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The synthesis of alumina nanowires on the surface of a porous alumina membrane R.S. McGrath, M. Misra, G.P. Sklar, and J.C. LaCombe Materials Science and Engineering, University of Nevada Reno, NV 89557-0136, U.S.A. ABSTRACT 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 10µm 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.

INTRODUCTION Much interest is being given to the use of anodized aluminum oxide (AAO) membranes for filtration purposes, as a possible substitute for current synthetic filtration techniques that mostly rely on nuclear tract etching of polymers to create a torturous path with uneven pore distribution [1]. AAO membranes can be grown in even matrices of parallel pores, all having similar pore diameters that are controlled by the electrolyte and anodizing voltage [3]. There has been much study done on the formation of these pores in the three most common electrolytes: sulfuric acid, oxalic acid, and phosphoric acid [3, 8-12]. Phosphoric acid as an electrolyte provides the self-ordering formation of relatively large pores (150-300nm diameter) compared to other more commonly studied electrolytes such as sulfuric or oxalic acids (15-50nm diameter). The concern with phosphoric acid is that the high voltages used result in a sensitive balance of anodizing conditions to achieve a well ordered structure without etching. The common remedy is to anodize at 0-3˚C [2], slowing down the reaction rate, and permitting maximum voltages (i.e. largest pores). As grown, these pores have a non porous barrier layer at the interface between the oxide and metal, attaching the oxide to the metal surface. Both the barrier layer and aluminum substrate are removed to make a through porous filtration membrane. Figure 1 shows the three layers of the oxide system: pores, barrier layer, and metal substrate. Three possible processes for the separation of the porous oxide which appear in the literature are: a voltage reduction sequence, chemical removal, and subsequent anodizing in a solution of 2,3-butadione. Voltage reduction is performed by a slow,

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