on the growth sequence of highly ordered nanoporous

On the growth sequence of highly ordered nanoporous anodic aluminium oxide M. Ghorbani a , F. Nasirpouri a, * , A. Iraji zad b , A. Saedi a a Department of Materials Science and Engineering, Sharif University of Technology, P.O. Box: 11365-9466, Tehran, Iran b Department of Physics, Sharif University of Technology, Tehran, Iran Received 7 October 2004; accepted 15 February 2005 Available online 14 April 2005 Abstract Anodic aluminium oxide films were fabricated by well known two-step anodizing process in oxalic acid electrolyte. The ordering characteristics (ordered pore domains, average pore diameter size and through-pore arrangement) of anodic aluminium oxide films, obtained in different growth sequences, were identified by microscopic analysis such as ex situ contact-mode atomic force microcopy and scanning electron microscopy. Flattened areas in which some pits are seen mostly cover the electropolished surface of alumin- ium. Single anodizing of aluminium produces a broad distribution of nanopore size, whereas induces a highly ordered hemispherical pattern, which plays the ordered nucleation sites for the second anodizing step. Moreover, a quasi-linear growth behavior exists for the ordered domain growth versus the duration of first step anodizing. The through-pore arrangement of ideally grown membranes is not influenced by increasing the duration of second step anodizing. Ó 2005 Elsevier Ltd. All rights reserved. Keywords: Nanomaterials; Nanoporous; Aluminium oxide; Anodic oxidation 1. Introduction Anodic aluminium oxide (AAO) has attracted much more interests recently due to its self-organizing nano- porous structure, introduced by Masuda and Fukuda [1]. This kind of self-organization of nanoporous anodic aluminium oxide is ba sed on naturally occurring long- range ordering, in which a highly regular poly-crystal- line pore structures occurs only for a quite small pro- cessing window, whereas an amorphous pore structure can be obtained for a very wide range of parameters without substantial change in morphology [2–7]. Over the last decade, these highly ordered nanoporous films have been used as templates for fabricating metal and semiconductor nanostructures in magnetic and opto- electronic applications [8–11]. On the mechanism of self-ordering in AAO nanopor- ous materials, so far OÕSullivan and Wood presented a model which was based on an electric field distribution at the pore tip. This model is able to give micro scopic explanations for the dependence of, e.g., pore diameters and inter-pore distances on applied voltage or electro- lyte composition, but cannot easily explain the self- ordering behavior. The self-organized arrangement of neighboring pores in hexagonal arrays can be explained by any repulsive interaction between the pores [12].A possible origin of these forces between neighboring pores is the mechanical stress, which is associated with the expansion during oxide formation interface and leads to form curved shape metal/oxide interface. It is claimed that the pores are formed during electropo lish- ing and/or ano dizing on the aluminium surface and can become hexagonal ly ordered at certain voltages and times of the init ial electropolishing [13] or by long-term anodization and reanodization [1] or also by a dynami c 0261-3069/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.matdes.2005.02.018 * Corresponding author. Tel./fax: +98 21 6005717. E-mail address: nasirpouri@mehr.sharif.edu (F. Nasirpouri). www.elsevier.com/locate/matdes Materials and Design 27 (2006) 983–988 Materials & Design process depending on the mobility of ions within the barrier oxide and of Al atoms within the metal [14]. The self-ordered ano dic aluminium oxide (AAO) membranes have been generally characterized by remov- ing the aluminium substrate and the structure has been observed from the bottom view of grown layer, in which the pore ordering can be achieved easily than the top surface of grown films which is more important in appli- cation [15–18]. In this study, we have investigated the topographical properties of AAO films, which contrib- ute into ordering the nanopore domains. Atomic Force Microcopy (AFM) is used as a useful method to control the nanopore topographical characteristics of anodic aluminium oxide. AFM allows the assessment of the ordering in nanopore domains, pore density and the external shape of AAO films. However, It must be taken into account that the internal diameter and shape of nanopores, which are defined as through-pore arrange- ment, cannot be evaluated by this method. This struc- tural characteristic of AAO membranes is easily characterized by scanning electron microscopy across the cleavage surfaces of films. 2. Method Two-step anodizing process was used to fabricate AAO templates with 40 nm pore diameter. High purity (99.999%) aluminium foils were annealed at 450 °C for 4 h to avoid remaining any residual stress in the alumin- ium substrates. Then, Al plates were electropolished in a mixture of perchlor ic acid (60%) and ethanol (1:4 in vol- ume) under 20 V below 5 °C for approximately 10 min. Anodization was conducted under constant cell poten- tial in oxalic acid electrolyte. The temperature of electro- lyte was maintained at 0 °C (between À2 and +2 °C) during anodization using a cooling system. However, the solution was stirred vigorously in order to accelerate the dispersion of the heat that evolved from the samples. The first and second anodization steps were conducted in the same condition as mentioned above. Meanwhile, the oxide layer formed in the first step was removed by wet chemical dissolution in a mixture of 0.2 M chro- mic acid and 0.4 M phosphoric acid at 60 °C for an appropriate time depending on the anodizing time. To facilitate the observation of pore arrangement on the Fig. 1. AFM images of electropolished surface of aluminium in 1 HClO 4 /4 EtOH below 5 °C for 10 min. Fig. 2. Topography of the single anodized aluminium in 0.3 M oxalic acid for 6 h at 40 V. 984 M. Ghorbani et al. / Materials and Design 27 (2006) 983–988 surface (topography), the samples were etched in 5%wt phosphoric acid in 35 °C for 30 min. In order to characterize the AAO films, the structural parameters includi ng ordered pore domains, average pore diameter size and through pore arrangement of AAO films, obtained in different growth sequences, were identified by microscopic analysis such as ex situ con- tact-mode AFM and scanning electron microscopy. The domain areas were determined by first outlining the boundaries of several domains on scanning electron microscopy (SEM ) micrographs, counting the number of pores for several domains, converting these numbers to areas, and finally averaging. Moreover, the through- pore configuration was observed across the fracture sur- face of grown films. 3. Results and discussion Fig. 1 illustrates AFM images of electropolished alu- minium surface in HClO 4 /EtOH solution. After electro- polishing, Al has an almost flat surface, exhibiting small etch pits and bumps, which could be seeds for pore nucleation [13]. Consequently, the electropolished alu- minium was anodized for first time. It has been shown [14] that the pits or pores nucleate on the natural barrier layer or in the bottom of porous layer during the ini- tial stages of anodization. However, it is assumed that the pits formed in the electropolishing contributing in the nucleation of pores on the aluminium in the order of 10 10 to 10 12 [18]. Fig. 2 shows the topography of anodized aluminium for 6 h in 0.3 M oxalic acid at Fig. 3. AFM images of aluminium surface after removing the first oxide layer in: (a) 2 lm · 2 lm and (b) 0.5 lm · 0.5 lm scan areas. M. Ghorbani et al. / Materials and Design 27 (2006) 983–988 985 0 °C, obtained by contact-mode AFM. As it can be seen, pores occur on the top surface randomly and have a broad size distribution. During the first step anodization process, the pores nucleate on the electropolished surface at almost random positions, i.e., lattice imper- fections or pits formed by electropolishing. As the anod- ization time is increasing the pores merge and form the curved metal/oxide interface due to stress inducing by volume expansion. Becau se of the random nucleation positions of initial pores, the hexagonally ordering of pores is just achieved in the first stages of anodizing at the bottom of porous oxide layer and cannot grow up to the thick anodized layers. After removing the first anodized oxide layer, the curved shape interface remains on the aluminium substrate. This structure is shown in Fig. 3, which demonstrates the AFM images of alumin- ium surface after removing the first oxide layer. The uni- form hemispherical shape of barrier layer covers the substrate surface. Anodizing the sample for the second time develops the pore growth exactly on the concave pattern created during the first step anodization. As the duration of the first step anodizing increases, the hexagonally or- dered pore areas, domains, in the bottom of porous oxide layer occur in the larger surfaces. In fact, nanop- ores exactly grow upon the relevant hemispheres and form direct pillars, which can be detected on top sur- faces of AAO films. Fig. 4 shows typical topography of AAO films, anodized in the different first step anodiz- ing times. The pore alignments are different in the do- mains and can be found out by domain boundaries, along which the pores gradually merge. Moreover, some other kinds of defects such as point defects and misfit dislocations can be seen in the topological studies. The misfit dislocation of the pores interrupts the periodic arrangement of the pores. As another important result, the domains size is changed as a function of time (Fig. 5). Two kinds of data fitting methods have been Fig. 4. SEM micrographs of identified ordered domains on top surfaces of AAO films obtained in oxalic acid after: (a) 4 h first step and 45 min second anodization step; (b) 15 h first step and 45 min second anodization step. Fig. 5. The ordered domain growth versus first-step anodizing time. Data represent the average domain size based on the identified areas of SEM micrographs. 986 M. Ghorbani et al. / Materials and Design 27 (2006) 983–988 applied to our experimental data. The linear method (R 2 = 0.9792) gives a function as: D = 0.55 t, where D is the domain size in square micrometer and t is the duration of the first step anodizing in hours. Li et al. [14] have shown a linear beha vior in their investigations. We have also fitted our da ta to the parabolic form: D = 0.52 t 0.7 with R 2 = 0.9988. This method provides a better fitting accuracy and is very similar to grain growth behavior in metals and alloys. For metals and al- loys, the driving force of grain growth is the grain boundary energy per unit area. For grain growth at a fixed temperature, the average radius R of the grain is a function of the time t: R = B n , where B is a tempera- ture-dependent parameter and n is about 0.4–0.5. As a result, we considered a quasi-linear growth of ordered domain size versus the first step anodizing time. It im- plies that the ordered domain size changes linearly in the short durations of first step anodizing and a para- bolic growth behavior exists in ordered domain size ob- tained by anodizing for long periods. Consequently, the effect of second step anodizing time has been investigated. In Fig. 6, the fracture sur- faces of highly ordered AAO films, obtained in different reanodizing time, elucidate the same through-pore arrangement. Thus, as the pore ordering takes place, increasing the second step anodizing time does not affect the achieved arrangement. The thicknesses of AAO films were measured approximately 1 and 6 lm after 45 min and 6 h anodizing in second step, respectively. 4. Conclusion Contact-mode atomic force microscopy confirmed the existence of concave pattern on the aluminium sub- strate after removing the first step oxide layer. More- over, the ordered domain size depends on the first step anodizing time as a quasi-linear behavior and the do- main size does not change significantly when the first step anodizing extends for very long periods. Also, thor- ough-pore arrangement of ideally grown of AAO films does not depend on second step anodizing time. Acknowledgment The authors wish to acknowledge the high technology center of Iranian ministry of industries for the financial support. References [1] Masuda H, Fukuda K. Ordered metal nanohole arrays made by a two-step replication of honeycomb structures of anodic alumina. Science 1995;268:1466–8. [2] Masuda H, Satoh M. Fabrication of gold nanodot array using anodic porous alumina as an evaporation mask. Jpn J Appl Phys 1996;35:L126–9. [3] Masuda H, Yada K, Osaka A. Self-ordering of cell configuration of anodic porous alumina with large-size pores in phosphoric acid solution. Jpn J Appl Phys 1998;37:L1340–2. [4] Masuda H, Hasegawa F, Ono S. Self-ordering of cell arrangement of anodic porous alumina formed in sulfuric acid solution. J Electrochem Soc 1997;144:L127–9. [5] Li AP, Muller F, Birner A, Nielsch K, Gosele U. Polycrystalline nanopore arrays with hexagonal ordering on aluminum. J Vac Sci Technol A 1999;17(4):1428–31. [6] Li AP, Muller F, Birner A, Nielsch K, Gosele U. Hexagonal pore arrays with a 50–420 nm interpore distance formed by self- organization in anodic alumina. J Appl Phys 1998;84(11):6023–6. [7] Li AP, Muller F, Gosele U. Polycrystalline and monocrystalline pore arrays with large interpore distance in anodic alumina. Electrochem Solid-State Lett 2000;3(3):131–4. [8] Mizeikisa V, Juodkazis S, Marcinkevicius A, Matsuo S, Misawa H. Tailoring and characterization of photonic crystals. J Photo- chem Photobiol C: Photochem Rev 2001;2:35–69. [9] Krauss TF, De La Rue RM. Photonic crystals in the optical regime, past, present and future. Prog Quant Electron 1999;23:51–96. [10] Metzger RM, Konovalov VV, Sun M, Xu T, Zangari G, Xu B, Benakli M, Doyle WD. Magnetic nanowires in hexagonally ordered pores of alumina. IEEE Trans Magn 2000;36(l):30–5. [11] Ji GB, Chen W, Tang SL, Gu BX, Li Z, Du YW. Fabrication and magnetic properties of ordered 20 nm Co–Pb nanowire arrays. Solid State Commun 2004;130:541–5. [12] Jessensky O, Muller F, Gosele U. Self-organized formation of hexagonal pore arrays in anodic alumina. Appl Phys Lett 1998;72(10):1173–5. Fig. 6. Through-pore arrangement of ideally grown AAO membranes in oxalic acid with 40 nm pore diameter after 6 h first step anodization and (a) 45 min and (b) 6 h in second step anodization. M. Ghorbani et al. / Materials and Design 27 (2006) 983–988 987 [13] Shimizu K, Kobayashi K, Skeldon P, Thompson GE, Wood GC. An atomic force microscopy study of the corrosion and filming behaviour of aluminium. Corros Sci 1997;39:701–18. [14] Li F, Zhang L, Metzger RM. On the growth of highly ordered pores in anodized aluminum oxide. Chem Mater 1998;10: 2470–80. [15] Sui YC, Cui BZ, Martmez L, Perez R, Sellmyer DJ. Pore structure, barrier layer topography and matrix alumina struc- ture of porous anodic alumina film. Thin Solid Films 2002;406: 64–9. [16] Xu T, Zangari G, Metzger RM. Periodic holes with 10 nm diameter produced by grazing Ar + milling of the barrier layer in hexagonally ordered nanoporous alumina. Nano Lett 2002;2(1):137–41. [17] Paredes JI, Martinez-Alonso A, Tascon JMD. Application of scanning tunneling and atomic force microscopies to the charac- terization of microporous and mesoporous materials. Micropor Mesopor Mat 2003;65:93–126. [18] Sui YC, Saniger JM. Characterization of anodic porous alumina by AFM. Mater Lett 2001;48:127–36. 988 M. Ghorbani et al. / Materials and Design 27 (2006) 983–988 . surface. Anodizing the sample for the second time develops the pore growth exactly on the concave pattern created during the first step anodization. As the duration of the. samples. The first and second anodization steps were conducted in the same condition as mentioned above. Meanwhile, the oxide layer formed in the first step