Lithium‐ion insertion in anodic TiO2 nanotubes resulting in high electrochromic contrast

The present work investigates lithium insertion into highly ordered nanotubular layers of TiO2 and its electrochromic effects. The nanotubes were formed by anodization of titanium in 1 M (H3PO4) + 1 M (NaOH) + 0.5wt% HF electrolyte at 20 V. This leads to nanotubular layers with a thickness of 1 ± 0.1 µm, individual tube diameter of 100 ± 10 nm and a tube wall thickness of 10 ± 2 nm. The cathodic behavior of tubes in the “as formed” amorphous phase and annealed to anatase were studied in a solution of 1 M LiClO4 in acetonitrile. Cyclic voltammograms and potential step experiments combined with reflectance measurements show for anatase nanotubes a well defined uptake potential for Li+ and a very strong and reversible electrochromic effect, while on the other investigated TiO2 structures the effects were much less pronounced. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)