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High-aspect ratio ordered nanomaterial arrays exhibit several unique physicochemical and optical properties. Porous anodic aluminum oxide (AAO) is one of the most typical ordered porous structures and can be easily fabricated by applying an electrochemical anodizing process to Al. However, the dimensional and structural controllability of conventional porous AAOs is limited to a narrow range because there are only a few electrolytes that work in this process. Here, we provide a novel anodizing method using an alkaline electrolyte, sodium tetraborate (Na2B4O7), for the fabrication of a high-aspect ratio, self-ordered nanospike porous AAO structure. This self-ordered porous AAO structure possesses a wide range of the interpore distance under a new anodizing regime, and highly ordered porous AAO structures can be fabricated using pre-nanotexturing of Al. The vertical pore walls of porous AAOs have unique nanospikes measuring several tens of nanometers in periodicity, and we demonstrate that AAO can be used as a template for the fabrication of nanomaterials with a large surface area. We also reveal that stable anodizing without the occurrence of oxide burning and the subsequent formation of uniform self-ordered AAO structures can be achieved on complicated three-dimensional substrates. [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]

• A chemical-resistant anodic aluminum oxide (AAO) was formed by anodizing in sodium tetraborate. • The AAOs were obtained in five typical acidic electrolytes and also in sodium tetraborate. • These anodized specimens were immersed in acidic H 3 PO 4 and alkaline NaOH aqueous solutions. • The AAO formed in sodium tetraborate exhibited the highest chemical resistance. • This can be attributed to their higher anodizing ratio and the formation of high-purity alumina. A porous anodic aluminum oxide (AAO) possessing higher chemical resistance in both acidic and alkaline solutions was fabricated by anodizing Al in an alkaline sodium tetraborate solution. The 5 N Al plates were anodized in five types of major acidic solutions (sulfuric, oxalic, phosphoric, chromic, and etidronic acids) and in alkaline sodium tetraborate solution to form AAOs. The anodized specimens were then immersed in a 0.52 M phosphoric acid solution (pH = 1.3) and a 2.5 M sodium hydroxide solution (pH = 13.8). Subsequently, the electrochemical impedance measurements and morphological characterizations of the AAOs before and after immersion were investigated. The time required for the complete dissolution of the AAOs formed in typical acidic electrolytes, including sulfuric, oxalic, phosphoric, and etidronic acids, increased linearly with the anodizing voltage. The AAO formed in chromic acid exhibited a higher chemical resistance than this linear relationship owing to the formation of high-purity alumina without incorporated anions. Moreover, the AAO formed in sodium tetraborate exhibited the highest chemical resistance in both acidic and alkaline solutions – approximately two times higher than that formed in etidronic and chromic acids. This can be attributed to their higher anodizing ratio and the formation of a high-purity alumina layer. [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]

Self-ordered periodic porous alumina with an undiscovered cell diameter was fabricated via electrochemical anodizing in a new electrolyte, phosphonic acid (H 3 PO 3 ). High-purity aluminum plates were anodized in phosphonic acid solution under various operating conditions of voltage, temperature, concentration, and anodizing time. Phosphonic acid anodizing at 150–180 V caused the self-ordering behavior of porous alumina, and an ideal honeycomb nanostructure measuring 370–440 nm in cell diameter was successfully fabricated on the aluminum substrate. Conversely, disordered porous alumina grew at below 140 V, and anodizing at above 190 V caused local thickening due to oxide burning. Two-step phosphonic acid anodizing allows the fabrication of high aspect ratio ordered porous alumina. HPO 3 2− anions originated from the electrolyte were incorporated into the porous oxide during anodizing. Consequently, a double-layered porous alumina consisting of a thick outer layer containing incorporated HPO 3 2− anions, and a thin inner layer without anions was constructed via phosphonic acid anodizing. [ABSTRACT FROM AUTHOR]

Anodic oxide fabricated by anodizing has been widely used for nanostructural engineering, but the nanomorphology is limited to only two oxides: anodic barrier and porous oxides. Therefore, the discovery of an additional anodic oxide with a unique nanofeature would expand the applicability of anodizing. Here we demonstrate the fabrication of a third-generation anodic oxide, specifically, anodic alumina nanofibers, by anodizing in a new electrolyte, pyrophosphoric acid. Ultra-high density single nanometer-scale anodic alumina nanofibers (1010 nanofibers/cm2) consisting of an amorphous, pure aluminum oxide were successfully fabricated via pyrophosphoric acid anodizing. The nanomorphologies of the anodic nanofibers can be controlled by the electrochemical conditions. Anodic tungsten oxide nanofibers can also be fabricated by pyrophosphoric acid anodizing. The aluminum surface covered by the anodic alumina nanofibers exhibited ultra-fast superhydrophilic behavior, with a contact angle of less than 1°, within 1 second. Such ultra-narrow nanofibers can be used for various nanoapplications including catalysts, wettability control, and electronic devices. [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: [•] 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]

Abstract: Nine kinds of Al alloys were anodized to form porous anodic oxide films, and then anodized specimens were immersed in 2-(2-(2-methoxyethoxy)ethoxy)ethanol (MEEE) and 2-(2-(2-butoxyethoxy)ethoxy)ethanol (BEEE) at 415K. Al–1.0%Sn–1.0%Bi alloy was corroded severely in both BEEE and MEEE, whereas other eight alloys showed no corrosion. The corrosion proceeded under the anodic oxide films through cracks formed in the film. Cathodic polarization in Cu electroplating solution after corrosion suggested that the crack formation during immersion in hot MEEE is due to thermal expansion of the substrate and Sn and Bi containing particles included in the anodic oxide film. [Copyright &y& Elsevier]

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. [Display omitted] [ABSTRACT FROM AUTHOR]

Nickel micro-pattern was fabricated on an insulating board via anodizing aluminum, laser irradiation, nickel electroplating, insulating board attachment and aluminum substrate dissolution. Aluminum specimens covered with porous type oxide films were irradiated with a pulsed Nd–yttrium aluminum garnet laser through a convex lens with 10 mm focal length in a nickel electroplating solution to remove the anodic oxide film. The oxide film was destructed with minimum 6 μm width at the focal position. After laser irradiation, a nickel metal layer was electrodeposited at only the laser irradiated area. The specimen was attached on an epoxy resin before dissolving aluminum substrate. A nickel micro-pattern with a 9 μm line width and 12 μm intervals was obtained on the insulating board. [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]

• 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]

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]

The fabrication of superhydrophilic and superhydrophobic aluminum alloys was achieved by pyrophosphoric acid anodizing and SAM (self-assembled monolayer) modification. The anodizing of three kinds of aluminum alloys, including 3004, 1N30, and 8021, in a concentrated pyrophosphoric acid solution resulted in the formation of numerous anodic alumina nanofibers. In addition, insoluble intermetallic compounds contained in the alloy matrix were exposed to the surface with increasing anodizing time, and nanofiber-tangled intermetallic particles also formed on the surface during anodizing. These anodized aluminum alloys exhibited a superhydrophilic behavior measuring less than 4° in the contact angle, and this superhydrophilicity was maintained via the long-term anodizing process. The nanofiber-covered aluminum alloys were immersed in fluorinated phosphonic acid SAM/ethanol solutions, thereby modifying SAMs on the anodic alumina nanofibers. The contact angle of the SAM-modified aluminum alloys increased with the immersion time and temperature of the SAM solution, and the surface was drastically shifted to superhydrophobicity, measuring more than 150°, from superhydrophilicity. However, exceeding 10 min in the anodizing process caused the contact angle to decrease and the gradual disappearance of hydrophobicity due to the formation of many hydrophilic intermetallic particles on the surface. The short-term pyrophosphoric acid anodizing and subsequent SAM modification are useful for the formation of various superhydrophilic and superhydrophobic aluminum alloys. [ABSTRACT FROM AUTHOR]

[Display omitted] • Aluminum plates were anodized in phosphoric acid and six phosphate solutions. • The current density increased with pH getting farther from neutral. • A typical porous alumina film was formed in acidic solutions. • Flat alumina films with and without a narrow porous layer were obtained in near-neutral solutions. • Alkaline solutions caused feathering of the pore wall and flattening of the barrier layer. 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. [ABSTRACT FROM AUTHOR]