Three-dimensional nanostructures from porous anodic alumina
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esearch Letters
Three-dimensional nanostructures from porous anodic alumina Maria R. Lukatskaya* and Yury Gogotsi*, Department of Materials Science and Engineering, A.J. Drexel Nanotechnology Institute, Drexel University, Philadelphia, Pennsylvania 19104 *Address all correspondence to Maria R. Lukatskaya and Yury Gogotsi at [email protected] and [email protected] (Received 31 January 2012; accepted 16 April 2012)
Abstract We report on the synthesis of unique tetragonal nanostructures from porous anodic aluminum oxide that enables geometrical threedimensional (3D) microdesign of the porous surfaces. Scanning electron microscopy study revealed self-organization of hollow oxide nanostructures into hierarchical arrays. The formation of these structures has a localized nature and is associated with local electrical break-down during hard anodization of aluminum in oxalic acid solution. Moreover, in this communication we propose the mechanism of the nucleation and growth of complex 3D structures from porous anodic alumina.
Anodization of aluminum for the protection and decoration of the metal surfaces has been widely used in industry since the beginning of the 20th century. The first patent on the porous anodic alumina was filed in 1898.[1] Various theories[2,3] attempted to explain the formation of porous structure and effort in this direction still continues.[4,5] In 1995, ordered anodic aluminum oxide (AAO) discovered by H. Masuda and K. Fukuda attracted the attention of the research community, owing to the possibility of obtaining self-organized porous films using two-step replication anodization.[6] This discovery led to a break-through in the synthesis of ordered structures with a very narrow pore size distribution. AAO membranes, owing to their straight pores with a controllable diameter, length and inter-pore distance, are widely used for template synthesis of nanowires,[7–9] nanorods[10,11] and nanotubes[12–14] with a customizable diameter and geometry[15]. Applications of 1D nanostructures have been reviewed in Refs.16–18. A number of articles have been published detailing the synthesis of porous films with either an ultra-small[19]
channel diameter or submicrometer pores, along with advanced nanochannel designs such as branches or periodical oscillations.[20–24] However, the architecture of porous AAO has so far been limited to the flat porous films. In this paper, we report the observation of complex three-dimensional (3D) structures from porous anodic alumina and suggest a mechanism for their formation. Aluminum foils (99.99%, 0.25 mm-thick, annealed, Alfa Aesar, Ward Hill, Massachusetts, USA) of 1 cm2 area were used as a starting material. Prior to anodization, foils were treated by 1 M NaOH solution for 15 min in order to remove a thin oxide layer from the surface. Anodization of aluminum was carried out for 1–3 h in a two-electrode cell in 250 mL of 0.3 M (COOH)2 + 3 M C2H5OH solution using a graphite rod as a counter electrode. Voltage was gradually increased at a rate of 1 V/s to the target voltag
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