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Electrochemical Synthesis and Structural Characterization of Titania Nanotubes

Abstract

Titania nanotubes have emerged as an exciting new material with a wide array of applications such as sensors, dye sensitized solar cells, and batteries due to their semi-conducting nature, high surface area, and distinct morphology. The nanotubes, synthesized electrochemically in a fluoride-containing electrolyte, are vertically aligned, close-packed, organized structures, with similar diameter and length. The formation mechanism responsible for the organized nanopore/nanotube arrays were examined by studying the effects of processing parameters (anodization voltage, synthesis time, electrolyte composition, substrate surface conditions, etc..) on the growth and structure of electrochemically synthesized titania. Characterization of the nanotube's crystal structure, morphology, and oxide composition were performed via cross-sectional and high-resolution transmission electron microscopy (TEM), micro x-ray diffraction (XRD), energy dispersive spectroscopy (EDS), and electron energy loss spectroscopy (EELS). Experimental results from the synthesis and characterization efforts lead to a novel planar-interface-breakdown model to describe the initiation of organized arrays of nanopores and nanotubes formed via anodization of titanium. It is proposed that the initiation step is triggered by compositional changes in the oxide and electrolyte, near the interface region, that break down the planar surface. In the electrolyte, the compositional changes are enhanced by ionic species, such as fluoride that form complexes with metal cations. In the oxide, the compositional gradient results from depletion of metal cations near the oxide/electrolyte interface. The proposed mechanism indicates that, in addition to the compositional gradient, the initiation of nanopores is controlled by the potential gradient in the oxide as well as the oxides dissolution rate. The initiation step is crucial not only for the growth processes that proceed during anodization, but also for the organization of the pores that result from synthesis. This mechanism, although formulated for the case of anodization of Ti, may be extended to other porous anodic oxide systems.

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