Hierarchically branched titania nanotubes with tailored diameters and branch numbers

Over the past decade, electrochemical anodization of self-organized TiO(2) nanotubes has been studied intensively with the main focus being on uniform diameters along the TiO(2) nanotube depth direction. In the present work, hierarchically branched TiO(2) nanotubes with tailored diameters and branch numbers are successfully achieved by adjusting the anodization voltage. Reducing the applied voltage by a factor of 1/√n causes a one trunk nanotube to diverge into n-branched TiO(2) nanotubes, whose diameters are 1/√n of the trunk nanotube diameter (n is an integer). Multiple layers of branched TiO(2) nanotubes are also obtained by further dividing the branched nanotubes when the applied voltage is further reduced step-by-step with a 1/√n factor. Enlargement and termination of TiO(2) nanotubes occur when the anodization voltage increases by √n times. Alternating increase and decrease in the applied voltage lead to a more sophisticated hierarchical structure of TiO(2) nanotubes. The fundamental understanding of these processes is discussed.