Use of Ionic Liquid in Fabrication, Characterization, and Processing of Anodic Porous Alumina
© to the authors 2009
Received: 19 February 2009
Accepted: 24 April 2009
Published: 8 May 2009
Two different ionic liquids have been tested in the electrochemical fabrication of anodic porous alumina in an aqueous solution of oxalic acid. It was found that during galvanostatic anodization of the aluminum at a current density of 200 mA/cm2, addition of 0.5% relative volume concentration of 1-butyl-3-methylimidazolium tetrafluoborate resulted in a three-fold increase of the growth rate, as compared to the bare acidic solution with the same acid concentration. This ionic liquid was also used successfully for an assessment of the wettability of the outer surface of the alumina, by means of liquid contact angle measurements. The results have been discussed and interpreted with the aid of atomic force microscopy. The observed wetting property allowed to use the ionic liquid for protection of the pores during a test removal of the oxide barrier layer.
KeywordsPorous alumina Anodization Galvanostatic Ionic liquids Wettability Roughness
Anodic porous alumina (APA [1–4]), also called porous anodic alumina (PAA) [5–9]), anodic aluminum oxide (AAO [10, 11]), or alumite [12, 13], is a form of Al2O3, which is deposited onto an aluminum (Al) foil working as the positively biased pole of an electrolytic cell [14–16]. Whereas in basic or neutral electrolytes (ELs) a compact alumina layer is grown, called “barrier” layer, in acidic ELs that can dissolve the oxide a porous alumina layer is grown, on top of a thin (10–100 nm ) barrier-type layer. Depending on the applications, pore ordering in APA can be necessary either on both sides such as for photonic crystals made by using APA as a lithographic mask , or only on one side such as for in situ photonic crystals made by incorporating materials on one APA surface [19–21], or on none of the two sides such as for filtering membranes [22–24], biosensor electrodes with enhanced surface area , and templates for the growth of separated metal nanowires [13, 25, 26], or supported oxide or polymer nanotubes [27, 28]. However, for several applications, it is desirable that the APA thickness h is comparatively high, h ≥ 100 μm. This can give the film the required robustness for use as either a standalone membrane in case of, e.g., battery separator  or lithographic etching mask [18, 29], or the required high aspect ratio (a.r. = h/d, where d is the pore diameter) when using the layer as a template for, e.g., nanowire electrodeposition (where a.r. ≥ 1,000 can be required [21, 30]). However, the film growth rate vg reported so far for conventional mild anodization (MA) of Al has been in the order of 0.03–0.1 μm/min , which makes the growth of thick APA quite time consuming. This slow growth hinders the advantage of the relatively cheap setup for the fabrication of APA, discouraging its use for both the development of prototype structures in the research academy, and for possible industrial fabrication processes. The current interest in speeding up the APA growth has recently found a viable way in the application of a single-step process combining MA with industrial hard anodization (HA) conditions , made possible by the protecting oxide layer grown during the preliminary MA phase.
Another possible route for fast APA fabrication could be the identification of proper additives, which can conveniently change the environmental conditions for anodization. Ionic liquids (ILs) are a class of solvents that have recently attracted a renewed interest as chemical additives in a number of reactions [32, 33]. To our knowledge, the only use of APA and IL system has been so far as an additive in the fabrication of cobalt nanowires inside an APA template . In this work, we report on the use of ILs in the fabrication and characterization of APA, starting from a well-known APA fabrication EL such as oxalic acid. The idea behind using IL additives in this system is that the convective flow of the IL component species can facilita