Porous Anodic Oxide Films Research Articles

Nature of instability in flow-driven porous anodic oxide.

Self-organized porous anodic oxide films are formed by the electrochemical oxidation of reactive metal aluminum in acidic solutions in which the oxide is soluble. Recently, viscous flow models have shown using linear stability analysis that the instability results from a trade-off between the destabilizing effect of viscous flow of oxide and the stabilizing effect of oxide formation, which provides the wavelength selection mechanism for pattern formation. Anion adsorption on surface growth sites causes nonuniform compressive stress at the oxide-solution interface, which drives the flow. This anodic instability is analogous to the classical Marangoni instability induced by surface tension gradients. In this work, nature of the instability beyond the stability threshold is determined using a weakly nonlinear analysis. For the growth of well-developed pores beyond the threshold, a subcritical nature of the instability is essential. However, our weakly nonlinear analysis shows that the solutions emerging from neutral stability are supercritical in nature at all wavenumbers for the practical range of anodizing control parameters investigated. We also determine the region where the model is Hadamard stable, a necessary condition for well-posedness.

Weakly Nonlinear Analysis of Flow-Driven Porous Anodic Oxide

Porous anodic oxide (PAO) films are formed by the electrochemical oxidation of metals such as titanium, aluminum, and other metals in acidic solutions. The optimal selection of parameters leads to the formation of submicron-sized pores arranged in a parallel hexagonal array within these films. Porous anodic oxide has enormous industrial applications in the development of functional nanodevices. The present study builds upon the flow-driven porous anodic oxide model proposed by Mishra and Hebert. They have shown through a linear stability analysis of the model that the inter-pore distance in PAO is successfully determined by the competition between stabilizing oxide formation and destabilizing viscous flow. In this work, we investigate the weakly nonlinear analysis of the model to determine the nature of instability arising in flow-driven porous anodic films beyond the stability threshold. For formation of well- developed pores, the nature of instability must be subcritical. However, the results of weakly nonlinear analysis show that the solutions resulting from neutral stability exhibit a supercritical nature for the practical range of anodizing control parameters as shown in Figure 1. Thus, in order to make accurate nonlinear predictions about the formation of pores, it is imperative to incorporate additional physics into the viscous flow model. We also determine the Hadamard stability criteria by tracking the neutral wavenumber to ascertain the regions of well- posedness of the viscous flow model. Figure 1

Isotopic Tracer Study of Initiation of Porosity in Anodic Alumina Formed in Chromic Acid.

In this paper, we focused on the initiation of porosity in the anodic alumina under galvanostatic conditions in chromic acid, using an 18O isotope tracer. The general concept of the initiation and growth of porous anodic oxide films on metals has undergone constant development over many years. A mechanism of viscous flow of the oxide from the barrier layer to the pore walls has recently been proposed. In this work, two types of pre-formed oxide films were analysed: pure Al2O3 formed in chromic acid, and a film containing As ions formed in a sodium arsenate solution. Both were anodized in chromic acid for several different time durations. Both pre-formed films contained the oxygen isotope 18O. The locations and quantities of 18O and As were analysed by means of ion accelerator-based methods supported by transmission electron microscopy. The significant difference observed between the two oxide films is in the 18O distribution following the second step of anodization, when compared with galvanostatic anodization in phosphoric or sulfuric acid reported in previous works. From the current experiment, it is evident that a small amount of As in the pre-formed barrier layer appears to alter the ionic conductivity of the film; thus, somehow, it inhibits the movement of oxygen ions ahead of advancing pores during anodization in chromic acid. However, anodising pure alumina film under these conditions does not enhance oxygen movement within the oxide layer. In addition, the tracer stays in the outer part of the growing porous oxide film. A lower-than-expected value for pure alumina enrichment in 18O in the pre-formed films suggests, indirectly, that the pre-formed film may contain hydrogen species, as well as trapped electrons, since no Cr is detected. This may lead to the presence of space charge distribution, which has a dual effect: it both retards the ejection of Al3+ ions and prevents O2- ions from migrating inward. Thus, the negative- and positive-charge distributions might play a role in the initiation of pores via a flow mechanism.

On the Scaleability of the Morphological Structure of Porous Anodic Aluminum Oxide Films

On the Scaleability of the Morphological Structure of Porous Anodic Aluminum Oxide Films

Self-Healing Coatings Consisting of an Outer Electrodeposited Epoxy Resin Layer and an Inner Porous Anodic Oxide Layer with Healing Agents for the Corrosion Protection of Al Alloys

Recently, new surface treatments for the corrosion protection of Al alloys by forming self-healing layers have attracted the attention of many researchers. The authors of this paper have previously developed self-healing polyurethane coatings with micro-capsules containing healing agents and porous anodic oxide films filled with healing agents. In this study, self-healing coatings consisting of an outer electrodeposited epoxy resin layer and an inner porous anodic oxide layer with healing agents were developed for the corrosion protection of Al alloys. The corrosion protection abilities of the self-healing coating were shown in Cu2+/Cl− solutions after damaging with indenters and were affected by freezing treatments and the tip angles of the indenter.

Construction of porous anodic oxide/Ce-MOFs film by induced electrodeposition and its corrosion resistance

Aluminum alloy has good expansion and low hardness. However, aluminum alloy is susceptible to corrosion in humid environments, limiting their application. In this experiment, we adopted an efficient and facile method to prepare porous anodic oxide (PAO) films and PAO/Ce-MOFs composite films. It can be characterized by SEM, EDS, FT-IR, XRD, XPS and other test methods. The corrosion resistance is studied by neutral salt spray test and electrochemical test. Electrochemical data show that the resistance of PAO/Ce-MOFs film is three orders of magnitude higher than that of aluminum alloy substrate, and the corrosion current density is three orders of magnitude lower than that of aluminum alloy substrate, reaching 1.621 × 10−8 A/cm2. The corrosion resistance efficiency is 99.92%. The salt spray experiment shows that the film can maintain the corrosion resistance for 30d. The results show that Ce-MOFs coating can effectively improve the corrosion resistance of aluminum alloy. Therefore, this study provides a new approach in the application of MOFs materials in the direction of anticorrosion.

The effect of electrolyte temperature on the growth, morphology, and properties of porous anodic tin oxide films

The effect of electrolyte temperature on the anodic formation of nanoporous tin oxide layers, their morphology and properties was studied in detail. Electrochemical synthesis of SnOx films was carried out in both acidic (H2C2O4) and alkaline (NaOH) electrolytes at various potentials. Differences in morphological features of as-generated films and the kinetics of their formation were carefully investigated from FE-SEM images. The effect of anodizing temperature was successfully verified revealing its undeniable importance when designing the anodization procedure. For instance, both in acidic and alkaline electrolytes, applying too low temperatures results in formation of cracked oxide layers with the least open pores or compact surface, while too high temperatures (≥40 °C) leads to obtaining films with clogged pores due to the partial etching of the material by the electrolyte. Anodizing temperature was also found to be a crucial parameter responsible for the type of morphology of the synthesized materials (compact or porous layers or even crater-like hierarchical structures) when the process is carried out at higher potentials (here 9 V). Finally, we have proved that the impact of electrolyte temperature on the crystallinity of the anodic tin oxide layers is negligible, whereas the photoelectrochemical activity is significantly influenced due to the tight relation with the morphology of as-synthesized materials.

Development of nano structures for solar driven desalination

Now-a-days there is shortage of pure water in several areas. So, the emerging technology solar driven desalination is one of the core practices that employ solar energy for fresh water making in sunny areas with lacking quality of water resources. In this work, the nano structure was developed using two step anodization procedure. Anodization was performed on pure aluminum foil (99.99 % purity) in 0.3 M of oxalic acid at constant voltage and effect of temperature on the morphology of formed porous anodic aluminum oxide (AAO) film was investigated. The temperature is varied between −5 to 25 °C. The characterization of the material is done using Scanning Electron Microscopy (SEM), Energy dispersive X-ray (EDAX), Ultraviolet-Differential reflectance spectroscopy (UVDRS). It is observed that, at −5 °C, micro indentations are formed on surface. At 5 °C, pores are randomly formed on the surface. At 15 °C, nano balls are formed on the surface. Whereas, at 25 °C, complete nano porous structure is formed. Hence, the formed nano structures were applied in solar driven desalination. In comparison to all other temperatures, we have observed that, the nano porous structure developed at 25 °C has 27 % of higher evaporation than the conventional.

Changes in the Structure and Corrosion Protection Ability of Porous Anodic Oxide Films on Pure Al and Al Alloys by Pore Sealing Treatment

It is well known that corrosion protection of pure Al is enormously improved by the formation of porous anodic oxide films and by pore sealing treatment. However, the effects of anodizing and pore sealing on corrosion protection for Al alloys are unclear, because the alloying elements included in Al alloys affect the structure of anodic oxide films. In the present study, porous anodic oxide films are formed on pure Al, 1050-, 3003- and 5052-Al alloys, and pore sealing was carried out in boiling water. Changes in the structure and corrosion protection ability of porous anodic oxide films on pure Al and the Al alloys by pore sealing, were examined by scanning electron microscopy (SEM) and electrochemical impedance spectroscopy (EIS). SEM observation showed that anodic oxide films formed on pure Al have a smooth surface after pore sealing, and that cracks are formed in anodic oxide films on 1050-, 3003- and 5052-aluminum alloys, after pore sealing. Corrosion protection after pore sealing increased with anodizing time on pure Al, but only slightly increased with anodizing time on the Al alloys.

Photoluminescence properties of anodic aluminium oxide films formed in a mixture of malonic acid and oxalic acid.

In this work, porous anodic aluminium oxide (AAO) films were fabricated by anodization in an electrolyte mixture with various concentration ratios of malonic acid and oxalic acid at room temperature. The photoluminescence (PL) properties of the AAO films before and after annealing from 300°C to 650°C in air or vacuum conditions were investigated, showing a strong PL band in the range 300-550 nm. We observed a weak PL in the AAO film formed in the malonic acid electrolyte, whereas the films fabricated using an electrolyte mixture showed strong PL emissions, exhibiting a maximum. The broad PL band was decomposed into three Gaussian sub-bands, in which the first two sub-bands could be attributed to the luminescence centre oxygen vacancies (F+ and F defect centres), whereas the latter transformed from malonic impurities and oxalic impurities. More interestingly, the redshift of the PL bands occurred with increasing oxalic acid concentration, and the PL wavelength and intensity could be modulated by varying the concentration ratios in the malonic acid and oxalic acid electrolyte mixture.

Nanomorphological changes of anodic aluminum oxide fabricated by anodizing in various phosphate solutions over a wide pH range

High-purity aluminum plates were anodized in phosphoric acid and six phosphate solutions over a wide pH range from acidic to alkaline. The current density corresponding to the growth rate of the anodic oxide was minimum when anodizing in near-neutral solutions and increased when the pH value increased or decreased. A typical porous anodic aluminum oxide (PAAO) film consisting of an outer porous layer and inner hemispherical barrier layer, which is based on the Keller–Hunter–Robinson (KHR) model, was formed in acidic solutions. At the pH values closer to the neutral region, a uniform barrier oxide film or a PAAO film with a narrow porous layer was produced owing to the low solubility of aluminum oxide. On the other hand, two types of characteristic PAAO films were formed during anodizing in alkaline phosphate solutions: one was a PAAO film consisting of an outer porous layer with feathered pore walls and inner hemispherical barrier layer, and the second was a PAAO film consisting of an outer porous layer with elongated pores and inner flat barrier layer. We showed that novel PAAO films, which are not based on the KHR model, can be prepared by anodizing aluminum in various alkaline phosphate solutions.

Nanoporous anodic alumina layers obtained from novel deep eutectic solvent formulations

ABSTRACT The possible fabrication of porous anodic oxide films on aluminium in ionic liquids based on choline dihydrogen citrate eutectic mixtures both with oxalic acid and isopropyl alcohol and ethylene glycol, has been investigated. The anodisation has been carried out in either potentiostatic or galvanostatic regime, at temperatures of 45–80°C, for different process durations. Quite compact, uniform anodic alumina layers have been obtained. Based on AFM and SEM investigations, pore diameters between 50 and 80 nm and interpore distances in the range of 160–200 nm have been estimated, with values influenced by the electrolyte type and anodisation conditions. The highest anodisation rate of about 0.4 µm min−1 has been determined by applying operation temperatures of 60°C. The recorded EIS spectra showed a pure capacitive behaviour and high anodic oxide resistances of 106–107 Ω cm2 order.

High-temperature annealing of porous anodic aluminium oxide prepared in selenic acid electrolyte

Selenic acid electrolyte provides porous anodic aluminium oxide (AAO) films with low porosity and high optical transparency. Here, the high-temperature behaviour of AAO obtained in a selenic acid electrolyte is reported for the first time. The crystallization of as-prepared amorphous AAO at about 800 °C is accompanied by the elimination of electrolyte impurities, an increase in pore diameter from 15 to 21 nm, and an increase in specific surface area to 55 m2·g−1 due to the mesopore formation inside the cell walls. The second crystallization stage at about 1150 °C results in the formation of coarse-grained corundum (α-Al2O3) films with a mean grain size of about 11 μm. Each of the grains consists of regularly arranged 27-nm-diameter pores, which are stable up to 1300 °C. AAO physicochemical properties depend on the annealing temperature. In particular, chemical stability is up to three orders of magnitude higher after two-step crystallization. Optical properties and thermal expansion of selenic-acid-made porous AAO films with different phase composition are quantified. The crystallization of as-prepared AAO to low-temperature Al2O3 polymorphs has a minor influence on optical transmittance and effective refractive index of AAO, whereas the coarse-grained structure of corundum causes an order of magnitude decrease in average transmittance in the visible region. The coefficient of thermal expansion increases by 25% to 8.5 × 10−6 K−1 for α-Al2O3 porous films compared to as-prepared AAO.

Effect of Heat Treatment on the Structure and Optical Properties of Porous Anodic Titanium Oxide Films

Effect of Heat Treatment on the Structure and Optical Properties of Porous Anodic Titanium Oxide Films

Fabrication of unique porous alumina films with extremely high porosity and an ultra-flat barrier layer by anodizing aluminum in sodium metaborate

Unique porous anodic aluminum oxide (PAAO) films with a completely flat barrier layer and an extremely porous loofah-like structure were fabricated by anodizing aluminum in an alkaline sodium metaborate solution. High-purity aluminum plates were anodized in a 0.3 M sodium metaborate solution at various applied voltages in the range of 0.1–200 V. A typical PAAO film with a spherical cap barrier layer was formed at voltages lower than 50 V, whereas a PAAO film with a flat barrier layer was formed at voltages higher than 100 V; this film formation was not based on the Keller-Hunter-Robinson model with the spherical barrier layer. The growth interface of the flat barrier layer exhibited an ultra-flat morphology with a minimum roughness value of 0.4 nm, which is much smaller than that of an electropolished aluminum surface. Such barrier layer morphology is expected to be formed by field-assisted dissolution without oxide flow. Although the current density slightly decreased with the applied voltage, a relatively higher current density of 6.8 Am−2 was still measured at the lowest voltage of 0.1 V. The alumina walls and the bottom barrier layer gradually thinned as the applied voltage decreased, and a loofah-like PAAO film with an ultra-high porosity of 0.93 was successfully fabricated at 0.1 V. The anodic oxide consisted of amorphous, anion-free aluminum oxide.

Electrochemical Synthesis of Crystalline Niobium Oxide

Features of creation of porous nanostructured oxides of transition materials on an example of niobium are considered. It has been experimentally shown that variation in anodizing modes makes it possible to obtain non-porous and porous amorphous anodic oxide films (AOF) and films of the crystalline type. It is determined that the process of AOF formation on niobium, as well as its structure and properties depend on such parameters as the type of electrolyte, anodizing voltage, activator concentration, the duration of the process. It is confirmed that the presence of an activator in the electrolyte is a necessary and decisive factor in the process of forming a nanostructured anode oxide layer. To obtain a nanostructured surface of niobium oxide, a necessary condition is the introduction of fluoride into the electrolyte, but also an important task is to determine the type of compound with which F– ions are introduced into the electrolyte. It has been experimentally determined that the optimal solution for the rapid growth of porous crystalline oxide is a solution consisting of a background electrolyte in the form of 1M H2SO4 with the addition of a fluoride ion activator in the form of 0.5M NaF. The increase in the activator accelerates the formation of the crystal structure on the surface of niobium. It is shown that higher voltage and longer anodizing time leads to an increase in the size of microcones and their number on the surface of niobium. Optimal for the formation of porous crystalline oxide is a voltage of 60 V in the electrolyte 1M H2SO4 + 0.5M NaF for 2 hours.

Kinetics of the Formation and Dissolution of Anodic Aluminum Oxide in Electrolytes Based on Sulfuric and Selenic Acids

Measurements were made to study the kinetics of the formation and dissolution of porous anodic aluminum oxide in electrolytes based on sulfuric and selenic acids at temperatures from 0 to 30°C. It was shown that there is a significant difference in the kinetics of the dissolution of anodic aluminum oxide at the bottom of the pores during its formation and throughout the walls of the porous structure after the termination of the anodizing. In the anodizing of aluminum in 2.0 M H2SO4 in kinetic mode, the apparent activation energy of the dissolution of anodic aluminum oxide at the bottom of the pores is 53.2 ± 1.8 kJ/mol, whereas in the chemical dissolution of the walls of the porous structure, this quantity is 83.3 ± 2.1 kJ/mol. The observed difference in the dissolution kinetics predicts the increase in maximum accessible thickness of the porous anodic aluminum oxide film with decreasing electrolyte temperature.

(Invited) Growth of Self-Ordered Porous Anodic Oxide Nanomaterials By Coupled Ion Migration and Viscous Flow

Porous anodic oxide (PAO) films are self-organized porous oxide nanomaterials formed by electrochemical oxidation. PAO includes porous anodic alumina and TiO2 nanotube layers. These films eventually evolve into a hexagonally-ordered array of parallel cylindrical pores or nanotubes of order 100 nm diameter, with a characteristic interpore spacing that scales directly with the cell voltage.We explore the roles of mechanical stress and oxide flow in self-ordering, through a modeling analysis of stress-driven viscous flow of oxide (1). The model is based on coupled viscous flow of oxide and stress- and electric field-driven migration of metal and oxygen ions (2). Stress at the film interfaces is generated by ion-transfer processes and relaxed by viscous flow, resulting in layers with elevated compressive or tensile stress within a few nm of the interfaces (3,4). Experimental evidence for these layers has been found from in situ stress measurements (5,6). We present linear stability analysis incorporating the surface stress boundary conditions in the coupled oxide flow-electrical migration model of Houser and Hebert (2).For porous alumina, the calculations reveal self-organized pattern formation with steady-state pore spacing-voltage scaling ratios that compare favorably to those found experimentally (1). The scaling ratios are independent of oxide viscosity or uncertainties in the values of any other model parameters. The stress driving oxide flow arises at the oxide-solution interface due to strong electrolyte anion adsorption, which blocks direct oxide deposition on the surface itself. New results for TiO2 nanotube layers are presented. Near-surface stress is not present in TiO2 nanotube layers (6); instead, compressive stress driving flow arises at the metal-oxide interface from the volume change upon conversion of metal to oxide.REFERENCES P. Mishra and K. R. Hebert, Electrochim. Acta, 340, 135879 (2020).J. E. Houser and K. R. Hebert, Nature Mater., 8, 415 (2009).K. R. Hebert, P. Mishra, J. Electrochem. Soc., 165, E737 (2018).K. R. Hebert, P. Mishra, J. Electrochem. Soc., 165, E744 (2018).Ö. Ö. Çapraz, P. Shrotriya, P. Skeldon, G. E. Thompson, K. R. Hebert, Electrochim. Acta, 167, 404 (2015).Q. Dou, P. Shrotriya, W. F. Li, K. R. Hebert, Electrochim. Acta, 292, 676 (2018).Q. Dou, P. Shrotriya, W. F. Li, K. R. Hebert, Electrochim. Acta, 295, 418 (2019).

The influence of water-induced crystallization on the photoelectrochemical properties of porous anodic tin oxide films

Nanoporous SnOx films with ultra-small channels (<20nm in diameter) were synthesized by one-step anodic oxidation of Sn foil in 1M sodium hydroxide. Then, as-prepared materials were immersed in distilled water for various durations (2–120h) to find out whether a simple soaking process can affect the photoelectrochemical performance of such kind of photoanodes. It was confirmed that during exposure to water, spontaneous crystallization of the amorphous SnOx matrix into rutile-type SnO2 occurs via a dissolution–redeposition process, which results in a gradual loss of the initial porous morphology and thinning of the anodic film. Moreover, due to the formation of less defective, more stoichiometric, and crystalline SnO2-x, gradual widening of the semiconductor band gap, the cathodic shift of the flat band and onset potentials, as well as decrease in the donor density are observed. All these factors are the reason for the significant deterioration of the photoelectrochemical performance of SnOx photoanodes during their exposure to water. Therefore, it was confirmed that even slight differences in the experimental procedure, especially sample washing, drying, and storage, can result in meaningful changes in the composition, and the photoelectrochemical properties of anodically generated tin oxide films.

The Confinement Effect of Angstrom-Sized Pores in Asymmetrical Membrane Constructed by Zeolitic Imidazolate Frameworks: Partially Dehydrated Ion Transport Performance.

The confinement effect in asymmetrical biological ion channels makes the state of molecules and ions differs from that in the external environment, and the mass transfer confined in the biological ion channels is in a single strand form. Herein, an asymmetrical membrane with angstrom-sized pores is constructed by growth of ZIF-90 membrane on the porous anodic aluminum oxide film. Due to the confinement effect of angstrom-sized pores of ZIF-90, ions transport through the pores of ZIF-90 suffer from multiple dehydration-hydration-dehydration process in the form of a single ionic chain. Molecular dynamics simulations imply that ions inside the pores of ZIF-90 are partially dehydrated. In alkaline condition, high rectification ratios of 237, 295, and 357 can be achieved in 10 × 10-3 m KCl, NaCl, and LiCl electrolyte, respectively. Besides, the strong electrostatic interaction between ions and the confined ZIF-90 pores makes the ions transport through the asymmetrical membrane suffer from an energy barrier, and the energy barrier is different with different ion species. This work helps to understand the ions transfer mechanism through angstrom-sized pores, which can provide guidance for the design of asymmetrical membrane and boost their applications.