Flexible self-organization of two size-scales oxide nanotubes on Ti45Nb alloy

We report on the growth of anodic oxide nanotubes grown on a simple Ti45Nb alloy that show self-organization on two size-scales. The well-ordered nanotube layers consist of large and small diameter tubes that are arranged in an alternating two-dimensional (2D) structure. This bimodal morphology and the thickness of the nanotube layer are affected by the electrochemical conditions used. The bimodal feature can be obtained over a wide potential range (between 15 and 55 V), resulting in a linear diameter variation from 80 to 330 nm for the larger and 50–180 nm for the smaller tubes. The organization into two size-scales takes place during the first hour of the anodization process. This result of flexible two size-scale self-organization using a simple two element alloy is of significant importance not only for fundamental research but also for specific applications that can benefit from bimodal properties.