Francesco Di Quarto

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 photoelectrochemical characterization of passive films on stainless steels

Abstract Passive films on stainless steels for sea water service have been studied by means of photocurrent measurements. Four experimental ELI (Extra Low Interstitital) ferritic stainless steels (types 25Cr-4Ni-4Mo) and a commercial superaustenitic stainless steel (type 20Cr-18Ni-6Mo-N) have been investigated. Differences in the photoelectrochemical behaviour have been attributed to the different nature of the passive films grown on ferritic or superaustenitic stainless steels. An interpretation is provided to explain the complex dependence of the photocurrent transients on the applied potential. Manuscript received 27 February 1986; in amended form 19 June 1986.

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.

Photoelectrochemical study of the amorphous-WO3-semiconductor–electrolyte junction

The photoelectrochemical behaviour of amorphous anodic films grown on tungsten has been studied. The wavelength of the incident light is shown to influence the photoresponse of the amorphous films. The experimental results are interpreted on the basis of the semiconducting properties of the film and by taking into account the various mechanisms of transport occurring in amorphous materials. At longer wavelengths a Poole–Frenkel mechanism of electrical conduction in the non-extended states of the amorphous semiconductor is invoked in explaining the transport of photoinjected carriers. At the shortest wavelengths a ‘free-carrier-like’ mechanism of transport of the photogenerated carriers is suggested. In each case a different electrode-potential dependence of the photocurrent is obtained experimentally.

Tensiostatic studies on formation and breakdown of anodic oxide films on tungsten in acidic chloride solutions

Abstract The anodic behaviour of tungsten in HCl and HCl + KCl solutions has been extensively studied. For each solution, the applied voltage was increased from 1 V up to the breakdown voltage. Values of the current and open circuit voltage decays were recorded. The HCl concentration was varied from 0.05 M to 12 M while appropriate addition to KCl was made to reach assigned molarities of the Cl − ion. The results are discussed and qualitatively interpreted in terms of the different influence of the H + and Cl − ions.

A phenomenological approach to the mechanical breakdown of anodic oxide films on zirconium

Abstract A phenomenological theory of the mechanical breakdown of films growing on valve metals during galvanostatic oxidation is presented and discussed in detail for ZrO2 anodic films. It is shown that the mechanical breakdown voltage, Vmb, can be linearly related to the logarithm of the anodizing current density both in the case of constant and variable critical thickness, Lc, at which the breakdown occurs. It is also shown that the Amb and Bmb parameters of the relationship: Vmb = Amb + Bmb log i are strictly related to the kinetic parameters of growth of the films in the different solutions. The expressions of Amb and Bmb parameters are derived for films grown in the presence as well as in the absence of space-charge effects. A quantitative check of the equations obtained is presented for different ZrO2/electrolyte systems.

Electrosynthesis of Ce–Co Mixed Oxide Nanotubes with High Aspect Ratio and Tunable Composition

Cerium oxide and cobalt oxides have attracted the interest of several researchers due to their potential application in several technological fields electrochromism, lithium batteries, catalysis, etc.. Ceria has been used as a promoter in the so-called “three-way catalyst” for the control of toxic emission from automobile exhaust. The promotion consists of the enhancement of the noble metal dispersion, as well as stabilization of the supporting medium toward thermal sintering. 1,2 A direct catalytic effect of CeO2 in chemical processes such as water–gas shift reaction or NOx decomposition has been also evidenced. 3,4

Physicochemical Characterization of Passive Films and Corrosion Layers by Differential Admittance and Photocurrent Spectroscopy

The stabilization of metallic surfaces against corrosion processes in natural and industrial environment rests on the onset of passivity condition with a subsequent drastic reduction of the corrosion rate of the underlying metallic substrate. In spite of a longstanding controversy it is now universally accepted that a passive metal is usually covered by a thin or thick external layer the whose physicochemical properties control the evolution of corrosion process as well as the possible reactions occurring at metal/oxide and oxide/electrolyte interfaces.1,2 In many cases of practical importance, passivity of metals is reached in presence of very thin (few nm thick) layer which makes the complete physico-chemical characterization a very complex task requiring the use of different powerful in situ and/or ex situ techniques. This is particularly true if we want to get information pertaining to the chemical composition, morphology, crystalline or disordered nature and solid-state properties of the passive layers.

Cerium Oxyhydroxide Nanowire Growth via Electrogeneration of Base in Nonaqueous Electrolytes

The preparation of compact cerium oxyhydroxide nanowires into anodic alumina membranes from cerium chloride in ethanol solution via electrogeneration of base is reported. Scanning electron microscopy analyses indicate that dense, well-aligned, and highly ordered nanowires can be formed in a wide range of applied potentials and current densities in alcoholic solution. The employment of water brings to a hybrid nanotube/nanowire structure, suggesting a key role of the electrolyte nature in determining the morphology of the deposit. Electrochemical findings and X-ray diffraction analysis have shown that nanowires are constituted by a Ce(III)/Ce(IV) oxyhydroxide that can be completely oxidized into CeO 2 by thermal treatment.

Electrochemical Tantalum Oxide for Resistive Switching Memories

Redox-based resistive switching memories (ReRAMs) are strongest candidates for the next-generation nonvolatile memories fulfilling the criteria for fast, energy efficient, and scalable green IT. These types of devices can also be used for selector elements, alternative logic circuits and computing, and memristive and neuromorphic operations. ReRAMs are composed of metal/solid electrolyte/metal junctions in which the solid electrolyte is typically a metal oxide or multilayer oxides structures. Here, this study offers an effective and cheap electrochemical approach to fabricate Ta/Ta2 O5 -based devices by anodizing. This method allows to grow high-quality and dense oxide thin films onto a metallic substrates with precise control over morphology and thickness. Electrochemical-oxide-based devices demonstrate superior properties, i.e., endurance of at least 106 pulse cycles and/or 103 I-V sweeps maintaining a good memory window with a low dispersion in ROFF and RON values, nanosecond fast switching, and data retention of at least 104 s. Multilevel programing capability is presented with both I-V sweeps and pulse measurements. Thus, it is shown that anodizing has a great prospective as a method for preparation of dense oxide films for resistive switching memories.