Self-organized TiO₂ nanotube (TNT) layers have attracted considerable scientific and technological interest over the past 17 years motivated for their wide range of applications including (photo-) catalysis, hydrogen generation and biomedical uses [1,2]. The synthesis of these TNT layers is carried out by electrochemical anodization of valve Ti metal substrates in various F^--containing electrolytes using a conventional 2-electrode set-up with the Ti substrate as anode and a Pt-foil as cathode.

Instead of using a conventional set-up, also bipolar electrochemistry can be employed for the anodization of valve metals [3-7]. In this set-up, the Ti substrate is placed in an electrolyte between two so-called feeder electrodes, which are connected to the potentiostat. Due to the electric field between the feeder electrodes, the Ti substrate is polarized. If the applied potential between the feeder electrodes is high enough, redox reactions can be driven on the Ti substrate. This method opens new doors for the preparation of TNT layers with gradients in dimensions (i.e. diameter and length of the nanotubes) [3-5] as well as for the preparation of TNT layers on Ti substrates, which cannot be directly connected to the potentiostat, e.g. Ti spheres [8].

In this presentation, the preparation of TNT layers on small Ti spheres [8] as well as on 3D printed Ti substrates [9] using bipolar electrochemistry will be shown and discussed. Furthermore, we will also show the possibility of using closed bipolar electrochemical cells, consisting of two closed half-cells with the Ti substrate as obstacle between the cells [10].

References:

[1] J.M. Macak et al., Curr. Opin. Solid State Mater. 2007, 11, 3.

[2] Lee, K. et al., Chem. Rev. 2014, 114, 9385.

[3] G. Loget et al, J. Mater. Chem A 2014, 2, 17740.

[4] G. Loget et al., Langmuir 2014, 30, 15356.

[5] W. Wei et al., Electrochim. Acta 2015, 176, 1393.

[6] H. Asoh et al., RSC Adv. 2016, 6, 90318.

[7] H. Asoh et al., J. Electrochem. Soc. 2018, 165, C295.

[8] H. Sopha et al., Electrochem. Commun. 2020, 111, 106669.

[9] H. Sopha et al., Nano Lett. 2021, 21, 8701.

[10] H. Sopha et al., ChemElectroChem 2021, 8, 3827.