ARTICLE pubs.acs.org/JPCC
The Influence of Pore Diameter on Bifurcation and Termination of Individual Pores in Nanoporous Alumina Olga Kopp, Monika Lelonek, and Meinhard Knoll* Institute for Physical Chemistry & CeNTech, University of Muenster, Heisenbergstr. 11, D-48149 Muenster, Germany ABSTRACT: We investigated for the first time the influence of pore diameter on the growth of nanoporous alumina on convex and concave aluminum surfaces. Due to pore bifurcation and termination, the pore diameter changed characteristically with increasing pore length. We observed clear indications that the growth of individual pores was influenced by neighboring pores. The self-organization on concave and convex substrates may be explained by repulsive interactions between the pores during growth.
1. INTRODUCTION Nanoporous alumina membranes are widely used as templates for both polymeric and metallic nanocomposites,19 and there is great interest in determining the effect of growth conditions on the resulting porous alumina. Alumina nanostructures are normally fabricated using an electrochemical etching process in which the pores grow perpendicular to a planar surface in a highly ordered hexagonal “honeycomb” structure.10 The remarkably parallel selforganization of the pores is the result of repulsive interactions between the pores during growth due to volume expansion.11 Throughout the oxidation process, the pore diameter and interpore distance remain constant, and both depend on reaction parameters such as the electrolyte (e.g., oxalic, sulfuric, or phosphoric acid1215). During the first seconds of anodic oxidation, a compact film of alumina (the barrier oxide) is generated.16 After this initial period, field-enhanced oxide dissolution at surface irregularities in the electrolyte/oxide interface is balanced by oxide growth at the metal/oxide interface.17,18 O2 and OH anions from the electrolyte migrate through the oxide layer to the bottom of the pores, while some of the Al3þ ions drift from the metal through the oxide into the solution at the top of the oxide,1821 contributing to the growth of this layer. Studies examining porous alumina structures typically employ planar surfaces. Our novel experiments on convex and concave surfaces reveal that bifurcation and termination of pores may occur during the growth phase,22 in contrast to earlier reports that bifurcation of pores was driven by a change in the electrolyte or applied voltage.2,23,24 Concave or convex substrate curvature strongly influences the growth rate of the oxide layer, with the difference in rate being a function of the radius of curvature under identical reaction conditions.26 In this work, we analyze for the first time the growth of individual pores under the influence of neighboring pores on concave and convex aluminum surfaces, observing both bifurcation and termination of pores. These results provide a deeper r 2011 American Chemical Society
insight into the mechanisms of pore bifurcation and termination during oxide growth. Oxide layers that undergo pore bifurcation or termination may be used as “bottom-up” fabrication templates for various polymer applications, or for the production of carbon nanotubes.25 They can also be used for transition between a nano- and microsized area, here in particular as a template for fillings with metal or polymers.
2. EXPERIMENTAL SECTION Two types of aluminum substrate were used for anodization in oxalic acid in order to demonstrate that the results are independent of the substrate’s manufacturer or fabrication process and may be applied to any aluminum undergoing oxidation. Thick Substrates. Thick aluminum foil samples (99.999%, 250 μm thick, ChemPur) were consecutively degreased in acetone, ethyl alcohol, and distilled water in an ultrasonic bath and then annealed under a nitrogen atmosphere at 400 °C for 3 h to enhance the grain size. The substrates were electropolished in a mixed solution of H3PO4, H2SO4, and CrO3 for 3 min at 80 °C. The concave and convex structuring was prepared by partially covering the aluminum with stripes of lacquer, electropolishing for 10 min to remove a layer of aluminum between the stripes to obtain a structure with sharp steps, and then removing the lacquer with acetone and electropolishing for 60 s to smooth the steps and form convex and concave areas. All polishing steps were performed in the same solution (H3PO4, H2SO4, and CrO3 at 80 °C). The dissolving rate of our mixture is much higher than the one in ethanol/perchloric acid. The rate is about 1.5 μm/min. The anodizing process was conducted under a constant cell potential of 40 V in 0.3 M oxalic acid at a temperature between 8 and 10 °C for 10 h. A subsequent pore-widening etching process was carried out in 5 wt % phosphoric acid at 25 °C for 20 min. Received: February 9, 2011 Revised: March 25, 2011 Published: April 07, 2011 7993
dx.doi.org/10.1021/jp201310f | J. Phys. Chem. C 2011, 115, 7993–7996
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Figure 3. Diverging pores on concave surfaces: (a) thin substrate; (b) thick substrate.
Figure 1. SEM pictures of alumina pore structures: parallel pores on a planar surface (a); diverging pores on a concave surface (b); converging pores on a convex surface (c); 3D view of diverging pores on a concave surface (d). Figure 4. Outer pore diameter d vs pore length L for diverging pores on concave surfaces.
Figure 2. Schematic of pore structure: (a) parallel pores; (b) diverging pores; (c) converging pores.
Thin Substrates. Soda-lime glass microscope slides (26 mm 76 mm 1 mm) were cleaned using deionized water in an ultrasonic bath for 10 min and then soaked in 1:2 H2O2(25%): H2SO4(97%) at 80 °C for 10 min. The slides were rinsed in deionized water for 10 min, dried under nitrogen, immersed in 1:1:8 H2O2(25%):HCl(30%):H2O at 80 °C for 10 min, rinsed in deionized water for 10 min, and finally dried under nitrogen. A 10 μm layer of aluminum (99.999%) was deposited (800 W, 5 105 mbar, 30 sccm Ar) using an LS 730 S DC sputtering system (Von Ardenne Anlagenbau GmbH, Germany). The layer was annealed in a tube furnace (TZF-12/100/900, CARBOLITE) under nitrogen at 400 °C for 3 h to increase the aluminum crystal grain size. Surface oxide was removed using 5% H3PO4 þ 20 g/L CrO3 (70 °C, 40 min).
Figure 5. Converging pores on a convex aluminum surface: thin substrate (a); thick substrate (b).
The surface curvature was obtained using a soft electropolishing procedure in a mixed solution of H3PO4, H2SO4, and CrO3 at 80 ( 1 °C. The applied potential was increased from 0 to 20 V over 10 s and maintained at 20 V for 20 s. The samples were rinsed in deionized water for 10 min and dried under nitrogen. The surface oxide was removed by immersion in 5% H3PO4 þ 20 g/L CrO3 at 70 °C for 40 min. A multistep anodic oxidation was conducted in 0.3 M oxalic acid at 40 V and a temperature of 10 ( 1 °C. The native oxide was 7994
dx.doi.org/10.1021/jp201310f |J. Phys. Chem. C 2011, 115, 7993–7996
The Journal of Physical Chemistry C removed using a solution of 5% H3PO4 þ 20 g/L CrO3 at 70 °C for 40 min. An initial oxidation was performed for 30 min, after which the oxide was again removed using the phosphoric acid/ chromium trioxide solution. The oxidation and removal cycle was repeated, followed by an anodic oxidation for 40 min and a pore-widening etching process in H3PO4 (5 wt %) at 23 °C for 40 min. For analysis using scanning electron microscopy, the samples were broken orthogonally and examined using a 1540 EsB SEM (Carl Zeiss SMT Ltd.) after the various preparation steps.
3. RESULTS AND DISCUSSION SEM images of nanoporous alumina grown on planar (a), concave (b), and convex (c) surfaces are presented in Figure 1. Pore development depends on the space available during growth. Parallel pores grown on a planar surface exhibited constant diameters and interpore distances similar to those commonly reported for anodization in oxalic acid. The diverging pores presented on concave
Figure 6. Outer pore diameter d vs pore length L for converging pores on a convex surface.
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substrates gained space during growth and increased in diameter before bifurcation, while converging pores on convex substrates decreased in diameter due to the loss of space before pore termination. A 3D view of diverging pores on a concave surface is given in Figure 1d. In order to provide a more detailed analysis, we defined several pore diameters (Figure 2). The outer diameter d is the main parameter to be analyzed, and is defined as the distance between the cell walls. do is the diameter at the oxide surface. The inner diameter depends on the amount of pore widening after growth. Therefore, we analyzed the outer diameter. Other critical diameters include the pore diameter at bifurcations (dmax) and at terminations (dmin). The typical outer pore diameter for planar surfaces anodized in oxalic acid is approximately 105 nm. Figure 3 contains SEM images from samples with concave surfaces and diverging pores. In the thin sample (Figure 3a), two pores (1 and 2) exhibit bifurcation. Another bifurcation appears in the thick oxide of Figure 3b. In this case, the bifurcation took place at a depth of approximately 12 μm. We analyzed the outer diameters of pores 1 and 2 from the thin sample in terms of pore length L (Figure 4). The diameters of the pores increased to 143 and 148 nm before bifurcation. Following bifurcation, the pore diameters were reduced to 72 and 74 nm. Because two pores must grow in the area previously available for a single pore, the diameter was reduced to 50 ( 3.6% of the pore diameter before bifurcation. A similar result was observed in the thick sample. At a depth of 12 μm, the pore diameter increased from 100 to 130 nm and then decreased to 65 nm following bifurcation. Again, the minimum pore diameter was 50 ( 5.8% of the maximum diameter. Pore bifurcation occurred once the pore diameter reached 130150% of its original size. At that point, the irregularities in the widened pore bottom refocus the field streamlines, initiating a field-enhanced dissolution of the oxide at the bottom of the pore and concluding with the development of pits that become the point of origin for the two new pores. The diameters of the newly formed pores also begin to increase due to interactions with neighboring pores. The maximum outer diameter of the pore at pore bifurcation dmax was independent of the radius of curvature and was equal to 140 ( 10 nm (140 ( 7.2%).
Figure 7. Converging pores on a convex surface with neighboring pores (thin substrates): outer pore diameter d vs pore length L (a); SEM image (b). 7995
dx.doi.org/10.1021/jp201310f |J. Phys. Chem. C 2011, 115, 7993–7996
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Figure 8. Converging pores on a convex surface with its neighboring pores (thick substrate): outer pore diameter d vs pore length L (a); SEM image (b).
In contrast to the pore bifurcation observed on concave surfaces, we found that pores in oxides with convex surfaces eventually terminated. In Figure 5a, three of the pores located in a thin substrate display pore termination during oxide growth (nos. 1, 3, and 6). Pore 1 is short (580 nm), pore 3 is intermediate in length (716 nm), and pore 6 has a length of 1150 nm. All have diameters of 100 ( 1 nm at the surface, with the diameter decreasing to 57.9 ( 1 nm (42.1 ( 1.7%) near the termination (Figure 6). A similar result occurred in thick substrates where the analysis was performed at a depth of approximately 12 μm (Figure 5b). Near the termination, the pore diameter dmin decreased to 55 ( 1 nm (Figure 6). These results were independent of the radius of curvature, and a pore termination was always observed once the outer pore diameter was reduced to 5060% of its original size. Five neighboring pores in thin substrates were analyzed (Figure 7). In pore 1, the diameter decreased from d0 = 101 nm to dmin = 59 nm and the pore terminated at a length of 580 nm. The diameter of pore 3 decreased from d0 = 100 nm to dmin = 58 nm, and pore termination occurred at a length of 720 nm. The termination of these pores exerted an influence on the growth of pores 2 and 4. As pore 1 stopped, pore 2 gained additional space, leading to a sharp increase in diameter from 75 to 107 nm. The diameter then decreased until pore 3 was terminated, after which pore 2 exhibited another cycle of increasing and decreasing diameter. Pore 3 exerted a similar influence on the growth of the neighboring pore 4. Similar results were observed in thick convex oxides (Figure 8). After termination of a pore, the diameter of neighboring pores initially increased to about 140 ( 10% of the original size and then decreased as the growth of converging pores again caused a reduction in available space.
After bifurcation, the diameter of the pores was reduced to approximately half of the diameter before bifurcation and then began to increase. Pores on convex substrates decreased in diameter during growth. As pores terminated, neighboring pores gained available space and their diameters sharply increased and then gradually decreased. The process was repeated if a second neighbor pore was terminated. The self-organization of the pores on concave and convex substrates may be explained by a repulsive interaction between the pores during growth. This is in good agreement with the remarkably parallel self-organization of the pores on planar surfaces, which may also be explained by repulsive interactions.11
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4. CONCLUSIONS We investigated the growth of individual pores in nanoporous alumina on Al substrates with concave or convex curvature, taking into account the influence of neighboring pores. We found that due to bifurcation and termination the pore diameter changed characteristically over the length of the pore. On concave substrates, the diameter of the growing pores increased until bifurcation took place. 7996
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