Bipolar anodization enables the fabrication of controlled arrays of TiO₂ nanotube gradients

Abstract

We report here a new concept, the use of bipolar electrochemistry, which allows the rapid and wireless growth of self-assembled TiO 2 NT layers that consist of highly defined and controllable gradients in NT length and diameter. The gradient height and slope can be easily tailored with the time of electrolysis and the applied electric field, respectively. As this technique allows obtaining in one run a wide range of self-ordered TiO 2 NT dimensions, it provides the basis for rapid screening of TiO 2 NT properties. In two examples, we show how these gradient arrays can be used to screen for an optimized photocurrent response from TiO 2 NT based devices such as dye-sensitized solar cells.

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

“…For comparison, tubes with a length and an aspect ratio of ~44 µm / ~317, and ~56 µm / ~337 were observed after an additional 1 hour at 100 V, reached with a sweep rate of 1 V/s and 10 mV/s, respectively. This shows that the nanotubes were growing faster at higher potentials, in line with Figure 4, and previous literature [38][39][40][41][42].…”

“…Subsequently the IR drop becomes lower for older electrolytes and the real potential on the working electrode increased. Since the diameter of the nanotubes strongly depends on the applied potential [38][39][40][41][42][43]46], the diameter of the tubes increased with increasing age of the electrolytes. Evidently, the electrolyte can actually be used several times without any significant issues of lower quality.…”

“…The influence of the anodization potential on the tube diameter and length was shown for early stage electrolytes approximately 10 years ago, for purely aqueous electrolytes [37,38], mixed water:glycerol electrolytes [39], as well as for some ethylene glycol based electrolytes [39][40][41][42]. Recently, Loget et al [40] used bipolar electrochemistry and prepared TiO 2 nanotubes on the bipolar electrode.…”

“…Recently, Loget et al [40] used bipolar electrochemistry and prepared TiO 2 nanotubes on the bipolar electrode. Due to the potential variation over the bipolar electrode, diameter and length of the nanotubes changed over the length of the electrode.…”

Hybrid Energy Storage Devices Based on Monolithic Electrodes Containing Well-defined TiO2 Nanotube Size Gradients

“…Although it has been reported that the nanotube dimensions (e.g., diameter and length) strongly influence their physicochemical properties, [21] fabrication and studies of the TiO 2 nanotube array thin films with size gradients have so far received relatively little attention [22][23][24]. The previous work in which TiO 2 nanotube size gradients were used to rapid screen the tube length dependence with respect to the photoelectrochemical properties clearly constitutes significant progress, [25] even though a small fraction of the nanotubes (i.e., the nanotubes with the optimal length) provided the majority of the photoelectrochemical effects. In the case of energy storage materials, it can be anticipated that the possibility to carefully control and alter the gradients (and the sizes of individual nanotubes within the gradients) would prove even more advantageous.…”