Cyclic voltammetry and scanning electrochemical microscopy studies of methylene blue immobilized on the self-assembled monolayer of n-dodecanethiol

Electron transfer (ET) kinetics through n-dodecanethiol (C₁₂SH) self-assembled monolayer on gold electrode was studied using cyclic voltammetry (CV), scanning electrochemical microscopy (SECM) and electrochemical impedance spectroscopy (EIS). An SECM model for compensating pinhole contribution, was used to measure the ET kinetics of solution-phase probes of ferrocyanide/ferricyanide (Fe(CN)₆^4−/3−) and ferrocenemethanol/ferrociniummethanol (FMC^0/+) through the C₁₂SH monolayer yielding standard tunneling rate constant () of (4 ± 1) × 10⁻¹¹ and (3 ± 1) × 10⁻¹⁰ cm s⁻¹ for Fe(CN)₆^4−/3− and FMC^0/+ respectively. Decay tunneling constants (β) of 0.97 and 0.96 Å⁻¹ for saturated alkane thiol chains were obtained using Fe(CN)₆⁴⁻ and FMC respectively. Also, it was found that methylene blue (MB) molecules are effectively immobilized on the C₁₂SH monolayer and can mediate the ET between the solution-phase probes and underlying gold substrate. SECM-mediated model was used to simultaneously measure the bimolecular ET between the solution-phase probes and the monolayer-immobilized MB molecules, as well as tunneling ET between the monolayer-immobilized MB molecules and the underlying gold electrode, allowing the measurement of k_BI = (5 ± 1) × 10⁶ and (4 ± 2) × 10⁷ cm³ mol⁻¹ s⁻¹ for the bimolecular ET and and (7 ± 3) × 10⁻² s⁻¹ for the standard tunneling rate constant of ET using Fe(CN)₆^4−/3− and FMC^0/+ probes respectively.     

Cyclic voltammetry and scanning electrochemical microscopy studies of the heterogeneous electron transfer reaction of some nitrosoaromatic compounds

The heterogeneous electron transfer reaction for the reduction of some nitroso aromatic derivatives in aqueous-alcoholic medium was studied on both mercury and glassy carbon electrodes (GCE) by using cyclic voltammetry (CV) and scanning electrochemical microscopy techniques (SECM).The nitrosoaromatic derivatives followed a two-electron two-proton mechanism producing a quasi-reversible overall process. This strongly pH dependent mechanism varied from ECCE mechanism at pH < 8.5 to ECEC mechanism at pH > 8.5.The apparent heterogeneous rate constant for the reduction of the nitroso derivatives was calculated using CV or SECM. The rate constant for the electron transfer process depends on the nature of the electrode material. The heterogeneous rate constant on the GCE is almost two orders of magnitude smaller than that on mercury electrode i.e. (3.4 ± 0.3) × 10⁻³ cm s⁻¹ on Hg and (7.0 ± 1.0) × 10⁻⁵ cm s⁻¹ on GCE, for the same nitroso compound and pH.The heterogeneous rate constant values were checked by comparison between experimental and simulated cyclic voltammograms.     

Interfacial electrochemistry and electrodeposition from some ionic liquids: In situ scanning tunneling microscopy, plasma electrochemistry, selenium and macroporous materials

In this paper we report on recent results from our group, namely on the interface ionic liquid/electrode, plasma electrochemistry and electrodeposition of selenium and of macroporous structures. Ionic liquids show an interesting and liquid dependent surface chemistry: in some liquids the long range “herringbone” superstructure of Au(1 1 1) is visible, in others it is not. Glow discharge plasmas can be employed as a contact free electrode to make nanoparticles in solutions, e.g. nanoparticles of germanium. Selenium can be electrodeposited from ionic liquids under environmental conditions in an open cell and both the red and the grey phases of selenium are feasible. With the help of self organized opal structures of polystyrene spheres macroporous materials of Ag, Al and conducting polymers can be made. The prospects and limits of ionic liquids in surface electrochemistry and electrodeposition are shortly discussed.     

Electrodeposition of Ti from TiCl4 in the ionic liquid l-methyl-3-butyl-imidazolium bis (trifluoro methyl sulfone) imide at room temperature: study on phase formation by in situ electrochemical scanning tunneling microscopy

Titanium was electrodeposited from a nominal 0.24 M TiCl₄ in l-methyl-3-butyl-imidazolium bis (trifluoro methyl sulfone) imide ([BMIm]BTA) at room temperature on a Au(1 1 1) substrate. The process of electrodeposition was studied by cyclic voltammetry, chrono amperometry and in situ scanning tunneling microscopy (STM). In a first step TiCl₄ is reacted to TiCl₂, which is subsequently reduced to metallic Ti. Two dimensional (2D) clusters form preferentially on the terraces in the under potential deposition range. 2D clusters presumably of TiCl₃ precipitates grow and coalesce to cover the whole substrate with a 2D film at a substrate potential below −1.1 V versus ferricenium/ferrocene ([Fc]⁺/[Fc]) redox couple. At a potential of −1.8 V a dense layer of three dimensional (3D) clusters of titanium of 1-2 nm thickness is formed. The features of the I-U tunneling spectra and the relative reduction of the effective tunneling barrier by 0.8 eV with respect to gold clearly indicate the metallic character of Ti deposits. Observation of circular holes on the Au(1 1 1) substrate after dissolution of the deposited Ti indicates the formation of Au-Ti surface alloying.     

The use of scanning electrochemical microscopy for the characterisation of patinas on copper alloys

The scanning electrochemical microscopy (SECM) was employed in the feedback mode for the visualisation of the local changes of morphology and reactivity, occurring on the surface of a quaternary copper-based alloy, due to exposure to environment. First, samples artificially aged by exposure to leaching acid rain were tested. The layer of corrosion products (“patina”) was investigated by performing SECM scans and approach curves, and the information provided by this electrochemical technique was compared with SEM–EDX–Raman characterisation. Successively, to highlight the early stages of localised corrosion processes, in situ examination of the surface exposed to an acidic environment was performed. The results show that SECM is a powerful characterisation tool of the deterioration process and is able to map the precursor sites of corrosion on the bronze surface.     

Characterization of coating systems by scanning electrochemical microscopy: Surface topology and blistering

Operation of the scanning electrochemical microscope used in feedback mode over a coated metal allows changes in the state of the coating surface to be monitored during immersion in aqueous electrolytes. This paper reports changes in the coating induced by specific anions in the electrolyte in situ during immersion. Significant surface roughening is observed for immersion times shorter than 1 day when the electrolyte contains chloride ions. This effect is also observed when the oxygen dissolved in the electrolytic phase is employed as redox mediator for SECM imaging. The coated system exposed to chloride-free electrolytes containing sulphate or nitrate maintains a featureless topography within the same time scale. The observed features are due to the nucleation and growth of blisters at the metal/coating interface induced by chloride ions in the environment. The implication is that ionic migration occurs simultaneously with the absorption of water by the coating already from the beginning of exposure to the aqueous environment. The unique role of chloride ions compared with sulphate or nitrate ions towards coating performance has been established at a very early stage following immersion of the sample.     

Uses of scanning electrochemical microscopy for the characterization of thin inhibitor films on reactive metals: The protection of copper surfaces by benzotriazole

Scanning electrochemical microscopy (SECM) was used to study the film formation of benzotriazole towards corrosion of copper. SECM was operated in the feedback mode by using ferrocene-methanol as redox mediator, and the sample was left unbiased at all times to freely attain its open circuit potential in the test environment. Following exposure to aggressive electrolytes the anticorrosion abilities of the layers were characterized by image analysis and by an electrochemical method derived from the experimental approach curves. Changes in the shape of the approach curves were clearly observed during the inhibitor film formation process. They showed the transition from an active conducting behaviour towards ferrocinium reoxidation typical of unprotected copper, to a surface exhibiting insulating characteristics when the metal was covered by a surface film containing the inhibitor. This supports that SECM is a practical technique in the investigation of corrosion inhibitor performance. However, a consistent tendency for the characterization of inhibitor film formation using SECM measurements in the positive feedback mode for the copper-benzotriazole system was only found if the experiments were conducted when the inhibitor molecule was not present in the test solution. That is, inhibitor molecules were found to interact not only with the copper surface during the monitoring process, but with the SECM tip as well, this effect being significantly enhanced when chloride ions were present in the electrolyte. Finally, a procedure to image the chemical activity of copper surfaces partially covered with the inhibitor film with SECM is proposed.     

Effect of cation structure on the electrochemical and thermal properties of ion conductive polymers obtained from polymerizable ionic liquids

Several onium cations having vinyl group formed ionic liquids after coupling with bis(trifluoromethanesulfonyl)imide. These monomers were polymerized, and the relation between onium cation structure and properties of thus polymerized ionic liquids was investigated. The polymerized ionic liquid having ethylimiadzolium cation unit showed the highest ionic conductivity of around 10⁻⁴ S cm⁻¹ at 30 °C among the obtained polymers reflecting the lowest glass transition temperature of −59 °C. These polymers were thermally stable and their decomposition temperatures were about 350 °C. The ionic conductivity of the polymerized ionic liquids decreased by both the addition of lithium bis(trifluoromethanesulfonyl)imide and the polymerization in the presence of cross-linker. However, the polymerized ionic liquid having 1-methylpiperidinium cation structure showed good lithium ion transference number of 0.43 at room temperature.     

Photoelectrochemical kinetics of Eosin Y-sensitized zinc oxide films investigated by scanning electrochemical microscopy under illumination with different LED

The overall efficiency of the light-induced charge separation in dye-sensitized solar cells depends on the kinetic competition between back electron transfer and dye regeneration processes by a redox electrolyte. In a previous study, the reduction of the intermittently formed photo-oxidized dye molecules by iodide ions in the electrolyte phase was investigated using the feedback mode of a scanning electrochemical microscope (SECM) and a quantitative model had been derived. Here we provide a more thorough experimental verification of this model by variation of the excitation wavelength, light intensities and mediator concentrations. Nanoporous ZnO/Eosin Y films prepared by self-assembly were used as model electrodes and were used with an iodide/triiodide electrolyte. The experimentally found effective rate constants could be related to the rate constant for the reaction of the dissolved donor with photo-oxidized Eosin Y bound to ZnO and the absorption spectrum of the dye and confirmed the assumption made in the derivation of the model. For the regeneration process of Eosin Y, a rate constant of k_ox with different light emitting diodes and light intensities is determined.     

Studying the localized deposition of Ag nanoparticles on self-assembled monolayers by scanning electrochemical microscopy (SECM)

The local deposition of Ag nanoparticles (NPs) on ω-mercaptoalkanoic acid, HS(CH₂)_nCO₂H, (n = 2, 10) self-assembled monolayers (SAMs) by scanning electrochemical microscopy (SECM) is reported. We found that the presence of a SAM had a pronounced effect on Ag deposition. Experiments were conducted by applying different potentials to an Au(1 1 1) substrate either in the presence of a constant concentration of Ag⁺ ions in solution (bulk deposition) or by generating a flux of Ag⁺ from an Ag microelectrode that was positioned close to the Au(1 1 1) substrate (SECM deposition). SECM was used for generating a controlled flux of silver ions by anodic dissolution of an Ag microelectrode close to the SAMs modified Au(1 1 1). We found that the shape of the NPs was affected by the length of the carbon-chain of the SAM. Tetrahedral NPs were obtained on bare Au(1 1 1) surfaces while rod like and cubic Ag NPs were deposited onto 3-mercaptopropanoic acid (MPA) and 11-mercaptoundecanoic acid (MUA) SAMs, respectively. The size and shape of the deposited NPs were influenced by the deposition potential.We conclude that the shape and distribution of locally deposited Ag NPs on Au(1 1 1) can be controlled by modification of the substrate with a SAM and through controlling the Ag⁺ flux generated by SECM.     

Comparison of electrochemical methods for triiodide diffusion coefficient measurements and observation of non-Stokesian diffusion behaviour in binary mixtures of two ionic liquids

Results of diffusion coefficient measurements of triiodide in a mixture of two ionic liquids (1-methyl-3-propylimidazolium iodide and 1-butyl-3-methylimidazolium tetrafluoroborate) at 25 °C are described in this paper. Four electrochemical methods for measuring diffusion coefficients of triiodide were evaluated for their reliability and performance, including impedance spectroscopy and polarization measurements at thin layer cells as well as cyclic voltammetry and chronoamperometry at microelectrodes of different radii. Viscosities of the blends were measured to investigate the transport behaviour of triiodide ions used in Grätzel-type dye-sensitized solar cells.     

Development of a Ag/Ag micro-reference electrode for electrochemical measurements in ionic liquids

We report on a novel miniaturized Ag/Ag⁺ reference electrode (RE) design suitable for electrochemical measurements in room temperature ionic liquids (RTILs). The electrode is based on capillaries with an outer diameter of 365 μm and contains a 10 mmol/l solution of a silver salt in a RTIL. The silver salt bears the same type of anion as the RTIL. While potential shifts of several hundred millivolts have been observed for common platinum or silver pseudo-reference electrodes, our Ag/Ag⁺ micro electrode provides a stable and reliable reference potential over a period of more than two weeks, if protected from light and stored in a nitrogen atmosphere. Due to the small dimensions of the RE, it can be placed close to the working electrode (WE) and it is well-suited for application in electrochemical micro cells as well as for potential-controlled in situ AFM, STM or electrochemical impedance measurements. The electrode characteristics were determined by voltammetric measurements on ferrocene and cobaltocenium hexafluorophosphate dissolved in a RTIL. The highest expected contamination of the sample with Ag⁺ ions was calculated and found to be below 4 ppm.     

Visualization of local degradation processes in coated metals by means of scanning electrochemical microscopy in the redox competition mode

Herein, the redox competition mode of scanning electrochemical microscopy (RC-SECM) has been applied to in situ monitor the local reactivity arising from a circular holiday operated in a painted metal. The metal-coating system consisted of a carbon steel substrate coated with an epoxy-polyamine polymeric film containing glass flakes as pigment. The present work demonstrates the possibility to use RC-SECM for investigation of the corrosion reactions occurring when protective coatings are applied on a metal surface without the addition of a redox mediator to the experimental system. Oxygen reduction was employed to monitor the reactive metal-polymer system, though the onset of a redox competition for this redox species between the SECM tip and the bare metal inside the holiday could be found in certain conditions. Whether attack predominated inside the holiday or the system became non-reactive depended on the composition of the test electrolyte, with borate ions acting as corrosion inhibitor.     

Precursor sites for localised corrosion on lacquered tinplates visualised by means of alternating current scanning electrochemical microscopy

In solutions of low conductivity and at high frequencies the impedance of a SECM tip-auxiliary electrode cell is dominated by the solution resistance between the tip and counter electrode. Alternating current scanning electrochemical microscopy (AC-SECM) utilises the effect of an increasing (decreasing) solution resistance as the SECM tip approaches an insulator (conductor) for mapping domains of different conductivity/electrochemical activity on surfaces immersed into electrolytes. In the present study, we employed AC-SECM in aqueous solutions to evaluate the integrity of the solid/liquid interface of lacquered tinplates as commonly used in industry to manufacture, i.e. food cans. Significant differences were determined between the AC response and the phase shift measured with the SECM tip above the intact coating and above defects where the surface of the steel base is exposed. This allowed with high lateral resolution to detect and to visualise artificial micro cavities which we consider as an experimental model of microscopically small precursor sites for localised corrosion.     

A mechanistic investigation of di-tert-butyl nitroxide using scanning electrochemical microscopy (SECM)

Cyclic voltammetry (CV) and scanning electrochemical microscopy (SECM) have been used to evaluate the electron-transfer mechanism of di-tert-butyl nitroxide (DTBN) in acetonitrile. The oxidation of DTBN is coupled to a rapid, irreversible chemical follow-up step that is difficult to characterize quantitatively with CV due to distortion of the voltammograms by solution resistance and mixed radial–linear diffusion within the scan rate region of interest. Collection efficiencies from the tip generation substrate collection (TGSC) mode of SECM were used to determine a rate constant of 21 s⁻¹ for the follow-up reaction. Collection efficiency versus distance plots obtained at 5 and 50 mM DTBN concentration are identical, confirming the first-order nature of the chemical reaction. Numerical simulation of linear scan voltammograms obtained at different tip/substrate distances provides a heterogeneous electron-transfer rate constant of 0.85 cm s⁻¹.     

Visualization of electrocatalytic activity of microstructured metal hexacyanoferrates by means of redox competition mode of scanning electrochemical microscopy (RC-SECM)

Transition metal hexacyanoferrates are versatile inorganic compounds widely employed for the assembling of sensors and biosensors in a variety of different electroanaytical applications. A modified version of the recently introduced redox competition mode of scanning electrochemical microscopy (RC-SECM) was exploited to visualize the local electrocatalytic activity of microstructured Prussian blue (PB) films towards the reduction of H₂O₂ with improved lateral resolution. The PB films were electrochemically deposited in a spot on glassy carbon surfaces using a droplet cell. The influence of the potential applied to the PB modified surface on the current at the SECM tip was evaluated when both the sample and the SECM tip were competing for H₂O₂ in solution. Thus, high local electrocatalytic activity is indicated by low currents at the SECM tip. The same strategy was successfully employed for the characterization of the performances of a biosensor employing the enzyme glucose oxidase (GOx) immobilized within a polymer hydrogel matrix on the top of PB-modified glassy carbon electrodes.     

Atomic force microscopy, scanning electron microscopy and electrochemical characterization of Al alloys, conversion coatings, and primers used for aircraft

Characterization of the metal–coating interface is crucial to the understanding and prediction of the performance of corrosion protective coatings. To date, such characterization has been incomplete and performed on a scale of measurement that gives little microscopic-scale information. Atomic force microscopy (AFM) and scanning electron microscopy (SEM) have the ability to provide such information. We present in this work the use of such methods to examine the interface between steel and marine coatings and to image the surface of untreated aircraft aluminum alloys. We have also imaged these aircraft alloys after chromate/phosphate pretreatment. AFM and SEM have also been used to investigate the changes in surface morphology, which accompany changes in the samples due to exposure. Electrochemical noise methods, electrochemical impedance spectroscopy measurements, and Prohesion^TM testing were performed in parallel with the AFM/SEM measurements. The results, along with implications for aircraft coatings, are discussed.     

Surface analysis with scanning electrochemical microscopy in the feedback mode: Monitoring of reactivity and pitting precursor sites on the Nd-Fe-B-type magnet

Scanning electrochemical microscopy (SECM) in the feedback mode was utilized for monitoring the surface reactivity and the localized corrosion processes occurring on the Nd_13.5Fe_79.5Si₁B₆ permanent magnet (intrinsically comprising iron inclusions). SECM imaging experiments, performed with the application of ferrocenecarboxylic acid as the mediator, revealed distinctly reactive areas at the magnet surface during early stages of its immersion in 0.1 mol dm⁻³ phosphate buffer (pH = 7). It was demonstrated that the iron inclusions were responsible for the existence of these reactive areas whereas the bulk material surface was practically unreactive (insulating). It was observed that the surface reactivity of the iron inclusions gradually decreased with time and after 7 h immersion the whole magnet surface became uniformly unreactive (insulating). The results were explained in terms of differences in the dynamics of the passive film formation on the iron inclusions and on the bulk material. Another factor that might be responsible for the observed non-uniform surface reactivity was also considered, namely, the differences in abilities to charge propagation through passive films existing on the iron inclusions and on the bulk material. The role of the iron inclusions as pitting precursor sites in the presence of chlorides in the phosphate buffer solution was also examined.     

Nanostructuration of ionic liquids in fluorinated matrix: Influence on the mechanical properties

In this work, a new method to prepare fluorinated coatings with mechanical properties enhanced has been developed. Pyridinium, imidazolium, and phosphonium ionic liquids have been synthesized and used as new synthetic building blocks in a polytetrafluoroethylene matrix. The strategy demonstrated using long alkyl chain cations provides an opportunity to prepare nanomaterials with a nanoscale structuration. The design of these new ionic and nanostructured materials is very dependent on the cation-anion combination of ILs. The morphology analyzed by transmission electronic microscopy (TEM) shows that it is clearly tuned by the chemical nature of ILs. The finest structuration leads to a dramatic compromise between stiffness and deformation of material. The small-angle X-Ray scattering (SAXS) shows the evolution of the ionic networks during the mechanical sollicitation.