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Porous anodic alumina layers with modulated pore diameters formed by sequential anodizing in different electrolytes
Author’s Accepted Manuscript Porous anodic alumina layers with modulated pore diameters formed by sequential anodizing in different electrolytes Leszek Zaraska, Marian Jaskuła, Grzegorz D. Sulka www.elsevier.com
PII: DOI: Reference:
S0167-577X(16)30273-7 http://dx.doi.org/10.1016/j.matlet.2016.02.113 MLBLUE20402
To appear in: Materials Letters Received date: 29 December 2015 Revised date: 25 January 2016 Accepted date: 21 February 2016 Cite this article as: Leszek Zaraska, Marian Jaskuła and Grzegorz D. Sulka, Porous anodic alumina layers with modulated pore diameters formed by sequential anodizing in different electrolytes, Materials Letters, http://dx.doi.org/10.1016/j.matlet.2016.02.113 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Porous anodic alumina layers with modulated pore diameters formed by sequential anodizing in different electrolytes Leszek Zaraska*, Marian Jaskuła, Grzegorz D. Sulka Department of Physical Chemistry and Electrochemistry, Faculty of Chemistry Jagiellonian University in Krakow, Ingardena 3, 30-060 Krakow, Poland
Keywords: porous alumina; nanopores; anodization; electrolyte change; modulated diameter
*Corresponding author. E-mail: [email protected]
Department of Physical Chemistry & Electrochemistry, Faculty of Chemistry Jagiellonian University in Krakow Ingardena 3, 30060 Krakow, Poland Tel: +48 12 663 22 64 Fax: +48 12 634 05 15
Abstract A high purity (99.999 %) Al foil was pre-textured by 1 h of anodic oxidation in 0.3 M oxalic acid at 45 V and room temperature, followed by the chemical etching of as formed irregular oxide layer. As prepared samples were used for fabrication of porous anodic aluminum oxide (AAO) layers with modulated pore diameters by a simple anodization carried out at the same anodizing conditions (potential and temperature) but in a sequentially changed electrolyte (0.3 M H2C2O4 and 0.3 M H3PO4). It was proved that anodization carried out in the oxalic acid result in a 1
relatively fast (~250 nm min-1) formation of porous Al2O3 structure with an average pore diameter of about 30 nm. On the other hand, anodic oxidation in the phosphoric acid electrolyte leads to a much slower (~40 nm min-1) formation of AAO layer with widen nanopores – about 45 nm in diameter. As a result, nanoporous Al2O3 layers with distinguishable nanochannel segments having different pore diameters were obtained. Moreover, no loss of hexagonal pore order was observed even when the anodization was carried out in phosphoric acid at the potential of 45 V which is far away from the self-ordering regime for this electrolyte (~195 V).
1. Introduction Recently, porous anodic aluminum oxide (AAO) formed by a two-step self-organized anodic oxidation of metallic Al has become one of the most popular template materials for nanofabrication [1]. Simplicity, flexibility and relatively low-cost of the synthetic procedure make AAO especially valuable for fabrication of various 1D nanostructures such as nanowires and nanotubes [1, 2]. In general, it is well known that a simple two-step potentiostatic or galvanostatic anodization in acidic electrolytes (e.g., H2SO4, H2C2O4 and H3PO4) can result in formation of the Al2O3 layer with a dense array of cylindrical, hexagonally arranged nanochannels with uniform pore diameter and interpore spacing, which depend on the anodizing conditions [3, 4]. However, it was proved that AAO templates with unusual and complex shape of channels can be also synthesized by anodic oxidation of Al substrates [5]. For instance, AAO layers with periodically modulated nanochannel diameter can be obtained by pulse techniques based on combination of alternating mild anodization (MA) and hard anodization (HA) pulses [6–8]. In addition, AAOs with multimodulated pore structures were fabricated by cyclic anodization [9]. This approach is based on the use of periodic anodic current waves with different 2
shapes, amplitudes, and periods [10]. On the other hand, Liu at al. successfully obtained the AAO film with a periodically modulated pore diameter by alternating anodization of Al in phosphoric and tartaric acid solutions under galvanostatic conditions [11]. It should be mentioned that anodization in both electrolytes exhibited approximately identical the best ordering voltage (195 V) under constant current density operation [11]. In addition, very recently we also confirmed that the nanoporous structure formed on Al during the first anodizing step carried out at a selfordering regime is initially replicated at the beginning of the second anodization, independently of the type of electrolyte used [12]. As a result, oxide layers with the same hexagonal pore arrangement and pore spacing but with different pore diameters were successfully obtained by anodizations performed at the same potential but in different electrolytes [12]. Therefore, here we present some results on fabrication of regular AAO structures with modulated pore diameters by anodization of the Al, pre-textured with hexagonal concaves, carried out in periodically changed electrolytes (H2C2O4 and H3PO4). We believe the proposed method could be a promising and extreamly simple alternative for pulse anodization techniques that often require sophisticated equipment (e.g., programmable powers supplies) and relatively complicated procedures. 2. Material and methods A previously described procedure of two-step anodization was employed for fabrication of AAO membranes [12] – for details see Figure 1. Briefly, a high purity Al foil (0.5 mm thick, 99.999% in purity, Goodfellow) was cut in specimens (1.0 × 2.5 cm), degreased in acetone and ethanol then dried. Before anodizing, electrochemical polishing in a mixture of perchloric acid (60 wt.%) and ethanol (1:4 vol.) was performed under the constant voltage of 20 V for 1 min at 10 °C (Figure 1A). Then, a working surface of the electrode was defined with an acid resistant paint (Protecting Lacquer Yellow, Enthone GmbH). The first anodization was carried out in a 0.3
3
M oxalic acid solution at the potential of the self-ordering regime (45 V) – see Figure 1B, followed by the chemical etching of as obtained irregular oxide in a mixture of 6 wt.% H3PO4 and 1.8 wt.% H2CrO4 at 40 °C for 12 h. Immediately after oxide removal, the samples were reanodized under the same anodizing potential but in sequentially changed electrolytes (0.3 M H2C2O4 and 0.3 M H3PO4) – Figure 1C. All anodizing conditions are collected in Table 1. The surface morphology of porous anodic alumina structures was studied using a Hitachi S-4700 field emission scanning electron microscope (SEM). Before SEM imaging a thin layer of Au was sputtered on all AAO samples. The structural features of alumina layers were estimated directly from SEM images by using the scanning probe image processor WSxM v.12.0 [13] and ImageJ 1.37v software [14]. Figure 1 Table 1 3. Results and discussion All structural features of as obtained porous anodic alumina layers determined directly from SEM images are collected in Table 1. SEM images of porous alumina layer formed after 5 min of anodic oxidation in 0.3 M H2C2O4 followed by 10 min of anodization in 0.3 M H3PO4 (see sample 1 in Table 1) are shown in Figure 2 A–C. A dense array of regularly distributed nanopores with an average diameter of about 30 nm can be seen on the top of as grown oxide (Figure 2A). On the other hand, two distinct layers can be recognized in the cross sectional views (Figures 2B and C). The outer oxide layer with a thickness of about 1.3 µm and an average nanochannel diameter of 30 nm was initially formed on the Al surface during 5 min of anodization in oxalic acid solution at 45 V (see OA in Figure 2B). Therefore, the estimated oxide growth rate under these conditions was about 250 nm min-1. A change of electrolyte into a 0.3 M 4
phosphoric acid solution resulted in much slower (~40 nm min-1) AAO growth rate and significantly widen nanopores – about 45 nm in diameter (see PA in Figure 2B and Figure 2C) but the same pore spacing. The significantly lower oxide growth rate observed in phosphoric acid is a direct consequence of much lower current density observed during anodization in this electrolyte (see ref. [12]) being a result of a lower electrical conductivity and higher pH values than those observed for the oxalic acid solution of the same concentration. Moreover, it is generally accepted that the pore diameter increases with increasing pH of the solution as a result of diminished rate of anodic oxide formation [15, 16]. Additionally, the incorporated phosphate anions can also enhance the chemical dissolution of as formed aluminum oxide [17]. Figures 2D– G show SEM images of porous alumina after two cycles of alternate anodizations in oxalic and phosphoric acid (see sample 2 in Table 1). Also in this case, OA and PA segments with an average pore diameters of about 30 and 50 nm, respectively can be easily recognized (Figures 2E–G). A slightly larger pore diameter was observed on the top side of the porous alumina layer when compared to that observed in sample 1. It is an obvious result of prolonged chemical etching of oxide in acidic environment. It is also noteworthy that the thickness of both PA segments is almost identical that indicates high reproducibility of the process. Figure 2 Figures 3A–D show SEM images of the AAO layer formed by 10 min of anodization in 0.3 M H3PO4 followed by 5 min anodizing in a 0.3 M H2C2O4 solution (for details see sample 3 in Table 1). As can be seen the outer oxide layer consists of hexagonally arranged nanopores with an average diameter of about 45 nm (Figure 3A) and the thickness of the layer initially grown in phosphoric acid is about 300 nm (PA layer in Figure 3B). The cross sectional images (Figures 3B–C) clearly demonstrate that the change of electrolyte to 0.3 M oxalic acid resulted in a much
5
faster growth of the anodic alumina layer with a significantly lower nanochannel diameter (~25 nm) – see OA layer in Figure 3B. Repeating the cycle of anodization in phosphoric and oxalic acid (sample 4 in Table 1), leads to formation of second PA and OA segments (see Figure 3F and 3H) with almost identical thicknesses. Also in this case, a slightly larger pore diameter in the outer OA segment (see Figure 3H) can be attributed to the longer duration of the chemical etching of pore walls by an acidic electrolyte. Figure 3 It is noteworthy that the regular, hexagonal pore distribution is maintained even if the anodization is carried out in phosphoric acid at the potential of 45 V which is far away from the self-ordering regime for H3PO4 (~195 V). However, we recently proved that this regular pore distribution is maintained only during an initial stage of the second anodizing, so the thickness of PA layers with well ordered and straight nanopores is limited to only few hundreds of nanometers. The prolonged anodization will lead to almost complete loss of the nanopore order and formation of characteristic serrated nanochannels [12, 18, 19]. 4. Conclusions In conclusion, nanoporous anodic alumina layers with modulated pore diameters were successfully obtained by anodic oxidation of Al, pre-textured with hexagonal concaves, carried out in sequentially changed electrolytes but at the same anodizing potential and temperature. Moreover, AAO segments formed during anodization in a particular electrolyte exhibit identical thicknesses and similar pore diameters that suggests high reproducibility of the process. We believe the proposed approach could be employed for formation of AAO layers with more complex internal pore structures. The thickness of individual segments can be easily tuned by varying duration of anodization in particular electrolyte, while a further widening of 6
nanochannels can be easily performed by chemical etching in H3PO4. Therefore, the proposed method can be a promising alternative for pulse anodization techniques typically used for formation of AAO layers with modulated nanochannel diameters.
Acknowledgements This work was made possible with assistance of the PL-Grid project, contract number: POIG.02.03.00-00-007/08-00, website: www.plgrid.pl. The project is co-funded by the European Regional Development Fund as part of the Innovative Economy program. The authors would like to acknowledge Ms. Iwona Sroka for the assistance in samples preparation.
References [1] G.D. Sulka, L. Zaraska, W.J. Stępniowski, Anodic porous alumina as a template for nanofabrication, in: H.S. Nalwa (Ed.), Encyclopedia of Nanoscience and Nanotechnology 2 nd Edition, American Scientific Publishers, Los Angeles, 2011, vol. 11, pp. 261–349. [2] W.J. Stępniowski, M. Salerno, Fabrication of nanowires and nanotubes by anodic alumina template-assisted electrodeposition, in: W. Ahmed, N. Ali (Eds.), Manufacturing Nanostructures, One Central Press, 2014, pp. 321–357. [3] G.D. Sulka, Highly ordered anodic porous alumina formation by self-organised anodising and template-assisted fabrication of nanostructured materials, in: Ali Eftekhari (Ed.), Nanostructured Materials in Electrochemistry, Wiley-VCH, Weinheim, 2008, pp. 1–116.
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[4] W. Lee, S-J. Park, Porous anodic aluminum oxide: Anodization and templated synthesis of functional nanostructures, Chem. Rev. 114 (2014) 7487–7556. [5] A.M.M. Jani, D. Losic, N.H. Voelcker, Nanoporous anodic aluminium oxide: Advances in surface engineering and emerging applications, Prog. Mater. Sci. 58 (2013) 636–704. [6] W. Lee, R. Ji, U. Gösele, K. Nielsch, Fast fabrication of long-range ordered porous alumina membranes by hard anodization, Nat. Mater. 5 (2006) 741–747. [7] G.D. Sulka, A. Brzózka, L. Liu, Fabrication of diameter-modulated and ultrathin porous nanowires in anodic aluminum oxide templates, Electrochim. Acta 56 (2011) 4972–4979. [8] G.D. Sulka, K. Hnida, Distributed Bragg reflector based on porous anodic alumina fabricated by pulse anodization, Nanotechnology 23 (2012) 075303. [9] D. Losic, M. Lillo, D. Losic Jr., Porous alumina with shaped pore geometries and complex pore architectures fabricated by cyclic anodization, Small 5 (2009) 1392–1397. [10] D. Losic, D. Losic Jr., Preparation of porous anodic alumina with periodically perforated pores, Langmuir 25 (2009) 5426–5431. [11] S. Liu, S. Tang, H. Zhou, C. Fu, Z. Huang, H. Liu, Y. Kuang, Fabrication of AAO films with controllable nanopore size by changing electrolytes and electrolytic parameters, J. Solid State Electrochem. 17 (2013) 1931–1938. [12] L. Zaraska, A. Brudzisz, E. Wierzbicka, G.D. Sulka, The effect of electrolyte change on the morphology and degree of nanopore order of porous alumina formed by two-step anodization, Electrochim. Acta, under review. [13] I. Horcas, R. Fernández, J.M. Gómez-Rodríguez, J. Colchero, J. Gómez-Herrero, A.M. Baro, WSXM: A software for scanning probe microscopy and a tool for nanotechnology, Rev. Sci. Instrum. 78 (2007) 013705 [14] W.S. Rasband, ImageJ, U. S. National Institutes of Health, Bethesda, Maryland, USA, 8
http://imagej.nih.gov/ij/, 1997–2014. [15] V.P. Parkhutik, I.V. Shershulsky, Theoretical modelling of porous oxide growth on aluminium, J. Phys. D: Appl. Phys., 25 (1992) 1258–1263 [16] K. Nielsch, J. Choi, K. Schwirn, R.B. Wehrspohn, U. Gösele, Self-ordering regimes of porous alumina: The 10% porosity rule, Nano Lett. 2 (2002) 677–680 [17] H. Takahashi, K. Fujimoto, H. Konno, M. Nagayama, Distribution of anions and protons in oxide films formed anodically on aluminum in a phosphate solution, J. Electrochem. Soc., 131 (1984) 1856–1861 [18] D. Li, Ch. Jiang, J. Jiang, J.G. Lu, Self-assembly of periodic serrated nanostructures, Chem. Mater. 21 (2009) 253–258. [19] D. Li, L. Zhao, Ch. Jiang, J.G. Lu, Formation of anodic aluminum oxide with serrated nanochannels, Nano Lett. 10 (2010) 2766–2771.
Figure captions: Figure 1. Schematic representation of the experimental procedure. Figure 2. SEM images of AAO layers formed by alternate anodizing in oxalic and phosphoric acid electrolytes: sample 1 (A–C) and sample 2 (D–G). Figure 3. SEM images of AAO layers formed by alternate anodizing in phosphoric and oxalic acid electrolytes: sample 3 (A–D) and sample 4 (E–H).
9
10
11
12
Table 1. Experimental conditions applied during both anodizing steps for different samples together with structural features of as obtained AAO layers; where: U – anodizing potential, T – anodizing temperature, t – anodizing duration, H tot – total thickness of AAO layer, H segm – thickness of AAO segment anodized in particular acid, Dp(top) – pore diameter at the top of the sample, Dp(segm) – nanochannel diameter in AAO segment anodized in particular acid. 2nd anodizing step Sample number
U [V]
T [°C]
Electrolyte 0.3 M H2C2O4
1
2
45
45
Structural features of AAO layer t [min]
H tot [µm]
5
20
H segm
Dp(top)
Dp(segm)
[µm]
[nm]
[nm]
1.3 1.7
0.3 M H3PO4
10
0.3 M H2C2O4
5
20
0.4
10
45 ± 3
1.3 3.0
0.3 M H3PO4
30 ± 2 32 ± 2
30 ± 2 35 ± 2
0.4
50 ± 3
13
3
4
45
45
0.3 M H2C2O4
5
1.1
30 ± 2
0.3 M H3PO4
10
0.4
50 ± 3
0.3 M H3PO4
10
20
0.3 1.4
45 ± 3 45 ± 3
0.3 M H2C2O4
5
1.1
25 ± 2
0.3 M H3PO4
10
0.3
45 ± 3
0.3 M H2C2O4
5
20
1.0 2.7
30 ± 2 47 ± 3
0.3 M H3PO4
10
0.3
45 ± 3
0.3 M H2C2O4
5
1.1
25 ± 2
Highlights 1. Porous alumina layers with modulated pore diameters were obtained by anodization. 2. Anodization was performed in sequentially changed electrolytes at the same voltage. 3. AAO segments with different pore diameters were formed in different electrolytes. 4. No loss of hexagonal pore arrangement was observed.
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PII: DOI: Reference:
S0167-577X(16)30273-7 http://dx.doi.org/10.1016/j.matlet.2016.02.113 MLBLUE20402
To appear in: Materials Letters Received date: 29 December 2015 Revised date: 25 January 2016 Accepted date: 21 February 2016 Cite this article as: Leszek Zaraska, Marian Jaskuła and Grzegorz D. Sulka, Porous anodic alumina layers with modulated pore diameters formed by sequential anodizing in different electrolytes, Materials Letters, http://dx.doi.org/10.1016/j.matlet.2016.02.113 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Porous anodic alumina layers with modulated pore diameters formed by sequential anodizing in different electrolytes Leszek Zaraska*, Marian Jaskuła, Grzegorz D. Sulka Department of Physical Chemistry and Electrochemistry, Faculty of Chemistry Jagiellonian University in Krakow, Ingardena 3, 30-060 Krakow, Poland
Keywords: porous alumina; nanopores; anodization; electrolyte change; modulated diameter
*Corresponding author. E-mail: [email protected]
Department of Physical Chemistry & Electrochemistry, Faculty of Chemistry Jagiellonian University in Krakow Ingardena 3, 30060 Krakow, Poland Tel: +48 12 663 22 64 Fax: +48 12 634 05 15
Abstract A high purity (99.999 %) Al foil was pre-textured by 1 h of anodic oxidation in 0.3 M oxalic acid at 45 V and room temperature, followed by the chemical etching of as formed irregular oxide layer. As prepared samples were used for fabrication of porous anodic aluminum oxide (AAO) layers with modulated pore diameters by a simple anodization carried out at the same anodizing conditions (potential and temperature) but in a sequentially changed electrolyte (0.3 M H2C2O4 and 0.3 M H3PO4). It was proved that anodization carried out in the oxalic acid result in a 1
relatively fast (~250 nm min-1) formation of porous Al2O3 structure with an average pore diameter of about 30 nm. On the other hand, anodic oxidation in the phosphoric acid electrolyte leads to a much slower (~40 nm min-1) formation of AAO layer with widen nanopores – about 45 nm in diameter. As a result, nanoporous Al2O3 layers with distinguishable nanochannel segments having different pore diameters were obtained. Moreover, no loss of hexagonal pore order was observed even when the anodization was carried out in phosphoric acid at the potential of 45 V which is far away from the self-ordering regime for this electrolyte (~195 V).
1. Introduction Recently, porous anodic aluminum oxide (AAO) formed by a two-step self-organized anodic oxidation of metallic Al has become one of the most popular template materials for nanofabrication [1]. Simplicity, flexibility and relatively low-cost of the synthetic procedure make AAO especially valuable for fabrication of various 1D nanostructures such as nanowires and nanotubes [1, 2]. In general, it is well known that a simple two-step potentiostatic or galvanostatic anodization in acidic electrolytes (e.g., H2SO4, H2C2O4 and H3PO4) can result in formation of the Al2O3 layer with a dense array of cylindrical, hexagonally arranged nanochannels with uniform pore diameter and interpore spacing, which depend on the anodizing conditions [3, 4]. However, it was proved that AAO templates with unusual and complex shape of channels can be also synthesized by anodic oxidation of Al substrates [5]. For instance, AAO layers with periodically modulated nanochannel diameter can be obtained by pulse techniques based on combination of alternating mild anodization (MA) and hard anodization (HA) pulses [6–8]. In addition, AAOs with multimodulated pore structures were fabricated by cyclic anodization [9]. This approach is based on the use of periodic anodic current waves with different 2
shapes, amplitudes, and periods [10]. On the other hand, Liu at al. successfully obtained the AAO film with a periodically modulated pore diameter by alternating anodization of Al in phosphoric and tartaric acid solutions under galvanostatic conditions [11]. It should be mentioned that anodization in both electrolytes exhibited approximately identical the best ordering voltage (195 V) under constant current density operation [11]. In addition, very recently we also confirmed that the nanoporous structure formed on Al during the first anodizing step carried out at a selfordering regime is initially replicated at the beginning of the second anodization, independently of the type of electrolyte used [12]. As a result, oxide layers with the same hexagonal pore arrangement and pore spacing but with different pore diameters were successfully obtained by anodizations performed at the same potential but in different electrolytes [12]. Therefore, here we present some results on fabrication of regular AAO structures with modulated pore diameters by anodization of the Al, pre-textured with hexagonal concaves, carried out in periodically changed electrolytes (H2C2O4 and H3PO4). We believe the proposed method could be a promising and extreamly simple alternative for pulse anodization techniques that often require sophisticated equipment (e.g., programmable powers supplies) and relatively complicated procedures. 2. Material and methods A previously described procedure of two-step anodization was employed for fabrication of AAO membranes [12] – for details see Figure 1. Briefly, a high purity Al foil (0.5 mm thick, 99.999% in purity, Goodfellow) was cut in specimens (1.0 × 2.5 cm), degreased in acetone and ethanol then dried. Before anodizing, electrochemical polishing in a mixture of perchloric acid (60 wt.%) and ethanol (1:4 vol.) was performed under the constant voltage of 20 V for 1 min at 10 °C (Figure 1A). Then, a working surface of the electrode was defined with an acid resistant paint (Protecting Lacquer Yellow, Enthone GmbH). The first anodization was carried out in a 0.3
3
M oxalic acid solution at the potential of the self-ordering regime (45 V) – see Figure 1B, followed by the chemical etching of as obtained irregular oxide in a mixture of 6 wt.% H3PO4 and 1.8 wt.% H2CrO4 at 40 °C for 12 h. Immediately after oxide removal, the samples were reanodized under the same anodizing potential but in sequentially changed electrolytes (0.3 M H2C2O4 and 0.3 M H3PO4) – Figure 1C. All anodizing conditions are collected in Table 1. The surface morphology of porous anodic alumina structures was studied using a Hitachi S-4700 field emission scanning electron microscope (SEM). Before SEM imaging a thin layer of Au was sputtered on all AAO samples. The structural features of alumina layers were estimated directly from SEM images by using the scanning probe image processor WSxM v.12.0 [13] and ImageJ 1.37v software [14]. Figure 1 Table 1 3. Results and discussion All structural features of as obtained porous anodic alumina layers determined directly from SEM images are collected in Table 1. SEM images of porous alumina layer formed after 5 min of anodic oxidation in 0.3 M H2C2O4 followed by 10 min of anodization in 0.3 M H3PO4 (see sample 1 in Table 1) are shown in Figure 2 A–C. A dense array of regularly distributed nanopores with an average diameter of about 30 nm can be seen on the top of as grown oxide (Figure 2A). On the other hand, two distinct layers can be recognized in the cross sectional views (Figures 2B and C). The outer oxide layer with a thickness of about 1.3 µm and an average nanochannel diameter of 30 nm was initially formed on the Al surface during 5 min of anodization in oxalic acid solution at 45 V (see OA in Figure 2B). Therefore, the estimated oxide growth rate under these conditions was about 250 nm min-1. A change of electrolyte into a 0.3 M 4
phosphoric acid solution resulted in much slower (~40 nm min-1) AAO growth rate and significantly widen nanopores – about 45 nm in diameter (see PA in Figure 2B and Figure 2C) but the same pore spacing. The significantly lower oxide growth rate observed in phosphoric acid is a direct consequence of much lower current density observed during anodization in this electrolyte (see ref. [12]) being a result of a lower electrical conductivity and higher pH values than those observed for the oxalic acid solution of the same concentration. Moreover, it is generally accepted that the pore diameter increases with increasing pH of the solution as a result of diminished rate of anodic oxide formation [15, 16]. Additionally, the incorporated phosphate anions can also enhance the chemical dissolution of as formed aluminum oxide [17]. Figures 2D– G show SEM images of porous alumina after two cycles of alternate anodizations in oxalic and phosphoric acid (see sample 2 in Table 1). Also in this case, OA and PA segments with an average pore diameters of about 30 and 50 nm, respectively can be easily recognized (Figures 2E–G). A slightly larger pore diameter was observed on the top side of the porous alumina layer when compared to that observed in sample 1. It is an obvious result of prolonged chemical etching of oxide in acidic environment. It is also noteworthy that the thickness of both PA segments is almost identical that indicates high reproducibility of the process. Figure 2 Figures 3A–D show SEM images of the AAO layer formed by 10 min of anodization in 0.3 M H3PO4 followed by 5 min anodizing in a 0.3 M H2C2O4 solution (for details see sample 3 in Table 1). As can be seen the outer oxide layer consists of hexagonally arranged nanopores with an average diameter of about 45 nm (Figure 3A) and the thickness of the layer initially grown in phosphoric acid is about 300 nm (PA layer in Figure 3B). The cross sectional images (Figures 3B–C) clearly demonstrate that the change of electrolyte to 0.3 M oxalic acid resulted in a much
5
faster growth of the anodic alumina layer with a significantly lower nanochannel diameter (~25 nm) – see OA layer in Figure 3B. Repeating the cycle of anodization in phosphoric and oxalic acid (sample 4 in Table 1), leads to formation of second PA and OA segments (see Figure 3F and 3H) with almost identical thicknesses. Also in this case, a slightly larger pore diameter in the outer OA segment (see Figure 3H) can be attributed to the longer duration of the chemical etching of pore walls by an acidic electrolyte. Figure 3 It is noteworthy that the regular, hexagonal pore distribution is maintained even if the anodization is carried out in phosphoric acid at the potential of 45 V which is far away from the self-ordering regime for H3PO4 (~195 V). However, we recently proved that this regular pore distribution is maintained only during an initial stage of the second anodizing, so the thickness of PA layers with well ordered and straight nanopores is limited to only few hundreds of nanometers. The prolonged anodization will lead to almost complete loss of the nanopore order and formation of characteristic serrated nanochannels [12, 18, 19]. 4. Conclusions In conclusion, nanoporous anodic alumina layers with modulated pore diameters were successfully obtained by anodic oxidation of Al, pre-textured with hexagonal concaves, carried out in sequentially changed electrolytes but at the same anodizing potential and temperature. Moreover, AAO segments formed during anodization in a particular electrolyte exhibit identical thicknesses and similar pore diameters that suggests high reproducibility of the process. We believe the proposed approach could be employed for formation of AAO layers with more complex internal pore structures. The thickness of individual segments can be easily tuned by varying duration of anodization in particular electrolyte, while a further widening of 6
nanochannels can be easily performed by chemical etching in H3PO4. Therefore, the proposed method can be a promising alternative for pulse anodization techniques typically used for formation of AAO layers with modulated nanochannel diameters.
Acknowledgements This work was made possible with assistance of the PL-Grid project, contract number: POIG.02.03.00-00-007/08-00, website: www.plgrid.pl. The project is co-funded by the European Regional Development Fund as part of the Innovative Economy program. The authors would like to acknowledge Ms. Iwona Sroka for the assistance in samples preparation.
References [1] G.D. Sulka, L. Zaraska, W.J. Stępniowski, Anodic porous alumina as a template for nanofabrication, in: H.S. Nalwa (Ed.), Encyclopedia of Nanoscience and Nanotechnology 2 nd Edition, American Scientific Publishers, Los Angeles, 2011, vol. 11, pp. 261–349. [2] W.J. Stępniowski, M. Salerno, Fabrication of nanowires and nanotubes by anodic alumina template-assisted electrodeposition, in: W. Ahmed, N. Ali (Eds.), Manufacturing Nanostructures, One Central Press, 2014, pp. 321–357. [3] G.D. Sulka, Highly ordered anodic porous alumina formation by self-organised anodising and template-assisted fabrication of nanostructured materials, in: Ali Eftekhari (Ed.), Nanostructured Materials in Electrochemistry, Wiley-VCH, Weinheim, 2008, pp. 1–116.
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[4] W. Lee, S-J. Park, Porous anodic aluminum oxide: Anodization and templated synthesis of functional nanostructures, Chem. Rev. 114 (2014) 7487–7556. [5] A.M.M. Jani, D. Losic, N.H. Voelcker, Nanoporous anodic aluminium oxide: Advances in surface engineering and emerging applications, Prog. Mater. Sci. 58 (2013) 636–704. [6] W. Lee, R. Ji, U. Gösele, K. Nielsch, Fast fabrication of long-range ordered porous alumina membranes by hard anodization, Nat. Mater. 5 (2006) 741–747. [7] G.D. Sulka, A. Brzózka, L. Liu, Fabrication of diameter-modulated and ultrathin porous nanowires in anodic aluminum oxide templates, Electrochim. Acta 56 (2011) 4972–4979. [8] G.D. Sulka, K. Hnida, Distributed Bragg reflector based on porous anodic alumina fabricated by pulse anodization, Nanotechnology 23 (2012) 075303. [9] D. Losic, M. Lillo, D. Losic Jr., Porous alumina with shaped pore geometries and complex pore architectures fabricated by cyclic anodization, Small 5 (2009) 1392–1397. [10] D. Losic, D. Losic Jr., Preparation of porous anodic alumina with periodically perforated pores, Langmuir 25 (2009) 5426–5431. [11] S. Liu, S. Tang, H. Zhou, C. Fu, Z. Huang, H. Liu, Y. Kuang, Fabrication of AAO films with controllable nanopore size by changing electrolytes and electrolytic parameters, J. Solid State Electrochem. 17 (2013) 1931–1938. [12] L. Zaraska, A. Brudzisz, E. Wierzbicka, G.D. Sulka, The effect of electrolyte change on the morphology and degree of nanopore order of porous alumina formed by two-step anodization, Electrochim. Acta, under review. [13] I. Horcas, R. Fernández, J.M. Gómez-Rodríguez, J. Colchero, J. Gómez-Herrero, A.M. Baro, WSXM: A software for scanning probe microscopy and a tool for nanotechnology, Rev. Sci. Instrum. 78 (2007) 013705 [14] W.S. Rasband, ImageJ, U. S. National Institutes of Health, Bethesda, Maryland, USA, 8
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Figure captions: Figure 1. Schematic representation of the experimental procedure. Figure 2. SEM images of AAO layers formed by alternate anodizing in oxalic and phosphoric acid electrolytes: sample 1 (A–C) and sample 2 (D–G). Figure 3. SEM images of AAO layers formed by alternate anodizing in phosphoric and oxalic acid electrolytes: sample 3 (A–D) and sample 4 (E–H).
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Table 1. Experimental conditions applied during both anodizing steps for different samples together with structural features of as obtained AAO layers; where: U – anodizing potential, T – anodizing temperature, t – anodizing duration, H tot – total thickness of AAO layer, H segm – thickness of AAO segment anodized in particular acid, Dp(top) – pore diameter at the top of the sample, Dp(segm) – nanochannel diameter in AAO segment anodized in particular acid. 2nd anodizing step Sample number
U [V]
T [°C]
Electrolyte 0.3 M H2C2O4
1
2
45
45
Structural features of AAO layer t [min]
H tot [µm]
5
20
H segm
Dp(top)
Dp(segm)
[µm]
[nm]
[nm]
1.3 1.7
0.3 M H3PO4
10
0.3 M H2C2O4
5
20
0.4
10
45 ± 3
1.3 3.0
0.3 M H3PO4
30 ± 2 32 ± 2
30 ± 2 35 ± 2
0.4
50 ± 3
13
3
4
45
45
0.3 M H2C2O4
5
1.1
30 ± 2
0.3 M H3PO4
10
0.4
50 ± 3
0.3 M H3PO4
10
20
0.3 1.4
45 ± 3 45 ± 3
0.3 M H2C2O4
5
1.1
25 ± 2
0.3 M H3PO4
10
0.3
45 ± 3
0.3 M H2C2O4
5
20
1.0 2.7
30 ± 2 47 ± 3
0.3 M H3PO4
10
0.3
45 ± 3
0.3 M H2C2O4
5
1.1
25 ± 2
Highlights 1. Porous alumina layers with modulated pore diameters were obtained by anodization. 2. Anodization was performed in sequentially changed electrolytes at the same voltage. 3. AAO segments with different pore diameters were formed in different electrolytes. 4. No loss of hexagonal pore arrangement was observed.
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