so-called bellshaped capacitance curves in agreement with the prediction of Kornyshev’s model were observed at polycrystalline Pt and Au electrodes by Ohsaka’s group.18 The potentials of capacitance maxima are assigned to the PZCs of these systems. On the other hand, the camel-shaped differential capacitance curves,

INTRODUCTION Because of their properties of a wide electrochemical window, low vapor pressure, and moderate solubility as well as accepted conductivity,1 room temperature ionic liquids, mainly imidazolium cation-based ionic liquids, have received increased attention in recent years in the field of electrochemistry for use in electrodeposition,2,3 electrocatalysis,4 and electrocapacitors.5 This has stimulated study to understand the structure of the electric double layer (EDL) at electrode/ionic liquids interface, where electron transfer reactions occur. Differential capacitance measurements is an important means of investigating the electrode/electrolyte interface, which can provide significant information about the EDL, such as the potential of zero charge (PZC) and adsorption behavior.6,7 Theoretical modeling of the EDL at electrode/ionic liquids interfaces is marked by Kornyshev’s formula.8 A bell-shaped or camel-shaped differential capacitance curve is predicted for ionic liquids with either a capacitance maximum or a local minimum close to the PZC of the system. Similar predictions are also deduced by Oldham9 and Lauw et al.10 Further molecular dynamic simulations (MDS) by Fedorov and Kornyshev11,12 demonstrate that when the size of the cation is remarkably different from that of the anion, the capacitance maximum shifts away from the PZC and the shape of the capacitance curve becomes more complicated. Monte Carlo simulations13 indicate that if ions have charged heads and neutral counterparts, such as alkylimidazolium cations, the differential capacitance curve presents camel-shaped owing to © 2012 American Chemical Society

Received: August 10, 2012 Revised: December 8, 2012 Published: December 10, 2012 205

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limidazolium hexafluorophosphate) and OMIPF6 (1-methyl-3octylimidazolium hexafluorophosphate) were purchased from IoLiTec in the highest available quality (99%). Prior to each measurement, ionic liquids were vacuum-dried for several hours at 80 °C in a glovebox filled with ultra pure N2 (99.999%) to remove the absorbed water, and the experiments were conducted under the protection of N2 atmosphere. Au(100) single crystal electrodes were purchased from MaTecK, Germany, and were subjected to electrochemical polishing and flame annealing in H2 followed by cooling under a N2 atmosphere prior to each experiment. Cyclic voltammetric measurements, single-frequency measurements, and EIS were performed on an Autolab electrochemical workstation (Eco Chemie, The Netherlands). In-situ STM measurements were carried out on a Nanoscope E STM instrument (Veeco, Santa Barbara, CA) under constant-current mode. Mechanically cut Pt/Ir tips were used, which were insulated by thermosetting polyethylene to reduce the faradaic current. Two platinum wires served as the counter and quasireference electrodes. All potentials reported in this paper are versus Pt quasi-reference electrode unless otherwise stated. A homemade cylindrical Kel-F cell25 was fabricated for electrochemical measurements, especially for EIS. The internal diameter and volume of the cell were 0.6 and 0.15 cm3, respectively. A Au(100) working electrode and a platinumsheet counter electrode were located at the two ends of the cylinder; a platinum wire acting as a quasi-reference electrode was placed between the working and counter electrodes. This configuration ensured the homogeneous current distribution of the working electrode surface, which is an important condition for obtaining reliable high-frequency impedance data. EIS data measured within a wide double-layer region were fitted using commercial ZsimpWin electrochemical impedance modeling software. For an ideal polarizable electrochemical interface, the series capacitance can be calculated directly from the imaginary part of impedance value (C = −(Z″2πf)−1, where Z″ is the imaginary part of the impedance and f is the frequency). This is the basis for single-frequency measurement, which considers capacitance to be independent of imposed frequency. However, for those systems showing capacitance dispersion, EIS data recorded in a wide range of frequency are more reliable, and the ideal capacitance is usually replaced by CPE in equivalent circuits. The impedance of CPE is given by the equation ZCPE = 1/Y0 × (jω)−n, where j is the imaginary number, ω is the angular frequency, n is a parameter that usually varies between 0 and 1, and Y0 is a constant in Ω−1 sn, which becomes equal to the differential capacitance when n = 1.14,32

predicted by Kornyshev in asymmetrical ionic liquids, were also observed on a GC electrode in a series of imidazolium cation and chloride anion-based ionic liquids by Lockett et al.19 The local minima of the capacitance curves decrease with the decrease of temperature, which make it reasonable that the local minima were assigned to the PZCs. Since the complexities of polycrystalline electrodes, such as the effect of capacitance dispersion,20,21 could increase difficulties in data analysis and theoretical modeling, the welldefined single crystal electrodes are advantageous to better understanding of the structures and properties of the EDL in ionic liquids. In our previous work,22 differential capacitance curve of Au(100) in BMIBF4 (1-butyl-3-methylimidazolium tetrafluoroborate) presented a bell-shaped feature similar to theoretical prediction.8 Moreover, in-situ scanning tunneling microscope (STM) revealed ordered adsorptions of anions and cations at the anodic and cathodic side of the maximum of the bell-shaped capacitance curve, respectively, while no immobilized adsorption of anions or cations was observed around the potential of the capacitance maximum. The results indicated that the PZC of the system locates at the potential close to the capacitance maximum, which correlates the features of capacitance curve with the adsorption of ionic liquids on the electrode surface. Further differential capacitance measurements on single crystal show that capacitance dispersion is observed even on a well-defined single crystal surface. For example, Lust et al. reported weak deviations from ideal polarizable interface of Bi(111)/EMI+ (1-ethyl-3-methylimidazolium)-based ionic liquids at low frequency,23 which could originated from the slow first layer reorganization process of EDL or partial charge transfer from ions to the electrode surface.24,25 In the system of Au(111) in contact with BMIPF6 (1-butyl-3-methylimidazolium hexafluorophosphate), a constant phase element (CPE) was introduced into the equivalent circuit to fulfill the deviated plots obtained from EIS by Kolb and co-workers.25 They employed CPE to model the kinetics of the restructuring of the interfacial region. More than one capacitive process was observed in the interfaces between single crystal surfaces and ionic liquids.24,26,27 Recently, Ohsaka’s group investigated interfacial structures at Au(111) electrode in three 1-alkyl-3methylimidazolium tetrafluoroborate-based RTILs by the measurements of capacitance−potential curves. They found that the curves vary significantly with changing the direction of potential scan and temperature.28 For better understanding the interface between an electrode and an ionic liquid, more studies on the interface at single crystal electrodes by combining differential capacitance measurements and surface-sensitive techniques, such as STM,29,30 are desired, which could benefit the development of the theory of EDL in ionic liquids. In this work, we study the EDL properties on Au(100) in contact with four imidazolium-based ionic liquids with different lengths of alkyl side chains of cations and attempt to explore the influence of cation size on the interfacial adsorptive structures. Both EIS and single-frequency measurements are used to investigate the double-layer capacitance and are compared with each other. In-situ STM is also applied to provide elaborate interfacial information in real space.

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RESULTS AND DISCUSSION Capacitance Measurement. BMIBF4. Au(100) in dried BMIBF4 has an electrochemical window of up to about 4 V33 with negligible current in a wide potential region between −2.5 and 1.4 V, as shown in Figure 1a. The cyclic voltammogram in the inset of Figure 1a presents only a small charging current within the region of electric double layer from 0 to −2 V with current density lower than 5 μA cm−2. Therefore, the measurements of differential capacitance are conducted in this potential region. In our previous work,22 we acquired differential capacitance curve of Au(100) in BMIBF4 by single-frequency measurement ( f = 18 Hz). Herein, in order to investigate the influence of frequency on the differential capacitance curve, we perform the

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EXPERIMENTAL SECTION BMIBF4 and BMIPF6 were synthesized according to a procedure described previously.31 PMIPF6 (1-methyl-3-propy206

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important to carry out EIS measurement and then compare the result with that obtained from single-frequency measurement. It should be pointed out that a number of EIS studies on the EDL have been carried out by using single-frequency measurements or recording complete impedance spectra so far. There exist different viewpoints on merits and drawbacks of the two methods. Therefore, it is meaningful to conduct experiments in the same system using two methods, and the experimental data may include some information for comparing the methods. Scanning from 105 to 1 Hz, a series of EIS data were recorded in the double-layer region upon cathodic potential excursion. The typical data are shown in Figure 2a,b. All impedance data were fitted to an R-CPE equivalent circuit. The value of the resistance is about 390 Ω and almost independent of electrode potential. The potential dependences of Y0 and n, as well as the differential capacitance curve obtained at 18 Hz, have been shown in Figure 3. The value of n varies between

Figure 1. (a) Cyclic voltammograms of BMIBF4 on Au(100) showing the electrochemical window of up to 4 V. Inset of (a) corresponds to the cyclic voltammogram at the potential region from 0 to −2 V. Sweep rate: 50 mV/s. (b) Differential capacitance curves of Au(100) in BMIBF4 obtained by single-frequency measurement at various frequencies. Amplitude: 5 mV.

measurements using a series of frequency from 10 to 104 Hz. Starting from the initial potential at −0.3 V, cathodic potential excursion was applied in the double-layer region and the differential capacitance was measured as a function of potential. As shown in Figure 1b, the values of the capacitance decrease with increasing frequency while keeping closer at a certain range of frequency. This tendency has also been observed in the system of Bi(111)/EMIBF4.23 Taking the value of capacitance maximum (Cmax) for example, once the frequency is located within the range of 20−400 Hz, Cmax remains 10 ± 0.2 μF cm−2. Even so, the change of differential capacitance of the upper and lower limits of frequencies still implies the existence of capacitance dispersion at the interface between Au(100) and BMIBF4 as in other systems.23,25 Therefore, the phase transitions and the adsorption of ions at the interface should be taken into account in this system. As a matter of fact, restructuring and reconstruction of Au(100) surface, as well as adsorption of both cations and anions, have been demonstrated by in-situ STM.22 To further explore the extent of capacitance dispersion at the interface between Au(100) and BMIBF4, it is

Figure 3. Potential dependence of coefficient Y0 (black solid triangle) and exponent n (red solid square) obtained by EIS as well as differential capacitance (blue solid line) of Au(100) in BMIBF4 obtained by single-frequency measurement at 18 Hz. Frequency: 105−1 Hz. Amplitude: 5 mV.

0.93 and 0.97, which is close to 1, indicating that the behavior of CPE resembles that of an ideal capacitance; i.e., the coefficient Y0 is comparative with actual interfacial capacitance.

Figure 2. Complex capacitance plots recorded in the “Au(100)/BMIBF4” system at 0 V (a) and −2 V (b). Fitting data are based on the equivalent circuit (c). Frequency: 105−1 Hz. Amplitude: 5 mV. 207

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two ionic liquids. Though the distinctness of differential capacitance curve in OMIPF6 is expected due to the notable alteration to the length of alkyl side chain of cations, further characterizations by surface-sensitive tools, e.g., in-situ STM34 and AFM,35 are required in order to understand the differential capacitance curve. The typical data of EIS in the three PF6-based ionic liquids are shown in Figure S1. The equivalent circuit of resistance in series with CPE element is also used to fit the recorded data. According to the fitting results, the sequence of resistance of all the three samples is as follows: PMIPF6 (∼400 Ω) < BMIPF6 (∼900 Ω) < OMIPF6 (∼2500 Ω). The conductivity of OMIPF6, OMIBF4, BMIPF6, and PMIBF4 is 0.026, 0.058, 0.146, and 0.35 S m−1, respectively.36−38 Compared with the above available values, the sequence of the obtained fit values is reasonable. It is also in consistent with the fact39 that the longer the alkyl side chain of cations is, the higher the viscosity of ionic liquid. The value of n, especially in PMIPF6, which keeps larger than 0.95 within the potential region under study (Figure S2), implies the weak capacitance dispersion as in BMIBF4. Interestingly, the comparability between differential capacitance curves (Figure 6a) and Y0−E curves (Figure 6b) is different for the three ionic liquids, in a sequence of BMIPF6 > PMIPF6 > OMIPF6. Similar to the result in BMIBF4, the Y0−E curve measured in BMIPF6 shows the obvious bell-shaped feature like differential capacitance curve, with a higher value of Y0 maximum than Cmax. However, in PMIPF6 or OMIPF6 the similarity between two curves becomes lower, and the value of Y0 increases with anodic potential excursion from −0.55 V, which leads to the ill-defined peak at about −0.5 V. In OMIPF6, in contrast to the wide capacitance peak at −1.65 V the Y0−E curve is almost flat at the same potential region. Briefly, in ionic liquids consisting of BMI+ cation, both differential capacitance curve and Y0−E curve display obvious bell-shaped feature with a peak at similar potential. However, in ionic liquids with PMI+ or OMI+ cations, the similarity between two curves becomes lower, and the rising of capacitance from about −0.5 V disturbs the bell-shaped feature. In addition, an extra capacitance peak appears at the potential as negative as −1.65 V in OMIPF6. Therefore, the length of alkyl side chain of cation might play an important role in the structure of EDL and thus the interfacial capacitance. In-Situ STM Characterization. To investigate the influence of the length of side chains on the adsorption of imidazolium cations on Au(100) surface, in-situ STM experiments are carried out in three PF6−-based ionic liquids. Since STM data of BMIPF6 have already been reported in our previous work22 together with those of BMIBF4, herein we mainly present STM data in PMIPF6 and OMIPF6. Within the chosen frequency window of EIS measurements, only very fast processes taking place on seconds up to several microseconds can be investigated. The process observed is most likely the charging of the electrochemical double layer via the bulk resistance. On the other hand, the STM results show the formation of adsorption layers consisting of IL cations. The adsorption of ions to the electrode surface should take place on time scales from seconds to minutes. Thus, the comparion between EIS data and STM data should be treated with great caution due to the discrepancy between the time scales of the two techniques. In this work, the STM data were just used to disclose the type of ions that adsorbed on electrode surface at different potentials. It is reasonable that structural change from the adsorption of anion to imidazolium cation should occur at

More remarkably, similar to the differential capacitance curve, the Y0−E curve also displays a bell-shaped feature with Y0 maximum around −0.5 V near the potential of Cmax though the value of Y0 maximum equaling 12 μF cm−2 sn−1 is a bit larger than Cmax of 10 μF cm−2. Significantly, the great similarity between the Y0−E curve and the differential capacitance curve, as well as the fact that the value of n is close to 1, indicates that the extent of capacitance dispersion is slight in the electrochemical interface of Au(100)/BMIBF4. This can be attributed to two major reasons. One is the employment of a single crystal electrode, and the other is the absence of strong specific adsorption. Consequently, from the experimental point of view, the differential capacitance curve and Y0−E curve are quite comparable in this system, which are obtained from singlefrequency measurement and EIS measurement, respectively. In BMIBF4, both Cdl and Y0 achieve maximum at the potential near PZC, which is consistent with theoretical prediction.8 PF6−-Based Ionic Liquids. To further investigate the capacitive properties of imidazolium-based ionic liquids, especially the influence of alkyl side chains of cations on the differential capacitance curve, single-frequency measurements, EIS, as well as in-situ STM studies have been systematically carried out in three PF6−-based ionic liquids with increasing length of alkyl side chains of cations, as shown in Figure 4.

Figure 4. Illustrations of chemical structures of (a) PMIPF6, (b) BMIPF6, and (c) OMIPF6.

Although the alkyl side chains of the three ionic liquids are different, they present similar electrochemical windows of about 3.5 V on Au(100) surface. As can be seen from Figure 5, due to the oxidation of either PF6− or Au electrode, positive excursion of potential beyond 1 V gives rise to ascending current representing anodic limit of the systems. Cathodic limit, which seems independent of the length of alkyl side chain of cations, locates at −2.5 V in all the three ionic liquids. In addition, all the three investigated ionic liquids display low current density of double-layer charging from 0 to −2 V; therefore, the singlefrequency measurements and EIS are performed in the potential range. Differential capacitance curves measured at 18 Hz in three PF6−-based ionic liquids are shown in Figure 6a. Similar to the curve in BMIBF4,22 the differential capacitance curve in BMIPF6 also exhibits a bell-shaped feature with a capacitance maximum of 9.3 μF cm−2 at around −0.65 V. When the length of alkyl side chain decreases from butyl to propyl, the differential capacitance curve in PMIPF6 shows peak of Cmax equaling 9.7 μF cm−2 at −0.78 V and rapidly rises with positive excursion of potential from −0.5 V. The rising of capacitance from about −0.5 V also appears in OMIPF6, which disturbs the bell-shaped feature at about −0.65 V. Interestingly, another wide capacitance peak of Cmax of 8.7 μF cm−2 can be observed at the potential as negative as −1.65 V in OMIPF6, which contains cations with alkyl side chain much longer than other 208

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Figure 5. Cyclic voltammograms of (a, b) PMIPF6, (c, d) BMIPF6, and (e, f) OMIPF6 on Au(100). Sweep rate: 50 mV/s.

BMIPF6 and BMIBF4.22 The stripes gradually increase and almost cover the whole surface after staying at −0.6 V for 13 min (Figure 7b). Resulting from surface etching accompanying adsorption of PMI+, some defects with atomic-scale depth appear on the Au(100) surface. As a matter of fact, the adsorption of PMI+ occurs at potential around the peaks of the Y0−E curve and differential capacitance curve, indicating that the PZC is likely to locate at the potential close to the peaks, which is in conformity with the results in both BMIPF6 and BMIBF4.22 It is also possible that there might exist a potential at which either specific adsorption is absent or an exchange between adsorbed ions and counterions takes place. The potential might also be called PZC and has been observed by us. The high-resolution STM image in Figure 7c discloses the typical micelle-like feature of imidazolium cations adsorbed on the Au(100) surface; that is, each row of the strip is composed of aligned PMI+ cations, with the large and bright spots being the imidazolium groups and the less intense parts being the propyl side chains. The width of the micelle-like structures in PMIPF6 is 0.85 ± 0.02 nm, which is smaller than that in BMIPF6, 1.23 ± 0.02 nm. The result is reasonable due to the shorter alkyl side chains of PMI+. Based on the above analysis, a structural model similar to BMI+ adsorption22 is proposed in Figure 7e. In the model, the double-row stripe lies along the √2 direction of the Au(100) surface with the theoretical width of 0.82 nm identical with experimental value within experimental error. The alignment of PMI+ within the row

Figure 6. Potential dependence of differential capacitance (a) and Y0 of CPE (b) in PMIPF6 (black), BMIPF6 (red), and OMIPF6 (blue) on Au(100).

potential close to the PZC of the system. Therfore, the structural alteration of the surface would be significant around the capacitance maximum if the maximum is close to the PZC and connected with adsorption/desorption of ions. Based on the above, in-situ STM measurements can disclose the structures of ionic liquids at electrified interfaces, which are useful for correlating the capacitance features of differential capacitance curves with the structure of the electric double layer. PMIPF6. As can be seen from Figure 7, a clean Au(100) surface is observed at −0.3 V (Figure 7a). Then a cathodic potential excursion to −0.6 V induces the adsorption of PMI+ cations in the form of perpendicular-oriented double-row stripes similar to the structure of BMI+ adsorption in both 209

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Figure 8. Sequence of STM images of Au(100) in OMIPF6 showing adsorption of OMI+ cations at (a) −0.4 V, (b) −0.6 V for 10 min, and (c) −0.9 V. (d) Proposed model of OMI+ adsorption structure. Scan size: (a, b) 100 × 100 nm2; (c) 20 × 20 nm2.

that the width of the micelle-like structure in OMIPF6 is 2.01 ± 0.02 nm, which is much larger than those in PMIPF6 and BMIPF6. A model is also proposed for OMI+ adsorption on the Au(100) surface as shown in Figure 8d, in which the width of the micelle equals to 2.04 nm. The adsorption of OMI+ also occurs at potential around the peaks of the Y0−E curve and differential capacitance curve, indicating that the PZC is likely to locate at the potential close to the peaks. So far, we have observed micelle-like structures on the Au(100)-(1 × 1) surface in four different imidazolium-based ionic liquids, demonstrating the generality of interfacial property for these systems. The PZC for the four interfaces should be around −0.6 V. However, different from the behaviors of the systems with shorter alkyl side chains, a phase transition at more negative potential can be observed in OMIPF6. At −1.2 V, both the number and the length of the stripes start to decrease as shown in Figure 9a,b. When the potential was further moved negatively, the ordered double-row stripes disapeared and the Au(100) surface was covered by a disordered adsorbate, leading

Figure 7. Sequence of STM images of Au(100) in PMIPF6 showing adsorption of PMI+ cations and the following reconstruction at (a) −0.3 V, (b) −0.6 V for 13 min, (c) −1.0 V, and (d) −1.6 V for 5 min. (e) Proposed model of PMI+ adsorption structure on Au(100)-(1 × 1). Scan size: (a, b, d) 100 × 100 nm2; (c) 20 × 20 nm2.

and the chain-to-chain arrangement between the rows provide the most efficient configuration for the PMI+ cations, which facilitates strong van der Waals interaction between the alkyl side chains and thus stabilizes the micelle-like structure. Further negative potential excursion to −1.6 V gives birth to the surface reconstruction from Au(100)-(1 × 1) to Au(100)hex identified by the appearance of reconstruction rows (Figure 7d). The orientation of the reconstruction rows deviates from the double-row stripes by 45°, which confirms the validity of the double-row stripe along the √2 direction of Au(100) in the proposed model. The reconstructed Au(100) surface with hexagonal arrangement cannot offer the adaptive substrate for micelle-like adsorption of PMI+ and lead to the disappearance of the perpendicular-oriented structures. OMIPF6. As can be seen from Figure 8, in OMIPF6 the imidazolium cations also form characteristic micelle-like structure on the Au(100)-(1 × 1) surface. A smooth Au(100) surface is observed at −0.4 V, on which there are some small Au islands as well as annealing-induced reconstruction rows (see Figure 8a). When the potential is negatively shifted to −0.6 V, some etched holes and perpendicular-oriented stripes gradually come into being on the surface, shown in Figure 8b. The process of the surface etching and the formation of micelle-like adsorption structures are ascribed to disorder−order transformation of adsorbed imidazolium cations and in line with that in other presented ionic liquids. High-resolution STM image (Figure 8c) reveals

Figure 9. Sequence of STM images showing the progress of adsorption structure on Au(100) in OMIPF6 at (a) −1.0, (b) −1.2, (c) −1.6, and (d) −2.5 V. Scan size: 100 × 100 nm2. 210

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observed due to the longer length of side chain and stronger interaction with Au(100) substrate. The process corresponds to the capacitance peak appears at −1.65 V, indicating the correlation between differential capacitance curve and STM result. It is noted, however, that information obtained from STM is limited in the region of compact layer. Therefore, studies using atomic force microscopy are highly desirable for further understanding of the structure of EDL in ionic liquids. Research toward this direction is currently in progress in our group.

to the relatively rough surface (Figure 9c). Since the phenomenon was not observed in the systems with shorter alkyl side chains, it is reasonable that the competition between electrostatic interaction and van der Waals interaction accounts for it. If the van der Waals interaction originated from the alkyl side chains is stronger enough, the OMI+ can form the ordered double-row stripes on Au(100) surface. When the electrode potential is moved negavtively and far from the PZC, the electrostatic interaction between the charged electrode surface and the ionic liquid will have an advantage over the interaction originated from the alkyl side chains, which results in the structural transition of OMIPF6. As a matter of fact, the rough surface begins to form at −1.2 V, which exactly is the potential corresponding to the onset of the wide differential capacitance peak with Cmax at −1.65 V, implying the mutuality between surface structure and interfacial capacitance. The rough surface would transform into reconstructed Au(100)-hex surface once OMI+ cations reduce at −2.5 V (Figure 9d). It should be pointed out that there are still some defects on the reconstructed surface due to the foregoing etching induced by OMI+ cations. It seems that in the case of OMIPF6 the reconstruction of the Au(100) surface is initialized once the cation is reduced at −2.5 V. This in not in line with the findings for the other systems PMIPF6/Au(100) and BMIPF6/Au(100) where the reconstruction becomes visible at potentials close to −1.6 V. An alternative explanation might be that in OMIPF6 surface reconstruction takes place at −1.6 V as well, where a phase transition of the adsorbed cation adlayer has been revealed by STM measurements, but that it cannot be observed since the OMI+ cation with its long and flexible alkyl chain is able to partially adapt the new underlying surface structure and therefore keeps being loosely adsorbed. According to the above STM results, the capacitance peak around −1.65 V in OMIPF6 originated from the order− disorder structure transformation of OMI + cations at sufficiently negative potential. In-situ STM characterizations reveal not only the elaborate adsorption structures of imidazolium cations on Au(100), which are dependent on the length of alkyl side chains, but also the possible origins of the complicated differential capacitance curves in ionic liquids.

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ASSOCIATED CONTENT

* Supporting Information S

Additional differential capacitance curves. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected] (J.Y.), [email protected] (B.M.); Ph +86-592-2186862; Fax +86-592-2186979. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Natural Science Foundation of China (NSFC no. 21033007), National Basic Research Program of China (973 Program, 2012CB932902), and the Natural Science Foundation of China (NSFC nos. 20973144, 20911130235, and 21021002). I am grateful to an anonymous referee for raising interesting points relating to the PZC, reconstruction, and phase transition.

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REFERENCES

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CONCLUSIONS The length of alkyl side chain of imidazolium cation plays an important role in the structure of EDL and interfacial capacitance. In ionic liquids consisting of BMI+ cation, differential capacitance curves and Y0−E curves of Au(100) display obvious bell-shaped feature. In ionic liquids with PMI+ or OMI+ cations, the rising of capacitance from about −0.5 V disturbs the bell-shaped feature. In addition, a new capacitance peak appears at the potential as negative as −1.65 V in OMIPF6. The EIS data demonstrate that capacitance dispersion is not too serious in the imidazolium-based ionic liquids on Au(100) single crystal surface, which is consistent with the fact that differential capacitance curves obtained by single-frequency measurements at 20−400 Hz are nearly coincident and totally present bell-shaped feature. In-situ STM characterizations reveal the generality of surface etching and micelle-like adsorption of imidazolium cations on Au(100), which occur at potential around the peaks of differential capacitance curves, indicating that the PZC should locate at the potential close to the peaks of the bell-shaped feature. In OMIPF6, an order−disorder transformation of OMI+ cation adlayer at sufficiently negative potential is 211

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