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Ultra-small nanopores obtained by self-organized anodization of aluminum in oxalic acid at low voltages
Materials Letters 111 (2013) 20–23
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Ultra-small nanopores obtained by self-organized anodization of aluminum in oxalic acid at low voltages Wojciech J. Stępniowski n, Małgorzata Norek, Marta Michalska-Domańska, Zbigniew Bojar Department of Advanced Materials and Technology, Faculty of Advanced Technologies and Chemistry, Military University of Technology, 2 Kaliskiego Str., 00-908 Warsaw, Poland
art ic l e i nf o
a b s t r a c t
Article history: Received 18 June 2013 Accepted 13 August 2013 Available online 22 August 2013
A simple method of fabrication of anodic aluminum oxide at low voltages (from 5 to 15 V) in 0.3 M oxalic acid is presented. To overcome the issues concerning the anode's burning, a high velocity stirring was applied. It allowed to achieve nanopores with ultra-small diameters (even 12 nm), interpore distance (even 31 nm) and tremendously high pore densities (up to 980 pores per 1 mm2) with simultaneous high porosity (up to 38%). Moreover, the presented approach (no modifier added to the electrolyte) resulted in the AAO with relatively well arranged ultra-small nanopores. & 2013 Elsevier B.V. All rights reserved.
Keywords: Nanostructures Nanopores Anodization Self-organization Anodic aluminum oxide
1. Introduction Recently, nanoporous anodic aluminum oxide (AAO) is one of the most commonly fabricated material with electrochemical techniques, due to its applications in template assisted nanofabrication [1]. It allows to fabricate hexagonally-arranged nanostructures made of diverse materials with emerging innovative properties owing to their small size. Close-packaging of anodic alumina ultra-small nanopores would be advantageous for template assisted fabrication methods. It would allow to obtain nanowires, nanotubes and nanodots with ultra-small diameters and enhanced properties linked to their dimensions. Pore diameter and interpore distance decrease linearly with anodizing voltage decrease. To obtain pores with possibly smallest diameters and interpore distances the lowest voltages have to be applied. Typically, the most suitable electrolyte for AAO formation with small pores is sulfuric acid and voltage ranging from 15 to 25 V [2]. The resulted pore diameter and interpore distance range from 20 to 25 nm and 44 to 65 nm, respectively. Ding et al. anodized aluminum in sulfuric acid at 10 V and obtained AAO with average pore diameter of 7.9 nm and interpore distance of 26.2 nm. In this case modifiers like aluminum sulfate and citric acid were added to the electrolyte to prevent anode from burning (anodic dissolution) at low voltages [3]. These results have been even commercialized [4].
n
Corresponding author. Tel.: þ 48 22 683 94 46; fax: þ48 22 683 94 45. E-mail address: [email protected] (W.J. Stępniowski).
0167-577X/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.matlet.2013.08.059
The major issue undertaken in this research is fabrication of anodic alumina with small nanopores via self-organized anodization in oxalic acid and investigation of the influence of operating conditions on geometrical features of the nanopores. In our study, a simple method of fabrication of anodic aluminum oxide is presented—to overcome the issues linked to the anode's burning, a high velocity stirring was applied and no modifiers were added to the electrolyte. 2. Materials and methods A high-purity 0.25 mm aluminum foil (Alfa-Aeasar) was cut into coupons (5.0 25 mm2). Next, aluminum was degreased in acetone and ethyl alcohol. Further, electropolishing was conducted in 1:4 volume mixture of 60% perchloric acid and ethyl alcohol. Platinum grid (6 cm2) was applied as a cathode, current density of the process was 0.5 A/cm2 and it lasted for 1 min. After electropolishing, the aluminum specimens were coated with acidresistant paint at the back and edges to prevent sample from “burning” during anodization. Working area of aluminum was 0.5 cm2. First step of anodization was conducted in 0.3 M oxalic acid at three different voltages (5.0, 10.0, and 15.0 V) and three different values of temperature (20, 30, and 40 1C). To prevent anode's burning, the solution during anodization was vigorously stirred (1000 rpm). Due to the high velocity stirring, local temperature and current density inhomogeneities, leading to anode's burning, have been minimized. Duration of the process was 30 min. Subsequently, as formed alumina was chemically removed in a stirred mixture of 6 wt% phosphoric acid and 1.8 wt% chromic
W.J. Stępniowski et al. / Materials Letters 111 (2013) 20–23
acid at 60 1C for 60 min. Next, aluminum was re-anodized at the same conditions as present during the first step of the process. The characterization of the nanoporous alumina was performed by using the Carl Zeiss Leo 1530 field emission scanning electron microscope (FE-SEM). The fast Fourier transform (FFT) of the FESEM images were calculated by using a scanning probe image processor WSxM v 5.0 [5,6]. Image analyses (pore diameter and pore density) were done with NIS-Elements AR 3.10 programme purchased from Nikon Company. Average pore diameter was evaluated from about 5.000 independent measurements for a given operating conditions of the process. Pore density (number of pores occupying 1 mm2) was estimated from six FE-SEM images of the same sample anodized at given operating conditions.
21
Interpore distance was evaluated from FFT as an inverse of a radial average of the FFT image of six FE-SEM images.
3. Results and discussion FE-SEM images of alumina formed at 30 1C in 0.3 M oxalic acid at low voltages show closely packed nanopores with small diameters (Fig. 1). Depending on temperature and voltage, pore diameter was ranging from 12 to 28.7 nm. It was found, that pore diameter increases linearly with voltage, as also observed in the application of higher voltages (Fig. 2a) [2]. Also temperature increase, enhancing reaction between the grown oxide and acidic
Fig. 1. FE-SEM micrographs with FFT images with their radial averages for aluminum anodized for 30-min in oxalic acid at 30 1C. Anodizing voltages was 5.0 V (A) and (B), 10.0 V (C) and (D) and 15.0 V (E) and (F).
22
W.J. Stępniowski et al. / Materials Letters 111 (2013) 20–23
Fig. 3. Porosity versus anodizing voltage.
Fig. 2. Pore diameter (a) and interpore distance (b) versus anodizing voltage.
electrolyte, provides wider pores formation. According to Ding et al. [3], one can obtain nanopores with diameter even below 10 nm in sulfuric acid with modifiers (aluminum sulfate and citric acid), but in this case nanostructures arrangement is poor probably due to the incorporation of citrate anions (especially for AAO formed at 2 V obtained nanostructures were rather nanotubular than nanoporous) [3]. In the case of anodization in 0.3 M oxalic acid at low voltages, pore diameters were greater, but of better arrangement (no modifiers added to the electrolyte). Moreover, Ding et al. obtained quite well-arranged AAO at 10 V and in our study, nanoporous alumina was obtained even at 5 V, providing AAO suitable for further template assisted nanofabrication. Martin et al. [7] reported fabrication of AAO with diameter of about 15 nm at 19 V, but a modifier (ethylene glycol) was being added to the electrolyte (sulfuric acid), to prevent the anode from burning. Pores with relatively small diameters were formed also by Sulka and Parkoła [2], however they applied greater voltages (from 15 to 25 V), which resulted in fabrication of AAO with greater pores than in our study (Fig. 2a). Close-packaging of the nanopores is a crucial
factor in template-assisted nanofabrication. Interpore distance translates into close-packaging: the smaller the interpore distance (pore center-to-pore center) the more pores are on the given area. Theoretically, interpore distance increases linearly with anodizing voltage increase [8] (Fig. 2b, Dc ¼ 2.5 U). In the case of AAO formed in 0.3 M oxalic acid at low voltages, the interpore distance was increasing with voltage and was independent of temperature, which is in agreement with the theory (Fig. 2b) [8]. The smallest values of interpore distance was obtained for AAO formed at the smallest voltage (5 V) and was about 32 nm. Sulka and Parkoła [2] and Martin et al. [7] obtained much greater values of interpore distances and the smallest ones were 44 and 50 nm. Ding et al. obtained AAO in oxalic acid at 2 and 10 V with interpore distance of 22 and 26 nm, respectively. However, a different mechanism of AAO growth at the smallest voltages was postulated, giving rise to deterioration of the final AAO structure arrangement [3]. The formed AAO, especially at 30 1C, has high porosity (Fig. 3). Porosity of AAO was estimated according to the following equation [8,9]: 2 Dp α ¼ 0:907 ð1Þ Dc where α is the porosity, Dp is the pore diameter and Dc is the interpore distance. The porosity values were up to 38%, which are much better than the ones achieved by Sulka and Parkoła [2], Ding et al. [3] and Martin et al. [7] (20%, 8% and 10%, respectively). In Fig. 4. evolution of pores density (number of pores occupying area of 1 μm2) is shown. Pores density is related to interpore distance, thus no temperature effect is expected [8,9]. A significant decrease of pores density with voltage's increase was observed. The obtained relation is an extrapolation of the trend observed for AAO formed in 0.3 M oxalic acid at greater voltages [9]. At 5 V even 980 pores occupy the area of 1 μm2, which is quite an impressive result as compared to the results obtained for instance by Martin et al. [7]. They reported twice smaller pores density (460 pores per 1 μm2) [7].
4. Conclusions A simple way of fabrication of anodic aluminum oxide at relatively small voltages and high temperatures was presented.
W.J. Stępniowski et al. / Materials Letters 111 (2013) 20–23
23
2. Pore diameter and interpore distance increase linearly with temperature, which is in agreement with the results obtained at greater voltages. 3. High porosity and pores density of AAO, competitive to the data obtained previously by other researchers, were accomplished. Fabrication of AAO in 0.3 M oxalic acid at low voltages and relatively high temperatures provides high porosity with simultaneous close-packaging of nanopores with small diameters. The AAO with such geometrical features could be very interesting for the production of highly ordered, ultra-small nanostructures, made of various material, desired in many fields of nanotechnology.
References
Fig. 4. Pore density (number of pores occupying 1 μm2) versus anodizing voltage.
No modifiers were applied to remove the heat from anode's surface. To prevent anode from burning, high velocity of stirring was applied in order to minimize local temperature and current inhomogeneities and remove necessity of electrolyte modification. The AAO with extremely small pores and relatively good arrangement was achieved. The major findings can be listed as follows: 1. AAO with ordered, extremely small pores (even 12 nm) and interpore distances (even 31 nm) was obtained.
[1] Sulka GD, Zaraska L, Stępniowski WJ. Anodic porous alumina as a template for nanofabrication. In: 2nd ed.. Nalwa HS, editor. Encyclopedia of nanoscience and nanotechnology, 11. Venice: American Scientific Publishers; 2011. p. 261–349. [2] Sulka GD, Parkoła KG. Anodising potential influence on well-ordered nanostructures formed by anodisation of aluminium in sulphuric acid. Thin Solid Films 2006;515:338–45. [3] Ding GQ, Yang R, Ding JN, Yuan NY, Zhu YY. Fabrication of porous anodic alumina with ultrasmall nanopores. Nanoscale Research Letters 2010;5: 1257–63. [4] /http://www.nano-star.com/en/Products.htmlS. [5] Horcas I, Fernández R, Gómez-Rodríguez JM, Colchero J, Gómez-Herrero J, Baro AM. WSXM: a software for scanning probe microscopy and a tool for nanotechnology. Review of Scientific Instruments 2007;78:013705. [6] WSxM©. 〈http://www.nanotec.es〉. [7] Martín J, Manzano CV, Caballero-Calero O, Martín-Gonzalez M. High-aspectratio and highly ordered 15-nm porous alumina templates. ACS Applied Materials and Interfaces 2013;5:72–9. [8] Sulka GD. Highly ordered anodic porous alumina formation by self-organized anodizing. In: Eftekhari A, editor. Nanostructured materials in electrochemistry. Weinheim: Wiley VCH Verlag GmbH & Co. KGaA; 2008. p. 1–116. [9] Stępniowski WJ, Bojar Z. Synthesis of anodic aluminum oxide (AAO) at relatively high temperatures. Study of the influence of anodization conditions on the alumina structural features. Surface and Coatings Technology 2011;206:265–72.
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Ultra-small nanopores obtained by self-organized anodization of aluminum in oxalic acid at low voltages Wojciech J. Stępniowski n, Małgorzata Norek, Marta Michalska-Domańska, Zbigniew Bojar Department of Advanced Materials and Technology, Faculty of Advanced Technologies and Chemistry, Military University of Technology, 2 Kaliskiego Str., 00-908 Warsaw, Poland
art ic l e i nf o
a b s t r a c t
Article history: Received 18 June 2013 Accepted 13 August 2013 Available online 22 August 2013
A simple method of fabrication of anodic aluminum oxide at low voltages (from 5 to 15 V) in 0.3 M oxalic acid is presented. To overcome the issues concerning the anode's burning, a high velocity stirring was applied. It allowed to achieve nanopores with ultra-small diameters (even 12 nm), interpore distance (even 31 nm) and tremendously high pore densities (up to 980 pores per 1 mm2) with simultaneous high porosity (up to 38%). Moreover, the presented approach (no modifier added to the electrolyte) resulted in the AAO with relatively well arranged ultra-small nanopores. & 2013 Elsevier B.V. All rights reserved.
Keywords: Nanostructures Nanopores Anodization Self-organization Anodic aluminum oxide
1. Introduction Recently, nanoporous anodic aluminum oxide (AAO) is one of the most commonly fabricated material with electrochemical techniques, due to its applications in template assisted nanofabrication [1]. It allows to fabricate hexagonally-arranged nanostructures made of diverse materials with emerging innovative properties owing to their small size. Close-packaging of anodic alumina ultra-small nanopores would be advantageous for template assisted fabrication methods. It would allow to obtain nanowires, nanotubes and nanodots with ultra-small diameters and enhanced properties linked to their dimensions. Pore diameter and interpore distance decrease linearly with anodizing voltage decrease. To obtain pores with possibly smallest diameters and interpore distances the lowest voltages have to be applied. Typically, the most suitable electrolyte for AAO formation with small pores is sulfuric acid and voltage ranging from 15 to 25 V [2]. The resulted pore diameter and interpore distance range from 20 to 25 nm and 44 to 65 nm, respectively. Ding et al. anodized aluminum in sulfuric acid at 10 V and obtained AAO with average pore diameter of 7.9 nm and interpore distance of 26.2 nm. In this case modifiers like aluminum sulfate and citric acid were added to the electrolyte to prevent anode from burning (anodic dissolution) at low voltages [3]. These results have been even commercialized [4].
n
Corresponding author. Tel.: þ 48 22 683 94 46; fax: þ48 22 683 94 45. E-mail address: [email protected] (W.J. Stępniowski).
0167-577X/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.matlet.2013.08.059
The major issue undertaken in this research is fabrication of anodic alumina with small nanopores via self-organized anodization in oxalic acid and investigation of the influence of operating conditions on geometrical features of the nanopores. In our study, a simple method of fabrication of anodic aluminum oxide is presented—to overcome the issues linked to the anode's burning, a high velocity stirring was applied and no modifiers were added to the electrolyte. 2. Materials and methods A high-purity 0.25 mm aluminum foil (Alfa-Aeasar) was cut into coupons (5.0 25 mm2). Next, aluminum was degreased in acetone and ethyl alcohol. Further, electropolishing was conducted in 1:4 volume mixture of 60% perchloric acid and ethyl alcohol. Platinum grid (6 cm2) was applied as a cathode, current density of the process was 0.5 A/cm2 and it lasted for 1 min. After electropolishing, the aluminum specimens were coated with acidresistant paint at the back and edges to prevent sample from “burning” during anodization. Working area of aluminum was 0.5 cm2. First step of anodization was conducted in 0.3 M oxalic acid at three different voltages (5.0, 10.0, and 15.0 V) and three different values of temperature (20, 30, and 40 1C). To prevent anode's burning, the solution during anodization was vigorously stirred (1000 rpm). Due to the high velocity stirring, local temperature and current density inhomogeneities, leading to anode's burning, have been minimized. Duration of the process was 30 min. Subsequently, as formed alumina was chemically removed in a stirred mixture of 6 wt% phosphoric acid and 1.8 wt% chromic
W.J. Stępniowski et al. / Materials Letters 111 (2013) 20–23
acid at 60 1C for 60 min. Next, aluminum was re-anodized at the same conditions as present during the first step of the process. The characterization of the nanoporous alumina was performed by using the Carl Zeiss Leo 1530 field emission scanning electron microscope (FE-SEM). The fast Fourier transform (FFT) of the FESEM images were calculated by using a scanning probe image processor WSxM v 5.0 [5,6]. Image analyses (pore diameter and pore density) were done with NIS-Elements AR 3.10 programme purchased from Nikon Company. Average pore diameter was evaluated from about 5.000 independent measurements for a given operating conditions of the process. Pore density (number of pores occupying 1 mm2) was estimated from six FE-SEM images of the same sample anodized at given operating conditions.
21
Interpore distance was evaluated from FFT as an inverse of a radial average of the FFT image of six FE-SEM images.
3. Results and discussion FE-SEM images of alumina formed at 30 1C in 0.3 M oxalic acid at low voltages show closely packed nanopores with small diameters (Fig. 1). Depending on temperature and voltage, pore diameter was ranging from 12 to 28.7 nm. It was found, that pore diameter increases linearly with voltage, as also observed in the application of higher voltages (Fig. 2a) [2]. Also temperature increase, enhancing reaction between the grown oxide and acidic
Fig. 1. FE-SEM micrographs with FFT images with their radial averages for aluminum anodized for 30-min in oxalic acid at 30 1C. Anodizing voltages was 5.0 V (A) and (B), 10.0 V (C) and (D) and 15.0 V (E) and (F).
22
W.J. Stępniowski et al. / Materials Letters 111 (2013) 20–23
Fig. 3. Porosity versus anodizing voltage.
Fig. 2. Pore diameter (a) and interpore distance (b) versus anodizing voltage.
electrolyte, provides wider pores formation. According to Ding et al. [3], one can obtain nanopores with diameter even below 10 nm in sulfuric acid with modifiers (aluminum sulfate and citric acid), but in this case nanostructures arrangement is poor probably due to the incorporation of citrate anions (especially for AAO formed at 2 V obtained nanostructures were rather nanotubular than nanoporous) [3]. In the case of anodization in 0.3 M oxalic acid at low voltages, pore diameters were greater, but of better arrangement (no modifiers added to the electrolyte). Moreover, Ding et al. obtained quite well-arranged AAO at 10 V and in our study, nanoporous alumina was obtained even at 5 V, providing AAO suitable for further template assisted nanofabrication. Martin et al. [7] reported fabrication of AAO with diameter of about 15 nm at 19 V, but a modifier (ethylene glycol) was being added to the electrolyte (sulfuric acid), to prevent the anode from burning. Pores with relatively small diameters were formed also by Sulka and Parkoła [2], however they applied greater voltages (from 15 to 25 V), which resulted in fabrication of AAO with greater pores than in our study (Fig. 2a). Close-packaging of the nanopores is a crucial
factor in template-assisted nanofabrication. Interpore distance translates into close-packaging: the smaller the interpore distance (pore center-to-pore center) the more pores are on the given area. Theoretically, interpore distance increases linearly with anodizing voltage increase [8] (Fig. 2b, Dc ¼ 2.5 U). In the case of AAO formed in 0.3 M oxalic acid at low voltages, the interpore distance was increasing with voltage and was independent of temperature, which is in agreement with the theory (Fig. 2b) [8]. The smallest values of interpore distance was obtained for AAO formed at the smallest voltage (5 V) and was about 32 nm. Sulka and Parkoła [2] and Martin et al. [7] obtained much greater values of interpore distances and the smallest ones were 44 and 50 nm. Ding et al. obtained AAO in oxalic acid at 2 and 10 V with interpore distance of 22 and 26 nm, respectively. However, a different mechanism of AAO growth at the smallest voltages was postulated, giving rise to deterioration of the final AAO structure arrangement [3]. The formed AAO, especially at 30 1C, has high porosity (Fig. 3). Porosity of AAO was estimated according to the following equation [8,9]: 2 Dp α ¼ 0:907 ð1Þ Dc where α is the porosity, Dp is the pore diameter and Dc is the interpore distance. The porosity values were up to 38%, which are much better than the ones achieved by Sulka and Parkoła [2], Ding et al. [3] and Martin et al. [7] (20%, 8% and 10%, respectively). In Fig. 4. evolution of pores density (number of pores occupying area of 1 μm2) is shown. Pores density is related to interpore distance, thus no temperature effect is expected [8,9]. A significant decrease of pores density with voltage's increase was observed. The obtained relation is an extrapolation of the trend observed for AAO formed in 0.3 M oxalic acid at greater voltages [9]. At 5 V even 980 pores occupy the area of 1 μm2, which is quite an impressive result as compared to the results obtained for instance by Martin et al. [7]. They reported twice smaller pores density (460 pores per 1 μm2) [7].
4. Conclusions A simple way of fabrication of anodic aluminum oxide at relatively small voltages and high temperatures was presented.
W.J. Stępniowski et al. / Materials Letters 111 (2013) 20–23
23
2. Pore diameter and interpore distance increase linearly with temperature, which is in agreement with the results obtained at greater voltages. 3. High porosity and pores density of AAO, competitive to the data obtained previously by other researchers, were accomplished. Fabrication of AAO in 0.3 M oxalic acid at low voltages and relatively high temperatures provides high porosity with simultaneous close-packaging of nanopores with small diameters. The AAO with such geometrical features could be very interesting for the production of highly ordered, ultra-small nanostructures, made of various material, desired in many fields of nanotechnology.
References
Fig. 4. Pore density (number of pores occupying 1 μm2) versus anodizing voltage.
No modifiers were applied to remove the heat from anode's surface. To prevent anode from burning, high velocity of stirring was applied in order to minimize local temperature and current inhomogeneities and remove necessity of electrolyte modification. The AAO with extremely small pores and relatively good arrangement was achieved. The major findings can be listed as follows: 1. AAO with ordered, extremely small pores (even 12 nm) and interpore distances (even 31 nm) was obtained.
[1] Sulka GD, Zaraska L, Stępniowski WJ. Anodic porous alumina as a template for nanofabrication. In: 2nd ed.. Nalwa HS, editor. Encyclopedia of nanoscience and nanotechnology, 11. Venice: American Scientific Publishers; 2011. p. 261–349. [2] Sulka GD, Parkoła KG. Anodising potential influence on well-ordered nanostructures formed by anodisation of aluminium in sulphuric acid. Thin Solid Films 2006;515:338–45. [3] Ding GQ, Yang R, Ding JN, Yuan NY, Zhu YY. Fabrication of porous anodic alumina with ultrasmall nanopores. Nanoscale Research Letters 2010;5: 1257–63. [4] /http://www.nano-star.com/en/Products.htmlS. [5] Horcas I, Fernández R, Gómez-Rodríguez JM, Colchero J, Gómez-Herrero J, Baro AM. WSXM: a software for scanning probe microscopy and a tool for nanotechnology. Review of Scientific Instruments 2007;78:013705. [6] WSxM©. 〈http://www.nanotec.es〉. [7] Martín J, Manzano CV, Caballero-Calero O, Martín-Gonzalez M. High-aspectratio and highly ordered 15-nm porous alumina templates. ACS Applied Materials and Interfaces 2013;5:72–9. [8] Sulka GD. Highly ordered anodic porous alumina formation by self-organized anodizing. In: Eftekhari A, editor. Nanostructured materials in electrochemistry. Weinheim: Wiley VCH Verlag GmbH & Co. KGaA; 2008. p. 1–116. [9] Stępniowski WJ, Bojar Z. Synthesis of anodic aluminum oxide (AAO) at relatively high temperatures. Study of the influence of anodization conditions on the alumina structural features. Surface and Coatings Technology 2011;206:265–72.