Barrier-oxide layer engineering of TiO2 nanotube arrays to get single- and multi-stage Y-branched nanotubes: Effect of voltage ramping and electrolyte conductivity

Single and multi-stage Y-branched TiO2 nanotube arrays (TNAs) have been fabricated by a voltage ramping down process using potentiostatic two-step anodization in 0.5 wt% hydrofluoric acid (HF)/glycerol (1:2 volume ratio) electrolyte. Initially, the voltage is kept at 40 V for 3 h and then it is ramped down to different voltages (e.g. 30 V, 34 V, 36 V, 38 V and 39 V) at a ramping rate of either −1.0 V s−1 or −0.5 V s−1 in one time and two-time aged electrolytes. The growth mechanism of Y-branching of TNAs is modeled and explained in terms of unequal interfacial movements of the two interfaces across the barrier oxide layer (BOL) under non-steady-state growth regime. The 'pinched off' area of the BOL at the nanotube propagation front can be controlled effectively with the relative ramping voltage levels and electrolyte's conductivity to obtain Y-branched TNAs.