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01/2012; , ISBN: 978-90-481-9752-1
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ABSTRACT: Redox-gated molecular junctions were obtained starting with a relatively large gap between two electrodes, in the micrometer range, followed by electrochemical polymerization of aniline. Polyaniline (PANI) grows from the tip side until it bridges the two electrodes. The resulting junctions were characterized electrochemically by following the variation of the tip-substrate current as a function of the electrochemical gate potential for various bias voltages and by recording their I(V) characteristics. The two electrodes make contact through PANI wires, and microjunctions with conductances around 10(-3) S were obtained. On the basis of a similar setup, PANI nanojunctions with conductances between 10(-7) and 10(-8) S were made, where the current appears to be controlled by fewer than 10 oligoaniline strands. Despite the small number of strands connecting the two electrodes, the junctions are highly stable even when several successive potential sweeps are performed. Comparison of the conductance measured in the oxidized and reduced states leads to an on/off ratio of about 70-100, which is higher than that reported for a single aniline heptamer bridging two electrodes, highlighting the interest of connecting a few tens of molecules using the scanning electrochemical microscopy (SECM) configuration. In some cases, the switching of the PANI takes place in several individual conductance steps close to that obtained for a single oligoaniline. Finally, starting with a microjunction and mechanically withdrawing the tip shrinks it down to the nanometer scale and makes it possible to reach the regime where the conductance is controlled by a limited number of strands. This work presents an easy method for making redox-gated nanojunctions and for probing the conductance of a few oligoanilines despite an initially large tip-substrate gap.
Analytical Chemistry 12/2011; 83(24):9709-14. · 5.86 Impact Factor
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ABSTRACT: The redox switching dynamics of poly(3,4-ethylenedioxythiophene) (PEDOT) in an acetonitrile solution and a room temperature ionic liquid, 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (EmiTFSI), are investigated by means of potential step experiments. Redox switching can be viewed as a phase transition in which the nucleation and growth processes occur. We have developed a phenomenological model allowing the determination of the kinetic parameters. Two limiting cases are shown as follows: (i) a progressive and (ii) an instantaneous nucleation. In all cases, the growth process is described in terms of a self-exchange electron transfer reaction. We show that the mechanisms depend upon the medium. In acetonitrile, progressive nucleation and growth occur during oxidation (p-doping), whereas nucleation is instantaneous in the reduction of the PEDOT film. On the other hand, instantaneous nucleation and growth mechanisms are observed for both oxidation and reduction in EmiTFSI. The difference in the mechanisms results from the ionic exchange process associated with electron transfer and the initial structure of the film (open or compact). The influence of the applied potential on the dynamics is analyzed for both media.
The Journal of Physical Chemistry B 01/2011; 115(2):205-16. · 3.70 Impact Factor
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ABSTRACT: Atomic contacts between two copper microelectrodes, with conductance close to the conductance quantum (G0), have been generated using simple electrochemical methods in aqueous media containing various types of surfactant. Neutral and cationic surfactant have only a small impact on the conductance with a preponderant value around 1 G0. Furthermore, varying the concentration of the neutral and cationic surfactants above or below the critical micellar concentration (cmc) during the growth process does not lead to any significant variation in the conductance. In contrast, atomic contacts generated in a solution containing an anionic surfactant below the cmc have unusual but reproducible properties. They have <1 G0 conductance and their conductance versus time behavior shows steps at fractional values of the quantum. I/V characterization shows ohmic behavior, supporting the formation of metallic contacts with quantized conductances. The low-conductance value (<1G0) is attributed to the occurrence of molecular adsorption at the contact. This specific effect seems to be closely related to the cmc. Indeed, when the anionic surfactant concentration is increased above the cmc, the contacts have conductances close to those in neutral or cationic surfactant solution.
The Journal of Physical Chemistry C 01/2011; 115(2):549-553. · 4.80 Impact Factor
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ABSTRACT: The electrochemical reduction of 4-nitrophenyl diazonium, NPD, in different ionic liquids presenting different viscosities has been investigated. The electrochemical studies show that the reduction of diazonium leading to the formation of its corresponding radical occurs whatever the viscosity of the grafting media. Following that, the presence of an organic layer attached to the electrode after electrochemical treatment was evidenced by cyclic voltammetry (CV) in acidic media thanks to the presence of nitro groups. Moreover, infrared spectroscopy (IR), X-ray photoelectron spectroscopy (XPS), and atomic force microscopy (AFM) confirm the presence of a nitrophenyl (NP) layer attached to the electrode material. Next, the examination of the electrochemical data through the measurement of the charge, corresponding to the reduction of the attached nitrophenyl (NP) moieties, shows that the surface concentration of NP, Γ(NP), decreases when the viscosity, η, of the grafting media increases. Additionally, in the case of the more viscous ionic liquid, N-tributyl-N-methylammonium bis(trifluoromethylsulfonyl)imide [Bu(3)MeN] [NTf(2)], a cosolvent has been added leading to fine decrease of the viscosity. The IR and CV investigations of the modified electrodes demonstrate the decrease of the amount of the attached molecules when the viscosity of the grafting media increases. In addition, a correlation between Γ(NP) as function of 1/η was observed. Finally, XPS and AFM experiments lead to an estimate of the thickness of the attached layer. As a result, both methods are in perfect agreement and thicknesses of 4 and 1 nm are measured after grafting in acetonitrile and in pure ionic liquid [Bu(3)MeN] [NTf(2)], respectively. By comparison with classical solvent, the use of viscous ionic liquid for the grafting leads to a decrease in the amount of the attached molecules and conduce to the formation of thinner or less dense layer.
Langmuir 11/2010; 26(23):18542-9. · 4.19 Impact Factor
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ABSTRACT: The displacement caused by an applied sinusoidal voltage and electrochemical impedance spectroscopy are used to analyze the electromechanical behavior of bucky-gel single-walled carbon nanotube actuators containing an ionic liquid. This behavior can be understood in terms of complex capacitance diagrams and the frequency dependence of the real and imaginary components of the capacitance. The complex power allows analyzing the relationship between energy storage and energy dissipation during the operation of the electrochemical actuator. Bucky-gel SWCNT actuators behave as supercapacitors. Accordingly, the energy−power plot or Ragone plot expresses the useable energy as a function of the power. In order to visualize the relationship between mechanical behavior and electrochemical properties, the strain−power plot is reported. The maximum power Pmax and specific energy densities Emax of the electrochemical actuator are calculated. The energy densities of bucky-gel SWCNT actuators containing an ionic liquid are of the same order of magnitude as those of natural muscles.
09/2010;
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ABSTRACT: Herein, we report the variation of localized surface plasmon resonance (LSPR) of gold nanoparticle (NP) arrays covered by poly(3,4-ethylenedioxythiophene) (PEDOT) as a function of the electronic state of the polymer. Giant shifts and fine-tuning of the LSPR of gold NPs surrounded by PEDOT/sodium docecyl sulfate have been achieved. The color variations of plasmonic/conducting polymer (CP) devices are given not only by changes of the optical properties of the CP upon doping but also by a close synergy of the optical properties of CP and NP. Such systems can considerably extend the field of CP-based electrochromic devices.
Journal of the American Chemical Society 08/2010; 132(30):10224-6. · 9.91 Impact Factor
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ABSTRACT: Electrochemical grafting of a water-insoluble diazonium salt in aqueous media onto an electrode surface was achieved by host-guest complexation. 1-(2-Bisthienyl)-4-aminobenzene (BTAB) was solubilized in a water/beta-cyclodextrin solution (beta-CD). The corresponding diazonium salt was generated in situ then electroreduced. This process leads to the attachment of bithiophene or short oligothiophene groups to the electrode surface. The modified surfaces were analyzed by cyclic voltammetry (CV), scanning electrochemical microscopy (SECM), X-ray photoelectron spectroscopy (XPS), infrared reflection absorption spectroscopy (IRRAS), and atomic force microscopy (AFM). The electrochemical investigations show that the water-based modified surface is similar to one generated in acetonitrile without beta-CD. Thus, the attached organic layer behaves like an electrochemical switch (above some threshold potential, a soluble external probe is oxidized, but the oxidized form cannot be reduced). The modified surfaces consist of grafted bisthienylbenzene (BTB) and cyclodextrins that can be removed from the surface. This procedure may be considered as a new means of creating a surface made of submicrometric holes in an organic semiconducting layer.
Journal of the American Chemical Society 02/2010; 132(5):1690-8. · 9.91 Impact Factor
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ABSTRACT: The functionalization of electrode materials through diazonium electroreduction using a heteroaromatic compound, without phenyl groups, has been investigated for the first time. The electrochemical reduction of 2-aminoterthiophenyldiazonium cation, generated in situ, coats the electrode (glassy carbon (GC), gold or platinum) with an ultrathin organic layer, shown by X-ray photoelectron spectroscopy (XPS) of that deposited on gold to consist of terthiophene or oligothiophene. The coating is electroactive at potential close to that of terthiophene in solution. The electrochemical response of the modified GC electrode in the presence of various reversible redox couples shows that the attached layer acts as a conductive switch. It behaves as a barrier to electron transfer when the standard redox potential is below 0.5 V/SCE; in this case diode-like behavior is observed. However, for more oxidizing redox probes the layer can be considered as transparent and no barrier effect is observed. The layer deposited on a platinum ultramicroelectrode (UME) behaves similarly to that obtained on the large GC electrode. Scanning electrochemical microscopy (SECM) can be performed using this electroswitchable modified platinum UME which can act as a filter toward competitive redox exchange pathways.
Journal of the American Chemical Society 09/2009; 131(41):14920-7. · 9.91 Impact Factor
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ABSTRACT: Ultrathin layers of thiophene derivatives were covalently attached to GC electrodes by electroreduction of
diazonium salts. The films are densely packed structures and are able to mediate electron transfer above a threshold voltage tuned by the nature of the grafted molecules onto the electrode. We investigate the electron transfer properties of these layers by using electroactive probes having various redox potentials. Voltammetry and SECM measurements clearly show that for redox probes with low redox potentials diode-like behavior is observed and the current can flow in only one direction across these organic layers whereas, when the potential of the external redox probe increases, the transparency of the layers toward electron transfer in both direction increases (within the time range investigated). This behavior is not compatible with an ECcat mechanism, with electroactive immobilized centers characterized by a single redox potential E°Im, and clearly demonstrates the switching of the layer conductivity. In this sense, these layers are better modeled as covalently grafted conducting oligomers densely packed on the surface and capable of changing their charge transfer characteristics upon charge injection (i.e., doping of the grafted conjugated oligomers). The charge transfer mechanism of these ultrathin films is thus very different from that most of SAMs bearing an electroactive group, in which electron tunnelling through the monolayer is the main charge transfer mechanism.
The Journal of Physical Chemistry C 01/2008; 112(47):18638-18643. · 4.80 Impact Factor
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ABSTRACT: Electroreduction of diazonium salts and electropolymerization of conductive polymers are two widely used
techniques for producing modified electrodes. In this work, these two techniques are combined in order to
functionalize glassy carbon electrodes. In a first step, a thin layer of diphenyl amine (DPA) is grafted through the electrochemical reduction of the 4-aminodiphenylamine (ADPA) diazonium salt. The DPA grafted layer has been studied using XPS and SECM. It appears to act as a switch giving diode-like behavior for probes with a redox potential below 0.4 V. As a consequence, aniline oxidation remains possible on this substrate, and polyaniline (PANI) can easily be deposited on the DPA-modified electrodes. The PANI films generated on such modified electrodes show electrochemical behavior similar to that of PANI electrodeposited on bare carbon electrodes but exhibit better chemical stability and high resistance to aggressive environments, that is, high temperatures and ultrasonic etching. Such results, associated with the scanning electrochemical microscopy response of the modified electrodes, strongly suggest that PANI is grafted covalently on the DPA layer. From a fundamental point of view, the results show that surface graft polymerization can be extended to surface graft electropolymerization. From a practical point of view, the method described in this work is a simple and efficient process for improving the stability of an electrodeposited conducting polymer, that is, polyaniline, on a carbon substrate and could be of considerable importance in the synthesis of PANI/carbon nanotube composites.
The Journal of Physical Chemistry C 01/2008; 112(41):16103-16109. · 4.80 Impact Factor
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ABSTRACT: The electrochemical behaviour of nanoporous TiO(2) in a room temperature ionic liquid (RTIL), 1-ethyl-3-methylimidazolium bis((trifluoromethyl)sulfonyl)amide (EMITFSI), was investigated by cyclic voltammetry (CV) and impedance spectroscopy. Exponentially rising currents in voltammetry were attributed to the charging/discharging of electrons in the TiO(2) film and a charge transfer mechanism. The main features of the voltammetry and impedance followed the same trends in the ionic liquid as in other organic solvents and also in aqueous electrolytes. In the presence of lithium ions, the onset potential of the charge accumulation increased due to the change of the initial position of the TiO(2) conduction band. The results show that substitution of organic solvents contained in solar cells, supercapacitors or other electrochemical devices is in general feasible, though requires some adjustment in the electrolyte composition for optimal performance.
Physical Chemistry Chemical Physics 05/2006; 8(15):1827-33. · 3.57 Impact Factor
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ABSTRACT: The electrochemical reduction of noble metal electrodes in the presence of redox ionic liquid, 1-ferrocenylethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide, [FcEMIM][TFSI], was investigated by cyclic voltammetry. Our experiments suggest the formation of metal with negative oxidation states, in the cases of platinum and gold electrodes [Mn-, FcEMIM+]. By analogy with the previous work, the formation of these phases is concomitant with the insertion of the supporting electrolyte; which correspond in our experimental condition to the redox cation of the ionic liquid. As an exciting result, the electrochemical investigations of the reduced electrode in electrolytic solution, containing solvent and supporting electrolyte, evidence the presence of the ferrocene groups at the electrode surface. Moreover, the reduced electrode exhibits the presence of the ferrocene even after, contact with air, after ultrasound, and after physical polishing, highlighting the large stability of this organo-metallic phases formed in this media. The AFM investigations demonstrate the morphological change of the platinum surface after the reduction process. Finally, our works bring a formal electrochemical proof of the presence of the ionic liquid cation inside the electrode material after the cathodic treatment in this media.
Electrochemistry Communications. 10(8):1205-1209.
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ABSTRACT: Electrochemical reduction of the 4-nitrophenyl diazonium salt in ionic liquid media has been investigated at carbon electrode. The ionic liquid chosen for this study was 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide, [EMIM][TFSI]. The cyclic voltammetry study demonstrated the possibility of the electrochemical grafting of the nitrophenyl groups onto carbon electrode after the reduction of its corresponding diazonium in ionic liquid. The electrochemical characterization of the modified electrode achieved on ionic liquid displays the presence of the nitrophenyl group at the carbon surface. Moreover, the surface concentration of the attached group obtained in this media was found to be around 1.7 × 10-10 mol cm-2, this value may suggest the possibility of the formation of monolayer. Furthermore, the characterization of the modified electrode in [EMIM][TFSI] showed the conversion of some NO2-phenyl groups to NHOH-phenyl. This observation could indicate the presence of surface interaction between the reduced NO2-phenyl and the ionic liquid cation, thanks to the presence of acidic proton in the ionic liquid cation.
Electrochemistry Communications. 10(7):1060-1063.
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ABSTRACT: Electrochemical oxidation of primary amine in ionic liquid media has been investigated. The ionic liquid chosen for this study was 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide. Two primary amine compounds are used for this study; 4-nitrobenzylamine and 2-aminoethylferrocenylmethylether. The oxidation of the amino compounds in ionic liquid conduces to the modification of the electrode surface. The modified electrodes were characterized by cyclic voltammetry and XPS analysis. Both techniques support the presence of an organic layer strongly attached onto the electrode surface. The surface concentration of the attached group obtained in this media was found to be around 1 to 3 × 10−10 mol cm−2. The use of ionic liquid as media for the grafting leads to decrease of the surface concentration of the grafted layer; and the formation of less dense layer compared with classical solvent such as acetonitrile.
Electrochemistry Communications.
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ABSTRACT: We discuss a model system of electroactive material containing heterogeneity or energy disorder, in which charge accumulation and transfer is governed mainly by chemical potentials of species in the solid material. The equivalent circuit for impedance spectroscopy is composed of parallel combination of the electrochemical capacitances of the phases and charge-transfer resistances between the phases. The origin of the circuit elements is analysed using the principle of detailed balance and the master equation, as a generalization of different systems, such as intercalation batteries, electrochromics and solar cells based on nanostructured semiconductors, and systems with traps. The Butler–Volmer type equation for interfacial charge-transfer at the solid-solid interface is derived. The model is extended with the relaxation of charge in one of the phases providing an inductive behaviour at high frequency. Experimental results of intercalation of Li into nanostructured TiO2, in the configuration TiO2/room temperature ionic liquid/PEDOT show the inductive behaviour.
Electrochimica Acta.
