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Alkaline corrosion-resistant porous alumina was fabricated by anodizing aluminum in etidronic acid, and its hydration behavior during pore sealing in boiling water was investigated. High-purity aluminum plates were anodized in sulfuric, oxalic, citric, and etidronic acid solutions. Anodizing with etidronic acid caused stable growth of a uniform, porous alumina layer at a high voltage of approximately 200 V. This porous alumina possessed a thick barrier layer with an inner layer of pure aluminum oxide and exhibited a 10-fold increase in the corrosion resistance in a 2.5 M sodium hydroxide solution. When the porous alumina film formed by sulfuric acid anodizing was immersed in boiling water, plate-like hydroxide scales rapidly formed on the whole surface, and the pores were sealed within 10 min. In the case of etidronic acid, the hydroxides formed at the bottom of the pores in the initial stage of immersion, and the thickness of the hydroxide layer gradually increased with the immersion time. The porous layer was completely sealed by long-term immersion. Although the barrier layer was reduced to approximately 80% of its original size due to hydration, a thick barrier layer was still maintained at the bottom of the pores after immersion. [ABSTRACT FROM AUTHOR]

A superhydrophilic aluminum surface with fast water evaporation based on nanostructured aluminum oxide was fabricated via anodizing in pyrophosphoric acid. Anodizing aluminum in pyrophosphoric acid caused the successive formation of a barrier oxide film, a porous oxide film, pyramidal bundle structures with alumina nanofibers, and completely bent nanofibers. During the water contact angle measurements at 1 s after the water droplet was placed on the anodized surface, the contact angle rapidly decreased to less than 10°, and superhydrophilic behavior with the lowest contact angle measuring 2.0° was exhibited on the surface covered with the pyramidal bundle structures. As the measurement time of the contact angle decreased to 200–33 ms after the water placement, although the contact angle slightly increased in the initial stage due to the formation of porous alumina, at 33 ms after the water placement, the contact angle was 9.8°, indicating that superhydrophilicity with fast water evaporation was successfully obtained on the surface covered with the pyramidal bundle structures. We found that the shape of the pyramidal bundle structures was maintained in water without separation by in situ high-speed atomic force microscopy measurements. [ABSTRACT FROM AUTHOR]

Highlights: [•] Growth behavior of anodic porous alumina formed in malic acid solutions was investigated. [•] Porous alumina was formed at grain boundaries during the initial stage of anodizing. [•] Growth region extended to the entire aluminum surface as the anodizing time increased. [•] Anodic porous alumina with cell diameters of approximately 300–800nm is formed. [ABSTRACT FROM AUTHOR]

• High-speed galvanostatic anodizing was achieved using a nanodimpled aluminum. • The peak voltage considerably decreased, whereas the plateau was unchanged. • The possible applied current density increased with the size of the nanodimples. • The speed of the anodizing process without oxide burning increased by five times. • This method is useful in various acidic electrolyte solutions at high voltages. Rapid formation of a porous alumina film without oxide burning was achieved by anodizing in etidronic acid at large current densities using a nanodimpled aluminum surface. After the electropolished aluminum specimens were galvanostatically anodized in a 0.3 M etidronic acid solution at 293 K, a uniform porous alumina film without oxide burning was formed at relatively low current densities of up to 20 Am−2. After the first anodizing process, an array of dimples was fabricated on the aluminum surface by oxide film removal in a chromic acid/phosphoric acid solution. After the nanostructured aluminum specimen was galvanostatically anodized once again under the same conditions, the possible applied current density without burning increased with the size of the nanodimples, and the current density during the high-speed anodizing process of the dimpled aluminum specimen increased by five times. Many pores grew on the whole surface of the aluminum dimples from the initial anodizing stage; then, pores that grew from the bottom of the dimples survived the anodizing process, and a clear porous alumina film was formed as the voltage reached the maximum value. [ABSTRACT FROM AUTHOR]

• Aluminum was galvanostatically anodized in ammonium carbonate solutions. • A micro- and nanoporous multi-layer structure grew by continuous visible sparking. • The multi-layer consisted of an outer crystalline oxide and inner amorphous oxide. • The nanopores of the inner oxide can be sealed by immersion in boiling water. A plasma electrolytic oxidation (PEO)/anodic aluminum oxide (AAO) multi-layer film was fabricated via one-step galvanostatic anodizing of high-purity aluminum in 0.3–2.0 M ammonium carbonate ((NH 4) 2 CO 3) solutions at 283–333 K and 25–400 Am−2. Anodizing at higher concentrations and higher temperatures caused the formation of relatively uniform anodic oxide film on the aluminum substrate. Characteristic voltage-time curves with two high-plateau voltages at approximately 250 V and 375 V were obtained during galvanostatic anodizing. A multi-layer structure of an outer PEO layer with a crystalline γ-Al 2 O 3 structure and an inner amorphous AAO layer with nano-cylinerical pores was formed by continuous visible sparking occurring after passing the first voltage plateau region. The whole aluminum surface was covered with the multi-layer structure after reaching the second voltage region. The thickness of the multi-layer increased with time via the further anodizing process at the second plateau voltage. Pore sealing of the inner nanoporous film was achieved by immersion post-treatment in boiling water. [ABSTRACT FROM AUTHOR]

Electrochemical and morphological characterization of the porous alumina formed by galvanostatic anodizing in etidronic acid under various operating conditions was performed. High-purity aluminum plates were anodized in 0.03–3 M etidronic acid solutions at 273–333 K and 0.25–500 Am−2 for up to 24 h. Galvanostatic anodizing in etidronic acid operated over a wide range voltage measuring from a few V to 246 V. The time required for the steady growth of porous alumina not only depends on the current density but also the temperature and the concentration of the electrolyte solution during galvanostatic anodizing. The average, maximum, and minimum cell sizes of the porous alumina were directly proportional to the anodizing voltage with a proportionality constant of 2.5, 3.5 and 0.7, respectively, and were independent of other parameters. The number density of the cell was also a function of the anodizing voltage and agreed with the theoretical value obtained for ordered porous alumina with an ideal honeycomb distribution. The maximum voltage measured during galvanostatic anodizing was linearly proportional to the plateau voltage with a proportionality constant of 1.4. Image 1 • Galvanostatic anodizing in etidronic acid operated at a maximum voltage of 246 V. • The time required for steady growth decreased with increasing current density. • The average cell size was directly proportional to the anodizing voltage. • The minimum and maximum cell sizes were also directly proportional to the voltage. • The number density of the cell was only a function of the anodizing voltage. [ABSTRACT FROM AUTHOR]

Graphical abstract Highlights • A thin layer of gold was coated on an aluminum dimple array and then heated. • The gold layer was transformed into numerous gold particles by the thermal treatment. • The transformation rate increases with the temperature of the thermal treatment. • Multiply-twinned particles were distributed on the surface. • The gold particle array exhibited excellent adhesion properties. Abstract Gold nanoparticles were fabricated on an ordered aluminum dimple array via aluminum anodizing, sputter coating, and thermal treatment, and the transformation behavior and nanostructural characterization were investigated in detail. Electropolished aluminum specimens were anodized in an oxalic acid solution under self-ordering conditions at 40 V, and then the porous alumina was selectively dissolved to expose an ordered aluminum dimple array with each dimple measuring 100 nm. A thin layer of gold was coated onto the dimple array, and a thermal treatment was subsequently performed. The gold layer was transformed into numerous particles by the thermal treatment due to dewetting. When the values of gold layer thickness, thermal treatment temperature, and thermal treatment duration were optimized, the gold particles were located at the bottom of each aluminum dimple. Consequently, multiply-twinned particles with polygonal and elliptical shapes were regularly distributed on the aluminum dimple array treated at 473 K. Although the rate of the transformation induced by dewetting increased with the temperature of the thermal treatment, non-uniform gold nanostructures were formed by extended thermal treatment at 873 K. The gold particles formed on the aluminum surface exhibited excellent adhesion upon ultrasonication. [ABSTRACT FROM AUTHOR]

The fabrication of submicrometer-scale ordered convex lens array was demonstrated through nanoimprint lithography using an aluminum dimple array mold fabricated by etidronic acid anodizing and selective oxide dissolution. Highly-pure aluminum plates were anodized in a 0.2 M etidronic acid solution at 260 V for the formation of ordered porous alumina. The anodized specimens were immersed in a 0.2 M CrO 3 /0.51 M H 3 PO 4 solution to dissolve the porous alumina, and an ordered dimple array measuring approximately 670 nm in dimple diameter was obtained on the aluminum surface. Phosphonic acid-based self-assembled monolayers (SAMs) were coated on the aluminum dimple array as a release agent for nanoimprint lithography. The shape of the aluminum dimple array mold was transferred to a UV curable photopolymer by nanoimprint lithography. An ordered convex lens array, which corresponded to the negative shape of the dimple array, was successfully obtained by the removal of curable polymer. However, the polymer surface was contaminated with phosphonate molecules due to the multilayered phosphonate films being formed on the aluminum mold. This contamination can be avoided by ultrasonication of the aluminum mold before nanoimprint lithography. [ABSTRACT FROM AUTHOR]

Advanced hard anodic alumina coatings measuring Hv = 610–769 on the Vickers hardness scale were obtained on an aluminum surface via aluminum anodizing using a new electrolyte, etidronic acid. The ordered porous alumina was fabricated by two-step etidronic acid anodizing at 260 V under self-ordering conditions, and pore-widening was carried out to control the porosity of the porous alumina. The Vickers hardness of the ordered porous alumina increased with decreasing diameter of the pores and porosity. Aluminum specimens were also anodized by the constant-current method under various concentrations, temperatures, and current densities. The Vickers hardness increased with decreasing concentration and temperature because chemical dissolution of the anodic oxide during anodizing was suppressed. A hard porous alumina measuring Hv = 610 was obtained by anodizing in a 0.05 M etidronic acid solution at 278 K and 5 Am − 2 . Subsequent thermal treatment caused the dehydration and corresponding hardening of the porous alumina, and a higher porous alumina hardness of Hv = 769 was successfully achieved by thermal treatment at 873 K for 12 h. [ABSTRACT FROM AUTHOR]

Anodizing of aluminum in an arsenic acid solution is reported for the fabrication of anodic porous alumina. The highest potential difference (voltage) without oxide burning increased as the temperature and the concentration of the arsenic acid solution decreased, and a high anodizing potential difference of 340 V was achieved. An ordered porous alumina with several tens of cells was formed in 0.1–0.5 M arsenic acid solutions at 310–340 V for 20 h. However, the regularity of the porous alumina was not improved via anodizing for 72 h. No pore sealing behavior of the porous alumina was observed upon immersion in boiling distilled water, and it may be due to the formation of an insoluble complex on the oxide surface. The porous alumina consisted of two different layers: a hexagonal alumina layer that contained arsenic from the electrolyte and a pure alumina honeycomb skeleton. The porous alumina exhibited a white photoluminescence emission at approximately 515 nm under UV irradiation at 254 nm. [ABSTRACT FROM AUTHOR]

A superhydrophilic aluminum surface covered by numerous alumina nanofibers was fabricated via pyrophosphoric acid anodizing. High-density anodic alumina nanofibers grow on the bottom of a honeycomb oxide via anodizing in concentrated pyrophosphoric acid. The water contact angle on the nanofiber-covered aluminum surface decreased with time after a 4 μL droplet was placed on the surface, and a superhydrophilic behavior with a contact angle measuring 2.2° was observed within 2 s; this contact angle is considerably lower than those observed for electropolished and porous alumina-covered aluminum surfaces. There was no dependence of the superhydrophilicity on the density of alumina nanofibers fabricated via different constant voltage anodizing conditions. The superhydrophilic property of the surface covered by anodic alumina nanofibers was maintained during an exposure test for 359 h. The quick-drying and snow-sliding behaviors of the superhydrophilic aluminum covered with anodic alumina nanofibers were demonstrated. [ABSTRACT FROM AUTHOR]

Self-ordered porous alumina was fabricated via anodizing in an acid salt electrolyte solution, sodium hydrogen sulfate (NaHSO 4 ). High-purity aluminum specimens were anodized in a NaHSO 4 solution under various operating conditions with adjusted concentrations, temperatures, applied voltages, and times. Self-ordering was achieved via NaHSO 4 anodizing at appropriate applied voltages ranging from 20 to 28 V, and ordered cell arrangements with the cell size of 55–77 nm were successfully fabricated. Sulfur atoms originating from the electrolyte anions were incorporated into the ordered porous alumina. A honeycomb distribution consisting of a thick outer layer with a high concentration of sulfur and a very thin inner skeleton with a relatively low concentration sulfur was formed via NaHSO 4 anodizing. Our results suggested that there are now many electrolyte options for the fabrication of self-ordered porous alumina with a wide range of nanosizes. [ABSTRACT FROM AUTHOR]

Ordered porous alumina (OPA) with large-scale circular and hexagonal pores was fabricated via etidronic acid anodizing. High-purity aluminum plates were anodized in 0.2–4.2 M etidronic acid solution at 145–310 V and 288–323 K. Self-ordering of porous alumina was observed at 165 V and 313 K in 4.2 M, at 205 V and 303 K in 1.0 M, and at 260 V and 298 K in 0.2 M, and the cell diameter was measured to be 400–640 nm. The ordering potential difference decreased with the electrolyte concentration increasing. OPA without an intercrossing nanostructure could be fabricated on a nanostructured aluminum surface via two-step anodizing. Subsequent pore-widening in etidronic acid solution caused the circular dissolution of anodic oxide and the expansion of pore diameters to 470 nm. The shape of the pores was subsequently changed to a hexagon from a circle via long-term pore-widening, and a honeycomb structure with narrow alumina walls and hexagonal pores measuring 590 nm in its long-axis was formed in the porous alumina. Transition of the nanostructure configuration during pore-widening corresponded to differences in the incorporated phosphorus distribution originating from the etidronic acid anions. [ABSTRACT FROM AUTHOR]

The formation behavior of anodic alumina nanofibers via anodizing in a concentrated pyrophosphoric acid under various conditions was investigated using electrochemical measurements and SEM/TEM observations. Pyrophosphoric acid anodizing at 293 K resulted in the formation of numerous anodic alumina nanofibers on an aluminum substrate through a thin barrier oxide and honeycomb oxide with narrow walls. However, long-term anodizing led to the chemical dissolution of the alumina nanofibers. The density of the anodic alumina nanofibers decreased as the applied voltage increased in the 10–75 V range. However, active electrochemical dissolution of the aluminum substrate occurred at a higher voltage of 90 V. Low temperature anodizing at 273 K resulted in the formation of long alumina nanofibers measuring several micrometers in length, even though a long processing time was required due to the low current density during the low temperature anodizing. In contrast, high temperature anodizing easily resulted in the formation and chemical dissolution of alumina nanofibers. The structural nanofeatures of the anodic alumina nanofibers were controlled by choosing of the appropriate electrochemical conditions, and numerous high-aspect-ratio alumina nanofibers (>100) can be successfully fabricated. The anodic alumina nanofibers consisted of a pure amorphous aluminum oxide without anions from the employed electrolyte. [ABSTRACT FROM AUTHOR]

The self-ordering behavior of anodic porous alumina that was formed by anodizing in selenic acid electrolyte (H2SeO4) at various concentrations and voltages was investigated with SEM and AFM imaging. A high purity aluminum foil was anodized in 0.1-3.0M selenic acid solutions at 273K and at constant cell voltages in the range of 37 to 51V. The regularity of the cell arrangement increased with increasing anodizing voltage and selenic acid concentration under conditions of steady oxide growth without burning. Anodizing at 42-46V in 3.0M selenic acid produced highly ordered porous alumina. By selective dissolution of the anodic porous alumina, highly ordered convex nanostructures of aluminum with diameters of 20nm and heights of 40nm were exposed at the apexes of each hexagonal dimple array. Highly ordered anodic porous alumina with a cell size of 102nm from top to bottom can be fabricated by a two-step selenic acid anodizing process, that includes the first anodizing step, the selective oxide dissolution, and the second anodizing step. [ABSTRACT FROM AUTHOR]

Abstract: The growth behavior of anodic porous alumina formed via anodizing in a new electrolyte, squaric acid (3,4-dihydroxy-3-cyclobutene-1,2-dione), is reported for the first time. A high-purity aluminum foil was anodized in a 0.1M squaric acid solution at 293K and a constant applied potential of 100-150V. Anodic oxides grew on the aluminum foil at applied potentials of 100-120V, but a burned oxide film was formed at higher voltage. Anodic porous alumina with a cell size of approximately 200–400nm and sub-100-nm-scale pore diameter was successfully fabricated by anodizing in squaric acid. The cell size of the anodic oxide increased with anodizing time because of the uneven growth of the porous layer. The anodic porous alumina obtained by squaric acid anodizing consists of amorphous Al2O3 containing 5-6 at% carbon from the electrolyte. [Copyright &y& Elsevier]

Highlights: [•] A new aluminum bulk micromachining through an anodic oxide mask was developed. [•] Laser irradiation of anodized aluminum results in the formation of a locally exposed aluminum pattern. [•] The anodic oxide acted as a resist mask during electrochemical etching. [•] A hemicylindrical microgroove with a flat surface was fabricated by electrochemical etching. [•] Free-standing oxides formed by an undercutting were easily removed by ultrasonication. [Copyright &y& Elsevier]

Abstract: An anodic alumina microlens array was fabricated by laser irradiation and electrochemical techniques. An aluminum specimen covered with a porous oxide film was irradiated with a pulsed Nd-YAG laser, and then electropolished to dissolve the aluminum substrate. A well-defined semi-elliptical micropore was formed on the aluminum by electropolishing. The immersion of the electropolished specimen in a CrO3/H3PO4 solution resulted in the dissolution of the remaining anodic oxide film. Subsequent re-anodizing enabled the formation of a characteristic meniscus-shaped oxide film on the micropore. A microlens array made of the thin anodic alumina film, which showed flexibility and heat resistance, was successfully fabricated by the lift-off of the anodic oxide. [Copyright &y& Elsevier]

• An aluminum surface was covered with numerous aluminum oxide nanowires by anodizing. • The nanowire morphology was precisely controlled by a two-step anodizing process. • Superhydrophobic aluminum surfaces were fabricated with self-assembled monolayers. • The slippery behavior can be controlled by the morphology of nanowires. • Opposite slipping behaviors were demonstrated by the water dropping experiments. The influence of the nanomorphology of alumina nanowires (ANWs) fabricated by a two-step anodizing process on the superhydrophobicity was investigated through advancing contact angle (ACA) and receding contact angle (RCA) measurements. Aluminum nanobowl specimens were anodized in pyrophosphoric acid to fabricate an ordered ANW structure with an average diameter of 7.1 nm, and the outermost surface of the ANWs was chemically bonded with fluorinated self-assembled monolayers. The growing ANWs bent immediately their own weight, and pyramidal ANW structures were formed as they joined the surrounding nanowires together. The ACA value increased with the number density of pyramidal ANW structures due to the reducing area fraction of ANWs, and an increased superhydrophobicity with a contact angle of approximately 165° was measured on the low-density pyramidal structure with a density of 8.1 × 1011 m−2. Additional anodizing led to complete nanowire bending; thus, the advancing contact angle decreased. The pyramidal nanowire structure exhibited a large slipping property with a contact angle hysteresis (CAH) < 10°, whereas the bent nanowire structure exhibited a decreased slipping property with a CAH > 100°. Superhydrophobic surfaces with opposite water slipping properties were demonstrated by a water dropping experiment. [ABSTRACT FROM AUTHOR]

The anodizing of aluminum in an alkaline sodium tetraborate (Na 2 B 4 O 7) solution was investigated with respect to the nanostructural characterization of anodic porous alumina. Electropolished aluminum specimens were galvanostatically anodized under various conditions in 0.1–0.5 M sodium tetraborate solutions at 293–353 K and a current density of 2.5–400 A m−2. Anodic oxide with numerous flower-like defects was formed by anodizing in a 0.1 M Na 2 B 4 O 7 solution due to the film breakdown with continuous visible sparking. On the other hand, a uniform porous alumina film without any breakdown was successfully obtained by anodizing a more concentrated solution than 0.3 M at 333 K. The anodic oxide was almost pure alumina and consisted of a thin outer layer with numerous small pits and a thick inner layer with typical porous alumina cells. The pore walls possessed continuous bumpy surfaces measuring 10–20 nm in roughness. As the temperature further increased to 353 K, the regularity of the porous alumina improved due to the high current density of more than 150 A m−2 during anodizing. Slippery superhydrophobic and sticky superoleophobic aluminum surfaces could be easily fabricated via high temperature anodizing and subsequent self-assembled monolayer modification. Unlabelled Image • Pure Al plates were galvanostatically anodized in an alkaline sodium tetraborate solution. • Porous alumina was formed in a high concentration solution at high temperature. • The pore walls possessed continuous bumpy surfaces. • The anodic oxide consisted of almost pure alumina. • The regularity was improved by the high current density anodizing at high temperature. [ABSTRACT FROM AUTHOR]