Atomic Layer Deposition AlO Coatings Significantly Improve Thermal, Chemical, and Mechanical Stability of Anodic TiO Nanotube Layers.

We report on a very significant enhancement of the thermal, chemical, and mechanical stability of self-organized TiO nanotubes layers, provided by thin AlO coatings of different thicknesses prepared by atomic layer deposition (ALD). TiO nanotube layers coated with AlO coatings exhibit significantly improved thermal stability as illustrated by the preservation of the nanotubular structure upon annealing treatment at high temperatures (870 °C). In addition, a high anatase content is preserved in the nanotube layers against expectation of the total rutile conversion at such a high temperature. Hardness of the resulting nanotube layers is investigated by nanoindentation measurements and shows strongly improved values compared to uncoated counterparts. Finally, it is demonstrated that AlO coatings guarantee unprecedented chemical stability of TiO nanotube layers in harsh environments of concentrated HPO solutions.