Self-Organized TiO2 Nanotube Arrays: Towards Improved Ordering

Abstract

Synthesis of highly-ordered nanostructures of valve metal oxides has recently attracted huge scientific and technological interest motivated by their possible use in many applications. The nanoporous Al2O3 – most established member of this group of materials – has been prepared by anodic oxidation of Al under suitable electrochemical conditions two decades ago into perfectly ordered, honeycomb-like porous structures (1). Owing to the flexibility of the pore diameter/length and the relative ease of the Al2O3 dissolution, its porous membranes have been since than widely used as template material of the choice for a range of materials (2-4). It is the TiO2 that has received the highest attention after Al2O3 motivated by its range of applications, including photocatalysis, water splitting, solar cells and biomedical uses. Very significant research efforts have led to reproducible synthesis of self-organized TiO2 nanotube layers by means of anodic oxidation, during which the starting Ti substrate is converted into highly-ordered nanotubular layer by anodization in suitable electrolytes (5-7). Although advancements in the anodic synthesis of self-organized TiO2 nanotube layers have been presented over past years (8), the degree of ordering has not reached so far the level known from porous alumina (1). Numerous factors influence the ordering and the homogeneity of the TiO2nanotube layers. In the presentation, we aim to demonstrate significant advancements in the ordering of anodic TiO2nanotubes compared to known state-of-art. In particular, we will show recent results on the extremely ordered nanotube arrays. In addition, utilizing AFM, SEM and optical measurements, we will show how to obtain via tailored anodization protocols a very decent degree of uniformity and homogeneity of the nanotube layers. Moreover, based on SEM, EBSD and TEM measurements, we will demonstrate how the Ti grain structure influences the lateral uniformity of the nanotube layers (9-11).