A review on highly ordered, vertically oriented TiO2 nanotube arrays: Fabrication, material properties, and solar energy applications

We review the fabrication, properties, and solar energy applications of highly ordered TiO2 nanotube arrays made by anodic oxidation of titanium in fluoride-based electrolytes. The material architecture has proven to be of great interest for use in water photoelectrolysis, photocatalysis, heterojunction solar cells, and gas sensing. We examine the ability to fabricate nanotube arrays of different shape (cylindrical, tapered), pore size, length, and wall thickness by varying anodization parameters including electrolyte concentration, pH, voltage, and bath temperature, with fabrication and crystallization variables discussed in reference to a nanotube array growth model. We review efforts to lower the band gap of the titania nanotubes by anionic doping. Measured optical properties are compared with computational electromagnetic simulations obtained using finite difference time domain (FDTD). The article concludes by examining various practical applications of the remarkable material architecture, including its use for water photoelectrolysis, and in heterojucntion dye-sensitized solar cells.