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Fabrication of porous alumina templates with a large-scale tunable interpore distance in a H2C2O4-C2H5OH-H2O solution LI Yi†, LING ZhiYuan, WANG JinChi, CHEN ShuoShuo, HU Xing & HE XinHua Department of Electronic Materials Science and Engineering, Institute of Materials, South China University of Technology, Guangzhou 510640, China

Highly ordered porous alumina templates with a large-scale tunable interpore distance (100̣445 nm) have been successfully fabricated under an electric field of 40̣180 V by modifying oxalic acid solution with adequate alcohol. The results under our experimental conditions show that the phenomena of burning and breakdown during the high-field anodization process can be avoided by adding a proper amount of alcohol to the oxalic acid solution. An excellent linear relationship between interpore distance and anodization voltage is obtained under 40̣170 V, and the maximum anodization voltage that could be used to avoid burning and breakdown is 180 V. anodization, porous alumina template, oxalic acid, alcohol, interpore distance

Since Keller et al.[1] first reported the fabrication process of the porous alumina template (PAT) by an electrochemical method in 1953, how to construct highly ordered PATs has been one of the most important problems in this field. In 1995, Masuda et al.[2] successfully prepared an ordered PAT by a two-step anodization method, which made it possible to be a template for the fabrication of highly ordered nanostructural arrays. Compared with other templates, such as porous polymer template[3] and colloidal crystal template[4], the PAT has so many advantages: adjustable pore size and interval, uniform tubes, highly ordered pores, a reliable fabricating process, low cost of production, good thermal and chemical stability. This makes it the preferred template to fabricate various ordered nanoarray materials widely used in many fields such as magnetism[5], energy storage[6], photocatalysis[7], photonics[8], and biosensor[9]. In the application to a porous template, the interpore distance and pore ordering of the PAT will directly influence the morphology and performance of the nanostructural arrays as prepared. So finding a proper method to fabricate highly ordered PATs with a large-scale tunwww.scichina.com | csb.scichina.com | ww.springerlink.com

able interpore distance has become a hot issue. At the present time, aluminum anodization is usually performed in various acid solutions, such as sulfuric, oxalic, and phosphoric acids. The typical interpore distances of the PATs fabricated in these three electrolytes are 13̣ ̣ 130 nm [10 14] , 70̣300 nm [13,15] , and 300̣500 nm[13,16,17], respectively. Compared with other electrolytes, fabricating PATs in the oxalic acid solution has many notable advantages: reliable fabricating technology, easily controlled process, and good reproducibility. So fabricating PATs with a large-scale tunable interpore distance in the oxalic acid electrolyte has a very good prospect in industrial applications. But at present the maximum interpore distance of the PAT fabricated in the pure oxalic solution as reported is 300 nm[15], which limits its applications to a certain extent. This paper presents a method to prepare highly ordered PAT with as large as 445 nm interpore distance under high anodization voltage by using an alcohol-modified oxalic acid Received July 3, 2007; accepted September 11, 2007 doi: 10.1007/s11434-008-0092-0 Corresponding author (email: [email protected])

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Chinese Science Bulletin | May 2008 | vol. 53 | no. 10 | 1608-1612

1 Experimental The high purity aluminum sheets (99.999ˁ) were annealed at 600ć under high vacuum to remove the residual stress; an ultrasonic cleaning process was conducted in acetone and alcohol separately for 10 min to remove the oil stain; and then the sample was electropolished in a mixture of C2H5OH and HClO4 (4:1 v/v) to smooth the surface. Anodization was divided into two parts with low and high anodization voltages respectively by using high purity graphite as cathode and aluminum sheets as anodes: (i) The first anodization was conducted under 40 V in 0.3 mol/L oxalic acid solution (0̣5ć) for 12 h. Subsequently, the sample was immersed into a mixture of H3PO4 (50 ml/L) and Cr2O3 (30 g/L) at 70ć for 5 min to remove the alumina membrane formed during the first anodization. The conditions of the second anodization were the same as the first step, except that it lasted for 24 h. (ii) The anodizations were performed under 115, 155, 160, 170 and 180 V separately in an alcohol-modified oxalic acid solution (0.3 mol/L H2C2O4C2H5OH = 41 v/v, ~10ć) for 2 h. All the as-prepared samples were immersed into a saturated CuCl2 solution to remove the residual aluminum. The morphology of the PATs’ barrier layer surface and cross-section was examined by a LEO1530 VP field emission scanning electron microscope (FE-SEM).

2 Results and discussion It is well-known that the PAT’s interpore distance iṇ creases with the anodization voltage[13 15]. So performing the aluminum anodization at a high voltage is an effective way to fabricate the PAT with a large interpore distance. In the pure oxalic acid solution, the maximum anodization voltage below which “burning and breakdown” can be avoided is 155 V as reported before[15]. When a high voltage is applied, there is always an extremely high current density during the aluminum anọ dization[14 16], and it is easy to induce the burning phenomenon[18,19]. The high current density generates a lot

LI Yi et al. Chinese Science Bulletin | May 2008 | vol. 53 | no. 10 | 1608-1612

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of heat at the same time, and a sharp temperature increasing at the PAT’s barrier layer can be obtained[16]. The acidic electrolyte has an aggravating corrosive action to the barrier layer under high temperature. As a result, the barrier layer becomes thinner and thinner, which makes its resistant voltage become lower. When the barrier layer thickness in certain regions of the PAT is reduced to a critical value, the electric breakdown happens. It is concluded that the current density and temperature during high-voltage anodization are two key factors to determine whether the burning and breakdown phenomena happen or not. In this work, the electrolyte was modified by adding a proper amount of alcohol into oxalic acid solution. The main considerations to choose alcohol are: (i) the temperature of the electrolyte can be lowered to 10ć by adding adequate alcohol which has an extremely low freezing point (114.3ć), and then the anodization process can be conducted at a lower temperature; (ii) the vaporization of alcohol (the boiling point of alcohol is 78.4ć) will take away most of the heat generated at the barrier layer and keep the temperature low under vigorous stirring[16]; (iii) the resistance of the oxalic acid solution can be increased by adding adequate alcohol which is a weak electrolyte, and consequently a lower current density and reduced caloric are obtained under the same anodization voltage; (iv) it also has so many merits like being nontoxic, inexpensive, easy for getting and recycling, all of which make it a preferred selection in the pollution-free commercial process. Using an alcohol-modified oxalic acid solution and applying voltage of 180 V, highly ordered PAT with a large interpore distance was successfully prepared, as shown in Figure 1. It can be seen that its hexagonal construction unit has a close-packed structure, and its average interpore distance is as large as 445 nm. The results clearly show that the burning and breakdown can be successfully avoided by adding a proper amount of alcohol to the electrolyte without destroying the pore ordering of the PAT as prepared. We also find that the burning and breakdown phenomena can still be effectively avoided even when the anodization is conducted in an electrolyte which has been used for several times, and the PATs as prepared have almost the same interpore distances, which indicates that the volatilization loss of alcohol during the anodization process is very little.

METALLIC MATERIAL

solution. This makes it possible to prepare highly ordered PATs with a large-scale tunable interpore distance and widen the scope of its applications greatly.

Figure 1 The morphology of the barrier layer of the PAT fabricated in an alcohol-modified oxalic acid solution under 180 V.

Furthermore, highly ordered PATs with a large-scale tunable interpore distance were prepared by changing the anodization voltage, as shown in Figure 2. It can be seen that all the as-prepared PATs have hexagonal construction units and a close-packed structure, their interpore distance increases with the anodization voltage (Figures 2(a)̣(c)), and the pipelines of the PATs obtained by high-voltage anodization have smooth walls, uniform shapes, almost the same spacing intervals, and

parallel to each other (Figure 2(d)). The measurement results show that the interpore distance of the PATs obtained under the anodization voltages of 40, 115, 155, 160 and 170 V is 102, 257, 346, 353 and 371 nm, respectively, as shown in Figure 3 (filled black squares). When the anodization voltage is lower than 170 V, the interpore distance (DInt) increases linearly with the anodization voltage (Ua), and the Dint versus Ua curve is fitted using the following equation: DInt = 2.1Ua+15.9 with applying voltages of 115̣170 V. This result is more or less different from that obtained in the pure oxalic electrolyte under high anodization voltage[15], which means that the PAT prepared in our experiment has a larger interpore distance than that as reported (open circles in Figure 3) when applying the same anodization voltage. Test results show that the current densities of these two processes are 42 and 105 A/m2[15] under 110 V respectively after anodization is conducted for 1 h. It is obvious that the resistance of the electrolyte increases and its freezing temperature decreases by adding adequate alcohol to the oxalic acid solution, which leads to a lower current density under the same voltage. The current density in the anodization process can be modi-

Figure 2 The morphologies of the barrier layer surface and cross-section of the PATs as prepared under (a) 40 V, (b) 115 V, and (c) (d) 160 V. Samples of (b)̣(d) were prepared by using an alcohol-modified oxalic acid solution, (a)̣(c) were barrier layer morphologies, and (d) was cross-section morphology. 1610


fied when applying the same anodization voltage by changing some conditions such as reaction temperature, electrolyte species and concentration, and the DInt of the PAT prepared decreases as the current density increases. Similar results have also been reported by Li et al.[16]. When the anodization voltage is 40 V, the theoretical value of the DInt calculated by the fitting equation is 100 nm which is nearly the same as that (102 nm) fabricated in the pure oxalic acid solution under the same voltage, and we find that the current density of this process is only 21 A/m2. It is concluded that the DInt increases linearly with anodization voltage if other conditions remain unchanged[13,15,20]. The current density under the same anodization voltage will be changed with other reaction conditions, and determined together by anodization voltage, reaction temperature, the area ratio of anode to cathode, electrolyte species and concentration. As a result, the DInt will no longer increase linearly with voltage and will be determined by anodization voltage and current density together[16]. Unlike the linear influence caused by anodization voltage, current density of1

The current density and temperature of the anodization process can be controlled by adding proper alcohol to the oxalic acid electrolyte, which is an effective way to avoid the burning and breakdown phenomena without damaging the morphology of the PATs as prepared. By using this method, we have successfully fabricated highly ordered PATs with a large-scale tunable interpore distance (100̣445 nm). The PAT’s interpore distance and anodization voltage exhibit an excellent linear relationship under 40̣170 V. The present work displays a very attracting prospect in related nanomaterial industry applications.

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Figure 3 The relationship of anodization voltage (Ua) and average interpore distance (DInt). The solid line represents the relation DInt = 2.1Ua+15.9. The data marked by filled black squares was obtained in our experiment, and the points marked by open circles represent the data in ref. [15].

ten has nonlinear impact on the DInt, and this impact is very little when the anodizations are conducted with relatively low current densities at the same voltage, even when other conditions like electrolyte concentration are changed[16]. Moreover, it is found that under our experimental conditions, the maximum anodization voltage that could be used to avoid burning and breakdown is 180 V under which the theoretical value of the DInt obtained by the fitting line is 394 nm. And there is a relatively large gap between this value and our actural measurement (445 nm). The possible reasons may be as follows: when the anodization is conducted under the critical voltage of breakdown, there is always a very fast PAT-growth rate in the anodization process accompanied with a lot of heat, and the alcohol in the electrolyte cannot cool the reaction system effectively, which leads to a severe temperature increase at the barrier layer of the PAT as prepared; the high temperature and fast growth rate of the PAT aggravate the volume expansion from aluminum to porous alumina, thus the self-organizing regime of the anodization process may be changed, which leads to the deviation at 180 V. Its detailed mechanism is still under research.

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Science in China Series E: Technological S c i e n c e s EDITOR YAN Luguang Institute of Electrical Engineering Chinese Academy of Sciences Beijing 100080, China

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