Anodic porous alumina has been widely investigated and used as a nanostructure template in various nanoapplications. The porous structure consists of numerous hexagonal cells perpendicular to the aluminum substrate and each cell has several tens or hundreds of nanoscale pores at its center. Because the nanomorphology of anodic porous alumina is limited by the electrolyte during anodizing, the discovery of additional electrolytes would expand the applicability of porous alumina. In this study, we report a new self-ordered nanoporous alumina formed by selenic acid (H2SeO4) anodizing. By optimizing the anodizing conditions, anodic alumina possessing 10-nm-scale pores was rapidly assembled (within 1 h) during selenic acid anodizing without any special electrochemical equipment. Novel sub-10-nm-scale spacing can also be achieved by selenic acid anodizing and metal sputter deposition. Our new nanoporous alumina can be used as a nanotemplate for various nanostructures in 10-/sub-10-nm-scale manufacturing.

Highly ordered anodic porous alumina with a large-scale cell diameter was successfully fabricated via anodizing in a new electrolyte, etidronic acid (1-hydroxyethane-1,1-diphosphonic acid). High-purity aluminum specimens were anodized in a 0.3 M etidronic acid solution under constant current density and voltage conditions. Etidronic acid anodizing at 210 to 270 V at the appropriate temperature caused the anodic porous alumina to exhibit self-ordering behavior, and periodic nanostructures measuring 530 to 670 nm in cell diameter were fabricated on the aluminum substrate. The self-ordering voltage and the corresponding cell diameter could be increased without burning by systematically increasing the stepwise voltage. Two-step etidronic acid anodizing without nanoimprinting can easily yield the formation of highly ordered anodic porous alumina with a large-scale cell diameter. A submicrometer-scale dimple array fabricated via etidronic acid anodizing and subsequent selective oxide dissolution gave rise to bright structural color with a rainbow distribution.Keywords: Anodizing; Etidronic Acid; Anodic Porous Alumina; Self-Ordering; Structural Color 3 IntroductionAnodizing aluminum and its alloys in acidic electrolyte solutions results in the formation of porous anodic oxide films (anodic porous alumina) with numerous nanometer-scale pores [1][2][3]. Most importantly, anodic porous alumina is self-ordered when anodized under the appropriate electrochemical conditions in a given acidic solution, and consequently, high-aspect-ratio anodic porous alumina with an ideal cell arrangement can be easily obtained [4][5][6][7][8][9]. In the self-ordering anodizing, it is well known that the cell diameter of the anodic porous alumina, D, is strongly related to the self-ordering voltage, U s : a) D = 50-60 nm at U s = 19-25 V for sulfuric acid [10,11], b) 100 nm at 40 V for oxalic acid [4,11,12] [35][36], and magnetic nanomaterials [37].Because the cell diameter of self-ordered porous alumina fabricated by typical anodizing in aqueous solutions is limited to approximately 500 nm at 200 V [5,18], larger cell diameters that correspond to visible-light wavelengths ranging between 500 nm and 800 nm are required to expand the applicability of porous alumina in the field of optics. Malic acid anodizing provides a relatively high anodizing voltage and, consequently, a larger cell diameter, D = 300-800 nm at 200-350 V [38,39]. However, to date, ordered anodic porous alumina has not been obtained by malic acid anodizing. As an alternative electrolyte for high-voltage anodizing, citric acid has also been investigated by several research groups [39][40][41][42]. Citric acid anodizing operates over an anodizing voltage range of approximately 260-540 V. However, non-uniform black oxides are easily formed by a phenomenon called "burning" during citric acid anodizing because of localized breakdown under high electric fields, and it is difficult to obtain high-aspect-ratio ordered porous alumina. Therefore, ethylene glycol mixture soluti...

The self-ordering behavior of anodic porous alumina that was formed by anodizing in selenic acid electrolyte (H 2 SeO 4 ) at various concentrations and voltages was investigated with SEM and AFM imaging. A high purity aluminum foil was anodized in 0.1-3.0 M selenic acid solutions at 273 K and at constant cell voltages in the range of 37 to 51 V. 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 42-46 V in 3.0 M selenic acid produced highly ordered porous alumina. By selective dissolution of the anodic porous alumina, highly ordered convex nanostructures of aluminum with diameters of 20 nm and heights of 40 nm were exposed at the apexes of each hexagonal dimple array. Highly ordered anodic porous alumina with a cell size of 102 nm 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.Key words: Aluminum; Anodizing; Anodic Porous Alumina; Selenic Acid IntroductionBarrier anodic oxide films and porous anodic oxide films on aluminum have been widely investigated by many researchers in the fields of surface finishing [1][2][3], electrolytic capacitor application [4][5][6], and micro-and nano-structure fabrication [7][8][9]. Recently, highly ordered anodic porous alumina with a cell size on the scale of 10-100 nm has been studied for potential use in various ordered-nanostructure applications [10][11][12][13][14][15][16][17][18][19][20][21]. Anodic porous alumina is typically fabricated on an aluminum substrate using electrochemical anodizing (or anodization) [22][23][24][25][26]. In several acidic electrolyte solutions, the porous alumina fabricated by anodizing is self-ordered when prepared at the appropriate electrochemical conditions, including appropriate concentrations, temperatures, and voltages (or electrochemical potentials) [27][28][29][30] [50], and acetylenedicarboxylic (HOOC-C≡C-COOH) [51] acids have also been reported as electrolytes used to fabricate porous alumina that has characteristic nanostructure morphologies.In addition to these acidic electrolytes, alkaline and neutral solutions used for porous alumina fabrication were reported by several research groups. Takahashi et al. reported that a porous anodic oxide film could be formed by anodizing in an H 3 BO 3 /Na 2 B 4 O 7 neutral borate solution at a high temperature [52]. Baron-Wiechec et al. investigated aluminum anodizing in a borax (Na 2 B 4 O 7 ) solution at 333 K and successfully obtained porous alumina [53]. Noguchi et al. investigated the anodizing behavior of aluminum in propylenediamine and choline alkaline solutions containing ammonium fluoride, ammonium tartrate, ammonium carbonate, and ammonium tetraborate, and obtained anodic porous alumina with nanopores that were 10-150 nm in diameter [54]. However, it is difficult to form a highly ordered porous alumina by anodiz...

Anodizing of aluminum and its alloys is widely investigated and used for corrosion protection, electronic devices, and micro-/nanostructure fabrication. Anodizing of aluminum in acidic solutions causes formation of porous aluminum oxide films, which consists of numerous hexagonal cells perpendicular to the aluminum substrate, and each cell has nanoscale pores at its center. Recently, highly ordered porous aluminum oxide has been widely investigated for various novel nanoapplications. In this review article, we introduce the fundamentals of anodic oxide films including barrier and porous oxides. Then, we summarize the electrolyte species used so far for porous oxide fabrication and describe the self-ordering conditions during anodizing in these electrolyte solutions. Fabrication of highly ordered porous oxides through the vertical section can be achieved by a two-step anodizing and nanoimprint technique. Various nanoapplications based on the ordered porous oxide are also introduced.

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.