Roles of Mechanical Stress and Lower-Valent Oxide in the Formation of Anodic TiO2nanotube Layers
Authors: Dou Qi , Shrotriya Pranav , Li Wenfang , Hebert Kurt 
Published: February 2020

Self-organized anodic oxide nanotube layers are produced by anodization of titanium in fluoride-containing electrolyte solutions that dissolve the oxide. Extensive research on TiO2nanotubes has explored applications such as solar cells, photocatalysis, batteries, and biomaterials. However, the growth mechanism of nanotube layers is still actively debated. Nanotube arrays are formed by a morphological instability after an initial period of conformal barrier oxide growth. Stress-driven flow of TiO2in nanotubes is suggested by evidence that the oxide volume expansion exceeds that expected from the stoichiometry of the anodization reaction (Pilling-Bedworth ratio) (1-3). Stress measurements during anodizing can reveal the mechanical driving force for oxide flow-assisted nanotube growth (4).
The initial growth or TiO2nanotube layers was investigated using in situ stress measurements by curvature interferometry (5). Anodization was carried out at constant potentials of 10 to 30 V in typical baths consisting of water and ammonium fluoride in ethylene glycol. Compressive stress was detected during initial growth of barrier oxide at times less than 2 min (Fig. 1). The initiation of nanotubes coincided with a tensile stress increase to about 20 min, followed by a large compressive stress change that continued for the duration of anodization and subsequently on open circuit. The tensile and second compressive transients were also observed on open circuit after interrupted anodization, when the interruption time was after the time of nanotube initiation (Fig. 1). Apparently, the tensile and second compressive transients during anodization are due to a chemical reaction between the solution and the oxide in the nanotube walls. Evidence for non-electrochemical reactions during anodization was also found in measurements of metal consumption, which revealed an apparent Ti oxidation valence of between 2 and 3 at times after nanotube initiation (2). From both types of experiment, we conclude that the direct product of anodization is an oxide with valence lower than 4, which is then chemically oxidized to TiO2in the tube walls by H+ions. The accompanying compressive stress increase is attributed to absorption of charge-compensating anions. Lower-valent Ti oxides play an important role in the electronic and catalytic properties of titanium oxide nanomaterials (6). The previously unrecognized involvement of such oxides in the anodization process itself may suggest new approaches to control the properties of nanotube layers.
REFERENCES

D. J. LeClere et al., J. Electrochem. Soc., 155, C487 (2008).

S. Berger et al., Electrochim. Acta, 54, 5942 (2009).

S. P. Albu and P. Schmuki, Electrochim. Acta, 91, 90 (2013).

Ö. Ö. Çapraz, P. Shrotriya, P. Skeldon, G. E. Thompson, K. R. Hebert, Electrochim. Acta, 167, 404 (2015).

Q. Dou, P. Shrotriya, W. F. Li, K. R. Hebert, Electrochim. Acta, 292, 676 (2018).

S. Mohajernia et al., Chem. Eur. J., 23, 12406 (2017).

ACKNOWLEDGMENT Qi Dou received support from the China Scholarship Council.
Figure 1. Force per width during anodization (solid lines) followed by open circuit (dashed lines). Anodization was at 20 V in ethylene glycol with 0.1 M NH4F and 1.6 wt.% H2O. The legend gives times of current interruptions.

Figure 1