Preparation of Porous Anodic Alumina with Periodically Perforated

Apr 24, 2009 - Mohammed Alsawat , Tariq Altalhi , Abel Santos , and Dusan Losic. The Journal of Physical Chemistry C 2017 121 (25), 13634-13644...
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Preparation of Porous Anodic Alumina with Periodically Perforated Pores Dusan Losic* and Dusan Losic, Jr. University of South Australia, Ian Wark Research Institute, Mawson Lakes, Adelaide, SA 5095, Australia Received December 28, 2008. Revised Manuscript Received April 13, 2009 Anodization of aluminum is an excellent nonlithographic alternative to conventional fabrication approaches for lowcost and large-scale synthesis of a variety of nanostructured materials. In this work, the preparation of anodic alumina oxide (AAO) with unique three-dimensional (3D) porous structures that consist of periodically perforated nanopores is reported. The fabrication method combines electrochemical anodization of aluminum and chemical etching. The key feature of this process is cyclic anodization where an oscillatory current signal was applied to create AAO with periodically shaped pore structures. Spatially specific dissolution of the pore walls was directed by modulated pore structures during chemical etching to generate hexagonally ordered arrays of holes with periodic distribution across the pore length.

Introduction The emerging field of nonlithographic nanofabrication has attracted considerable attention in recent years for engineering of nanomaterials with arrayed nanostructures.1 A number of new fabrication techniques that approach the limits of conventional photolithography have been introduced, including copolymer block lithography, vapor-based deposition methods, colloidal methods, sol-gel, template synthesis, and self-assembly.2-5 Among them, the self-ordering process based on electrochemical anodization of metals such as aluminum, titanium, and silicon is one of the most popular nonlithographic approaches for the design of novel nanostructured materials and devices with new properties and functions.6,7 Anodization of aluminum is generally referred to as the process of self-driven formation of cylindrical and vertically aligned arrays of pores with nanoscale diameters.6,8 Numerous studies over the years have explored anodization conditions such as voltage, current, electrolyte composition, concentration, temperature, and prepatterning of surface to achieve a self-ordering regime and highly ordered anodic alumina oxide (AAO) pore structures.9-13 AAO has attracted considerable interest for applications in template synthesis, filtration, catalysis, sensing, *Corresponding author: E-mail address: [email protected]. Tel.: +618 8302 6862. Fax: +618 8302 3683. (1) Chik, H.; Xu, J. M. Mater. Sci. Eng., R 2004, 43, 103–138. (2) Fisher, A.; Kuemmel, M.; Jarn, M.; Linden, M.; Boissiere, C.; Nicole, L.; Sanchez, C.; Grosso, D. Small 2006, 2, 569–574. (3) Colfen, H.; Mann, S. Angew. Chem., Int. Ed. 2003, 42, 2350–2365. (4) Jung, J. M.; Kwon, K. Y.; Ha, T. H.; Chung, B. H.; Jung, H. T. Small 2006, 2, 1010–1015. (5) Losic, D.; Mitchell, J. G.; Voelcker, N. H. Chem. Commun. 2005, 39, 4905– 4907. (6) Diggle, J. W.; Downie, T. C.; Goulding, C. W. Chem. Rev. 1969, 69, 365–405. (7) Thompson, G. E.; Wood, G. C. Nature (London) 1981, 290, 230–232. (8) Thompson, G. E. Thin Solid Films 1997, 297, 192–201. (9) Jessensky, O.; Muller, F.; Gosele, U. J. Electrochem. Soc. 1998, 145, 3735– 3740. (10) Li, F. Y.; Zhang, L.; Metzger, R. M. Chem. Mater. 1998, 10, 2470–2480. (11) Masuda, H.; Fukuda, K. Science 1995, 268, 1466–1468. (12) Ono, S.; Saito, M.; Ishiguro, M.; Asoh, H. J. Electrochem. Soc. 2004, 151, B473–B478. (13) Nielsch, K.; Choi, J.; Schwirn, K.; Wehrspohn, R. B.; Gosele, U. Nano Lett. 2002, 2, 677–680. (14) Martin, C. R.; Kohli, P. Nat. Rev. Drug Discovery 2003, 2, 29–37. (15) Xian, Y. Z.; Hu, Y.; Liu, F.; Xian, Y.; Feng, L. J.; Jin, L. T. Biosens. Bioelectron. 2007, 22, 2827–2833. (16) Chen, W.; Yuan, J. H.; Xia, X. H. Anal. Chem. 2005, 77, 8102–8108.

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photonics, energy storage, cell culturing, and drug delivery.14-21 In general, three well-studied growth regimes using conventional, so-called mild anodization (MA) in H2SO4, H2C2O4, and H3PO4 are accepted as the optimal conditions for AAO fabrication.9-13 To solve the problem of very slow anodization by MA, typically requiring a fabrication time of several days, a new method called hard anodization (HA) has recently been introduced to considerably speed up the process.22-24 More recently, several new approaches have been reported for engineering of various nanostructures using self-ordered nonlithographic porous alumina.24-27 AAO templates with complex architectures were fabricated and used in combination with other fabrication methods, which opened new prospects in the controlled synthesis of a variety of nanostructured materials with advanced electric, magnetic, and optical properties.28-31 Several groups have reported the fabrication of AAO with “Y”-branched, partially branched, and multiple connected pores, created by reducing the voltage during anodization, where the number of branches correlates to the changes in the anodization voltage.25,32-34 (17) Losic, D.; Shapter, J. G.; Mitchell, J. G.; Voelcker, N. H. Nanotechnology 2005, 16, 2275–2281. (18) Schneider, J. J.; Engstler, J. Eur. J. Inorg. Chem. 2006, 9, 1723–1736. (19) Losic, D.; Cole, M. A.; Dollmann, B.; Vasilev, K.; Griesser, H. J. Nanotechnology 2008, 19, 245704. (20) La Flamme, K. E.; Popat, K. C.; Leoni, L.; Markiewicz, E.; La Tempa, T. J.; Roman, B. B.; Grimes, C. A.; Desai, T. A. Biomaterials 2007, 28, 2638–2645. (21) Kang, H. J.; Kim, D. J.; Park, S. J.; Yoo, J. B.; Ryu, Y. S. Thin Solid Films 2007, 515, 5184–5187. (22) Lee, W.; Ji, R.; Gosele, U.; Nielsch, K. Nat. Mater. 2006, 5, 741–747. (23) Li, Y.; Ling, Z. Y.; Chen, S. S.; Cwang, J. Nanotechnology 2008, 19, 225604. (24) Chu, S. Z.; Wada, K.; Inoue, S.; Isogai, M.; Yasumori, A. Adv. Mater. 2005, 17, 2115–2119. (25) Meng, G. W.; Jung, Y. J.; Cao, A. Y.; Vajtai, R.; Ajayan, P. M. Proc. Natl. Acad. Sci. U. S. A. 2005, 102, 7074–7078. (26) Yamauchi, Y.; Nagaura, T.; Ishikawa, A.; Chikyow, T.; Inoue, S. J. Am. Chem. Soc. 2008, 130, 10165–10170. (27) Ho, A. Y. Y.; Gao, H.; Lam, Y. C.; Rodriguez, I. Adv. Funct. Mater. 2008, 18, 2057–2063. (28) Chen, W.; Wu, J. S.; Xia, X. H. ACS Nano 2008, 2, 959–965. (29) Li, J.; Papadopoulos, C.; Xu, J. Nature (London) 1999, 402, 253–254. (30) Li, L.; Pan, S. S.; Dou, X. C.; Zhu, Y. G.; Huang, X. H.; Yang, Y. W.; Li, G. H.; Zhang, L. D. J. Phys. Chem. C 2007, 111, 7288–7291. (31) Li, L.; Koshizaki, N.; Li, G. H. J. Mater. Sci. Technol. 2008, 24, 550–562. (32) Zhang, L.; Cheng, B.; Shi, W. S.; Samulski, E. T. J. Mater. Chem. 2005, 15, 4889–4893. (33) Wang, B.; Fei, G. T.; Wang, M.; Kong, M. G.; Zhang, L. D. Nanotechnology 2007, 18, 365601. (34) Krishnan, R.; Thompson, C. V. Adv. Mater. 2007, 19, 988–992.

Published on Web 4/24/2009

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Figure 1. Fabrication scheme of AAO with periodically perforated pores (nanopores with nanoholes), which combines electrochemical anodization and chemical etching. (a) Fabrication of AAO with modulated pore structures by cyclic anodization of aluminum and followed by removal of aluminum with (b) chemical etching of AAO by 5% H3PO4 that includes the following steps: (c) the pore opening, (d) the pore widening, and (e) the hole opening inside of pores. A top view model of a single pore is presented in the insets.

Geometrically shaped aluminum substrate has been used by Zakeri et al. to prepare AAO with nonlinear and dendritic pores.35 A method that includes several consecutive anodization and chemical etching steps has been introduced by Ho et al. to prepare AAO with multilayered and hierarchical three-dimensional (3D) nanostructures.27 Krishnan and Thompson used a stepwise anodization with two different electrolytes to prepare AAO with two-step diameter pores termed as “nanofunnels”.34 Combined conventional MA and HA processes have been applied by Lee et al. for the fabrication of modulated pore structures with periodic changes in diameter using pulsing anodization.36 Recently, we introduced a new anodization process, termed cyclic anodization for controlled 3D structuring of AAO with shaped pore geometries.37 The method is based on slow and oscillatory changes of anodization conditions that combines MA and HA. By applying continuous oscillatory current signals during anodization, with different shapes (profiles), amplitudes, and periods, the fabrication of AAO with shaped internal geometry of pores is demonstrated.37 In this work we report the fabrication of unique “nanopores with nanoholes” AAO architecture that consists of nanopores interconnected with ordered arrays of nanoholes. The fabrication approach combines electrochemical anodization of aluminum and chemical etching. The scheme is outlined in Figure 1. The core part of this approach is the synthesis of AAO with modulated pore structures using the cyclic anodization approach (Figure 1a). The subsequent chemical etching of these AAOs creates holes inside of the pores as a result of spatially specific dissolution of the oxide layer between pores (the wall), directed by the periodically shaped structures formed by cyclic anodization (Figure 1b-e). This work is primarily motivated by the prospect of these unique porous structures for the development of novel and inexpensive molecular sieves and filtering devices. In addition, their applications for template synthesis of a variety of nanostructured materials with distinctive optical, electrical, and magnetic properties are also anticipated. (35) Zakeri, R.; Watts, C.; Wang, H.; Kohli, P. Chem. Mater. 2007, 19, 1954– 1963. (36) Lee, W.; Schwirn, K.; Steinhart, M.; Pippel, E.; Scholz, R.; Gosele, U. Nat. Nanotechnol. 2008, 3, 234–239. (37) Losic, D.; Lillo, M.; Losic, D.. Small 2009; doi: 10.1002/smll.200801645.

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Results and Discussion A cross-sectional scanning electron microscopy (SEM) image of AAO pore structures and corresponding current-time signals applied during cyclic anodizaton of aluminum in 0.1 M H3PO4 using galvanostatic mode is presented in Figure 2a. To achieve pore nanostructuring of AAO and control the shape, length, and periodicity of shaped nanostructures, using cyclic anodization, it is essential to optimize the anodization conditions by adjusting the applied current signal, including profile, amplitude, and period to combine the contributions of HA and MA conditions during a single cycle.37 Figure 2a shows a typical example of fabricated pore structures when an asymmetrical anodization current signal (sawtooth) with amplitudes of Imin = 5 mA cm-2 to Imiax = 120 mA cm-2 is applied. The image confirms modulated morphology of AAO with periodically structured pore geometry with a longitudinal feature length of 600-800 nm and diameters of 50-200 nm. Apart from the differences of internal pore geometry, AAO fabricated by cyclic anodization shows topography of barrier oxide layer on the bottom surface (Figure 2b) with ordered hexagonal organization, which confirms the same topography as the AAO fabricated by conventional anodization procedures. When AAO with modulated pore structures was exposed to chemical etching in H3PO4 (5 wt % at 35 °C), for a certain time (130-150 min), periodic perforations of the cell wall between pores were created as a result of dissolution of the internal oxide layer (Figure 2c,d). A unique 3D nanoporous AAO architecture, which combines nanopores and nanoholes, and a significant increase of internal porosity was fabricated. These nanoholes with dimensions of 60-100 nm in diameter were located periodically inside of the pore with a longitudinal period of 600-800 nm across the pore length at each plane of the hexagonal pore cell (Figure 2d). SEM image shows that a very thin oxide layer remains between pores, which was sporadically broken as a result of the extended etching. The cross-section images confirm a periodic and longitudinal distribution of holes with the same interdistance inside of the pores (Figure 2c). The model of a pore with holes (top view and cross-section) is illustrated (Figure 2e) showing the location and spatial organization of holes in more detail. A similarity with the corresponding image obtained from initial AAO fabricated by cyclic anodization is evident. Some of DOI: 10.1021/la804281v

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Figure 2. Fabrication of AAO with “nanopores with nanoholes”. (a) AAO with modulated pore structures fabricated by cyclic anodization of aluminum in 0.1 M H3PO4. Corresponding current signal applied during anodization process is shown at the top. (b) Bottom surface of fabricated AAO with typical topography of barrier oxide layer. (c,d) AAO with periodic nanoholes inside of pores (cross-section and top view) created by chemical etching of AAO in H3PO4 (5 wt %, 35 °C) for 140 min. (e) Model of the top view of hexagonal array of AAO pores (right) showing the location (arrows) where the barrier oxide layer in the pore wall was removed. A model of a single AAO pore structure (profile) showing the arrays of holes located at each plane of the hexagonal AAO cell (left).

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Figure 3. The formation of holes in a pore. (a) Cross-section SEM image of AAO with typical modulated pore structures. Bar scale is 500 nm. (b) SEM image of pore structure (left) fabricated by a single current cycle (right) which includes MA and HA. Bar scale is 100 nm. (c) Scheme of the pores and pore wall during the dissolution process. The pore widening and the location where the holes were created during the etching process are marked (dots and arrows).

the wall structures can be seen (Figure 2c) disconnected from the neighboring junctions of the pore walls. This is a result of the cleavage process to obtain a cross-section for SEM imaging. However, we should state that the entire AAO structure was selfstanding and solid after chemical etching, and allowed for handling in the same way as common AAO membranes. Chemical etching using control samples of AAO with flat pores fabricated by conventional anodization, either HA or MA showed thinning of the pore walls but did not show the pore perforation effect (Supporting Information, Figure S1). If the etching time was extended, the pore walls break down, and depending on the cleavage/breakage in the nanopore walls, different nanostructures such as tips or brushes were formed. Thus, it is reasonable to conclude that the morphological origin directed by the modulated pore features of AAO fabricated by cyclic anodization is a key feature for the creation of nanopores with nanoholes structures. The cross-sectional images (Figure 2a,c) confirm that holes in pores are formed at periodic locations where the pore diameter is largest. The pore wall at these locations is the thinnest, and the oxide layer is expected to dissolve earlier than at any other parts, thus creating the holes. AAO with modulated pore structures was formed by cyclic anodization as a result of changing the anodization conditions during a current cycle to include MA and HA anodization.37 This method allows routine fabrication of AAO with desired thickness, and complex pore architectures. SEM image of typical selfstanding AAO with modulated pores fabricated by cyclic anodization using 30 cycles is shown in Figure S2a-c (Supporting Information). To better understand the formation of periodically modulated pores and the formation of holes within the pore, the single current cycle and corresponding pore structure formed by this signal is presented in Figure 3a,b. Three characteristic parts of the pore structure can be recognized that correspond to the different values of current signals or anodization modes. The smallest pore diameter and shortest part corresponds to the minimum current of the applied cycle and MA anodization condition, and the largest diameter part corresponds to the maximum current and HA anodization. The slope in the current between these two modes, we termed transitional anodization (TA), corresponds to the condition, which determines the shape of the modulated pore segment.37 However, we should state that the process of oxide growth and dissolution during this intermediate condition is still not well understood.37 The corresponding model of the pore widening and the hole opening of single pore structure is presented in Figure 3c to better clarify the relation between modulated pores and holes. The parts of pore formed by periodic HA anodization are locations where the holes are created and where extensive dissolution during chemical etching occurs. Langmuir 2009, 25(10), 5426–5431

Recent studies confirmed that oxide layers of AAO fabricated by MA and HA anodization have different properties such as thickness, ion conductivity, and chemical composition, as a result of different growth rates and the incorporation of anions from electrolytes during anodization.18,22-24,38 Therefore, in addition to the morphological factors, the influence of different dissolution properties (rate) of the MA and HA oxide layer can be another factor that promotes the spatially selective etching of the pore walls. In this work, to fabricate “nanopores with nanoholes” structures, we applied chemical etching of AAO in H3PO4 controlled by time. We selected the optimal etching time between 130 and 150 min based on the dissolution rate of the bottom oxide layer (2.5 nm/min), determined from experimental protocol by the twoelectrode method and SEM imaging, described in our previous work (Figure S3a, Supporting Information).39 The etching of AAO with modulated pores was performed from the bottom side, and, after the pore-opening step, the continuing etching process involves the dissolution of the wall between pores (pore widening). It is desired to terminate the etching process at the point when the holes are opened, before breakage of the walls, but in experimental procedures where chemical etching is controlled by time, it is difficult to precisely determine this point (Figure 3c). The consequences are particularly critical when etching conditions (temperature, H3PO4 concentration) are changed. Examples of AAO fabricated using the same etching time (140 min) but with higher concentration of H3PO4 (10%), and higher temperature (40 °C) are presented in Figure 4a. AAO with nanostructured alumina pillars was formed, and these structures are obviously generated as a result of dissolution of the oxide layer between holes and the separation of the neighboring junctions of the pore walls. These pillar-like structures with lengths of several microns were decorated with characteristic periodic and half-circular features with size and distribution similar to the hole structures seen in Figure 2c,d. Some of these structures were collapsed or grouped into bunches. Although, it was not the intention to make these structures, this result shows that another attractive structural feature of AAO can be obtained by this approach. Another example to demonstrate the impact of etching time using the same etching condition (5% H3PO4, 35 °C) but with a longer time (220 min) is shown in Figure S4 (Supporting Information). AAO with completely dissolved pore structure was obtained, which confirms that extensive etching after hole opening is equally destructive. To address the problem of controlled hole opening and to make fabrication of AAO with perforated nanopores more (38) Han, C. Y.; Willing, G. A.; Xiao, Z. L.; Wang, H. H. Langmuir 2007, 23, 1564–1568. (39) Lillo, M.; Losic, D. J. Membr. Sci. 2009, 327, 11–17.

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Figure 4. (a) AAO with nanostructured alumina pillars with periodic circular shapes fabricated by extensive chemical etching (10 wt % H3PO4, 40 °C); (b,c) Scheme of the model (top view and cross-section) describing the formation of these nanostructured pillars as a result of the removal of the internal barrier layer between holes by the etching process.

reproducible, we combined the etching process with the pore opening detection method using two-electrode electrochemical detection, we recently introduced for controlled fabrication of free-standing AAO membranes.39 This method allows the precise detection of the point when the barrier layer at the bottom of pores is opened during chemical etching. Although this method provides useful information about progress of the oxide dissolution process, it does not provide information about the hole opening point (Figure S3b, Supporting Information). However, when we applied this method from the top of AAO, rather than from the bottom, and terminated etching 15 min after the pore opening point, we found AAO pore structures similar to those we observed in Figure 2 c,d. (Figure S3c, Supporting Information). If we assumed that the thickness of the pore wall (D = 2d ) is double the thickness of the bottom barrier layer, then the pore opening point can be used as a reasonably good practical indicator for controlling the etching process. Further study is underway on the implementation of this method to control the hole-opening process and achieve reproducible fabrication of these 3D nanoporous AAO structures. The unique architecture of fabricated AAO with an array of periodic nanoholes inside of pores is believed to have interesting properties that have potential to be applied as molecular sieving devices. Following the similarity of these porous structures with pillar-based nanostructures, whose sieving properties based on hindered transport of molecules with different molecular sizes are confirmed by several groups, we believe that a similar strategy can (40) Han, J.; Fu, J.; Schoch, R. B. Lab Chip 2008, 8, 23–33.

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be used with this matrix.40,41 If the diameter of the holes (in pores) is comparable to the size of macromolecules, then these structures could act as entropic barriers in an electric-field partitioning process.39,40 The main advantage of this porous matrix in comparison with existing molecular sieving devices reported in literature is their inexpensive fabrication using low-cost materials (aluminum foil) and setup (power supply). The 3D nanoporous structure with very large surface area, adjustable surface chemistry, and biocompatibility make this AAO matrix a desirable porous platform for drug delivery applications. Nanoporous materials have been extensively explored in recent years for drug delivery as implantable devices to achieve sustained release, and the particular advantage of this matrix is based on its large loading capacity and the capability to accommodate water insoluble therapeutics in the literature such as drugs, proteins, and genes, including drug-carriers.42,43 In summary, a low-cost fabrication approach for creating AAO with unique 3D ordered porous architecture of nanopores perforated with arrays of nanoholes was demonstrated. The nanoholes are located at the pore walls, hexagonally ordered, and periodic in distribution across the length of the pore. The method is based on chemical etching of AAO substrates with modulated pore structures prepared by cyclic anodization using oscillatory current signals. This fabrication procedure combines anodization and chemical etching, and offers a promising (41) Schoch, R. B.; Han, J.; Renaud, P. Rev. Mod. Phys. 2008, 80, 839–883. (42) Ainslie, K. M.; Desai, T. A. Lab Chip 2008, 8, 1864–1878. (43) Prasad, S.; Quijano, J. Biosens. Bioelectron. 2006, 21, 1219–1229.

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alternative for the creation of complex and ordered 3D porous structures of AAO for diverse applications, especially in the development of new molecular separation and drug delivery devices.

Experimental Section A high purity (99.997%) aluminum foil with 0.1 mm thickness supplied by Alfa Aesar (USA) was used for fabrication of AAO. The foil was cleaned in acetone and then electrochemically polished in a 1:4 volume mixture of HClO4 and C2 H5 OH by a constant voltage of 20 V for 2 min to achieve a mirror finished surface. Two-step anodization was performed using an electrochemical cell equipped with a cooling stage at a temperature of -1 °C. The first anodization step was performed under 40 V for 6-8 h in 0.3 M H2C2O4. Afterward, the formed porous oxide film was chemically removed by a mixture of 6 wt % of H3PO4 and 1.8% chromic acid for a minimum of 3-6 h at 75 °C. The second anodization step was performed by cyclic anodization in 0.1 M H3PO4 at -1 °C using a personal computer controlled power supply (Agilent, USA) and Labview based software (National Instruments, USA).37 Anodization in H3PO4 rather than in H2C2O4 was applied during the second anodization step because this acid provides AAO with larger pores and more suitable for SEM visualization. Galvanostatic cyclic anodization was performed using continuous current cycles with sawtooth profile, amplitudes Imin = 5-10 mA to Imax = 100-120 mA cm-2, periods t = 0.25-1 min and 20-50 cycles. These parameters were

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adjusted to correspond to the desired anodization condition and modes (MA, TA, and HA) during the cyclic process. Galvanostatic mode was applied instead of potentiostatic mode, as it is more controllable and provides more reproducible fabrication results. Current-time and voltage-time signals were continuously recorded during the anodization process. After anodization, the remaining Al layer was removed from the AAO films using CuCl2/HCl solution. Chemical etching of AAO in 5% H3PO4 at 25 °C was performed for 130-150 min to achieve the removal of the bottom barrier layer of AAO (pore opening), and hole opening inside of the pores. The etching process was controlled by time, but the process was also additionally monitored with the method for controlled dissolution of the bottom barrier layer using a twoelectrode system.39 SEM Philips Xl-30 was used to characterize the pore structures of fabricated AAO.

Acknowledgment. The authors acknowledge the financial support of the Australian Research Council (DP 0770930) and the University of South Australia for this work. Supporting Information Available: SEM image of chemical etching of AAO with straight pores. SEM images of AAO with modulated pores fabricated by cyclic anodization. The description of two-electrode detection of pore opening and chemical etching. Scheme of experimental setup and current curves of monitoring of etching process. SEM image of AAO fabricated by longer etching time. This material is available free of charge via the Internet at http://pubs.acs.org.

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