Enhanced stabilisation of tetragonal (t)-ZrO2 in the controlled nanotubular geometry†
Precise control of the structure of nanogranular materials over different polymorphs is directly related to the manifestation of the desired and resultant properties. The room temperature phase stabilisation of nanocrystalline tetragonal (t)-ZrO2 has been a controversial topic in the literature. Here, we report that the stabilisation of t-ZrO2 is enhanced with the tubular geometry at the nanoscale and that it can be manipulated by carefully selecting the initial structures as-grown. ZrO2 nanotubes (ZNTs) produced via template-directed atomic layer deposition (ALD) techniques were in the growth temperature range of 150 through 250 °C, followed by thermal treatment up to 700 °C. The resulting phases of the ZNTs (i.e., tetragonal and/or monoclinic ZrO2) were strongly affected by the interplay between the original deposition and post-annealing temperatures. We further elucidated that both the initially grown phases and the types of crystalline nuclei in the as-deposited nanotubes determine the final microstructures. This observation gives us an unambiguous clue to understanding the controversial results on the phase stabilisation of nanocrystalline t-ZrO2. Calculation results also support that the confinement within the thin wall layers is pronounced during nucleation and growth, which results in the enhanced stabilisation of t-ZrO2. The present results provide us with an insight to the stabilisation of nanocrystalline t-ZrO2 and with a strategy on how to tailor the structures of ZrO2 nanostructures in fine-tuning material properties.