Formation of Vertically Oriented TiO₂ Nanotube Arrays using a Fluoride Free HCl Aqueous Electrolyte

Described is the synthesis of TiO2 nanotube array thin films by anodization of Ti foil in an aqueous HCl electrolyte. This process represents an alternative electrolyte that can be utilized instead of fluoride-containing electrolytes. Nanotube arrays up to 300 nm in length, 15 nm inner pore diameter, and 10 nm wall thickness were obtained by using a 3 M HCl aqueous electrolyte for anodization potentials between 10 and 13 V. An anodization voltage of up to 20 V could be used if it was increased stepwise with a resulting nanotube length of approximately 600 nm; however, the resulting nanotubes are not as well-ordered as those fabricated by a constant anodization voltage. The addition of a low concentration of H3PO4 as a buffering medium in the concentration range 0.01−0.1 M expands the anodization voltage up to 14 V but the architectures formed more closely resemble rods than tubes.