# Porous Alumina Films with Width-Controllable Alumina Stripes

- Kai Huang
^{1}Email author, - Shi-Ming Huang
^{2}, - Lin Pu
^{2}, - Yi Shi
^{2}, - Zhi-Ming Wu
^{1}, - Li Ji
^{1}and - Jun-Yong Kang
^{1}

**Received: **8 June 2010

**Accepted: **5 August 2010

**Published: **21 August 2010

## Abstract

Porous alumina films had been fabricated by anodizing from aluminum films after an electropolishing procedure. Alumina stripes without pores can be distinguished on the surface of the porous alumina films. The width of the alumina stripes increases proportionally with the anodizing voltage. And the pores tend to be initiated close to the alumina stripes. These phenomena can be ascribed to the electric field distribution in the alumina barrier layer caused by the geometric structure of the aluminum surface.

## Keywords

## Introduction

Nanostructured materials exhibit interesting properties in a wide range of spectra including catalytic activity [1], optical properties [2] and magnetic properties [3]. Nanoporous materials with ordered structures have attracted increasing attention in recent years due to their possible utilization as templates for the organization of nanosize structures [4, 5]. One approach to the fabrication of such templates is using anodic porous alumina, which is prepared by the anodic oxidation of aluminum in various acidic electrolytes [6–9]. The degree of the ordering of the pores configuration at the surface of the porous alumina films is low because the pores develop randomly at the initial stage of the anodization. With the growth of self-organized pores, a densely packed hexagonal pore structure is established gradually. An explosion of porous alumina research was ignited once the capability of producing a nanohole array with excellent regularity was established by Masuda et al. [6]. Highly ordered anodic alumina films have been mostly achieved by the two-step anodizing the aluminum after electropolishing in perchloric acid–alcohol solution. The highly ordered anodic alumina films play important roles in many fields such as photonic crystals and magnetic interaction [10–14]. Recently, another anodic porous photoelectric material, the porous TiO_{2}, acquired highly ordered structure through the two-step anodizing procedure after electropolishing [15].

In order to improve the degree of order, shorten the fabrication time and fabricate non-hexagonal structure porous alumina films, surface prepatterning by different physical techniques has been found by Masuda et al. [16–18]. This plays its role at the nucleation stage of pore formation as the pores are initiated directly at the troughs of prepatterned surface. It suggests that the nanoscale surface morphology or nanotexture of the aluminum surface before anodizing is important for further pore formation stage. In most works, pores are initiated directly at the troughs of the pretextured surface. But in some cases, the pores are also initiated at unexpected sites between prepatterned troughs [19, 20].

Here, we investigate the surface morphology of porous alumina films after the aluminum anodizing procedure on the electrochemically polished aluminum surface. The morphology of a cellular aluminum surface fabricated by the electropolishing process can be transferred to the anodic alumina surface. There exist alumina stripes without pores on the surface of porous alumina films. And the stripes comprise cells [21]. The width of stripes increases proportionally with the anodizing voltage. The pores present on the remaining part of surface tend to initiate close to the stripes rather than the middle of the center of the cell. These can be ascribed to the electric field distribution in the alumina barrier layer caused by the geometric structure of the aluminum surface.

## Experimental Section

Porous alumina films were made by anodizing aluminum foils (99.999%, 0.5-mm thick). After annealing under N_{2} ambient at 450°C for 4 h, aluminum foil was ultrasonically degreased in acetone for 5 min and then it was electrochemically polished in a mixture solution of HClO_{4} and C_{2}H_{5}OH with a volume ratio of 20:80 at a current density of 100mAcm-2 for several times (first time 2 min, second time 5 min and third time 15 min). The polished aluminum foil was placed on the anode (Cu plate) and anodized in certain electrolyte at 5°C for 10 min. A Pt wire served as the cathode. The anodizing voltages are 30, 40, 50 and 60 V in 0.3 M oxalic acid (sample A, B, C and D, respectively) and 5, 15 V in 15wt% sulfuric acid. The morphology of the polished aluminum surface was measured using AFM (NanoScope IIIa). The sample morphology was observed using a SEM (LEO 1530VP).

## Results and Discussion

*R*=

*R*

_{1}(denoted as region

*R*

_{1}) and a positive curvature region on which

*R*=

*R*

_{2}(denoted as region

*R*

_{2}). It can be deduced from the AFM image that the

*R*

_{1}is much bigger than

*R*

_{2}. During the first few seconds of the constant voltage anodizing, the current decreases rapidly. A thin barrier layer–type alumina forms at this stage [21]. The thickness of the barrier layer is about 1.4 ×

*V*nm, where

*V*is the applying anodizing voltage. The sketch of the barrier layer has been shown as Fig. 4b. According to the Gaussian’s law, the electric field intensity of the point with a distance

*r*from point

*A*can be approximatively regarded as , where σ is the electric charge density of point

*A*, ε is the dielectric constant of alumina and

*r*

_{ a }is the curvature radius on point

*A*. The geometry indicates that on the electrolyte/alumina interface,

*r*

_{ a }≪

*r*, so it can be deduced that . Therefore, the pores cannot be initialized on the region

*R*

_{2}. And then, region

*R*

_{2}forms the alumina stripes. Where at the tie point of region

*R*

_{1}and region

*R*

_{2}, the intensity of the electric field can be approximatively regarded as . Under the static electric field assumption, the electric charge density on the region

*R*

_{2}is much bigger than on the region

*R*

_{1}. Therefore, the electric field intensity is much bigger on region

*R*

_{2}(

*E*

_{2}≫

*E*

_{3}). Thus, we can obtain the reason why the pore density of the region close to the stripes is much higher than the region in the middle of the cells. Therefore, pores tend to be initialized at the tie point of the region

*R*

_{1}and region

*R*

_{2}, i.e., the brink of the stripes. It can also explain the pore-in-pore structures referred by Zhu et al. [24]. We can see from the geometric structure that the width of the alumina stripes is proportional to the thickness of the barrier layers or the anodizing voltage as shown in Fig. 1.

*R*

_{2}/1.1 nm/V), it can be deduced that the barrier layer forms the structure as Fig. 5c. Under the static electric field assumption, the electric charge density on the region

*R*

_{2}is much bigger than on the region

*R*

_{1}. Therefore, the electric field intensity is bigger on region

*R*

_{2}. The thickness of the barrier can be estimated by the equation ∫

*E*·d

*l*=

*V*. Thus, we can draw the conclusion that the barrier layer on region

*R*

_{2}is much thinner than that on region

*R*

_{1}. Along with the anodizing proceeding, microcracks caused by the stress and the heat on region

*R*

_{2}can be much easier to impenetrate the hole barrier layer and then form the pores. That is, in the case of a rather small anodizing voltage, the pores tend to initiate on the positive curvature region i.e., on the stripes.

## Conclusions

In conclusion, porous alumina films were fabricated by anodizing from films of aluminum after an electropolishing procedure. Alumina stripes can be distinguished on the surface of the porous alumina. The width of the alumina stripes increases proportionally to the anodizing voltage. And the pores tend to be initiated close to the alumina stripes. These phenomena were ascribed to the non-uniformity of the electric field caused by the non-planar anodizing procedure of aluminum. We believe that this mechanism is not a special case but just surface geometric structure related. And further, it can result in a new method for prognosticating the surface structure of the porous alumina films.

## Declarations

### Acknowledgments

The authors are grateful to the Natural Science Foundation of Fujian Province of no. 2008J0029.

**Open Access**

This article is distributed under the terms of the Creative Commons Attribution Noncommercial License which permits any noncommercial use, distribution, and reproduction in any medium, provided the original author(s) and source are credited.

## Authors’ Affiliations

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