Growth Of Porous Anodic Alumina Research Articles

Influence of Ethanol on Porous Anodic Alumina Growth in Etidronic Acid Solutions at Various Temperatures.

Etidronic acid, used in aluminum anodization, has a great potential for the fabrication of porous anodic alumina (PAA) with large cell sizes (>540 nm). PAAs are particularly suited to applications in optics and photonics where large-scale periodicity corresponding to visible or infrared light is needed. Additionally, such PAAs should be characterized by long-range pore ordering. However, to obtain regular pore arrangement in an etidronic electrolyte, the anodization should be performed at high electric fields using relatively high temperatures, which makes the process challenging in terms of its stability. To stabilize the process, the electrolyte can be modified with ethanol. In this work, the impact of ethanol on pore geometry and a level of pore ordering is systematically analyzed. It is shown that the additive tends to reduce pore ordering. Moreover, by changing the anodizing temperature and the amount of ethanol, it is possible to tune the porosity of the PAA template. At 20 °C, porosity drops from 14% in PAA grown in a pure water-based electrolyte to ca. 8% in PAA fabricated in the 1:3 v/v EtOH:H2O electrolyte. The larger PAA thickness obtained for the same charge density strongly suggests that PAA formation efficiency increases in the 1:3 v/v EtOH:H2O mixture.

Effect of Aluminum Anode Temperature on Growth Rate and Structure of Nanoporous Anodic Alumina

In the present study, we investigated the effect of an anode temperature on current transient process during porous anodic alumina growth and morphology of the anodic layers. Alumina films were formed in a 0.4 M oxalic acid at a constant voltage mode and electrolyte temperature. The temperature of the Al anode was controlled by thermoelectric Peltier element and varied in the range of 5 °C–60 °C. Surface morphology of both sides of anodic films and their cross-sections were analyzed by scanning electron microscopy (SEM) with subsequent statistical analysis of the SEM images by ImageJ software. It was found that when anode temperature was increased from 5 °С to 50 °С the pores diameter and interpore distance has not changed, but the porous structure became more ordered. According to these results, the rate of chemical dissolution of the barrier layer and pore walls did not depend on the anode temperature. At the anode temperature of 60 °С, pores diameter has increased 1.7 times and there was a distortion of the ordering of porous cells. It was concluded that the temperature difference between the aluminum substrate and electrolyte is an important parameter affecting the formation of ordered structure of nanoporous anodic alumina.

Growth of Porous Anodic Alumina on Low-Index Surfaces of Al Single Crystals

The pseudoepitaxial growth of amorphous anodic alumina with ordered porous structure within single crystal grains of aluminum substrates is an amazing feature of the self-organization process, which occurs during anodization. Here, we used single crystal Al(100), Al(110), and Al(111) substrates to inspect the effect of aluminum crystallography on anodization rates and the morphology of the resulting alumina films grown under different anodization conditions. The difference in the kinetics of porous film growth on various substrates is described in terms of the activation barrier of aluminum atom release from the metal surface to the oxide layer. Scanning electron microscopy and small-angle X-ray scattering are applied for quantitative characterization of different kinds of ordering in anodic alumina films. The highest number of straight channels was found in porous anodic alumina grown on Al(100) substrates, whereas Al(111) was proved to induce the best orientational order in anodic alumina with the forma...

Constitution of Anodizing Current and Effects on the Growth of Porous Anodic Alumina

Field-assisted dissolution theory was the generally accepted mechanism for pore formation and development in porous anodic aluminum oxide (AAO). However, field-assisted dissolution rate or dissolution current has never been reported. The quantitative relationship between the length of AAO and total anodizing current Jtotal is unclear, because the constitution of Jtotal is hard to be determined in situ. In order to make clear of the constitution of Jtotal and effects on AAO growth, different anodizing processes in two electrolytes (0.3 M and 0.7 M H2C2O4) are explored under constant current rather than constant voltage. Quantitative relationship between AAO lengths (y) and total anodizing currents (x) were investigated. Two fitting equations in 0.3 M and 0.7 M H2C2O4 are y = 0.92(x−4.58) and y = 1.00(x−5.05), respectively. The interesting results indicate that the Jtotal is composed of ionic current Jion and electronic current Je, the pore development and the length of AAO are attributed to the Jion rather than the dissolution current, the corresponding Je = 4.58 and 5.05 mA/cm² contribute to the formation of oxygen bubbles which act as models for the pore formation in 0.3 M and 0.7 M H2C2O4, respectively. This is an efficient and simple method to determine the kinetics of porous anodic oxides.

The Role of Stress in the Self-Organized Growth of Porous Anodic Alumina.

Ridges and depressions were formed on the barrier layer during chemical and physical etching of porous anodic alumina (PAA) from the bottom side, indicating nonuniform etching rate around each cell. These behaviors cannot be explained solely by the well-known composition variation, but were in line with the hexagonal distribution of stress within the barrier layer of each cell. Such stress variation should be attributed to the interactions of neighboring cells undergoing volume expansion. These interactions could account for the self-organization of PAA.

A stochastic route to simulate the growth of porous anodic alumina

Porous anodic alumina (PAA) film is composed of highly ordered and controllable structures, and their extensive application requires the understanding of their growing mechanism. Herein, we present a localized oxidation model to unravel the phenomena of PAA growth, showing that random processes converge into an ordered formation due to the unique characteristics of ion transport confinement in alumina. The anodizing voltage shows a quadratic relationship with barrier layer depth. In addition, we predict the furcate conditions of PAA and the voltage threshold to produce a PAA by our model.

Thorough electrochemical kinetic and energy balance models clarifying the mechanisms of normal and abnormal growth of porous anodic alumina films

Thorough kinetic and energy balance models of growth of porous anodic alumina were formulated. Insight into film growth mechanisms showed that all local processes interact with those in the rest surface and the processes rates and parameters vary along Al surface, imperceptibly in normal and perceptibly in abnormal film growth. Al was anodised at 0.51 and 1.02M H2SO4, 0–25°C and anodic potential 23.5–25V. The normal growth or abnormal one presenting island-like and strong burning were discerned and related with peculiar chronoamperometric plots. Island-like burning emerges at random places in the two Al faces above threshold conditions after a current density boundary in the second transient stage. Among this stage and quasi-steady state the plots show intense maxima and poor reproducibility. More afar from them this burning near a side is transformed to strong at corresponding positions in Al faces at this stage after a higher current density boundary. Plots rise fast coming from fast rise of current in burnt area due to avalanche-like hastened processes. Burning is a joint kinetic, Joule heating, thermochemical and heat transfer local effect yielding higher local current density and real anodising temperature. It emerges after the self-regulated mechanism of normal film growth collapses when local starting parameters, like thickness of barrier layer and/or attached electrolyte layer, etc. distributed in Al surface exceed limiting values which vary with conditions, experimental details and position. New methods to avoid burning can be elaborated that are vital for proper Al anodising.

Effect of ethanol on the fabrication of porous anodic alumina in sulfuric acid

Porous anodic alumina (PAA) films with small pore diameters and interpore distances are significantly important in many practical applications such as isolation of pathogens, catalysis and molecular sieving. This kind of PAA films is generally prepared in diluted sulfuric acid, and the film growth is very slow due to the low anodizing potential and current density. Many efforts were done to enhance the PAA formation, and the majorities focused on increasing growth rate by increasing the anodizing potential, which also increases the pore diameter and interpore distance of the obtained PAA. In our paper, the influence of ethanol addition in electrolyte of sulfuric acid was studied in details. Results show that the addition of ethanol can maintain the low self-ordering potentials in sulfuric acid and at the same time significantly increase the growth rate of PAA. With a series of ethanol addition, it was found that with the 10% ethanol addition, the growth rate increases by 5 times from 1.6μm/h to 8μm/h at 2°C and 25V, and there is a nonlinear relationship between ethanol addition amount and PAA growth rate. These findings are of great significance for further investigation on PAAs with ultrasmall nanopores, as well as for PAA commercialization.

High-field anodization of aluminum in concentrated acid solutions and at higher temperatures

Previous studies suggested that high-field anodization of aluminum could be realized only in relatively dilute acid at a low temperature to avoid the occurrence of undesired breakdown during anodization process. According to the model of avalanche breakdown, we proposed that the key to preventing the breakdown is to reduce the barrier-layer thickness of porous anodic alumina (PAA). Since the barrier-layer thickness of PAA decreases with increasing the acid concentration and temperature, the breakdown phenomenon during high-field anodization should not be occurred in concentrated acid solutions and at higher temperatures. Anodization behaviors of aluminum in concentrated oxalic acid solutions and at higher temperature were studied in detail. The results indicated that in the oxalic acid solutions of higher concentrations (>0.3M) and at higher temperatures (16–40°C), stable high-field anodization can be realized without the occurrence of breakdown and well-ordered PAA films can be available. Moreover, the well-ordered PAA film can be obtained even without stirring of the electrolyte. Our findings offer not only a simple and fast fabrication process of PAA templates but also an opportunity to fully understand the breakdown and the self-ordering mechanism during the growth of PAA.

Porous Anodic Alumina Growth in Borax Electrolyte

Ion beam analysis and electron microscopy have been employed to investigate the formation of anodic films on aluminum in a borax electrolyte under potentiostatic conditions. The results indicate the initial growth of a barrier film, containing boron species, followed by growth of boron-free alumina, containing pores. The efficiency of porous film formation is ∼45%, owing to the loss of Al3+ species to the electrolyte. 18O tracer is retained in the film as pores develop, which is suggested to be related to the inward transport of the O2− ions in the growing alumina. In contrast, boron species are lost from the film, which is attributed to the field-assisted dissolution of the alumina as pores develop in the boron-containing region.

Anisotropic Growth of Porous Anodic Alumina and Its Potential Application in Three-Dimensional Nanostructure Construction

The porous anodic alumina was fabricated by means of electrochemical oxidation using Al foil as a substrate in oxalic acid under constant anodic voltage at room temperature. Prior to anodization, the substrate surface with roughness in microscale was obtained through electrochemical treatment. The tube growth on the substrate was investigated using a scanning electron microscope (SEM). The results showed that the tube growth behavior of the porous anodic alumina was strongly dependent on the surface roughness. In presence of sphere-shaped cavities in microscale (diameter of 5~30 μm) on the surface of the substrate, the nanotubes were growing essentially away from the center of curvature of each cavity, forming individual growing regions separated by visible interface. The tube branching and tube cutoff were evidently observed in the growing regions and along the interface, respectively. Anisotropic growth of porous anodic alumina suggested its potential application in three-dimensional nanostructure construction.

In situ Optical Emission Spectrometry during the Initiation and Growth of Porous Anodic Alumina

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Modeling the Potential Distribution in Porous Anodic Alumina Films during Steady-State Growth

Porous anodic alumina (PAA) films, formed by anodic oxidation in acidic solutions, contain hexagonal arrays of parallel cylindrical pores, with pore diameter and spacing between ten and several hundred nanometers. Simulations were developed for the electrical potential distribution in the film during steady-state PAA growth, and used to calculate the rates of metal-film and film-solution interface motion. In particular, a model using the assumption of no space charge (Laplace’s equation) and one based on the current continuity equation, in each case coupled with high-field ionic conduction, were evaluated with respect to the requirement that the interface profiles are time invariant. Laplace’s equation, on which prior simulations of PAA growth were based, yielded unrealistic behavior with highly nonuniform interface motion, suggesting the presence of significant space charge. In contrast, interface motion predicted by the current continuity equation was uniform, except near convex ridges on the metal-film interface between pores. To fully rationalize the steady-state PAA geometry, phenomena other than conduction should be considered, which are able to provide inhibition of the oxidation rate on these ridges.

Self-aligned tantalum oxide nanodot arrays through anodic alumina template

A novel strategy for fabricating the ordered nanodot arrays of tantalum oxide with anodic alumina film is proposed to serve the template. Anodic oxidation of Al on TaN film has been performed in oxalic acid and sulfuric acid electrolytes at various applied voltages for porous alumina formation. Anodising reaction proceeds in the sequence of growth of porous anodic alumina when the aluminum layer is consumed up to the underlying TaN, and the growth of tantalum oxide under the bottoms of the alumina pores occurred simultaneously. The nanostructure of the nanodot arrays was studied by scanning electron microscopy, and the chemical composition of nanodots was analyzed by X-ray photoelectron spectroscopy. The nanodots are composed of nonstoichiometric TaO x . The nanodot diameter demonstrated here ranges between 15 nm and 70 nm and density ranges between 10 11/cm 2 and 10 10/cm 2.

Nucleation and growth of the nanostructured anodic oxides on tantalum and niobium under the porous alumina film

Anodic oxidation of Ta–Al (aluminium deposited on tantalum) and Nb–Al (aluminium deposited on niobium) has been performed in organic and inorganic acid electrolytes for porous alumina formation. Arrays of tantalum nanoscale oxide ‘hillocks’ and niobium oxide ‘goblets’ derived from the anodised Ta–Al and Nb–Al bilayer samples have been investigated by scanning electron microscopy, transmission electron microscopy and X-ray photoelectron spectroscopy depth profiling. Anodising proceeds in the sequence of growth of porous anodic alumina and, when the aluminium layer is consumed up to the underlying metal, growth of anodic tantalum (niobium) oxide under the bottoms of the alumina pores. The oxidation of the underlying metal results from metal ions migrating outward and oxygen ions transported through, and released from, the alumina barrier layer, which dissolves at the tantala/alumina interface, i.e. without tantalum being in direct contact with the electrolyte. The shape and mutual arrangement of the anodic oxide nanostructures depend on the nature of the underlying metal, anodising solution, and are consistent with the difference between the resistivities of the tantalum (niobium) oxide formed and the barrier layer of the overlying alumina cells, which is influenced by incorporated electrolyte-derived species.