Highly self-assembled nanotubular aluminum oxide by hard anodization
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Se-Yeon Jung and Tae-Yeon Seong Department of Materials Science and Engineering, Korea University, Seoul, Korea
Sungho Jina) Materials Science and Engineering, University of California, San Diego, La Jolla, California 92093 (Received 8 June 2010; accepted 17 September 2010)
Anodized aluminum oxide (AAO), well-known hexagonally ordered vertical pore nanostructure, can be altered to form nanotubular AAO arrays potentially having many favorable properties due to its large surface area and unique geometry. We present here a creation of novel nanotubular AAO structure by the hard anodization technique. Because of the guided formation of void channels at triple cell junctions during the course of the controlled anodization process, periodically spaced-apart aluminum oxide nanotubular geometry could be achieved over large areas. Further separation to well-defined individual AAO nanotube arrays was obtained when etched in a mixed CuCl2/HCl solution during Al substrate removal. Nanotubular geometry AAO with periodic and mechanically robust structure can be useful not only for biomedical applications such as to enhance cell adhesion and viability or drug delivery vehicles, but also as a large-surface-area catalyst support or sensor elements.
I. INTRODUCTION
Since the discovery of self-ordered porous alumina in 1995,1 anodized aluminum oxide (AAO) has become one of the most popular periodic templates to create lowdimensional materials such as nanodots, nanowires, and nanotubes for various nanotechnology applications.2,3 More recently, some efforts have been made to use AAO as a material for implant surface coatings on Ti-based substrates for enhanced bone in growth,4 stent coating,5 and drug delivery6 because of its pored structures together with the biocompatible properties of aluminum oxide. It is well known that ordered porous AAO can be obtained in broad processing windows. Masuda and coworkers7 reported ordered porous AAO in several selfordering regimes, i.e., 25 V in sulfuric acid, 40 V in oxalic acid,1 and 195 V in phosphoric acid.8 In addition, Lee and co-workers9–13 recently reported a new self-ordering regime: 110~140 V in malonic acid and 110~150 V in oxalic acid, which drew renewed attention to a hard anodization (HA) process associated with high anodic currents and formation of unique nanoarray structures. In addition to regular porous AAO, various novel AAO nanostructures were reported in the literature such as modular,10 serrated,14 perforated,15 pore size-oscillated,16 a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2010.6 186
J. Mater. Res., Vol. 26, No. 2, Jan 28, 2011
http://journals.cambridge.org
Downloaded: 02 Jun 2014
and bimodal17 geometries by using modified anodizing parameters. HA techniques with high current densities involved, have several key advantages over conventional mild anodization (MA), such as an ultrafast growth rate and improved controllability, and hence can provide highly ordered AAO structures that could not be obtained previously using MA te
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