A Comparative Study of Anodic Aluminum Oxide Preparation for Synthesis of Nanomaterials

Abstract

A template-based synthesis of nanomaterials is one of the most simple and low-cost fabricating way. The template-assisted method, which employs porous anodic aluminum oxide (AAO) templates, allows for a precise control of the morphology and dimensions of deposited nanowires [1, 2]. Nanoporous Al2O3 can be prepared by a two-step anodization of aluminum in acidic electrolytes. During the anodization, Al foil [2] or Al film sputtered on conducting substrates [3] is subjected to anodic oxidation. The anodization of Al foil results in the Al2O3/Al substrate, which is not suitable to carry out a direct electrodeposition into the pores, due to the presence of the non-conductive oxide barrier layer at the oxide/metal interface. To enable a direct electrodeposition in the Al2O3/Al substrate, the barrier layer at the pore bottom has to be removed. There are two basic methods used for the removal of the barrier layer: wet chemical etching [1, 2] and electrochemical [4–8]. The first one, the wet chemical etching requires additional steps, which include: dissolving the aluminum layer remaining after anodization of the Al foil, pore opening and widening in a phosphoric acid solution to obtain a through-hole membrane, and the sputter deposition of the conductive metal layer that serves as a working electrode during electrodeposition. Drawbacks of this method are: the use of harmful chemicals (HgCl2, CuCl2) [2] or concentrated acid solutions (H3PO4, HCl) [1, 2], and multi-stage process (loss of the material at each stage). The alternative method is based on the electrochemical reduction of the barrier layer. Due to the fact that the thickness of the barrier layer depends on the anodizing potential, a gradual decrease in the applied potential leads to a significant reduction of the barrier layer thickness. As a result of this treatment we obtain AAO, with dendritic channels with a very thin barrier layer at the pore bottoms. Such a structure allows to conduct the direct electrodeposition on the aluminum substrate without any additional steps. An important advantage of this method is the fact that the potential reduction is carried out in the same electrolyte in which the anodization was realized. Therefore, the potential reduction can be performed immediately after the anodizing process. On the other hand, there is a new electrochemical method developed by Lira and Paterson [7] for AAO detachment from the aluminum substrate. It was found that a large voltage pulse applied to the sample results in simultaneous membrane detachment and pore opening. The voltage detachment method has significant advantages over the typically used chemical etching, e.g., it is faster, and environmental friendly. As-obtained AAO membranes are suitable for electrodeposition of metallic nanostructures. Herein, we report on investigation of electrochemical methods for the AAO membrane preparation and their optimization toward further metallic nanostructure deposition. Highly ordered nanoporous AAO templates were fabricated by a two-step self-organized anodization of aluminum carried out in oxalic and sulfuric acids solutions. In the first method, the barrier layer thinning was done by applying an exponentially decreasing voltage at the end of anodization process. Depending on the function used for the reduction of the barrier layer and its thickness, the dendrite-like structures of the barrier layer were observed at the cross-sections. In the second method, a modified voltage detachment method in a mixture of HClO4 and ethanol was used. Different pulses were applied for the detachment of alumina formed in oxalic and sulfuric acids.