In this work, strong beneficial effects of thin and uniform TiO2 coatings within TiO2 nanotube layers for photocurrent generation and photocatalytical degradation of methylene blue are demonstrated for the first time. TiO2 nanotube layers were coated by TiO2 of various thicknesses (from 2.8 nm to 22 nm) using atomic layer deposition (ALD) and compared with TiO2 nanotube layers decorated by TiO2 nanoparticles (using established TiCl4 treatment) and with blank (uncoated) layers. By means of photocurrent measurements and cyclic voltammetry, it is demonstrated that the most efficient charge carrier separation can be achieved for TiO2 nanotube layers with an optimal ALD TiO2 coating thickness ≈11 nm. Significant differences in flatband potentials and carrier density among all nanotube layers were revealed by Mott-Schottky measurements. Photocatalytic decomposition rates for methylene blue solutions were significantly enhanced for ALD TiO2 coated TiO2 nanotube layers compared to their uncoated or TiO2 nanoparticles - decorated counterparts. A perfect agreement in trends was obtained for photocurrent and photocatalytic results

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).

The utilization of the anodic TiO₂ nanotube layers, with uniform Al₂O₃ coatings of different thicknesses (prepared by atomic layer deposition, ALD), as the new electrode material for lithium-ion batteries (LIBs), is reported herein. Electrodes with very thin Al₂O₃ coatings (∼1 nm) show a superior electrochemical performance for use in LIBs compared to that of the uncoated TiO₂ nanotube layers. A more than 2 times higher areal capacity is received on these coated TiO₂ nanotube layers (∼75 vs 200 μAh/cm²) as well as higher rate capability and coulombic efficiency of the charging and discharging reactions. Reasons for this can be attributed to an increased mechanical stability of the TiO₂ nanotube layers upon Al₂O₃ coating, as well as to an enhanced diffusion of the Li⁺ ions within the coated nanotube layers. In contrast, thicker ALD Al₂O₃ coatings result in a blocking of the electrode surface and therefore an areal capacity decrease.

In the present work we report on the influence of the age of ethylene glycol-based electrolytes on the synthesis of self-organized TiO2 nanotube layers. Electrolytes of different ages, defined by the total duration for anodization, were explored in order to get an insight into how the tube structure changes with the electrolyte age. The results show a strong dependence of the electrolyte age upon the nanotube length and diameter — a phenomenon surprisingly not discussed in existing literature. When fresh electrolytes are employed, nanotube arrays with a high aspect ratio are received, while in older electrolytes (i.e. already used for anodization) the nanotube arrays exhibit low aspect ratios. This is a very important aspect for the reproducible synthesis of the nanotube layers. Moreover, the effect of the potential on the nanotube dimensions was investigated. Linear dependence of the diameter upon the potential was observed. Last, but not least, the influence of a potential change towards the end of the anodization time was studied. By sweeping the potential to 100 V, or to 5 V and keeping this for 1 h after applying a constant potential of 60 V for 4 h, nanotubes underwent interesting morphological changes. In particular, when slow sweeping from 60 V to 5 V was carried out, small nanotubes grew in the gaps between the initial nanotubes. Interestingly, these nanotube layers showed lower adhesion to the underlying substrates