Growth and Characterization of Anodic Films on Al-Nb Alloys

Abstract The anodizing behaviours of sputtering-deposited aluminium, niobium and Al-Nb alloys, containing 0.4, 7.5, 21, 40 and 55 at.% niobium, have been examined in 0.1 M ammonium pentaborate electrolyte with interest in the morphology, structure and electronic properties of the anodic oxides. Transmission electron microscopy revealed amorphous oxides, containing units of Nb2O5 and Al2O3, with formation ratios intermediate between those of anodic alumina and anodic niobia. Photocurrent spectroscopy provided increased understanding of the electronic properties of the anodic films, suggesting the formation of “mixed oxides” with insulating behaviours. The estimated band gap values are correl…

A photoelectrochemical investigation of conversion coatings on Mg substrates

Abstract The structure, morphology and composition of conversion coatings grown in stannate bath on pure Mg were studied using potential–time, polarization curves, X-ray diffraction, scanning electron microscopy and photocurrent spectroscopy. The coating is manly constituted by crystalline magnesium–tin hydroxide, whose morphology and distribution depends on the conversion bath composition and temperature. The photoelectrochemical investigation allowed to estimate the band gap value of MgSn(OH) 6 and flat band potential. A sketch of the metal/passive film/electrolyte junction formed during conversion on the metal substrate is reported to account for the overall photoelectrochemical behaviou…

Photoelectrochemistry in corrosion studies: achievements and perspectives

Anodic alumina membranes as template for the synthesis of 1-D metal oxide and hydroxide nanostructures

Anodic alumina membranes with highly ordered cylindrical pores and tuneable geometry have been prepared (pore diameters=20−200 nm; pore density 1012-1014 pores/m2; thickness: 20-100 mm) by controlling the anodizing process of aluminum in phosphoric, oxalic and sulphuric acid. The influence of different parameters (initial treatment of aluminium surface, composition of electrolyte, temperature and applied potential) on the final characteristics of the membranes have been investigated. The use anodic alumina membranes as template for the electrosynthesis of some metal hydroxides and oxides 1-D nanostructures (nanowires and nanotubes) will be also proved.

Identification of passive layers on Co in Alkaline solutions by photocurrent spectroscopy

The photoelectrochemical behavior of passive films on Co has been studied as a function of the polarizing voltage and electrolyte composition. Passive layers formed at 0 V (standard hydrogen electrode, SHE) in 0.1 M NaOH consisted of Co(OH) 2 , whose bandgap value has been found to be 1.85 eV. Higher bandgap values (2.75 eV) have been measured for passive films formed in borate buffer at 0 V (SHE), which are mainly consist of CoO. The Eg values have been related to the film composition on the basis of a correlation between the bandgap of passive films and the electronegativity of their constituents.

Light induced electropolymerization of poly(3,4-ethylenedioxythiophene) on niobium oxide

Abstract The photoelectrochemical polymerization of poly(3,4-ethylenedioxythiophene), PEDOT, was successfully realized on anodic film grown to 50 V on magnetron sputtered niobium. Photocurrent Spectroscopy was employed to study the optical properties of Nb/Nb 2 O 5 /PEDOT/electrolyte interface in a large range of potential, and to get an estimate of the band gap and flat band potential of both the oxide and the polymer. Scanning Electron Microscopy was used to study the morphology of PEDOT. Both the optical and morphological features of the photoelectrochemically grown polymer were compared with those showed by PEDOT electropolymerized on gold conducting substrate.

Electronic properties and corrosion resistance of passive films on austenitic and duplex stainless steels

Abstract Passive films were grown at constant potential in acidic (pH∼2) and alkaline (pH∼13) solutions on chromium, AISI 304L, AISI 316L and Duplex stainless steels. Passive films on chromium grow following a high field mechanism considering the presence of dissolution phenomena. According to the photoelectrochemical characterization, passive films on Cr have a bandgap of 3.4 eV when formed in acidic solution, and of 2.4 eV when formed in alkaline solution due to the formation of Cr(OH)3. These films result to be poorly stable against anodic dissolution due to a very anodic flat band potential. Conversely, impedance and photoelectrochemical measurements proved that passive films on stainle…

Qualitative Models for the Photoresponse and Capacitance of Annealed Titania Nanotubes

Physicochemical characterization of annealed TiO2 nanotubes (TNTs) was conducted by using photocurrent spectroscopy and differential capacitance techniques. It has been shown that the geometry and architecture of nanotubes determine how photogenerated electrons and holes are separated and transferred. Photocurrent generation in TNTs is a consequence of two phenomena; drifting of holes into the electrolyte and diffusion of electrons toward the substrate. These two processes have been shown to be independent of the anodic polarization. The capacitance of TiO2 nanotubes is also affected by their geometry. In anodic potentials, with respect to the flat band potential of the underlying barrier l…

Photocurrent spectroscopy of thin passive films

Publisher Summary This chapter reviews theoretical background on the photoelectrochemistry of metal and semiconductor electrodes on which photocurrent spectroscopy (PCS) relies, by focusing particularly on new features that are typical of the photoelectrochemical behavior of thin passive films and usually absent in the behavior of bulk crystalline semiconductors. The chapter also highlights the advantages of PCS in getting in situ information on the structure of the metal-passive-film-electrolyte systems and shows a more recent quantitative use of this technique in characterizing the composition of passive films. The formation of a protective film on a metallic surface is a key step in esta…

Oxide anodes in electro-organic oxidation. Oxidation of maleic addon tungsten oxide anodes

The electrochemical oxidation of maleic acid on tungsten anodes has been investigated. Glyoxal and carbon dioxide were the main products together with tartaric acid and acetaldehyde. Glyoxal is also obtained as the main product from the oxidation ofd-tartaric acid. Under the same conditions succinic acid is completely oxidized to carbon dioxide and water. The anodic dissolution of tungsten and the oxidation of water to oxygen become predominant in the final stages of the electrolyses.

Electronic Properties of Thermal Oxides on Ti and Their Influence on Impedance and Photoelectrochemical Behavior of TiO2 Nanotubes

Thermal oxidation of titaniumwas carried out at 350◦C, 450◦C, and 550◦C for 2 h or 12 h.X-rayDiffraction and Raman Spectroscopy suggest that the thermal oxides are scarcely crystallinewhen the annealing temperature is low, while both anatase and rutile are present for high annealing temperature and time. Photoelectrochemical measurements allowed estimation of a bandgap decreasing from 3.35 eV to 3.15 eV with increasing annealing temperature. The impedance spectra confirmed the formation of n-type semiconductors, with an impedance strongly decreasing on going from a reverse bias toward a forward bias regime. TiO2 nanotubes grown by anodizing Ti in NH4F and water containing ethylene glycol so…

Phosphomolybdic acid and mixed phosphotungstic/phosphomolybdic acid chitosan membranes as polymer electrolyte for H2/O2 fuel cells

Abstract Flat, free-standing phosphomolybdic acid and mixed phosphotungstic/phosphomolybdic acid chitosan membranes were prepared by in-situ ionotropic gelation process at room temperature on porous alumina support firstly impregnated by heteropolyacid. Scanning electron microscopy revealed the formation of compact and homogenous polymeric membranes, whose thickness resulted to be dependent on reticulation time, and almost independent on the employed heteropolyacid nature and concentration. X-ray diffraction and Fourier transform infrared spectroscopy evidenced the formation of crystalline membranes without appreciable concentration of unprotonated NH 2 groups and heteropolyacid ions with p…

Electronic properties and mobile defects distribution in amorphous semiconducting passive films

A study of the electronic properties of thin (drop 25 nm) a-WO3 and a-Nb2O5 is presented. Based on theory of amorphous semiconductor Schottky barrier the fitting of admittance curves in a large range of electrode potential (around 9 V) and a.c. frequency (100 Hz - 10 kHz) is performed. A density of electronic state distribution (DOS) is derived, which mimics the mobile defects distribution suggested by the classical high field model of oxides growth.

Amorphous semiconductor—electrolyte junction. Energetics at the a-WO3—electrolyte junction

In order to elucidate the influence of thickness and amorphous structure on the kinetics of electron exchange with redox couples in solution, a critical re-examination of the energetics at the amorphous anodic WO3 films (a-WO3)—electrolyte junction has been performed, based on a recent theory of amorphous semiconductor (a-SC) Schottky barrier. The admittance study of the barrier performed in a large interval of electrode potential at changing frequency and film thickness allowed the determination of the energy levels as well as the distribution of localized electronic states within the mobility gap of the films. The new energetic picture derived is able to explain some features of the kinet…

Influence of the crystallization process on the photoelectrochemical behaviour of anodic TiO2 films

On the basis of kinetic and photoelectrochemical studies we show that the formation of amorphous or strongly disordered TiO2 films on electropolished titanium rods can occur upon anodization in 0.5 M H2SO4 solution in a range of thickness which depends on the anodization rate. This finding is confirmed both by the changes in the shape of the photocurrent vs. potential curves with the energy of the incident photons, and by the impedance behaviour of the junction. Our data indicate that TiO2 films having different degree of disorder are formed depending on the anodization rate and oxide thickness. Crystalline films are formed at very low growth rates since very low thicknesses. Amorphous or s…

ChemInform Abstract: Photoelectrochemical Study of Electrochemically Formed Semiconducting Yttrium Hydride (YH3-x).

Abstract The first photoelectrochemical study of semiconducting YH 3− x films formed by etching bulk Y metal in 0.5 M H 2 SO 4 solution is reported. The formation of semiconducting hydride having an indirect optical band gap, E g opt , of about 2.35 eV is confirmed by in situ photocurrent spectroscopy. The photoelectrochemical behaviour of such a phase was investigated both in alkaline and in acidic solutions. The flat band potential was estimated to be U fb =−1.25 V/NHE, independent of pH.

Red Shift in the Light Absorption Threshold of Anodic TiO2 Films Induced by Nitrogen Incorporation

Abstract Titanium foils were anodized in ammonium containing and ammonium free solutions in order to check the possibility of inducing nitrogen incorporation into anodic TiO 2 films. XPS spectra confirmed the presence of O-Ti-N bonds on the surface of the anodic films prepared in ammonium biborate electrolytes. In order to evidence the effect of nitrogen incorporation on the light absorption threshold, photoelectrochemical behavior of as-anodized and high temperature annealed films as a function of the anodizing electrolyte composition were investigated. A photocurrent tail at energies lower than the mobility gap of TiO 2 appeared for those films prepared in ammonium containing electrolytes…

Semiconductor electrochemistry approach to passivity and passivity breakdown of metals and metallic alloys

AbstractA critical appraisal of the use of the theory of semiconductors in characterising passive films on metals and alloys is provided, with special emphasis on the use of Mott-Schottky theory for the location of characteristic energy levels of the passive film-electrolyte junction. Some inconsistencies between theory and experimental results in the case of thin passive films are discussed together with possible alternative ways for overcoming such problems. The role of semiconducting properties in determining the pitting behaviour of passive films on W in solutions containing halide is presented and discussed. The validity of a recently proposed correlation between the solid state proper…

Electrochemically prepared oxides for resistive switching memories

Redox-based resistive switching memories (ReRAMs) are the strongest candidates for next generation nonvolatile memories. These devices are commonly composed of metal/solid electrolyte/metal junctions, where the solid electrolyte is usually an oxide layer. A key aspect in the ReRAMs development is the solid electrolyte engineering, since it is crucial to tailor the material properties for obtaining excellent switching properties (e.g. retention, endurance, etc.). Here we present an anodizing process as a non vacuum and low temperature electrochemical technique for growing oxides with tailored structural and electronic properties. The effect of the anodizing conditions on the solid state prop…

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

Abstract 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 th…

Nanoporous alumina membranes filled with solid acid for thin film fuel cells at intermediate temperatures

Thin film fuel cells have been fabricated by impregnation of inorganic porous membranes with inorganic proton conductor. Anodic alumina membranes (50 μm thick and pore diameter of 200 nm), filled with CsHSO4 salt have been used as protonic conductor in a hydrogen-oxygen fuel cell working between 423 and 443 K in dry atmosphere. Polarization curves at 433 K showing ohmic control with open circuit values near 0.8 V and short circuit current around 8 mA cm−2 have been obtained. Possible causes of degradation as well as alternative routes to overcome some of the problems encountered with this approach will be reported. Keywords: Solid acid, Anodic alumina membranes, Pore filling, Thin film fuel…

Physicochemical characterization and photoelectrochemical analysis of iron oxide films

Iron oxide films with a nanoporous structure were grown by anodizing sputter-deposited Fe in a fluoride containing ethylene glycol solution and annealed under air exposure at different temperatures. X-ray diffraction and Raman spectroscopy allowed to identify the presence of hematite and/or magnetite after thermal treatment for films annealed at T ≥ 400 °C under air exposure. According to GDOES compositional depth profiles, the thermal treatment sensitively reduced the amount of fluoride species incorporated into the film during the anodizing process. A band gap value of ~2.0 eV was estimated for all the investigated layers, while a flat band potential dependent on both the growth condition…

ChemInform Abstract: A Photocurrent Spectroscopic Study of the Initial Stages of Anodic Oxide Film Formation on Niobium.

Abstract An extensive electrochemical and photoelectrochemical investigation has been carried out with very thin anodic oxide layers grown on niobium metal in sulphuric acid solutions. From the experiments the presence of an initial suboxide layer on the metal surface was inferred. Upon anodic polarization a strongly non-stoichiometric pentoxide film is formed, which is subsequently oxidized to the normal amorphous pentoxide phase (a-Nb 2 O 5 ). The use of photocurrent spectroscopy (PCS) allowed this oxidation process to be followed at various potentials and polarization times. Moreover, from the experimental results we obtain information both on the composition and the solid-state structur…

Erroneous p-type assignment by Hall effect measurements in annealed ZnO films grown on InP substrate

We report on incorrect carrier type identification achieved by Hall effect measurements performed on ZnO films grown by pulsed laser deposition on InP substrates and subsequently annealed for 1 h at 600 C in air. While Hall measurements, after post-growth annealing, reveal a change in the electrical properties of the films, from n-type to p-type, both photocurrent-based and standard C V measurements performed on the same samples show no change in the native n-type doping of the ZnO films. A possible interpretation of the two results is reported. In particular, p-type conductivity observed by Hall effect may be ascribed to a highly conductive thin layer formed during the annealing process at…

Structural Analysis and Photocurrent Spectroscopy of CCCs on 99.99% Aluminum

A characterization of chromate conversion coatings (CCCs) formed in the presence and in the absence of accelerator (ferroferricyanide redox couple) has been performed by various techniques (transmission electron microscopy, TEM, glow discharge optical emission spectrometry, GDOES, X-ray absorption near-end structure, XANES, and photon correlation spectroscopy). The results of a detailed investigation on morphological, compositional, and solid-state properties of freshly converted aluminum samples at different immersion times (30 s-90 min) are reported. The TEM and GDOES data suggest the presence of iron-cyanide species only in the external layer of CCC of nearly constant thickness. The XANE…

Photoelectrochemical study of electrochemically formed semiconducting yttrium hydride (YH3−x)

Abstract The first photoelectrochemical study of semiconducting YH 3− x films formed by etching bulk Y metal in 0.5 M H 2 SO 4 solution is reported. The formation of semiconducting hydride having an indirect optical band gap, E g opt , of about 2.35 eV is confirmed by in situ photocurrent spectroscopy. The photoelectrochemical behaviour of such a phase was investigated both in alkaline and in acidic solutions. The flat band potential was estimated to be U fb =−1.25 V/NHE, independent of pH.

Anodic oxide films on tungsten—I. The influence of anodizing parameters on charging curves and film composition

Abstract The formation of anodic films on tungsten has been studied in 1N solutions of H 2 PO 4 , H 2 SO 4 , HNO 3 , HClO 4 and HCl at different temperatures. The shape of the anodic charging curves depends on the electrolyte composition and on the temperature. The chemical composition of the films is strongly influenced by the working temperature. The first step of the anodization is the formation of a barrier film of amorphous WO 3 . The thickening of the oxide layer occurs on account of the WO 3 dissolution and following precipitation of sparingly soluble oxides on the electrode surface.

Characterization of the Solid State Properties of Anodic Oxides on Magnetron Sputtered Ta, Nb and Ta-Nb Alloys

Tantalum oxide, niobium oxide and Ta-Nb containing mixed oxides were grown by anodizing sputter-deposited Ta, Nb and Ta-Nb alloys of different compositions. A photoelectrochemical investigation was performed in order to estimate the band gap and the flat band potential of the oxides as a function of their composition. The band gap of the investigated Ta-Nb containing mixed oxides changed monotonically between those estimated for Ta2O5 (4.1 eV) and Nb2O5 (3.4 eV) and in agreement with a proposed correlation between the Band gap of an oxide and the difference of electronegativity of the oxide constituents. From the differential capacitance curves recorded in a wide range of electrode potentia…

ChemInform Abstract: Electrical Breakdown and Pitting in Anodic Films on Tungsten in Halogen Ion-Containing Solutions.

Abstract The systematic investigation of the anodic behaviour of W in halogen ion-containing solutions reveals noticeable differences in the presence of different anions. Strong generalized dissolution is observed in fluoride solutions, the oxide growth being hindered at low anodizing current densities. Sparking phenomena occur in the presence of Br− and I− anions as in nitrate and sulphate solutions. Only in Cl− containing solutions is the growth of the anodic films limited by the occurrence of pitting phenomena at a critical thickness of the oxide. The laws of dependence of the phenomenon on the experimental parameters and the influence of ferrous ions on the voltage at which pitting occu…

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

Abstract 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 …

Physicochemical characterisation of thermally aged anodic films on magnetron sputtered niobium

The influence of thermal aging, at intermediate temperature (1h at 250°C) and in different environments, on the electronic and solid-state properties of stabilized 160 nm thick amorphous anodic niobia, grown on magnetron sputtered niobium metal, has been studied. A detailed physicochemical characterisation of the a-Nb2O5/0.5M H2SO4 electrolyte junction has been carried out by means of photocurrent and electrochemical impedance spectroscopy as well by differential admittance measurements. A change in the optical band gap (3.45 eV) of niobia film has been observed after aging (3.30 eV) at 250°C in air for 1 hour. A cathodic shift (0.15-0.2 Volt) in the flat band potential of the junction has …

Initial Surface Film on Magnesium Metal. A Characterization by X-ray Photoelectron Spectroscopy (XPS) and Photocurrent Spectroscopy (PCS)

Abstract A detailed investigation of the initial film grown on mechanically polished Mg electrodes has been carried out by ex situ X-ray Photoelectron Spectroscopy (XPS) and in situ Photocurrent Spectroscopy (PCS), allowing to reach a detailed picture of the passive layer structure. The XPS data show that the films formed soon after mechanical treatment and immersion in aqueous electrolyte have a bilayer structure, consisting of an ultra-thin MgO inner layer (∼2.5 nm) and a Mg(OH) 2 external layer. The thickness of the Mg(OH) 2 layer is a function of immersion time and solution temperature. After mechanical treatment and immersion in aqueous solution at room temperature, the MgO/Mg(OH) 2 la…

Fabrication and Characterization of Chitosan-Heteropolyacid complex as membranes for low temperature H2-O2 fuel cell

In this work we describe an easy procedure to fabricate homogeneous CS-HPA polyelectrolyte films using phosphotungstic acid (PTA) as cross-linking agent. The re action between CS chains and PTA is controlled in order to allow fabricating PEC thin films, that can be easily peeled off from the support, cut to any size and shape, whose thickness can be controlled by setting reticulation and time and/or chitosan concentration

Photoelectrochemical and XPS characterisation of oxide layers on 316L stainless steel grown in high-temperature water

Passive films on AISI 316L stainless steel were grown by exposure in high temperature (300 °C and 150 bar) water. X-ray photoelectron spectroscopy was employed to study their composition as a function of immersion time. A photoelectrochemical investigation, supported by electrochemical and impedance measurements, allowed to get information on the solid-state properties of the investigated layers. The experimental results suggest the formation of a stratified layer with an outer iron-rich layer and an inner Cr-rich oxide layer, whose relative thickness and composition are dependent on the immersion time.

Electrochemical fabrication of metal/oxide/conducting polymer junction

After discovery of conducting polymers and the possibility to modify their electrical properties from insulating to metallic like behavior by doping and a careful choice of the processing conditions, a large amount of research effort has been devoted to the theoretical understanding of their solid state properties as well as to exploit the possible application of conducting polymers in many technological fields including large area organic electronics, polymer photovoltaic cell, and sensors. 1-4 Organic thin film transistors appear very promising devices for the development of low cost, flexible, and disposable plastic electronics. In order to reduce the operating voltage it has been sugges…

Amorphous semiconductor-electrolyte junction

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 den…

Photocurrent spectroscopy in passivity studies

The aim of this article is to present photocurrent spectroscopy as useful in situ technique for the physicochemical characterization of passive films and corrosion layers. The response of (both amorphous and crystalline) semiconductor/electrolyte junction under irradiation is treated and discussed in order to get information about solid-state properties such as band gap and flat band potential. The possibility to use Photocurrent Spectroscopy (PCS), in a quantitative way, to get information on the composition of corrosion layers is discussed through a semiempirical correlation between the band gap of the oxides (or hydroxides) and the difference of electronegativity of their constituents. F…

Investigation of amorphous oxide film-electrolyte junctions by AC techniques

Current AC (alternating current) techniques are used often to characterize the energetics at a semiconducting solid phase/electrolyte interface. For thin layers having a strongly disordered or amorphous structure (such as oxide-passive layers anodically grown on valve metals), interpretative models currently used for crystalline semiconductors may produce misleading data. A new interpretation of the admittance data, based on recent models for amorphous semiconductors (a-Sc) Schottky barriers, is presented for passive films of Nb, W and Ti. The physical bases of the model are presented as well as its advantages and disadvantages. The new theory views the solid/electrolyte interface more sati…

Semiempirical Correlation between Optical Band Gap Values of Oxides and the Difference of Electronegativity of the Elements. Its Importance for a Quantitative Use of Photocurrent Spectroscopy in Corrosion Studies

A semiempirical correlation between the optical band gap of binary oxides and the difference of electronegativity between the oxygen and metallic elements (Pauling's extraionic energy) is proposed. In the frame of the proposed correlation an estimate of the repulsive term in the total lattice energy of ionic oxides is obtained in very good agreement with the existing data. An extension of the correlation to the ternary oxides and hydroxides is proposed by using the concept of average cationic or anionic group electronegativity. The usefulness of the proposed correlation for the in situ characterization of passive films on metals and alloys by photocurrent spectroscopy is illustrated by repo…

Effect of Composition on the Photoelectrochemical Behavior of Anodic Oxides on Binary Aluminum Alloys

The photoelectrochemical behavior of anodic films on Al alloys, containing titanium, tantalum, and tungsten (valve metals), has been studied as a function of alloy composition and anodizing conditions. Photocurrent spectroscopy has been used to get information on bandgap and the flatband potential values of different mixed oxides. Both insulator-like and semiconducting behavior has been observed for anodic oxides grown on Al-W and Al-Ti alloys dependent on alloy initial composition. Optical bandgap values, E opt g , of different oxides are in accordance with predictions based on the correlation between E opt g and the difference of electronegativities of the oxide constituents, indicating p…

The effect of thickness on the composition of passive films on a Ti–50Zr at% alloy

Abstract Anodic films were grown potentiodynamically in different electrolytes (pH = 1–14) on a Ti–50Zr at% cast alloy, obtained by fusion in a voltaic arc under argon atmosphere. The thickness of the films was varied by changing formation potential from the open circuit potential up to about 9 V; growth was followed by 30 min stabilization at the forming potential. Films having different thicknesses were characterized by photocurrent spectroscopy (PCS) and electrochemical impedance spectroscopy (EIS). Moreover, film composition was analyzed by X-ray photoelectron spectroscopy (XPS). Regardless of the anodizing conditions, passive films on the Ti–50Zr at% alloy consist of a single layer mix…

Electronic properties of Al-Nb mixed oxide as a function of their composition

Influence of initial treatments of aluminium on the morphological features of electrochemically formed alumina membranes

Abstract The fabrication of alumina membranes by electrochemical oxidation of annealed aluminium was investigated. Porous layers were grown in 0.4 M H3PO4 at −1 and 5 °C at different anodising times. The morphology of the outer surface was found to be dependent on temperature and charge density, with both determining the extent of chemical dissolution of the anodic oxide. The inner-surface morphology was found to depend on the applied voltage only. The chemical dissolution rate of anodic oxide grown on annealed aluminium was found to be lower than that formed on unannealed aluminium under otherwise identical conditions. Such a difference in behaviour is explained in terms of a higher finish…

ChemInform Abstract: Electrical and Mechanical Breakdown of Anodic Films on Tungsten in Aqueous Electrolytes.

Abstract Different types of breakdown are reported to occur during the galvanostatic growth of WO3 films in different aqueous electrolytes. Stresses inside the growing film cause the occurrence of cracks at a critical thickness which varies with the anodizing solution. The electrical breakdown is caused by avalanche ionization of the electronic current inside the film. The influence of the different experimental parameters on both the mechanical and the electrical breakdown voltages is discussed. For the electrical breakdown a model is proposed which explains the dependence of the sparking voltage on the electrolyte resistivity by assuming a double layer effect on the oxygen evolution react…

Poster Presentation

Improvement in the performance of low temperature H2-O2 fuel cell with chitosanephosphotungstic acid composite membranes

Abstract Free-standing chitosan/phosphotungstic acid polyelectrolyte membranes, prepared by ionotropic gelation on alumina porous supports, were employed as proton conductor in low temperature H 2 –O 2 fuel cell. A drying step on glass substrate was introduced in the fabrication procedure to reduce shrinkage and consequent corrugation. Membranes were tested with electrodes prepared according to different procedures and with two different Pt loadings, namely 0.5 and 1 mg cm −2 . Both the investigated kinds of electrodes allowed to get very promising power peaks of 550 mW cm −2 in spite of the different Pt content. The polarization curves and the electrochemical impedance spectra suggest that…

A semiempirical correlation between the optical band gap of hydroxides and the electronegativity of their constituents

In analogy with previous results on anhydrous oxides, a correlation is proposed between the optical band gap of hydroxides and the electronegativities of their constituents. Based on the experimental results on passive hydrated layers on metals obtained in our laboratory and the literature data, it is found that the hydroxide band gap varies with the square of the difference between the electronegativities of the metallic cation and the hydroxyl group. Like in the case of anhydrous oxides, two different interpolation lines have been found forsp-metal andd-metal hydroxides, respectively. The proposed correlations predict semiconducting or insulating behavior even for the most electronegative…

Photoelectrochemical Evidence of Cu2O/TiO2 Nanotubes Hetero-Junctions formation and their Physicochemical Characterization

Cu2O/TiO2 nanotubes heterojunctions were fabricated by electrochemical deposition of cuprous oxide on TiO2 nanotubes arrays grown by anodizing. X-ray diffraction and Raman Spectroscopy analysis allows for identification of Cu2O, whose morphological features were studied by Scanning Electron Microscopy as a function of the charge circulated during the electrodeposition step. Photoelectrochemical measurements in aqueous solutions evidenced a red shift of the light absorption threshold of TiO2 nanotubes due to the presence of cuprous oxide even for very low circulated charges, while electrochemical impedance measurements proved a significant reduction of the electrode impedance due the presenc…

Photoelectrochemical characterization of amorphous anodic films on Ti-6at.%Si

Abstract The solid state properties of anodic films grown galvanostatically on sputtering-deposited Ti–6at.%Si alloys were studied as a function of the formation voltage (5–40 V). From the photocurrent spectra a band gap of ∼3.4 eV was estimated for all the investigated thicknesses, which is almost coincident with the value measured for amorphous TiO 2 . The photocharacteristics allowed to estimate the flat band potential of the films, which resulted to be more anodic for thicker layers and allowed to evidence a change from n-type semiconducting material to insulator by increasing the formation voltage. A dielectric constant of ∼31 was estimated by differential capacitance measurements. The…

Photoelectrochemical monitoring of rouging and de-rouging on AISI 316L

Electrochemical conditions for inducing rouging on surface of AISI 316L in quasi neutral aqueous solution are studied. Potentiostatic polarization at 0.6 V vs. SSC at pH ∼ 7 allowed growth of colourless passive films with a band gap slightly lower than that estimated for the oxide grown on the SS surface by air exposure due to chromium dissolution. Under stronger anodic polarization (UE = 1.5 V vs. SSC) coloured passive films are formed, mainly constituted by iron oxide according to their band gap (Eg = 2.0 eV). Etching in citric acid at 60 °C results to be effective in removing rouging.

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 analy…

ChemInform Abstract: 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 resp…

Resistive switching behaviour in ZnO and VO 2 memristors grown by pulsed laser deposition

The resistive switching behaviour observed in microscale memristors based on laser ablated ZnO and VO 2 is reported. A comparison between the two materials is reported against an active device size. The results show that devices up to 300 × 300 μm 2 exhibit a memristive behaviour regardless of the device size, and 100 × 100 μm 2 ZnO-based memristors have the best resistance off/on ratio.

Template electrosynthesis of La(OH)3 and Nd(OH)3 nanowires using porous anodic alumina membranes

High quality arrays of Ln(OH)3 (Ln = La, Nd) nanowires have been successfully fabricated for the first time by an electrochemical process using anodic alumina membrane templates. A physico-chemical characterisation of electrodeposited hydroxides has been carried out by different techniques (XRD, SEM and EDX). The results show that the synthesized nanostructures are crystalline, dense, continuous, well aligned, and with high aspect ratio, suggesting further development of possible applications in the lanthanide family species. Keywords: Nanowires, Anodic alumina membranes, Hydroxide electrodeposition, Template, Lanthanide compounds

Photoelectrochemical investigation of passive layers formed on Fe in different electrolytic solutions

Abstract The passive films formed on Fe in different electrolytic solutions, spanning almost the overall pH range, and with different growth procedures were investigated systematically by photocurrent spectroscopy. The potentiodynamic growth curves are compared both in aerated and de-aerated electrolytes and a potentiostatic growth procedure is also employed. For high positive formation potentials, similar anodic spectra are recorded in all solutions giving an optical gap very close to that expected for crystalline Fe2O3. The origin of photocurrent spikes is also investigated and the effect of the formation potential upon the measured absorption threshold is discussed taking into account th…

Room temperature electrodeposition of photoactive Cd(OH)2 nanowires

Cd(OH)2 nanowires (NWs) were successfully prepared by room temperature electrogeneration of base using Cd(NO3)2 aqueous electrolyte and Anodic Alumina Membrane (AAM) as template. Cd(OH)2 films have been also deposited on tin-doped indium oxide (ITO) for comparison. SEM analysis shows high quality deposits made of closely packed nanowires (NWs) into AAM and uniform flake-like surface on ITO. XRD analysis reveals that Cd(OH)2 films on ITO are polycrystalline, while the nanowires grow along the preferential directions [100] and [110]. Photoelectrochemical measurements show that Cd(OH)2 NWs are photoactive materials with indirect and direct band gap of 2.15 and 2.75 eV, respectively. Keywords: …

The influence of surface treatment on the anodizing of magnesium in alkaline solution

Abstract The mechanism of magnesium anodizing after mechanical polishing or HF pickling was studied. X-ray Photoelectron Spectroscopy and Photocurrent Spectroscopy were employed to characterize both the initial films formed on Mg soon after surface treatments and the anodic films grown in strongly alkaline solutions. Electrochemical and capacitance measurements were performed in order to study the oxidation process and to check if the kinetic of growth fits the Mott–Cabrera high field model. From Tafel plots it was possible to estimate the activation distance, while the simulation of the barrier layer growth as a function of the potential at high scan rate allowed to estimate the other kine…

Anodization and anodic oxides

Anodizing is a low-temperature, low-cost electrochemical process allowing for the growth, on the surface of valve metals and valve metal alloys, of anodic oxides of tunable composition and properties. This article is an overview on theoretical aspects concerning the general aspects of the kinetics of growth of barrier and porous anodic oxides and some of their present and possibly future technological applications of anodic oxides. The first part of the article is devoted to anodic oxide growth models, from Guntherschulze and Betz work (in 1934) to the more recent results on barrier and porous oxide films. The second part is focused on industrial processes to fabricate anodic oxides and the…

From ceria nanotubes to nanowires through electrogeneration of base

The preparation of Ce(OH)3/CeO2 nanostructures (NSs) through electrogeneration of base into anodic alumina membranes was studied. The effects of solvent (alcohol and/or water), Ce3? partner anion nature (chloride or nitrate) and concentration, applied potential or current density in driving the morphology toward nanowires (NWs) and/or nanotubes (NTs) was described. The structural analysis performed by X-Ray Diffraction and Raman Spectroscopy allowed to evidence that the presence of Ce(IV) into the nanostructures strongly depends on the oxygen content in the growing environment.

A Photoelectrochemical Study of Passivating Layers on Nickel

Anodic Electro Deposition of CeO2 and Co-Doped CeO2 Thin Films

CeO2 and Co containing CeO2 thin films were deposited on indium tin oxide and stainless steel by anodic electrodeposition. Scanning electron microscopy showed that the films are flat and show globular morphology and cracks resulting from volume shrinking. According to XRD and Raman Spectroscopy pure ceria layers are crystalline, while the presence of Co induces the formation of amorphous films. The good adhesion and the compactness allowed the photoelectrochemical characterization of the films. A band gap value of 2.9 eV was estimated for CeO2, while slightly higher values (̃3.0 eV) were estimated for Co containing films. A mechanism for ceria anodic electrodeposition is proposed and discus…

Photoelectrochemical evidence of nitrogen incorporation during anodizing sputtering--deposited Al-Ta alloys.

Anodic films were grown to 20 V on sputtering-deposited Al–Ta alloys in ammonium biborate and borate buffer solutions. According to glow discharge optical emission spectroscopy, anodizing in ammonium containing solution leads to the formation of N containing anodic layers. Impedance measurements did not evidence significant differences between the dielectric properties of the anodic films as a function of the anodizing electrolyte. Photoelectrochemical investigation allowed evidencing that N incorporation induces a red-shift in the light absorption threshold of the films due to the formation of allowed localized states inside their mobility gap. The estimated Fowler threshold for the intern…

Behavior of alloying elements during anodizing of Mg-Cu and Mg-W alloys in a fluoride/glycerol electrolyte

Anodizing of sputtering-deposited magnesium and Mg-0.75at.%Cu and Mg-1.23at.%W alloys has been carried out in a fluoride/ glycerol electrolyte. The aims of the study were to investigate the enrichment of alloying elements in the alloy immediately beneath the anodic film and the migration of alloying element species in the film. The specimens were examined by electron microscopy and ion beam analysis. An enrichment of copper is revealed in the Mg-Cu alloy that increases with the anodizing time up to ∼6×1015 Cu atoms cm-2. Copper species are then incorporated into the anodic film and migrate outwards. In contrast, no enrichment of tungsten occurs in the Mg-W alloy, and tungsten species are im…

Investigation of the kinetics of growth of anodic oxide films on niobium by galvanostatic and tensiodynamic experiments

Abstract The kinetics of growth of anodic oxide films on niobium has been investigated by interferometric methods. Tensiodynamic experiments allow the growth of the films at constant rate. Photoeffects during the growth of the films under absorbed light give additional information on the growth process.

Growth and Characterization of Anodic Films on Scandium

The anodic behavior of Sc in a slightly alkaline aqueous solution is studied. Electrochemical and capacitance measurements suggest that passive films can be formed on the scandium surface under a high electric field. The formation of these layers occurs at low faradaic efficiency due to oxygen evolution. Photoelectrochemical experiments suggest the formation of a barrier layer with a thickness high enough to hinder external electron photoemission processes and allow the estimation of the bandgap of the films as a function of their formation voltage. The estimated bandgap values were lower than that reported for Sc2O3, suggesting the formation of hydrated phases and/or of a strongly oxygen d…

Amorphous semiconducting passive film-electrolyte junctions revisited. The influence of a non homogeneous density of state on the differential admittance behaviour of anodic a-Nb2O5

Abstract An analysis of the electronic properties of amorphous semiconductor-electrolyte junction is reported for passive films grown on Nb in alkaline solution and in a large range of thickness (~20nm ÷ ~250nm). A modelling of electronic density of state (DOS) has been carried out by fitting EIS spectra, at different potentials and in a range of frequencies (0.1 Hz ≤ f ≤100 kHz), and differential admittance (DA) data of a-Nb 2 O 5 /El interface. The fitting of EIS and DA curves was performed by using the theory of amorphous semiconductor Schottky barrier and a non-homogeneous DOS distribution.

Anodic oxide films on tungsten—II. The morphology and dissolution of the films

Abstract Scanning electron microscopy was used to investigate the morphology of anodic oxide films on tungsten, obtained in various conditions of anodization. Studies were made of the growth of porous films, whose thickness increases with time and depends upon the current density. Temperature and electrolyte composition influence the film morphology. Gravimetric measurements of film dissolution at 70°C show that after a transient time, the rate of metal dissolution and that of film formation coincide. The porous films thicken because tungsten dissolves as WO 2 2+ and precipitates as WO 3 .H 2 O.

The Amorphous Semiconductor Schottky Barrier Approach to Study the Electronic Properties of Anodic Films on Ti

A detailed study of the electronic properties of thin (>20 nm) anodic TiO2 potentiostatically grown on titanium in two different solutions is presented. The results show that the nature of the anodizing solution affects the electronic properties of the anodic film and, more specifically, the density of electronic states (DOS) distribution. Different DOS were derived from the experimental data analyzed according to the theory of amorphous semiconductor (a-SC) Schottky barrier. It is shown that the usual non-linear and frequency dependent Mott-Schottky plots are in agreement with expected theoretical behavior of a-SC Schottky barrier.

Electromechanical coupling in anodic niobium oxide: Electric field-induced strain, internal stress, and dielectric response

Seemingly, contradictory results have been reported so far for electrostriction in anodic oxides. Furthermore, no definitive agreement could be obtained with theory. In this paper, in situ techniques are combined to elucidate electrostriction in anodic niobium oxide. The dependence of strain, internal stress, and dielectric constant on the electric field is measured by, respectively, spectroscopic ellipsometry, curvature, and impedance measurements. The through-thickness strain is tensile and proportional to the square of the electric field. The in-plane internal stress is compressive and proportional to the square of the electric field at low field values. The internal stress is predicted …

INFLUENCE OF THE ANODIZATION CONDITIONS ON THE ELECTRONIC PROPERTIES AND CRYSTALLOGRAPHIC STRUCTURES OF THE CORROSION LAYERS ON TUNGSTEN. A PHOTOELECTROCHEMICAL APPROACH.

ABSTRACT The influence of morphology, composition and crystallographic structure on the photoelectrochemical behaviour of anodic oxide films on tungsten obtained in various conditions of anodization has been investigated. Different photocurrent spectra and absorption edges were obtained for each type of film. Optical band gaps ranging between 2.55 eV and 3.15 eV were determined for crystalline and amorphous WO 3 films grown in different conditions. The low quantum efficiency of the anodic films must be attributed to the presence of an amorphous (a-W0 3 ) film which controls the transport of the injected photocarriers.

Comments on 'formation and characterization of nanotubes of La(OH) 3 obtained using porous alumina membranes'

New experimental evidence concerning the formation of La(OH)3 nanowires on anodic alumina membranes by cathodic polarization in 0.05 M lanthanum nitrate solution is provided to further support the conclusions previously reached in our work (Bocchetta et al 2007 Electrochem. Commun. 9 683-8) and recently criticized by González-Rovira et al (Nanotechnology 2008 19 495305). Some unconvincing aspects of the paper of González-Rovira et al, according to which the same electrochemical process should lead to the formation of hydroxycarbonate nanotubes, are also discussed.

A photocurrent spectroscopic investigation of passive films on chromium

Abstract Photoelectrochemical experiments have been carried out in order to identify the passive layers grown on chromium metal in different conditions. Changes in the values of the band gap of the films have been related to changes of composition, due to the different water content of the films.

Photoelectrochemical Synthesis of Polypyrrole on Anodic Ta[sub 2]O[sub 5] Films

Polypyrrole film was photoelectrochemically grown on insulating Ta 2 O 5 anodic film in acetonitrile solution. A characterization by photocurrent spectroscopy (PCS) of metal/oxide/polypyrrole interface was carried out. The PCS results suggest that a metallic-like PPy is formed under illumination at constant anodic potential. By polarizing the polypyrrole at cathodic potentials a photocurrent spectrum typical of p-type semiconducting film was recorded. A scanning electron microscopy study of PPy surfaces solution side and oxide side allowed us to obtain information on the morphology of the polymer as well as a rough estimate of the film thickness and of the diameter of PPy globules at the tw…

Electrochemical Fabrication and Physicochemical Characterization of Metal/High-k Insulating Oxide/Polymer/Electrolyte Junctions

Photoelectrochemical polymerization of poly(3,4-ethylenedioxythiophene), PEDOT, was successfully realized on anodic film grown to 50 V on magnetron sputtered Ti-6 atom % Si alloys. Scanning electron microscopy allowed us to evidence formation of compact and uniform polymer layers on the oxide surface. Photoelectrochemical and impedance measurements showed that photopolymerization allows one to grow PEDOT in its conducting state, while a strong cathodic polarization is necessary to bring the polymer in its p-type semiconducting state. Information on the optical and electrical properties of metal/oxide/polymer/electrolyte junctions proves that PEDOT has promising performance as an electrolyte…

Influences of Structure and Composition on the Photoelectrochemical Behaviour of Anodic Films on Zr and Zr-20at.%Ti

Abstract A photoelectrochemical investigation on anodic films of different thickness grown on sputter-deposited Zr and Zr–20 at.%Ti was carried out. The estimated optical band gap and flat band potential of thick ( U F  ≥ 50 V) anodic films were related to their crystalline structure and compared with those obtained for thinner ( U F  ≤ 8 V/SCE) anodic oxides having undetermined crystalline structure. The E g values obtained by photocurrent spectroscopy were also compared with the experimental band gap estimated by other optical ex situ techniques and with the available theoretical estimates of the zirconia electronic structures in an attempt to reconcile the wide range of band gap data rep…

A photoelectrochemical study on anodic tantalum oxide films

Both anodic and cathodic photocurrents have been detected on tantalum oxide films depending on polarization. The optical band gap, E g opt , has been derived for anodic films grown in different conditions as well as for native oxides. Cathodic photocurrents at hv<E g opt have been attributed to electron injection at the metal/oxide interface. A change in the sign of the photocurrent with the wavelength of the incident light has been observed near to the flat band potential. The latter has been estimated from the fitting of the photocurrent vs potential curves

Photoelectrochemical study on anodic aluminum oxide films. Internal photoemission processes at the metal-oxide interface

A photoelectrochemical investigation has been carried out on aluminum oxide films grown anodically at constant rate up to different thicknesses. Depending on the potential both anodic and cathodic photocurrents were observed at photon energies well below the optical bandgap expected for these layers. This finding is explained with the presence of internal photoinjection processes both for electrons and holes from the base metal into the oxide film. The analysis of the photocurrent spectra has given the threshold energies for both processes. The effect of the image force at the metal/oxide interface has been taken into account in order to derive the mobility gap of the films and the energeti…

Chitosan-phosphotungstic acid complex as membranes for low temperature H2-O2 fuel cell

Abstract Free-standing Chitosan/phosphotungstic acid polyelectrolyte membranes were prepared by an easy and fast in-situ ionotropic gelation process performed at room temperature. Scanning electron microscopy was employed to study their morphological features and their thickness as a function of the chitosan concentration. The membrane was tested as proton conductor in low temperature H 2 –O 2 fuel cell allowing to get peak power densities up to 350 mW cm −2 . Electrochemical impedance measurements allowed to estimate a polyelectrolyte conductivity of 18 mS cm −1 .

Anodizing of aluminium and AA 2024-T3 alloy in chromic acid: Effects of sulphate on film growth

Chromic acid anodizing is important for the corrosion protection of aerospace aluminium alloys. Previous study has demonstrated that SO42 − impurity in the chromic acid affects the film growth on aluminium at a voltage of 100 V. The present work further investigates aluminium and extends the study to industrial anodizing conditions (Bengough-Stuart (B-S) process) and to the AA 2024-T3 alloy. It is shown that SO42 − concentrations between ~ 38–300 ppm reduce the film growth rate for aluminium anodized at 100 V in comparison with an electrolyte than contains ≤ 1.5 ppm SO42 −, whereas ~ 1500–3000 ppm SO42 have an opposite effect and lead to an unstable pore diameter. Under the B-S process, the…

Physicochemical characterization of passive films on niobium by admittance and electrochemical impedance spectroscopy studies

An analysis of the electronic properties of amorphous semiconductor-electrolyte junction is reported for thin (D ox < 20 nm) passive film grown on Nb in acidic electrolyte. It will be shown that the theory of amorphous semiconductor-electrolyte junction (a-SC/EI) both in the low band-bending and high band-bending regime is able to explain the admittance data of a-Nb 2 O 5 /El interface in a large range (10 Hz-10 kHz) of frequency and electrode potential values. A modelling of experimental EIS data at different potentials and in the frequency range of 0.1 Hz-100 kHz is presented based on the theory of amorphous semiconductor and compared with the results of the fitting of the admittance data…

Tailoring of the Solid State Properties of Al–Nb Mixed Oxides: A Photoelectrochemical Study

Al–Nb containing mixed oxides were grown by anodizing sputter-deposited Al–Nb alloys of different compositions. A photoelectrochemical investigation was carried out in order to estimate the band gap, flat band potential, and conductivity type of these oxides as a function of their composition. The dependence of the band gap on the composition of mixed sp–d metal oxides has been rationalized by using a semiempirical correlation between the difference of electronegativity and band gap of oxides proposed in the literature some years ago and recently tested for regular d–d metal mixed oxides. The band gap increase observed as a function of Al content into the oxides seems mainly depending on th…

A tensiostatic study of the anodic behaviour of tungsten in acid solutions

Abstract Tensiostatic anodization of tungsten has been studied in 1 N solutions of H 2 SO 4 , HClO 42 , HNH 2 SO 3 and HCl at 25 °C. For several voltage values, the curves of current vs . time and open-circuit voltage vs . time have been obtained. All results indicate that the oxide films produced during the polarization are slowly dissolved in the acid solutions. The HCl, and to a lesser extent the HNH 2 SO 3 solutions, show a more aggressive behaviour. For values of the applied voltage above about 50 V, gas evolution begins. In the case of HCl solutions, the gas is Cl 2 and the electrode is rapidly corroded.

Nanoscale membrane electrode assemblies based on porous anodic alumina for hydrogen–oxygen fuel cell

In this paper, we demonstrate that nanoscale membrane electrode assemblies, functioning in a H 2/O 2 fuel cell, can be fabricated by impregnation of anodic alumina porous membranes with Nafion® and phosphotungstic acid. Porous anodic alumina is potentially a promising material for thin-film micro power sources because of its ability to be manipulated in micro-machining operations. Alumina membranes (Whatman, 50 μm thick, and pore diameters of 200 nm) impregnated with the proton conductor were characterized by means of scanning electron microscopy, X-ray diffraction, and thermal analysis. The electrochemical characterization of the membrane electrode assemblies was carried out by recording t…

Assessment on the use of the amorphous semiconductor theory for the analysis of oxide films

Abstract Although the theory of Schottky barrier in amorphous semiconductors is generally accepted, the limits of validity of such theory have not yet been explored. The classic semi-analytical solution is obtained under the constraint of constant electronic density of states (DOS) distribution in the mobility gap. In order to take into account the presence of a DOS variable in energy, a semi-empirical corrective power law was introduced in this paper. It is shown that the equations derived for thick films maintain their validity also in the case of thin films, provided that the space charge region width remains lower than 70% of the whole film thickness. A new expression based on the use o…

Recent advances in photocurrent spectroscopy of passive films

Abstract The quantitative application of photocurrent spectroscopy (PCS) for the in-situ determination of the composition of passive films and corrosion layers is reviewed in the light of recent theoretical advances, that have allowed to relate the measured optical gaps to the Pauling electronegativities of the film components. The correlations derived are tested versus recent experimental results regarding mixed oxides, anhydrous passive films on metallic alloys and hydroxide layers. The effect of the eventual long-range disorder into the passive film on the optical band gap values is also discussed. New experimental evidence reported for mixed d,d-metal oxides and passive films on sp,d-me…

Formation of anodic films on sputtering-deposited Al–Hf alloys

Abstract The growth of barrier-type anodic films at high efficiency on a range of sputtering-deposited Al–Hf alloys, containing from 1 to 95 at.% Hf, has been investigated in ammonium pentaborate electrolyte. The alloys encompassed nanocrystalline and amorphous structures, the latter being produced for alloys containing from 26 to 61 at.% Hf. Except at the highest hafnium content, the films were amorphous and contained units of HfO 2 and Al 2 O 3 distributed relatively uniformly through the film thickness. Boron species were confined to outer regions of the films. The boron distributions suggest that the cation transport number decreases progressively with increasing hafnium concentration i…

Photocurrent Spectroscopy Applied to the Characterization of Passive Films on Sputter-Deposited Ti-Zr Alloys

Abstract A photoelectrochemical investigation on thin (⩽13 nm) mixed oxides grown on sputter-deposited Ti–Zr alloys of different composition by air exposure and by anodizing (formation voltage, UF = 4 V/SCE) was carried out. The experimental results showed that the optical band gap, E g opt , increases with increasing Zr content in both air formed and anodic films. Such behaviour is in agreement with the theoretical expectation based on the correlation between the band gap values of oxides and the difference of electronegativity of their constituents. The flat band potential of the mixed oxides was found to be almost independent on the Ti/Zr ratio into the film and more anodic with respect …

Dielectric Properties of Al-Nb Amorphous Mixed Oxides

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Kinetics of coloration of anodic electrochromic films of WO3·H2O

Polycrystalline layers of WO3·H2O are obtained by anodization of tungsten in 1 N H2SO4 at 70° C. The cathodic reduction of these layers in acid solutions causes the formation of blue WO3−x·H2O (0<x⩽:0.12) oxides. The kinetics of coloration are investigated by galvanostatic and potentiostatic techniques. The experimental results are compared with the theoretical data obtained by solving the diffusion equation for a constant flow of oxygen vacancies and for a time-dependent surface vacancy concentration. Except in the initial stage of coloration, the process controlling rate can be ascribed to the diffusion of oxygen vacancies from the oxide-electrolyte interface into the bulk of the layers. …

Electrodeposition of CeO2 and Co-Doped CeO2 Nanotubes by Cyclic Anodization in Porous Alumina Membranes

An anodic electrodeposition process is proposed to prepare CeO2 and Co-doped CeO2 nanotubes. Anodic alumina membrane is used as template and linear sweep voltammetry is employed to allow the formation of nanotubes without alumina dissolution. SEM micrographs showed large arrays of well defined and aligned NTs, which resulted to be crystalline soon after deposition according to XRD diffraction patterns and Raman Spectroscopy.

Photo-electrochemical and impedance investigation of passive layers grown anodically on titanium alloys

Abstract The anodic behaviour of two titanium cast alloys, obtained by fusion in a voltaic arc under argon atmosphere, was analyzed in aerated aqueous solutions having different pH values. In all solutions the alloys, having nominal compositions Ti–50Zr at.% and Ti–13Zr–13Nb wt.%, displayed a valve-metal behaviour, owing to the formation of barrier-type oxide films. Passive films, grown potentiodynamically up to about 9 V, were investigated by photocurrent spectroscopy (PCS) and electrochemical impedance spectroscopy (EIS). These passive layers show photoactivity under anodic polarizations, with optical gaps close to 3.55 and 3.25 eV for the binary and the ternary alloy, respectively, indep…

Influence of synthesis conditions on the performance of chitosan–Heteropolyacid complexes as membranes for low temperature H2–O2 fuel cell

Flat, free-standing chitosan/phosphotungstic acid (PTA) polyelectrolyte membranes were prepared by in-situ ionotropic gelation process at room temperature on porous alumina support firstly impregnated by H3PW12O40. Scanning electron microscopy revealed the formation of compact and homogeneous membranes, whose thickness resulted to be dependent on chitosan concentration and reticulation time. X-ray diffraction and Fourier transform infrared spectroscopy (FTIR) evidenced the formation of almost amorphous membrane without appreciable concentration of not protonated NH2 groups and PTA3- ions with preserved Keggin structure. Membranes were tested as proton conductor in low temperature H2-O2 fuel…

A Simple Model for the Photoelectrochemical Behavior of Corrosion Layers with Variable Hydration Degree

Abstract With the goal of quantitatively applying photocurrent spectroscopy to corrosion studies, the photoelectrochemical behavior of layered passive films having a decreasing hydration/hydroxylation degree, from the solution interface toward the metal interface, has been modeled. The model makes use of the correlations between optical gap and electronegativity of the constituents previously proposed for surface oxides and hydroxides. In this paper, the model was tested successfully against the experimental data pertaining to three high band gap insulating oxide-forming metals, like Zr, Al, and Y. In the first case, the fitting of the experimental spectra was compatible with the occurrence…

Recent advances on physico-chemical characterization of passive films by EIS and differential admittance techniques

Abstract Thin Nb2O5 anodic films (∼20 nm thick) grown in phosphoric acid solution have been characterised by EIS and differential admittance study in a large range of potential and frequency. The overall electrical behaviour has been interpreted by means of the theory of amorphous semiconductor Schottky barrier in presence of a non-constant density of states (DOS). A comparison of DOS for films grown in different electrolytes is reported.

Formation and photo-electrochemical characterization of anodic films on valve-metal alloys

The formation of anodic films on valve metals alloys has been studied as a function of alloy composition and anodizing conditions. Photocurrent Spectroscopy technique (PCS) has been used to determine the band gap (E g) and the flat band potential (Vfb,) of oxides grown on these alloys. The formation of insulating-like or semiconducting oxide films has been evidenced as a function of initial alloys composition. The composition of "mixed oxide" has been related to the measured optical band gap values on the basis of experimental results and previous proposed correlation.

Influence of Anodic and Thermal Barrier Layers on Physicochemical Behavior of Anodic TiO2 Nanotubes

Electrochemical and photo-electrochemical behavior of self-organized TiO2 nanotubes formed in organic solvents have been studied by taking into account the formation of new barrier layers beneath nanotubes either due to the anodic polarization in aqueous solutions or air exposure during high temperature annealing. It has been shown that before annealing, electrochemical and photoelectrochemical answers are dominantly controlled by the physicochemical properties of the anodic barrier layer. Annealing in air at sufficiently high temperatures changes the initial amorphous structure of as-prepared nanotubes and forms a new oxide layer below them due to thermal oxidation of underneath titanium. …

The influence of nitrogen incorporation on the optical properties of anodic Ta2O5

Abstract Anodic oxides were grown on sputter-deposited Ta in different aqueous solutions. A photoelectrochemical investigation was performed in order to estimate the band gap of the films as a function of the anodizing bath composition and formation voltage, i.e. thickness. Photoelectrochemical results provided evidence of sub-band gap photocurrent for films formed in a bath containing ammonium ions at pH 9. Elemental depth profiles obtained by glow discharge optical emission spectroscopy revealed the presence of nitrogen species in the outer part of the anodic films, which is bonded to Ta according to XPS analysis. A mechanism of nitrogen incorporation is proposed in order to account for t…

The effect of hydration layers on the anodic growth and on the dielectric properties of Al2O3 for electrolytic capacitors

Hydrous films were grown on high purity and cubicity Al foils for electrolytic capacitors in deionized water, ethylene glycol - deionized water and in glycerol - deionized water for different immersion times. According to the X-ray diffraction patterns the hydration treatment allowed to grow a pseudo boehmite layer on Al surface whose morphology is appreciably affected by the bath composition. Capacitance measurements and photoelectrochemical findings suggest that a more compact barrier layer forms during the immersion in alcohol containing solutions. The hydration in water allowed to save energy and to prepare more blocking oxide films. The beneficial effect of hydration in hot water on th…

Photocurrent spectroscopy study of passive films on hafnium and hafnium–tungsten sputtered alloys

Abstract Anodic and air-formed films on sputtered Hf and W–Hf alloys of different composition have been investigated by Rutherford back scattering, photocurrent spectroscopy (PCS) and transmission electron microscopy (TEM) techniques. In alkaline solutions the PCS data suggest the formation on Hf metal of a duplex layer with anodic hafnia covered by an external layer of composition close to HfO(OH) 2 . This last compound is also present on Hf air-formed films. In acidic solutions the initial oxy-hydroxide film disappears at high anodising potentials ( V f >10 V). In the case of W–Hf alloys films of different composition and semiconducting behaviour are formed by air exposure or by anodising…