Fabrication of Self-Ordered Porous Alumina via Etidronic Acid Anodizing and Structural Color Generation from Submicrometer-Scale Dimple Array

Abstract

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...

Characterization of nanopores arrangement of anodic alumina layers synthesized on low-(AA1050) and high-purity aluminum by two-step anodizing in sulfuric acid with addition of ethylene glycol at low temperature

“…Depending on the anodization electrolyte, it is possible to obtain AAO membrane with cell diameter in the range of 25-75, 75-125, 375-525 nm in sulphuric, oxalic and phosphoric acid solution as electrolyte, respectively. According to research published recently, it is possible to extend this range up to 530-670 nm in cell diameter by using etidronic aqueous solution as novel self-ordering electrolyte and anodizing Al foils at voltage in the range 210-270 V at 0-60°C [20]. What is more, additions of various modifiers to well know electrolytes [21] or to the new ones are recently reported in numerous papers [20,[22][23][24].…”

“…Typically, the two-step anodization of aluminium is conducted in sulfuric [14,15], oxalic [16,17] and phosphoric [18,19] acid aqueous solution at voltages in the range of [15][16][17][18][19][20][21][22][23][24][25] and 120-195 V, respectively. Depending on the anodization electrolyte, it is possible to obtain AAO membrane with cell diameter in the range of 25-75, 75-125, 375-525 nm in sulphuric, oxalic and phosphoric acid solution as electrolyte, respectively.…”

Manufacturing of highly ordered porous anodic alumina with conical pore shape and tunable interpore distance in the range of 550 nm to 650 nm

“…1. Anodization of aluminum in etidronic acid electrolyte at various conditions was studied in detail in the literature [27]. The anodization starts at lower voltage (80 V in our case) and the voltage is immediately increased to a target value within 2 min to 3 min.…”

“…Since the heights of pillars are very important at longer wavelength region, the larger area of the cone base makes it possible to obtain better graduation of refractive index along the pore walls for relatively high pores. Self-ordered PAA with D c > 500 nm and parallel pores (constant pore diameter throughout the PAA thickness) have previously been synthesized [27][28][29][30].…”

Morphological and chemical characterization of highly ordered conical-pore anodic alumina prepared by multistep citric acid anodizing and chemical etching process

“…Etidronic acid anodizing was conducted according to the approach presented elsewhere [5,6]. In this approach, the anodization starts at relatively low voltage (80 V) which is subsequently increased to a target value.…”