Fabrication of self-ordered nanohole arrays on Si by localized anodization and subsequent chemical etching

Nanohole arrays with a 60 nm hole periodicity were fabricated on a Si substrate by the anodization of an aluminum film sputtered on a Si substrate in sulfuric acid and subsequent chemical etching. The transfer of the nanoporous pattern of anodic alumina into the Si substrate was achieved by the selective removal of silicon oxide, which was produced by the anodic oxidation of the underlying Si substrate through the anodic porous alumina used as a mask.     

Synthesis of highly ordered nanopores on alumina by two-step anodization process

Highly ordered anodic alumina was produced, on RF sputtered aluminium on a conductive glass substrate, by two step anodizing process in 0.4 M sulphuric acid at constant cell potentials of between 5 and 25 V and at a constant current density of 20 mA cm⁻². The temperature was kept constant at 15 °C during both anodization processes. The effects of the anodizing potential, current density, and time on the pore diameters were established. Longer anodization periods result in wider irregular pores with reduced porosity for both constant potential and constant current density anodization processes. The current density increases with increasing constant anodizing potential and generally remains constant with time after a sharp rise. Potential drop during constant current density anodization behaves in a similar manner. We confirm that sulphuric acid has a self-ordering potential of 25 V above which burning occurs.     

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.     

Surface treatment of iron by electrochemical oxidation and subsequent annealing for the improvement of anti-corrosive properties

Corrosion resistance of iron oxides on iron foils prepared by anodization, annealing or a combination of both was characterized by electrochemical methods. Even though iron oxide film with a thickness of more than 2 μm could be prepared by single anodization, corrosion resistance deteriorated because the oxide film was in the amorphous phase and contained many defects. Corrosion resistance of iron oxides was also not enhanced by single annealing. Conversely, combination of anodization and subsequent annealing led to a positive shift of the corrosion potential in the Tafel plot, indicating that corrosion resistance was improved. Formation of thicker oxide during anodization was associated with a more positive shift in corrosion potential after annealing. Electrochemical impedance spectroscopy showed that the slowest charge transfer was observed in oxide films grown by a combination of anodization and annealing. We found that the optimum annealing temperature of anodic films in terms of the most positive shift of E_corr was 500 °C.     

Effects of ordering quality of the pores on the photoluminescence of porous anodic alumina prepared in oxalic acid

Photoluminescence (PL) of the anodic alumina has been studied and related with quality of hexagonal ordering of the pores. The photoluminescence excitation (PLE) spectra have been successfully de-convoluted into primarily two sub-bands with peak positions at about 355 and 395 nm and maximum emission at about 450 and 500 nm, respectively; the former being assigned to F⁺ centers and the latter to the F-centers. A red shift in the PLE takes place, at a given anodizing voltage, when the quality of the hexagonal pore ordering deteriorates with an increase in number density of defects, i.e., pentagons and hexagons with missing pores. The metallic hills at these defects change the curvatures of the metal-oxide and the oxide-electrolyte interfaces that could affect the field distribution and hence the stress-state and other characteristics of the oxide at the defects. This allows a comparatively larger concentration of F centers (395 nm band), causing a red shift in the PLE with increase in defect density.     

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.     

Characteristics, apatite-forming ability and corrosion resistance of NiTi surface modified by AC anodization

NiTi samples were anodized in the non-sparking regime using AC voltage in a solution containing calcium and phosphate ions (solution Ca-P). The as-anodized samples were subsequently treated hydrothermally in water (sample A-W-NiTi) or in solution Ca-P (sample A-CaP-NiTi). Thin-film X-ray diffractometry (TF-XRD) analysis confirmed the existence of anatase in the hydrothermally treated samples, but not in the as-anodized sample, while hydroxyapatite (HA) was detected only in sample A-CaP-NiTi. Cross-sectional micrograph by scanning-electron microscopy (SEM) revealed that the thickness of the modified surface layer formed on sample A-CaP-NiTi was ∼200 nm. X-ray photoelectron spectroscopy (XPS) analysis showed that the Ni concentrations at the surface of sample A-W-NiTi and sample A-CaP-NiTi were in the order of 0.4 and 0.3 at.%, respectively, which were about an order of magnitude lower than that for bare NiTi. Both Ca and P were present in the surface layer on as-anodized NiTi and sample A-CaP-NiTi, but negligible on sample A-W-NiTi, as determined from XPS composition depth profiling. Immersion tests in a conventional simulated body fluid (SBF) of the Kokubo type to study apatite-forming ability showed that growth of apatite was induced on A-W-NiTi and much more abundantly on A-CaP-NiTi, but not on bare NiTi and as-anodized NiTi, suggesting that the presence of anatase and HA is favorable for apatite growth. The apatite-forming ability of the samples in the present study may be ranked in ascending order as: bare NiTi < As-anodized NiTi < A-W-NiTi < A-CaP-NiTi. Polarization tests in Hanks’ solution recorded significant increase in corrosion resistance due to anodization and further increase was obtained via hydrothermal treatment. The present study thus shows that anodization followed by hydrothermal treatment is a simple method to form a potentially bioactive and at the same time corrosion resistant surface layer on NiTi.     

Nanoscale islands and color centers in porous anodic alumina on silicon fabricated by oxalic acid

Anodic oxidation of Al film on silicon substrate in oxalic acid is investigated through the j–t curves and its photoluminescence (PL). Their growth is analyzed with three typical stages according to j–t curve, which is agreed with the growth of nanoscale SiO₂ islands at the interface between Al film and Si substrate. The violet and blue peaks of PL were due to F⁺ and F centers, respectively. The evolvement from F⁺ to F centers during the late stage of anodization was revealed by the PL behavior at different stage.     

Bi3.25La0.75Ti3O12 (BLT) nanotube capacitors for semiconductor memories

We report results of fabrication and examination of Bi_3.25La_0.75Ti₃O₁₂ (BLT) ferroelectric nanotubes. BLT nanotubes are suggested for developing 3D ferroelectric nanotube capacitors which could be used in high-density memory applications. BLT nanotubes were prepared by template-wetting process using polymeric sources where anodic aluminum oxide had been used as a template. After annealing, tubular BLT structures were crystallized inside the pores of the template. By selective etching of the template, released BLT nanotubes have been obtained. Crystallization and nucleation of the nanotubes were analyzed by XRD and FE-SEM techniques.     

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.     

Ultrasonic degradation of oxalic acid in aqueous solutions

This paper describes the ultrasonic degradation of oxalic acid. The effects of ultrasonic power, H₂O₂, NaCl, external gases on the degradation of oxalic acid were investigated. Reactor flask containing oxalic acid was immersed in the ultrasonic bath with water as the coupling fluid. Representative samples withdrawn were analysed by volumetric titration. Degradation degree of oxalic acid increased with increasing ultrasonic power. It was observed that H₂O₂ has negative contribution on the degradation of oxalic acid and there was an optimum concentration of NaCl for enhancing the degradation degree of oxalic acid. Although bubbling nitrogen gave higher degradation than that for bubbling air, both gases (for 20 min before sonication and during sonication together) could not help to enhance the degradation of oxalic acid when compared with the degradation without gas passage.     

Nanoporous alumina membrane prepared by nanoindentation and anodic oxidation

The fabrication of nanopatterned surfaces at large scale attracts the interest of research groups from a wide range of areas as biotechnology, nanoelectronics and nanomagnetism. An extended method to pattern the surface in the nanoscale is the fabrication of ordered arrays of nanoelements based on porous templates as Nanoporous Anodic Aluminium Oxide (NAAO). One of the challenges of the NAAO fabrication, based on self-organized methods, is the control of the symmetry and lattice parameter of the ordered nanoporous films. In this work, we present a combined method based on Atomic Force Microscopy (AFM) nanoimprint and anodic oxidation of Al surface. AFM nanoindentations substitute the first anodization process and even more important, allow us to control the symmetry and the lattice parameter of the ordered arrays. In addition, by using AFM nanoimprint method it is possible to select the region were the ordered alumina grows. We demonstrate that square nanoporous arrays of alumina with lattice parameter of 105 nm can be obtained by this method.     

Formation of ZnO nanowires during short durations of potentiostatic and galvanostatic anodization

Different behaviors during the formation of ZnO nanowires from a Zn foil by either potentiostatic or galvanostatic anodization are described herein. During the initial stage, 4.5 fold fewer nucleation sites are created in potentiostatic mode than galvanostatic mode. However, the nucleation sites continuously increase as anodization proceeds in the potentiostatic mode, whereas the number of nucleation sites is determined at the beginning of anodization in galvanostatic mode. Overall, the total number of nanowires produced is almost identical. The growth rate of nanowires is 0.04 μm/s for both modes. Based on the findings during anodization of a Zn foil, a sputtered film of Zn is anodized. In galvanostatic mode, the growth of well-defined nanowires is similar to that observed during anodization of the foil, whereas sparse growth of nanowire bundles is observed in potentiostatic mode. The formation of unevenly grown nanowires is ascribed to the surplus growth species, which are intensively deposited on a few nucleation sites.     

Ultrasound-assisted conversion of tannic acid to gallic acid as a strategy to obtain value-added products

In this work, ultrasound was applied for the conversion of tannic acid into gallic acid using only diluted H₂O₂ as reagent. Experiments were carried out using several types of ultrasonic horns operating at 20 kHz (VC750W processor). The following experimental conditions were evaluated: H₂O₂ concentration (0.2 to 8.5 mol L⁻¹), horn type (10 to 25 mm of diameter), ultrasound amplitude (20 to 70%), sonication time (10 to 45 min), tannic acid concentration (170 to 1360 mg L⁻¹), and reaction temperature (50 to 90 °C). Gallic acid production was monitored with ultra-performance liquid chromatography with high-resolution time-of-flight mass spectrometry (UPLC-ToF-MS). The isolated gallic acid was confirmed with nuclear magnetic resonance (¹H and ¹³C NMR). It is important to emphasize that this study was developed as a proof of concept to demonstrate the potential of ultrasound for tannic acid conversion into gallic acid using just diluted H₂O₂. Under selected conditions gallic acid production yield was 128 ± 4 mg g⁻¹ of initial tannic acid (using 170 mg L⁻¹ of tannic acid as starting material). Reaction time was set as 30 min, which was carried out using 1 mol L⁻¹ H₂O₂ and ultrasound amplitude of 50% at 90 °C. At silent conditions (mechanical stirring, from 100 to 1000 rpm), gallic acid production was halved (less than 78 ± 4 mg g⁻¹ of initial tannic acid).     

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.     

Optimization of effective parameters in the synthesis of nanopore anodic aluminum oxide membrane and arsenic removal by prepared magnetic iron oxide nanoparicles in anodic aluminum oxide membrane via ultrasonic-hydrothermal method

In this study, a new anodized aluminum oxide (AAO) nanostructure membrane was synthesized by anodization process under a constant voltage, in oxalic acid solution that was improved with trace amounts of sulfuric acid at room temperature. The effect of various parameters on the morphology of the synthesized nanostructures such as voltage, electrolyte composition, anodization time and type of stripping solution were investigated. According to the results, corrosion of the walls, size regularity, diameter and number of the pores increased in the presence of sulfuric acid (0.018 mol.L⁻¹). Nitrogen adsorption-desorption analysis confirmed significant porosity, array and uniformity of the pore size in the synthesized nanoporous membrane. A new modification method was used based on ultrasonic-hydrothermal method to modify the synthesized AAO with Fe₃O₄/SiO₂ nanoparticles for metals and metalloids removal from aqueous solution. In this method, Fe₃O₄/SiO₂ nanoparticles were placed very regularly and uniformly on the surface and inside the pores. This modification was confirmed by characterization techniques. The modified AAO@Fe₃O₄/SiO₂ membrane showed excellent results for removing arsenic from aqueous media.     

Dissolution behaviour of the barrier layer of porous oxide films on aluminum formed in phosphoric acid studied by a re-anodizing technique

Chemical dissolution of the barrier layer of porous oxide films formed on aluminum foil (99.5% purity) in the 4% phosphoric acid after immersion in 2 mol dm⁻³ sulphuric acid at 50 °C has been studied. The barrier layer thickness before and after dissolution was determined using a re-anodizing technique. A digital voltmeter with a computer system was used to record the change in the anode potential with re-anodizing time. It has been found that the barrier layer material may consist of two or three regions according to the dissolution rate. The barrier oxide contains two layers at 35 V: the outer layer with the highest dissolution rate and the inner layer with low dissolution rate. The barrier oxide contains three layers at 40 V and above it: the outer layer with high dissolution rate, the middle layer with the highest dissolution rate and the inner layer with low dissolution rate. It has been shown that there is a dependence of the dissolution rate on the surface charge of anodic oxide film. Annealing of porous alumina films for 1 h at 200 °C leads to disappearance of layers with different dissolution rates in the barrier oxide. We explained this phenomenon by the absence of the space charge in the barrier oxide of such films.     

Dual effects of ultrasound on fabrication of anodic aluminum oxide

Ultrasound has been proven to enhance the mass transfer process and impact the fabrication of anodic aluminum oxide (AAO). However, the different effects of ultrasound propagating in different media make the specific target and process of ultrasound in AAO remain unclear, and the effects of ultrasound on AAO reported in previous studies are contradictory. These uncertainties have greatly limited the application of ultrasonic-assisted anodization (UAA) in practice. In this study, the bubble desorption and mass transfer enhancement effects were decoupled based on an anodizing system with focused ultrasound, such that the dual effects of ultrasound on different targets were distinguished. The results showed that ultrasound has the dual effects on AAO fabrication. Specifically, ultrasound focused on the anode has a nanopore-expansion effect on AAO, leading to a 12.24 % improvement in fabrication efficiency. This was attributed to the promotion of interfacial ion migration through ultrasonic-induced high-frequency vibrational bubble desorption. However, AAO nanopores were observed to shrink when ultrasound was focused on the electrolyte, accompanied by a 25.85 % reduction in fabrication efficiency. The effects of ultrasound on mass transfer through jet cavitation appeared to be the reason for this phenomenon. This study resolved the paradoxical phenomena of UAA in previous studies and is expected to guide AAO application in electrochemistry and surface treatments.     

Influence of defects on the ordering degree of nanopores made from anodic aluminum oxide

Anodic aluminum oxide (AAO) templates with highly ordered nanoporous structure were fabricated by means of the electrochemical anodization under the constant anodic voltage and electrolyte temperature. The dependence of the ordering degree of nanopores on the point defects, dislocation configuration and grain boundary of aluminum is qualitatively analyzed. Experiment results show that the size of the ordered region of nanopores depends strongly on the point defects, dislocation cell configuration.     

Ultrasound-assisted intensified crystallization of L-glutamic acid: Crystal nucleation and polymorph transformation

In this paper, the crystallization of L-glutamic acid with application of ultrasound was explored in detail, including the process of nucleation, polymorphic control and polymorphic transformation. The induction time and metastable zone widths (MSZWs) were measured with and without ultrasound during the nucleation process. The induction time and MSZWs were decreased by ultrasound and the induction time was further decreased by higher ultrasonic power. The calculated nucleation parameters (such as interfacial energy, critical nucleus size and critical Gibbs energy) showed an obvious decrease in the presence of ultrasound, indicating that the nucleation was enhanced with application of ultrasound. By adjusting the ultrasonic power in the quench cooling process, the difference in nucleation temperatures would determine the distribution of polymorphs. In further, the polymorphic transformation was investigated quantitatively, and to the best of our knowledge, it was the first time to study the transformation kinetics with ultrasound using Avrami-Erofeev model. In the transformation process, the crystallization mechanism of the stable form was modified by ultrasound. The ultrasound eliminated the nucleation element in the rate-limiting step and facilitated the crystal growth of stable form. Thus, the ultrasound has a profound influence on L-glutamic acid crystallization.