The anodizing is a high-voltage electrochemical conversion process that forms barrier-type oxides or nanoporous/nanotubular layers on valve metals and alloys mainly depending on the composition of electrolyte used.[1] The key to achieve the ordered nanoporous/nanotubular structures is a displacement of the film material above the original surface position due to synergistic effect of pits generation (field assisted oxide dissolution), stress generated at the metal-oxide interface including electrostriction and plastic oxide flow switching the growth of the barrier-film to nanotube/nanopore. Typically, the TiO₂nanotube array formed in organic electrolytes such as glycerol, dimethyl sulfoxide, ethylene-glycol is obtained in the form of close packed structure in which the nanotubes nearly stick together with the tube walls. A fundamentally different nanostructure from that is obtained in diethylene-glycol electrolyte. The nanostructure is characterized by free standing nanotubes separated by an interconnecting space much bigger than the thickness of nanotube wall (Fig. 1). [2,3] Such geometry gives possibility for deposition of secondary materials using electrochemical processes in view of improved wetting behavior of the nanostructure. [4] Herein we report on how to control the synthesis of nanotubes to achieve unique oxide geometries, we discuss functional properties of nanostructures and report an examples of composites formed between oxide layer and Si, CdSe, Au, Pt. An examples of the nanostructured energy storage devices will be demonstrated.[4]

References:

[1] D. Kowalski, D. Kim and P. Schmuki, Nano Today, 8(2013) 235.

[2] D. Kowalski, J. Mallet, J. Michel, and M. Molinari, J.Mater.Chem.A 3(2015) 6655.

[3] D. Kowalski, J. Mallet, S. Thomas, J. Rysz, B. Bercu, J. Michel and M. Molinari, Electrochim. Acta, (2016), 10.1016/j.electacta.2016.02.213.

[4] D. Kowalski, and P. Schmuki, Chem. Comm., 46(2010) 8585.

Fig. 1 SEM and TEM images for TiO₂ nanotubes formed at 60V and 70V for 16h in diethylene glycol electrolyte containing 0.1M HF and H₂O 2-4 wt.%. The right figure shows STEM and TEM images for Si electrodeposited on TiO₂ nanotubes and applied as negative electrode in lithium ion battery.