Effect of anodic oxygen evolution on cell morphology of sulfuric acid anodic alumina films

Journal of Solid State Electrochemistry - The purpose of this work was to study and analyze the effect of electrolyte temperature and anodization voltage on cell morphology of thin films of...     

Behavior of acid species during heat treatment and re-anodizing of porous alumina films formed in malonic acid

In the present work infrared spectroscopy, photoluminescence spectral measurements and the potenthiodynamic technique for studying the effect of treatment temperature on compositional and electronic properties of malonic acid alumina films were used. In the course of our studies, it has been proven that heat treatment of malonic acid films at temperatures from 250 up to 400 °C leads to considerable changes in the photoluminescence properties and voltammetric response during their potentiodynamic re-anodizing. We suggest that defects, such as electron traps, in this type of porous anodic films are caused by the atoms of hydrogen (one or two) escaping from the CH₂ groups of the malonic acid species as a result of the heat treatment. The sites of such defects provide pathways for easy electron migration under a high electric field increasing electroconductivity of anodic alumina films. On the contrary, no structural defects responsible for enhanced electroconductivity are observed during thermal splitting of oxalate groups in the oxalic acid alumina films.     

Electronic properties of electrolyte/anodic alumina junction during porous anodizing

The growth of porous oxide films on aluminum (99.99% purity), formed in 4% phosphoric acid was studied as a function of the anodizing voltage (23-53 V) using a re-anodizing technique and transmission electron microscopy (TEM) study. The chemical dissolution behavior of freshly anodized and annealed at 200 °C porous alumina films was studied. The obtained results indicate that porous alumina has n-type semiconductive behavior during anodizing in 4% phosphoric acid. During anodising, up to 39 V in the barrier layer of porous films, one obtains an accumulation layer (the thickness does not exceed 1 nm) where the excess electrons have been injected into the solid producing a downward bending of the conductive and valence band towards the interface. The charge on the surface of anodic oxide is negative and decreases with growing anodizing voltage. At the anodizing voltage of about 39 V, the charge on the surface of anodic oxide equals to zero. Above 39 V, anodic alumina/electrolyte junction injects protons from the electrolyte. These immobile positive charges in the surface layer of oxide together with an ionic layer of hydroxyl ions concentrated near the interface create a field, which produces an upward bending of the bands.     

Photoluminescence properties of heat-treated porous alumina films formed in oxalic acid

Photoluminescence and optical properties of as-anodized and heat-treated at 500 °C porous alumina films formed in a 0.3 M oxalic acid at 40 V have been studied. The FTIR indicates that the oxalate ions are embedded in the anodic alumina as chelating bidentate structures and further heating up to 500 °C does not cause any change in ion coordination. The results of time-resolved spectroscopy show the presence of two luminescence centers both in the as-anodized and heat-treated anodic alumina films with lifetimes of about 0.25 and 4.0 ns. The F⁺-centers in anodic alumina are responsible for the luminescence peak at about 420 nm, with a lifetime of about 4.0 ns. The luminescence peak at about 480 nm, with lifetime of about 0.25 ns, can be attributed to the luminescence of carboxylate ions existing in bulk of anodic alumina.     

Effect of heat treatment on the structure of incorporated oxalate species and photoluminescent properties of porous alumina films formed in oxalic acid

The present work focuses on the use of IR spectroscopy and photoluminescence spectral measurements for studying the treatment temperature effect on the compositional and luminescent properties of oxalic acid alumina films. In line with the recent researches we have also found that heat treatment of porous alumina films formed in oxalic acid leads to considerable changes in their photoluminescence properties: upon annealing the intensity of photoluminescence (PL) increases reaching a maximum at the temperature of around 500 °C and then decreases. IR spectra of as-grown and heat-treated films have proved that PL emission in the anodic alumina films is related with the state of ‘structural’ oxalate species incorporated in the oxide lattice. These results allowed us to conclude that PL behavior of oxalic acid alumina films can be explained through the concept of variations in the bonding molecular orbitals of incorporated oxalate species including σ- and π-bonds.     

Dissolution behavior of anodic oxide films formed in sulfanic acid on aluminum

Chemical dissolution of the barrier layer of porous oxide films formed on an aluminum foil (99.5% purity) in 1.5 M sulfanic acid after immersion in a 2 mol dm⁻³ sulphuric acid at 50 °C was studied. The barrier layer thickness before and after dissolution was determined using a re-anodizing technique. Re-anodizing was conducted in 0.5 mol dm⁻³ H³BO³/0.05 mol dm⁻³ Na²B⁴O⁷ solution. We found that the change in the porous oxide growth mechanism was observed at the anodizing voltage of 30 V. Taking into account this result chemical dissolution behaviour of the barrier layer of porous films formed at 20 V and 36 V and also the influence of annealing of oxide films at 200 °C were studied. We showed the interplay between the dissolution rates and charge distribution across the barrier layer. We conclude that the outer and middle layers have negative space charges and the inner layer has positive space charges.     

Impurity-defect structure of anodic aluminum oxide produced by two-sided anodizing in tartaric acid

Porous aluminum oxide is prepared in a 0.4 M aqueous solution of tartaric acid by two-sided anodizing. Fourier Transform IR spectroscopy (FTIR) data reveal the presence, in the alumina, of unoxidized tartarate ions, as well as products of their partial (radical organic products and CO) and complete (CO₂) oxidation. Carboxylate ions and elemental carbon contained in the anodic oxide impart a gray color to the films.     

Photoluminescent behavior of heat-treated porous alumina films formed in malonic acid

In the present work IR spectroscopy, electron probe microanalysis (EPMA) and photoluminescence (PL) spectral measurements were applied to study the effect of treatment temperature (T) on compositional and luminescent properties of malonic acid alumina films. Our studies have shown that the heat treatment of anodic alumina films at investigated temperatures from 100 up to 700 °C changes their photoluminescence spectra considerably. An increase in T results in the PL intensity growth. When reaching its maximum at 600 °C the luminescence intensity then decreases drastically with further T growth. The films heat-treated at 500 and 600 °C demonstrate asymmetrical PL band with Gaussian peaks at 437 and 502 nm. We proved that the malonic acid species incorporated into the alumina bulk during the film formation are responsible for photoluminescence band with its peak at 437 nm.     

Investigations on current transients in porous alumina films during re-anodizing using the electrochemical quartz crystal microbalance

Current transients and mass variations in as-prepared and heat-treated anodic alumina films were measured during re-anodizing by means of voltammetry and electrochemical quartz crystal microbalance (EQCM), respectively. Aluminum electrodes (100 nm) on quartz crystals were prepared by thermal evaporation. Anodic alumina films were formed on the surface of Al electrodes in aqueous solutions of oxalic (0.3 M) and phosphoric (0.6 M) acid in the potentiostatic regime. The EQCM experiments did not detect an overshoot in the mass variation of the Al electrode during re-anodizing of heat-treated anodic alumina films. The observed current overshoot in transients proved the presence of electrons and electron holes injected from the contacts in the bulk of the oxide. This can be explained by the emergence of excess electrons in the barrier layer of the alumina films due to a change in the mobility of the electrons.