Self-Ordered Anodic Alumina with Continuously Tunable Pore Intervals from 410 to 530 nm

Abstract

We report a "mild anodization" (MA) process using aluminum oxalate (Alox) as an additive to suppress breakdown of porous anodic alumina (PAA) in the electrolyte of phosphoric acid at high potentials and comparatively high temperatures. It is shown for the first time that continuously tunable pore intervals (D(int)) from 410 to 530 nm with ordered hexagonal pore arrangement can be controlled by varying the concentrations of phosphoric acid and Alox at anodization voltages (U(a)) from 180 to 230 V, far beyond the U(a) in the single electrolyte of phosphoric acid or oxalic acid. The fabricated PAA films are uniform without any burning spots, and the anodization temperature can be increased to 10-20 degrees C with a much higher growth rate of PAA films than that at a low temperature. Meanwhile, a typical two-step anodization process could also be performed under our conditions. Our results could not only extend the applications of PAA templates but also facilitate understanding of the effects of anions in the process of anodic oxidation.

Mechanisms of Nanoporous Alumina Formation and Self-organized Growth

“…It was found that higher U, larger J, and faster AAO growth rate are the typical characteristics of Fig. 1.13 The D int -U curve of MA processes in commonly used electrolyte systems: the data marked by green squares were reported in [24,26,39,42], by orange circles were reported in [44], by red circles were reported in [22,24,26,40,46,47], by blue triangles were reported in [26,41,45] conventional HA processes. However, although various HA processes have been investigated by industry, the cell uniformity and pore ordering of the resulting porous AAO have been unsatisfactory for their applications in nanotechnology [51,52] 1-10°C) have been realized by using a pre-anodized electrolyte.…”

“…To obtain larger D int , Sun et al proposed a phosphoric acid based MA process, an additive of aluminum oxalate (Alox) was applied to suppress burning or breakdown of porous AAO membranes during anodization under high U. Using this method, highly ordered porous AAO membranes with continuously tunable D int from 410 to 530 nm can be obtained by varying U from 180 to 230 V [45]. Moreover, considering that highly ordered porous AAO can be obtained in oxalic acid electrolyte, other organic acid electrolytes have been tried in MA processes.…”

Ordered arrays of polymeric nanopores by using inverse nanostructured PTFE surfaces

“…Ordered AAO templates were prepared by a two-step electrochemical anodization of aluminum [13,37]. An aqueous solution of phosphoric acid (1 wt.…”

“…The temperature was carried out at 4.5 ºC for 6 h. Aluminum oxalate was also added to the solution (0.01 M) for stabilizing the reaction as reported elsewhere [37]. Then, the first anodic layer was removed by chemical etching in a mixture of phosphoric acid (7 wt.…”

Electrochemical methods for template-assisted synthesis of nanostructured materials

“…Therefore, the use of the HA processes provides extra degrees of freedom for the manufacture of patterned NAMs with tunable geometries. The HA technique has been successfully performed in several electrolytes comprising sulphuric (Chu et al, 2005) and oxalic acids (Lee et al, 2006a), together with mixtures of these acidic electrolytes (Li et al, 2007;Almasi-Kashi et al, 2007), which contained different additives such as ethanol (Li et al, 2006(Li et al, , 2007 and Al sulfate or oxalate (Sun et al, 2010) to stabilize the anodization process and avoid the dielectric breakdown and burning of the HA alumina membranes caused by the high current densities (about 175 mA/cm 2 ) achieved during these processes. Alternatively, a preanodization step under MA conditions, followed by a linear sweep of the anodization voltage to reach potentiostatic HA conditions, is sometimes used as a strategy to circumvent dielectric breakdown phenomena (Lee et al, 2006a;Santos et al, 2011a).…”