Ionic Liquid

Article pubs.acs.org/JPCC

Long Distance Electron Transfer at the Metal/Alkanethiol/Ionic Liquid Interface Victoria A. Nikitina,*,†,‡ Alexander V. Rudnev,†,§ Galina A. Tsirlina,‡ and Thomas Wandlowski† †

Department of Chemistry and Biochemistry, University of Bern, Freiestrasse 3, CH-3012 Bern, Switzerland Department of Electrochemistry, Moscow State University, Leninskie Gory 1/3, 119071 Moscow, Russian Federation § A. N. Frumkin Institute of Physical Chemistry and Electrochemistry,Russian Academy of Sciences, Leninskii pr. 31, 119991, Moscow, Russian Federation ‡

S Supporting Information *

ABSTRACT: The rate constants of simple electron transfer (ET) reactions in room temperature ionic liquids (ILs) available now are rather high, typically at the edge of experimental accuracy. To consider ET phenomena in these media in view of theory developed earlier for molecular solvents, it is crucial to provide quantitative comparison of experimental kinetic data for certain reactions. We report this comparison for ferrocene/ferrocenium reaction. The ET distance is fixed by Au surface modification by alkanethiol self-assembled monolayers, which were characterized by in situ scanning tunneling microscopy. The dependence of ln kapp on barrier thickness in the range of ca. 6−20 Å is linear, with a slope typical for the same plots in aqueous media. This result confirms diabatic mode of Fc oxidation at long distance. The data for shorter ET distances point to the adiabatic regime of ET at a bare gold surface, although more detailed computational studies are required to justify this conclusion.

1. INTRODUCTION Reactant−electrode distance is a crucial parameter responsible for transmission coefficient, work terms, and solvent reorganization energy for heterogeneous electron transfer (ET) reactions. Due to the large number of parameters and their interdependence, a comparative experimental study of electrochemical reactions at various fixed electrode-reactant distances is an attractive approach to verify the available theoretical models.1 Rate constants reported for ferrocene (Fc) oxidation in room temperature ionic liquids (ILs) at bare electrode surfaces scatter significantly2−6 (typically in the range of 1−2 orders of magnitude, depending on the technique applied). Slowing down the ET rate by means of increasing electrode−reactant separation can provide much higher accuracy. As the first attempts to model ET in ILs in the framework of traditional ET models developed for polar solvents have already been undertaken,7,8 precise kinetic data is absolutely essential for further progress in this area. Deep understading of key parameters for such modeling requires special computational efforts. In our recent study9 we compared ET in acetonitrile and [bmim][BF4] IL on the basis of molecular dynamics (MD) and quantum mechanical (QM) computational results and found that the factors determining the ET rate in these two solvents differ significantly. We showed that reorganization energy in ILs is lower than that in AN, and Marcus simple dielectric model fails to correctly predict the magnitude of the ET solvent activation energy in IL. © 2014 American Chemical Society

We also found that the solvent effective frequency in ILs is not proportional to the solvent first relaxation time, but is significantly higher (for the adiabatic case). In order to provide a test for our computational model, precise information on the rate constant value and its distance dependence is crucial. This paper presents the data for traditional experimental arrangement widely used for metal/solution interfaces,10 the artificial barrier layers between electrode and reactant, to ET in ILs. The properties of these layers are still poorly understood in IL media.11−15 Good electrochemical stability of octadecanethiol in 1-butyl3-methylimidazolium hexafluorophosphate IL was reported.13 Reductive desorption for a series of n-alkanethiols was investigated in four ionic liquids14 revealing the trend of cathodic shift of the desorption peak with the increase of the alkanethiol chain length. However, the lack of microscopic and electrochemical information on the capacitance and stability potential range prevents accurate characterization of barrier properties. The data on ET at alkanethiol barrier layers in ILs are also rather scarce. According to ref 16, suppression of Fc reaction in 1-butyl-3-methylimidazolium tetrafluoroborate [bmim][BF4] is less pronounced than the suppression of 4-OH-TEMPO oxidation (unfortunately, no quantitative information is Received: June 5, 2014 Revised: June 30, 2014 Published: June 30, 2014 15970

dx.doi.org/10.1021/jp5055843 | J. Phys. Chem. C 2014, 118, 15970−15977

The Journal of Physical Chemistry C

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electrode was used for high scan rate voltammetry of Fc at the bare Au surface. Impedance spectra were collected in 1 Hz−100 kHz frequency range with ac amplitude of 0.005 V and fitted to the Randles equivalent circuit (for C6SH and C8SH, points corresponding to 1−10 Hz were neglected in the fitting procedure). We note that rate constants estimates using Nicholson technique or impedance spectroscopy are not corrected for work terms, as no reliable models currently exist to estimate electrostatic double layer correction and specific adsorption energy corrections in ILs. Thus, the experimental rate constant values are regarded as apparent rate constants kapp. 2.4. STM. The STM measurements were carried out with a PicoSPM system (Molecular Imaging) in a sealed, argon-filled chamber. Au(111) disc electrode (1 cm diameter) was flame annealed, cooled in Ar, kept in 10 mM HCl for 15 min for lifting reconstruction and smoothing the surface,18 rinsed thoroughly with water, dried with argon, and placed into a container with assembly solution. The STM liquid cell was mounted on top of the electrode via an O-ring (Kalrez). The working area was 0.36 cm2. Pt wires served as counter and quasi-reference electrodes. The STM tips were electrochemically etched gold wires (0.25 mm diameter) coated with polyethylene.

provided). High-pressure electrochemical strategy was applied to study ferrocene oxidation reaction both on bare11 and alkanethiol-modified11,16 gold electrodes in 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([bmim][NTf2]) IL. Changeover of the diabatic regime to the adiabatic one at electrode−reactant separations shorter than 8 Å (alkanethiols with 2, 3, and 4 methylene units) was reported.16 However, barrier layer properties in the applied IL were not characterized, so the rate constant values can be overestimated because of possible reactant permeation into defective regions of the barrier layer. To our knowledge, no systematic data on long distance ET rates are currently available for redox reactions at barrier layers in ILs. Due to lack of distance and solvent dependences of the rate constants, the quantitative kinetic analysis of long-range ET is troublesome. Our current goal is to characterize the capacitive and ET barrier properties of alkanethiol adlayers on polycrystalline (polyAu) and Au(111) for a wide range of alkyl chain lengths C6−C18, in order to obtain quantitative information regarding the rate constant value for the oxidation of ferrocene as a model reactant.

2. EXPERIMENTAL METHODS 2.1. Materials. Ionic liquids 1-butyl-3-methylimidazolium tetrafluoroborate ([bmim][BF4], Aldrich, >97%) and 1-n-hexyl3-methylimidazolium tris(pentafluoroethyl)trifluorophosphate ([hmim][FEP], Merck, >99%) were dried with activated 3 Å molecular sieves at 70−80 °C for 48 h in vacuo (13 mbar) following the procedure proposed in ref 17. Figure S1 in the Supporting Information (SI) demonstrates the quality of IL purification. Potential window satisfies currently existing standard for basic electrochemistry in ILs (see SI for details). Ferrocene (Fc, Aldrich, >98%) was used as received. Alkanethiols (CnSH, n = 6, 8, 11, 16, 18) were obtained from Aldrich and used as received. Absolute ethanol (Merck, p.a.) was used for the preparation of alkanethiol solutions. 2.2. Solution Preparation and Electrode Modification. Ten millimolar solutions of Fc were prepared by dissolving the reactant directly in 1 mL of dried IL under argon atmosphere. Due to the high volatility of Fc, heating of the solution was not applied. Barrier layers of alkanethiols at Au electrodes were formed by immersing a freshly flame annealed and cooled with argon Au bead electrode in 1.0 mM ethanolic solution of the respective alkanethiol in a sealed container at 60 °C for 12−16 h. The samples were removed from the solution after incubation, rinsed with absolute ethanol, dried in a stream of argon, and subsequently transferred into the electrochemical or scanning tunneling microscopy (STM) cell. 2.3. Electrochemistry. Electrochemical measurements were carried out in a single compartment three-electrode cell with a working volume of 0.3 cm3, containing Pt wires as counter and quasi-reference electrodes (QRE). The stability of this quasi-reference and its potential in Fc/Fc+ reference scale are reported in the SI and illustrated by Figure S2. The drift of the QRE potential never exceeded 5 mV in the course of experiment. The working electrodes were polycrystalline Au beads, cut in half and polished to mirror finish to expose a diskshaped areas of 3.0−3.5 mm2. The measurements were performed in hanging meniscus configuration. The potentiostat was a software controlled Autolab PGSTAT30 system (Eco Chemie BV, Netherlands). A 12.5 μm diameter Au micro-

3. RESULTS AND DISCUSSION 3.1. Specific Features of Alkanethiol Barrier Layers in IL. The traditional analysis of ET kinetics across Au/alkanethiol interface rests on the assumption of completely insulating monolayer. This is true for the layers of low defectiveness, which exclude either solvent or reactant permeation into the monolayer. Permeation, if any, would result in a marked increase in both capacitance and rate constant values. To avoid misleading discrepancies between the experimental conditions and model interpretations, information on the potential dependent structure and stability of the monolayers is of crucial importance. The stability of barrier layers is illustrated by Figure S3. The reductive desorption potential shifts negatively with the increase of alkanethiol chain length, which is also typical for aqueous systems.19 All studied barrier layers are quite stable in the region of Fc formal potential (0.12 V vs Pt QRE), and cathodic desorption onset is rather negative for all the series. More crucial for further Fc oxidation study is surely the behavior of alkanethiol layers at potentials more positive than 0.2 V versus Pt. The features of C6SH and C8SH oxidation are observed already at 0.3 V in [bmim][BF4], when for C11SH, C16SH, and C18SH the onset of thiol oxidation is much more positive (up to 0.5−0.6 V vs Pt). Fortunately, this circumstance does not affect the quality of kinetic data, as, for short-chain thiols, polarization curves are only informative at low overvoltage. In [hmim][FEP] IL, even the shortest thiol monolayers are stable up to 0.5−0.6 V versus Pt. To study capacitance behavior of alkanethiol adlayers, we limited ourselves by potential ranges, in which all alkanethiols undergo neither cathodic nor anodic desorption. Cyclic voltammograms (CVs) of alkanethiol-modified Au in ILs (Figure 1) are reversible in these ranges. The shorter the alkyl chain, the more pronounced is the asymmetry of CVs. This is similar to behavior of alkanethiols in aqueous solutions of Cl−, ClO4−.20 For [bmim][BF4] (Figure 1a), the capacitance is higher at more positive potentials, when it is higher at more 15971



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under study, extracted from cyclic voltammetry, CCV, and from impedance spectra, Cimp, at the formal potential of Fc+/Fc couple (E f = 0.12 and 0.06 V in [bmim][BF4] and [hmim][FEP]) and minimal capacitances from cyclic voltammetry, CminCV (at −0.8 to −0.5 V in [bmim][BF4] and at 0−0.4 V in [hmim][FEP]). Formal potential of Fc+/Fc couple is assumed to correspond to the region of positive surface charges,9 and thus anion permeation into the monolayer is expected. For the shortest alkanethiols, C6SH, C8SH, and C11SH, the values of CCV at Ef exceed the values reported for aqueous solutions containing F− and Cl− anions.20 The capacitance value of Au/C6SH in [bmim][BF4] is 30 times higher than that in F− and 15 times higher than that in Cl− containing solutions. These deviations are less pronounced in [hmim][FEP]. Even the lowest capacitances, as extracted from CVs, exceed the values reported for aqueous systems. For C16SH and C18SH alkanethiols the capacitance values are in reasonable agreement with the values for aqueous systems. It is evident that medium effects on the capacitance can be ignored only for the longest alkanethiols, when for shorter chains the capacitance in ILs is dramatically higher than in aqueous solutions. Moreover, the lowest possible capacitances in ILs are still higher even than the highest values in water known for chloride solutions. Possible reason is larger size of ions in ILs, which is comparable with the size of short alkanethiols under study. In this case, one can assume that even slight penetration can affect the capacitance dramatically, but this does not mean immediate loss of barrier properties in relation to interfacial ET. Indirectly, these high capacitances put some thoughts about specific adsorption of IL ions on gold as a driving force for penetration of ions into compact layers. Alkanethiol layers were imaged in situ by STM at −0.3 V in [bmim][BF4] (i.e., in the region of increased capacitance) in order to reveal the effect of IL on the monolayer structure. Figure 2 shows the images of C8SH and C11SH adlayers assembled on Au(111) electrode. C11SH adlayer on Au(111) represents close packed structure (Figure 2b). Similar structure is found for C16SH (see Figure S4 in the SI). The vacancy

Figure 1. Cyclic voltammograms of CnSH-modified Au polycrystalline electrodes in [bmim][BF4] (a) and [hmim][FEP] (b). Scan rate is 10 mV·s−1.

negative potentials for [hmim][FEP] (Figure 1b). This difference can be understood if one assumes partial penetration of IL ions into the space between alkyl chains. If it is the case, the asymmetry is determined by the relative sizes of anion and cation in IL. According to presented CVs, penetration is more pronounced for shorter chains. For capacitances C of alkanethiol-modified electrodes in aqueous solutions, linear dependence of 1/C on the layer thickness r is widely accepted.10 The most pronounced deviations from linearity were reported for alkyl chains shorter than 10 units in chloride solutions (just for these systems, CVs asymmetry was reported earlier): capacitance values appeared to be anomalously high as compared to capacitances in fluoride solutions.20 The capacitance abnormality for short-chain alkanethiols is frequently attributed to the permeation of cations and anions of the supporting electrolyte into the monolayer structure.21−23 Such permeation is more likely to occur at the defective sites, but is not ruled out for a defect-free monolayer.24 Table 1 lists capacitance values for the two ILs Table 1. Capacitances of Alkanethiol Adlayers (in μF·cm−2) Extracted from Cyclic Voltammetry (CCV) and from Impedance Spectra (Cimp) at the Formal Potential of Fc+/Fc Couple (Ef) and Minimal Capacitances from Cyclic Voltammetry (CminCV) CnSH

RTIL

CCV at Ef

Cimp at Ef

CminCV

C6SH

[bmim][BF4] [hmim][FEP] [bmim][BF4] [hmim][FEP] [bmim][BF4] [hmim][FEP] [bmim][BF4] [hmim][FEP] [bmim][BF4] [hmim][FEP]

60 8.0 40 3.2 2.3 2.5 0.9 1.0 0.8 0.9

4.9 3.1 2.8 2.3 1.5 1.2 1.0 1.0 0.86 -

9.5 8.0 5.0 3.2 1.8 2.5 0.9 1.0 0.8 0.9

C8SH C11SH C16SH C18SH

Figure 2. In situ STM images of C8SH (a,c), C11SH (b,d) adlayers on Au(111) obtained at −0.3 V in [bmim][BF4]. Frame sizes are (a,b) 300 × 300 nm2 and (c,d) 15 × 15 nm2. 15972



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islands are easily recognized, which is typical for thiol adlayers on gold substrates.25 The depth of vacancies amounts to 0.25− 0.5 nm. A closer examination reveals ordered domains of C11SH. Note that ordered lattice is also visible within vacancy islands (Figure 2d). The assembly of C6SH and C8SH (according to the protocol described above) leads to formation of close-packed adlayers, where ordered domains coexist with disordered areas (Figures 2a and S4 in the SI). Figure 2c shows the boundary between ordered and disordered domains in the C8SH adlayer. The less ordered structure of C6SH and C8SH adlayers seems to result from penetration of IL anions and cations into the monolayer, which is reflected in higher capacitance values. We should note that the defectiveness of monolayers is likely to be even higher at more positive potential of ferrocene oxidation, as is indicated by the rise in capacitance. The influence of the monolayers defectiveness on the barrier properties was further tested in kinetic experiments, highly sensitive to adlayer defects. 3.2. The Effect of Barrier Layers on Fc Oxidation. 3.2.1. Ferrocene at bare Au surface. Voltammetry of 10 mM solutions of Fc in [bmim][BF4] and [hmim][FEP] reveals reversible behavior in both ILs with peak-to-peak separations close to 60 mV at 10 mV/s scan rate (Figure S2 in the SI). High scan rates could not be applied in experiments with macroelectrodes due to high values of uncompensated ohmic resistance, so gold microelectrode (12.5 μm diameter) was used for high speed voltammetry of Fc in ILs. At 120 V·s−1, the peak current did not exceed 5 nA, and no correction for uncompensated resistance was required. Diffusion coefficients (voltammetry at low scan rates, 5−300 mV·s−1) were calculated to be (5.4 ± 0.6) × 10−8 and (5.8 ± 0.8) × 10−8 cm2·s−1 in [bmim][BF4] and [hmim][FEP] ILs, respectively. This correlates with the similarity in the viscosity values of the ILs.26 Nicholson technique27 was applied to calculate the apparent rate constants kapp from voltammetric data collected in the range of scan rates v 10−100 V·s−1 (Section S2, Figure S5 in the SI). In [bmim][BF4] medium, the apparent rate constant for the Fc oxidation at Au microelectrode was calculated to be 0.009 ± 0.001 cm·s−1 under assumption that transfer coefficient equals 0.5 and diffusion coefficients of the oxidized and reduced species are equal. Although it is impossible to estimate both rate constant and transfer coefficient α using Nicholson analysis, our assumptions do not introduce significant uncertainties into the calculated values. The rate constants determined under assumption of α equal to 0.5 are related to zero overvoltage, so the decrease of α in accordance with nonlinear activation energy dependence on the overvoltage is excluded. At zero overvoltage, α deviates from 0.5 only when the reaction terms demonstrate the pronounced deviation from symmetric parabolas, which is the case for reactions with high and asymmetric innersphere reorganization energy or with bond cleavage.28 For Fc reactions, these reasons can be solidly excluded. Figure S5 in the SI shows high scan rate voltammetry of Fc at Au microelectrode in [bmim][BF4] IL. 3.2.2. Kinetics of ET at Blocked Au Surfaces. Figure 3 shows cyclic voltammograms of modified Au polycrystalline electrode in Fc-containing [bmim][BF 4 ], and Figure 4 collects representative impedance Nyquist plots for comparison (ohmic resistance is subtracted to simplify the comparison) and the equivalent circuit used to fit the spectra. Plots in Figure 6a−e show sharp increase of the charge-transfer resistance Rct upon increasing the alkanethiol chain length and the

Figure 3. Cyclic voltammograms of alkanethiol-modified Au polycrystalline electrodes in 10 mM Fc solution in [bmim][BF4]. Scan rate 10 mV·s−1.

simultaneous decrease of the monolayer capacitance C. CVs and impedance spectra for Fc reaction in [hmim][FEP] can be found in the SI (Figures S7, S9). Fitting parameters of the spectra are also placed in the SI (Table S2). Impedance spectra were registered in the vicinity of formal potential, assumed to be the same for bare and modified gold. This assumption could only be verified for quasi-reversible CVs obtained for shorter alkanethiols. For C6SH- and C8SH-modified electrodes, the difference of Fc+/Fc formal potential from that for bare Au surface did not exceed 10 mV, which only slightly exceeds the experimental accuracy of potential measurement. A considerable potential shift is unlikely for longer alkanethiols. The reason for this shift can only be the change of solvation shell when the reactant and/or product find themselves partially in the external region of the layer. In view of capacitance data presented above, this is more probable for shorter alkanethiols. Thus, practically constant formal potential for these thin barrier layers demonstrates that the change of solvation shell is negligible. CVs of C6SH- and C8SH-modified electrodes are characteristic for diffusion-controlled processes (peak-to-peak separation does not exceed 65 mV at 10−100 mV·s−1 scan rates). Cyclic voltammograms of C11SH-modified Au electrodes in [bmim][BF4] show no scan rate dependence up to 0.2 V, which is indicative of kinetically limited process. Still, the extremely narrow overvoltage range does not allow construction of informative Tafel plots for ET at the C11SH-coated electrode. Fast scan rate voltammetry was not applied to measure rate constants for Fc oxidation at C6SH-, C8SH-, and C11SHmodified electrodes due to errors associated with Ohmic resistance compensation at high scan rates. Kinetic information for these systems was obtained exclusively from impedance measurements at zero overvoltage. For C16SH- and C18SH-coated electrodes, the kinetic limitations last up to 0.4 and up to 0.5 V versus Pt, respectively. Convolution of linear sweep voltammograms for longer alkanethiols was performed in order to correct measured current for mass transfer contribution29 (see Figure S8 for the results of the convolution procedure). From the linear fragments of Tafel plots of corrected current (Figure S10), the values of the logarithm of the exchange current densities i0 were extracted and recalculated into kapp. Transfer coefficients for available overpotential intervals were close to 0.5. The accuracy was much higher for longer alkanethiols. The same trends are observed for the voltammetry in [hmim][FEP]. 15973



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Figure 4. Nyquist plots for CnSH modified Au electrodes in [bmim][BF4] ionic liquid (a−e) and (f) the equivalent circuit used for the fitting procedure (Rsol and Rct are solution and charge transfer resistances respectively, Cdl is the double layer capacity mentioned above as Cimp, and W and is Warburg impedance).

Table 2 lists all kapp values obtained by means of two techniques, and Figure 5 demonstrates distance dependence of kapp. Table 2. Rate Constants (kapp, cm·s−1) for the Oxidation of Fc at Alkanethiol-Modified Au Electrodes in the Two ILs under Studya [bmim][BF4] alkanethiol C6SH C8SH C11SH C16SH C18SH a

[hmim][FEP]

kapp (impedance)

kapp (CV) [slope, mV]

kapp (impedance)

kapp (CV) (slope, mV)

(5.0 ± 1.2) × 10−3 (8.9 ± 1.0) × 10−4 (1.0 ± 0.2) × 10−5 (2.6 ± 0.8) × 10−8 (3.0 ± 1.5) × 10−9

-

(2.7 ± 1.4) × 10−3 (2.7 ± 0.8) × 10−4 (6.2 ± 1.5) × 10−6 (7.2 ± 2.0) × 10−8 -

-

(3.3 ± 0.8) × 10−8 [0.158] (3.0 ± 0.6) × 10−9 [0.170]

-

Figure 5. Plots of the ln kapp for the Fc+/Fc system against the thickness of the SAM (r). Thin dash lines constrain the interval of possible slopes β known for ET through bond and through space.30 The available series for Fe(CN)6,3−31 and Ru(NH3)63+32 in aqueous solutions together with data for Fc in [bmim][NTf2]16 are also plotted for comparison. Dash-dotted line represents an extrapolation of the kapp value measured at bare Au surface to the distances, where reaction proceeds in adiabatic regime. The symbol sizes exceed the size of the error bars.

(9.0 ± 1.8) × 10−8 [0.172] -

Tafel plot slopes are given in parentheses for C16SH and C18SH.

The plot in Figure 5 confirms that in spite of relatively high capacitance and partially defective structure observed in electrochemical and STM experiments, alkanethiol layers in ILs keep barrier properties in relation to Fc reaction. Sharp dependence of reaction rate on the electrode-reactant distance is observed for r > 8 Å. This type of distance dependence is characteristic for diabatic ET regime, which assumes weak electrode-reactant electronic coupling. For this diabatic limit, the standard ET rate constant is proportional to electronic transmission coefficient, which is far below unity. In our recent computational study,9 we found that transmission coefficient for Fc in [bmim][BF4] at a model Au cluster is only weakly dependent on the electrode−reactant separation up to the distances of 6.0−6.5 Å and demonstrates a sharp exponential decay at larger distances. As we did not take into account tunneling through alkanethiol chains, our computational results

can be only regarded as semiquantitative. In spite of this, the agreement in the predicted and experimental rate constant distance dependence is good. As well as transmission coefficient, rate constant varies exponentially with the electrode-reactant separation r, which is controlled in our study by the monolayer thickness: k0 = kr = r0e−βr

(1)

where kr=r0 is the extrapolated value of the rate constant to the distance corresponding to n = 0 and β is the exponential decay coefficient. For other model parameters (e.g., reorganization energy and works of reactant approach, now included in kr=r0), 15974



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the distance dependence is less pronounced. To a first approximation these parameters can be treated as distanceindependent. The slopes of ln kapp versus r dependences are 1.24 and 1.04 Å−1 for [bmim][BF4] and [hmim][FEP], respectively. Similar slopes were obtained for model reactants [Ru(NH3)6]3+ and [Fe(CN)6]3− in aqueous solutions29,32 and for Fc oxidation in [bmim][NTf2]16 (also shown in Figure 5 for comparison). Thus, distance dependence is mostly affected by tunneling through spacer molecules, with only minor effect of acceptor nature and medium. This is not surprising because very close values of the slopes are known for distance dependences of alkanethiols molecular conductivity.33 In spite of higher stability and lower capacitance values of short chain alkanethiol adlayers in [hmim][FEP], the slope of ln kapp versus r dependencies is slightly lower than that for [bmim][BF4]. In addition, the rate constant value in [hmim][FEP] at C16SH-modified electrode is higher than that in [bmim][BF 4]. Unfortunately, we failed to obtain well reproducible data for Au/C18SH in [hmim][FEP] (the rate constants had the same order as for Au/C16SH, or were even higher). However, the above-mentioned facts already evidence that the structure of long chain alkanethiol adlayers in [hmim][FEP] is more defective compared to that in [bmim][BF 4] and [bmim][NTf2] ILs, probably due to more pronounced ion permeation. As the sizes of [hmim]+ and [FEP]− ions are larger than the sizes of [bmim]+, [NTf2]− and [BF4]− ions, we suggest that the enhanced penetration of ions into the monolayers and the corresponding increase in defectiveness is determined by the dispersion interactions between alkyl chains of longer alkanethiols and long alkyl chain of [hmim]+ and/or ethyl tails of [FEP]−. For the shortest alkanethiols (C6SH and C8SH) kapp values are rather close to the value at bare gold. Corresponding region of r < 8 Å (distance-independent rate constant) can be conventionally assigned to adiabatic ET (strong electronic coupling, transmission coefficient close to unity). The similarity in the rate constant values in the two ILs at longer distances evidence the lack of ET rate solvent dependence, which is a feature of diabatic ET character. In contrast, in the limit of strong electrode-reactant coupling, the ET rate is known to be controlled by the dynamic properties of the solvent, the feature typical for adiabatic ET. In particular, in frames of the Debye model of dipole liquid, the relation between the ET rate constant k0 and the solvent longitudinal relaxation time τL is assumed to hold:1 k0 ∼ τL−1

reorganization energy values and work terms for the solvents do not differ significantly. Rate constant values reported for the oxidation of Fc in different ILs are summarized in Table 2 and also plotted in Figure 6. It is not easy to estimate the accuracy of the rate

Figure 6. Rate constants for Fc oxidation at bare Au. Data for organic solvents is taken from refs 34−36. For ILs open and filled symbols refer to longitudinal relaxation times τL calculated from the first (the longest) and second relaxation times, respectively (τ1, τ2 in Table 3). The symbol sizes exceed the size of the error bars.

constant values obtained for various ILs by means of different techniques, and the whole set of scattering data points has to be analyzed in order to get insight into the nature of slow relaxation controlling ET event. ILs are solvents with very complex dielectric behavior and 3−4 relaxation times are commonly observed in the dielectric spectra.37 In our previous study, we concluded that first and second relaxation times of [bmim][BF4] mainly contribute to the effective solvent frequency value. These two relaxation times (Table 3) were Table 3. Rate Constant Values (kapp) for Fc Oxidation at Bare Electrodes and First and Second Relaxation Times of ILs (τ1, τ2) IL

kapp, cm s−1

technique

0.078 ± 0.014 7.5 × 10−3

SECMa CV

electrode

[emim] [NTf2]

(2)

The lack of the rate constant distance dependence is traditionally interpreted as the manifestation of the adiabatic ET mechanism. However, as we found earlier in our computational study9 the calculation of the solvent effective relaxation time for ILs is not straightforward. We showed that for [bmim][BF4] the effective solvent relaxation time τL is much higher than the corresponding parameter, calculated from the slowest relaxation time. Moreover, as the effective frequencies in the adiabatic and diabatic limits differ by more than 2 orders of magnitude, it becomes impossible to unambiguously distinguish between these two ET regimes.9 However, traditional treatment of the data for rate constants in molecular solvents implies a construction of kapp versus τL dependence.34−36 The linearity of this plot is considered to be a diagnostic feature of the adiabatic ET mechanism, if the

[bmim] [NTf2] [hmim] [NTf2] [omim] [NTf2] [bmim] [BF4] [bmim] [PF6]

τ1, τ2 ps

reference

261

2 3

Pt 0.21 0.01−0.02 0.039 ± 0.005

HSCEb CV, imp. SECM

Pt

482

4 5 2

6.0 × 10−3 0.018 ± 0.002

CV SECM

Au Pt

22.638 -

11 2

0.011 ± 0.001

SECM

Pt

-

2

0.01 ± 0.005

CV

Au

1140

2.5 × 10−3

CV

Pt

73.137 1406

24.238

this work 6

38.837 a

Scanning electrochemical microscopy. electrode. 15975

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theory formalism for the description of ET kinetics in polar solvents is applicable to ILs in general, the calculation of key model parameters requires special modeling efforts. Solvent reorganization energy and solvent effective frequency values in ILs cannot be estimated in the framework of models, worked out for the simplest Debye solvents and habitually used for molecular solvents, in spite of their complexity. Due to a pronounced difference of solvent model parameters in ILs and molecular solvents, the linearity of the k app versus τ L dependence cannot be regarded as a manifestation of adiabatic ET regime. To have better understanding of the influence of the ionic permeability on the ET rate in ILs, a “microscopic” level investigation is required. In the next communication, we shall report the results of molecular dynamics studies of the ionic permeation of IL ions into alkanethiol monolayers on gold.

considered for each IL when calculating the longitudinal relaxation time values τL. [Longitudinal relaxation time τL was calculated as (εopt/εs) τn, where εopt and εst represent optical and static dielectric constants of the IL, and τn is the time, corresponding to the nth relaxation of the medium.]38,37 Correspondingly, two points (solid and open) can be found in Figure 6 for each IL, when only one point (solid black squares) is given for each molecular solvent. The values of kapp in ILs are significantly lower than those in molecular solvents, which is not surprising because of much slower relaxations of ILs (all points for ILs are located at the right side of the plot, i.e., at longer relaxation times). Note that in less complex molecular solvents, τeff has a simple meaning of the reciprocal frequency of Debye type solvent dipole reorientations. The nature of the ET-controlling microscopic events is different in ILs due to general complexity of their dielectric behavior. Figure 6 shows extrapolation of kapp vs τL trend for molecular solvents toward characteristic relaxation times of ILs. If we formally use the first (the slowest) relaxation time of [bmim][BF4],37 the rate constant (green open symbol) is only slightly higher than that expected from extrapolation of the straight line (Figure 6). Thus, the extrapolation of kapp to slower relaxation, not to faster, gives the same general trend for molecular and ionic solvents (both for [bmim][BF4] and [bmim][NTf]2). The analysis of kinetic data reported for other ILs is less transparent due to lower accuracy of rate constant values. However, the observed tendency has to be considered carefully before stating the adiabatic ET character. Straightforward estimates suggest that ET in ILs is controlled by the slowest relaxation processes (often attributed to the reorientation of the dipolar cations39). Our model calculations9 suggest that for the adiabatic case the reorganization energy in [bmim][BF4] is significantly lower than that in molecular solvent acetonitrile (0.45 and 0.69 eV in IL and acetonitrile, respectively). This difference is sufficient to provide a deviation in the linear trend in Figure 6. The difference in the work terms in IL was also found to be very pronounced. Thus, the observed linearity of the plot in Figure 6 might originate from the compensation of different factors, affecting the rate constant value. Same consideration is valid for molecular solvents. Still, a detailed study of ET activation parameters for longdistance ET is required to build a self-consistent description of electrochemical ET in ILs in the framework of theoretical models.

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ASSOCIATED CONTENT

S Supporting Information *

Additional CV, STM, and EIS data. 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]; Tel +7(495)9391321. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Swiss National Science Foundation and partly by RFBR 12-03-31748. A.R. also acknowledges the financial support by FP7 project ACMOL (618082).

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REFERENCES

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4. CONCLUSIONS Basically, the permeability of alkanethiol layers on gold for the components of ILs is found to be more pronounced than that for the ions in electrolyte solutions in molecular liquids. In spite of this trouble (manifesting itself in higher capacitance values and in its potential dependence), the layers can solidly play the role of barriers for ET. For not too short alkyl chains the distance dependence of the Fc rate constant demonstrates the same slope as the majority of reactions of coordination compounds in aqueous media. Despite the linear trend, observed in k app versus τ L dependence for the Fc reaction at nonmodified Au, the ET regime in ILs cannot be unambiguously determined. We suggest that simple models should be carefully tested before application to ET in IL due to very complex dielectric behavior and specific reaction layer structure in this type of solvents. In our previous study,9 we demonstrated that although Marcus 15976



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