C. Mathieu

Surface modification of platinum and gold electrodes by anodic oxidation of pure ethylenediamine

A permanent modification of Pt and Au electrode surfaces was observed after the oxidation of pure neutral or acidic ethylenediamine (EDA). Platinum and gold were coated by an organic compound strongly tied through oxygen atoms to metal atoms. The presence of water is of importance: the larger the water concentration the less stable the coating. This phenomenon was studied by coupling electrochemistry, in situ EQCM and ex situ XPS chemical analysis for Pt.

Pore Formation on n-InP(100) in Acidic Liquid Ammonia at 223 K

For the first time, pore formation on n-InP(100) has been carried out by galvanostatic treatments in acidic liquid ammonia at 223 K. Voltage oscillations correlated to a specific current line oriented pore morphology have been evidenced by scanning electron microscopy. Whatever the anodic charge, a constant pore depth was formed (2-3 μm). Porous layers have been characterized by ex situ photoluminescence measurements that have revealed a dead layer behavior. This work demonstrates the crucial role of interfacial phenomena illustrated by the use of this uncommon nonaqueous electrolyte.

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.

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.

Taking advantage of liquid ammonia to control the surface modification of silicon electrodes

Abstract This work shows the advantages of using a non-aqueous solvent such as ammonia, which can be used at cryogenic temperatures and can be prepared anhydrously, to control the state of a silicon electrode surface in contact with the electrolyte. Depending on the in-situ experimental conditions, it was possible to change the oxidized material surface (native oxide) to an oxide-free surface and conversely. An original technique for pretreatment of the electrodes in a solvated electron solution in ammonia was used to corroborate the results obtained with the usual treatment, HF under an inert atmosphere. The influence of a superficial oxide layer and the growth of this layer under various conditions, was studied carefully by following the variations of two experimental parameters: the flatband potential and the photocurrent onset potential. By changing the pH, in liquid ammonia, the growth of the superficial oxide layer could be completely controlled; moreover, the oxide could be obtained either by a chemical reaction with traces of water or by a photoelectrochemical route.

Porous Anodic Etching of p ‐ Cd1 − x Zn x Te Studied by Photocurrent Spectroscopy

Anodic treatment of p-Cd 1-x Zn x Te (x=0,0.05, and 0.12) in 0.5 M H 2 SO 4 leads to the formation of a network of pores which can extend more than 100 μm below the top surface. Photocurrent spectroscopy reveals the presence of Te° on the surface of the pores. The Te° film is on average thin (≤5 A), in contrast to the thicker film obtained by photoanodic etching of n-CdTe. The photocurrent spectra arc affected not only by the absorbance of Te°, hut also by recombination in the porous layer.

Oxygen reduction mechanisms at p-InP and p-GaAs electrodes in liquid ammonia in neutral buffered medium and acidic media

Abstract The use of buffered neutral liquid ammonia medium for the first time provided evidence for a current doubling effect on p-InP during oxygen photoreduction. Unlike results in aqueous medium, a common mechanism of O2 reduction was observed at p-InP and p-GaAs electrodes. When protons were added to the solution, two different oxygen reduction mechanisms occurred at these electrodes. This study emphasizes the important results of the hydrogenated p-InP surface and revealed that liquid ammonia (at 223 K) was perfectly appropriate to understand the mechanism of O2 reduction at InP electrodes.

Using Pt microelectrodes in liquid ammonia for studying proton reduction

Abstract The electrical response of Pt microelectrodes, with a radius of 10 μm, was studied in liquid ammonia (−50°C). The question of a suitable time scale for steady state measurements in liquid ammonia is important. The proton reduction currents were measured for different scan rates. A suitable experimental time scale of 20 mV s −1 was determined to reach a stationary state. The diffusion coefficient of protons in liquid ammonia was deduced from these electrochemical results: D (NH 4 + ) NH 3 =(3.8±0.4)×10 −9 m 2 s −1 .

n-Butylamine as solvent for lithium salt electrolytes. Structure and properties of concentrated solutions

Abstract Physical characterizations of some lithium salt solutions in n-butylamine, such as apparent molar volumes, viscosity, vapour pressure measurements, solvation numbers by NMR spectroscopy, electrical conductivities, electroactivity range … have been carried out showing that this amine is a good solvent for lithium batteries. Three salts were used, LiNO3, LiTrif and LiTFSI. Solutions of lithium triflate or LiTFSI are stable toward lithium and possess a good conductivity (better than 3 × 10−3 S cm−1 at + 20 °C) for concentrations far from the saturation (the maximum of conductivity laying within the salt mole fraction interval 0.1–0.2 for LiSO3CF3 · xn-Bu) and an electroactivity range of 4.5 V at a smooth Pt electrode.

Study of a thin anodic oxide on n-InP by photocurrent transient, capacitance measurements and surface analysis

Abstract Investigation of the anodic oxide on n-InP has been made with an approach coupling photocurrent transient and capacitance measurements in the dark, associated to chemical surface analysis by photoelectron spectroscopy (XPS). Growth, formation and stability of anodic oxides obtained on n-InP were investigated under illumination at pH 9. A potentiostatic mode has been preferred to explore the chemical evolution of the interface. This study shows that the decay of the photocurrent transients, the modification of the capacitances and the chemical evolution of the surface composition are totally related. The growth of a thin and stable oxide provides a blocking behaviour of the oxidised n-InP toward the photo-transfer. Moreover, our study evidenced a formation and growing of a mixed In-P oxide layer, in three stages.