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).
In the present work we report on the influence of the age of ethylene glycol-based electrolytes on the synthesis of self-organized TiO 2 nanotube layers. Electrolytes of different ages, defined by the total duration for anodization, were explored in order to get insight about 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 phenomena 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 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 one hour after applying a constant potential of 60 V for 4 hours, 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 nanotubes layers showed lower adhesion to the underlying substrates.
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.
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