Fig. 4. (a) Experimental equipment of accelerated anodic oxidation of TiO2 nanotube
arrays. (b) and (c) Schematic representation of the acceleration mechanisms.
When the anodization was conducted in the electrolyte containing
Na2CO3, the H+ generated during the Ti oxidation at the oxide
layer/Ti substrate would move quickly towards the graphite cathode
under the assistance of the electric field. Hence, the chemical
reaction between CO3
2− and H+ mainly occurred around the cathode
(exhibiting as the bubbles) and only a small part of H+ reacted
with CO3
2− in the vicinity of anode and elsewhere. The parallel
experiment was taken in the electrolyte without Na2CO3 and
almost no bubbles could be observed anywhere. Fig. 4(b) and (c)
showed the mechanisms of the Na2CO3 addition accelerating the
formation of TiO2 nanotube arrays. Timely elimination of H+ by
the CO3
2− (forming the gas CO2) resulted in the drastic reduction
of the H+ concentration situated at the interface of oxide
layer/Ti substrate[26] and would in turn accelerate the Ti substrate
oxidation, causing the forward movement of the interface
into the substrate. Therefore, TiO2 nanotube arrays could grow with
a straightforward rate.
4. Conclusions
In summary, the normal anodization method for the synthesis
of TiO2 nanotube arrays was exhausted with the local acidification
at the pore tips during the entire reaction process as well as
the high-speed of fluoride chemical dissolution of the formed TiO2
nanotubes assisted by theH+. These two factors resulted in the sluggish
formation of TiO2 nanotubes. We tackled these two obstacles
by using the alkaline additiveNa2CO3 into theNH4F/ethylene glycol
electrolyte and the experiment achievement proved our hypothesis.
Thisphenomenonwasmainly due to the opportune suppression
of the local H+ concentration fluctuation, resulted from the spontaneous
ion reaction between the H+ and CO3
2−.
Acknowledgements
This work was supported by the National Natural Science Foundation
of China (Nos. 91023030 and 51072044), the Specialized
Research Fund for the Doctoral Program of Higher Education (No.
20100111110012) and the International Scientific and Technological
Cooperation Project of Anhui Province (No. 10080703017). The
authors would like to acknowledge Mingkuang Kong for the FESEM
measurements. And we also would like to thank the editor and the
reviewers for their kindness, patience and scientific evaluations.
References
[1] T. Maschmeyer, M. Che, Angew. Chem. Int. Ed. 49 (2010) 2–6.