Porous aluminum oxide membranes with a complete and even covering of alumina nanowires were formed in a one-step anodization process in dilute phosphoric acid electrolyte. The anodizing conditions can be adjusted to start forming alumina wires that originate on the surface of the porous alumina layer at the triple junction points (the edges of the hexagonal inter-pore structure where three pores meet). The wires tangle together as they become longer; eventually creating a tangled mesh layer above the porous oxide layer. SEM micrographs of the oxide cross section show tapered wires that are approximately 2 to 10 m long, depending on anodizing time, and range in width from a few nanometers to 50nm. The aluminum substrate can be chemically removed and the alumina barrier layer dissolved to leave a free standing porous alumina membrane with very high surface area alumina wires on one face. Some possible future applications of this high surface area structure involve filtration of liquids and gasses, combined with chemical functionalization on the large surface area.
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McGrath, R.S., Misra, M., Sklar, G.P. et al. The synthesis of alumina nanowires on the surface of a porous alumina membrane. MRS Online Proceedings Library 876, 13 (2005). https://doi.org/10.1557/PROC-876-R1.3