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Preparation of self-ordered nanoporous anodic aluminum oxide membranes by combination of hard anodization and mild anodization
Thin Solid Films 552 (2014) 75–81
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Thin Solid Films journal homepage: www.elsevier.com/locate/tsf
Preparation of self-ordered nanoporous anodic aluminum oxide membranes by combination of hard anodization and mild anodization Jia Liu a,b, Shu Liu b, Haihui Zhou a,b, Congjia Xie b, Zhongyuan Huang b, Chaopeng Fu b, Yafei Kuang b,⁎ a b
State Key Laboratory for Chemo/Biosensing and Chemometrics, Hunan University, Changsha 410082, China College of Chemistry and Chemical Engineering, Hunan University, Changsha 410082, China
a r t i c l e
i n f o
Article history: Received 25 March 2013 Received in revised form 13 December 2013 Accepted 13 December 2013 Available online 18 December 2013 Keywords: Anodic aluminum oxide templates Two-step anodization Hard anodization Mild anodization
a b s t r a c t Anodic aluminum oxide (AAO) is one of the most important templates for fabrication of nano-materials. In this paper, we present a fast two-step anodization method to prepare self-ordered AAO films. The first step is hard anodization (HA) in H2SO4 and the second step is mild anodization (MA) in H2C2O4. HA in H2SO4 at a wide range of anodization voltage provided a fast formation of ordered arrays of pits on Al sheet, and in the second step, arrays of pits on Al substrate affected nucleation initially, and the anodization voltage of H2C2O4 MA determined the degree of AAO later. Then the best anodization voltage of H2C2O4 MA was validated to be around 40 V. The improved combination of HA and MA not only shortens the time of the first anodization step compared to conventional two-step anodization which was reported by Masuda and Fukuda (1995), but also keeps a highly ordered AAO within large area. Moreover, the relationship between the size of arrays of pits formed in H2SO4 HA and degree of order of AAO formed in H2C2O4 MA was investigated in detail. © 2014 Elsevier B.V. All rights reserved.
1. Introduction Ordered nanochannel-array materials have attracted much attention over the last decades due to their broad applications in synthesis of nano-structured magnetic, electronic and optoelectronic materials [1–3]. Anodic aluminum oxide (AAO) films as templates have played an important part in preparing ordered nanochannel-array materials. Ordered AAO films have specific structure parameters with closepacked hexagonal cells, which can be controlled by changing anodization conditions. Many works have been done on fabrication of highly ordered AAO templates in the past decades [4–8]. Masuda H et al. employed the method of two-step mild anodization (MA) and etching to prepare a highly ordered AAO film in oxalic acid solution [4,5]. In the two-step anodization, it cost 10 or more hours at 40 V in oxalic acid solution to form a relatively thick AAO film on the Al substrate for the first step, and ordered arrays of pits were obtained after removing the AAO film. Secondly, the obtained Al sheet was anodized again and as a result a highly ordered AAO film with enough mechanical robustness and high aspect ratio was obtained due to the induction of the arrays of pits left on Al substrate. Without induced by ordered arrays of pits on Al substrate, it usually needs a very long time for selfadjustment from disordered to a highly ordered AAO. So, it takes a long time for the first step in Masuda H's two-step anodization. Meanwhile, the highly ordered cells appear only in a quite narrow voltage window, which leads the interpore distance or cell size to be confined to some fixed values in mild anodization (MA). For example, the best ⁎ Corresponding author. Tel.: +86 0731 88821603; fax: +86 0731 88713642. E-mail address: [email protected] (Y. Kuang). 0040-6090/$ – see front matter © 2014 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.tsf.2013.12.023
ordering anodization voltage in oxalic acid solution is about 40 V. Thus, there is a great limitation in Masuda H's two-step anodization process to form ordered arrays of pits on Al. In the method of etching, the area of a highly ordered AAO domain can only reach to square millimeter level and the aspect ratio of the formed AAO is low. The high cost and time consuming preparation process limited practical application of AAO. The key of two-step anodization method is to obtain highly ordered arrays of pits on Al substrate. Chu et al. [6] employed high electric field to fabricate a highly ordered AAO rapidly in aged sulfuric acid solution. Lee W et al. [7–9] used a thin pre-existed film (about 400 nm in thickness) forming on the Al substrate to avoid local catastrophe, then prepared a highly ordered AAO film in oxalic acid or sulfuric acid solution through high electric field. High electric field led to rapid formation of self-ordered AAO. However, it also caused high mechanical stress in AAO, bringing the boundedness in practical applications. Fast fabrication of a highly ordered AAO in aged sulfuric acid solution by hard anodization (HA) provides an improved way for the first step to prepare highly ordered arrays of pits on Al substrate in two-step anodization process. In this work, we present a fast and improved two-step anodization method to prepare a highly ordered AAO. Fig. 1 shows the schematic diagram for the synthetic process. For the first step, HA in sulfuric acid is employed for less than 30 min to achieve ordered arrays of pits on Al substrate. For the second step, MA is conducted in oxalic acid. The relationship between the behavior of the second anodization step in oxalic acid and pits left on Al substrate after the first anodization step is also discussed. The improved combination of HA and MA not only shortens the time of the first anodization step compared to Masuda
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There are three methods to fabricate a highly ordered AAO in H2SO4 hard anodization (HA): (1) anodization under potentiostatic mode [6]; (2) anodization under galvanostatic mode [10]; and (3) anodization under potentiostatic mode of Al substrate with a thin pre-existed film (about 400 nm in thickness) [7,9]. Compared with methods (1) and (2), method (3) can avoid phenomena of burning and cracking, and the operation of method (3) is simpler. So method (3) was employed in this work.
Fig. 1. Schematic diagram for the synthetic process.
H's two-step anodization [4] and keeps a highly ordered AAO within large area, but also decreases the high mechanical stress in AAO formed in HA. The process is characterized by low cost, time-saving and wide preparation parameter scope. 2. Experimental section 2.1. Fabrication of a highly ordered AAO in sulfuric solution 2.1.1. Pretreatment High purity aluminum plates (99.999%, 20 mm × 10 mm × 0.5 mm, Beijing Research Institute for Nonferrous Metals, China) were used as working electrodes. Prior to anodization, all the Al sheets were degreased by ultrasonic in ethanol for 5 min and then electrochemically polished in a mixture solution of 65% HClO4 and 99.5% ethanol (VHClO4/Vethanol = 1/4) with vigorous stirring at 0 °C and 20 V for 5 min to achieve mirror finished surfaces in order to eliminate the influence of natural oxide film on the aluminum surfaces.
2.1.2. Anodization 0.3 M and 0.04 M sulfuric acid solutions were employed under potentiostatic mode with U from 35 V to 70 V. For 0.3 M sulfuric acid solution the voltage was used below 40 V, and for 0.04 M sulfuric solution the voltage was used above 40 V. Anodization was performed in the corresponding sulfuric acid solution at 5 ± 2 °C with vigorous magnetic and compressed air stirring. A powerful cooling system and a large electrolysis bath (1 L) were used to maintain the low temperature needed for the high-field anodization. The aluminum sheets were anodized at 25 V (MA) for 10 min in the corresponding solution to generate a thin porous alumina surface layer which could avoid the burning or cracking in subsequent step. U was gradually increased to Utarget of 35 V to 70 V at a rate of 0.1 V s− 1. Subsequently, the as-anodized sample was immersed into a mixture of phosphoric acid (6 wt.%) and chromic acid (1.8 wt.%) at 70 °C for 5 h to remove the alumina layer. 2.2. Fabrication of ordered AAO in oxalic acid solution After removing the AAO formed in sulfuric acid, the second anodization step was conducted on the aluminum sheet in 0.3 M oxalic acid at 17 ± 2 °C with a voltage of 30 V, 32 V, 38 V, 40 V, 44 V and 52 V, respectively. Next, the sample was immersed in a mixture solution of 0.2 M CuCl2 and 6.1 M HCl to remove the underlying Al substrate. Finally, a free-standing AAO film was obtained. The barrier layer and cross-sectional morphologies of the as-prepared AAO film were
Fig. 2. FESEM images of the bottom surface of AAOs prepared in 0.3 M H2SO4 at 40 V for different time. (b) 10 min (j decreases to 125 mA cm−2), (c) 30 min (55 mA cm−2) and (d) 60 min (40 mA cm−2). (a) is the relationship between current density and time in H2SO4 HA at 40 V.
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Fig. 3. FESEM images of AAOs formed in H2SO4 HA at various potentials for 30 min. (a) 35 V, (b) 55 V, (c) 60 V and (d) 65 V. (e) is the relationship between cell size and anodization voltage of H2SO4 HA.
Fig. 4. FESEM images of AAOs prepared in H2SO4 at 50 V for 30 min (60 mA cm−2). (a) is a typical bottom surface and (b) is the highly ordered arrays of pits on Al substrate after removing AAO.
characterized by a field emission scanning electron microscopy (SEM, Hitachi S-4800). 3. Results and discussion 3.1. Fabrication of various sizes of highly ordered pits on Al substrate Under hard anodization condition, the samples present plastic deformation, accompanying many cracks and burns on the surface [7–9]. For example, when the anodization voltage increases to 50 V, AAO exhibits a dark gray color, with a bunch of gas bubble holes on the surface. In the anodization process, when U reaches to Utarget, j firstly increases to jmax as shown in Fig. 2a, and then decreases exponentially, stable anodization begins. Fig. 2b, c and d shows typical bottom surface images of AAO films prepared in 0.3 M sulfuric acid solution at 40 V for 10 min (125 mA cm− 2), 30 min (55 mA cm− 2) and 60 min (40 mA cm− 2), respectively. It can be observed from Fig. 2b, c and d that the degree of order of AAO has no obvious change with time prolonging, and the cell size also keeps consistent with each other in Fig. 2b, c and d. The cell size here always keeps around 80 nm, differing from the result reported by Lee W that cell size of AAO increased from 64 nm to 78 nm when current density decreased from 130 mA cm − 2 to 40 mA cm − 2 [9]. The result provides a firm evidence that highly ordered arrays of pits can be formed fast on Al substrate in H2SO4 HA. Fig. 3 shows SEM images of bottom surface of highly ordered AAOs formed in H2SO4 HA at various anodization voltages and the relationship between cell size and anodization voltage. As shown in Fig. 3,
AAO films formed in H2SO4 HA present a high degree of order, and cell size and anodization voltage present a linear relationship with a slope of about 1.9 nm V−1. Junctions of three cell boundaries could be easily corroded by the mixture solution of HCl/CuCl2 [11], also it can be found that voids appear in the junctions of three cell boundaries which exhibit weak cell junctions. Fig. 4a shows the morphology of bottom surface of the AAOs formed at 50 V in H2SO4 HA, which exhibits a high degree of order. Fig. 4b shows the highly ordered pits on Al
Fig. 5. Relationship between cell size and anodization voltage of H 2 C 2 O 4 MA. The correlation coefficient estimated from the linear regression is ζMA = 2.5 nm V−1.
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Fig. 6. FESEM images of AAOs prepared in 0.3 M H2C2O4 at 17 ± 2 °C at the matched voltages. (a), (b), (c) and (d) are the bottom surface images of AAOs prepared at 30 V, 32 V, 44 V and 52 V in H2C2O4, which are induced by the arrays of pits formed in H2SO4 at 35 V, 40 V, 55 V and 65 V respectively.
Fig. 7. FESEM images of AAOs prepared in 0.3 M H2C2O4 at 17 ± 2 °C at the matched voltage. (a), (b) and (c) are the bottom surface images of AAOs prepared at 38 V in H2C2O4 for 1 h, 4 h and 8 h respectively, which are induced by the arrays of pits formed in H2SO4 at 50 V. (d) is the cross-section image corresponding to (c).
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Fig. 8. FESEM images of AAOs prepared in 0.3 M H2C2O4 at 17 ± 2 °C and 40 V for 16 h for the second anodization step. (a), (c), (e) and (g) are the top surface images of the AAOs induced by the arrays of pits formed in H2SO4 at 35 V, 40 V, 50 V and 55 V respectively. (b), (d), (f) and (h) are the bottom surface images corresponding to (a), (c), (e) and (g) respectively.
substrate. The degree of order of pits corresponds to that shown in Fig. 4a after removing the AAO film. It can be noted that the ordering anodization voltage window of H2SO4 HA is much wider than that of H2C2O4 MA and ordered AAO can be obtained in H2C2O4 by anodizing the Al sheets with the arrays of pits.
3.2. Preparation of a highly ordered AAO by two-step anodization under condition of HA–MA Fig. 5 shows that the cell size is also proportional to the anodization voltage of H2C2O4 MA with a slope of ζMA = 2.5 nm V−1, which is
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larger than that in H2SO4 HA. So the effect of anodization voltage on cell size in H2SO4 HA is different from that in H2C2O4 MA. According to Figs. 3e and 5, we can determine the relationship between the size of arrays of pits formed on Al substrate in H2SO4 HA and the cell size of AAO formed in H2C2O4 MA. According to the changing regularities of size of arrays of pits formed in H2SO4 HA and cell size of AAO formed in H2C2O4 MA shown in Figs. 3e and 5, it is known that the size of pits formed in H2SO4 HA at 35 V, 40 V, 50 V, 55 V and 65 V approximately equals to the cell size of AAO formed in H2C2O4 MA at 30 V, 32 V, 38 V, 44 V and 52 V respectively. And so we prepared AAO films in H2C2O4 at five matched states respectively. Fig. 6 shows bottom surface images of AAOs formed at four different voltages of the five matched states. Although the corresponding pits on Al substrate presented a high degree of order, the poorly ordered AAOs were obtained at the four voltages. Therefore, the preparation of a highly ordered AAO in H2C2O4 MA not only depends on the guidance of ordered pits on substrate, but also the voltage of H2C2O4 MA. The experiment results show that the best ordering voltage of H2C2O4 MA is around 40 V. Fig. 7a, b and c shows the bottom surface images of AAO films prepared in 0.3 M H2C2O4 at 17 ± 2 °C and 38 V for 1 h, 4 h and 8 h respectively, which are induced by the pits formed at 50 V in H2SO4. These SEM images show that there is no obvious change in degree of order of AAO, which is much better than that of AAOs formed at the four voltages in Fig. 6. The arrays of channel of AAO grow straightly, as shown in Fig. 7d, and it exhibits a high aspect ratio of 90. Accordingly, AAO with a high-aspect-ratio can be obtained by anodizing Al substrate with highly ordered pits in H2C2O4 at the matched voltage around 40 V. Fig. 8a, c, e and g presents the surface images of AAOs prepared in 0.3 M H2C2O4 at 17 ± 2 °C and 40 V, which are induced by pits formed in H2SO4 at 35 V, 40 V, 50 V and 55 V respectively. The cell sizes in (a), (c), (e) and (g) are 75 nm, 80 nm, 94 nm and 110 nm respectively, which just coincide with the cell sizes of AAOs formed in H2SO4 at
35 V, 40 V, 50 V and 55 V respectively (shown in Figs. 2 to 4). The degree of order of the AAOs is just one-to-one correspondence. It can be noted that, at the early stage of anodization in H2C2O4, the anodization process is extremely affected by the highly ordered arrays of pits left on the Al substrate after the first anodization step. The subsequent anodization voltage has little influence on the growth of AAOs. After the early stage, the process of anodization is mainly affected by the subsequent anodization voltage. Panels b, d, f and h in Fig. 8 are the bottom surface images of AAOs formed in H2C2O4 at 40 V for 16 h, corresponding to Fig. 8a, c, e and g respectively. After a relative long time of adjustment, all the cell sizes of AAOs, determined by the subsequent anodization voltage, are 107 nm. The aforementioned results indicate that, the pits on Al substrate have a great effect on the early growth stage of AAOs in H2C2O4 MA, the cell size of AAOs is the same as the size of pits, but after a long time of self-adjustment of AAOs, the structure and cell size of AAOs are determined by the anodization voltage of H2C2O4 MA. As Fig. 8 shows, the cell sizes of AAOs formed in H2C2O4 for 16 h are almost the same though the difference in the sizes of pits on Al substrate is about 40 nm. Masuda H et al. [5] has proved that, if the size of pits did not match with the anodization voltage, ordered growth of AAO could not maintain any more. Fig. 9a, b, c and d shows the bottom surface images of AAOs prepared in 0.3 M H2C2O4 at 40 V for 1 h, 4 h, 8 h and 16 h respectively, which are induced by pits formed in H2SO4 at 40 V. The size of highly ordered arrays of pits on the Al substrate remains at 80 nm. The cell sizes of subsequent AAOs formed in H2C2O4 are 107 nm, which is much different from the size of arrays of pits. In Fig. 9a, the ordered area is less than 1 μm2, and there exists many defects. The area reaches to about 1 μm2 in Fig. 8b with less defects than in Fig. 9a. The ordered area in Fig. 9c is bigger than that in Fig. 9a and b, and the quantity of defects in Fig. 9c decreases. Finally, the area reaches to several μm2 or level of mm2 in Fig. 9d. These results indicate that, with the adjustment of AAO at the early stage of anodization, the ordered
Fig. 9. FESEM images of AAOs prepared in 0.3 M H2C2O4 at 17 ± 2 °C and 40 V for (a) 1 h, (b) 4 h, (c) 8 h and (d) 16 h respectively, which are induced by pits formed at 40 V in H2SO4.
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area without defects enlarges, but the defects can still be observed at the edge of the ordered domain. 4. Conclusions In summary, a highly ordered AAO was obtained in H2SO4 HA at a wide range of anodization voltage in a short time, and then highly ordered arrays of pits were obtained on the Al sheet after removing the AAO, which can induce the growth of ordered AAO in H2C2O4 MA. At the early stage of anodization in H2C2O4, the growth of AAO is obviously induced by the arrays of pits left on the Al sheet. But after a long time of self-adjustment of AAO, the structure and cell size of AAO are determined by the anodization voltage of H2C2O4 MA. After the Al sheet was anodized for about 10 min or more in H2SO4 at around 50 V, AAO with a good degree of order and high aspect ratio could be prepared at around the best ordering anodization voltage (40 V) in H2C2O4. This time-saving, low-cost and simple process is proved to be a promising way to prepare AAO with potential applications in fabrication of nanomaterials.
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Acknowledgment This work was supported by the National Natural Science Foundation of China (Grant Nos. 51071067, J1103312, J1210040, 21271069, 51238002).
References [1] [2] [3] [4] [5] [6] [7] [8]
R.J. Tonucci, B.L. Justus, A.J. Campillo, C.E. Ford, Science 258 (1992) 783. T.W. Whitney, J.S. Jiang, P.C. Searson, C.L. Chien, Science 261 (1993) 1316. P.L. Lu, S.H. Charap, IEEE Trans. Magn. 30 (1994) 4230. H. Masuda, K. Fukuda, Science 268 (1995) 1466. H. Masuda, H. Yamada, M. Satoh, H. Asoh, M. Nakao, Appl. Phys. Lett. 71 (1997) 2770. S.Z. Chu, K. Wada, S. Inoue, M. Isogai, A. Yasumori, Adv. Mater. 17 (2005) 2115. W. Lee, R. Ji, U. Gösele, K. Nielsch, Nat. Mater. 5 (2006) 741. W. Lee, K. Schwirn, M.S. Pippel, R. Scholz, U. Gösele, Nat. Nanotechnol. 3 (2008) 234. [9] K. Schwirn, W. Lee, R. Hillebrand, M. Steinhart, K. Nielsch, U. Gösele, Acs Nano 2 (2008) 302. [10] K. Lee, Y. Tang, M. Ouyang, Nano Lett. 8 (2008) 4624. [11] S. Zhao, K. Chan, A. Yelon, T. Veres, Adv. Mater. 19 (2007) 3004.
Contents lists available at ScienceDirect
Thin Solid Films journal homepage: www.elsevier.com/locate/tsf
Preparation of self-ordered nanoporous anodic aluminum oxide membranes by combination of hard anodization and mild anodization Jia Liu a,b, Shu Liu b, Haihui Zhou a,b, Congjia Xie b, Zhongyuan Huang b, Chaopeng Fu b, Yafei Kuang b,⁎ a b
State Key Laboratory for Chemo/Biosensing and Chemometrics, Hunan University, Changsha 410082, China College of Chemistry and Chemical Engineering, Hunan University, Changsha 410082, China
a r t i c l e
i n f o
Article history: Received 25 March 2013 Received in revised form 13 December 2013 Accepted 13 December 2013 Available online 18 December 2013 Keywords: Anodic aluminum oxide templates Two-step anodization Hard anodization Mild anodization
a b s t r a c t Anodic aluminum oxide (AAO) is one of the most important templates for fabrication of nano-materials. In this paper, we present a fast two-step anodization method to prepare self-ordered AAO films. The first step is hard anodization (HA) in H2SO4 and the second step is mild anodization (MA) in H2C2O4. HA in H2SO4 at a wide range of anodization voltage provided a fast formation of ordered arrays of pits on Al sheet, and in the second step, arrays of pits on Al substrate affected nucleation initially, and the anodization voltage of H2C2O4 MA determined the degree of AAO later. Then the best anodization voltage of H2C2O4 MA was validated to be around 40 V. The improved combination of HA and MA not only shortens the time of the first anodization step compared to conventional two-step anodization which was reported by Masuda and Fukuda (1995), but also keeps a highly ordered AAO within large area. Moreover, the relationship between the size of arrays of pits formed in H2SO4 HA and degree of order of AAO formed in H2C2O4 MA was investigated in detail. © 2014 Elsevier B.V. All rights reserved.
1. Introduction Ordered nanochannel-array materials have attracted much attention over the last decades due to their broad applications in synthesis of nano-structured magnetic, electronic and optoelectronic materials [1–3]. Anodic aluminum oxide (AAO) films as templates have played an important part in preparing ordered nanochannel-array materials. Ordered AAO films have specific structure parameters with closepacked hexagonal cells, which can be controlled by changing anodization conditions. Many works have been done on fabrication of highly ordered AAO templates in the past decades [4–8]. Masuda H et al. employed the method of two-step mild anodization (MA) and etching to prepare a highly ordered AAO film in oxalic acid solution [4,5]. In the two-step anodization, it cost 10 or more hours at 40 V in oxalic acid solution to form a relatively thick AAO film on the Al substrate for the first step, and ordered arrays of pits were obtained after removing the AAO film. Secondly, the obtained Al sheet was anodized again and as a result a highly ordered AAO film with enough mechanical robustness and high aspect ratio was obtained due to the induction of the arrays of pits left on Al substrate. Without induced by ordered arrays of pits on Al substrate, it usually needs a very long time for selfadjustment from disordered to a highly ordered AAO. So, it takes a long time for the first step in Masuda H's two-step anodization. Meanwhile, the highly ordered cells appear only in a quite narrow voltage window, which leads the interpore distance or cell size to be confined to some fixed values in mild anodization (MA). For example, the best ⁎ Corresponding author. Tel.: +86 0731 88821603; fax: +86 0731 88713642. E-mail address: [email protected] (Y. Kuang). 0040-6090/$ – see front matter © 2014 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.tsf.2013.12.023
ordering anodization voltage in oxalic acid solution is about 40 V. Thus, there is a great limitation in Masuda H's two-step anodization process to form ordered arrays of pits on Al. In the method of etching, the area of a highly ordered AAO domain can only reach to square millimeter level and the aspect ratio of the formed AAO is low. The high cost and time consuming preparation process limited practical application of AAO. The key of two-step anodization method is to obtain highly ordered arrays of pits on Al substrate. Chu et al. [6] employed high electric field to fabricate a highly ordered AAO rapidly in aged sulfuric acid solution. Lee W et al. [7–9] used a thin pre-existed film (about 400 nm in thickness) forming on the Al substrate to avoid local catastrophe, then prepared a highly ordered AAO film in oxalic acid or sulfuric acid solution through high electric field. High electric field led to rapid formation of self-ordered AAO. However, it also caused high mechanical stress in AAO, bringing the boundedness in practical applications. Fast fabrication of a highly ordered AAO in aged sulfuric acid solution by hard anodization (HA) provides an improved way for the first step to prepare highly ordered arrays of pits on Al substrate in two-step anodization process. In this work, we present a fast and improved two-step anodization method to prepare a highly ordered AAO. Fig. 1 shows the schematic diagram for the synthetic process. For the first step, HA in sulfuric acid is employed for less than 30 min to achieve ordered arrays of pits on Al substrate. For the second step, MA is conducted in oxalic acid. The relationship between the behavior of the second anodization step in oxalic acid and pits left on Al substrate after the first anodization step is also discussed. The improved combination of HA and MA not only shortens the time of the first anodization step compared to Masuda
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There are three methods to fabricate a highly ordered AAO in H2SO4 hard anodization (HA): (1) anodization under potentiostatic mode [6]; (2) anodization under galvanostatic mode [10]; and (3) anodization under potentiostatic mode of Al substrate with a thin pre-existed film (about 400 nm in thickness) [7,9]. Compared with methods (1) and (2), method (3) can avoid phenomena of burning and cracking, and the operation of method (3) is simpler. So method (3) was employed in this work.
Fig. 1. Schematic diagram for the synthetic process.
H's two-step anodization [4] and keeps a highly ordered AAO within large area, but also decreases the high mechanical stress in AAO formed in HA. The process is characterized by low cost, time-saving and wide preparation parameter scope. 2. Experimental section 2.1. Fabrication of a highly ordered AAO in sulfuric solution 2.1.1. Pretreatment High purity aluminum plates (99.999%, 20 mm × 10 mm × 0.5 mm, Beijing Research Institute for Nonferrous Metals, China) were used as working electrodes. Prior to anodization, all the Al sheets were degreased by ultrasonic in ethanol for 5 min and then electrochemically polished in a mixture solution of 65% HClO4 and 99.5% ethanol (VHClO4/Vethanol = 1/4) with vigorous stirring at 0 °C and 20 V for 5 min to achieve mirror finished surfaces in order to eliminate the influence of natural oxide film on the aluminum surfaces.
2.1.2. Anodization 0.3 M and 0.04 M sulfuric acid solutions were employed under potentiostatic mode with U from 35 V to 70 V. For 0.3 M sulfuric acid solution the voltage was used below 40 V, and for 0.04 M sulfuric solution the voltage was used above 40 V. Anodization was performed in the corresponding sulfuric acid solution at 5 ± 2 °C with vigorous magnetic and compressed air stirring. A powerful cooling system and a large electrolysis bath (1 L) were used to maintain the low temperature needed for the high-field anodization. The aluminum sheets were anodized at 25 V (MA) for 10 min in the corresponding solution to generate a thin porous alumina surface layer which could avoid the burning or cracking in subsequent step. U was gradually increased to Utarget of 35 V to 70 V at a rate of 0.1 V s− 1. Subsequently, the as-anodized sample was immersed into a mixture of phosphoric acid (6 wt.%) and chromic acid (1.8 wt.%) at 70 °C for 5 h to remove the alumina layer. 2.2. Fabrication of ordered AAO in oxalic acid solution After removing the AAO formed in sulfuric acid, the second anodization step was conducted on the aluminum sheet in 0.3 M oxalic acid at 17 ± 2 °C with a voltage of 30 V, 32 V, 38 V, 40 V, 44 V and 52 V, respectively. Next, the sample was immersed in a mixture solution of 0.2 M CuCl2 and 6.1 M HCl to remove the underlying Al substrate. Finally, a free-standing AAO film was obtained. The barrier layer and cross-sectional morphologies of the as-prepared AAO film were
Fig. 2. FESEM images of the bottom surface of AAOs prepared in 0.3 M H2SO4 at 40 V for different time. (b) 10 min (j decreases to 125 mA cm−2), (c) 30 min (55 mA cm−2) and (d) 60 min (40 mA cm−2). (a) is the relationship between current density and time in H2SO4 HA at 40 V.
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Fig. 3. FESEM images of AAOs formed in H2SO4 HA at various potentials for 30 min. (a) 35 V, (b) 55 V, (c) 60 V and (d) 65 V. (e) is the relationship between cell size and anodization voltage of H2SO4 HA.
Fig. 4. FESEM images of AAOs prepared in H2SO4 at 50 V for 30 min (60 mA cm−2). (a) is a typical bottom surface and (b) is the highly ordered arrays of pits on Al substrate after removing AAO.
characterized by a field emission scanning electron microscopy (SEM, Hitachi S-4800). 3. Results and discussion 3.1. Fabrication of various sizes of highly ordered pits on Al substrate Under hard anodization condition, the samples present plastic deformation, accompanying many cracks and burns on the surface [7–9]. For example, when the anodization voltage increases to 50 V, AAO exhibits a dark gray color, with a bunch of gas bubble holes on the surface. In the anodization process, when U reaches to Utarget, j firstly increases to jmax as shown in Fig. 2a, and then decreases exponentially, stable anodization begins. Fig. 2b, c and d shows typical bottom surface images of AAO films prepared in 0.3 M sulfuric acid solution at 40 V for 10 min (125 mA cm− 2), 30 min (55 mA cm− 2) and 60 min (40 mA cm− 2), respectively. It can be observed from Fig. 2b, c and d that the degree of order of AAO has no obvious change with time prolonging, and the cell size also keeps consistent with each other in Fig. 2b, c and d. The cell size here always keeps around 80 nm, differing from the result reported by Lee W that cell size of AAO increased from 64 nm to 78 nm when current density decreased from 130 mA cm − 2 to 40 mA cm − 2 [9]. The result provides a firm evidence that highly ordered arrays of pits can be formed fast on Al substrate in H2SO4 HA. Fig. 3 shows SEM images of bottom surface of highly ordered AAOs formed in H2SO4 HA at various anodization voltages and the relationship between cell size and anodization voltage. As shown in Fig. 3,
AAO films formed in H2SO4 HA present a high degree of order, and cell size and anodization voltage present a linear relationship with a slope of about 1.9 nm V−1. Junctions of three cell boundaries could be easily corroded by the mixture solution of HCl/CuCl2 [11], also it can be found that voids appear in the junctions of three cell boundaries which exhibit weak cell junctions. Fig. 4a shows the morphology of bottom surface of the AAOs formed at 50 V in H2SO4 HA, which exhibits a high degree of order. Fig. 4b shows the highly ordered pits on Al
Fig. 5. Relationship between cell size and anodization voltage of H 2 C 2 O 4 MA. The correlation coefficient estimated from the linear regression is ζMA = 2.5 nm V−1.
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Fig. 6. FESEM images of AAOs prepared in 0.3 M H2C2O4 at 17 ± 2 °C at the matched voltages. (a), (b), (c) and (d) are the bottom surface images of AAOs prepared at 30 V, 32 V, 44 V and 52 V in H2C2O4, which are induced by the arrays of pits formed in H2SO4 at 35 V, 40 V, 55 V and 65 V respectively.
Fig. 7. FESEM images of AAOs prepared in 0.3 M H2C2O4 at 17 ± 2 °C at the matched voltage. (a), (b) and (c) are the bottom surface images of AAOs prepared at 38 V in H2C2O4 for 1 h, 4 h and 8 h respectively, which are induced by the arrays of pits formed in H2SO4 at 50 V. (d) is the cross-section image corresponding to (c).
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Fig. 8. FESEM images of AAOs prepared in 0.3 M H2C2O4 at 17 ± 2 °C and 40 V for 16 h for the second anodization step. (a), (c), (e) and (g) are the top surface images of the AAOs induced by the arrays of pits formed in H2SO4 at 35 V, 40 V, 50 V and 55 V respectively. (b), (d), (f) and (h) are the bottom surface images corresponding to (a), (c), (e) and (g) respectively.
substrate. The degree of order of pits corresponds to that shown in Fig. 4a after removing the AAO film. It can be noted that the ordering anodization voltage window of H2SO4 HA is much wider than that of H2C2O4 MA and ordered AAO can be obtained in H2C2O4 by anodizing the Al sheets with the arrays of pits.
3.2. Preparation of a highly ordered AAO by two-step anodization under condition of HA–MA Fig. 5 shows that the cell size is also proportional to the anodization voltage of H2C2O4 MA with a slope of ζMA = 2.5 nm V−1, which is
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larger than that in H2SO4 HA. So the effect of anodization voltage on cell size in H2SO4 HA is different from that in H2C2O4 MA. According to Figs. 3e and 5, we can determine the relationship between the size of arrays of pits formed on Al substrate in H2SO4 HA and the cell size of AAO formed in H2C2O4 MA. According to the changing regularities of size of arrays of pits formed in H2SO4 HA and cell size of AAO formed in H2C2O4 MA shown in Figs. 3e and 5, it is known that the size of pits formed in H2SO4 HA at 35 V, 40 V, 50 V, 55 V and 65 V approximately equals to the cell size of AAO formed in H2C2O4 MA at 30 V, 32 V, 38 V, 44 V and 52 V respectively. And so we prepared AAO films in H2C2O4 at five matched states respectively. Fig. 6 shows bottom surface images of AAOs formed at four different voltages of the five matched states. Although the corresponding pits on Al substrate presented a high degree of order, the poorly ordered AAOs were obtained at the four voltages. Therefore, the preparation of a highly ordered AAO in H2C2O4 MA not only depends on the guidance of ordered pits on substrate, but also the voltage of H2C2O4 MA. The experiment results show that the best ordering voltage of H2C2O4 MA is around 40 V. Fig. 7a, b and c shows the bottom surface images of AAO films prepared in 0.3 M H2C2O4 at 17 ± 2 °C and 38 V for 1 h, 4 h and 8 h respectively, which are induced by the pits formed at 50 V in H2SO4. These SEM images show that there is no obvious change in degree of order of AAO, which is much better than that of AAOs formed at the four voltages in Fig. 6. The arrays of channel of AAO grow straightly, as shown in Fig. 7d, and it exhibits a high aspect ratio of 90. Accordingly, AAO with a high-aspect-ratio can be obtained by anodizing Al substrate with highly ordered pits in H2C2O4 at the matched voltage around 40 V. Fig. 8a, c, e and g presents the surface images of AAOs prepared in 0.3 M H2C2O4 at 17 ± 2 °C and 40 V, which are induced by pits formed in H2SO4 at 35 V, 40 V, 50 V and 55 V respectively. The cell sizes in (a), (c), (e) and (g) are 75 nm, 80 nm, 94 nm and 110 nm respectively, which just coincide with the cell sizes of AAOs formed in H2SO4 at
35 V, 40 V, 50 V and 55 V respectively (shown in Figs. 2 to 4). The degree of order of the AAOs is just one-to-one correspondence. It can be noted that, at the early stage of anodization in H2C2O4, the anodization process is extremely affected by the highly ordered arrays of pits left on the Al substrate after the first anodization step. The subsequent anodization voltage has little influence on the growth of AAOs. After the early stage, the process of anodization is mainly affected by the subsequent anodization voltage. Panels b, d, f and h in Fig. 8 are the bottom surface images of AAOs formed in H2C2O4 at 40 V for 16 h, corresponding to Fig. 8a, c, e and g respectively. After a relative long time of adjustment, all the cell sizes of AAOs, determined by the subsequent anodization voltage, are 107 nm. The aforementioned results indicate that, the pits on Al substrate have a great effect on the early growth stage of AAOs in H2C2O4 MA, the cell size of AAOs is the same as the size of pits, but after a long time of self-adjustment of AAOs, the structure and cell size of AAOs are determined by the anodization voltage of H2C2O4 MA. As Fig. 8 shows, the cell sizes of AAOs formed in H2C2O4 for 16 h are almost the same though the difference in the sizes of pits on Al substrate is about 40 nm. Masuda H et al. [5] has proved that, if the size of pits did not match with the anodization voltage, ordered growth of AAO could not maintain any more. Fig. 9a, b, c and d shows the bottom surface images of AAOs prepared in 0.3 M H2C2O4 at 40 V for 1 h, 4 h, 8 h and 16 h respectively, which are induced by pits formed in H2SO4 at 40 V. The size of highly ordered arrays of pits on the Al substrate remains at 80 nm. The cell sizes of subsequent AAOs formed in H2C2O4 are 107 nm, which is much different from the size of arrays of pits. In Fig. 9a, the ordered area is less than 1 μm2, and there exists many defects. The area reaches to about 1 μm2 in Fig. 8b with less defects than in Fig. 9a. The ordered area in Fig. 9c is bigger than that in Fig. 9a and b, and the quantity of defects in Fig. 9c decreases. Finally, the area reaches to several μm2 or level of mm2 in Fig. 9d. These results indicate that, with the adjustment of AAO at the early stage of anodization, the ordered
Fig. 9. FESEM images of AAOs prepared in 0.3 M H2C2O4 at 17 ± 2 °C and 40 V for (a) 1 h, (b) 4 h, (c) 8 h and (d) 16 h respectively, which are induced by pits formed at 40 V in H2SO4.
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area without defects enlarges, but the defects can still be observed at the edge of the ordered domain. 4. Conclusions In summary, a highly ordered AAO was obtained in H2SO4 HA at a wide range of anodization voltage in a short time, and then highly ordered arrays of pits were obtained on the Al sheet after removing the AAO, which can induce the growth of ordered AAO in H2C2O4 MA. At the early stage of anodization in H2C2O4, the growth of AAO is obviously induced by the arrays of pits left on the Al sheet. But after a long time of self-adjustment of AAO, the structure and cell size of AAO are determined by the anodization voltage of H2C2O4 MA. After the Al sheet was anodized for about 10 min or more in H2SO4 at around 50 V, AAO with a good degree of order and high aspect ratio could be prepared at around the best ordering anodization voltage (40 V) in H2C2O4. This time-saving, low-cost and simple process is proved to be a promising way to prepare AAO with potential applications in fabrication of nanomaterials.
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Acknowledgment This work was supported by the National Natural Science Foundation of China (Grant Nos. 51071067, J1103312, J1210040, 21271069, 51238002).
References [1] [2] [3] [4] [5] [6] [7] [8]
R.J. Tonucci, B.L. Justus, A.J. Campillo, C.E. Ford, Science 258 (1992) 783. T.W. Whitney, J.S. Jiang, P.C. Searson, C.L. Chien, Science 261 (1993) 1316. P.L. Lu, S.H. Charap, IEEE Trans. Magn. 30 (1994) 4230. H. Masuda, K. Fukuda, Science 268 (1995) 1466. H. Masuda, H. Yamada, M. Satoh, H. Asoh, M. Nakao, Appl. Phys. Lett. 71 (1997) 2770. S.Z. Chu, K. Wada, S. Inoue, M. Isogai, A. Yasumori, Adv. Mater. 17 (2005) 2115. W. Lee, R. Ji, U. Gösele, K. Nielsch, Nat. Mater. 5 (2006) 741. W. Lee, K. Schwirn, M.S. Pippel, R. Scholz, U. Gösele, Nat. Nanotechnol. 3 (2008) 234. [9] K. Schwirn, W. Lee, R. Hillebrand, M. Steinhart, K. Nielsch, U. Gösele, Acs Nano 2 (2008) 302. [10] K. Lee, Y. Tang, M. Ouyang, Nano Lett. 8 (2008) 4624. [11] S. Zhao, K. Chan, A. Yelon, T. Veres, Adv. Mater. 19 (2007) 3004.