Improved two-step anodization technique for ordered porous anodic aluminum membranes

https://doi.org/10.1016/j.jelechem.2011.02.008Get rights and content

Abstract

We report on an improved two-step anodization technique through combining the first hard anodization in C2H2O4 with the second mild anodization in H3PO4, which successfully overcome the drawbacks of irregular top surfaces in the conventional two-step hard anodization in C2H2O4 and disordered pore arrays in the two-step mild anodization in H3PO4. The key success of our method is the strong guidance effect of the first hard anodization on the second mild anodization. Highly-ordered (both top and bottom surfaces) porous anodic aluminum (PAA) membranes with interpore spacing from 220 to 350 nm have been realized under anodizing voltages from 100 to 150 V. The interpore spacing is only determined by the first anodizing voltage, while the pore diameter can be manipulated in the second step by adding ethanol in the H3PO4 electrolyte, changing the H3PO4 bath temperature, and altering the second anodizing voltage. The bath temperature for the steady growth of ordered structures can be expanded up to 20 °C, from which the average activation energy can be yielded. The present novel two-step anodization approach is simple, efficient, and cost-effective. It expands the self-ordering regime of PAA membranes, which is of great value for applications in diverse areas of nanotechnology.

Highlights

► Ordered PAA membranes (Dint from 220 to 350 nm) have been fabricated. ► Dint depends linearly only on the first anodizing voltage (U1). ► Activation energy (Ea) is yielded from the growth rate versus temperature. ► Addition of ethanol decreases growth rate and pore opening. ► Decoupled second voltage influences pore opening with constant Dint.

Introduction

While the anodic aluminum has been used as anticorrosion or decoration coating to improve the mechanical properties of aluminum, the porous anodic aluminum (PAA) is now widely employed in nanoscience and nanotechnology as membrane for its self-ordered uniform cylindrical pore size and pore density. PAA has been extensively used as template, mask, or host materials to synthesize various nanostructures in the form of nanopores [1], [2], nanowires [3], nanotubes [4], [5], and nanodots [6], which are all functional materials for developing nanoscale devices. Besides, PAA has also been employed in the field of studies on fluidic dynamics in nanoholes [7], [8], separation filters [9], DNA translations [10], and photonic crystals [11]. These versatile applications benefit from the special physical characteristics of the PAA films, which can be fabricated through relatively simple anodization processes.

However, problems still exist in the current fabrication techniques of the PAA films. In general, there are two typical methods for the fabrication of PAA films: a pre-patterned anodization [12] and self-organized two-step anodization [1]. The pre-patterned anodization leads to an ideal hexagonal, square, and triangle arrangement of pores in the final structure [13], and the size of mold/stamp used for aluminum pre-patterning can be designed by the need of PAA. Nevertheless, the preparation of the master mold is time consuming and is usually based on expensive lithography techniques [12]. On the other hand, the two-step anodization is much simple and low-cost. But regular self-ordered pore structures occur only in quite small processing windows. The difficulty lies in the limited structural features of PAA such as pore diameter (DP), interpore spacing (Dint), and regularity, which strongly depend on the chosen electrolyte and anodizing conditions. Self-ordered pore formation can be obtained for three major regimes at the following anodizing voltages: 25 V at H2SO4 for Dint = 60 nm (DP = 22 nm), 40 V at C2H2O4 for Dint = 100 nm (DP = 45 nm), and 195 V at H3PO4 for Dint = 500 nm (DP = 176 nm) [1], [14], [15], [16], [17]. When the anodization of aluminum is carried out outside those self-ordering regimes, the degree of PAA regularity will decrease significantly.

It is therefore essential to form well-ordered PAA films with a wide range of pore diameter and interpore spacing, which can be manipulated in various ways. The importance of high electric field for self-ordering of anodic porous alumina has been noticed and high electric field can induce self-ordering pore arrays [18]. Hard anodization has been proposed to expand the ordered regime in H2SO4 at 27–80 V for Dint from 72 to 145 nm (DP from 22 to 50 nm) [19], and in C2H2O4 at 100–150 V for Dint from 220 to 300 nm (DP from 40 to 60 nm) [20]. Hard anodization in H3PO4 is different from those in H2SO4 and C2H2O4, which maintains the anodizing potential at a self-ordering voltage of 195 V while modulating the current density (1500–4000 A/m2) to obtain highly-ordered pore arrays [21]. What is more, highly-ordered cell arrangements of porous alumina films were realized in malonic acid at 120 V and tartaric acid at 195 V having 300 nm and 500 nm pore intervals, respectively [22].

However, the PAA films obtained in H2SO4 using hard anodization have poor mechanical properties because of a high density of cracks and structural defects, which are not stable enough for real-life application [19]. Hard anodization in C2H2O4 results in PAA films with self-ordered array of uniformly sized parallel channels with cylindrical shape closed at the pore bottoms [20], where irregular top surface with “pore in pore” structures [23] cannot be exclusive in C2H2O4. To obtain ordered arrays with both regular top and bottom surfaces, the irregular top surface needs to be detached by ion beam polishing which is time consuming and difficult to control.

We find alternating the electrolyte type in the second anodization can reduce the current density to be small enough to avoid the irregular top surface, where the self-arrangement effect is weak and the guidance effect of the first anodization will be the leading factor to keep ordered pore arrays [24]. On that basis, we propose an improved two-step anodization, taking advantages of hard anodization for high growth rate and ordered bottom pore arrays in C2H2O4 for the first step and mild anodization for regular top arrays in H3PO4 for the second one. We have demonstrated that well-ordered PAA films with Dint from 220 to 350 nm (DP from 90 to 250 nm without pore broadening after pore formation) can be fabricated at anodizing voltages of 100–150 V, taking advantage of the guidance effect of the first hard anodization for the ordered pore arrangement in the second mild anodization of H3PO4, and working temperature as high as 20 °C to enhance the growth rate. We have carried out a detailed investigation on the effects of ethanol percentage, bath temperature, and decoupled anodizing voltages on the formation of PAA films, from which three easy methods to manipulate the pore diameter/interpore spacing have been found.

Section snippets

Materials

High purity (99.999% purity) aluminum foils with a thickness of 0.2 mm were used as the starting material. Prior to anodizing a piece of round aluminum foil with 20 mm diameter was degreased in acetone and rinsed in distilled water. A working surface of the sample was 1.5 cm2 and the rest of aluminum plate was insulated.

PAA films formation under conventional two-step approach

For the normal two-step anodization, the first anodization was carried out in 0.3 M C2H2O4 with a 1:4 volume mixture of ethanol and water as solvent at −5 °C, 140 V for 20 min or 5 wt.%

Limitation of the conventional two-step approach

Fig. 1 presents the FE-SEM results and the corresponding current density–time characteristics of the PAA films fabricated under the normal two-step of both the first and the second anodization in the same C2H2O4 (a–c) or H3PO4 (d–f) electrolytes. Although the PAA films obtained by the traditional two-step anodization in C2H2O4 have uniform and ordered bottom surfaces and cross-sections (Fig. 1a), there are disordered top surfaces with special “pore in pore” structures as demonstrated in Fig. 1

Improved two-step approach

Fig. 2a–c shows the schematic diagram of the proposal with the first and second anodization in different electrolytes. An alumina membrane was formed with small porosity, irregular top surface but ordered bottom arrays and cross-sections (Fig. 2a and d) by the first anodization in C2H2O4. The preformed oxide was removed leaving ordered concaves on aluminum (Fig. 2b and e). The second anodization was carried out in H3PO4 on the basis of these concave structures where ordered pore arrays were

Conclusions

In this study, we have addressed the limitation of the PAA membranes with interpore spacing from 220 to 300 nm yielded by the conventional two-step anodization approach [20], where hard anodization in C2H2O4 results in irregular top surface with “pore in pore” structure and mild anodization in H3PO4 leads to pore arrays with both irregular top and bottom surface. An improved two-step anodization technique has been proposed taking advantages of the first hard anodization in C2H2O4 and second mild

Acknowledgements

This work was supported by the National Major Basic Research Project of 2010CB933702 and the Natural Science Foundation of China under Contract 10734020.

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