Anne-Marie Gonçalves

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.

Growth of Anodic Oxides on n-InP Studied by Electrochemical Methods and Surface Analyses Correlation Between Oxidation Methods and Oxide Growth

Growth, formation, and stability of anodic oxides obtained on n-InP were investigated by coupling electrochemical methods and X-ray photoelectron spectroscopy (XPS) analyses. Photocurrent transients and capacitance measurements performed before and after the semiconductor surface oxidation exhibit new electrical interfacial properties, whereas XPS analysis gives access to chemical composition and estimation of oxide layer thickness. In this work, using a galvanostatic method, oxidation of the InP surface has been studied at pH 9 for two current densities: 0.2 and 12 mA cm 2 . Using transient photocurrent and Mott-Schottky behavior as in situ probes, we have pointed out several steps for the InP oxidation process, correlated to a gradual oxide coverage which is evidenced by XPS characterization. The current density chosen to perform the oxidation governs the resulting chemical composition, texture, and electrical properties of the oxide. For low current density, the anodic mechanism varies progressively from a pure semiconductor oxidation process to a solvent oxidation contribution. For high current density, this trend disappears whereas semiconductor oxidation continues to take place.

Interfacing a heteropolytungstate complex and gelatin through a coacervation process: design of bionanocomposite films as novel electrocatalysts

Bionanocomposite films on glassy carbon electrodes (GCEs) have been prepared by a very straightforward and reliable method based on a layer-by-layer deposition of polyoxometalate (POM) (i.e. [BW12O40]5−) and gelatin solutions. The strong immobilisation of [BW12O40]5− on the surface of the GCE results from electrostatic interactions between gelatin and POM, as evidenced in a hybrid hydrogel prepared by a coacervation process. The GCE has also been modified by imidazolium based ionic liquids in order to increase the charge transfer rate. Two modified electrodes with one or two POM layers have been prepared by this strategy. When compared to other POM-based modified electrodes, the novel electrodes with two POM layers exhibit excellent electrocatalytic performance for nitrite detection at pH 3 in terms of sensitivity (868 mA M−1 cm−2) and linear range of nitrite concentrations (50–600 μM).

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 .

Liquid ammoniates: nonaqueous electrolytes for electrochromism

New conductive materials involving Cu + ions are proposed for electrochromic devices. They are liquid ammoniates which can be used from room temperature up to 80°C, and whose conductivity and p(NH3) can be modulated according to requirements. The synthesis does not require strong acids or complexing agents such as halide ions. The principle of operation of the electrochromic material is the reduction of Cu + to Cu at the transparent cathode while Cu + is oxidized to Cu 2 + (thus developing a blue color) at the transparent anode. The bleaching is realized by the reverse reactions.

Uncommon behavior of the p-GaAs electrode during the reduction of oxygen in liquid acidic ammonia

In liquid acidic ammonia, the mechanism of oxygen reduction begins as in acidic aqueous solutions, by the successive contribution of the conduction band and then of the valence band (doubling effect). However, unlike in the acidic aqueous solutions, soon after the oxygen photoreduction, an activation process of the p-GaAs surface is initiated, tied to the presence of protons in the solution. This activation of the surface allows the passage from a reduction mechanism via a conduction band to a reduction mechanism via a valence band (oxygen reduction in the dark).

Electrochemical and Photo-Electrochemical Properties of Porous n-InP Layers

In this paper, we investigate the properties of porous structures anodically grown onto n-InP (100) in 1 M HCl. The relation between growth parameters and the pore morphology is firstly reported. In situ electrochemical characterizations show that the pore formation strongly influences the physical properties of n InP surfaces. Capacitance measurements reveal a modification of the electronic distribution but no variation of the flat band potential. Photocurrent spectra performed during the pore growth are strongly modified: uniform increase of the photocurrent followed by a decrease with narrowing of the spectrum is observed, red shift. Finally ex situ photoluminescence experiments carried out onto porous films show both a quantum size effect and a dead layer behavior.