M.C. Bernard

Electrochromic Reactions in Manganese Oxides I . Raman Analysis

Like nickel oxide, manganese oxide is a widely studied material in the primary batteries field. The reactions taking place during voltametric cycling of manganese oxides can be determined using in situ Raman spectroscopy. The main difficulty for the oxide identification is to obtain relevant Raman reference spectra because of the many possible compounds and, for some of these compounds, of their instability in the laser beam. As a consequence, several modifications of different tetra-, tri- and divalent manganese oxides and oxyhydroxides were carefully studied. The electrochromic behavior of three types of manganese oxides, two prepared by thermal oxidations and the other by electrochemical deposition, were then compared. The presence of nonstoichiometry in the pristine material was necessary to obtain a reversible electrochromic effect. The reaction during electrochromic cycling is more complicated than a simple passage from MnO[sub 2] to MnOOH.

Preparation, characterization and photocatalytic activity of nanocrystalline thin film TiO2 catalysts towards 3,5-dichlorophenol degradation

Both opaque and transparent TiO2 nanocrystalline thin films were developed on glass substrates by applying dip coating and doctor-blade deposition techniques, using titanium(IV) butoxide and Degussa P25 TiO2 powder as precursor and starting material, respectively. Atomic force microscopy (AFM) and scanning electron microscopy (SEM) evaluated the surface characteristics of the films. Results on their structure and crystallinity were obtained by means of X-ray diffraction and Raman spectroscopy. The catalytic activity of the films towards photodegradation of 3,5-dichlorophenol (3,5-DCP) pollutant was examined and their efficiency was compared to that of the TiO2 powder (slurry) suspensions. Pseudo-first-order photodegradation kinetics were observed and the reaction constants were determined. It has been shown that the film photocatalysts can efficiently decompose the pollutant, although relatively higher decomposition rates were observed with the commercial starting powder. Differences in the film efficiencies can be attributed to differences in their grain size, surface roughness and fractal parameters. No altering on the doctor-blade films surface characteristics was observed for several hours of cyclic operation during which their photocatalytic efficiency remained remarkably stable.

Polyaniline Layer for Iron Protection in Sulfate Medium

Polyaniline (PANI) has been electrodeposited on iron in oxalic medium in order to evaluate the protective character of this polymer. PANI has been found to be efficient for corrosion protection during at least 10 h in a pH 4.5 sulfate medium. Interaction between the passive layer and polymer has been studied using spectroelectrochemical techniques such as Raman spectroscopy and reflectance measurements.

Sensitization of TiO2 by Polypyridine Dyes Role of the Electron Donor

Dye-sensitized photoelectrochemical cells (DSSC) are characterized by electrochemical impedance spectroscopy (EIS) and Raman spectroscopy during their polarization. Cells realized with a dye recently synthesized in one of our laboratories, containing two terpyridyl (terpy) ligands, are compared with cells using commercial dyes (Ru535 and Ru620) containing isothiocyanates and either bipyridyl (bpy) or terpy ligands. Here, two points are emphasized, first, the role of the functional group (carboxylate or phosphonate) which ensures the linkage to TiO 2 and, second, the role of the redox couple (I /I - 3 ) present in the electrolyte which can react with the dye D to give unwanted intermediate species. Two species, each of them giving a characteristic Raman band in the low wavenumber range, are characterized by Raman spectroscopy. The first of these species is triiodide; the nature of the second one, which directly implies the oxidized form of dye, D + , is discussed. During the DSSC functioning, EIS allows one to discriminate three potential ranges, the photocurrent plateau, the recombination range, and the direct current range when the voltage decreases from anodic to cathodic. The second intermediate exists only in the photocurrent plateau, while I - 3 exists also in the recombination range. These results do not depend on the nature (bpy or terpy) of the ligand.

Electrochromic properties of TiO2 anatase thin films prepared by a dipping sol–gel method

Abstract Electrochromic TiO 2 anatase thin films on ITO were prepared by the sol–gel dipping method using a solution of titanium tetraisopropoxide, diethanolamine and ethanol. The films were transparent in the visible range and can be colored in a solution of LiClO 4 in propylene carbonate. The transmittances of the colored films were found to be strongly dependent on the Li + inserted charge. Combining the experimental data obtained from in situ Raman and in situ transmittance spectra with the data from chronoamperometic measurements, it was demonstrated that the fully colorated state of the TiO 2 anatase films is Li 0.5 TiO 2 with a crystalline structure of Imma space group symmetry. In the Raman spectra this coloration state exhibits five characteristic bands at 176, 224, 316, 531 and 629 cm −1 .

Protection of Iron Against Corrosion Using a Polyaniline Layer: I. Polyaniline Electrodeposit

Polyaniline (PANI) has been studied for some years in order to protect mild steels against corrosion in the absence of inorganic conversion layers. During the electropolymerization of PANI film, iron is passivated; the subsequent protection is dependent on the nature of this passive layer, and PANI acts as a stabilizer of this layer. The best conditions for PANI electrodeposition are discussed here, using a simple electrochemical test to characterize the passivity breakdown.

Study of CdS Epitaxial Films Chemically Deposited from Aqueous Solutions on InP Single Crystals

Epitaxial growth of cadmium sulfide on InP single crystals is achieved by chemical bath deposition (CBD) in ammonia solutions at near room temperature. A better understanding of the correlations between the deposition parameters (temperature, bath composition) and the epitaxial quality is obtained by using electron diffraction and transmission techniques, x-ray diffraction, in combination with Raman spectroscopy. They are supplemented by electrochemical impedance and photocurrent experiments which give information on energetic structures between InP and CBD-CdS. Direct relations between the substrate properties and the growth habits of the CdS film (hexagonal vs. cubic, epitaxial vs. polycrystalline) are found.

Direct Low-Temperature Deposition of Crystallized CoOOH Films by Potentiostatic Electrolysis

c DPC/SCCME, Centre dEtudes Nucleaires de Saclay, 91131 Gif-sur-Yvette Cedex, France Cobalt oxide films were deposited onto nickel electrodes from 25 up to 90°C at a constant potential of 0.5 Vvs. SCE from a pH 7.4 nitrate solution containing 0.09 mol L 21 Co~II!. Above 60°C, scanning electron microscopy and X-ray diffraction ~XRD! experiments reveal that the films are constituted of crystallized grains. At 90°C the crystallite size is measured atca. 34 nm. XRD experiments show that the crystallized product is composed of CoOOH, and Raman spectroscopy shows the formation of this compound between 40 and 90°C. Between room temperature and 90°C, the deposition rate is increased by an order of magnitude and the deposit crystallization has a favorable effect on the faradaic deposition efficiency. It may also favor the mechanical stability in temperature of this deposit when used as cobalt oxide-based protective coating of the molten carbonate fuel cell nickel cathode.

In Situ Raman Study of the Corrosion of Zinc‐Coated Steel in the Presence of Chloride I . Characterization and Stability of Zinc Corrosion Products

The different corrosion products formed on zinc specimens (pure or industrially electrodeposited) during exposure to solutions containing two concentrations of chloride were identified by Raman spectroscopy ; the crystallographic nature of these products was checked by x-ray diffraction and their morphology was described by scanning electron microscopy. The conditions of formation of hydroxycarbonates and hydroxychlorides were described, as well as the influence of pH on the growth of the different modifications of Zn(OH) 2 . The cathodic or anodic nature of the mechanisms leading to the stabilization of the different products was particularly emphasized.

Surface modification and photosensitisation of TiO2 nanocrystalline films with ascorbic acid

Abstract Semi-transparent microporous TiO 2 films on SnO 2 -coated conductive glass were prepared by applying a sol–gel process. Surface topography analysis was conducted with atomic force microscopy, which revealed a highly textured and rough surface, possessing a high capacity for adsorption of a diverse variety of compounds. Surface modification of the films with ascorbic acid results in the formation of a stable, coloured surface complex which red shifts the absorption threshold of TiO 2 , enhancing utilisation of the solar spectrum. Upon illumination in a liquid electrochemical cell, ascorbic acid injects electrons into the conduction band of the semiconductor, giving rise to electrical current. The incident-photon-to-current efficiency presents a maximum at 415xa0nm (5.29%).