Michel Herlem

pH Sensing at Pt Electrode Surfaces Coated with Linear Polyethylenimine from Anodic Polymerization of Ethylenediamine

A novel pH sensor using a smooth Pt electrode coated with an electrically insulating polymer is addressed in this study. We present a new electrochemically modified Pt electrode. Its modification results from the anodic oxidation of pure ethylenediamine, charged with lithium trifluoromethanesulfonate, which leads to the formation of a polymer coating in one step. The assembly of the electrode surface coated with electropolymerized ethylenediamine acts as a transducer of the electrode potential vs. the pH value in aqueous solutions. It has been shown that the anodic oxidation of pure ethylenediamine yields polyethylenimine which is a high impedance polymer containing amino groups sensitive to H + concentration. Since the reproducibility of polymer-based pH sensors is a controversial subject, we examined both the effect of the electrode-bias time, which can be linked to the polymer thickness, and the aging of the polymer on the potentiometric response of the sensor. The pH sensor we propose here has a quasi-Nernstian behavior. It is reliable for some weeks in the pH range from 3 to 11 in aqueous media.

Surface modification of p-Si by a polyethylenimine coating: influence of the surface pre-treatment. Application to a potentiometric transducer as pH sensor

p-Si electrodes coated with linear polyethylenimine (L-PEI) allow the fabrication of a pH sensitive film for potentiometric transducers. The coating is realized in one step through the anodic oxidation of pure ethylenediamine (EDA) charged with 0.1 M LiCF3SO3 (Lithium Triflate). Such an electrochemical procedure leads to the thickness control of the coating. The best silicon surface pre-treatment before any coating is obtained with potassium dichromate in sulfuric acid, which leads to OH-terminated p-Si. This pre-treatment allows a uniform thin coating. In this work, the thickness is 2.6 nm. The pH response is high and close to 50 mV per pH unit.

Direct Electrocatalytic Activity of Capped Platinum Nanoparticles toward Oxygen Reduction

This paper reports on the original and promising electrocatalytic properties of thiol-capped platinum nanoparticules toward O2 reduction in acidic medium. The nanoparticles were initially functionalized by 4-mercaptoaniline, and were further derivatized with 2-thiophenecarbonyl chloride leading to high stabilization of the organic crown. Ultrathin films of these particles were built up by the LB technique on gold. Their electrochemical behavior was studied. These assemblages were also characterized by X-ray photoelectron spectroscopy before and after oxygen reduction experiments. An electrocatalytic activity was established without any activation treatment. Our results show that the organic crown of the nanoparticles does not prevent their electrochemical activity

The photodissolution of InP

The linear dependency of the n-InP photocurrent on the level of illumination is indepedent of the solution composition, the type of illumination, the exposed crystalline face, or the level of doping. The photodissolution mechanism (for photocurrents between 1.1 and 65 mA cm−2) utilizes six charges to dissolve one molecule. In the absence of an insoluble oxide layer, the ratio between photogenerated holes and injected electrons is independent of the exposed crystalline face, the photon flux, the solution composition and the level of doping. Solutions containing oxygen bearing mineral anions (such as ClO−4, H2PO−2, or SO−24) are predisposed to the formation of an insoluble oxide layer on the surface of the InP under strong illumination.

Are reactions in superacid media due to protons or to powerful oxidising species such as SO3 or SbF5

Abstract The concept is questioned that in superacid media hydride ions are abstracted from saturated hydrocarbons solely. Apparently there are two other processes by which carbonium ions can be generated from alkanes in superacids, viz ., oxidation by SO3 or HSO3F and oxidation by SbF5.

New handy relationship between the conductivity of concentrated nonaqueous electrolyte solutions and the dielectric constant and viscosity of the solvents

Abstract For nonaqueous electrolyte solutions, we correlated the equivalent conductance Λ 0 at infinite dilution and the conductivity maximum κ MAX with only two intrinsic parameters of the pure solvents: the dielectric constant and the viscosity. On the basis of two new handy empirical formulas, predictions of Λ 0 and κ MAX can now be made for a given salt in any solvent on the basis of only one Λ 0 or κ MAX measurement in only one solvent.

Spectroscopic and ab initio study of polymeric films used as chemical sensors

Electrochemical oxidation of ethylenediamine leads to the coating of different electrode surfaces by a thin polymeric film. This film was characterized as a polyethylenimine one using both spectroscopic methods such as infrared-attenuated total reflection or infrared absorption spectroscopy and ab initio calculations. More, since the polymeric coating changes the electrochemical behavior of different surfaces, this film can be used successfully to make new chemical sensors.

Cathodic Decomposition of n‐InP during Hydrogen Evolution in Liquid Ammonia

In acidic liquid ammonia solutions, the cathodic decomposition of n-InP occurring during hydrogen evolution is similar to that in aqueous systems. This decomposition results in the formation of an indium layer on the surface for a cathodic bias in the potential range where protons are reduced. A characteristic anodic peak is observed in the following positive-potential scan and is associated with the anodic dissolution of the surface indium. The identity of this peak was verified by the electrochemical behavior of InP and smooth Pt electrodes coated with an indium film, prepared by the reduction of In{sup 3+} on the electrodes in liquid ammonia. These results suggest that a strong interaction of hydrogen atoms with the InP surface takes place during the first step of the reduction of protons, regardless of their solvation (H{sub 2}O or NH{sub 3}). The hydrogen evolution reaction is of prime importance in electrochemical research. The knowledge of each step of the reaction is necessary to understand the behavior of electrode material, e.g., semiconductors (SC) and metals. This reaction is important in the conversion of solar energy to useful energy.

Electrodeposition of Keggin-Type Heteropolyanions on Different Electrode Surfaces from Nonaqueous Media

It is demonstrated that films of heteropolyanions (HPAs) can be electrodeposited onto different metallic electrode surfaces in nonaqueous media such as N,N-dimethylformamide (DMF) or N-methyl-2-pyrrolidinone (NMP) by reduction of Keggin-type (NEt 4 ) 3 PMo 12 O 40 . For both DMF and NMP, electrodeposition gives rise to a coating. In DMF, the electrodeposition takes place during the first and second reduction steps of HPA, while in NMP, the electrodeposition takes place only during the second reduction step, which demonstrates that electrodeposition strongly depends on the solvent nature, possibly because of a process of coadsorption of the solvent during electrodeposition.

In situ modification of the energetic structure of the n-GaP/NH3 junction in the presence of solvated electrons

Band-edge shifts of the n-GaP/electrolyte junction, when the pH and solution redox potential vary, are demonstrated in situ. Large variations (14 pH units and more than 2 V) of these parameters are obtained by using liquid ammonia solutions (neutral and basic) in the presence and absence of solvated electron species. The semiconductor band-edge position was deduced from Mott-Schottky curves in the following ammonia solutions: neutral, solvated electron, basic, basic + solvated electron, basic and basic + solvated electron. The transformation of a solution to the next one was obtained in situ, i.e. without removing the electrode from the solution. Large band-edge shifts could be demonstrated, up to ΔVfb = −1.8 V between the first solution (neutral) and the last one (basic + solvated electron). A reversible band-edge shift was observed between the basic and basic solvated electron solutions. The photoelectrochemical study revealed classical semiconductor/electrolyte behaviour with large photovoltages (up to 1.75 V in the solvated electron solution). The GaP/solvated electron junction behaviour gives some information to the more general investigation concerning the contact of a semiconductor and a redox system, the potential of which is located within the material conduction band.