Atomic Layer Deposition for Coating of High Aspect Ratio TiO₂ Nanotube Layers

We present an optimized approach for the deposition of Al₂O₃ (as a model secondary material) coating into high aspect ratio (≈180) anodic TiO₂ nanotube layers using the atomic layer deposition (ALD) process. In order to study the influence of the diffusion of the Al₂O₃ precursors on the resulting coating thickness, ALD processes with different exposure times (i.e., 0.5, 2, 5, and 10 s) of the trimethylaluminum (TMA) precursor were performed. Uniform coating of the nanotube interiors was achieved with longer exposure times (5 and 10 s), as verified by detailed scanning electron microscopy analysis. Quartz crystal microbalance measurements were used to monitor the deposition process and its particular features due to the tube diameter gradient. Finally, theoretical calculations were performed to calculate the minimum precursor exposure time to attain uniform coating. Theoretical values on the diffusion regime matched with the experimental results and helped to obtain valuable information for further optimization of ALD coating processes. The presented approach provides a straightforward solution toward the development of many novel devices, based on a high surface area interface between TiO₂ nanotubes and a secondary material (such as Al₂O₃).