High carrier density and capacitance in TiO2 nanotube arrays induced by electrochemical doping

The paper describes the electronic charging and conducting properties of vertically oriented TiO2 nanotube arrays formed by anodization of Ti foil samples. The resulting films, composed of vertically oriented nanotubes approximately 10 mu m long, wall thickness 22 nm, and pore diameter 56 nm, are analyzed using impedance spectroscopy and cyclic voltammetry. Depending on the electrochemical conditions two rather different electronic behaviors are observed. Nanotube array samples in basic medium show behavior analogous to that of nanoparticulate TiO2 films used in dye-sensitized solar cells: a chemical capacitance and electronic conductivity that increase exponentially with bias potential indicating a displacement of the Fermi level. Nanotube array samples in acidic medium, or samples in a basic medium submitted to a strong negative bias, exhibit a large increase in capacitance and conductivity indicating Fermi level pinning. The contrasting behaviors are ascribed to proton intercalation of the TiO2. Our results suggest a route for controlling the electronic properties of the ordered metal-oxide nanostructures for their use in applications including supercapacitors, dye-sensitized solar cells, and gas sensing.