Effect of electrolyte age and potential changes on the morphology of TiO2 nanotubes

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

Self-organized double-wall oxide nanotube layers on glass-forming Ti-Zr-Si(-Nb) alloys

In this work, we report for the first time on the use of melt spun glass-forming alloys - Ti75Zr10Si15 (TZS) and Ti60Zr10Si15Nb15 (TZSN) - as substrates for the growth of anodic oxide nanotube layers. Upon their anodization in ethylene glycol based electrolytes, highly ordered nanotube layers were achieved. In comparison to TiO2 nanotube layers grown on Ti foils, under the same conditions for reference, smaller diameter nanotubes (~ 116 nm for TZS and ~ 90 nm for TZSN) and shorter nanotubes (~ 11.5 μm and ~ 6.5 μm for TZS and TZSN, respectively) were obtained for both amorphous alloys. Furthermore, TEM and STEM studies, coupled with EDX analysis, revealed a double-wall structure of the as-grown amorphous oxide nanotubes with Ti species being enriched in the inner wall, and Si species in the outer wall, whereby Zr and Nb species were homogeneously distributed

Comparison of photoelectrochemical performance of anodic single- and double-walled TiO2 nanotube layers

In this work, the photoelectrochemical response of single-walled (SW) and double-walled (DW) TiO2 nanotube (TNT) layers is presented. TNT layers were grown on Ti substrates by anodization in two different ethylene glycol-based electrolytes to obtain ~5 and ~15 μm thick TNT layers. The inner shell of the TNT was quantitatively removed via a mild pre-annealing followed by a selective chemical etching treatment in piranha solution. All TNT layers were investigated for their photoelectrochemical response in the ultraviolet and near visible spectral range. Significantly enhanced photocurrent densities were revealed for the SW-TNT layers. This is ascribed to improved charge carrier separation along the tube walls due to the lack of the C- and F-rich inner shell removed by etching. Keywords: Titanium dioxide, Nanotubes, Single-walled, Double-walled, Photoelectrochemistr

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 a high aspect ratio (≈180) anodic TiO2 nanotube layers using 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 sec) of the trimethylaluminium (TMA) precursor were performed. Uniform coating of the nanotube interiors was achieved with longer exposure times (5 and 10 sec), 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 towards the development of many novel devices, based on a high surface area interface between TiO2 nanotubes and a secondary material (such as Al2O3)