Unique AAO films with adjustable hierarchical microstructures

Abstract

Anodic aluminium oxides (AAO) films with adjustable hierarchical microstructures were synthesized via a simple unsteady state anodization process. Moreover, novel nano-/submicron-/micron-porous AAO films can be obtained by further film removal and low voltage anodization processes.

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Aluminium anodization processes and anodic aluminium oxides (AAO) have been investigated and used in numerous fields for about a century.^1–4 Among them, porous AAO films which contain a large quantity of nanochannels have attracted extensive attention.² However, the nanochannels are disordered, limiting their practical applications to a certain extent. To solve this problem, a pre-patterning process (i.e., the first anodization) has been applied for the formation of highly ordered pits on the aluminium surface, and then ideally ordered porous AAO films can be obtained by the second anodization.³ At present, diverse functional nanostructures have been synthesized by using porous AAO films as preferred templates.^3c,5 In addition, they can also be applied as functional materials for many applications, e.g., spintronics, biotechnology, MEMS.⁶

Although porous AAO films can be used in many fields, some drawbacks still exist: their geometrical structures are relatively simple (i.e., honeycomb structure with a round straight channel in each cell), and can merely get nanoscale structural parameters, which greatly restrict their use in template synthesis of various scale functional materials.^3,4 Considering that great interest has arisen in the development of hierarchical materials in these decades,⁷ finding proper methods for the fabrication of porous AAO films with complex hierarchical nano-/submicron-/micron-scale structures is significant. To solve the problem, many pre-patterning processes have been applied to form diverse submicron- or micron-scale patterns on the aluminium surface before the anodization, including chemical etching process, focused ion beam (FIB) method, stamp-assisted fabrication, etc.⁸ However, these methods are either difficult to control the patterns or expensive and unavailable to the researchers. Considering that the first anodization and the following AAO removing process is reliable, readily available and low cost, developing proper pre-patterning methods based on the anodization process to form diverse submicron-/micron-scale patterns on aluminium surface is significant from the viewpoint of both scientific research and commercial applications.

In the present work, a simple, effective, and inexpensive method has been realized for the fabrication of complex AAO films with diverse submicron-/micron-scale structures at the barrier layer, based on the unsteady state anodization process and oxalic acid-based electrolyte system. Close-packed, interlinked or separated clusters of the structural cells can be alternatively formed by different competitive growth modes. Furthermore, novel nano-/submicron-/micron-porous AAO films have also been fabricated, which opens a door for the simple template synthesis of complex hierarchical functional materials.

In former research, most porous AAO films were fabricated by steady state anodization processes: the growth of all the structural cells is almost synchronous and regional ordering of the cells (i.e., similar to the honeycomb structure) can be formed by self-organized mechanism.^3,4 To obtain different microstructures, an unsteady state anodization process has been applied.⁹ Previous studies showed that the process can be induced by high anodization voltage (U_a), and a maximum U_a of 400 V has been realized in 0.3 M oxalic acid electrolyte.^9a However, it is difficult to apply a U_a of over 400 V under current anodization conditions. It has been known that the anodization voltage and current density (i_a) can influence the anodization process and the structural parameters of the AAO films to a large extent. Therefore, the setting i_a during the constant current voltage increasing process has been increased to 600 A m⁻², and the AAO film has been successfully fabricated under 500 V (0.3 M oxalic acid electrolyte) with a final i_a of 180 A m⁻² (Fig. S1†). Fig. 1 shows the barrier layer side of the as prepared AAO film. It can be seen that the shape of the structural cells is irregular, which is quite different from those obtained by steady state anodization processes.^3,4 In addition, the barrier layer surface is rough, and ordered regions can not be observed. There are many micron-scale irregular banded structures distributes on the barrier layer surface, which are made up of submicron-scale structural cells (the lower left insert in Fig. 1), SEM images with more details can also been seen in Fig. S2.† Considering that the composition of the electrolyte can have a great impact on the anodization process and the as-prepared AAO films, an equal volume of ethanol has been added to the 0.3 M oxalic acid electrolyte for a comparative investigation. Measurement results show that the anodization time is shorter (∼40 min in Fig. S3†) than that (∼66 min) shown in Fig. S1,† which means that the adding of ethanol is useful for obtaining a higher U_a under the same anodization conditions.^4g Fig. 2 shows the barrier layer of the as prepared AAO film which has a quite different structure with that shown in Fig. 1. There are many micron-scale island structure clusters distribute on the barrier layer surface. The clusters are made up of submicron-scale structural cells, and there are many relatively smooth regions around the clusters (the inserts in Fig. 2), SEM images with more details can also been seen in Fig. S4.† It is interesting that AAO films with three different types of submicron-/micron-scale structures, i.e., close-packed (Fig. S5†), interlinked (Fig. 1 and S2†) and separated (Fig. 2 and S4†) clusters of the structural cells can be alternatively formed by controlling the anodization conditions. Based on the above results and our former investigations, here we develop a model to illustrate the growth process of the as-prepared AAO films.

Fig. 1 SEM images of the AAO film (voltage increasing rate: 1 V s⁻¹, setting current density: 600 A m⁻², 0.3 M oxalic acid electrolyte), the lower left and upper right inserts are enlarged images of the barrier layer surface and the cross-section, respectively, scale bars = 10 μm (main image) and 2 μm (inserts).

Fig. 2 SEM images of the AAO film (voltage increasing rate: 1 V s⁻¹, setting current density: 600 A m⁻², 0.3 M oxalic acid electrolyte–ethanol = 1 : 1 v/v), the lower left and upper right inserts are enlarged images of the barrier layer surface and the cross-section, respectively, scale bars = 10 μm (main image) and 2 μm (inserts).

Fig. 3 shows the structural model of the as-prepared AAO films. It has been known that the competitive growth mode can be induced by a high enough U_a, thus resulting in the occurring of unsteady state anodization processes.^9a In this case, many competitive growth starting sites (CGSS) can be formed in the AAO film (the arrows in Fig. 3(a)), and close-packed clusters with hemispherical shape can thus be obtained based on the CGSS, corresponding to the AAO film fabricated under 400 V in 0.3 M oxalic acid electrolyte (Fig. S5† and 3(a)).^9a When the U_a is increased to 500 V, the competitive growth process can be further enhanced in the same electrolyte. Under this condition, more CGSS can be formed, which means that there is a shorter average distance between adjacent CGSS (Fig. 3(b)). As a result, the clusters closely adjacent with each other, thus forming interlinked clusters of the structural cells at the barrier layer surface (Fig. 1, S2† and 3(b)). In previous studies, it has been found that the high voltage anodization processes can be more stable by modifying the electrolyte with ethanol.^3c,10 Therefore, the competitive growth process will be inhibited in the modified electrolyte (0.3 M oxalic acid electrolyte–ethanol = 1 : 1 v/v), and less CGSS can be formed, thus resulting in a relatively farther average distance between adjacent CGSS (Fig. 3(c)), and forming separated clusters at the barrier layer surface (Fig. 2, S4† and 3(c)).

Fig. 3 The structural model of AAO films with three different types of submicron-/micron-scale structures, (a) close-packed clusters, (b) interlinked clusters, (c) separated clusters, the arrows represent the competitive growth starting sites (CGSS).

In order to obtain hierarchical porous AAO films, the anodized aluminium sheets were immersed in a mixture solution of CrO₃ (30 g L⁻¹) and H₃PO₄ (50 mL L⁻¹) at ∼70 °C for 10 h to remove the as-prepared AAO films, and pre-patterned aluminium substrates can be obtained with unique submicron-/micron-scale pits on their surface (Fig. S6† and the grey parts shown in Fig. 3). By using a low voltage anodization (40 V in 0.3 M oxalic acid electrolyte for 8 min) and a subsequent pore widening process, a hierarchical porous AAO film with novel nano-/submicron-/micron-scale structures can be fabricated (Fig. 4). It can be seen that the micron-scale pit is made up of many submicron scale pits (Fig. 4(a)–(c)), and there are lots of nanopores embedded in them (Fig. 4(b)–(e)), which agrees well with the structure at the barrier layer surface (Fig. 4(f)).

Fig. 4 SEM images and schematic diagram of the hierarchical porous AAO film with novel nano-/submicron-/micron-scale structures, the first anodization conditions correspond to the sample shown in Fig. 2: (a) the porous surface; (b) the corresponding schematic diagram; (c) the enlarged image of the white rectangular region in (a); (d) the enlarged image of the white rectangular region in (c); (e) the tilting porous surface; (f) the barrier layer surface, scale bars = 4 μm (a), 1 μm (c) and 200 nm (d)–(f).

In summary, we have fabricated AAO films with diverse submicron-/micron-scale structures at the barrier layer or with novel nano-/submicron-/micron-porous structures, by using a simple anodization method. The structure of the as-prepared AAO film can be regulated by varying the anodization conditions. The unique porous AAO films can be used as templates for the synthesis of various complex hierarchical functional materials, which may have diverse potential use in many related fields, such as bionics, catalysis, sensors, etc.

Acknowledgements

This work was supported by National Natural Science Foundation of China (51202071), Specialized Research Fund for the Doctoral Program of Higher Education (20120172120009), Guangdong Natural Science Foundation (S2011040005425), Fundamental Research Funds for the Central Universities (2012ZZ0004), and SRP of South China University of Technology.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Experimental details, diagrams and SEM images of other samples. See DOI: 10.1039/c4ra13076f

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