EIS study of photo-induced effets on self-organized TiO2 nanotubular arrays
Abstract
Two different morphologies of nano-structured titanium dioxide – a nanotubular layer and a compact layer were obtained by anodization of titanium in fluoride-based baths, and the photo-induced effects of these layers were investigated by voltammetry and electrochemical impedance spectroscopy (EIS). The first layer showed long–lasting photo-induced modifications after UV illumination, whereas, in the case of the compact layer, no longlasting UV-induced modifications were observed. Before light exposure, in the nanotubular layer, only the bottom of the tubes were electro-active and contributed to the conduction of the layer. Moreover an exponential distribution of surface states could be evidenced. After UV exposure, the surface states were filled by the photo-generated electrons, leading to activation of the walls of the tubes and to a hundred fold increase of the space charge layer capacitance. This capacitance increase was attributed to an increase of the active surface in the layer but also to an increase of the charge carrier density.
Keywords: titanium dioxide, anodization, nanotubular structure, compact layer, electrochemical impedance spectroscopy, surface states