Comparative Electrochemical Investigations in Ionic Liquids and

Article pubs.acs.org/JPCC

Comparative Electrochemical Investigations in Ionic Liquids and Molecular Solvents of a Carbon Surface Modified by a Redox Monolayer Joanna Jalkh, Yann R. Leroux, Corinne Lagrost, and Philippe Hapiot* Institut des Sciences Chimiques de Rennes (Equipe MaCSE), CNRS, Université de Rennes 1, UMR 6226, Campus de Beaulieu, 35042 Rennes Cedex, France ABSTRACT: Electrochemical properties of carbon surfaces modified by a covalently attached redox monolayer (alkyl-ferrocenyl) have been investigated in three different classical room temperature ionic liquids (RTILs): BMIPF6, BMINTf2, Me3BuNNTf2, and in two common molecular solvents (ethanol and CH2Cl2) for evidencing special aspects of the electronic transfer in RTILs. If the redox activity and the stability of the layer are globally preserved in the RTILs, a considerable decrease of the amount of active redox groups is observed when the layer is examined in the RTILs. The oxidation potentials are only slightly higher in the RTILs contrarily to the charge transfer kinetics that decrease by a factor of 10−20 as measured for the rates between the attached redox moieties and the substrate. These behaviors that appear as possible limitations for practical applications of such systems in RTILs could be ascribed to specific ionic associations and a large reorganization of the layer during its charging process.

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INTRODUCTION The preparation of robust modified surfaces with a molecular organization presents a growing interest for many applications ranging from analytical chemistry, molecular electronics, or more recently in charge storage devices as supercapacitance.1−3 In relation with these applications, use of room temperature ionic liquids (RTILs)4,5 has been largely considered for both homogeneous and interfacial electrochemical processes as these media are attractive for electrochemical applications owing to their unique physicochemical properties.6−8 They possess a negligible vapor pressure, high chemical and thermal stability, and, more specifically for applications related to electrochemical devices, they play the role of solvent and of the supporting electrolyte allowing applications in electrochemical detection, sensors ,or actuators as done in organic solvents but with a higher stability of the device. These possibilities have encouraged numerous investigations combining modified surfaces and RTILs to characterize the interfaces at the modified surfaces/RTILs. In this field, most fundamental studies have concerned the behavior of self-assembled thioalkyl layer (SAMs) on metal for improving their preparations9 and stability in RTILs.10,11 The charge transfer processes at gold/ alkyl-ferrocene SAMs have been examined in classical RTILs as a function of the alkyl chain length, and different relaxation processes were evidenced for the kinetic control of the electronic transfer.12 In more recent investigations, the electrochemical responses of the alkyl-ferrocene SAMs have been compared in the RTILs and in aqueous media, revealing some puzzling observations.13,14 It was notably observed that the electrochemical response is considerably affected by the size of the anion of the RTILs and by the density of redox groups attached on the surface and that only a fraction of the © 2014 American Chemical Society

ferrocenyl groups remains active in the RTILs. For practical applications, carbon substrates are receiving more and more attention for designing functional interfaces,1−3,15 and there is a growing literature about the use of carbon material and ionic liquids particularly for improving energy storage devices16 or sensors.17 It is known that ionic liquids affect the electron transfer characteristics on modified surfaces, RTILs were used for improving the interfacial charge transport with large redox biomolecules (see for example ref 18 and quoted references) or with immobilized species for the example of the behavior of adsorbed anthraquinoyl groups.19 Despite this large literature, little is known about the basic properties of covalently bound electroactive monolayer in RTILs, especially how basic properties like stability or electrochemical behavior evolve when passing from a classical organic solvent to a RTIL as it was done for SAMs adsorbed on metal. The functionalization of carbon interface with an organic layer requires special strategies.20 Among them, a popular approach for preparing robust modified carbon surface is the electrografting of aryldiazonium salts that relies on the electrochemical generation of reactive phenyl radicals.1−3 This procedure has been used for immobilizing a wide range of functional groups on carbon substrates and was recently extended to the preparation of monolayer using special aryldiazonium precursors.21 A cleavable bulky tri(isopropyl)silyl group is introduced on an ethynyl-aryldiazonium salt to limit the extension of the layer during the grafting. After deprotection of the grafted deposit, an Received: September 14, 2014 Revised: November 3, 2014 Published: November 6, 2014 28640

dx.doi.org/10.1021/jp5092976 | J. Phys. Chem. C 2014, 118, 28640−28646

The Journal of Physical Chemistry C

Article

Scheme 1. Modification Procedure of the Carbon Surface

RTILs were synthesized and purified according to a previously described procedure.26 For all syntheses, the RTILs were purified by repeated washing with H2O and filtered over neutral alumina and silica. Prior to each experiment, vacuum pumping carefully dried RTIL overnight and then dried over molecular sieves at least for 24 h. Experimental Setup and Electrochemical Procedure. Modification steps were performed with Autolab PGSTAT 12 (Metrohm) and a conventional three-electrode system, comprising a 3 mm diameter glassy carbon electrode as a working electrode, a platinum wire as the auxiliary electrode, and a SCE electrode as reference. Cyclic voltammetry of the modified surfaces were performed at 25 °C using a homemade potentiostat equipped with a positive feedback compensation device27 and a three-electrode setup comprising the modified glassy carbon electrode as a working electrode, a platinum wire as an auxiliary electrode, and an Ag/AgCl wire as a pseudoreference electrode. All potentials were standardized versus the decamethylferrocene/decamethylferrocenium couple (Me10Fc/Me10Fc+) that was directly added in the solution at the end of the experiments and for all the different media. The total surface coverage of active ferrocene centers were derived from faradaic charge according to the following equation: Γ = Q/nFA where n is the number of exchanged electrons, F is the Faraday constant, A is the geometric surface of the glassy carbon electrode, and Q is the faradaic charge calculated by integration of the area under the voltammetric peaks at low scan. Errors on Γ values were estimated to be lower than 15% (3 measurements on the same sample). Estimations of the electronic rate constants for the immobilized ferrocene, kel, and for the Me10Fc/Me10Fc+ couple, ks, were achieved by considering a simple Butler−Volmer Law for the electron transfer and transfer coefficient α equal to 0.5 applied to an immobilized species. Each cyclic voltammogram was measured at least three times with the same sample. Numerical simulations were performed using the KISSA-1D software package (KISSA Software for simulation of electrochemical reaction mechanisms of any complexity) and using the default parameters for adsorbed species.28,29 Errors on ks were estimated about 20−30%.

active monolayer of ethynylaryl moieties remains available for a postfunctionalization, using the selective “click” coupling (Cu(I)-catalyzed Huisgen 1,3-dipolar cycloaddition). 22,23 Using an azido-alkylferrocene (Fc-C11-N3) for the purpose of the present study, it becomes possible to prepare a dense electroactive monolayer of alkyl-ferrocene attached on the carbon surface similar to a functional thioalkyl SAM deposited on metal but with the difference that the redox moieties are covalently linked to the substrate (see Scheme 1). Once the molecules are covalently attached onto the carbon substrate, they could not diffuse and stay in a sort of frozen arrangement.24 In the present article, we have examined the redox behavior of the immobilized redox monolayer in three different RTILs (see Scheme 2) as well as in ethanol and dichloromethane to Scheme 2. Different RTILs Used in the Study

highlight the consequences of using a RTIL and a redox active layer. In this purpose, the same sample of a modified carbon surface is successively examined in the different media, which is a prerequisite for such comparative study.

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EXPERIMENTAL SECTION Chemicals and Reagents. Commercially available reagents were used as received without further purification. 4((Triisopropylsilyl)ethynyl) benzenediazonium tetrafluoroborate (TIPS-Eth-ArN2+) was prepared according to a previously described procedure.21 11-Azidoundecanoylferrocene (Fc-C11N3) was synthesized according to a published procedure.25 Tetrabutylammonium fluoride (TBAF) was purchased from Fluorochem. Lithium perchlorate (LiClO4) and tetrabutylammonium hexafluorophosphate (TBAPF6) were purchased from Aldrich. Ethylenediaminetetraacetic acid (EDTA), ascorbic acid, and copper sulfate were purchased from Alfa Aesar. 28641



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Electrode Modification. Scheme 1 presents the general principle of the modification process in which the anchoring monolayer was prepared by electroreduction of the protected aryldiazonium salt using cyclic voltammetry. The first modification step is the preparation of the anchoring monolayer by electroreduction of TIPS-Eth-ArN2+ groups on a carbon electrode from a solution of 10−2 mol L−1 TIPS-Eth-ArN2+ and 0.1 mol L−1 TBAPF6 in acetonitrile using five cycles between 0.6 and −0.75 V vs SCE at a scan rate of 50 mV/s. This protected surface was then rinsed with acetone and stirred in a THF solution for 20 min. Deprotection of the silyl group was performed by immersing the electrodes in a stirred solution of 0.1 M TBAF in tetrahydrofuran (THF) for 20 min. Hence, a carbon surface modified by a covalently bound ethynylbenzene monolayer (H-Eth-GC) is obtained. The second step is the coupling of the azido C11-alkylferrocenyl (Fc-C11-N3) molecules to H-Eth layers by click chemistry: the modified electrodes were immersed in a 5 mL solution of Fc-C11-N3 (1 mg) in THF and 2.5 mL of an aqueous solution of copper sulfate (0.01 mol L−1). The solution was degassed with argon for 15 min, then 2.5 mL of an aqueous solution of L(+)-ascorbic acid (0.01 mol L−1) containing 80 mg of sodium bicarbonate (NaHCO3) was added dropwise. The reaction mixture was left stirring under argon for 1 h. The resulting electrode was rinsed thoroughly with acetone and stirred in a saturated EDTA solution for 10 min to remove residual copper.

Figure 1. Cyclic voltammetry of carbon substrates modified with the ferrocene-C11 layer recorded in RTILs (BMINTf2 (red), Me3BuNNTf2 (purple), and BMIPF6 (green)) and in organic media (ethanol (+0.1 mol L−1 LiClO4) (blue) and CH2Cl2 (+0.2 mol L−1 TBAPF6) (magenta). scan rate = 0.2 V s−1.

modified surface from one solvent to another one and back. This highlights the good stability of these long chain ferrocene surfaces in the molecular solvents and the RTILs. As expected for a surface-immobilized redox couple, we found that the peak currents vary linearly with the scan rates in the range of 0.02−1 V s−1 (see examples in Figure 2). For the experiments performed in the ionic liquids (Figure 2c,d), deviations due to the residual ohmic drop only appear for the curves recorded at higher scan rates, indicating that curves recorded at the lowest scan rates are not considerably affected by the residual ohmic drop and could be considered for detailed analysis.27 Concerning the thermodynamics of the electron transfer, standard redox potentials, E°, were estimated by the half-sum, E1/2, between the oxidation and reduction peaks and standardized versus the Me10Fc/Me10Fc+ couple. The question of an absolute reference electrode in ionic liquids is still an open question, but protected ferrocenes, as the Me10Fc/Me10Fc+ couple used in this study, are generally considered to be unaffected when changing the media and served as internal reference.6,7 In this framework, our data shows a negligible variation between E° measured in the molecular solvents and those recorded in BMIPF6 and Me3BuNNTf2, E° in BMINTf2 being only slightly higher (see Table 1). E° values depend on both the cation and anion of the RTIL as previously observed in studies of redox species in solution.6,7 Observations are totally different when examining the currents; one could notice that the intensities of the faradaic currents are much lower in the RTILs than in the two molecular solvents. This decrease of the redox activity of the layer in RTILs is not due to a chemical degradation of the layer as the same electrochemical response is reobtained when the surface is re-examined in the original molecular solvent and after the experiments were performed in the RTILs (variation less than 10%). We could also observe that the peak currents are much broader in the RTILs than in the molecular solvents, which is indicative of large interactions between the immobilized redox groups.30 It is thus more interesting to compare the transferred charge Q measured by integration of the faradaic current than the peak current itself. We derived the surface coverage of grafted electroactive ferrocene moieties, Γ = Q/FA (Γ represents the amount of active immobilized redox groups, A is the geometric surface of the electrode, and F the Faraday number). In ethanol and in CH2Cl2, Γ values are similar and close to 9 × 10−10 mol cm−2. This value indicates a full coverage of the electrode considering the size of the

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RESULTS AND DISCUSSION As explained in the introduction, we chose to attach a ferrocene molecule linked with a long hydrocarbon chain (C11) to the anchoring monolayer (Scheme 1). This permits to mimic the long alkyl chains used for SAMs on metallic interfaces but with a frozen position of the attachment of the layer on the substrate.2,21 Ferrocene couple is a classical electrochemical probe that has been considered for many basic investigations in solution of RTILs.6,7 The electroactivity of the carbon surface modified with the alkyl-ferrocene was investigated in three different RTILs and two classical molecular solvents (ethanol and dichloromethane) chosen as references of classical solvents. In these experiments, the surface serves as an electrode and the electron transfer at the carbon/ferrocene monolayer/RTIL junctions is examined by cyclic voltammetry at different scan rates. In all experiments, we used a homemade potentiostat equipped with a positive-feedback compensation device to limit the possible artifacts arising from the high resistivity of the RTILs and resulting ohmic drop.27 Bis(trifluoromethanesulfonyl)-imide (NTf2−) and PF6− anions were considered for the same butyl-methylimidazolium cation (BMI+) or trimethylbutylammonium cation (Me3BuN+) to examine the effect of the anion of the RTILs on the electrochemical response. The weakly basic NTf2− exhibits an extensive charge delocalization within the S−N−S backbone, which decreases interactions in these salts, accounting for their low viscosity and good conductivity. On the contrary, RTILs with fluorinated anions like PF6− are more viscous and less conductive media.4,6,7 Figure 1 displays voltammograms of one modified surface (the same sample is used for one series of experiments) recorded at 0.2 V s−1 in blank solutions. In all media, the curves exhibit well-defined current peaks corresponding to the reversible oxidation of the immobilized ferrocene groups. We observed a full reproducibility of the voltammograms upon cycling in the different media and after transferring the 28642



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Figure 2. Variation with scan rates between 0.1 and 1 V s−1 of the cyclic voltammograms of the grafted monolayer: in molecular solvents (a) CH2Cl2 (+0.2 mol L−1 Bu4NPF6) and (b) ethanol (+0.1 mol L−1 LiClO4); and in RTILs (c) BMINTf2 and (d) BMIPF6.

Table 1. Estimated Half-Sum Potential (E1/2) and Surface Coverage (Γ) in Different Media; Comparisons with the Viscosity η and the Apparent Static Dielectric Constants ε of the RTILs

a

solvent

CH2Cl2 (+0.2 mol L−1 Bu4NPF6)

ethanol (+0.1 mol L−1 LiClO4)

BMINTf2

BMIPF6

Me3BuNNTf2

E1/2 (V) Γ (10−10 mol cm−2) kel (s−1)a ks (Me10Fc) (cm s−1)a,b η (cp)c εc

0.46 8.5 ± 0.3 30 fast

0.33 9.4 ± 0.5 20 0.1

0.52 5.9 ± 0.4 3 0.0025 52 11.5

0.46 5.4 ± 0.5 0.4 0.0012 308 11.4

0.47 6.1 ± 0.4 0.6 0.0023 98 10

Uncertainties on k are around ±30%. bks for the Me10Fc/Me10Fc+ couple on the blanked carbon substrate. cFrom ref 6 and 31.

ferrocene and the roughness factor of the carbon materials.24 Notice that these Γ values are estimated using the geometric area that is much smaller than the effective surface area. For the same modified electrode, Γ values around (5−6) × 10−10 mol cm−2, which is about half the values measured in the molecular solvents, are then derived in the three considered RTILs. Such values remain sufficient for easy electrochemical detection, but they indicate that only a fraction of the attached ferrocene groups participates in the redox reaction. As indicated in the introduction, comparable lowering of Γ was observed for alkyl-ferrocene SAMs on gold surface when passing from an aqueous media to similar RTILs.13,14 For these systems, authors reported that Γ are lower in NTf2-based ionic liquids than in BF4-based ionic liquids and that the decrease depends on the density of attached ferrocene groups.14 They proposed that the steric hindrance of the anion of the RTIL impedes the compensation of the positive charges on the ferrocenium formed upon oxidation of the layer, thus limiting the amount of efficient ferrocenyl group that could be oxidized as it was reported in aqueous media when changing the size of the anion of the electrolyte.32,33 With our layer and comparing data in PF6− and NTf2− based ionic liquids, no clear experimental tendency was observed between RTILs for the evolution of Γ values when changing the anion of the RTIL.

Kinetic Studies. As exemplified on Figures 1 and 2, cyclic voltammograms recorded in all media show the classical symmetrical peaks of an immobilized redox system with a peakto-peak potential, ΔEp, that increases with the scan rate.25 ΔEp are much larger even for the lowest scan rates in the RTILs (Figure 2c,d) than in the molecular solvents (Figure 2a,b), indicating much slower electron transfer kinetics when the layer is examined in the RTILs. For example at low scan rate (0.2 V s−1), ΔEp are found to be in the order of 10 mV for the voltammograms recorded in CH2Cl2. This value increases to 80 mV in BMINTf2, 235 mV in BMIPF6, and 195 mV in Me3BuNNTf2. Considering a simple Buttler−Volmer law for the electron transfer kinetics,31 a transfer coefficient α equal to 0.5, and that the influence of the residual uncompensated ohmic drop is negligible at low scan rate in our experimental conditions, ΔEp values correspond to apparent electronic exchange standard rate constants, kel, equal to 30, 3, 0.4, and 0.6 s−1, respectively, in CH2Cl2, BMINTf2, BMIPF6, and Me3BuNNTf2, highlighting the slowness of the electron exchange in the RTILs. Decrease of kel when passing from an organic solvent to a RTIL is a common behavior in solution that has been reported for many redox couples.6,7,34 The amplitude of the decrease was related to the charge delocalization inside the solute but also depends on the cation and anion of the RTIL and more generally on the associations 28643



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Scheme 3. Possible Structural Changes and Ion-Pairing Associations of the Ferrocene Monolayer in RTILs

and long-distance order existing in the neat RTIL.34 For ferrocene or large organic redox couples where the positive charge is delocalized in the oxidized form of the redox couple (see, for example, the anthracene/anthracene cation radical couple),35 the kinetic rates typically diminish by a factor of 10− 50 when passing from a molecular solvent to an RTIL (for similar RTILs than used in that work).6,35,36 To illustrate that no special behavior occurs in our considered RTILs, we have estimated the apparent standard rate constants, ks, of the charge transfer for the Me10Fc/ Me10Fc+couple measured in the same experimental conditions and on the same carbon substrate before modification. As expected kinetics decreases around 50 were observed confirming the classic properties of the considered RTILs versus charge transfer kinetics. In solution, the lower kinetics is explained by strong ion pairing associations between the ions of the RTILs with the redox couple and solvation-ordering of the media in RTILs.6 This leads to higher solvent reorganization and slower double-layer interface relaxation, which results in apparent slow heterogeneous kinetics because of the higher reorganization energy upon electron transfer.37 The amplitude of the variation of kel appears in the same range for the oxidation of ferrocene in solution and for our immobilized alkyl-ferrocene, suggesting a similar cause for the largest part of the decrease. We could observe in Table 1 that the kel variation follows the viscosity η of the RTILs, this last parameter being correlated to the ordering inside the neat RTILs.6 Indeed, a change of charge in the redox layer must be compensated by a counteranion that also implies a change in the local order of the ionic liquid. It is likely that such association would also result in folding of alkyl chain that could also make some of the redox centers less accessible as illustrated in Scheme 3.13,14 However,

even if we do not have a clear-cut answer to the cause of the phenomenon, as a major effect, it remains that only a part of the redox groups is available for the redox process in the RTILs. Additionally, we could notice that some asymmetry exists in the voltammogram recorded in the different RTILs (the peak shape for the oxidation is different from that of the reduction) contrarily to those recorded in ethanol and CH2Cl2 that are perfectly symmetric. Upon oxidation, anions of the RTILs are required to compensate the positively charged ferrocenium ions (see Scheme 3). The bulky anions of the RTILs could not easily insert in a dense layer of ferrocenyl groups limiting the quantity of available redox moieties as it was evidenced when the anion is a surfactant in an aqueous media.32 In that case, a rigid ionic layer having strong ion pairing is formed at the interface providing a solid-like monolayer, which slows down the electron transfer in RTILs.33 However, no clear tendency was observed with the dielectric constant of the RTILs for Γ and kel values. If it is difficult to get a final answer from our sole experiments, these observations support the occurrence of a strong restructuration of the layer upon charge transfer due to ion-pairing in the RTIL associations that follow a different pathway during the oxidation and the reduction processes. We could also underline that these behavior are specific to the RTILs and are not visible when the redox layer is examined in a molecular solvent containing the same anion (PF6−) as electrolyte (see Figure 1a for experiments performed in CH2Cl2 + 0.2 mol L−1 Bu4NPF6). This supports a fundamental characteristic of ionic liquids that ions in RTILs are certainly not isolated species, which probably explains that their insertion inside the redox layer is rendered much more difficult because of a larger effective radius. 28644



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(10) Li, J.; Shen, Y.; Zhang, Y.; Liu, Y. Room-Temperature Ionic Liquids as Media to Enhance the Electrochemical Stability of SelfAssembled Monolayers of Alkanethiols on Gold Electrodes. Chem. Commun. 2005, 360−362. (11) Oyamatsu, D.; Fujita, T.; Arimoto, S.; Muankata, H.; Matsumoto, H.; Kuwabatta, S. Electrochemical Desorption of a SeffAssembled Monolayer of Alkanethiol in Ionic Liquids. J. Electroanal. Chem. 2008, 615, 110−116. (12) Khoshtariya, D. E.; Dolidze, T. D.; Van Eldik, R. Multiple Mechanisms for Electron Transfer at Metal/Self-Assembled Monolayer/Room-Temperature Ionic Liquid Junctions: Dynamical Arrest versus Frictional Control and Non-Adiabaticity. Chem.Eur. J. 2009, 15, 5254−5262. (13) Sun, Q.-W.; Murase, K.; Ichii, T.; Sugimura, H. Electrochemical Behavior of Ferrocenylthiol/Alkanethiol Binary SAM in Ionic Liquids. ECS Trans. 2009, 16, 575−581. (14) Sun, Q.-W.; Murase, K.; Ichii, T.; Sugimura, H. Anionic Effect of Ionic Liquids Electrolyte on Electrochemical Behavior of Ferrocenylthiol/Alkanethiol Binary SAMs. J. Electroanal. Chem. 2010, 643, 58−66. (15) McCreery, R. L. Advanced Carbon Electrode Materials for Molecular Electrochemistry. Chem. Rev. 2008, 108, 2646−2687. (16) Simon, P.; Gogotsi, Y. Capacitive Energy Storage in Nanostructured Carbon-Electrolyte Systems. Acc. Chem. Res. 2013, 46, 1094−1103. (17) Ho, T. D.; Zhang, C.; Hantao, L. W.; Anderson, J. L. Ionic Liquids in Analytical Chemistry: Fundamentals, Advances, and Perspectives. Anal. Chem. 2014, 86, 262−285. (18) Loget, G.; Chevance, S.; Poriel, C.; Simonneaux, G.; Lagrost, C.; Rault-Berthelot, J. Direct Electron Transfer of Hemoglobin and Myoglobin at the Bare Glassy Carbon Electrode in an Aqueous BMI.BF4 Ionic-Liquid Mixture. ChemPhysChem 2011, 12, 411−418. (19) Ernst, S.; Aldous, L.; Compton, R. G. The Voltammetry of Surface Bound 2-Anthraquinonyl Groups in Room Temperature Ionic Liquids. Chem. Phys. Lett. 2011, 511, 461−465. (20) Devadoss, A.; Chidsey, E. D. Azide-Modified Graphitic Surfaces for Covalent Attachment of Alkyne-Terminated Molecules by Click Chemistry. J. Am. Chem. Soc. 2007, 129, 5370−5371. (21) Leroux, Y. R.; Fei, H.; Noël, J.-M.; Roux, C.; Hapiot, P. Efficient Covalent Modification of a Carbon Surface: Use of a Silyl Protecting Group To Form an Active Monolayer. J. Am. Chem. Soc. 2010, 132, 14039−14041. (22) Rostovtsev, V. V.; Green, L. G.; Fokin, V. V.; Sharpless, K. B. A stepwise huisgen cycloaddition process: copper(I)-catalyzed regioselective ″ligation″ of azides and terminal alkynes. Angew. Chem., Int. Ed. 2002, 41, 2596−2599. (23) Tornoe, C. W.; Christensen, C.; Meldal, M. Peptidotriazoles on Solid Phase: [1,2,3]-Triazoles by Regiospecific Copper(I)-Catalyzed 1,3-Dipolar Cycloadditions of Terminal Alkynes to Azides. J. Org. Chem. 2002, 67, 3057−3064. (24) Leroux, Y. R.; Hapiot, P. Nanostructured Monolayers on Carbon Substrates Prepared by Electrografting of Protected Aryldiazonium Salts. Chem. Mater. 2013, 25, 489−495. (25) Hoertz, P. G.; Niskala, J. R.; Dai, P.; Black, H. T.; You, W. Comprehensive Investigation of Self-Assembled Monolayer Formation on Ferromagnetic Thin Film Surfaces. J. Am. Chem. Soc. 2008, 130, 9763−9772. (26) Bonhôte, P.; Diaz, A. P.; Papageorgiou, N.; Kalyasundaram, K.; Grätzel, M. Hydrophobic, Highly Conductive Ambient-Temperature Molten Salts. Inorg. Chem. 1996, 35, 1168−1178. (27) Garreau, D.; Savéant, J.-M. Linear Sweep Voltammetry. Compensation of Cell Resistance and Stability. Determination of the Residual Uncompensated Resistance. J. Electroanal. Chem. 1972, 35, 309−331. (28) Amatore, C.; Klymenko, O.; Svir, I. A New Strategy for Simulation of Electrochemical Mechanisms Involving Acute Reaction Fronts in Solution: Application to Model Mechanisms. Electrochem. Commun. 2010, 12, 1165−1169.

CONCLUSIONS Covalently bonded ferrocene molecules were efficiently grafted on carbon surfaces with long alkyl chain linkers. This provides a monolayer having a remarkable robustness allowing kinetic and thermodynamic studies in RTILs and molecular solvents (ethanol and dichloromethane). It is clear that these results would require additional investigations to be fully understood notably theoretical calculations to verify the proposed hypothesis and to extend the conclusion to other RTILs. On the whole, a similar behavior is observed for this layer on carbon with what was previously reported for alkylthiolferrocene layer deposited on gold. Electron transfer kinetics is much slower in the RTILs than in the molecular solvents. This amplitude of the decrease is comparable with the variations observed when comparing the electronic kinetics of a redox couple in a molecular solvent and in RTILs but would probably originate from an important reorganization of the layer upon charge transfer. The main difference between the molecular solvent and RTILs is that only a part of the redox moieties remains active in these last media. This phenomenon combined with a slow down of the interfacial electron transfer could be a major difficulty for practical applications, as, for example, in electro-analytical systems or energetics devices as the global efficiency of the systems will be related with the charge transfer rates. When such immobilized redox systems are combined with the use of a RTIL, the choice of the anion−cation of the RTIL combination remains of primary importance.

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AUTHOR INFORMATION

Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS Prof. Irina Svir and C. Amatore (Ecole Normale Supérieure, Paris) are warmly thanked for the possibility of using the KISSA Software.

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Comparative Electrochemical Investigations in Ionic Liquids and

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