Atomic Layer Deposition Al₂O₃ Coatings Significantly Improve Thermal, Chemical, and Mechanical Stability of Anodic...

We report on a very significant enhancement of the thermal, chemical, and mechanical stability of self-organized TiO₂ nanotubes layers, provided by thin Al₂O₃ coatings of different thicknesses prepared by atomic layer deposition (ALD). TiO₂ nanotube layers coated with Al₂O₃ 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 Al₂O₃ coatings guarantee unprecedented chemical stability of TiO₂ nanotube layers in harsh environments of concentrated H₃PO₄ solutions.