Facile Fabrication of Ordered Multi-Tiered Hierarchical Porous Alumina Nanostructures with Multiple and Fractional Ratios of Pore Interval toward Multifunctional Nanomaterials

Abstract

Conventional nanoporous anodic alumina films are composed of one-tiered pore arrays that can only render to one-dimensional (1D) nano-structures if they are used as nano-templates. Hence, three-dimensional (3D) nanoporous alumina films are desirable to dramatically increase the variety of nanostructures by template-fabrication for various potentially enhanced performances. Herein we report a facile approach to fabricate a variety of ordered multi-tiered 3D porous alumina nanostructures with multiple integer (m = 2, 3, 4, 5, 7, 9, and 10) and fractional ratios (n = 1/2 and 1/3) of the pore interval and pore diameter among different tiers on ITO/glass and Al sheets. The multi-tiered nanostructures and the pore interval ratios of the neighboring tiers can be accurately and independently controlled by a tailored multiple stepwise anodization, successively in appropriate acidic electrolytes of sulfuric, oxalic, and/or phosphoric solutions at designed voltages of 15-180 V, corresponding to the multiple or fractional ratios of pore interspacing. Moreover, the multi-tiered hierarchical nanoporous alumina films on ITO/glass substrates exhibited a lower transmittance compared with that of the single-tiered films with ordered cylindrical pores, which can be attributed to the diffusive reflection from the tier interface with different pore intervals. In past decades, porous anodic alumina (PAA) films have attracted considerable attention in both scientific and technical applications owing to the unique geometric structure with ordered hexagonal arrays of cylindrical nano-pores, similar to a nano-scaled honeycomb.1-4 In recent years, PAA nanostructures have been extensively used as a core nanotemplate to fabricate various one-dimensional (1D) functional nanostructures, such as nanowires 5-9 and nanotubes, 10-14 which can be used in a wide variety of applications including DNA and gas sensors, electro-chromic devices, light-emitting diodes, field emitters, super-capacitors, nano-electronic devices, and nano-generators. In order to facilitate the functional integration of nanostructures, template synthesis is evolving towards the fabrication of nanomaterials with increasing geometric complexity, i.e., going from two-dimensional (2D) to three-dimensional (3D) nanostructures. However, a major challenge in obtaining 3D nanostructures is the prior fabrication of ordered 3D nanoporous template with independently controllable size, spacing, position, and shape of the pore arrays as required for various applications.Many studies so far have attempted to fabricate various porous alumina nanostructures with variable pore diameters using tailored anodization methods assisted by chemical dissolution at different operating temperatures. Lee at al. reported first on the fabrication of ordered PAA films with variable pore diameters from 40 to 59 nm by combining mild anodization (MA) with hard anodization (HA) in oxalic acidic electrolytes. 15 This approach was further developed to achieve 3D PAA films with shaped pore geometries in periodic...

Recent Advances in Nanoporous Anodic Alumina: Principles, Engineering, and Applications

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Nanoporous anodic alumina-based iontronics: fundamentals and applications

“…69 Another strategy to generate NAA with asymmetrical nanopore architectures such as hierarchical (Fig. 4a), [70][71][72] funnel-like (Fig. 4b), [73][74][75][76][77][78] and multi-layered (Fig.…”

High-speed galvanostatic anodizing without oxide burning using a nanodimpled aluminum surface for nanoporous alumina fabrication

“…Electrochemical anodizing of aluminum in suitable acidic solutions produces a porous alumina coating that possesses a characteristic self-organized hexagonal nanostructure [1,2]. Various unique characteristics, such as an improved corrosion resistance [3][4][5][6][7][8], wear resistance [9], Vickers hardness [10][11][12], coloring [13][14][15][16], and self-ordering of a nanoporous structure [17][18][19][20][21][22][23], can be produced on the aluminum surface by anodizing. Galvanostatic anodizing is usually employed for industrial processes due to easy handling of the electric current and production of a porous alumina film, and a porous alumina film can be rapidly formed by anodizing at high current density conditions.…”