Voltammetry and impedance spectroscopy investigations an object becomes as much increases its surface/volume ratio until reaching the point in which that object becomes "all surface". Besides, it is the surface and not the bulk material that interacts with the surrounding environment, hence by altering the surface in a controlled manner we can modulate the properties of the material towards that environment, i.e., chemical reactivity and physical properties. However, to develop materials with new or improved properties, it is necessary to know how to shape the matter at the nanoscale level. Nanostructuring involves a broad range of surface modification procedures, one of the most interesting is the introduction of arrangements of functional molecules on the surface. The polymerization and adsorption of thiol or disulfide compounds are well-known methods. Nevertheless, they have some drawbacks, the main is the relatively weak surface-molecule bond strength, which can make the system unstable. In this context, a covalent bond would be desirable.

Diazonium salts were discovered during the diazotization of aryl amines for the synthesis of azo dyes by the German scientist Peter Greiss in 1858. However, it was not until the year 1992 when Savéant et al. demonstrated a new reaction of these compounds: they reported the "Covalent modification of carbon surfaces by grafting of functionalized aryl radicals produced from electrochemical reduction of diazonium salts".

This method for surface modification leads to strong C-C covalent bonds between the aryl groups and carbon surfaces. Since the large existing variety of aryldiazonium compounds, a wide range of modifications are possible. Furthermore, the organic layer could be post-functionalized by conventional chemical reactions to suit different applications. The applications that currently attract more attention are those related to analytic and biotechnological purposes and materials development for energy storage devices. These various applications fundamentally require a good physicochemical characterization of the surfaces and the understanding of the interfacial processes.

This thesis focus on surface modification at the monolayer, near-monolayer and multilayer level (films of less than 10 nm-thickness) by aryldiazonium electro-reduction, with particular attention to the electron charge transfer process at the electrochemical interface between the modified surfaces (typically carbon and occasionally gold substrates) and a condensed phase constituted by the solvent and the electrolyte. Three different electrochemical methods have mainly been used throughout this thesis, cyclic voltammetry, electrochemical impedance spectroscopy and scanning electrochemical microscopy. This work is within the framework of the fundamental research and contributes to the study of the electron transfer through modified carbon or gold surfaces of electroactive species immobilized on the electrode surface or in solution, and to the understanding of their physicochemical properties and applications.

The thesis is structured in the following manner: Chapter 1 reviews the current literature regarding the use of aryldiazonium salts for surface modification, focusing on the electrochemical reduction of aryldiazonium salts. Their synthesis and chemical reactivity are briefly overview, and different strategies for accurately functionalize surfaces by controlling the thickness and distribution of the molecules on the surfaces are described more in depth. The post-functionalization of the resulting aryl layers are commented, with particular emphasis in "click" chemistry procedures. Thereafter, the objectives are summarized in more detail before presenting the three chapters that refer to the studies involved in the thesis. All three are connected by the use of aryldiazonium salts at different levels of layer thickness, from the monolayer to the multilayer, electroactive and non-electroactive layers with and organized or dendritic structure. Note that each chapter begins with a discussion of the literature related to the corresponding topic investigated.

Chapter 2 is the first work completed in relation with the thesis, focusing on the influence of the solvent in the electron transfer of the ferrocene-terminated C 11 -alkyl monolayer on carbon electrodes. A protection-deprotection strategy allows the formation of a covalently attached monolayer, which is post-functionalized in a second step with an alkyl ferrocene-terminated electroactive chain. Cyclic voltammetry and electrochemical impedance spectroscopy experiments allow us to study the electrochemical properties of the electron transfer in different media and to evaluate its applicability for increasing the charge density for energy storage applications.

Chapter 3 being the second work of the thesis and focused on the electro-reduction of a tetrahedral-shape pre-organized aryldiazonium salt, leading in this case to the formation of a non-electroactive ultrathin film (few layer-thickness) on gold and carbon electrodes. The organic layer shows very particular properties towards the electron transfer of ferrocene and derivatives in solution that have been investigated by cyclic voltammetry. The investigation of the electrochemical properties is complemented with a complete characterization of the modified surfaces by means of X-Ray Photoelectron Spectroscopy (XPS), Atomic Force Microscopy (AFM), Infrared (IR) spectroscopy and ellipsometry.

Chapter 4 is the final study of the thesis, focusing on the use of scanning electrochemical microscopy for the detection of reactive intermediates during oxygen reduction reaction on platinum ultramicroelectrodes. A foot-printing approach is used where the interaction with sensitive organic aryl layers evidences the production of reactive oxygen species. The role of different sensitive layers, applied potentials and electrolytes are investigated. Finally, the general conclusions and future perspectives are detailed.

Chapter 1

Aryldiazonium salts Abstract

Surface modification with aryl layers containing different molecules or functional components has become increasingly popular in recent decades since it allows the development of materials with new or improved properties. This raising interest is mainly related to the ability to control the modification at the molecular level, and also to the development of new techniques that let characterize the molecular constructions, i.e., XPS or AFM. The level of sophistication of molecular designs and the level of knowledge related to layer fabrication have increased and as a consequence also the potential applications in different areas of science. This chapter discusses the literature about aryldiazonium salts for surface modification applications. Particular attention is paid to the electro-reduction of these compounds and the existing strategies for controlling the thickness of the organic layers during the electro-grafting process. The most relevant characterization techniques and the post-functionalization procedures are described, emphasizing the "click" chemistry since it was used throughout this thesis.

keywords: aryldiazonium salt, synthesis, electrochemistry, multilayer, monolayer, click chemistry, characterization

Introduction

Diazonium salts were discovered in 1858 by the German chemist Johann Peter Griess during the diazotization of amines for the synthesis of azo dyes. [START_REF] Griefs | Vorläufige notiz über die einwirkung von salpetriger säure auf amidinitro-und aminitrophenylsäure[END_REF] Their structure is RN 2 + X -where R can be an aryl or alkyl group and X -a weak nucleophilic organic or inorganic anion. Although it is theoretically possible to make a wide variety of combinations of R and X, the nature of the two groups strongly influences the final stability of the salt, resulting in some cases unstable or even explosive. For example, while alkyls diazonium salts are hazardous to be synthesized in the laboratory, aryldiazonium salts are much more stable because of the conjugation between the aromatic ring and the diazo group. The combination with a stabilizing counterion makes possible the isolation of aryldiazonium salts in crystalline form. In fact, some diazonium tetrafluoroborates are commercially available. [2] Figure 1.1: Structure of aryldiazonium salts Aryldiazonium salts are under-used despite their vast synthetic potential. [START_REF] Patai | The Chemistry of Diazonium and Diazo Groups[END_REF] Publications reporting recent incidents handling benzene diazonium chloride intermediates have contributed to their bad reputation among chemists. [4][5][START_REF] Sheng | Reactive chemical hazards of diazonium salts[END_REF] However, different aryldiazonium salts were reported being the stability their main feature. Tetrafluoroborates are the most common counterions in synthesis, [7][START_REF] Cygler | The crystal structure of benzenediazonium tetrafluoroborate, C 6 H 5 N 2 + BF 4[END_REF][START_REF] Allongue | Covalent modification of carbon surfaces by aryl radicals generated from the electrochemical reduction of diazonium salts[END_REF][START_REF] Chaussé | The electrochemical reduction of diazonium salts on iron electrodes. the formation of covalently bonded organic layers and their effect on corrosion[END_REF][START_REF] Leroux | Design of robust binary film onto carbon surface using diazonium electrochemistry[END_REF] but other stable salts have been demonstrated, i.e., hexafluorophosphates, [START_REF] Galli | Radical reactions of arenediazonium ions: An easy entry into the chemistry of the aryl radical[END_REF] disulfonimides [START_REF] Barbero | Halodediazoniations of dry arenediazonium o-benzenedisulfonimides in the presence or absence of an electron transfer catalyst. Easy general procedures to prepare aryl chlorides, bromides, and iodides[END_REF] and carboxylates, [START_REF] Colas | An efficient procedure for the synthesis of crystalline aryldiazonium trifluoroacetates-synthetic applications[END_REF] and more recently tosylates, [START_REF] Filimonov | Unusually stable, versatile, and pure arenediazonium tosylates: their preparation, structures, and synthetic applicability[END_REF] which can be stored at room temperature. This stability can be explained by the very high affinity between the anion and the cation. [START_REF] Romming | The structure of benzene diazonium chloride[END_REF] In this context, different procedures have been reported for the synthesis of aryldiazonium salts. [START_REF] Zollinger | Diazo Chemistry I: Aromatic and Heteroaromatic Compounds[END_REF] The most commonly used consists in the diazotization of a primary amine with a source of nitrite in the presence of an acid such as HBF 4 , which provides protons and the necessary counterions. The temperature is kept under 0 • C to avoid the decomposition of the product into nitrogen and the aryl cation, which can undergo secondary reactions with the nucleophiles present during the reaction.

Scheme 1.1: Reaction mechanism for the synthesis of aryldiazonium salts

As shown in Scheme 1.1, the mechanism of diazotization involves the nucleophilic attack of a primary aromatic amine, in this case aniline, on the nitrosonium ion (NO + ) generated in situ from the mixture of sodium nitrite and acid. In the acidic solution, the N-nitrosamine undergoes protonation to yield the corresponding aromatic diazonium salt. The presence of electron withdrawing substituents attached to the aromatic ring make the amine considerably less nucleophilic, and in consequence, less reactive toward the diazotization. Salts resulting from this reaction can be isolated and stored for future use; water insoluble aryldiazonium salts are often obtained in a good yield. [START_REF] Zollinger | Diazo Chemistry I: Aromatic and Heteroaromatic Compounds[END_REF] In an even more straightforward approach, aryldiazonium salts are synthesized in situ in aqueous or organic media and used without isolation. [START_REF] Bélanger | Electrografting: A powerful method for surface modification[END_REF] In this way, the handling of hazardous solids is avoided and the aryldiazonium cation in situ generated can be directly used, taking advantage of its relative stability in ice-cold acidic or MeCN solutions. [START_REF] Baranton | Electrochemical derivatization of carbon surface by reduction of in situ generated diazonium cations[END_REF][START_REF] Baranton | In situ generation of diazonium cations in organic electrolyte for electrochemical modification of electrode surface[END_REF] Both approaches, the isolation and in situ generation of aryldiazonium salts, have been widely used in surface electrochemistry.

Surface modification by aryldiazonium salts

Different chemical and electrochemical approaches have been developed for surface modification. [START_REF] Bélanger | Electrografting: A powerful method for surface modification[END_REF] In general, chemical procedures requires the immersion of the surface in a solution containing a molecule capable of interacting spontaneously with the surface, the paradigm that exemplifies this process is the chemisorption of organothiolsself-assembled monolayers (SAMs) on gold surfaces (Scheme 1.2). [START_REF] Ulman | Formation and structure of self-assembled monolayers[END_REF] Although they are perfect models for fundamental research, from a technological point of view they have some drawbacks, the most important is the relatively weak surface-molecule bond strength, which makes the system unstable facing possible applications. [START_REF] Shewchuk | Comparison of diazonium salt derived and thiol derived nitrobenzene layers on gold[END_REF] In this context, a covalent bond would be desirable.

The electro-reduction of aryldiazonium salts is nowadays a common technique for surface functionalization. [START_REF] Delamar | Covalent modification of carbon surfaces by grafting of functionalized aryl radicals produced from electrochemical reduction of diazonium salts[END_REF] The electro-reduction leads to surfaces being modified by organic films of thickness that vary from a monolayer [START_REF] Nielsen | Electrochemical approach for constructing a monolayer of thiophenolates from grafted multilayers of diaryl disulfides[END_REF][START_REF] Combellas | Sterically hindered diazonium salts for the grafting of a monolayer on metals[END_REF][START_REF] Lee | Covalently anchored carboxyphenyl monolayer via aryldiazonium ion grafting: A well-defined reactive tether layer for on-surface chemistry[END_REF][START_REF] Fontaine | Ionic liquid viscosity effects on the functionalization of electrode material through the electroreduction of diazonium[END_REF][START_REF] Mattiuzzi | Electrografting of calix[4]arenediazonium salts to form versatile robust platforms for spatially controlled surface functionalization[END_REF] to thicker films in the range of 5 to 100 nm, [START_REF] Cao | Electroreduction of viologen phenyl diazonium salts as a strategy to control viologen coverage on electrodes[END_REF][START_REF] Bousquet | Redox grafting of diazotated anthraquinone as a means of forming thick conducting organic films[END_REF] and even up to a few micrometers. [START_REF] Adenier | Formation of polyphenylene films on metal electrodes by electrochemical reduction of benzenediazonium salts[END_REF] Conductors and semiconduct-Scheme 1.2: Representation of an alkanethiol self-assembled monolayer on gold formed spontaneously by chemisorption and organized into more or less large ordered domains; an alkanethiol can be thought of as containing three parts: a sulfur binding group (head group) for attachment to a noble metal surface, a spacer chain (typically made up of methylene groups), and a functional group (X) in the tail.

ors surfaces could be easily modified by electrochemically-induced aryldiazonium reduction leading to a fast modification (seconds-minutes) of the substrate. [START_REF] Hicks | Tailoring the electrochemical properties of carbon nanotube modified indium tin oxide via in situ grafting of aryl diazonium[END_REF] Furthermore, insulators [START_REF] Pinson | Attachment of organic layers to conductive or semiconductive surfaces by reduction of diazonium salts[END_REF] could also be modified by adding chemical reductants in solution, [START_REF] Gohier | Tunable grafting of functional polymers onto carbon nan-200 otubes using diazonium chemistry in aqueous media[END_REF] by the formation of diazotes [START_REF] Troian-Gautier | Controlled modification of polymer surfaces through grafting of calix[4]arene-tetradiazoate salts[END_REF] or by spontaneous grafting [START_REF] Mesnage | Spontaneous grafting of diazonium salts: Chemical mechanism on metallic surfaces[END_REF] which makes this technique very versatile. Since the flexibility of the synthetic methods previously described, a wide variety of surfaces bearing functional groups can be achieved. For all these reasons, the electro-reduction of aryldiazonium salts was proposed for a wide number of applications, ranging from corrosion protection, [START_REF] Combellas | Spontaneous grafting of iron surfaces by reduction of aryldiazonium salts in acidic water: Applications to the inhibition of iron corrosion[END_REF] active plasmonic devices, [START_REF] Schaming | Tunable plasmon resonance of gold nanoparticles functionalized by electroactive bisthienylbenzene oligomers or polythiophene[END_REF] supercapacitors [START_REF] Menanteau | Enhancing the performance of a diazonium-modified carbon supercapacitor by controlling the grafting process[END_REF] and molecular diodes, [START_REF] Yan | All-carbon molecular tunnel junctions[END_REF][START_REF] Sayed | Bilayer molecular electronics: All-carbon electronic junctions containing molecular bilayers made with "click" chemistry[END_REF] to name a few.

Electrochemical reduction of aryldiazonium salts 1.2.1.1 Reduction at the electrode surface

Dediazoniation is one of the most important reactions of aryldiazonium salts. [START_REF] Galli | Radical reactions of arenediazonium ions: An easy entry into the chemistry of the aryl radical[END_REF] It occurs by loss of the diazo group either by a heterolytic or homolytic mechanism to generate the corresponding aryl cation (Ar + ) or the aryl radical (Ar • ). As far as the electrochemical reduction of aryldiazonium salts is concerned, we will be only interested in the homolytic pathway.

ArN

+ ≡N + e -→ Ar • + N 2 (1.1)
Reduction of aryldiazonium salts at the surface of an electrode follows the homolytic pathway. The electro-reduction of an aryldiazonium cation was demonstrated by Andrieux and Pinson [START_REF] Andrieux | The standard redox potential of the phenyl radical/anion couple[END_REF] to be a concerted process with the cleavage of dinitrogen and generation of the corresponding aryl radical (Equation 1.1). The latter, extremely reactive, can then react with the surface. This step is particularly efficient since the aryl radicals are produced in the vicinity of the electrode surface.

First evidence of aryl radicals production by electro-reduction of aryldiazonium salts was reported by [START_REF] Elofson | The polarographic reduction of diazotized aromatic amines[END_REF]. [START_REF] Elofson | The polarographic reduction of diazotized aromatic amines[END_REF] The reduction of diazotized amines in acid solution on a dropping mercury electrode was investigated by polarography at increasing concentration of aryldiazonium salt. It was found a non-linear relation between the current of the first reduction peak and the concentration of the aryldiazonium salt. The current of the first reduction peak reached a constant value, indicating, as authors stated, an absorption process as result of the one-electron reduction of the aryldiazonium salt to a free radical, which was stabilized by interaction on the mercury electrode surface.

Aryl-mercury derivatives were found after by coulometry analysis of the mercury pool.

Some objections can be raised given the tendency of mercury to adsorb species; however, later works by Elofson [START_REF] Gadallah | Arylation of aromatic compounds by electrochemical reduction of benzenediazonium tetrafluoroborate in aprotic solvents[END_REF][START_REF] Elofson | Pschorr reaction by electrochemical generation of free radicals. i. phenanthrene synthesis[END_REF] demonstrated the aryl radical production in MeCN solution during electro-reduction of aryldiazonium salts. Radical production was indirectly demonstrated through Pschorr reaction, which consists of an intramolecular substitution of aromatic compounds via aryldiazonium salts. The dediazoniation of the aryldiazonium compounds was electrochemically promoted by electrolysis, and the expected reaction products were confirmed by melting point, infrared and mass spectral analysis. A few years later, aryl radical production was confirmed by direct spectroscopic detection during electrochemical reactions. [START_REF] Bard | Application of spin trapping to the detection of radical intermediates in electrochemical transformations[END_REF] 1.2.1.2 Other methodologies of reduction Electrochemical reduction of aryldiazonium salts requires a specific experimental setup.

Usually, an aryldiazonium salt is dissolved (10 -2 to 10 -3 mol L -1 ) in an aprotic solvent containing the electrolyte; a standard three electrode setup is used in CV or chronoamperometry, where the working electrode is the surface that will be modified. The working electrode is dipped into the electrochemical cell containing the aryldiazonium solution and polarized for a few seconds or minutes using a potentiostat. However, it is not always possible to electrically connect the material to be functionalized, either because it is not a conductor like organic polymers or dielectrics, or because it is not "elec-trically connectable" to a potentiostat like gold nanoparticles or carbon black powders.

The reduction can then be carried out by adding a reducing agent (i.e., cuprous salt, ascorbic acid or ferrocene) in the solution containing the diazonium salts. [START_REF] Galli | Radical reactions of arenediazonium ions: An easy entry into the chemistry of the aryl radical[END_REF] In this case, the concentration of aryldiazonium salt in solution should be higher than that of electrochemical experiments, around 0.1 mol L -1 . The reduction of aryldiazonium salts can also be spontaneous, that means, without adding reducing agents or applying an electrochemical potential. In this way, spontaneous modifications of metal surfaces such as aluminium alloys or copper have been demonstrated. [START_REF] Hurley | Covalent bonding of organic molecules to Cu and Al alloy 2024 T3 surfaces via diazonium ion reduction[END_REF] This spontaneous process is based on the relative potential of the metal surface when is dipped into the aryldiazonium salt solution. It is also possible to promote the reduction of aryldiazonium salts thermally by heating the solution slightly, [START_REF] Barrière | Covalent modification of graphitic carbon substrates by non-electrochemical methods[END_REF] or using light to induce the photochemical decomposition of the aryldiazonium salt. [START_REF] Wataru | Photochemistry of the Diazonium and Diazo Groups, chapter 9 in Diazonium and Diazo Groups[END_REF] These alternative methodologies for reducing aryldiazonium salts are out of the scope of this thesis, which will follow the most straightforward path of the electrochemical reduction.

Electrochemical behavior of diazonium salts

In 1992, Savéant et al. [START_REF] Delamar | Covalent modification of carbon surfaces by grafting of functionalized aryl radicals produced from electrochemical reduction of diazonium salts[END_REF] reported a new reaction of aryldiazonium compounds:

the "Covalent modification of carbon surfaces by grafting of functionalized aryl radicals produced from electrochemical reduction of diazonium salts". A strong covalent bond between the grafted molecule and the carbon surface was observed. This was later confirmed by spectroscopic analyses for carbon surfaces, [START_REF] Liu | Reactions of organic monolayers on carbon surfaces observed with unenhanced raman spectroscopy[END_REF][START_REF] Saby | Electrochemical modification of glassy carbon electrode using aromatic diazonium salts. 1. Blocking 202 effect of 4-Nitrophenyl and 4-Carboxyphenyl groups[END_REF][START_REF] Kariuki | Nucleation and growth of functionalized aryl films on graphite electrodes[END_REF][START_REF] Bahr | Functionalization of carbon nanotubes by electrochemical reduction of aryl diazonium salts: a bucky paper electrode[END_REF][START_REF] Brooksby | Electrochemical and atomic force microscopy study of carbon surface modification via diazonium reduction in aqueous and acetonitrile solutions[END_REF] and also semiconductors, [START_REF] Villeneuve | Electrochemical formation of close-packed phenyl layers on si (111)[END_REF][START_REF] Allongue | Phenyl layers on h-si (111) by electrochemical reduction of diazonium salts: monolayer versus multilayer formation[END_REF][START_REF] Stewart | Direct covalent grafting of conjugated molecules onto si, gaas, and pd surfaces from aryldiazonium salts[END_REF] gold [START_REF] Bernard | Organic layers bonded to industrial, coinage, and noble metals through electrochemical reduction of aryldiazonium salts[END_REF][START_REF] Lyskawa | Direct modification of a gold electrode with aminophenyl groups by electrochemical reduction of in situ generated aminophenyl monodiazonium cations[END_REF] and iron [START_REF] Combellas | Spontaneous grafting of iron surfaces by reduction of aryldiazonium salts in acidic water: Applications to the inhibition of iron corrosion[END_REF] surfaces. Theoretical calculations support the experimental conclusions, especially regarding the covalent nature of the bonding between the aryl group and metal surfaces. [START_REF] Jiang | Structure and bonding between an aryl group and metal surfaces[END_REF] Other evidences that support the covalent bonding layer-surface are its resistance to ultrasonication in different organic solvents, [START_REF] Allongue | Covalent modification of carbon surfaces by aryl radicals generated from the electrochemical reduction of diazonium salts[END_REF] and its stability at high temperatures. [START_REF] Toupin | Thermal stability study of aryl modified carbon black by in situ generated diazonium salt[END_REF] These results are incompatible with a molecule-surface interaction type Van der Waals forces or hydrogen bonds.

Scheme 1.3 summarizes the process of surface modification by electrochemical reduction of aryldiazonium salts. Figure 1.2a shows the voltammogram registered for the electroreduction of p-ethynylbenzenediazonium tetrafluoroborate in MeCN + 0.1 mol L -1 nBu 4 NPF 6 on a GC electrode. It shows a broad irreversible reduction peak at + 0.17 V (vs SCE), which evidence an irreversible reaction compatible to the cleavage of dinitrogen during the electron transfer; a strong inhibition of the electrochemical signal is observed after the first cycle. After modification, no electro-activity of solution-phase species is observed (Figure 1.2b). This behavior is characteristic of the deposition of an insulating organic film, leading to a blocked electrode.

Scheme 1.3: Surface modification by electro-reduction of aryldiazonium salts. One of the main characteristics of aryldiazonium salts is that they are very easily reducible compounds; this is related to the strong electron-withdrawing properties of the diazo group. The incorporation of substituents to the aromatic ring of the aryldiazonium salts affects their half-wave reduction potential. [START_REF] Elofson | Substituent effects in the polarography of aromatic diazonium salts[END_REF] In general, while the incorporation of electron-withdrawing groups facilitates the reduction (the half-wave reduction potential shifts to positive potentials), the incorporation of electron donating groups makes the aryldiazonium salt more difficult to be reduced due to the stabilization of the starting diazonium salt (Table 1.1).

As shown in Table 1.1, the strong electron-withdrawing effect of the nitro group on the aromatic ring allows the electro-reduction of the substituted benzenediazonium salt at very positive potentials, while the substitution with alkyl groups such as -CH 3

shifts the half-wave reduction potential to negative potentials in around 220 mV. In any case, the range of potential where the electro-reduction of aryldiazonium salts oc- [START_REF] Pinson | Attachment of organic layers to conductive or semiconductive surfaces by reduction of diazonium salts[END_REF] to clarify some aspects about this peculiar electrochemical behavior of aryldiazonium salts. Unlike aryldiazonium salts, aryl halides have never shown grafting properties towards the electrode surface. They undergoes a first one-electron reduction (through a concerted mechanism or not) to aryl radical, depending on the lifetime of the aryl radical it can diffuse to the bulk of the solution where reacts with the solvent, or stays at the electrode surface, where undergoes a second one-electron reduction to aryl anion (Scheme 1.4).

The different electrochemical behavior of aryl halides and aryldiazonium salts can be explained by the aryl radical generated from the starting aryl halide, which is very easily reduced to aryl anion and has no opportunity to react with the electrode surface. In consequence, no electrografting is observed for this family of compounds. On the contrary, the electrochemical behavior of diazonium salts is related, as stated before, with the electron-withdrawing character of the diazo group, and also with the mechanism of the electron transfer. The concerted mechanism ensures the formation of the aryl radical just on the electrode surface. This aryl radical, unlike the aryl radicals generated from the one-electron reduction of aryl halides compounds, can be reduced to aryl anion but at much more negative potential than that of the starting aryldiazonium The molecular structure of an aryl multilayer is the consequence of a series of processes on the electrode surface that are summarized in the Scheme 1.5. The organic layer results from the attack of the aryl radicals on the electrode surface (1), and on the aryl groups already attached to the surface (2). A third possible process was first reported by Bélanger et al. [START_REF] Saby | Electrochemical modification of glassy carbon electrode using aromatic diazonium salts. 1. Blocking 202 effect of 4-Nitrophenyl and 4-Carboxyphenyl groups[END_REF]. It involves the reaction between an aryldiazonium molecule in solution and an intermediate cyclohexadienyl radical on the film surface (3). This process is not kinetically favored and leads to the formation of azo bonds (-N=N-) inside the layer. [START_REF] Doppelt | Surface modification of conducting substrates. existence of azo bonds in the structure of organic layers obtained from diazonium salts[END_REF] Therefore, the structure of multilayers could be described as a polyaryl structure covalently bonded to the electrode surface, [START_REF] Delamar | Modification of carbon fiber surfaces by electrochemical reduction of aryl diazonium salts: Application to carbon epoxy composites[END_REF][START_REF] Boukerma | X-ray photoelectron spectroscopy evidence for the covalent bond between an iron surface and aryl groups attached by the electrochemical reduction of diazonium salts[END_REF][START_REF] Laurentius | Diazonium-derived aryl films on gold nanoparticles: Evidence for a carbon-gold covalent bond[END_REF] with the possible presence of azo bonds inside the aromatic structure.

Indeed, aryl multilayers do not show a well-defined structure at the molecular level and they lack of ordered molecular domains. The growth of the aryl layer has been described as a nucleation process; this means that the attack of the aryl radical is kinetically more favored on an already grafted phenyl group than on the electrode surface. [START_REF] Kariuki | Nucleation and growth of functionalized aryl films on graphite electrodes[END_REF][START_REF] Laforgue | Characterization of the deposition 204 of organic molecules at the surface of gold by the electrochemical reduction of aryldiazonium cations[END_REF] During the growth of the multilayer, the transfer of electrons between the surface of the electrode and the aryldiazonium molecules in solution often becomes difficult, until no electron transfer occurs through the organic layer (See Figure 1.2a where no electron transfer was observed after the first reduction cycle of p-ethynylbenzenediazonium salt).

Most probably, the growth continues through a radical chain mechanism on the layer, although experimentally it has been observed that the organic layer thickness is limited, and the growth is therefore a self-limiting process. [START_REF] Chehimi | Aryl Diazonium Salts: New Coupling Agents in Polymer and Surface Science[END_REF] This limiting growth of the organic layers was also observed even when aryl radicals are generated in the solution,

i.e., by adding a reducing agent. However, the concurrence of side reactions like dimerization of the aryl radicals in solution, or deactivation of the aryl radicals by reaction with the solvent could explain the formation of thinner films in these conditions. [START_REF] Podvorica | Spontaneous grafting of diazoates on metals[END_REF] Very thick aryl layers (micrometer level) can be achieved by incorporating a redox active molecule to the structure. This strategy ensures the transport of electrons through the organic layer and its growth, allowing the reduction of the aryldiazonium salts in solution to continue (Scheme 1.6). For example, the grafting of aryldiazonium salts containing electroactive moieties such as nitrobenzene, anthraquinone or benzophenone led to thick conducting aryl films ranging from 20 nm to around 1 µm-thickness on carbon and gold surfaces. [START_REF] Ceccato | Using a mediating effect in the electroreduction of aryldiazonium salts to prepare conducting organic films of high thickness[END_REF] Some aryldiazonium molecules without incorporating electroactive molecules also allow the formation of very thick films. This is the case

Aryl monolayers

The electro-grafting of aryldiazonium salts routinely leads to the formation of disordered aryl multilayers. At the molecular level, the lack of a well-defined structure and the difficulties of controlling the polymerization process on the electrode surface are significant drawbacks of the electro-grafting method. The formation of well-organized monolayers is rather tricky to implement and requires strict control of the experimental conditions or the use of strategies based on molecular engineering. Thickness and surface coverage (Γ, units mol cm -2 ) are often used to characterize the monolayers.

Experimental values of surface coverage are usually estimated from CV experiments and compared with a standard molecular systems such as the 4-nitrophenyl layer with Γ = 12.5x10 -10 mol cm -2 . [START_REF] Chehimi | Aryl Diazonium Salts: New Coupling Agents in Polymer and Surface Science[END_REF] This surface coverage corresponds to a close-packed monolayer of 4-nitrophenyl molecules. Regarding the thickness, the height of a phenyl monomer is around 0.6 nm and therefore the thickness of an aryl monolayer must be around that value. [START_REF] Leroux | Efficient covalent modification of a carbon surface: Use of a silyl protecting group to form an active monolayer[END_REF] Different strategies have been proposed for controlling the thickness of aryl multilayers during the electro-grafting process. [START_REF] Combellas | Steric effects in the reaction of aryl radicals on surfaces[END_REF] Some experimental parameters were demonstrated to influence the thickness of the organic layer such as the time of electrolysis and the potential applied, [START_REF] Allongue | Phenyl layers on h-si (111) by electrochemical reduction of diazonium salts: monolayer versus multilayer formation[END_REF][START_REF] Anariba | Mono-and multilayer formation by diazonium reduction on carbon surfaces monitored with atomic force microscopy "scratching[END_REF] the nature of the electrode surface [START_REF] Adenier | Formation of polyphenylene films on metal electrodes by electrochemical reduction of benzenediazonium salts[END_REF] and the chemical nature and structure of aryldiazonium salts. [START_REF] Mattiuzzi | Electrografting of calix[4]arenediazonium salts to form versatile robust platforms for spatially controlled surface functionalization[END_REF] During the electro-grafting, the control of the time of electrolysis and the potential applied is equivalent to control the charge. For example, by controlling the charge, 4-bromophenylbenzene monolayers were directly achieved by electro-reduction of aryldiazonium salts on H-terminated Si [START_REF] Marković | Oxygen reduction reaction on Pt and Pt bimetallic surfaces: A selective review[END_REF] surfaces. [START_REF] Allongue | Phenyl layers on h-si (111) by electrochemical reduction of diazonium salts: monolayer versus multilayer formation[END_REF] Experimentally, it was observed a critical value of charge at which the polymerization process started (the system enters in a multilayer regime); this "critical charge" was observed to be dependent on the potential applied. It was found that the optimal experimental conditions were those where the potential applied was equal to the reduction peak potential of the aryldiazonium salt, and the charge was close to the critical value. Note that on carbon or metal surfaces, the layer grows following a nucleation mechanism. Therefore, a limitation of the charge or the potential applied during the electro-grafting could result in a nonhomogeneous coverage or partial modification of the surface. Hence, this is not always an adequate strategy.

The influence of the applied potential on the thickness of different aryl layers was studied by McCreery et al. [START_REF] Anariba | Mono-and multilayer formation by diazonium reduction on carbon surfaces monitored with atomic force microscopy "scratching[END_REF] Aryldiazonium salts of biphenyl (BP), nitrobiphenyl (NBP)

and nitroazobenzene (NAB) were reduced on PPF electrodes by a single voltammetric scan from + 0.4 V to 0, -0.2, -0.3, -0.4, and -0.6 V vs Ag/Ag + in an aprotic electrolyte solution. Thickness values of the resulting layers were estimated by AFM "scratching" experiments. Furthermore, all the salts were reduced by consecutive voltammetric scans from + 0.4 V to -0.6 V to investigate the effect of the time as an additional parameter (Table 1.2).

As shown in Table 1.2, the thickness increases monotonically with the cathodic potential. Usually, the time of electrolysis (or no. of cycles) is not a determinant parameter if we consider the electro-grafting as a self-limiting process. In this case, the thickness of the organic films increased with the no. of cycles until reaching multilayer values:

2.3 nm (BP), 4.2 nm (NBP) and 6.4 nm (NAB) at ten reduction cycles. In this case, the growth of the multilayers is consistent with previous publications, which reported conducting properties of these films. Thus, the electrical properties of some organic films make challenging to obtain monolayers by simple control of the experimental conditions. Other approaches to control the thickness of the organic film are the use of radical scavengers [START_REF] Menanteau | Electrografting via diazonium chemistry: from multilayer to monolayer using radical scavenger[END_REF][START_REF] Menanteau | Evidence of monolayer formation via diazonium grafting with a radical scavenger: electrochemical, AFM and XPS monitoring[END_REF] or ionic liquids as electrolyte media [START_REF] Fontaine | Ionic liquid viscosity effects on the functionalization of electrode material through the electroreduction of diazonium[END_REF][START_REF] Shul | Electrochemical functionalization of glassy carbon electrode by reduction of diazonium cations in protic ionic liquid[END_REF][START_REF] Carvalho Padilha | Functionalization of carbon materials by reduction of diazonium cations produced in situ in a brønstedt acidic ionic liquid[END_REF] during the electrografting process. Radicals scavengers are used during the reduction of the aryldiazonium salt to prevent the polymerization process, "playing" with the different kinetic that the attack of the aryl radicals shows on the electrode surface (grafting process) and the already grafted groups (polymerization process). The influence of the ionic liquids during the electro-reduction of aryldiazonium salts was investigated separately by Lacroix, Bélanger, and Lagrost. They found a correlation between the viscosity of the ionic liquid and the surface coverage of the organic layer and reported self-limiting grafting of different aryldiazonium salts with values of thickness close to that of a monolayer for higher viscosity systems. Although these two strategies have proved to be useful for controlling the growth of the organic layer during the electro-grafting, a priori the resulting surface is not sufficiently well-organized for developing, i.e., molecular electronic devices or other applications, where nano-structuring surfaces are required.

In this context, different strategies based on molecular engineering have been demonstrated to allow the formation of well-ordered nano-structured monolayers on carbon and metals by electro-grafting of aryldiazonium salts. For example, the vertical propagation of the layer can be controlled by incorporating very bulky groups on the aromatic ring in the meta-or para-position of the aryldiazonium molecule. This strategy based on a steric hindrance effect makes possible to obtain monolayers. [START_REF] Combellas | Sterically hindered diazonium salts for the grafting of a monolayer on metals[END_REF] However, the deposited layer is chemically inert (i.e., using 3,5-bis-tert-butylbenzene diazonium) and it is no longer possible to perform a post-functionalization reaction on this monolayer (Scheme 1.7).

Another strategy was developed this time allowing the post-functionalization of the monolayer. It consists in using an aryldiazonium salt having a bulky protective group, which is chemically or electrochemically cleavable after grafting. [START_REF] Leroux | Efficient covalent modification of a carbon surface: Use of a silyl protecting group to form an active monolayer[END_REF][START_REF] Malmos | Using a hydrazone-protected benzenediazonium salt to introduce a near-monolayer of benzaldehyde on glassy carbon surfaces[END_REF] Daasbjerg and co-workers first introduced cleavable groups in aryldiazonium salts. [START_REF] Nielsen | Electrochemical approach for constructing a monolayer of thiophenolates from grafted multilayers of diaryl disulfides[END_REF] Thin films of arylthiophenolate ions were prepared on carbon substrates through a two-step procedure of multilayer formation-degradation (Scheme 1.8), which consisted on the grafting of a diaryl disulfide multilayer (ArS-SAr) and its subsequent electrochemical reduction to thiophenolates (ArS -). This idea evolved into a protection-deprotection strategy. [START_REF] Malmos | Using a hydrazone-protected benzenediazonium salt to introduce a near-monolayer of benzaldehyde on glassy carbon surfaces[END_REF] Benzaldehyde near-monolayers were prepared by electro-grafting of an hydrazone-protected aryldiazonium salt on carbon. After grafting, the acid hydrolysis of the hydrazone gave rise to the monolayer. The protective group fulfills here three primary electro-grafting reaction allowing a post-functionalization of the organic layer previous deprotection step. Bartlett and co-workers [START_REF] Chrétien | Covalent tethering of organic functionality to the surface of glassy carbon electrodes by using electrochemical and solid-phase synthesis methodologies[END_REF] used this same approach for the preparation of anthraquinone monolayers on carbon.

The protection-deprotection strategy was later finely improved. [START_REF] Leroux | Efficient covalent modification of a carbon surface: Use of a silyl protecting group to form an active monolayer[END_REF][START_REF] Leroux | Nanostructured monolayers on carbon substrates prepared by electrografting of protected aryldiazonium salts[END_REF] Well-organized ethynylaryl monolayers were prepared by reduction of protected ethynyl benzenediazonium salts on carbon electrodes and subsequent deprotection of the ethynyl function. Three protective groups with increasing steric hindrance were investigated, trimethylsilyl (TMS), triethylsilyl (TES) and tri(isopropyl)silyl (TIPS). In this case, they prevent the polymerization of the aryl layer, but also allow the structuring of the surface at the nanometric level. After the deprotection step, the ethynylaryl monolayer showed a retained structure according to the size of the protective group (Scheme 1.9).

As shown in the Scheme 1.9, the deprotection gave rise to a regular distribution of this approach, i.e., carboxyphenyl monolayers [START_REF] Lee | Covalently anchored carboxyphenyl monolayer via aryldiazonium ion grafting: A well-defined reactive tether layer for on-surface chemistry[END_REF], amine-terminated monolayers [START_REF] Lee | Amine-terminated monolayers on carbon: Preparation, characterization, and coupling reactions[END_REF] and binary monolayers [START_REF] Lee | Mixed monolayer organic films via sequential electrografting from aryldiazonium ion and arylhydrazine solutions[END_REF]. Indeed, this protection-deprotection strategy allows the formation of functionalized-aryl monolayers and the control of the spatial distribution of the molecules on the electrode surface at the nanoscale level. Moreover, this method is extendable to other surfaces than carbon.

The surface nano-structuring can also be accomplished by grafting of pre-organized entities in a bottom-up approach. A recent example of surface structuring by electroreduction of a pre-designed aryldiazonium salt was published by Lacroix et al. [START_REF] Nguyen | Unprecedented self-organized monolayer of a ru (ii) complex by diazonium electroreduction[END_REF] The electro-reduction of an in situ generated diazonium salt derived from a 3D ruthenium (II) complex on ITO (iridium tin oxide) and HOPG (highly oriented pyrolytic graphite) led to a self-organized monolayer of ruthenium complex. XPS analysis confirmed the covalent bonding of the organic layer to the surfaces, and scanning tunneling microscopy (STM) evidenced a regular distribution of the molecules. Other types of pre-organized diazonium salts are molecular macrocycles such as calixarenes or cyclodextrins. [START_REF] Mattiuzzi | Electrografting of calix[4]arenediazonium salts to form versatile robust platforms for spatially controlled surface functionalization[END_REF][START_REF] Yang | The application of β-cyclodextrin derivative functionalized aligned carbon nanotubes for electrochemically dna sensing via host-guest recognition[END_REF] For example, the formation of calix [4]arene monolayers covalently attached to carbon or gold surfaces was demonstrated by electro-reduction of the corresponding aryldiazonium salt. [START_REF] Mattiuzzi | Electrografting of calix[4]arenediazonium salts to form versatile robust platforms for spatially controlled surface functionalization[END_REF] The particular structure of these macrocycles, rigid and well-defined, with the presence of nanometric cavities, have made them especially suitable for sensing applications. [START_REF] Troian-Gautier | Controlled modification of polymer surfaces through grafting of calix[4]arene-tetradiazoate salts[END_REF] Scheme 1.9: Modification of carbon surfaces by adjustable-porous ethynyl monolayers.

From "Nanostructured monolayers on carbon substrates prepared by electrografting of protected aryldiazonium salts" by

Post-functionalization procedures

Aryldiazonium salts offer a wide variety of possibilities regarding the functional groups or molecular systems that can be introduced to modify the physicochemical properties of the materials. [START_REF] Dequaire | Biotinylation of screen-printed carbon electrodes through the electrochemical reduction of the diazonium salt of p-aminobenzoyl biocytin[END_REF][START_REF] Downard | Electrochemically assisted covalent modification of carbon electrodes[END_REF][START_REF] Blankespoor | Dense monolayers of metal-chelating ligands covalently attached to carbon electrodes electrochemically and their useful application in affinity binding of histidine-tagged proteins[END_REF][START_REF] Corgier | A versatile method for direct and covalent immobilization of dna and proteins on biochips[END_REF][START_REF] Gooding | Advances in interfacial design for electrochemical biosensors and sensors: Aryl diazonium salts for modifying carbon and metal electrodes[END_REF] Aryldiazonium salts can be synthesized already bearing the chemical function, or structure that provides the properties sought for the surface. This is perfectly exemplified by the one-step fabrication of superhydrophobic metal surfaces through spontaneous grafting of 4-tetradecylbenzene diazonium cation in MeCN solution. Figure 1.4 shows contact angle images of water drops on the modified metal surface; a minimum interaction evidences its hydrophobicity. A long hydrophobic carbon chain was part of the starting aryldiazonium molecule, allowing the surface modification by one-step grafting. However, it is difficult to immobilize complex molecular systems in a single stage.

Procedures involving two or three stages are the most reported. [START_REF] Leroux | Efficient covalent modification of a carbon surface: Use of a silyl protecting group to form an active monolayer[END_REF][START_REF] Malmos | Using a hydrazone-protected benzenediazonium salt to introduce a near-monolayer of benzaldehyde on glassy carbon surfaces[END_REF] In general, they consist in two steps, the electro-grafting of the functionalized aryldiazonium salt and the chemical or electrochemical post-functionalization. In between, an additional step of deprotection could be required. Some strategies already described for building controlled molecular structures involve the use of protective groups, which mainly allow the surface nano-structuring and avoid the polymerization of the aryl layer. Pro- tective groups can also perform other functions related with the chemoselectivity in a subsequent chemical reaction or simple protect the chemical function during the electro-grafting. In this case, a third step of protection is required.

Different chemical reactions have been described for post-functionalizing aryl layers.

Savéant et al. [START_REF] Allongue | Covalent modification of carbon surfaces by aryl radicals generated from the electrochemical reduction of diazonium salts[END_REF] proposed different methodologies to obtain amino-terminated layers on carbon and their post-functionalization. For example, amino-terminated layers were achieved by electro-reduction of 4-nitrobenzene diazonium cations on carbon surfaces in aprotic solution; the nitro groups on the surface were electrochemically reduced to NH 2 by electrolysis in an aqueous protic solution, and subsequently, the amino groups were post-functionalized by chemical reaction with an epoxy derivative (Scheme 1.10). This procedure was first described for developing enhanced epoxy composite materials. [START_REF] Barbier | Electrochemical bonding of amines to carbon fiber surfaces toward improved carbon-epoxy composites[END_REF] The same two-step approach was used to obtain an electroactive layer of viologen on carbon through nucleophilic substitution on grafted 4-(chloromethyl)phenyl by the corresponding pyridine derivative. [START_REF] Holm | Nucleophilic and electrophilic displacements on covalently modified carbon: introducing 4, 4 -bipyridinium on grafted glassy carbon electrodes[END_REF] Since viologen is an electroactive compound, the modification could be easily characterized by electrochemical methods. Other reactions for post-functionalization of grafted aryl layers are i.e., the Suzuki coupling reaction, [START_REF] Coulon | Surfacemodified carbon felts: possible supports for combinatorial chemistry[END_REF] Michael addition reactions, [START_REF] Nguyen | Electrochemistry and reactivity of surface-confined catechol groups derived from diazonium reduction. bias-assisted michael addition at the solid/liquid interface[END_REF], the carboxyl-to-amine crosslinking, [START_REF] Liu | Diazonium salts: Stable monolayers on gold electrodes for sensing applications[END_REF] and click chemistry reactions. [START_REF] Evrard | Electrochemical functionalization of carbon surfaces by aromatic azide or alkyne molecules: A versatile platform for click chemistry[END_REF] Click chemistry reactions come from the idea of emulating the chemical bond in nature, which is preferentially heteroatomic. Thus, proteins, nucleic acids, or polysaccharides are the result of the condensation of small sub-units linked together by carbon-heteroatomic bonds. From here, the term "click chemistry" was coined [START_REF] Kolb | Click chemistry: Diverse chemical function from a few good reactions[END_REF] Scheme 1.10: Electrochemical reduction of nitro groups and post-functionalization with epichlorhydrin to β-hydroxyamino derivative. From "Covalent modification of carbon surfaces by aryl radicals generated from the electrochemical reduction of diazonium salts" by Savéant et al., J. Am. Chem. Soc., 119, p. 206. Copyright 1997 ACS.

and a series of criteria were established that define this set of reactions. Briefly, click chemistry reactions must be modular, wide-ranging, stereospecific with high yield and generate only harmless byproducts. Regarding the experimental conditions, the simplicity of the process, the reagents availability, the harmlessness of the solvents and the robustness of the reaction towards changes in the conditions are key factors. In this context, it was reported the copper (I)-catalyzed Huisgen 1,3-dipolar cycloaddition [START_REF] Rostovtsev | A stepwise huisgen cycloaddition process: Copper (i)-catalyzed regioselective "ligation" of azides and terminal alkynes[END_REF] as a member of the family of click chemistry reactions. It consists of the regioselective reaction of terminal alkynes (-C≡CH) and azides (-N=N + =N -) to give highly stable triazole products. The reaction is regioselective in the presence of copper (I)-catalyst, which is generated in situ from a copper (II) salt and a reducing agent, typically ascorbic acid. It is a very robust reaction that can be carried out at room temperature in water, without special precautions.

The Cu(I)-catalyzed Huisgen 1,3-dipolar cycloaddition was first introduced by Chidsey and co-workers [START_REF] Collman | clicking" functionality onto electrode surfaces[END_REF][START_REF] Collman | Mixed azide-terminated monolayers: A platform for modifying electrode surfaces[END_REF] as a general methodology for surface post-functionalization of self-assembled monolayers on gold, and later on, it was used to post-functionalize aryl layers with electroactive ferrocene moieties on carbon surfaces (Scheme 1.11

Characterization of modified surfaces

Numerous techniques of analysis (i.e. optical, spectroscopic, electrochemical or by imaging) allow the characterization of surfaces modified by electrochemical reduction of aryldiazonium salts; we discuss some of them here.

Chemical analysis of modified surfaces

X-ray photoelectron spectroscopy (XPS) is a spectroscopic technique widely used in the surface analysis since it provides quantitative information about the elemental composition and chemical and electronic state of the elements within a material. It is typically accomplished, in high vacuum or ultra-high vacuum conditions, by irradiating the sample surface with a mono-energetic beam of X-rays which produces the emission of photoelectrons that are detected in an electron energy analyzer. This technique allows surface chemical analysis to a depth of about 10 nm. Using an X-ray source, the electrons associated with each atom can be subtracted from the attraction of the nucleus and emitted in the form of photoelectrons. The binding energy they had before leaving the atom is characteristic of the emitting atom and its chemical environment.

For one chemical element, we can distinguish several contributions. An XPS spectrum is composed of peaks at different binding energies (in eV) and intensities, which varies as a function of the number of photoelectrons emitted. The processing of the data obtained allows a kind of elemental analysis of the modified surfaces. [START_REF] Hofmann | Auger-and X-ray Photoelectron Spectroscopy in Materials Science: A User-Oriented Guide[END_REF][START_REF] Van | X-ray Photoelectron Spectroscopy: An Introduction to Principles and Practices[END_REF] As example, the XPS analysis of a 4-nitrophenyl modified GC electrodes shows, prior reduction of the nitro groups on the surface, a peak at 406 eV corresponding to the nitrogen of the nitro groups; [START_REF] Gui | A comparative study of electrochemical reduction of 4-nitrophenyl covalently grafted on gold and carbon[END_REF] the presence of other nitrogen species with lower binding energy at 400 eV suggests the existence of azo groups inside the organic layer.

After electrochemical reduction of the nitro groups, many different nitrogen-derived functions are found: nitro groups, nitroso groups, hydroxilamino and amino groups.

The presence of nitro groups on 4-nitrophenyl modified surfaces can be also evidenced by polarization modulation IR reflection absorption (PMIRRAS). [START_REF] Allongue | Covalent modification of carbon surfaces by aryl radicals generated from the electrochemical reduction of diazonium salts[END_REF][START_REF] Adenier | Covalent modification of iron surfaces by electrochemical reduction of aryldiazonium salts[END_REF] The two characteristics absorption bands associated to the anti-symmetric and symmetric vibrations of the nitro group in nitrobenzene are found at 1485 and 1265 cm -1 .

Thickness of organic films

The thickness of the organic layers grafted on a surface is often estimated by spectroscopy ellipsometry (SE) or atomic force microscopy (AFM).

SE is an optical technique which allows the non-destructive characterization of thin

films by investigating the dielectric properties (complex refractive index) of the sample.

It measures changes of the polarization of the light that is reflected or transmitted from the sample and, it can be used to characterize film thickness with angstrom resolution, or i.e., determine the roughness and optical constants of materials. [START_REF] Fujiwara | Spectroscopic Ellipsometry: Principles and Applications[END_REF] AFM is a versatile and atomic-resolution type of scanning probe microscopy useful in the characterization of surface morphology and interaction forces. Images are ac-quired by raster scanning a small area of the sample with a sharp probe attached to a cantilever. Typically, this micro-cantilever can be operated in three different modes:

non-contact mode, contact mode, and tapping mode. Contact and tapping mode are the most frequently used in surface characterization. The first allows, i.e., the atomic manipulation of the sample surface in a controlled way by well setting the forces between the tip and the sample; and the second, also called semi-contact mode, keeps the probe close enough to the sample surface for acquiring high-resolution data while preventing the cantilever tip to impact the surface. [START_REF] Tsukruk | Scanning Probe Microscopy of Soft Matter: Fundamentals and Practices[END_REF] For the thickness measurement, "scratching" experiments are performed. [START_REF] Anariba | Mono-and multilayer formation by diazonium reduction on carbon surfaces monitored with atomic force microscopy "scratching[END_REF] The organic layer is intentionally destroyed by contact with the tip, which is operated in contact mode. By analyzing the AFM image, a height profile is extracted that allows the estimation of the thickness.

Compactness of organic films

Another important parameter is the compactness of the grafted layers, that means, how closely-packed is the organic layer, its homogeneity and the possible existence of pinholes or defects. The electrochemical methods are very suitable for this purpose, in particular, cyclic voltammetry. The electrochemical response of redox probes in solution is usually used to detect and monitor the formation and "quality" of aryl films on the electrode surface. Usually, the cyclic voltammetry of a redox probe in solution is recorded before and after modifying the electrode surface with the organic layer. It is important to choose the redox probe in solution carefully. In general, we can distinguish two different types of redox probes: outer-sphere redox probes and inner-sphere. The former are insensitive to most surface defects since they do not "interact" with the surface for transferring the electrons. [Ru(NH 3 ) 6 ] 3+/2+ and Fc +/0 are part of this group, their electron transfer is not inhibited by, i.e., the presence of a monolayer; the electron transfer, in this case, occurs by electron tunneling. On the contrary, inner-sphere redox probes require, as dopamine, to be adsorbed on the surface for transferring the electrons. Therefore, DA is sensitive to the surface chemistry, and a non-defective mono-and multilayer inhibit its electron transfer. If the grafted layer is not compact, the surface electrode can be "observed" through the defects, and the redox probe undergoes electron transfer. DA is a redox probe especially suitable for detecting pinholes or defects on the layers. [START_REF] Mccreery | Advanced carbon electrode materials for molecular electrochemistry[END_REF] Other electrochemical methods such as SECM can provide electrochemical information, as well as topographic information (i.e., homogeneity or existence of defects). In general, all the microscopies (AFM or STM) are useful techniques for studying the compactness of grafted aryl layers and other surface properties. [START_REF] Allongue | Phenyl layers on h-si (111) by electrochemical reduction of diazonium salts: monolayer versus multilayer formation[END_REF][START_REF] Bernard | Organic layers bonded to industrial, coinage, and noble metals through electrochemical reduction of aryldiazonium salts[END_REF] 1

.6 Conclusion

Surface modification and nano-structuring with aryl layers containing different functional groups have become increasingly popular since these functional groups could confer new or enhanced properties to the materials or can be used as anchoring points for post-functionalization procedures. In this context, the electro-reduction of aryldiazonium salts is a straightforward approach for the preparation of functional aryl layers. It allows the modification by covalent bonding of a wide range of materials such as carbon, metals or semiconductors. Unlike chemisorbed systems, covalently bonded aryl layers show good thermal stability and resistance to different organic media.

However, electro-reduction of aryldiazonium salts routinely leads to the formation of disordered aryl multilayers, and at the molecular level, they lack a well-defined structure. These main drawbacks have been faced in the last decades; different strategies have been proposed for controlling the growth of the organic layer during the electrografting and for nano-structuring the electrode surface. Some experimental parameters such as the transferred charge during the electro-grafting process influence the thickness of the organic layer, but it is not possible to prepare well-structured layers only by controlling the experimental conditions. In this direction, the introduction of cleavable bulky protective groups as substituents in meta-or para-position on the aryl ring of the aryldiazonium salt prevents the polymerization of the aryl layer, allow the surface nano-structuring and, previous deprotection step, the introduction of molecular structures. The post-functionalization opens the door to a wide number of applications: development of (bio)sensors, molecular electronic devices, photo-electroactive layers for electrochemical photosynthesis, surfaces with particular properties such as catalytic, bactericide and hydrophobic among others. Many chemical reactions allow this post-functionalization process, but undoubtedly the most attractive are the "click" chemistry reactions. They allow the modular coupling of reagents with high yield and selectivity under mild conditions. Without a doubt, the continual development of more sophisticated molecular designs is joined to the improvement of the techniques for surface characterization at the nanoscale, mainly the spectroscopies such as XPS and the surface enhanced infrared techniques and the microscopies such as AFM, STM, and SECM, to name a few.

Objectives

The review of the literature has demonstrated that aryldiazonium salts are ideal building blocks for designing surfaces with tailored properties. This is mainly due to their great structural diversity, their ability to covalently modify different conductor and semiconductor surfaces using relatively simple and fast procedures, and the progress of these modification methods towards nanometric control of the arrangement of molecules on the surface. These advances are also linked to the improvement and development of new surface characterization techniques, allowing a more accurate understanding of the molecular constructions. Thus, the level of sophistication of molecular designs and the level of knowledge related to layer fabrication have increased and, as a consequence, also the potential applications in different areas of science.

This thesis therefore focuses on aryl layers and the surface modification at the monolayer, near-monolayer and multilayer level (films of less than 10 nm-thickness) by aryldiazonium electro-reduction, when the electroactive molecule is attached to the surface or in solution, and with particular attention to the electron transfer process at the electrochemical interface between the modified surfaces (typically carbon or gold) and a condensed phase constituted by the solvent and the electrolyte. For the modification and characterization of aryl layers, and to observe the electrochemical properties of these modifications, three different electrochemical methods have mainly been used: cyclic voltammetry, electrochemical impedance spectroscopy and scanning electrochemical microscopy. This work is within the framework of the fundamental research and contributes to the study of the electron transfer through modified carbon or gold surfaces of electroactive species immobilized on the electrode surface or in solution, and to the understanding of their physicochemical properties and applications.

In this context, three are the main objectives of this thesis that are translated in three different studies, all them linked by the use of the aryldiazonium salts. First, the investigation of the electrochemical properties of the ferrocene-terminated C 11alkyl on-carbon monolayers in different solvents, and its applicability for increasing the charge density in charge storage devices such as supercapacitors. Monolayers of ferrocene derivatives attached to the electrode surfaces are very interesting systems for understanding fundamental aspects of electron transfer reactions at the interface electrode/solution, since they allow the control of different parameters such as the concentration of electroactive molecules or their organization on the electrode surface. [START_REF] Leroux | Efficient covalent modification of a carbon surface: Use of a silyl protecting group to form an active monolayer[END_REF] In particular, the condensed-phase redox reaction of ferrocene to ferrocenium in organic or aqueous solutions is especially interesting. It occurs with little or no molecular reorganization, [START_REF] Astruc | Why is ferrocene so exceptional?[END_REF] and in turns allows the investigation of the effect of the reorganization of the solvent during the electron transfer reaction, which should be a determinant factor under these conditions. [START_REF] Yang | Electron exchange between ferrocene and ferrocenium ion. Effects of solvent and of ring substitution on the rate[END_REF][START_REF] Gennett | Role of solvent reorganization dynamics in electron-transfer processes. theory-experiment comparisons for electrochemical and homogeneous electron exchange involving metallocene redox couples[END_REF] Thus, we will investigate this modification on carbon electrodes in a series of solvents covering a wide range of dielectric constants and determine the optimal reaction medium to maximize the energy storage in both the electrochemical double layer and the faradaic reaction. For this purpose, we will use two complementary methods in electrochemistry, cyclic voltammetry and electrochemical impedance spectroscopy.

Second, we are interested in evaluate how the introduction of pre-organized aryldiazonium molecules could affect the organization, structure and chemical composition of the resulting aryl layer in classical conducting surfaces such as carbon and gold, and in turns how the organic layer organization influence the electron transfer of redox probes in solution. There are only a few examples in the literature of preorganized aryldiazonium salts in which a bottom-up electrochemical approach is used for fabricating well-organized surfaces, the publications of Lagrost et al. [START_REF] Mattiuzzi | Electrografting of calix[4]arenediazonium salts to form versatile robust platforms for spatially controlled surface functionalization[END_REF][START_REF] Troian-Gautier | Controlled modification of polymer surfaces through grafting of calix[4]arene-tetradiazoate salts[END_REF] regarding the calixarenes and a recent publication of Lacroix et al. [START_REF] Nguyen | Unprecedented self-organized monolayer of a ru (ii) complex by diazonium electroreduction[END_REF] where a symmetrical heteroleptic Ru(II) complex was synthesized and deposited on surface by diazonium electro-reduction process, leading to a self-organized monolayer of ruthenium complex molecules. In this context, our purpose is the electro-grafting of the previously synthesized tetrahedral-shape preorganized Tetrakis(4-phenyldiazonium tetrafluoroborate)methane (C(ArN 2 + ) 4 ) on carbon and gold surfaces, and its physicochemical and electrochemical characterization. The study will be accomplished by cyclic voltam-metry of ferrocene and ferrocene derivatives in solution; the influence of the electrolyte in the oxidation of ferrocene will be also evaluated. For completing this investigation, a full physicochemical characterization of the aryl layer will be performed by means techniques such as XPS, AFM and SE among others.

Finally, we focus on the detection of intermediate species during oxygen reduction in aqueous media on platinum electrodes. Oxygen reduction in aqueous media is of high interest from a technological and biological point of view. The ORR is one of the most important cathodic process in fuel cells and a difficult challenge in electrocatalysis since a four-electron transformation from oxygen to water is desirable. [START_REF] Nørskov | Origin of the overpotential for oxygen reduction at a fuel-cell cathode[END_REF][START_REF] Marković | Oxygen reduction reaction on Pt and Pt bimetallic surfaces: A selective review[END_REF] Also in the body, ORR is the most important reaction in life processes such as biological respiration. Oxygen reduction takes places naturally in the body but, during oxidative stress periods in the cell, high concentrations of reactive oxygen species are generated, this species are related with inflammatory processes and cell damage but also with cell protection strategies against bacteria. [START_REF] Halliwell | Free Radicals in Biology and Medicine[END_REF] The generally admitted description of ORR in water as a simple competitive pathway process between 2-electron and 4electron reduction is often inadequate. Electron transfer steps could be accompanied by homogeneous or heterogeneous chemical steps and oxygen-oxygen bond breaking leading to possible numerous intermediates. [START_REF] Sawyer | How super is superoxide?[END_REF] This chemical reactions are usually as fast that the detection and characterization of intermediates by classical electrochemical methods remains difficult. Our aim here is the determination of intermediates during the ORR on platinum by SECM using a foot-printing strategy. [START_REF] Noël | Evidence for OH radical production during electrocatalysis of oxygen reduction on Pt surfaces: consequences and application[END_REF] This approach is based on the use of sensitive surfaces to evidence the formation of reactive oxygen species during the ORR. The role of different sensitive layers, applied potentials and electrolytes will be investigated. Much significant electron transfer reactions take place in condensed phase. [START_REF] Marcus | Electron transfers in chemistry and biology[END_REF] It is therefore important to understand how the environment affects the electrochemical properties of redox couples. An especially interesting type of condensed-phase redox reactions is the oxidation of ferrocene to ferrocenium cation in organic or aqueous electrolyte solutions. It occurs with little or no molecular structural changes, [START_REF] Yang | Electron exchange between ferrocene and ferrocenium ion. Effects of solvent and of ring substitution on the rate[END_REF][START_REF] Gennett | Role of solvent reorganization dynamics in electron-transfer processes. theory-experiment comparisons for electrochemical and homogeneous electron exchange involving metallocene redox couples[END_REF] and it is therefore expected a dominant influence of the solvent reorganization on the electron transfer properties. For example, polar solvents have been shown to have a strong influence on the properties of electrochemical systems. [START_REF] Heitele | Dynamic solvent effects on electron-transfer reactions[END_REF] This influence comes from the different solvation energy of reagents and products, and often results in changes of reaction rates of several orders of magnitude. These effects derived from the reaction conditions in the equilibrium are well established, while those derived from the non-equilibrium are not as much, i.e. the effects derived from the solvent dynamics during the reaction or how fast the solvent responds to a change in the charge distribution during the course of a reaction. Polar solvent dynamics and electron transfer reactions have been extensively studied over the past few decades. [START_REF] Zusman | Outer-sphere electron transfer in polar solvents[END_REF][START_REF] Calef | Classical solvent dynamics and electron transfer. 1. Continuum theory[END_REF][START_REF] Hynes | Outer-sphere electron-transfer reactions and frequency-dependent friction[END_REF][START_REF] Sumi | Dynamical effects in electron transfer reactions[END_REF][START_REF] Sparpaglione | What is the solvent longitudinal time scale controlling electron transfer rates?[END_REF] As far as our study is concerned and in general, we will consider that the more polar the solvent, the stronger the interaction between polar molecules and the faster the solvation response of the solvent.

From here, monolayers of ferrocene derivatives attached to the electrode surface are very interesting systems. From the earlier works by Chidsey, [START_REF] Chidsey | Coadsorption of ferrocene-terminated and unsubstituted alkanethiols on gold: electroactive self-assembled monolayers[END_REF] many different studies has been carried out concerning self-assembled monolayers on gold [START_REF] Ulman | Formation and structure of self-assembled monolayers[END_REF] and equivalent systems on carbon, [START_REF] Bélanger | Electrografting: A powerful method for surface modification[END_REF] often derived from electrografting of aryldiazonium and post-functionalization. These systems are very interesting for understanding the fundamental aspects of electron transfer reactions at the electrode/solution interface, since they allow controlling the structure on the electrode surface, [START_REF] Hapiot | Molecular nano-structuration of carbon surfaces through reductive diazonium salts grafting[END_REF] for example, the distance ferrocene-surface by the length of the alkyl chain [START_REF] Downard | Potential-dependence of self-limited films formed by reduction of aryldiazonium salts at glassy carbon electrodes[END_REF] and the ferrocene surface concentration. [START_REF] Leroux | Nanostructured monolayers on carbon substrates prepared by electrografting of protected aryldiazonium salts[END_REF]. In this type of system, electron transfer reactions involve changes in the charge of species immobilized on the organic layer. Such is the case of the electrochemical oxidation of ferrocene (non-charged species) to ferrocenium cation (positively charged species). Changes in the charge of surface species open the door to different types of electrostatic interactions, which may affect the rate or driving force of the electrochemical reaction.

The electron transfer of self-assembled ferrocene monolayers on gold are the most investigated systems concerning the influence of the dielectric environment on the electron transfer properties. [START_REF] Rowe | Redox and ion-pairing thermodynamics in selfassembled monolayers[END_REF][START_REF] Creager | Redox properties of ferrocenylalkane thiols coadsorbed with linear n-alkanethiols on polycrystalline bulk gold electrodes[END_REF][START_REF] De Long | Ionic interactions in electroactive self-assembled monolayers of ferrocene species[END_REF][START_REF] Shimazu | Electrochemical quartz crystal microbalance studies of self-assembled monolayers of 11-ferrocenyl-1-undecanethiol: structure-dependent ion-pairing and solvent uptake[END_REF][START_REF] Cruanes | Characterization of charge transfer processes in self-assembled monolayers by high-pressure electrochemical techniques[END_REF][START_REF] Viana | Redox induced orientational changes in a series of short chain ferrocenyl alkyl thiols self-assembled on gold (111) electrodes[END_REF][START_REF] Valincius | Anion effect on mediated electron transfer through ferrocene-terminated self-assembled monolayers[END_REF][START_REF] Kazakevičienė | Mediated oxidation of ascorbic acid on a homologous series of ferrocene-terminated self-assembled monolayers[END_REF][START_REF] Norman | Microcantilevers modified with ferrocene-terminated self-assembled monolayers: Effect of molecular structure and electrolyte anion on the redox-induced surface stress[END_REF][START_REF] Dionne | Redox-induced ion pairing of anionic surfactants with ferrocene-terminated self-assembled monolayers: Faradaic electrochemistry and surfactant aggregation at the monolayer /liquid interface[END_REF] The influence of the anions (from the electrolyte) on the electron transfer properties, [START_REF] Valincius | Anion effect on mediated electron transfer through ferrocene-terminated self-assembled monolayers[END_REF][START_REF] Dionne | Redox-induced ion pairing of anionic surfactants with ferrocene-terminated self-assembled monolayers: Faradaic electrochemistry and surfactant aggregation at the monolayer /liquid interface[END_REF] the changes in the structure of the monolayer, [START_REF] Viana | Redox induced orientational changes in a series of short chain ferrocenyl alkyl thiols self-assembled on gold (111) electrodes[END_REF][START_REF] Kazakevičienė | Mediated oxidation of ascorbic acid on a homologous series of ferrocene-terminated self-assembled monolayers[END_REF][START_REF] Norman | Microcantilevers modified with ferrocene-terminated self-assembled monolayers: Effect of molecular structure and electrolyte anion on the redox-induced surface stress[END_REF][START_REF] Popenoe | Infrared spectroelectrochemical characterization of ferrocene-terminated alkanethiolate monolayers at gold[END_REF] and the mass transport through the monolayer [START_REF] Shimazu | Electrochemical quartz crystal microbalance studies of self-assembled monolayers of 11-ferrocenyl-1-undecanethiol: structure-dependent ion-pairing and solvent uptake[END_REF] have been widely investigated. Ion-pairing reactions affect the rate of electron transfer reactions in solution [START_REF] Marcus | Properties of Solvents[END_REF] and onto the electrode surface [START_REF] Savéant | Effect of ion pairing on the mechanism and rate of electron transfer. Electrochemical aspects[END_REF]. The first evidence of ion pairs formation was reported during the study of electron exchange reactions between metallocenes redox couples such as ferrocene or cobaltocene; [START_REF] Yang | Electron exchange between ferrocene and ferrocenium ion. Effects of solvent and of ring substitution on the rate[END_REF][START_REF] Gennett | Role of solvent reorganization dynamics in electron-transfer processes. theory-experiment comparisons for electrochemical and homogeneous electron exchange involving metallocene redox couples[END_REF] This was attributed to the formation of ion pairs between the anions and the charged cobaltocenium cations in solution. If we switch from an homogeneous redox reaction to an heterogeneous, and from redox species in solution to redox immobilized species on the electrode surface, and try to find bibliographic references regarding studies performed in organic solvents, we would realize that they are not abundant. As we said before, the most studied systems have been the ferrocene-terminated monolayers on gold in aqueous electrolyte solutions. Despite that, a set of interesting conclusions can be derived from these experiments in aqueous solutions with SAMs, which could assist us during the discussion of our results. surface. The anion sequence investigated comprises a wide range of hydration energies in water. There is a strong influence of the anion on the electrochemical response of the system; both the oxidation peak potential of ferrocene and the current peak intensity are affected. It is also evident that the peak shape and the reversibility of the system are influenced. Thus, anions with a large hydrophobic character, which are therefore less solvated in water, give rise to narrower and sharper peaks, and the redox potential of ferrocene shifts to less positive potentials (ferrocene is more easily oxidized). In contrast, highly solvated anions in water, such as fluoride, result in wider peaks and more positive redox potentials. The reversibility of the system is affected and also the transferred charge during the electron transfer. This last point should be highlighted, the surface concentration of ferrocene depends on the anion employed.

Therefore, for less solvated anions (hexafluorophosphate, perchlorate, tetrafluoroborate and even nitrate), the surface coverage of ferrocene was between 4.2x10 -10 and 4.4x10 -10 mol cm -2 , while for anions such as chloride the value obtained was 2.7x10 -10 mol cm -2 . Authors remarked the correlation between the hydration energy of the anions in water and the surface coverage of ferrocene, which decreased with the solvation of the anion. [START_REF] Valincius | Anion effect on mediated electron transfer through ferrocene-terminated self-assembled monolayers[END_REF] The authors completed the study with the estimation of the kinetic rate constant of the ferrocene-mediated oxidation of ascorbic acid in different electro-lyte solutions. Authors reported rate constant values around 300 M -1 s -1 in solutions containing poorly solvated anions, while a value around 4000 M -1 s -1 was reported in fluoride solution, a more than 10-fold increase in the electrocatalytic rate constant. The lyte employed was 1 mol L -1 HClO 4 in aqueous solution. According to previous observations, [START_REF] Valincius | Anion effect on mediated electron transfer through ferrocene-terminated self-assembled monolayers[END_REF] when the environment of the ferrocenium cation upon oxidation allowed the formation of ion pairs with the perchlorate anion, narrower peaks were obtained and the oxidation peak potential of ferrocene shifted to smaller values, being the ferrocene easily oxidized. This was observed for the shortest chains of alkanethiols, which allowed the contact between the electrolyte solution and the ferrocene on the layer. On the contrary, when the environment of the ferrocenium cation was modified by introducing alkanethiol chains longer than that containing the ferrocenium cations, it was observed a shift of the oxidation peak potential of ferrocene to more positive values and the voltammetric peaks became broader. This shift of the redox potential to positive values was explained by the authors as a consequence of the destabilization of the ferrocenium cations upon oxidation by an increasing alkane-like environment created by the coadsorbed alkanethiol chains. In this case, the variation of redox potential of ferrocene with the concentration of electrolyte was studied. It was found a linear correlation between potential and the logarithm of the concentration of perchlorate anions in solution. This linear correlation was also observed when the electrolyte was hexafluorophosphate. Authors conclude that this linear correlation between potential and the logarithm of the concentration of electrolyte indicates a possible reaction mechanism, where the reaction proceeds via electron transfer and ion-pairing reaction between the counterion (X -) in solution and the ferrocenium cation upon oxidation.

Note that the ion-pairing interaction is electrostatic in nature, and in some cases these ionic interactions could have some covalent bonding character depending on the value of the equilibrium constant (K ip ). See Equations 2.1 and 2.2 bellow (the asterisk indicates that the species are attached to the electrode surface).

In this case, the ferrocene surface coverage was approximately the same in all cases (Figure 2.1b), around 2.4x10 -11 mol cm -2 . The most plausible explanation is that there have been no changes at the molecular level in the structure of the monolayer since the electrolyte remains the same for all experiments, and therefore no loss of charge density was observed during electron transfer. However, the kinetics of the system seems to be affected by the increase of the peak width for the most unfavourable situation, where the ferrocenium cations on the layer are surrounded by long alkyl chains. No further conclusions were provided by the authors concerning the kinetics of the system. [START_REF] Rowe | Redox and ion-pairing thermodynamics in selfassembled monolayers[END_REF] These two examples summarize well everything contained in the literature. [START_REF] Rowe | Redox and ion-pairing thermodynamics in selfassembled monolayers[END_REF][START_REF] Creager | Redox properties of ferrocenylalkane thiols coadsorbed with linear n-alkanethiols on polycrystalline bulk gold electrodes[END_REF][START_REF] De Long | Ionic interactions in electroactive self-assembled monolayers of ferrocene species[END_REF][START_REF] Shimazu | Electrochemical quartz crystal microbalance studies of self-assembled monolayers of 11-ferrocenyl-1-undecanethiol: structure-dependent ion-pairing and solvent uptake[END_REF][START_REF] Cruanes | Characterization of charge transfer processes in self-assembled monolayers by high-pressure electrochemical techniques[END_REF][START_REF] Viana | Redox induced orientational changes in a series of short chain ferrocenyl alkyl thiols self-assembled on gold (111) electrodes[END_REF][START_REF] Valincius | Anion effect on mediated electron transfer through ferrocene-terminated self-assembled monolayers[END_REF][START_REF] Kazakevičienė | Mediated oxidation of ascorbic acid on a homologous series of ferrocene-terminated self-assembled monolayers[END_REF][START_REF] Norman | Microcantilevers modified with ferrocene-terminated self-assembled monolayers: Effect of molecular structure and electrolyte anion on the redox-induced surface stress[END_REF][START_REF] Dionne | Redox-induced ion pairing of anionic surfactants with ferrocene-terminated self-assembled monolayers: Faradaic electrochemistry and surfactant aggregation at the monolayer /liquid interface[END_REF][START_REF] Jalkh | Comparative electrochemical investigations in ionic liquids and molecular solvents of a carbon surface modified by a redox monolayer[END_REF] Investigations performed coupling surface enhanced spectroscopic techniques and electrochemical quartz crystal microbalance experiments allow monitoring possible structural changes on the ferrocene-terminated organic layers such as modifications in the bonding strengths and the mass changes that occurs at the electrode surface during the electron transfer reaction. [START_REF] De Long | Ionic interactions in electroactive self-assembled monolayers of ferrocene species[END_REF][START_REF] Shimazu | Electrochemical quartz crystal microbalance studies of self-assembled monolayers of 11-ferrocenyl-1-undecanethiol: structure-dependent ion-pairing and solvent uptake[END_REF][START_REF] Kazakevičienė | Mediated oxidation of ascorbic acid on a homologous series of ferrocene-terminated self-assembled monolayers[END_REF] These type of coupling experiments on SAMs confirmed the ion pairs formation between ferrocenium cations upon oxidation and poorly solvated counterions in solution; the ion-pairing does not seem to be accompanied by incorporation of the solvent in the organic layer; these investigations also suggests that the ion pairs formation is a reversible process. The same trend regarding the effect of anions in solution was observed in viologen-terminated monolayers (SAMs), where a strong electrostatic interaction occurred between the perchlorate anions and the viologen dications that was attributed to the formation of ion pairs (See Scheme 2.2). [START_REF] De Long | Ionic interactions play a major role in determining the electrochemical behavior of self-assembling viologen monolayers[END_REF] In favorable conditions, a stoichiometric association of the anions in solution and the ferrocenium cations on the organic layer was suggested by Norman et al. [START_REF] Norman | Microcantilevers modified with ferrocene-terminated self-assembled monolayers: Effect of molecular structure and electrolyte anion on the redox-induced surface stress[END_REF] For example, in the case of hydrophobic hexafluorophosphate and perchlorate anions in aqueous solution was suggested a reversible stoichiometric ion-pairing reaction with the ferrocenium cations after oxidation.

Note how the formation of ion pairs affects the kinetics of mediated or catalyzed reactions. We have already explained that the formation of ion pairs decreases the rate constant of the ferrocene-mediated oxidation of ascorbic acid in perchlorate aqueous solutions. [START_REF] Valincius | Anion effect on mediated electron transfer through ferrocene-terminated self-assembled monolayers[END_REF] In the context of the study of electron transport through conducting polymers, a decrease in the charge transport rate was suggested by Savéant to occur as a consequence of the ion-pairing reaction between anions in solution and the fixed redox charged sites on a polymer structure, which are responsible of the charge transport through the polymer. [START_REF] Saveant | Electron hopping between localized sites: effect of ion pairing on diffusion and migration; general rate laws and steady-state responses[END_REF][START_REF] Saveant | Electron hopping between localized sites: Coupling with electroinactive counterion transport[END_REF] Environments with low values of dielectric constant or low solvating abilities favour the electrostatic interaction of opposite charged ions.

In the case of ferrocenium ions, this should result in a decrease of their free energy by formation of ionic pairs, which in turn must increase the activation energy towards electron transfer catalytic reactions.

From the literature concerning the behavior of ferrocene terminal monolayers in different electrolytic media, we can conclude that the formation of ion pairs between the ferrocenium cations upon oxidation and the anions in solution constitutes the principal explanation for the electrochemical behavior of ferrocene-terminated SAMs.

Redox active molecules for improving the charge density storage on carbon materials

Carbon materials are widely considered for electrochemical supercapacitor applications especially because of their low price, high active surfaces, and the possibility of chemical functionalization. [START_REF] Conway | Electrochemical Supercapacitors: Scientific Fundamentals and Technological Applications[END_REF][START_REF] Taberna | Electrochemical characteristics and impedance spectroscopy studies of carbon-carbon supercapacitors[END_REF][START_REF] Toupin | Performance of experimental carbon blacks in aqueous supercapacitors[END_REF][START_REF] Simon | Materials for electrochemical capacitors[END_REF][START_REF] Segalini | Qualitative electrochemical impedance spectroscopy study of ion transport into sub-nanometer carbon pores in electrochemical double layer capacitor electrodes[END_REF][START_REF] Cougnon | Impedance spectroscopy study of a catechol-modified activated carbon electrode as active material in electrochemical capacitor[END_REF] Improvements of the charge storage have been obtained by an adequate choice of the carbon material (See for example the "Simon-Gogotsi" effect related to size pore influence on the capacitance [START_REF] Simon | Materials for electrochemical capacitors[END_REF]) but also by the introduction of mineral redox active compounds or by grafting of organic molecules. [START_REF] Miller | Fundamentals of electrochemical capacitor design and operation[END_REF][START_REF] Pognon | Performance and stability of electrochemical capacitor based on anthraquinone modified activated carbon[END_REF][START_REF] Fic | Interfacial redox phenomena for enhanced aqueous supercapacitors[END_REF][START_REF] Wang | A review of electrode materials for electrochemical supercapacitors[END_REF] In an electrochemical supercapacitor, the charge storage is due both to a "real" electrochemical double layer capacitance (namely movement of ions at the carbon/solution interface) and faradaic processes. They are often qualified as pseudo-capacitor because the charge/discharge process is not purely capacitive. [START_REF] Brousse | To be or not to be pseudocapacitive?[END_REF] The obvious advantage is a considerable increase of the charge density that could be stored in the materials due to additional faradaic contributions and purely capacitive processes. More recent papers have addressed the question of the coupling between both capacitive and faradaic processes in carbon materials showing all the interest of these hybrid approaches.( [START_REF] Cougnon | Impedance spectroscopy study of a catechol-modified activated carbon electrode as active material in electrochemical capacitor[END_REF][START_REF] Brousse | To be or not to be pseudocapacitive?[END_REF] and references therein) Many different electrodes have been proposed; among the numerous possibilities of modifications, the functionalization of carbons by small redox molecules is currently the subject of large interest. [START_REF] Brousse | Grafting of quinones on carbons as active electrode materials in electrochemical capacitors[END_REF] In the literature, among the possibilities of small redox groups for modifying carbon materials, investigators often favor the use of multielectronic redox couples like quinones or catechols because each redox unit could store more than one charge per redox units. [START_REF] Naoi | Electrochemistry of poly (1,5diaminoanthraquinone) and its application in electrochemical capacitor materials[END_REF][START_REF] Kalinathan | Anthraquinone modified carbon fabric supercapacitors with improved energy and power densities[END_REF] Derivatives are often anthraquinones but several other quinones have also been considered. These redox couples are two-electron systems, which is an advantage because two electrons could be stored by redox units. However, quinones are not real 2 e -systems but are 2 e -+ 2 H + systems (or 2 e -+ 1 H + at high pH) and their electrochemistry follows complicated mechanism schemes, and their reversibility depends on solvent and proton availability. The reversibility of the system is generally only achieved in aqueous solvents and for a very specific range of pH. Even in the most favorable conditions, the global redox kinetics remains slow (see the work of E. Laviron [START_REF] Laviron | Electrochemical reactions with protonations at equilibrium: Part VIII. The 2e, 2H + reaction (nine-member square scheme) for a surface or for a heterogeneous reaction in the absence of disproportionation and dimerization reactions[END_REF][START_REF] Laviron | Electrochemical reactions with protonations at equilibrium: Part X. The kinetics of the p-benzoquinone/hydroquinone couple on a platinum electrode[END_REF][START_REF] Laviron | Electrochemical reactions with protonations at equilibrium: Part XIII. Experimental study of the homogeneous electron exchange in quinone/dihydroquinone systems[END_REF]) or at least much slower than a fast redox couple like ferrocene/ferrocenium that present little reorganization upon charge transfer, and in turn could be used in many different electrolyte solutions.

Improvements of the carbon-based supercapacitors performance have been obtained by an adequate choice of the carbon material considering a thin control of the structure and porosity of the material itself. [START_REF] Largeot | Relation between the ion size and pore size for an electric double-layer capacitor[END_REF] To keep this structural advantage, it is interesting to retain the initial structure of the carbon material and not to block or limits the transport of ions from the solution to the carbon substrate by introducing a blocking chemical functionalization. As we stated before, the faradaic process could be introduced by deposition of a thin film of an electroactive molecule by simple adsorption or grafting, but the modification should not alter the double layer capacitance to maximize the synergy between the faradaic and double layer capacitive processes. In this direction, it was recently published by Cougnon et al. [START_REF] Touzé | Improvement of electrochemical performances of catechol-based supercapacitor electrodes by tuning the redox potential via different-sized o-protected catechol diazonium salts[END_REF] a protection-deprotection strategy for a controlled grafting of cathecol moieties on a microporous carbon material for supercapacitor electrodes working in 1 mol L -1 H 2 SO 4 . The modification method is based on the spontaneous grafting of catechol aryldiazonium salts bearing protecting groups such as the bulky tri(isopropyl)silyl group. The use of protective groups allows to retain or modulate the porous structure of the material, which is modified only externally, where the catechol moieties (after deprotection step) are in turn electroactive by direct contact with the solvent containing the protons necessary for the redox process. Authors reported an increase in the specific charge and a positive shift in the redox potential which together result in a better specific energy performance of the system.

From here, we propose the evaluation of the C 11 -alkyl ferrocene-terminated monolayer on edge plane pyrolytic graphite electrodes. [START_REF] Leroux | Design of robust binary film onto carbon surface using diazonium electrochemistry[END_REF][START_REF] Leroux | Efficient covalent modification of a carbon surface: Use of a silyl protecting group to form an active monolayer[END_REF][START_REF] Leroux | Nanostructured monolayers on carbon substrates prepared by electrografting of protected aryldiazonium salts[END_REF] The general interest of this modification is the formation of a robust covalently attached ferrocene-terminated monolayer where the ferrocene molecules, in optimized conditions, exhibit fast charge transport from the carbon substrate to the redox moieties; other important features of this modification are its flexibility and the presence of size-controlled nanometric channel (pores) for ion passage, allowing the combination of the fast faradaic process with the mass transport through the organic layer, which is required for purely capacitive processes. The modification procedure is based on the electrografting of a protectedethynylaryldiazonium salt that, after deprotection step, is modified by click chemistry with an azido-ferrocene derivative. The use of the bulky tri(isopropyl)silyl group as protecting group limits the grafting process to a monolayer and allow a spatially controlled distribution of the molecules on the electrode surface. The size of the protecting group determines the size of the pores on the organic layer. The resulting ferrocene monolayer on carbon is equivalent to those obtained by adsorption of alkanethiol ferrocene derivatives on gold surface (self-assembled monolayers), with the difference that the former is attached to the carbon surface by covalent bonding. To our knowledge, there is no literature related with the use of these on-carbon pseudo "self-assembled monolayers" for energy storage purposes.

Study of electrochemical capacitors: cyclic voltametry and electrochemical impedance spectroscopy

The electrodes characteristics for electrochemical supercapacitors applications are usually investigated by cyclic voltammetry and electrochemical impedance spectroscopy.

Such combine approach has shown its interest in the investigations of modified carbon materials and they are probably the most common methods employed for determining the specific charge storage of these systems. [START_REF] Taberna | Electrochemical characteristics and impedance spectroscopy studies of carbon-carbon supercapacitors[END_REF][START_REF] Segalini | Qualitative electrochemical impedance spectroscopy study of ion transport into sub-nanometer carbon pores in electrochemical double layer capacitor electrodes[END_REF][START_REF] Cougnon | Impedance spectroscopy study of a catechol-modified activated carbon electrode as active material in electrochemical capacitor[END_REF] Cyclic voltammetry provides information corresponding to the integrated charges. If such quantities are less detailed information than the corresponding differential data extracted by EIS, they are also closer to what is required by users in real applications. [START_REF] Brousse | To be or not to be pseudocapacitive?[END_REF] Charge storage is normally expressed in terms of specific charge (in A s g -1 ), or capacitance (in F), which is usually standardized according to the surface area or mass of the active carbon material. The capacitance is directly determinable as a differential quantity from CV experiments in absence of faradaic contributions (Equation 2.3). In this way, the electrode can be understood as a RC series circuit where R is the resistance of the solution and C is the capacitance of the double layer. The magnitude of the RC product is known as the time constant (τ ), which limits the useful range of scan rates. The time constant could be understood as the charging time of the capacitor.

[162]

C = dq/dE = idt/dE = i/(dE/dt) = i/ν (2.
3)

The Equation 2.3 shows the expression to determine the differential capacitance from CV experiments in absence of electron transfer reactions. The double layer capacitance (C) is the current intensity (i) at a given potential (E) divided by the scan rate (ν).

In the case of an interfacial charging process in which the capacitance is not constant over a given voltage window, for example in the case of a redox reaction, an apparent capacitance can be determined from CV data as follows (Equation 2.

4).[143]

C = t 1 t 0 idt ∆V = ∆Q ∆V (2.4)
The integral of the current intensity (i) between the time t 0 and t 1 is the area under the curve current vs time that corresponds with the charge stored (∆Q) expressed in Coulombs (C); and the charge divided by the voltage window ∆V is the global capacitance expressed in Farads (F). This apparent capacitance contains the contribution of the faradaic reactions to the capacitance of the double layer.

Values of apparent capacitance can be also derived from impedimetric experiments.

The super-capacitance of an electrode modified with an electroactive species can be treat as an apparent capacitance that would contain all the non-capacitive terms. [START_REF] Taberna | Electrochemical characteristics and impedance spectroscopy studies of carbon-carbon supercapacitors[END_REF][START_REF] Bueno | Elucidating capacitance and resistance terms in confined electroactive molecular layers[END_REF] The apparent capacitance C(ω) is thus simply derived from the complex impedance Z(ω).

Z(ω) = (jωC(ω)) -1 (2.5)
For a real electrode, its apparent capacitance contains both a real (C'(ω)) and an imaginary (C"(ω)) part:

C(ω) = C (ω) -jC (ω) (2.6)
Note that for a pure double layer capacitance, the imaginary part is equal to zero. The imaginary part shows the non-ideality of the system in terms of complex capacitance.

The real and imaginary parts could be derived from the complex impedance by the following transformations. [START_REF] Taberna | Electrochemical characteristics and impedance spectroscopy studies of carbon-carbon supercapacitors[END_REF] C

(ω) = -Z (ω) ω|Z(ω)| 2 (2.7) C (ω) = Z (ω) ω|Z(ω)| 2 (2.8)
At low frequencies, C'(ω) corresponds to the apparent capacitance of the modified electrode, including the faradaic and non-faradaic contributions; the time constant (τ ) is experimentally derived from the characteristic frequency (ω) in rad s -1 (ω = 1/RC) that is 2π times the value of frequency (f 0 ) in Hz at the maximum of the plot C"(ω) vs frequency. Note that ω = 2πf and τ is defined in terms of ω, not f 0 . [START_REF] Conway | Electrochemical Supercapacitors: Scientific Fundamentals and Technological Applications[END_REF] We highlight this aspect since it is common in the literature not to consider the factor 2π for determining τ . [START_REF] Taberna | Electrochemical characteristics and impedance spectroscopy studies of carbon-carbon supercapacitors[END_REF] Figure 2.2 shows as example the real and complex capacitance vs frequency plots for a RC circuit, where the values of capacitance and resistance are previously known.

From the experimental impedance raw data and using the Equations 2.7 and 2.8, the real and complex values of capacitance are calculated and plotted vs frequency.

From the Figure 2.2a, it is estimated the experimental value of capacitance for the RC circuit; from the frequency at the maximum in the fication should be prevented in order to maximize the synergy between faradaic and non-faradaic processes. Procedures for a controlled modification of the carbon-based materials are available. For example, the protection-deprotection modification procedure, based on the grafting of redox active molecules bearing bulky protective groups, has been shown to be useful in preserving the double layer capacitive properties of the initial carbon material. In connection with this scheme, we propose the evaluation of on-carbon ferrocene-terminated monolayers. This electroactive monolayers show fast electron transfer rates and reversibility in a wide range of electrolytes and solvents, and size controlled nanometric access channels or pores for ion passage from the solution to the carbon surface. CV and EIS are commonly used to study the performance of new materials and modifications, and for determining the capacity for charge storage of the system. Three different methodologies are proposed in the literature where the electrode is understood as a RC series circuit, and characteristic parameters such as capacitance, resistance and time constant of the system can be determined.

Results and Discussion

Cyclic voltammetry investigations

As depicted in Scheme 2.3, the pyrolytic carbon electrodes were modified by electrografting of 4-(tri(isopropylsilyl)ethynyl)benzenediazonium tetrafluoroborate (TIPS-Eth-ArN 2 + ). After deprotection, the ethynyl layer was functionalized by click chemistry with (11-azidoundecyl)ferrocene following the previously published procedure (See Experimental Section, Preparation of modified substrates). [START_REF] Leroux | Nanostructured monolayers on carbon substrates prepared by electrografting of protected aryldiazonium salts[END_REF] At this point, we obtained a dense layer of ferrocenyl moieties that are linked to the carbon material with C 11alkyl link. Such layer could be compared with a self-assembled thiol-alkyl monolayer on gold (SAMs) but with the difference that the substrate is carbon and that the layer is attached on the surface by covalent bond. It allows a distance control between the functional groups and the density of the layer. The electrochemical behavior of the ferrocene monolayer was studied by cyclic voltammetry in each electrolyte solution at increasing scan rates (from 0.1 to 0.9 V s -1 ); a new modified electrode was prepared each time, thus avoiding a possible "memory" effect of the organic layer when is plunged into different electrolyte solutions. Cyclic voltammograms of Figure 2.3 show the response of the modified electrodes in all the organic electrolytes at scan rate 0.2 V s -1 ; the response in aqueous solution + 0.1 mol L -1 KPF 6 is a particular case and will be discuss separately towards the end of this section. Figure 2.3a shows the response in all organic electrolytes; a reversible electrochemical system is visible that corresponds to the reversible oxidation of the grafted ferrocene groups. The peak potential varies linearly with the scan rate in agreement with a grafted electrochemical system (Figures 2. which show a linear relation as expected for a surface-confined redox system. [START_REF] Savéant | Elements of Molecular and Biomolecular Electrochemistry: An Electrochemical Approach to Electron Transfer Chemistry[END_REF] In all media, the voltammograms exhibit well-defined current peaks corresponding to the reversible or quasi-reversible oxidation of the ferrocene centers. Deviations corresponding to the ohmic drop appear for the curves registered at higher scan rates, above 0.5 4a andb). This ohmic drop was more important at high scan rates in low dielectric constant solvents, such as CH 2 Cl 2 and EtOH, given that no ohmic potential drop correction was applied during the experiments. In any case, curves registered at low scan rates are not importantly affected by ohmic drop, and thus they will be considered for detailed analysis. As shown in Table 2.1, the electron transfer of the ferrocene-terminated C 11 -alkyl monolayers has been investigated in five different organic solvents containing in all cases 0.1 mol L -1 of electrolyte, thus maintaining a constant value of ionic strength in all the solutions. The influence of the solvent in an electron transfer reaction usually involves a differential solvation of reactants and products. In our case, the electron transfer involves the oxidation of a non-charged species to form a charged species (Equation 2.9, the asterisk indicates an adsorbed species). Upon oxidation, it exist the possibility of association (ion-pairing reaction) between the * Fe(Cp)

V s -1 (Figures 2.

Cyclic voltammograms on

+ PF 6 - * Fe(Cp) 2 + PF 6 - K 1 (2.10) * Fe(Cp) 2 + + ClO 4 - * Fe(Cp) 2 + ClO 4 - K 2 (2.11)
In the literature, the redox potential of ferrocene immobilized on gold surfaces was found to be dependent of the concentration of anions from the electrolyte in solution. Therefore, according to a possible mechanism of electron transfer followed by an ion-pair association between the ferroceniums and the counterions, we can derive an expression from the Nernst equation that relates the apparent standard potential (E 0 ') of ferrocene with the concentration of ion in solution as follows.

[125]

E 0 = E 0 + 2.303 RT nF log (Γ F c + ) T otal Γ F c K -2.303 RT nF log a X - (2.12)
where (Γ F c + ) Total = Γ F c + + Γ F c + X -and the ion pair formation constant (K) is derived from the Equations 2.10 or 2.11, being X -= PF 6 -or ClO 4 -and a X -the anion activity in solution.

K = Γ F c + X -/ Γ F c + a X - (2.13)
The shape and width of the voltammetric peaks for immobilized redox species are sensitive to interactions with their environment. [START_REF] Lane | Electrochemistry of chemisorbed molecules. I. Reactants connected to electrodes through olefinic substituents[END_REF][START_REF] Laviron | Surface linear potential sweep voltammetry: Equation of the peaks for a reversible reaction when interactions between the adsorbed molecules are taken into account[END_REF][START_REF] Brown | Cyclic and differential pulse voltammetric behavior of reactants confined to the electrode surface[END_REF][START_REF] Lenhard | Chemically modified electrodes. 13. Monolayer /multilayer coverage, decay kinetics, and solvent and interaction effects for ferrocenes covalently linked to platinum electrodes[END_REF] As shown in Table 2.1, all electrolyte solutions, with the exception of the anodic process in ethanol, showed both in oxidation and reduction FWHM values higher than 90.6 mV that is the theoretical value expected for a one-electron process where ferrocenes have equivalent environments and minimal interactions between them. [START_REF] Trasatti | Acquisition and analysis of fundamental parameters in the adsorption of organic substances at electrodes[END_REF][START_REF] Bard | Electrochemical Methods: Fundamentals and Applications[END_REF] Values above the theoretical one have been attributed to electrostatic effects consequence of neighbouring charged species, suggesting some heterogeneity or interaction among attached ferrocene centers. Regarding the anodic peak widths, no deviation from a Nernstian behaviour was observed in ethanol and only a small deviation was observed in CH 2 Cl 2 . On the contrary, all registered cyclic voltammetry waves in MeCN, DMSO and PC solutions are substantially broader (from 112 to 138 mV). This significantly larger anodic FWHM in MeCN, DMSO and PC reflects the predominance of repulsive interactions between the ferroceniums cations (positively charged) upon oxidation; a probable consequence of the non-ionic interaction between the ferroceniums and the anions from the electrolyte in these polar solvents.

The ion-pairing formation could explain the narrower and sharper peaks for the oxidation of ferrocene in CH 2 Cl 2 and EtOH electrolyte solutions, and the broader peaks in MeCN, DMSO and PC where the ion-pairing is not favored since the anions are highly solvated by these polar organic solvents (See ??). Also, that could explain why for MeCN, DMSO and PC electrolyte solutions the anodic and cathodic full peak widths are almost equals, only a negligible variation was observed from the anodic to the cathodic FWHM values. However, an increase of the FWHM was observed in the case of At this stage, it is interesting to analyze the transferred charge during the electron transfer by studying the ferrocene surface coverage (Γ Fc ), which was calculated from Γ = Q/nFA (where n is the number of electrons in the electron transfer, F is the Faraday constant and A the surface area of the electrode). Considering the geometrical surface area of the pyrolytic graphite electrode, we derived the surface coverage of ferrocene in each electrolyte solution. First, we observed that the ferrocene surface coverage varies with the reaction medium when it would be expected to be conserved, since there are the same electroactive centers in all cases; and second, the values of surface coverage are unusually higher than those expected for a monolayer on a flat surface.

For example, the surface coverage was 12.1x10 -10 mol cm -2 in CH 2 Cl 2 containing 0.1 mol L -1 nBu 4 NPF 6 and 13.0x10 -10 mol cm -2 in EtOH containing 0.1 mol L -1 LiClO 4 , which are about 3 times higher than the value reported by Leroux [START_REF] Leroux | Efficient covalent modification of a carbon surface: Use of a silyl protecting group to form an active monolayer[END_REF] for the ferroceneterminated C 1 -alkyl monolayer on a glassy carbon electrode in EtOH containing 0.1 mol L -1 LiClO 4 (Γ Fc = 4.4x10 -10 mol cm -2 ).

The first observation concerns the decrease of charge density or surface coverage when passing i.e., from the ethanol electrolyte solution to dimethyl sulfoxide. This latter showed a surface coverage almost three times lower than ethanol, when it would be expected the charge density to be constant since the population of ferrocenes is approximately the same between experiments. This particular feature was previously observed in equivalent systems like ferrocene-terminated monolayers on gold surface (SAMs) [START_REF] Valincius | Anion effect on mediated electron transfer through ferrocene-terminated self-assembled monolayers[END_REF][START_REF] Dionne | Redox-induced ion pairing of anionic surfactants with ferrocene-terminated self-assembled monolayers: Faradaic electrochemistry and surfactant aggregation at the monolayer /liquid interface[END_REF] and carbon. [START_REF] Jalkh | Comparative electrochemical investigations in ionic liquids and molecular solvents of a carbon surface modified by a redox monolayer[END_REF] Although currently, there is not a conclusive explanation for the decrease in charge density (or coverage) in the literature, there are some evidences that could be consider for the discussion.

Many attempts have been made to correlate the chemical composition and structure of ferrocene-terminated monolayers with their redox behavior, most of them focus on the role of the anions from the electrolyte and their ability to ion-pairing with the ferrocenium ion after oxidation. [START_REF] Valincius | Anion effect on mediated electron transfer through ferrocene-terminated self-assembled monolayers[END_REF][START_REF] Norman | Microcantilevers modified with ferrocene-terminated self-assembled monolayers: Effect of molecular structure and electrolyte anion on the redox-induced surface stress[END_REF][START_REF] Dionne | Redox-induced ion pairing of anionic surfactants with ferrocene-terminated self-assembled monolayers: Faradaic electrochemistry and surfactant aggregation at the monolayer /liquid interface[END_REF][START_REF] Popenoe | Infrared spectroelectrochemical characterization of ferrocene-terminated alkanethiolate monolayers at gold[END_REF] One experiment described in the literature is specially interesting in our discussion. [START_REF] Valincius | Anion effect on mediated electron transfer through ferrocene-terminated self-assembled monolayers[END_REF] It concerns the study of the electrochemical behavior of Fc(CH 2 ) 4 COO(CH 2 ) 9 SH coated-gold electrodes in different electrolyte solutions with anions having different solvation energies in water, when the solvent was always water. In our case, instead of to change the anion, we chose two hydrophobic anion electrolytes with similar solvation properties and ionic sizes when the organic solvent varies, from very low dielectric constant and non-polar to high dielectric and polar media. Despite the obvious differences, both experiments are comparable in general terms as both are a solvent-related anion solvation problem.

Valincius et al. [START_REF] Valincius | Anion effect on mediated electron transfer through ferrocene-terminated self-assembled monolayers[END_REF] observed a strong influence of the anion on the electrochemical response of the system; both the oxidation peak potential of ferrocene and the current peak intensity were affected by the media. It was also influenced the peak shape and the reversibility of the system (See Figure 2.1a). Anions with a large hydrophobic character, which are therefore less solvated in water, gave rise to narrower and sharper peaks, and the redox potential of ferrocene shifted to less positive potentials (ferrocene was more easily oxidized). In contrast, highly solvated anions in water, such as fluoride, resulted in wider peaks and more positive redox potentials. In turn, the surface concentration of ferrocene was dependent on the anion employed, as we observed when we change the organic solvent and in turns the solvation of the anions and the extent of the ionpairing reaction. Valincius et al. observed that the less solvated the anions are, the higher the surface concentration of ferrocene; the same thing was observed in our case.

Infrared spectroscopy experiments confirmed the formation of ion pairs between poorly solvated anions and ferrocenium ions upon oxidation; no evidence of ion pair formation was observed in solutions containing ions strongly solvated by the solvent.

Other experiments described in the literature showed that if the media remains the same (electrolyte and solvent), and the dielectric environment of the ferrocene is altered by introducing, for example, non-electroactive co-adsorbed alkyl chains around the ferrocene groups (Figure 2.1b), the ferrocene surface concentration is not affected, but the kinetics and thermodynamics of the process are affected. [START_REF] Rowe | Redox and ion-pairing thermodynamics in selfassembled monolayers[END_REF] This suggest that the loss of charge density could be related to changes at the molecular level in the structure of the monolayer, as the reaction medium varies. However, the literature does not contain definitive evidence to support these possible reorganization of the monolayer as result of the electrochemical oxidation of ferrocene. Changes in the bonding strength between the alkyl chain and the gold surface have been observed, [START_REF] Popenoe | Infrared spectroelectrochemical characterization of ferrocene-terminated alkanethiolate monolayers at gold[END_REF] the reorientation of the ferrocene molecules during the electron transfer has also been proposed, [START_REF] Viana | Redox induced orientational changes in a series of short chain ferrocenyl alkyl thiols self-assembled on gold (111) electrodes[END_REF] or even a severe restructuring of the organic layer. [START_REF] Norman | Microcantilevers modified with ferrocene-terminated self-assembled monolayers: Effect of molecular structure and electrolyte anion on the redox-induced surface stress[END_REF] Finally, a value of 0 mV is expected for the peak-to-peak separation at low scan rates, as diffusion does not play a role with regard to species adsorbed onto the electrode surface. [START_REF] Bard | Electrochemical Methods: Fundamentals and Applications[END_REF] However, experimentally this value differs from zero and increases with the scan rate. In our case, the greatest values of peak-to-peak separation correspond to the solvents with low-intermediate dielectric constants (CH 2 Cl 2 and EtOH), which are around 40 -60 mV; while high dielectric constants media showed values ranging from 15 to 25 mV, that is the case of MeCN, DMSO and PC. It is well known that solvents with low dielectric constants raise the resistance of the electrochemical cell, [START_REF] Bard | Electrochemical Methods: Fundamentals and Applications[END_REF] then peakto-peak distances larger than 0 mV are not a surprising result, which is not contrary to the conclusion that the redox species are adsorbed onto the electrode. [START_REF] Laviron | Surface linear potential sweep voltammetry: Equation of the peaks for a reversible reaction when interactions between the adsorbed molecules are taken into account[END_REF] From the experimental data in Table 2. A second experimental observation is related with the high values of surface coverage obtained for ferrocene on pyrolytic graphite electrodes (Table 2.1). As we stated before, the value reported in the literature for the ferrocene-terminated C 1 -alkyl monolayer onto a glassy carbon electrode in EtOH solution containing 0.1 mol L -1 LiClO 4 was Γ Fc = 4.4x10 -10 mol cm -2 . [START_REF] Leroux | Efficient covalent modification of a carbon surface: Use of a silyl protecting group to form an active monolayer[END_REF] The experimental value that we found on the same medium for the ferrocene-terminated C 11 -alkyl monolayer onto pyrolytic graphite was around 3 times larger than that reported. This large surface coverage is not surprising since the polished-edge plane pyrolytic graphite electrode showed a high roughness.

We modified different carbon substrates showing an increase in roughness (PPF, GC and PG), and studied the system by cyclic voltammetry in CH 2 Cl 2 solution containing 0.1 mol L -1 nBu 4 NPF 6 . AFM topographic images of bare pyrolyzed photoresist film (PPF), glassy carbon (GC) and edge plane pyrolytic graphite (PG) electrodes were registered and the roughness values corresponding to their surfaces were estimated.

Alkyl-ferrocene monolayer on different carbons

Three different carbon materials were modified to evidence the influence of the surface roughness on the ferrocene surface coverage (Γ Fc ). Pyrolyzed photoresist films were fabricated in the laboratory following previously reported procedures. [START_REF] Brooksby | Electrochemical and atomic force microscopy study of carbon surface modification via diazonium reduction in aqueous and acetonitrile solutions[END_REF] PPF is a carbon material structurally and electronically similar to GC, and unlike GC electrodes, it does not require a previous polishing process. [START_REF] Ranganathan | Photoresist-derived carbon for microelectromechanical systems and electrochemical applications[END_REF] GC electrodes are widely used in electrochemistry because of their good characteristics such as its extreme resistance to chemical attacks. Its structure has been described as a fullerene-related microstructure comprising interwoven ribbons of graphitic planes with the contribution of sp 2and sp 3 -hybridized carbon atoms. [START_REF] Harris | Fullerene-related structure of commercial glassy carbons[END_REF] Routinely in our laboratory, GC electrodes are polished with a grinding paper made in silicon carbide (grain size 5 µm) and ultrapure water. Edge plane PG electrodes and GC have some common features such as high chemical inertness and high electrical conductivity. Edge plane PG electrodes have a polycrystalline sp 2 -carbon structure consisting of a perpendicular arrangement of graphite sheets. [START_REF] Mccreery | Advanced carbon electrode materials for molecular electrochemistry[END_REF] The root mean square (rms) roughness of all bare carbon materials was estimated from their AFM topographic images (Figure 2 As shown in Table 2.2, the maximal ferrocene surface concentration was found for the modified-edge plane PG electrode (12.1x10 -10 mol cm -2 ), which showed the larger value of surface roughness. This surface coverage, as we stated before, is around 5 times larger than that expected for a perfectly flat surface such as PPF (2.3x10 -10 mol cm -2 ), and 3 times larger than the value registered at a GC electrode (4.6x10 -10 mol cm -2 ). The surface coverage obtained at the PPF is accordant with the theoretical value that can be calculated for a monolayer of ferrocene molecules on a flat surface, this value is around 2x10 -10 mol cm -2 considering a size of the protecting group (TIPS) of around 9.2 Å diameter. The surface coverage obtained at the GC electrode is in agreement with the literature (4.4x10 -10 mol cm -2 in EtOH solution + 0.1 mol L -1

LiClO 4 ). [START_REF] Leroux | Efficient covalent modification of a carbon surface: Use of a silyl protecting group to form an active monolayer[END_REF] A close-packed monolayer was obtained onto GC, and a dense layer was indeed immobilized on the pyrolytic graphite electrode. The low value of full peak width obtained at PPF indicates small interactions between the ferrocene moieties on the surface layer, being smaller than that value expected for an adsorbed system that follows the Langmuir isotherm model. We can thus conclude that the layer on PPF is not a crowded monolayer of ferrocenes, as expected considering the previous nanostructuring of the surface with the bulky protecting groups.

Further examination of the voltammetric curves shown in Figures 2.3 and 2.6 indicates that the double layer capacitance changes significantly after oxidation of the ferrocene moieties. For example, by calculating the differential capacitance from the I-E plot shown in the Figure 2.6b at a potential before and after the electron transfer, we observe at 0.15 V a capacitance of 61 µF cm -2 and at 0.60 V, 127 µF cm -2 . The double-layer capacitance was double after the reversible oxidation of all the available ferrocenes on the layer.

As we observed in The increase in the double layer capacitance after oxidation could be explained by two main reasons: first, spatial reorganization of the organic layer after oxidization or with the applied potential; and second, the stoichiometric oxidation of the available ferrocenes to ferrocenium ions on the organic layer, creating cationic sites, in turn compensated with the anions in solution. As we stated before, spectroelectrochemical analysis only showed slight variations in the bonding strength of SAMs on gold surfaces;

and no influence of the applied potential was observed by in situ IRRAS analysis on the ferrocene-terminated layers. [START_REF] Popenoe | Infrared spectroelectrochemical characterization of ferrocene-terminated alkanethiolate monolayers at gold[END_REF] On the contrary, the literature extensively associate the reversible and stoichiometric oxidation of ferrocene monolayers with the formation of ion pairs between the anions and the ferrocenium cations. [START_REF] Shimazu | Electrochemical quartz crystal microbalance studies of self-assembled monolayers of 11-ferrocenyl-1-undecanethiol: structure-dependent ion-pairing and solvent uptake[END_REF][START_REF] Viana | Electrochemical quartz crystal microbalance study of self-assembled monolayers and multilayers of ferrocenylthiol derivatives on gold[END_REF] The stoichiometric oxidation of the available ferrocenes in CH 2 Cl 2 and EtOH electrolyte solutions to ferrocenium ions could increase the double-layer capacitance by a factor of two, considering that the redox reaction is the transition from an uncharged species to another with a charge of +1. Thus, some correlations with the surface coverage and the double-layer capacitance are observed by carefully considering the data in Table 2.3. For example, the surface concentration of ferrocene increased from PPF to GC in a factor of two, and we observed an increase in a factor of two in the double-layer capacitance before and after redox reaction; and from GC to PG, the surface concentration increased in a factor of 2.6, while the capacitance increased in a factor of almost 2.2.

Discrepancies in the case of PG are expected since, as we demonstrated, the effective surface of the edge plane PG electrodes widely differs from its geometric surface area, unlike PPF and GC electrodes. On the other hand, the electrolyte solutions in which it was observed a loss in charge density (MeCN, DMSO and PC) showed capacitance ratios below two, being compatible with a partial oxidation of the ferrocenes available in the organic layer. This reinforces the hypothesis of a partial oxidation of the available ferrocenes in MeCN, DMSO and PC electrolyte solutions, and could explains the loss in charge density in these media. A partial oxidation of ferrocenes are probably derived from the possible participation of different ferrocene populations with different dielectric environments (ferroceniums associated and not associated with the anions PF 6 and ClO 4 -), being some redox units disable or disfavoured for the electron transfer.

From here, we can conclude that the stoichiometric oxidation of the available ferrocenes in CH 2 Cl 2 and EtOH electrolyte solutions, related with the ion-pairing reaction in these media probably seems responsible for the 2-fold increase in the double layer capacitance after ferrocene oxidation. Although the spatial reorganization of the layer upon oxidation is still a possibility, the hypothesis of a partial oxidation of the available ferrocenes in MeCN, DMSO and PC electrolyte solutions, probably derived from the possible participation of different ferrocene populations, ferroceniums associated and not associated with the anions, is reinforced from these results.

Asymmetry in aqueous electrolyte solution

The electrochemical behavior of the ferrocene-terminated C 11 -alkyl monolayer on pyrolytic graphite in an aqueous 0.1 mol L -1 KPF 6 electrolyte solution is independently considered since it showed a particular feature, which was not previously observed in organic electrolyte solutions. The cyclic voltammograms for the oxidation of the ferrocene-terminated layer in aqueous solution are shown in Figure 2.7a at scan rate 0.5 V s -1 , and at increasing scan rates from 0.1 to 0.9 V s -1 in Figure 2.7b. The system is immobilized in the electrode surface; a linear correlation was observed between the anodic and cathodic current peak intensities and the scan rate (Figure 2.7c).

The voltammogram of the ferrocene-terminated monolayer shows a different behavior from the forward to the backward scan in aqueous 0.1 mol L -1 KPF 6 electrolyte solution (Figure 2.7a). The anodic peak resembles a diffusion process, while the cathodic resembles the reduction of an adsorbed species with the symmetrical shape commonly observed in adsorbed redox processes. Table 2.4 lists the parameters that allow us to characterize the system in aqueous solution. We observed that the redox reaction is a quasi-reversible process with a half-wave potential of 0.35 V vs FcMeOH/FcMeOH + .

Full peak width values for both oxidation and reduction indicate strong interactions between the electroactive groups in the organic layer. If we consider that the reaction follows a mechanism of electron transfer and ion-pairing formation (Equations 2.9 and 2.10), where the electron transfer is fast (k ap s = 10 3 s -1 and α = 0.5) and the ionpairing equilibrium constant (K ip ) is around 10 -4 , and performed a cyclic voltammetry simulation only modifying the equilibrium rate of the ion-pairing reaction by adjusting the values of the forward and backward reaction rates, we observed that the same kind of asymmetry appears in the anodic scan (note that the simulation was performed in reduction) when the rate of association between the Fe(Cp) 2 + and the PF 6 -decreases (Figure 2.8). This result reinforce the hypothesis of an EC mechanism of reaction where the redox reaction is followed by an ion-pairing interaction.

Impedance spectroscopy investigations

The electroactive ferrocene-terminated C 11 -alkyl monolayer on edge plane pyrolytic graphite electrodes has been considered for charge storage applications. Our goal is to evaluate how an organized alkyl-ferrocene layer could increase the pseudo-capacitance of carbon materials. In this purpose, impedance spectroscopy investigations were performed using edge plane pyrolytic graphite electrodes (PG electrodes). Even if such material is not classical in the field of supercapacitors, they are commonly used for preparing enzymatic sensors because of their high active surface. [START_REF] Banks | Edge plane pyrolytic graphite electrodes in electroanalysis: An overview[END_REF] The edge plane PG electrodes show a roughness ranging from the micrometer to the nanometer level, and they could be considered as a reasonable model of 3D carbon material. Concerning the alkyl-electroactive monolayers, its use on carbon has not been tested before with this purpose. Ferrocene-terminated C 11 -alkyl monolayers present some important features as flexibility and channels for ion passages. Flexible organized monolayers on carbon should limit the blockage of the carbon surface, allowing the ionic transport to the electrode surface required for the "pure" double layer capacitance, while an increase of the charge density is possible by introducing electroactive molecules on the layer surface. Alkyl-ferrocene has the disadvantage of being monoelectronic but the great advantage to be fast and usable in different electrolytes. This is why we have chosen to evaluate the possibilities of alkyl-ferrocene immobilized systems.

From here, the edge plane pyrolytic graphite electrodes were modified by electrografting of 4-(tri(isopropylsilyl)ethynyl) benzenediazonium tetrafluoroborate (TIPS-Eth-ArN 2 ).

Considering the geometrical surface area of the electrode, from the integration of the faradaic peak current, we derived a surface coverage around 12 10 -10 mol cm -2 . It shows that a dense layer was indeed immobilized on the carbon surface. The corresponding EIS responses of the electrode are shown on Figure 2.9 before and after carbon modification when the solvent is CH 2 Cl 2 ; we performed the EIS experiments at the two potentials that are marked by the lines on the CV of Figure 2.9a.

EIS of the bare pyrolytic graphite electrode in CH 2 Cl 2 are shown on Figure 2.9. We performed these experiments at two different potentials that are marked by the lines on the CV: -0.08 V and 0.30 V vs the FcMeOH/FcMeOH + couple. As observed on the EIS spectra at -0.08 V vs FcMeOH/FcMeOH + , the response does not correspond to a pure capacitance behavior. (For a pure capacitance in an RC scheme, the phase-angle changes from 90 • at low frequencies to 0 • at high frequencies). [START_REF] Barsoukov | Impedance Spectroscopy: Theory, Experiment, and Applications[END_REF] This is visible on the phase variation at low frequencies or on the corresponding Nyquist plot showing the variation of the imaginary part Z" as function of the real part Z' of the complex impedance. This observation is common for carbon electrodes and is generally ascribed to the heterogeneity of the surface both in terms of topological structures and chemical groups on the surfaces. [START_REF] Cougnon | Impedance spectroscopy study of a catechol-modified activated carbon electrode as active material in electrochemical capacitor[END_REF][START_REF] Randin | Differential capacitance study on the edge orientation of pyrolytic graphite and glassy carbon electrodes[END_REF] As shown on the AFM images (Figure 2 From this experiment, it is important to note that there is no major change in the EIS response of the pyrolytic edge planes carbon functionalized electrode. One could just observe that the phase is slightly more constant at low frequency and closer to what is expected for a pure capacitor. This shows that the grafting does not modify the capacitive contribution and that the main geometric structure of the surface is kept. This observation falls in lines with the AFM images of Figure 2.10. The large roughness of the edge planes pyrolytic graphite limits the resolution but the images are similar before and after grafting. The small differences in the EIS could be ascribed to a chemical homogenization due to the modification of the surface groups by the phenyl derivatives. [START_REF] Leroux | Design of robust binary film onto carbon surface using diazonium electrochemistry[END_REF] Considering a simplified impedance RC circuit, we derived the apparent resistance and capacitance of our electrodes as R = 69 and 66 Ω cm 2 , and C = 52 and 73 µF cm -2 for the bare and functionalized PG electrodes, respectively (Tables 2.5 and 2.6).

When the same experiments are performed at 0.30 V, no change in the EIS spectra are observed for the bare PG electrode, whereas on the functionalized electrode at this potential, i.e. when the ferrocenyl groups are oxidized, a large decrease of the impedance is visible (Figure 2.9d). To quantify such variation, instead of the raw impedance data, it is convenient to treat the super-capacitance of the modified electrode as an apparent capacitance that would contain all the non-capacitive terms. [START_REF] Taberna | Electrochemical characteristics and impedance spectroscopy studies of carbon-carbon supercapacitors[END_REF][START_REF] Cougnon | Impedance spectroscopy study of a catechol-modified activated carbon electrode as active material in electrochemical capacitor[END_REF][START_REF] Bueno | Elucidating capacitance and resistance terms in confined electroactive molecular layers[END_REF] The apparent capacitance C(ω) is thus simply derived from the complex impedance as Z(ω) = (jωC(ω)) -1 . For a real electrode, its apparent capacitance contains both a real and an imaginary parts: C(ω) = C'(ω) -jC"(ω). Note that for a pure capacitance, the imaginary part is equal to zero. The imaginary part shows the non-ideality of the system in terms of complex capacitance. The real and imaginary parts could be derived from the complex impedance by the Equations 2.7 and 2.8. By analogy with the experiments performed with modified electrodes, the impedance data of the nonmodified electrodes were recorded at the same fixed potentials, one around 0 V, and another around the potential at which the ferrocene ET takes place on modified electrodes and different electrolytes.

95

Variations of the apparent capacitance are shown on Figure 2.11 for the two applied potentials. As first observation, a large increase of the real part capacitance occurs at frequency around 2 Hz when the ferrocenyl moieties are oxidized (i.e. at E applied = 0.30 V). On the contrary, the non-faradic process (at E applied = -0.08 V) tends to be less frequency-dependent but still shows a small increase around 30 Hz. Basically, the system oscillates between two states; it shows a capacitor-like behavior at low frequencies and a resistor at high frequencies, in between, it behaves like an R-C circuit. Figure 2.11b presents the evolution of the complex capacitance vs frequency; the complex capacitance goes through a maximum at the frequency point defined as characteristic frequency, ω (ω = 1/RC) or time constant, τ (τ = RC). [START_REF] Conway | Electrochemical Supercapacitors: Scientific Fundamentals and Technological Applications[END_REF][START_REF] Evans | Differential capacity measurements at carbon electrodes[END_REF] Results of Figure 2.11 are very similar to the EIS spectra reported for a well-organized ferrocene-undecanethiol film deposited on gold (frequency and half peak width) [START_REF] Bueno | Capacitance spectroscopy: A versatile approach to resolving the redox density of states and kinetics in redox-active self-assembled monolayers[END_REF] confirming the analogy of the obtained layer on pyrolytic carbon and a self-assembled monolayer on gold. As C(ω) is a complex function, it could also be presented as a Nyquist plot by analogy with what is done for the impedance Z(ω). Such plots provide a better estimation of the capacitance by a simple fitting of the equivalently generated Nyquist plot with a semi-circle variation. Figure 2.12 shows the corresponding Nyquist capacitive plots, without (Figure 2.12a) and with the contribution of the faradaic process to the capacitance (Figure 2.12b). The value of capacitance is thus given by the diameter of the semicircle of the Nyquist capacitive spectrum and is estimated by a semi-circle fitting as seen on the full lines. From these experiments, we derived a capacitance of 73 µF cm -2 at -0.08 V with a characteristic time τ = 0.0048 s and a much higher capacitance of 1475 µF cm -2 at + 0.30 V and a corresponding characteristic time of τ = 0.097 s (Table 2.6). R is obtained from the magnitude of Z at high frequency (see Figure 2.9d) and does not appreciably change with or without the faradic contribution (R = 66 Ω cm 2 ). In other words, τ varies linearly with the apparent capacitance measured at -0.08 V and + 0.30 V respectively. It means that in this electrolyte (CH 2 Cl 2 ), the charging time of the modified electrode is indeed controlled by the resistance at the carbon materials/solution interface and not by the electron transfers kinetics to the ferrocene moieties that is much faster, and thus that energy efficiency at short times depends on the electrolyte. This conclusion falls in line with fast electrochemical measurement concerning long alkyl-ferrocene-thiol layer on gold for which the rate constants of the electron transfer are in the 10 6 s -1 range. [START_REF] Amatore | Ultrafast voltammetry for probing interfacial electron transfer in molecular wires[END_REF] To get more insights on this limitation due to the media, we performed detailed electrochemical studies of the functionalized electrode in different common electrolytes.

The impedances variations are shown on Figure 2.13 and the derived capacitances and characteristics times on Table 2.6 for the ferrocene-terminated C 11 -alkyl monolayer modified-edge plane PG electrode in different 0.1 mol L -1 electrolytes. For all media, the modified electrode/electrolyte behaves as a RC circuit. Similarly to the previous data obtained when the modified electrode is studied in CH 2 Cl 2 , R is not affected by the presence of the faradaic process indicating that the charging time is controlled by the ohmic drop and not by the charge transfer kinetics. It is also noticeable that besides the variations of R, the apparent capacitance when the faradaic process is involved changes a lot with the electrolyte. In a practical view, ethanol or acetonitrile could be seen as the best compromise between the largest capacitance and the shortest charging times. In these electrolytes, the capacitance is only a bit smaller, no more than 30 % smaller than in CH 2 Cl 2 , but with a much lower charging time. If water is required for the application, the modification is still valuable as it permits a large increase of the charge density with a short response time.

Finally, it is useful to compare the different methodologies for characterizing the modified electrode. In the case of an interfacial charging process in which the capacitance is not constant over a given voltage window, the global capacitance could be estimated from cyclic voltammetry data as it was previously proposed (see the Introduction for details about the different methodologies for determining the capacitance). [START_REF] Brousse | To be or not to be pseudocapacitive?[END_REF] The effective capacitance is calculated from the integral of the curve current vs time that is the area under the curve that corresponds with the charge stored, and the charge is simply divided by the voltage variation to derive the capacitance. Integration was made on the whole scan from -0.1 to 0.6 V vs the FcMeOH/FcMeOH + couple (∆E = 0.7 V). This value provides an efficient storage capability of the modified carbon.

The derived data for bare and modified PG electrodes are reported in Table 2.5 and 2.6 and are obviously lower than the values derived by EIS but show the interest of the modifications that indeed increase the effective charge density storage.

Concerning the efficiency of ferrocene-terminated C 11 -alkyl monolayer for improving the pseudo-capacity of the material, it is useful to compare the value of 2.6 estimated by CV with related literature data where small redox molecules are used for improving the pseudo-capacitance. A simple way to make comparisons is by considering the increase of capacitance before and after modification. Indeed, the characteristics of pseudocapacitor are generally reported in capacitance per mass (F g -1 ) or charge per mass unit as our results are reported in capacitance per surface unit (F cm -2 ), which is adapted to a planned electrode. After the grafting of the alkyl-ferrocene layers and optimization of the electrolyte/solvent, we observed an increase of 6.6 in EtOH and 7.8 of the average pseudo-capacitance. By comparison, the best reported increases after the introduction of small redox molecules like quinones on pristine carbons materials are around 2 -2.5. [START_REF] Brousse | Grafting of quinones on carbons as active electrode materials in electrochemical capacitors[END_REF] The comparative large increase obtained with alkyl-ferrocene monolayer arises from the structure and the electrochemical characteristics of alkyl-ferrocene but also because such monolayer is compatible with a large choice of electrolytes and solvents.

Indeed as seen on 2.6, the faradaic pseudo-capacitance in water is around half of those in CH 2 Cl 2 or EtOH showing the interest of an optimization of the solvent. At this stage, the origin of this variation of the pseudo-capacitance concerns mainly the faradaic process as the double layer does change in a much lower extend. We could ascribe this variation of the pseudo-capacitance to a differential solvation of the electrolyte anions in the organic solvents, which produces variations in the extent of the ion-pairing reaction that follows the ferrocene electron transfer in a possible EC mechanism of reaction.

As already reported in the literature, the introduction of a redox active monolayer on a carbon capacitor is a convenient way for improving the global charge density of these materials. In this field, we show the interest of using an organized long chain alkyllayer with a fast charge transfer redox moieties like ferrocene for increasing the charge capacity in active carbon materials. After modification and optimization of the solvent, we observed a considerable increase of the charge density storage by a ratio around 7 which is higher than most of the reports of literature. Of course, such results still need to be tested with other carbon materials but they already show the potential interest of such monolayer in charge storage application. As for a "classical" capacitance, the characteristic time of the layer depends on the electrolyte, since the charge transfer kinetics from the carbon to the active alkyl-ferrocene moiety is very fast and not a limiting factor. Moreover, we highlight the importance of the solvent not only for the RC time but also for maximizing the value of the faradaic charge. Because of the versatility of the carbon modification process, similar alkyl-redox monolayers could be considered for the modification of many different types of nano-structured carbon as those presenting a large specific surface and controlled porosity. Redox groups other than ferrocene, as those presenting higher electronic stoichiometry could also been used but at the condition that their global charge transfer kinetics remains fast and allows a large choice of solvents.

of a 3 mm-diameter edge-plane pyrolytic graphite (PG) or a 2 mm-diameter glassy carbon (GC) disk electrodes as working electrode, a platinum wire as the auxiliary electrode, and a saturated calomel electrode (SCE, KCl sat.) as the reference. The latter was isolated from the working solution employing a salt bridge, which contained the electrolyte solution. When necessary, a conventional gas inlet was used for bubbling argon inside the electrochemical cell to displace oxygen, which is chemically reactive and electrochemically reducible. Occasionally, low roughness and flat carbon surfaces were required as working electrodes. In this case, pyrolyzed photoresist films (PPF) were homemade fabricated according to described methods [START_REF] Del Campo | Fabrication of PPF electrodes by a rapid thermal process[END_REF]. Since the larger surface area of these flat working electrodes, a platinum mesh was used as the counter electrode instead of a platinum wire.

Methodology

Cyclic voltammetry and impedance measurements. I-E plots of bare and (11azidoundecyl)ferrocene modified-edge plane PG electrodes were registered at increasing scan rates from 0.1 to 0.9 V s -1 in different electrolyte solutions: 0.1 mol L -1 nBu 4 NPF 6 in CH 2 Cl 2 , MeCN, DMSO and PC, 0.1 mol L -1 LiClO 4 in EtOH, and 0.1 mol L -1 KPF 6 in water. Fresh modified-PG electrodes were used for each electrolyte, avoiding a possible "memory" effect of the organic layer. The experimental sequence followed for each electrolyte solution was: modification of the carbon surface (PPF, GC or edge-plane PG electrodes), voltammetric study, and impedimetric study. This sequence allowed the correlation of the data extracted from CV and EIS. At the end of each experiment, the voltammogram of the immobilized system was registered in CH 2 Cl 2 + 0.1 mol L -1 nBu 4 NPF 6 as reference, and FcMeOH was added to the studied electrolyte solution to standardize the potential according to the FcMeOH/FcMeOH + couple. All voltammograms were registered without ohmic drop correction. From the I-E plots, since the modification is a surface immobilized system, it must follow the Langmuir isotherm model. [START_REF] Laviron | Surface linear potential sweep voltammetry: Equation of the peaks for a reversible reaction when interactions between the adsorbed molecules are taken into account[END_REF][START_REF] Bard | Electrochemical Methods: Fundamentals and Applications[END_REF] The electrochemical parameters extracted from the I-E curves were: E pa , E pc , I pa , I pc , anodic and cathodic FWMH (full width at half maximum) and ∆E p . Other parameters calculated from the experimental data were the surface coverage of ferrocene (Γ Fc ), and the half-wave potential (E 1/2 ). The reversibility of the system was studied through the ratio of the current peak intensities (I pa /I pc ). The characteristic kinetic parameters, the apparent electron transfer rate constant (k ap s ) and the electron transfer coefficient (α) of the ferrocene electron transfer could not be determined. Electrochemical impedance measurements were performed, after cyclic voltammetry experiments, in each electrolyte solution when the working electrode was a bare edge-plane PG or an (11-azidoundecyl)ferrocene modified-edge plane PG electrode. The ac frequencies for impedance experiments ranged from 10 kHz to 0.01 Hz, with an amplitude of ±10 mV. Lissajous curves were monitored at each frequency to ensure that no changes in the linearity of the circuit occur during the experiment.

Preparation of modified substrates

• Electrochemical grafting of the protected aryldiazonium salt.

Surface modification of all working electrodes (PPF, GC and edge-plane PG electrodes) was performed by cyclic voltammetry (five consecutive cycles from + 0.6 to -0.8 V vs SCE at scan rate 0.05 V s -1 ,) in an acetonitrile solution of 10 -2 mol L -1 aryldiazonium salt TIPS-Eth-ArN 2 + + 0.1 mol L -1 nBu 4 NPF 6 . After grafting, the electrodes were thoroughly rinsed with acetone and dried under argon. Figure 2.14 shows the voltammograms for the electrochemical grafting of the aryldiazonium salt TIPS-Eth-ArN 2 + . We observed the usual electrochemical response; an irreversible reduction of the aryldiazonium salt on carbon surface and the inhibition of the electrochemical signal after the first reduction cycle.

At this point, a well-structured monolayer of protected ethynylaryl groups is attached to the carbon substrate by covalent bond. • Deprotection of the ethynyl function.

The deprotection of the ethynyl function was performed by immersion of the grafted carbon electrodes in a solution of tetrahydrofuran (THF) containing 0.05 mol L -1 of tetrabutylammonium fluoride (TBAF). [START_REF] Green | Protective Groups in Organic Synthesis[END_REF] The electrodes were kept in this solution around 20 minutes under vigorous stirring at room temperature.

It is important to use small stirrers since they can approach the electrode surface enough, and guaranty a continuous flux of the reactive solution.

• Click chemistry.

After deprotection, the ethynylaryl monolayer onto the carbon surface was postfunctionalized by Cu(I)-catalyzed Huisgen 1,3-dipolar cycloaddition with (11azidoundecyl)ferrocene. The ethynylaryl modified-electrodes were immersed in a 1:1 water:THF solution of CuSO 4 • 5 H 2 O (10 -2 mol L -1 ) and L(+)-ascorbic acid (2x10 -2 mol L -1 ) and vigorous stirred in presence of (11-azidoundecyl) ferrocene (1 mg = 1 µL) during 1h. [START_REF] Leroux | Efficient covalent modification of a carbon surface: Use of a silyl protecting group to form an active monolayer[END_REF] Before adding the ascorbic acid, the oxygen should be removed from the reaction cell by bubbling argon during around 15 -20 min.

After click-chemistry, the alkyl-ferrocene modified electrodes were stirred in pure tetrahydrofuran to remove any residue from the reaction mixture. dichloromethane electrolyte solution. In yellow the integration limits used to calculate the area between the baseline fixed at 0 current and the anodic curve. The potential window (∆E) was 0.7 V for all electrolyte solutions. Origin software (version 7.5) was used to integrate all current vs time plots.

From EIS experiments

The real (C') and complex (C") capacitances were determined from the impedance raw data according to the Equations 2.7 and 2.8, and standardized using the geometrical surface area of the carbon electrode (0.071 cm 2 for the edge plane pyrolytic graphite electrode). The global apparent capacitance was estimated from the diameter of the semicircle of the resulting curve fitting in the Nyquist capacitive plot (Figure 2.16a); the time constant was derived from the characteristic frequency at the maximum of the plot complex capacitance vs frequency (Figure 2.16b). Figure 2.15b shows the two potentials at which the impedance raw data were acquired in dichloromethane electrolyte solution, -0.08 V and + 0.30 V vs FcMeOH/FcMeOH + couple.

Nyquist capacitive plots were fitted using the Equation 2.14, which is the equation of the circle centre (x0, y0) and radius r; complex capacitance vs frequency plots were fitted using the log-normal distribution included in the ORIGIN 7.5 equation database.

y = y0 + [r 2 -(x -x0) 2 ] 1/2
(2.14) techniques. [START_REF] De Long | Ionic interactions in electroactive self-assembled monolayers of ferrocene species[END_REF][START_REF] Kazakevičienė | Mediated oxidation of ascorbic acid on a homologous series of ferrocene-terminated self-assembled monolayers[END_REF][START_REF] Norman | Microcantilevers modified with ferrocene-terminated self-assembled monolayers: Effect of molecular structure and electrolyte anion on the redox-induced surface stress[END_REF][START_REF] Popenoe | Infrared spectroelectrochemical characterization of ferrocene-terminated alkanethiolate monolayers at gold[END_REF] In general, the formation of ion pairs is favored in an environment in which the anions are poorly solvated, i.e., in solvents with low values of dielectric constant. [START_REF] Reichardt | Solvents and Solvent Effects in Organic Chemistry[END_REF] The association of the counterions with the ferrocenium ions upon oxidation have been demonstrated to stabilize the cation, therefore displa-cing the redox potential of ferrocene to lower values. [START_REF] Rowe | Redox and ion-pairing thermodynamics in selfassembled monolayers[END_REF] Additionally, it was found a linear correlation between the redox potential of ferrocene and the logarithm of the concentration of PF 6 -and ClO 4 -in solution, which suggest a possible reaction process via electron transfer and ion-pairing reaction between the anions in solution and the ferrocenium cations upon oxidation. [START_REF] Valincius | Anion effect on mediated electron transfer through ferrocene-terminated self-assembled monolayers[END_REF][START_REF] Dionne | Redox-induced ion pairing of anionic surfactants with ferrocene-terminated self-assembled monolayers: Faradaic electrochemistry and surfactant aggregation at the monolayer /liquid interface[END_REF][START_REF] Savéant | Effect of ion pairing on the mechanism and rate of electron transfer. Electrochemical aspects[END_REF] From the experimental data: From the impedimetric investigations, we can conclude that the introduction of a redox active monolayer on a carbon capacitor is a convenient way for improving the global charge density of these materials. After grafting of the alkyl-ferrocene layers and optimization of the electrolyte/solvent, we observed a considerable increase of the charge density storage by a ratio around 7 which is higher than most of the reports of literature. [START_REF] Brousse | Grafting of quinones on carbons as active electrode materials in electrochemical capacitors[END_REF] Indeed, such results are limited by the carbon material employed, which shows a much lower contribution to the double layer capacitance than those porous carbon-based materials commonly used in real applications. Therefore, this modification still need to be tested with other carbon materials to really show the potential interest of such monolayer in charge storage applications. The most significant outcome of this work is that the dynamics of the charge storage process is governed by the internal series resistance of the cell and not by the kinetic of the faradaic process, since the charge transfer kinetics from the carbon to the active alkyl-ferrocene moiety is very fast and not a limiting factor. Moreover, we highlight the importance of the solvent not only for the RC time but also for maximizing the value of the faradaic charge.

Chapter 3

Molecular sieving and current rectification properties of thin organic films.

Abstract

A bottom-up electrochemical approach for fabricating well-organized surfaces was developed. Carbon and polycrystalline gold substrates were modified by electroreduction of a tetrahedral-shape preorganized aryldiazonium salt resulting in an ultrathin organic film. A complete physicochemical characterization of the modified surfaces was performed using cyclic voltammetry (CV), X-Ray Photoelectron Spectroscopy (XPS), Atomic Force Microscopy (AFM), Infrared (IR) spectroscopy and ellipsometry. The tetrahedral molecular geometry of the aryldiazonium molecule led to the formation of an organic film which showed a retained structure capable of molecular sieving and rectifying the current towards the electron transfer of redox probes in solutions.

keywords: molecular sieving, current rectification, molecular electronics, aryldiazonium salts, ultrathin films, nano-electrochemistry Preorganized aryldiazonium salt and its electrochemical behavior

Background

Aryldiazonium salts are ideal building blocks for designing surfaces with tailored properties since their almost infinite structural diversity and their electrochemically-assisted bonding ability to modified conducting substrates. [23-30, 80, 123] One paradigm in preorganized aryldiazonium salts is the quad molecule calixarene (Figure 3.1). Its particular geometry allows a fine spatial control during the electrografting process and leads to stable and closely packed monolayers, which can be post-functionalized with different purposes. [START_REF] Mattiuzzi | Electrografting of calix[4]arenediazonium salts to form versatile robust platforms for spatially controlled surface functionalization[END_REF] In this direction, the ability to control the local topology of electrode surfaces by introducing aryldiazonium pre-structured molecules offers the possibility of determining how geometric factors affect the electron transfer of redox probes in solution or the ionic molecular transport through the organic layer. Thus, the raised interest in molecular surface nanostructuring lies not only in its potential applications such as biosensors or molecular electronics, but also in the framework of the fundamental research. For example, the developing of controlled molecular structures at the electrode surface allows the modulation of the interfacial properties such as mass transport that in turn are linked to the electron transfer properties. Therefore, by modulating the surface properties we can deepen the knowledge of interfacial phe- nomena and electronic transfer mechanisms. From this point of view, electrochemical systems with molecular sieving and current rectification properties are of high interest.

Electrochemical current rectifiers (ECR) were originally reported by Murray et al. [START_REF] Abruña | Rectifying interfaces using two-layer films of electrochemically polymerized vinylpyridine and vinylbipyridine complexes of ruthenium and iron on electrodes[END_REF]. Based on the simultaneous and sequential electrochemical polymerization of two ruthenium and iron complexes on platinum, the films generated in a two-step process showed a diode-like behavior while the I-E response of the copolymer, simultaneously deposited on the electrode, was the addition of the two redox components. The twostep process led to a two-layer film in which the oxidation state of the outer layer was electron-transfer-dependent on the inner polymer. In this way, even though neither the starting electroactive monomer nor the electrode material were individually a semiconductor, the modified electrode showed rectifying properties.

ECR consist of surface-confined electron transfer mediators, redox molecules, allowing unidirectional current flow through the interface. [START_REF] Liu | Rectification behaviors based on redox-active molecular systems[END_REF] Before reviewing the different approaches reported in the literature regarding ECR systems, it is convenient to clarify that redox-active polymers constitute a broad field that falls into at least three different polymer categories, π-conjugated electronically conducting polymers, ion-exchange polymers, and redox-linked polymers. All of them share the ability to pass electrical current, and they fundamentally differ in how the electron conduction takes place.

Conducting polymers yields electronic properties similar to those observed in metals, where the electrons are delocalized in the conducting band. On the contrary, electron transport in redox polymers occurs through the redox centers, and the driving force behind the "diffusion" of electrons from the electrode surface to the film-solution interface is the concentration of oxidized and reduced redox sites, which are highly influenced by the ion motion inside the polymeric structure. All cases have been extensively covered in the literature. [START_REF] Andrieux | Catalysis of electrochemical reactions at redox polymer electrodes. Kinetic model for stationary voltammetric techniques[END_REF][START_REF] Chidsey | Electroactive polymers and macromolecular electronics[END_REF][START_REF] Mao | In situ measurement of the conductivity of polypyrrole and poly[1-methyl-3-(pyrrol-1-ylmethyl)pyridinium] + as a function of potential by mediated voltammetry. Redox conduction or electronic conduction?[END_REF][START_REF] Mao | Polypyrrole-based anion-exchange polymers[END_REF][START_REF] Teasdale | Conductive Electroactive Polymers: Intelligent Polymer Systems[END_REF] Since the earliest works on electroactive polymers, an extensive number of publications reporting different strategies to realize ECR appeared. Examples include electrochemical rectification on redox-active self-assembled monolayers (SAMs) including ferrocene terminated-monolayers [START_REF] Alleman | Electrochemical rectification at a monolayer-modified electrode[END_REF], gold nanoparticles [START_REF] Shein | Formation of efficient electron transfer pathways by adsorbing gold nanoparticles to self-assembled monolayer modified electrodes[END_REF], the more complex approach of incorporating ferrocenyl dendrimers as redox mediators for current rectification towards redox species in solution [START_REF] Liu | Rectification behaviors based on redox-active molecular systems[END_REF][START_REF] Oh | Electrochemical rectification using mixed monolayers of redox-active ferrocenyl dendrimers and n-alkanethiols[END_REF], or electroactive biological molecules [START_REF] Liu | Electrochemical current rectification at bio-functionalized electrodes[END_REF],

to name a few. For example, the latter consisted in the bio-functionalization of an undecanethiol modified-gold electrode by physisorption of a hemoprotein, which made possible the current rectification by inhibition of the direct electron transfer between the redox probe (ferro/ferricyanide) in solution and the surface of the electrode. An EC mechanism describes the charge transfer between the redox centers of the protein and the redox probe in solution that involves an outer-sphere electron transfer (3.1) and a bimolecular chemical reaction (3.2).

Fe 3+ (hemin) + e - Fe 2+ (heme) E (3.1)

Fe 2+ (heme) + Fe 3+ (ferri) → Fe 2+ (ferro) + Fe 3+ (hemin) C (3.2)
This is one of the common schemes for ECR design. Organic layers deposited on conducting substrates to prevent the direct access of the redox probe in solution to the electrode surface, and an electrode-confined electroactive molecule that mediates the electron transfer between the redox probe and the electrode. The direction in which the current flows is given by the relative potentials of the mediator and the redox probe, and the electron transfer between the surface of the electrode and the mediator occurs by electron tunneling through the organic layer.

Despite the versatility and robustness of the aryldiazonium modifications, only very few examples of ECR fabricated by electro-reduction of aryldiazonium salts were demonstrated. Indeed, the reduction of classical aryldiazonium ions, as the most commonly used p-nitrophenyl diazonium ion, leads to the formation of multilayered dendritic in-sulating films which limits the mechanism of electron transfer by tunneling [START_REF] Pinson | Attachment of organic layers to conductive or semiconductive surfaces by reduction of diazonium salts[END_REF][START_REF] Saby | Electrochemical modification of glassy carbon electrode using aromatic diazonium salts. 1. Blocking 202 effect of 4-Nitrophenyl and 4-Carboxyphenyl groups[END_REF][START_REF] Gui | A comparative study of electrochemical reduction of 4-nitrophenyl covalently grafted on gold and carbon[END_REF].

However, the reduction of specifically designed aryldiazonium salts, such as biphenyl or nitrobiphenyl, was first reported by McCreery et al. [START_REF] Solak | Modified carbon surfaces as "organic electrodes" that exhibit conductance switching[END_REF] and further demonstrated by Lacroix et al. [START_REF] Fave | Tunable electrochemical switches based on ultrathin organic films[END_REF][START_REF] Fave | Electrochemical switches based on ultrathin organic films: From diodelike behavior to charge transfer transparency[END_REF] incorporating, in this case, redox units of thiophene to yield organic films having ECR properties.

Ultrathin films (around 5 nm thickness) of two different thiophene derivatives, 1-(2bisthienyl)benzene (BTB) and 1-(2-thienyl)benzene (TB), were achieved by grafting of their corresponding aryldiazonium salts. [START_REF] Fave | Tunable electrochemical switches based on ultrathin organic films[END_REF] Studies carried out by CV showed an electrochemical response according to a diode-like behavior towards ferrocene and decamethylferrocene in solution (ferrocene is shown in Figure 3.2a). No electrochemical signal was observed in the range of potential where ferrocene is classically reversibly oxidized on a bare electrode. However, at the potential where charges are injected into the organic films, or in other words, the potential at which the films are oxidized, the irreversible oxidation of ferrocene was observed. Further studies about the electrochemical behavior of the two thiophene films were performed with two different redox probes, 1,4-amino-diphenylamine and thianthrene.

The former shows two reversible anodic peaks, at 0.35 and 0.83 V (Figure 3.2b) and the latter, one reversible anodic peak at 1.13 V on a bare glassy carbon electrode. [START_REF] Fave | Electrochemical switches based on ultrathin organic films: From diodelike behavior to charge transfer transparency[END_REF] It was found that thiophene films behave as current rectifiers towards redox probes in solution when the standard potential of the redox probe (E 0 ) is lower than the standard potential of the redox film (E 0 f ilm ), such was the case with ferrocene and decamethylferrocene. On the contrary, if E 0 > E 0 f ilm , the rectification feature disappears and the redox probes behave as expected on a bare electrode. Such was the case of thianthrene, which showed a reversible electron transfer both on modified and bare glassy carbon electrodes. Regarding the electron transfer mechanism, electron tunneling or electron transfer through pinholes in the organic films were discarded since the thickness of the organic films were around 4-5 nm, and images registered by AFM and SECM showed defect-free densely packed structures. Hence, authors state that the changes in the conducting properties observed throughout the experiments suit well to a model based on conducting polymers, since conducting polymers can switch their conductivity upon charge injection.

Molecular sieves and current rectifiers are two interlinked fields since it is possible to rectify currents by controlling the permeability of ions across the interfaces, with a physical mechanism, different from conventional electrochemical diodes. Thus, an asymmetric permeation of ions across an interface, intentionally design for this propose, could lead to rectification of current. [START_REF] Yamamoto | Electrochemical mechanism of ion current rectification of polyelectrolyte gel diodes[END_REF] The design of molecular sieves has attracted the attention of many research groups as they find a broad number of applications such as water desalination, [START_REF] Zhu | Synthesis and seawater desalination of molecular sieving zeolitic imidazolate framework membranes[END_REF] gas separation [START_REF] Ghoufi | Electrically induced breathing of the MIL-53(Cr) metal-organic framework[END_REF] or as membrane in fuel cells. [START_REF] Duke | Carbonised template molecular sieve silica membranes in fuel processing systems: Permeation, hydrostability and regeneration[END_REF] Molecular sieves can be realized using many different materials as for example vertically aligned mesoporous silica films, [START_REF] Vilà | Molecular sieving with vertically aligned mesoporous silica films and electronic wiring through isolating nanochannels[END_REF] porous covalent-organic-frameworks, [START_REF] Kandambeth | Selective molecular sieving in self-standing porous covalent-organic-framework membranes[END_REF] metallopolymeric films, [START_REF] Bélanger | Perfect" electrochemical molecular sieving by thin and ultrathin metallopolymeric films[END_REF] graphene-based [START_REF] Koenig | Selective molecular sieving through porous graphene[END_REF] or biological-based materials. [START_REF] Seo | Simple fabrication of molecular sieving carbon for biogas upgrading via a temperature controlled carbonization of Phyllostachys pubescens[END_REF] Regarding this specific application, the use of aryl-diazonium salts is limited to the functionalization of already existing molecular sieves for tuning their pore sizes and their chemical nature. [START_REF] Li | Surface functionalization and pore size manipulation for carbons of ordered structure[END_REF][START_REF] Wang | Sulfonated ordered mesoporous carbon as a stable and highly active protonic acid catalyst[END_REF] In this study, we are interested in evaluate how the introduction of pre-organized aryldiazonium molecules could affect the organization, structure and chemical composition of the resulting aryl layer deposited on classical conducting surfaces such as carbon and gold, and in turns how the organic layer organization influence the electron transfer of redox probes in solution. In this context, our purpose is the electro-grafting of the previously synthesized tetrahedral-shape preorganized tetrakis(4-phenyldiazonium tetrafluoroborate)methane (C(ArN 2 + ) 4 ) on carbon and gold surfaces, and its physicochemical and electrochemical characterization. The study will be accomplished by cyclic voltammetry of ferrocene and ferrocene derivatives in solution; the influence of the electrolyte in the ferrocene electron transfer will be also evaluated. For completing this investigation, a complete physicochemical characterization of the aryl layer will be performed by means techniques such as XPS, AFM and SE among others.

Results and Discussion

Characterization of the organic films

The tetrakis(4-phenyldiazonium tetrafluoroborate)methane C(ArN Table 3.1 summarize the results obtained for modified gold and carbon electrodes.

Similar thickness film values were obtained on gold and carbon electrodes when CV or chronoamperometry was used. Despite both techniques, SE and AFM "scratching" experiments, were in good agreement between them, for the discussion, we will only consider the thickness values estimated by SE since the possible damage of the substrate during the AFM "scratching" experiment increases the uncertainties in the measurements.

As shown in Table 3.1, modifications performed by chronoamperometry (at constant [START_REF] Shein | Formation of efficient electron transfer pathways by adsorbing gold nanoparticles to self-assembled monolayer modified electrodes[END_REF][START_REF] Lhenry | Tunneling dendrimers. Enhancing charge transport through insulating layer using redox molecular objects[END_REF] and it may be difficult to differentiate between direct electron transfer by pinholes or defects when using outer-sphere redox probes. Hence, we will only discuss here the experimental data obtained from carbon electrodes modified by chronoamperometry, which led to a 4.1 nm films where tunneling electron transfer could be neglected. [START_REF] Lhenry | Tunneling dendrimers. Enhancing charge transport through insulating layer using redox molecular objects[END_REF] The molecular structure of tetraphenylmethane has been already reported in the literature. [START_REF] Claborn | Calculations of optical properties of the tetraphenyl-X family of isomorphous crystals (X = C, Si, Ge, Sn, Pb)[END_REF] Based on the reported crystallographic structure, we derived a thickness of 1.1 nm for a monolayer of tetraphenylmethane. It is clear from these results that multilayered films were obtained both on gold and carbon substrates, with thicknesses equivalent to two monolayers for the thinnest film and up to 3-4 monolayers for the thickest one. -294 eV, 59.9 %), gold Au 4f (84 -88 eV, 38 %), oxygen O 1s (533 eV, 0.9 %) and nitrogen N 1s (398 -404.5 eV, 1.2 %). The presence of intense Au 4f photoelectrons peaks was in agreement with the formation of thin organic layers onto the gold substrate. The main contribution in the XPS analyses was due to carbon with a C 1s peak, that was fitted to three components centered at 285 eV (C-C/C-H), 286.5 eV (C-N, ∼ 10 % of the whole carbon signal) and 291.2 eV (π-π*) for the shake-up satellite transition (Figure 3.6a). The presence of the C 1s shake-up satellite definitively supports the attachment of aromatic aryl groups onto the surface. The N 1s spectrum could be decomposed into two components: a main component at 399.7 eV attributed to (-N=N-) azo linkage; [START_REF] Gam-Derouich | Electrografted aryl diazonium initiators for surface-confined photopolymerization: A new approach to designing functional polymer coatings[END_REF][START_REF] Schmidt | Azobenzene-functionalized alkanethiols in self-assembled monolayers on gold[END_REF][START_REF] Ulrich | X-ray spectroscopy characterization of azobenzene-functionalized triazatriangulenium adlayers on Au(111) surfaces[END_REF][START_REF] Gahl | Charge-transfer dynamics in azobenzene alkanethiolate self-assembled monolayers on gold[END_REF] and a small component corresponding to a π-π* shake-up satellite transition at 402.8 eV. [START_REF] Nakayama | XPS study of oriented organic molecules[END_REF] There was no signal located at ∼ 404 eV, indicating that there were no remaining diazonium functions within the layer (Figure 3.6b). From these experiments, it could be concluded that the organic films were composed of carbon atoms in aryl groups and that there was a minimal amount of nitrogen atoms inside the layers. By considering the starting molecule, the atomic N/C ratio was equal to 32 % whereas this ratio decreases to 2 % in the deposited organic films, corresponding to a minimal amount of azo bridges present within the film. Similar qualitative observations and conclusions can be drawn on XPS experiments performed on carbon substrates functionalized by chronoamperometry (Figure 3.7).

As a complementary characterization technique, infrared absorption spectroscopy (IR)

experiments were also performed on freshly prepared gold samples confirming that diazonium functions do not remain inside the films. As discussed by Kanoufi et al. [START_REF] Hazimeh | Radical chemistry from diazonium-terminated surfaces[END_REF] or Locklin et al. [START_REF] Marshall | Reductive electrografting of benzene (p-bisdiazonium hexafluorophosphate): A simple and effective protocol for creating diazoniumfunctionalized thin films[END_REF], the post-functionalization of deposited organic films containing diazonium functions could be performed using the Gomberg-Bachmann reaction [START_REF] Gomberg | The synthesis of biaryl compounds by means of the diazo reaction[END_REF] allowing aryl-aryl coupling. The reaction consists of plunging the modified surfaces into a solution containing an aromatic molecule just after the grafting. We used the ferrocene molecule in these experiments since it could be easily detected by an electrochemical method. No electrochemical signal of grafted ferrocene was evidenced after the post-functionalization experiments confirming the absence of diazonium functions in the deposited organic films (Figure 3.8). In work described by Locklin et al. [START_REF] Marshall | Reductive electrografting of benzene (p-bisdiazonium hexafluorophosphate): A simple and effective protocol for creating diazoniumfunctionalized thin films[END_REF], the electro-reduction of p-benzene(bisdiazonium) ions led to two irreversible reduction peaks. The authors ascribed these two reduction peaks to a sequential reduction of the two diazonium functions of the p-benzene(bisdiazonium) molecule and, they also stated that a diazonium monolayer was achieved by reducing the molecule at a potential above the first reduction peak (and below the second reduction peak), hence allowing the post-functionalization of the monolayer by Gomberg-Bachmann reaction.

In our experiments, only one reduction peak was observed indicating that all diazonium functions are reduced at the same potential and explaining why no remaining active diazonium functions are constituting the film. Further, no electrochemical activity was detected when cyclic voltammetry of modified substrates was performed in a pure electrolytic acetonitrile solution (Figure 3.8).

The charge transfer properties of the deposited organic thin films were examined by CV on a modified GC electrode in aqueous solution using dopamine (DA) and ferrocenemethanol (FcMeOH). DA undergoes a reversible oxidation to o-quinone in acidic medium but it needs to be absorbed onto the electrode surface for rapidly transferring its charge, which makes DA a very sensitive and suitable redox specie for probing the quality of ultrathin organic films (presence of pinholes or defects), by discriminating between direct and indirect charge transfer (Figure 3.9a). [START_REF] Mccreery | Advanced carbon electrode materials for molecular electrochemistry[END_REF][START_REF] Duvall | Control of catechol and hydroquinone electrontransfer kinetics on native and modified glassy carbon electrodes[END_REF] FcMeOH also undergoes a reversible oxidation in aqueous medium (typically 0.1 mol L -1 KNO 3 ).

FcMeOH was used for testing the blocking properties of the film toward the electron transfer since it can transfer electrons by tunneling. That means, FcMeOH is hardly affected by a monolayer and does not require to be in contact with the electrode surface for transferring its charge (Figure 3.9b). [START_REF] Mccreery | Advanced carbon electrode materials for molecular electrochemistry[END_REF] As shown in Figure 3.9, after electro-grafting of C(ArN 2 + ) 4 (red curves), DA oxidation is totally inhibited. This shows that the deposited organic film was homogeneous, without defects, and effectively blocks the passage of dopamine from the solution to the substrate. However, FcMeOH oxidation remained observable (Figure 3.9b) but became irreversible, highlighting some unexpected electrochemical current rectification properties of the deposited organic film towards this redox probe in solution. This suggests that FcMeOH oxidation occurs via indirect charge transfer (tunneling). Considering that the layer thickness is estimated to be thicker than 4 nm, this also indicates that the redox probe must penetrate inside the deposited organic thin film before charge transfer through tunneling. From these experiments, two specific properties of the ultrathin organic film were evaluated: (i) molecular sieving properties, where diffusion of the redox probe through the film is dependent on their size and (ii) current rectification properties. 11c) was totally inhibited on the organic film. Moreover, the ratio of the anodic peak current intensities (modified surface/ bare surface) was measured and, it was observed that by increasing the steric hindrance of the ferrocene derivatives, the ratio of anodic current monotonically decreased from 0.89 for Fc until the total suppression of the electron transfer for Me 10 Fc (Table 3.2 and Table 3.3).

Molecular sieving properties

From these individual experiments, we could also explain such decreases in current by the variation of the potential applied to the modified working electrode as the substitution on the ferrocene probe changes the size of the molecule but also its halfwave potential (E 1/2 ). In the ferrocene series, substituents on the cyclopentadienyl ring affect the E 1/2 of the species. While electron withdrawing groups shift the potential in the anodic direction (to more positive potentials), electron donating groups such as -CH 3 shift the potential in the cathodic direction (to more negative potentials). This is the case, i.e., of Me 10 Fc which is much more easily oxidized than ferrocene.

Indeed, the potential applied to the electrode could induce structural changes in the organic layer and consequently affects the passage of the redox probes through the film and the kinetics of the redox processes. [START_REF] Traulsen | The effect of electrical polarization on electronic structure in LSM electrodes: An Operando XAS, RIXS and XES study[END_REF] In order to clarify this point, two other ferrocene derivatives were considered, 1,1'-di(methylphenyl)ferrocene ((PhMe) 2 Fc) and 1-(biphenylethynyl)ferrocene (Ph 2 EFc), both with higher steric hindrance than Fc but with a half-wave potential 40 mV below and 130 mV above that of Fc respectively (Table 3.2 and Figure 3.12).

Electrochemical current rectification properties

Most of the electrochemical current rectifiers are based on electrode-confined electroactive molecules which mediate the electron transfer between the redox probe in solution and the electrode surface. The direction in which the current flows depend on the relative potentials of the mediator and the aqueous-phase redox specie. In this way, rectification systems were achieved by functionalization of surfaces with redox entities.

In the present study, the rectification effect has a different origin since no redox entities were present within the deposited organic thin film as evidenced by the different characterization techniques (XPS, IR, CV). Additional CV experiments were performed to identify the origin of this phenomenon. Fresh modified GC electrodes were sequentially immersed in MeCN (+ 0.1 KPF 6 ) solutions with an increasing concentration of Fc from 1 mM to 10 mM. Additionally, the 1 mM Fc solution was studied at different scan rates from 0.1 to 1 V s -1 .

As shown in figures 3.14 and 3.15, the current is limited by the mass transport (diffusion), showing an anodic peak current intensity that varies linearly with the bulk concentration of the redox probe (Figure 3.14a) and with the square root of the scan rate (Figure 3.15). Hence, an EC mechanism of electron transfer in which electroactive surface-confined species act as mediator was discarded. This was in agreement with the previous results regarding the characterization of the deposited organic films. All modified electrodes showed irreversible oxidation of Fc independently of the concentration of the redox probe (Figure 3.14b) and the scan rate (data not shown) in the range studied. From the I p = f (ν 1/2 ) plot for a modified carbon substrate, it was not possible to determine with accuracy the diffusion coefficient of Fc through the organic film, since that would require scan rates above four million volts per second to reduce the diffusion layer to the 4 nm-thickness of the organic film. [START_REF] Macdonald | Transient Techniques in Electrochemistry[END_REF] However, the decrease in the slope of the I p = f (ν 1/2 ) plot from the bare to the modified electrode (Figure 3.15) evidence the difficulties of the redox probe to penetrate the organic layer. The redox probe needs to diffuse inside the pores of the organic layer before the charge transfer. A change of the charge of the redox probe is accompanied by variations of the electrolyte composition present inside the film as reported for charge/discharge processes of conducting polymers where it is possible to control the electrochemical response by changing the size of the anion in polyaniline. [START_REF] Lapkowski | Control of polyaniline electroactivity by ion size exclusion[END_REF] By analogy with our case, this should create movements of cations and anions entering or leaving the film by the same pore channels than those used by the probe. It is thus interesting to evaluate how the size of the supporting salts constituting the electrolyte could affect the electrochemical response of the probe. In that purpose, a series of supporting salts presenting different size of cation or anion was considered (Figure 3.16). In the first series of experiments, the nature of the electrolyte cation was considered when the anion is remaining the same. Potassium hexafluorophosphate (KPF 6 , Figure 3.16a), tetraethylammonium hexafluorophosphate (nEt 4 NPF 6 , Figure 3.16b) and tetraoctylammonium hexafluorophosphate (nOc 4 NPF 6 , Figure 3.16c) were chosen as supporting salts containing monovalent cations having different size. As previously observed on modified substrates, the ratio of the anodic peak current intensity decreased, and Fc underwent an irreversible or quasi-irreversible anodic electron transfer in all the electrolyte solutions. Even if the cation size does not seem to affect the irreversibility of the system, the bulkiest is the cation, the larger is the decrease of the ∆I a (Table 3.4).

As shown in Figure 3.17 and Table 3.4, this trend was also observed on gold-modified electrodes. By considering that ferrocene is oxidized once it is inside the film, its oxidation should be accompanied by repulsion of cations that were "trap" inside the film to maintain charge balance. Considering that the pore size of the deposited organic films is fixed, the bigger is the cation, the more difficult is the ejection of this cation from the organic film to the solution, resulting in smaller oxidation current. This phenomenon thus supports the sieving properties of the organic films. In the second series of experiments, the nature of the anion was evaluated keeping the same cation. Tetrabutylammonium chloride (nBu 4 NCl, Figure 3.16d), tetrabutylammonium tetrafluoroborate (nBu 4 NBF 4 , Figure 3.16e) and tetrabutylammonium tetraphenylborate (nBu 4 NBPh 4 , Figure 3.16f) were chosen as supporting salts containing monovalent anions with different sizes. For all electrolytes, we observed a decrease of the ratio of the anodic peak current intensities (∆I a ) of Fc and a quasi-irreversibility of the redox signal on modified carbon substrates. However, the irreversibility of the electron transfer was more pronounced for the bulkiest anions (Table 3.4). Table 3.4 summarizes the results obtained on both modified-carbon and modifiedgold surfaces towards Fc electron transfer in electrolyte solutions carefully selected for their size. Note that it was not possible to use nBu 4 NCl as supporting electrolyte for experiments performed on gold substrates as the chloride anions are oxidized before Fc redox probe. From this series of experiments, we conclude that for all the electrolyte solutions and modified substrates investigated, the ratio of the anodic peak current intensities of Fc decreased and, this decrease was affected by the size of the counterion, being more pronounced the larger was the counterion. An irreversible electron transfer was observed for Fc in the cation series (K + , nEt 4 N + and nOc 4 N + ), but this feature did not seem to be affected by increasing the size of the cation. On the contrary, a quasi-irreversible electron transfer was observed for Fc in the anion series (Cl -, BF 4 -, and BPh 4 -) and, in this case, the reversibility of the system was more influenced by the size of the anion. Since the thickness of the organic film is around 4 nm, it is evident that the electron transfer of Fc requires the previous diffusion of the redox probe inside the layer for exchange electrons with the electrode surface by tunneling.

The oxidation of Fc to Fc + creates a positive net charge within the organic film, which should be compensated, following the law of electroneutrality of the materials, by anion entrance in the film or by cation expulsion until the net charge balance. The larger the counterion, the more difficult is the diffusion through the pores of the film, resulting in smaller oxidation or reduction peak current. This is in agreement with the previously observed molecular sieving properties. Regarding the current rectification properties of the organic film toward redox probes in solution, an asymmetrical distribution of charge within the film caused by the limitation in movement of the ions entering and leaving the organic layer is responsible for this specific property, and that molecular sieving and current rectification properties are intimately linked.

Experimental section 3.3.1 Reagents

All commercially available reagents were used as received.

Non-commercially reagents were synthesized in the laboratory:

• Tetrakis(4-phenyldiazonium tetrafluoroborate)methane (C(ArN 2 + ) 4 ) was synthesized from the corresponding commercial amine following the classical procedure of diazotisation of aromatic amines.

• Ferrocene derivatives:

-1,1'-Di(methylphenyl)ferrocene ((PhMe) 2 Fc) -1-(Biphenylethynyl)ferrocene (Ph 2 EFc)

Electrochemical measurements

Instrumentation

All the electrochemical measurements were performed using a standard three-electrode setup at a controlled potential with a potentiostat Autolab PGSTAT 302N (Metrohm).

The set of electrodes consisted of a 2 mm-diameter polycrystalline gold (Au) or glassy carbon (GC) disk electrodes (IJ Cambria) as working electrode (surface area 0.03 cm 2 ), a platinum wire as the auxiliary electrode, and a saturated calomel electrode (SCE, KCl sat.) as the reference. The latter was isolated from the working solution employing a salt bridge, which contained the electrolyte solution. When necessary, a conventional gas inlet was used for bubbling argon inside the electrochemical cell to displace oxygen, which is chemically reactive and electrochemically reducible. Occasionally, low roughness and flat carbon and gold surfaces were required as working electrodes. In this case, pyrolyzed photoresist films (PPF) were homemade fabricated according to described methods [START_REF] Del Campo | Fabrication of PPF electrodes by a rapid thermal process[END_REF], and gold wafer substrates were purchased from Aldrich (surface area 3 cm 2 ). Since the larger surface area of these flat working electrodes, a platinum mesh was used as the counter electrode instead of a platinum wire.

Methodology

Preparation of modified substrates. Surface modifications were performed by cyclic voltammetry (ν = 0.05 V s -1 , five consecutive cycles from + 0.6 to -0.8 V vs SCE)

or fixed potential chronoamperometry (300 s at -0.8 V vs SCE) in a saturated solution

(< 5x10 -4 mol L -1 ) of C(ArN 2 + ) 4 + 0.1 mol L -1 nBu 4 NPF 6 in acetonitrile (MeCN),
previously deaerate with argon (20 min). The modified electrodes were rinsed with a copious amount of MeCN and acetone, stirred 20 min in THF solution and dried under argon. Before modification, the disc electrodes were polished successively with 2400 and 4000 silicon carbide abrasive discs and ultrapure water and thoroughly rinsed with ultrapure water and acetone. In case of flat working electrodes, no polish treatment is necessary before modification, PPF electrodes were rinsed with acetone and dried under argon and, gold substrates were cleaned using piranha solution, thoroughly rinsed with ultrapure water and acetone, and dried under argon.

Cyclic voltammetry measurements. I-E plots of GC and Au electrodes were registered at scan rate 0.1 V s -1 in different redox probe and electrolyte solutions, before and after modification by both CV and chronoamperometry. Five different 10 -3 mol L -1 redox probe solutions + 0.1 mol L -1 KPF 6 in MeCN were studied (redox probes are listed in Table 3.5); and also six different solutions of 10 -3 mol L -1 ferrocene + 0.1 mol L -1 electrolyte in MeCN (electrolytes are listed in Table 3.6). From the I-E plots, two characteristic parameters of the voltammogram were determined, the peak potential (E p ) and the peak current (I p ); the half-wave potential (E 1/2 ) was estimated by the half-sum of the anodic and cathodic peaks. Note that when the diffusion coefficients of the oxidized and reduced species are equal, E 1/2 = E 0 . [START_REF] Savéant | Elements of Molecular and Biomolecular Electrochemistry: An Electrochemical Approach to Electron Transfer Chemistry[END_REF]. To compare the results, the peak current was normalized to 1 (in arbitrary units(a.u.)) an plotted versus applied potential (in volts(V)), which was standardized versus ferrocene as an internal standard. All potentials refer to the SCE reference potential unless otherwise stated. The thickness of the organic films was measured by Spectroscopy Ellipsometry with an αSE ellipsometer (J.A. Woollam, Co.) using a model-based analysis approach. Prior to modification with the aryldiazonium molecule, the SE data of the bare substrate were registered to build a substrate optical model which represents the nominal structure of the non-modified material; the refractive index (n) and the extinction coefficient (k) were 0.4 and 3.5 for gold, and 2.4 and 1.1 for bare PPF electrodes respectively.

This substrate model was used after modification to establish the contribution of the substrate to the SE experimental data and thus, estimate the thickness of the organic layer according to the Cauchy model. Finally, the goodness of the fit was quantified by the Mean Squared Error (MSE) to validate our results. In all cases, the fitting reproduced the structures and features of the SE experimental data. The values of thickness obtained are the average of ten measurements performed on three different substrates (PPF and gold wafer electrodes).

X-ray Photoelectron Spectroscopy

X-ray photoelectron spectroscopy data were collected with a Kratos Axis Nova spectrometer using the Al Kα X-ray source working at 1486.6 eV and using a spot size of 0.7x0.3 mm 2 . Survey spectra (0 -1000 eV) were acquired with an analyzer pass energy of 160 eV (0.5 eV/step); high resolution spectra used a pass energy of 40 eV (0.1 eV/step). Binding energies were referenced to C1s peak at 285 eV. The core level spectra were peak-fitted using the CasaXPS software, Ltd. (version 2.3.18).

Atomic Force Microscopy

Atomic force microscopy experiments were performed on modified PPF and modified gold wafer substrates using a NT-MDT Ntegra microscope. Topographic images were obtained in semi-contact mode using ACTA tips (AppNano Inc.) for which the resonance frequency is around 300 kHz. Scratching experiments were performed with the same tips to estimate the thickness of the organic layers according to the McCreery AFM "scratch" procedure. [START_REF] Anariba | Mono-and multilayer formation by diazonium reduction on carbon surfaces monitored with atomic force microscopy "scratching[END_REF] The term "AFM scratching" is used here to describe intentional damage to an organic layer grafted on a relatively hard substrate. These experiments were obtained by scanning the surface in contact mode on a 500 × 500 nm square with a force setpoint sufficient to remove the organic layer. After scratching, the damaged surface imaged in semi-contact mode presented a square-shaped zone surrounded by swept amounts of matter. The images were treated using Gwyddion open-source software.

[184] The thickness of the organic layer was extracted from the depths of the scratched zones averaged on different profiles. Scratching experiments were also performed on bare Au and PPF surfaces to evaluate the eventual damaging of the substrate. In this case, the heights of the scratched zones were measured and subtracted to those obtained on the modified surfaces to obtain corrected organic layer heights, taking into account the substrate damaging. Furthermore, the roughness of the organic films deposited on gold and carbon substrates was estimated, according to the standard ISO 4287/1-1997, as the average of the measured height deviations taken within the image (Root Mean Square (RMS) roughness).

Conclusion

The electro-reduction of a tetrahedral-shape pre-organized tetra-aryldiazonium salt, tetrakis(4-phenyldiazonium tetrafluoroborate)methane C(ArN 2 + ) 4 , led to the deposition of ultrathin organic films on gold and carbon substrates. Various characterization techniques as XPS, IR and CV showed that all diazonium functions are reduced in the organic layer, which appears to be mainly composed of carbon and hydrogen atoms.

Molecular sieving properties were evidenced by cyclic voltammetry of solution-phase redox probes in aqueous and organic solutions. Substrates modified by this technique also behave as electrochemical current rectifiers (ECR). Due to the lack of redox relay in the layer, it appears to be different from the classical ECR as described by Murray et al. [START_REF] Abruña | Rectifying interfaces using two-layer films of electrochemically polymerized vinylpyridine and vinylbipyridine complexes of ruthenium and iron on electrodes[END_REF] The transport of the redox molecules and the counterions through the layer has been evidenced as responsible of the electrochemical current rectification properties of the modified materials. Both molecular sieving and electrochemical current rectification properties are related to the specific organization of the grafted ultrathin organic layer. Observation of both molecular sieving and electrochemical current rectification properties has never been observed previously for organic films electro-generated by the reduction of aryldiazonium ions that do not present this tetrahedral pre-organization. [START_REF] Combellas | Steric effects in the reaction of aryl radicals on surfaces[END_REF][START_REF] Paulik | Grafting aryl diazonium cations to polycrystalline gold: Insights into film structure using gold oxide reduction, redox probe electrochemistry, and contact angle behavior[END_REF][START_REF] Yang | Effects of surface monolayers on the electrontransfer kinetics and adsorption of methyl viologen and phenothiazine derivatives on glassy carbon electrodes[END_REF][START_REF] Raicopol | Organic layers via aryl diazonium electrochemistry: Towards modifying platinum electrodes for interference free glucose biosensors[END_REF] Chapter 4

Intermediates detection during multielectronic oxygen reduction reaction on modified carbon materials: a SECM footprinting strategy.

Abstract

The oxygen reduction reaction (ORR) and its catalysis are of general importance in

Background

Scanning Electrochemical Microscopy (SECM) [START_REF] Bard | Scanning electrochemical microscopy. Introduction and principles[END_REF][START_REF] Kwak | Scanning electrochemical microscopy. Theory of the feedback mode[END_REF] has already demonstrated all its interest for probing at the local scale a variety of biological, chemical and electrochemical processes occurring at interfaces. [START_REF] Polcari | Scanning electrochemical microscopy: A comprehensive review of experimental parameters from 1989 to 2015[END_REF] SECM in feedback mode provides a chemical view of a surface from the solution side and is complementary to other electrochemical methods such as cyclic voltammetry. [START_REF] Kwak | Scanning electrochemical microscopy. Theory of the feedback mode[END_REF] SECM was used with success for the study of electrocatalysis phenomena and for what concerns the present study, the catalysis of oxygen reduction reaction (ORR). The contributions of Bard and co-workers are noteworthy in this regards; [START_REF] Liu | Scanning electrochemical microscopy. 45. Study of the kinetics of oxygen reduction on platinum with potential programming of the tip[END_REF][START_REF] Fernández | Optimization of "wired" enzyme O 2 -electroreduction catalyst compositions by scanning electrochemical microscopy[END_REF][START_REF] Fernández | Thermodynamic guidelines for the design of bimetallic catalysts for oxygen electroreduction and rapid screening by scanning electrochemical microscopy[END_REF][START_REF] Fernández | Pd-Ti and Pd-Co-Au electrocatalysts as a replacement for platinum for oxygen reduction in proton exchange membrane fuel cells[END_REF][START_REF] Weng | Combinatorial biomimetics. Optimization of a composition of copper (II) poly-L-histidine complex as an electrocatalyst for O 2 reduction by scanning electrochemical microscopy[END_REF][START_REF] Walsh | Rapid screening of bimetallic electrocatalysts for oxygen reduction in acidic media by scanning electrochemical microscopy[END_REF][START_REF] Fernández | Characterization and theory of electrocatalysts based on scanning electrochemical microscopy screening methods[END_REF][START_REF] Sánchez-Sánchez | Scanning electrochemical microscopy. 60. Quantitative calibration of the SECM substrate gen-eration/tip collection mode and its use for the study of the oxygen reduction mechanism[END_REF] mainly focusing on the SECM rapid-screening of catalysts arrays for the optimization of oxygen reduction in fuel cells applications.

Oxygen reduction in aqueous media is therefore of high interest from a technological and biological point of view. ORR is one of the most important cathodic process in fuel cells and a difficult challenge in electrocatalysis. [START_REF] Nørskov | Origin of the overpotential for oxygen reduction at a fuel-cell cathode[END_REF][START_REF] Marković | Oxygen reduction reaction on Pt and Pt bimetallic surfaces: A selective review[END_REF] Further, ORR is an important reaction in life processes such as biological respiration. Oxygen reduction takes places naturally in the body but, during oxidative stress periods in the cell, high concentrations of reactive oxygen species are generated, this species are related with inflammatory processes and cell damage but also with cell protection strategies against bacteria. [START_REF] Halliwell | Free Radicals in Biology and Medicine[END_REF] In aqueous medium, ORR occurs mainly by two pathways: the These reactive oxygen species are known for their high reactivity; the molecular orbital diagram of these species can help to understand their reactivity. Scheme 4.2 reveals that oxygen is a biradical with 2-electrons, each located in a different π * orbital with parallel spins. This is the most stable state of O 2 and its reactivity is limited by a spin restriction, that means, oxygen can act as an oxidizing agent but the most organic molecules are non-radical diamagnetic molecules (with pairs of electrons with opposite spins), and therefore they cannot fill the vacancies in the π * orbitals of oxygen.

Oxygen is thus unable to efficiently oxidize organic molecules, and as a consequence of the spin restriction, it preferentially accepts electrons one by one during redox reactions. Then oxygen easily reacts with species containing unpaired electrons such as transition metals, and by single electron transfer with other radical species. [START_REF] Halliwell | Free Radicals in Biology and Medicine[END_REF][START_REF] Krumova | Chapter 1. Overview of Reactive Oxygen Species[END_REF] The product of the one electron reduction of oxygen is the anion radical superoxyde (O 2 • -), who has a diminished radical character since only one unpaired electron is then in the anti-bonding orbital (Scheme 4.2). By reducing superoxide radical to hydrogen peroxide, an additional electron goes into the anti-bonding orbital. Since extra electrons are located in the anti-bonding orbital, the strength of the oxygen-oxygen drops, making hydrogen peroxide more easily cleavable, i.e., in presence of transition metals via Fenton reaction [START_REF] Brillas | Mineralization of 2,4-D by advanced electrochemical oxidation processes[END_REF] or by UV-induced homolytic fission of the O-O bond [START_REF] Hambly | Laser flash photolysis of hydrogen peroxide to oxidize protein solvent-accessible residues on the microsecond timescale[END_REF] to yield hydroxyl radicals (Scheme 4.1). Therefore, the one-electron reduction product of hydrogen peroxide is the radical (HO • ), who is one of the most reactive oxidizing agent known, with a positive redox potential of 2.80 V vs SHE. [START_REF] Brillas | Electro-Fenton process and related electrochemical technologies based on Fenton's reaction chemistry[END_REF] Its steady-state concentration is effectively zero; with a short lifetime, it rapidly reacts as soon as it is formed with molecules in its proximity or by abstraction of H • from water, propagating itself in aqueous solution. Rate reaction constants of HO • with small organic molecules Concerning the electrochemistry of these reactive oxygen species, the behavior of superoxyde is well described in organic aprotic media, [START_REF] Sawyer | How super is superoxide?[END_REF][START_REF] Andrieux | Mechanism of superoxide ion disproportionation in aprotic solvents[END_REF] where a mechanism of reaction was proposed. [START_REF] Andrieux | Mechanism of superoxide ion disproportionation in aprotic solvents[END_REF] The reactivity of superoxide is sensitive to the solva-tion of O 2

•but even more important is the stabilization of peroxide by protons. In aprotic media, the peroxide dianion O 2 2 -, removed from an ionic environment and non-coordinated to any metal ion, is highly unstable species. Thus the likelihood that superoxide will act as a one-electron oxidant under conditions where an isolated peroxide dianion is the product is nil (See Figure 4.1 for the reversible electro-reduction of oxygen to superoxyde on gold; the voltammogram is perfectly reversible since superoxyde is stabilized in dried aprotic media such as DMSO or DMF). On the other hand, when protons or metals ions are available to stabilize the peroxide anion, the thermodynamics changes and oxygen is reduced to peroxide in a 2-electron process. [START_REF] Andrieux | Mechanism of superoxide ion disproportionation in aprotic solvents[END_REF]

O 2 •-+ H + → HO 2 • C (4.2) HO 2 • + O 2 •-→ O 2 + HO 2 - DISP (4.3) HO 2 -+ H + → H 2 O 2 (4.4)
Despite extensive studies over the past decades, the mechanistic aspects of the O 2 electrode reactions in aqueous solution are not yet understood. As we previously stated, transient species are generated in the multielectronic pathway from oxygen to water, and these species cannot be followed by classical electrochemical techniques. In a previous publications, we have shown that reactive oxygen species could be detected by means SECM and a sensitive layer in DMF after adding phenol. [START_REF] Noël | Evidence for OH radical production during electrocatalysis of oxygen reduction on Pt surfaces: consequences and application[END_REF] The SECM tip is maintained in the proximity of the sensitive surface to trap the reactive intermediate.

In a second step, the same tip serves to read the modifications of the surface that are created after reaction with the intermediates. In previous work, we used ferrocene as redox mediator and a carbon surface modified by a polyarene layer. Observed modifications rely on a change of the sensitivity of the layer and its conducting properties, and this change was ascribed to the ROS formation and its interaction with the organic layer; ROS like HO • since very few intermediates are able to significantly react with a polyaryl layer. However, many questions remain about the exact nature of the intermediate and the mechanism as the detection remains indirect. To add in the complexity of the observation, it was recently shown that ORR is often accompanied by the dissolution of the Pt tip electrode as it was demonstrated by the detection of Pt nanoparticles during ORR on a Pt electrode. [START_REF] Percival | Cathodically dissolved platinum resulting from the O 2 and H 2 O 2 reduction reactions on platinum ultramicroelectrodes[END_REF][START_REF] Bae | Dissolution of Pt during oxygen reduction reaction produces Pt nanoparticles[END_REF] Concerning the production of ROS, they have been efficiently used in environmental applications [START_REF] Brillas | Electro-Fenton process and related electrochemical technologies based on Fenton's reaction chemistry[END_REF][START_REF] Sirés | Electrochemical advanced oxidation processes: Today and tomorrow[END_REF] or in local patterning of surfaces. [START_REF] Ktari | Patterning of polystyrene by scanning electrochemical microscopy. Biological applications to cell adhesion[END_REF][START_REF] Ktari | Local oxidation of polystyrene by scanning electrochemical microscopy[END_REF][START_REF] Shiku | Microfabrication of alkylsilanized glass substrate by electrogenerated hydroxyl radical using scanning electrochemical microscopy[END_REF] In this context and as extension of previous works, our aim here is the determination of intermediates during the ORR on platinum by SECM using a foot-printing strategy. This approach is based on the use of sensitive surfaces to evidence the formation of reactive oxygen species during the ORR. The role of different sensitive layers, applied potential and electrolytes are investigated. In this conditions, thick organic layers with strong blocking properties towards the electron transfer of redox probes in solution are obtained. The literature already mentions the formation of multilayers by reduction of these compounds. [START_REF] Leroux | Efficient covalent modification of a carbon surface: Use of a silyl protecting group to form an active monolayer[END_REF][START_REF] Combellas | Steric effects in the reaction of aryl radicals on surfaces[END_REF] In any case, we studied their blocking properties by cyclic voltammetry and their homogeneity and roughness, and thickness, by AFM and spectroscopic ellipsometry respectively. spectively, which correspond to more than 10 monolayers (Table 4.1). In the case of the p-(ethynyl)phenyl multilayer, its thickness is in good agreement with the value reported in the literature for the same molecule and carbon surface, but obtained by "scratching" AFM experiments (6 nm-thickness). [START_REF] Leroux | Efficient covalent modification of a carbon surface: Use of a silyl protecting group to form an active monolayer[END_REF] Therefore, both layers have similar surface features and blocking properties towards the electron transfer of redox probe in solution.

Concerning the structure and according to the characterization data and the literature, [START_REF] Bélanger | Electrografting: A powerful method for surface modification[END_REF][START_REF] Chehimi | Aryl Diazonium Salts: New Coupling Agents in Polymer and Surface Science[END_REF] in both cases a polyaromatic structure is obtained. In the case of the p-(ethynyl)phenyl multilayers, the structure integrates acetylene groups. This feature is expected to confer a differential sensitivity of this multilayer towards highly reactive oxygen species in solution. The p-(methyl)phenyl-based structure could be also described as a polyaromatic multilayer, but in this case, it integrates the methyl groups that were originally in para-position in the aryl ring of the corresponding aryldiazonium salt. It is known that methyl terminal groups offer few opportunities for organic chemistry compared to acetylene terminal groups. [START_REF] Garrat | Alkylations of alkynyl carbanions[END_REF] Then a different behavior could be expected in the presence of oxidants. phenyl multilayers on PPF, we perform the SECM experiments. For reactive intermediates detection during the oxygen reduction reaction, we followed a foot printing strategy based on the use of SECM in feedback mode and with the test surface in unbiased conditions. [START_REF] Noël | Evidence for OH radical production during electrocatalysis of oxygen reduction on Pt surfaces: consequences and application[END_REF] The experiment proceeds as follow: the tip electrode is place at the vicinity of the organic layer (5 µm from the organic layer) using a redox mediator; when the tip electrode is in position, the redox probe solution is replaced with the target solution where the oxygen reduction reaction will be performed; during this replacement, the electrochemical cell is rinsed several times with ultrapure water and the target solution. The potential to reduce oxygen is applied to the tip electrode, and the interaction between the electro-generated species and the organic layer occurs. The local transformations of the layer are "read" with a redox mediator such as ferrocenedimethanol (Fc(MeOH) 2 ). The effects of the oxygen reduction on the organic layer can be evidenced either moving the tip in the solution to register an image in the x-y plane (the image is registered before and after ORR for comparisons); or registering the approach curves before and after ORR. In this case, different types of approach curves could be obtained, from positive to negative feedback, which sense the ability of the redox mediator to pass through the layer, providing a view of the modification.

The use of SECM in a foot-printing configuration for detecting intermediates during ORR is extended here to different aqueous electrolyte solutions as a continuation of the investigations previously carried out in our group in different organic solvents. [START_REF] Noël | Evidence for OH radical production during electrocatalysis of oxygen reduction on Pt surfaces: consequences and application[END_REF] Two different aqueous electrolyte solutions have been considered for this study, 0.01 mol L -1 KOH and 0.01 mol L -1 KPF 6 solutions. The low concentration of electrolyte tries to decrease the production of secondary species derived from an interaction between the electrolyte and the intermediates electro-generated during the reduction of oxygen and extends the lifetime of the radicals in solution. The reduction of oxygen was performed with the concentration of oxygen naturally dissolved in these media that was constant during the experiment under the laboratory conditions of pressure and temperature. The experimental concentration of oxygen was derived from the diffusion limited current at the platinum ultramicroelectrode during the reduction of oxygen by cyclic voltammetry in the reaction media (Figure 4.5). The steady-state limit current at a microdisc electrode is related with the concentration of the electroactive species by the equation: i lim = 4nFDaC where a is the radius of the platinum UME (5 µmradius), C is the concentration of oxygen and D is the diffusion coefficient of oxygen (1.97x10 -5 cm 2 s -1 from the literature [START_REF] Ferrell | Diffusion coefficients of nitrogen and oxygen in water[END_REF][START_REF] Davis | The solubility and diffusion coefficient of oxygen in potassium hydroxide solutions[END_REF]). It was estimated a concentration of oxygen 8.50x10 -4 mol L -1 in 0.01 mol L -1 KOH and 8.92x10 All potentials studied by SECM in each media are marked in the voltammograms as blue solid squares.)

Each applied potential studied by SECM is a sequence of two pulses in chronoamperometry. Following a previously described procedure, [START_REF] Noël | Evidence for OH radical production during electrocatalysis of oxygen reduction on Pt surfaces: consequences and application[END_REF] the platinum ultramicroelectrode is subjected during a total of 20 seconds to an applied potential by a series of two square wave pulses in chronoamperometry from 0 to E applied (vs Ag/AgCl). The tip electrode is moved to different points (eight in total) in an x-y plane maintaining constant the distance tip-organic layer at 5 µm. The only parameter that varies is the potential applied, which sequentially shifts to more negative values as marked in the voltammograms (See Figure 4.5).

As shown in Figure 4.6, before oxygen reduction, both organic multilayers showed an insulating behavior, blocking the electron transfer of the ferrocene derivative in solution; KPF 6 solution (g,h). Y-axes display the normalized tip current (i/i inf ); the background current was subtracted for ease of comparison.

the two organic layers showed a very homogeneous electrochemical response, with no surface defects or large current variations in both the neutral and the basic solution.

After performing the reduction of oxygen at different applied potentials and at different points on the surface of the organic layer, it was observed no modification in the case of the p-(ethynyl)phenyl multilayer that remains totally insulating, still blocking the electron transfer of ferrocenedimethanol in solution; this was observed in both neutral and basic solutions. In the case of the p-(methyl)phenyl multilayer, modifications are visible after exposure to reactive oxygen species both in neutral and basic solution, when the applied potentials were the half-wave potentials or those corresponding to the plateau of the oxygen reduction (-0.6 and -0.8 V vs Ag/AgCl in the case of the KPF 6 solution, and -0.4 V and -0.6 V vs Ag/AgCl in the case of the KOH solution); but also, for applied potentials in the anodic region, where in view of the cyclic voltammetry curves, no electron transfer reaction occurs and the current intensities were almost zero (a few tenths of a nanoamper). To rule out possible artifacts such as the displacement of the oxygen reduction potential during the experiment, the voltammogram of oxygen reduction was registered at the beginning and at the end of the experiment; there were no significant differences in the half-wave potential of oxygen that justified the modification at anodic potentials on the organic layer observed both at neutral and basic pH. For a easier comparison between experiments, the current-distance profiles should be examined (Figure 4.7). which the current profiles were extracted; 1 corresponds to cathodic potentials with E applied < 0 V and 2 to anodic potentials and zero with E applied 0 V. The extent of the modification in the p-(methyl)phenyl multilayer is much more important in alkaline medium than in neutral solution; the current intensities associated to the electron transfer of Fc(MeOH) 2 is more than three time larger when the applied potentials are those in the plateau of the ORR. A common feature is that no modification occurs when the applied potential is around 0 V vs Ag/AgCl in both alkaline and neutral media.

The differential behavior between the p-(ethynyl)phenyl and the p-(methyl)phenyl multilayers could be explained in terms of reactivity towards the hydroxyl radicals probably generated as intermediate during the oxygen reduction on platinum. The former did not undergo modification after ORR at any potential or reaction media, while the later was affected in both media and almost at all applied potentials except around 0 V. Indeed, the oxygen reduction in aqueous solution is a multielectronic process that leads to the formation of radical and non radical species; radicals such as superoxyde

(O 2 • -), perhydroxyl (HO 2
• ) and hydroxyl (HO • ), and non radicals such as hydrogen peroxide (H 2 O 2 ). [START_REF] Krumova | Chapter 1. Overview of Reactive Oxygen Species[END_REF] However, hydroxyl radicals are the most reactive among them and must be preferentially produced on a platinum surface. There are three possible reactions between HO • and organic molecules: the abstraction of H atoms, the electrophilic addition on unsaturated bonds, and the electron transfer process. From pulse radiolysis experiments, it have been proposed different reaction mechanisms towards hydroxyl radicals for systems that could be assumed to be close or equivalent to those investigated, acetylene [START_REF] Anderson | Reactions induced by hydroxyl radical attack on acetylene in aqueous solution. A pulse radiolysis study[END_REF][START_REF] Senosiain | The reaction of acetylene with hydroxyl radicals[END_REF] and methane [START_REF] Anbar | Reactivity of aliphatic compounds towards hydroxyl radicals[END_REF] molecules in an aqueous basic solution. The reaction between the HO • and acetylene mainly proceeds by electrophilic addition to the carbon-carbon triple bond. The addition product is the enol form of the formylmethyl radical that is stabilized by resonance and will dissapears bimolecularly by reaction with water. [START_REF] Anderson | Reactions induced by hydroxyl radical attack on acetylene in aqueous solution. A pulse radiolysis study[END_REF][START_REF] Senosiain | The reaction of acetylene with hydroxyl radicals[END_REF] However, hydrogen abstraction is the dominant reaction pathway for the attack of HO • on saturated hydrocarbons. [START_REF] Anbar | Reactivity of aliphatic compounds towards hydroxyl radicals[END_REF][START_REF] Kopinke | What controls selectivity of hydroxyl radicals in aqueous solution? Indications for a cage effect[END_REF] For example, the reaction between hydroxyl radicals and methane involves the abstraction of H atoms that results in the formation of CH 3 • radicals, [START_REF] Anbar | Reactivity of aliphatic compounds towards hydroxyl radicals[END_REF] which are species much more reactive than the stabilized enol intermediates and could therefore induce other radical reactions (propagation reactions) in the organic layer, thus contributing to its further oxidative degradation. Additionally to the oxidation of the substituents, HO • radicals can also attack the aromatic ring to form phenolic adducts. In the case of the para-substituted methylbenezene molecules, the electrophilic attack to the ring is kinetically favored (k = 2.62x10 9 M -1 s -1 ) compared to the abstraction of H atoms in the substituent (k = 1.43x10 8 M -1 s -1 ). [START_REF] Anbar | Reactivity of aliphatic compounds towards hydroxyl radicals[END_REF][START_REF] Anbar | The reactivity of aromatic compounds toward hydroxyl radicals[END_REF] In connection with the modification of the p-(methyl)phenyl multilayers observed in an aqueous KPF 6 solution after ORR, it has recently been published a dissertation where an starting insulating polyphenylene multilayer, grafted on gold from its corresponding benzene aryldiazonium cation, was used with the same SECM approach and under similar experimental conditions that those used in this experiments. [START_REF] Hartmann | Advanced electrochemical analysis: Boron-doped diamond electrodes, electrocatalytic films, and combined SECM -SPR[END_REF] A platinum tip (25 µm-diameter) was approached to the vicinity of the polyphenylene layer using ferrocene methanol in 0.1 mol L -1 KPF 6 solution; without changing the solution, a potential of -1 V vs Ag/AgCl was applied during 20 seconds to the platinum tip.

Changes on the organic layer before, during and after reducing oxygen were monitored by surface plasmon resonance (SPR). Changes in the minimum angle of reflection were registered 2 seconds after the reaction started; but the greatest modification of the organic layer was recorded once the working electrode was turned off, a decrease in the SPR signal of 82 % was then recorded. This behavior was attributed to a radical reaction induced in the organic layer after the attack of the reactive oxygen species during the oxygen reduction reaction. SECM approach curves were also registered before and after modification; prior ORR, a negative feedback indicated the blocking properties of the organic layer towards the electron transfer of ferrocenemethanol in solution, while after ORR, a positive feedback was registered. This indicates that the redox probe was able to permeate the organic layer and undergo the electron transfer.

The study confirmed our previous observation in 0.01 mol L -1 KPF 6 solution with the p-(methyl)phenyl multilayer at applied potentials at the plateau of the oxygen reduction. In fact, a very good reproducibility was achieved in this medium allowing patterns to be drawn on the organic layer ( of "radical scavenger"; while p-(methyl)phenyl and polyphenylene multilayers appear to be damaged by species generated during the reduction of oxygen in platinum.

The modification of the p-(methyl)phenyl multilayer was greater in the alkaline solution than in KPF 6 . Two factors must be considered, on the one hand, the variation in pH, and on the other hand the electrolyte. In 0.01 mol L -1 KOH, the pH was around bond breaking leading to numerous intermediates. [START_REF] Andrieux | Mechanism of superoxide ion disproportionation in aprotic solvents[END_REF] In any case, some mechanistic consideration could enlighten the discussion.

In alkaline solutions, a 1-electron mechanism from O 2 to superoxyde O 2 •has been proposed; [START_REF] Sawyer | How super is superoxide?[END_REF] superoxide exhibits a significant lifetime since high concentrations of OH -promote the existence of superoxide according to the Equation 4.5. [START_REF] Sawyer | How super is superoxide?[END_REF][START_REF] Zhang | Electrochemistry of oxygen in concentrated naoh solutions: solubility, diffusion coefficients, and superoxide formation[END_REF] At

Besides of these considerations, many other parameters such as the nature of the electrode material and the electrolyte affect the mechanism of oxygen reduction, and thus the nature of the intermediates. For example, platinum is widely used as component in the cathode of fuel cells to speed up the ORR and promote the direct 4-electron pathway to water; [START_REF] Song | Electrocatalytic oxygen reduction reaction[END_REF] however, the catalysis of the ORR in platinum surfaces also depends on the state of the surface (platinum single crystal, facets, alloys with other metals, pre-treatments of the surface, adsorption of anions from the electrolyte among others); [START_REF] Marković | Oxygen reduction reaction on Pt and Pt bimetallic surfaces: A selective review[END_REF][START_REF] Sawyer | Electrochemistry of dissolved gases: II. Reduction of oxygen at platinum, palladium, nickel and other metal electrodes[END_REF][START_REF] Gómez-Marín | Oxygen reduction reaction at Pt single crystals: A critical overview[END_REF][START_REF] Briega-Martos | Effect of pH and water structure on the oxygen reduction reaction on platinum electrodes[END_REF] not all platinum surfaces have the same catalytic activity. The amount of H 2 O 2 produced, as well as the overall reaction kinetics, are sensitive to the platinum surface structure. For example, a larger proportion of H 2 O 2 was formed during the oxygen reduction on polycristalline platinum when the preferential surface orientation was (100) than when it was (111) (See [START_REF] Spendelow | Electrocatalysis of oxygen reduction and small alcohol oxidation in alkaline media[END_REF] and references therein for role of the Pt surface structure in the catalysis of ORR).

In this context, to assess the role of hydrogen peroxide in the modification observed in the p-(methyl)phenyl multilayer at those potentials corresponding to the plateau of the ORR, we performed an experiment where two fresh organic layers (the p-(ethynyl)phenyl and the p-(methyl)phenyl multilayers) just grafted on PPF surfaces were kept during approx. 30 seconds in a 30 % H 2 O 2 solution (Figure 4.9). We re- gistered the approach curves before and after the treatment with H 2 O 2 at different points on the organic layers. As shown in Figure 4.9, for both organic layers before and after treatment in H 2 O 2 , the approach curves show a negative feedback, indicating that the electron transfer of the redox mediator is hindered by the organic layer. Note that plots registered before and after are overlayed; no significant differences were observed in either of the two layers after exposure to a 30 % H 2 O 2 solution.

These findings suggest that hydrogen peroxide may not be directly involved in the damage of the p-(methyl)phenyl multilayer during the ORR on the Pt UME. Nevertheless, it has been previously proposed in the literature that hydrogen peroxide can undergo reduction at metals such as platinum or be catalytically decomposed to HO • in presence of transition metals such as iron (II). [START_REF] Percival | Cathodically dissolved platinum resulting from the O 2 and H 2 O 2 reduction reactions on platinum ultramicroelectrodes[END_REF][START_REF] Sirés | Electrochemical advanced oxidation processes: Today and tomorrow[END_REF][START_REF] Haber | The catalytic decomposition of hydrogen peroxide by iron salts[END_REF] As it is known, hydrogen peroxide is a weak oxidant, while HO • is one of the most active oxidants known. [START_REF] Brillas | Electro-Fenton process and related electrochemical technologies based on Fenton's reaction chemistry[END_REF] Although the exact identity of the species involved in the layer modification cannot be derived from these series of experiments, preliminary investigations carried out in 0.01 mol L -1 KPF 6 aqueous solution with gold and carbon ultramicroelectrodes on the methylaryl multilayer, reveal a possible non-catalytic mechanism of hydroxyl radical production.

To develop a further understanding about the nature of the species responsible of the layer modification future works can consider experimental conditions in which the 2-electron pathway for reduction of oxygen is favored. Oxygen under certain conditions is preferentially reduced to H 2 O 2 , specifically, at carbon electrodes or at pre-reduced Pt surfaces in acidic aqueous solution. [START_REF] Lingane | Chronopotentiometric study of oxygen reduction at a platinum wire cathode[END_REF][START_REF] Liu | High-yield electrosynthesis of hydrogen peroxide from oxygen reduction by hierarchically porous carbon[END_REF] An open question arising from the reactivity observed with carbon and gold electrodes is whether traces of ferrocene dimethanol possibly present in the target solution (0.01 mol

The most favourable experimental conditions for an efficient electro-Fenton process are: non-catalytic cathodic materials such as carbon derivatives, graphite or mercury, where the production of hydrogen peroxide is maximized, ambient temperature, acidic medium (pH 2 -4), and the presence of metal catalysts such as salts of iron (II) and

(III), copper (I), cobalt (III) among others; although the salts of iron (II) and (III) are the best catalysts since they act efficiently at lower concentrations, typically 10 -4 mol L -1 . [START_REF] Brillas | Electro-Fenton process and related electrochemical technologies based on Fenton's reaction chemistry[END_REF] In our case, the experimental conditions are not the most favorable for an electro-Fenton process, with a pH around 6 in the KPF 6 solution, where platinum and gold electrodes, unlike carbon, can be considered catalytic for a direct conversion of oxygen to water following a 4-electron pathway, and with no metals in solution, only possible traces of ferrocene from the redox mediator used to approach the tip to the surface. Other sources of metal ions such as iron, copper, cobalt, chromium and nickel were previously discarded by ICP-MS analysis of the electrolytes (nBu 4 NPF 6 and KPF 6 ), reagents (4-(methyl)benzenediazonium tetrafluoroborate) and water used for the modification of the PPF surfaces and during the SECM experiments; traces of iron, copper and nickel were only found in concentrations below 0.02 ppm in the electrolyte KPF 6 . In any case, production of H 2 O 2 by the reduction of oxygen develops in most cathodes materials, but even so, a source of metal ions is required for its decomposition.

Concerning the possible catalytic activity of ferrocene and its derivatives towards the decomposition of H 2 O 2 by a Fenton-like reaction, some recent works have considered this approach for the removal of methylene blue and other organic molecules such as antibiotics from aqueous solutions. [START_REF] Wang | Ferrocene-catalyzed heterogeneous Fenton-like degradation of methylene blue: Influence of initial solution pH[END_REF][START_REF] Lin | Ferrocene-functionalized graphitic carbon nitride as an enhanced heterogeneous catalyst of Fenton reaction for degradation of Rhodamine B under visible light irradiation[END_REF][START_REF] Divyapriya | Ferrocene functionalized graphene based electrode for the electro-Fenton oxidation of ciprofloxacin[END_REF] However, from these publications the role of ferrocene as a catalyst cannot be asserted since other side processes or reactions such as hydrogen evolution, reduction of hydrogen peroxide in the cathode, decomposition of hydrogen peroxide by applying a source of light, or adsorption of the organic molecule coexist during its degradation, which has been inadequately attributed to the production of hydroxyl radicals by catalytic hydrogen peroxide decomposition in the presence of ferrocene. Considering the stability of ferrocene, [START_REF] Astruc | Why is ferrocene so exceptional?[END_REF] and the fact that if it was present, it was at concentrations certainly lower than those detectable by electrochemical means on an ultramicroelectrode; its participation in the modification of the methylaryl multilayer seems unlikely. Nevertheless, to avoid the possible contribution of a Fenton-like reaction to the production of hydroxyl radicals during the ORR, future works should consider the use of aqueous soluble non-metal mediators.

Finally, in relation to the modification observed on the methylaryl multilayer at anodic applied potentials in both electrolyte media, in particular at + 0.6 V, + 0.4 V and + 0.2 V vs Ag/AgCl in 0.01 mol L -1 KOH and at + 0.6 V and + 0.

Methodology

Operational modes in SECM. Different SECM operational modes have been described. [START_REF] Bard | Scanning Electrochemical Microscopy[END_REF] The two most commonly used depend essentially of how the probe is used: either the probe electrode detects one or more redox species or it generates the electroactive species. The first mode is called generation/collection (GC) since it consists in recording the oxidation (or reduction) current of a redox species produced by the substrate. In this case, the mode is called "substrate generation/tip collection" (SG/TC). [START_REF] Bard | Scanning Electrochemical Microscopy[END_REF] This mode has a lower resolution since it depends on the diffusion of the species from the substrate to the tip UME. However, it has found its utility i.e.

for analyzing biological cells (surface topography, respiration, electron transfer and membrane transport among others) on inert substrates. [START_REF] Amemiya | Biological applications of scanning electrochemical microscopy: chemical imaging of single living cells and beyond[END_REF] It is also possible to use the substrate as collector, that means, the tip UME generates an electroactive species that is detected by the substrate. This is called "tip generation/substrate collection" (TG/SC). This mode requires a bipotentiostat and it allows, i.e., the study of homogeneous reactions in solution. [START_REF] Zhou | Scanning electrochemical microscopy. 16. Study of second-order homogeneous chemical reactions via the feedback and generation/collection modes[END_REF] The second mode is called "feedback" mode and it is the most commonly routine for surface analysis. [START_REF] Pust | Investigation of localized catalytic and electrocatalytic processes and corrosion reactions with scanning electrochemical microscopy (SECM)[END_REF] As far as this thesis is concerned, this is the most suitable experimental approach for interrogate the organic layer before and after performing the oxygen reduction. For measurements in feedback mode, one redox form of a reversible redox couple is added to the solution (typically in a concentration of 10 -3 mol L -1 ). For further explanation, we assume that the reduced form of the redox couple is in solution, therefore the applied potential at the tip UME is that at which the reduced form is oxidized under diffusion-controlled conditions. The applied potential is selected from a previous cyclic voltammogram of the reduced form. Routinely, a steady-state cyclic voltammogram is recorded to obtain the diffusion-controlled current i inf in the bulk solution that is calculated from the equation i inf = 4nF DaC * β(RG). Here n is the number of transferred electrons, F is the Faraday constant, D is the diffusion coefficient, and C * is the bulk concentration of the electroactive species; the typical values of the factor β(RG) at a microdisk electrode are around 1.02 and 1.04 for a RG parameter ranging between 5 and 10. [START_REF] Lefrou | Analytical expressions for quantitative scanning electrochemical microscopy (SECM)[END_REF] This value (i inf ) serves as a reference value for measurements close to the sample surface. Often a measurement starts moving slowly the tip UME towards the surface for registering an approach curve, where the tip current is function of the distance tip/surface. The feedback mode can therefore be used in two different ways in electrochemical microscopy: either the distance tip/surface is fixed then the surface is scanned in order to obtain a chemical image of it; or the current is measured as a function of the tip/surface distance in order to obtain a SECM approach curve. One of the strengths of this microscopy is the analysis of these approach curves. If the tip UME approaches an insulating surface, the surface hinders the diffusion of the reduced form towards the tip UME, and in turn the current registered on the tip decreases with the distance (Figure 4.4b). This scenario is described as "negative feedback". On the contrary, if the UME moves towards a conducting surface, the reduced form can be regenerated by redox reaction on the substrate and as a consequence the current at the tip UME increases with the distance (Figure 4.4c). This phenomenon is described as "positive feedback". Note that the regeneration of the mediator at the substrate surface can occur in three different ways: redox reaction (O + ne -= R); local modification of the substrate such as local reduction, similar to those phenomena observed in corrosion;

and, a mediated process with consumption of O at the substrate (See [START_REF] Kwak | Scanning electrochemical microscopy. Theory of the feedback mode[END_REF][START_REF] Pust | Investigation of localized catalytic and electrocatalytic processes and corrosion reactions with scanning electrochemical microscopy (SECM)[END_REF][START_REF] Lefrou | Analytical expressions for quantitative scanning electrochemical microscopy (SECM)[END_REF] for a quantitative approach to the feedback mode).

Methodology in SECM. To investigate the oxygen reduction by SECM we used a foot-printing strategy. This approach is based on the use of a sensitive surface to evidence the formation of reactive oxygen species. Different polyaromatic layers can be covalently bounded on carbon substrates by electroreduction of aryl diazonium salts, bearing different chemical functions that in turn will provide a differentiated reactivity to the the organic layers towards oxidants in solution. In this Chapter 4, we follow a foot printing strategy based on the use of SECM in feedback mode when the modified substrate (the modified PPF with the corresponding polyaromatic layer) is not electrically connected. Briefly, three are the main steps that describe the foot printing strategy: positioning of the tip UME at 4 -5 µm to the organic layer, oxygen reduction on the surface of the tip UME, and "reading" of the organic layer after performing the reaction (Scheme 4.5). The tip UME is placed in the close proximity of the organic layer by registering approach curves with the aid of a redox probe in solution; from the approach curves, the distance tip-substrate is derived. Once the tip UME is in position, the solution with the redox probe is replaced with the target solution for oxygen reduction; during this replacement, the electrochemical cell is thoroughly rinsed to remove possible pollutants that could affect the experiment. The potential to reduce oxygen is applied to the tip; this potential is previously established by registering the cyclic voltammogram of oxygen reduction in the medium under investigation. To evidence the local transformations, the organic layer is "read" with an indifferent redox mediator; the impact of the reactive oxygen species on the organic layer can be assert either moving the tip in the solution to register an image of the area in the x-y plane (the image should be registered before and after exposure by comparison); or registering the approach curves before and after ROS interaction. Different types of approach curves

Conclusion

In aqueous media, ORR is generally described as a mixture of 2-and 4-electron reduction processes leading to the formation of H while the later was affected in both media and almost at all applied potentials, anodic and cathodic, except around 0 V. This differential behavior could be attributed to the "radical scavenger" properties of the unsaturated bonds integrated in the ethynylaryl multilayer, which seem to prevent or delay the oxidative damage of the polyaromatic structure during the exposition to oxygen reactive species, unlike that observed in the methylaryl multilayer.

Concerning the influence of the electrolyte and the pH, low concentrations of electrolyte were expected to avoid side reactions of the reactive oxygen species electro-generated on the platinum electrode. Two different aqueous electrolyte solutions with different pH values were considered. The larger damage on the methyl layer was observed when the electrolyte was KOH (pH ∼ 10). It is difficult to extract conclusions regarding the effect of the pH but from the literature, [START_REF] Sawyer | How super is superoxide?[END_REF][START_REF] Sawyer | One-electron mechanism for the electrochemical reduction of molecular oxygen[END_REF] and despite that different mechanisms of reaction have been described depending on the pH of the aqueous solutions, pulse radiolysis experiments indicate a total conversion of oxygen to hydrogen peroxide between pH values from 2 to 5, and a yield of 80 % in the case of alkaline solutions (pH ∼ 11). These considerations should be taken with caution given the widely demonstrated influence of the nature of the electrode on the catalysis of the oxygen reduction. [START_REF] Marković | Oxygen reduction reaction on Pt and Pt bimetallic surfaces: A selective review[END_REF][START_REF] Gómez-Marín | Oxygen reduction reaction at Pt single crystals: A critical overview[END_REF] Although from these series of experiments, the oxidizing species responsible of the damage on the methyl layer cannot be derived, the direct participation of H 2 O 2 was discarded; this is consistent with its moderate oxidizing properties. At those potentials corresponding to the plateau of the ORR, preliminary investigations carried out in 0.01 mol L -1 KPF 6 aqueous solution with gold and carbon ultramicroelectrodes on the methylaryl multilayer, revealed a possible non-catalytic mechanism of hydroxyl radical production as damage to the organic layer was also observed when the working electrode Third and finally, an important time period of this thesis has been dedicated to the study of the catalysis of oxygen reduction in aqueous media on polycrystalline platinum electrodes. The technique and methodology used are adapted to the objective previously defined as the determination of reactive oxygen species during the ORR and the impact of parameters such as the applied potential and the reaction media during the ORR. To evidence the production of highly reactive species, we used the SECM imaging approach, which is well adapted to the study of oxygen catalysis as evidenced by the extensive literature on the subject, as it allows a rapid screening at the local level of samples evidencing their topography and electroactivity in one time experience. The experimental approach, previously developed in the group, is known as "foot-printing". It is based on the use of different organic layers that are sensitive to the presence of oxidizing species. The role of different organic layers was investigated. SECM experiments reported differential behaviours between ethynylaryl and the methylaryl multilayers, which may be explained in terms of their reactivity towards hydroxyl radicals. Preliminary experiences with gold and carbon electrodes evidence a possible non-catalytic production of hydroxyl radicals that presumably are the only species with sufficient oxidizing ability to react with polyaromatic layers as those studied here. No significant differences were observed when the reaction medium was modified; however, an interesting observation was the modification of the methylaryl layer at applied potentials corresponding to the anodic region, where no electro-activity was observed by cyclic voltammetry. The overall observations here reported have not been fully understood and many questions remain about the exact nature of the intermediate and the mechanism as the detection remains indirect. Thus, future work may consider characterization of the chemical composition of the layer after exposure to oxidizing species and ensure radical production in the anodic region, i.e., by using radical scavengers in solution. Catalytic or non-catalytic production of these radicals could be considered in future investigations under rigorous control of the experimental conditions. Dans ce manuscrit, nous avons démontré que les propriétés électrochimiques de sondes redox en solution ou greffées à la surface d'un matériau peuvent être modulées par l'introduction de films organiques, permettant la réalisation de recherches fondamentales dans des champs disciplinaires aussi important que le stockage de l'énergie ou de la catalyse.

Introduction 1

 1 Aryldiazonium salts 1.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.2 Surface modification by aryldiazonium salts . . . . . . . . . . . . . . . 1.2.1 Electrochemical reduction of aryldiazonium salts . . . . . . . . . 1.2.1.1 Reduction at the electrode surface . . . . . . . . . . . 1.2.1.2 Other methodologies of reduction . . . . . . . . . . . . 1.2.2 Electrochemical behavior of diazonium salts . . . . . . . . . . . 1.3 From multilayer to monolayers . . . . . . . . . . . . . . . . . . . . . . . 1.3.1 Aryl multilayers . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.3.2 Aryl monolayers . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.4 Post-functionalization procedures . . . . . . . . . . . . . . . . . . . . . 1.5 Characterization of modified surfaces . . . . . . . . . . . . . . . . . . . 1.5.1 Chemical analysis of modified surfaces . . . . . . . . . . . . . . 1.5.2 Thickness of organic films . . . . . . . . . . . . . . . . . . . . . 1.5.3 Compactness of organic films . . . . . . . . . . . . . . . . . . . 1.6 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Objectives 2 Alkyl-ferrocene monolayer carbon in different solvents.

Figure 1 . 2 :

 12 Figure 1.2: (a) I-E plots of 10 -3 mol L -1 p-ethynylbenzenediazonium tetrafluoroborate in MeCN + 0.1 mol L -1 nBu 4 NPF 6 on a GC electrode (2 mm-diameter). Scan rate 0.05 V s -1 . (b) I-E plots of 10 -3 mol L -1 Fc in MeCN + 0.1 mol L -1 nBu 4 NPF 6 before (blue) and after (red) electrografting of p-ethynylbenzenediazonium tetrafluoroborate on a GC electrode (2 mm-diameter). Scan rate 0.1 V s -1 .

  Scheme 1.4: Mechanism of reduction of aryl halides compounds. From "Attachment of aromatic layers to conductive or semi-conductive surfaces by reduction of diazonium salts" by Pinson and Podvorica, Chem. Soc. Rev., 34, p. 431. Copyright The Royal Society of Chemistry 2005.

1. 3 From multilayer to monolayers 1 . 3 . 1

 3131 Scheme 1.5: Mechanism of surface modification by electro-reduction. Adapted from "Steric Effects in the reaction of aryl radicals on surfaces" by Podvorica et al., Langmuir, 25, p. 287. Copyright 2009 ACS.

Scheme 1 . 6 :

 16 Scheme 1.6: Mediated electron transfer through redox active-aryl films. The electroactive groups incorporated in the organic film allow the transfer of charge between the surface of the electrode and the electroactive molecules in solution, allowing the process of polymerization to very thick films (microns). From "Using a mediated effect in the electroreduction of aryldiazonium salts to prepare conducting organic films of high thickness" by Daasbjerg et al., Chem. Mat., 23, p. 1551. Copyright 2011 ACS.

Figure 1 . 3 :

 13 Figure 1.3: (a) I-E plots of 2x10 -3 mol L -1 K 3 [Fe(CN) 6 ] in 0.1 mol L -1 KCl aqueous solutions and (b) 2x10 -3 mol L -1 Fc in MeCN solution + 0.1 mol L -1 nBu 4 NPF 6 on a bare gold electrode (1), and modified C 6 H 5 BF 4 -gold (2) and C 6 H 4 NO 2 BF 4 -gold (3) electrodes. Scan rate 0.2 V s -1 . Adapted from "Formation of polyphenylene films on metal electrodes by electrochemical reduction of benzenediazonium salts" by Podvorica et al., Chem. Mat., 18, p. 2025. Copyright 2006 ACS.

  Scheme 1.7: Formation of multilayers and monolayers. Adapted from "Sterically hindered diazonium salts for the grafting of a monolayer on metals" by Podvorica et al., J. Am. Chem. Soc., 130, p. 8576. Copyright 2008 ACS.

  Scheme 1.8: Formation-degradation procedure. Adapted from "Using a hydrazoneprotected benzenediazonium salt to introduce a near-monolayer of benzaldehyde on glassy carbon surfaces" by Daasbjerg et al., J. Am. Chem. Soc., 131, p. 4930. Copyright 2009 ACS.

Figure 1 . 4 :

 14 Figure 1.4: Contact angle images of a 4-tetradecylphenyl-iron surface. From "Hydrophobization of metal surfaces by covalent grafting of aromatic layer via aryldiazonium chemistry and their application in the fabrication of superhydrophobic surfaces" by Pan et al., Chem. Lett., 36, p. 1312. Copyright 2007 The Chemical Society of Japan.

  Figure 1.5: (a) AFM "scratching" image and (b) height profile of a 4-nitrobenzene film on PPF. From "Design of robust binary film onto carbon surface using diazonium electrochemistry" by Leroux et al., Langmuir, 27, 11222-11228. Supporting Information. Copyright 2011 ACS.

Figure 2 .

 2 Figure 2.1 shows I-E plots of two different experiments performed with ferroceneterminated SAMs in aqueous electrolyte solutions. Figure 2.1a shows the effect of the anion on the oxidation of ferrocene from a Fc(CH 2 ) 4 COO(CH 2 ) 9 SH coated-gold

Figure 2 . 1 :

 21 Figure 2.1: (a) I-E plots of Fc(CH 2 ) 4 COO(CH 2 ) 9 SH immobilized on gold in aqueous solutions containing different anions (0.1 mol L -1 ) + 0.01 mol L -1 sodium phosphate (pH 7.0). (b) I-E plots in 1 mol L -1 HClO 4 aqueous solution of electrodes coated from ethanol solutions containing FcC 6 SH and n-alkanethiols: (A) n-butanethiol; (B) n-hexanethiol; (C) n-octanethiol; (D) n-decanethiol; and (E) n-dodecanethiol. Scan rate 0.1 V s -1 . From "Anion effect on mediated electron transfer through ferroceneterminated self-assembled monolayers" by Valincius et al., Langmuir, 20, p.6632. Copyright 2004 ACS; and from "Redox and ion-pairing thermodynamics in self-assembled monolayers", by Creager et al., Langmuir, 7, p.2310. Copyright 1991 ACS.

  use of spectroscopic techniques (Fourier transform surface-enhanced Raman spectroscopy) coupled with CV allowed the authors to analyze the organic layer in structural terms. Therefore, infrared spectroscopy confirmed the formation of ion pairs between poorly solvated anions in water and ferrocenium ions upon oxidation. No evidence of ion pair formation was observed in solutions containing ions with high hydration energies in water. Regarding the influence of the anion on the rate constant of the ferrocene-mediated ascorbic acid oxidation, authors pointed out to possible changes at the molecular level in the organic layer (Scheme 2.1). No explanation was provided for the shift in the redox potential of ferrocene.

Figure 2 .

 2 Figure 2.1b shows the electrochemical response of mixed self-assembled monolayers constituted by ferrocenehexanethiol (Fc(CH 2 ) 6 SH) and n-alkanethiols with an increasing chain length. The experiment consisted in modulate the dielectric environment of the ferrocene attached to the tail of the hexanethiol chain by changing the length of a co-adsorbed n-alkanethiol chains from four to twelve carbons.[125] The electro-

Ferrocene-terminated monolayers

  Scheme 2.2: Consecutive two-electron reduction of methyl viologen.

Figure 2 .Figure 2 . 2 :

 222 Figure 2.2: Evolution of the real (a) and complex (b) capacitance with the frequency for a real RC circuit, which values of capacitance and resistance are previously known: 0.47 µF and 1600 Ω.

Scheme 2 . 3 :

 23 Scheme 2.3: General procedure for modifying carbon materials through a protectiondeprotection strategy. On the right, the resulting ferrocene-terminated C 11 -alkyl monolayer (n = 9) covalently attached to the surface.

4

 4

  as example). Cyclic voltammograms on Figure 2.4 show the response of the modified electrode in MeCN and DMSO containing 0.1 mol L -1 nBu 4 NPF 6 and their corresponding current peak vs scan rate plots,

Figure 2 .

 2 3a show the response in all the organic electrolyte solutions. A rapid inspection of the responses shows that there is a significant differ-ence between the media with low-intermediate and high dielectric constants. We can distinguish two groups according to the electrochemical response, on the one hand, the solvents with low-intermediate dielectric constants (CH 2 Cl 2 and EtOH) gave rise to narrower peaks and lower oxidation peak potentials (Figure2.3b), while the solvents with high dielectric constants (MeCN, DMSO and PC) showed broader peaks and the ferrocene moieties were oxidized at more positive potentials (Figure2.3c). Different parameters characterize the electron transfer of the ferrocene monolayer: the anodic and cathodic peak potentials (E pa , E pc ), the anodic and cathodic current peak intensities (I pa , I pc ), the full peak width at half maximum current (FWHM) and the peak-to-peak distance (∆E p ). Other parameters of interest are calculated from the experimental data: the ferrocene surface coverage (Γ Fc ) derived from the integration of the faradaic peak current at low scan rates, considering the geometrical surface area of the pyrolytic graphite electrode, and the half-wave potential (E 1/2 ) as the half-sum of the anodic and cathodic peak potentials. All the parameters that characterize the electron transfer of ferrocene are summarized in Table2.1.

Figure 2 . 3 :

 23 Figure 2.3: (a) I-E plots for the oxidation of the ferrocene-terminated C 11 -alkyl monolayer onto a pyrolytic graphite electrode in all organic electrolytes: 0.1 mol L -1 nBu 4 NPF 6 + CH 2 Cl 2 (black), MeCN (dark blue), DMSO (red) and PC (cyan), and 0.1 mol L -1 LiClO 4 + EtOH (green). Solvents of low (b) and high (d) dielectric constant separately. Scan rate 0.2 V s -1 . All potentials are referred to FcMeOH/FcMeOH + as internal standard.

Figure 2 .

 2 Figure 2.4: I-E plots for the oxidation of ferrocene-terminated C 11 -alkyl monolayer onto a pyrolytic graphite electrode (3 mm-diameter) in MeCN (a) and DMSO (b) organic solvent + 0.1 mol L -1 nBu 4 NPF 6 . Scan rates from 0.1 to 0.9 V s -1 without iR compensation. All potentials are referred to FcMeOH as internal standard. Corresponding anodic and cathodic peak current (I) vs scan rate plots for MeCN (c) and DMSO (d) electrolytic solutions and their linear fits (blue and red lines respectively).

CH 2

 2 Cl 2 and EtOH from the anodic to the cathodic process, indicating the probably dissociation of the ion pairs * Fe(Cp) 2 + PF 6 -or * Fe(Cp) 2 + ClO 4 -during the reduction of the ferrocenium ions to ferrocene; this dissociation does not take place in MeCN, DMSO and PC electrolyte solutions since there was no previous ion-pairing interaction. This could explain the no substantial differences observed between the anodic and cathodic FWHM, concluding that the environment of the electroactive groups on the organic layer hardly changed during the electron transfer in MeCN, DMSO and PC electrolyte solutions. All these observations concerning the full peak width are supported by the literature where it have been observed by CV the increase of the peak width of oxidation of ferrocene when the anions (from the electrolyte) in solution are highly solvated by the solvent, and therefore the formation of ion pairs was not favored.([131, 132, 134] and other references therein) As shown in Figure 2.3, the peak intensities showed a strong dependence on the reaction medium. Thus, solvents with the lower dielectric constants (CH 2 Cl 2 and EtOH) presented well-defined peaks and greater current intensities, while solvents with the higher dielectric constants (MeCN, DMSO and PC) showed broader peaks and lower current intensity values. It was discarded that the observed differences in current intensities were due to a degradation of the organic layer in MeCN, DMSO and PC solutions. A control experiment was performed in which the voltammogram of a modified electrode was sequentially registered in all the organic electrolyte solutions, using the electrochemical response of ferrocene in CH 2 Cl 2 as reference. The characteristic electrochemical response of ferrocene in CH 2 Cl 2 was always recovered after each solvent. Hence, this diminution of the current peak intensities in MeCN, DMSO and PC is not consequence of the degradation of the organic layer in these media. The smaller cur-rent peak intensities in MeCN, DMSO and PC electrolyte solutions are connected by the kinetics of the system with the larger peak widths observed in these media. These observations go in the direction of a possible influence of the medium on the kinetics of the reaction. The kinetics of the reaction seems to decrease with the greater solvation of the anions by polar solvents; on the contrary, kinetics is favored in environments where the anions are poorly solvated (CH 2 Cl 2 and ethanol), resulting in narrower and more intense peaks. At this point we can point out that both the kinetics and thermodynamics of the system are affected by the different solvation of the hydrophobic anions in the organic solvents, which in turn is related to the formation of ion pairs in non-polar or water-like environments. Concerning the reversibility of the electron transfer, the ratios of anodic and cathodic current peak intensities (I pa /I pc ) were calculated for each electrolyte solution. (I pa /I pc ) values close to the unit or slightly below were obtained in CH 2 Cl 2 and EtOH electrolyte solutions, indicating that the oxidation of ferrocene was reversible in these media. In MeCN, DMSO and PC solutions, quasi-reversible electron transfers took place. In terms of stability, it was registered a 25 % loss of faradaic current after 1000 charge/discharge cycles in CH 2 Cl 2 + 0.1 mol L -1 nBu 4 NPF 6 solution. Certainty, aprevious process of purification and drying of the organic solvent would be advisable to improve the stability of the system, avoiding possible side reactions with impurities during the electron transfer.

  1 and a review of the literature, we can conclude that both the kinetics and thermodynamics of the ferrocene-terminated C 11 -alkyl monolayer electron transfer are influenced by the electrolyte solution, in particular for the solvent employed. A differential solvation of the hydrophobic anions in the organic solvents produces variations in the extent of the ion-pairing reaction between the anions in solution and the ferrocenium cations upon oxidation. As consequence of the ion-pairing reaction in CH 2 Cl 2 and EtOH electrolyte solutions, the redox potential of ferrocene shifts to more negative potentials as predicted by the Nernst equation considering that the reaction proceeds via electron transfer and ion-pairing reaction. This shift in the potential could be attributed to the stabilization of the electrogenerated ferrocenium cations, contributing in turn to the integrity of the organic layer. The values of full width at half maximum (FWHM) obtained in CH 2 Cl 2 and EtOH electrolyte solutions, especially in EtOH, for the oxidation of the ferrocene-terminated C 11 -alkyl monolayer indicate that ferrocenes on the surface have equivalent environments, and therefore all the available surface ferrocenes are reversible oxidized at the same halfwave potential yielding sharp and well-defined peaks without loss in the transferred charge. In this case, the ion-pairing reaction between the ferroceniums and the anions could be reversible and stoichiometric (1:1) as suggested elsewhere in the literature.On the contrary, in MeCN, DMSO and PC electrolyte solutions, the positive shift of the potential is understood as a partial or lack of ion-pairing interaction between the anions and the ferroceniums, since the anions are strongly solvated in these media.A partial compensation of the charge on the layer upon oxidation (non-stoichiometric ion-pairing reaction) could lead to different ferrocene populations with different dielectric environments, which is compatible with the larger values of FWHM obtained in these electrolyte solutions. Concerning the loss of charge density observed through the analysis of the surface coverage in these organic electrolyte solutions (MeCN, DMSO and PC), as a hypothesis, the possible participation of different ferrocene populations in electron transfer, having different dielectric environments, could lead to different redox potentials and kinetics of reaction, being some redox units disable or disfavoured for the electron transfer.

  .5). The roughness values found were 0.067 nm (PPF), 0.982 nm (GC) and 72 nm (edge plane PG). Images show a very smooth and homogeneous PPF surface (Figure2.5a), with a maximum height difference of 0.425 nm. Scratches created during the polishing process are visible on the GC sample (Figure2.5b). The polishing increases the surface roughness and introduces defects on the electrode surface, being the maximum height difference 15 nm. In the sample corresponding to the edge plane PG (Figure2.5c), we observe the perpendicular stacking of graphite sheets characteristic of this type of material. It shows a highly irregular and rough surface, with a distribution of heights spanning from the micro to the nanometric range. As consequence, the effective electrode surface area of a edge plane PG electrode is much larger than the corresponding geometric one. This

Figure 2 .

 2 Figure 2.5: AFM images of bare PPF (a), GC (b) and edge plane PG (c) electrodes.

Figure 2 .

 2 Figure 2.6: I-E plots for the oxidation of ferrocene-terminated C 11 -alkyl monolayer onto (a) PPF, (b) GC, and (c) edge plane PG electrodes in CH 2 Cl 2 solution + 0.1 mol L -1 nBu 4 NPF 6 . Scan rate 0.1 V s -1 without iR correction.

Figure 2 . 7 :

 27 Figure 2.7: I-E plots for the oxidation of the ferrocene-terminated C 11 -alkyl monolayer on a pyrolytic graphite electrode in an aqueous 0.1 mol L -1 KPF 6 electrolyte solution at scan rate 0.1 V s -1 (a), and at increasing scan rates from 0.1 to 0.9 V s -1 (b) without iR correction. (c) Anodic and cathodic peak current intensities (I) vs scan rate plots in aqueous solution and their linear fits (blue solid lines). All potentials are referred to the FcMeOH/FcMeOH + couple.

Figure 2 . 8 :

 28 Figure 2.8: Simulation for a fast electron transfer where k ap s = 10 3 s -1 and α = 0.5 followed by an ion pairing reaction with K ip = 10 -4 , and (a) kf = 10 M -1 s -1 ; k b = 10 5 M -1 s -1 , (b) k f = 1 M -1 s -1 ; k b = 10 4 M -1 s -1 , and (c) k f = 0.1 M -1 s -1 ; k b = 10 3 M -1 s -1. Scan rate 0.1 V s -1 and Γ Fc = 10 -10 mol cm -2 . Note that the simulation was performed in reduction. Simulations carried out with the free-available software package MECSim.[175]

  .10), edge planes pyrolytic graphite presents numerous fissures and pores and the electrode behaves as a collection of impedances with different values and time constants leading to the response observed in EIS. Roughness (rms) values were estimated from the AFM topographic images of a bare edge plane PG electrode and a ethynylaryl-modified PG electrode (after deprotection step). The roughness values were 72.38 nm and 88.21 nm.

Figure 2 . 9 :

 29 Figure 2.9: (a) I-E plots of a bare (•) and a ferrocene-terminated C 11 -alkyl monolayermodified ( ) pyrolytic graphite electrode in CH 2 Cl 2 + 0.1 mol L -1 nBu 4 NPF 6 solution at scan rate 0.1 V s -1 . Two vertical dashed lines mark the two different fixed potentials at which EIS data were acquired, -0.08 V and + 0.30 V. (b) Nyquist and (c,d) Bode diagrams of a bare ( , ) and a ferrocene-terminated C 11 -alkyl monolayer-modified ( ,•) pyrolytic graphite electrode in CH 2 Cl 2 + 0.1 mol L -1 nBu 4 NPF 6 solution at -0.08 V ( , ) and + 0.30 V ( ,•). Geometric area PGE = 0.071 cm 2 . All potentials are referred to FcMeOH as internal standard.

Figure 2 .

 2 Figure 2.10: AFM topographic 3D-images of a (a) bare edge plane PG electrode and (b) ethynylaryl-modified PG electrode (after deprotection step).

Figure 2 . 11 :

 211 Figure 2.11: Evolution of the (a) real and (b) complex capacitance versus frequency for the ferrocene-terminated C 11 -alkyl monolayer modified-edge plane PG electrode in CH 2 Cl 2 + 0.1 mol L -1 nBu 4 NPF 6 solution at two different fixed potentials, -0.08 V ( ) and 0.30 V (•) vs FcMeOH/FcMeOH + couple.

Figure 2 . 12 :

 212 Figure 2.12: Nyquist capacitive plots of the ferrocene-terminated C 11 -alkyl monolayer modified pyrolytic graphite in CH 2 Cl 2 + 0.1 mol L -1 nBu 4 NPF 6 solution at two different fixed potentials, (a) -0.08 V and (b) 0.30 V vs FcMeOH/FcMeOH + couple. Lines correspond to semi-circle curve-fittings.

Figure 2 .

 2 Figure 2.13: (a,c) Phase-angle and (b,d) log |Z| versus log frequency of the ferroceneterminated C 11 -alkyl monolayer modified-edge plane PG electrode in (•) CH 2 Cl 2 , ( ) MeCN, ( ) DMSO and ( ) PC + 0.1 mol L -1 nBu 4 NPF 6 , in ( ) EtOH + 0.1 mol L -1 LiClO 4 , and in ( ) water + 0.1 mol L -1 KPF 6 at (a,b) an applied potential where there is no faradic contribution and (c,d) at an applied potential where the ferrocenyl moieties are oxidized.

Figure 2 .

 2 Figure 2.14: I-E plots for the reduction of 10 -2 mol L -1 TIPS-Eth-ArN 2 + tetrafluoroborate in acetonitrile solution + 0.1 mol L -1 nBu 4 NPF 6 on an edge-plane PG electrode (a) and on a PPF (b). Electrografting conditions: five consecutive voltammetric cycles from + 0.6 to -0.8 V vs SCE at scan rate 0.05 V s -1 . The reduction peak potentials were: E pc = + 0.13 V vs SCE on the edge-plane PG electrode, and E pc = + 0.08 V vs SCE on PPF.

Figure 2 .

 2 Figure 2.15: (a) I-t and (b) I-E plots of C 11 -alkyl ferrocene monolayer on pyrolytic graphite electrode in CH 2 Cl 2 + 0.1 mol L -1 nBu 4 NPF 6 . Scan rate 0.1 V s -1 .

Figure 2 .2. 4 Conclusion

 24 Figure 2.16: (a) Nyquist capacitive plot of C 11 -alkyl ferrocene monolayer on PG electrode in CH 2 Cl 2 + 0.1 mol L -1 nBu 4 NPF 6 solution at potential applied + 0.30 V and its fitting (blue solid line). The radius of the curve fitting was 737.4 µF cm-2, and thus the capacitance of the corresponding system was 1475 µF cm-2 that corresponds to the diameter of the semicircle. (b) Complex capacitance vs frequency of C 11 -alkyl ferrocene monolayer on PG electrode in CH 2 Cl 2 + 0.1 mol L -1 nBu 4 NPF 6 solution at -0.08 V and its fitting (blue solid line). The complex capacitance reached its maximum value at 30.2 Hz, giving a value of ω = 190 rad s -1 (ω = 2πf). The time constant is defined as the inverse of the angular frequency (τ = 1/ω = RC), τ = 0.0053 s.

•

  Thermodynamics. It was observed a shift to more positive potentials of the halfwave potential of ferrocene in MeCN, DMSO and PC electrolyte solutions, where the anions are strongly solvated, and in turn limiting the ion-pairs formation between anions and the ferrocenium cations upon oxidation. On the contrary, the ion-pairing in CH 2 Cl 2 and EtOH stabilized the ferrocenium cations, and probably also the structure of the monolayer, displacing the half-wave potential of ferrocene to more negative values (ferrocene was easily oxidized in these media). Thus, the largest positive shift of the half-wave potential was observed in DMSO, with a difference of 130 mV over EtOH.• Kinetics. The different peak shape and width and current peak intensities observed between the group constituted by CH 2 Cl 2 and EtOH and the group constituted by MeCN, DMSO and PC indicates that the kinetics of the electron transfer is also influenced by the different solvation energies of the anions in the organic solvents. In CH 2 Cl 2 and EtOH, where the ion pairs formation is favored by the nature of the solvents, narrower and sharper peaks with larger current peak intensities were observed. Also, the lower peak widths suggest an equivalent dielectric environment and minimal interactions of the ferrocenium cations upon oxidation. In MeCN, DMSO and PC, dipolar solvents with high dielectric constant where the anions are highly solvated, it was observed broader peaks and smaller current peak intensities that reflects the predominance of repulsive interactions between the ferrocenium cations, and some heterogeneity, with the possible existence of different ferrocene populations with different dielectric environments affecting the kinetics of the electrochemical reaction.Concerning the loss of charge density observed in MeCN, DMSO and PC, as hypothesis, the possible participation of different ferrocene populations in the electron transfer, having different dielectric environments, could lead to different redox potentials and kinetics of reaction, being some redox units totally disable or disfavoured for the electron transfer. However, a possible restructuring of the organic layer or reorientation of the ferrocenium cations after oxidation have been suggested in the literature and can not be discarded from these experiments.

Figure 3 . 1 :

 31 Figure 3.1: Representation of calix[4]arene tetra-aniline undergoing in situ transformation to diazonium salts for electrografting. Adapted from "Electrografting of calix[4]arenediazonium salts to form versatile robust platforms for spatially controlled surface functionalization", by Mattiuzzi et al.,Nat. Commun., 3, p.3. Copyright 2012 Macmillan Publishers Limited.

Figure 3 . 2 :

 32 Figure 3.2: I-E plots of 0.01 mol L -1 Fc (a) and 0.01 mol L -1 1,4-amino-diphenylamine (b) in acetonitrile + 0.1 mol L -1 LiClO 4 on a (1) bare glassy carbon electrode, and a (2) 1-(2-bisthienyl)benzene (BTB), and (3) 1-(2-thienyl)benzene (TB) glassy carbon modified electrode. Scan rate 0.1 V s -1 . Adapted from "Electrochemical switches based on ultrathin organic films: from diode-like behavior to charge transfer transparency" by Lacroix et al., J. Phys. Chem. C, 112, p.18640. Copyright 2008 American Chemical Society; and from "Tunable electrochemical switches based on ultrathin organic films", by Lacroix et al., J. Am. Chem. Soc., 129, p.1890. Copyright 2007 American Chemical Society.

2 + ) 4 ( 123 Figure 3 . 3 :

 2412333 Figure 3.3: (a) Schematic representation of tetrakis(4-phenyldiazonium tetrafluoroborate)methane (C(ArN 2 + ) 4 ). (b) Optimized 3D structure of the molecule C(ArN 2 + ) 4 . (c) I-E plots for the electrochemical grafting of C(ArN 2 + ) 4 in MeCN organic solution + 0.1 mol L -1 nBu 4 NPF 6 on a GC electrode. Scan rate 0.05 V s -1 .

Figure 3 . 4 :

 34 Figure 3.4: AFM images show the topography of the organic films deposited on gold (a) and PPF (b) substrates.

Figure 3 .

 3 Figure 3.5a shows the optimized 3D image of a structural unit corresponding to two monolayers of C(ArN 2 + ) 4 ) covalently bonded to a carbon (or gold) substrate.

Figure 3 . 6 :

 36 Figure 3.6: C 1s (a) and N 1s (b) XPS spectra and deconvoluted peaks recorded for a gold electrode modified by chronoamperometry.

Figure 3 . 7 :

 37 Figure 3.7: C 1s (a) and N 1s (b) XPS spectra and deconvoluted peaks recorded for a carbon electrode modified by chronoamperometry.

Figure 3 . 8 :

 38 Figure 3.8: (a) Post-functionalization experiment using ferrocene as modifier in a Gomberg-Bachmann reaction. I-E plots recorded in MeCN + 0.1 mol L -1 nBu 4 NPF 6 solution on a GC electrode modified by grafting of C(ArN 2 + ) 4 . First scan (black curve), 5 th scan, 10 th scan, 15 th scan, 20 th scan, 25 th scan (black dash curves) and 30 th scan (red curve). Scan rate 0.1 V s -1 . (b) I-E plot recorded in MeCN + 0.1 mol L -1 nBu 4 NPF 6 solution on a GC electrode modified by grafting of C(ArN 2 + ) 4 . Scan rate 0.1 V s -1 . The organic film was not electroactive in the range of potential studied.

Figure 3 . 9 :

 39 Figure 3.9: I-E plots of (a) 10 -3 mol L -1 dopamine + 0.1 mol L -1 H 2 SO 4 aqueous solution and (b) 10 -3 mol L -1 FcMeOH + 0.1 mol L -1 KNO 3 aqueous solution on a bare GC electrode (blue) and on a carbon electrode modified by chronoamperometry with C(ArN 2 + ) 4 (red). Scan rate 0.1 V s -1 .

Ferrocene (Fc), dimethylferrocene (Me 2

 2 Fc) and decamethylferrocene (Me 10 Fc) were chosen as redox probes with increasing steric hindrance to probe the molecular sieving properties of the deposited organic thin films in MeCN. Their molecular volumes were estimated from DFT calculations (Figure 3.10 as example), and their electrochemical behaviors are shown in Figure 3.11. As previously observed for FcMeOH in aqueous solution, Fc and Me 2 Fc (Figures 3.11 a and b) showed a totally irreversible oxidation in MeCN organic solution, whereas the oxidation of Me 10 Fc (Figures 3.

Figure 3 . 10 :

 310 Figure 3.10: Optimized geometries of Fc (a), Me 2 Fc (b), and Me 10 Fc (c).

Figure 3 .

 3 Figure 3.11: I-E plots of 10 -3 mol L -1 (a) Fc, (b) Me 2 Fc and (c) Me 10 Fc in MeCN + 0.1 mol L -1 KPF 6 solution on a bare GC electrode (blue) and on a carbon electrode modified by chronoamperometry with C(ArN 2 + ) 4 (red). Scan rate 0.1 V s -1 . Potentials are referred to Fc as internal standard.

  Figures 3.12 a and c show the I-E plots of the ferrocene derivatives (PhMe) 2 Fc and Ph 2 EFc. In both cases, an irreversible anodic electron transfer was observed on modified carbon substrates, and also values of ∆I below that of Fc were registered, 0.73 and 0.79 respectively for (PhMe) 2 Fc and Ph 2 EFc. Similar experiments were performed on gold substrates modified by chronoamperometry (Table3.4 and Figure3.13). All modified surfaces show similar trends: (i) a decrease of the oxidation peak current intensity with increasing the size of the redox probe associated with (ii) a totally irreversible anodic process. All these results demonstrate that the decrease of the oxidation peak current intensities is correlated to the size of the redox probes and not to the potential applied. Thus, the electro-generated organic thin films of tetrakis(4-phenyldiazonium tetrafluoroborate)methane present sieving properties.

Figure 3 .

 3 Figure 3.12: I-E plots of 10 -3 mol L -1 (a) (PhMe) 2 Fc, (b) Fc and (c) Ph 2 EFc in MeCN + 0.1 mol L -1 KPF 6 solution on a bare GC electrode (blue) and on a carbon electrode modified by chronoamperometry with C(ArN 2 + ) 4 (red). Scan rate 0.1 V s -1 . Potentials are referred to Fc as internal standard.

Figure 3 .

 3 Figure 3.13: I-E plots of 10 -3 mol L -1 (a) Fc, (b) Me 2 Fc, (c) Me 10 Fc, (d) (PhMe) 2 Fc, (e) Fc and (f) Ph 2 EFc in MeCN + 0.1 mol L -1 KPF 6 solution on a bare gold electrode (blue) and on a gold electrode modified by chronoamperometry with C(ArN 2 + ) 4 (red). Scan rate 0.1 V s -1 . Potentials are referred to Fc as internal standard.

Figure 3 .

 3 Figure 3.14: (a) Variation of the anodic peak current intensity with the concentration of Fc from 10 -3 to 10 -2 mol L -1 on a GC electrode modified by chronoamperometry with C(ArN 2 + ) 4 . Linear fit in red (R-square = 0.9993). (b) Corresponding I-E plots of Fc at different concentrations in MeCN + 0.1 mol L -1 KPF 6 . Scan rate 0.1 V s -1 . Potential refers to Fc as internal standard.

Figure 3 . 15 :

 315 Figure 3.15: Variation of the anodic peak current intensity of a 1 mM Fc solution (in MeCN + 0.1 mol L -1 KPF 6 ) with the square root of the scan rate (from 0.1 to 1 V s -1 ) on a bare GC electrode (black squares) and on a GC electrode modified by chronoamperometry with C(ArN 2 + ) 4 (empty squares). Linear fit in red for the bare GC electrode (R-square = 0.9997), and in blue for the modified one (R-square = 0.9998).

Figure 3 .

 3 Figure 3.16: I-E plots of 10 -3 mol L -1 Fc in MeCN solution + 0.1 mol L -1 a) KPF 6 , b) nEt 4 NPF 6 , c) nOc 4 NPF 6 , d) nBu 4 NCl, e) nBu 4 NBF 4 and f) nBu 4 NBPh 4 on a bare GC electrode (blue) and on a carbon electrode modified by chronoamperometry with C(ArN 2 + ) 4 (red). Scan rate 0.1 V s -1 . Potentials are referred to Fc as internal standard.

Figure 3 .

 3 Figure 3.17: I-E plots of 10 -3 mol L -1 Fc in MeCN solution + 0.1 mol L -1 a) KPF 6 , b) nEt 4 NPF 6 , c) nOc 4 NPF 6 , d) nBu 4 NBF 4 and e) nBu 4 NBPh 4 on a bare gold electrode (blue) and on a gold electrode modified by chronoamperometry with C(ArN 2 + ) 4 (red). Scan rate 0.1 V s -1 . Potentials are referred to Fc as internal standard.

  natural and industrial processes. The generally admitted description of ORR in water as a simple competitive pathway process between 2-electron and 4-electron reduction is often inadequate. Electron transfer steps could be accompanied by homogeneous or heterogeneous chemical steps and oxygen-oxygen bond breaking leading to possible numerous intermediates. This chemical reactions are usually as fast that the detection and characterization of intermediates by classical electrochemical methods remains difficult. Our aim here is the determination of intermediates during the ORR on platinum by SECM using a foot-printing strategy. This approach is based on the use of sensitive surfaces to evidence the formation of reactive oxygen species during the ORR. The role of different sensitive layers, applied potential and electrolytes are investigated. keywords: oxygen reduction, intermediates, electrochemical microscopy, aryl multilayer, platinum, aqueous electrolyte Pattern obtained on a methylaryl multilayer after ORR

4 -

 4 Scheme 4.1: ROS formation through chemical and electron transfer reactions. Adapted from "Chapter 1: Overview of reactive oxygen species, in Singlet oxygen. Vol. 1: applications in biosciences and nanosciences", by K. Krumova and G. Cosa. Copyright 2016 The Royal Society of Chemistry.

  Scheme 4.2: Molecular orbital diagrams for ground-state O 2 and ROS. Adapted from "Chapter 1: Oxygen. Introducing oxygen toxicity and reactive species, in Free radicals in biology and medicine, 5th Ed.", by B. Halliwell and J. M. C. Gutteridge. Copyright 2015 B. Halliwell and J. M. C. Gutteridge.

Figure 4 . 1 (

 41 on the right) shows the reversible one-electron reduction of oxygen to superoxide in dried DMSO, and its conversion to a 2-electron process when a small amount of weak acid (phenol) is added. The mechanism proposed was as shown in Equations 4.1 -4.4, where the protonation of the superoxide radical is followed by the disproportionation of the resulting perhydroxyl radical by another molecule of O 2• -, to finally yield hydrogen peroxide.[START_REF] Sawyer | How super is superoxide?[END_REF][START_REF] Andrieux | Mechanism of superoxide ion disproportionation in aprotic solvents[END_REF] O 2 + e -→ O 2

Figure 4 . 1 :

 41 Figure 4.1: Steady-state voltammogram for the electro-reduction of oxygen to superoxyde in dried DMF + 0.1 mol L -1 nBu 4 NPF 6 aerated solution (blue) and oxygensaturated solution (red) registered with a 12.5 µm-diameter gold UME at 0.02 V s -1 (left). Cyclic voltammetry of oxygen (2.1x10 -3 mol L -1 ) in DMSO in absence (a) and presence (b) of phenol (0.1x10 -3 mol L -1 ) on a 3 mm-diameter carbon disk electrode at 0.2 V s -1 (right). This experiment from "Mechanism of superoxide ion disproportionation in aprotic solvents", by Savéant et al., J. Am. Chem. Soc., 109 (12), p.3769. Copyright 1987 by ACS.

Figure 4 .

 4 Figure 4.3 shows the electrochemical characterization of the polyaromatic layers with dopamine and ferrocene in solution. After modification of the carbon surface with p-(methyl)phenyl and p-(ethynyl)phenyl diazonium salts, it was observed a complete inhibition of the electron transfer of dopamine and ferrocene. This indicates that both layers are multilayers, free of defects or pinholes, and they are thick enough to inhibit the electron transfer of ferrocene by tunneling. In connection with the electrochemical behavior observed, an homogeneous and defect-free covering was evidenced by AFM imaging on both modified PPF surfaces (Figure 4.4). Changes of topography and surface roughness were observed according to the growth of disorder and dendritic organic multilayers. Analyses of thickness by ellipsometry yielded similar values for both p-(methyl)phenyl and p-(ethynyl)phenyl multilayers, 5.6 nm and 6.8 nm re-

Figure 4 . 2 :

 42 Figure 4.2: Structure of the aryldiazonium molecules: on the left, the p-(methyl)benzene diazonium; on the right, the p-(ethynyl)benzene diazonium molecule.

Figure 4 . 3 :

 43 Figure 4.3: I-E plots of (a,b) p-(ethynyl)phenyl multilayer and (c,d) p-(methyl)phenyl multilayer on glassy carbon in an (a,c) aqueous solution of 10 -3 mol L -1 dopamine + 0.1 mol L -1 H 2 SO 4 and (b,d) an acetonitrile solution of 10 -3 mol L -1 ferrocene + 0.1 mol L -1 nBu 4 NPF 6 before (blue line) and after (red line) electro-grafting the carbon electrode by chronoamperometry (at -0.8 V vs SCE during 300 seconds). Scan rate 0.1 V s -1 .

Figure 4 . 4 :

 44 Figure 4.4: AFM topographic images of (a) bare PPF, (b) p-(methyl)phenyl-modified PPF and (c) p-(ethynyl)phenyl-modified PPF.

  -4 mol L -1 in 0.01 mol L -1 KPF 6 . Values of oxygen solubility in aqueous electrolyte solutions from the literature range between 10 -3 and 10 -6 mol L -1 , depending on the partial pressure of oxygen and the concentration of electrolyte at 298 K.[START_REF] Lingane | Chronopotentiometric study of oxygen reduction at a platinum wire cathode[END_REF][START_REF] Narita | Solubility of oxygen in aqueous electrolyte solutions[END_REF]

Figure 4 .

 4 Figure 4.5: Steady-state cyclic voltammetry of oxygen in (a) 0.01 mol L -1 KOH and (b) 0.01 mol L -1 KPF 6 aqueous solution on a platinum ultramicroelectrode (5 µmradius). Blue solid squares mark the sequence of applied potentials studied by SECM. Potentials referred to Ag/AgCl. Scan rate 0.02 V s -1 .

Figures 4 .

 4 Figures 4.6 a, b, c and d shows the SECM experiments performed in 0.01 mol L -1 KOH, before oxygen reduction (Figures 4.6 a and b) and after oxygen reduction (Figures 4.6

c

  and d); Figures 4.6 e, f, g and h shows the SECM experiments performed in 0.01 mol L -1 KPF 6 , before oxygen reduction (Figures 4.6 e and f) and after oxygen reduction (Figures 4.6 g and h). Therefore, SECM images on the left show all the experiments performed on the p-(ethynyl)phenyl multilayer, and figures on the right show all the experiments on the p-(methyl)phenyl multilayer.In each media, we studied the effect of the applied potential during the ORR; the half-wave potential of oxygen reduction in the basic solution was -0.28 V vs Ag/AgCl, and in the neutral solution (0.01 mol L -1 KPF 6 ), it was -0.44 V vs Ag/AgCl. Note that the surface of the platinum ultramicroelectrode is not a single-crystal surface; the electrodes are routinely polished with silicon carbide abrasive paper and ultrapure water. We chose a sequence of applied potentials covering a wide range from oxidation to reduction until reach the plateau of the ORR. (See Figure4.5 that shows the cyclic voltammetry for oxygen reduction in KOH and KPF 6 solutions; after oxygen reduction, a sharp increase in the cathodic current is clearly observed as a result as a result of the evolution of molecular hydrogen.

Figure 4 . 6 :

 46 Figure 4.6: SECM images (400x200 µm) registered with a Pt UME (5 µm-radius) in 10 -3 mol L -1 Fc(MeOH) 2 as redox mediator in 0.1 mol L -1 H 2 SO 4 before (a,b,e,f) and after (c,d,g,h) performing the oxygen reduction in the vicinity (5 µm from the surface) of the p-(ethynyl)phenyl multilayer (c,g) and p-(methyl)phenyl multilayer (d,h) sequentially at different fixed potentials from + 0.6 V to -0.6 V vs Ag/AgCl in 0.01 mol L -1 KOH solution(c,d) and from + 0.6 V to -0.8 V vs Ag/AgCl in 0.01 mol L -1 KPF 6 solution (g,h). Y-axes display the normalized tip current (i/i inf ); the background current was subtracted for ease of comparison.

Figure 4 . 7 :

 47 Figure 4.7: Current-distance profiles of (a) p-(ethynyl)phenyl multilayer and (b) p-(methyl)phenyl multilayer on PPF after ORR in 0.01 mol L -1 KOH (red and black lines) and 0.01 mol L -1 KPF 6 (blue and green) solutions. Profiles drawn from Figures 4.6 c, d, g and h corresponding with the lines 1 and 2 marked in the figures. Data analysis was performed with Gwyddion software.[184]

Figure 4 .

 4 7a corresponds to the current intensity profiles of the p-(ethynyl)phenyl multilayer in the two aqueous electrolyte solutions after ORR. As previously indicated, no modification of the organic layer is observed under any of the experimental conditions studied, applied potential and electrolyte solution. On the contrary, the methylaryl multilayer showed a different behavior towards reactive oxygen species in solution, and this behavior varies with both the applied potential and the electrolyte solution.

Figure 4 .

 4 7b shows the extent of the modification on the p-(methyl)phenyl multilayer after ORR in KOH (black and red lines) and KPF 6 (blue and green lines). The numbering corresponds to lines 1 and 2 in the Figure 4.6 from

Figure 4 . 8 )

 48 . Notice that ferrocene dimethanol is well adapted as mediator to long time SECM experiments as it electrochemical response is highly stable. As seen on Figure4.8, clear spots are visible after exposition to O 2 reduction where ferrocene dimethanol shows a faster electron transfer, being able to pass through the organic layer. Hence, the unsaturated bonds integrated in the p-(ethynyl)phenyl multilayer seem to prevent or delay the oxidative damage of the polyaromatic structure during the exposition to oxygen reactive species, acting as a sort

Figure 4 . 8 :

 48 Figure 4.8: SECM 3D-images (140x150 µm) registered with a Pt UME (5 µm-radius) in 10 -3 mol L -1 Fc(MeOH) 2 as redox mediator + 0.1 mol L -1 H 2 SO 4 before (a) and after (b) performing the oxygen reduction in the vicinity (5 µm from the surface) of the p-(methyl)phenyl multilayer sequentially (sequence of nine points on the organic layer) at -0.8 V vs Ag/AgCl (10 s x 2 x -0.8 V) in 0.01 mol L -1 KPF 6 solution. Y-axes display the normalized tip current (i/i inf ); the background current was subtracted for comparison.

Figure 4 . 9 :

 49 Figure 4.9: SECM approach curves registered with a Pt UME (5 µm-radius) in 10 -3 mol L -1 Fc(MeOH) 2 as redox mediator + 0.1 mol L -1 H 2 SO 4 on PPF surfaces modified with (a) p-(ethynyl)phenyl multilayer and (b) p-(methyl)phenyl multilayer before (black) and after (blue) treatment with a 30 % H 2 O 2 solution. Note that plots registered before and after are totally overlayed.

  4 V vs Ag/AgCl in 0.01 mol L -1 KPF 6 (See Figures4.6 d and h), it is well known that hydroxyl radicals are directly generated from the electrochemical oxidation of water.[START_REF] Sirés | Electrochemical advanced oxidation processes: Today and tomorrow[END_REF] However, no electron transfer occurs at these applied potentials when observing the corresponding cyclic voltammetry plots (See Figure4.5); only a few tens of nanoampers were registered at these applied potentials. During the investigation of the influence of the applied potential on the modification of the organic layers, this unexpected damage observed on the methylaryl multilayer at anodic applied potentials a priori cannot be justify by an electrochemical production of reactive oxygen species during the oxidation of water at the platinum ultramicroelectrode. This observation is not yet understood; the influence of the concentration of oxygen in the aqueous solutions should be further considered, i.e. in an experimental approach allowing the total elimination of dissolved oxygen in the solution, or the use of radical scanvengers non electroactive in this range of potential (from + 0.2 V to + 0.6 V) to assert the formation of radicals during the anodic scan.

•

  For PPF surface modification. All the surface modifications were performed using a standard three-electrode setup at a controlled potential with a potentiostat Autolab PGSTAT 302N (Metrohm). The set of electrodes consisted of a small square (15x15 mm 2 ) of pyrolyzed photoresist film (PPF) as working electrode, a platinum mesh as the auxiliary electrode, and a saturated calomel electrode (SCE, KCl sat.) as reference. The latter was isolated from the working solution employing a salt bridge, which contained the electrolyte solution. When necessary, a conventional gas inlet was used for bubbling argon inside the electrochemical cell to displace oxygen. Pyrolyzed photoresist films (PPFs) were homemade fabricated according to described methods[START_REF] Del Campo | Fabrication of PPF electrodes by a rapid thermal process[END_REF]. See Procedure of PPF fabrication in this Section.• For SECM experiments. SECM experiments were performed with a three-electrode setup, comprising a 10 µm-diameter Pt UME (IJ Cambria) with a RG = 7, a Pt wire as the auxiliary electrode, and a homemade Ag/AgCl (0.1 mol L -1 KCl) electrode as reference. Figure4.10 shows SEM images of a just polished platinum UME embedded and sealed in a quartz glass body. A homemade SECM equipment was used comprising the positioning elements, translator stages, motor controllers, data acquisition system, and software written by Wittstock et al.[START_REF] Wittstock | Investigation of ion-bombarded conducting polymer films by scanning electrochemical microscopy (SECM)[END_REF] that provides complete control over all of the above elements and also, over an adjustable stage for the UME tilt angle correction (See Scheme 4.3 for a typical scanning electrochemical microscope with the essential components). SECM images and approach curves were recorded using a CHI910 potentiostat from CH Instruments. Additional experiments were performed using as WE a gold SECM tip of 12.5 µm-diameter (IJ Cambria) and a carbon fiber disc tip of 10 µm-diameter (Kation Europe).

Figure 4 . 10 :

 410 Figure 4.10: SEM images of a 10 µm-diameter disk-shaped Pt UME. SECM UMEs are typically characterized by the radius of the active electrode area (a) and a shape factor RG = r g /a where r g is the radius of the insulating sheath.

Scheme 4 . 3 :

 43 Scheme 4.3: Representation of a scanning electrochemical microscope with four main components: a low current bipotentiostat for precision at both the probe and substrate, a high resolution XYZ positioning system for accuracy using stepper and piezoelectric motors for coarse and smooth movements, imaging resolution varies depending on the small scale low micrometer to nanometer probe (SECM tip), and a computer system for data acquisition is required. From "Scanning Electrochemical Microscopy: A Comprehensive Review of Experimental Parameters from 1989 to 2015", by Polcari et al.,Chem. Rev., 116, p.13235. Copyright 2016 ACS.

Scheme 4 . 4 :

 44 Scheme 4.4: General principle of the feedback mode. (a) Steady-state diffusion-limited current with the tip UME in the bulk solution (far away from the substrate); (b) hindered diffusion with the UME in the vicinity of an insulating substrate (negative feedback); (c) mediator regeneration by a heterogeneous reaction at the conducting substrate (positive feedback). From "Investigation of localized catalytic and electrocatalytic processes and corrosion reactions with scanning electrochemical microscopy (SECM)", by Wittstock et al.,Phys. Chem. (Munich), 222, p.1469. Copyright 2008 by Oldenbourg Wissenschaftsverlag.

2 O 2

 22 and H 2 O. However, such global reaction schemes do not reflect the mechanistic pathways especially for what concerns the transient reactive oxygen species, which are key intermediates even if not explicitly considered. In this context, our aim was the detection of intermediates during the ORR on platinum by SECM using a foot-printing strategy.[114] The role of different sensitive layers, applied potential and electrolytes were investigated. Two different polyaromatic multilayers were successfully grafted by aryldiazonium chemistry on PPF surfaces, the p-(ethynyl)phenyl multilayer and the p-(methyl)phenyl multilayer. Both multilayers were characterized showing similar thickness values and blocking properties towards the electron transfer of ferrocene and dopamine in solution; no electron transfer was observed and the absence of defects or pinholes was also confirmed by AFM imaging experiments. From the SECM experiments, a differential behavior between the p-(ethynyl)phenyl and the p-(methyl)phenyl multilayers was observed and explained in terms of their reactivity towards the hydroxyl radicals probably generated as intermediate during the oxygen reduction on platinum. The former did not undergo modification after ORR at any potential or reaction media,

  was gold and carbon. The unfavored experimental conditions for an electro-Fenton process in KPF 6 solution (pH ∼ 6), the lack of metals able to catalyze the decomposition of H 2 O 2 and the unlike catalytic activity of traces of ferrocene dimethanol towards the decomposition of H 2 O 2 are contrary to the hypothesis of the non-catalytic production of hydroxyl radicals from H 2 O 2 electro-generated on platinum, gold and carbon. However, further confirmation of these preliminary results is required and future investigation could consider experimental conditions in which the 2-electron pathway for reduction of oxygen is favored, and also the use of aqueous soluble non-metal mediators to avoid the possible contribution of a Fenton-like reaction to the production of hydroxyl radicals during the ORR.An unexpected modification of the methylaryl multilayer was registered in both elec-trolyte solutions at applied potentials in the anodic region, where no electroactivity was observed by cyclic voltammetry experiments. The participation of radical species in the damage of the organic layer at these anodic applied potentials should be further investigated since a priori, this observation is not compatible with the electro-generation of any reactive oxygen species during i.e. the oxidation of water. In this regard, further experiments could consider the influence of the concentration of oxygen in the aqueous solutions and the use of radical scavengers non-electroactive in this range of potential to assert a possible formation of radicals during the anodic scan.based on carbon modification with quinones. Further investigation could consider the evaluation of this modification with other carbon materials more adapted for real applications in electrochemical supercapacitors and charge storage devices.Second, we have demonstrated the interest of introducing aryldiazonium molecules with a pre-organized structure by electro-reduction of the tetrahedral-shape pre-organized tetra-aryldiazonium salt, tetrakis(4-phenyldiazonium tetrafluoroborate)methane. Its electro-reduction led to the deposition of an ultrathin film on gold and carbon surfaces, which was further confirmed by XPS, AFM, SE and CV experiments. This was a first occurrence where both molecular sieving and electrochemical current rectification properties towards the electron transfer of redox probes in solution has been observed for non-electroactive organic films electro-generated by the reduction of aryldiazonium ions. Only a few examples in the literature concerning the electrochemical reduction of aryldiazonium salts consider this bottom-up approach for structuring surfaces. Due the remarkable properties observed here, more efforts could be oriented to the synthesis and study of pre-organized systems based on aryldiazonium salts for surface modification.

2. 4 2

 42 Cyclic voltammetry of ferrocene-terminated C 11 -alkyl monolayers onto pyrolytic graphite in MeCN and DMSO electrolyte solutions at different scan rates and their corresponding current versus scan rate plots . . . . 2.5 AFM images of bare PPF, GC and edge plane PG electrodes . . . . . . 2.6 Cyclic voltammetry of ferrocene-terminated C 11 -alkyl monolayers onto PPF, GC, and edge plane PG electrodes in CH 2 Cl 2 electrolyte solution 2.7 Cyclic voltammetry of ferrocene-terminated C 11 -alkyl monolayer on pyrolytic graphite in aqueous electrolyte solution at different scan rates and its corresponding current versus scan rate plot . . . . . . . . . . . . . . 2.8 Cyclic voltammetry simulation for an immobilized electroactive species that undergoes fast electron transfer followed by an ion-pairing reaction 2.9 Cyclic voltammetry of bare pyrolytic graphite and ferrocene-terminated C 11 -alkyl monolayer modified pyrolytic graphite in CH 2 Cl 2 electrolyte solution, and Nyquist and Bode diagrams of bare pyrolytic graphite and ferrocene-terminated C 11 -alkyl monolayer modified pyrolytic graphite in CH 2 Cl 2 electrolyte solution at two different fixed applied potentials . . 2.10 AFM topographic 3D-images of a bare edge plane PG electrode and an ethynylaryl-modified PG electrode (after deprotection step) . . . . . . . 2.11 Evolution of the real and complex capacitance versus frequency for the ferrocene-terminated C 11 -alkyl modified-edge plane PG electrode in CH Cl 2 electrolyte solution at two different fixed potentials . . . . . . . 2.12 Nyquist capacitive plots of the ferrocene-terminated C 11 -alkyl modified pyrolytic graphite in CH 2 Cl 2 electrolyte solution at two different fixed potentials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2.6 Resistance, capacitance and time constant of ferrocene-terminated C 11alkyl modified PG electrodes in all electrolyte solutions . . . . . . . . . 2.7 Structural formula and purity grade of all organic solvents investigated 3.1 Thickness values and standard deviation obtained by SE and AFM for the organic films (from the electro-grafting of C(ArN 2 + ) 4 ) on wafer gold and PPF surfaces . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2 Half-wave potential (E 1/2 ), ratio of the anodic peak current intensities (∆I) and steric hindrance of the different ferrocene derivatives . . . . . 3.3 Variation of the anodic peak potential (∆E p ) and ratio of the anodic peak current intensities (∆I) of different ferrocene derivatives between bare and modified carbon and gold electrodes by chronoamperometry with C(ArN 2 + ) 4 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.4 Ratio of the anodic peak current (∆I a ) and cathodic peak current (∆I c ) intensities of Fc in MeCN solution + 0.1 mol L -1 electrolyte on carbon and gold substrates modified by chronoamperometry with C(ArN 2 + ) 4 . 3.5 Structure and chemical formula of ferrocene and its derivatives . . . . . 3.6 Structure and chemical formula of electrolytes . . . . . . . . . . . . . . 4.1 Roughness and thickness values of bare PPF, and PPF modified with the p-(methyl)benzene and p-(ethynyl)benzene diazonium salts . . . . . de surfaces par des films organiques constitués de différentes molécules ou groupements fonctionnels est devenue de plus en plus populaires ces dernières décennies, puisqu'elle permet le développement de matériaux avec des propriétés nouvelles ou améliorées. Cet intérêt croissant est lié principalement à la possibilité de contrôler la modification à l'échelle moléculaire, mais aussi au développement de nouvelles techniques permettant la caractérisation de ces films, i.e. XPS ou AFM. Le niveau de sophistication des molécules synthétisées ainsi que le niveau de connaissance lié à la fabrication de ces couches organiques ont augmentés considérablement, tout comme le nombre applications potentielles dans des disciplines scientifiques très variées. Les applications les plus développées actuellement sont celles en lien avec l'analytique, les biotechnologies et le développement de matériaux pour des dispositifs de stockage de l'énergie. Ces différentes applications requièrent inévitablement une excellente caractérisation physicochimique des surfaces étudiées et la compréhension des phénomènes interfaciaux.Cette thèse est dédiée à l'étude du transfert électronique au travers de différentes couches organiques, générées par électro-réduction de sels d'aryles diazonium, d' épaisseurs variables (monocouches, couches ultraminces et multicouches) où l'espèce électroactive est soit attachée à la surface ou en solution. Trois techniques électrochimiques différentes ont été utilisées au cours de cette thèse: la voltammétrie cyclique (CV), la spectroscopie d'impédance électrochimique (EIS) et la microscopie électrochimique (SECM). Le travail entrepris dans cette thèse est un travail de recherche fondamentale et contribue à l'étude du transfert électronique à l'interface électrode/solution, et à la compréhension de leurs propriétés physicochimiques. Ce manuscrit est structuré comme suit: le premier chapitre présente la littérature actuelle concernant l'utilisation de sels d'aryles diazonium pour la fonctionnalisation de surfaces, et plus particulièrement utilisant la réduction électrochimique de ces sels. Leurs synthèses et réactivités chimiques sont rapidement décrites. Les différentes stratégies existantes pour fonctionnaliser ces surfaces avec précision (contrôle de l' épaisseur et de l'organisation moléculaire) sont décrites plus en détails. Les procé-dures servant à post-fonctionnaliser ces couches organiques sont aussi décrites, en se concentrant principalement sur celles employant la «Click Chemistry». Ensuite, les objectifs sont résumés plus en détail avant de présenter les trois chapitres décrivent et commentent les différentes études entreprises pendant cette thèse. Elles ont toutes en lien l'utilisation de l'électro-réduction de sels d'aryles diazonium pour former des couches organiques de différentes épaisseurs, allant de la monocouche aux multicouches, qu'elles soient électroactives ou non et possédant ou non une structure organisée. Le deuxième chapitre est dédié à l'étude de l'influence du solvant sur le transfert électronique de monocouches d'alkylferrocène greffées sur surfaces carbonées. La stratégie de protection-déprotection, développée au laboratoire ces dernières années, permet le greffage covalent de monocouches fonctionnelles, qui sont post-fonctionnalisées dans un second temps par une chaine alkyl portant un groupement ferrocényl terminal électroactif. Les propriétés électrochimiques (transfert électronique) de ces surfaces fonctionnalisées ont été étudiées par voltammétrie cyclique et spectroscopie d'impédance électrochimique. Ces études ont été menées dans différents solvants organiques et en solution aqueuse, couvrant une large gamme de constantes diélectriques (de 9 à 80 environ). Nous avons considéré leur utilisation pour augmenter la densité de charge dans des dispositifs de stockage de l'énergie. Les études en voltammétrie cyclique nous ont permis de conclure que la formation de paires d'ions entre l'électrolyte (les anions) et les cations ferrocéniums, générés après oxydation des groupements ferrocényls greffés, influençait fortement la réponse électrochimique du système. La cinétique et la thermodynamique du transfert électronique de la monocouche d'alkyl-ferrocène sont toutes deux influencées par le solvant utilisé, au travers des différences de solvatation existant entre les anions PF 6 -et ClO 4 -. Ceci se traduit par une variation dans la formation de paires d'ions entre les anions en solution et les cations ferrocéniums. D'après les expériences d'impédance, nous pouvons conclure que l'introduction d'une monocouche électroactive à la surface de substrats carbonés est un moyen très efficace d'augmenter la densité de charge globale du matériau. Après greffage de la monocouche d'alkyl-ferrocène et optimisation du système électrolyte/solvant, nous observons une augmentation très importante du stockage de densité de charges du matériau d'un facteur 10, ce qui est bien plus important que la plupart des résultats obtenus dans la littérature. Le troisième chapitre se concentre sur l'électro-réduction d'un sel d'aryle diazonium pré-organisé de forme tétraédrique, aboutissant dans ce cas au dépôt d'un film organique ultrafin non-électroactif sur or et sur carbone. Ce film organique présente une structure spécifique avec des propriétés particulières vis-à-vis d'électrolytes et sondes redox en solution. Des propriétés de tamisage moléculaire ont pu être mises en évidence par voltammétrie cyclique de sondes redox en solutions aqueuses et organiques. Les matériaux fonctionnalisés par cette méthode se comportent comme des rectificateurs de courant électrochimique (ECR). Il apparait que le processus de rectification soit différent de ceux décrits dans la littérature, impliquant l'utilisation de relais redox dans la couche organique modifiant le matériau. Le transport des sondes redox et des ions (de l'électrolyte) au travers du film organique électro-déposé a été démontré comme étant responsable des propriétés de rectification du courant électrochimiques du matériau ainsi fonctionnalisé. Les propriétés de tamisage moléculaire et de rectification du courant électrochimique ont toutes deux pour origine l'organisation spécifique du film organique électro-greffé. L'observation conjointe de ces deux propriétés n'a jamais été observée précédemment pour des films organiques électro-générés par la réduction de sels d'aryles diazonium et ne présentant pas cette pré-organisation en forme de tétraèdre. Les investigations des propriétés électrochimiques ont été complétées par en ensemble de techniques de caractérisation de surface : spectroscopie de photoélectrons X (XPS), microscopie à force atomique (AFM), spectroscopie IR et ellipsométrie. Le quatrième et dernier chapitre de cette thèse présente l'étude et l'utilisation de la microscopie électrochimique (SECM) pour la détection d'intermédiaires pendant les réactions de réduction du dioxygène (ORR) sur ultramicroélectrode (UME) de platine. Ces intermédiaires sont très réactifs et sont très importants, car nous les retrouvons dans beaucoup de processus naturelles et industrielles. Une stratégie «d'empreinte» est utilisé ici, où l'interaction de ces intermédiaires vis-à-vis de deux différentes couches organiques sensitives permet de mettre en évidence la production d'espèces réactives lors de la réduction du dioxygène. Le rôle du potentiel appliqué ainsi que de l'électrolyte ont été étudiés. Un effet protecteur dû à la présence de groupements insaturés acétyléniques dans la couche organique semble protèger cette dernière de sa dégradation vis-à-vis d'espèces hautement réactives, formées lors de la réduction du dioxygène.

  Characterization of the organic films . . . . . . . . . . . . . . . 3.2.2 Molecular sieving properties . . . . . . . . . . . . . . . . . . . . 3.2.3 Electrochemical current rectification properties . . . . . . . . . . 3.3 Experimental section . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3.1 Reagents . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Methodology . . . . . . . . . . . . . . . . . . . . . . . 3.3.3 Surface characterization . . . . . . . . . . . . . . . . . . . . . . 3.3.3.1 Spectroscopy Ellipsometry . . . . . . . . . . . . . . . . 3.3.3.2 X-ray Photoelectron Spectroscopy . . . . . . . . . . . 3.3.3.3 Atomic Force Microscopy . . . . . . . . . . . . . . . . 3.3.4 Density Functional Theory Calculations . . . . . . . . . . . . . 3.4 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Background . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Modification of surfaces constitutes a field of research of great interest. Nowadays, science and technology advance into the nanoscale level and, it is a fact that as smaller

	CONTENTS	iii
	3.3.2.2 4 Intermediates detection during oxygen reduction reaction: Introduction
	a SECM footprinting strategy
	4.1 Conclusion and Perspectives
	Bibliography
	List of figures
	List of tables
	List of schemes
	Résumé en Français

2.1 Background . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1.1 Influence of the solvent/electrolyte in the electrochemical properties of ferrocene-terminated monolayers . . . . . . . . . . i 3 Current rectification and molecular sieving properties of thin organic films 3.1 Background . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2 Results and Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2.1 3.3.2 Electrochemical measurements . . . . . . . . . . . . . . . . . . . 3.3.2.1 Instrumentation . . . . . . . . . . . . . . . . . . . . . 4.2 Results and Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2.1 Characterization of the polyaromatic layers . . . . . . . . . . . . 4.2.2 Detection of reactive oxygen species . . . . . . . . . . . . . . . . 4.3 Experimental section . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.3.1 Reagents . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.3.2 Electrochemical measurements . . . . . . . . . . . . . . . . . . . 4.3.2.1 Instrumentation . . . . . . . . . . . . . . . . . . . . . 4.3.2.2 Methodology . . . . . . . . . . . . . . . . . . . . . . . 4.3.3 Pyrolytic photoresist films fabrication . . . . . . . . . . . . . . . 4.3.4 Ag/AgCl quasi-reference electrode fabrication . . . . . . . . . . 4.4 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Table 1 .

 1

1: Effect of the substituent on the half-wave potential (E 1/2 ) of aromatic diazonium salt. From "Substituent effects in the polarography of aromatic diazonium salts" by Elofson et al., J. Org. Chem., 34, p. 856. curs (usually from + 0.2 to -0.8 V vs SCE in aprotic solvents) is low by comparison with other families of organic compounds. Precisely, the comparison of the electrochemical behavior of aryldiazonium salts with a different family of organic compounds, the aryl halides, allowed Pinson and Podvorica

Table 1 . 2

 12

: Films thickness. Adapted from "Mono-and multilayer formation by diazonium reduction on carbon surfaces monitored with atomic force microscopy "scratching" by McCreery et al., Anal. Chem., 75, p. 3842. Copyright 2003 ACS.

Table 2 .

 2 1: Characteristic parameters for the oxidation of ferrocene-terminated C 11alkyl monolayer onto a pyrolytic graphite electrode in different electrolyte solutions.Values of half-wave potential (E 1/2 ), full anodic and cathodic peak width (FWHM), peak-to-peak potential (∆E p ), ratio of anodic and cathodic current peak intensities (I pa /I pc ), and ferrocene surface coverage (Γ) were derived from the experimental data at low scan rates. Values of dielectric constant ( ) and dipolar moment (µ) are also listed. LiClO 4 nBu 4 NPF 6 nBu 4 NPF 6 nBu 4 NPF 6

	Solvents	CH 2 Cl 2	EtOH	MeCN	DMSO	PC
	+ 0.1 mol L -1 nBu 4 NPF 6 E 1/2 / V a 0.27	0.23	0.34	0.36	0.33
	FWHM(pa) / mV	96	84	128	112	138
	FWHM(pc) / mV	110	94	130	118	140
	∆E p / mV	58	44	24	15	24
	I pa /I pc	0.97	0.94	0.84	0.91	0.87
	Γ x10 -10 / mol cm -2	12.1	13.0	10.9	4.8	7.4
	b,c	8.93	24.55	35.94	46.45	64.92
	µ c / Debye	1.14	1.66	3.92	4.06	4.94

a All potentials are referred to FcMeOH/FcMeOH + couple. b Values of dielectric constant ( ) at 298 K for the organic solvents, without electrolyte. c From

[START_REF] Reichardt | Solvents and Solvent Effects in Organic Chemistry[END_REF][START_REF] Marcus | Properties of Solvents[END_REF][START_REF] Weissberger | Organic solvents: Physical properties and methods of purification[END_REF]

  DMSO, and PC gave half-wave potential values higher than 0.30 V. Thus, ferrocene is more readily oxidized in CH 2 Cl 2 and EtOH solutions. The positive shift in the half-wave potential of ferrocene observed in MeCN, DMSO and PC electrolyte solutions could be explain by the stronger solvation of the anions PF 6 -or ClO 4 in these polar solvents. The high solvation energies of these anions in polar solvents limit the formation of ion pairs between them and the ferrocenium cations upon oxidation. On the contrary, the anions are poorly solvated in CH 2 Cl 2 and EtOH electrolyte solutions; the ion pairs formation between the anions and the ferroceniums is therefore favored. The ion pairs formation stabilizes the electrogenerated cations, and probably also stabilizes the structure of the monolayer, doing the electron transfer of ferrocene thermodynamically easier in these non-polar (CH 2 Cl 2 ) and water-like (EtOH) media.

		2	+ and the anions, PF 6	-, or
	ClO 4	-in the case of the ethanol solution (Equations 2.10 and 2.11).
	As shown in Table 2.1, the variation of the half-wave potentials in the different elec-
	trolyte solutions reached its maximum difference between EtOH and DMSO, with a

positive shift of 130 mV from EtOH to DMSO. The electrolytes with low-intermediate dielectric constants outputted values below 0.30 V while the group of electrolytes con-stituted by MeCN, These observations are in agreement with the literature.[108, 131] Ion-pairing reactions involving anions such as PF 6 -or ClO 4 -are favored in non-polar or water-like solvents, where these hydrophobic anions are poorly solvated; also, the positive shift of the half-wave potential was previously observed in ferrocene-terminated monolayers on gold when the ferrocenium cation was not stabilized by ion-pairing formation with the anions from the electrolyte.[125] Note that solvents with low-intermediate dielectric constants allow the formation of ion pairs, unlike high dielectric constant solvents.[136] * Fe(Cp) 2 * Fe(Cp) 2 + + e -E 0 (2.9) * Fe(Cp) 2 +

Table 2 .

 2 2: Characteristic parameters for the oxidation of the ferrocene-terminated C 11alkyl monolayer onto PPF, GC and edge plane PG carbon electrodes in CH 2 Cl 2 solution + 0.1 mol L -1 nBu 4 NPF 6 . Values of E 1/2 , FWHM, ∆E p , I pa /I pc , Γ were calculated from the experimental data at scan rate 0.1 V s -1 . Surface geometric areas and roughness values for all carbon materials are highlighted.

	Carbon material	PPF	GC	PG
	Surface area / cm 2	0.42 0.031 0.071
	Roughness / nm	0.067 0.982	72
	E 1/2 / V a	0.36 0.36 0.37
	FWHM(pa) / mV	46	80	96
	∆E p / mV	15	21	58
	I pa /I pc	0.99 1.01 0.97
	Γ x10 -10 / mol cm -2	2.3	4.6	12.1

a All potentials are referred to SCE.

Table 2

 2 .3, an increase in the double layer capacitance was registered upon oxidation of the layer-bound ferrocenes. The double layer capacitance values increase by two in electrolytes where the redox reaction was reversible and the ion pairing was stoichiometric (CH 2 Cl 2 solution + 0.1 mol L -1 nBu 4 NPF 6 and EtOH + 0.1 mol L -1 LiClO 4 ), and this increase was observed in all modified carbon materials (PPF, GC and PG electrodes). On the other hand, in the electrolytes in which a partial oxidation of the ferrocenes and the loss in charge density occurs (MeCN, DMSO and PC solutions + 0.1 mol L -1 nBu 4 NPF 6 ), a value below two is observed.

Table 2 . 3

 23

	PC	nBu 6 4 NPF	PG	86	154	1.79
	DMSO	nBu 6 4 NPF	PG	90	132	1.47
	MeCN	nBu 6 4 NPF	PG	139	258	1.86
	EtOH	4 LiClO	PG	164	328	2.00
	CH 2 Cl 2	nBu 6 4 NPF	PG	123	267	2.17
	CH 2 Cl 2	nBu 6 4 NPF	GC	61	127	2.08
	CH 2 Cl 2	nBu 6 4 NPF	PPF	30	60	2.00
	Solvent	+ 0.1 mol L -1	Material / cm 2	C before / µF cm -2	C after / µF cm -2	C ratio

: Double-layer capacitance before (C before ) and after (C after ) ferrocene oxidation on the organic layer. Capacitance ratios were calculated as C ratio = C after /C before for all organic electrolytes and carbon materials. Electrode geometric surface areas were considered for standardizing the capacitance values.

Table 2 .

 2 4: Characteristic parameters for the oxidation of ferrocene-terminated C 11alkyl monolayer onto a pyrolytic graphite electrode in an aqueous 0.1 mol L -1 KPF 6 electrolyte solution. Values of E 1/2 , FWHM, ∆E p , I pa /I pc , and Γ were calculated from the experimental data at scan rate 0.1 V s -1 . Values of dielectric constant ( ) and dipolar moment (µ) are also listed.

	Solvent	Water
	+ 0.1 mol L -1	KPF 6
	E 1/2 / V a	0.35
	FWHM(pa) / mV	132
	FWHM(pc) / mV	120
	∆E p / mV	44
	I pa /I pc	0.93
	Γ x10 -10 / mol cm -2	7.3
	b,c	78.36
	µ c / Debye	1.85

a Potential referred to FcMeOH/FcMeOH + couple. b Values of dielectric constant ( ) at 298 K without electrolyte. c From

[START_REF] Reichardt | Solvents and Solvent Effects in Organic Chemistry[END_REF][START_REF] Marcus | Properties of Solvents[END_REF][START_REF] Weissberger | Organic solvents: Physical properties and methods of purification[END_REF]

Table 2 .

 2 5: Resistance, capacitance and time constant of bare PG electrodes.

		CH 2 Cl 2 EtOH MeCN DMSO PC Water	69 45 10 50 59 9	0.0036 0.0040 0.0007 0.0051 0.0061 0.0012	52 89 71 101 104 133	69 45 10 49 59 9	0.045 0.050 0.0008 0.0051 0.0070 0.0012	65 110 79 104 118 128	49 79 110 86 79 153
			R / Ω cm 2	τ / s	C / µF cm -2	R / Ω cm 2	τ / s	C / µF cm -2	C / µF cm -2
	Applied potential	/Electrolyte		Bare electrode around 0 V a		Bare electrode	around peak potential a		peak potential a From CV around	a

Table 2

 2

	11 -alkyl modified PG electrodes. .6: Resistance, capacitance and time constant of ferrocene-terminated C	Applied potential	/Electrolyte CH 2 Cl 2 EtOH MeCN DMSO PC Water	66 39 11 41 63 14 R / Ω cm 2	Modified electrode 0.0048 0.0034 0.0010 0.0036 0.0050 0.0011 without faradaic τ / s	80 80 73 86 88 88 C / µF cm -2	62 13 Modified electrode 65 39 11 45 R / Ω cm 2	0.039 0.008 with faradaic at peak potential 0.097 0.049 0.010 0.017 τ / s	610 590 1480 1270 930 410 C / µF cm -2	From Cyclic	Voltammetry C / µF cm -2	with faradaic

Table 3 .

 3 1: Thickness values and standard deviation (in nanometers) obtained by SE and AFM for the organic films on gold and carbon substrates. Two deposition methods are compared, CV and chronoamperometry (abbrev. Chrono in the table).

		GOLD (in nm)	CARBON (in nm)
		Thickness-SE Thickness-AFM	Thickness-SE Thickness-AFM
	CV	2.4 ± 0.1	2.4 ± 1.0	2.2 ± 0.2	1.7 ± 0.4
	Chrono	4.2 ± 0.1	4.3 ± 1.1	4.1 ± 0.1	3.6 ± 0.5
	applied potential -0.80 V vs SCE) gave rise to thicker films than those obtained by CV,
	both on gold and carbon substrates. The organic film thickness was 2.4 and 2.2 nm on
	gold and carbon electrodes modified by CV, and 4.2 and 4.1 nm on gold and carbon
	electrodes modified by chronoamperometry, respectively. Considering electron transfer
	properties, a 2 nm thick film should allow electron transfer by tunneling,

Table 3 .

 3 2: Half-wave potential (E 1/2 ), ratio of the anodic peak current intensities (∆I) and steric hindrance of the different ferrocene derivatives. Steric hindrance of each redox probe was calculated by DFT and defined as the molecular volume of each molecule. It has to be used with precaution as the geometry of the probes are different. Optimized geometries of each redox probe are displayed in the supplementary information section (Figures 3.10).

	Redox probes	Me 10 Fc Me 2 Fc Fc (PhMe) 2 Fc Ph 2 EFc
	E 1/2 vs Fc/Fc + / V	-0.51	-0.11	0	-0.04	0.13
	∆I	-	0.81	0.89	0.73	0.79
	Steric hindrance a / Å3	740	397	382	796	724

a

Table 3 .

 3 3: Variation of the anodic peak potential (∆E p ) and ratio of the anodic peak current intensities (∆I) of different ferrocene derivatives between bare and modified carbon and gold electrodes by chronoamperometry. Errors correspond with the % SD of three different carbon and gold electrodes modified in an intraday experiment.

		carbon	gold
		∆E p (mV)	∆I	∆E p (mV)	∆I
	Fc	47 ± 11	0.89 ± 3	33 ± 9	0.96 ± 3
	Me 2 Fc	63 ± 13	0.81 ± 2	61 ± 8	0.83 ± 6
	Me 10 Fc	-	-	-	-
	(PhMe) 2 Fc	92 ± 10	0.73 ± 3	79 ± 15	0.75 ± 8
	Ph 2 EFc	75 ± 8	0.79 ± 3	64 ± 8	0.79 ± 7
	Figures 3.12

Table 3 .

 3 4: Ratio of the anodic peak current (∆I a ) and cathodic peak current (∆I c ) intensities of 10 -3 mol L -1 Fc in MeCN solution + 0.1 mol L -1 electrolyte on carbon and gold substrates modified by chronoamperometry with C(ArN 2 + ) 4 . Errors correspond with the % SD of three different carbon and gold electrodes modified in an intraday experiment. NPF 6 0.81 ± 8 0.79 ± 4 nBu 4 NBF 4 0.57 ± 6 0.62 ± 11 nOc 4 NPF 6 0.76 ± 8 0.80 ± 10 nBu 4 NBPh 4 0.54 ± 6 0.58 ± 12

	∆I a

Table 3 .

 3 5: Structure and chemical formula of ferrocene and its derivatives

	Redox Probes	Structure	Formula
	Ferrocene (Fc)		C

10 H 10 Fe Dimethylferrocene (Me 2 Fc) C 12 H 14 Fe Decamethylferrocene (Me 10 Fc) C 20 H 30 Fe 1,1'-Di(methylphenyl)ferrocene ((PhMe) 2 Fc) C 24 H 24 Fe 1-(Biphenylethynyl)ferrocene (Ph 2 EFc) C 24 H 19 Fe 142

Table 3 .

 3 6: Structure and chemical formula of electrolytes

	Electrolytes	Cation	Anion	Formula
	Potassium
	hexafluorophosphate	K +		KPF 6
	(KPF 6 )
	Tetraethylammonium
	hexafluorophosphate
	(nEt 4 NPF 6 )

C 8 H 20 F 6 NP Tetraoctylammonium hexafluorophosphate (nOc 4 NPF 6 ) C 32 H 68 F 6 NP Tetrabutylammonium chloride (nBu 4 NCl) Cl - C 16 H 36 ClN Tetrabutylammonium tetrafluoroborate (nBu 4 NBF 4 ) C 16 H 36 BF 4 N

Table 4 .

 4 1: Roughness and thickness values of the p-(methyl)phenyl-modified PPF (Me-Ar-PPF) and p-(ethynyl)phenyl-modified PPF (Eth-Ar-PPF) and the bare PPF for comparison. Values are the average of three different measurements performed in three different samples.

	Surface properties	Bare PPF Me-Ar-PPF Eth-Ar-PPF
	Roughness -AFM / nm	61 ± 3	132 ± 8	154 ± 10
	Thickness -SE / nm	-	5.6 ± 0.7	6.8 ± 0.3
	4.2.2 Detection of reactive oxygen species
	Once we have our test surfaces, the two different p-(methyl)phenyl and p-(ethynyl)

  10, while in 0.01 mol L -1 KPF 6 was around 6. Considering mechanistic aspects of ORR in aqueous solution, the reduction of oxygen proceeds mainly by two pathways, (i) the 2-electron reduction of O 2 to H 2 O 2 , and (ii) the 4-electron process from O 2 to H 2 O. In aprotic solvents such as dimethyl sulfoxide and alkaline solutions, the 1-electron reduction of O 2 to superoxyde O 2 •could take place.[113] However, this generally admitted description of ORR in water is often inadequate. Electron transfer steps could be accompanied by homogeneous or heterogeneous chemical steps and O -O

  2•by H 2 O could occurs with a kinetic rate constant of about 200 M -1 s -1 ; at pH 11, an 80 % conversion to H 2 O 2 was observed.[START_REF] Sawyer | One-electron mechanism for the electrochemical reduction of molecular oxygen[END_REF] On the contrary, the lifetime of superoxide in presence of protons is too short and should dissapears by protonation to form perhydroxyl radical (HO 2 • ) that in turn undergoes disproportionation to H 2 O 2 . The values of rate constant for the disproportionation step estimated by pulse radiolysis technique range around 10 6 M -1 s -1 at pH from 2 to 5, being more kinetically promote at neutral pH with a rate constant of 21x10 6 M -1 s -1 .[265]; a complete conversion in a 2-electron process to H 2 O 2 was observed.

	2 O 2	•-+ 2 H 2 O	H 2 O 2 + O 2 + 2 OH -

L

  -1 KPF 6 ) during the ORR were able to mediate the decomposition of H 2 O 2 to HO • by Fenton-like reaction, and in turn damage the methylaryl multilayer. The electro-Fenton reactions have been widely studied in the framework of the electrochemical advanced oxidation processes (EAOPs) for waste water remediation, and there is a consensus in the literature on the role of hydroxyl radicals as the primary oxidizing agent involved in the destruction of the organic matter.[START_REF] Walling | Intermediates in the reactions of Fenton type reagents[END_REF][START_REF] Bautista | An overview of the application of Fenton oxidation to industrial wastewaters treatment[END_REF][START_REF] Cañizares | Synthesis of novel oxidants by electrochemical technology[END_REF] It is very well established the production of HO • radicals from the in situ electrogenerated H 2 O 2 in presence of metals by electro-

	Fenton reaction in the cathode (Equations 4.6 and 4.7).[244, 276]
	O 2 + 2 H + + 2 e -	H 2 O 2	(4.6)
	Fe

2+ + H 2 O 2 → Fe 3+ + HO • + HO -

avec de nouvelles propriétés, il est nécessaire de savoir comment moduler la matière à l'échelle nanométrique. La nanosctructuration peut impliquer une multitude de procédures de fonctionnalisation de surfaces; une des plus intéressantes étant la possibilité de pouvoir arranger de façon organisée des molécules fonctionnelles sur une surface.La polymérisation et l'adsorption de thiols ou composés dithiolés sont des méthodes de fonctionnalisation de surfaces bien connues. Néanmoins, elles ont des inconvénients; et le plus important est surement la faible énergie de liaison existant entre la surface et la molécule immobilisée, qui peut rendre le système instable. Dans ce contexte, l'utilisation de liaisons covalentes est souhaitable.Les sels d'aryles diazonium sont des briques élémentaires idéales pour la conception de surfaces possédant des propriétés spécifiques de par leurs diversités structurelles et leur capacité à modifier des surfaces conductrices par électrochimie. La modification

Acknowledgements

First and foremost, I would like to thank Dr. Philippe Hapiot and Dr. Yann Leroux for their supervision during the course of my thesis at Rennes. In particular, I wish to thank Dr. Hapiot for his guidance and patience, the valuable time dedicated to our discussions had helped me develop personally as well as professionally. I am especially grateful to Dr. Leroux, without his support this work would not have been possible.

Je voudrais lui remercier pour sa présence sans faille, et en même temps, pour me

Experimental section 2.3.1 Reagents

All commercially available reagents were used as received.

Organic solvents were used without further purification (See Table 2.7 below). Table 2.7: Structural formula and purity grade of all organic solvents All aqueous solutions were made employing ultrapure water (18.2 MΩ-cm).

Non-commercially reagents were synthesized in the laboratory:

• 4-(tri(isopropylsilyl)ethynyl) benzenediazonium tetrafluoroborate (TIPS-Eth-ArN 2 + ) was synthesized according to the literature. [START_REF] Leroux | Design of robust binary film onto carbon surface using diazonium electrochemistry[END_REF] • (11-azidoundecyl)ferrocene was synthesized following a previously published procedure. [START_REF] Casas-Solvas | Convenient methods for the synthesis of ferrocene-carbohydrate conjugates[END_REF] 2.3.2 Electrochemical measurements

Instrumentation

All the electrochemical measurements were performed using a standard three electrode setup at a controlled potential with a potentiostat Autolab PGSTAT 302N (Metrohm)

equipped with an ADC10M and a FRA2 module. The set of electrodes consisted All the modification steps (grafting, deprotection and click chemistry) were characterized by cyclic voltammetry using the oxidation of outer-sphere redox probes such as potassium ferrocyanide (K 4 [Fe(CN) 6 ] • 3 H 2 O) or ferrocene (Fe(Cp) 2 ) in solution. The oxidation of the redox probes on the different modified-electrode surfaces provides information about the blocking properties of the layer towards the electron transfer of the redox probe in solution, and allows us to follow the modification procedure. The electrochemical grafting of the aryldiazonium TIPS-Eth-ArN 2 + tetrafluoroborate, the deprotection of the ethynyl functions and its post-functionalization by click chemistry using an azido-ferrocene derivative is a highly reproducible procedure, which has been extensively optimized since its publication in the year 2010. [START_REF] Leroux | Efficient covalent modification of a carbon surface: Use of a silyl protecting group to form an active monolayer[END_REF] As previously observed, the ferrocene oxidation (in MeCN solution + 0.1 mol L -1 nBu 4 NPF 6 ) was totally inhibited when the working electrode was TIPS-Eth-PG. After deprotection step, the oxidation of ferrocene on the modified H-Eth-PG showed the exactly same voltammetric response that on a bare PG electrode. In other words, the oxidation of ferrocene, when the working electrode was H-Eth-PG, was fast and reversible. This behavior was expected since access channels (pores) to the electrode surface appears after deprotection, allowing the electron transfer of ferrocene.

Interfacial apparent capacitance

Review the introduction to this chapter for more detail about the different electrochemical methods for determining the capacitance of an interface electrode/electrolyte solution, having the electrode a bare or modified surface.

From CV experiments

The global apparent capacitance was determined from cyclic voltammetry data according to the Equation 2.4 as charge stored divided by the potential window. Figure 2.15a

shows, as example, the current intensity vs time for C 11 -alkyl ferrocene monolayer in

Density Functional Theory Calculations

The conformation of all ferrocene derivatives were optimized using density functional theory (DFT) calculations at the B3LYP/6-31G* level using the Gaussian 09 Revision B.01 package with the default parameters.

[225] The molecular volume is computed as the volume inside a contour of 0.001 electron/bohr3 using a Monte-Carlo method of integration and the SCF density. The number of points per bohr3 is 100 with a cutoff = 10-4 and used with the "Vol=Tight" option for a better accuracy that is better than 10 %.

[226]

4.3 Experimental section

Reagents

All commercially available reagents were used as received.

Non-commercially reagents were synthesized in the laboratory:

• 4-(ethynyl)benzenediazonium tetrafluoroborate (Eth-ArN 2 + BF 4 -) was synthesized as follows, from the corresponding commercial amine following the classical procedure of diazotisation of aromatic amines: 4-(ethynyl)aniline (10 -2 mol L -1 ) was dissolved in 7.5 mL of aqueous HBF 4 25 % wt solution (3x10 -2 mol L -1 ).

After cooling in an ice bath, sodium nitrite (1.5x10 -2 mol L -1 ) was added to the solution dropwise under vigorous stirring. After reaction and filtering, the diazonium salt was washed with 5 % NaBF 4 , cold water, cold methanol and diethyl ether. All aqueous solutions were made employing ultrapure water (18.2 MΩ-cm).

are obtained (from positive to negative feedback) that sense the ability of the probe to pass through the layer, providing a view of the modifications. Preparation of modified substrates. All PPFs were modified in a 10

Pyrolytic photoresist films fabrication

The fabrication of pyrolyzed photoresist films (PPFs) consists of a thermal processing of a standard positive photoresist (AZ4562, supplied by Microchemicals), which was previously deposited by spin coating (6000 r.p.m. during 30 s) over a silicon wafer (Si (100)) and heated in an inert atmosphere (95 % Ar/ 5 % H 2 ) at rate 10 • C/min up to a temperature of 1100 • C. For complete graphitization of the photoresist material, the annealing was performed during 1 h. [START_REF] Del Campo | Fabrication of PPF electrodes by a rapid thermal process[END_REF] Finally, the PPFs with typical dimensions of 1.5 x 1.5 cm 2 were cleaned in an ultrasonic bath in acetone, methanol and 2-propanol, then dried under Argon and stored at room temperature for later use. The resulting PPFs show electrochemical properties close to that observed in glassy carbon electrodes, and low resistivities of around 10 -3 Ω cm.

Ag/AgCl quasi-reference electrode fabrication

Ag/AgCl quasi-reference electrodes were fabricated by electrolysis of a 0.1 mol L -1 aqueous solution of KCl when the working electrode was a silver wire, the counter electrode was a platinum mesh and the reference a saturated calomel electrode (SCE, KCl sat.). Both the reference and the counter were isolated from the working solution employing a salt bridge containing the electrolyte solution. The electrolysis was performed during 1 h at a fixed potential of 0.1 V vs SCE, according to the potential required to cover the silver wire used as working electrode with an homogeneous film of AgCl (Equation 4.8). After electrolysis, the silver surface has darkened and lost its characteristic shine. The resulting quasi-reference Ag/AgCl electrodes were tested with a voltmeter against a SCE (KCl sat.) in an 0.1 mol L -1 KCl aqueous solution, being the difference in potential of around 44 mV. [START_REF] Ives | Reference Electrodes. Theory and Practice[END_REF] AgCl(s) + e - Ag(s) + Cl -E 0 = 0.22 vs SHE (4.8)

Conclusion and Perspectives

During this thesis, we have demonstrated the interest and versatility of aryldiazonium salts in fundamental research of phenomena such as the study and optimization of electron transfer properties at the electrode/solution interface and in applications such as oxygen reduction catalysis and electrochemical energy storage systems, which are fields of great interest nowadays. We have shown how the surface properties of materials such as carbon can be modulated by electrochemical reduction of aryldiazonium salts. First, with the introduction of an on-carbon ferrocene-terminated electroactive system with a monolayer type organization. In this case, we have observed the great influence of the reaction media on the electron transfer kinetics and thermodynamics of the immobilized electroactive molecule. This influence was demonstrated to be linked to a differential solvation of the different organic solvents on the electrolyte in solution and, therefore, to the possibility of this latter to form ionic pairs with the oxidized form of ferrocene. The resistance, apparent capacitance and time constant of the immobilized electroactive system on carbon were successfully determined from electrochemical impedance measurements. The reaction medium was optimized to maximize the energy storage of the mixed capacitive system, consisting of the electrochemical double layer and the faradaic contribution. We have demonstrated that a controlled modification of the carbon surface allows maximizing the synergy between the inherent-material properties for energy storage (electrochemical double layer) and the contribution of the ferrocene electron transfer to the charge storage system, achieving improvements of charge density far superior to those reported in the literature for equivalent systems

List of Figures Abstract: It is the surface, not the bulk material that interacts with the surrounding environment; hence by altering the surface in a controlled manner we can modulate the properties of the material towards its environment. Aryldiazonium salts are suitable to tailor the surface properties since their structural diversity and their electrochemically-assisted bonding ability to modified conducting surfaces. This thesis focuses on the study of the electron transfer through different aryl layers by aryldiazonium electro-reduction at three different thickness levels, monolayer, near-monolayer, and multilayer, when the electroactive molecule is attached to the surface or in solution. Three different electrochemical methods have been used throughout this thesis, CV, EIS and SECM. The first study of this thesis focused on the investigation of the electrochemical properties of alkyl-ferrocene on-carbon monolayers in different solvents and its evaluation for improving the global charge density of carbon materials for energy storage applications. The second study used a bottom-up approach for the fabrication of wellorganized surfaces. Carbon and gold substrates were modified by electro-reduction of a tetrahedral-shape preorganized aryldiazonium salt resulting in an ultrathin organic film that showed molecular sieving and current rectification properties towards redox probes in solution. The third study then focused on the oxygen reduction reaction and its intermediates, which are of general importance in natural and industrial processes. Detection of intermediates was achieved by SECM in a foot-printing strategy based on the use of different sensitive aryl multilayers. The role of the applied potential and electrolytes was investigated. Here we have demonstrated that the electrochemical properties of redox probes attached to a surface or in solution can be modulated by introducing aryl layers allowing fundamental research investigations of interest in fields such as energy storage and catalysis.

List of Tables