Fabrication of Anodic Porous Alumina by Squaric Acid Anodizing

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 0.1 M squaric acid solution at 293 K and a constant applied potential of 100-150 V. Anodic oxides grew on the aluminum foil at applied potentials of 100-120 V, but a burned oxide film was formed at higher voltage. Anodic porous alumina with a cell size of approximately 200-400 nm 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 Al 2 O 3 containing 5-6 at% carbon from the electrolyte. Fig. 1). The morphology of anodic porous alumina, especially its cell size (identical meaning to interpore distance) and pore diameter in the porous layer, is limited by the electrolyte and voltage (electrochemical potential) applied during anodizing [23,24]. Fig. 1 summarizes the anodizing electrolytes that have been reported to date for porous alumina fabrication. Sulfuric (H 2 SO 4 ), oxalic ((COOH) 2 ), and phosphoric (H 3 PO 4 ) acids are well-known, self-ordering anodizing solutions for porous alumina fabrication, and highly ordered porous alumina can be obtained by anodizing in these solutions at constant voltages of 18-25 V, 40 V, and 160-195 V, respectively [24-29]. Malonic acid (HOOC-CH 2 -COOH) and tartaric acid (HOOC-(CHOH) 2 -COOH), typical dicarboxylic acids, have also been reported to support the fabrication of highly ordered porous alumina fabrication at 120 V and 195 V, respectively [30,31]. In addition, several other electrolytes, such as chromic (H 2 CrO 4 ) [32,33], malic (HOOC-CH(OH)-CH 2 -COOH) [23,34], citric (HOCO-CH 2 -C(OH)(COOH)-COOH)) [23], and glycolic (CH 2 OH-COOH) [23] acids, have been reported as anodizing electrolytes to date. However, the formation of thick porous alumina with straight nanopores by anodizing in chromic acid is difficult because of branching and colony-forming nanopores [35]. Although citric acid anodizing has already been used for nanostructure fabrication [36], other organic electrolytes, including malic and glycolic acid solutions, cause the formation of non-uniform anodic porous alumina [23,34]. Therefore, five self-ordering electrolytes, namely, sulfuric, oxalic, and phosphoric acids, are generally selected for anodic porous alumina fabrication by many researchers.In recent years, new anodizing procedures using an effective experimental approach and optimal anodizing conditions have been reported by several researchers to control the nano-scale features of anodic porous alumina. Lee et al. studied the rapid formation of ordered porous alumina by a hard anodizing procedure involving a powerful cooling stage and obtained an ultra-high aspect ratio (>1,000) of anodic porous alumina by anodizing in oxalic acid at 120-150...

Fabrication of anodic porous alumina via anodizing in cyclic oxocarbon acids

“…Therefore, croconic acid anodizing is also useful for surface finishing and nanostructuring applications. Based on the experimental results obtained in our previous study [54] and those obtained in this work, the cyclic oxocarbon acids, squaric, croconic, and rhodizonic acid, form suitable electrolytes for anodic porous alumina fabrication. Thus, heptagonic acid (Fig.…”

“…Very recently, we discovered a new organic electrolyte for anodic porous alumina fabrication, squaric acid (3,4-dihydroxy-3-cyclobutene-1,2-dione) [54], which is different from the carboxylic acids described above. Squaric acid is currently the most popular cyclic oxocarbon acid used in organic chemistry and is a strong diacid composed of two acidic hydroxyl groups with low dissociation constants of pKa 1 = 0.5 and pKa 2 = 3.5 [55].…”

“…Squaric acid is currently the most popular cyclic oxocarbon acid used in organic chemistry and is a strong diacid composed of two acidic hydroxyl groups with low dissociation constants of pKa 1 = 0.5 and pKa 2 = 3.5 [55]. Squaric acid anodizing is performed at voltages of 100-150 V under typical anodizing conditions [54]. An anodic porous alumina film with micrometer-scale thickness, submicron-scale-cell size, and sub-100-nm-scale pore diameter can be successfully obtained by squaric acid anodizing.…”

Porous Aluminum Oxide Formed by Anodizing in Various Electrolyte Species

“…Cyclic oxocarbonic acids including squaric acid with a four-membered ring, croconic acid with a five-membered ring, and rhodizonic acid with a six-membered ring have recently been reported as useful electrolytes for the formation of porous oxide films [87,88]. The porous oxide films can be obtained under the following electrochemical conditions: squaric acid at 100-120 V, croconic acid at 90-125 V, and rhodizonic acid at 80-160 V. The growth behavior of the porous oxide formed using these oxocarbonic acids is still unknown, and it is interesting to understand the growth behavior for the formation mechanism of porous oxides, although these organic electrolytes are expensive and not suitable for industrial applications.…”

Growth behavior of anodic oxide formed by aluminum anodizing in glutaric and its derivative acid electrolytes

“…The ideal cell arrangement of porous alumina can be achieved via sulfuric, selenic, and phosphoric acid anodizing, whereas poorly ordered porous alumina is fabricated via chromic acid anodizing [22,23]. Cyclic oxocarbonic acids such as squaric [24], croconic, and rhodizonic[25] acid were very recently determined to be suitable organic electrolytes for fabricating porous alumina, although the details of the growth behavior are still unknown. Carboxylic acids can also be employed as suitable electrolytes for fabricating porous alumina, including oxalic [26,27] Typical dicarboxylic acids and their corresponding acid dissociation constants, pKa 1 and pKa 2 , are summarized in Table 1[ [40][41][42].…”