S. Piazza

Photocurrent Spectroscopic Investigations of Passive Films on Chromium

A detailed photoelectrochemical study was carried out in order to identify the nature and the properties of the passive layers on chromium under different experimental conditions. Different compositions of the films, related to various hydration degrees of the Cr(III) oxides, have been inferred on the basis of the bandgap values measured at changing pH, electrode potential, and polarization time. By comparison with air‐grown films, it has been concluded that anhydrous is never formed in acidic solution. The water content in the passive film increases at decreasing potentials and for long polarization times. In some cases, especially at neutral pH values, passive films possess a duplex structure, consisting of two layers with different hydration degrees. The photoelectrochemical study suggests an insulating or a slightly p‐type behavior of the passive layers.

A Photocurrent Spectroscopic Investigation of Passive Films on Ferritic Stainless Steels

Etude experimentale de la nature des films passifs croissant sur trois aciers inoxydables ferritiques de compositions differentes immerges dans une solution a 3,5% NaCl a la temperature ambiante

Amorphous semiconductor—electrolyte junction. Impedance study on the a-Nb2 O5—electrolyte junction☆

A systematic study of the impedance behaviour of the anodic niobium oxide film/aqueous electrolyte interface was carried out using the lock-in technique at different signal frequencies. The dependence of both components of the impedance on the electrode potential and on frequency is analysed by taking into account the amorphous nature of the films. The lack of long-range order in these oxide layers modifies the physical picture in respect to the case of single crystal semiconductors. A new equivalent circuit has been assumed, based on recent theory of an amorphous semiconductor Schottky barrier. Such a new approach allows the characterization of the interface and the determination of the electronic states distribution within the oxide gap. Possible mistakes arising from the use of models valid for crystalline semiconductors to study these kinds of interfaces are pointed out. The effect of annealing of the films under argon atmosphere as well as the possible nature of the defects in Nb2O5 anodic films are discussed.

Photoelectrochemical study of the corrosion product layers on copper in weakly acidic solutions

Abstract The nature of the surface layers grown on copper electrodes in free corrosion conditions in unbuffered aerated Na 2 SO 4 solutions (pH = 3.0–6.0) was investigated by photoelectrochemical methods. The formation of cuprous oxide has been revealed by means of a careful study of the photocurrent spectra obtained in situ . The growth of n -Cu 2 O has been observed to occur with or without Cu 2+ ions in solution in a narrow range of pH around five. More acidic solutions seem to favour the formation of p -Cu 2 O both in absence as well as in presence of Cu 2+ ions in solution ([Cu 2+ ] = 0.05 M). The addition of chloride ions in variable amounts ([Cl − ] ⩽ 0.1 M) seems to affect mainly the corrosion rate of copper electrodes at low concentrations ([Cl − ] ⩽ 10 −2 M). At higher chloride concentrations changes in the semiconducting properties as well as in the nature of the corrosion layers can occur. The presence of complex transients in the photocurrent vs time plots has been related to the presence of chloride ions in solution.

In situ characterization of passive films on Al-Ti alloy by photocurrent and impedance spectroscopy

Abstract The anodic behaviour of an Al-Ti alloy (Ti-48Al-1V, atomic %) was investigated in different aqueous electrolytes. In all cases the alloy was passive owing to the growth of a barrier-like oxide film according to the high field mechanism. The study of the growth curves suggests partial dissolution of the film during the formation process in acidic solution. The kinetic parameters for film formation have been estimated in neutral solutions and the dielectric constant of the passive layer was roughly estimated. The in-situ characterization of the passive film revealed a n-type behaviour only for very low thicknesses, whilst thicker films showed insulator-like characteristics. The analysis of the photocurrent spectra suggests that the composition of the surface oxide does not change in a large thickness interval since the very thin initial layers formed by simple immersion of the alloy in solution. From the values of the threshold energy derived from both anodic and cathodic spectra recorded in different conditions it is postulated that this passive film consists of a mixed oxide of the main components of the alloy. A more complex behaviour was observed at higher film thicknesses, which reveals some modification in the properties of the surface layer.

The photoelectrochemistry of thin passive layers. Investigation of anodic oxide films on titanium metal

A photoelectrochemical investigation has been performed on thin TiO2 films grown anodically in 0.5 M H2SO4 solution at high growth rates. The shape of the photocurrent vs. potential curves under monochromatic irradiation (photocharacteristics) depends on the photon energy of the incident light at energies above the optical band gap of the films (3.25 ± 0.05 eV). This finding has been explained by considering the presence of geminate recombination of the photogenerated electron-hole pairs. In order to fit the experimental photocharacteristics, an expression for the photocurrent is proposed which takes into account the low drift range of photocarriers and possible recombination in the space-charge region. Moreover, an exponential potential distribution in the film was assumed from the theory of the amorphous semiconductor (a-SC) Schottky barrier. The results of theoretical simulations suggest that the behaviour of TiO2 passive films can be explained in the frame of the theory of a-SC, taking into account the influence of the photon energy on the thermalization length of the photocarriers.

Template electrosynthesis of CeO2 nanotubes

Nanotube arrays of CeO2 were produced in a single step by potentiostatic electrochemical deposition from a non-aqueous electrolyte, using anodic alumina membrane templates. The CeO2 nanotubes showed a polycrystalline structure, and they were assembled in the membrane nanochannels. The nanotubes had somewhat uniform diameters, with an average external value of about 210 nm, and a maximum length of about 60 µm; the latter parameter was controlled by the electrodeposition time. Each single nanotube was found to consist of crystalline grains having a size of about 3 nm. Raman analysis shows that these CeO2 nanotubes are suitable for catalytic applications.

Photoelectrochemical characterization of thin anodic oxide films on zirconium metal

Abstract The effect of metal surface preparation on the properties of thin oxide films grown on zirconium in different electrolytes was investigated by photocurrent spectroscopy. Both passive layers grown by free corrosion of the samples in a solution and thin oxides grown anodically at a constant rate were investigated. The photoelectrochemical results give a complex picture of the interface, being influenced by the metal surface preparation, the solution pH and the electrode potential. A duplex structure of the films has been suggested on the basis of the photocurrent spectra, with an external hydrous layer (amorphous or strongly defective) having an optical gap ( E g 2 ~ 3 eV) lower respect to that of the underlying anhydrous oxide ( E g 1 = 4.65 ± 0.1 eV). The photoresponse of the hydrated phase was strongly influenced by the initial surface treatment of the base metal. Both anodic and cathodic photocurrents were recorded to the latter being originated by external (very thin films) or internal (thicker films, anodic oxides) electron photoemission phenomena which take place under cathodic polarization of the metal/oxide/electrolyte interface.

Amorphous semiconductor-electrolyte junction: Influence of the wavelength and oxide thickness on the photocharacteristics of a-Nb2O5 anodic films

Abstract The photoelectrochemical behaviour of amorphous anodic films on niobium (a-Nb 2 O 5 ) grown in a wide range of thicknesses (20 ⩽ d ox ⩽ 25 nm) is presented. The influence of the wavelength, light intensity and film thickness on the photocharacteristics of the a-Nb 2 O 5 /electrolyte junction is investigated. Expressions for the photocurrent curves under steady-state conditions are derived by assuming a variable efficiency of photocarrier generation and different distributions of the electrical potential inside the amorphous films. The influence of the light intensity on the photocharacteristics and the existence of a sub-band-gap photoresponse are explained by assuming a finite density of states within the mobility gap of a-Nb 2 O 5 films.

Breakdown Phenomena During the Growth of Anodic Oxide Films on Zirconium Metal Influence of Experimental Parameters on Electrical and Mechanical Breakdown

Different aspects of the breakdown phenomena observed during the growth of anodic films on zirconium metal in aqueous solutions have been investigated. Two different mechanisms of breakdown are suggested as a result of the experimental observations. A mechanical breakdown, which is dependent upon the logarithm of the anodizing current density, has been observed in carbonate and sulfate solutions at low concentrations of salt or aci and/or low anodizing current densities. The appearance of small fissures in the ZrO/sub 2/ films is related to this type o breakdown. An electrical breakdown was observed in sulfamic acid solutions as well as in sulfate and carbonate solutions as high salt or acid concentrations and/or high current densities. In the case of electrical breakdown, the first spark voltage showed a significant dependence upon the value of current density only in carbonate and sulfate anion containing solutions. A possible explanation for this dependence is suggested in agreement with Christovs model, taking into account also the incorporation of anions into the films. The different influences of the electrolytic solutions upon both types of breakdown are presented and discussed.