Synthesis and Characterization of Controllably Branched Anodized Aluminum Oxide Pores

Well-ordered anodized aluminum oxide (AAO) is a widely known porous material which has been used in a variety of applications. One of the advantages of AAO is its tunable pore diameter and length, which are controlled by the anodizing conditions.  Additionally, more complex structures can be produced by modifying the anodization and etching conditions. Here we present a technique for introducing branch points into the pores which allows for controllable branching in sections of the pores while maintaining straight, hexagonally arrayed, regular structures throughout the rest of the alumina membrane. 

We also demonstrate that this method can be integrated with a strategy for barrier layer thinning and removal which allows for the use of the underlying aluminum as a current collector. This allows these 3-D networks to be used as electrodes, without having to withstand the normal processing route, which requires wet etches to remove the aluminum and then remove the barrier layer, followed by vacuum deposition of a new current collector. This process is difficult to carry out for very thin membranes, since AAO is brittle and prone to cracking and shattering. This new combination of techniques allows the production and electrochemical characterization of very thin (>3um) porous networks, ranging from well-ordered 1D channels to highly branched 3D networks. 

First, we demonstrate electrodeposition into these pores, to confirm the opening of the barrier layer, and to demonstrate that the branched pores are accessible to electrolyte. Then, a variety of networks of different thicknesses and numbers of branch points are characterized by electrochemical impedance spectroscopy. Since this method allows branch points to be added at various intervals along the pores, networks with different numbers of branch points as well as with different spacing between the branch points can be analyzed as well. This study is focused on understanding the movement of electroactive species through porous networks of increasing complexity, as well as demonstrating a new potential template for electrodeposition of 3D electrodes.