Bipolar Anodization Enables the Fabrication of Controlled Arrays of TiO2   Nanotube Gradients
  We report here a new concept, the use of bipolar electrochemistry, which allows the rapid and wireless growth of self-assembled TiO2 NT layers that consist of highly defined and controllable gradients in NT length and diameter. The gradient height and slope can be easily tailored with the time of electrolysis and the applied electric field, respectively. As this technique allows obtaining in one run a wide range of self-ordered TiO2 NT dimensions, it provides the basis for rapid screening of TiO2 NT properties. In two examples, we show how these gradient arrays can be used to screen for an optimized photocurrent response from TiO2 NT based devices such as dye-sensitized solar cells. 
G. Loget, S. So, R. Hahn, P. Schmuki
cond-mat.mes-hall cond-mat.mtrl-sci