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Removal of Barrier Oxide in the Anodized Aluminum Oxide Nanotemplates N. Tsyntsaru1,2 1

Institute of Applied Physics, Academy of Sciences of Moldova, Chisinau, MD-2028, Academiei 5, Moldova 2 Vilnius University, Faculty of Chemistry, Vilnius, LT-03325, Naugarguko 24, Lithuania

Abstract— Aluminum as other valve metals possesses an important feature: it can form on the surface a porous structure – aluminium anodized oxide (AAO), under proper anodization conditions (voltage, pH, temperature, and etc). In order to be used as a template obtained AAO should undertake the barrier oxide removing step, which will ensure the appropriate electrical contact for material deposition. Thus, in this study we show the simple and unique method for barrier oxide thinning/removing by anodizing solution itself, in comparison with conventional procedure used. The more precise control of barrier oxide removing/thinning was achieved in the same solution used as for anodisation, but at the temperature of 50 °C. Keywords— aluminium anodized oxide, barrier oxide layer, zincate treatment, anodic solution treatment.

I. INTRODUCTION Aluminum and its alloys are important industrial materials due to their low weight and good corrosion resistance because of surface passivation. Aluminum possesses a unique feature, under proper conditions it can form on the surface a porous structure. This feature is known since 1960s and the as-prepared porous material is called anodized aluminum oxide (AAO). The size of the pores can be well controlled within micrometer and nanometer scales by careful selection of the processing conditions (temperature, voltage, type of electrolyte and electrolyte concentration, etc.). Due to the rapid progress in the nanotechnology, the fabrication of AAO attracts the attention of researchers as it provides convenient and cheap way to produce nanowires with diameters ranged from several tenths of nanometers. The shape of pattern can be replicated using a templating method, wherein a material is printed, pressed or grown against the voids of a template. Often the template is then removed, leaving the inverse of its pattern; this pattern can be used as a template to achieve a replica of the original. Templating is a relatively fast, cheap and reproducible technique for the preparation of polymeric surfaces [1]. Various materials can be used as templates for nanostructures forming, such as lotus leaves, masters prepared by lithography to inorganic membranes. Woo Lee et al. [2] prepared AAO membranes and used them as templates to form nanopatterns by heating and pressure-driven nanoimprinting. Sun et al. Neto et al. [3] introduced the electrodeposition to prepare dense, aligned metal nanowires (nano-

carpets) in the alumina membrane template, followed by dissolution of the membrane. Masuda and Fukuda [4] proposed and successfully realized a two-step procedure to fabricate alumina membranes with regular pores throughout the entire membranes. Now this method became as standard procedure to prepare the highly ordered AAO structures. The polishing of aluminum before anodizing is a one of the most important steps in the successful preparation of highly ordered AAO structures. This can be achieved by means of mechanical, chemical, or electrochemical polishing, and among which, the electrochemical polishing in a 60% HClO4+C2H5OH (1:4, volume ratio) solution at 10 ℃ and 500 mA/cm2 for 1 min is most commonly used to prepare extremely smooth Al surfaces before anodizing. The anodization is commonly carried out in sulfuric, oxalic or phosphoric acids with voltages of 19–25 V, 40–60 V and 160–195 V, respectively [5]. In two-step anodization, during first anodization the non-uniformly ordered pore structure on the surface is formed. By removing this nonregular oxide layer, a periodic concave pattern is formed on the aluminum surface, and these hollows play role of centers for consequent act as seeds in a further anodization process. The second anodization is carried out at the same anodizing potential as that used for the first anodization, and well-ordered AAO surfaces are formed. In order to be used as a template the obtained AAO should undertake the barrier oxide removing step, which will ensure the reliable electrical contact required for material deposition. This work is devoted to the fabrication of AAO nanotemplate by anodizing in sulfuric acid and to the investigation of the barrier oxide thinning/removing procedures by simple and unique method using anodizing solution itself. II. MATERIALS AND METHODS The first and second anodizing steps have been performed in a 20 wt% sulphuric acid solution using a two electrode electrochemical cell setup equipped with a magnetic stirrer rotating at 500 rpm. The temperature was kept at ~ 0 – 1oC; anodization potential 21 V. After each anodization step, the subsequent electropolishing and oxide removal steps, the influence of the condition on the each anodization step was controlled by means of SEM.

© Springer Science+Business Media Singapore 2016 V. Sontea and I. Tiginyanu (eds.), 3rd International Conference on Nanotechnologies and Biomedical Engineering, IFMBE Proceedings 55, DOI: 10.1007/978-981-287-736-9_29

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Following the second anodization, the non-conductive barrier oxide dissolution, closing the pore bottoms and hindering the direct electrochemical growth of metallic nanostructures within the template, was performed using several procedures: 1. Treatment in the zincate solution; 2. Treatment in the acidic CoCl2 solution; 3. Treatment in the anodization solution. The zincate treatment was performed in the solution: 120 g\l NaOH + 20 g\l ZnO + 2 g\l FeCl3. The treatment with acidic solution of cobalt chloride (0.1 M CoCl2 + 0.001 M nitric acid) was performed at pH 2. The overview of the conditions applied during barrier layer dissolution is provided in Table 1.

related to the types of electrolyte, concentration, temperature and applied voltage. Meantime, the most popular model for the self-ordering of pores in AAO is based on mechanical stress associated with expansion of aluminum during the oxide formation. This is the cause of a repulsive force between neighboring pores, which leads to the self-ordering process [8]. Mainly, the following chemical processes dominate in the anodization of the alumina membranes [9-12]: (1) Al3+ ions form at the metal/oxide interface and distribute in the oxide layer near the oxide/metal interface: Al → Al 3+ + 3e −

(2) The electrolysis of water (a water-splitting reaction) occurs at the pore bottom near the electrolyte/oxide interface:

Table 1 The description of samples Sample number 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11.

Description Zincate treatment, ultrasonic, 20 sec, 1 °C Zincate treatment, ultrasonic, 40 sec, 20 °C Zincate treatment, ultrasonic, 20 sec, 20 °C Zincate treatment, ultrasonic, 5 sec, 20 °C CoCl2 treatment, pH 2 60 min Treatment by anodization solution, 120 min, 1 °C Treatment by anodization solution, 30 min, 30 °C Treatment by anodization solution, 60 min, 30 °C Treatment by anodization solution, 120 min, 30 °C Treatment by anodization solution, 8 min, 50 °C Treatment by anodization solution, 15 min, 50 °C

2 H 2O → 2O 2 − + 4 H + Barrier oxide layer, nm 31 Membrane dissolved Membrane dissolved 22.5

15.7 Membrane dissolved 12.7 Membrane dissolved

III. RESULTS AND DISSCUSIONS The formation of porous alumina depends upon the current density or voltage employed for anodizing, the temperature, pH and composition of the electrolyte. These parameters determine the extent of chemical interaction between the anodic alumina and the electrolyte during anodizing. Such interaction is accelerated by the electric field [6]. After anodization, the films comprise a barrier layer next to the metal and an outer barrier layer, in which the pores, orientated normal to the film surface [7]. The topography of the AAO film as well as the formation rate of the oxide layer depends on the anodization condition which is closed

(2)

(3) In the presence of electric field, the O2−ions migrate within the barrier layer from the electrolyte/oxide interface to the oxide/metal interface, and react with the Al3+ ions there forming Al2O3: 2 Al 3+ + 3O 2− → Al2O3

(3)

(4) Then electric-field enhances oxide dissolution at the electrolyte/oxide interface:

20 28.7 28.5

(1)

Al2O3 + 6 H + → 2 Al(3aq+ ) + H 2O

(4)

In the anodization process of the porous alumina membranes, a balance exists between the electric-field-enhanced oxide dissolution at the electrolyte/oxide interface and the formation of oxide at the oxide/metal interface. This balance is crucial to the formation of the porous alumina membranes, because it keeps the constant thickness of the barrier layer during whole duration of anodization process and hence allows steady-state pore propagation into the Al. Knowledge on the AAO formation including barrier oxide layer is important step for visualization of reaction occurring during barrier oxide layer dissolution. Our investigations on zincate treatment (Table 1, Fig. 1) gave unsatisfactory results. Treatment at low temperatures (1°C) gives small growing of barrier layer instead of dissolution. Increasing the temperature promotes the rate of dissolution reaction resulting in complete dissolution of AAO, if the process is undertaken longer than 20 sec. Treatment at shorter times and 20 °C gives better results (Fig. 1), but still is difficult to achieve a good control over the dissolution process, even in the presence of ultrasonication. Ultrasonication is a widely used process for equilibrating concentration differences and allows a controlled homogeneous dissolution of pore walls and bottoms, most probably

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Removal of Barrier Oxide in the Anodized Aluminum Oxide Nanotemplates

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Treatment by anodization solution, 120 min, 1 °C

Zincate treatment, ultrasound, 20 sec, 1 °C

Zincate treatment, ultrasonic, 20 sec, 20 °C

CoCl2 treatment, pH 2 60 min



Zincate treatment, ultrasonic, 5 sec, 20 °C

Fig. 1 SEM images of the treated barrier oxide layers after zincate treatment.

by keeping an even concentration gradient throughout the entire pore structure. The treatment performed in the acidic solution of the active metal as cobalt also did not result in some visible dissolution of the barrier oxide layer, but some tinning has been noticed (Fig. 2) in accordance with equation (4). As acidic solution gives some tinning of the barrier oxide layer it was decided to explore the possibility to remove the barrier oxide layer in the same solutions used for aluminium anodization. Thus, several conditions were investigated varying the temperature and time (Fig. 2). The results show


Fig. 2 SEM images of the treated barrier oxide layers after treatment by


anodization solution.

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that increasing in temperature promotes the reaction of dissolution, equation (4) and retards the rate of the reaction of alumina formation, equation (3). It was found a reasonable dissolution rate of barrier oxide layer and practical removing it at the temperature of 50 °C. Nevertheless, some additional research needs to be performed in order to establish a good reproducible control over the dissolution rate of the non-conductive barrier oxide layer of AAO nanotemplate. IV. CONCLUSIONS In this study the influence of different treatments was explored in order to perform dissolution of non-conductive barrier oxide layer of anodic aluminum nanotemplate: in the zincate solution, in the acidic solution of CoCl2, in the anodization solution. The experiments carried out reveal that treatment in the zincate and anodic solutions are the best options for barrier oxide layer removing/tinning, but treatment in the zincate solution is hardly controlled, because the dissolution rate is quite high: just several seconds. The treatment in the solution used for anodization can be the good option in view of performing the process under controllable conditions. Also, this simple method by treatment in the anodized solution itself provide a good option to conventional procedure, e.g. to the treatment by phosphoric acid. Nevertheless, some additional study on the efficiency of the ratio between treatment time and temperature are needed in order to determine optimal rate of dissolution with less damage of the template itself.

ACKNOWLEDGMENT The authors acknowledge funding from FP7 Oil&Sugar project (295202). Also, partial funding was granted by the Research Council of Lithuania (MIP-031/2014) and Moldavian national projects (14.02.121A), (14.819.02.16F).

CONFLICT OF INTEREST The authors declare that they have no conflict of interest.

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