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A facile approach to formation of through-hole porous anodic aluminum oxide film
Materials Letters 59 (2005) 40 – 43 www.elsevier.com/locate/matlet
A facile approach to formation of through-hole porous anodic aluminum oxide film Yanchun Zhao, Miao Chen*, Yanan Zhang, Tao Xu, Weimin Liu State Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou 730000, PR China Received 8 September 2004; accepted 10 September 2004 Available online 2 October 2004
Abstract Highly ordered through-hole anodic porous alumina film was fabricated using a facile single side and two-step anodization approach without any special pretreatment. The as-fabricated anodic aluminum oxide (AAO) film has a mean pore diameter of 80 nm and the interpore distance is ~150 nm. The pore density can be high as 1.01010 cm 2. During the processes of removal of aluminum substrate and pore opening, a simple setup was used. The evolution of surface morphology of the AAO film was characterized by scanning electron microscopy (SEM). The fabricated pore-through AAO film can then be used as templates for growth of well aligned nanowire, nanotube and nanodevice. D 2004 Elsevier B.V. All rights reserved. Keywords: Template synthesis; Porous anodic alumina; Anodization
1. Introduction Anodic aluminum oxide (AAO) films in the form of selfordered honeycomb array of uniformly sized parallel channels are attractive for many technical applications in the rapidly growing nanotechnology field due to their high pore density [1–4]. The pore diameter distribution in such films is a function of the film preparation and is typically close to monodisperse. AAO films grown in acid electrolytes possess hexagonally ordered porous structures with pore diameters ranging from below 10 to 200 nm, pore lengths from 1 to over 100 Am, and pore density in the range of 109 to 1012 cm 2. These unique structure properties and their thermal and chemical stability make AAO films ideal templates for the fabrication of uniform nanoscale structures and masking. Thus, by filling the pores of the AAO template, arrays of aligned nanowires or nanotubes uniform in diameter and length, are obtained reproducibly and economically [5–7].
* Corresponding author. Tel.: +86 931 4968189; fax: +86 931 8277088. E-mail address: [email protected] (M. Chen). 0167-577X/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2004.09.018
In the past decade, there has been a particular focus on the fabrication of AAO film with ordered array of holes [8– 10]. Masuda et al. advocated a two-step oxidation technology [11] and molding [12] process to prepare highly ordered pore arrays. Recently, pore nucleation and growth of AAO with square cells were fabricated using imprinting process [13]. However, two main factors should be born in mind for successful fabrication of perfect AAO. The first factor involves the pretreatment including annealing and polishing of the aluminum sheet, which is considered to be essential for the preparation of ordered pores [13,14], these two processes make the fabrication process tedious and boring. The second factor is the removal of an oxide barrier layer between the porous alumina and the aluminum base to ensure the pores open-through. Up to now although many efforts have been made to deal with these problems [15–17], the detaching and pore-opening processes are still a challenge and are difficult to do. In this paper, we reported a facile procedure to fabricate through-hole AAO template. The pretreatment processes including annealing and electrochemical polishing of the aluminum base were not necessary. Simple degreasing and oxide removal pretreatment was enough for obtaining clean and flat Al species that were essential for symmetrical
Y. Zhao et al. / Materials Letters 59 (2005) 40–43
Fig. 1. Setup for anodic oxidation of aluminum sheet.
distribution of applied electric field on the aluminum surface in the followed anodizing process for achieving ordered pores. A schematic representation of the setup for the anodization and the whole procedure was shown in Figs. 1 and 2. The as-fabricated AAO film has a mean pore diameter of 80 nm. The evolution of surface morphology of the AAO film was characterized by scanning electron microscopy (SEM).
2. Experimental Single side of high-purity aluminum sheet was anodized by a regulated DC power supply in a 0.3 M oxalic acid solution. The aluminum substrate were first degreased in acetone for 6 h and followed by 180 s of ultrasonic cleaning. Then the samples were rinsed with distilled water and etched in 3.0 M NaOH until bubbles over the surface occurred and finally the samples were rinsed with distilled
Fig. 2. Schematic diagram of the preparation procedure for the formation of through-hole AAO film. (A) Formation of the porous alumina layer after the first anodization; (B) removal of the porous alumina layer; (C) formation of ordered porous alumina layer after the second anodization; (D) removal of the aluminum layer; (E) removal of the bottom layer; (F) setup for the detaching and pore-opening processes.
41
water. Anodization was performed in 0.3 M H2C2O4 at either 0 or 15 8C, in a constant-temperature bath. For obtaining pores on a single side, experiments were carried out using a two-electrode setup (Fig. 1) from a vigorously stirred solution of 0.3 M oxalic acid. The cleaned aluminum sheet was used as the anode and Pb foil as the cathode. The first anodization lasted 3 h and the second 8 h. After the first anodization, the strip-off process was carried out in a mixture solution (0.4 M H3PO4+0.2 M H2CrO4) at 80 8C for 1 h. The exposed and well-ordered concave patterns on the aluminum substrate acted as self-assembled mask for the second anodization process. After the second anodization, the remaining aluminum substrate was removed in a CuCl2based solution (100 ml of HCl (38%)+100 ml H2O+3.4 g of CuCl2d H2O) at room temperature for about 10 min. The bottom of the pores was subsequently opened by 5% H3PO4 at room temperature for 2 h. Both detaching and poreopening processes were carried out in a setup described in Fig. 2F.
3. Results and discussion Alumina film was fabricated by anodizing aluminum metal in oxalic acid solution at the voltage of 60 V and 0 8C. The current densities (using geometric area of aluminum exposed to solution) distribution curves as a function of the anodic oxidation time were reported in Fig. 3. The solid and dashed curves were for the first and second oxidation process, respectively. The current densities for the first and second anodization were divided into three regions: formation of the barrier oxide layer, initiation of pore formation and steady state pore growth (formation of the porous oxide layer) [18]. Although the same current behaviors for the first and second anodic process were similar, there were several important differences between the results observed in the first and second anodic oxidation. First, for the second anodic oxidation, the time for reaching
Fig. 3. Current–time anodization curve for anodization in 0.3 M H2C2O4 at 60 V and 0 8C (constant-voltage mode).
42
Y. Zhao et al. / Materials Letters 59 (2005) 40–43
the lowest and constant current was significantly shorter than those for the first oxidation. Second, the minimum current density was larger for the second oxidation than for the first oxidation. The longer time for reaching the steadystate condition in the first oxidation was simply due to the fact that at the beginning the porous structure was not yet formed, accordingly, the time for pores nucleation and growth for the first oxidation was longer than that in the second oxidation process. After first oxidation and removing the initial alumina film, the remained aluminum layer was textured with pits corresponding to the base of the pores from the first anodization step. The textured surface remained at the bottom of each curvature, where the resistance was lowest and electric field was highest, so the pore nucleation was easier on a textured surface. In addition, the thermal effect, high electric field and the change of the diffusion mechanism from planar diffusion to spherical diffusion accelerated diffusion rate of protons to the oxide/ solution interface, which resulted in great acceleration of both the barrier layer dissolution and the growth of pores [10]; for the same reason, the time for reaching a steady state was shortened. Under constant potential condition, the observed large current density in the second oxidation step was also owing to the large textured surface area left by the dissolution of previous porous structure. Fig. 4 showed an SEM micrograph of the surface view of AAO after the second anodization at 60 V in 0.3 M oxalic acid solution at 0 8C. A well-ordered array of nanopores was
observed. The diameter of the pores is ~80 nm and the interpore distance is ~150 nm. The pore density can be high as 1.01010 cm 2. Previous researchers [12,19,20] reported that such well-ordered nanopore arrays could be obtained from the aluminum sheets with tedious pretreatments or using pre-textured aluminum. However, our results showed the tedious pretreatments such as annealing and electropolishing were not necessary and simple pretreatment including only degreasing and oxide removing was enough. After the second oxidation, the detaching and poreopening processes were directly carried out in the side of aluminum substrate. Compared with the commonly used HgCl2 solution, an environmentally friendly CuCl2-based solution was used to etch away the aluminum substrate. The advantages of the use of the CuCl2-based solution were much faster and no hazardous products (that is, Hg) will be produced. Fig. 4B showed the bottom surface images of the AAO film. The results showed distinctly that the opened pores with an ordered honeycomb structure could be observed. The pore diameter is ~80 nm and the interpore distance is ~150 nm, which were consistent with the pore diameter and interpore distance measured at the top surface (Fig. 4A). During the detaching and pore-opening processes, previous researchers [9,14] reported using a protective coating layer for the fabrication of through-hole AAO films. Although there was no protective residue left on the AAO film following its removal, the operation to peel off the protective layer was not easy since the AAO film was
Fig. 4. SEM images of anodic alumina films after the second anodization at 60 V in 0.3 mol/l H2C2O4 at 0 8C. (A) Top surface; (B) bottom surface; (C) crosssection surface.
Y. Zhao et al. / Materials Letters 59 (2005) 40–43
43
Fig. 5. SEM image showing the highly ordered porous structure after the second anodization at 50 V in 0.3 mol/l H2C2O4 at 15 8C. (A) High magnification; (B) low magnification.
fragile. Our results showed that the detaching and poreopening processes were easy to control using a simple setup (Fig. 2F). The through-hole porous AAO film with aluminum base, which remained at the edge, was obtained after the detaching and pore-opening processes. Thus, the aluminum base supported at the edge of AAO film made the fragile AAO film easier to handle. Fig. 4C depicted a crosssection view of the AAO with pores parallel to each other and perpendicular to the surface of the film. A highly ordered hexagonal arrangement of the pores was obtained under appropriate anodization condition. Fig. 5 showed the SEM image of ordered porous AAO film which was anodized in 0.3 M H2C2O4 at 50 V and 15 8C. The average pore diameter and interpore distance are 76 and 130 nm, respectively. The thickness of the asfabricated AAO film is ~50 Am. This film enabled one to make thin cylinders of uniform dimensions with controlled diameter as small as few tens of nm, and the through-hole porous AAO films can then be used as host for growth of nanowire, nanotube and nanodevice through the template synthesis.
pore-through AAO film can then be used as templates for growth of nanowire, nanotube and nanodevice.
4. Conclusion
[8] [9] [10] [11] [12]
We have successfully prepared through-hole porous anodic alumina film using two-step anodizing method without any special pretreatment (e.g., annealing and electropolishing) in the present work. Experiment results showed that highly ordered and through-hole porous AAO film with honeycomb structure can be obtained through a facile approach. An environmentally friend CuCl2-based solution was used to etch away the aluminum substrate. During the processes of removal of aluminum substrate and pore opening, a simple setup was used. The aluminum base supported at the edge of the through-hole porous AAO film made the fragile AAO film easier to handle. The fabricated
Acknowledgements We acknowledge the 973 project of the Ministry of Science and Technology (Grant No. 2003CB716200) for financial support. The authors would like to thank researcher Jia-Zheng Zhao of Lanzhou Institute of Chemical Physics of the Chinese Academy of Sciences for enthusiastic help.
References [1] [2] [3] [4] [5] [6] [7]
[13] [14] [15] [16] [17] [18] [19] [20]
J.W. Diggle, T.C. Downie, C.W. Goulding, Chem. Rev. 69 (1969) 365. C.R. Martin, Chem. Mater. 8 (1996) 1739. M.M. Lohrengel, Mater. Sci. Eng., R 11 (1993) 243. G. Schmid, J. Mater. Chem. 12 (2002) 1231. C.R. Martin, Science 266 (1994) 1961. Z. Wang, M. Chen, H.L. Li, Mater. Sci. Eng., A 328 (2002) 33. Y.C. Zhao, M. Chen, W.M. Liu, X. Liu, Q.J. Xue, Mater. Chem. Phys. 82 (2003) 370. H. Masuda, K. Fukuda, Science 268 (1995) 1466. T. Xu, G. Zangari, R.M. Metzger, Nano Lett. 2 (2002) 37. J. Yuan, F.Y. He, D.C. Sun, H.X. Xia, Chem. Mater. 16 (2004) 1841. H. Masuda, M. Satoh, Jpn. Appl. Phys. 35 (1996) L126. H. Masuda, H. Yamada, M. Satoh, H. Asoh, M. Nakao, T. Tamamura, Appl. Phys. Lett. 71 (1997) 2770. H. Asoh, S. Ono, T. Hirose, M. Nakao, H. Masuda, Electrochim. Acta 48 (2003) 3171. O. Jessensky, F. Muller, U. Cosele, Appl. Phys. Lett. 72 (1998) 1173. H. Lira, L. De, R. Paterson, J. Membr. Sci. 206 (2002) 375. T.T. Xu, R.D. Pinter, R.S. Ruoff, Langmuir 19 (2003) 1443. R.C. Furneaux, W.R. Rigby, A.P. Davison, Nature 337 (1989) 147. D.A. Brernon, M.B. Barela, M.E. Piyasena, G.P. Lopez, P.B. Atanassov, Chem. Mater. 16 (2004) 682. F. Li, L. Zhang, R.M. Metzger, Chem. Mater. 10 (1998) 2470. H. Asoh, K. Nishio, M. Nakao, T. Tamanura, H. Masuda, J. Electrochem. Soc. 148 (2001) B152 – B156.
A facile approach to formation of through-hole porous anodic aluminum oxide film Yanchun Zhao, Miao Chen*, Yanan Zhang, Tao Xu, Weimin Liu State Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou 730000, PR China Received 8 September 2004; accepted 10 September 2004 Available online 2 October 2004
Abstract Highly ordered through-hole anodic porous alumina film was fabricated using a facile single side and two-step anodization approach without any special pretreatment. The as-fabricated anodic aluminum oxide (AAO) film has a mean pore diameter of 80 nm and the interpore distance is ~150 nm. The pore density can be high as 1.01010 cm 2. During the processes of removal of aluminum substrate and pore opening, a simple setup was used. The evolution of surface morphology of the AAO film was characterized by scanning electron microscopy (SEM). The fabricated pore-through AAO film can then be used as templates for growth of well aligned nanowire, nanotube and nanodevice. D 2004 Elsevier B.V. All rights reserved. Keywords: Template synthesis; Porous anodic alumina; Anodization
1. Introduction Anodic aluminum oxide (AAO) films in the form of selfordered honeycomb array of uniformly sized parallel channels are attractive for many technical applications in the rapidly growing nanotechnology field due to their high pore density [1–4]. The pore diameter distribution in such films is a function of the film preparation and is typically close to monodisperse. AAO films grown in acid electrolytes possess hexagonally ordered porous structures with pore diameters ranging from below 10 to 200 nm, pore lengths from 1 to over 100 Am, and pore density in the range of 109 to 1012 cm 2. These unique structure properties and their thermal and chemical stability make AAO films ideal templates for the fabrication of uniform nanoscale structures and masking. Thus, by filling the pores of the AAO template, arrays of aligned nanowires or nanotubes uniform in diameter and length, are obtained reproducibly and economically [5–7].
* Corresponding author. Tel.: +86 931 4968189; fax: +86 931 8277088. E-mail address: [email protected] (M. Chen). 0167-577X/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2004.09.018
In the past decade, there has been a particular focus on the fabrication of AAO film with ordered array of holes [8– 10]. Masuda et al. advocated a two-step oxidation technology [11] and molding [12] process to prepare highly ordered pore arrays. Recently, pore nucleation and growth of AAO with square cells were fabricated using imprinting process [13]. However, two main factors should be born in mind for successful fabrication of perfect AAO. The first factor involves the pretreatment including annealing and polishing of the aluminum sheet, which is considered to be essential for the preparation of ordered pores [13,14], these two processes make the fabrication process tedious and boring. The second factor is the removal of an oxide barrier layer between the porous alumina and the aluminum base to ensure the pores open-through. Up to now although many efforts have been made to deal with these problems [15–17], the detaching and pore-opening processes are still a challenge and are difficult to do. In this paper, we reported a facile procedure to fabricate through-hole AAO template. The pretreatment processes including annealing and electrochemical polishing of the aluminum base were not necessary. Simple degreasing and oxide removal pretreatment was enough for obtaining clean and flat Al species that were essential for symmetrical
Y. Zhao et al. / Materials Letters 59 (2005) 40–43
Fig. 1. Setup for anodic oxidation of aluminum sheet.
distribution of applied electric field on the aluminum surface in the followed anodizing process for achieving ordered pores. A schematic representation of the setup for the anodization and the whole procedure was shown in Figs. 1 and 2. The as-fabricated AAO film has a mean pore diameter of 80 nm. The evolution of surface morphology of the AAO film was characterized by scanning electron microscopy (SEM).
2. Experimental Single side of high-purity aluminum sheet was anodized by a regulated DC power supply in a 0.3 M oxalic acid solution. The aluminum substrate were first degreased in acetone for 6 h and followed by 180 s of ultrasonic cleaning. Then the samples were rinsed with distilled water and etched in 3.0 M NaOH until bubbles over the surface occurred and finally the samples were rinsed with distilled
Fig. 2. Schematic diagram of the preparation procedure for the formation of through-hole AAO film. (A) Formation of the porous alumina layer after the first anodization; (B) removal of the porous alumina layer; (C) formation of ordered porous alumina layer after the second anodization; (D) removal of the aluminum layer; (E) removal of the bottom layer; (F) setup for the detaching and pore-opening processes.
41
water. Anodization was performed in 0.3 M H2C2O4 at either 0 or 15 8C, in a constant-temperature bath. For obtaining pores on a single side, experiments were carried out using a two-electrode setup (Fig. 1) from a vigorously stirred solution of 0.3 M oxalic acid. The cleaned aluminum sheet was used as the anode and Pb foil as the cathode. The first anodization lasted 3 h and the second 8 h. After the first anodization, the strip-off process was carried out in a mixture solution (0.4 M H3PO4+0.2 M H2CrO4) at 80 8C for 1 h. The exposed and well-ordered concave patterns on the aluminum substrate acted as self-assembled mask for the second anodization process. After the second anodization, the remaining aluminum substrate was removed in a CuCl2based solution (100 ml of HCl (38%)+100 ml H2O+3.4 g of CuCl2d H2O) at room temperature for about 10 min. The bottom of the pores was subsequently opened by 5% H3PO4 at room temperature for 2 h. Both detaching and poreopening processes were carried out in a setup described in Fig. 2F.
3. Results and discussion Alumina film was fabricated by anodizing aluminum metal in oxalic acid solution at the voltage of 60 V and 0 8C. The current densities (using geometric area of aluminum exposed to solution) distribution curves as a function of the anodic oxidation time were reported in Fig. 3. The solid and dashed curves were for the first and second oxidation process, respectively. The current densities for the first and second anodization were divided into three regions: formation of the barrier oxide layer, initiation of pore formation and steady state pore growth (formation of the porous oxide layer) [18]. Although the same current behaviors for the first and second anodic process were similar, there were several important differences between the results observed in the first and second anodic oxidation. First, for the second anodic oxidation, the time for reaching
Fig. 3. Current–time anodization curve for anodization in 0.3 M H2C2O4 at 60 V and 0 8C (constant-voltage mode).
42
Y. Zhao et al. / Materials Letters 59 (2005) 40–43
the lowest and constant current was significantly shorter than those for the first oxidation. Second, the minimum current density was larger for the second oxidation than for the first oxidation. The longer time for reaching the steadystate condition in the first oxidation was simply due to the fact that at the beginning the porous structure was not yet formed, accordingly, the time for pores nucleation and growth for the first oxidation was longer than that in the second oxidation process. After first oxidation and removing the initial alumina film, the remained aluminum layer was textured with pits corresponding to the base of the pores from the first anodization step. The textured surface remained at the bottom of each curvature, where the resistance was lowest and electric field was highest, so the pore nucleation was easier on a textured surface. In addition, the thermal effect, high electric field and the change of the diffusion mechanism from planar diffusion to spherical diffusion accelerated diffusion rate of protons to the oxide/ solution interface, which resulted in great acceleration of both the barrier layer dissolution and the growth of pores [10]; for the same reason, the time for reaching a steady state was shortened. Under constant potential condition, the observed large current density in the second oxidation step was also owing to the large textured surface area left by the dissolution of previous porous structure. Fig. 4 showed an SEM micrograph of the surface view of AAO after the second anodization at 60 V in 0.3 M oxalic acid solution at 0 8C. A well-ordered array of nanopores was
observed. The diameter of the pores is ~80 nm and the interpore distance is ~150 nm. The pore density can be high as 1.01010 cm 2. Previous researchers [12,19,20] reported that such well-ordered nanopore arrays could be obtained from the aluminum sheets with tedious pretreatments or using pre-textured aluminum. However, our results showed the tedious pretreatments such as annealing and electropolishing were not necessary and simple pretreatment including only degreasing and oxide removing was enough. After the second oxidation, the detaching and poreopening processes were directly carried out in the side of aluminum substrate. Compared with the commonly used HgCl2 solution, an environmentally friendly CuCl2-based solution was used to etch away the aluminum substrate. The advantages of the use of the CuCl2-based solution were much faster and no hazardous products (that is, Hg) will be produced. Fig. 4B showed the bottom surface images of the AAO film. The results showed distinctly that the opened pores with an ordered honeycomb structure could be observed. The pore diameter is ~80 nm and the interpore distance is ~150 nm, which were consistent with the pore diameter and interpore distance measured at the top surface (Fig. 4A). During the detaching and pore-opening processes, previous researchers [9,14] reported using a protective coating layer for the fabrication of through-hole AAO films. Although there was no protective residue left on the AAO film following its removal, the operation to peel off the protective layer was not easy since the AAO film was
Fig. 4. SEM images of anodic alumina films after the second anodization at 60 V in 0.3 mol/l H2C2O4 at 0 8C. (A) Top surface; (B) bottom surface; (C) crosssection surface.
Y. Zhao et al. / Materials Letters 59 (2005) 40–43
43
Fig. 5. SEM image showing the highly ordered porous structure after the second anodization at 50 V in 0.3 mol/l H2C2O4 at 15 8C. (A) High magnification; (B) low magnification.
fragile. Our results showed that the detaching and poreopening processes were easy to control using a simple setup (Fig. 2F). The through-hole porous AAO film with aluminum base, which remained at the edge, was obtained after the detaching and pore-opening processes. Thus, the aluminum base supported at the edge of AAO film made the fragile AAO film easier to handle. Fig. 4C depicted a crosssection view of the AAO with pores parallel to each other and perpendicular to the surface of the film. A highly ordered hexagonal arrangement of the pores was obtained under appropriate anodization condition. Fig. 5 showed the SEM image of ordered porous AAO film which was anodized in 0.3 M H2C2O4 at 50 V and 15 8C. The average pore diameter and interpore distance are 76 and 130 nm, respectively. The thickness of the asfabricated AAO film is ~50 Am. This film enabled one to make thin cylinders of uniform dimensions with controlled diameter as small as few tens of nm, and the through-hole porous AAO films can then be used as host for growth of nanowire, nanotube and nanodevice through the template synthesis.
pore-through AAO film can then be used as templates for growth of nanowire, nanotube and nanodevice.
4. Conclusion
[8] [9] [10] [11] [12]
We have successfully prepared through-hole porous anodic alumina film using two-step anodizing method without any special pretreatment (e.g., annealing and electropolishing) in the present work. Experiment results showed that highly ordered and through-hole porous AAO film with honeycomb structure can be obtained through a facile approach. An environmentally friend CuCl2-based solution was used to etch away the aluminum substrate. During the processes of removal of aluminum substrate and pore opening, a simple setup was used. The aluminum base supported at the edge of the through-hole porous AAO film made the fragile AAO film easier to handle. The fabricated
Acknowledgements We acknowledge the 973 project of the Ministry of Science and Technology (Grant No. 2003CB716200) for financial support. The authors would like to thank researcher Jia-Zheng Zhao of Lanzhou Institute of Chemical Physics of the Chinese Academy of Sciences for enthusiastic help.
References [1] [2] [3] [4] [5] [6] [7]
[13] [14] [15] [16] [17] [18] [19] [20]
J.W. Diggle, T.C. Downie, C.W. Goulding, Chem. Rev. 69 (1969) 365. C.R. Martin, Chem. Mater. 8 (1996) 1739. M.M. Lohrengel, Mater. Sci. Eng., R 11 (1993) 243. G. Schmid, J. Mater. Chem. 12 (2002) 1231. C.R. Martin, Science 266 (1994) 1961. Z. Wang, M. Chen, H.L. Li, Mater. Sci. Eng., A 328 (2002) 33. Y.C. Zhao, M. Chen, W.M. Liu, X. Liu, Q.J. Xue, Mater. Chem. Phys. 82 (2003) 370. H. Masuda, K. Fukuda, Science 268 (1995) 1466. T. Xu, G. Zangari, R.M. Metzger, Nano Lett. 2 (2002) 37. J. Yuan, F.Y. He, D.C. Sun, H.X. Xia, Chem. Mater. 16 (2004) 1841. H. Masuda, M. Satoh, Jpn. Appl. Phys. 35 (1996) L126. H. Masuda, H. Yamada, M. Satoh, H. Asoh, M. Nakao, T. Tamamura, Appl. Phys. Lett. 71 (1997) 2770. H. Asoh, S. Ono, T. Hirose, M. Nakao, H. Masuda, Electrochim. Acta 48 (2003) 3171. O. Jessensky, F. Muller, U. Cosele, Appl. Phys. Lett. 72 (1998) 1173. H. Lira, L. De, R. Paterson, J. Membr. Sci. 206 (2002) 375. T.T. Xu, R.D. Pinter, R.S. Ruoff, Langmuir 19 (2003) 1443. R.C. Furneaux, W.R. Rigby, A.P. Davison, Nature 337 (1989) 147. D.A. Brernon, M.B. Barela, M.E. Piyasena, G.P. Lopez, P.B. Atanassov, Chem. Mater. 16 (2004) 682. F. Li, L. Zhang, R.M. Metzger, Chem. Mater. 10 (1998) 2470. H. Asoh, K. Nishio, M. Nakao, T. Tamanura, H. Masuda, J. Electrochem. Soc. 148 (2001) B152 – B156.