H. Cachet

Chemical bath deposition of CdSe layers from Cd(II)-selenosulfite solutions

Abstract The chemical bath deposition of CdSe layers from cadmium acetate + sodium selenosulfite solutions was studied by means of quartz crystal microbalance (QCM) experiments, combined with electron microscopy observations and energy-dispersive X-ray analysis. Nanocrystalline deposits with the cubic blende structure are obtained showing confinement effects. From mass vs. time measurements and modeling of the time derivative, it is established that the nucleation and growth process occurs via two steps: the first corresponds to an instantaneous nucleation of cylindrical nuclei, with two-dimensional growth until complete coverage of the substrate; the second consists of a three-dimensional instantaneous nucleation and growth process occurring on sites randomly distributed on the evolving surface. The addition of silicotungstic acid to the deposition bath is shown to influence the growth kinetics of the CdSe deposit. From analysis of the kinetic data, and from the transmission and scanning electron microscope images, it is suggested that the chemical deposition of CdSe occurs through the aggregation of colloids and not according to an atom-by-atom mechanism.

Relation between morphology and conductivity in TiO2 nanotube arrays: an electrochemical impedance spectrometric investigation

Two types of TiO2 nanotubular arrays were obtained by anodisation of a titanium foil, in two different solutions containing fluoride ions. For the first type which has rough tube walls, impedance measurements in the dark showed the presence of a localised surface state which was related to adsorbed molecular water. Under UV illumination, this adsorbed molecular water was photo-dissociated. Moreover, an increase of 2 orders of magnitude for the limiting capacitance of the space charge layer was observed, simultaneously with the disappearance of the localised state and with a 100-time increase of the carrier density associated with hydrogen insertion. The second type of layer was characterised by smoother tube walls, a high doping level (1020u2009cm−3) in the dark, a lack of localised states and no long-lasting photo-induced effect. In this case, the width of the space charge layer became rapidly higher than the half-thickness of the tube walls, when the applied potential increased. Therefore, the walls were progressively depleted under anodic polarisation, passing from a situation where the tubes were totally active in the cathodic range towards a situation where the contribution of the tube walls could be neglected.

Characterization of deposits by direct observation and by electrochemical methods on a conductive transparent electrode. Application to biofilm and scale deposit under cathodic protection

Abstract A new device is proposed where the electrode is a transparent electrode in modified SnO 2 . With this new electrode the reduction of oxygen occurs in a way similar to a gold electrode. Simultaneously the oxygen current is recorded versus time and picture of the interface itself is taken with a digital camera through the electrode. An image analysis is performed to determine the coated area and the number of objects by surface unit. Under cathodic protection, the reduction of oxygen induces an increase of the interfacial pH and then formation of scale deposit. The usual chronoamperometric analysis was completed by an in situ observation of the scale deposit. In natural water the biofilm development was studied by electrochemical method to determine the biofilm thickness and this analysis is completed by a direct observation through the electrode. A good correlation between the two techniques was found.

Dependence of the flat-band potential of n-type GaAs on the redox potential in methanol and acetonitrile

Abstract The electrochemical behaviour of n-type GaAs has been studied in non-aqueous media. The results obtained indicate that the Fermi level of the semiconductor is pinned. Redox couples whose potentials are more negative than −0.3 V vs. Ag 0 /Ag + in CH 3 CN (in methanol, −0.2 V vs. Ag 0 /Ag + in CH 3 OH) produce a negative displacement of the flat-band position in both solvents. Results are interpreted by an equilibration between the redox potential of the solution and surface states situated above the valence band edge of the semiconductor. Couples whose potentials are more positive than the valence band edge (as determined in the redox-free supporting electrolyte) inject holes into the valence band and produce a positive flat-band shift leading to the equilibration of the valence band edge with the potential of the redox reagent. The latter behaviour is observed only in CH 3 CN.

Photoelectrochemical behaviour of GaAs modified by electrodeposition of heteropolyanions

Abstract A reductive deposition of 18-tungsto-diphosphate heteropolyanions (HPA) P 2 W 18 O 6− 62 is performed at n and p -GaAs surfaces. It is found by electrical measurements ( I–V , impedance) and surface observations (SEM, TEM) that the treatment is more efficient and durable when it is performed at pH = 0. Photocapacitance measurements in that case reveal that the HPA surface modification induces surface states in the lower and the upper part of the band gap of GaAs. Such a distribution is able to explain both the observed improved H 2 evolution at n and p -GaAs and the better stability of photoanodes.

Films and powders of fluorine-doped tin dioxide

Publisher Summary This chapter reviews the up-to-date developments and applications of fluorine doped tin dioxide materials that have attracted the attention of a large number of research groups from several disciplines. The electrical conductivity can be largely improved and stabilized by doping with foreign impurities. Fluorine is incorporated into the SnO2 lattice without generating large densities of electronic gap states. Fluorine incorporation into SnO2 matrix has been studied by many authors on the basis of analytical techniques, especially by resonant nuclear analysis (RNA). Even though highly doped, the semiconducting behavior of tin dioxide is maintained: the consequence is a large overvoltage pointed out for oxygen evolution, which allows the use of tin oxide films as anode for the electrochemical oxidation of organic pollutants and wastewater treatment based on generation of hypochlorous species. The synthesis of FTO films and powders is first discussed. Structural and electrical properties of SPD tin oxide and fluorine incorporation into FTO materials are described in the chapter. Electrochemical applications and other applications are also detailed.

Orientation-dependent electronic properties of n-type GaAs in contact with various redox systems in acetonitrile and methanol

The flatband potential of n-GaAs/non-aqueous electrolyte contacts has been measured as a function of (i) the redox energy level of the electrolyte (over 2 V in acetonitrile and over 1.3 V in methanol) and (ii) the crystallographic orientation, considering the (100), (111)As, (111)Ga and (110) faces. The experimental conditions have been selected to prevent oxide formation. A strong Fermi level pinning (FLP) is observed for the (100) and both (111) faces due to the same surface state distribution for a given solvent. For acetonitrile (methanol), this distribution is localized near one-third of the gap (0.1 eV) from the valence band edge. In comparison with the (100) face, both (111) faces exhibit a flatband potential negatively shifted by 0.5-0.7 V. This shift can be correlated with the piezoelectric property of the (111) direction. On the contrary, for the (110) face there is no FLP. The flatband potential is independent of the redox level when this latter is varied over all the GaAs gap. This means that the interactions between the redox species and the GaAs surface are sufficiently weak to hold the ideal character of the (110) surface, i.e. without surface states as for the cleaved surface in UHV conditions.

Photoinduced metal deposition on sprayed SnO2 films

La photodeposition de palladium ou de platine par rayonnement UV sur une couche mince doxyde detain est examinee

Surface evolution of n-GaAs in contact with acetonitrile solutions studied by electrochemical impedance spectroscopy

Abstract Electrochemical impedance spectroscopy (EIS) is used to study oxide film formation onto an n-type GaAs electrode in contact with an acetonitrile solution, in the case where the redox reaction rate is mass-transport limited. In the dark and under anodic polarization, the n-GaAs surface is oxidized leading to dispersed flatband potential determinations. Under white light illumination and anodic polarization, a similar situation occurs. Surface film formation can be studied by electrochemical impedance spectroscopy. The shape of impedance diagrams strongly changes with the applied potential. The observed behaviour is well accounted for by considering that the oxide film is stable at high positive polarization, but it is electrochemically decomposed at more negative potential. In practice, it can be considered that an oxide-free surface is obtained when the GaAs electrode is polarized at a sufficiently negative potential for oxide to be reduced. The stability of such a “reduced” GaAs surface at more positive potential will depend on the redox reaction rate, on the residual water trace content and on the electrode potential.

pH and redox potential dependence of the flatband potential of n-type GaAs in methanol

Previous studies concerning the behaviour of n-type (100) GaAs in organic media have shown a dependence of the flatband potential V~b on the solutions redox potential [1,2]. Results obtained in CH3OH and CH3CN redox solutions indicated an almost complete Fermi level pinning (FLP) of the semiconductor. In these studies, pH effects were omitted, because CH3CN is an aprotic solvent and, in the case of CH3OH, very small pH variations were registered upon the addition of the different redox couples used. These variations, excluding two couples, did not exceed two pH units. As Vro shifts ranged over 0.7-0.8 V, the pH effect on the Vfb shift was considered to be negligible. To study the combined effect of a solutions redox potential and pH on the flatband potential displacement of an n-GaAs (100) surface, measurements were made in CH3OH solutions with different pH values. The pH of the solution was varied between 0.7 and 12 in methanol. The flatband potential was determined in different solutions before and after the addition of a given redox couple. These results are presented in the following communication.