Transparent highly ordered TiO2 nanotube arrays via anodization of titanium thin films

Titanium thin films, 400 nm to 1000 nm thick, fabricated by radio frequency (rf) sputter deposition are anodized in an electrolyte containing acetic acid and hydrofluoric acid to form optically transparent films of highly ordered titania nanotube arrays. Real-time monitoring of the anodization current, at a fixed potential, is used to controllably eliminate the Ti layer underneath the titanium oxide nanotube array without disturbing the architecture. Fabrication variables critical to achieving the transparent nanotube-array film include annealing temperature of the anodized, initially amorphous nanotube array and Ti-film sputter deposition variables, including rate, film thickness, and substrate temperature. Structural investigations on the transparent nanotube arrays reveal only the presence of the anatase phase even after annealing at 500 degrees C. In contrast, both rutile and anatase phases were observed in films with a metal layer underneath the nanotubes and annealed in an oxygen ambient above 430 degrees C. Rutile growth occurs at the nanotube-metal interface as metal oxidation takes place during annealing. The average refractive index of the transparent nanotube-array film is found to be 1.66 in the UV-vis range, with a calculated porosity of 67 %; the bandgap is determined as 3.34 eV, with a bandgap tail extending to 2.4 eV.